Process for producing wet process phosphoric acid from phosphate rocks

Abstract

Phosphoric acid is produced by the wet process from phosphate rock containing fluochlorapatite. The rock is ground in a crusher (10) to a size such that from about 50% to about 90% by weight will be passed by a -200 mesh U.S.S. sieve before being passed into a digestor (14) where it is mixed with sulfuric acid and recycled phosphoric acid to form a slurry. A suitable reductant such as sulphur dioxide is added to the slurry to maintain the oxidation-reduction potential in the range from about +150 mv to about +750 mv. The gypsum by-products are then removed from the slurry at filter (16) and a portion of the acid is recycled to digestor (14). The remaining acid is either concentrated and further processed to make fertilizers or is further processed via path (20) to remove aluminum and/or iron impurities. Select amounts of fluoride and sodium containing compounds are added to the acid in a stirred reactor (30) to cause precipitation of aluminum, principally as Na.sub.2 AlF.sub.5. After about 15 to about 60 minutes of stirring at a temperature below about 70.degree. C., the precipitate is removed at filter (32) where the acid is either processed to make fertilizer or subjected to additional processing to remove iron contaminants. Iron contaminants are removed from the filter acid or from aluminum removed acid from filter (32) by adding sufficient ammonia to the acid in a stirring reactor (40) to cause precipitation of iron principally as NH.sub.4 Fe.sub.2 H.sub.8 (PO.sub.4).sub.5.AH.sub.2 O, to reduce the level of iron to desired levels. After aging in the reactor from about 2 to about 24 hours at a temperature below the boiling point of the acid, the precipitate is removed at filter (42) and the resulting iron reduced acid is concentrated and subjected to conventional processing for making fertilizers.