Abstract:
A magnetic separator for a mineral feed comprising, a rotatable drum having an attraction surface, an inlet for receiving a stream of the mineral feed and directing it past the rotating drum, means for generating a magnetic field to attract magnetic components in the feed to the attraction surface, a first zone for take off of non-magnetic components of the mineral feed, and a second zone for take off of the magnetic components, wherein the magnetic field generating means are adapted to subject the magnetic components to at least one reversal of polarity of the magnetic field as the drum rotates.

Description:
FIELD OF THE INVENTION 
   This invention relates to a method for separating magnetic material from a feed. It also relates to a magnetic separator construction for performing the method of the invention. 
   BACKGROUND OF THE INVENTION 
   In industries such as the mining industry where it is necessary to employ crushers to crush rocks, it is common place for pieces of magnetic material forming part of the crushing equipment to break off and be mixed with the crushed rock. This magnetic material, if it is left mixed with the crushed rock feed, can cause substantial harm in any downstream processing of the crushed rock feed. 
   In many instances, magnetic separators are therefore employed immediately downstream of crushers to remove any magnetic material whether it has been broken off from the crushers during the crushing operation or incorporated from any other source. 
   In the gold mining industry in particular, rock is crushed whilst being mixed with water to form a slurry. This slurry may then be introduced into a magnetic separator and allowed to pool in contact with an attraction surface such as a drum. A magnet sitting behind the opposite surface of the drum attracts magnetic particles from the slurry. As the drum rotates beyond the magnet, these magnetic particles fall off into a collection area. 
   Whilst magnetic separation of slurries in this fashion has proven to be reasonably effective, particularly where the particles comprising the slurry are relatively fine, there are a number of problems associated with this technique. 
   For instance, where are a substantial number of coarse particles in the slurry, eg. particles in a range 5 mm to 50 mm, it has been found that this technique is not effective because the step of pooling the slurry causes the coarse particles and slurry to clump and hence interfere with flow of slurry past the rotating drum. 
   Even in situations where the particles are relatively fine, there may be difficulties associated with clumping of magnetic material on the attraction surface when it passes the edge of the magnet with the result that the magnetic material does not separate cleanly from the drum as it rotates. 
   A further concern relates to the fact that particles of magnetic material attached to the drum by virtue of their strong attachment can entrain a proportion of non-magnetic material from the slurry. As a result, this non-magnetic material may be harvested together with the magnetic material rather than in a separate stream free of magnetic material. Thus, valuable components of the slurry, eg. gold may be lost. 
   It is an object of the invention to provide a method and apparatus which addresses one or more of the aforesaid disadvantages. 
   DISCLOSURE OF THE INVENTION 
   The invention provides in one aspect a method of removing magnetic material from a feed comprising subjecting the feed to a magnetic field to attract the magnetic components of the feed to an attraction surface and, subjecting the magnetic components attracted to the attraction surface to at least one reversal of polarity of the magnetic field before separating a stream of the magnetic components from the attraction surface. 
   Suitably, the magnetic components are subjected to a plurality of reversals of polarity of the magnetic field. 
   The stream of magnetic components may be separated from the attraction surface by gradually decreasing the strength of the magnetic field used to attract the magnetic components to the attraction surface. 
   In a particular aspect of the invention, the attraction surface may comprise a rotating drum. The feed may comprise a slurry. The slurry may comprise a water based slurry. 
   In another aspect the invention provides a magnetic separator for a mineral feed comprising,
         a rotatable drum having an attraction surface,   an inlet for receiving a stream of the mineral feed and directing it past the rotating drum,   means for generating a magnetic field to attract magnetic components in the feed to the attraction surface,   a first zone for take off of non-magnetic components of the mineral feed, and   a second zone for take off of the magnetic components,   wherein the magnetic field generating means are adapted to subject the magnetic components to at least one reversal of polarity of the magnetic field as the drum rotates.       

