WO2001093978A1

WO2001093978A1 - Method and apparatus for froth deaeration

Info

Publication number: WO2001093978A1
Application number: PCT/AU2001/000663
Authority: WO
Inventors: Stephen William Riley
Original assignee: M.I.M. Holdings Limited
Priority date: 2000-06-05
Filing date: 2001-06-05
Publication date: 2001-12-13
Also published as: AUPQ796900A0

Abstract

Froth (23) in a minerals separation flotation process is deaerated to improve the efficiency of pump (18) by rotating on an impeller (7) in a cylindrical sump (4) feeding the pump. The impeller has low profile blades (11) which when rotated at specified tip speeds and using specified impeller to sump diameter ratios D1:D2, effectively deaerate the froth giving improved pumping efficiency and reducing oxidation effects.

Description

TITLE: METHOD AND APPARATUS FOR FROTH DEAERATION FIELD OF THE INVENTION

The present invention relates to a method and apparatus for froth deaeration, and in particular, relates to a method and apparatus for deaerating froths containing entrained mineral particles having diameters primarily less than 25 microns.

The invention has been developed primarily for use with froths hi mineral separation flotation processes containing mineral particles of these sizes, and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use. BACKGROUND

Mineral processing plants commonly employ flotation cells in order to separate different types of minerals. In such a cell, air is pumped into the slurry (a mixture of water and particulates) and becomes entrained in the form of tiny bubbles. When this happens, the hydrophobic particles stick to the air bubbles and float to the top of the slurry as a froth (containing water, particulates, chemical additives such as frothing agents, and entrained air). The hydrophilic particles, on the other hand, tend to remain as a slurry. Thus, the flotation cell effectively separates the hydrophobic particles into a froth and the hydrophilic particles remain as a slurry.

Once the particulate mineral froth has been separated from the slurry, it then needs to be pumped downstream of the flotation cell to the next treatment or storage site. Prior art methods of pumping this froth are both difficult and inefficient because of the three-phase nature of froths (water, solids, and entrained air). In slurries, the fluid is comprised mainly of water and particulates, Ijg-fb nf.-sMii-ete are incompressible. When air is entrained in significant amounts, say aoove 25% oy Volume the entire fluid becomes compressible, and therefore does nou icaci wcfrl to t ϋ pressure increases generated in conventional slurry pumps. In addition, entrained air from the froths is commonly released into the chamber of centrifugal pumps which causes cavitation and significantly reduces the pumping efficiency.

Furthermore, in flotation pulps where air is injected into the slurry there are induced oxidation/reduction reactions. In particular, minerals such as pyrite (iron sulphide) react to produce iron oxides and iron hydroxides. In sulphide flotations, such as that of the applicant's McArthur River Mine, the oxidation of these sulphide materials results in a reducing environment which leads to the depression of value sulphide flotation. This oxidation is further enhanced by the fine particles which have a large surface area. Minimising the particle contact with oxygen can reduce the extent of this oxidisation. There is a chemical series in which different metal sulphide minerals oxidise in preference, of which the waste sulphide mineral pyrite is one of the most reactive. Although, as with all real process, this series doesn't always hold true for mixed mineral systems. It is widely practiced in the mining industry to add pre-conditioning stages immediately after size reduction (milling) and prior to reagent addition and flotation, where oxygen/air/oxidising reagents are added to speed up the oxidation of pyrite. It follows that the continued exposure of the flotation concentrate to oxygen will result in increased oxidation of reactive minerals such as pyrite (waste) and galena (value) which can reduce flotation performance. It is therefore desirable to reduce the air content of the particulate mineral froth, which reduces the amount of oxidation that occurs. THE INVENTION

In one aspect, the present invention provides a method of handling froth in a flotation process comprising the steps of subjecting the froth to the action of rotating blades having characteristics designed to shear the froth in such a manner that gas entrained in the froth is released by the shearing action of the blades, removing gas so released from the froth, and feeding the resulting deaerated froth to a pump.

Preferably, the blades are part of an impeller arranged to rotate within a vessel adapted to receive the froth. Preferably, the impeller is arranged to rotate about a substantially vertical axis and the vessel is substantially cylindrical in configuration having a substantially vertical axis substantially aligned with the axis of rotation of the impeller.

Preferably, the ratio of the diameter of the impeller to the diameter of the vessel is between 25:100 and 75:100. More preferably, the ratio of the diameter of the impeller to the diameter of the vessel is between 35:100 and 50:100.

Preferably, the vessel is provided with a gas outlet located above the impeller.

Preferably, the impeller is rotated at blade tip speeds between 5 meters per second and 40 meters per second. More preferably, the impeller is rotated at blade tip speeds between 11 meters per second and 14 meters per second.

