Purification of molybdenum trioxide calcine by selective leaching of copper with HCl–NH4Cl

The leaching of Mo, Cu and Fe from MoO3 calcines (59–61% Mo) containing 1–2% Cu, 1–2% Fe and other impurities produced from roasting molybdenite (MoS2) concentrate was examined using water and a series of chloride lixiviants, including NH4Cl (0.6 M), HCl (0.115–1.40 M) and a mixture of HCl (0.70 M) + NH4Cl (0.1–0.93 M) at a solid/leachant (w/v) ratio of 1:1 and within the temperature range 25–70 °C. Stabcal modelling showed that there are many compounds formed from Cu, Fe and Mo co-exist in the solution. Optimisation studies show that HCl solutions of concentrations ≥ 0.35 M are capable of dissolving > 80% of Cu within 1 h. The addition of NH4Cl (0.30–0.93 M) to 0.7 MHCl reduces the dissolution of Mo to < 0.5%, due to the formation of a precipitate containing mainly NH3(MoO3)3, as confirmed by X-ray diffraction. A mixture of 0.7 M HCl and 0.6 M NH4Cl at ambient temperatures removes Cu to < 0.5% within 2 h, as required for the production of Fe–Mo alloys at Kwangyang Ferro Alloys Co. (Korea). Although not required, Fe was removed to < 0.6% whereas most other metal impurities were removed to < 0.1%. Pilot plant trials over 7 campaigns, each treating 2.0–2.2 tonnes of MoO3 calcine containing 2.1% Cu using 1.44 M NH4Cl and 0.97 M HCl at an ambient temperature yielded final products containing 0.22% Cu as required. Mass balance calculations (closure to 96–99%) of the pilot plant trials confirm that the Mo loss during leaching is around 0.5%.