If you had to pick a few technologies that have had a tremendous effect on modern society, the refining of iron and steel would have to be somewhere near the top of the list. Iron and steel show up in a huge array of modern products. Automobiles, Trains, Bridges, Tools, almost all depend on iron and steel.
Steel is a form of Iron with most impurities removed. . When Iron is used to make steel, the product is an even harder and tougher metal compound. Steel is formed by treating molten (melted) iron with intense heat and mixing it (alloying) with carbon
Steel is manufactured using two basic methods:
Integrated Steel Plant - from raw materials, including iron ore, limestone, and coke by the blast furnace and basic oxygen furnace methods; or
Mini Mills - from recycled steel via the electric arc furnace method.
Integrated steel plants are large factories with a complex structure and a broad range of products, and are designed for high capacity output.
Mini mills, on the other hand, are less complex, are usually limited in their range of products, and have production levels that are comparatively low. Some of these mini mills specialize in high valued finished products, such as superior quality stainless steel or special alloys.
The modern trend is to have concentrated values in one facility, essentially creating critical bottlenecks and potentially long Business Interruption periods. This trend, which could result in major losses, is caused by the necessity of the steel industry to become increasingly efficient and competitive. In the past, capital was separated between two or three sites, rather than just one.
Another manufacturing trend to improve efficiency is to optimize industrial process flow by dedicating one plant to a single type of product, as well as utilization of just-in-time production techniques. In less industrialized countries, despite the lack, in some instances, of modern high rate production equipment, the reliability of production is often ensured by a very high degree of redundancy and back-up capabilities.
The Reduction of Iron Ore
The starting material for the production of steel is iron ore. First the ore must be prepared mechanically in various processes known collectively as "dressing". First the size of the ore is reduced in crushers and grinding mills.The opposite process of agglomeration involves sintering and pelletizing. In the sintering process fine ore is fed into a furnace, where it fuses together to form largish lumps called sinter.
In the pelletizing process extra fine ore is converted into pellets, which are then baked. In these dressing processes the iron ore attains the size that is needed for further processing.
After mechanical dressing, oxygen is separated from the iron in a chemical process called reduction. This requires a suitable reducing gas. The reaction takes place in a blast furnace, direct reduction plant, or smelting reduction plant.
The blast furnace process is the most significant in terms of output. In the past 20 years or so, direct reduction has experienced an upswing. Smelting reduction plants still bear many features of prototypes and have little significance in terms of output.
The Production of Pig Iron in The Blast Furnace
The charge for the production of pig iron in the blast furnace is mainly dressed iron ore, limestone, and coke. Coke is manufactured by degasifying coal at a temperature of about 950°C in a coking plant, which comprises a large number of heated coke ovens. The gas released in the coke ovens is collected, purified, and fed into the steel plant's gas network.
The blast furnace is a continuously operating shaft furnace with a thick refractory lining, which is kept cool by large amounts of cooling water. The water is passed through cooling elements installed between the furnace shell and the refractory lining. The reducing gas is produced by burning coke in the lower section of the blast furnace.
The charge is fed into the upper section of the blast furnace (Height: 50–70 m; max. diameter: approx. 15 m; useful volume: up to about 5,000 m3)– called the throat – using conveyor belts or skips. It is preheated and dried and then in countercurrent to the reducing gas gradually descends into the hotter sections of the furnace, where the latent heat of fusion for the pig iron and the reducing gas are generated in the coke-burning process.
The production of pig iron by smelting reduction
In the smelting reduction process, liquid pig iron is produced as in a blast furnace, but the reducing gas is produced by means of coal gasification using oxygen as the agent. The production of coke in cost-intensive coking plants is not necessary. There are several smelting reduction processes, some of which are still at the development or testing stage.
The products at the end of the first processing stage are liquid pig iron and solid sponge iron. Both contain large amounts of accompanying elements or impurities and these are removed in the next stage, which is refining.
This requires large amounts of pure oxygen, which is produced at the steel plant in air separation plants. Of the many types of chemical reaction, decarbonization is the most important. The product at the end of the second processing stage is liquid steel.
The refining process takes place either in a converter working on the principle of basic oxygen steelmaking or in an electric-arc furnace. In terms of production volume, basic oxygen steelmaking is the predominant type of process. Besides pig iron and sponge iron, other materials such as fluxing agents, alloying additions, and scrap are needed as charges. Smelting time at modern refining plants is below one hour.
