There are many pollution problems faced by the steel industry and are not confined to any one processing stage. As the size of the industry is large, pollution control is a costly proposition. Major pollution sources and types in iron and steel industry are presented.
How to Control Air Pollution in Iron and Steel Industry:
There are emissions to the atmosphere in the form of dust, fume and steam, acid emissions, fugitive emissions and toxic gases.
ADVERTISEMENTS:
Dust-granular particulates arise mainly in mining, crushing and screening operations and may also spread during transportation and are released at points of belt transfer or blow down by winds from storage heaps and blending beds.
Large quantities of fumes may be created due to high temperatures in high intensity oxygen steel refining processes or in electric arc furnaces. The size of dusts and fumes vary depending on the source of emission. Flow sheet for steel plant is shown in Fig. 30.1.
Oxides of nitrogen and sulphur and to a much lesser extent fluorides and chlorides may be released as they are present in the materials being burnt.
About 99% of the total fumes and dust generated in steel-making process escape as fugitive emissions whereas slags also lead to release of fumes in the form of iron oxide, kish (graphite), soot and silica.
Coke ovens are another major source of emissions. About 50 harmful substances are emitted in the atmosphere (some are concentrated and harmful), e.g. benzopyrene and unsaturated hydrocarbons. Flow sheet for Coke ovens is shown in Fig. 30.2.
During refining of pig iron into steel, large quantities of carbon-monoxide are produced. Byproduct coke ovens gas contains CO and hydrogen.
Difficult and serious problems faced fall into two broad categories, viz.:
(i) Cleaning of air from hard substances.
(ii) Cleaning of air from gaseous substances.
ADVERTISEMENTS:
Two types of cleaning systems, dry and wet cleaning from hard substances are practised. In dry cleaning the following control systems are used, viz., inertial dust catchers, cyclones, electro-static precipitators and different types of cloth filters.
While in wet cleaning scrubbers, wet cyclones and various kinds of venturis are used. Wet method of gas cleaning is used such production where the cleaning is done from gases containing explosive grade substances.
Table 30.1 highlights some specific harmful substances from different production shops.
All kinds of emission can be grouped into two categories, viz.:
(i) Organised emissions are arises from installations for process gas discharge through dusts supplied to gas cleaning plants and in turn through stacks.
(ii) Secondary emissions are created as a result of spreading of emission due to non-sealing.
The most acute problem in the whole of steel industry is from secondary emissions. Steel industry has been able to a large extent reduce the emission of dust. However, less effort has been made against controlling the gaseous pollutants.
Carbon-monoxide control has till date no abatement system. A modern sinter plant with a surface of 400 m2 emits some 2 million m3/h of waste gas with about 1% CO into the atmosphere. Dilution and dispersion is achieved through tall chimneys commonly used in sintering plants.
For removal of Sulphur Oxides, efficient processes are being used in Japan and consist of washing the fumes with a lime or a basic solution (calcium or ammonium hydroxide) and of obtaining as a byproduct gypsum or ammonium sulphate.
The desulphurisation yield is about 90% for an initial content of about 400 ppm SO2. The level of SO2 in exhaust gases from sintering plants (400 ppm) is comparable to that from coal an oil-fired power stations after desulphurisation and much lower than those power stations which do not desulphurise their stack gases.
Scrubbing techniques for nitrogen oxides exist in Japan but have only begun to be applied on an industrial scale in some coking or sintering plants.
It appears preferable to seek alternatives of improving process techniques (better regulation of combustion in reheating furnaces) rather than to utilise sophisticated techniques for NOx removal.
Certain classical particulate removal techniques seem adequately efficient for the trapping of the gaseous components of fluoride (HF), e.g., dissolution in vent scrubbers, trapping in dust cake on bag filters.
How to Control Water Pollution in Iron and Steel Industry:
Many operations within steel-making complex require large quantities of water and is in direct contact with process materials, e.g., cooling and purification of coke oven gas can lead to pollutants such as tar oils, ammonia, phenols, cyanides, thiocyanates and thiosulphates. This polluted water required extensive purification before it can be recycled or discharged to the environment or to a local sewage treatment plant.
