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					Assessment and Modelling of Chromium Release
    in Minerals Processing Waste Deposits

                   Joachim Petersen

             Thesis Presented for the Degree of


         in the Department of Chemical Engineering


                       March 1998

The minerals processing industry is by far the largest generator of mineral solid wastes,
which are commonly stored in large scale landfill deposits. The potential environmental
impact of these is directly linked to the time-dependent process of leachate generation
within these deposits. Rainwater draining through the porous matrix of a deposit creates
a slowly moving aqueous environment within the deposit. Heavy metal species that may
be contained in trace amounts in the waste material can be mobilised into the aqueous
phase by various chemical reactions and be transported by mechanisms of diffusion and
convection to the base of the deposit and from there further into the surrounding

Laboratory assessment methods aim to provide indicators to the leachate generation
potential of a particular waste material, often based on “worst case” assumptions, but
generally fail to offer a meaningful appreciation of the time-dependent leach behaviour
of the material in a full scale deposit. This is to a large part due to the lack of a thorough
description - in terms of a rigorous mathematical model - of the leachate generation
process itself.

Such a model is developed in the present work, building on an existing model for heap
leaching, which, conceptually, is very similar to the leachate generation process. The
model is based on the continuity equation formulated for reaction-diffusion processes at
the level of an individual porous particle and for convection-dispersion transport at the
bulk level. This is combined with a number of reaction models, both kinetic rate
expressions and thermodynamic equilibrium models, to describe the release process of
individual species at the solid liquid interface and also within the aqueous phase. The
model has been translated in the WASTESIM computer code within which waste
material and disposal scenario are characterised by a number of parameters, such as
those describing reaction modes and constants, particle size and pore diffusion effects as
well as bed transport and saturation. The program was found to be a versatile tool for
modelling a wide range of multi-species, multi-reaction deposit and batch leach

However, for modelling real waste materials the model parameters have to be
established from a systematic laboratory investigation. An assessment methodology is
proposed which aims to combine lysimeter studies with bench scale leach and physico-
chemical characterisation experiments to enable determination of all model parameters
entirely on the basis of laboratory experiments and validate them at this level against the
results from independent lysimeter studies with the modelling tool. It is argued that, if
all model parameters are validated at the laboratory scale in this way, modelling of full
scale scenarios involving the same waste material can be conducted with some

This approach has been put to the test with two waste materials from the ferro-alloy
industry - a furnace emission control dust and a smelter slag. The contaminant species of
particular interest for both these materials was chromium, especially Cr(VI), and
therefore it was the release behaviour chromium on what much of the work presented
herein has focused. The aqueous and environmental chemistry of chromium is
extensively reviewed and, as a side aspect, the long-term atmospheric oxidation of
Cr(III) to Cr(VI) has been positively identified by experimental work with a third
chromium-containing waste material.

The two test materials have been subjected to intensive characterisation in terms of
column and batch leach experiments, adsorption studies, column tracer studies and
physical characterisation experiments. The results are carefully interpreted with a view
to establishing a complete set of parameters to simulate the leachate generation
behaviour with respect to chromium species in a deposit scenario. It is demonstrated that
the modelling tool can in fact also be used for the interpretation of batch leach data
through curve fitting exercises.
For both materials the WASTESIM code, calibrated with parameters established entirely
through the laboratory experimentation, has been used to simulate the leach curves of
two independent lysimeter experiments, which are then compared to the measured data.
In both cases the modelled and measured curves compared reasonably well and in most
regards discrepancies can be explained by insufficient characterisation in the bench-
scale experiments. The overall approach is therefore seen as valid in principle, but it is
acknowledged that further experimental work and model development would be needed
to take account of the remaining discrepancies.

Two aspects were found to be particularly significant. The first relates to slow reaction
mechanisms, which may go unnoticed in short-term laboratory experiments, but may
become significant in full scale deposits given their long life-span. The slow
atmospheric oxidation of chromium is a point in case. The second aspect relates to the
hydro-dynamic characterisation of flow through unsaturated beds. Both model and
laboratory assessment methods are insufficiently developed to account for effects such
as dead pore diffusion and a distribution of flows.

