The first stage in the process of regeneration involves passing a dilute solution of acid, usually hydrochloric, through the cation bed. After the cation resin has been regenerated,the anion resin is regenerated by passing a dilute solution of caustic (sodium hydroxide) through the anion resin bed. As a result, the cation resin is reconverted to the hydrogen form and the anion resin to the hydroxyl form.
The split stream process thus produces soft water with reduced alkalinity and consequently with a lower dissolved mineral content.
The service run continues as long as
- Soft water is obtained from the softener
- Cation-free water is obtained from the HI Unit, or
- Designed throughputs are achieved through the softener or HI Unit.
When sulphuric acid is used as a regenerant for the HI unit, the waste effluent consists of the sulphate salts of calcium, magnesium and sodium. As calcium sulphate has very limited solubility, care has to be taken to maintain the designed concentrations of sulphuric acid to avoid the precipitation of calcium sulphate in the ion exchange bed.
With hydrochloric acid (HCl) as a regenerant the HCl solution is injected normally at uniform concentration of 5% w/v.
When sulphuric acid (H2SO4) is used as a regenerant, the solution of H2SO4 is injected at a concentration ranging from 1.5% w/v to 5% w/v. The lower concentration is used when the influent water has a high calcium content. To conserve water during the injection step the concentration of H2SO4 solution is progressively increased in steps as calcium is depleted from the bed.
The HI unit can operate as a downflow or an upflow unit with coflow or counterflow technique of regenerant injection .
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