This is an in-depth research article from which you can learn about the various methods used for controlling air pollution in Industries :- 1. Foundries 2. Lead and Zinc Mining Activities 3. Copper Industries 4. Iron and Steel Industries.
How to Control Air Pollution in Foundries?
In foundry, atmospheric pollutants may be classified broadly into particulate matter and gases. Dust, fly-ash etc., are particulate matter while sulphur dioxide and carbon monoxide are main gaseous pollutants.
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During numerous stages of foundry operations, dust in large quantities is generated when preparing moulding and core sand, melting the metal, pouring the moulds, knocking out the prepared moulds, loading and unloading of dusty materials, leakages from gas pipes etc. The supreme superseding factor is use of compressed air gases for various application of foundry operations, such as cleaning of body, moulds, cores, patterns which are the main source of evolution of dust.
Apart from the above said pollutants, there are other factors such, as noise pollution and water pollution. Formaldehyde, phenol, furfural, cyanides, nitrogen compounds also pollute water and soil. Even waste and either acidic or alkaline also cause degradation of surface water and soil.
The noise level in foundry is in moulding/core making and knockout and in fitting department is considerably high and more so beyond 85/90 dB.
Control of Pollution in Foundries:
Close capture of ventilation reliably controls air pollution. The hood receives and collects the pollutants by either of the two capture principles, namely the hood is located directly above the source and the fumes rise in the thermal draft with the air current and are collected. In second case, the suction at the hood collects the pollutants;
Application of Close Capture Ventilation System of Foundry Pollution Control:
In the induction furnace, the hood system is designed to capture emissions during melt-down operations should be mounted directly on and bolted to the furnace using a transit baseplate for isolation of the hood from the furnace. Swing away back doors can be provided accessible to the furnace charging, slagging and metal inspection, this will do so through the hood-side lid door.
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There should be a side exhaust with two swivel connections and a pair of telescoping duct worthy for volume adjustment accommodating the tilting operation of the furnaces for pouring the molten metal from the furnace to the ladle.
On electric arc furnace used for grey iron steel manufacture, a large plenum or mixing chamber will connect to two pieces telescoping duct section with swivels.
The swivel assembly on the tilt center line, serves the four separate/individual hoods which are isolated/activated by automatically controlled dampers:
1. A suspended side hood for charging.
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2. An electrode hood for melt-down.
3. Spout hood for tapping.
4. A slag door for slagging/oxygen lancing operations.
Molten Metal Pouring Operations:
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Especially where toxic fumes are being generated locally low Canopy hoods should be mounted onto the ladle bail. The capture is thus localised and the exhaust requirement is maintained at a minimum. It becomes permanent part of the ladle bail assembly and could be moved from one point to another on the shop.
A great deal of dust is produced by the knocking out of castings and cores and by the cleaning of castings. When the castings are knocked out above stationary riddle the main mass of the sand falls through into a hopper or receptacle under the riddle and then onto the used sand conveyor belt. Dust is given off above the riddle and the fine dust is carried upwards by the convection currents over the hot castings. Dust is also given off under the riddle by the tailing materials.
Air Pollution Control by Aeration and Dispersion:
Proper aeration of the shop and dispersion of harmful substances by high stack are important in air pollution control. Aeration is natural ventilation system in which air contaminated with pollutants is exhausted from the building by its replacement/displacement by supply of fresh air current.
The outside air helps in dilution of the pollutants and ensures that the temperature in the working zone remains within predictable limits. As much as few million cubic meter per hour can be achieved in the foundry building by this method most economically.
Proper dispersion of various pollutants over a wider area is most important for limiting the ground level concentration. The pollutants emitted are released to the atmosphere through high stacks and get dispersed there and are carried away in the down wind direction.
Therefore the ground level concentration of the pollutants depends upon the prevailing wind speed atmospheric stability and pollutants discharge data. Aeration in the shop building takes place due to the differences in the temperature of external and internal air and wind speed pressure. Temperature difference and wind speed can cause the transfer of enormous quantities of air changes without expenditure of mechanical energy.
Dust, aerosols, carbon monoxide, sulphur dioxide are the main pollutants released from foundry stacks. Usually the pollutants are raised from stack initially and then carried away in the downward direction. The Object of the stack is to prevent ground level concentration at any point to rise above tolerable levels.
Thus, the tall stack is the tool which can be used to reduce the ground level concentration of air pollutants by discharging them at height where sufficient dispersion will take place so that ground level concentration at any point from the base of stack will not exceed allowable level.
The knowledge of the type of smoke plume under given conditions and the degree of dispersion that may be expected, enables one the design stack of proper height and to select satisfactory site for industrial plants.
Other Dust/Aerosol Pollutant Control Measure in Foundry:
i. Substitution and Process Change:
A less toxic form of the substitution may be substituted for a more toxic area, e.g., it has been found Olivine sand is not fibrogenic to animals or the human being. Hence, substituting of Olivine sand in place of river sand during moulding process can be considered. Similarly steel shots blasting operation is less harmful in comparison to sand blasting. However, hazards of silica too exists with the shot blast.
It may be noticed that this is general state of affair of having located exhaust fans much above the level of worker’s breathing zone. Hence, whenever the fans are in operation current of air passes from below upwards, creating clouds of dust around the worker’s breathing zone, which is harmful. Therefore exhaust ventilation should be planned at the level slightly below said zone.
iii. Other Dust Control Measures:
The efforts should be to control the dust at the source of generation, hence in the sand preparation plant, dust can be kept at minimum by continuous and uniform spray of water and other suitable fluid with a view to smoothening and suppressing the dust clouds. It may be advisable to cover the sand conveyor system or keep it underground.
Good housekeeping and high degree of cleanliness of the foundry floor must be maintained. Sand and dust should not be left lying on the floorings. As this is frequently agitated by the workmen as well as machines thereby suspending the already settled dust. Vacuum cleaning and wet cleaning of the floor must be carried out at regular intervals.
One point which is often saliently noticed is the cleaning the moulds, cores and other surfaces with the help of high pressure (85 psi and above) compressed air hose. This is at times dangerous, because it causes agitation and suspension of the settled dust which is usually inhaled by the workmen. Instead, negative pressure suction for cleaning purposes should be practiced for cleaning of above nature.
Otherwise the combination of both positive pressure source and negative pressure source can be attempted so that the dust so generated by the positive pressure source can be sucked in by the negative pressure source and channelised to a safe locality. It must be borne in mind that the positive pressure ventilation should not be resorted to unless alternative is possible and should be sparingly used.
