The centrifugal elevator invented and patented by the writer is designed for the purpose of conveying the tailings of a gold dredge to such a distance that they shall not impede the dredging operations. Formerly this was usually accomplished by means of a belt of buckets travelling round a ladder set at an angle of about 30 degrees to the horizontal, but this meant heavy wear and tear on the travelling buckets and the tumblers actuating them. The centrifugal elevator throws the tailings through the air to the required height and distance, and it is manifest that: during their flight from the elevator to the heap there is absolutely no wear and tear, only the attraction of gravity and the resistance of the air has to be overcome. All, then, there is left to be considered is how to throw the stuff with the minimum of power and wear on the elevator.
The elevator consists of a cam-shaped steel casting with a disc of mild steel plate on either side. These side plates project a few inches beyond the tips of the beaters, in order to prevent odd stones from straying in an undesired direction ; they serve also to prevent extra large stones from reaching the beaters, for they slide off the rims into the water. The two beater
surfaces are faced with renewable strips of hard, tough steel; the ends of these strips project through slots in the side discs and are secured with bolts to angle steels rivetted to the outside surface of the plates. Care must be taken that the material to be projected is distributed as evenly as possible over the whole surface of the beaters—not only over the breadth, but also over the depth—for if the stuff receive the blow from the metallic surface, it will be thrown more efficiently than if it is hit second-hand, as it were, through the stone and silt behind it. To attain this clean blow it is requisite to consider the speed of the elevator, generally 240 revolutions per minute, i.e., 4 per second; there being two beater surfaces, this means an interval of of a second between each beater, so the tailings must fall on to the elevator with a speed of a little less than the depth of the beater per eighth of a second. If this speed be exceeded some of the stuff will strike the cam boss and not be projected. The beater surfaces are radial, less power being required than if they were not so, also the wear is less, as the stuff does not drag back over the surface. The position of the feed chute should be such that the delivery end overlaps the tip of the beater by about two inches ; when the latter is horizontal the chute should be set at an angle of about 50 degrees from the horizontal. The resistance offered by the air to the revolving drum is slight, for the air between the discs is imprisoned, and revolves with the drum.
To arrive at the weight elevated per indicated horse power (neglecting friction and air resistance), divide 33,000 by the height in feet it is required to stack, and the result is the lbs. weight delivered per minute to the stack. If the tailings do not contain much more than thirty per cent, of silt, the height to which the elevator will stack them can be ascertained by the laws of falling bodies, thus:
Velocity in feet per second of beater tips/32 = Time in seconds a body would have to fall to acquire the velocity.
Then height of stack = 16 x (time in seconds)².
The above gives the height very accurately when the major part of the material is stone, but with sand it is evident the result will not be so good, and it would not be safe to guarantee a height with fine silt of more than one-half of that height. If an excessive quantity of water is allowed to fall along with the sand and stone on to the elevator, it not only increases the power required in the proportion its weight bears to the rest of the stuff, but it also forms a spray in front of the machine which the sand cannot travel freely through.
Regarding cost of upkeep, the average is about ten shillings per week, being confined to the replacement of beater bars. The machine is frequently driven by a small separate engine, but it is more satisfactory to drive with the main engine of the dredge, as the latter is usually a compound engine and consequently more economical. When driving from the main engine, which, when the dredge is in stiff ground, is liable to draw up at times, a driving pulley connected to the drum shaft after the style of a free wheel should be used, in order to allow the elevator to revolve when these reductions in speed occur.
The advantages of the centrifugal as compared with the ladder elevator are:
- Reduced first cost and upkeep, and longer life.
- Smaller pontoons, as the biggest centrifugal yet made only weighed 6 tons, and was for dredge with 7 ft. buckets, and elevated 60 feet.
- Stoppages for elevator repairs entirely done away with, as new strips can be fitted during the stop for oiling round.
- Smaller driving power and consequent saving of fuel.