The Blast Furnace

In Europe, iron had been manufactured in many small and very dispersed furnaces. There was a big demand for a more effective manufacturing process. The answer was the blastfurnace, and the production became a multi-stage process  

Using larger furnaces and increased air supply mainly achieved an increase of the produced iron quantity. In Germany the volume of the blast furnaces, Sttickofen, increased from ca. 1.5 m to ca. 4 m in the period 1500-1700 and the daily output grew from 1200 to 2000 kg. Around 1830 a blast furnace produced 3000-4000 kg of pig iron a day. For each tonne of iron 1500-2000 kg charcoal was needed. 

The furnace was fed from above with iron ore, limestone and charcoal. Air was blown into the lower parts of the furnace. Then carbon monoxide CO was formed, which reduced iron oxide to solid iron sponge in the middle of the furnace. In the lower part the iron absorbed carbon and melted as pig iron with 4% carbon. 

The high-carbon iron from the blast furnace could in fact be directly used for one type of final product cast components. Castings were made either directly from the blast furnace or from iron remelted in special low cupola furnaces, sometimes placed far from the blast furnace, even within the cities. 

The consumption of cast iron rose considerably with the frequency of wars and the mechanization of the armies. It is said that during the Thirty Years War the Cathohc army under Tilly sent about 15 000 cannon balls of cast iron every day into Magdeburg at the siege of this town in 1631. Iron cannons were cast directly from the blast furnace. These pieces were inexpensive - compared to bronze cannons - but very heavy. They were not used for light field artillery but for permanent installations in forts and on ships. 

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Pure iron is a silvery white, relatively soft metal and is rarely used commercially. Typical properties are Hsted in Table 1. Electrolytic (99.9% pure) iron is used for magnetic cores (2) (see Magnetic materials, bulk). Native metallic iron is rarely found in nature because iron which commonly exhibits valences of +2 and +3 combines readily with oxygen and sulfur. Iron oxides are the most prevalent form of iron (see Iron compounds). Generally, these iron oxides (iron ores) are reduced to iron and melted in a blast furnace. The hot metal (pig iron) from the blast furnace is refined in steelmaking furnaces to make steel... 

In 1979, there were 168 blast furnaces in the United States, most located in Pittsburgh and Chicago, and these produced ca 8 x 10 t of pig iron. By 1992, most of the blast furnaces in the Pittsburgh area had disappeared. Only 44 blast furnaces were operating in the United States, producing ca 4.7 x lO t of pig iron. The drop in pig iron production can be attributed to decreased and more efficient use of steel products, competition from steel imports, and rapid growth of scrap-based steelmaking. 

The Utah deposit is located in southwestern Utah near Cedar City. The iron ore deposits are of contact metamorphic origin. The cmde ore contains 35 to 65% iron, primarily in the form of magnetite and goethite. Mining is done by the open pit method. The cmde ore is cmshed, screened at —75 mm (—200 mesh size) and shipped as lump ore containing 54% iron. The ore is rescreened at the steel mill to produce lump ore (10—64 mm) for the blast furnace and sinter feed (0—10 mm) for the sinter plant. 

Blast Furnace. The blast furnace is the predominant method for making iron. Estabhshed for centuries as the premier ironmaking process, blast furnace ironmaking both enabled and profited from the Industrial Revolution. Although the fundamental principles of operation are unchanged, the blast furnace has evolved into a highly efficient and productive process. 

Thermochemistry. From an overall heat and mass balance point of view, the main chemical reactions of the blast furnace include oxidation of carbon in the zone in front of the tuyeres (raceway) to give CO plus heat. 

After it leaves the stoves, the hot blast enters a large refractory-lined busde pipe to distribute the gas evenly around the furnace. Multiple connecting pipes (tuyere stock) direct the hot blast to the blowpipes. At the ends of the blowpipes are the tuyeres, water-cooled copper no22les set into the refractory lining of the blast furnace. 

Opera.tlon, Because of the long residence time of the materials (8—10 h), the blast furnace process can exhibit considerable inertia, and control is usually appHed where the goal is maintaining smooth, stable input conditions. One of the most important aspects of blast furnace control is supply of consistent quaUty raw materials, which is why there is a strong emphasis on quaUty control at coke plants, peUeti2ing plants, and sinter plants (see Quality ASSURANCE/QUALITY control). 

The most common method of converting iron ore to metallic iron utilizes a blast furnace wherein the material is melted to form hot metal (pig iron). Approximately 96% of the world s iron is produced this way (see Iron). However, in the blast furnace process energy costs are relatively high, pollution problems of associated equipment are quite severe, and capital investment requirements are often prohibitively expensive. In comparison to the blast furnace method, direct reduction permits a wider choice of fuels, is environmentally clean, and requires a much lower capital investment. 

