EPA-600/2-77-105
June 1977
Environmental Protection Technology Series
..
'
AMMONIUM-CARBONATE LEACHING OF METAL
FROM WATER-TREATMENT SLUDGES
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-105
June 1977
AMMONIUM-CARBONATE LEACHING OF METAL
VALUES FROM WATER-TREATMENT SLUDGES
by
J. B. Hallowell
E. S. Bartlett
R. H. Cherry, Jr.
Battelle Columbus Laboratories
Columbus, Ohio 43201
Grant No. R-803787-01
Project Officers
D. L. Wilson
Industrial Environmental Research Laboratory
C. J. Rogers
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268.
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
J.3. Environmental Protection
^ '' Dearborn St-eet,'R»«« '
. ..go, IL 60604
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DISCLAIMER
This report has been reviewed by the Industrial Environmental Research
Laboratory, Cincinnati, U.S. Environmental Protection Agency, and approved
for publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute endorsement
or recommendation for use.
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used, the related pollutional impacts on our environment and even on our
health often require that new and increasingly more efficient pollution con-
trol methods be used. The Industrial Environmental Research Laboratory--
Cincinnati (lERL-Ci) assists in developing and demonstrating new and improved
methodologies that will meet these needs both efficiently and economically.
As a part of these kinds of activities, this study was undertaken specif-
ically to explore and to develop processes based on the ammoniacal leaching of
metal-bearing sludges that are produced during wastewater treatment opera-
tions in the metal-finishing (and related) industries. The major objective
was to eliminate or to reduce sufficiently the heavy-metal content of the
sludge so that it would no longer constitute a potential ecological hazard.
Removal of copper and nickel from the sludge constitutes the first step in the
recovery and recycle of these important metal commodities. The results
reported herein can be utilized by those who need to remove copper and nickel
from mixed-metal-bearing sludges for the purposes of detoxification and
resource recovery.
For further information on this subject, contact the Industrial Pollution
Control Division, Metals and Inorganic Chemicals Branch.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
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PREFACE
Many of the potential hazards accompanying the unrestricted disposal of
wastes from biological and industrial processes have been recognized for many
years. Only in recent years, however, have the complexity and intensity of
the problems involved in waste management and disposal reached the point
where the quality of the air-land-water environment is critically impaired
in many geographical areas. As efforts to curb pollution have been intensi-
fied, legal standards have been made more strict.
Industrial wastes have contributed heavily to the degradation of the
quality of man's environment, and many industries have invested considerable
effort to abate pollution. The waterborne wastes produced in metal-finishing
operations, particularly, are such that complex and costly methods have been
needed to prevent the contamination of water supplies with the toxic com-
pounds used in these operations.
The specific steps involved in the treatment of metal-finishing wastes
vary considerably from installation to installation. However, the general
treatment consists of (1) separation of grease and oil, (2) reduction of
chromates, (3) destruction of cyanides by oxidation, (4) neutralization of
the waste streams, and (5) separation and disposal of the sludges. The
metals initially present in the wastewaters are precipitated as hydroxides
and appear in the sludges.
One of the most troublesome and costly aspects of the treatment of
metal-finishing wastes is the final disposal of the metal-bearing sludges
that appear in Step (5). Increasing congestion of urban areas in which many
of the larger plating facilities are located, together with the accompanying
increases in the cost of land, adds to the problems involved in ultimate
disposal.
These sludges may contain large quantities of relatively valuable
metalsnickel, chromium, cadmium, titanium, etc.--which often are in short
supply. The values of the nickel and chromium lost in plating waste in the
United States has been estimated at 25 to 30 million dollars per year (1975).
Sludges selected from various steps in the waste-treatment procedure may con-
tain 10 to 15 percent of nickel, chromium, etc., on a dry basisconcentra-
tions greater than those found in most ores containing these metals. The
economic feasibility of recovery would be largely dependent on the size of
the plating facility, the cost of the primary raw materials used in the
plating operations, and the cost of alternative methods of sludge disposal,
e.g., lagoons, landfills, etc.
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Because of the importance of eliminating potential waste-control prob-
lems, while concurrently recovering valuable materials, an investigation of
the possible recovery of metals from sludges produced during the treatment
of metal-finishing wastes was undertaken. Additional considerations in this
undertaking were that this is a general problem in the metal-finishing indus-
try and a large segment of the industry would realize benefits from a suc-
cessful study.
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ABSTRACT
This project was undertaken to explore and develop processes based on
ammoniacal leaching to recover metal values from metal-finishing wastewater
treatment sludges. The objective was to eliminate or to reduce sufficiently
the heavy metal content of the sludge so that it would no longer constitute a
potential ecological hazard, thus greatly reducing the cost of sludge disposal
Metallic copper and nickel were to be recovered by electrodeposition from the
filtered ammoniacal leachant and chromium (as chromate compound) from the
depleted sludge by a carbonate roast followed by water leaching. The market
value of these recovered materials would at least partially offset the costs
of the stripping and metal recovery, and at best would net a profit.
Early leaching experiments revealed greater difficulty than expected in
obtaining satisfactory depletion of the sludge. Consequently, experimental
efforts were concentrated upon optimizing tha leaching process. Preferred
leaching conditions were defined. With two-stage leaching at 50 C in concen-
trated ammonium carbonate, the copper-plus-nickel content can be reduced to a
level of less than 1 percent from an original content on the order of 10 per-
cent. By roasting the depleted sludge with soda ash, at least 80 percent of
the chromium can be recovered in water-soluble form. Controlled-potential
electrodeposition was shown to be capable of separately winning copper and
nickel values from the pregnant ammoniacal leaching liquor.
A process flow sheet is presented, and process economics are reviewed.
The economic evaluation indicates that it is probable that these operations
will be a net expense. This net expense falls within the very wide range of
costs reported for sludge disposal. The process studied would be an alterna-
tive only if costs for disposal of sludge rose to the highest part of the
current range.
This report was submitted in fulfillment of Grant No. R803787-01 by
Battelle's' Columbus Laboratories under the sponsorship of the U.S.
Environmental Protection Agency. This report covers a period from June 1,
1975, through October 31, 1976, and work was completed as of July 31, 1976.
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CONTENTS
Foreword iii
Preface iv
Abstract vi
Figures ix
Tables x
Acknowledgment xi
1. Introduction 1
2. Conclusions 3
3. Recommendations 5
4. Materials and Equipment 6
5. Experimental Procedures 9
Sludge Preparation 9
Leaching Procedures 9
Analytical Procedures 12
Chromium Oxidation 17
Recovery of Metal Values 18
Electrolysis 18
Cementation Studies 19
Precipitation with Sodium Aluminate 19
Tests for Acid teachability of Sludge after Treatment
to Remove Copper and Nickel 19
6. Results and Discussion 20
Leaching Studies 20
Sludge Pretreatment Effects 20
Leachant Effects 21
Temperature Effects 23
Leaching Time Effects 23
Aeration and C0£ Sparging Effects 24
Liquor-Enrichment Study 25
Leaching of Oxides and Hydroxides 25
Staged Leaching Studies 26
Discussion of Leaching Studies 28
Chromium Oxidation Study 29
Recovery of Metal Values 30
Electrolysis 30
Cementation 31
Precipitation with Sodium Aluminate 32
Leaching of Metals after Treatment of Sludge
to Remove Copper and Nickel 32
7. Practical Perspectives and Economics 34
vii
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CONTENTS
Appendixes
A. Data and Analysis Tabulations 40
B. Estimation of Plant Costs 52
vm
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FIGURES
Number Page
1 Flowsheet for plant cost estimation 36
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TABLES
Number Page
1 Oxide and Hydroxide Descriptions 8
2 Results of Sludge Analysis 10
3 Analytical Procedures for Copper Determinations 13
4 Analytical Procedures for Nickel Determinations 14
5 Analytical Procedures for Chromium Determinations 16
6 List of Major Equipment Items, Delivered Costs,
and Total Capital Cost Estimates 37
7 Categories of Daily Operating Costs and Credits 39
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ACKNOWLEDGMENTS
The cooperation of U.S. Gauge Division of Ametek, Sellersville,
Pennsylvania, is gratefully acknowledged. In particular, Messrs. C. E. Rausch,
L. Barthold, and K. Scheffler provided access to the plant, information on the
origin and nature of the sludge, and samples of the sludge used in this study.
Mr. William Marcovich provided continuing interest and encouragement of this
study from its inception.
The program was guided and monitored by Mr. D. L. Wilson of the
Industrial Pollution Control Division of the Industrial Environmental Research
Laboratory and Mr. Charles J. Rogers of the Solid and Hazardous Waste Research
Division, Municipal Environmental Research Laboratory, both at Cincinnati.
The support of the program was shared by those respective organizational
units.
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SECTION 1
INTRODUCTION
The treatment of wastewaters from 15 to 20,000 metal-finishing facili-
ties in the U.S. generates large amounts of sludge that require disposal.
The sludges are neutralized prior to disposal and contain concentrations of
heavy metals according to (1) the metals, alloys, and finishing operations
conducted in the plants and (2) the specifics of wastewater treatment or
management practiced by the plants. Some of these heavy metals are ecologi-
cally objectionable. Further, they represent wasted resources, which, if
recovered, would have substantial market value.
Both the ecological penalties and the financial incentive for recovery
of any heavy metals contained in such sludges depend upon the concentration
of the individual metals contained. Sludges that may contain percentages of
metals such as copper, nickel, cadmium, or chromium would probably benefit
by further treatment to reduce the level of these ecologically objectionable,
or even hazardous, components to a level of less than 1 percent. If this
could be achieved in a manner that would allow reclamation of the metals in
marketable forms, the return from sale or reuse of recovered material might
be used to offset the cost of the required additional disposal procedures
or safeguards.
Chromium, copper, and nickel are three heavy metals common at levels up
to 10 or 15 percent (dry) in many sludges derived from metal-finishing opera-
tions. Two of these, copper and nickel, are recoverable from low-grade
resources by ammoniacal leaching.*
Laboratory studies have shown that the metal values extracted from
metal-finishing sludges by ammoniacal leaching can be recovered by electroly-
sis.** Furthermore, because chromium, which is most often associated with
copper and/or nickel in metal-finishing sludges, is not complexed during
ammoniacal leaching, pregnant ammoniacal leach liquors should be relatively
* Hewedi, M. A., and L. R. Engle. The NH3-C02-H20 System at Atmospheric
Pressure in Nonferrous Extractive Metallurgy. International Symposium
on Hydrometallurgy. AIME, Chicago, Illinois, 1973.
** Tripler, A. B., R. H. Cherry, Jr., and G. R. Smithson, Jr. Reclamation
of Metal Values from Metal-Finishing Waste Treatment Sludges. Report
EPA-670/2-75-018. Battelle's Columbus Laboratories, April, 1975. 89 pp.
