Lithium recovery from synthetic geothermal brine using electrodialysis method

The demand of lithium in the global market is experiencing a significant increase. The electric vehicle era is the driving force of this lithium increase phenomenon. Although the demand of lithium continues to increase every year, the available lithium resources are still not able to meet the demand, so that lithium resources with much greater potential are being considered. The main objective of this study is to extract lithium from a primary resource, geothermal brine, with a practical and environmentally friendly method. Research on the extraction of lithium resources from synthetic geothermal brine with a specific lithium composition using the electrodialysis (ED) method has been carried out. The ED device used is provided with electricity and is operated using temperature variations (30°C and 40°C) and variations in electric voltage (2 V and 4 V). The highest flux is achieved at an operating temperature of 40°C and a power supply voltage of 4 V.


Introduction
Lithium is a mineral resource that plays a critical role in various aspects of people's lives in the world, such as industry and daily life. Lithium has been widely used in making glass, ceramics, batteries, and other industries. Lithium has various uses, but its abundance in nature is only 0.0018% [1].
The demand for lithium in the global market is experiencing a rapid increase due to its increasing use. The need for lithium is expected to continue to grow and dramatically in the coming years as various lithium batteries are the most promising candidates for powering electric vehicles [2]. Lithium consumption in the world continues to increase, reaching more than 100,000 tons of lithium carbonate per year. It is estimated that by 2025 lithium consumption can get more than 160,000 tons of lithium carbonate per year [3]. Currently, 2/3 of the world's lithium is obtained from brine extraction, half a million liters of brine per ton of lithium carbonate [4]. Although the demand for lithium is huge, lithium resources are still unable to meet this demand, so that a much larger lithium resource is being considered.
Lithium extraction from nature has become a trend in the lithium extraction industry due to its low cost and abundant availability. Lithium reserves are currently estimated at 14 million tons; 70-80% of 2 this lithium is stored in salt lake brine, geothermal brine, seawater, and solid lithium contained in lithium ore [5].
Geothermal brine is one of the lithium resources which has considerable potential. Geothermal brine is saltwater from within the earth and contains many minerals because it flows between the rocks in the earth. Geothermal fluids contain various minerals due to contact between hot fluids and rocks [6].
Even though seawater is the largest producer of lithium, the concentration in seawater is still very small, namely 0.1-0.2 ppm. Meanwhile, from a technological point of view, geothermal brine has the most significant potential, with a concentration of 10-15 ppm [7]. The geothermal water has great potential to be a source of clean energy and lithium in the future [8]. Geothermal fluids carry lithium in minerals as it reaches the earth's surface. One of the studies using geothermal brine for lithium extraction is a study conducted by Siekierka et al. [7] using the hybrid capacitive deionization method, which produces lithium with a concentration of 73%.
Electrodialysis is a process in which ions are transferred through the membrane due to the difference in potential electrical energy given and the flow of electric current [9]. In general, ED experiments are carried out in an electrodialysis stack, which is alternately arranged between a cation exchange membrane (CEM) and anion exchange membrane (AEM), anode electrode (such as an iridium coated titanium electrode), and a cathode electrode (such as a stainless-steel electrode) [10]. Several studies have shown that Li + can penetrate the monovalent ion exchange membrane from the brine (feed) into the concentrate (purified solution) under an electric driving force during the process. In contrast, other ions such as Mg 2+ and Ca 2+ do not penetrate the monovalent ion exchange membrane and stay in the bait [11]. Therefore, ED shows excellent potential in lithium extraction, especially the separation of lithium in brine.
The ED method is often applied in brackish water desalination, sewage treatment, boiler feed water, and process water. The ED method can also separate a mineral from other unwanted ions, as done by Bunani et al. [12] regarding the extraction of boron and lithium metals using the ED system. Nie et al. [10] also conducted a study on separating Li + ions from Mg 2+ ions with various Li + / Mg 2+ ratios using the electrodialysis method. Other studies on lithium recovery using the ED method have also been carried out, such as Zhao et al. [13]. This research discusses the effect of temperature on the lithium recovery process. Another important research on electrodialysis has also been conducted by Hoshino [14]. In this study, an ionic liquid-based membrane was used to separate lithium from seawater.
