Recovery of hydrogen fluoride from metallic fluoride salts

In a method for recovering hydrogen fluoride containing phosphate values, reported by analysis as P.sub.2 O.sub.5, from a metallic fluoride salt mixture comprising at least about 7% by weight, on a dry basis, fluorine and analyzing to contain from about 4% to about 25% by weight, on a dry basis, P.sub.2 O.sub.5, the method comprising the steps of:

• introducing such metallic fluoride salt mixture and sulfuric acid in an amount in excess of that required for reaction with the metallic fluoride salt mixture into a digestion stage maintained at an elevated temperature sufficient for reaction therein of the metallic fluoride salt mixture with sulfuric acid and reacting the sulfuric acid with the metallic fluoride salt mixture to release a gas comprising hydrogen fluoride and phosphate values and produce a slurry comprising a residual liquid including introduced unreacted sulfuric acid and residual solids resulting from reaction of the metallic fluoride salt mixture with sulfuric acid;
• and
• withdrawing the released gas and slurry from the digestion zone, the improvement comprising the steps of:
• introducing water to the digestion zone;
• and
• maintaining the concentration of water and sufficient excess unreacted sulfuric acid in the digestion zone such that the released gas comprising hydrogen fluoride withdrawn from the digestion zone is substantially free of phosphate values and has a weight ratio of hydrogen fluoride to P.sub.2 O.sub.5 of greater than 100.

In the method for recovering hydrogen fluoride containing phosphate values, reported by analysis as P.sub.2 O.sub.5, from a metallic fluoride salt mixture comprising metallic cations reactive with sulfuric acid, at least about 7% by weight, on a dry basis, fluorine and analyzing to contain from about 4% to about 25% by weight, on a dry basis, P.sub.2 O.sub.5, the method comprising the steps of:

• introducing such metallic fluoride salt mixture and sulfuric acid to a digestion zone maintained at a temperature sufficiently high for the sulfuric acid to react with the metallic fluoride salt mixture to produce a distillate comprising hydrogen fluoride and phosphate values and a slurry comprising a residual liquid including sulfuric acid and residual solids resulting from reaction of the metallic fluoride salt mixture with sulfuric acid;
• and
• withdrawing the distillate and slurry from the digestion zone, the improvement comprising the steps of:
• introducing water to the digestion zone and
• maintaining the concentration of water and sulfuric acid in the digestion zone at a level such that the distillate comprising hydrogen fluoride and phosphate values withdrawn from the digestion zone is substantially free of phosphate values and has a weight ratio of hydrogen fluoride to P.sub.2 O.sub.5 of greater than 100 and such that Ci is from about 20 to about 70, R is from about 1 to about 15 and Cf is from about 25 to about 90, wherein:
• Ci=the amount of sulfuric acid and phosphoric acid theoretically present in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in excess of the amount of sulfuric acid required for reaction with the reactive metallic cations in the digestion zone, where Ci is in units of percent by weight in the liquid phase;
• R=Ci/HFi, where HFi=the theoretical concentration of hydrogen fluoride in the liquid in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in units of percent by weight in the liquid in the digestion zone;
• and
• Cf=the actual concentration of sulfuric acid and phosphoric acid in the residual liquid in units of percent by weight in the residual liquid.

A method for producing substantially anhydrous hydrogen fluoride having a hydrogen fluoride to P.sub.2 O.sub.5 weight ratio greater than 10,000 from a feed mixture comprising phosphate values analyzed as P.sub.2 O.sub.5 and, on a dry basis, from about 10 to about 35% by weight hydrogen fluoride, where the weight ratio of hydrogen fluoride to phosphate values as P.sub.2 O.sub.5 in the feed mixture is from about 100:1 to less than 10,000:1, the remainder of the feed mixture comprising substantially water, the method comprising the steps of:

• (a) rectifying the feed mixture in a first rectification zone to produce a substantially azeotropic mixture of water and hydrogen fluoride, the azeotropic mixture containing the phosphate values;
• (b) introducing the azeotropic mixture to a second rectification zone for rectification of the azeotropic mixture therein to produce a vapor stream enriched with hydrogen fluoride;
• (c) withdrawing the vapor stream enriched with hydrogen fluoride from the second rectification zone;
• and
• (d) introducing sufficient sulfuric acid to the second rectification zone such that the vapor stream withdrawn from the second rectification zone comprises at least 80% HF and has a hydrogen fluoride to P.sub.2 O.sub.5 weight ratio of greater than 10,000:1.

