Abstract:
A centrifugal separator of the type having migration, retention and lip zones of the interior surface of the rotating bowl has a variable diameter lip to increase the amount of target material retained in the retention zone. The lip zone&#39;s variable diameter is provided by a flexible cylindrical member having an inner diameter which is selectively and uniformly decreased or increased by variation of the pressure applied to the member. By progressively decreasing the inner diameter of the lip zone the volume of retained material is increased over the cycle of the separator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. provisional patent application No. 60/672,024 filed Apr. 18, 2005, which is pending. 
    
    
     TECHNICAL FIELD 
     The present invention relates to centrifugal concentrators of the rotating bowl type for the separation of solids of higher density such as gold, iron or tin from a slurry containing solids of a lower density and liquid and more particularly to such centrifugal concentrators of the batch type. 
     BACKGROUND 
     The problem of separating particles of high density such as gold, iron or tin from tailings and other slurry streams has attracted a great many attempted solutions. The problem is that of separating small particles of higher density from a slurry containing water and particles of lower density. One approach has been to use the centrifugal force created in a rotating bowl to separate the high density particles from the lower density slurry. In the past this had been generally done by placing obstructions such as ribs in the path of the rotating slurry to trap the heavier particles. A problem with this method was that where the slurry contained fine, dense gangue particles, the grooves or depressions designed to retain the concentrate would rapidly pack with the gangue particles. The problem of packing has been partially solved by the present inventor&#39;s batch centrifugal concentrator which is the subject of U.S. Pat. No. 4,824,431. In that centrifugal concentrator, there are no obstacles to the flow of the slurry in the rotating drum. The slurry is delivered to the vicinity of the bottom of the rotating drum and travels up the smooth interior surface of the drum. The interior surface has three continuous zones: a migration zone, a retention zone above the migration zone, and an inwardly-inclined lip zone above the retention zone. The respective lengths and inclinations of the zones are selected to produce flow conditions in which less dense particles are expelled from the drum while denser particles migrate to and are retained in the retention zone. 
     The result is that an enriched layer of concentrate accumulates in the retention zone without the use of ridges or grooves which may become packed. 
     In operation of the present inventor&#39;s above-described centrifugal concentrator, which is a batch device, and uses a fixed lip to define the retention zone, on start-up, primarily gangue particles immediately accumulate against the wall of the retention zone. The target particles then gradually accumulate in a shallow layer inward of the first layer of relatively barren gangue particles at a depth where centrifugal force holds the denser target particles against the underlying layer, but the lighter gangue particle are swept away. If the concentration cycle is started with a very shallow lip, however, there is significantly less opportunity for non-target, or gangue material to deposit in the retention zone. In that case however, the retention zone has limited volume and quickly overflows so it is necessary to stop the machine to empty it at that point. 
     SUMMARY OF INVENTION 
     It has been discovered that the performance of centrifugal concentrators in capturing fine target materials can be improved by providing a bowl design in which the diameter of the lip zone can be varied over time. In this way the geometry of the retention zone can be varied to reduce the initial accumulation of barren materials and later increase the volume available for additional accumulation of target materials. In this way a greater quantity of higher grade material can be obtained from the operating cycle. 
     The invention provides a concentrator for separating particulate material of higher specific gravity from particulate material of lower specific gravity comprising: (a) a hollow drum having an open end and interior surface; (b) means for rotatably supporting the drum on an axis; (c) drive means for rotating the drum about the axis; and (d) material supply means to deliver the particulate material into the end of the drum spaced from the open end; wherein the interior surface of the drum comprises a migration zone, a retention zone above the migration zone and an inwardly inclined lip zone above the retention zone, and where the respective lengths of the migration, retention and lip zones and the relative degrees of inclination of the migration and lip zones are selected to provide a sufficient component of force on the particulate material to expel the lighter particulate material from the drum and to permit the heavier particulate material to be retained in the retention zone, and wherein the lip zone has a variable diameter provided by a flexible cylindrical member having an inner diameter which is selectively and uniformly decreased or increased by variation of the pressure applied to the member. Preferably the rate of decrease of the diameter is controlled through control of a supply of pressurized fluid such as compressed air. 