ED membranes are divided into two types, namely anion and cation membranes. Both types of membranes have a smooth flat surface with a plastic-like appearance. The membrane is waterproof because the membrane is reinforced with synthetic fibers. Other characteristics of the ED membrane are low electrical resistance, resistance to fouling, insoluble in water, resistance to pH conditions 2-9, and durability (10-15 years) [15].
A study on ED conducted by Ji et al. [16] showed that the membrane used is resistant to strong acids and bases, has an electrical resistance between 2.4-3 ohm.cm 2 , with a thickness of 0.13-0.15 mm, and can operate at temperatures up to 40°C. In other studies, it was also shown that the specifications for the membrane used were 0.183 mm, with a conductivity of 8.3 Siemens / m and a capacity of 0.89 meq/g [17]. There is a slight difference between the membranes that have been discussed and the characteristics of the membranes used in the research conducted by Gmar and Chagnes [18]. The characteristics of the membrane used in this study include a thickness between 0.125-0.2 mm, a resistance of 2.8-3.5 ohms.cm 2 , and a capacity of 0.74-162 meq/g. Electrodialysis has the same principle of electric flow as the electrolysis system. Electrodialysis is performed using a direct current (DC) source and cannot be done with an alternating current source. Electrodialysis undergoes breaking down an electrolyte due to an electric current as occurred in the electrolysis process. There is a change in electrical energy into chemical energy in electrodialysis cells, and the reaction does not occur spontaneously [19].
Research conducted by Chen et al. [20] uses electricity with a constant voltage of 5V. In a study conducted by Ji et al. [16], the power voltage used is 3-8V. Whereas in the research conducted by Song and Zhao [17], the electric voltage used was 3.5V. Voltage drop is the amount of the final measured voltage drop compared to the initial or planned voltage. The voltage drop is caused by several factors, including the amount of electric current and the ED system's internal resistance. The voltage drop is an important parameter that affects the performance of the ED process.
According to Zhou et al. [21], the voltage drop across one pair of membranes is 0.5-2 V/cp. Research conducted by Melnikov et al. [22] stated that the voltage drop is another factor besides the conductivity of the solution, which causes an increase in the consumption of electrical energy. The membrane resistance represents the resistance of the polymer matrix to ionic transport currents. Considerable membrane resistance can increase voltage drop [23].
Another possible phenomenon is concentration polarization. Concentration polarization is caused by the rejection of the membrane against the solute so that partial permeation will occur. The solution that cannot pass as a permeate will accumulate [9]. The concentration polarization can cause changes in solution conditions such as increasing pH, scaling, decreasing current efficiency, and water dissociation reactions [24].
Research conducted by Gmar and Chagnes [18] shows an effect of concentration polarization on research results. In that study, there was a reduction in efficiency from 25% to only 8% due to concentration polarization. Also, Nie et al. [10] show that there is a possibility of concentration polarization due to the use of high power during the process. Vermaas et al. [25], in their research, states that spacers that are shaped like nets can reduce the occurrence of concentration polarization but can increase the pressure drop and electrical resistance. Another negative impact is a reduction in the active surface area of the membrane due to shadow effects.
In this study, the emphasis is on the extraction of lithium from geothermal brine. Studies related to the extraction of geothermal brine using the electrodialysis (ED) method aimed at obtaining lithium resources with higher concentrations. Synthetic brine geothermal solution was used because it was assumed that the silica in the solution had been removed by other methods such as precipitation. Without silica, the fouling phenomenon will occur more slowly so that the observations of the research results will be more precise. The general objective of this study was to determine the best operating conditions during the lithium extraction process from geothermal brine using electrodialysis (ED). The specific purpose of this research is to determine the effect of variations in temperature and voltage on the lithium extraction process from geothermal saltwater using the electrodialysis (ED) method on the resulting lithium concentration.

Tools and material
The primary tool is an electrodialysis device. While the electricity used is direct electricity (DC). The ion exchange membrane is made of polyethylene and is manufactured by HARMOTECH ® . Before using the membrane, it must be immersed in 5% w/w NaCl brine for 24 hours. The maximum operating conditions in the use of this membrane are at a temperature of 45°C, a pressure of 0.5 MPa, and a pH of 1-12.      [26].
The silica composition in the study was neglected because it was assumed that the silica had been removed before being sent to the electrodialysis system. Silica removal aims to reduce the speed of the fouling phenomenon so that the analysis of the research results can be seen clearly.

