WO2018076062A1

WO2018076062A1 - Improvements to agitators

Info

Publication number: WO2018076062A1
Application number: PCT/AU2017/051179
Authority: WO
Inventors: Ben Nathan Gablonski; Tobin Giles Robinson
Original assignee: Berg Engineering Pty Ltd
Current assignee: Berg Engineering Pty Ltd
Priority date: 2016-10-26
Filing date: 2017-10-26
Publication date: 2018-05-03
Anticipated expiration: 2019-04-26

Abstract

An impeller for an agitator or the like is disclosed. The impeller has a central component which is operable to rotate when the impeller is in use, and at least one blade which is initially formed separately from the central component but which is mounted to the central component when the impeller is assembled. Each blade is mounted to the central component in such a way that, except for the blade itself, when the blade is mounted to the central component, other parts and components involved in mounting the blade to the central component do not create substantially any shaped features which give rise to voids or low-pressure regions in their wake when the impeller spins.

Description

IMPROVEMENTS TO AGITATORS

TECHNICAL FIELD

[0001] The present invention relates to agitators.

[0002] One application in which agitators are used is in the extraction of metals such as gold from ores. The present invention will therefore be described mainly with reference to this application. However, it is to be clearly understood that this is merely for illustrative purposes, and actually the invention is by no means limited to agitators used in the extraction of gold (or indeed any particular metal or ore). The present invention could therefore find use in a range of other applications as well, including applications where agitators are used but which are different or unrelated to the mineral processing type application referred to for illustrative purposes below.

BACKGROUND

[0003] In many gold deposits around the world, the gold ore exists as a refractory ore such as (or including) pyrite and/or arsenopyrite. Refractory ores, including pyrite and arsenopyrite which are sulphide ores, trap/encapsulate extremely fine (often submicron sized) gold particles within them, and this prevents or reduces the effectiveness of cyanidation as a means for gold recovery directly from such ores. These refractory ores must therefore generally be pre-treated, and the purpose of the pre-treatment is to enable adequate gold recovery to be achieved by subsequent cyanidation (i.e. the pre-treatment makes acceptable gold recovery from these ores by later cyanidation possible). The pre-treatment typically involves a pressure oxidation (PO) process. The pressure oxidation pre-treatment typically occurs in an autoclave, where high purity oxygen mixes with an aqueous slurry of the sulphide ore, at high pressure (HP) and elevated temperature. By way of example (although without limitation) some autoclaves used in practice for the pre-treatment of gold-containing sulphide ores operate at temperatures around 190°C-230°C and pressures around 40 bar (roughly 40 atm). In any case, during the pressure oxidation process, the sulphides are oxidised by high-purity oxygen, and this breaks the sulphides down to a solution phase consisting of metal sulphate compounds and sulphuric acid. The gold locked in the original sulphide material is typically completely liberated, allowing very high gold recovery to be achieved when the product is subsequently treated by cyanidation.

[0004] The autoclaves in which the above pressure oxidation processes take place are typically large pressure vessels. By way of example (albeit without limitation) many autoclaves used in practice for performing the above pre-treatment processing have a capacity of tens, or even hundreds, of cubic metres. As depicted in Figure 1 , the basic processing circuit consists of a feed system to supply ground ore in a slurry to the autoclave. In the schematic example in Figure 1 (and this is often the case in practice too) the autoclave (or each autoclave if there is more than one) is divided into a series of compartments, typically by titanium walls which decrease in height towards the autoclave's discharge end. In each compartment there is an agitator (or there may be multiple or a series of agitators in each compartment) to mix, disburse oxygen and facilitate the oxidation reactions.

[0005] As shown schematically in Figure 1 (although this is generally also how it is in practice) each agitator in the autoclave has (or it includes) an impeller 1. Each impeller 1 is itself mounted on the rotating vertical (drive) shaft 2. Hence, each impeller 1 is driven (i.e. it is rotated) by the rotation of a vertical shaft 2. Although figure 1 shows each impeller 1 being mounted at the bottom of the respective vertical shafts 2, the impellers are not necessarily located at the bottom of the shafts. Further, it may be possible to have more than one impeller mounted on each shaft. For example, each shaft may have two impellers mounted to, with one impeller being vertically spaced from the other impeller.

[0006] However, a problem is considered to exist, insofar as these forms of agitators are concerned, namely that the impeller can be subject to high levels (or a high/rapid rate) of metal loss which can lead to the destruction and failure of the impeller blades (and the agitator generally). In order to understand this further, it is useful to discuss the way in which the agitator impellers are typically/traditionally constructed.

[0007] As mentioned above, in many agitators, the agitator impeller is mounted on, and it is driven/rotated by, a vertical shaft. There is generally a central component or "disc" (or some other component that performs a similar function, and any such component may hereinafter be considered to be encompassed by the terms "central component" and/or "disc"), and this "disc" is mounted on the end of the vertical drive shaft, such that the disc therefore becomes oriented generally horizontally relative to the vertical shaft. The individual blades that form the (normally vertically-oriented) blades of the impeller are attached on/near or relative to the circumferential outer/perimeter edge of the disc.

[0008] The way in which individual blade components 9 are often attached to the outside of the disc 8 can be appreciated from Figure 3. Note that, in Figure 3, the blade component 9 and the disc 8 are both severely damaged/destroyed. Nevertheless, the way in which an individual blade component 9 is attached to the outside of the disc 8 can still be made out. As can be seen in Figure 3, the vertical blade portion 9a (i.e. the portion of the blade component 9 that actually functions as an impeller blade) is itself welded or otherwise formed together with a horizontal attachment plate 9b, and the attachment plate 9b is in turn connected to the disc 8 by bolts 9c. It will be understood that the bolts 9c extend through holes (not shown) in the attachment plate 9b and also through or into holes (not shown) in the disc 8 itself. The disc 8 with a number of the blade components 9 thus attached thereto at evenly spaced locations around the perimeter of the disc 8 (such that the vertical blade portions 9a of the respective blade components 9 form the "blades" of the impeller) are then often coated (i.e. the disc 8 with all of the blade components 9 attached thereto are all coated, in effect, as a single component) with a durable (e.g. wear resistant, etc) coating. This coating is done in an attempt to increase the wear resistance, and hence the operational life, of the impeller.

[0009] However, as can also be seen in Figure 3, and also in Figure 2 and Figure 4, the blades 9a, and also the disc 8, etc, can be subject to high levels of wear, which can ultimately lead to the destruction of the blades and/or the disc, as clearly shown in these Figures.

[0010] It is thought that it would be desirable if this metal loss on the agitator impeller (or on the different parts of it, such as the disc, blades, the connections therebetween, etc), could be reduced so that the operational life of the agitator may be increased, and so that the duration of periods between autoclave shutdowns for repair and/or maintenance may be increased. If this could be achieved, the benefits in terms of increased production, and reduced costs (or reduced downtime losses), are readily apparent.

[0011] It is to be clearly understood that reference anywhere herein to any previous or existing devices, apparatus, products, systems, methods, practices, publications or to any other information, or to any problems or issues or causes thereof, does not constitute an acknowledgement or admission that any of those things, whether individually or in any combination, formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.

SUMMARY OF THE INVENTION

[0012] In one form at least, the present invention provides an impeller for (or of) an agitator or the like, the impeller including a central component which is operable to rotate when the impeller is in use (the central component may be operable for connection to or with a drive shaft or whatever other component drives the rotation of the impeller), and at least one blade which is initially formed separately from the central component but which is mounted to (normally an outer periphery of) the central component when the impeller is assembled, wherein each blade is mounted to the central component in such a way that, except for the blade itself, when the blade is mounted to the central component, other parts and components involved in mounting the blade to the central component do not create substantially any shaped features which give rise to voids or low-pressure regions in their wake when the impeller spins (the impeller typically spins in only one direction).

