EPA-670/2 74 001
January 1974
                         Environmental Protection Technology Series
Carbonate Bonding
of Taconite Tailings

                                   Office of Research and Development
                                   U.S. Environmental Protection Agency
                                   Washington, D.C 20460

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             RESEARCH REPORTING SERIES
Research  reports of the  Office  of   Research  and
Monitoring,   Environmental Protection Agency, have
been grouped into five series.  These  five  broad
categories   were established to facilitate further
development   and  application   of    environmental
technology.    Elimination  of traditional grouping
was  consciously  planned  to  foster   technology
transfer   and  a  maximum  interface  in  related
fields.   The five series ares

   1.  Environmental Health Effects  Research
   2.  Environmental Protection Technology
   3.  Ecological Research
   ft.  Environmental Monitoring
   5.  Socioeconomic Environmental Studies

This report  has been assigned to the ENVIRONMENTAL
PROTECTION    TECHNOLOGY   series.     This   series
describes   research   performed  to  develop  and
demonstrate    instrumentation,    equipment    and
methodology   to  repair  or  prevent environmental
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.
                  EPA REVIEW NOTICE
This report has been reviewed by the Office of Research and
Development, EPA,  and approved for publication.  Approval
does not signify that the contents necessarily reflect the
views and policies of the Environmental Protection Agency,
nor does mention of trade names or commercial products consti-
tute endorsement or recommendation for use.

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                                                      EPA-670/2-7^-001
                                                      January,
         CARBONATE BONDING OF TACONITE TAILINGS
            Paul J.  LaRosa, K. A. Ricciardella
                            and
                       R.  J. McGarvey
                  Contract No. 68-01-0195
                 Program Element No.  IBBOlj-O
                        Project Officer
                         Ronald D. Hill
                Mining Pollution Control Branch
             National  Environmental Research Center
                     Cincinnati, Ohio   ^5268
                          Prepared for

               OFFICE  OF RESEARCH AND  DEVELOPMENT
              U. S. ENVIRONME1NTAL PROTECTION AGENCY
                     Washington, D. C.   20^60
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price $1

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                              ABSTRACT
The carbonate bonding process consists of mixing a suitable material
with water and lime hydrate, compacting the mixture, and reacting it
with carbon dioxide-rich gas to form a coherent structure bonded by a
matrix of calcite crystals.  A laboratory study has indicated that
taconite tailings could be carbonate bonded to form an effective road
paving or brick-making material.  In general, the compressive strength
of carbonate bonded taconite tailings increased with increasing lime
hydrate content, reaction time, and carbon dioxide concentration in
the reaction gas.  In addition, air and water permeabilities, freeze-
thaw resistance, and flexural strengths of carbonate bonded taconite
tailings were found to be comparable to concrete.  Scale-up of the
laboratory studies to demonstrate paving applications in small plots
was hampered by a failure to obtain sufficient compaction.  The results,
however, did confirm the laboratory study findings.

Possible applications of the carbonate bonding process utilizing taconite
tailings are road building, formation of aggregate, and brickmaking.
An approximate cost comparison suggests that the road construction
application is an economical alternative to conventional road building
materials and techniques.

This report was submitted in fulfillment of Contract No. 68-01-0195 by
Applied Technology Corporation under sponsorship of the Environmental
Protection Agency.  Work was completed as of December, 1973.
                              ii

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                              CONTENTS

                                                                Pag
Abstract	^
List of Figures	iv
List of Tables	   v
Acknowledgements.  .  .  	   .

SECTIONS
   I Conclusions.  	   i

  II Recommendations	   3

 III Introduction  	   5

  IV Experimental  Studies (Briquettes)	   7
      Raw Materials  Used	   7
      Carbonate Bonding Technique and Apparatus 	   8
      Experimental Program	10
      Effect of Lime Hydrate on Compressive Strength	10
      Effect of Water Content on Compressive Strength 	  n
      Effect of Curing Time on Compressive Strength 	  12
      Effect of Carbon Dioxide Concentration on
        Compressive  Strength	„  15
      Effect of Formation Load on Compressive Strength	17
      Lime Hydrate Study	17
      Air and Water  Permeability of Carbonate Bonded
        Taconite Tailings 	  	  21
      Freeze-Thaw  Test on Carbonate Bonded Taconite
        Tailings	22
      Flextural Strength Tests on Carbonate Bonded
        Taconite Tailings	  24
      Subsequent Experimentation	24
   V Experimental  Study (Bricks)	25
      Carbonate Bonding of Taconite Tailings as
        15 cm Cubes	i	25
  VI Taconite Test Plots	30

 VII Application of  Carbonate Bonded Taconite Tailings	33
      Process Description	38
      Suggested Operating Parameters	39
      Process Economics	40
VIII Appendices	,	43

      Theoretical  Consideration of Carbonate Bonding	43
      Determination  of Air Permeability	47
      Determination  of Water Permeability 	  51
      Freeze-Thaw  Test	53


                               iii

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                                FIGURES
                                                                 Page
 1.  Carbonate Bonding Reaction System	9
 2.  Effect of Lime Hydrate Content on Compressive Strength ... 11
 3.  Effect of Moisture Content on Compressive Strength Using
       15 Weight Percent Lime Hydrate	.12
 4.  Effect of Moisture Content on Compressive Strength Using
       20 Weight Percent Lime Hydrate	13
 5.  Effect of Curing Time on Compressive Strength.	14
 6.  Reduced Compressive Strength vs Curing Time	15
 7.  Effect of Carbon Dioxide Concentration on Compressive
       Strength	16
 8.  Effect of Reaction Time on Compressive Strength	16
 9.  Effect of Formation Load on Briquette Compressive Strength . 17
10.  Effect of Lime External Surface Area on Compressive
       Strength .	20
11.  Compressive Strength vs Freeze-Thaw Cycles 	 .23
12.  Sectioned Bricks Reacted with 100 Percent C02	 26
13.  Sectioned Bricks Reacted with 20 Percent C02	26
14.  Carbonate Bonded Depth vs Time for 15 cm Bricks	27
15.  Effect of Formation Load on Depth of Carbonate Bonding of
       15 cm Bricks	29
16.  Configurations of Plots 1 and 2	31
17.  Configuration of Plots 3, 4, and 5 .	32
18.  Location of Sampling Sites in Each Test Plots	34
19.  Compressive Strength vs Density for Plots 3, 4, and 5. . . .36
20.  Carbonate Bonding Model- 	 46
21.  Air Permeability Apparatus 	 48
22.  Water Permeability Apparatus	52
                              iv

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                                TABLES
                                                                   Page
 1  Screen Analysis of Taconite Tailings ..............  7
 2  Chemical and Screen Analysis of Lime Hydrates Tested ......  7
 3  Compressive Strength of the Lime Hydrate Briquettes
       (dry C02> .......................... 18
 4  Compressive Strength of the Lime Hydrate Briquettes
       (wet C02) .......................... 19
 5  Air Permeability of Taconite Tailings Briquettes ........ 21
 6  Water Permeability of Taconite Tailings Briquettes ....... 22
 7  Plot Thickness ......................... 30
 8  Plot C02 Addition Rate ..................  .  .  . 33
 9  Test Results for Plots 3, A, and 5 ............... 35
10  Paving Cost ............ . ............. 40
11  Paving Cost for Carbonate Bonding ............... 41
12  Paving Cost .......................... 41

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                           ACKNOWLEDGEMENTS
The support of the project by the Environmental Protection Agency and
the help provided by Dr. James M. Schackelford and Mr.  Ronald D.  Hill,
Project Officers, is acknowledged with sincere thanks.
                            vi

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                             SECTION I

                            CONCLUSIONS
Carbonate bonding consists  of mixing a material with water and lime hydrate,
compacting the mixture,  and reacting it with carbon dioxide-rich gas to
form coherent structures bonded by a matrix of calcite crystals.  This
study has shown that taconite tailings can be used in the carbonate bonding
process.   To minimize pollution of Lake Superior by taconite tailings,  the
taconite  can be carbonate bonded rather than stockpiled and utilized as
a road building material, road aggregate, and/or in the preparation of bricks,

The following conclusions have resulted from this study:

     1.  In the laboratory, compressive strengths in the order
     of 281 kg/cm  (4000 psi) were obtained from carbonate
     bonded taconite tailings.  For these studies, taconite
     tailings containing 3  to 5 percent moisture were mixed
     with 20 percent lime hydrate and reacted with carbon dioxide-
     rich gas.   The compressive strength of the carbonate
     bonded taconite tailings increased with increasing lime
     hydrate content, formation load, carbonate bonding reaction
     time, and carbon dioxide concentration of the reaction gas.

     2.  Air and water permeabilities of carbonate bonded
     taconite tailings were found to be comparable to those
     of concrete.

     3.  A lime hydrate study indicated that dolomitic lime
     hydrates with high external surface areas yield the highest
     compressive strengths.

     4.  A simplified model of carbonate bonding was developed
     and indicated that the progression of the carbonate bonding
     interface is related to the  square root of time.  A
     brick study (15 cm cubes) confirmed the model and yielded
     compressive strengths  for the bricks in the order of 225 kg/cirr
     (3200 psi).  Although  no economics were generated, it
     appears that taconite  tailings could be utilized in the prepar-
     ation of bricks.

     5.  A plot study involving the carbonate bonding of several
     1.2 metre (4 ft) squares was hampered by inclement weather
     and insufficient compaction.  The results agreed with
     smaller scale study work and indicated that a single source
     of carbon dioxide at the center could carbonate bond a 1.2 metre
     (4 ft) square.  However, due to uneven compaction resulting
     from roller/plot size  limitations, the compressive strength
     of the plot decreased  at locations away from the center of
                                -1-

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the plot.  Further work Is required to develop adequate
techniques to control carbon dioxide injection, moisture
content of taconite tailings, and uniform compaction before
this process can be utilized for road paving applications.

6.  If the adequate paving techniques mentioned above
do not result in increased labor or equipment costs over
conventional paving techniques, an approximate cost comparison
indicates that carbonate bonded taconite tailings are
comparable, and in some cases, less costly than conventional
road construction materials.  For secondary road construction,
an 8-inch layer of carbonate bonded taconite tailings would
cost approximately $11.31 per square yard, as compared to
$11.50 per square yard for a comparable bituminous pavement
with base course-  In the construction of first class roads,
carbonate bonding offers a cost saving of approximately
$2 per square yard as compared to concrete.

7.  Based on the above economic comparison and the results
of the laboratory study, there could be an incentive to
utilize the carbonate bonding process to minimize stock-
piling of taconite tailings and reduce its harmful ecological
effects.
                          -2-

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                             SECTION II

                           RECOMMENDATIONS
Because of the  encouraging  results  of  this  study,  ATC  believes  that
additional carbonate bonding  development work is warrented.   Such work
must be conducted on a  larger scale under more controlled conditions.
It is recommended that  the  following demonstration studies be conducted
in the following sequence:

     1.  A study should be  conducted to ascertain  the  techniques
     and economics of brick preparation using taconite tailings.
     This study would include some  basic experimentation but
     would be more orientated toward preparing preliminary
     specifications for a brick making facility.   This application
     may be of  particular interest  to  U. S.  AID programs in
     developing countries.

