IN-SITU REMEDIATION OF BAUXITE RESIDUE (BR) BY SULFURIC ACID NEUTRALIZATION AND BIPOLAR-MEMBRANE ELECTRODIALYSIS PART 1: PH REBOUND AND PHYSICAL PROPERTIES OF NEUTRALIZED RESIDUE

The authors acquired a process idea of in-situ remediation of BR by integrating bauxite residue storage area (BRSA) with CSIRO’s soda recovery process (WO 2012/145797), which comprises the following steps: (1) neutralizing BR with sulfuric acid (sodium of DSP is extracted as sodium sulfate into the leachate), (2) sending the neutralized BR (NBR) slurry to BRSA equipped with under-drainage system after the solid-liquid separation, (3) washing the deposited BR with rainfall, (4) collecting liquors from solid-liquid separation step, runoff and drainage, (5) purifying the collected liquors, (6) splitting sodium sulfate in the liquor into caustic soda and sulfuric acid by bipolar-membrane electrodialysis. The recovered caustic soda is returned to the alumina refinery and the sulfuric acid is recycled to the BR neutralization step. If this process works well, it is expected that the in-situ neutralization of BR will be applied economically to all refineries processing bauxites from low to high silica in composition without causing contamination at the refineries by impurity sulfate in the returned liquor.

Rebound phenomenon of pH during neutralization of BR with sulfuric acid was studied to determine the optimum pH at neutralization. It can be concluded that neutralization at pH 4.5 is optimum to make the NBR in the range of pH 6 to 8 for the safe and easily re-vegetated BRSA achieving maximum recovery of sodium from BR. About 60% of sodium in BR was recovered as sodium sulfate by the neutralization. Water permeability was measured by column test. It was observed that the water permeability of BR did not change by neutralization at the pH range of 4~6 but significantly deteriorated at pH <3. It was found that NBR was amended by washing from extremely saline to slightly- or non-saline soil (EC1:5<1mS/cm) with pH ~8. These results suggest that the proposed process can be feasible for in-situ remediation of BR. Further studies of BRSA such as consolidation of NBR and properties of runoff and drain recovered from BRSA, which will be strongly affected by the local weather conditions, are required.