What is the purpose of adding phosphate solution to boiler feedwater?

Sodium phosphate is commonly used; it is a white solid with a melting point of 1340°C. It is soluble in water, and upon dissolution it decomposes into disodium hydrogen phosphate and sodium hydroxide, resulting in a strongly alkaline solution. The sodium phosphate method is suitable for boilers without water-cooled walls, where the hardness of the feed water is less than 3.5 mmol/L, and the ratio of magnesium hardness to total hardness is less than 0.5 ； Boilers with an evaporation capacity of less than 4 t/h that have no external treatment, or boilers at any pressure that do have external treatment. This method can effectively prevent the formation of calcium and magnesium scale, but it cannot prevent the formation of iron scale. Generally, in a dosing tank, deionized water is used to dissolve solid sodium phosphate into a concentration of 5% to 8%, and this solution is then added directly to the boiler water using a piston pump. Phosphates can also be added to the boiler feed water, which carries them into the boiler. The dosage of chemicals when the boiler starts operating is calculated using relevant formulas.

Phosphates are added to prevent the formation of calcium scale, as calcium is the main scaling agent in water. When the furnace water contains a sufficient concentration of PO43-, and under certain pH conditions, Ca2+ reacts with PO43- to form basic calcium phosphate. The reaction equation is as follows: 10 Ca2+ + 6 PO43- + 2 OH- = Ca10(PO4)6(OH)2. Basic calcium phosphate does not form scale in boilers; instead, it appears in the form of sludge that can be removed from the boiler along with the furnace water. Adding phosphates to the boiler water is a remedial and preventive measure to prevent harm caused by calcium ions that leak during the desalination process. Obviously, the fundamental measure should be to improve the quality of water desalination treatment, and to prevent as much as possible the entry of scaling substances into the desalinated water (although it is inevitable that trace amounts of Ca2+ will leak through); since even if scale is formed, an excessive amount of it can affect the quality of the steam. The amount of phosphate added is determined by the concentration of scaling substances in the furnace water. Due to the high quality of the deionized water, the concentration of Ca2+ that leaks in is extremely low, so only a small amount of phosphate needs to be added. Excessive phosphates in boiler water are not only useless but also harmful, as they not only lead to waste of chemicals but also increase the total salt content in the boiler water. An increase in the total salt content can lead to salt deposition in the boiler tubes; it may also cause numerous bubbles to form in the waste drum, resulting in steam containing water, an increase in the salt content of the steam, and a decline in the quality of the steam. Furthermore, due to the high concentration of PO43-, it may react with trace amounts of Mg2+ in the boiler water to form magnesium phosphate precipitates. The reaction equation is as follows: 3Mg2+ + 2PO43- = Mg3(PO4)2↓. Mg3(PO4)2 has very low solubility in hot water, and it can adhere to the inside of the boiler tubes, resulting in secondary scale formation. Currently, the control range for the PO43- concentration in the boiler water of large domestic chemical plants is generally 2–10 PPm. Some believe that this control parameter is on the high side, as large-scale plants achieve high water treatment quality with very little scaling material; moreover, experiments have been conducted to reduce the phosphate concentration, with good results. Some foreign factories specify that a PO43- concentration controlled between 0.5 and 5 PPm is referred to as low-phosphate treatment. If all the added phosphates are trisodium phosphate (Na3PO4), there is a drawback. Because Na3PO4 is a sodium salt that easily forms fusible deposits on high-temperature furnace tubes, it precipitates on the walls of these tubes at high temperatures, releasing free sodium hydroxide. Its precipitation reaction is as follows: Na3PO4 + 0.15H2O = Na2.35H0.15PO4↓ + 0.15NaOH. When trisodium phosphate undergoes precipitation, free sodium hydroxide appears in the liquid phase within the boundary layer of the furnace tube wall. When the boiler water containing free sodium hydroxide becomes highly concentrated at the tube walls, it causes alkaline corrosion of those tube walls. To prevent the alkaline corrosion of the tube walls by the boiler water, the traditional method of treating the boiler water solely with trisodium phosphate was modified, and a new phosphate treatment method was developed and gradually refined, known as the \"harmonized PH-phosphate treatment\" method. The characteristic of this method is that, along with the addition of sodium phosphate, disodium hydrogen phosphate (Na2HPO4) is added in an appropriate proportion, so as to ensure an adequate concentration of PO43- in the furnace water without the formation of free sodium hydroxide. Disodium hydrogen phosphate can react with sodium hydroxide, thereby eliminating free sodium hydroxide: Na2HPO4 + NaOH = Na3PO4 + H2O, which prevents alkaline corrosion of the furnace tube walls. To ensure an appropriate pH level of the boiler water, the amounts of disodium hydrogen phosphate and trisodium phosphate added must be kept within a certain range. Generally, the aforementioned ratio is determined by the molar ratio of Na+ to PO43- in the furnace water. Experience has shown that the optimal range for the Na+/PO43- molar ratio is 2.2 to 2.85. When the analysis shows that the Na+/PO43- ratio is close to or greater than 2.85, an appropriate amount of disodium hydrogen phosphate should be added ； When the Na+/PO43- ratio is close to 2.2, an appropriate amount of sodium hydroxide should be added to keep the concentration of PO43- in the furnace water relatively stable.

The calcium and magnesium hardness in boiler feed water undergo chemical reactions or concentrate and crystallize under high-temperature conditions, forming insoluble scale that adheres firmly to the heating surfaces of the boiler. This scale is a poor conductor of heat, which hinders heat transfer; in severe cases, it can lead to boiler tube failures. Additionally, it induces and exacerbates chemical corrosion of the metal beneath the scale, resulting in serious damage. Although the boiler’s condensate water and feedwater undergo strict softening and desalination treatments, a small amount of calcium and magnesium hardness still enters the boiler water. If this hardness is not treated, it can lead to scaling, posing a threat to the safe operation of the boiler. Currently, adding phosphates to the boiler water is the most suitable treatment method; the reaction is as follows: 10Ca2++6PO43-+2OH- → Ca10(OH)2(PO4)6 (calcium hydroxyposphate). Calcium hydroxyposphate is a soft type of sludge that can be easily removed through the boiler’s blowdown process, and it does not adhere to the inside of the boiler to form scale. Application range: Water treatment for boiler systems with high pressure and higher parameters. Boiler water phosphate addition system: In addition to adding trisodium phosphate directly to the solution tank as shown in the system diagram for preparing the phosphate solution, it is also possible to use a separate 4m3 dissolution tank; the solution is first circulated and stirred using a transfer pump before being added to the solution tank. The amount of phosphate added can be controlled manually or automatically: based on the sampling signals from the boiler water phosphate level meter, the microcomputer processes these signals and controls the dosage of the phosphate addition pump to achieve automatic dosing.