1. Introduction
In the early industrial development, lithium (Li) was used in many industries. For example, lithium acted as a fluxing agent in the ceramic industry and as a deoxidizer and dechlorination agent in the metallurgical industry. In addition, lithium has attracted more and more attention, worldwide, in the recent years, because of its application in the battery industry [
1,
2]. Lithium is mainly supplied to electronic industrial products, especially the power battery market. Lithium-ion batteries (LIBs) are widely used in electric vehicles and hybrid vehicles, due to their high energy density and high tolerance of a wide range of temperatures [
3,
4].
According to the report of the US Geological Survey [
5,
6,
7,
8,
9], the market for lithium is mainly 39% for the battery industry, 30% for the ceramics and glass industry, and 8% for the lubrication industry. Lithium is usually extracted from brine [
10,
11,
12] and mineral [
13] and the world’s annual crude lithium production is about 35,000 tons. Among all countries, Australia and Chile are the largest and second largest crude lithium exporters, respectively. In order to meet the needs of the battery industry, lithium production increased by, approximately, 12% in 2016. It is expected that the lithium-ion battery industry will flourish in the future and the demand for lithium metal will become more and more urgent. Therefore, it is important to recycle lithium metal or its compounds from LIBs. It not only achieves the goal of waste reduction but also improves the secondary resource utilization of waste LIBs.
In the LIBs, lithium, cobalt (Co), nickel (Ni), and manganese (Mn) are the main materials that are needed to recycle. Among them, Lithium is usually obtained in the form of lithium carbonate. Lithium carbonate has become a very important material in recent years. The global demand for lithium carbonate has gradually increased, due to its versatility, and the price will rise significantly in the future [
14]. According to the report of Sociedad Quimica y Minera de Chile, the demand for lithium carbonate will increase six hundred thousand tons to eight hundred thousand tones. For industrial activities, lithium carbonate is not only used as cathode materials in the LIBs but is also used to create other compounds, such as lithium chloride (LiCl), lithium bromide (LiBr) and lithium oxide (Li
2O). LiCl, LiBr, and Li
2O all can be raw materials for other industries. For example, LiBr can be used as an absorbent and a refrigerant. On the other hand, in the medical industry, lithium carbonate can also be used as a treatment for bipolar disorder [
15,
16]. Due to the unlimited development of lithium carbonate, a lot of countries try various methods to purify lithium carbonate to apply in different industries.
In the purification process, the methods used mainly are the causticization method, electrolysis, recrystallization, hydrogenation–decomposition, and the hydrogenation–precipitation method [
17]. As the hydrogenation–decomposition method generates little waste of materials and solutions and can achieve a higher purification of lithium carbonate, it was chosen for this experiment. During the decomposed process, calcium and lithium, both, separate out in the form of carbonates, due to their common properties of precipitation with an increase in temperature. In addition, sodium is also a critical problem in the purification process. To deal with these problems, the ion-exchange resin was chosen to remove the impurities. Comparing the literature, it could be observed that as a resin, IRC-748 has a great adsorption efficiency on Ca
2+, but insufficient efficiency on Na
+; Dowex G26 has the better effect on both Ca
2+ and Na
+. In order to reduce the impurities, efficiently, Dowex G26 was used in this experiment. To sum up, the hydrogenation–decomposition method and the Dowex G26 resin were used to get a high purity of the lithium carbonate.
2. Materials and Methods
2.1. Materials
The sulphate solutions came from a recycling LIBs waste cathode materials, which were done by previous research; their content is shown in
Table 1 [
18]. Sodium carbonate (Na
2CO
3) was purchased from Nihon Shiyaku Reagent, Tokyo, Japan (NaCO
3, 99.8%), for the chemical precipitation. CO
2 was purchased from Air Product and Chemical, Taipei, Taiwan (CO
2 ≥ 99%), to carry out the hydrogenation–decomposition method. Dowex G26 was obtained from Sigma-Aldrich (St. Louis, MO, USA) and was used as a strong acidic cation exchange resin, to remove impurities. Multi-elements ICP standard solutions were acquired from AccuStandard, New Haven, Connecticut State, USA. The nitric acid (HNO
3) and sulfuric acid (H
2SO
4) were acquired from Sigma-Aldrich (St. Louis, MO, USA) (HNO
3 ≥ 65%) (H
2SO
4 ≥ 98%).
2.2. Equipment
The materials were analyzed by energy-dispersive X-ray spectroscopy (EDS; XFlash6110, Bruker, Billerica, MA, USA), X-ray diffraction (XRD; DX-2700, Dangdong City, Liaoning, China), scanning electron microscopy (SEM; S-3000N, Hitachi, Tokyo, Japan), and inductively coupled plasma optical emission spectrometry (ICP-OES; Varian, Vista-MPX, PerkinElmer, Waltham, MA, USA). In order to control the hydrogenation temperature and heating rate, a thermostatic bath (XMtd-204; BaltaLab, Vidzemes priekšpilsēta, Rīga, Latvia) was used to heat the lithium carbonate slurry and the lithium bicarbonate solutions. On the other hand, a stainless-steel machine which was a closed, eight hundred milliliters cylinder, was designed, in which CO2 could be added from the top. Moreover, an electric blender was appended to make the CO2 dissolve into the lithium carbonate slurry. To calculate the amount of CO2 aeration, Flowmeter (GP5700; SIARGO, Lexington, MA, USA) was used in this experiment.