   The inlet may direct the mineral feed past the rotating drum by flowing it over the rotating drum. 
   The magnetic field may be generated by one or more magnets arranged behind the attraction surface. The magnet may comprise a permanent magnet or an electromagnet. It may comprise a combination of these two types of magnets. 
   The magnet may comprise a plurality of individual magnet segments. The magnet segments may be arranged in an arc generally following the interior surface of the rotating drum. One or more of the segments may have a polarity reversal compared with an adjacent segment. The magnetic strength of the segments may be varied. For example, the strength of the segments at the beginning of the arc where the feed first contacts the rotating drum and hence the attraction surface may comprise segments of higher magnetic strength. Towards the end of the arc, the segments may decrease in magnetic strength. Furthermore, towards the end of the arc, the segments may increase in separation from the attraction surface of the drum so as to decrease the strength of the magnetic field for attracting magnetic components to the drum and allowing the magnetic components to be separated more readily. 
   The magnetic separator may include sizing means. The sizing means may be arranged to separate the non-magnetic components into a fine stream and a coarse stream. Thus, the first outlet of the separator may be split into two outlets, namely a fines outlet and a coarse outlet. 
   The sizing screen may be mounted on a cover. The cover may be tiltable with respect to a housing forming part of the separator so as to allow access to the sizing screen. The sizing screen may be mounted in such a fashion as to allow its direction to be reversed. 
   Preferred aspects of the invention will now be described with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a side on x-ray elevation of a magnetic separator constructed in accordance with the invention; 
       FIG. 2  shows an end on elevation of the separator of  FIG. 1 ; 
       FIG. 3  shows a side on elevation of the separator of  FIG. 1  with cover opened; 
       FIG. 4  shows an enlarged view of the rotating drum within the separator of  FIG. 1 ; 
       FIG. 5  shows an exploded view of the rotating drum used in the separator of  FIG. 1 ; 
       FIG. 6  shows a partial plan view of a mesh screen used in the separator of  FIG. 1 ; 
       FIG. 7  shows an elevational view of the screen of  FIG. 6 ; and 
       FIG. 8  shows an enlarged sectional view of the circled part of the mesh screen of  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The various elements identified by numerals in the drawings are listed in the following integer list. 
   INTEGER LIST 
   
       
       
         
             1  Magnetic separator 
             3  Housing 
             4  Cover 
             5  Inlet 
             7  Drum assembly 
             9  Motor 
             11  Gear box 
             13  Drive shaft 
             15  Launder 
             16  Launder 
             17  Launder 
             20  Outlet 
             21  Outlet 
             22  Outlet 
             24  Support stand 
             26  Flow plate 
             27  Feed cover 
             28  Drum 
             30  Flange 
             31  Cover 
             32  Bearing 
             33  Magnet assembly 
             34  Outer plate 
             35  Primary magnet element 
             36  Secondary magnet element 
             37  Tertiary magnet element 
             38  Opening 
             39  Mounting plate 
             40  Stub shaft 
             41  Hub 
             42  Secondary stub shaft 
             45  Mounting block 
             46  Cross member 
             47  Frame member 
             50  Shaft 
             51  Bearing 
             52  Spring 
             53  Pneumatic cylinder 
             55  Sieve assembly 
             57  Side wall 
             58  Screen 
             59  Frame 
             60  Mesh elements 
             61  Support members 
             62  Gap 
             63  Front face 
             64  Rear face 
         
       
     
