There is great potential in the iron and steel industry to use residues in a resource-efficient and environmentally sensitive manner.
In 2010, five million tons of iron and steel products were produced in Sweden that generated approximately two million tons of waste products.
Metallurgical slag accounted for about half of the amount and the rest consisted of dust, sludge and residues from coke ovens. Slag is the active component of the metallurgical process and provides the steel with its desired properties. The amount of slag is equal to the amount of steel produced.
Where do all these residues go? Is there any beneficial use of so much material?
We posed the question to Peter Solomon who is communications director of the trade organization Jernkontoret.
“38 percent of the by-products arising from steel production are reused externally in various applications, 42 percent is re-used in enterprises and 20 percent ends up in landfills,” says Solomon, referring to the association’s new manual: “The steel industry makes more than steel.”
“Steel companies are becoming more imaginative when it comes to materials used in various applications. In the mid 2000’s, 35 percent went to landfills but that has now fallen to 20 percent. If you go further back in time, the share of landfill deposits was even higher.”
Waste or resource?
In one project, Swedish iron and steel companies sorted their waste into by-products and waste. The products identified have been registered under the comprehensive chemicals legislation REACH.
“Steel companies have done extensive surveys of slag from an environmental and health perspective. The results from 2010 show that nearly three-quarters of the slag has been identified as a by-product and thus can be put on the market,” says Solomon. “This is good for the environment with a view to increasing resource efficiency and at the same time it is something that provides new business opportunities for the steel industry.”
Many uses for slag
A good third of the slag that is used by companies is consumed in various internal applications. Of the slag that leaves the mill, most of it ends up in road or other construction activities.
The important characteristics in such contexts are the slag’s bearing, durability and binding capacity. The opportunities to build strong structures with slag aggregates are good. Compared to most natural materials slag is metallurgic-ally controlled during the manufacturing process. Slag is used in the following applications:
- Road construction – Different types of slag can be used in road surface, reinforcement and protection layers. An interesting application is the use of slag in asphalt (see below).
- Construction materials for covering landfills – Slag can be very persistent and some have properties similar to cement. Research and full-scale experiments show that they have the necessary characteristics to be used in landfill construction.
- Soil improvers – The fine grinding of crystalline air-cooled slag yields a KRAV environmentally certified soil conditioner. The product counteracts acidity and acts as lime, which raises the pH levels of the earth. The silicon in the slag is good for plants and the soil structure.
- Other uses – Crushed or finely ground slag can be used for example as a material for equestrian tracks, in pastures and in farmyards and in concrete production, and as a binder.
Paved with good properties
Steel slag is used in many countries in asphalt pavements but in Sweden, this is not yet widespread. Slag’s characteristics include high stability, good wear and friction characteristics and noise reduction, which makes it extremely suitable as a component to asphalt. Slag’s basic properties also mean that the binder in asphalt (bitumen) has better adhesion properties compared to conventional aggregates.
Compared with traditional asphalt, slag has better wear resistance, torsion and tensile strength and stability. It is therefore ideal for use in traffic circles, or where braking and acceleration occurs, or on congested roads.
The material is also less sensitive to water temperature changes above and below zero.
Another good feature of slag is related to the particle shape. The result is a noise canceling effect, something that is particularly important in urban areas.
Slag asphalt also has a tendency to “self-heal” minor cracks. It is slightly more expensive than regular asphalt but in terms of life expectancy it can be cheaper. One disadvantage is that it can be darker than regular asphalt.
Slag from Ovako Steel is used in several places in Sweden with good results. The project has been carried out in collaboration with NCC Roads and was launched in 2005.
Another interesting possibility is that the slag asphalt can improve air quality. There are indications that the wear and tear of the road caused by the use of studded tires in the winter may be lower on roads with slag asphalt.
Studded tires create wear and tear on the roads and cause the spread of harmful substances into the environment. What qualities slag asphalt has on the wear and tear on roads will be clarified in the context of one of Jernkontoret technology meetings during the autumn / winter 2012
The article was published in September 2012