AU757040B2 - Intertank screen sealing system - Google Patents

Description

WO 00/09915 PCT/AU99/00649 -1-

TITLE

Intertank Screen Sealing System FIELD OF THE INVENTION The present invention relates to the field of hydrometallurgical processing, and in particular to improvements in apparatus for leaching/adsorping/adsorbing values.

BACKGROUND ART A typical mineral processing plant utilising leach/adsorption/adsorbtion processes for extracting metal values from a finely ground ore slurry consists of a series of leach/adsorption/adsorbtion tanks. The tanks are generally interconnected with launders to allow the slurry to proceed from tank to tank. Each tank is fitted with an intertank screen to retain the adsorpbent/adsorbent, and an agitator to ensure uniform mixing. A wiper is associated with the intertank screen to prevent the intertank screen from becoming clogged by the absorbent.

Pregnant slurry is introduced into the first tank and flows (either via gravity, pump or a combination of both) through the tank train. The adsorpbent/adsorbent is introduced into the last tank of the train and advanced counter current to the slurry flow via airlift or pump to next upstream tank either batchwise or continuously.

The adsorpbent/adsorbent is retained by the intertank screen within the upstream tank while the slurry gravitates back to the downstream tank. The counter current process is repeated until the adsorpbent adsorbent reaches the first tank, from where it is removed via airlift or pump for stripping. The slurry which discharges from the last tank is directed to tails (waste disposal.

The purpose of the intertank screen is therefore twofold: to facilitate the counter current movement of the adsorpbent adsorbent; and to prevent the loss of loaded adsorpbent adsorbent to each downstream tank in turn and ultimately to tails disposal.

WO 00/09915 PCT/AU99/00649 -2- Historically, there have been several versions of intertank screens which have been used at some point or another, including but not limited to static screen, Kambalda screen, vibrating screen, trommel, air swept cylindrical with bottom discharge, mechanically swept cylindrical with bottom discharge (also commonly known as a North Kalgurli screen and its various adaptations thereof), and mechanically swept cylindrical with top discharge (and its various adaptations thereof).

Irrespective of type however, all the intertank screens have been fitted with a filter screen generally referred to as a screen cloth (woven wire, wedgewire, or similar, of either stainless steel or polyurethane construction) whose aperture is significantly less than that of the adsorpbent adsorbent, thus retaining the adsorpbent adsorbent, while permitting the finely ground ore slurry to pass through.

To prevent clogging of the screen cloth, the cloth has usually been gently swept (agitated) with either air and or mechanical devices.

More recently, the industry has tended towards top discharge, mechanically swept, cylindrical screens in which the screen assembly is hinged to the intertank launder and a seal provided between the mating flanges of the screen assembly and launder, as a consequence of the weight of the screen assembly acting about the hinge point. There are however, perceived difficulties with retaining an effective seal and removing the screen from the tank for maintenance purposes with such an arrangement.

This invention seeks to provide an alternate sealing system between the screen assembly and launder to prevent the recirculation and/or loss of adsorpbent adsorbent.

Other advantages as a result of the invention, in particular with regard to ease of installation and removal for maintenance will also be apparent from the following description.

WO 00/09915 PCT/AU99/00649 -3- Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

DISCLOSURE OF THE INVENTION In accordance with one aspect of the invention there is provided a screen assembly intended to be received through an outlet in a leach tank, said outlet being accessible from above for vertical insertion and extraction of said screen assembly, said screen assembly being of proportions generally commensurate with the size of said outlet to allow ready insertion therethrough to a working position, said screen assembly having a filter screen extending in a position below said outlet (when in said working position) to allow undersize material to flow therethrough while retaining oversize material, and a seal means located around its periphery in a position to sealingly interfere with said outlet.

Preferably said screen assembly includes a flange fixed at a position commensurate with the position of said outlet (in said working position) to locate and/or carry said seal means.

Preferably said seal means extends peripherally beyond said flange to sealingly interfere with said outlet.

