Management of bottom ash in Europe
Materials separated from the residue of household waste incineration can be used in construction and metals production, thereby increasing recycling and reducing the need for production of ‘new’ substrates. Lenka Muchovà and Peter Rem report
Municipal solid waste is increasingly being pre-separated and incinerated in Europe, resulting in a flow of 20million metric tonnes per year of municipal solidwaste incinerator (MSWI) bottom ash. The largest producers bottom ash are France and Germany (Figure 1). Themanagement of this residue from incineration of MSW is not yet regulated at a Europe-wide level and so every country has its own system, which relates to its market possibilities and which is formalized in legislation and regulations, such as leaching tests and leaching limits.
Figure 1. Amounts of municipal solid waste land filled and incinerated in 2005 for EU countries1 Click here to enlarge image
In Switzerland, for example, bottom ash is mainly landfilled, whereas it is used as road filler in the Netherlands and Denmark — both these countries have extensive legislation controlling the application of bottom ash in infrastructure. However, in many European countries, efforts are under way to improve the quality of bottom ash in such a way that it canbe used for building applications instead of being landfilled, and to optimize the level of metal scraprecovery.
Bottom ash has a high recycling potential for secondary building material products and metals (ferrous metals, non-ferrous metals and precious metals). European bottom ash contains approximately 2million metric tonnes of iron scrap and 400,000 tonnes of non-ferrous metals, which corresponds to 1% of European iron and steel production and non-ferrous metals production.
What is bottom ash?
Bottom ash is by far the largest residue fraction (20%—25%) after the incineration of household waste. Due to the conditions of incineration, the structure of the waste is broken down into the various materials, but temperatures are generally not high enough for metals and slag to combine back into aggregated particles. Therefore over 80% of the metals are free. Most incinerators are of the stoker furnace type, which produce relatively course bottom ashes with about 50% of the material larger than 2 mm. This allows for a relatively easy separation of metals, minerals and organics. A major problem is that bottom ash is usually quenched in water before further processing. This step considerably complicates classification into particle size fractions, a step that is necessary prior to materials separation. The average composition of bottom ash is shown in Table 1.
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The major component of bottom ash is aggregate (stone, glass, ceramics) which has similar engineering properties to primary building materials (gravel, sand) and is therefore often used in infrastructural projects. The value of the material ranges from a maximum of 60% of the primary material with which it competes, to negative values if the material has to be isolated to protect the environment. The steel scrap from bottom ash typically contains residual stone, and about 1% of copper and zinc from electric appliances and batteries, which reduces the value of the scrap by € 20—40/tonne with respect to clean scrap. The main non-ferrous metal is aluminium, which extends through the entire particle size range of the ash. The remaining metals are a mixture of copper, brass, zinc, lead, stainless steel and precious metals. Most of the precious metals exist as alloys from jewellery in the 2—6 mm fraction and as remainders of electronic components in the 0—2 mm fraction2. The price of various metals has increased in the past few years, and so the recovery of metals from bottom ash is an increasingly interesting option.
Treatment and utilization inEurope
MSWI bottom ash treatment in Europe usually involves one of four basic solutions:
- Landfilling, possibly after partial recovery of ferrous and non-ferrous metals (>10 mm) by dry physical separation (magnetic and eddy current separators) (Switzerland, Italy).
- Partial recovery of ferrous and non-ferrous metals (>10 mm) by dry physical separation, followed by use of the residue in infrastructure (the Netherlands, Germany, France).
- Deep dry recovery of metal particles (>4 mm) by dry physical methods, followed by use of the residue in infrastructure (Denmark).
- Wet treatment of the bottom ash to remove organics and metals down to 0.1 mm, producing at the same time a clean aggregate for the building industry (the Netherlands).
Bottom ash is commonly separated by dry physical methods (coarse screening, size reduction, magnetic separation, eddy current separation) and using aging or washing to stabilize the residue in terms of leaching capacity. France is a good example with some of its incinerators separating the ferrous metals directly on site. The country has over 50 bottom ash management facilities for treatment and maturing. The facilities screen and crush the ash and use sifters to eliminate light, unburned fractions, magnetic sorting to recover ferrous metals and eddy current separation to recover aluminum and other non-ferrous metals. Off-site, secondary separation facilities recover another 5.3% of ferrous metals and 0.5% of non-ferrous metals3. The total French production of recovered ferrous metals from bottom ash was 301,000 tonnes in 2002.
Above Pilot plant for wet physical separation of bottom ash with a capacity of 50t/h. Click here to enlarge image
In Germany bottom ash is also treated by magnetic separation and eddy current separation. After the separation the residue is sprayed with water and stored in an enclosed facility for about three months. It is believed that after this treatment the bottom ash is stabilized and can be used for road construction. Other EU member countries are using the same or similar procedures to improve the quality of bottom ash residue.
Danish facilities are able to separate a higher proportion of metalsfrom bottom ash. In 2007, the Melgaard company recovered 1.5% of non-ferrous metals. Fine metals, up to 4 mm, are separated bydry physical separation after intensive screening, depending on the moisture content of the ash. The coarse magnetic fraction is separated by hand to recover copper from electric appliances. The residue is then used as road filler if the leaching values are within the required limits.
