METHOD AND APPARATUS FOR WASHING HEMIHYDRATE GYPSUM FILTER IN MANUFACTURE OF PHOSPHORIC ACID BY WET PROCESS

PURPOSE:To prevent reduction in the concn. of phosphoric acid, to reutilize waste water after washing, and to improve a washing effect by continuously washing the filter cloth of a belt filter used in the manufacture of phosphoric acid by a wet process with a phosphoric acid soln. and misty washing water in order. CONSTITUTION:In the manufacture of phosphoric acid by a wet process, phosphate ore is decomposed with sulfuric acid or a mixed acid consisting of sulfuric acid and phosphoric acid, and the resulting slurry is filtered with a hemihydrate gyspum filter to separate and recover phosphoric acid. At this time, a belt filter 1 composed of a belt 2 and filter cloth 3 above the belt 2 is used as the hemihydrate gypsum filter. The belt 2 and the cloth 3 are moved together, and after filtering off hemihydrate gypsum, the cloth 3 is separated from the belt 2 at a lower position. The belt 2 and the cloth 3 are separately washed and circulated. The cloth 3 is washed from both front and rear sides with a phosphoric acid soln. spouted from slits cut in pipes 21, 21 for washing filter cloth in the longitudinal direction and openings pierced at both ends of the slits. The cloth 3 is then washed with a small amount of misty washing water from spray nozzle pipes 22, 22 for washing filter cloth. Waste liquor after washing is returned to the system through drip pans 8, 9.