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Hot metal and crude steel production
Steel is mainly produced via two different process routes: the path from “iron ore to steel” and the path from “scrap to steel”. With the iron-ore based route, hot metal is mostly produced from iron oxide ores in blast furnaces, and more rarely in smelting reduction plants, and is processed to make crude steel in oxygen converters. Sponge iron (direct reduced iron, DRI; hot briquetted iron, HBI) produced from iron ore in direct reduction plants is converted to crude steel in electric arc furnaces. With the scrap-based route, crude steel is produced by recycling steel scrap in electric arc furnaces.
Steel is a modern material that offers versatile use and is still undergoing continuous further development. Thus the European steel industry produces about 2,500 standardised steels as well as a wide range of customer-specific steel grades. Each year, the compositions of about 100 steel grades are adapted to the increasing demands. Up to 30 entirely new steel grades are developed annually. Steel is used in almost all important industrial sectors, e.g. apparatus and machine construction, bridge construction, structural engineering, energy and environmental technology, transport and traffic, the packaging industry, etc.
Levels of regional and national steel production and steel use depend on population numbers and on the state of technical and economic development. World crude steel production rose from 40 million tonnes in 1900 to about 1.55 billion tonnes in 2012. In the international ranking Germany is in seventh place with 42.7 million tonnes annual production, while China is the world’s largest steel producer with 716.5 million tonnes. Steel is the number one material – and will remain so in this century.
Hot metal production in blast furnaces
In the case of the process route via the blast furnace, liquid hot metal is initially produced from iron ores, additives as well as coke and other reducing agents (such as coal, oil or gas). The hot metal is converted to crude steel in the downstream oxygen converter steelworks.
A blast furnace is a shaft-shaped aggregate that operates according to the “counter-current streams” principle. The coarse-grained input materials, coke and the burden (iron ore + additives) are charged from top via the furnace throat, while the reducing gas flows in the opposite direction of the sinking bulk material from the lower part of the blast furnace.
Preparation of blast furnace charge materials
Preparation of the charge materials is important for ensuring an even gas flow throughout the column of bulk material in the blast furnace. Iron ores are used in the form of lump ores, sinter and pellets. Lump ores are natural useable coarse grained ores extracted at iron ores mines. Preparation and enrichment processes undertaken at the iron ore mines to increase the iron content, however, result in increasingly fine grained ores that must be granulated. This is achieved by means of pelletising and sintering.
In the case of pelletising, the finest of ores (the pellet feed) and concentrate with grain sizes of well below 1 mm are formed into small balls with diameters of about 10 – 15 mm. For this purpose, the ore mixture is moistened and a binding agent added. The “green pellets” are then formed in rotary drums or on rotary tables. These green pellets are dried and burned for strengthening at temperatures of over 1000°C. This can take place in shaft furnaces, rotary kilns or on a travelling grate. Pelletising plants are generally located at the iron ore producers.
Sintering (caking) is carried out using strand sintering plants with strand widths of over 4 m and strand lengths of over 100 m. During the sintering process a mixture of ore fines with coke breeze, additives, recycling materials and return fines placed on a circulating grid, called the sintering strand, and the coke breeze on the surface is ignited in an ignition furnace using gas flames. A flow of gas or air is passed through the mixture from above by means of suction applied from below. A hot front thus passes over the band length of the approx. 500 mm layer, causing a caking of the mixture to create coarse lumps of ore. Sinter plants are located on the grounds of steel producers, near the blast furnaces.
All iron ores contain oxygen that must be removed through reduction in the blast furnace process. Carbon is used for this process. The most important carbon carrier is blast furnace coke that is nowadays produced in modern environmentally friendly coking plants. “Coking” is the heating of coal in coking chambers in the absence of air. Whereby volatile constituents – such as coke oven gas, tar, benzene, hydrogen sulphide and ammonia – are expelled and collected for various uses.
The blast furnace process
Hot air (at 1200°C) is injected into the lower part of the blast furnace, where the carbon in the coke is gasified with the oxygen in the air to produce the reducing gas carbon monoxide, generating temperatures of up to 2,200°C. This gas rises, binding the oxygen in the iron oxides and produce carbon dioxide, thus reducing the ore. The rising gases heat the charge lying above them. The accompanying elements of the input material form a liquid slag and can thus be separated out. Hot metal and slag gather in the lower area of the blast furnace and leave the furnace at a temperature of about 1500°C via a tap hole there (which must be opened). The hot metal and slag are separated via a refractory lined runner system and directed to the hot metal torpedo ladle and the slag ladle.
Other carbon carriers, such as coal, oil, gas or specially prepared old plastic are also injected into the blast furnace as an alternative to coke in order to optimise the process and reduce production costs. Operation of a blast furnace entirely without coke, however, is not possible. The coke retains its solid structure in the area of the blast furnace in which the ores become soft and melt, thus ensuring an even gas flow and acting as a supporting matrix for the solid charge column above.
Large blast furnaces have hearth diameters of about 15 m and a total volume of approximately 6,000 cubic metres. They produce about 13,000 tonnes of hot metal per day or more than 4 million tonnes annually. This necessitates the movement and feeding-in of very large material flows every day, e.g. 20,800 tonnes of iron ores, 4,300 tonnes of coke, 1,900 tonnes of coal for injection, and 12 million cubic metres of air that is heated to over 1200°C in hot-blast stoves. In addition, 3,600 tonnes of slag are produced every day, largely used as a raw or building material in the cement industry or for road construction, and 18 million cubic metres of blast furnace gas is also produced which, after cleaning, is used to generate energy.
The campaign life of a blast furnace, i.e. the length of time until its refractory lining needs to be completely renewed, is currently 15 to 20 years.
Iron production using direct or smelting reduction
Processes have also been developed to reduce ores without using coke. These processes are grouped together as either direct reduction or smelting reduction.
In the case of direct reduction, no liquid hot metal is produced because the process operates at lower temperatures than in the blast furnace. The ores are only exposed to oxygen, and the gangue content of the ores remains in the so-called sponge iron (DRI; HBI). In most direct reduction processes the production of reducing gas takes place by converting natural gas into hydrogen and carbon monoxide. The sponge iron is mainly used for steel production in electric arc furnaces.
Two steps are required with the smelting reduction method. First the ore is reduced to sponge iron and this is then converted to a blast-furnace-like form of hot metal through the use of coal and oxygen. Of the various smelting reduction processes available, only the Corex and Finex processes have so far been used industrially.
Both process types are restricted to particular regions and plant configurations for reasons of cost, and can by no means achieve the production performance of a large blast furnace.
The pig iron contains unwanted accompanying elements such as carbon, silicon, sulphur and phosphorus. These components are removed in a so-called oxygen steel converter through the injection of oxygen, whereby the impurities are oxidised and floated on the liquid metal bath in the form of slag.
The blowing process lasts about 20 minutes and generates a lot of heat. The converter is therefore charged with up to 25% scrap in order to cool down the reaction heat. The addition of lime supports slag formation. After completion of the blowing process, the slag and pig iron are separated by tapping the convertor. The converter is tilted into the tapping position for this purpose. The melt is guided into a steel ladle via the tap hole. The slag is retained in the converter and subsequently re-utilised.
The largest converter vessels worldwide currently contain up to 400 tonnes of crude steel. In addition to the pure oxygen top-blowing process (LD converters), converters with blowing from below (OBM converters) or with combined blowing from above and below (K-OBM converters) are also used nowadays. Inert gases, acting as stirrers, can be inserted from the bottom or the side walls during the blowing process to improve mixing in the converter vessel.