Purification and conversion of phosphoric acid to ammonium phosphates

Abstract

A method of purifying filter-grade wet-process phosphoric acid, then converting the purified phosphoric acid directly to ammonium phosphate without first concentrating the phosphoric acid. The wet-process phosphoric acid is treated with a mixture of ammonia or an ammonium salt and acetone near or at the boiling point of the mixture. Solids are separated from the phosphoric acid liquid phase. The solids are a mixture of metal phosphates, gypsum, and fluosilicates. Adhering phosphoric acid is washed free of the solids with fresh acetone. Wash liquor is combined with purified phosphoric acid. Gaseous ammonia is added to the phosphoric acid-acetone extract in the ratio necessary to produce either monoammonium phosphate (MAP) or diammonium phosphate (DAP). Ammonium phosphate is filtered from the acetone-water mother liquor, then, depending on the level of ammonium phosphate left in the mother liquor and the type of ammonium phosphate produced (MAP or DAP), the mother liquor is treated in one of several ways. If DAP is produced and the amount of DAP left dissolved in the mother liquor is sufficiently low, the acetone is distilled from the mother liquor and recycled and the aqueous waste either discarded or used for filter cake wash water in the phosphoric acid plant. If necessary, the amount of ammonium phosphate left with the aqueous waste can be reduced to nil by adding lime to the mother liquor. This converts ammonium phosphate to dicalcium phosphate and ammonia. The precipitated calcium phosphate is separated and returned to the phosphoric acid plant. Ammonia and acetone are co-distilled and return to the process.