ZA200500298B - Winning of metals. - Google Patents

Description

PCT/ZA2003/000145

ELECTROWINNING OF METALS

. This invention relates to the winning of metals.

Background to the Invention

In large-scale gold mining and gold extraction operations the crushed ore 1s leached in a sodium cyanide solution which will then contain typically 6-8 ppm gold.

This solution is passed through columns containing activated carbon which preferentially adsorbs the gold. After loading is complete the columns are eluted with a hot caustic solution to yield a gold eluate solution containing typically 650 ppm gold. The carbon is heated to regenerate it from time to time. The eluate then passes through electro winning cells where the gold is electro deposited on to cathodes of knitted stainless steel wool. The loaded cathodes are removed periodically and manually washed using high pressure water jets to dislodge a gold sludge. This is filtered, after which the filter cake is smelted and cast into impure gold bars. These bars are sent 10 a refinery, which is generally at a remote location, for further purification. In recent times the electro winning also been carried out in so-called "sludge reactors” which provide for the electro winning and sludge dislodgment steps to be carried out in situ and automatically. The product remains a gold sludge, which is filtered and smelted as before. It is often difficult to assay this gold sludge accurately unless it is smelted so that without smelting the mine cannot say with precision how much gold is sent to the refinery.

Further problems also occur in small-scale gold mining operations where the amalgamate the gold. In any event the value of the gold product which is sold is . generally very low.

It is an object of the invention to provide an improved electro winning method for metals and apparatus for this purpose and in particular but not limited to such method and apparatus for electro winning gold.

According to one aspect of the invention there is provided an electro winning cell comprising a housing; a first electrode within the housing; second electrode means within the housing and including collector electrode means electrical connections to the electrodes and switching means for connection to the positive and negative terminals of a power source, the switching means being arranged so that in use the first electrode may be connected selectively to the negative and positive terminals and that the second electrode means will be connected to the other of such terminals, Thus when the first electrode acts as a cathode the second electrode switching means acts as an anode.

Conveniently the second electrode means comprises a second electrode and the collector electrode means, and the switch means is arranged so that (a) when the first electrode is connected to act as a cathode, the second electrode is connected to act as an anode and the collector electrode means are isolated and (b) when the first electrode is connected to act as an anode, the second electrode is isolated and the collector - 20 electrode means acts as a cathode. However if desired the second electrode means may comprise a single electrode which acts as the collector electrode when cathodic.

The first electrode means is preferably a high surface area member. It preferably } comprises a first electrically conductive member and a second member which has a high surface area material which latter is often does not have good electrically conductive characteristics. The first member is preferably a mesh or expanded metal and is herein referred to as a "meshed member". The second member is hereinafter referred to as a pored member. The first member is preferably in close electrical and physical relationship with the second, pored, member.

The cell is used in combination with a source of electric current.

According to another aspect of the invention there is provided a method of electrowinning metal, comprising passing a solution loaded with metal into a cell in the combination as set out above; connecting the first electrode to the negative terminal of the source of electrical power so that it acts as a cathode, connecting the electrode means to the positive terminal of the source of electrical power so that it acts as an anode treating the solution to load the first electrode with metal and thereafter connecting the first electrode to the positive terminal of the electric power source and the electrode means to the negative terminal so that the metal loaded on to the first electrode is transferred to the collector electrode means.

The cell is preferably drained after the loading operation and an electrowinning solution, which is preferably specially made up for this purpose, is introduced for use . 20 in the electrowinning operation.

In the drawings: -

Figure 2 is a diagrammatic section through a secondary cell of the invention which is an electro-forming cell,

Figure 3 is a section on line 3 - 3 of Figure 2, ) Figure 4 is an enlarged detail of the first electrode of the cell of Figure 2,

Figure 5 is a diagrammatic view of a switching arrangement for controlling the electrodes of the cell of Figure 2,

Figure 6 is a view similar to Figure 2 of a first modified cell of the invention,

Figure 7 is a view similar to Figure 2 of a second modified cell of the invention,

Figure 8 is a horizontal section through the second modified cell,

Figure 9 is a graph showing the residual gold concentration in the solution, and

Figure 10 is a similar graph on a log scale.

Fluid flow diagram

Referring now to Figure 1 there is shown the fluid flow diagram for an electro winning assembly 10 of the invention. This particular electro winning arrangement 10 is intended to win gold from an eluate being a gold-bearing cyanide solution.

