1 Introduction.

Steel, because of its versatile properties and its recycling possibilities, is the basic material for sustained development in modern industrial society. It enjoys a broad range of uses in almost all important sectors of industry, such as apparatus and machinery manufacture, bridge building, steel-framed building construction, power and environmental engineering, transportation, and the packaging industry, to name just a few.

The level of steel making and steel utilisation of a country or region depends on the demographic evolution and on the technical and economic state of development. In the period from 1900 to 2005 , world crude steel production rose from 40 mill. t to more than 1,1 bill. t. Germany ranks in sixth place with an annual output of 44,7 mill. t in 2005, while the EU economic area was the world's second largest steel producer until 2002 . China took over first that position in 2002. Steel is, and will continue to be, the no. 1 material in this century with the best price/performance ratio.

Iron ores are primary raw materials and are imported predominantly from Brazil, Canada, Australia and Sweden. The use of steel scrap to make steel is one of the oldest methods of recycling.

2 Process routes

2.1 Iron ore — blast furnace — converter process route

With this process route, iron ores, fluxes and coke as well as other reductants such as coal, oil, gas or processed waste plastics are firstly reduced in blast furnaces into hot metal, which is then converted into crude steel in downstream basic oxygen furnace (BOF) steel plants.

A blast furnace is a shaft-like unit that operates according to the countercurrent principle. The coarse-grained charge materials (coke and burden, i.e. iron ore + fluxes) are charged via the furnace top, while reducing gas flows upward, counter to the descending burden.

2.2 Preparation of the blast furnace charge materials

An important aspect of the blast furnace process is the preparation of the charge materials, in order to ensure the gas flow through the burden column. Iron ores are charged in the form of lump ores, sinter and pellets. Lump ores are naturally mined ores that are crushed and screened to a certain grain size before their use. However, as a result of preparation and enrichment processes in the iron ore mines to increase the Fe content, very fine-grained ores increasingly accumulate which have to undergo agglomeration. This is done by means of pelletising and sintering.

Pelletising involves the forming of ore fines (pellet feed) and concentrates with grain sizes of well under 1 mm into pellets measuring around 10 to 15 mm in diameter. To do this, the ore mix is moistened and a binding agent added. The "green" pellets are then formed in rotating drums or on rotary discs . These green pellets are dried and indurated at temperatures of more than 1000°C. This can take place in shaft or rotary furnaces or on a travelling grate. Pellet plants are generally located at the iron ore producers.

The sintering (= agglomeration) is performed at strand sintering plants, where the strands can measure more than 4 m in width and over 100 m in length. Sintering involves charging a mix of ore fines together with coke breeze, fluxes, in-plant returns and return fines onto a circulating grate, or sinter strand, and igniting the coke breeze contents in the surface by means of gas flames in an ignition furnace. A stream of gas or air is drawn from top to bottom through the mix. A flame front thus passes through the roughly 500 mm thick layer over the strand length and agglomerates the mix into coarse lumps of ore. Sintering plants are located in proximity of the blast furnaces on the works sites of the steel producers. Pelletising and sintering plants can produce around 6 million tonnes of pellets and sinter, respectively, per year.

The most important carbon carrier is blast furnace coke, which nowadays is produced in modern, environmentally friendly coking plants. Understood by coking is the heating of coal in coking chambers, closed off from the outside air, in the course of which the volatile constituents such as coke oven gas, tar, benzol, hydrogen sulphide and ammonia are expelled, collected and recycled for other uses.

2.3 Crude steel production

The hot metal contains spurious tramp elements such as carbon, silicon, sulphur and phosphorus. These constituents are removed in BOF steel plant converters.

From hot metal comes crude steel. The impurities are oxidised in converters by top-blowing oxygen through a water-cooled lance. Certain quantities of scrap, accounting for as much as 25% of the total charge, are added as cooling agents, since the oxidation process generates a strong amount of heat.

Added along with hot metal and scrap are lime, for slag forming purposes, and alloying agents. The blowing process takes some 20 minutes. Also practised nowadays besides pure top-blowing with oxygen is combined blowing, in which inert stirring gases or oxygen is additionally injected through the converter bottom.