Recovery of aluminium from municipal incinerator bottom ash
Each year around the world, millions of tonnes of rubbish are generated and find their way to waste tip mountains or sent to landfill – even merely dumped by the roadside. But, this waste can obversely be regarded as a valuable resource for metals and materials recovery.
Extraction of metals from the residue following incineration of municipal waste derives not only environmental benefits but also real economic advantage. Although the recovery of scrap metal from this residue, so-called Bottom Ash, has been employed for many years, the increased interest and development in this process began only in the early 1990s driven by various factors: A significant increase in the amount of waste incinerated; emerging technologies for the recovery of non-ferrous metals, and a relative increase in metal prices.
In a world full of products of innovation, such as highly engineered materials, synthetic chemical compounds and complex products such as electrical and electronic equipment, the natural environment remains paramount, and the concept of total recycling becomes vital, albeit at the same time, more complicated and convoluted.
Recycling does have its limits – it is not a goal in itself, but a means to an end in maximising overall performance. The main benefits of recycling are environmental protection, decreasing the need to extract and produce virgin materials, and reducing energy requirements and large-scale emissions. In addition, overall resource availability of is enhanced by recycling. Traditionally, modern solid waste management addressed public health and environmental protection and incidentally local resource scarcity, occurring for instance in war times or during the pre-industrial era. Recently, global-scale issues such as climate change, resource efficiency and overall sustainability have become increasingly important for the global waste management community.
In terms of recycling and resource efficiency: it is time for a change from bulk quantity to specific quality. A high level of municipal solid waste (MSW) collected for potential recycling does not necessarily translate into maximum resource efficiency or sustainability. It is the value of the processing that counts. Considering the current levels of performance for recycling of MSW, in affluent countries, high rates of metal recovery have been achieved, starting from levels of less than 10% wt. Huge efforts over the last 30 years have realised growing recycling indices, including composting, with Eurostat, the statistical office of the European Union, reporting a 42% wt. average for the EU-27 in 2011, and numbers close to 70% wt. for top performing countries. Also, rates close to 90% wt. have been recorded for certain materials such as aluminium, and even higher levels are being sought. As a real bonus, recycling metals from bottom ash genuinely makes positive savings in Greenhouse gas emissions.
The Metal Packaging and Recycling Industry, the Recycling Industry of bottom ash and the Waste-to-Energy (WtE) sector jointly support the European Commission’s approach to boost quality recycling and markets for secondary raw materials in the Circular Economy Package. These related industries have robustly supported the EC’s proposal to include recycling of metals from WtE bottom ash in the recycling performance indices in the Waste Framework Directive and the Packaging Waste Directive, while meeting certain quality criteria. This will provide additional incentives for WtE operators to recycle even more metals from bottom ash and also help to reduce GHG emissions that would be generated during the production of primary metals. The GHG savings/credits from metal recycling from WtE bottom ash amount to some 2,000 kg CO2 equiv/tonne of recycled metals.
In Europe, metal recycling from bottom ash saves the equivalent of some 3.2 million tonnes of GHG emissions per year. While collection and separation at source is vital for the quality recycling of most waste streams, small and in particular very thin metal packaging items can sometimes be difficult to screen out and therefore remain in the residual waste fractions sent for incineration. Fortunately, metals such as steel and aluminium can still be recycled from the incinerator bottom ash, while maintaining their material quality and value. The combustion process does not change the material properties or the value of the remaining metals, which can therefore be further used again as secondary raw material, so replacing virgin metal and avoiding the related GHGs. After steel, aluminium is the most abundant metallic component of bottom ash. Neither steel nor aluminium are degraded during the combustion process. Aluminium melts and re-solidifies as small droplets which can then be easily sorted and recycled as secondary raw material, and used for example in new castings for the automotive industry, such as engine blocks. The separated ferrous fraction is directly used for the production of new cast iron and steel. The inert fraction, forming the rest of the bottom ash, once stabilised, can be used as secondary raw material, for example in construction.
Incinerator bottom ash aggregate is processed from the material discharged into the combustion residue grate of Municipal Solid Waste (MSW) incinerators. One company, WRT Ltd in the UK has over 20 years experience in supplying processing equipment, which involves screening the material, separating the ferrous and non-ferrous metals for external recycling, re-screening, then further metal removal before final product stockpiling.
The scrap metal recycling rate depends on the waste composition and the technology used – in the latest facilities this can reach more than 80%. Metal recycling from incinerator bottom ash should not be disadvantaged in comparison to recycling from other sources. The remaining residues after the bottom ash sorting phase from WtE plants should be considered as valuable secondary material fit for recycling within the European Circular Economy. Member States should be able to report these extra quantities in their annual recycling reports to the EU statistical services, in order to meet the new and ambitious targets indicated in the waste proposals of the revised EU Circular Economy Package.
In 2014, around 88 million tonnes of waste were treated in Waste-to-Energy plants in Europe with incineration producing around 18 million tonnes/yr of bottom ash. Important quantities of metals and minerals are present in these residues remaining after the combustion process, offering many opportunities for recycling.
Bottom ash comprises inert, non-combustible materials: sand, stone, ash – together with an important 10-12% metal content, including up to 4% aluminium. For example, in 2014, some 20,000 and 17,000 tonnes of aluminium were recovered from bottom ash in the Netherlands and in France, respectively.
The scrap metal recovery rate depends on the waste composition and the technology used, but in best case scenarios the recovery rate can reach about 80% of the total metals content of the bottom ash. To extract the valuable metals, bottom ash is processed either on site or in specialised facilities. Usually, the first step is to separate the ferrous metals using a powerful magnet device, and then bottom ash is sorted into different fractions based on the size of the residues. This allows for a more accurate separation of the various non-ferrous metals using the eddy current separation technique. This involves a conveyor belt that ejects non-ferrous metals such as aluminium at different distances according to their reaction to changing magnetic fields.
The remaining inert fraction of the residue, after metal recovery can be used as construction material, such as for road building or noise barriers, as an aggregate for concrete, or as an alternative to using sand and gravel.
Innovative separation plant for bottom ash: Dutch companies, AEB Amsterdam and Inashco, have developed an innovative separation plant for bottom ash. This enables recovery of a maximum of valuable raw materials, such as aluminium and zinc from incinerator residues, while also providing a sustainable product alternative for gravel and sand.
In the Netherlands, all operators of Waste-to-Energy plants have signed a “Green Deal Bottom Ash” with the Dutch Government. The main items of this agreement are that more than 75% of all non-ferrous metal fragments greater than 6 mm present in the bottom ash must be recovered – and, as from 2020, granulates must be sufficiently sterile that they can be applied for all potential applications. Through this prime example of a Public-Private partnership, the Dutch Government and the Waste-to-Energy operators have been able to successfully process bottom ash, for both metal recovery and also to produce a clean and totally applicable secondary building material.
Sources and for more information:
European Aluminium – www.european-aluminium.eu
Confederation of European Waste-to-Energy Plants – www.cewep.eu
European Suppliers of Waste-to-Energy Technology – www.eswet.eu
Fédération Internationale du Recyclage – www.fir-recycling.com
Municipal Waste Europe – www.municipalwasteeurope.eu