Extraction of heavy metals from industrial phosphoric acid in a transverse flow hollow fiber membrane contactor

Phosphoric acid is produced mainly in the so-called wet processes, in which phosphate ore reacts with sulfuric acid to form phosphoric acid and calcium sulfate solids. With the phosphate ore, impurities enter the production process. Some of these impurities mainly end up in the product acid, others mainly in the calcium sulfate. The aim is to develop an in-line technique for the removal of impurities from the product acid, and simultaneously avoid their incorporation into the calcium sulfate solids. This paper describes the treatment of industrial phosphoric acid in transverse flow hollow fiber membrane contactors and the feasibility of these contactors for the treatment of a phosphoric acid/calcium sulfate slurry. Mercury, copper, lead, and cadmium were extracted from an industrial phosphoric acid by Cyanex 302 (bis(2,4,4-trimethylpentyl)thiophosphinic acid) in kerosene in a contactor. The permeability was 3 × 10^-6 m/sec. With the same contactor, erbium, and dysprosium were extracted by di(2-ethylhexyl)phosphoric acid in kerosene. The permeability was 2-6 × 10^-8 m/sec. Experiments had to be terminated, because of leakage of organic phase into the aqueous phase. Vibrations due to pulsation of the tube pumps are thought to be the main reason for this leakage. Problems with respect to organic phase leakage due to e.g., the formation of eddies caused by the transverse flow could not be determined. The feasibility of hollow fiber membrane contactors to remove impurities from the slurries depends strongly on the particle size of the solids and the required pitches to avoid clogging.