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CHAPTER# 04 Dusts

CHAPTER# 04 Dusts. Dynamic behavior of air-borne particulate matter: Dust is the second category of air-borne contaminant which is of major concern to the mining engineer Dust have much in common with gases in modes of occurrence, behavior, and control

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CHAPTER# 04 Dusts

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  1. CHAPTER# 04Dusts • Dynamic behavior of air-borne particulate matter: • Dust is the second category of air-borne contaminant which is of major concern to the mining engineer • Dust have much in common with gases in modes of occurrence, behavior, and control • Aerosols: Air-borne suspension of particulate matter are termed aerosols • Principles of Dynamic behavior of air-borne dust: • Particulate matter, whether liquid or solid, exhibits remarkably similar behavior when air-borne

  2. CHAPTER# 04Dusts • The dust Particles of either pathological or combustible consequence are predominantly below 10 microns(µ) in size (10µ =0.0004in) • Particles larger then 10µ are unlikely to remain long in suspension in air streams of even moderate velocity • Industrial and mine dust characteristically have a mean in range of 0.5-3µ. Chemical activity in increases with decreasing particle size • The dust below 10µ in size, which are of serious consequence in air hygiene, have no significant weight or inertia and hence remain suspended in atmosphere • The control of fine dust( below 10µ) that have become air-borne requires control of air stream in which they are suspended. This is the basic concept of dust control

  3. CHAPTER# 04Dusts • Classification of dusts: Dusts are classified on basis of their harmful physiological effects or explosive properties • Pulmonary Dusts (harmful to respiratory system) • Silica (quartz, chert) • Silicates (asbestos, talk, mica, sillimanite) • Metal fumes (nearly all) • Beryllium ore • Tin ore • Iron ore (some) • Carborandum • Coal (anthracite, bituminous)

  4. CHAPTER# 04Dusts • Toxic dusts (poisonous to body organs, tissue, etc) • Ores of beryllium, arsenic, lead, uranium, radium, thorium, chromium, vanadium, mercury etc. (principally the oxides and carbonates) • Radioactive dusts (injurious because of radiation) • Ores of uranium, radium, thorium • Explosive dusts ( combustible when air-borne) • Metallic dusts (magnesium, aluminum, zinc, tin , iron) • Coal (bituminous, lignite) • Sulphide ores • Inert dust (no harmful effects)

  5. Physiological effect of pulmonary dusts: • Human respiratory system: • The nasal passage and oral open in to the trachea (wind pipe) in throat. • The trachea in turn leads to the bronchial ramus. • These tubes conduct air into right and left branch. • Each branch sub divides into many smaller tubes the bronchioles. • The bronchioles end in small sacs, the alveoli. • The oxygenation of the blood takes place in alveoli.

  6. Physiological effect of pulmonary dusts: • The respiratory has may built in safeguard to protect the tissues from the harmful effects of dusts. • Upper reparatory system—a system of filters. • Hairs in nasal passage filters out larger particles (over 5µ) • Mucus membrane, lining the nose and throat, traps still more particles larger and smaller. • In the trachea and , hair like cilia sweep medium sized particles (5-10µ) upward with a vibratory motion into throat where they are swallowed.

  7. Physiological effect of pulmonary dusts: • The secondary line of defense against small particles is elimination. • In the alveoli, a tiny jelly like wandering cells called phagocytes await the particles which reach the lungs. • Mostly 1u size particles reach alveolar spaces. • The phagocytes envelop any foreign matter and move it to drainage vassals called lymphatic. • These vessels empty into lymph nodes. The sedimentation chamber of the body waste system and are eventually discharged.

