A sluice is generally defined as an artificial channel through which flows controlled amounts of water. In gold placering, the sluice includes sluice-boxes which collect the gold by means of various configurations of riffles, corrugations, mats, expanded metal, or the like, which trap the heavier particles while allowing the waste to continue through.
An important part of any sluicing operation is its water supply, and where water is not plentiful, pumps, pipelines, or even dams with special head-gates may be required.
Small-scale sluicing by hand methods has been called quite appropriately shovelling -into-boxes. In contrast, in ground sluicing, usually a more efficient operation, most of the excavation is accomplished by the action of water flowing openly over the materials to be mined. In either case, the materials pass through a sluice, where gold is collected behind riffles. A variation of the sluicing technique, where water is stored and released against or across the materials intermittently, is called booming.
The sluice-box in its simplest form might be a 12-foot-long plank of 1- by 2-inch pine lumber, to which sides about 10 to 12 inches high are nailed, with braces secured at several places across the top. Larger sluices can be made with battens to cover joints between boards where gold might slip out, and with braces built around the outsides of the box for greater rigidity. To provide for a series of boxes, the ends should be bevelled or the units tapered so that one will slip into the other in descending order and form a tight joint. Four to eight such boxes in series would be a typical installation. Two men hand-shovelling into sluice-boxes can wash 5 to 10 times as much gravel as could be put through a rocker in a day. The slope of the sluice and the supply of water must be adjusted so that the gravel, including larger cobbles, will keep moving through the boxes and on out. Slopes of 4 to 12 inches per 12-foot box are normal, but if water is in short supply the slope may be increased. Trestles are necessary to support the boxes over excavated ground, galleys, or swales.
Inside the boxes, various kinds of riffles may be employed, depending upon availability of material and personal preference. The riffles, which go on the bottom, are usually set crosswise in the box, but they can also be effective when placed lengthwise, the concentrates settling between them. They may be of wood, or of strap or angle iron, or a combination of the two. Straight, round poles or a pattern of square blocks or stones can serve for riffles. Rubber or plastic strips have even been used. Durability is important for prolonged operations, so wood may be armoured with metal. Expanded metal, heavy wire screen, or cocoa mats make good riffles for collecting fine gold.
A common height for riffles is 1-1/2 inches; they may be placed from one-half to several inches apart. Fastening the riffles to a rack, which is then wedged into place in the box, permits their removal. A tapered shape on the cross riffle, with the thinnest edge to the bottom, tends to create an eddying action that is favourable for concentration. Another way to achieve this eddying action is to cant the riffle or even just the top of the riffle. Burlap or blanket material is commonly placed under the riffles to help in collecting fine gold.
Sluice cleanups should be made at fairly regular intervals. After running clear water until the sluice is free of gravel, riffles are removed in sections starting at the upper end. With a thin stream of water, the lighter of the remaining material is washed to the sections below. Mercury may be used to amalgam fine gold from concentrate, but care must be taken to prevent escape of the mercury. The gold, heavy sands, and amalgam, if mercury has been used, can be panned or cleaned up in a rocker to obtain a final concentrate or amalgam.
Feeding the Sluice
It is common in a small operation, when feeding the sluice, to place a heavy screen or closely spaced bars of some sort across the section where the gravels enter, to eliminate the larger particles, which are probably barren anyway. The screen or bars (a "grizzly") should be sloped so the oversize material rolls off to the side. The size of mesh or spacing will depend upon the gradation of feed, but would generally be in the range of 1/4 to 1 inch, with 3/8 inch being a common size. In larger operations a rotating screen, or trommel, might be used. In a ground sluicing operation, possibly all materials would be run through the sluice-boxes. Provisions must be made for removing the oversize material, and, if required, stacking it away from the work area.
If the gravel contains much clay it may be desirable to use a puddling box at the head of the string of sluice-boxes. This may be any convenient size--for instance, 3 feet wide by 6 feet long, with 6- to 8-inch sides. The clayey material is shovelled into this box and broken up with a hoe or rake before being allowed to pass into the sluice. The importance of this step is that if allowed through the sluice, the unbroken clay lumps may pick up and carry away gold particles already deposited.
