Overview in Europe
Bottom ash (BA) from municipal solid waste incineration (MSWI) has a high potential for the recovery of valuable secondary materials due to the conditions of incineration. This is because the structure of the waste is broken down into various materials, but temperatures are generally not high enough for metals and slag to combine back into aggregated particles. The bottom ash contains valuable metals and aggregates which could be alternatively used by building industries if they are environmentally safe.
Overview of European MSWI BA treatment systems:
- Land filling, possibly after the partial recovery of ferrous and non-ferrous metals (>6-8 mm) by dry physical separation e.g magnetic and eddy current separators (Switzerland, Italy).
- Partial recovery of ferrous and non-ferrous metals (>6-8 mm) by dry physical separation, followed by use of the residue in infrastructure (Germany, France).
- Deep dry recovery of metal particles (>4 mm) by dry physical methods, followed by use of the residue in infrastructure (Melgaard, Denmark).
- Advanced dry recovery of metal particles (>1 mm) by advanced dry and wet physical methods (Inashco, the Netherlands).
- Wet treatment of BA (Indaver, Belgium).
- Dry physical separation of particles <5mm (KEZO, Switzerland); prior to this system of separation, the incineration process must be adjusted in order to generate very dry BA. This plant is able to separate particles down to 0.2-0.5 mm and residue fraction can be sent to a landfill.
- Dry physical separation by advanced treatment steps for particles down to 0.5 mm (DHZ AG, Switzerland).
Bottom ash is mostly separated by dry physical methods (coarse screening, size reduction, magnetic separation, eddy current separation) and by using aging or washing to stabilize the residue in terms of leaching capacity.
Many countries are trying to find ways to bring the amount of leaching down to the proposed level. Several alternatives have been studied in recent years, mainly focusing on the immobilization of mobile metals and other elements. Some immobilization processes occur at low temperatures (natural weathering, carbonation, reaction with phosphate, etc.) and others at high temperatures (clinker production, vitrification, sintering, calcination, co-smelting with glass and artificial stone formation). However, immobilization strategies that have been tested on BA 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 are possible options.