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3ics R. I. 3465 UNITED STATES DEPARTMENT OF THE INTERIOR Harold L. Ickes, Secretary BUREAU OF MINES John W. Finch, Director KENT STATE UNIVERSITY / 3 1850 022 840 046 September 1939 REPORT OF INVESTIGATIONS METHODS OF ROCK-DUSTING AMERICAN COAL MINES KENT STATE UNiVER US.. DOCS. BY J. J. Forbes SCF-FA-0603 After this report has served your purpose and if you have no further need for it. please return it to THE BUREAU OF MINES. USING THE OFFICIAL MAILING LABEL ON THE INSIDE OF THE BACK COVER. / R. I. 3U65, September 1939 REPORT OF INVESTIGATIONS UNITED STATES DEPARTMENT OF THE INTERIOR - BUREAU OF MINES METHODS OF ROCK-DUSTING AMERICAN COAL MINESi/ By J. J. Forbes INTRODUCTION Investigations by the. Bureau of Mines and by mining-research depart ments of European countries have established definitely the value of rock dusting to prevent or limit the propagation of coal-mine explosions. Its effectiveness in this connection has been demonstrated not only by research but by actual coal-mine explosions in which hundreds of lives have been saved because of its presence. In a general way this paper gives pertinent facts concerning rock dusting practices in the United States. Information has been obtained from many reliable sources, and various coal-mining companies have furnished data on cost and other details. HISTORY OF ROCK-DUSTING IN THE UNITED STATES The use of rock dust to prevent or limit coal-dust explosions in bituminous-coal mines was first suggested in England in 1891 "by Sir William Garforth, who had noted that the Altofts mine explosion in 1887 nt penetrated any of the naturally shale-dusted passages. Rock-dusting has been advocated by the Bureau of Mines since 1910, but it was not until. 1923 and 192'4 that the mine operators in the United States considered it seriously, although a few mines had begun to use it experimentally as early as 1911 and 1913* The' first mine in the United States to experiment with rock dust was the Delagua mine of the Victor-American Fuel Co., Delagua, Colo., which in 1911 applied rock dust on the main trolley haulage entries by a locally developed machine consisting of an electrically driven blower and a funnel-shaped hopper mounted on a mine truck. During 1915* the Pittsburgh Coal Co. began rock-dusting experiments in two of its mines by applying limestone dust to the ribs and roof by hand. Following an explosion in one of its mines in Illinois in 1917* the Old Ben Coal Corporation began rock-dusting its mines. 1) The Bureau of Mines will welcome reprinting of this paper, provided the following footnote acknowledgment' is used: "Reprinted from Bureau of Mines Report of Investigations 3^65." Presented at the Fifth International Conference of Directors in Mine Safety Research, Pittsburgh, Pa., Sept. 25-30* 1939* 2/ Supervising engineer, Safety Division, Bureau of Mines, U. S. Depart ment of the Interior, Pittsburgh, Pa. ' CONFERENCE POSTPONED RFCAUSE OF CONDi fIONS IN EUftOPE R. I. 3465 Interest in rock-dusting American coal mines was stimulated by inves tigation of rock-dusting methods in England and Europe by G. S. Rice in 1923 and earlier, and meetings in 1924 with coal-mining operators in the various coal-mining States, at which the necessity for reck-dusting was stressed, resulted in the rock-dusting of a number of mines. INVESTIGATIONS ON ROCK-DUSTING BY THE BUREAU OF MINES Since its organization in 1910, the Bureau of Mines has advocated rock-dusting as a method of preventing or limiting coal-dust explosions. At first it was suggested as an alternative fbr watering, but finally in 1924, after many experiments with watering, salt solutions, and rock dust, the latter was declared to be the most reliable method known for successfully combatting the hazard of coal-dust explosions. Research work on rock-dusting as a means of preventing coal-dust explosions has been conducted by the Bureau nearly 30 years, and during that time many valuable data have been obtained and published. The explosibility of various sizes of coal dust from many coal beds has been determined, as well as the quantity of rock dust necessary to prevent ignition of coal dust and the propagation of an explosion. Research work relative to rock-dusting has been carried on by the Bureau in cooperation with the Carnegie Institute of Technology and the Advisory Board of Operators and Engineers of Western Pennsylvania. Results of this work were published in 1925 in a bulletin on "Method and Costs of Rock-Dusting Bituminous Coal Mines." STATE-LAW REQUIREMENTS FOR ROCK-DUSTING In addition to the laws enacted by various coal-mining States re quiring rock-dusting, ono of the most helpful features in the advancement of rock-dusting has boon the reduction by accident-liability insurance companies of the premium paid by coal companies on mines using rock dust. ' Eleven of the 25 coal-mining States have ohacted laws requiring rock-dusting in some form or other. In general, however, these laws are inadequate and in some States they are indefinite and leavo the question of rock-dusting to the discretion of the mine inspector* Only throe States (Nov/ Mexico, Utah, and Wyoming) have enactod laws that adequately define and require rock-dusting. In 10 of tho States water may bo used for sprinkling' roadways and workings in lieu of rock-dusting, Utah being the only State that requires rock-dusting in both nongassy and gassy mines regardless of moisture. .. . T Ala Col Ind Ker Nev Oh: Ok! Pe: ' Ut 7239 - 2- 72 R. I. 3^65 TABLE 1 -- State mine law recruirements for rock-dusting and watering Alabama: Water or rock dust is required in any mine containing coal dust that will ignite or explode. Colorado: Water or rock dust (a minimum of 60 percent inert material) is required when in the opinion of the chief mine inspector it is necessary. Indiana: In mines not maintained wet, rock dust must be applied. A minimum of 55 percent inert material is required. Rockdust barriers may be used in isolated panels. Monthly sampling is required. Kentucky: All dry and dusty operating sections must be watered or rock-dusted. . New Mexico: All mines must be rock-dusted unless in muddy condition at all times. All entries, slopes, and aircourses in nongassy mines, and in gassy mines all entries, slopes, aircourses, rooms, crosscuts, and pillar workings must be rock-dusted to within 4o feet of the face. Minimum of 55 percent inert material required. Where rock-dusting on roadways is not practicable, watering is permitted. Sampling at 90-day intervals with record of such sampling and analysis is required. Barriers are required