Typically, the froth comprises a particulate mineral froth containing entrained particles where more than 80% of the entrained particles have a diameter of less than

25 microns. More typically, more than 80% of entrained particles have a diameter less than

12 microns.

Preferably, the blades have a low profile with a radial length to height ratio between 100:1 and 100:12.

More preferably, the blades have a radial length to height ratio between 100:8 and 100:12. In a further aspect, the present invention provides apparatus for deaerating froth, including a containing vessel having a froth receiving inlet, a plurality of blades adapted to be driven through froth contained within the vessel so as to impart a deaerating shearing motion to the froth, and an air outlet adapted to vent released air from the vessel.

Preferably, the blades are part of an impeller arranged to rotate within the vessel.

Preferably, the impeller is arranged to rotate about a substantially vertical axis and the vessel is substantially cylindrical in configuration having a substantially vertical axis substantially aligned with the axis of rotation of the impeller.

Preferably, the ratio of the diameter of the impeller to the diameter of the vessel is between 25:100 and 75:100.

More preferably, the ratio of the diameter of the impeller to the diameter of the vessel is between 35:100 and 50:100. Preferably, the air outlet comprises an open top to the vessel.

Alternatively, the air outlet comprises an upwardly extending conduit in communication with the cylindrical wall of the vessel.

Preferably, the blades have a low profile with a radial length to height ratio between 100:1 and 100:12. More preferably, the blades have a radial length to height ratio between 100:8 and 100:12.

Preferably, each blade is formed from a first elongate plate-like strip arranged to lie in a substantially horizontal plane in use, and a second substantially vertical strip extending upwardly from the horizontal strip over at least part of the length of the horizontal strip. Alternatively, the vertical strip extends over substantially the entire length of the horizontal strip.

THE PREFERRED EMBODIMENT

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic vertical section through a first embodiment of the froth deaerator of the present invention;

Figure 2 is a plan view of the impeller used in the froth deaerator of Figure 1;

Figure 3 is a plan view of the second embodiment of the impeller of the present invention;

Figure 4 is a side view of the impeller of Figure 3;

Figure 5 is a similar view to Figure 1 showing a second embodiment of the froth deaerator of the present invention;

Figure 6 is a perspective view of a prior art pitched-blade impeller; and Figure 7 is a perspective view of a prior art flat-blade impeller.

Although it is well known to use impellers rotated within slurries or other bodies of liquid for various purposes in the generation of froths in mineral separation flotation techniques or in other configurations requiring processing of froths, such known impellers are very different from those proposed for use in the apparatus and method of the current invention.

Referring to the drawings, Figure 6 shows a prior art pitched blade turbine 1 commonly used to agitate solid-liquid suspensions. In this embodiment, the blades 2

are pitched at an angle of 45°. Such a turbine is inappropriate for deaerating froths for

two reasons. Firstly, the blades 2 do not impart enough of a shearing force to allow the entrained air bubbles to be released from the froth 23. Secondly, because of the high profile of the blades, a significant amount of power is needed to turn the turbine

1 through the solid-liquid suspension, resulting in inefficient power usage.

Turning to Figure 7, a prior art flat-blade turbine 3, used for agitating solid- liquid suspensions, is shown. This turbine is also unsuitable for deaerating froths, for similar reasons to those mentioned in relation to the pitched blade turbine.

The present invention results from a realisation by the inventor that an impeller could be designed, which was significantly different from any prior art impeller and which would enable froths to be deaerated. Such deaeration not only reduces the volume of matter to be pumped, but also improves the efficiency of the pump by removing a considerable proportion of the compressible component of the fluid, as well as reducing the oxidation reactions noted above.

Turning to Figure 1, there is shown a first embodiment of the froth deaerator of the present invention. The froth deaerator includes a sump 4 having a froth receiving inlet 5 at its upper end and a slurry discharging outlet 6 at its lower end.

The deaerator has a rotatable, froth deaerating impeller 7 located within the sump, between the inlet 5 and the outlet 6. The deaerator also includes an air discharge vent

8 located above the impeller for discharging released air from the sump, to the atmosphere. In this embodiment, the opening at the top of the sump 4 serves as both the froth receiving inlet 5 and the air discharge vent 8.

