AU712511B2 - A launder arrangement, settling tank and a process for removing surface froth - Google Patents

Description

WO 97/17123 PCT/AU96/00707 A LAUNDER ARRANGEMENT, SETTLING TANK AND A PROCESS FOR REMOVING SURFACE

FROTH

FIELD OF THE INVENTION The present invention relates to a launder arrangement, a settling tank, a froth treatment device for a separation device and a method of operating same. Such separation devices include thickeners and clarifiers.

BACKGROUND OF THE INVENTION Thickeners and clarifiers are gravity liquid/solid separation devices and are typically used in process or chemical plants to separate the components of slimes, slurries and sludges.

Thickeners and clarifiers generally include a circular tank in which rotates a set of slowly moving rake arms mounted close to the tank floor. A feed stream comprising a mix of solids and liquid is fed into the tank. Under the influence of gravity, the solids gradually settle to the bottom of the tank. The rake arms are generally driven by a drive assembly which may be supported above the tank either by a bridge spanning the tank or particularly in the case of large diameter tanks, by a centre column or pier. As they rotate, the rake arms slowly move the settled solids to an outlet in the tank floor. The liquid component of the feed stream is removed from the tank in any suitable manner, such as by overflow into an open launder which surrounds the tank at the top of the tank wall and removed through an outlet pipe.

In an ideal operating thickener or clarifier all the solids settle to the bottom of the tank and the supernatant liquid flowing into the launder is free of solids.

When operating thickeners or clarifiers, some of the solids, particularly the finer particles, may not settle to the bottom of the tank but may float on or be suspended by the liquid. This is particularly true if a froth forms on the liquid surface during the thickening or clarifying process as the froth forms a support blanket for the finer solids. In most cases, it is desirable that the liquid overflowing into the launder be collected and returned to the plant for further reuse. However, if a layer of froth, which contains suspended solids, exists on the liquid surface, the froth will flow into the launder, resulting in the removed liquid containing suspended solids. This may render the removed liquid unsatisfactory for re-use. The solids suspended in the removed liquid may also represent a loss of product or may cause problems with equipment as the liquid is reused within the plant. The normal preventative solutions to overcome the problem are either high pressure liquid sprays, such as a froth suppression fluid or water or other liquid, or a froth baffle near the launder or a combination of both. The high pressure sprays are directed onto the tank surface to break up the froth and to sink the solid particles. While this can be effective, there is high maintenance to keep all the sprays unblocked and operating properly due to the general contamination levels of plant process liquids. If one spray stops working then there is a passage open to allow solids to flow to the launder. It is generally not practicable to spray a froth suppression fluid on the tank liquid SUBSTITUTE SHEET (Rule 26) WO 97/17123 PCT/AU96/00707 2 surface, as the chemicals in the fluid may affect other processes once the liquid is returned for re-use in the plant.

The froth baffle is only effective if maintained properly without any gaps in the baffle and if the froth loading is low. At high froth loadings the froth may flow under or over the baffle and into the launder.

This invention is directed towards removing surface from a mixture (such as a solid/liquid mixture in a setting tank) in an efficient and economical manner.

SUMMARY OF THE INVENTION The invention provides a launder arrangement for removing surface froth from a mixture in a settling tank, said launder arrangement including: *a first launder adapted to be located near an upper surface of the mixture, and having an entry point for receiving surface froth from the mixture and a launder outlet for removal of said surface froth; *a second launder, having inlet means for receiving liquid from the mixture, and outlet means, for removing liquid from the second launder, said inlet means being positioned lower than the entry point of the first launder; and *Baffle means for inhibiting surface froth entering the second launder.

Preferably, the inlet means of the second launder is located in a lower portion of the second launder. The lower portion is preferably the base of the second launder. Preferably the inlet means are spaced apart holes in the base of the second launder.

The second launder is preferably located adjacent a sidewall of the tank. It is preferred that the second launder is positioned radically inwardly of the sidewall and extends around the circumference of said sidewall.

It is further preferred that the first launder be located adjacent the second launder. Preferably, the first launder is positioned radially inwardly of the second launder and may be substantially concentric with the second launder. An outer wall of the first launder and an inner wall of the second launder may be a common wall. The top of this common wall is preferably higher than the entry point of the first launder.

The baffle is preferably located radially inwardly of the inlet means of the second launder. The baffle may include a flange extending downwardly from the first launder, the second launder or from the common wall.

