Environmental Performance of Iron and Steel Plant
Environmental concerns associated with various processes of the iron and steel plant relate to mainly (i) atmospheric emissions, (ii) waste water and liquid effluents discharges, and (iii) solid wastes generation and disposal (Fig 1). There are regulatory requirements which are to be complied. Environmental performance of the plant greatly impacts the natural environment around the plant. Environmental management practices are followed by the plant management to address to the environmental concerns and to take reasonable and practical measures to preserve and enhance the quality of the environment which is affected by the plant. Environmental management practices normally ensure that the plant achieves environmental performance results which are superior to the environmental standards set by the regulatory authorities.
Fig 1 Environmental concerns of a steel plant
The major activities and the main processes in an integrated iron and steel plant having blast furnace (BF) – basic oxygen furnace (BOF) process route, which are of relevance to the environmental performance and the associated environmental releases are described below.
Raw materials handling and storage
The main environmental issue relating to raw materials handling and storage is the fugitive emission of particulate matter arising from material transfers, dumper and other vehicular traffic, and wind erosion of the piles of the raw material storage. A secondary concern is the suspended solids and, in some cases, oil, contained in the runoff water from the storage areas.
Fugitive emissions of particulate matter are normally controlled by spraying stockpiles with water or crusting agents and ensuring that roads and wheels of dumpers and other vehicles are kept clean. The water runoff is generally directed to a waste water treatment plant.
Coke making
Coke making represents perhaps the greatest environmental concern for a steel plant. Although there has been good progress, coke making emissions continue to be a target of environmental regulating authorities. Continual efforts to reduce emissions are necessary in this area.
Coke oven emissions can be intermittent or continuous. The combustion stack emissions resulting from the under-firing process are continuous and include carbon monoxide (CO), carbon dioxide (CO2), oxides of sulphur (SOx), oxides of nitrogen (NOx), and particulate matter. The levels of each of these emissions depend on the fuel used and the performance of the combustion control system. Intermittent emissions arise from a multitude of sources including the charging operation of the coke ovens, pushing of coke from the ovens, transport of hot coke, coke wet quenching operations, and process leaks from coke oven doors, topside lids, off takes, and collector mains. Intermittent emissions include particulate matter and a wide range of hydrocarbons including benzene and PAH (polycyclic aromatic hydrocarbons). Coke ovens are normally the major source of PAH emissions in the steel plant.
Potential sources of benzene emissions include leaks from process pumps, valves, vents, storage tanks, and associated equipment and the handling of light oil (a mixture of benzene, toluene, xylene, and other hydrocarbons) recovered from the coke oven gas (COG). Cooling towers used for process cooling water can also be a substantial source of benzene emissions if there are leaks in the heat exchangers.
A large volume of process waste water is generated at a well-controlled coke oven plant out of which substantial volume is generated as waste ammonia liquor from moisture contained in the charged coal. The balance is from the steam used in distilling ammonia from the waste liquor, light oil recovery, and other processes. Coke making waste water contains significant levels of oil and grease, ammonia, nitrogen, cyanides, thiocyanates, phenols, benzenes, toluene, xylene, other aromatic volatile components, and polynuclear aromatic compounds. Waste waters also contain trace amounts of the toxic metals antimony, arsenic, and selenium. The amount of each pollutant generated depends on the by-product process, specific facility equipment, practices, and the range of constituents in the coals used.
The residues collected from coal tar recovery, tar storage tanks, light oil processing units, wastewater sump, and naphthalene collection and recovery falls into hazardous category.
Various regulatory authorities have either revised or in the process of revising coke oven plant air emission norms for doors, off-takes, lids, coal charging, and collection mains that call for significantly more stringent emission requirements. In addition, standards are also likely to be proposed for pushing, quenching and battery combustion stack emissions and tighter effluent requirements for coke making waste waters.
Iron ore sintering
Sintering plant emissions are primarily generated from raw materials handling operations, which result in airborne dust and from the combustion reaction taking place on the sintering machine (wind box exhaust), sinter discharge (associated sinter crushers and hot screens), and the sinter cooler and cold screen. Combustion gases from the combustion reaction contain dust entrained directly from the sintering machine along with the products of combustion such as CO, CO2, SOx, NOx, and particulate matter. Wind-box exhaust is the primary source of particulate emissions – mainly iron oxides, sulfur oxides, carbonaceous compounds, and chlorides. Fluorides, ammonia, and various metal compounds may also be present. The concentrations of these substances vary with the quality of the fuel and raw materials used and the conditions of the combustion. Atmospheric emissions also include volatile organic compounds (VOCs) formed from volatile material in the oily mill scale and coke breeze etc., and dioxins and furans formed from organic material under certain operating conditions. Metals are volatilized from the raw materials used, and acid vapours are formed from the halides present in the raw materials. At the discharge end, emissions are mainly iron and calcium oxides.
