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The alkali-acid combination leaching has the characteristics of reacting with most minerals from various grades coal and high safety but the removal of silicic minerals as quartz and aluminosilicate is very limited. To further remove quartz and aluminosilicate from coal, we optimized the samples size, alkali leaching concentration, temperature and contract times of leaching process, and designed a new demineralization process of flotation pretreatment followed by alkali-acid combination leaching. On leaching process the percentage demineralization was 86.81, but appending the flotation column pretreatment the percentage demineralization reached 94.51. To investigate the removal mechanism of quartz and aluminosilicate in coal, the mixture coal samples of quartz–coal and aluminosilicate–coal was carried out by combination leaching process. Leach samples characterization of raw coal, flotation pretreatment coal, and mixture coal were done by XRD, ICP-OES, and SEM-EDS analysis. The result of the analysis showed that production of quartz and aluminosilicate in alkali leaching tend to adsorb on the surface of coal, then these products form new impurities in acid leaching. Flotation removes part of the dissociated silicic minerals in advance, which reduces the adhesion of soluble silica-containing sodium salt produced by alkali leaching to the coal surface and thus leads to the reduction of ash content.

Preparation of low silicon ultra clean coal by flotation pretreatment followed by alkali-acid leaching combined process