Removal of silicon from CFB-derived fly ash leachate in the context of phosphorus recovery

Abstract

High concentrations of dissolved silica in the acid leachate impose two major challenges on precipitation based recovery of phosphorus (P). Firstly, co-precipitation of colloidal silica in the acidic regimes decreases the purity and value of precipitated P-products. In addition, silica scaling on internal surfaces of equipment is also a problematic issue in industrial operations. Therefore, removal of dissolved silica prior to P-recovery process minimizes the risks of Si-contamination in P-products and Si-scaling. In the present study, silica removal was achieved by accelerated silica polymerization with higher acidity and ionic strength of mineral acid, which also assisted the leaching of CFB-derived fly ash, by aging of the leachate solution. The effects of acid concentration, temperature, and stirring on silica removal rate in acid leachate were also investigated. Higher concentrations of H+ and Cl− and higher temperature significantly reduce the silica removal time. HCl was more suitable for improving leachability of phosphorous and precipitating silica in comparison with HNO3. The silica removal method discussed in this work has been shown to be capable of removing >98% of dissolved silica from the acid leachate solution.