Application of 1-alkyl-3-methyl-imidazolium ionic liquids in the oxidative leaching of sulphidic copper, gold and silver ores

The application of 1-butyl-3-methyl-imidazolium hydrogen sulphate (bmim+(HSO4−)) ionic liquid and close analogues is described as a solvent medium (either as a neat liquid and or as aqueous mixtures) for the leaching of gold, silver, copper and base metals from sulphidic ores using thiourea (mainly) in the presence of iron(III) as oxidant. Initially pure pyrite and chalcopyrite leaching was studied. Copper extraction from chalcopyrite was much more efficient than iron extraction in the ionic liquid medium at 70 °C, increasing from 55 to 87% as the ionic liquid composition increased from 10% w/w in water to 100%. This compared with only 23% Cu extraction by 1 M H2SO4 under equivalent conditions. Gold recovery was > 85% from both a synthetic oxidic ore and a natural sulphidic ore in the presence of iron(III)/thiourea at 20–50 °C in the ionic liquid. Silver recovery from the natural sulphidic ore (> 60%) was significantly higher compared with that leached with an aqueous acid medium (< 10%). The effect of varying the alkyl chain length in the (n-alkyl) methyl-imidazolium cation and of varying the anion in bmim+X− liquids (X− = Cl−, CH3SO3−, N(CN)2−, HSO4−) on metal ion recovery from ore was also examined and showed that the bmim+(HSO4−) liquid was the most effective solvent medium. Oxygen solubility in several bmim+X− liquids is also reported, and is similar to that of oxygen in water. Eight other S-containing leaching agents, in addition to thiourea, were explored for metal ion extraction with iron(III) as oxidant in the bmim.(HSO4) ionic liquid system. Apart from thiourea, only 2-mercaptothiazoline showed promising behaviour.