Preferably said seal means comprises a seal formed of a resiliently deformable material.

Preferably said seal is sandwiched between said flange and a clamping member, said seal also extending peripherally beyond said clamping member to sealingly interfere with said outlet.

Preferably said clamping member is of a configuration substantially commensurate with the extent and configuration of said annular flange.

WO 00/09915 PCT/AU99/00649 -4- Preferably said clamping member is divided into a plurality of portions, for ease of handling.

Preferably the uppermost of said clamping member and said flange has the greater overall external transverse size. Since the seal is sandwiched between said flange and the clamping member, and also extends peripherally beyond both to sealingly interfere with said outlet, the insertion of the screen assembly causes the seal to interfere with the wall(s) of the outlet and deflect upwards, while the seal can readily deflect in the opposite direction allowing for easy removal of the screen assembly from the outlet.. As a result of reduced clearance between the uppermost member (of the clamping member and the flange) and the outlet, the sealing properties are improved. In a screen assembly where the clamping member is located above the flange, and where difficulty is encountered with obtaining a seal between the screen assembly and the outlet, new clamping members may be fabricated with a greater overall diameter, to improve the sealing characteristics.

As an alternative to the arrangement where the seal extends peripherally beyond said flange to sealingly interfere with said outlet, said flange is arranged to mate with a corresponding internal flange located at said outlet.

In the above alternative arrangement, the seal means may comprise machined mating surfaces of the flanges, a formed surface to create a line seal located on one or both contacting surfaces of the flanges, or a discrete seal which can be located between the flanges.

Preferably said screen assembly is supported relative to said outlet from a position located above said outlet. In this manner, the only contact with said screen assembly below the level of the slurry (ie below the "water-line") is the contact between the seal and the outlet.

Preferably said outlet is of cylindrical shape and has a vertical axis, said screen assembly has a transverse diameter generally commensurate with the diameter of said outlet (proximal to said outlet when said screen assembly is in the working WO 00/09915 PCT/AU99/00649 position) to allow ready insertion therethrough to said working position, and said seal is also of annular configuration to sealingly interfere with said outlet (in said working position).

Preferably said filter screen is of cylindrical configuration, and extends around the periphery of said screen assembly, below said outlet (in said working position).

Preferably said flange is in the form of an annular step.

Preferably said clamping member is of generally annular configuration, substantially commensurate with the extent and configuration of said flange.

In accordance with a second aspect of the invention there is provided in a leach tank, an outlet being accessible from above for vertical insertion and extraction of a screen assembly as hereinbefore described, said outlet leading to a launder.

Preferably there is included a channel portion located between said outlet and said launder, said channel portion having a narrow end connecting with said outlet, and a wide end accessible from above to allow removal and replacement of said screen assembly from and to said leach tank, said channel portion tapering generally between said narrow end and said wide end.

Preferably said launder is fluidly connected to said channel portion through the wall thereof. In this manner, the slurry will exit upwardly from the screen assembly and enter the channel portion and flow across and out through said launder.

Preferably said channel portion is of frusto-conical configuration.

In accordance with a third aspect of the invention there is provided in a leach tank, a screen assembly intended to be received through an outlet in said leach tank, said outlet being accessible from above for vertical insertion and extraction of said screen assembly, said outlet leading to a launder through which fluid may be discharged after passing through said screen assembly, said screen assembly WO 00/09915 PCT/AU99/00649 -6being of proportions generally commensurate with the size of said outlet to allow ready insertion therethrough to a working position, said screen assembly having a filter screen extending in a position below said outlet (when in said working position) to allow undersize material to flow therethrough while retaining oversize material, and a seal means located between said outlet and said screen assembly to seal between said leach tank and said launder.

Preferably said seal means comprises a seal formed of a resiliently deformable material.

Preferably said outlet includes an inwardly extending flange to carry said seal, said seal extending inwardly beyond said flange to sealingly interfere with said screen assembly.

Preferably said seal is sandwiched between said flange and a clamping member, said seal also extending inwardly beyond said clamping member to sealingly interfere with said screen assembly.

