The residual stream of municipal waste, i.e. what has not been selectively collected at source, is sent to waste incineration plants with energy recovery. This is usually a mixture of combustible fractions (plastics, paper and cardboard, textiles), some bio-waste, glass and ceramics, metals and fine mineral material.
The better the separate collection, the less biowaste and metals end up in the ‘residual’. Modern characteristics of the composition of the mixed waste bin show the great potential for improving segregation at source and explain where recyclables still come from in the residual stream.
At what stage are metals recovered and how does the technology work?
The key separation of metal raw materials takes place after incineration, from the furnace slag (English term: incineration bottom ash). Ferrous metals are captured with magnetic separators and non-ferrous metals such as aluminium, copper and zinc are recovered with eddy current separators. Sensor sorting, which improves recovery from fine fractions, is also increasingly used.
Screening ‘before the grate’ is mainly for process safety, not recycling. Such a treatment model is described in both Best Available Techniques reference documents and technology reviews.
How much metal is in the slag and how effective is the recovery?
| Indicator | Value or scope | Explanation |
|---|---|---|
| Furnace slag contains a measurable proportion of metals, including non-ferrous metals. | Most often, between 5 and 15 per cent of the slag weight is metals, of which non-ferrous metals are usually between 1 and 5 per cent; aluminium predominates among these. | These ranges are given by scientific and industry reviews on furnace slag treatment. |
| Modern lines allow a very high recovery of ferrous metals and increasingly better recovery of non-ferrous metals. | Typically, the recovery of ferrous metals reaches about 85-95 per cent of the content in the slag, and non-ferrous metals about 40-65 per cent; recovery of fine fractions further improves the results. | These findings corroborate recent summaries of literature and industry practice. |
| The scale of the slag and metal stream recovered after incineration is increasing, as illustrated by national and European data. | In the European Union, as well as in Norway and Switzerland, the order of 18-20 million tonnes of slag is produced annually; in the Netherlands, around 130,000 tonnes of ferrous metals and around 47,000 tonnes of non-ferrous metals were recovered in 2020. | The data show a growing contribution of slag to the secondary raw materials market, although the share of the overall secondary metals market remains limited. |
What is the treatment of slag and the path of metals back into the economy?
Once selected from the grate, the slag is cooled and stabilised, then screened into fractions and directed to the separation line. Subsequent stages remove ferrous metals using the magnetic method, non-ferrous metals using the eddy current method and then use sensor sorting to capture fine grains.
After purification, the metal fractions are sold to recycling plants and further to steel mills, aluminium alloy foundries and facilities for refining copper and other metals. As a result, the secondary raw material is returned to circulation and replaces some of the primary raw materials.
Dust and residues from flue gas cleaning – a more difficult fraction with increasing potential
Unlike furnace slag, dust and flue gas cleaning residues contain higher concentrations of salts and heavy metals and are therefore often classified as hazardous waste. Stabilisation and controlled disposal is the standard.
At the same time, technologies for the recovery of metals, especially zinc, as well as copper and silver, using chemical leaching and electrochemical methods, are developing more rapidly. Pilot implementations and first realisations confirm the direction, although their viability depends on ash composition and market conditions.
Legal and technical framework – The European Union as a reference point
Waste incineration plants and slag treatment facilities operate under the regime of the 2010 Industrial Emissions Directive and the Best Available Techniques Conclusions for Waste Incineration adopted by a European Commission implementing decision in 2019. The reference document and the conclusions cover not only the thermal process, but also waste acceptance, slag management and metal recovery. The requirements are transferred to the integrated permits of the installations.
Regional differences
| Region | Image and practice regarding waste incineration and furnace slag | A hard example or figure to illustrate the scale |
|---|---|---|
| Europe | Europe has the most developed waste incineration market with energy recovery and mature furnace slag treatment lines, including ferrous metal separation with magnets, non-ferrous metals with eddy current separators and increasingly sensor sorting of fine fractions. In parallel, there are quality requirements for slag aggregates. | The Netherlands has industry targets for the minimum level of recovery of non-ferrous metals from slag, and state reports confirm that the slag material meets standards for reuse as an aggregate replacement in concrete and road construction. |
| North America | North America has fewer facilities than Europe, but operators are extensively upgrading post-combustion metal separation lines, investing in fine metal separation equipment and dedicated slag treatment plants, increasing revenue and reducing residue storage. | Publicly available industry material and operator reports describe an increase in recovery through metal separation facilities and the construction of incineration slag treatment plants. |
| Asia | Asia operates very large volumes of incineration, particularly in Japan, and Singapore is developing a programme to reuse recycled furnace slag as a building material under the NEWSand initiative, for which interim environmental standards have been introduced. | Singapore has published interim environmental standards for the use of recycled furnace slag and gasification slag as a substitute for fine aggregates in concrete and road foundations. |
| Oceania | Oceania is entering a phase of commercial deployments of slag treatment and metal recovery; state regulations are clarifying the mode of approval for the reuse of slag materials, and the first specialised facilities are emerging. | In Australia, in the state of Victoria, an ‘A16’ permit is required for the use of furnace slag or aggregate made from slag, and in New South Wales a ‘warrant and exemption’ system for resource recovery is used. |
| Africa | Africa is only just establishing infrastructure in this area; the first large-scale facilities demonstrate technical and energy feasibility and set the stage for slag and ash regulation. | The incineration plant in Addis Ababa, Ethiopia, is designed to process around 1 400 tonnes of waste per day and to generate around 185 gigawatt hours of electricity per year. |
| Latin America | Latin America is at the pilot implementation and investment preparation stage; project documentation describes pathways for the recovery of metals from slag and possible uses of the mineral fraction of slag as aggregate. | Industry reports indicate that the use of processed slag as a road or construction material is being considered and implemented at pilot project level. |
What about the ‘share’ of recovered metals in the real input of smelters?
Metals recovered from furnace slag are a locally noticeable source of secondary raw material, but against the background of the overall secondary metals market – especially aluminium – they are still a fraction feeding smelters and foundries.
From the point of view of a circular economy, more important than the percentage share is the fact that these metals are realistically replacing part of the primary raw materials, reducing the pressure on extraction and the accompanying emissions.
Sources:
– European Commission Implementing Decision of 12.11.2019 establishing conclusions on best available techniques for waste incineration; reference document for waste incineration (BREF Waste Incineration 2019).
– Scientific and industry reviews on the metal content of incineration slag and the effectiveness of recovery, including the contribution of non-ferrous metals and the effects of recovery of fine fractions.
– Estimates of European volumes of slag and flue gas treatment residues and national compilations for the Netherlands (volumes of metals recovered from slag in 2020).
– Environmental standards and information on the NEWSand programme in Singapore (interim standards for the use of recycled slag as a building material).
– Regulatory framework for the reuse of slag materials in Australia: the ‘A16’ permits in the state of Victoria and the ‘orders and exemptions’ regime in New South Wales.
– Information on the Addis Ababa incinerator in Ethiopia (capacity and energy production).
– Residual and recycling waste composition studies in Ireland (conclusions on the quality of separate collection and the raw material content of the residual stream).
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