Cyanides, fluorides, lead, zinc and dust particles compounds emanating from blast furnaces can find their way into cooling and cleaning waters because of direct contact with gases. Casting, scale, lubricating oils and hydraulic fluids can also contaminate the water. Similar pollution can also arise from rolling mills coating operations and scarfing.
The risk of water pollution from steel industry depends to a large extent on water management plan adopted.
The pollution can be of the following kinds, viz.:
(i) Thermal (increase in the temperature of water used).
(ii) Suspended matter (including oil and tar).
(iii) Toxic substances (phenol, cyanide, heavy metals) etc.
(iv) Waste acids, solutions and rinse waters from finishing processes.
Approximately 70% of the water used for steel industry is spent for cooling purposes alone and contains a heat equivalent of 75 kwh/tonne of crude steel processed. Water must be cooled and techniques are available and are relatively well established. The smaller volume can easily be discharged without placing thermal load on the receiving water body.
Particulate matter in the form of iron oxide, coke particles, oxide scales, oil and greases is likely to contaminate the water in circulation used for direct cooling and its removal is necessary to prevent blockages, etc., and also prior to its discharge as an effluent.
Coke oven blast furnace gas washing, steel-making, fume cleaning and some of the product finishing operations create toxic effluent problems including trace compounds and some of them are bio-accumulative chemicals (Polycyclic aromatic hydrocarbons, benzene, chlorophenols).
Waters from one circuit can be used as the input to another, without extensive treatment when the second circuit does not require very pure water supply.
It may be worthwhile to consider substitution of indirect cooling systems for direct cooling systems. These are capital intensive but the recurring expenditure on treatment of water can be considerably reduced.
Dilution by rainfall and site drainage waters is useful but can cause surge problems in water handling. Provision should be made for emergency storage and treatment for effluents, e.g. should damage occur to biological treatment systems due to sudden release of cyanide or other toxic material, and then the plant should have sufficient storage space and emergency treatment or dilution facilities.
Pickling is a major operation in steel mill which presents a considerable environment concern. Table 30.2 gives the characteristics and other details concerning wastewaters and Table 30.3 gives wastes from still plant.
Suggested Approach for Various Sectors in an Integrated Steel Industry:
A single operation of pig iron production requires about 150 tons of water and 3.5 tons of air to produce one ton of iron. Likewise, every tonne of steel produced gives about 20 tonnes of liquid wastes, 5 tonnes of waste gases and 1 ton of solid wastes. Primary pollutants in waste gases are iron oxide, dust and oxides of sulphur, carbon etc. of these, dust is by far the most important air pollutant in the industry.
Solid waste mainly comprises of slags from blast furnace and steel melting shops and dust separated in gas cleaning plants and ventilation systems. Typical emission from an integrated steel plant is shown in Table 30.4. The pollution problems and the associated treatment methods have been suggested and are indicated sector-wise for the iron and steel industry.
1. Transport/Handling of Raw Materials:
Raw material transport by rail, road and water, loading/unloading; belt transport; coal washing.
Suggested Treatment:
If material is received in moist condition, no precaution needed; For dry material, use water curtain or de-dusting by evacuation to a bag filter while unloading; Extensive enclosure of receiving hopper necessary;
Minimum height of fall to avoid wind entrainment; Mobile equipment to be avoided, tyred vehicle cause (salt and cement) may get contaminated. For proper care use bucket conveyor unloaders with water sprays; Chemical sealing if found suitable.
2. Bedding and Blending of Ore:
Large beds for greater homogenisation of composition; Blend recovered and placed on belt for storage; it aids in further blending.
Suggested Treatment:
Binding agent in the water may be necessary; Ensure proper wetting and use detergents, if need be; Large enclosures and evacuation at high rates at transfer points; Bag filters for cleaning gas; Spray installation at transfer points; Recovery of particulate – laden waters for treatment if necessary. Plantation in and around to arrest dust emission.
3. Sintering/Pelletising of Iron and Steel:
Suitability of fine ore in Blast Furnace; Pelletising with binder and rolled in drums/pans, Indurated at high temperature and cooled; for sintering blending of fines with coarser granular ores, flux mixed with coke breeze and heated; sizing.
