Silica can not be removed by exchange of cation - hydrogen or exchange of sodium zeolite. Silica removal can be performed partially in the process of lime - soda, either in cold or hot. Silica is highly undesirable impurity because it can cause formation of scale which attached very strong.
Silica removal from boiler water can be done by using dolomite lime or active magnesia in the softener. If using coagulation and previous deposition, some silica will can be removed by coagulate ferry. This substance is very suitable if concentration of silica is in additive water.
This method can not remove all soluble silica, but may reduce its concentration until sufficiently low so that blowdown disposal in steam boiler can be used to prevent scale formation inside boiler when it's done properly. The most commonly treatment that is used to produce water that contains very little silica is demineralization.