Study On Extraction Of Rare Earth Elements From Coal Fly Ash By Alkali Fusion-Acid Leaching

Coal fly ash is the main solid residue for coal combustion,which is rich in many important metal elements.Studies have shown that coal and CFA contain about 50million tons of key elements including rare earth elements,which is equivalent to nearly 50%of rare earth mineral reserves.Therefore,it is of great significance to study the extraction of rare earth from fly ash for the recycling of resources and the safety of the strategy of rare earth.In this paper,the process and reaction mechanism of extracting rare earth from coal fly ash are systematically studied,mainly including:Firstly,the modes of occurrence of rare earth elements in fly ash was studied.Using mathematical statistics and SPSS analysis,it is found that rare earth elements are positively correlated with Al and Si,while negatively correlated with Fe.The results of screening test showed that the total contents of rare earth elements increased with the decrease of size,and the contents of+100μm particles were the lowest,387.03 ppm;The contents of particles-10μm can reach 429.67 ppm at most.The results of sequential chemistry extraction procedure show that the main carriers of rare earth elements in coal fly ash are silicate and aluminosilicate minerals,accounting for 77.90%,followed by sulfide minerals,accounting for only 11.98%.Other existing forms of rare earths account for about 10.12%.Based on the above correlation analysis,it can be concluded that most rare earth elements in fly ash is combined with silicate and aluminosilicate.The mechanism of the alkali fusion process was studied by the alkaline flux action analysis and kinetic analysis,and the effects of various conditions on the leaching efficiency of the alkali fusion process were systematically studied.The corresponding temperatures of different alkali fusion reactions were observed by thermogravimetric analysis,and the products were analyzed by XRD.It was found that Na₂CO₃,Na₂O₂,NaOH and KOH can destroy the structure of coal fly ash and convert it into soluble aluminosilicate and other silicate minerals,thus effectively improving the leaching efficiency of rare earth elements.KOH has the best effect,the leaching efficiency of Y,La,Ce,Pr and Nd can reach 85.47%,64.70%,77.22%,74.19%and 69.38%,respectively.However,NaCl and Ca（OH）₂ cannot destroy the structure of coal fly ash and have little effect on leaching.Taking Na₂CO₃ as alkali flux,the optimal test conditions of alkali fusion are obtained through conditional experiments:the temperature is 860℃,the time is 30min,the mass ratio of Na₂CO₃to coal fly ash is 1:1,and the leaching efficiency of Y,La,Ce,Pr and Nd can reach85.17%,47.21%,53.41%,53.24%and 58.38%respectively.The Kissinger method was used to calculate the kinetic parameters of alkali fusion reaction.The linear correlation（R²）was 0.948,the activation energy E of alkali fusion was 895.52kJ.mol^-1,the pre-exponential factor A was 3.47×1041 min^-1,and the reaction rate constant K was 1.68 min^-1.The E-pH diagram shows that,in the acid leaching system,Y³⁺and Ce³⁺can exist stably in the thermodynamic region in the solution with high H⁺concentration,while Ce⁴⁺cannot exist stably.The optimum experimental conditions were obtained by the conditional experiment and the orthogonal test.The stirring strength was 400rpm,the concentration of HCl was 3 mol.L^-1,and the ratio of solid mass to liquid volume was 1:20 g:mL.The leaching efficiency was the best,and the leaching efficiency of total rare earth elements reached 72.78%.The kinetics of the leaching process was studied by the shrinkage nucleus model.The results show that the value of R² is higher in the results of the fitting of the 1-（1-α）^1/3=k₁t equation with the kinetic data,and the correlation coefficient is 0.975.Above,it indicates that the model of leaching kinetics of the rare earth elements conforms to the shrinkage nucleus model.