| Diffusion dialysis is a spontaneous membrane separation process drivern by concentration gradient. Due to its simple operation, low energy consumption, without secondary pollution and so on, diffusion dialysis has been exploited to extract various acids and alkalies from waste solutios in steel production, titanium white, rare earth industry, as well as tungsten industry. In addition, because it is easily to be integrated with other processes, there are many application examples of the integration, and they learn from other’s strong points to offset one’s weakness to achieve better effect. What is more, due to the rapid development of social economy and the stong environmental protection consciousness, diffusion dialysis has been paid more and more attention. Although, diffusion dialysis has been widely used and developed, but with the increased application, some shortages appeared. Aiming at the shortages, this paper will study on the relevant theory and application of diffusion dialysis process.This dissertation consists of seven chapters.The1st chapter carrys on the brief overview of chemical industry, membrane separation technology and so on, also summarizes the diffusion dialysis process, mainly including:the principle, ion exchange membrane, membrane module, theoretical model and application field as well. Then leads to the deficiency of diffusion dialysis and starts this paper.The2nd chapter treats of the application of diffusion dialysis to deal with simulated chemosynthesis aluminum foil wastewater. The results show that the acid recovery ratio and aluminum leakage ratio decreased with an increase in the flow rate intensity, and an appropriate flow rate intensity ratio (diffusate:dialysate) is found to be approximately1.0. The HC1concentration in feed is observed to have little influence on the diffusion performance, while the acid recovery ratio and aluminum leakage ratio increase as the AlCl3concentration in feed increases. More than95%HCl are recovered from the simulated wastewater which contains2.12mol/L HCI and0.8mol/L AlCl3at the flow rate intensity of0.48L/(m2·h). When compares with plate-and-frame diffusion dialysis membrane module, the sprial wound diffusion dialysis membrane module has a higher acid recovery ratio, lower metal leakage ratio, higher loading density, and similar time to reach the equilibrium. Preliminary economic evaluation reveals that an investment in this process could be recovered within16.5months.Chapter3deals with the integration of diffusion dialysis and conventional electrodialysis to recover hydrochloric acid from simulated chemosynthesis aluminum foils. Results shows that the integration of diffusion dialysis and conventional electrodialysis is a feasible and effective approach to recover the hydrochloric acid. The dialysate flow rate and conventional electrodialysis current are adjustable to achieve a compatibility and operational uniformity between diffusion dialysis and conventional electrodialysis. When the dialysate flow rate is0.60L/(m2·h), ti is10min, and conventional electrodialysis current is2A, the average acid recovery ratio and average aluminum leakage ratio are74.9%and12.2%respectively, while the energy consumption is only0.41kw·h. The results confirm that such integration process is not only a cost-effective process compared with an individual diffusion dialysis process, but also an environment friendly process with little water consumption. Especially, due to the concentrating by conventional electrodialysis, the recovered acid can be reused in the productive cycle directly.The focus of the4th chapter is adopting the additional weak-electric field to improve the performance of diffusion dialysis process. Firstly, mode of applying electric field is discussed, results show that the mode that anode contacted with dialysate compartment is more effective, and results are explained through Nernst-Plank equation. Secondly, effect of electric field on odd pieces of membranes DD is studied, results illustrate that the more the pieces of membranes, the weaker the effect of electric field on DD performance. Thirdly, effect of electric field on even pieces of membranes DD is investigated, results reveal that overall dialysis coefficient has almost nothing to do with electric field. At last, energy consumption and current efficiency are investigated, and differences between electrically assisted diffusion dialysis and electro-electrodialysis are stated. But in industrial production, DD membrane stack usually contains many repeating units composed of many pieces of membranes, so it can come to a conclusion that EADD has some limiting factors and bottlenecks in practical application. However, EADD, with novel functions, has good prospects for development. And more efforts need to be contributed to bring this process to sustainable practice.The5th chapter disposes the quantification of diffusion dialysis process with single NaCl solution. Firstly, the volume of water which permeates from water side to feed side is quantified detailedly, and results show that diffusate solution volume changes over time according with the characteristics of quadratic polynomial (y＝ax2+bx+c). Secondly, Permeability coefficients of membrane to H2O (PH2O、P’H2O) and NaCl (PNaCl P’NaCl) are used to quantify diffusion dialysis process. Results show that with the increase of feed concentration, P’NaCl and PNaCl decrease, while P’H2o and PH2O increase. Furthermore, when modifying factor λ=3.30and0=2.37, comparisons of theoretical and experimental concentrations of NaCl in diffusate solution illustrate that both P’NaCl and P’H2O are satisfactory coefficients for characterizing the diffusion dialysis process. In addition, the mathematical model discussed in this study can provide an effective method to deal with the mass transfer process of diffusion dialysis easily and conveniently, especially, is helpful to select an optimum ion exchange membrane to operate diffusion dialysis.The6th chapter handles the quantification of diffusion dialysis process with the single HCl solution. Firstly, the volume of water which permeates from water side to feed side is quantified detailedly, and relevant equation is used to fit the variation curve of water volume; chemical potential difference of solution is used as the mass transfer driving force in the transformative Fick’s laws, results show that as the HCl concentration increases, PHCl and PH2O increase.The7th chapter summarizes the chapters above and hopes for prosperity of diffusion dialysis.In conclusion, this paper is basic research, and there is much to do before putting diffusion dialysis to industrialization. Other than scaling up the experiments of practical feed, more efforts have to be taken to collect the strength from the government, academia and industry. It is necessary to improve the recovery performance and treatment effeciency by developing the new excellent ion exchange membranes and membrane modules. In view of the inherent characteristics of diffusion dialysis process, we believe that the technology will be developed better. |
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