ASBESTOS
Introduction
1. Asbestos is available in nature as a
mineral silicate of calcium and magnesium found in the form of very thin fibres
in the veins of metamorphic rocks.
2. There are
six types of asbestos: actinolite, amosite, anthophyllite, crocidolite,
tremolite, and chrysolite. The first five types are known as amphiboles. They
are characterized by having very strong and stiff fibres, which makes them a
serious health hazard. Amphibolic asbestos fibres can penetrate body tissue,
especially in the lungs, and eventually cause tumours to develop. The sixth
type of asbestos, chrysotile is known as a serpentine. Its fibres are much
softer and more flexible than amphibolic asbestos, and they do less damage to
body tissue. All six types of asbestos are composed of long chains of silicon
and oxygen atoms, locked together with various metals, such as magnesium and
iron, to form the whisker-like crystalline fibres that characterize this
mineral.
2. Chrysotile
is the most commonly used type of asbestos and accounted for about 98% of the
worldwide asbestos production in 1988. It is usually white, and is sometimes
known as white asbestos, although it can also be amber, gray, or greenish in colour.
Most chrysotile fibres are about 0.25-0.50 in (6.4-12.7 mm) long and are
usually added to concrete mixes to provide reinforcement. Only about 8% of
chrysotile fibres are long enough to be spun into fabric or rope.
3. Amosite,
sometimes called brown asbestos, accounted for about 1% of worldwide production
in 1988. It often has a light brown tinge, but is also found in dark colours,
as well as white. Amosite has coarse fibres that are about 0.12-6.0 in
(3.0-152.0 mm) long. The fibres are difficult to spin into fabric or rope and
are mostly used as an insulating material, although that use is banned in many
countries.
4. Crocidolite,
sometimes called blue asbestos, accounted for the remaining 1% of world-wide
production. It has a bluish tinge, and its fibres are about 0.12-3.0 in
(3.0-76.0 mm) long. Crocidolite has very high tensile strength and excellent
resistance to chemicals. One of its uses is as reinforcement in plastics.
5. The other
three types of asbestos—anthophyllite, actinolite, and tremolite—have no
significant commercial applications and are rarely mined.
General Characteristics
6. Following
are the general properties and characteristics of asbestos:
a.
It has low water permeability, is vermin proof and
resists rot.
b.
It is brown gray and white in colour.
c.
It is good for heat sound and electric insulation
d.
It is resistant to acids and alkalis.
e.
Its fibres are capable of being woven into fabrics.
f.
It is highly durable and is strong.
g.
Its specific gravity is 3.10.
h.
Its melting point is 120-150 ºC.
i.
It acts as reinforcing material when mixed with ordinary
Portland cement.
The Manufacturing
Process
7. Asbestos
deposits are found underground, and the ore is brought to the surface for
processing using conventional mining practices. Chrysotile asbestos is usually
found near the surface and can be accessed with an open-pit mine. Other
asbestos deposits are found at varying depths and may require tunnels as deep
as 900 ft (300 m) to gain access.
8. Asbestos fibres
are formed by the gradual growth of mineral crystals in cracks, or veins, found
in soft rock formations. The crystals grow across the vein, and the width of
the vein determines the resulting asbestos fibre length. Because the minerals
come from the surrounding rock, the chemical composition of the fibres is
similar to the rock. As a result the asbestos must be separated from the rocky
ore using physical methods, rather than the chemical methods sometimes used to
process other ores.
9. Here are
the steps used to process the chrysotile asbestos ore commonly found in Canada:
a. Mining
i.
Chrysoltile asbestos deposits are usually located using a
magnetic sensor called a magnometer. This method relies on the fact that the
magnetic mineral magnetite is often found near asbestos formations. Core
drillings are used to pinpoint the location of the deposits and to determine
the size and purity of the asbestos.
ii.