There are more sophisticated ways of dust control such as electro-static precipitators. However, it has limitations such as sustained and delicate maintenance practices and high running cost and difficulties of achieving high degree of effectiveness and efficiency.
iv. Segregation/Enclosure of the Process:
Isolation and enclosures of the dusty section/process together with proper local ventilation is an important step in controlling the dust. The process like abrasive blasting, dry grinding sand plant are potentially dangerous, hence should be dealt with this technique. Sand preparation and mould box conveyor, belt conveyors may be totally enclosed.
Environmental monitoring for periodic assessment of the quality and quantity of the total and respirable dust in the environment will definitely provide basis for applying the control measures and assessing the efficiency of the existing control measures.
Automation is known for its attribute of naturally eliminating human exposure to optimum extent. Therefore mechanisation of process which evolves lot of dust like transportation of sand by conveyor belts, use of moulding machines instead manual procedures should be adopted.
Pollution Control Measures at the Source in Foundries:
To collect dust for stack gases and to prevent fire and contamination of the plant, spark arrestor of wet type or dry type is advised for installation on the cupola stack. The arrestor is expected to control the emission of dust.
Further, air pollution from cupola can be reduced by using the following methods:
1. By making use of quality raw material and with proper treatment thereof.
2. By improving the process and making design modification of cupola, if necessary.
3. By using unconventional fuel instead of conventional coke containing high ash content.
In the arc furnace clean and heavy melting scrap charge is advised. The dust evolution is expected to reduce to half by half by these measures.
In the Induction furnace too, the furnace using clean scrap found to have evolved reduced emission of particles and iron oxide.
In the knock-out grid, the type of exhaust hood has a great effect on the efficiency of dust extraction. The most effective would be one having grating from all sides and with apertures for admitting mould boxes for removal of castings. It must be sufficient to draw off 5000-8000 meter of air per hour per meter of grate area.
Where the castings may be knocked out of the moulds and loaded on to an underground conveyor, the dust laden air may be drawn off from below where stripping of boxes are done. The use of lateral hood is recommended. With such lateral hoods the amount of dust in the air is ranging from 1.2 kg to 1.5 kg/m3.
In sand handling system, the dust elimination can be better achieved by creating negative pressure. The dust is maximum at various transfer points located at sand mixtures, flat sieves, belt conveyors etc.
While pouring the moulds by liquid metal, carbon monoxide is evolved and continues even during cooling and upto solidification.
It is therefore necessary to have mechanised ventilation such as man- coolers to ensure air changes as per requirement:
1. In case it is possible to use fabric arrestor type of dust collector for shot blasting chamber, it will reduce the dust concentration to desired value. The recommendation of mechanical pre- cleaners to reduce the dust loan on collector is often made.
2. While performing the welding operation, to remove the smoke and fumes the exhaust booth should be provided.
In heat treatment furnace, the pollutant carbon monoxide and sulphur dioxide can be dispersed into upper atmosphere by making provisions of high stack and also ground level concentration can be brought to permissible limits.
In pattern shop, the central exhaust system with a mechanical type of dust collector such as a cyclone, would be sufficient to eradicate particulate matter because of wooden dust. The built up hood arrangement for dust collection may be provided on wood working machines such as saws, planers, disc-sanders, drilling machine etc.
Use of Personal Protective Equipment’s for Pollution Control in Foundry:
The personal protective equipment is the second line of defence. Situations requiring the use of personal protective equipment is dangerous to life, may at times be immediate and the work cannot be performed without the use of right type of respiratory equipment. This assumes utmost importance when there is an emergency repairs such as attending to leakage or bursting; resulting into release of high concentrations of gases, liquids, vapours, fumes or mists.
Use of suitable respiratory personal protective equipment is also required with a view to carrying out inspection or cleaning inside the closed vessels or other confined spaces, suspected of containing or having contained toxic vapours, gases, fumes etc.
It also cannot be ruled out that certain usual and normal operations such as spray painting, shot blasting inside the chamber, cleaning of drain lines, constant use of respirator is a constraint, but there are number of operations wherein the respiratory equipment is used intermittently during the shift.
Design, selection, operations and constructional features of respiratory equipment plays very important part in air pollution control in foundry.
In the control of pollution, use of air purifying, breathing apparatus as a personal protective equipment is prescribed. Amongst these, the Canister gas mask is most predominant. The Canister is designed for a specific contaminant and it is therefore very necessary to use the appropriate Canister. Apart from proper label of the Canister, it is standard practice that Canisters are given a particular colour coding specified under BIS: 8318:1977.
It is likely that the colour coding may be different in Canisters imported from the different nations and hence care is necessary. The expiry date marked on the Canister should be properly and cautiously seen. Only valid one should be used. This is in view of the fact that chemicals may lose their effectiveness over a period of time. Canister mask can be used in the atmosphere not containing more than 2 per cent by volume of contaminant.
The life of Canister will depend on the type of Canister, the concentration of the gas in which it is used and the activity of the wearer. It is important to keep a record of the duration to the Canister used from time to time and also to replace the same when not in use.
Special mention is required to be made in respect of the personal protective equipment for the operator in sand blasting chamber as it needs a protective helmet of a type approved by the chief inspector of factories and the helmet should supply air at a rate of not less than six cubic feet per minute and such air should be free from fumes or mist of mineral.
Suitable dust proof goggles and boots should also be provided to operator. These are all statutory provisions.
How to Control Air Pollution from Lead and Zinc Mining Activities?
Environmental Problems Faced due to Lead and Zinc Smelting:
Lead-Zinc mining activities have so far been limited to underground operations. Open cast mining as envisaged for lead-zinc will have a great impact on the environment. There are also lead-zinc ore deposits in various parts of the world which need a careful environment management plan to be established before exploitation is attempted in such ecologically sensitive hilly terrains.
The environmental impact problems in the mining industry are generally classified as follows:
1. Visual impact.
2. Subsidence impact.
3. Noise pollution.
4. Ground vibration.
5. Air pollution.
6. Water pollution.
7. Deforestation.
It is now obligatory to prepare a proper environment management plan before any mining project is allowed.
1. Visual Impact:
The visual impact of a mine relates to changes in the topography caused by surface excavations, dumping of waste rock, ore and other overburden and improper impounding of waste slurries.
On abandonment, such mine areas appear as sore scars on the land surface unless due care is taken, right in the beginning for preserving top soil which can be used for land reclamation.
