Municipal authorities must comply with the European legislation on waste management and therefore move away from direct landfilling to waste treatment. As natural resources become more and more scarce, prices rise dramatically, in particular for non-ferrous metals (aluminium, copper, zinc, brass, etc.). Thus, investments in sorting equipment can be considered economically attractive for the mechanical-biological treatment plant (MBT) operators that perform the municipal solid waste treatment. In that way, sorted metal products may be used as secondary raw materials in various metallurgical processes.
To do so, the different metals and alloys have to be fully separated from each other in the waste stream. Embedded impurities have to remain at low levels (iron below 1%, copper below 0.35 % and zinc below 0.35 % when considering aluminium, for instance).
Within current MBT plants, separating devices combine both mechanical separating and bio separating processes. The mechanical part consists of several processes such as screening or wind-sifting. Magnetic separation and eddy-current separation are applied to the extraction of ferrous and non-ferrous metals respectively. Both separation techniques are physical and therefore non-selective techniques and share comparable shortcomings. Firstly, compound materials cannot be detected as such (e.g. particles with low shares of magnetic iron, such as wood with nails, are extracted into a ferrous sorting product) resulting in impurities. Moreover, in contrast to magnetic separation, which aims at magnetic and therefore ferrous metals, eddy-current separation aims at the extraction of electric conductors, which applies to a variety of metals and therefore produces a product consisting of different metals and alloys.
The separation into the different metals and alloys based only on physical properties is strongly limited, so that other properties such as visual appearance have to be taken into account in order to produce high-quality products. Consequently, shipment of non-ferrous concentrates to low-income countries for manual sorting, which provides for complex sorting decisions, occurs, because manual sorting is economically challenging in the member states of the EU and if applied copes with the problem of low yields.
To overcome these drawbacks, two types of sensor technologies are combined and perform simultaneous sorting. An X-ray detector determines the density of materials (assuming the band width is specific to every material, especially when focussing on non-ferrous metals) and an electromagnetic sensor coupled with a near-infrared detector differentiates organic impurities and composites from metals. A full scale demonstration plant in Salzgitter (Germany) has been funded by the EU CIP Programme (a grant funding programme) Programme and built; hence the technology Readiness Level is 9 on the TRL scale. This plant has demonstrated how this sensor sorting technology (assisted with a final manual quality control) can effectively and efficiently sort non-ferrous metals. A recovery ratio of about 98 % of aluminium compounds, aluminium profiles, copper, brass and zinc has been achieved. As a result, recovering non-ferrous metals into secondary raw materials reduces the carbon dioxide-equivalent emissions of metal production by at least 4.
Due to the required throughput to establish a profitable process, the present process is a centralized solution to the purification of non-ferrous concentrates coming from different sources (e.g. several MBT plants) and cannot be considered as a solution to implement within every MBT. A combined business evaluation has been carried out together with a sensitivity analysis, which has shown that the success of the process depends heavily on the profile of the input materials. As this is a result of waste management (particularly collection and treatment schemes) which vary between member states, the process is not per se suitable for all EU countries, and the input materials should be subject to characterization when planning a plant. Profitability can only be achieved if the right balance of non-ferrous metals reaches a critical value. Furthermore, as matter stands, a final manual quality control is still necessary in order to score the above-mentioned yields at high product qualities due to the great diversity of the materials to be treated. However, the effort for the manual quality control is very low in comparison with manual sorting.