The water softening method is based on the principle of solubility product, adding lime and other agents to the water as needed to react with calcium and magnesium ions to form insoluble precipitates CaCO3 and Mg(OH)2. Lime softening is the process of digesting lime into milk of lime and adding it to raw water. CaCO3 and Mg(OH)2 formed by the reaction with bicarbonate under high pH conditions are both insoluble compounds and can be precipitated from water. Precipitate out. But the permanent hardness and negative hardness in the water cannot be removed by lime treatment, because the permanent hardness of magnesium reacts with the negative hardness and slaked lime.

The permanent hardness of magnesium is all converted into the same amount of permanent hardness of calcium with high solubility, while the negative hardness is converted into the same amount of sodium hydroxide and alkalinity, so the alkalinity in the water is not removed.

The original water suspended matter is less than 100mg/L, the oxygen consumption is less than 5mg/Lo2, and the alkalinity is greater than 2mmol/L, no coagulant needs to be added frequently, and coagulant can be added regularly. When lime is treated, coagulants such as FeS04·7H20, polymeric ferric sulfate, etc. can be considered at the same time. When using lime softening pretreatment, it is theoretically impossible to obtain effluent water with a hardness less than 40mg/L (CaC03). Because when pH = 10.8, the solubility of Mg(OH)2 is 10mg/L, CaCO3. Its solubility is about 30mg/L. In addition, both CaCO3 and fly (OH)2 tend to form supersaturated solutions. The agent softening method can also achieve the purpose of desiliconization by adsorption or condensation of silicon; this is a non-deep desiliconization method, which generally can remove 60%-90% of colloidal silicon.

There are two types of lime treatment: cold method and thermal method, which are carried out in sedimentation tank and clarification tank. When lime is softened, the sedimentation tank and clarification tank must have mechanical mud scraping devices.

  • Cold method.
  • Thermal method. Carry out under warming conditions. When lime is softened and silicate is required to be removed, magnesium oxide (chalcanthite) or dolomite powder can be added, and the raw water should be heated to further reduce the colloidal silicon content. Thermal treatment can use UHM type sludge suspension to run at 40±1℃.

When lime-sodium carbonate (soda ash) is used to remove the hardness of non-carbonate, the temperature can be at room temperature or heated to 49 ℃ or above 98 ℃ (using steam mixing heating, and at the same time have the effect of deoxygenation, suitable for Small-capacity, low-pressure boiler make-up water treatment). After lime softening treatment, two main precipitates, Mg(OH)Z and CaCO3, are formed. Mg(OH)Z is loose in nature, small in density and flocculated; CaCO3 is dense in nature and dense in spherical particles.

1. Water quality changes after lime treatment

  1. Alkalinity and free C02. Under given temperature conditions, with sufficient lime dosage, the alkalinity in the water is theoretically determined by the pH and the residual amount of Ca2+, and the residual amount of Ca2+ is determined by whether the C 203 is completely precipitated and separated. The alkalinity of deep well water after lime treatment accords with theoretical calculation. Regarding the crystallization conditions of surface water sediments, due to factors such as changes in water composition and the inhibitory effect of organic substances on the precipitation process, the alkalinity after lime treatment is much greater than the theoretical calculation value. The greater the residual amount of Ca’10 (when the raw water contains greater non-carbonate hardness), the smaller the residual carbonate alkalinity and total alkalinity. If ferrous sulfate is used as a coagulant at the same time, it will also reduce the alkalinity of carbonate in the water.
  2. Instability. During the lime treatment process, due to the Cat+ and M expansion+ residues of the effluent clams are in an unstable supersaturated state, secondary precipitation may occur in the subsequent pipelines and equipment; the instability of the water (indicated by △A) That is, it refers to the reduced value of the alkalinity of the water and the corresponding hardness. Generally, the instability △A=0.05-10.1 mmol/L, and the maximum allowable value △A=0.15mmol/L.
  3. Suspended matter. The content of suspended solids in the effluent water of the adjusted clarifier is generally less than 10mg/L, and the limit allowable content should not exceed 20mg/L.
  4. Oxygen consumption (COD). Under normal natural water conditions, the oxygen consumption reduction value after lime or lime agglomeration treatment is 25%-40%.
  5. Hardness. The hardness of clarified water (Hc) is determined by the residual non-carbonate hardness and the dosage of coagulant, and its calculation formula is Hc=Ho-Ay+Ac+K(mmol/L)
    In the formula, Ho—Total hardness in raw water, mmol/L;
    AY-the alkalinity of raw water, mmol/L;
    Ac clarified water residual alkalinity, mmol/L;
    K—Cohesive dosage (ferrous sulfate), rnmol/L (when not added, this item is zero).
  6. Silica ((SiO2). The removal rate of SiO2 after lime treatment is 30%-3500; the residual Si after lime-coagulation-magnesium agent desiliconization is 1-1.5mg/L, colloidal silicon can be removed 60% to 90%.
  7. The solid S (mg/L) is dissolved in the water after lime treatment.

2. Lime softening treatment equipment-vortex reactor

Vortex reactor, also called fast reactor, is a kind of cyanide suitable for lime softening reaction. The advantage is that the residence time of water in the reactor is short, the flow rate is fast, and the equipment is small in size. The applicable conditions of the fast reactor are: low suspended solids and low oxygen consumption in raw water, carbonate hardness not less than calcium hardness (rnmol/L); magnesium hardness in raw water less than 20% total hardness (mmol/L). High-purity lime should be used to prepare milk of lime with a concentration of 2%. It is best to pre-install marble or quartz sand filler (d=0.10-0.5mm) before operation. The filling height is 1/3^1/2 cone height from the bottom of the body. The granular substance can act as a contact agent and accelerate the reaction. Crystal, the reaction product is a calcium carbonate crystal with a diameter of 2 to 3 mm, and the pH value of the effluent is controlled between 9.5-10.3. The effluent turbidity and supersaturation of the reactor are high. Practice has proved that the supersaturation of Caz+ is 0.2~0.3mmol/L (calculated as 1/2 Ca’+). In order to prevent the post-filter from fouling, it is necessary to consider adding As a measure to adjust the pH value by acid, it is also suggested to use the reactor as the filter medium of the post filter to further reduce the supersaturation of water. The gravity reactor can be equipped with an air separator before the raw water enters. The effluent of the vortex reactor should be controlled at 2PM, and the amount of lime is appropriate at this time; the slag of the vortex reactor is relatively pure and granular CaCO3, and the density of 2.58g/cm3 can be effectively used.

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