     The invention further provides a method of operating a centrifugal concentrator to maximize the recovery of target particles by varying the diameter of the lip zone over time and in particular by controlling the rate of decrease of the diameter of the lip zone over the centrifuging cycle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In drawings which illustrate a preferred embodiment of the invention: 
         FIG. 1  is a side elevation view of the centrifuge of the invention; 
         FIG. 2  is a vertical cross-section of the invention taken along lines  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a vertical cross-section of the rotor of the invention; 
         FIG. 4  is a detail cross-sectional view of the variable lip assembly as shown in  FIG. 3 ; 
         FIG. 5  is a top view of the rotor bowl; 
         FIG. 6  is a vertical cross-section of a second embodiment of the invention taken along lines  2 - 2  in  FIG. 1 ; and 
         FIG. 7  is a detail of the cross-sectional view of the variable lip assembly area B as shown in  FIG. 6 . 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     With reference to  FIGS. 1 and 2 , the centrifuge of the invention is designated by reference numeral  10 . It has a support frame  12 , a shroud  14  consisting of a cylindrical wall  16 , a shroud lid  18 , tailings launder  20 , access door  21  and tailings discharge port  22 . Mounted in frame  12  is drive motor  24 . The shroud lid  18  has openings for a slurry feed pipe  26  and inspection ports  28 . An outer lower flange  30  of shroud lid  18  is bolted to a flange on cylindrical wall  16 . 
     Rotor  32  is mounted in the frame  12  by bearing assemblies  34  in cartridge  33 . The rotor has a sheave  36  which is driven by a belt (not shown) driven by drive motor  24  and protected by belt guard  38 . Rotor  32  has a rotor bowl  40  of the same general type disclosed in this inventor&#39;s U.S. Pat. No. 4,824,431 in that, rather than relying on obstructions to the slurry flow in the surface of the rotor bowl  40 , the inner surface of rotor bowl  40  forms three zones: a migration zone, a retention zone and a lip zone, which cause the denser, target particles from the slurry flow to be concentrated in the retention zone in the manner described in U.S. Pat. No. 4,824,431, which is incorporated herein by reference. A rotor baffle  38  ( FIG. 3 ) is provided on the center of the floor of rotor bowl  40  which has three or four upstanding vanes to assist in the rotation of the slurry. Also mounted in frame  12  around the rotor  32  is concentrate launder  42  with a concentrate discharge port  44 . Water is introduced into concentrate launder  42  through pipe  46 . Discharge ports  48  in rotor bowl  40  carry concentrate into concentrate launder  42 , with assistance of water introduced through pipe  50  and spray nozzles  52 . 
     Rotor bowl  40  is formed of a steel lower bowl section  54  lined with rubber  55  and variable diameter lip assembly  56  shown in more detail in  FIG. 4 . Lip assembly  56  comprises steel upper and lower air manifolds  60  and  62  bolted together by bolt  61  and which form cavity  64  and air gap  90 . Lower air manifold  62  is in turn bolted to lower bowl section  54  by bolts  63  through flange  65 . Lip ring  78  is bolted to upper air manifold  60 . Contained within cavity  64  is cylindrical muscle member  75  made preferably of 60 Durometer rubber, preferably about 1.25 inches thick. Cylindrical protective sheath  72 , also preferably formed of 60 Durometer rubber extends around the upper interior surface of bowl  40  and has flange  74  secured in place between lip ring  78  and upper air manifold  60  and has flange  76  secured in place between flange  65  and lower air manifold  62 . Bosses  77  and  79  on ring  78  and lower air manifold  62  assist in securing the flanges  74 ,  76 . Cylindrical protective sheath  72  has an outer cylindrical rib  87  which extends into gap  90 . 
     Compressed air is supplied to the lip assembly in essentially the same way as compressed air is provided to the flow control valves in the present inventor&#39;s U.S. Pat. No. 5,462,513 which is incorporated herein by reference. A compressed air storage tank (not shown) provides compressed air through a stationary supply line  67  to rotating union  66 . Rotating union  66  provides the compressed air from stationary supply line  67  to the hollow center  43  of rotor shaft  35  which in turn communicates with rotating supply lines  68  via channels  58  without loss of pressure. Rotating supply lines  68  provide compressed air to channels  82  which communicate with circumferential channel  83 . Compressed air relief channels  84 ,  86  communicate with the outside atmosphere to release air pressure built up in chambers  85 ,  89 . 
