The Process Optimization And Mechanism Of Extraction Of Al₂O₃ From Fly Ash Or Coal Gangue With Na₂CO₃

Fly ash and coal gangue as the bulk of industrial solid waste are generated in the total volume of nearly 15 million tones.Due to the limited utilization methods,the huge storage has caused serious environmental and social problems.As substitutes for bauxite resources,extracting alumina from fly ash/coal gangue has been widespread concern.Sodium carbonate activation and acid leaching technology have overcame the technical bottleneck of the poor activity of fly ash/coal gangue,and the dissolution percentage of Al₂O₃ in the acid solution is above 90%.However the vast consumption of sodium carbonate still limited the industrial application.The optimization of the process of Na₂CO₃ activation and acid leaching technology were mainly studied,according to the adjustment of molar ratio of Al/Si in raw materials.This paper discussed the impact of pre-desilication of fly ash and red mud synergistic activation of fly ash/coal gangue on Na₂CO₃ consumption and Al₂O₃ extraction.The main research contents and results are as follows:?1?This paper discussed the process conditions of pre-desilication of fly ash,and the pre-desilication mechanism was studied by the X-ray diffraction?XRD?and scanning electron microscopy-energy spectrum analysis?SEM-EDS?.It was investigated that the impact of pre-desilication of fly ash on the change of the Al/Si desilication ash,Al₂O₃ extraction and Na₂CO₃ consumption.The results showed the removal of SiO₂ was up to 33.4%under the optimum conditions of desilication of which was at 100?,for 1.5 h,20%?co?sodium hydroxide solution and liquid-solid ratio of 1:1.By XRD and SEM-EDS analysis,the results showed that the main mechanism of desilication is the dissolution of amorphous SiO₂ in sodium hydroxide solution,while the formation of the hydroxy sodalite may reduce desilication efficiency in the process of the reaction.Under the optimized conditions of pre-desilication,fly ash mixed with desilication ash by Al/Si mole ratio of 1 and molar ratio of Na/Al of 1:1?1.5:1,the extraction of Al₂O₃ reached more than 87.3%,and the consumption of Na₂CO₃ was reduced by 43.8%?65.6%.?2?The impact of red mud on activation of fly ash/coal gangue were discussed,the activation mechanism is analyzed by X-ray diffraction?XRD?and thermogravimetric-differential thermal analysis?TG-DSC?.The results showed that the red mud synergistic activation of fly ash/coal gangue significantly reduced the consumption of assistant Na₂CO₃.When the mole ratio of Na/Al/Si in the mixed sample was 1:1:1?1.5:1:1,the dissolution rate of alumina in the mixed sample was more than 80%,the consumption of Na₂CO₃ was reduced by 59%?68%.By TG-DSC and XRD analysis,the results showed that the temperature lower than 600?,red mud didn't basically react with fly ash or coal gangue and sodium carbonate,and the calcium nepheline and hydrated garnet in red mud pyrolyzed;at the temperature of 600?850?,red mud reacted with fly ash/coal gangue and sodium carbonate.Pre-desilication of fly ash and red mud deployment significantly reduced the consumption of sodium carbonate,however,the employment of desilicification fly ash or red mud introduced large amounts of impurities,such as sodium,iron and calcium,which brought much more difficulties to separate the impurities from Al,further influencing the quality of the Al and Si products prepared from fly ash or coal gangue.Therefore,more studies should be developed to remove impurities and prepare high purity of products.