Patent Publication Number: US-2011067568-A1

Title: Apparatus and method for mechanical deaeration

Description:
FIELD OF THE INVENTION 
     The present invention relates to deaerating liquids and in particular to an apparatus and method for deaerating or separating entrained air or froth from liquid suspensions or pulps. It has been developed primarily for use in thickeners, clarifiers, or concentrators and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use. 
     BACKGROUND OF THE INVENTION 
     The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field. 
     Thickeners, clarifiers and concentrators are typically used for separating solids from liquids and are often found in the mining, mineral processing, food processing, sugar refining, water treatment, sewage treatment, and other such industries. 
     These devices typically comprise a tank in which solids are deposited from suspension or solution and settle toward the bottom as pulp or sludge to be drawn off from below and recovered. A dilute liquor of lower relative density is thereby displaced toward the top of the tank, for removal via an overflow launder. The liquid to be thickened is initially fed through a feed pipe or feed line into a feedwell disposed within the main tank. The purpose of the feedwell is to ensure relatively uniform distribution and to prevent turbulence from the incoming feed liquid from disturbing the settling process taking place within the surrounding tank. 
     In cases where the feed liquid comprises entrained air, such as flotation concentrate, it is normally at least partially aerated. The air bubbles, if allowed to pass from the feedwell into the main tank, tend to produce a considerable amount of relatively stable froth on the surface of both the feedwell and the thickener. This froth can contain a significant proportion of entrained solids and thereby tends to reduce the separation efficiency of the thickener, and contaminates the dilute liquor. In addition, air bubbles can become trapped in the sludge, resulting in slower settling rates and lower underflow densities, both of which reduce separation efficiency further still. A further problem is that the froth leaves solid particulates in the overflow and these particulates eventually deposit in storage tanks or dams, which consequently must be frequently cleaned to remove accumulated sedimentation and contaminants. The particulates also contaminate the process water for the plant, as the dilute liquor is generally recycled for this use. This increases plant costs in the additional maintenance of the storage tanks or dams, and the removal of solid particulates from the process water. 
     One solution for this problem has been to provide a deaeration unit for separating froth from the feed liquid before it is fed into the separation device. This deaeration unit has a cyclonic separator, which generates a centrifugal vortex that separates partially aerated liquid into a froth or gas component and a deaerated liquid or sludge component. The froth or gas component is removed from the deaeration unit as an overflow stream while the deaerated liquid or sludge component leaves as an underflow stream that is subsequently fed into the separation device. 
     Whilst this solution has proved effective in reducing the amount of froth that is generated in the separation device, it has several limitations. First, the partially aerated feed liquid has to be pumped into the unit at high pressure, around 100 kPa, to generate a sufficiently powerful vortex to separate froth from the feed liquid. This means that a pumping system and its associated plumbing is required to be installed and maintained in the plant. Second, the maximum capacity of this deaeration unit is around 80 m 3 /hr. This places an upper limit on the throughput of feed slurry that can be processed by a single deaeration unit. For example, to process 400 m 3 , which is a typical amount of feed slurry, five such deaeration units are required. 
     It has also been found that to increase the capacity of these deaeration units would require a higher flow velocity to generate the vortex, meaning that higher pressure must be generated from the pumping system, rendering such upscaling uneconomical. In addition, using a pump system in the deaeration unit conflicts with many types of separation devices, such as thickeners and clarifiers, which prefer using a gravity feed for the incoming slurry to save on costs in installing and maintaining a pumping system. 
     It is an object of the invention to overcome or ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided an apparatus for deaerating a feed liquid comprising a liquid suspension or pulp, the apparatus comprising a feed conduit to convey the feed liquid into a separator, the separator comprising a mechanical agitator for inducing a rotational flow of the feed liquid in a separation chamber such that the rotational flow generates a centrifugal vortex to separate the feed liquid into a first component consisting essentially of froth or gas and a second component consisting essentially of deaerated liquid or sludge, the separator further comprising a device for controlling the location of the vortex in the separation chamber. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. 
