I find this stuff fascinating. I probably don't have too much company in this. It is probably a personality defect, like having one leg much longer than the other, good for mountain work going one way on one side of the valley but not much else.
If one-quarter-inch lag screws are driven into the bottom of each rocker about 5 inches from each side of the spike and the heads are allowed to protrude from the wood, a slight bump will result as the machine is worked back and forth. This additional vibration will help to concentrate the gold. If screws are used, metal strips should be fastened to the bed-plates to protect the wood.
Operation.
Gravel is shoveled into the hopper and the rocker is vigorously shaken back and forth while water flows over the gravel. The slope of the rocker is important for good recovery. With coarse gold and clay-free gravel, the head bed plate should be 2 to 4 inches higher than the tail bed plate. If the material is clayey, or if fine gold is present, lessen the slope to perhaps only an inch.
The rate of water flow is also important. Too much water will carry the gold through the rocker without settling, and too little will form a mud that will carry away fine gold. Water may be dipped in by hand, or fed with a hose or pipe (Photo 2). It is important to maintain a steady flow of water through the rocker. When all the material that can pass through the screen has done so, the screen is dumped and new material added and washed. The process continues until it is necessary to clean the apron. Frequent cleanups, on the order of several times a day, are necessary for maximum recovery.
For cleanup, the apron is removed and carefully washed in a tub. The riffles are cleaned less frequently, whenever sand buildup is heavy. After cleanup, the rocker is reassembled and processing resumed. The collected concentrates are further re- fined, usually by amalgamation or panning. Mercury is sometimes added to the riffles to collect fine gold. Two people operating a rocker and using 100-800 gallons of water can process 3 to 5 cubic yards of material in 10 hours.
The capacity of rockers may be increased by using a power drive set for forty 6-inch strokes per minute. A power rocker operated by two men can process 1 to 3 cubic yards of material per hour. (This was the method used on dredges in AK, and Yk. It probably lost most of the fine gold if this memoir is correct. See below.)
The rocker is an improvement over the gold pan, but is limited by the need for frequent cleanups and poor fine-gold recovery.
Rockers are not widely used today.
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 (Photo 4). In Russia, some dredges use sluices with continuously moving rubber matting for fine-gold recovery (Zarnyatin and others, 1975).
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. <nb>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% then sluices were cleaned every 2 hours (Zamyatin and others, 1975). This may be because the rotor is only efficient when it is less constrained.
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 availability. 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 initial investment. |
