The Cassiar Asbestos mill, with the mountains in the back-
ground.

Truck-loads of packaged asbestos fibre leaving the plantsite.

ducts is affected by pressure from other fans in the
system, which cause major dust dispersions. When
mill fans are shut down for weekly maintenance, the
push system discharges dust into the air through
all openings.

The second alternative was to install a pull-through
system. Instead of blowing dusty air through the filter,
fans are used to draw clean air through the filter
fabric. It was desirable to install fan plenums and
dust collector units at the top of the mill, as they
would be closer to the 7th-floor cyclones, which handle
over 85% of the mill air. However, because it is a
timber structure, the 1970 mill could not support the
extra weight. A separate building was constructed in
1977 to supply 480,000 cfm for process and environ-
mental air in the 1970 mill. The adjacent location added
an approximate 2-in. loss to the proposed system. An
additional floor was added to the 1970 mill to house
the air plenums collecting air from the cyclones and
dust risers. Three plenums feed an air manifold,
which distributes the dusty air to eight 54-in.-dia-
meter downcomers to the mill air building. Dust-
laden air passes through two settling chambers be-
fore being discharged to nine hoppers under the au-
tomatic shaker-type bag filter. The dust collected
from the bags falls into the hopper, from which it is
then conveyed by screw conveyors and belts to a pug
mill, where it is wetted and discharged to tailings.
With the high settlement rate in the plenums, grain
loadings of two grains per cu. ft are expected — or 2

tons per hour of collected dust. The air is generated
by four Chicago blower fans of 120,000-cfm capacity
each, operated at a negative pressure of 16 in. water
gauge. The clean air is distributed through the fans
into a vertical clean air plenum, where it is returned
through louvered opening to each floor or discharged
to the atmosphere as desired. The ground floor of the
mill air building houses the mill maintenance shop.
The second floor contains the quality control labo-
ratory and the mill offices.

Mill Modifications

With the connection of the new mill air building,
additional air is available for processing and for en-
vironmental dust control. Mill circuits in the 1970 mill
were redesigned to make use of the additional air to
improve working conditions. The ducts and aspirating
lines were rerouted to eliminate long horizontal runs
and permit additional dust take-offs. Dust take-offs
have been revised to reduce pick-up velocities to 300
to 400 ft per minute. This aids in keeping transport
velocities low and reduces loss of fibre values within
the system.

Leaks in ducts and chutes are a major source of the
environmental contamination within the mill. High
conveying velocities in ducts of the aspirating and
dust collection system cause high wear and result in
short duct life. Various liners, including Ni-hard and
rubber, have been used to overcome these problems.
Recent experience has shown the superiority of cera-
mic liners, and new installations are being equipped
with these liners at elbows and transition areas.
Screen enclosures have been modified based on oper-
ating experience at Cassiar and other asbestos mines.
These modifications are designed to more effectively
contain the dust normally associated with the gyrating
screens at both feed and discharge points. Similar
modifications have been designed on inspection door
and conveyor enclosures.

In order to maintain an effective dust system, long
horizontal lines have been avoided. During 1977, eight
vertical dust risers were located in close proximity
to dust sources to reduce the horizontal distances.
These risers extend from the ground floor to the dusty
air plenums on the 8th floor.

Tailings Disposal

As 90% of the mill feed as well as 30% of the con-
centrator feed is discarded as waste, material handling
of dry, finely ground rock is another major environ-
mental problem. Various methods have geen tried to
wet the tailings for dust control.

The only successful treatment to date has been to
collect all the waste components in one bin and treat
them at a controlled rate through a pug mill fitted
with water sprays. Some problems are encountered
with the automation of sprays, which causes sticking
and freezing to the exposed conveyor belt.

The milling process at Cassiar has, within the last
two years, undergone major redesign and modification
of machinery and processes. The improvements in en-
vironmental control and production facilities will
maintain and improve Cassiar’s position in the supply
of quality asbestos fibre.

Projected improvements and capital expenditures
over the next three years are designed for environ-
mental improvement, increased recovery from the mill
feed, and improvement of fibre quality and customer
service.

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