Dissolved alkaline earth metals together with the dissociation forms of carbonic acid form the hardness in water. The main hardness formers are the cations of Calcium and Magnesium. The anions relevant for hardness formation are the carbonates of carbonic acid.
Boiler scale consists mainly of a stable crystal form of calcium carbonate (Calcite). Scale occurs when saturation limits are exceeded. Less stable crystal forms like Aragonite are also possible.
Ion exchangers remove Calcium and Magnesium cations by substituting them with 2 moles Sodium anions from table salt. Calcium- and Magnesiumcarbonates are turned do Sodiumcarbonates. Calcium- and Magnesiumchlorides leave the process with the effluent of the ion exchanger.
Physical treatment of hardness aims at forming less stable crystal forms like Aragonite without removing hardness chemically.
Theory Water hardness
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