Two routes to producing iron, the direct and indirect, are recognised in iron metallurgy. The direct route is often called the "bloomery process" and the indirect route is that which utilizes a blast furnace. The differences between the two processes are many, but the most significant is that direct or bloomery smelting produces a soft, heterogeneously carburized, solid-state mass of iron known as a "bloom," while the indirect or blast furnace process produces a hard, high-carbon but brittle molten iron that can then be cast. Both products then needed further fining treatments.
In ancient China, it was the blast furnace, cast iron method that was the primary means of iron and steel production. The earliest cast iron objects appeared in China no later than the Spring and Autumn period (eighth-century BC) (Han 2000, 1178). In contrast, for example, to family-based iron smelting activities in Britain in the second century BC (Cunliffe 1991, 454), in ancient China, ca. 117 BC, salt and iron production were monopolized by the state (Wang 1958; Wagner 1993, 2008). The iron smelting activities in ancient China at this time were large-scale and well controlled by the central government. [End Page 93]
Blast Furnace Operation Pdf Download
The furnaces that produced cast iron in ancient China are called "iron blast furnaces" or "blast furnaces." A blast furnace produces iron with a high carbon content and therefore a lower melting point. The iron forms in the furnace in the molten state and is then cast, either as "pigs" (ingots) for later remelting, or, as in early European practice, directly into moulds of the intended products. A blast furnace is vastly more efficient than a bloomery furnace, and in modern industry virtually all iron is produced in blast furnaces (Wagner 1993, 178).
The blast furnaces discovered in China have been dated to as early as the third-century BC, and are inherently oriented towards large-scale production. It is almost technically impossible to produce less than perhaps a half-ton of iron per day, and efficiency rises dramatically as production increases. Efficiency also requires that the furnace be operated continuously over long periods; under pre-modern conditions, perhaps a week or a month at a time. The operation of a blast furnace thus requires a greater degree of reliability in all aspects of production, especially with respect to fuel and ore sources, labour, markets, and transportation (Wagner 1993, 239). The original size of the Western Han blast furnace from Guxingzhen, Zhengzhou, Henan Province was reconstructed according to the ruined foundation as being about 2.7 to 4 meters in diameter and over 6 meters high (Yang 1982, 87).
The structure of these fining furnaces at Xuxiebian is similar to a modern-type fining furnace which was popular for a long time in Shanxi Province (Yang 1982) (Figure 12:a). However, this modern type has a contracted furnace mouth (C-shaped rather than U-shaped in section), while all of the Xuxiebian type furnaces have a U-shaped section. The contracted mouth is an advanced structure which directs the hot blast from the furnace upwards rather than directly against the metalworkers. Otherwise, it is necessary to build another wall against the furnace mouth (no evidence of which was found near the furnaces at Xuxiebian) or the workers must stop pumping the bellows when the materials in the furnace needed to be stirred, which reduces operating efficiency.
Fourthly, although no blast furnace or blast furnace foundation was discovered at Xuxiebian, as evidenced by the high temperature-type B and D slags, we believe that blast furnaces and pig iron smelting were undertaken at Xuxiebian. In addition, based on the amount of furnace bricks and the volume of smelting waste deposits discovered at the site, there was likely more than one blast furnace here operating at the same time. The furnaces were possibly located on the south side of the site (Figure 14).
Smooth and uniform movement of burden materials downward and movement of furnace gases in the upward direction is very important for a stable and efficient operation of the blast furnace (BF). For ensuring this, a lot of work has been carried out in the recent past. This includes (i) improvement in the characteristics of the burden materials, (ii) improvement in the furnace charging system, (iii) improvement in the BF cooling system, (iv) adequate automation and control of the BF operation to eliminate human errors, and (v) improvement in the furnace operating procedures. In spite these improvements, BF does not always run as smoothly as the casual observer can be led to believe and irregularities during the operation do occur. However, the furnace irregularities are not as frequent as they were in former years, but still the upsets in the BF operations are there which cause considerable concern and frequently need quick thinking and the use of good judgement and skill as well as timely corrective actions on the part of the operator to prevent serious trouble.
