Abstract:
Method and apparatus for separating components of slurries by gravity settling thereby forming a thickened slurry and a clarified liquid. The apparatus comprises a vessel for decanting a volume of slurry, the vessel having a top and an interior formed by a side wall and a bottom wall for holding the slurry, a slurry inlet means, an outlet for the clarified liquid near the top of the vessel, and a slurry withdrawal apparatus for removal of the thickened slurry from the vessel at or near the bottom wall thereof. The slurry withdrawal apparatus physically engages a portion of the slurry within the vessel interior and transports it through a vessel outlet. The apparatus may include an elongated, rotatable, open spiral-shaped element extending a distance into the vessel from outside near the bottom wall of the vessel, the open spiral-shaped element being in direct and open communication with the interior of the vessel over at least a significant portion of the distance, and a rotational drive mechanism for rotating the open spiral-shaped element, at least intermittently.

Description:
This application claims priority to U.S. Provisional Application No. 60/710,455, filed Aug. 23, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus capable of producing highly viscous slurries in industrial processes, as well as withdrawing and transferring such slurries once formed. More particularly, the invention relates to the thickening, withdrawal and transfer of slurries that are so viscous that they cannot be removed from a vessel simply by draining or even by conventional pumping techniques. 
     2. Discussion of Prior Art 
     The present invention is described in the following with particular reference to the treatment of “red mud” which is an aqueous mineral slurry produced during the extraction of alumina from bauxite by the Bayer process. However, such description is just by way of illustration and the present invention may be used to thicken, withdraw and transfer slurries and muds of various kinds, particularly, although not exclusively, those having clay-sized particles and yield pseudo-plastic properties produced by any industrial process. It should also be noted that, in the following description, the term “mud” is used to mean the same as “slurry”. 
     During operation of the Bayer process, there are various stages in which red mud is introduced into a vessel and treated for a variety of procedures, such as clarification, washing and thickening of the mud. During such procedures, the mud thickens (i.e. the slurry is separated in a higher solids content fraction) towards the bottom of the vessel to form a thickened bed below a clarified liquor, and the mud is normally displaced or “activated” within the bed by means of a rotating rake or set of arms. Such activation can further increase the thickening of the mud at the lower end of the vessel so that, at the very bottom of the vessel, a highly viscous mud can be formed that is extremely difficult to remove from the vessel on a continuous or intermittent basis. In some parts of the vessel, the thick and highly viscous bed of mud may become stagnant or inactive, making it even more viscous and difficult to extract. 
     The treatment of red mud in this way is shown, for example, in U.S. Pat. No. 4,830,507 which was issued on May 16, 1989 to Peter F. Bagatto, et al. and is assigned to Alcan International Limited, and also in U.S. Pat. No. 5,080,803 which was issued on Jan. 14, 1992 to Peter F. Bagatto et al. and is assigned to the same assignee. 
     It has also been observed that when a highly viscous bed of mud forms towards the lower end of the vessel and this mud is being extracted from or near the bottom section of the vessel using a suction pump or similar device, a preferential path of lower viscosity mud (or diluted mud from an upper section) tends to form within the highly viscous bed of mud, leaving higher solids concentration mud un-extracted and stagnant. The phenomenon is informally called “rat-holing” or “doughnut formation” and is undesirable. 
     The consequence of this situation is that, although some gravity settling vessels can produce highly viscous mud, the resulting mud of high solids content becomes partly diluted by the creation of a preferential path of extraction, so that what is actually extracted is a more dilute mud. This dilution phenomenon induces instability with respect to the concentration and viscosity of the mud being extracted from that vessel and hence introduces severe control difficulties. 
     The problem of removing highly viscous slurries or muds from vessels of this kind is specifically addressed in U.S. Pat. No. 6,340,033 which was issued on Jan. 22, 2002 to Ronald Paradis et al. and is assigned to Alcan International Limited. The solution to the problem described in this patent involves using a pump or impeller to withdraw slurry from a vessel, subjecting it to high shear, and returning it to the vessel at a point somewhat displaced from the point of slurry withdrawal. The high shear applied to the slurry reduces the viscosity of the slurry (which is referred to as shear-thinning) and thus creates a supply of mud of reduced viscosity. The mud of reduced viscosity, upon re-entry into the vessel, creates a stream of mud within the vessel that entraps particles or clumps of slurry of higher viscosity that are thereby removed from the vessel, recirculated and themselves reduced in viscosity. During the recirculation process, some of the slurry of reduced viscosity is removed and transferred to a different location, thereby continuously withdrawing mud from the system. 
     While this is an effective solution to the problem of removing slurry of high viscosity from a vessel, it has the disadvantage that the slurry thus removed is necessarily of somewhat reduced viscosity and has to be allowed to settle and stand if a higher viscosity material is required. In many instances, higher viscosity slurry is desirable because it has many of the properties of a solid. Thus, it is at least partially self-supporting when dumped at a land-fill site or other location and can therefore be stacked at a greater height than slurry of low viscosity which tends to flow and dissipate when dumped. Slurry of high viscosity can also be transported on an open conveyor belt, an open truck, or the like, and there is always the option of subjecting it to high shear by means of a pump or impeller, when desired, so that it can be pumped through pipes to another location. Moreover, U.S. Pat. No. 6,340,033 does not fully address the “rat-holing” issue discussed earlier when the mud becomes very thick. 
