Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by Subject "Alloys"

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    The recovery of platinum group metals from low grade concentrates to an iron alloy using silicon carbide as reductant
    (Stellenbosch : Stellenbosch University, 2014-12) Malan, Willem du Toit; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: In this study, SiC reduction of Rowland and Easterns LG (Low Grade) concentrates was investigated. The purpose of the study was to investigate the feasibility of SiC as reductant with respect to metal fall, PGM grade in the alloy, slag composition, Cr solubility and overall PGM recovery. The integration of such process in the current matte-based collection process was also investigated. Currently, the matted-based collection process is most widely used for PGM recovery, but because PGM containing concentrates are becoming more enriched with UG2 (Upper Group 2) LG concentrates, it is expected to be integrated or replaced with an alloy collection process. This kind of process offers greater flexibility to the different types of ore that could be used. The process is chromium tolerant and environmentally friendly. For this purpose Rowland and Easterns UG2 LG Concentrate samples from Lonmin Western Platinum Limited were analysed with XRD, XRF and ICP-MS and it was found that SiO2 and MgO are the most abundant oxides and Pd is the most abundant element from the PGMs. Sulphide bearing minerals such as chalcopyrite were detected in low concentrations (below 1 %) and Cr2O3 concentrations are between 2 – 4 %. The FeO/SiO2 ratio was lower in Rowland LG concentrate. SiC reduction of Rowland and Easterns concentrate was done at 1600℃. Reductant to concentrate ratios for laboratory scale experiments were ranged from 2.5 to 3.5 kg SiC / 100 kg concentrate. SiC reduction of Rowland concentrate had different reduction times. The duration of reduction experiments ranged from 30 - 180 min. PGM recoveries from SiC reduction of Rowland concentrate were very poor (below 10 %) and Fe recoveries were lower than 50 %. A slag viscosity at the end of the melt of more than 4 poise was responsible for poor phase separation. SEM images revealed metal prills entrained in the slag phase instead of settling and combining to the alloy globule at the bottom of the crucible. However, PGM recoveries from SiC reduction of Easterns concentrate was significantly better. More than 85 % of Ir and Pd and almost 60 % of Pt were recovered in a test with a reductant to concentrate ratio of 3.5 kg SiC / 100 kg Easterns concentrate. Fe recovery was also the highest at 66%. Cr and Si concentrations were below 5 % in total. The slag viscosity at the end of melt was calculated to be less than 4 poise and a SEM image of a slag sample revealed few entrained metal prills. After the above findings on the importance of viscosity, it was decided to increase the FeO content in the initial concentrate charge in order to decrease slag viscosity, increase metal fall (PGM collecting phase) and further increase PGM recovery. Peirce-Smith converter slag was used for this purpose. A test was conducted with the addition of 10 kg converter slag / 100 kg Easterns concentrate. The reductant to concentrate ratio was kept at 3.5 kg SiC / 100 kg Easterns concentrate. The results revealed that Ir and Pd recoveries were more than 95%, while Pt recovery was almost 70%. Fe recovery increased to 76 %. On the basis of the results from the test, an optimum feed ratio between Easterns LG concentrate, Rowland concentrate and Peirce-Smith converter slag was calculated. Thermodynamic phase equilibrium calculations predicted that the concentrate charge should consist of 60 - 80% Easterns concentrate with a slag addition of 30 – 40 kg converter slag / 100 kg LG concentrate. SiC reduction of this optimum LG concentrate charge is expected to recover more than 90% of all PGMs. Cr and Si concentrations in the alloy will be below 1 % in total. The amount of converter slag as an addition will be however limited by final PGM grade in the alloy, furnace slag quantities recycled and slag resistivity required in the alloy furnace. The effectiveness of SiC as reductant was also compared to C reduction. C reduction of an optimum concentrate charge had a marginally higher metal fall at the same reductant to concentrate ratio than SiC reduction of an optimum concentrate charge. However, gas emissions are on average 3 times higher for C reduction of a concentrate charge and C reduction requires at least 300 MJ more to smelt 1 ton of LG concentrate than SiC reduction. This is mostly due to C reacting endothermically with FeO to produce Fe(l) and CO(g) in contrast to SiC reacting exothermically with FeO to produce Fe(l), SiO2(l) and CO(g). Integrating SiC reduction of LG concentrates into the existing smelting route at Lonmin was also proposed through a process flow diagram. From an economic point of view, it was found that SiC reduction of 1 ton of LG concentrate charge with a converter slag addition requires almost 700 MJ more than the smelting of a UG2 blended concentrate to produce a matte phase. However it must be taken into account that the sulphide rich layers in the Bushveld complex are being depleted rapidly and alternative processes such as SiC reduction and alloy collection process will be utilized faster than expected. Moreover, gas emissions from reductive smelting is considerably lower, hence it is a more environmentally friendly process. Finally, from the findings of this study, it could be said that base metals and PGMs could be recovered in an iron alloy from SiC reduction of LG concentrate with converter slag additions. Therefore integrating such a process into the matte-based collection process could be considered as a future alternative to smelting UG2 LG concentrates.