Activation Technology For Aluminum Recovery From Coal Spoil Through Acid Leaching Route

It is a high value-added process to produce aluminiferous chemicals through kaolinite-rich coal spoils using the acid leaching route. Activation technologies are always used for elevating chemical reactivity of inert coal spoils. However, the mechanism and influence factors for coal spoil activation have not been clearly understood. This research was designed to investigate the thermal and mechanical activation of coal spoil systemically. The mechanism and influence factors for coal spoil activation were discussed. The influence of coal content on thermal and mechanical activation of coal spoil was studied. The key factors restricting the industrialization of coal spoil activation technology were pointed out and a novel technology was proposed.In the thermal activation section, the relationship between the structure variation of minerals and the chemical reactivity of thermally treated coal spoil and the influences of coal content, particle size and atmosphere on thermal activation of coal spoil were investigated. The reactivity of calcined coal spoil mainly depended on metakaolinite formed from the decomposition of kaolinite. The reactivity of calcined coal spoil decreased with the decomposition of metakaolinite. During the calcination process of coal spoil, kaolinite underwent three stages including dehydroxylation, segregation and recrystallization. The coal content in coal spoil accelerated the dehydroxylation of kaolinite due to the combustion of volatile matter and fixed carbon. The activation energy of the transformation from metakaolinite to crystalline phase was consequently reduced. The controlling step of kaolinite dehydroxylation changed from the chemical reaction to the diffusion. Temperature gradients along the radial direction were presented in coal spoil particles during calcination due to the combustion of coal content. The decomposition degree of kaolinite varied along the radial direction of coal spoil particles. This variation increased with the particle size, which resulted in lower average reactivity of calcined particles. Under the condition of fast calcination the dehydroxylation of kaolinite in coarse coal spoil particles could produce water vapor pressure so that the phenomena occurred including the variation of thermal transformation of kaolinite, the recrystallization of metakaolinite and an up to 400?decrease in the formation of mullite. The calcination of coal spoil at N2 and CO2 atmosphere could eliminate the influence of coal combustion on kaolinite transformation in fine coal spoil particles and CO2 atmosphere could inhibit further decomposition of metakaolinite.In the mechanical activation section, the relationships among particle size, mineral structure and reactivity of mechanically treated coal spoil were investigated. The influence of coal content on mechanical activation of coal spoil was analyzed. During the mechanical activation of coal spoil, kaolinite structure was distorted continuously and resulted in three Al coordination modes of Al?, Al?and Al?. The hydroxyls in kaolinite were removed from the Al-O octahedron to form water, which was subsequently coordinated or hydrogen-bonded to the active surface of kaolinite. Coordinated water involved in the coordination of Al in [AlO5(H2O)] which led to larger amounts of more active Al?than that in metakaolinite. Mechanical activation of coal spoil depended on milling time, milling medium weight and milling method (dry/wet). Three phenomena including crushing of large particles, agglomeration of fine particles and crushing of agglomerated particles were presented in all mechanical treatments. The agglomeration of fine particles was associated with the destruction of kaolinite structure. The mechanochemical effect of dry milling on kaolinite in CS was not as strong as that in kaolin due to the grinding aid effect of coal content, however, was stronger in wet milling process. Mechanical treatments could effectively activate aluminate silicates such as kaolinite and muscovite in coal spoil and were therefore more applicable than thermal treatments.The reasons for low reactivity and unstable quality of coal spoil calcined in conventional calcination process were discussed. A novel process for coal spoil calcination was proposed. A concept design for alumina production from coal spoil (10,000 t/a) was accomplished.