Hydrothermal decomposition of K-feldspar in KOH–NaOH–H2O medium

•Hydrothermal decomposition experiments of K-feldspar in KOH–NaOH mixed solution were studied.•Decomposition products depended on compositions of mixed alkaline solution and temperature.•Higher NaOH concentration was beneficial to increase crystal size.•The conversion of K-feldspar to kalsilite or hydroxyl-cancrinite was a dissolution–precipitation process.

The hydrothermal decomposition of K-feldspar in mixed alkaline solutions containing NaOH and KOH at a concentration of 8.5 M was studied. As the mole fraction of KOH (xKOH = KOH/(KOH + NaOH) in mole) in the mixed alkaline solution decreased, metastable kalsilite, kalsilite and hydroxyl-cancrinite crystallized progressively at 240 °C. The metastable kalsilite disappeared when reaction temperature increased from 240 °C to 280 °C, while the crystallization zone of kalsilite was extended from xKOH = 0.8–0.3 to xKOH = 1.0–0.4, and the crystallization zone of hydroxyl-cancrinite was enlarged from xKOH = 0 to xKOH = 0.2–0. The K-feldspar dissolved under the action of OH− and released K+, Al(OH)4− and H2SiO42 −. The kalsilite or hydroxyl-cancrinite was precipitated from solution.