It is caused in mining operations and results in extensive degradation of natural environment which can affect ground water regime, surface drainage pattern and other man-made structures like highways, buildings, bridges, etc.
It is a major problem associated with mining and mineral processing operations involving blasting, drilling, movement of mining equipment, ventilation fans, and heavy earth moving machinery, crushing and grinding equipment, large vacuum pumps, compressors and generators. Care must be taken for sound proofing each installation to the correct tolerable noise levels particularly where people have to work very close to such noise generating equipment. They also have to be provided with proper ear muffs to avoid noise related occupational health problems.
It is caused by blasting, deployment of heavy mobile equipment, rock bursts, etc. and can result in structural damages on the surface. Careful planning has to be done at site surface installations away from areas influenced by ground vibrations.
It is caused by mining operation like drilling, blasting, mucking and movement of ore, crushing and screening of ores, as these create dusty conditions. Inhalation of the fine particulate matter so generated results in its deposition in the respiratory system proves injurious to health. It is therefore, necessary that mine workers wear protective masks when actually carrying out the mining operations.
In major installations, proper ventilation is to be provided in the building with a scrubbing arrangement to contain the dust at the permissible level in the ambient air and in the emissions let off through the chimney to avoid its effect on the adjoining vegetation and water bodies.
It needs check through proper treatment of mine water and reclamation of water is to be done to the maximum possible extent to keep surface discharges minimal. The contaminated water is recirculated for use.
It is a major problem in the mining area particularly where open cast mining is to be done. The growth of a mining hamlet, even in a remote area, brings in urbanisation and a pressure for consumption of wood, unless the colony is provided with alternative fuel supply arrangements.
The formation of surface roads and mine roads essential for mining, invariably affect the environment and ecology of the area. Proper plans for afforestation/reforestation are needed to be drawn as far as practicable to prevent soil erosion.
Environmental Problems and Control Measures in Lead and Zinc Smelting:
The processes of lead and zinc smelting from sulphide concentrates have inalienable environmental problems relating to air and water pollution. The first step in the smelting process is the conversion of the sulphides into the oxides.
In the case of zinc, roasting of concentrates is carried out to produce rich sulphur dioxide bearing gases, while the zinc sulphide minerals are transformed into the zinc oxides. The sulphur dioxide gases are cleaned, cooled, and converted into sulphuric acid to minimise SO2 emissions into atmosphere.
In lead smelting, the lead sulphide minerals are sintered in the first stage. But sulphur dioxide strength in the off gas is low and requires very special treatment for production of sulphuric acid. The handling of concentrates, sinter and calcines results in emission of dust and particulate matter in plant areas.
Good ventilation and scrubbing is necessary to control ambient air quality and to provide protection to the men. However, the process of cleaning ambient air, by ventilation and scrubbing, results in liquid acidic effluents which need to be treated for neutralisation.
The downstream hydro-metallurgical treatment for zinc leaves behind leach residues and other process wastes, which will have to be stockpiled carefully after treatment, as in the case of the mill tailings in the mine, to prevent surface water and ground water contamination.
The lead blast furnace, where coke is used as a reductant, generates poisonous carbon monoxide, which needs careful monitoring of the atmosphere to prevent leakages and keep concentration of emissions well within the threshold limits.
The solid residues like slag from smelting operations are relatively unharmful, since they can be stored safely to prevent any water contamination. Melting and casting of zinc, lead, and cadmium demand adequate ventilation to keep away metallic fumes from work place. Noise pollution in the industrial installations of the smelter can be kept at the desired decibel levels by providing proper insulation.
Some of the measures taken for prevention and control of pollution in lead-zinc smelters are:
1. Modern gas cleaning system consisting of cyclones, scrubbers and electrostatic precipitators are used to control the emission of particulate matter in roasting plants of zinc smelters.
2. Dust recovery systems consisting of multicyclones, venturi scrubbers, tray separators, electrostatic precipitators have been provided for cleaning gases in lead plants.
3. Emission of sulphur dioxide from roasting and sintering operations are controlled by providing suitable sulphuric acid plants. Some of the sulphuric acid plants are based on modern technology of Double Conversion-Double Absorption (DCDA) to increase conversion efficiency.
4. Cyclones are used for removal of dust and soda ash scrubbing and absorption of SO2 from gases evolved in the roasting process.
5. Modern effluent treatment plants based upon the process of two stage precipitation and filtration for recovery of acidic and metallic constituents, have been provided at the zinc smelters.
6. Most of floors and drains carrying untreated effluent in the plants should be lined with flexy vinyl material, which is impervious in nature. This reduces seepage of such effluents to underground water bodies.
7. A mercury removal plant at smelter should control movement of mercury to the effluents and sulphuric acid.
8. Other solid residues, slag etc., are stored in separate areas and are given to possible users in the nearby areas.
9. In addition to the facilities available for control of pollution, all the units should have a well-developed environmental monitoring cell fully equipped with modern instruments like continuous pulsed fluorescent SO2 analysers, high volume samplers, ion specific meters and electrodes, besides other common laboratory equipment. Regular and independent monitoring of gaseous emissions, effluents and well water in the vicinity, enables the management to exercise an effective control over emissions.
Threshold Limits and Health Effects of Lead and Zinc Smelting:
Effects of lead and zinc on human health have been well documented. Cognizable symptoms of poisoning are known to occur in some individuals when levels of lead in blood stream exceed 70-80 mg/100 ml.
Workers exposed to lead fumes or lead compounds especially in battery and paint industries are likely to have more intake of lead mainly by inhalation and to some extent by ingestion due to inadequate hygienic conditions. On an average 30 per cent of inhaled lead is absorbed into the body. About 79-80 per cent of this is excreted through urine and 15 per cent as gastro intestinal excretion.
Zinc is less toxic. It is an essential metal for normal development of bulk aquatic and terrestrial plants. Use of zinc sulphate as a fertiliser to overcome zinc deficiency in acidic soils is well known.
Concern about cadmium arose during 1950’s in Japan, where high intake of cadmium was found to be responsible for the disease ‘Itai-Itai’. However, this painful disease has not occurred in other areas having cadmium pollution. Accumulation of cadmium in the body over a long period may result in impairment of the renal function. Threshold limit for cadmium as dust or fume is 50 mg/m3 for 40 hr./week exposure.
Most of the intake of these metals by living organisms is through drinking water. Due to the toxic nature of these metals and being hazardous to health, their discharge by industries need to be controlled very carefully. The cadmium is recovered in a separate column along with the cadmium sponge from the cadmium plant treating dust from the hot gas precipitator of sinter plant.
