Diffusion dialysis is used in the metal-processing industry for the regeneration of used pickling acids. It replaces the retardation process - an ion exchange process - where high acid losses and insufficient separation of the metal salts make retardation uneconomical.

In the diffusion dialysis process, ion exchange membranes are used in the form of a membrane stack, separating two different liquids: the used acid and demineralised water. The concentration gradient between the two solutions is the driving force of the process. The special anion exchange membranes with positive surface charges in their polymer structure, facilitate the diffusion of dissociated acid (anions) through the membrane, wheras the cations (metals are retained by their positiv charges. Hydrogen ions are the exception to the rule here; their slight size means that they can pass through the membrane together with the anions.

The membranes are installed in a membrane stack similar to a filter press. Membrane frames positioned between the membranes form cells with the corresponding spacings and ensure that the membranes are propper supported. The demineralised water and the used acid pass through the cells alternately in counterflow. The demineralised water adopts the free acid, and the resulting diffusate is returned to the pickling process. The other by-product - the dialyzate - is usualley fed to a neutralization process. In certain cases, this by-product can also be reused or retreated.


For years, the diffusion dialysis has been being used with great succes for the recovery of sulphuric acid, hydrochloric acid, nitric acid and hydrofluorid acid. It is possible to extract 80 - 85 % of free acids from used pickling from used pickling solutions and to recycle these to the pickling process in a purifed form with approx. 5 % metal contamination.

dd schema

In practice, the filtered, used acid and water is pumped into two head tanks from where they flow through the membrane stack.

Flow meters and level indicators function as the necessary control elements for the individual tanks. In this way, the process can be easily automated so that constant control by operating staff ist not necessary.
Proven aplications of the process

  • Rercovery of mixed acids from stainless steel pickling processes (HF/HNO3)
  • Rercovery of H2SO4/HNO3 or H2SO4/HCl from pickling plants for nonferrous metals, such as zinc and aluminium
  • Rercovery of  von H2SO4, HNO3 und HCl from steel- and nonferrous metal pickling plants     

Advantages of the process

  • Very low energy requirement
  • Drastic reduction in fresh acid requirements
  • Drastic reduction inneutralization and landfill costs
  • Considerable reduction in pollutant freight
  • Fully automatic operation
  • Very low maintenance costs
  • Long membrane service life
  • High economic efficiency
  • Short amortization period