Manufacture of sodium tripolyphosphate from wet acid

A process producing sodium tripolyphosphate from wet acid comprising:

• (a) introducing said wet acid containing fluorides into a pretreatment stage, adding silicon dioxide and sodium hydroxide or a sodium salt and precipitating insoluble fluorides,
• (b) separating the insoluble fluorides from the resulting pretreated wet acid,
• (c) introducing the pretreated wet acid of step (b) into a solvent extraction zone with a substantially water insoluble organic phosphate extractant and extracting phosphoric acid values from such pretreated wet acid,
• (d) stripping the extracted phosphoric acid values from the loaded organic phosphate extractant of step (c) with an aqueous stripping solution, and recovering a purified phosphoric acid,
• (e) adding the purified phosphoric acid to a make up stage and reacting it with sodium hydroxide or a sodium salt to produce a sodium orthophosphate liquor having a Na:P mole ratio of about 1.67:1,
• (f) introducing the sodium orthophosphate liquor into a heating zone to form sodium tripolyphosphate and recovering same,
• (g) passing gases evolved in the heating zone through a scrubbing zone containing a scrubber liquor and scrubbing fluoride values and entrained sodium tripolyphosphate from said gases into said scrubber liquor,
• (h) recycling scrubber liquor to the make up stage,
• (i) removing a bleed stream from said recycling scrubber liquor and introducing said bleed stream into said pretreatment stage in amounts sufficient to prevent fluoride impurities from preciptitating in said recycling scrubber liquor.