JP2922290B

The direct hydrothermal production of high purity potassium silicate solutions having a high SiO2: K2O molar ratio by reaction of a silicon dioxide source with aqueous potassium hydroxide solutions is made possible by using a silicon dioxide source that contains a sufficient fraction of cristobalite phase, or by conditioning other crystalline forms of silicon dioxide by heating at or above 1100 DEG C., but below the melting point of silica, before the hydrothermal treatment. Preferably the potassium hydroxide solution has a concentration range of 10 to 40% by weight, and the reaction is carried out in a closed pressure reactor at temperatures of 150 DEG to 300 DEG C. and under saturated steam pressures corresponding to those temperatures.