Diffusion dialysis is a membrane separation process used to regenerate acids or bases, for example.
The process is based on two different fluids flowing through two channels which are separated by a semi-permeable membrane.
In a simplified way, mass transport occurs from concentrated to diluted fluids until the concentration difference has been balanced out. With a selective membrane, certain components can be "separated out" from the concentrated fluid by increasing the resistance for other components.
The special feature of acid diffusion dialysis is the membrane used.
If acid is to be recovered from old process baths, an anion exchange membrane is used, through which only anions (negatively charged ions) can diffuse. Since the acid cation H+ is very small, it is carried along in anion diffusion, while the much larger metal cations are withheld. This selective membrane keeps two channels separate.