Mentioned in the previous article we can use membrane electrolysis method to remove the free alkali in sodium tungstate. Next, let's take a look at another kind of new method--ion exchange membrane diffusion dialysis method. Membrane separation technology is a contemporary new chemical separation technology. Compared with conventional separation methods, membrane process has the advantages of low energy consumption, high single-stage separation efficiency, simple process and no pollution. It is the important advanced technology to solve the problem of contemporary energy, resources and environment. What’s more, it will have a far-reaching influence on the industry technical transformation for the 21st century. The basic principle of diffusion dialysis method recycling free alkali from crude sodium tungstate solution is showed in the figure below.

Diffusion dialysis method to realize the separation of salt and alkali is based on the concentration of a solution of membrane on both sides of the poor, such as from sodium aluminate solution by diffusion dialysis method of recovery of sodium hydroxide, the process does not need to plus direct current (dc) to provide the driving force, the dealkalization energy consumption should be lower than the membrane electrolysis method. So the diffusion dialysis method of removing free alkali from coarse sodium tungstate solution, for further reducing the consumption of tungsten smelting process is promoted. Using cation exchange membrane, membrane on both sides of the sodium tungstate solution and recycle fluid respectively. By dry crude sodium tungstate solution of sodium ion concentration high recovery of sodium ion concentration in liquid, under the effect of concentration, sodium ions will be through a cation exchange membrane, migration from crude sodium tungstate solution to recovery liquid. For small hydroxyl ion radius, high motion mobility, with sodium ions migrate to recycle chamber, so as to keep a neutral solution. While tungsten acid radical ion radius is bigger, basically do not transfer or rarely transfer, thus to realize the separation of sodium hydroxide and sodium tungstate. Technology free alkali concentration of sodium tungstate solution can be dropped to a lower level by using diffusion dialysis. Therefore, the ion exchange membrane diffusion dialysis method can effectively removing free alkali from sodium tungstate solution.

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