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The Basic Oxygen Steelmaking (BOS) Process By John Stubbles, Steel Industry Consultant
THE MANUFACTURING PROCESS
Iron ore is converted to steel via two main steps. The first involves the production of molten iron and the second is that of actual steel manufacture. The details of these steps are outlined below.
Step 1 - The production of molten iron
The Primary Concentrate is mixed with limestone and coal and heated. The iron oxides are reduced in the solid state to metallic iron, which then melts, and the impurities are removed either as slag or gas. The flow diagram for this process is shown in Figure 1.
The multi-hearth furnaces
There are four multi-hearth furnaces, each of which feeds a rotary kiln. The furnaces preheat the materials fed into the rotary kiln and reduce the amount of volatile matter present in the coal from about 44% to about 9%. This is important because the large volumes of gas produced during the emission of the volatile matter would otherwise interfere with the processes in the rotary kiln.
There are 12 hearths in each furnace and the feedstock passes down through these. In the first three hearths, hot gases from the lower stages preheat the material in the absence of air to about 450 degree C . Air is introduced in hearths 4 to 9 to allow combustion of the volatile material, so as to increase the temperature to about 650 degree C. The supply of air is adjusted to control the percentage of residual volatiles and coal char in the product. In the final hearths (10 - 12) the char and the primary concentrate equilibrate and the final temperature is adjusted to 620 degree C . The total residence time in the multi-hearth furnace is 30 - 40 minutes.
The multi-hearth furnaces also have natural gas burners at various levels. These are used to restart the furnace after shutdown and to maintain the temperature if the supply of materials is interrupted.
The waste gas from the multi-hearth furnace contains water vapour and other volatile compounds from the coal (e.g. carbon dioxide, carbon monoxide and other combustion products) as well as suspended coal and primary concentrate dust particles. These solids are removed and returned to the furnace. This gas along with gas from the melter (mainly carbon monoxide) is mixed with air and burnt. The heat so produced is used to raise steam for the production of electricity. As well as providing a valuable source of energy, this combustion of the waste gases is necessary to meet emission controls.
The Rotary Kilns
There are four rotary kilns. Here about 80% of the iron of the primary concentrate is reduced to metallic iron over a 12 hour period. The kilns are 65 m long and have a diameter of 4.6 m, closely resembling those used for cement production.
The pre-heated coal char and primary concentrate from the furnaces is mixed with limestone and fed into the kiln. In the first third of the kiln, known as the pre-heating zone, the feed from the multi-hearth furnace is further heated to 900 - 1000 degree C . This increase in temperature is partly a result of the passage of hot gases from further along the kiln and partly a result of the combustion of the remaining volatile matter in the coal.
The last two-thirds of the kiln is known as the reduction zone, and is where the solid iron oxides are reduced to metallic iron. In this region the air reacts with the carbon from the coal to produce carbon dioxide and heat:
C + O2 → CO2 ∆H = -393 kJ mol-1
The carbon dioxide then reacts with more carbon to produce carbon monoxide, the principal reductant, in an exothermic reaction:
C + CO2 → 2CO ∆H = +171 kJ mol-1
Some of the carbon monoxide burns with the oxygen to produce heat, whilst the remainder reduces the magnetite1to iron in a reaction that is almost thermochemically neutral.
In a similar manner the titanomagnetite is reduced to iron and titanium dioxide. The product from the kiln is known as Reduced Primary Concentrate and Char (RPCC) and is nearly 70% metallic iron. Unchanged ore, unburnt char, titanium oxide and coal ash accout for the rest of the mixture. This hot (950 degree C ) mixture is then discharged to the melters.
In this latter part of the kiln the temperatures can reach 1100 degree C . Higher temperatures would lead to an increase in the percentage reduction of the concentrate, but unfortunately they also produce accretions of solids on the walls of the kiln which reduce its efficiency and damage the refractory lining.
Air is injected into the kiln at nine evenly spaced points along its length. In the kiln the limestone is converted to lime (calcium oxide) which then acts as a flux in the melters. The waste gases from the kiln are scrubbed to remove solids and burnt to remove any flammable compounds before being vented to the air. There are currently plans to use the energy from this process for the co-generation of electricity.
The hot reduced primary concentrate from the kilns is fed into two melters. These are about 27 m by 12 m and hold a total charge of 1000 tonnes of iron and 900 tonnes of slag. Lime and primary concentrate may also be added to control the composition of liquid iron in the melter. The lime reacts predominantly with sulfur from the coal. Power is supplied by three continuously renewed carbon electrode pairs, which pass a large three-phase a.c. current through the contents of the melter. The potential difference across an electrode pair is 300 V and current is typically 60 kA.
The temperature in the melters rises to 1500 degree C , and this causes the reduced primary concentrate to melt and form two layers. The lower layer is of molten iron with some elements, especially carbon, dissoved in it. The upper layer is liquid oxide slag and this supports the solid feed. During the melting process reduction of the remaining iron containing compounds occurs. The electrodes are immersed in the molten slag and, becasue its electrical resistance is much greater than that of iron, most of the heat is generated in this layer.
One problem affecting the melters is that the refractory lining is subject to attack by the molten slag. In order to combat this, the solid feed in introduced around the perimeter of the melter to provide a protective barrier. The gas produced in the melter is mainly carbon monoxide and this presents both toxic and explosion hazards. It is recovered and burnt for co-generation of electricity.
Molten iron and slag are both tapped periodically by drilling a hole through the refractory sidewalls at special tapping points, higher up for the slag and lower down on the opposite side for the molten iron. The slag from the melter is approximately 40% TiO2, 20% Al2O3, 15% MgO, 10% CaO and 10% SiO2 with smaller amounts of sulfides and oxides of iron, manganese and vanadium.
After cooling and the recovery of the metal trapped in it, the slag is sold. It is mainly used as a substitute for quarried rock in applications such as road construction.
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