SULPHUR DIOXIDE (SO2)

Sulphur dioxide is a colorless, suffocating, irritating gas, with a strong, pungent, sulphurous smell. It is sometimes given off by explosives, especially in the blasting of certain sulphide ores, and may be present when there are gob fires in which iron pyrite is burning. The specific gravity of sulphur dioxide at 760 mm. (sea level) and 0° C. (32° F.) is 2.2638, and its weight in grams per liter under the same conditions is 2.9266,51 or 0.18309 pound per cubic foot. Its boiling point is 78.5° C.52 Sulphur dioxide is very poisonous, even when present in small percentages. It is very irritating to the eyes and respiratory passages and therefore is intolerable to breathe before it reaches concentrations that are dangerous.

HYDROGEN (H2)

Hydrogen is colorless, odorless, and tasteless. It is produced by incomplete combustion in explosions, mine fires, and firing blasting explosives. The specific gravity of hydrogen at 760 mm. (sea level) and 0° C. (32° F.) is 0.0695, and its weight in grams per liter under the same conditions is 1.6398,53 or 0.00562 pound per cubic foot. Its boiling point is 252.7° C.54 Air that contains 4.1 to 74 per cent of hydrogen will explode violently if ignited. Hydrogen will explode when the oxygen content is as low as 5 per cent.55

NITROGEN (N2)

Nitrogen is held in coal to a very limited extent. Accumulation above the normal amount in mine air is due chiefly to the removal of oxygen from the air by the coal. The specific gravity of nitrogen at 760 mm. (sea level) and 0° C. (32° F.) is 0.9674, and its weight in grams per liter under the same conditions is 1.2507,56 or 0.07809 pound per cubic foot. Its boiling point is 195.8° C.57 It is not combustible and will not support combustion. Nitrogen is physiologically inert and is harmless to man. It has no effect other than to act as a diluent of the oxygen in air.

DIFFUSION OF GASES

In still air gases will stratify according to their specific gravities. Where conditions are such that gases stratify readily they diffuse or mix relatively slowly. According to Graham's law the rate of

diffusion of air and gas or of different gases is inversely proportional to the square root of their specific gravities. Under certain conditions, such as in air currents or around mine fires, gases become readily diffused or mixed and once diffused will not stratify again. This fact is brought out rather strikingly by conditions sometimes found in mines where carbon dioxide admixed with oxygen and nitrogen may be of a specific gravity lighter than air. Especially is this noteworthy where the mixture is comparatively hot and there may be near-by circulating air which is cooled. In a number of instances bureau engineers have observed in metal mines that a candle or carbide light burned readily on the floor, yet was extinguished as soon as it was placed near the roof or "back," and that the flame-extinguishing atmosphere was a combination of carbon dioxide and nitrogen with a deficiency of oxygen.

ROCK GAS

In some metal-mining districts, notably Cripple Creek, Colo., and Tintic and Park City, Utah, as well as in Oklahoma, Nevada, California, and other States, gas, commonly called "rock gas," 58 issues from the rock strata into the mines. Many men have been overcome and a considerable number killed by this gas, which apparently consists largely of nitrogen and carbon dioxide; consequently, it is deficient in oxygen and produces death by suffocation.

FIRE DAMP

Fire damp is a term often applied to methane but more properly to a mixture of methane and air; the mixture becomes explosive when it contains approximately 5.0 to 15.0 per cent of methane.59

An open-flame lamp, lighted match, cigarette lighter, or spark from a locomotive trolley, open motor, telephone magneto, or broken incandescent electric lamp will ignite the explosive mixture.

The addition of relatively high percentages of carbon dioxide to the methane-air mixture or a deficiency of oxygen in the mixture tends to narrow the explosive range of methane.60 Both conditions affect chiefly the upper explosive limit, rather than the lower, so that no practical value should be placed on either, or both, as a factor of safety until the oxygen has been reduced below 12.1 per cent.61

5 McElroy, G. E., Rock Strata Gases in Mines of the East Tintic Mining District, Utah: Repts. of Investigations, Serial 2275, Bureau of Mines, 1921, 3 pp. Denny, E. H., Marshall, K. L., and Fieldner, A. C., Rock Strata Gases of the Cripple Creek District and Their Effect on Mining: Repts. of Investigations, Serial 2865, Bureau of Mines, 1928, 24 pp.

50 See footnote 22.

60 See footnote 22.

See footnote 22.

AFTERDAMP

Afterdamp is a gaseous product of a mine fire or an explosion of fire damp or coal dust.62 It consists of carbon dioxide, water vapor (quickly condensed), nitrogen, oxygen, carbon monoxide, a small amount of hydrocarbons, hydrogen, and smoke. Small quantities of creosote and benzol, ethylene, and other products of heating coal are also formed and are largely responsible for the characteristic odor of afterdamp. As a rule it consists mainly of carbon dioxide, carbon monoxide, low oxygen, and high nitrogen and is irrespirable to the extent of causing unconsciousness and death, unless greatly diluted with fresh air. The effect of afterdamp when breathed is usually due to the amount of carbon monoxide it contains. The physiological effect of carbon monoxide has been discussed previously in this publication.

