FALCONBRIDGE IN SUBBURY: SELECTIVE MINING AT CRAIG

Falconbridge’s fifteenth mine in the Sudbury Basin and tenth on the North Rim, the Craig, was drilled during the 1970s, disclosing several zones of good grade nickel-copper ore. Shaft-sinking was started in 1982 but didn’t get very far before work was stopped and the project put on hold. A recession and low metal prices were the cause; indeed, this was the time when many analysts questioned if Falconbridge could survive as a corporate entity. It did, of course, with Bill James as the new president.

By 1984, circumstances had improved and the Onaping mine, also closed in 1982, was reopened and the decision made to develop and mine one of the Craig zones.

Craig has nine known ore zones. They are situated about one mile from Onaping’s main shaft and all of the development carried out to date has been from the 2,050-ft., 3,050-ft. and 4,025-ft. levels (615, 915 and 1,207 metres respectively). Craig’s own shaft did not get under way again until January, 1990 and it won’t be commissioned until early 1992.

Craig ore reserves are understood to be 16.5 million tons at a grade in the vicinity of 1.7% nickel and 0.6% copper. The nickel grade was quoted several years ago at 2%, but this appears high for a mill head and The Northern Miner Magazine’s estimate takes into account the Falconbridge statement that “the Craig mine will be providing 60% of Sudbury nickel production in 1994.” The company itself was not prepared to elaborate.

Development and initial stoping continue from the main Onaping shaft while the Craig shaft and a ventilation system, comprising four bored raises, are being pushed ahead. The shaft is a concrete lined opening, 20.5 ft. (6.25 metres) in diameter and will bottom at a depth of 4,920 ft. (1,500 metres). Dynatec Mining is the shaft-sinking contractor and, with the exception of certain items, its practice resembles that of J.S. Redpath. Where Dynatec differs is in the use of a cactus grab and the unusually large 14-ton bucket for mucking. It is so large, in fact, that concrete is delivered to the shaft forms by the bucket rather than by the more conventional slickline. In the drilling phase, an 8-inch cut-hole is drilled with a dedicated in-the-hole machine. The remainder of the round is drilled by two separate hydraulic drills. The shaft bottom is drilled off full-face using a burn cut, and an advance of 15.4 ft. (4.7 metres) per day is being made.

When completed, the shaft will be equipped with a large, 80-man, double-deck cage (8.5 by 17 ft.) and a 22-ton skip. The skip is an unusually long 40 feet. North American and European manufacturers are currently bidding for the new skip hoist. The cage hoist is a 15-ft.-diameter double drum unit, previously used at White Pine, Mich. It is powered by a 4,500-hp dc motor and coils a 2.25-inch-diameter hoisting rope.

The new shaft is scheduled for completion January 1992 and all ore and waste hoisting and servicing will be carried out through this shaft in early 1993. Until that time, the main Onaping shaft will continue to be used.

Setting a Record

The four raises being driven probably represent a record. Few raises of such length and proximity have been bored in hard rock anywhere else in North America. All four raises are for ventilation with one as intake and three for exhaust. Each is being bored in 2,000-ft. (600-metre) lifts in diameters of 10, 11 and 12 ft. Redpath Raiseboring is working in close co-operation with Falconbridge personnel on this project. The raiseborer itself, a Robbins 97RL, was in fact puchased by Falconbridge. It has a thrust of 1.5 million pounds and is powered by a 400-hp dc motor. A pilot hole of 13.75 inches in diameter is first drilled and then reamed to the final diameter. Pilot hole deviation is usually less than 1.5%. Reaming advance ranges between 10 and 16 ft. (three and 4.8 metres) per 8-hour shift.

About 90% of Craig’s ore occurs in the so-called “late granite breccia,” just below the norite contact of the Sudbury Igneous Complex. The ore occurs in east-west-striking zones dipping 40deg to 50deg to the south with a plunge of 55deg to the southwest. The zones pinch and swell from a few inches to 250 ft. (75 metres) along both strike and downdip. The principal minerals are the typical Sudbury sulphides, pyrrhotite, pentlandite and chalcopyrite, occurring as irregular masses, blebs and stringers in the granite breccia. The remaining 10% of the ore is found in an overlying norite breccia and there the mineralization tends to be more erratic, both in grade and in continuity.

Falconbridge has had many years of experience mining at depth in the hard, crystalline rocks of the Sudbury Complex and high rock stresses are the norm in this environment. Planning the mining of Craig was thus preceded by an extended period of study.

Two distinct mining methods were promoted and both have since been introduced underground. The first method is post-pillar cut-and-fill. It will be used primarily in the narrow and erratic ore zones that carry the bulk of Craig’s ore reserves.

Where open spans exceed 36 ft. (11 metres), a grid of post pillars 16×16 ft. (5×5 metres) is established to support the back as permanent pillars. The width of the permissible open span is recalculated for each ore zone. Also noteworthy is the orientation of the opening with reference to the strike of the zone. The question arises, Does the opening run along the strike or at right angles to it? The ore contained in the pillars is not recoverable and, under extreme conditions, will account for 10% of the calculated ore reserves.

High-grade Millfeed

After all the ore has been taken at a given horizon, the stope is filled to within nine feet (three metres) of the back with uncemented tailings or waste rock and the next breast taken 16 ft. (five metres) in height. Fresh pillars are cut on top of those from the previous lift, thereby providing continuous rigid support from the original footwall. The method allows the use of high-productivity equipment in an environment that requires the miner to chase irregular mineralization. There is some loss of ore in pillars, but the method produces a relatively high grade of millfeed.

The second mining method is applicable to the larger and more regular zones. It has the advantage that it can be started in mid-zone; in other words, the same oreshoot can be worked simultaneously from two or more horizons, leaving a recoverable sill between the working horizons. The method also provides superior ground control. The method, called rib pillar cut-and-fill, mines the ore in two passes. At the first pass, panels are driven 32 to 40 ft. (10 to 12 metres) wide, leaving 32-ft. (10-metre) rib pillars between. After a lift has been completed, mucked out and rockbolted, cemented tailings or cemented rock fill is introduced. The purpose of the cement additive is to create a more rigid fill to improve recoveries when the time comes to extract the pillars in the second phase of mining.

Rib pillars will be extracted on completion of primary mining and while no data have been published so far, studies are continuing on the several techniques that are available.

Craig is an intensively mechanized mine and when full production is reached in 1994, it will be hoisting 1.3 million tons per year, or 3,600 tons per day (1.2 million tonnes and 3,300 tonnes respectively).

Mobile equipment include seven twin-boom electric-hydraulic jumbos, 15 diesel load-haul-dump machines of 5-, 6- and 8-yd. capacity and four 26-ton trucks, with the possibility of 40-ton trucks in the future.