Efficient battery recycling starts with controlled solvent removal

Recycling lithium-ion batteries calls for increasingly advanced process technology. Besides recovering valuable metals such as lithium, nickel, cobalt and manganese, controlling electrolyte residues is becoming more and more important.

A major challenge arises when battery cells are opened or shredded. The electrolyte contains organic solvents, including ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC).

These substances can evaporate when battery cells are opened, shredded and processed. Without controlled capture, solvents are lost and emissions and additional safety risks arise.

A combination of vacuum drying, controlled condensation and closed powder handling offers a way to tackle these challenges on an industrial scale.

Why electrolyte solvents are a challenge

The electrolyte in lithium-ion batteries usually consists of a lithium salt, such as LiPF₆, dissolved in a mixture of organic carbonates. The individual solvents have very different physical properties.

The linear carbonates DMC, EMC and DEC in particular are relatively volatile. When battery cells are opened or shredded, these solvents can easily enter the gas phase.

EC and PC are considerably less volatile at ambient temperature, but depending on the process conditions they may also need to be removed.

The challenge is therefore not only to remove the electrolyte from the battery material, but also to capture the released solvents in a controlled way.

Vacuum drying: removing solvents at lower temperatures

One possible process step is vacuum drying of the shredded battery material. Lowering the pressure in a closed dryer lowers the boiling point of the solvents present. Volatile components can then be removed at lower process temperatures.

This offers several advantages:

• Lower thermal load on the material.
• Controlled removal of volatile solvents.
• Possibility of recovery through condensation.
• Fewer uncontrolled emissions.
• Better control of the drying process.

Depending on the composition of the battery material and the required residual concentrations, a temperature range of about 40–60 °C can be used as a starting point for a feasibility study.

The actual process conditions must be determined experimentally. The required temperature, vacuum pressure and residence time depend on the battery chemistry, particle size, amount of electrolyte and the required residual moisture and residual solvent concentrations.

From evaporation to controlled solvent recovery

Evaporating solvents is only one part of the process. What happens to the vapour stream afterwards is at least as important.

A closed vacuum drying installation can be connected to a condenser or cold trap. Solvent vapours are cooled and converted into liquid. A possible process layout consists of four steps:

1. Controlled opening and shredding. Battery cells are opened or shredded under controlled, dry and suitably inert conditions, taking account of residual electrical energy, fire hazard and possible emissions.
2. Vacuum drying. The shredded material is fed into a closed dryer. Under reduced pressure, volatile electrolyte components are removed at a controlled temperature.
3. Condensation and recovery. The resulting vapour stream is routed to a suitable condenser. The temperature of the condensation system is matched to the composition of the vapour stream and the required recovery. The condensate composition can then be determined analytically, for example by gas chromatography (GC).
4. Further material processing. Once the volatile components have been removed, the material can be processed further. Depending on the chosen recycling route, extraction processes such as supercritical CO₂ processes can be applied.

An important point of attention is the lithium salt LiPF₆. In contact with moisture, this salt can form HF (hydrogen fluoride) under certain conditions. Moisture control, material compatibility and suitable process conditions are therefore essential.

Industrial scale-up: from lab trial to continuous processing

What seems relatively simple on a laboratory scale becomes a complex process question on an industrial scale. The quantity of material increases, the composition varies, and heat and mass transfer become decisive for process efficiency.

For industrial applications, the following parameters are important:

• Capacity and batch size of the dryer.
• Heat transfer to the battery material.
• Mixing and homogeneity during drying.
• Vacuum performance and vapour removal.
• Condensation capacity and solvent recovery.
• Residual solvent concentration after drying.
• ATEX and process safety concept.

An agitated vacuum dryer can be interesting when the material needs to be mixed during drying to achieve even heat transfer.

The final choice of installation should be based on representative drying trials with the actual battery material.

Black mass: a new challenge for powder handling

Once the electrolyte residues have been removed, a material stream remains that can be separated further and processed into black mass. Black mass usually contains fine powder particles with valuable battery materials, including compounds of nickel, cobalt, manganese and lithium. The exact composition depends on the battery chemistry.

This raises a second important process question: how is this fine material safely removed from the dryer and conveyed to the next process step?

Black mass is not an ordinary industrial powder. It can pose health and environmental risks and, depending on its composition and residues, specific fire and explosion risks. The transfer therefore deserves particular attention.

Closed filling and conveying of black mass

After drying, the material must be discharged in a controlled way, without unnecessary exposure of operators or the environment. A closed filling system can combine several functions:

• Closed discharge from the vacuum dryer.
• Controlled transfer to an intermediate container.
• Dust control during filling.
• Reduced product loss.
• Safe connection to further processing.
• Containment suited to the specific risk profile.

Depending on capacity, drums, closed containers or big bags with suitable connection technology can be used. Cleaning, inspection and maintenance of product contact parts must also be part of the design.

With fine metal-containing powders, it is also important to look beyond dust control at the specific dust properties, ignition sensitivity and possible explosion risks. An ATEX classification or protection concept must therefore be based on a separate risk assessment and relevant material data.

The complete process line makes the difference

In battery recycling it is tempting to treat vacuum drying, solvent recovery and powder handling as separate technical disciplines. In reality they are closely connected.

An efficient drying process has little value if the vapours are not captured in a controlled way. And successful solvent recovery does not automatically mean the dried material can be discharged safely.

The integration of drying technology, condensation, closed conveying and containment ultimately determines the performance of the complete production line.

Hecht Technology works in powder handling and containment and, within the Hecht organisation, has access to industrial drying technology and process engineering, including through Bachiller. This allows process questions around vacuum drying, material transfer and closed filling to be addressed from one integrated approach.

Conclusion

Drying black mass is an important process step in lithium-ion battery recycling. By combining vacuum drying with controlled condensation, volatile electrolyte solvents can be removed and potentially recovered. Processing the fine, metal-containing black mass then calls for a closed, well-designed powder handling process.