Granted Patent B2

US 7,371,278 · App. 11/014,998 · Granted May 13, 2008

Abstract

Claims (36)

1. Process for stabilizing a calcined β-hemihydrate plaster comprising the steps of:

a) providing a heated hemihydrate plaster preferably at temperatures over 100° C.;

b) feeding the heated plaster in a moistening device having walls heated to at least 100° C.;

c) injecting water and/or steam into the moistening device in conditions that such not yet moistened surfaces of the plaster are exposed to the injected water and/or steam;

d) maintaining an atmosphere in the moistening device at a level of the dew point in the range of 75 to 99° C.;

e) feeding the moistened blend into a curing device;

f) maintaining an atmosphere in the curing device above 75° C. for at least 3 minutes;

g) feeding the moistened and cured blend into a drying device; and

h) drying said moistened and cured blend.

2. The process of claim 1 , further comprising the steps of:

i) grinding the dried product; and/or

j) cooling the dried product, where the cooling can take place before or after the grinding step, if any.

3. The process of claim 2 , wherein the grinding and cooling are combined into a single process step.

4. The process of claim 2 , where the drying device is an indirectly heated device and the product temperature is 80 to 110° C.

5. The process of claim 2 , where the drying device is an air heated device and the product temperature is 50 to 95° C.

6. The process of claim 2 , where the drying device is an indirectly heated device and the product temperature is 95 to 105° C.

7. The process of claim 2 , where the drying device is an air heated device and the product temperature is 60 to 80° C.

8. The process of claim 1 , where the plaster of step a) has a temperature of 100 to 135° C.

9. The process of claim 1 , where the walls are heated at a temperature of 100 to 150° C.

10. The process of claim 1 , where water and steam are injected by one or several two phase nozzles.

11. The process of claim 1 , where water and/or steam is sprayed on a cascading plaster powder.

12. The process of claim 1 , where water and/or steam is sprayed on a fluidized bed of plaster powder.

13. The process of claim 1 , where the free moisture, based on the weight of the hemihydrate, is from 3 to 12%.

14. The process of claim 1 , where the dew point of step d) is 80 to 95° C.

15. The process of claim 1 , where the curing step f) is carried out while maintaining an atmosphere in the range of 75 to 99° C.

16. The process of claim 15 , where the temperature of step f) is 80 to 95° C.

17. The process of claim 1 , where the curing time is 4 to 15 minutes.

18. The process of claim 1 , wherein the moistening and curing are performed in the same apparatus.

19. The process of claim 1 , where the thus-obtained plaster is free of Anhydrite III and substantially free of dihydrate.

20. The process of claim 1 , where the thus-obtained plaster has a LOI of 6.2 to 8% calculated on a 100% purity.

21. The process of claim 1 , where the thus-obtained plaster has a d50 of 30 to 100 μm or a d50 of 20 to 30 μm or a d50 of 15 to 22 μm or a d50 of 10 to 20 μm, or is bimodal with a first peak at 3 to 10 μm and a second peak at 20 to 60 μm.

22. The process of 1 , where the plaster originates from FGD gypsum or any other chemical by-product gypsum.

23. The process of claim 1 , where the free moisture, based on the weight of the hemihydrate, is from 3.5 to 6%.

24. The process of claim 1 , where the curing time is 5 to 10 minutes.

25. The process of claim 1 , wherein the heated hemihydrate plaster is provided at a temperature over 1000° C.

26. The process of claim 1 , wherein the thus-obtained plaster has a LOI of 6.2 to 7.3%, calculated on 1000% a purity.

Assignments (10)