Important Consideration:
Fines generated –
(a) Crushing/grinding,
(b) Grinding for pelletisation,
(c) Cooling/crushing/screening sinter,
(d) Cooling and screening pellets;
Fugitive dust in pellet plant; Emission of gaseous and liquid fluorine compounds and oil as fuel, SO2/SO3; while fumes due to K2SO4/Na2SO4; Stack emissions may contain upto 1% CO and difficult to remove by incineration; If sintering materials contain lubricants/soluble oils (rolling mill waste), emissions will be visible and may contain hydrocarbon; Large fans create noise.
Suggested Treatment:
Fugitive dust (a) Recovery by suction hood installation and bag filters/electrostatic precipitation for dry material only, (b) Wet material requires no such precautions, (c) Energy saving by recycling clean heated air to ignition hood on sinter strand.
Stack Emissions:
(a) Normally not necessary to treat stack gases than to remove dust,
(b) CaO/SiO2 ratio important. Low ratio may require desulphurisation of gases,
(c) CaO/SiO2 > 2, difficult to apply electrostatic precipitators for fame removal,
(d) High SOx – scrubbing with alkaline liquids (milk of lime). Expensive, fouling and disposal may create environmental problems. SO2 converted to gypsum (saleable),
(e) High fluorine – wet scrubbing or contact with alumina/lime. High basility leads to low emission,
(f) NOx removal – catalytic converter (expensive),
(g) Particulate removal by water scrubbing or electrostatic precipitators,
(h) Cyclones for coarse grit removal,
(i) Alkalies can cause problems with precipitators and tend to clog riddles and other mechanisms,
(j) Dust to be dumped if recycles not possible,
(k) Oily scale from rolling mills to be treated and not recycled to sinter plant.
Water:
(a) Removal of suspended solids before recirculation,
(b) Lime sludges may create problems
(c) Scrubbing with water for particulate, removals may require treatment for oil, lead and zinc compounds.
4. Coking:
Charging of blended coal in fine form to refractory lined oven; Recovery of bent by dry cooling units/tower; Quenching with water, drying on wharf, screening and transport to BF; Recovery of ammonia, benzol, xylene/toluene, tar, pitch and tar acids; If by-products not recovered because of economics and environmental considerations, there is loss of source.
Important Consideration:
Dust in runoff waters from bedding/blending operation; Dust and organic emissions, sulphurous fumes, carcinogenic materials due to improper sealed doors and lids; Fugitive emissions of grit, smoke and combustion products, emission of steam with grit during wet quenching; Fire breeze and dust of dry quenched materials; SOx/H2S; Water contains phenols, NH3, tarry residue, etc.; Coke ovens and by-products recovery are the most difficult installations to control.
Suggested Treatment:
Charging –
(a) Gravity charging through several holes creates fume, dust, flame and toxic gases. Doubling collecting mains allows greater suction by steam or high pressure liquid ejectors or breach pipes between ovens reduce emissions or stage or sequential gravity charging to contain emission,
(b) Adequate provision of capturing devices for fugitive emissions.
Coke Pushing:
(a) Travelling hood with belt low maintenance cost but requires sound engineering to achieve higher suction,
(b) Problems are associated with, if ovens are pushed too soon and coal is insufficiently cooked,
(c) Mobile hood linked to feed duct and dust arrectment,
(d) Coke side enclosure;
Quenching:
(a) Rapid addition of limited water to hot coke to reduce temperature with internal bent for drying it up and reduction in water quantities for treatment,
(b) Steam generated can be used for bent recovery (capital intensive),
(c) Dry product is friable and dust problems.
Hoods and fans to reduce emissions at transfer points.
Doors and Lids:
(a) Effective seats on doors, lids and caps for coke oven plants,
(b) Long length contact of is suggested to distortion by heat Hand/mechanical cleaning difficult,
(c) High pressure jets to clean this area,
(d) Flexible edged seats between the door and oven is usual. Cranes to be used,
(e) Leaks to be corrected immediately otherwise, situation deteriorates badly. Record-keeping and good maintenance is the only solution.
Stack Emission:
(a) Heating gases for coke oven are usually clean, based on pre-cleaned blast furnace/natural or coke oven gas,
(b) Contamination due to gases evolved by carbonised coal due to leaks through cracks,
(c) Silica welding techniques to rectify cracks to reduce emissions and to improve heating,
(d) NOx reduction by control of combustion condition or by reduction by ammonia in presence of catalyst (capital intensive,
(e) SOx.