[0013] In the above impeller, the central component may (and often will) comprise a substantially circular and generally flat disc.

[0014] Each blade may have a substantially planar shape with opposing faces on either side, and each blade may be operable to be mounted to the disc such that the plane of the blade is perpendicular to the plane of the disc, or otherwise (e.g. if the disc does not have a single, easily identifiable "plane") such that the plane of the blade is parallel to the disc's axis of rotation. In other embodiments, one or more or each of the blades may be mounted at any angle to the disc.

[0015] On each blade, the face on one side of the blade may comprise a first face and the face on the opposing side of the blade may comprise a second face, and the blade may be able to be mounted to the disc such that the first face forms the blade's leading face (sometimes referred to as the high pressure face) when the impeller rotates and the second face forms the blade's trailing face (sometimes referred to as the low pressure face), or such that the second face forms the blade's leading face when the impeller rotates in the first face forms the blade's trailing face. Preferably the design will be such that a blade which is initially mounted to the disc in one said orientation can be disconnected and (re-)mounted to the disc in the other said orientation.

[0016] Each blade may incorporate an at least generally elongate cutout. The said cutout may open through one edge of the blade and the cutout may have opposing sides. The blade may be mounted to the disc by sliding the blade onto the outer edge of the disc such that a portion of the disc becomes located in between the opposing sides of the cutout in the blade (or, in other words, such that one of the opposing sides of the cutout in the blade becomes positioned generally above or on the upper side of the disc, and the other of the opposing sides of the cutout in the blade becomes positioned generally below or on the underside of the disc).

[0017] In one embodiment, the inner end of the cutout or opening may be spaced from an outer edge of the disc when the blade is mounted to the disc.

[0018] At each location on the disc where a blade can mount to the disc, a radially oriented channel may be formed in one or both of an upper (top) and lower (underside) portion or surface of the disc, and when a blade is mounted to the disc at a said location, one of the opposing sides of the cutout in the blade may slide into the (or each) said channel in the disc. More preferably, at each location on the disc where a blade can mount to the disc, a radially oriented channel may be formed in both an upper (top) and lower (underside) portion or surface of the disc, and when a blade is mounted to the disc at a said location, one of the opposing sides of the cutout in the blade may slide into the channel on the upper side of the disc and the other of the opposing sides of the cutout in the blade may slide into the channel on the lower side of the disc. In one embodiment, the radially oriented channels may be formed on one side of the disc only.

[0019] At each location where a blade mounts to the disc there may also be an indented or recessed cavity (a "pocket") in the upper surface of the disc. The pocket may connect with and form an extension of the radial channel at that location on the upper side of the disc, laterally to one side of the said channel. The impeller may further incorporate one or more components operable to be inserted into the pocket for the purpose of securing the blade to the disc. In this regard, the impeller may incorporate a wedge component and a lock component which are operable to be inserted into the pocket for the purpose of securing the blade to the disc. To secure a blade on the disc, the blade may first be slid onto the disc, the wedge component may then be inserted into the pocket and slid across within the pocket towards the blade, and the lock component may then be inserted into the pocket behind the wedge component, such that the wedge component secures the blade relative to the disc and the lock component secures the wedge component relative to the disc.

[0020] At least one of the opposing sides of the cutout in the blade may include a space or opening or recessed portion, and at least a portion of the wedge component may be shaped to engage with or (at least partially) insert into said space or opening or recessed portion to thereby secure the blade relative to the disc when the lock component is fully inserted into the pocket.

[0021] The lock component may have a threaded hole therein, and there may also be a threaded hole in the disc within the pocket beneath where the lock component becomes positioned. After the lock component is inserted into the pocket behind the wedge component, a countersunk bolt may be screwed first into the lock component and then further down into the hole in the disc, such that the lock component becomes bolted firmly to the disc. As the lock component becomes more firmly secured to the disc, this may also cause the lock component to push or otherwise urge the wedge component laterally within the pocket toward the blade, thereby causing the wedge component to engage more firmly with the blade which may in turn help to secure the blade.

[0022] When the blade is fully mounted to the disc, there will preferably be substantially no openings or spaces in between the wedge component, the lock component the countersunk bolt, the blade and the disc. Furthermore, when the blade is fully mounted to the disc, the upper surfaces of the wedge component, the lock component and the counter sunk bolt will preferably be substantially flush with an upper surface of the disc. [0023] When the blade is fully mounted to the disc, a small opening may remain in the blade, the said opening extending fully through the blade (i.e. through its full thickness) and the opening may be located radially outward of the perimeter of the disc.

[0024] In another form, the present invention provides an agitator incorporating an impeller as described above.

[0025] In yet another form, the present invention provides a central component of an impeller for an agitator or the like, wherein the central component has at least one location where an impeller blade can be mounted thereon, and at each location where a blade can mount to the central component, a radially oriented channel is formed in one or both of an upper (top) and lower (underside) portion or surface of the central component, the said channel(s) being such that a blade having an elongate cutout therein can be slid onto to the central component and one of the sides of the cutout in the blade slides into the (or each) said channel in the central component.

[0026] In another form, the present invention provides a blade as described herein. It will be appreciated that the blades may be sold as separate items to replace worn or damaged blades on the impeller. In one embodiment, the blade may have a substantially planar shape with opposing faces on either side, and an at least generally elongate cutout opening through one edge of the blade. In some embodiments, the inner end of the cutout may be spaced from an outer edge of the disc when the blade is mounted to the disc. The blade may be formed by machining or by casting. The term "cutout" is used to refer to the shape of the opening in the blade and it does not require that the opening be formed by cutting out or removing material from the blade. The cutout may be formed, for example, by use of a casting mould having the shape of the cutout present therein.

[0027] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0029] Figure 1 : Basic schematic illustration of a processing circuit, including the autoclave, used for pressure oxidation pre-treatment of refractory gold ores. [0030] Figure 2: Photographic illustration of the destruction, through wear, of the blade component and also the disc of an agitator impeller - photo taken from behind (i.e. on the the rearward side of) the blade shown

[0031] Figure 3: Photographic illustration of the destruction, through wear, of the blade component and also the disc of an agitator impeller (the same one as in Figure 3) - photo taken from in front (i.e. on the the forward-facing side) of the blade shown

[0032] Figure 4: Photographic illustration of the destruction, through wear, of the disc (or the central component that functions as the disc) of an agitator impeller - the (four) agitator blades are not pictured although the damaged regions where they would have been are plainly evident

[0033] Figure 5: Schematic illustration of the mechanism by which it is thought that potentially damaging voids or regions of low pressure may be formed near (or behind) certain parts or shaped features of an agitator impeller (or parts/components of the impeller)

[0034] Figure 6: Schematic views (top and underside perspective views) of an agitator impeller disc, in each case with blades attached to the disc, in accordance with an embodiment of the invention

[0035] Figure 7: Schematic view (top perspective view) of an agitator impeller disc, with blades attached to the disc, in accordance with a slight variant on the embodiment in Figure 6 - specifically a variant having eight blades rather than four - but the way in which the blades attach to the disc is the same

[0036] Figure 8: Close up view of the arrangement used for attaching (and clamping) each blade to the disc in the embodiments in Figure 6 and Figure 7

[0037] Figure 9: Close up view, simillar to Figure 8, but from behind the blade and showing an opening that may be present in the blade and the way this may allow angled discharge (or passage of fluid/slurry from in front of the blade and at an angle) into the region behind the blade

[0038] Figure 10: Wireframe outline of the shape of one of the blades in the embodiment in Figure 6 to Figure 9

[0039] Figure 1 1 : Close up view of one of several such cutouts or indentations (one for each blade) formed in the undersaide of the disc, each of which is configured to assist with directing the flow of oxygen bubbles onto the leading face of the following impeller blade [0040] Figure 12: Schematic (close-up and partially cross-sectional) view of another alternative mechanism (i.e. alternative to the embodiment in Figure 6 to Figure 8) for attaching a blade to the central "disc" component of an impeller-like agitator

DETAI LED DESCRI PTION

[0041] Before discussing the embodiments of the invention shown in Figure 6 to Figure 12, it may be useful first to mention that present invention was conceived, if not entirely, then at least in part based on the realization (or based on the considered belief following investigation of actual failed impeller parts as well as the application of theory) that a very significant (if not the primary or sole) cause of the wear related failure (and also the cause of the speed or rate at which this occurs) in the agitator impellers and the parts thereof, as shown for example in Figure 2 to Figure 4, is due to a phenomenon generally referred to as cavitation.