     2.  A study should be  conducted to determine  how  moisture
     cnntent, carbon dioxide  injection, and compaction can be
     controlled under field conditions for  road paving applications.

     3.  It is  recommended  that a large plot be paved  to
     develop adequate techniques and to demonstrate the commercial
     utility of the process as a road  building technique. For
     demonstration purposes,  the test  plot  should  be a minimum
     of 6 m by  15 m  (20 ft  x  50 ft) to permit the  utilization
     of commercial-sized  compaction equipment. Based  on the
     results from the demonstration plot, an engineering evaluation
     of the process would be  made with pertinent recommendations
     as to further application.

     4.  If items 2 and 3 are successful, it is recommended
     that the process be  demonstrated  by construction  of a
     secondary  road in  the  vicinity of a  taconite  tailings pile.
     This demonstration would reduce the size of the selected
     taconite tailings  pile,  provide additional roads  for the
     community, establish road building costs, and develop road
     building techniques  for  application of the carbonate bonding
     process.

     5.  A study should be  conducted to determine  the  technical
     and economic feasibility of manufacturing aggregate using
     carbonate  bonding  taconite tailings.   This aggregate may
     have applications  in road or building  construction.

     6.  As part of any of  the above projects, a study should
     be undertaken to determine the feasibility of producing
     lime hydrate suitable  for carbonate bonding  from limestone
     available  near  taconite  tailings  sources. The purpose
     of this work would be  to minimize the  cost of transporting
     lime hydrate to  the  point of use.

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ATC strongly recommends the brick study (1) above, because it believes
that this application of the process can be implemented in a relatively
short period of time with a minimum of development work.  Brick manu-
facturing equipment should be relatively simple, employing a brick
press, standard drying and mixing equipment, a carbonate bonding chamber,
and CO- gas generator.
                          -4-

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                             SECTION III

                            INTRODUCTION
The Lake Superior District is a major source of iron ore for the United
States.  The ores mined in this region vary widely in mineralogy,
chemical composition, and marketability.  Prior to 1950, mining was
restricted to marketable lump or easily beneficiated ores.  However,
as reserves dwindled, attention was focused on the recovery of iron from
low grade taconite in the 1950's.  Despite their low iron content (25
to 35 percent total iron as mined), taconite ores could be beneficiated
to increase the iron content and produce a high quality blast furnace
feed.

Taconite is a hard, dense, fine grain rock that normally contains
about 40 to 55 percent silica.  To produce an acceptable iron ore
concentrate, the silica must be removed prior to shipment.  Typically
this is accomplished by grinding the taconite ore to 200 mesh or finer
and then separating the tailings from the iron oxide by magnetic
separation or flotation techniques.  These tailings or gangue produced
in the beneficiation process are extremely fine and represent a source
of water pollution.  The finely sized tailings which enter Lake Superior
either through the discharge of process water or storm water run-off,
remains in suspension for extended periods of time.  This brings about
an upset in the natural environment of the lake.  The taconite tailings
being finely sized, hard, and sharp edged cause multiple minute abrasions
on the interior and exterior of fish or similar aquatic life.  Additionally,
the huge surface area of the finely sized particles tend to enhance
the solution of trace elements which may induce undesirable side chemical
reactions.   The esthetic appearance of the lake is also destroyed because
of the cloudy haze created by the particles held in suspension.  For
these reasons, methods are being sought to utilize taconite tailings
and, thereby, prevent or minimize this serious form of water pollution.

This study involves the investigation of a process in which taconite
tailings can be profitably transformed into a concrete like material.
This material can be used as a filler in the construction of carbonate
bonded road pavement and in the manufacture of low-cost bricks and similar
products.  If successful, there would be sufficient incentive to utilize
the taconite tailings and not permit run-off to Lake Superior.

In this process, lime hydrate is blended with taconite tailings and water
and then reacted with carbon dioxide to form strong calcium carbonate
(calcite) crystals.  The calcite crystals act as a cementing matrix that
turn the taconite tailings into a coherent and impervious structure.
The carbonate bonding process relies on the simple reaction that occurs
between carbon dioxide and lime hydrate:

                     Ca(OH)2 + C02 = CaCO  + H.O
                             -5-

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This reaction is well known and occurs naturally upon the exposure of
lime hydrate to the atmosphere.  However, the rate of conversion of
lime hydrate to calcium carbonate is relatively slow because of the
small percentage of carbon dioxide contained in the atmosphere.  Although
the reaction in air is slow, the reaction proceeds rapidly in con-
centrated carbon dioxide.  Because of their high strength, calcite
crystals afford an opportunity for employing lime hydrate as a cementing
material to bind taconite tailings into a coherent structure.  A more
detailed description of carbonate bonding can be found in the Appendix.

The majority of experimentation prior to this work has been conducted
using coal refuse rather than taconite tailings as the filler material.
(For reference see EPA report entitled, "Carbonate Bonding of Coal
Refuse", 14010 FOA 02/71.)

This present study was undertaken to determine the operating parameters
for generating a strong carbonate bonded taconite tailings structure
and to determine the feasibility of the carbonate bonding process for
the paving of roads.  In particular, the effects of different brands
of lime hydrate with varying lime hydrate contents, moisture content,
partial pressure of carbon dioxide in the carbonating gas and carbonation
(reaction) time were determined.  In addition, air and water permeability,
freeze-thaw resistance, and flexural strength tests were determined for
carbonate bonded taconite tailings.
                           -6-

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                             SECTION IV

                   EXPERIMENTAL STUDY (BRIQUETTES)
RAW .MATERIALS USED

Fine taconite tailings were obtained from the Reserve Mining Company*,
Silver Bay, Minnesota.  The screen analysis for these taconite tailings
is presented in Table 1.

                              Table 1
                 Screen Analysis of Taconite Tailings

                             Cumulative Weight, %
                                     96.7
                                     89.0
                                     79.6
                                     68.2
                                     61.8
                                     43.6
                                     30.5
                                     24.1

The external surface area of the taconite tailings was determined using
an air permeability technique in which the Carman-Kozeny equation was
used to relate the external surface area to the pressure drop of air
passing through a packed bed of taconite tailings.  This procedure is
presented in detail in the Appendix.  The average external surface area
of the taconite tailings was determined to be 552.4 cm^/gram.  Table 2
shows the chemical analysis and screen analysis of the lime hydrates
tested in this study.  Initial experiments were performed using Ohio
Super Spray* while latter experiments were performed to investigate
the effects of the various lime hydrates of Table 2 on carbonate bonding
properties.

                              Table 2
        Chemical and Screen Analysis of Lime Hydrates Tested
Chemical Analysis
Sample
Pfizer Spray
Tiger Finish
National Hydrate
Mercer Lime
Marble Cliff
Ohio Super Spray
Manufacturer
Pfizer Co.
Pfizer Co.
National Lime
Mercer Lime &
Sciota Lime &
Ohio Lime Co.
& Stone Co.
Stone Co.
Stone Co.
CaO
43.82%
42.15%'
48.19%
73.63%
70.00%
49.20%
MgO
31.03%
29.19%
33.82%
0.61%
5.00%
34.00%
Ca(OH)9
36.86%
37.82%
49.86%
96.07%
92.00%
65.00%
*Mention of trade names does not imply endorsement nor recommendation by
either Applied Technology Corporation or the Environmental Protection
Agency
                           -7-

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                          Table 2 Continued

                U. S. Mesh Percent/Passing         External Surface Area
   Sample         100 Mesh   200 Mesh  325 Mesh   	cm2/gram	
Pfizer             100.00%    90.86%    54.13%            5432
Tiger Finish       100.00%    85.85%    54.86%            4659
National Hydrate   100.00%    98.84%    85.39%            6404
Mercer Lime        100.00%    99.91%    99.21%            2492
Marble Cliff        99.50%    94.00%    63.45%            5698
Ohio Super Spray   100.00%    99-78%    99.64%            5330

CARBONATE BONDING TECHNIQUE AND APPARATUS

The preparation of taconite tailings for carbonate bonding consisted of
mixing in sufficient water to bring the water content to a specified value.
This mixture was allowed to stand to permit an equilibration of water
throughout the mixture.  Lime hydrate was then thoroughly blended into
the taconite tailings.  To permit a rapid scanning of parameters and to
minimize the quantities of materials and size of the carbonate bonding
apparatus, the lime hydrate-taconite tailings mixture was placed in a
small die and compacted to yield briquettes 5 cm diameter by 5 cm high.
Compaction loads varying from 35 kg/cm2 to 281 kg/cm2 (500-4000 psi) were
used to form the briquettes of these experiments.  In the commercial
application of the carbonate bonding process, lower compaction loads in
the order of 70 kg/cm2 (1000 psi) would be used.  It was decided to use
higher compaction load for experimental purposes so that the briquettes
would not have loose edges and would be of exact dimensions.  The briquetting
pressure was then removed and the soft, easily crushed briquette was
placed in a reactor and contacted with a carbon dioxide-rich gas for a
specified time.

The carbonate bonding reactor is shown schematically in Figure 1.  Tanks
of carbon dioxide and nitrogen were used as the source of the carbonating
gas constituents.  Experiments were run with carbonate bonding gas
containing 15 to 100 percent carbon dioxide concentrations.  The carbon
dioxide and nitrogen were individually metered,  combined in a mixing
chamber, and admitted to the carbonate bonding reactor.   The reactor
consisted of a plexi-glass tube, 9.52 cm I.D. by 61 cm long (3.75 x
24 inches).  The formed briquettes were placed on a metal grate which
slid  into the reactor tube.  Six reactor tubes were used to react several
batches of briquettes in a single operation.


To carbonate bond the briquettes they were placed in the reactor tubes.
The flow  rates of nitrogen  and carbon dioxide were then adjusted to the
desired flows.   During this initial adjustment,  the flow of the gas
mixture bypassed the reactor tubes.  Once the desired gas mixture was
obtained, a stopwatch was started and the bypass valve closed causing
the gas mixture  to enter the reactor tubes.

When the  specified carbonate bonding reaction time was reached, the
carbon dioxide was cut off  and nitrogen was allowed to flow through the
                               —8—

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Rubber Stopper
                    Plexiglass Tube
    \
                              V
Gas Outlet
                               V
                     Grate
                                   Briquette
                                                     Glass Beads
Carbon Dioxide
Rich Gas Inlet
                         Reactor  System
              Reactor    \
              Reactor
         «	1 Reactor
              By Pass
         *  1 Reactor   1—2
Portable
R6tameter«__|  Pa_^,   \   7
Line to
Check
Individual  p"-.             Valve
Gas Flowg
                                                           -X-
                                                       Carbon  Dioxide
                                                           Tank
                                                       *	21
                                                       '   n^
                                                        Nitrogen
                                                          Tank
              FIGURE 1.  CARBONATE  BONDING  REACTOR SYSTEM

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reactor tubes for several additional minutes to flush out all remaining
reaction gas.  The carbonate bonded taconite tailings briquettes thus
formed were removed and either tested for their compressive strength or
subjected to other tests.

