The concept is to remove all ions and to replace them with OH^- and H⁺. These ions react with each other and help drive the ion exchange. You want the two different ion exchange resins in close proximity during the exchange but have to separate them for regeneration.

A mixed bed exchanger cannot be regenerated easily because the reagents for one resin would put the other into an undesirable form. For example, contact with sodium hydroxide would convert one resin to the desired hydroxyl form while putting undesired sodium ions on the other. The resins must be separated so that each may be regenerated. This is accomplished by designing a process that has resins with different sedimentation characteristics. If the densities or particle sizes are sufficiently different, upflow through the bed will classify the resins with the one that settles more rapidly underneath the other. Careful hydraulic balancing allows one reagent to suspend and regenerate the lower resin while another reagent suspends and regenerates the upper resin.

Animation (wait to see entire cycle)

Continue to Improvements of Beads.