Advanced Mixing Technologies for Efficient Lithium Processing

In lithium processing, advanced mixing technologies — applied across the crystallization, carbonization and decomposition steps — can serve as critical enablers in meeting efficiency, scalability and sustainability demands

Lithium carbonate (Li₂CO₃) is a key raw material in the production of lithium-ion batteries, which power everything from smartphones to electric vehicles. As demand for these batteries grows, so does the need for high-purity lithium compounds. Producing battery-grade lithium carbonate requires a multi-step purification process (Figure 1) that includes crystallization, carbonation and decomposition (Figure 2). Each of these steps involves complex multiphase interactions that must be carefully controlled to ensure product quality and process efficiency.

Mixing represents a core unit operation within lithium purification processes. Whether suspending solids, dispersing gases or controlling temperature gradients, mixing plays a central role in determining the success of lithium purification. Poor mixing can lead to uneven crystal growth, incomplete reactions and energy inefficiencies. Conversely, well-designed mixing systems can enhance mass and heat transfer, reduce fouling and facilitate continuous processing.

This article provides a detailed overview of how advanced mixing technologies can be applied to lithium purification. It examines the role of mixing in crystallization and the design of draft-tube baffle (DTB) crystallizers. It also focuses on carbonation and gas-liquid reactors, as well as the decomposition step, where lithium bicarbonate is converted back into high-purity lithium carbonate.

 

Primary crystallization of crude Li₂CO₃

The initial crystallization of lithium carbonate typically occurs after lithium sulfate (Li₂SO₄) has been converted to Li₂CO₃ via soda ash (Na₂CO₃) addition. This primary crystallization step is designed to recover lithium carbonate from solution while minimizing the incorporation of impurities, such as sodium, calcium and magnesium. The process must be carefully controlled to avoid excessive nucleation, which can lead to fine particles that are difficult to separate and wash. Supersaturation for inducing initial crystallization is achieved by the evaporation of water.

Advantages of the DTB crystallizer design. DTB crystallizers are specifically engineered to meet the demanding requirements of continuous industrial crystallization processes, particularly where precise control over crystal size distribution, product purity and process stability is essential. Their defining feature is an internal circulation loop, created by a central draft tube and an axial-flow impeller, which ensures intensive mixing and uniform conditions throughout the crystallizer (Figure 3). This design minimizes concentration and temperature gradients, thereby promoting consistent supersaturation levels — a critical factor for controlled crystal growth and reduced secondary nucleation. One of the most significant advantages of the DTB configuration is its ability to perform in-situ classification. Coarse crystals can be selectively withdrawn from the bottom of the crystallizer, while finer particles are recirculated for further growth. This continuous separation mechanism enhances product uniformity and reduces the need for downstream classification equipment.

Thermal homogeneity is another key benefit of the DTB design. Whether the crystallization is driven by cooling, evaporation or reaction, the efficient mixing ensures small temperature gradients within the DTB crystallizer, which is essential for maintaining stable process conditions and avoiding localized supersaturation or premature nucleation. Furthermore, DTB crystallizers are highly scalable. Their modular design and predictable hydrodynamics allow for straightforward scaleup from laboratory or pilot-scale units to full industrial installations. This makes them a preferred choice in many industries, such as pharmaceuticals, chemicals and battery materials, where consistent product quality and process reliability are paramount.

Mixing requirements and design of DTB crystallizers. The performance of a DTB crystallizer is closely tied to the design of the mixing system. The impeller must generate sufficient axial flow to drive circulation through the draft tube while minimizing shear that could damage growing crystals. At the same time, an efficient operation is desired to reduce energy consumption. Several impeller designs are used in DTB crystallizers, including pitched-blade turbines, hydrofoil impellers and proprietary axial-flow designs. There are commercially available impellers specifically designed for use with DTB crystallizers to achieve high pumping rates at low power input. In general, the best principle is to select an impeller that provides strong axial flow, low shear and high hydraulic efficiency.

The calculation of the required hydraulic power input (P_hydr) for DTB crystallizers operating with pure liquids follows the same principles as pump design. It is based on the total pressure drop (∆p_total) across the system and the required circulation flowrate (q̇) and can be found in Equation (1).

 

P _hydr = ∆ p _total × q̇(1)

 

This hydraulic power then serves as the basis for determining the power required from the impeller (P_imp), considering the impeller’s efficiency (η), as shown in Equation (2).

 

P_imp = P_hydr/η (2)

 

The efficiency itself depends on the impeller type, the ratio of impeller diameter to draft-tube diameter, and most significantly, on the blade geometry. Another important factor in the design is the pressure drop coefficient (ζ), which accounts for flow resistance caused by internal components, changes in flow and other structural elements. The pressure drop is then calculated by Equation (3) using the pressure drop coefficient, the density of the liquid (ρ) and flow velocity inside the draft tube (w _DT).

