PLACER RECOVERY by W.A. McCARTER

Dawson City,

Yukon

Presented at the D.I.A.N.D. - K.P.M.A. Placer Mining Short Course, Whitehorse, Yukon, April 20-22, 1982

The placer mining industry has recently been rejuvenated due to the increuse in the price of gold. The most common gold recovery system used is, however, the sluice box, which has not changed significantly for many years. This paper outlines the functions and limitations of sluice boxes, and describes other gravity concentrating equipment developed in the tin and mineral sands industries.

The Sluice Box

The sluice box has been in use for centuries. Nature demonstrates the simplest form of sluice box, in stream channels. Discontinuities in the stream bottom act as riffles, concentrating gold and other heavy minerals, and creating placer deposits.

Over the years, man has improved upon the stream with respect to the materials of manufacture, shape of channel, and method of riffling in sluice boxes, but the principles of operation remain the same. The classic sluice box is an open channel with a riffled bottom. Transverse angle iron riffles, as shown in Figure 1, are the most common.

Figure 1: Cross Section of riffles in sluice box. Yukon Placer Mining, 1978-82, p. 46-49

Expanded metal and some type of matting are usually installed below the riffles. The gravels to be processed are mixed with water at the head of the sluice, in what is commonly called the dump box. The resulting slurry then runs down the sluice and the gold, being heavier than the other minerals present, settles in between the riffles. Such a simple and robust system is appreciated by miners as it will run a long time without mechanical breakdown.

There are, however, problems, such as inherent downtime. The sluice box is a batch-type machine. It must be shut down periodically so that the heavy concentrates can be removed from the riffles for further processing. Depending upon the operation, this lost working time can amount to a high percentage of the available hours.

It is found in practice that the more often the concentrate is removed from the sluice, the more efficient is the recovery. Accordingly,the ideal concentration method implements continuous cleaning.

Even if a sluice were cleaned continuously, however, it would still not catch all the gold. When the gold grains are course and round, they are effectively concentrated, but when the gold grains are fine or flat, the recovery leaves much to be desired.

To understand why a sluice box loses gold, the method by which it works must be studied. Sediment transport in water is a field which has come under intense research. The results can be readily applied to the sluice box.

Placer gravels are made up of many different sizes of purticles varying in specific gravities. Particle sizes in standard Tyler screen mesh sizes, and mm are listed in Table I for reference.

Table I-. Particle sizes Mesh Aperture Mesh Aperture Wmber (mm) Mimber (mm)

The rate at which a particle falls through water is predicted by either Stoke's or Newton's Laws. Figure 2 shows that large particles of low specific gravity fall at the same rate as small particles of high specific gravity. The rate of fall of particles is also affected by their shapes. Flat grains fall more slowly than do spherical ones. Figure 3 illustrates the effects of shape on settling velocities of gold particles less than 1 mm (.04 inches) in diameter. Specific gravity, grain size, and grain shape are thus all important in determining the rates of fall of particles.

Figure 2: Chart showing settling velocities in water of gold, galena, pyrite, and quartz grains of various sizes and shapes (after Hague, 1940).

Water flowing smoothly and slowly has laminar flow. In effect,it has different layers or laminae of Water moving at different velocities built up upon each other. In the rate of fall of particles, it is as if the water were still. Any particle dropped into the water will eventually touch the bottom.

When the flow velocity of water is increased, the flow becomes turbulent. Eddies in the flow can hold small particles in suspension. As the flow velocity is further increased, the size of the particles which can be held in suspension also increases.

The high flow velocity of water used to move rocks through sluice boxes causes losses of gold. Some of the finest gold particles are carried by turbulence through the sluice box and into the tailings.

Part of the cause of this is that gold is hydrophobic.It is not wetted by water. If there are air bubbles present in the slurry, the gold will tend to adhere to them. If the gold is fine-grained enough, the air bubbles will float it to the surface where it will stay, and be carried out of the sluice box. Turbulent flow in the sluice box is therefore not totally desirable.

Figure #3: Chart Showing settling velocities in water of gold grains of various degrees of flatening, and quartz. (after T0urtelot, 1968)

Other problems arise when the flow velocity is decreased. The most obvious problem is the loss of the ability of the flow in the sluice box to transport large rocks. Either they must be removed by hand, or they must be screened off.

In addition, the gold must be able to penetrate the grains between the riffles in order to be caught. Turbulence in the flow stirs or "boils" the grains between the riffles, keeping them in a semi-fluid condition. If large grains are screened off, and the flow velocity reduced too much, the grains between the riffles begin to pack solid. When this occurs, the gold remains in the main stream of the sluice box, and is carried out the end.

