EPA 530-R-95-040
PB 95-260287
LONG TERM DISSOLUTION TESTING OF MINE WASTE
Report to the United States Environmental Protection Agency
Grant Number: X-8200322-01-0
Kim Lapakko
Jennifer Wessels
David Antonson
Minnesota Department of Natural Resources
Division of Minerals
500 Lafayette Road, Box 45
St. Paul, MN 55155-4045
March 1995
U S. Environmental Protection Agency
Region 5, library (PL-!2J)
77 West Jackson Boulevard, 12th Floor
Chicago, it 60604-3590
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Disclaimer
This document was prepared by the Minnesota Department of
Natural Resources, Division of Minerals, under a grant from the
U.S. Environmental Protection Agency. Although the Environmental
Protection Agency supported the development of this document and is
making this report available to the public, the information
presented here is solely the product of the authors and does not
necessarily reflect the Agency's position or policy.
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TABLE OF CONTENTS
List of Tables iii
List of Figures v
List of Appendices viii
^ 1. INTRODUCTION 1
!Q 2. OBJECTIVES 2
^ 3. BACKGROUND CHEMISTRY 2
fK 4. METHODS 4
4.1. Mine Waste Samples 4
V 4.1.1. Sample Procurement 4
?j 4.1.2. Sample Screening 4
~~ 4.1.3. Sample Preparation and Solid-Phase Analysis 5
4.2. Procedures 8
4.2.1. Wet-Dry Cycle Test 8
4.2.2. Elevated Temperature Test - 9
4.2.3. Particle Size Experiment - 9
4.3. Drainage Analysis 11
4.4. Calculations 11
5. RESULTS 12
5.1. Mine Waste Particle Size, Chemistry, Mineralogy 12
5.2. Wet-Dry Cycle Test 13
5.2.1. Introduction 13
5.2.2. Acid Producer (TL5) 14
5.2.3. Non-Acid Producers, NP[(Ca/Mg)CO3] Depleted 15
5.2.4. Non-Acid Producers, NP[(Ca/Mg)CO3] Near Depletion 15
5.2.5. Non-Acid Producers, NP[(Ca/Mg)CO3] Remaining and Controlling Drainage
Quality (TL1, TL3, TL6) 16
5.3. Elevated Temperature Test 17
5.3.1. Introduction 17
5.3.2. Strong Acid Producers: TL5, RK4, TL4 18
5.3.3. Moderate (RK3) and Mild (TL2) Acid Producers 20
5.3.4. Intermittent Acid Producers: RK1, RK2 20
5.3.5. Non-Acid Producers: TL1, TL3, TL6 21
5.4. Particle Size Experiment 23
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TABLE OF CONTENTS
(continued)
6. SUMMARY 26
7. CONCLUSIONS 28
ACKNOWLEDGEMENTS 30
REFERENCES 31
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LIST OF TABLES
1. Samples selected for predictive testing 33
2. Average and standard deviation for sulfur and carbon dioxide content of triplicate
sample splits 34
3. Sample mass and rinse volume for the Particle Size Experiment 35
4. Particle size distribution for mine waste samples used in the Wet-Dry Cycle and the
Elevated Temperature Tests 36
5. Chemical analysis for sulfur, carbon dioxide, and major components in mine waste
samples 37
6. Mine waste mineralogy: Sulfur-bearing minerals 38
7. Mine waste mineralogy: Carbonate minerals and neutralization potential present as
calcium and magnesium carbonate (NP[(Ca/Mg)COj]) 39
8. Mine waste mineralogy: Rock forming minerals 40
9. Solid phase analysis for the Particle Size Experiment: Sulfur and carbon dioxide
contents as a function of particle size 41
10. Wet-Dry Cycle Test summary 42
11. Rates of release of sulfate, calcium plus magnesium, calcium, and magnesium for the
Wet-Dry Cycle Test 43
12. Percent depletion of acid production potential (APP) and neutralization potential
(NP[(Ca/Mg)COjJ) for the Wet-Dry Cycle Test for weeks 0 - 132 45
13. Comparison of APP and NP[(Ca/Mg)CO3] release in the Elevated Temperature (weeks
1 - 130/131) and the Wet-Dry Cycle (weeks 1 - 132) Tests 46
14. Elevated Temperature Test summary 47
15. Rates of release of sulfate, calcium plus magnesium, calcium, and magnesium for the
Elevated Temperature Test 48
16. Percent depletion of acid production potential (APP) and neutralization potential
(NP[(Ca/Mg)COj]) for the Elevated Temperature Test 50
iii
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LIST OF TABLES
(continued)
17. Comparison of Elevated Temperature Test empirical neutralization potential (ENP) and
neutralization potential present as calcium and magnesium carbonate (NP
[(Ca/Mg)COJ) 51
18. Percent moisture content of the covered reactors as a function of particle size at day
seven of the drying cycle for the Particle Size Experiment 52
19. Classification of samples based on drainage quality and neutralization potential
(NP[(Ca/Mg)CO3J) remaining in the Wet-Dry Cycle and in the Elevated Temperature
Tests 53
IV
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LIST OF FIGURES
1. Experimental apparatus (reactor) for the Wet-Dry Cycle and the Elevated Temperature
Tests, and the 75 g samples of the Particle Size Experiment 54
2. Drainage sample collection from large experimental reactor for the Particle Size
Experiment 55
3. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL5 in the Wet-Dry Cycle Test (weeks 1-132) 56
4. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK1 in the Wet-Dry Cycle Test (weeks 1-132) 57
5. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK2 in the Wet-Dry Cycle Test (weeks 1-132) 58
6. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK3 in the Wet-Dry Cycle Test (weeks 1-132) 59
7. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK4 in the Wet-Dry Cycle Test (weeks 1-132) 60
8. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL4 in the Wet-Dry Cycle Test (weeks 1-132) 61
9. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL2 in the Wet-Dry Cycle Test (weeks 1-132) 62
10. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL1 in the Wet-Dry Cycle Test (weeks 1-132) 63
11. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL3 in the Wet-Dry Cycle Test (weeks 1-132) 64
12. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL6 in the Wet-Dry Cycle Test (weeks 1-132) 65
13. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL5 in the Elevated Temperature Test (weeks 1-131) 66
14. Scanning electron microscope (SEM) photograph of leached feldspar grain from TL5
in the Elevated Temperature Test (at week 26) 67
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LIST OF FIGURES
(continued)
15. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK4 in the Elevated Temperature Test (weeks 1-130) 68
16. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL4 in the Elevated Temperature Test (weeks 1-131) 69
17. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK3 in the Elevated Temperature Test (weeks 1-130) 70
18. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL2 in the Elevated Temperature Test (weeks 1-131) 71
19. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK1 in the Elevated Temperature Test (weeks 1-130) 72
20. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from RK2 in the Elevated Temperature Test (weeks 1-130) 73
21. Scanning electron microscope (SEM) photograph of leached feldspar grain from a
1.63% S sample similar to RK2 (olivine gabbro) at 289 weeks in a separate dissolution
experiment 74
22. SEM photograph of leached pyroxene (augite) grain from a 1.63% S sample similar
to RK2 (olivine gabbro) at 289 weeks in a separate dissolution experiment 75
23. SEM photograph of leached mica (biotite) grain from a 1.63% S sample similar to
RK2 (olivine gabbro) at 289 weeks in a separate dissolution experiment 76
24. SEM photograph of leached amphibole (probably hornblende) grain from a 1.63% S
sample similar to RK2 (olivine gabbro) at 289 weeks in a separate dissolution
experiment 77
25. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL1 in the Elevated Temperature Test (weeks 1-131) 78
26. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in drainage
from TL3 in the Elevated Temperature Test (weeks 1-131) 79
27. pH and concentrations of net alkalinity, sulfate, calcium, and magnesium hi drainage
from TL6 in the Elevated Temperature Test (weeks 1-131) 80
vi
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LIST OF FIGURES
(continued)
28. SEM photograph of leached pyroxene (hedenbergite) grains from TL6 at 173 weeks
in the Elevated Temperature Test 81
29. Explanation of box plot figure 82
30. Drainage pH and rates of sulfate and calcium plus magnesium release as a function of
particle size from the Particle Size Experiment: RK3 (weeks 15-30) 83
31. Drainage pH and rates of sulfate and calcium plus magnesium release as a function of
particle size from the Particle Size Experiment: RK4 (weeks 15-30) 84
32. Drainage pH and rates of sulfate and calcium plus magnesium release as a function of
particle size from the Particle Size Experiment: RK5 (weeks 15-30) 85
VII
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LIST OF APPENDICES
(separate volume)
Appendix A ~ Solid Phase Characterization
Appendix B -- Wet-Dry Cycle Test
Appendix C -- Elevated Temperature Test
Appendix D -- Particle Size Experiment
Appendix E -- Quality Assurance and Control
Appendix F ~ Contract and Modifications
vni
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1. INTRODUCTION
Kinetic testing is one tool used for the prediction of mine waste drainage quality. In these tests
mine waste samples are subjected to dissolution in the laboratory, and drainage quality observed
is used to predict the quality of drainage from mine wastes in the field. This concept has been
likened to metallurgical testing of drill core samples to predict metal recovery from an ore body
(Lapakko 1990a). However, mine waste drainage quality prediction may be more complicated
than metallurgical testing due to, among other factors, the long time over which mine waste
drainage quality is of concern (Lapakko 1990a). Lawrence et al. (1989) recognized the difficulty
in using short duration tests to predict drainage quality over a longer time period, stating "The
prediction of long term weathering characteristics of a tailing or waste rock will always have
some uncertainty factor if the prediction test is carried out on a practical time scale in the
laboratory."
The drainage from sulfidic rock (or tailings) will become acidic if the rate of acid production
exceeds the rate of acid neutralization. Drainage generated in a kinetic test may be acidic
immediately or within the test duration, and the sample can be classified as an acid producer.
If the drainage is not acidic for a specific test duration (e.g. 12 weeks, 20 weeks, 40 weeks,
etc.), it might remain so if the test were continued indefinitely. That is, the drainage would
remain neutral until all the acid-producing minerals were depleted. However, drainage could
become acidic if the test were continued for a longer period. This would occur if the acid-
neutralizing minerals were depleted while acid-producing minerals remained and oxidized.
Consequently, a kinetic test on a sample that produces non-acidic drainage does not necessarily
indicate that the sample will not produce acid during the decades and centuries following mine
closure.
If a rock contains at least a moderate potential to neutralize acid (neutralization potential or NP)
its initial drainage will probably be neutral, since the dissolution of minerals contributing to NP
will neutralize the acid produced by iron sulfide oxidation. As acid production continues (i.e.,
iron sulfide oxidation continues) the rate of acid neutralization may decrease with an attendant
decrease in drainage pH. tThe period between the initiation of dissolution and acidification of
drainage has been referred to as the lag period. The rate of acid neutralization will decrease if
the effective acid-neutralizing minerals approach depletion, or if coatings form on the surface
of these minerals. If the NP is depleted or rendered unreactive while a significant amount of
reactive iron sulfide minerals remains, the drainage will become acidic. In contrast, if coatings
form on the iron sulfide minerals and the rate of iron sulfide oxidation is adequately inhibited,
the drainage will not acidify.
Empirical demonstration of a lag period may require an extended period of experimentation
(Hedin and Erickson 1988; Miller and Murray 1988). Neutral drainage could be generated over
the relatively short duration of a test but, over a longer period, the neutralization potential could
be depleted with resultant drainage acidification. Thus, the rates of acid production and
neutralization can be determined from kinetic tests, but it may be time consuming to
experimentally determine if the acid neutralizing capacity will be depleted before the acid
producing capacity.
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The time required to deplete the acid production potential (APP) and the NP has been estimated
using the APP and NP of the mine waste and the rates of acid production and acid neutralization
observed in kinetic tests (Lapakko 1990b). This estimation was used to predict the acidification
of drainage under laboratory conditions. The estimation neglected the effects of coating
formation on acid- producing and acid-neutralizing minerals, since it is presently not possible
to quantitatively model this formation with the accuracy required to predict the effects on
drainage quality. Consequently, the quality of drainage from some abandoned mine wastes over
the long term can be assessed only through extended dissolution studies.
The present study employed long term laboratory studies to examine the dissolution of
abandoned mine wastes and the consequent drainage quality. Two dissolution experiments
conducted under a previous grant (Agreement #CX-816270-01-0) were continued beyond the 20-
week time frame initially funded to obtain a test duration of 132 weeks. The Wet-Dry Cycle
Test was conducted under approximately ambient indoor conditions, and the Elevated
Temperature Test was conducted in an oven maintained at 100°C. The present study also
examined the effects of waste rock particle size on drainage quality.
Additional information on the four waste rock (RK1 - RK4) and six tailings (TL1 - TL6) samples
examined in this study is presented in Lapakko (1993). In addition to the results of the initial
testing, the reader is referred to Lapakko (1993) for information on mine waste dissolution
chemistry which may facilitate the uninitiated reader's understanding of the present report.
Selected information from the earlier report is reiterated for the convenience of the reader. A
literature review on predictive testing for mine waste drainage quality is also available (Lapakko
1991).
2. OBJECTIVES
The objectives of this study were as follows.
1. Provide a description of longer term dissolution of mine wastes,
2. Provide data which will facilitate interpretation of shorter term predictive tests,
3. Examine the extent to which acid-producing and acid-consuming components of mine
waste will dissolve in the laboratory, and
4. Examine the effect of particle size on the dissolution of the mine waste.
3. BACKGROUND CHEMISTRY
The majority of acid production by mine wastes is due to oxidation of iron sulfide minerals. As
indicated by reaction 1 (Nelson 1978) and reaction 2 (Stumm and Morgan 1981). Two moles
of acid are produced for each mole of sulfur oxidized.
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FeS(s) + (3/2)H2O + (9/4)O2(g) = FeOOH(s) + 2H+(aq) + SO42'(aq) [1]
FeS2(s) + (5/2)H2O + (15/4)O2(g) = FeOOH(s) + 4H+(aq) +2SO42'(aq) [2]
Dissolution of sulfate minerals such as melanterite and jarosite will also produce acid. It should
be noted that the solubility of jarosite is slight, except at low pH. As was the case for the
sulfide minerals, the dissolution of melanterite yields two moles of acid per mole of sulfate
dissolved. In contrast, the dissolution of jarosite yields 1.5 moles of acid per mole of sulfate
dissolved. The dissolution of sulfate minerals such as gypsum (CaSO4.2H2O), anhydrite
(CaSO4), or barite (BaSO4) will not produce acid.
The most effective minerals for neutralizing (consuming, buffering) acid are those containing
calcium carbonate and magnesium carbonate, examples of which are calcite, magnesite,
dolomite, and ankerite (CaCO3, MgCO3, CaMg(CO3)2, CaFe(CO3)2, respectively). Dissolution
of calcium and magnesium carbonate components neutralizes acid (reactions 3-6). Reactions 3
and 5 are dominant above approximately pH 6.3, while reactions 4 and 6 are dominant below
this pH.
CaCO3(s) + H+(aq) = HCCV(aq) + Ca2+(aq) . [3]
CaCO3(s) + 2H+(aq) = H2CO3(aq) + Ca2+(aq) [4]
CaMg(C03)2(s) + 2H+(aq) = 2HCO3Xaq) + Ca2+(aq) + Mg2+(aq) [5]
CaMg(C03)2(s) + 4H+(aq) = 2H2C03(aq) + Ca2+(aq) + Mg2+(aq) [6]
/
If both iron sulfide minerals and calcium carbonate or magnesium carbonate minerals are present
in mine wastes, the net reaction of the mine waste can be expressed as the sum of acid
producing and acid neutralizing reactions. For example reaction 7 is the sum of reactions 2 and
3. The reaction represents concurrent pyrite (FeSj) oxidation and calcite (CaCO3) dissolution
releasing sulfate, calcium, and alkalinity to solution, and generating FeOOH(s) as a solid-phase
reaction product.
FeS2(s) + (5/2)H2O + (15/4)O2(g) + 4CaCO3(s) =
FeOOH(s) + 2SO42-(aq) + 4Ca2+(aq) + 4HCO3-(aq) [7]
When both iron sulfide and calcium/magnesium carbonate minerals are present, the drainage will
remain neutral to alkaline as long as the rate of acid neutralization equals or exceeds the rate of
acid generation. If the reactive iron sulfide minerals are depleted or rendered unreactive while
calcium/magnesium carbonate minerals remain, the drainage will not acidify. If the carbonate
minerals are rendered unreactive or depleted while reactive iron sulfide minerals remain, the
drainage will acidify. The time required to deplete the neutralizing minerals has been referred
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to as the "lag time" to acid production. Additional discussion on the lag time and its importance
is presented in the introduction.
Dissolution of silicate minerals such as anorthite (reaction 8, Busenberg and Clemency 1976)
and forsterite (reaction 9, Hem 1970) can also neutralize acid, but their dissolution rate and
associated rate of acid neutralization is very slow in the neutral pH range. These minerals
dissolve more rapidly as pH decreases and, therefore, provide more acid neutralization under
acidic conditions. The rate of acid production must be relatively slow for this dissolution to
maintain drainage pH in the neutral range.
CaAl2Si2O8(s) + 2H+(aq) + H2O = Ca2+(aq) + Al2Si2O5(OH)4(s) [8]
Mg2SiO4(s) + 4H+(aq) = 2Mg2+(aq) + H4SiO4(aq) [9]
4. METHODS
4.1. Mine Waste Samples
4.1.1. Sample Procurement
Western Governors' Association (WGA) member states and mining operations within these states
were asked to provide mine waste samples with the following attributes. First, samples with
field drainage quality data were preferred. Second, samples with a marginal potential for acid
production were desirable. (To use only samples with high sulfur to carbonate ratios would be
of limited value since all tests would probably predict acid drainage. Similarly, samples with
very low sulfur to carbonate ratios would be of limited value since all tests would probably
predict alkaline drainage.) Thirdly, a variation in mineralogy and petrology among the samples
was viewed as beneficial. In response to these requests, 16 tailing samples and 20 rock samples
were sent to the Minnesota Department of Natural Resources (MN DNR).
4.1.2. Sample Screening
Of the 36 samples, 16 were of limited value due to lack of field data, similarity with other
samples, reported sulfur and carbonate contents which were widely disparate (see parenthetical
comment in section 4.1.1), and/or excessive oxidation of the sulfides originally present. The
remaining 20 samples (12 tailings and 8 rock samples) were sent to Lerch Brothers, Inc.
