IN STONE CRUSHERS
IMK SEnate House CAmpus
- 1. INTRODUCTION
Stone Crushing Industry is an important industrial sector in the country engaged in producing crushed stone of various sizes depending upon the requirement which acts as raw material for various construction activities such as construction of Roads, Highways, Bridges, Buildings, Canals etc. It is estimated that there are over 12,000 stone crusher units in India. The number is expected to grow further keeping in view the future plans for development of infrastructure of roads, canals and buildings that are required for overall development of the country. In India, the Stone Crushing Industry sector is estimated to have an annual turnover of Rs. 5000 crore (equivalent to over US$ 1 billion) and is therefore an economically important sector. The sector is estimated to be providing direct employment to over 500,000 people engaged in various activities such as mining, crushing plant, transportation of mined stones and crushed products etc. Most of these personnel are from rural and economically backward areas where employment opportunities are limited and therefore it carries greater significance in terms of social importance in rural areas. It is a source of earning for uneducated poor unskilled rural people.
The stone crusher is one such industry that exists in the vicinity of almost all major cities/towns throughout the country in all the states because the construction activities go on throughout the country. As transportation of stone over long distances adds to cost of the crushed stone products, the crushers need to be necessarily located nearer to the demand centers such as Cities, Bridges, Canals etc. Stone Crushers also need electricity supply and large number of man power for its operation. It also needs access roads for the movement of mined stone as well as crushed stone products. It is for these reason that most Stone Crushers are located along the periphery of Cities or in the vicinity of major construction projects. In most cases the Stone Crushers come up in clusters of number of units ranging from five to fifty in one cluster. The crushers are located nearer to the source of raw material such as Stone mines, River Beds etc.
These stone crushers though socio-economically an important sector, gives rise to substantial quantity of fine fugitive dust emissions which create health hazards to the workers as well as surrounding population by way of causing respiratory diseases. The dust also adversely affects visibility, reduces growth of vegetation and hampers aesthetics of the area. In order to prevent/control these emissions, CPCB has already evolved Emission Standards and guidelines in 1989, which has been notified under Environment (Protection) Act, 1986 by Ministry of Environment & Forests vide Notification No. G.S.R. 742(E) dated 30th August 1990 & S.O. 8(E) dated December 31, 1990 based on techno-economic feasibility to achieve the standards. But over the years, as the need for more effective control and enforcement has been felt and to provide more specific guidelines to the stone crushers to enable them control emissions satisfactorily, CPCB has signed a Memorandum of Undertaking (MoU) with National Productivity Council (NPC) and commissioned the study with the work of reviewing the existing standards, guidelines and sitiing criteria and to evolve a Comprehensive Industry Document (COINDS) for Stone Crushers. This report is an outcome of the in-depth studies carried out by NPC jointly with CPCB on a representatives cross section of Stone Crushers throughout the country and a series of meeting held with State Pollution Control Boards, Stone Crushers Associations / Stone Crushers unit representatives etc..
- 2. Typical Set-up & Sizes of Stone Crushers
There are large variations in the types of stone crusher setup across the country depending on geographical locations, type of demand for crushed products, closeness to urban areas, type of raw
TYPICAL SET-UP AND SIZES OF STONE CRUSHERS
material, availability of plant and machinery locally etc. Primarily the stone crusher industry sector could be divided in three categories small, medium & large.
2.1 Small Size Stone Crushers
There are different types of small crushers in various states with a production capacity ranging from 3 to 25 TPH. Typically, the units having only one Jaw type crusher used as primary or secondary crusher along with one or maximum 2 screens are grouped as small stone crushers. A few most representative types of small crushers are:
- Crushers with completely manual breaking/feeding/ retrieving/stocking operation with single jaw and single rotary screen.
- Crushers with manual breaking manual/gravity feeding belt conveyors single jaw & rotary screen type.
- Mechanical/Gravity feeding, single jaw with vibratory type screens with belt conveyors.
- Single Jaw Bucket Elevator, Elevated vibratory screen with storage bunkers type compact units.