[0042] According to Wikipedia (https://en.wikipedia.org/wiki/Cavitation; September 2016):

Cavitation is the formation of vapor cavities in a liquid - i.e. small liquid free zones ("bubbles" or "voids") - that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shock wave.

Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal causing a type of wear also called "cavitation". [A] common example of this kind of wear [is] to pump [and other] impellers...

In devices such as propellers and pumps [and also impellers in other applications], cavitation [can] cause a great deal of noise, damage to components, vibrations, and a loss of efficiency ...

When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby often creating spots of high temperature and emitting shock waves...

Although the collapse of a small cavity is a relatively low energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller [or impeller] or pump's lifetime.

After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits increase the turbulence of the fluid flow and creates crevices that act as nucleation sites for additional cavitation bubbles. The pits also increase the components' surface area and leave behind residual stresses. This makes the surface more prone to stress corrosion.

[0043] It is therefore thought that it would be desirable if metal loss such as that shown in Figure 2 to Figure 4, which is thought to be caused at least partly (probably significantly) by cavitation, could be reduced or if the rate of the metal loss caused by this could be reduced. [0044] One strategy that is often employed to prevent or reduce cavitation, at least in other applications different to the autoclave pressure oxidation application presently discussed, is to increase the overall pressure in the system. This can help to prevent cavitation because increasing the pressure in the system causes the difference between the static pressure and the vapor pressure of the liquid to increase, and as a result the acceleration (or whatever it is) that causes the local pressure drop (and which could otherwise lead to cavitation) is then not enough to reduce the localised pressure in the liquid to below the liquid's vapor pressure, and as localised vaporization is essentially what causes cavitation, preventing this can also (at least help to) prevent cavitation. However, increasing the overall pressure in the system as a means for combatting cavitation may not be possible in the autoclave pressure oxidation application being discussed here because the pressure at which the autoclave operates is already very high, so further pressure increases may not be achievable (or the effect of this, if any, may be limited, given how high the operating pressures already are).

[0045] Another strategy that is sometimes employed to prevent or reduce cavitation, at least in other applications different to the autoclave pressure oxidation application presently discussed, is to reduce the temperature. This is because vapor pressure in fluids generally increases as temperature increases, and vice versa. Accordingly, if the fluid temperature is reduced such that the vapor pressure of the fluid is also reduced, (assuming static pressure remains unchanged) it follows that the difference between the static pressure and the vapor pressure of the liquid again increases, and as a result the acceleration (or whatever it is) that causes the local pressure drop (and which could otherwise lead to cavitation) is then not enough to reduce the localised pressure in the liquid to below the vapor pressure, and again as localised vaporization is what causes cavitation, preventing this can also (at least help to) prevent cavitation. However, reducing temperature as a means for combatting cavitation also may not be possible in the autoclave pressure oxidation application discussed here because, for instance, it may often be that certain of the oxidation (or other chemical) reactions and processes involved may need to operate at or above certain temperatures, or within certain temperature ranges, or they may be temperature sensitive.

[0046] Accordingly, some other alternative means for reducing cavitation is thought to be needed, and to this end the focus of the present invention is on certain aspects of the physical/mechanical design of the agitator impeller itself, and the various parts thereof.

[0047] More specifically, it is thought that it would be desirable, to combat cavitation, if the design of an agitator impeller could be modified or changed, e.g. as compared to the conventional designs in Figure 2 to Figure 4, in a such way that eliminates or reduces (where possible) parts or shaped features, particularly on the impeller disc and also in the connection between blades and the disc, where the shape of said parts or features is such that, when the impeller is spinning in the slurry (inside the autoclave), a void or low pressure region is formed behind or near the said part or feature. In other words, it is thought that it would be desirable if the presence of these parts or shaped features (or their impact), which give rise to voids or low- pressure regions when the agitator impeller is spinning, could be reduced.

[0048] What is meant in the previous paragraph can perhaps be further understood with reference to the schematic illustration in Figure 5. Figure 5 may be understood to be a cross- sectional view through some part (or a portion of one or more parts) of an impeller disc, e.g. like the disc 8 in Figure 3, or it could be a cross-sectional view through a part such as one of the attachment plates 9b in Figure 3, or it could even be a cross-sectional view through a combination of these parts such as one of the attachment plates 9b and the disc 8 to which it is joined, etc. In any case, the grey shaded area in Figure 5 represents the material inside the part (i.e. inside the disc 8, or inside the attachment plate 9b, or inside whatever the depicted part happens to be). A portion of the outer surface of the said part (whatever it is) is represented in Figure 5 by a thin black line along the edge of the shaded grey region. The curved/wavy lines in Figure 5 (indicated collectively as S) represent the flow of fluid/slurry over or past the outer surface of the depicted part of the agitator impeller within the autoclave.

[0049] It should be mentioned here that, in the autoclave application discussed herein, it is actually the agitator impeller which spins, thereby agitating the relatively more stationary slurry. And in fact, this motion of the impeller is represented in Figure 5 by the arrow (I). However, for the purpose of visualising what is occurring in Figure 5, it is useful to ignore the arrow (I) and consider a frame of reference that is fixed on the impeller part. In such an "impeller-fixed" frame of reference, the impeller part (schematically illustrated in Figure 5) may be considered to actually remain stationary (i.e. it is unmoving relative to the frame of reference), and in this frame of reference it is the slurry which is moving (flowing) over and relative to the said part. Describing the curved lines S as representing a "flow" of slurry over the said impeller part also makes more sense from this point of view (i.e. to describe this as a "flow" makes more sense in this "impeller-fixed" frame of reference). The direction of the flow of slurry, if an impeller-fixed frame of reference is taken in Figure 5, is in the opposite direction to arrow (I).

