CARBON MONOXIDE IN METAL MINES

9

down. In the fumes from some fires in metal mines, especially fires. that have been burning for several years, there are likely to be dangerous percentages of sulphur gases even when the sulphur content of the burning material is low. The gases from these fires usually consist of sulphur dioxide with relatively small amounts of hydrogen sulphide and possibly, under certain conditions, sulphur trioxide. In some iron ore mines with adjacent black-shale strata spontaneous fires give off fumes containing hydrogen sulphide and sulphur dioxide as well as carbon monoxide.

Sometimes sulphur gases are liberated in coal and metal mines directly or indirectly from waters released in mining; as a rule, these gases are hydrogen sulphide and possibly sulphur dioxide in small amounts; other sulphur-gas combinations may also occur. They are rarely dangerous to life, although frequently they affect the eyes of workers, causing temporary blindness from swelling or various kinds of inflammation.

CARBON MONOXIDE IN METAL MINES

Probably more than 90 per cent of the fatalities from fires 10 in the metal mines of the United States (the total lives so lost amounts to several hundred) have been due to carbon monoxide, and the main source of carbon monoxide in more than 70 per cent of these fires, as well as those with property loss only, has been timber, although there are generally numerous other possible sources, such as oil, explosives, and many other combustible materials brought in for various purposes.

11

BURNING MATERIALS

Scarcely a metal mine in the United States is free from fire hazards, although probably 90 per cent of the operators would vigorously deny such a statement. Some of the heaviest losses of life in metal-mine fires have resulted from the burning of less than a railroad freight car or only a few cords of timber. Also, the burning of a box of explosives or a barrel of oil in the confined places so numerous in metal mines readily gives off enough deadly fumes, largely carbon monoxide, to kill several hundred men if they were trapped in the poorly ventilated places so frequently found in the unventilated mines, which include by far the greater number of the metal mines of the United States, as well as in the relatively well-ventilated metal mines.

COMBUSTIBLE ROCK MINERALS

In addition to the fire hazard due to combustible materials brought into the mine (timber, explosives, fuse, oil, etc.), fires in some metal mines are caused by combustible material in the strata. Occasionally

10 Pickard, B. O., Lessons from the Fire in the Argonaut Mine: Tech. Paper 363, Bureau of Mines, 1926, 39 pp.

Harrington, D.. Lessons from the Granite Mountain Shaft Fire, Butte: Bull. 188, Bureau of Mines, 1922, 50 pp.

Harrington, D., Pickard, B. O., and Wolflin, H. M., Metal-Mine Fires: Tech. Paper 314, Bureau of Mines, 1923, 20 pp.

11 Gregory, F. C.. Fires and Fire Prevention in Lake Superior Mines: Inf. Circ. 6073, Bureau of Mines, 1928, 17 pp.

explosive gas such as methane accumulates and is ignited. In some regions carbonaceous shales or slates adjacent to the mineral-bearing strata fire spontaneously or are ignited by an open light or other cause. In other metal mines the sulphur content of the ore or of the walls is so high that fire is spontaneous, especially when the ore is crushed during mining operations. Fires may also be started in the broken sulphide by an open light or other ignition. Whatever the source of a mine fire the fumes almost invariably contain carbon monoxide, which generally is present in dangerous percentages.

EXPLOSIVES

Gases evolved by any of the explosives used in metal-mine blasting probably contain carbon monoxide, although those given off by some types of explosives under certain conditions contain comparatively little. If an explosive has partly deteriorated or partly frozen or if it burns rather than explodes, carbon monoxide as well as other dangerous gases are likely to be given off in dangerous quantities.

The fuse used almost universally in metal-mine blasting evolves material quantities of carbon monoxide in burning. Fuse in socalled "tight" places with small cross-sectional area and little or no ventilation is particularly dangerous. It seems strange that electrical blasting is not employed more widely in metal mines, as it is much safer, more efficient, and decidedly more applicable to the faulty ventilation practice so general in metal mines. In some poorly ventilated metal mines analysis has revealed carbon monoxide in small but readily recognizable percentages in the general atmosphere of the working faces.

In some metal mines, as well as in some nonmetallic underground properties, internal-combustion engines used underground for haulage, hoisting, or pumping give off carbon monoxide. Gasoline or similar drive for pumps or hoists has no place underground and should not be allowed there; gasoline trucks should not be allowed to go into mines to pull out mineral products, and the use of steam locomotives, occasionally found in underground workings, should not be permitted in any underground mining work, even in so-called fresh air.

