How are Sulphates Reacted on by Internal Treatment?
High temperatures in the boiler water reduce the solubility of calcium culphate and tend to make it precipitate out directly on the boiler metal as scale. Consequently calcium sulphate must be reacted upon chemically to cause a precipitate to form in the water where it can be conditioned and removed by blow-down. Calcium sulphate is reacted on either by sodium carbonate, sodium phophate or sodium silicate to form insoluble calcium carbonate, phosphate or silicate. Magnesium sulphate is reacted upon by caustic soda to form a precipitate of magnesium hydroxide. some magnesium may react with silica to form magnesium silicate. Sodium sulphate is highly soluble and remains in solution unless the water is evaporated almost to dryness.
