MONDAY, AUGUST 22, 2005, P.M. SESSION 1: INTERNATIONAL SYMPOSIUM ON COMPUTATIONAL ANALYSIS IN HYDROMETALLURGY (35TH ANNUAL HYDROMETALLURGY MEETING) COMPUTATIONAL FLUID DYNAMICS AND DATA ANALYSIS Sponsor(s): Hydrometallurgy Section, The Metallurgical Society of CIM Room: Herald Chairmen: M. DRY, University of Toronto, Canada and D. DIXON, University of British Columbia, Canada PAPER 1.1—14:00 HYDRODYNAMIC MODELLING OF HYDROMETALLURGICAL UNIT OPERATIONS. M.P. SCHWARZ, CSIRO Minerals, Australia CSIRO Minerals, in association with the AJ Parker CRC for Hydrometallurgy, has an active program of hydrometallurgical research in which physical and computational modelling are used to better understand and improve a range of different unit operations. The modelling is based on flow simulation (Computational Fluid Dynamics or CFD), but incorporates other physical and chemical phenomena of importance, such as droplet break-up and coalescence, species transport and reaction. This paper reviews some of the major projects that have been undertaken at CSIRO Minerals and will illustrate the critical importance of hydrodynamics to the performance of unit operations and the improvements that can be made to performance using such modelling techniques. Unit operations that are discussed include solvent-extraction, gravity thickeners, flotation cells, bio-heap leaching and stirred tanks.

PAPER 1.2—14:25 APPLICATION OF CFD MULTIPHASE MODELLING TO HYDROMETALLURGICAL PLANTS. L.M. OSHINOWO, Hatch Associates Ltd, Canada The incorporation of advanced analysis tools, such as CFD, into the process plant design process has become possible through advances in commercially available computational fluid dynamics software and faster computers. Multiphase modeling is now typically employed in the design phase of process plant design at Hatch. The multiphase models include Lagrange-Euler and Euler-Euler treatment of the multiple phases. Additionally, mixture models with interface-sharpening algorithms are applied to the prediction of free-surface flows in complex geometry. CFD is applied to applications where traditional design rules are not applicable. The additional rigor of CFD modeling in 2D and 3D allows process engineers to come closer to realizing true virtual plant design. The objective of this paper is to outline the benefits and pitfalls in the application of multiphase modeling for rigorous design. Several application examples in the hydrometallurgical field will be presented.

PAPER 1.3—14:50 PRELIMINARY RESULTS OF A HYDRODYNAMIC STUDY ON A PLASMA-LIFT REACTOR. L. MUNHOLAND, P. QUINTAL and G. SOUCY, Université de Sherbrooke, Canada This work presents early results from the first study to optimize the hydrodynamics of a Plasma Lift Reactor (PLR). The PLR is designed to break down organic contaminants in aqueous solutions. New experimental results are presented for air/water tests and for plasma degradation of organic contaminants in Bayer liquor. In addition, a multiphase Computational Fluid Dynamics (CFD) model has been developed which predicts the hydrodynamic behavior in the PLR. These results of this work suggest the PLR is effective at reducing contaminants in the Bayer liquor and that the CFD model adequately describes the hydrodynamic flow.

COFFEE BREAK—15:15-15:45 PAPER 1.4—15:45 MODELLING VARIABLY SATURATED FLOW IN POROUS MEDIA FOR HEAP LEACH ANALYSIS. D. MCBRIDE, M. CROSS, N. CROFT, C.R. BENNETT, University of Greenwich, United Kingdom and J. GEBHARDT, Process Engineering Resources Inc., U.S.A. A comprehensive model of solution flow through heap leach systems requires a numerical solution with the ability to predict variably saturated-unsaturated flow through porous media domains that contain materials with spatially variable properties. The types of flow associated with stockpile leach processes lead to flow problems, such as infiltration into dry soil, drainage, perched water tables and flow through heterogeneous materials. Computational methods for modelling this class of flow behaviour are conventionally based on the classical Richards’ equation.