EXPERIMENTAL PROGRAM

The structural integrity of carbonate bonded taconite tailings is a
prime consideration for any of its possible applications.  3Unce the process
is being investigated as a possible road pavement, the compacted layer
must have sufficient strength to withstand heavy vehicular traffic.
Consequently, in this study, it was necessary to select one strength
property as representative of the structural integrity of the resulting
carbonate bonding material.   For this study, compressive strength was
selected because of its applicability toward the intended use of the car-
bonate bonding process and because of its ease of determination.  To
determine the compressive strength of carbonate bonded briquettes, a
briquette was placed in a press and the press plates closed until a small
load was applied to the briquette.  The press operator then applied a
slow but continuous rate of loading on the briquette until failure occurred.
The load at failure was recorded and used to calculate the compressive
strength of the briquette.

The laboratory experimentation can be divided into two categories:  those
experiments designed to investigate the effects of various carbonate
bonding parameters (lime hydrate content, moisture content, etc.) on
the compressive strength and those experiments which were used to determine
the physico-chemical properties of carbonate bonded taconite tailings.
Initially, Ohio Super Spray lime hydrate was used to prepare the carbonate
bonded taconite tailings.  Prior experimentation with coal refuse suggested
that this lime hydrate would be suitable.  Later in the experimental program
other lime hydrates were tested.

EFFECT OF LIME HYDRATE CONTENT ON COMPRESSIVE STRENGTH

To determine the effect of lime hydrate content on briquette compressive
strength, briquettes were prepared in which the lime hydrate content was
varied between 5 and 20 weight percent.  The water content was held con-
stant at 3 weight percent for these tests.  After blending, the taconite-
lime hydrate mixture was charged to the five cm diameter mold and compressed
at 84 kg/cm2 (1200 psi) to yield a 5 cm high cylindrical briquette.  The
briquettes were then reacted for six hours using pure carbon dioxide,
removed from the reactor and subjected immediately to a compressive
strength determination.

The results are shown in Figure 2 where the briquette compressive strength
is shown as a function of lime hydrate content.  Figure 2 clearly indicates
that the compressive strength increases as the lime content is increased.
                               -10-

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     PS I

    3000
    2500 ••  176
oo
c
.2
4J
w
a
a.
e
2000
    1500
    1000
     500
        kg/cm^

        211
                               Conditions:  3% water
                                            6  hr reaction time
                                          100% C02
                                          Zero Curing Time
141
        105
         70
         35
                     L
                                                       I
          0           5           10          15          20

                      Lime Hydrate, weight percent

         FIGURE 2—EFFECT OF LIME HYDRATE CONTENT ON COMPRESSIVE STRENGTH
 The  increase in compressive  strength with lime hydrate content is expected
 since more lime hydrate  is available for bonding.   However,  briquettes
 made with 40 percent  lime hydrate  developed cracks and had essentially
 no compressive strength.  These  cracks developed because the carbonate
 bonding  reaction is highly exothermic.   This exothermic reaction causes
 rapid vaporization of moisture which creates high  internal pressure and
 cracks the briquettes.   Consequently, there is a practical limit to the
 lime hydrate content  that can be used effectively- to increase the strength
 of carbonate bonded taconite tailings.

 This initial test established that subsequent testing would be done using
 either 15 or 20 weight percent lime hydrate.

 EFFECT OF WATER CONTENT  ON COMPRESSIVE STRENGTH

 The  effect of water content  on carbonate bonded compressive strength is
 shown in Figures 3 and 4.  In these experiments, (Figure 3), the taconite
                               -11-

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 tailings contained 15 percent lime hydrate and a formation load of
 70 kg/cm   (1000 psi) was used.  The briquettes were reacted for six
 hours and  then cured for 100 hours.  The maximum compressive strength
 was achieved at 3 percent moisture as is seen in Figure 3.
      PSI   kg/cm
    3000
    2500-• 17
to  2000
 4)
CO


 *
T<
 ID
 05
 «l
 M

 &
 o
1500


1000


 500
       211
                                    Conditions:  6 hr reaction time
                                              100% CO,,
                                              100 hours curing time
               2     4     6     8     10     12    14   16

                    Percent Moisture in Taconite Tailings

     FIGURE 3--EFFECT OF MOISTURE CONTENT ON COMPRESSIVE STRENGTH USING
                      15 WEIGHT PERCENT LIME HYDRATE
Additional experiments were done using 20 percent lime hydrate, 70 kg/cm
formation load, six hour reaction time, and 100 hour curing time.
These experiments using 20 percent lime hydrate are shown in Figure 4.
As seen, a maximum compressive strength of 211 kg/cm2 (3000 psi)
was achieved at 3 to 5 percent moisture.  At a moisture content greater
than 5 percent, the compressive strength dropped sharply.

Based on these results, the optimum compressive strength is obtained
with 3 to 5 percent moisture.  The significance of this result is
that since the taconite tailings are produced using a flotation process,
they must be dried to a 3 to 5 percent moisture before they can be used
in the carbonate bonding process.

EFFECT OF CURING TIME ON COMPRESSIVE STRENGTH

As shown in Figures 3 and 4, the maximum compressive strength obtained
for the taconite tailings briquettes was 197 to 211. kg/cm2 (2800-3000 psi)
with 20 percent lime hydrate and 3 to 5 percent water.  The resulting
                                -12-

-------
    PSI    kg/on
   3000 -• 211
   2500 -• 176
•g  2000
60
C
0)
   1500 -•
3  1000
&
    500
                                         Conditions:   6 hr reaction time
                                                   100% CO,
                                                   100'houPS  curing time
35
               2     46     8    10    12     14

                Percent Moisture in Taconite Tailings
                                             16
     FIGURE 4—EFFECT OF MOISTURE CONTENT ON COMPRESSIVE STRENGTH
                      USING 20 WEIGHT PERCENT LIME HYDRATE

                                  -17-
compressive strengths were lower when compared with prior experimental
results using other fill materials  (coal refuse).  Carbonate bonded
coal refuse yielded compressive strengths in the order of 281 kg/cm'.
Consequently, experiments were conducted to determine if there is a
curing time effect on compressive strength.

To determine the effect of curing time on compressive strength, carbonate
bonded briquettes were prepared in  the manner described previously except
that following reaction with carbon dioxide, the briquettes were permitted
to cure at room temperature in the laboratory, for periods up to 700 hours.
The compressive strength was then determined as a function  of the time
elapsed after carbonate bonding was completed.  This was done using
taconite tailings with 3 percent moisture and 20 pe.rcent lime hydrate.
                               -13-

-------
For these experiments, 100 percent carbon dioxide  gas was  reacted with
the briquettes  for six hours.  Compress!ve strengths were  determined  at
various times after carbonate bonding was completed up  to  a  total time
of 700 hours.   In Figure 5 the compressive strength is  shown as  a function
of curing time.
    PSI   kg/cm
    4000T 282
    3000 - -
   60
   c
   4)
   l-i
   u
   o:
   CO
   m
   4)
   M
    12000
    1000
            Conditions: 3% moisture
                       20% lime hydrate
                        6  hr reaction time
                      100% COo
                             /  9
                       84 kg/cm^ formation
                                  load
                    200
  i
400
  I
600
800
                            Curing  Time, Hours
          FIGURE  5--EFFECT  OF  CURING  TIME  ON COMPRESSIVE STRENGTH
The compressive strength increased significantly during the first
300 hours and continued to gradually increase to a relatively constant
value after 700 hours.  The taconite tailings compressive strength
increased from approximately 239 kg/cm2 (3400 psi) after 190 hours to
274 kg/cm2 (3900 psi) after 700 hours.

In order to make these findings generally applicable  (as will be shown
later), Figure 6 was prepared.  Figure 6 is a plot of the reduced compressive
strength, as a function of the curing time.  The reduced compressive
                               -14-

-------
strength is defined as S/Sf where S = compressive strength at any time,
and Sf = compressive strength at steady state conditions (700 hours).
    U-l
   w
   CO
    «>1.0._
    c
    2
    JJ
    CO
   •H

   SO. 75
   M
   I"
   O
   o
   •o
   
-------
    PS I   kg/cin
   3000 -  211
4-1
6C
e
»
M
4-1
CO
09
09

2
   2000
   1000
141
Conditions:  3% moisture

            20% lime hydrate

             6 hr reaction time
           190 hr curing time
            70 kg/cm.2 formation load
                   20
                      40
            60
80
100
                           C02 in Reaction  Gas,  %


  FIGURE 7--EFFECT OF CARBON DIOXIDE CONCENTRATION ON COMPRESSIVE STRENGTH





    PSI   kg/cm2
4000 '
4J
00
5 3000-
14
4J

01
^J
^H
ra
to
0)
Q. 2000 '
I
o
1000
• 282
1
• 211 	 j" 	 . — 	
»— — "" "^

+
Conditions: 3% moisture
20% lime hydrate
15% CO
141 190 hrs curing time
70 kg/cm^ formation load

' r ~T 	 1
5 10 15 20
                        Reaction Time, hours


      FIGURE 8--EFFECT OF REACTION TIME  ON  COMPRESSIVE STRENGTH
                            -16-

-------
EFFECT OF FORMATION LOAD ON COMPRESSIVE STRENGTH
Formation load is an important factor in the carbonate bonding of
taconite tailings.  A study was conducted to determine the effect of
the initial formation load (the load applied during briquette preparation)
on briquette compressive strength.  Briquettes were formed at 35, 70,
140, and 281 kg/cm2, and reacted with 100 percent carbon dioxide gas for
six hours and cured for 190 hours.  Figure 9 shows that briquette
compressive strength increased from 140 to 323 kg/cm2 as the formation
load was increased from 35 to 281 kg/cm2.  It is, therefore, advantageous
to compact the lime hydrate-taconite tailings mixture to as high a
degree as possible.  Formation loads of 70 to 141 kg/cm2 (1000-2000 psi)
are possible using commercially available equipment.

     PSI  kg/cm2
   4000 - - 282
 CO
 a
 S 3000
 4-1
 CO
 0)
 •H
 m
 CO
 
-------
and typical analyses for each of the lime hydrates.  Since the five
lime hydrate samples had different calcium oxides contents and different
external surface areas, the Ohio Super Spray was used as a standard.
In prior experiments, 20 weight percent of Ohio Super Spray was used;
consequently, the other five lime hydrate samples were adjusted to yield
approximately the same weight ratio of calcium oxide to taconite tailings
as used with Ohio Super Spray.  The calcium oxide to taconite tailings
weight ratio varied between 0.11 and 0.12 for the various lime hydrates.

Briquettes were prepared using 3 percent water and an initial formation
load of 84 kg/cm  (1200 psi).  The briquettes were then reacted for six
hours using 100 percent dry carbon dioxide and allowed to cure for 190 hours
prior to determining the compressive strength.  The compressive strengths
for the various lime hydrate-taconite tailings briquettes were then
adjusted to a steady state value using Figure 6.  The compressive strengths
at 190 hours and at steady state (700 hours) of the various lime hydrates
briquettes are listed in Table 3.  The results of Table 3 indicate that
Pfizer's spray lime hydrate yielded the highest compressive strength
(337 kg/cm2 [4800 psi]) and that Ohio Super Spray and National Hydrate
were next best at 278 and 260 kg/cm2 (4000 and 3700 psi), respectively.