Recommendations for further development work should focus on these two aspects and
on expansion of the approach to heavy metal species other than chromium. It is hoped
that the modelling and assessment methodology will ultimately find welcome
application in the environmental risk assessment of mineral processing waste disposal
                               Table of Contents

Abstract                                                              iii

Acknowledgements                                                      vii

Table of Contents                                                    viii
List of Figures                                                      xiii
List of Tables                                                        xx

Nomenclature                                                         xxii

1. Introduction                                                        1

1.1 Background                                                         1
1.2 Scope of Work                                                      4
1.3 Thesis Structure                                                   8

2. Chromium in the Environment                                       11

2.1 Environmental Chemistry of Chromium                               13
       2.1.1 Occurrence, Uses and Toxicity of Chromium                13
       2.1.2 Aqueous Chromium Chemistry: Speciation and Solubility    16
       2.1.3 Adsorption of Chromium from Aqueous Solution             23
       2.1.4 Aqueous Electrochemistry of Chromium                     28
       2.1.5 Cr(III) Oxidation in the Presence of Oxygen              30
       2.1.6 Cr(III) Oxidation by Manganese Dioxide                   34
       2.1.7 Cr(VI) Reduction                                         36
2.2 Chromium Cycles in Natural Environments                           38
       2.2.1 Chromium Cycle in Soils                                  38
       2.2.2 Chromium Cycle in Natural Waters                         41
       2.2.3 Imbalances                                               44
2.3 Experimental Study of Chromium Oxidation by Atmospheric Oxygen    46
       2.3.1 Oxidation in Aqueous Solution                            48
       2.3.2 Oxidation of Solid Chromic Oxide                         55
       2.3.3 Long-term Cr(VI) Oxidation In A Column Experiment        59
2.4 Closure                                                           66
3. Management of Solid Minerals Processing Wastes                             69

3.1 Wastes, Waste Disposal and Pollution - A Conceptual Introduction          70
3.2 Aspects of Waste Disposal Practise in the Minerals Processing Industry    76
       3.2.1 Introduction                                                     76
       3.2.2 Site Selection                                                   80
       3.2.3 Liners                                                           81
       3.2.4 Leachate Collection Systems                                      84
       3.2.5 Monitoring Wells                                                 86
       3.2.6 Rock and Slag Deposits                                           87
       3.2.7 Tailings Impoundments                                            89
       3.2.8 Mixed Deposits                                                   91
       3.2.9 Ash Deposits                                                     92
       3.2.10 Waste Stabilisation/Solidification                              94
       3.2.11 Site Closure                                                    97
3.3 Chromium Containing Wastes: Types, Treatment and Pollution                98
       3.3.1 Waste Types and Origin                                           99
       3.3.2 Chromium Removal From Waste Waters                              103
       3.3.3 Pre-disposal and In-Deposit Treatment Practices                 106
       3.3.4 Recycle Technologies                                            107
       3.3.5 Pollution Case Studies                                          109
       3.3.6 Clean-up and Remedial Technologies                              112
3.4 Closure                                                                  115

4. Development of A Model to Simulate Leachate Generation                    117
and Transport in Solid Waste Deposits

4.1 Hydro-transport Through Porous Media                                     118
4.2 Contaminant Transport in Aqueous Flow                                    127
4.3 Reactions Leading to Contaminant Release or Removal                      134
4.4 Review of Existing Hydro-transport Models                                139
4.5 The Dixon Heap Leach Model                                               148
       4.5.1 Reaction Modelling                                              149
       4.5.2 The Particle Pore Model                                         150
       4.5.3 Bulk Transport Model                                            153
       4.5.4 Some Critical Comments                                          155
4.6 Formulation of the Fundamental Waste Leach Model                         159
       4.6.1 Particle Level Model                                            159
       4.6.2 Particle Surface Model                                          160
       4.6.3 Bulk Flow Transport Model                                       166
       4.6.4 Batch Model                                                     169
       4.6.5 Boundary and Initial Conditions                                 171
4.7 Multiple Reaction Modelling                                              173
       4.7.1 A General Reaction Mechanism                                    173
       4.7.2 Reaction Rates                                                  175
       4.7.3 Reaction Models                                                 178
       4.7.4 Modelling Pore, Surface and Bulk Reactions                      183
4.8 Model Sensitivity Study Using WASTESIM                                        188
       4.8.1 Computer Simulations Based on the Model Equations                    190
       4.8.2 A Base Case for Model Evaluation                                     194
       4.8.3 Batch Studies: Particle Size, Size Distribution and Pore Diffusion   196
       4.8.4 Testing of Reaction Types and Their Influence on Leach Scenarios     199
       4.8.5 Column Studies: Bed Residence Time, Bed Saturation and
             Dispersion                                                           206
4.9 Summary and Further Aspects                                                   210
       4.9.1 Summary                                                              210
       4.9.2 Model Limitations                                                    213