A method for producing substantially anhydrous hydrogen fluoride having a hydrogen fluoride to P.sub.2 O.sub.5 weight ratio greater than 10,000 from a feed mixture comprising phosphate values analyzed as P.sub.2 O.sub.5 and from about 20 to about 35% by weight hydrogen fluoride, where the weight ratio of hydrogen fluoride to P.sub.2 O.sub.5 in the feed mixture is from about 100:1 to less than 10,000:1, the remainder of the feed mixture comprising substantially water, the method comprising the steps of:

• (a) introducing the feed mixture to a rectification zone for rectification of the feed mixture therein to produce a vapor stream enriched with hydrogen fluoride;
• (b) withdrawing the vapor stream enriched with hydrogen fluoride from the rectification zone;
• and
• (c) introducing sufficient sulfuric acid to the rectification zone such that the vapor stream withdrawn from the rectification zone comprises at least 80% HF and has a hydrogen fluoride to P.sub.2 O.sub.5 weight ratio of greater than 10,000:1.

A method for recovering substantially anhydrous hydrogen fluoride having a hydrogen fluoride to P.sub.2 O.sub.5 weight ratio greater than 10,000 from a metallic fluoride salt mixture comprising metallic cations reactive with sulfuric acid, at least about 7% by weight, on a dry basis, fluorine and from about 4% to about 25% by weight phosphate values as P.sub.2 O.sub.5, on a dry basis analysis, comprising the steps of:

• (a) introducing such metallic fluoride salt mixture to a digestion zone;
• (b) introducing to the digestion zone water and sulfuric acid and maintaining the digestion zone at an elevated temperature for release of a gas comprising hydrogen fluoride and phosphate values from the metallic fluoride salt mixture in the digestion zone, thereby leaving a slurry for removal from the digestion zone, the slurry comprising a residual liquid including introduced water and introduced sulfuric acid and residual solids resulting from reaction of sulfuric acid with the metallic fluoride salt mixture, wherein sufficient water and sufficient sulfuric acid are introduced to the digestion zone such that Ci is from about 20 to about 70, R is from about 1 to about 15, and Cf is from about 25 to about 90, where
• Ci=the amount of sulfuric acid and phosphoric acid theoretically present in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in excess of the amount of sulfuric acid required for reaction with the reactive metallic cations in the digestion zone, where Ci is in units of percent by weight in the liquid phase;
• R=Ci/HFi, where HFi=the theoretical concentration of hydrogen fluoride in the liquid in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in units of percent by weight in the liquid in the digestion zone;
• and
• Cf=the actual concentration of sulfuric acid and phosphoric acid in the residual liquid in units of percent by weight in the residual liquid;
• (c) withdrawing released gas from the digestion zone;
• (d) removing slurry from the digestion zone;
• (e) introducing withdrawn released gas to a rectification zone for rectification of the gas therein to produce a vapor stream enriched with hydrogen fluoride;
• (f) introducing sufficient sulfuric acid to the rectification zone such that the vapor stream withdrawn from the rectification zone comprises at least 80% HF and has a hydrogen fluoride to P.sub.2 O.sub.5 weight ratio of greater than 10,000:1;
• and
• (g) withdrawing the vapor stream enriched with hydrogen fluoride from the rectification zone.

