These impurities and requirements in mines environment is discussed below:
1. Oxygen (O2):
Oxygen is a colorless, odorless, and tasteless gas with a specific gravity of 1.1047 (heavier than air = 1), a typical miner at rest consumes about 0.005 litres of oxygen every second due to respiration. Similarly, a miner exposed to moderate and extreme workloads consumes 0.03 litres and 0.05 litres of oxygen every second respectively due to his breathing.
Oxygen can cause a dangerous condition in a mine only by its absence. Air which has been stagnant for a considerable period may have much of its oxygen removed by the oxidation of metals and minerals and the decaying of timber, and some or all of it may be replaced by carbon dioxide.
Such a mixture of gas which is deficient of oxygen, but is neither poisonous nor explosive, is called black-damp in underground mine environment and ventilation. Normal air contains 21% of oxygen by volume (23% oxygen by weight).
Mine safety laws in India require a TLV greater than 19% for oxygen. This means that any part of a given mine must have an oxygen concentration of at least 19% in the mine air/atmosphere.
The physiological effects of staying/working in an atmosphere deficient in oxygen are tabulated below:
The percentage of oxygen present in an atmosphere can be estimated by using the principle of electrochemical method, paramagnetic method or the flame safety lamp. In the electrochemical method, very small concentrations of the gas are detected by its influence on the output from an electrochemical cell. The MSA Oxygen Indicators are based on this principle and use fuel cells as oxygen sensors.
The flame safety lamp is also used to find out the percentage of oxygen in the atmosphere. In this method, the length of the flame produced by the lamp because of the burning of the gas is used. Accordingly, the oxygen percentage can be found out by noticing the flame height.
2. Nitrogen (N2):
Nitrogen is a colorless, odorless and tasteless gas. It is slightly lighter than air with a specific gravity of 0.967. It is inert and does not support life or ignition. It has low solubility in water.
There are three major sources of nitrogen in mines:
a. Production by the decomposition of organic substances.
b. Production from blasting using explosives (1 kg of nitroglycerine releases 0.135 m3 of nitrogen).
c. Production from the strata through cracks.
Nitrogen has no known harmful effects on the human system but a higher concentration of nitrogen leads to deficiency of oxygen in the mine air. Thus, increase in nitrogen concentration indirectly leads to the physiological effects caused by a lack of oxygen on humans.
3. Carbon Dioxide (CO2):
Carbon dioxide is colorless, odorless and has a slightly acidic taste. The specific gravity of carbon dioxide is 1.519 which is almost one-and-a-half times that of the specific gravity of air. That is why carbon dioxide is found in low-lying areas in the mines. It is fairly soluble in water and forms carbonic acid when dissolved in water.
Its solubility in water increases with decrease in temperature. An increase in inhaled CO2 and subsequent reaction with water in the blood forms carbonic acid (H2CO3), which then dissociates into hydrogen ions [H+] and bicarbonate [HCO3]. The excess CO2 shifts the equilibrium towards the creation of more hydrogen ions, thus creating an acidic environment. That is why the pH of the blood becomes less than 7.35 in such conditions.
Carbon dioxide occurs both in coal and metal mines. It is produced from a variety of sources including strata emissions, oxidation of carbonaceous materials, internal combustion engines, blasting, fires, explosions and respiration. A mixture of carbon dioxide and nitrogen in which the concentration of CO, may vary from almost negligible to 20%, is known as black damp.
Blackdamp is usually heavier than air, but becomes lighter when the percentage of CO, in it falls below 5.25%. Carbon dioxide liquefies at -5°C under a pressure of 31.4 bars. If it is further cooled, it solidifies into dry ice which derives its name from the fact that it evaporates in air without melting (undergoes sublimation process).
Carbon dioxide dilutes oxygen in air and acts as a stimulant to the respiratory and central nervous system. Diffusion of gas in bloodstream is rapid and it affects the rate and depth of breathing.
The physiological effects of carbon dioxide are tabulated below:
As per Indian Standards (DGMS), the CO2 concentration should not be allowed to exceed 0.5 % (5000 ppm) in air.
All active workings in coal mines must be ventilated by an air current containing not less than 19.5 % of Oz and not more than 0.5% of CO2. If we need to calculate the air quantity required only to satisfy the respiratory requirement, we need to consider both the cases i.e., the oxygen consumed by each worker and the CO, produced by him. Since, CO2 is a contaminant, accurate measurement of quantity needed is a must for the safety of the workers. Table given below is useful in calculating quantity of air requirements under different conditions.
