Mining the bottom ash mountains

Date: January; 19; 2014 | Author:
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Afatek is responsible for one-third of Denmark's remining operations. The company processes 200,000 tons of bottom ash annually and reuses all of it. In just a few years, the recovery of iron and metal will reach up to 90% of the bottom ash's content, which presents both huge environmental and economic benefits.

The pen has run out of ink, which means it's time to toss it in the waste basket, right? But what if, instead, you sat down and took the pieces apart? You would then have the plastic cap, the stainless steel spring and the brass tip, and you would then be able to properly dispose of the pen's small parts at the recycling station.

Or, imagine this: at a construction site, an employee needs to dispose of his used safety shoes. He takes the shoes apart and makes sure that the steel toe pieces and soles are recycled.


Waste combustion is effective sorting

For better or worse, this is seldom what happens in reality. The pen and safety shoes, along with countless other multi-material products, end up in waste baskets and are taken to the combustion plant, where plastics, textiles and rubber are burned to create electricity and heat, leaving behind metal in the bottom ash. The pen's brass tip is trapped in the whirlpool machine, which captures fragments as small as 2 mm. The brass ends up with copper, zinc and aluminum pieces in the 9,000 tons of metal that are recovered annually by fine combing the slag from Danish waste-to-energy plants. An additional 38,000 tons of iron and 552,000 tons of bottom ash rubble are recovered and used for roadway construction.

"We truly have the world's best sorting machine here," says Jens Kallesøe, CEO of Afatek A/S. The company is owned by five waste-to-energy plants on the Danish island of Zeeland, and it currently processes one-third of all slag produced in Denmark. 

Modern mining operations. The sorting machine's sieves divide the bottom ash into coarse, medium-grain and fine-grain fractions, from 50 mm down to 2 mm. Metals are processed and recovered. Minerals become bottom ash rubble. Recovery of the bottom ash is thus 100%. 

A half millimeter provides 90%

From the conference room on Selinevej in Copenhagen there is a view of the lunar landscape of sorted bottom ash mountains. Each year, 200,000 tons of bottom ash pass through sieves, magnets, sensors and whirlpool machines, which, together, recover between 75 and 80% of the metal contained in the bottom ash. Some pieces are even as small as 2 millimeters. In the future, the percentage of recovered metal is expected to reach 90%, that is, once a three-year test period has fine-tuned the design and technology to the point that grain sizes of a half millimeter can be extracted. As such, even the tiniest copper wire and the finest drop of smelted aluminum will be included in the mix.

The test plant is up and running and currently performs the follow-up work on Afatek's previous development project, which resulted in a 50% increase in metal recovery. The project's success can be attributed to new technology, which identifies and captures stainless steel that is not caught by magnets or whirlpool machines, but which relies on sensors, computers and air nozzles that blow the steel out of the slag. The new technology also made it possible to sort down to the current 2 mm fragments. 

Mountains of sorted bottom ash. This is what our waste looks like ‒ that is, after its energy has been extracted at a waste combustion plant and it has been carefully sorted. 

Bottom ash research pays off

"Twenty percent of the waste that ends up at waste-to-energy plants becomes base ash and bottom ash. That is a large portion of the waste, and we want to be sure that it receives a due amount of attention so that it can be properly processed – both for the environment and the economy," says Kallesøe. The development project has led to significant results on both fronts. The recovery percentage has increased considerably, and this in turn can be seen in the company's earnings figures.

"The increase in earnings makes up for the initial investment in the development project, as well as for the contract from 2011 that gave us the most advanced technology. As a result, our current costs related to processing bottom ash are absolutely competitive," explains Kallesøe. The fact that the CEO knows the recovery percentage, the potential of the bottom ash and the methods by which even more of that potential can be utilized, it is also a result of Afatek's development project, which involved measuring bottom ash, recovery and operations over the course of three years.

