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15. Sources and Methods of Dust Control M. P. J. SANDYS Formerly Assistant Group Environmental Engineer, Anglo-American Corporation of S.A. Limited J. H. QUILLIAM Formerly Chamber of Mines Research Organisation Page 1 Introduction.............................................................................................................................................. 2 Basic principles in the control of dust...................................................................................................... 3 General preventative measures............................................................................................................... 3.1 Removal of personnel .......................................................................................................................... 3.2 Prevention of formation of dust at its source....................................................................................... 3.3 Dilution by ventilation............................................................................................................................ 3.4 Use of respirators................................................................................................................................. 4 Main sources of dust and methods of control in underground operations............................................. 4.1 Blasting.................................................................................................................................................. 4.2 Cutting and loading............................................................................................................................... 4.2.1 Cutting and loading - coal................................................................................................................. 4.2.2 Loading - metalliferous mines........................................................................................................... 4.3 Raise and blind hole boring equipment................................................................................................ 4.4 Drilling................................................................................................................................................... 4.4.1 Drilling - metalliferous ...................................................................................................................... 4.4.2 Drilling - coal..................................................................................................................................... 4.4.3 Drilling - diamonds............................................................................................................................ 4.5 Transportation of rock.......................................................................................................................... 4.6 Tipping and transfer points.................................................................................................................. 4.7 Dust raised in travelling ways, tractor/haulage roads and intake airways........................................... 5 Main dust-producing operations on the surface of mines....................................................................... 5.1 Transfer points ..................................................................................................................................... 5.2 Screens ................................................................................................................................................ 5.3 Crushers................................................................................................................................................ 5.4 Rotary filters ........................................................................................................................................ 5.5 Workshops............................................................................................................................................ 5.6 Grading rooms....................................................................................................................................... 5.7 Assay crushers..................................................................................................................................... 5.8 Smelting operations.............................................................................................................................. 6 Classification of dust extraction plants.................................................................................................... Bibliography................................................................................................................................................ 379 380 380 380 380 381 381 382 382 382 382 383 383 384 384 386 386 387 389 389 389 389 389 390 391 391 391 391 392 392 394 1 INTRODUCTION Dust is formed at just about all mining and rock handling processes. Due to the effects of pneumoconiosis and more particularly silicosis, it is probably one of the most severe and costly occupational health problems in the mining industry. Medically no entirely satisfactory answer has been found either in the case of treatment or prevention. It remains therefore in the field of mining ventilation to 379 380 Environmental Engineering in S.A. Mines 1. prevent the dust from being airborne or if this fails to 2. reduce as far as possible the exposure of the worker to the airborne dust. If both these measures cannot be satisfactorily applied then recourse to respirators is the only other alternative. Coal dust explosions are known to have cost the lives of many miners to date. This is obviously an added incentive to control dust in collieries. 