49. Iron and steel
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2. Environmental impacts and protective measures
2.1 Sintering / pelletising plants
2.2 Blast furnaces
2.3 Direct-reduction plants
2.4 Crude steel production
2.5 Steel forming
2.6 Foundry and forging operations
3. Notes on the analysis and evaluation of environmental impacts
4. Interactions with other sectors
5. Summary assessment of environment relevance
Statutory provisions, regulations
This environmental brief covers iron and steel production and processing with the following activities:
- sinter, pellet and sponge-iron production
- pig iron, cast iron and crude steel production (including continuous or strand casting)
- steel forming (hot and cold)
- foundry and forging operations.
The above activities are carried out in an integrated ironworks or sometimes in separate locations.
After delivery and pretreatment of the ore in the ore preparation, sintering and where applicable pelletising plant, pig iron is smelted in the blast furnace with the addition of coke and admixtures; coke supplies the energy and reduces the ore to pig iron. In the converting mill the molten pig iron is refined to form crude steel by top blowing or purging with oxygen and the addition of scrap. Crude steel is also produced from scrap in electric furnaces, sometimes with the addition of pig iron, ore and lime. The crude steel is either continuously cast as blanks or, after casting as slab ingots or blocks in permanent moulds, rolled in the hot rolling mill to form sheets, billets or profiles. Further processing takes place in the cold rolling mills and forges. Continuous casting which already represents 90% of German and 60% of worldwide steel production improves crude steel utilisation by some 10%, saves energy by rolling operations and reduces the production scrap yield in steel and rolling mills per tonne of finished steel by more than 50%.
The direct-reduction process represents an alternative to traditional steel production. With the addition of reduction gas, e.g. from natural gas or coal, sponge iron is produced as a solid, porous product from which crude steel is then refined in the electric furnace, often with the addition of scrap. 90% of sponge iron is produced by the gas-reduction process.
Cast iron smelting takes place in the cupola furnace, with increasing use of induction furnaces.
Moulds and cores are required for the shaped casting of cast iron; these are mostly of sand but frequently contain an organic binding agent.
The following are classified as major units:
sintering plants 20,000 t/day
blast furnaces 12,500 t/day
steel converters 400 t holding capacity
electric furnaces (arc) 250 t holding capacity
cupola furnaces 70 t/h
induction furnaces 30 t/h
In many countries steel is extensively produced from scrap in electric furnaces.
Since iron and steel production is predominantly based on pyrometallurgical processes, air pollution is a primary consideration. In addition to a multitude of gaseous air impurities, dusts play a special role, not only because they occur in large quantities but also due to the fact that the dusts contain some hazardous substances affecting both man and the environment, e.g. heavy metals. Due to the use of coolant water and wet separation methods, problems of maintaining water purity also occur. Continuous casting plants require high specific water quantities from which the wastewater is considerably contaminated with oil. Casting without spray-water cooling relieves the load on water resources.
Metallurgical processes also produce slags which should be recycled wherever possible. Where no effective recycling and final dumping facilities exist, dusts and sludges separated from the waste gas cleaning systems represent potential pollutants of the ground and water environments.
In blast furnace plants and converting mills, also in rolling mills and forging works, noise and vibration protection is of fundamental importance. Foundries produce large amounts of waste from used sand, broken cores and cupola slag.
For reasons of ecology and economy, work is taking place worldwide on process methods which permit the use of coal instead of coke and the extensive use of lump ore instead of sinter or pellets. This would enable coking and sintering plants to be dispensed with as emission sources in a metallurgical plant.
Other developments concern the casting of rolling feed stock in approximately final dimension form. Shortening the process chain permits reductions in energy requirements, residual substances, waste and emissions.
2. Environmental impacts and protective measures
2.1 Sintering / pelletising plants
Sintering plants form lumps of fine ore prior to introduction into the blast furnace and recycling of ferriferous residues (waste materials). Sintering is the traditional method of treating residual and waste materials from the smelting plant. Factors determining the limits include the zinc concentration, because zinc in the sinter contributes in the blast furnace to the formation of scaffolding with impaired gas distribution.
