Quality Control

With the highly competitive nature of today’s
market and its demand for high standards of quality
and uniformity, it becomes quality control’s function
to ensure that all fibre shipments are within the
narrow limits specified for all grades produced. A
close continuous liaison among mine, mill and quality
control is required to ensure that properly blended ore
is delivered to meet the demands of the production
schedule. Quality control must also maintain a relation-
ship with the sales department to assist in customer
problems, which may include fibre quality, bag
weights and packaging.

During the course of its work, quality control uses
many different tests and procedures in both wet and
dry tests, such as Bauer McNett, T & N Classifier,
filtration rate, wet volume and buoyancy, surface area,
Quebec Standard Test, dry bulk, colour, magnetic, Ro-
tap, grit determination and the modified Suter Webb
test.

To maintain fibre quality, regular samples are
taken of both the finished product and the fibre at
various stages of the milling process. The maintenance
of uniformity in fibre quality is of prime importance,
and this is maintained by setting all fibre specifica-
tions within narrow limits demanded for maximum
and minimum shipping standards.

Fibre samples are taken immediately before the
bagging operation on each grade. These samples are
composited and tested for various fibre qualities. The
fibre is tested for dry screen analysis, wet screen
analysis, fibre length, -200 mesh content and surface
area. In the spinning grades, length of fibre is an
important characteristic and a small sample is taken
and physically combed on the modified Suter Webb,
with the fibres separated in various lengths. The dry
tests, although more relative than absolute, are used
as a quick test of fibre quality and production control.
Wet screen analysis, which is the better evaluation of
fibre length and dust content, is a longer process.
For cement fibre grade, the more important fibre qual-
ities are strength, fibre length, filtering rate and dust
content. Filtration rates are important, because fast-
filtering fibre will allow faster production in the
customers’ plants.

Facilities exist at the plant to fabricate asbestos
cement plaques, which are used to evaluate the action
of Cassiar fibre in an asbestos cement product. Basic
research has been done in this field to determine the
optimum condition for the various cement fibres to
yield ultimate strength.

Air-Handling System

The total air handled in the milling process is over
660,600 cfm, representing 80 tons of air for each ton
of fibre produced. Approximately 80% of the air is
used for processing; 20% is used for environmental
dust control.

CONCENTRATOR AND DRYER CIRCUIT

Process and hygiene air in the concentrator and
dryer circuit is generated by seven individual fans
aspirating from seven individual pulse-type fabric
filter collectors. The fans are located on the clean-air
side of the dust collectors. All transfer points and
sereen enclosures are fitted with dust take-offs having
a pick-up velocity of 300 ft (91.5 m) per minute.

Air, used in the drying circuit both for drying the
ore and dust control, is drawn through three pulse-

FIGURE 11—Strapping machine with a one-ton unit of

cement (AK) fibre. .

type dust collectors equipped with Nomex filter fab-
rics. This allows the operating temperature to exceed
400°F. The bag collectors are insulated to maintain
the temperature above the dew point. The total air
handled in the dryer and concentrator circuits is
160,000 cfm.

MILL CIRCUIT

More than 500,000 cfm of air is required for the mill
operation. This air is generated by approximately 40
fans pulling air from individual cyclones assisted by
five booster fans to maintain a negative pressure in-
side the gathering plenums. The system now used at
Cassiar is designated as a push-pull air-handling sys-
tem. The individual fans create a negative pressure
from the pick-up point to the inlet of the fan. From
this point, the fan pushes air toward booster fans
which are operating at the end of major plenums. The
inlet to the booster fans may be slightly negative or
positive, depending on air conditions and conditions
in the dust filter. The booster fans push air at a posi-
tive pressure of up to 6-in. water gauge through
fabric filters located in the 1954 mill ground floor; air
must find its own way through various openings to
the aspirating point.

Modifications to the original fabric air filter system
installed in 1956 have permitted an increase in its
capacity from 230,000 to the present 500,000 cfm.
This has been achieved by additional units, fabric
selection and increased cloth area with the use of
smaller-diameter filter tubes. Increased air-to-cloth
ratio has resulted in a high pressure drop across the
filtering units.

In 1975, additional air was required to improve
product transport and to meet tighter environmental
regulations in regard to air-borne asbestos dust. Stu-
dies were made of various alternatives, with the most
obvious being expansion of the present system. This
was ruled out on environmental considerations due to
the location of the fabric filter on the bottom floor
of the original mill. In most mills the mill air is re-
circulated, and the air returning to the top floor of
the 1970 mill has to pass all floors, where contamina-
tion could be picked up and dispersed over a wide area.
Velocity in stairways is excessive. There is difficulty
in balancing the fan system with different fan charac-
teristics. With the pressure system, any circuit that
is required to be isolated for the purpose of repairing