                              Table 3
        Compressive Strength of the Lime Hydrate Briquettes*
                                                           o
                                Compressive Strength, kg/cm
        Lime Hyjdrate              190 hours     700 hours
        Ohio Super Spray            239            278
        Pfizer Spray                295            337
        Tiger Finish                140            162
        National Hydrate            225            260
        Marble Cliff  ,               77             88
        Mercer Lime                  22             26

        *Using dry CO-

In the actual operation of the carbonate bonding process, a water bearing
flue gas containing carbon dioxide will be used to carbonate bond the
lime hydrate taconite mixture.  Therefore,- an experiment was conducted
using briquettes made from the six lime hydrate samples and water saturated
100 percent carbon dioxide.  The briquettes were placed in the reaction
chamber and carbon dioxide gas bubbled through a column of water.  After
a reaction time of six hours, the briquettes were removed and allowed to
cure for 190 hours.  The compressive strength was then determined for the
briquettes.  The results of these experiments are shown in Table 4.
                            -18-

-------
                              Table 4
         Compressive Strength of the Lime Hydrate Briquettes*
                                                             2
                                  Compressive Strength, kg/cm
         Lime Hydrate               190 hours     700 hours
         Ohio Super Spray            276            315
         Pfizer Spray                176            201
         Tiger Finish                148            169
         National Hydrate            223            255
         Marble Cliff                 84             96
         Mercer Lime                  35             40

         *Using water saturated CO,.

In comparing Table 3 (dry C02) with Table 4 (wet C02>, it is seen that the
Compressive strength of each lime hydrate increased using a water saturated
carbon dioxide except for the Pfizer Spray which decreased from 337 to
201 kg/cm2 at steady state.  Also, by using a water saturated carbon
dioxide gas, the Ohio Super Spray briquettes increased from 278 to 315 kg/cm2.
Thus, under the more representative carbonate bonding condition of water
saturated carbon dioxide, Ohio Super Spray* is the most suitable lime
hydrate.  In a commercial application of the carbonate bonding process,
carbon dioxide will be formed by burning a fuel and water vapor will be
present in the gas.

There are other factors besides the calcium oxide content of lime
hydrate which can affect the extent of the carbonate bonding reaction and
the resulting briquette compressive strength.  The external surface
area of the lime hydrate plays an important role because as the surface
area increases, more carbonate bonding reaction area is available to yield
a denser and stronger limestone matrix.  Thus, external surface area
which is a function of particle size and the lime hydrate manufacturing
procedure is indicative of the resulting carbonate bonding strength.  This
is shown in Figure 10 where the steady state compressive strength of each
lime hydrate from Table 3 (dry carbon dioxide) is plotted versus the
external surface area of the lime hydrate per gram of taconite tailings.
In Figure 10, compressive strength increases substantially with increasing
external surface area.  Other lime hydrate factors influence compressive
strength.  One such factor which probably affects strength is the lime
hydrate's MgO content.  As seen in Tables2 and 3, the more successful lime
hydrates tended to have higher MgO contents.  For example, Marble Cliff
lime hydrate which has an external surface area comparable to National
Lime Hydrate (5698 cm2/gram compared to 6404 cm2/gram) yielded a con-
siderably lower compressive strength probably because of its low,er MgO
content (5 percent as compared to 34 percent for National Lime Hydrate).
*Mention of trade names in the report does not imply endorsement or
recommendation by either Applied Technology Corporation or the Environmental
Protection Agency.
                                -19-

-------
While determination of the exact reasons why one lime hydrate is more
suitable than another is beyond the scope of study, it can be stated that
dolomitic lime hydrates with high external surface areas yield the highest
compressive strengths.

         PSI   kg/cm2

        5000  -|-  352
  CM

  0)
  6
  o
  A
  4J
  cc
  c
  w
  
-------
to weathering.  If carbonate bonded taconite tailings cannot maintain
structural integrity during repeated freezing and thawing, then it
cannot be an effective construction material.  The properties that were
investigated and will be presented in the next section are air and water
permeability of taconite tailings, the compressive strength after repeated
freeze-thaw cycles, and flexural strength.

AIR AND WATER PERMEABILITY OF CARBONATE BONDED TACONITE TAILINGS

A search of the ASTM standards did not yield an applicable standard
determination for air and water permeability, therefore, special
equipment and procedures were established for these determinations based
on the commonly accepted theory of permeability.  The equipment and
procedures are discussed in the Appendix.  Where applicable, cement
briquettes were used as a basis for comparison.  These cement briquettes
were prepared using one part (by weight) Portland Type One cement, 2.75 parts
Ottawa Sand and 1/2 part water.  The cement and sand were dry mixed, the
water added slowly, and the wet mixture blended thoroughly.  The cement
briquettes were allowed to cure 28 days.

The experimental procedure to determine air permeability consisted of
using a taconite tailings briquette as a plug in a sealed chamber.  The
chamber pressure was then determined as a function of time.  This is an
unsteady state operation in which the pressure driving force is continually
changing with time.  The mathematical treatment required to obtain the
air permeability is presented in the Appendix.  The  air permeability is
defined as the volumetric flow rate of air  (m3/hr) per cross-sectional
flow area  (m2) per unit pressure driving  force  (kg/m2/m).

In this test  the air permeability of Ohio Super Spray lime hydrate, Pfizer
Spray lime hydrate, and National Lime Hydrate taconite tailings briquettes
were determined.  For comparative purposes,  the air  permeability of a
cement briquette of the same dimensions as  the  taconite tailings briquettes
was also determined.  The results are presented in Table 5 where the air
permeability, and the ratio of the permeability of the taconite tailings
briquettes to the cement briquette are presented.

                              Table 5
         Air Permeability of Taconite Tailings Briquettes

Lime Hydrate             Air Permeability*     Permeability Ratio**
Ohio Super Spray              1.01xlO~b                1.6
Pfizer Spray                  l.OlxHT6                1.6
National Hydrate              0.63xlO~6           _    1.0
Cement                        0.63xlO~6
*m3/(hr-m2-(kg-m2/m])

**Air Permeability of Lime-Hydrate Taconite Briquettes
                Air Permeability of Cement
                              -21-

-------
The water permeability of taconite tailings briquettes was determined in
much the same manner as air permeability and is discussed in the Appendix.
Water permeability is the mass flow rate of water per cross-sectional
flow area per unit pressure driving force.  The water permeability of
Ohio Super Spray, Pfizer Spray, and national Hydrate taconite tailings
briquettes is shown in Table 6.  Also shown in Table 6 is the water
permeability ratio defined as the water permeability of taconite tailings
divided by the water permeability of cement.  It is interesting to note
the varitLons in water permeability of the lime hydrate taconite tailings
briquettes.  The Ohio Super Spray compares quite well with cement in the
prevention of water permeation.  The Pfizer Spray and the National Lime
taconite tailings briquettes proved to be inferior in the prevention of
water permeation and would probably not provide  an adequate seal against
water permeation.  On the other hand, the Ohio Super Spray-taconite
briquette would provide an adequate seal against water permeation.

                              Table 6
           Water Permeability of Taconite Tailings Briquettes

                                                Permeability
            Lime Hydrate      Permeability*       Ratio**
            Ohio Super Spray    9.8xlO~-LU           2.3
            Pfizer Spray        6.7xlO~8          160.0
            National Hydrate    9.4xlO~9           22.0
            Cement              3.9xlO"10

            *Water Permeability in kg/(hr-m2-[kg/m2/m])
           **Permeability of lime taconite/permeability of cement

The air and water permeability results obtained are somewhat inconsistent.
The order of permeability for the various lime hydrates should be the
same for air and water permeation, as both permeabilities are a function
of porosity.  As a possible explanation for the varying permeabilities of
the lime hydrates, it is postulated that water is reacting with the lime
hydrate to change its porosity whereas air would not be expected to react.
This would account for the significant differences in water permeability
shown in Table 6 for the various lime hydrates.  Nevertheless, the
permeability results are still applicable.

FREEZE-THAW TEST ON CARBONATE BONDED TACONITE TAILINGS

Freeze-thaw tests were performed on briquettes formed using Ohio Super
Spray,  Pfizer Spray and National Hydrate.   The freeze-thaw test was
based on the ASTM designation:  C290-67 entitled "Standard Method of Test
for Resistance of Concrete Specimens to Rapid Freezing and Thawing in
Water."  The freeze-thaw test, as modified for this study, is discussed
in the Appendix.  The objective of this test was to determine compressive
strength of the lime-hydrate taconite tailings briquettes as a function of the
number of freeze-thaw cycles for the three lime hydrates briquettes.  As
seen in Figure  11,  the compressive strength of the briquettes tend to
                              -22-

-------
stabilize at a relatively constant value after five freeze-thaw cycles.
After 40 cycles, briquette compressive strength varied between 169 kg/
cm2 (2400 psi) for National Hydrate to 211 kg/cm2 (3000 psi) for Ohio
Super Spray.

It must be noted that subjecting a composite material of small dimensions
(five cm diameter by five cm high briquettes) to 40 freeze-thaw cycles is
an extreme test.  The ASTM standard test method contemplates the use of
specimens not less than 7.6 cm nor more than 12.7 Cm i-n width and depth
and not less than 35.6 cm nor more than 40.1 cm in length.  The normal
erosion of surface edges which occurs during freeze-thaw testing does not
seriously affect large specimens, whereas for small briquettes the effect
on strength is quite dramatic as evidenced by the results of this study.

                                  Conditions: 3% moisture
                                            100% C02
                                              6  hr reaction time
                                            190  hr curing  time
                                             84  kg/cm2 formation
                                                         load
282 -i
"B
^ (
00
43 [
00
c
0)
Vl
4J
0)
3141-
0)
i-
o
o
70-
1 O - Ohio Super Spray 2C
\ A- Pfizer Spray 20.7 v
>A Q - National Hydrate 15
1 Vn n
\\


N.








it
3^
                           3"  ^b    2§    36
                             Freeze-Thaw Cycles
             FIGURE 11— COMPRESSIVE STRENGTH VS. FREEZE-THAW CYCLES
An examination of the lime hydrate taconite tailings briquettes after  40  cycles
revealed an erosion of about 0.6 cm of material almost completely
around the edges of the Pfizer lime and National Hydrate.  The  edges on
the Ohio Super Spray lame hydrate tatonite  tellings briquetts revealed less than
                           -23-

-------
0.3 cm. material loss.  This reduced area on which the compressive
strength test load was applied caused the briquettes to fail at lower
loads and indicates a lower compressive strength than those based on the
original 20.3 cm  area.  For comparison, cement briquettes had an initial
compressive strength of 140 kg/cm2 which reduced to 84 kg/cm2 after 40
freeze-thaw cycles.  An examination of the cement briquettes after 40
cycles revealed an erosion of about 0.3 cm of material aim o s t  com-
pletely around all edges.