Metallic lead produced in the furnace is tapped periodically from its bottom together with the slag formed from the gangue constituents of the charge. Separation of the lead bullion from the slag is carried out in a fore hearth after which the slag is granulated. The lead bullion after decopperisation is electrolytically refined to 99.99 per cent purity. The copper drosses from decopperisation will be processed to produce copper through a hydro- metallurgical route.
Facilities for processing of anode slimes and drosses to recover silver, gold, bismuth, arsenic, antimony and tin are being considered. Mercury will be scrubbed from the sintering off-gases as mercury sludge, to be processed further to produce metallic mercury.
Air Pollution Control Systems:
Two types of gaseous emissions in the sintering process may be identified:
(i) Process gas from the desulphurising operation containing amongst other things, sulphur dioxide and
(ii) Hygiene ventilation air from the work places and containing dust and fume.
Process gas contains carbon monoxide and carbon dioxide and the hygiene ventilation air contains dust and fume of lead, zinc and cadmium metal/oxides. Elaborate gas cleaning and hygiene ventilation systems have been proposed for meeting the laid down standards and maintaining safe work atmosphere.
Sinter plant rich gases will be cleaned by combination of hot electro-static precipitators, venturi scrubbers and wet electro-static precipitators. The lean gases from tail end are recirculated in sinter plant to enrich SO2 concentration upto 6 per cent. A DCDA sulphuric acid plant is installed for converting SO2. The plant meets the environmental standards for gaseous emissions of SO2 as per BIS-635-1972 to a limit of 4 kgms of SO2, per tonne of sulphuric acid produced.
Furnace gases, after condensation of zinc vapours, are scrubbed clean in a tower and passed through a Theisen disintegrator to remove all particulate matter to yield clean low calorific value off gas (18-23 per cent CO) to be utilised as a fuel in pre-heating of coke and air and in zinc refluxing. The surplus gas will be used to operate a boiler to generate steam for providing power. A flare stack provided would burn any surplus LCV gas after utilisation so that no CO is allowed to join the atmosphere.
Bag houses are used for cleaning dry gases and venturi scrubbers for wet gases with large particulates. There is a main stack (about 60 m tall) for acid plant. The height and design of various stacks are such that ground level concentration of the gaseous pollutants remain within the allowable limits prescribed by the Air (Control & Prevention of Pollution) Act 1981. Lead content in the ambient work place will not exceed 150 mcg/nm3. The individual stack dust emissions will not exceed 50 mg/nm3 with a lead content of not more than 11.5 mg/nm3.
Mercury removal plant should be installed to keep mercury below 1 ppm in the acid and liquid effluents so as to avoid mercury passage to the bio-cycle through acid being used for fertiliser manufacture.
In the case of lead refinery, generally pyre and electro-routes are preferred. Electro-refining is opted whereby the dust/fume emissions will be minimised.
In-Plant Pollution Problems:
Monitoring of in-plant atmosphere is carried out continuously using static sampling equipment at different locations. A 300 meter wide green belt is developed around the smelter complex to facilitate monitoring pollution effect.
Inhalation of poisonous chemicals like sodium cyanide and copper sulphate and the deposition of toxic metals like lead and zinc in lungs, blood and kidneys are harmful for workers. Proper safety measures should be taken in the plant for handling of chemicals and provision of safety appliances to workers made besides installation of efficient exhaust systems for reagent preparation sections.
Dust is generated during crushing and screening operations which can cause damage to lungs. It is suppressed by water sprinkler at material transfer points. Also, an integrated dust collection unit like cyclone and scrubber should extracts dust efficiently before discharge through chimneys in crushing plants. For minimising noise level, suitable rubber liners should be provided for equipment and chutes. Ball mill trommels are covered besides use of ear plugs and muffs by the operators working in critical areas.
No effluent should be allowed to flow out of the mill premises and where ever possible should be recycled back to process streams. Effluents generated due to power failures and at the time of shutdown of the plants, are allowed to settle in pits before discharging to drains. These pits should be emptied regularly. Noise level, quantity and quality of effluents and dust levels should be monitored regularly and measures taken to improve the system.
How to Control Air Pollution from Copper Industries?
A typical flow sheet for copper extraction is given in Fig. 17.8. The extraction of copper ore generates considerable quantity of solid waste. After the ore is taken out, the ore is crushed, milled and subjected to floatation for separation of copper rich minerals. Bulk of the tonnage treated is separated as waste material in the form of slurry which needs disposal. The copper rich phase, called concentrate, is then treated in a smelter to produce copper. The problem at this stage is the disposal of sulphur containing gases to prevent pollution.
The impure copper, called anode copper is refined in electrolytic cells to produce 99.99 per cent pure copper. However, in this process the impurities get accumulated in the electrolyte. A part of the electrolyte has therefore, to be discarded to maintain a low level of impurities. The disposal of this blend electrolyte is a major problem for the plant.
The electro-refined copper (called cathode) is melted and cast into wire bars. During this melting, some amount of fumes are generated which also cause pollution. These fumes are, however, recovered easily in electrostatic precipitators, bag houses or wet scrubbers without causing much problem.
The copper wire bar is then rolled into wire rods of 3/8″ and is pickled to remove the copper oxide scale. The dissolved copper oxide in the form of copper sulphate is another source of pollution. However, this solution is electrolysed to remove copper and the acid is recycled for pickling. Some sludge, a very small quantity, is easily neutralised and discarded.
Sulphur Problem:
Most of the copper in the earth’s crust occurs as complex sulphides. The desired mineral after concentration is treated through pyro-metallurgical route. In smelting, the concentrates burn which give rise to formation of sulphur dioxide. The sulphur dioxide containing gases can cause damage to vegetation and create extensive ecological problems in the long run.
The problem is to fix sulphur, in a form like liquid sulphur dioxide or sulphuric acid or any other useful sulphate economically. The air pollution laws world over are forcing the copper industry to take necessary measures in this direction regardless of economics. The main technological constraint facing the industry is that a rich steady stream of SO2 is not available for plant operations.
As such off gases available cannot be economically used for sulphuric acid production. Moreover, the conventional reverberatory furnaces or even flash furnace-converter combinations need stringent operational controls to supply a high grade SO2 bearing gases.
