Since the solubility of ammonium paratungstate increases rapidly at the high pH necessary to avoid molybdenum contamination, when very small molybdenum levels are desired, it may be desirable to leave some tungsten in solution, filter out the initial ammonium paratungstate precipitate, and recycle the filtrate. For example, if the precipitation is performed at a pH slightly above 8.5, 85 percent to percent of the tungsten will be precipitated as pure ammonium paratungstate. Further precipitation of tungsten may be obtained by boiling the filtrate in an open vessel. During this evaporation step ammonia and water are removed and the pH of the filtrate drops progressively to approximately 7.0. The molybdenum'content of the ammonium paratungstatethat is precipitated between the'pl-l values of 8.5 and 7.0'will rise from approximately 0.02 percentv t percent. This secondary precipitate will contain as high as 13 percent of the initial molybdenum. This material may then be recycled to a new batch of concentrated digester solution where it will raise the tungsten concentration. The filtrate resulting from this secondary precipitation step will contain approximately 2 percent or the initial tungsten and 92 percent of the initial molybdenum. If molybdenum is a desired byproduct, it may be obtained by any of a number f methods from this resulting filtrate.
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