Method for jointly reducing COD (Chemical Oxygen Demand) and hardness of chloride ions and calcium and magnesium in circulating water

Abstract

The invention relates to the technical field of circulating water treatment, in particular to a method for jointly reducing COD (Chemical Oxygen Demand) and hardness of chloride ions and calcium and magnesium in circulating water. Circulating water firstly passes through a multi-medium filter to reduce the turbidity in the circulating water, then the circulating water enters an anion exchange column, chloride ions, sulfate radicals and other inorganic anions and oxalate, formate, benzoate radicals, acetate and other organic anions contributing COD in the circulating water are replaced with hydroxyl radicals, and at the moment, the pH of outlet water of the circulating water is 13-14; effluent of the exchange column enters a sloping plate sedimentation tank, 4% sulfuric acid is added into an inlet of the sedimentation tank to adjust the pH to be 7-8.5, and effluent of the sloping plate sedimentation tank is purified circulating water. According to the method, the scaling possibility of the circulating water is effectively reduced, the water quality of the circulating water can be remarkably improved, for example, after the circulating water is adsorbed by the ion exchange column, the COD removal rate is greater than 85%, the chloride ion removal rate is higher than 80%, and after the circulating water is settled by adding sulfuric acid into the inclined plate settling tank, the calcium and magnesium hardness removal rate is not lower than 50%.

Description

Technical Field

[0001] The invention relates to the technical field of circulating water treatment, and in particular to a method for jointly reducing COD, chloride ions, and calcium and magnesium hardness of circulating water. Background Art

[0002] In industrial production, circulating water systems are widely used. However, in the long-term use of circulating water, the presence of calcium, magnesium ions, suspended matter, chloride ions, sulfate ions and other substances in the water can easily lead to system scaling. Affected by chloride ions, calcium and magnesium hardness, the circulating water has to add a large amount of new water and discharge the same volume of stored water. The main source of circulating water replenishment is industrial new water. As the circulating water rate increases, the COD in the replenishment water increases, and the concentration of COD and calcium and magnesium ions increases accordingly. The increase in calcium and magnesium hardness will lead to scaling of equipment such as pipes and heat exchangers in the system. Scaling will not only reduce the heat exchange efficiency of the equipment and increase energy consumption, but also may affect the normal operation of the equipment and shorten the service life of the equipment; pollutants such as COD in the water will also have adverse effects on the circulating water system and the surrounding environment; excessive Cl- concentration in the circulating water will greatly increase the corrosion rate of each heat exchanger in the system, and will also cause irreversible corrosion damage to the pipes and valves in the system, causing huge economic losses.

[0003] Existing methods for reducing calcium and magnesium hardness in circulating water include bypass RO membrane method, electrochemical method and high-density sedimentation by adding sodium hydroxide.

[0004] Although adding oxidizing chemicals such as sodium hypochlorite can reduce the COD concentration in the circulating water, it cannot reduce other indicators at the same time and will increase chloride ions in the circulating water or bring in other impurities.

[0005] Ro membrane can effectively remove salt from circulating water, but the investment is high and the preconditions are harsh. The water production rate is generally around 85%, the amount of concentrated water produced is large, and the subsequent treatment of concentrated water is difficult.

[0006] The electrochemical method can effectively prevent the scaling problem of circulating water, but it cannot effectively reduce the chloride ions in the circulating water and cannot solve the corrosion problem of the heat exchanger caused by high chloride ions in the circulating water.

[0007] Adding sodium hydroxide to reduce the calcium and magnesium hardness in the circulating water is simple and effective, but it also introduces new sodium ions into the circulating water, and the chloride ions in the circulating water cannot be reduced.

[0008] Existing circulating water treatment methods often have problems such as poor treatment effect, complex equipment, and high cost. Therefore, developing a complete set of equipment that can simultaneously reduce the concentrations of chloride ions, COD, and calcium and magnesium ions can effectively improve the above working conditions, and has important practical significance for efficient, economical and reliable circulating water anti-scaling and purification technology. Summary of the invention

[0009] The purpose of the present invention is to provide a method for jointly reducing COD, chloride ions, calcium and magnesium hardness of circulating water, effectively removing scaling substances in circulating water, reducing pollutant content, reducing the risk of scaling in circulating water and improving water quality.

[0010] The present invention is achieved through the following technical solutions: The present invention adopts a circulating water anti-scaling method combining a multi-media filter, an anion exchange column and a high-density inclined plate sedimentation tank, wherein the multi-media filter, the anion exchange column and the high-density inclined plate sedimentation tank are connected in sequence; the three steps rely on each other, and can simultaneously reduce the problem of calcium and magnesium ion scaling in the circulating water and the problem of chloride ion corrosion on the heat exchanger, and can effectively reduce the COD in the circulating water.