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free of chromium interference during electrolysis for copper or nickel. This
would not be the case, for example, in acid leaching, which solubilizes most
metal values (including chromium). Preference for ammoniacal leaching is
based in part on the knowledge that most sludges tend to be basic in charac-
ter (neutralized with lime--metals are probably contained in hydroxide form)
and acid leaching would require considerable acid waste in counteracting the
contained calcium before leaching would proceed at lower pH levels. Ammonium
chemicals are relatively expensive, but the ammonia values would not be con-
sumed chemically and would be recycled in a leaching scheme designed around
their use.
The original objectives of the present program were to (1) demonstrate
that rapid and complete leaching of copper and nickel values could be accom-
plished from a typical metal-finishing sludge, (2) establish procedures and
demonstrate the direct recovery of metallic copper and nickel values from the
pregnant leach liquor by electrolysis, (3) investigate the applicability of a
carbonate-roast/water-leach treatment for recovery of chromium after ammonium
carbonate leaching of copper and nickel, (4) combine operations in an inte-
grated process amenable to portable demonstration, and (5) estimate the pro-
cess costs. Underlying these objectives was the assumption that the process-
ing would reduce the copper, nickel, and chromiu.n levels of the sludge to a
point where the residuals of these metals would not be environmentally
objectionable.
The results of early experiments with leaching indicated considerably
greater difficulties in obtaining satisfactorily complete leaching of the
contained copper and nickel values than had been anticipated. Without at
least 90 percent reduction in the original copper and nickel contents of the
sludge, achievement of Objectives (2)-(5) was deemed academic. Consequently,
when this problem was not resolved in early experiments, a revised scope was
requested and approved by EPA. With this revision, the primary objective
became the demonstration of at least 90 percent removal of the contained cop-
per and nickel values from the sludge by leaching with ammonium carbonate.
It was towards this objective that experiments in the later stages of this
program were directed.
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SECTION 2
CONCLUSIONS
Experimental studies in this program were concentrated on defining an
ammoniacal leaching practice that would maximize the return of copper and
nickel values from metal-finishing sludges to the leach solution, while at
the same time minimizing the dissolution of chromium values. In relation
to this primary objective, the following conclusions are presented, with the
general assumption that the specific sludge examined is representative of
copper-, nickel-, and chromium-bearing sludges.
(1) The raw sludge is best leached without resorting to drying
or grinding before leaching. Drying and grinding of the
sludge before leaching undesirably increases the fraction
of contained chromium that dissolves during leaching with
ammonium carbonate.
(2) The preferred leachant is ammonium carbonate. This salt
is conveniently added to mixtures of water and sludge.
The strength of the aqueous phase is maintained at a
level of approximately 10 percent contained NH3 for best
results.
(3) Leaching extractions are most rapid where leaching is
conducted at a temperature of 50 C.
(4) The return of nickel to the aqueous phase is inhibited
(probably by the pH of the leach solution) when the
aqueous phase is rich in copper, and vice versa. For
this reason, leaching of sludges containing both cop-
per and nickel is best conducted in two stages, each
comprising 2 to 3 hours of leaching time. Interstage
processing involves the separation of the liquid and
solid phases, so that the subordinate metal during
the first stage of leaching is allowed to dominate
reactions in the second stage.
(5) No advantages were found to support the use of either
ammonium sulfate leaching or ammonium hydroxide forti-
fication of the ammonium carbonate leaching solution.
(6) Ammonium carbonate leaching is a practical method for
reducing the copper and nickel contents of sludges to
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levels of less than 1 percent, probably to levels on the
order of 1/2 percent or slightly lower. (Original sludge
samples contained 5-9 percent copper and 0.8-1.2 percent
nickel by weight on a dry basis.) However, some residual
copper and nickel values in sludges are apparently refrac-
tory against aqueous complexing in ammoniacal solutions.
Based upon the results of these limited studies, it is further con-
cluded that chromium values in the sludge can be largely removed by roasting
with sodium carbonate, followed by water leaching of the fused mass to
return soluble chromates or dichromates for recovery from the aqueous leach
solution. Studies were not sufficiently extensive to define precisely the
recovery limits; an 80 percent reduction in the chromium content of the
sludge was demonstrated. For sludges containing more than 10 percent chro-
mium, this recovery level constitutes a significant resource of chromium for
recycling.
The recovery of the metals copper and nickel from pregnant aqueous
liquors was studied only briefly. The technical feasibility of recovering
metals from ammoniacal solutions, either by electrowinning or by cementation
with zinc, was indicated.
Treatment of the raw, dried sludge used in this experimental program
by ammonium carbonate leaching to remove copper and nickel appears to have
reduced the susceptibility of the treated residue to further attack, both by
simulated acidic rainwater and by organic acids, such as acetic acid. The
evidence in support of this conclusion is derived from a single set of leach-
ing experiments. The acceptability in terms of reduction of hazard or of
insult to the environment cannot be judged until suitable regulations appli-
cable to hazardous materials containing heavy metals are promulgated.
.The economics of such a metals recovery operation were examined in terms
of a projected plant assumed to process 100 tons per day of raw (i.e., 20
percent solids) sludge and recover copper, nickel, and anhydrous chromic
acid as principal products. The capital cost of such a plant was estimated
to be in the range of 3-5 million dollars and the daily operating costs,
after credits were allowed for sale of the copper, nickel, and 003, were
estimated to amount to a net outlay. This cost was compared to the results
of a survey of sludge treatment and disposal costs which reported costs of
from $6/ton to $60/ton for treatment and disposal, depending on the type or
method of disposal, the nature of the waste, geographical location, and
general variations among disposal contractors and local regulations. It
was concluded that the cost associated with the process, while not repre-
senting a profit-making situation, might become an alternative of choice
should either the prices of metals increase or the disposal of the raw
sludge become the subject of stringent regulation because of the metals
content.
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SECTION 3
RECOMMENDATIONS
The major finding of this study is that the treatments investigated can
be used to lower the copper, nickel, and chromium in metal-finishing sludges
from a level on the order of 10 to 20 percent to a residual level on the
order of 1 percent. This reduction would undoubtedly significantly decrease
the risk of groundwater contamination associated with sludge storage.
Estimates of the costs of processing indicate that, under current condi-
tions, the net cost of processing in a projected plant falls within the very
wide range of current disposal costs. This wide range of disposal costs pre-
vents any firm recommendation regarding economic feasibility of the process
studied. Future events that could change this point of view would be (1)
disproportionate increases in the prices of chromium, nickel, and/or copper,
or shortages of these metals, and (2) increased costs associated with the
disposal of sludges, which would be substantially decreased if metal values
were removed using the process studied.
In view of these factors, it is recommended that the results of this
study be periodically evaluated in terms of regulations concerning the desig-
nation and disposal of hazardous wastes and further development studies be
keyed to the needs indicated by those regulations.
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SECTION 4
MATERIALS AND EQUIPMENT
The experimental work described in this report was conducted on samples
of sludge extracted from the sludge storage lagoon of the Ametek U.S. Gauge
plant at Sellersville, Pennsylvania. Brass, steel, and copper-nickel alloys
are the principal materials processed by Ametek. Operations that contribute
metal values to the sludge include pickling, bright dipping, deburring, and
electroplating (chromium, cadmium, tin, lead, nickel, and cyanide zinc).
The sludge is largely generated in the wastewater treatment processes,
which include collection, neutralization (with lime), cyanide destruction,
and chrome reduction. Treated wastes are settled in one of two wet lagoons,
from which the sludge is removed periodically to the storage lagoon.
An initial 18 kg sample of sludge was collected in July, 1975, and
transported to Battelle for initial experiments. Solids content was deter-
mined to be about 20 percent. Analyses showed the dried and blended solids
to contain 17.2 Cr, 8.7 Cu, and 1.1 Ni (percents by weight). A second sam-
ple of 213 kg was taken and shipped to Battelle in September, 1975. This
was dried at nominally 110 C, resulting in a solids weight of 46 kg. Ana-
lytical results for elements of particular interest contained in the blended
dry sludge were (percents by weight) 12.8 Cr, 5.8 Cu, and 0.91 Ni, with
standard deviations (at least six observations) of 0.82, 0.24, and 0.07,
respectively. Supplementary analysis of the dried sludge indicated (percent
by weight) 6.7 Si, 5.7 Ca, 4.1 Al, 2.0 Na, 1.6 Fe, 0.9 Mg, and 2.5 C.
The dried sludge was further characterized to determine its water solu-
bility. Samples were stirred in demineralized, double-distilled water for
3 hours, followed by filtration and analyses. It was determined that 10 to
11 percent of the dried sludge was water soluble or was volatilized during
drying of the filter cake at 105 C. Analyses of the filtrate indicated a
total solids content (evaporated at 103 C) of about 9.7 percent, with solute
analyses as follows:
Solute Specie Content, gm/100 gm sludge
Sulfate 6.3
Sodium 2.1
Calcium 0.65
Chloride 0.23
Magnesium 0.20
Organic carbon 0.05
Silicon 0.01
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Solute Specie Content, gm/100 gm sludge
Inorganic carbon 0.008
Chromium 0.007
Potassium 0.007
Nitrate-nitrite nitrogen 0.003
Nickel 0.001
Cu, Mn, P, Fe, Zn <0.001 ea
Total identified 9.6
Thus, the heavy metals contained in the sludge are virtually insoluble in
water.
A special series of leaching experiments was conducted on cupric, nick-
elous, and chromic oxides and hydroxides. These were purchased as fine pow-
ders, except for chromic hydroxide, which was available only as a relatively
coarse powder. Sources and purities are indicated in Table 1. These mate-
rials were used in the condition in which they were received.
Chemicals used in all leaching experiments were usually reagent grade.
Lot assay data were available for ammonium carbonate, ammonium hydroxide,
and ammonium sulfate, which were the principal leachate components explored.
Leaching and wash media were prepared using demineralized, double-distilled
water. Reagents used in analyses of filtrates, sludge, and filter cakes for
copper, nickel, and chromium were prepared and standardized at Battelle
using reagent grade chemicals. Periodic analytical checks were made using
atomic absorption equipment by TraDet, a local commercial testing laboratory.
No specific equipment was required for the performance of this program.
Leaching and limited electrolytic studies were conducted in conventional
laboratory glassware and employed available equipment (stirrers, rectifiers,
ovens, pressure filter, etc.).
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TABLE 1. OXIDE AND HYDROXIDE DESCRIPTIONS
CuO
Cu(OH)2
NiO
Ni(OH)2
Cr203
Cr(OH3)
Supplier
Mallinckrodt
Atomergic
Chemicals
Baker
Alfa-Ventron
Baker
Alfa-Ventron
Grade
Reagent
"99%"
Reagent
Reagent
C.P.