The reduction of iron ore is accompHshed by a series of reactions that are the same as those occurring in the blast furnace stack. These include reduction by CO, H2, and, in some cases soHd carbon, through successive oxidation states to metallic iron, ie, hematite [1309-37-17, Fe202, is reduced to magnetite [1309-38-2], Fe O, which is in turn reduced to wustite [17125-56-3], FeO, and then to metallic iron, Fe. The typical reactions foUow. 

DRI, in peUet/lump or HBI form, can be added to the blast furnace burden to increase furnace productivity and reduce coke requirements. It can be used for short-term increases in blast furnace output when a faciUty is short of hot metal during times of high steel demand, or when one of several blast furnaces is down for a reline. It also can be justified if the increased output is sufficient to allow operation of fewer blast furnaces long-term. 

Use of a blast furnace is preferred if a regular supply of a charge of coarse and consistent quaUty is available. However, the blast furnace is not suitable for treating finely divided feed material. 

Reduction to Liquid Metal. Reduction to Hquid metal is the most common metal reduction process. It is preferred for metals of moderate melting point and low vapor pressure. Because most metallic compounds are fairly insoluble in molten metals, the separation of the Hquified metal from a sohd residue or from another Hquid phase of different density is usually complete and relatively simple. Because the product is in condensed form, the throughput per unit volume of reactor is high, and the number and si2e of the units is rninimi2ed. The common furnaces for production of Hquid metals are the blast furnace, the reverberatory furnace, the converter, the flash smelting furnace, and the electric-arc furnace (see Furnaces, electric). 

The molten slag and the molten Hon, called hot metal or pig Hon, ate tapped from the hearth of the blast furnace. A modem blast furnace yields 5000—9000 t/d of Hon. The compositions of the pig Hon and the slag are determined by the furnace temperature, the composition of the ore, and the added flux. Pig Hon always contains 3.5—4.5 wt % carbon, variable amounts of siHcon, manganese, sulfur, and phosphoms. 

Pig iron and iron and steel scrap are the sources of iron for steelmaking in basic-oxygen furnaces. Electric furnaces have rehed on iron and steel scrap, although newer iron sources such as direct-reduced iron (DRI), iron carbide, and even pig iron are becoming both desirable and available (see Iron bydirectreduction). In basic-oxygen furnaces, the pig iron is used in the molten state as obtained from the blast furnace in this form, pig iron is referred to as hot metal. 

Pig iron consists of iron combined with numerous other elements. Depending on the composition of the raw materials used in the blast furnace, principally iron ore (beneficiated or otherwise), coke, and limestone, and the manner in which the furnace is operated, pig iron may contain 3.0—4.5% carbon, 0.15—2.5% or more manganese, as much as 0.2% sulfur, and 0.025—2.5% phosphoms siUcon can be as low as 0.15% with modern techniques and is almost always less than 0.8%. Sulfur, phosphoms, and siUcon can be reduced significantly by treating the hot metal between the blast furnace and the steelmaking vessel. During the steelmaking process, many but not all solutes are reduced, often drastically. 

Methods exist to make impure iron direcdy from ore, ie, to make DRI without first reducing the ore in the blast furnace to make pig iron which has to be purified in a second step. These processes, generally referred to as direct-reduction processes, are employed where natural gas is readily available for the reduction (see also Ironbydirectreduction). Carbonization of iron ore to make iron carbide as an alternative source of iron units is in its infancy as of the mid-1990s but may grow. 

The prime requirement of any carbonaceous material used in the blast furnace hearth wall or bottom is to contain Hquid iron and slag safely within the cmcible, throughout extended periods of continuous operation, often up to 15 years. 

For practical reasons, the blast furnace hearth is divided into two principal zones the bottom and the sidewalls. Each of these zones exhibits unique problems and wear mechanisms. The largest refractory mass is contained within the hearth bottom. The outside diameters of these bottoms can exceed 16 or 17 m and their depth is dependent on whether underhearth cooling is utilized. When cooling is not employed, this refractory depth usually is determined by mathematical models these predict a stabilization isotherm location which defines the limit of dissolution of the carbon by iron. Often, this depth exceeds 3 m of carbon. However, because the stabilization isotherm location is also a function of furnace diameter, often times thermal equiHbrium caimot be achieved without some form of underhearth cooling. 

Refractories for Cupolas. In many ways, the use of carbon cupola linings has paralleled the appHcation of carbon in the blast furnace. 

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