Negative to Positive Sorting Reaching for the stars with SATURN

Due to the change of emphasis from landfilling to waste treatment, which is implicated by the Landfill Directive, more and more waste is undergoing treatment processes.

Started in 2009 in Salzgitter, Germany the SATURN project set about demonstrating how sensor based sorting technologies can be optimised to recover significant volumes of increasingly valuable non-ferrous metals from mixed municipal solid waste. Bastian Wens reports on the results.

Due to the change of emphasis from landfilling to waste treatment, which is implicated by the Landfill Directive, more and more waste is undergoing treatment processes. The driving force behind the SATURN project, which stands for Sensor-sorting Automated Technology for the advanced Recovery of Non-ferrous metals from waste, has been this prevailing EU waste legislation, as well as economic factors.

Mechanical Biological Treatment (MBT) is one of the most commonly used treatment processes for Mixed Municipal Solid Waste (MMSW) and is a substantial part of waste management in many EU countries today, with rising significance in the future.

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In addition to the increasing use of MBT technology, since 2003 the prices for Non-Ferrous metals (NF-metals) has risen steadily (despite the financial crisis in 2008/2009) making sorting equipment economically attractive for plant operators - as demonstrated by the Fact sheets on German MBT plants 2010/2011. These show an increasing number of separation units for NF-metal extraction being installed in German MBT plants in recent years.

Furthermore, reduced landfilling and greater treatment and separation of waste enables the environmental impact of waste management to be minimised. Table 1 displays the energy savings as well as the CO2-savings by using secondary raw materials.

It becomes clear that aluminium recycling bears the highest relative and total energy saving potential. Considering the limited possibilities for the purification of aluminium during metallurgical processing and the resulting high requirements for purity of secondary raw materials by the market, the importance of producing high-quality secondary aluminium at high yields is becoming more and more significant.

Funded by the European Commission's Competitiveness and Innovation Framework Programme (CIP), SATURN has been designed to demonstrate how sensor sorting technology can efficiently separate NF-metals from municipal solid waste. Having started in August 2009, the project is coming to an end in early May 2012, having planned, established and operated a full scale demonstration plant at the premises of its partner, recycling company Metall-Konzentrat und Recycling GmbH (Mekon) in Salzgitter, Germany.

NF-metals are extracted using sensor sorters with electromagnetic sensors Credit: TITECH

Scientific examination and project coordination was carried out by the Department of Processing and Recycling (I.A.R.) of RWTH Aachen University, while the sensor based sorting equipment was provided and optimised by TITECH, Germany. Aachen based pbo Ingenieurges, a waste management consultancy, was the partner responsible for the process design and UK based Envirolink Northwest, has been supporting the project with dissemination activities and is also looking at the environmental performance and the business plan.

SATURN's processes focus on NF-concentrates from the mechanical or mechanical-biological treatment (both are referred to here as MBT plants) of municipal solid waste (MSW), with the main focus on MMSW. NF-metals are extracted from waste streams using eddy-current separators and sometimes sensor sorters equipped with electromagnetic sensors (EM sensors). Both separation techniques share two main characteristics with regard to the sorting products:

Little or no differentiation between different metals and alloys Non-metallic impurities 

Depending on the process design, non-metallic residues usually range from 20% to 40% but can also be above 60%. Aluminium profiles, which relate to thick-walled pieces that are usually made from cast aluminium, display the main component. Thin-walled aluminium products, usually wrought aluminium qualities, are comprised by the different fractions displayed in the left diagram. Besides the light metals, heavy metals are also present, with the major components of brass and zinc, both being found in comparable quantities, usually between 4% and 12%. Copper is usually found in concentrations below 5%.

NF-concentrates from the UK and the Czech Republic have also been studied and were found to be suitable input materials for the process established. Differences were found in the share of thin-walled aluminium fractions, which exhibit substantially higher concentrations in NF-concentrates analysed from MBT plants in the UK. The reason for this is seen to originate from the different collection schemes that direct these products to the separate collection of light packaging materials. Furthermore, beverage cans are taken back in refund systems nationwide in Germany. Since the NF-concentrates from MBT plants show input materials with considerable levels of impurities and a large variety of products, application beyond the borders of Germany as well as to other concentrates, e.g. from the treatment of separately collected wastes is therefore generally given.

In order to create metal products that can be used as secondary raw materials in metallurgic processes, the different metals have to be separated and impurities decreased to an acceptable level. Limitations on aluminium products with regard to alloying elements are especially important. Many commonly used aluminium cast alloys exhibit iron below 1%, copper below 0.35% and zinc below 0.35% or lower. The variety of products and the requirements on high product qualities determine many factors of influence that either qualify products or disqualify metal products (especially metal composites) for reuse, further treatment or disposal.

