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Secondary refining process control of brine 1 The significance of secondary refining of brine The ion membrane electrolysis process has strict requirements on the quality of the incoming brine. The first reason is that the ion exchange membrane used in electrolysis can pass through a large number of cations, such as Ca2+, Mg2+, Fe2+, Ba2+, Ni2+, Sr2+, etc. These metal ions and OH- ions in the cathode chamber generate insoluble hydrogen. The oxides precipitate and block the micropores of the membrane, causing the ion exchange performance of the membrane to decrease, the cell voltage to increase, and the current efficiency to decrease, eventually leading to membrane damage. Second, the brine contains components such as free chlorine and ClO-, which will cause poisoning and failure of the chelating resin used in secondary brine refining. If the primary brine filter uses a carbon sintered tube filter, it will also reduce the performance of the carbon sintered tube filter. The high standard quality of secondary refined brine is the primary condition to ensure the safe, stable and efficient operation of the ion membrane electrolyzer. When the ion exchange membrane is running, the total content of Ca2+ and Mg2+ in the brine is required to be less than 20×10-9, and the correct operation of the filter and resin tower system is the core to ensure the quality of secondary refined brine. The total mass concentration of Ca2+ and Mg2+ in brine refined by ordinary chemical methods can only be reduced to about 10 mg/L. To make it lower than 20×10-9, the solid suspended solids (SS) content in the brine must first be reduced to less than 1 mg/L, and then chelating resin can be used to absorb it to reach the standard. The secondary refining of brine is based on the primary refining, and further reduces the concentrations of Ca2+, Mg2+, etc. in the brine to the required range by adsorbing with chelating resin. 2 Types and working mechanism of chelating resin 2.1 Types of chelating resin There are two types of chelating resin commonly used in secondary brine refining, one is iminodiacetic acid type and the other is aminophosphoric acid type. The principle of selecting chelating resin is to comprehensively consider several aspects such as large exchange capacity, high regeneration efficiency, small volume change, small resistance drop, strong oxidation resistance and price economy. The chelating resin used by Taishan Salt Chemical Company is Japan's CR-II (iminodiacetic acid) resin. 2.2 Ion exchange principle and regeneration process of chelating resin Chelating resin is also a kind of ion exchange resin. It can adsorb metal ions and form a ring structure (such as chelate), so it is called chelating resin. The iminodiacetic acid-type resin used by the company is formed from a central ion and two ligands. Among them, M is the central (metal) ion; N and O are atoms that can provide a shared electron pair, and the central ion forms two coordination bonds with it, forming a stable ring structure under appropriate conditions. The formation and dissociation of complexes are two mutually opposing but dependent processes. On the one hand, the central ion combines with the complexing agent through coordination bonds to form a complex, which shows a certain chemical attraction; on the other hand, due to the contradictory movements within the complex, part of them will dissociate and show a certain chemical repulsion. Under certain external conditions (such as pH value, temperature, concentration), a relative equilibrium state is reached. Changing the conditions will destroy this balance. The regeneration of chelating resin is carried out based on this principle. Taking iminodiacetic acid type chelating resin as an example, the theory and practice of complexes show that its complexing (chelating) ability of metal ions changes with changes in pH value. The lower the pH value, the weaker the complexing ability; the higher the pH value, the stronger the complexing ability. On the other hand, different metal ions have different complexing abilities with chelating resins. Those with strong complexing ability can still complex (such as mercury) when the pH value is low; while those with weak complexing ability can only complex at higher pH values. As acidity increases, the following equilibrium shifts to the right. In actual production, about 5% hydrochloric acid is added to "clean" the resin. When the pH value is 1, almost all the originally complexed metal ions are "washed out". The resin at this time is called "H" type. Add 4% NaOH solution to the "H" type resin that has "eluted" metal ions and adjust its pH value to 14. Due to the large reduction in H in the solution, the following equilibrium shifts to the right. The resin returns to its pre-adsorption state. The absorption, desorption and regeneration of chelating resin are carried out based on this principle. When 1-valent and 2-valent ions coexist in the solution, this resin selectively absorbs 2-valent ions. The absorption capacity is as follows: Hg2+>Cu2+>pb2+>Ni2+>Cd2+>Zn2+>Co2+>Mn2+>Ca2+>Mg2+>Ba2+>Sr2+>Na+ 3 Salt water secondary refining process. The resin tower secondary refining brine production process can be divided into a three-tower process and a two-tower process. Among the major ion membrane caustic soda production technology and equipment suppliers, Beijing Chemical Machinery Factory, Japan Chlorine Engineering Company and Asahi Kasei Company all use the three-tower process for secondary refining of brine; Italy's Woody Company, Germany's Wood Company and the American Western Company use the two-tower process. The three-tower process ensures that two towers operate in series when one cation exchange tower is regenerated to meet the process requirements for secondary purified brine in the electrolysis process. Taking the three-tower process production as an example, the primary filtered brine is acidified by adding acid to adjust the pH value to 9±0.5, and then enters the primary filtered brine tank. The primary filtered brine is pumped to the plate brine heat exchanger to preheat to (60±5)°C, and then enters three cation exchange towers. The secondary refined brine flowing out from the ion exchange tower flows into the secondary refined brine tank, and then sent to the electrolysis unit using the secondary refined brine pump. The waste liquid generated during the regeneration of the ion exchange tower flows into the regeneration wastewater pit. After neutralization, the waste liquid is sent to the primary brine process by the regeneration wastewater pump for salt conversion. The three-tower production process flow is shown in Figure 1. 4. The influence of process control on the operation of the resin tower. In addition to the resin itself, the adsorption capacity of the chelating resin is also related to the temperature, pH value, brine flow, Ca2+, Mg2+ content and other factors of the brine. Chelating resins have different internal structures and different exchange capacities, but the changing trends in flow rate, temperature, and pH value are the same. Therefore, it is necessary to strengthen the control of various process control indicators to ensure that the quality of the brine entering the tank is qualified. 4.1 The chelating reaction between temperature chelating resin and calcium and magnesium is carried out at a certain temperature. When the temperature is high, the chelating reaction speed is fast and the resin has a long service life. However, if the brine temperature is too high (greater than 80°C), the strength of the resin will decrease and the breakage rate will increase, which will cause irreparable damage to the resin. To ensure the good performance of the resin, the temperature of the brine entering the resin tower should be controlled at 55 to 65°C. 4.2 pH value At a certain pH value, calcium, magnesium, etc. exist in the form of ions, which facilitates chelation removal by the resin. When the pH is 11, magnesium ions easily generate Mg(OH)2 colloidal precipitate, which will block the resin pores after entering the resin tower. * * The exchange capacity of the resin is reduced, and at the same time, the brine entering the resin tower is deflected and the pressure drop is increased, resulting in incomplete removal of calcium ions in the brine and an increase in the calcium and magnesium content in the secondary brine. Therefore, the pH value of salt water should be controlled at 9.0±0.5. 