Integrated Process for Selective Purification and Production of High-Purity Lithium Carbonate from Aral Sea Brine

1. Introduction

Lithium carbonate is one of the key materials used in lithium-ion battery fabrication. However, there are several conditions governing lithium extraction besides lithium concentration from the brine. Naturally, a wide range of ions that should be efficiently separated from lithium, including the following characteristics occurring in brines: magnesium, calcium, potassium, sodium, chloride, and sulfate [1,2]. At the same time, the process should avoid the use of excessive amounts of chemicals and the generation of waste. In addition, it should not influence the quality of the product. It is especially difficult in the case of low concentrations of lithium and higher concentrations of the ions mentioned above that can have an impact on the consumption of carbonates, crystal formation, and filterability [3,4].

The Aral Sea brine is an example of an interesting lithium source. It is characterized by high salinity and complex ionic composition: 0.11 g/L lithium, 54.92 g/L sodium, 3.67 g/L potassium, 0.80 g/L calcium, 10.25 g/L magnesium, 70.90 g/L chloride, and 22.11 g/L sulfate, resulting in 130.9 g/L total dissolved solids. These ions include lithium as well as all interfering elements needed for obtaining the lithium carbonate [5,6,7].

Earlier research dealing with the problem of lithium extraction usually considered purification and Li₂CO₃ crystallization as two separate procedures. While purification aims at removing magnesium, calcium, and sulfate, crystallization uses synthetic solutions for analysis of effects of mixing and concentration [8]. While the approach can provide an understanding of the mechanism involved, it does not cover all aspects of working with natural brine. Techniques useful in synthetic solutions cannot be applicable in the case of natural brine due to the presence of magnesium, calcium, and sulfate, which can result in precipitate formation, changes in concentration, agglomeration of particles and lowering of the product purity [9,10].

Recent research in this area shows that pre-removal of magnesium, potassium and sulfates makes Aral Sea brine suitable as a feedstock for Li₂CO₃ crystallization, while simultaneously producing useful phosphate and sulfate products [7]. An analysis of the studies presented shows that the use of chemicals such as Na₂HPO₄, CaCl₂ or Na₂CO₃ makes it possible to obtain lithium carbonate with a purity of 99.5%, using the stages of purification of the salt brine followed by crystallization [7,11,12,13].

To achieve the goal of obtaining lithium carbonate for use in lithium-ion batteries, the following extraction criteria are required:

(i) To achieve the objective of selective co-precipitation of magnesium potassium phosphate and sulfate followed by crystallization of Li₂CO₃; (ii) To investigate the effects of dosing, pH, residence time, mixing, and feeding rate of Na₂CO₃ on process efficiency; (iii) To assess the physical and chemical properties of byproducts and lithium carbonate; and (iv) To identify problems to be solved before scaling up the process.

2. Materials and Methods

2.1. The Concept of Using Brine as a Raw Material and Technological Process

Aral Sea brine was used as the feedstock for the initial stage of purification; its chemical composition is given in Table 1.

The solution contained 0.11 g/L lithium, 54.92 g/L sodium, 3.67 g/L potassium, 10.25 g/L magnesium, 70.90 g/L chloride, and 22.11 g/L sulfate, where the majority of the components were either chlorides or sodium sulfate. For the separation of Mg²⁺, K⁺ and SO₄^2- ions from the brine, precipitation was performed using a stirred reactor. The main chemicals used for this process included sodium hydrogen phosphate, calcium chloride, and sodium hydroxide. The best results were obtained when the stirring speed was kept constant at 100 rpm and pH = 10.

2.2. Selective Removal of Mg²⁺, K⁺ and SO₄^2-

The 100 mL of the Aral Sea brine underwent treatment in the stirred reactor. Precipitation of Mg²⁺ and K⁺ as KMgPO₄ type of phosphate compounds was carried out using sodium phosphate (Na₂HPO₄), adjustment of the pH to about 10 was done using sodium hydroxide (NaOH) and the removal of sulfate as calcium sulfate dihydrate (CaSO₄∙2H₂O) was achieved using calcium chloride (CaCl₂). The mixing was done at 100 rpm, and times for the precipitation reactions ranged from 10 to 30 minutes.

The general reaction involved is as follows: MgCl₂ + KCl + LiCl + Na₂HPO₄ + NaOH → KMgPO₄ ↓ + 3NaCl + LiCl + H₂O; and Na₂SO₄ + CaCl₂ + 2H₂O → CaSO₄∙2H₂O ↓ + 2NaCl. These are process-level reactions and not complete chemical speciation, where the ion activities, hydration and other competing equilibria will affect the phase distribution.

The precipitates obtained were dried at 100 ℃ to constant mass. XRF determined the concentration of cations, whereas that of chloride, sulfate and phosphate ions was determined spectrophotometrically. Phase analysis of the solid products was done using X-ray diffraction and EDX/EDAX.

