BACKGROUND OF THE INVENTION

1. . Field of the Invention

This invention relates to a mixer for tanks.

The invention particularly relates, but is not limited to, a horizontal mixer for slurry tanks, or other tanks through which process fluids flow, and where the homogeneity of the slurry or process fluids is preferabl to be improved.

2. Dictionary

Throughout the specification, the following terms will be used:

(a) "slurry tank" shall be used to include any tank, channel or container through which process fluids may flow, where such fluids may contained solid partials in suspension;

(b) "blade" shall be used to include blade, paddle, vane or like mixing device mounted on, or integral with, a rotatable mixer shaft; and

(c) "motor" shall be used to include a motor (e.g. electric, hydraulic or pneumatic), an engine (e.g. diesel, gas or gasoline) or like power source; which may be connected to the mixer either directly, or indirectly via a transmission mechanism.

3. Prior Art

The conventional means of mixing a tank containing a liquid or a suspension of solid particles in a liquid (slurry) is to mount a mixer in the tank such that the shaft of the mixer is substantially vertical. The mixing device typically consists of a set of vanes or curved plates connected to the vertical shaft. The vertical shaft may be fitted alternatively (i) through a rotating seal in the bottom of the tank or (ii) may simply be inserted from above the level of the fluid in the tank. The rotational motion of the mixer will always lead to some degree of rotational motion being imparted to the fluid being mixed, even if the vessel is baffled; and this rotational motion creates a centrifugal force on the particles suspended in the fluid, and therefore particles with a larger mass and/or lesser drag coefficient will tend to migrate to the regions of the tank remote from the rotational axis of the mixer itself.

This separation of the generally larger and/or heavier particles from the smaller and/or lighter particles, due to centrifugal action, creates a segregation of material in the tank, rather than a full homogenisation of the slurry in the tank.

In a single phase fluid, the centrifugal forces generated in the fluid due to the mixing or stirring, are of no consequence; but in the case of a slurry (solid in liquid) in which the particles have a range of particle sizes and/or densities different from that of the fluid, the stirring (as described) will always induce a degree of segregation in the slurry when the distribution of the solids within the tank is considered.

Gravitational force may also contribute to segregation.

Segregation of particles within the tank will lead, in general, to segregation within an outflow stream from the tank.

The sampling of slurry process flows is an important industrial process. The sampling is necessary for determination of, or tracking the variation in, the composition of the particles in the slurry and/or for the determination of the solids content of the process flow.

A sample is taken from the flowing slurry by some means and this sample is deemed to be representative of the entire slurry in the flow. Both the mass fraction of total solids in the slurry as well as the composition of the solids in the slurry is important. However, if the sample is taken in a manner that reflects the non-homogeneity (heterogeneity) of the slurry in the tank, or flowing from the tank, the sample will be biased with respect to both solids mass fraction and solids composition. The sampling with then be unsuccessful and provide incorrect and misleading results which will in turn lead to economic loss.

Following the accepted modern theory of sampling (Gy, P M 1982, Sampling of Particulate Materials - theory and practice, Elsevier), the only unequivocal means of collecting a representative sample of a process flow, which is subject to some degree of compositional inhomogeneity, is to sample the entire process flow in an 'equiprobable' or 'mechanically correct' manner.

The following definition of "equiprobable sampling" applies strictly to a sampler having coplanar cutter edges that are driven at constant velocity in a plane substantially perpendicular to the motion of the stream to be sampled. It is to be understood that the sampler passes from a location entirely outside the stream of material, then through the stream, to a location remote from the first one which is entirely outside the stream of material.

Definition: "A sampler that samples in a manner that provides an 'equiprobable' sample is such that, for an arbitrary trajectory within the process flow, the time interval between the passage of the leading and trailing edges of the sampling device through the trajectory is a constant".

If sampling is carried out with such a device, then process flow inhomogeneity cannot lead to a biased sampling result.

More complex definitions of mechanically correct sampling may be defined for various sampler geometries.

The mechanically correct sampling of a process flow implicitly assumes that sample increments will be collected at various times during the sampling period. However, it has been suggested by some that it is possible to correctly sample a process flow by collecting a portion or fraction of the process flow continuously.

It is an obvious assumption behind such a suggestion that the portion of the process flow collected is representative of the entire process flow. If there is persistent segregation of the process flow, this principle is in error and will lead to bias in the sampling results. However, there is little quantification of this logical conclusion concerning the circumstances of sampling simply due to the very substantial cost of such quantification. No prior art has been discovered in this context.

Summary of the Invention

It is an object of the present invention to provide a mode of mixing to a process stream which is intended to create homogeneity (i.e. lack of segregation) in the direction at right angles to the flow of process slurry or fluid.

It is a preferred object of the present invention to avoid the segregation of the slurry or process fluid in a direction at right angles to the flow.

It is a further preferred object to provide a mixing device that can homogenise the slurry to the extent that no sampling bias can be detected.

It is a still further preferred object to provide a mixing device for a flow of process material through a conduit upstream of the sampling device,

It is a still further preferred object of the present invention to overcome the fault of conventional mixers when applied to the homogenisation of a slurry process flow in a conduit or channel, where the rotation of the mixer is such that the particles tend to be thrown to the sides of the flow, causing segregation in a direction at right angles to the axis of the mixer.