Most chrysotile asbestos mining operations are conducted
in an open-pit mine. A spiralling series of flat terraces, or benches, are cut
into the sloping interior sides of the pit. These are used both as a work
platform and as a roadway for hauling the ore up and out of the pit. The
asbestos ore deposits are loosened from the surrounding rock by careful
drilling and blasting with explosives. The resulting rocky debris is loaded
into large rubber-tired haul trucks and brought out of the mine. Some
operations use an excavation technique called block caving, in which a section
of the ore deposit is under-cut until it crumbles under its own weight and
slides down a chute into the waiting haul trucks.
b. Separating
i. The
ore contains only about 10% asbestos, which must be carefully separated from
the rock to avoid fracturing the very thin fibres. The most common method of
separation is called dry milling. In this method, the primary separation is
done in a series of crushing and vacuum aspirating operations in which the
asbestos fibres are literally sucked out of the ore. This is followed by a
series of secondary separation operations to remove rock dust and other small
debris.
ii. The ore is fed into a jaw crusher,
which squeezes the ore to break it up into pieces that are 0.75 in (20.0 mm) in
diameter or less. The crushed ore is then dried to remove any moisture that may
be present.
iii. The
ore falls on the surface of a vibrating 30-mesh screen, which has openings that
are 0.002 in (0.06 mm) in diameter. As the screen vibrates, the loosened
asbestos fibres rise to the top of the crushed ore and are vacuumed off.
Because the crushed ore is much denser than the fibres, only the very smallest
rock particles get vacuumed off with the asbestos.
iv. The very fine silt and
rock particles that fall through the vibrating screen are called throughs or
tailings and are discarded. The crushed ore pieces that remain on the screen
are called overs and are moved to the next stage of processing.
v. The
crushed ore from the first screen is fed through a second crusher, which
reduces the ore pieces to about 0.25 in (6.0 mm) in diameter or less. The ore
then falls on another vibrating 30-mesh screen and repeats the process
described in steps 3 and 4.
vi. The process of crushing and vacuum aspiration of the asbestos
fibres is repeated twice more. Each time the pieces of ore get smaller until
the last asbestos fibres are captured and the remaining ore is so small that it
falls through the screen and is discarded. This four-step process also
separates the asbestos fibres by length. The longest fibres are broken free
from the surrounding rock in the first crusher and are vacuumed off the first
screen. Shorter length fibres are broken free and captured on each successive
set of crushers and screens, until the shortest fibres are captured on the last
screen.
vii.
The asbestos fibres and other material captured from each
screen are carried suspended in a stream of air and run through four separate
cyclone separators. The heavier debris and rock dust particles fall to the centre
of the whirling air stream and drop out the bottom of the separators.
viii.
The air then passes through four separate sets of
filters, which capture the different length asbestos fibres for packaging.
USES
10. Historically,
asbestos is best known for its resistance to flame and its ability to be woven
into cloth. Because of these properties, it was used to make fireproof stage
curtains for theatres, as well as heat-resistant clothing for metal workers and
fire fighters. It is used in the form of paper and mill board’s asbestos is
used for low voltage insulation more modern applications of asbestos take
advantage of its chemical resistance and the reinforcing properties of its fibres
to produce asbestos-reinforced cement products including pipes, sheets, and
shingles used in building construction. Asbestos is also used as insulation for
rocket engines on the space shuttle and as a component in the electrolytic
cells that make oxygen on submerged nuclear submarines. Much of the chlorine
for bleach, cleansers, and disinfectants is produced using asbestos products.
11. Various
products for use in buildings are made with a mixture of 8 to 20% of asbestos
with Portland cement. These products are called ‘asbestos cement products’.
12. Asbestos
cement products are light in weight, strong, durable, water proof and resist
corrosion. These can be sawn, drilled, nailed and screwed. It is rolled into
various forms such as sheets, tiles and pipes for drainage e.t.c.
Applications
- Asbestos Cement Sheets
Asbestos cement paste
obtained by mixing 15% asbestos fibres with cement and mixed with water is
pressed under grooved and toothed rollers so as to give a series of waves or
corrugations. Corrugations are given to increase strength and rigidity of
sheets and ensure quick flow of rain water.
These are light, durable,
impervious and fire resisting commonly used for roofing big halls, factories,
workshops. These provide a reasonably priced roof covering which is a
non-conductor of heat and is durable. A.C. sheets require no maintenance.
There are three types of A.C. sheets
1.
Everite ‘Big Six’.
2.