Even under extremely unfavorable test conditions, the carbonate bonded
taconite tailings briquettes yielded compressive strengths in the order
of 169 to 211 kg/cm2.  Consequently, it should withstand the effects of
severe changes in temperature.  Ohio Super Spray  lime hydrate yielded
the highest compressive strength (211 kg/cm2) after 40 freeze-thaw cycles.

FLEXURAL STRENGTH TESTS ON CARBONATE BONDED TACONITE TAILINGS

A study was conducted to determine the flexural strength of the three
superior lime hydrates, Ohio Super Spray, Pfizer Spray and National
Hydrate.  In the flexure test, a load is applied at the center of a bar
that is supported near its two ends.  As the load increases, the specimen
bar exceeds its elastic limit and failure occurs.  The load applied at
failure is calculated in kilograms per square centimeter.  Several bars
measuring 2.54 cm square by 10.16 cm long were prepared using the three
lime hydrates.  The flexural strength of taconite tailing bars using
Ohio Super Spray, Pfizer Spray and National Hydrate were 35, 31 and 24
kg/cm2 (500, 440 and 340 psi), respectively.  The flexural strength of
concrete is normally about one-tenth its compressive strength.  The
highest flexural strength for the lime taconite tailings bars was about
one-eighth their compressive strength.  Both the Ohio Super Spray and
Pfizer Spray limes exceeded the flexural strength of concrete.

SUBSEQUENT EXPERIMENTATION

As a result of the briquette laboratory study, it was decided to select
Ohio Super Spray as the lime hydrate to use in subsequent brick and plot
testings.  Except for the air permeability test, the Ohio Super Spray
taconite tailings briquettes were superior to the other selected lime
hydrates.
                             -24-

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                             SECTION V

                   EXPERIMENTAL STUDY (BRICKS)


CARBONATE BONDING 01* TACONITE TAILINGS AS 15 cm CUBES

In the Appendix entitled "Theoretical Considerations of Carbonate Bonding"
a simplified model of carbonate bonding is presented.  This model
considers that the progression of an interface separating a carbonate
bonded zone from an unreacted zone in a slab of lime hydrate bearing
material is controlled by the diffusion of carbon dioxide into the slab.
The model predicts that the progress of the carbonate bonded interface
is related to the square root of time:

                          Z  -  \J4kt                           (1)
            t
     where Z  - the depth of the carbonate bonded
                 layer into the slag at any time
           k  = constant
           t  = time

Furthermore, the model predicts that the rate of diffusion of C02 is
directly proportional to the concentration of the C02 in the reaction
gas and for a given reaction time the depth of the carbonate bonded
zone will be directly proportional to the C©2 concentration in the
reaction gas.  The model was primarily developed to ascertain the required
carbonate bonding time for plot experimentation.  Plot experimentation
involved the carbonate bonding of plots, 1.2 metres square, and will be
discussed in a later section.

To check the model and ascertain the required carbonate bonding time
for plot experimentation, 15 cm taconite cubes were carbonate bonded
at various CO^ concentrations and carbonate bonding times, using Ohio
Super Spray lime hydrate.  These experiments were conducted by placing
the pressed cube in a sealed container which had inlet and outlet
valves.  C02 was introduced and the reaction took place over the entire
surface of the brick.  After the allotted time, the bricks were cut in
half and the extent of carbonate bonding measured.

Figure 12 shows a cross-sectional view of two bricks, reacted with
100 percent C02 for times of six and ten hours.  After a total reaction
time of six hours, the carbonate bonded interface progressed to a
depth of 5.7 cm.  After ten hours reaction time, the entire brick was
reacted (7.6 cm progression).  Another series of experiments were
conducted to simulate carbonate bonding with flue gas.  A gas mixture
containing 20 percent C02 and 80 percent N2 was used for ten and twenty
hour periods.  These bricks are shown in Figure 13 where it is seen
that the carbonate bonded interface progressed a distance of 1.6 cm
and 2.4 cm for reaction times of ten and twenty hours, respectively.
                             -25-

-------
       15 cm
        3.8 cm
              — E
              1 u

               oo


              I"
  Fully Reacted
                       ^^•B
3
u

in
6 hours reaction time

shaded portion — unreacted
                                  I-*-
                                          15 cm
Fully Reacted
              10 hours reaction time
    FIGURE 12—SECTIONED BRICKS REACTED WITH 100% C0r
       15  cm
       12  cm
       Unreacted

        Portion


              ///
     Fully Reacted
 10 hours  reaction time
                   15 cm
                                           10cm
                     Y/7,
                ' / Unreactea ,
            t
                Fully Reacted
                                     g

                                    m
            20 hours reaction time
    FIGURE  13—SECTIONED BRICKS REACTED WITH 20% CO,
                       -26-

-------
The results of the above experiments were plotted in Figure 14 as the
carbonate bonded depth versus the square root of the carbonate bonding
time.  For both the '20* 'and 100 percent C02 cases, the curves are straight
which infers that equation (1) is a valid representation of the progression
of the carbonate bonded interface.  That is, the progression of the car-
bonate bonded layer is directly proportional to the square root of carbon-
ate bonding time as predicted by the model.
  Inches   cm
       3.. 7.5
        -'  5
   •o
   •3
    G
    O
    0)
    4J
    flj
    C
    O
    •s
    «
    O
I..  2.5
                                                             20% CO,
                     I
                                                  I
                    1           2           345
                                                   1/2
                                                                  1/2
                           Square Root of Time, Hr

        FIGURE 14--CARBONATE BONDED DEPTH VS TIME FOR 15 cm BRICK
The slope of the lines shown in Figure 14 are 2.1 and 0.55  cm/(hr)
for the 100 and 20 percent CC^ cases'.  The ratio of  these slopes is
4.8 which is very close to mass-transfer driving force  ratio:
                           100%
                            20% C0
                                 100-0*
                                  20-0
     where  CR =  concentration of C0£ in  the reaction  gas
            C_ =  concentration of C02 at  the carbonate bonding
                 interface
          *C     is assumed  to be zero
                            -27-

-------
Thus, as Indicated by the model, the carbonate bonding depth at any
given time is directly proportional to the CC>2 concentration in the
gas provided similar formation loads are used.  Figure 14 can be
used to estimate the required carbonate bonding time for a lime
hydrate bearing slab of specified thickness.

A study was undertaken to evaluate the effect of formation load on the
depth of carbonate bonding using the 15 cm cubes.  Cubes were prepared
using 20 percent Ohio Super Spray lime hydrate, 3 percent moisture,
100 percent carbon dioxide gas, and reacted for six hours.  Formation
loads in the range of 35 to 105 kg/cm2 (500-1500 psi) were used.  After
reaction, the carbonate bonded cubes were cut in half and the depth of
the carbonate bonded layer measured.  The objective was to determine if
there was any significant change in the carbonate bonding rate due to
porosity changes in the taconite tailing-lime hydrate structure which
would affect the diffusion of carbon dioxide gas.  The formation loads
selected are considered to be those obtainable using commercial equipment.

The results of this study are shown in Figure 15 where the depth of
the carbonate bonded layer is shown as a function of the formation load.
Cubes formed at 35 kg/cm  had an average depth of reaction of 4 cm
whereas cubes formed at 105 kg/cm2 had an average reaction depth of
3.8 cm.  Considering the experimental error involved in these measurements,
it appears that formation load is not a significant factor—at least for
the range of formation load studied.  However, while formation load may
not have a significant effect on the progression of a carbonate bonding
layer, it has a very definite effect on compressive strength (see
Briquette Study — Figure 9).

The compressive strength of the sectioned cube (Figure 12, ten hour
reaction time, fully carbonated) was determined to be 226 kg/cm2
(3200 psi) using a formation load of 54 kg/cm2 (770 psi).  This is sig-
nificant in that taconite tailings bricks could be formed at formation
loads higher than 84 kg/cm2 (1200 psi) to yield bricks as strong or
stronger than concrete.  This would be another outlet for the use of
taconite tailings.
                               -28-

-------
  Inches

     2.0
     1.5  -•
+J

g
IX
0)
     0.5
    cm

    5.0
             3.8
-- 2.5
    1.3
                     35
                     500
                        70
105
141 kg/cm
                       1000         1500


                          Formation Load
            2000 PSI
           FIGURE  15  -  EFFECT OF FORMATION LOAD  ON DEPTH OF CARBONATE

                                BONDING OF 15  cm BRICKS
                                    -29-

-------
                              SECTION VI

                         TACONITE TEST PLOTS
Based on the results of the bench-scale tests, Ohio Super Spray lime
hydrate at 20 weight percent, a 3 percent moisture content, and a
100 percent carbon dioxide reaction gas were selected for the plot study.
Using these conditions, several test plots were prepared.  The purpose
of the plots was to investigate various carbon dioxide addition
techniques under conditions more nearly approaching road building
techniques.

Five 1.2 metre square plots were prepared.  Figures 16 and 17 show
the configuration of these plots.  Each was formed such that the top
surface was at ground level with 10.1 cm of compacted lime hydrate-
taconite mixture carbonate bonded.  Various carbon dioxide addition
techniques were explored.  A summary of the base material and lime
hydrate taconite mixture thickness is presented in Table  7  for the
five plots.

                              Table 7
                           Plot Thickness

                                     Compacted Lime      Plot
                                   Hydrate-Taconite    Thickness,
  Plot             Base              Thickness, cm        cm	
    1    Fine gravel (5.1 cm layer)        10.1          15.2
    2    None                              10.1          10.1
    3    Fine gravel (5.1 cm layer)        10.1          15.2
    4    Coarse gravel (5.1 cm layer)      10.1          15.2
    5    Sand (5.1 cm layer)               10.1          15.2

Plot 1 had a 0.64 cm diameter perforated hose positioned in a pattern
as shown in Figure 16.  On top of the hose a 5.1 cm layer of gravel
was spread.  The lime hydrate-taconite mixture was then spread over the
gravel and compacted to a thickness of 10.1 cm.

Plot 2 also shown in Figure 16 was similar to Plot 1 except that no gravel
was used and the total plot thickness was 10.1 cm.  Plots 3, 4, and 5 used
different base materials as shown in Table 7.  These plot configurations
are shown in Figure 17 where it is seen that carbon dioxide was admitted
at the center for all three plots.

