Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
1. GENERAL CONSIDERATIONS
Equipment used for the recovery of placer gold has changed very little over the years and, in general, remains relatively simple. Most devices employ some form of riffled surface to hold the gold or other heavy mineral after it has been separated from the valueless material. The actual separation relies on the ability of heavy minerals to resist the action of moving water while the lighter materials are carried away. In dry washers where a current of air is used as the transporting medium, the same principle applies. Although many have tried, no one to date has devised a gold-saving device or system which can economically replace the ordinary riffled sluices or placer jigs used on today's dredges or in other comparable large-scale placer operations. It is true that sluices may lose some of the fine gold but this is normally offset by their low operating cost and their high unit capacity which combine to return the greatest dollar profit.
When selecting a machine for washing and concentrating placer samples, the first consideration should. be whether or not it will indicate the commercially recoverable gold content of the sample. Other desirable features would be:
- Low first cost.
- Easy to maintain or repair in the field. Easy to transport and set up for operation.
- Will accept bank-run material without pre-screening.
- Will thoroughly wash and reject the oversize material.
- Makes efficient use of water.
- Will efficiently process small as well as large samples.
- Will effectively reduce the sample to a small-volume residue or concentrate.
- Can be quickly and completely cleaned between samples.
- Is time-tested and accepted by knowledgeable mine operators or engineers familiar with placer sampling requirements.
It should again be stressed that no dredge or other large-scale placer equipment saves 100 percent of the values and because of this it is important that the sample washing process indicate the actual returns to be expected from a commercial operation. In this connection it is noteworthy that the pan, rocker, and the sluice when used by experienced placer operators fulfill this requirement.
2. MINERS' PAN
The miners' pan, better known as a gold pan, is perhaps the most widely used device for washing small placer samples and from the standpoint of simplicity it has no peer. A separate article describing its use and manipulation will be found in Part V under the heading of "PANNING." In the hands of an expert the pan is both a versatile and a highly efficient concentrator well suited to washing small amounts of gravel but where individual samples weigh more than about 30 pounds, or where a large number are to be washed, something which provides a greater throughput with less expenditure of time and labor is needed.
3. SLUICE BOX
Another widely used sample washing device is the ordinary sluice box which in its smaller form is sometimes loosely but erroneously called a "long tom". A sluice in its simplest form is no more than an elongated, rectangular trough fitted with transverse cleats or some other form of riffled bottom. It is operated by essentially allowing a stream of water to carry the sands and gravel over the riffles which, in turn, detain any gold or heavy minerals as they settle to the bottom. Small sluices of the type used for sampling are commonly 8 or 12 inches wide with 6 or 8-inch sides and are usually 6 to 12 feet in length. Construction details and the materials used are largely a matter of personal choice, but the simpler ones are no more than an open-ended trough made of planed 1-inch lumber and provided with cross bracing where needed. Some samplers prefer a sluice made of heavy-gauge sheet metal (made rigid by bracing) and others prefer exterior-grade plywood painted with marine varnish or spar. The longer sluices are usually sectionalized to facilitate transportation.
The primary function of a riffle is to retard heavy minerals giving them a chance to settle and, at the same time, to provide pockets in which the values are retained. A well designed and properly working riffle will create eddies along its downstream edges and the so-called "boiling" action in these eddies will do two things. First, it will prevent packing of the black sand or other material caught by the riffle, and, second, it will provide a classifying effect which further concentrates the gold and heavy minerals. While this boiling action must be strong enough to prevent packing it must not be so strong that flake or fine gold cannot settle. It should be obvious that proper riffle action is the key to good recovery. An easily built and effective riffle suitable for use in a small sampling sluice can be made from 1/2" x 1/2" wood strips placed across the bottom of the sluice at right angles to the flow, with 3/4" spaces between each riffle bar. The boiling action can be improved by undercutting the downstream face of each bar on a 30-degree bevel. The individual riffle bars are tacked to wooden side rails and the whole assembly held in place by means of cleats and wedges as shown in Figure 17. The riffle assemblies are made a littler narrower than the inside width of the sluice and of convenient length.
Heavy wire screen of the type used for screening gravel, and expanded metal lath are sometimes used as riffles in small-size sluices, particularly where much fine gold is present. This type of riffle is usually installed over burlap mats, cocoa matting, or similar rough-surfaced fabric which helps hold the fine gold. Because burlap and cocoa matting are difficult to clean, ordinary canvas should be substituted in sampling sluices. It will be noted that metal webs forming the diamond-shaped openings of expanded metal lath have a noticeable slope in one direction. When the expanded metal is placed in the sluice so this slope leans downstream, small eddies form beneath the over-hangs and make excellent gold catchers. Expanded metal riffles do not have a large holding capacity, that is, they may soon fill with concentrate, but this is seldom a problem in sampling work where close watch and frequent clean-ups should keep the riffles working efficiently. Hungarian-type riffles, such as those shown in Figure 17, can hold a comparatively large amount of concentrate and for this reason may be preferred where the gravel contains much black sand. On the other hand, expanded metal riffles leave a minimum amount of concentrate to be panned at the end of a sample run and this time-saving feature makes them easy to clean up. Many engineers compromise by equipping the upper 2 or 3 feet of a sampling sluice with Hungarian-type riffles and the remainder with expanded metal lath.