2 BASIC PRINCIPLES IN THE CONTROL OF DUST in order to control dust adequately it is essential to know how, when and where it is produced and to measure that portion which becomes airborne. Methods of measurement, which are not necessarily the same as those required for estimating the health hazards associated with dust, are described elsewhere. Dust clouds are never static, the natural forces constantly changing the concentration, the size distribution and the physical and chemical characteristics of dust. Factors such as sedimentation, impingement, diffusion, humidity and condensation play a considerable part in preventing the build-up of dust throughout the mine from becoming cumulative. However, these factors are usually not nearly sufficient in themselves, and it becomes necessary to employ "artificial" methods of control. Generally, there are four rules regarding dust control which should be followed if the problem is to be tackled effectively. These are as follows: (a) Keep the dust production to a minimum and prevent it from contaminating the atmosphere by controlling it at its source; (b) dilute it as rapidly as possible; (c) filter it; (d) remove persons from high dust concentrations. In most mines excessive dust exposure is the result of failure to take adequate steps under these four headings. Dust control to be effective requires constant supervision and regular maintenance of the service equipment. 3 GENERAL PREVENTATIVE MEASURES The basic ways in which high dust exposures can be controlled or prevented are given below. It should be borne in mind, however, that "prevention is better than cure". 3.1 Removal of personnel This is the most effective way of preventing exposure to dust but generally the least practicable. It is applied mainly after blasting by the insistence on a minimum re-entry period, by arranging a fixed blasting time for each working place so that other workers are not exposed to the blasting dust and fumes, and by ensuring that blasting takes place only at the end of the shift when most other workers have already been withdrawn. When on-shift or mutiple-shift blasting is permitted, removal of personnel is impossible and the dust and fumes must either be taken directly to surface or the dust filtered out and the fumes diluted. Other ways in which workers are kept out of dusty air is by arranging that men travel in downcast shafts and by having all underground waiting places in fresh air. 3.2 Prevention of formation of dust at its source Initially all efforts should be concentrated on the prevention of the formation and liberation of dust at the source. With minor exceptions, water is used for this purpose. If used in excess it results in flooding, overloading of pumping systems, messy conditions in flat mining, washing away of "fines" Sources and Methods of Dust Control 381 containing valuable minerals (this is a problem in gold mines). Excess water usage has led to dangerous mud-rushes when water is drained through ore-pass systems. In hot or deep mines this water can constitute an additional heat source and is often cooled to improve environmental conditions. When wetting down, a suitable spray nozzle should be used to ensure the spread of water over a greater area and to prevent the settled dust from being stirred up. Laboratory experiments have indicated that a moisture content of the rock of only 1 per cent by mass produces a very significant reduction in dust production when compared with rock being transported under dry conditions. As it is difficult to maintain a moisture content of 1 per cent under conditions encountered underground an optimum moisture content of about 5 per cent should be aimed at for minimum dust production during transportation. It is important that the water used for dust suppression, particularly in drilling and in water-blasts, should be as clean as possible as the evaporation of dirty water can release considerable quantities of dust. This is particularly important in gold mines when service water pressures are relatively high. Special considerations for collieries Water infusion has had varying measures of success in reducing the respirable dust levels in coal mines. It is widely practised in some countries, namely Germany, France and Belgium. . Depending on the permeability of the coalbed infusion pressures may range from 2 000 kPa to 34 000 kPa. Not less than 7 of water per ton of coal and as much as 20 per ton may be necessary. It must be noted that water infusion in some instances is primarily used for the control of methane emission - the gas being displaced and caused to migrate away from the face area. Caution must therefore be exercised in this regard. Notwithstanding the fact that wetting agents reduce surface tension, no chemical preparation that has been tested to date on South African coal mines has been found to be effective in controlling the production of dust at the source. It appears that the time factor is most important if wetting agents are to be effective and is of the order of minutes rather than seconds. Further disadvantages at present are the high cost of material and spray equipment and the technical problems of reliable, consistent dosage. 