     Preferably, the vortex locating device controls a start point of the vortex. Preferably, the position of the vortex locating device is adjustable. 
     Preferably, the vortex locating device has a shape such that its transverse cross-section complements the cross-sectional shape of the separation chamber. Preferably, the vortex locating device is substantially circular. In one preferred form, the vortex locating device is a substantially horizontal circular disc. 
     Preferably, the mechanical agitator comprises a rotor mounted to a drive shaft and a drive mechanism for rotating the drive shaft so that the rotor induces the rotational flow in the separation chamber. 
     Preferably, the vortex locating device axially displaces a start point of the vortex from the rotor. Preferably, the vortex locating device is provided adjacent or on the drive shaft. Preferably, the vortex locating device extends substantially perpendicular to the drive shaft. Preferably, the vortex locating device has a diameter equal to or less than diameter of the rotor. 
     Preferably, the rotation of the rotor defines a shape that substantially complements the cross-sectional shape of the separation chamber. 
     Preferably, the rotor comprises a plurality of rotor blades. Preferably, the rotor blades are equidistant to each other. Preferably, the rotor blades extend substantially horizontally and vertically in the separation chamber. In one preferred form, the rotor blades define at least one V-shape or U-shape in the vertical plane. In another preferred form, the rotor blades define at least an X-shape in the horizontal plane. 
     Preferably, the separation chamber is substantially frusto-concial in shape. Alternatively, the separation chamber is substantially cylindrical in shape. In another preferred form, the separation chamber is partly cylindrical and partly conical in shape. 
     Preferably, the feed conduit is configured to permit a gravity feed of the feed liquid. 
     Preferably, the first component leaves the separator as an overflow stream. Preferably, the separation chamber comprises an upper outlet for the first component. In one preferred form, the upper outlet is located centrally about the drive shaft. Preferably, the second component leaves the separator as an underflow stream. Preferably, the separation chamber comprises a lower outlet for the second component. The overflow and underflow may be directed to separate downstream process units. More preferably, the underflow stream is directed as a feed stream into a separation device. The separation device is preferably a thickener. 
     According to a second aspect, the invention provides a method for deaerating a feed liquid comprising a liquid suspension or pulp, the method comprising the steps of conveying the feed liquid into a separation chamber, mechanically agitating the feed liquid to induce a rotational flow, such that the rotational flow generates a centrifugal vortex to separate the feed liquid into a first component consisting essentially of froth or gas and a second component consisting essentially of deaerated liquid or sludge, and controlling the location of the vortex in the separation chamber with a vortex locating device. 
     Preferably, the vortex locating step comprises controlling a start point of the vortex. Preferably, the vortex locating step comprises adjusting the position of the vortex locating device. 
     Preferably, the method further comprises the step of forming the vortex locating device such that its transverse cross-section complements the cross-sectional shape of the separation chamber. Preferably, the vortex locating device is substantially circular in shape or is a substantially horizontal circular disc. 
     Preferably, the mechanical agitating step comprises rotating a rotor about a drive shaft to induce the rotational flow of the feed liquid. 
     Preferably, the vortex locating step comprises axially displacing a start point of the vortex from the rotor. Preferably, the vortex locating step comprises locating a vortex start point adjacent or on the drive shaft. 
     Preferably, the method further comprises the step of forming the rotor such that rotation of the rotor defines a shape that substantially complements the shape of the separation chamber. 
     Preferably, the feeding step comprises feeding the feed liquid under gravity into the separation chamber. 
     Preferably, the method further comprises the steps of removing the first component as an overflow stream and removing the second component as an underflow stream. Preferably, the method further comprises the step of directing the overflow and underflow streams to separate downstream process units. 