(Hibbing, MN) to be analyzed for sulfur, sulfate, and carbon dioxide. These values were used
to estimate the acid production potential (APP) and neutralization potential (NP). Based on these
values, 10 samples which exhibited a range of sulfur and carbonate contents were selected for
submittal to tests for prediction of mine waste drainage pH (table 1).
The samples selected for predictive testing were relatively fresh, that is, they were not highly
oxidized due to environmental exposure. Samples RK1 and RK4 had been exposed to the
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environment for less than two months. Sample RIG had been in a rock pile for two to three
years. Sample RK2 had been stockpiled for about 15 years, but the rock particles collected were
fairly large and were crushed prior to distribution for analysis and predictive testing.
Consequently, most of the rock surface was relatively fresh.
The ages of tailings TL1, TL4, and TL5 are unknown, but their appearance and chemistry
suggest they have not been extensively weathered. Sample TL3 had been in the tailings basin
less than two months, while for TL6 this period was between zero and two years. Sample TL2
may have been in the basin for up to ten years. However, this sample was collected from a
depth of about ten feet and appeared to be relatively unoxidized.
4.1.3. Sample Preparation and Solid-Phase Analysis
The waste rock samples, as received in five-gallon buckets, were all fairly coarse (approximately
minus six inches), and were crushed to a nominal minus-one-inch size to obtain homogeneous
samples. One of the tailings samples (TL4) was crushed to eliminate clumps and the consequent
sample inhomogeneity. For the remaining tailings samples, subsamples were split from the
samples as received using ASTM method E877-82 (Lerch Brothers, Inc.). To quantify the
compositional variation introduced by the splitting procedure, the sulfur content (ASTM E395
using a Dietert furnace) and carbon dioxide content (ASTM E350-89C) of three sample splits
were determined. The standard deviations for the sulfur and carbon dioxide determinations on
the triplicate splits ranged from 0 to 6.8% of the mean value, except for the carbon dioxide
content of samples RK2 and TL4 (50% and 17%, respectively; table 2).
The various splits were distributed for particle size distribution analysis (Lerch Brothers),
chemical analysis (Lerch Brothers and Bondar-Clegg and Company Ltd., Ottawa, Ontario),
mineralogical analysis (Midland Research Center, Nashwauk, MN; previously Hanna Research
Center), Acid-Base Accounting (Sobek et al. 1978) by the Minnesota Department of Natural
Resources, Division of Minerals (MN DNR, Hibbing, MN), and dissolution testing by the MN
DNR (Babbitt, MN).
Lerch Brothers analyzed samples for sulfur, sulfate, and carbon dioxide. Sulfur was determined
with a carbon rod furnace (Dietert) using ASTM 395 and sulfate was determined following a
sodium carbonate leach. Carbon dioxide was analyzed using a gas evolution method (ASTM
E350-89C). Bondar-Clegg analyzed the samples for silicon, major metal components (Al, Ca,
Fe, K, Mg, Mn, Na, P, Ti), trace metals of regulatory interest (Ag, As, Ba, Cd, Cr, Cu, Mo,
Ni, Pb, Zn), and a set of miscellaneous trace metals included in the Inductively Coupled Plasma
Emission Spectrophotometer (ICP) analytical package (Bi, Co, Ga, La, Li, Nb, Sc, Sn, Sr, Ta,
Te, V, W, Y, Zr). Silicon and the major metal components were extracted using borate fusion
and analyzed using Direct Current Plasma. The remaining metals were extracted using a
solution of HF, HC1O4, HNO3, and HC1 and analyzed by ICP.
Mineralogical analyses were conducted by Louis Mattson of the Midland Research Center in
Nashwauk, MN. X-ray diffraction (XRD, Phillips Electronic Instruments Inc.) was used in
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conjunction with chemical analyses for mineral identification as well as for determination of the
approximate modal composition. This analysis was applied to the bulk sample and to a heavy
mineral concentrate which was separated using a Haultain Superpanner (Infrasizers Ltd.). The
heavy mineral concentrate (specific gravity greater than about 3.5) was analyzed to more
accurately identify the sulfides, siderite, and other heavy minerals present. This was necessary
since the sulfide content of some samples was quite low.
The carbon dioxide content was used to determine the total carbonate mineral content. The
carbonate minerals present were identified by XRD, and checked by scanning electron
microscopy (SEM, Amray model 1200B) and energy dispersive spectroscopy (EDS, Noran
Instruments model 2010). For some samples, optical microscopy (standard Zeiss petrographic
microscope) was used for additional verification. The relative amounts of carbonate minerals
present were determined by interpretation of the major peak heights based on the analyst's
twenty years of experience. The formula for dolomite used by Midland Research Center was
30% CaO, 22% MgO, and 48% CO2, which represents equal molar amounts of calcium
carbonate and magnesium carbonate. The corresponding contents for ankerite were 30%, 15%,
and 45%, with the remaining 10% composed of iron oxide. The values for dolomite
composition are theoretical while those for ankerite are "typical" values. The potential error in
the XRD mineral determinations was estimated as 20%-.
The extent of sulfide and carbonate mineral liberation was determined by wet screening on 100,
270, and 500 mesh sieves, and using optical microscopy to examine the fractions separated. The
coarse waste rock fragments were examined with the unaided eye, a hand lens, and/or a
binocular microscope to qualitatively assess the surface area of sulfide and carbonate minerals
available for reaction in the large particles.
Acid-base accounting (ABA, Sobek et al. 1978) was conducted on all samples. Sulfur content
was determined by Lerch Brothers, Inc. and neutralization potential was determined by the MN
DNR Hibbing Laboratory. These analyses were also conducted on one of the duplicate samples
after 24 weeks of the Wet-Dry Cycle Test and after 24 (waste rock) or 26 weeks (tailings) of
the Elevated Temperature Test.
Selected samples were analyzed to identify 1) non-carbonate minerals which dissolved to
neutralize acid and 2) potential coating formation on sulfide or carbonate mineral surfaces.
Samples of TL5, RK1, RK2, and TL6, both unleached and leached for 24 or 26 (TL6) weeks
in the Elevated Temperature Test, were analyzed using XRD and SEM. Also analyzed were a
TL6 sample leached for 173 weeks in the Elevated Temperature Test, and a sample which was
similar to RK2 and had been leached for 289 weeks at room temperature.
The X-ray patterns of leached samples were compared to those generated by the unleached
samples. Specific feldspar and pyroxene minerals were identified, as well as the percentages
of these minerals and their physical characteristics (e.g., particle size). The +200 mesh fraction
of the leached samples was separated by wet sieving and grams were selected for examination
by SEM. However, the only -1-200 mesh particles in the leached RK1 sample were
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agglomerates of mineral grains, therefore SEM analysis was conducted on a bulk mount of this
sample.
In order to increase the probability of detecting changes in primary silicate minerals, a more
highly weathered sample was selected for analysis. XRD analysis of the unleached rock
indicated it was mineralogically similar to RK2 (olivine gabbro). The leached sample (1.63%
S) was taken from a previous Wet-Dry Cycle experiment after 289 weeks of dissolution. At this
time the drainage pH was between 4.0 and 4.1, and the non-carbonate acid neutralization was
estimated to be equivalent to that resulting from dissolution of 1.1 grams of calcite (measured
as 0.75 grams after 150 weeks of dissolution).
Waste rock samples RK3, RK4, and RK5 were selected for the Particle Size Experiment. RK3
and RK4 were the same solids as in the preceding two tests. RK5 was chosen since it was
mineralogically similar to, and more readily available than, RK2; it did have a higher sulfur
content than RK2 (1.63% vs 0.64%). After receipt for the initial experimental work, these
rocks were crushed to a nominal minus-one-inch size to ensure the distribution of
compositionally uniform samples (Lapakko 1993). The six size fractions arbitrarily selected for
examination were: -270 mesh, +270/-100 mesh, +100/-35 mesh, +35/-10 mesh, -1-10 mesh/-
0.25 inch, and +0.25/-0.75 inch. The corresponding sizes in millimeters were: d < 0.047 mm,
0.047 < d < 0.149 mm, 0.149 < d £ 0.5 mm, 0.5 < d ^ 2.0 mm, 2.0 < d <, 6.25 mm,
6.25 < d < 19.05 mm.
Size fractions were separated by subjecting rock samples (150 - 200 g) to 15 minutes on a ro-tap
apparatus loaded with U.S. standard mesh sieves. (Due to a missing 10 mesh sieve of
compatible size, the -10/ +12 mesh was separated from the -1/4-inch/ -I-12 mesh fraction by hand
sieving.) After the separation, the various size fractions were wet sieved by placing
approximately 150 g of solid on the plus size sieve and rinsing it repeatedly with tap water,
agitating the rock while rinsing. When the water ran clear, the rock was rinsed with distilled
water. The size fractions of RK3 and RK4 rocks were then oven dried at about 38°C, and the
RK5 rock was air dried. The drying times ranged from one to three days. Portions of each size
fraction were retained for dissolution testing to assess the effect of particle size on waste rock
dissolution.
A split of each particle size was submitted to Midland Research Center for chemical and
mineralogical analysis. Sulfur, sulfate, and evolved carbon dioxide were determined for each
size fraction. In an attempt to identify and quantify the relative surface areas of acid-producing
and acid-consuming minerals, all particle size fractions were analyzed microscopically. As an
alternative method the various size fractions were leached with 6N HC1, and the leachate was
analyzed for sulfur, iron, calcium, magnesium, and silicon. Additional X-ray patterns were
obtained for RK5 to confirm similarity to RK2 and to identify potential acid neutralizing phases.
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4.2. Procedures
4.2.1. Wet-Dry Cycle Test
For the Wet-Dry Cycle Test (Lapakko 1988) rock samples were crushed to -100 mesh, tailing
sample TL4 was crushed to eliminate clumps and the consequent sample inhomogeneity (see
section 4.1.3), and the remaining tailing samples were run as received. Samples, run in
duplicate for the first 24 weeks, were placed'into the upper segment, or reactor, of a two-stage
filter unit (figure 1). (After 24 weeks, one of each pair of duplicate reactors was terminated and
the leached sample was subjected to ABA analysis, as described in section 4.1.3.) A glass fiber
filter, onto which the sample was placed, rested on a perforated plastic plate within the reactor.
Prior to the inception of the experiment all samples were rinsed with three distilled water
volumes of 200 rnL, to remove oxidation products which accumulated during sample storage.
To each reactor, 200 mL of distilled water was added and allowed to drain overnight through
the mine waste sample. Once the experiment started, single 200-mL rinses were repeated
weekly for 132 weeks.
The volume of rinse water recovered was determined by weighing. The drainage was analyzed
on site to determine pH, alkalinity (if pH > 6.3) or acidity, and specific conductance. Samples
were then filtered for subsequent determination of sulfate, calcium, and magnesium
concentrations. Samples taken for metal analyses were acidified with 0.2 mL AR Select nitric
acid (Mallinckrodt) per 50 mL sample.
Between rinses the solids were retained in the reactors and stored in an uncovered box to further
oxidize. A thermostatically controlled heating pad was placed beneath the box to control
temperature. The box was stored in a small room equipped with an automatic humidifier and
an automatic dehumidifier, to maintain a stable range of humidity. Temperature and relative
humidity were monitored two to three times a week. The temperature ranged from 19.4 to
30.0°C, with an average of 25.3°C and a standard deviation of 1.92°C (n = 451). The relative
humidity ranged from 30.0 to 68.0%, with an average of 52.3% and a standard deviation of
5.79% (n = 434, see also appendix B).
Experimental modifications were necessary for some of the samples, due to accretion of the
solids and/or low flow through the solids bed. These problems were apparently due to both
particle size and mineralogic factors. During the drying cycle the grams of samples RK1, RK3,
and RK4, tended to cement and subsequently the solid bed would crack. To achieve more
uniform flow through the bed it was necessary, prior to rinsing, to remove the consolidated
solids from the reactor and break them up, or mix the solids with a stainless steel spatula in the
reactor. Despite a slight cementation of the TL4 grains, these solids were not mixed during the
first 63 weeks; the solids were mixed in subsequent weeks to increase flow through the bed.
Despite these preparations there were tunes when all of the rinse water did not pass through the
solids, and it was necessary to decant from the top of the TL4 tailings bed and add it to that in
the receiving flask. The volume decanted from RK3 and RK4 was quite small, approximately
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2% of the total sample volume, while the corresponding percentages decanted for RK1 and TL4
were about 15% and 21%, respectively.
4.2.2. Elevated Temperature Test
For the Elevated Temperature Test (modified from Renton 1983; Rentonet al. 1985, 1988), rock
samples were crushed to -100 mesh, tailings sample TL4 was crushed (see section 4.1.3), and
the remaining tailings samples were run as received. Samples, run in duplicate for the first 24
or 26 weeks, were placed into the upper segment, or reactor, of a two-stage filter unit (figure
1). After 24 (waste rock) or 26 weeks (tailings), one of each pair of duplicate reactors was
terminated, and the leached sample was subjected to ABA analysis, as described in section 4.1.3.
Prior to the inception of the experiment all samples were rinsed with three distilled water
volumes of 200 mL, to remove products which accumulated from oxidation during sample
storage. The solids were subsequently rinsed every two weeks for 130 to 131 weeks. To each
reactor 200 mL of distilled water, heated to 85 °C, was added and allowed to drain overnight
through the mine waste sample. (The water was heated to simulate the soxhlet extraction rinsing
used by Renton (1983).) The procedure was repeated on the following day for additional
recovery of the oxidation products. Specific conductance was determined for each of the two
samples. The two samples were composited, weighed to determine total volume, and analyzed
on site for pH, alkalinity (if pH > 6.3) or acidity, and specific conductance. Samples were then
filtered for subsequent analysis of sulfate and metals. Metals were acidified with 0.2 mL AR
Select nitric acid (Mallinckrodt) per 50 mL sample.
The solids were retained in the reactors and stored in a Thelco Precision Scientific oven.
Temperature ranged from 82.0°C to 120.0°C, averaging 97.3°C, with a standard deviation of
4.07°t: (n = 473, see also appendix C). During the drying cycle the grains of samples RK1,
RK3, and RK4, tended to cement and crack. Consequently, it was necessary to either remove
the consolidated solids from the reactor and break them up, or mix the solids with a stainless
steel spatula in the reactor prior to rinsing. Despite a slight cementation in the grains of TL4,
the solids were not mixed during the first 59 weeks. The degree of TL4 cementation increased
and the solids were mixed in subsequent weeks to increase flow through the bed. Despite these
preparations there were times when all of the rinse water did not pass through RIG, RK4, and
TL4, and it was necessary to decant from the top of the bed. The decant water was then added
to that which passed through the solids. The volume decanted from RK3 and RK4 was less than
3.5 % of the total sample volume collected. Decanting contributed about 25 % of the total sample
volume from TL4 in this test.
4.2.3. Particle Size Experiment
For the Particle Size Experiment, the -270, +270/-100, and + 100/-35 mesh fractions were
leached with the apparatus and methods used for the Wet-Dry Cycle Test (figure 1). For larger
size fractions (+35/-10 mesh, +10 mesh/-0.25 inch, and +0.25/-0.75 inch) a clear cylindrical
acrylic reactor was used. The reactor measured 4.0 inches in diameter, 7.5 inches in height,
and was equipped with a 1/8-inch outlet port and a cover with a 1/8-inch vent hole. The solids
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rested on a glass fiber filter placed on the bottom of the reactor (figure 2). The mass of rock
used in the larger reactors varied with particle size, as did the volume of distilled water added
to the reactors for the weekly rinses (table 3). Due to the limited mass of RIG available, the
+0.25 inch/-0.75 inch fraction of this rock was omitted, and 500 g rather than 1000 g of the
+ 10 mesh/-0.25 inch fraction was used in the experiment.
At the start of the experiment (week 0) the solids were rinsed between four and seven times to
remove reaction products which had accumulated since the samples were rinsed during sieving.
The rinse water was analyzed for specific conductance to provide an indicator of the decreasing
masses of oxidation products removed from the solids (see appendix D for results).
Subsequently, the solids were rinsed weekly for 30 weeks with a volume sufficient to completely
submerge the solids and then allowed to drain freely (table 3). One of the reactors ( + 1/4 inch/-
3/4 inch RK4) drained rapidly after week 19, apparently due to a hole in the glass fiber filter,
and the reactor outlet was plugged while the rinse water was added. After 10 minutes the water
was allowed to drain into the receiving flask. The volume of drainage and the volume of rinse
water retained by the solids were determined by weighing. The drainage was analyzed on site
to determine pH, alkalinity (if pH > 6.3) or acidity, and specific conductance. Samples taken
for sulfate and metals analysis were filtered through a 0.45-micron filter. Metals were acidified
with 0.2 mL AR Select nitric acid (Mallinckrodt) per 50 mL sample.
Between rinses the solids were stored in the reactors in a room in which temperature and
humidity were controlled. The smaller reactors were stored in the same box used to store the
Wet-Dry Cycle Test reactors. For the larger reactors, a thermostatically controlled heating mat
was placed onto each of two shelves and covered with a piece of perforated tag board. Each
of the larger reactors was supported 3.8 cm (1.5 inches) above the tag board by a pair of
wooden spacers. The humidity was maintained within a stable range with an automatic
humidifier/dehumidifier. Temperature and relative humidity were monitored three to four times
weekly. The temperatures for the box, lower shelf, and upper shelf were fairly uniform, with
mean values of 24.4°C, 24.1°C, and 23.8°C, respectively. Relative humidity was more
variable, with corresponding average values of 57%, 52%, and 51% (see also appendix D).
Between week 0 and week 1 the reactors were left uncovered and weighed on a daily basis to
determine the variation in water retained over time. Due to the wide discrepancy of drying
among reactors, it was determined that the covers be left on the reactors between rinses, except
for reactors containing the -270 fractions of RK3 and RK4, which would not drain under wet
conditions. The water remaining on top of these solids was pipeted from the reactor and added
to the drainage before analysis. The percentage of the total volume decanted from the -270 mesh
fractions of RK3 and RK4 was approximately 62% and 39%, respectively. It should be noted
that, based on the weekly weights and the presence of moisture on the inside walls of the
covered reactors, it was assumed that the relative humidity was near 100%. To determine the
influence of the apparatus and procedure on drainage quality, 300 milliliters of distilled water
was added weekly to a large reactor equipped with a glass fiber filter to produce a blank sample.
