**1. Introduction**

### *1.1. General Introduction*

Metal resources currently in use can be divided into mine resources and renewable resources. Urban mining means that metal resources, which are industrial raw materials, are widely distributed in the form of products or waste, which means that they exist quantitatively on a mine scale. These are renewable resources. The amounts of waste catalyst, waste lithium batteries, and electronic waste are steadily increasing, and among them, the amount of waste lithium batteries is greatly increased. As the demand for lithium, a raw material that is a key component of electric vehicle batteries, is increasing, technology for recovering it from waste resources is required [1,2]. The market demand for electrolytes that make up lithium-ion batteries is expected to have an annual average growth rate of 42% from 2019 to 2025 [3]. Lithium is the main material of the fourth industrial revolution. Lithium-based batteries are rechargeable chemical batteries with excellent technical performance and high working voltage and energy density [4]. In addition, the charging and discharging life cycle is long, and they can be implemented with light weights, so the demand in the electric vehicle market has greatly increased [5]. In the Republic of Korea, because the amount of lithium is very small, the country is dependent on imports, so if the supply is insufficient compared to global demand, the domestic industry will be affected. So there is a need for a method to maintain an internally stable lithium supply and demand.

The amount of lithium in saltwater is about twice as large as that in ore, and many studies have been conducted on recovering lithium from brine [6–10]. In order to recover lithium from brine, a purification process is required due to the large amount of impurities. The concentration of lithium is also low, so the energy consumption required is high and the process takes a long time [6–12]. Lithium carbonate is mainly used in industry, and in order to prepare lithium carbonate from a lithium sulfate solution, the concentration of lithium ions in the lithium sulfate solution must be at least 10 g/L [13]. In general, the recovery of lithium from brine can be done by commercialized processes with natural evaporation and evaporative concentration, but this has the disadvantage of low energy efficiency and recovery. Lithium recovery by solvent extraction is effective at a lithium concentration of 1.0 g/L or more, and there are increased process cost and environmental pollution due to the use of organic extractant [11,14,15]. Electrodialysis (ED) is a process used to concentrate the desired component or separate it from impurities by selectively passing ions through an ion exchange membrane in an electric field. It is environmentally friendly because almost no byproducts are generated during the process and no additional chemical treatment is required to reuse the reagents afterward. Research using ED to recover lithium has been reported, and ion exchange membranes used for ED have been developed. Most of the previous studies use seawater or brine as a raw material and contain about 200 ppb lithium and impurities (potassium, magnesium, and sodium) [6–9].

Therefore, in this study, seawater or brine was not used as a raw material, but electrodialysis was used to concentrate lithium in the raffinate after the hydrometallurgy of cobalt and nickel. A basic study was conducted to concentrate high concentration lithium from the waste solution containing about 3000 ppm lithium.

#### *1.2. Theoretical Background of Electrodialysis*

In electrodialysis, when an electric field is applied, cations move to the cathode and anions move to the anode by an electric potential gradient, and ions are selectively passed through an ion exchange membrane that has a charge, thereby selectively concentrating a target component or separating impurities. At this time, the cation and the anion exchange membranes form a pair, and they can be effectively concentrated by being installed in several pairs. Figure 1 shows the ion exchange principle and solution flow in a general electrodialysis apparatus. The raw material solution is introduced into the solution chamber, and ions move to be divided into a diluted or a concentrated solution. When a membrane and laminar flow boundary layer is formed, an increase in current density can rapidly decrease ions in the boundary layer, generating concentration polarization. When the concentration polarization increases, the electrical resistance increases and the required power increases rapidly, which can cause decomposition of water. The pH change of the solution generated by the electrolysis of water can damage the ion exchange membrane or cause scaling. In addition, a spacer, which is an insulating material, is placed between the membranes to prevent short circuit between them. This spacer can form turbulence that reduces polarization due to concentration gradient. As a main variable in research and at industrial sites, there is a water migration phenomenon from the dilution chamber to the concentration chamber. If the water transfer rate is high, the ion concentration ratio can be reduced, so the effect on water movement cannot be ignored. Although osmotic pressure due to a concentration difference is hardly generated while an electric field is applied to the ion exchange membrane, metal ions and hydrated water molecules can move together by electrolytic osmosis to increase the water transfer [16,17].

Generally, high volume ratio concentration (HVRC) and multistage concentration (MSC) are the methods of concentrating target components using ED. HVRC is a method of concentrating a target component by using the volume difference between a concentrate and a diluent. In this study, two or more stages of ED were performed through a continuous process, and the MSC method was used for the concentrated solution at high concentration. Usually, the optimal conditions in the first stage are confirmed by the HVRC method and concentrated to a high concentration by the MSC method [17]. In a previous study by the authors, the conditions for lithium concentration in waste liquid containing

lithium were established by HVRC, and in this study, a high concentration of lithium was also achieved by MSC.

**Figure 1.** Schematic diagram of the electrodialysis process for the ion transfer.
