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Scale Removal Titanium Electrode Reference Video

China’s industrial water consumption accounts for about 20% of the country’s total water consumption, second only to agricultural water consumption. One of the industrial circulating cooling water consumption is very large, in a variety of industrial production process is very wide range of uses, such as petrochemical, metal smelting, power generation and other industries, accounting for about 80% of the total amount of industrial water.
The harm of scaling in the circulating cooling system mainly has the following aspects:
(1) scale is mostly a bad conductor, in the circulation process will affect the heat transfer, indirectly affect the production efficiency, or may cause an explosion and other dangerous events;
(2) Scale will reduce the water flow in the pipeline runner area, increase fluid resistance, resulting in increased energy consumption;
(3) If the scale deposits too much, you must stop to clean up, which will reduce the equipment running time to reduce efficiency. Therefore, whether from the perspective of normal production or the pursuit of economic benefits, to solve the circulating cooling water scaling problems are urgent, while saving water resources and ensure industrial safety is of great significance.
Electrochemical descaling technology has the advantages of no additional chemicals, green and non-polluting, easy to adjust, simple process structure, automation and so on, which is a new type of environment-friendly process.
In electrochemical descaling, the decomposition of water near the anode produces oxygen and acidic environment, while hydrogen and alkaline environment are produced near the cathode. In the alkaline environment, hardness ions such as calcium, magnesium, and carbonate ions react chemically to form water-insoluble calcium carbonate (CaCO3) and magnesium carbonate (MgCO3), and at the same time, hydroxide ions (OH-) generated by electrolysis of water combine with magnesium ions in the water to generate magnesium hydroxide (Mg(OH)2), which is subsequently removed from the water by the precipitate. In addition, the anode oxidation reaction generates H2O2, O3, ClO- and other strong oxidizing substances, which can play a good role in sterilization and algaecide and COD removal.
Although electrochemical descaling technology has many advantages, but the traditional electrochemical descaling technology there are still many problems that need to be improved:
1, due to the cathode generated OH- will be under the action of electric field force and the anode generated H + neutralization reaction, resulting in OH – utilization rate is low, low descaling efficiency;
2, the cathode in the generation of OH- at the same time can also provide deposition sites, so the non-homogeneous precipitation-oriented traditional electrochemical descaling technology on the cathode of the effective area of the higher requirements;
3, a single electrochemical descaling technology is mainly for the electrodeposition effect, it is difficult to meet the demand for water after a single treatment, so it is necessary to consider coupling with other technologies. In response to the above problems, many scholars in recent years have carried out adequate research. This paper summarizes and analyzes the H+-OH- separation technology, cathode design and electrochemical descaling coupling process according to the literature reports in recent years, and finally discusses the challenges that the electrochemical descaling technology may face in terms of commercialization and proposes the directions and opportunities for the future related research.

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Fig. 1 Diagram of the reaction mechanism:
(a) Ion exchange membrane electrochemical descaling process, the
(b) Bipolar membrane electrochemical descaling mechanism.
(c) Highly efficient split-flow bipolar electrolytic descaling system.
(d) Flow of scale and acid-base ions in two electrochemical systems:
(d1) electrochemical descaling system without CCM.
(d2) Electrochemical descaling system with CCM
In the electrochemical descaling process, the role of membrane materials is varied and critical, and they mainly play the role of isolating cathodes and anodes and ion selection.
Currently, the most commonly used membrane is ion exchange membrane, which is divided into cation exchange membrane and anion exchange membrane, and their main characteristic is the selective permeability to specific types of ions. Cation exchange membranes allow only cations (e.g. Ca2+, Mg2+, etc.) to pass through them, while anion exchange membranes allow only anions (e.g. CO32-, SO42-, etc.) to pass through them.
Electrochemical reactions occur in the vicinity of the electrodes. In addition to allowing specific ions to pass through, the ion exchange membrane prevents the mixing of other substances, such as preventing H2 and OH- generated in the cathode area from mixing with O2 and H+ generated in the anode area, thus improving the safety and descaling efficiency of the system. When a cation exchange membrane is used, the pH of the cathode chamber rises rapidly because OH- cannot pass through the cation exchange membrane, which contributes to the removal of hardness examples.
When the anion exchange membrane is used, Ca2+ and Mg2+ will migrate directionally towards the cathode and gather on the surface of the anion exchange membrane facing the anode side; while OH- and HCO3- in the electrolyte, will migrate directionally through the anion exchange membrane towards the anode, and react with the aggregated Ca2+ and Mg2+ on the other side of the membrane to form CaCO3 and Mg(OH)2. In the membrane-based segregated electrolytic cell In a membrane-based separated electrolyzer, the neutralization reaction of H+ and OH- is slower than in an unseparated electrolyzer, so preventing the neutralization reaction of H+ and OH- does not depend entirely on the selectivity of the ion exchange membrane. Meanwhile, since ion exchange membranes are expensive and the cations and anions accumulate near the ion exchange membranes during their migration to the cathode and anode under the action of the electric field force during a long period of operation, which can lead to the clogging of the ion exchange membranes, which can further lead to the increase of the energy consumption, a non-selective, porous, and sturdy separator material can be used as an alternative to ion exchange membranes as the separator material for electrolytic baths. liu et al. When hydrophilic polytetrafluoroethylene microfiltration membrane (PMM) was selected as the separation material, Ca2+, Mg2+, H+ and HCO3-, CO32-, OH- in the circulating cooling water could easily pass through the PMM, which made the anode and cathode chambers electrically neutral.
Although PTFE membrane has good chemical stability, excellent temperature resistance and other characteristics. However, it still has problems such as difficult fabrication, high membrane resistance, and easy adhesion of air bubbles. Therefore, in response to the problems of ion exchange membranes and PTFE membranes, Li et al. developed a new type of domain-limited crystalline membrane, which can use the high alkalinity environment in the cathode region to induce the precipitation of hardness ions near the diaphragm, and then accelerate the removal of hardness in the circulating cooling water. In the experiment, it was found that the domain-limited crystallization membrane could attenuate the effect of concentration polarization on mass transfer, and the formation of high pH and slow water flow environment was very conducive to scale precipitation.

