Application Of Membrane-Electrolysis In A Novel Alumina Process Consist Of Kali-Bleaching And NaHCO₃ Decomposing

A recently proposed alumina process in which bauxite is bleached by kaline but the NaAlO2 solution is decomposed by NaHCO3 is thought to increase the production twice. And membrane electrolysis of the Na2CO3 solution resulted from decomposition into NaOH and NaHC03 is the key technology for accomplishing the novel alumina process. This thesis focused on the problems happened in membrane-electrolysis of Na2CO3 solution and the efforts to solve them.First of the problems is that the resulted Na2CO3 solution after decomposition must be throughtly purified before entering into the electrolysis cell, for prolonging the life of ion-exchanging membrane and decreasing the cell voltage. Secondly, the electrolysis conditions, such as the material of anode and cathode, the concentration of electrolytes, temperature and current density, should be optimized to obtain the lowest cell voltage. Thirdly, the whole process including kali-bleaching, decomposing NaAlO2 by NaHCO3, and electrolysis of Na2CO3 into NaOH and NaHCO3 should be connected with each other in larger scale. The study results demonstrated that the concentration of impurities of Al, Ca, Fe, Mg, Si in the decomposition solution could below 3 mg/L after being purified by a sequence treatment:pre-filtering, advanced decomposed by NaHCO3,0.25μm filtering and finally ion-exchanging. Selection of electrodes and optimization of conditions were done on a cell with electrode area of 120cm2. The whole process was tested on a larger cell with electrode area of 1100cm2 under conditions as following:Ti-Ir anode, Ni-Ru cathode,1.5M purified Na2CO3 solution as anolyte,5.5M NaOH solution as catholyte, the temperature is kept at 70±5℃. The whole process was continuously operated for 5 days, the cell voltage was 2.53V and the average current efficiency was 97%.