Technical
Assay
Weight Percent
79.5 Cu
64.4 Cu
77.9 Ni
61.6 Ni
68.4 Cr
38.5 Cr
Basis
< 0.5 percent
impurities
99 percent Cu(OH)2
99.1 percent NiO
"Typical assay"
Assumption
76.2 percent Cr(OH)3
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SECTION 5
EXPERIMENTAL PROCEDURES
SLUDGE PREPARATION
To facilitate quantitative evaluation of results, both sludge samples
were dried at HOC to remove free water. The dried sludge samples were
then crushed, ball-milled (dry) and tumble-blended (as two lots) in a
plastic barrel. Analytical details of these lots are given in Table 2.
For later experiments, two nominal 1-kg samples of the Lot 2 dried
sludge were further wet ball-milled in a ceramic mill. Each sample was
ground with 2-1/2 liters of water. The resulting slurries appeared to be
quite stable suspensions and were determined (by redrying at 110 C) to con-
tain 28.5-29 percent solids as expected. Analyses on these slurry samples
were compatible with those of the Lot 2 dried sludge from which they were
prepared, except for nickel. For the first of the slurry samples (Sample
No. 1), nickel was analyzed to be 1.3 weight percent [with a standard devia-
tion of 0.009 (four analyses)], which was somewhat greater than the expected
value for the Lot 2 dried sludge. Nickel in slurry Sample No. 2 was analyzed
to be 0.96 percent, in accord with the Lot 2 value.
LEACHING PROCEDURES
Leaching was conducted in 2 or 3-liter glass beakers. For leaching of
the raw sludge (Lot 1 with 19.5 percent solids was used for these early
experiments), 500-gram samples of sludge (97.5 grams of solids) were weighed
immediately after homogenizing with a paint stirrer. For leaches of dried
sludge, 100-gram samples were weighed when withdrawn from the plastic stor-
age barrel in which the dried sludge had been tumble-blended. Leaches con-
ducted on the wet-ball-milled sludge slurries used 350-gram samples contain-
ing 102.5 grams of solids. These samples were weighed immediately after
stirring of the slurry batch for homogenization. The sample to be leached
was placed in a beaker, which usually contained a predetermined amount of
water. The leachate and its method of addition varied for the numerous
experiments that were conducted. In all cases, the weight ratio of solids
to be leached to leaching solution was within the range of 1:9 to 1:14.
Nominally, leaches comprised 100 grams of solids and one liter of leachant.
Leachant make-up and strengths were as follows:
(1) Premixed (NH4)2C03 plus NH^OH to result in a leachant
strength of 6:5 contained NHo:C0> in percent by weight.
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TABLE 2. RESULTS OF SLUDGE ANALYSIS*
Lot 1
Lot 2
Percent Solids
Copper, Wei
95 percent
Nickel , Wei
95 percent
n
X
s
ght Percent, Dry Basis
n
X
s
confidence range
ght Percent, Dry Basis
n
X
s
confidence range
3
19.5
0.03
3
8.7
0.2
7.84 - 9.56
3
1.09
0.02
1.00 - 1.18
4
21.75
1.25
6
5.8
0.235
5.19 - 6.
9
0.91
0.07
0.75 - 1.
41
07
Chromium, Weight Percent, Dry Basis
95 percent
n
X
s
confidence range
3
17.2
0.27
16.0 - 18.3
6
12.8
0.82
10.7 - 14.
9
* In this table: n = number of observations; x = mean value;
and s = standard deviation.
10
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(2) Same as above, but at a strength of 11:9.
(3) Same as above, but at a strength of 25:14. (This was
nearly saturated.)
(4) NH4OH at two strengths:
(a) Concentrated (29 weight percent NH.J
(b) Dilute (6 weight percent NH.J.
(5) (NH4)2C03 at three strengths:
(a) Dilute (5 weight percent NH-)
(b) Saturated at room temperature
(c) Supersaturated at room temperature (9 weight per-
cent NH~). This was used in two forms: premixed
and as a direct addition to a stirred sludge-water
mixture.
(6) Premixed (NH^SCty (200 g) and NH4OH (40 ml of 29 per-
cent NH3) to a leachant volume of 1 liter. Strength
was 5 weight percent
For leaching experiments conducted at elevated temperature which used
a direct addition of (NH^)?^ as the leachant, the water-sludge slurry was
usually heated to about 5 degrees warmer than the planned leaching tempera-
ture to accommodate the loss of temperature upon addition of the (1^4)2003,
which dissolves endothermically. In all cases, the water-sludge mixture was
thoroughly stirred with a propeller to effect a vortex that extended nearly
to the bottom of the beaker. This stirring commenced prior to the addition
of the leachant and continued throughout the experiment. Stirring was inter-
rupted only briefly to allow the taking of grab samples for intermediate
analyses during some of the experiments. Leaching time was measured from the
time of leachant addition to the time the stirrer was shut off. Upon con-
clusion of leaching, the mixtures were pressure filtered. Beaker contents
were chased with distilled water into the filtration vessel. The usual time
lapse between the end of leaching and the beginning of pressure filtering
was not greater than 10 minutes. Leaching times ranged from 2 to 64 hours.
For staged leaching experiments, most of the wet filter cake was trans-
ferred to a beaker containing 200 ml of water, and the filter paper was
rinsed in a second beaker. Then the contents of the two beakers were com-
bined for the next stage of leaching.
During leaching, beakers were covered with a hinged plexiglass lid
to prevent splash losses. Holes for the stirrer shaft and a thermometer or
thermocouple (used with a recorder for extended leaches) were provided.
11
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For leaching experiments where grab samples were taken, the stirrer was
stopped, half of the lid raised, and the sample (nominally 80 ml) was dipped
from the top of the mixture, the lid closed, and the stirrer started again.
This procedure normally took about 10 seconds. The grab samples were then
gravity filtered, and filtrates were analyzed for copper, nickel, and chro-
mium. In a few cases, the residues were dried, weighed, and analyzed as
well.
In addition to chemical analyses of filtrates and selected head stock
and filter cakes, data that were regularly obtained were filtrate (including
wash water) volume and weight and wet and dry filter-cake weights (with
proper tares for vessels and papers). Final filtrate volume measurements
were accurate to roughly 1 percent, and weights were accurate to within
about 1/2 percent, with some uncertainty attached to the filter paper tare
value.
Intermediate data relating to analyses and calculations based on grab
samples were less accurate because of the crudeness in estimating volume of
the leaching mixture. The leaching beakers were graduated in 100 ml incre-
ments. Graduation marks were probably accurate to about ± 10 ml. Estimates
of mixture volume (made while the stirrer was off) were to the nearest 10 ml
and were probably accurate to only ± 10 ml. Frequently, a layer of 1 to 5
cm of foam, not counted in the mixture volume, was present. No allowance
was made for the uncertain volume of the suspended solids, which was probably
on the order of 20 ml. Inaccuracies due to foaming and solids would tend to
offset each other. Because of these uncertainties, intermediate data are
probably only accurate to about ± 30 ml, or roughly 3 percent of the volume
of the mixture. These inaccuracies, however, did not influence the conclu-
sions drawn on the basis of the data generated. Calculations describing the
cumulative leaching action in experiments where grab samples were taken
included progressive consideration of metal values removed in the grab sam-
ples, both in solution and as suspended solids (the latter prorated except
where filter residues were weighed and analyzed).
ANALYTICAL PROCEDURES
The procedures used in the determination of concentrations of copper,
nickel, and chromium in samples of sludges, filtrates, and filter cakes are
given in Tables 3, 4, and 5. The results obtained with these methods were
checked periodically by submitting duplicate samples for atomic absorption
analyses at a commercial laboratory. Good checks were obtained.
All analyses were conducted at least in duplicate. Thus, each reported
analysis was the average of two or three individual results and had a sample
standard deviation associated with it. During this program, evaluations of
these standard deviations were made wherein the standard deviations were
assumed to be normally distributed and were pooled. At least ten analyses
of each type were considered. The pooled values, with their associated
degrees of freedom, were used to estimate the variability expected at the 90
percent level of significance. For copper and nickel, analyses of solids
12
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TABLE 3. ANALYTICAL PROCEDURES FOR COPPER DETERMINATIONS
For sludge, a filter residue 0.2-gram samples, weighed to nearest
0.1 mg.
For leach filtrate--2 ml (pipette) add 25 ml H20--boil gently for 5
minutes with chips.
1. Add 5 ml concentrated HC1 and 2 ml concentrated HN03 in a 250 ml
Erlenmeyer flask.
2. Digest slowly and evaporate to dryness (brown color).
3. Add 3 ml concentrated HC1 and evaporate to dryness again to drive off
the last of the nitrates.
4. Cool and add 80 ml of H20.
5. Make faintly ammoniacal by adding 1:1 NH^H from a burret.
6. Add 10 ml concentrated acetic acid and boil for 1 minute (use boiling
chips).
7. Cool ; then 3 minutes before ready to titrate, add one porcelain spoonful
of sodium fluoride and one spoonful of potassium iodide. Let stand for
1 minute.
8. Add 3 ml starch indicator just before beginning titration; titrate with
0.02 N sodium thiosulphate to pale blue end point.
Percent copper in residue or sludge:
= 6.354 x ml of NapSgOo x normality of
weight of sample, g
= 63.54 x ml NagS203 x normality of NapSpQ.,
volume of sample, ml
13
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TABLE 4. ANALYTICAL PROCEDURES FOR NICKEL DETERMINATIONS
For sludge in filter residue - 0. 5-gram samples, weighed to nearest
0.1 mg.
For leach filtrate - 20 ml (pipette) add 25 ml HLO - boil gently for
5 minutes with chips.
1. Put samples in 400 ml beaker and add 30 ml 1:1 HLSO. and 4 ml
concentrated
2. Fume for 10 minutes at high heat.
3. Cool and add 3 ml concentrated HN03.
4. Boil to drive off reddish brown fumes until white fumes of S03 start to
evolve.
5. Cool and add 100 ml hLO and stir (heat if necessary) to dissolve salts.
6. Filter through a Wattman 541 paper. Wash 5 times alternately with 5%
HC1 and hot water. Discard paper and precipitate.
7. To the filtrate and washings, add 25 ml citrate-sulfate solution.*
8. Warm on hot plate to about 50 C and add 25 ml dimethyl glyoxime**
solution.
9. Add concentrated NH40H until faintly ammoniacal (blow and smell) to
precipitate nickel .
10. Digest 30 minutes at 60 C, then for 3 hours or longer at room temperature.
11. Filter through weighed sintered-glass crucible. Wash at least 5 times
with cold water.
12. To filtrate, add 5 ml dimethyl, glyoxime solution to certify complete
precipitation.
13. Dry crucible and precipitate at 130-150 C for 1 hour.
14. Cool in a dessicator and weigh.
* 200 g citric acid monohydrate in 1 liter dilute (1:9) H2S04.