Other preferred objects of the present invention will become apparent from the following description.

The solution to the problem of segregation in the flow at right angles to its average velocity component is to employ a mixer which has an axis at right angles to the average flow and which rotates about this axis.

In one aspect, the present invention resides in a mixer for a flow of process fluid in a slurry tank, the mixer including:

a mixer shaft, connectable to a motor as a power source; a plurality of mixing units mounted on, or integral with, the mixer shaft, each mixing unit having a plurality of blades extending there from; wherein: the mixer shaft is rotatably mountable in the slurry tank with a rotational axis substantially at right angles to an average velocity of the flow.

Preferably, the rotational axis is substantially horizontal; and the mixer shaft extends substantially transversely across the entire flow.

Preferably, the mixing units are provided along the whole length of the mixer shaft, so that mixing of the process fluid is effected across the entire flow, and solids suspended in the flow are not thrown towards one end of the mixer shaft or the other.

Preferably, each mixing unit has a hub non-rotationally mounted on the mixer shaft; and

the blades extend substantially radially from the hubs.

In one preferred embodiment, opposite faces of the blades lie in planes parallel to a plane passing through the rotational axis of the mixer shaft.

In a second preferred embodiment, the opposite faces of the blades lie in planes inclined to a plane passing through the rotational axis of the mixer shaft.

Preferably, the mixer units are provided on the mixer shaft to be substantially mechanically symmetric about a centre-line of the flow to avoid lateral segregation of the solids in the flow.

Preferably, the mixing units are provided so that mixing of the flow is primarily in the vertical direction, to maintain the solids suspended in the slurry and/or to reduce a tendency for the solids to settle to the bottom wall of the slurry tank. Preferably, the mixing units are rotatably driven by the mixer shaft so that the blades create turbulent eddies in the flow.

In a second aspect, the present invention resides in a slurry tank including:

a tank interior, defined by one or more side- or end walls and a bottom wall, having an inlet and an outlet, through a process fluid flows;

a mixer as hereinbefore described received in the tank interior; and a motor operably connected to rotatably drive the mixer; so arranged the mixer shaft, or a driveshaft of the motor connected to the mixer shaft, passes through one of the side- or end walls of the slurry tank.

Preferably, the mixer shaft or the driveshaft is provided with a slurry seal in the side- or end wall; and

optionally, the mixer shaft is rotatably supported at a non-driven end.

Preferably, a substantially vertical transverse baffle is interposed in the tank interior between the inlet and the mixer, the transverse baffle being spaced above the bottom wall to provide a flow path for the flow of the process liquid to the mixer.

Preferably, the output of the slurry tank incorporates an overflow weir.

Brief Description of the Drawings

To enable the invention to be fully understood, preferred embodiments will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side elevational view of a slurry tank provided with a horizontal mixer in accordance with a first embodiment of the present invention;

FIG. 2 is a schematic sectional end view of the slurry tank, taken on line 2 - 2 on FIG. 1 ; and FIG. 3 is a schematic sectional end view, as for FIG. 2, of a slurry tank provided with a horizontal mixer in accordance with a second embodiment of the present invention.

Detailed Description of the Invention

FIGS. 1 and 2 illustrate a first embodiment of the invention.

Without limiting the scope of the invention, the invention will be described with reference to the flow of process slurry S through a zone or tank of a specific volume, just upstream of a sampling device (not shown).

FIGS. 1 and 2 illustrate a slurry tank 10 has a pair of end walls 11 , 12, a pair of side walls 13, 14, and a bottom wall 15.

The slurry tank 10 is equipped with an (internal, substantially vertical) transverse baffle 16, and has an overflow weir 17 extending from the end wall 12.

The transverse baffle 16 interconnects the side walls 13, 14; but is spaced above the bottom wall 15 to provide a flow path 18 for the slurry S between the transverse baffle 16 and the bottom wall 15. The transverse baffle 16 is provided upstream of the horizontal mixer 20, to be hereinafter described; so the incoming flow SI of the slurry S is between the end wall 11 and the vertical baffle 16.

The mixed (i.e. homogeneous) flow SM of the slurry S over the overflow weir 17 is directed to a sampling device SD connected to outflow conduit 19.

The slurry S is contained within the slurry tank 10 to a level SL.

The horizontal mixer 20 is installed adjacent the bottom wall 15 of the slurry tank 10 upstream of the overflow weir 17 and downstream of the transverse baffle 16.

The horizontal mixer 20 is driven by a geared motor 30 connected to a horizontal driveshaft 31 , where the horizontal driveshaft 31 passes through a slurry seal 41 , such as is used in a centrifugal pump, the slurry seal 41 being provided in a base plate 42 of a housing 40 sealably mounted on the side wall 13 of the slurry tank 10.

The housing 40 has a removable cover 43, which provides a mounting for the motor 30, and a bearing assembly 44 for the horizontal driveshaft 31

The motor 30 may be optionally powered through a variable frequency drive, permitting adjustment of the speed of the horizontal mixer 20 to suit the nature of the solids to be suspended in the slurry S as a result of the turbulence generated by the horizontal mixer 20 in the slurry tank 10.

