Only 10 percent residues remain after the incineration. Most of this is made up of the slag produced by the combustion process. Molten metals are extracted from the slag and reused. Another product obtained is boiler and filter dust from the flue gas cleaning process.
The waste material is almost completely burned up by the time it reaches the end of the incineration grate. The only end product of combustion that remains is slag, which falls into a water bath through openings in the grate, where it can cool off (wet slag remover).
The high combustion temperature ensures full inclusion of the ingredients formerly contained in the waste.
The slag contains glass, metal and ceramic residual material, molten together by the high temperatures. Slag is a versatile end product. It has to however be prepared appropriately before reuse.
The first step involves washing out the easily soluble salts in a water bath. The material is then broken up, followed by removal of suspended matter. The material is then aged for at least three months on an open-air trapezoidal heap. The metal fraction is stored separately and sent for recycling.
Treatment of slag is regulated by the specifications of the German Working Group of the Federal States on Waste Issues (LAGA) and is required to fulfil certain criteria. Material intended for recycling has to comply with various product characteristics required for quality assurance.
The material may only be used if this is the case. There are various different ways in which the product can be used in construction:
- Bearing layer under concrete, asphalt or paving
- Road embankment substructure
- Mineral surface cover for noise prevention barriers
- Integration in waste dump areas as equalising layer between waste and surface sealing
Using slag helps to conserve natural resources such as gravel and sand as well as reducing intensive landscape consumption. Being absolutely reaction-free, toxic leakage water or gases are no longer emitted.
Flue gas cleaning residues are made up of filter dusts. These pollutants are produced by the incineration of waste. Since the pollutants are concentrated in this residue, it requires special monitoring. Filter dust is used to fill hollow spaces in old mines.
Old salt domes are commonly used, because their special properties allow the formation of a firm geological barrier. The cavities located at great depths associated with shut-down mines also represent a potential danger for any towns situated above. This is why backfilling is obligatory by law.
Cavities are filled up with filter dust as well as with power plant residues such as boiler ash and other industrial residues. This conserves natural resources. Adequate capacities for many years are available at present. Pollutants are withdrawn from the environmental cycle permanently in these absolutely reaction-neutral surroundings.
Backfilling Conducted By Certified Specialist Companies.
Certified recycling companies have become specialised in safe backfilling. A tough mixture is obtained by a specific combination of dusts, boiler ash and brine. This is carefully pumped into the cavities. Once the mixture has hardened, it is as strong as concrete and the cavity is adequately supported.
As opposed to other mines, salt domes do not tend to form cracks. The salt walls enclose the reaction-free material so safely that no contact to the outside world can take place any more. The water cycle within the mine is closed. That is why there is no danger of liquids having come into contact with the dusts reaching the environment.
Alternatively, the dusts are put in big-packs and used to fill hollow spaces underground.