Blast Furnace Products

As well as containing iron at around 40 to 45 per cent, matte contained about 12 per cent lead, most of the copper from blast furnace feed, about half the zinc and a substantial proportion of the silver. Matte was initially roasted in open heaps or stalls to remove sulfur and was then recycled to the blast furnace, but later reverberatory roasting furnaces were used. A shaft kiln was used at the Harz smelter in Germany. 

The recycle of roasted matte to the blast furnace resulted in the progressive enrichment of the copper content of matte, until it reached a grade where it could be processed to blister copper in a converter. Table 2.1 shows the composition of matte produced after five recycling stages from a smelting operation around the 1890s (Hoffman, 1899). 

Zinc proved to be a particularly troublesome problem and many techniques were developed to remove zinc from blast furnace feed. Mineral separation techniques using gravity had limited effectiveness for some ores, and in particular cases it was necessary to leach zinc from roasted ores 

Speiss was the other significant blast furnace product from earlier smelting operations. It contained significant amounts of entrained particulate lead as well as silver and gold. Speiss was often roasted in heaps or a calcining furnace and recycled to the blast furnace. At the Trail smelter in Canada it was treated in a bottom blown converter with the addition of molten lead. The lead captured most of the silver and gold and no doubt significant amounts of arsenic were volatilised into the gas stream. 

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The contents of the cmcible are tapped through a Roy tapper (8,9) iato external settlers for layer separatioa. The tapper removes blast furnace products as they are made, giving a more uniform blast-furnace performance. A typical buUion analy2es in wt %, 1.0—2.5 Cu, 0.6—0.8 Fe, 0.7—1.1 As,... 

These factors have prompted two principal thrusts in ironmaking development. First, progress continues to be made in increasing blast furnace productivity and in decreasing coke rates. Coal (qv) injection to replace coke units has assumed a prominent role. Coal replaces coke on a nearly 1 1 mass basis, and coal injection rates of up to 250 kg/t of hot metal (thm) have been achieved. Injection of oxygen and other reductants besides coal are expected to be used more extensively. Increased additions of scrap, DRI, and HBI are expected to play a significant role in efforts to boost productivity and decrease coke rates. 

Blast furnace production of iron allows the hot, newly reduced product to trickle through the bed of heated coke to the hearth. Since carbon is somewhat soluble in molten iron, pig iron usually contains from 3 to 4.5% carbon. It also contains smaller percentages of other reduced elements such as silicon, phosphorus, manganese, etc., generated by the same reducing processes that yielded the iron (Table 14.3). Primarily from the effect of the high-carbon content on the iron crystal structure, the blast furnace product is brittle, hard, and possesses relatively low-tensile strength. Hence the crude pig iron product of the blast furnace is not much used in this form. 

Blast furnace productivity increased by the use of sinter. In some parts of the world, nearly all ore is sintered. Sintering provides the charge sizing that iron melters had long wanted for their furnaces. 

Experience with oxygen enrichment of blast air is illustrated in Figure 5.6 (Fern and Jones, 1980), which shows that furnace capacity is increased by five times the oxygen percentage increase. A two per cent enrichment from 21 per cent to 23 per cent will raise furnace lead production by ten per cent. Blast furnace production is thus directly proportional to the oxygen flow rate given a constant tuyere volume and other furnace parameters. 

The blast furnace had a large productive capacity but the product, if not used for cast components, was initially of limited value. Because of its high carbon content, 3-4%, it was brittle and impossible to forge. Iron wire, sheets or needles could not be made of it At the end of the Middle Ages people said, it was better in earlier years . Then it was possible to make armor, chain mail and helmets of the soft iron that was produced in the small furnaces by direct reduction. From the blast furnace product it was, however, impossible. 

In 1990, U.S. coke plants consumed 3.61 x 10 t of coal, or 4.4% of the total U.S. consumption of 8.12 x ICf t (6). Worldwide, roughly 400 coke oven batteries were in operation in 1988, consuming about 4.5 x 10 t of coal and producing 3.5 x 10 t metallurgical coke. Coke production is in a period of decline because of reduced demand for steel and increa sing use of technology for direct injection of coal into blast furnaces (7). The decline in coke production and trend away from recovery of coproducts is reflected in a 70—80% decline in volume of coal-tar chemicals since the 1970s. 

