Flotation tank impeller

An impeller for use with apparatus to enhance mixing of a gas and a liquid, as used for example in froth flotation apparatus, having a substantially upright portion (11) substantially parallel to the axis of rotation (17), and a plurality of vanes (12) extending outwardly therefrom, at least some of the vanes (12) having an upper part (19) adapted to induce liquid flow generally downwardly towards the impeller and a lower part (20) adapted to induce liquid flow generally upwardly towards the impeller, wherein the impeller further comprises barrier means (14) at least partially dividing the upper and lower parts, and a gas input situated on the same side of the barrier means (14) as the lower part (20). The barrier means (14) may comprise a flange like member in a plane generally perpendicular to the axis of rotation (17) and may intersect, or be integral with at least part of the vanes.

FIELD OF THE INVENTION
 This invention relates to an impeller primarily but not exclusively for use
 with apparatus adapted to enhance mixing of a gas and a liquid, as used
 for example in froth flotation apparatus for separation of substances.
 DESCRIPTION OF THE PRIOR ART
 Froth flotation is a process currently used in both mineral processing to
 separate required mineral particles from gangue in a mineral ore slurry
 and in other processing systems when a separation of one liquid type or
 types can be made from another liquid or liquids. The apparatus utilised
 comprises a tank for holding the slurry, an impeller for inducing flow of
 the slurry and dispersing a gas therein, a static diffuser arrangement
 located outwardly of the impeller and a means to introduce the gas,
 commonly compressed air, into the slurry.
 It will be appreciated that the term "mixing" used herein should be read as
 relating to dispersion of a gas in a liquid, true homogeneity not
 necessarily being achieved by introduction of the gas into the liquid.
 Furthermore, the term liquid will hereinafter he used although it will be
 appreciated that during use of the apparatus the liquid may contain solid
 matter and thus constitute a slurry, and the term slurry should be
 understood to comprise a suspension of solid particles in a liquid in
 general, and not be limited to any particular mineral suspension.
 The mechanism of the extraction process is known in the art, and will
 therefore not be described here in any great detail, although an outline
 of the principal when used in mineral processing is to cause the required
 mineral particles to adhere to bubbles of the gas, which then rise to the
 surface of the liquid to form a froth which is removed. A plurality of
 such tanks may be provided in series with the liquid passing through each
 in turn, such that any required minerals not extracted in a given tank may
 be subjected to a further extraction process in the next tank in the
 series, or a single tank housing a plurality of gas/liquid mixing
 apparatus may he provided.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide an improved impeller
 which enhances the above mixing process and improves the efficiency of the
 extraction process.
 According to a first aspect of the invention there is provided an impeller
 for rotation about a substantially upright axis for use with apparatus
 adapted to enhance mixing of a gas and a liquid, the impeller having a
 substantially upright portion substantially parallel to the axis, and a
 plurality of vanes extending outwardly therefrom, at least some of the
 vanes having an upper part adapted to induce liquid flow generally
 downwardly towards the impeller and a lower part adapted to induce liquid
 flow generally upwardly towards the impeller, wherein the impeller further
 comprises barrier means at least partially dividing said upper and lower
 parts, and a gas input situated on the same side of the barrier means as
 the lower part.
 Preferably, the barrier means extends outwardly from the substantially
 upright portion.
 The barrier means preferably comprises a flange-like member having upper
 and lower faces, at least one face of which is flat or substantially flat.
 Preferably the lower face is the flat or substantially flat face.
 Preferably the flange-like member is substantially planar. Conveniently the
 flange-like member lies in a plane substantially perpendicular to the axis
 of rotation. The flange-like member may conveniently be disc shaped.
 The substantially upright portion may comprise a cylinder-like portion
 wherein the vanes extend outwardly from the outer surface of the
 cylinder-like portion. Conveniently, the vanes extend radially outwardly
 from the cylinder-like portion. The vanes may be secured to the outer
 surface of the cylinder-like portion, or may pass partly therethrough.
 The substantially upright portion preferably has at least one passage
 therein. Conveniently, the or each passage lies substantially parallel to
 the axis of rotation of the impeller.
 A lower end of the substantially upright portion is preferably located
 below the plane of the barrier means.
 Conveniently, the or each passage has an input end adapted to receive at
 least one gas and an output end adapted to emit the or each gas to the
 tank. Preferably the input end is adapted to be connected, in use, to a
 gas source which is at greater than atmospheric pressure.
 Conveniently, the output end is situated at or near the lower end of the
 substantially upright portion.
 As an alternative, the substantially upright portion may be substantially
 hollow. Preferably the substantially upright portion comprises a tube. The
 tube may also have an input end and an output end analogous to those
 described above.
 Conveniently, only a part of the vanes engage the substantially upright
 portion.
 The barrier means preferably comprises an annulus, the inner rim of which
 is located substantially adjacent the outer surface of the tube.