A method for recovering hydrogen fluoride substantially free of phosphate values reported by analysis as P.sub.2 O.sub.5 from a metallic fluoride salt mixture comprising metallic cations reactive with sulfuric acid, and at least about 7% by weight, on a dry basis, fluorine and analyzing to contain from about 4% to about 25% by weight, on a dry basis, P.sub.2 O.sub.5, comprising the steps of:

• (a) introducing such metallic fluoride salt mixture to a digestion zone;
• (b) introducing to the digestion zone water and sulfuric acid and maintaining the digestion zone at an elevated temperature sufficiently high to release a gas comprising hydrogen fluoride substantially free of phosphate values from the metallic fluoride salt mixture in the digestion zone and leave a slurry for removal from the digestion zone, the slurry comprising a residual liquid including introduced water and introduced sulfuric acid and residual solids resulting from reaction of sulfuric acid with the metallic fluoride salt mixture;
• (c) withdrawing released gas from the digestion zone;
• (d) removing slurry from the digestion zone;
• wherein the quantity of water and sulfuric acid introduced to the digestion zone are selected to substantially satisfy the equations:
• ##EQU4## where J=1-Ci/Cf
• HF.sub.v =the concentration of HF in the released gas withdrawn from the digestion zone in % by weight;
• Y=the percent by weight of the fluorine in the metallic fluoride salts introduced to the digestion zone in the released gas withdrawn from the digestion zone;
• US=the solids contents of the slurry, % by weight;
• K=0.095565e (0.075435Cf);
• W=with respect to the metallic fluoride salts introduced to the digestion zone, the mass ratio of fluorine represented as HF to calcium oxide represented as CaSO.sub.4 ;
• Ci=the amount of sulfuric acid and phosphoric acid theoretically present in the digestion zone, after introduction of the sulfuric acid and water to the digestion, zone in excess of the amount of sulfuric acid required for reaction with the reactive metallic cations in the digestion zone, where Ci is in units of percent by weight in the liquid phase;
• R=Ci/HFi, where HFi=the theoretical concentration of hydrogen fluoride in the liquid in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in units of percent by weight in the liquid in the digestion zone;
• and
• Cf=the actual concentration of sulfuric acid and phosphoric acid in the residual liquid in units of percent by weight in the residual liquid.

In a method for recovering hydrogen fluoride containing phosphate values reported by analysis as P.sub.2 O.sub.5 from a metallic fluoride salt mixture contained in the waste stream of a wet process phosphoric acid facility and comprising metallic cations reactive with sulfuric acid wherein the metallic fluoride salts comprise at least about 7% by weight, on a dry basis, fluorine and analyzing to contain from about 4% to about 25% by weight, on a dry basis, P.sub.2 O.sub.5, the method comprising the steps of:

• introducing such metallic fluoride salt mixture and sulfuric acid to a digestion zone maintained at a sufficiently high temperature for production therein of a distillate comprising hydrogen fluoride and phosphate values and the production of a slurry comprising a residual liquid including introduced sulfuric acid and residual solids resulting from reaction of the metallic fluoride salt mixture with sulfuric acid;
• and
• withdrawing the distillate and slurry from the digestion zone, the improvement comprising the steps of:
• introducing water to the digestion zone;
• and
• maintaining the concentration of water and sufficient sulfuric acid in the digestion zone at a level such that the distillate comprising hydrogen fluoride withdrawn from the digestion zone is substantially free of phosphate values and has a weight ratio of hydrogen fluoride to P.sub.2 O.sub.5 of greater than 100 and such that Ci is from about 20 to about 70, R is from about 1 to about 15, and Cf is from about 25 to about 90, where
• Ci=the amount of sulfuric acid and phosphoric acid theoretically present in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in excess of the amount of sulfuric acid required for reaction with the reactive metallic cations in the digestion zone, where Ci is in units of percent by weight in the liquid phase;
• R=Ci/HFi, where HFi=the theoretical concentration of hydrogen fluoride in the liquid in the digestion zone, after introduction of the sulfuric acid and water to the digestion zone, in units of percent by weight in the liquid in the digestion zone;
• and
• Cf=the actual concentration of sulfuric acid and phosphoric acid in the residual liquid in units of percent by weight in the residual liquid.