The term respiratory quotient is defined as the ratio of carbon dioxide expelled to the oxygen consumed by person in the process of respiration.
The presence of CO2 is usually detected from oxygen depletion indicated by the extinguishing of oil lamps at 17-17.5% O2 CO2 also turns lime water milky and this property can be used in detection of CO2 in mines. Optical methods like non-dispersive infrared gas analyzers can also be used in finding out the concentration of CO2 Portable instruments are also used to estimate CO2 concentration. Hand held detectors based on the principle of colorimetric indication are also used for finding our CO2 percentage.
These detectors have detector tubes which are filled with an indicating chemical substance. When a sample of air is drawn through a detector tube, this substance changes its color over a length. This length is proportional to the concentration of CO2. The concentration of gas can be read off on the concentration scale printed on the tube.
4. Carbon Monoxide (CO):
CO is a colorless tasteless odorless gas with a specific gravity of 0.972.Its specific gravity is almost equal to that of air and therefore it exists at all levels in an underground opening. It burns with a blue flame and is explosive in presence of air at concentrations between 12.5% and 75%.
Though CO is an inflammable gas, this fact is of no practical importance in mining because it never occurs in significant concentration to burn or to cause an explosion. The ignition temperature of CO is 873K. CO is produced by the incomplete combustion of carbonaceous materials. It is also produced by internal combustion engines, blasting and spontaneous combustion in coalmines.
It can also be generated as a component of water gas (mixture of CO and H2), when water is applied to coal for controlling the fire. That is why it is advisable not to apply water at the centre of coal fire because it will lead to the formation of hydrogen and CO, and because of the formation of hydrogen which is an explosive gas, this fire will become more violent.
CO is considered as the most dangerous gas in mines because of the following reasons:
a. It is highly toxic in nature.
b. As it is colorless tasteless and odourless; it is not noticed easily.
c. As its specific gravity is almost equal to the specific gravity of air, it exists at all the levels in an underground opening. Hence the chances of its inhalation are very high.
As per Indian Standards (DGMS), the CO concentration should not be allowed to exceed 0.005 % (50 ppm) The TWA for CO is 0.005% and STEL is 0.04% (400 ppm).
Hemoglobin present in human blood has 300 times more affinity towards CO than O2.The new substance formed by the combination of CO and hemoglobin is known as carboxy-haemoglobin. This is relatively stable and accumulates in the bloodstream. This results in a reduction in the number of red cells for carrying oxygen to vital parts of the body. Thus the physiological effects of CO arise because of the reduction in oxygen supply to vital parts of the body. The symptoms depending upon the saturation of blood by carboxy-haemoglobin are given below in Table.
CO.Hb – Carboxyhemoglobin
Carbon monoxide imparts a bright pink color to blood, and the patient poisoned by CO has a typically pink color as a result of this. The best remedy for CO poisoning is to quickly expose the patient to fresh air and provide him pure oxygen. The patient should be covered by blankets so that he can be kept warm. Black coffee is also very useful. The property of blood turning pink by absorbing carbon monoxide has been used for testing the gas.
The air with CO contained in it is passed through a light straw colored solution of blood and the coloration produced is compared with a standard color chart calibrated for different colorations of the gas. This method gives a good accuracy within the range of 0.01 to 0.2% CO.
Modern CO detectors, tubes and ammonium-palladium-complex colorimetric detectors use detector tubes containing suitable gels such as alumina, silica gel, iodine pentoxide and fuming sulphuric acid soaked in pumice stone, silica gel impregnated with palladium sulphate and ammonium molybdate etc.
The CO is detected using the principle of the charges in the:
1. Color of the chemical present in the detector tube.
2. Length of the color of the detector tube.
Warm blooded birds like munia/canary or mouse are also used for detecting CO as they are affected sooner than human beings by CO. Only fresh birds are used in this method as repetition of same bird may lead to the acclimatization of the bird to the low percentages of CO. There are no immediate signs of distress observed when birds are exposed to 0.1% of CO.
But at 0.15% of CO, a bird shows distress (pronounced chirruping and loss of liveliness) in 3 minutes. And at 0.3% of CO in air, the bird shows almost immediate distress and falls off its perch (when a bird perches on something such as a branch, it lands on it and stands there) in 2-3 minutes.