Preferably said clamping member is of a configuration substantially commensurate with the extent and configuration of said annular flange.

Preferably said clamping member is divided into a plurality of portions, for ease of fitting and handling.

Preferably the uppermost of said clamping member and said flange has the smaller overall internal transverse size. Since the seal is sandwiched between said flange and the clamping member, and also extends peripherally beyond both to sealingly interfere with said screen assembly, the insertion of the screen assembly causes the seal to interfere with the screen assembly, and deflect in one direction, while the seal can readily deflect in the opposite direction allowing for easy removal of the screen assembly from the outlet.. Due to the reduced clearance between the lowermost member (of the flange and the clamping member) and the outlet, the sealing properties are improved. In an outlet where the clamping member is located below the flange, and where difficulty is WO 00/09915 PCT/AU99/00649 -7encountered with obtaining a seal between the screen assembly and the outlet, new clamping members may be fabricated with a smaller overall internal diameter, to improve the sealing characteristics.

Preferably said screen assembly is supported relative to said outlet from a position located above said outlet. In this manner, the only contact with said screen assembly below the level of the slurry (ie below the "water-line") is the contact between the seal and the outlet.

Preferably said outlet is of cylindrical shape and has a vertical axis, said screen assembly has a transverse diameter generally commensurate with the diameter of said outlet (proximal to said outlet when said screen assembly is in the working position).to allow ready insertion therethrough to said working position, and said seal is also of annular configuration to sealingly interfere with said screen assembly (in said working position).

Preferably said filter screen is of cylindrical configuration, and extends around the periphery of said screen assembly, below said outlet (in said working position).

Preferably said flange is in the form of an annular step.

Preferably said clamping member is of generally annular configuration, substantially commensurate with the extent and configuration of said flange.

Preferably there is included a tapering channel portion located between said outlet and said launder, said tapering channel portion having a narrow end connecting with said outlet, and a wide end accessible from above to allow removal and replacement of said screen assembly from and to said tank.

Preferably said launder is fluidly connected to said tapering channel portion through the wall thereof. In this manner, the slurry will exit upwardly from the screen assembly and enter the conical channel portion and flow across and out through said launder.

WO 00/09915 PCT/AU99/00649 -8- Preferably said tapering channel portion is of frusto-conical configuration.

BRIEF DESCRIPTION OF THE DRAWINGS Three preferred embodiments of the invention will now be described in the following description, with reference to the drawings in which: Figure 1 is a perspective view of a screen assembly, conical channel portion and launder according to the first embodiment; Figure 2 is a cross-section side elevation of the screen assembly, conical channel portion and launder of figure 1; Figure 3 is a cross-section side elevation of detail of the seal utilised in the first embodiment; Figure 4 is a cross-section side elevation of detail of a seal utilised in an alternative second embodiment; and Figure 5 is a cross-section side elevation of detail of a seal utilised in an alternative third embodiment.

BEST MODE(S) FOR CARRYING OUT THE INVENTION Referring to figure 1, the first embodiment is a screen assembly 11 and discharge 13 for a leach tank in a carbon-in-pulp gold extraction plant. It will of course be understood that while this embodiment is intended for use in carbon-in-pulp gold extraction, it may be used in any slurry hydrometallurgical application utilising leaching/adsorption/adsorbtion processes. As described in the preamble, the leach tank will be one of a series (often of six or seven), in which pulp moves progressively through activated carbon resident in each leach tank, the screen assembly 11 being provided with a filter screen 15 to retain the activated carbon in the leach tanks, while the finer slurry passes through the filter screen 15 and into the discharge 13, for transfer to the next leach tank in the series.