The processes developed have covered the following alternatives:
1. Retrofitting/adopting old reverberatory process with top oxygen lancing.
2. Use of tonnage oxygen in furnaces to give richer gases (Oxygen enrichment of air in flash furnaces).
3. Develop new pollution effective continuous technologies giving a steady stream of high SO2 bearing gases (e.g., Mitsubishi process).
4. Develop new control equipment for sulphur recovery.
While the first three options concern the modification of the metallurgical plant, the fourth option concerns the development/use of technologies in the sulphur recovery plants.
The costs and benefits of different technologies are decided by various costs of investment under local set of conditions. But the present trend is to use high tonnage of oxygen plant or install new technologies involving use of tonnage oxygen (either enrichment or pure oxygen). The advantages of using this are very obvious.
The gases generated have a very high sulphur dioxide content. This could with, lesser quantity of gases, make the SO2 abatement much easier and less expensive. Even from process and operational point of view, higher production capacities and better thermal efficiencies are possible with the use of oxygen.
Today the sulphuric acid plant, far from being a by-product unit, has become a critical factor which effects the smelter operation. Often the smelter has to be shut down if the sulphuric acid plant is not operated efficiently. It is no wonder that such an extreme step is warranted as a drop of 5 per cent efficiency for say a 600 MT/day sulphuric acid plant would introduce about 30 MT of sulphur bearing gases, primarily sulphur trioxide, in the atmosphere.
Whereas the technology for producing sulphuric acid from sulphur dioxide gases is well known, many new modified processes have been opened up in this area.
Some of these are:
1. Double catalysis double absorption technique.
2. Production of sulphur dioxide liquid by refrigeration.
3. Production of elemental sulphur by reducing the sulphur dioxide by oil or natural gas.
4. Production of a rich stream of sulphur dioxide and mixing it with the weak stream to produce sulphuric acid economically.
5. Removal of SO2 through scrubbing.
The factors which decide the choice of these technologies are:
1. Nature and continuity of gas stream.
2. Location of markets for the by-product.
3. Cost of operation (especially energy).
4. Environmental control standards and penalties.
5. Reliability of technology and availability of basic raw materials (in some cases). inspect
Simultaneously, attempts have been made to add some new technologies to the existing plants all over the world to control the emission limits.
Some of the ways in which the weak gas streams can be controlled are:
1. Gas blending—use of sulphur burner.
2. Removal of SO2 through absorption process.
3. Concentration of SO2 through a regenerative process.
Control of emission limits, in fact, is a vast area of technology.
However, some of the few measures taken are:
1. Sulphur dioxide absorption process employing ammonia.
2. Battersea process where very dilute aqueous solution of alkaline salts is used to remove SO2 by solution and oxidation.
3. Cyclic lime process where flue gases are washed with a slurry of calcium sulphate in water, which is kept alkaline by addition of lime. SO2 is absorbed and the salts of calcium are precipitated.
Sulphur Dioxide Emissions from Copper Smelters and their Control:
One of the serious problems facing the copper smelting industry is control of gaseous SO2 emissions. Sulphur is a major component in the concentrate fed to the smelting processes varying in the general range of 25-35 per cent. The mass ratio of S to Cu varies between 0.8-1.6 in copper concentrates.
Sulphur also occurs in all fossil fuels used in copper smelting. During various unit operations, sulphur is oxidised in SO2, although small amount of SO3 may be formed under certain conditions of temperature and oxygen partial pressure. SO2 may also get oxidised in the atmosphere to form sulphates and particulate forms of sulphur compounds. These are major contributors to air pollution.
SO2 emissions, through the formation of acids and other compounds in the atmosphere can cause damage to human health, vegetation and property. High levels of sulphate concentrations and long term exposure to sulphates are said to aggravate asthma, lung and heart diseases. Sensitive vegetation can be severely damaged even by low levels of SO2 in the atmosphere.
Studies made in USSR during the past decade showed that pine trees growing in an atmosphere having SO2 concentrations of 500 mg/m3 had a growth loss of 48 per cent in comparison with pine trees growing in an atmosphere free of SO2.
Many materials such as metal surfaces, marble, brick, stone work, plastics, rubber, paper become discoloured and brittle when exposed to SO2. Thus buildings, bridges, steel girders, automobiles and highways are all affected by excessive SO2 emissions.
Air Pollution Control Regulations:
Many countries where copper smelters are located have formulated strict air pollution control standards for SO2 both for ambient air quality and for allowable emissions and there is increased awareness regarding the harmful effects and dangers of such pollution. In USA, primary and secondary standards have been established.
Primary standards (80 mg/m3 annual arithmetic mean) which protect the public health, define how clean the ambient air must be so that it will not be harmful to human health. Secondary standards (60 mg/m3 annual arithmetic mean), which protect the public welfare, define how clean the air must be in order to protect against the known or anticipated effects of air pollution on property, materials, climate, economic values and personal comfort.
The conventional pyro-metallurgy of copper involves a number of unit operations such as roasting, smelting and converting. The resulting strength and volume of the gas streams emitted by each of these units differ considerably. Table 17.1 gives comparative gas strengths in some typical unit operations and different smelting technologies.
The SO2 contained in off-gases is at the high end of a range in processes where oxygen enrichment is used. Figures at the low end of a range of Table 17.1 represent the worst case of ingress of dilution air. Processes employing oxygen smelting have a gas strength around 80 per cent SO2. Because converting is a batch process having two distinct blowing cycles, the gas flow is discontinuous and varies in strength within the range shown in the Table 17.1.
As removal of sulphur from the molten bath progresses, the SO2 content of the gas falls and approaches zero for each individual converter. In a multi-unit operation, fluctuations in gas strength are evened out by careful scheduling of converter operations which enables a gas of nearly constant volume and strength (5 per cent SO2) to be delivered to an acid plant.
Emission Control Technologies:
It is first necessary to remove the dust or particulate content as completely as possible before recovery of SO2 from the smelter gases. In older smelters, primary collection of dust is effected in settling chambers or balloon flues where the low gas velocity and hence long residence time, allows gravity settling of large particles.
In modern smelters, high velocity flues and cyclones have replaced the balloon flues. Final gas cleaning in the smelter is carried out in electrostatic precipitators, where over 99 per cent collection efficiency is achieved.
SO2 emission control technology in the copper smelting industry has been developed essentially to treat two types of gas streams:
(i) Concentrated gas streams which arise from fluid bed roasters, primary smelting furnaces and converters where the gas strength is more than 4.5 per cent SO2; and
(ii) Dilute gas streams (containing generally less than 2 per cent SO2) which arise from multi-hearth roasters, reverberatory furnaces, fugitive emissions and tail gases.