[0050] In any case, it is next important to recognise that the impeller part (or combination of parts) in Figure 5 (whatever it is) has, as part of its overall shape, a raised or protruding portion R. This raised portion R in Figure 5 could be, for example, the head of one of the bolts 9c in Figure 3, in which case the following non-raised or relatively lower or recessed portion L shown in Figure 5 would be the upper surface of the attachment plate 9b though which the bolt extends. Or, the raised portion R in Figure 5 could be a part (the edge) of one of the attachment plates 9b, and in that case the following non-raised or relatively lower or recessed portion L could be the upper surface of the disc 8 on which the attachment plate 9b sits. These are just examples, and Figure 5 could also be interpreted as representing a cross-sectional view through some other parts as well. Regardless, the reason it is significant to mention the relatively raised or protruding portion R (whatever it is) is because, if one continues to view Figure 5 using the impeller-fixed reference frame discussed above, then as the slurry flows over the raised portion R (and in particular as it flows over the rearward/downstream edge thereof Ro), the flow must bend or curve so as to move down or inward towards the part in order to then flow along the relatively lower or recessed surface of portion L that follows the raised portion R in the flow direction. The curvature of the lines S in Figure 5 shows generally how the flow is caused/required to curve due to the shape of raised or protruding portion R. However, one very important thing to be aware of is that, given the speed at which the impeller is rotating (and hence the speed at which the slurry may be considered to be flowing over the raised portion R and over the edge R₀ in the impeller-fixed frame of reference) it is not possible for the flow to bend/turn quickly enough for slurry to flow directly/straight down/along the rearward/downstream face R-i of the raised portion R. Therefore, because the flow cannot bend quickly enough to flow down/along this rearward face Ri , consequently the flow may be said to "separate" (at least somewhat) from the surface of the part(s) as it passes over the edge R₀ thereby causing a void V (or at least a region of significantly reduced pressure compared with the rest of the slurry) to be formed behind (i.e. in the wake of) the rearward edge face R-i . And it is thought to be voids or low pressure regions, formed in this general manner, which can ultimately lead to (or these can at least significantly contribute to) the kinds of cavitation that is in turn thought to be a major cause of the damage (or of the accelerated rate of the kind of damage) depicted in Figure 2 to Figure 4.

[0051] Accordingly, what paragraph [0047] above is intended to convey is that it may be beneficial to eliminate or reduce (where possible) the existence or presence (or the influence/affect) of parts or shaped features, particularly on the impeller disc and also in the connection between impeller blades and the disc, where the shape of the said part or feature is (or it would otherwise be) such that, when the impeller is spinning, a void or region of low pressure (V) is (or it would otherwise be) formed in the general manner (or based on the same mechanism) as explained above and represented schematically in Figure 5.

[0052] It is also worth noting that Figure 5 may actually be viewed from different perspectives, or in different orientations. For instance, Figure 5 can be interpreted as a schematic illustration that is "looking" horizontally into the cross-section of an impeller part, such that the black line represents a portion of the vertically upper surface of the said part, with the slurry S flowing over and along (i.e. above) the said vertically upper surface. However, in an alternative perspective, Figure 5 can also be interpreted as a schematic illustration that is "looking" vertically down into the cross-section of the impeller, such that the black line represents a horizontally outer or perimeter edge surface of the said impeller part, with the slurry S (located horizontally outward thereof) flowing along and around the outside thereof. Figure 5 could actually also be interpreted as a cross section through impeller part(s) taken in some other plane or orientation. In any case, the point is, the mechanism depicted in Figure 5, which can lead to the formation of voids or low-pressure regions (and hence potentially to cavitation) can operate in any orientation where a void or low-pressure region is formed on the downstream side of a raised or protruding part or feature. And as a result, it is not just on the vertically upper surface, or on the horizontally outer edge, etc, of the impeller where it is necessarily important to eliminate or reduce parts or shaped features that cause (when the impeller is spinning) a void or low pressure region to be formed behind or near the said part or feature. Rather, this is potentially important on many outer surfaces of the impeller, with one important exception to this being the impeller blades themselves. In order for the blades to operate and function - i.e. in order for them to mix/stir the slurry, disburse the introduced high- purity oxygen and facilitate or promote the oxidation reaction within the autoclave - the blades may or should have (and for the purposes of the present invention each blade is assumed to have) a generally planar configuration with one side/face of the blade pointing forward (the "leading" face) such that this face effectively "strikes" the slurry face-on as the impeller rotates within the autoclave (or in other words the angle of attack of this leading face of the blade is 90° or thereabouts), and each blade also has a rear face which is effectively parallel to the leading face but on the rear/back/downstream/trailing side of the blade.

[0053] It is next important to emphasise that, although a number of embodiments of the invention are described below which seek to implement the above (i.e. the embodiments discussed below are intended to change or modify certain aspects of an agitator impeller design in accordance with the design objectives discussed above, namely eliminating or reducing the presence of parts or shaped features which can give rise to voids or low-pressure regions in their wake), the scope of the invention is not necessarily limited to the particular embodiments discussed below. The invention therefore also potentially extends to other embodiments or means by which these design objectives may be implemented or put into effect. All such other embodiments and means therefore also fall within the scope of the invention.

[0054] Turning now to Figure 6, this Figure contains (on the left) a top perspective view and (on the right) an underside perspective view of an agitator impeller disc, in each case with blades attached to the disc, in accordance with one possible embodiment of the invention. As can be seen in the top and underside views in Figure 6, the agitator impeller in this particular embodiment has four blades attached to the central disc. Turning next to Figure 7, this Figure shows a slightly different embodiment in which the agitator impeller has eight blades. However, the different number of blades (and the consequent difference in the spacing between blades, etc) is the only real difference between the embodiment in Figure 6 and the embodiment in Figure 7. The configuration of the individual blades themselves, and the way in which the blades connect to the disc, etc, is the same in both Figure 6 and Figure 7, and consequently these two Figures may be considered to really relate to one and the same embodiment Gust with different numbers of blades) and they therefore will not be described as separate embodiments. Further variants of this embodiment could also be made with other different numbers of blades.

[0055] As shown in Figure 6 and Figure 7, the impeller includes a main central component 100, and just like in conventional agitator impellers used in the autoclave applications presently discussed, the main central component 100 in Figure 6 and Figure 7 takes the form of a round (circular) and substantially flat disc. The disc 100 itself will typically be made from steel or other appropriate metal. An appreciation for the size of the disc 100 (i.e. a general appreciation for how big it might be), for an impeller used in the kind of refractory ore pressure oxidation autoclave applications presently discussed, may be gleaned from earlier Figures. Basically, for an impeller used in these applications it is anticipated that the disc may have a diameter ranging from as small as a few centimetres (say 10 or 20 cm) possibly up to a few meters. However, it is to be clearly understood that, as mentioned above, the present invention may also find use in a range of other applications which are different or unrelated to the presently-discussed autoclave applications. And given that the invention may be used on impellers in a range of other applications, it follows that the size of the impellers may also vary (according to the requirements in the different applications). Thus, the invention is not necessarily limited to impellers of any particular size.

[0056] The central disc 100 has a large through-hole 102 in the centre as well as a number (in this case eight) of smaller "planet" through-holes 104 arranged at equally-spaced locations around the large central hole 102. These holes 102 and 104 may exist (at least one of their functions may be) to help in facilitating attachment of the disc 100 to an associated drive shaft (not shown) or the like which in turn drives the rotation of the disc 100 (and therefore the rotation of the whole impeller generally).

[0057] Turning now to the impeller blades, there are four of these in Figure 6, and eight in Figure 7. However, in both Figures the blades themselves, and the way in which they attach to the disc 100, are identical. Individual blades are designated by reference numeral 200 in Figure 6 to Figure 10. The overall configuration of each blade 200 is generally that of a planar, rectangular plate (albeit quite a thick plate made from steel or some other suitable metal). Each blade 200, once it is installed/mounted on the disc 100, has a forward or "leading" face 202 and a rear/back face 204 (see Figure 6). As mentioned above, when a blade 200 is mounted to the disc 100, the blade 200 consequently becomes oriented such that its leading face 202 (and in fact the plane of the blade 200 overall) is substantially perpendicular to the plane of the disc 100. If the disc 100 is oriented substantially horizontally within the autoclave, it follows that each blade 200 mounted to the disc will be oriented in a substantially vertical manner.