These complex decisions often rely on manual sorting, whereas high quality products require positive sorting, which means that only particles that are classified as target materials are extracted. On the contrary, in negative sorting mode the target material is purified by the extraction of impurities. To put magnitude in context, Table 2 gives an example of the number of particles that constitute NF-concentrates. In a range of 30 mm to 120 mm, aluminium particles exhibit an average weight of 12g to 52g. The heavy metal particles have an average weight between 70g and 90g. With 10 workers at a rate of 0.5 grabs/sec, the throughput of such a plant would be 0.77 Mg/h assuming a yield of 100%. In reality, this leads to reduced yields and losses to the process residues. However, it is clear that positive manual sorting requires extensive use of labour.

 

SATURN aims to help replace manual sorting with sensor sorting equipment Credit: TITECH

Manual sorting in high-income countries, such as many of the EU's Member States, has disadvantages such as limited yield and relatively high operational expenditures. As a result, shipment of NF-concentrates to low-income countries for manual separation occurs, directing these secondary raw materials out of the EU.

The approach of SATURN is the replacement of the manual sorting step by adapted sensor sorting equipment and the moderate deployment of manual labour for the quality control of the sorting products. The result is a shift from positive to negative sorting, so that the number of grabs can be lowered substantially.

The key elements of the process are the sensor based sorting machines, which have been adapted to the input material in the demonstration plant in Salzgitter. Both sorters are ribbon type automatic sorters, which were provided by TiTech.

The X-Tract machine measures the absorption of x-rays, thus being able to determine the density of materials. Figure 2 shows the bandwidth of the NF-metals in focus. As can be seen, aluminium alloys lay in a range between 2.6 kg/m³ and 2.9 kg/m³. Compound materials or other factors such as inclusion of other materials (e.g. stones or organic materials) alter the overall density of the particles, which prohibits the exact measurement of the material density of the main metal of a particle. A separation between different aluminium alloys is hardly possible as it requires exact density measurement. The same is true for brass and copper or zinc and tin. The large differences between aluminium and the heavy metals however, leads to highly efficient separation of aluminium from the heavy metals.

The second machine is equipped with an EM Sensor and a near infrared (NIR) spectrometry. The main object is the differentiation between organic impurities and composite metals. Using the information from both sensors simultaneously, it can differentiate between particles only containing low grades of organic impurities and particles that exhibit higher levels of impurities to a limited extent. The complexity caused by the large diversity of products and materials in the NF-metals has not yet been fully implemented in sorting algorithms, so that a manual quality control is required.

Information from both sensors was used simultaneously to differentiate particles Credit: TITECH

With the final justification of the sensor sorters established during continuous optimisation a recovery of 98.4% of Al compounds, Al profiles, Copper, Brass and Zinc has been measured in test runs with real scale throughputs.

Preliminary life cycle analyses suggest that the use of the secondary raw materials produced reduces the CO2-equivalent emissions of metal production by at least four times. Under current market conditions it is estimated to reach the break-even point at an annual throughput roughly between 2600 tonnes to 8000 tonnes, depending on input material and site-specific circumstances.

As the concentration of NF-metals is in the range of 0.5% in MMSW, this relates to a minimum MBT capacity of between 520,000 tonnes and 1.6 million tonnes, which makes clear that this solution can only be seen as a centralised treatment of NF-concentrates from different sources and not as a solution to upgrade NF-concentrates at each MBT plant.

  A recovery of 98.4% of compounds, brass and zinc has been measured in test runs Credit: TITECH

The final results of the project will be presented at the Saturn final workshop taking place alongside the International Sensor Sorting Conference in Germany on April 19th, 2012 in Aachen (more information can be found at http://bit.ly/AdGSxP.

Bastian Wens is project engineer for SATURN and an academic at the Department of Processing and Recycling, RWTH Aachen University. e-mail: wens@ifa.rwth-aachen.de
The SATURN project has been funded under the CIP Eco-innovation Pilot and market replication projects (Call Identifier: CIP-EIP-Eco-Innovation-2008). Project coordinator: Professor Thomas Pretz, project manager: Kate Hornsby. e-mail: hornsby@ifa.rwth-aachen.de
For more information on SATURN and for details of the SATURN Workshop, please visit www.saturn.rwth-aachen.de or e-mail: saturn@ifa.rwth-aachen.de

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