Weak Acid Cation

Certain waters that contain a high percentage of hardness associated with alkalinity can be economically treated by passage through a weak acid cation resin. By definition, the weak acid resin will remove Ca⁺⁺, Mg⁺⁺, and Na⁺ which enters the bicarbonate (HCO₃)^- form. Because most industrial water sources contain some noncarbonate hardness (CaSO₄, etc.), it is necessary to follow the weak acid cation unit with a strong acid cation unit to achieve truly demineralized water.

Weak and strong acid cation resins can be placed in different vessels or they can be placed in two distinct layers in the same vessel. The regeneration efficiency of a weak acid resin is very high compared to that of a strong acid resin. Therefore, it is possible to utilize the regenerant acid stream from the strong acid unit to regenerate the weak acid unit. When weak and strong acid cation resins are loaded into the same vessel, the strong acid resin settles on the bottom of the unit after backwash because of the density difference between the two resins. Because the weak acid resin contains some strong acid sites, after regeneration with sulfuric acid, a 10% brine solution must be passed through the unit. The brine solution exhausts any strong acid sites in the weak acid resin and regenerates the strong acid resin in the sodium form. If this is not done when raw water enters the weak acid resin, noncarbonate hardness exchanges at the strong acid sites. FMA exits the weak acid resin and prevents the exchange of residual noncarbonate hardness in the strong acid resin. Normally, a weak acid resin produces FMA for 40-60% of its service cycle. This combination would not be suitable for higher pressure boiler applications because of the presence of excess sodium in the effluent from the sodium-form strong acid resin.

Regeneration of a weak acid cation resin with sulfuric acid must be carefully monitored to insure that the acid concentration during the regeneration does not exceed 0.7%. Higher concentrations of sulfuric acid can react with the Ca⁺⁺ in the exchange sites of the exhausted resin and result in the precipitation of calcium sulfate (CaSO₄). Calcium sulfate, or gypsum, is insoluble even in the concentrated form of many acids. Often, mechanical removal is the only satisfactory way to rid the resin of this contaminant. From an operational standpoint, it is objectionable because it produces a pressure drop across the unit.