Precipitation Behavior Of Tricalcium Aluminate In Sodium Aluminate Solution With High Molar Ratio

With the bauxite decline in China,the problem of resource waste caused by traditional Bayer process in treating low grade bauxite is becoming more and more prominent,which has become one of the most important issues for China’s alumina industry to develop new process of alumina with high efficiency and low consumption.This paper is based on the new process of treating low grade bauxite by one-step alkali digestion method based on andradite,with chemical reagents as raw material,using XRF,XRD,SEM-EDS,FT-IR,Mastersizer 3000 and other modern testing methods,and systemly studied the precipitation process of tricalcium aluminate in high molar ratio of sodium aluminate solution system,microstructure and dynamics behavior of tricalcium aluminate,as well as dissolution process in sodium carbonate solution,the theoretical basis and process control mechanism for the formation and transformation process of tricalcium aluminate are established.The main results are showed as follows:The precipitation behavior of tricalcium aluminate in sodium aluminate solution with high molar ratio of 5 was studied by single factor control variable method.In the range of C/A(the weight ratio of CaO and Al2O3)=1.2～1.8,the content of tricalcium aluminate showed a trend of decreasing firstly and then remaining unchanged with the increase of C/A.When C/A increased from 1.4 to 1.6,the(211)and(611)crystal surfaces of tricalcium aluminate crystals were transformed to(420)and(521)crystal surfaces,and the overall content of tricalcium aluminate was relatively unchanged.When T=60～95℃,the main products were calcium aluminate,followed by calcium carbonate and calcium hydroxide.The content of tricalcium aluminate was the best at 80℃.In the range of t=0.5～4.0 h,the content of tricalcium aluminate increases firstly and then remains unchanged,the content of tricalcium aluminate was the largest at t=2.0 h,the content does not change much with the prolonged reaction time.The optimal process conditions for the precipitates of tricalcium aluminate in the sodium aluminate solution were finally determined:C/A was 1.4,the reaction temperature was 65℃,the reaction time was 2.0 h,and the content of tricalcium aluminate was 92.1%.The precipitation behavior of tricalcium aluminate in sodium aluminate solution containing carbonate or silicate ions was studied.The relative content of calcium carbonate increased obviously with the increase of carbonic alkali concentration in the solution.The content of silica in solution is within the range of 0～2 g·L-1,which will have an impact on the content and quality of synthetic products.Therefore,the concentration of carbonaceous and silicon dioxide in sodium aluminate solution should be strictly controlled in practical production.The formation kinetics and mechanism of tricalcium aluminate were systematically studied,and the evolution behavior of each phase in the precipitation process was described in combination with the changes of particle size,crystal structure and microstructure.The formation of tricalcium aluminate mainly involves the formation of calcium hydroxide and the nucleation and growth of tricalcium aluminate.With the increase of reaction time,the content of tricalcium aluminate increased.The generation kinetics calculation of tricalcium aluminate showed that the formation process was controlled by unreacted shrinking nucleus model,and the reaction activation energy was 6.03 kJ·mol-1,and the kinetic equation was 1-2a/3-(1-a)2/3=110.6r0.4texp(-6030.7/RT).The dissolution behavior of tricalcium aluminate in sodium aluminate solution containing sodium carbonate was studied.Tricalcium aluminate has good activity,easy to be dissolved by sodium carbonate solution,making alumina into the solution and realizing high extraction of alumina.The increase of carbon concentration is beneficial to the increase the dissolution rate and decrease the average particle size.The molar ratio had little effect on the dissolution rate and average particle size of tricalcium aluminate.With the increase of dissolution time and temperature,the average particle size of tricalcium aluminate decreases gradually.The optimal process conditions for the dissolution of tricalcium aluminate in sodium carbonate solution were determined:CNK=145g·L-1,and CNC=14 g·L-1,αK=2.22,the liquid-solid ratio L/S=5,dissolution temperature T=180℃,time t=1 h.At this point,the dissolution rate of tricalcium aluminate can achieve 92%,solid product after dissolution is cubic calcium carbonate.