The electro winning assembly includes a primary electro winning tank array 100 and a set of secondary cells 200. In addition there is a liquid flow circuit and ancillary devices being a switch arrangement 400 and 450 being a lifting arrangement. All the abovementioned parts will be described more fully below.

The first electro-winning tank array 100 is in the form described in International ) Publication No. WO 01/051685 (the contents whereof is included in this specification

The tank array 100 comprises four tanks 102, of which only two 102.1 and 102.2, are shown; the tank 102.1 being shown in somewhat more detail. Each . tank 102 is provided with an inlet line 104 connected to a header feed line 106. The inlet line 104 opens to the lower end of the tank 102 and is provided with a valve 108 5 controlling the connection to the header feed line 106. Each tank 102 further has an outlet 110 at its upper end for the treated or spent solution. This outlet 110 leads to a line 112 which splits into two lines 114 and 116, one (114) of which is connected to the inlet line 104 of the next tank 102 of the array and the other (116) of which is connected to a drain line 118. Control valves 120 and 122 are provided respectively in the lines 114 and 116.

In each tank 102 are up to four electro winning cells 124 each including a cathode assembly located above a discharge pipe 126.

An ultrasonic resonator 128 is provided for causing the gold particles formed on the cathodes to be shaken free and to be discharged through the discharge pipe 126 as a gold sludge. The discharge pipes 126 are each connected through a valve 130 to a collector pipe 132. As is apparent from the above, further details of the tank array are to be found in the above International Publication.

The collector pipes 132 of each of the various tanks 102 are connected to their respective two feed lines 134 and 136. These two feed lines 134 and 136 are connected

C20 respectively through valves 138 and 140 to one of two inlets of two final electro winning ) cells 200 of the invention to be described.

PCT/ZA2003/000145

Each secondary electro winning cell 200 has a central outlet leading to a re- circulating line 142. These re-circulating lines 142 are connected via valves 144 through ] a pump 146 which re-circulates the liquid from the cell 200 either (a) back to the tank 102; (b) to a holding tank or reservoir 148 through return lines 150 which each split into two sections 152 and 154 controlled respectively by valves 158 and 160 or (c) delivers the spent solution to a drain line 188 via drain valves 178 and 130.

The reservoir 148 is connected back to the secondary cells 200 through two lines 164 and 166 each controlled by a valve 168 and 170 and leading to the second inlet to the secondary cell 200.

It will be noted that there are four sub-assemblies 12 each comprising a first electro winning tank 102, a pair of main electro winning cells 200 and the associated piping, valves etc.

The secondary cell 200

The secondary cell 200 comprises a housing 204 (see Figure 2) having a base 206, a top part or roof 208 and a generally cylindrical body 210.

The body 210 of the housing 204 is provided at its lower and upper ends with flanges 212 and 214. The lower flange 212 is connected by bolts 216 to the base 206.

The upper flange 214 is received within a ring clamp 218 into which is also received . the edge of the top part 208 of the housing 204. "O"-rings 220 and 222 are provided respectively between the flanges 212 and 714 and the base 206 and top part 208 to seal for this purpose a shaft 224 is provided connected to an appropriate rotating motor (not ’ shown).

There is a central outlet 226 provided in the base 206 connected to the recirculating line 142. A further outlet 228 is provided in the base 206 for recovery of the gold form as will be described. Two inlet ports 230 and 232 are provided in the sides 210 connected respectively to the lines 134 or 136 and 164 or 166. Within the housing 204 is a central porous polypropylene cylinder 234 surrounding the outlet 226; first, second and third electrodes 236, 238 and 240 and collector electrodes 242.

There are electrical connections 244, 246 and 248 passing through the base 206 and being connected respectively to the first, second and third electrodes 236, 238 and 240. A further electric connection 250 is connected to the collector electrodes 242.

A control or switch system 400 (see Figure 5) is provided for the electric connections 244, 246, 248 and 250 to connect them as will be described to the positive and negative terminals 402, 404 of a rectified power source 406 (which as described below may provide a pulsed power supply).

