most sophisticated industrial inorganic chemistry:
refining of platinum group metals ore concentrate
The figure below shows a typical assay of a South African platinum group metals ore concentrate obtained as the filter cake after sulfuric acid leach (according to the Sherrit process) of a Merensky reef converter matte.
In fact the Sherrit process results in two completely different behaving concentrates which are blended normally by concentrate producers for fast value recovery. The larger fraction (L) is resulting as a direct residue of multiple pressure leach, the minor one (M) from the recovery of rhodium dissolved during acid leach.
More attention should be paid in the future to completely avoid fraction M.
The total precious metal content of the concentrate is between 40 and 50%. The higher the precious metal content in the feed the less reactive is the concentrate (which is bad for pgm refining but saves some transport cost for those refiners who send their materials overseas).
The ore concentrate is dissolved in hydrochloric acid applying appropriate oxidants. Other acids than hydrochloric acid are not applicable in technical plants due to environmental control of the process.
Normally more than 98% of Pt Pd and Au, more 90% of Rh, Ir and Ru should be dissolved during dissolution; Ag is left undissolved as AgCl. But due to mistreatment of Sherrit fraction M the dissolved fraction may decrease to 20% Rh, 50% Pd, 80% Pt and less than 10% Ru resulting in a dramatic reduction of first pass yield and cash lock-up in a pgm refinery.
residual treatment and opm activation
The filter cake from dissolution mainly consists of AgCl, SiO2, Cr2O3 and opm oxides. After optional separation of SiO2 (in case of high quantities) silver is separated and passed to the silver refining line. Concerning value silver is of neglectable importance in a pgm refining factory; of course it is recommended not to invest in a Ag refining line just because of these small quantities in pgm concentrates.
The remaining opm rich material is fed to a special melting process in which opm's are activated again for a second acid leach.
Ru Os distillation
The presence of ruthenium interferes the quality of palladium, platinum, iridium solvent extraction and rhodium precipitation. The same is true for classical old fashioned more pollutive process routes based on precipitation of Pd, Pt, and Ir as their low soluble hexachloro complex compounds. Therefore in pgm refinery circuits Ru is separated as early as possible below 50 ppm.
Hence the filtrate after dissolution is passed to the separation of ruthenium and osmium by distillation and reactive absorbtion; most refiners don't keep trace of the later. The feed may be processed in the same equipment as the leach step (under approppriate conditions). Special attentencion has to be paid for design and construction materials of this process because the volatile Ru compound might cause explosions and the volatile Os compound is very poisonous.
The distillate/absorbate itself is passed to the ruthenium refining line comprising again distillative equipment for the separation of Os from the bulk of ruthenium under appropriate conditions.
As Se Te Fe separation
The pregnant solution after Ru Os separation is further treated to remove As Se Te Fe by precipitation. This process is necessary only for high amounts of these elements, not necessarily for the ore concentrate mentioned above. This combined removal of impurities cares for
- elemination of harmful components (As) in the final waste water volume
- deminishing of components which may prolongate rhodium recovery and purification in the rhodium refining line (Se Te)
- separation of elements blocking or reducing the capacity of platinum solvent extraction (Fe)
The disadvantage of this process is that some Pt and Pd is precipitated too if this process is not controlled carefully. Therefore eventually the separated solid has to be treated to recover a small percentage of platinum and palladium locked in this cycle.
In total this process step means a considerable upgrade in purity of the pregnant liquor with positive influence on first pass yield of Pd, Pt, Rh and recovery time of rhodium
From the filtrate of (hazardous) metal waste removal gold is separated by reductive precipitation. The quality of the primarily separated material is >99.5% (good delivery). One more dissolution (aqua regia etc.) and reduction will yield gold >99.95% up to >99.99% (fine gold) especially if Se and Te have been removed thoroughly in the step before.
It is possible to remove the gold from the main stream by solvent extraction too (HEV CHEMICALS is supplying extractants for this process) but it is not recommended in the pgm ore concentrate refining cycle because the amount of gold is quite low; hence this process would be rather cost-ineffective.
palladium solvent extraction
From the filtrate of gold precipitation after adjusting appropriate feed conditions palladium is separated by solvent extraction applying DnHS or DnOS as the extractant (HEV CHEMICALS is supplying the extractants for this process). Old-fashioned ineffective refineries still use the traditional much more pollutive precipitation process entraining for instance large amounts of ammonia or other pollutants into the estuary.
It depends upon the scale of operation whether the solvent extraction plant is designed as a continuous or batch process. In either case the complete solvent extraction process comprises
- extraction; palladium is transferred into the organic phase
- scrubbing; reduction/removal of coextracted impurities from the extract
- stripping; transfer of Pd back into a second pure aqueous phase for palladium recovery and further processing
- solvent make-up; cleaning the organic before extraction again
The purity of palladium in the strip solution is about 99%, main impurities are silver and platinum. The process is very effective in the rejection of non-precious metals.
platinum, iridium solvent extraction
Similar like palladium, platinum and iridium are separated from the raffinate of the palladium extraction. Of course the extractants, conditions and equipments differ. For low concentrations of iridium it is not recommended to extract the metal form the main stream. In this case it is more efficient to concentrate the main stream before iridium extraction or separate iridium from platinum within the platinum extraction cycle or further downstream in the platinum refining line.
The purity of the platinum leaving the solvent extraction cycles is about 99.5%. Main impurities are iridium, Se and As.
Rhodium is separated from the raffinate of the platinum or iridium extraction by precipitation applying a highly selective organic under appropriate conditions.
The purity of the dissolved rhodium precipitate is >99%, main impurities are iridium and bismuth.
platinum group metals trace recovery
Finally traces of platinum group metals being present in the barren liquor from time to time on a level of 5 - 10 ppm are recovered by electrolytic cementation and sulfide precipitation if necessary.