Text 3. Hydraulic elevators

Hydraulic elevators as usually employed in connection with hydraulic mining. They are used in deep alluvial deposits, occurring in comparatively flat country and lying below the natural drainage level of the district where an adequate supply of pressure water is available and where dredging is not possible. They are also employed where the grade is not sufficient to use sluice boxes and to provide dumping room in placer mining, where the natural fall for the disposal of tailings is lacking. The gravel broken down by the giants, together with the spent giant water and seepage is delivered to the bottom the elevator by means of ground sluices or bedrock sluices, and is elevated to a sluice placed at a suitable height above ground level, so as to obtain the required grade and dump room.

By means of hydraulic elevators, gravel to a total depth of about 90 feet has been successfully worked, but a depth of about 25 feet is the most favorable; the ground to be worked should be relatively free from big boulders and tree stumps, as these not only hinder work but may require blasting and breaking up, which adds to the cost of operation.

Fig.41 shows the construction of a Heady hydraulic gravel elevator. The elevator utilizes the velocity of the ascending water, caused by the head or pressure, to lift or elevate the gravel and water; the water under pressure ascends through the nozzle and in passing through the throat, which has a restricted area, sucks up water and gravel from the intake or ground section, the whole of which is forced up the upraise or upcast pipe by the force of the jet into the elevated sluice box. The inclination of the pipe upraise varies from аs much as 40 to 90º, but is usually set at an inclination from 60 to 70º; the mouth of the elevator is placed in a sump excavated in bedrock, which should be at least 10 feet square by 5 feet deep, and into this the spoil is washed by the giants, although it is better practice to feed the material directly into the intake or ground section of the elevator, as this reduces the suction head, which should be kept аs low as possible

It is advisable to fit a grating or grizzly over the end of the sluice, which delivers the material to the ground section or intake of the elevator, and the bars of the grizzly should be so spaced that the distance between them is 1 inch less than the diameter of the throat. Liners reinforce the entrance section and the throat as these take the most severe wear.

Fig. 41. Hendy hydraulic gravel elevator

Water required. It is usually stated that the elevator nozzle water will lift 0,5 times its own volume of outside water, and thus half the supply of water under pressure is allotted to the elevator and the remaining half to the giants; the elevator should be large enough to lift all that the giants can excavate, and thus if 50% of the water supply is allotted, to the elevator and 50% to the giants, to obtain the greatest efficiency it is necessary to see that the parts of the elevator are correctly proportioned with regard to existing conditions - that is, the sizes of the nozzles of the elevator and the giants should he such that they discharge these amounts, in which case the elevator will lift all that the giants discharge (fig. 42).