     European Patent Publication EP 0 019 538 A1 issued in the name of Alsthom-Atlantique SA, uses a spiral-shaped element to assist the removal of slurry from a tank. The spiral-shaped element is positioned below the tank and acts to remove slurry exiting the interior of the tank through a narrow central opening. The element is largely confined within a closely-surrounding tube or cylinder and only its distal end is aligned with the central opening of the tank. Such an arrangement is likely to be of little use for removing slurry of very high viscosity because such slurry would not flow easily through the narrow central opening provided in the bottom wall of the tank. 
     Accordingly, it would be advantageous to provide equipment that can generate, on a consistent basis, a highly viscous mud having a high solids concentration, minimizing any internal dilution due to preferential path within the thick mud bed, and without substantially varying the viscosity of the slurry during the removal process. 
     SUMMARY OF THE INVENTION 
     In exemplary embodiments of the present invention, thickened slurry of high solids content produced from a slurry introduced into a vessel acting as a gravity settler is removed from the vessel by a procedure in which a portion of the slurry of high solids content is physically engaged by a removal element located permanently or temporarily within the interior of the vessel (i.e. the part of the vessel where the thickened slurry of high solids content is initially formed) and is transported by the element through an outlet provided for the slurry of high solids content. Reliance on gravity draining through the outlet and the use of suction pumps, impellers, and the like to remove the slurry of high solids content from the vessel can thereby be avoided in whole or in part, and any tendency of the slurry of high solids content to bind or bridge at the outlet is overcome. 
     By the term “physically engaged” we mean that one or more parts of the removal element contact the portion of thickened slurry of high solids content within the interior of the vessel in such a way that the portion is moved in the vessel upon operation of the element (e.g. by rotation or translation) towards and through the outlet without applying undue shear to the thickened slurry. 
     Certain exemplary embodiments can provide an improvement to gravity settler design comprising a slurry vessel in which at least a component of a slurry of high solids content accumulates at the lower end of the vessel without any significant internal dilution, and a slurry withdrawal apparatus for removal of the slurry of high solids content from the vessel. The slurry withdrawal apparatus may comprise an elongated, rotatable, open-spiral-shaped element extending, at least intermittently, for a distance into the interior of the settler vessel from the outside at the lower end of the vessel, the open-spiral-shaped element being in direct and unconfined communication with the interior of the vessel, at a position where the slurry of high solids contents accumulates, over at least a majority of its length when extending fully into the vessel. The slurry withdrawal apparatus also preferably includes a rotational drive mechanism for rotating the open-spiral-shaped element, at least intermittently. 
     Other exemplary embodiments provide a method of generating, on a consistent basis, a slurry of very high solids content and of withdrawing the slurry of very high solids content from a vessel. The method comprises introducing into a vessel containing a slurry of very high solids content an elongated open-spiral-shaped element exposed to the slurry over at least a very large section of the length thereof, the spiral-shaped element being introduced through a wall of the vessel near the bottom, and operating the spiral-shaped element to withdraw the slurry from the vessel. 
     The invention may be used with mineral slurries, especially red mud from bauxite extraction, as well as with slurries of other kinds. Certain embodiments of the invention can be also used with slurries or muds having high sand content (i.e. particles larger than usual for slurries by an order of magnitude at least) without encountering difficulties. 
     By the term “open-spiral-shaped element” we mean to include any kind of elongated element having a longitudinal axis that is preferably straight and this is made up of one or more components having vanes, flutes or constituent parts that create a helical path for slurry to follow and translate from one end to another as the element rotates, or that allow the element to bore into the slurry with minimal slurry displacement, as the element is rotated and inserted into a body of the slurry. Generally, when the spiral-shaped element has vanes, the vanes are orientated at an angle (e.g. at right angles) to the line of motion, and have uniform spacing (pitch). Such an element is described as “open” when the helical path, i.e. the spaces between the vanes, flutes or constituent parts, are open to the exterior of the element (the interior of the vessel) so that mud or slurry can enter along an exposed length of the element. The element is exposed, unshielded, unconfined or not blocked by any other member, at least over a majority of the length of the member and at least from one side (region of the circumference). This allows unconfined and unrestricted access of the slurry to the helical path defined by the element at least along the majority of its length (more than 50%) within the vessel and preferably along its entire length within the vessel. The slurry preferably has access to the open-spiral-shaped element such that, as slurry is withdrawn from the vessel, more slurry may descend around the element solely under the effects of gravity and any suction developed by the slurry withdrawal. The access to the element in this way should not encounter any constriction or choke points that cause bridging or blocking of the slurry flow as it advances into contact with the element. The slurry should therefore not be caused to pass through narrow openings before reaching the element from the interior of the vessel. Slurry of high viscosity will normally flow under the effects of gravity if there are no confining surfaces or articles to restrict the downward flow. 