The first launder may have a base which includes at least one trough portion and a launder outlet located in said at least one trough portion. The base of the first launder preferably has a series of trough portions and peak portions. Preferably, these trough and peak portions are WO 97/17123 PCT/AU96/00707 3 joined by inclined portions which are adapted to direct surface froth towards the trough portions.

The launder arrangement described above may also include at least one spray means adapted to spray fluid into the first launder to direct any surface froth towards said launder outlet.

Preferably, there will be several spray means located above the first lauder. Preferably, spray means are located directly above each peak portion of the base of the first launder. Preferably, the launder outlet of the first launder is positioned lower than the second launder.

The invention further provides a settling tank for separating a mixture of solids and liquids, said settling tank including a launder arrangement as described above.

According to a further aspect of the invention, there is provided a settling tank for separating a mixture of solids and liquids, including: a first launder adapted to be located near an upper surface of the mixture, having an entry point for receiving surface froth from the mixture and a launder outlet for removal of said surface froth; tank outlet means for removal of liquid from the mixture, said tank outlet means being positioned lower than the entry point of the first launder; and baffle means for inhibiting surface froth from entering the tank outlet means.

Preferably, the tank outlet means is located on a sidewall of the tank and the baffle means is located radially inwardly of the tank outlet means.

Preferably, the tank outlet means includes a second launder having inlet means for receiving liquid from the mixture, and outlet means for removing liquid from the second launder. The inlet means is preferably in a lower portion of the second launder. The said lower portion is preferably the base of the launder, and preferably the inlet means is spaced apart holes therein.

The invention, also provides a process for removing surface froth from a mixture in a settling tank having a launder arrangement as described above, said process including the steps of: adjusting the surface of the mixture to a height (or maintaining it at a height) level with or slightly above the entry point of the first launder; allowing surface froth to overflow into the first launder; allowing liquid to pass from the mixture into the tank outlet means; and removing surface froth from the first launder; The height of the mixture may be adjusted or maintained by controlling the flow of liquid through the tank outlet means.

The process may further include the step of spraying the first launder to direct any surface froth gathered into the first launder towards the launder outlet.

WO 97/17123 PCT/AU96/00707 4 The invention also provides a method of operating a separation device said separation device including a settling tank, a first launder and tank outlet means as described above, said method including the step of controlling the outlet flow rate of liquid from said tank outlet means, to prevent liquid or surface froth from entering said first launder; Preferably, a second step in the method includes decreasing the outlet flow from said tank outlet means to thereby increase, relative to an entry point to said first launder, the level of liquid in said settling tank, so that surface froth will flow into said first launder.

Preferably, a third step in the method includes the spraying of liquid into said first launder to direct surface froth gathered in said first launder towards a launder outlet of said first launder.

Preferably, the fourth step in the method includes the drawing off of surface froth from said first launder so that the froth can be further processed.

Preferably said second, third and fourth steps are performed either periodically, intermittently, or after detection of froth on the surface of said settling tank.

Preferably the fifth step in the method is to deliver the surface froth from said first launder to a collection sump.

Preferably a sixth step in the method includes dissolving the surface froth or entraining the solid particles suspended by the froth into a liquid.

Preferably a seventh step includes either dewatering the liquid in order to obtain dissolved or suspended solids or returning the liquid, with solid particles entrained, back to the settling tank.

Preferably, said tank outlet means includes a second launder as described above.

The invention further provides a froth treatment device including a reservoir having entry means for receiving surface froth from a settling tank, said device also including froth suppression means to dissolve said froth or to entrain solid particles suspended by the froth into said liquid.

Preferably, said suppression means includes spray means.

Preferably, said froth treatment device can be associated with a pump means to pump said liquid (with dissolved froth and solid particles) to another location.

Preferably, said pump is associated with level switching means to activate and deactivate said pump according to the level of liquid in said device.

Preferably, said froth treatment device is associated with a selection means so that the location to which said liquid (with dissolved froth and solid particles) is to be pumped can be selected according to need.

Preferably, said selection device can select between at least two locations, namely either a settling tank or a dewatering/filtering location.

WO 97/17123 PCT/AU96/00707 One advantage of the launder arrangement described above is that the amount of liquid which is captured by the first launder (which is intended to receive surface froth is minimised. This is because the first launder can be used intermittently or only when the froth levels require it. This helps to ensure that there is sufficient liquid for normal operating procedures and minimises the amount of liquid lost or removed when removing the surface froth.

