Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to a screening device for separating
suspended solids from a liquid.
Metal values may be extracted from their ores by finely
grinding the ore, and then leaching the ore with a suitable
leaching solution. For example, gold values may be extracted
from their ores by leaching finely ground gold-bearing ore with
a cyanide solution. The product of the leaching step is a
metal-rich leach solution or pulp known as a pregnant solution
or pulp. The metal values may be recovered from the pregnant
leach solution or pulp using various techniques. One widely
used technique is to capture the metal values on a suitable
support such a activated carbon by the so-called carbon-in-pulp
method.
The carbon-in-pulp method may be carried out in a series of
tanks with the pulp flowing in one direction under the
influence of gravity and the carbon flowing counter-current to
the pulp flow. The lean pulp is withdrawn from one end of the
series of tanks while the loaded carbon is withdrawn from the
other end.
A screening device is commonly provided at the outlet from each
tank, which has a covering mesh or screen to enable the pulp to
pass through, but not the carbon. The screens tend to become
clogged with carbon during use. Many methods have been proposed
and are used for unclogging the screens during use. One such
method is to sweep the screens with air. This method is not
very efficient and is costly in energy. Problems are
encountered with proper air distribution and the method
necessitates the use of a multiplicity of air injection nozzles
which are subject to wear and choking with solids. Furthermore,
it is only possible to attain relatively low throughputs,
typically 60m3~h.m2. It is also known to sweep the screens
mechanically or to cause the screens to vibrate. These methods
again have not proved very efficient.
SUMMARY OF IHE _NY~NTION
According to the invention there is provided a screening device
for separating suspended solids from a liquid in a container,
comprising a screen element which has a mesh structure and
which i5 adapted to be at least partially submerged in the
container between an inlet to and an outlet from the container;
at least one agitator arranged to be driven past a surface of
the screen element to create turbulence in the liquid adjacent
the surface; and at least one impeller arranged to be driven
together with the or each agitator to cause the liquid to flow
past the surface.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial sectional side view of a screening device
according to the invention; and
Figure 2 is a partial sectional top view of the device of
Figure 1.
DESCRIPTIO~ 0~ ~ PREFER~E~ ~MBODI~E~T
Figure 1 shows a screening device, generally designated by the
numeral 14, located with its lower end submerged in a
gold-bearing pulp 12. The pulp 12 is contained in a large
container or tank (not shown). The screening device 14 has a
central housing 16 which defines a vertical bore 18 in which a
shaft 20 is rotatably mounted. The shaft is supported near its
upper end on bearings 22 and 24, and is arranged to be rotated
by an electric motor 26 via a belt drive 28 mounted at the
upper end of the screening device. The bore 18 has an opening
54 and an optional overflow pipe 56.
At the lower end of the screening device is a screen element 30
which is in the form of a hollow cage. The screen element 30
has an inner wall 32 which is generally frusto-conical in
shape, and an outer wall 34 which is right cylindrical in
shape. The inner wall 32 and the outer wall 34 are coaxial, and
comprise a relatively coarse grid covered with a fine mesh.
Seen from above, the screen element has an annular cross
section which decreases towards the bottom of the element. The
interior of the screen element 30 is in communication with the
interior of the housing 16, which defines an outlet flow path
from the tank or container.
Towards the lower end of the shaft 20 are mounted impellers 36
and 38. Each impeller comprises four equally radially spaced
blades which extend laterally from the shaft 20 and which are
inc]ined at an angle of 45 relative to the axis of the shaf-t
20. An agitator assembly, shown more clearly in Figure 2, is
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also attached to the shaft 20. A circular plate 40 i8 fixed to
the shaft 20 a-t the same heigh-t as the impeller 36. Eight
agitator elements 42, comprising sections of angle iron, extend
downwardly from the plate 40 to respective radially extending
legs 44 which extend from a hub 46 at the bottom end of the
shaft 20. The agitator members 42 are angled so as to be
parallel to the inner surface 32 of the screen element 30. The
radial legs 44 extend beyond the outer wall 34 of the screen
element 30 and carry respective vertical agitator members 48
which extend parallel to the outer surface 34 of the screen
element 30. Strengthening hoops 50, 52 and 58 reinforce the
agitator assembly.
As can be seen more clearly in Figure 2, adjacent agi-ta-tor
elements 42 are rotated through 180 about their respective
longitudinal axes, and the orientation of adjacent vertical
agitator members 48 is similarly reversed. On one flange of
each agitator member 42, 48 and running the entire length
thereof is a reinforced flat rubber blade 51, one edge of which
extends beyond the edge of the respective flange.
The operation of the screening device is as follows. Gold
bearing pulp is introduced to the tank or container via an
inlet 60, where it is mixed with carbon present in the tank.
The shaft 20 is caused to rotate, and the impellers 36 and 38
cause a flow of pulp through the center of the screen element
30 in the direction of the arrows.The pulp is drawn into the
central bore 18 through the opening 54 and passes down through
the centre of the screen element 30 back into the tank or
container, flowing from the apical end to the basal end of the
inner wall 32. Screened liquid which has passed through the
mesh of the screen element 30 passes via the interior of the
housing 16 and an outlet (not shown) to a successive container
or pipeline. Clearly, suspended particles smaller than the mesh
size will pass through the screen element. Solid material in
the pulp tends to collect on the inner and outer mesh walls of
the screen element 30 as liquid crosses the mesh into the
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interior of the screen element. However, the agita-tion caused
by the rotating agitator elements 42, 48 tends to dislodge this
solid matter by creating pressure waves or -turbulence adjacent
the surfaces. The rubber blades 51 pass close to the respective
surfaces of the screen element but do not contact the surfaces.
The exact configuration of the blades, their distance from the
walls of the screen element, and the rotational speed of the
agitator assembly will depend on the characteristics of the
pulp in the tank or container.
If the orienta-tion of the blades of the impellers 38 and 40 is
reversed, or if the direction of rotation of the shaft 20 is
reversed, the pulp will flow through the screening device in
the opposite direction to that shown by the arrows in Figure 1.
In the event that the shaft 20 is rotated too rapidly, causing
an excessive flow of pulp through the central bore 18, an
overflow pipe 56 diverts the excess flow back into the tank or
container, and prevents the pulp from rising up to the level of
the bearings 22 and 24.
It has been found in use, however, that clogging of the inner
surface 32 of the screen element 30 is minimised by causing the
pulp to flow downwards through the screen element 30 as
described earlier.
It has been found that the improved pulp flow caused by the use
of the impellers 36 and 38 together with the agitator assembly
improves the throughput of the screening device over prior
devices.
