Incinerator bottom ash, also known as IBA, is a by-product of residual waste incineration with or without energy recovery that collects at the bottom of municipal incinerators. Approximately 20 per cent of waste material sent for incineration is recovered as IBA, which can be processed to recover valuable metals, with the remainder being used for construction purposes.
In the recovery process, before IBA is deemed suitable for recycling, it is sampled and measured to ensure that it is non-hazardous and compliant under the European Waste Code 19 01 12. When the material is classified as hazardous, it then requires further treatment and stricter controls for disposal.
Once sent for reprocessing, the IBA is passed along on conveyor belts through large magnets that remove the ferrous IBA elements, particular steel. Following this, non-ferrous ash is also separated for onward reprocessing. This is passed through eddy current separators, inductive sorting and x-ray technology to separate the different types of non-ferrous metal, including zinc, brass, copper and aluminium (which makes up the large fraction, due to the prevalence of aluminium in packaging that does not get initially separate in the recycling process).
These recovered metals, which can be as small as a few millimetres, are then sent onward for further processing, to be melted down for subsequent use in metal products, such as copper pipes, brass taps, aluminium vehicle engines or zinc oxide (in paints). These can be recovered in significant amounts, with an estimated 1,000 tonnes of aluminium recovered from IBA in Wales every year.
The remaining, largest fraction of the IBA is then used for construction as a secondary aggregate, principally as a sub-base for highways, car parks and other constructions for vehicle traffic. When it is bound with cement, the use of IBA can be extended and is particularly suited to road paving. The use of IBA in construction aggregates displaces the use of other materials for use in aggregate, potentially reducing the carbon footprint if used locally.
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