The impeller 7 is mounted on a vertical shaft 15 forming an axis of rotation of the impeller and the sump 4 is typically cylindrical in configuration. The impeller is rotated about the shaft 15 and arranged such that the vertical axis of the cylindrical sump 4 is substantially aligned with the axis of rotation 15 of the impeller. As shown in Figures 1 and 2, the first embodiment of the impeller includes a central mounting boss 9 with eight blades 10 extending outwardly from the boss in an evenly spaced circumferential array. Each of the blades is substantially flat and has an upwardly extending protrusion or flange 11 at its tip. In this manner, each blade is formed from a first elongate plate-like strip 10 arranged to lie in a substantially horizontal plane in use, and a second substantially vertical strip 11 extending upwardly from the horizontal strip 10 over at least part of the length of the horizontal strip. As can be clearly seen in Figures 1 and 2, in the first embodiment of the invention the vertical strip extends inwardly from the tip of the blade to a position partway along the radial length of the blade.

As shown in Figure 3, a second embodiment of the impeller has eight circumferentially spaced blades 10, each having a strip base 12 and a vertical strip flange 13. The flanges on each of the blades extend from the central mounting boss 9 to the tips 14 of the blades as shown in Figure 4. Various alternative embodiments of impeller are also envisaged, including, but not limited to low profile impellers having:

(a) more or fewer blades;

(b) a disc, rather than blades; or

(c) alternative arrangements of protrusions 11 or flanges 13, protruding from the upper or lower sections of the impeller.

Whilst a number of different styles of low profile impeller may be successfully used to remove air from mineral particulate froths 23, the inventor has found that impellers having a radial length to height ratio ranging between 100:8 and 100:12 achieve the best results, although some effect is achieved with ratios between 100:1 and 100:12. In addition, whilst a number of speeds of rotation are possible, the inventor has found that the best results are achieved when the impeller is rotating at tip speeds of between 11 meters per second and 14 metres per second. Tip speeds between 5 meters per second and 40 meters per second are also believed to have some effect. The inventor has also found that the ratio of the impeller diameter Dl to the internal containing vessel diameter D2 (in this embodiment, the internal sump diameter) has an optimum range of between 25:100 to 75:100. Whilst alternative ratios are envisaged, the inventor has found that when the ratio (D1:D2) gets much larger than 75:100, the impeller impedes the flow of the froth/slurry. At the other end of the spectrum, when the ratio is much less than 25:100, the impeller 7 is not able to remove sufficient entrained air from the froth 23 and convert it into slurry 24.

It has been found that the impeller can work quite efficiently at a ratio of impeller diameter Dl to vessel diameter D2 of 35:100.

Furthermore, the inventor has also found that the angle of the impeller blades 10 to the drive shaft or axis of rotation 15 is also significant. The best results are achieved when the blades 10 extend from the central mounting boss 9 at an angle of

between 75° and 90° to the axis of rotation 15.

In the first embodiment of the froth deaerator of the present invention, shown in Figure 1, the impeller 7 is positioned within the sump 4 used to feed a pump 18, near the top 16 of the cone-shaped base 17 of the sump. The inventor has found that, at this level, the impeller 7 is able to achieve the best deaeration results.

The inventor has also achieved superior results when the impeller is positioned below the natural froth/liquid interface 25 of the slurry 23. Dotted line A in Figure 1 indicates the maximum height the froth 23 reaches when the impeller is rotating at its optimum speed. Dotted line B in Figure 1 indicates the maximum level the froth reaches when the impeller is turned off. As shown, when the impeller is off, the froth simply overflows the sump 4. As shown in Figure 1, the froth deaerator of the present invention is preferably used in combination with a conventional slurry pump 18 (such as a Warman pump) which pumps the slurry 24 in a traditional fashion, once it has been deaerated. The inventor has found that significant savings in pump power usage and cost have been achieved by using the deaerator of the present invention. In one alternative embodiment, the impeller 7 and the slurry pump 18 are powered by the same motor (not shown), leading to additional power reductions.

Referring now to Figure 5, there is shown a second embodiment of the deaerator of the present invention. This second embodiment is an in-line deaerator in which the sump has been replaced by a substantially vertical pipe or conduit 20 and in which the air discharge vent 8 is positioned in a cylindrical wall 21 of the pipe. The air discharge vent 8 is positioned above the impeller 7 so that released air can escape through the vent. The discharge vent communicates with an air discharge passage 22 which extends upwardly and outwardly from the pipe 20 to facilitate the discharge of released air. The inventor has found that the best results are achieved when the air

discharge passage 22 extends from the pipe at an angle of between 45° and 90° to the

horizontal. The angle from the horizontal is needed to prevent the froth 23 from escaping up the discharge passage. Whether the first embodiment (sump 4) or second embodiment (pipe 20) is used, the deaerator of the present invention is preferably used in accordance with a method which includes the following steps:

(a) feeding the particulate mineral froth 23 into the sump/pipe through the froth receiving inlet 5;

(b) allowing the froth to come into contact with the rotatable impeller 7;

(c) shearing and compressing the froth 23 by rapidly rotating the impeller through the froth to produce:

(i) released air 26 in the form of small (less than 2mm diameter) entrained air bubbles; and

(ii) slurry 24, containing less than 25% entrained air by volume;

(d) venting the released air 26 via the air discharge vent 8; and

(e) discharging the slurry 24 via the slurry discharging outlet 6. Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

By removing a significant proportion of the air from the froth before it reaches the pumping chamber, the pumps downstream of the deaerator are able to operate more efficiently. In addition, by reducing the overall volume of the froth to be pumped, smaller pumps, requiring less power, can be installed. The present invention therefore enables a user to save on both capital and operating expenses in the handling and pumping of froth/slurry mixtures.

