Limitations of Diffusion Dialysis

A necessary condition of dialysis is that the solute concentration in the recovery stream must be lower than in the feed stream in order to provide a driving force for diffusion. This is not a real limitation in applications where the diffusing solute is a waste that can be easily discarded. But this condition can be a limitation when the diffusing solute is the desired product, because the product is often recovered at a low concentration. Fortunately, the acid from steel pickling solution can be recycled to the pickling bath at the concentration at which it was recovered. Another limitation is that the nondiffusing solutes are left in the original solution in a slightly diluted state, which means that the waste volume can be considerable.

The selectivity of diffusion dialysis membranes for rejecting metal ions is influenced by the ionic charge on the metal ion. Metal ions with multiple positive charge are rejected more efficiently than ions with a single charge. However, zinc and some other metal ions form complexes with the anions of the acid. In HCl solutions, zinc forms ZnCl3~ and ZnCll- complexes that behave as anions in the anion exchange membrane. These complex ions do not diffuse through the membrane as readily as Cl3 ions do, but they diffuse much faster than Zn2 + ions. However, zinc does not form a complex in H2SO4 solution, so zinc is rejected quite well in the sulfate system.

All of the halogens form complexes with some metals. Chloride complexes of Cu, Ga, Fe (ferric forms a much stronger chloride complex than ferrous), V and Zn have been reported. The existence of a chloride complex does not necessarily mean that HCl cannot be recovered from the metal salt by diffusion dialysis. Since the chloride complexes are rather bulky, they do not pass through the anion exchange membranes as easily as chloride ions do.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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