Process for purifying molybdenum trioxide

A method for preparing ammonium molybdate having low impurities aluminum, calcium, copper, magnesium, and iron from an impure concentrate of molybdenum trioxide comprising contacting said concentrate with an aqueous solution comprising about 4.2 moles per liter of nitric acid, and about 1.75 moles per liter of ammonium nitrate to solubilize less than about 1.5 percent of the molybdenum values from said concentrate and solubilize a major portion of said impurities, wherein during contacting from about 0.85 parts to less than about one part by weight aqueous solution is present per part of impure concentrate, washing the resulting concentrate, digesting said resulting concentrate with an aqueous solution of ammonium hydroxide at suitable concentrations to solubilize substantially all of the molybdenum values present in said resulting concentrate as ammonium molybdate and form an ammonium molybdate solution, adding ammonium sulfide to said ammonium molybdate solution in an amount sufficient to convert substantially all of the soluble copper values present in said solution to an insoluble copper sulfide, separating said ammonium molybdate solution from an insoluble residue, and contacting said resulting ammonium molybdate solution with a chelate cation exchange resin in ammonium form, removing ammonia to adjust the pH of said ammonium molybdate solution of from about 6 to about 8 to convert substantially all of the soluble aluminum values to an insoluble aluminum hydroxide, separating said resulting ammonium molybdate solution from a resulting insoluble residue, and concentrating said resulting ammonium molybdate to form ammonium molybdate having low impurities, aluminum, calcium, copper, magensium, and iron.