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

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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

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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)

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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)

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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.

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The respiratory has may built in safeguard to protect the tissues from the harmful effects of dusts.

1. Upper reparatory system—a system of filters.2. Hairs in nasal passage filters out larger particles

(over 5µ)3. Mucus membrane, lining the nose and throat,

traps still more particles larger and smaller. 4. In the trachea and , hair like cilia sweep

medium sized particles (5-10µ) upward with a vibratory motion into throat where they are swallowed.

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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.

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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

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• Silicatosis, by other silicates• Siderosis, by iron or iron ores• Anthracosis, by coal, both

bituminous and anthracite, suspected also to contain free silica

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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

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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

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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)

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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

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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)

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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.

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1. 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.

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Anthracite coal is non explosive at normal temperature.

Moisture and ash reduces explicability.2. Particle size: finer the dust the most explosive it would be. Coal dust over 20 mesh (850 u) enter into

explosion.3. 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.

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4. Flammable gases: The presence of methane gas

enhances the ease of coal dust.5. Relative humidity of air: Practically no effect on coal dust

explosion.

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Threshold limits: The following threshold limits (MAC)

have been established for mineral dusts

High silica (>50%), asbestos……….. 5 mppcfMedium silica ( 5-50%), talk, mica…. 20 mppcfLow silica (<5%), cement, slate, nuisance.. 50

mppcfTotal all dust ……………………………. 50 mppcf

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Dusts Minimum concentration, g/cu m

Ignition temperature Co

Minimum pressure, psi

Aluminum 25 645 89

Iron 250 425 36

Magnesium 20 530 72

Titanium 45 480 52

Zinc 480 600 36

Coal 35 610 46

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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).

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Toxic or nuisance compound Mg.cu.m

Mercury 0.1

Lead 0.2

Antimony, arsenic, barium 0.5

Manganese 6.0

Iron , zinc, magnesium oxide, molybdenum

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Operation Time, min Concentration, mppcf

Number * Time

Tramming 200 3.5 700

Drawing chutes 80 18.5 1496

Dumping cars 115 11.2 1288

Maintenance 35 2.7 94

Idle 50 2.0 100

Total 480 3678

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Solution: Avg. Shift Exposure =3678/480=7.7 mppcf

Air born dusts determinations: Air born dusts are determined in following

manner Sampling: The following instruments are

used for sampling1. Standard impinger 2. Midget impinger3. Konimeter 4. Filter paper sampler 5. Molecular filter sampler 6. Electrostatic precipitator7. Thermal precipitator 8. Radiation survey meter

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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.

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• Liquid used is mostly distilled water or alcohol.

• Liquid should not be a solvent for dusts.• A 10 minutes sampling period is usually

adequate.

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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.

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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.

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Solution: Avg. field counts: (72+83+71+75+80)/5=76-

8=68 Concentration=(4*68*22)/(100*12)= 5.0 mppcf

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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.

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OPERATION PRIMERY SOURCE SECANDARY SOURCE

Blasting + (major source) +

Cutting, continuous mining

+ _

Drawing chutes _ ( minor source) +

Drilling + 0

Mucking, loading _ +

Blowing holes 0 ( negligible source) +

Dumping cars _ +

Slushing _ +

Belt transfer, discharging

0 _

Haulage 0 _

Timbering 0 _

Barring down 0 _

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 dustAir cleaning with dust collector

SuppressionInfusion with water or stream in advance of miningAllying with water or foam in sprayTreating settled dust with deliquescent chemicals

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Isolation:Restricted blasting or off-shift blastingEnclosure of operation Local exhaust systems

Dilution:Local dilution by auxiliary ventilation Dilution by main ventilation air streamRock dusting to dilute combustible content of

settled dust

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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.

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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.

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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.

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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.

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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

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CHAPTER# 04 END

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