Process for making sodium tripolyphosphate from wet process phosphoric acid

A process for producing sodium tripolyphosphate from wet process phosphoric acid comprising the steps of:

• (a) reacting said phosphoric acid with a sodium compound and forming a reaction mixture comprising an aqueous solution of Na.sub.2 HPO.sub.4 and NaH.sub.2 PO.sub.4 having a pH above 6 with silicon and magnesium impurities contained therein;
• (b) adding a fluoride compound to said reaction mixture;
• the amount of said fluoride compound added being sufficient so that the resulting molar ratio of F:(Si+Mg) in said reaction mixture is at least about 5:1;
• (c) precipitating a major portion of said magnesium in said reaction mixture;
• (d) separating said precipitated magnesium from said reaction mixture, thereby forming a clarified aqueous solution;
• (e) heating said clarified aqueous solution at temperatures in the range of about 400.degree. C. to about 700.degree. C. for sufficient time in order to,
• (i) volatilize and remove a major portion of the combined sum of fluoride and silicon in said clarified solution from said clarified solution;
• and
• (ii) form a solid sodium tripolyphosphate product;
• and
• (f) recovering said solid sodium tripolyphosphate product.