Abstract:
This invention relates to a method of electrowinning metal from an eluate containing the metal in solution and includes the steps of feeding the eluate into a closed tank which includes a suitably electrified anode and cathode, electrolytically depositing metal from the eluate onto the cathode and at least periodically causing the deposited metal to be dislodged from the cathode for removal from the tank. Preferably the metal is dislodged from the cathode by movement of the cathode in the eluate in the tank. The invention further extends to an electrowinning cell for carrying out the method of the invention with the anode being in the form of a metal cylinder in which the cathode is rotatably located.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of electrowinning metals such as gold, silver and the like from a metal-rich electrolyte and to an electrowinning cell for use in carrying out the method. 
     BACKGROUND TO THE INVENTION 
     Electrowinning cells for the recovery of gold are well known and consist fundamentally of a tank in which a static sandwich arrangement of alternate electrically connected anode and cathode assemblies are located in a flow path between the electrolyte inlet to the tank and an outlet weir from the tank. 
     In use, in one form of electrowinning, a gold rich eluate is fed into the tank through its inlet to pass between the electrified anodes and cathodes in the tank and from the tank through its outlet. In doing so a gold rich sludge is electrolytically built up on the low adhesion material of the cathodes and in time gravity separates from the cathodes to settle in a sump or gold trap in the base of the tank from where it is periodically removed. 
     The cathodes in the cells generally consist of steel wool which is trapped between perforated sheets which are made from a suitable plastics material and the anodes each consist of a composite unit which is made up of stainless steel strips. 
     For security reasons, the cell tank is closed by a lockable lid. 
     Although electrowinning cells of the above type are reasonably efficient their gold recovery rate is slow and a large percentage of gold remains trapped in the cathode wool which necessitates regular removal of the cathodes from the tank for maximum gold recovery by means of a calcine or acid treatment process. The removal of the gold laden cathodes from the tank is labour intensive and poses severe gold theft security problems. 
     SUMMARY OF THE INVENTION 
     A method of electrowinning metal such as gold, silver and the like from an eluate containing the metal in solution according to the invention includes the steps of feeding the eluate into a closed tank which includes a suitably spaced and electrified anode and cathode, electrolytically depositing the metal from the eluate onto the cathode and at least periodically causing the deposited metal to be dislodged from the cathode for removal from the tank. 
     The deposited metal may be dislodged from the cathode by movement of the cathode in the tank. 
     The cathode may be moved in the tank in any suitable manner, for example, by vibration, rapping, oscillation, rotation and the like or by any combination of these movements. 
     The method may further include the step of depositing metal which is dislodged from the cathode in a metal trap which is located on or is in communication with the base of the tank and from which the metal is periodically removed through a security protected outlet. 
     The method may include the step of feeding eluate which has been exposed to the cathode from the tank through a settling tank, gravity separating particulate metal from the eluate in the settling tank and periodically removing the separated metal from the settling tank. 
     After a predetermined operating period the barren eluate is drained from the cell system and the cathode is spray cleaned by means of liquid spray nozzles in the tank to remove gold particles which are trapped on or in the cathode. 
     An electrowinning cell for electrowinning metal such as gold, silver and the like from a metal rich eluate according to the invention includes a tank having an eluate inlet and outlet, a suitably insulated anode and a suitable electrowinning cathode in the tank in the eluate flow path through the tank between its inlet and outlet and means for dislodging metal which has been electrolytically deposited on the cathode from the cathode during use of the cell. The metal dislodging means may be a device or arrangement for moving the cathode in the tank. In addition to the cathode movement generating device or arrangement, the metal dislodging means could include a liquid spray jet arrangement. 
     The electrowinning cell could be any conventional cell which includes the cathode moving and the liquid spray arrangement of the invention for dislodging the metal from the cathode. In a preferred form of the invention, however, the electrowinning cell tank is for practical purposes permanently closed and the cell includes an axially rotatable shaft which is located in and projects from the tank with the cathode attached to the shaft for rotation with the shaft in the tank, means for rotating the shaft, an anode in the tank on the outside of a path circumscribed by the cathode during its rotation in the tank and suitable electrical connections for connecting the cathode and the anode to a suitable electrical supply. 
     The shaft rotating means may be adapted periodically to reverse the direction of rotation of the shaft and so the cathode in the tank. Preferably, the tank is a cylindrical vessel, and in one form of the invention the anode is a tubular sleeve which is circular in cross-section and which is concentrically located in the tank, the shaft is coaxially located in the tank with one end portion passing through an end wall of the tank and the cathode is a cathode panel which is attached to the shaft for rotation in the tank at least partially within the anode. 
     The cathode may, and preferably does, consist of a plurality of cathode panels which project radially from the shaft. 
     In another form of the invention the anode consists of two sheet metal cylinders which are concentrically spaced from and attached to each other and the cathode is cylindrical in shape and rotatably located between and spaced from the anode cylinders. 
     The cathode panels and the cylindrical cathode conveniently include a mat of stainless steel wire wool or knitted mesh which is sandwiched between suitably rigid open mesh material. The anode cylinders, in both forms of the cell, may be made from a suitable stainless steel. 
