Sampling of Dust DUST in MINES -5

8/8/2019 Sampling of Dust DUST in MINES -5

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SAMPLING OF DUST

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Suitable preventive and suppressive

measures for allaying dust in amine

it is essential to have a suitable device to

estimate orsample air-borne dust likelyto be breathed by miners.


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The following facts must be considered inthe choice of a sampling instrument:

Knowledge of the necessary dustconcentration in the dangerous size range.

Mass concentration of the respirablefraction of dust is widely acceptedrelevant standard.

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But MAC is not an agreed international

standard

as conclusive correlation betweendifferent dust parameters and incidence of

pneumoconiosis uniformly applicable to all

types of dusts is not established yet,

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particle concentration with particle-size-frequency distribution and surface areaconcentration still continue to be assessed

Also the sample should be able to give anassessment of the dangerous component of dustas regards composition.

This necessitates adequate volume of sample forchemical and mineralogical analysis.

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The duration of the sample should be suchas to give a true picture of dustiness ofany mining operation or place or work with

optimum number of samples to be taken. The sample should be continuous over at

least a working shift.

The sample should be representative ofthe dust cloud in the breathing zone of theworker.

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SAMPLING INSTRUMENTS

The sampling instrument should be easily

portable and

robust for rugged underground conditions and

require minimum maintenance

need minimum operational skill

self-contained power supply

render the measurement of dust in as little time,as possible without involving too much

accessory work.

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DURATION AND INTERVAL OF

SAMPLING Dust generation in any mining operation

varies a great deal with time

the variation is greatly irregular due toirregularities in normal cycles of operation.

the time and duration of sampling should

be very carefully chosen to give a fairly

accurate estimation of the dustiness of any

particular operation over a long period.

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A true estimate can however be obtained by acontinuous sampling device working over thewhole shift or even over several shifts.

From this a true shift mean or, in general terms,a true period mean can be obtained.

A mean close to the true period mean canbe obtained by taking a large number of short-

duration samples over the period, the deviation approaching zero as the number

approaches infinity but it is impracticable.

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Snap samples or samples taken over a short

duration are liable to deviate a great deal from

the true average at a source dust production

owing to large variations in the rate of dustproduction at the source and

to get a close approximation to the true mean,

a very large number of samples will be

necessary .

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Studies suggest that a sampling period of

ten minutes will reasonably represent the

true mean with the result falling within

20% or the average in two out of three

cases.

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POSITION OF SAMPLING

intermittent dust production in any mining

operation and

the irregular nature of air movementgoverning the dust distribution at the face,

it is difficult to select the correct part of the

face where a true representative sample

can be obtained

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Samples taken at different sections of theworking face near the source of production of

dust show wide variation .The variations gets gradually attenuated as thesampling points are receded farther along thereturn airway,

This is due to the longitudinal and lateral mixingof the dust in the air-stream,

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If the Reynolds number characterizing the air-flow is well above the critical number which isgenerally the case with any ventilation system,complete mixing takes place at a distance, of

five to ten roadway diameters It is possible to get a more average dust sample

beyond the mixing zone than near the source,

Any temporary variation in dust production at the

source tends to be attenuated with increasingdistance from the source,

Due regard of course is to be paid to anyadmixing with dust from other sources.

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With velocities prevalent in mine airways,gravitational settling of dust is slow,particularly so for the pathogenic size of

< 5 m and it is possible to get arepresentative sample at a distance of

6 to15 m away from the face.

uniformity or distribution in an air-streamis dependent on the uniformity of flow inthe airway.

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for getting a representative sample, it isnecessary to collect the sample in a straightposition on of the air way at least ten diameters

away from any bend or major obstruction.

Another factor which should be borne in mindwhen collecting a sample of dust from air is that

the sampling velocity should be as far aspracticable, equal and equidirectional to thevelocity of air from which the sample is beingcollected.

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this is essential for minimizing the loss ofparticles by inertial separation before entryinto the sampler.

The best sample having direct relevance tohealth hazard is one collected at the breathingpoint of the worker.

This has led to the development personal dustsamplers which collect a sample of therespirable dust fraction over a sufficiently longperiod, usually a shift

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METHODS OF SAMPLING

Based on the principle of operation,the present-

day air-borne dust sampling methods can be

classed as follows:

(a) filtration,

(b) sedimentation.

(c) inertial precipitation,

(d) thermal precipitation, (e) electrical precipitation ,and

(f) optical methods based on light scattering,

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Filtration has been the earliest method ofsampling.

Sugar tubes - had only 87 % filtrationefficiency by weight for silica dust and afair amount of fine dust of the respirable

size range escaped collection

(Later replaced by konimeters.)