at or near entrance to panels or on main entries and aircourses to isolate panel or section. Ohio: A mine or section thereof containing accumulations of coal dust is required to be rock-dusted, sprinkled, or otherwise treated at discretion of inspector. Oklahoma; Dry and dusty' sections of mines are required to be sprinkled or rock-dusted at discretion of mine inspector. Pennsylvania In such portions of dry and dusty mines where explosive gas can be detected by safety lamp, rooms and entries must be moistened by water. In lieu of water, rock dusting may be substituted" or may be used in conjunction with water. A minimum of 65 percent inert material is required. Record of sampling and analysis is required at 60-day intervals. Watering and rock-dusting are re quired at discretion of State mine inspector. Utah: 7239 Rock-dusting is required in all active working sections of mines to last crosscut and 50 feet in all rooms and pillar workings. In nongassy mines, 55 percent incom bustible material is required, and in gassy mines 65 per cent. When dust becomes below minimum, the section must be fenced off. Barriers are required in worked-out sections. 3- - / R. I. 3465 CABLE 1. - State mine law requirements for rock-dusting and watering - Coat1! West Virginia: Requires dry dust to be removed as far as practicable, and all dry-', dusty sections watered or rock-dusted.- Wyoming: . ' All coal mines, unless consistently damp in judgment of State inspector, must sprinkle dry and dusty places twice a week. In lieu of sprinkling, mine may be rock-dusted* Rock dust is to be maintained at a minimum of 60 percent inert, with 10 percent increase with each 1 percent methane*. The State inspector is required to :sanq>le and analyze dust* Barriers are permitted in trackless entries. PROGRESS. IN ROCK-DUSTING .. Accurate records showing the progress of rock-dusting in American mines are not available for the years prior to 1926. Not much consideration was~given to rock-dusting by coal-mine operators before 1923; however, by 1926 at least 177 minos in the United States were using rock dust, and by 1927 the number had increased markedly to 463 mines. "By 1933* the number of rock-dusted mines had decreased to 382, but by 193& it had increased again to 462* The fact that in 1936 (tho last year for which statistics are available) only 462 mines reported the use of rock dust shows that the progress of rock-dusting is by no means satisfactory. The favorable side of the record shows that the annual output of the mines using rock dust has increased from a low of 88,660,000 tons in 1932 (the lowest depression year) to l6l,000,000 tons in lS'jS; the mines using rock dust during 1936 produced more than 37 percent of the country's bituminous coal. The fact that mines using rock dust, represent but 7*5 percent of the total number of operating nines, yet produced 39*3 percent of the coal and employed 32*5 percent of the underground employees in 1936 indicates that it is chiefly the larger-capacity mines that are using rock dust. Mines in IS States used rock dust to some extent during 1938, but it ms used most widely in Utah; about 90 percent of the State's produc tion (including all rail-shipping mines) was from mines using rock dust. Wyoming, Now Mexico, Alabama, and West Virginia rank next to Utah in this respect. West Virginia reported 179 mines (33*^ percent) rock-dusted during 1938, compared to 29 mines rock-dusted in 1928. Although many Pennsylvania mines use rock dust, the State Condensation Rating and Inspection Bureau has given compensation credit to only 8 mines as being rock-dusted in . accordance with its standards. 7239 -4- R. I. 3465 Although progress in rock-dusting in coal mines in the United States has improved considerably since 1923 when scareely a dozen mines used rock dust, the 462 mines rock-dusted in 193^ represent only a small percentage of the total number of mines in operation. The application of rock dust to underground surfaces in American mines has been an important factor in preventing widespread explosions; however, while some of the most dangerous mines have been rock-dusted fairly adequately, in other mines rock dust has been applied without regard to the amount required to prevent propagation of flame. SPECIFIGATIOITS Specifications^J for rock-dusting were formulated by the American Engineering Standards Committee in 1927 and no changes have been made since then, although a committee of the American Institute of Mining and Metallurgical Engineers is now considering revision of these standards. The principal requirements are: Size of dust. - The finer the. dust, the more easily is it raised into the air with coal dust to prevent propagation of an explosion. All of the dust should pass through a 20-mesh sieve, and at least 50 percent should pass through' a 200-mesh sieve. (If a coarser dust is used, the amount applied must bo proportionately larger.) Tests conducted by the Bureau of Mines show tt&t finer dust is ad vantageous but that there is no additional advantage in having it finer than 70 percent through 200-mesh, Character of dust. - The dust preferably should contain not more than 5 percent combustible matter nor more than 25 percent free silica* The rock dust should be as light in color as possible. Enough dust must be present to maintain the incombustible matter at or above 55 percent. Where methane is found in the ventilating current, the amount of incombustible material specified must be raised approximately 10 percent for each 1 percent of gas. Redusting. - After dusting, samples should be taken at regular intervals and analyzed to determine whether or not additional rock-dusting is necessary. . The maximum permissible percentage of combustible matter varies with the rank of the coal. Each coal bed has its own limits. Moisture. - The dust should not absorb moisture from the air to such an extent as to cake and destroy its effectiveness as a dry dust. lj &ice, Gr. S., Sayers, R. R., and Harrington, D.f Rock-Dusting in Coal Mines: Bureau of Mines Inf. Circ. 6030, 1927, 3 PP 7239 .1 - 5- R. I. 3^65 TABLE 2. - Summary of data for mines in United. States using rock dust Humber Percent Production mines of all from mines Year using underground using rock mines using rock dust, dust rock dust short tons 1926 I77 - -- Percent of total produc tion from underground mines using rock dust - Percent of total under ground em ployees in mines using rock dust -- Percent of total man hours worked in mines using rock dust -- Rock dust used, pounds T* Rock dust used per ton of coal mined in rock-dusted mines, pounds - 1927 463 6.5 137,280,000 24.0 21.3 -- 1930 474 8.4 l4g,634,912 33.2 27.5 30.1 137,738,000 0.93 1931 449 8.3 115.904,819 31.9 26.8 29.1 119,621,000 1.03 1932 393 7.5 88,632,327 30.6 25*7 27.1 74,938,000 .85 1933 382 7.2 96,691,753 30.7 ' 25.4 27.4 73,150,000 .76 ! O 193^ 4i4 1935 4i3 1936 462 112,300,000 33*2 6.9 116,600,000 33.4 7.5 161,000,000! 