Precipitation of Mg 3 (AsO 4 ) 2 for the removal of arsenic (As) from a molybdenum oxide processing plant liquor containing 70.9 g/L Mo(VI) and 469 mg/L As(V) was performed.. The Stabcal software was used to model the speciation and solubility equilibria, and to identify the pH conditions at which optimum precipitation can be carried out. The optimum pH range for As(V) removal is between pH 7.5 to 10.2. This avoids the need to adjust the liquor pH to affect the precipitation. By adding magnesium chloride or sulphate at a Mg:As molar ratio of at least 2:1, As(V) could be removed to less than 5 mg/L at pH 10.2 resulting in a pure Mo(VI) liquor from which a high purity product of 99.9% MoO 3 could be produced by acidification.

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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