The combustible components of the gas are carbon monoxide and hydrogen, but combustion (heat) value varies because of dilution with carbon dioxide and with nitrogen. The gas has a low flame temperature unless the combustion air is strongly preheated. Its use has been limited essentially to steel (qv) mills, where it is produced as a by-product of blast furnaces. A common choice of equipment for the smaller gas producers is the WeUman-Galusha unit because of its long history of successful operation (21). 

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. 

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. 

Over 95% of the world s DRI production is consumed in electric arc furnace steelmaking. The remaining 5% is spHt among blast furnaces, oxygen steelmaking, foundries, and ladle metallurgy (qv) faciUties. 

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. 

Pyrometa.llurgica.1 Methods. To prepare blast furnace bulhon for commercial sale, certain standards must be met either by the purity of the ores and concentrates smelted or by a series of refining procedures (r6—r8,r20,r21). These separated impurities have market value and the refining operations serve not only to purify the lead, but also to recover valuable by-products. 

Reverberator Furnace. Using a reverberatory furnace, a fine particle feed can be used, the antimony content can be controlled, and batch operations can be carried out when the supply of scrap material is limited. However, the antimony-rich slags formed must be reduced in a blast furnace to recover the contained antimony and lead. For treating battery scrap, the reverberatory furnace serves as a large melting faciUty where the metallic components are hquefted and the oxides and sulfate in the filler material are concurrently reduced to lead metal and the antimony is oxidized. The furnace products are antimony-rich (5 to 9%) slag and low antimony (less than 1%) lead. 

The principal U.S. lead producers, ASARCO Inc. and The Doe Run Co., account for 75% of domestic mine production and 100% of primary lead production. Both companies employ sintering/blast furnace operations at their smelters and pyrometaHurgical methods in their refineries. Domestic mine production in 1992 accounted for over 90% of the U.S. primary lead production the balance originated from the smelting of imported ores and concentrates. 

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). 

Carbon and Graphite. Carbon (qv) and graphite [7782 2-5] have been used alone to make refractory products for the lower blast furnace linings, and electrodes for steel and aluminum production. They are also commonly used in conjunction with other refractory raw materials. These materials are highly refractory nonwettable materials and are useful refractories in nonoxidizing environments. Carbon blacks are commercially manufactured, whereas graphite for refractory use has to be mined. 

The commercial production of silicon in the form of binary and ternary alloys began early in the twentieth century with the development of electric-arc and blast furnaces and the subsequent rise in iron (qv) and steel (qv) production (1). The most important and most widely used method for making silicon and silicon alloys is by the reduction of oxides or silicates using carbon (qv) in an electric arc furnace. Primary uses of silicon having a purity of greater than 98% ate in the chemical, aluminum, and electronics markets (for higher purity silicon, see Silicon AND SILICON ALLOYS, PURE SILICON). 

The largest use for calcium carbide is in the production of acetylene for oxyacetylene welding and cutting. Companies producing compressed acetylene gas are located neat user plants to minimize freight costs on the gas cylinders. Some acetylene from carbide continues to compete with acetylene from petrochemical sources on a small scale. In Canada and other countries the production of calcium cyanamide from calcium carbide continues. More recentiy calcium carbide has found increased use as a desulfurizing reagent of blast-furnace metal for the production of steel and low sulfur nodular cast iron. 

Anthracite. Anthracite is preferred to other forms of coal (qv) in the manufacture of carbon products because of its high carbon-to-hydrogen ratio, its low volatile content, and its more ordered stmcture. It is commonly added to carbon mixes used for fabricating metallurgical carbon products to improve specific properties and reduce cost. Anthracite is used in mix compositions for producing carbon electrodes, stmctural brick, blocks for cathodes in aluminum manufacture, and in carbon blocks and brick used for blast furnace linings. 

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