Environmental Code of Practice for integrated steel mills: section 2

Section 2: Operational Activities

This section describes the major activities involved in the operation of integrated steel mills. It is not intended to be an all-inclusive list of operational activities of potential environmental significance; nor are all activities and techniques necessarily applicable to all mills. Rather, the intent is to identify the nature and scope of the activities addressed in the Code with emphasis on those activities that relate to the environmental concerns and mitigative measures that are discussed in Section 3 and 4.

The major activities and processes of relevance to the Code and the associated environmental releases are illustrated in Figures 2.1, 2.2, and 2.3.

Figure 2.1 Cokemaking and Blast Furnace Ironmaking and Related Environmental Releases

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Cokemaking and Blast Furnace Ironmaking and Related Environmental Releases

Figure 2.2 Steelmaking and Hot Forming and Related Environmental Releases

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Steelmaking and Hot Forming and Related Environmental Releases

Figure 2.3 Cold Forming and Finishing and Related Environmental Releases

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Cold Forming and Finishing and Related Environmental Releases

2.1 Raw Materials Handling and Storage

Iron-bearing raw material, usually agglomerated in the form of pellets, is transported to the steel mill by lake carriers or, in special circumstances, by rail and blended for storage in outdoor stock piles. Coal is handled in a similar manner. Fluxing materials include limestone and dolomite for ironmaking, and burnt lime, burnt dolomite, fluorspar, and silica for steelmaking. Limestone and dolomite are usually transported by rail or truck for storage in outdoor stockpiles, while steelmaking fluxes are usually transported by truck and stored in enclosed silos.

Steel scrap may be classified as home or “revert” scrap (generated from the steel plant operations), and purchased scrap. The availability of home scrap has been decreasing as a result of the application of new technologies aimed at increasing productivity. Purchased scrap may be classified as “prompt” industrial scrap returned directly from customers, lower-grade scrap such as shredded automobiles or turnings, mixed scrap from miscellaneous sources, obsolete scrap from the demolition of buildings or other structures, stainless steel scrap, and alloy scrap. Purchased scrap is usually transported by rail or truck and is usually stored outdoors.

2.2 Cokemaking

The primary function of coke in the blast furnace is to reduce iron oxide to iron metal chemically. Coke also acts as a fuel, provides physical support and allows the free flow of gas through the furnace. Coal cannot fulfil these functions as it softens and becomes impermeable under smelting conditions; therefore, it must be converted to coke by heating to ~1300°C in an oxygen-free atmosphere for 15-21 hours. Only certain coals with the right plastic properties, for example coking or bituminous coals, can be converted to coke and, as with ores, several types may be blended to improve blast furnace productivity, extend coke battery life, etc.

A coke oven battery may contain 40 or more refractory-walled coke chambers interspaced with heating chambers referred to as “flues.” Each coke chamber typically measures 0.4-0.6 metres wide, 4-7 metres high, and 12-18 metres long and is fitted at both ends with removable full-height doors. From a coal-charging car that runs along the top of the battery, coal is charged through three or four holes approximately 300 mm in diameter located above each coke chamber. Once charged, the coal is levelled, the doors and charge lids are sealed, and heating (referred to as under-firing) commences. Distillation products in the form of tar and coke oven gas (COG) driven off during the coking process are collected in mains that run the full length of the battery and are transported to the by-products plant. When the heating cycle is complete, the oven is isolated from the main, the end doors are removed, and solid coke is pushed into a “quench-car.” The quench-car travels along the side of the battery to the quench tower, where fresh water or recycled water is sprayed onto the hot coke to reduce its temperature to approximately 200°C.

The coke is crushed and screened and transported to the blast furnace. The oversize coke is returned to the crusher while the undersize coke, known as coke breeze, is recycled to the coke ovens, used as fuel in a sinter plant, or sold.