10
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Two peripheral experiments were conducted to examine the effects of the reactor type used and
to compare results from covered and uncovered reactors. The -I-100/-35-mesh size fraction of
each rock was run in both a small (75 g) and large reactor (225 g) to determine if reactor type
had any effect on drainage quality. Covered and uncovered reactors were compared using the
+ 1/4 inch/-3/4 inch RK4 and the +270/-100 mesh, +35/-10 mesh, and +1/4 inch/-3/4 inch
fractions of RK5. Results of these experiments are presented in appendix D.
4.3. Drainage Analysis
The drainage quality samples were analyzed on site to determine pH, alkalinity (if pH > 6.3)
or acidity, and specific conductance. An Orion SA 720 pH meter, with a Ross combination pH
electrode (8165), was used for pH determinations. Alkalinity and acidity were analyzed using
standard titration techniques (APHA et al. 1992). A Myron L conductivity meter was used to
determine specific conductance. Most of the sulfate and all metals analyses were conducted at
the MN DNR Minerals laboratory in Hibbing. Except for Particle Size Experiment samples
after week 16, sulfate was determined at the MN DNR laboratory using an HF Scientific DRT-
100 nephelometer for the barium sulfate turbidimetric method (APHA et al. 1992). Sulfate
determinations on samples collected from the Particle Size Experiment after week 16 were
analyzed using ICP, at Midland Research Center. Calcium and magnesium were determined
with a Perkin Elmer 603 atomic absorption spectrophotometer in flame mode. Data were
checked by examining concentration variation over time, relationships of concentrations with
conductance, and charge balances. Samples for which concentrations were anomalous were
reanalyzed. If reanalysis was not possible (for example, due to inadequate sample volume) the
anomalous values were deleted from the data tables and footnoted.
4.4. Calculations
The neutralization potential present as calcium carbonate and magnesium carbonate
(NP[(Ca/Mg)CO3]) expressed as kg/t CaCO3 was calculated as indicated below. Error bars for
NP[(Ca/Mg)CO3] = 10 x (%CaCO3) + 11.9 x (%MgC03) [10]
the values determined were established by different methods, all of which accounted for error
contributed by splitting and determination of total carbon dioxide content. For most of the
samples (RK1, RK2, RK4, TL1, TL2, TL4, TL5, TL6) the upper error bar was determined by
assigning the maximum potential carbonate content, as indicated by carbon dioxide
determination, to neutralizing minerals. The lower error bar for these samples (minimum
amount of carbonate occurring with calcium and magnesium) was calculated as the minimum
total carbonate content minus the maximum possible carbonate occurring with iron. For all of
these samples except TL6, the maximum iron carbonate content was calculated based on the
presence of less than 0.5% siderite (i.e., not detected) in the heavy mineral fractions. For TL6
a maximum iron carbonate content of 0.5% in the entire sample was used. For samples
containing substantial siderite, RIG and TL3, a potential 50% error in calcium carbonate and
magnesium carbonate analysis was used.
11
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The masses of sulfate, calcium, and magnesium released were calculated as the product of the
observed concentration in the drainage and the drainage volume. Missing concentrations were
estimated by quadratically smoothed interpolation of the previous and subsequent concentrations.
For each reactor sulfate, calcium, and magnesium release rates were calculated for numerous
periods. Cumulative sulfate release over time was plotted for each reactor. Periods of linear
sulfate release were selected based on visual examination of the plots, and the release rate for
each period was determined by linear regression. For calcium and magnesium release rates,
linear regression analyses were conducted over the same periods.
Empirical neutralization potentials (ENP) were calculated to determine the acid-neutralizing
mineral dissolution prior to drainage pH decreasing below 6 and remaining in this range. The
acid-neutralizing mineral dissolution was calculated as the sum of the cumulative calcium and
cumulative magnesium released (expressed as kg/t CaCO3) prior to the point at which drainage
pH decreased below pH 6. The release was calculated both including and excluding the calcium
and magnesium attributed to reaction products accumulated during sample storage. If the pH
of drainage from a solid never decreased permanently below 6, the ENP was reported as
"greater than" the sum of the total calcium and magnesium release for the period of record. The
pH of drainages from RK1 and RK2 in the Elevated Temperature Test decreased below 6 for
periods of 52 and 32 weeks, respectively, and then increased above pH 6. The ENP values for
these solids were calculated as the total calcium and magnesium released prior to these extended
periods.
5. RESULTS
/
5.1. Mine Waste Particle Size, Chemistry, Mineralogy
The particle size distributions of the various samples used in the Wet-Dry Cycle and Elevated
Temperature Tests are presented in table 4. The sulfur content of the samples, most of which
occurred as sulfide, ranged from 0.46% to 5.81 %. Carbon dioxide concentrations, which reflect
the carbonate mineral content, ranged from virtually zero to about 4% (table 5). The major
metals present in the samples, in a general order of decreasing abundance, were silicon,
aluminum, iron, potassium, magnesium, and calcium.
The predominant sulfur-bearing minerals in the samples were pyrite and pyrrhotite, and
marcasite was detected in four samples (table 6). Lesser amounts of trace metal sulfides were
detected in all samples. The most frequently encountered carbonate minerals were calcite,
dolomite, and siderite (table 7). Ankerite, magnesite, and possibly rhodochrosite were each
detected in one or two samples. Quartz, feldspar, and mica were the major rock-forming
minerals (table 8).
The sulfide and carbonate minerals in all the tailings samples were well liberated due to the
small particle size. Visual inspection of the waste rock samples, as received, for assessing the
availability of sulfide and carbonate mineral surfaces indicated that RK1 was partially oxidized.
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Most of the sulfides occurred on fracture surfaces and would, therefore, be readily available for
oxidation even in relatively large rock particles. The sulfide minerals in RK2 were reported as
"fine grained and [to] occur included in or interstitial to relatively coarse grained rock forming
minerals." The sulfide minerals on the surface of larger rock particles would be available for
oxidation, but those within the rock matrix would oxidize very slowly.
RK3 was reported to be "generally friable"; that is, the rock is amenable to physical breakdown.
The occurrence of both coarse and fine grained sulfide minerals was both disseminated and in
veinlets. The physical breakdown of this rock would leave the sulfide minerals available for
oxidation. Both the sulfide and carbonate minerals in RK4 occurred with quartz in veinlets.
However, the sulfide veinlets were "relatively open and porous" as opposed to the "tight"
veinlets containing carbonate minerals. This suggests that in larger rock particles, the sulfide
minerals would be accessible to air and water, and therefore available for oxidation. In contrast,
the tight structure of the carbonate veinlets would limit the reactivity of the carbonate minerals
present in larger rock particles.
For each of the three rock samples used in the particle size experiment, sulfur and carbonate
contents varied with particle size (table 9). No pattern to the variation was obvious.
5.2. Wet-Dry Cycle Test
5.2.1. Introduction
Mine waste drainage quality prediction has at times been empirically based on the quality of
drainage observed in kinetic tests, such as the Wet-Dry Cycle Test. That is, it is assumed that
the quality of drainage generated in the kinetic test simulates the quality of drainage from mine
wastes in the field. However, due in part to the relatively short test duration, the quality of
kinetic test drainage from some samples may not be representative of that generated over the
long period following mine abandonment. In particular, calcium and/or magnesium carbonate
minerals may survive throughout a kinetic test, with their dissolution maintaining a non-acidic
drainage. However, over a longer period these acid neutralizing minerals may be depleted or
rendered essentially unreactive by coatings, while acid producing minerals remain and oxidize,
with a consequent acidification of drainage.
Samples from the Wet-Dry Cycle Test were classified based on the observed drainage pH and
concentrations of alkalinity, sulfate, calcium, and magnesium. The classifications were
discussed hi light of calculated residual solid-phase calcium and magnesium carbonate
(NP[(Ca/Mg)CO3]). The resultant groups were as follows.
Acid producers (TL5)
Non-acid producers with
NP[(Ca/Mg)CO3] depleted (RK1, RK2)
NP[(Ca/Mg)CO3] near depletion (RIG, RK4, TL2, TI>t)
NP[(Ca/Mg)CO3] remaining and controlling drainage quality (TL1, TL3, TL6)
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The relative amount of calcium carbonate and magnesium carbonate minerals present in a sample
was reflected by its drainage composition. Samples with an abundance of these minerals
produced drainage with elevated pH and concentrations of calcium, magnesium, and alkalinity
despite continued iron sulfide mineral oxidation (see reaction 7). As the calcium carbonate and
magnesium carbonate mineral dissolution continues and these minerals approach depletion,
drainage pH and concentrations of calcium, magnesium, and alkalinity tend to decrease. Caution
must be used when interpreting drainage quality data since decreases in carbonate mineral
dissolution may also be caused by decreasing iron sulfide oxidation and the attendant acid
production. Furthermore, iron sulfide oxidation rates, as indicated by sulfate release, typically
decreased over time in the Wet-Dry Cycle Test. The presence (or absence) of calcium and
magnesium carbonate minerals qualitatively implied by drainage quality was compared to the
residual NP[(Ca/Mg)CO3] calculated from solid-phase analysis and the mass of calcium and
magnesium released.
In addition to drainage quality and NP[(Ca/Mg)CO3], acid production potential (APP), empirical
neutralization potential (ENP), and release rates of sulfate, calcium, and magnesium are also
presented to describe the dissolution of the samples. For convenience of data presentation the
APP values presented were calculated based on total sulfur content (Sobek et al. 1978), and
slightly exceed the more accurate APP values calculated based on sulfide sulfur. However, the
extent of overestimation is small since almost all of the sulfur was present as sulfide (table 5).
The ENP represents the observed potential of the sample to neutralize acid, as indicated by
calcium and magnesium release. Wet-Dry Cycle Test results are summarized in table 10.
5.2.2. Acid Producer (TL5)
/
All of the carbonate NP was depleted from TL5 (158 kg/t CaCO3 APP, 15 kg/t CaCO3
NP[(Ca/Mg)CO3]), the only sample which generated drainage pH values below 6.0 (figure 3).
The TL5 drainage pH was above 6.0 for the week 0 rinses, decreased to a typical range of 2.7-
2.8 between weeks 20 and 60, and subsequently remained between 2.8 and 3.0. Sulfate
concentrations decreased until week 90 and then plateaued at about 100 mg/L. At this time the
rate of iron sulfide oxidation, as inferred by sulfate release, was 0.21 millimoles/week (tables
10, 11). Concentrations of calcium and magnesium decreased and plateaued at low levels (figure
3), reflecting the depletion of the dolomite and magnesite reported to be present (table 12). The
continued slow release of calcium and magnesium was apparently due to the dissolution of
noncarbonate host rock minerals.
The acidic drainage is consistent with the presence (as indicated by the sulfur remaining) and
oxidation of iron sulfides after the depletion of NP. By the end of the 132-week experiment
58% of the sulfur originally present remained. At this time the NP[(Ca/Mg)CQj] had long been
depleted. Comparison of calcium release with solid-phase chemistry (table 5) indicates that
virtually all of the calcium (including that present in noncarbonate minerals) was leached from
the sample by week 132.
14
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The high rate of acid production (inferred by the sulfate release rate) and low rate of acid
neutralization (inferred from the sum of calcium and magnesium release rates) were consistent
with the low drainage pH. Over the last third of the period of record, the rate of sulfide
oxidation, and attendant acid production, was 2 to 30 times that of the other samples (tables 10,
11). In contrast, the rate of calcium and magnesium release from TL5 (along with RKl and
RK2) was the lowest of the samples examined (tables 10, 11).
5.2.3. Non-Acid Producers, NP[(Ca/Mg)CO3] Depleted (RKl, RK2)
The small amount of calcium and magnesium carbonates initially present in samples RKl and
RK2 also appeared to have been depleted (respectively 14, 20 kg/t CaCO3 APP; 3, 1 kg/t
CaCO3 NP[(Ca/Mg)C03]). Next to TL5 these samples produced the lowest pH values in the
Wet-Dry Cycle Test. The pH values observed were initially near 7.5, decreased below 7.0 after
25 to 45 weeks of dissolution, plateaued in the range of 6.5 to 7.0, and increased slightly over
the last 40 to 50 weeks of the experiment. The release of sulfate, calcium, magnesium, and
alkalinity decreased over time (figures 4, 5; table 11).
The temporal trends of pH, alkalinity, calcium, and magnesium in these two drainages are
consistent with decreasing calcium/magnesium carbonate mineral dissolution. The decrease in
calcium/magnesium carbonate mineral dissolution was apparently due to depletion of the
carbonate minerals initially present. This contention is supported by mass release calculations,
which indicate the NP[(Ca/Mg)CO3]) was depleted by week 132 (table 12).
The rates of sulfate release, and attendant acid release, from RKl and RK2 (along with TL1)
were the lowest observed, reflecting the low sulfur content of these samples (tables 10, 11). The
dissolution of silicate minerals may have neutralized some, if not all, of the acid released at
these slow rates. (For additional discussion of silicate mineral dissolution, see the last paragraph
of section 3.) These samples contained 24% and 54% feldspar, respectively, and lesser amounts
of chlorite and mica; respective pyroxene and olivine contents of 18% and 11% were reported
for sample RK2 (table 8). The rates of acid neutralization by silicate mineral dissolution were
enhanced by the fine nature of these samples, 63 to 85 percent of which were less than 0.053
mm in diameter (table 4). Furthermore, the particle size reduction required for the experiment
artificially enhanced the rate of acid neutralization relative to that which would occur for coarser
waste rock particles in the field. Whereas particle size reduction also may have increased the
iron sulfide mineral area, the extent of this increase was apparently less than that for the silicate
minerals. This is consistent with mineralogical analyses which indicate that the silicate mineral
grains in this rock are larger than the sulfide mineral grains.
5.2.4. Non-Acid Producers, NP[(Ca/Mg)CO3] Near Depletion (RK3, RK4, TL2, TL4)
Drainage quality data also indicated that calcium and magnesium carbonate NP was nearing
depletion for samples RIG, RK4, TL2, and TIA (respectively 51, 91, 47, 72 kg/t CaCO3 APP;
5, 32, 16, 6 kg/t CaCO3 NPKCa/MgXTOj]). Although these samples did not produce acidic
drainage, the drainage pH, alkalinity, and (except for TL2) calcium and magnesium
15
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concentrations tended to decrease near the end of the period of record (figures 6, 7, 8, 9). This
was also the case for the rate of calcium plus magnesium release relative to sulfate release (table
11). These trends suggest that these samples may be nearing the end of their "lag period." The
NP[(Ca/Mg)CO3] remaining in samples RK3, TL2, and TL4 is consistent with this contention.
The rates of sulfate release from these four samples at the end of the experiment was two to
seven times that for RK1 and RK2 (tables 10, 11). Unlike RK1 and RK2, noncarbonate host
rock mineral dissolution may not be capable of neutralizing the acid produced at these higher
rates. Consequently, drainages from these samples may acidify when the calcium and
magnesium carbonates are depleted.
At week 132 the calculated NP[(Ca/Mg)CO3] of RK3 was no more than about 1 kg/t CaCO3.
The rate of acid production, as indicated by sulfate release, from this sample was about twice
that from RK1 and RK2. Assuming some calcite remains in this sample, the more rapid rate
of acid production observed may lead to acidification of the drainage from this sample. That
is, the dissolution of host rock minerals may not be fast enough to neutralize the acid produced.
The pH of drainage from RK4 decreased over time, a trend which was most pronounced from
80 to 130 weeks (figure 7). Drainage alkalinity decreased from 25 to 5 mg/L as CaCO3 over
this one-year period, which is also consistent with decreasing availability of carbonate minerals.
As further support for this contention, calcium and magnesium release rates decreased over time,
a trend which would be expected to accompany the depletion of dolomite from the sample
(figure 7, table 11). Furthermore, the molar rates of calcium and magnesium release were
approximately squal throughout the experiment (table 11), which supports the contention that
their release was due to dissolution of dolomite (CaMg(CO3), see reactions 5, 6). Whereas the
release rate of sulfate also decreased, the extent of decrease was not as great as that for calcium
and magnesium. The NP[(Ca/Mg)CO3] of this sample remaining after 132 weeks of dissolution
was calculated as 10 to 15 kg/t CaCO3. The calcium and magnesium carbonates remaining may
have been rendered unreactive due to coating by iron oxyhydroxide precipitates. The apparent
remaining NP[(Ca/Mg)CO3] may also be an artifact of cumulative errors in solid-phase and
drainage analyses, as well as sample splitting.
Decreases in drainage pH, alkalinity, and the ratios of calcium plus magnesium to sulfate release
were also observed for samples TL4 and TL2 after about 100 weeks of dissolution (figures 8,
9). The trends for these samples were more subtle than those observed for RK4. The remnant
NP[(Ca/Mg)CO3] values for TL4 and TL2 after 132 weeks of dissolution were calculated as 0
to 2 kg/t CaCO3 and 0 kg/t CaCO3, respectively (table 10). Drainage from these samples may
acidify upon dissolution of the small amount of calcium and magnesium carbonates remaining.
5.2.5. Non-Acid Producers, NP[(Ca/Mg)CO3] Remaining and Controlling Drainage
Quality (TL1, TL3, TL6)
The drainage quality from the remaining three samples, TL1, TL3, and TL6 (respectively 30,
68, 182 kg/t CaCO3 APP; 19, 19, 46 kg/t CaCO3 NP[(Ca/Mg)CO3]), showed no indication of
calcium and magnesium carbonate depletion (figures 10, 11, 12). The quality of drainage from
16
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these samples was relatively constant over time. Drainage pH typically ranged from 7.5 to 8.0,
with values from TL1 on the lower end of the range and those from TL3 on the upper end.
Drainage pH from TL6, ranging from 7.1 to 7.95, was more variable than that from TL1 or
TL3. Drainage alkalinities from TL1, TL3, and TL6 varied within ranges of 10-20, 50-60, and
35-50 mg/L as CaCO3, respectively. The values for TL1 appeared to decrease slightly over
time.
Release of calcium and magnesium from the solids was fairly constant. Substantial magnesium
release was observed only for sample TL3, which contained 1.9% ankerite (tables 10, 11). The
low magnesium release from TL1 is interesting since it was reported to contain 0.4% dolomite
(table 10). Preferential dissolution of calcite is one reasonable explanation for the apparent slow
dissolution of dolomite. Furthermore, additional examination of the XRD data indicated that the
dolomite content may have been erroneously elevated due to interference from a secondary
feldspar peak. The ratio of calcium plus magnesium to sulfate release for these three samples
tended to increase over the course of the experiment. This implies the calcium/magnesium
carbonate mineral dissolution rate increased relative to the sulfide mineral oxidation rate.