Sintering plants produce the following emissions:
Waste gases and dust containing components with potential environmental relevance:
SO2, NOx, CO2, HF, HCl, As, Pb, Cd, Cu, Hg, Tl, Zn
Of dust components, the heavy metals lead, cadmium, mercury, arsenic and thallium have the greatest environmental relevance where these are present in the charge materials. The relevance of anthropogenic heavy metal emissions is based less on their overall emission rate than in high localised mass flow densities or concentrations. The iron and steel industries are among those industries in whose vicinity the highest immission rates of heavy metals occur in the air and ground.
Dust is separated and returned to the sinter process in gas cleaning systems, normally electrostatic precipitators. In continuous operation the dust content of clean gas is between 75 and 100 mg/m3. Heavy metal, e.g. lead, enrichment in the sinter plant dust is possible with continuous recycling. Dust with heavy concentrations of lead and zinc should be conducted to a zinc and lead recovery system. In the case of stoppages of the sintering belt due to faults, care must be taken to ensure that the gas cleaning system continues to operate at maximum possible separation capacity. In addition to sintering belt dedusting, modern sintering plants also have room dedusting whereby dust-laden waste air from transfer stations, chutes, crushers etc. is cleaned by a hot sieve system.
Depending on the composition of charge materials, inorganic gaseous fluoride and chloride compounds as well as sulphur dioxide and nitrous oxides are emitted. Sulphur dioxide emission can be significantly reduced by using coke with a low sulphur content. The emission of gaseous pollutants can also be reduced by increased lime dosing. This results in problem substances being transferred to the separated dust. Where regional conditions and process engineering do not permit these measures, wet-process desulphurisation systems offer a means of reduction; in this case some problem substances are transferred to the wastewater. On account of the large gas volumes - up to 10 E 6 m3/h - only partial waste gas desulphurisation can take place. For this reason preference should be given to primary measures. Concentrations in cleaned waste gas are around 500 mg/m3 sulphur dioxide.
With respect to noise impact, a distinction is made between the noise immissions of operations to the neighbourhood and the effect on the staff at their work-places. Principal noise sources of the sintering plant include the large fans for drawing air through the sinter cakes, cooling the sinter and dedusting. Crushing and screening stations should be housed in solidly constructed buildings whose walls restrict the propagation of sound. Possible noise reduction measures are silencers in the air supply and discharge pipelines, also the encapsulation of individual units. The acoustic power immission level is used to evaluate the noise radiated to the open air by the plant. The acoustic power level of a noise source is a distance-dependent parameter; for sintering plants without silencers on supply and discharge air pipelines it can be as high as 133 dB(A) and for those with silencers 124 dB(A). With very good acoustic planning and execution an immission level of around 40 dB(A) can be achieved at a distance of 1,000 m from the individual noise sources. If this target cannot be achieved, protection of the residential area adjacent to the sintering plant is only possible by noise protection measures on the propagation path, e.g. a noise abatement wall. Measures for optimising noise protection are to be considered in parallel with the planning of the production unit.
By encapsulation and the separate installation of principal noise sources it is also possible to protect the work places. The typical noise level in the sintering hall is between 83 and 90 dB(A); attention must be paid to the use of personal noise protection because long-term exposure to an acoustic power level in excess of 85 dB(A) results in serious hearing impairment. The wearing of safety helmets and shoes also helps reduce industrial accidents. Staff in work-places particularly exposed to dust, gases, noise and heat are to have regular preventive medical examinations by works doctors.
In pelletising plants, fine ores are mixed with additives and water to form green pellets which are burned in pellet incinerators on travelling grates. The dust-laden waste gases are cleaned in dedusting plants, usually electrostatic precipitators. The filter dust is re-used. Pelletising plants are associated with lower dust and gas emissions than sintering plants. In contrast to sintering, pelletising is mainly performed at the ore mine.
2.2 Blast furnaces
The blast furnace is a countercurrent reactor loaded or charged from the top with layers of feed and coke, the molten pig iron and slag being drawn off from below. Hot air is injected in the opposite direction from the bottom of the furnace. Residual materials (waste) such as oily metal chips and oily rolling scale can be introduced after sintering.
