ApplicationsIncinerator Bottom Ash and Slag
Main subjects of this page:
- Information on bottom ash and slag
- Conventional processing and utilisation
- Ressource extraction with the IRRT process
The value of bottom ash is still not recognised. It often contains many valuable raw materials that can be extracted easily and extremely profitably using modern processes. Nevertheless, in most cases it is disposed of at great expense.
What Are Bottom Ash and Slag?
Grate ash – also known as »municpal solid waste incineration ash« (»MSWI bottom ash« for short) – is produced during the thermal treatment of household waste. If other waste – such as commercial waste or processed substitute fuel – is incinerated instead of household waste, the term "incinerator bottom ash" (»IBA« for short) is generally used.
Contents
- What Are Bottom Ash and Slag?
- Why Should Bottom Ash Be Processed?
- Which Raw Materials Can Be Obtained Using the IRRT Process?
- How Does the Processing of Bottom Ash Work?
- What are Boiler Ash and Fly Ash?
Can these also be recycled? - Should Bottom Ash Be Treated Fresh or after Drying?
If the waste is heated above the softening point during the process, the powdery bottom ash turns into a doughy mass. In this state, it is called “slag”. When slag cools down, it undergoes a kind of fossilisation (sintering) and becomes difficult to treat. Depending on whether the materials fall through the incineration grate during incineration or are discharged into the outlet system, they are also referred to as »grate fall-through« or »grate discharge«; together they form grate ash and slag. Sometimes, the grate fall-through even contains organic material and is fed back into the incineration process.
Why Should Bottom Ash Be Processed?
Metals and Scrap Iron
Bottom ash and slag are valuable sources of raw materials. After all, not all parts of household waste are turned into ash during thermal treatment. Metals (ferrous and non-ferrous), glass and ceramics in particular are retained. The amount of these materials contained depends heavily on the mechanical processing prior to incineration. The more extensive this was, the fewer recyclable materials are ultimately found in the incinerator bottom ash. The recycling of valuable materials such as metal is particularly worthwhile in two respects: firstly, separating and processing these materials is often much more economical than mining new raw materials. Secondly, this process not only conserves resources, but is also more climate-friendly and sustainable.
Substitute Building Material Production and Raw Material Extraction
Depending on the material composition, either high-quality raw materials or cheap substitute building materials can be obtained from bottom ash and slag. Substitute building materials are easy to produce using conventional methods. They are often used as a base layer for construction projects, which was long regarded as a better alternative to landfill. In the meantime, however, the processes for treating bottom ash and slag have advanced to such an extent that targeted processing by means of material analyses should definitely be examined. With the IRRT process, for example, we offer an extremely economical way of marketing up to 80 % of the dry mass as legally recognised raw materials.
Which Raw Materials Can Be Extracted Using the IRRT Process?
In addition to the recovery of the majority of all metals, the IRRT process offers the possibility of producing two marketable raw materials:
How Does the Processing of Bottom Ash Work?
The best possible type of reprocessing depends on the material composition. This determines whether the special IRRT procedure can be used or whether conventional reprocessing is more suitable.
IRRT Procedure
The IRRT process (Innovative Resource Recovery Technology) relies on three newly developed process steps to improve the physical properties of the waste incineration plant ash and to release the inclusions in the sintered slag.
Conventional Treatment
The conventional treatment aimes at separating the metals that are still present and preparing the ash as a base layer. Overbelt magnets and screening systems are therefore generally used. The high pH value, abrasiveness and moisture of the input material also place special demands on the technology to be used – we at Eggersmann have the expertise to meet these high requirements.
Are You Interested in Processing Your Bottom Ash?
We Will Be Happy to Advise You!
What are Bottom Ash and Fly Ash? Can These also Be Recycled?
IBA or bottom ash is also called "grate ash" because it falls through the grate into the ram deslagger during incineration. There it is cooled in a water bath. However, some of the ash also rises with the hot air in the combustion chamber during combustion. The ash that accumulates in the boiler's empty flue is known as "boiler ash". However, particularly fine ash particles travel even further and reach the flue gas cleaning system. These filter dusts are known as "fly ash". The boiler ash and the fly ash from waste incineration plants are usually heavily contaminated with pollutants. Fly ash in particular is considered hazardous waste and is therefore subject to particularly strict regulations. Recycling is currently not possible.
Should the Bottom Ash be Treated Fresh or after Drying?
Both approaches have advantages and disadvantages:
Dried/Aged Bottom Ash
Advantages
- Cooled and therefore safe to process
- The low residual moisture reduces carry-over rates and makes mechanical treatment overall more efficient
- The pH value and certain physical building properties improve during storage
Disadvantages
- Certain materials such as aluminium oxidise during storage and, depending on their size, may dissolve completely
- Storage requires a lot of space and is therefore very cost-intensive
- High dust generation during processing
Fresh Bottom Ash
Advantages
- Recyclable materials such as aluminium hardly have time to oxidise and can still be recovered
- Hardly any dust during processing
- Expensive interim storage can be completely dispensed with
Disadvantages
- Glowing embers that have not yet cooled down in the water bath pose a risk to the process technology
- The material is still very sticky and the carry-over rates are therefore particularly high
- The material is still highly abrasive and causes high wear – the highest quality standards must therefore be met
We can realise systems for both approaches.
With the innovative IRRT process, you can combine the advantages of both approaches. It is therefore superior to traditional methods.
Reference Plants for Bottom Ash and Slag Treatment
Plant for Processing Grate Ash
Singapore
Engineering and implementation planning, procurement, manufacture and delivery of the entire plant technology as well as installation and commissioning.