Fig. 2 (a) Continuous electrolyzer structure for anode boundary layer extraction, the
(b) Reaction mechanism diagram of electrochemical separation descaling process with nylon mesh.
(c) Reaction mechanism diagram of electrochemical descaling promoted by bubble movement and water diffusion
The H+ generated at the anode and OH- generated at the cathode during the water electrolysis process are initially confined within the boundary layer on the electrode surface (thickness less than 100 μm), and then H+ and OH- migrate to the cathode and anode respectively under the action of electric field force, and are eventually consumed in the neutralization reaction in the native solution. Inspired by this property, Ba et al. concluded that when a tubular porous titanium filter is used as the anode, pumping the anode boundary layer solution at an appropriate rate prevents the anode-generated H+ from diffusing into the body solution.
The subsequent OH- produced by the cathode then raises the pH of the body solution and facilitates the removal of hardness ions from the solution. Since this process has to extract the H+-rich boundary layer solution from the tubular anode, the anode is always in a highly acidic zone, and prolonged operation may shorten the life of the anode and increase the operating cost.
The random diffusion of H2 bubbles generated due to water electrolysis can lead to turbulence in the flow regime near the cathode, which in turn generates turbulence, while water currents have also been found to accelerate the diffusion of ions. Taking advantage of these properties, it was deduced that the diffusion of OH- from the cathode surface could be accelerated and the effective separation of OH- and H+ could be achieved using bubble diffusion and water movement.Kang et al. separated the electrolyzer into anode and cathode chambers by employing a sandwich sandwich structure of a mesh cathode, a nylon mesh, and a mesh anode. During continuous operation, OH- is rapidly pushed from the cathode surface to the cathode chamber by the movement of water flow and H2 bubbles generated by cathode hydrolysis. As a result, a large alkaline zone is formed in the cathode chamber, which enhances the homogeneous precipitation of hardness ions. As the reactor needs to restore the electroneutrality, the nylon mesh structure in this system does not prevent the migration of OH- from the cathode chamber to the anode chamber and the migration of hardness ions from the anode chamber to the cathode chamber. Long-term operation will lead to nylon mesh fouling and clogging.
Mao et al. developed a reactor with a hollow cylinder on the upper part and a hollow inverted cone on the lower part, in which the mesh cathode, the center tube and the mesh anode were placed horizontally concentrically from the center outward, and the water flowed into the reactor from the inner part of the center tube, firstly passed through the mesh circular cathode, and then flowed through the mesh annular anode, and then overflowed out of the upper end of the reactor. During the operation of the reactor, the H2 bubbles generated by water electrolysis move upward and the influent water flow downward, and the average pH of the center tube can reach 10.6 within 3 min. However, due to the parallel placement of the anode and cathode in this reactor, which leads to a higher tank voltage inside the reactor, more electrical energy is consumed during the electrochemical descaling process. At the same time, because of the water flow vertically through the porous cathode, long-term operation will inevitably lead to scaling caused by clogging, making the voltage further increase.
Through the above analysis, it can be seen that the use of membrane materials can efficiently realize the separation of H+ and OH-, and thus improve the efficiency of electrochemical descaling, but the higher cost of membrane materials, poorer anti-pollution and other characteristics have led to its difficult to be applied in large-scale in the actual production. Although the new membrane-free electrochemical descaling reactor developed in recent years separates OH- without using membrane materials and improves the descaling efficiency, it inevitably brings the problems of complex reactor and high energy consumption, and its application in practice still needs to be further verified.

