In the course of the electrification and digitalisation of our everyday lives, the number and mass of electrical devices are inevitably increasing. This increase also leads causally to an increased volume of WEEE. The following figure shows the forecast of the worldwide volume in millions of tonnes. The data shows a projected increase in WEEE mass of around 60% within 15 years.
WEEE contains many valuable metals such as gold, palladium, silver and also copper with up to 22, 7, 313 and 44 g/t respectively . These must be recycled for economic and ecological reasons. The WEEE-NET9 project is investigating several waste streams from electrical appliances. Specific flowsheets for mechanical treatment and subsequent post-treatment by bioleaching are designed and evaluated.
Mechanical processing is an important instrument for producing highly enriched material concentrates, which are further processed in subsequent steps, for example by metallurgical processes . The basic principle of mechanical processing is to separate each material from the rest on the basis of specific physical properties such as magnetic susceptibility, settling velocity, electrical conductivity, etc.. The most important prerequisite is the complete liberation of all materials. This means that there must be no more compounds between different materials. This can be achieved either by targeted disassembly or by adapted comminution of the pieces. Depending on the feed material, a variety of different machines are available for this purpose, which achieve the desired result through different stress mechanisms such as impact, pressure, tearing, etc.. Too much comminution or the wrong stress leads to unnecessary energy consumption, wear of the machines or even to a deterioration of the sorting properties of the materials. There is also a lot to consider when sorting the materials. Several material or particle properties always have an influence on the respective sorting principle. Furthermore, the properties are usually distributed and superimposed on each other. Different materials can also have the same properties. For example, not only iron but also cobalt and nickel are magnetic. 
Figure 1 shows a possible flowsheet for the mechanical pre-processing of mobile phones. The pre-treatment in which the batteries were removed and the telephones were pre-shredded is not included. All products shown are to be understood as intermediate products. It is already apparent that it is a multi-stage interconnection of different process steps to merely produce material concentrates. And the smaller and more complex electrical devices become, the more complex the recycling process is.
Figure 1: flow chart – processing of mobile phones
Further mechanical processing of the products is also possible, but the economic benefit must be investigated by means of a cost-benefit analysis. It would be conceivable, for example, to separate the products < 3.15 mm with an air-sorting table. The dust < 1 mm could be further separated by a wet high gradiant matrix separator and/or flotation. Most of the valuable metals mentioned are found in the printed circuit boards. Accordingly, they can be found in the PCB products or have already been successfully liberated and are enriched in the dust product < 1 mm. The following analyses and further experiments must show this.
This is exactly the advantage of the approach from the project to combine mechanical treatment with bioleaching for the treatment of WEEE. Enriching the valuable metals finely distributed on the printed circuit boards is mechanically simple, but extracting them is complex. This is where the special microorganisms come into play, which can extract the corresponding valuable substances from the complex compounds through their natural metabolic processes. This technology is still comparatively new but offers many possibilities. The use for secondary raw materials is still in the research stage, as corresponding publications show .
- Chancerel, P., et al., E-scrap metals too precious to ignore. Recycling International, Nov, 2008: p. 42-45.
- Bigum, M., L. Brogaard, and T.H. Christensen, Metal recovery from high-grade WEEE: A life cycle assessment. Journal of Hazardous Materials, 2012. 207-208: p. 8-14.
- Schubert, G., Zerkleinerungstechnik für nicht-spröde Abfälle und Schrotte. AT. Aufbereitungs-Technik, 2002. 43(9): p. 6-23.
- Hopfe, S., et al., Leaching of rare earth elements from fluorescent powder using the tea fungus Kombucha. Waste Management, 2017. 62: p. 211-221.