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Essentially, a gravity table is a mechanized gold pan, that operates with a high degree of efficiency and continuously. The table is comprised of a deck, in somewhat of a rectangular shape, covered with riffles (raised bars running perpendicular to the feed side of the table), mounted in a near flat position, on a supporting frame that allows the table to slide along the long axis of the table. The mechanism is attached to the table, and it moves the table along the long axis a distance adjustable between ½" and 1" and then back to the starting position between 200 and 300 times per minute. This reciprocal movement is faster on the reverse stroke than it is on the forward stroke. This shaking movement helps transport the concentrates or heavy material to the concentrate end of the table. A very important operating variable of a shaking table is the tilt adjustment. Normally, the feed side is lower, and the concentrate end is higher on the table, which creates an upward slope where the heavy material will ascend, while the light density material will not, and consequently, flow over the riffles. The tailing (low density) side is usually near level to lower than the feed side. Feed is introduced to the feed box in a slurry at 25% solids, and is distributed along the feed side of the table by the feed trough. Additional wash water is added ad the end of the feed trough, and distributed along the remainder of the length of the table by a wash water launder. About 75% of the water is added to the feed and the remaining 25% added in the wash water launder. this will normally maintain a good water balance. The amount of wash water is another important variable in operating concentrating tables. Too much will wash the concentrates over the riffles and into the tailings. The gravity concentrating tables remove the high density material from the low density material, since the high density material will reside behind the riffles and allow the low density material to flow over the top of the riffles with the wash water, to the tailings discharge. A comment on the art of operating gravity tables, is that the optimum operating settings must be obtained experimentally, by making minor adjustments to the air flow, end tilt, stroke length and frequency and the side tilt of the table. Generally speaking, the frequency and stroke relationship are similar to screens, short stroke, high frequency is better for fines (-80 mesh), while a shorter stroke and lower frequency is better for coarse material (1/8" to 80 mesh). The riffle is always taller on the feed side of the table, and decreases in height as they progress towards the tailings side of the table. This allows for the quick separation of the larger high density material, and allows more residence time for the more difficult finer high density particles to separate from the finer low density material. Theory of gravity concentrating tables Essentially, gravity tables are Flowing Film Concentrators. Flowing film concentrators have a thin "blanket" of water flowing across them, into which particles of differing density are subjected. The primary physical interactions are when the particles initially penetrate the flowing film of water. Smaller particles of the same specific gravity (SG) will travel further than large particles. The film of water has various velocities based on the distance from the water's surface. The highest velocity is the layer of water just below the surface of the water, and the lowest velocity layer, next to the deck surface, is not moving at all. In between these layers are differing velocities, based upon the distance from the water's surface. On a table, with particles of mixed densities, layers of material form, a particle in suspension will be subjected to a greater force the nearer it is to the surface of the water, and will cause it to tumble over those at greater distances from the surface. The combination of the particles tumbling and sliding, the flowing stream with differing velocities, will cause the bed of solids to dilate, and will allow high SG particles to find their way down through the bed of light SG particles, and eventually the low SG particles will work their way to the top, where they will be carried along by the swifter flowing water. In all gravity separations, a difference in specific gravity of the materials needs to be significant, at 1 or greater. I.e., a 2.2 SG material will usually separate from a 3.2 SG material. 
Now introduce the pattern of raised ridges (riffles) across the length of the table to the flowing film separator, and the higher density particles will stay behind the ridge, since they are closest to the bottom, and will follow the ridge down the slope to the discharge, with the residence time giving the water flowing across the ridge more time to remove any low SG particles trapped in the high SG particle bed behind the ridge of the table. Since the water is flowing perpendicular to the ridges or riffles of the table, the low SG material will be washed over the top of the ridges and off the tailings discharge side of the table. Also, particles of the same SG, the large particles will descend faster, due to the force of the water affecting the vertical movement of a large particle less than it would a small particle. This is compounded by the lower amounts of force acting in the horizontal direction as the particle sinks in the water, creating an accelerating rate of descent for the larger particle. Efficiency How efficient is a gravity concentrating table compared to a gold pan? Well, using a full sized table with 64 square feet of working area, a average bed depth of ½ inch, with a average SG of solids of 1.7, and a packing factor of 0.60, there is 100 pounds of ore on the table at any given period of time. The average residence time of ore on a table is 7 minutes for 100 pounds of ore. Most gravity tables have a sand deck for treating 1/8" to 80 mesh material and a slime deck for treating -80 mesh material. The appropriate deck should be used when treating ores. Most used involve the sand deck. Now, for a gold pan, a very good panner can take a pan with 15 pounds of ore in it and pan it in 10 minutes. That is 1.5 pounds per minute, versus 100 pounds in 7 minutes, or 14.3 pounds per minute. The table is 10 times faster than a gold pan using theoretical situations. In reality it is more like 20-30 times more efficient than a gold pan. In gold and mineral applications, a full sized gravity table, like the No. 6 Deister Table, will handle 1 ton per hour of sand feed, and less for slime feed, sometimes only 1/2 TPH. Information provided by Charles Kubach, Mining and Mineral Processing Engineer Reference Material, Deister Concentrating Company |
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