“THE CASSIAR STORY”
Milling

David C. Cook, General Mill Superintendent,
Cassiar Asbestos Corporation Limited,
Cassiar, B.C.

Abstract

The milling complex, situated just east of the Cassiar
townsite, receives raw ore from the tramline and processes
it into 105,000 tons per year of milled asbestos fibre. The
ore is passed through a concentrator, where a barren rock
fraction is removed. The concentrated ore, representing 70%
of the feed, is dried, stored and then milled into nine separ-
ate fibre classifications. The finished product is packaged into
100-Ib bags and loaded into trucks in one-ton units. New
air-handling facilities were added in 1977 to provide additional
air for environmental control.

Introduction

THE ASBESTOS ORE, delivered by the tramline at
the rate of 300 tons per hour, is received through the
discharge terminal into the concentrator building
(Figs. 1 and 2) where the processing of the ore into
the finished product begins. Milling consists of con-
centration, drying, storing and the actual separation
of the fibre from the rock by means of screening,
aspiration and fiberization. The aspirated fibre is
cleaned and graded by means of horizontal gyratory
screens and rotary dedusters (paddle trommels).

The mill produces nine direct-milling grades, clas-
sified according to length and dust content, five of
them being produced concurrently. Special fibres,
which are dictated by customer preference, number
over twenty, and are variations of the nine basic
grades. After classification, the fibre is packaged in
100-lb woven polyethylene bags, sewn, palletized and
strapped into one-ton packages. The strapped pallets
are stored for shipment by truck to Whitehorse, Y.T.,
or Fort Nelson, B.C.

The mill complex is comprised of several intercon-
nected, but distinct, structures (Fig. 3). The original

David C. Cook was born in Newcastle,
New Brunswick, attended Mount Alli-
son University and graduated from
Queen’s University, Kingston, Onta-
rio, in mining engineering. He worked
at Sigma Gold Mines Ltd. and Algoma
Ore Properties. In 1951, he joined
Canadian Johns-Manville Co. Ltd. as
= mining engineer and worked in vari-
- ous positions at Asbestos, Quebec, and
- Matheson, Ontario. He joined Cassiar

“=< in 1963 as assistant mill superinten-
dent, fen somed as mine manager with Indusmin Limited
at Nephton, Ontario. In 1973, he joined United Asbestos
as mine manager.

Mr. Cook returned to Cassiar in 1977 as special project
manager and is now general mill superintendent. He is a
CIM Member and a member of the Association of Profes-
sional Engineers of Ontario.

Keywords: Cassiar Mine, Asbestos, Milling, Concentrators,
Dryers, Ventilation, Tailings disposal.

mill building, with the associated dryer and dry-rock
storage structure, was built in 1954 and produced two
grades of fibre. It had a daily capacity of 150 tons of
ore, which was trucked directly from the mine. The
mill was gradually expanded until 1970, when a 132- by
128-ft addition was built. It was then producing a total
of eight fibre grades, four concurrently, and handling
3000 tons of ore per day.

The 1970 laminated wood mill building was designed
to:

(1) obtain improvements in packaging and material
handling ;

(2) improve the production of spinning and cement
grades;

(3) allow the production of an additional fibre grade;
(4) improve environmental conditions.

The mill air building was built in 1977 to improve
environmental conditions.

Milling Principles

Recovery of asbestos fibre is based on the physical
differences between the fibre and the host serpentine
rock. The fibre and rock are chemically alike and, in
the rock state as mined, of equal specific gravity. Only
by making use of the fibrous nature of the asbestos,
through the process of fiberization to change its ap-
parent specific gravity, can it be effectively separated
from the host rock.

The value of the fibre is primarily dependent upon
length (Fig. 4), so the milling process is designed to
separate the fibre from the rock as early as possible
in the circuit to prevent fibre degradation.

In order to maintain fibre quality, care must be
taken through the mining and milling operation to
preserve fibre length and to create a minimum amount
of damage and dust impregnation in the fibre. Long
spinning fibre is the highest quality and is released
early in the milling process; therefore, the ore must
be treated carefully until the aspiration from the in-
itial mill screens. This is achieved by minimal hand-
ling, gentle crushing, and drying at controlled tem-
peratures and moisture content.

As all the separation is physical, contamination by
any fibrous materials, such as wood, paper or other
light material, which could be aspirated with the fibre,
has to be eliminated at the source. The ore delivered to
the mill is passed over large-mesh screens to remove
the longer fibre before any fiberization. All fibre is
aspirated through hoods to cyclone collectors. The
cyclones make use of centrifugal force to separate the
fibre from the aspirated air. The fibre is discharged
through rotary valves to horizontal cleaning screens,
where the fine rocks are dropped out. Fibre is further
cleaned and classified with the selection of screen
meshes on horizontal gyratory and horizontal cylin-
drical screens equipped with counter-rotating beaters.
These trommel dedusters are under negative pressure
and the fine dust, released by the action of the beaters,
is removed to the dust system.

Final aspiration is through air classifiers in which
the last traces of fine rock particles still remaining
in the fibre are removed.

Fibre grade is controlled by ore selection in the pit,
blending from storage, and circuit and screen mesh
changes.

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