Screening device

A screening device for immersion in a vessel containing a pulp or slurry, said device comprising a screening chamber having at least one side wall defined by a screen and closed at the top and bottom said screen being located to be immersed in the pulp or slurry, a discharge pipe located within the screening chamber having an open lower end and positioned close to the bottom and an upper end opening into a discharge outlet external to said vessel said discharge outlet being located below the surface level of the pulp or slurry and above the level of the bottom of the chamber, the capacity of the discharge outlet being such as to maintain a flow rate through the screening chamber and discharge pipe to maintain the solids in the pulp or slurry passing through the screening device in suspension.

This invention relates to an improved screening device. 
In many hydrometallugical and other industrial operations there is often a 
need to screen solids from liquids or classify suspsended solids in 
liquids, pulps or slurrys. 
In the past screening has been done by simply inserting the screen in the 
flow or by pumping the total material over a vibrating screen or 
alternatively a DSM wedgebar screen or sieve bend. Such devices have 
presented the following dificiencies: 
1. Screens inserted in the flow in general have a low capacity and rapidly 
block or "blind"; 
2. To pass the total flow over an external screen requires the use of a 
pump to transport the slurry to the screen. 
The carbon in pulp (C.I.P.) process for the recovery of gold from solution 
involves the use of cyanide to dissolve the gold from gold bearing ore 
whereby the dissolved gold values are subsequently adsorbed onto activated 
carbon granules which are suspended or freely mixed with the gold ore and 
cyanide slurry. After a suitable time has been allowed for adsorption the 
carbon is removed, or alternatively the slurry is allowed to pass through 
a screen to a subsequent stage. Adequate adsorption generally requires at 
least four stages of reaction which requires several screen separations of 
the slurry. 
It has been found that open launder screens function reasonably at 
relatively low flow rates but require constant attention to avoid blocking 
and to prevent the heavier sands from settling. 
It is an object of the present invention to provide a screening device 
which has a relatively high flow rate and in which the likelihood of 
sanding and blocking of the screen is reduced. 
It is a further object of the invention to provide a screening device which 
can be readily removed for routine maintenance and replacement without the 
need for shutting down the operation of the vessel in which the screen is 
placed. 
In one form the invention resides in a screening device for immersion in a 
vessel containing a pulp or slurry, said device comprising a screening 
chamber having at least one side wall defined by a screen and closed at 
the top and bottom said screen being located to be immersed in the pulp or 
slurry, a discharge pipe located within the screening chamber having an 
open lower end and positioned close to the bottom and an upper end opening 
into a discharge outlet external to said vessel said discharge outlet 
being located below the surface level of the pulp or slurry and above the 
level of the bottom of the chamber, the capacity of the discharge outlet 
being such as to maintain a flow rate through the screening chamber and 
discharge pipe to maintain the solids in the pulp or slurry passing 
through the screening device in suspension. 
According to a preferred feature of the invention said screening chamber is 
substantially cylindrical, the one wall comprising the cylindrical wall. 
According to a further preferred feature of the invention the discharge 
pipe opens through the top of said screening chamber. 
According to a further preferred feature of the invention clearance means 
is provided for causing a flow of air bubbles over the exterior of the 
screen. 
According to a preferred feature of the previous feature said clearance 
means comprises a plenum chamber at the bottom of said screening chamber, 
source of air connected to said plenum chamber and provided with a set of 
outlets located around the base of the screen. 
According to a preferred feature of the invention the device is adapted to 
be located in a weir to extend downwardly therefrom said weir comprising 
said discharge outlet.

