Study On The Leaching Behavior Of Al³⁺ From Pulverized Coal Furnace Coal Fly Ash By Sintering With Concentrated H₂SO₄ And The Adsorption Properties Of Residue

Coal fly ash is a type of solid wastes that was produced in coal-firing power plants.The utilization of coal fly ash to prepare profitable materials or extraction useful elements from coal fly ash is of great importance in the solid waste field.In this paper,the alumina was extracted from Junggar pulverized coal furnace coal fly ash in view of its high content of alumina,and the strategy was implemented via the sulfuric acid sintering-water leaching method.The solid residue after Al³⁺-leaching,which is mainly amorphous Si O₂,was demonstrated to be a type of porous materials which looks forwards applications in waste water treatment.In the first part,the influence of sintering and leaching procedures on the extraction efficiency of Al³⁺was investigated.Results showed that the extraction yield of Al₂O₃ was as high as 86.00%,when the ratio of H₂SO₄:CFA was 1.2,the sintering temperature was 300oC,the sintering time was 110 min,the water leaching temperature was 90oC,and the water leaching time was about 1.0 h.The phase transformation and microstructure of materials including pristine coal fly ash,the calcined clinker and solid residue were studied by means of XRD,PLM,EPMA,SEM and TEM.It was revealed that mullite in coal fly ash reacted with concentrated H₂SO₄ easily to dissociate Al³⁺.The presence of Cr-corundum in coal fly ash was also found,which made it possible to extract the element Cr.The kinetics of the concentrated H₂SO₄ sintering process was derived to follow a shrinking mechanism to decompose the coal fly ash,and its rate control step was determined by the internal diffusion hindrance from the preformed layer（Al₂（SO₄）₃）.Owing to the large specific surface area of the solid residue which was obtained after water-leaching of Al³⁺,the residue was considered as adsorbent to remove organics from waste water,e.g.,methylene blue and crystal violet.Effects of the amount of adsorbent,the initial concentration of dye,the pH values of waste water,and duration on the adsorption were investigated.Results showed that the coal fly ash residue was an efficient adsorbent for the removal of both dyes.The adsorption of methylene blue reached equilibrium within 360 min,which was faster than the adsorption of crystal violet（480 min）.The adsorption data for methylene blue and crystal violet by coal fly ash residue were found to be consistent with the Langmuir and Freundlich isotherm adsorption models,respectively.By fitting the kinetic data,both of methylene blue and crystal viole adsorbed on the coal fly ash residue follow a second-order kinetic model.Multiple use of the coal fly ash residues for removal of methylene blue and crystal violet suggests that the material can be recycled.After four recycles,the removal efficiencies of the adsorbent can still reach 89.57%and88.87%,respectively.