The principal emissions, residues and waste materials are:
- top gas, with the following potentially environmentally relevant components: CO, CO2, SO2, NOx, H2S, HCN, CH4, As, Cd, Hg, Pb, Ti, Zn
- top gas dust (dry) from the gas cleaning plant with high iron contents (35 - 50%)
- slag with the following major components : SiO2, Al2O3, CaO, MgO
- sludge from the waste gas cleaning system
- wastewater from the waste gas cleaning system, with the pollutants: cyanides, phenols, ammonia
- dust from the casting house dedusting system.
The waste gases from the blast furnace are pretreated in mass force separators (dust catchers or cyclones) and, in a second stage, finally cleaned with a high pressure scrubber or wet electrostatic precipitators. Clean gas dust concentrations from 1 to 10 mg/m3 are achieved.
Other dust emissions in the blast furnace area, particular from the burdening process, pig iron desulphurisation and the casting house must also be identified and cleaned.
Dust formation ("brown fume") in the casting house affects not only the neighbourhood but also, to a considerable extent, the workplaces. Efficient casting house dedusting systems which intercept process waste gases and peripheral emissions at the taphole, runners and cut-off points and separate dusts in horizontal electrostatic precipitators can achieve clean gas dust concentrations significantly under 50 mg/m3 (best values 7 and 12 mg/m3 and dust emission factors between 0.020 and 0.028 kg/t pig iron in blast furnace plants with a capacity of 4,000 to 6,000 t/day). As a replacement for the standard collection and cleaning methods, trials are currently in progress with the suppression of "brown fume" through inertisation with nitrogen.
In the dedusting of pig iron desulphurisation, clean gas dust concentrations of 50 mg/m3 are adhered to in both calcium carbide and soda desulphurisation, using radial flow scrubbers or electrostatic precipitators.
The top gas contains between 10 and 30, though possibly as much as 60 g/m3 dust with 35 to 50% iron, i.e. 30 to 80 kg/t pig iron, in older plants 50 to 130 kg/t pig iron. The dust is separated in the dry state in mostly multistage separators, from where it goes to the sintering plant and from there back to the blast furnace.
In view of the zinc and lead content and other factors, the top gas scrubbing water sludge must be disposed of by dumping, unless there is a special hydro-cyclone separation system. With higher concentrations, it should be transferred to a non-ferrous metal works. Recycling in this way would leave the blast furnace process practically free of residues. Dumping involves the risk of leaching and hence penetration of the soil and groundwater by compounds of zinc, lead and other heavy metals. The dump must be permanently and verifiably sealed and the seepage water must be collected and chemically processed. The special requirements imposed on such a dump must be laid down in the project planning stage.
Slag produced by the blast furnace process accounts for roughly 50% of the overall waste materials from pig iron and steel production. This slag is mostly used in road-building. Part of the molten slag is granulated by quenching in water. This so-called slag sand is also used in road-building. Part is used to produce iron slag Portland cement and blast furnace cement. Quenching and granulating releases carbon monoxide and hydrogen sulphide. The wastewater has an alkaline reaction and contains small quantities of sulphide.
Slag heaps sometimes produce seepage water with high levels of dissolved sulphides and strong alkaline reaction, posing a hazard for the groundwater. Slag heaps must be sealed and any seepage water must be treated.
Wastewater is generated by top gas scrubbing and simultaneous wet dedusting. The wastewater is normally clarified in settling tanks and, where necessary, gravel bed filters and recirculated. The wastewater contains suspended matter (dust) and sulphides, cyanides, phenols, ammonia and other substances in dissolved form. The last three substances must be removed from the wastewater using appropriate physical and chemical treatment processes.
The top gas can be used as a fuel for heating purposes within the works, in view of its high carbon monoxide content due to the reducing atmosphere in the blast furnace, though this will inevitably result in the formation of carbon dioxide, with its climatic implications.
Excessive levels of sulphur dioxide and nitrous oxide gases can be reduced by flue gas desulphurisation and denitrification.