  8. CHAPTER# 04Dusts • Physiological effect of pulmonary dusts: • It effect on the followings • The human respiratory system: A dust may produce a fibrous or nonfibrous respiratory diseases, the collective name of such diseases is called “ Pneumoconiosis” • The dusts and their effects on human body are • Silicosis (miners’ phthisis), by free silica • Silicotuberculosis (complication of tuberculosis by silica • Asbestosis, by asbestosis

  9. CHAPTER# 04Dusts • Silicatosis, by other silicates • Siderosis, by iron or iron ores • Anthracosis, by coal, both bituminous and anthracite, suspected also to contain free silica

  10. CHAPTER# 04Dusts • Harmful effect on lungs: the following symptoms may be seen due to continuous and prolong exposures to dusts • Shortness of breath • Labored breathing • Chest pains • Coughing • Loss of body strength • Spitting blood

  11. CHAPTER# 04Dusts • Physiological effect of other dusts: • Toxic dusts harms the organs (stomach, liver, kidney, etc), of the body • Radioactive dusts, causes lung cancer, radiation damage to the body • Lead and radium ore dusts, causes irritation and damage eyes or skin

  12. CHAPTER# 04Dusts • Factors which determine dust harmfulness to human: • The Factors which determine dust harmfulness to human are • Composition • Chemical • Mineralogical • Concentration • One number basis: units are million of particles per cu ft of air (mppcf) in united states and particles per cc(ppcc) abroad (1mppcf=35ppcc)

  13. CHAPTER# 04Dusts • On weight basis: units are g per cu m(mg/1) or oz or gr per cuft (1gr/cuft=2.3mg/cum • Particles size • Exposure time • Individual susceptibility

  14. CHAPTER# 04Dusts • Explosive Dusts and Factors in Ignition: • A dusts explosion consists of a sudden pressure rise caused by the very rapid combustion of air born dusts • Ignition of combustible dusts occur in following ways • Initiation by flame or spark • Propagation by a gas explosion or blasting • Spontaneous combustion • Electric arcs (in coal mines)

  15. Explosive Dusts and Factors in Ignition • Coal Dusts: • Common explosive dusts encountered are bituminous coal dusts. • Coal dusts can explode even in the absence of gas. • In addition the following physical and chemical factors influence the expansibility.

  16. Composition : • volatile content of coal is the principal determinant. • Explicability increases linearly to about 25% ( medium-volatile coal). • High rank coal ( low volatile content) are least susceptible. • If the Volatile content divided by volatile content +fixed carbon exceeds 0.12, then the dust is potentially explosive.

  17. Anthracite coal is non explosive at normal temperature. • Moisture and ash reduces explicability. • Particle size: • finer the dust the most explosive it would be. • Coal dust over 20 mesh (850 u) enter into explosion. • Concentration: • Minimum airborne dust concentration to be explosive is 0.035 oz per cu ft. • This corresponds to 35,000 mppcf, if the mean size is 3u.

  18. Flammable gases: • The presence of methane gas enhances the ease of coal dust. • Relative humidity of air: • Practically no effect on coal dust explosion.

  19. CHAPTER# 04Dusts • Threshold limits: • The following threshold limits (MAC) have been established for mineral dusts • High silica (>50%), asbestos……….. 5 mppcf • Medium silica ( 5-50%), talk, mica…. 20 mppcf • Low silica (<5%), cement, slate, nuisance.. 50 mppcf • Total all dust ……………………………. 50 mppcf

  20. Explicability of coal and Metallic Dusts

  21. Threshold limits for coal and radioactive minerals Threshold for coal dusts adopted by the US Bureau of mines Average shift exposure--------20 mppcf Single operation exposure----40 mppcf Radiation : MAC for radioactive mineral is based on radiation emission and its daughter products. It is set as 300 micro micro curies per liter (uu/l).