The porous polypropylene cylinder 234 serves as a filter for gold particles as will be described to inhibit or prevent them from escaping through the outlet 226. ) The first, second and third electrodes 236, 238 and 240 are mounted on the ) base 206. Their upper ends are received in grooves in the top plate 208 in such a way that the top plate can rotate relative to them.

The first electrode 236 is a high surface area member. It comprises a dimensionally stable good electrical conductivity "meshed" spiral member 252 . comprising expanded metal mesh or wire cloth and a "pored” high surface area mater- ial 254. The spiral member 252 comprises titanium which is coated with iridium oxide.

The “pored" high surface area material 254 comprises porous carbon fibre felt. The "pored” high surface area material may however comprise infer alia stainless steel wool or graphite felt. The titanium metal mesh and the porous fibre material are wound toge- ther in alternate spirals in the form of a "Swiss Roll" about the cylinder 234 (as is shown in Figure 4) and are in close physical and electrical proximity. Because of the close physical and electrical proximity of the mesh 252 and the "pored" high surface area material 254 there is a good electrical connection with the latter which improves its operation as an electrode. Other "pored" high surface area materials that can be used are mentioned below.

The first electrode 236 is wound closely around the cylinder 234 so that the latter traps the metal sludge in such a way that it, the sludge, is in electrical contact with the electrode 236.

The second electrode 238 surrounds the first electrode 222 in coaxial spaced relationship therewith. It comprises a metal oxide-coated expanded titanium mesh.

The third electrode 240 also iridium oxide coated titanium mesh coaxially surrounds the second electrode 238 leaving a fairly large annular space 260 there- : between.

There are four collector electrodes 242 depending from the top 208 of the ) housing 204 and being equi-spaced within the annular space 260. Each collector electrode 242 comprises a frusto conical polished stainless steel] member 262 with a domed lower end 264 and a small cone angle. Its upper end 266 is cylindrical. Each collector electrode 242 passes closely and in sealing relationship with an insulating block 268 through which it can be withdrawn as will be described. At its upper end and above the block 268, the collector electrode 242 has a flange 270 above which is an upper extension 272 having a peripheral recess 274. The electrical connection 250 is connected to the connector electrode 242 through the insulating block 268.

The electrical control system 400 includes first, second and third switches 408, 410 and 412. These may be electronic switches but for convenience they are shown as simple mechanical switches.

The switch 412 has a switch member 426 connected to the electrode 250. It has one terminal 428 connected to the negative terminal 404 of the power source 406. ts second terminal 430 is a blank terminal.

The switch members 414, 420 and 426 are connected together to move from a first position to a second position. In the first position, the switch member 414 is connected to the negative terminal 404 of the power source 406; the switch member 420 is connected to the positive terminal 402 of the power source 406 and the switch © 20 member 426 is connected to the blank terminal 430. In the second position, the switch member 414 is connected to the positive terminal 402; the switch member 420 is connected to the blank terminal 424; and the switch member 426 is connected to the switches are in the first position, the first and third electrodes 236 and 240 will act as cathodes and the second electrode 238 will act as an anode. The collector electro- des 242 will be disconnected. When the switches are in their second position, the first and third electrodes 236 and 240 will act as anodes and the collector electrodes 242 will act as cathodes. The second electrode 238 will be disconnected. In a third position of the switch members the electrodes are all disconnected.

The lifting mechanism

The lifting mechanism 450 comprises 2 pair of jaws 452 . These jaws 452 are at the end of a lazy tongs mechanism 454 actuated by a cylinder 456 to open and close the jaws 452 in such a way that they are able to engage in the recess 274 and thereby grip the collector electrode 242. A lifting cylinder 458 serves to lift the cylinder 456 when the jaws 452 engage the electrode 242 and thus to lift this electrode 242.

Operation of the apparatus

The sub-assemblies 12 of the electro winning assembly 10 are each operated as follows: - gold loaded eluate comprising about 650 ppm of gold is fed into the tank 102.

The valves 130, 138, 144 and 158 are opened so that the solution can circulate through the tank 102 to one of the secondary cells 200. The switches are moved into the first position. The gold in the solution 170 in the tank 102 is deposited as particles on the cathode assemblies of the electro winning cells 124. Residual gold also comes out of solution in the secondary cell 200 and is deposited on the cathodes 236 and 240. After a substantial amount of gold has been deposited as aforesaid and the gold content of the particles off the cathode assemblies and the gold is conveyed as a gold sludge to the secondary cells 200. The gold particles from the gold sludge will now been trapped on . the cathodes 236 and 240 and such further gold as may have been in solution is also so deposited.