However, the governing equations are highly non-linear, difficult to solve, and require iterative numerical solution methods. The pressure-based form of the Richards’ equation suffers from poor mass balance, while the mixed form can possess convergence difficulties. An adaptive transformed mixed algorithm is described, which reduces the nonlinearity of the problem, optimizes the time step size, and provides a fast, numerically robust scheme that significantly reduces computation (or CPU) time. This method is shown to give fast, accurate solutions on a number of complex flow problems. The utility of this algorithm is illustrated through its implementation within the PHYSICA computational modelling software, which incidentally, provides a framework for a comprehensive heap leach model.

PAPER 1.5—16:10 FAULT DIAGNOSIS IN METALLURGICAL PROCESS SYSTEMS WITH SUPPORT VECTOR MACHINES. G.T. JEMWA and C. ALDRICH, University of Stellenbosch, South Africa Fault detection and identification are major challenges in process engineering and manufacturing and the key component of abnormal event management systems. Timely detection, diagnosis and rectification of abnormal or faulty process conditions can lead to savings of billions of dollars in equipment damage and lost productivity, not to mention the prevention of injury and loss of human life associated with industrial accidents. A major contributing factor to current losses in industry is the reliance on human operators to interpret high-frequency samples from hundreds or thousands of variables simultaneously. As a result, the automation of fault detection and diagnosis is seen as crucial to the successful implementation of abnormal event management, the need for which is becoming all the more urgent given the increased complexity associated with modern industrial plants. In this paper, a methodology for process monitoring that uses support vector methods to extract nonlinear features from data is discussed and applied in the diagnostic monitoring of an industrial liquid-liquid extraction column.

PAPER 1.6—16:35 ANALYSIS OF ELECTROCHEMICAL NOISE DATA WITH PHASE SPACE METHODS. C. ALDRICH, B.C. QI and A.J. MAKOKA, University of Stellenbosch, South Africa Reliable monitoring of corrosion is vitally important in the combat against corrosion and over the last two decades the interpretation of electrochemical noise has received considerable attention as a means to discriminate between different types of corrosion. To this end, many different analytical methods have been proposed, such as power spectral density analysis and analysis via various other stochastic criteria. However, although many corrosion systems are nonlinear and therefore not readily interpretable by the abovementioned analytical approaches, few studies in the analysis of electrochemical noise to date have exploited advances made in the analysis of nonlinear systems. In this paper, the use of phase space methods to analyze and detect change in corrosion systems via electrochemical noise is considered. In particular, the electrochemical noise data are used to reconstruct trajectories in the so-called phase space of the system and changes in the topology of these trajectories or attractors as represented by correlation dimension curves are consequently interpreted as an indication of change in the dynamics of the system.

TUESDAY, AUGUST 23, 2005, A.M. SESSION 10: INTERNATIONAL SYMPOSIUM ON COMPUTATIONAL ANALYSIS IN HYDROMETALLURGY (35TH ANNUAL HYDROMETALLURGY MEETING) SIMULATION I Sponsor(s): Hydrometallurgy Section, The Metallurgical Society of CIM Room: Herald Chairmen: L. GUNNEWIEK and L. OSHINOWO, Hatch Associates Ltd, Canada PAPER 10.1—9:00 A DYNAMIC CIP SIMULATION USING MATLAB SIMULINK. J.W. COETZEE and H.E. DEIST, Cape Technikon, South Africa Carbon-in-pulp (CIP) and Carbon-in-leach (CIL) processes are used extensively in the gold mining sector to recover aurocyanide from solution. This paper discusses the modelling of these operations, using various simplifying assumptions, in order to simulate the effect of changing operating conditions on the dynamics of the system. It is assumed that the adsorption and leaching rates are a function of concentration difference between the source phase and recipient phase. The simulation was set up using Matlab’s Simulink interface. Plant variables are solved using a medium scale Newtonian search algorithm in Matlab, where the differences between the actual and predicted plant outputs are minimized on a continuous time-line.