     Preferably, the method further comprises the step of directing the underflow stream into a separation device. Preferably, the separation device is a thickener 
     In the preferred embodiments of both aspects, the invention is used for removal of froth and air from a feed slurry before it is fed into a thickener. The thickener preferably comprises a tank in which a dispersed solid component tends to settle from solution or suspension toward a lower region of the tank to be drawn off from below whilst a relatively dilute liquor is thereby displaced toward an upper region of the tank for separation via an overflow launder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of an apparatus for deaerating a feed liquid according to a first embodiment of the invention; 
         FIG. 2  is a perspective schematic view of the mechanical agitator used in the deaeration apparatus of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of an apparatus for deaerating a feed liquid according to a second embodiment of the invention; 
         FIG. 4  is a perspective schematic view of the mechanical agitator used in the deaeration apparatus of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of an apparatus for deaerating a feed liquid according to a third embodiment of the invention; 
         FIG. 6  is a cross-sectional view of an apparatus for deaerating a feed liquid according to a fourth embodiment of the invention; 
         FIG. 7  is a cross-sectional view of an apparatus for deaerating a feed liquid according to a fifth embodiment of the invention; and 
         FIG. 8  is a perspective schematic view of a mechanical agitator for use in the deaeration apparatus according to the invention. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     A preferred application of the invention is in the fields of mineral processing, separation and extraction, whereby finely ground ore is suspended as pulp in a suitable liquid medium such as water at a consistency which permits flow, and settlement in quiescent conditions. The pulp is settled from the suspension by a combination of gravity with chemical and/or mechanical processes. The pulp gradually clumps together to form aggregates of larger pulp particles as it descends from the feedwell towards the bottom of the tank. This is typically enhanced by the addition of flocculating agents, also known as flocculants, which bind the settling solid or pulp particles together. These larger and denser pulp aggregates settle more rapidly than the individual particles by virtue of their overall size and density relative to the surrounding liquid, gradually forming a compacted arrangement within a pulp bed at the bottom of the tank. 
     Referring to  FIGS. 1 and 2 , an apparatus  1  for deaerating a feed liquid comprising suspension or pulp according a first embodiment of the invention is illustrated. The apparatus  1  comprises a feed conduit in the form of an inlet  2  to convey the feed liquid  3  into a centrifugal-type separator  4 , which has a mechanical agitator  5  for inducing a rotational flow of the feed liquid in a substantially frusto-conical separation chamber  6 . The separator  4  separates the feed liquid  3  into a first component  7  consisting essentially of froth or gas and a second component  8  consisting essentially of deaerated liquid or sludge. 
     The feed inlet  2  receives the feed liquid  3  by a gravity flow from an upstream process, and may be provided with a valve assembly (not shown) to regulate the flow of the feed liquid. In other embodiments, the feed liquid is pumped through the feed inlet  2  into the apparatus  1 . It will also be appreciated by one skilled in the art that instead of a feed inlet, the feed conduit may comprise a feed line, channel (open or closed) or trough upstream of the apparatus  1 . 
     The froth component  7  leaves the separator  4  as an overflow stream through an upper outlet  9  centrally located at the top of the separation chamber  6  while the deaerated component  8  leaves the separator as an underflow stream through a lower outlet  10 . The overflow and underflow streams from the separator  4  may be directed to separate downstream process units (not shown). 
     The mechanical agitator  5  comprises a rotor  11  mounted to a drive shaft  12  and a drive mechanism  13  for rotating the drive shaft (as shown by arrow  14 ). The rotor  11  induces rotational flow of the feed liquid  3  in the separation chamber  6  to create a centrifugal vortex  15  that separates the feed liquid into the froth component  7  and the deaerated liquid or sludge component  8 . The rotor  11  comprises four rotor blades  16  equidistantly spaced to define an X-shape when viewed in the horizontal plane. 
     A device  17  for controlling the location of the vortex  15  in the separation chamber  6  is provided on the drive shaft  12  in the form of a substantially horizontal disc. The disc  17  locates the vortex  15  so that its start point  18  begins adjacent or on the upper surface  19  of the disc, as best shown in  FIG. 2 . As a consequence, the vortex  15  is axially displaced above the rotor  11 , ensuring that formation of the vortex  15  is controlled and kept confined within an upper portion of the separation chamber  6 . This prevents the vortex  15  from extending past the rotor  11  and towards the bottom  20  of the separation chamber  6 , which would contaminate the deaerated liquid or sludge component  8  by re-aerating it. The disc  17  extends substantially perpendicular to the drive shaft  12 , has a diameter approximately equal to the diameter of the rotor  11  and has a circular transverse cross-section to complement the circular cross-sectional shape of the separation chamber  6 . 
