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How Steel Is Made : Carbon Scrap
 

Integrated and minimill steel producers consume scrap in their steelmaking processes. The minimills use scrap as a primary feed; the integrateds use it as a supplemental feed for 10-40% of the basic oxygen furnace inputs. The demand for scrap rises cyclically with steel production and secularly with the addition of minimill capacity. The demand for scrap with low amounts of residual elements rises as the minimills continue to move into higher-valued products. Moving up the product quality spectrum requires either purer and more consistent ingredients or extended cooking and refining time; the former is usually cheaper. To make a better cake, use better ingredients. Sorting scrap into various grades is essential to ensure that only the elements desired in the steel are introduced into the melting process.

Nonoxidizable resid-ual elements are watched carefully, as they cannot be removed from the electric furnace melt. Residuals that are frequently monitored include nickel, copper, chrome, molybdenum, and tin. Usually, nickel-bearing scrap will be used in the electric furnace of a stainless steel producer that wants nickel in the final product.

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Grades of Scrap

Low Residual Scrap

Low residual scrap comes in five grades. No. 1 bushelings are a clean steel scrap under one foot in any dimension. No 1 bushelings include sheet clippings and stampings, which are the waste trim material resulting from the fabrication of steel products and may not include old auto stock or coated material . New black sheet clippings are essentially waste created when processing hot rolled material. Black sheet clippings should be no longer than 8 feet by 18 inches and must also be free of old auto stock and coated material.No. 1 bundles are scrap that has been compressed or hand bundled. No. 1 bundles may include chemically detinned material but not old auto stock or coated material. Shredded clippings are miscellaneous scrap cut into smaller pieces. Shredded scrap is a more homogeneous, magnetically separated iron and steel scrap originating from automobiles, unprepared No. 1 steel, and miscellaneous scrap. All of the above, except for shredded scrap, are referred to as prompt scrap or waste from the fabrication of steel products. Another source of lower residual scrap, home scrap, is internally generated by a steel company. Examples of home scrap would be side trim, end trim, and poor-quality production recycled back into the melt system. The availability of prompt scrap and home scrap will depend primarily on activity levels. That is, the supply from these scrap sources is not price elastic. Indeed, as yields have improved over time the generation of home scrap has diminished, forcing the integrateds into the scrap market in a bigger way.

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Scrap Substitutes

The search for scrap substitutes is really a search for consumable iron units and new ways to convert those iron units into steel. Only when scrap prices had risen above $110 per ton in the last cycle did the known but experimental scrap substitutes become economically feasible for commercialization. Each of the developing scrap substitutes involves processing virgin iron ore in such a way as to make it usable for at least part of the input into an electric arc furnace. The iron-ore- cooking process uses chemical energy from coal or natural gas but in a furnace configuration that is much lower cost than a blast furnace/basic oxygen furnace (BOF) configuration. As the minimills use more iron-ore-based inputs, they become more like an integrated producer, particularly if they own an interest in the manufacturer of the scrap substitute. The value of each alternative iron source or scrap substitute depends on its contribution to steel-making process in the electric arc furnace. For example, a high amount of metallization (iron) in the reduced-iron product could allow the minimill to spend less time refining the metal and/or allow the furnace to use more low-cost, high-residual scrap as a blend. Steelmakers will alter the ingredients in their furnaces, both electric and blast, to optimize the productivity versus cost equation. The required amounts of electrode, oxygen, time, temperature, power, and metallic source will be chosen to maximize profitability at a given quality

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Direct Reduced Iron (DRI)

Direct reduced iron is a product made by converting iron ore into purer, metallic iron without actually melting the ore. There are sev-eral specific process designs to achieve this general end that are differentiated by either the furnace type(s), the reductant fuel (coal or natural gas), and the form of iron ore input (lump, pellet, fines).Although there are many ways to make direct-reduced iron products, the method using a moving-bed-stack furnace, natural gas, and iron ore pellets has become the most common and is often referred to as DRI. This DRI is made as pellets of  crushed iron ore are fed into a tall, gravity-fed stack furnace. The burning of pre-heated gases in the furnace leads to chemical reactions, which drive away the impurities and most of the oxide from the ore. This “reduces” the iron ore to a purer form much like a blast furnace would, though the furnace does not get hot enough to melt the iron. The DRI output is then sold as pellets or tumbled with binders and compressed into small briquettes. The briquettes are called hot briquetted iron or HBI. While some steel has been made from 100% DRI, more of-ten we hear of DRI pellets or HBI replacing 20-30% of the charge or input feed into an electric arc furnace (with the rest being scrap steel). An advantage of the DRI process outlined above is that it produces a compound of iron and oxygen without impurities. However, the oxide remaining in the DRI leads to two of DRI’s disadvantages: first, the need to use more energy in the electric arc furnace to drive off the oxygen; and second, higher transportationcosts, as the DRI will create a lot of heat or burn if it reoxidizes when mixed with any moisture. Briquetting the DRI or coating it first (like an M&M™) ship-ping will reduce these transportation problems. A third disadvantage of DRI as far as potential domestic producers are concerned, is that it uses natural gas, a more expensive energy source in the United States. Imported DRI is being used today to replace 10-15% of the scrap charge in several electric arc furnaces. The price to the steel mills of this scrap substitute ranges from $110 to $135 per ton.