     Further according to the invention the cell includes suitably positioned liquid spray nozzles in the tank which are directed to spray jets of liquid onto the or each cathode panel as it is rotated in the tank and liquid supply lines to the nozzles. 
     The shaft rotating means is conveniently a motor which is attached to the tank end from which the steel shaft projects. The motor may be connected to the shaft through a suitable gearbox. 
     Further according to the invention the eluate inlet to the tank is above the anode and cathode in the tank and the tank outlet is located in its base with the cell including a metal trap which is in communication with the tank outlet below the tank base. The cell conveniently includes a valve for shutting the tank outlet from the metal trap and a security protected metal outlet valve from the trap from which the metal sludge is periodically withdrawn. 
     The cell of the invention may include a settling tank, on the outside of the cell tank, into which eluate which has passed through the cell tank may be fed from bottom to top to a line for recirculating the eluate through the cell tank feed system while particulate metal in the eluate settles to the bottom of the settling tank in use. The settling tank preferably includes a gold trap as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is now described by way of example only with reference to the drawings in which: 
     FIG. 1 is a partially diagrammatic sectioned side elevation of one embodiment of the electrowinning cell of the invention, 
     FIG. 2 is a plan view of the FIG. 1 cell shown sectioned on the line  2 — 2  in FIG. 1, 
     FIG. 3 is a fragmentary exploded perspective view illustrating the manner of construction of the cathode panels of the electrowinning cell of FIGS. 1 and 2 of the invention, 
     FIG. 4 is a view similar to FIG. 1 of a second embodiment of the electrowinning cell of the invention, and 
     FIG. 5 is a plan view of the FIG. 4 cell shown sectioned on the line  5 — 5  in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The electrowinning cell of the invention is shown in FIGS. 1 and 2 of the drawings to include a tank  10 , an anode  12 , a drive shaft  14 , a cathode  16 , a drive arrangement  18  for the shaft  14 , water jet manifolds  20 , a settling tank  22  and gold traps  24 . 
     The tank  10 , in this embodiment of the invention, is in the form of a cylindrical vessel which is closed top and bottom. The upper end of the tank is releasibly bolted to the side wall of the tank to be, for all practical purposes except maintenance, a permanent closure to the tank. The tank includes an eluate inlet  26  and an outlet  28  which is located in the downwardly curved base of the tank. 
     The anode  12  is in the form of an open ended cylinder which is made from a suitable stainless steel and which is concentrically located in the tank  10  by suitable insulators  30 . 
     The drive shaft  14  is axially located in the tank  10  in bearings in the upper end cap of the tank with its free end on the outside of the tank being connected to the drive arrangement  18 . The drive arrangement  18  consists of a suitably sized electric motor  32  and a gearbox  34  to which the shaft  14  is attached. Preferably the drive arrangement  18  is adapted, in any known manner, periodically to reverse the direction of rotation of the shaft  14  and so the cathode  16 . 
     The cathode  16 , in this embodiment, consists of four cathode panels  36  which each include, as shown in FIG. 3, a central mat  38  which is made from a suitable stainless steel wool or knitted mesh, two mesh elements  40  which could be made from stainless steel wire, a suitable plastics material or the like and between which the mat  38  is sandwiched. In this embodiment of the invention, the cathode consists of four electrically connected panels  36  which are attached in the radially projecting arrangement shown in FIG. 2, to the shaft  14 . The shaft  14  is suitably insulated from a lower bearing holder in the drawing by a bearing which is made from electrically insulating material and from the gearbox drive shaft by a coupling which is electrically insulating. 
     The water jet manifolds  20  pass through the end closure of the tank  10  and are located between the inner surface of the anode  12  and the outer edges of the cathode panels  36  as shown in FIG.  1 . Each of the water jet manifolds carries a plurality of spaced nozzles  42  which are directed into the tank to apply water jet streams at high pressure to the highly exposed surfaces of the cathode panels as they are rotated in the tank as will be explained below. 
     The water jet manifolds  20  are shown connected on the outside of the tank  10  through on/off valves, not shown, to a source of water under pressure. 
     The inlet to the settling tank  22  is connected, as shown in FIG. 1, to a tube  44 , which is open to the cell tank outlet  28 , by a tube  46  which enables eluate which has left the tank  10  to be pressure fed into the settling tank and from its outlet back through the cell feed system to the tank inlet  26 . 
     The gold traps  24  are located in the cell system as illustrated in FIG. 1 with a first of the gold traps in alignment with the tube  44  and a second in alignment with an inlet/outlet tube  50  from the settling tank  22 . Each of the gold traps includes an upper valve  52  for shutting the traps off from the cell system and lower outlet valves  54  which are security protected by key operated locks, remote control solenoids or the like which are not shown in the drawings. 
     The anode  12  is connected, through an insulator which passes through the wall of the tank  10  to a connector  55  to which the positive pole of the DC supply to the cell is connected in use. The drive shaft  14  and so the cathode  16  are connected to a suitably insulated wiper  56  which is engaged with the shaft  14 , as shown in FIG. 1, for connection to the negative pole of the DC supply to the cell. 
     In use, the shaft  14  is rotated to cause the cathode to be rotated within the anode in the tank and gold rich eluate, from a carbon in pulp circuit, is fed into the tank to fill the cell tank  10  and the settling tank  22 . With the electrical supply to the anode and cathode of the cell activated, soluble gold in the eluate is liberated from the eluate conventionally according to the following reactions: 
      0 2 +4H + +4e→2H 2 O  (1) 
     
