The treatment of reducing the content of calcium and magnesium ions in water to make hard water into soft water is called water softening. The main methods are as follows:
Reaction when boiling temporarily hard water:
Ca(HCO3)2 =CaCO3 ↓+H2O+CO2↑
Mg(HCO3)2 =MgCO3↓ +H2O+CO2↑
Because CaCO3 is insoluble and MgCO3 is slightly soluble, magnesium carbonate can also react with water to form more insoluble magnesium hydroxide under the condition of further heating:
MgCO3 +H2O = Mg(OH)2 ↓+CO2↑
It can be seen that the main components of scale are CaCO3 and Mg(OH)2
The classic water quality treatment method in industry is the chemical softening method, such as adding lime (CaO) and sodium phosphate. Adding lime can cause the carbon dioxide, calcium bicarbonate and magnesium bicarbonate in the water to form calcium carbonate and magnesium hydroxide precipitation. For hard water with large permanent hardness, an appropriate amount of soda ash can be added. The amount of lime added during softening, according to experience, needs to add 10 grams of pure calcium oxide for every one degree of temporary hardness reduction in 1000 liters of water.
During the reaction, magnesium is precipitated in the form of magnesium hydroxide, and calcium is precipitated in the form of calcium carbonate.
Mg2+(aq)–lime-soda method–> Mg(OH)2(s)
It uses ion exchangers to exchange ions in water with diffusible ions in ion exchangers to soften water. Most beverage water uses organic synthetic ion exchange resins as ion exchangers.
When processing water, first let the water pass from the top of the yang column, so that the metal ions in the water are adsorbed by the cation exchange resin, and the hydrogen ions in the cation exchange resin are exchanged into the water; then pass through the negative column to make the water in the water The anions are adsorbed by the anion resin, and the anion resin exchanges hydroxide ions into water, and ionizes the hydrogen to synthesize water to purify the water.
The electrodialysis method uses the selective permeability of the anion and cation exchange membranes to the ions in the water under the action of an external DC electric field, so that the anion and cation in the water move to the anode and cathode through the anion and cation exchange membranes respectively, so as to achieve the purification effect. This technology is often used to prepare primary pure water from tap water.
It uses pressure as the driving force to increase the pressure of the water to overcome the osmotic pressure, so that the water passes through the functional semi-permeable membrane for desalination and purification. The reverse osmosis method can also remove colloidal substances, and the utilization rate of water can reach more than 75%; the reverse osmosis method has large water production capacity, simple operation, and can effectively make the water purified to meet national standards.