Pollution blocking and treatment of suspended solids
The suspended solids in the raw water will block the pores in the resin layer, thus increasing its flow resistance, increasing the operating pressure drop, and covering the surface of the resin particles, thus reducing the working exchange capacity of the resin.
In order to prevent the pollution of suspended solids, it is mainly to strengthen the pretreatment of raw water to reduce the content of suspended solids in water. Wedge wire filter is a good choice.
In order to remove the suspended matters accumulated in the resin layer, the methods such as increasing the number and time of backwashing or using compressed air scrubbing can be adopted.
The requirements of common chemical desalting system for the influent suspended solids are generally as follows:
Chemical desalination unit suspended solids (mg / L) strong acid positive (downstream regeneration) < 5 strong acid positive (convection regeneration) < 2 strong acid positive (floating bed) < 2 strong acid positive (downstream) → strong acid positive (floating bed) < 5 positive double bed, double chamber bed ＜ 2 positive double chamber floating bed ＜ 2 weak acid positive (downstream) → strong acid positive (downstream) ＜ 5 weak acid positive (downstream) → strong acid positive (floating bed) ＜ 5.
Iron pollution and treatment
Iron pollution may occur in both cation and anion resins.
The appearance of the contaminated resin is dark brown, which can turn black in severe cases. In general, when the iron content of every 100g resin exceeds 150mg, it should be treated.
The existence of iron can accelerate the degradation of anion resin.
In the use of cation resin, most of the iron ions brought by the raw water are in the form of Fe2 +. After they are absorbed by resin, some of them are oxidized to Fe3 +. When they are regenerated, they cannot be completely exchanged by H +, so they stay in the resin and cause iron pollution.
When iron salt is used as coagulant, some alum will be brought into the cation bed, which will be accumulated on the surface of resin layer by filtration.
Sometimes the iron content of anion resin is many times larger than that of cation resin. The iron of anion resin mainly comes from regeneration liquid.
During regeneration, alum will be dissolved by acid solution to make it Fe3 +, and some of it will be absorbed by cation resin, causing iron pollution. A large amount of Fe3 + in industrial hydrochloric acid will also cause iron pollution to the resin.
The cation resin used for sodium ion exchange is more easily polluted by iron.
The caustic soda produced by general diaphragm method contains 0.01% – 0.03% Fe2O3 and 6-7mg / L NaCl O3.
Such caustic soda will generate ferrate (FeO4) when it contacts with iron containers and pipes (without anticorrosive coating) during storage and transportation. When ferrate enters the anion bed with alkali solution, it will decompose due to the decrease of pH value. The reaction formula is as follows:
2FeO42－ ＋ 10H＋ → 2Fe3＋ ＋ 2／3 O2 ＋ 5 H2O
Fe 3 + further formed Fe (OH) 3, which attached to anion resin particles, resulting in iron pollution.
After the resin is polluted by iron, it can not be removed in the general regeneration process, so it must be cleaned with hydrochloric acid.
The common cleaning method is to use 10% HCl solution. Before this method, the corrosion resistance of the exchanger equipment must be checked, otherwise, hydrochloric acid with inhibitor must be used.
Enter 10% HCl solution equivalent to 0.5 times of the resin bed volume from the top of the resin bed (considering the residual water in the resin bed, keep the concentration of HCl solution), drain the water equivalent to the residual water in the bed from the bottom of the resin bed, stir the solution, and contact with the resin for 12 hours. Drain out the acid, wash from top to bottom, and then back wash for 30 minutes, remove the loose material, and then regenerate the resin bed and put it into operation.
The measures to prevent iron pollution of resin are as follows:
- Reduce the iron content of inlet water of cation bed. The groundwater with high iron content should be treated by aeration and manganese sand filtration. For surface water with high iron content or when using iron salt as coagulant, alkaline agent, such as Ca (OH) 2 or NaOH, should be added to improve the pH value of water and prevent iron ions from entering the cation bed.
Necessary anti-corrosion measures shall be taken for the pipeline and storage tank transporting high iron content raw water to reduce the iron content of raw water.
The storage tank and transmission pipeline of caustic soda for anion bed regeneration shall be rubber lined for corrosion protection to reduce the iron content of the alkali regeneration liquid.
When the iron content of the resin exceeds 150g / GR, pickling should be carried out.
Pollution and treatment of calcium sulfate
When sulfuric acid is used to regenerate calcium cation resin, if the concentration of the regenerated solution is too high or the flow rate is too slow, at the place close to the resin particles, the solubility product of the regenerated Ca2 + and SO42 – concentration in the solution exceeds CaSO4 will produce CaSO4 precipitation, which will be attached to the resin particles.
It is not only difficult to clean after regeneration, but also there is hardness in the washing solution, which will affect the progress of the ion exchange reaction.
During operation, it will also Dissolving in the outlet water can increase the hardness content and reduce the exchange capacity of the cation bed.
The solubility product of calcium sulfate at 25 ℃ is 2000 ppm, which decreases with the increase of temperature, so it is difficult to remove.