When the gold content of the solution has dropped to a sufficient level, the secondary cell 200 is drained of solution. It is then flushed with water and/or with a sodium hypochlorite solution. After flushing, a made up gold loaded electro forming solution consisting of 8 to 40 gram gold/litre is fed into the reservoir 148. The switches are moved to the second positions. The electro winning solution is now circulated through the secondary cell and reservoir 148 via lines 142, 150, 154 and 164. The gold on the first and third electrodes 236 and 240 (which now act as anodes) in passed Into solution and then deposited on to the polished stainless steel member 262 of each of the collector electrodes 242.

The gold builds up on the member 262 to form an integral stable gold electro form 276 which because of the shape of the member 262 is in the form of a thimble.

When this is of an appropriate size, the switches are moved in into their third, isolated, position. The cell is then drained with the electroforming solution recovered for re-use.

The electro-forms 276 are now recovered as follows. The jaws 452 engage the collector electrode 242 and the cylinder 458 is now actuated to lift the electrode 242 a small amount, which may be as little as 1 mm. The base of the electroform : "thimble" 276 engages the block 268 and is thus moved off the steel part 262 and falls through the opening 228 from whence it is conveyed to a collection position (not position and the jaws 452 release the collector electrode 242. The top member 208 is now rotated by the rotating motor until the next collector electrode 242 is located above the opening 228 so that it may be lifted by the lifting mechanism 450 as described above. When all the "thimbles" 276 have been collected, the cell is again flushed, reconnected to the electro winning tank 102 the operation is repeated.

When one of the secondary cells is in the metal collection phase and in the circulating mode with the tank 102, the other secondary cell is in the stripping/electroforming mode and the two cells operate alternately.

A first modified secondary cell

Referring now to Figure 6 there is shown details of a modified secondary cell 300 of the invention. This cell 300 comprises a housing 310 having the same liquid connections as the housing 210. Within the housing 310 is a porous polypropylene cylinder 312 which extends from the top plate 314 to the base 316 of the housing 310.

A first electrode 318 is wound on to this cylinder 312 in a manner identical to the first 236 of the first embodiment. Centrally located within the cylinder 312 is a cylindrical titanium rod 320 which is coated with ruthenium oxide. The rod 320 is carried by a central conical insert 322 in the top plate 314 with "O"-rings 324 and 326 to provide adequate sealing between the insert 322 and the rod 320 and top plate 314 respectively. Electric connectors 328 and 340 are provided so that (a) the first electrode 318 can act as a cathode during the electro winning operation (when the solution is circulating to the tank) and as an anode during the plating operation (when the solution is circulating to the reservoir) and (b) the rod acts as an anode during the

The modified secondary cell 300 operates within the sub-assembly 12 in the ) analogous manner to that described above. During the plating operation the gold is deposited on the rod 320 which is now cathodic. When the rod 320 is fully loaded it is removed from the secondary cell 300 and subjected, at a suitable location, to an electro-refining operation. When stripped of gold, it is returned to the secondary cell 300 for further operations. It will be understood of course that there will be a number of such rods 320 provided to ensure continuous operation of the secondary cell.

Alternative method of operation of the cell 300.

The secondary cell 300 may also be used, especially in small-scale field operations, to refine gold in situ. This is done as follows. After the high surface area first electrode 318 has been loaded with gold, the cyanide solution is drained from the cell and recycled to its reservoir. The cell 300 is then flushed with a water/sodium hypochlorite solution to remove vestiges of cyanide in the cell and on the electrodes.

The cell 300 is then filled with a gold chloride/hydrochloric acid solution. The first electrode 318 is now made anodic with respect to the electroforming rod 320.

Essentially pure gold is electroformed on to the rod 320. Impurities such as silver, platinum group metals and lead are precipitated out as a metal sludge, which can be conveyed to a suitable location for refining, if desired.

For this refining operation the central electroplating rod 320 may be replaced by an electro forming electrode or mandrel such as the collector electrode 262 of the } embodiment described with reference to Figure 2. If desired, even more complex electro forming electrodes or mandrels may be provided instead of the rod 320.

