Coking plant
SSAB uses both pulverized coal and coke for producing hot metal. SSAB metallurgical processes have coking plants in Luleå and Oxelösund. Injection coal and coke
are the reducing agents in the blast furnace process.
The heart of the process in the coking plant is the coking battery comprising a number of tall, narrow ovens. Coking is a dry distillation process, i.e., combustion without access to oxygen. The coal is charged by “coal machines” above the oven battery.
The ovens have brick partitions in which the heating wall channels are heated by the gas generated in the coking battery itself, possibly mixed with blast furnace gas. The coal is heated in the narrow, airtight ovens until it is in an almost flowing, plastic form.
The elements that are to be removed will then be gasified. The process takes about 18 hours. The temperature is above 1,000°C, and the coal is converted to 75 percent coke and 25 percent gas. The coking plant has a number of processes in which the gas is cleaned in several stages and many raw materials are recovered.
The most important is the cleaned gas that supplies energy to the processes of the coking plant itself and to other users, such as the blast furnace. But raw materials are also recovered for the chemical process industry, such as sulphur in desulphurizing, fertilizers for agriculture, tar and asphalt. The end customers of the coking plant include manufacturers of perfumes and pharmaceuticals.
Blast furnaces – a smelting reduction process
About 100 years ago, Sweden had 120 blast furnaces. Combined, they jointly produced just over half a million tonnes of hot metal per year. Today, Sweden has three blast furnaces, all of which are owned by SSAB.
The largest is in Luleå, and this alone produces 2.5 million tonnes of hot metal on an annual basis. There are two smaller blast furnaces in Oxelösund. SSAB’s total hot metal capacity is just over 4 million tonnes.
All Swedish hot metal is produced using pellets from the LKAB ore deposits in Lapland. The LKAB pellets are produced from magnetite ore and have a high iron content - more than 66 percent. LKAB utilizes the properties of its magnetite ore in its pellets, so that a lower amount of energy is needed for producing the blast furnace pellets. The high purity of the LKAB pellets provides SSAB with benefits in hot metal production.
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Reducing elements, additives
The oxygen in the ore must be removed for converting the ore pellets into hot metal in the blast furnace, which is known as reduction.
The blast furnace process is a melting reduction process. In the iron ore, the iron is bound to oxygen as magnetite, with the chemical formula Fe3O4. Fe is derived from the Latin name of iron, ferrum, and O stands for oxygen. The numerals specify the number of atoms of each element in magnetite ore.
A reducing agent that will combine with the oxygen at high temperature must be added. Carbon, with the chemical symbol C, is used as reducing agent in the form of coke and injection coal.
Hot metal flow around the clock
The blast furnace is charged – continuously, around the clock – from the top with iron ore pellets, coke and additives.
The coke supports the large column of ore and coke inside the blast furnace. The particle size of the coke allows the molten hot metal to trickle down and the gas to rise through the blast furnace. Some of the coke can be replaced by injecting pulverized coal together with the blast air. The blast air is supplied at high pressure though a number of large nozzles under the broad belly of the blast furnace, where pulverized coal is injected at the same time.
The blast air nozzles are known as tuyeres. The blast air is heated in tall brickwork towers, cowpers or heaters, using energy recovered from the blast furnace gas and coke gas.
The reduction process takes place inside the refractory-lined blast furnace, where the temperature is 2,200°C in the hottest zone. The melting point of iron is 1,535°C, but the carbon lowers the melting point to below 1,200°C.
When the iron has been reduced and melted, it trickles down and is collected in the bottom of the blast furnace, which is known as the hearth. The blast furnace is tapped at a uniform rate. Tapping takes place during about 2 hours and is then interrupted for 40-50 minutes before the next tapping.
But what happens to the material that has been reduced – the oxygen and the carbon?
They combine into a gaseous form, i.e., carbon monoxide CO and carbon dioxide CO2, which is discharged from the blast furnace through large gas pipes to a gas treatment plant. The carbon monoxide is rich in energy, and the blast furnace gas is recovered as energy for the processes of the blast furnace itself and for other energy customers in the steelworks, and also for power generation and for district heating (see the section on Energy).
SSAB participates in research into how carbon monoxide can be recycled and used in the reducing process. At the same time, carbon dioxide will have to be separated in the future for storing in underground caverns. This EU-supported research project is known as Ulcos, is supported by the EU and is run in the LKAB pilot blast furnace at Mefos in Luleå. The project may lead to future ultra-economical blast furnaces. When the hot metal is tapped from the blast furnace, the slag is also discharged.
The slag consists mainly of silicon and limestone. The silicon is a residue from the ore gangue – the rock that surrounds the ore veins in the mine - and the ash from the coke and coal.
Limestone is added to the blast furnace process in order to collect silicon and other undesired substances to form a slag. The slag floats on top of the molten hot metal, is separated by the skimmer and then tapped subsequently. After it has cooled and solidified, the slag is recovered as blast furnace slag.