2.3. Experimental Procedures
2.3.1. Chemical Precipitation
The metal which was dissolved in the liquid phase was selectively precipitated, in the solid phase, by the chemical reaction in this process. The lithium was precipitated by carbonate sodium as shown in Equation (1).
The lithium carbonate was recovered after filtration and washed with hot water, to remove the residual impurities. In this procedure, lithium carbonate would precipitate when the temperature increased. However, the content of nickel ion would decrease intensely. Due to the different characteristics of solubility, the composition of lithium carbonate would change, as well.
Table 2 shows the metal composition of the lithium carbonate obtained, after the chemical precipitation by inductively-coupled plasma optical emission spectrometry (ICP-OES; Varian, Vista-MPX, PerkinElmer, Waltham, MA, USA).
2.3.2. Lithium Carbonate Slurry
Enough lithium carbonate was obtained in order to make lithium carbonate slurry, in the first process. However, the amount of the water would affect the hydrogenation and decomposition processing. If the amount of water was low, it would not be possible to add enough CO2 to the slurry. If there was too much water, it would cause more energy consumption in the decomposition procedure. On account of these conditions, 10 g lithium carbonate with 400 milliliters of deionized water were mixed to make our lithium carbonate slurry.
2.3.3. Hydrogenation Processing
After making the lithium carbonate slurry, it was first put into the thermostatic bath to maintain its temperature. Subsequently, it was poured into the aeration and stirring device and the CO
2 was added into it. Equation (2) shows the reaction for the dissolution of the CO
2 into the slurry.
In this procedure, the CO2 aeration and hydrogenation temperature were controlled and the pressure was kept at 0.2 MPa. The parameters for the CO2 aeration (1 L–2.5 L) and hydrogenation temperature (26.5 °C–30 °C) were set up. The initial weight of lithium carbonate and deionized water was 410 g. On adding the CO2 into the slurry, it turned into the lithium bicarbonate solution. However, it still had some lithium carbonate, which did not change into lithium bicarbonate. By filtering the remain lithium carbonate solid, we could weigh the lithium bicarbonate solutions. At last, the value we got the yields of the lithium bicarbonate solutions and this has been shown in this study.
2.3.4. Ion-Exchange
Dowex G26 is a strongly acidic resin which was used to exchange cations ions efficiently. Dowex G26 was used to remove the Ca2+ and Na+ which were the two most abundant impurities in the lithium bicarbonate solutions, in this process. The adsorption isotherms, described by means of the Langmuir and Freundlich isotherms, were used to investigate the ion-exchange behaviors of Ca2+. With the parameters of pH value, the Ca2+, and Na+ adsorption efficiencies were compared, without high pH values, because the Ca2+ would precipitate when it combined with OH−. On the other hand, the effect of the reaction time was also compared. The parameters of the ion-exchange experiments, the pH values (1–8) and reaction times (2 min–1024 min), were set. After removing the impurities, the content of the lithium bicarbonate solutions was investigated in this study.
2.3.5. Decomposition Processing
After removing the impurities in the lithium bicarbonate solutions by Dowex G26, lithium carbonate was needed to precipitate from the lithium bicarbonate solutions. Equation (3) shows the reaction through which the lithium carbonate was acquired, by heating the lithium bicarbonate solutions.
The heating rate was an important factor to control the impurities in this procedure. If the heating rate was too high, the lithium carbonate crystal would contain other ions. If the heating rate was too low, the energy consumption would increase. In order to compare the effect of the heating rate, the solutions were put into a thermostatic bath, the parameters (0.5 °C/min–1 °C/min) were set up and the solutions were heated to 90 °C. The whole process of experiment is shown in
Figure 1.
4. Conclusions
This study proposed a hydrometallurgical way to recover lithium carbonate, effectively, from the sulphate solutions. Calcium and sodium could be separated, effectively, with pH 7 and a 4 min reaction time in the ion-exchange step. In the hydrogenation–decomposition procedure, yields were the highest with 1.75 L of CO2 aeration, 27.5 °C of hydrogenation temperature, and 1 °C of the heating rate; purity was highest with a 0.5 °C heating rate, with the Dowex G26 resin. With these conditions, the lithium carbonate product could be acquired by the drying treatment, at 90 °C and its purity and recovery rate were almost 99.9% and 87.6%. In comparison with other methods, this method produced less waste and energy consumption, less use of chemicals, and could efficiently purify the lithium carbonate from the sulphate solutions.