Diffusion dialysis is a membrane separation process. It has been successfully used for many years for the separation and recovery of acids from dissolved metal-bearing solutions. Diffusion is the spontaneous movement of a material from an area of high concentration to an area of lower concentration. Driven by the concentration difference, the movement of material will continue on its own until the concentration difference no longer exists. Dialysis is the separation of molecules due to the differences in the rate of movement of the molecules through a semi-permeable barrier.

In the recovery of acids with diffusion dialysis, an anion exchange membrane acts as a semi-permeable barrier placed between a flowing water stream and a flowing acid with dissolved metal solution. The anion exchange membrane has fixed positive charge located on its surface. These positive charge locations attract the negatively charged anions in solution that come in close contact with the anion exchange membrane surface.

In the case of sulfuric acid anodize baths the overwhelmingly predominant anion is the sulfate ion, SO4. As these sulfate ions in the sulfuric acid anodize solution are attracted to the membrane they are also driven by the concentration difference to diffuse across the membrane to the water side.

Simultaneously, the thermodynamic Law of Electroneutrality (in solution total charge must balance to zero) requires that the transference of every sulfate ion, which carries two negative charges, be accompanied by the transference of two positive charges.

Positively charged ions, such as Al +3 or other metal ions, are strongly inhibited from crossing the positively charged membrane because of the repulsion be­tween like charges. The hydrogen ion, present in the acid solution as H3O +1 ions, or protonated water, is also positively charged, but is able to cross the membrane with very little hindrance. This is due, in part, to the high concentration of hydrogen ion in the acid solution and also, in part, because of the highly associated nature of water, which allows the hydrogen ion to effectively delocalize its charge.

The net effect is that the rate of diffusion of an acid across the membrane is an order of magnitude greater than that of the dissolved metal. Finally, by causing the flow of the acid solution to be in the opposite direction to the flow of water (counter-current flow), optimal advantage of the necessary con­centration gradients can be realized. The results are that the water entering the diffusion dialysis system exits as a metal-depleted recovered acid solution and that the acid solution entering the diffusion dialysis system exits as an acid-depleted dissolved metal-bearing solution.

After desired results are achieved, little maintenance or no maintenance at all is required on the unit. The metering pumps are typically automatically operated 24 hours per day, seven days per week. The system has built in alarms to prevent overflowing of the liquids in the holding chambers at all times. We advise inspecting the filters and replacing them as often as necessary. If the bath has a lot of particulates it may make sense to replace the filters more often. Mech-Chem Associates offers a full line of replacement and spare parts.

The membrane life in a typical diffusion dialysis system from Mech-Chem is between 5-10 years: 5 years for strong, oxidizing acids such as nitric and hydrofluoric, up to 10 years for hydrochloric and sulfuric acids.

If the acid chemistry is the same, then one unit can be plumbed to multiple baths. It is not recommended to switch from one type of acid to another, such as hydrochloric to nitric. This is because the membrane acts like ion-exchange resin and will retain the dissolved acids on the membranes unless a special cleansing step is undertaken.

Mech-Chem Associates offers complete design, build, installation, and start-up of diffusion dialysis systems, specifically designed to meet your needs.