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