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 ½” x ½” wood strips placed across the bottom of the sluice at right angles to the flow, with ¾” 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 little 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 overhangs 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 ½-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 1½ 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 1½ 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.
AN ABSOLUTE MUST FOR THE BEGINNER
The sluice box is an absolute must for the beginner. It is a simple, yet effective, tool which allows the operator to process a great deal more material than would ever be possible by simply using a gold pan. Our Proline sluice boxes are built with the same quality as found in our powered equipment. We use only the highest quality aluminum and all steel parts are plated for corrosion resistance. Ribbed matting is installed at the front of the sluice and aids in quick inspection for gold.
Perhaps the most unique feature found on our sluices is that the expanded wire is inset onto the bottom of the riffles, and is welded in place, making cleanup much easier. These sluices are available in small, medium and large. The medium and large are equipped with large handles for easy.
Basically a sluice box in it’s simplest form is a man-made creek ted. In the writer’s opinion it falls short of a creek bed in the following ways. It does not have the natural swirls, eddies, drops, shallows, turbulent areas, calm areas, change of flow nor length – all of which will drop various types, sizes and shapes of gold during all the millions of combinations possible with the aforementioned factors. Any system with specific gravity as its deciding factor tries to create a laminar flow in order to concentrate the denser particles. In a sluice box, as in most gravity systems, removal of oversize particles is essential to recovery of the finer fractions of valuable product for laminar separation is impossible with much oversize in the gangue. Now what happens to particles entering the system?
- The large and dense particles (nuggets) have the ability to physically displace smaller particles and larger less dense particles so they drop out almost immediately.
- The smaller but still very dense particles also drop out very quickly – but generally a little further down the box.
- While this has been happening the total gangue and water are picking up speed. How many times have you heard the old saw “We know we are catching all the gold because after the first five feet there is practically no gold in the sluice box”?
- As the speed increases, all laminar flow is destroyed by the accompanying turbulence.
- At same point speed and turbulence combined stops all concentration and everything continues through the box. At this point it is no longer a sluice box, it is a transfer box.
- Any particles, whether they are gold or just rock, that do lodge in a sluice box usually very quickly become imbedded with black sand and none but the larger particles of gold can penetrate this non-fluidized bed. A simple rule with almost any 1 g device is that if you are losing black sand, you are losing fine gold.
- The foregoing are the reasons that virtually 97-98% of all gold caught in a sluice is caught in the first 3-5 feet.
Critical to this whole separation system is the shape of the gold particles. As a general role it runs this way.
The larger the particle-The easier to catch The lesser the aspect ratio-The easier to catch The smaller the particle-The harder to catch The greater the aspect ratio-The harder to catch See Figure 1.
If you have large particles with small aspect ratio, you have to work to lose them. If you have small particles with large aspect ratios, you’d better have a good device to catch them and that does not include a sluice box.
It is the writer’s opinion that within a very few minutes of operational start up a sluice box is decreasing its efficiency and shortly thereafter starts to select and retain only on the basis of size (nugget). The greater the black sand content the quicker this happens. If the black sand is accompanied by large amounts of clay, impacting and blinding occurs, which accelerates this loss of efficiency.
The start of decrease can be as little as 10 minutes and again it is the writer’s opinion that the retention of fine gold is almost non-existent in most sluice boxes after four hours and I am being generous at that.