The calculated NP[(Ca/Mg)CO3] remaining in the samples after 132 weeks of dissolution ranged
from 0 kg/t CaCO3 for TL3 to about 13 kg/t CaCO3 for TL1 (tables 10, 12). The drainages
from TL3 and TL6 would likely acidify sooner than that from TL1. Relative to TL1, the small
amount of NP[(Ca/Mg)COj] remaining in samples TL3 and TL6 would be depleted rapidly due
to the high rates of acid production associated with these., samples (tables 10, 11). Upon
depletion of the calcium and magnesium carbonates the drainages would acidify.
5.3. Elevated Temperature Test
5.3.1. Introduction
The objective of subjecting mine waste samples to higher than ambient temperatures is to
accelerate the rate of sulfide oxidation and consequent acid generation. This will, in turn,
accelerate the dissolution of acid-neutralizing minerals. The acceleration of mineral dissolution
reduces the time required for drainage quality prediction. This reduced experimental duration,
as well as the reduction in labor achieved by rinsing samples every other week rather than every
week, are advantages of this test over the Wet-Dry Cycle Test. A disadvantage is that the
drainage quality generated at the higher temperatures may not simulate that hi the environment.
For example acid-producing reactions may be disproportionately accelerated or decelerated
relative to acid-neutralizing reactions at the elevated temperature. Rates of mineral coating
reactions may also by also be disproportionately affected relative to other reaction rates. Other
deviations from field conditions could also be introduced by the elevated temperature.
The elevated temperature accelerated sulfide mineral oxidation in six of the ten samples. For
these samples the total APP release, as indicated by sulfate release, in the Elevated Temperature
Test was about two to six times that in the Wet-Dry Cycle Test. However, APP release from
17
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samples TL3, TL5, and TL6 was similar in the two tests, while that from RK2 in the Elevated
Temperature Test was only 70% of that in the Wet-Dry Cycle Test (table 13).
Acceleration of sulfide mineral oxidation increased the rates of acid-neutralizing mineral
dissolution, as indicated by the NP[(Ca/Mg)COj] release in the two experiments (table 13). The
acceleration of acid-neutralizing mineral dissolution was further exemplified by the fact that
seven samples (all except TL1, TL3, TL6) produced some acidic drainage in the Elevated
Temperature Test. The acidification of drainage implies neutralization potential depletion in
these seven samples. In contrast, only TL5 yielded acidic drainage in the Wet-Dry Cycle Test.
Based on the drainage quality in the Elevated Temperature Test, samples were classified as
strong acid producers (TL5, RK4, TL4);
moderate acid producers (RIG);
mild acid producers (TL2);
intermittent acid producers (RK1, RK2); and
non-acid producers (TL1, TL3, TL6).
5.3.2. Strong Acid Producers: TL5, RK4, TL4
The three strong acid producers in the Elevated Temperature Jest were, in decreasing order of
acid generation, TL5, RK4, and TL4. The APP of these samples ranged from 72 to 158 kg/t
CaCO3 and NP[(Ca/Mg)CO3] values ranged from 6 to 32 kg/t CaCO3 (tables 7, 14). The pH
of drainage from TL5, RK4, and TL4 decreased for periods of 14, 30, and 130 weeks, reaching
minimum values of 2.6, 3.0, and 3.1, respectively. The acidic drainages reflected rates of acid
generation (indicated by sulfate release) which exceeded rates of acid neutralization (indicated
by calcium plus magnesium release; tables 14, 15).
The pH of drainage from TL5 was acidic throughout the test and remained below 3.3 after week
2, reaching a minimum of 2.56 at week 14 (figure 13). Sulfate release was elevated during the
first 30 weeks, concurrent with the period of lowest pH values, and may have been the result
of oxidation of the marcasite present in the sample (table 6). Subsequently, sulfate
concentrations plateaued at about 80 mg/L. Calcium and magnesium concentrations in the
drainage decreased during the initial 30 weeks and plateaued at low levels (figure 13).
The acidic pH observed at week 2 suggests that the 1.1 % dolomite and 0.2% magnesite initially
present had been depleted. The acid neutralization implied by calcium and magnesium release
(empirical neutralization potential, ENP) at week 0 was equivalent to that provided by 8 kg/t
CaCO3, about haft the 15 kg/t NP[(Ca/Mg)COj] determined based on solid phase analysis.
Calcium and magnesium concentrations in the TL5 drainage decreased during the initial phase
of dissolution, indicating a decrease in solid-phase sources of these metals (figure 13). These
concentrations began to plateau at about week 22. At this time the ENP was calculated as 16
kg/t CaCO3, a value closer to the NP[(Ca/Mg)CO3].
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The generation of acidic drainage despite the calculated presence of calcium/magnesium
carbonates may have been solely an artifact of cumulative error introduced by solid-phase
analyses, sample splitting, and aqueous-phase analyses. Two other scenarios, or a combination
of these scenarios, are also possible. First, some of the calcium/magnesium carbonate minerals
may have remained after week 0, dissolving during the subsequent 22 weeks at a rate which was
too slow to offset the rapid rate of acid production. Second, calcium/magnesium carbonate
mineral dissolution may have neutralized acid produced during sample storage prior to week 0,
and the calcium and magnesium released formed sulfate minerals. These minerals then dissolved
through week 22. More detailed assessment of the drainage quality data, in conjunction with
chemical equilibrium considerations and additional solid-phase analyses, may identify the
cause(s) of this apparent anomaly.
After 131 weeks of leaching, the total sulfur content of the sample was 2.8 percent, indicating
a substantial potential for acid production remained. Since sulfate initially present was released
in the initial stage of the experiment, virtually all of the sulfur was present as sulfide. The rate
of sulfate release at this time was the highest of the 10 samples examined (tables 14, 15). The
slow calcium and magnesium release was apparently due to dissolution of non-carbonate host
rock minerals.
TL5 had the highest degree of sulfide oxidation of the samples examined and was, therefore, the
most likely sample to manifest evidence of silicate mineral dissolution. The unleached sample
contained 36% potassium feldspar and 16% sodium feldspar. For comparison, a sample leached
for 26 weeks in the Elevated Temperature Test was examined by XRD and SEM. XRD patterns
for the leached TLS sample were not markedly different from those of the unleached sample.
However, SEM examination revealed pitting on feldspar surfaces (figure 14). Numerous grains
were examined and all manifested similar pitting. This indicates that feldspars dissolved, with
attendant acid neutralization, during 26 weeks of the Elevated Temperature Test. This
dissolution neutralized only a fraction of the acid produced, as indicated by the highly acidic
drainage.
RK4 and TL4 also produced acidic drainages over the majority of the test, with minimum values
around pH 3. The time required to deplete the NP from RK4 and TL4 (and produce acidic
drainage) was longer than that for TLS, due largely to a higher initial NP and a slower rate of
acid production, respectively. At the end of the period of record, the rates of sulfate release
from these two samples were lower than only that of TLS. The pH of drainage from RK4
decreased below 6.0 at week 16 (figure IS). Prior to this time the calcium and magnesium
release was equivalent to the dissolution of 27 kg/t CaCO3. This empirical neutralization
potential (ENP) is about 30% lower than the NP[(Ca/Mg)CO3] determined based on the RK4
mineralogy.
The pH of drainage from TL4 decreased steadily from near 8 at week 2 and, with one exception,
remained below 6.0 after week 34. The ENP was calculated based on the calcium and
magnesium release through week 34. The value of 4.2 kg/t CaCO3 agreed reasonably well with
the 6 kg/t CaCO3 NP[(Ca/Mg)COJ determined based on the TL4 mineralogy (tables 14, 17).
19
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Drainage pH continued to decrease and reached the low 3's at week 131. The pH decline after
week 70 was concurrent with an increase in the sulfate release rate and a decrease in the rates
of calcium and magnesium (figure 16, table 15). The increase in sulfate release was likely due
to the commencement of bacterially catalyzed ferric iron oxidation of the iron sulfides as pH
decreased (Nordstrom 1982; Kleinmann et al. 1981; Singer and Stumm 1970).
5.3.3. Moderate (RIG) and Mild (TL2) Acid Producers
Based on the minimum drainage pH values observed in the Elevated Temperature Test, RK3 and
TL2 (respectively 51, 47 kg/t CaCO3 APP; 5, 16 kg/t CaCO3 NP[(Ca/Mg)CO3]) were classified,
respectively, as moderate and mild acid producers. Qualitatively, the variation of drainage pH
from these samples was similar to that observed for the strong acid producers. However, the
minimum pH values generated by RK3 and TL2 (3.5 and 4.7, respectively) were higher than
those observed for the strong acid producers by one and two units, respectively.
Drainage pH from RK3 decreased to and remained below 6.0 after week 18. At this time the
acid neutralization implied by calcium plus magnesium release was equivalent to the dissolution
of 4.9 kg/t CaCO3. This ENP is in good agreement with the NP[(Ca/Mg)COj] of 5 kg/t CaCO3.
The drainage pH continued to decrease, with values around 4 over the last ten weeks of the test
(figure 17). The pH of drainage from RK3 was slightly higher than values from the strong acid
generators due to a lower rate of sulfide oxidation and attendant acid production (tables 14, 15).
During the test the sulfur content of the sample decreased from 1.6% to 1.2% (tables 14, 16).
The pH of drainage from TL2 decreased below 6.0 after week 51. At this time the sum of
calcium and magnesium release was equivalent to an NP release of 20 kg/t CaCO3, somewhat
higher than the measured NP[(Ca/Mg)CO3] of 16 kg/t (tables 14, 16). The drainage pH did not
decrease much below 6.0, ultimately oscillating between 5.0 and 5.7 (figure 18). The rate of
acid production, as inferred by sulfate release, was comparable to that for more acidic samples.
However, the rate of acid neutralization was considerably higher than the range for the more
acidic samples. Indeed the final rate of sulfate release (acid production) approximated the rate
of calcium plus magnesium release (acid neutralization), which is consistent with the near neutral
drainage pH (tables 14, 15). Dissolution of feldspars, which comprised 30% of the sample, may
have contributed to the calcium release in the latter stages of the experiment, although it is also
possible that some calcite remained. Over the course of the test the sulfur content was reduced
from 1.5% to 0.9%.
5.3.4. Intermittent Acid Producers: RK1, RK2
The temporal variation of drainage pH from RK1 and RK2 (respectively 14, 20 kg/t CaCO3
APP; 3, 1 kg/t CaCO3 NP[(Ca/Mg)CO3]) was distinctly different from that of the samples
discussed previously. Whereas the pH of drainage from these samples initially decreased from
the mid-sevens to values below six, drainage pH subsequently increased and plateaued near pH
6 (figures 19, 20). The ENP for samples RK1 and RK2 was calculated based on calcium and
magnesium release during weeks 0-6 and weeks 0-66, respectively. Drainage pH decreased to
20
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pH 6 or less for periods of 52 and 32 weeks, respectively, and then increased above pH 6.
Respective ENP values of 2.5 and 2.9 kg/t CaC03 were determined for RK1 and RK2, and
agreed fairly well with the NP[(Ca/Mg)CO,] values of 3 and 1 kg/t CaCO3 determined based
on carbonate mineralogy (table 7). Sulfate concentrations decreased over time and plateaued at
low levels, reflecting sulfate release rates which were the lowest of the samples examined.
Apparently dissolution of noncarbonate host rock minerals was fast enough to neutralize the acid
produced by the slow rate of iron sulfide oxidation in these two samples. Feldspar was a major
component of both RK1 (14% potassium feldspar (microcline), 10% sodium feldspar) and RK2
(4% potassium feldspar, 22% sodium feldspar, 28% calcium feldspar). In addition RK1
contained 14% chlorite, and RK2 contained 18% pyroxene and 11% olivine (table 8).
Magnesium release accounted for a substantial fraction of the major cation release (tables 14,
15), suggesting that dissolution of one or more of these minerals contributed to acid
neutralization. No etch pits or other dissolution features were detected by SEM analysis of a
sample of RK1 leached for 24 weeks in the Elevated Temperature Test, reflecting the low extent
of dissolution. Since the extent of dissolution of RK2 at this time was similar to that of RK1,
it was not analyzed by SEM.
In order to increase the probability of detecting changes in primary silicate minerals, a sample
leached for 289 weeks in a separate dissolution experiment at room temperature was subjected
to SEM analysis. The unleached sample (1.63% S) was mineralogically similar to RK2 (olivine
gabbro). The pH of drainage from the sample was between 4.0 and 4.1 and the non-carbonate
acid neutralization was estimated to be the equivalent of dissolution of 1.1 grams of calcite
(measured as 0.75 grams after 150 weeks of dissolution). This was approximately two and six
times the neutralizing mineral (including calcium and magnesium carbonates) dissolution of RK1
and RK2 after 24 weeks in the Elevated Temperature Test.
SEM examination of the feldspars present in the rock leached for 289 weeks revealed mild
pitting and iron oxide coating of feldspar grains (figure 21). These features are consistent with
acidic drainage contacting the feldspar mineral surface, feldspar dissolution with attendant pH
elevation, and consequent iron oxyhydroxide precipitation. Evidence was also observed for
dissolution of pyroxene (augite), mica (biotite), and amphibole (probably hornblende) present
in the sample (figures 22, 23, 24). It is assumed that dissolution of these minerals also
neutralized acid and contributed to the release of calcium and magnesium from RK1 and RK2.
However, SEM analysis did not detect etch pits on silicate minerals present in RK1 because they
had not dissolved extensively. Dissolution was limited by the low sulfur content of the samples
and the short time of dissolution. Since the molar release of calcium from RK2 was roughly
twice that of magnesium, dissolution of calcium feldspar and clinopyroxene (the major silicate
minerals) apparently exceeded that of the olivine, mica, and amphibole.
5.3.5. Non-Acid Producers: TL1, TL3, TL6
The quality of drainage from TL1, TL3, and TL6 (respectively 30, 68, 182 kg/t CaCO3 APP;
19, 19, 46 kg/t CaCO3 NP[(Ca/Mg)CO3]) indicated that the acid generated as a result of iron
21
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sulfide oxidation was neutralized by dissolution of calcium/magnesium carbonate minerals.
Sulfate concentrations in the three drainages tended to decrease over time. With the exception
of four values from TLl, the pH of drainage from these samples exceeded 6 (figures 25, 26,
27). Furthermore, at the end of the period of record typical alkalinity concentrations from TLl,
TL3, and TL6 were 10, 40, and 20 mg/L as CaCO3, respectively. These nonacidic qualities,
in conjunction with the elevated release of calcium and magnesium (tables 14, 15), are consistent
with the presence and dissolution of calcium/magnesium carbonate minerals.
The ultimate rates of sulfate release from TLl and TL6 were similar in magnitude to those
observed for acid producing samples (tables 14, 15). Despite these rapid rates of acid
production the calcium and magnesium carbonates present were not depleted, although four pH
values below 6.0 were observed in drainage from TLl. While the rate of iron sulfide oxidation
for TLl was accelerated by almost six times in the Elevated Temperature Test, the extent of
sulfate release from samples TL3 and TL6 in the Elevated Temperature Test was approximately
equal to that in the Wet-Dry Cycle Test (table 13). Since these two samples produced circum-
neutral drainage in the Wet-Dry Cycle Test, nonacidic drainage would also be expected from
these two samples in the Elevated Temperature Test.
The mass release of calcium and magnesium from these three samples indicates the carbonate
mineral neutralization potential was near depletion after 131 weeks (table 16). Indeed, the mass
of calcium and magnesium released from TL3 was 130% to 170% of that reported as occurring
in the solid phase as carbonate minerals. The corresponding range for TL6 was 90% to 170%
(table 16). Since 0.6% S to 4.0% S remained in the samples, additional iron sulfide oxidation,
and the attendant acid production, after depletion of the calcium/magnesium carbonate minerals
could lead to acidification of these drainages.
Sample TL6 was selected for additional dissolution and solid-phase analysis to 1) determine if
drainage acidification was imminent; 2) clarify the contradictory implications on
calcium/magnesium carbonate mineral presence based on a) drainage quality and b) solid phase
calcium/magnesium carbonate content and mass calcium and magnesium release; and 3) identify
non-carbonate minerals, if any, contributing to acid neutralization. The pH of drainage from
TL6 decreased below 6.0 at week 163 and reached 3.45 at week 173. At this time the
dissolution of this sample was discontinued.
The elevated alkalinity and calcium (and magnesium for TL3) concentrations, as well as pH, in
the drainage clearly indicates the dissolution of calcium carbonate minerals until about week 163.
The mass release calculations indicated calcium and magnesium release from TL6 (and TL3)
may have exceeded the calcium and magnesium reported present as calcium/magnesium
carbonate minerals in the solid phase. This suggests that non-carbonate minerals were dissolving
to release calcium and/or magnesium, neutralize acid, and maintain the drainage pH in the
neutral range.
Further examination revealed both 1) error in evolved carbon dioxide analysis and/or splitting
and 2) some non-carbonate mineral dissolution. Reanalysis of the unleached TL6 sample
22
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revealed an evolved carbon dioxide content of 2.8%, about 40% higher than the value originally
determined (2.01%). This suggests that the NP[(Ca/Mg)CO3] was actually 40% higher than
originally calculated, and reduces the calculated calcium and magnesium release at week 131 to
65% to 120% of that occurring in the solid phase as carbonate minerals.
Examination of the TL6 sample after 173 weeks of dissolution indicated that noncarbonate
dissolution probably neutralized some acid. SEM analysis of the leached solids revealed some
of the clinopyroxene minerals, which comprised the majority of the sample, were altered to the
point of being friable (figure 28). It must be noted that no alteration was detected on some
grains. Most of the grains examined were of the hedenbergite-diopside series (CaFeSLO6-
CaMgSi2O6; 43% hedenbergite with 21 % CaO and 2% MgO, 12% diopside with 25% CaO and
17% MgO). Energy dispersive spectroscopy (EDS) indicated that the grain in figure 28a had
a relatively high iron content, suggesting that calcium was preferentially leached. EDS analysis
of the needle-like crystals in figure 28b indicated a composition approximating that of the
pyroxene in the sample, although these crystals appear to be secondary, possibly zeolites.
In summary, the SEM analyses indicate that dissolution of some of the clinopyroxene in this
sample occurred by week 173. Some of this dissolution occurred under acidic conditions from
week 163 to 173. During this period pH was below 6.0, and calcium concentrations increased
by 50% and magnesium concentrations doubled over those observed from week 151 to 161.