39*3 28.4 <r\1r0l ''T 32.5 29.8 29.9 35-7 104,234,000 99,188,000 153,686,000 93 .85 95 7239 6- - --J O K a Hj H- P* O H- O' H- o' - CT5 ^C0 4OpjPjpct* !I--111--1 M M t--l TABLE 3. - Roclc-dusting in West Virginia R. I. 3U65 Rock dust used per ton of Year Rock dust coal mined in used, rock-dusted tons mines, pounds 193S 32,495.00 -- 1937 i93S , 1939^/ 40,775*72 42,69s.83 12,587.62 1.297 1.562 1.596 1/ January to March. Mines using rock dust 133 162 179 16S Rock dust used per mine, tons 244 245 239 Commercial mines 5I+2 549 536 - It is common for interior surfaces of coal mines in many parts of the United States to "sweat" during the summer months as moisture in the air is deposited on the roof, ribs, and timber. In some mines the rock dust on the ribs and roof becomes "slimy" mud while in others it may be dry or moist. This absorption of moisture virtually destroys the effect of the rock dust. If coal dust, usually difficult to wet, is mixed with rock dust the entire mixture becomes wet; a thin layer of coal dust settling later on the surface of the wet rock dust is likely to become wet also, but the top of a thick layer of coal dust may remain dry. On drying, some dusts remain caked and will not disperse readily, if at all, while othersvirtually regain their original dispersibility. Sampling. - Passageways should be sampled at regular intervals to give an accurate cross section of the condition of the mine with respect to dust. Analysis of dust sample. - A comparatively simple fire analysis may be used to determine the total combustible matter, except when limestone is used. The Bureau of Mines modification of the Taffand volumeter may be used to determine the combustible content of any kind of rock dust and is accurate within practical limits. Extent of rock-dusting. - The Bureau of Mines recommends that all coal mines except anthracite be rock-dusted in every part and that rockdust barriers be used as a secondary precaution but not in lieu of general dusting. TYPES OP DISTRIBUTORS Mechanical rock-dust distributors were assembled as early as 1911, in which year W. J. Murray, of the Victor American Puel Co., developed, a rock-dust distributor employing a fan, hopper, and flexible hose. This machine was used in the Delagua mine in Colorado. In 1914 the Bureau of Mines assembled a fan, nozzle, hopper, and rubber hose and mounted them on a truck, which was used principally for demonstration purposes. 7239 -7- R. I. 3465 When, rock-dusting was actively revived in the United States in about 1924, a number of coal companies built rock-dust distributors in their own shops. One of the first commercial distributors was jjlaced on the market in June 1924. Others followed shortly afterward, but only three companies now are actively merchandising rock-dusting machines that have been approved by the Bureau of Mines. The first permissible machine - that is, approved by. the Bureau of Mines as safe in a gassy or dusty atmosphere - was approved'November 5> 1926, and a second machine was approved on December 28, 1926. Shortly afterwards another machine, known as a high-pressure model, was developed to force dust through a hose so that trackless entries could be rockdusted, The first high-pressure machine was approved on July 2, 1927* Two types of rock-dust distributors are used in the United States. Low-pressure dusters are used for application of dust on mine openings having tracks, and high-pressure machines arc used principally for dusting areas not accessible to the low-pressure machines. Construction of both types of machine is similar in many respects. A modified V-shaped hopper of varying length' contains the dust. Conerally, a screw conveyor in the bottom of the hopper carries the dust to the front end of the hopper and allows it to fall into a mixing chamber. The fine dust tends to form an arch, and consequently a constant feed is not always obtained; this con dition is overcome by using agitators. One type of distributor uses a steel shaft 1-5/8-inch in diameter provided with a number of steel prongs. As the shaft revolves, the prongs cause the dust to fall onto the screw .conveyor. Other models have two screw conveyors rotating in a direction opposite to that of the feed conveyor. Some of the earlier models of low-pressure dusters deliver dust into an airduct at the bottom of the machine along the full length of the hopper. Back pressure was formerly prevented by placing a cover on tho machine. The later type machines use the mixing chamber, which is placed at the front end of the hopper. As tho dust falls into the chamber, it is mixed with the air and is forced through the discharge in a fluid mass. Low-Pr0ssure Distributors A number of low-pressure machines still in use are provided with a flexible wire-reinforced hose about 4 to 6 inches in diameter and approxi mately 4 feet long. A handle near the discharge end allows the operator to direct the air-dust stream towards the roof and rib or crosscuts to obtain suitable coating of the surfaces. Usually enough dust spills to coat the roadway. Another low--pressure duster is provided with a fixed discharge nozzle having two modified cones, one placed within the other in such a manner that dust is discharged around the periphery at the base of the two cones. A modified fishtail discharge nozzle is employed by a third type of lowpressure duster. This machine is designed to make two trips over the same 7239 - 3- H. I. 3465 entry; on the first trip the dust is directed approximately at the junc tion of the roof and one rib, and the second trip is made with the nozzle pointed toward the junction of the roof and the other rih. Air for low-pressure dusters generally is provided hy a fan capable of developing from 10 tol5 inches water-gage pressure. The discharge in low-pressure machines ranges from 50 to 200 pounds of dust per minute. High-Pressure Distributors High-pressure distributors use 3"inch fire hose ranging in length from 25 to 500 feet. In one mine, however, up to S00 feet of hose is used. The distributor stops opposite an opening in a stopping, the hose is carried through the opening, and a man directs the air-dust stream onto the roof and ribs, enough dust being spilled to coat the bottom. Some of the machines employ a rotary-type valve to prevent back pressure from blowing the