PAPER 10.2—9:25 A FLOWSHEET-LEVEL APPROACH TO MODELLING AQUEOUS PHASE EQUILIBRIA AND SPECIATION IN A CIP/CIL GOLD PLANT. S. MUNRO and J. RUMBALL, CSIRO Minerals, Australia Operating CIP/CIL gold recovery plants in the arid regions of Western Australia face unique challenges due to water and ore quality issues. The presence of cations of Mg, Fe and Cu metals in water supplies and leached sulphide ores can have considerable effect on plant consumption of lime and cyanide, due to pH buffering and complexation effects. Reagent costs are critical to many of these operations. In this paper, computational modelling is used to analyse a number of process options to mitigate such issues. This requires careful consideration of the chemistry of the process system, and the ability to predict accurately the response of speciation and precipitation reactions to changing process conditions. The Mg-Fe-Cu-CN-H2O system is specifically modelled in this application. The Gibbs Free Energy Minimisation thermodynamic framework has been selected to model equilibrium phenomena. Furthermore, this approach has been extended to the process flowsheet level, incorporating multiple unit operations and recycle streams in a steady-state mass balance model. The Gibbs equilibrium composition module of HSC Chemistry software has been successfully integrated into the commercial flowsheeting package SysCAD. Consideration of the Gibbs Energy Minimisation capabilities of other flowsheeting packages has also been given. This approach has been successful in simulating the aqueous process chemistry typical of such plants. It has resulted in a predictive capability where equilibrium states respond appropriately to process changes. Potential cost savings may be identified by examining alternative process conditions to minimise overall reagent consumption. This methodology may be extended to other hydrometallurgical plants where issues of solution ideality, temperature and other equilibrium processes are sufficiently understood.

PAPER 10.3—9:50 DYNAMIC MODELLING OF THE SHERRITT AMMONIA LEACH PROCESS. N. DHADLI, G. FREEMAN, Sherritt International Corporation, Canada and K. NIKKHAH, AMEC Mining and Metals Consulting, Canada Ammoniacal pressure leaching operations have been in commercial use at the Fort Saskatchewan refinery of Sherritt International Corporation since 1954. Recent improvements have significantly increased the capacity of this circuit. The computer modeling of this process has proved to be a valuable tool in facilitating the evaluation of different operating scenarios in an effective manner. Reagent response in the ammonia leach of nickel and cobalt sulphides is examined using the dynamic process simulation capabilities of IDEAS™ software. The dynamic simulation enables accurate measurement of the sensitivity of the process to variables such as temperature, feed rate and ammonia concentration as well as compartmental analysis of metals concentration, pulp density, vent gas composition, and the formation of specific complexes. The complexity of the ammonia leach process makes it a logical choice for evaluation by computational analysis. The methodology for dynamic modeling of a pressure leach process is presented.

COFFEE BREAK—10:15-10:45 PAPER 10.4—10:45 MATHEMATICAL MODELLING OF THE SHERRITT-GORDON AMMONIACAL PRESSURE LEACHING PROCESS. T.M. WOODWARD, P.A. BAHRI, Murdoch University, Australia and D.G. DIXON, University of British Columbia, Canada A comprehensive mathematical model of the Sherritt-Gordon ammoniacal pressure leaching process at Kwinana Nickel Refinery has been constructed. The process is comprised of six autoclaves functioning as a three-stage leach circuit, operating over the temperature and pressure ranges of 85–95˚C and 750–1000 kPa, respectively. The process chemistry forms a complex series-parallel reaction network, characterised by: (1) the gas-liquid mass transfer and reduction of oxygen, (2) the oxidation of eight distinct minerals, (3) the oxidation of sulphur, and successive reactions involving various intermediately oxidised sulphur species, and (4) the oxidation of Fe(II) to Fe(III), and subsequent precipitation as a hydrated ferric oxide. Material balances were developed for species in all three phases, and an energy balance to account for all reactions and phase conversions. The multiple convolution integral (MCI) statistical reactor model was employed for the scale-up of particle kinetics. The model has been validated against plant data, and simulation results are in agreement with plant performance.

PAPER 10.5—11:10 METSIM® AS A MASS BALANCING TOOL FOR PLATINUM-GROUP METAL CONCENTRATORS. C. PANAOU, M. BELLINO, Hatch Africa, South Africa and M. GREYLING, Anglo Platinum, South Africa Commonly the most frequent method of calculating the overall mass balance of a multi mineral platinum-group metal (PGM) concentrator circuit in Southern Africa has been through the use of spreadsheets, typically Microsoft EXCEL®. While there are some advantages in using spreadsheet based mass balances during the conceptual design phase of the project, their use becomes limited in the detailed design phase of the project. This paper discusses the successful application of METSIM® for mass balancing of a PGM concentrator and highlights the benefits gained during the conceptual and design phase of the project.