Due to a) the extensive weathering of some grains revealed by SEM examination and b) the
calcium release in excess of that present in carbonate minerals prior to drainage acidification
(calculated using the calcium/magnesium carbonate content based on reanalysis of evolved
carbon dioxide), some of the clinopyroxene dissolution apparently occurred prior to acidification.
This dissolution would neutralize acid produced by iron sulfide oxidation while drainage pH
exceeded 6.0. Thus, some of the clinopyroxene present in sample TL6 may have contributed
to the effective neutralization potential of the sample. Adjusting the solid-phase carbonate
mineralogy based on the reanalysis for evolved carbon dioxide, the neutralization provided by
clinopyroxene dissolution was estimated as equivalent to that by dissolution of about 0.5 g
CaCO3. Thus, neutralization by the clinopyroxene, which comprised 55% of the sample, was
relatively small, representing at most 10% to 15% of the ENP. Additional study of this
phenomenon is warranted.
5.4. Particle Size Experiment
Particle size reduction is often a necessity for predictive dissolution testing of waste rock.
Unfortunately the quality of drainage, in particular drainage pH, from small waste rock particles
may not accurately simulate the quality of drainage from operational scale waste rock. Iron
sulfide oxidation and calcium/magnesium carbonate mineral dissolution are responsible,
respectively, for the generation and neutralization of acid. The rates of these reactions are
generally proportional to the surface area of the respective mineral available for reaction.
Particle size reduction may result in preferential enhancement of acid-producing or acid-
neutralizing mineral surface areas. This in turn will affect the relative rates of acid generation
and acid neutralization and, consequently, drainage pH.
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The effect of particle size on drainage quality was examined using RK3, RK4, and RK5, which
was mineralogically similar to RK2 but with a slightly higher sulfur content (0.8% vs 0.6% S).
Larger particles of each rock type were crushed and sieved to obtain the various size fractions.
The sulfur and NP contents of the size fractions varied, introducing an additional variable. This
variable was normalized in calculations by expressing APP and NP release as a percent of that
originally present. The amount of moisture retained in reactors also tended to increase as
particle size decreased, due in part to the increased influence of capillary forces in the smaller
particles (table 18). The influence of this variable is addressed qualitatively. Boxplot figures,
which are explained in figure 29, are used to present the pH data for the Particle Size
Experiment.
The pH values for drainages from the various particle size fractions of RK3 were typically
between 7 and 8 during the 30-week experiment, with no strong trend with respect to particle
size. With the exception of the largest particle size fraction ( + 10 mesh/-l/4 inch), from which
drainage pH was the lowest, drainage pH decreased slightly as particle size decreased (figure
30). The rate of sulfate release, reflecting iron sulfide oxidation and the consequent acid
production, was slightly higher at the small particle sizes. In contrast, the rate of calcium plus
magnesium release, reflecting carbonate mineral dissolution and acid neutralization, increased
substantially as particle size decreased (figure 30). This suggests that the available sulfide
mineral surface area increased slightly and the available calcium/magnesium carbonate mineral
surface area increased substantially as particle size decreased. The release rates of calcium,
magnesium, and sulfate from the finest fraction may have been limited by inefficient transport
of reaction products from the solids, as indicated by the low flow through the fine-grained
solids. Over the course of the experiment, the total drainage volume from the minus 270 mesh
fraction was only 27 percent of that from the 4-2707-100 mesh fraction (see section 4.2.3).
The reduction of RK3 particle size preferentially accelerated the dissolution of the acid-
neutralizing carbonates relative to the oxidation of iron sulfides. Coarse and fine grained sulfide
minerals were present in this rock, occurring both as disseminated grains and in veinlets. Since
the sulfate release rate increased only slightly as particle size decreased, it was concluded that
the available sulfide mineral surface area increased only slightly as particle size decreased. It
must be noted that the sulfate release for the -270 fraction may have been limited by reaction
product transport due to limited flow through the solids bed (see section 4.2.3.) rather than the
sulfide oxidation rate. This relationship between sulfide surface area and particle size would be
expected if the majority of the sulfides were present in veinlets along fracture planes.
In contrast, the calcium and magnesium release rate tended to increase as particle size decreased.
This indicates that the calcium/magnesium carbonate mineral surface area increased as particle
size decreased. Thus, the size reduction of RK3 particles tended to preferentially enhance
calcium/magnesium carbonate dissolution relative to iron sulfide oxidation. In other words the
reduction of RK3 size for predictive testing would tend to underestimate the tendency for large
operation scale waste rock to produce acid. (Drainage pH did not increase strictly with calcium
and magnesium release due to differences in the mass of solids used and the volume of drainage
generated.)
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The drainages from the two largest size fractions of RK4 were acidic, while the pH of drainage
from the smaller size fractions typically ranged from 7 to 8 (figure 31). The rates of sulfide
oxidation for the larger particles were faster than those for the smaller particles and exceeded
the corresponding rates of carbonate mineral dissolution. In contrast, the rate of sulfide mineral
oxidation for the smaller particles was less than the rate of carbonate mineral dissolution.
Both the sulfide and carbonate minerals in RK4 occurred in veinlets, very possibly along fracture
planes. However, the sulfide veinlets were "relatively open and porous" and, therefore, were
accessible to air and water even in the larger particles. That is, the available sulfide surface area
was relatively constant among the size fractions examined. In contrast, the carbonates were
present in "tight" veinlets. The available carbonate mineral surface area in the larger particles
may have been limited by this mode of occurrence. As a result, the dissolution of carbonates
in the larger particles may have been inhibited to the extent that they could not dissolve fast
enough to neutralize the acid produced by the oxidation of iron sulfides. The acidic conditions
may have given rise to bacterial mediation of the sulfide mineral oxidation, thereby accelerating
the oxidation rate. In the smaller particle size fractions, a greater extent of the carbonate
mineral surface area was available for reaction. Consequently, the carbonate mineral dissolution
was adequately rapid to neutralize the acid produced by the iron sulfide mineral oxidation.
Physical factors associated with the finer size fractions may have enhanced the interaction of acid
generated and alkaline components present. Reduction of particle size decreases the distance
between sulfide and carbonate mineral grains, that is, there is more intimate contact between
carbonates and sulfides. The increased moisture retention by finer particles may have further
enhanced the interaction of acidic and alkaline components by providing a transport medium.
For example, acid generated at sulfide mineral surfaces can diffuse through the water to reach
calcium carbonate surfaces and be neutralized. These factors allow neutralization reactions to
occur more or less continuously. In contrast, with larger particles neutralization reactions would
occur only if sulfide grains and calcium/magnesium carbonate grains were adjacent or when
rinse water was added. For example, the +35/-10 RK4 fraction produced neutral drainage from
week 1 to week 30, when the moisture content in the cell was 12.9 percent. However, at week
0 when the solids were essentially dry, a drainage pH of 3.92 was produced. During the 30-
week experiment, the moisture content of the +35/-10 fraction was roughly 2.5 times values
observed for the larger particle size fractions which produced acidic drainage.
Furthermore, with the more intimate contact of sulfide and carbonate mineral contact and
elevated moisture content, acid is more likely to be neutralized before acidic microenvironments
develop to a great extent. This in turn would inhibit bacterial acceleration of oxidation rates.
Elevating the pH near sulfide minerals also enhances the precipitation of iron oxyhydroxides on
the sulfide mineral surfaces, thereby inhibiting the rate of oxidation.
The pH of drainage from the RK5 samples decreased as particle size decreased (figure 32).
Release rates of sulfate, calcium, and magnesium increased as particle size decreased (figure 32).
However, the rate of sulfate release increased to a greater extent, causing the decrease in pH
with decreasing particle size. The sulfide minerals in this rock occurred included in or
25
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interstitial to relatively coarse grained rock forming minerals. It is assumed that both the
fraction of total surface area occupied by iron sulfide minerals (fj and that occupied by acid
neutralizing minerals (fj was fairly constant over the particle sizes examined. If this is the case,
the reduction of particle size does preferentially enhance the gross rates of acid production or
acid neutralization.
The net rate of acid production by any solid can be expressed as
P = A K, fa - A Kn fn = A (K. f, - K,, fj [10]
P = net rate of acid production (moles per unit time);
A = total surface area (square meters).
K,,, K,, = rates of acid production and acid neutralization, respectively (moles per unit
time per square meter); and
fa, fn = fractions of surface area occupied by iron sulfide and acid neutralizing
minerals, respectively (dimensionless).
As can be seen from the right hand term in the equation, the net rate of acid production is
proportional to the total surface area. The observed increase in the difference between the gross
rates of acid production and neutralization (A K. fa - A K,, fj and decrease in drainage pH with
decreasing particle size (figure 32) are consistent with this explanation. Also implied by this
approach is that if the volume of water rinsing reaction products from rock surfaces remains
constant, drainage pH will decrease with increasing numbers of particles (i.e., reactive surface
area increases).
6. SUMMARY
The major objectives of this project were to describe the long term dissolution of mine waste,
and to describe the effect of particle size on the quality of drainage from waste rock. Tests at
room temperature (Wet-Dry Cycle Test) and at 100°C (Elevated Temperature Test) were
conducted for 130-132 weeks to describe the quality of drainage from four waste rock and six
tailing samples which had been physically, chemically, and mineralogically characterized. The
APP, based on total sulfur content (almost all of the sulfur in the samples was present as
sulfide), of the samples ranged from 14 to 182 kg/t CaCOj and the acid neutralization potential
present as calcium carbonate and magnesium carbonate (NP[(Ca/Mg)COj]) ranged from 1 to 46
kg/t CaCO3. In addition to pH, concentrations of alkalinity, sulfate, calcium, and magnesium
in the drainages were determined. The rates and extents of iron sulfide and calcium/magnesium
carbonate mineral dissolution were calculated based on the masses of sulfate, calcium, and
magnesium released in the drainage from the samples. At both room temperature and 100°C,
the rates of sulfide oxidation and calcium carbonate and magnesium carbonate dissolution
generally decreased over time. General classifications of the samples based on drainage quality
generated in the Wet-Dry Cycle and Elevated Temperature Tests are presented in table 19.
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The fraction of initial sulfur content released during the Wet-Dry Cycle Test was 44 percent for
TL5 and ranged from 6 to 25 percent for the remaining nine samples. The fraction of
NP[(Ca/Mg)C03] released was 100 percent for TL5, and apparently for RK1 and RK2 as well.
TL5 (158 kg/t CaCO3 APP, 15 kg/t CaCO3 NP[(Ca/Mg)CO3]) was the only sample which
produced acidic drainage in the Wet-Dry Cycle Test, with a typical drainage pH between 2.6
and 3.0. The low drainage pH indicated that iron sulfide minerals were oxidizing in the absence
of calcium/magnesium carbonate minerals. Apparently the dissolution of silicate minerals
present in RK1 and RK2 (14, 20 kg/t CaCO3 APP; 3, 1 kg/t CaCO3 NP[(Ca/Mg)CO3],
respectively) maintained drainage pH values between 6.5 and 7.0 despite a continued slow rate
of acid production. The dissolution of the host rock silicates (per unit mass rock) was
accelerated by the fine panicle size used in the tests.
The fraction of NP[(Ca/Mg)CO3] released from the remaining samples ranged from 30 percent
to close to 100 percent. The declining pH (although usually above 7) and alkalinity of drainage
from four of the samples, as well as the mass calcium and magnesium released, indicated that
their calcium and magnesium carbonate minerals were near depletion at the end of the
experiment (RIG, RK4, TL2, TL4: 51, 91, 47, 72 kg/t CaCO3 APP; 5, 32, 16, 6 kg/t CaCO3
NP[(Ca/Mg)CO3]). The final three samples (TL1, TL3, TL6: 30, 68, 182 kg/t CaCO3 APP;
19, 19,46 kg/t CaCO3 NP[(Ca/Mg)CO3]) generated drainage with typical pH values in the range
of 7.5 to 8.0 and alkalinities in the range of 10 to 40 mg/L as CaCO, near the end of the
experiment. It was concluded that substantial calcium/magnesium carbonate and iron sulfide
minerals remained in these samples at the end of the experiment (tables 12, 19). Over a longer
experimental time frame, continued iron sulfide oxidation and the resultant acid production by
these seven samples could deplete the NP[(Ca/Mg)COj] with an attendant acidification of
drainage. That is, the alkaline drainage observed during the test would probably not simulate
the long term drainage quality from the mine waste in the field.
The uncertainty regarding the ultimate drainage quality from these samples even after 132
weeks, underscores the difficulty in empirically simulating (in dissolution tests) the long term
drainage quality from mining wastes with even modest neutralization potentials (typically less
than 20 kg/t CaCO3 for the solids examined). The results generated demonstrate the long kinetic
test duration required to surpass the lag period for samples with even small amounts of
neutralization potential.
For six of the ten samples examined, the rate of sulfide oxidation in the Elevated Temperature
Test (100°C) was two to six times that at ambient temperatures in the Wet-Dry Cycle Test. The
NP[(Ca/Mg)CO3] was depleted from five of these six samples. The NP[(Ca/Mg)COj] was also
depleted from TL5 (which also produced acidic drainage in the Wet-Dry Cycle Test) and RK2.
For five (RIG, RK4, TL2, TLA, TL5) of the aforementioned seven samples, the neutralization
potential depletion was inferred by the permanent decrease of drainage pH below 6. The pH
of drainage from the remaining two samples (RK1 and RK2, from which the NP[(Ca/Mg)COj]
was also depleted in the Wet-Dry Cycle Test) decreased below 6 for 32 to 42 weeks and then
rose above pH 6. The amount of calcium and magnesium released in the drainages from these
samples indicated that the calcium and magnesium carbonates had been depleted from the solids.
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It is assumed that dissolution of noncarbonate host rock minerals neutralized the acid produced
in the latter stages of the test.
The pH (and alkalinity) of drainage from TL1, TL3, and TL6 indicated that some
NP[(Ca/Mg)CO3] remained in these samples at the end of the test. The pH of drainage from
TL1 was below 6 on four occasions, suggesting the calcium and magnesium carbonates may
have been near depletion. The pH of drainages from TL3 and TL6 were typically in the middle
to upper sevens, indicating pH control by calcium/magnesium carbonates. Since sulfides present
in these three samples continued to oxidize, the neutral drainage generated during the test does
not necessarily simulate the long term drainage quality which would occur in the field. That is,
continued dissolution may deplete the calcium and magnesium carbonate minerals, with
consequent drainage acidification.
Field scale waste rock size must typically be reduced in size for predictive dissolution testing.
Since this reduction may bias predictive test results, the effects of particle size on drainage
quality were examined using three different rock types: a quartz latite diatreme with coarse and
fine iron sulfides occurring both as disseminated grains and in veinlets (RIG); a pyritized
mudstone with sulfides occurring in "relatively open and porous veinlets" and carbonate minerals
occurring hi "tight" veinlets (RK4); and an olivine gabbro with sulfides occurring as
disseminated grains (RK5).
The effect of particle size on drainage pH was apparently controlled by the changes in available
sulfide mineral surface area relative to calcium/magnesium carbonate mineral surface area. As
the particle size of RK3 was reduced, drainage pH remained relatively constant, but the
dissolution rate of acid-neutralizing minerals increased while that of acid-producing minerals
remained relatively constant. This suggests that the reduction of particle size would, over the
short term, produce drainage less acidic in nature than that which would be generated by large
particles. This general trend was demonstrated more explicitly by RK4. The largest size
fractions of this rock generated drainage pH values in the low three's, while the pH of drainage
from the smaller size fractions was almost always above seven. With RK5, a gabbro containing
disseminated sulfides, both acid production and acid neutralization increased as particle size
decreased. Since the rate of acid production exceeded the rate of acid neutralization, pH
decreased with particle size. Field pH data for this rock were more closely simulated by the fine
particles.
7. CONCLUSIONS
1. An extended duration is required for ambient temperature kinetic dissolution tests is
required to deplete even a moderate (and available) neutralization potential in mine waste
samples. Of the ten samples examined in the present study, the neutralization potential
present as calcium and magnesium carbonate minerals (NP[(Ca/Mg)CQj]) in four samples
did not exceed 6 kg/t CaCO3 and values for only two samples exceeded 20 kg/t CaCO3.
Despite the relatively small neutralization potentials, the calcium and magnesium carbonate
28
-------
minerals were depleted from only three of the samples over the course of the 132-week
dissolution experiment. The pH from drainage from one of these three samples (TL5:
5.05% S, 16 kg/t CaCO3 NP[(Ca/Mg)CO3) was below 6.0 at week 1 and was below three
for most of the test. Drainage pH from the other two samples remained above pH 6 due
to acid neutralization by silicate mineral dissolution, which was enhanced due to the small
particle size used in the experiment.
2. Even if mine waste samples generate neutral pH drainage over the course of relatively long
dissolution tests, it cannot be concluded that these samples will generate similar drainage
in the field after mine closure. The neutralization potential was not depleted from seven
of the samples which generated neutral pH drainage for 132 weeks. Sulfide minerals in the
samples continued to oxidize over the entire period of record, as indicated by sulfate
concentrations in the drainages. Whereas these seven samples may continue to produce
nonacidic drainage, additional dissolution could deplete their neutralization potential while
iron sulfide minerals remain and oxidize, with a consequent acidification of drainage.
3. Dissolution testing at higher temperature generally accelerated rates of sulfide oxidation and
carbonate mineral dissolution and, therefore, decreased the time required to deplete
neutralization potentials. For six of the ten samples, rates of iron sulfide oxidation (and the
resultant acid generation) at 100°C were two to six times those at ambient temperature.
4. The Elevated Temperature Test will reduce the time required for predictive testing on some
samples. The acceleration of sulfide oxidation produced by elevating the reaction
temperature reduced the time required for neutralization potential depletion. In the Elevated
Temperature Test, the NP[(Ca/Mg)CO3] was depleted from a total of seven samples,
including the three for which neutralization potential depletion was observed at ambient
temperature. The four additional samples produced drainage pH below six after periods
("lag times") of 14 to 55 weeks in the Elevated Temperature Test.
5. Although the rates of sulfide mineral oxidation generally decreased over the course of
experimentation at both ambient and elevated temperature, sulfate release continued
throughout the duration of both experiments. At the end of the ambient temperature
experiment, sulfate release rates were typically 30 to 60 percent of those near the beginning
of the experiment. The corresponding range for the Elevated Temperature Test was 10 to
30 percent.
