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

Transcription

1 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 2.1 General Properties 3.0 CARBON DIOXIDE 3.1 General Properties 3.2 Physiological Effects 3.3 Permissible Concentration 3.4 Detection of CO2 4.0 CARBON MONOXIDE 4.1 General Properties 4.2 Permissible Concentration 4.3 Physiological Effects 4.4 Detection of Carbon Monoxide REFERENCES Page 1 of 12

2 1.0 OXYGEN (O2) 1.1 General Properties Oxygen is a colorless, odorless, and tasteless gas with a specific gravity of Its molecular weight is 32. A typical miner at rest consumes about 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its oxygen removed by the oxidation of metals and minerals and the decaying of timber, and some or all of it may have been replaced by carbon dioxide.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 ofat least 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 atmosphere deficient in oxygen (McPherson, 1993) O 2 Conc. Effect 19% Flame height on flame safety lamp (to be discussed latter) gets reduced by 50% 17% Noticeable increase in rate of breathing. Increase in the concentration of carbon dioxide in the atmosphere 15% Spinning sensation in head and increased heartbeat. 13-9% Disorientation, fainting, vomiting sensation, headache, blue lips, Page 2 of 12

3 and coma may occur 7% Coma, convulsions (violent uncontrollable contractions of muscles) or death may occur Below The conditions may eventually lead to death 6% 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 for estimating oxygen percentage (after, Ramlu, 1991) The whole setup is encapsulated in an inert 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 Page 3 of 12

4 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 the spheres 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 Page 4 of 12

5 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 oxygen percentage can be found out by noticing the flame height. More details on flame safety lamp will be discussed in latter lectures. 2.0 NITROGEN (N 2) 2.1 General Properties Nitrogen is a colorless, odorless and tasteless gas. It is slightly lighter than air with a specific gravity of 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 m 3 of 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 (CO 2) 3.1 General Properties Carbon dioxide is colorless, odorless and has a slightly acidic taste (CO 2 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-anda-half times that of the specific gravity of air. That is why carbon dioxide is Page 5 of 12

6 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 CO and subsequent reaction 2 with water in the blood forms carbonic acid (H CO ), which then dissociates 2 3 into hydrogen ions [H + - ] and bicarbonate [HCO ]. The excess CO2 shifts the 3 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}. CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - 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 2 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 2 in it falls below 5.25%. Carbon dioxide liquefies at - 5 C 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). 3.2 Physiological Effects 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. Page 6 of 12

7 Table 2 Physiological effects of carbon dioxide (Source) CO 2 Conc. Effects 0.03% No effect- it is the normal concentration of CO 2 in air 0.50% Lung ventilation gets increased by 5% 2.00% Lung ventilation gets increased by 50% 3.00% Lung ventilation gets doubled, heavy breathing or panting on exertion 5% to 10% Violent panting leading to fatigue from exhaustion(extreme fatigue), headache 10% to 15% Intolerable panting, severe headache, rapid exhaustion and collapse 3.3 Permissible Concentration As per Indian Standards (DGMS), the CO 2 concentration should not be allowed to exceed 0.5 %. Federal regulation require that all active workings in coal mines must be ventilated by an air current containing not less than 19.5 % of O 2 and not more than 0.5% of CO 2. 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 2 produced by him. Since, CO 2 is a contaminant, accurate measurement of quantity needed is a must for the safety of the workers. Table 3 given below is useful in calculating quantity of air requirements under different conditions. Page 7 of 12

8 Table 3 Respiratory requirements under different situations (Source) Type of activity Respiration rate (Breaths/min) Air inhaled /respiration (10 3 mm 3 ) Air inhaled (10-4 m 3 /s) O 2 consumed (10-5 m 3 /s) At rest moderate Vigorous work Respiratory quotient 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 CO 2 The presence of CO 2 is usually detected from oxygen depletion indicated by the extinguishing of oil lamps at %. CO 2 also turns lime water milky and this property can be used in detection of CO 2 in mines. Optical methods like non-dispersive infrared gas analyzers can also be used in finding out the concentration of CO 2. Portable instruments are also used to estimate CO 2 concentrations. Hand held detectors based on the principle of colorimetric indication are also used for finding out CO 2 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 CO 2. 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 CO CARBON MONOXIDE (CO) 4.1 General Properties Page 8 of 12

9 CO is a colorless and odorless gas with a specific gravity of 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 H 2), 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 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 %. 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). Page 9 of 12

10 4.3 Physiological Effects Hemoglobin present in human blood has 300 times more affinity towards CO than O 2.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 carboxyhaemoglobin are given in Table 4 Table 4 Physiological effects of CO poisoning (McPherson, 1993) Blood Symptoms saturation % CO.Hb 5-10 Slight loss of concentration Sensation of tightness across forehead, slight headache Throbbing headache, judgment impaired Severe headache, dizziness, disorientation, dimmed vision, nausea(vomiting), possible collapse Increased probability of collapse, rise in rates of pulse and respiration, convulsions Coma, depressed pulse and respiration, possible death 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 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 Page 10 of 12

11 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-palladiumcomplex 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 Page 11 of 12

12 REFERENCES Bolz, R. E. and Tuve, G. L., eds., Handbook of Tables for Applied Engineering Science, 2 nd ed. 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 Ventilation and Air Conditioning ; John Wiley & Sons, New York. McPherson, M. J. (1993); Subsurface Ventilation and Environmental Engineering ; Chapman & Hall, London. Misra G.B. (1986); Mine Environment and Ventilation ; Oxford University Press, Calcutta, India. Ramlu, M. A. (1991); Mine fires, Explosions, Rescue, Recovery and Inundations ; Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi. Vutukuri, V. S. & Lama, R. D. (1986); Environmental Engineering in Mines ; Cambridge University Press, Cambridge. Page 12 of 12