Diffusion dialysis

The term and process, „dialysis“ are closely linked to the most important medical use of membranes for the treatment of renal failure, namely kidney dialysis or haemodialysis. In this process, toxins and body waste are transported, by a concentration gradient, out of the blood through a membrane into an isotonic salt solution. The membranes used in this case have open pores. In diffusion dialysis, the potential of a concentration difference is also instrumental for mass transfer but pore-free ion exchange membranes are used in this case. Hence, free strong acids (acid dialysis) can be recovered by using anion exchange membranes and free strong bases (base dialysis) by using cation exchange membranes.

The dialysis process is applied for the recovery of free acids or bases from spent treatment bath solutions in surface finishing and textile processes. The spent bath solution is separated in a chamber in counter-current flow and via an ion exchange membrane and is then routed to a second chamber through which water flows. The free acid or base is able to permeate while the salts are retained. Depending on the process design, up to 95% of the free acid and up to 80% of the base can be recovered in this way and returned to the production process, thus saving chemicals needed for the finishing baths or wastewater treatment. However, the important advantage is that process efficiency in production is increased. The individual parameters for the bath solutions are stabilised which results in a constant uniform effect of the bath solution on the workpieces. These aspects have particular significance for example in anodising baths (surface optics) or in alkaline baths for the chemical milling of aluminim.

New membranes, which have high chemical and thermal resistance, have been developed for use in acid dialysis, thus allowing troublesome process solutions to be treated also. When applied at industrial level, these membranes can be easily cleaned because of their improved resistance to oxidation as well as their high chlorine tolerance.

New membrane modules combined with highly permeable membranes have been developed to reduce investment costs. In addition to these, hybrid processes combining diffusion dialysis and nanofiltration have been developed for operations with a low salt concentration and high volume flows, hence reducing investment costs effectively. Typical applications for this hybrid process can be found in acid as well as base recovery.