     Preferably, in certain embodiments, the open-spiral-shaped element extends into the vessel (which is normally cylindrical) horizontally along a radius of the vessel through an opening in a sidewall of the vessel, but this is not essential. For example, the element may be displaced from, but arranged parallel to, a central diametrical line of the vessel. The opening through which the element enters the vessel may be provided in the side wall or a sloping part of the lower wall of the vessel and preferably has a generally horizontal orientation. The vessel is therefore not normally provided with a central vertically-disposed opening or drain as has been conventional in settlers of this kind. Essentially, the open-spiral-shaped element enters the vessel interior at the height of the slurry layer of high solids content and mechanically withdraws the slurry from the vessel without first requiring the slurry to pass through a restricted slurry outlet or an inlet of a conventional pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing the yield stress of a particular slurry plotted against the solids content of the slurry to illustrate different kinds of slurry that can be obtained; 
         FIG. 2A ,  FIG. 2B  and  FIG. 2C  are each photographs of slurry extracted from settling apparatus, the slurries being of different yield stress values and solids content; 
         FIG. 3  is a simplified cross-section of a slurry treatment vessel provided with slurry withdrawal apparatus according to one embodiment of the present invention; 
         FIG. 4  is a perspective view of an interior of a vessel similar to that of  FIG. 3 , but having a trough in the bottom wall of the vessel housing a spiral-shaped slurry withdrawal element; 
         FIG. 5  is a simplified cross-section of a slurry treatment vessel and slurry withdrawal apparatus according to another embodiment of the invention; and 
         FIG. 6  is a view similar to  FIG. 5 , but showing another step in the operation of the apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To facilitate an understanding of the present invention, it may first be useful to describe the types of slurry with which the present invention may be employed. In order to describe the rheological properties of slurries, a graph similar to the one shown in  FIG. 1  is often employed. This is a graph of yield stress versus the percentage of solids in a particular slurry (in this case, a “red mud” produced during the extraction of alumina from a given bauxite ore body). Other red muds or other mineral slurries will have different yield stress values at various solids contents, but most will have a curve of similar shape. 
     As can be seen from the graph, the yield stress of the slurry has only a very low value at a solids content of less than 40%. 
     For slurries with higher solids content, the yield stress increases gradually at first (Slope  1  in the figure—a slope of approximately 1 or less). Slurries of this kind are referred to as “dilute slurries” and an example of such slurries is the mud obtained from a conventional wide and flat bottom thickener. An illustration of such slurries is shown in  FIG. 2A  and it will be seen that, upon being left unsupported on a flat surface, the slurry immediately flattens into a pool. For slurries of this kind, the slump or slump ratio is in the order of 0.1 or less. In this regard, it should be noted that viscosities of slurries and muds are often assessed by carrying out slump tests in which the mud is packed into a standard cylinder having an open bottom and top resting on a support surface. The cylinder is then removed upwardly and the height and width of the remaining pile are measured after a given time. The unsupported slurry will slump to some extent. A stiffer or more viscous slurry will slump less than a less viscous slurry and thus will have a greater “slump height” or “slump ratio” (ratio between height and width at the base of the cone). 
     Referring again to  FIG. 1 , for slurries with solids contents of about 45 to 52%, the slope increases more rapidly (Slope  2  of the figure—a slope ranging from about 10 to 20). This corresponds to slurries referred to as “paste slurries” and an illustration of such a slurry is shown in  FIG. 2B . This slurry was obtained from a deep thickener, e.g. as described in U.S. Pat. No. 4,830,507. The slump ratio is approximately between 0.2 and 0.5 and it can be seen that there is a considerable spread at the base. 
     The slope of the curve of  FIG. 1  starts to increase dramatically from about 52% solids onwards (Slope  3  of the figure—a slope essentially above 20). This region corresponds to slurries referred to as “solid pastes.” An illustration of such a slurry is shown in  FIG. 2C  which is a solid paste extracted from a thickener in accordance with the present invention. The slump ratio of this sample is approximately 1.25 (any ratio above about 0.5 is considered to indicate a solid paste). 
     Dilute slurries do not require any specialized means of extraction and transfer easily from thickeners or other vessels. A normal centrifugal pump is sufficient for such transfer. Paste slurry may require the use of specialized equipment and techniques, e.g. as disclosed in U.S. Pat. No. 6,340,033. On the other hand, solid pastes cannot be removed from a thickener using a suction pump and certainly will not flow from an outlet on their own merely under gravity. The present invention is intended most preferably for use with both paste slurries and solid pastes, but especially with the latter. 