6. The majority of acid neutralization potential in the samples examined occurred as calcium
and magnesium carbonate minerals. The empirical neutralization potential (ENP) observed
in the Elevated Temperature Test quantified the content of minerals that would dissolve and
maintain a drainage pH of at least 6.0. The ENPs of the seven samples which produced
drainage pH values below 6.0 were in reasonably good agreement with the
calcium/magnesium carbonate content of the samples.
29
-------
7. Some acid neutralization by silicate mineral dissolution was observed. For two samples
(RK1, RK2) the silicate mineral dissolution was fast enough to neutralize the slow rate of
acid production by the small amount of iron sulfides present (0.46% S, 0.64% S). The
drainage from a third sample (TL6, 5.81% S) eventually acidified; however, mass release
calculations and SEM examination of leached grains indicate that some of the clinopyroxene
(hedenbergite) present dissolved to neutralize acid. Quantitatively, the neutralization
supplied by the 40 g of clinopyroxene present was small, equivalent to that supplied by
dissolution of approximately 0.5 g of calcite.
8. The drainage quality beyond the dissolution test duration can be approximated using the
initial content of calcium and magnesium carbonates and acid producing minerals, and the
rates of calcium, magnesium, and sulfate release.
9. Extreme care must be taken when splitting samples for analysis and experimental use. The
carbon dioxide evolved from samples subjected to dissolution testing was reanalyzed.
Although the difference from the mean for the pairs of analyses was generally small, with
a median value of 11%, one value was reported as 32%. Such differences can introduce
substantial error in NP determination. Post-test verification of solid-phase analyses is
recommended to limit the introduction of such error.
10. The effects of rock particle size on drainage pH were variable. The fine fraction of one
rock type produced the lowest drainage pH, while the largest size fraction produced the
lowest drainage pH for a second rock type. With a third rock type there was little effect
of particle size on drainage quality. The effect for a particular rock type was apparently
due to the influence of particle size reduction on the relative abundance of iron sulfide and
carbonate mineral surface areas.
ACKNOWLEDGEMENTS
Solid-phase chemistry was detennined by Bondar-Clegg and Company Ltd. (Ottawa, Ontario)
and Louis Mattson of Midland Research determined mineralogical composition. Kate Willis,
with assistance from Anne Jagunich and Cal Jokela, conducted the dissolution experiments. Al
Klaysmat and Jean Matthew analyzed drainage samples for calcium, magnesium, and sulfate.
Midland Research analyzed Particle Size Experiment Samples collected after week 16 for sulfate.
Funding for the project was provided by die U.S. Environmental Protection Agency under Grant
X-820322-01-0.
30
-------
REFERENCES
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Environment Federation. 1992. Standard methods for the examination of water and wastewater,
18th ed. American Public Health Association, Washington, D.C.
Busenberg, E., Clemency, C. 1976. The dissolution kinetics of feldspars at 25°C. Geochim.
Cosmochim. Acta, 40. p. 41-49.
Hedin, R. S., Erickson, P. M. 1988. Relationships between the initial geochemistry and
leachate chemistry of weathering overburden samples. In Proc. 1988 Mine Drainage and
Surface Mine Reclamation Conference, vol. 1. BuMines 1C 9183. p. 21-28.
Hem, J. D. 1970. Study and interpretation of the chemical characteristics of natural water.
Geological Survey Water-Supply Paper 1473, Washington, D.C. 363 p.
Kleinmann, R. L. P., Crerar, D. A., Pacelli, R. R. 1981. Biogeochemistry of acid mine
drainage and a method to control acid formation. Mining Eng., vol. 33. p. 300-306.
Lapakko, K. A. 1993. Evaluation of tests for predicting mine waste drainage pH: Report to
the Western Governors' Association. Minnesota Department of Natural Resources, Division of
Minerals, St. Paul, MN. 76 p. plus appendices.
Lapakko, K. 1991. Mine Waste Drainage Quality Prediction: A Literature Review. First
Draft. MN Dept. Natural Resour. St. Paul, MN. 50 p.
/
Lapakko, K. A. 1990a. Solid phase characterization in conjunction with dissolution
experiments for prediction of drainage quality. la Proc- Western Regional Symposium on
Mining and Mineral Processing Wastes, F. Doyle (ed.), Soc. for Mining, Metallurgy, and
Exploration, Inc., Littleton, CO. p. 81-86.
Lapakko, K. A. 1990b. Regulatory mine waste characterization: A parallel to economic
resource evaluation. Irj Proc. Western Regional Symposium on Mining and Mineral Processing
Wastes, F. Doyle (ed.), Soc. for Mining, Metallurgy, and Exploration, Inc., Littleton, CO. p.
31-39.
Lapakko, K. A. 1988. Prediction of acid mine drainage from Duluth Complex mining wastes
in northeastern Minnesota. la Proc. 1988 Mine Drainage and Surface Mine Reclamation
Conference, vol. 1. BuMines IC9183. p. 180-190.
Lawrence, R. W., Ritcey, G. M., Poling, G. W., Marchant, P. B. 1989. Strategies for the
prediction of acid mine drainage. la Proc. of the Thirteenth Annual British Columbia Mine
Reclamation Symposium. June 7-9, 1989, Vernon, British Columbia, p. 52-67.
31
-------
Miller, S. D., Murray, G. S. 1988. Application of acid-base analysis from base metal and
precious metal mines. In Proc. 1988 Mine Drainage and Surface Mine Reclamation
Conference, vol. 1. BuMines 1C 9183, p. 29-32.
Nelson, M. 1978. Kinetics and mechanisms of the oxidation of ferrous sulfide. Ph.D. Thesis,
Stanford University, Palo Alto, CA.
Nordstrom, D. K. 1982. Aqueous pyrite oxidation and the consequent formation of secondary
iron minerals. In Acid Sulfate Weathering. Soil Sci. Am. Madison, WI.
Renton, J. J., Rhymer, T. E., Stiller, A. H. 1988. A laboratory procedure to evaluate the acid
producing potential of coal associated rocks. Mining Science and Technology, 7. p. 227-235.
Renton, J. J., Stiller, A. H., Rhymer, T. E. 1985. Evaluation of the acid producing potential
of toxic rock materials. Jn Stopping Acid Mine Drainage: A New Approach. West Virginia
Geological and Economic Survey, p. 7-12.
Renton, J. J. 1983. Laboratory studies of acid generation from coal associated rocks. In
Proceedings of Surface Mining and Water Quality, Clarksburg, W. Va. May 26, 1983.
Singer, P. C., Stumm, W. 1970. Acid mine drainage: The rate determining step. Science,
167. p. 1121-1123.
Sobek, A. A., Schuller, W. A., Freeman, J. R., Smith, R. M. 1978. Field and laboratory
methods applicable to overburden and minesoils. EPA 600/2-78-054. 203 p.
Stumm, W., Morgan, J. J. 1981. Aquatic chemistry - an introduction emphasizing chemical
equilibria in natural waters. John Wiley & Sons, Inc. p. 470.
32
-------
Table 1. Samples selected for predictive testing.
U)
Solid _ Host Rock
TyP* Total
RK1 Au pyritized mudstone 0.46
RK2 Cu-Ni troctolite-gabbro 0.64
RK3 Au quartz latile 1.63
RK4 Au adularized and pyritized mudstone 2.91
TL1 Mo metasomatised aplite/andesite 0.%
TL2 Mo quartz monzonite w/quartz porphyry 1 .49
TL3 Au sideroplessite quartz schist (?) 2.19
TL4 Cu hydrothermal porphyry (?) 2.30
TL5 Au latile 5.05
TL6 Au (pyrrhotite) skarn 5.81
"?' implies uncertainty
1 Determined as difference between total sulfur and sulfate.
2 APP = Acid Producing Potential in tons CaCOj/1000 tons
1 Mod. APP = Acid Producing Potential in tons CaCO3/1000
Sulfur (%)
Sulfate
0.04
0.01
0.03
0.09
0.06
0.04
0.07
0.20
0.20
0.63
rock = 31
tons rock
4 NP = Neutralization Potential in tons CaCOj/1000 tons rock = 22.73
5 Net NP in tons CaCOj/1000 tons rock = NP - Mod. APP.
* Field = drainage quality observed in the field
Sulfide1
0.42
0.63
1.60
2.82
0.90
1.45
2.12
2.10
4.85
5.18
.25 x Total
= 31.25 x
x COj.
CO2
(%)
0.11
0.03
1.41
1.42
0.87
0.80
4.06
0.25
0.65
2.01
Sulfur.
Sulfide.
APP2
14
20
51
91
30
47
68
72
160
180
Mod.
APP1
13
20
50
88
28
45
66
66
152
162
NP<
3
0.7
32
32
20
18
92
6
15
46
Net
NP5
-10
-19
-18
-56
-8
-27
26
-60
-137
-116
Field6
no data
acid
acid
acid
neutral
neutral
neutral
neutral
acid
neutral
-------
Table 2. Average and standard deviation for sulfur and carbon dioxide content of triplicate
sample splits.
Solid
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
Sulfur Content (%)
A
0.45
0.63
1.61
2.87
0.91
1.48
2.19
2.14
5.43
6.00
B
0.47
0.64
1.75
2.97
0.98
1.48
2.21
2.23
4.84
6.05
C
0.45
0.64
1.53
2.88
0.98
1.50
2.16
2.28
4.98
5.37
Ave1
0.46
0.64
1.63
2.91
0.96
1.49
2.19
2.22
5.08
5.81
SD2
0.012
0.006
0.111
0.055
0.040
0.012
0.025
0.071
0.308
0.379
Carbon Dioxide (%)
A
0.12
0.02
1.30
1.38
0.90
0.75
4.18
0.18
0.68
1.99
B
0.11
0.03
1.44
1.47
0.84
0.84
4.00
0.24
0.64
2.10
C
0.11
0.05
1.48
1.42
0.86
0.82
4.00
0.25
0.64
1.95
Ave
0.11
0.03
1.41
1.42
0.87
0.80
4.06
0.22
0.65
2.01
SD
0.006
0.015
0.095
0.045
0.031
0.047
0.104
0.038
0.023
0.078
1 Ave = average (mean)
2 SD = standard deviation
34
-------
Table 3. Sample mass and rinse volume for the Particle Size Experiment.
Solid
RK3
RK4
RK5
Particle Size
(mesh)1
-270
+270/-100
+ 100/-35
+ 35/-10
+ 10/-W"
-270
+2707- 100
+ 100/-35
+35/-10
+ 10/-K"
+ %"/-%"
-270
+270/-100
+ 100/-35
+35/-10
+ 10/-K"
+ KV-V -
Mass
(g)
75
75
75
1000
500
75
75
75
1000
1000
1000
75 -
75
75
1000
1000
1000
Rinse Volume
(mL)
200
200
200
400
200
200
200
200
400
400
400
200
200
200
300
300
300
Unless otherwise indicated.
35
-------
Table 4. Particle size distribution for mine waste samples used in the Wet-Dry Cycle and the
Elevated Temperature Tests.
Sample
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
+ 100 M
(+0.149 mm)
0.00
0.02
0.05
0.00
75.49
27.57
0.59
27.10
31.08
22.57
+270 M
(+0.053 mm)
14.80
37.10
29.35
11.60
11.69
26.65
31.22
23.16
21.10
31.41 *
+500 M
(+0.025 mm)
15.18
26.20
19.42
13.65
4.14
14.61
29.90
12.08
12.10
18.31
-500 M
(-0.025 mm)
70.02
36.68
51.18
74.75
8.68
31.17
38.24
37.66
35.72
27.71
36
-------
Table 5. Chemical analysis for sulfur, carbon dioxide, and major components in mine waste samples.
Solid
RK1
RK2
RIO
RK4
TL1
TL2
TL3
TL4
TL5
TL6
concentrations in percent
Sror
0.46
0.64
1.63
2.91
0.96
1.49
2.19
2.30
5.05
5.81
SO.asS
0.04
0.01
0.03
0.09
0.06
0.04
0.07
0.20
0.20
0.63
S!
0.42
0.63
1.60
2.82
0.90
1.45
2.12
2.10
4.85
5.18
CO,
0.11
0.03
1.41
1.42
0.87
0.80
4.06
0.25
0.65
2.01
AIA
15.20
17.20
12.80
11.10
11.30
9.91
5.28
14.40
13.60
4.59
CaO FejO, K2O MgO MnO Na/)
0.51 6.27 3.24 3.61 0.03 1 .19
7.79 14.52 0.61 7.90 015 259
0.28 4.59 5.84 0.53 0.71 010
1.27 5.67 5.46 1.57 0.07 006
2.05 2.87 5.98 1.47 0.07 135
1.66 4.17 4.53 1.11 0.08 034
1.60 21.94 0.95 4.13 0.57 019
0.35 4.12 4.84 0.67 0.04 029
0.33 7.35 6.70 0.78 0.01 184
14.90 22.30 1.31 4.92 027 027
P,0,
004
023
0 17
025
025
021
006
Oil
<001
021
SiO2
60.50
45.73
69.30
6640
69.80
74.98
56.65
6880
6240
47.60
TiO,
0.73
1.54
0.21
055
052
026
0.24
0.35
029
030
LOI
663
0.15
401
573
2.40
270
7.59
424
509
2.63
'1 Oldl
97 96
9842
98 54
98 13
98.06
99 96
99 19
98.21
98 40
99 30
-------
Table 6. Mine waste mineralogy: Sulfur-bearing minerals (values in weight percent).
Sulfur-Bearing Minerals
Pyrile S
Marcasile S
Pyrrhotiie S
Pendandile Ni.S
Mackinawiie Ni.S
Anenopyrite As.S
Maucherite Ni.As.Ag
Tennantite Cu.As.S
Proustilc Ag.As.S
Chalcopyrite Cu.S
Cubanile Cu.S
Bornite Cu.S
Chalcociie Cu.S
Covellite Cu.S
Stibniie Sb.S
Tetrahedrite Cu.Sb.S
Galena Pb.S
Sphalerite Zn.Cd.S
Acanthite Ag.S
Gypsum SO4
Anhydrite SO4
Barite Ba,SO4
Jarosite SO4
Melanterite SO4
RK1
0.74
0.02
0.03
-
-
7
-
0.02?
-
7
-
-
?
?
?
<0.01?
<0.01?
0.02
-
-
-
7
0.1?
0.2
RK2
-
-
0.64
0.18
<0.01
-
0.01
-
-
0.27
0.67
0.04
-
-
-
-
-
-
-
-
-
-
-
0.1?
RK3
2.43
-
-
-
-
0.01
-
-
-
0.04
-
-
-
-
-
<0.01
0.25
0.79
<0.01
-
-
0.3
-
-
RK4
5.16
0.16
-
-
-
0.02?
-
7
-
<0.01
-
-
0.01
<0.0|
0.01
7
<0.01
0.02
i
0.5
-
7
-
0.2?
TL1
1.58
-
-
-
-
-
-
-
-
0.04
-
-
7
7
-
-
-------
Table 7. Mine waste mineralogy: Carbonate minerals and neutralization potential present as
calcium and magnesium carbonate
Calcite
Dolomite
Ankente
Siderue2
Rhodochrosite
Magnesite
RKl
o
i
.
0.3
-
-
RK2
RK3
RK4
TL1
TL2'
TL3
Carbonate Minerals (weight percent)
0.1
-
-
-
-
-
0.5
-
-
3.2
i
-
,
3.0
-
-
-
-
1.5
0.43
.
-
<0.1
-
0.7
0.83
.
0.2
.
-'
0.2
-
1.9
8.3
.
-
TL4
TL5
TL6
0.6
-
.
.
.
-
.
1.10
.
.
.
023
4.6
.
.
.
.
Carbonate (percent)
Total CaCO,4
Total MgCO,4
0
0
0.1
0
0.5
0
1.61
1.39
1.71
0.18
1.13
0.37
1.23
0.60
0.60
0
0.59
0.74
460
0
NP[(Ca/Mg)CO,], kg/t CaCO,
NP[(Ca/Mg)COd
Error Interval
3s
0-2.9
1
0-2.3
5
1.9-9.7
32
28-37
19
17-23
16
13-21
19
8.1-33
6
3.7-6.6
15
9.8-17
46
35-52
'?" indicates the mineral was probably present but not precisely identified by XRD.
"-' indicates the mineral was not present in the sample.
' Trace amounts of copper carbonates were noted in TL2.
1 Some magnesium may be associated with siderite.
3 Secondary feldspar XRD peak may make false contribution to dolomite peak.
4 Values represent all calcium or magnesium associated with carbonate.
' Calculated as CO, content (in percent) x 22.73.
39
-------
Table 8. Mine waste mineralogy: Rock forming minerals (values in weight percent)1.
Mineral
Quartz
Feldspar
Mica
Chlorite
Amphibole
Pyroxene
Olivine
Stilpnomelane
Serpentine
Kaolinite
Clay (15 angstroms)
Iron Oxides/ Ilmenite
RK1
24
24
6
14
-
-
-
-
-
2
29
<1
RK2
.
54
4
2
4
18
11
-
<1
-
-
4
RK3
41
29
12
2
-
-
-
-
-
8?
-
7
RK4
34
29
4
2
-
-
-
-
-
2
19
1
TL1
38
39
14
1
-
-
-
-
-
-
-
2
TL2
53
30
10
-
-
-
-
-
-
-
-
2
TL3
42
12
10
14
3
-
-
2
-
-
-
-
TL4
45
13
30
-
-
-
-
-
-
6
-
1
TL5
21
52
10
-
-
-
-
-
-
6?
-
1
TL6
12
5
2
.
3
55
-
-
-
-
-
-
"?" indicates the mineral was probably present but not precisely identified by XRD.
"-" indicates the mineral was not present in the sample.
' Samples may also contain accessory to trace amounts of one or more of the following minerals: rutile, sphene,
garnet, epidote, graphite, tourmaline, scheelite, topaz, apatite, fluorite.
40
-------
Table 9. Solid phase analysis for the Particle Size Experiment: Sulfur and carbon dioxide
contents as a function of particle size.
Solid
RK3
RK4
•
RK5
Particle Size
(mesh)1
-270
-100/+270
-35/ + 100
-10/+35
-W/ + 10
-270
-100/-f-270
-35/ + 100
-10/+35
-K"/ + 10
-*"/+*"
-270
-100/+270
-35/ + 100
-10/+35
-%"/ + 10
-*"/+%"
S (total)
0.95
1.98
2.34
1.90
1.50
2.83
3.01
2.61
2.79
3.52
3.46
1.51
0.87
1.29
0.88
0.83
0.93
Assays (%)
S as SO4
0.16
0.04
0.03
0.05
0.04
0.28
0.08
0.07
0.11
0.13
0.14
0.08
0.08
0.09
0.07
0.06
0.03
CO2
0.96
1.32
1.60
1.73
1.46
0.88
1.64
1.68
1.40
1.27
1.89
0.32
0.10
0.06
0.05
0.09
<0.01
Unless otherwise indicated.
