Colliery Gases - Carbon Dioxide or Carbonic Acid Gas (not Carbonic Acid)

Occurrence

Carbon dioxide occurs in mine air and also in black-damp and after-damp. It is produced by the breathing of men and animals, burning of lamps and candles, oxidation of coal and carbonaceous shale, decay of timber, blasting operations, explosions of fire-damp and coal dust, and the action of some pit waters on certain rocks. Since the gas is r~ times as heavy as air it is found in dip workings and in disused wells. Where ventilation is insufficient it tends to accumulate near the floor and is usually found in old workings. As in the case of fire-damp, accumulations are only possible when the gas is being given off at a greater rate than it can diffuse, and therefore it must creep or gravitate downwards because of its weight.

Properties

Carbon dioxide is a heavy, colourless, and odourless gas with an acid taste when in large quantities. (The sharpness of mineral waters is due to this gas.) It does not burn and does not support combustion. The gas is a narcotic poison over 15%, and extinguishes flame when present in air to the extent of 15% or more.

Effects of Carbon Dioxide

% of Carbon Dioxide	% of Atmospheric Air	Effect on Man	Effect on Flame
1	99	No appreciable effect	No appreciable effect
3	97	Breathing slightly more difficult	Slightly dim
5 to 6	95 to 94	Decided panting and headache	Flame dim
10	90	Severe distress	Flame very dull
15	85	Partial loss of consciousness with narcotic poison effects	Flame extinguished
18	82	Suffocation and death	-
25	75	Death in short time	-

Effects of Carbon Dioxide with Corresponding Shortage of Oxygen

% of Carbon Dioxide	% of Oxygen	% of Nitrogen	Effect on Man	Effect on Flame
0.6	20.4	79	Deeper breathing	No appreciable effect
1.0	20	79	Panting on exertion	Flame rather dull
2.0	19	79	Decided panting headache	Flame dull and smoky
3.0	18	79	Severe panting, headache, face very flushed	Flame extinguished
5	16	79	Severe distress	-
9	12	79	Unconsciousness and death	-
10	11	79	Death in a few seconds	-

The last table has been included to show that the combined effects of carbon dioxide and a shortage of oxygen are much more intense than either of the two conditions alone, and further, this is the kind of air which is breathed out from the lungs and is given off from the burning of lamps and oxidation of coal; in fact, with the inclusion of fire-damp it is the kind of atmosphere which would be produced in mines if they were not efficiently ventilated (see "Black-damp "). Unless actual death has taken place, a man rendered unconscious by carbon dioxide or a shortage of oxygen or both can be rapidly restored to consciousness by artificial respiration in fresh air or oxygen.

The Coal Mines Act limits the percentage of carbon dioxide to 1.25% except in mines that have been exempted by the Mines Department on the ground of liability to spontaneous combustion, provided that certain conditions are complied with.

Detection of Carbon Dioxide

The gas may be detected by its action on the flame of the safety lamp; a dull and smoky flame would indicate at least 2%, and men must be withdrawn where the amount exceeds 1 �%.

A reliable test for carbon dioxide is obtained when the gas is passed through lime-water. The liquid becomes milky and chalk is precipitated (deposited). This effect can be obtained by breathing exhaled air from the lungs into lime-water, due to the presence of carbon dioxide in exhaled air.

The Estimation of Carbon Dioxide in air is more accurately done by chemical analysis; the gas is absorbed by caustic potash or caustic soda, and the reduction in volume measured in a given sample.