Recovery of hydrofluoric acid from fluosilicic acid with high pH hydrolysis

A method for recovering solid amorphous silica from a fluosilicate salt comprising the steps of:

• (a) reacting a fluosilicate salt, M.sub.2 SiF.sub.6, where M is selected from the group consisting of sodium, potassium, and ammonium, with MOH to form a first alkaline aqueous slurry having a pH of from about 11 to about 14 and comprising precipitated fluoride salt MF and dissolved M.sub.2 SiO.sub.3 ;
• (b) separating precipitated fluoride salt from the first alkaline aqueous leaving a first aqueous slurry solution comprising M.sub.2 SiO.sub.3 ;
• (c) combining the first aqueous solution with fluosilicic acid or M.sub.2 SiF.sub.6 or both for forming a second alkaline aqueous slurry comprising silica and the fluoride salt MF;
• wherein the pH of the second alkaline aqueous slurry is maintained at a value greater than 7 and up to about 9, the water content of the second alkaline aqueous slurry is maintained sufficiently low such that the slurry is at least saturated with the fluoride salt, and the temperature of the second alkaline aqueous slurry is maintained substantially equal to its boiling point so that the second alkaline aqueous slurry contains precipitated amorphous silica;
• and
• (d) separating precipitated amorphous silica from the second alkaline aqueous slurry.

A method for recovering hydrogen fluoride and solid amorphous silica from phosphate plant scrub liquor comprising at least 15% by weight fluosilicic acid and at least 1000 parts per million P.sub.2 O.sub.5, comprising the steps of:

• (a) combining the scrub liquor with sodium sulfate at about ambient temperature to form solid sodium fluosilicate in a purification zone;
• (b) recovering such sodium fluosilicate;
• (c) reacting recovered sodium fluosilicate with sodium hydroxide in a first precipitation zone to form a first alkaline aqueous slurry having a pH of from about 11 to about 14 and comprising dissolved sodium metasilicate and precipitated sodium fluoride;
• (d) recovering precipitated sodium fluoride from the first alkaline aqueous slurry in a first separation zone leaving an aqueous solution of sodium metasilicate;
• (e) combining the aqueous solution of sodium metasilicate with the scrub liquor and sodium fluosilicate in a second precipitation zone to form a second alkaline aqueous slurry comprising silica and dissolved sodium fluoride;
• wherein the pH of the second alkaline aqueous slurry is maintained at a value greater than 7 and up to about 9, the water content of the second alkaline aqueous slurry is maintained sufficiently low such that the second slurry contains solid sodium fluoride, and the temperature of the second alkaline aqueous slurry is maintained substantially equal to its boiling point so that the second slurry contains precipitated amorphous silica;
• (f) adding sufficient water to the second alkaline aqueous slurry in a dissolving zone to dissolve substantially all of the solid sodium fluoride;
• and thereafter;
• (g) separating precipitated amorphous silica from the second alkaline aqueous slurry leaving an aqueous solution of sodium fluoride;
• (h) evaporating water from the aqueous solution of sodium fluoride in an evaporation zone to form a third aqueous slurry comprising solid sodium fluoride and a saturated solution of sodium fluoride;
• (i) recycling at least a portion of the evaporated water to the dissolving zone;
• (j) passing the third aqueous slurry from the evaporation zone to a second separation zone to recover solid sodium fluoride from the third aqueous slurry leaving a saturated solution of sodium fluoride;
• (k) introducing at least a portion of the saturated solution of sodium fluoride from the second separation zone to the second precipitation zone;
• (l) reacting recovered solid sodium fluoride with sulfuric acid in a hydrogen fluoride generation zone maintained at a temperature of at least about 80.degree. C. to form hydrogen fluoride and sodium sulfate;
• (m) introducing at least a portion of the sodium sulfate formed in the hydrogen fluoride generation zone to the purification zone;
• and
• (n) recovering hydrogen fluoride formed in the generation zone.

A method for recovering sodium fluoride and solid amorphous silica from an aqueous solution comprising at least 15% by weight fluosilicic acid comprising the steps of:

• (a) reacting fluosilicic acid in such an aqueous solution with sodium sulfate to form sodium fluosilicate in a purification zone;
• (b) reacting the formed sodium fluosilicate with sufficient sodium hydroxide in a precipitation zone to form a first alkaline aqueous slurry having a pH of at least about 11 and up to about 14, the first slurry comprising dissolved sodium metasilicate and precipitated sodium fluoride;
• (c) recovering precipitated sodium fluoride from the first alkaline aqueous slurry leaving an aqueous solution comprising sodium metasilicate;
• (d) combining the aqueous solution of sodium metasilicate with fluosilicic acid or sodium fluosilicate or both in a reaction zone to form a second alkaline aqueous slurry comprising sodium fluoride and precipitated, filterable amorphous silica by the steps of:
• (i) maintaining the pH of the second alkaline aqueous slurry at a value greater than 7 and up to about 9;
• (ii) maintaining the water content of the second alkaline aqueous slurry sufficiently low such that the second slurry is at least saturated with sodium fluoride;
• and
• (iii) maintaining the temperature of the second alkaline aqueous slurry substantially equal to its boiling point;
• (e) filtering precipitated amorphous silica from the second alkaline aqueous slurry leaving an aqueous solution of sodium fluoride;
• and
• (f) recovering sodium fluoride from the aqueous solution of sodium fluoride.