Aluminum Extraction From Coal Fly Ash And Preparation Of Adsorbent With The Acid Leaching Residue

With the increase in total energy consumption,the demand for coal decreases annually.As the main source of energy consumption,coal still accounts for more than half of total energy consumption,though its demand is continuously decreasing.Therefore,the output of coal fly ash（CFA）has increased,according to the government’s investigation.The utilization rate of CFA has decreased.However,its utilization value is generally low.The content of alumina（Al₂O₃）in CFA is about15%～50%.Hence,CFA can be used as low-grade bauxite to recycle aluminum,which has broad application prospects.Meanwhile,it is also essential to develop a highly-valued utilization method of the corresponding acid leaching residues in order to avoid secondary pollution in the process.In this study,the NaOH sintering-acid leaching method was used to extract Al from coal fly ash.The optimal conditions of the aluminum extraction process were explored,and the sintering activation mechanism was discussed according to composition and functional group changes of the sintered products.When the sintering temperature was550°C,the NaOH/CFA（wt./wt.）was 1.40,the mechanical excitation time was 30 min,the sulfuric acid concentration was 30 wt.%,and the sintering time was 10 min,the aluminum leaching efficiency was higher than 95.00%.Mullite and amorphous aluminosilicate react with NaOH,and their alumina octahedron transforms into alumina tetrahedron during the sintering activation process of coal fly ash.Meanwhile,alumina tetrahedron combine with the silicate tetrahedron to form aluminosilicates with 4-number ring and 6-number ring units,such as faujasite and nepheline.During the acid leaching process,faujasite and nepheline react with sulfuric acid.Aluminum element exists in the acid leaching solution in the form of ions after the acid leaching process,and the nepheline phase is more conducive to the acid leaching process of the sintered product.In order to avoid solid wastes caused by the NaOH sintering-acid leaching method,mesoporous silica was prepared by a sol-gel method using acid leaching residues after extraction as raw material.FT-IR was used to monitor the process of preparing materials and analyze the mechanism of mesoporous silica preparation.When the modulus ratio was 3,the silicon concentration was 45 g·L^-1,and the aging pH was 7,the maximum specific surface area of mesoporous silica was 1039.32 m²·g^-1,the average pore diameter was 3.09 nm,and the pore volume was 0.87 kg·m^-3,and pores were narrow and uniform.The preparation of mesoporous silica by sol-gel method using acid leaching residue as raw material was mainly divided into three steps:hydrolyzing,polymerizing,and aging.Methylene blue was used as the adsorbate to determine the adsorption performance of mesoporous silica.When the pH was 11,the adsorption temperature is 293.15 K,the initial methylene blue solution concentration was 200 mg·L^-1,and the dosage of mesoporous silica was 1 g·L^-1,the adsorption capacity of the material for methylene blue was 189.10 mg·g^-1,the removal rate was 94.55%.The adsorption process of MB using mesoporous silica was spontaneous and exothermic,which meant that temperature increase was not beneficial to adsorption,and the disorder of the solid-liquid interface decreased after the adsorption of MB.The adsorption process was physical adsorption mainly by electrostatic attraction and hydrogen bond,and the adsorption process was reversible.