Formation And Transformation Behavior Of Andradite

With the competition of alumina enterprises and 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,simulating the desilicon reaction system of bauxite by pure reagent,using XRF,XRD,SEM-EDS and other modern testing methods system was studied the synthesis of sodium ferrite,the evolution of ore facies,microstructure and dynamics of CaO-Na2O-Al2O3-H2O,CaO-Na2O-Al2O3-SiO2-H2O,desilication product in actual dissolution of bauxite system,the theoretical basis and practical guiding method for the formation and transformation process of andradite are established.The main results are showed as follows:The sintering behavior of sodium ferrite in high temperature was studied by single factor control variable method.In the range of N/F(molar ratio of Na2O to Fe2O3)=0.9?1.1 the content of sodium ferrite gradually increases in the phase compositions,which indicates that,the purity of sodium ferrate increases with the increase of N/F.The alumina digestion rate of bauxite with sodium ferrate is gradually increased,and the content of alkali is almost the same,indicating that the activity of sodium ferrate is improved with the increase of N/F.In the range of T=850?1000?,the phase composition of sodium ferrate does not change,and the purity of sodium ferrate is not affected in the temperature range.The alumina digestion rate of bauxite dissolved is relatively high and basically unchanged,and the content of the alkali is very low,which indicates that the activity of sodium ferrate is similar and higher in the temperature range.In the range of t=0.5?2.0h,the content of sodium ferrite in the phase composition of sodium ferrate is the highest at t=1.0h,and the corresponding purity of sodium ferrate is the highest.With the sintering time prolonged,the alumina digestion rate of bauxite is high and slightly decreased,and the content of alkali in red mud is very low,indicating that the sodium ferrate activity is higher in the time range.The reaction behavior of sodium ferrite in the CaO-Na2O-Al2O3-H2O and CaO-Na2O-Al2O3-SiO2-H2O system was studied systematically.After the reaction with the mixing of lime and sodium ferrite in aqueous solution,sodium hydroxide solution,the solid phase was composed of Ca(OH)2 and Fe2O3,sodium ferrite is involved in the reaction of hydrolysis to NaOH and Fe2O3 There is no 3CaO·Fe2O3·6H2O in solid phase,and the andradite cannot be generated by the way of the crystallization water in 3CaO·Fe2O3?6H2O after SiO2 replaced by SiO2 in the form of 3CaO·Fe2O3·6H2O.After the reaction with the mixing of lime,sodium ferrite and sodium silicate in aqueous solution,sodium hydroxide solution,the solid phase was composed of Ca(OH)2,Fe2O3 and CaSiO3,no andradite.In the reaction of sodium aluminate solution,andradite and hydrate garne were formed in the reaction solid phase,and the presence of Al2O3 in solution was a necessary condition for the formation of andradite.The formation mechanism and dynamics of andradite were studied,the morphological behavior of various objects in the formation process is illustrated by the micromorphology of granular crystal structure.The formation process of andradite mainly experienced the dissolution of sodium aluminosilicate hydrate,and Fe2O3 replaced the Al2O3 in hydrated garnet,and the SiO2 implant into hydrated garnet.With the increase of the reaction temperature and the extension of reaction time,the crystallinity of andradite,the surface area and silicon saturation coefficient increased,and the lattice constant grain size decreased continuously.The dynamics of andradite shows that its forming process is controlled by internal diffusion,and the reaction activation energy is 7.38 KJ/mol.The rate equation of the reaction is 1-2a/3-(1-a)2/3=13 18.18xexp(-7.38/RT)xt.The behavior of andradite in A/S=3.5 low grade bauxite actual reaction dissolution system was studied.When the reaction conditions in the iron and aluminum than F/A(Fe2O3 and Al2O3 mass ratio)is 0.5,calcium iron than C/F(CaO and the molar ratio of Fe2O3)is 4.5,heavy alkali concentration NK=240 g/L,?K(Na2O and the molar ratio of Al2O3)is 30,dissolution temperature T=260?,dissolution time T=1h,the liquid-solid ratio L/S=4,the content of Na2O in red mud is less than 0.5%,mainly for a large amount of andradite and a small amount of hydrated garnet.The actual dissolution rate of alumina is greater than 83%.The saturation coefficient of silicon in calcium and iron eclogite is around 1.65.The crumby hydrated garnet has been replaced by Fe2O3 as the geometrical appearance rule and crystal good spherical andradite granules.The results of andradite are consistent with the results of the pure system reaction of the sodium silicate solution.