Ion Exchange Membranes-based DD, ED And EED Processes For The Separation Of Sodium Aluminate Solution

Alumina is produced mainly through the Bayer process, in which large amounts of alkaline solution, i.e. sodium aluminate solution containing NaOH and NaAl(OH)4, are produced. At present, the sodium aluminate solution is treated mainly by seeding and stirring procedures to form Al(OH)3 precipitate, and then the supernatant can be used as the mother liquor in the Bayer circuit. However, the high concentration of NaOH in the sodium aluminate solution seriously lowers gibbsite growth rate and decreases production efficiency in the precipitation process. Besides, the purity of NaOH in the mother liquor is not high, which is disadvantageous for its circular usage. To resolve the above problems, ion exchange membranes-based diffusion dialysis (DD), electrodialysis (ED) and electro-electrodialysis (EED) are proposed for separating sodium aluminate solution before seeding and precipitation procedures, so that the concentration of NaOH in the solution can be decreased. The gibbsite growth rate and production efficiency, as well as the purity of NaOH in the mother liquor can then be enhanced. Firstly, in DD process, the Na+ions can be transported through cation exchange membrane (CEM) from feed to water compartment, and the OH- ions are also transported to keep electric neutrality of the solution. However, the Al(OH)4-ions can be retained by the CEM due to its large size. Hence, the NaOH and NaAl(OH)4 in sodium aluminate solution can be separated. Secondly, in ED process, the Na+ions can be migrated through CEM from the feed to the recovery solution under the direct current, while both the OH-and Al(OH)4-ions can be migrated through AEM. Nevertheless, the Al(OH)4-ion is of larger size and lower diffusion activity (1/3 the value of OH-), and its migration is more difficult. Accordingly, it is feasible to separate sodium aluminate solution via ED. Lastly, in EED process, H2O is hydrolyzed to produce OH-ions. The Na+ions in the feed in the anode compartment can be migrated through CEM to the cathode compartment and combine with the OH- ions, so that NaOH is produced and recovered. This dissertation investigates the effect of different membrane processes and experimental parameters on separation efficiency. It comprises five chapters, and the contents are detailed as following.Chapter 1 begins with a brief introduction of alumina production through the Bayer process. Then the deficiency of the Bayer process is also pointed out, for which ion exchange membranes-based DD, ED and EED processed are proposed. Lastly, the principle and application of each process is introduced in detail, and the feasibility of the sodium aluminate solution separation is also evaluated.In chapter 2, sodium aluminate solution is separated through DD and ED. The effect of operating time on DD performance and the effect of feed concentration and current density on ED performance are investigated. Besides, the membrane fouling during the separation process is studied. The results show that after 4 h of DD operating no obvious membrane fouling is observed, Al(OH)4-leakage ratio (ηAl(OH)4) is as low as 0.6% and the caustic ratio (αk) of the recovery solution is high (17.8). But the alkali recovery ratio (ηOH-) is relatively low (6.3%) since the driving force is concentration difference rather than pressure or electric potential difference. In ED process,ηOH- is enhanced significantly. When the OH-concentration in the feed chamber is～1.5 mol/L and the current density is 350 mA/cm2, the ηOH- can reach up to 48.0%; the ηAl(OH)4- and αk are 12.6% and 6.3, respectively, and can meet the needs of industrial production; the current efficiency and energy consumption are 62.5% and 12.43 kW h/kg, respectively. Overall, the DD process has advantages of low energy consumption, low membrane fouling and environmental friendliness, but has a disadvantage of low ηoH-; while the ED process is more feasible to separate sodium aluminate solution in industrial production. The reason is that ED process has high treatment capacity and can achieve a high ηOH- in short-term. Nevertheless, the ηOH- is still not sufficient, and the energy consumption is relatively high in above experiments. Besides, repeated batch experiments (RBEs) are needed for investigating stability of the ED process systematically. Hence, a further study is carried out in subsequent sections.In chapter 3, firstly, the ED membrane stack configuration (MSC) is optimized to achieve high ηOH- and low energy consumption. Secondly, RBEs are carried out to investigate the effect of erosion on membrane stability. And the performances of self-prepared AEM AM-QP-30 and commercial AEM FQB are also investigated. Lastly, an acid method is proposed to eliminate the membrane fouling during the RBEs. The results indicate that the optimal MSC is three repeating units stack; the membrane fouling can be effectively eliminated by the acid method; and the ED performances are excellent and stable. For instance, the alkali recovery ratio (ηOH-) can reach up to 64.9-68.5%, and the energy consumption can be reduced to 7.29-7.65 kW h/kg. The performance of AM-QP-30 membrane is superior to commercial FQB membrane. As a result, a stable ED process with high separation efficiency can be achieved after the optimization of MSC, membrane and experimental conditions.Sodium aluminate solution can be separated directly and rapidly via ED process, but the ηAl(OH)4- will be increased rapidly in the latter stage of the separation. Hence, it is critical to control the ηAl(OH)4- for achieving a higher ηOH-. In contrast, the separation rate of EED process is relatively low since hydrolysis of H2O is needed to produce OH-. But EED process has an outstanding merit, i. e. zero ηAl(OH)4-. Therefore, EED can be used for further separation of the solution, so as to enhance ηOH-. Accordingly, in chapter 4 a coupling process of ED and EED is proposed to separate NaOH from sodium aluminate solution to achieve high separation efficiency. This chapter begins with a study of single ED or EED process. The effects of current density, membrane category and effective membrane area on separation efficiency are investigated. The main results are listed as following. The optimal membrane is CMV/AMV. The optimal current density of ED and EED are 45-60 and 30 mA/cm2, respectively. Then the coupling process is carried out to optimize the ED operating time and the ED current density. For instance, the ED operating time is optimized as 120 min when the ED current density is 60 mA/cm2. Under these conditions, the ηOH- can achieve a high value of 90.9%, the ηAl(OH)4 decreases to a low value of～5% and the energy consumption is only 2.25 kW h/kg. Accordingly, high separation efficiency and low energy consumption can be achieved by the coupling process. If the coupling process is applied for the separation of sodium aluminate solution produced in the Bayer process, the gibbsite crystal growth rates and the yield of Al(OH)3 can be enhanced substantially, which is advantageous to the enhancement of the production efficiency. Besides, the abstracted NaOH solution with high concentration and purity can be directly used as the mother liquor for circuit usage and the production cost can be saved.The chapter 5 is a summary of the full text of this dissertation. The ion exchange membranes-based DD, ED and EED processes are feasible for the separation of the sodium aluminate solution, and high separation efficiency can be achieved in experimental research. Further studies are needed for their scale-up application, including scale formation on the membrane, increase of the membrane process stability, reduction of the energy consumption, and enhancement of the treatment capability. Besides, the membrane fouling of the complex organic and/or inorganic components in the Bayer liquors should be decreased to improve the IEM processes performance.