Manufacture of synthetic gypsum from limestone and sulphuric acid

Gypsum is an essential raw material in the manufacture of cement. South African natural gypsum deposits are generally low grade and need benefication by means of washing. If upgrading is not possible due to non-availability of water, natural deposits become uneconomical because of very long transport distances. After investigating the economics of using treated phosphogypsum, Pretoria Portland Cement (PPC) decided to manufacture gypsum by reacting limestone with sulphuric acid. Limestone is ground to a powder, sulphuric acid is diluted from 98 to 65% and the reactants introduced into a mixer. The gypsum is cured in a store for two days and then transferred to the cement works for use in the milling process. The gypsum has proved to be eminently suitable for cement set control purposes.