Preparation of Lithium Carbonate from Manganese-Containing Desorption Solution from Salt Lakes via an Organophosphoric Acid Extraction System

Abstract

Adsorption is a popular method for the recovery of low-grade lithium. It is a low-cost and highly efficient way to treat solutions with low lithium concentrations. The impurity content determines the industrial application. This study investigated a novel strategy to remove divalent cations from a desorption solution containing Mg²⁺, Ca²⁺, and Mn²⁺, generated by a manganese absorbent using an organophosphoric acid, followed by precipitation of lithium carbonate from the concentrated raffinate by evaporation. Di(2-ethylhexyl)phosphoric acid (P204) was selected as the preferred extractant. The saponification method and degree of saponification were determined, and the extraction parameters (pH, extractant concentration, and phase ratio) were investigated. A three-stage countercurrent extraction process was tested. Removal efficiencies of Mg²⁺, Ca²⁺, and Mn²⁺ from the manganese-containing desorption solution exceeded 99%, leaving <1.0 mg/L divalent cations in the raffinate. The raffinate was evaporated and concentrated to >23 g/L lithium. The total concentration of divalent cations in the lithium-rich solution was approximately 10.0 mg/L. Further conversion with sodium carbonate was carried out to prepare a battery-grade lithium carbonate product with a purity of 99.83%. The present work may provide a novel means of lithium recovery from a manganese-containing desorption solution.

1. Introduction

With the rapid development of electric vehicles, lithium has become an important strategic resource in various countries. The current demand for lithium products exceeds supply, so the development of lithium extraction technologies is an important consideration in the exploitation of lithium resources [1,2,3]. Owing to continuing exploration, measured and indicated worldwide lithium resources have substantially increased and total about 115 Mt. Of these, China has 6.8 Mt, accounting for less than 6% of the world’s lithium reserves (Mineral Commodity Summaries 2025, United States Geological Survey). Salt-lake brines account for more than 85% of China’s lithium resources. These are mainly distributed in Qinghai and Tibet; China’s most important salt lakes are Qarhan, West Taijinar, East Taijinar, and Big Chaidan [4]. Salt-lake brine in the Qinghai region has a very high magnesium content, and its Mg/Li ratio is up to hundreds of times that of similar foreign salt lakes [5], which leads to great difficulty in the recovery of lithium. China’s lithium extraction technology has only recently been initiated and is still in the early stages of development [6,7,8,9].

At present, the membrane separation [10,11], adsorption [12,13], and extraction [14,15] methods and a variety of coupled-process technologies have been applied to the extraction of lithium resources from the brines of Qinghai salt lakes. Owing to their different brine compositions, the development of an appropriate lithium extraction technology, known as the “One Lake, One Strategy” [16,17], has been adopted. Most salt-lake brines have a low lithium content, making it difficult to extract and prepare lithium salt products using only a single process [18]. The separation of Li⁺ is very challenging because Li⁺ and Mg²⁺ have similar hydration radii and chemical properties [19]. A combined adsorption–membrane separation has become the main process for lithium extraction from the Qinghai salt lakes [20,21]. In the wake of the rapid development of lithium battery materials, the demand for lithium salt products is increasing, and extracting lithium from old brine in salt lakes can no longer meet current development needs. The original brine extraction technology may not be the most economical method for lithium extraction. Therefore, it is imperative to develop more affordable lithium extraction technologies [22].

Adsorption is a popular method for the recovery of low-grade lithium. It is a low-cost and highly efficient treatment for desorption solutions with a low lithium concentration, and the final impurities present determine its industrial application. Of the inorganic metal-based adsorbents used for lithium extraction, manganese-containing adsorbents have a high adsorption capacity and have been explored for industrial applications in lithium extraction from salt-lake brine [23]. However, compared with other methods, such as adsorption, precipitation, and membranes, solvent extraction is a low-energy continuous operation with high efficiency for separating and extracting Li⁺ from complex brines [24]. A combination of adsorption and extraction methods can provide a new and efficient approach to lithium extraction, which could effectively alleviate the problems caused by the inevitable dissolution of manganese-containing adsorbents [25]. For this reason, this study developed a salt-lake lithium extraction technology for preparing lithium carbonate by combining adsorption and extraction, using the desorption solution obtained from the adsorption method as the raw material. This work provides a new research idea and scheme for lithium extraction from the Qinghai salt lakes [26,27,28].

This investigation proposes a novel strategy for the efficient removal of divalent cations from a desorption solution containing Mg²⁺, Ca²⁺, and Mn²⁺, generated by a manganese absorbent using an organophosphoric acid (OPA) and precipitation of lithium carbonate from the concentrated raffinate by evaporation. The type of extractant, saponification method, degree of saponification, extractant concentration, initial aqueous phase pH, and phase ratio were first optimized to remove Mg²⁺, Ca²⁺, and Mn²⁺ divalent cations. A lithium-rich raffinate (>20 g/L Li) was obtained by evaporation and concentration. High-purity lithium carbonate was then prepared by conversion using sodium carbonate. The overall experimental program is shown in Figure 1.

