2.3 Materials management
2.3.2 Management of production residues
The integrated steelmaking process produces a wide range of materials, including steel as a main product. Other material outputs from the steelmaking processes are blast furnace slag and various steel slags, ammonium sulphate, sulphur or sulphuric acid from the COG desulphurisation plant, coke tar, coke pitch and (crude) benzene from the coke oven gas by- product plant and other materials such as iron scrap.
Some of these materials are often used as raw materials in other sectors. Through process optimisation, including maximising the internal recirculation of carbon and iron-bearing dusts, residues that are surplus to the requirements of the integrated steelworks production processes are minimised. A range of uses, including a variety of recovery processes, has been developed for such materials, resulting in a relatively small proportion of total residues requiring disposal. In addition to the management of materials arising within the processes, integrated steelworks receive residues, including wastes, from other installations and sectors.
Figure 2.11 shows a typical example of the management of production residues such as by- products and wastes in an integrated steelwork.
Source: [ 243, Eurofer 2007 ]
2.3.2.1 Internal use of integrated steelworks residues
Most of the residues arising within an integrated steelworks have a high content of iron, carbon, calcium and other useful components and can replace primary raw materials such as iron ores, coal, slag formers, heavy oil and cokes. Indeed, the management of residues in an integrated steelworks is characterised by the application of advanced integrated processing techniques designed to retain, as much as possible, all of the useful components contained in residues within the steelmaking process.
The identification and separate collection by material categories (e.g. by chemical composition, particle size, or oil content) are necessary preliminary conditions to ensure proper use inside the steelworks without negative effects in terms of production efficiency, product quality and environmental protection.
The iron-bearing fraction of the various types of crude slag, iron- and carbon-bearing dusts and sludges from gas cleaning systems, used oil, new scrap and scale can be recirculated back through sinter plants, pellet plants, coke ovens, blast furnaces, and BOF plants. The fine materials tend to be recirculated through the sinter plant, whereas coarser materials are more often returned to the blast furnace or BOF plant. In order to return certain fractions to the blast furnace and BOF plant, briquetting plants are often employed, where all sorts of fine graded residues are combined into cold bonded bricks/briquettes that are easier to handle and can be used elsewhere within the process. In particular, the dusts or sludges from the steelmaking plant can be briquetted or pelletised and afterwards used in the steelmaking plant when the non- ferrous content is not too high.
In Section 2.5.4.4 specialised recycling facilities for iron-rich residues are described. Some of these facilities make the direct recovery of liquid iron possible; others serve as a pre-treatment stage to reuse the residues in the blast furnace or the electric arc furnace. The presence of high concentrations of unwanted compounds such as alkalis, heavy metals and mineral oil sets limits on the recycling of iron-rich residues.
Table 2.7 shows a compilation of different uses for steel slags in Europe.
Table 2.7: Use of steel slags in Europe
Use of steel slags (1) % Amount
Cement production 1 152 Road construction 45 6840 Hydraulic engineering 3 456 Fertiliser 3 456 Internal recycling 14 2128 Interim storage 17 2584 Final deposit 11 1672 Other 6 912 Total 100 15200
(1) ‘Steel slags’ refers to slags from BOF, secondary metallurgy and EAF.
NB: Figures are from 2004 and correspond to 12 EU member states (AT, BE, DE, DK, ES, FR, FI, LU, NL, UK, SE, SK).
2.3.2.2 External applications of integrated steelworks residues
Zinc-rich sludges and dusts are generated during BOF gas cleaning and BF gas cleaning. Nevertheless, the zinc content is not high enough to make reuse economical. Only some of these sludges and dusts can be used, so almost all iron and steel plants have a large deposit of zinc- rich sludges and wastes.
If the non-ferrous metal content of the dusts or sludges arising in the integrated steelworks is sufficiently high to ensure its technical and economic feasibility, some non-ferrous metals can be recovered in external metal production and recycling plants. For example, steelmaking dusts with enriched zinc concentrations can be used as a raw material within the zinc sector instead of zinc ores.
2.3.2.3 Disposal of integrated steelworks residues
Small parts of the overall quantity of residues from an integrated steelworks have no economic use (either internally, within the installation, or externally) and some disposal is inevitable. Materials usually requiring disposal include:
• fine dust/sludge from BF gas cleaning
• runner refractory rubble from the BF
• fine dust from BOF gas scrubbing (if a wet cleaning process is used)
• in some cases, the dust which contains high amounts of alkali chlorides and heavy metal chlorides from the last field of electrostatic precipitators, bag filters or scrubbers used to treat the off-gas from sinter strands.
Many integrated steelworks have their own internal, certified landfill facilities whilst other companies rely on external landfill sites. In all cases, landfill sites must be authorised to receive the particular wastes.