Some of the silica may be retained in soluble form in the sodium aluminate solution, so it is usually necessary to subject the solution to some treatment to remove as much of the silica as possible before proceeding with the precipitation of the alumina by CO2 gas.
Various methods were tried for the purification of the solution and the results of these tests were already informed in the Journal of the Japanese Ceramic Association vol. 53. No. 620 (1944), December.
Here, only the results of studying on the mechanism of the reaction that all of the silica may be removed from the solution by merely digesting it with adequate quantity of lime at ordinary pressure, will be described.
The experiments pointed out that the removal of all of the silica from the solution was caused only by the formation of an insoluble calcium alumino-silicate, so the complete desilication might be impossible even when sodium silicate soln were digested with large quantity of lime.