       
         2H 2 O+2e→2OH − +H 2   (2) 
       
     
     
       
         Au(CN) 2   − +→Au+2CN −   (3) 
       
     
     although the reduction of other metallic ions (such as Ag(CN) 2   −  and Cu(CN) 2   3   −  may also be important if they are highly concentrated. 
     Thermodynamically the reduction of oxygen is the most favourable. 
     The significant reactions at the anodes are: 
     
       
         2H 2 O→4H + O 2 +4e  (4) 
       
     
     
       
         CN − +2H 2 O→H + +NH 3 +CO 2 +e  (5) 
       
     
     2CN − →(CN) 2 +2e (Catalyzed by copper in solution) being the oxidation of water to oxygen gas and the oxidation of cyanide to ammonia and carbon dioxide or to cyanogen. The evolution of oxygen is the predominant reaction. 
     The gold particles which are liberated from the eluate are electrolytically deposited on the surfaces of the cathode panels  36  to form a gold rich sludge on the panels. 
     The rotation of the cathode panels in the eluate causes eluate turbulence against the faces of the panels which causes the gold sludge to be dislodged from the panels far sooner than is the case if the panels had been static and dislodgement of the gold sludge is purely dependant on gravity separation of the sludge from the panels. The gold sludge which is dislodged from the cathode panels gravitates through the eluate onto the base of the tank  10  and from there into the gold trap  24  below the tank outlet  28  through the open valve  52 . The eluate is recirculated from the tank  10  through the settling tank  22  and back to the feed line to the inlet  20  of the tank  10 . In the passage of the eluate through the settling tank  22  gold particles in the eluate, which were not deposited onto the cathode panels  36 , gravitate out of the electrolyte in the settling tank  22 , through the tube  50 , the open valve  52  and into the gold trap  24  beneath the settling tank. The gold trapped in the gold traps  24  is removed from time to time by closing the valves  52  and opening the valves  54 . 
     The exposed combined cathode area in the tank  10 , in the cell of this embodiment of the invention, is about 3 2  m and the cell is designed to operate according to the following operating parameters: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 i. 
                 Reactor Current 
                 600 to 850A 
               
               
                 ii. 
                 Reactor Voltage (measured across busbars) 
                 4 to 7v 
               
               
                 iii. 
                 Electrolyte pH value 
                 Above 12 
               
               
                 iv. 
                 NaOH concentration 
                 Above 0, 4% 
               
               
                 v. 
                 Electrolyte conductivity 
                 Above 1, 66 Sm −1   
               
               
                 vi. 
                 Electrolyte resistivity 
                 Below 60 OHM cm 
               
               
                 vii. 
                 Flow Rate 
                 200-400 l/min 1   
               
               
                 viii. 
                 Temperature 
                 Above 20° C. 
               
