The Study Of Remove Aluminum Of Na₂CO₃in The New Process Of Alumina Production By Alkali-dissolution And Carbonization-precipitation

The process for production of alumina from bauxite ores by membrane electrolysis of sodium carbonate is a new process. The new process is not limited by the equilibrium of Na2O-Al2O3-H2O system in Bayer process and the production efficiency was therefore enhanced by110%. It has the advantage of low energy consumption and would be more potential to produce the qualified gibbsite. But the purity of resultant sodium carbonate solution is critical for membrane electrolysis to efficiently produce sodium hydroxide and bicarbonate. To reduce electricity consumption and increase the durability of cation ion-exchange membrane, metal element impurities in the resultant sodium carbonate solution are required to be as low as possible.In this study, the test methods the aluminum concentration in sodium carbonate solution were investigated. The influence of aluminum concentration in sodium carbonate solution on the membrane electrolysis process was also studied. Both the methods of deep carbonate decomposition and electrosorption of aluminum from resultant sodium carbonate solution on activated carbon fibers (ACFs) were investigated. The main conclusions are as follows:1. Graphite furnace atomic absorption method is not suitable for the determination of aluminum content in sodium carbonate solution, Chromazurine S spectrophotometric method can be used to determination of sample with aluminum content greater than2.7μg·L-1in sodium carbonate solution.2. Increase in aluminum concentration would cause an increase in the voltage, decrease in current efficiency and increase in aluminum content on ionic membrane. When aluminum concentration reduced to0.1mg L-1, it has little influence on the membrane electrolysis.3. During the decomposition of the sodium aluminate with sodium bicarbonate, the aluminum can be reduced to1mg·L-1by carefully controlling conditions including the dropping speed of NaHCO3, rotate speed and temperature.4. The adsorption of aluminum on ACFs can be enhanced by anodic polarization. The adsorption follows the first-order kinetics and the adsorption rate constant is linearly related to voltage. The adsorption fits the Langmuir isotherm better than the Freundlich and indicated that the adsorption occurs mainly in monolayer. Increasing aluminum concentration, increasing pH of the solution and decreasing sodium carbonate concentration resulted in greater amount of aluminum adsorbed during the electrosorption process. And the aluminum concentration can be reduced to0.3mg L-1by this method.