PAPER 10.6—11:35 STUDY AND DEVELOPMENT OF LEACHING PROCESSES WITH THE AID OF MATHEMATICAL MODELING. E.M. VIGDORCHIK, E.E. ZHMARIN and Y.M. SHNEERSON, Gipronickel Institute, Russia One of the Gipronickel Institute activities is research and development of customized leaching technologies. The work is performed in several steps. Leaching kinetics are studied on a lab-scale according to a special procedure. Experimental data are processed using mathematical models of batch and continuous processes. Reliability of kinetic data and preliminary calculations is verified experimentally in laboratory reactors of various capacities. If necessary, certain parameters of the mathematical model are adjusted after pilot plant tests in continuous mode (4-reactor train, each with 25 L capacity). The specified kinetic characteristics along with the mathematical model are used for further development of industrial leaching technology.

TUESDAY, AUGUST 23, 2005, P.M. SESSION 21: INTERNATIONAL SYMPOSIUM ON COMPUTATIONAL ANALYSIS IN HYDROMETALLURGY (35TH ANNUAL HYDROMETALLURGY MEETING) SIMULATION II Sponsor(s): Hydrometallurgy Section, The Metallurgical Society of CIM Room: Herald Chairmen: V. PAPANGELAKIS, University of Toronto, Canada and J. DUTRIZAC, CANMET, Canada PAPER 21.1—14:00 DATA RECONCILIATION IN HYDROMETALLURGY: APPLICATIONS TO LEACHING OF CLAY AND COPPER SOLVENT EXTRACTION. C. BAZIN, K. EL-OUASSITI, D. HODOUIN and M. ZOUADI, Laval University, Canada Data reconciliation techniques are used to attenuate the effect of measurement errors and estimate unmeasured variables. This paper illustrates the application of data reconciliation to hydrometallurgy processes using measurements from leaching and solvent extraction obtained from laboratory experiments. The application of data reconciliation provides an efficient way to detect gross and systematic errors and produces reliable performance indices for process evaluation.

PAPER 21.2—14:25 COMPUTATIONAL POWER AND ION-EXCHANGE MODELLING. A. NESBITT and J. ABRAHAMS, Cape Peninsula University of Technology, South Africa Over the past half-century, researchers have commented on the complexities of ion-exchange processes, but have been forced to use gross assumptions to simplify the associated mathematics sufficiently for mathematical models to be of practical use, given the available computational power. This paper reviews the development of increasingly complex models as computational power has increased over the past half-century, from the direct application of Fick’s Law in 1947, through to today’s models requiring finite-element methods. The assumptions required for and the associated limitations of the various models are presented and the associated computational complexities are discussed. A case study is presented in which a continuous loss of capacity in an acid resin was observed over about sixty cycles of loading and elution. Resin poisoning does not explain this observation. This paper presents the capacity loss data, speculates on possible causes of the phenomenon, discusses why conventional kinetic and equilibrium mechanisms cannot explain it and concludes that the need to develop a more rigorous model is apparent. The significance of this observation is that standard tests done for a new resin application could significantly overestimate the commercial performance of the resin concerned, highlighting the importance of computer simulation in this field of process engineering.

PAPER 21.3—14:50 WATER MANAGEMENT SIMULATION FOR THE POGO GOLD MINE. L. GORMELY, B. NETHERY, G. BECKSTEAD, AMEC Americas Ltd., Canada, R. ZIMMER, Teck Cominco Ltd., Canada and K. HANNEMAN, Teck-Pogo Inc., Canada As part of the EIS and permitting activities for the Pogo Mine in Alaska, a risk assessment study for the water management plan was carried out. The basis for the study was a site water volume and contaminant balance model. Probabilistic modeling was carried out using the @Risk Monte Carlo simulation package to investigate the frequency, quantity, and quality of discharges under varying assumptions. The results were used in discussions with regulatory officials as part of the permitting process for the project. The simulation allows probabilistic modeling of key environmental parameters based on input frequency distributions involving both quality and quantity of flow. When combined with predictions of water treatment plant capacities and removal efficiency, this was very useful for defining the likelihood of important regulatory outcomes. Based on the model, certain project modifications were made. Using the simulation as a basis, ongoing dialog with regulators led to consensus that all potential risks have been considered and mitigated for the Pogo Mine according to their likelihood and severity, and that regulatory requirements would be achieved. The paper describes the model and some of the applications for which it was used.

