Demineralization Process Continuation

Ion exchange between the solid exchanger and the water containing the electrolytes takes place without structural changes in the solid material (the solid does not go into solution). The ions in the solution rapidly diffuse into the molecular network of the exchanger to reach the exchange sites where interchange of the ions occurs. The ions in the exchanger similarly diffuse out of the exchanger and into the solution.

The ion exchange demineralization takes place with an equilibrium (reversible) reaction. They may be expressed in simple form by the following 2 equations:

Cation exchange:

Z *a(+) + b(+) ===> Z*b(+) + a(+)

Z is the matrix and anionic fixed site of the cation exchanger, and a(+) and b(+) are two cations.

Anion exchange:

A*c(-) + d(-) ===> A*d(-) + c(-)

A is the matrix and cationic fixed site of the anion exchanger, and c(-) and d(-) are two anions.

In the previous equilibria equations, the reverse processes may take place in the exchanger as during regeneration. The direction the reaction will go depends mainly on the affinity of the resin for the various ions in the water or solution. Ion exhange equilibria are often expressed in terms of "selectivity coefficients" K

K = ([a(+) in resin]*[b(+) in solution]) / ([b(+)in reesin]*[a(+) in solution])

This applies only to ions of equal valences.

Now you are familiar with the lengthy process of water purification applied to water used in the semiconductor industry, lets do some mass transfer problems.

2007 Arizona Board of Regents for The University of Arizona