is large it is covered with dry boards and the dust is placed upon these boards. This method of distribution insures that practically all of the dust is free to be raised into the air by the force of the advaric

ing explosion.

SIZES OF DUST USED.

The coal dust used in the explosibility tests is ground in the grinding house at the experimental mine from run-of-mine coal. The coal is first crushed in a hammer crusher having a screen with iV-inch slots in it. It is then passed through a pulverizer which has beaters revolving at a very high velocity. The fine material is elevated by an air current to dust collectors, from which the dust is drawn off as needed. The strength of the elevating air current can be modified by changing the size of the inlet openings, thus regulating the size of the dust particles that are carried out. Early in the series of tests a definite size of inlet opening was adopted, and this size was used hi the preparation of the dust used. Only occasionally was any change necessary to insure about the same percentage of fine sizes in the various dusts tested. In some of the tests the dust was used as it came from the crusher, no pulverizing being done. In still other tests some of the pulverized dust was mixed with the coarse dust in such proportions that the mixture would have a certain predetermined percentage of 200-mesh dust. The shale dust used in any particular test was generally prepared similarly to the coal dust for the same test.

MIXING AND DISTRIBUTION OF DUST IN STANDARD TESTS.

TUe coal dust and shale dust are weighed out in such relative proportions that a mixture is given having the percentages desired for the test planned. A total of 50 to 100 pounds of the two constituents are placed in a chamber of a mixing apparatus consisting of a metal can with a dust-tight cover, which is then revolved for a sufficient number of turns to mix thoroughly the constituents. The dust is then dumped from the mixing chamber and a known weight shoveled into cans. The covered cans, each containing about 150 pounds of dust, are taken into the mine and placed at measured intervals. Usually the mixing and distribution of dust take place the same day

the test is made.

SAMPLING OF DXTSTS.

Samples of each size of the coal dust and shale dust used are taken before the dusts are mixed. Screen tests and analyses are made of these samples. Two or more samples of the mixed dust are taken from the shelves in the mine just before firing the igniting shot and are analyzed to determine whether the mixture as actually tested is the same as that planned. The analyses were made at the Pittsburgh station under the direction of A. C. Fieldner, in charge of the chemical research laboratories. Most of the screening tests were made at the experimental mine; in the earlier tests the screens were shaken by hand but later the screens were shaken by a mechanical shaker.

Some trouble was encountered in determining the percentage of 200-mesh dust. Three screens, all nominally 200-mesh, were used during the test period. Screen A, used during the early part of the period, was later found to have holes larger than the standard size, and Screen B was found to have holes too small. A third screen, C, was obtained later which was tested and found to be correct and was used as the standard in all subsequent tests. The results with all three screens are given for some of the dust, and from this, comparison of the three screens can be made and some idea gained of the probable error in the early series in which only the A screen was used or in those tests where the B screen was used.

CHAPTER V.—EXPLOSIBILITY TESTING OF DIFFERENTSIZED PITTSBURGH DUSTS.

PRELIMINARY CONSIDERATIONS.

In making determinations of the relative explosibility of the dusts of various kinds of coal, finely ground or pulverized dust has generally been used. The percentages of fine sizes—that is, of dust passing through 100-mesh and 200-mesh screens—vary only slightly in the pulverized dusts, so that variations in results of tests are principally due to other factors affecting the explosibility. It was recognized in the course of these investigations that the relative behavior in an explosion of dusts pulverized by machinery was not necessarily identical with the behavior of dusts from the same coals made incidentally in mining operations; that is, in mining operations all coals do not break alike. Some coals, like the subbituminous and splint coals, tend to break cubically and do not make fine dust, whereas other friable coals, like the smokeless coals of West Virginia, make much fine dust. This condition is reflected in freedom from disastrous explosions of some mining districts as in those first named, while many explosions have in the past occurred in districts in which friable coals are mined. When the coal dust is produced by crushers and grinders or pulverizers, the natural structure of the coal fragments is broken down, and the coal dusts thus produced are highly explosive, like the pulverized dust of subbituminous coals, and the relative behavior of the various pulverized dusts depends, as will be hereinafter indicated, on characteristics other than the structure, such as content of volatile combustible and the ash and moisture ratios.

Accordingly after standard methods had been determined upon, using pulverized dust, it was considered necessary to make tests upon dusts of different natural sizes, first of Pittsburgh coal and subsequently of dusts of various coal beds in different mining districts.

The reader who compares the explosibility limits and effects produced in these experimental mine tests with those obtained by foreign stations, must bear in mind that size of dust is one of the most important factors and as between different kinds of bituminous dusts, is the most important factor.

