Supercritical ethanol drying of zinc borates of 2ZnO·3B 2O 3·3H 2O and ZnO·B 2O 3·2H 2O

Abstract

Supercritical ethanol (SCE) drying of zinc borates was investigated to obtain dry zinc borate nanoparticles and to elucidate possible chemical interactions between zinc borates and ethanol. Commercial and synthesized 2ZnO·3B 2O 3·3H 2O and ZnO·B 2O 3·2H 2O samples were dried by both conventional and SCE drying methods and the products were compared. Zinc borates were characterized using X-ray powder diffraction (XRD), thermo gravimetric analysis (TGA), Fourier transform infrared (FTIR) analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and helium pycnometer. Zn and B contents of samples were also determined by analytical titration. It was found that zinc borate of ZnO·B 2O 3·2H 2O decomposed completely into zinc oxide and boric acid. However, zinc borate having oxide formula of 2ZnO·3B 2O 3·3H 2O decomposed partially to form anhydrous zinc borate, zinc oxide, water and boric acid during the SCE drying carried out at 250 °C and 6.5 MPa. Boric acid and water were extracted from both of zinc borates by supercritical ethanol. The extraction of boric acid by ethanol in the SCE drying of zinc borates is an important result which can be utilized in the production of boric acid from water insoluble natural minerals, e.g. colemanite. The boric acid–ethanol solution could be directly utilized in the synthesis of ethyl borate.