     It will be appreciated that the vortex locating device  17  need only maintain a sufficient distance between the rotor  11  and the start point  18  of the vortex  15  to ensure that the vortex does not extend past the rotor. Thus, the vortex locating device  17  can be positioned axially lower on the drive shaft  12  so that the vortex  15  is not confined in the upper section of the separation chamber  6  but extends further down and occupy more volume of the separation chamber. 
     Although adjusting the rpm (revolutions per minute) of the rotor  11  could also control the vortex, it will be appreciated that the disc  17  provides a more convenient and efficient means for controlling the location of the vortex  15 . 
     In operation, the feed inlet  2  feeds aerated slurry  3  into the separation chamber  6 , preferably tangentially. The drive mechanism  13  and the drive shaft  12  rotate the rotor  11  so as to induce a rotational flow of the slurry  3  that develops into a centrifugal vortex  15  initiating from the vortex locating disc  17 . The vortex  15  separates the feed slurry  3  into the froth component  7  and the deaerated liquid or sludge component  8 . Due to its lighter density, the froth  7  migrates upwardly in the separation chamber  6  and is removed through the upper outlet  9  as an overflow stream. The deaerated liquid or sludge  8 , due to its heavier density, migrates downwardly towards the bottom  20  of the separation chamber  6  and is removed through the lower outlet  10  as an underflow stream. 
     The centrifugal-type separator  4  is particularly efficient in separating froth from partially aerated pulps by centrifugal forces and/or “shearing” to remove the air bubbles from the solid particles. The proportion of deaeration of the feed liquid can be controlled as appropriate by varying several operating parameters of the centrifugal separator  4 , including the diameter of the separator, the separator length, the angle of the separator barrel, the size of the feed conduit, the feed density, throughput of feed liquid into the separator and the speed of rotation of the rotor. With a partially aerated feed liquid, and appropriately tuned operating parameters, a relatively small overflow stream can be produced with the apparatus  1  which contains the vast majority of the froth, leaving a proportionately large volume of deaerated underflow liquid having a density similar to that of the feed liquid. 
     A deaeration apparatus  21  according to a second embodiment of the invention is illustrated in  FIGS. 3 and 4 , where corresponding features have been given the same reference numerals. In this embodiment, the separator  22  has a separation chamber  23  with an upper cylindrical section  24  and a lower conical section  25 . The mechanical agitator  26  has a rotor  27  within the lower conical section  25  and a vortex locating disc  28  axially displaced from the rotor to lie within the upper cylindrical section  24 . 
     Referring  FIG. 4 , the configuration of the mechanical agitator  26  is shown in more detail. The rotor  27  has two rotor blades  29 , each having an angled blade section  30  and a substantially vertical blade section  31 . The rotor blades  29  define a V-shape in the vertical plane such that in use the rotation of the rotor  27  defines a shape or volume that substantially complements the shape of the lower conical section  25 . That is, when the rotor  27  rotates about the drive shaft  12 , it defines a substantially conical volume of revolution  32  to complement the shape of the lower conical section  25 . This maximises the area of the separation chamber  23  above the vortex locating disc  28  that is subjected to the vortex  15 , thus maximising separation of the feed liquid  3  into its froth and deaerated components. The shape of the lower conical section  25  also assists the creation of the vortex  15  due to its shape. The vortex locating disc  28  is positioned on the drive shaft  12  to limit the vortex  15  to substantially within the upper cylindrical section  24  and has a circular cross-sectional shape that to complement the circular cross-sectional shape of the upper cylindrical section of the separation chamber  23 . In addition, the vortex locating disc  28  has a diameter that is less than the diameter of the rotor  27 , as defused by the rotor blades  29 . 
