Selective recovery of phosphorus as AlPO4 from silicon-free CFB-derived fly ash leachate

Abstract

The prospect of phosphorus (P) recovery from siliceous fly ash was investigated. The phosphorus content in the pristine fly ash was 1.21%. Obtaining pure phosphorus products from fly ash is very challenging because of high concentration of other elements, silicon (Si) at 17.3% being the major contaminant. The fly ash was fractionated with sieve size of 125 μm to concentrate the phosphorus in the small-size fraction, which also facilitated the removal of 78% of silica (Si) in the solid phase. The fractionated fly ash was treated with 8 M HCl in order to remove 98% of Si by aging (5 h) of leachate until precipitation of Si-gel, and a phosphorus-rich solution is obtained. Iron (Fe) is also considered an impurity in the recovered P-product and its co-precipitation is prevented by the use of EDTA as Fe-masking reagent at [EDTA]/[Fe] ratio of 1. About 70% of Fe is retained in the solution during recovery of phosphorus with aluminum at pH 4. Amorphous white solid was recovered containing 18.08% P, this is 15 times high than that in the pristine fly ash used as raw material in this recovery process.