Reactive phase behavior of aluminum nitrate in high temperature and supercritical water

Batch hydrothermal reaction of Al(NO₃)₃ in water was investigated at temperatures up to 421 °C and pressures up to 850 MPa by direct visual observation of reactive solutions in a 50-nL microreactor. A number of gelatinous phase transitions were observed that seem to play a key role in boehmite formation. At high Al(NO₃)₃ concentrations (0.32 M) and high densities (1060 kg/m³), high pressure induced formation of a gel-like phase and the phase formed seemed to be much less reactive than solutions at lower densities (400-670 kg/m³). At densities closer to the critical density of water, as heating occurred, the gelatinous material precipitated and redissolved into solution prior to intense precipitation around the apparent critical point. A model for the aluminum hydroxocomplexes in solution was developed which showed that cationic species were the main species present at high-density conditions. Nitrate ion seems to play a key role in the gel-like phase formation and boehmite particle production.