 

∆p_liquid = ζ × ρ /2 × w _DT ² (3)

In lithium carbonate crystallization, solids suspension is a critical requirement. The presence of solids increases the pressure drop and affects the flow regime. To account for this, an additional, empirically determined term is included in the hydraulic calculations (∆ p_solid), as illustrated in Equations (4) and (5). This term is based on the difference between liquid and particle density (∆ρ), the draft-tube length (L), flow velocity in the draft tube, solid volume concentration (c_V) and hindered settling velocity of the particles (w_ss).

∆p_total = ∆p_liquid + ∆p_solid (4)

 

∆p_solid = ƒ(∆ρ,  L,  w_DT,  c_V,  w_ss) (5)

Together, these interdependent parameters form the foundation for designing DTB crystallizers. In summary, the DTB crystallizer is a cornerstone technology in the production of high-purity lithium salts. Its unique design enables precise control over crystallization conditions, supports continuous operation and delivers consistent product quality. When combined with a well-engineered mixing system and robust process control, the DTB crystallizer becomes a powerful tool capable of meeting the stringent specifications for lithium carbonate required by the battery industry.

 

Carbonation reactor

In the production of battery-grade lithium carbonate, the carbonation reactor plays a critical role in the purification process. This reactor facilitates the conversion of lithium carbonate, which is only sparingly soluble in water, into lithium bicarbonate (LiHCO₃), a more soluble intermediate. This transformation enables the selective removal of insoluble impurities before the lithium is re-precipitated in a subsequent decomposition step.

Process chemistry and function. The carbonation reaction is a gas-liquid-solid process in which solid lithium carbonate reacts with carbon dioxide (CO₂) in aqueous suspension to form dissolved lithium bicarbonate:

 

Li₂CO₃ (s) + H₂O + CO₂  → 2LiHCO₃ (aq)

 

The reaction enhances the solubility of lithium ions in the aqueous phase, thereby facilitating the separation of insoluble impurities, such as silicates and alkaline earth-metal compounds (for example, calcium and magnesium salts). The resulting purified lithium-containing solution is subsequently subjected to a carbonation process to precipitate high-purity lithium carbonate. This carbonation step is typically conducted in a continuous stirred-tank reactor (CSTR) or a cascade of multiple CSTRs. Reactor design must ensure efficient gas-liquid mass transfer, homogeneous solid suspension and precise thermal regulation to maintain optimal reaction kinetics and product quality.

Mixing requirements and impeller design. Efficient mixing is essential for the carbonation reactor to achieve high conversion rates and consistent product quality. The primary mixing tasks include:

• Dispersing CO₂ gas into fine bubbles to maximize interfacial area
• Maintaining suspension of lithium carbonate solids to ensure complete reaction
• Promoting uniform temperature and concentration distribution throughout the reactor volume

To meet these requirements, various impeller configurations can be employed. A common approach is to use a combination of a gas-dispersing and a self-inducing impeller (Figure 4, left). Employing a self-inducing impeller enhances mass transfer. Internal gas recirculation reduces the need for external gas input and typically improves energy efficiency. This improvement in gas-liquid interaction is often quantified by the k_La value (volumetric mass-transfer coefficient), which combines the liquid-side mass-transfer coefficient (k_L) and the specific interfacial area (a). A higher k_La indicates more efficient gas transfer into the liquid phase, which is clearly critical for the carbonation reactor. The k_La value can be calculated by an empirical equation (Equation (6)) and, in a stirred reactor, depends on the type of gas dispersing-impeller or setup (c₁), specific power input (P/V) and superficial gas velocity (v_sg). Note that c₁, x and y are empirically determined. Due to the internal gas recirculation of self-inducing impellers, v_sg is increased, which in turn leads to a higher k_La value. Alternatively, systems without gas recirculation may use a combination of a dispersing impeller and a mixed-flow impeller (Figure 4, right). The choice of impeller setup depends on factors such as reactor size, desired gas throughput, and process constraints. Figure 5 shows the increased k_La values of the self-inducing setup compared to a setup with a mixed-flow impeller.

 

k_La= c₁ ×(P/V)^x × v_sg ^y (6)

 

Influence of operating and reactor parameters on carbonation efficiency.The efficiency of the carbonation reaction for lithium carbonate purification is governed by a complex interplay of operating and reactor design parameters. Key factors, such as CO₂ partial pressure, directly affect the solubility of carbon dioxide, thereby impacting the reaction rate. Temperature also plays a crucial role, with lower values favoring the stability of lithium bicarbonate and minimizing its thermal decomposition. In stirred reactors, the specific power input (P/V) enhances gas dispersion and solid suspension, although it must be optimized to avoid excessive energy consumption.

For industrial-scale applications, carbonation reactors are typically arranged in cascades of CSTRs. This setup enables staged control of the reaction and allows for a more compact reactor design compared to a single large vessel. Each reactor in the cascade must be carefully designed to ensure optimal mass transfer, effective suspension of solids and precise thermal management. Among the available configurations, reactors equipped with self-aspirating impellers stand out for their ability to enhance mass transfer while maintaining energy efficiency, making them particularly suitable for high-throughput and high-purity lithium carbonate production.