Fig. 3 (a) Schematic diagram of the operating principle of coupled cathode, the
(b) Mechanism diagram of three-dimensional cathode electrochemical descaling reaction, the
(c) Schematic diagram of multistage reactor electrochemical descaling device, the
(d) Fence cathode electrochemical descaling reaction mechanism.
During the electrochemical descaling process, the scale layer gradually covers the entire cathode surface as the reaction proceeds. This will reduce its working area, hinder the electrolysis reaction and mass transfer process, and increase the electrode resistance, leading to a decrease in the hardness removal rate and an increase in energy consumption.
Periodic cleaning of the cathode is therefore required, which can further increase operating costs. In electrochemical installations, in order to achieve good treatment performance, the cathode usually needs to be designed to achieve a larger working electrode surface area, which in turn improves the efficiency of electrochemical descaling.
Different cathode materials have a greater impact on scale deposition during the electrochemical descaling process, with mirror stainless steel cathodes showing the highest descaling performance, followed by plain stainless steel, nickel plate, brushed stainless steel and titanium plate. In addition, the mesh cathode has a higher hardness deposition rate than the flat cathode due to its larger specific surface area and rougher surface. Numerous researchers have focused on increasing the effective area of the cathode.
Luan et al. studied the electrochemical descaling efficiency of multilayer mesh stainless steel cathodes with different mesh counts, and Yu et al. designed a multistage reactor with Z-shaped flow by staggering the electrodes up and down, which can increase the contact area between the solution and the electrodes and improve the hardness removal efficiency by decreasing the internal dead zone. Compared with the two-dimensional cathode, the three-dimensional cathode can provide more deposition sites for the non-homogeneous precipitation of CaCO3 due to its larger specific surface area, and thus has a higher hardness removal effect, and improves the OH- utilization to a certain extent due to the slower diffusion of OH- into the bulk solution. The results show that the three-dimensional cathode system can inhibit the neutralization reaction between H+ and OH-, and has a higher current efficiency compared with the two-dimensional cathode system.
Although all of the above studies increased the effective area of the cathode and enhanced the reactor scale removal efficiency, the cathode still faces the problem of cathode cleaning due to scale deposition on the cathode, which is unable to maintain the long-term efficient hardness removal. Therefore Yang et al. designed a new fence cathode for hardness removal from solution. The cathode is covered with a layer of carbon felt on the mirror stainless steel plate cathode, which utilizes the porous structure inside the carbon felt to delay the diffusion of OH- generated by cathode water reduction into the body solution, and at the same time provides a large number of deposition sites for scale deposition, at which time an alkaline atmosphere is formed inside the carbon felt that is conducive to the removal of hardness ions.

Fig. 4 (a) Reaction flow diagram of electrochemical descaling-filtration-crystallization coupled system, the
(b) Reaction mechanism diagram of electrochemical reverse pole descaling-filtration-crystallization coupled system, the
(c) Schematic diagram of electrochemical accelerated precipitation-microfiltration coupling system device.
(d) Schematic diagram of the membrane deposition electrodialysis process device.
Since a single electrochemical descaling technology only has the role of electrochemical precipitation, the removal of hardness ions in the solution is relatively limited, the coupling process can take advantage of the complementary nature of the different technologies to improve the removal efficiency, reduce operating costs, expand the scope of application, and enhance the role of the effluent water quality. Therefore there have been numerous studies in electrochemical descaling coupling process has made efforts.
Electroflocculation is a promising method for removing hardness from water. The flocs produced during electroflocculation can provide a large number of precipitation regions for hardness removal, which will break through the limitations of the cathodic zone for electrochemical descaling. However, the slow hydrolysis reaction severely limits the formation of flocs, which in turn inhibits hardness removal, while dissolved Al3+ causes secondary contamination.Yu et al. proposed to significantly improve the efficiency of electroflocculation for hardness removal by introducing membrane polarization to catalyze the dissociation of H2O. However, the anode is constantly consumed during the electroflocculation process and a large amount of flocculated sludge is generated, which will undoubtedly increase the operating cost.
During the electrochemical descaling process, the scale cannot be completely precipitated and adsorbed on the cathode, but some of the scale is scattered in the reactor, which is easy to cause the phenomenon of antisolvation affecting the descaling effect, so Guo et al. proposed an electrochemical filtration crystallization coupling system coupled with a filtration crystallization module after the electrochemical descaling process. It can not only effectively intercept the scale particles in the system effluent, but also provide a large number of deposition sites for the homogeneous precipitation of CaCO3, so that the circulating water can be further purified. However, the filtration crystallization system in a long time after the operation, porous materials intercept adsorption of a large number of scale particles will be blocked, and its use of a reasonable way of regeneration needs to be the next key consideration.
During the accelerated nucleation of scale particles, small scale particles with a diameter of < 0.15 mm are formed due to homogeneous precipitation of the particles in the bulk solution. Microfiltration is therefore a very effective solid-liquid separation method that has a small footprint, is easy to operate and manage, and provides a large surface area for CaCO3 nucleation and growth, thus accelerating crystallization and improving descaling efficiency.
The electrodialysis process also enhances the removal of hardness ions from the solution, which is driven by an electrical potential difference. In order to overcome the problem that conventional electrochemical descaling techniques are less effective in removing hardness ions from water while removing other ions, Yu et al. developed a membrane deposition electrodialysis process, which combines conventional electrochemical precipitation with electrodeposition to remove ions from solution, thus contributing to the realization of zero discharge.

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