COFFEE BREAK—15:15-15:45 PAPER 21.4—15:45 HEAPSIM – UNRAVELLING THE MATHEMATICS OF HEAP BIOLEACHING. N. OGBONNA, J. PETERSEN, University of Cape Town, South Africa and D.G. DIXON, University of British Columbia, Canada Although heap bioleaching has been recognized as an economic alternative for treating low grade mineral ores, the underlying physical, chemical and biological processes involved are complex. A detailed investigation into the dynamics of heap bioleaching processes has provided much insight into the underlying complexities, and has led to the development of a sophisticated modeling tool – HeapSim. This tool can be of great help in the design and operation of heap bioleaching processes. In this paper, the mathematical model implemented in HeapSim is examined. The model takes into account mineral kinetics, particle level effects, bacterial growth, oxidation and adsorption, gas absorption, pore diffusion, bulk advection, gas balance and heat conservation. Three case studies where the model has been applied are briefly discussed.

PAPER 21.5—16:10 CURRENT DISTRIBUTION IN MODERN COPPER REFINING. N.J. ASLIN, W. WEBB, Xstrata Technology, Australia and D. STONE, P.I. International, U.S.A. In today’s modern copper electro-refineries, increasingly higher average current densities are being employed. With these increases many refineries are approaching their limiting current density. The nearness of the average operating current density to the limiting current density has placed increasing emphasis on the need to maintain an even current distribution. This paper explores the importance of maintaining even current density and discusses the factors, processes and practices that are necessary to achieve and maintain high quality production at high operational intensity.

WEDNESDAY, AUGUST 24, 2005, A.M. SESSION 32: INTERNATIONAL SYMPOSIUM ON COMPUTATIONAL ANALYSIS IN HYDROMETALLURGY (35TH ANNUAL HYDROMETALLURGY MEETING) CHEMICAL AND PHYSICAL MODELING I Sponsor(s): Hydrometallurgy Section, The Metallurgical Society of CIM Room: Herald Chairmen: D. DIXON, University of British Columbia, Canada and D. BERKLEY, H.G. Engineering, Canada PAPER 32.1—8:30 A COMPREHENSIVE MODEL FOR CALCULATING PHASE EQUILIBRIA AND THERMOPHYSICAL PROPERTIES OF ELECTROLYTE SYSTEMS. P. WANG, A. ANDERKO, R.D. YOUNG and R.D. SPRINGER, OLI Systems Inc., U.S.A. A thermodynamic model has been developed for calculating phase equilibria and other properties of multicomponent electrolyte systems. The model has been designed to reproduce the properties of both aqueous and mixed-solvent electrolyte systems ranging from infinite dilution to solid saturation or pure solute limit. The model incorporates formulations for the excess Gibbs energy and standard-state properties coupled with an algorithm for detailed speciation calculations. The excess Gibbs energy model consists of a long-range interaction contribution represented by the Pitzer-Debye-Hückel expression, a second virial coefficient-type term for specific ionic interactions and a short-range interaction term expressed by the UNIQUAC equation. The accuracy of the model has been demonstrated for common acids and bases and for multicomponent systems containing aluminum species in various environments.

PAPER 32.2—8:55 CHEMICAL MODELLING IN HYDROMETALLURGY USING OLI. H. LIU, V.G. PAPANGELAKIS and J.F. ADAMS, University of Toronto, Canada Chemical modelling is becoming increasingly useful in the development, analysis, design, and control of hydrometallurgical processes. The paper presents the development of reliable databases for chemical modelling of selected hydrometallurgical systems using state-of-the-art software, such as OLI. The modelling strategies are also demonstrated. The high pressure acid leaching (HPAL) of laterites, oxygen solubility in the oxidative leaching process of zinc sulfide, gypsum solubility and lead chemistry in mixed sulfate-chloride solutions are presented as case studies. This on-going work attempts to bridge the gap between theoretical research and industry needs.