dust bade into the hopper. Other machines have the hopper covered to accomplish the same purpose. Usually, a blower replaces the'Tan on the low-pressure machines. Pressures developed range from 30 to l66 inches water gage, the higher pressure being developed with 500 feet of hose. The machines generally deliver 50 to 100 pounds of dust per minute. . Semiportable Distributors The advent of mechanical mining has presented new rock-dusting problems because track models cannot be used with conveyors. To meet this problem, two manufacturers have developed a small machine modeled after the larger ones. The pressure developed ranges from 3--1/2 to 1-1/2 inches water gage. Each machine uses 4 feet of 4-inch reinforced hose; the capacity ranges from 30 to 50 pounds of dust per minute. On slopes where electric power is not available, two types of rock-- dusting machines are used. One consists of a conventional track-type machine in which power'is provided by means of a chain attached to the axle and to the blower, so that wheh the machine moves, the blower is operated, providing necessary air for the dust. One coal company lias constructed a truck on which storage batteries are placed to provide the electric power necessary to operate the dust distributor. In some of the mines in the State of Washington an injector-type nozzle has been developed that greatly increases the induced air flow. A small hopper placed near the inlet of the injector allows dust to be fed into the device. Compressed air flowing through the injector blows the dust from the discharge end, and this allows the faces of working places to be rock-dusted where electric power is not available. 7239 - 9- R. I. 3465 METHODS OF ROCK-DUSTING Two complementary methods of roclc-dusting are employed in American Mines - general rock-dusting by machine andfrequently, supplementary dusting by hand. Procedure before initial dusting varies to a marked degree. In many mines no effort is made to remove dust, pieces of coal, rock, and other materials from the road bed, ribs, and timbers. In many of the better-maintained mines the loose material is loaded from the road bed before the rock dust is applied. Several mines have attempted, also, to remove surplus dust from the ribs and roof by washing them thoroughly with water from a hose, and in at least one mine by using a specially constructed high-pressure sprinkling car designed to throw a number of strong streams of water against the roof and ribs. Recently, in one mine, after an explosion had thrown dust on ribs, roof, and timbers, excess dust was dislodged with compressed air before rock dust was applied. The most rapid retrogression of rock dust generally occurs on main haulage roads and on secondary haulage roads whore animal haulage is em ployed. Naturally, entries having track in them were the first to be coated with rock dust, and the application formerly was began at the drift mouth or the shaft bottom and progressed inby. Present-day high- pressure machines are capable of dusting trackless entries as well as regular haulage entries. . Fixed-nozzle machines proceed along the entry slowly against the air current so that the dust will be blown away from the operators. Generally the machine is stopped under high places in the roof to allow the dust to reach the high surfaces. Machines with flexible, nozzles may be stopped under high roof and opposite crosscuts so that the dust stream may be directed at the surfaces of these openings. In most up-to-date mines all entries now are rock-dusted before the track is removed. A large percentage of rock-dusted mines apply a thick coat 0# rock dust before the track is removed, and this may remain effect ive for several years. Frequently g to 10 pounds of dust per linearfoot of entry is applied in coal 5 to 7 feet high in entries 12 to l6 feet wide. Samples collected 4 and 5 3rears later have shown the incom bustible content in such trackless entries to bo above the lower recom mended limit. It should be emphasized, however, that mines whore this condition ms observed use water freely at face regions for allaying coal dust raised into the air during mining operations. Generally, the applicat ion of rock dust on haulagc'roads is approximately 3 pounds per linear foot, ranging from 1-1/2 to 6-1/2 pounds per linear foot of entry. Many mines had miles of entry in which thore was no track when rock dusting was initiated. At first these entries were protected only with 7239 - 10 - R. I. 3465 rock-dust barriers, but it soon became evident that all open areas should be rock-dusted thoroughly. A few companies had these entries rock-dusted by hand. This proved a slow and expensive process and other methods were tried. One coal company had the trackless entries swept with brooms. Openings were then made in stoppingsand a steel door was placed on each side of the opening. A short length of hose ms carried through the opening to the entry beyond and, with the rock-dust distributor on the haulage track, dust was blown through the hose. Dust was applied to the surface near the end of the hose andthen discharged into the air, which carried and deposited it along the trackless entries. Although some dust was carried for considerable distances, the distribution was not entirely satisfactory. The need for better distribution was met by manufacturers who developed the high-pressure machine capable of blowing dust through 500 to 800 feet of flexible hose. . Methods of making openings in stoppings for the hose vary, but in general pipes 4 to 6 inches in diameter are built into the stopping. Each ond is threaded .and a cap is screwed over one end. The hose is drawn through the pipe, and the ribs, roof, and floor of the trackless entries arc coated with rock dust when necessary. Recently, in dusting trackless entries in a relatively snail mine, 20-pound (per yard) rails were laid in the entries, using steel ties, and a low-pressure rock-dusting machine drawn by a storage-battery loco motive was used to rock-dust these entries thoroughly. This method, however, is costly and would not be feasible in many mines owing to falls in return airways and trackless entries. While it is desirable from a safety and health standpoint to pull the rock-dust distributor against the air current, it is not always possible to do so because the air travels in on one or more entries and out on parallel entry or entries; consequently, it is necessary to pull tho machine in the direction of the air or to turn the distributor around. In some mines this is done by using a "Y" in the track. Several mines have provided rigid couplings and push the