The leaching of Mo, Cu and Fe from MoO 3 calcines (59-61% Mo) containing 1-2% Cu, 1-2% Fe and other impurities produced from roasting molybdenite (MoS 2 ) concentrate was examined using water and a series of chloride lixiviants, including NH 4 Cl (0. 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 MoO 3 calcine containing 2.1% Cu using 1.44M NH 4 Cl and 0.97M 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%. Research Highlight We determine the process chemistry of leaching Cu from MoO3 calcine to reduce its content to <0.5% A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2  The optimum leaching conditions to treat calcines containing 1% Cu (0.6M NH4Cl, 0.7M HCl at ambient temperature) were adopted to remove impurities to <0.5% Cu, <0.6% Fe and <0.1% for other impurities  The dissolution of Mo by acid was minimized by the precipitation of ammonimum molybdate  Stabcal was used to model the speciation of the dissolution process.  Pilot plant trials were conducted to treat 14 tonnes of calcines to confirm the results before plant operations were implemented.

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