Text 2. Placer mining вy excavating machinery
Many alluvial deposits, which had been considered too poor to be worked by hydraulics or dredging, or even by sluicing methods, have now been rendered workable by the use of excavating machinery, employing portable washing plants. Excavating machinery employed usually consists of dragline excavators, power shovels, cableways, power scrapers.
Generally speaking, of the four types of excavating machinery enumerated above, the dragline excavator is most widely used and known, and will therefore be described in some detail with reference to its use in alluvial mining.
The dragline is practically identical with the mechanical shovel as far as base, revolving frame, and main machinery units are concerned, but the digging part of the machine is entirely different. The job of the machine is much longer than the jib of the shovel and is also usually held inclined at a smaller angle to the horizontal. A heavy digging bucket is suspended on a steel rope running over a sheave at the point of the jib; a second steel rope, known as the "drag-rope", is connected with this bucket by means of chains and shackles, and the motive parts of the machine are arranged so that the main engine power can be used for either pulling the drag-rope, which enters the machine through a fairlead suspended at about the junction of the jib with the front end of the machine, or alternatively, it can hoist the bucket by pulling on the rope passing over the jib point (fig. 1).
Two further functions of the motive power of the machine are to propel the machine, which is usually mounted on caterpillar tracks, giving it the ability to negotiate difficult ground or steep slopes, and, secondly, to rotate the upper part of the machine with respect to its base.
The bucket, which is provided with digging teeth, is so shaped and attached to the hoist and drag-ropes that, when it is lowered and a pull is applied to the drag-rope, a powerful digging action is exerted by the teeth of the bucket so that it fills as it is dragged towards the machine. When the bucket is full the hoist rope is actuated, while still retaining a pressure on the drag-горе. This raises the bucket clear of the excavation and the machine then swings to the point where it is to dispose of the material, which is then dumped by releasing the pull on the drag-rope.
Expressed roughly, the machine has the ability to dig from a point rather farther away from its centre line than the length of its jib and to dump the material dug in any direction from its centre line at a radius up to this length. The machine can dig up to a depth which varies with the material, the nature of the out, and the length of the jib.
Fig.1. Ruston-Bycyrus dragline excavator
It is the long working and dumping reach of the dragline which makes it an instrument of such potential value for mining alluvial deposits, and when it is recognized that the machine covers all the ground which lies in the circle described by the point of its jib, for the purpose of either digging or dumping, the value of using as long a jib as possible becomes apparent, since the area will vary as the square of the jib length. The dragline has the advantage of standing at work on the original level ground, backing away from the excavation dug behind it and to its sides.
The method of mining given below allows the simultaneous mining of overburden and pay gravel, that is one bucket full of overburden can be dug and rejected, the next bucket being in the pay gravel and loaded into the hopper. Almost any property will have a definite downstream boundary, and fairly well-defined side boundaries can be drawn, which determine the limits between payable and unpayable ground. A commencement is made at the downstream boundary of the property, and provision must be made to dispose of the tailings from the first series of cuts across the property. This usually can be done by leading them into the stream bed below the property, and in this connection the tailings that will have to be disposed either below or on the downstream boundary are only that proportion of the pay gravel that will wash down the launder, which is contained in the first series of cuts across the property. These cuts will be about 100 to 150 feet long, according to the size of the machine.
The overburden and stones will remain in the out, and extra ground will not be required to dispose of this material downstream. If it is not possible to lead the tailings off the property downstream, it will usually be found that there is either a piece of unpayable ground adjacent, on to which they can be led, or else a small piece at the downstream end of the property would be sacrificed temporarily for a tailings dump. For the purpose of the drawings reproduced here, it has been assumed that there is ground available at the bottom end of the property and for the first series of cuts off the property, a small cuts bund 3 or 4 feet high has first been dug by the machine, the width of the property, and the tail end of the launder resting on this bank and discharging just over it.
A first cut, as shown in fig. 2, is commenced on the downstream boundary, with its side limit resting оn the boundary line between payable and unpayable ground, previously discovered by prospection. The sketches have been drawn to represent the extreme and most difficult conditions which a ½ yard machine with a 10-foot jib can handle, and these consist of digging ground to a total depth of 20 feet with a maximum content of 13 feet of overburden and stones rejected. This machine can handle suitable ground to any less depth and proportion of overburden, stones, and pay ground than the figures just named. For the ground named, the 10 - feet jib machine can make a cut approximately 100 feet long and 30 feet wide with steep slopes, while remaining at a constant dumping distance from the hopper. This is achieved by moving the machine on its caterpillars on a path shaped as an arc of a circle, having its centre in the middle of the hopper.