Coke Oven Waters:
(a) Gases scrubbed with recirculating water to contain NH3 and other water soluble components,
(b) Steam distillation to recover free NH3
(c) Fixed ammonia removal by addition of alkali
(d) Line slurries create problems in preparation, handling and control. Fouling by precipitation of insoluble Ca salts. Coating of particles with tar and oils. Disposal of lime sludge becomes difficult and operation is expensive,
(e) NH3 can be converted to sulphate, phosphate or anhydrous NH3. Incineration of NH3 when fertiliser is not required which creates NOx problems. Temperature to be kept 1000°C,
(f) NH3 concentration of 50-300 mg/l with phenols, etc. in water treatment by biological reactor system before final discharge Activated sludge system effective for phenols/thiocyanates.
Removal of NH3 is tricky and requires nitrification/denitrification biological system. pH control necessary for nitrification step. Possibility of using mixed liquor recirculation between aerated and anoxic regimes.
Reduces demands for neutralisation. Sludge disposal to be planned properly. Activated carbon with an oxidising agent (Ozone) can be a future possibility for total destruction of organics.
5. Lime Burning:
Soft burnt quality lime necessary; kilns/shaft for heating to remove CO2; closeness of lime burning to steel plant reduces the re-carbonising/slake formation.
Suggested Treatment:
Provision of hoods at transfer points; Aspiration to ensure capture of dry dust; Covered conveyors/seal truck/transfer cars; Stack gases treated to minimise combustion products and fine dust by wet scrubbing/bag filters; Pneumatic transfer, if fine powdered lime can be injected into the furnace.
6. Reduction:
Direct Reduction:
Normally followed by electric arc steel making; Removal of O2 from iron in solid state by gaseous or combustible reductant, (a) Shaft kilns (Natural gas), (b) Rotary kilns (coal/oil) (c) Fluidised bed (Gaseous reductants).
Important Consideration:
Rotary Kilns –
(a) Dust and gases containing sulphur through stacks; shaft kilns;
(b) Natural gas if reformed by steam, control problems are related to combustion, emission and boiler blow down liquids,
(c) NOx/SOx depends on the materials burnt, flame temperature and stoichiometry,
(d) Flue gas with low NOx, if process gas used NH3 reduces formation of NOx,
(e) Fine fume may be prepared for reduction by cold bond briquetting/pelletising with lime.
Suggested Treatment:
Direct reduced fines which may be soluble, Fines from pellets and hot briquetted materials; overheating, if storage in large heaps; H2S/mercaptans to be removed by activated carbon, ZnO or molecular sieves, if material gas is to be used (Catalytic reforming units); SOx emission low in flue gases, NOx low because of low temperature; S forms H2S in reactor reduction zone and removal in part in quench tower and CO2 removal (if CO2 absorption is practised); Coal reduction gives stack gases, viz. CO, CO2. H2, Steam, SOx, NOx, and CnHn.
Heavy metals/fluorides depending on grade of ore; Limestone holds sulphur and discharge CaS in Char. Stack contains low S; Shaft type processes with high recirculation of natural gas based reductant and relatively clean; In kilns, exit gases burned followed by wet scrubbing or electrostatic precipitation. Scrubber product recycled or dumped; Scrubbing effluents contain S-Compounds with pH 2.3/4.5 and needs neutralisation; Total sludge amounts to 600 kg/t of DRI and dust in the gas stack is about 150 mg/Nm3.
7. Blast Furnace Smelting:
Refractory lined shaft; Aerated air injected in the bottom of furnace. Coke burns creating reducing conditions; Solid slag passes down and separated; Molten iron tapped and passes to receiving ladles to steel plant.
Important Consideration:
Blast furnace gas is a resource; Cleaning through dust, catcher, cyclones, scrubbing/electrostatic precipitator; Use of dry electrostatic precipitator bag filters followed by pressure recovery turbine (recent development); Cleaning uses large quantities of water and contains suspended/dissolved solids, phenols, CN and NH3; Recycling of materials from steel-making to BF via sinter plant, dust recovered contains Zn; Measures for Zn removal expensive otherwise land fill usual for Zn-rich fractions; Pressure fluctuations leads to release of dust and CO.
