We have summarized the different procedures described in the literature for the production of phosphoric acid of which the solid phase is either the dihydrate, the alphahemihydrate or the anhydrite.
In order to establish the operational parameters for the attack of the phosphate by sulfuric acid, it was necessary to examine the transition speed and the duration of the stability of the alpha-hemihydrate in the system calcium sulfate-phosphoric acid - water in function of the temperature and the phosphoric acid concentration and to establish the equilibrium diagram basis for those results. The objective of our study was to examine the characteristics of the alpha-hemihydrate which is used as plaster and which is formed by the attack of phosphate with sulfuric acid.
Thus, we have:
- examined the factors affecting the crystal dimensions and their mechanical properties
- compared the properties of the alpha-hemihydrate “plaster” produced by the attack of phosphate by
- 1) Beta-hemihydrate “plaster” which is formed by dehydration of dihydrated sulfate by baking.
- 2) Alpha-hemihydrate “plaster” which is yielded by the dehydratation of calcium sulfate dihydrate in salt solution with or without the addition of auxiliaries.
In the workshop, one could, after filtration and washing, transform the alphahemihydrate into plaster plates or dry it preferably by a flashing procedure (by ascension tube) (40).