Carbon monoxide concentrations in the workplace pose a particular problem. Where top gas pipes are not perfectly leakproof there is a danger of poisoning with possible fatal consequences for workers present at the furnace throat. Close attention must also be paid to CO concentrations by carrying out measurements and ensuring that protective breathing equipment is worn during repair and maintenance work on shut-down blast furnaces or gas cleaning systems.
Protective equipment for blast furnace workers includes fireproof clothing, breathing equipment and ear protectors, depending on where they are working; protective helmets and safety footwear must be worn in all areas.
Noise in blast furnace plants comes mainly from the combustion air fans and the charging process; also there is the noise generated upon changeover from blast to heating operation. Suitable abatement measures include silencers, enclosure of the furnace throat or encapsulation of all valves and shields. The noise level from the blast furnace plant is in the range of 110 to 125 dB(A); the level of background noise in the immediate vicinity may be 75 to 80 dB(A). Possible noise reduction measures should be selected as early as the blast furnace planning phase. Their effect can be determined by advance calculation, taking care to ascertain the significance of the emission sources (plant sections and operating processes). One should preferably begin by damping or eliminating occurrences and noise sources which arise only periodically.
2.3 Direct-reduction plants
Direct reduction plants function according to a variety of methods, e.g. with shaft furnaces or rotary tube furnaces which are similar to blast furnaces. In the former, the top gas is scrubbed and then enriched with natural gas and used for heating; in the latter, the gas is not used unless steel and rolling mills are available for this purpose. If this is the case, the gas should be burnt provided the CO content is sufficiently high. The waste gas flow is cleaned by mass force separators (dust chambers) for preliminary separation and then by fabric filters. Sulphur dioxide emissions may occur in the solids reduction process, depending on the sulphur content of the coal used.
2.4 Crude steel production
Excessive carbon content impeding further processing of the pig iron and substances influencing the quality of crude steel, such as silicon, phosphorus or sulphur, are either expelled in gaseous form or slagged during the steel production. The following emissions occur in the steel works:
- waste gases and dust containing components with potential environmental relevance: CO, NOx, SO2, F, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Si, Tl, V, Zn ammonia, phenol, hydrogen sulphide and cyanide compounds may occur, depending on the process.
- dust from waste gas cleaning
In the steel works dust is formed mainly due to the top-blowing or through-blowing with oxygen necessary for oxidation. The solids content of the waste gases from the oxygen converter is between 5 and 50 g/m3. They contain finely dispersed evaporation products of iron oxides and primary anoxide ("brown fume"); also sulphur and phosphorous compounds, fluorine compounds and, where fluxing agent is used, silicon tetrafluoride.
Specific dust masses are approximately as follows:
- electric furnace: 2 - 5 kg dust per tonne crude steel
- bottom blowing converter oxygen bottom metallurgy (OBM) 5 - 10 kg dust per tonne crude steel
- top blowing converter (LD and LDAC process) 15 - 20 kg dust per tonne crude steel
Gases occurring in addition to carbon monoxide include inorganic fluorine compounds with the addition of fluorspar, also small quantities of sulphur dioxide and nitrous oxides, nitrous oxide formation being significantly higher in electric furnaces than in the blowing converter.
A technical solution exists for the collection and cleaning of the process gases from the converter. A fixed or lowerable hood over the converter prevents the intake of large quantities of infiltrated air or the escape of converter gases. The gas is subsequently dedusted by a wet or dry process. Wet dedusting takes place in a two-stage operation by a combined wet scrubber and wet electrostatic precipitator. For dry dedusting, dry electrostatic precipitators are used, designed to resist internal pressures up to 2 bar (due to risk of deflagration). The clean gas concentrations are under 50 mg/m3 dust and under 500 mg/m3 sulphur dioxide. A value of under 400 mg/m3 nitrous oxide cannot be continuously maintained. Maintenance of the separation equipment is important in order to achieve an adequate continuous level of separation. Dry dedusting is advantageous as the yielded dust can be returned to the converter after hot briquetting.
