Aluminum Extractions by the Alkali Method Directly from Alkali-Acid (NaOH-HCl) Chemical Deashing of Coals

An advanced alkali-acid (NaOH-HCl) chemical method was used to deash aluminum-rich coals (ARCs) with a high ash content of 27.47 wt% to achieve a low ash content of 0.46 wt%. In the deashing process, aluminum in the coal ashes was dissolved in both alkali solutions and acid solutions. The deashing alkali solutions with dissolved coal ashes were regenerated by adding CaO, and the resulting precipitates were added with sodium bicarbonate for aluminum extraction. High temperatures increased aluminum extraction, and excessive sodium bicarbonate addition decreased aluminum extraction. The deashing acid solutions were concentrated by evaporation, and silica gels formed during the process. The obtained mixtures were calcinated at 350 °C for the decomposition of aluminum chlorides, and soaked with water at 60 °C to remove the soluble chlorides. For the insoluble oxides after soaking, diluted alkali solutions were used to extract the aluminum at 90 °C, and aluminum extraction failed due to the formation of albite in the presence of sodium, aluminum and silicon elements as proved by XRD and SEM/EDS. When silica gels were separated by pressure filtering, aluminum extraction greatly increased. Aluminum extractions were accordingly made in the form of sodium aluminate from the deashing solutions of coals, which could be advantageous for sandy alumina production.