GB1002722A - Improvements in and relating to the dehydration of gypsum - Google Patents

Abstract

<PICT:1002722/C1/1> Calcium sulphate hemihydrate is prepared by calcining gypsum at atmospheric pressure under a partial water vapour pressure p calculated from p=aC+b to give physical properties of selected predetermined value C, where a and b are constants characteristic of the natural gypsum. The physical properties may be heat of solution, density, Vicat set time; or may be consistency, set time, compressive strength of a mixture of hemihydrate, sand, water and a retarder. The process is carried out for certain values of p, and the values C of the various properties of the hemihydrate formed are measured. Diagrams of p versus C are drawn up, from which values of p corresponding to required values of C can be determined directly. The apparatus has an inner column 2 of temperature resistant glass pipe units, heated by air cabinet 3, the temperature of which is controlled by electric heaters 6, while air circulation is maintained by blower 21. Air preheated at 9, is introduced into extension 11 and is allowed to reach 140 DEG to 150 DEG C.; plate 23 is then removed; the gypsum charge, of particle size 100-325 mesh, introduced, whence it falls into the fluidization zone above plate 14. The charge is heated in the air stream to 100 DEG C. when steam is added to the air stream at 12, and calcination is effected at 100-140 DEG C. Effluent gases pass through 15, condenser 16, to atmosphere at 17. The pressure drop across plate 14 and the gypsum charge is maintained constant by control of air and steam flow. From the total air flow and total volume of condensate in 16, the average ambient water vapour is calculated. The product is aged to hydrate anhydrite by exposure for a week in air at ambient conditions, for 3 days at 70 DEG F. and 50% humidity or for 14 days at 70 DEG F. and 65% humidity.