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Ritchie S.,Hatch Ltd. | Eksteen J.J.,Lonmin Western Platinum Ltd | Eksteen J.J.,Stellenbosch University
Minerals Engineering | Year: 2011

The presence of chrome in electric arc furnaces smelting platinum group metals (PGM's) has a number of potentially negative consequences. In cases where the slag chrome content is above the saturation limit the existence of near-stagnant conditions near the slag/matte interface increases the risk of chromite spinels settling and consolidating into a "mushy layer", a three-phase suspension of slag, chromite and matte. The hold-up of matte above the elevation of the slag/matte interface can lead to the attack of freeze linings and copper cooling elements potentially causing failures of the furnace lining and significant downtimes as well as major safety risks. This paper investigates the relationship between typical furnace operating parameters and the behavior of the slag bath with respect to the formation of the "mushy" layer at the slag/matte interface using computational fluid dynamics (CFD). The extent of the potential "mushy" layer is seen to increase with decreasing electrode immersion and furnace power. Electrode immersion is, however, a considerably stronger driver is this regard. The CFD modelling results have aided in selecting appropriate furnace electrode immersion/power combinations intended to minimize "mushy" layer formation. © 2010 Elsevier Ltd. All rights reserved.

Eksteen J.J.,Lonmin Western Platinum Ltd | Eksteen J.J.,Stellenbosch University
Minerals Engineering | Year: 2011

High matte temperatures can be related to numerous catastrophic furnace failures in the platinum group metal (PGM) industry where chromite-rich upper group 2 (UG2) concentrates are smelted. Chromite rich concentrates require high slag temperatures as well as sufficient mixing to suspend the chromite spinel particles in the slag and prevent settling in a so-called "mushy" layer consisting of a three phase emulsion of slag, matte and chromite particles. To achieve sufficient bath mixing and to melt and suspend chromite spinel build-up, high hearth power densities are utilised. However, high hearth power densities in conjunction with a heat-isolating concentrate layer, leads to high side wall heat fluxes which motivated the use of intensive cooling in the furnace side wall so that a slag freeze lining can be formed. If matte temperatures are above the slag liquidus temperature, any matte that comes into contact with the freeze lining can destroy the freeze lining. Moreover, if the matte temperature exceeds ca. 1500 °C, chemical thermodynamics indicate that matte has the ability to sulfidise MgO-FexO-Cr2O 3 refractories, leading to rapid wear of refractories exposed to high temperature flowing matte. Models are derived for the concentrate-to-matte and slag-to-matte droplet heat transfer. Calculations using the derived models, physical properties and furnace operating conditions give realistic matte temperatures and show that matte temperatures rapidly increase as the concentrate bed becomes matte drainage rate limiting. It is shown that for each concentrate blend mean particle size and mineralogy, there is a maximum smelting rate above which the concentrate bed becomes rate limiting with regards matte drainage, thereby significantly contributing to matte preheating, prior to further heat absorption from the slag layer. © 2010 Elsevier Ltd. All rights reserved.

Chibwe D.K.,Stellenbosch University | Akdogan G.,Stellenbosch University | Bezuidenhout G.A.,Lonmin Western Platinum Ltd | Kapusta J.P.T.,3BBA | And 3 more authors.
Journal of the Southern African Institute of Mining and Metallurgy | Year: 2015

Peirce-Smith converters (PSCs) are extensively used in the copper, nickel, and platinum group metals industries. The typical converting operation involves lateral purging of air into molten matte through a bank of tuyeres. This blowing operation occurs at low pressure from the blowers, resulting in a bubbling regime that is considered inefficient from both a process and an energy utilization perspective. Inherent drawbacks also include recurrent tuyere blockage, tuyere punching, and low oxygen efficiency. Western Platinum embarked on a full-scale industrial evaluation of generating a jetting regime by using sonic injection. Prior to industrialscale tests, a numerical assessment to ascertain the feasibility of implementing sonic injection into a PSC was conducted. The work included flow characterization at high-pressure injection achieving sonic velocity at the tuyere exit. The 2D and 3D simulations of the three-phase system were carried out using the volume of fluid method together with the RKE turbulence model to account for the multiphase and turbulent nature of the flow. This paper discusses the key findings in understanding plume extension, velocity distribution, shear wall stress analysis, and phase distribution characteristics in the system. Plant trials are also discussed with reference to the commercial aspects of a full-scale implementation of sonic injection in the smelter. © The Southern African Institute of Mining and Metallurgy, 2015.

Malan W.,Stellenbosch University | Akdogan G.,Stellenbosch University | Bradshaw S.,Stellenbosch University | Bezuidenhout G.A.,Lonmin Western Platinum Ltd
Journal of the Southern African Institute of Mining and Metallurgy | Year: 2015

The purpose of the study was to investigate the feasibility of SiC reduction of low-grade concentrates from Lonmin's Rowland and Easterns operations with respect to metal fall and PGM recovery. These concentrates are rich in SiO2 and MgO with low concentrations of chalcopyrite and Cr2O3. Pd is the most abundant of the PGMs. SiC reduction of samples was conducted at 1600°C with 2.5-3.5 kg SiC per 100 kg concentrate. PGM recoveries for Easterns concentrate were better than for Rowland. More than 85% of the Ir and Pd and almost 60% of the Pt were recovered with 3.5 kg SiC per 100 kg concentrate. SEM of slag samples showed little entrainment of metallic prills compared to Rowland samples. This was attributed to the relatively higher melt viscosities of the Rowland concentrate. In order to decrease slag viscosity and to enhance PGM recovery, the FeO content of the Easterns concentrate was increased with the addition of 10 kg converter slag per 100 kg concentrate. Ir and Pd recoveries were increased to about 95%, while Pt recovery was around 70%. On the basis of these results an optimum feed ratio between Easterns and Rowland concentrates and converter slag is proposed. Carbothermic reduction of the optimum charge was also compared to SiC reduction. Carbothermic reduction yielded a marginally higher metal fall; however, the calculated gas emissions and energy requirements were higher than for SiC reduction. © The Southern African Institute of Mining and Metallurgy, 2015.

Chibwe D.K.,Stellenbosch University | Akdogan G.,Stellenbosch University | Eksteen J.,Lonmin Western Platinum Ltd
Metallurgical and Mining Industry | Year: 2011

Pyro-metallurgical processes are multiphase in nature involving gas-liquid-solid interactions. In the Peirce-Smith converter operation, the additions of cold solids in liquid matte in the form of fluxing agents (silica sands) for slag liquidity, process scrap and reverts for temperature control is a common practice. It is reasonable to postulate that with such practice, solid-liquid mass transfer step may play an important role in the performance and attainment of liquid bath homogeneity of the process. In this work, solid additions were simulated with sintered benzoic acid compacts spatially positioned in a 1:5 water model of a Peirce-Smith converter. Water and kerosene were used to simulate matte and slag respectively. Solid-liquid mass transfer was characterized by experimentally determined mass transfer coefficient, K (ms-1) values of benzoic acid sintered compacts and calculated dimensionless turbulence characteristic, Tc values. The mass transfer coefficients and dimensionless turbulence characteristic values were highest at the bath surface and near plume region. The values decreased in identified dead zones in the regions close to the circular side walls of the model. The results revealed that the mass transfer coefficients and turbulence characteristics were different with respect to different submergence levels of the compacts. These findings lead to the conclusion that the fluid flow was stratified within the vessel. © Metallurgical and Mining Industry, 2011.

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