5. Hydrogen Sulphide (H2S):
H2S is a gas with a smell similar to that of a rotten egg and has a sweetish taste with a specific gravity of 1.175. It burns with a light blue flame and is soluble in water. It is also known as Stink Damp. It is combustible and is explosive over a wide range of its concentration from 4.3 % to 45.5 %. It is more poisonous than CO and has a TWA value of 10 ppm and an STEL value of 15 ppm. H2S is not very common in mines and usually occurs in firedamp and gob fires in sulphurous coal.
It is formed naturally by bacterial/chemical decomposition of organic compounds and is often detected near stagnant pools of water in underground mines. It may also occur in natural gas and petroleum reserves and migrate through the strata in a weakly acidic water solution.
In metal mines it is produced by the action of acidic water on iron pyrites which can be represented by the equation:
Hydrogen sulfide has a very low odor threshold, with its smell being easily sensed by the human nose at concentrations well below 1 part per million (ppm) in air. The odor increases as the gas becomes more concentrated, with the strong rotten egg smell recognizable up to 30 ppm.
Above this level, the gas has a sickeningly sweet odor up to around 100 ppm. However, at concentrations above 100 ppm, a person’s ability to detect the gas is affected by rapid temporary paralysis of the Olfactory nerves in the nose, leading to a loss of the sense of smell. This means that the gas can be present at dangerously high concentrations, with no perceivable odour.
Prolonged exposure to lower concentrations can also result in similar effects of olfactory fatigue. This unusual property of hydrogen sulfide makes it extremely dangerous to rely totally on the sense of smell to detect the presence of the gas.
The physiological effects of H2S poisoning are given below in Table:
A person who recovers from H2S poisoning may have conjunctivitis and bronchitis for a long period after recovery.
Although hydrogen sulfide is an extremely poisonous gas, miners are rarely affected by it. This is mainly because it seldom occurs in dangerous concentrations and also a very small concentration can be easily detected by virtue of smell.
H2S is easily detected by its smell at very low concentrations of up to 0.000075% (0.75 ppm). Another detection test for this gas is done by exposing a filter paper soaked in lead acetate solution to an atmosphere containing H2S gas. The filter paper turns brown and then black if the concentration of the gas is sufficiently high. H2S detector is an accurate instrument for detecting H2S.
It consists of a glass tube filled with white granules of activated aluminum oxide coated with silver cyanide. When gas containing H2S is drawn through the tube, the gas combines with silver cyanide forming black silver sulphide which turns the granules black. The percentage is calculated with the help of a scale placed along the side of the tube which measures the length up to which the change of color has taken place.
6. Nitrous Fumes (NOx):
Nitrous fumes are rarely found in mines and mainly consist of nitric oxide (NO) nitrogen dioxide (NO2) and nitrogen tetra-oxide (N2O4). The original product is nitric oxide (NO) which quickly combines with oxygen to form nitrogen dioxide (NO2), which has a pungent smell like that of fuming nitric acid.
As NO2 cools down it is slowly converted to nitrogen tetra-oxide (N2O4), which is a colorless gas. The specific gravity of NO is 1.036, specific gravity of NO2 is 1.519 and specific gravity of N2O4 is 1.588. The nitrous fumes are highly soluble in water. These fumes are very poisonous and TWA value for NO is 25 ppm, for NO, is 50 ppm and for N2O4 is 3 ppm.
The nitrous fumes are mainly produced by explosives containing nitroglycerine and in exhaust fumes of diesel locomotives. They are also found in shafts and tunnels where heavy shot firing takes place in a confined place.
The brown fumes of NO2 get dissolved in water and produce nitrous acid (HNO3) and nitric acid (HNO3). These acids cause irritation and corrosive effect on eyes and respiratory system.
Various physiological effects of these fumes at different concentrations are given below in Table:
The first-aid for NO2 poisoning includes:
a. Administration of oxygen to the affected person,
b. The person should rest and should not be allowed to move,
c. The person should be kept warm so that body temperature doesn’t fall to a lower value.
The common test for nitrous fumes is to soak a filter paper with starch and potassium iodide solution and expose it to the air containing nitrous fumes. The paper turns blue by the liberation of iodine. Tube type detectors are also % available which indicate the percentage of nitrous fumes by the length of change in color in the tube. For example tubes manufactured by Drager use diphenyl benzidine as the reactant.
7. Sulphur Dioxide (SO2):
This is a colorless gas with a very suffocating odor and a specific gravity of 2.264. It is highly soluble in water. The TWA value for this gas is 2 ppm and STEL value is 5 ppm. It is found in smaller quantities in afterdamp (Afterdamp is a mechanical mixture of gases found in a mine after an explosion.