A second modified secondary cell. . A second modified secondary cell 600 is shown in Figures 7 and 8. It comprises a generally parallelipipedal housing 602 having sides 604 and 606, ends 608 and 610, a base 612 and a top plate 614. Three central form discharge ports 616 are provided in the base 612 in the position to be described. Two inlet ports 618 are provided at the ends of one side 604 and an outlet port 619 is provided centrally of the other side 606.

Porous polypropylene sheets 620 on either side of the outlet port 619 extend from the base 612 to the top plate 614 and from one side to the other to serve as filters in the same way as the filter 234. A pair of first electrodes 622 extending the whole height and width of the housing 602 between the sides 604 and 606 are provided respectively on the outside of each sheet 620 and in close proximity thereto so that so that each filter sheet traps the metal sludge in such a way that it, the sludge, is in electrical contact with the electrode 622. Each electrode comprises a pair of identical flat metal meshes 624 clamping closely between them a high surface area material 626. Each mesh 624 comprises titanium which is coated with iridium oxide. The high surface area material 626 comprises any one of the materials described above.

On either side of each of the first electrodes 622 and in spaced relation thereto are identical second and third electrodes 628. These electrodes 628 comprise a metal oxide-coated expanded titanjum mesh. They are located inside of the inlet ports 618.

There are three equi-spaced collector electrodes 630 depending from the top plate 614 of the housing 602 being located on a line mid-way between the ends 608 electrodes 242 described above. The form discharge ports 616 are aligned with the ) electrodes 242.

Lifting means, not shown but similar to the lifting means 450, are provided to lift the collector electrodes 630 for the same purpose as described above.

Electric connections 632 are provided for the electrodes in a manner analogous to that described for the secondary cell 200. An electrical control system (not shown) similar to the system 400 is provided. It comprises three switches. The first switch connects the first electrodes 622 either to the positive or negative terminals of a pulsing

DC electric source. The second switch (i) connects the second and third electrodes 628 to the positive terminal when the first electrodes are connected to the negative terminal and (ii) isolates these electrodes 628 when the first electrodes 622 are connected to the positive terminal. The third switch (i) connects the collector electrodes 630 to the negative terminal when the first electrodes are connected to the positive terminal, and (ii) isolates these collector electrodes 630 when the first electrodes 622 are connected to the negative terminal.

The secondary cell 600 operates in the same manner as the secondary cell 200.

After the electroforms or thimbles are formed, the cell 600 is drained. The lifting mechanism now lift the collector electrodes 630 to strip them of the electro forms which now are collected through the drain ports 616.

Size of the secondary cells and collector electrodes.

The various secondary cells 200, 300 and 600 may have any capacity as desired.

In the particular embodiments the cell 200 has a capacity of about 10 litres and the cell 300 has a capacity of about 2 litres. The cell 600 has a capacity of about 12 litres.

The collector electrodes 242 each have an axial length of about 200 mm and an average diameter of 17 mm.

Examples

Six examples of use of the cell 200 to remove gold from solution are now described. In each example a single secondary cell 200 was used to treat gold loaded solution. The examples were runs carried out seriatim in the same ceil. As the quality of gold was limited, the electro won gold from all the examples was loaded on to only one high surface area electrodes 236. The solution was circulated in a circuit containing the reservoir 148 by the pump 146. The primary tank array was not used. Eighteen litres of each solution were used. Six gold bearing solutions were used, four, numbered 1A, 2A, 3A and 4A were obtained from a South African Gold Mine A and two numbered 5B and 6B were obtained from a South African Gold Mine B.

A seventh example deals with the electro forming operation. Only one collector electrode 242 was connected to the switching circuit 400 i.e. the gold was electro formed on only one collector electrode and this electrode was masked off so that the gold would be electro formed on a section of the collector electrode. Further as

This was necessary because of the limited amount of gold present. The cathode current : density is expressed in terms of the geometrical, not actual, surface area of the high surface area cathode i.e. the mesh and the high surface area material, and of the collector cathode in example 7.

In the first six examples the plating power supply was a direct current supply.

The runs took place for a period of time as mentioned and the gold in solution was periodically measured. The results of such measurement is given in Table 1.

Example 1

In this example, solution 1A was used with a starting gold content of 103 ppm. The solution was heated to 21°C. The voltage applied was 6,5 volts. The current density was 2A/dm2. The solution was circulated at 32 litres / minute. The total time of the run was 360 minutes.