2. Experimental Section

2.1. Materials and Equipment

The manganese-containing solution used in this study was sourced from the desorbent after the adsorption of lithium by a manganese adsorbent from the brine of Qarhan Salt Lake in West Qinghai, China. The chemical composition of the desorption solution is shown in

Table 1. Composition of desorption solution.

Ions	Li+	Na+	K+	Mg2+	Ca2+	Mn2+	B	Cl−	pH
Mass Conc. g·L−1	1.296	0.025	0.057	0.308	0.317	0.217	0.002	8.793	3.18
Molar Conc. mol·L−1	0.187	1.1 × 10−3	1.5 × 10−3	0.013	7.9 × 10−3	3.9 × 10−3	1.8 × 10−4	0.248

. The OPA extractants, P204 (di(2-ethylhexyl) phosphoric acid, C₁₆H₃₅O₄P), CYANEX 272 (C272; bis(2,4,4-trimethylpentyl)phosphinic acid, C₁₆H₃₅O₂P), and P507 (2-ethylhexyl phosphonic acid mono-2-ethylhexy ester, C₁₆H₃₅O₃P) (Figure 2) were of 98% purity (Aladdin Industry Corporation, China) and were used without further purification. The organic phases used in the experiments were formulated from these OPA extractants using sulfonated kerosene as the diluent. The aqueous solutions were prepared with deionized water. All inorganic reagents were of analytical grade.

The main instruments used were as follows: an oscillator (SR-2DW; TAITEC, Tokyo, Japan), centrifuge (JIDI-20D, Guangzhou Jidi Instrument Co., Ltd., Guangzhou, China), X-ray diffractometer (XRD) (X’ Pert Pro, Panaco, The Netherlands), scanning electron microscope (SEM) (JEOL, JSM-5610LV, Tokyo, Japan), inductively coupled plasma optical emission spectrometer (ICP-OES; Thermo Fisher ICAP 7000, Waltham, MA, USA), analytical balance (AL204, Mettler Toledo, Greifensee, Switzerland), and pH meter (Seven Excellence; Mettler Toledo, Greifensee, Switzerland).

2.2. Experimental Procedure

In the extraction experiments, 0.1 mol/L OPA diluted with sulfonated kerosene was used as the organic phase. The organophosphorus acid was saponified with different concentrations of NaOH solution to varying degrees. The organic and aqueous phases were mixed in a separatory funnel at a specified organic-to-aqueous volume ratio (O/A) and shaken for 10 min at room temperature. After the separation of the phases, the aqueous portion was removed, appropriately diluted, and measured for the content of metal ions using ICP-OES. The organic phases were separated by centrifugation followed by complete stripping with 6 mol/L HCl and dilution to determine the cation content in the resulting acid solution by ICP-OES.

2.3. Three-Stage Countercurrent Extraction Procedures

The three-stage countercurrent extraction procedures followed the procedure described in our previous work [29]. The countercurrent extraction procedures were accomplished at a laboratory scale using a separatory funnel, as shown in Figure 3 [30]. P204 (0.07 mol/L) dissolved in sulfonated kerosene and the desorption solution without a pH adjustment were used as the organic and aqueous phases, respectively.

2.4. Analysis and Calculation

C_o and C_e (g/L) are the initial and equilibrium concentrations of metal ions in the aqueous phase, respectively. The extraction efficiency (E) is the ratio of the mass of the metal extracted into the organic phase to the initial mass of the metal in the aqueous phases, given by the following expression:

$$E\left(\%\right)={\displaystyle \frac{C_o-C_e}{C_o}}\times 100$$

(1)

The degree of saponification was calculated according to:

$$Saponification degree\left(\%\right)={\displaystyle \frac{C_{NaOH}\times V_{NaOH}}{C_{OPA}\times V_{org}}}\times 100$$

(2)

when 10 mol/L NaOH is reacted with an OPA extractant during saponification, a homogeneous phase is formed (instead of two separate phases), and the saponified solution is generally not separable [31]. Theoretically, the water generated by the saponification reaction has little impact on the volume of the original extractant, with an error within 1%; therefore, the generated water and saponified extractant are regarded as a single extraction organic phase. The error generated in this process is identified as a systematic error, and its impact on the calculation of subsequent extraction results has a constant value.

