Application effect of PAC in water treatment of thermal power plant

1. Pre-treatment of make-up water

Natural water bodies often contain mud, clay, humus and other suspended matter and colloidal impurities and bacteria, fungi, algae, viruses and other microorganisms, they have a certain stability in water, is the main cause of water turbidity, color and odor. These excessive organic substances enter the ion exchanger, contaminate the resin, reduce the exchange capacity of the resin, and even affect the effluent quality of the desalting system. Coagulation treatment, settlement clarification and filtration treatment is to remove these impurities as the main purpose, so that the content of suspended matter in the water is reduced to less than 5mg/L, that is, to get clarified water. This is called water pretreatment. After pretreatment, the water can be used as boiler water only when the dissolved salts in the water are removed by ion exchange and the dissolved gases in the water are removed by heating or vacuuming or blowing. If these impurities are not removed first, subsequent treatment (desalting) cannot be carried out. Therefore, the coagulation treatment of water is an important link in the water treatment process.

The pretreatment process of thermal power plant is as follows: raw water → coagulation → precipitation and clarification → filtration. The coagulants commonly used in the coagulation procedure are polyaluminum chloride, polyferric sulfate, aluminum sulfate, ferric trichloride, etc. The following mainly introduces the application of polyaluminum chloride.

Polyaluminum chloride, referred to as PAC, is based on aluminum ash or aluminum minerals as raw materials, at high temperature and a certain pressure with alkali and aluminum reaction produced polymer, raw materials and production process is different, product specifications are not the same. Molecular formula of PAC [Al2(OH)nCI6-n]m, where n can be any integer between 1 and 5, and m is the integer of cluster 10. PAC comes in both solid and liquid forms.


2.Coagulation mechanism

There are three main effects of coagulants on colloidal particles in water: electrical neutralization, adsorption bridging and sweeping. Which of these three effects is the main one depends on the type and dosage of coagulant, the nature and content of colloidal particles in water, and the pH value of water. The mechanism of action of polyaluminum chloride is similar to that of aluminum sulfate, and the behavior of aluminum sulfate in water refers to the process of Al3+ producing various hydrolyzed species.

Polyaluminum chloride can be regarded as various intermediate products in the process of hydrolysis and polymerization of aluminum chloride into Al(OH)3 under certain conditions. It is directly present in water in the form of various polymeric species and A1(OH)a(s), without the hydrolysis process of Al3+.


3. Application and influencing factors

1. Water temperature

The water temperature has obvious influence on the coagulation treatment effect. When the water temperature is low, the hydrolysis of the coagulant is more difficult, especially when the water temperature is lower than 5℃, the hydrolysis rate is slow, and the flocculant formed has loose structure, high water content and fine particles. When the water temperature is low, the solvation of colloidal particles is enhanced, the flocculation time is long, and the sedimentation rate is slow. The research shows that the water temperature of 25~30℃ is more suitable.

2. pH value of water

The hydrolysis process of polyaluminum chloride is a process of continuous release of H+. Therefore, under different pH conditions, there will be different hydrolysis intermediates, and the best pH value of polyaluminum chloride coagulation treatment is generally between 6.5 and 7.5. The coagulation effect is higher at this time.

3. Dosage of coagulant

When the amount of coagulant added is insufficient, the remaining turbidity in the discharge water is larger. When the amount is too large, because the colloidal particles in the water adsorb excessive coagulant, the charge property of the colloidal particles changes, resulting in the residual turbidity in the effluent increases again. The coagulation process is not a simple chemical reaction, so the required dosage can not be determined according to the calculation, but should be determined according to the specific water quality to determine the appropriate dosage; When the water quality changes seasonally, the dosage should be adjusted accordingly.


4. Contact medium

In the process of coagulation treatment or other precipitation treatment, if there is a certain amount of mud layer in the water, the effect of coagulation treatment can be significantly improved. It can provide a large surface area, through adsorption, catalysis and crystallization core, improve the effect of coagulation treatment.

Coagulation precipitation is a widely used method for water treatment at present. Polyaluminum chloride industry is used as a water treatment flocculant, with good coagulant performance, large floc, less dosage, high efficiency, fast precipitation, wide application range and other advantages, compared with the traditional flocculant dosage can be reduced by 1/3~1/2, the cost can be saved 40%. Combined with the operation of valveless filter and activated carbon filter, the turbidity of raw water is greatly reduced, the effluent quality of the desalt system is improved, and the exchange capacity of the desalt resin is also increased, and the operating cost is reduced.

Post time: Mar-22-2024