       FIG. 3  of the accompanying drawings shows, in simplified form, an apparatus  10  used for treating red mud slurry during operation of the Bayer process, e.g. for washing and thickening of the red mud by gravity settling. 
     The apparatus  10  includes a settler vessel  12  in the form of an open-topped tank having a side wall  14  and a flat bottom wall  16 . The side wall  14  includes a tapering section  18  at the lower end  20  of the vessel. The apparatus includes a feed well  22  through which slurry is introduced into the vessel with minimal disturbance of the volume of liquid  24  already present in the vessel. The feed well surrounds a central vertical shaft  26  of a raking device  28  (stirrer) which is rotated about its central vertical axis in the direction of arrow A as shown in the figure. The raking device  28  includes upwardly sloping arms  30  arranged in a V-shape and upright stirrer elements  32  supported by a horizontal arm  34 . As the solid particles of the mud settle by gravity towards the bottom of the vessel, water is squeezed from between the solid particles with the assistance of the raking device  28 , and the mud acquires a greater solids content and a higher viscosity as it approaches the bottom of the vessel. The water expelled from the solid particles forms a clarified liquid  33  that exits the vessel via an upper outlet  35 . 
     In a raked region  36 , the viscosity of the mud is reduced by virtue of the shear-thinning properties of this kind of mud, but beneath the margins of the raking device  28 , a region  38  or bed of thickened and unraked mud of high solids content and high viscosity tends to form and build up (as indicated by the dashed line). The viscosity of the thickened mud can be extremely high, for example it may have an initial yield stress of 30 Pa or more, and more probably 50 Pa or more, generally 500 Pa or more, normally 1000 Pa or more, or even 3000 Pa or more. 
     It is to be noted that the term “initial yield stress” in this context means the minimum force per unit area required to initiate the movement or displacement of a given slurry from the state of rest. It is a measurement used in the industry as an indication of the viscosity of the mud, but it is not a true measure of viscosity itself. The viscosity of a pseudo-plastic material varies with the applied shear caused by mixing or turbulence. 
     The thickened red mud produced in the illustrated apparatus may have a solids content of more than 56 weight percent, and normally more than 57 weight percent, for example 57.9 weight percent solids or more. Red mud of this consistency cannot be removed by gravity, for example by providing a conventional outlet at the central point of the bottom of the vessel and allowing the mud to drain out. It is even difficult or impossible to remove mud of this consistency by means of a suction pump or impeller, even when resort is made to the invention of U.S. Pat. No. 6,340,033 mentioned above. Mud of this consistency is solid paste of the type described above. 
     In the illustrated apparatus, the mud of high solids content and viscosity in unraked region  38  is removed by means of an elongated, rotatable, open-spiral-shaped element  40  extending into the vessel from the outside through an opening  50  in the tapered region  18  of the side wall  14  at the lower end  20  of the vessel. The element  40  preferably extends into the tank by a distance x such that the free (distal) end  42  of the element  40  is positioned directly beneath the shaft  26  at the center of the vessel  12 . The slurry of high viscosity enters between spiral vanes  48  of the element  40  and is removed from the vessel  12  through opening  50  (which accordingly acts as a slurry outlet) by rotation of the element  40  around its longitudinal axis  44  in the direction of arrow B as shown in  FIG. 3 . The element consequently physically engages portions of the slurry of high viscosity in the interior of the vessel and withdraws them from the vessel through the restricted outlet  50 . 
     The element  40  as shown is in the form of an Archimedes screw, i.e. a solid longitudinal shaft having one or more encircling spiral vanes, but it could be a spiral element of another form, e.g. an element lacking a central shaft (as if produced by twisting a flat strip or a rod having a propeller-like cross-section) around its longitudinal axis. It is to be noticed that the outer surface of element  40  is positioned within, and is completely open to and in direct communication with, the interior  46  of vessel  12  at the height of the region  38 , so that its spiral coils or vanes  48  are exposed to and in contact with the mud of high viscosity along the full length x of insertion of the element into the vessel. The element  40  is essentially completely buried within the mud preferably without contact with the mud of reduced viscosity in the stirred region  36 . The spaces between the vanes are unconfined (i.e. they are not obstructed, shielded or covered by other parts of the apparatus) and are hence open to the interior of the vessel and may be directly loaded with slurry at all points where contact with the slurry is made. 
     It has been found that, when employing such an arrangement within a body of a slurry material of high viscosity and density (particularly a slurry paste or solid paste), the slurry surrounding the element  40  appears to confine slurry positioned between the vanes  48  thus causing the slurry to remain in contact with the element, and causes the material to be conveyed longitudinally. This ensures that the material passes through opening  50  in the side wall of the vessel as the element is rotated, and preferably into an external chamber or tube (not shown in  FIG. 3 ) from which it can be transferred away from the apparatus. As slurry is removed in this way, more slurry is forced between the vanes  48  of the element  40  by virtue of the weight and pressure of the surrounding slurry. In effect, slurry confined between the vanes is constrained to move axially with the rotating element  40 , while more slurry enters between the vanes to replace the slurry withdrawn from the vessel. 