** 10 g dimethyl glyoxime in 1 liter absolute alcohol.
14
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TABLE 4. (Continued)
Percent nickel in residue or sludge
= 20.32 x grams dimethyl-glyoxime nickel precipitate
weight of sample, g
g/1 nickel in solution (filtrate)
= 203.2 x grams dimethyl-glyoxime precipitate
volume of sample, ml
15
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TABLE 5. ANALYTICAL PROCEDURES FOR CHROMIUM DETERMINATIONS
For sludge or filter residue - 0.2-gram samples, weighed to
nearest 0.1 mg
For leach filtrate - 20 ml (pipette) add 25 ml HLO - boil gently for
5 minutes with chips.
1. Put samples in a 400 ml beaker and add 20 ml H9SO. (1:1), 3 ml
concentrated H3P04. ^ ^
2. Fume for 10 minutes and cool.
3. Add 3 ml concentrated HN03 and boil to drive off reddish brown fumes of
nitrogen.
4. Cool and add 100 ml of H20.
5. Add 5 ml of 2% AgNO., and 20 ml of 10% ammonium persulfate and boil for
10 minutes.
6. Cool and with a pipette add 50 ml of 0.1 N ferrous ammonium sulfate
solution.
7. Titrate excess of the ferrous ammonium sulfate with 0.1 N KMnO, until
a change in color persists for 2 minutes.
A = ml KMnO. required to react with total amount of ferrous ammonium
sulfate^added.
B = ml KMnO. used in back titration.
Percent chromium in residue = 1-733 x normality of KMn04 x (A-B)
weight of sample, g
g/1 chromium insolution (filtrate) = 17.33 x normality of KMn04 x (A-B)
volume of sample, ml
16
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exhibited about twice the variability as was associated with analyses of
filtrates, as indicated below.
Expected (90 percent significance)
± Variability, percent of
reported value
Element Soli dIs Solutions
Cu 11 5
Ni 11 6
Cr 5 6
Reasons for these variations were not determined but are believed to be
related to sampling or analysis techniques. It is important to be aware of
these variabilities, or limitations on accuracy, in considering the experi-
mental results and in arriving at conclusions based on them.
CHROMIUM OXIDATION
Metal-finishing sludges are the product of waste-treatment practices
that include operations designed to reduce chromium to the chromic state
(insoluble in water) from the chromate or dichromate state (water soluble).
Chromic chromium is reportedly unaffected by leaching in ammonium salts, and
chromium thus largely remains with the solid residue after leaching, effect-
ing a desirable segregation of metal values. Because of the high level of
chromium in many sludges, its recovery would be economically desirable.
A series of roasts of high-chromium leaching residues in the presence
of soda ash (N32C03) was conducted to briefly evaluate the feasibility of
chromium recovery via reoxidizing it to the +6 valence state and leaching
the product in water. In these studies, the residue from a 20-hour leach
of raw sludge from Lot 1 (17.2 percent Cr) was used. Before roasting, the
residue was determined by analysis to have a chromium concentration of 22.2
weight percent on a dry basis.
The roast/leach procedure for chromium conversion and solution was as
follows:
(1) Two grams of dried residue was blended with 3-1/2 grams
of N32C03 in a crucible. (This represents about a 50 per
cent excess of the stoichiometric requirement for
formation from ^03.)
(2) The crucible and its contents were brought to a red heat,
and the fused mass was maintained at temperature for 1
hour.
(3) After cooling, the fused mass was broken up and dissolved
in distilled water.
17
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(4) The mixture was filtered through a slow paper and the
residue washed until the filtrate was colorless.
(5) Filtrate and wash were combined and diluted to 500 ml.
Chromium analyses were made on this diluted volume.
This experiment was conducted in duplicate.
RECOVERY OF METAL VALUES
One can visualize a general type of process for the leaching of metal
values from a sludge in which leach liquor is circulated continuously between
the leach vessel and a process subsystem for the removal of one or more of
the metals dissolved in the liquor. The leach liquor is returned to the
leach vessel after passing through this subsystem. After the original sludge
has been depleted of its content of heavy metals, the treated, detoxified
sludge is discharged. The process sequence is then repeated.
Candidate subprocesses for the removal of metals from the leach liquor
include, but are not necessarily limited to, the following:
Electrolysis to win metals from ammonium carbonate leach
liquors
Cementation of metals from ammonium leach liquors
Selective precipitation by the addition of appropriate
reagents to the leach liquor.
The first two of these candidate subprocesses offer the advantage that
metal values are produced as such; they are described under Metal Winning
Experiments.
The last of these candidate subprocesses offers some possible advan-
tages, such as a quick, simple means of removing from the leach liquor dis-
solved metals (e.g., copper and nickel), which display apparent antagonistic
effects in terms of mutual solubility in the leach liquor.
Electrolysis
Electrowinning experiments for the recovery of copper and nickel were
conducted before actual leaching filtrates were available. Synthetic
sludges were prepared by precipitating salts of copper, nickel, chromium,
zinc, and iron as hydroxides in aqueous solution, and precipitation of alu-
minum as the phosphate. The precipitates were then leached in 1M (NH^COs
to simulate the complexing action expected during leaching and filtered. If
all the precipitates had returned to solution upon treatment with ammonium
carbonate, concentrations, in g/1, would have been 4 Cu, 0.4 Ni, 10 Cr,
0.5 Zn, 2 Fe, and 5 Al. The pH of the filtrates (six solutions with varying
combinations of metals were explored) ranged from 8.75 to 9.40, and conduc-
tivities ranged from 84,000 to 93,000 ymho-cm.
18
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Electrolysis experiments were conducted on the above filtrates in a
three-compartment cell, with catholyte and anolytes separated by a medium-
pore glass frit. Active electrodes were of platinum. A calomel electrode
was contained in the third cell, and a capillary bridge referenced this
electrode to the catholyte. Experiments were conducted at controlled poten-
tials designed to deposit copper at the higher potential and nickel (plus
zinc) at a somewhat lower potential.
Observations in these preliminary studies were largely qualitative and
concerned the color and nature of deposits as the electrolytes were depleted.
The dependence of current density upon solute concentration for the deposit-
ing species was observed.
Cementation Studies
Samples of leaching filtrates were subjected to cementation. In these
beaker studies, metallic zinc or iron was added to the filtrate with the
intent of replacing the contained copper and nickel in solution by a more
electronegative metal. The resulting solutions were filtered and analyzed
for copper and nickel. Additional details are given in the Results Section.
Precipitation with Sodium Aluminate
Ammonium carbonate leach liquor containing copper, nickel, and chromium
was treated with an aqueous solution of sodium aluminate. The sodium alumi-
nate solution was prepared by adding aluminum powder to a strong sodium
hydroxide solution until all evidence of hydrogen evolution ceased. The
sodium hydroxide solution was prepared by adding approximately 5 grams of
reagent-grade solid sodium hydroxide to 100 ml of demineralized water. The
results are presented in Section 6.
TESTS FOR ACID LEACHABILITY OF SLUDGE AFTER
TREATMENT TO REMOVE COPPER AND NICKEL
It was judged necessary to obtain an experimental indication of the
effectiveness of the ammonium carbonate leaching of copper and nickel in
producing a treated sludge that would be less subject to attack by the
environment and one which therefore would be more acceptable in terms of
insult to the environment. Samples of dried, treated residue from Experi-
ments 35B and 36B (after two-stage ammonium carbonate leaching of the origi-
nal sludge were exposed for 24 hours to
(1) Simulated (sulfuric) acid rainwater (pH 4)
(2) Simulated acidic environment in a landfill (1 percent by
volume glacial acetic acid in water).
For purposes of comparison, two samples of untreated dried sludge
(which had not been leached with ammonium carbonate) were exposed to these
same acid environments. The results of these exposures are described in
Section 6.
19
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SECTION 6
RESULTS AND DISCUSSION
LEACHING STUDIES
Leaching studies were conducted to evaluate the influence of (1) pre-
treatment of sludge, (2) leachant type and strength, (3) leaching tempera-
ture, (4) leaching time, (5) aeration and C02 sparging, (6) loading of the
leach liquor, (7) oxide vs. hydroxide forms of contained metals, and (8)
staged leaching. Detailed descriptions of the individual experiments are
tabulated in Appendix A, Table A-l.
The principal criteria for evaluation were chemical analyses of fil-
trates to determine the degree of extraction from the sludge of copper,
nickel, and chromium during the leaching experiments. Analytical data
obtained on filtrates and residual solids are listed in Appendix A, Table
A-2. Quantitative data calculated from measurements and analyses are pre-
sented in Table A-3, and the results of recovery calculations and accounta-
bilities are reported in Table A-4.
As discussed previously, it was the objective of these studies to maxi-
mize the amounts of copper and nickel extracted (dissolved in the pregnant
leach liquor), while minimizing the extraction of chromium. Judgmentally,
it was assumed that at least 90 percent extraction of both copper and nickel
would be required for returns to be considered satisfactory.
Sludge Pretreatment Effects
Four forms of the Ametek sludge were examined in the various leaching
experiments. These were (1) the raw sludge, which contained about 20 per-
cent solids', (2) sludge that had been dried at 110 C, then comminuted and
blended; (3) dried sludge that had been ball-milled with water to create a
slurry suspension; and (4) dried sludge that had been washed with water to
remove about 10 percent of its weight as water-soluble material, followed by
filtering, redrying, and grinding.
The single experiment with sludge which had been water-washed to remove
water-soluble constituents suggested that water-washing was detrimental to
both nickel and copper recoveries. The percentages of the copper, nickel,
and chromium which were dissolved during leaching (relative to the amounts
which were in the sludge before leaching), expressed as percent extracted,
were as follows:
20
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Copper: 34 percent
Nickel: 11 percent
Chromium: 4 percent
Other sludge pretreatments evaluated in leaches at room temperature showed
no significant effects on extractions, but some weak trends may be present,
particularly relative to an undesirable increase in chromium solubility
resulting from reslurrying.
Raw Dried Reslurried
Sludge Sludge Dried Sludge
Experiment numbers* 1,2,3 8,9,11,15,16,17,18 29
Copper
Average, percent 57 53 56
Standard deviation 4.2 6.3
Nickel
Average, percent 19 30 19
Standard deviation 7.1 10.6
Chromium
Average, percent 2 7 15
Standard deviation 2.1 4.7
This trend (increasing chromium extraction) was also supported by other
direct comparisons (Experiment 32 vs. Experiment 33 and Experiment 19 vs.
Experiments 28 and 30). Relative to copper and nickel extractions, condi-
tioning of the sludge is of no value.
Leachant Effects
Among early investigations, the effects of the strength of ammonium
carbonate-ammonium hydroxide (AMC-AMH) were explored in leaches done at room
temperature. Strengths can be categorized as weak (with a 6:5 weight per-
cent ratio of contained NH3:COo value), moderate (11:9), and strong (25:14).
The effects of leachant strength upon extraction were
Weak Moderate Strong
Experiment numbers 1,2,3,15 8,9,11 17
Copper
Average, percent 52 56 59
Standard deviation 9.7 7.2
* Details of experimental conditions are given in Appendix A.