The first embodiment shown at FIGS. 1 to 3 comprises a weir box 18 which is 
located in a tank 19 containing a slurry or pulp the level of which is 
maintained at a level below the top of the weir 19 and such that a head 
"h" is developed between the surface of the slurry and the bottom wall of 
the weir box 18. The weir box 18 extends beyond the side wall of the tank 
19 and at its outermost extremity is formed with an open wall to provide a 
discharge outlet 20. A screen 11 which is of substantially cylindrical 
shape in which the cylindrical walls are formed of a suitable mesh or 
wedgebar, is supported from the bottom of the weir box 18 to depend 
downwardly therefrom. The bottom of the screen is closed by an imperforate 
plate 13 while the top of the screen 11 is partially closed by an annular 
imperforate plate 12 which is fixed to the upper end of the screen 11. The 
top plate is formed with radially extending lugs at its outer periphery 
such that when the screen is in position in the weir box 18 the lugs 15 
are engaged with complementary flanges 16 formed around the aperture in 
the base of the weir box 18. The top plate 12 is fitted with handles 17 
which facilitate the removal and location of the screen in the base of the 
weir box 18 and rotation of the screen such that the lugs 15 engage or 
disengage from the flanges 16. The plate 12 also supports a discharge pipe 
14 which extends downwardly from the top plate 12 to a point adjacent the 
bottom plate 13. 
In use the slurry in the tank 19 is caused to flow through the screen walls 
of the screen 11 to the interior of the screen by virtue of the head "h" 
provided between the top surface of the slurry A and the upper end of the 
discharge pipe 14. On the entry of the slurry to the interior of the 
cylindrical screen it is caused to flow downwardly to the lower end of the 
discharge pipe 14 through the discharge pipe 14 into the weir box and out 
of the weir box through the discharge outlet 20. The diameter of the 
discharge pipe 14 in relation to the head "h" between the surface of the 
slurry and the bottom of the weir box 18 is sufficient to ensure that the 
flow rate of material through the interior of the screen and the discharge 
pipe eliminates sanding or settling of the solids suspended in the slurry 
at the bottom of the screen 11. This is further facilitated by the 
relatively close spacing between the lower end of the discharge pipe 14 
and the bottom plate 13. 
In one example of the embodiment it has been found that satisfactory 
results can be obtained by using a screen of 300 mm diameter and having 
the length of 2 m with a screen slot width of 850 microns. The diameter of 
the discharge pipe was of the order of 75 mm and in use a flow rate of 27 
cubic meters per hour or 0.45 cubic meters per minute was produced in a 
slurry having a density of 50% solids in a carbon in pulp processing 
plant. 
It will be appreciated that in the arrangement described above with the 
lower end of the discharge pipe closely adjacent the bottom plate of the 
screen, the static pressure on either side of the screen is substantially 
constant so that the pressure drop and flow rate through the screen is 
also substantially constant irrespective of depth. 
In addition it will be appreciated that the means of locating the screen in 
the bottom of the weir box 20 described above need not be limiting on the 
scope of the invention and that it may be of advantage to provide a 
locking means which located above the upper end of the screen 11 and above 
the surface level of the slurry in the weir box 18. 
A particular advantage of the screen of the first embodiment relates to the 
ready installation, removal or replacement of the screen. If the screen is 
readily removed for routine maintenance or replacement there is not the 
need to stop the operation of the whole plant to which the tank 19 
relates. This contrast to plants utilising launder screens which are fixed 
to the walls of the vessels and which necessitate the shutting down of the 
plant or at least the tank to effect repairs. 
The second embodiment of the invention as shown at FIG. 4 comprises a 
screen 11 of the similar configuration to that of the first embodiment 
except that the screen is supported within the slurry by means other than 
a weir box. In addition the discharge pipe 14 extends beyond the upper 
surface of the screen 11 and is provided with a discharge outlet 20 which 
opens into the discharge pipe 14 substantially at the surface level of the 
slurry but is inclined downwardly to discharge at a level equal to or 
lower than the top plate 12 of the screen. 
According to a preferred feature of the second embodiment which if desired 
may also be a feature of the first embodiment a plemun chamber 113 is 
formed at the bottom of the screen 11 and defines the bottom of the 
screen. The plenum chamber 113 has an inlet having a pipe 115 connected 
thereto which extends concentrically through discharge pipe 14 and is 
connected to a source of compressed air. The outer circular periphery of 
the plenum chamber 113 is provided with a plurality of valves which may be 
of any suitable type to allow for air flow from the plenum chamber 113 
into the slurry such that air bubbles are caused to rise over the exterior 
surface of the screen in order to displace any solids which may have 
collected on the exterior surface of the screen and thus maintain a clean 
screen surface. 
The screen of the present invention offers practical separation of fine 
solid slurries as in gold cyanidation processes from course solid 
particles (as in C.I.P. processing) or for any alternative processes for 
example uranium extraction by resin-in-pulp technology because it avoids 
the additional handling of the slurry including the course particles 
(carbon resin, etc.) and it avoids much of the attrition loss which other 
high rate screening processes involve. It also avoids the blocking and 
sanding problems which are current with simple screens and launder 
screens. 
If desired the flow rate through the screen may be increased by the use of 
a small impeller located in the discharge pipe or the air lift therein. 
Whilst the present invention has been described with particular reference 
to the screening of pulps and slurries it is equally applicable to screens 
used in classifying material according to maximum size rather than 
utilizing cyclones, hydrocyclones or other classifiers.