Steel production residues
The making of iron and steel generates residual products, namely by-products and waste. In Sweden, no less than 80 per cent of the residual products are either reused as raw materials for new steel production or sold on for other areas of application. The Swedish steel industry’s vision is for none other than socially useful products to leave the company premises.
The amount of the steel production residues, i.e. by-products and waste, reflect the amount of produced steel. Iron and steel production in Sweden generate almost two million tonnes of residual products (2018). This total can roughly be divided into three groups:
- 44 % is used externally, e.g. sold on as products.
- 34 % is used internally, e.g. reused as a raw material in the production processes.
- 22 % is waste and sent to landfill
Furthermore, iron and steel scrap also arise in connection with the production and processing of the end products. This recycled scrap metal is reused within companies as iron raw material in the production process or else the scrap is sold on.
Read more about scrap in the section Recycling of iron and steel
At the iron-ore based plants, above all, a large portion of the residual products can be fed back into the processes. The steel producer SSAB, for example, has developed techniques for reintroducing residual products in the form of cement-bonded briquettes, see illustration above. The briquettes may comprise the following: blast furnace flue dust, revert scrap and mill scale sludge. These materials render a raw material for the blast furnace that includes iron, limestone and carbon.
No less than 80 per cent of the residual products are utilised
Examples of areas of application for residual products are:
- Metals industry – different kinds of raw material
- Road construction and the building industry – ballast (fill materials), steel slag asphalt, binder material and production of cement
- Agriculture and animal husbandry – soil improvers and horse riding surfaces
- Electronics industry – ferrites
- Chemical industry – different kinds of raw materials and fertiliser production
Examples of residual products
Slag has several important functions in the production processes for iron and steel. Just over one half of the residual products are metallurgical slags; with blast furnace slag being the largest item.
Read more about slag.
Dust and sludge from flue gas purification is another major item amongst the residual products.
Read more dust and sludge
Mill scale is the thin oxide coating formed when hot steel comes in contact with air and then flakes off during working of the steel. Mill scale consists of oxides of iron, above all, but also the metal alloys that are included in the steel of which the mill scale is formed. Chemically, the mill scale can be compared with iron ore and can frequently be used in similar applications, e.g. as a raw material on production of pig iron, ferrous alloys and cement.
Iron oxide, iron sulphate and metal hydroxide sludge arise on pickling with different types of acid. The iron oxide that is obtained on recycling of hydrochloric acid may, under certain conditions, be sold to the electronics industry as ferrite. Iron sulphate, which is a by-product on the regeneration of sulphuric acid (pickling liquor) is used for water purification.
The recycling of the metals from metal hydroxide sludge which is formed during pickling of stainless steel has engaged the interest of many researchers and companies. To find a good method has been shown to be difficult, despite the fact that the metals such as chromium and nickel have significant value. One method, developed within the company Outokumpu, involves the sludge with the metals being dried and then reintroduced to the production process. A further advantage of this method is that the sludge can then replace part of the steel plant’s use of fluorspar, a virgin mineral imported from China.
Shavings and filings are obtained as a residual product on surface grinding of steel in order to remove surface defects.
Crushed-brick scrap is obtained on demolition of the furnace lining.
Ferric chloride, zinc, tar, benzene and sulphur are also examples of residual products.
20 per cent goes to landfill
Within the steel industry, the companies have traditionally had their own landfill sites but some companies have decided now to dispense with own landfills. This is a result of a conscious plan for the residual product work. The materials that cannot be utilised or sent to landfill internally are sent to the municipal landfill sites or to other companies that take charge of the waste.
Waste amounts shall be kept to a minimum, so far as this is reasonable in technical and economic terms. The assessment of the best handling of the waste must be carried out in each individual case, taking full account of the environmental, technical and economic factors. At times, it is neither desirable nor environmentally intelligent to actually cut the amount of waste: due to the flue gas purification within the steel industry becoming more efficient, the amounts of separated waste dust have increased, which in itself is positive from the environmental viewpoint (cleaner air) even though this means increased waste amounts.
Despite the capture of increased amounts of waste from dust, the trend is clear. The amount of material going to landfill is constantly decreasing.
Research and development
The steel industry is investing considerable resources in attempting to increase the utilisation of different residual products. Research projects are run both in-house and jointly as part of Jernkontoret’s Nordic steel research, within technical area TO 55, Steel production residues.
Read more about Jernkontoret’s technical area TO 55, Steel production residues