Selective removal of arsenic(V) from a molybdate plant liquor by precipitation of magnesium arsenate

Precipitation of Mg3(AsO4)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 and 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% MoO3 could be produced by acidification.

Research highlights
► The removal of As(V) from the Mo(VI) stripped liquor produced from a solvent extraction circuit is essential for the production of high purity MoO3.
► The removal is affected by adding Mg(II) chloride or sulphate into the liquor at a Mg(II):As(V) molar ratio of at least 2:1 causing the precipitation of crystalline Mg3(AsO4)2.
► The precipitation could remove As(V) at ambient conditions as long as the solution pH is maintained between pH 8 and 10. At pH 12, all Mg(II) will precipitate as magnesium hydroxide, making the removal process ineffective.
► In plant practice, the ammonia stripped liquor of pH 10.2 does not need further pH adjustment. At optimum conditions, almost all As(V) could be removed to less than 5 mg/L, to form a well crystalline product of Mg3(AsO4)2 which can be easily separated by filtration. This results in the production of arsenic free liquors which on acidification could yield a high purity commercial product of 99.9% MoO3.