21
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Weak Moderate Strong
Nickel
Average, percent 18 29 23
Standard deviation 5.5 3.2
Chromium
Average, percent 26 7
Standard deviation 1.7 3.2
These data suggest a slight inferiority in copper and nickel recoveries
associated with the use of the weak leachant.
Leaches conducted with only ammonium hydroxide (AMH) (Experiments 12
and 13) resulted in the extraction of the following percentages of metals:
Average, Standard
percent Deviation
Cu 34 6.4
Ni 6 2.1
Cr 2 1.4
AMH is clearly less effective than AMC-AMH, regardless of strength. Room
temperature leaches with only ammonium carbonate were conducted using a
strong (supersaturated) mixture. Results for Experiments 16, 18, and 29
were
Average, Standard
percent Deviation
Cu 57 1.5
Ni 31 13.7
Cr 14 2.1
These results for copper and nickel are comparable to those for "moderate"
or "strong" AMC-AMH solutions. The nickel extractions varied from 19 to 46
percent; however, note the high standard deviation values. For AMC leaching,
the chromium removed from the sludge was undesirably high.
Ammonium sulfate leaches were conducted at room temperature (Experiment
10) and at 50 C (Experiment 37B). In neither case were extractions promis-
ing. Statistics are
Average, Standard
percent Deviation
Cu 31 2.8
Ni 8 0.7
Cr 0.6 0.6
22
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AMC, AMH, and AMC-AMH leaches exhibit a pH of about 10 or higher on
these copper-rich sludges. As a consequence, the leaching of nickel may be
difficult. AMS leaches proceed at a lower pH, which should be conducive
to improved nickel extraction, but such a result was not observed.
Best results in terms of copper and nickel extractions were obtained
with moderate or strong AMC-AMH or the strong AMC leachants.
Temperature Effects
Leaches were conducted at room temperature and at moderately elevated
temperatures (50-70 C) using strong AMC or AMC-AMH leachants on either dried
sludge or slurries. The average percentage of metals extracted were
Elevated
Room Temperature
Temperature (50-70 C)
Experiment numbers 16,17,18,29 19,20,28,30
Copper
Average, percent 58 82
Standard deviation 1.5 10.1
Nickel
Average, percent 32 38
Standard deviation 9.9 9.8
Chromium
Average, percent 11 19
Standard deviation 4.0 7.5
From this comparison, it is apparent that leaching at elevated temperature
is beneficial to copper extraction and probably also to nickel extraction.
Chromium segregation appears to be lessened at elevated temperatures, but
the elevated temperature results for chromium are biased by a proportionately
greater number of leaches conducted on slurries, rather than dried sludge.
In general, this analysis indicates a strong preference for performing
leaches at modestly elevated temperature, e.g., 50 C.
Leaching Time Effects
Several sets of experiments were conducted illustrating the effects of
leaching time. One comparison between the second and third experiment (room
temperature, dilute AMC-AMH) indicates modest improvement in copper and
nickel extractions attributable to longer leaching time. The percentage of
metals extracted were
Experiment number 3 2
Leaching time, hours 20 3
Copper extraction, percent 60 54
23
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Nickel extraction, percent 24 14
Chromium extraction, percent 1 Low
In concentrated leachants at room temperature, the following comparisons are
possible of the percentages of metals extracted:
Leaching time, hours 3 64
Experiment numbers 16,17,18 32,33
Copper
Average, percent 58 68
Standard deviation 1.2 2.1
Nickel
Average, percent 33 38
Standard deviation 11.9 0.7
Chromium
Average, percent 10 15
Standard deviation 3.5 4.2
For leaches conducted at 50 C, the following comparison is valid:
Experiment number 19 34
Leaching time, hours 3 64
Copper extraction, percent 90 70
Nickel extraction, percent 49 38
Chromium extraction, percent 12 8
From these results, leaching at room temperature for times substantially
longer than 3 hours appears to result in a modest increase in extraction of
copper and nickel, but at 50 C, the opposite conclusion appears warranted.
Analyses of grab samples taken at 16 and 40 hours during the 64-hour leach-
ing experiment conducted at 50 C (Experiment No. 34) indicated that nickel
extraction would have been maximized if leaching had been terminated at
about 16 hours. For copper, extraction would have been best if leaching
were stopped at about 40 hours. The respective extractions would have been
56 and 92 percent. These results are compatible with a postulated progres-
sive loss of NH3 values by volatilization from the leaching mixture during
extended leaching at 50 C.
Overall, it appears that extending the leaching time beyond 3 hours
does result in improved extractions of copper and nickel. The improvements,
however, are marginal and probably are not sufficient to justify the added
costs that would be involved in substantial increases in leaching time
beyond the nominal 3-hour value.
Aeration and C02 Sparging Effects
Three experiments (Experiments 8, 9, and 11) were conducted with bub-
bling of airorC02 through the stirred mixture while leaching proceeded. The
objective was to promote the higher valence states in the contained copper
24
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and nickel values in the sludge, hopefully, for significant improvements in
extraction. This leaching variation was ineffective, as indicated below by
percent extractions which do not differ from leaches in which no sparging was
used.
Copper
Average, percent 52
Standard deviation 1.5
Nickel
Average, percent 34
Standard deviation 8.2
Chromium
Average, percent 8
Standard deviation 4.6
Nickel extraction was highest (41 percent) in the C02-sparging experiment,
but chromium solubility was also undesirably high. Aeration or sparging was
concluded to be ineffective.
Liquor-Enrichment Study
In early leaching studies, copper extraction was less than expected;
despite reports of AMC-AMH leachant being able to support levels of at least
50 g/1 of copper in ore leaching, there was doubt concerning whether or not
this was true in the leaching of sludge. To test this, an experiment
(Experiment 6) was conducted wherein the same leach liquor was used to leach
three fresh samples of dried sludge. Each stage of this experiment was
designed to maintain the liquid:solids weight ratio at 9:1. Leached solids
were filtered and washed after each stage. The filtrates and wash stock
were combined for analysis. The combined filtrates (including a total of
1042 g of wash, some of which was retained in the wet filter cakes) weighed
1692 g and comprised a volume of 1.51 liters. Analyses showed concentra-
tions (g/1) of 11.6 Cu, 0.50 Ni, and 1.6 Cr for contents of 17.5 g Cu,
0.76 g Ni, and 2.4 g Cr. Based on the volume of leachant alone (assuming
complete washing and subtracting the wash volume not used as leachant), con-
tents of at least 21 g Cu, 0.91 g Ni, and 2.9 g Cr are supported by the
leachant. These support levels appreciably exceed actual returns found in
leaching experiments. It was concluded that leachant saturation was not
limiting recoveries to any significant degree.
Leaching of Oxides and Hydroxides
When a number of leaching experiments failed to produce what could have
been considered satisfactory extractions, it became desirable to evaluate
leaching reactions of known oxide and hydroxide forms of the metals copper,
nickel, and chromium, both individually and in selected combinations. To
this end, eight experiments (Experiments 21-27 and 31) were conducted at
50 C using the strong (supersaturated) AMC leachant. Cupric oxide (Experi-
ment No. 21) was observed to leach slowly but did return 49 percent of the
copper value in a 3-hour leach. Nickelous oxide (Experiment No. 22) and
25
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chromic oxide (Experiment No. 23) were not leached to any significant degree
(< 5 percent recoveries). The corresponding hydroxides of copper (Experiment
No. 24) and nickel (Experiment No. 26), when leached individually, were com-
pletely complexed in 2 hours or less (100 percent recoveries). Chromic
hydroxide was not affected (Experiment No. 25).
For Experiment 27A, the combined copper and nickel contents (125 g)
exceeded the amount that could be supported by the initial amount of AMC
added. After leaching for 3 hours, virtually all of the copper had been
complexed, but very little of the nickel was returned to solution. The mix-
ture was not filtered but was stored for 21 days. Following this, addi-
tional water (0.6 1) and AMC (333 g) were added, the mixture was stirred and
heated to 50 C, and leaching was continued for 3 more hours. This resulted
in a substantial, but still inadequate, increase in nickel extraction (to 46
percent). At this point, the mixture was filtered and the wet filter cake
combined with fresh leachant. Based on filter cake analysis, solids after
the first leaching contained about 2 percent copper, 8 percent nickel, and
25 percent chromium. The final leach extracted 64 percent of the contained
nickel but only 29 percent of the contained copper. (Chromium returns to
solution were < 5 percent.)
In Experiment 31, 32.2 g'Cu and 6.2 g Ni, as hydroxides, were leached at
50 C in strong AMC. The leachant in the 9:1 solids:liquid mixture was more
than adequate to support full extraction of the contained metal values.
Extraction of copper was very good (95-100 percent), but nickel remained
largely with the solids and extraction was < 5 percent.
In these experiments, and in corroborative qualitative leach tests on
10-gram samples, it was determined that the leaching of copper proceeds at
a pH value of about 10, and leaching of nickel in AMC solutions proceeds best
at somewhat lower pH (8 to 9). So long as copper predominates in the mix-
ture, the pH will exceed a value conducive to good nickel recoveries. Con-
versely (Experiment 27B), when nickel predominates, its extraction is
improved, but,at the lower pH level, extraction of copper is inhibited.
These results, in particular, indicated the desirability of multistage
leaches.
Staged Leaching Studies
Five staged leaching experiments were performed. Two of these were
three-stage leaches conducted at room temperature in medium-strength AMC-AMH
leachants (Experiments 5 and 7). For these, the filter cakes from the first
and second stages were dried and ground to provide head stock for the second
and third stages. Net recoveries for these leaches are summarized below in
terms of percentages of available metal which was extracted into the leach
liquor:
Copper
Average, percent 80
Standard deviation 6.4
26
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Nickel
Average, percent 38
Standard deviation 1.1.3
Chromium
Average, percent 15
Standard deviation 4.2
Extractions of copper and nickel were modestly improved over single leaches
for 3 hours at room temperature but on a par with single leaches conducted
at 50 C. Chromium extraction followed this increasing trend (undesirably).
Three variants of staged leaches were conducted at 50 C. These were
(1) a two-stage leach, 6 hours per stage (Experiment 35); (2) a two-stage
leach, 2 hours per stage (Experiment 36); and (3) a two-stage leach, 2 hours
per stage, with AMS as the leachant for the second stage (Experiment 37).
