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GASES THAT OCCUR IN METAL MINES

By D. HARRINGTON 2 and E. H. DENNY 8

METAL-MINE AND COAL-MINE ATMOSPHERES

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When the word “gas” is mentioned in connection with mining almost invariably it is inferred that the explosive gas, methane, is in mind and that the reference is to coal mining. Although methane is likely to occur in any coal mine, the metal mines of the world contain by far a greater variety of gases than do the coal mines. Often the gases in metal mines are as dangerous as the gases likely to be found in coal mines, and sometimes are even more deadly.

A coal mine is more likely to contain methane than a metal mine. In rare instances it may contain high percentages of nitrogen or carbon dioxide or it may be deficient in oxygen where the air is confined in a blind end or in a sealed region. Much more rarely relatively small percentages of ethane or other hydrocarbons with an odor of ether, gasoline, or kerosene, or minute percentages of hydrogen sulphide are found in a coal mine. During a coal-mine fire or explosion carbon monoxide content is likely to be high (4 per cent or more), and if the coal contains much sulphur the fire fumes may contain some sulphur dioxide.

GASES POSSIBLE IN METAL MINES

Methane has been found in a number of metal mines and occurrences have been attended by explosions; its limits of inflammability are 5 to 14 per cent in air. Some metal mines contain appreciable percentages of the deadly explosive hydrogen, whose limits of inflammability are 4 to 74 per cent in air. In some metal-mining regions carbon dioxide flows into the mine, fills the workings, and overflows like water. In other regions high-temperature gases containing a mixture of gases of sulphur (sulphur dioxide, hydrogen sulphide, and possibly sulphur trioxide), carbon dioxide, and nitrogen are found at the working faces; in some high-sulphide metal mines the deadly hydrogen sulphide is found in lethal proportions at blasting time. Carbon monoxide constitutes a decidedly dangerous day-by-day hazard in connection with metal-mine blasting because of the heavy charges of dynamite employed and the use of fuse which in burning gives off much carbon monoxide. The hazard of asphyxial gases is accentuated because of the relatively small openings where metal-mine blasting is done and the inefficient ventilation of the ordinary metal mine.

1 Work on manuscript completed July 2, 1931. 2 Chief engineer, safety division, U. S. Bureau of Mines. 8 District mining engineer, U. S. Bureau of Mines, Denver, Colo.

In most metal mines large quantities of dynamite, fuse, and detonators are stored underground; when a fire occurs in an explosives magazine in which dynamite is stored the gases given off, largely carbon monoxide and oxides of nitrogen, are dense and deadly. In addition, the oxygen content is often very low, as the underground metal-mine explosives magazine is usually a dead end, with practically no ventilation. In long-standing metal-mine fires, which are rather widely distributed over the United States, the nature of the gases produced depends upon the ores and other materials involved in the fire region. Gases from metal-mine fires include hydrogen sulphide, sulphur dioxide, carbon monoxide, carbon dioxide, methane, and hydrogen, as well as oxides of arsenic, antimony, etc.; moreover, there is frequently a definite deficiency of oxygen.

In addition to the variety of gases found in metal mines, the effect of gas occurrence is often accentuated by high temperatures due either to the natural heat of surrounding strata, to heated rock from sealed or partly sealed mine fires, or to some form of oxidation or other chemical reaction. Not infrequently the gas condition or the hightemperature condition is complicated by the intrusion of finely divided dusts from drilling, blasting, shoveling, and other operations. Some of these dusts are soluble and some insoluble; both are dangerous, especially if present in the mine air in finely divided form and in large quantities.

METHANE OR HYDROGEN IN METAL MINES OR TUNNELS

Notwithstanding the fact that to the metal miner methane is almost invariably associated with coal mines, scarcely a year passes without one or more ignitions of methane in other types of mine workings, including those where gold, silver, copper, iron, zinc, lead, limestone, salt, mercury, potash, and other metallic or nonmetallic substances are produced, and frequently in tunnels or shafts driven in or around cities for water or similar purpose.

In gold-silver properties in Gilpin County, Colo., numerous ignitions of methane have resulted from open lights, especially while long-abandoned shafts were being unwatered. Similar methane ignitions have occurred while long-abandoned timbered shafts in the Butte (Mont.) copper-silver-zinc mining district were being unwatered. In both these districts the strata are distinctly igneous, no shale or other carbonaceous formations being present. The methane undoubtedly was formed by slow decomposition of timber under water, similar to the formation of marsh gas in swamps. Methane ignitions have also occurred in the Michigan copper and iron mines while long-abandoned timbered mines were being unwatered, and as usual the open light has been the igniting agency. In unwatering any kind of mine, coal or metal, long filled with water, precautions should be taken to guard against the occurrence of explosive gas.

In the Grass Valley (Calif.) gold-mining district, where carbonaceous shales are found in conjunction with or close to the goldbearing measures, some rather extensive and troublesome accumulations of methane have been found.

Several ignitions of methane have occurred in the old Joplin leadzinc district of Missouri and in the newer Picher (Okla.) district as well; in fact, methane ignitions have occurred in all three States comprising the tri-State lead-zinc district. Methane has been found also in at least one iron ore mine in Alabama and during some quicksilver mining in California. Methane explosions have been started by open lights or matches in salt mines in New York and in Louisiana. Disastrous explosions have been started by ignitions of gas in asphalt mines in Oklahoma and in gilsonite mines in Utah.

In fighting partly sealed metal-mine fires of long standing methane has been found, even when the entire region consisted of igneous rock and no sedimentary strata were present; undoubtedly it comes from some high-temperature reactions between the timber in the fire area and the water introduced to fight the fire. Methane has been found in limestone mines or other similar noncoal-mining operations in sedimentary formations.

Hydrogen has been found in gases coming out of drill holes in South African gold mines and in a potash mine in New Mexico; this gas also has been found, usually in rather small quantities and proportions, in the fumes from very active metal-mine fires, especially when water is introduced into fire areas where some of the strata are heated to incandescence.

Recently, while drilling, a mixture of hydrogen, methane, ethane, and nitrogen in explosive proportions was encountered in New Mexico in a shaft being sunk through formation carrying potash and soda. The gas pressure was high enough to prevent drilling, and it had to be blown off from the drill holes for many hours before work could be resumed.

Methane occurrence in metal mines is by no means confined to the United States. From time to time the foreign technical press has described methane occurrences, explosions, or ignitions in European, African, and other metal mines, the most recent being in the gold mines of South Africa.

Details connected with the occurrences of methane in metal mines are omitted here; details for some have not been published. The reader is referred to published data available.4

GAS EXPLOSIONS IN TUNNEL WORK

So many hazardous situations due to the occurrence of methane have arisen during the driving of water or other tunnels in or near our large cities and during the sinking of shallow shafts for bridges and other foundations and so few precautions have been taken against possible disasters in connection with this work that some drastic action should be taken to make tunnel driving and excavating for deep foundations safer, with particular reference to lighting,

1.

4 Harrington, D., An Unusual Hazard in Reopening Long-Flooded Timbered Metal Mines : Rept. of Investigations 2255, Bureau of Mines, 1921, 3 pp.

Pickard, B. O., and Gardner, E. D., Methane in California Gold Mines : Rept. of Investigations 2303, Bureau of Mines, 1921, 6 pp.

Annual Report of the Government Mining Engineer of the Union of South Africa, 1927, pp. 67-68.

Report of the State mine inspector of Kansas, 1928.

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