[0011] The specific steps include: Filtration stage: The circulating water first enters the multi-media filter. By controlling the water flow rate, the water is fully in contact with the filter medium in the filter, effectively reducing the turbidity of the circulating water and removing large suspended solids and colloids. The multi-media filter is backwashed regularly to maintain its filtration performance.

[0012] Ion exchange stage: The filtered water enters the anion exchange column, and the water flow rate and residence time are adjusted to ensure that the anion exchange reaction is fully carried out. When the anion exchange column reaches saturation, it is regenerated with 4% sodium hydroxide solution. The regenerated waste liquid enters the multi-effect evaporation system for treatment to produce salt substances.

[0013] Sedimentation stage: The high pH water flowing out of the anion exchange column enters the high-density inclined plate sedimentation tank. 4% sulfuric acid is added at the inlet of the sedimentation tank to adjust the pH value to an appropriate range. The sedimentation effect of the inclined plate is used to further remove impurities such as tiny particles, calcium and magnesium ions in the water to produce purified circulating water. The sediment at the bottom of the sedimentation tank is cleaned regularly.

[0014] The water inlet flow rate of the above-mentioned multi-media filter is controlled to be 2-6BV / h.

[0015] The water inlet flow rate of the above anion exchange column is controlled to be 2-6 BV / h.

[0016] Sulfuric acid solution is added to neutralize the pH value and produce calcium sulfate precipitation to reduce the hardness of the circulating water.

[0017] The specific working principle is: the circulating water first passes through a multi-media filter to reduce the turbidity in the circulating water, and then enters the anion exchange column, in which the chloride ions, sulfate ions, and other inorganic anions and oxalate ions, formate ions, benzoate ions, acetate ions and other organic anions that contribute to COD in the circulating water are replaced by hydroxide ions. At this time, the pH of the circulating water outlet is 13-14; the outlet water of the exchange column enters the inclined plate sedimentation tank, and 4% sulfuric acid is added at the inlet of the sedimentation tank to adjust the pH to 7-8.5. The outlet water of the inclined plate sedimentation tank is the purified circulating water.

[0018] According to experimental demonstration, after adsorption by ion exchange column, COD removal rate is greater than 85%, chloride ion removal rate is higher than 80%, and after sedimentation by inclined plate sedimentation tank with sulfuric acid, calcium and magnesium hardness removal rate is not less than 50%.

[0019] Compared with the prior art, the present invention has the following advantages: The present invention forms a complete circulating water treatment system by combining a multi-media filter, anion exchange column and a high-density inclined plate sedimentation tank, which can remove scaling substances and reduce pollutant content in circulating water from multiple aspects. It not only effectively reduces the possibility of scaling of circulating water, but also significantly improves the water quality of circulating water. For example, after adsorption by ion exchange column, COD removal rate is greater than 85%, chloride ion removal rate is higher than 80%, and after precipitation by sulfuric acid in inclined plate sedimentation tank, calcium and magnesium hardness removal rate is not less than 50%. DETAILED DESCRIPTION

[0020] The technical solution and effects of the present invention are further described below through specific embodiments. Example 1

[0021] The present invention adopts a circulating water anti-scaling method which combines a multi-media filter, an anion exchange column and a high-density inclined plate sedimentation tank.

[0022] In the circulating water system of the 6.05 coke oven production area of ​​Jinding Steel Coal Coking Co., Ltd., a new bypass multi-media filter was built, and the outlet was connected to a newly added DN200 pipe of about 10 meters. The inlet pipeline of the anion exchange column was connected to the outlet of the existing multi-media filter, and the outlet pipeline of the anion exchange column was connected to the inlet of the high-density sedimentation tank.

[0023] Multi-media filter: It includes a tank body, which is filled with quartz sand, anthracite and other filter media from bottom to top. There is a water inlet on the top of the tank body and a water outlet on the bottom. The circulating water enters from the water inlet and is intercepted and adsorbed by the filter media to reduce the turbidity of the circulating water and remove impurities such as suspended matter and colloids in the water.

[0024] Anion exchange column: connected to the outlet of the multi-media filter, the column is filled with anion exchange resin. After the water is treated by the multi-media filter, it enters the anion exchange column, and the anions such as chloride ions and sulfate ions in the water exchange with the exchangeable ions on the resin and are replaced by hydroxide ions, so that the pH of the circulating water outlet is raised to 13-14.

[0025] High-density inclined plate sedimentation tank: connected to the outlet of the anion exchange column, an inclined inclined plate group is set in the sedimentation tank. A dosing device is set at the inlet of the sedimentation tank for adding 4% sulfuric acid.

[0026] The water inlet flow rate of the above-mentioned multi-media filter is controlled at 2-6 BV / h; the water inlet flow rate of the anion exchange column is also controlled at 2-6 BV / h.

[0027] Sulfuric acid solution is added to neutralize the pH value and produce calcium sulfate precipitation to reduce the hardness of the circulating water.