Strong AMC was the leachant for both stages in Experiment 35 and the first
stage of Experiment 37. Medium strength (11:9) AMC-AMH was the leachant for
Experiment 36. In these experiments, filter cakes from the first stage were
not dried but were directly returned to water baths for the second stage of
leaching. The results of these experiments are summarized as follows in
terms of the percentages of available metal extracted from the solids into
the leach liquor:
Experiment
Number Type Cu Ni Cr
35 6-hr 92 64 34
36 2-hr 90 53 26
37 2-hr, AMC/AMS 84 56 16
Summary Statistics
Copper
Average, percent 87
Standard deviation 4.2
Nickel
Average, percent 58
Standard deviation 5.7
Chromium
Average, percent 25
Standard deviation 9.0
In summary, the second-stage AMS leach of Experiment 37 was ineffective, as
indicated in earlier discussion of leachant type. There appeared to be a
slight advantage of extended time (Experiment 35 vs. Experiment 36), neglect-
ing the possible effect of leachants on copper and nickel extractions, but
the desired chromium segregation was also degraded by the increased leaching
time (or higher C02 content in the leachant). Overall, two-stage leaching
27
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resulted in satisfactory copper extractions. Nickel extractions, although
improved, were less than satisfactory, according to the criterion set for
this study.
Discussion of Leaching Studies
In summary, the preferred leaching conditions defined in these studies
are as follows:
Sludge pretreatment--none
Leachant--ammonium carbonate or ammonium carbonate-ammonium
hydroxide
Leachant strengthabout 10 weight percent contained NH3
Leaching temperatureabout 50 C
Leaching proceduretwo stages of leaching, with mechanical
agitation. Each stage should be conducted for 2 to 3
hours. Supernatant liquid from the first stage should be
removed prior to conducting the second stage.
Based on evaluation of the results obtained in this study, this processing
of the Ametek sludge would be expected to return at least 90 percent of the
copper, about 60 percent of the nickel, and less than 10 percent of the
chromium to the leaching liquids. Solid residues, after leaching, would con-
tain about 0.5 percent each of copper and nickel and about 12 percent chro-
mium. These values may be compared to the initial raw sludge, containing
approximately 6 percent copper, 1 percent nickel, and 13 percent chromium.
Reasons for not being able to extract all of the copper and nickel from
the Ametek sludge are not known. However, based on the observations of the
degrees and time-dependence of extractions of copper and nickel, it is specu-
lated that these metal values may be contained partly as hydroxides and
partly as oxides in the Ametek sludge. Crudely estimated, copper is sug-
gested to exist as about 70 percent hydroxide and 30 percent oxide and nickel
on the order of 50:50. If this is reasonably accurate, then some fraction of
the contained metal values would be refractory regarding ammoniacal leaching,
and limits would exist to the complete extraction of metal values from this
sludge by this method. The low degrees of extraction may also have been
dependent on some unquantified effects of the drying and grinding of most of
the sludge used for leaching experiments. Drying the sludge at 110 C may
have altered the constitution of the original precipitates produced during
waste-water treatment, possibly toward a more refractory form. The effects
of lagoon storage prior to sampling also may be speculated as affecting the
form in which the metals are present.
The Ametek sludge was the only material upon which studies were con-
ducted. Other copper- and nickel-rich sludges may respond differently to
AMC or AMC-AMH leaching. However, it seems likely that, in the treatment of
any sludge, some limits to the recovery of metal values by ammoniacal leach-
ing are to be expected. A question of controlling importance would be: Is
the extent of extraction that is practical sufficient to reduce the amounts
of residual metals to innocuous levels so as to permit virtually unmonitored
28
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and much less expensive sludge disposal? This question is addressed in
greater detail in Section 7 of this report.
It had been expected that the chromium extraction by an ammoniacal
leaching solution would be negligible, thus allowing virtually complete
segregation of chromium to the residual solids. This situation would sim-
plify operations for recovery of copper and nickel from the pregnant liquor,
as well as the extraction and recovery of chromium in further treatment of
the residue. The ideal chromium segregation appeared to be closely
approached in experiments on raw sludge. However, experimental leaching of
dried and ground sludge, and particularly of the reslurried, ball-milled
sludge, produced less favorable results, and appreciable amounts of chromium
were extracted by the leaching solution. Chromium is not strongly complexed
by the NH3 liquid, and chromic hydroxide is too weakly hydrolizable to
account for the high extractions observed. The most logical explanation of
the higher-than-expected extractions of chromium is the likelihood of the
presence of chromate or dichromate ions, which are readily soluble. If, in
chromium-reduction treatment of waste water to produce the trivalent state,
particles of sludge are generated that retain an enclosed nucleus of a chro-
mate compound, this nucleus could be exposed in the process of regrinding the
dried sludge, and particularly in the process of ball-milling the reslurried
dried sludge. This hypothetical explanation is in harmony with observations,
but whether or not it has a basis in fact is unknown. An argument against
this theory was the result of water-washing of the dried sludge described in
Section 4 of the report, where only 70 ppm of the 12-13 percent chromium con-
tained was water soluble. This reported value for chromium, however, was
suspect, as the water-wash filtrate was amber in color, which suggests a
much higher chromium content. To check this, the filtrate was reanalyzed
and was found to contain about 0.7 percent chromium, suggesting that about 5
or 6 percent of the chromium in the dried sludge was soluble in water.
CHROMIUM OXIDATION* STUDY
As described in Section 4 of this report, duplicate 2-gram samples of
the dried filter cake from leaching Experiment 2 were roasted with an excess
of sodium carbonate, and the fused mass was water leached to dissolve the
converted chromates.
Analysis of the starting filter cake indicated that each 2-gram sample
of this material would contain 0.44 g of chromium. Duplicate analyses on
each of the two tests indicated that the amount of chromium dissolved was
0.34 g, with a standard deviation of only 0.0064 g. Accordingly, the extrac-
tion of chromium from the leached sludge was indicated to be 77 ± 2 percent
at the 95 percent level of significance.
This was considered to be an adequate demonstration of feasibility, and
no further investigation of the influence of processing variables was con-
ducted.
29
-------
RECOVERY OF METAL VALUES
Electrolysis
Leaching filtrates prepared from simulated sludges containing copper,
nickel, and zinc were evaluated. In addition to these metals, some fil-
trates contained chromium, iron, aluminum, and other trace metal ions as
impurities carried over from leaching of prepared simulated sludges. The
filtrate-electrolyte contained ammonium carbonate at a level about equivalent
to a dilute AMC leaching liquor (1 molar solution).
The significant differences in behavior during electrolysis were
observed for the various solutions examined. There was no apparent effect
of varying impurity types or levels.
The open-circuit voltages of the solutions were measured and were about
+ 0.6 V with respect to the calomel electrode. Current-voltage traces indi-
cated three reduction reactions occurring at overpotentials of
(1) + 0.55 to + 0.34 V, the range for copper reduction
(2) + 0.11 to -0.34 V, the range for nickel and zinc reduction
(3) -0.28 to -0.78 V, where hydrogen was evolved at the working
electrode.
In the experimental electrodeposition of copper from these solutions,
the current densities (indicative of deposition rates) were inversely related
to the concentration of copper in solution. Thus, in electrolytes contain-
ing, for example, 6 grams per liter of copper, a current density of 20 mA/cm2
was observed, but at a copper concentration of 0.6 g/1, current density was
about 2 mA/cm2. For electrodeposition of nickel and zinc (which are both
deposited in the same potential range), current densities were not dependent
upon concentration but remained quite constant at about 2 mA/cm2 over a con-
centration range of 0.1 to 0.01 mole per liter (about 6 and 0.6 g/1, respec-
tively). Current densities were diffusion limited, so that, even with stir-
ring, deposition efficiencies would remain somewhat less than 100 percent.
One of the filtrates, prepared from a simulated sludge that, in addition
to copper, nickel, and zinc, contained from about 0.1 to 0.6 g/1 of tin,
lead, calcium, zirconium, magnesium, and titanium, as well as major chromium,
iron, and aluminum contaminants, was used for more detailed electrolysis
experiments. This solution was electrolyzed at four different potentials,
each potential level corresponding to a current plateau on the current-
voltage curves. Deposition times ranged from 10 minutes to 2-1/2 hours.
The results were as follows:
(1) At + 0.45 volt and 10 mA/cm2, the cathode was unchanged;
metal was not deposited.
30
-------
(2) At -0.24 volt and 50 mA/cm2, a dark red deposit adhering to
the platinum substrate was obtained, confirming copper depo
sition. The deposit was completely stripped from the plati
num in 1 :1
(3) At -0.50 volt and 60 mA/cm2, a red, powdery copper deposit
was obtained, which could be stripped from the substrate.
Nickel and/or zinc may have been codeposited in small
amounts (< 1-16 percent).
(4) At -0.56 volt and 4 mA/cm2, nickel was deposited in a
dense, adherent form from the 0.1 M Ni solution. The
deposit did not dissolve in cold 1:1
These results indicate the technical feasibility of electrolytic
recovery of metallic copper and nickel from pregnant liquors from ammonium
carbonate leaching. Analyses of the deposits and definitive quantitative
experiments were not conducted in deference to the more pressing problem of
attempting to define satisfactory leaching methods.
Cementation
Samples of filtrate from leaching Experiment 35A were used for prelimi-
nary cementation experiments designed to cement copper and nickel from solu-
tion with zinc. This filtrate contained 3.7 g/1 of copper and 0.37 g/1 of
nickel. All cementation experiments were conducted at room temperature.
In the first experiment, 1 gram of zinc strip was added to 20 ml of
filtrate (contents = 0.074 g Cu, 0.007 g Ni) and allowed to stand for about
3 hours. The zinc strip lost 0.4 gram in this treatment. Solids that accu-
mulated on the bottom of the beaker were copper colored; the residue was fil-
tered, dried, and weighed. Its weight was approximately 0.1 gram, or
slightly greater than the combined copper and nickel content of the original
sample. The filtrate was almost colorless instead of the deep blue color
associated with copper- or nickel-laden pregnant liquors.
In a second experiment, 200 ml of the filtrate was used to completely
dissolve 4 grams of zinc strip in 3 hours. The resultant mixture was fil-
tered. Analysis of the rosy-colored filter residue showed that all of the
copper originally in solution had been replaced by the zinc. The blue color,
typical of copper- or nickel-ammino complexes, was gone from the filtrate,
which was light green in color.
A third experiment was similar to the first, except that 1 gram of zinc
powder was used for cementation. Rapid reaction was indicated by frothing,
and, within a few minutes, the typical copper-nickel blue color had dis-
appeared from the solution.
An attempt to cement with iron was totally unsuccessful. (Iron will not
replace copper or nickel in arnmino complexes.)
31
-------
It was the intent of these preliminary studies to confirm the technical
feasibility of the recovery of metallic copper from ammoniacal leach liquor
by cementation, and this was successfully accomplished. Quantitative data
necessary for an assessment of economic viability were not obtained.
Precipitation with Sodium Aluminate
An alternative method of removing the copper, nickel and chromium from
the ammonium carbonate leach liquor is to precipitate the metals with sodium
aluminate. In a preliminary test, sodium aluminate was added to a sample of
leach liquor by stirring in NaOH and aluminum metal powder simultaneously.