Combustion gases are generally cleaned in electrostatic precipitators (ESPs), which significantly reduce dust emissions but have minimal effect on the gaseous emissions. In some sinter plants, cyclones are used in place of ESPs but they have lower particulate collection efficiency. Water scrubbers, which are sometimes used in sinter plants, usually have lower particulate collection efficiency than ESPs but higher collection efficiency for gaseous emissions. Significant amounts of oil, if present in the raw material feed can create explosive conditions in the ESP. Sinter crushing and screening emissions are usually controlled by ESPs or fabric bag filters. Wastewater discharges, including runoff from the materials storage areas, are treated in a wastewater treatment plant.
Handling of other materials as well as coke and flux crushing operations also generate emissions. Fabric bag filters are used to capture particulate matter generated during these processes. This dust is recycled as feedstock to the sinter plant
Solid wastes generated during the sintering process include refractories, dust and sludge generated by the treatment of emission control system water in cases where a wet emission control system is used. Wet sludge is normally dried and briquetted along with the dust and is recycled back in the sinter making process.
Undersize sinter generated in the sinter screening section of the plant is normally recycled back in the sinter making process.
Waste waters are generated from the use of wet air pollution control scrubbers to clean the wind-box and discharge ends of sinter machines. Wet-scrubber waste water treatment includes removal of heavy solids by sedimentation process and recycling of clarifier or thickener water overflows as well as metals precipitation treatment for blow downs. Some sinter plants are operated with once-through water in the wet scrubber wastewater treatment plants. The principal constituents can include suspended solids, oil and grease, ammonia-nitrogen, cyanide, phenolic compounds, and heavy metals, such as lead, zinc, arsenic, cadmium, copper, chromium, and selenium.
Blast furnace iron making
Particulate emissions are primarily generated in the cast house of the blast furnace during the casting of liquid iron and slag. During casting operation of blast furnace, liquid iron and slag flow out of a tap hole in the cast house into runners that lead the hot metal to hot metal ladles and slag to the slag processing unit. When the liquid iron and slag contact air, particulate emissions are generated. Emissions also are generated during opening and closing of the tap hole, and during the use of oxygen lance to open a clogged tap hole. When the blast furnace is casting, iron oxides, magnesium oxide, and kish (graphite flakes) are generated as particulate matter. Casting emissions are captured in a high canopy or local hood and exhausted to a cleaning device, generally a bag house with fabric bag filters.
Emissions at the blast furnace shop also arise primarily from materials handling operations of the blast furnace burden in the stock house. These material handling operations result into airborne dust which is normally handled through fabric bag filters.
Variable quantities of hydrogen sulphide (H2S) and sulphur dioxide (SO2) are emitted from slag cooling and treatment. The control of these emissions is generally carried out through the process change or the operating practices. Some emissions, including particulate matter, sulphur oxides (SOx), and other gases, are generated on an intermittent basis when de-sulphurization is practiced. Here emission control is usually by fabric bag filter. Some fugitive emissions, including iron oxides and graphite flakes, occur during hot metal transport to the steel melting shop.
Waste water effluents arise from blast furnace gas cleaning and slag cooling and processing operations. Waste water is normally recirculated after treatment and the bleed stream is treated to remove solids, metals, and oil prior to discharge. The waste waters are typically treated by clarifiers or thickeners to remove suspended solids, and the overflows are recycled to the gas scrubbers.
Slag is the main solid by-product. It can be processed in a variety of ways which include slag granulation or air cooling. Air cooled slag is usually crushed and screened. The granulated slag is sold as a by-product, primarily to the cement industry for the manufacture of BF slag cement. Crushed air cooled slag is used in road making and the construction industry.
Sludge from the gas cleaning system can be recycled to the sinter plant or is sent to a solid waste disposal site.
Steel making
Emissions are generated during each of the five major BOF steelmaking and refining operations namely (i) charging, (ii) melting, (iii) refining, (iv) tapping, and (v) slag and muck handling. The most significant emissions from BOF steelmaking occur during the oxygen blow period. The principal compounds generated are iron oxides and lime.
The particulate-laden combustion gases and fumes (a very fine iron oxide) created during oxygen blow periods are removed from the BOF by evacuation through a large collection main. Primary emissions of gas and particulate matter are collected in a hood above the mouth of the basic oxygen furnace during oxygen blowing. These emissions include CO, CO2, iron oxides, and other metal oxides. Fugitive emissions emerge from the mouth of the BOF during oxygen blowing but are minimized in modern steel plants by the use of a close-fitting hood. The primary emissions are usually controlled by a wet scrubbing system, although a few steel plants use an electrostatic precipitator.
Charging and tapping emissions are controlled by a variety of evacuation systems and operating practices. Charging hoods, tap side enclosures, and full furnace enclosures are used to capture these emissions. The particulate emissions are then exhausted to the gas cleaning plant.
The fugitive emissions from hot metal transfer and charging, scrap handling and charging, ore and flux handling, oxygen blowing, tapping, and slag handling are usually collected by locally placed hoods and cleaned in fabric bag filters.
Minor emissions of particulates arise from ladle metallurgy processes and vacuum degassing. These are usually collected and cleaned by fabric bag filters. Some wastewater effluent is also generated by the degassing process and is treated with other wastewater effluents. The wastewater effluent from gas scrubbing is recycled and the bleed stream is treated to remove suspended solids and oil and to control pH.
The main solid wastes include steel scrap, steel skulls, slag, waste refractories, and muck. Other solid wastes include the wastewater treatment sludge and dust from dry dust collectors. The steel skulls are recycled after lancing and cutting. Muck and slag are processed for removal of steel scrap. This steel scrap is also recycled in the BOF. Slag is crushed and screened for recycling, and other solid wastes are recycled, where appropriate, or disposed for landfill.
Continuous casting
Air emissions of particulate matter and metals arise from the transfer of liquid steel to the mould and from the cutting to length of the product by oxy-fuel torches.
Continuous casting machines normally include two separate closed-loop non-contact cooling water systems for mould cooling and machine cooling and also direct contact water systems for spray and mist cooling. The mould cooling water system is used to cool the mould, while the machine cooling water system is used to cool all other mechanical equipment. Direct-contact water systems are used for spray cooling of the steel as it exits the mould, at the gas cutting torches to control fume generation, and for flushing mill scale down the flume beneath the run out table.
The principal pollutants are total suspended solids, oil and grease, and low levels of particulate metals. As with vacuum degassing, chromium, copper, and selenium may be found in continuous casting waste water. Waste water treatment includes scale pits for mill scale recovery and oil removal, mixed media or single media filtration, and high rate of recycle.
Hot rolling
Hand scarfing or machine scarfing of semi-finished steel to remove surface defects generates particulate and gaseous emissions. Those from hand scarfing are localized and generally minor in comparison to those from machine scarfing, which are typically controlled with local exhaust hoods and wet or dry cleaning systems. The scarfing process volatilizes the steel at the surface of the steel product, creating a fine iron oxide fume. Major pollutants emitted during scarfing include iron and other oxides (FeO, Fe2O3, SiO2, CaO, and MgO). Machine scarfing operations generally use an electrostatic precipitator, scrubber, or water spray chamber for particulate control while most hand scarfing operations are not controlled.
Air emissions from hot rolling include gases generated by the combustion of fuel in the reheating furnaces and VOCs from rolling and lubrication oils.
Waste water effluents are generated from the high-pressure water descaling of the hot steel as well as from water used for roll cooling during rolling and also from water used for on line heat treatment of rolled products. This waste water can contain suspended solids, oil, and grease.
Solid waste is primarily waste iron oxides recovered from the descaling and wastewater treatment operations and includes oil and grease. The principal pollutants in these wastewaters are total suspended solids, cyanides, dissolved iron, hexavalent and trivalent chromium, and nickel.
Pickling and cleaning of steel
The major air emissions are acid aerosols from the acid pickling operations and the acid regeneration plant. Acid pickling waste waters include spent pickling acids, rinse waters, and pickling line fume scrubber waste waters, acid regeneration plant scrubber, and alkaline cleaning waste water. Spent pickle liquor is a hazardous waste because it contains considerable residual acidity and high concentrations of dissolved iron salts. Pickling performed prior to coating may use a mildly acidic bath, which is not being considered as a hazardous waste.
Acid pickling rinse water discharges can be minimized by counter flow cascading and, in some cases, recycling to the acid regeneration plant. The waste water effluents contain suspended solids, oil and grease, metals and acids. Waste water effluent from alkaline cleaning is treated in a waste water treatment facility.