Methodology
At the initial stage, the system was operated at a temperature of 40°C. Then the operating voltage was changed until it reached the desired voltage variation (2V and 4V). Then the geothermal brine flowed  on the side of the feed stream. Meanwhile, the recovery solution was flowed on the concentrate side to carry the lithium ions that escaped due to the difference in electric potential. The parameters to be measured were the solution flux. In the second experiment, the geothermal brine temperature was changed until it reached the desired temperature variation. (30°C and 40°C) and ran like the first try. In the second experiment, the used electric voltage was 4V because this condition was the optimum condition based on the previous first experiment.

Mathematical Models
The research was based on a review of the mass transfer theory in membranes in the electrodialysis (ED) process. As shown in Figure 3, the mass transfer mechanism that occurs is as follows, 1. Mass transfer of lithium ions towards the membrane surface on the feed side 2. The mass transfer of lithium ions from the feed side of the solution towards the concentrate side is due to the potential difference. 3. Transfer of lithium ions from the membrane surface of the concentrate side to the recovery solution.
Where Co represents the ion concentration in bulk and Cm represents the ion concentration in the layer near the membrane. In the ED system, the flux equation as a function of time can be formulated as in Equation 1 [25]: Where, = ion flux = effective membrane surface area = volume of solution ′ = time

Voltage variation
The first research was conducted in various operating conditions where 2V and 4V varied the voltage. The study was performed at an operating temperature of 40°C so that the experimental results were obtained Figure 4. The study was conducted using crossflow and co-current flow. The flux in the membrane expresses the amount of permeate that can be passed by the membrane per unit area and per unit time. Flux is often used as an indicator of membrane performance quality.  Figure 4. Effect of operating voltage on the lithium uptake rate. Figure 4 shows that the highest flux for all temperature variations is at the beginning of the measurement where the membrane used has not been fouled, which is 26 MLH for 2 V operating voltage and 37 MLH for 4 V operating voltage. Over time the flux tends to decrease because the membrane used during the study was getting clogged. The lowest flux in all types of operating voltage variations in the research that has been carried out is at a value of 23 MLH, wherein in these conditions, the flux decline has become very slow.
In addition, Figure 4 shows that the higher the applied voltage, the higher the flux generated. This phenomenon happens because the higher the voltage, the greater the electricity delivered and the faster the redox reactions in the system. Ohm's law shows that the electric voltage is directly proportional to the electric current; the greater the voltage, the greater the electric current delivered from the anode to the cathode [24]. So based on the solution flux from each voltage variation, it can be concluded that the best condition in the first study was at a voltage of 4 V because it produced the largest solution flux compared to the lower voltage condition.

Temperature variation
The second study was carried out in various operating conditions where 30°C and 40°C varied the operating temperature. The electric voltage used in the second experiment is 4V because this condition is the best based on the first experiment. Figure 5 shows that temperature plays an essential role in returning lithium using an electrodialysis system. Figure 5. Effect of operating operation temperature on the lithium uptake rate. Figure 5 shows that the highest flux for all temperature variations is at the beginning of the measurement where the membrane used has not been fouled, which is 26 MLH for an operating temperature of 30°C and 37 MLH for an operating temperature of 40°C. Over time the flux tends to decrease because the membrane used during the study is getting clogged. The lowest flux in all types of operating voltage variations in the research carried out is at an estimated value of 22-23 MLH, where the flux decline has become very slow at that condition.
In addition, Figure 5 shows that the higher the temperature, the faster the lithium recovery rate. This phenomenon is because the molecules in the solution move faster at higher temperatures. The faster the  molecules move; the more lithium ions move to the concentrate side. Vice versa, at a lower operating temperature, the lithium recovery rate is also slower. Besides that, the membrane's pores will enlarge at higher temperatures so that more solution escapes [13]. So based on the solution flux from each temperature variation, it can be concluded that the best condition in the first study was at an operating temperature of 40°C because it produced the largest solution flux compared to other lower temperature conditions.