1.2 Mine Gases - Gases in Mine Air I

8/10/2019 1.2 Mine Gases - Gases in Mine Air I

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LECTURE 2

THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS:

1.0 OXYGEN

1.1 General Properties

1.2 Physiological Effects

1.3 Detection of Oxygen

2.0 NITROGEN


3.0 CARBON DIOXIDE


3.2 Physiological Effects3.3 Permissible Concentration

3.4 Detection of CO2

4.0 CARBON MONOXIDE


4.2 Permissible Concentration4.3 Physiological Effects

4.4 Detection of Carbon Monoxide

REFERENCES


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1.0 OXYGEN (O2)


Oxygen is a colorless, odorless, and tasteless gas with a specific gravity of

1.1047. Its molecular weight is 32. 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timber, and some or all of it may have been replaced by carbon dioxide.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.

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 ofatleast 19% in the mine air/atmosphere. In former USSR, the TLV for oxygen

was 20% while it is 19.5% in the USA.

1.2 Physiological Effects: The physiological effects of staying/working in

an atmosphere deficient in oxygen are given in Table 1

Table 1 Physiological effects of staying/working in an atmospheredeficient in oxygen (McPherson, 1993)

O2Conc. Effect

19% Flame height on flame safety lamp (to be discussed latter) getsreduced by 50%

17% Noticeable increase in rate of breathing. Increase in theconcentration of carbon dioxide in the atmosphere

15% Spinning sensation in head and increased heartbeat.

13-9% Disorientation, fainting, vomiting sensation, headache, blue lips,


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and coma may occur

7% Coma, convulsions (violent uncontrollable contractions ofmuscles) or death may occur

Below6%

The conditions may eventually lead to death

1.3 Detection of Oxygen

a. 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 Indicator Model 244, Auer Oxygen

Indicator Model P etc. are based on this principle and use fuel cells as oxygen

sensors. The cells contain a gold cathode and a lead anode in a basic

electrolyte (Fig. 1).

Fig. 1 Electrochemical cell using electrochemical method forestimating oxygen percentage (after, Ramlu, 1991)

The whole setup is encapsulated in aninert plastic which is sealed off. A thin

membrane is used to protect the sensing electrode and it allows oxygen to

get diffused into the cell. The oxygen gets dissolved in the electrolyte and

reacts with the gold cathode. Thus the gold cathode becomes positively


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charged. The lead anode becomes negatively charged through the formation

of lead oxide and water. A current proportional to the oxygen concentration

flows between the cathode and the anode which is used as a measuring

signal.

b. In the paramagnetic method, as oxygen is a paramagnetic gas, it acts like

a magnetic dipole in a magnetic field. A weak permanent magnet having a

dumb-bell shape is suspended in a magnetic field against a light torque (Fig.

2). At first, the spheres are kept in balance in an inhomogeneous magnetic

field. When oxygen molecules having a large magnetic susceptibility flow

there, the molecules are pulled towards the stronger magnetic field zone and

the spheres are moved away from the zone. The resulting deviation of thespheres is detected with the light source, reflecting mirror and light receiving

element, and a current is flowed through the feedback loop to control so that

the spheres can return to the initial balanced state. The current flowing

through the feedback loop is proportional to oxygen concentration. Thus,

oxygen concentration gets converted into electric signal.

Fig. 2 Paramagnetic method for estimating oxygen


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c. 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 oxygenpercentage can be found out by noticing the flame height. More details on

flame safety lamp will be discussed in latter lectures.

2.0 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 usually inert and does not support

life or ignition. It has low solubility in water. There are three major sources of

nitrogen in mines:

Production by the decomposition of organic substances

Production from blasting using explosives (1 kg of nitroglycerine

releases 0.135 m3of nitrogen)

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.

3.0 CARBON DIOXIDE (CO2)


Carbon dioxide is colorless, odorless and has a slightly acidic taste (CO2 is

sometimes called carbonic acid gas because with water it forms carbonic

acid). The specific gravity of carbon dioxide is 1.519which is almost one-and-

a-half times that of the specific gravity of air. That is why carbon dioxide is


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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 CO2and subsequent reaction

with water in the blood forms carbonic acid (H2

CO3

), 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 (see equation below). That is why the pH of the blood

becomes less than 7.35 in such conditions}.