     Even though there may be some localized shear force applied to the slurry material as it is acted on by the element  40  (e.g. in a thin layer where the mud contacts the material of the vanes), this does not produce a dramatic or unacceptable reduction of the overall viscosity of the slurry material as it is removed from the vessel. Without wishing to be bound by theory, some degree of shear-thinning may be helpful to act as a lubricant between the slurry and the vanes (thereby allowing longitudinal movement of the slurry trapped between the vanes rather than mere rotation in concert with the element). However, it is desirable to rotate the element  40  fairly slowly to avoid substantial shear-thinning of the slurry and to avoid undue compression or further de-watering of the slurry. The actual rotational speed considered desirable in a particular case depends on the size and pitch of the vanes  48 , as well as the nature of the slurry. Normally, it is desirable not to rotate the element  40  at more than 130 rpm. The flow rate of the slurry is usually linear with the rotational speed of the element  40 , provided any outlet tube attached to the opening  50  has essentially the same diameter as the element  40 . 
     While the element  40  employed in  FIG. 3  is of constant diameter along its full length, the element may (if desired) be tapered inwardly towards the free end  42  to ensure a uniform rate of extraction along the full length of the element. 
     It should also be noted that more than one extraction point can be provided around the vessel  12 , each provided with its own spiral-shaped element  40  in order to increase the rate of extraction of the slurry and to minimize the regions in which inactive slurry may build up. Such extraction points may be arranged at 90° to each other or arranged at other angles to best suit the design of the raking device  28  that pushes the material towards the extraction points. Further, the (or each) element may be positioned off-radius if desired. 
     The illustrated embodiment thus employs a spiral-shaped element that is fully exposed to the interior of the tank (i.e. is unconfined) at least along a substantial portion of its length (e.g. at least 20% or at least 25% of its length). More preferably, the element is fully exposed to the interior of the tank for at least a majority (50% or more) of the distance x between the free end  42  of the element  40  and the vessel wall  18 , and even more preferably at least (in increasing order of preference) 55, 60, 65, 70, 75, 80, 85, 90, 95%, and most desirably 100% of the distance x. Thus, ideally, the spiral-shaped element is fully exposed to the interior of the vessel along its full length. 
     As already noted, in the embodiment shown in  FIG. 3 , the spiral-shaped element  40  is open to the interior of the vessel (and hence to the slurry material of high viscosity) from all sides of the element (i.e. the entire 360 degrees of the circumference of the spiral-shaped element is directly exposed to, and positioned within, the interior of the vessel). However, as will be apparent from additional embodiments described below, it is only necessary to expose one side (e.g. an elongated strip preferably on the upper side) of the circumference of the spiral-shaped element to the interior of the vessel for the apparatus to be effective, for example by positioning the element within an open-topped trough of rectangular plan view formed in the bottom wall of the vessel. However, the lateral width of the trough (the open top) must preferably be wide enough to allow the slurry of high viscosity to enter the trough without significant restriction or confinement and be withdrawn by the spiral-shaped element. Such an arrangement is shown in more detail in  FIG. 4 . 
       FIG. 4  shows the interior of a vessel  12  having a side wall  14  and a bottom wall  16  similar to that of  FIG. 3 , except that, in the embodiment of  FIG. 4 , the side wall  14  does not taper inwardly towards the bottom wall  16  which is flat and horizontal. As in the previous embodiment, the vessel is provided with raking apparatus  28  including a vertical shaft  26  and stirrer arms  34  (which are horizontal in this embodiment). 
     The bottom wall  16  has a diametrically-arranged trough  52  extending completely between opposite parts of the side wall of the vessel. The trough  52  has an open top  51  and contains an open-spiral-shaped element  40  extending the full length of the trough  52 . In this embodiment, the spiral-shaped element includes two coaxial and co-extensive, mutually telescoped, corkscrew-shaped spiral members  53  and  54  of different diameters. Both these members are of the open spiral type having an open axial core (corkscrew type). The smaller-diameter member  53  extends through and along the core of the larger-diameter member  54 , as shown. A support  56  is connected to a rotational device (not shown) outside the vessel that is capable of rotating the two members  53  and  54  at the same or different rotational speeds in the same or different directions. This design and arrangement is found particularly effective for removing slurry of very high viscosity from the vessel because the larger-diameter member  54  acts as a distribution/homogenization device that may create an acceptable degree of shear-thinning and acts as an arch-breaker. The member moves the slurry to its center as well as along its length. The smaller-diameter member  53  carries out the extraction of the slurry, so the larger-diameter member feeds slurry to the smaller-diameter member. The combined element  40  is particularly effective when the members  53  and  54  are rotated at different speeds in the same direction. Ideally, there is a fixed ratio of rotation between the two members so that if one member is speeded up to increase the rate of slurry extraction, the other member also speeds up to the same extent. When the members  53  and  54  rotate at different speeds, there is little possibility of the screws filling up with slurry and turning as a whole rather than moving longitudinally. Normally, the member of smaller-diameter is rotated at speeds up to about 130 rpm and the member of larger diameter is rotated at speeds of up to about 8 rpm. 
