Process for producing wet process phosphoric acid from phosphate rocks containing fluochlorapatite and related minerals

A process for producing phosphoric acid from phosphate rock containing fluochlorapatite, comprising the steps of:

• (a) crushing the fluochlorapatite containing phosphate rock to a size such that from about 50% to about 90% by weight thereof will pass a -200 mesh U.S.S. sieve;
• (b) mixing the rock crushed in step (a) with sulfuric acid and recycled phosphoric acid to form a slurry;
• (c) filtering the resulting slurry of step (b) to remove gypsum by-products therefrom to obtain a phosphoric acid;
• and
• (d) adding sufficient reductant to a portion of the phosphoric acid obtained in step (c) to be recycled to the digestor to maintain the oxidation-reduction potential of the slurry formed in step (b) from about +150 mv to about +750 mv.

A process for producing phosphoric acid from phosphate rock containing from about 0.2% to about 2.4% chloride as fluochlorapatite, from about 0.4% to about 9.5% Fe.sub.2 O.sub.3 as hematite and geothite, from about 0.2% to about 3.0% Al.sub.2 O.sub.3 as crandellite, from about 0.1% to about 0.35% MnO.sub.2 as pyrolusite, and from about 0.4% to about 3.0% TiO.sub.2 as ilmenite, comprising the steps of:

• (a) crushing the phosphate rock to a size such that from about 50% to about 90% by weight thereof will pass a -200 mesh U.S.S. sieve;
• (b) mixing the rock crushed in step (a) with sulfuric acid and recycled phosphoric acid to form a slurry;
• (c) adding a reductant to the slurry formed in step (b) to maintain the oxidation-reduction potential in the range from about +150 mv to about +750 mv, wherein said reductant is metallic iron, aluminum or zinc, sodium sulfite, ammonium bisufite or sulphur dioxide;
• (d) adding sufficient active silica to the digestor slurry obtained in step (b) to maintain the F/SiO.sub.2 weight ratio of said slurry between 1.9 and 2.3;
• (e) filtering the resulting slurry of step (d) to remove gypsum by-products therefrom to obtain a phosphoric acid;
• (f) treating the phosphoric acid obtained in step (e) to remove excess aluminum by adding sufficient soluble sodium and fluoride containing compounds to said phosphoric acid to precipitate aluminum as a crystalline salt, when the aluminum content of the acid obtained in step (e) is unacceptably high;
• (g) removing the aluminum precipitate from the solution of step (f) to obtain an aluminum removed phosphoric acid;
• (h) treating the phosphoric acid obtained in step (e) to remove excess iron by adding sufficient ammonia to the filtered acid to precipitate iron as a crystalline salt, when the level of iron in the acid obtained in step (e) is unacceptably high;
• (i) removing the iron precipitate from the solution of step (h) to obtain an iron reduced phosphoric acid;
• and
• (j) concentrating the acid obtained in steps (e)-(i) to a value of about 40% to about 54% P.sub.2 O.sub.5.

A process for reducing the aluminum content of phosphoric acid having a P.sub.2 O.sub.5 concentration from about 17% to about 54%, wherein the ratio of Al.sub.2 O.sub.5 to MgO is at least 5 to 1, comprising the steps of:

• (a) adding sufficient sodium and fluoride containing compounds to the phosphoric acid to precipitate aluminum as a crystalline salt principally in the form of Na.sub.2 AlF.sub.5, wherein the amount of fluoride added is determined by the relation X=1.86 (Al.sub.2 O.sub.3)+1.90 (SiO.sub.2)-(F) and the amount of sodium containing material (Y) is determined by the relation Y=0.90 (Al.sub.2 O.sub.3)+0.76 (SiO.sub.2)-(Na) where Al.sub.2 O.sub.3 is the amount of alumina in the phosphoric acid, SiO.sub.2 is the amount of soluble silica in the phosphoric acid, F is the total amount of fluoride already present in the phosphoric acid and Na is the total amount of sodium present in the phosphoric acid;
• (b) stirring the mixture obtained in step (a) from about 15 to about 60 minutes at a temperature less than about 70.degree. C.;
• and
• (c) filtering the aluminum precipitate from the mixture of step (b) to obtain an aluminum removed phosphoric acid.

A process for reducing the iron content of phosphoric acid to a desired level, comprising the steps of:

• (a) adding sufficient ammonia to the phosphoric acid to precipitate iron as a crystalline salt, principally in the form of NH.sub.4 Fe.sub.2 H.sub.8 (PO.sub.4).sub.5.4H.sub.2 O, where the amount of ammonia to be added is determined by the relation:
• ##EQU5## where A is the amount of ammonia required, I.sub.i is the initial amount of iron present (as Fe), I.sub.f is the final amount of residual iron desired (as Fe) and W is the total weight of the acid to be treated;
• (b) aging the mixture obtained in step (a) from about 2 to about 24 hours at a temperature from about 25.degree. C. to about the boiling point of said acid;
• and
• (c) filtering the iron precipitate from the mixture of step (b) to obtain an iron reduced phosphoric acid.