The metals used in batteries are a valuable, finite resource that are not readily available in Europe. There is therefore a huge desire to recycle as much as possible. Researchers at Chalmers University of Technology in Sweden have come up with a new way of recycling the metals found in rechargeable batteries, with less harmful effects for people and the environment, whilst maintaining the same level of efficiency. Their study investigates how fossil-based chemicals used in metal recovery can be replaced with alternatives produced from renewable biomass.

A rise in global energy consumption and the need to use more sustainable energy systems wherever possible is driving an increasing demand for energy storage systems, such as batteries. At the same time, the need to recover and recycle the metals used in batteries – including copper, cobalt, lithium and manganese – is also increasing. These materials are essential for the green transition, and several of them are included in the EU Critical raw materials act.

Critical raw materials are “raw materials of high economic importance for the EU, with a high risk of supply disruption due to the concentration of their sources and the lack of any good, affordable substitutes”. For example, China supplies 100 per cent of the EU’s demand for heavy rare earth elements. The EU is working to diversify and secure its supply of critical raw materials, and recycling is playing a key role.

Batteries require high degree of purity

To make metal recovery both efficient and economically viable, metals must be separated and purified before they can be reused. The production of batteries and other high-value products often requires metals of a high degree of purity.

In some cases, using higher-purity raw materials can lead to the exclusion of less favorable materials (for both the environment and human safety), such as mercury. For example, in the past, the shelf life of non-rechargeable batteries was extended by adding mercury to the zinc electrode. However, with higher-purity zinc, it is possible to produce an equally stable battery that is free from mercury.

“If we do not separate and purify materials during recycling, their quality will gradually deteriorate. Ultimately, we risk ending up with materials that can no longer be used in advanced applications, and the whole purpose of recycling is lost,” says Mark Foreman, Associate Professor at the Department of Chemistry and Chemical Engineering at Chalmers.

Alternatives for existing production lines

Solvent extraction is a widely used method (read more below) for separating and purifying metals in battery recycling, as well as in mining, the nuclear industry and in other industrial sectors. Today, the diluents used in these processes are typically produced from fossil-based feedstocks.

“In our study, we wanted to demonstrate that renewable biomass, for example, by-products from the forestry industry, can be used to produce alternative diluents. In this case, we investigated two aromatic compounds that could also be used directly in existing industrial production lines,” says Daniel Keywan Hoffmann, PhD student at Chalmers and first author of the study.

The study shows that the aromatic compounds perform just as well as conventional commercial alternatives in the extraction of several important metals. Furthermore, they could be implemented directly in existing industrial production lines.

“It is expensive for industry to rebuild factories or invest in entirely new infrastructure to improve sustainability. If the existing processes and equipment can be used while simply switching to a significantly safer chemical, the barrier to change becomes much lower and far less expensive,” says Daniel Keywan Hoffmann.

The aromatic compounds are safer to handle

Large-scale metal recovery operations use substantial quantities of diluents, which often need to be handled by people, so safety considerations are of particular importance. The researchers found that the two aromatic compounds used in the study have higher flash points and lower volatility than several commercially used alternatives. This means a lower risk of fire and reduced exposure to hazardous substances for workers in recycling facilities.

Some commercial chemicals used frequently for these processes today are particularly potentially harmful, since they form a group of neurotoxins when they degrade. These neurotoxins can have harmful effects in the brain and nervous system of humans and animals, and many conventional diluents are converted into these in the body. The new aromatic compounds which have been tested in this study cannot form these neurotoxins when they degrade.

“If we can achieve the same performance as current processes while reducing risks to people and the environment, that represents a significant benefit for everyone,” says Mark Foreman.

Aim to inspire industry

The researchers emphasise that manufacturing processes would need to be optimised, and the availability of renewable feedstocks increased, to make the approach cost-effective.

“We hope our work can inspire industry to think differently. Sustainable alternatives do not necessarily require starting from scratch. In many cases, replacing certain chemicals may be enough,” says Daniel Keywan Hoffmann.

More about the study:

• Read the study in RSC Sustainability: Safer aromatic process diluents for solvent extraction of critical metals from spent batteries
• Liquid–liquid extraction, also known as solvent extraction, uses an organic phase consisting of:
  a) A complex-forming molecule (extractant) that binds the metal to be extracted.
  b) A diluent in which the extractant is dissolved, such as kerosene.
• The primary role of the diluent is to dissolve the extractant and create a usable organic phase, as extractants cannot generally be used on their own.