The COG that is driven off during the coking process is a complex mixture containing hydrogen, methane, carbon monoxide (CO), carbon dioxide (CO2), water vapour, oxygen, nitrogen, hydrogen sulphide, cyanide, ammonia, benzene, light oils, tar vapour, naphthalene, Polycyclic Aromatic Hydrocarbons (PAHs), many other hydrocarbons, and condensed particulate. Prior to distribution as a fuel gas, the COG is usually processed in a by-products plant where some of the components (e.g., benzene, toluene, xylene, sulphur, ammonia, and tar) are removed and collected for subsequent sale.

2.3 Sintering

Sintering involves the heating of fine iron ore with flux and coke fines or coal to produce a semi-molten mass that solidifies into porous pieces of sinter with the size and strength characteristics necessary for feeding into the blast furnace. Moistened feed is delivered as a layer onto a continuously moving grate or "strand." The surface is ignited with gas burners at the start of the strand, and air is drawn through the moving bed causing the fuel to burn. Strand velocity and gas flow are controlled to ensure that "burn through" (i.e. the point at which the burning fuel layer reaches the base of the strand) occurs just prior to the sinter being discharged. The solidified sinter is then broken into pieces in a crusher and is air-cooled. Product outside the required size range is screened out, oversize material is recrushed, and undersize material is recycled back to the process. Sinter plants that are located in a steel plant recycle iron ore fines from the raw material storage and handling operations and from waste iron oxides from steel plant operations and environmental control systems. Iron ore may also be processed in on-site sinter plants.

2.4 Ironmaking

Ironmaking is a smelting process in which iron-bearing material is reduced from the oxide form to produce liquid iron. This smelting process is carried out in a blast furnace, which is a refractory-lined shaft furnace. The blast furnace is a closed system into which iron-bearing materials (iron ore, sinter, and pellets), fluxing additives (slag formers such as limestone or dolomite), and a reducing agent (coke) are continually added to the top of the furnace shaft through a charging system that prevents the escape of blast furnace gas. A hot air blast, usually enriched with oxygen and auxiliary fuels (such as oil, natural gas, or pulverized coal), is injected from the bottom of the furnace, providing a counter-current stream of reducing gases. The air blast reacts with the coke to produce carbon monoxide (CO), which in turn reduces iron oxide to iron.

The liquid iron is collected in the hearth along with the slag, and both are tapped on a regular basis. The liquid iron flows in refractory troughs and is collected in a torpedo-shaped rail car for transport to the steelmaking shop. The liquid iron is often treated to remove part of the unwanted substances such as sulphur or phosphorus.

The remaining slag flows in other refractory troughs to the slag yard where it is either water-cooled or pelletized. After cooling, the water-cooled slag is crushed for use as a construction material. The blast furnace gas is collected at the top of the furnace and cleaned for use as a fuel for the blast furnace, coke ovens, or steam-generating boilers.

2.5 Steelmaking

The basic oxygen steelmaking process is used in Canadian integrated steel plants to produce most of their steel. Dofasco Inc. also uses an electric arc furnace to produce part of its steel.

Basic oxygen steelmaking involves the conversion of iron from the blast furnace into steel through the injection of pure oxygen into the liquid iron bath to remove carbon, silicon, and other elements. This conversion takes place in a refractory-lined, pear-shaped vessel. One-quarter to one-third of the furnace charge is steel scrap that is charged into the furnace before the liquid iron is poured from a ladle into the furnace. Fluxes such as burnt lime or dolomite are added to the bath to produce slag. Alloying materials are added to alter the composition of the steel. During the process, carbon in the iron is oxidized and released as CO and CO2. Silicon, manganese, and phosphorus are also oxidized and captured in the slag formed by the fluxes. The reactions between silicon, carbon, and oxygen are strongly exothermic and cause the temperature in the vessel to rise. Once the final steel composition and temperature are achieved, the liquid steel is tapped into a refractory-lined ladle for transfer to the ladle metallurgy station, vacuum degassing, or the continuous casting machine.

