The recovery of platinum group metals from low grade concentrates to an iron alloy using silicon carbide as reductant

Malan, Willem du Toit (2014-12)

Thesis (MEng) -- Stellenbosch University, 2014.


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.

AFRIKAANSE OPSOMMING: In hierdie studie, word SiC reduksie met Rowland en Oostelikes LG (Lae Graad) konsentrate ondersoek. Die doel van die studie was om die doeltreffendheid van SiC as reduktant te ondersoek met betrekking tot metaalval, PGM graad in die allooi, slaksamestelling (spesifiek word daar gekyk na Cr oplosbaarheid) en algehele PGM herwinning. Die integrasie van die proses in die huidige mat-gebaseerde versamelingproses word ook ondersoek. Tans word die mat-gebaseerde versamelingproses die algemeenste gebruik om PGM'e te kollekteer, maar omdat PGM konsentrate al hoe meer verryk word met UG2 (Upper Group 2) LG konsentrate, word daar verwag dat dit geïntegreer of vervang gaan word met 'n allooi-versamelingproses. Hierdie tipe proses bied groter buigsaamheid om die verskillende reekse van erts wat gebruik kan word. Die proses kan ook chroom hanteer en is omgewingsvriendelik. Vir hierdie doel was Rowland en Oostelikes UG2 LG konsentraatmonsters van Lonmin Western Platinum Limited ontleed met XRD, XRF en ICP -MS en met die ontleding was daar gevind dat SiO2 en MgO die volopste oksides was en dat Pd die volopste elemente van die PGMe was. Sulfiedminerale soos chalkopiriet is in lae konsentrasies opgespoor (minder as 1%) en Cr2O3 konsentrasies is tussen 2-4 %. Die FeO/SiO2 verhouding was laer in Rowland konsentraat. SiC reduksie van Rowland en Oostelikes konsentrate is teen 1600 ℃ uitgevoer. Die reduktant tot konsentraat verhouding vir laboratoriumskaal eksperimente het gewissel van 2.5 – 3.5 kg SiC / 100 kg konsentraat. SiC reduksie van Rowland LG konsentraat het verskillende reduksie tye gehad. Die duur van die reduksie eksperimente het gewissel van 30-180 min. PGM herwinning van SiC reduksie met Rowland konsentreer was baie laag (onder 10 %) en Fe herwinning was minder as 50%. 'n Slakviskositeit aan die einde van die smelt was hoër as 4 poise en was verantwoordelik vir die swak skeiding van fases. SEM beelde het gewys dat fyn metaalstukkies opgehou was in die slakfase in plaas daarvan dat dit vestig en kombineer met die allooibolletjie aan die onderkant van die smeltkroes. In teenstelling was die PGM herwinning van SiC reduksie met Oostelikes konsentraat aansienlik beter. Meer as 85 % van Ir en Pd was herwin en byna 60% van Pt was herwin tydens 'n toets met 'n reduktant tot konsentraat verhouding van 3.5 kg SiC / 100 kg Oostelikes konsentraat. Fe herwinning was 66% en was ook die hoogste van al die eksperimente. Cr en Si konsentrasies was minder as 5 % in totaal. Die slakviskositeit aan die einde van smelt was bereken en is minder as 4 posie. 'n SEM beeld van 'n slakmonster het baie min vasgevangde metaalstukkies getoon. Na afloop van die bogenoemde bevindinge oor die belangrikheid van viskositeit, was daar besluit om die FeO inhoud van die aanvanklike konsentraat te verhoog. Dit was gedoen om die slakviskositeit te verminder, die metaalval (PGM kollektering fase) te verhoog en sodoende die PGM herwinning verder te verbeter. Vir die doel was Peirce -Smith omskakelaarslak gebruik. ʼn Toets was uitgevoer met die toevoeging van 10 kg omskakelaarslak / 100 kg Oostelikes konsentraat. Die reduktant tot konsentraat verhouding was behou by 3.5 kg SiC / 100 kg Oostelikes konsentraat. Die resultate het getoon dat meer as 95 % van Ir en Pd herwin was, terwyl byna 70 % Pt herwin was. Die Fe herwinning het toegeneem tot 76%. Op grond van die resultate van die toets, is 'n optimale verhouding tussen Oostelikes konsentraat, Rowland konsentraat en Peirce -Smith omskakelaarslak bereken. Termodinamiese modellering voorspel dat die begin LG konsentraat voer uit 60 – 80 % Oostelikes konsentraat moet bestaan, met 'n slak toevoeging van 30 – 40 kg omskakelaarslak / 100 kg LG konsentraat. Daar word verwag dat meer as 90 % van PGM'e herwin sal word vanaf SiC reduksie met ʼn optimum LG konsentraat voer. Cr en Si konsentrasies in die allooi sal minder as 1% in totaal wees. Die hoeveelheid slak wat bygevoeg kan word sal egter beperk word deur die finale PGM graad in die allooi, oond-slak hoeveelhede wat herwin kan word en slakweerstand wat benodig word in die allooi-oond. Die effektiwiteit van SiC as reduktant is ook vergelyk met C reduksie. C reduksie van ʼn optimale konsentraatvoer het 'n effens hoër metaalval wanneer dieselde reduktant tot konsentraat verhouding behou was vir SiC reduksie van 'n optimale konsentraatvoer. Gas hoeveelhede van C reduksie is gemiddeld 3 keer meer en vereis te minste 300 MJ meer om 1 ton LG konsentraat te smelt. Dit is hoofsaaklik te wydte aan C wat endotermies reageer met FeO om Fe (l) en CO (g) te produseer, in teenstelling met SiC wat eksotermies reageer met FeO om Fe (l), SiO2(l) en CO (g) te produseer. Integrasie van SiC reduksie met LG konsentrate in die bestaande smeltroete by Lonmin Western Platinum Limited is ook voorgestel deur 'n proses vloeidiagram. Uit 'n ekonomiese oogpunt, is daar gevind dat SiC reduksie van 1 ton LG konsentraatvoer met 'n omskakelaarslak byvoeging ongeveer 700 MJ meer benodig as om 1 ton UG2 gemengde konsentraat te smelt en 'n mat-fase te produseer. Dit moet wel in ag geneem word dat die sulfied ryk lae in die Bosveld-kompleks vinnig uitgeput word en dat hierdie alternatiewe prosesse soos SiC reduksie en ʼn allooiversameling proses vinniger as verwagtend benut sal word. Verder, gas hoeveelhede van SiC reduksie is laer en daarom is dit 'n meer omgewingsvriendelik proses. Ten slotte, vanaf die bevindinge van hierdie studie, kan dit gesê word dat basismetale en PGM'e in 'n ysterallooi herwin kan word deur middel van SiC reduksie met LG konsentrate en die toevoeging van omskakelaarslak. Daarom kan die integrasie van so 'n proses in die huidige mat-gebaseerde versameling proses beskou word as 'n alternatief vir die toekoms om UG2 LG konsentrate te smelt.

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