     The members  53  and  54  are rotated in such a direction as to move the slurry to the left as shown in  FIG. 4  where a slurry exit  50  is located. The members thereby cause the slurry to be transferred through the outlet and hence removed from the vessel. In an alternative embodiment, the support  56  may be positioned outside the vessel so that the slurry may be drawn to the right along the channel  52 . This has the advantage of avoiding exposure of seals and the like in the support  56  to the full pressure of the slurry in the tank. 
     As noted, the rectangular top  51  of the trough  52  is wide enough and long enough to allow the slurry of high viscosity to descend into the trough under the effect of gravity and the pressure of the surrounding slurry. There is therefore no choke point or confined outlet to cause the slurry material to bind or bridge at the entrance  51  and, in effect, the trough forms a part of the vessel interior as a layer of the slurry of high viscosity forms directly within the trough. As can be seen, in this embodiment, the entrance  51  to the trough is wider than the width of the larger-diameter member  54 . In practice, it is found that the width of the trough  52 , and the width of the entrance  51 , should be at least equal to the diameter of the largest part of the spiral-shaped element and preferably at least one and half times that diameter. The overall area of the entrance  51  of the trough should preferably be at least (1.5 times the outer diameter of the spiral-shaped element)×(50% of the radius of the vessel at the bottom). 
     Ideally, the trough  52  has vertical sides, or sides that are steeply inclined (either inwardly or outwardly towards the bottom), to prevent bridging of the slurry descending into the trough. Also, the trough depth should preferably be the same as the diameter of the element  40 , or only slightly larger in order to avoid the formation of a zone of inactive slurry beneath the element  40 . 
     By locating the spiral-shaped element  40  in the trough  52  formed in the lower wall  16  of the vessel, stirrer arms  34  may be positioned closer to the bottom wall  16  than in the embodiment of  FIG. 3 , thereby minimizing the build-up of inactive mud above the lower wall  16  of the vessel and confining it more specifically to the trough  52 . 
     In the above embodiments, slurry of high viscosity is removed from the vessel by the rotational action of the spiral-shaped element  40  which withdraws the slurry from the interior of the vessel between the vanes of the element as the element is caused to rotate in place on a continuous basis. 
     In an alternative embodiment of the present invention, slurry is withdrawn by first inserting the spiral-shaped element into the vessel while causing it to rotate (so that it “drills into” the slurry of high viscosity without causing substantial displacement) and then physically withdrawing the spiral-shaped element loaded with slurry from the vessel without allowing the element to rotate, so that a plug or cylinder of the slurry of high viscosity positioned between the vanes of the spiral-shaped element is withdrawn from the vessel en masse. This is illustrated in more detail in  FIGS. 5 and 6 . 
     In  FIG. 5 , vessel  12  is similar to that of  FIG. 4  as it has a trough  52  positioned beneath and communicating with the bottom wall  16  of the vessel at an entrance  51 . The trough  52  contains a spiral-shaped element  40  extending completely across the vessel floor when fully inserted. The spiral-shaped element  40  is attached to a rotatable rod  60  positioned within a withdrawal chamber  62 . The rotatable rod  60  extends at its opposite end through an end wall  64  of the withdrawal chamber  62  (via a sliding seal) and is connected to a motor  66  used to intermittently rotate the rotatable rod  60  around its longitudinal axis, which in turn intermittently rotates the spiral-shaped element  40  about its longitudinal axis. The motor  66  is mounted on a track  68  and can be reciprocated back and forth along the track by means of a pneumatic or hydraulic ram  70  or by a mechanical or electrical drive (not shown). 
     With the spiral-shaped element  40  in the position shown in  FIG. 5  (already drilled into the mud), rotation of the spiral-shaped element is terminated and the ram  70  is operated to withdraw the motor  66  backwardly along the track  68  so that the rod  60  and the spiral-shaped element  40  are moved to the right in the drawing.  FIG. 6  shows the same apparatus with the spiral-shaped element moved fully to the right where it is positioned fully within chamber  62 . Once in this position, rotation of the rod  60  and spiral-shaped element  40  is commenced and the ram  70  moves the motor  66  forwardly at a rate suitable to allow spiral-shaped element  40  to bury itself within the slurry of high viscosity positioned within the trough  52 . Ideally, the rotational speed of the element  40  and the translational speed of insertion are matched to minimize disturbance of the slurry in the vessel and applied shear force. The spiral-shaped element  40  has an exposed tip  42  at its free end that allows the element to burrow into the slurry of high viscosity in the vessel in a manner similar to the operation of a drill or corkscrew. Slurry already between the vanes of the element from a previous operational cycle remains in the chamber  62  as the element drills into fresh slurry in the vessel itself. Once in the position shown in  FIG. 5 , rotation is terminated as explained above and the cycle is repeated. The movement of the spiral-shaped element to the right in  FIG. 5  causes a plug or cylinder of the slurry associated with the spiral-shaped element  40  to be drawn bodily into the chamber  62  and slurry material already in the chamber from a previous operational cycle is forced out of an outlet  72  from the chamber in the direction of arrow C. Ideally, there should be very little free space between the element  40  and the adjacent walls of the chamber  62  so that the element acts like a piston to drive slurry out of the chamber. 
