Purifying phosphogypsum for cement manufacture

Abstract

Phosphogypsum available as a by-product from the wet process phosphoric acid industry in India contains various impurities which, when used in cement manufacture, impair the strength of the cement. Such methods as simple washing with water or wet sieving of phosphogypsum, heating to hemihydrate-anhydrite stage followed by washing or neutralization with lime, and treating phosphogypsum with a mixture of sulphuric acid and silica or hot aqueous ammonium sulphate solutions have been suggested for reducing the amount of impurities or making them ineffective. In the investigation reported here, treatment of phosphogypsum with aqueous ammonium hydroxide solutions was tried to purify phosphogypsum and render it suitable for cement manufacture. The treatment converts phosphatic and fluoride impurities into water-soluble ammoniated compounds which can be removed with a stream of water. The results of chemical and physical tests and differential thermal analysis of the phosphogypsum with and without the ammonium hydroxide treatment confirmed the efficacy of the treatment for purifying phosphogypsum. The treated phosphogypsum was found to have comparatively smaller amounts of impurities that the untreated material. The cement produced was found to have strength characteristics similar to those produced using mineral gypsum.