The horizontal mixer 20 shown has a plurality (e.g. nine) mixing units 21 assembled on a hexagonal mixer shaft 22.

Each mixing unit 21 has a hub 23 with three, radially-extending, flat, mixing blades 24; where the mixing blades 24 of adjacent mixing units 21 are offset by 60 degrees (60°) - see FIG. 1 - to provide a more even mixing action. Each mixing blade has opposite flat side faces 25, 26, which lie parallel to a plane 27 passing through the axis of rotation 28 of the mixer shaft - see FIG. 1.

The number of such mixing units 21 can be varied to suit the lateral dimension of the slurry tank 10.

The number of paddles or mixing blades 24 on the mixing units 21 (e.g. 2 to 8); and their angular offsets with respect to each other; may be varied without departing from the general concept of the invention.

The role of the mixing units 21 is to pump and create turbulence within the slurry S in the slurry tank 10, thereby mixing the slurry both vertically and laterally across the width of the slurry tank 10. Thus the horizontal mixer 20 increases the homogeneity of the flow of mixed slurry SM relative to the flow of the incoming slurry SI.

The mixer shaft 22 may be coupled at one end to the horizontal driveshaft 31 via engagement in a complementary (hexagonal or screw- threaded) recess 32 in the inner end of the horizontal driveshaft 31; while the other end of the horizontal mixer shaft 22 may be rotatably journalled in a bearing assembly 29 mounted on the side wall 14 of the slurry tank 10.

In the horizontal mixer 120 of the second embodiment illustrated in FIG. 3, some, or all of the adjacent mixing units 121 have mixing blades 124 whose surfaces 125, 126 do not lie in, but are inclined to, a plane parallel to a plane passing through the axis of rotation of the mixer shaft 122.

Such mixing units 121 will produce a pumping action towards one end of the mixer shaft 122 or the other. To ensure that there is no overall motion of the slurry S to one end or the other of the mixer shaft 122 (and thereby, of the slurry tank 1 10), the pumping action can be opposed by arranging the adjacent mixing units 121 with the faces 125, 126 of the mixing blades 124 alternatively-handed. It is preferable to have equal numbers of the mixing units 121 pumping in both directions to avoid lateral segregation of the solids in the slurry S.

In an alternative embodiment not illustrated, the drive motor 30, 130 of the horizontal mixer 20, 120 may be magnetically coupled to the mixer shaft 22, 122, obviating the need for a slurry seal 41 for the horizontal driveshaft 31 , 131 passing through the side wall 14, 14 of the slurry tank 10, 1 10.

The end of the mixer shaft 22, 122 remote from the driven end may be supported in an appropriate manner to prevent whipping of the mixer shaft 22, 122.

While each embodiment as described and illustrated has a single horizontal mixer 20, 120 in the slurry tank 20, 120, the skilled addressee will appreciate that two or more such horizontal mixers 20, 120 may be provided in the slurry tank 20, 120; where the horizontal mixers 20, 120 have the same / different numbers of mixing units 21 , 121 ; with the same different numbers of mixing blades 24, 124 at the same / different angular offsets; and the mixer shafts 22, 122 are driven at the same / different rotational speeds.

As hereinbefore described, it is preferred that the mixer primarily mixes the process fluid in a vertical direction. While it is preferred that the mixer 20, 120 rotates in the direction of the arrow in FIG. 1 , so that the mixer moves the slurr S in the direction concurrent with the flow of the slurry through the flow path 18, this is not essential.

It has been demonstrated in experimental work (see Schubert, H, Aufbereitung Fester Mineralischer Rohstoffe. Vol. I to III, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig., 1984 to 1989), that to suspend solids in a stirred tank, such as used in the froth flotation of minerals, it is necessary to put a certain amount of energy into the fluid in the form of turbulent eddies. Depending on the size and density distributions of the solids particles, the required energy dissipation in the fluid may vary, but in general is of the order of a few kilowatts per cubic meter.

Utility of the Invention

While there is no substitute for a sampler that takes an equiprobable sample from the process flow, as defined above, the practical circumstances Of sampling (due to the large mass flow rate of the process flow or to the tendency of the.solids in the process flow to settle out or 'sand up' or fill up the tank ahead of the sampling point) may dictate the use of a sampler that is not equiprobable and instead relies on process flow homogenisation in a specific plane upstream of the sampler.

The utility of the invention derives from the fact that rather than increasing particle segregation within a process flow of slurry stirred or agitated with a conventional mixer, the horizontal mixing device disclosed herein provides mixing without throwing larger heavier particles to one side or the other side of the flow. If a sampling device which intercepts some of the stream all of the time is placed downstream of the mixing zone, the sampler can be presented with a much more homogeneous flow than is possible with conventional mixers. The skilled addressee will appreciate that the present invention provides a simple, elegant solution to the problems with the known mixers hereinbefore described in the Prior Art.

Various changes and modifications may be made to the embodiments described and illustrated without departing from the present invention.