To prepare the plots, the lime hydrate-taconite mixture was spread over
the base material (gravel or sand with carbon dioxide carrier gas hose
embedded) and compacted with a small (low load generating) motorized roller.
The compacted mixture was then carbonate bonded using cylinders of carbon
dioxide gas according to the flow rate schedule shown in Table 8.  An
excess (assuming no leaks) of C0_ (16-24 hr period) was added to insure
full carbonate bonding.  Table 8 was prepared using the theoretical car-
bonate bonding model developed in this work in which the depth of carbonate
                                 -30-

-------
               Top View - Plots 1 & 2

             M	1'2m    	M
r~n r
i i i
i i !
1 '
i ' i
1 i i
.1 I i
i i 	 j
"1
i
•
I
j
1
N
               CO  out
                                       6
                                       CN
CO,
\
in
Hose Pattern for
 plots 1 & 2
 (0.64 cm O.D.)
Ground
 Level
              Side View - Plot 1
                                Hose
              Side View - Plot 2
                                           Plastic Lining
Ground 	 ^ _ >• —
Level
4
o o ° "" o ^H
— Taconite
— Gravel
        /
                                           Plastic Lining
                                         Taconite
                               Hose
  FIGURE 16—CONFIGURATION OF PLOTS 1 & 2
                        -31-

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             Top View - Plots 3, 4, & 5
             L«	 1.2m   	+. \
                             1.3 cm
                              I.D.
                              Pipe
                      C02 in
                                       6
                                       CM
Ground
 Level
              Side View - Plot 3
                            Pipe
  Taconite

  Fine Gravel
Ground
 Level
              Side View - Plot 4
                            Pipe
  Taconite
                                        Coarse Gravel
Ground
 Level
              Side View - Plot 5
                         O
                           Pipe
-Taconite

-Sand
     FIGURE 17—CONFIGURATION OF PLOTS 3, 4, & 5
                           -32-

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bonding as a function of time was determined.  The data generated in
the brick study was used to evaluate the various constants in the
resulting expressions.

                              Table 8
                        Plot C02 Addition Rate
                    Time                    C02 Flow Rate
                   Period                   (liter/hour)
                   0-1/2 hr                       4950
                   1/2-2                          1700
                   2-4                             850
                   4-16                            340
                   16-24                           142

Unfortunately,  prior to preparing these outdoor plots, it had rained
intermittently for several weeks and the ground became saturated with
water.  During compaction, the roller had to be driven over the
saturated ground and onto the lime hydrate-taconite mixture.   While
the roller was compacting the plots, the surrounding ground was giving
way and insufficient compaction of the plots resulted.  In addition,
because of the area limitations of the plots (1.4 metres square) a
sufficiently large roller could not be used to generate formation
loads in the order of 70 kg/cm2 (1000 psi).   The weight of the motorized
roller was approximately 1360 kgs (3000 Ib).  Each of the two rollers
of the motorized roller was 0.61 meter (2 ft)  long.  Assuming a contact
area of 0.64 cm (0.25 inches) for each roller, the resulting formation
load was estimated to be 35 kg/cm^ (500 psi).   Thus, in essence, ATC
could not apply the desired formation load (70 kg/cm2 [1000 psi]) to
form these plots.   After all five plots were compacted and reacted, they
were allowed to cure to permit the strength of the lime-taconite mixture
to reach steady state.  The plots were then sampled at several locations
and tested for percent carbonation, compressive strength, and density.

The percent carbonation was defined as:
                                          WF
               Percent Carbonation = (1 - — ) 100

     where W  = initial weight fraction of lime hydrate
                 in the mixture

           W  = final weight fraction of unreacted lime
                 hydrate in the carbonate bonded material

Percent carbonation was determined by reacting a sample of the carbonate
bonded taconite with hydrochloric acid and measuring the amount of
carbon dioxide evolved.  From the known amount of lime hydrate used and
the amount of carbon dioxide evolved following reaction, the percent
carbonation was determined.

Figure 18 shows the location of the sampling sites.  Samples from
locations Al to A3 were tested for percent carbonation and compressive
strength whereas apparent densities were determined from the samples
at locations Bl to B3.
                                   -33-

-------
               Top View of Plot
                                  B.
        FIGURE 18—LOCATION OF SAMPLING SITES IN EACH TEST PLOT
 Attempts were made to take samples  from Plots  1  and 2  but  the  plots  had
 not bonded well (probably due  to  rainfall  and  insufficient compaction)
 and core samples could not be  taken.   Samples  were obtained from Plots 3,
 4,  and 5.  In Table 9 is  shown the  compressive strength and density
 for  the  various  samples of Plots 3 to 5.  Examination of the results
 in Table 9  and the sampling locations of Figure 18  indicate that the
 compressive strength  increases as the sampling location approached the
 center of the plot  (where  the single carbon dioxide source was located).
 At first, it appeared reasonable to say that most of carbonate bonding
 occurred at the  C02 source  (center of the plot).  Upon closer examination,
 however,  the percent  carbonation (extent of carbonate bonding) was not
 significantly different (18 to 20 percent) at  the various sampling
 sites.   In  all probability, the center of the plot  was formed with a
 higher formation load which yielded higher compressive strength.  This is
 somewhat  substantiated by  the increase in density observed  as the
 center of the plot is  approached.  Thus, the strength of the carbonate
 bonded plot is strongly dependent on the formation  load.

 Since the percent  carbonation did not vary significantly across  the
 plot, it can be  said  that  the single source of C02  whether  located in
 fine gravel, coarse gravel, or sand did not significantly affect
 the  distribution of C02«   The fact that this relatively even distribution
-of C02 results  (even  with  a single source of C02 in a 1.2 metre  square)
                               -34-

-------
Ui
I
               Plot
                                                    Table  9


                                      Test Results for Plots 3. 4. & 5
Location
Al
A2
A3
Al
A2
A3
Al
• A2
A3
Percent Carbonation
20
20
20
20
19
20
20
18
18
Compress ive Strength
Kg/cm2 (psi)
105
84
0
84
70
0
70
70
0
2
Location
Bl
B2
B3
Bl
B2
B3
Bl
B2
B3
Density, s/cc
2.73
2.49
2.21
2.51
2.20
2.09
2.25
2.13
2.02
                 1  Tested for percent carbonation and compressive strength


                 2  Tested for density

-------
is significant in that it demonstrates the feasibility of road building
using perforated tubing at 1.2 metre (4 foot) centers (road building
applications will be discussed in a subsequent section).

In Figure 19 the compressive strengths of sample "A" are plotted versus
the density of corresponding "B" samples (data from Table 9).  There
is a relationship between density and compressive strength; the compressive
strength increases as the density increases.  Since the percent carbonation
is approximately the same, it can again be deduced that the formation
load must have varied at the different locations.
               1OO
                                               -H-
          CM
            00
            00
            c
            Q)
            M
            4J
            (O
            0)
            •H
            n
            to
            0)
                75  -
                                            Percent Carbonation
                                                18-20%
                              I
                              2.2
   T            |
   2.4         2.6
Density, gra/cc
2.8
              FIGURE 19—COMPRESSIVE  STRENGTH VS DENSITY FOR PLOTS 3,4,& 5
The compressive strength of samples taken near  the  center of  the plot
where it may be assumed that the roller formation load was 35 kg/cm^
(500 psi) agrees with the briquette formation load  affect illustrated    0
as Figure 9.  Figure 9 predicts a compressive strength of about 84 kg/cm*"
(1200 psi) and plot compressive strengths between 70.  and 105  kg/cn^
                                   -36-

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(1000 and 1500 psi) were achieved.  It might be deduced that if higher
formation loads were possible for plot preparation, compressive strength
comparable to those obtained in the briquette study would have been
achieved.

In summary, the plot results are encouraging in that they correlate well
with the briquette study.  Unfortunately, due to plot/roller size
limitation and inclement weather, the desired and commercially obtainable
formation load could not be realized.  Considerably more work will be
required to develop carbonate bonding road building techniques under
actual field conditions.
                            -37-

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                            SECTION VII

         APPLICATION OF CARBONATE BONDED TACONITE TAILINGS
The primary objective of this study was to determine the feasibility
of utilizing taconite tailings as a fill material in the carbonate
bonding process.  The results of this study show that carbonate bonded
taconite tailings have the necessary strength properties for paving
applications and the necessary sealant properties to effectively prevent
permeation of water which causes degradation of paved roads.  The
carbonate bonded process utilizing taconite tailings could be employed
to (1) construct secondary roads utilizing the formed carbonate taconite
tailings as is, (2) provide a strong inexpensive base material for
major road construction, (3) provide a first class road when used in
conjunction with a thin layer of asphalt, (4) provide aggregate as a
base material, and (5) make bricks.  These applications have the advantage
that taconite tailings can be used up, thereby, reducing stockpiles
and minimizing their polluting effect.

PROCESS DESCRIPTION

Although no work has been completed on larger scale field testing of
the carbonate bonding process employing taconite tailings, the following
proposed commercial concept appears feasible for the construction of
roads.  It is to be noted that suitable techniques will have to be
developed to permit carbonate bonding under inclement weather conditions.
The dried taconite tailings material is mixed with lime hydrate and
water (if necessary) in the proper proportions to make a homogeneous
mixture of lime hydrate and taconite tailings.  A perforated plastic
pipe is placed at 1.2 metre centers on the base of the road bed.  Gravel
or sand can be placed over the road bed and over the perforated hose.
The lime hydrate taconite mixture is then spreadover thehose which is expendable
and a roller is driven over the mixture to compact it.  After compaction
the perforated hose is attached to a portable carbon dioxide generator
which can be a burner of coal, oil, or any other carbonaceous fuel which
generates products of combustion containing carbon dioxide.

The flue gases are pumped under pressure into the perforated plastic
hose.  The conversion of lime hydrate to carbonate once completed
forms a road bed that will be surface' hardened with a concrete like
covering.  At present, the effect of combustion products other than
carbon dioxide and water vapor on the carbonate bonding reaction is not
known.  Since these gases will consist mainly of nitrogen, unburnt
oxygen, and hydrocarbons, it is anticipated that there will be little or
no effect.  In this study, experiments conducted with water saturated
carbon dioxide indicate that wet gas increased the lime hydrate taconite
compressive strength.
                               -38-

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As is evident from the foregoing description, the process can use
existing road grading equipment, requires no unusually skilled labor,
and can use locally available materials (with the exception of the
lime hydrate) thus minimizing material handling.  To carry out the
process, the following items of equipment will be required:

     1,  Expendable perforated plastic hose
     2.  Earth moving and compacting equipment
     3.  A portable carbon dioxide generator
     4.  Large capacity mixers (cement mixing truck)

The earth moving, compacting, and mixing equipment are not unique and
are considered part of the normal inventory of maintenance construction
equipment.  The portable carbon dioxide generator can also be readily
fabricated.  The equipment would essentially consist of a burner and
a blower.  The burner which can operate on a convenient fuel (coal, fuel
oil, bottled gas, or other carbonaceous fuels) would generate products
of combustion containing 15 to 20 percent carbon dioxide.  These flue
gases would be compressed and introduced into the perforated hose.  If
rapid setting of the carbonate bonded material is required, concentrated
carbon dioxide can be generated by the addition of an amine absorber
to the above equipment.  Economics will dictate the type of carbon dioxide
generator.

SUGGESTED OPERATING PARAMETERS

The economics of carbonate bonded taconite tailings depend primarily
on four controllable factors:  moisture content of the taconite tailings,
quantity of lime hydrate required, carbon dioxide concentration, and
the carbonation time.

The moisture content of the taconite tailings has been shown to be an
important factor.  To obtain maximum compressive strength, the moisture
content of the taconite tailings must be in the range of 3 to 5 percent.

This study has shown that a maximum of 20 percent lime hydrate will yield
compressive strengths approaching 316 kg/cm^ (4500 psi) for taconite
tailings.  Should lower compressive strength be acceptable, the amount
of lime hydrate can be decreased accordingly.