CO2+ H

2OH

2CO

3H

+

+ HCO3

-

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 CO2may vary from almost negligible to 20%, is known as

black damp. Blackdamp is usually heavier than air, but becomes lighter when

the percentage of CO2 in it falls below 5.25%. Carbon dioxide liquefies at -

5C under a pressure of 31.4 bar. 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 given in Table 2.


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Table 3 Respiratory requirements under different situations

(Source)

Type ofactivity

Respirationrate

(Breaths/min)

Air inhaled/respiration

(103

mm3

)

Airinhaled

(10-4

m3/s)

O2consumed

(10-5

m3/s)

Respiratoryquotient

At rest 12-18 377-705 0.82-2.18

0.47 0.75

moderate 30 1476-1968 7.64-9.83

3.3 0.90

Vigorous

work

40 2460 16.4 4.7 1.00

The term respiratory quotient is defined as the ratio of carbon dioxide

expelled to the oxygen consumed by person in the process of respiration.

3.4 Detection of CO2

The presence of CO2 is usually detected from oxygen depletion indicated by

the extinguishing of oil lamps at 17-17.5%. CO2 also turns lime water milky

and this property can be used in detection of CO2in 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

concentrations. Hand held detectors based on the principle of colorimetric

indication are also used for finding out CO2percentage. 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.

M.S.A. Universal Tester, Drager Multigas Detector and Auer Gas tester are

commonly used in detection and estimation of CO2.

4.0 CARBON MONOXIDE (CO)



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CO is a colorless and odorless gas with a specific gravity of 0.972.As can be

seen, 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 leadto the formation of hydrogen and CO, and because of the formation of

hydrogen which is a explosive gas, this fire will become more violent.

CO is considered as the most dangerous gas in mines. This is because of the

following reasons:

It is highly toxic in nature.

As it is colorless and tasteless, it is not noticed easily.

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.

4.2 Permissible Concentration

As per Indian Standards (DGMS), the CO concentration should not be allowed

to exceed 0.005 %. The TWA for CO is 0.005% and STEL is 0.04% as per the

recommendations of the American Conference of Governmental Industrial

Hygienists (ACGIH) and US National Institute of Occupational Safety and

Health (NIOSH).


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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 in Table 4

Table 4 Physiological effects of CO poisoning (McPherson, 1993)

Bloodsaturation% CO.Hb

Symptoms

5-10 Slight loss of concentration

10-20 Sensation of tightness across forehead, slight headache

20-30 Throbbing headache, judgment impaired

30-40 Severe headache, dizziness, disorientation, dimmed vision,nausea(vomiting), possible collapse

40-60 Increased probability of collapse, rise in rates of pulse andrespiration, convulsions

60-70 Coma, depressed pulse and respiration, possible death

70-80 Fatal- will lead to death

4.4 Detection of Carbon Monoxide

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 remedyfor 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


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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, like Hoolamite tubes and M.S.A. ammonium-palladium-

complex colorimetric detectors (Fig.3) 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:

o the change in the color of the chemical present in the detector tube

o the change in the 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.

Fig. 3 MSA CO detector with detector tubes


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REFERENCES

Bolz, R. E. and Tuve, G. L., eds., Handbook of Tables for Applied EngineeringScience,2nded. CRC Press, Cleveland

Deshmukh, D. J. (2008); Elements of Mining Technology, Vol. II; Denett& Co., Nagpur, India.

Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (1982); Mine Ventilationand Air Conditioning; John Wiley & Sons, New York.

McPherson, M. J. (1993); Subsurface Ventilation and EnvironmentalEngineering; Chapman & Hall, London.

Misra G.B. (1986); Mine Environment and Ventilation; Oxford UniversityPress, Calcutta, India.

Ramlu, M. A. (1991); Mine fires, Explosions, Rescue, Recovery andInundations; Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi.

Vutukuri, V. S. & Lama, R. D. (1986); Environmental Engineering in Mines;Cambridge University Press, Cambridge.
http://www.google.co.in/search?tbo=p&tbm=bks&q=inauthor:%22Jan+M.+Mutmansky%22http://www.google.co.in/search?tbo=p&tbm=bks&q=inauthor:%22Jan+M.+Mutmansky%22

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1.2 Mine Gases - Gases in Mine Air I