3.3 Dilution by ventilation This is essential when measures to suppress the dust at its source have failed. Obviously, if the quantity of air passing a given source of dust is increased the dust concentration will be reduced. If carried to excess the resultant air velocity in some cases may even be responsible for stirring up dust. Pick-up velocities for coal and quartz dust are given below Dry Semi Dry Particle Size-Micrometre 75-105 35-75 10-35 75-105 35-75 10-35 Quartz Coal Pick-up Velocities - m/s 6,3 5,3 5,3 4,2 3,1 3,2 7,4 6,3 6,3 5,3 4,2 4,2 3.4 Use of respirators In certain operations conducted in underground workings and on surface plant personnel are exposed to high concentrations of dust for only short periods of time, and fully adequate dust control measures cannot be implemented. Such underground operations may include, for example, blowing-over operations in sinking shafts, operations at shaft loading boxes and transfer points and blowing-out of electric motors. On 382 Environmental Engineering in S.A. Mines surface plants high dust concentrations may be formed at blow-off points on rotary filters, ore grading rooms, crushing plants and in handling of dry lime. In such instances respirators should be used as a means of protection only if dust control systems are not practical. No known type of respirator can be worn for long periods of time without being a cause of some degree of discomfort to the wearer. Respirators must be light, easy to wear and be able to withstand strenuous conditions. They need not necessarily be highly efficient provided they can reduce the concentrations of dust inhaled to relatively harmless levels. A respirator with an efficiency of some 90 per cent could still provide the necessary protection whilst making the breathing effort much easier than would be the case with a respirator of higher efficiency, thus ensuring it would be worn for the peak dust period. The selection of a respirator should be based on: 1. Type of dust, i.e. organic or toxic. 2. High dust retention efficiency on the respirable fraction (95% for Silica) combined with a relatively low initial resistance of the order of 150 Pa. 3. Wearability such as comfort, mass, face fit and vision. Closed circuit breathing systems tend to be cumbersome and they can reduce worker mobility and should be considered only for abnormal circumstances. The airstream helmet provides a solution to the problem of airborne dust inhalation. Up to 90% of particulate matter is filtered from the environmental air with no extra breathing effort and with no remote air supply. Electrical power at a safe low voltage is supplied to the fan motor via a flexible cable from a lightweight sealed and rechargeable battery pack. This pack can be clipped to a belt or carried in an overall pocket. 4 MAIN SOURCES OF DUST AND METHODS OF CONTROL IN UNDERGROUND OPERA TIONS 4.1 Blasting Blasting, particularly in hard rock, produces enormous quantities of dust, fumes and poisonous gases in concentrations which have been shown to be higher than those produced by any other mining process. Exposure of personnel to this dust and gas must be prevented. This is normally done by removing men from the area before blasting and clearing the area of dust and fumes before the men return. This is comparatively easy to control in metal mines in which only one main shift is worked during twenty-four hours. In development ends the amount of dust produced by the blast can be controlled to a certain extent by choice of the pattern of the round, the method of blasting, the material used for tamping and the type of explosive used. The practical aim should, however, be the complete replacement by fresh air of the air contaminated by dust in whatever time is available before re-entry. Intervals of re-entry are controlled by: (a) The type of mine, e.g. metalliferous, diamond, coal. (b) The method of mining giving rise to the source of dust. (c) The ventilation system. Water-blasts are used before and after a blast in development ends. These reduce the amount of dust and noxious gases present in the air to a limited extent. The main value of the water-blast is that when subsequent watering-down is undertaken on re-entry less dust is produced. 4.2 Cutting and loading 4.2.1 Cutting and loading - coal During cutting and mechanical loading high dust levels can be encountered unless water spray equipment is provided. This equipment should comprise an integral part of cutters, loaders and continuous miners and should operate automatically when the unit is in operation. Water control valves should be inter-connected with the mechanism operating the cutting or loading movement. Sources and Methods of Dust Control 383 The type of nozzle and water pressure are important factors. The minimum water quantity required for dust control is 20 per ton mined excluding wastage. The control of dust at continuous miners is a well recognized problem. The effectiveness of machine mounted scrubbers, fan and venturi spray arrangements has recently been confirmed in tests conducted locally. In these tests high pressure sprays effectively suppressed dust at source and simultaneously induced airflow which scoured the face of dust and methane. The machine mounted scrubber further reduced dust levels and improved face ventilation. Sharp cutters and picks not only improve cutting efficiency but significantly reduce dust generation. Regular maintenance and replacement is essential. Machines should be kept cutting in coal. It is known that cutting in stone can produce up to 20 times more dust than cutting coal. There are obviously many more reasons why this practice should be avoided. 