Review of Key Parameters of Lithium Precipitation Process and Purity Control for Lithium Carbonate

Global Business, BICHEM Group

Lithium Carbonate (Li₂CO₃) is the major row material for lithium batteries, glass-ceramic and pharmaceutical industry. The lithium precipitation process directly impacts the purity and yield of finished products. During lithium precipitation, chemical precipitation is applied to convert lithium ions into solid lithium carbonate in which reacting conditions and operating parameters are crucial to ensure the quality of products. This essay will systematically analyze crucial parameters of lithium precipitation and methods of controlling purity.  

I. The basic principle of lithium precipitation and the formula of lithium precipitation reaction are explained as following two paths:

1. Direct Carbonization:

LiCl or LiOH+CO₃^2- → Li₂CO₃   + other byproducts. At high temperatures (e.g., ≥95°C), lithium ions react with carbonate ions in an alkaline environment (e.g., high-concentration Na2CO3 solution) to form insoluble lithium carbonate precipitate.

2. Indirect precipitation method (using ammonium carbonate as an example):

LiCl + (NH₄)₂CO₃ → Li₂CO₃  + 2NH₄Cl

When using ammonium carbonate as the precipitating agent, the reaction rate is relatively slow. Extended reaction time (60–120 minutes, optimal at 90±10 minutes) is required to ensure complete precipitation.

II. Optimization of Lithium Precipitation Reaction Conditions

1. Optimal Temperature Range

The temperature for lithium carbonate precipitation must be adjusted based on raw material characteristics and process objectives, with the core range concentrated between 70–95°C.

· High-temperature conditions (90–95°C): In processes like lithium feldspar purification, maintaining 90–95°C accelerates the Li⁺–CO₃²⁻ reaction rate, reduces impurity encapsulation, and enhances lithium precipitation efficiency. For example, decarbonation of lithium precipitation mother liquor requires heating to 70°C to promote CO₂ volatilization.

· Temperature Effects on Impurities: Low temperatures (<70°C) may cause incomplete precipitation of Li₂O, increasing lithium loss in the mother liquor. Excessively high temperatures (>95°C) may exacerbate nucleation phenomena, leading to impurities like sodium and magnesium incorporating into the crystal structure.

2. pH Control Strategy

pH in the lithium precipitation system requires staged adjustment to balance reaction efficiency and impurity removal:

· Precipitation Phase pH: When sodium carbonate concentration is controlled at 1.5–2 mol/L, the reaction solution pH must be maintained between 8.4 and 9.2. This range ensures preferential precipitation of Li₂CO₃ while suppressing co-precipitation of impurities like Na⁺ and SO₄²⁻.

· Decarbonization and Mother Liquor Treatment: After lithium precipitation, adjust the mother liquor pH to 6–7 by adding sulfuric acid to release residual CO₂. Subsequently, raise the pH to 12.5 for calcium-magnesium complexation, utilizing NaOH neutralization to precipitate metal ions.

· Washing Stage pH: The pH of the washing mother liquor typically ranges from 10 to 12 and requires neutralization treatment to meet discharge standards.

III. Key Technologies for Purity Enhancement

1. Impurity Removal Processes

· Physical-Chemical Combination Methods:

Washing and Centrifugation: Crude lithium carbonate undergoes agitation washing with 95°C pure water to remove surface-adsorbed Na⁺, SO₄²⁻, etc. After centrifugal separation, the mother liquor is recovered for soda ash preparation.

Chelation Precipitation: Add EDTA (e.g., 0.8 kg/20,000 L reactor) to chelate Ca²⁺ and Mg²⁺ ions, then adjust pH with sodium hydroxide to achieve coprecipitation.

Stepwise Precipitation Method: The Zhabuye Salt Lake process employs chemical stepwise alkalization. First, Fe³⁺, Al³⁺, Ca²⁺, and other impurities are removed. Finally, lithium is precipitated using ammonium carbonate, achieving a purity of 99.90%.

2. Fine-Tuning of Reaction Conditions

· Feed Method and Agitation Parameters:

Sodium carbonate must be added slowly via drip feeding to prevent abnormal crystal nucleation caused by localized supersaturation. Simultaneously, agitation speed is controlled at 300-500 rpm to ensure uniform mixing while minimizing mechanical fragmentation.

The molar ratio of sodium carbonate to lithium solution is recommended at 1.05-1.2 (30%-50% excess CO₃²⁻), ensuring complete reaction while minimizing lithium loss in the mother liquor.

3. Post-Treatment Purification Techniques

· Three-stage CO₂ purification: Adjust pH to neutral by bubbling CO₂, precipitating trace impurities (e.g., Mg²⁺, Cl⁻) as carbonates. Final product purity exceeds 99%, though ion exchange is required to further remove sulfate ions.

· Membrane Separation and Ion Exchange: For trace impurities in leaching mother liquor (e.g., Na⁺ < 0.2 wt%), nanofiltration membranes can retain macromolecular impurities, or strongly acidic cation exchange resins can adsorb metal ions.

IV. Quality Control and Process Improvement Directions

1. Analytical Testing Methods

· Routine Testing: Gravimetric analysis for lithium content with ±0.5% error, suitable for crude product screening; Atomic Absorption Spectroscopy (AAS) detects ppm-level impurities, while ICP-MS analyzes trace elements (e.g., Fe, Al < 10 ppm).

· Morphology and Structural Characterization: SEM for grain morphology observation, XRD for purity verification, and IR spectroscopy to confirm characteristic peaks of Zabuye-type lithium carbonate.

2. Existing Challenges and Optimization Pathways

Suppression of Eutectic Phenomena: Reduce sodium-lithium eutectic formation by lowering the Li₂O/CO₃²⁻ ratio in mother liquor (e.g., <0.7) or introduce seed crystal methods to control nucleation growth rates.

Mother Liquor Recycling: Concentrate lithium-precipitated mother liquor after decarbonation and impurity removal. Secondary lithium precipitation can elevate overall lithium recovery rates to over 95%.

Conclusion

Optimizing lithium carbonate precipitation requires systematic integration of temperature, pH, feed control, and multi-stage purification technologies. Current best practices are: Temperature: 90-95°C for lithium precipitation, 70°C for decarbonation; pH: 8.4-9.2 for reaction system, 6-7 for mother liquor treatment; Purity control: Stepwise precipitation + three-stage purification + membrane separation, combined with ICP-MS online monitoring, achieving >99.9% purity targets. Future development should focus on intelligent process parameter interlocking systems to address raw material fluctuations and environmental requirements.