The initial and final metal contents of the filtered liquor were as follows:
Metal Concentration, mg/1
Metals Initial Final Removal Efficiency
Copper 3780 1.1 99.99+
Nickel 350 2.8 99.2
Chromium 1380 2.1 99.8
Several mechanisms for the precipitation are possible. The copper,
nickel, and chromium may have come down as aluminates, if such exist, and are
insoluble. Alternatively, the carbonate ion present in the ammonium carbon-
ate solution may precipitate A1(OH)3 by the reaction
2NaA102 + C02 + 3H20 -> 2A1(OH)3 + Na2C03 (I).
The precipitated aluminum hydroxide should be an excellent adsorbent for
metal ion species in solution. This suppositionand the suggestion that
carbonate ion plays an important role in this chemistryis supported by the
fact that no precipitation occurs from metal-bearing leach liquors which con-
tain only ammonium hydroxide (but no carbonate ion) and which are treated
with sodium aluminate solution. The addition of solid ammonium carbonate to
the ammonium hydroxide leach liquor which contains sodium aluminate causes an
immediate precipitation reaction, with removal of the characteristic blue
color from the liquor.
LEACHING OF METALS AFTER TREATMENT OF
SLUDGE TO REMOVE COPPER AND NICKEL
Residues from the two-stage, ammonium-carbonate leaching Experiments 35A
and 35B, and for dried sludge, were analyzed as follows:
Residue Residue Dried, Unleached
35B 36B Sludge
Copper, percent 1.6 1.3 5.9
Nickel, percent 0.59 0..70 1.3
Chromium, percent 13.2 13.0 12.3
32
-------
Five grams each of the dried residues from Experiments 35B and 36B were
stirred with 200 ml of each of two acid media.
(1) Sulfuric acid at pH 4, which simulates an action somewhat
more severe than by exposure of the treated residue to
very heavy rainfall in the northeastern United States.
(2) One percent by volume glacial acetic acid, which repre-
sents an attempt to simulate worst-case conditions in a
nonsegregated landfill which is producing a variety of
organic acids.
Five-gram samples of untreated dried sludge also were exposed to each acid
under identical conditions. Analyses of the resulting solutions after
removal of solids are as follows:
Metal Concentration, mg/1 in
Sulfuric Acetic Acid,
Solid Acid, 1 percent by
Used Metal pH 4 volume
Residue 35B Copper 0.08 14.9
Nickel < 0.05 5.0
Chromium 5.4 6.6
Residue 36B Copper 0.05 10.0
Nickel < 0.05 4.8
Chromium 3.2 6.9
Dried sludge Copper 0.20 463.0
Nickel 0.45 51.0
Chromium 2.4 106.0
As expected, the exposure to organic acetic acid, which is a ligand-
former, shows high levels of metals leaching from dried sludge. What was
not expected was the remarkable diminution of metals Teachability after car-
bonate leaching for all three metals.
Ammonium-carbonate treatment of dried sludge, as in Experiments 35 and
36, reduced the susceptibility to acid rainwater leaching of copper and
nickel from two to four times, as reflected by this preliminary experimental
exposure. The opposite is true in terms of the Teachability of the chromium
upon exposure to simulated acid rainwater; chromium Teachability increased
approximately 50 to 100 percent. It should be noted that no attempt was
made to remove chromium from dried sludge by leaching in ammonium carbonate;
rather, the removal of chromium from the original sludge would be the sub-
ject of further experimental treatments. It is expected that reduction of
the chromium content of dried sludge would produce rainwater-resistant resi-
due, as in the case of copper and nickel.
33
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SECTION 7
PRACTICAL PERSPECTIVES AND ECONOMICS
Studies performed on this program have defined, at least to a first
order, the levels of sludge detoxification with regard to copper, nickel,
and chromium that can be expected via ammoniacal leaching and soda ash roast-
ing. An order-of-magnitude reduction of these metals in sludge is achiev-
able. They also have indicated that the recovery of metal values (or chemi-
cal value in the case of chromium) is technically feasible.
At present, deposits of such sludges of varying quality exist in stor-
age sites at many of the larger industrial metal-finishing organizations.
Smaller operations often dispose of sludge by contract haulage to landfills,
where allowed by state regulations. Because of near-term environmental
regulations and the expectation of more stringent future promulgations, the
status of sludge deposits will change. It is certain that existing sludge
heaps will not disappear, and, in the near term, they may reasonably be
expected to grow.
The potential ecological threat of sludge piles is unknown, but cer-
tainly varies on an individual basis, depending on storage-site construction
(e.g., whether or not lagoons are lined), stratigraphy and hydrology of the
areas where sludge sites are located, and the nature of the sludge deposits
themselves. Sludge deposits represent a possibility of future hydrological
contamination via leaching and seepage. (Some states, in recognition of this
risk, have invoked measures to prevent seepage, such as requiring the lining
of storage lagoons with plastic.)
The reduction of metal contents of sludge would have a cost associated
with it. Based on the indicated processes (two-stage leaching with electro-
lytic recovery of copper and nickel, chromium oxidation and recovery as
chromic acid anhydride), a flow sheet was derived and costs were estimated
on the basis of a plant to process 100 tons per day of raw sludge containing
an estimated 20 tons of solids comprising 1.2 tons of copper, 0.22 tons of
nickel, and 2.6 tons of chromium. The cost estimate was developed on a
"bare bones" basis, and little detail was developed.
The hypothetical plant included the following considerations:
(1) The plant was considered to consist of operations to recover
copper, nickel, and chromium.
(2) The process was considered to include
34
-------
(a) An ammonium-carbonate leach system to extract copper
from the sludge with the copper being recovered from
this first-stage leach liquor by electrolysis
(b) A second-stage ammonium-carbonate leach to extract
nickel from the sludge, with the nickel being
recovered from the second-stage leach liquor by
electrolysis
(c) A sodium-carbonate roast of the solid residue, fol-
lowed by a water leach to recover chromium as a solu-
tion of sodium chromate, which was further processed
to produce anhydrous chromic acid.
(3) It was assumed that 90 percent of the copper was recovered,
60 percent of the nickel was recovered, and 90 percent of
the chromium was recovered.
(4) The plant was assumed to operate 24 hours per day.
(5) It was estimated that the capital investment cost for such a
plant would be in the range from $3 million to $5 million.
(6) It was estimated that the plant would recover, on a daily
basis, 1.08 tons of copper (as metal), 0.13 ton of nickel
(as metal), 4.5 tons of chromic acid, and 3.4 tons of by-
product sodium sulfate.
(7) It was assumed that the above materials could be marketed
at 80 to 100 percent of the currently quoted prices of
commercial materials.
The process operations and materials flows, on which the cost estimate
was based, are indicated in Figure 1. The copper and nickel recovery cir-
cuits reflect the parameters developed during this study, and the chromium
recovery sequence reflects the findings of this program and current indus-
trial practice for the production of anhydrous chromic acid (003). A fuller
discussion of the operating conditions and assumptions used to develop the
flowsheet and the associated cost are given in Appendix B.
On the basis of the flowsheet, a list of required equipment and sizes
was developed and costed. The list of major equipment items is given in
Table 6, along with the estimated cost of the various items. In view of the
preliminary nature of the cost estimate, the method used to estimate the
total capital cost of the plant was to apply a multiplying factor to the
"delivered cost of equipment". The cost factor usually used in this method
for "chemical-process" or "solids-liquids" types of plants is 3.5.* Because
* Guthrie, K. M., "Process Plant Estimating, Evaluation and Control",
Craftsman Book Company of America, Solana Beach, California, 1974; "Cost
Engineering" October, 1965 (Industrial Research Service, Inc.; Dover, New
Hampshire).
35
-------
Evaporator] Water £-- Decant
1 to
I Recycle
"V°4
FIGURE 1. FLOWSHEET FOR PLANT COST ESTIMATION
36
-------
TABLE 6. LIST OF MAJOR EQUIPMENT ITEMS, DELIVERED COSTS,
AND TOTAL CAPITAL COST ESTIMATES
Equipment Items
Copper and nickel leach and electrolysis
Leach tanks, 4000 gal, steel
Plate and frame pressure filters, 70 gpm
Circulating pumps, 70 gpm
Bucket elevators, 20 ft high
Electrolysis tanks, 6200 gal, steel
Ammonia stripper
Carbonate roast and chromate leach
Bucket elevator, 20 ft
Rotary kiln, 80 tons/day
Waste heat boiler and baghouse
Rod mill , 80 tons/day
Water leach tank, 4000 gal
Bucket elevator, 20 ft
Plate and frame pressure filter, 70 gpm
Chromate liquor storage tank, 4000 gal
Chromic, acid production
Evaporator, single stage
Reaction vessel, 200 gal
Settler
Triple effect evaporator
Thickener (Na2S04)
Drum dryer
Reaction vessel
Flaker
Total delivered equipment cost, 1975 $
Total plant capital investment low (
Total plant capital investment high
Number
of
Items
4
4
8
4
4
1
1
1
-
1
1
1
1
1
1
1
1
1
1
1
1
1
factor = 2
(factor =
1975
Delivered
Equipment
Cost
Each
$
6,960
22,050
525
3,375
9,930
60,000
3,375
1,020,000
150,000
4,080
6,960
3,375
22,050
6,960
6,000
1,500
3,000
7,500
2,250
1,500
1,500
3,000
.0)
3.5)
Delivered
Equipment
Cost
All Items
(1975 $)
27,840
88,200
4,200
13,500
39,720
60,000
3,375
1,020,000
150,000
4,080
6,960
3,375
22,050
6,960
6,000
1,500
3,000
7,500
2,250
1,500
1,500
3,000
$1,476,510
$2,953,020
$5,167,785
37
-------
of the relatively small size of this plant and the possibility that costs of
installation, construction, and support facilities might be minimized in a
smaller plant operation, capital investment costs were estimated using fac-
tors of 2.0 and 3.5 on the delivered equipment costs. The corresponding
estimated capital costs were about $3.0 million and $5.2 million (see Table
6). Correspondingly, two sets of operating costs were developed since some
categories of operating costs (e.g., interest depreciation, etc.) depend on
capital investment. The two sets of operating costs are shown in Table 7.
The daily operating costs are also expressed in terms of dollars per ton of
feed and cents per gallon of feed. In both cases of estimated capital
investment levels, the costs show an operating loss.
The preceding exercise in costing has certain considerations implicit
such as the advantage of scale in the selection of the plant size, the
assumption of no cost for the feed material sludge, and the assumption of a
10-year supply of sludge (i.e., about 350,000 tons).
However, the resulting net cost of processing, about $6-17 per ton, may
be compared with the existing costs of disposal of sludges to the land.
