Decomposition of supersaturated sodium aluminate solution
摘要
The decomposition of supersaturated sodium aluminate solution in the process of alumina production was reviewed. Some fundamental problems, such as the structure of supersaturated sodium aluminate solution and the growth unit of gibbsite precipitation as well as the decomposition mechanism of sodium aluminate solution, were investigated by model analytic technique and theoretical calculation. It is found that the main Al-contained species is S4 symmetric [Al(OH)4(H2O)4]-. [Na+(H2O)4*Al(OH)-4] is found to form in intermediate concentrated solution while [Na+(H2O)2*Al(OH)-4] in highly concentrated solution. Meanwhile, it is proved that [(H2O)2Al(OH)4]- is the basic growth unit of gibbsite and [Al6(OH)18(H2O)6] is a favorable growth unit. Based on above results and ab initio quantum mechanical calculation and transition state theory, a reaction pathway of the decomposition of supersaturated sodium aluminate solution was put forward, in which the transformation of pentacoordinate aluminates ion to hexacoordinate basic growth unit as well as the formation of circled growth unit [Al6(OH)22(H2O)2]4- are the controlling steps.