4.2.2 Loading - Metalliferous Mines Track and tyre mounted loaders are widely used for mucking blasted ore. Water sprays are most commonly used to control dust from this source. It is essential to re-apply water as the muck pile is removed. Adequate ventilation is necessary to dilute any dust that is liberated. The minimum lateral velocity required to remove dust is 0,5 m/s. The exhaust system of ventilation can be used to limit the exposure of persons in very dusty operations. This system is particularly useful in diamond mines where water decomposes the Kimberlite and dry mining is consequently practiced. 4.3 Raise and Blind Hole Boring Equipment Big hole boring machinery has been in use in South Africa since 1969. With this type of equipment, and because of the high rate at which rock is broken, high dust concentrations can be encountered. In the case of raise boring machines, during the pilot hole operations, rock chips and dust are created by the crushing and rolling action of the tricone drill bit. The most widely practised method of removing the chips from the pilot hole is by means of compressed air injected through the centre of the hollow drill string. The compressed air forces the chippings from the bottom of the hole into the annulus created between the drill pipe and the larger diameter bored hole. They then proceed up the annulus to the top of the hole and are directed into a discharge pipe. The rotating drill pipe is sealed at the top of the hole by means of a light fitting rubber seal, known as the blooie seal, which prevents the rock chips and dust from being blown out of the hole onto the machine thus ensuring that they all pass into the discharge pipe and are removed from the system at some remote point. Dust from the pilot hole is controlled by the addition of water to the compressed air before it enters the centre of the drill pipe. Generally only 0,75 /s of water is required for complete dust control. For the drilling of very deep pilot holes water under pressure is used instead of compressed air as the flushing medium because normal mine air pressure is inadequate. In both cases if the "blooie seal" is maintained in good condition, dust from the pilot hole need not be a hazard. See Figure 15.1. In the reaming cycle, Figure 15.2, potentially large quantities of dust can be produced by the action of the reamer cutters. However once the reamer has bored a short distance up the raise, dust control is generally not a difficult problem because the muck pile can be built up to mask the bottom of the raise, in which case care should be exercised to ensure that it does not build up too far and consolidate in the raise. The "collaring in" of the reamer is the most difficult period because at this time the cutting face is completely open and considerable attention is necessary to dispel the dust harmlessly. This is accomplished by means of a water blast from a hose sprayed continuously against the cutting face. Some reamer manufacturers have provided spray systems on the cutterhead with water supplied from the machine via the centre of the drill string. In the case of blind hole boring dust is suppressed at the cutterhead by means of a dual water feed. A water hose supplies a series of sprays mounted on the underside of the adaptor plate. From this position the sprays are able to wet all of the material passing through the muck aperture. 384 Environmental Engineering in S.A. Mines Figure 15.1 Dust control during pilot hole drilling for a raise borer A second hose supplies water through the centre of the gear box to the tricone bit at the front of the hole. This water cascades down and washes the chips away from the cutter head. This method has proved to be effective in minimizing dust in the vicinity of the machine to the extent that dust from drilling has never been considered to be a problem provided the water feed to the cutter head is maintained. As an additional safeguard against the liberation of dust, an exhaust system and filter can be employed which draws air in through the discharge hopper as shown in Figure 15.3. 4.4. Drilling 4.4.1 Drilling - metalliferous A large quantity of rock is pulverised to form dust during the drilling process and if this dust becomes airborne it constitutes a major dust hazard of underground mining. Potentially, the rotary-percussive type of machine-drill in general use in South African gold mines presents a much greater dust hazard than, say, a rotary diamond drill, because with the former type of drill all the rock from the hole is pulverised, whereas with the latter a solid core is generally formed; nevertheless, the modern rotary-percussive machine, if maintained in good condition and properly used, has been shown to produce very little airborne dust from the rock except during the comparatively short period of "collaring". The most important means of preventing the entry into the atmosphere of dust which is produced by drilling is the use of adequate quantities of water. The water must be at such a pressure (not less than 150 kilopascals at the drill) that a sufficient quantity is provided to keep the rock surface wet all Sources and Methods of Dust Control 385 BROKEN ROCK PILE Figure 15.2 Reaming with a raise borer