SMALL SCALE RECOVERY EQUIPMENT
Much of the equipment described in this section has been used for centuries. Many variations of the basic designs have been used throughout the years. Some are more efficient than others. Most have low capacity and do not efficiently recover fine gold. Only the most useful, simple, inexpensive, or easily constructed of these old but practical devices are described.
Perhaps the oldest and most widely used gold concentrator is the gold pan. Although available in various shapes and sizes, the standard American gold pan is 15 to 18 inches in diameter at the top and 2 to 2 1/2, inches in depth, with the sides sloping 30-45 degrees. Gold pans are constructed of metal or plastic (Photo 1) and are used in prospecting for gold, for cleaning gold-bearing concentrates, and rarely, for hand working of rich, isolated deposits.
A gold pan concentrates heavy minerals at the bottom while lighter materials are removed at the top. The basic operation of a pan is simple, but experience and skill are needed to process large amounts of material and achieve maximum recovery. Panning is best learned from an experienced panner, but the general principles and steps are outlined below.
For maximum recovery, the material to be panned should be as uniform in size as possible. Panning is best done in a tub or pool of still, clear water. First, fill the pan one-half to threefourths full of ore or concentrate. Add water to the pan or carefully hold the pan under water and mix and knead the material by hand, carefully breaking up lumps of clay and washing any rocks present. Fill the pan with water (if not held underwater) and carefully remove rocks and pebbles, checking them before discarding. Tilt the pan slightly away and shake vigorously from side to side with a circular motion while holding it just below the surface of the water. Removal of lighter material is facilitated by gently raising and lowering the lip of the pan in and out of the water. The pan may be periodically lifted from the water and shaken vigorously with the same circular motion to help concentrate materials. Large pebbles should be periodically removed by hand. Panning continues until only the heaviest material remains. Gold may be observed by gently swirling the concentrate into a crescent in the bottom of the pan. Coarse nuggets are removed by hand, while finer grained gold may be recovered by amalgamation. An experienced panner can process one-half to three-quarters of a cubic yard in 10 hours.
Panning was widely used as a primary recovery method in the early days of mining. However, the process is extremely limited, as only coarse gold is recovered, while very fine particles are usually washed away with the gravel. Only small amounts of gravel can be processed, even by the most experienced panners. Today the gold pan is used mostly for prospecting or for cleaning concentrate. Its low price, immediate availability, and portability make it an essential tool for the prospector or miner.
One of the first devices used after the gold pan was the rocker. The rocker allowed small operators to increase the amount of gravel handled in a shift, with a minimum investment in equipment. Rockers vary in size, shape, and general construction, depending upon available construction materials, size of gold recovered, and the builder’s mining experience. Rockers generally ranged in length from 24 to 60 inches, in width from 12 to 25 inches, and in height from 6 to 24 inches. Resembling a box on skids or a poorly designed sled, a rocker sorts materials through screens. (Figure 1).
Rockers are built in three distinct parts, a body or sluice box, a screen, and an apron. The floor of the body holds the riffles in which the gold is caught. The screen catches the coarser materials and is a place where clay can be broken up to remove all small particles of gold. Screens are typically 16 to 20 inches on each side with one-half inch openings. Fine material is washed through the openings by water onto an inclined apron. The apron is used to carry all material to the head of the rocker, and is made of canvas stretched loosely over a frame. It has a pocket, or low place, in which coarse gold and black sands can be collected. The apron can be made of a variety of materials: blanket, carpet, canvas, rubber mat, burlap or amalgamated copper plate. Riffles below the apron help to collect gold before discharge.
Figure 2 shows a portable rocker that is easily built. The six bolts are removed to dismantle the rocker for easy transportation. The material required to construct it is given in the following tabulation:
A. End, one piece 1 in. x 14 in. x 16 in.
B. Sides, two pieces 1 in. x 14 in. x 48 in.
C. Bottom, one piece 1 in. x 14 in. x 44 in.
D. Middle spreader, one piece 1 in. x 6 in. x 16 in.
E. End spreader, one piece 1 in. x 4 in. x 15 in.
F. Rockers, two pieces, 2 in. x 6 in. x 17 in. (shaped)
H. Screen, about 16 in. square outside dimensions with screen bottom. Four pieces of 1 in x 4 in. x l5 1/4 in. and one piece of screen 16 in. square with 1/4 in. or 1/2 in. openings or sheet metal perforated by similar openings.
K. Apron, made of 1 in. x 2 in. strips covered loosely with canvas. For cleats and apron, etc., 27 feet of 1 in. x 2 in. lumber is needed. Six pieces of 3/8 in. iron rod 19 in. long threaded 2 in. on each end and fitted with nuts and washers.
L. The handle, placed on the screen, although some miners prefer it on the body. When on the screen, it helps in lifting the screen from the body.
If l- by 14-inch boards cannot be obtained, clear flooring tightly fitted will serve, but 12 feet of 1- by 2-inch cleats in addition to that above mentioned will be needed. A dipper may be made of no. 2 1/2 can and 30 inches of broom handle. Through the center of each of the rockers a spike is placed to prevent slipping during operation. In constructing
riffles, it is advisable to build them in such a way that they may be easily removed, so that clean-ups can be made readily. Two planks about 2 by 8 by 24 inches with a hole in
the center to hold the spike in the rockers are also required. These are used as a bed for the rockers to work on and to adjust the slope of the bed of the rocker.
The parts are cut to size as shown in Figure 2. The cleats on parts A, B, C, and D are of1- by 2-inch material and are fastened with nails or screws. The screen (H) is nailed together and the handle (L) is bolted to one side. Corners of the screen should be reinforced with pieces of sheet metal because the screen is being continually pounded by rocks when
the rocker is in use. The apron (K) is a frame nailed together, and canvas is fastened to the bottom. Joints at the comers should be strengthened with strips of tin or other metal.
Parts are assembled as follows:
Place bottom (C), end (A) with cleats inside, middle spreader (D) with cleat toward A, and end spreader (E) in position between the two sides (B) as shown. Insert the six bolts and fasten the nuts. Rockers (F) should be fastened to bottom (C) with screws. Set apron (K) and screen (H) in place, and the rocker is ready for use. 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.
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. 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 refined, 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. 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.