In commercial-scale placer operations, mercury is usually placed in the riffles to assist holding the gold over extended periods of time but in small-scale sampling work where clean-ups are frequent, mercury is not needed for this purpose and is seldom used.
It should be noted that when a particle of gold is "wetted" by mercury, the mercury actually penetrates the surface and causes the gold to become brittle. Depending on the size of the gold particle and length of exposure, the penetration may be superficial or complete. The ductility is not restored when the mercury is removed with acid and in the case of small gold particles, a delicate crystalline structure is often induced. It can be seen that amalgamation within the sampling sluice would either impair or prevent a later study of particle size, the surface texture, or other physical characteristics of the gold, any of which could prove important in a placer investigation.
The feed and water flow should be regulated to maintain proper riffle action and it can be said generally that where this is not done more fine gold will be lost by its inability to penetrate packed riffles, than will be carried off by suspension in the water. The quantity of water required varies considerably according to the character of material being washed and the rate of feed, the type of riffle, width and grade of sluice, etc. Because of these variables the water requirement is difficult to predict but as a rough guide, a minimum flow of about 50 gallons per minute should be provided for a 6 or 8-inch sampling sluice. Unlike panning or rocking, the water is seldom reclaimed or reused and, for this reason, the water requirement for a typical sampling operation can easily be several thousand gallons per day. It should be apparent that the sluice is best suited to testing stream placers. Supplemental equipment is often needed. This may be a water pump, a puddling box or tub, or some kind of screening arrangement. Because of the relative short length of the usual sampling sluice, cemented gravel or gravel containing much clay must be thoroughly broken up in a puddling box or tub before being fed into the sluice. Screening out the plus 1/2-inch rocks ahead of the sluice will conserve water and will generally improve the entire operation.
The slope or grade of a sluice depends on a number of factors including rock size and shape, the amount and composition of sand, type of riffle and depth of water flow. In each case the proper grade must be determined by trial and this can be best done by initially setting the sluice on a grade of about 1 inch fall per foot of length and then adjusting the grade as found necessary. When in doubt it is better to have the grade too steep than too flat, as too flat a grade will result in sanding of the riffles which in turn will impair their ability to recover fine gold. Where fine gold is to be saved the depth of flow over the riffles should be as shallow as possible while still sufficient to carry off the pebbles and maintain a loose bed between the riffles. To do this the sluice grade is steepened and it can be said generally that the grade for very fine gold should be steeper than for coarser colors. Increasing the grade will, to a point, offset a deficient water supply but, in any case, the riffle action should tell the operator when the proper balance has been reached.
The daily capacity for a sampling sluice varies widely with the type of gravel, degree of cementation, amount of clay, etc. These factors individually and collectively determine the amount of material that a man can prepare for washing in a day's time and, in many cases, the preparation time will exceed the actual washing time. Under favorable conditions with an efficient sluice set-up, two men can handle 1 to 3 cubic yards per day taking into account time needed for clean-ups between samples, processing the sluice concentrate, logging sample data, etc.
When the mineral examiner washes a placer sample in a small sluice box and fails to find the amount of gold anticipated by the property owner or vendor, it is sometimes argued that he failed to recover the hoped-for value because his sluice was too small or the sample was put through too fast. While it is true that a sluice box crowded beyond its optimum capacity will lose some of the gold, a quick look at the facts will usually show that there is little or no room for argument in most cases.
For example, modern gold dredges provide about 10 square feet of riffle area for each hourly yard of material passing over the gold-saving tables. By direct comparison it can be shown that it would be necessary to feed an 8-inch x 10-foot sampling sluice at a rate of 1 1/2 cubic yards per hour to attain this degree of riffle loading. But experience tells us that the rate of feed for a sampling sluice of this size is more likely to be on the order of 1 1/2 cubic yards per day rather than per hour. It can be seen that in a sampling sluice the riffle area provided for each unit of sample material is considerably greater than the riffle area provided in standard mining practice. In other words, the small sampling sluice usually favors the sample. This is borne out by long experience which shows that a properly constructed and carefully operated sampling sluice will save all of the gold or other heavy minerals which can be economically recovered by standard placer methods.
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