     It is to be noted that the arrangement shown in  FIGS. 5 and 6  involves pulling of the stationary spiral element  40  out of the vessel after insertion with rotation. However, a pushing action could also be employed in the present invention. That is to say, the element  40  may be driven from the left in  FIGS. 5 and 6  and pushed into the chamber  62  still positioned on the right hand side. The element is then rotated as it is pulled out of the chamber into the vessel. 
     In the embodiment of  FIGS. 5 and 6  of the present application, it is found that the energy required to introduce the spiral-shaped element  40  into the trough  52  filled with slurry of high viscosity is relatively small because of the screw-like movement of the element as it is introduced causing little displacement of the slurry. This has the advantage of introducing minimum disturbance to the internal structure of the slurry. Then, by pulling the spiral-shaped element out of the vessel without any rotation, but with the power of a ram  70 , the slurry maintains its undisturbed internal network structure and hence its original viscosity value. By repeating this cycle several times, the slurry of high viscosity is pushed into pipe  72  and may be transferred to another vessel, a transportation device or directly to a disposal site. 
     Normally, the apparatus can be operated with up to 30 complete strokes per minute but this may clearly be varied to suit the size and type of equipment, and type of slurry, etc. 
     It will be appreciated that the open-spiral-shaped element  40  of the present invention, particularly that of the embodiments of  FIGS. 5 and 6 , should be a screw-like device that can bury itself into a body of material with minimal disruption of the material effected. There are several designs of spiral-shaped element that may accomplish this or even multi-component elements as shown, for example, in  FIG. 4 . 
     The apparatus of the present invention is capable of conveying a slurry of high viscosity up to 100 meters or even more from a vessel, particularly in the embodiments of  FIGS. 5 and 6 . 
     As noted the apparatus shown in  FIGS. 5 and 6  is in simplified form and, in reality, would likely be more complex as will be apparent to a persons skilled in the art. For example, it may be desirable to avoid using the housing of the motor  66  to transmit force from the ram  70  to the rod  60 . Instead, the ram  70  may be connected directly to the rod  60  and gear arrangement used to allow the motor to rotate the rod. 
     In all of the above embodiments, the spiral-shaped element is operated horizontally. This is usual but not essential. For example, in the embodiment of  FIG. 3 , the spiral-shaped element  40  may be arranged to extend into the vessel  12  parallel to the tapering part  18  of side wall  14 , especially if it remains buried in mud of high solids content. It is also conceivable, but not currently preferrable, that the spiral-shaped element could be arranged to extend vertically into the vessel from below through an opening in the bottom wall, provided the element does not penetrate completely through the layer of slurry of high viscosity. 
     The apparatus of the present invention is usually, although not necessarily, employed with vessels of 8 meters or more (ideally 12 meters or more) in diameter and the length of full insertion of the spiral-shaped element into the vessel is normally at least about one third of the vessel diameter, more preferably half of the vessel diameter, and even the complete vessel diameter (as shown in  FIG. 3 ). 
     While it is normally desirable to operate the apparatus to avoid changing the viscosity of the slurry by much as it is withdrawn from the vessel, the pitch of the spiral-shaped element (e.g. the number of vanes per unit length) and its speed of rotation may alternatively be chosen to vary the viscosity and speed of delivery of the slurry exiting the apparatus. The motor used to rotate the spiral-shaped element, particularly in the embodiment of  FIGS. 3 and 4 , may be a variable speed motor so that the speed of rotation can be adjusted by an operator or computer on site to produce a slurry of desired exit viscosity. 
     The present invention may be employed with slurries having initial yield stress values of at least 30 Pascals (more preferably at least 50 Pascals) and also up to several thousand Pascals. While the slurries with which the present invention is used are generally shear-thinning, this is not essential. For example, muds having a high sand content may not have shear-thinning properties, but may still be used with the present invention. Slurries or pastes from many industrial processes may also be used where feed material is ground to a fine size prior to the extraction or recovery of a desired material, e.g. tailings produced during the extraction of gold, copper, zinc and lead. 
     The present invention is described in more detail with reference to the following Example which should not be considered as limiting the scope of the invention. 
     COMPARATIVE EXAMPLE 1 
     A test was carried out in a deep thickener (12 meters in diameter) of the kind described in U.S. Pat. No. 4,830,507 modified to include a spiral-shaped removal element as shown in  FIGS. 5 and 6  of the accompanying drawings. The thickener was also equipped with a conventional centrifugal pump with a recirculation system at the underflow as described in U.S. Pat. No. 6,340,033. 
     The thickener was fed with a bauxite residue slurry (red mud) at a flow rate of 500-550 m 3 /h. The slurry had a solids content of 100-150 g/l (dry basis) for a total feed rate of 55 to 60 t/hr. 
     The slurry was extracted by means the centrifugal pump with recirculation. The results are summarized in Row 1 of Table 1 below: 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Characteristics of Mud at the Underflow of an Industrial thickener 
               