 Conveniently, the radius of the annulus is less than the dimension of the
 vanes in a radial direction. Such a direction will henceforth be referred
 to as the width of the vanes, and, correspondingly, the dimension
 perpendicular to the width, and thus parallel with the axis of rotation of
 the impeller, will be referred to as the height.
 The upper part of the vanes preferably has a top edge and inner and outer
 side edges, wherein the inner side edge abuts or passes into the
 substantially upright portion, and wherein the outer side edge is remote
 from the substantially upright portion. The outer side edge of the upper
 part sweeps through the liquid in the tank in a rotational manner in order
 to induce flow of the liquid. The top edge of the vane and the outer side
 edge of the upper part of the vane may meet substantially at right angles
 to each other to define a corner of the vane.
 Alternatively however, the top edge of the vane and the outer side edge of
 the upper part of the vane may be joined by an intermediate edge.
 The intermediate edge may be generally linear, or may be of curved
 configuration. Preferably, the curved configuration is concave.
 The lower part of the vane has a bottom edge, and inner and outer side
 edges. Preferably, the inner side edge is located at or near the axis of
 rotation. Accordingly, where there are a plurality of such vanes, the
 inner side edges of each preferably meet at or near the axis of rotation.
 There is preferably an intermediate edge between the bottom edge and outer
 side edge of the lower part of the vane. Conveniently, the intermediate
 edge is of curved configuration suitable to induce flow of the liquid
 generally upwardly towards the impeller. Preferably, the curved
 configuration is concave.
 Alternatively, the substantially upright portion may comprise a
 cylinder-like portion wherein the vanes extend outwardly therefrom but do
 not engage the outer surface of the cylinder-like portion. In such a case,
 the cylinder-like portion may have associated therewith a flange,
 henceforth referred to as a drive flange for the sake of clarity. The
 drive flange is preferably of annular configuration. Preferably, the
 barrier means is attached to or integral with the drive flange. In this
 alternative, the barrier means comprises a first part attached to or
 integral with the drive flange, and a second part extending radially
 outwardly from the first part.
 Preferably, the second part engages at least some of the vanes. Preferably,
 both the first and second parts are substantially annular, having
 substantially flat lower faces. In this alternative, the distance between
 opposite inner side edges of the upper parts of the vanes is preferably
 greater than the diameter of the cylinder-like portion. Conveniently, said
 distance is greater than the diameters of both the drive flange and the
 first part of the barrier means. The inner side edges of the lower parts
 of the vanes, in this alternative, do not meet at or near the axis of
 rotation. There is preferably provided a central portion which engages
 said inner side edges.
 Conveniently, the central portion comprises a cylinder, the outer surface
 of which engages said inner side edges. The diameter of said central
 portion is preferably greater than the diameter of the substantially
 upright portion.
 It is further envisaged that there may be provided baffle means between
 adjacent vanes of the impeller. The baffle means may have a concave outer
 surface.
 In an alternative arrangement, the substantially upright portion may
 comprise a cylinder like portion the ends of which are sealed or
 substantially sealed. Such a cylinder like portion may be solid. In such
 an arrangement, a lower end of the cylinder like portion may be attached
 to, or integral with, the vanes at a position below the plane of the
 barrier means. In this arrangement, there may be further provided an outer
 pipe, preferably concentric with the cylinder like portion, which does not
 rotate with the vanes. Preferably, gas is emitted to the impeller through
 the outer pipe, which has an open lower end. The lower end of the outer
 pipe is also preferably located below the plane of the barrier means.
 According to a second aspect of the invention, there is provided a method
 of mixing a gas and a liquid, comprising the steps of introducing the gas
 to the liquid, and inducing flow of the gas and liquid to enhance mixing
 thereof by use of an impeller in accordance with the first aspect of the
 invention.

DETAILED DESCRIPTION
 Referring to FIGS. 1 to 3, an impeller 10 is shown comprising a
 substantially upright portion which in this example is a hollow tube 11.
 There are provided eight outwardly extending vanes 12 which are attached
 to the outer surface 16 of the hollow tube by any suitable means, for
 example welding. The hollow tube extends approximately mid-way along the
 vanes in a vertical direction. There is provided near the lower end 13 of
 the tube 11 a flange 14 which is of annular configuration, wherein the
 inner rim 15 of the flange is located adjacent the outer surface 16 of the
 tube 11. It will be appreciated that whilst the figures show only a
 section of the flange, the flange extends through 360.degree. about the
 axis of rotation 17 of the impeller. The flange 14 is generally planar and
 the underside of the flange 18 is flat. The vanes comprise an upper
 portion 19 and lower portion 20 defined by upper, lower and side edges 21,
 22, 23 and 24 respectively. As shown in FIGS. 1 to 5, the upper and side
 edges meet substantially at right angles to define a corner 25 whereas the
 lower and side edges are joined by a concave formation 26 formed within
 the vane. However, as shown in FIG. 6, the upper and side edges may also
 be joined by a concave formation 25a, analogous to lower concave formation
 26.