[0058] As discussed further below, the configuration of each blade (and in particular the shape of the blade) is such that each blade is (at least approximately) symmetrical about a hypothetical horizontal centreline C (see Figure 7). This (at least approximate) symmetry allows a given blade to be installed on the disc 100 "either way". In other words, each blade can be installed so that either one of its faces becomes the "leading" face 202, and naturally the opposing face then becomes the rear face 204. The symmetry also means that it is possible for blades to be disconnected/detached from the disc 100, "flipped over" and then reinstalled, so that the face of the blade that was initially/previously the leading face 202 becomes the rear face 204, and vice versa. This will be discussed further below.

[0059] It can be seen that each of the blades 200 also has an elongate, overall basically U- shaped slot or cutout 300 formed therein (see Figure 7 and Figure 10). In each case (i.e. in each blade 200), the slot 300 actually extends into the blade from one of the blade's short side edges, and the overall slot 300 extends into the blade 200 for approximately half the width of the blade (with the width being the blade's longest dimension). And as can be understood from Figure 6 and Figure 7, each blade 200 is mounted to the disc 100, basically, by sliding the blade 200 onto the disc 100 such that the disc 100 becomes received between the opposing sides of the slot 300 in the blade.

[0060] However, it is important to understand that the slot 300 in each blade in this embodiment is not merely a simple, straight-sided U-shape. This can be better appreciated from Figure 10. As can be seen from Figure 10, on each blade 200, the distance between the opposing sides of the slot 300 is widest near the innermost end of the slot (i.e. near the inner end that forms the curved portion of the slot's overall general U-shape). However, in other portions of the slot, closer to the side edge of the blade that the slot 300 opens through, the opposing sides of the slot are generally closer together.

[0061] More specifically, in Figure 10 the respective opposing sides of the slot 300 are labelled 310 (this will be referred to as the first side) and 320 (this will be referred to as the second side). Note that neither side of the slot 300 is referred to as the upper or lower side. This is because, as mentioned above, the blade 200 can be installed on the disc 100 either way around, in which case either the first side 310 or the second side 320 can become the vertically upper side of the slot when the blade is installed, and the other the lower side, depending on which way around the blade is installed on the disc 100.

[0062] In any case, as can be seen in Figure 10, there is a vertically indented or cutout or recessed portion 312 in the first side 310. This will be referred to as a notch 312. There is also an identical notch 322 in the second side 320. Whilst these cutouts or recessed portions 312 and 322 will be referred to as notches, they could each alternatively be viewed simply as the horizontal gap or opening or space that exists between the protruding or upstanding block-like portions on each respective side of the slot 300. These block-like portions on either side of the slot 300 will be referred to as "posts", and the posts on the first side 310 are labelled 314 and 316, and the posts on the second side 320 are labelled 324 and 326.

[0063] In order to understand more fully how a blade 200 is installed on, and secured to, the disc 100, it is next important to be aware that there are actually channels (guide channels) that are cut or otherwise formed into the upper and lower surfaces of the disc 100 at each of the locations where a blade 200 is to be installed on the disc 100. These guide channels, which allow and help a blade 200 to be mounted at each blade mounting location on the disc 100, are not independently shown or labelled in the Figures that show this embodiment, although the ends of these channels at one location where a blade connects to the disc can just be made out inside the dashed circle in Figure 9. As can be appreciated from Figure 9, the guide channels just referred to, which are formed in the upper and lower surfaces of the disc, effectively form intended or recessed channels in the respective upper and lower surfaces of the disc that extend into the disc from the disc's outer/perimeter side edge. The width of the respective guide channels is the same as (or very slightly/minimally greater than) the thickness of the blade 200. The depth of each channel, one on either side of the disc at each location where a blade mounts to the disc, is somewhat greater than the vertical height of the respective posts 314, 316, 324, 326 on the blade 200. However, more importantly than this, at each location where a blade is to be installed, the thickness of the disc 100 in between the opposing channels (i.e. the thickness of the material of the disc in between each pair of opposing channels) is the same as (or very slightly/minimally less than) the distance between posts 316 and 326, and between post 314 and 324, respectively, on the blade 200.

[0064] Accordingly, it will now be understood that, when a blade 200 is to be mounted on the disc 100, the blade 200 is positioned adjacent one of the sets of channels (the channels exist only at locations where blades are to be mounted on the disc), and the blade 200 is then slid onto the disc 100 in such a way that the above-mentioned posts, which form part of the slot 300 in the blade, insert into the channels on either side of the disc. It will be understood that when the blade 200 is first being slid onto the disc 100, it will be the outmost posts 314 and 324 on the blade that first insert into and slide along the respective channels in the upper and lower sides of the disc. Then, as the blade 200 slides further onto the disc 100, the inner posts 316 and 326 will also become received in and they too will slide along/within the channels in the disc. The blade 200 will continue to slide inwards onto the disc 100 until the outermost posts 314 and 324 on the blade reach (and collide with) the innermost/terminal ends of the respective channels (i.e. these ends of channels prevent the blade 200 from sliding any further onto the disc 100).

[0065] This therefore explains how each blade is "slid onto" the disc 100 initially. However, as will be recalled, the overall agitator impeller (of which the disc 100 and the various blades 200 all form part) is a fast-rotating device. Accordingly, a means is needed to prevent each blade from simply sliding (or being flung violently) back off the disc by centrifugal force as the disc 100 rotates. This will be discussed below.

[0066] In order to understand the way in which each blade is secured on the disc (and hence prevented from sliding back off the disc, including when the disc is rotating), it is next important to note that, at each location where a blade is to be attached to the disc, there is an additional indented or recessed rectangular cavity (a "pocket") formed in the upper surface of the disc. In each case (i.e. at each location where a blade is to attach to the disc) this pocket actually connects with and forms a rectangular extension of the channel on the upper side of the disc, extending laterally to one side of the said channel. Note that, in each case, the pocket is located some distance back in from the outer perimeter edge of the disc. As for the channels themselves (described above), the rectangular pockets which form lateral extensions of each respective channel on the upper surface of the disc are not independently shown or labelled in the Figures, but their configuration and the purpose they serve will be readily understood from the Figures and the discussion below.

[0067] Each one of the abovementioned pockets is sized and shaped to receive two blade- securing components, namely a wedge component 410 and a lock component 420. Numerous of these wedge components 410 and lock components 420 (one of each for securing each blade 200 to the disc 100) can be seen in Figure 6 and Figure 7; however the way in which these operate to secure a blade 200 to the disc 100 can be most easily appreciated from Figure 8. Referring to Figure 8, once the blade has been slid onto the disc (as described above), in order to secure the blade 200 on the disc 100, the wedge component 410 is first inserted ("dropped") into the then upwardly-open pocket beside the blade in the upper surface of the disc. After it is inserted into the pocket, the wedge component 410 can then be slid laterally within the pocket toward the blade 200, so that the wedge component 410 slides into the above- mentioned channel, and so that at least part of the wedge component 410 moves into the channel underneath the blade. More specifically, the wedge component 410 is sized and shaped such that, not only does it fit snugly between the side edges of the pocket, but also so that when it is then slid across within the pocket so that at least part of it moves into the channel underneath the blade, a portion of the wedge component 410 then inserts snugly (and is received within) the notch 312/322 that exists between the posts 314/324 and 316/326 on the blade. Note that, whether the wedge component 410 inserts into the notch 312 that exists between the posts 314 and 316 on the first side 310 of the blade slot 300, or alternatively into the notch 322 that exists between the posts 324 and 326 on the second side 320 of the blade slot 300, depends on which way around the blade is installed on the disc (and hence whether it is the first side 310 or the second side 320 of the slot 300 that is positioned on the upper side of the disc 100). Regardless, it should be noted that the shape of the respective notches 312 and 322 (i.e. the width of these between their respective posts, and also their depth/height) is configured so as to snugly receive a portion of the wedge component 410. And in fact, these various parts and features are all shaped and sized such that the said portion of the wedge component 410 does not simply slide easily into the notch (whichever one it is) in the blade, but rather the said portion of the wedge component 410 must be forced into the said notch in the blade (once the blade has already been slid onto the disc). The wedge component 410 may even be provided with one or more angled or sloping portions (not illustrated) the function of which may be to only allow the said portion of the wedge component 410 to insert into the notch in the blade if a sufficient force is applied to "wedge" it in there (hence the name of the wedge component 410).

