Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing
Patent
1984-11-20
1986-05-13
Heller, Gregory A.
Chemistry of inorganic compounds
Phosphorus or compound thereof
Oxygen containing
423166, 423319, 423321R, 423555, C01B 2516
Patent
active
045885702
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for preparing phosphoric acid and calcium sulfate, notably as .alpha.-hemihydrate of calcium sulfate, II-anhydride of calcium sulfate, or a mixture of both these types of calcium sulfate, according to which calcium phosphate is subjected to an attack by a mixture of sulfuric and phosphoric acid.
Belgian Patent No. 638,739 relates to a method according to which the calcium sulfate is first precipitated under dihydrate form which is converted after removing the generated phosphoric acid, by increasing the temperature and acidity, into hemihydrate.
This known method has the advantage relative to the so-called "dihydrate" method, according to which the calcium sulfate is directly separated in the form of unusable gypsum, that the calcium sulfate thus obtained in hemihydrate form contains less than 0.15% P.sub.2 O.sub.5 on dry material at 150.degree. C., which brings a total extraction yield markedly higher than the one of the dihydrate method. Moreover a simple lime-neutralizing of the low free acidity contained in this hemihydrate makes it usable as such, for the plaster industry, and as setting regulator in the cement industry.
Indeed approximately five tons gypsum are produced for every ton P.sub.2 O.sub.5 in the acid and for methods ending with an unsalable gypsum, this enormous amount gypsum has to be disposed of and stored in a suitable location, which consequently brings substantial expenses to cover the conveying, storage and pollution control costs.
Moreover as regards the possibility of using the residual hemihydrate produced according to the method described in Belgian Patent No. 638,739, other additional advantages may further be mentioned.
The hemihydrate produced by the method described in said Belgian Patent contains about 6.2% crystallizing water, while the dihydrate produced by other methods, such as the so-called "dihydrate" method; or resulting from a purifying operation contains about 20% crystallizing water. There results therefrom a lowering of the conveying costs and during the calcination, a lowering of the amount of water to be vaporized by about 0.166 tons per ton calcium sulfate, which thus brings a substantial energy saving, notably in the plaster industry and in the cement works, which require the calcination of the calcium sulfate to the hemihydrate or anhydrite state.
A second advantage of the hemihydrate method according to this Belgian Patent lies in the fact that by the natural re-hydrating to the expense of the accompanying moisture thereof, the hemihydrate which normally contains at the production, about 20 to 25% total water, is completely hydrated back after a stay of about three weeks on a storage area without any outside action and does only contain at this time, but about 5% moisture.
With respect to the gypsum produced with the dihydrate method for example, or resulting from a purifying operation, this re-hydrated product is dry and consequently very easy to handle, which lowers as much the conditioning and metering costs.
Finally a third advantage of the method according to this Belgian Patent relative to the dihydrate method lies in the fact that the obtained hemihydrate lies in the .alpha.-crystalline form, to the contrary of the hemihydrate produced by calcination of natural gypsum or phospho-gypsum produced by the other known methods, which is present in the .beta.-crystalline form. This feature is very advantageous because the .alpha.-hemihydrate has much better mechanical properties than .beta.-hemihydrate. The .beta.-hemihydrate can be converted into .alpha.-hemihydrate but by means of a costly process based on a discontinuous treatment inside an autoclave.
However according to the method described in this Belgian Patent, it is impossible to produce directly a phosphoric acid with a high P.sub.2 O.sub.5 content.
Indeed the nefarious influence on the shape of the gypsum crystals, of the P.sub.2 O.sub.5 content from the liquid phase, limits this latter one to 35% P.sub.2 O.sub.5 approximately, and as most conventional use
REFERENCES:
patent: 1836672 (1931-12-01), Larssen
patent: 3416887 (1968-12-01), Matsubara et al.
patent: 3632307 (1972-01-01), Van Es et al.
patent: 4101638 (1978-07-01), Inoue et al.
patent: 4136151 (1979-01-01), Beaumont et al.
Chemical Abstracts, vol. 80, No. 16, 4/22/74 (Columbus, Oh.) p. 128, Abstract No. 85239j, JP, A, 4870094 (Okamura, et al).
Davister Armand L.
Dubreuco Andre R.
Thirion Francis A.
Heller Gregory A.
Prayon Development
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