machine. Rooms generally arc not rock-dusted, out a few mining companies rov quire rooms to be dusted by hand after each cut of coal has been loaded . out. Each working place is provided with a sack of rock dust, and aftor cleaning up a cut of coal, miners are required to throw the dust onto the ribs and roof by hand. In a few mines, the advance development in rooms is rock-dusted by machine once each week. Hand-dusting is not as efficient as machine dusting. Frequently the quantity applied by hand is much less than that applied by machine and the cost of application is considerably higher. The dust thrown by hand generally is not as well distributed and does not adhere as well as dust applied by machine; moreover, the force of the air-dust stream often re moves and mixes the dust on the ribs with the rode dust. 7239 11 - R. I. 3465 The application of noncombustible dust at the face to guard against dust ignitions is not practicable during the cutting and loading of coal, but dust raised during mining operations may be allayed through judicious application of water at the coal face. Methods of allaying coal dust in mines are discussed in a paper by D. Harrington. A few mines have used calcium chloride along the floor, especially along manways where mules and horses travel daily. The hygroscopic property of the salt tends to moisten the dust and prevent rapid retro gression of the rock-dusting. One mine in Colorado applies natural salt water to road beds to help allay dust. Several companies in the eastern and also far western part of the United States "sprinkle" or wet the road beds at intervals to prevent dust from being raised into the air. Some mining companies endeavor to maintain compact roadways by frequently cleaning up the coal that falls from tops of loaded coal cars during transportation and by subsequently watering such roadways.. Where coal is gathered from producing entries by animals, largo quantities of dust are formed and thrown into the air by the animals stepping on and pulverizing the coal that spills along the roadways. This situation pre sents a serious problem, especially in mines where watering is not done to allay dust in face regions or where water is not used to wet such roadways. . Generally, the best practice is to clean roadways and apply rock dust. Some mining companies ballast such roadways with ashes or other inert material to assist in maintaining enough incombustible matter on them to prevent propagation of an explosion. qUAITTITY OF ROCK DUST USED The quantity of rock dust that must be used for initial dusting for any particular stretch of passageway depends on several factors, such as (l) character of coal,. (2) size of particles of coal dust, (3) percentage of inert matter naturally present, and (4) quantity of road and rib dust before rock-dusting. The quantity of rock dust that must be used for redusting is determined by sampling at regular intervals along passageways. Analysis of samples will indicate whether additional rock-dusting should be done and whether cleaning up is necessary before redusting. Frequency of redusting will depend on the number and extent of the dust-producing and disseminating processes in the mine, including spilling along haulageways and the amount of float dust deposited on the haulageway surfaces and in the air ways. Sprinkling of loaded trips eliminates much of the coal-dust deposit along haulageways, and sprinkling at face regions and use of water on mining machines, mechanical loaders, and conveyors eliminates much of the float-dust deposit in rooms as v/oll as in haulage and aircourse entries. According to table 2, the quantity of rock dust actually used in mines of the United States over a J~yoa.r period averaged about 0.9 pound per ton of coal mined in rock-dusted mines. The average use of this small 7239 - 12 B. I. 3465 amount of rock dust bears out previous statements that mines are not being rock-dusted adequately. Recent investigation of a number of individual rock-dusted mines in various parts of the country revealed that as much as 4.5 pounds of rock dust per ton of coal mined have been used by certain mines for 1 year. The quantity of rock dust necessary per linear foot of entry depends on the cross-sectional area of the entry. In the investigation referred to, it was found that 3*5 pounds of rock dust per linear foot was being used in one mine with entries 55 feet in cross-sectional area; while another mine, with an entry area of J2 feet, was using 6.9 pounds per linear foot of entry. ROCK-DUST BARRIERS Rock-dust barriers have been recommended by the Bureau of Mines as a secondary means of defense and not as a substitute for general rock dusting. Before development of a machine for coating trackless entries, barriers were fairly generally thought to be necessary, especially at the entrances to panels and side entries. Concentrated types of barriers have been recommended by the Bureau of Mines, but few if any are used in commercial mines. V-trough barriers were adopted to some extent shortly after 1924 but generally were placed in little--traveled parts of the mine, and the absorption of moisture . caused the rock dust to cake or oven to become soaked with moisture, and dispersion in event of an explosion was doubtful in some cases and impos sible in others. Material spalling from the ribs frequently fouled the end of the troughs, and fallen roof often was found on top of the barriers. When practical and economical rock-dusting of open areas was developed, most of the mining companies abandoned the use of' barriers. At present a company in Washington, one in Utah, and a mine in Illinois have a new V-trough barrier, and a few are to be found in mines in West Virginia and other coal-mining States. A barrier using rock dust in bags was developed and installed recently by a large coal-mining company in Illinois. Details of the barrier are given in Bureau of Mines Report of Investigations 3*ML1 (1938), "Tests of a Barrier Using Rock Dust in Paper Bags." The attitude taken by most of the large coal-mining companies is that the barriers may become ineffective at any time, due to falls of roof or rib, and no longer afford protection; consequently rock-dusting of open areas is held to be the most effective practice at the present time. 7239 - 13 - R. I. 3465 SAMPLING Samples are not collected in most rock-dusted mines unless the State laws require it* On the other hand, a few large progressive com panies collect samples regularly. The Bureau of Mines ha3 "been largely instrumental in initiating rock-dusting in the United States and lias given instructions for sampling