Example 2

In this example, solution 2A which is identical to solution 1A was used in a 240 minute run. The average temperature of the run was 43°C. The average voltage was 4,2 volts. The current density and flow rate were the same as in example 1.

Example 3

In this example a solution 3A containing 349 ppm of gold was used in a run of 280 minutes. The average temperature of the solution of the run was 43°C. The average voltage 4,8 volts. The average current density was 4 A/dm? and the flow rate 32 litres per minute.

Example 4

A solution 4A containing 342 ppm gold was subjected 10 a Tun of 315 minutes. The solution was heated to 34°C and power was supplied at 3.8V and 2 A/dm?. The flow rate was 16 litres per minute.

Example 5

A solution 5B containing 895 ppm gold was subject to a run of 305 minutes. The solution was heated to 44°C. The same power supply as mentioned in example 4 was provided. The flow rate was 32 litres per minute.

Example 6

A solution 6B containing 164 ppm gold was subject to a run of 240 minutes. The solution was heated to 45°C. The average voltage 4.5 volts. The average current density was 2 A/dm’ and the flow rate 32 litres per minute.

Example 7

The cell 200 was used to produce an electro form. The high surface area electrode 236 was loaded with gold as described in the previous examples. By calculation the total amount of gold amounted to 32 grams of gold. After draining of the housing as discussed above, the electro forming solution was introduced into the circuit. The ~electroforming solution used was 18 litres of a gold eluate solution from mine B, which contained about 200 ppm gold. To this solution was added sodium cyanide (10 g/0), di-potassium hydrogen phosphate (4 g/f), sodium hydroxide (5 g/f) and a proprietary : brightener (Sml/ ?). The high surface area electrode was made anodic relative to the collector electrode. The power was supplied as a pulse supply being twenty current density of about 0,3 A/dm?. Because of the low concentration of electro forming solution, the operation was very lengthy taking approximately forty eight . hours.

As a result of the electroforming operation, the high surface area electrode was stripped of gold and a matt self sustaining gold alloy tapered tube 276 of gold was formed weighing about 8 grams. It had an average internal diameter of 16 mm, an axial length of about 30 mm and a wall thickness of approximately 0,3 mm.

PCT/ZA2003/000145

Table 1

Example Elapsed Residual Av. Voltage Current Flow Rate

Time Gold Temp (volts) Density (£/min) . (Minutes) (ppm) Deg C (A/dm? 1A 0 103,0 21 6,5 2 32 91,5 82,2 78,5 56,6 60 36,7 120 14,8 240 1,05 360 0,08 2A 0 103,0 43 4,2 2 32 ) 98,5 15 56,0 30 26,8 60 6,7 120 0,96 240 0,34 3A 0 349.0 43 4,8 4 32 95 1,4 180 0,68 280 0,35 4 A 0 342,0 34 3,8 2 16 30 133,0 90 14,5 150 0,91 315 1,25 5B 0 895,0 44 3,8 2 32 5 797.0 19 510,0 30 251,0 90 0,16 190 0,30 305 1,06 6B 0 164,0 45 4,5 2 32 5 154,0 16 106,0 40 45,7 72 12,9 en : 7 no

PCT/ZA2003/0001453

Results

Electrowinning

The results of using the cell 200 to extract gold from various gold eluates from two South African gold mines are given in Table 1 and plotted graphically in Figures 9 and 10. It will be appreciated that content of the solution in parts per million is the converse of the amount of gold that is recovered by the electro winning operation.

As will be seen from Table 1 and Figure 9, in all cases gold extraction is very effective, the residual gold concentration being reduced to less than 1 ppm, in times ranging from about 1,5 to 4 hours, reflecting gold extractions of >99%. In Figure 10, the logarithm of the residual gold concentration is plotted as a function of time, with the residual gold concentration also being shown (plotted on a logarithmic scale). As can be seen from Figure 10 in the region down to about 1 ppm gold a linear relationship between the log of the gold concentration and the elapsed time apples.

The times required to reach a value of 1 ppm residual gold concentration under the different conditions can be found by the intersections of the graphs with the abscissa.