Concentrated Gas Streams:
Concentrated SO2 gas may be captured and fixed as elemental sulphur or alternatively used in the production of liquid SO2 or sulphuric acid.
Weak Gas Streams:
These originate from the following sources:
1. Process gas streams (SO2 content 0.5 to 2 per cent) such as from multiple hearth roasters, reverberatory furnaces, fire refining furnaces etc.
2. Tail gas emissions are from sulphur fixation plants treating, in general, concentrated gas streams.
3. Fugitive emissions (1 to 2 per cent of the total S) arising from transfer of hot calcine from multiple hearth roasters to smelters and leakage through furnace refractories.
Technology for the control of weak SO2 gas streams has been largely developed for thermal power plants, and has not yet become popular in metallurgical industry for the following reasons:
1. The flue gas desulphurisation (FGD) systems are not yet commercially proven.
2. A stable product generally cannot be produced economically.
3. Systems are expensive to install.
4. SO2 control conditions are not that stringent at some of the places.
The major source of weak uncontrolled SO2 emissions from smelters is the off gas generated by reverberatory furnace. For any given plant, this can be from 9 to 34 per cent of the total sulphur input depending on whether the furnace is charged with calcine or concentrate, respectively. Construction, operation and maintenance of reverberatory furnace are important factors in concentrating SO2 content of the gas.
Minimising dilution effects by sealing all openings in the furnace, oxygen enrichment of combustion air and more uniform charging practice can increase the gas strength to about 2.5 per cent SO2. Using oxygen enrichment to 60 per cent O2 in air can increase the reverberatory furnace gas strength to between 5.8-7.3 per cent SO2.
In the context of control of weak gas emissions the following two approaches should be adopted:
1. Increasing the concentration of SO2, by using a regenerative system for subsequent processing to sulphuric acid, elemental sulphur or liquid SO2. Absorbing media used are MgO, citric acid, low temperature water or ammonium bisulphite. The Cominico process which uses ammonium bisulphite as absorbent can achieve high efficiencies of SO2 removal over a wide range of SO2 concentrations well within that encountered by copper reverberatory furnaces.
2. Non-regenerative absorption system based on neutralising SO2 by scrubbing to produce a stable waste product. Important scrubbing systems developed are the lime scrubbing, Palabora scrubbing and double alkali processes. In lime scrubbing system, the inlet gas containing 2.5 to 3.2 per cent SO2 is washed and cooled in sea water fed gas coolers thereby forming sulphurous acid which then reacts with lime to form calcium sulphite slurry.
This is then oxidised to yield calcium sulphate (gypsum) which can be utilised for cement manufacture. In Palabora process, the hydro-separator over-flow from the concentrator, which contains alkaline calcium and magnesium carbonates, is used as a scrubber. Vallerite containing around 22.9 per cent Cu is recovered. Several technologies go under the designation of the double alkali process.
One is the ‘concentrated double alkali process’ which is installed at various small industrial steam plants. The scrubbing medium ‘sodium sulphite’ is converted to sodium bisulphite by reaction with SO2. It is then reacted with slaked lime to produce calcium sulphite which is centrifuged to a fine cake with 60-70 per cent solids.’ Sodium-lime double alkali’ process uses lime to regenerate the scrubber and also produces a waste calcium sulphite cake.
How to Control Pollution from Iron and Steel Industries?
The iron and steel industries usually involve large corporations. The industries range from new modern facilities to those built early in the century. In most of the countries the older facilities seem to be dominating. The use of somewhat outdated facilities, compared to those of postwar II facilities, has led to the difficulties in controlling emissions.
Pollution Caused by Iron and Steel Industries:
Japanese and Germans chose to install small units using the most modern technologies after the war. Since then, they have continued to expand these facilities and are now of a mammoth magnitude. The large plants require enormous volumes of water and air to produce iron and steel.
The five major stages of processing include:
1. Coke making.
2. Iron ore concentration.
3. Blast furnace treatment.
4. Steel production.
5. Rolling and other shaping processes.
The pollutants generated are very diverse and encompass the full range: air and water pollution and solid wastes. Because of the complexity of the steel industry, the associated environment problems for each process will be considered separately in this section.
Air pollution is the release of waste gases or odours from a biological or chemical process which contain substances which can be considered harmful to human life and comfort, either because they are toxic, reduce the oxygen available for sustaining life or because they are aesthetically undesirable.
Sources of Air Pollution in an Integrated Steel Plant:
The air pollutants may be divided into two major categories:
1. Suspended air pollutants or particulates in air.
2. Gaseous air pollutants such as sulphur oxides, nitric oxides, carbon monoxide and hydrocarbons.
Table 17.2 indicates sources of particulate emissions in an integrated steel plant.
Approximate amounts of dust emissions in certain areas of an integrated steel plant are given in Table 17.3.
Air Pollution Control Practices and Equipment:
Ground level concentration of contaminant is now recognised by air pollution technologists as the controlling factor rather than the older concept of contaminant concentration in a stack.
The dust generated in the sintering plant can be returned to the process. Because of this, most plants are atleast equipped with cyclones. Dry type cleaners are best suited for the cleaning because the sulphur content of gas streams can lead to corrosion problem in wet system. Cyclones, electro-static precipitators, venture scrubbers and bag houses are used in various combinations at the various points of emission.
Gaseous and particulate matter released as a by-product in coking operation except that which escapes from ovens to the atmosphere are conveyed in ducts to a coal chemical processing plant for recovery of chemicals. Emissions occurring from handling operations present dust contaminant problems which are difficult to control. Emissions during charging can be minimised by minimising the openings through which smoke can escape and by creating a slight vacuum inside the oven during charging so that air flows into the openings instead of smoke having come out.
The quenching of coke produces a rising cloud of steam in the chimney which lifts coke dust into the atmosphere. Most of this dust appears to fall out in the vicinity of the quench tower. Baffles installed in a quench tower can reduce the dust emission into the atmosphere by about 75 per cent. A schematic of typical operations of iron and steel processes along with pollution status.
iii. Blast Furnace:
Blast Furnace gases contain about 0.2 tonne of dust per tonne of pig-iron produced.