[0068] In any case, during the process of blade installation, once the blade 100 has been slid fully onto the disc (as described above), the wedge component 410 is then inserted into the pocket and slid laterally in the pocket, at least so that the said portion of the wedge component 410 touches or comes into light/initial contact with the blade 200 (from above it will be understood that the portion of the wedge component 410 will initially come into contact with a portion of the notch (312 or 322) and/or with one or both of the posts (314/316/324/326). When the wedge component 410 has been slid laterally within pocket to at least touch the blade like this, it (the wedge component 410) will then be located at least partly underneath the blade, and consequently it will have moved over laterally within the pocket far enough to leave room for the lock component 420 to then be inserted into the pocket as well, behind the wedge component 410. However, it is to be understood that the amount of room that remains within the pocket, after the wedge component 410 has been slid across into contact with the blade, is only just enough to accommodate the lock component 420. Accordingly, when the lock component 420 is then inserted into the pocket behind the wedge component 410, it (the lock component 420) is snugly received therein, and together the wedge component 410 and the lock component 420 substantially fill the pocket leaving no gaps around the outside, etc.

[0069] Next, it will be noted from Figure 8 that the lock component 420 has a circular hole 422 formed therein. The holes 422 in a number of the other visible lock components 420 can also be made out in other Figures. This hole 422 in each lock component 420 is, in fact, threaded (or at least a portion of it is threaded), and the hole 422 is therefore designed to receive a counter sunk bolt (not shown). However, in addition to the hole 422 in the lock component 420, there is also a threaded hole (not shown) formed in the floor of the pocket, immediately beneath the location where the hole 422 in the lock component 420 becomes positioned when the lock component 420 is inserted into the pocket. Therefore, when the lock component 420 is inserted into the pocket (which is after the blade has been mounted on the disc, and the wedge component 410 has been inserted into the pocket and slid across, etc, as discussed above) the counter sunk bolt (not shown) can then be inserted and screwed into the hole 422. As the bolt is screwed into the hole 422, the bolt will initially "screw down" into the lock component 420; however with further rotation the bolt will then also continue down and a portion of it will also screw into the threaded hole in the floor of the pocket. Thus, the bolt effectively causes the lock component 420 to be screwed/bolted directly to the main body of the disc 100 itself. In addition to this, it should be noted that as the bolt is screwed down, and more specifically as it is screwed into the threaded hole in the bottom of the pocket (i.e. as it screws into the body of the disc 100 itself) this causes the lock component 420 to be pressed more and more firmly against the body of the disc 100 (i.e. the lock component 420 will be drawn more firmly down into the pocket). And furthermore, the configuration of the respective wedge component 410 and lock component 420 (e.g. through an angled camming action between slopping contacting edges thereof) will typically be such that, as the lock component 420 is drawn more firmly down into the pocket (i.e. as it presses down more firmly against the disc 100 itself) this in turn causes the wedge component 410 to be pushed further in the lateral direction, thereby forcing ("wedging") the wedge component 410 more firmly underneath the blade 200 and/or causing the above-mentioned portion of the wedge component 410 to engage more firmly with, or to insert further and more firmly into, the notch (312 or 322) and/or between the posts (314 and 316, or 324 and 326).

[0070] The way in which the mechanism discussed above secures the blade 200 to the disc 100 may now be readily understood. Firstly, the reason why the blade 200 (if properly secured in the manner discussed above) is prevented from simply sliding or flying off the disc 100 when the disc rotates is because, when the blade is so secured, even though centrifugal forces acting on the blade still try to force the blade to slide off the disc, nevertheless this is prevented because the wedge component 410 is forced into the notch (312 or 322) of the blade, and consequently sliding movement of the blade 200 in an outward direction relative to the disc 100 is prevented because the relevant inner post (316 or 326) on the blade collides with the wedge component 410. At the same time, a portion of the wedge component 410 also remains in contact with the enclosing solid wall of the pocket (which is formed in the upper surface of the disc). Consequently, the wedge component 410 is itself unable to move at all in a radially outward direction, meaning that the blade 200 is secured radially in place on the disc 100. It should also be noted that the blade is prevented from rattling around in its mounted location on the disc, partly because the width of the channels into which the blade 200 slides when it is initially slid onto the disc 100 are the same with (or only very slightly wider) than the width of the blade itself, but also because, if the blade is properly secured in place by the wedge component 410 and the lock component 420, the forcible engagement of the wedge component 410 with the blade also helps to hold the blade securely in place (and prevents it from rattling around, etc).

[0071] Note that, at each location on the disc where a blade is to be mounted, the pocket used in securing the blade at that location is formed in the upper surface of the disc on the forward side of the location where the blade will be mounted. This is important because when the impeller spins, the main force that is created on the blade (namely by the leading face of blade contacting and pushing through the slurry ahead of it) tends to push back on the blade, thereby attempting to twist the blade in a direction (or about an axis) as indicated by arrow T in Figure 7. However, even if the blade does twist (or try to twist) or bend in this way, the effect of this is that the inner portion of the blade tends to move forward (or tries to move forward) slightly in the direction of disc rotation, which in turn tends to wedge the blade (and specifically the notch and/or the posts etc) more firmly against the wedge component 410.

[0072] At this point it is important to take note of a very important design aspect in the embodiment described and shown in Figure 6 to Figure 10, namely that when the wedge component 410 and the lock component 420 are inserted into the pocket to secure the blade 200 on the disc 100, as explained above, the upper faces of both the wedge component 410 and the lock component 420 are substantially flush (or very close to it) both with one another and also with the flat upper surface of the disc 100. Furthermore, in relation to the bolt which is used to secure the lock component 420 down into the pocket, when this bolt is fully "screwed in", the upper face of the bolt head will also sit substantially flush (or very close to it) with the upper surface of the lock component 420. Therefore, when a blade 200 is properly secured on the disc 100 in the manner described above, the parts and the mechanism by which this is achieved do not have any parts or shaped features which give rise to any significant voids or low-pressure regions when the agitator impeller is spinning. This is in stark contrast to, for example, the regions of low pressure created behind the upstanding bolt heads in the prior agitator impeller design shown in Figure 3, which (it is thought) are a significant cause of the major wear-related damage shown in that Figure and also in Figure 2.