2.2 Medium Size Crushers
Typically those crushers having than one crusher i.e., one primary and one secondary or one/two primary & two secondary crushers along with one or more vibratory screens are categorized as medium size crushers. The mined stones are transported mostly by trucks/dumpers and unloaded in to elevated stone wells. The stones are fed by gravity to primary crushers. The Crushed stones are conveyed to vibratory screens. The screened products of various sizes are conveyed to stock piles by belt conveyors. The oversize is returned to secondary crushers for further crushing and back to the vibratory screen. Such type of medium type crushers has a production capacity in the range of 25 – 100 TPH.
2.3 Large Stone Crushers
Those crushers typically having two or more numbers each of primary, secondary & tertiary type crushers with at least 2 or more vibratory screens with mechanized loading, unloading conveying operation s and producing more than 100 TPH crushed stones are categorized as large crushers. Typically large stone crusher are owned by bigger construction companies like L&T, Dodsal Construction, etc who have their own large construction projects demand. Such crushers have a capacity range of 100 –200 TPH. These type of crusher generally have their own/leased open-cast stone mines and a fleet of mechanical mining equipment , trucks and dumpers, loaders etc. These crushers involve high capital investment and mostly operate round the clock. All conveying operations are done through proper belt conveyors.
- 3. ENVIRONMENTAL PROBLEMS IN STONE CRUSHERS
3.1 Sources of Emissions
All quarrying and stone processing operations including surface mining, crushing, screening, material handling and transfer operations are potential sources of particulate emissions. These sources may be categorised as either process sources or fugitive dust sources. Process sources include those sources for which emissions are amenable to capture and subsequent control. Fugitive dust sources generally involve the re-entrainment or settled dust by wind or machine movement. Factors effecting emissions from either source category include the type, quantity and the moisture content of the rock processed, the type of equipment and operating practices employed as well as topographical and climatic factors.
The typical emission sources during stone processing are given in the table below.
||Fugitive Dust Source
||Blasting, Loading and Hauling.
||CrushingScreeningConveyor transfer points
Emission during Mining Activity
Fugitive Emissions are generated during various mining activities such as drilling, blasting, excavation, breaking and loading etc. In some bigger mines the primary blasting emissions are quite substantial over a large area. The emissions during secondary blasting are also substantial but lesser compared to primary blasting. These emissions last only for a few minutes. There is hardly any controls adopted world over for controlling blasting emissions.
Emission during Transportation
During transportation of mined stones by heavy vehicles like trailers, trucks, dumpers, fugitive dust emission occur due to movement of heavy vehicles on earthen roads. The quantity of emissions depends on various aspects like climatic conditions, moisture in the soil, speed of the vehicle, frequency of the vehicles etc. Generally such emissions can be controlled to a great extent by spraying water on these roads intermittently.
Emission during Crushing Operation
During crushing operation, generation of particulate emissions is inherent and the emissions are most apparent at crusher feed and discharge points. The greater the reduction in size during subsequent crushing stages from primary, secondary to tertiary crushing, the higher the emissions. Primary jaw crushers produce more dust than comparable gyratory crushers because of the bellows effect of jaw and because gyratory crushers are usually choke-fed, thus minimising the open spaces from which dust may be emitted. For subsequent reduction stages, cone or roller type crushers produce more fines as a result of attrition and consequently generate more dust.
Emissions During screening
In the screening section, the mixture of stones is classified and separated according to size. Generally the screening efficiency is considered to be in the range of 60 to 75%. The screening equipments commonly used include grizzlies, shaking screens, vibrating screens and revolving screens. Although screening may be performed wet or dry, dry screening is the most common. Dust is emitted from screening operations as a result of the agitation of dry stone. The screening of fines produces higher emissions than the screening of coarse sizes. Also screens agitated at large amplitudes and high frequency emit more dust than those operated at small amplitudes and low frequencies.
