The agent softening method is the process of using the principle of chemical precipitation, that is, the process of removing the hardness contained in the water by forming insoluble compounds under the action of appropriate agents according to the principle of solubility product. The most commonly used method in water treatment is the combined precipitation of calcium and magnesium ions, followed by the precipitation of hydroxides of metal ions.
Without requiring the removal of alkalinity, caustic soda can completely replace lime to remove temporary hardness. While removing temporary hardness, caustic soda can also remove an equimolar amount of permanent hardness. Caustic soda can remove the permanent hardness of magnesium most directly.
High purity. The purity of lime is low, which will lead an increase in the dosage, and impurities will settle into the sludge discharge system and increase the sludge discharge. The purity of industrial caustic soda is mostly above 99%, which will reduce unnecessary sludge production.
High effective utilization rate.Because lime is slightly soluble in water, it is added in the form of emulsion, and the effective utilization rate is low. The part remaining in the supernatant will increase the consumption of acid when the pH value adjusts, and will dissolve into the water in the form of Ca2+ to increase the permanent hardness of the water. While caustic soda has good solubility, fast reaction speed, complete reaction, the effective utilization rate is almost 100%. No excess sediment is produced as sludge.
Conversion of CO2.The combination of CO2 in water and caustic soda will generate Na2CO3, which can also be used to remove permanent hardness in water.
Removal of magnesium hardness.When using lime to remove the permanent hardness of magnesium, the permanent hardness of magnesium will first be converted into the permanent hardness of calcium. This part of permanent hardness also needs to be removed with Na2Co3, which increases the consumption of additional agents. In addition, the whole reaction generates calcium carbonate while generating magnesium hydroxide, which will increase the amount of sludge discharged. If caustic soda is used, magnesium hydroxide can be directly generated, reducing the consumption of chemicals and the output of sludge.
Removal of metal ions. In order to achieve the effect of rapid precipitation in the process of removing alkalinity, it is generally necessary to add iron salt or aluminum salt as a coagulant. This part of the coagulant will react with lime to form the permanent hardness of calcium, which need to be removed with Na2Co3. But if using caustic soda, it can directly generate iron and aluminum hydroxides, reducing the consumption of chemicals and the output of sludge.
Removal of permanent hardness.Lime has absolutely no effect on the removal of permanent hardness. Caustic soda will generate Na2CO3 when removing temporary hardness, which can be used to remove part of permanent hardness.