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 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
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
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
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.
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