gravimetric analysis paper
TRANSCRIPT
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Gravimetric Analysis
Stanley Mumbo Mayonde
1 Eric Ngandu
2 Kashala Kamalonga
3
1 Occupational Health and Safety Department - Kansanshi Mining PLC.
2Environmental Engineer, Occupational Health and Safety Department - Kansanshi Mining
PLC
3 Occupational Health and Safety Department - Kansanshi Mining PLC
Abstract
Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative
determination of an analyte based on the mass of a solid. Materials hazardous to health
often occur in the workplaces such as the mine in the form of aerosols. The term ‘aerosol’
is used to describe any suspension of particles in air, whether they constitute dust, fibers,
fume, smoke or liquid droplets. Most aerosols consist of a wide range of particle diameters.
Kansanshi Mining PLC prioritizes safety of the employee above anything else. It is for this
reason that the mine has carried out a comprehensive risk assessment to identify the
Homogeneous exposure groups and Hazards associated with them. The availability of the
Occupational Hygiene Lab ensures that the dust management through gravimetric analysis
is achieved with great precision and minimum erroneous results.
The analysis is achieved through the use of equipment that is specifically designed for the
purpose. The equipment consists of sampling train and others such as the high precision
weigh balance.
During the analysis, a quality check is made during the pre and the post sampling. Prior to
sampling, sufficient sample collection media should be placed in suitable containers, with
lids ajar and left to equilibrate overnight in the balance room.
The type of sampler to be used depends on many factors including the size fraction of
interest and the suspected aerosol concentrations. In most cases an accurately measurable
mass may be collected for long-term samples (greater than 4 hours) using a sampler that
operates in the region of 2 l.min-1
. A higher flow rate sampler will provide a lower Limit
Of Detection (LOD) for any given sampling period and may be preferred for shorter
sampling periods or if the aerosol concentration is low.
To calculate the volume of the air sampled, VS, in m3, multiply the mean volumetric flow
rate in cubic meters per minute (liters per minute divided by 1000) by the sampling time in
minutes.
Therefore, the measured aerosol concentration, C, in mg.m-3
, can be calculated according
to the following equation:
Where:
M1 = mass of filter before sampling (mg)
M2 = mass of filter after sampling (mg)
B = average mass change of reference filter (mg)
VS = volume of air sampled (m3)
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Introduction
Present in the workplace are substances that are in the form of aerosols. ‘The term aerosol
is used to describe any suspension of particles in air, whether they constitute dust, fiber,
fume, smoke or liquid droplets’1.
Determining aerosol size fraction is a critical issue to carefully consider, the aerosol size
fraction relate to the region of the respiratory tract that they deposit. Exposure to an aerosol
in form of dust needs to be quantified in an effort to implement control measures that will
eliminate or reduce exposure. In line with First Quantum Minerals Limited Corporate
Occupational Health and Safety Policy, which considers the safety and health of its
employees to be of utmost importance, Kansanshi Mine has implemented a program to
manage dust exposure of its employees. The technique in use at Kansanshi to achieve this
is Gravimetric Analysis. Gravimetric analysis describes a set of methods in analytical
chemistry for the quantitative determination of an analyte based on the mass of a solid.
Since its introduction in the early 1980s, gravimetric analysis gives the most accurate and
representative results; that is if done correctly, in comparison to the sugar tube method and
the konimeter method.
To ensure that the process is done to international best practice Kansanshi Mine has
adopted international sampling methods NIOSH 75002, NIOSH 7300
3, NIOSH 500
4 and
NIOSH 6005.
Applying these methods; the total inhalable dust and respirable dust are collected and
analyzed.
Inhalable dust approximates to the fraction of airborne material that enters the nose and
mouth during breathing, and is therefore available for deposition in the respiratory tract.
The respirable dust on the other hand approximates to the fraction of airborne material that
penetrates to the gas exchange region of the lung.
Gravimetric Analysis Process at KMP
At KMP a dedicated weighing room has been set up with the following factors into
consideration.
Set up at in an uncontaminated location, dust free environment and away from
activity or equipment that can cause Vibration.
No windows fitted to the weighing room to avoid draughts and the ingress of dust.
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Before entrance to the weighing room are two access rooms. The first being the
“dirty” room and the second, the clean room adjacent to the weighing room. All
rooms are fitted with self-closing doors.
Fitted with Air-conditioning system to maintain a constant temperature and
humidity.
The weighing room is dedicated to the weighing and preparation of filters only.
Weighing Room Equipment
The weighing room is equipped with the following best practice standard equipment:
A weigh balance with the capacity to measure to the 5th
of a decimal point i.e.10
micrograms. The balance is correctly leveled and set on rigid specially designed
balance table together with a properly earthed anti-static mat.
An additional anti-static mat properly earthed, for disassembly of cassettes and
handling of filters is set on a separate table.
A filter stabilization chamber is placed enclosing the weigh balance and is also earthed.
The main aim of earthing the equipment and accessories is to dump as much as possible
the static charge to earth.
The weighing room is also equipped with accessories such as suitable spring loaded
tweezers for handling filter media.
Diagram 1
Only trained and certified personnel are allowed access and use of the Laboratory to ensure
accuracy of results obtained from the weighing room.
The gravimetric analysis process starts with the preparation of sampling media up to the
calculation of concentration from the mass of dust obtained.
Weigh Balance
0.01mg sensitivity.
Filter stabilization
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Before we can discuss this process systematically it is essential to note that the selection
of a particular filter media type for a specific application is invariable the result of a
compromise of many factors. These factors include analytical requirements, the ability of
the filter to retain its filtering properties, cost, availability, collection efficiency and
physical integrity under the ambient sampling conditions.
At KMP the Cellulose Nitrate for both mass determination and analysis is currently in use.
The filter is first placed in the filter stabilization chamber at least 24hours before taking an
initial weight, the filter is given a unique identification number e.g. Filter A. See diagram 2
below:
Diagram 2
Pre-Filter weighing. After 24hrs Filter A is weighed 3 times and an average of the three
weights is recorded; three (3) reference (blank) filters are also weighed three times each,
and their respective average weights are recorded, from which a final average of the three
averages is recorded. The reference filters as well are given unique identification number
e.g. Ref1, Ref2, Ref3. The reference filters are left in the acclimatisation chamber until the
measurement of the final weight for Filter A. The weight difference of the reference filters
will work as a correction for Filter A. The correction takes in to consideration the moisture
gain or loss of Filter A.
Preparing the Filter and Filter cassette for sampling 6
The filter cassette holds the filter securely in place during sampling. The cassette consists
of an inlet section, an outlet section, and a middle ring. The cassette, with all three
sections, can be used with the inlet in place (closed face) or with the inlet removed (open
face) depending on the sampling method.
Reference Filters
in Stabilization
Chamber
Filters for sampling
in Stabilization
Chamber
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When sampling for Inhalable dust a cyclone is not used and used when sampling for
respirable dust.
Without Cyclone (Inhalable dust sampling) Diagram 3 below 6
To load the cassette, place a cellulose support pad or stainless steel screen in the outlet
section of the cassette and then the appropriate filter (conditioned and weighed according
to the method used). Add the extension cowl or middle ring if required, close the cassette
firmly with the inlet section, and insert the plugs into the inlet and outlet.
.
Diagram 3
Air Flow
Cassette Outlet
Support Pad
Filter
Cassette Ring (Middle)
Cassette Ring (Inlet)
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With Cyclone (Respirable dust sampling) Diagram 4 below
To load the cassette, place a cellulose support pad in the outlet section of the cassette, add
the appropriate filter, insert the middle ring, and place the cyclone securely into this ring.
Diagram 4
Cassette Outlet
Support Pad
Filter
Cassette Ring (Middle)
Air inlet
Grit Pot
Air Flow
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After the filter has been assembled with the cassette with or without the cyclone; a
sampling train should now be prepared consisting of a pre calibrated personal sampling
pump, flexible tygon tubing and a filter cassette holder.
Pre - sampling pump calibration.
At KMP the Air – Check XR5000 is in use. The pump is run for 5 minutes before
calibrating as a pump warm up measure. Deciding the flow rate to be set on a pump should
be in line with the sampling /analytical method for the sampling being undertaken. The
NIOSH standards discussed earlier gives the flow rates used at KMP.The cyclones in use
are the SKC Aluminum cyclone and the Higgins Dewell cyclones which meets the
ACGIH/ISO/CEN curve (solderholm convention).
With the representative sampling medium in line the pump is calibrated using a pump
calibrator to the flow rate specified in the sampling/ analytical method for the sampling to
be done.
A sampling train is assembled see diagram 5 below:
Diagram 5
The sampling train is now ready for sampling; the pump is fit on an employee of the
scheduled Homogenous Exposure Group, with the pump clipping on the employee’s belt
or clothing and the sampling head placed on the employees breathing zone and the
employee will carry on with their normal working pattern. See diagram 6 below:
Rubber Tubing with
Adapter
Filter Cassette
Cyclone
Grit Pot
Cyclone Holder
Tygon Tubing
Personal Sampling Pump
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Diagram 6
Homogenous exposure group (HEG) is a group of workers who experience exposures
similar enough that monitoring exposures of any representative subgroup of workers
provides data useful for predicting exposures of the remaining workers.
Monitoring of exposures in this case is planned according to defined HEGs.
The sampling period is dependent on the analytical method being applied making certain
that no dust overload on the filter and sampling for a representative duration of the
employee’s shift i.e. Six (6) to Eight (8) hours of the employees 8 hours per day and 40
hours per week.
Post - sampling pump calibration.
After sampling period has elapsed the pump and sampling head are retrieved from the
employee and prepared for post flow rate calibration. Post flow rate calibration is done in
the same way as the pre sampling calibration.
The average of the pre calibration flow rate and the post calibration flow rate will be
recorded as the flow rate for the period (shift) and multiplied by the period of sampling T
(min) to get the volume V (liters).
The sampling head is disassembled separating the filter cassette and placing the cassette
inlet back with caps on; the filter cassette is carefully transported to the weighing room.
Breathing
Zone
Sampling
Head
Pump with
tygon
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Post-Filter weighing
With the filter cassette transported to the weighing room; the filter is carefully removed
from the cassette; put on a petri dish and placed in the filter stabilisation chamber for 24
hours before taking the final weight.
After 24 hour of filter stabilization the refere nce filters (Ref1, Ref2, Ref3) are weighed in
the same way as at pre weighing and the mass recorded as Post weight of reference filter.
The filter (Filter A) is also weighed in the same manner as at pre weighing and the weight
recoded as post weight.
The filter is placed back in the petri dish and stored. If further analysis is required it will be
shipped in special carrying case.
It is important to clean the sampling accessories (cassettes, cyclone, grit pot and tubing)
thoroughly in readiness for the next sampling cycle. An ultrasonic cleaner is used to clean
all the accessories at KMP. The tubing is not necessarily cleaned after every sampling but
can be cleaned at intervals.
Results
The formula below is used to calculate the concentration of the dust in the sampled
atmosphere:
Where:
F1 = mass of filter before sampling (mg)
F2 = mass of filter after sampling (mg)
B = average mass change of reference filters (mg)
VS = volume of air sampled (m3)
When the concentration is calculated; the time weighted average is also calculated as
below:
Time weighted average 8 hrs is the concentration of airborne contaminants which has been
weighted for 8 hours. 8 hours representing an employee’s work shift per day and 40 hours
per week.
Below is a table representing some results obtained from actual sampling and gravimetric
analysis and analytical analysis by X- Ray Powder diffraction adopting NIOSH 75002
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Sample Number
HEG No
Parameter
Mass
(Mg)
Concentra
tion
(mg/m3)
TWA
(mg/m3)
%Quartz
OEL
(mg/m3)
150203KMP01011001PN5590
1001
PNORR
0.46
0.43
0.38
Not
Analyzed.
5.00
150304KMP010116002SM5558
16002 PNORR 0.18 0.19 0.15 Not
Analyzed
5.00
1506080102031419001DS0182
1419001 PNORR 0.24 0.53 0.22 Not
Analyzed
5.00
150511010203149002PN0091
149002
PNORR 1.77 2.535 1.933 Not
Analyzed
5.00
150511010203149002DS0093
149002
PNORR 1.92 2.710 2.072 Not
Analyzed
5.00
150205KMP010123005EN5576
23005
Silica
Quartz
(Respirable)
0.0062
0.01 0.01 5.6 0.05
150205KMP010123006EN5597
23006
Silica
Quartz
(Respirable)
0.0055 0.01 0.001 7.1 0.05
150311KMP01016003EN5560
6003
Silica
Quartz
(Respirable
0.0075 0.01 0.01 2.9 0.05
150318KMP01012003EN5559 2003 Silica
Quartz
(Respirable
0.02 0.02 0.02 11 0.05
150318KMP01012002EN5583 2002 Silica
Quartz
(Respirable
0.026 0.03 0.02 8.3 0.05
Table 1
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Conclusion
The gravimetric analysis technique for analysing employee’s exposure to dust at KMP is an
effective technique that is accurate and quantitative. It is a critical tool that is vital in
updating the baseline risk assessment.
The monitoring is on-going providing information for the determination of the hazard
profile in terms of dust exposure.
Results obtained are useful in determining the control measures to be implemented at the
place of work and if required the type of respiratory protection to be issued to employees.
The dust control methods implemented include:
The prevention of dust formation at source by the use of water suppression.
Dust extraction and filtration by use of Local exhaust ventilation.
Work procedures and methods to separate workers from dust, such as ventilated
control cabins and appropriate personal protective equipment.
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References
1. NIOSH Aerosol Research Overview
2. NIOSH 7500 SILICA, CRYSTALLINE, by XRD (filter redeposition)
3. NIOSH 7300 ELEMENTS by ICP (Nitric/Perchloric Acid Ashing)
4. NIOSH 0500 PARTICLES NOT OTHERWISE REGULATED, TOTAL
5. NIOSH 0600 PARTICLES NOT OTHERWISE REGULATED, RESPIRABLE
6. SKC sample set up guide Publication 1166 REV 1303