Strengthening desilication of coal fly ash by alkaline leaching with the addition of ethylene diamine tetraacetic acid

Pre-desilication, as a treatment to improve the mass ratio of alumina to silica of aluminosilicate materials, is one of the essential procedures for alumina extraction from coal fly ash (CFA). In this work, a new method for strengthening desilication efficiency of CFA was proposed by alkaline leaching with the addition of ethylene diamine tetraacetic acid (EDTA). The effects of alkaline leaching conditions on the desilication efficiency of CFA were investigated; meanwhile, the strengthening pre-desilication mechanism was further clarified by means of scanning electron microscopy-energy dispersive spectroscopy, laser particle size analysis, x-ray diffraction and nuclear magnetic resonance, etc. The results showed that the desilication efficiency of CFA was significantly increased from 38 % to 50 % with the addition of EDTA during the alkaline leaching process. The mass ratio of alumina to silica of desilicated CFA was increased from 1.41 to 1.66, accordingly. During this process, the dis-solved Al(OH)4- in the desilicated solution would be complexed by the additional EDTA, which makes it not to largely consume the dissolved H2SiO42-to form an insoluble by-product of hydroxysodalite (Na8Al6-Si6O24(OH)2(H2O)2). Both the reduction of cumulative amount of hydroxysodalite on the desilicated CFA and the continuous exposure of new reaction interface on the desilicated CFA contribute to the improvement of desili-cation efficiency.