The Worst Case Loss Scenario is usually based on a blast furnace explosion, with the potential collapse of refractory lining and consequential structural damage. The liquid pig iron in blast furnaces and smelting reduction plants rereaches temperatures of about 1,600 to 1,650°C. Furnaces containing liquid metal must be monitored for "hot spots". A furnace breakthrough or a spill of fluid pig iron during transportation can cause a major conflagration. The intense heat radiated by steel spillages can also cause damage.
The next severe loss scenario is a fire in the largest or most critical rolling mill. When a fire occurs in a rolling mill, the concern is not so much the mill itself, but rather the large motors, electrical cable trays, cable tunnels, and cable cellars associated with the motor generator sets, which have a relatively long lead time to replace and can generate long Business Interruption periods.
Explosions may also occur in oxygen generating plants, coke ovens, steam generators, annealing ovens, and furnaces. In basic oxygen steel-making vessels, the escape of lance and gas hood cooling water can damage the vessel's refractory and hood integrity.
Fires can occur in control rooms, electrical switch rooms, cable tunnels, cable vaults, Motor Control Centre rooms, transformers, lubricating oil rooms, desulphurization filter bag houses, and coal tar storage tank farms.
The off-gases from the converters are very corrosive and have a high dust content. The intermitting operation of converters entails high levels of mechanical and thermal stress for the downstream off-gas boilers and this can result in damage.
In addition, since blast furnaces, electric furnaces, open hearth furnaces, and basic oxygen furnaces all hold several hundred tons of molten metal, major fire damage could result if this molten metal comes in contact with combustible construction (roofs, floors, walls of the building which contains the furnace, or adjacent control rooms of sheds); flammable or combustible hydraulic oils (from exposed hydraulic oil lines and reservoirs used to tilt the furnace); or ordinary combustibles in the vicinity (exposed electrical cables, wood pallets, scrap lumber, or empty paper bags which held furnace materials that may, inadvertently, have been thrown into the furnace pit area).
In continuous casting plants it is possible for the liquid steel to break out while the still unfrozen strand is being withdrawn. Surrounding plants are then exposed to the hazard of fire and radiant heat.Steam or gas turbine-generators and large capacity compressors may suffer machinery breakdown, while dust accumulations on cable trays may cause collapse or electrical damage.
In regard to Business Interruption, bottlenecks are found primarily on main transformers, arc furnaces, or ladle furnaces. Replacement time for a furnace is generally estimated at 18 months, while 9 months is required to replace hydraulic oil control and casting lines. The loss of critical control rooms, electrical rooms, coal conveyors, or junction towers can also generate relatively long Business Interruption periods.
Since steel producers are facing a rising demand for zero defect products from the aeronautics, automobile, food packaging etc., quality controls are widespread along the process. As a result, any damage to the manufacturing equipment which affects the tolerance of the equipment or finished product could lead to several weeks of business interruption.
Risk Management Recommendations
In rolling mills, sprinklers are often needed in rooms housing the lubricating oil systems, as well as large electrical rooms, cable tunnels, and cable cellars.
Sponge iron re-oxidizes at temperatures of between 150 and 230°C and on contact with water. Heat is released and there is an increased risk of fire. Sponge iron must therefore be given special protection by being roofed over during storage and transportation.
Some of the fire-proof bricks used in refractory linings like dolomite are very delicate. If they are stored improperly, they may absorb moisture from the atmosphere and disintegrate in a matter of days. Particular attention must therefore be given to protecting them during storage. Long periods of storage on construction sites should be avoided. Fire clay bricks are less of a problem. Refractory linings must be dried carefully in accordance with a set schedule.
Gaseous extinguishing systems are usually recommended in non-occupied rooms, such as small electrical rooms, as well as concealed spaces with relatively high combustible loading. These areas include false floors and ceilings containing a high density of cables with combustible insulation.
Detection alone is generally recommended in concealed spaces with relatively low combustible loading, and in normally occupied areas, such as control rooms. In addition, it is strongly recommended to adequately seal cable and duct penetrations through walls and floors in order to prevent fire spreading from one area to another.
The level of detection or protection required depends on the continuity of combustibles, the combustible loading, and the congestion (access and visibility factor) within the process units. This should be investigated on a case-by-case basis during field surveys.
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