 The inner side edges 23 of the vanes are provided with a recess 28 below
 the flange to provide a space 29 around the axis 17. The hollow tube 11 is
 provided at its end remote from the impeller with gas input means (not
 shown) which admits gas under super atmospheric pressure to the tube 11
 where the gas is emitted at its lower end 13.
 In use, the impeller shown in FIG. 1 is incorporated in apparatus as shown
 in FIG. 3. The apparatus comprises a tank 40 for containment of the liquid
 41 to be mixed with the gas. The gas is introduced to the liquid through
 the tube 11 from a source remote from the tank (not shown). The tube 11 is
 rotated about the axis 17 in the direction shown by arrow A by any
 suitable means, for example an electric motor (also not shown). Of course,
 the direction of rotation could equally be the opposite of that shown by
 arrow A. There are provided in the tank, froth baffles 42 which serve to
 retain the froth 50 above each apparatus when there are more than one
 apparatus in a tank and to facilitate removal of the required mineral
 particles. The impeller 10 rotates and draws liquid towards the impeller
 creating flow paths as illustrated by the arrows L. The gas, usually air,
 is emitted to the liquid at the end of the tube 11 and is guided by the
 barrier means initially to a position generally indicated at 43.
 Accordingly, the liquid flow path and the gas meet at the area 43 which
 permits of effective aeration and accordingly efficient froth production.
 It will be appreciated that there may be provided more than one apparatus
 in a given tank, the apparatus being arranged either adjacent to each
 other in series along the length of a tank or in a series of individual
 connected tanks.
 There are provided diffusers 44 in a generally circular arrangement around
 the impeller 10. The diffusers serve to further mix the air and liquid,
 and to ensure a radial flow of air and liquid out from the impeller. The
 diffusers are maintained in position by webs 45 which extend downwardly at
 approximately 45.degree. from a substantially cylindrical cover 46 which
 is attached to a support member above the tank (now shown).
 The provision of the flange on the impeller significantly increases the
 mixing of the gas and liquid and thus improves the efficiency of froth
 creation and mineral extraction.
 FIG. 4 illustrates an alternative embodiment of the invention. In this
 embodiment the impeller 10 comprises a substantially upright portion which
 again is a hollow tube 11, but the diameter of which is less than that of
 the hollow tube shown in FIGS. 1, 2 and 3. At the lower end of the tube 11
 there is provided a drive flange 60 having a diameter less than the
 distance between opposite edges 61 of the upper parts of the vanes 12. The
 barrier means 14 again comprises a flange but has a first part 62 attached
 to the drive flange 60 and a second part 63 extending outwardly from, and
 attached to, the first part. The second part 63 engages the vanes 12 in a
 slot-like manner and the vanes are thus held secure in relation to the
 hollow tube 11. In the drawing, the drive flange 60 and first part 62 are
 shown as being secured together by bolts 70 although it will be
 appreciated that they may be attached to each other by any suitable means.
 As also shown in FIG. 1, there is provided a space 29 about the axis 17
 beneath the barrier means 14. In this example, however, the lower parts 71
 of the vanes do not meet the axis 17 but there is provided a central
 portion 64 in the form of a cylinder 65 having closed upper and lower
 surfaces 66 and 67. It will be appreciated however that the upper and
 lower surfaces could alternatively be open so as to provide a hollow
 central portion.
 Fluid flow baffles 68 are provided between the vanes 12. The baffles have
 an outer concave surface.
 The fluid flow baffles serve to guide liquid flow to the area 43 as shown
 in FIG. 3.
 Referring to FIG. 5, an alternative embodiment is shown wherein the
 substantially upright portion 80 comprises a rotatable drive shaft which
 is sealed at its lower end 81 and is attached to the vanes in the region
 generally indicated at 82. In this arrangement, there is also provided an
 outer pipe 83 through which gas is passed downwardly and which is emitted
 to the impeller at the region generally indicated at 84. The flange 14 is
 separate from the outer pipe and secured to the vanes. It will be
 appreciated that whilst this embodiment differs from those previously
 described, the desired effect of aeration is obtained in a manner
 substantially similar to that previously described. Furthermore, it will
 be appreciated that the outer pipe 83 at its upper regions may support the
 diffusers in substantially the same way as shown in FIG. 3 by reference
 numerals 45 and 46.
 The features disclosed in the foregoing description, or the accompanying
 drawings, expressed in their specific forms or in terms of a means for
 performing the disclosed function, or a method or process for attaining
 the disclosed result, as appropriate, may, separately or in any
 combination of such features, be utilised for realising the invention in
 diverse forms thereof.