[0028] When the COD of circulating water is higher than 60 mg / L or the Cl- is higher than the warning value, open the inlet and outlet valves of the multi-media and anion exchange column, and the flow rate is 30-80 m 3 / h One cycle runs for 8-20 hours, with a processing capacity of about 640m 3 , entering the filtration stage and ion exchange stage. Filtration stage: The circulating water first enters the multi-media filter. By controlling the water flow rate, the water is fully in contact with the filter medium in the filter, effectively reducing the turbidity of the circulating water and removing large suspended particles and colloids. The multi-media filter is backwashed regularly to maintain its filtration performance. Ion exchange stage: The filtered water enters the anion exchange column. By adjusting the water flow rate and residence time, the anion exchange reaction is ensured to be fully carried out. When COD is reduced to below 40 mg / L, the Cl- concentration can theoretically drop by 80 mg / L. After three consecutive cycles, the backwashing procedure begins.

[0029] The effluent from the anion exchange column is pumped into a high-density sedimentation tank. An automatic sulfuric acid dosing device is installed at the front end of the sedimentation tank to add 4% sulfuric acid at the inlet of the sedimentation tank to control the pH at 7.5-8.5. Mud is discharged regularly at the bottom of the sedimentation tank. The effluent from the high-density sedimentation tank is the finished water.

[0030] Backwash: After the anion exchange column is saturated with adsorption, close the circulating water inlet and outlet valves, open the alkali wash inlet valve, and turn on the pneumatic diaphragm pump to pump 32% sodium hydroxide solution to the adsorption tower. The amount of 32% sodium hydroxide is 2 tons (1.5 m 3 ), open the desalination membrane inlet valve to overflow the exhaust port, open the compressed air valve to start bubbling mixing for 2 hours.

[0031] Rinse twice, 15 m each time 3, rinsing time 2 hours. The solution after the second rinse is recycled for the next rinse. Each rinse needs to be discharged 15m 3 Waste liquid.

[0032] The device generates 30 m every 4 days of operation. 3 Sodium hydroxide waste liquid. 15 m 3 The sodium hydroxide solution can be saved for next use, and the remaining 15 m 3 The waste liquid is a mixed solution of sodium chloride and sodium hydroxide with a concentration of 2-3%.

[0033] Average waste liquid output 5 m 3 / day, the theoretical water production rate of the device is 97.7%.

[0034] After adsorption by ion exchange columns, the COD removal rate is greater than 85%, the chloride ion removal rate is higher than 80%, and the calcium and magnesium hardness removal rate is not less than 50% after sedimentation in the inclined plate sedimentation tank with sulfuric acid.

[0035] The combined device has a compact structure and small footprint, and is suitable for circulating water systems of different sizes. Resources are used rationally during the treatment process, such as the treatment of anion exchange column regeneration wastewater to produce salts, which has good economic benefits. The treatment method is simple to operate, easy to control, low in operating cost, has good environmental benefits, and reduces pollutant emissions.

Claims

1. A method for jointly reducing COD, chloride ions, calcium and magnesium hardness of circulating water, characterized in that The following steps are involved: A multi-media filter, anion exchange column and a high-density inclined plate sedimentation tank connected in sequence; Filtration stage: The circulating water first enters the multi-media filter. By controlling the water flow rate, the water is fully in contact with the filter medium in the filter, the turbidity of the circulating water is reduced, large suspended particles and colloids are removed, and backwashing is performed regularly to maintain its filtration performance; Ion exchange stage: The filtered water enters the anion exchange column, and the water flow rate and residence time are adjusted to ensure that the anion exchange reaction is fully carried out. When the anion exchange column is saturated, it is regenerated with 4% sodium hydroxide solution. The regenerated waste liquid enters the multi-effect evaporation system for treatment to produce salt substances; Sedimentation stage: The water flowing out of the anion exchange column enters the high-density inclined plate sedimentation tank. 4% sulfuric acid is added at the inlet of the sedimentation tank to adjust the pH to 7-8.

5. The sedimentation effect of the inclined plate is used to further remove tiny particles and impurities in the water. The clean water flows out from the outlet at the top of the sedimentation tank and returns to the circulating water system.

2. A method for jointly reducing COD, chloride ions, calcium and magnesium hardness of circulating water according to claim 1, characterized in that The water inlet flow rate of the multi-media filter is controlled to be 2-6BV / h.

3. The method for jointly reducing COD, chloride ion, calcium and magnesium hardness of circulating water according to claim 1, characterized in that The water inlet flow rate of the anion exchange column is controlled to be 2-6 BV / h.

4. The method for jointly reducing COD, chloride ion, calcium and magnesium hardness of circulating water according to claim 1, characterized in that Sulfuric acid solution is added to neutralize the pH value and produce calcium sulfate precipitation to reduce the hardness of the circulating water.