Polarity reversal electrochemical process for water softening
Introduction
Recirculating cooling water, applied in many industries such as power, metallurgical and petroleum industry [1], [2], [3], usually has a high content of hardness ions. These high concentration hardness ions may form scale deposits on the surface of heat exchangers, resulting in the decrease of flow rate and heat transfer efficiency [4], [5], [6], [7], [8], [9]. Various techniques are proposed and used to prevent scaling. One of them is ion exchange. Although ion exchange has high efficiency, it requires frequent chemical regeneration, which is not only laborious but also environmentally unfriendly due to generating a large volume of waste solutions of acids or salts [10], [11], [12]. Another common technique is lime softening. It is simple in methodology. However, it is not environmentally friendly likewise [13], [14].
Electrochemical precipitation in water softening has drawn widely attention in recent years. It has many advantages such as environmental friendliness, no additional chemicals, and convenient process control [15], [16]. The hardness ions is removed by generating a high pH environment at the cathode surface by the following cathodic reactions:O2 + 2H2O + 4e− → 4OH−2H2O + 2e− → H2 ↑ + 2OH–
The precipitate is induced by the following reactions:HCO3– + OH– → CO32– + H2OCO32– + Ca2+ → CaCO3 ↓Mg2+ + 2OH– → Mg(OH)2 ↓
Most of the deposits adhere to the cathode, leading to the deactivation of cathode and the increase in electrical resistance. Therefore, it is necessary to remove the scale deposits from the cathode surface regularly to ensure the water softening efficiency and to decrease the energy consumption.
Periodic mechanical scraping is the most prevalent technique used for removing the scale layer [17]. It has been applied in scale detachment for decades because of its easy operation. Whenever scale detachment is necessary, the elastic scraper moves down tardily, scraping the scale deposits from the internal surface of the circular shaped cathode. However, the interelectrode gap would significantly increase due to the elastic scraper installing between the electrodes. As a result, energy consumption increases, and the effective area of cathode decreases, resulting in the degradation of water softening performance. So if a simpler and more convenient cathode cleaning method could be used in electrochemical reactor, the higher water softening efficiency and the lower energy consumption would be accomplished.
In this work, we used polarity reversal as a scale detachment method in electrochemical precipitation process. Polarity reversal has been used in various technologies such as electrodeionization and electrodialysis to restrain scale formation and to enhance performance [18], [19], [20]. When polarity of the electrodes is reversed, the chemical reactions at the electrodes are reversed accordingly. The original cathode with scale layer turns into anode to produce hydrogen ions. The hydrogen ions tend to react with calcium carbonate and magnesium compounds inside the scale layer, resulting in the detachment of the whole scale layer. Therefore, polarity reversal can be an effective method for cathode cleaning. In addition, the device configuration can be simple without any extra equipment for scale detachment. Hence, the interelectrode gap decreases notably, giving rise to a higher hardness removal efficiency and a lower energy consumption. The major objectives of this work are to examine the water softening and scale detachment performance, to evaluate the operational stability of the electrochemical process, and to examine the electrochemical properties of the DSA used.
Section snippets
Experimental setup
The experimental setup sketch is shown in Fig. 1. The working volume of the reactor was 100 mL. The IrO2 and RuO2 coated sheet titanium DSA (Baoji Zhiming Special Metals Company, Shaanxi Province, China) were used as an anode and a cathode, fixed in the interior of the reactor. The dimension of the DSA was 210 × 70 × 2 mm. A silicone spacer was placed between the anode and the cathode to keep an interelectrode gap of 5 mm. The central part of the silicone spacer was empty to ensure not covering
Effluent quality variation of the water softening process
The electrochemical process was tested for hardness removing. The feed solution passed continuously through the reactor. Fig. 2 shows the removal efficiency variations of Ca2+, Mg2+ and total hardness of the effluent in an operational cycle. The Ca2+ removal efficiency was only 10.6% initially and increased rapidly to 20.9% within 1.67 h. Then it increased slowly and reached to the maximum value of 26.5% at 4.67 h. Finally, it decreased gradually to 16.5%. Unlike the variation tendency of Ca2+,
Conclusions
Polarity reversal was used for removing scale from cathode surface successfully in the electrochemical precipitation process. High hardness removal efficiency and effective scale detachment were accomplished. After scale detachment by polarity reversal, the cathode could still keep its ability to remove hardness ions. The precipitation rate was 53.9–73.4 g/h/m2. The energy consumption was 17.3–23.6 kWh/kg CaCO3 and the total hardness removal efficiency was 16.4–21.4%. The results of cyclic
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