TABLE 1.-Maximum allowable concentrations of (industrial) atmospheric contaminants, 1956-Continued After more than 25 years of general underground experience in mines of all types and from extensive study of dusts for 8 years, Harrington 23 concluded that: Any dust insoluble in the fluids of the respiratory passages and in sufficiently finely divided form to float in the air and be breathed by underground workers will ultimately be harmful to health if the dust is in the air in large quantities and if breathed by workers for considerable periods of time. This applies to nonmineral as well as mineral dusts or mixtures of them and includes coal dust or mixtures of coal and other dusts. There are also definitely harmful mine dusts which are soluble, and some dusts experts appear to believe that so-called insoluble dusts under certain conditions become soluble and are harmful only when soluble. Silica dusts are stated to be the most harmful of the ordinary dusts to breathe, and considerable quantities of free silica (SiO2) and com 23 Harrington, D., Dust and the Health of Miners: Pres. at AIME meeting, New York, N. Y., February 1924, 5 pp. bined silica are likely to be found in the dust of most hardrock mines. The percentages of various constituents of rock from mines where dust is known to have caused silicosis are listed in table 2. The analyses in the table show only that silica is a principal constituent of the rocks at most metal and nonmetal mines. In only one of the samples has the total silica content been separated into free silica and silicates. Exact analyses have little importance because of local variations of rock in mines and because the fine, suspended atmospheric dust does not ordinarily have the same percentages of constituents as the rock from which it comes. DUST PRODUCED BY MINING OPERATIONS Nearly all of the ordinary mining and milling processes tend to produce or disseminate dust unless water is applied or the material and surroundings are naturally wet. Certainly blasting, shoveling, hauling, timbering, dry drilling, chute loading, and other activities in dry metal or nonmetallic mines are likely to make and stir up dust. The amounts of dust produced by various mining operations have been compared by the Bureau of Mines in investigations made at the Mount Weather, Va., testing adit. Samples of atmospheric dust were collected during the cycle of drilling, blasting, and mucking in a drift face. The working face, in hard basalt, was ventilated only by the natural movement of air, except after blasting and during mucking, when 1,100 cubic feet of air per minute was brought to within 50 feet of the face through canvas tubing. The dust concentrations are given in table 3. 24 The Bureau of Mines has made studies of the dust concentrations during normal activities in many mines; the amounts listed in table 4 are more or less representative of the amounts of dust associated with various operations. DETERMINATION OF DUST CONCENTRATIONS Because the range in dust concentrations in mining and treating ores and minerals is very great, the instruments used in sampling dusts should operate with efficiency in both high and low dust concentrations. These devices must also be capable of extracting reasonably accurate samples of dusts, which may vary greatly in size of particles. The instruments now in use are not ideal, but they are adequate for the purposes for which dust sampling is done, and one or another can be selected, depending on whether a routine check on dust-control methods or a detailed dust survey is being made. The impinger apparatus, shown in figures 4 and 5, has been employed as standard apparatus for dust surveys. This instrument and the technique of sampling and enumerating described by the Bureau of Mines 25 26 have been found of practical value for study 24 Johnson, J. A., and Agnew, W. G., Relative Air Dustiness During Cycle of Operations at Mount Weather Testing Adit: Bureau of Mines Rept. of Investigations 3453, 1939, 7 pp. 25 Brown, C. E., and Schrenk, H. H., A Technique for Use of the Impinger Method: Bureau of Mines Inf. Circ. 7026, 1938, 20 pp. 20 Brown, C. E., Midget Microprojector for Dust Determinations: Bureau of Mines Rept. of Investigations 3780, 1944, 14 pp. 1 Sayers, R. R., Silicosis Among Miners: Bureau of Mines Tech. Paper 372, 1925, pp. 6–7. TABLE 3.-Dust produced during work at face Face idle.. Charging holes. After blasting, 5 minutes, not ventilated. After blasting, 60 minutes, ventilated. After blasting, 140 minutes, ventilated. Mucking, wet, ventilated. Operation 15.9 21.9 1 1.9 ing the dust problem of mining operations. Other devices that also sample dusts over a continuous period are the electrical precipitator, the paper thimble, the electrostatic precipitator, and the hot-wire thermal precipitator. The konimeter, which samples dust by impinging air at a high velocity against a sticky glass plate, has been described in detail by the Bureau of Mines.27 The konimeter has been found to be efficient and may be employed for concentrations not exceeding 35 million particles per cubic foot of air; however, this limit usually is suffi 27 Littlefield, J. B., Brown, C. E., and Schrenk, H. H., Technique for Routine Use of the Konimeter: Bureau of Mines Inf. Circ. 6993, 1938, 6 pp. |