Phosphogypsum radioactivity and its purification

Phosphogypsum is a waste product in phosphoric acid production using dihydrate wet phosphoric process. The radioactivity of the phosphogypsum is a consequence of the presence of radionuclides resulting from uranium radioactive decay. Presence of uranium in some phosphorites and its concentration depend on their source and variety whether they are primary or secondary can be 1- 400 ppm. Radioactivity determined for phosphogypsum confirmed the presence of radionuclides 40 K and 226 Ra. The concentration of 40 K in phosphogypsum is < 10 mBq/g while the concentration of 226 Ra of 580 +- 5 Bq/kg exceeds MAC for Ra. For this reason two methods of purification of raw phosphogypsum have been developed. According to the first method, phosphogypsum containing radioactive material is calcined to from hemihydrate crystals carrying radioactive contaminants. In the second method anhydrite crystals are obtained through dehydration of radiation containing phosphogypsum in strong sulfuric acid; a portion of anhydrite crystals containing radionuclides is converted to substantially radiation free gypsum. (author)