2.3. Lithium Carbonate Precipitation from Purified Brine

The purified brine, which contained 0.200 wt.% LiCl, was then subjected to Na₂CO₃ addition in a stirred semi-batch crystallizer. The brine charge was kept at 300 g at a constant temperature of 50 ℃. The addition of sodium carbonate was carried out at mixing speeds of 400, 500 and 600 rpm. The temperature was selected in the range of 25-50 ℃. Due to the lower solubility of Li₂CO₃ at 50 ℃ than at 25 ℃, 50 ℃ was selected as the optimal temperature.

In-line nucleation and growth measurements were conducted using FBRM. The pH and conductivity were measured along with the FBRM data. The characterization of the Li₂CO₃ phase was done using X-ray Diffraction (XRD), while the product morphology was characterized through Scanning Electron Microscopy (SEM), and lithium recovery was determined through the mass of the precipitate.

3. Results and Discussion

3.1. Chemical Composition of Raw Aral Sea brine

From the analysis of the raw brine, it is clear that the lithium is present in a matrix composed of NaCl, MgCl₂, and Na₂SO₄. In addition, the significantly higher amount of magnesium as compared to lithium causes problems since the magnesium consumes alkalinity and carbonate, leading to the creation of unwanted components and reducing selectivity in the precipitation of Li₂CO₃. Sulfate causes additional problems through influencing the precipitation of calcium and magnesium and raising the salinity of the solution. Therefore, purification becomes necessary before the crystallization of Li₂CO₃ can be achieved.

3.2. Phosphate Precipitation Removes Mg²⁺ and K⁺ While Producing a Recoverable by-Product

The effectiveness of the removal of magnesium and potassium was estimated according to the amount of Na₂HPO₄ used and the time taken to effect removal. Magnesium was removed effectively to 97.1%, while potassium was effectively removed to 96.3%, using 1.72 g of Na₂HPO₄ in a time period of 30 minutes. The combined removal of the two ions caused the production of a solid that resembles KMgPO₄ instead of the individual solids. It is worth noting that these solids have uses as fertilizer minerals rather than being classified as waste.

Figure 1. Integrated process concept for Aral Sea brine purification, evaporation/concentration and Li₂CO₃ precipitation. L and S denote liquid and solid streams, respectively.

Figure 1. Integrated process concept for Aral Sea brine purification, evaporation/concentration and Li₂CO₃ precipitation. L and S denote liquid and solid streams, respectively.

A pH of about 10 is chemically reasonable, as phosphate speciation and magnesium hydrolysis favour precipitation in the alkaline region.

However, excessive hydroxide addition can introduce complications. It may compete with phosphate, promote the formation of magnesium hydroxide, increase sodium contamination, and complicate control over by-product composition.

Figure 2. Effects of reagent dose and residence time on removal of (a) Mg²⁺, (b) K⁺ and (c) SO₄^2- from Aral Sea brine. The data identify 1.72 g Na₂HPO₄ and 0.58 g CaCl₂ as effective operating points at 30 min.

Figure 2. Effects of reagent dose and residence time on removal of (a) Mg²⁺, (b) K⁺ and (c) SO₄^2- from Aral Sea brine. The data identify 1.72 g Na₂HPO₄ and 0.58 g CaCl₂ as effective operating points at 30 min.

3.3. Sulfate Removal as Gypsum Complements Phosphate Purification

Adding CaCl₂ helps in removing the sulfate through the formation of calcium sulfate dihydrate (CaSO₄·2H₂O). The maximum sulfate removal efficiency recorded was 93.4% in the presence of CaCl₂ due to the precipitation of the sulfate as CaSO₄·2H₂O. Optimal sulfate removal of 93.4% was achieved by using a CaCl₂ concentration of 0.58 g with a mixing time of 30 minutes. Gypsum crystals were formed as per the X-ray diffraction analysis. This process is followed by phosphate precipitation, in which phosphate reacts with magnesium and potassium while CaCl₂ reacts with sulfate.

Without following the order of reactions, there will be reactions between calcium and phosphate, as well as sulfate/phosphate, which will affect the further carbonate precipitation process. Therefore, careful reagent addition order and filtration are required. The removal product consists of a complex fertilizer that has a crystal fraction of CaSO₄·2H₂O and an amorphous phosphate fraction.

3.4. Elemental Composition of Impurity-Removal Solids

EDAX showed that the co-treated precipitate contained PO₄^3-, Mg²⁺, SO4^2-, Ca²⁺ and K⁺, with significant amounts of Ca and sulfate when CaCl₂ was added (Figure 3).

These results support the proposed removal pathway and lead to the simultaneous precipitation of the double salts MgKPO₄ and CaSO₄·2H₂O.

3.5. Purified Brine Enables Controlled Homogeneous Li₂CO₃ Crystallization

The addition of Na₂CO₃ to the system, after the preliminary removal of significant interfering ions, heralded the formation of Li₂CO₃ via controlled crystallization. The pH displayed an immediate jump and then proceeded to increase more slowly as carbonate interacted with lithium. Conductivity showed an increase due to sodium and chloride concentration accumulation, and thereafter stabilized after the majority of the reaction. The events during minutes 19, 24, and 42 point out three different stages corresponding to carbonate addition, growth of the crystals, and reaching equilibrium.

