Abstract:The stability of calcium oxalate is critical for the removal of sodium oxalate from sodium aluminate solutions. This study investigated the behavior of calcium oxalate in sodium aluminate solution containing sodium carbonate. Results show that calcium oxalate can be converted to tricalcium aluminate hydrate (TCA) and calcium carbonate in sodium aluminate solution and sodium carbonate solution, respectively. Elevating temperature, extending residence time, or increasing caustic soda concentration enhances the conversion ratio of calcium oxalate in sodium aluminate solution; as a consequence, anti-causticisation occurs. Stability of calcium-containing compounds in sodium aluminate solution containing sodium carbonate differs from that in sodium aluminate solution or sodium carbonate solution. Na2CO3 in aluminate solution accelerates the transformation of calcium oxalate; thus, alumina is lost because of 4CaO·Al2O3·CO2·11H2O and TCA formation. Calcium carbonate, 4CaO·Al2O3·CO2·11H2O and calcium oxalate can change into TCA in sodium aluminate solution at elevated temperature. Calcium oxalate remains relatively stable in dilute aluminate solution within a short residence time at low temperature. Thus, a novel process for removal of sodium oxalate by lime causticisation was presented and employed in an alumina refinery in China.

Key words: calcium oxalate; sodium aluminate solution; sodium carbonate; lime causticisation; activity coefficient