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Other soluble filtering media like potassium

nitrate, salicylic acid etc. were tried to improve

filtration efficiency.

Non soluble filters tried were of cotton ,wool,flannel etc.,

synthetic membrane filters having pore size

down to. 0.01m have been developed. Paper thimbles are claimed to have 98 %

filtration efficiency.

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Attempts have been made to obtain the massconcentration in the respirable size range from dustsamples collected on soluble filters, by dissolving thefilters and separating the < 5m particles by

sedimentation. This does not give the picture of the mass concentration

of the respirable fraction in the air-borne dust since theaggregates present in the air-borne dust cloud which arenormally treated as single particles of the aggregate size

in the respiratory system, Dust particles break up in the process of solution and

separation and so an overestimation of the massconcentration of the respirable fraction is obtained.

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Particle concentration

Particle concentration and size distribution havebeen determined by spreading a certain amountof dust collected on the filter on a glass slide in a

solution of Canada balsam in xylol (the dust getsfixed in Canada balsam when the xylolevaporates ) and counting it under amicroscope.

But proper dispersal of dust on a microscope slide by

this method is difficult and errors creep in due to bothsmall particles flocculating and aggregates breaking up.

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Other Instruments based on the principle

of filtration include the Soxhlet and Gothe

filters which use paper thimbles,

Siter filter using a synthetic membrane

filter and the microsorban filter using a

low-resistance soluble filter.

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Filtration has become important in recent timesdue the adoption on of the mass concentrationof the respirable size fraction as the relevantdust parameter signifying health hazard.

The sampling instruments commonly known asgravimetric dust samplers

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Gravimetric dust samplers

use an elutriator of gravitational settling orcyclone type which separates out dust particlesabove respirable size range

the respirable size fraction is collected on amembrane or glass fibre-paper filter in a suitableholder.

The filter can be weighed before and after

sampling to get the mass of dust collected. Today matched-weight filters are available which

obviate double weighing.

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These are of two main types:

(a) those using a gravitational-settling type

of elutriator and

(b) those using cyclones for elutriation.

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Respirable dust monitorRDM-201

This is a gravimetric sampler

developed in the U.S.A. which gives anautomatic digital readout of the mass

concentration of the respirable dust by a

beta-ray absorption technique.

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The instrument comprises a cyclone elutriator whichseparates the coarse fraction and a filter which collectsthe respirable fraction.

The mass, of dust deposited on the filter perunit area is obtained from the absorption of

radiations from a carbon-14 source as measured by a

Geiger counter.

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Inertial Precipitation

Dust sampling instruments using inertial precipitation arebased on three principles:

impaction.

impingement and

centrifuging.

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impaction.

Certain volume of mine air is made to

impinge on a glass slide coated with a

suitable dust collecting adhesive.

The dust which gets deposited on the slide

is counted under a microscope which, in

most makes, forms an integral part of the

instrument

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A measured quantity of the dust-laden airfrom the nozzle impinges on the bottom ofthe flask and the dust is collected in the

collecting fluid. A portion of the dust-laden fluid is filledinto a

counting cell usually 1 mm deep which is

then covered with a cover slip. The cell is allowed to stand for about half

an hour to allow the dust particles to settleto the bottom or the cell and counted

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centrifuge is based on the principle of

centrifuging. This instrument collects a

size-graded spectrum of dust, but its high

speed of rotation confines it to the

laboratory only

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THERMAL PRECIPAITATION

This method utilizes the principle that when abody surrounded by dusty air is heated, a dust-free zone is produced around the hot body,

the extent of the dust-free zone depending onthe temperature gradient between the hot bodyand the surrounding air.

If such a zone is intercepted by two glass cover

slips and a current of dusty air allowed to enterthe space between them the dust in the air getsdeposited on the cover slips where it remainsattached by molecular attraction

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The dust particles collected on the cover slips is

are counted under the microscope

The thermal precipitator has a high collection

efficiency. The low velocity of sampling does notbreak up aggregates and a fairly representative

sample over a reasonably long duration is

obtained. .

Microscopic counting of the dust

particles is tedious and subject to personal error

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Automatic electronic devices have been

developed for counting and sizing of dust

samples collected by the thermal precipitator.

A diffraction size-frequency analyzer developed

has been claimed to give an accurate measure

of the size distribution of the dust sample as well

as the respirable surface area of the

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Electrical Precipitation

The electrical precipitator essentially consists ofa charging wire maintained at a high negativepotential of about 12000 volts and surrounded

by an earthed concentric cylinder Dust-laden air is drawn through the cylinder bya fan at a constant rate.

The dust particles when passing through the

instrument, get charged and are drawn to andprecipitated on the inner surface of the earthedcylinder..