 

Decomposition reactor

Following the carbonation step in lithium carbonate purification, the decomposition reactor plays a pivotal role in converting dissolved lithium bicarbonate back into solid lithium carbonate. This thermal process not only completes the purification cycle, but also enables the recovery and reuse of CO₂, contributing to the overall efficiency and sustainability of the production process.

The decomposition reaction is thermally driven and involves heating the lithium bicarbonate solution to a temperature at which CO₂ is released and lithium carbonate precipitates. The simplified reaction is shown here:

 

2LiHCO₃ (aq.) → Li₂CO₃ (s) + H₂O + CO₂₍g)

 

This step is typically carried out in a CSTR or a cascade of such reactors, depending on the required throughput and conversion efficiency. The reactor must be designed to ensure uniform heating, effective degassing and controlled crystallization.

Another important aspect of the lithium purification step is the integration of CO₂ recovery. This requires a well-designed gas-handling system. In some configurations, the CO₂ is directly routed from the decomposition reactor to the carbonation reactor, where it is reintroduced into the process via a gas-inducing impeller. This closed-loop approach reduces CO₂ consumption and improves process economics.

Mixing requirements and impeller design. One of the key challenges in the decomposition process is managing the release of CO₂ gas. The degassing rate is strongly influenced by the intensity of agitation. Sufficient mixing is required to promote bubble formation and disengagement, while also maintaining a homogeneous temperature profile throughout the reactor. The use of high-efficiency impellers with high local power input has proven effective in this context (Figure 6). For example, commercial impellers have been applied in decomposition reactors to provide the necessary energy input, while the optimized blade shape strongly reduces wear rates.

During decomposition, the solution is heated. Heating must be carefully controlled to avoid localized boiling, which can lead to fouling or scaling on heat-transfer surfaces. Reactor designs frequently incorporate a heated jacket or internal heat exchangers to deliver the necessary thermal input. Depending on the plant infrastructure and operating conditions, the heating medium is selected accordingly. At high altitudes, where the boiling point of water is reduced, the heating system must be adapted accordingly. As lithium carbonate precipitates, the reactor must also function as a crystallizer. This dual role requires that the mixing system not only supports degassing but also maintains the solids suspended and promotes uniform crystal growth. The precipitated lithium carbonate must exhibit a narrow particle-size distribution and be devoid of inclusions or agglomerates to ensure optimal downstream processability. These morphological characteristics are critical for enhancing the efficiency of subsequent solid–liquid separation steps, such as filtration and drying, and for maintaining consistent product quality in high-purity applications.

 

In summary

The growing demand for high-purity lithium carbonate, driven by the rapid expansion of lithium-ion battery technologies, places increasing emphasis on the efficiency, scalability and sustainability of lithium purification processes.

In the crystallization stage, DTB crystallizers have proven essential for achieving controlled supersaturation, uniform crystal growth and consistent product quality. The internal circulation loop and axial-flow mixing design of DTB systems support thermal and concentration homogeneity, enabling the production of lithium carbonate with a narrow particle-size distribution and minimal impurities. The integration of solid-suspension modeling and hydraulic-power calculations further enhances the reliability and scalability of these systems.

In the carbonation reactor, lithium carbonate is converted into soluble lithium bicarbonate, which requires a distinct set of mixing capabilities. Here, gas-liquid-solid interactions dominate, and the reactor must facilitate rapid CO ₂ absorption, solids suspension and temperature control. To enhance mass transfer and reduce CO ₂ consumption, gas-inducing impellers have proven highly effective in promoting overall reactor performance. Additionally, the reactor’s configuration, including cascade operation, further contributes to its performance and adaptability.

In the final decomposition step, lithium bicarbonate is thermally converted back into lithium carbonate while releasing CO₂. These reactors must balance effective degassing, controlled crystallization and heat management. The integration of CO₂ recovery into the process loop not only improves resource efficiency but also supports a more sustainable operation.

By leveraging optimized mixing technologies within their crystallization, carbonation and decomposition strategies, chemical engineers can ensure the reliability, efficiency and sustainability of lithium refining operations. ■

Edited by Mary Page Bailey

Acknowledgement

All images provided by EKATO

Authors

Marc Labusch is head of the EKATO RMTs Reaction Engineering Group (Hohe Flum Strasse 37, D-79650 Schopfheim, Germany; Email: marc.labusch@ekato.com). He joined EKATO in 2019, focusing on multiphase reactor engineering, especially for hydrogenation applications. Labusch holds a Ph.D. in technical chemistry from University Duisburg-Essen in Germany.

 

Wolfgang Keller is head of the EKATO RMTs R&D Department (Hohe Flum Strasse 37, D-79650 Schopfheim, Germany; Email: wolfgang.keller@ekato.com). He has more than 20 years of professional experience in process engineering and development, especially focused on polymer and minerals processing applications. Keller holds an M.S. degree in chemical process engineering from the University of Karlsruhe in Germany.