Method for the preparation of phosphoric acid and calcium sulphate
AssigneePrayon Development
InventorsArmand L. Davister, Andre R. Dubreuco, Francis A. Thirion
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Abstract
The present invention provides a continuous method for preparing phosphoric acid and calcium sulphate by reacting calcium phosphate with a mixture of sulphuric acid and phosphoric acid. Our method uses at least three successive crystallization steps: a first step of reacting the phosphate with the mixture of phosphoric and sulphuric acids and forming a-hemihydrate calcium sulphate, a second step of recrystallizing the a-hemihydrate into dihydrate and a third step of transforming the dehydrated calcium sulfate into a hemihydrate or anhydrite II or a mixture of the two. Where desired recycling of the products from the above steps can be used for increasing the efficiency of the above method.
Claims33
A continuous method for preparing phosphoric acid and calcium sulfate, by reacting calcium phosphate with a mixture of sulfuric and phosphoric acids, said method comprising at least three succeeding crystallizing steps, (a) a first step of reacting the phosphate with said mixture of sulfuric and phosphoric acids at a temperature of from 70.degree. to 100.degree. C., with a P.sub.2 O.sub.5 concentration comprised between 38 and 50%, in the presence of sulfuric acid with a concentration of between 0.25 and 2.5%, to provide a solid content in the mixture of between 20 and 38%, and with a reaction time of between 2 and 4 hours, to produce .alpha.-hemihydrate calcium sulfate and separating the produced phosphoric acid therefrom;
- (b) a second step of reacting the hemihydrate at a temperature of from 40.degree. to 80.degree. C., changing the concentration in the liquid phase of P.sub.2 O.sub.5 to between 15 and 35%, and of the sulfuric acid between 0.5 and 17.5%, with a solid content of between 22 and 62%, and a reaction time of between 4 and 16 hours, to obtain a dihydrated calcium sulfate;
- and (c) a third step of reacting the dihydrate at a temperature of from 60.degree. to 100.degree. C., retaining in the liquid phase a P.sub.2 O.sub.5 content of from 15 to 34% and a sulfuric acid concentration of from 7 to 25%, and a solid content of between 20 and 34%, a reaction time from 0.5 to 3 hours to transform the dihydrate into a form selected from the group consisting of .alpha.-hemihydrate, II-anhydrite, and mixtures thereof.
A continuous method for preparing phosphoric acid and calcium sulfate, according to which calcium phosphate is subjected to an attack by a mixture of sulfuric and phosphoric acids, said method comprising at least three succeeding crystallizing steps:
- (a) a first of said steps attacking the phosphate with said mixture of sulfuric and phosphoric acids under the following reaction conditions:
- a temperature in the range of substantially 70.degree. to 100.degree. C.;
- a P.sub.2 O.sub.5 concentration in the liquid phase comprising a range of approximately 38 to 50%, an H.sub.2 SO.sub.4 concentration in the liquid phase comprising approximately 0.25-14%;
- and a reaction time between substantially two and four hours, so that the calciuim sulfate crystallizes substantially in .alpha.-hemihydrate form;
- at the end of this first step, the produced phosphoric acid being separated from the hemihydrated calcium sulfate and a part of the separated acid being cycled back to the attack of the calcium phosphate;
- (b) a second of said steps subjecting the hemihydrated calcium sulfate to the following modified reaction conditions:
- a temperature in the range of substantially 40.degree. to 80.degree. C.;
- a P.sub.2 O.sub.5 concentration in the liquid phase of approximately 15 to 34%;
- an H.sub.2 SO.sub.4 concentration in the liquid phase extending over the range between approximately 0.5 and 17.5%;
- a solid content in the calcium sulfate pulp being between substantially 22 and 62%;
- and a reaction time between approximately four and sixteen hours;
- so that the hemihydrate-.alpha. recrystallizes into substantially a dihydrate form;
- and
- (c) a third of said steps comprising contacting the obtained dihydrate with a concentrated sulfuric acid and another part of the phosphoric acid produced and extracted at the end of the first step;
- and agitating the formed pulp, under the following reaction conditions:
- a temperature in the approximate range between 60.degree. and 100.degree. C.;
- concentrating a P.sub.2 O.sub.5 in the liquid phase from approximately 15 to approximately 34%;
- concentrating an H.sub.2 SO.sub.4 in the liquid phase from approximately 7 to approximately 25%;
- a solid content between 20 and 34% and a reaction time of between approximately 0.5 and three hours;
- combining these reaction conditions to transform the dihydrate into one of hemihydrate .alpha., anhydrite II, or into a mixture of hemihydrate .alpha. and anhydrite II, to obtain calcium sulfate during this third step which is subjected to a separation of the liquid phase which is cycled back to at least one of the preceding steps.
Dependencies are inferred from the claim text.