Application of phosphate in water treatment
Phosphate is mainly used as a corrosion inhibitor and scale inhibitor in cooling water and boiler water treatment, and its mechanism varies according to different varieties, uses and conditions of use. Due to factors such as corrosion and scale inhibition and environmental protection, phosphate treatment technology has undergone continuous reform. With each improvement, it has solved some technical problems in control.
Water treatment can improve the recycling rate of industrial water, which accounts for a large proportion of water consumption. Many chemicals are used in water treatment, and phosphate is still an important chemical product selected in water treatment. Their role in water treatment varies depending on the purpose of treatment, the quality of the water being treated, and the treatment method. Industrial circulating cooling water and boiler water treatment are two major areas of phosphate in water treatment applications, and the amount of cooling water in food, textile, paper, chemical, petroleum, steel and machinery industries is very high. On average, it accounts for 67% of total water use. Among them, the chemical, petroleum and steel industries account for about 85% ~ 90%. Boiler water treatment is an important means to ensure the thermal efficiency of the boiler, prevent accidents, extend the service life, and ensure the safe operation of the boiler. At present, more than 400,000 industrial boilers in China use phosphate treatment to treat furnace water.
Corrosion inhibition mechanism and scale inhibition mechanism of phosphate
Phosphate is mainly used as corrosion inhibitor and scale inhibitor in cooling and boiler water treatment. Its mechanism of action is corrosion inhibition mechanism and scale inhibition mechanism.
Corrosion inhibition mechanism.
The corrosion inhibition mechanism of phosphate as a corrosion inhibitor varies according to its variety, use and use conditions. In circulating cooling water treatment, polyphosphate is used more. Various polyphosphates have good corrosion inhibition effect on carbon steel. The corrosion of carbon steel in water is caused by microbatteries, and its electrochemical reaction formula is:
Anode reaction: Fe Fe2++2e
Cathodic reaction: O2+2H20+4e 40H-
This mechanism holds that when water has a certain concentration of Ca2+ or other divalent metal ions, the polymeric phosphate ion forms a positively charged coordination ion with Ca2+, and this coordination ion exists in the aqueous solution in a colloidal state. When steel is corroded in water, the anode reaction product Fe2+ diffuses and moves in the direction of the cathode, and the positively charged calcium polyphosphate coordination ion in the colloidal state can then complexe with Fe2+ to generate a coordination ion with calcium iron polyphosphate as the main component, which is deposited on the cathode surface by corrosion current to form a precipitated film film. This film has a certain compactness, which can block the diffusion of dissolved oxygen to the cathode, that is, it inhibits the cathodic reaction of the corroded battery, thereby inhibiting the entire corrosion reaction.
Scale inhibition mechanism
Polyphosphate is often used as a scale inhibitor in circulating cooling water treatment. Orthophosphate is mostly used as a scale inhibitor for boiler water treatment. The two play a role in their respective occasions according to different scale inhibition mechanisms. The scale in the cooling water is mainly calcium carbonate scale, calcium phosphate scale, calcium silicate scale and magnesium silicate scale. When there is no excess PO3-4 or SiO2 in the cooling water, calcium phosphate scale and magnesium silicate scale are not easy to form, and the easiest to generate calcium carbonate scale is calcium carbonate scale. Adding a small amount of polyphosphate to the water can prevent calcium carbonate from precipitating and precipitating. The reason why calcium carbonate can be effectively prevented from precipitation is that polyphosphate effectively controls the rate of crystal nucleus formation. In addition, polyphosphate can also chelate Ca2+, Mg2+ plasma. Formation of monocyclic or bicyclic chelates. This disrupts the normal growth process of crystals such as calcium carbonate, thereby preventing the formation of calcium carbonate scale.
The furnace water treatment of the boiler is often used as trisodium phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate. The main reaction and exert it as an antiscalant by different mechanisms
As follows:
3Ca(HCO3)2+2Na3PO4=Ca3(PO4)2↓+3Na2CO3 +3CO2↑+3H2O
3CaSO4+2Na3PO4=Ca3(P04)2↓+3Na2SO4
↓+3Na2CO3 +3CO2↑+3H2O
3Mg(HCO3)2+2Na3PO4=Mg3(PO4)2
3MgSO4+2Na3PO4=Mg3(PO4)2
↓+3Na2SO4
When the water temperature is quite high and there is enough alkalinity, calcium phosphate will turn into hydroxyapatite Ca10(OH)2(PO4)6 in the form of water residue,
The reaction is as follows:
10Ca3(PO4)2.H2O+6NaOH=3Ca10(OH)2(PO4)6+2Na3PO4+ 10H2O
The generated water slag can be discharged to the outside of the furnace with the boiler blowdown.
While magnesium phosphate is easy to form secondary scale, and it is difficult to remove by mechanical means when adding - quantitative silicate in furnace water, magnesium phosphate can continue as follows
React:
Mg3(PO4)2+2Na2SiO3+2NaOH+H2O=3MgO.2SiO2.2H2O+2Na3PO4
The resulting serpentine (3MgO.2SiO2.2H2O) is a sludge that does not adhere to metal walls. For alkaline water quality, in order to avoid excessive alkalinity in the furnace water, sodium dihydrogen phosphate or disodium hydrogen phosphate can be used instead of trisodium phosphate
The role of phosphate in water treatment
Whether phosphate plays a corrosion-inhibiting or scale inhibition role in the water treatment process depends on the composition of the specific formula, the conditions of use and the amount of phosphate. Such as polyphosphorus to play a corrosion inhibiting effect. High concentration, the acid acts as a scale inhibitor at low doses.
Role in circulating cooling water treatment
The treatment method of polyphosphate as the main corrosion inhibitor has been widely used in the treatment of circulating cooling water at home and abroad. At present, due to the corrosion and scale inhibition effect and environmental protection factors, countries are developing and improving phosphorus-based formulas.
Replace polyphosphate with phosphate
One of the obvious drawbacks of polyphosphate when used is that it hydrolyzes into orthophosphate, causing calcium phosphate deposits in cooling water, which in turn promotes corrosion under scale. Replacing polyphosphate with phosphate compounds is an effective way to improve the performance of phosphorus-based formulations. Recommended phosphate dosage [2]2~20mg/L.
Prevent calcium phosphate deposition with a new phosphorus-containing polymer
On the basis of continuing to use polyphosphate, a new agent with a special effect on preventing the deposition of calcium phosphate is added, which not only achieves the corrosion inhibition effect but also plays a role in scale inhibition. For example, organic phosphonates not only have obvious low restriction effect on calcium, magnesium and other divalent ions, but also have synergistic effects on other agents, they have the characteristics of non-toxic, non-polluting, small dosage, not easy to hydrolyze and good stability at high temperature and high pH.
At present, phosphorus-containing polymers are also being studied in China, and a small number of varieties have been put into production (such as phosphorus-containing polyacrylic acid), containing phosphorus acrylic acid, acrylamide, acrylic acid
Many varieties such as copolymers of hydroxypropyl ester have yet to be developed.