Emissions During material handling
In the material handling section, various handling devices like feeders, belt conveyors, bucket elevators and screw conveyors are used to transport crushed materials from one point to another. Particulates may be emitted from any of the material handling operations. Most of the emissions occur at transfer points. Since transport of material on the conveyor causes little disturbance of air and emissions that occur due to the wind are judged to be minimum. The transfer points include transfers from a conveyor on to another, into a hopper and on to a storage pile. The amount of uncontrolled emissions depends on the size distribution of the material handled, the belt speed and the free fall distance.
3.2 Nature and Spread of Emissions
The dust is generated primarily due to size reduction and handling of the stones at various stages. The major source of dust generation is during size reduction in the primary, secondary & tertiary crushers. The fines in the dust generated increases with subsequent stages of crushing i.e., more fines are generated in secondary crusher as compared to primary crusher. The dust is also generated during handling of stones, especially at the point where the stones fall through a height from places like one belt to another or from belt to hopper or stock piles etc. During vibratory screen large quantity of fine dust is emitted.
Coarser part of the dust settles down within the premises but the finer particles get air borne and get carried away with wind to longer distances. The actual area of the source of the dust generation is quite small (about 0.5 to 1 square meter)at each source, but as the dust rises it spreads and typically the area in which it spreads is more than 10 –15 times larger than the area of actual emissions at about 3 to 4 metres height. This rising dust column shifts from left to right or north to south etc. depending on wind direction and current and eventually it gives an appearance that the dust is emitted from almost every inch of area in the stone crusher.
3.3 Factors that Influence Emissions
Factors affecting emissions that are common to most operations include moisture content in the rock, type of rock processed, type of equipment, and operating practices employed. These factors apply to both process (primary) sources and fugitive (secondary) sources in quarry and plant operations.
Depending on geographic and climatic conditions, the inherent moisture content or wetness of quarried rock may range from nearly zero percent to several percent. The effect of moisture content is especially important during quarrying, material handling, and initial plant operations such as primary crushing. Surface wetness causes fine particles to agglomerate or adhere to the faces of larger stones, resulting in a dust suppression effect. However, as new fine particles are created by crushing and attrition and since moisture content is reduced by evaporation, this suppressive effect diminishes in later stages.
The type of rock processed is also important. Soft rock produces a higher percentage of fines than do hard rocks because they are more friable. Therefore, processing of soft rocks has greater potential for emissions.
The type of equipment and operating practices employed also affect uncontrolled emissions. Equipment selection is based on a variety of parameters, including quarry characteristics; rock type
SOURCES OF DUST GENERATION IN STONE CRUSHERS
processed, and desired end products. Emissions from process equipment such as crushers, screens and conveyors are generally a function of the size distribution of the material, and the amount of mechanically induced velocity imparted to it.
3.4 Environmental, Health and Safety Problems due to Emissions
Some percentage of the fugitive dust emissions may get settled down within the unit premises it self, but a substantial percentage of airborne emissions are carried away to the surroundings by wind currents. Dust that settles within the plant gets air borne again due to vehicular movement or by wind and acts as a secondary emission source. Dust settled over the equipments may cause rapid wear and tear of the rotating parts and may lead to frequent breakdowns and higher maintenance costs.
Dust emissions affect the climate, damage the material, human health and vegetation. When the total amount of particulates in the atmosphere increases, particulates may absorb incoming solar radiation, causing an increase in the atmospheric and land surface temperature. With the deposition of aerosols on materials, especially buildings, although little damage is caused to the materials, the effect is expensive to remove deposited particulates which damages vegetation by preventing them from photo synthesis. The physical properties of atmospheric particulates affect human health either by allowing penetration of the lung and causing irritation to the internal membrane, or by transporting absorbed toxic gases and vapors deeper into the lung than they would normally travel. The work place environment at stone crusher sites contain millions of suspended mineral particles of respirable size that get deposited in lungs following inhalation.