Blast furnace gas is cleaned in three stages:
1. Preliminary Cleaning – Settling chamber or dry type cyclones.
2. Primary Cleaning – Wet scrubbers.
3. Secondary Cleaning – Electrostatic precipitators or high energy scrubbers. Open hearth furnace
iv. Open Hearth Furnace:
The small size of the particles emitted from open hearth furnace requires high efficiency collection equipment such as venturi scrubbers and electrostatic precipitators.
v. Basic Oxygen Furnace:
The basic oxygen furnace creates more emissions than the open hearth furnace and the particles are smaller. All basic oxygen furnaces are generally equipped with high efficiency electro-static precipitators or venturi scrubbers. In some countries fabric filters have also been in use.
Electric arc furnaces are becoming more popular for many metal melting operations. Particulate emissions from electric arc furnaces are difficult to collect because of their small size and because of a strong tendency to adhere to fabric surfaces, a high angle of repose and high resistivity.
However, over 95 per cent collection can be achieved with appropriate hooding and high efficiency collection equipment. The characteristically small particle size of electric arc furnace fume precludes the use of dry centrifugal collectors, settling chambers etc. High efficiency scrubbing systems, electrostatic precipitators and bag houses are used to control fumes in electric arc furnaces.
The majority of the world investment in air pollution control has been directed mostly towards the collection of suspended particles. The reason for this could be that collection techniques for these are simpler and also that such collecting equipment has been well developed and proven. When compared to particulate air pollutants, the gaseous air pollutants are more dangerous.
The gaseous air pollutants are sulphur oxides, nitrogen oxides, hydrogen sulphide, carbon monoxide and carbon-dioxide etc. In the steel industry coke-oven gas and Blast furnace gas are cleaned for their further use in the plant. In recent years more attention has been paid to control the emissions of these gaseous pollutants particularly SOx and NOx into the atmosphere by adopting desulphurising and denitration technology.
Pollution in the Conventional Iron and Steel Making Processes and an Alternative Supplement:
The classical route of steel making in an integrated iron and steel plant continues to be the main method of production of steel. However, this route suffers from certain drawbacks like environmental pollution mainly due to units like coke ovens, sinter plants, etc., the economic viability at large capacities and dependence on superior grades of raw materials.
While quite a good number of pollution control measures are available for these plants, the application of such measures pose economic and infrastructural problems. Further, the industry faces pollution problems because of yet another inherent factor on account of its large scale of operations.
In order to overcome these problems and at the same time, sustain a balanced growth of the industry, a suitable alternative could be found in a ‘Direct Reduction’ process using non-coking coals. This process would supplement the production of iron through the blast furnace. Some aspects of the pollution in an integrated iron and steel plant and the techno-economics of a suitable direct reduction alternative, therefore need to be discussed.
Pollution Due to Steel Industry:
The development of steel industry is associated with a cumulative injury to the environment and the quality of life through pollution of air, water and land. Noise and thermal pollution are also felt almost throughout any integrated steel plant.
It is an acknowledged fact that steel industry discharges sufficient quantities of toxic chemicals such as acid, ammonia, phenols, tar acids, cyanide, chromium, sulphur-containing compounds and others. Some of these pollutants deplete oxygen from the water and destroy the aquatic life necessary for ecological balance.
Even when present in trace quantities, some substances render the atmosphere toxic. The qualitative and quantitative nature of the pollutants, present in trace quantities, comes under the purview of trace quantity engineering.
The important air pollutants in any integrated steel plant will be smoke, dust, fume SO2 and H2S.
The location and climatic condition around the industry have a fundamental influence on the transport and diffusion of air pollutants. Among these, meteorological factors, wind direction and speed, duration, frequency and intensity of rainfall, humidity and temperature are of significance.
In order to combat the menace of air pollution and control harmful fumes and other gaseous discharges produced by the processing and production units in an integrated steel plant, several types of equipment are being employed.
Air and gas cleaning equipment such as dust collectors, cyclones, settlers and scrubbers, smoke stacks with leak proof vents and shutters, quenching baffles and towers, fans, precipitators, suction and vibrating screening devices, filters, diffusers, desulphurisers, absorbers, spray washers etc. are being extensively used throughout. If proper maintenance and repair of this equipment is not undertaken, the cleaning efficiency will be low thus permitting more pollutants into the atmosphere.
Corrosion of Air Pollution Control Equipment:
Corrosion resistance of materials of construction has an important bearing on the design of air pollution control equipment. This is particularly important in view of the fact that corrosion is accelerated by high gas inlet temperature, high velocities etc., commonly encountered. New methods like use of activated carbon in air pollution control techniques may result in a better environment.
Noise Pollution and Thermal Pollution:
Noise pollution also involves energy waste in a large scale activity, like waste heat, that causes thermal pollution, and waste light. Noise is now being recognised as a contributor to a wide variety of human disorders and it is desirable to incorporate noise control equipment at the design stage itself.
The very fact that about 4 million kilo calories (gaseous fuels) are required to produce an ingot tonne of iron suggests that the enormous amount of heat energy involved causes thermal pollution. Modern technological methods in waste heat recovery and insulation would help in solving not only thermal shock problems, but would result in conservation of energy as well.
Conditions for Direct Reduction:
In countries like India (and some others like Brazil, New Zealand, etc.) there are large reserves of iron ores and non-coking coals but natural gas or oil are in short supply. The total reserves of iron ore in India are estimated to be over 23000 MT, spread over different parts of the country.
Indian iron ores, though rich in iron (62-66 per cent) and relatively in-expensive, have an unfavourable alumina to silica ratio of 1.8 to 2.5: 1. Such high ratios not only result in high alumina slags but also give rise to problems for silicon control in Blast furnaces.
Compared to the abundant reserves of iron ore, coking coal in India is in short supply. Although the total coal reserves are quite sizable (about 1,10,000 MT, only about 25 per cent are of prime, medium and semi-coking variety, which occurs predominantly in the eastern part of the country.
Even these meagre and localised coking coal deposits are high in ash content (25-28 per cent or even more). To make this high ash coal amenable to coke manufacture, it has to be washed to 17-18 per cent ash level prior to carbonisation. As such after accounting for mining and washery losses, the usable coking coal reserves of all the three varieties are not expected to be over 6000 MT.
Energy Requirement Considerations:
The world coal deposits represent the largest reservoir of energy still available to mankind. Today, industrial growth of a country depends largely upon sufficient supply of energy at reasonable price levels. While natural gas and oil represent only a small part of the world energy resources and that their availability will become limited in near future.
By the year 2010, the gas resources are expected to be reduced by 23 per cent and oil by 43 per cent, while the coal reserves will be reduced by only 1.4 per cent. Increased use of solid fuels, therefore becomes mandatory and this trend has already set-in-since the oil crisis in 1973.