the time, so that the rock is actually broken under a film of water. Breaking rock under water is a highly efficient way of preventing the dust produced from becoming airborne, but an inadequate film of water, or the presence of air bubbles, will permit the dust to enter the atmosphere. Most modern drilling machines therefore, are so designed that a flow of air-free water is provided at the bottom of the hole being drilled. This does not, however, prevent dust from entering the air during the initial collaring period when the water is not confined to the hole and the dust cannot be so easily trapped. Various means have been tried to prevent the escape of dust during collaring, ranging from simple hand-held sprays to elaborate types of suction traps round the end of the drill steel, but no single method has been found to be very efficient. 386 Environmental Engineering in S.A. Mines STAEHlt ?ING PINS If some of the compressed air operating the drill leaks into the front head of the drill and escapes down the drill steel, it will cause dry drilling and carry out of the hole the dust thus created. Also some of this compressed air will escape through the front head release ports and in doing so will atomise some of the water also in the front head. This atomised water, which forms a fog at the front head release ports of many rock-drills, evaporates rapidly and if the water is dirty, as it often is, many dust particles will remain in the air. The use of the sealed-spline rock-drill reduces the amount of dust produced at this source considerably. The piston of such a rock-drill is designed so that the splines on the back of the piston are not exposed while the air adjacent to it is under presssure. , This forms a seal which prevents excessive leakage of compressed air into the front head of the machine. 4.4.2 Drilling - coal Hand held electric drills are often used which have no axial water feed through the drill stem. Duff collectors are used with these machines to reduce dust liberation. An external water spray has recently been perfected which dramatically reduces dust from dry drilling in coal. 4.4.3 Drilling - diamonds As mentioned previously Kimberlite decomposes rapidly when wetted. Two techniques have been successfully employed to control dust during drilling as described below. Foam drilling The objectives here are: (a) Reduce the required volume of water. (b) Minimize hole erosion. (c) Amalgamate fines and keep drill chips dry. . Sources and Methods of Dust Control 387 (d) Keep drill-steel dry and ensure that particles do not bind on the steel. (e) Disperse drill cuttings homogeneously throughout the hole to prevent sticking and (f) Ensure laminar flow characteristics thus providing an efficient flushing system. An effective foam generating system is shown in Figure 15.4. 2% by volume of foaming additive is mixed into the water supply. Foam is generated at the needle valve shown and is piped into the rock drill. About 2,0 of water is used to drill 1 hole representing some 90% reduction in water consumption. The standard rock-drill was modified to incorporate a drill steel retainer and a larger more robust water tube. A recess was also machined into the shank of the drill steel to accommodate a rubber seal through which the water tube enters the shank. Dust ejection, conveying and disposal system for dry drilling In this method dust is captured at the collar of the drill hole in a rubber cowl which is connected with a plastic hose to a pipe range and a filtering and disposal system as shown in Figure 15.5. The dust collecting package can be trolley mounted for mobility or fixed mounted as shown. 4.5 Transportation of Rock Increased mechanisation in the handling of rock has become a major source of dust. In South African gold mines at present there are almost as many scrapers used as there are rock-drills, and a large number of mechanical loaders are also in common use. Control of the dust produced by these devices is difficult. Good ventilation is of course essential, unless exposure to the dust can be avoided, and the controlled use of water has been shown to help considerably. Figure 15.4 Chemical solution supply system for foam drilling 388 Environmental Engineering in S.A. Mines 1) 1018 MSP Blower duty 0,45mJ/s at 15kPa 2) 2 x 390mm diameter cyclones 3) Expansion box 4) Motorised flap valve 5) Vehicle container bucket 6) 150mm diameter galvanised spiral vent.ducting 7) 100 mm diameter plastic hose 8) Suction cowl 9) Drill (Jack hammer or rotary drill) 10) Cyclone discharge to R.A.W. Figure 15.5 Dust conveying system for any drilling Scraping, or slushing, in particular is a major cause of dust generation underground. Dust is generated mainly by the abrasion of the footwall and/or rock on the footwall by the scraper blades. It has been shown that dust concentrations produced by gully scraping could be reduced considerably by:-- (i) using high-capacity shovels of as light a construction as possible, consistent with operating requirements, (ii) reducing scraper speeds to approximately 0,6 m/s or less, and (iii) using scraper shovels in tandem as required for the desired tonnage. As air which flows along scraper gullies often finds its way to nearby working faces, it is essential to reduce the amount of dust produced during scraping operations. Broken rock should be wetted down thoroughly before being moved and maintained in a wet condition. To overcome the shortcomings of conventional methods of dust