Several other trends were observed:

Comparing Examples 1 and 2, higher operational temperatures improved the Kinetics of gold extraction significantly, and also meant a lower voltage requirement. ’ Comparing Examples 3 and 4, a higher cathode current density improved the kinetics -- ~¢ =alA aviraction. but with a higher voltage requirement; the lower flow-rate

PCT/ZA2003/000145

Comparing Examples 2, 6,and 5 in Figure 6, it is evident that at the same temperature (about 44C), flow-rate (32 l/min) and cathode current density (2A/dm2), a : higher initial gold concentration implies a longer time to reduce that concentra- tion to below 1 ppm gold — as might be expected.

In some cases (Examples 4 and 5) very long extraction times resulted in a small amount of gold going back into solution. This may be ascribed to the fact that under the operating conditions employed, resistance heating within the cell had caused the overall temperature to tise Over prolonged times (0 well over 50°C, resulting in gold on the high surface area electrodes re-dissolving in the hot cyanide solution.

Similarly, in Example 5 in which the undiluted head sample was used, the initial gold concentration analyzed at 895 ppm gold, as against about 659 ppm gold for the original head sample from gold mine B. This discrepancy may again be explained by the re-dissolution of finely dispersed gold present on the elec- trodes 236 and 238. This gold would be there from previous examples and as the circulating cyanide solution was pre-heated to operating temperature Over several hours before the commencement of Example 5, that gold would re- dissolve and register as a higher value than would have been anticipated.

It may be noted that the slopes of the graphs in Figure 9 are dependent upon the kinetics, the steeper the gradient, the faster the kinetics.

Electroforming vo iN ha annreriate that in commercial use, the electroforming solution which

PCT/ZA2003/000143 and might contain proprietary additives to achieve a good integral fine-grained electrodeposit at relatively fast deposition rates (up to 100 micrometers/hour are : possible). In such cases each thimble would typically have a wall thickness of 2 mm and a mass of 450 grams. Because of the impracticability of using such a solution in the presence of such a small amount of electro-won gold - any effects would be swamped - the electroforming solution used was, as described, 18 litres of a gold eluate solution from mine B, which contained about 200 ppm gold with the additives mentioned.

Initially electroforming was carried out at 40-45°C, at about 0,15 A/dm2 (DC).

The electro form thus produces appeared dark friable and rough. The change to 2 pulsed current supply as mentioned improved the quality of the electroform produced.

As the gold concentration of the electroforming solution in the examples was very low this meant very long deposition times at very low curTents, and a really good deposit was hardly to be expected. Nevertheless, using pulse plating a good electroform was obtained at an average current density of 0,3 A/dm? and a peak current density of 3

A/dm? (ON time:20 milliseconds, OFF time:180 milliseconds). The cathode current efficiency in this case was about 40%, but of course this would decrease with decreasing gold concentration. A subsequent (final) electroform was analyzed for gold content as was the electroforming solution at that stage. The gold content of the electroform was 89,3%, the balance being mainly silver. The gold concentration in the electro- forming solution was now 437 ppm gold, an increase from the starting value of +200 ppm gold, and the free cyanide content was 3,531 grams/litre. The total free cyanide added originally was about 5,3 grams/litre (10g NaCN/¢). Thus the inference - \ st +tanm walnc ic that some gold on the electrodes had gone into solution and

PCT/ZA2003/000145 still remained to be plated out. At that stage the total recovery of gold as an electroformed product was about 75%, excluding the silver content of the electroform.

In commercial practice, where gold is being continually collected, and masses of a kilogram, or more, are present on the high surface area electrodes, there would be no problem about having a high gold concentration electroforming electrolyte in place.

In that case the cost of the circulating load of gold would not be prohibitive in relation to the value of the gold recovered. Essentially the gold in that solution would not be consumed, the high concentration present merely serving to promote a high rate of electroforming.

Materials from which the electrodes may be constructed

The materials from which the various parts are made are described above.

These materials and other materials are mentioned in the following paragraphs.

The electrode which acts as an anode during the loading operation (i.e. the electrode 238 in the embodiment of Figure 2 and the electrode 628 in the embodiment of Figures 7 and 8), may comprise expanded metal mesh typically of stainless steel or of titanium coated with iridium oxide, ruthenium oxide, or similar such as tantalum oxide.