Dangerous near the furnace, but no serious problem for air pollution; Emission of particulates and fumes including alkalioxides (slag), ZnO and smoke (tar-combustion) or resins (refractory clays) from BF cast house; Molten slag and water in contact may form H2S; Electric furnace smelting produces similar pollution.
Suggested Treatment:
Charging – (a) Fugitive emissions controlled through use of closed conveyors/evacuation through hoods to bag filters;
Tapping:
(a) Covered hoods at transfer points to bag houses,
(b) Combustion of gas or other methods of producing a non-oxidising atmosphere can be introduced to control pollution,
(c) Runner side or roof extraction, provide high extraction volumes for good working conditions;
Water Treatment:
(a) Top gas scrubber water requires cleaning to avoid fouling when water is re-circulated to BF,
(b) CO2 lowers pH in scrubbing water. Lime adjusts pH and forms Ca(HCO3), or CaCO3,
(c) Competing reactions with Zn/NH4-salts leave Ca in solution. Ca(HCO3)2 reaction may not be complete. Liberate CO2 in cooling towers. High pH forms Ca/Zn precipitate (Scale fouling pipe work/cooling tower)
(d) Fungal growth in recirculating water,
(e) Method includes settling tanks, filter, chemical treatment (coagulation), clarifiers, chlorination and carbon adsorption,
(f) Low organics (biological treatment difficult). Chlorination for low levels of pollutant makes carbon adsorption necessary,
(g) Once through system if salt water available using thickener/chemical treatment. Chemical control of effluent difficult,
(h) Coke oven/BF effluent can be treated together.
It is preferable to treat BF separately.
Accepts only slow changes –
(i) Wide variations of pollutant concentration particularly CN because of changes in operating conditions of BF,
(j) Sinter plant scrubber and BF waters are compatible.
Slag:
(a) Necessary to collect steam over the beds,
(b) A condensing chimney can be used where continuous granulation operation exists,
(c) Use of strong oxidant in water to control H2S odour,
(d) Control of pH in water can also minimise odour.
8. Hot Metal Pretreatment:
Removal of undesirable elements; Desiliconisation, Desulphurisation, Dephosphorisation where silicon removed injection of Na2CO3 producing corrosive fumes.
Important Consideration:
Desiliconisation may create fumes exceeding the capacity of side extraction units. Increase fan capacity; Dephosphorisation, large fumes because of addition of Na2CO3 (volatile at high temperature); Slag removal difficult. Calcium carbide for desulphurisation may create explosion (contact with water); Slags with Na2CO3 are corrosive (refractory attack possible).
Suggested Treatment:
Provide high extraction capacity; Lime creates binding when bag filters are used; Special attention in recovering waters from washing of gas from Na2CO3 extraction facilities; Alkaline waters separated for neutralisation of acid solutions from other parts of the plant; Recover/regenerate Na2CO3 from slag.
9. Steel-Making:
Oxygen Furnace:
Hot metal from BF and scrap; Refractory line (Basic) vessel (Magnesia, Dolomite with Carbon, Tar/Pitch); CO/CO2 produced and mouth of vessel laden with iron-oxide fumes; O2/Secondary O2 used to convert CO to CO2; Gases produced may be burnt or used as fuel; Hot metal refined to steel and slag/steel separated; Basic process due to lime for holding S/P; Slag-rich in iron-oxide/lime.
Suggested Treatment:
Desulphuriser slags to be skimmed prior to charging hot metal. Provide evacuation hood over slag and to capture graphite emission (High air flows); Dust (open combustion) by electrostatic precipitation or high intensity scrubbing. Bag houses can be used; Secondary fumes (fugitive) by local hoods; Tapping fumes contained by blanket flame or inert gas around the stream; Fugitive dust from fluxing agents. Provision of capture/cleaning in bag houses necessary; Large converters closed combustion needed, reduces the total amount of gas to be treated (fumes).
Compact control equipment like high intensity scrubber/horizontal electrostatic precipitator; Energy saved by recovering off gases; Water from scrubbers treated through thickener (polymer addition)/vacuum treated.
