Ore passes are simply vertical or inclined openings for transfer of broken ore to the haulage system. They may be raises in the ore or in the walls of the deposit or timbered openings through filled stopes and, as previously stated, usually terminate at the bottom in some form of loading chute.
They must be steep enough to insure that the ore will run freely and usually are inclined at angles ranging from 50° to vertical. The inclination often is determined largely by the dip of the ore body, especially if it is a narrow, tabular one, but the physical characteristics of the broken ore also must be considered. Most ores will run well at 60° to 65°. Some hard, dry, lumpy ores will run at angles as low as 45° if the cross-sectional area of the pass is large, but most ores will not. Moist, sticky ores are especially troublesome; the fines often will build up on the bottoms of flat raises and, if the raise is too steep, will pack owing to the superincumbent load, if much ore is allowed to accumulate, that is, unless the raise is kept nearly empty. In vertical or steep ore passes, chunks of hard ore falling considerable distances are likely to break the chute gates unless some ore is left in the bottom at all times. The fall in long ore passes may be broken by short offsets or by reversing the dip. Thus, it is common practice to drive an ore-pass raise at 50° to 60° for 50, 75, or 100 feet and then “knuckle back” on a slope in the opposite direction. This often assists materially in reducing wear on ore-pass linings and preventing breakage of chutes and gates.
The type of lining will depend upon the nature of the surrounding material (whether rock, ore, or waste filling), the characteristics of the material to be handled, and the total quantity of ore to be transferred during the life of the pass. In hard firm ore and rock, ore and waste passes often are unlined.
In soft ore, lined sets or cribbing may be employed to support the surrounding ground and to take the wear. The lining usually consists of planking laid the long way—that is, in the direction of movement of the ore—and is secured to the timber sets or cribbing by spikes. Falling chunks often cause trouble by striking the planking a glancing blow and knocking it off, and under such conditions it may be necessary to employ lag screws instead of spikes. An alternative to plank lining employed successfully under these conditions at some mines is to use heavy sawed timber for cribbing, the timbers being laid “skin-to-skin” to form a smooth inside face. If the life of the ore pass is to be long and a large tonnage is to be handled through it, the cost of concrete lining may be justified. This would seldom be the case for individual stope chutes but may be for main ore transfers that are to be in service for a number of years. Sometimes T-rails instead of planking are laid lengthwise of the ore pass to take up the wear and protect the timbers.
The ore passes should be of cross-sectional area adequate for the tonnage to be handled and the size and character of the ore. If too small, chunks are likely to wedge themselves between the sides and block the pass. Since vexatious and costly delays and broken chutes often result from “hung” ore passes, it is poor economy to restrict their size. It is customary to carry a manway along one side of the ore pass as a separate compartment of a two-compartment raise, and access to points where the ore pass becomes blocked may be had from the manway.
Waste passes for introduction of stope filling often are driven from one level to the next before stoping operations are begun but may be driven from the backs of the stopes after they have been mined up part way. As with ore passes, they may be timbered or not, depending on the character of the ground, but passes to individual stopes are used for a comparatively short time (until the stope is mined out and filled), and permanence of construction usually is not required. As with ore passes, the inclination and size must be adapted to the nature of the filling material.
The passes may terminate in the stope at a chute or may be open at the bottom, depending on the method of distributing the fill in the stopes. If it is to be spread by natural gravity flow or with scrapers, the passes are open; but if it is to be drawn into stope cars for distribution, chute gates must be used. The chutes in this case should be of one of the simpler forms such as the stop-board type, easily and quickly installed, since they must be moved frequently as the stope advances upward.
Underground Transfer Points
As previously pointed out, considerable savings can sometimes be made in cost of development if only certain levels are connected with the shaft and intermediate levels are used solely for ore extraction and transfer purposes. Even though all the levels connect with the shaft, savings can often be made by providing for hoisting only from one or more main haulage levels to which the ore from intermediate levels is transferred through ore passes. In this way the cost of installing, operating, and maintaining ore-storage and skip-loading pockets on the intervening levels is eliminated, and additional savings may be possible through concentration of main-line haulage on one or relatively few levels.
This practice requires that one or more main ore passes be provided for transferring the ore from the intermediate or ore-extraction levels to the haulage level below. The position of these passes will obviously be determined by the relative positions of the ore bodies, intermediate levels and shafts, or other main outlets of the mine. Since each transfer of ore costs money, it is desirable to handle it the minimum number of times; hence, wherever possible, ore passes connecting each extraction level directly with the main haulage level should be provided.
A practice that has become more and more common in recent years is to provide a main ore pass paralleling the shaft, into which ore can be dumped on any ore-extraction level and drawn off at any hoisting level below. In addition, where a filled-stope method of mining is employed it is advantageous to have a main waste pass into which waste may be dumped and from which it may be drawn on each level. Even if other than a filling method is employed, a main waste pass paralleling the shaft may be advantageous. If a filling system is used the waste pass or passes may be either close to the shaft on all levels or some distance from the shaft and centrally situated with respect to the stoping areas.
Figure 75 shows ore- and waste-pass systems paralleling the hoisting shafts, by which ore may be dumped, and waste may be either dumped or drawn off on every connecting level.
It should be noted that the fall of ore and waste is broken at each level by an offset in the pass or by “knuckling back” on the dip; this also keeps the passes close to the shaft and at the same time at an angle from the vertical.
Figure 76 shows an ore- and waste-pass system in which there is a main ore pass and one main waste pass approximately parallel to the shaft and other main waste passes (shown in heavy black) are provided in the stoping areas.
Main ore passes often remain in service for years and large tonnages may be handled through them. For this reason considerable expense may be justified for providing permanent resistant linings with chutes and gates designed for optimum operating efficiency. In addition to their other functions, main ore and waste passes serve a useful purpose in providing storage reservoirs. Thus in the event of delays on the haulage level or in the shaft or of break-downs in the milling plant, they may obviate suspension of mining in the stopes by reason of their capacity to store the broken ore until normal operations can be resumed. Likewise, if for any reason stoping must be discontinued for a short time, they may serve to provide ore for the mill during the interim.
Stope-filling material often is required in large quantities at regular or irregular intervals. To provide this when needed and thus avoid delays in the stopes, main waste passes can be utilized for accumulation of waste during periods when the demand for waste is small.
At the Hollinger mine in Ontario a system of ore and waste passes parallels the Central shaft from surface to the skip-loading pockets below the 2,750-foot-level station, and another system parallels the No. 19 shaft from the 2,600-foot level to loading pockets below the 3,800-foot level. Ore can be dumped into the Central shaft system on any level from the 2,750 up, and waste can be dumped into or drawn from the waste pass at any level. The ore-pass system at this shaft together with the storage pockets has a capacity of 25,000 tons, enough for about a 5-day run for the mill. The waste-pass system has a storage capacity of 6,500 tons.