3.5 Noise Pollution Problems
There are a number of sources from which high noise level are generated, some continuously and some intermittently. The vibratory screen is the most predominant source of continuous noise. Especially vibratory screens are operated at higher frequency and without enclosures can give rise to abnormally high noise levels. Intermittent noise level is also generated at the crusher during the time of the breaking of stones. Intermittent noise is also generated during un-loading and loading operations. Belt conveyor movement is also a source of continuous noise, especially the ill-maintained and cheaper end conveyor system make more noise.
- 4. Emission reduction techniques adopted in crushers
Diverse particulate emission sources in stone processing operations have resulted in the use of a variety of control methods and techniques. Dust suppression techniques are the most commonly used. They are designed to prevent particulate matter from becoming air borne and are applicable to both process and fugitive dust sources. Particulate emissions such as those generated by crushing operations can be captured in collection systems. The applicable control options for various types of emission sources are listed in the table below:
- 1. Control of Mining/ Quarrying Operations
i) Control of drilling operations
For controlling emissions during drilling operations, generally two methods are adopted, water injection and aspiration to control device. In water injection, water or water plus a wetting agent or surfactant, usually a liquid detergent is forced into the compressed air screen that flushes the drill cuttings from the hole and the moisture missed dampens the stone particles which causes them to agglomerate. In the dry collection system, a hood encircles the drill rod at the whole collar and vacuum capture emissions which are vented through flexible duct to a control device such as cyclones or fabric filters, at times preceded by a settling chamber. Air volumes required for effective control may range from 850 to 3400 m3/hr depending on the type of rock drilled, hole size and penetration rate.
ii) Control of blasting operations
During blasting operations, no effective method is available for controlling particulate emissions. Good blasting practices are generally employed to minimize noise, vibration and air shock and the blasting operations are scheduled to occur only during conditions of no wind and no inversion potential so as to reduce the impact of emissions substantially.
iii) Control of quarry loading operations
The particulate emissions from the loading operations are controlled by using water trucks equipped with hoses or portable watering systems to wet down the piles prior to loading.
iv) Control of hauling operations
The emissions from hauling operations are proportionate to the condition of the road surface and the volume and speed of vehicular traffic. The control measures include methods to improve road surfaces or suppress dust and operational changes to minimise the effect of vehicular traffic. The various treatment methods applied to control fugitive emissions from haul roads include watering, surface treatment with chemical dust suppression, soil stabilisation and paving. The road dust is also controlled commonly by treatment with oil usually supplemented by watering. Waste oils such as crankcase drainings are spread over roadways at a rate of about 0.23 liters/m2 of roadways, but such oil treatment is used judiciously to avoid slippery and dangerous conditions due to excessive application. Other treatments such as application of hydroscopic chemicals such as organic sulphonates and calcium chloride spread directly over unpaved road surfaces are used where these chemicals dissolve in the moisture absorbed from the air and form a clear liquid i.e. resistant to evaporation. An alternative to surface treatment is soil stabilisation where water dilute emulsions of either synthetic or petroleum resins are used which act as adhesive or binder. Apart from the above measures, some operational measures such as reduction of traffic volume and control of traffic speed are taken to reduce emissions from the hauling roads.
- 2. Control of plant operations (Crushing, Screening and Conveying)
The emissions from the plant operations contain a multiplicity of dust producing points including numerous crushers, screens, conveyor transfer points and storage facilities. Control methods generally applied to control plant generated emissions include wet dust suppression, dry collection and a combination of the two. Wet dust suppression consists of introducing moisture into the material flow to restrain fine particulate matter from becoming air borne. Dry collection involves hooding or enclosing dust producing points and exhausting emissions to a collection device. In combination systems, both methods are applied at different stages throughout the process. Completely enclosing process equipment is another very effective technique.
i) Wet dust suppression systems
Typically the wet dust suppression system contains basic components such as a dust control agent, proportioning equipment, a distribution system and control actuators. A pump is required to provide adequate pressure. Distribution is accomplished by spray headers fitted with pressure spray nozzles. The nozzle types used are hollow cone, solid cone or fan type depending on the spray pattern desired. Screen filters are used to prevent nozzle plugging.
Spray actuation and control are important to achieve effective control and to reduce waste and undesirable muddy conditions especially when the material flow is intermittent. Spray headers at each application point are normally equipped with an on – off controller i.e. interlocked with a sensing mechanism so that sprays will be operating only when material is flowing.
ii) Dry Collection Systems
Particulate emissions generated from crushers, screens, conveyor transfer points and bins are at times controlled by capturing and exhausting emissions to a collection device. Collection systems consist of hoods and enclosures to confine & capture emissions and ducting & fans to convey the captured emissions to a collection device where they are removed before the air stream is exhausted to the atmosphere. For the collection system to be effective in preventing emissions from being discharged to atmosphere, its hooding and ducting must be properly designed and balanced. Process equipment should be enclosed as completely as practicable, yet allow excess room for routine maintenance and inspection requirements. In general, a minimum in draft velocity of 1 m/s is maintained through all open hood areas. The conveying velocities range from 18 to 23 m/s.
The most commonly used dust collection device in the crushed stone industry is the fabric filter or bag house equipped with mechanical shaker type or pulse jet type cleaning mechanism and normally equipped with cotton sateen cloth bags. The air to cloth ratios generally employed range from 2:1 to 3:1 for mechanical shaker and higher ratios of 6:1 to 10:1 for pulse jet type bag filters are employed. Other collection devices such as cyclones and scrubbers are used for primarily controlling coarse particles of 20 micron or higher size in the past, but these devices are not used currently in the industry.
iii) Combination Control Systems
The wet dust suppression and dry collection techniques are often used in combination to control particulate emissions from stone crushers. The wet suppression is generally used to control emissions at the primary crushing stage and at subsequent screens, transfer points and crusher feeds. The dry collection is generally used to control emissions from the remaining points such as secondary and tertiary crusher discharges where new dry stone surfaces and fine particulates are formed. A large portion of the fine particulates is removed by dry collection but subsequent dust suppression applications become more effective with the minimum of added moisture.
- 3. Control of fugitive dust sources
Significant fugitive dust emissions, seen as visible emissions, result during formation of new aggregate piles and erosion of previously formed stock piles during which emissions are generated by wind blowing across the streams of falling stone and segregating fine particles from coarse particles. Emissions are also generated when the falling stone impacts on the piles. Control methods include wet dust suppression and devices designed to minimise the free fall distance to which the material is subjected, thus lessening its exposure to wind. Control devices include telescopic chutes, stone ladders and hinged boom stacker conveyors. Stone ladders consist of a section of vertical pipe into which stone from the stacking conveyor is discharged. Watering is the most commonly used technique for controlling wind blown emissions from active stockpiles. Locating stockpiles behind natural or artificial wind breaks (walls) also aids in reducing wind blown dust.
Fugitive dust emissions are also generated from conveying operations for which the dust control alternatives include chemical suppression and covering. Fugitive emissions are also generated during transfer of fine materials from stockpiles into open dump trucks. The dust formation may be reduced by keeping the stones wet on the stockpiles and the loaded buckets are emptied as close as possible to the truck beds. At some installations water spray systems are used to wet the stone in the truck while loading.
Fugitive dust emissions are also generated from the plant yard areas due to vehicular traffic and wind. These emissions generally are not controlled at crushed stone plants. It can be minimised through good house keeping practices. Street sweeping equipment has been effective for paved or other smooth yard surfaces.
4.5 Environmental Impacts Of Various Control Technologies
Application of emission reduction systems are associated with both beneficial and adverse impacts on air, water, solid waste, energy and noise that may be directly or indirectly attributed to the operation of these emission control systems.