  22. Threshold limits for toxic or nuisances dusts

  23. Example 4-1: given the survey results below, find the avg. shift exposure. Solution: Avg. Shift Exposure = 3678/480=7.7 mppcf

  24. CHAPTER# 04Dusts • Air born dusts determinations: • Air born dusts are determined in following manner • Sampling: The following instruments are used for sampling • Standard impinger • Midget impinger • Konimeter • Filter paper sampler • Molecular filter sampler • Electrostatic precipitator • Thermal precipitator • Radiation survey meter

  25. Midget impinger: • Small scale version of the standard impinger. • Inefficient for fine dusts. • Relatively low in sampling rate. • It is safe, portable, easy to use and needs no external force. • Operation: • Air is drawn into nozzle impinges at high velocity against the liquid in the flask. • Dusts are deposited and air exhausted.

  26. Liquid used is mostly distilled water or alcohol. • Liquid should not be a solvent for dusts. • A 10 minutes sampling period is usually adequate.

  27. Quantitation: • Quantitation: • Determining under microscope, how much dusts or how many particles, concentration, particles size. • Two methods for Quantitation. • Number bases • Weight bases • Number bases is preferred for evaluating a pulmonary hazard while the weight base is preferred fro toxic, radioactive, or explosive dusts. • Number base is usually applied for impinger,, Konimeter, molecular filter and thermal precipitator. • Weight Quantitation is used fro for filter paper and electrostatic precipitator.

  28. Quantitation: • For midget impinger ( sampling rate=0.1cfm) the equation is • Concentration = 4(avg. of five field counts)(dilution)/(100*sampling time), mppcf • Example : • Field counts: 72, 83, 71, 75, and 80 • Sampling period: 12 minutes • Dilution: 22 ml • Avg. blank count: 8 • Required: avg. dust concentration.

  29. Solution: Avg. field counts: (72+83+71+75+80)/5=76-8=68 Concentration=(4*68*22)/(100*12)= 5.0 mppcf

  30. Dusty operation and sources of dusts • The mining operation which are responsible for dustiness are classified according to source, in order of decreasing importance . • Almost all activities are responsible for air pollution. • Primary source: • if an operation produces or creates a dust. • Secondary source: • if an operation disperse dust. • + symbol for major source and – symbol for minor source.

  31. Engineering dust control • Engineering dusts control: • Prevention • Avoidance by modifying operation or improving practice • Reducing amount formed with properly maintained equipment • Removal • Clean up workings to eliminate settled dust • Air cleaning with dust collector • Suppression • Infusion with water or stream in advance of mining • Allying with water or foam in spray • Treating settled dust with deliquescent chemicals

  32. Engineering dust control • Isolation: • Restricted blasting or off-shift blasting • Enclosure of operation • Local exhaust systems • Dilution: • Local dilution by auxiliary ventilation • Dilution by main ventilation air stream • Rock dusting to dilute combustible content of settled dust

  33. Engineering dust control • Cardinal rule: “prevent dust from becoming airborne” • Once the soled contaminant becomes airborne, the control task becomes difficult and costly. • Ventilation: • Either auxiliary or main ventilation system for control of dusts. • Velocity range of air is 10 to 30 fpm. • But some times 50 to 100 fpm is maintained as a safety factor. • The upper velocity is determined by comfort and economic limit.

  34. Engineering dust control • In main airways the velocity is less than 800 fpm. • Usually restricted to a maximum of 300 to 400 fpm. • High velocities raise the dust. • Allaying with water: • Wetting muck piles prior to loading. • Spraying loaded mine cars prior to dumping. • Suppressing airborne dust is a difficult task. • Special mist type nozzles are used to produce finely atomized spray of water.

  35. Engineering dust control Water coagulates with dust particle and increase its weight. High pressure up to 600 psi produce smaller particles. Surface tension can be reduced by additives in the water. Water infusion: A technique used in advance of mining. Water is injected in to coal face trough long drill holes.

  36. Four to six holes per face are sufficient. Length of holes 20 to 60 ft. Liquid infuse in to the seam along crakes and fissures.

  37. CHAPTER# 04Dusts • Medical and legal means of dust control: • There are three methods of medical control with which the engineer should be acquainted • Physical examination • Respirator • Aluminum therapy

  38. CHAPTER# 04 Dusts CHAPTER# 04 END

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