“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. — 38 —