| With the vigorous development of China’s alumina industry,China’s own bauxite is increasingly unable to meet industrial needs,and the dependence on imported bauxite has increased year by year.Since foreign bauxite is dominated by gibbsite-type ore and has a high content of organic matter,together with flocculants,crystallization aids,and anti-foaming agents added in the production process,these organic impurities enter the solution and are accumulated in the process.This will seriously affect the production of alumina,in which the effect of oxalate is particularly obvious,it will lead to equipment scarring,reduce the seed decomposition rate,reduce the alumina production and quality.At present,domestic and foreign experts have proposed many methods for efficiently removing oxalate from sodium aluminate solutions,of which the most potential is adsorption and chemical precipitation.In this paper,the influencing factors of adsorption of oxalate by Al(OH)3,the adsorption kinetics,and the analysis of adsorption models were systematically studied.After that,The formation kinetics of calcium aluminate and calcium oxalate in the process of chemical precipitation was studied.And the formation and transformation mechanism of calcium oxalate under the Na2O-Al2O3-H2O system were studied.The results will provide certain technical support and theoretical basis for alumina production.The main research results are as follows:The effect of sodium oxalate concentration and Al(OH)3 particle size on adsorption were studied.It was found that the adsorption process is a two-velocity process consisting of rapid diffusion and slow diffusion,with the increasing concentration of sodium oxalate and thining of Al(OH)3 particle size,the adsorption speed rate and adsorption rate were significantly increased.The process of Al(OH)3 adsorb sodium was in accordance with the Freundlich model.The adsorption kinetics was in accordance with the pseudo-second order kinetics equation.The adsorption behavior is multi-layer adsorption,and there are both physical adsorption and chemical adsorption.The adsorption rate equation at different sodium oxalate concentrations was:2.0 g·L-1,t/qt=0.892t+1.883;4.0 g·L-1,t/qt=0.303t+0.211;6.0 g·L-1,t/qt=0.147t+0.015.The kinetics of the formation of calcium aluminate and calcium oxalate were studied.It was found that the reaction of calcium oxide and sodium aluminate under dilute solution conditions was mainly controlled by chemical reaction.The controlling factor was temperature,and the rate equation of calcium aluminate was 1-(1-α)1/3=1.16×107exp[50470/(RT)];The reaction of calcium oxide and sodium oxalate is mainly controlled by internal diffusion.The main control factor is the stirring speed,and the rate equation of calcium oxalate was:1-2α/3-(1-α)2/3=1.39×108exp[72750/(RT)];After that,the formation rule of calcium oxalate under Na2O-Al2O3-H2O system was studied.It was determined that at the temperature of 50℃ and the stirring speed 200 r·min-1,the calcium oxalate formation rate reached 90.33%,the alumina loss rate was 31%.The formation and transformation of calcium oxalate were further studied.It was found that when the solution contains C2O42-an relatively stable hydrocalumite containing C2O42-will be generated first,and when the solution contains a large amount CO32-,the oxalate formation rate will be greatly reduced. |