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This instrument, like the filtration devices, has alarge sampling capacity and is suitable forcollecting large quantities of dust for chemical

analysis. The mass concentration can also be determinedby noting the difference in weight of the cylinderbefore and after collection of dust.

The instrument has a high collecting efficiency,but the high voltage used in it makes itunsuitable for use in coal .

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Gravimetric dust samplers utilizing

electrical precipitators have

been developed where the coarse fraction

of the air-borne dust is first removed in a

cyclone separator and the respirable

fraction is collected in the electrical

precipitator for estimation of massconcentration by differential weighing

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Optical Method

This method utilizes the property of scattering oflight by suspension of fine particles

For particles large enough compared to thewave length of light i.e., particles above 1m indiameter

the intensity of scattered light is roughly given bythe relation

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where

Is - intensity of scattered light,

1o intensity of incident light,

N number of particles per unit

volume,

D diameter of particles The equation shows that the intensity of

scattered light is proportional to the surface area

of the particles, a fact well borne out in practice.41


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OPTICAL

Holographic System

A different optical system which has been

developed for counting and size analysis of a

suspended cloud of aerosols consists of apulsed ruby laser illuminating the particles

The light diffracted from the cloud forms ring-like

wave interference patterns on a photographic

film which gives a permanent record of these

patterns called the hologram

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This can be magnified 300 times by aVidicon camera tube and televisionmonitor.

Individual particles can be focused anddisplayed on a screen for visual analysis.

However, the existing instruments are

capable of recording particles above3

msize only and hence are not suitable formine-dust analysis.

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Location of dust sampling

For fixed point 'area sampling' (also

sometimes referred to as static sampling),

the location is so chosen as to obtain

representative concentration

of respirable dust associated with dust

generating sources.

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The National Dust Prevention Committee

for coal mines in India by its

recommendation of 1993(21) gave

sampling locations for area sampling in

(a) board and pillar,

(b) long wall,

(c) drivage, and

(d) opencast workings and surface plants .

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Board and Pillar workings

(i) Intake airway of a working district within 20 to30 m of immediate out-bye ventilationconnection from the first working face

(ii) Return airway of a working district within 20to 30 m of immediate out bye ventilationconnection from the last working face

Where there is more than one return airway, all

the return airways are to be sampled. iii) Working place or face where dustconcentration is likely to be maximum .

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(iv) Resting place of workers.

(v) 3 to 5 m on the return side of other placeswhere substantial dust is generated, e.g.

loading/transfer point (vi) Main return airway of the mine.

Long wall workings 30 m out bye of the face inthe intake airway

(ii) at the centre of the face and (iii) 50 to 70 m out bye of the face in return

airway

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Drivages or long headings

Sampling shall be done in the drivages

when they are driven more than 30 m or

when the system of working is fully

established to enable collection of a

representative sample.

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Opencast workings / surface

plants-

(i) Samples should be taken at the working

places near workers.

(ii) In surface plants like coal handlingplant (CRP), crusher house, siding and

other places n processing plants where

dust is generated.

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In opencast workings, sampling bypersonal dust samplers is considered

more reliable than sampling by fixed-point

samplers.

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The frequency of respirable dust sampling

is specified as once at least every six

months in CMR 1957.

The maximum interval between sampling

is reduced to three or one month when the

dust concentration exceeds fifty or seventy

five percent respectively of the allowablevalue

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4.0 Sampling procedure

4.1 Places to be sampled

4.1.1 General:As required under Reg. 123(3)(a) & (c) of CMR 1957, all the workplaces in a

mine, where respirable dust is evolved, shall besampled to ascertain the air borne dustconcentration of that area and the dust exposureprofile for different categories of work personsworking in that area.

In view of the above Manager of every mineshall identify such places in the mine.

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Mine air of that area shall be sampled by'static sampling' method by a fixed typesampling instrument.

This shall be cross checked by measuringthe dust exposure of selected workers,whose exposure is deemed to berepresentative of their groups, by 'portal toportal personal monitoring' by usingpersonal dust samplers

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4 1 2 Fixed point or static sampling


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4.1.2 Fixed point or static sampling

for underground workings

The following places shall be sampled byfixed type samplers orstatic samplers:

(a) For B & P orR& P workings : Thesampler shall be positioned on the returnside of the point of dust generation (and

within 1m of the normal working positionof, but not behind the operator or worker)in the following places-

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Within 30m out bye of the first working

face in the intake side of the district.

