Large-scale production of alkali-resistant Shirasu porous glass (SPG) membranes: Influence of ZrO2 addition on crystallization and phase separation in Na2O–CaO–Al2O3–B2O3–SiO2 glasses; and alkali durability and pore morphology of the membranes

To develop the large-scale production of alkali-resistant Shirasu porous glass (SPG) membranes based on phase separation and subsequent acid leaching, the crystallization and phase separation in mother glasses in the NaO–CaO–Al2O3–B2O3–SiO2 system containing ZrO2 were investigated. A crystalline phase of wollastonite (CaSiO3) was formed in the ZrO2-free mother glass heat-treated at approximately 900 °C, corresponding to the glass-shaping temperature. This crystallization was induced by the phase separation in the mother glass. The crystallization tendency was reduced by adding ZrO2 to the mother glass due to an increase in the glass viscosity, and there was no crystallization in the mother glass with 3.5 mol.% ZrO2. SPG membranes containing 3 mol.% ZrO2 were successfully prepared from the mother glass with 3.5 mol.% ZrO2 by the phase separation of the mother glass at 700–760 °C and subsequent acid leaching with a 0.5 mol dm−3 hydrochloric acid. The membranes had an interconnected pore structure over a wide pore diameter range of 0.1–10 μm. The membrane pore diameter increased with increasing heat-treatment temperature and holding time during the phase-separation process for the mother glass. The alkali resistance of the membranes was about 3.5 times higher than that of conventional SPG membranes.