Sluice

A sluice is defined as a channel through which controlled amounts of diluted slurry. Sluice box and riffles are one of the oldest forms of gravity separation equipment. The size of this equipment is variable and range from small and portable metallic models to large units. Small models are employed for prospecting purposes. In general, sluices can be made of wood, aluminum, steel or plastic. A typical sluice section is 12 feet long and one feet wide. As a rule, a long narrow sluice is more efficient than a short wide one. The sluice must slope 4 to 12 inches per 12 feet depending on the water available, the coarse particle to be processed and the possible gold particle size.

The riffles in a sluice retard material flowing in the water that forms the sand bed that traps heavy particles and created turbulence. This turbulence causes heavy particles to tumble and repeatedly exposes them to the trapping medium. An overhanging lip known as Hungarian riffle increases the turbulence behind the riffle that agitates the sand bed improving gold recovery. Riffles are fabricated of wood, rocks, rubber, steel and they are 1-1/2 inches high, placed from one half inch to several inches apart. The riffles are commonly fastened to a rack that is wedged into the sluice so that they can be easily removed. Many years ago, quicksilver was added to riffles to facilitate fine gold recovery, but its release into the environment make this practice dangerous.

Riffles

Riffles and other material are used to line sluices for enhanced recovery. In the past, carpet, corduroy, burlap and denim were all employed to line sluices to aid in the recovery of gold. Long-strnad Astro Turf carpet, screens and rubber mats are employed today for the same purpose. In Russia, some dredges use sluices with continuously moving rubber matting for fine gold recovery. Extensive use of sisal clothes as sluice boxes liners to concentrate gold was employed in mining operations in Africa. The main problem is to use very opened sisal clothes than can work properly for coarse gold, but it is unlikely efficient for medium and fine gold particles. This material costs around US$ 4/m2. It is matter of trying different types in order to get the most appropriate according to the gold ore to be processes.

• Slurry, steady and pre-screened at 5 mm
• Percent solids, 15%. Clayed material needs special conditions
• Flow velocity, depend on box width and slope
• Stream depth, 20-30 mm
• Slope, 10-25 degrees
• Length, 2-5 m
• Width, depend on flow speed
• Water, 30-70 m3/h/m

To perform efficiently, a sluice needs large amounts of water. Enough water must be added to the feed to build up a sand bed in the bottom of the sluice. For maximum recovery, the flow should be turbulent, yet not forceful enough to wash away the sand bed. Russian studies have shown that recovery increases with the frequency of cleaning. For clean up, clear water is run through the sluice until the riffles are clear of gravel. A pan or barrel is placed at the discharge end to prevent loss of concentrate. Starting from the head of the sluice, riffles are carefully washed into the sluice. According to the model, any bottom covering is removed and washed into a separate container. Clean up continues until all riffles are washed. Large particles of gold should be removed by hand, and then the concentrate is washed out of the sluice or dumped into an appropriate container. The concentrate can be smelted or cleaned more by panning. After clean up, the sluice is reassembled and more ore is processed.

Riffles separate heavy and light particles

Gold recovery with sluices can vary depending on several factors. Fine gold losses can be minimized by cleaning up more frequently, reducing the speed of the slurry flow to around 0.50 to 0.80 m/s., and decreasing the feed size by performing a previous screening. Some experienced operators have increased recovery by adding a liner to the sluice to trap fine gold and other have lengthened sluices to increase the residence time and the trapping area.

Sluices are used due to their low cost and availability. They need little supervision and maintenance, they can tolerate large fluctuations in feed volume; they are portable and can be achieved acceptable gold recoveries. Unfortunately, there some disadvantages such as very fine particles are not recovered, needs frequently clean up, and is important to have a big source of water. Some manufacturers offer sluice boxes fabricated for specific conditions.