Aluminum extraction from coal ash by a two-step acid leaching method

Abstract

To improve aluminum extraction efficiency, a two-step acid leaching process was proposed to dissolve aluminum and aluminum-bearing crystals with sulfuric acid. The first leaching residue (FLR) was obtained through direct leaching of coal ash with sulfuric acid. After roasting a mixture of the FLR and sodium carbonate, the aluminum-bearing crystals in FLR were completely destroyed while sodium silicate was generated. Then the roasted products were leached with water to dissolve the sodium carbonate. X-ray diffraction (XRD) analysis indicated that the water leaching residues (WLR) mainly contained the amorphous phase, which allowed easy aluminum extraction in the second leaching step. In the first leaching test with 10 mol/L sulfuric acid at 120 °C in 2 h and a solid to liquid ratio (S/L) of 1:2, aluminum dissolution from raw coal ash reached 81.72%. In the second leaching step with 5 mol/L sulfuric acid, the aluminum dissolutions from WLR were 87.31% at 20 °C and 99.06% at 100 °C. Total aluminum dissolutions were more than 97%. Moreover, the leaching process produced two by-products, sodium silicate solution and amorphous silicon dioxide, which contained nearly all the silicon from the coal ash.