4.3 Brine flow rate The supply of brine is determined based on the electrolysis production capacity, the selection of the resin tower and the resin filling amount in the tower. The flow of brine entering the resin tower depends on the size of the resin tower and the required cycle time. If the flow of brine is too large, the residence time in the resin will be shortened, causing the brine to short-circuit in the resin tower, and the calcium and magnesium ions in the treated brine will be unqualified; if the flow of brine is reduced, the service time of the resin will be extended, but a larger resin tower will be required. It is generally required that the brine flow rate should be less than 40 m3/h, and the optimal flow rate is 20 m3/h. 4.4 Ca2+ and Mg2+ concentration in brine. The adsorption capacity of calcium and magnesium ions in brine by the chelating resin tower increases as the concentration increases. However, when the mass concentration of Ca2+ and Mg2 exceeds 10 mg/L, the resin's ability to remove calcium and magnesium ions decreases as the concentration of calcium and magnesium ions increases. This is because the exchange volume of the chelating resin is certain. The calcium and magnesium ions in the brine have no time to exchange and are brought into the secondary brine, causing the calcium and magnesium content in the secondary brine to increase. 4.5 Free chlorine in brine Free chlorine is extremely oxidizing and can easily destroy the structure of the chelating resin, causing irreversible poisoning of the resin. The resin performance drops sharply and cannot chelate calcium and magnesium ions. Therefore, it is required that the brine must not contain free chlorine. 5. Regeneration process of chelating resin tower. The regeneration of chelating resin tower resin requires the following steps: (1) Program change. This step is the regeneration preparation time, and the reaction will stop after 30 seconds. (2) Drainage. The liquid in the tower is discharged with process air. (3) HCl and NaOH tank detection. This step is performed simultaneously with the previous step of drainage. (4) Backwash. Pure water is introduced from the bottom of the tower for backwashing, which can loosen the resin layer, and the waste water is discharged from the pipe in the middle of the tower. (5) Bubble. Process air is introduced from the bottom of the tower to loosen the resin layer. (6) Let stand. The floating resin is left to rest. (7) Cleaning. In the above drainage step, the brine has been discharged, but the salt (particles) in the resin have not been completely discharged. If it is not washed away, it will exist in the resin. Therefore, rinse with water to replace the salt in the resin particles with water. Pure water flows through the middle of the tower and is discharged from the bottom of the tower. (8) Backwash. In order to wash out the suspended solids deposited on the top and inside of the resin layer when feeding, pure water is introduced from the bottom nozzle in a countercurrent manner and discharged from the top nozzle of the tower. (9) Let stand. The floating resin is left to rest. (10) HCl regeneration. 4% hydrochloric acid enters downstream from the central nozzle and is discharged from the bottom of the tower. (11) Hydrochloric acid discharge. Some regeneration liquid may remain in the resin layer. In order to make full use of it, pure water can be introduced from the middle pipe after hydrochloric acid regeneration and discharged from the bottom of the tower. (12) Drainage. Use process air to discharge the wash water with a small amount of hydrochloric acid. (13)NaOH regeneration. 5% NaOH enters countercurrently from the bottom of the tower and is discharged from the pipe in the middle of the tower. (14) Wash with water. There is some NaOH regeneration liquid remaining at the bottom of the tower. In order to make full use of it, after the NaOH regeneration is completed, pure water will be introduced from the bottom of the tower and discharged from the middle of the tower. (15) Bubble. There will be some NaOH regeneration liquid remaining in the resin layer. To make full use of it, process air should be introduced from the bottom. (16) Let stand. The floating resin is left to rest. (17)NaOH emission. There may be some NaOH regeneration liquid remaining in the resin layer. In order to make full use of it, super-refined brine is introduced from the middle pipe and discharged from the bottom of the tower. (18) Bubbling. During the above process, the discharge of ultra-refined brine causes the resin particles to shrink, causing the resin surface to be uneven. If operated in this state, the brine will flow to one side. After the liquid is discharged, process air is introduced from the bottom of the tower to flatten the resin surface. (19) Let stand. The floating resin is left to rest. (20) Feed preparation. In preparation for operation, the tower is filled with ultra-refined brine. The entire process of resin tower regeneration is controlled by PLC and DCS. 6 Issues that should be paid attention to in the operation and regeneration of the resin tower 6.1 Resin filling amount The resin filling amount has been strictly specified in the design and cannot be easily changed. The filling amount must be guaranteed to be at a given value. If resin loss occurs, it should be added in time to ensure that there is a sufficient amount of absorbing resin in the tower. 6.2 Resin tower pressure difference control If the pressure difference in the resin tower is too high, a large amount of resin will be broken, and it also indicates that there is a large amount of broken resin or impurities such as cellulose and SS in the tower. Therefore, it is stipulated that the pressure difference between the inlet and outlet brine of the two towers cannot exceed 0.1 MPa. When the pressure difference rises, the backwash intensity should be increased to wash out impurities such as broken resin to reduce the pressure difference, or the resin tower should be disassembled to find out whether the specific cause is blockage of the distributor in the tower or excessive impurities. 6.3 Decreased resin performance: The ability of resin to absorb precious metal ions is stronger than its ability to absorb calcium and magnesium ions. When heavy metal ions are adsorbed by the resin, normal regeneration steps can no longer restore the adsorption capacity of the resin and affect the absorption of calcium and magnesium ions. After the resin has been used for a long time (two or three months), according to the principle of desorption, use 2 to 3 times the amount of hydrochloric acid and 2 times the amount of NaOH for regeneration, which can remove most of the heavy metal ions and restore the resin to more than 90% of its normal adsorption capacity. Since the resin is oxidized by ClO- in the brine, the adsorption capacity is permanently reduced. At this time, even if double the amount of regeneration is used, the adsorption capacity cannot be restored. The solution should be to replace the resin. 6.4 The time for adding pure water and acid-base concentration must be accurate. If the amount of acid-base used does not meet the requirements, the resin regeneration will be incomplete and the regeneration effect will not be achieved. If the amount of acid-base is too high, the performance of the resin will decrease. Even the resin will be damaged and lost. Therefore, during the resin tower regeneration operation, special personnel should be assigned to monitor and record each step in the regeneration process and the completion time of each step of the regeneration operation. 6.5 Regularly check the instrument valves. During the resin tower regeneration process, frequent switching of the surrounding control valves will cause tightness to decrease or malfunction, resulting in brine leakage and resin loss. The control valves around the resin tower should be checked regularly and any problems found should be dealt with or replaced. 7 Conclusion To maintain good performance of chelating resin, you must be familiar with the ion exchange principle and process of the resin, analyze and summarize the factors that affect the performance of the chelating resin, so as to reasonably control and implement optimization plans. Only when on-site workers fully understand the regeneration principle and the control conditions of the regeneration steps can they correctly judge abnormal situations and handle them promptly and correctly. Therefore, on-site worker training must be strengthened. At the same time, strict process control is required to produce high-quality secondary refined brine and ensure the efficient operation of the ion membrane electrolyzer. See if it helps you.

