Research On The Solid-liquid Separation In The Sub-melton Salt Process For Alumina Production

Due to the problem of the solid-liquid separation of leach slurry in the production for alumina by the sub-molten salt method,studies on the viscosity of sodium aluminate solution with high alkali concentration and the separation behaviors of the leach slurries produced with bauxite and the Bayer red mud by the Sub-molten Salt method have been carried out.The methods of improving separation behavior were put forward.The following achievements and innovative progresses were obtained:（1）The viscosity of sodium aluminate solution with high alkali concentration was systematically measured for the firs time.The effects of temperature,alkali and alumina concentration on the viscosity were studied.The empirical equation of the sodium aluminate solution viscosity suitable for the sub-molten salt system was obtained by experiments.（2）The filtration behaviors of leach slurry produced by the Sub-molten Salt process and the Bayer process with low lime were studied,and compared with that of the Bayer process.Results indicated that the leach slurry produced by the Sub-molten Salt process was the most difficult to be separated due to its high filtration resistance.The average cake resistance was as high as 2.76×10¹¹（Δp/p₀）⁰.52.52 m·kg^-1,and the dynamic viscosity of the filtrate was 2.5 mPa·s at 105°C.The Bayer process with low lime showed high performance on the exaction of alumina and the separation of red mud simultaneously.（3）The effects of lime and calcination for bauxite on the filtration behaviors of the leach slurry were investigated.The filtrate capacity of the leach slurry produced from bauxite by the sub-molten salt process was increased by 4 times with 12%wt lime.The filtrate capacity can be improved dramatically by roasting bauxite.The best conditions was determined by experiments.When the bauxite was calcined at 900°C for 1 h,the filtrate capacity can be improved to 1.17 m³·m^-2·h^-1.It’s the reason for improving the filtration behaviors that the impurity with colloidal properties was eliminated and the size of red mud was increased.（4）The solid-liquid separation behaviors of the leach slurry produced from the Bayer red mud by the Sub-molten Salt process was studied.When the pressure filtration was conducted,the highest filtrate capacity was 1.05 m³·m^-2·h^-1 in this paper.The average cake resistance was 1.64×10¹¹（Δp/p₀）^0.38 m·kg^-1.The best conditions for flocculation sedimentation were that the caustic concentration was diluted to 420 g·L^-1,and the setting temperature was 100°C.When the flocculant Nalco 85035 was used with the amount of 40 g·t^-1,the setting velocity was 1.33 m·h^-1 after 5 min.The suitable separation methods can be chosen based on the equipment leve and economic cost in industrial production.