PAPER 32.3—9:20 SPECIATION FOR AQUEOUS SYSTEMS – AN EQUILIBRIUM CALCULATION APPROACH. H.-H. HUANG, L.G. TWIDWELL and C.A. YOUNG, Montana Tech, U.S.A. Equilibrium calculational software programs can be effectively utilized to model important parameters in aqueous and aqueous/solid systems, e.g., activity or concentration, solubility, temperature, solution pH, solution potential, and ligand complexation. In this regard, the calculational program STABCAL will be demonstrated. Illustrative examples for the creation of diagrams such as potential-pH, and activity or concentration-pH will be presented to demonstrate the fundamentals in real hydrometallurgical systems. Speciation, titration and mixing will be used to simulate or predict operational processes. Berkeley Pit water from a flooded abandoned copper open-pit mine will be used to illustrate the modeling process. According to analytical results the water contains about 20 important constituents. In addition to measured potential and dissolved oxygen, there are two distinctive oxidation/reduction couples. Concentration and other experimental parameters are seldom matched to each other and 12 different conditions have been identified. Water for each condition must be speciated in order to find the best-fit model. The STABCAL program will be used for the modeling calculations and the implication of the water chemistry to defining treatment opportunities will be discussed.

PAPER 32.4—9:45 THERMODYNAMICALLY CONSISTENT VERSUS COMPLETELY EMPIRICAL PROPERTY MODELS IN PROCESS SIMULATION. M.W. WADSLEY, Austherm Pty. Ltd., Australia Physical and chemical property models are widely used in process engineering. There exist, in the published literature, completely empirical models of “thermodynamic properties”, that is, property models that are not consistent with the laws of thermodynamics. There also exist property models that purport to be thermodynamically consistent

but are flawed in their implementation. There are also published property models that are consistent with the laws of thermodynamics but that do not reproduce property values with an acceptable accuracy. A practicing engineer might be asked to assess very different property models for which there are a variety of claims. This paper discusses features of thermodynamic and empirical property models that may assist in their practical evaluation and implementation.

COFFEE BREAK—10:10-10:40 PAPER 32.5—10:40 MODELLING OF PHASE TRANSFORMATIONS AND EQUILIBRIA OF CALCIUM SULFATE HYDRATES IN CONCENTRATED AQUEOUS CHLORIDE SOLUTIONS USING THE OLI SOFTWARE. Z. LI and G.P. DEMOPOULOS, McGill University, Canada The solubilities of calcium sulfate in HCl-CaCl2-H2O solutions up to 100°C were successfully modelled with the aid of OLI software after bringing proper modification to it. Modelling, more specifically, was carried out with the help of the Environmental Simulation Program, V-6.6.0.4 (www.olisystems.com). New model parameters were obtained by regressing the solubility data. A private databank was established by adding the newly defined model parameters and the calcium sulfate hemihydrate phase that is not included in the original OLI database. The McGill-modified OLI software was successfully applied not only to modelling solubilities but also the transformation of the various CaSO4 phases, namely dihydrate, hemihydrate and anhydrite. In this paper, the modelling methodology will be discussed along with a comparison of the capabilities of the newly developed model vs the original OLI model.

PAPER 32.6—11:05 MODELLING THE HIGH PRESSURE LEACHING OF LATERITE USING ASPEN PLUS AND OLI. M. DRY, V. PAPANGELAKIS and M. CHEUNG, University of Toronto, Canada The high pressure acid leaching of laterite was modelled and simulated using a combination of two commercially available software packages, namely Aspen Plus 11.1 and the OLI Stream Analyzer. New kinetic parameters for the acid-controlled metal dissolution and precipitation reactions at temperature were first determined from previous experimental data on the leaching of limonite. A grain model with cylindrical pellets was used to model the solidliquid dissolution reactions. Precipitation reactions were modeled via aqueous equilibrium chemistry. A dedicated thermodynamic database previously developed at the University of Toronto was used in conjunction with the OLI software to model the solution chemistry and calculate the concentration of hydrogen ions in solution at the leaching temperature. Mass balances were handled by Aspen Plus. The simulation gave good predictions of nickel dissolution under industrial and laboratory conditions, for the dissolution of limonite and limonite/saprolite blends.

