cient in determining whether or not a hazard exists. Samples obtained by the konimeter are grab samples, whereas those taken with the impinger are of larger volumes of air over a longer period of time. Dust-determination apparatus now available requires considerable training to use, and the laboratory processes involved are not simple. However, once a program of dust control has been adopted, a fairly simple routine sampling method will serve to keep a check on existing conditions. Certainly, atmospheric dust that can be seen does not require continued sampling and analysis. Wet Methods DUST CONTROL Water and sometimes other liquids are employed to control dust by suppressing it at its point of origin, by removing it from the air surrounding the source, and by preventing settled dust from being stirred up. Water applied in drilling, loading, handling, and crushing rock or ore kills the dust to the extent of wetting accomplished; with some materials and very fine sizes of dust, other liquids may have to be put into the water to increase the wetting action. In some operations it may be found necessary to prevent dust-laden mist from escaping into the air, since dust-laden vapor may evaporate, leaving dust on the walls or in the air. Inhalation of such vapors introduces the contained dust into the lungs, sometimes in harmful amounts. Where mixtures of air and water pass through drill steel, dust escapes into the surrounding air, although in much smaller amounts than in dry drilling. Where water alone is used to clear the hole while drilling, very little dust is introduced into the air. Longhole diamond drilling is less dusty than ordinary drilling, as the holes are deep and more water is used.28 Dust is washed out of the air by sprays or atomizers operated when excessive quantities are being produced, as at a face after blasting. Fine sprays or curtains of mist are probably more effective in removing microscopic particles than heavier streams, but the degree of improvement may be less than is anticipated. However, water sprays do reduce the dustiness, and the method is particularly useful underground. A tank car with spray nozzles for allaying dust in haulage drifts is shown in figure 6, and an automatically controlled spray for wetting the tops of loaded trips is pictured in figure 7. Figure 8 shows a water curtain in a drift, and figure 9, misting nozzles installed at the outlet of an exhaust-air duct to settle dust in mines. Wetting surfaces on which dust settles to cake and hold fine particles or washing down these surfaces to remove accumulations will prevent redispersion of such dust into the air. Figure 10 is a view of a shrinkage stope where a pile of broken ore is being wet down with a hose stream. The details of an air-water blast for use in drifts and crosscuts during and after blasting are given in figure 11, and in figure 12 the device is shown installed at the face of a drift. 28 Johnson, E. W., and Cash, F. E., Diamond Drilling of Blast Holes, Lake Superior District Iron-Ore Mines: Bureau of Mines Inf. Circ. 7317, 1945, 11 pp. The interest in dry-dust collectors lapsed with the general adoption of wet drilling in metal and nonmetallic mining and other hard-rock operations. The recent advent of rock bolting for roof and wall support, with its attendant drill-dust problem, particularly in coal mines, has renewed interest in the dry dust collector, and several have been developed for use in connection with roof drilling in coal mines. Informal tests of some collectors by the Bureau of Mines aided in their development, and the Bureau has issued approval requirements under which dry-dust collectors may be approved for all types of rock drilling in coal mines. The work on dry-dust collectors, reviewed by Berger in 1952,29 described the types of collectors and stressed that proper maintenance and use of dry-dust collectors are essential if adequate protection against dust exposure is to be obtained. Fixed installations, such as crushers or screens, where an unusual amount of dust is made, may be provided with exhaust systems that remove the major part of the dust directly from the unit so that it does not spread into the surrounding air. Dust-collecting systems 29 Berger, L. B., Progress in Development of Dry Dust Collectors: Pres. at meeting of Coal Min. Sec., Nat. Safety Cong., Chicago, Ill., Oct. 22, 1952, 11 pp. Berger, L. B., Types of Dust Collectors Used in Conjunction With Roof-Bolt Drilling: Pres. at meeting of Coal Min. Inst. America, Pittsburgh, Pa., Dec. 12, 1952, 9 pp. FIGURE 10.-Watering in Shrinkage Stope. have been successfully applied to surface plants at many mines; for example, the installation shown in figure 13 removes the dust from over a conveyor belt in a shaft house. Percussion-type drills are rarely operated dry when used underground, except when blockholing and sometimes when collaring holes. These practices create objectionable health conditions, because the dust-laden atmosphere usually is not removed by adequate ventilation. In the interest of health, dry drilling should not be done under any circumstances at an underground location. Devices de 386913 0-57-4 |