[0073] There are also a number of other design features in the embodiment shown in Figure 6 to Figure 10 which have not so far been discussed. One of these relates to the innermost portion of the slot 300 in the blade 200. The innermost portion of the blade slot 300 is the portion that forms the curved end of the slot's overall general U-shape. It was mentioned above that the distance between the opposing sides of the slot (310 and 320) is greatest in this innermost portion of the slot 300. In any case, the significance of this innermost portion of the blade slot 300 is that, even when the blade 200 is slid fully into place on the disc 100 (and even if the blade is secured in place in the manner described above) nevertheless the innermost portion of the blade slot 300 does not come into contact with the outer perimeter edge of the disc 100. Rather, the innermost portion of the blade slot 300 creates an opening or hole in between the outermost perimeter edge of the disc 100 and the central body of the blade 200. This hole is labelled with reference numeral 500 in a number of the Figures. And in fact, as specifically illustrated in Figure 9, the function of the said hole 500 is that, when the agitator impeller is spinning and the blade 200 is consequently moving rapidly through the slurry, this hole 500 allows a small amount of the slurry immediately ahead of the advancing blade to flow through the said hole 500 and into the region immediately behind (i.e. in the wake of) the blade 200 as the blade passes. What this does (or is thought to do) is to effectively cause at least some slurry to flow into the region in the wake of the blade; whereas if the hole 500 were not there and no slurry could flow (effectively through the blade) into the said wake region a significant void or low-pressure region would be created in the wake region. Thus, the purpose of the hole 500 is to (it is hoped) reduce, at least somewhat, the magnitude of the pressure drop that is created by the rapid movement of the blade through the slurry, in the region immediately behind (i.e. in the wake of) the moving blade. And as will be appreciated from the explanations given above, including with reference to Figure 5, this may in turn help to prevent cavitation (or at least reduce the amount or severity of cavitation) in the region behind the blade, which is thought to be a major cause for the damage to the rear of the blade shown for example in Figure 2.

[0074] Another of the design features in the embodiment in Figure 6 to Figure 10 which has not so far been discussed is several the cutouts which are formed on the underside of the disc 100, and which help to ensure that oxygen that is introduced into the autoclave beneath the impeller disc becomes directed appropriately. One of these cut-outs is shown clearly in Figure 11 , but they are also clearly visible in other figures. As can be seen, these cutouts are outwardly sloping in that, at their inner ends, each cutout merges/joins with the main underside face of the disc 100. However, each respective cutout becomes more deeply indented into the underside of the disc 100 as it extends radially outwards. This means that, for any oxygen bubbles introduced or existing underneath the impeller near the centre of the impeller, as these move radially outward beneath the disc they will likely be collected by or caught in one of these cutouts (remember the disc is spinning so the cutouts will be sweeping around). Once caught in a cut-out, the oxygen bubbles will then continue to move out towards the outer edge of the disc, but as the oxygen bubbles pass out into the space radially outward from the disc, because of the way the cutout is indented, the bubbles enter the region radially outward from the disc not at the vertically lower level of the disc's main underside, but rather somewhat further up towards the vertical middle of the disc (and hence closer to the vertical middle of the blades). Directing the oxygen effectively towards the centre of the blades helps to ensure that the oxygen is well mixed, etc, to promote the oxidation reaction in the autoclave. Another point to note is that, on each of these cutouts, the forward/leading side edge of the cutout is sloped, but the rearward/trailing side edge is a sharp/90^o edge. The purpose for the sharp/90^o rear/trailing edge is to trap oxygen bubbles against that edge so that that the bubbles are channelled radially outward toward the outside of the disc rather than slipping black along underneath the underside of the disc. However, a significant reason why the/leading edge on each cutout is not a sharp/90^o edge, but rather is an angled/tapering edge, is to prevent the creation of a void or low-pressure region in the wake/rear of that edge that might otherwise lead to cavitation and damage to the underside of the disc.

[0075] Having described the particular embodiment shown in Figure 6 to Figure 10, one point that should be made is that whilst one of the significant aims with this embodiment is to minimise the effect or severity of cavitation (and the wear and damage cavitation causes), it is not expected that cavitation can be eliminated or prevented completely. Indeed, one region where it is thought that cavitation may still occur to some degree is in the region behind (or on the planar rear face) of each blade. And therefore, even if cavitation in this region is also lessened or reduced (which hopefully it will be due to e.g. the hole 500, etc, as described above) nevertheless cavitation may with time still cause wear on the rear face of the blade. It is largely for this reason that the present invention contemplates, and indeed it provides a design that facilitates, detachment/removal and reattachment of blades. Indeed, as mentioned above, the configuration of each blade (and in particular the shape of the blade) is such that each blade is (at least approximately) symmetrical about a hypothetical horizontal centreline C (see Figure 7). This symmetry allows a given blade to be installed on the disc 100 "either way" (and the way in which a blade can be installed on the disc either way will now be more fully understood). Thus, each blade can be installed so that either one of its faces becomes the "leading" face 202. But the symmetry also means that it is possible for blades to be disconnected/detached from the disc 100 (this is done by reversing the installation process above), and then "flipped over" and reinstalled, so that the face of the blade that was initially/previously the leading face 202 becomes the rear face 204, and vice versa. And it will now be understood that one of the reasons why this (i.e. detaching, flipping and reinstalling the blade the other way around) may be done is if the blade has undergone some degree of cavitation-related wear on one face (the current rear face), but not to a sufficient degree that the blade is destroyed or no longer useful. In these circumstances, the present invention allows that blade to be removed, flipped over (i.e. so that the slightly damaged rear face becomes the leading face of the blade, and the potentially less damaged formerly leading face becomes the new rear face) such that the blade may continue in use, rather than simply being discarded or left in place and to fail more quickly due to the increased rate at which wear occurs once cavitation has begun.

[0076] Turning now to Figure 12, this figure illustrates an alternative mechanism (i.e. alternative to the embodiment in Figure 6 to Figure 8) for attaching a blade to the central "disc" of an impeller-like agitator. In Figure 12, only a single blade 2000 is shown, and only a small portion of the disc 1000 (to which the blade 2000 attaches) is shown. The blade 2000 shown in Figure 12 differs somewhat in shape from the blade 200 in the previous embodiment above. For instance, whereas the blade 200 in the above embodiment had a generally rectangular shape overall, the blade 2000 in Figure 12 has had its inner corners "cut off" to form slightly inwardly tapering inner corners. The slot in the blade 2000 (i.e. which slots onto the disc 1000) also has a somewhat different configuration, and the innermost region of the slot in the blade 2000 does not leave any space or opening (i.e. nothing equivalent to form a hole like the hole 500 above) which would allow slurry to flow in between the blade and the disc when the impeller is spinning. The slot in the blade 2000 (at least as it is shown in Figure 12) is also not symmetrical about the horizontal centreline of the blade, which means that in this particular embodiment (at least the way it is depicted in Figure 12) the blade cannot simply be detached, flipped and reinstalled the other way around. Of course, those skilled in the art will readily appreciate how the blade slot in the embodiment in Figure 12 could be easily modified (compared with that shown in Figure 12), specifically to make the slot symmetrical about the blade's horizontal centreline, and this would in turn enable to blade to be "flipped" and used either ways.

[0077] Nevertheless, despite the differences mentioned in the previous paragraph, there are also a number of similarities between the embodiment in Figure 6 to Figure 8 and the embodiment in Figure 12 in terms of the way in which the blade is secured to the disc. For instance, in both embodiments, the opposing sides of the slot in the blade are each received in channels on the upper and lower faces of the disc. In fact, in Figure 12, one of these channels - one on the upper side of the disc - is visible and labelled as Y. The equivalent channels that perform the same function in the earlier embodiment were not actually illustrated in any of the earlier Figures.

[0078] Perhaps the most significant difference between the embodiment in Figure 6 to Figure 8, and the embodiment in Figure 12, in terms of the way in which the blade is secured to the disc, lies in the fact that the embodiment in Figure 12 uses only a single blade-securing component 4000 (this is instead of the two components, namely the wedging component 410 and lock component 420, used in the previous embodiment). In the embodiment in Figure 12, the single blade-securing component 4000 is a generally circular cylindrical component; however the upper face of the component 4000 is sloping/angled such that the component 4000 is thicker on one side and thinner on the other.