Riffle scour is another problem caused by high flow velocities, as gold that has been caught in the riffles is brought by turbulence back into the main flow, from which it can be carried into the tailings.

This presents a dilemma. If the velocity of the water is increased, gold is lost to turbulence. If the velocity of the water is decreased, gold is lost due to rapid filling of the riffles.

Optimum recovery is achieved when the flow rate is just fast enough to keep the rif f les from packing. Classification of gravel prior to sluicing through the use of a grizzly, screens, or punch-plate is one way to optimize gold recovery. Other types of equipment which are more effective than the sluice box in the recovery of fine gold may also be used.

The Jig

In a jig, the optimum condition of having the flow velocity reduced to nil, while maintaining the fluid condition of material trapped by riffles is achieved. The pulsating action of the jig lifts the entire bed of particles off the screen surface. As the stroke reverses, the bed tends to fall bottom layer first, next layer next, and so on. This dilation of the bed effectively fluidizes it so that the high specific gravity particles can sink through it and be concentrated.

Figure 4: Cross-sectional diagram of a jig, showing the paths of particles of various densities.

The jig is a continuously cleaning device as small particles pass through the screen and are collected in the hutch. Nuggets are caught on top of the screen. Particles larger than 2 cm (.75 inches) have a high enough settling rate that they are not discarded to tailings, so they clog and reduce the capacity of the machine. Classification of the feed is necessary to keep a jig operating at optimal efficiency.

Although the jig, shown in Figure 4, is a better recovery mechanism than the sluice box, it is not perfect. Fine gold is lost due to the low residence time in the jig and the back flow of water from the hutch upward through the bed.

The Pinched Sluice

A high capacity concentrator that will catch fine grained gold is called the "pinched" sluice. It uses another concentrating approach to improve upon the sluice box.

Figure 5: Plan and Sectional diagrams of a pinched sluice, showing the separation of high density material from low and medium density material at the end of the sluice

Particles on the bed of a sluice box have a lifting force on them similar to areodynamic force. The velocity of water is higher at the top of the particle than at the bottom. This velocity shear creates a lower pressure area at the top of the particle that "lifts" it up. The particle is then ccirried downstream until it again hits bottom.

This process is called saltation. It is repeated over and over until the particle leaves the sluice box. Particles of high specific gravity have lower lifting force per unit mass and tend to stay close to the bottom, producing a layered effect.

If the pinched sluice has laminar flow, the layers become well defined, with virtually all the heavy minerals on the bottom.

When such a sluice is narrowed or "pinched" at the downstream end, the depth of the flow is increased making the layers easier to separate into concentrate and tailings.

Particles larger thcin 1.65 mm (.06 inches) tend to roll rather than saltate. This destroys the layering effect, making classification necessary to keep the layers stable.The pinched sluice must be operated with an even, dense feed. If this is done, very high recovery rates may be achieved.

Recovery Rates

Recovery rates of fine grained gold were determined Wang (1979) for a conventional sluice box, a jig, a shaking table (a fine gold recovery device not discussed here). The higher recovery rates of fine-grained gold of the jig and shaking table compared to the sluice box are illustrated in Figure 6.

Figure 6: Recovery Rates of fine-grained gold for sluice box, jig, and shaking table (after Wang 1979).

There are many other devices available which are capable of concentrating heavy minerals. Each has its own size range of capabilities. In every case, it is necessary to feed the device in an appropriate manner to obtain optimum recovery. In most cases, this means classifying the feed material. Figure 7 indicates the size range applicability of commercial gravity concentrating units.

Figure 7: Grain size range applicability of commercial gravity concentrating units.

The aggregate industry has been in the business of classifying gravel for many years. The technology needed for application to the placer mining industry is readily availible. In these days when placer deposits with coarse-grained gold have been exhausted, and fine-grained gold is what remains, the classic sluice box must be modified in order to obtain the highest rate of gold recovery.

References

Hague, J.M., 1940. The Recovery of Lode Gold in Jigs. Eng. and Min. Journ., Vol. 141, No. 4, p. 40-45.

Tourtelot, H.A., 1968. Hydraulic Equivalence of Grains of Of Quartz and Heavier Minerals, and Implications for the Study of Placers. U.S.G.S. Prof. Paper 594-F, 13 p.

Wang, W., 1979. A Study on Methods for Fine Placer Gold Processing. Non-Ferrous Metals, No. 4, p. 6-12 (Chinese).

Wang, W., and Poling, G.W., 1981. Methods for Recovering Fine Placer Gold. Paper presented at Sixth Annual District 6 Meeting, Can. lnst. Min. Metall., Victoria, B.C.