Doctoral Degrees (Chemical Engineering)
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Browsing Doctoral Degrees (Chemical Engineering) by Author "Bezuidenhout, Gert Adrian"
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- ItemPyrometallurgical refining of high grade PGM leach residues prior to precious metals separation(Stellenbosch : Stellenbosch University, 2014-12) Bezuidenhout, Gert Adrian; Bradshaw, S. M.; Akdogan, G.; Eksteen, Jacques J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Primary platinum mining companies use a complex multistage recovery and refining process. The removal of base metals in the base metal refinery (BMR) leaves a residue that requires a number of hazardous and expensive unit operations to prepare the platinum group metal (PGM) concentrate for final separation and refining. In the PGM industry, hydrometallurgy is used almost exclusively to refine material with a PGM content of more than a couple of per cent. This thesis proposes and investigates an alternative pyrometallurgical refining method to remove contaminants from a high grade PGM residue (e.g. residue from a BMR after the bulk of the base metals has been removed), whilst minimising aqueous wastes and potential PGM losses. The hypothesis behind the use of a pyrometallurgical process was the effective separation of a number of elements from the PGMs through the use of only a few processing steps. The noble nature of PGMs, i.e. their resistance to oxidation and low vapour pressure at high temperature, allows for effective separation. The theoretical feasibility of the concept was explored using thermochemical modelling. Modelling suggested that the use of a controlled atmosphere roasting step, followed by smelting and atomisation, could lead to a technically feasible process to separate PGMs from other metals. Throughout this thesis, thermochemical modelling was shown to be a useful tool to interpret and understand elemental behaviour across the roasting and smelting steps. It was experimentally illustrated that roasting of PGM residues in an oxidising environment at low temperatures (700 °C - 900 °C range) will selectively vaporise the volatile oxides of S, Se, Te and As (to varying degrees), that would otherwise be quite stable in the alloy or matte phase during a melt at low oxygen partial pressure. Arsenic volatilisation proceeded only partially (30 wt% to 60 wt%), probably due to the formation of a temperature stable species of arsenate. Osmium volatilisation was not properly described by modelling and proceeded only partially (30 wt% to 70 wt%), possibly due to the presence of Osmium in a solid solution phase that depresses the activity and correspondingly the fugacity. Although a number of the PGMs oxidise in the roasting temperature range, their vapour pressures do not allow measurable losses to the gas phase (apart from Osmium that is not recovered in most precious metal refineries). Almost all the PGMs reported to two separate solid solution phases during roasting. The smelting step allows the PGMs to dissociate from oxygen, alloy and melt, without the addition of a collector. The liquidus temperature was shown to be strongly influenced by the concentrations of impurities (S, Se, Te, As, Fe, Ni and Cu) in the feed to the melt and ranged between 1 235 °C and 1 510 °C. The most important variable necessary to avoid non-PGM elements (especially As, Pb, Fe and Ni) from joining the alloy phase is control of the partial oxygen pressure in the melt. Au losses due to volatilisation of species such as AuS, AuSe and AuTe were shown possible at higher temperatures, with 21% recovery loss measured at 1 700 °C in the presence of 6% Se, Te and S. Ru losses (ranging from 9% to 39%) across the smelting step were shown to be sensitive to the combined Fe, Ni and Cu content of the feed, the melting temperature and the partial oxygen pressure across the melt. With Cu addition (to achieve Cu content in the alloy phase >60% by weight), melting temperatures as low as 1 200 °C is sufficient to collect PGMs and separate them from the slag phase. However, Ru is insoluble in the Cu-rich alloy and varying Ru recoveries were measured, pointing to a possible loss mechanism. After casting and solidification of the melt, the alloy phase can be separated from the slag phase by mechanical means. Atomisation is necessary to break the alloy phase into fine particulate that allows improved leaching kinetics. From experimental test work and literature studies, it appears that fine particulate sizing (D50 of 20 μm) can be achieved at high pressure requirements (in excess of 400 bar). The necessity of re-melting the alloy phase before atomisation allows the opportunity for a high temperature treatment step. It was experimentally shown that near complete volatilisation of Pb and Bi can be achieved if the alloy is kept at 1 700 °C for 30 minutes in an inert environment. Significant Ag losses to the vapour phase were also measured across this high temperature treatment step. This study fundamentally and practically illustrated that it is possible to use pyrometallurgy to separate PGMs from a large number of metals/oxides/amphoterics present in leach residues. By effectively using a combination of roasting and smelting, it is possible to upgrade the PGM content of a pressure leach residue from low forty per cent to high eighty per cent, with low PGM losses. The proposed roasting, smelting and atomisation processes were able to remove the bulk of S, Se, Te, Bi, Pb, oxides (such as SiO2 and Cr2O3) and partially remove Os, As, Fe, Ni and Cu. It was experimentally shown that the PGM containing alloy is amenable to leaching in a chlorine environment as used in a precious metal refinery, but more work is necessary to achieve near complete dissolution. This investigation spanned a number of processing steps and measured the associated impact of each on multiple elements, and in that regard it does not follow a classical or typical doctoral thesis approach. However, a field of research has been studied that enjoys very little publication, apart from specialist alloys or specialist characteristics. This is in part due to the proprietary nature of PGM refining. This study contributes to the existing scientific knowledge base, but also to the process engineering knowledge base of the behaviour of high grade PGM residues at high temperatures.