MISCELLANEOUS GASES IN METAL MINES

In addition to carbon monoxide, the fumes from a metal-mine fire under certain conditions may contain hydrogen sulphide, sulphur dioxide, methane, or hydrogen, as well as other gases. If the ore contains antimony or arsenic the fire fumes will give off oxides of these metals; in a few instances it was believed that certain excessively corroding fumes forced out of a highly heated low-grade sulphide ore deposit were sulphur trioxide. If the fire occurs in a confined region the oxygen content may be much less than 10 per cent and the carbon dioxide or nitrogen content relatively high.

If the fire consumes explosives or the boxes, paper, and sawdust in which explosives are wrapped and shipped, the fumes are likely to contain not only high concentrations of carbon monoxide and very low concentrations of oxygen but also high concentrations of various oxides of itrogen, the breathing of which is dangerous. There is

PROPERTIES OF GASES

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good reason to believe that metal-mine fires involving explosives or explosives containers give off fumes in quantities that can not be handled adequately by any type of gas mask; hence, only up-to-date oxygen breathing apparatus should be worn if respiratory protection is sought. Fires involving heavily insulated electric power cables or wires also give off fumes that contain high percentages of poisonous gases, including carbon monoxide.

PROPERTIES OF GASES

NITROGEN

Pure nitrogen gas is a hazard to life; it will suffocate a human being in much the same way that water does. When mixed with oxygen in the approximate percentages of 79 to 21, as in ordinary air, it acts to dilute the oxygen.

CARBON DIOXIDE

Pure carbon dioxide will asphyxiate a person. Its property of stimulating breathing is utilized in certain resuscitation apparatus.12 Such stimulation in ordinary work tends to impair the efficiency of the worker, as he utilizes energy needlessly. If the air contains considerable carbon dioxide, the physical effect is definitely detrimental, particularly if the worker breaths the contaminated air over a long period. If the air contains 0.5 per cent of carbon dioxide, a man breathes deep and fast; with 2 per cent he breathes approximately 50 per cent more air than he would normally; with 5 per cent he breathes three times as much air as he would normally; and with 10 per cent he can breathe only a short time. Inhaling large amounts of carbon dioxide prevents the blood from giving off the gas as an elimination product. When resting a man can breathe 2 per cent of carbon dioxide in otherwise pure air for several hours without ill effects; 3 to 4 per cent of the gas causes an oppressed feeling and produces shortness of breath or panting; and 5 to 6 per cent soon exhausts him so that he can not continue work effectively. The Bureau of Mines recommends that mine air be considered unfit for men to breathe if it contains more than 1 per cent of carbon dioxide.

OXYGEN

Various parts of a mine may be deficient in oxygen (that is, contain less than the normal amount found in the atmosphere), a condition usually associated with the presence of carbon dioxide. The United States Bureau of Mines recommends that mine air be considered unfit for men to breathe if it contains less than 19 per cent of oxygen. A lighted candle is extinguished when oxygen in the air falls below 17 per cent. Work may be continued in air containing less than 17 per cent, but below 13 per cent the blood is unable to absorb oxygen fully; when the oxygen content of the air drops much below 10 per cent the judgment is impaired and delirium may follow.

12 Burrell, G. A., Robertson, I. W., and Oberfell, G. G., Black Damp in Mines: Bull. 105, Bureau of Mines, 1916, 88 pp.

Under certain conditions unconsciousness is the result, and when the oxygen content is less than 5 per cent paralysis follows quickly. Generally, when a man goes from good air to an atmosphere very deficient in oxygen, he drops almost instantly, without warning. A lighted carbide lamp is extinguished when the oxygen in the air falls below approximately 13 per cent. It seems evident that a man can not be expected to work efficiently and continue in good health if his body is undergoing slow oxygen starvation.

METHANE

In its physiological effects methane closely resembles nitrogen; hence it is not harmful when inhaled, unless, because of its presence, the oxygen content of the air is lowered beyond the safe amount. In normal air mixtures of 5 to 14 per cent of methane and air are explosive if brought in contact with an igniting medium, such as an open light or an electric arc.

