Battery recycling processes are commonly a combination of three different fields of expertise: mechanical, pyrometallurgical and hydrometallurgical unit processes. Nowadays, especially the recycling of consumer grade batteries, which greatly vary in shape, size and chemistry, rely on the combination of mechanical, pyrometallurgical and hydrometallurgical unit process. By using pyrometallurgical methods it is easy to recover the most valuable metals, all the while polymers burn and possible hazardous compounds evaporate into gas washers. The issue of this method of approach is that some of the metals are lost to phases whose treatment is difficult and not profitable. This way uneconomic side streams are created from which the metal recovery will be difficult and unprofitable. It is not possible to have your cake and eat it, e.g. recover cobalt from metal phase and leave several other metals into slag, destined for landfill. From market perspective, this is sustainable but not from the point of view of circular economy.
Batteries in general are not designed for recycling. Their structure is compact and composition extremely heterogeneous. Having plastics end up in to leaching processes is a problem which can be solved by using pyrometallurgical methods: the plastics are simply burnt. When concerned with modern, especially with small consumer batteries this is a viable approach to metals recovery in primary process.
Specifically, at Aalto University, we are studying processes that would omit this pyro stage. We are investigating methods that could be applied in economically and environmentally sustainable way, recovering additional metals which are currently being lost into various waste streams. Once you omit the pyrometallurgical stage of processing, mechanical handling of the battery waste will be extremely important part of process design as it is the phase that dictates what is possible for hydrometallurgist to achieve. As part of this mechanical processing investigation on industrially crushed battery waste, we have studied the size distribution of particles, along with how chemical elements are distributed as a function of particle size. We have also studied the density distribution of major battery elements and if density-based separation of components would be possible. Additionally, we determined the density of aluminum and copper electrode foils and whether it would be possible to separate the foils via density separation.
The most valuable and therefore interesting material in a battery resides on electrodes as fine powder. Commonly, on anode, there is a copper foil, covered in graphite into which lithium intercalates during charging cycle. On cathode, aluminum foil exists, covered in active electrode material, such as lithium cobalt oxide (LiCoO2). Active materials are deposited to the surface of the foils by using polymer adhesives, such as polyvinylidene fluoride (PVDF). By sieving, it is possible to concentrate the valuable materials into underflow, enabling more efficient hydrometallurgical processing of the waste material.
In our hydrometallurgical experiments battery waste was dissolved in strong mineral acids, such as sulfuric and hydrochloric acid and weak organic acids such oxalic and malic acid. By combining mechanical processing to hydrometallurgical process we are able to produce a solution enriched in dissolved active material compounds. The future challenges lie in efficient, economic and environmental recovery of dissolved elements.
However, one of the more vital and potential research issues are the plastics ending up into hydro processes and the lack of methods for its efficient separation. The plastic removal is challenging, especially from the crushed battery waste, as the polymeric separator is difficult to treat. (Fig. 1, Down Right). Conversely, nowadays batteries itself have so varied shapes and chemistries that non-destructive disassembly may well be practically impossible. Plastics are harmful to the aqueous processes, and can e.g. clog equipment. After processing, the residues must also be washed and dried before disposal processes.
* Upper left corner: copper anode foil with graphite
* Lower left corner: aluminum anode foil with lithium cobalt oxide active material
* Upper right corner: random assortments of separator, casing and cap pieces
* Lower right corner: Polymeric separator which resides between anode and cathode in the battery
The efficiency at which batteries and their materials are recycled in the future is not only depended on the development of metallurgical processes. Changes must occur at societal and individual level. Without guiding law enforcement, we are expecting such hypothetical breakthrough into waste management that the profitability of process would be equal or better to what has been before. In this way, it is highly unlikely that the goals associated with concepts of circular economy can be achieved. Instead, we should aim to develop processes that balance economics and environmental aspects into efficient process. This is the equation that we are trying to improve in order to be able to move towards more efficient hydrometallurgical processing of battery waste.
Antti Porvali, Doctoral Student, Hyrometallurgy and Corrosion
Mari Lundströ, Assistant professor, Hydrometallurgy and Corrosion