It has been found that adequate carbonate bonded taconite tailings
strengths are achieved when a 20 percent carbon dioxide gas mixture is
used for reaction times of ten hours.  Assuming that faster set times
are not required, a simple inexpensive carbon dioxide gas generator can
be used to yield carbon dioxide concentrations of about 15 to 20 percent.
Faster set times will require the use of carbon dioxide generators
producing a higher purity gas.
                            -39-

-------
PROCESS ECONOMICS

Costs for using carbonate bonding as a method of road construction can
only be approximated as full scale paving tests have not been performed.
These costs can be generated by considering the paving costs for con-
ventional materials shown in Table 10.

                               Table 10
                        Paving Cost, $/sq. yard

   Construction Material      Material in Place*   Labor**   Materials**
   4-inch rigid concrete            11.5             7.3        4.2
   6-inch rigid concrete            11.3             7.2        4.1
   8-inch rigid concrete            13.9             8.8        5.04

   4-inch sub-base                   2.0
   6-inch sub-base                   2.4
   8-inch sub-base                   2.1

   6-inch Bituminous                 4.0             0.7        3.3
    pavement w/base course
   9-inch Bituminous                11.5             2.1        9.4
    pavement w/base course
   12.5-inch Bituminous             13.0             2.4       10.6
    pavement w/base course

     *Pennsylvania Department of Transportation, Bulletin 50-
       Construction Cost Catalog (Sept., 1973).
     **Labor/material =1.75 (concrete paving), 0.22 (asphalt
       paving), Popper, H., "Modern Cost Estimating Techniques,"
       McGraw-Hill Co., New York, p. 101 (1970)

 As this information has been obtained from the above indicated source, it
 is presented in the units of the source.  For comparative purposes, carbonate
 bonding costs are presented in the same units.

 As stated previously, labor costs using carbonate bonding are not
 available.  Although the techniques and equipment required for paving
 with the carbonate bonding process will probably not differ significantly
 from asphalt paving, the labor cost associated with drying, mixing, and
 carbon dioxide generation will be greater than those of asphalt paving.
 Therefore, for a conservative cost comparison, the carbonate bonding
 labor cost has been assumed to be the same as for concrete (Table 10).
 Paving costs for carbonate bonding were then developed on this basis and
 are shown in Table 11. For the material costs of Table 11, it was assumed
 that taconite tailings are available at no cost, with the cost of
 transportation, drying, mixing, etc., included in the labor cost.  Lime
 hydrate was assumed to be $25 per ton (bulk) delivered, and 20 percent
 lime hydrate was used in the fill material.  In Table 11, the lime hydrate
 cost, the cost of perforated hose, and cost of fuel required to generate
 carbon dioxide are presented as a function of thickness.  The cost of
 fuel is directly proportional to the carbonate bonding thickness.  To

                                 -40-

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determine the cost of perforated hose, it was assumed that the hose
would be located on four foot centers in the pavement, and for thicknesses
greater than six inches, two layers of hose would be required.

Table 12 was prepared to illustrate road construction costs of conventional
paving materials, carbonate bonded taconite tailings, and combinations
of both using the paving costs of Tables 10 and 11.

                              Table 11
            Paving Cost for Carbonate Bonding. $/sq. yard


  Thickness
Total Cost
8.6
9.0
11.3
Labor Cost*
7.3
7.2
8.8
Material Cost
Lime Hydrate Hose**
0.92 0.12
1.38 0.12
1.85 0.24
    4 inch
    6 inch
    8 inch

     *Labor cost for concrete, Table 10
    **Hose cost is $0.04 per linear foot


For secondary road construction, Table 12, the application of carbonate
bonded taconite tailings offer no significant cost advantage or disadvantage
over conventional materials.  That is, despite the much higher assumed
labor costs for carbonate bonding, total paving costs for secondary
roads are comparable to conventional paving materials,

For first class roads, Table 12, carbonate bonded taconite tailings offer
a definite cost advantage over concrete.  When carbonate bonded taconite
tailings are used as a sub-base in conjunction with a bituminous surface,
the paving costs obtained are comparable to conventional concrete/sub-base
construction costs.

                              Table 12
                       Paving Cost. $/sq. yard

                                                           Carbonate Bonding
Secondary Road                                              Cos t^ Advantage
(1) 9 inch bituminous         8 inch carbonate
     w/base            11.5    bonded layer      11.3             $0.2
(2) 12.5 inch bitum-          6 inch bituminous
     inous w/base      13.0    w/6 inch carbonate                  0.0
                               bonded base       13.0
                              -41-

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                       Table 12 — Continued
Primary Road
(1) 6 inch concrete
     w/6 inch base
(2) 4 inch concrete
     w/8 inch base
(3) 6 inch concrete
     w/6 inch base
(4) 8 inch concrete
     w/6 inch base
                              Carbonate Bonding
                               Cost Advantage
      6 inch carbonate
13.7   bonded layer w/6 inch
       base               11.4
13.6  6 inch bituminous w/
       6 inch carbonate
13.7   bonded base        13.0
      6 inch bituminous w/
15.8   6 inch carbonate
       bonded base        13.0
      6 inch bituminous w/
       6 inch carbonate
       bonded base        13.0
$2.3


 0.6


 0.7


 2.8
Although the above comparisons are estimates of paving costs, the
indication is that taconite tailings can be economically utilized as
a road construction material.  Not only does the cost of utilizing car-
bonate bonded taconite tailings as paving material compare favorably
with conventional materials, but utilization of this waste material would
minimize disposal (stockpiling) and reduce its harmful ecological effects.
                              -42-

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                             APPENDIX
 THEORETICAL CONSIDERATIONS OF CARBONATE BONDING

 In the carbonate bonding process lime hydrate, water, and filler material
 are blended, compacted, and reacted with carbon dioxide to form a strong
 calcium carbonate crystal matrix.  This reaction can be represented simply
 as follows :

                      CaO + C0  - + CaC0                       (1)
 The relationship between the compressive strength of carbonate bonded
 materials and process operating variables is as yet not full understood.
 Prior experimentation on soils, clays, coal refuse, and sand indicates
 that compressive strength increases as:

           1.  The particle size of the fill material and lime
      hydrate decreases

           2.  An optimum water content is selected for each
      specific fill material

           3.  The external surface area of the fill material
      increases

           4.  The carbon dioxide partial pressure in the car-
      bonating gas is balanced so that the rate of heat generation
      within the composite equals the rate of heat loss to the
      surrounding gas phase

           5.  The time of carbonation increases, and

           6.  The lime hydrate concentration increases

Decreasing the particle size (along with a wider particle size distribution)
of the fill material decreases the porosity of the composite which is to
be carbonate bonded.  Decreasing the porosity yields a more dense composite
and enhances the formation of a continuous calcite matrix.  As the
compressive strength of the composite is mainly a function of the continuity
of the calcite crystal structure, a minimum porosity condition increases
the strength of the carbonate bonded material.

As the surface area of the lime hydrate increases , its rate of solution
in water is increased.  This causes the carbon dioxide to react, with a
saturated lime hydrate solution and results in a rapid precipitation of
a fine calcite crystal matrix.  The rapid formation of a multitude of
crystals enhances additional crystaline growth so that bridging and growth
can occur between crystals with a resulting increase in strength.
                             -43-

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 In general, the greater the external specific surface area of the fill
 material, the greater will be the rugosity or roughness of the surface
 of the particles.  This condition provides numerous bridging channels
 between particles for the growth of calcite crystals and increases the
 strength of the carbonate bonded material.

 The effect of the partial pressure of carbon dioxide in the carbonating
 gas on the carbonation reaction is directly related to the gas phase mass
 transfer of carbon dioxide to the liquid film.   Prior experimentation
 has shown that the rate of the carbonate bonding reaction is directly
 proportional to the partial pressure of carbon  dioxide.   Since the car-
 bonate bonding reaction is highly exothermic, the heat released during
 this reaction can be controlled by varying the  partial pressure of car-
 bon dioxide in the gas phase.   Depending upon the water content, the
 thermal characteristics and packing of the fill material, it is possible
 to crack the resulting carbonate bonded material by causing the reaction
 to occur too rapidly.   Due to a high carbonation rate,  sufficient heat
 is generated to produce steam within the interior of the composite.   If
 the packing is too dense,  rapid steam evolution from the carbonate bonded
 material is inhibited  and  can result in sufficient internal pressure to
 crack  or rupture the composite.   Consequently,  the partial pressure of
 carbon dioxide must be optimized so that internal pressure cracks are
 not generated and the  strength characteristics  of the carbonate bonded
 material can be maintained.

 A  simplified model of  carbonate bonding has  been developed.   The model
 considers  that the progression of an interface  separating a carbonate
 bonded zone from an unreacted  zone  in a slab of lime hydrate bearing
 material is controlled by  the  diffusion of C02  into the  slab.   That  is,
 the  carbonate bonding  reaction (1) ,  is  assumed  to be instantaneous  and
 irreversible  with no CC>2 beyond  the moving interface.  The  model  is  shown
 in Figure  20.   As  seen, C02  (component  A)  is in contact  with a slab  of
 lime bearing  material  (component  B,  CaO) which  is  infinitely long and
 deep.   The  distance measured from the  C02-solid interface  (the  slab  sur-
 face)^ is represented by Z.  Z1 (t) designates a  parallel  interface  separating
 a  region of totally  carbonate  bonded material (CB  =  0 when  0 <  Z £  Z'(t)) from
 a  region having no C02  (component A).   In  the latter region,  the  concentration
 of CaO  at  time  t  is still equal to the  concentration at  time  0  (CB =  CR (0)
when Z'(t)    Z    infinity).  The  concentration  of  C02 at  Z =  0  is  assumed
 to be constant  and equal to CA (0) whereas CA =  0  at Z =  Z'(t), since
 the carbonate bonding  reaction is assumed  instantaneous.  The distance
 Z'(t) is a  function of time (t) since the boundary between A  and B advances
into the slab as B is  consumed in the reaction.   The differential equation
representing  the diffusion of  C02 into the slab  is
                         DAS
                v             0 Z       0 < Z £ Z

      where D^ is the diffusivity of C02 in the slab.




                                 -44-

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The solution of equation (2) has the form:
                  CA                *  (    z    )
                        = a  + a  erf
                                                                  (3)
      where a. , a? are integration constants.

It can be shown that equation  (3) can yield the movement of the  carbonate
bonded interface as a function of time.  The resulting expression of  Z' (t)
with time is:

                  Z1  .=  \j4Kt                                     (4)

      where R is an integration constant.