     The second embodiment of the invention works in substantially the same manner as is described in relation to the first embodiment of  FIGS. 1 and 2 . That is, aerated feed slurry  3  is gravity or pump fed into the separation chamber  23 , preferably tangentially, via the feed conduit or inlet  2 . The drive mechanism  13  and the drive shaft  12  rotate the rotor  27  so as to induce a rotational flow of the slurry  3  within the separation chamber  23  to develop a centrifugal vortex  15  initiating from the vortex locating disc  28 . Due to the complementary shapes of the volume  32  and the lower conical section  25 , the maximum amount of slurry is subjected to the rotation of the rotor  27 , and therefore the rotational flow and vortex  15 , thus enhancing the efficiency of the deaeration process. The vortex  15  separates the feed slurry  3  into the froth component  7  and the deaerated liquid or sludge component  8 . The froth  7  migrates upwardly for removal through the upper outlet  9  as an overflow stream. The deaerated liquid or sludge  8  migrates downwardly for removal as an underflow stream through the lower side outlet  10 , positioned at the bottom of the upper cylindrical section  24  above the conical section  25 . A drain  33  removes any residual deaerated liquid or sludge  8  that is not captured by the lower side outlet  10  and is combined with the underflow stream before entering the thickener. 
     A third embodiment of the invention is illustrated in  FIG. 5 , where corresponding features have been given the same reference numerals. In this embodiment, the deaeration apparatus  40  has a separator  41  with substantially conical separation chamber  42  and a mechanical agitator  43 . A rotor  44  has two linear rotor blades  45  that define a V-shape in the vertical plane that complements the vertical cross-section of the conical separation chamber  42 . As in the second embodiment, when the rotor  44  rotates about the drive shaft  12 , it defines a substantially conical volume of revolution  46  to complement the shape of the conical separation chamber  42 , thus maximising the feed slurry that is subjected to the vortex  15  and consequently separation of the feed liquid  3  into its froth and deaerated components. The third embodiment operates in substantially the same manner as described in relation to the second embodiment of  FIGS. 3 and 4 , and thus it is not necessary to repeat the description of its operation. 
     A fourth embodiment of the invention is illustrated in  FIG. 6 , where corresponding features have been given the same reference numerals. In this embodiment, the deaeration apparatus  50  has a separator  51  with substantially cylindrical separation chamber  52 , instead of a frusto-conical chamber, and the mechanical agitator  5  of the first embodiment of the invention. In use, the rotor  11  defines, by way of its rotation, a cylindrical volume of revolution that complements the shape of the cylindrical separation chamber  52 . This maximises the area of the separation chamber  52  above the vortex locating disc  17  that is subjected to the vortex  15 , thus maximising separation of the feed liquid  3  into its froth and deaerated components. The third embodiment operates in substantially the same manner as described in relation to the first embodiment of  FIGS. 1 and 2 , and so a detailed description will not be repeated. However, due to the complementary shape of the volume defined by rotation of the rotor  11 , more slurry  3  is subjected to the rotational flow, thus improving the efficiency of the deaeration process. 
     A fifth embodiment of the invention is illustrated in  FIG. 7 , where corresponding features have been given the same reference numerals. In this embodiment, the deaeration apparatus  60  has a separator  61  with a substantially frusto-conical separation chamber  62  and a mechanical agitator  63 . A rotor  64  has four rotor blades  65 , each with a substantially horizontal blade section  66  and an angular blade section  67 . Each pair of diametrically opposing rotor blades  65  define two generally U-shapes in two vertical planes perpendicular to each other. The U-shapes complement the vertical cross-sectional shape of the bottom section  68  of the separation chamber  62 , so that the rotor  64  defines a frusto-conical volume of revolution  69  that also complements the frusto-conical shape of the bottom section  68 . Again, this increases the amount of slurry  3  that is subjected to the rotational flow and thus the deaeration process. A vortex locating device  17  is in the form of a substantially horizontal disc to complement the transverse cross-sectional shape of the separation chamber  62 , as well as having a diameter less than the diameter of the rotor  64 . The fifth embodiment operates in substantially the same manner as described in relation to the second embodiment of  FIGS. 3 and 4 , and thus it is not necessary to repeat the description of its operation. 