A survey of charges made by waste-disposal contractors performed in 1973
for the Office of Solid Waste Management Programs of the U.S. EPA reported
costs ranging from $6 per ton to $60 per ton.* The range of variation of
these costs reflects differences in methods of treatment and disposal, dif-
ferences in the management of the disposal sites, and differences in over-
head and profit margins. It may be further noted that special charges, in
addition to those constituting the above range, were made for the disposal of
wastes containing hexavalent chromium or cyanide. These costs, supplemented
by more current information developed on a subsequent study for OSWMP, pro-
duced an average disposal cost (in 1975 dollars, for comparison with the plant
operating costs) of $33 per ton, with the same range of costs still persist-
ing.** Thus, it may be considered that the projected plant operating cost
of $6-17 per ton may be compared with the alternative cost of sludge disposal
ranging from $6 to $60 per ton, averaging $33 per ton.
This cost comparison allows the speculation that, if uniform regulations
were promulgated which made the sludge residue from the projected process
acceptable, the process might well be the alternative of choice for the
treatment of sludges, prior to their disposal to the land.
* Unpublished survey by Versar, Inc., Springfield, Virginia, for the Office
of Solid Waste Management Programs, U.S. EPA, Washington, D.C.,
September 10, 1974.
** "Assessment of Industrial Hazardous Waste Disposal Practices -
Electroplating and Metal Finishing Industry", Final Report on Contract
68-01-2664, to the Hazardous Waste Management Division of the Office of
Solid Waste Management Programs, U.S. EPA; September 30, 1976.
38
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TABLE 7. CATEGORIES OF DAILY OPERATING COSTS AND CREDITS
Cost Category
Labor (eight men/shift, $8/hr)
Chemicals and materials
Energy
Electrical
Fuel
Maintenance (2 percent of capital)
Insurance (1 percent of capital)
Taxes (3 percent of capital)
Interest (8 percent of capital)
Depreciation
Credits
1 .08 tons of copper
0.13 ton of nickel
4.5 tons of CrOo
3.4 tons of Na2S04
Total credits
Net operating costs
(Low
Capital
Investment)
$1,536
2,635
261
1,262
169
84
253
675
844
$7,719
$1,261
443
5,220
187
$7,111
-$ 608
~ -$6.00/ton
~ 3<£/gal
(High
Capital
Investment)
$1,536
2,635
261
1,262
295
148
443
1,181
1,477
$8,795
$1,261
443
5,220
187
$7,111
-$1,684
~ -$17.00/ton
~ 8-l/2<£/gal
39
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COMMENTS ON CALCULATION METHODS FOR TABLE A-3
i-1
1. Cumulative grams to filtrate = C- x V. + £ 0.08 Cn
I | _ II
n=l
where C-j = concentration, g/1 at time i
V- = volume, in 1., at time i
i-1 1
£ 0.08 Cn = summation of dissolved metal removed in prior grab samples.
n=l
In the case of two or 3 stage leaches, contents of pregnant liquors were
added together for calculations, except for experiment 27B, which was in-
dependently calculated.
2. In leaches of sludge where more than 3 grab samples were removed, recovery
calculations were based on head stock contents less the estimated (pro-
rata) progressive removal of solid values removed with grab samples.
3. Accountability calculations also consider losses of solids values removed
in grab samples, along with estimated or analyzed filtering losses of
solids (incomplete recovery of filter cake) for multistage leaches.
48
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51
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APPENDIX B
ESTIMATION OF PLANT COSTS
The following paragraphs give the major considerations and assumptions
on which the plant flowsheet and costs were developed.
The overall considerations for flowsheet development were that the input
to the plant would be 100 tons per day of sludge, containing 20 percent
solids. The output of the plant was assumed to be copper and nickel metals
and chromium in the form of anhydrous chromic acid, 003. The solids were
assumed to have a composition similar to the sludge used in this study,
namely 6 percent copper, 1.1 percent nickel, and 13 percent chromium (weight
percents on a dry basis).
COPPER AND NICKEL LEACHING
Leaching conditions were assumed on the basis of the experimental find-
ings of this study. Each leach was assumed to take 3 hours, using a leach
solution containing 9 weight percent ammonia (NH3) and 12 weight percent C02
and a liquid-to-solid ratio of 9 to 1. Leaching was assumed to be done on a
batch basis, and two leach circuits were assumed to operate in series, each
accommodating five 3-hour leach cycles in 24 hours. The leach tanks were
sized to a 4000-gallon capacity and are constructed of mild steel, with
covers. Agitation and heat were considered to be supplied by process steam.
PRESSURE FILTRATION
It was assumed that the copper leach liquor would be filtered in pres-
sure filters throughout the leaching cycle, with the cake recirculated to
the leach tank, and the filtrate to the electrolysis tanks. Recirculation
was assumed to occur three times per leach cycle, or at a rate of about 70
gallons per minute. The pressure filter was assumed to operate at a rate of
30 gallons per hour per square foot and produce filter cake containing 50
percent solids.
COPPER ELECTROLYSIS
The copper electrolysis was assumed to operate at a current density of
15 amperes per square foot, at a current efficiency of 50 percent. It was
52
-------
estimated that 130 cathodes would be required, each 3-1/2 x 4 feet in a tank
containing approximately 6,000 gallons of solution.
NICKEL ELECTROLYSIS
The nickel leach, filter, and electrolysis circuits were considered to
be the same as for the copper circuit.
AMMONIA STRIPPER
Because of the introduction of 80 tons of water per day into the leach
circuit with the raw sludge, it was assumed that a leach liquor bleed stream
would be steam-stripped to produce an NH3-C02 concentrate for recycle and
reconstitution of the solution added to the copper (and nickel) leach tanks.
The stripping operation was assumed to be 97 percent effective.
LEACH REAGENT LOSSES AND MAKE-UP
The stripped leach solution represents losses of ammonia and C02 in
solution since the stripper is assumed to operate at less than 100 percent
efficiency. The other loss of reagents was assumed to occur in the filter
cake from the nickel circuit, which was considered to be 50 percent by weight
solids with the balance being leach solution. It was estimated that 2.2 tons
per day of make-up ammonia would be required and would be supplied in the
form of purchased anhydrous ammonia. Also, it was estimated that 2.9 tons/
day of CO? would be required to be replaced and would be obtained from
cleaned flue gas from the carbonate roasting furnace.
CARBONATE ROAST FURNACE
Based on industrial process descriptions, it was assumed that the charge
to the carbonate roasting furnace (which was assumed to be a rotary furnace)
would contain 40 short tons of filter cake per day, along with 10 tons per
day of lime (CaO), 12.5 tons per day of sodium carbonate (Na2C03J, and
recycled residue to total 80 tons per day input. The rotary furnace was
assumed to be equipped with a waste-heat boiler and a baghouse to control
pollution, produce process steam, and permit reclamation of C02 for use in
the leach circuit (as needed).
CHROMATE LEACHING
Chromate leaching was assumed to be done at a liquid-to-sol ids ratio of
5 to 1 and to produce a liquor which was 20 percent by weight sodium chromate.
The filtration of the chromate liquor was assumed to produce a filter cake of
50 percent solids which represents the residue of the process.
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CONVERSION OF CHROMATE LIQUOR TO CHROMIC ACID
The process was assumed to be that described for current commercial
practice. The sodium chromate solution is evaporated to 40 weight percent
sodium chromate, reacted with sulfuric acid to produce sodium dichromate and
sodium sulfate, evaporated again to precipitate the sodium sulfate, which is
assumed to be dried and sold at the rate of 3.4 tons/day. The sodium dichro-
mate, concentrated to 85 percent by weight, is reacted with sulfuric acid and
separates into chromic acid anhydride (Cr03; 4.5 tons per day), which is sold,
and sodium bisulfate (5.4 tons per day), which is assumed to be wasted.
The production of chromic acid consumes 6.6 tons per day of sulfuric
acid and represents the second major fuel consumption component because of
the evaporation of some 30 tons of water per day.
BASIS OF COSTS FOR EQUIPMENT AND REAGENTS
Most of the equipment costs were developed using information from the
reference "Process Plant Estimating, Evaluation, and Control", K. M. Guthrie,
Craftsmen Book Company of America, Solana Beach, California, 1974. A few
equipment costs were developed using recent design data and costs from con-
current studies by co-workers. Reagent costs were taken from the "Chemical
Marketing Reporter" of July 5, 1976, and included the following values:
Ammonia
Lime
Sodium carbonate
Sulfuric acid
Chromic acid
Sodium sulfate
$185/ton
$30/ton
$60/ton
$50/ton
$0.58/pound
$55/ton
Prices for copper and nickel were obtained from "American Metal Market"
for July 11, 1976, as $0.73/pound for copper and $2.20/pound for nickel. A
factor of 1.33 was applied to these prices to estimate the costs of fabri-
cated starting sheets for the electrolysis bath.
All equipment costs were converted to 1975 dollars using cost indexes
from "Chemical Engineering", July 5, 1976, page 7. Factors relating "fob"
equipment costs to installed costs were taken from Guthrie (cited above).
Operating costs were estimated using allowances as being used in currently
published cost estimates for maintenance, taxes, insurance, and interest;
these and the assumed 10-year, straight-line depreciation rate are indicated
in the table of operating costs and its footnotes.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-105
3. RECIPIENT'S ACCESSION NO.
*' WlMON mM^CARB^NATE LEACHING OF METAL VALUES FROM
WATER-TREATMENT SLUDGES
5. REPORT DATE
June 1977 issuing date
6, PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
J.B. Hallowell, E.S. Bartlett, and R.H. Cherry, Jr.
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle Columbus Laboratories
505 King Avenue
Columbus, Ohio ^3201
10. PROGRAM ELEMENT NO.
1 RR6in-
11. CONTRACT/GRANT NO.
R 803787-01
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Lab-Cin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This project was undertaken to explore and develop processes based on ammonia-
cal leaching to recover meta'i values fro:.i metal-finishing wastewater treatment
sludges. The objective was to eliminate or to reduce sufficiently the heavy metal
content of the sludge so that it would no longer constitute a potential ecological
hazard, thus greatly reducing the cost of sludge disposal.
Experimental efforts were concentrated upon optimizing the leaching process.
Preferred leaching conditions were defined. With two-stage leaching at 50 C in con-
centrated ammonium carbonate, the copper-plus-nickel content can be reduced to a
level of less than 1 percent from an original content on the order of 10 percent. By
roasting the depleted sludge with soda ash, at least 80 percent of the chromium can
by recovered in water-soluble form. Controlled-potential electrodeposition was
shown to be capable of separately winning copper and nickel values from the pregnant
ammoniacal leaching liquor.
A^process flow sheet is presented, and process economics are reviewed. The
economic evaluation indicates that it is probable that these operations will be a
net expense- The process studied would be an alternative only if costs for disposal
of sludge rose to the highest part of the current range.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Metal finishing
Plating
Sludge
Industrial Waste Treatment
Leaching
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
b.IDENTIFIERS/OPEN ENDED TERMS
19. SECURITY CLASS (This Report)
UNCLASSIFIED
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
c. COSATI Field/Group
13B
21. NO. OF PAGES
67
22. PRICE
EPA Form 2220-1 (9-73)
55
S. GOVERNMENT PRINTING OFFICE:l977-757-056/6433 Region No. 5-I I
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