Transfer, charging and mixing processes produce random dust emissions which may pose a considerable nuisance for the neighbourhood. Clean gas contents of 10 mg/m3 can be maintained by a waste gas collection system with a collection rate of 90% and a downstream separator using fabric filters or horizontal electrostatic precipitators.
Proposals for the use of a process-dependent control and instrumentation system for reducing specific waste gas quantities must be examined with respect to system requirements such as robustness, error detection ability and ease of maintenance.
Since waste gas collection is difficult with Siemens-Martin furnaces while the furnace is in operation, the solution is to convert to electric furnaces. In addition to lead and zinc, chromium, nickel and vanadium occur in the dust if electric furnaces are used to produce fine steels. Certain chromium compounds in the form of breathable dusts have proved to be carcinogenic.
A full doghouse enclosure is necessary to achieve 95% collection of the waste gases occurring with electric furnaces during charging, smelting and casting. Fabric filters permitting clean gas dust concentrations of under 20 mg/m3 are used for dust separation.
When the converter is in operation, large amounts of carbon monoxide are produced which should be transferred for controlled burning in a torch or in a boiler with energy conversion, so as to avoid excessive air burdens (immissions). A potential source of polyhalogenated dibenzodioxin and furan emissions (though not currently thought to pose a major risk) is the recycling of iron scrap in electric steelmaking plants. Large quantities of iron scrap contaminated with halogen compounds and the operating conditions give rise to the formation of these substances. Initial random sample checks yielded emission concentrations of the order of a few nanograms. A comprehensive measuring programme is being prepared. Careful selection and preliminary sorting of iron scrap is currently a practicable way of minimising carcinogenic emissions. Processes for separating health-endangering dioxins and furans are currently being developed. Current trials of activated charcoal adsorption filters and their separation capabilities are being followed with close interest.
The wastewater from wet dedusting is clarified in a hydrocyclone or settling tank and recirculated. The separated sludge is dewatered by a vacuum drum filter and returned to the blast furnace via the sintering plant. Attention must be paid to the zinc content of the sludge upon recycling. Slag produced in steel works is used in road construction or processed into fertilisers.
Loud noise is generated in converting steel works by high-powered fans and dedusting systems and in electric furnaces by the arcing and transformer. Noise levels in electric steelmaking plants without noise reduction measures is between 117 and 132 dB(A), and around 100 dB(A) with noise reduction.
Noise reduction measures can include:
- arc soundproofing
- smaller apertures in the furnace shell
- encapsulation of the furnace
- acoustic separation of the furnace bay from adjacent bays
- increasing the soundproofing of bay walls
- silencers on air intakes and outlets
- slow-running cooling air fans
- enclosure of individual systems
- avoiding free-fall of scrap upon loading and charging.
Very high peak noise levels can occur during smelting, especially with wet scrap. Highly automated modern plants have control rooms which provide effective protection against noise at the workplace. The protective measures mentioned under 2.2 also apply to workplaces in steelworks.
2.5 Steel forming
The following emissions and residues occur with forming (shaping) of crude steel into rolled steel:
- oily rolling scale
- waste gases from the furnace
- oily wastewater
- wastewater from the waste gas cleaning
During the production of steel plate, the following are produced:
- oily wastewater
- waste air from the pickling baths
- spent pickling solutions
- sulphuric and hydrochloric acid
- or nitric and hydrofluoric acids
The most prolific residue produced in hot rolling mills is rolling scale. The specific mass is 20 to 70 kg/t finished steel. Scale comprises mainly iron oxides (70 - 75) and can therefore be utilised in the blast furnace. Finer components must first be sintered or pelletised. Oily scale with a small percentage of oil from the machinery lubricants can be freed of oil by combustion or by alkaline wet scrubbing. To avoid polluting the subsoil with oil, oily scale should not be dumped.
Wastewater is produced in the hot rolling mill by
- transport of the scale to the wastewater treatment system
- alkaline washing of the oily scale.
The scale-water mixture is separated in settling tanks and gravel filters (sometimes with the addition of flocculation agents). Floating rolling oil and grease is skimmed off and the settled or filtered scale is dewatered and transferred to the sintering plant. The clarified wastewater is recirculated.