Afterdamp contains nitrogen and carbon dioxide as the chief constituents along with carbon monoxide, methane, water vapour, hydrogen, oxygen and small quantities of H2S and SO2. A typical composition of afterdamp may be- CO-1.5%, CO2-8%, O2-S%, CH4-0.3%, H,-0.2%, N2-85%) in some coal mines; but occurs abundantly in sulphide ore mines with fires. Blasting in rich sulphide ores also produces large quantities of SO2 and H2S. It is also released from the exhaust of internal combustion engines.
SO2 is an irritating and toxic gas. At low concentrations it causes intense burning sensation to the eyes and the respiratory tracts. Table below the gives the physiological effects at various concentrations of sulphur dioxide.
SO2 can easily be detected by its odor even at low concentrations of 0.0005% (5 ppm). However, tube- type detectors using iodine, starch, and potassium iodide as reagent are available for more accurate estimation.
It is colorless, odorless, tasteless and non-toxic in nature. It is the lightest of all gases. It has specific gravity of 0.07. This makes it rise to the roof.
The sources of hydrogen in underground mines include:
(i) Charging of batteries
(ii) Action of acids on metals
(iii) Action of water on hot coal (as water gas) or even some of the hot minerals
(iv) Rarely as strata gas and in afterdamp
The explosibility curve of hydrogen is shown above in Fig. As can be seen from the figure, the explosibility range of hydrogen is 4-74% in air. Also, it burns/ignites at 480° C, lower than that of methane. It ignites at oxygen concentration in air as low as 5%. Methane requires at least 12% of oxygen in air for ignition. Further, hydrogen requires almost half of the energy required by methane to ignite. This makes hydrogen a very explosive gas.
9. Methane (CH4):
Methane is a colorless, odorless and tasteless gas. This gas is lighter than air as its specific gravity is equal to 0.559. That is the reason why it tends to rise to the roof of a mine working. Methane becomes liquid below 112 K and solidifies below 90.5 K. Methane gas is poorly soluble in water, but is soluble in organic solvents like alcohol and ethers. This property of methane is utilized during drainage of coal bed methane. It burns with a blue flame and produces carbon dioxide and water as products.
This exothermic reaction is a chain reaction and under suitable conditions gets self-accelerated and leads to an explosion.
The combustion of methane in air can be represented as follow:
The ignition temperature of methane is roughly 650°C. This means that only very hot spark or flame will be able to ignite methane. When methane, even though dilute, is brought in contact with a flame, it burns. When the concentration of methane in air is low, the methane molecules are far apart from each other.
Therefore burning of one molecule of methane does not create sufficiently high temperature so as to ignite the molecule adjacent to it. With increase in the concentration of methane, each molecule of methane is able to ignite the molecule adjacent to its which spreads the flame rapidly throughout the mixture. In such condition an explosion takes place and such a mixture of methane and air is called an inflammable mixture.
If the mixture contains too much methane, the oxygen percentage in the mixture is reduced which causes reduction in the rate of combustion which in turn causes insufficient development of heat to propagate a flame and therefore explosion does not take place.
When wire gauze is held horizontally in a jet of methane, it will be possible to lit the gas above the wire gauze and it will not burn below the gauze or vice-versa. This is because; the wire gauze carries away the heat from the flame so rapidly that the temperature on its other surface is reduced to below 650°C. Due to this, the methane on that side does not ignite.
Explosibility Curve of Methane:
There is proper oxygen balance with methane content of 9.8 % by volume in the air. Because of this reason, at this concentration of methane i.e., 9.8 %, the mixture is most explosive. The explosions caused due to methane gas are not as violent as those of commercial explosives.
This is because of the density difference. The density of methane-air mixtures is around 1.15 kg/m3 whereas the density of gun powder is 1000 kg/ m3 and for nitroglycerine it is 1600 kg/m3. The explosible range for methane in air is 5 to 15 % by volume. Most explosive mixture of methane gas occurs at 9.8 %. The lower flammable limit of methane gas is almost constant whereas the upper limit reduces with decrease in the oxygen percentage in the air.
Lag on ignition is an important characteristic of methane gas. Methane gas starts burning only after absorbing 92.53 KJ/mol heat. Thus ‘lag on ignition’ is defined as the time interval between the exposure of “CH4 – Air” to an igniting source to the appearance of flame. This lag on ignition is dependent on the temperature of the igniting source. As for instance, at 650 °C, the delay is around 10 seconds, at 1000 °C, it is 1 second and 1200 °C it is 1/15th of a second.