Impact on Air
Impacts on air emissions associated with the application of the alternative emissions reduction systems for the control of particulate emissions from both process and fugitive dust sources are discussed here. Because emissions form fugitive dust sources are typically large in area and are discharged directly to the atmosphere in an unconstrained manner rather than through a stack, such a quantitative measurement of these emissions is considered to be difficult, if not impossible. Similarly because of the nature of wet dust suppression systems, no data are available that permit a quantitative comparison of the control capabilities of wet systems versus dry collection systems on process sources. Typically the uncontrolled emissions from 200 to 600 tons/hr. crushers range between 1000 to 3000 kg./hr which after reduction by over 99% in dry type collection systems such as bag filters reduce to as low as 3 to 6 kg/hr
Impact on Water Pollution
Dry collection control techniques generate no water effluent. When wet dust suppression techniques are used, the water is absorbed by the material processed and therefore, does not produce any water effluent either. The application of air pollution control technology in the stone crushers has little impact on water quality.
Impact on Solid Waste Disposal
The method of disposal of quarry, plant and dust collector waste materials depends somewhat upon state and local government and corporate policies. When dry type bag filter systems are used, the solid waste (dust) collected can be sold or used for a variety of purposes. Collected fines are sometimes disposed of in an isolated location in the quarry if no market is available which generally in the range of 0.5% of the total production. If such waste piles are properly controlled by any of the techniques discussed above, no subsequent air pollution problems develop. Thus, the solid waste generated by the application of dry collection methods can be dispersed without any adverse impact on the environment. When wet dust suppression is used, no solid waste disposal problem results over that produced by normal operation.
Impact on Energy Consumption
Application of the alternative control techniques in stone crushers necessarily results in an increase in energy consumption over that required to operate a plant without air pollution control. Typical energy consumption with various types of air pollution control technologies used in different capacity stone crushers is given in the table below.
The application of dry collection system results in the highest increase in energy usage. Typically for a 200 TPH plant, the increase in energy consumption with dry system is about 103 HP whereas in wet type suppression system, the increase is only 2 HP and in a combination of wet and dry system, the increase is about 23 HP. This shows the impact of increase in energy consumption by adopting wet dust suppression systems is negligible whereas it is substantial in the range of 16 to 17% when dry control systems are adopted.
Impact on Noise
Allowable noise levels and employee exposure times are specified by the mining enforcement and safety administration department. These limits require that potential noise problems be assessed and sound dampening equipment be installed as required. By application of either of the above air pollution control techniques, no significant noise impact is anticipated.
Review of Dust Containment Enclosures
The enclosures are commonly found for the crushing equipment’s and the vibratory screens. Some units have also provided enclosures to belt conveyors. The appropriateness of the type of enclosures provided is discussed below.
a) Enclosures for Crushing & Screening Equipment’s and Constraints Reported
In some crushers the crushing equipment’s (primary or secondary) are enclosed inside a big shed. The shed is closed from 3 sides and top but leaving the front end completely open. This type of shed cannot stop wind currents carry the emission away from the crusher and therefore such enclosure does not serve any useful purpose unless enclosed from all sides. The constraints were reported in completely enclosing the shed such as during maintenance it is required to take out large components using cranes, loaders etc and this becomes difficult if the front access is closed. The general practice is to remove some sheets from the enclosure to create an opening to take out material during repairs. These removed sheets never get replaced again anticipating the occurrence of the similar problem again. This ultimately results into partial enclosures.
b) Constraints in Enclosing Crusher/Screen Discharge Areas
At the discharge of screen and crushers some stones spill over from the belt conveyor and get accumulated on the floor. This spilled over material needs to be periodically removed or else the piled up material beyond the height of the system drive component may hamper the operation of the belt conveyors etc. Such spillages are so frequent and large in quantity that almost once in a day it needs to be cleaned. This spilled material also contains large quantity of stone dust. It is due to this reason, in order to provide easy access for removal of material, one side of the enclosure is generally left open or even if a door is installed it always remains in open condition.
