Question

an ion-exchange softener removes calcium hardness using a sodium-form resin. after prolonged operation, the resin becomes exhausted. which procedure restores its exchange capacity?

wash with lime slurry
backwash with clean water only.
regenerate the resin using a concentrated sodium chloride solution.
raise the ph above 11 using sodium hydroxide

Explanation:

Identify the mechanism of ion-exchange softening

$$ \text{Resin-Na}_2 + \text{Ca}^{2+} ightleftharpoons \text{Resin-Ca} + 2\text{Na}^+ $$

Determine the regeneration process

$$ \text{Resin-Ca} + 2\text{Na}^+ (\text{excess}) ightleftharpoons \text{Resin-Na}_2 + \text{Ca}^{2+} $$

Match with the correct option

$$ \text{Regenerant} = \text{Concentrated NaCl solution (brine)} $$

Answer:

• Wash with lime slurry
• Backwash with clean water only.
• Regenerate the resin using a concentrated sodium chloride solution. (Correct answer)
• Raise the pH above 11 using sodium hydroxide