3. Results and Discussion

3.1. Effect of NaOH Concentration on Saponification Method

The effect of different concentrations of NaOH on the degree of saponification of the OPA extraction systems was first investigated [32]. Taking P204 as an example, when the degree of saponification was equal to the molar concentration of OPA (referred to as 100% saponification), different concentrations of NaOH solution were used as saponification agents. The experimental phenomena after saponification followed by 12 h of standing are shown in Figure 4. When using 1.0 mol/L or 0.1 mol/L NaOH for saponification, a turbid phase was initially generated, and the interface only appeared after a long period of standing, which is not conducive to timely separation. In contrast, the use of too low a NaOH concentration for saponification can cause changes in volume, which is not conducive to subsequent extraction processes. However, when using 10.0 mol/L NaOH, the system did not exhibit phase separation and formed a transparent phase without the above-mentioned phenomena. Therefore, when choosing 10.0 mol/L NaOH as the saponification agent, saponification will occur according to the expected extent, which complies with the experimental requirements.

3.2. Screening of Organophosphorus Acid Extractants

The organophosphorus acids P204, P507, and C272 are widely used as extractants for the removal of divalent ions due to their high affinity for Mg²⁺. Examples are the separation of low concentrations of Mg²⁺ from lithium-containing solutions [33], synergistic solvent extraction of Ca and Mg from concentrated Ni sulfuric acid solutions using C272 [34], and high-efficiency removal of Ca and Mg from a lithium-concentrated solution via countercurrent extraction using di(2-ethylhexyl)phosphinic acid [35]. The pK_a values of P204, P507, and C272 differ, with values of 2.75, 3.30, and 3.73, respectively. This results in a decrease in H⁺ dissociation ability across the sequence and, hence, differences in their extraction ability for divalent cations. Therefore, this study investigated the removal of divalent ions using these three types of organic phosphates.

The experimental extraction results for the P507, P204, and C272 extraction systems at different degrees of saponification are shown in Figure 5. When using the same concentration of OPA (0.1 mol/L), the ability to extract divalent cations gradually increased with the degree of saponification. The divalent cations were completely removed at 70% saponification. The influence of extraction efficiency and extractant structure was weak. All three OPA extractants achieved good extraction efficiency with respect to divalent cations, provided the extent of saponification was precisely controlled. Considering that P204 is the most widely used and least expensive extractant [36], we chose this as the extractant for subsequent experiments.

The saponification process is considered a neutralization reaction between a monobasic strong acid, such as an OPA, and sodium hydroxide. The reaction rate is very fast, and the reaction is simple:

(3)

3.3. Effect of pH on Extraction Efficiency of Li⁺, Mg²⁺, Ca²⁺, and Mn²⁺ from Desorption Solution Using P204

The effect of the pH of the desorption solution on the extraction efficiencies of Li⁺, Mg²⁺, Ca²⁺, and Mn²⁺ are shown in Figure 6. The results show that the extraction efficiency of all ions was relatively low at pH 1; at pH > 2, the extraction efficiencies of Mg²⁺, Ca²⁺, and Mn²⁺ exceeded 95%, while that of Li⁺ was about 10%. If the pH of the desorption solution was further increased, the extraction efficiencies remained unchanged, indicating that the pH had little effect on the extraction efficiency. Therefore, there was no need to adjust the pH, and the as-received desorption solution was directly used for subsequent extraction experiments.

3.4. Determination of Extractant Concentration

The effect of the extractant concentration was investigated using 70% saponified P204 and with no pH adjustment of the desorption solution. The results are shown in Figure 7. As the concentration of P204 increased, the extraction efficiencies of Ca²⁺, Mg²⁺, and Mn²⁺ first increased and then stabilized, but that of Li⁺ increased sharply under high extractant concentrations. To minimize the loss of Li⁺ and remove only the divalent cations, we chose 0.07 mol/L P204 as the preferred extractant concentration, under which the conditions of the extraction efficiencies of Ca²⁺ and Mn²⁺ exceeded 95%, Mg²⁺ extraction efficiency was 80%, and Li⁺ loss was 7%.

3.5. Determination of Phase Ratio

The effects of different phase ratios on the extraction efficiencies of Li⁺, Ca²⁺, Mg²⁺, and Mn²⁺ are shown in Figure 8. The results indicate that as the O/A ratio decreased, the extraction efficiencies of Li⁺, Ca²⁺, Mg²⁺, and Mn²⁺ correspondingly decreased. The extraction efficiency of the divalent cations Mg²⁺, Ca²⁺, and Mn²⁺ decreased sharply when the phase ratio (O/A) was greater than 1/2, so we chose the phase ratio 1/1 as the best comparison condition.