BLACK DAMP

63

Black damp is a term generally applied to carbon dioxide, but this is a misapplication. Generally it is a mixture of nitrogen and carbon dioxide. Average black damp contains 10 to 15 per cent of carbon dioxide and 85 to 90 per cent of nitrogen. There may also be a small percentage of oxygen and methane present in the mixture. Black damp may also be formed by mine fires and the explosions of fire damp and coal dust in mines, and in such instances may or may not contain small percentages of carbon monoxide. It is also found in abandoned mine workings and in wells. It is an atmosphere depleted of oxygen rather than an atmosphere containing an excess of carbon dioxide.

SMOKE

Smoke consists of exceedingly small particles of solid and liquid matter suspended in the atmosphere. These particles are composed chiefly of soot or carbon, together with tarry substances, mostly liquid hydrocarbons. Smoke is irritating to breathe, but is not in itself asphyxiating to any considerable degree. Asphyxiating and irritating gases and vapors, carbon monoxide, and products of distillation generally accompany smoke, and persons overcome thereby often attribute the effects to the visible smoke.

METHODS OF DETECTING MINE GASES

Various methods and devices are in use for detecting mine gases. Primarily the detection of black damp or an atmosphere deficient in oxygen, methane, and carbon monoxide is the most important

during normal mining conditions and rescue and recovery operations. The detection of these gases will be discussed in detail later. Other gases that may be found in mines, such as hydrogen sulphide, oxides of nitrogen, sulphur dioxide, etc., usually occur in small quantities and unless present in amounts large enough to be detected by smell require collection and the analysis of air samples. The collection and analysis of these gases require special processes entirely different from the usual procedure in alayzing ordinary mine gases; therefore a chemist of considerable experience should perform the work.

DETECTION OF METHANE

There are five generally accepted methods for detecting methane. Four of these use devices that can easily be taken into a mine, and the other is ordinarily not used within mines. These methods are: 1. Use of a permissible or approved flame safety lamp.

2. Use of an approved Burrell indicator for combustible gases in air.

3. Use of an approved Martienssen methane detector.

4. Use of an approved Union Carbide Co. methane-indicating device.

5. Collection of air samples and determination of the amount of methane by a combustion analysis with a volumetric gas-analysis apparatus. This is accomplished by taking a known amount of air sample, burning the methane in the presence of oxygen, and calculating the percentage of the resultant methane from the contraction in volume produced by the combustion. In addition to these methods for detecting methane there is a more elaborate device known as a continuous methane recorder. This apparatus, when placed in a return air current, will automatically determine and continuously record the amount of methane in the air.

DETECTION OF METHANE BY FLAME SAFETY LAMP

Flame safety lamps have been used in gassy mines since the Davy lamp was introduced more than 100 years ago. During the last few years the flame safety lamp has been rapidly supplanted as a device for illuminating the miner's working place by the electric lamp described later, although the ordinary electric safety lamp does not give any aid in determining the methane content of the air. In the United States the electric cap lamp has been adopted by approximately onehalf of those employed in coal mines. Flame safety lamps can do more than the name indicates.

Paul, J. W., Ilsley, L. C., and Gleim, E. J., Flame Safety Lamps: Bull. 227, Bureau of Mines, 1924, 212 pp.

First, if an explosive mixture of gas and air is entered during testing there is no great danger of the mixture exploding provided the lamp is correctly assembled and used carefully; however, many gas ignitions have occurred through misuse of flame safety lamps. The lamps should be introduced into an explosive mixture only where absolutely necessary.

Second, in the hands of a careful, experienced man a flame safety lamp indicates the presence of gas below the explosive limit and thus can be used to indicate the approach of a possibly unsafe condition. Third, flame lamps will not burn in an atmosphere deficient in oxygen (oxygen below 16 per cent), therefore the careful user is warned of such deficiency in time to withdraw to a place of safety. Flame safety lamps are still used to some extent in the United States for general illumination and almost universally for detecting gas. Most of the coal-producing States require fire bosses to use these lamps in gassy mines, and the regulations of several States specify that a given number be kept for emergency service.

The Davy lamp was first put in general use in the mines of England during January, 1816. Although Davy is generally credited with developing the first safety lamp, Clanny and Stephenson were also actively engaged in work of this kind at about the same time. From the history of flame safety lamps it seems that all three men contributed in part those principles upon which flame safety lamps are now constructed. Clanny is generally credited with inclosing the lamp flame in a case, forming a combustion chamber; Davy with the safe confinement of the flame by the cooling effect of a wire gauze; and Stephenson with the safe confinement of the flame by holding the burned air in the upper portion of the lamp. Although the Davy lamp is still used to a limited extent, it has features that are inherently dangerous, and it has been generally replaced with the designs that have been evolved from it. The safety lamp of to-day is far better in every way than the Davy lamp. The modern lamp gives five to ten times as much light, and it is far safer when exposed to gas. There have been a large number of different kinds of flame safety lamps designed, many of which have never developed beyond the model stage, while many others are being widely used. In the United States there have been possibly a half dozen different makes that have been extensively used. The flame safety lamp is safe only when used by a competent man who has not only good judgment and good eyesight but who is conscientious and careful enough to maintain the safety lamp in safe condition and use it with care and discretion. No flame safety lamp should be allowed in any coal mine or in any place where explosive gas may be expected, unless the lamp is of the permissible magnetically locked variety.