Hearth of Blast Furnace

Finally, not the least important development in refractories was the introduction of carbon blocks to replace fireclay (compositions similar to kaolinite) refractories in the hearths of blast furnaces making pig iron. Early experience was so successful that the all carbon blast furnace seemed a possibility. These hopes were not realized because later experience showed that there was sufficient oxygen in the upper regions of the furnace to oxidize the carbon and hence preclude its use there. 

Keywords vanadium, distribution ratio, blast furnace, hearth of blast furnace... 

Carbon refractories. Carbon blocks used in the construction of blast-furnace hearths are made from dense coke possibly mixed with anthracite. Suitable grain-size fractions (up to 5—15 mm) are mixed with tar to a plastic mass which is shaped at elevated temperature by pressing or ramming. The products are fired without air at about 1400 °C in saggers covered with fine-grained coke. The carbonized tar produces a firm bond between the grains. Granular mixes arc also used in certain applications. 

Carbon Refractories. These refractories, consisting almost entirely of carbon, are made from a mixture of graded coke, or anthracite, pitch and tar the shaped blocks are fired (packed in coke). The fired product has an apparent porosity of 20-25% crushing strength 50-70 MNm-2 R.u.L. (350 kPa), 1700°C thermal expansion (0-1000 C), 0.65%. The principal use is in the lining of blast furnaces, particularly in the hearth and bosh (cf. plumbago). 

The blast furnace consists of a vertical shaft in which lead oxide contained in sinter is reduced to lead metal by a counter-current flow of gas rich in carbon monoxide. Elevated temperatnres are required and the necessary heat and CO are generated by the combustion of coke with an air blast at the base of the shaft. Coke is nsed to provide a non-fnsible support for the charge and a free open stmctnre to facilitate uniform gas flow np throngh the shaft. Reduction of free PbO in the sinter by gaseous CO occurs from the solid state in the npper shaft. The sinter melts in the lower part of the shaft and the bulk of the contained lead, in the form of glassy silicates, is rednced to lead metal as it flows down over the hot coke bed. Fnrther rednction can take place in the slag pool contained in the hearth of the furnace. 

Secondary lead smelting is predominantly based on the treatment of scrap lead-acid batteries, as detailed in Chapter 10. A wide range of smelting methods are used for the treatment of secondary lead, originally based on traditional smelting methods such as the hearth and blast furnace, feeding whole batteries, but are changing under stricter environmental controls to the use of prior separation and specialised reduction in rotary furnaces or newer electrochemical techniques. 

A typical blast furnace installation is shown in Fig. 8.1 and the evolution of blast furnace sizes in Fig. 8.2 [3]. As seen, recent developments have tended to produce larger furnaces, especially those with larger hearth diameters. Modern blast furnaces have working volumes ranging from 60,000-150,000 ft (1080-4500 m ) and may produce up to 10,000 tons of hot metal per day. 

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. 

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. 

The principles pertaining to carbon blast furnace hearths apply as well to submerged-arc furnace hearths. In some processes, such as in d-c arc furnaces, the electrical conductance of carbon is a most important factor. The long life of carbon linings in these appHcations is attributable to carbon s exceptional resistance to corrosive slags and metals at very high temperatures. 

FIG. 23-43 Reactors for solids, (a) Temperature profiles in a rotary cement lain, (h) A multiple hearth reactor, (c) Vertical lain for lime burning, 55 ton/d. (d) Five-stage fluidized bed lime burner, 4 by 14 m, 100 ton/d. (e) A fluidized bed for roasting iron sulfides. (/) Conditions in a vertical moving bed (blast furnace) for reduction of iron oxides, (g) A mechanical salt cake furnace. To convert ton/d to kg/h, multiply by 907. 

The blast furnace (Fig. A, opposite) remains the basis of ironmaking though the scale, if not the principle, has changed considerably since the eighteenth century the largest modem blast furnaces have hearths 14 m in diameter and produce up to 10000 tonnes of iron daily. 

Gestell, n. frame, stand, support, base, rack, bed, mount (of a blast furnace) hearth, gestem, adv. yesterday, gestemt, a. starred. 

Solid effluents arising from metallurgical operations occur principally in two forms fine particulate solids or dusts, and solid wastes. As an example, blast furnace gas may contain up to 170 kg of dust per ton of pig iron produced. Suitable methods must be devised for processing the solid effluents for two reasons (i) to prevent pollution of the environment and (ii) to recover their valuable content, if any. As far as the latter is concerned, reference may be drawn, as an example, to the recovery of rhenium from the exit gas from molybdenite roasting in a multiple-hearth furnace. 

In the blast furnace reduction slag-making materials are also added together with a small amount of iron, the function of which is to reduce any sulphide which remains, to the product of the roasting operation to produce a sinter. The sinter is then reduced with coke in a vertical shaft blast furnace in which air is blown through tuyeres at the bottom of the shaft. The temperature in the hearth where metal is produced must be controlled to avoid the vaporization of any zinc oxide in the sinter. The products of this process are normally quite complex, and can be separated into four phases. Typical compositions of these are shown in Table 13.1. 

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