Mineral Processing and Extractive Metallurgy: Transactions of the Institutions of Mining and Metallurgy

The smelting of nickel laterite ores to ferronickel alloy is unique in extractive metallurgy. It treats feed that is very low grade with respect to the target metal and, as a result, produces much more waste slag than valuable metal product. The energy consumption per tonne of product is therefore high and requires sustained research and design development in an effort to improve the economics of laterite smelting. In this work, the main characteristics of nickel laterite smelting are reviewed, and then a simple and transparent computational thermodynamics model of the electric furnace smelting step is developed. This model predicts the nickel grade, nickel recovery and FeO content of the slag as functions of the iron recovery to ferronickel satisfactorily. It correctly predicts that the carbon and silicon contents in ferronickel increase sharply at high iron recoveries. However, in common with more sophisticated models, it incorrectly predicts the iron recovery at which this increase is observed in practice. It is concluded that the model provides an accessible and a satisfactorily accurate vehicle for understanding the relationships between process variables and process outcomes during nickel laterite smelting.

Previous work showed that manganese ore reduction rates are strongly influenced by the extent of slag phase formation. In this work, the effect of ore composition on slag formation during manganese oxide reduction was predicted using thermochemical calculations; FactSage 6·4 was used to calculate the equilibrium phase relations in the oxide system MnO–SiO₂–CaO–MgO–Al₂O₃. Practically observed differences in ore composition, even within the same orebody, are predicted to cause significant differences in slag formation during reduction, with large differences in ore reducibility expected.

In this study, a mathematical model was developed to describe the progress of the successive gas–solid reactions occurring in a porous pellet and the model was applied to hydrogen reduction of molybdenum disulphide in the presence of lime. To carry out a realistic modelling for the multi-step reactions, effect of the structural changes and non-isothermal conditions during the reaction within the pellet has been taken into account. The effect of structural change was implemented in the modelling through variation of the solid reactant grain sizes and effective diffusion. The results predicted by the model were evaluated and validated by comparing with the experimental data. Structural changes and non-isothermal conditions had no significant effect on the modelling results. Therefore, deviation of the model prediction from the experimental data can be ascribed to the side reaction occurring within the pellet. To compensate the effects of the side reaction, adapted parameters were obtained through the experimental data.

The effect of adding black copper, originating from treating waste electrical and electronic equipment (WEEE), to a Peirce-Smith converter has been investigated by using a thermodynamic process model. The model was formulated, by the authors, in an earlier publication and expanded, in the present work, to include the minor elements antimony and bismuth. The results show that the model describes the distribution of Bi well, whereas the distribution of Sb is not described as well and should only be used for trends. Addition of black copper lowers the removal of Bi and Sb compared to a converter cycle without addition. To maintain a good removal of Bi and Sb, black copper should be added as early as possible during a converter cycle.

This study provides an assessment of the high temperature thermochemical reactions of ilmenite and chromite with sulphur and uses the information to analyse the possibility of selective sulphidation of chrome-bearing spinels as a new route for chromite removal from ilmenite concentrates. The work includes both systematic thermodynamic assessment and targeted experimental investigations. Thermodynamic calculation results studying the effect of reactants composition, temperature and different sulphur sources (H₂S and elemental S) showed that chromite can be selectively sulphidised at a controlled atmosphere of partial pressure of oxygen (pO₂) below 10⁻¹⁰ atm and partial pressure of sulphur (pS₂) above 10⁻⁶ atm. The addition of carbon with sulphur was found to be useful for the chromite sulphidation reaction. The optimum quantity of sulphur reactant to carbon was found to be 3: 1 (in mole) for 1 mol of chromite. Sulphidation experiments on a mixture of natural ilmenite and chromite at 1100°C for 5 h using 5%H₂S as a sulphur source showed that the ilmenite was preferentially sulphidised first, which was also in a good agreement with the thermodynamic assessment. Sulphidation of a naturally occurring chrome spinel contaminated ilmenite concentrate showed that the degree of weathering also played a role in the sulphidation of the chrome spinel. It was also concluded that H₂S is not suitable for selective sulphidation of chrome spinel from the ilmenite concentrate and that tightly controlled pS₂ and pO₂ conditions are required.

The Archimedean single bob technique was employed to secure density data in the range of 1350–1710°C for selected molten slag compositions in the CaO–SiO₂, CaO–SiO₂–MgO, CaO–SiO₂–Al₂O₃, and CaO–SiO₂–MgO–CaF₂ systems. In each system, the measurements typically revealed minor to negligible density variations across the temperature range. In cases where density variations were apparent, density was observed to slightly decrease as temperature was lowered. The density variations between the different slag systems are significant, with lower densities generally associated with the more acidic (higher per cent silica) compositions.

The beneficiation of graphite is very costly and energy intensive and can necessitate multiple processing steps, often including flotation. Products have to satisfy very stringent quality criteria. To decrease beneficiation costs, a careful characterisation of feed and concentrate materials is needed. This study elucidates the additional benefit of methods of automated SEM-based image analysis, such as mineral liberation analysis (MLA), in addition to ‘traditional’ methods [optical microscopy and X-ray powder diffraction (XRD)] for the analyses of graphite raw materials and processing products. Owing to the physical and chemical properties of the mineral graphite, samples require delicate sample preparation as well as particular backscattered electron (BSE) imaging calibration for automated image analysis. These are illustrated in this study. The results illustrate that SEM-based image analysis of graphite feeds and concentrates can provide accurate and reliable information for the graphite beneficiation process. This applies to both mineralogical characteristics and process relevant parameters.