Gravity concentration by Sluice Boxes

Gravity concentration: The gold mining industry has recently been rejuvenated due to the increase in the price of gold. The most common gold recovery system used in many plants, however, some equipment such as the sluice box, has not changed significantly for many years. These lines outline the functions and limitations of sluice boxes, and describe other gravity concentrating equipment.

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 4.1, are the most common.

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 more often the concentrate is removed from the sluice; the more efficient is the recovery. Accordingly, the ideal concentration method implements continuous cleaning.