Another advantage is that the invention provides a relatively efficient froth removal system which can be activated intermittently or continuously to remove surface froth when it appears, rather than letting the froth build up to unacceptable levels, as has been the case in the prior art systems.

A further advantage of the launder arrangement of this invention is that it can be installed relatively easily and inexpensively into existing settling tanks.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a separation device of the prior art, in cross section; Figure 2 is a plan view of the apparatus of figure 1; Figure 3 is a cross section through the launder of the apparatus of figure 1; Figure 4 is a cross section through the launder of the present invention; Figure 5 is an isometric view of a part of the launder of figure 4; and Figure 6 is a froth collection sump and tank as used with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Illustrated in Figures 1, 2 and 3 is a separation device of the prior art. These are also known as thickeners or clarifiers and are continuous gravity liquid/solid separation devices used in process or chemical plants to separate the components of slimes, slurries and sludges. The separation device 1 of the prior art generally consists of a circular tank 31 which contains a mixture of solids and liquids and in which rotates a set of slowly moving rake arms 9 mounted close to the tank floor 30. The rake arms 9 are driven by a drive assembly 7 which is supported above the tank 31 either by a bridge 6 spanning the tank (or in the case of large diameter tanks by a centre column or pier which is not illustrated).

As the rake arms 9 rotate the rake blades 13 (which are attached to the rake arms 9) slowly move settled solids to an outlet 10 in the tank floor 30. This outlet 10 can be at either the centre of the tank 31 or at the tank wall 32 (in the case of a tank with a flat or substantially flat floor).

In an ideal operating condition, all solids entrained or suspended in the mixture 5 settle to the tank floor 30 and liquid flows into the launder 2, free of solids.

WO 97/17123 PCT/AU96/00707 6 A feed stream comprising a mix of solids and liquid is fed through a pipe 3 (or an open launder) into a feedwell 4, which is mounted at the top of the tank 31. The feedwell 4 is supported from the bridge 6. When the tank 31 is full with mixture 5, the influence of gravity causes the solids component of the mixture 5 to gradually settle to the tank floor 30. The liquid component of the mixture 5 overflows into an open launder 2 which surrounds the tank 31 adjacent the top of the tank wall 32. The liquid flows around the launder 2 and then exits the launder 2 through outlet 11.

In some cases of operation of the separation device 1, some of the solids, particularly the finer particles, may not settle to the tank floor 30, but may float or be carried by the liquid as it moves toward and into the launder 2. This is particularly true if a froth (not illustrated) forms on the liquid surface due to air entrainment from previous processes. The surface froth forms a support blanket for the fine solids and inhibits them from settling.

In most cases the overflowing liquid is collected via outlet 11 and returned to the plant for further reuse.

It is generally not practical to spray a froth suppression fluid on the surface of the mixture 5 as the chemicals may affect other processes once the liquid is returned for reuse in the plant.

Solids entrained in the liquid, which exit via outlet 11, may be a loss of valuable product, may contaminate the liquid or may cause problems with equipment when the liquid is reused within the plant.

Prior art solutions to overcome this problem use either high pressure liquid sprays 15 or a froth baffle 13 near the launder 2 or a combination of both. The high pressure sprays 15 are fitted to a manifold 14 and are directed onto the feed stream surface to break up froth and to sink the solids particles. While this can be effective there is a high maintenance cost to keep all the sprays unblocked and operating properly due to the general contamination levels of plant process liquids. If one spray stops working then there is a passage open to allow solids to flow to the launder 2.

A froth baffle 13 is attached to launder 2 and is only effective if maintained properly without any gaps in the baffle and if the froth loading is low. At high loadings the froth may flow under and over the baffle and into the launder 2.

Figure 3 is a cross section of the prior art launder 2 (from figure 1) which includes baffle 13.

The liquid 5 from the mixture 5, when the level in the tank 31 is sufficiently high, flows over the wall 2A of launder 2. The support 16 keeps the baffle 13 a sufficient distance away from wall 2A and allows liquid 5A on the left of baffle 13 to pass under the baffle 13, leaving any surface froth (and thus solids) to the left of baffle 13. The liquid 5A, now on the right of baffle 13, can flow over wall 2A and into the launder 2. This liquid 5A then leaves the separation device 1, via outlet 11.

WO 97/17123 PCT/AU96/00707 7 Figures 4 and 5 illustrate an embodiment of the invention wherein an inner launder 19 and an outer launder 17 are arranged adjacent the top of the tank wall 32.