The parts list for items noted and illustrated in the embodiments is given in the following table: screen assembly 11 circular conical base member 49 discharge 13 upward blind holes 51 WO 00/09915 PCT/AU99/00649 -9filter screen 15 circular ribs 17 vertical bar 19 bolts 21 annular transverse flange 23 seal 25 structural pipe members 27 platform 29 central axial tube 31 drive shaft 33 electric motor/gearbox 35 wiper 37 spaced vertical arms 39 upper hoop 41 lower hoop 43 radial arms 45 coupling assembly 47 sparge pipes 53 outlet 55 (of discharge 13) frusto-conical channel 57 (of discharge 13) launder 59 (of discharge 13) wall 61 of frusto-conical channel 57) central tubular conduit 62 clamping member 63 nuts bolts 67 flared portion 69 (of discharge 13) apertures 71 plate 73 pair of rods u-shaped bracket 77 pivot 79 threaded actuator 81 second seal means 83 The screen assembly 11 is of cylindrical shape along its vertical axis, and the filter screen 15 is supported on a plurality of spaced circular ribs 17. The filter screen 15 comprises a stainless steel woven mesh, 20 x 20 mesh (strands per inch) having a wire strand thickness of 0.45 mm and aperture size of 820 micron. The filter screen 15 is secured and tensioned in place by a vertical bar 19 which is received in a recess (not shown), so that as bolts 21 holding the vertical bar 19 are tightened, the filter screen 15 is tensioned across the vertical ribs.

Referring to figure 2, at the top of the filter screen 15, there is provided an annular transverse flange 23, extending around the cylindrical surface of the screen assembly 11, which receives a seal 25 (see figure 3 for finer detail). Above the central tubular conduit 62, four structural pipe members 27 extend to a platform 29, which sits atop the discharge 13 (and leach tank), supporting and locating the screen assembly 11 relative to the discharge 13.

A central axial tube 31 runs from the top of the screen assembly 11, to the bottom WO 00/09915 PCT/AU99/00649 thereof, and receives a drive shaft 33 connected to a 3kW electric motor and gearbox assembly 35 configured to run the drive shaft 33 at approximately revolutions per minute. The drive shaft 33 is connected to a wiper 37. The wiper 37 has three spaced vertical arms 39 supported between upper and lower hoops 41 and 43 respectively, and has three radial arms 45 which connect to the drive shaft from the lower hoop 43. The drive shaft 33 has a coupling assembly 47 located at a position above the slurry level in the leach tank. There are no bearings or guides located lower down the drive shaft 33. This minimises maintenance which immersion might bring about. There is a clearance of approximately 30mm between the arms 39 and the filter screen 15. The purpose of the wiper 37 is to disturb activated carbon which settles against the filter screen under influence of the flow of slurry. The agitation prevents the filter screen from becoming clogged with activated carbon. The screen assembly 11 is closed at the bottom by a circular conical base member 49, which includes three upward blind holes 51 (only one of which is shown), for receiving the upstanding legs of a support stand when the screen assembly is removed for maintenance. Finally, sparge pipes 53 allow sparging down to dislodge any sedimentation from the inside of the base member 49, that may occur especially if the leach tank has been standing idle.

The discharge 13 includes an outlet 55 of circular tubular configuration which connects at its upper end with the narrow end of a frusto-conical channel 57. A launder 59 (which is an open topped conduit) extends from the wall 61 of the frusto-conical channel 57 to carry slurry from the leach tank, to the next leach tank stage, for further processing. The flow path of slurry which has passed through the filter screen 15 is indicated by double line arrows in figure 2. Internally, the screen assembly 11 includes a central tubular conduit 62 extending therewithin toward the base 49 thereof, to ensure that the region near the base 49 has flowing slurry. This prevents the slurry settling as a sediment in the base 49 of the screen assembly 11.

