Elina Huttunen-Saarivirta, Marjaana Karhu, Timo Kinos, Pertti Lintunen and Tarja Laitinen
VTT Technical Research Centre of Finland Ltd
World annual steel production in 2017 was about 1690 million tonnes (with the contribution of China by approximately 50%) . This value covers the manufacturing of all types of steels: carbon steel, alloy steels and stainless steels and is by far the greatest figure among the metallic materials. For comparison, the global cement production was roughly 2.5 times that of steel production. 70% of the produced steel used iron ore as the raw material, whereas scrap-based steel accounted for the remaining 30%. It is easy to imagine that the worldwide consumption of steel is constantly growing, due to increase in population and overall standard of living. Luckily, iron ore is a relatively abundant resource in Earth, thus we still have reserves left to cover the demand , and the recycling rate is expected to rise, as well.
On average, the production of one tonne of steel results in the range from 200 kg (steel making in electric arc furnace) to 400 kg (iron making in blast furnace/basic oxygen furnace) of by-products . Currently, one of the greatest challenges in the area is carbon dioxide emissions related to steel making; these form one group of by-products, and the exact amount of released carbon dioxide emissions is dependent on the used technology . However, steel industry here in Northern Europe is a forerunner in environmental responsibility, aiming at significant reduction in the emissions by using hydrogen as the reducing agent instead of carbon. Other gaseous by-products from steel making are, once cleaned, typically used to generate steam and electricity.
The main solid by-products in iron and steel making are slags, accounting for the majority of side streams by mass (depending on literature: from 70 to 90%). Additionally, dusts and sludges are created. According to World Steel Association , more than 400 million tonnes of iron and steel slag is produced each year. Compositionally, the slags are a mixture of silica, calcium oxide, magnesium oxide, and aluminium and iron oxides. Previously slags were landfilled as useless by-products, whereas today they are recognized as products with clear market value. Iron making slags are compositionally more homogeneous than those in steel making; these are available on market in various forms, air-cooled, granulated and pelletised, and used widely as construction aggregate, in cement and concrete production and, e.g., in road bases. Steel production slags are chemically more heterogeneous, and approximately 50% of the recovered slag is used in the construction of roads.
As compared to landfilling, the utilisation of steel industry slags in construction industry: road bases, asphalt, cement and concrete making may be seen as a high-value end use. However, there are also applications, in which the physical properties, e.g., ability withstand elevated temperatures, of the slags may be made use of. VTT Technical Research Centre of Finland Ltd currently develops in EU-RESLAG and SA-CloseLoop projects refractory ceramics that are manufactured from steel industry slag, a so-called secondary raw material. In the CloseLoop project, the focus is on improved fundamental understanding on the causalities between the raw material characteristics and the properties of produced refractory ceramics.
Ferrochrome slag and a refractory ceramic specimen manufactured using the slag as aggregate. Photo: Marjaana Karhu.