     Referring to  FIG. 8 , a mechanical agitator  70  is illustrated for use with the embodiments of the invention, where corresponding features have been given the same reference numerals. In this embodiment of the mechanical agitator  70 , the vortex locating device  71  is arranged at the base of the drive shaft  12  adjacent the rotor  11 . This results in the maximum possible area of the separation chamber being used to generate the vortex  15  and thus minimise any “dead” areas of slurry  3  that may be within the separation chamber. This configuration of the mechanical agitator is applicable to the apparatuses previously described in relation to  FIGS. 1 ,  3 ,  5 ,  6  and  7 , but is particularly useful for the deaeration apparatuses of  FIGS. 1 and 6 . 
     In the preferred embodiments of the invention, the underflow stream from the lower outlet  10  feeds the deaerated liquid or sludge from the centrifugal separator  4 ,  22 ,  41 ,  51  and  61  to a thickener (not shown). This obviates the problem of accumulation of excess froth in the thickener and the associated feedwell, which in prior art devices significantly reduces the efficiency of the thickening process. The overflow stream from the upper outlet  9  is fed to a launder (not shown), where is can be broken down with fine water spray jets (not shown). This produces a third component consisting essentially of liquid from the spray jets mixed with the liquid from the collapsed froth, which may be combined with the underflow liquid downstream of the centrifugal separator and thence fed to the thickener, or else recycled to the feed liquid upstream of the centrifugal separator. 
     Whilst a single separator is illustrated in the preferred embodiments, it will be appreciated that a plurality of separators connected in series, parallel or a combination of both, may also be used depending upon the throughput, the degree of separation required, and other variables. However, it is preferred that the separator is upscaled in capacity to meet the required throughput of feed liquid that needs to be processed. 
     Of course, the centrifugal separator arrangement need not necessarily be applied only to thickeners, since the principle of deaeration performed by the centrifugal separators may be used in numerous other applications. There is also no specific requirement to recombine the overflow from the centrifugal separator with the underflow or with the feed material. The separated streams may simply be directed to discrete downstream process units as required. 
     In other embodiments, the position of the vortex locating device is adjustable upwardly or downwardly on the drive shaft. This additionally provides more control of the location of the vortex within the separation chamber and allows the amount of deaeration to be controlled within the apparatus, in conjunction with other operational parameters. Other embodiments use vortex locating devices of differing shapes, such as square, rectangular, triangular or other polygonal shapes. While the preferred embodiments of the invention have been described using vortex locating devices having a diameter equal to or less than the diameter of the rotor, vortex locating devices having diameters greater than the rotor diameter can also be used. 
     One skilled in the art will appreciate that the rotor configuration can be varied according to the shape of the separation chamber and is not limited to the configurations illustrated in the described embodiments. 
     It will also be appreciated by one skilled in the art that the invention provides a useful apparatus for mechanically deaerating liquids, especially liquid suspensions or pulps, thus reducing or substantially eliminating the harmful effects of froth in the subsequent separation processes conducted downstream of the deaeration apparatus. 
     Moreover, the illustrated deaeration apparatuses according to embodiments of the invention avoid the operational restrictions and additional expense involved with the installation and maintenance of the cyclonic-type centrifugal separators in the prior art. Consequently, the invention permits the deaeration apparatus to be scaled up to increase its capacity without requiring significant power to generate the centrifugal force required in cyclonic separators. For example, where five cyclonic centrifugal separator units would have been required to process 400 m 3  of feed slurry, only a single deaeration apparatus according to the invention needs to be installed. In addition, the invention permits a simple means of feeding of the feed liquid or slurry by gravity, rendering it compatible with the majority of separation devices and facilitating retrofitting to existing plants and avoiding the use of pumps. Consequently, maintenance and installation costs for the deaeration apparatus are significantly less than the associated installation and maintenance cost for a comparable cyclonic separator unit. The increased capacity of the deaeration apparatus of the invention, coupled with its lower installation and maintenance costs, results in improved production efficiency in separation devices employing such apparatuses. In all these respects, the invention represents a practical and commercially significant improvement over the prior art. 
     Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.