It will be understood that the only fluid flow path from the leach tank to the discharge 13 must be through the filter screen 15, or there will be a loss of loaded activated carbon from the leach tank. To prevent loss of loaded activated carbon WO 00/09915 PCT/AU99/00649 -11 in this manner, the seal 25 extends from the flange 23, and sealingly contacts against the inner wall of the outlet 55. A clamping member 63 (refer to figure 3) of configuration commensurate with the external periphery of the flange 23 extends with the flange and is fastened with nuts 65 and bolts 67 (spaced evenly around the flange) to sandwich the seal 25 and thereby secure it in place. The clamping member 63 is divided into four equal arcuate portions (not shown), to reduce the weight of each piece for ease of handling, fitting, and removal. This also allows for removal and fitting of the clamping member, in circumstances where the structure of the screen might prevent removal of a clamping member of unitary construction.

The seal 25 is cut from soft natural rubber sheet of thickness between 8mm and 12mm (or thicker if it is desired to increase the sealing force), and is cut to a diameter exceeding the diameter of the outlet 55 by approximately 40mm. It will be understood that changes may be made to the seal 25 as required, depending on the size of the space to be bridged between the flange 23 and the outlet The seal may be made from any resilient material that is inert to the reagents present in the leach tank, however, soft natural rubbers are the most preferred materials for the seal.

The screen assembly 11 is designed to be lifted out of the discharge 13 by a hoist. At the lower end of the outlet 55 there is provided a flared portion 69 which serves as a guide for removal of the screen assembly 11, preventing damage to the filter screen 15 that might otherwise occur should the filter screen 15 become misaligned during removal. On lifting, the seal to the outlet 55 is broken, and the screen assembly 11 may be placed on a support stand for maintenance.

To refit the screen assembly 11, it is lowered vertically in place via the frustoconical channel 57 (which is open at the top to receive the screen assembly 11).

The frusto-conical shape of the channel 57 assists in guiding the screen assembly 11 into the outlet 55. Once the platform 29 is located on the top of the discharge 13, the seal 25 will seal with the outlet 55 (since the parts are dimensioned to locate the seal 25 vertically coincident with the outlet 55). It will be understood that where it is desired to site the platform 29 on the leach tank rather than on top WO 00/09915 PCT/AU99/00649 -12of the discharge 13 (top of the channel 57), it is a simple matter to fabricate the screen assembly 11 with structural pipe members 27 (or other structure of the required length so that the seal 25 is in correct alignment with the outlet To facilitate start up (and enhanced operability), the base member 49 includes drainage means to allow drainage of any sediment present, which would otherwise settle in the base member. The drainage means comprises four apertures 71 which are closed by a flat circular stainless steel plate 73. The plate 73 has a central aperture through which the drive shaft 33 extends. The plate 73 is supported by a pair of rods 75, which extend with and on either side of the central axial tube 31. The rods 75 are fixed to a u-shaped bracket 77, mounted on a pivot 79, and moveable up and down by operating a threaded actuator 81.

Movement of the rods 75 downward, drops the plate 73, allowing any fluid and sediment to drain from the base member 49.

The second embodiment shown in figure 4 differs from the first embodiment in that the clamping member 63 has an outer edge 65 which extends closer to the wall of the outlet 55, reducing the rubber flexibility and hence creating a greater sealing force, causing the seal to get a greater "purchase" on the wall. The arrangement of the second embodiment should not interfere greatly with the force required to break the seal when removing the screen assembly 11, since the distance between the flange 23 and the wall of the outlet 55 is the same as in the first embodiment. In both embodiments, the distance between the flange 23 and the wall of the outlet 55 is in the order of 20mm. The second embodiment can be employed as a "fix" where due to unforseen problems an adequate seal cannot be obtained with the arrangement of the first embodiment. In such circumstances, it is a simple matter to fabricate new clamping members 63 with the required increased diameter, to replace the original clamping members.

The third embodiment differs from the first embodiment in that there is provided a double seal arrangement, a second seal means 83 having like numbered parts and construction as the first seal means, the second seal means 83 being located below the first seal means. This embodiment would be utilised where operational difficulties which might lead to failure of either of the first or second embodiments WO 00/09915 PCT/AU99/00649 -13would be expected to be encountered. As will be understood, one of the features of the larger diameter in the clamping member 63 in the first (upper) seal means may be utilised in further embodiments where difficulties are encountered in the field, where due to unforseen problems an adequate seal cannot be obtained.

