Table of Contents
Gravity usually is employed wherever possible to move broken ore from stopes to the haulage levels, and ore-passes from the stopes terminate at the bottom in chutes, by means of which the flow of the ore into cars or onto conveyors can be controlled. An exception is where scraper haulage is employed on the level or on transfer levels, in which instance the raises are open at the bottom and the falling ore simply piles up in the scraping drift, as previously described.
Loading chutes are constructed in various ways and consist essentially of a framework of timber, steel, or concrete with gates or other means provided for holding back the column of broken ore in the raise above.
Chute gates are of several different types and have been discussed by Jackson and Knaebel in an earlier Bureau of Mines publication.
Loading of cars from chutes is the first operation in underground haulage, and it is therefore appropriate to discuss chutes under the heading of “Transportation.”
Following is a classification of underground chutes and gates that includes the principal types in common use.
1. Platform chutes.
A. Chinaman chutes.
B. Scraper loading chutes.
2. Simple stopboard gates.
A. Cleated; boards between sides of chute.
B. Projecting; boards supported by iron brackets.
3. Vertical overcut or “guillotine.”
A. Operated by hand lever.
C. Rack-and-pinion type.
4. Sliding undercut gates.
5. Hinged gates.
A. Hinged-lip apron.
B. Bottom-discharge door; toggle or levers and counterweight.
C. Front-discharge door; toggle or levers and counterweight.
6. Butterfly gate.
7. Arc-type gates.
A. Overcut gates.
a. Flat door.
b. Curved door; (1) convex face against ore, (2) concave face against ore.
B. Undercut gates.
a. Convex side against ore.
b. Concave side against ore.
8. Finger chutes.
9. Curtain type (flexible door).
Platform or “Chinaman” chute
A common form of platform or “Chinaman” chute is illustrated in figure 69 at A. Strictly speaking, it is not a chute at all, the ore is held back by an offset, as shown, a car is run underneath, and the ore is shoveled or scraped into it through an opening made by removing boards or lagging over the drift timbers. It has the advantage of simplicity, low first cost, and ease of installation. Large chunks can be rolled to one side and broken, and there is no obstruction at the bottom of the raise behind which they can wedge and cause a “hang-up” of ore. In addition, an opportunity is afforded for sorting waste on the platform. Figure 69, B, shows a steel cantilever loading slide with a short chute for loading cars with a scraper.
Stopboard chutes probably are used more widely than any other type, particularly for handling small or moderate tonnages of ore and waste. They are simple in construction and cheap to install. Stopboard chutes are shown in figure 69 at C, D, and E. The ore is drawn by raising one or more of the stopboards. The form shown at C is a cleated chute and that at D is of the simplest bracket-supported construction. The latter was employed at the Ahmeek mine in Michigan, where the ore was mined in flat-dipping open stopes and the back stands well, requiring little support along haulageways. The form shown at E is one developed after long experience at the Hollinger mine in Ontario, where the ore breaks in various sizes from fines to coarse lumps. When stopboards become worn or broken they can be replaced easily and cheaply. Large chunks may cause trouble, and in removing them fines may run out, bury the car, and cover the track. Sometimes a narrow lipboard is set in brackets to hold back fines after the chute is closed, thus assisting in keeping the track clean. When hang-ups are blasted the boards and chute timbers frequently are broken. In general, stopboard chutes are best suited to conditions where the ore breaks fairly small but does not pack, where the total tonnage to be drawn through each chute is not so great that it will be worn out and have to be replaced one or more times, and where rapid loading is not required. In very heavy ground they have one advantage over steel doors, as they can be trimmed or otherwise altered more readily so as to permit their continued operation when damaged by ground pressure and movement.
Vertical overcut or guillotine-type gate
The vertical overcut or guillotine-type gate is often employed underground, the more elaborate forms on main ore passes, for opening and closing shaft pockets and skip-measuring pockets, and in connection with other more or less permanent or long-life installations. They are especially adapted to handling fine or medium-size material. Lumpy ore may block them and allow fines to run out underneath unless a lipboard or apron also is provided. Warping of the door or springing of the frame may cause them to bind, and therefore they must be installed in a substantial framework so that distortion will be prevented. When the nature of the material is suitable for handling with this type of gate, rapid loading is possible. Figure 70 shows small, hand-operated, guillotine gates at A and B as installed to handle comparatively small tonnages. The lower one at A was used for loading a skip in an inclined shaft and is provided with a short hinged apron to throw the ore farther out from the lip. When folded back, this apron serves to retain fines after the gate is closed. The upper gate in the same figure is operated by a rack and pinion. The gate at B is operated by a lever arm, which is simply used to pry up the gate. For rapid handling of larger tonnages, gates of this type usually are operated by compressed-air cylinders. The cylinder is mounted above the chute and raises and lowers the gate by means of a piston attached to a bridle on the top of the gate. It is apparent, however, that considerably more headroom is required than is available in most mine drifts, and the air-operated guillotine gate is more often employed on skip pockets and large main ore passes than on loading chutes in the drifts.
