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Les gaz les plus dangereux dans l’exploitation minière

Chart Industries | 9 novembre 2023

Il va sans dire que nous avons besoin d’air pour vivre et respirer.

L’air que nous respirons à la surface est un mélange de plusieurs gaz, dont l’oxygène, l’azote, l’argon, le dioxyde de carbone et d’autres gaz en quantités infimes. Nous respirons plus facilement lorsque 21 % d’oxygène est présent dans l’air.

When other gases contaminant the air, the oxygen levels drop, and that is when the trouble begins.

The air in mines can be contaminated by the presence of other gases such as carbon monoxide, hydrogen sulfide, methane, and excess of carbon dioxide. Due to being in a confined space these gases are not always able to disperse and can therefore build up in the mine, and due to their combustible, explosive, or toxic qualities this is a serious issue. These other gases are often referred to as mine damps. This name comes from the German word Dampf meaning “vapour”.

Rather than one particular gas they are a toxic or explosive mixture of different gases that have a varying effect on human health and mine safety.

These damps are produced or released during mining operations including drilling and blasting, by mining machinery such as diesel and gasoline motors, and by other means such as the decay of timbers, the aftereffects of mine fires, and chemical processes like oxidation.

The most efficient way of preventing these gases in mines is the incorporation of high quality mining ventilation systems as well as the use of early detection devices.

A dangerous mine atmosphere is one that is toxic or explosive and there are several damps that create this kind of atmosphere. They are:

• Firedamp
• Black damp
• White damp
• Stinkdamp

Firedamp - Methane Gas (CH4)

Firedamp is a mining term for a set of explosive gases found in mines. It’s mostly made up of methane and methane is often an interchangeable term when miners talk about firedamp. Methane (CH4) is a colourless, odourless, highly flammable, and highly explosive noxious gas. It occurs naturally in coal seams and shale deposits and is a major component of the natural gas that we burn for energy.

Methane gas accumulates in “pockets” of the coal and adjacent strata naturally over millions of years and can be released as the coal is mined. As the pockets are penetrated by the mining machinery the gas seeps into the pit where explosive mixtures of methane can form.

Methane has a density relative to air of 0,55. Because it’s much lighter than air it tends to accumulate at higher levels within enclosed spaces where little ventilation exists to disperse it.

Methane is only combustible at levels between 4 and 16%. Below that range there isn’t enough to be ignited, and above that range the mixture is too dense to be explosive. The riskiest level of methane in the air is said to be 9,5% where it finds the perilous balance. All it takes is a naked flame or a spark from a machine to cause an explosion when these levels of methane are in the air.

Methane in sufficient quantities is also capable of displacing oxygen from the air and can cause asphyxia in humans who breathe it in. When humans do not receive enough oxygen to the brain it can cause headaches, nausea, dizziness, and can ultimately result in death when the oxygen concentration drops below 6%. It makes sense then to firstly avoid the build-up of this highly flammable gas. This is done through quality mine ventilation.

Find out how Howden have taken an evolutionary leap forward in the concept, design, engineering, equipment and control of mine ventilation

And secondly, any dangerous gases must be detected and monitored. This was often done through the use of a testing flame. Originally a candle with a naked flame would be an indicator of an unsafe atmosphere depending on the change to the flame tip.

As you can imagine, a flame is a pretty clear means of ignition, so when it got too close to the gas explosions would take place. Following from this, the safety lamp was developed to provide the coal mines with a means of testing for gases while still being able to operate in potentially flammable or explosive air.

Black Damp - Carbon Dioxide (CO2)

Black damp is the mining term for the suffocating mixture of carbon dioxide and other unbreathable gases that can build-up in mines causing poisoning, asphyxiation, and ultimately death if left untreated. Carbon dioxide (CO2) is a colourless, noxious gas that only gives off a slight acrid smell at higher concentrations making it very hard to identify through human senses.

While not toxic in itself, once carbon dioxide is mixed with the air it reduces the available oxygen one can breathe. The early symptoms of blackdamp (drowsiness, dizziness, and light-headedness) are easily mistaken for simple fatigue however lethal doses will cause asphyxiation that can easily result in death within seconds.

If there is 18% carbon dioxide in the air it can kill, levels as low as 3% can cause breathing difficulties with symptoms worsening as the percentage increases.