Overflow recycled (10% bleed may be needed). Treatment of bleed water for pH correction. Suspended solids can be precipitated and vacuum dried; Basic problem with dust is its association of Zn; Liquid slag (to be solidified) for recovery of scrap.
10. Electrical Furnace:
Scrap/dried reduced iron used; Refractory and water-lined chamber; Graphite electrodes with high power arc discharges; Slag/steel separated; Gases combusted/cleaned/ejected into atmosphere, DC electrodes/plasma gun to melt scrap are under development. Similar pollution; Induction furnaces are of lower capacity foundries than steel-making plants.
Important Consideration:
Slag/metal cause fugitive dusts/fume; Air aspirated into duct to burn CO;
Basic problems (Temperature) –
(a) Mild oxidising atmosphere (melt down). Sufficient suction can be applied (little emission),
(b) Reducing slag operation, little air permitted. Full available suction from stack cannot be applied (fugitive emission increases),
(c) Carbon removal stage.
Difficulty in matching the suction with the emission rates (increase in emission); Coolant water (walls/hood) required is large. Treatment necessary to avoid corrosion/fouling. Once through cooling expensive. Deposits/sludges to be handled.
Suggested Treatment:
Gas and Fume:
(a) Primary fugitive emissions,
(b) Point source hood or large roof monitors or both (cost intensive),
(c) Electrostatic precipitator roof monitor (hot plume of air from furnace to provide motion of dust through it,
(d) Gas volumes to be treated are large. High intensity scrubbers not advocated, but bag filters are generally used (explosion possibility),
(e) Heat exchange approach (difficult due to intermittent nature of process) or admixture with fresh air,
(f) Spark box between furnace and precipitator/bag house,
(g) Eliminate control of combustion by movement of aspiration gap through a combustion chamber so that air/gas flows can be matched by a controller,
(h) Partial enclosure of the entire furnace for controlling fumes and noise from arc furnaces. Dusty work environment can be a possible disadvantage,
(i) Preheating creates excessive pollution due to oils, etc. in the scrap.
Waters:
(a) High intensity scrubbing water requires similar treatment as in BOS; Wastes; (b) S/ Fluorine compounds create fumes. Fluorine may enter into water, (c) Recycling of solid wastes with Zn/Pb/Cd/Cr and alkalies difficult, (d) Under special cases, such materials can be fed to direct reduction kilns, (e) Dumping when leaching of heavy metals (Ni/Cr) is not possible.
11. Open Hearth/Other Furnaces:
Unit Operation/Unit Process:
Shallow bath (refractory) charged with hot metal/scrap/flux; High intensity flame at high temperature (tar/oil-fuel); Older system, evolution of fumes low; Modern plant with high intensity, needs gas cleaning system; UHP electric furnace appears to be economical to smelt scrap. OHF may not be used.
Important Consideration:
Dust emission (handling/charging raw materials); High SOx in stack gases (fuel-S); NOx because of high intensity flame and increases with O2.
Suggested Treatment:
Advanced pollution control technology not developed because OHF are not in vogue; similar control as with oxygen steel-making; Old plants can use electrostatic precipitators/roof monitors; Wet precipitators create wastewater with S, F, NO3 and Zn (Galvanised scrap). BOF treatment method is recommended; Tandem process uses efficiently heat by higher production rates. Emissions similar to OHF.
Control of NOx/duct; Electro slag steel melting (high quality steel/cladding rolls) uses slags with calcium fluoride (viscosity). SOx, NOx, dust and F could be anticipated; new furnaces (Enemy optimising) can use gases from hearth to preheat scrap/iron; submerged oxygen injection reduces tap to tap times with smaller quantities of oil. NOx/SOx emissions have reduced;
12. Casting:
Ingot:
Liquid steel through ladle into teeming bay, poured into moulds and solidified; Soaking pits heated for rolling purposes; Ingot moulds of pig iron prepared in foundery; Unserviceable moulds (recycle) added to scrap.
Important Consideration:
Generation of fumes as liquid steel passes into moulds through various means; Moulds with tarry compounds (surface quality of ingots) give fumes; Bottle top moulds lowers risk of explosion; Free cutting steel, Pb/S are added. Toxic and emission control necessary.
