Removal of carbonate and oxalate pollutants in the Bayer process using thermal and chemical techniques

• Removal efficiency of carbonate and oxalate pollutants has been increased using thermal and chemical techniques.
• Ba2Al2O5, Ba2Al4(OH)16 and BaAl2O4, calcium aluminate (Ca3Al2(OH)12) and (Na3PO4)-lime were separately used.
• The effects of temperature, time, caustic soda, Al2O3 and carbonate were investigated on the pollutants removal efficiency.

In the Bayer process, diasporic bauxite is dissolved in a caustic soda solution in the presence of lime at high temperature. During this process, concentration of carbonate and oxalate gradually increases causing problems in the alumina refining process. In this work, removal efficiency of the carbonate and oxalate pollutants from sodium aluminate solution has been increased using thermal and chemical techniques. The pollution of sodium aluminate solution has been reduced by heating the bauxite and removing the carbonate and oxalate by applying a chemical method. Bauxite heating leads to the organic compounds and carbonates in magnesite and calcite minerals exit as CO2, preventing addition of these substances into the sodium aluminate solution. In the method of chemical removal of the carbonate and oxalate soluble in sodium aluminate liquor, barium aluminate with structures of Ba2Al2O5, Ba2Al4(OH)16 and BaAl2O4, calcium aluminate (Ca3Al2(OH)12) and sodium phosphate (Na3PO4)–lime were separately used. As a result, these substances were sedimented as carbonate or barium oxalate, calcium and apatite. The results also showed that soda was retrieved as sodium aluminate or sodium hydroxide. Investigating the effect of temperature, time, concentration of caustic soda, Al2O3 and sodium carbonate revealed that using Ba2Al4(OH)16 and the heating methods are the most efficient technique to remove the carbonate and oxalate, respectively.