Chemistry of the direct acid leach, calsinter, and pressure digestion-acid leach methods for the recovery of alumina from fly ash

The chemistry of three methods for the recovery of alumina from fly ash is reviewed. Fly ashes are characterized with respect to both physical and chemical properties, and differences in ashes derived from eastern and western coals are identified. Aluminum solubilization from fly ashes is explained in terms of the solid phases present, which are related to the basic element content of the ash. The aluminum solubilized in the Direct Acid Leach method is from the amorphous aluminum-containing phases while aluminum in crystalline mullite remains insoluble and can be dissolved by acid only after a vigorous treatment with a basic reagent. A lime source was employed in the Calsinter method while caustic was used in the Pressure Digestion-Acid Leach method. These methods yield high aluminum solubilization from both eastern and western fly ashes.