The major sources of solid wastes are iron oxide from the acid regeneration process and sludge from waste water treatment facilities.
Cold rolling
Air emissions from cold rolling are primarily VOCs from rolling and lubrication oils. Some minor emissions result from the combustion of annealing furnace fuel.
Process waste waters from cold rolling operations result from the use of synthetic or animal fat based rolling solutions, many of which are proprietary. The rolling solutions may be treated and recycled at the mill or used on a once through basis and discharged to a waste water treatment system, or handled as some combination of the two.
Waste water effluents are generated from rolling oil-filtering systems, leaks, and spills and include oil and minor amounts of suspended solids.
The principal pollutants are suspended solids, oil and grease (emulsified), and metals (lead and zinc for carbon steels, and chromium and nickel for specialty and stainless steels). Trace chromium may also be a contaminant from cold rolling of carbon steels caused by wear on chromium-plated work rolls. Toxic organic pollutants, including naphthalene, other polynuclear aromatic compounds, and chlorinated solvents, have also been found in cold rolling wastewaters. Conventional treatment of cold rolling wastewaters includes chemical emulsion breaking, dissolved gas flotation for gross oil removal, and co-treatment with other finishing wastewaters for removal of toxic metals.
Coating
Significant emissions can occur in coating lines at each major stage namely (i) pre-treatment, (ii) coating, and (iii) post-treatment, depending upon the product or process. At the pretreatment stage, emissions are limited to alkali mist/fume, acid mist/fume, dust, and sometimes VOCs depending upon the cleaning process (alkaline cleaning, acid pickling, mechanical cleaning such as brushing, abrasive blast cleaning and buffing, or combinations thereof), special cleaning reagents (solvent cleaners, emulsion cleaners, etc.) and other processes used (such as conversion coating). These emissions are collected by local exhaust fume extraction systems and scrubber cleaning systems.
At the coating stage, emissions are limited to products of combustion, metal/metal oxide mists, fumes and powders (from molten metals in hot-dip pots), VOCs and product of combustion (from organic coating lines with baking ovens), as well as acid/electrolyte mists/fumes (from electroplating lines). Products of combustion are carried away with the flue gas and are typically well controlled. VOCs and mists/fumes are collected by local exhaust fume extraction systems and scrubber cleaning systems. Metal/metal oxide mist/fume/powders from molten metal hot-dip pots may be collected with local exhaust fume extraction systems and bag houses.
At the post-treatment stage, the emissions are limited to fumes and dust. These are generally collected by local exhaust fume extraction systems, together with scrubber cleaning systems, if required. The control techniques for removing pollutants and maintaining emission standards include packed towers and wet scrubbers, as well as bag house dust collectors. Removal effectiveness in excess of 95 % is common. In general, available technologies are quite sufficient to control emissions from various coating operations.
Waste water effluents include wet scrubber discharge and electrolytic coating process waste water and rinse water. These effluents contain suspended solids, metals, and acids and are treated in a waste water treatment plant before discharge. Waste water effluent and rinse water that contain hexavalent chromium from chromium coating facilities are treated by an ion exchange process, and the chromic acid is recycled.
At the pretreatment stage, wastewaters include rinse waters from various sub-stages (e.g., alkaline cleaning, acid pickling, mechanical cleaning, and solvent cleaning etc.), overflow and spills from various sub-stages, and various fume scrubber wastewaters. At the coating stage, wastewaters come from the following.
Quench tanks (in hot-dip metallizing and organic coating lines)
Rinse tanks (in all coating lines)
Over-flow, drag-out and recirculating tanks ( mainly from electroplating lines)
Scrubbers
At the post-treatment stage, waste water comes from various rinse tanks, make-up water tanks, spills and over flow as well as fume scrubbers. Process water and wastewater flow at each stage and sub-stage are dependent on the product and process used. The typical pollutants are suspended solids, oil and grease, and heavy metals such as Fe, Pb, Zn, Cr, Cd, Ni, and Ba etc., depending on the coating, base metal, and process.
Acidic alkaline rinse waters are generally neutralized separately before being mixed with other rinse water effluents. These are then sent to a central water treatment plant. The available technologies are adequate in meeting the effluent discharge regulations for existing plants.
Solid wastes include zinc dross, tin oxide, tank sludges, and water treatment sludges. The zinc dross and tin oxide are sold, and other solid wastes are used in landfills.