The alkaline scrubbing water from the scale scrubber contains an oil emulsion which must be broken down with chemicals. The water contains oil and chemical residues. It should be transferred to a biological filter plant. The recovered oil can be processed and in certain cases re-utilised in the rolling mill.
In the cold rolling mill the steel plate is descaled in a pickling bath before further processing. Hence, no solid waste (scale) is produced in the actual cold rolling process.
With cold rolling, wastewater occurs due to contamination of water with rolling oils (mineral oils, palm oil) and from the pickling. The rolled down plates are once more pickled with acid and electrolytically degreased prior to tinning or galvanising.
Wastewater treatment requirements in rolling mills depend on the type and extent of recycling and the quality of the receiving body of water. Regular monitoring of wastewater values is necessary.
The oil-water emulsions produced by the cold rolling process must be chemically treated (flocculation with ferrous salt and lime). The oily sludge must be incinerated and the ashes transferred to the sintering plant. Oil separated from the emulsion can be used for secondary lubrication purposes.
To protect soil and groundwater from unwanted discharges, a waste disposal and re-utilisation record should always be kept for emulsions, mixtures of mineral oil products and mineral oil sludges.
Spent steel pickling agents contain mainly ferrous salts. These can be separated and sold (for production of pigments, precipitation agents for clarification processes, sulphuric acid). The remaining pickling agent must be neutralised with lime milk. The resulting hydroxide sludges are placed in drying beds or preferably dewatered with filter presses. Before dumping, the leachability and stability of these residues must be checked to ensure they are suitable for final dumping. If the solids content exceeds 40%, the residues should be taken to the sintering plant.
The acid pickling water must be neutralised and the coagulated hydroxide sludges separated in clarifying tanks. The clarified wastewater can be re-used (must be neutralised with acid); sludges must be placed in a suitable, sealed dump.
Special hoods are used to eliminate oil mist in rolling mills; it is separated by a mechanical preliminary separator combined with a downstream electrostatic precipitator.
The effective noise level generated by hot and cold rolling mills is 95 - 110 dB(A). In a rolling mill the noise level, e.g. 5 metres from the open bar steel train, is 106 dB(A) and in a pipe steel rolling mill, near the tube straightening machine, as much as 124 dB(A).
To protect workplaces from noise, the plant is extensively automated and provided with appropriate control rooms. These can be well insulated against noise. Ear protection should be worn at workplaces with high levels of noise.
2.6 Foundry and forging operations
Smelting takes place in cupola furnaces (shaft furnaces) and electric melting furnaces. Gaseous emissions from smelting are: carbon monoxide, sulphur dioxide, fluorine compounds and nitrous oxides; those from casting are: phenol (briefly), ammonia, amines, cyanide compounds and aromatic hydrocarbons (traces).
Dust occurs in foundries during e.g. preparation of the moulding sand and core sand, manufacture of sand moulds and cores, in casting, cooling of castings, knocking out moulds and with the surface treatment of parts of moulds, known as fettling. Fabric filters have proved effective for reducing dust emissions. These have permitted the achievement of concentrations of under 10 mg/m3 in the clean gas from sand preparation dedusting systems. Optimum fine dust separation with fabric filters can help reduce toxic emissions, e.g. nickel, during fettling.
Dust occurring in cupola furnaces during smelting is intercepted by wet type dedusters or filtering separators. With cold blast cupola furnaces with smelting capacities below 10 t/h, wet dedusters are increasingly being replaced by fabric filters with preliminary separators. Clean gas dust concentrations of under 20 mg/m3 are being adhered to. Fluorine emissions can also be reduced by dry absorption using hydrated lime.
It is essential to intercept emissions in all operating phases, including blowing and melting-down.
With hot blast cupola furnaces with smelting capacities exceeding 10 t/h, operators have managed to obtain clean gas dust concentrations of 20 mg/m3, with blowing and melting-down as well, using optimised wet type dedusters in combination with primary measures on the cupola furnace. An enclosed forehearth feed bay also contributes to low-emission operation.
The use of induction crucible furnaces is increasing; with these, emissions from the crucible opening are intercepted by an extraction system.
