Air Gravity Concentrating Tables

Air Gravity Concentrating Tables

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Essentially, a air gravity table is similar to a mechanized gold pan, that operates with a 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. Instead of water as the medium, air is used and is continuously injected through the porous bed of the table.

The mechanism is attached to the table, and it moves the table along the long axis a distance adjustable between " 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 a air 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 near level to lower than the feed side. Another important variable in air table operation is the volume of air, and this is typically adjusted by a series of valves, or plate type regulators, allowing more or less air to flow to the deck. It is important to have a uniform flow of air across the deck, to prevent "blow outs", which is why multiple air regulator points are provided for air tables. 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).

Feed is introduced to the feed box in a narrow size range. For air tables to function effectively, the feed needs to be in a narrow size range, usually with a ratio of 2.8:1, from the smallest particle to the largest particle. The maximum particle size is about 1/8" and the typical fine size is normally 60 mesh that can be separated on a air table. 60 mesh is 0.0098 inches. So, if 60 mesh material were to be separated, the feed size range would be from 0.027 inches, or about 25 mesh, to 60 mesh. 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. Air tables can also separate, somewhat, on particle shapes, also, as differing particle shapes react differently in the rising air columns. Generally, these will show up in the middlings (discharge between tailings and concentrate).

There has been very few successful applications of gold recovery, using air tables. Even though the difference in SG is great, the gold, during crushing, becomes flat and forms little aerodynamic wings, that tend to float across the riffles to the light density tailings side of the table. Air tables have been successful in many mineral separations, such as tungsten, fluorite, garnet and they work well in coal.

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.

Air tables work similar to wet gravity tables, in that the material is fed perpendicular to the riffles, and the high density material remains behind the riffles, and the fluidizing air columns rise through the bed of material, relative to Stokes Law and Hindered Settling. This causes settling at differential rates, where the light density material flows above and over the riffles, while the high density material stays close to the desk surface, and follows the riffles to the concentrate discharge.

Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference material from Taggart  

Diagrams showing air table deck loading, adjustments.

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