The ZIX-Zak ion Exchange Process

Revolutionary water recovery solutions

When the ion exchange plant is run sequentially i.e. first through all the catex CSTR’s and then through the anex CSTR’s the level of free H⁺ in the final catex CSTR is high enough to interfere with the complete absorption of sodium ions and that stops all the Na⁺ being removed from the aqueous phase. This results in the weak-based anex resin not being able to remove the equivalent amount of anions and the final water then contains a residual amount of least exchangeable ions, usually sodium and chloride – sometimes as high as 800 mg/ltr.


To remove the build-up of H⁺ in the catex line the acid radicals HCl and HSO₄⁻ are removed by routing the aqueous phase in a zig-zag fashion from catex line to anex line. This process is thus named the ZIX-Zak process and is patented in many parts of the world. The result is that when the ZIX-Zak process is used, the resultant water quality is the equivalent of demineralized water with a zero conductivity


To have saleable agricultural end products the regenerations are done by nitric acid for the catex resin and ammonia for the anex resin. As these reagents have different ionic strengths (Nitric acid having strong ionic strength and ammonia being weak) the resins must have suitable exchange values, thus, the catex resin should be a strong-based cation resin and the anex resin should be a weak-based resin.

Strong-based catex resins operate in the acidic range and absorb most cations onto the resin in exchange for H⁺ that was placed on the resin during regeneration, so the solution being treated will become steadily more acidic as the absorption of cations proceeds. This has the effect of making it difficult to remove all the Na⁺ below 400mg/ltr.


Weak-based resins operate in the mildly acidic range and absorb anions as an acid radical i.e. chloride will be exchanged as HCl and sulfate as HSO₄⁻ in exchange for OH⁻ ions from the ammonia solution used for regeneration. So neutral salts, like NaCl and Na₂SO₄, cannot have the anion removed and will end up in the final product water about equimolal to the residual cations left from the catex extraction process.


To overcome this TBT has created the ZIX-Zak process, which eliminates most of the excess H⁺ in the catex line by moving the aqueous phase from the catex line and back to the anex line several times. With the greatly reduced free H⁺ level operating in the catex line, sodium can be removed quantitatively allowing all the anions to be removed by the anex resin resulting in a product water with low residual total dissolved solids (TDS).

Successful Performance of the ZIX-Zak ion exchange process on different waters processed by Trailblazer's Pilot Scale ion exchange plant.


The ZIX-Zak process has worked successfully with feed water up to 50 000 mg/ltr and can still make water of minimal conductivity.

How the ZIX-Zak

process works