"The bottom ash is never the same. It varies over time, from plant to plant, and from periods of crisis to economic growth in society. That's why numerous measurements are necessary over a long period of time," Kallesøe explains. 

50 million garbage bags contain bottom ash to be recovered

Afatek is accustomed to fluctuations in the content of bottom ash. However, there is no sense of fear at the prospect of more sorting at the source and the recovery of resources outside of the waste-to-energy plants.

"Our primary job is to make bottom ash rubble, and that is something we can do regardless of bottom ash quality and resource strategies. At the most, there will be a smaller percentage of metals," says Kallesøe, who goes on to explain that most metal waste will consist of the heaps of pots, pans and kitchen sinks that make their way to the plant. Today these items constitute only 0.2% of the total bottom ash, whereas the number of coils from chargers will most likely continue to grow as we purchase more and more consumer electronic devices.

"It's not because there are a lot of people throwing electronic devices into garbage cans; rather, mistakes are sometimes made in the 50 million annual household disposals across Zeeland," says the CEO, who needs only to glance outside the window to confirm that chargers, pots, eyeglasses and coins sometimes end up in the garbage can – along with ballpoint pens and safety shoes, naturally.

"We sort it all at the waste-to-energy plant, all the way down to the tiny screws in my glasses and the stainless steel used in the soles of safety shoes," concludes Kallesøe. 

Bottom ash statistics, 2013

Total bottom ash in Denmark:

600,000 tons

Recovery: 100%
- Bottom ash rubble: 552,000 tons
- Iron: 38,000 tons
- Metals: 9,000 tons

Value of iron and metals:    

117,000,000 Danish kroner (DKK)

15,683,000 Euro

About Afatek A/S

  • Joint-stock company owned by I/S Vestforbrænding, I/S Nordforbrænding, I/S AffaldPlus, I/S KARA/NOVEREN and I/S REFA.
  • Processes 200,000 tons of bottom ash annually.
  • Represents 1/3 of the market for bottom ash processing.
  • Has four sites located near the cities of Slagelse, Næstved and Copenhagen.
  • Development projects managed in partnership with the Danish Environmental Protection Agency, the Danish Construction Association, the Technical University of Denmark, the Danish Technological Institute and ZAR in Switzerland. 

Recycling of bottom ash

  • The bottom ash undergoes a preliminary sorting process. Fragments larger than 50 mm are separated and subsequently sorted by hand.
  • Sieves in the metal sorting machine divide the slag into coarse, medium-grade and fine-grain fractions, from 50 mm down to 2 mm.
  • The whirlpool machines for each fraction trap the metal pieces as small as 2 mm.
  • The upper belt magnets capture iron-containing fragments as small as 2 mm.
  • Sensors are used to detect stainless steel in fragments as small as 8 mm.
  • The rest consists of minerals that are used to make bottom ash rubble with a grain size that matches that of base course. Bottom ash rubble is used in roads and replaces the excavation of gravel from Danish gravel pits.
  • Iron and metal fractions are processed by a sub-contractor, so that the content of stainless steel (e.g.) in the fraction ends up being around 95%. Stainless steel makes up 1% of the bottom ash. The content of metals in the fraction end up at 90%. Metals constitute 3% of the bottom ash's make-up.
  • After processing, the iron and metals are sold on the market. The recovered iron and metal thus replace the extraction of iron and metal from mining.
  • A lifecycle analysis performed as part of a Ph.D. study at the Technical University of Denmark in partnership with Afatek's development projects have shown that the environment benefits from the extraction of iron and metal from bottom ash, as opposed to mines. 

The processed fraction of stainless steel. Bottom ash from waste combustion plants contains only 1% stainless steel. In the sorted and processed fraction, stainless steel makes up 95%. 

Brass, copper, zinc, aluminum in the fully processed fraction of mixed metals, which here constitute 90%. The metals make up only 3% of the bottom ash from waste combustion plants. 

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