[0079] The way in which the blade is secured to the disc in the embodiment in Figure 12 will now be discussed. Firstly, the blade-securing component 4000 is inserted into the circular pocket E in the upper face of the disc 1000, which is designed to receive it. And importantly, in order to allow the blade to be subsequently mounted on the disc, the blade-securing component 4000 should initially be oriented/rotated within the pocket E such that the thinner side of the component 4000 is located in the region where the pocket E cuts into the channel Y. Next, the blade is slid onto the disc in much the same way as in the previous embodiment. That is, the blade 2000 is first positioned adjacent the set of the blade-receiving channels (including channel Y), and the blade 2000 is then slid onto the disc 1000 in such a way that the opposing sides of the slot in the blade insert into the channels on either side of the disc. Importantly, when the blade-securing component 4000 is oriented with its thinner side located in the region where the pocket E cuts into the channel Y, the blade-securing component 4000 does not impede (i.e. no part of it impedes) the blade 2000 from sliding onto the disc 1000. And finally, in order to then secure the blade on the disc, the disc-securing component 4000 is rotated (this can be done by inserting a tool into the hole in the centre of the component and turning it) so that the thicker side of the component 4000 rotates into the region where the pocket E cuts into the channel Y. This in turn causes the thicker side of the blade-securing component to become physically wedged underneath the blade 2000, thereby securing the blade to the disc.

[0080] It is to be noted that the embodiment depicted in Figure 12 is generally similar to the earlier embodiment discussed above in that, when a blade 2000 is properly secured on the disc 1000 in the manner described above, the parts and the mechanism by which this is achieved do not have any major parts or shaped features which give rise to any very significant voids or low- pressure regions when the agitator impeller is spinning.

[0081] In the present specification and claims (if any), the word 'comprising' and its derivatives including 'comprises' and 'comprise' include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0082] Reference throughout this specification to One embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0083] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.

Claims

1. An impeller for an agitator, the impeller including a central component which is operable to rotate when the impeller is in use, and at least one blade which is initially formed separately from the central component but which is mounted to the central component when the impeller is assembled, wherein each blade is mounted to the central component in such a way that, except for the blade itself, when the blade is mounted to the central component, other parts and components involved in mounting the blade to the central component do not create substantially any shaped features which give rise to voids or low-pressure regions in their wake when the impeller spins.

2. The impeller as claimed in claim 1 , wherein the central component comprises a substantially circular and generally flat disc.

3. The impeller as claimed in claim 2, wherein each blade has a substantially planar shape with opposing faces on either side, and each blade is operable to be mounted to the disc such that the plane of the blade is perpendicular to the plane of the disc, or otherwise such that the plane of the blade is parallel to the disc's axis of rotation.

4. The impeller as claimed in claim 3 wherein, on each blade, the face on one side of the blade comprises a first face and the face on the opposing side of the blade comprises a second face, and the blade can be mounted to the disc such that the first face forms the blade's leading face when the impeller rotates and the second face forms the blade's trailing face, or such that the second face forms the blade's leading face when the impeller rotates and the first face forms the blade's trailing face.

5. The impeller as claimed in claim 4, wherein a blade which is initially mounted to the disc in one said orientation can be disconnected and (re-)mounted to the disc in the other said orientation.

6. The impeller as claimed in any one of claims 2 to 5, wherein each blade incorporates an at least generally elongate cutout, wherein the said cutout opens through one edge of the blade and the cutout has opposing sides, and the blade is mounted to the disc by sliding the blade onto the outer edge of the disc such that a portion of the disc becomes located in between the opposing sides of the cutout in the blade.

7. The impeller as claimed in claim 6 wherein, at each location on the disc where a blade can mount to the disc, a radially oriented channel is formed in one or both of an upper (top) and lower (underside) portion or surface of the disc, and when a blade is mounted to the disc at a said location, one of the opposing sides of the cutout in the blade slides into the (or each) said channel in the disc.

8. The impeller as claimed in claim 6 wherein, at each location on the disc where a blade can mount to the disc, a radially oriented channel is formed in both an upper (top) and lower (underside) portion or surface of the disc, and when a blade is mounted to the disc at a said location, one of the opposing sides of the cutout in the blade slides into the channel on the upper side of the disc and the other of the opposing sides of the cutout in the blade slides into the channel on the lower side of the disc.

9. The impeller as claimed in claim 7 or 8 wherein, at each location where a blade mounts to the disc there is an indented or recessed cavity (a "pocket") in the upper surface of the disc, the pocket connecting with and forming an extension of the radial channel at that location on the upper side of the disc, laterally to one side of the said channel.

10. The impeller as claimed in claim 9, wherein the impeller further incorporates one or more components operable to be inserted into the pocket for the purpose of securing the blade to the disc.

1 1. The impeller as claimed in claim 10, wherein the impeller incorporates a wedge component and a lock component which are operable to be inserted into the pocket for the purpose of securing the blade to the disc.

12. The impeller as claimed in claim 1 1 wherein, to secure a blade on the disc, the blade is first slid onto the disc, the wedge component is then inserted into the pocket and slid across within the pocket towards the blade, and the lock component is then inserted into the pocket behind the wedge component, such that the wedge component secures the blade relative to the disc and the lock component secures the wedge component relative to the disc.

13. The impeller as claimed in claim 12, wherein at least one of the opposing sides of the cutout in the blade includes a space or opening or recessed portion, and at least a portion of the wedge component is shaped to engage with or (at least partially) insert into said space or opening or recessed portion to thereby secure the blade relative to the disc when the lock component is fully inserted into the pocket.

14. The impeller as claimed in claim 13, wherein the lock component has a threaded hole therein, and there is also a threaded hole in the disc within the pocket beneath where the lock component becomes positioned, and after the lock component is inserted into the pocket behind the wedge component, a countersunk bolt is screwed first into the lock component and then further down into the hole in the disc, such that the lock component becomes bolted firmly to the disc.

15. The impeller as claimed in claim 14 wherein as the lock component becomes more firmly secured to the disc, this also causes the lock component to push or otherwise urge the wedge component laterally within the pocket toward the blade, thereby causing the wedge component to engage more firmly with the blade which in turn helps to secure the blade.

16. The impeller as claimed in any one of claims 1 1 to 15 wherein, when the blade is fully mounted to the disc, there are substantially no openings or spaces in between the wedge component, the lock component the counter sunk screw, the blade and the disc.

17. The impeller as claimed in any one of claims 1 1 to 16 wherein, when the blade is fully mounted to the disc, the upper surfaces of the wedge component, the lock component and the counter sunk screw are substantially flush with an upper surface of the disc.

18. The impeller as claimed in any one of claims 1 1 to 17 wherein, when the blade is fully mounted to the disc, a small opening remains in the blade, the said opening extending fully through the blade (i.e. through its full thickness) and the opening being located radially outward of the perimeter of the disc.

19. An agitator incorporating an impeller as claimed in any one of the preceding claims.

20. A central component of an impeller for an agitator, wherein the central component has at least one location where an impeller blade can be mounted thereon, and at each location where a blade can mount to the central component, a radially oriented channel is formed in one or both of an upper (top) and lower (underside) portion or surface of the central component, the said channel(s) being such that a blade having an elongate cutout therein can be slid onto to the central component and one of the sides of the cutout in the blade slides into the (or each) said channel in the central component.

21. The impeller as claimed in claim 6 wherein, at each location on the disc where a blade can mount to the disc, a radially oriented channel is formed in one of the upper (top) and lower (underside) portion or surface of the disc, and when a blade is mounted to the disc at a said location, one of the opposing sides of the cutout in the blade slides into the channel on the disc.

22. A blade having a substantially planar shape with opposing faces on either side, and an at least generally elongate cutout opening through one edge of the blade. A blade as claimed in claim 22 wherein the inner end of the cutout is spaced from outer edge of the disc when the blade is mounted to the disc.