in mines to determine the effectiveness of the rock dust. An effort should he made to obtain true average conditions, no dust should be taken from recesses in the ribs such as hitches, from places where rock dusting machines have been-filled, nor from those where the machine has discharged for a long time in one spot. Some companies establish sampling stations and place boards painted with the station number on the rib on the haulage entries. Samples are collected on parallel entries at points approximately opposite the sampling station* She stations usually are placed at definite intervals of 500, 1,000, or 1,500 feet. The stations frequently are placed just outby switches, at the bottom of dips, and at other points where spilled coal quickly lowers the incombustible content of the dust on the surround ing surfaces. Mining companies do not sample in producing rooms or chambers to any appreciable extent; it is the exception rather than the rule. Sandies are collected at various intervals based on knowledge of conditions. Places near faces mined mechanically, at shaft bottoms, and similar points niay be sampled every 2 weeks. Haulage roads are sampled every month, every 2 months, or every 3 months, whereas trackless entries are likely to be sampled only onco or twice a yea.r. Occasionally the points of sampling are selected by the foremen or by the safety engineer after a preliminary visual inspection of the dust condition of the mine surfaces. Retrogression of rock dust in trackless entries in most instances is considerably less than on haulage roads, but where mechanical loading is employed the excess dust is very likely to be carried into the return airways and the rock dust contaminated. In one mine, 2 weeks after dusting a trackless entry near coal faces where mechanical loaders were operating, the white rock dust was virtually hidden by black coal dust. Special cases like these require a change in the regular sampling intervals. Virtually all sampling consists of brushing all loose dust from the surface sampled to a width of 6 inches or a multiple of this width. Whero the dust is deep, the sample is taken only from the top inch. If considerable sand has accumulated at or near tho rail, care is takon not to include the sand or to overbalance the sample with a preponderance of sand. 7239 - 14 - R. I. 3465 Two types of sampling kits are available. One consists of a wedge shaped scoop, the top of which contains a 10-mesh screen. The dust is sifted through to the interior of the scoop; the oversize is rejected; the minus-10-inch material is drawn off through an opening about 1 inch square at the back of the scoop, which is 6 inches wide. A 2- or 3"fnch paint brush will dislodge all loose dust and brash it into the scoop satisfactorily. . Another kit consists of a 2- or 4-inch paint brush, a 6-inch scoop, and a telescopic box about 8 inches square and 3 inches deep. The.smaller section has a 10-mesh screen in the bottom. The dust is sifted through this screen. The minus-10-mesh material usvially is placed in cardboard containers of Suitable size. If the sarrple contains more dust than can be placed in the container, the dust is mixed thoroughly and quartered according to standard sampling practice. Entries containing timber or steel crossbars are sampled by brushing the dust from the timbers and crossbars; the very fine dust lodged here is inportant, as it is most likely to be thrown into suspension by an explosion. The amount of roof dust is considerably less than that from the ribs and road; consequently, the rib and roof dust generally is included in one sample and the road, dust in another. Some coal companies require the collection of a complete "round11 or peripheral sample, and in Pennsylvania, where the mining lav/ requires that "a representative sample of dust" be "gathered at each sanpling point from the roof, sides, and floor", a peripheral sanple usually is collected, based on a strict interpretation of the law* That a true picture of dust conditions is not given by a peripheral sample is indicated by table 4. TABLE 4. - Unweighted arithmetical average of rib roof and road dust Sample Rib and roof dust.................. Road dust................................. Arithmetical average...... Peripheral................................ Percent incombustible matter 93 95 92 87 32 67 49 34 62 81. 71 6l 62 ___ 50___ 33...... The unweighted arithmetical average shows the average of the rib-, roof-, and road-dust samples if the same weight of material were collected in each sample. This is not the true average, but would be the average if the peripheral samples were taken to an equal depth from the ribs, roof, and road. In the. first group, the road dust contains about a third as much incombustible matter as the rib- and roof-dust sample, and the unweighted average is 62 percent; however, the peripheral sample contains 43 percent, showing that more road dust than rib and roof dust had been collected* Actually, the sampler collected a thin coating from the road 7239 - 15 - 3. I. 3l`r&5 dust, so that the incombustible matter apx>roaches the unweighted average more closely than any. other peripheral sample's* The amount of dust in the average road-dust sample is about 10 to 15 times as much as in the average roof- and rib-dust sample. The effect of such large quantities of road dust on the peripheral samples i3 shown in the tabulation, the road-dust and peripheral samples agreeing relatively closely* Therefore, it can be seen readily that a peripheral sample does not give a true indication of the condition of the roch-dustcd surfaces, and to determine the true condition, roof and rib'dust should be sampled separately from the road dust. ' In making an explosion-hazard examination, the Federal Bureau of Mines collects representative samples of dust at carefully selected locations. The road dust is sanplod separately from the rib and roof dust, and all of the dust in each.sample is sent to the coal laboratory for analysis. Where no rock dust has been applied, a proximate analysis is made; and in samples from rock-dusted areas, moisture, incombustible matter, and carbon dioxide arc reported. The weight and size of dust in each sample is recorded so that a weighted'average of incombustible matter may be calculated from the roof-, rib-, and road-dust samples at each sampling point. The degreo of hazard then may be estimated from the size of the dust and from the combustible matter present. . Field samples are taken to a laboratory or to a sampling room at the mine and rescreened through a 20-mesh sieve. Wet or moist dust in the mine often