Is the double drum hoist still viable at depth? Recent experiences
CIM Bulletin, Vol. 97, No. 1083, 2004
Agnico-Eagle, LaRonde Division, operates the deepest single-lift shaft in North America. Its Penna shaft successfully introduced lower rope safety factor hoists to Canada based on a code of practice developed in South Africa.
End loads permitted on hoisting ropes are legislated and fall into two broad categories. Rope selection can be based on fixed factors, such as a capacity factor of 7.5 and a safety factor of 5.0, or based on a formula where the factor depends on the suspended rope length.
The Penna production hoist rope is based on Quebec regulation 288.1, which permits the rope to have a minimum safety factor of 25 000/(4 000+L) where L is the suspended length.
Penna uses a flattened strand 57.15 mm (2 1/4 in.) rope. The rope has a breaking force of 2630 kN and a mass of 12.6 kg/m.
The end load envelopes graph shows various hoisting plants plotted against the end load envelopes for the Penna rope. The fixed factor envelope dips steeply from 1450 m onward. The length dependent factor envelope slopes gently from surface. Until recently, the only way of increasing the end load in a compartment was to double the number of ropes per conveyance, i.e. install a Blair multi-rope (BMR) hoist.
Three hoisting plants are shown in the upper right hand corner of the figure with suspended lengths of 3 km. They are the deepest single-lift shafts in South Africa, all are BMR installations, and all operate at the lower rope selection factor. Partial end load envelopes are shown for each installation. It is interesting to note that a single Penna rope, using the lower rope selection factor, has a greater end load capacity than two of the deepest BMR hoists.
Rope life is very system-dependent as all hoisting systems represent a compromise between capital, expected operating cost, the “as built” system, and the maintenance effort. Triangular strand rope lives can be maintained and possibly enhanced with lower factors. The first rope set at Penna achieved an enviable 284 000 skips and 5.6 Mt, the second pair 126 000 skips and 3.0 Mt. The second rope was discarded due to a worsening of an initial defect.
The introduction of the lower rope selection factor formula has opened the door for double drum hoist in Canada to compete at depths where a BMR hoist was required under fixed rope factors. This new development will encourage Canadian miners to stick with double drum hoists instead of introducing BMRs to their operations.
The Ontario government recently introduced a safety factor based on the suspended rope length. End loads available in Quebec and Ontario for the lower rope selection factor are not always identical for the same suspended length. Ontario operators should therefore not rely on the graph shown as being representative of their situation.
The renewed interest in deeper orebodies and consequent deeper shafts is pushing the industry to revisit the regulations
concerning end loads for sinking plants. Deep shafts need to be sunk rapidly to enhance project economics.
A lower rope selection factor for hoisting, combined with lower rope selection factors for shaft sinking, makes the double drum hoist a cost-effective and viable option for deep shafts.
Hoisting, Deep shaft, Safety, Regulations