Caustic soda consumption is a significant production cost when high silica bauxite is processed in an alumina refinery. The authors have developed a new process to recover soda from desilication product (DSP) in bauxite residue (BR), which comprises the following steps: (1) leaching DSP by sulphuric acid to extract soda as sodium sulphate, (2) purification of the leached liquor and (3) splitting the sodium sulphate into caustic soda and sulphuric acid by electrochemical processing. The recovered NaOH is returned to the alumina refinery and sulphuric acid is recycled to the soda recovery leaching process.
Optimised acid leaching and liquor purification processes have been established for synthetic DSP, lime-free BR and lime-containing BR using sulphuric acid, without troublesome leaching of silica. A soda extraction extent of up to 80% was achieved by stoichiometric consumption of sulphuric acid to form sodium sulphate with DSP or lime-free BR. For the lime-containing BR, ∼1.6 times more acid was consumed in leaching because of lime-acid reactions. The resultant liquor contained 50∼120 g/L Na2SO4 and <10 mg/L (each) of impurities (Al, Si, Ca, Mg, Fe, Ti).
Sodium sulphate solution (250 g/L), as model (recycled) liquor was dialysed by two-compartment system. 160 g/L NaOH was recovered with electric power consumption of less than 1500 kWh/t-NaOH.
The leached residues after soda recovery showed a low and stable pH and, of course, lower soda. This is expected to be of great benefit to long term residue management.