               
                 ix. 
                 Mass s/s knitted mesh or wool 
                 ± 18, 0 kg 
               
               
                   
               
             
          
         
       
     
     These conditions will allow the cell to recover 60-75 percent of the gold in solution entering the cell (i.e. if simultaneous samples of electrolyte entering and leaving the cell are collected then 100×(1−C out/C in) should be greater than 60, where Cin and Cout are the concentrations of gold entering and leaving the reactor. For Cin greater than 200 p.p.m. somewhat lower extraction recoveries are to be expected. 
     When the electrowinning cell of the invention has completed a predetermined period of operation the electrical supply to the anode is terminated. With the gold rich sludge removed from the traps  24 , the valves  52  and  54  of both traps are opened to drain the now barren eluate from the tanks  10  and  22 . The valves  54  are now closed. The cathode is rotated in the now dry tank  10  and its panels  36  are water spray washed by means of the nozzles  42  to dislodge gold sludge which has been trapped in the low adhesion stainless steel knitted mesh or wool of the cathode panels from the panels to gravitate, as described above, into the gold traps  24  from which the sludge is removed. This step in the electrowinning process of the invention results in optimum gold recovery from the system without having to remove the cathode panels from the tank  10  except for very occasional internal maintenance or repair of the tank and its components. The security of the system may further be enhanced by automatically discharging gold sludge from the gold trap valves  54  at predetermined intervals into sealed sludge containers. 
     The bulk of the components of the embodiment of the electrowinning cell illustrated in FIGS. 4 and 5 are the same as those of the FIG. 1 embodiment with these components having the same reference numbers as those used in the description of the cell of FIGS. 1 to  3  and therefore require no further explanation. 
     The fundamental differences between the two embodiments of the cell of the invention lies in the cathodes and anodes of the two cells. 
     The anode  57  of the cell of FIGS. 4 and 5 is a composite anode consisting of two anode cylinders  58  and  60  which are held concentrically together by a floor  62  which is fixed to and extends between them. The anode floor  62  slopes downwardly from the lower edge of the cylinder  58  to the edge of the cylinder  60 . The anode cylinder walls are holed for the passage of eluate through and between them and the floor  62  is holed, as seen in FIG. 5, against the cylinder  60  to enable gold rich sludge to gravitate out of the space between the cylinders. The anode cylinder  58  is held in the tank in the same manner as that of the FIG. 1 anode by means of insulated spacers  30  which are shown in FIG.  5 . The anode cylinder  58  includes a vertical row of holes which are in register with the water spray nozzles  42  to enable the water sprays to reach and penetrate the cathode during the water jet washing cycle of operation of the cell. 
     The cathode  64  in this embodiment of the invention is cylindrical and of the same construction as the cathode panels of FIGS. 1 and 2. The cathode  64  is located in the gap between the anode cylinders with its outer surface spaced from the cylinders as shown in the drawings. The cathode is held in position and rotated between the anode cylinders by cross beams  66  which are attached to the cathode and its drive shaft  14 . 
     To stir the eluate in the otherwise largely dead space in the anode cylinder  60 , the drive shaft  14  carries a paddle or agitator arrangement  68 . On rotation of the paddle arrangement the eluate on the inside of the anode cylinder  60  is stirred and a percentage of the stirred eluate is caused to enter the cathode space between the anode cylinders through the holes in the wall of the cylinder  60 . 
     Another difference between the two cells is that the cell of the second embodiment includes water spray nozzles  42  which are downwardly directed onto the cathode from an overhead ring manifold  68  in addition to the nozzles which are located in its side wall. 
     The cell of FIGS. 4 and 5 functions in the same manner as that of FIGS. 1 and 2 with the gold sludge being turbulence stirred from the surfaces of the cathode as it is rotated. The dislodged sludge particles gravitate onto the anode floor and from the floor through the holes in it onto the base of the tank  10  and from there into the trap  24 . 
     Yet another small difference between the two cells is that the water jet manifold  20  of the FIGS. 4 and 5 embodiment is situated on the outside of its tank  10  as opposed to inside it as is the case with the FIG. 1 cell.