This property of methane gas is utilized in designing of permitted explosives which are prescribed for use in underground coal mines. These permitted explosives produce a flame of a very short duration. In this time period/duration, methane doesn’t get enough heat required for its ignition. Thus the permitted explosives can be used safely in underground coal mines. The presence of hydrogen or other gases reduces the ignition lag.
In India, coal mines are classified into three categories based on methane emission.
The details of this classification are given below in Table:
As can be seen from above Table for underground coal mines to come under Degree – II gassy mine category, both the “percentage of inflammable gas in general body of air” as well as “rate of emission of methane gas in m3/ton of coal raised” must be satisfied.
For underground coal mines to come under Degree – II gassy mine category, any one of the conditions i.e., “percentage of inflammable gas in general body of air” or “rate of emission of methane gas in m3/ton of coal raised” is sufficient.
Methane is usually found in coal mines and sometimes in the tunnels passing through carbonaceous shale. Methane is the major constituent of firedamp [a mixture of air which mainly consists of methane (80-96%) with other minor gases such as nitrogen, ethane, carbon dioxide etc.], which is a common gas mixture in mines.
The vegetable matter composed mainly of cellulose and lignin is decomposed by bacterial action which produces methane and carbon dioxide. That is why methane is sometimes called as marsh gas. The process continues and in the early stage of coalification, much of methane escapes because of poor confinement but largely retained in higher rank coal. Coal has large number of small pores and correspondingly high internal surface area. Porosities of coal may vary from 1 % to more than 20 %.
Methane exists in coal in two different forms which are referred as free gas and adsorbed gas. In free gas, the methane molecules are free to move within the pores. In adsorbed state, the methane molecules are adsorbed on to the coal surface. Around 95 % of the total methane content in coal is in adsorbed state which is due to very high gas pressure.
Methane is not the only gas that is trapped in the coal seam or strata. The other gases which are also found in coal seams are carbon dioxide, carbon monoxide, nitrogen, etc. Of these, carbon dioxide is the most important one. It is the carbon dioxide which is a major constituent in coals of lower grade, while methane is the major constituents in the higher rank coal.
For example, in lignite, bituminous and sub-bituminous coal, carbon dioxide is the most significant one. However, in anthracite (which is the highest rank coal) methane has major part compared to carbon dioxide. One tone of anthracite may have approximately 765 m3 of methane and 565 m3 of carbon dioxide.
During coalification, a fraction of these gases escape out. The amount of gas retained per ton of coal is called seam gas content (includes all gases). It is generally expressed in m3/ton. Another important term to know is, the specific methane emission, defined as the volume of methane emitted per ton of coal. It is used as an index for/of seam gas content.
Methane is emitted mainly from coal seams and associated strata in an underground mine. Methane in the coal seam has considerable pressure. But the mine workings have pressure equal to that of atmospheric pressure. Hence the adsorbed gas in the coal gets desorbed because of the lowering of pressure when the rock containing the gas is drilled/ cut.
Methane tends to migrate to low pressure zones. Speedy migration of methane takes place through macro cracks and fissures and slow migration takes place by permeation through microspores and micro cleavages. Major channels for gas migration are faults and joints. Dykes and sills also provide a path for methane migration.
When coal is cut, transported etc., desorption of methane occurs. The size of coal produced has a great influence on the amount of gas emitted at the face. Large lumps of coal release only 2 % of their total gas content in the first 10 minutes whereas coals of 0.25-1.0 mm size release 40 % and coal of size less than 0.25 mm release 66 % of their total gas content.
Some of the Indian laws regarding the methane concentration are:
(i) Methane concentration should not exceed 0.75 % in the return of a ventilation district.
(ii) Methane concentration should not exceed 1.25% in any part of the underground mine.
(iii) Electric supply should be cut-off from the district if the methane concentration exceeds 1.25%. Also workers should be withdrawn from the place.
(iv) Charging, stemming or firing of shot holes are not allowed in areas where methane is found /detected.
(v) In gassy mines, where electricity is used, detection/testing of methane is carried on the intake side of the first working face and on the return side of the last working face in a district as per the following rule-
Methane in mine air can be detected either by using chemical analysis in laboratory or by using flame safety lamp and special instruments called methanometers. Using safety lamp or methanometers, methane can be detected on the spot in underground.