PAPER 32.7—11:30 CHEMICAL TITRATION SIMULATION – AN EQUILIBRIUM CALCULATION APPROACH. H.-H. HUANG, L.G. TWIDWELL, C.G. ANDERSON and C.A. YOUNG, Montana Tech, U.S.A. An equilibrium calculational computer program (STABCAL) can be used for titration simulation. The program can handle various forms of titrants: solid, aqueous, gaseous or another solution, and various type of reactions: reductive, oxidative and adsorption. Background chemistry, mathematical considerations, and program development will be illustrated. Also, simple titration applications such as how to determine the acidity of aqueous solutions and alkaline sulfide leaching of gold are presented. Processing of an acid mine water from the Berkeley Pit in Butte, Montana has been used for illustration purposes. The treatment studies included: cementation, sulfidization, and neutralization. The calculational results for the neutralization treatment using various alkaline reagents are compared to actual experimental data and potentially appropriate processes are discussed.

WEDNESDAY, AUGUST 24, 2005, P.M. SESSION 41: INTERNATIONAL SYMPOSIUM ON COMPUTATIONAL ANALYSIS IN HYDROMETALLURGY (35TH ANNUAL HYDROMETALLURGY MEETING) CHEMICAL AND PHYSICAL MODELLING II Sponsor(s): Hydrometallurgy Section, The Metallurgical Society of CIM Room: Herald Chairmen: P. SCHWARZ, CSIRO Minerals, Australia and M. DRY, University of Toronto, Canada PAPER 41.1—14:00 MODELLING A SEMI-BATCH REACTIVE PRECIPITATION OF FERRIHYDRITE. T.S. LI, D. ILIEVSKI, CSIRO Minerals, Australia and W.R. RICHMOND, Curtin University of Technology, Australia The reactive precipitation of ferrihydrite was studied in a laboratory semi-batch precipitator, where a highly concentrated sodium hydroxide solution was fed into an iron nitrate solution. Instantaneous hydrolysis of iron[III] takes place when the two reagents are mixed generating supersaturation, and leading to immediate precipitation of ferrihydrite via a primary nucleation mechanism then followed by limited crystallite growth to 3 to 7nm. A mathematical model of the semi-batch laboratory precipitator was used to simulate the pH responses during the precipitation of ferrihydrite at different reagent feed rates. The model comprised of interconnected compartments of averaged hydrodynamic conditions, incorporating physically meaningful turbulent mixing time constants. The precipitation process is described by the solute concentration conservation equation incorporating a postulated ferrihydrite primary nucleation model. It was found that the semi-batch precipitator model predicted the experimental pH response reasonably well. The experimental results demonstrated that the reagent feed rate has a critical effect on the course of the precipitation of ferrihydrite, i.e. it is a rate determining step given the fast hydrolysis and precipitation kinetics.

PAPER 41.2—14:25 POINT OF ZERO CHARGE (PZC) AND DOUBLE LAYER ADSORPTION – AN EQUILIBRIUM CALCULATION APPROACH. H.-H. HUANG, L.G. TWIDWELL and C.A. YOUNG, Montana Tech, U.S.A. Solid particles, especially oxides/hydroxides, develop an electrical charge on their surface when submerged in aqueous solution. Parks was able to relate PZC (point of zero charge) of an oxide to its minimum solubility and isoelectric point. Dzombak has compiled an extensive database for adsorption reactions on ferrihydrite surfaces using the double layer theory. The equilibrium calculational program, STABCAL has been used to extend the considerations of Parks and Stumm. The PZC for many hydroxides, complex oxides, and non-oxides have been investigated and the results are reported in the literature. Removal of arsenic and metals by adsorption on ferrihydrite as the substrate has been computed using STABCAL and the computational results are compared to experimental data. Absorbents other than ferrihydrite such as aluminum hydroxide are also discussed.

