Recovery of Valuable Metals (Rare-Earths) from Phosphate Fertiliser Waste (Phosphogypsum)

Abstract

AbstractManufacturing of phosphate-based fertiliser mainly requires phosphoric acid, which is produced from phosphate ores through digesting the same by using sulphuric acid. The main waste produced in manufacturing of phosphoric acid is phosphogypsum (CaSO4.2H2O) that is moved to the identified disposal area from the plant. Based on the adopted methods of phosphoric acid production, the resultant phosphogypsum waste mainly comprises gypsum (calcium sulphate dihydrate) or calcium sulphate hemi-hydrate, besides small amounts of silica, fluoride compounds and unreacted phosphate rock. During manufacturing of phosphoric acid, REE preferentially concentrates in phosphogypsum waste. Approximately 70–85% of REE contained in phosphate rocks are transferred to phosphogypsum waste, whereas the remaining rare-earths go with the phosphoric acid. Notably, compared to REE content in ore minerals, the average content of REE in phosphogypsum waste is rather low. Nevertheless, it is significant in view of its huge quantity, as manufacturing of 1 tonne of phosphoric acid generates approximately 4.5–5.5 tonnes of phosphogypsum waste. It is possible to easily take out nearly 50% rare earths from phosphogypsum waste through leaching by sulphuric acid (0.1–0.5 M). From the leached liquor rare earths may be separated by precipitation or solvent extraction. Considerable scope is evident for value addition during complete cycle of phosphate ore processing right from its mining to application in fertilisers to agriculture to creation of wealth from its waste (phosphogypsum) and finally reusing waste as land-filling material for reclamation of land and thereby supporting afforestation, gardening and land beautification. There is a need to carry out comprehensive characterisation of phosphatic rocks and phosphorite deposits of India not only to assess their suitability for fertiliser industry, but also for the possible recovery of valuable metals from them and their waste products like phosphogypsum.KeywordsREE recoveryValuable metalsPhosphatic rockPhosphate fertiliser wastePhosphogypsum