An additional advantage of the present invention is that the froth is not damaged during the deaeration process, since no new chemicals are added which would adversely effect the downstream processes. Furthermore, the removal of a significant proportion of the entrained air reduces the amount of oxidation that can occur with the entrained solids. Such oxidation is usually detrimental to downstream flotation or treatment of the entrained solids, as it reverses the induced hydrophobicity of the valuable minerals.

Claims

CLAIMS:-

1. A method of handling froth in a flotation process comprising the steps of subjecting the froth to the action of rotating blades having characteristics designed to shear the froth in such a manner that gas entrained in the froth is released by the shearing action of the blades, removing gas so released from the froth, and feeding the resulting deaerated froth to a pump.

2. A method as claimed in claim 1, wherein the blades are part of an impeller arranged to rotate within a vessel adapted to receive the froth.

3. A method as claimed in claim 2, wherein the impeller is arranged to rotate about a substantially vertical axis and the vessel is substantially cylindrical in configuration having a substantially vertical axis substantially aligned with the axis of rotation of the impeller.

4. A method as claimed in claim 3, wherein the ratio of the diameter of the impeller to the diameter of the vessel is between 25:100 and 75:100.

5. A method as claimed in claim 4, wherein the ratio of the diameter of the impeller to the diameter of the vessel is between 35:100 and 50:100.

6. A method as claimed in any one of claims 2 to 5, wherein the vessel is provided with a gas outlet located above the impeller.

7. A method as claimed in any one of claims 2 to 6, wherein the impeller is rotated at blade tip speeds between 5 meters per second and 40 meters per second.

8. A method as claimed in claim 7, wherein the impeller is rotated at blade tip speeds between 11 meters per second and 14 meters per second.

9. A method as claimed in any one of the preceding claims, wherein the froth comprises a particulate mineral froth containing entrained particles where more than 80% of the entrained particles have a diameter of less than 25 microns.

10. A method as claimed in claim 9, wherein more than 80% of entrained particles have a diameter less than 12 microns.

11. A method as claimed in any one of claims 2 to 10, wherein the blades have a low profile with a radial length to height ratio between 100:1 and 100:12.

12. A method as claimed in claim 11, wherein the blades have a radial length to height ratio between 100:8 and 100:12.

13. Apparatus for deaerating froth, including a containing vessel having a froth receiving inlet, a plurality of blades adapted to be driven through froth contained within the vessel so as to impart a deaerating shearing motion to the froth, and an air outlet adapted to vent released air from the vessel.

14. Apparatus as claimed in claim 13, wherein the blades are part of an impeller arranged to rotate within the vessel.

15. A method as claimed in claim 14, wherein the impeller is arranged to rotate about a substantially vertical axis and the vessel is substantially cylindrical in configuration having a substantially vertical axis substantially aligned with the axis of rotation of the impeller.

16. Apparatus as claimed in claim 15, wherein the ratio of the diameter of the impeller to the diameter of the vessel is between 25:100 and 75:100.

17. Apparatus as claimed in claim 16, wherein the ratio of the diameter of the impeller to the diameter of the vessel is between 35:100 and 50:100.

18. Apparatus as claimed in any one of claims 13 to 17, wherein the air outlet comprises an open top to the vessel.

19. Apparatus as claimed in any one of claims 15 to 17, wherein the air outlet comprises an upwardly extending conduit in communication with the cylindrical wall of the vessel.

20. Apparatus as claimed in any one of claims 13 to 19, wherein the blades have a low profile with a radial length to height ratio between 100:1 and 100:12.

21. Apparatus as claimed in claim 20, wherein the blades have a radial length to height ratio between 100:8 and 100:12.

22. Apparatus as claimed in any one of claims 14 to 21, wherein each blade is formed from a first elongate plate-like strip arranged to lie in a substantially horizontal plane in use, and a second substantially vertical strip extending upwardly from the horizontal strip over at least part of the length of the horizontal strip.

23. Apparatus as claimed in claim 22, wherein the vertical strip extends over substantially the entire length of the horizontal strip.