Polyaluminum chloride production from mullite-type coal fly ash by alkali desilication and high-pressure HCl leaching

In this paper, an alkali/acid procedure of coal fly ash (CFA) treatment was developed and analyzed: a desilication method involving high concentration of NaOH and high-pressure HCl leaching of mullite-type CFA. Effects of NaOH concentration (425-450 g/L), of the solid-to-liquid (S/L) ratio (10-15), and of desilication duration (20-60 min) on the dissolution of amorphous silica from the surface of aluminosilicate spheres without precipitation of sodalite were evaluated. High-pressure and atmospheric-pressure HCl leaching of desilicated CFA was analyzed by machine learning with artificial neural networks. The extraction efficiency of Al in this procedure could exceed >83.5% at 175 degrees C, HCl concentration of 281 g/L, an S/L ratio of 10, and tau = 90 min or 69.9% at 103 degrees C, HCl concentration 281 g/L, S/L ratio 10, and tau = 48 h. Chemical compositions of the poly- aluminum chloride solution derived from desilicated CFA and of the FeCl(3 )solution obtained from the magnetic fraction of CFA were compared with China state standards. Sodalite and nanosilica powders resulting from alkali leaching were studied by laser diffraction, X-ray diffraction, and scanning electron microscopy. The findings can facilitate the development of an effective technology for CFA utilization in China and possibly in other countries.