Suggested Treatment:
Hoods with high suction in mould are necessary; Access to teeming stage a problem (Ladle to pass over the top of the mould); Side extraction with mobile hoods necessary.
13. Continuous Casting:
Liquid steel in ladle into open bath of refractory (Tundish), tapped to a defined height of liquid in tundish; Liquid flows into moulds (Copper), solidifies (intensive water cooled) and carried through series of guiding rollers. Water spraying further cools it; Cutting (Sheer/flame).
Important Consideration:
Continuous process least polluting; Powder addition to tundish (exposure to worker); Oils (mould lubrication on billet castor) results in fumes; Flame cutting operation creates fumes. Large air flows needed and difficult to fix efficient equipment; Iron powder is used for steel cutting. Bag house may be fixed; Water contains scale suspended solids and lubricating oils.
Suggested Treatment:
Waters cascade from mould cooling circuit (highest quality) through other circuits to strand quenching (lowest quality). Settling tanks/clarifiers are required to remove oil/SS. Pressure filters/DAF/Pressure ultrafiltration or vacuum floatation are also necessary; Consistency in water temperature through cooling towers; A bleed (treatment plant) to avoid build-up of fouling materials; Control of addition of chemicals (biological growth/precipitation/corrosion) before release to environment.
14. Rolling:
Scarfing:
Temperature equalisation in soaking pits, ingots rolled to semi-products and cooled to room temperature; Defects inspected for surface flows/cracks to reduce rolling problems; Scarfing by oxyacetylene flame over flow regions; direct charging uses materials hot from rolling/continuous casting for reheating/temperature equalisation furnaces. On line scarfing can be done. Reduces energy losses; Good quality surfaces can help in occasional scarfing (off line).
Important Consideration:
Hand scarfing minor fumes (little dust); Automatic scarfers must be controlled for their effect on environment (0.5% of steel removed-melted droplets/iron oxide/combustion products); Automatic scarfers better than band scarfer (exposure to workers of fume/oxide dust).
Suggested Treatment:
Workers to be supplied with protective equipment including earmuffs. Provision of air-conditioned environment/remotely operated devices; Suction hoods for fumes and then to electrostatic precipitation/high intensity scrubbing; to develop automatic devices for defect detection which will remove hand scarfing and in line scarfing can be achieved.
15. Hot Rolling:
Prior to rolling, continuous casting to be reheated; heated ingots are rolled to semis; Rolling reduces thickness/sometimes width; Hot rolled product after shearing/cutting. Some plate mills equipped with surface preparation/heat treatment facilities.
Important Consideration:
Cold material heating (furnaces) requires energy;
Emission depends on fuel –
(a) Oil leads to SOx (S), NOx (temperature), Low NOx burners being developed,
(b) Natural gas leads to lower air pollution,
(c) Induction heating (low energy input) for correction to temperature profiles in products;
Reheating creates oxidation of products, scales are formed. Roiling breaks in scales and flows with waters to cool the rolls; Flakes of iron oxide dust in last strand requires capture.
Suggested Treatment:
Mill scale (magnetite) low in oils recycled to similar plate; Water may contain oil and scale. Scale may therefore contain oil. Difficulties on sinter strand (visible emission-problems in electrostatic precipitators). Better to be sold; Direct reduction, with scale can be mixed with other wastes/ores (fuel/reductant in kilning operations); Oil from mill scale removed by solvent extraction/extraction in a critical fluid (CO2) (costly); Dumping recommended provided ground water pollution is eliminated; Largest consumer of water but most of it is recycled.
Two distinct water lines recommended:
(a) One treating indirect cooling water and requiring minimal care/treatment (motor-room, reheating furnaces, control rooms, etc.)
(b) Direct cooling waters (scale breakers/flushing, roll cooling, crop pit, scarfing machine, hot run table, down coiler, hot and cold saws).
Treatment through clarifier/filters. Solids to be vacuum filtered.
16. Cold Rolling/Forming:
Flat products processed by cold rolling after acid treatment (pickling) for thin scale removal; Cold mill may be reversing or tandem train design; annealing; Shape control, slitting, shearing and forming for sale.
