To prevent the calcium scale generating in boiler steam drum , in addition to that the quality of water supply shall be treated and guaranteed, usually some chemical agents shall be added to the boiler water, in order to make the calcium ions (residual in the make-up water or leaking from the condenser) entering into the boiler along with the water supply not form scale and slag,  the scale and slag shall be discharged along with the boiler blowdown. In the power plant boiler, the chemical agent that is most appropriate for dosing treatment is phosphate. The method of adding phosphate to boiler is called treatment method of phosphate. Under the special conditions, the treatment method of phosphate not only can prevent the scale of calcium, but also can prevent the function of alkaline corrosion. 

Principle: Add a certain amount of phosphate to boiler water to make the boiler water maintain a certain amount of phosphate radical. Under the condition of boiling alkaline (PH value of boiler water generally is within the range of 9-11): 

The calcium and phosphate radical in boiler water react: (10 Ca2 + + 6 PO43 - + 2 OH -  > Ca10 (OH) 2 (PO4) 6), (Calcium phosphate  ), which is a kind of water slag,  it is easily drained with the boiler blowdown and will not stick on the boiler and become scale. (zjl)