It will be understood that an alternative to embodiments having variations in the clearance between the screen assembly 11 and the wall of the outlet 55 may be to increase the thickness of the seal With the arrangement of the screen assembly 11 and the outlet 55, there are many advantages over previously known or published techniques. The seal is readily and easily engineered and calculated. The effecting of sealing of the seal is repeatable after successive removal and replacement of the seal assembly, and finally, the seal is easily maintained and replaced. In the most preferred embodiments, all serviceable components either sit above the slurry level in the leach/adsorption/absorbtion tanks, or are removed to a position above the slurry level when the screen assembly 11 is removed for servicing. This means that in preferred embodiments, there is no need to drain a leach tank in order to effect servicing of a screen assembly.

When compared with the screen assemblies of the type which are hinged to a launder, a simple hoisting operation is required to completely remove the screen assembly 11 of the invention from a leach tank, whereafter the screen assembly can be worked on in a location away from the leach tank. In addition, the screen assembly may be replaced in its entirety, without problems associated with hinge alignment and sealing.

It should be appreciated that the scope of the invention is not limited to the specific embodiments described, and that changes may be made that will fall within the scope of the invention.

Claims (28)

1. A screen assembly intended to be received through an outlet in a leach tank, said outlet being accessible from above for vertical insertion and extraction of said screen assembly, said screen assembly being of proportions generally commensurate with the size of said outlet to allow ready insertion therethrough to a working position, said screen assembly having a filter screen extending in a position below said outlet (when in said working position) to allow undersize material to flow therethrough while retaining oversize material, and a seal means located around its periphery in a position to sealingly interfere with said outlet.

2. A screen assembly as claimed in claim 1 including a flange fixed at a position commensurate with the position of said outlet (in said working position) to locate and/or carry said seal means.

3. A screen assembly as claimed in claim 1 or 2 wherein said seal means extends peripherally beyond said flange to sealingly interfere with said outlet.

4. A screen assembly as claimed in any one of the preceding claims wherein said seal means comprises a seal formed of a resiliently deformable material. A screen assembly as claimed in any one of claims 2 to 4 wherein said seal is sandwiched between said flange and a clamping member, said seal also extending peripherally beyond said clamping member to sealingly interfere with said outlet.

6. A screen assembly as claimed in claim 5 wherein said clamping member is of a configuration substantially commensurate with the extent and configuration of said annular flange.

7. A screen assembly as claimed in claim 5 or 6 wherein said clamping member is divided into a plurality of portions, for ease of handling. WO 00/09915 PCT/AU99/00649

8. A screen assembly as claimed in any one of claims 5 to 7 wherein the uppermost of said clamping member and said flange has the greater overall external transverse size.

9. A screen assembly as claimed in any one of claims 5 to 8 wherein where the seal extends peripherally beyond said flange to sealingly interfere with said outlet, said flange is arranged to mate with a corresponding internal flange located at said outlet. A screen assembly as claimed in claim 9 wherein the seal means comprises machined mating surfaces of the flanges, a formed surface to create a line seal located on one or both contacting surfaces of the flanges, or a discrete seal which can be located between the flanges.

11. A screen assembly as claimed in any one of the preceding claims wherein said screen assembly is supported relative to said outlet from a position located above said outlet.

12. A screen assembly as claimed in any one of the preceding claims wherein said outlet is of cylindrical shape and has a vertical axis, said screen assembly has a transverse diameter generally commensurate with the diameter of said outlet (proximal to said outlet when said screen assembly is in the working position) to allow ready insertion therethrough to said working position, and said seal is also of annular configuration to sealingly interfere with said outlet (in said working position).

13. A screen assembly as claimed in claim 12 wherein said filter screen is of cylindrical configuration, and extends around the periphery of said screen assembly, below said outlet (in said working position).

