# Research on a Reductive Deep Chlorine Removal Process for Breaking Through the Solid Film Barrier
## 1 Introduction  
As one of the important non-ferrous metals, zinc has a wide range of industrial applications and significant economic value [1, 2]. Hydrometallurgical zinc refining is the method predominantly used in the zinc smelting process, accounting for 80% of the world's zinc production [3]. However, with the acceleration of industrialization and the overexploitation of mineral resources, the quality of ones has gradually declined, leading to a significant increase in the impurity content in zinc ores, especially the content of chlorine [4, 5]. In the hydrometallurgical zinc refining process, the continuous accumulation of chloride ions can corrode equipment and electrodes [6, 7]. In an acidic environment, the high concentration of chloride ions easily reacts with the PbO\({}_{2}\) protective layer on the anode, leading to anode degradation [8]. Additionally, the generated Pb\({}^{2+}\) can reactwith chloride ions to form PbCl\({}_{2}\), which not only affects the stability of the electrodes but also reduces the purity of the final zinc ingots, thereby impacting product quality [9; 10]. Therefore, effectively controlling the chloride ion concentration in the hydrometallurgical zinc refining system has become a key step in ensuring process stability and product quality. At present, there have been extensive studies on chlorinated wastewater treatment at home and abroad, and the main methods include ion exchange [11; 12], adsorption [13; 14], electrochemical dechlorination [15; 16; 17], chemical precipitation, etc. Although ion exchange does not introduce new impurities, it generally shows poor dechlorination performance and requires a long processing time. Adsorption methods can efficiently remove chloride ions but often suffer from high adsorbent loss rates. Electrochemical dechlorination offers excellent removal efficiency. In recent studies, [PERSON] et al. [18] conducted an electrochemical chlorine evolution reaction (CER) using Ti/RuO\({}_{2}\) electrodes, which enhanced the chloride removal efficiency and reduced long-term chlorine release. [PERSON] et al. [19] developed conductive RO membranes (CNT-RO) embedded with carbon nanotubes, which were effective in reducing membrane degradation through the electrochemical reduction of chlorine, significantly improving chlorine resistance and desalination efficiency. However, the relatively high energy consumption of this method limits its feasibility for long-term industrial application. The chemical precipitation method is often used as the main method for removing chloride ions because of its easy operation and fast effect, and common precipitation methods include silver chloride precipitation [7], bismuth oxychloride precipitation [20], and cuprous chloride precipitation [21]. However, the price of silver and bismuth is usually high, which makes the dechlorination cost of the chemical precipitation method high and limits its application. Currently, companies typically use copper slag produced during the hydrometallurgical zinc refining process for dechlorination [22]. However, the copper slag dechlorination reaction is a typical solid-liquid reaction. In practical applications, when copper slag is left for a long time, a solid film forms on its surface, which hinders the mass transfer process. Meanwhile, the product CuCl tends to accumulate on the surface of the solid reactant, covering the active sites and further hindering the reaction. In addition, the copper slag dechlorination reaction makes a special redox reaction, and is characterized by a low reaction-driving force and incomplete reactions. These factors lead to low dechlorination efficiency, long dechlorination times, and high consumption of copper slag in this process [23]. Therefore, it is of great significance to explore a more efficient and rapid dechlorination method. In response to the issues existing in current dechlorination processes, a new method of deep dechlorination using ascorbic acid to reduce Cu\({}^{2+}\) is proposed in this study.