Continuous process for preparing phosphoric acid and calcium sulphate
AssigneePrayon Developpment, Societe Anonyme
InventorsArmand L. Davister, Francis A. Thirion
Life of this patent
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Abstract
There is described a process which comprises adjusting on the one hand the flow rates of a circulating mixture relative to a base flow rate according to very specific specifications, and on the other hand so adjusting the reaction conditions in succeeding reaction zones as to obtain a production phosphoric acid with a P.sub.2 O.sub.5 concentration between 33 and 52%, and a H.sub.2 SO.sub.4 concentration between 0.25 and 2.5%, and calcium sulphate in the form of dihydrate, hemihydrate, anhydrite or a mixture of at least two said crystalline forms.
Claims36
A method for preparing phosphoric acid and calcium sulphate containing II-anhydite as a by-product, wherein, in a mixture containing calcium sulphate flowing through a sequence of reaction zones, calcium phosphate is subjected to an attack by a mixture of sulfuric and phosphoric acids, and at least part of the production phosphoric acid is removed, which comprises adjusting the circulating mixture flow rates relative to a mixture base flow rate which corresponds to the flow rate being discharged from the reaction zones, in conformity with the following specifications:
- a general circulation through a majority of the reaction zones with a rate lying between 300 and 4000% of the base flow rate,
- a circulation through a vacuum cooling zone with a rate lying between 2000 and 4000% of said base flow rate,
- local circulations in the reaction zones with rates each lying between 500 and 2000% of the same base flow rate,
- the sum of the general circulation and the local circulation being higher than or equal to 2500% of the base flow rate in every zone where reactants are being added, as well as in that zone which directly follows each of said zones in the general circulation circuit, and retaining in a first part of the reaction zones wherein tricalcic phosphate is fed,
- a temperature lying between
- 75.degree. and 85.degree. C., and a sulfuric concentration between 0.25 and 0.85%, so as to obtain the substantially complete attack of said tricalcic phosphate, and retaining in a subsequent second part communicating with the first reaction zone part, a temperature lying between 65.degree. and 75.degree. C., and a sulfuric concentration lying between 1 and 2.5%, so as to solubilize P.sub.2 O.sub.5 combined with Ca.sup.++ ions, in the form of HPO.sub.4.sup.-- and/or H.sub.2 PO.sub.4.sup.-, the reaction conditions being selected and combined within said limits so as to obtain on the one hand a production phosphoric acid having a P.sub.2 O.sub.5 concentration lying between 33 and 52%, and a H.sub.2 SO.sub.4 concentration lying between 0.25 and 2.5%, and on the other hand calcium sulphate as by-product, (a) essentially in dihydrate form by re-crystallizing .alpha.-hemihydrate or by unchanged crystallizing by selecting and combining within said reaction limits, a temperature within a range lying between 30.degree. and 80.degree. C., a H.sub.2 SO.sub.4 concentration between 4 and 15% and a P.sub.2 O.sub.5 concentration between 15 and 20%, (b) essentially in II-anhydrite form by re-crystallizing dihydrate by selecting and combining within said reaction condition limits, a temperature between 30.degree. and 75.degree. C. with a P.sub.2 O.sub.5 concentration between 15 and 20% and a H.sub.2 SO.sub.4 concentration between 12 and 15%, or (c) essentially in hemihydrate form either by recrystallizing dihydrate or by an unchanging crystallizing of .alpha.-hemihydrate by selecting and combining within said reaction condition limits, a temperature between 75.degree. and 95.degree. C. with a P.sub.2 O.sub.5 concentration between 22 and 35% and a H.sub.2 SO.sub.4 concentration between 4 and 15%, the II-anhydrite content lying in the above cases (a) and (c) between 2 and 30% of the production calcium sulphate.
The method as defined in claim, 19 which further comprises when the calcium sulphate to be pulped back contains essentially dihydrate, operating during the second step under the following reaction conditions:
- a temperature lying within a range between 70.degree. and 85.degree. C., a P.sub.2 O.sub.5 concentration lying within a range between 15 and 25%, a sulfuric acid concentration lying within a range between 5 and 15%, a solid content lying within a range between 20 and 45%, and a residence or reaction time lying within a range between 0.5 and 3 hours, said reaction conditions being selected and combined within said limits so as to retain substantially unchanged the crystalline form of the pulped-back calcium sulphate.
Method for preparing phosphoric acid and calcium sulphate as by-product, wherein in a mixture containing calcium sulphate flowing through a sequence of reaction zones, calcium phosphate is subjected to an attack by a mixture of sulfuric and phosphoric acids, and part at least of the production phosphoric acid is removed, which comprises adjusting the circulating mixture flow rates relative to a mixture base flow rate which corresponds to the flow rate being discharged from the reaction zones, in conformity with the following specifications:
- a general circulation through a majority of the reaction zones with a rate lying between 300 and 4000% of the base flow rate,
- a circulation through a vacuum cooling zone with a rate lying between 2000 and 4000% of said base flow rate,
- local circulations in the reaction zones with rates each lying between 500 and 2000% of the same base flow rate,
- the sum of the general circulation and the local circulation being higher than or equal to 2500% of the base flow rate in every zone where reactants are being added, as well as in that zone which follows directly each said zones in the general circulation circuit, dividing these reaction zones in at least two succeeding steps by subjecting during a first step, in a mixture flowing through a sequence of reaction zones, calcium phosphate to an attack by a mixture of sulfuric and phosphoric acids while removing part at least of the production phosphoric acid and separating calcium sulphate, and retaining in said first step a temperature of 75.degree. to 85.degree. C. and a sulfuric concentration of 0.25 to 0.85%, and in a subsequent second step, substantially all the calcium sulphate produced during said step, to a treatment under the following reaction conditions:
- a temperature lying between 65.degree. and 75.degree. C.,
- a phosphoric acid with a P.sub.2 O.sub.5 concentration lying between 14 and 38%,
- a sulfuric concentration of between 1 and 2.5%,
- a solid concentration lying between 20 and 62%, and
- a residence or reaction time between 1 and 18 hours;
- a substantial part at least of that calcium sulphate being separated after the first step is returned in suspension in at least one reaction zone which is not crossed by the general circulation, so as to be able to control therein the reaction conditions substantially independently from the conditions prevailing in the other reaction zones crossed by said general circulation, said reaction conditions being so selected and combined within said limits as to have the II-anhydrite content lying between 2 and 95% of the resulting calcium sulphate
Dependencies are inferred from the claim text.