Effect of temperature and alumina/caustic ratio on precipitation of boehmite in synthetic sodium aluminate liquor

In Bayer process gibbsite (alumina trihydrate, Al2O3·3H2O) is precipitated from sodium aluminate liquor for production of alumina by calcination. Precipitation of boehmite or alumina monohydrate (Al2O3·H2O) is an alternative method for production of alumina. Boehmite is also a good precursor material for production of different activated aluminas. At present this boehmite precipitation process is in the early stage of development. This process if developed would reduce the energy consumption for making alumina. The present work is aimed to carry out boehmite precipitation under atmospheric pressure conditions. Parameters such as alumina/caustic (A/C) ratio and temperature are varied to precipitate out boehmite from synthetically prepared sodium aluminate liquor. It was found that boehmite precipitation could be possible at normal pressure only when boehmite seed was added to the supersaturated sodium aluminate solution keeping the alumina/caustic ratio at either 1.1 or 1.0 and temperature at ≥ 85 °C. By decreasing the A/C ratio to 0.95 and below, it was possible to precipitate out boehmite at 60 °C.