The remedial actions for removal of hanging in the BF are (i) use of large lump lime stone, calcining of which in the BF produces CO2 (carbon di-oxide) which forces the solution loss reaction to take place and improves the permeability of the bed, and (ii) reduction in the blast temperature and pressure so as to improve the distribution and flow of gases in the furnace. In case of prolonged heavy hanging, the pressure of the hot blast is brought down drastically for a few moments. The shock created due to this sudden reduction in the pressure of hot blast makes the furnace slip. This slip is normally heavy and hence, this remedial action is to be carried out only after tapping the furnace when the furnace hearth has minimum of liquid metal and slag. In an extreme case, a persistent hanging can be cured by blowing down the furnace to bosh level and filling it with coke blank.
The term scaffolding is used when accretions or scabs build up on the furnace walls and cause a decrease in the cross sectional area of the stack of the BF. Scaffolds are normally made up of a solid shell on the inner side of the BF and a layer of loose burden material between this shell and the wall of BF. Scaffolding can occur relatively at the higher level of the stack of the blast furnace or relatively low in the stack, near the top of the bosh. It is difficult to generalize the types of scaffolds since there is very little in common between the structure and location of scaffolds from different BFs. However, scaffolds can be generally arranged in two groups. These groups are (i) laminated scaffolds, and (ii) non laminated scaffolds. Scaffolds with laminated structure consist of alternate layer of metallic iron (Fe) and burden rich in alkalis. Scaffolds can cause hanging in the BF. Typical formation of a large scaffold in a BF is shown in Fig 1.
The place where the scaffold is located depends on the agglomerating material, adhering material, burden materials, furnace operation, and furnace constructional features such as cooling elements and lining material. It can be located at various levels in the BF such as the shaft, the bosh, or the belly.
The phenomenon of channeling happens when the ascending gases in the furnace does not properly get uniformly distributed both radially and circumferentially in the furnace and find a passasge of least resistance. The different causes for channeling to occur in the blast furnace are charging of excessive fines, improper distribution of the burden material inside the furnace and high level of liquid iron and liquid slag in the hearth. Channeling upsets the heating and reduction processes which in turn affects the quality of the hot metal.
In case of fines charging, the channeling leads to the increase of the heat load at the walls of the BF which results in an unstable BF operation and reduction in the production. Due to the fines, the ascending gasses gets diverted from the area and channel around the fines. This diversion of the ascending gases upset the preheat of the materials and the reduction process. It causes unscheduled bleeder opening, off chemistry of the hot metal, unstable production of the BF and reduction in the furnace productivity. If the channeling can be predicted effectively, then the BF heat load can be reduced by improving the quality of the raw materials or by adjusting the BF operation.
Slag break outs are normally not as serious as iron break outs, because there are not as much danger from explosions as in the case when liquid iron and water come into contact. With either type of breakout, it is necessary, if at all possible, to open the tap hole and drain out as much liquid material as possible, and to take the furnace off blast.
The causes of the bosh breakouts are (i) by conditions inside the furnace, such as high pressure of blast, very heavy slips, or severe working on the hearth walls, all of which can lead to breakout, (ii) breaking of the hearth bands, ejection of the cooling plates, or parts of brickwork between the band and the plate, or (iii) cracking and opening of the bosh cooling staves.
During the past few years serious breakouts have occurred more frequently at the hearth than at the bosh and the tuyere breast. In fact this has always been so, but with small amount of hotmetal in the hearth, the breakouts were not necessarily serious, specially as the blast pressure has not being high. With increasing tonnage and fast driving, the breakouts have assumed serious proportions, sometimes wrecking the furnace, occasionally costing lives, and almost always causing bad messes, delays, and inconvenience.
During the regular operation, BF normally provides warning signals before the furnace shows the symptoms of chilling. The warning signals normally consist of (i) reduction in wind volume and slow burden movement due to the furnace running cold, (ii) frequent hanging and slipping in the furnace, (iii) temperature of the tapped hot metal and liquid slag is lower than the normal temperature, (iv) tapped liquid slag is viscous and not freely moving in the slag runner, (v) water coming out from the tap hole, (vi) blocking of tuyeres and blow pipes with slag or slag-metal mixture, (vii) excessive build-up of hot metal and slag in the furnace due to either insufficient draining of the hot metal and slag during tapping and / or delay in opening of the tap hole, and (viii) very less coke in the dead man area. When the furnace starts giving warning signals, it is necessary to take remedial actions to avoid approaching of the BF towards a chilling. The corrective actions are several but it is advisable to run the furnace on the hotter side by increasing the coke in the charge. 2ff7e9595c
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