In the manufacture of Keene's cement from calcined calcium sulphate containing iron as an impurity, the iron is converted into a colourless salt by treatment with a suitable reagent such as ammonium sulphate, ammonium arsenate, ammonium phosphate, phosphoric acid or sulphuric acid. 1-3 per cent of the reagent may be added to the hydrated calcium sulphate before calcination. When temperatures in excess of 800 DEG C. are employed, however, the reagent is not added until after calcination, in order to prevent re-formation of red iron oxide; the hot mass is rapidly cooled by the addition. The calcium sulphate employed may be that obtained as a by-product in the production of superphosphates by treatment of phosphate rocks with sulphuric acid. Iron may be present as an impurity in the rocks or may enter during the treatment. Residual phosphoric and sulphuric acids present in the by-product calcium sulphate may be removed by neutralization or washing; this is unnecessary when calcination is effected at the higher temperatures. The cement composition is completed by the addition of an accelerator such as 2 per cent of potassium alum or a mixture of 1 per cent of aluminium sulphate and 1 per cent of potassium sulphate.
展开▼