     An electrical control  88 , preferably a programmable logic controller, is provided which controls the supply of compressed air to the lip assembly. It thereby controls the rate at which the diameter of the lip is reduced. 
     In operation, initially no air pressure is applied to channel  83  and the lip assembly is configured as shown in  FIG. 4 . Motor  24  is activated to rotate the rotor  32 . The slurry feed is introduced to the spinning rotor  32  through feed pipe  26 . Centrifugal forces cause the slurry to climb up the inner surface of the rotor bowl  40  before being expelled into tailings launder  20  and thence out of the machine through discharge port  22 . Concentrate collects along the wall surface of the retention zone as indicated at “C” in  FIG. 2 . As the process advances, a layer of heavier concentrate builds up on the surface of the bowl  40 . After an appropriate delay, which may be very short, when the retention zone has been filled with target concentrate, electrical control  88  causes an appropriate level of air pressure to be introduced to channel  83 . This causes muscle member  75  to extend into gap  90  and press against outer rib  87  of protective sheath  72 , causing the inner surface of protective sheath  72  to extend inwardly in a uniform way circumferentially, thereby reducing the upper diameter of bowl  40  and forming a dam or weir. The reduced lip diameter causes an increased capacity in the retention zone and allows further layers of target concentrate to build up. By further increasing the compressed air supply to channel  83 , the build up of target concentrate in the retention zone can be maximized. The reduction in diameter of the lip can proceed in a step-by-step progression with discrete reductions, or continuously. Once the maximum amount of retained concentrate is reached, generally after a few minutes, the rotation of bowl  40  is significantly reduced, and water is sprayed through nozzles  52  to wash the concentrate into launder  42  and out discharge pipe  44 . The maximum amount of variability of the diameter at protective sheath  72  will vary depending on the size of the bowl. Typically for a bowl  40  which has a diameter at its mouth of 1 meter it will be approximately 20 to 30 mm. but could be significantly larger or smaller for particular applications. 
     Compressed air may leak around muscle member  75  into gap  90  and into the spaces  85 ,  87  between protective sheath  72  and upper and lower air manifolds  60  and  62 , thereby causing undesirable discontinuity in the surface of sheath  72 . To relieve this pressure, pressure relief channels  84 ,  86  are provided and vented to the atmosphere. While compressed air has been disclosed, other compressible or non-compressible gases or fluids such as hydraulic fluid may be used as the source of pressure. 
     The programmed rate and degree of reduction of the lip diameter will vary depending on the nature of the slurry and the size of rotor bowl  40 . In some cases it may be uniform. In other cases it may increase rapidly at first followed by a decrease in the rate of reduction. Or it may decrease slowly at first and then more rapidly. 
     While rubber has been disclosed as the preferred material for sheath  72 , other flexible materials, synthetic or natural, such as polyurethane will be suitable provided they are sufficiently durable to withstand the harsh environment, and converge uniformly under the application of pressure. 
     A second embodiment of the invention is illustrated in  FIGS. 6 and 7 . In this case the muscle member  87  is dispensed with in order to provide a greater radial displacement of the flexible variable lip  100 , illustrated in dotted outline at maximum radial extension at  102  from the rotor wall  104 . Again the variable lip has flanges  106 ,  108  which are secured to the rotor  110  by bolts  112  into upper clamp ring  114  and lower clamp ring  116 . Annular clamp plate  118  covers the heads of bolts  112 . In this embodiment compressed air is delivered through supply line  68  through delivery aperture  120  directly into cavity  122  behind the flexible variable lip  100 . Otherwise the mode of operation is as indicated above. 
     As will be apparent to those skilled in the art, various modifications and adaptations of the structure above described may be made without departing from the spirit of the invention. While the preferred embodiment has been described in the context of the separation of higher density particles from a slurry, it will be apparent to those skilled in the art that the invention has similar application in the separation of any two flowable substances of differing density, whether solid particles from solid particles, liquid from liquid or solid particles from liquid.