41
-------
Table 10. Wet-Dry Cycle Test summary.
Solid
RK1
RK2
RK3
RK4
TL1
TU
TL3
TU
TL5
TL6
Initial
Sulfur
<*)
0.46
0.64
1.63
2.91
0.%
1.49
2.19
2.30
5.05
5.81
Initial
Sulfidc
<*)
0.42
0.63
1.60
2.82
0.90
1.45
2.12
2.10
4.85
5.18
Carbonate Mineral
Content1 (%)
C
tr
0.1
0.5
-
1.5
0.7
0.2
0.6
-
4.6
DAS
tr - 0.3
_ _
- 3.2*
3.0
0.4* -
0.8* -- 0.2
1.9 8.3
1.1* --
APP
(kg/t CaCOj)
«o2
14
20
51
91
30
47
68
72
160
180
'm
12
16
47
76
27
42
57
67
91
140
NP |(Ca/Mg)CO,]
(kg/t CaCOj)
<0
3'
1
5
32
19
16
19
6
15
46
•m'
0
0
0-1
10-15
12-13
0
0
0-2
0-7
0-9
ENP4
X3-4)
X3-4)
X4-5)
X17-22)
X6-7)
X17-19)
> (2 1-25)
X4-7)
0-6
> (37-65)
S04
0093
0.11
0.20
0.40
0.093
0.65
0.47
0.20
2 8
1 4
Release Rates (IIU)
(fimol/g- wk)
Ca + Mg
0.15
0.15
0.21
0.41
0.43
1.4
1.5
0.28
0.15
2.2
Ca
0.053
0.080
0.080
0.21
0.40
1.3
0.82
0 15
0.067
2.1
Mg
0 II
0.067
0 15
0.20
0027
0093
067
0 13
0.080
0.093
t, «= at time equals x. in weeks
1 C = calcite; D - dolomite; A = ankerite; S = siderite; tr = trace; - = not detected.
2 Based on total sulfur.
1 NPl(Ca/Mg)COjJ at 132 weeks calculated by subtracting calcium and magnesium release from NP[(Ca/Mg)CO,| at I0. Minimum value includes release from
weeks 0-132. while maximum value excludes the release attributed to removal of oxidation products generated prior to week 0 (weeks 0-5).
4 ENP = Empirical Neutralization Potential. Total NP release prior to the point at which the drainage pH dropped below and remained below 6.0. For solids
where the drainage pH did not remain below 6.0, NP was reported as greater than the NP release at the end of the test. The lower end of the range includes
release from weeks 0-5, while the upper end of the range excludes release from weeks 0-5.
5 Non-standard neutralization potential calculation, see table 7 for additional detail.
* See table 7 for additional detail.
-------
Table 11. Rates of release of sulfate, calcium plus magnesium, calcium, and
the Wet-Dry Cycle Test (Rocks RK1 - RK4).
magnesium for
Solid
RK1
RK2
RK3
RK4
Penod
(weeks)
0-3
4-59
60-132
0-24
25-83
84-132
0-6
7-25
26-105
106-132
0-4
5-40
41-122
123-132
No. of
Measured
Values
6
56
73
27
59
49
9
19
SO
27
7
36
82
10
Sulfate
m"
0.086
0.013
0.007
0044
0.017
0.008
0.110
0.024
0.011
0.015
0.444
0.022
0.085
0.030
r2"
0.528
0.975
0.993
0.962
0.993
0.992
0.877
0.987
0.992
0.995
0.868
0.947
0.991
0.993
Calcium + Magnesium
m
0.090
0.027
0011
0037
0.020
0.011
0.115
0.036
0.021
0.016
0.498
0.080
0.117
0.031
r2
0.941
0.994
0.992
0.960
0.994
0998
0.937
0.998
0.988
0.998
0.916
0.996
0.978
0.995
Calcium
m
0.032
0.010
0004
0024
0.010
0006
0.072
0.025
0.011
0.006
0.253
0.045
0.065
0.016
r2
0.933
0.988
0.989
0.958
0.9%
0.999
0.944
0.998
0.976
0.995
0.899
0.998
0.975
0.9%
Magnesium
m
0.057
0.017
0.008
0.013
0.010
0.005
0.043
0.011
0.010
0.011
0.245
0.035
0.052
0.015
r
0.945
0995
0993
0958
0.989
0988
0.923
0.998
0.995
0.999
0.933
0.994
0.983
0.994
Ca + Mg
SO.
1.047
: 07^
1 571
0 841
1 1"
1 375
1 046
1 500
1.909
1.067
1.122
3.636
1 377
1.033
m - slope - release in minol/week
r - squared correlation coefficient
43
-------
Table 11 (con'l).
Rates of release of sulfate, calcium plus magnesium, calcium, and
magnesium for the Wet-Dry Cycle Test (Tailings TLl - TL6).
Solid
TLl
TL2
TL3
TL4
/
TL5
TL6
Period
(weeks)
0-1
2-32
33-132
0-1
2-109
110-132
0-1
2-10
11-20
21-132
0-4
5-19
20-85
86-103
104-132
0-21
22-85
86-132
0-4
5-19
20-59
60-132
No. of
Measured
Values
4
31
100
4
108
23
4
9
10
112
7
15
66
18
29
24
64
47
7
15
40
73
Sulfate
m'
0.088
0.018
0.007
0.184
0.012
0.049
0.320
0.159
0.084
0.035
0.117
0.027
0.010
0.007
0.015
0.661
0.361
0.209
2.187
0.313
0.153
0.103
r1"
0.183
0.992
0.992
0.499
0.979
0.994
0.182
0.989
0.973
0.998
0.597
0.997
0.986
0.994
0.997
0.997
0.997
0.999
0.783
0.987
0.999
0.997
Calcium +• Magnesium
m
0 168
0.044
0.032
0 381
0.098
0.104
0.551
0.242
0.159
0.112
0.189
0.046
0.025
0.018
0.021
0.297
0.029
0.011
2.880
0.428
0.226
0.167
r
0.305
0.998
0997
0.574
0.996
1.000
0.363
1.000
0.990
1.000
0.773
0.997
0.999
0.993
0.999
0.931
0.950
0.989
0.829
0.988
1.000
0.999
Calcium
m
0.135
0041
0030
0.334
0.091
0.097
0.345
0.149
0.099
0.062
0.180
0.039
0.014
0.008
0.011
0.102
0.006
0.005
2.807
0.412
0.216
0.160
r1
0.282
0.999
0.999
0557
0.998
1.000
0.381
1.000
0.988
1.000
0.774
0.993
0.994
0.994
0.999
0.666
0.945
0.941
0.833
0.988
1.000
0.999
Magnesium
m
0033
0.003
0 002
0047
0007
0.007
0.207
0.094
0.059
0.050
0.009
0.007
0.011
0.009
0.010
0.195
0.023
0.006
0.074
0.016
0.010
0.007
r2
0425
0 946
0 937
0 708
0 951
0.984
0 334
1.000
0.993
0.998
0.741
0.982
0.994
0.991
0.997
0.995
0.950
0.999
0.568
0.990
0.997
0.999
Ca-t-Mg
SO.
1 909
T .\.\f\
4 5'!
: 071
8 167
: 122
1.722
1.522
1 893
3.200
1.615
1.704
2.500
2.571
1.400
0.449
0.080
0.053
1.317
1.367
1.477
1.621
m » slope - reletie in mmol/week
r1 - squared correlation coefficient
44
-------
Table 12. Percent depletion of acid production potential (APP) and neutralization potential
(NP[(Ca/Mg)CO3]) for the Wet-Dry Cycle Test for weeks 0 - 132.
Initial Total , ... . nn . nn n . .
_ ,, Initial APP APP Released
Sulfur
Solid
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
% kg/t
0.46
0.64
1.63
2.91
0.96
1.49
2.19
2.30
5.05
5.81
CaCO, mg as CaCO,
14.4 216
20.0 269
51.0 303
90.9 1150
30.0 193
46.6 315
68.4 848
71.9 341
158 5030
182 3390
APP Percent
Depletion1
weeks weeks
0-5 0-132
8.6 20
4.2 18
3.4 7.9
5.0 17
3.2 8.6
2.3 9.0
5.4 17
3.1 6.3
5.5 43.0
11 25.0
APP Remaining
kg/t CaCO,
12
16
47
76
21
42
57
67
91
140
Init
Solid
kg/t
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
• i K™ Calcium
131 NP Released
CaCO, mg
3 45.3
1 63.4
5 89.4
32 356
19 199
16 526
19 434
6 158
15 264
46 1870
Magnesium Np Released
Released
mg mg as CaC03
48.9 314
28.9 277
41.9 396
176 1610
12.2 546
25.7 1420
197 1890
32.5 528
163 1330
58.2 4900
NP Percent
Depletion1
weeks weeks
0-5 0-132
34.9 140
66.1 370
30.0 105
14.0 67.3
7.74 38.3
13.2 118
24.1 133
45.7 117
65.1 118
61.8 142
NP Remaining2
kg/t CaCO3
-0.15 to -1.2
-2.0 to -2.7
1.3 to -0.27
15 to 11
14 to 12
-0.77 to -2.9
-1.7 to -6.2
1.7 to -1.0
7.1to-2.7
9.1 to -19
1 Weeks 0-5 represent release attributed to rinsing off of oxidation products accumulated during sample storage prior the
beginning of the experiment. Weeks 0-132 represent release attributed to mineral dissolution plus release from weeks 0-5.
2 Ranoe of denletion renreaents weeks 6-132 ID weeks 0-132: negative sixn indicates calcium and maEnesium release exceeded
that reported with solid phase carbonate analysis
45
-------
Table 13.
Comparison of APP and NP[(Ca/Mg)CO3J release in the Elevated Temperature
(weeks 1 - 130/131) and the Wet-Dry Cycle (weeks 1 - 132) Tests.
APP Release
(kg/t CaC03)
Wet-Dry Elevated
Cycle Temperature
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
1.9
3.3
3.0
13
1.7
3.5
8.3
2.7
63
33
8.0
2.2
12
47
9.7
16
9.3
6.6
65
33
Ratio1
4.2
0.67
4.0
3.6
5.7
4.6
1.1
2.4
1.0
1.0
NP[(Ca/Mg)CO3] Release
(kg/t CaCO3)
Wet-Dry Elevated
Cycle Temperature
3.7
3.5
4.5
20
6.2
18
* 22
5.2
11
49
8.3
3.7
8.3
31
17
25
28
5.8
11
55
Ratio1
2 2
1.1
1.8
1.6
2.7
1.4
1.3
1.1
1.0
1.1
Ratio of Elevated Temperature Test release to Wet-Dry Cycle Test release.
46
-------
Table 14. Elevated Temperature Test summary.
Initial
Solid Sulfur
(*)
RK1
RK2
RK3
RK4
TL1
TL2
TU
TU
TL5
TL6
«. = .,
1 C =
» Roc.
0.46
0.64
1.63
2.91
0.96
1.49
2.19
2.30
5.05
5.81
time equals
= calcite; D
»;! nn friral c
Initial
Sulfide
0.42
0.63
1.60
2.82
0.90
1.45
2.12
2.10
4.85
5.18
x, in weeks
= dolomite;
Carbonate Mineral
Content1 (%)
C D A S
tr tr .-- 0.3
0.1
0.5 - -- 3.2*
3.0
1.5 0.4*
0.7 0.8* -- 0.2
0.2 - 1.9 8.3
0.6
1.1* --
4.6
A = anker ite; S = siderite; tr
APP
(kg/i CaCO,)
(16-18)
17-20
> (25-32)
3-6
0-8
> (42-79)
Release Rates (1I3U)
(fiinol/g- wk)
S04
0.11
0093
044
1.5
052
0.60
0 16
0.87
2.3
0.87
Ca + Mg
Oil
0 13
0.19
0.19
0.99
0.64
1.5
0.24
0.11
1.7
Ca
0.040
0.093
0.053
0.080
0.96
0.52
0.81
0.12
0.027
16
Mg
0.067
0040
0.13
0.11
0013
Oil
0.71
Oil
0.080
0 12
J NPj(Ca/Mg)CO3] at 130 weeks calculated by subtracting calcium and magnesium release from NP|(Ca/Mg)CO,| at t,,. The minimum value includes release from
weeks 0-130, white the maximum value excludes the release attributed to the removal of oxidation products generated prior to week 0 (weeks 0-6).
4 ENP = Empirical Neutralization Potential. Total NP release prior to the point at which the drainage pH dropped below and remained below 6.0. For solids
where the drainage pH did not remain below 6.0. NP was reported as greater than the NP release at the end of the test. The lower end of the range includes
release from weeks 0-6, while the upper end of the range excludes release from weeks 0-6.
1 Non-standard neutralization potential calculation, see table 7 for additional detail.
* See table 7 for additional detail.
-------
Table 15. Rates of release of sulfate, calcium plus magnesium, calcium, and magnesium for
the Elevated Temperature Test (Rocks RK1 - RK4).
Solid
RK1
RK2
RK3
RK4
Period
(weeks)
0-14
16-22
24-28
30-52
54-66
68-130
0-4
6-24
26-62
64-76
78-82
84-130
0-8
10-28
30-54
56-130
0-18
20-28
30-88
90-130
No. of
Measured
Values
10
4
3
12
T
32
5
10
19
7
2
25
7
10
13
38
12
5
30
21
Sulfate
m'
0.227
0082
0.101
0.029
0.016
0.008
0.084
0.017
0.013
0.010
0.013
0.007
0.287
0.107
0.071
0.033
0.906
0.308
0.173
0.109
r2"
0.990
0.989
0930
0996
0.997
0.985
0.813
0.992
0.992
1.000
1.000
0.999
0.952
0.972
0.994
0.994
0.994
0.989
0.998
0.993
Calcium -(-Magnesium
m
0.236
0.076
0.108
0.037
0.021
0.008
0 125
0.028
0.026
0.014
0.022
0.010
0.221
0.085
0.052
0.014
1.045
0.282
0.040
0.014
r1
0.992
0.993
0.913
0.989
0.979
0.985
0.894
0.999
0.990
0.963
1.000
0.998
0.963
0.976
0.994
0.971
0.989
0.981
0.934
0.999
Calcium
rn
0.098
0.031
0044
0016
0010
0.003
0.087
0.019
0.015
0.009
0.011
0.007
0.142
0.041
0.015
0.004
0.561
0.179
0.022
0.006
r2
0.993
0992
0914
0.985
0963
0.985
0.908
0.998
0.993
0.953
1.000
0.999
0.969
0.940
0.990
0.985
0.990
0.979
0.899
0.999
Magnesium
m
0 139
0045
0064
0021
0.011
0.005
0.038
0.009
0.011
0.005
0.011
0.003
0.079
0.044
0.037
0.010
0.485
0.103
0.018
0.008
i1
0.991
0 994
0911
0 991
0988
0.984
0861
0.997
0.978
0.977
1.000
0.996
0.951
0.988
0.995
0.960
0.986
0.984
0.968
0.999
Ca+Mg
SO.
1.040
0 927
1 069
1 2~6
1 313
1 000
1.488
1.647
2.000
1.400
1.692
1.429
0.770
0.794
0.732
0.424
1.153
0.555
0.231
0.128
m - slope » release in moot/week
1 r2 * squared comUoon coefficient
48
-------
Table 15 (con't). Elates of release of sulfate, calcium plus magnesium, calcium, and
magnesium for the Elevated Temperature Test (Tailings TLl - TL6).
Solid
TLl
TL2
TL3
TL4
TL5
TL6
Period
(weeks)
0-2
4-8
10-18
20-45
47-65
67-131
0-2
4-12
14-30
32^*5
47-63
65-131
0-4
6-24
26-51
53-63
65-131
0-2
4-6
8-20
22-77
79-131
0-8
10-22
24-28
30-47
49-131
0-2
4-18
20-30
32-63
65-131
No. of
Measured
Values
4
3
5
14
10
33
4
. 5
9
8
9
34
5
10
14
6
34
4
2
7
29
27
7
7
3
10
42
5
7
6
17
34
Sulfate
ra'
0.086
0.016
0.069
0085
0.060
0.039
0.180
0.053
0.290
0.124
0.079
0.045
0.466
0.160
0.041
0.026
0.012
0.216
0.030
0.018
0.014
0.065
0.725
1.730
0.590
0.212
0.169
2.879
0.449
0.239
0.126
0.065
r2"
0.230
0.997
0.976
0.992
0.990
0.993
0.535
0.995
0.983
0.987
0.998
0.996
0.717
0.994
0.995
0.995
0.976
0.301
1.000
0.987
0.987
0.997
0.926
0.994
0.999
0.956
0.996
0.694
0.999
0.974
0.984
0.993
Calcium + Magnesium
m
0.172
0.058
0.130
0.145
0.098
0074
0390
0.189
0.475
0.184
0.099
0.048
0.618
0.309
0.153
0.127
0.114
0.310
0.057
0.051
0.037
0.018
0.498
0.200
0.080
0.033
0.008
3.759
0.741
0.366
0.263
0.131
r2
0.358
1.000
0.991
0.999
0.993
0.994
0.643
0.994
0.9%
0.998
0.997
0.994
0.777
0.998
0.995
0.997
0.972
0.428
1.000
0.9%
0.978
0.905
0.766
0.978
0.999
0.997
0.968
0.690
0.996
0.989
0.999
0.992
Calcium
m
0.151
0.055
0.122
0.139
0.095
0072
0.358
0.169
0.439
0.166
0.085
0.039
0.396
0.202
0.086
0.071
0.061
0.295
0.054
0.040
0.020
0.009
0.265
0.048
0.020
0.007
0.002
3.645
0.705
0.347
0.250
0.122
i2
0.373
1.000
0.990
0.999
0992
0.994
0.644
0.993
0.9%
0.998
0.997
0.992
0.795
0.990
0.994
0.998
0.969
0.430
1.000
0.989
0.971
0.915
0.576
0.902
0.955
0.904
0.917
0.696
0.996
0.989
0.999
0.991
Magnesium
m
0.021
0.003
0.008
0.005
0.004
0.001
0.033
0.020
0.036
0.019
0.013
0.008
0.223
0.107
0.066
0.056
0.053
0.015
0.003
0.011
0.017
0.008
0.233
0.152
0.061
0.026
0.006
0.115
0.035
0.019
0.014
0.009
r1
0.276
0.964
0.997
0.990
0986
0.998
0630
0999
0.997
0.991
0.999
1.000
0.746
0.992
0.995
0.997
0.975
0.376
1.000
0.988
0.985
0.893
0.948
0.990
0.999
0.995
0.978
0.469
0.993
0.987
0.998
0.999
Ca + Mg
SO,
2.000
3.625
1 884
1.706
1 633
1 897
2 167
3 566
1 638
1.484
1.253
1.067
1.326
1.931
3.732
4.885
9.500
1.435
1.900
2.833
2.643
0.277
0.687
0.116
0.136
0.156
0.047
1.306
1.650
1.531
2.087
2.015
49
-------
Table 16. Percent depletion of acid production potential (APP) and neutralization potential
(NP[(Ca/Mg)C03J) for the Elevated Temperature Test for weeks 0 - 130/131.