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Replenish: Regarding the amount of acid and alkali used for regeneration, please refer to http://bbs.hcbbs.com/thread-503651-1-1.html Resin Tower Regeneration Usage Calculation Table (Beijing Chemical Machinery)

What about temperature? Are there any requirements for regeneration temperature?

This post was last edited by lujianfei79 on 2009-11-19 16:59 "The chelating reaction between chelating resin and calcium and magnesium is carried out at a certain temperature. When the temperature is high, the chelating reaction speed is fast and the resin has a long service life. However, if the brine temperature is too high (greater than 80°C), the strength of the resin will decrease and the breakage rate will increase, which will cause irreparable damage to the resin. To ensure the good performance of the resin, the temperature of the brine entering the resin tower should be controlled at 55 to 65°C. ” The third floor has explained it very clearly. If not, there will be no heat exchanger behind the 154 pump. Especially now that it is winter, if the insulation measures are not in place, the set temperature of the heat exchanger outlet should be adjusted upwards appropriately. 6#wu * nren

Resin Tower Regeneration Usage Calculation Table (Beijing Chemical Machine) I can’t download this file. I want to check it. I hope someone who can download it can send me a copy. The email address is 398594095@qq.com. Thank you.

Is the inlet water temperature too high? Generally, the inlet water is controlled at 35-40 degrees Celsius.