The most common gravity separation device used in placer mills, sluices are simple to construct, yet effective heavy mineral recovery tools. Sluice design is quite diverse, and opinions differ widely with respect to capacity, riffle design, and recovery.
In general, capacities and performances vary with box width and slope, gold particle size, nature of feed, and availability of water. Sluices are primarily used for rough concentration and are capable of processing poorly sorted feeds. As with other methods, however, recovery is related to the degree of previous sizing.
Sluice design can be quite complex but usually is a matter of trial and error. Several basic principles typically apply. Width is determined by the maximum and minimum volume of water available, the size and quantity of oversize feed that must be transported, and the slope.
Length depends principally on the character of the gold. Coarse gold and granular gold settle quickly and are easily held in-the riffles, while fine gold and porous gold may be carried some distance by the current. velocity of the water is controlled primarily by the slope. In general, the sluice should be con- structed and installed so that water flowing through the box will transport oversized material and prevent sand from packing the riffles.
If the surface of the water flowing through the sluice is smooth, the bottom of the sluice is probably packed with sand, allowing little gold to be saved. The desired condition occurs when waves form on the surface of the water flowing through the sluice, and these waves, along with the wave-forming ridges of material on the bottom of the sluice, migrate upstream. This indicates an eddying or boiling activity on the lee side of the ridges, which maximizes gold recovery and tailings transport. Consequently, the sluice attains maximum efficiency when riffle overloading is incipient.
Sluices are generally considered to be high-capacity units, with a 12-in-wide sluice box capable of handling 15 yd3/h if sufficient water is available. A 24-in-wide sluice can handle up to 40 yd3/h, and 48-in-wide sluices have reportedly processed up to 200 yd3/h. Of course, a sluice will handle as much gravel as the operator wants to push through it. However, to ensure reasonable recovery, capacity is limited by box width and slope, water availability, and feed characteristics.
Feed slurry densities are highly variable and range from 1% to 35% solids by weight, averaging 10%. Water use can be reduced significantly if the larger of the oversize is eliminated from the feed. Sluices require no power to operate unless a pump is needed to transport water or slurry. One disadvantage of the sluice is the necessity to halt operations in order to recover concentrates