makes screening difficult with a 20-mesh sieve* Wet material collected is air-dried before final sampling. The incombustible content is determined with the volumeter in nest instances, although calcination is used by .a few companies, in which case separate detormina- tion of CO2 is necessary if the real condition is to be ascertained. , Practice of reporting results of sampling varies according to State laws or company regulations. At some mines the safety engineer collects and analyzes samples or supervises these operations; he then determines where redusting is required. In Pennsylvania a special book is provided by the State, and results must bo entered and countersigned as required for fire bosses and mine foremen. ' The best practice, followed by many of the larger companies, is to make a permanent record in a bound book and to send copies of the results to the superintendent, mine foreman, and safety engineer. This procedure provides a triple check on the existing status as to dust and insures that required action will be taken to corroct substandard conditions. TYPICAL BOCK-LUSTED MIKE The Bureau of Minos recommends what are considered ideal conditions for rock-dusting coal mines, but no one mine follows the program completely. One mine, considered to be 100-perccnt roclc-dusted by the Pennsylvania Department of Minos, presents a typical example of what the industry considers to be full protection by rock- dust. 7239 - 16 - begur. side minii the 1 firs to b dust cour and dusl carl del: fir bef in tab sua anc du! of of ab la bu Pe be Ar A1 a S r 7 R. I. 3465 Entries are cleaned of combustible material before rock-dusting is begun. In rooms, waste material is gobbed on one side, but the opposite side of the track is kept free from excess duct and lumps. Several mining machines are equipped with tanks to hold water to be applied on the cutter bars during cutting of coal. Back entries are swept with brooms before they are dusted for the first time by blowing dust into the air at break-throughs, allowing dust to be carried by the air stream. Haulage entries and rooms in this mine were rock-dusted by a rode dusting machine in 1924 and 1925* Dust ms blown into the main air courses, but it 'ms thought that not enough dust wan deposited on roof and ribs to give full protection. In December 1937 a high-pressure rock dusting machine was purchased and dusting of tradcless entries ms begun. Limestone dust is jmrehased with a guarantee of 9S percent calcium carbonate and a fineness of 85 percent through 200-mosh. The dust is delivered in paper bags containing 80 pounds each. The rock-dusting crew consists of two men, one of whom is a certified fire boss. He enters a working place and makes an examination for gas before the rock-dusting machine enters. The two rock-dusting machines in use at the mine are permissible, but the management does not wish to take unnecessary risks, and this is excellent safety procedure. The crew applies dust about 3 days per week. Average coverage of surface with rock dust for one 7"hour shift is 350 foot of trackless entry and 6,600 feet on haulage roads. This also includes rooms, which are dusted by machine. Ho hand-dusting is done at this mine. Some sections of the roadmy are wet, but these sections are rock-dusted when the rest of the area is dusted. The main air courses were rock-dusted with a high-pressure distributor about 2 years ago and still are well coated. A 6-inch pipe line has been laid on the bottom of the road bed since the application ms finished,, but about 3/4 inch of air-borne coal dust has settled on the pipe. Perhaps the extensive application of rock dust helps to offset any air borne coal dust in these entries. The mine foreman is charged with designating points to be sampled. An engineer collects the samples and determines the incombustible content. About 10 samples are collected each week. The results are noted in the special book provided by the State, and a report is made direct to the 'superintendent, with a copy each to the general superintendent and the safety engineer. The mine foreman is required to countersign the record book. 7239 - 17 - R. I. 3465 A sample of the record in the hook, selected at random, is given in tables 5 and 6. . Samples were collected on all parallel entries approximately opposite one another; peripheral samples were taken, hence the same analyses for bottom, roof, and. ribs in each sample# The report on barriers shows the condition of each to be good. The State law allows substitution of rock-dusting for barriers, and at present virtually all barriers have been abandoned in favor of rock-dusting in the trackless entries. Data are not available on the application of rock dust on a basis of pounds per linear foot; however, during the calendar year 1933, 444 tons of limestone dust was applied, the equivalent of 1.36 pounds of dust per ton of coal mined. The total cost for labor and materials was less than 1 cent per ton for rock-dusting during 1938. COST OF ROCK-DUSTIHO To a large extent, the cost of rock-du3ting depends on the quantity of dust used per iincar foot of entry or per ton of coal mined. For com parative purposes, the cost of rock-dusting per ton of coal mined probably is the most equitable.basis. The cost of rock-dusting in 27 widely scattered mines is shown in table 7* Most of these are large mines and their rock-dusting program is- more consistent than that of the average rock-dusted mine. Therefore, these cost figuros do not represent the cost of rock-dusting in the ' average mine in the United States, but represent the cost in mines that are more concerned with safety than is the average mine. The cost of rock-dusting ranges from $0.0019 to $0.01S4 (an average of $0.00S9) per ton of coal mined. The cost per linear foot of entry ranges from $0.0110 to $0.64l7 (an average of $0.0229 Per foot). Not a great many data are available concerning cost of hand dusting. However, cost data from one mine on the hand dusting of 1,015 feet of 6- by 10-foot entry show that the cost of applying 8.1 pounds of dust per linear foot of entry was $0.0467. At another mine the cost of applying by hand 4*1 pounds of dust per linear foot was $0.0326. The cost per linear foot of applying 3*4 pounds of rock dust in trackless entries with a high-pressure dust distributor through a 4-inch hose is reported by another company as $0.0326. . 