HYDROGEN SULPHIDE

Hydrogen sulphide is one of the most poisonous gases known; fortunately only traces are ordinarily found in mine operation. In some respects it is more poisonous than hydrogen cyanide. In small proportions its odor of spoiled eggs is noticeable, but in heavy concentrations the organ of smell appears to be paralyzed. When it is detected ventilation should be improved at once. Several instances are on record of hydrogen sulphide filtering through sands and muds in tunneling and shaft-sinking operations, overcoming the workers. Numerous fatalities in atmospheres connected with high-sulphur petroleum production and refining amply testify to the extreme danger of this gas. Hydrogen sulphide inhaled in large amounts results in immediate asphyxiation; is in small amounts it results in inflammation of the eyes, signs of inflammation of the respiratory tract, and sometimes in bronchitis and pneumonia. Subacute poisoning can be produced by long exposure to concentrations as low as 0.005 per cent; immediate collapse usually is the result when one is exposed to 0.06 to 0.1 per cent of the gas; and death ensues quickly when one is exposed to more than 0.1 per cent.

SULPHUR DIOXIDE

Sulphur dioxide has the characteristic odor of burning sulphur matches. It is irritating to the eyes and respiratory passages, and 1 part in 500 is intolerable to breathe. As previously mentioned, the gas is occasionally found in metal mines as a result of oxidation or burning of sulphides and is also formed as an oxidation product in blasting. Sulphur dioxide is a hazard, chiefly in blasting operations and in mine fires; in such instances it is likely to be present in combination with other irritating gases, the combined effect sometimes resulting in irritation of bronchial passages and lungs, with fatal results a few hours after exposure.

13 Sayers, R. R., Mitchell, C. W., and Yant, W. P., Hydrogen Sulphide as an Industrial Poison: Rept. of Investigations 2491, Bureau of Mines, 1923, 6 pp.

PRECAUTIONS AGAINST GASES FOUND IN METAL MINES 13

CARBON MONOXIDE

Carbon monoxide when inhaled replaces the oxygen in the red corpuscles of the blood; consequently, a man may be overcome by a relatively small amount. Therefore 0.02 per cent is the upper limit that should be permitted in any part of a mine atmosphere; 0.1 per cent causes severe symptoms in 30 to 60 minutes; and 0.5 per cent is likely to be fatal if breathed for a short period. The gas is colorless, tasteless, and odorless. Carbon monoxide in dangerous quantities is also formed from metal-mine fires owing to incomplete combustion of timbers or other combustible material. The Bureau of Mines has numerous records of men being killed by carbon monoxide because they returned too soon to unventilated places after blasting.

OXIDES OF NITROGEN

Oxides of nitrogen are formed in dangerous amounts when high explosives burn. Miners generally speak of such gases as having a characteristic "burned-powder " odor. As little as 0.01 per cent of oxides of nitrogen may cause dangerous illness if breathed for a short time, and 0.07 per cent is fatal if breathed for 30 minutes. A Bureau of Mines burning test of gelatin dynamite of 40 per cent strength gave among the various gases evolved 11.3 per cent of nitric oxide and 0.6 per cent of nitrogen peroxide.15 The bureau has the record of a case in which explosives detonated incompletely and in large part burned, giving off brownish yellow fumes; 13 men were affected by the smoke and fumes from the explosive, but all revived upon reaching fresh air. Within a short time, however, symptoms of bronchial and lung irritation developed, and in 3 days 9 of the 13 men had died; the remaining 4 took several months to recover.

PRECAUTIONS AGAINST GASES FOUND IN METAL MINES

It is apparent that numerous gases are found in metal mines and that many are dangerous to health or to life; it is equally apparent that most of the noxious gases that afflict the metal miner are those from fires and from explosives. The gases that flow from the strata or are due to chemical action between the rock and water of air exposed by mining operations are serious hazards in some localities, but mine fires and hazards from explosives fumes involve practically all types of underground mining operations in the world. Hence, if the metal-mining industry would realize that every mine presents a fire hazard and would take common-sense, readily available precautions against it, the greatest danger from gases in and around metal mines would be avoided.

FIRE PRECAUTIONS

Foremost among the precautions that should logically be taken are the elimniation of open lights and smoking; the limitation and other

14 Sayers, R. R., and Yant, W. P., The Pyrotannic Acid Method for the Quantitative Determination of Carbon Monoxide in Blood and in Air: Tech. Paper 373, Bureau of Mines, 1925, 18 pp.

15 Hall, Clarence, and Howell, S. P., The Selection of Explosives Used in Engineering and Mining Operations: Bull. 48, Bureau of Mines, 1914, 50 pp.