Study On The Leaching Behavior Of Coal Fly Ash Clinker In Sodium Silicate Solution

To alleviate the supply-demand imbalance of bauxite in China’s alumina production industry and the environmental problems caused by the massive accumulation of coal fly ash in China,our group proposed a coal fly ash reduction-oxidation roasting-alkali leaching desilication process.It has been experimentally demonstrated that coal fly ash clinker obtained by reduction-oxidation roasting of coal fly ash and iron oxide mixture at1100℃ can leach more than 95%of silica in 100 g/L sodium hydroxide solution,and the resulting high aluminum-silica ratio leaching slag can recover aluminum by Bayer process.In addition,sodium silicate solutions can increase the modulus of silica by dissolving it or decrease it by increasing crystal decomposition.Therefore,in this thesis,the reaction behavior of coal fly ash clinker in sodium silicate solution was investigated using low modulus sodium silicate solution as a bleaching agent,and a two-stage leaching method was proposed to obtain a relative leaching rate of more than 90%.The main research work and conclusions are as follows.（1）Thermodynamic calculations show that free silicon oxide in coal fly ash clinker can be dissolved in sodium silicate solution,while aluminum oxide in fly ash clinker reacts with sodium silicate solution to form sodium aluminosilicate hydrate.At the same time,the experiments proved that the reduction-oxidation roasting-alkali leaching desilication process is not suitable for the desilication of fly ash with high calcium content.Preliminary kinetic studies show that the silica leaching process in coal fly ash clinker is controlled by interfacial chemical reactions.（2）The reaction pattern of fly ash clinker in sodium silicate solution was revealed.The leaching temperature,the concentration of sodium silicate solution,initial modulus,and theoretical target modulus had a strong influence on the relative leaching rate.Increasing the leaching temperature and sodium silicate solution concentration,decreasing the initial modulus and theoretical target modulus of sodium silicate solution all had a facilitating effect on the leaching process while increasing the ball milling,adding sodium salt,and changing the thermal history of the solution had little effect on the final leaching.（3）The conditions for the generation of sodium aluminosilicate hydrate from fly ash clinker in sodium silicate solution were clarified.The pure material roasting experiments showed that when the oxidation roasting temperature was increased to 1100℃,the alumina in the clinker was transformed into more stableα-alumina,and the roasted clinker was reacted in sodium silicate solution with Na₂O concentration of 100 g/L and modulus of 1.2 at 110℃ for 3h,and no obvious sodium aluminosilicate hydrate was generated.（4）The two-stage countercurrent leaching method was proposed to simultaneously obtain high Al-Si ratio concentrate and high modulus sodium silicate solution.Under the conditions of leaching temperature not exceeding 110℃,initial modulus 1.0,and Na₂O concentration 100 g/L,the relative leaching rate of fly ash clinker reached more than 90%after two-stage counter-current leaching;the modulus of leaching solution reached2.02,which can be recycled after crystal decomposition.