Pyrometallurgical upgrading of PGM-rich leach residues from the Western Platinum Base Metals Refinery through roasting

The production of Platinum Group Metals (PGMs) normally entails the smelting of PGM flotation concentrates, converting of the furnace matte and removal of the bulk of the Ni, Cu, Co, S and Fe through atmospheric and pressure leaching in a base metals refinery to produce a PGM-rich concentrate. A number of impurities, mostly Se, Te, As, Bi, Os and Pb, are not removed significantly during the oxidising leach process in sulphuric acid media. In addition slag inclusions in matte leads to contamination of the PGM residues with silica, fayalite, magnetite and trevorite phases. Furthermore some Cu, Ni, Fe and S also remain. For this reason a typical Precious Metal Refinery (PMR) feed material contains less than 65% PGMs. The PMR is based on a chloride process and requires contaminants to be within narrow specification limits to prevent the formation of PGM residues that must be reprocessed or tolled, leading to poor first pass metal efficiencies and extending the duration of the production pipeline for efficient recovery. A process has been developed to significantly upgrade the BMR leach residues through pyrometallurgical processing, which include a multistep process of roasting under oxidising atmospheres, a two-step smelting process of the roasted calcine (with engineered slag chemistry and slag-refractory interactions) and subsequent atomisation of the molten alloy which can be fed as a slurry into the HCl/Cl2 dissolution reactors in the precious metals refinery. These pyrometallurgical steps upgrade the BMR residue from a 45–50% grade up to an alloy grade of ca. 90% PGMs, whilst removing the most deleterious elements with major process impacts on the PMR. This paper will focus primarily on the roasting step and it will investigate the thermochemical and mineralogical changes occurring during roasting. These changes were evaluated through a combination of thermochemical modelling and experimental investigation. The roasting step needs to be in an oxidative environment in order to achieve the vapourisation of Se, Te, As, Os and S. The speciation of PGMs and their vapourisation behaviour are presented, as well as the sensitivity of precious metals deportment to changes in roast conditions.http://www.elsevier.com/locate/minenghb2014ai201

The determination of acceptable Hippopotamus (Hippopotamus, Amphibius, Linn.) densities in the Crocodile River, outside the Kruger National Park

A research report submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree Master of Science in Quantitative Conservation Biology, 1993The aim of the study was to provide a basis for the determination of acceptable hippo densities along the Crocodile River, outside the Kruger National Park. Hippo densities have to be acceptable to land-user~, who suffer hippo damage, and to the conservation authority, responsible for the resources along the river. Hippo nllmbers and distribution in the study area were determined by using a combination of aerial census and ground count. The hippo population number was estimated at 88 animals. Hippo distribution was influenced by flow speed of water. No hippos were observed in river sections with steep gradients, (>6.0 m/km) , while major herds were observed at low gradients «3.5 m/km). A mean density of 0.6 hippo per km was calculated, with a maximum of 2,5 hLppoe Zkm at Kaapmuiden, next to the Kruger National Park. TPA Nature Conservation records on hippo complaints were analyzed, and no relationship between the occurrence of hippo damage and hippo density could be found. There were strong indications that hippo damage are caused by individual problem hippo , The majority of land-users (92 %) supported the conservation of hippos. The size of the area available to hippo was determined for distances of 5 kn. up- and downriver from the major hippo herds. hippo could move away from the river was 336 restriction when compared to the generally assumed hippo grazing range of 3.2 km. Hippo access to grazing was assessed by determining the effective grazing area.MT201

Variance propagation in toll smelting operations treating multiple concentrate stockpiles

Metallurgical accounting for toll treatment smelters pose a number of challenges. Multiple feed stockpiles have to be accounted for as part of the monthly recovery estimation. Moreover, multiple metallurgical intermediates make up part of the in-process inventory, which also influence the monthly recovery calculation. Errors propagate from the measurements of volumes, assays, moisture fractions and bulk densities in the various material types through multiple steps up to the point where the final recovery is calculated. These errors contribute non-linearly to the variance in the final recovery estimate. This paper develops the mathematical formulation for variance propagation in toll smelting operations, including the effect of in-process inventory, assuming measurement biases have been eliminated beforehand. Operational data from a Southern African copper smelter is used for a case study. The method of propagation of variance showed that uncertainties in stockpile assays were the main contributors to variance in the recovery estimate. Variance in the volume and bulk density uncertainties contributed a secondary, yet significant, proportion to the overall recovery variance. It was determined, for the given case study, that the recovery variance depended on the calculation method used and that variance propagation via the two-product formula was smaller than recovery variances calculated via the standard recovery formula. However, the probability that the two product formula will give inaccurate (versus imprecise) results is significantly more due to the practical difficulty of equiprobable sampling of tailings streams from smelter plants (which includes materials such as slags and flue dust). Recommendations are made on how to achieve a reduction in overall uncertainty for toll treatment smelters. © The South African Institute of Mining and Metallurgy, 2006.Revie

The Equilib-ARMAX approach to the dynamic modelling of the melt metallurgy in DC plasma arc smelting operations

A methodology is presented and evaluated to model the metallurgical dynamics of DC plasma arc smelting, using high carbon ferrochrome production as a case study. The model type is called an Equilib-ARMAX model as it integrates fundamental thermodynamic models with dynamic models. The Equilib-ARMAX model takes into account the inherently uncertain nature of smelter plant data, as well as the process dynamics and the moving chemical equilibrium target. The model structure, modelling performance, and model limitations of this model type are presented. The model could predict the metallurgical dynamics well within the inherent uncertainty of the process data. © 2006 Elsevier Ltd. All rights reserved.Articl

