The consumption habits in modern lifestyles are causing a huge worldwide waste problem. The increase in the amount and diversity of waste materials generated by human activity, and their potentially harmful effects on the environment and public health, have led to an increasing awareness of the urgent need to adopt sensible methods for safe disposal of waste. While there is an obvious need to minimize the generation of waste, and to reuse and recycle it, technologies for recovery of energy from waste also play a vital role in mitigating the problems. Besides recovery of energy, these technologies reduce the overall quantity of waste requiring final disposal. Incineration of municipal waste converts the waste into incinerator bottom ash, flue gases, particulates and heat, which can in turn be used to generate electricity.
About 60 million tonnes of MSW is incinerated in the European Union annually. MSW incinerators produce approximately 20 million tonnes of bottom ash – which is the end-of-the-line product of incineration. After incineration the bottom ash remains on the plant site of the incinerators. Up to now, the waste incineration sector has had no perfect solution to realize the full potential of its major residual stream – incinerator ash. Following combustion this slag typically contains 10% of ferrous and 3.5% of non-ferrous metals.
Each year in the Netherlands, aluminum, copper, silver and other metals worth some €45 million (US$67.3 million) are lost during incineration. Until recently, 80% of the non-ferrous metals were not recycled. This is because about half of all bottom ash is extremely fine; hindering what should be a simple separation process.
Current situation
The bottom ash can either be sent to landfill or treated for recycling. Conventional treatment is typically able to recover metal particles larger than 10–15 mm from it, leaving the majority of the non-ferrous metals as contaminants in the mineral fraction. The possibility of these remaining fine metal particles leaching is a significant environmental problem which calls for preventive measures. Isolation of the bottom-ash layer – by sealing it off with impermeable foils such as bituminous material or concrete, or by a near watertight covering – is commonly necessary if the material is sent to landfill. If bottom ash is used as a low-grade building material, the extra costs for leach-prevention are significant. This generally results in a negative net value of bottom ash.
EU directives encourage the recycling of MSW and discourage the use of landfill sites. Fading borders between European countries demand a unification of the policy on waste management. At the present time, the Netherlands is one of the leading member states in waste management policy. In fact, there is great similarity between Dutch and overall European policy in this field, demonstrating that current Dutch legislation and policy might be indicative of the future policies of other EU countries.
In the Netherlands (and Germany), landfilling MSW that can be incinerated is prohibited. Moreover, Dutch authorities are aiming for a useful application of 83% of the annual MSW generated in 2012. Most national authorities discourage landfilling of MSW by increasing gate fees. In countries where landfill is still the most common treatment for MSW, there is an urgent need to adjust the management programme. Building municipal solid waste incinerators will most likely be a part of this.
The ideal solution
The Delft University of Technology has many years of experience researching the characteristics of waste incinerator ash and the development of suitable recycling technologies. Having extensively explored the field of wet processing techniques, the Delft University of Technology recently revealed a breakthrough dry recycling technology which is simple, robust, effective, and environmentally and economically unbeatable. ‘The new technology is cost saving, separates the small metal and mineral particles extremely efficiently and decreases the environmental charges considerably. The new separation technology offers a solution for the current issues,’ says Dr P Rem from the Faculty of Civil Engineering and Geosciences at the university.
A prototype drew the attention of Fondel Development BV, a subsidiary of the Fondel Group. ‘We are continuously looking for new and innovative ways to efficiently recycle waste. This technology is unique because in the short term major economic revenue can be obtained, and at the same time a huge step is taken to recycle 100% of waste into valuable products,’ says Jaap Vandehoek, director of Fondel Development. The Fondel Group and Delft University jointly founded a company named Inashco to further develop this technology. Inashco stands for Incineration Ash Company.
 Figure 1. A diagram of the Inashco process |
With Inashco technology, the slag can be economically processed to recover these metals and remove contaminants in order for the remainder to then be used as an improved aggregate for the construction sector. With conventional technologies, only a small amount of metal (particles larger than 12 mm) can be extracted. Inashco has developed an advanced process able to recover metal particles down to 1 mm. Inashco can separate out the tiniest metal and mineral fractions from this ash, proving that even the dregs of the waste chain are still precious.
The technology does not only maximize the economic benefits, it also is the most environmental-friendly process available. Direct handling of bottom ash prevents dust and gas emissions, and minimizes chemical reactions such as leaching of heavy metals and the corrosion of aluminium metal particles, resulting in a higher quality of aluminium scrap product.
The Inashco process
The process consists of a concentrator and an upgrading facility. The concentrator is a patented dry technology which is able to efficiently recover metal particles down to 1 mm from bottom ash. It is a compact (semi) mobile installation producing a non-ferrous metal concentrate, a steel scrap concentrate and – due to its high efficiency – mineral aggregate in various size classes with immediate added value for the building and construction sector.
The technology works like this: the moist bottom ash is sized by ordinary screens at a size typically between 12–20 mm. Steel and non-ferrous metals are recovered from the coarse (+12 to +20 mm) product as usual. The remainder, without any form of drying, is brought into contact with the Inashco concentrator. The very fine metal contaminants (-1 mm) and mineral particles (-2 mm) are separated from the bottom ash in a physical classification step without the use of water, chemical additives or drying energy.
 The Inascho process is one of the most environmentally friendly available |
The advantage of this technology is it can be used as an extension to existing installations, meaning it can be added to conventional technology (screens, magnets and eddy current separators) to improve the non-ferrous metal recovery by 50%–100%.
As well as the Inashco concentrators that are operated on site at waste incinerators and dedicated bottom ash recyclers, Inashco operates a centralized process called the upgrading facility for the further separation and cleaning of the non-ferrous metal concentrate into pure secondary metal products, like copper, zinc, brass, lead, gold and silver. This metal can easily be sold to scrap dealers.
The cleaned mineral aggregates from bottom ash for concrete products have been researched by an independent committee called CUR in the Netherlands since 2008. Based on the results of tests carried out by the CUR-Committee, it is expected that 20% of the aggregates currently used in concrete in the Netherlands may be substituted with cleaned bottom ash. Beside the CUR tests, promising ongoing research by leading concrete producers is also being done for concrete tiles and concrete bricks.
Future challenges
Besides the research programme by the Delft University of Technology, which aims to further develop and improve the process and its products, the cementation or binding ability of incineration bottom ash is also being investigated in several parts of the world. A few universities have reported that it is possible to use incineration bottom ash as a type of cement. The idea is to produce simple concrete products (i.e. concrete tile and bricks) out of milled incineration bottom ash. Inashco is monitoring these developments.
 Fine mineral product, at -2 mm and +2 mm |
Another future challenge for resource recycling is the increasing social awareness of environmental issues, such as pollution and safety. This applies to both consumers and companies. In terms of incinerator ash management this calls for optimizing metal recovery, minimizing the period of storage, minimizing dust and gas emissions and finding a sound closed loop solution for the 85% of non metals in bottom ash. Based on this breakthrough dry recycling technology, Inashco has set up a unique service package enabling the 100% recycling of bottom ash from waste incineration to usable products.
Marc de Keizer is the technology sales manager of Inashco BV.
e-mail: marc.dekeizer@inashco.com