             
          
           
               
                   
                   
                 Solids 
                   
                   
               
               
                 Row 
                 Sampling 
                 concentration 
                 Initial Yield 
               
               
                 No. 
                 location 
                 (%) 
                 Stress (Pa) 
                 Comments 
               
               
                   
               
             
          
           
               
                 1 
                 Exit of 
                 49.8 
                 65 
                 Thick mud requiring 
               
               
                   
                 centrifugal 
                   
                   
                 recirculation to be 
               
               
                   
                 pump (with 
                   
                   
                 extracted 
               
               
                   
                 recirculation) 
               
               
                 2 
                 Exit of screw 
                 50.0 
                 270 
                 Thick mud requiring 
               
               
                   
                   
                   
                   
                 recirculation to be 
               
               
                   
                   
                   
                   
                 extracted 
               
               
                 3 
                 Exit of screw 
                 51.1 
                 475 
                 Thick mud requiring 
               
               
                   
                   
                   
                   
                 recirculation to be 
               
               
                   
                   
                   
                   
                 extracted 
               
               
                 4 
                 Exit of screw 
                 56.1 
                 2900 
                 Mud too thick to be 
               
               
                   
                   
                   
                   
                 recirculated 
               
               
                 5 
                 Exit of screw 
                 56.6 
                 4300 
                 Mud too thick to be 
               
               
                   
                   
                   
                   
                 recirculated 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 1 
     A test was carried out in a pilot deep thickener (0.6 meter in diameter by 1.5 m in height) modified to include a spiral-shaped removal element as shown in  FIG. 4 . 
     The thickener was fed via a pump with a bauxite residue slurry (red mud) at a flow rate of 1 L/min. The slurry had a solids content of 100 g/l (dry basis) for a total feed rate of 6 kg/hr. 
     The slurry was extracted by means of the spiral-shaped removal element. The solids concentration at the underflow was constant at 52.2% with a slump ratio of 0.5. 
     EXAMPLE 2 
     The procedure of Comparative Example 1 was repeated, except that the slurry was extracted by the spiral-shaped element. The results are summarized in Row 2 of Table 1. It can be seen that the mud maintains the same solids concentration, but in this case has a much higher yield stress (270 Pa). 
     The procedure was again repeated with a mud of higher solids content (51.1%), and the measured yield stress is significantly higher (475 Pa). The results are summarized in Row 3 of Table 1. 
     The slurries obtained in both of these cases are examples of paste slurries (less than 500 Pa) that could also be extracted by the recirculating pump. The difference is that the yield stress of the mud at the outlet of the “screw pump” (i.e. the pump in accordance with the invention) is about four times higher than the yield stress of the “equivalent” mud coming out of a conventional centrifugal pump. 
     EXAMPLE 3 
     The procedure of Comparative Example 1 was again repeated with slurries of even higher solids content (56.1 and 56.6%), and slurries of extremely high yield stress were obtained (2900 and 4300 Pa). The results are summarized in Rows 4 and 5 of Table 1. These slurries are examples of solid pastes and they could not be extracted by any other means than the spiral shaped element.