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How it works

Diffusion dialysis is a membrane separation process suitable for the recovery of free acids and bases from electrolytic solutions or to selectively replace ions in such a solution. Spiraltec GmbH is the only company in the world offering spiral-wound membrane modules suitable for countercurrent flow operation.

Driving force of the separation process can be regarded to the concentration gradient between two different fluids. Mass transfer takes place from concentrated to diluted fluids. A selective membrane can be used to "extract" components from the concentrated fluid by increasing the resistance for other components.

For high recovery rates, feed solution (e.g. a mixture of acid and metal salts) and the deionized water are flowing in countercurrent mode, similar to a heat exchanger.

To guarantee the performance of diffusion dialysis in electrolytes, an ion-selective dense membrane can be used. Thanks to a cooperation with FUMATECH BWT GmbH, in place since 2015, Spiraltec is the exclusive partner for the latest generation of ion-conducting flat membranes. Through this cooperation, we can rely on the support of FUMATECH BWT GmbH also for all our R&D projects regarding membranes.

Advantages of the process in treatment
of process solutions

Very low energy consumption
Highest separation efficiency for metal salts
Highest salt concentration in waste water
Lowest amount of acid in waste water
Large reduction in acid/alkali demand
Very low neutralization and landfill costs
Constant bath conditions

Advantages of spiral wound membrane modules in diffusion dialysis applications

Low maintenance times when replacing membranes
Low space requirement due to high packing density
Highest transport rates in relation to the installed membrane area


Acid diffusion dialysis uses an anion exchange membrane to separate two channels. For efficient acid recovery, feed solution (from the process) and deionized water are passed through the channels in countercurrent flow. Because of the positive surface charge of the membrane, acid anions can diffuse through the selective membrane, and are then absorbed by the water. Due to the very small size of the acid cation H+, it is entrained in the anion diffusion, while the much larger positively charged metal ions are retained. The spiral membrane module WD achieves recovery rates of up to 95% of the free acid with metal retention of up to 99%.


In caustic diffusion dialysis, a cation exchange membrane is used. For efficient caustic recovery, the feed solution and the deionized water are also passed in countercurrent flow. The caustic cations can diffuse through the selective membrane due to its negative surface charge. Due to the small size of the leach anion OH-, it is entrained in the cation diffusion, while larger anions are retained. It has been shown in practice that recovery of caustic solutions with cation exchange membranes works, even though the OH- ions are much larger than the H+ ions that are co-diffused during acid diffusion dialysis. However, the diffusion rates are lower than in acid diffusion dialysis. In caustic recovery, the spiral membrane module WD achieves up to 70% recovery of free caustic with simultaneous salt retention of up to 99%.

Overview fields of application







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