The measures to prevent calcium sulfate precipitation are to reduce the concentration of sulfuric acid in the regeneration solution and to speed up the flow rate of the regeneration solution.
It can also adopt step-by-step regeneration method to gradually increase the concentration of regeneration liquid and slow down the regeneration flow rate.
Once it is found that the resin precipitates with calcium sulfate, the most commonly used method at present is to backwash with a large amount of soft water, and then wash repeatedly with ~ 10% HCl (3 bed volumes) at 2.0 L / h / L. however, it should be noted that the dissolution rate of HCl and calcium sulfate is very slow, so it needs to be cleaned many times.
Another method is to use EDTA sodium salt, but the price is very high, and it is exothermic reaction, so attention should be paid when using.
Pollution and treatment of silicon
The pollution of silicon compounds occurs in the strong alkali anion exchanger, especially in the equipment and system where strong and weak anion resins are used together. The result often leads to the decrease of the efficiency of silicon removal in the anion exchanger.
Due to improper regeneration of strong alkali resin in anion bed, failure of resin regeneration in time or incomplete regeneration of anion resin, silicic acid will polymerize in resin particles, which is difficult to regenerate. This phenomenon is the accumulation of silicon in resin and does not belong to silicon pollution.
Silicon pollution refers to the silicate that has been regenerated from the resin in the regeneration process. Due to the reduction of pH value of the regeneration solution, a large amount of silicic acid is separated out in the colloidal state. In serious cases, the regeneration solution can become jelly like and be coated on the resin surface, affecting the exchange capacity of the resin, and causing the increase of SiO2 content in the effluent.
In general, silicon pollution will not occur in fixed bed and moving bed.
Silicon pollution mainly occurs in the single bed of convection regeneration with high ratio of silicon content to total anion content (excluding alkalinity) in raw water, especially in the equipment and system of combined application of weak and strong anion exchange resin.
Caustic soda can be used for cleaning silica pollution.
The recommended dosage is 130-160 g / L, the concentration is 2.0%, and the treatment temperature is 50 ℃ – 60 ℃.
The resin bed must be soaked first. If the condition is not allowed, the solution can pass through the resin bed at a flow rate of 2 bed volumes / hour. The key of this method is to keep a high temperature and contact time.
The main measures to prevent silicon pollution include:
- The anion bed shall be regenerated in time after failure, and it shall not be standby in failure state.
- The regenerated alkali liquor shall be heated, the temperature of type I resin shall not be higher than 40 ℃, and that of type II resin shall not be higher than 35 ℃.
- Reduce the concentration of regeneration solution to 2% NaOH.
- The flow rate of regeneration liquid shall not be less than 5 m / h, but the time of entering regeneration liquid shall not be less than 30 min.
- In the joint application system, it is necessary to ensure that the weak resin fails first in design.
Oil pollution and treatment
The pollution of mineral oil to resin is mainly adsorbed on the framework or coated on the surface of resin particles, resulting in the pollution of resin micropores, resulting in the decrease of resin exchange capacity and periodic water production.
The sources of mineral oil are:
- The mineral oil infiltrated into the ground is brought into the exchanger with the raw water.
- When the steam is used to heat the raw water, the oil is brought into the raw water along with the steam.
- The oil fired boiler uses steam atomized oil. When the oil pressure is higher than the steam pressure, the heavy oil (or crude oil) leaks into the steam and enters the condensate demineralization system through the condenser.
- The oil in the production process of the refinery or chemical plant leaks into the raw water through the steam system. When the oil content in the feed water of chemical demineralization equipment is 0.5mg/l, the resin will be polluted by oil within a few months.
Methods of treating oil polluted resin:
First of all, the source of the oil should be checked quickly, and the faults should be eliminated to prevent the oil from leaking in. If necessary, the oil accumulated in the equipment shall be cleaned. Slightly polluted resin does not need to be treated, and its exchange capacity can be recovered gradually in multiple regeneration. For severely polluted resin, proper treatment method shall be selected through small-scale test.
- Circulating cleaning with NaOH solution
Use 8% – 9% NaOH solution at 38-40 ℃, pass through anion bed and cation bed from alkali tank (about 10m3), and then return to alkali tank for circulation cleaning (specific time is determined by small-scale test), supplement NaOH solution, keep the solution concentration, use the emulsification of NaOH on mineral oil, and remove oil stains.
Cleaning with solvent
Petroleum ether or 200 solvent gasoline can be used to clean the resin, which should be strictly fireproof during cleaning.
Cleaning with solvent and surfactant
20 kg of No.200 solvent gasoline and TX-10 (non-ionic type, full name is PVC octane phenol) with 20% resin volume are used. After being added into the exchanger, the temperature is kept at 45-50 ℃, and then the oil-free compressed air is used for stirring and scrubbing. After 30 minutes, 200 kg of TX-10 surfactant is added, and the oil is emulsified by continuous stirring.
Resin trapper related information: Resin Trap Filter.