be added in amounts of from about 0 1 to about 1 and preferably from about 0 3 to 0 5 active chlorine calculated on the amount of calcium sulphate the calcium sulphate can be purified by fractionation preferably in a cyclone either prior or subsequent to the bleaching step in order to remove undesired particles after the bleaching the calcium sulphate is washed with water for example on a filter or in a column in order to remove chlorine residue chlorides and oxidizing compounds the invention is described in greater detail in the following example thereof in this example waste gypsum from the manufacture of phosphoric acid was used as the raw material the raw material is suspended in water in a container 1 to a dry content of about 15 percent by weight and the ph value of the slurry is adjusted to a value below 3 this value is a critical one because at higher ph values discoloring impurities will adhere to the gypsum particles gypsum of the type used in this embodiment includes as impurities in addition to organic material heavily discolored gypsum calcium fluoride and silicates from previous processes when the ph value for the slurry exceeds 3 acidification is preferably carried out with sulphuric acid the resultant slurry is then fractionated in a hydrocyclone 2 whereby a fraction having a particle size of from about 25 to 100 u03bc was recovered if the particle size is thus below 25 u03bc it has been found that dewatering in a subsequent process step would be rendered substantially more difficult the fraction including particles with a size above 100 u03bc.