Important Consideration:
Pickling acid baths (H2SO4/HCl) at high temperatures for scale removal; Oil coating to avoid rusting; Acid mist during pickling, contained through enclosed system or using hoods over tanks.
Acid laden air to be water washed before release; Rinsing water, spills, overflows/other discharges from pickling tanks and fume stack effluent needs treatment and pickled wastewater, Oil-water emulsion leads to high BOD, SS and free oil problems.
Unpleasant odour due to biological growth on oils; Gases (H2/O2) produced during electrolytic gases to be controlled; Stainless steel grades require HF and HNO3 for pickling and abrasives may be used to remove refractory oxides.
Suggested Treatment:
Primary steeling tank for oil skimming (Cold rolling forming); Lime and flocculent treatment tanks for finely divided/emulsified solids and oils prepared with addition of alum and acids; Major difficulty in oil removal and to obtain solids that can be filtered;
Neutralisation, rapid/slow agitation in tanks with addition of flocculents/polymers, scum skimming and final adjustment are basic methods; Scum can be incinerated after dewatering; Ultra filtration (possibility) for oil recovery/recycle can be considered.
Oil Removal:
(a) Skimming (0.25 h, HRT depending on waste),
(b) Filtration (beds blind quickly resulting in frequent back wash). Costly (filter material/energy),
(c) Flotation (dispersed air broth). Air/vacuum flotation achieves high degree of separation. Energy efficient;
Neutralisation:
(a) Possibility of neutralising pickling wastewater with alkaline electrolytic cleaning wastewater,
(b) Use of acid effluents to break oil-water emulsion,
(c) Neutralising pickling wastewater with lime. Producing sludge which after dewatering can be disposed off on land. Sludges containing heavy metals (Passivation baths) should be treated to avoid remobilisation of metals.
Recovery of Acids:
(a) H2SO4 recovery leads to production of FeSO4 .7H2O, sell FeSO4. 7H2O (weed killer, water treatment plant for flocculation, dephosphorisation treatment), excess FeSO4 to be dumped under conditions that its solution will not contaminate environment,
(b) HCl pickling regeneration techniques recovers iron as pure and fine iron oxide which can be recycled within the steel plant or sold to pigment users or grown with specific magnetic properties (ferrites – coating recording tapes/computer memories).
17. Coatings:
Continuous coating limes to apply protective and decorative coatings of Zn, Sn, Al plastic and paints. Some plants have terne (Pb), brass, Cr and Cd coating operations; Zn can be deposited electrolytically (Process is becoming sophisticated); Galvanising by electrolytically or hot dipping in liquid metal (alkaline cleaning, pickling, bright annealing, cooling, coating, induction heating, water quenching, cooling and shearing); Tinning (alkaline cleaning, rinsing, pickling, plating with tin, rinsing, chemical treatment, rinsing, drying, oiling and coiling.
Important Consideration:
Degreasing leads to emission of chlorinated hydrocarbons; Acids can be released into atmosphere; Solvents released during plastic/paint coatings; dilute rinsing waters create some problems, chromating/Phosphating create problems. Cr (+6) to be converted to Cr (+3).
Suggested Treatment:
Neutralisation and correct oxidation state before releasing to environment, particularly for Cr (+6); Cr (+6) rinse water sent to reduction tank where acid and SO2 (recycled pickle) to reduce it to Cr (+3); these can join wastes from fume.
Scrubber and other rinse waters to give approximate neutralised water. Neutralisation is completed after oil skimming of addition of lime/flocculants.
Separation of solids possibly in a clarifier and vacuum filter; Sludge (high Cr, etc.) from tinning/Cr-plating to be sold; Toxic metal removal by alkaline compounds like lime NaOH; Hydroxides to be converted to insoluble carbonates by CO2, Lime helps in removal of phosphorus/fluoride; Soluble sulphides (Na2S) for removal of many heavy metals.
Final pH adjustment necessary; use of Activated Carbon provided oils/grease and suspended solids have been removed and can be regenerated or dumped after use; Plastic/painting coating lines should have suction hoods and led off to a combustion facility; Difficulty is in disposal of waste tin/drums containing plasticisers/glues/paints, solvents etc. in excess of their use and must be incinerated.