  
   Referring to  FIGS. 1 to 4 , there is shown a magnetic separator generally designated  1  having a housing  3  provided with a tiltable cover  4 . The magnetic separator is mounted on a support stand  24 . 
   The top of the housing is provided with an inlet for a feed such as a slurry of crushed rock with water. 
   The inlet is located above a drum assembly  7 , the inlet being arranged so that slurry being fed to the magnetic separator via the inlet falls upon the flow plate  26  and then flows down over the drum  28  forming part of the drum assembly. A feed cover  27  is provided at the upper part of the magnetic separator. 
   A motor  9  acting through gear box  11  and drive shaft  13  is arranged to rotate the drum assembly. 
   Three launders  15 ,  16  and  17  are provided in series beneath the drum to receive various streams coming off the drum, namely a non-magnetic fines stream for the launder  15 , a non-magnetic coarse stream for the launder  16  and a magnetic stream for launder  17 . The launders have the outlets  20 ,  21  and  22  respectively. 
   The drum assembly shown in detail in  FIG. 5 , comprises a stainless steel drum  28  which provides an attraction surface to which magnetic particles in the feed are attracted. 
   Opposite sides of the drum are provided with a circumferential flange  30 . A cover  31  is provided on each of the opposite ends of the drum. Each cover  31  is connected to the respective flange and bearings  32  fit over the openings  38  in the covers  31 . 
   As can be seen in  FIG. 5 , the bearing  32  on the right hand side of the drawing fits over the stub shaft  40  which is secured to the magnet assembly  33 . In turn, the stub shaft  40  is fixed to a frame member  47  of the magnetic separator. The connection between the two is such as to secure the magnet assembly  33  against rotation. 
   Looking at the opposite side of the drawing in  FIG. 5 , it can be seen that the drive shaft  13  by connection with the outer plate  34  and cover  31  rotationally drives the drum. 
   A short secondary stub shaft  42  which is not visible in  FIG. 5  but it is located in line with the bearing  32  on the left side of the drawing of  FIG. 5  projects into the bearing  32  and provides support for the magnet assembly  33  in association with the support provided by the stub shaft  40  on the opposite side of the magnet assembly. The drive shaft  13  is also mounted via the mounting block  45 . The mounting block  45  is mounted via a cross member  46  extending between frame members  47 . 
   The magnet assembly  33  comprises a number of magnet elements which are mounted on the mounting plates  49 . 
   The magnet elements comprise a number of primary magnet elements  35  which are of maximum strength followed by a number of secondary magnet elements  36  of the same or lesser strength than primary magnet elements  35 . In turn, even weaker tertiary magnet elements  37  are arranged beyond the primary and secondary magnet elements. 
   It can be seen that the magnet elements define an arc which follows the interior outline of the drum  28  with the exception that the final few tertiary magnet elements gradually extend away from the inner drum surface as shown more clearly in  FIG. 4 . 
   A number of the primary and/or secondary magnet elements have their magnetic field directions reversed with respect to their more adjacent elements for purposes to become apparent. 
   The cover  4 , is mounted via the shaft  50  and bearings  51  on the frame members  47 . Springs  52  are arranged to urge the cover to the closed position shown in  FIG. 1 . A pneumatic cylinder  53  is arranged to pivot the cover  4  to the open position shown in  FIG. 3 . The cover includes a sieve assembly generally designated  55 . This comprises side walls  57  on opposite sides of a screen  58 . 
   The screen  58  shown in more detail in  FIGS. 6 to 8 , comprises a number of mesh elements  60  running parallel to each other and supported by a number of support members  61 . The mesh elements and support members are in turn supported at the ends by the frame  59 . The mesh elements are separated by a gap  62  which defines the desired sizing required for the feed. It should be noted that the front face  63  of the mesh elements is broader than the rear face  64  in order to reduce the likelihood of particles “hanging up” within the gap between the mesh elements. 
   The construction of the cover and associated sieve assembly is such that the direction of the screen  58  can be reversed by opening the cover, lifting out the screen and simply turning it around. This has the advantage that wear and tear on the leading edges of the mesh elements can be shared between both sides of the mesh elements rather than a single side as would be the case with a fixed mesh screen. Thus the longevity of the screen is substantially enhanced. 
   In normal operation of the magnetic separator, a water-based slurry of feed containing entrained magnetic components enters through inlet  5  and is allowed to flow across the flow plate  26  onto the outer surface of the stainless steel drum  28  in proximity to the first of the primary magnet elements. 
   The strong magnetic field of the primary magnet elements attracts magnetic materials to the outer surface of the rotating drum and so the attracted magnetic materials rotate with the drum in an anticlockwise direction. 
   As the drum continues to rotate, the magnetic particles attracted to the outer surface of the drum are subjected to a reversal in magnetic polarity as they pass by various of the magnet elements having polarity reversals. This has the effect of agitating the attracted magnetic components sitting on the drum surface. As a result, entrained non-magnetic material is shaken free of the magnetic components and falls off the drum. As the magnetic materials continue to rotate with the drum towards the secondary and tertiary magnet elements, the degree of magnetic attraction is gradually decreased with the result that the magnetic materials fall off the drum quite readily as the surface of the drum passes beyond the last of the magnetic elements. The fact that the magnetic attraction is gradually decreased by reducing the strength of the magnet elements and also by increasing their distance from the drum surface means that the magnetic components do not tend to clump or ball up and hence fall off the drum surface in a more controlled manner. When they fall off the drum surface, they are collected by the launder  17  arranged so as to collect the magnetic materials and direct them through the outlet  22 . 
   The non-magnetic materials fall onto the sieve assembly  55  and are screened so that the fines are directed into launder  15  and coarse non-magnetic elements are directed into launder  16 . The launders  15  and  16  are associated with outlets  20  and  21  respectively. 
   Whilst the above description includes the preferred embodiments of the invention, it is to be understood that many variations, alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the essential features or the spirit or ambit of the invention. 
   It will be also understood that where the word “comprise”, and variations such as “comprises” and “comprising”, are used in this specification, unless the context requires otherwise such use is intended to imply the inclusion of a stated feature or features but is not to be taken as excluding the presence of other feature or features. 
   The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge in Australia.