Figure 4. Time evolution of pH and conductivity during Na₂CO₃-based homogeneous precipitation of Li₂CO₃ from purified Aral Sea brine at 400-600 rpm.

Figure 4. Time evolution of pH and conductivity during Na₂CO₃-based homogeneous precipitation of Li₂CO₃ from purified Aral Sea brine at 400-600 rpm.

One essential outcome of the experiments was that the stirring speed seemed to exert a more substantial influence on the size distribution of Li₂CO₃ particles than the Na₂CO₃ feed rate. More specifically, the increase in stirring speed from 400 to 600 revolutions per minute was reflected in the shift of the size distribution properties of Li₂CO₃ particles toward smaller particles.

High mixing intensity increases the overall homogeneity of the solution, reduces concentration gradients, limits uncontrolled aggregation, and increases the collision of crystals that break up weak clusters. The Na₂CO₃ feed rate has a small net effect, since a higher feed rate leads to larger aggregates. It can be seen from Figure 5 that the size of the particles precipitated at 500 rpm is larger than that of the particles precipitated at 600 rpm. The crystals obtained as a result of uniform precipitation were studied by X-ray fluorescence analysis. According to the results of the analysis, it was determined that Li₂CO₃ precipitated in the precipitate. Figure 6 shows a micrograph of lithium carbonate and its elemental composition.

The results of elemental analysis show that lithium carbonate contains 18.4% lithium, 65.15% oxygen, and 16.0% carbon, which is consistent with the composition of the compound Li₂CO₃.

3.6. Phase Purity and Morphology of Li₂CO₃

The fact that Li₂CO₃ is the primary crystalline phase was verified by XRD analysis, which correlates with the information on ICDD card 00-022-1141. SEM imaging (Figure 7) indicated the formation of clusters made of thin and ellipsoidal layers – a sign of prior crystallization and agglomeration of the resulting particles. The best parameters were discovered to be 600 rpm and 50 ℃ using 4.53 ml/min of Na₂CO₃, resulting in 99.5% purity of the product.

As a result of the reaction carried out at 25°C and 50°C, the precipitated Li₂CO₃ crystals appear in the form of ellipsoidal and sheet-like clusters, as shown in SEM images.

To determine the physical and chemical properties of the obtained lithium carbonate, its X-ray diffraction pattern and IR spectra were also fully studied.

As a result of the studies, it was found that in the X-ray diffraction pattern of lithium carbonate (Figure 8), diffraction maxima such as 4.16, 2.91, and 2.80 Å represent Li₂CO₃, while in the IR spectrum, absorption bands belonging to the carbonate group (Figure 9) are found to be 1437.03 cm^-1 and 869.93 cm^-1.

3.7. Integrated Mechanism

The method functions by gradually decreasing chemical interference. The source brine has high amounts of magnesium, potassium, and sulfate, which cause carbonate to precipitate and affect the purity of Li₂CO₃. The process of using Na₂HPO₄ helps in separating magnesium and potassium from the brine as a solid phosphate precipitate, thus reducing the level of ions which utilize carbonate. The other part of the procedure employing CaCl₂ helps in sulfur precipitation as gypsum, which reduces the sulfur content as well and gives a solid by-product. Afterwards, the lithium-rich liquid is treated with carbonate in a controlled manner, while stirring at 600 rpm prevents clogging and allows obtaining smaller salt crystals. In this way, purification and crystallization are dependent on one another, as the purification process determines the chemical conditions while mixing influences the shape of the crystals.

3.8. Practical Implications and Scale-Up Constraints

There exist several advantages of the technique mentioned in the previous section. It utilizes readily available chemicals, works at moderate temperatures, enables obtaining Li₂CO₃ of high purity, and allows converting some impurities into useful minerals. Moreover, it does not depend solely on long-term solar evaporation for brine concentration.

4. Conclusions

The two-step approach has been developed in this study to convert brine from the Aral Sea, which has high contents of magnesium, potassium and sulfate, to produce lithium carbonate suitable for battery production. With the help of Na₂HPO₄ and CaCl₂, 97.1% of magnesium, 96.3% of potassium, and 93.4% of sulfate were removed, which are the major ions hindering the production of lithium carbonate. Solid residues contain gypsum and phosphate, which can potentially serve as minerals or fertilizers. The next stage featured a significant role of stirring speed compared to the rate of feeding Na₂CO₃ for controlling the particle size of Li₂CO₃, as the run with the stirring speed of 600 rpm led to obtaining particles of a smaller size. An increase in the rate of feeding Na₂CO₃ resulted in aggregation of crystals due to a local increase in concentration. The proposed method allows recovery of lithium carbonate with a purity of 99.5%, typical for rechargeable batteries, at a precipitation temperature of 50 ℃. Therefore, the purification stage and crystallization stage of the process are directly connected and mutually influence each other.