Table 17.6 shows various alternate processes (both solid and gaseous fuel-based) along- with their operating temperatures. The operating temperature seldom exceeds 1200°C which means that, because of the fairly low temperature of operation, such processes require relatively lower levels of energy input.
An integrated Steel plant is a large industrial complex involving different types of process operations from coke making to steel finishing leading to different types and quality of pollutants.
Most of the steel plants have been provided with necessary control facilities during their installation but, due to ageing of the equipment and changes in the quality of raw material and intermediate products and changes in the environmental protection regulations from time to time, it has become necessary either to modify the equipment or to modify the technology to comply with the statutory regulations.
Emphasis is now being given to the conservation of resources like air and water, and stringent regulations have been laid down in western countries for the quantity and quality of polluted water discharged into the rivers.
Recycling has been one of the many processes adopted to reduce pollution in the steel industry. Over the years, several methods involving recycling of air, gas, water, solid wastes with or without pre- treatment have been practiced.
The air pollution recycling aspects are discussed below:
Dust emissions are inherent in most processes of the steel industry. Dust laden gases are emitted in the operation of coke ovens, sintering plants, Blast furnaces, steel making furnaces, etc. Some of these gases also contain certain gaseous pollutants which need to be controlled before their emission to the atmosphere.
ii. Sintering Plant:
Sinters of different quality are produced for feeding into the Blast furnaces for making iron. Due to several advantages of sinters over conventional iron ore, most of the steel plants have switched over to increased quantity of sinter in the burden of the Blast furnaces. Sintering process involves heating a mixture of iron ore fines, coke, fluxes etc. in suitable proportions on a sinter bed.
The heat energy is supplied by the fuel gases and coke present in the feed material. During the process of sintering, considerable quantity of dust is generated. The dust is separated from the flue gases using cyclones, venturi scrubbers and/or electrostatic precipitators. It is estimated that nearly 15 to 20 gm/m3 of dust is generated during sintering and 200-250 mg/m3 in cooling of sinter.
The stack gases after removal of dust contain 17-18 per cent of oxygen besides CO, NOx, SO2, etc. In order to reduce the total quantity of gases and dust emitted to the atmosphere, it has been found beneficial to partially recycle these stack gases in the sintering process itself. It has been found that upto 30 per cent of the total stack gas volume can be recycled without affecting the quality of sinter.
In one case, it was observed that the dust emission was reduced by nearly 30 per cent with comparative reduction of CO and NOx, emissions, though however a marginal increase of SO2 was observed. In Kokura Plant at Japan, it was found that upto 40 per cent of stack gas volume could be recycled back into the system resulting in reduction of dust content by 25 per cent and NOx by 3-5 per cent.
Due to the decrease in the total volume of the gas, the downstream units like ESP, cyclones, bag fitters etc. can be comparatively smaller. It is observed that nearly 20-25 per cent of heat input to sintering process is carried away along with waste gases. Nearly 40 per cent of the waste heat can be recovered economically by recycling process gases.
iii. Steel Making Operation:
The use of tonnage oxygen for steel making has revolutionised the process of conversion of iron into steel. The development of oxygen steel making process like the LD process has drastically changed steel making technology in the last 30 years.
The use of oxygen in converter produces large quantities of waste gases which contain considerable amount of dust and these need to be removed before the gases are let off to the atmosphere. In addition to the dust, the gases contain high percentage of carbon monoxide depending on the type of gas cleaning process adopted.
Normally two types of processes are adopted for the gas cleaning:
1. Combustion of waste gases followed by dust removal.
2. Dust removal without combustion of waste gases.
Obviously the quantity of gases treated in the second case will be less and smaller gas cleaning equipments will be necessary. The carbon monoxide content in the waste gases can be as high as 70 per cent which can be economically used as a fuel (calorific value 2100 Kcal/m3) in the steel plants.
However this suffers from a drawback that the generation of the gases is not uniform throughout the steel making cycle. But, in steel plants having three or four converters, the gas generation will be more or less uniform and the waste gases after cleaning can be utilised as fuel gases in other units of steel plant.
Toxicity of Wastes:
Human illness characterised by diarrhoea, mouth sores, dark urine and burning mouth have been reported when individuals have consumed water polluted with phenol. The toxic effect of phenol may not be so acute with regard to human beings but its presence even in traces in potable water is objectionable, simply because on chlorination it forms chlorophenols which impart unpleasant taste to water and makes it carcinogenic.
When 3″ carps were exposed to phenol concentration of 400 mg/l, they have been killed within an hour. In the activated sludge process, receiving the waste, phenol inhibits cyanide oxidation, thiocyanate oxidation and ammonia oxidation.
Cyanide is an extremely toxic substance. In mammalian system, hydrocyanic acid prevents the process of oxidation in the tissue cells. It paralyses the respiratory center of the brain cells. The lethal effect of cyanide ion is due to the inability of the tissue cells to utilise oxygen.
Cyanide is known to be an inhibitor at the terminal oxidation of the cytochrome (cytochrome oxidase). Cyanide forms stable complex with cytochrome oxidase. This complexation results in loss of the enzyme activity resulting in death. Fish has high sensitivity to cyanide.
Chronic absorption of thiocyanate may cause dizziness, skin eruptions, cramps, vomiting, nausea and mild to severe disturbances of the nervous system. Thiocyanate containing water upon chlorination or irradiation releases the cyanide by splitting the sulphur moiety. It inhibits the absorption of iodine by thyroid gland. It is toxic to fish life also.
iv. Ammonia:
Free ammonia has been found to be toxic to fish life even at concentration levels of 2.3 mg/l. It is also toxic to human beings at higher concentrations. The toxicity of ammonia increases with pH because at higher pH, most of the ammonia remains in the gaseous form.
Air Pollutants and their Effects on Human:
1. Toxicological and Sensory Effects on Human:
(a) Acute toxicity.
(b) Chronic toxicity.
(c) Irritation of eyes, lungs, skin etc.
(d) Odour.
2. Effects on Plant and Animal Life:
(a) Toxic to agricultural crops, timber, grass etc.
(b) Toxic to livestock and wild life.
3. Effects on Materials and Facilities:
(a) Soiling.
(b) Corrosive to metals, minerals, plastics, textiles and paints.
(c) Effects on electrical systems.
4. Optical and Aesthetic Effects:
(a) Visibility reduction.
(b) Visible plume and coloured atmosphere.
5. Modification of atmospheric chemical and physical properties