Abstract: A process is disclosed for the hydrothermal production of potassium silicate solutions with high SiO.sub.2 :K.sub.2 O molar ratios, by the hydrothermal reaction of quartz sand with aqueous potassium hydroxide solutions at temperatures in the range of 150.degree. to 300.degree. C. at the pressures of saturated water vapor corresponding to these temperatures in a pressure reactor, which is characterized in that the potassium silicate solutions obtained thereby, which have SiO.sub.2 :K.sub.2 O molar ratios of less than 2.75:1, are then reacted with a quartz tempered at temperatures in the range of over 1100.degree. C. up to melting point, the temperatures and pressures also being maintained in the ranges named.

Abstract: The direct hydrothermal production of high purity potassium silicate solutions having a high SiO.sub.2 : K.sub.2 O 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.degree. 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.degree. to 300.degree. C. and under saturated steam pressures corresponding to those temperatures.

Abstract: The direct hydrothermal production of high purity sodium silicate solutions having a high SiO.sub.2 : Na.sub.2 O molar ratio by reaction of a silicon dioxide source with aqueous sodium hydroxide solutions, or with aqueous sodium silicate solutions having a lower SiO.sub.2 : Na.sub.2 O molar ratio, 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.degree. C., but below the melting point of silica, before the hydrothermal treatment. Preferably the sodium hydroxide solution has a concentration range of 10 to 50% by weight, and the reaction is carried out in a closed pressure reactor at temperatures of 150.degree. to 300.degree. C. and under saturated steam pressures corresponding to those temperatures.