Sliding undercut gates
Sliding undercut gates (fig. 70, C), operated by compressed air, are not employed extensively but overcome two of the objections to guillotine gates. First, the gate cuts upward through the stream of ore when it closes, effectually stopping the flow of fines; and since
the top of the chute is open, large chunks are merely raised and tumbled one way or the other (back into the chute or into the car). Second, no additional headroom is required for installing the air cylinder. On the other hand, fines can hardly be prevented from working in between the gate and the guides and between the gate and the bottom of the chute, causing the gate to bind.
Hinged gates or doors sometimes are used as lip aprons to catch fines or to throw the material farther out, as in figure 70, A. Figure 70, D, shows a front-discharge toggle gate hinged at the top. Figure 70, E, shows the Baltic hinged gate, which, when dropped, rests on top of the car and serves as a continuation of the chute bottom. When raised, it folds back against the broken ore column. It is operated by a sweep or lever, as shown.
Figure 71, A, shows a butterfly-type gate that originated in South Africa. To open it, the stop pin is withdrawn, and the pressure of the ore causes the door plate to swing outward at the bottom; the flow of the ore keeps it open until the loader, having reinserted the stop pin, strikes the under side of the projecting open gate depressing the inside half of the door until the flow of ore catches it and swings it shut against the pin.
Arc gates are of two principal different types—overcut and undercut.
Overcut arc gates
Overcut arc gates work best with fine or medium-size dry material. With coarse ore, chunks may lodge under the gate and permit fines to pass because of its failure to close tightly. They are employed extensively, however, and are operated by hand, as at B (fig. 71), or by compressed air, as at C. The one at B is not installed as it should be for greatest ease in opening and closing, however. The overcut gate can be operated more easily by hand if the bottom edge of the arc, when in contact with the bottom of the chute, is slightly above the axis of rotation.
Undercut arc gates
Undercut arc gates are especially adapted to handling large quantities of coarse, hard material. They can be opened and closed quickly, and closing is not hindered by the lodging of chunks under the edge. Figure 72, A, shows an undercut arc counterweighted to reduce the effort required for hand operation. Two loaders and one locomotive engineer regularly loaded, trammed, and dumped 300 tons of coarse rock from five chutes of this type in 2½ hours. With this type of gate stop, logs are essential to hold back the column of ore. Figure 72, B, illustrates a more elaborate gate operated by compressed-air cylinders. One cylinder operates the undercut gate, and two larger cylinders, one on each side of the chute, operate a check gate (dotted fines). The check gate is of heavy steel construction and is hung on a shaft set between the side posts of the chute. This arrangement was adopted at the Alaska-Juneau mine, where large tonnages were handled through each chute, the life of each installation was long, and the expense was therefore justified. Other forms of undercut gate are employed; in general, those of the arc type are suitable for rapid drawing of large tonnages of chunky ore.
Finger gates (fig. 73) consist of a number of arms or fingers separately suspended on a horizontal shaft above the chute opening, each finger being free to move independently of the others. The simplest form is shown at A and is operated by a hand fine or by a hand winch. The fingers are pivoted at the knuckle, and the weight of the long (horizontal) arm closes the short arm against the ore. A pull on the rope serves to open all the fingers, but if a lump lodges behind one or more of them they may be pried up separately with a bar, releasing the lump without disturbing the others. A board may be inserted behind the lip angle irons to hold back fine material. A finger gate actuated by an air jack is shown at B (fig. 73). It may be noted that the gate shown at A requires considerable head room and that at B requires room on the opposite side of the drift.
Finger chutes are suitable for handling coarse, blocky ore, and their chief advantage lies in the means afforded for removing large chunks, one at a time (by lifting one or more fingers separately), without causing a run of muck. They are relatively high in cost, cumbersome, and require considerable room for installation.
Curtain or flexible-door gate
The curtain or flexible-door gate is illustrated diagrammatically in figure 74. This is known as the Cramp chain gate and is named for the inventor, D. L. Cramp, mechanical superintendent of the Lake Shore mine, Kirkland Lake, Ontario. It is understood to be protected by patents. It has proved its adaptability to loading either fine or chunky ore, or both together, requires no extra room for its installation, has a long life, and does not require frequent repairs. The balls and chains can be taken off readily and reinstalled quickly on another chute. They are not injured by chunks that may fall against them, because of their flexibility, and for the same reason chunks can be blasted in the chute without damage. There is ample space between the chains for inserting a bar. To open, the balls, which are attached to a yoke, may be raised by hand rope, hand winch, or air piston. Upon closing, the balls, by reason of their shape, fall into position alongside one another and prevent fine material from passing. The balls may be pried up, one at a time, for releasing chunks, as with the finger chutes. A rather flat chute bottom is desirable, although angles of 45° have been satisfactory with some types of rock.
Cost of Installation of loading chutes
Table 31 gives typical costs of building and installing loading chutes. The costs shown cover single chutes and are to be considered for the most part as approximate and not directly comparable, since they were obtained from various sources without reference to any standard form for segregation of cost items. Thus, in some cases cost of cutting out ground for timbers was included, whereas in others this item may have been omitted.