Thus the model predicts that the progression of the  carbonate bonded
interface will be related to the square root of time.  It_can also be
shown that the average rate of diffusion of components A,N, , up  to
time T is:
                       2CA  (0)
                       erf
                                                                  (5)
This infers that  the rate of diffusion of CC^ is  directly proportional
to the concentration of  the C02 in  the reaction feed  gas, CA(0).   For a
given reaction time t, the thickness of  the  carbonate bonded  zone  will
also be directly  proportional  to
                                   -45-

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        Gas
CA-CA<°>
                         Taconite Slab
                    Carbonate


                    _   .  ,
                    Bonded



                    Region
                 2=0
                                        I  Vcc.o-S(0)
        '
tt    ,
uncarbonate

Bonded
        .   _  .
        |   Region


        |
*  -2-S2	,,
           FIGURE 20.   CARBONATE BONDING MODEL
                                        -46-

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DETERMINATION OF AIR PERMEABILITY

The air permeability of taconite tailings briquettes was determined
using the apparatus schematically represented in Figure 21.  The
apparatus consists of a 5 cm I.D. by 53 cm high plexi-glass chamber
on the bottom of which is cemented a taconite tailings briquette.
The top of the chamber was sealed with a plexi-glass plate containing
a pressure gauge.  The chamber was pressurized with gas and the change
in chamber pressure with time was measured to determine the permeability
of the taconite tailings briquette.  An initial chamber pressure of
1.4 kg/cm^ (20 psig) was used.  For an experimental run, the chamber
pressure was allowed to decrease to a pressure of 0.07 kg/cm2 (1 psig)
before termination.  Several tests were performed to determine the air
permeability which was defined as the volumetric flow rate of air (m3/hr)
per cross-sectional flow area (m^) per unit pressure driving force
(kg/m2/m) .

The determination of air permeability is based on the fact that when
a pressure difference exists across a porous solid, a flow of gas through
the solid will occur.  This is not diffusional flow in the usual sense
but it can be described using the methods of diffusion.  For the permeation
of gas through a taconite tailings briquette, where the diameter of the
capillaries is small and the gas velocity through the solid is small,
flow will be streamline and can be described by Porseuille's law for
a compressible fluid obeying the perfect gas law:
                       N =  — -  P  (P  - P ")
                       N   32ulRT   a ^1    2}
     where ?a =  (P^P


To facilitate discussion, the following nomenclature will be used

     A   Area through which gas is flowing
     d   Diameter of a capillary
     g   Conversion factor
     1   Length of capillary
     N   Rate cf diffusion
     n   Moles
     P   Pressure
     PI  Gas pressure in pressure vessel
     Pp  Atmospheric pressure
     P   Gas pressure in pressure vessel at time zero
     R   Universal gas constant
     T   Absolute Temperature
     t   Time
     u   Viscosity
     V   Volume of pressure vessel
     v   Volume
     Z   Height of Briquette
                                   -47-

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  Pressure
  Gauge
                                   Valve
 Chamber
 Sealant
FIGURE 21.  AIR PERMEABILITY APPARATUS
                       -48-

-------
The perfect gas expression which relates the pressure, temperature, and
volume of a perfect gas is expressed as

                            Pv = nRT                            (2)

The experimental procedure used to determine the permeability of
taconite tailings briquettes consisted of using a briquette as a plug
in a pressure vessel (Figure 21) and determining the pressure within the
vessel as a function of time.  This is an unsteady state operation in
which the pressure driving force is continually changing with time;
therefore, a mass balance on the gas contained in the vessel yields

                            gr  =  -NA                          (3)

     where A is the area through which gas flow is occurring

Expressions (1) to (3) form the basis of the mathematical treatment
required to determine permeability.  Substituting equation  (1) into
(3) yields


               dn      d gA           -
               dt  ~   32 ulRT     a   1


Taking the differential of the ideal gas law, equation  (2), with dv = 0
results in dn = Vdp /RT and substituting this expression into  (4)
yields
     where K = A d2g/(32 Vul)

Rearranging equation  (5) results in the following expression relating
the pressure within the vessel P..  to  time  t
                                                                 (6)
     where P  is the initial pressure in the vessel  at  time t = 0.
                                   -49-

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Integrating the above expression between the indicated limits yields
the following relationship between the chamber pressure P, and t:

                     (P +P )/(P -P )
Expression (7) is used to determine K since experimentally P- is known
as a function of time.  The- parameter K is determined as the slope of
(P2 + PQ/P2 - PQ)/(P2
                                                    vs
By definition, the permeability of a porous solid is the rate of diffusion
per unit pressure gradient across the solid.  The permeability P can be
defined using the following expression for which P has the units of
m^/hr - m^ - (kg/m^/m) and z is the depth of the porous solid in the
direction of flow.
                         (N RT/P^/CPj^ - P2/z)                    (8)
Substituting expression  (1) into  (8) yields  a  relationship between  the
parameter K and permeability:
                    P -    •                                      (9)
Since K is known from  (7) and V, z, and A are known,  the permeability
can be established using expression (9).
                                    -50-

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DETERMINATION OF WATER PERMEABILITY

Water permeability is the mass flow rate of water per cross-sectional
flow area per unit pressure driving force.  The water permeability of
taconite tailings was determined using the apparatus shown in Figure 22.
This apparatus consists of a 5 cm I.D. by 61 cm high plexi-glass primary
chamber sealed on top with a plexi-glass plate containing a valve and on
the bottom with a taconite tailings briquette.  An additional chamber
was attached to the primary chamber and located beneath the briquette
to collect the water permeating through the briquette and to prevent
unrealistic permeation rates due to convective air flows about the exposed
surface of the briquette.

In addition, water permeating through carbonate bonded taconite tailings
will probably enter into surroundings which will normally be saturated
with water vapor.  The lower chamber, therefore, provides a water vapor
saturated atmosphere to better simulate actual conditions and provide
more meaningful results.  The lower chamber contains a small hole on its
side to vent evaporated water and to prevent a buildup of pressure.

The water permeability test was conducted by initially filling the
primary chamber with water to a level of 48 cm and, on a daily basis,
measuring the amount of water lost due to permeation.  After the quantity
of water loss was determined, this amount was added back into the primary
chamber to maintain a constant pressure driving force.

This test was run for 35 days and an average rate of water permeation
determined.  For all practical purposes this test was run on a steady
state basis in which the water pressure driving force was constant;
consequently, the water permeability of taconite tailings can be directly
calculated since the flow area of the briquette, the thickness of the
briquette, the constant water pressure driving force, and the average
rate of water permeation are known.
                         -51-

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Valve
~i
Plexiglass
Tube ^
Briquette
~— —
Vent
•
k.
»
•••
-'

• •*.
«•

— «
C 	
-4 Water Level
Primary
**^" Chamber
	 Sealant
Secondary
Chamber
I
FIGURE  22.  WATER PERMEABILITY APPARATUS
                        -52-

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FREEZE-THAW TEST

The freeze-thaw test was based on the ASTM test C290-67 entitled "Standard
Method of Test for Resistance of Concrete Specimens to Rapid Freezing
and Thawing in Water".  The freeze-thaw,as modified for this study, is as
follows.

Freeze-thaw experiments were conducted using a refrigerator in which the
refrigeration area was at 2 to 5°C and the freezing compartment at
-18 to -16°C.  To initiate the freeze-thaw test, eight briquettes each
of Ohio Super Spray, National Hydrate, Pfizer Spray with taconite briquettes
were soaked in water for one hour and then placed in the freezing compartment
for four hours.  The freeze-thaw test required a rigid schedule in order
to make the results meaningful.  The schedule was as follows:
                          Time                  Event
                         9:00 a.m.     Remove from freezing compartment,
                                       Place in thawing water.
     Complete Cycle     10:00 a.m.     Remove from thaw water.  Place
                                       in freezing compartment.

                         2:00 p.m.     Remove from freezing compartment.
                                       Place in thawing water.
     Complete Cycle      3:00 p.m.     Remove from thawing water.
                                       Place in freezing compartment.
                         7:00 p.m.     Remove from freezing compartment.
                                       Place in thawing water.
     Complete Cycle      8:00 p.m.     Remove from thawing water.  Place
                                       in freezing compartment.

A freeze-thaw cycle  consisted of one thawing operation and one freezing
operation.  After five of these cycles, one of each of the three different
kinds of briquettes was removed from the system and allowed to air dry
for one to two days.  The compressive strength of these samples was then
determined.  This procedure was repeated at every five cycles up to
forty cycles.  The data obtained was the compressive strength as a function
of the number of freeze-thaw cycles for each kind of briquette.
                          -53-

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  SELECTED WATER
  RESOURCES ABSTRACTS
  INPUT TRANSACTION FORM
        I. Report No.
  4.  Title

     CARBONATE BONDING OF TACONITE TAILINGS
  7.  Anthoi(s)
  LaRosa, Paul J., Ricciardella, Kenneth A., McGarvey, Ronald J.
  9.  Organization
  Applied Technology
  135 Delta Drive
  Pittsburgh, PA   15238
3. Accession No.

w

J/ Report Date
f.  f '  •
A. Performing Organization
   Report No.  •  ,
10. Project No.
                            11.  Contract f Grant No.

                                68-01-0195
                            13.  Type of Report and
                               Period Covered
  IS.  Supplementary Notes
                     „         .,-....            .     .
                     Environmental Protection Agency report number
                     EPA-670/2-7^-001, January,
  16.  Abstract
  Carbonate bonding consists of mixing  and compacting a suitable material with water
  and lime hydrate, and then reacting it with a carbon dioxide-rich gas to form
  a coherent  structure bonded by a matrix  of calcite crystals.  Carbonate bonding of
  mining refuse can minimize the pollution associated with refuse  stockpiling and
  disposal.

  A laboratory study has indicated that taconite tailings can be carbonate bonded to
  form an effective road paving or brick-making material.  In general, the compressive
  strength of carbonate bonded taconite tailings increased with increasing lime
  hydrate content, reaction time, and  carbon dioxide concentration in the reaction  gas,
  Air and water permeability, freeze-thaw  resistance, and flexural strength of
  carbonate  bonded taconite tailings were  found to be comparable to concrete.  Scale-
  up of  the  laboratory studies to demonstrate paving applications  in small plots was
  hampered by a failure to obtain sufficient compaction.  The results, however, did
  confirm  the laboratory study findings.
  Possible  applications of the carbonate bonding process utilizing taconite tailings
  are road building, formation of  aggregate, and brickmaking.   An approximate  cost
  comparison suggests that the road construction application is an economical  alter-
  nat-lvp to  conventional road buildingjnaterials and techniques.	
  17a. Descriptors

   Carbonate Bonding*, Taconite Tailings1? Brickmaking*,  Road Construction*,
   Pollution Abatement*, Aggregate*
  17b. Identifiers

   Carbonate Bonding*,  Taconite Tailings*




  17c.CO WRR Field & Group
  18. Availability
  Abstractor p. j,  LaRosa
    ^'.%!;,>-••.*" ^"t':«irl'-;"V",;;"'^^•   i   i ™^^^—-—  _..     ->•• iv ii i
    ^l|S-Jfoi'iife^-v Send To:
    ijij^iHi	
    ,v^-,~*^*. ^';,V, ,'., WATER RESOURCES SCIENTIFIC INFORMATION CENTER
    <_,..f'-t~r"™,;l -'-',.lj U.S. DEPARTMENT OF THE INTERIOR
    •r/i. --^i --t'^.l^V/J..:/ WASHINGTON. D. C. 20240
             _
/ast/tut/oa Applied  Technology Corporation, Pgh.,  PA
WRSIC 102 (REV. JUNE 1971)


 *U.S. GOVERNMENT PRINTING OFFICE:1974  546-316/260 1-3
                                                                                   6PO 9!3.2«f

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