The calcium and magnesium hardness in the boiler feedwater will undergo chemical reactions under high temperature environment, or concentrate and crystallize to form water-insoluble scale, which is firmly attached to the heating surface of the boiler. This scale is a poor conductor of heat and will hinder heat conduction. In severe cases, a boiler tube explosion accident may occur. In addition, scale will induce and aggravate chemical corrosion of the metal under the scale, which is quite serious. Although the boiler condensate and feed water have been strictly softened and desalted, there is still a small amount of calcium, Magnesium hardness enters the boiler water. If this part of the hardness is not treated, scaling will also pose a threat to the safe operation of the boiler. At present, adding phosphate to furnace water is the most suitable treatment method, and the reaction is as follows:

10Ca + 6PO4 + 2OH Ca (OH) (PO4) (basic calcium phosphate)

Alkaline calcium phosphate is a kind of soft water slag, which is easy to be removed with the boiler blowdown, and will not adhere to the furnace and become scale.