Computational fluid dynamic modelling of two phase flow in a hydrocyclone

Computational fluid dynamic (CFD) modelling is used to research the complex flow structures that exist in a hydrocyclone. By simulation of a two phase (water and air) flow system, the internal flow and multiphase interactions are investigated. The suitability of CFD modelling as a design tool is further evaluated by examining the effect of varying device dimensions. Three hydrocyclone geometries, used in previous studies, are specified. A transient simulation approach, which employs the Reynolds Stress Model as turbulence model and the Volume of Fluid model as multiphase model, is followed. Results are validated qualitatively against experimental measurements from the previous studies. © The Southern African Institute of Mining and Metallurgy, 2008.Articl

An integrated thermochemical-systems approach to the prediction of matte composition dynamics in an Ausmelt® nickel-copper matte converter

The Ausmelt® converter is being introduced into the South African platinum industry as an alternative to the traditional Pierce Smith converter process to convert a nickel-copper matte containing platinum group metals (PGMs). The semi-fundamental modelling technique used involved developing a complete thermochemical equilibrium database of the system, and then using this in conjunction with linear systems models to predict the dynamic iron and sulphur concentrations in the matte. The thermochemical equilibrium database was developed using the FACT thermochemical simulation software. The generated equilibrium database was used to train and validate feedforward neural networks. This technique allowed the database to be used online to predict the matte equilibrium composition. The equilibrium matte concentrations of iron and sulphur predicted by the neural network were then used as exogenous inputs into two linear systems models used to predict the actual iron and sulphur matte concentrations. It was found that the models characterised the dynamic behaviour of the sulphur and iron well. © 2002 Elsevier Science Ltd. All rights reserved.Conference Pape

Factors influencing base metal recovery from waste reverberatory furnace slags in a 50 kVA laboratory DC plasma arc furnace

The recovery of cobalt and copper from waste slag was studied in a plasma DC-arc furnace in reducing conditions. Three fluxes, CaO, CaF2, and TiO2, were investigated for their influence on selective recovery of cobalt into an iron-rich alloy. A further investigation on the influence of furnace power on the reduction of slag to recover cobalt to the alloy showed that a change in power setting had an unexpected effect on the recovery of metals to the alloy, in that selectivity of base metal recovery over iron recovery decreased at an increased power level.Articl

Dynamic structures in variance based data reconciliation adjustments for a chromite smelting furnace

The characterisation of furnace product materials is discussed in relation to their impact on the material balance reconciliation. It was found that the melts from open arc furnaces, as used in the chromite smelting industry, is not as homogenous as assumed. It was found that, for silicon in the ferrochrome melt, the spatial composition variance could be related to the degrees sub cooling relative to the alloy liquidus. The spatial variances in the furnaces were therefore incorporated in the variance-based reconciliation. The use of different data reconciliation techniques as a tool towards furnace control in the pyrometallurgical industry is discussed. Both the Lagrange multiplier, as well a direct method using the generalised reduced gradient method were evaluated. The adjustments to the measurements, which may be viewed as a combination of structural and random error in the measurements as well as systematic bias, were found to have dynamic structure. A comparison of time series models of the adjustments and their Fourier power spectra, has shown that any given adjustment is auto-correlated with previous historic values, even though the reconciliations were performed independently. Systematic biases were also apparent in the measured data, which were identified and subsequently confirmed. © 2002 Elsevier Science Ltd. All rights reserved.Conference Pape

Calculation of FeO-TiO2-Ti2O3; liquidus isotherms pertaining to high titania slags

During the production of pure TiO2 for the pigment industry, ilmenite, containing 35-60% TiO2, is reduced to high titania slag, containing 70-95% 'TiO2' and pig iron. The high titania slag produced, consists mainly of titanium in different oxidation states and FeO in equilibrium with liquid iron. To predict the phase and chemical reaction equilibria in a furnace, a suitable solution model is required with accurately determined binary interaction parameters, as slag melts tend to behave very non-ideally. This paper investigates two popular slag models, namely, modified quasi-Chemical (MQC) model and the cell model and provides the binary interaction parameters based on recently published data of the ternary system and critically assessed published unary and binary data. Very few modelling results have been published on the ternary Ti-Fe-O system pertaining to high titanium slags. The solution models, with their regressed interaction parameters, can subsequently be implemented in commercial software of the slag-metal phase equilibria to predict how much and which type of solids will precipitate for a given bulk slag chemistry and operating temperature or, alternatively, above which temperature should a furnace be operated to avoid excessive solids precipitation. © The South African Institute of Mining and Metallurgy, 2005.Revie

The role of slag modifiers on the selective recovery of cobalt and copper from waste smelter slag

Slag cleaning is important from both an economical and environmental perspective. Dump slags from copper smelting prove to be a valuable source of base metals, but the abundance of iron in the slag leads to significant amounts of iron reporting to the alloy phase during carbothermic reduction, which poses a significant cost to remove in subsequent hydrometallurgical processes. A carbothermic reduction of nonferrous smelter slag was conducted under the influence of three different slag modifiers to investigate the recovery of cobalt (and copper) versus iron recovery. The test work was performed in two campaigns; one campaign was performed under a constant oxygen potential of 4.33 × 10-9 atm, while the other set of test work was performed using graphite under a pure argon atmosphere. It is shown in both cases that, out of these slag modifiers used, TiO2 had a more selective recovery of cobalt compared to CaO and CaF2. It appears that TiO2 reduces the activity coefficient of the FeO in slag due to its strong affinity for FeO in the slag phase. © 2002 Elsevier Science Ltd. All rights reserved.Conference Pape