FIGURE 9— Bauer air classifiers in the cement (AK) fibre
circuit.

screens is the first lift into the cement fibre grade. All
screens in the rock circuit are double decked, with
aspiration from the top deck. The oversize from the
third set of screens after aspiration goes to three ver-
tical impact mills called ‘fiberizers’. These mills re-
ceive rock by gravity, and hammers attached to a ver-
tical shaft throw the rock against serrated liners of
manganese steel. The impact releases fibre from the
host rock, which is aspirated from the screen receiving
the fiberizer discharge. The process of screening,
aspiration and fiberization is carried on through two
additional stages in the primary rock circuit on pro-
gressively finer mesh screens.

The aspirated fibre from the rock circuit is dis-
charged from the cyclone to cleaning screens. A typical
flowline would have the cyclone discharging to a series
of rotary paddle trommels, commonly known as de-
dusters. These machines consist of a horizontal cylin-
drical screen, 36 inches in diameter and approximately
9 ft long, in a dust-tight enclosure and under negative
pressure.

The fibre is introduced into one end of the hori-
zontal revolving screen (Fig. 8) and is subject to the
action of paddle arms rotating in the opposite direc-
tion. The paddles beat the fibre against the screen
and dust aspiration from the top of the enclosure

FIGURE 10— Packaged asbestos after pressure packing.

removes the liberated dust. The shorter fibres fall
through the screen and are conveyed to subsequently
shorter fibre grades. The oversize of the screen, which
has been cleaned and graded, is conveyed onto addi-
tional horizontal gyratory Hall screens, and then as-
pirated to the final product cyclone. This process of
cleaning fibre is repeated with some variation for each
of the five basic fibre grades. In all grades, except
the spinning fibre, the final aspiration is through air
classifiers of one type or another (Fig. 9). These
classifiers complete the removal of the fine grit which
has not been cleaned in the screening process.

The fibre aspirated to the final product collector is
discharged into a 6-ton live-bottom bin, which is used
for final blending before bagging. The live-bottom bins
discharge via screws to weigh hoppers, where the fibre
is weighed into 100-Ib portions prior to delivery to the
bagging machines. Fibre, which has an approximate
density of 5 to 10 lb per cu. ft at this stage, is dis-
charged into a vertical chamber, where it is compressed
by a hydraulic plunger or ram. The pressure-packed
fibre, which then has a density of approximately 40 Ib
per cu. ft, is ejected through a horizontal spout into
a woven polyethylene bag, the approximate package
size being 24 by 16 by 8 in. The actual size of the
package varies according to the grade and openness
of the fibre. The fibre package is then sewn, sorted
into various grades, palletized into one-ton lots,
strapped and sent to fibre storage to await test results
prior to shipment (Figs. 10 and 11).

Specialty Grades

Fibre as milled has a specific surface area of ap-
proximately 4000 cm? per gram. Manufacturing pro-
cesses in both spinning and cement fibre plants require
fibre openness from 4000 to 40,000 cm?® per gram.
Within the milling circuit, facilities are provided on a
small scale to treat a portion of the basic fibre grades
to produce a more highly opened specialty grade. This
opening process is achieved by passing the fibre
through high-speed, specially designed impact fans.
The opening is done only after fibre specifications
have been met on basic grade, preserving the fibre
quality and preventing degradation by inclusion of
rock and dust particles.

— 42 —