Effects of temperature and initial molar ratio of Na₂O to Al₂O₃ on agglomeration of fine Al(OH)₃ seed in synthetic Bayer solution

Abstract

Fine Al(OH)₃ crystals were aggregated from supersaturated aluminate solution in the batch reaction tanks. By means of laser particle size analyzer and scanning electron microscopy, the influences of temperature and initial molar ratio of Na₂O to Al₂O₃ (α _K) on agglomeration of fine seed in Bayer process were investigated. The results show that agglomeration is almost finished in 8 h, and seeds with size less than 2 μm are easily aggregated together, and almost disappear in 8 h under the optimal process conditions. In the aluminate solution with the same moderate initial α _K, when the reaction temperature reaches 75 °C, the secondary nucleation does not occur, and the effect of agglomeration is better. And at the same reaction temperature, when the initial α _K is 1.62, the initial supersaturation of aluminate solution is moderate, the binders on the surfaces of the seed are enough to maintain the agglomeration process, and the agglomeration degree is better. From SEM images, agglomeration mainly occurs in the fine particles, the combinations among the fine particles are loose and the new formed coarse crystal shapes are irregular.