Everite standard: Sheets 0.76m wide, 6.5m thick and 1.1m
to 3m in length. Overall depth of corrugations is 2.56 to 2.85m. There are 10 ½
corrugations per sheet at a pitch of 5.4cm.
3.
Turnal-Trafford sheets: 1.12m wide, 1.3 to 3m long and
6.5mm thick. Every sheet has 4-5cm deep corrugations alternating with flat
portions. Pitch of corrugations is 34 cm. Excellent roof covering specially for
large spans.
- Asbestos Cement Flat
Sheets
These
may be compressed or uncompressed and are intended for interior and exterior
use. The compressed sheets are stronger than the uncompressed ones. Nominal
thickness of these sheets are: 5, 6, 8, 10 and 15mm with + 5 % tolerance. Their
nominal lengths are 2400, 1800 and 1200 mm; and the width is 1200 mm with
tolerance of + 5 mm in each case.- Unreinforced Corrugated
Asbestos Cement Sheets
These are designed for roof
coverings and walls of different types of buildings and for decorative and
other purposes.
Asbestos Fibre Roof
- Asbestos Cement Boards
These are made by moulding
under pressure a mix of ordinary Portland cement and asbestos fibre. The boards
are usually 1.22 m wide and 3-6 mm in thickness and up to 2.44 m long. It has
dense hard surfaces. The exposed surface is finished smooth. The sheets may be
in the colour of plane cement or any other colour, with plain finish or with
tiles cut on its surface or with marble finish. The sheets can be sawed and
nailed with ease. These provide a nice wall covering with the resistant
properties. Asbestos is used for electric insulation and heat insulation
The above diagram shows typical locations for materials
containing asbestos. A full and detailed survey is required to identify all
asbestos containing materials
Quality Control
13. Asbestos fibres
are graded according to several factors. One of the most important factors is
their length, since this determines the applications where they may be used
and, therefore, their commercial value.
14. The most
common grading system for chrysotile asbestos fibres is called the Quebec
Standard dry classification method. This standard defines nine grades of fibres
from Grade 1, which is the longest, to Grade 9, which is the shortest. At the
upper end of the scale, Grades 1 through 3 are called long fibres and range
from 0.74 in (19.0 mm) and longer down to 0.25 in (6.0 mm) in length. Grades 4
through 6 are called medium fibres, while Grades 7 through 9 are called short fibres.
Grade 8 and 9 fibres are under 0.12 in (3.0 mm) long and are classified by
their loose density rather than their length.
15. Other
factors for establishing the quality of asbestos fibres include tests to
determine the degree of fibre separation or openness, the reinforcing capacity
of the fibres in concrete, and the dust and granule content. Specific
applications may require other quality control standards and tests.
Health and
Environmental Effects
16. It is now
generally accepted that inhalation of asbestos fibres can be associated with
three serious, and often fatal, diseases. Two of these, lung cancer and
asbestosis, affect the lungs, while the third, mesothelioma, is a rare form of
cancer that affects the lining of the thoracic and abdominal cavities.
17. It is also
now generally accepted that different types of asbestos, particularly the
amphiboles, pose a greater health hazard than chrysotile asbestos.
18. Finally,
it is recognized that other factors, such as the length of the fibres and the
duration and degree of exposure, can determine the health hazard posed by
asbestos. In fact some studies have shown that some asbestos-induced lung
cancers only occur when the exposure is above a certain level of concentration.
Below that threshold, there is no statistical increase in lung cancer over that
found in the general population.
19. Although
not everyone agrees with these findings, overall concerns about the potential
adverse health effects of inhaling asbestos fibres have led to stricter
regulations on the amount of airborne asbestos allow-able in the workplace.
These regulations vary from one country to another, but they all mandate
significantly lower levels than previously found. In the United States, the
Occupational Health and Safety Administration (OSHA) set the maximum
permissible exposure to fibres longer than 0.005 mm at 0.2 fibres/cubic
centimetre during an eight hour workday or 40-hour work week.
20. Airborne
asbestos levels in the general environment outside the workplace are many times
lower and are not considered a hazard.
The Future
21. Asbestos
is still an important component in many products and processes, although its
usage is expected to remain low in the United States. The stricter exposure
regulations and improved manufacturing and handling procedures now in place are
expected to eliminate health problems associated with asbestos.