14. A screen assembly as claimed in claim 12 or 13 wherein said flange is in the form of an annular step. WO 00/09915 PCT/AU99/00649 -16- A screen assembly as claimed in any one of claim 12 to 14 wherein said clamping member is of generally annular configuration, substantially commensurate with the extent and configuration of said flange.

16. In a leach tank, an outlet being accessible from above for vertical insertion and extraction of a screen assembly as claimed in any one of the preceding claims, said outlet fluidly communicating with a launder.

17. A leach tank as claimed in claim 16, including a channel portion located between said outlet and said launder, said channel portion having a narrow end connecting with said outlet, and a wide end accessible from above to allow removal and replacement of said screen assembly from and to said leach tank, said channel portion tapering generally between said narrow end and said wide end.

18. A leach tank as claimed in claim 16 or 17 wherein said launder is fluidly connected to said channel portion through the wall thereof.

19. A leach tank as claimed in claim 17 or 18 wherein said channel portion is of frusto-conical configuration. In a leach tank, a screen assembly intended to be received through an outlet in said leach tank, said outlet being accessible from above for vertical insertion and extraction of said screen assembly, said outlet leading to a launder through which fluid may be discharged after passing through said screen assembly, said screen assembly being of proportions generally commensurate with the size of said outlet to allow ready insertion therethrough to a working position, said screen assembly having a filter screen extending in a position below said outlet (when in said working position) to allow undersize material to flow therethrough while retaining oversize material, and a seal means located between said outlet and said screen assembly to seal between said leach tank and said launder. WO 00/09915 PCT/AU99/00649 -17-

21. A leach tank as claimed in claim 20 wherein said seal means comprises a seal formed of a resiliently deformable material.

22. A leach tank as claimed in claim 20 or 21 wherein said outlet includes an inwardly extending flange to carry said seal, said seal extending inwardly beyond said flange to sealingly interfere with said screen assembly.

23. A leach tank as claimed in any one of claims 20 to 22 wherein said seal is sandwiched between said flange and a clamping member, said seal also extending inwardly beyond said clamping member to sealingly interfere with said screen assembly.

24. A leach tank as claimed in claim 23 wherein said clamping member is of a configuration substantially commensurate with the extent and configuration of said annular flange. A leach tank as claimed in claim 23 or 24 wherein said clamping member is divided into a plurality of portions, for ease of fitting and handling.

26. A leach tank as claimed in any one of claims 23 to 25 wherein the uppermost of said clamping member and said flange has the smaller overall internal transverse size.

27. A leach tank as claimed in any one of claims 20 to 26 wherein said screen assembly is supported relative to said outlet from a position located above said outlet.

28. A leach tank as claimed in any one of claims 20 to 27 wherein said outlet is of cylindrical shape and has a vertical axis, said screen assembly has a transverse diameter generally commensurate with the diameter of said outlet (proximal to said outlet when said screen assembly is in the working position).to allow ready insertion therethrough to said working position, and said seal is also of annular configuration to sealingly interfere with said screen assembly (in said working position). WO 00/09915 PCT/AU99/00649 -18-

29. A leach tank as claimed in claim 28 wherein said filter screen is of cylindrical configuration, and extends around the periphery of said screen assembly, -below said outlet (in said working position). A leach tank as claimed in claims 28 to 29 wherein said flange is in the form of an annular step.

31. A leach tank as claimed in any one of claims 28 to 30 wherein said clamping member is of generally annular configuration, substantially commensurate with the extent and configuration of said flange.

32. A leach tank as claimed in any one of claims 20 to 31 wherein there is included a tapering channel portion located between said outlet and said launder, said tapering channel portion having a narrow end connecting with said outlet, and a wide end accessible from above to allow removal and replacement of said screen assembly from and to said tank.

33. A leach tank as claimed in claims 32 wherein said launder is fluidly connected to said tapering channel portion through the wall thereof.

34. A leach tank as claimed in claim 32 or 33 wherein said tapering channel portion is of frusto-conical configuration. A screen assembly and outlet substantially as herein described, with reference to any one of the embodiments.