The slag is tapped into a slag pot, air-cooled, and transported to the slag yard. After cooling, the solid slag is removed from the slag pot and broken into pieces. A magnet removes steel that is present in the slag, and the slag is crushed and screened. The steel is recycled to steelmaking, and the slag is sold as construction material.

Most modern steel plants increase productivity by using the basic oxygen furnace for the melting phase and a ladle metallurgy facility for the final refining and alloying phase. In some cases the steel ladle is taken to a vacuum degassing station where the gas content of the molten steel is reduced for quality requirements.

2.6 Continuous Casting

Over 97% of Canadian steel production is continuously cast into semi-finished products including slabs, blooms, billets, or beam blanks depending on the finished product and metallurgical and rolling requirements; the balance is cast into moulds to produce ingots.

In the continuous casting process, a ladle containing liquid steel is positioned over a refractory-lined vessel called a tundish into which steel is tapped to a predefined level. The steel flow can be shrouded by refractory tubes to minimize contact with air. Stoppers or sliding gates in the base of the tundish are opened to control the flow of the liquid steel into one or more water-cooled oscillating copper moulds. A solid shell forms around the steel in contact with the mould and the shell. The molten core is withdrawn through the bottom of the mould and carried through guiding rollers where solidification is completed with the help of water sprays. The solidified steel is subsequently cut to length with either mechanical shears or a flame-cutting torch depending on the thickness of the steel.

2.7 Hot Forming

In many modern plants the continuous cast product is transported hot to a reheat furnace to ensure that it is at the uniform temperature required to meet the hot forming specification. Prior to hot forming, surface imperfections may be removed by scarfing the surface with an oxy-fuel flame or by mechanical means. Modern steelmaking practices aim to minimize surface imperfections and eliminate this operation. Hot forming changes the shape and metallurgical properties of the steel slab, bloom, billet, or beam blank by compressing the hot metal between electrically powered rolls. The rolls for bar, wire rod, or structural shapes (long products) have indentations to change the shape of the steel progressively to the final desired form. The rolls for sheet, strip, and plate (flat-rolled) products are flat or have a small contour to form the flat surface of the final product.

Following the hot forming operation the product may be processed in finishing operations. These include roller straightening and cut-to-length operations for long products, sheet products and plate products, and edge trimming and coiling for strip. Some of the strip in coil form is sent to cold forming for further processing.

2.8 Cold Forming

Some hot-formed products, primarily flat-rolled products (steel strip or sheet), undergo further processing by cold forming. The first process is acid pickling to remove the oxide coating that forms during hot forming. The steel strip or sheet is then cold reduced by compression between rolls to the required thickness and specifications. The material may have its metallurgical properties altered by annealing. Some flat-rolled products have a final pass in a temper mill to meet flatness and surface hardness specifications.

2.9 Pickling and Cleaning

The oxide coating on the surface of hot-formed flat-rolled product is removed by passing the steel strip through an acid pickling operation followed by a rinse operation to remove any trace of acid. Hydrochloric acid is the most common acid used; however, some plants use sulphuric acid.

Waste acid pickling liquor is processed in an acid regeneration plant to regenerate the acid for reuse and to recover iron oxide for recycling or sale.

Alkali or solvent cleaning is used to remove oil that remains on the product from the cold forming operation prior to annealing or coating to prevent surface staining or contamination.

2.10 Coating

Coatings are applied to the steel strip for protection and decoration. They may be metallic including zinc, tin, nickel, aluminum, lead, zinc/aluminum alloys, and chromium, or non-metallic including paints, polymers, varnish, and lacquer. Metallic coatings are applied by hot dipping the strip or sheet into a molten bath of coating metal, in the case of zinc and zinc/aluminum alloy coatings, or by electro-deposition using the product as an electrode, in the case of zinc, nickel, tin, and copper coatings. Non-metallic coatings are normally organic compounds in the form of powders, paints, films, and liquids and are applied by brushing, rolling, spraying, or immersion.

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