Usually, the shovelling -in method proceeds as follows: After the boxes are set, shovelling begins at an advantageous point. Experienced miners work out the ground in regular cuts and in an orderly fashion. Enough faces are provided so that shovellers will not interfere with one another. Provision is made to keep bedrock drained, and boulders and stumps are moved a minimum number of times. Cuts are taken of such a width and length that shovelling is made as easy as possible. The boxes are kept as low as possible so a minimum lift of gravel is necessary. At the same time an adequate slope must be maintained for the gravel to run through the boxes under the limitations of the available water. Allowance for dump room must also be provided at the tail end of the sluice. Leaks in the sluice are stopped promptly, and shovelling is done in such a manner that the sluice does not become clogged nor does water splash out (Water in the pit hampers shovelling.)
All material of a size that will run through the sluice is shovelled in, and the oversize material is thrown to one side. Boulders from the first cut should be stacked outside the pit, on barren ground if possible. The width of a cut is usually limited to the distance a man can shovel in one operation. When shovelling from more than several feet away, it is best to set boards above and on the opposite side of the box; this increases the efficiency of the shovellers. The greatest height a man can shovel into a box is 7 to 8 feet, and above 5 or 6 feet the efficiency of the shoveller is markedly reduced. If the gravel is over 3 or 4 feet deep, it usually is excavated in benches to facilitate digging and to permit the upper layers to be raised a minimum shovelling height. Where the gravel is shallow, wheelbarrows may be used. Another way is to shovel the gravel onto a conveyor belt that discharges into a trommel, discarding the oversize material and running the undersize material through the sluice. Where two or more persons are working in the same cut, the height of succeeding benches is governed by the character of the material being dug and the distance the gravel has to be lifted.
The sluice may be maintained on the surface of unworked ground or supported on bents on the opposite side of the cut. After the first cut the boulders are thrown onto the cleaned-up bedrock. Where cuts are run on both sides of the sluice, the boxes are supported on bents as the ground underneath them is dug out. At other places the boxes may be set on bedrock and the dirt may be shovelled into the head of the sluice from short transverse cuts at the upper end of the pit. Work usually begins at the lower end of a deposit so that bedrock may be kept drained, and then proceeds across the deposit by regular cuts. The length and order of the cuts will depend upon local conditions. As heavy sands and gravel build up deposits between the riffles in the sluice, it may be necessary to stir these up to prevent packing and the consequent override of gold particles. A tined implement such as a pitchfork is often convenient for this. Larger stones that lodge in the sluiceway may be similarly removed.
The quantity of water available will influence the scale of operations and the size of sluice used. A minimum flow of 15 to 20 miner's inches (170 to 225 gallons per minute) is required for a 12-inch-wide sluice-box with a steep grade. Smaller flows than this can be utilized by storing the water in some kind of reservoir and using the supply intermittently. A common practice followed where the quantity of water is limited is to use a grizzly or screen over the sluice to eliminate oversize material and thus increase the duty of the water. Reduction in the amount of material to be treated by first running it through a trommel to wash and screen out the coarse size is another effective way to lower the water requirements.
Water usually is conducted via ditch to the sluice. However, if the ground is rich enough it may be practicable to pump water for the sluice. The feasibility of obtaining a gravity flow should first be investigated, as the expense of pumping may be more than the cost of a long ditch, when the cost is distributed over the yardage of gravel moved. A suitable number of sluice-boxes or some other removal system may be used to transport the tailings to a dumping ground away from the working area. A tailings or settling pond may be required to maintain downstream water quality.
Ground sluicing utilizes the cascading effect of water to break down the gravel; hence, the requirements for water are much greater. The chief application of ground sluicing is to streambed deposits. Pipelines, flumes, or ditches would be necessary if ground sluicing were applied to gravels higher up on banks or terraces, and the larger scale hydraulic methods would then become more favourable. If booming is to be done, a dam and reservoir are needed. The dam is usually equipped with a gate mechanism that permits either automatic or manual control and quick release of the impounded water for maximum washing effect. The water may be passed over the upper face of a gravel bank or diverted against the bottom in order to undercut and carry away the gravel as the face of the bank breaks down. All materials are channelled toward the sluice.
The natural flow of a stream can be used by diverting the current with boards or simply with piled boulders. "Shears" can be constructed of 1- or 2-inch-thick boards 12 feet long nailed to pairs of tripods so that the boards slope back from the water flow at an angle of about 60 degrees. The tripods are built in such a way that boulders can be piled inside the base to hold them in place. A row of these shears may be used to divert the force of the water against a bank, or two rows may be used to form a flume.
The seasonal nature of stream flow in different areas must be kept in mind when planning any placer operation. State and Federal agencies can provide information on stream runoff for many of the more important streams, information which will indicate the limitations in water supply that might be expected due to seasonal changes.
From: Bureau of Mines Information Circular 8517 by J.M. West, released in 1971.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.