In a hydrothermal process for preparing potassium silicate solutions with a high SiO2:K2O molar ratio, crystalline silicon dioxide is reacted with an aqueous solution of potassium hydroxide. The crystalline silicon dioxide used is quartz tempered at temperatures above 1100°C up to its melting point. This tempered quartz is then reacted with an aqueous solution of potassium hydroxide at a concentration between 10 and 40 wt.% in a closed pressure reactor, at temperatures between 150 and 300°C and saturated water vapour pressures corresponding to these temperatures.
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