A powder diffraction study of problems in platinum group metal extraction.
The base metal and platinum group metal extractive processes for two South African refineries were examined using x-ray powder diffraction. Base metal refinery matte contained nickel sulphide (Ni3S2), copper sulphide (Cu2S), nickel-copper alloy (Ni-Cu), iron oxide (Fe3O4) - nickel iron oxide (NiFe2O4) and nickel oxide (NiO). Nickel iron oxide and cobalt sulphide (CO3S4) were found in matte aeration cavities. Matte fast cooling caused alloy zoning. Alloy stoichiometry and weight percentages of matte constituents were calculated. Magnetic separation of the alloy phase was unfeasible due to high bulk and poor separation from Ni3S2. Sulphuric acid matte leaching produced residues containing copper sulphide (Cul.8S) and nickel sulphides (Ni3S4, NiS). Potassium cyanide residue leaching produced NiS-rich residues, while thiourea / hydrochloric acid leaching produced residues almost free of copper sulphide. PGM concentrates were examined. Base metals occurred as: copper sulphate pentahydrate; copper sulphate hydroxide hydrate (CU4SO4(OH)6'H2O); copper sulphide (CuS); copper platinum (Cu3Pt); nickel oxide (NiO); nickel iron oxide (NiFe2O4). Lead occurred as lead sulphate and silicon as quartz and enstatite ((Mg,Fe,Al)SiO3). Heating the concentrate to 260°C with sulphuric acid converted copper sulphide to copper sulphate. PGMs were mostly metallic. Overdrying concentrates caused pgm sintering. Drying with sulphuric acid reduced sintering. PGM oxidative leach solubility was examined. Platinum sulphide (Pt,Pd)S caused low platinum and palladium solubility. (Ru,Rh,Ir,Pt)AsS caused low rhodium and ruthenium solubility. Platinum-rhodium (Rh0.57Pt0.43), ruthenium-osmium (~Ru16Os), iridosmine (Os,Ir,Ru) also caused low pgm solubility. Silver chloride remained in residues. Concentrate overdrying produced insoluble sintered platinum, palladium monoxide, and ruthenium dioxide. In minor concentrates sintered platinum, rhodium selenide (RhSe2+x), ruthenium dioxide and possibly palladium monoxide lowered pgm solubility. Precipitates and salts were examined. Iron precipitated as iron oxide hydroxide (B-FeOOH); gold as metallic gold; lead as lead chloride. Common salts were: sodium chloride; sodium iron hydroxide sulphate hydrate (Na2Fe(SO4)2(OH)'3H2O); ammonium chloride; sodium carbonate monohydrate. PGM-Iead fusion was examined. Fusion produced the insoluble alloy Pb(Pd,Pt)3. Ruthenium dioxide reacted with lead carbonate forming lead ruthenium oxide (Pb2Ru2O6.5). Nitric acid insoluble residues also contained lead sulphate and iron oxide (Fe3O4). This study demonstrated that solution problems can be understood by identifying the crystalline insoluble phases in intractable residues.