Carbon dioxide forms through the decomposition of organic materials, such as rotting mine timbers, as well as through human and animal respiration. The coal itself, once exposed to the air of a mine, will begin to absorb oxygen and exude carbon dioxide. With a heavier density relative to normal air (1,53) carbon dioxide will accumulate in lower-lying areas within the mine.

The accumulation of black damp in a mine is caused predominantly by poor ventilation however other factors can be attributed such as the temperature, the amount of exposed coal, and the type of coal.

The fact that blackdamp will always be present where coal is present and that it is hard to identify and distinguish between fatigue, increases the risk that warning signs will be missed allowing blackdamp to cause serious harm within seconds.

Therefore prevention of black damp and identification of the dangerous gases is essential for mine safety.

White Damp & Afterdamp - Carbon Monoxide (CO)

White damp is a mixture of poisonous gases found in coal mines and is predominantly made up of carbon monoxide (CO). It is colourless, odourless, and tasteless making it very hard for a human to detect. It is commonly referred to as a “silent killer”.

Carbon monoxide is a product of the incomplete combustion of carbon. In coal mines, large quantities of CO is generated during the oxidation of coal, and during mine fires or explosions. It will then be present in what miners call afterdamp - the resulting noxious gases given off by these fires, explosions, or blasting.

Carbon monoxide is extremely toxic - it is absorbed by the haemoglobin in the blood blocking the ability of the haemoglobin to absorb and carry oxygen around the body, in turn, the body will begin to shut down. Carbon monoxide is also cumulative meaning that a person can be exposed for a number of short periods to no apparent ill effect, however, with each interaction the person will become more and more susceptible to its effect. If its levels get as high as 0,2%, death will take place within one or two hours.

It is also highly flammable and explosive in mixtures with air between 12,5 and 74% with the most explosive concentration being 29%. And flames won’t extinguish in its presence as it would with carbon dioxide. It is therefore one of the most dangerous gases found in a mine, and one of the most difficult to detect.

Infamously, the domestic canary was used as an early warning against any noxious gases in the past. This is because carbon monoxide affects small animals more quickly than humans, and when exposed to carbon monoxide the bird would fall from their perch alerting the miners to move to safety.

There is now modern equipment that can easily detect carbon monoxide without any cost to life.

Stinkdamp - Hydrogen Sulfide (H2S)

Stinkdamp is the mining term given to hydrogen sulfide (H2S) due to its characteristic smell of rotten eggs.

Hydrogen sulfide is a highly poisonous, flammable, colourless, and as mentioned, pungent-smelling gas. It has only ever been found in trace amounts in mines - but it can be lethal in even small concentrations. It is produced from the decomposition of iron pyrites in a mine due to the presence of water and how they interact.

As mentioned H2S is a highly toxic gas capable of causing death in humans via asphyxiation. In lower levels, it will irritate the eyes, nose, and throat, as it increases it has a narcotic effect on the nervous system causing headache, dizziness and difficulty breathing. At higher levels of exposure H2S suppresses oxygen in the blood and tissues resulting in paralysis of the respiratory system and ultimately death in severe cases. Prolonged low exposure can lead to bronchitis, pneumonia, migraine, and loss of motor control.

It can also form flammable mixtures in the air in the range of 4,5 - 45% where any sort of ignition would cause an explosion. With a heavy density of 1,9 it can accumulate in low points in the mine that are poorly ventilated.

As always, the best way to prevent stinkdamp is proper ventilation and methods to test for the gases.

White Damp & Afterdamp - Carbon Monoxide (CO)

The most effective, safe, and efficient way of keeping mines free of dangerous gases is done through quality mining ventilation systems.

Mine ventilation provides a safe flow of air through the workings of a mine at a measurable volume that dilutes and removes any build-up of dangerous gases and other chemical and physical contaminants that may be present, as well as regulating the temperature.

To create a thorough safe mining environment prevention must be supported by detection. Detection of dangerous gases in mines has changed and developed over the years with some techniques becoming traditions ingrained in the mining culture.

The use of the canary has become a symbol for gas testing in mines, along with a testing flame, and the further developed flame safety lamps.

Did you know? A man, known as a fireman, penitent, or monk would edge forward with a candle on the end of a stick to test for the gases. If there was an explosion he was simply to keep his head down to allow it to pass over him - maybe not the safest means of detection.

While the canaries have gotten off lucky, and some safety lamps are still used today, there are now a sufficient range of detection devices, and means to detect gas including air samples, gas monitors and chemical analysis. No matter your requirement, at Howden we have a Total Mine Ventilation Solution for you: