Gravimetric Analysis Paper

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)

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.

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

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

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)

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

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

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

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

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

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.

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

Gravimetric Analysis Paper

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