3.6. Removal of Mn²⁺, Ca²⁺, and Mg²⁺ by Three-Stage Continuous Countercurrent Extraction Process

For the complete removal of the Mn²⁺, Ca²⁺, and Mg²⁺ divalent cations, a three-stage continuous countercurrent extraction process was carried out using the desorption solution shown in Table 1 as the aqueous phase and 0.07 mol/L P204 as the organic phase. The experimental process was consistent with the description shown in Figure 3. Measurements of the Li⁺, Mn²⁺, Ca²⁺, and Mg²⁺ concentrations in the aqueous phases following eight contact cycles were used to determine the results shown in Figure 9. The extraction efficiencies of Mn²⁺, Ca²⁺, and Mg²⁺ significantly improved in three stages of countercurrent extraction, exceeding 99.5%, and Li⁺ was almost completely rejected. The composition of raffinate is shown in

Table 2. Composition of brine at different stages.

Ions (g·L−1)	Li+	Na+	K+	Mg2+	Ca2+	Mn2+	B	Cl−	pH
Desorption solution	1.296	0.025	0.057	0.308	0.317	0.217	0.002	8.793	3.18
Raffinate	1.288	1.010	0.049	0.001	N.D.	N.D.	0.004	8.781	5.87
Lithium-rich solution	23.61	20.14	1.050	0.008	0.002	N.D.	0.034	130.2	5.11

N.D.: Not detected.

: the Li⁺ concentration remained at approximately 1.30 g/L, but Mn²⁺, Ca²⁺, and Mg²⁺ were reduced to <1 mg/L. Quantitative separation of the divalent cations from Li was achieved.

3.7. Preparation of Li₂CO₃

The P204 extraction system has very good selectivity for divalent metal ions. The divalent cations were essentially completely removed from the raffinate, with only 0.0010 g/L of all divalent cations detected in the lithium-rich solution. Thus, we were able to directly utilize the lithium-rich solution to prepare a high-purity lithium carbonate product. The raffinate was first concentrated to achieve a Li⁺ content of approximately 23 g/L. The composition of the lithium-rich solution obtained after concentration is shown in Table 2.

To prepare the Li₂CO₃, the lithium-rich solution was placed in a thermostatic water bath at a preset temperature of 80 °C. Sodium carbonate solution (2.0 mol/L) was used as the precipitant at 110% of the stoichiometric requirement and slowly added dropwise. The resulting Li₂CO₃ precipitate was filtered and washed three times with deionized water at 80 °C to remove residual sodium and potassium, then dried under vacuum at 100 °C for 24 h. X-ray diffraction (XRD) and scanning electron microscopy analyses of the Li₂CO₃ are illustrated in Figure 10. The XRD patterns are in accordance with the standard card for Li₂CO₃ (PDF#22–1141) [37]. Microstructural analysis showed that the particle surface was smooth and the morphology regular. The purity of the Li₂CO₃ product exceeded 99.5%, and the yield was 78.7%. The main components of the Li₂CO₃ product are shown in

Table 3. Composition of Li₂CO₃ product (mass%).

Species/%	Li2CO3	Na	K	Mg	Mn	Cu	Ca	Cd	Ni
Sample	99.83	0.0045	<0.001	<0.0006	<0.0001	0.0003	0.021	<0.0001	<0.0001
Species/%	Al	SO42−	Si	Zn	Pb	Cl−
Sample	<0.0002	<0.005	0.0056	0.0003	<0.0004	0.0022

.

Based on the above experimental results, we propose a process flow diagram for the recovery of Li⁺ from manganese-containing desorption solutions from salt lakes using an OPA extraction system, as shown in Figure 11. Using this process, we can obtain a raffinate with the complete removal of divalent cations. The process comprises one saponification stage, three stages of extraction, and one stripping stage. The raffinate can be further concentrated, evaporated, and converted using sodium carbonate to obtain a high-purity lithium carbonate product.

4. Conclusions

This study utilized a coupled adsorption–extraction process to produce lithium carbonate from a manganese-containing desorption solution from Qarhan Salt Lake in Qinghai, China, to achieve the goal of extracting and separating lithium with high efficiency. A battery-grade lithium carbonate product was successfully prepared. Quantitative removal (>99%) of the divalent cations Mg²⁺, Ca²⁺, and Mn²⁺ present in the manganese-containing desorption solution was achieved using P204 under the following extraction conditions: OPA = 0.07 mol/L, 10 mol/L NaOH as the saponification agent, 70% saponification, initial aqueous phase pH = 3.18, O/A = 1/1, and phase contact by shaking for 10 min at room temperature. The total amount of divalent cations in the raffinate was <1.0 mg/L. The raffinate solution was evaporated, concentrated, and further converted with sodium carbonate to prepare a battery-grade lithium carbonate product with a purity of 99.83%. Solvent extraction with P204 was proven to be a feasible and promising method for the quantitative removal of divalent cations from a manganese-containing desorption solution.