Such spillages happen due to systems faults or design lacunae such as misalignment and improper angles of discharge and belt conveyors, use of locally available material like used truck tyre tread pieces joined together to form the belt conveyor, lack of provision of rubber skirting etc. The possible solution to overcome this problem could be to provide well designed layout, proper/standard belt conveyor material, rubber skirting on the side of the belts, proper alignment and angle of the crusher discharge chute and belt conveyor etc
The product spillages also occur due to holes formed in the discharge chute of crushers and screens at the point of free fall of stones as the steel plate gets punctured due to wear and tear and
PREVALENT DUST CONTROL MEASURES (ENCLOSURES)
eventually gives way for spillages. This could be minimized by adopting a step wise design of the chutes to reduce free fall height or by strengthening the steel at the point of impact. In the step design, a layer of stones always remains above the steps and the free fall impact is taken by these accumulated stones which saves the steel from direct wear and tear and increases its life.
c) Enclosures for Belt Conveyor & Dust Hopper and Constraints Reported
Some crushers use Jute Cloth in place of GI steel sheets as material of construction for enclosures to reduce the cost. But wind current can pass through jute cloth enclosure thereby reducing the usefulness of the enclosure in terms of containment of dust. At times plastic/PVC sheets are used for enclosure especially for the belt conveyors and vibratory screens but it gets torned frequently due to high velocity wind currents and therefore not effective enclosures.
Many units provide an enclosed hopper for collection and storage of less than 6mm size dust product, but a large opening is kept at the point of discharge from
Review of Dust Suppression Arrangements
Most of the SCUs were found to have some water spray arrangement, though not satisfactory. The water is generally drawn from nearby bore-well and stored in a water tank. The water tank is generally located at an elevated level where raw material is unloaded. Water is supplied by gravity, through GI pipes, to various locations where dust is generated. Generally, simple holes are made in the pipes through which water is sprinkled. Some Crushers have installed “domestic showers” to get better spray. None of the unit was found to have filtering arrangements to clean the water. In such cases, in the absence of adequate pressure the spray formation is not appropriate and thereby to achieve the same effect as spray by nozzle more quantity of water is required to be sprayed, which again is unacceptable as it may increase moisture in the product excessively affecting quality adversely. Sprays are required to generate large number of fine water droplets which in turn come in contact with large number of fine dust particles, ideally each dust particle should get in contact with each water particle, only then the dust gets suppressed, otherwise suppression effect would only be partial.
a) Typical arrangement adopted for Dust Suppression:
Following arrangements generally exist in stone crushers.
- A bore well as source of water
- A domestic water storage tank, placed at an elevated level.
- GI pipes for water circulation by gravity
- Generally holes are made in the pipes through which water get sprinkled on stone, some units have domestic showers for spraying.
- Some units have additional arrangements such as Water Sprinklers around the plant, along with a pump and G.I. Pipeline network in and around the plant.
b) Sprinkler Arrangement
Some stone crusher have installed water sprinklers all around the plant premises which spray water at a height of about 5-7 meters in 360 degrees periodically. Such system consumes a large quantity of water and if operated continuously it can create muddy conditions all over the plant due to accumulation of water in low lying areas. With regard to effectiveness of dust suppression, as discussed earlier the airborne dust emissions spread to a very large area as it raises and the sprinklers can spray water covering a small percentage of this area and therefore the airborne emissions hardly get suppressed by sprinklers. On the other hand the sprinklers wet the floor area as well as the stock piles and hauling roads and therefore help minimize only the secondary fugitive dust emissions but are ineffective in suppressing/controlling primary process dust emissions which are air borne.
Local Problems In Implementation Of Pollution Control Measures
Discussions were made with number of stone crusher owners to know and learn about the local problems faced by them in controlling dust emissions. A few of the reported problems are highlighted below.
- High investment cost for providing dust containment like G.I. sheet enclosure for Vibratory Screen, Belt Conveyors etc for small size SCU’s, for Medium and Large size SCU’s it is not.
- Huge quantity of water has been used for suppression of dust resulting in poor product quality forcing SCU to abandon it
- Lack of appropriate nozzles for proper spray resulting in excess moisture added to the product and thereby increasing the water consumption
- Lack of knowledge of pollution control measures by SCUs
- Lack of availability of water for dust suppression forcing the SCU’s to purchase the same at higher cost
- Financial constraints to implement the pollution control measures for small units