When using electric furnaces, which produce significantly lower dust emissions than cupola furnaces, values of 20 mg/m3 are possible using filtering separators. Additional emissions of hydrochloric acids, soot and traces of organic compounds (possibly dioxins) occur when smelting large amounts of scrap mixed with oil, paints and plastics. A high-performance wet scrubber must be used under these operating conditions.
Highly odorous substances such as formaldehyde, phenols and ammonia occur in foundries for small castings for which moulds are produced according to the cold-box, hot-box or Croning process. In addition to the odour nuisance, these substances are also health hazards. As formaldehyde and high ammonia concentrations are suspected carcinogens, steps must be taken to reduce these. Emissions can be reduced by a counter-current scrubber with a phosphoric acid solution. The scrubbing fluid is recirculated and continuously treated.
Waste gases with inorganic compounds occur during core production, including core sand mixing. The waste gases must be cleaned with a wet scrubber and in particular the amount of amines in the waste gas must be under 5 mg/m3.
The sludge-water mixture resulting from wet dedusting, which may contain substances hazardous to health and the environment such as cadmium, lead and zinc, is neutralised. The precipitated solids are separated from the water by sedimentation. The scrubbing water is recirculated. Before dumping the sediment, which may contain phenols from the moulding sand binders, it must be tested for leachability and treated if necessary. In a suitably modified process, part of the wastewater flow can be evaporated and the circuit largely, closed, thereby considerably reducing the scrubbing water requirement.
The moulds are made of moulding sands with approx. 4 to 10% binder (clays, cement, organic materials, hardenable plastics, soda, water glass etc.). They are usually used once and then broken up. The used sands can be treated and re-used as components in clay-bonded mould production.
The ambient noise levels in foundries can reach 120 dB(A). Noise sources include loading operations, mixing, dedusting systems, fettling bays, sand preparation, conveyors and fans. Noise reduction measures include enclosed hall designs, installation of fans in enclosed rooms and silencers on air intakes and outlets. Machine soundproofing measures are especially necessary in the moulding, core and fettling shops. Measurements made over an 8 hour shift have yielded workplace noise levels of 106 dB(A) in the moulding shop, 99 dB(A) in the core shop and 103 dB(A) in the fettling shop. Principal noise sources affecting workplaces are: jolt moulding machines, vibratory grates, swing conveyors, fettling machines, impact pneumatic tools, grinders, fans, compressors and conveyors.
Appropriate noise protection measures in the workplace include encapsulation of noisy machines, separation of noisy machines from other parts of the shop and avoidance of manually operated machines. Personal ear protection must of course be worn. Monitoring is imperative.
Waste gases are expelled from the furnace in forges. Emissions can be controlled by using gas as a fuel. A forge must be regarded as an industrial installation as regards production of wastewater and waste materials.
The ambient noise level in a forging shop with e.g. 6 hammers (impact energy 0.6 to 1.3 Mpm) is 112 dB(A). The background noise level due to heating furnaces, fans etc. is already 90 to 100 dB(A); to this must be added the pulsating noise of the forging machinery. Forging hammers are louder than mechanical and hydraulic presses. It is important to maintain a safe distance between the forge and purely residential areas. This distance must be calculated and allowed for in the planning where a reasonable noise level cannot be achieved in the near vicinity through noise reduction measures in the works. The maximum noise level in the workplace of a drop hammer (1,500 kg tup weight) is 120 dB(A). That of an electric forging hammer in the workplace (tup weight 275 kg) is 97 dB(A). The interior noise level in forging shops is normally above 90 dB(A).
Possible noise reduction measures include reducing structure-borne noise by modifying the forging force curve, reducing the propagation of the structure-borne noise, encapsulating work room openings, reducing the noise from pneumatic control systems, placing silencers on air relief pipelines and using multiple tube nozzles for descaling. The wearing of personal ear defenders should be obligatory and should be monitored.
Besides noise, forging also produces vibrations. Measures to reduce vibration include the definition, at the planning stage, of suitable foundation designs, with appropriate vibration insulation at the time of installation. Vibrations in the neighbourhood must be below the threshold of perceptibility.
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