WHAT CONSTITUTES COAL DTJST.

In the comparison of naturally or incidentally made dusts of coal mines it is first essential to know what constitutes dust, and particularly coal dust. This question was considered in Bulletin 203B under

• Rice, Q. 8., and others, The exploslbility of coal dust: Bull. 20, Bureau of lilnes, 1911, p. 33. 55617°—22 14 187

the heading "Coal dust and its origin and distribution," wherein it is stated: "Coal that will pass through 100-mesh screens is frequently accepted as representing mine dust." Tests were made in the Pittsburgh gallery 40 to determine how coarse coal dust could be and yet propagate an explosion. The tests were made successively on pure coal dusts in four sizes: (1) Through 80 and over 100 mesh; (2) through 60 and over 80 mesh; (3) through 40 and over 60 mesh; (4) through 20 and over 40 mesh. The fine dust made by abrasion in the process of screening and handling was floated off by stirring with a jet of compressed air. The results showed ready propagation of sizes (1), (2), and (3) and partial propagation of (4); the belief was expressed that, according to the indications, propagation of (4) might have been obtained with a larger initiating charge than that used (2J pounds of black powder). Accordingly the senior author, in charge of the investigation, recommended tentatively that 20-mesh be accepted as the dividing line between fine coal and dust, and stated: "It must not be inferred that larger particles of coal may not be a factor in a mine explosion once fairly started. The assumption as to the size (20-mesh) therefore refers only to the initial stage of an explosion."

It should be noted also that in the tests referred to the finer sizes of coal dust were excluded in each test, whereas if the fine dust were included it is probable that dust coarser than 20-mesh might be a factor in a strong, prolonged explosion in which coarse dust might be carried along in the active combustion zone.

The term "combustion zone" is here used in the sense of a fastmoving flaming body that constitutes the constantly progressing locus of violent chemical reactions of any individual branch of an •explosion, traveling through a mine entry or a room filled with air, dust and gases.

In the tests at the experimental mine on the influence of. different sizes of dust on their relative explosibility it was therefore deemed best to include the several screen sizes in any one sample up to the maximum screen size selected for the particular test.

ADOPTION OF 20-MESH AS COARSEST LIMIT FOR DUST TESTING.

It was decided in beginning this series of tests to try coal particles as large as would pass through a screen with 1-inch square openings. Later it was found that for the length of the standard test zone particles larger than those passing through a 20-mesh wire screen (openings between wires about 1/30 inch square) had no appreciable influence upon the explosions, so 20-mesh dust was accepted as the upper or larger limit in the size of dust. Any particles over this size

« Kloe, O. S., work cited, p. 42.

was rejected in making subsequent standardized tests" of cxplosibility. It became evident from screening tests that in artificially prepared dust — that is, dust made by pulverizing machinery — the percentages of the different screen sizes finer than the coarsest size included in the sample under test varied, even with dust of the same kind of coal; the variation was probably due chiefly to differences in the time of exposure in the grinding and pulverizing machines, but also to variations in conditions of the machines. Accordingly it became evident that numerous road-dust and rib-dust samples of different mine dusts should be gathered in order to determine the average proportion of each smaller size, so that artificially made dust, prepared from the large coal sample from that mine, be made to conform as nearly as possible in respect to the proportions of fine sizes therein for the primary explosion testing. Later, in each series of tests relating to the particular mine, specific quantities of rock dust were added successively until ignition or propagation, as the test might be, was no longer obtained, thus fixing what is termed the limits of explosibility for those dusts.

A large number of samples were gathered from Illinois mines for sizing tests.41

After discarding the dust coarser than 20-mesh, the aver

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Figure 28.—Average sizing test curves for road and rib dusts from 100 Illinois mines.

age of the remaining finer sizes

showed 37.2 per cent through

100-mesh and 25.3 per cent

through 200-mesh for the road

samples. The average for the

rib samples was 44.8 per cent through 100-mesh and 31.1 per

cent through 200-mesh. The curves of these average values are

shown in figure 28.

Figure 29 shows how nearly one of the standard sizes adopted for testing in the mine agrees with a single sample taken from one of the Illinois mines. The curve on the right is for the mine sample and that on the left is the dust prepared for test at the experimental mine. The curves are very close together for the fine sizes, which are the most important. The coarser sizes, where the curves separate slightly, are of relatively less importance.

As any dust or mixture of dusts consists of different proportions of each of the various sizes present, even when all the dust will pass

11 See Clement, J. K., and Scholl, L. A., Jr., The Inflammability of Illinois coal dusts: Bull. 102, Bureau of Kines, 1916, 74pp.

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