Effect Of Anionic Impurities On The Precipitation Of Sodium Aluminosilicate Hydrate

In the Bayer process,when the sodium aluminate solution is diluted and precipitated,it is easy to hydrolyze and cause loss of alumina,so it is the focus of research to how to decrease the hydrolysis loss in Bayer process.In this paper,the composition of industrial sodium aluminate solution was simulated to study the influence of different times and different anion impurity and its content on the precipitation activity of sodium aluminosilicate hydrate in the process of low temperature Bayer process and provide theoretical support for reducing the actual hydrolysis loss in Bayer process,the main contents of this paper are as follows:Sodium aluminosilicate solution at the reaction temperature of 145?,the main phases of sodium aluminosilicate hydrated are zeolite-A,zeolite and sodalite.The zeolite-A is produced only at the beginning of the reaction,its content is 5.6%,its micro-morphology is cube,its crystallinity is 73.3%,its unit cell parameter is 1.2340nm,which is higher than 0.8979nm of zeolite and 0.8883nm of sodalite.When the reaction time was 60min,89%SiO2 reacted to form sodium aluminosilicate hydrated,and the reaction reached the equilibrium stability.With the extension of reaction time,the particle size and crystallinity of sodium aluminosilicate hydrated were increased,when the reaction time is 60min,the particle size parameter d(0.5)is 43.337?m,when the reaction time was 240min,the particle size parameter d(0.5)rose to 46.694?m,the particle size increased 3.357?m,the crystallinity of the zeolite increases by about 2%,the crystallinity of the sodalite increases by about 1%;The morphology of the sodium aluminosilicate hydrate at different time was formed by many concentric circular lamellar structures.The reaction time was 15min to 240min,and the thickness of the circular lamellar structure increased from 75nm to 180nm.When the desilicication reaction time is prolonged,the particle size of the sodium aluminosilicate hydrate increases,zeolite-A formation and disappearance,the crystallinity of the zeolite and the sodalite increases,and the precipitation activity of the sodium aluminosilicate hydrate decreases,which is helpful to improve the stability of sodium aluminate solution.The inorganic anion impurity Cl-,SO42-and CO32-can promote the desilicication reaction of sodium aluminosilicate solution,meanwhile,the particle size of desilicication products can be reduced and the crystallinity of the products decreased.When the concentration of Cl-,SO42-and CO32-in solution increased from 10g·L-1 to 20g·L-1,the particle size parameter d(0.5)decreased by about 5?m,6?m and 4?m,respectively;When the concentration of Cl-in the solution increased from 5g·L-1 to 20g·L-1,the main phase of the sodium aluminosilicate hydrate was zeolite,sodalite-Cl and sodalite,and the crystallinity decreased about 2%,5%and 3%;When the concentration of SO42-in solution increased from 5g·L-1 to 20g·L-1,the main phase of the sodium aluminosilicate hydrate was noselite and sodalite,the crystallinity of each phase decreased by about 2%and 1%;When the concentration of CO32-in solution increased from 5g·L-1 to 20g·L-1,the main phase of the sodium aluminosilicate hydrate was zeolite and sodalite-CO32-,the crystallinity of each phase decreased by about 4%and 2%;Compared with the same conditions,inorganic anion impurity will not affect the main micro-morphology of sodium aluminosilicate hydrate the main,will only affect the thickness of circular lamellar structure;However,Cl-rich whiskers were formed in the microstructure of desilicication product with NaCl concentration of 10g·L-1.When the concentration of the three inorganic anions in the solution increased from 5g·L-1 to 20g·L-1,the particle size of sodium aluminosilicate hydrate was reduced and the crystallinity of each phase decreased,and the precipitation activity of the sodium aluminosilicate hydrate increases,which is not helpful to improve the stability of sodium aluminate solution.The organic anion impurity C2O42-and CH3COO-,can reduced the particle size of the desilicication product and decreased the crystallinity.When the concentration of C2O42-and CH3COO-was increased from 5g·L-1 to 20g·L-1?the particle size parameter d(0.5)of sodium aluminosilicate hydrate decreased about 18?m and 3?m,respectively.When the concentration of C2O42-in solution increased from 5g·L-1 to 20g·L-1,the main phase of sodium aluminosilicate hydrate was zeolite and sodalite,and the crystallinity of each phase decreased by about 8%and 5%.When the concentration of CH3COO-in the solution increased from 5g·L"1 to 20g·L-1,the main phases of the sodium aluminosilicate hydrate were zeolite and sodalite-CH2OH,the crystallinity of each phase was reduced by about 3%and 1%.Compared with the same conditions,organic anion impurity will not affect the main micro-morphology of sodium aluminosilicate hydrate the main,will only affect the thickness of circular lamellar structure;When the concentration of the two organic anions in the solution increased from 5g·L-1 to 20g·L-1,the particle size of sodium aluminosilicate hydrate was reduced and the crystallinity of each phase decreased,and the precipitation activity of the sodium aluminosilicate hydrate increases,which is not helpful to improve the stability of sodium aluminate solution.When the sodium aluminosilicate solution contains 10g·L-1 of Cl-,SO42-,and CO32-,each anion impurity is involved in desilicication reaction and enters into the molecular structure of sodium aluminosilicate hydrate.The addition of sodium aluminosilicate hydrate seeds can increase the crystallinity of each phase of the sodium aluminosilicate hydrate,and decrease the activity of sodium aluminosilicate hydrate,which is helpful to improve the stability of sodium aluminate solution.