prevention in scraper gullies, a device known as a "drip-feed" has been used. It consists of a length of hose, of either rubber or plastic, with holes or slots at approximately 150 mm intervals along its length. The length of the hose should be sufficient to cover the width of the scraper gullies. One end of the hose is sealed and the other connected to a water supply. The drip-feed is mounted across the scraper gully in such a position that it is clear of the scraper rope and is situated at points at which the broken rock normally accumulates during scraping operations, that is, near the top of the scraper gully and at points at which the rock is transferred from one scraper shovel to another. In a long gully there should be drip-feeds at, say, 45 to 70 m intervals. Access to stope faces which have been blasted is usually limited due to the accumulation of broken rock, making effective wetting-down a difficult task. Consequently, wetting-down is often inadequate. High dust concentrations are, therefore, frequently generated when the scrapers start operating. Sources and Methods of Dust Control 389 4.6 Tipping and transfer points When ore is handled underground, it often falls either over rock surfaces or through the air. The greater the impact, the greater the amount of dust produced, and in certain situations such as at large tips and belt transfer points high dust concentrations may be generated in spite of the measures taken to keep the material wet. These high dust concentrations are usually caused by the displacement of air from the orepass or bin by the falling material. The mass and height of fall are major influencing factors. At these tipping or transfer points it is usual to downcast the air and filter it before the air rejoins the main circuit. To effectively downcast air the tip or bin opening must be minimised and the downcast velocity should be greater than 0,5 m/s. Tips are often located in main airways and it is preferable to keep lateral velocities as low as possible to avoid dust entrainment. This can be accomplished by routing air through alternative airways. Many types of filter are used, but by far the most common is the fabric bag filter. During 1980, out of a total of 1 573 filter units used underground in 38 South African gold mines, 88 per cent were of the fabric bag filter type, with a total capacity of over 9 000 m3/s. The tendency of late is toward reverse pulse filters which feature automatic cleaning. Most of these filters were installed at tips. Anti-static fabrics only should be used when filtering explosible dusts. 4.7 Dust raised in travelling ways, tractor/haulage roads and intake airways Vehicular and pedestrian traffic in main intake airways can be the cause of a high level of dust contamination. The use of water may not always resolve this problem because of the frequency with which it must be applied and the need in deep level mining to maintain low relative humidities in the intake air. Calcium chloride in solution and also an emulsified oil have been used with some considerable effect in allaying the dust on the footwall or floor of these airways. Obviously success will depend on (a) the concentration of the solution; (b) the amount of traffic; (c) the frequency of application, and (d) relative humidity of ambient air, in the case of calcium chloride. Intermittent use of fog nozzles has been found to be effective in controlling dust in tractor roads on collieries. 5 MAIN DUST-PRODUCING OPERATIONS ON THE SURFACE OF MINES 5.1 Transfer Points If the material is wet the dust produced is minimal. The exceptions are large tips and high-drop transfer points. The handling of dry materials invariably produces dust levels which must be controlled. See section 4.6. Some useful design data for extraction hoods at belt transfer points are given in Figure 15.6. 5.2 Screens Stationary or vibrating screens are in common use for size classification of the ore mined. Large amounts of dust are released if the material is allowed to dry, but if it is wet, screening does not usually present a hazard. If dust levels are high screens are closed in and exhaust ventilation used with a wet scrubber. 390 Environmental Engineering in S.A. Mines Conveyor transfer less than 1,0 m fall.For greater fall provide additional exhaust at lower belt at (A) 50mm clearance for load on belt Detail of belt opening DESIGN DATA Enclose to provide 0,75 to 1,0 m/s in draft at all openings. Minimum Q = 0,5m3/s/m belt width for belt speeds under 1,0 m/s = 0,75m3/s/m belt width for belt speeds over 1,0 m/s and for magnetic sepa rators. Duct velocity = 18m/s minimum Entry loss = 0,25VP Chute to belt tranfer and conveyor transfer, greater than 1,0m fall Use additional exhaust at (A) for dusty material. Belt width 300-900mm Q= 0,35 m3/s above 900mm Q=0,5m3/s Figure 15.6 Dust control at belt transfer points 5.3 Crushers Where the ore must be crushed large quantities of dust are produced. Most crushers are supplied with exhaust ventilation, the method applied depending on the type of crusher, size and location. To prevent contaminating the atmosphere the wet scrubber type of filter is generally used. Enclosures are also effective and sometimes essential. Sources and Methods of Dust Control 391 5.4 Rotary Filters Considerable dust can be released at the blow-off points of rotary filters. Here the pulp is filtered from the solution containing the ore, and the "cake" of finely-divided rock is blown off the filter by