Coal fly ash cleaner utilization by ferric oxide assisted roasting – leaching silica: Recycling lixiviant by seeded precipitation of leachate

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Recycling lixiviant from sodium silicate solution is a key to the cleaner utilization of aluminosilicate solid wastes such as coal fly ash and coal gangue via ferric oxide assisted roasting - alkali leaching silica process. This work focuses on seeded precipitation of higher-modulus sodium silicate solution to obtain recyclable spent solution with lower-modulus. The results show that adding silica hydrate seeds can induce the decomposition of sodium silicate solution to obtain amorphous silicate hydrate and low-modulus spent solution. The SiO2 precipitation yield, precipitation rate and the spent solution modulus can respectively reach up to 108.26 g/L, 72.17 % and 0.86, under the appropriate conditions: initial modulus 3.09, total Na2O concentration 50 g/L, seed amount 350 g/L-liquor, temperature 50 ℃, and duration 6 h. Meanwhile, the average particle size of precipitation product is ~ 56.20 µm. Moreover, the structural evolution of silicates during the precipitation implies that silica hydrate mainly precipitates from high-polymerized silicates. The precipitation product was further characterized as opal-A by XRD, chemical composition, TG-DSC and FT-IR analyses. This work may be beneficial either to improving the ferric oxide assisted roasting - alkali leaching silica process for treating aluminosilicate solid wastes or to developing a new technology for preparing opaline silica hydrate.