A sluice is generally defined as an artificial channel through which controlled amounts of water flow. Sluice box and riffles are one of the oldest forms of gravity separation devices used today. The size of sluices range from small, portable aluminum models used for prospecting to large units hundreds of feet long. Sluice boxes can be made out of wood, aluminum, plastic or steel. Modern sluices are built as one unit although sluices formed in sections are still used. A typical sluice section is 12 feet long and one foot wide.
As a rule, a long narrow sluice is more efficient than a short wide one. The sluice should slope 4 to 18 inches per 12 feet, usually 1-1/8 to 1-3/4 inches per foot, depending on the amount of available water, the size of material processed, and the size of gold particles. The riffles in a sluice retard material flowing in the water, which forms the sand bed that
traps heavy particles and creates turbulence. This turbulence causes heavy particles to tumble, and repeatedly exposes them to the trapping medium. An overhanging lip, known as a Hungarian riffle, increases the turbulence behind the riffle, which agitates the sand bed, improving gold recovery. Riffles can be made of wood, rocks, rubber, iron or steel, and are generally 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. Mercury may be added to riffles to facilitate fine gold recovery, but its escape into the environment must be prevented.
In addition to riffles, other materials are used to line sluices for enhanced recovery. In the past, carpet, courdoroy, burlap, and denim were all used to line sluices to aid in the recovery of fine gold. Long-strand Astro-Turf carpet, screens, and rubber mats are used today for the same purpose. In Russia, some dredges use sluices with continuously moving rubber matting for fine-gold recovery.
To perform efficiently, a sluice needs large amounts of clean water. Enough water should 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 cleanups. On one dredge, gold recovery was 90% for 12 hour cleanups, and increased to 94% when sluices were cleaned every 2 hours.
For cleanup, 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 removed and carefully washed into the sluice. Any bottom covering is removed and washed into a separate container. Cleanup continues until all riffles are removed and washed. Large pieces of gold should be removed by hand, then the concentrate is washed out of the sluice or dumped into a suitable container. The collected concentrate may be sent to a smelter, but is usually further concentrated by panning, tabling, or a variety of other methods, including resluicing.
After cleanup, the sluice is reassembled and more material is processed. Gold recovery with sluices can vary depending on a number of factors. Fine gold losses can be minimized by cleaning up more frequently, reducing the speed of the slurry flow to 2 to 3 feet per second, and decreasing the size of the feed, usually by screening. Some operators have increased recovery by adding a liner to the sluice to trap fine gold, and others have lengthened sluices to increase the square footage of particle trapping area.
Overall, sluices are widely used today due to their low cost and availabiity. They have many advantages. They require little supervision and maintenance; they can tolerate large fluctuations in feed volume; they are portable; properly operated, they can approach a gold recovery of 90%; and they entail a minimal
Disadvantages include: very fine particles of gold are not effectively recovered; frequent cleanups are required; sluices can not operate when being cleaned; and large volumes of clean wash water are needed. Although some manufacturers offer sluice boxes, the majority of those in use are fabricated for specific operations, usually by local firms or by the individual mining company.
Among the many variants of the sluice, the long tom and the dip box are included here because of their simplicity and potential usefulness. The long tom is a small sluice
that uses less water than a regular sluice. It consists of a sloping trough 12 feet long, 15 to 20 inches wide at the upper end, flaring to 24 to 30 inches at the lower end. The lower end of the box is set at a 45 degree angle and is covered with a perforated plate or screening with one-quarter- to three-quarter-inch openings. The slope varies from 1 to 1-1/2 inches per foot.
The long tom uses much less water than a sluice but requires more labor. Material is fed into the upper box and then washed through, with water. An operator breaks up the material, removes boulders, and works material through the screen. Coarse gold settles in the upper box and finer gold in the lower. The capacity of a long tom is 3 to 6 yards per day. Other than using less water, advantages and disadvantages are the same as for sluices.
The dip-box is a modification of the sluice that is used where water is scarce and the grade is too low for an ordinary sluice. It is simply a short sluice with a bottom of I by 12 inch lumber, with 6-inch-high sides and a 1 to 1-1/2 inch end piece. To catch gold, the bottom of the box is covered with burlap, canvas, carpet, Astro-Turf or other suitable material. Over this, beginning 1 foot below the back end of the box, is laid a strip of heavy wire screen of one-quarter-inch mesh. Burlap and the screen are held in place by cleats along the sides of the box. The box is set with the feed end about waist high and the discharge end 6 to 12 inches lower. Material is fed, a small bucketful at a time, into the back of the box. Water is poured gently over it from a dipper, bucket, or hose until the water and gravel are washed out over the lower end. Gold will lodge mostly in the screen. Recovery is enhanced by the addition of riffles in the lower part of the box and by removal of large rocks before processing. Two people operating a dip box can
process 3 to 5 cubic yards of material a day. As with a sluice, fine gold is not effectively recovered.
Sluices and related devices were commonly used in the early days of placer mining. Today, sluices are important in a large number of systems, ranging from small, oneperson
operations to large sand and gravel gold recovery plants and dredges. Recent innovations, such as the addition of longstrand Astro-Turf to riffles and the use of specially designed screens, have resulted in increased recovery of fine and coarse gold. Sluices are inexpensive to obtain, operate, and maintain. They are portable and easy to use, and they understandably play an important role in low-cost, placer-gold-recovery operations, especially in small deposits.