means of compressed air. This can produce high dust concentrations especially if the air pressures used are too high, but the dust can be controlled by exhaust ventilation. Observations of the blow-off have indicated that while the cake is dislodged within less than a second, compressed air continues to pass through the interstices of the cloth and to atomise the sludge. During the initial dislodging period, the cake loosens and, before dropping away, actually acts as a protective blanket preventing atomised sludge from being blown from the cloth into the atmosphere. The blow-off period can be limited and confined to the nett time required for dislodging the cake, thus reducing the amount of dust, by means of automatic cut-off valves attached to the compressed air supply at each filter. This method of control is commonly referred to as the "snap blow-off". 5.5 Workshops Dust conditions in workshops have been found to vary widely. The presence of forge fires, open hearth fires, compressed air hammers, cutting machines, oxy-acetylene torches, electric arc welding plants and the conducting of a number of operations not only cause local contamination but also give the general atmosphere a very mixed dust and smoke content. Oxy-acetylene torches and electric arc welding result in contamination of the air by metal fumes which often produce high dust concentrations. The silica content of the dust is generally low except where work on, or salvage of, underground equipment is the major operation. Welding fumes are invariably present in most workshops, and fixed or adjustable exhaust hoods fitted over the workshop benches, as well as flexible exhaust hoods with magnetic collars, should be used to draw fumes away from the operators. As helmets must be worn by welders for protection of the eyes, a small volume of air at a positive pressure can be supplied to the inside of the helmet by means of a tube encircling the lens holder. This protects the wearer from fumes, and also provides relief from the radiant heat produced by oxy-acetylene or arc welding units. 5.6 Grading Rooms Samples of crushed ore in the form of sludge are taken at regular intervals for grading in order to check on milling efficiencies. These samples are dried, mixed, weighed and graded. As the grading is done in sealed Tyler screen units no dust is released. The mixing process can release high concentrations of dust which can only be controlled if the operations are done in a ventilated fume cabinet. 5.7 Assay Crushers Rock samples for assay purposes have to be crushed and pulverized and this produces high dust levels. Such dust must be controlled as near as possible to the point at which it is produced and before being dispersed into the atmosphere. In practice the source of dust should be totally enclosed and placed under a negative air pressure or alternatively, an exhaust hood installed near the source of the dust. The emphasis on the design of such dust control systems should be placed on exhausting sufficient air to create velocities in the zone to be ventilated that will control the dust-bearing air currents before the breathing zone of the operators is reached. 392 Environmental Engineering in S.A. Mines 5.8 Smelting Operations Hoods are normally fitted to the furnaces and the air exhausted is filtered by means of scrubbers or preferably bag filters to collect the gold, silver, lead and zinc constituents before being exhausted to atmosphere by means of stacks. Generally where dusts (such as manganese and lime) are being transported and handled exhaust ventilation is resorted to. 6 CLASSIFICATION OF DUST EXTRACTION PLANTS A wide variety of dust extraction plants is available and it is not the intention in this chapter to describe them in detail. Reference can be made to manufacturers' specifications for this purpose. The table below is offered as a guide for the selection of any type of plant and the principle of operation is indicated by the diagrammatic sketches in Figs. 15,7, 15,8 and 15,9. Type of Plant Size Range Jim Efficiency % on Respirable Fraction Scrubber Flannel bags Cyclones Venturi scrubbers Electrostatic precipitators 3 to 100 + 0,1 to 100 + 3 to 100 + 0,01 to 100 + 0,01 to 100 + 70 99 + 70 90 + 90 to 95 *Can be high when reverse pulse cleaning is used. Air Velocity m/s 2 0,025 to 0,125 20 60 to 120 1 to 2 Dust Load Low Low* High High High Pressure Drop kPa 0,025 to 1,25 0,25 to 0,75 0,75 to 1,25 2,5 to 5,0 0,05 to 0,125 Clean air k Figure 15.7 Flannel bag filter Sources and Methods of Dust Control 393 Figure 15.9 Electrostatic filter 394 Environmental Engineering in S.A. Mines BIBLIOGRAPHY Guide to the Prevention and Suppression of Dust in Mining, Tunnelling and Quarrying. Published by International Labour Office, Geneva. Control of Harmful Dust in Coal Mines Published by the National Coal Board. Development of Optimized Diffuser and Spray Fan Systems for Coal Mine Fan Ventilation. Prepared for the U.S. Bureau of Mines by Foster-Miller Associates, Inc. Waltham Mass. Industrial Ventilation - A Manual of Recommended Practice. American Conference of Governmental Industrial Hygienists. Rabson, S. R., Quilliam, J. H., and Goldblatt, E. The elimination of nitrous fumes from blasting gases. J. S. Afr. Inst. Min. Metall. vol. 61, 1960. p. 152-181. Respirable Dust Control. Proceedings Bureau of Mines Technology Transfer Seminars, Pittsburg, Pa Sept. 21, 1976 & St. Louis, MO, Sept. 23, 1976. 1C 8753 Bureau of Mines Info Circular/1977.