The outer launder 17 is for the collection of the clean liquid while the inner launder 19 is for the collection and transportation of the froth and floating particles and any liquid associated therewith.

The outer launder 17 has inlets 18 in the base 33 of the launder 17. The size and number of the inlets 18 will depend on the required design flow of liquid for each separation device 1. An outlet pipe 11 fitted with a control valve 24 is connected to the outer launder 17 to control the rate of liquid leaving the outer launder 17. Control valve 24 can be any form of flow restriction such as a conventional valve, a dart valve, adjustable weir or any other type of valve. From control valve 24 liquid is carried away by piping (not illustrated) into a collection tank and/or pump (not illustrated).

Between launders 17 and 19 is a common wall 20 which preferably extends higher than wall 21 of inner launder 19. Outlet pipes 23 are fitted to the inner launder 19 to allow the collected surface froth to leave the inner launder 19. To assist in movement of the surface froth in the inner launder, there are at least four outlet pipes 23. The number of outlet pipes, 23, may depend on tank diameter and length of launder. The bottom 19a of the inner launder 19 is sloped toward each of the outlets to form a series of alternating peaks 19(c) and troughs 19(b) to which the outlets 23 are in fluid communication. This is best illustrated in Figure A baffle plate 22 projects downwardly from the common wall At positions around the inner launder 19 there are located spray manifolds 24 and spray nozzles to direct the froth to the outlets pipes 23. The spray manifolds 24 are generally located above launch 19 just above the height of common wall 20 to stop froth entering launder 17.

The spray manifolds 24 are preferably located immediately over the peaks 19(c), but may also be located at other locations along the inner launder 19.

The froth can be separately piped from the outlet pipes 23 into a froth collection sump 26 as illustrated in figure 6. The froth collection sump 26 of figure 6 includes froth dissipation or suppression sprays 28 which spray the froth to help to dissipate or suppress the froth and thus entrain the solid into liquid for further processing which will be discussed below. The sump 26 also includes two level switches 29, which are bubble or float switches or some other kind of liquid level switch, one at a high level mark and the other at a low level mark. When liquid reaches a high level mark the switch at that location senses the level and activates the pump 27 to decrease the liquid level until the switch at the lower level also detects the level and switches the pump off. It is desirable to keep a sufficient head or height of liquid above the inlet to the pump 27 to ensure that mainly solid and liquid are pumped out of the sump 26 and that there is no entrainment of air in the liquid. The pump 27 is also positioned to pump from the base so that any solid which may have settled will be removed first. The liquid and any entrained solid WO 97/17123 PCT/AU96/00707 8 from the collection sump 26 can be switched to either proceed back to the separation device 1 or into a dewatering/filtration process (not illustrated) so as to recover the solid contained in the liquid.

The following describes the operation of the invention.

The removal of the froth from the separation device can either be a continuous operation or a batch operation. The removal of froth by means of a batch operation is one of the advantages which result from the above described embodiment.

In a continuous operation, valve 24 is partially closed to raise and maintain the level of the mixture 5 in the tank 31 so that the mixture level is slightly higher than wall 21. Once the mixture level in the tank 31 is higher than wall 21 surface froth and some liquid will flow from tank 31 into the inner launder 19, then into the outlet pipes 23 and then into the collection sump 26. The flow of surface froth and some liquid into the inner launder 19 will continuously remove froth from the surface of the mixture 5. Sprays 25 will operate to continuously flush the froth inside the inner launder 19 toward the outlet pipes 23.

In a batch operation, valve 24 is opened so that the level of mixture 5 in the tank 1 is dropped below the level of wall 21 thereby preventing surface froth from flowing into inner launder 19.

With the level of mixture 5 below the height of wall 21, liquid will still flow under baffle 22 up through inlets 18 and into outer launder 17. From outer launder 17 the liquid will flow into outlet pipe 11 and through valve 24.

At a preset time interval or from an input from some form of froth measurement device, valve 24 will partially close causing the level of mixture 5 in tank 1 to rise until it is slightly above the height of wall 21 thereby discharging surface froth into inner launder 19. At this point, the batch operation is similar to the continuous operation described above. After a preset time interval or from an input from some form of froth measurement device (not illustrated), valve 24 will again open to lower the level of mixture 5 in tank 31 below the height of wall 21. The discharge of froth into inner launder 19 will cease but liquid will still flow upward into outer launder 17 etc. The previously discharged froth will flow into the collection sump 26 where froth is suppressed. Level switches (not illustrated) will cycle pump 27 on and off, to maintain the required level of froth in the sump 26.