30m out bye of the last working face in the

return side of the district

Working faces

Loading / Unloading / Transfer points

Bunkers / Chutes

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b) For Longwall working

30m outbye of the face in the intakeairway

10m from the intake end at the face

10m Inbye from the return end of the faceAt the- centre of the face

30m outbye of the face in the return

airway Loading / Unloading / Transfer points

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c) ForMechanized Long wall gate

roads

10m out bye of the face

30m out bye of the face

Loading / Unloading / Transfer points Bunkers / Chutes

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4.1.3 PersonalDust Sampling for

underground workings In addition to the static sampling as above, dust exposure to the

following categories of persons shall be determined by personaldustsampler

Loaders

Shotfirers and helpers

Drillers / Dressers

Operators and helpers of loading machines (SDL, LHD, Shuttle carsloaders or other mechanical loaders)

Operators of Cutter loaders ( Shearer, Plough, Continuous miners,

Road headers etc., Longwall face-crews

Conveyor. bunker or chute operators

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4.1.4 Fixed point or static sampling for Opencast

workings surface operations

Direction of air current should be kept in mindand the instrument be placed on the return sideof the 'point of dust generation.

It is suggested that more than one fixed-point

sampler shall be installed along thecircumference of concentric circles of 5m & 10mradius around the point of equipment generatingdust.

The sampler shall be installed along thedirection of wind or air current and the samplersmay change their location during sampling withchange of direction of the air current.

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However, in opencast workings, sampling

by personal samplers is considered more

reliable than sampling by fixed samplers.

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5PersonalDust Sampling for open

cast working and other operations

Dust exposure of the following categories

of persons shall be determined personal

dust sampler :.

Drillers & helpers

Operators of excavators/loaders (Shovel,

Back-hoe, Dragline, Front-end loaders)

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Dumper & Tipper operators & helpers

Dozer/Grader operators & helpers

CHP crews, crusher operators & helpers,wagon loaders

Any other machine operators.

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Sampling with MRE or its equivalent should be carriedout with air inlet of the Instrument facing the air currentkeeping the instrument in horizontal position,

if the velocity of air is less than 4 metre per second

and perpendicular to the air current, if the velocity of air is more than 4 metre per second. The

instrument shall be placed at about breathing levelwith normal posture of the operator and located centrally,and away from the side, as far as practicable.

Safety of the instrument and interference of the normalwork shall be kept in mind for positioning the instrument.

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4.3 Frequency of dust sampling

(a)As required under Reg. 123(3) (a) of CMR1957, every place as mentioned above shall be

sampled by fixed point samplers, at least oncein every six months.

Provided that if any measurement of any workplace shows the concentration in excess of 50%of the permissible limit, the subsequentmeasurement shall be carried out at intervalsnot exceeding 3 months.

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Provided that if any measurement of' anyworkplace shows the concentration in excess of75% of the permissible limit, the subsequentmeasurement shall be carried out at intervals

not exceeding one month (b) Such measurement shall also be carried out

immediately upon the commissioning of anyplant, equipment or machinery or upon theintroduction of any new work practice or upon

any alteration therein thatis likely to bring about any substantial change inthe levels of airborne respirable dust.

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(c) If respirable dust concentration exceeds the

permissible limit, then the area shall be sampled

again for 5 times in the next 7 successive normal

working shifts according to the provision of Reg.123(4) of CMR 1957 and continued till the

permissible concentration comes within limit

Once the respirable dust concentration falls to

permissible limit or below, the places shall besampled as required under clause 4.3(a).

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4.4 Sampling by personal dust sampler

shall also be carried on along with

fixed point sampling to have a cross

check.

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5.0 Recording of results

All results of measurements of air borne respirable dust and all therelevant particulars shall be recorded in accordance with Reg.123(3) (f) of CMR 1957.

5.1 A plan in a suitable scale should/be maintained showing clearlyall the places where sampling of air borne dust has been done.Date 0f last sampling should be indicated on toe plan,

5.2Results of air-borne dust survey shall be maintained in theformat per annexure I & II.

5.3Recordof respirable dust content and the Quartz content shallmaintained in the enclosed format and kept in a bound paged book

5.4Details of the weighing balance used shall be recorded alongsampling data.

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6.0 Preservation of filter papers

The filter papers used for a particular

sampling shall be preserved along with its

serial number and date of sampling.

These filter papers be. preserved at least

for a period of 3 years and shall be readily

available to the Inspector of Mines if so

desired

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7.0 Report on Air Borne Dust

Survey

After conducting the air borne dust survey as per

the above guide report shall be prepared giving

the following details

(i) Introduction (ii) Methodology of dust sampling

(a) Sampling strategy for static sampling

(b) Sampling strategy for personal sampling

(c) Instrumentation

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iii) Observation

(a) Tables showing the results of air bornedust survey for static and personal

sampling.(b) Maximum dust concentration and Time

Weighted Average( TWA ) for differentlocations.

iv) Conclusions.

v) Recomendations

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Sampling of Dust DUST in MINES -5