| 摘要 |
<p>Parent patent described the mfr. of high quality plaster starting from gypsum obtd. in the treatment of phosphates. In the present invention, the crude gypsum is subjected to a two phase heat treatment, possibly after washing:- (a) dehydration for 0.5-1.5 h at 110-150 degrees C, esp. 120-135 degrees C.; (b) conditioning for 0.16 - several hours at a constant temp. between 130-180 degrees C., esp. 140-160 degrees C. under a steam pressure >=100 torr, pref. 150-250 torr; and an additive (I) is added during treatment. The additive (I) is pref. added at the end of step (a); or at the start or during step (a) which pref. also uses a steam pressure >=100 torr, esp. 400-500 torr. Additive (I) pref. forms CaO during heat treatment; or is another alkaline earth oxide (II) or forms such oxide (II) during heat treatment. In the main patent, additive (Ie was pref. CaO or Ca(OH)2 whereas the present invention covers the use of e.g. Ca CO3; CaCL2; CaBr2; Ca(NO3)2, 4H2O; or MgCl2, 6H2O or BaCl2, 2H2O forming the corresp. oxides during the heat treatment (a, b). The impurities in the plaster are essentially Ca HPO4, so the plaster has a setting time and mechanical strength similar to that of natural plaster.</p> |