When the feed stream to the separation device 1 is stopped, liquid will continue to flow under baffle 22 and upward through inlets 18 and into outer launder 17 until such time as the level of mixture 5 in tank 1 is below the inlets 18.

Continuous baffle 22 will prevent any froth floating on the top of mixture from flowing into inlets 18 and hence into inner launder 17.

The foregoing describes one embodiment of the present invention and modifications by those skilled in the art can be made thereto without departing from the scope of the present invention.

Claims (24)

1. A launder arrangement for removing surface froth from a mixture in a settling tank, said launder arrangement including: a first launder adapted to be located near an upper surface of the mixture and having an entry point for receiving surface froth from the mixture, and a launder outlet for removal of said surface froth; a second launder, having inlet means for receiving liquid from the mixture, and outlet means for removing liquid from the second launder, said inlet means being positioned lower than the entry point of the first launder; and baffle means for inhibiting surface froth from entering the second launder.

2. A launder arrangement according to claim 1, wherein the inlet means of the second launder is located in a lower portion of the second launder.

3. A launder arrangement according to claim 2, wherein the lower portion is a base of the second launder.

4. A launder arrangement according to claim 2, wherein the inlet means are spaced apart holes in the base of the second launder.

A launder arrangement according to any one of claims 1 to 4, wherein the second launder is located adjacent a sidewall of the tank.

6. A launder arrangement according to claim 5, wherein the second launder is positioned radially inwardly of said sidewall and extends around the circumference of said side wall.

7. A launder arrangement according to claim 6, wherein the first launder is located adjacent the second launder.

8. A launder arrangement according to claim 7, wherein the first launder is positioned radially inwardly of said second launder and is substantially concentric with the second launder.

9. A launder arrangement according to claim 7 or claim 8, wherein the first and second launders are separated by a common wall, the top of which is at least as high as the entry point of the first launder.

10. A launder arrangement according to any one of claims 1 to 9, wherein the baffle means is located radially inwardly of the inlet means of the second launder.

11. A launder arrangement according to claim 10, wherein the baffle means includes a flange extending downwardly from the first launder, the second launder or the common wall. CE\99197020.9

12. A launder arrangement according to any one of claims 1 to 11, wherein a base of the first launder includes at least one trough portion and a launder outlet located in said at least one trough portion.

13. A launder arrangement according to any one of claims 1 to 12, wherein the base of the first launder includes a series of trough portions and peak portions.

14. A launder arrangement according to claim 13, wherein said base includes inclined portions joining the trough portions and peak portions, said inclined portions being adapted to direct surface froth towards said trough portions.

A launder arrangement according to any one of claims 1 to 14, further including at least one spray means adapted to spray fluid into the first launder to direct any surface froth towards said launder outlet.

16. A launder arrangement according to claim 15, wherein the spray means are located above each peak portion of claim 13 or claim 14.

17. A launder arrangement according to any one of claims 1 to 16, wherein the launder 15 outlet of the first launder is positioned lower than the second launder. g* 0. 0

18. A settling tank for separating a mixture of solids and liquids, said settling tank including a launder arrangement according to any one of claims 1 to 17 and a tank outlet means. .0 0 0

19. A process for removing surface froth from a mixture in the settling tank according to 20 claim 18, said process including the steps of: adjusting the surface of the mixture to a height, or maintaining it at a height, level with or above the entry point of the first launder; Soallowing surface froth to overflow into the first launder; removing the surface froth from the first launder; allowing liquid from the mixture to pass into the second launder; and removing the liquid from the second launder.

A process according to claim 22, wherein the height of the mixture is adjusted or maintained by controlling the flow of liquid through the tank outlet means.

"21. A process according to claim 22 or claim 23, including the step of spraying the first launder to direct any surface froth gathered in the first launder towards the launder outlet.

22. A process according to any one of claims 22 to 24, including the step of dissolving or transferring the surface froth from the first launder to a collection sump. CE\99197020.9 11

23. A process according to any one of claims 22 to 25, including the step of entraining the surface froth into a liquid and returning it to the settling tank.

24. A launder arrangement for removing surface froth from a mixture in a settling tank, said launder arrangement being substantially as hereinbefore described with reference to Figure 4 or Figure 00 0 *00 9 S e