PAPER 41.3—14:50 MICROMIXING IN THE GIBBSITE CAUSTIC ALUMINATE PRECIPITATION SYSTEM. T.S. LI, D. ILIEVSKI and I. LIVK, CSIRO Minerals, Australia Micromixing of the fluid and the particulate phases in a continuous gibbsite precipitator was investigated for three different micromixing states, namely maximum mixedness (MM), complete segregation (CS), and maximum mixedness of the fluid but complete segregation of the particulates (MMF-CSP). The influence of the micromixing on continuous precipitation of gibbsite was studied at different precipitation conditions via simulation. Three different cases were considered with respect to the precipitation kinetics: (1) gibbsite secondary nucleation is favoured with slow crystal growth and agglomeration rates; (2) gibbsite crystal agglomeration and growth are dominant mechanisms with secondary nucleation suppressed; and (3) gibbsite crystal growth is the only relevant phenomenon. Simulation results for the cases (1) and (3) showed that the crystalline products for both the MM and MMF-CSP micromixing states were very similar, but the product obtained from the CS micromixing state was considerably different. This indicates that at those precipitation conditions the fluid micomixing has the dominant effect on the product properties, and the particulates micromixing effect is insignificant. However, when operated under the conditions of fast crystal growth and agglomeration kinetics, i.e. case (2), both the fluid and the particulates micromixing were found to affect the precipitation process, resulting in a significantly different product.

COFFEE BREAK—15:15-15:45 PAPER 41.4—15:45 MATHEMATICAL MODEL OF THE AGUILARITE (AG4SES) LEACHING PROCESS IN THE PRESENCE OF OTHER REFRACTORY SILVER PHASES. R.M. LUNA-SÁNCHEZ and G.T. LAPIDUS, Universidad Autónoma Metropolitana, Mexico In the present study, a series of cyanidation experiments were performed on a bulk sulfide concentrate which contained silver in four distinct phases, to determine its extraction rate, varying the oxygen concentration. The results showed that three of the four phases were refractory, aguilarite (Ag4SeS) being the only leachable phase, and that the effect of the oxidant concentration is minimal. Mathematical simulations were performed, adjusting only the Damköhler number to adequately fit the experimental data. This model, and the mechanism implied, was compared with that found for acanthite (Ag2S) in a previous study, and the differences between the two discussed.

PAPER 41.5—16:10 MATHEMATICAL MODEL FOR THE LEACHING OF ACANTHITE (AG2S) IN THIOSULFATE SOLUTIONS. J. TREJO-GALLARDO and G.T. LAPIDUS, Universidad Autónoma Metropolitana, Mexico A phenomenological model is presented for leaching silver from silver sulfide contained in mineral particles with ammoniacal thiosulfate solutions of copper in stirred reactors. Experimental results were used to adjust the kinetic parameters of the model, such as the reaction order and velocity. The incorporation of the cuprous sulfide-silver sulfide equilibrium at the reaction interface allowed the prediction of the conversion limits for different reagent concentrations. The information obtained from the reactor model was later integrated into a heap leaching simulation, whose results adequately describe tendencies observed in column tests, with respect to variations in copper and thiosulfate concentrations.

PAPER 41.6—16:35 MATHEMATICAL MODELLING OF OXIDATIVE LEACHING OF NICKEL CONCENTRATE MAGNETIC FRACTION. E.E. ZHMARIN, E.M. VIGDORCHIK, M.I. KALASHNIKOVA, Y.M. SHNEERSON, Gipronickel Institute, Russia, and Y.V. DEGTIAREV, Severonickel Plant, Russia Separation of a magnetic fraction from nickel concentrate following matte flotation, and its treatment in a separate circuit enable reduces precious metals losses. The magnetic fraction is treated in two stages of oxidative leaching: atmospheric and pressure leaching. A study of the atmospheric stage, using mathematical modelling, is described in this paper. The mathematical model and experimental data were used to carry out industrial process calculations. The model enabled extrapolation of the results of batch laboratory test to industrial-scale continuous processing and the selection of optimal process conditions and large-scale equipment. In December 2004, commercial oxidative leaching tests were performed on the magnetic fraction at the Severonickel Plant (Monchegorsk, Russia). A comparative analysis of the mathematical modelling results and the actual process parameters is presented.