3^65 7239 IS - H . ; p* i 3 M T d- 3 ct* CD R. I. 3465 Date 9/1/3S By whom collected J.H.T. 9/S/3S J.H.T. __________i ' i^ri-TTa lABBE 5 - Record of rock dust analyses Heading and location #4 east at 7320 #5 east a-t 7350 #6 east at 7325 #7 cast at 7310 #1 north at 500 #2 north at 515 #3 north at 490 #4 north at 495 #5 north at 525 #6 north at 510 if7 north at 530 #1 entry, 1 south at 4o #1 entry, 1 south at 2040 #1 entry, 1 south at 4000 #2 entry, 1 south at 4o #2 entry, 1 south at 2000 #2 entry, 1 south at 4o40 #3 entry, 1 south at 4o #3 entry, 1 south at 2040 #3 entry, 1 south at 4i00 #4 entry, 1 south at 40 #4 entry, 1 south at 2000 Date analyzed ' 9/1/32 9/1/38 9/1/38 9/1/38 9/1/38 9/1/38 9/1/38 9/1/38 9/1/32 9/1/32 9/1/32 9/2/32 9/2/32 9/2/32 9/2/32 9/2/38 9/2/32 9/2/32 9/2/32 9/2/32 9/2/32 9/2/32 Percent of incom'Dus tihle Bottom g6 Roof g6 Sides 36 70 70 70 66 66 66 64 64 3* 65 65 65 64 64 64 72 72 g6 36 36 70 70 70 72 72 72 60 60 60 60 60 60 72 72 72 go go 30 go go SO gg gg gg 76 76 76 70 70 70 52 52 58 76 76 76 32 32 82 25 25 25 7239 - 19 - R. I. 3^65 IABLS 6. - Record of condition of rock-dust barriers Date in spected 9/15/33 9/15/33 9/15/33 9/15/33 97.15/33 By whom inspected R.E.J. Location of harrier #4 entry of 2 south #0 entry of 2 south #2 in 5 east #3 in 6 east #4 in 7 east . Condition of harrier Dust Frame Mechanism Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good TABLE 7 - Cost of rock-dusting in 27 widely scattered mines Mine A B C D E F G H I J Z L M Av* 7 mines Av. 4 mines N 0 P Average State West Virginia do. do. do. do Pennsylvania . do. do. do. do. do. do. do Utah Illinois Wyoming New Mexico Washington do. - Co st Per ton Per lin of coal ear foot mined of entry ~$o.oogo $0.0275 .0184 .0150 -- .0050' - .006S .0174 ,oog6 - .0080 .0130 -- .0071 .0131 .0134 ,0220 .0165 .0288 .0094 .0181 .0019 .0110 .0090 -- .oo44 .oo4o .0031 -- .0089 .0250 .0417 .0250 .0229 Area of entries, sq..ft. 72 - 96 84 72 72 72 55 71 p: Rock dust used per linear ft.of entry .pounds 6.25 3.5 2.5 1.2 - 1.3 3.1 4.5 1.9 6.9 - 3.53 1.0 - 96 . - 54 54 -- ' - 3.5 4.1 2.5 2.5 2.5 . AVAILABILITY OP MATERIAL FOR ROCK-DUSTINCS As general use of limestone or gypsum in rock-dusting coal mines de-i pends to some extent on the cost, and the cost in turn depends chiefly on transportation, a survey was made some years ago by the Bureau of Mines of sources of these materials close to coal mines or on railway lines that would provide, at low freight rates, ready access to coal fields. The re of ad st cc tfc of we ir rc si tl hi hi Sc w< ii mi f; i: c a m d 7239 I - 20 - 7 of the United States showing locations of quarries and giving names and addresses of companies operating quarries. The survey, shows that lime stone or gypsum is readily accessible to all the coal fields of the country;.suitable rock-dusting material is more readily available today than when this survey was made. INSTANCES WHERE ROCK DUST IS BELIEVED TO HAVE STOPPED OR LIMITED EXPLOSIONS Notwithstanding the inadequacy of rock-dusting in most of the mines of the United States, numerous 5.nstances are on record indicating the . well-founded basis of the claims regarding the effectiveness of rock dust in stopping the propagation of explosions. Bureau of Mines records during the past 9 years show that in 60 rock-dusted mines where explosions occurred, rock dust is credited with stopping or limiting the explosions in 26 of them. In several instances the element of luck unquestionably was present, in that the explosion happened to occur in one of the few sections of the mine where rock-dusting had been done. No accurate figure is available on the number of lives saved in these 26 explosions, but it is almost certain that several hundred were.saved and much property damage avoided because of the rock dust used in these mines. ' CONCLUSIONS 1.. Although the number of mines using rock dust is increasing and more: rock dust is being used, rock-dusting practices in tho United States fall far short of giving absolute protection against coal-dust explosions. 2. State laws regulating the safety of mines arc, in general, entirely inadequate with respect to rock-dusting. 3. The Bureau of Mines recommends that all coal mines except anthra cite be rock-dusted in every part and that rock-dust barriers be used as a secondary precaution but not in lieu of general rock-dusting. 4. Machinery for distributing rock dust, as well as analytical equip ment for inspection purposes, has reached a fairly advanced stage of development. . 5. .Limestone or gypsum is roadil5r accessible to all coal fields of the United States, and this material can be delivered to the mines for rock-dusting purposes at a reasonable cost. Bowles, Oliver, Sources of Limestone, Gypsum, and Anhydrite for Busting Coal Mines to Prevent Explosions: Bureau of Mines Bull. 247, 1925, 70 pp. ' 7239 21 - R. I. 3465 6. The average cost of rock-dusting in 27 mines where the practice is considered good amounts to $0.0089 per ton of coal mined, or $0.0229 per linear foot of entry. 7. During the past 9 years rock dust is credited with having stopped or limited explosions in 26 of 60 partly rock-dusted mines and thereby unquestionably saved the lives of several hundred persons. Although the Bureau of Mines has advocated the use of rock dust as a preventive of widespread explosions since its establishment in 1910, comparatively few mines in the United States today are adequately rockdusted. Of the country*s approximately 6,000 bituminous- and lignitic-coal mines, probably fewer than 100 are adequately rock-dusted. This situation exists because State laws requiring protection of lifo against explosions are inadequate and the coal operator is inclined to take chances in the hope that he will not have the misfortune to suffer an explosion in his mine. The cost of rock-dusting is by no means prohibitive when the value of the protection it affords is taken into consideration, and unquestionably rock-dusting of coal mines of the United States should be more universal than it is at present. A CKUOWIEBGMEHT The author expresses his appreciation to the various coal-mining companies for data furnished concerning cost of rode-dusting; also to engineers of the Bureau of Mines for data furnished, and especially to, W. J. Fene and C. W. Owings for assistance in the preparation of this paper. 7239 22 - After this report has served your purpose and if you have no further NEED FOR IT, PLEASE RETURN IT TO THE BUREAU OF MINES. THE USE OF THIS MAILING LABEL TO DO SO WILL BE OFFICIAL BUSINESS AND NO POSTAGE STAMPS WILL BE REQUIRED. UNITED STATES DEPARTMENT OF THE INTERIOR BUREAU OF MINES OFFICIAL BUSINESS RETURN PENALTY LABEL THIS LABEL MAY BE USED ONLY FOR RETURNING OFFICIAL PUBLICATIONS. THE ADDRESS MUST NOTBE CHANGED PENALTY FOR PRIVATE USE TO AVOID PAYMENT OF P03TAGE. SSOO BUREAU OF MINES, WASHINGTON, D. C