Solid
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
Initial Total
Sulfur
%
0.46
0.64
1.63
2.91
' 0.96
1.49
2.19
2.30
5.05
5.81
Initial APP APP Released
kg/t CaCO3 mg as CaC03
14.4 676
20.0 198
51.0 1010
90.9 3780
30.0 802
46.6 1280
68.4 979
71.9 654
158 5290
182 3760
APP Percent
Depletion1 ^
weeks weeks
0-6 0-130
18 63
4.1 13
6.1 27
9.9 55
4.0 36
3.3 37
8.8 19
3.6 12
6.8 45
15 28
iPP Remaining
kg/t CaCOj
5.4
17
38
41
19
30
55
63
88
130
Splid ~
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
Initial NP
kg/t CaCOj
3
1
5
32
19
16
19
6
15
46
' Weeks 0-6 represent releas
exnerimenL Weeks 0-130
Calcium Magnesium p
Released Released K««sea
mg mg mg as CaCO,
114 97.7 685
78.7 23.4 292
131 90.6 699
555 275 2520
531 14.7 1380
704 49.5 1960
576 240.4 2430
177 39.2 602
284 159 1360
2250 80.4 5940
NP Percent
Depletion1
weeks weeks
0-6 0-130
82 , 305
79 390
52 187
29 105
10 97
19 160
37 170
51 130
72 120
81 170
reoresent release attributed to mineral dissolution phis release from weeks 0-6.
NP Remaining
kg/t CaCOj
-3.7to-6.1
-2.1 to -2.9
-0.76 to -4.3
7.5 to -1.6
2.5 to 0.55
-7.0 to -10
-6.2 to -13
1.0 to -2.0
7.7 to -3.2
4.1 to -33
irior to the beginning of the
Range of depletion represents weeks 8-132 to weeks 0-132; negative sign indicates that calcium and magnesium release exceeded that
reported with solid phase carbonate analysis.
50
-------
Table 17. Comparison of Elevated Temperature Test empirical neutralization potential
(ENP) and neutralization potential present as calcium and magnesium carbonate
(NP [(Ca/Mg)C03]).
Solid
RK1
RK2
RK3
RK4
TL1
TL2
TL3
TL4
TL5
TL6
NP [(Ca/Mg)C03]
(kg/t CaCO3)
3
1
5
32
19
16
19
6
15
46
ENP
(kg/t CaC03)
0-2.5
2.0-2.9
2-4.9
13-22
> (16-18)
17-20
> (25-32)
3-5.9
0-7.7
> (42-79)
Week
61
66'
18
14
1312
51
1312
55
0
1312
1 Drainage pH dropped below pH 6.0, but did not remain below 6.0 for duration of test.
2 Drainage pH remained above 6.0 for duration of test.
51
-------
Table 18. Percent moisture content of the covered reactors as a function of panicle size at
day seven of the drying cycle for the Particle Size Experiment.
Solid
RIG
RK4
RK5
Particle Size
(mesh1)
+270/-100
+ 100/-35
+ 35/-10
+ 10/-K"
+270/-100
+ 100/-35
+35/-10
+ 10/-K"
+ W"/-H"
-270
+270/-100
+ 100/-35
+357- 10
+ 10/-K"
+ H"M4"
Mass
(g)
75
75
1000
500
75
75
1000
1000
1000
75
75
75
1000
1000
1000
Mean Volume
(mL)
30.4
26.5
186
44.2
37.1
36.0
148
52.4
46.6
24.3
25.0
25.6
122
31.5
17.6
Moisture2
(%)
28.8
26.1
15.7
8.1
33.1
32.4
12.9
5.0
4.5
24.5
25.0
25.5
10.9
3.1
1.7
1 Unless otherwise indicated.
2 Percent moisture is reported as the mean volume of water retained in the reactors through week 30.
52
-------
Table 19. Classification of samples based on drainage quality and neutralization potential
(NP[(Ca/Mg)C03]) remaining in the Wet-Dry Cycle and in the Elevated
Temperature Tests.
Drainage pH
pH < 3.5
3.5 < pH <, 4.5
4.5 < pH < 6.0
occasionally below 6.0
6.5 < pH < 7.0
Wet-Dry Cycle Test
Elevated Temperature Test
NP[(Ca/Mg)CO3] Depleted
TL5
none
none
none
RK1, RK2
RK4(3.7)1, TL4(2.4), TL5 (1.0)
i
RK3 (4.1)
TL2 (4.6)
RK1 (4.2), RK2 (0.68)
none
NP[(CayMg)CO3] Nearing Depletion
6.5 <£ pH
pH decreasing
RK3, RK4, TL2, TL4.
TL1 (5.6Y
NP[(Ca/Mg)CO3] Remaining and Controlling Drainage Quality
7.2 £ pH <8.0
TL1, TL3, TL6
TUd.l)2, TL6(1.0)2
1 The parenthetical values represent the ratio of sulfate release (weeks 1-130/132) in the Elevated Temperature
Test to that in the Wet-Dry Cycle Test.
2 NP[(Ca/Mg)COJ calculated as nearing depletion.
53
-------
Figure 1. Expenmental apparatus (reactor) for the Wet-Dry Cycle and the Elevated Temperature
Tests, and for the 75 g samples of the Particle Size Experiment.
1.6 micron glass
fiber filter
Perforated
plate
54
-------
Figure 2. Drainage sample collection from large experimental reactor for the Particle Size
Experiment.
Vent hole
(1/8")
Lid
Glass, fiber
filter
4.0 in.
Sample bottle
(500ml)
7.5 in.
Perforated
wood base
Outlet pipe
(0.125 in)
55
-------
Figure 3 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL5 in the Wet-Dry Cycle Test (weeks 1 - 132).
20
T ' T
• calcium
O magnesium
40
for weeki 60-132:
0.1 < (Cat < 0.6
0.5 < (Mil < 2.5
60 80 100
Time, weeks
120
140
56
-------
Figure 4 pH and concentrations of net alkalinity, sulfate, calcium, and magnesium in
drainage from RK1 in the Wet-Dry Cycle Test (weeks 1 - 132).
• calcium
O magnesium
140
Time, weeks
57
-------
Concentration. mg/L
Sulfate. mg/L
pH
T)
OQ'
C
oo
« o
«, o
•-I P
o o
Bn
«v»
— D
0
O
>-*i
p
o ST
o
rt"
s
Ul C
- B
I (a
P
c: o-
¥ B
(»
OQ
P
B
Net alk. mg/L as CaCO,
-------
Concentration. mg/L
Sulfate. mg/L
C/l
OQ
c
£
O
B
p-
n
I
o
n
D
O
rt
O
o
D
a
r>
rr
l»
>—
5-
S- 2.
v> &
- B
I
(U
O
B
l»
00
O
Net alk. mg/L as CaCO
-------
Concentration. mg/L
Sulfate. mg/L
Tl
Brt
*•
— . D
g
s1
C/l *-*
s
B
to
OQ
0
n
a
5
Net alk. mg/L as CaCO
3
-------
Figure 8 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL4 in the West Dry Cycle Test (weeks 1-132)
I
Q.
8.0
7.5
7.0
6.5
60
40
o
«•»
cd
1 20
Cfl
0
35
_ o
21
O
o
§ 7
0
9 9,
,99
o ooe«< b
net alkaliity
PH
20 40 60 80
Time, weeks
60
40
BO
E
20 ^
«
0
• calcium
O magnesium
100 120
140
61
-------
Figure 9 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL2 in the Wet-Dry Cycle Test (weeks 1 - 132).
X
a
9.5
8.5
7.5
6.5
60
40
ti
4_l
Cd
I 20
V3
0
50
1 40
6
S 30
_o
<_>
ca
2 20
o
o
§ 10
U
•" T
0
,,7 y •
;'• ;' / ®
net alkalinity
, .
k'•%'"'a.-''« "•
J L.
20 40 60 80
Time, weeks
60
o
U
ea
40 "
CQ
20
0
SO
E
CO
u
• calcium
O magnesium
100
120
140
62
-------
Figure 10 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL1 in the Wet-Dry Cycle Test (weeks 1 - 132).
^b12
s
I 9
4-1
09
~ 6
4
o 3
U
i
O
cj
(4
ao
E
- 10 J
U
Z
0
• calcium
O magnesium
20 40 60 80
Time, weeks
63
100
120
140
-------
Concentration. mg/L
Sulfate. mg/L
pH
T1
OQ
c
S 5.
? s
o a
s s
H 5
5-
I cu
°l
o 5;
2- "^
«
en
— O
r» r>
c
B
(u
p
O.
B
p>
OQ
C
3
Net alk. mg/L as CaCO
3
-------
Figure 12 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL6 in the Wet-Dry Cycle Test (weeks 1 - 132).
8.0
• calcium
O magnesium
for week» 65-132:
33 < (Cal < 45
0.6 < (Mfl < 1.2
-------
Figure 13 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL5 in the Elevated Temperature Test (weeks 1 - 131).
i 1 1
• calcium
O magnesium
for weeks 65-131:
0.2 < (Cal < 0.6
0.4 < (Mgl < 1.1
10
30 40
50 60 70 80 90 100 110 120 130
Time, weeks
66
-------
Figure 14. Scanning electron microscope (SEM) photograph of leached feldspar grain
from TL5 in the Elevated Temperature Test (at week 26).
This grain displays etch pits and dissolution due to extended leaching (marked
by arrows on photo).
67
-------
Figure 15 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from RK4 in the Elevated Temperature Test (weeks 1 - 130).
• calcium
O magnesium
for weeks 76-130:
0.9 < (Cal < 2.0
0.9 < (Mgl < 2.1
60 80 100
Time, weeks
68
120
140
-------
Concentration. mg/L
Sulfate. mg/L
p
*
OQ
c
S pc
OQ
n>
p
O-
-i O
O P
B S
H S
5' g
^ *°
S" 2,
A
O
ja
•-»
n
o
ta
£L
o
*•*•
»=
C3
P
i B
rt
i/>
C
Net alk. mg/L as CaCO
3
-------
Concentration. mg/L
Sulfate. mg/L
K>
O
u*
O
O O
O
O
K>
O
O
pH
o
c
O
O
o
O
C/J OJ
O P
(u
B
s
O P
B
— . O
p p
s
s.
•H 5"
§ £
•o
«= ^
3 «
S-
l/l
g
p
o.
i B
(u
1 — OQ
1>J (3
CD r»
: — • c«
£'
S
O
O
o
o
o
o
Net alk. mg/L as CaCO
3
-------
Concentration. mg/L
Sulfate. mg/L
OQ
P
to
OQ
n
O
B
o
B
m
P
O-
o
P
g
n •—-
O- O
c
3
s
(U
1 S
^— rt
i>j ">
^T c'
' B
Net alk. mg/L as CaCO
3
-------
Concentration. mg/L
Sulfate. mg/L
OQ
c
oo
a
2
-1 O
o a
B 2
?a S
J^ S
^— «-»
— 5'
a a
H "
a 3
XJ
« l«
-i C
0 Si
it
s- g
^ £X
7 3
— OQ
o CB
c
3
Net alk. mg/L as CaCO
3
-------
Figure 20. pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
in drainage from RK2 in the Elevated Temperature Test (weeks 1 - 130).
9
8
7
X
a.
4
50
40 J
J
\
00 ,n
g 30
«
3 20
x>
O
CJ
CO
O
oo
en
00
-20 E
«
-40 -
z
-60
V
• calcium
O magnesium
140
-------
Figure 21. Scanning electron microscope (SEM) photograph of leached feldspar grain from
a 1.63% S sample similar to RK2 (olivine gabbro) at 289 weeks in a separate
dissolution experiment.
This feldspar grain displays an "iron-stained" surface and mild pitting creating an
irregular, deteriorated surface (lighter color on upper half of photo).
74
-------
Figure 22.
SEM photograph of leached pyroxene (augite) grain from a 1.63% S sample
similar to RK2 (olivine gabbro) at 289 weeks in a separate dissolution
experiment.
Lower photo is enlargement of portion marked with arrow on upper photo. This
enlarged area shows pitting and dissolution of augite grain.
75
-------
Figure 23.
SEM photograph of leached mica (biotite) grain from a 1.63% S sample similar
to RK2 (olivine gabbro) at 289 weeks in a separate dissolution experiment.
This grain displays surface and edge dissolution and has little physical integrity.
The light (white) spots are dissolution products. The areas noted by the arrows
are more dense concentrations of the dissolution pits.
76
-------
Figure 24. SEM photograph of leached amphibole (probably hornblende) grain from a 1.63 %
S sample similar to RK2 (olivine gabbro) at 289 weeks in a separate dissolution
experiment.
This grain displays an iron-rich coating over a partially dissolved, friable ulterior.
The parallel, light areas of the grain (as indicated by the arrows) are the areas of
iron enrichment.
77
-------
Concentration. mg/L
Sulfate. mg/L
pH
OQ
c
*- K)
o o
u»
o
o o
K»
o
-tk.
o
oo
O
O
O
OO
-J
oo
O
^1 ^^
B °
o
o
o
5T
00
O
O
O
K)
O
IB
OQ
rt
O P
0 S
=•
5f o
rt ~,
tn p
3 s
5? S
B ^
•a
A V)
*"* E-
c •"
^T r-»
O ^
S
& S
O-
B
P
CT
l/>
c'
B
I
K»
O
K>
O
Net alk. mg/L as CaCO,
-------
Concentration. mg/L
Sulfate. mg/L
pH
IB
OQ
n
o
0
c
•-I
CD
•o
sc
o
o
p
o
H 5
5" |
? S.
« r^
P> to
r-t- •—'
o p^*
p\ (^
•H 5
B ^"
T3
n vi
i^ P^
tr «>
C to
>-i «-»
o
-------
Figure 27 pH and concentrations of net alkalinity, sulfate. calcium, and magnesium in
drainage from TL6 in the Elevated Temperature Test (weeks I - 131).
150
• calcium
O magnesium
for weeks 67-131:
1.0 < (Mil < 1.6
SO 60 70 80 90 100 110 120 130
Time, weeks
80
-------
Figure 28.
28a.
28b.
SEM photograph of leached pyroxene (hedenbergite) grains from TL6 at 173
weeks in the Elevated Temperature Test.
The grain in 28a is extremely friable due to extended leaching.1 The particles in
the lower left corner crumbled off of the grain as it was moved. The fibrous
blades (noted by the arrow) in photo 28b are secondarily deposited minerals.
81
-------
Figure 29. Explanation of box plot figure.
The box plot is comprised of the central box, the whiskers, and the outliers.
• Within the central box, the median of the data set is depicted by the center horizontal line,
and the lower and upper hinges are depicted by the other two horizontal lines of the central box.
The median splits the ordered data set in half, and the hinges split those two resulting halves in
half again (i.e. the three horizontal lines in the central box represent the 25th, 50th, and 75th
percentiles of the entire data set). The distance between the two hinges is called the H-spread
(If).
• The whiskers (the two vertical lines) represent the range of values that fall within 1.5 H-
spreads of the two hinges.
• The outliers represent values that fall outside of the inner and outer fences. Asterisks
represent those values that lie outside of the inner fences but within the outer fences. The inner
and outer fences are defined as:
inner fence =
outer fence =
lower hinge - 1.5H
upper hinge + 1.5H
lower hinge - 3H
upper hinge + 3H
Whisker 1
Upper Hinge » ' — i
Median —
i
Lower Hinge 1
H
i
1 .3(ri)
3
3(H)
Outer Fence
82
-------
Figure 30. Drainage pH and release rates for sulfate and calcium plus magnesium
as a function of particle size from the Particle Size Experiment: RK3 (weeks
15-30).
5. 3
8f\
.0
.5
Q.
7.0
6.5
a f\
i i i i i
T
.
1 1 ' T
T • 1
T 1
i— —
' Vi _, ,_. . -1-
1 i
1
o
*
i i i i i
eo
"o
g
Is
J
£
*o
2
£
u.o
0.5
0.4
0.3
0.2
0.1
rt n
i i i i i
•so4
o O Ca+Mg
_
_
o
o
• • 0
• • ° -
1 1 1 1 1
Particle size fraction
83
-------
Figure 31. Drainage pH and release rates of sulfate and calcium plus magnesium
as a function of particle size from the Particle Size Experiment: RK4 (weeks
15-30).
X
Q.
3
»
00
\
"o
S
"w
CO
e«
4)
13
M
<*-
0
•*••
rt
*
^
8
7
6
5
4
3
2
4
3
2
1
0
i i i i i i
1 1 I T I 1 "" 1 T
i * ^
_
•*
_
-*- •*• -
1 1 1 1 1 1 ...
' ' I 1 1 1 1 1
• SO4
O Ca+Mg
0
^
o
0
_ —
o
o
° • •
•
1 1 1 1 1 1
Jt» .^ ..*r* ..,A« .^A'' . ..i^1"
A^
Particle size fraction
84
-------
Figure 32. Drainage pH and release rates of sulfate and calcium plus magnesium
as a function of particle size from the Particle Size Experiment: RK5 (weeks
15-30).
7
6
—
a
5
1
.0
tt 0.8
"o
J 0.6
W3
e«
| 0.4
O
« 0.2
ed
0.0
1 1 1 1 1 1
*
T L. | |
T T T x
_J . — .
1
1
i
- T 1 *
, 1 I " 1 L
4
• so4
0 O Ca+Mg
o
o
8
«Q \QP ifi \Q \|* ^K^'"
1^' vV^' >^'" AQl A'
> LI jf\ JT j ^ > ^>
Particle size fraction
85
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floe
Chicago, II 60604-3590
------- |