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Circofer and Circored

Circofer and Circored are two reduced-iron processes developed by Lurgi. Both use a series of fluidized bed reactors to reduce iron ore fines. Circofer uses coal as a fuel and Circored uses natural gas. Because the Circored output is expected to have no carbon, a bit more heat will be required in the steelmaking furnace, although the material will not have to be treated for shipment. Cleveland Cliffs and LTV are in a joint venture with Lurgi to develop in Trinidad the first commercial Circored facility.

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Corex

The Corex process is described as a set of coke ovens and blast furnaces combined in one unit that is environmentally attractive. Corex uses lump ore and pellets, and produces 93% pure DRI. Noncoking coal is the reductant. The advantage of the Corex process is that it produces molten pig iron without any waste gangue and that the excess heat can be used to generate electricity. A Corex plant in South Africa has been operating overrated capacity since 1989.South Korea’s Pohang Iron and Steel Co. has a 600,000-700,000 ton per year unit, double the size of the South African plant. In the United States, Geneva Steel has joined Air Products and Centerior Energy in a Department of Energy project to demonstrate the commercial viability of the Corex process in the United States. Geneva may partially be motivated by the aging of its coke ovens, its noncoking coal properties, and the potential of eventually replacing its blast furnaces with iron from a successful Corex facility.

Many other observers feel, however, that the Corex process, except under specific circumstances, is a more expensive approach to supply raw iron than existing coke-based blast furnace steelmaking. A process similar to Corex developed in Russia is called Romelt

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Fastmet

In the Fast Metalization (Fastmet) process, finely ground iron ore fines and pulverized coal are formed into pellets. The pellets are dried and then placed in a rotating furnace (a rotary hearth furnace). Within six to ten minutes (other direct reduction processes can take four to eight hours), 90-95% of the material is converted into metallic iron. One advantage of the Fastmet process is that it uses coal rather than natural gas, and therefore, may be the iron-reduction process used where coal is plentiful and cheap, and natural gas is expensive. A second advantage is that domestic production could also allow for the direct hot charging of this material into the electric arc furnace. Three disadvantages of the Fastmet product are

  • it also must be processed for shipping (coated) so that it does not pick up any water (it is unstable and could burn);
  • the output contains more sulphur and phosphorus (both less desirable) than other reduced-iron products; and
  • it requires more energy or a longer heat time in the electric furnace be-cause the product contains 5-10% gangue (the waste portion of the ore). The Fastmet process is under development by Midrex and its parent, Kobe Steel.
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Finmet and Fio

Finmet and Fior are gas-based processes developed by Voest-Alpine/Fior de Venezuela and Exxon, respectively. In both processes, iron ore fines (lower cost than lump ore or pellets) are cooked in a series of four fluidized bed reactors.

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Iron Dynamic

The Iron Dynamics (IDI) process for direct-reduced iron is being commercialized by Steel Dynamics. The IDI process involves forming a cake of coal and iron ore fines, which is then passed through a furnace on a large turntable called a rotary hearth. The reduction process is quite quick—it is com-pleted in a matter of minutes rather than hours. However, because the coal will frequently bring with it undesirable sulphur, a second-stage cooking process in a submerged arc furnace can be used to drive off the sulphur in the steel. A sub-merged arc furnace is like an electric arc furnace, using electrical energy.

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Iron Carbid

The direct-reduced-iron process referred to as iron carbide uses hydrogen and carbon gases (natural gas), as opposed to coal, to reduce iron in a fluidized bed reactor. The required purifying chemical reactions take place as heated gas is forced through a bed of finely ground ore, which is stirred vigorously, giving the effect of fluid movement. The reactor itself can be 40 feet in diameter and 60 feet high with a rapidly swirling gas/ore mixture. The processing system is designed to extract oxygen and other impurities from the ore so that iron and carbon remain. The resulting product is 80-90% iron carbide (Fe3C), has an 8% carbon content, is stable for shipping, and requires no pelletizing. The extra carbon in the iron carbide can be used to generate heat in the electric arc furnace and help save electricity. It is currently estimated that iron carbide could replace 20-25% of the scrap charge into an electric furnace, although additional injection equipment would have to be installed.

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Other

Other specific reduced iron processes and their features include DIOS(Japanese, two-stage furnace, coal based, uses fines); Hismelt (developed by CRA and Klockner, coal based, uses fines, shaft furnace); HYL (Hylsa process, natural gas based, pellets or lump ore, shaft furnace); Inmetco (Inco process,pelletized iron ore with coal, natural-gas-fired rotary hearth furnace); Midrex (natural gas, pellets or lump ore, shaft furnace); Purofer (Thyssen-Hutte, natural gas, lump ore, shaft furnace); and SL/RN (international joint venture, coal based,lump ore or pellets, tilted rotary hearth furnace.

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