Text 2. Sluicing
Alluvial mining, in general, consists of the following stages:
clearing the area of bush, timber, etc and the excavation of drains, for drainage purposes;
excavation of overburden and gravel, and cleaning up of the bedrock;
transport of the gravel to the treatment plant;
treatment of the gravel, for the recovery of its mineral content;
the "clean-up".
These operations should proceed systematically and in a predetermined order, to avoid any possible break in the sequence of work, which might lead to loss of time, reduction of output, and a loss of efficiency in general.
For working shallow alluvial deposits, sluicing is the simplest and cheapest method to employ; sluice boxes are easily constructed, easily transported from place to place, give a fair output, and are simple in operation, considerations which are of importance in any country, where transport is a difficult problem, and where raw unskilled native labor only is available.
Sluice Boxes
Sluice boxes are made in 12 to 16 feet long sections, the former size being most common for ordinary sluicing, the side and bottom boards varying in thickness from 1to 1 1/2 inches by 12 to 16 inches wide, a width of 12 inches being considered the minimum that should be employed. Bottom boards are sawn 2 to 4 inches narrower at one end that the other, to enable series of sluice boxes to telescope one into the other (fig. 23).
Sluice boxes may be rigged up on the ground, in ditches, or elevated on trestles, but they must be so arranged that there is a slope or grade throughout the line of boxes. Sluices are usually braced by means of cross and side braces (fig. 24). In the construction of boxes, particular attention should be paid to the joint between the side and bottom boards; in the author’s opinion more fine gold is liable to be lost in a vertical joint (fig. 24, a) than in a horizontal joint (fig. 24, б).
Timber sluices have a short life, but they are cheap, in some districts steel sluices are used. A general rule is to construct the sluice boxes as simply as possible, as a complicated lay-out requires more time for rigging up, besides being more difficult of operation with unskilled labor. In the bottom of the sluice boxes riffles are placed at right angles to the major axis of the box to catch the heavy minerals in their descent down the box.
Briefly stated, the operation of sluicing is as follows: water for washing is brought to the head of the sluice, by means of a Jeat or flums, and gravel, which is dumped in, is well washed and puddled against the flow of water by means of a shovel or sluice fork, all large pebbles and stones being forked cut, when thoroughly clean, and discarded. The current of water carries the fine sands along to the discharge end of the box, where they are discharged as tailings, whilst the heavier minerals and sand are caught behind the riffles and deposited, when, after a certain period, they are cleaned up and washed for the recovery of the mineral content.
Fig. 23. Telescoping sluice boxes
Fig. 24. Joint between side and bottom boards
It should be mentioned that the sluice fork should always be employed to fork out large stones and pebbles from the sluice boxes, and that a shovel should never be used for this purpose, as the latter may scrape up material containing fine gold or other minerals, which would be discarded with the stones and pebbles, resulting in the loss of the mineral contents.
Grade of a Sluice Box
It depends on the quantity of water available for washing and on the character of the gravel to be treated, no fixed rule can be given for determining the grade at which sluice boxes have to be set, as this can only be determined by experiment. Generally speaking, for a given class of material, steep grades are necessary if the water supply is small, whereas the grade can be flattened if sufficient water is available; on the other hand, too steep a grade will result in very fine gold being carried away and lost, and too flat grade will cause the "packing" of sand behind the riffles in the sluice boxes, which will also result in the loss of gold, as it will have no chance of settling.
The grade of a sluice box is usually expressed in inches per12-foot box, unless otherwise specified; thus a grade of 6 inches means a slope of 6 inches per 12 feet. For box sluices the grade varies from 6 to 20 inches per 12-foot box, but for average conditions 6- to 8-inch grades appear to be satisfactory. However, as stated above, the selection of the right grade is a matter of experiment. When a line of boxes has been rigged up, pannings of the tailings being discharged over the end of the tailings sluice should be taken hourly, and should any trace of gold be visible in the pan the grade should be slightly flattened, and this procedure should be repeated until no gold is visible in the pan; the grade thus obtained will be the grade required for the particular material being sluiced.