High technologies increasingly require metals, semiconductors and rare earths in the required purity. Electromagnetic fields open up new possibilities for the development of innovative separation processes for the separation of material mixtures to provide these important resources. The extraction and processing of raw materials and the recycling of materials are playing an increasingly important role in the supply of our local economy. Already today, the demand for non-ferrous metals is covered to about 50% of scrap metal. In addition, the need to recycle electronic waste creates a need for suitable separation technologies.
The upsurge in renewable energy technologies has led to a rapidly increasing demand for photovoltaic solar cells. Silicon wafers are produced with an enormous consumption of energy (~ 100 kWh/kg). A major amount of this energy is spent for the production of the silicon feedstock. Usually, the silicon is cast in form of large ingots. Still to date, the majority of the thin wafers are sliced by wire sawing. After the sawing step, only 45%-50% of the silicon feedstock finally ends up in a wafer. The remaining 50%-55% is lost in the block cutting process (tops, tails and slabs) and the biggest portion (up to 40%) is lost as sawing slurry in the wafer saw process and is currently not recoverable. According to the current production processes, this equals to about 140,000 tons of discarded silicon per year.
The HZDR played a leading role in the European project SIKELOR, whose goal was the development and testing of new technologies for the recovery of saw waste from silicon production. This research was carried out together with the industrial partners GARBO (Cerano, Italy) and EAAT (Chemnitz, Germany) as well as the Universities of Padua (Italy) and Greenwich (UK).
The project performed a combination of numerical simulation, physical modeling, and demonstration experiments. The research activities considered all process steps as compaction, melting, purification and casting. Four strategic tasks have been tackled:
- Controlling the fluid flow: The contaminant particles suspended in the silicon melt are affected by the electromagnetically driven flow, transient drag, buoyancy, surface tension, turbulent fluctuations, evaporation, and the local electromagnetic pressure. The combination of physical and numerical modeling provides insight into the underlying mechanisms. Optimized magnetic fields parameters have been identified.
- Electromagnetic design and implementation: The achievement of optimized combinations of induction heating, electromagnetic stirring and electromagnetic separation is a challenging engineering task. Two different magnetic field types are required for heating and separation (alternating magnetic field, AMF) and electromagnetic stirring (traveling magnetic field, TMF). Creating these by separate coil systems seems impossible due to the space limitations and the mutual electro-magnetic interference (crosstalk). A sophisticated solution for an inverter delivering a current comprising the superposition of two harmonics of different frequencies was realized in the project. The necessity of matching the respective impedances of the loads required an intensive collaboration between the partners developing the power supply and the partners concerned with the overall design and the implementation of the coil system for the melt treatment.
- Reclaiming silicon product: The existing proprietary compaction process had to be improved. The focus was on decreasing the surface-to-volume ratio and the treatment of the very small size silicon particles. Upon completion of the pilot plant, compacted feedstock needed for crystallization experiments was produced in the first test production cycle using an advanced treatment developed within this project. The new production line demonstrated the feasibility and the reliability of the process, thus opening the path for exploitation and commercialization.
- Crystallization of silicon into ingot casts: An experimental Demonstrator was build for casting of the silicon ingots ready to be sliced into wafers. This unique facility is equipped with tailored magnetic systems for controlling the fluid flow and the separation of impurities during crystallization under the influence of external fields. Both heating and melting will be achieved using the electromagnetic induction heating. The AMF also generates the Leenov-Kolin force for separating the SiC inclusions. Particle capturing is supported by TMF-driven electromagnetic stirring for transporting the particles into regions where separation is highly efficient.