Many countries are trying to find ways to bring the amount of leaching down to prescribed levels. Several alternatives have been studied in the past few years, mainly focusing on the immobilization of mobile metals and other elements. Some of the immobilization processes proceed at a low temperature (natural weathering, carbonation, reaction with phosphate, etc.) and others at a high temperature (clinker production, vitrification, sintering, calcination, co-smelting with glass and artificial stone formation). However, immobilization strategies that have been tested on bottom ash did not completely meet the required criteria, in terms of leaching values. Outside Europe, expensive thermal treatment or a combination of separation and high temperature processing is considered an option.
Products from bottom ash
The recycling of bottom ash is not regulated at the EU level and therefore the market for the stony fraction is determined by local legislation. Countries like Belgium, the Netherlands, France, Germany and Denmark have developed and adjusted their civil and environmental regulations so as to allow the use of bottom ash residue in road construction4. The alternative of land filling in Europe varies in cost between € 35—80 per tonne of bottom ash. Recycling of the stony fraction is complicated due to variations in the safety and quality of the residue. Many countries have very strict leaching limits and therefore the utilization of bottom ash is very limited. The largest fraction (aggregate) is usually used for a variety of building applications (foundation, road construction, concrete, bituminous concrete aggregates, asphalt, etc.). The fine fractions of bottom ash are somewhat pozzolanic as a result of the incineration process. A new application related to this property is the use of finely crushed aggregate as cement replacement.
Left to Right The heavy non-ferrous metal from bottom ash can be used in smelting -Another use for the products of bottom ash is in the production of concrete blocks Click here to enlarge image
The other main products are steel and non-ferrous scrap. The steel scrap is usually sold to shredders to upgrade the scrap for smelting. The non-ferrous scrap is sold to sink-floaters to produce aluminum and heavy metal fractions for the various types of non-ferrous smelters. Now that increasingly finer non-ferrous fractions are produced, with relatively high concentrations of heavy non-ferrous alloys, it may become viable for bottom ash treatment plants to separate the heavy alloys from the aluminum in a jig or a kinetic gravity separator. The resulting heavy 4—8mm fraction can then be sold directly to the copper smelter.
In the Netherlands, projected changes in legislation have created a challenge to improve the separation of bottom ash. As a result, a wet physical separation pilot plant was built in Amsterdam. The wet technology can recover virtually all metals (see Table 2) and remove organic and fine particles from the residue. This combination of screening, metals recovery and washing considerably improves the quality of the residue (see Table 3) with the result that it can be sold for a positive price. The recovery of precious metals from the 0—6 mm fraction is another important economical aspect where the wet system outperforms the dry system. New developments in sink-floating using a magnetic density separator (MDS)5 provide the possibility for relatively small processors, like bottom ash treatment plants, to concentrate precious metals and produce non-ferrous metal fractions that are directly suitable for smelting.
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The high recovery of metals is essential for the quality of the stony product if it is to be used for high value construction applications, because metals are problematic even at very low concentrations. The most difficult metals are wires and stainless steel parts. The mechanical properties of the aggregate are usually not a problem.
Wet physical separation shows many positive economical and environmental contributions; therefore this system is already a step ahead in terms of sustainable principles.
The advanced dry and wet treatment technologies that are now offered on the market have a significant positive impact on the economics and recycling efficiency of MSWI bottom ash. In the short term, the choice for wet or dry will be determined by the local market for granulate and sand versus the cost of landfilling the residue or of the isolation measures required for using it in roads. In the somewhat longer term, the superior metal recovery, particularly in terms of copper and precious metals, the smaller amount of residue and the flexibility with respect to the moisture content of the input ash may become crucial points in favour of wet treatment technologies.
Lenka Muchova is a PhD student at TU Delft, The Netherlands
Peter Rem is assistant professor at TU Delft, The Netherlands
- EU statistics, (2007). Structural Indicators: Municipal Waste, http://Europe.eu.int.
- Muchová, L., and Rem, P., (2006). Pilot plant for wet physical separation of MSWI bottom ash, Conference proceeding MMME 2006, Cape Town (South Africa), 14-15 November, pp.12
- Autret, E., Berthier, F., Luszezanec, A., Nicolas, F., Incineration of municipal and assimilated wastes in France: Assessment of latest energy and material recovery performances. 2007. Journal of Hazardous Materials, B 139 (2007), pp. 569-574.
- Van Gerven, T., 2005. Management of incinerator residues in Flanders (Belgium) and in neighbouring countries. A comparison., Waste Management, Vol. 25, 75-87pp.
- Bakker, E.J., Muchová, L., Rem, P.C. (2007). Separation of precious metals from MSWI bottom ash, Conference proceeding from 6th international industrial mineral symposium, 1-3 February, Izmir, Turkey, pp.6.
- Muchová, L., Rem, P., Van Berlo, M. 2007. Innovative Technology for the Treatment of Bottom Ash. Conference proceeding from ISWA/NVRD World Congress 2007, Amsterdam, The Netherlands, 24-27 September 2007.