5. An Integrated Waste Assessment Methodology:                                    217
Experimental Methods and Analytical Tools

5.1 Methodological Approach to Waste Leachibility Assessment                      218
       5.1.1 Review of Existing Waste Assessment Methods                          219
       5.1.2 The Integrated Waste Assessment Methodology                          223
5.2 Material Characterisation Methods                                             227
5.3 Lysimeter Studies                                                             228
5.4 Batch Leach Experiments                                                       234
       5.4.1 Methods and Types of Batch Leach Experiments                         235
       5.4.2 Interpretation of Batch Leach Experiments                            241
5.5 Cr(VI) Adsorption Studies                                                     246
       5.5.1 Experimental Methods                                                 247
       5.5.2 Fitting Adsorption Isotherms                                         249
5.6 Hydrodynamic Characterisation Using Tracer Studies                            251
       5.6.1 Residence Time Distributions                                         251
       5.6.2 Experimental Set-up of Tracer Studies                                254
       5.6.3 Two Flow Analysis Using WASTESIM                                     255
5.7 Column Modelling                                                              259
5.8 Overview of Experimental Studies                                              260
       5.8.1 Origin of Materials                                                  260
       5.8.2 Overview of Experimental Work                                        262
       5.8.3 Presentation of Experimental Data                                    265
5.9 Closure                                                                       266

6. Experimental Study 1: Metallurgical Dust                                       269

6.1 Origin and Characterisation of the Material                                   270
6.2 Lysimeter Studies                                                             278
       6.2.1 Experiment MD1                                                       278
       6.2.2 Experiment MD2                                                       282
       6.2.3 Summary                                                              285
6.3 Bench Scale Studies                                                     286
       6.3.1 Acid and TCLP Leaches                                          286
       6.3.2Water Wash                                                      289
       6.3.3 Alkali Leaches                                                 291
       6.3.4 Cr(VI) Extraction Leach                                        292
       6.3.5 Cr(VI) Adsorption Studies                                      293
       6.3.6 Reaction Modelling                                             298
6.4 Bed Diffusion Experiment and Modelling                                  304
6.5 Column Tracer Study                                                     309
6.6 Column Modelling                                                        316
       6.6.1 Modelling of MD1 Data                                          317
       6.6.2 Modelling of MD2 Data                                          321
6.7 Closure                                                                 324

7. Experimental Study 2: Low Carbon Slag                                    327

7.1 Origin and Characterisation of the Material                             328
7.2 Lysimeter Studies                                                       333
       7.2.1 Experiment LCS1                                                334
       7.2.2 Experiment LCS2                                                338
       7.2.3 Summary                                                        342
7.3 Bench Scale Studies                                                     343
       7.3.1 Acid and TCLP Leaches                                          344
       7.3.2 Water Wash                                                     346
       7.3.3 Alkali Leaches                                                 349
       7.3.4 LCS Long-term Leach                                            350
       7.3.5 Cr(VI) Extraction Leach                                        352
       7.3.6 Cr(VI) Adsorption Studies                                      353
       7.3.7 Reaction Modelling                                             356
7.4 Particle Pore Diffusion and Modelling of Kinetic Constant               358
       7.4.1 Particle Pore Diffusion Experiment                             359
       7.4.2 Establishment of Diffisivity and Kinetic Reaction Parameters   361
7.5 Column Tracer Study                                                     366
7.6 Column Modelling                                                        372
       7.6.1 Modelling of LCS2 Data                                         373
       7.6.2 Modelling of LCS1 Data                                         377
7.7 Closure                                                                 381

8. Conclusions                                                              385

8.1 Summary                                                                 387
8.2 Concluding Discussion                                                   396
8.3 Recommendations for Further Work                                        404
8.4 Future Applications of the Model and the Assessment Methodology         407
References                                                                    411


A. Analytical Equipment and Methods

B. An Introduction to WASETSIM, Model Solution Algorithms & Numerical Methods

C. Table of Contents - Data Files and WASTESIM Code

A 3.5” disk, labelled DATA & WASTESIM, containing all files listed in Appendix C is
supplied in the sleeve in the back-cover of this volume

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