Text 3. Power shovels
Power shovels are used extensively for alluvial mining, but generally speaking they do not compare favorably with dragline excavators, owing to the fact that usually a haulage system has to be employed in conjunction with them, which adds to the cost. For instance, the shovel is employed to excavate the material, which is then loaded into trucks for transport to the washing plant, a method which involves the use of trucks, rails, and perhaps locomotives, all of which increase the expenditure, and necessitate the working of rich alluvial ground, to stand the extra cost per yard. It is possible for the shovel to deliver the excavated material to a storage bin, but this entails additional headroom, which is not always available and increases the first cost. However, where rich deposits occur on hillsides, where the water supply is inadequate, mechanical shovels used in conjunction with a haulage system is an economical method to employ.
Gravel banks, for shovel mining, should be at least 15 feet high, as otherwise frequent moves of the shovel will be required, which leads to costly delays.
The base, revolving frame, and main machinery units of the shovel are similar to those of the dragline excavator, but the digging unit is entirely different, and consists of a jib or boom, which rotates horizontally, carrying the dipper or bucket and its arm, supported by a derrick, which moves in a vertical plane; by a special device, such as rack and head and tail ropes, the action being such that whilst one drum is winding in its rope, the other is free and is rotated by the pull of the paying out rope. The hoists themselves may be driven by electric or compressed-air motors or by oil engines, which range in power from 10 to 50 horsepower, depending on conditions (fig. 6, 7).
In commencing operations, is advisable to start at the downstream boundary of the property, provision being made for the proper disposal of tailings from the sluice boxes or washing plant. When stripping overburden the scraper equipment is so arranged that in making the first cut the overburden is dumped well beyond the pay boundary, thus ensuring that no payable ground is covered over, and for subsequent cuts the scraping plant can be arranged so that the overburden is disposed of in the previously worked out ground. When excavating gravel, the scraper may be hauled up an incline to dump its contents over a grizzly into a hopper, which feeds the sluice boxes or washing plant, or it
Fig. 6. Ruston steam shovel with 3 ½ cu. yd. bucket
Fig. 7. Types screpers
may discharge the excavated material into trucks, for transport to the treatment plant; however, as before, haulage of excavated material should be avoided as far as possible, as the plant required adds to the cost.
The scraper may be hauled up an incline to dump its contents over a grizzly into a hopper, which feeds the sluice boxes or washing plant, or it may discharge the excavated material into trucks, for transport to the treatment plant; however, as before, haulage of excavated material should be avoided as far as possible, as the plant required adds to the cost.
After a paddock has been worked by means of a scraping plant, it is usually necessary to make a final clean up of the bedrock by manual labor. Fig. 8-10 show a lay-out for a scraper installation.
Fig. 8. Lay-out for scraper installation, using double-drum type Holman Scrapehauler
Fig. 9. Lay-out for scraper installation, using «Tandem» dolman Scraphauler
Fig. 10. Typical three-drum lay-out for alluvial deposit
A series of sheave anchorages are fixed at convenient points on the opposite side of the deposit to the Scrapehauler, and the scraper drawn backwards and forwards by the tail rope, which passes through a sheave on one of the anchorages; on its forward trip, the scraper digs into the gravel and the load thus obtained is hauled to the discharge terminal and dumped into hopper or into trucks, as the case may be. When bedrock is reached, the path of the scraper is altered by moving the tail sheave a few feet farther along towards the next anchorage, and this process is repeated until the whole section has been worked out, when a move is made to the next site, where the above operations are repeated.
Fig. 9 shows another lay-out for a scraper installation, employing a "Tandem" Scrapehauler instead of the double-drum type.
When the deposit to be scraped is extensive and the depth to bedrock not great, the delays necessitated by moving the tail sheave may reduce yardage and increase cost; to over- come these drawbacks, three-drum scraping is employed. In this system the position of the tail sheave is controlled by the operator of the Scrapehauler. Fig. 10 shows a typical lay-out for three-drum scraping.
At the corners farthest away from the Scrapehauler, two posts are placed in position and suitably anchored; to the right-hand post a single rope sheave is attached, whilst the post on the left carries two separate sheaves or else on double-pulley sheave. Three ropes lead off from the drums of the Scrapehauler - viz., (I) the head rope direct to the bridle at the front of the scraper, (2) the tail rope, over one of the pulleys of the double sheave, through the travelling sheave to the chains at the back of the scraper, and (3) the third rope, over the second pulley of the double sheave, through the travelling sheave, around the pulley of the sheave attached to the right-hand post, and back to the travelling sheave, to the frame of which it is fixed.
To change the path of the scraper from left to right, is only necessary to release the brake holding the third rope drum, and engage the clutch to wind in the rope, which causes the travelling sheave to move to the right; when the desired position is obtained, the clutch is disengaged and the brake re-applied; when the opposite effect is required - i.e., to change the path from right to left - the tail rope is wound in, the clutch and brake of the third rope drum being disengaged, thus causing the travelling sheave to shift to the left. At the desired point, the clutch of the tail rope is disengaged and the brake of the third rope drum applied.