Recovery of fine aluminum hydroxide with high whiteness index from low quality bauxite using caustic roasting and water leaching

Abstract

A bauxite of low quality containing 6.75% reactive silica and 37.4% Al2O3 as gibbsite was roasted with additives including NaOH (caustic/total alumina molar ratio of 2-4:1) and CaO (CaO/total SiO2 molar ratio of 0.5-3:1) at 300-1000 °C, producing calcines containing sodium aluminate (Na2O.Al2O3), silicates and hematite. Optimum conditions included roasting at 350-400 °C with CaO added at a CaO/total SiO2 molar ratio of 1:1 for 2-3 h. Dissolving the calcine in water at 90 °C produced a sodium aluminate liquor from which aluminum hydroxide (Al2O3.3H2O - alumina trihydrate or ATH) was precipitated. The addition of CaO reduced the silica extraction to <15% at optimum conditions, whereas the total organic carbon(TOC) was ∼200 mg/L as carbon in all cases. Losses of alumina and caustic were experienced, as different phases of Al-Ca-Si or Na-Ca-Al-Si minerals were formed during roasting and/or leaching. The extracted alumina in the leached solution were precipitated at 60 °C for 20 h to produce a fine aluminum hydroxide having D50 of 1.9-2.8 urn with a whiteness index of 96.8-97.5, better than commercially available products.