Effect Of Roasting Activation On Acid Leaching Performance Of Coal Fly Ash

Coal fly ash as thermal power plants to capture coal combustion flue gas,and electrostatic,mechanical sedimentation and other ways to recover the micro-fine solid particles,its production increased year by year,but the annual comprehensive utilization rate of no significant increase,and the use of low value.At present,the use of fly ash is concentrated in building materials,construction and other aspects,but the use of high aluminium fly ash in this way will undoubtedly cause a waste of resources.The aluminium content of high alumina fly ash exceeds 38%,and its recovery as a low-grade bauxite can not only achieve high value utilization of fly ash,but also alleviate the problem of shortage of bauxite resources in China.In this paper,the extraction of aluminium from fly ash by roasting activation-sulphuric acid leaching method was proposed using high alumina fly ash from Shuozhou No.2 power plant as raw material.The main studies are as follows:（1）According to the thermodynamic theory,the feasibility of activating fly ash by roasting with sodium carbonate or sodium hydroxide was analysed and explored,and the results showed that sodium carbonate gradually reacted with mullite above 700 K to produce Na Al Si O₄,and sodium hydroxide gradually reacted with mullite above 400 K to produce Na Al Si O₄and Na Al O₂.Comparative analysis by SEM and XRD of raw materials and clinker showed that mullite obtained from fly ash is almost fully transformed into sodium aluminosilicate and sodium metaaluminate.The optimal process conditions for the activation phase:1:1 material ratio of sodium carbonate to fly ash,roasting temperature of 900℃and roasting time of 2 h,under which the aluminium leaching rate was 90.3%.Sodium hydroxide to fly ash material ratio 1.2:1,roasting temperature of 600℃and roasting time of 90 min,under this condition,the aluminium leaching rate was 81.5%.（2）According to the thermodynamic theory,the feasibility of acid leaching of activated clinker with sulphuric acid was analysed and explored.The results showed that under room temperature conditions xanthite and sodium meta-aluminate could react with sulphuric acid to produce aluminium sulphate,and the optimum process conditions for the acid leaching experiment were obtained through single-factor experiments and orthogonal experiments:the mass fraction of H₂SO₄was 30%,the acid leaching temperature was 90℃,the acid leaching time was 120 min,and the liquid-solid ratio was 5:l.At this time,the leaching rate of aluminium from sodium carbonate activated fly ash reached 93.2%and that of aluminium from sodium hydroxide activated fly ash reached 92%.The kinetic analysis shows that the process of sulphuric acid leaching of Al from fly ash activated clinker is controlled by an interfacial chemical reaction with an activation energy E_a=25.187 KJ·mol^-1and a pre-exponential factor A=22.778.This is in accordance with the kinetic equation:1-（1-x）^1/3（28）22.778exp[-25187/（RT）]t.（3）By comparison,sodium carbonate activation is better,but requires higher temperature and longer time,while sodium hydroxide reaction requires lower temperature and shorter time,but the amount of helper is greater,and sodium hydroxide as a strong alkali requires more equipment,also has the possibility of pollution,and its price is also more expensive than sodium carbonate.From the analysis of the chemical composition of the acid leach slag,it is clear that sodium carbonate activation has the best leaching effect on aluminium elements,while sodium hydroxide activation has a more pronounced leaching effect on aluminium,iron and titanium elements.