The electrode which acts as a cathode during the loading operation (i.e. the electrodes 236 and 240 in the embodiment of Figure 2, the electrode 318 in the sw oe Timwn A the electrode 622 in the embodiment of Figures 7 and 8) may

PCT/ZA2003/000143 wire cloth each coated with iridium oxide, ruthenium oxide, or similar such as tantalum oxide, or else stainless steel expanded mesh or wire cloth, and/or (b) and the "pored” . material is reticulated vitreous carbon, carbon or graphite felt, coated titanium wool or coated open cell titanium foam, or similar materials such as packed carbon granules.

Preferably as described each such electrode comprises both these two types of material.

Because this meshed member of the electrode is of this construction and in particular because it is coated as aforesaid it can be used both as cathode and as an anode in prolonged service. The use of the "pored” material is of importance because of its very high surface are enabling it to remove metals at very low concentrations.

It will be understood too that these electrodes may be of any desired and applicable shape in addition to the shapes described above.

The collector electrodes may comprise polished stainless steel, as described, or titanium or which are of ruthenium oxide coated titanium, when required as electro-refi- ning anodes in a chloride medium. These electrodes, which as described are preferably conical in shape to facilitate electroform removal, may be of other shapes if desired.

The filter material may be of porous polypropylene, as described, but may be any other suitable material.

General : It is appreciated that certain features of the invention, which are, for clarity, n Aecrrihed in the context of separate embodiments may also be provided in combination

PCT/ZA2003/000145 brevity described in the context of a single embodiment may also be provided separately or in any suitable combination.

Modifications

The invention is not limited to the precise constructional details hereinbefore described and illustrated in the drawings. For example the secondary cells may be used in certain circumstances without there being a primary tank array, i.e. the secondary cells will receive the eluate direct from the leaching operation. Furthermore the electro winning arrangements as described above may be used for electro winning other metals such as silver, copper, nickel, zinc, and cobalt, and, by separating the anode and cathode compartments by means of a suitable membrane, the platinum group metals.

It may also be used to remove unwanted metals from liquids being returned to nature.

The sizes of the housings and the collector electrodes may vary as desired.

Where other metals are electro won the electrowinning solution may comprise the anions e.g. sulphate of the metal being won. 5976

Claims (1)

1. PCT/ZA2003/000145

   Claims 1 An electro winning cell comprising a housing a first electrode within the housing second electrode means within the housing and including collector electrode means electrical connections to the electrodes and switching means for connection to the positive and negative terminals of a power source, the switching means being arranged so that in use the first electrode may be connected selectively to the negative and positive terminals and that the second electrode means will be connected to the other of such terminals. 2 A cell as claimed in claim 1 wherein the second electrode means comprises a second electrode in addition 10 the collector electrode means, and wherein the switch means is arranged so that (a) when the first electrode is connected to act as a cathode, the second electrode is connected to act as an anode and the collector electrode means are isolated and (b) when the first electrode is connected to act as an anode, the second electrode is isolated and the collector electrode means acts as a cathode. 3 A cell as claimed in claim 1 wherein the second electrode means comprises a single electrode which acts as the collector electrode when cathodic.

   PCT/ZA2003/000143

   4 A cell as claimed in claim 2 further comprising a third electrode having electrical connections which are in electric communication with the electrical connections to the : first electrode.

   A cell as claimed in claim 2 or claim 4 wherein the first and second electrodes are cylindrical in shape and are arranged with the first electrode coaxially located within the second electrode. 6 A cell as claimed in claim 5 wherein the collector electrode means is located within the housing outside the second electrode. 7 A cell as claimed in claims 4, 5 and 6 wherein the third electrode is cylindrical in shape and co-axially surrounds the second electrode. 8 A cell as claimed in claim 7 wherein the collector electrode means is located between the second and third electrodes. 9 A cell as claimed in claim 2 or any claim dependant thereon wherein the collector electrode means comprises a plurality of collector electrodes.

   A cell as claimed in claim 9 wherein the collector electrode means comprises . four collector electrodes. 11 A cell as claimed in claim 10 wherein the collector electrodes are equi-spaced Coe 4 ites emare hotween the second and third electrodes.

   PCT/ZA2003/000145

   12 A cell as claimed in claim 3 or in claims 9, 10 or 11 wherein the or each collector electrode is mounted in a holder so as to be axially movable therein and - comprises a tapered body. 13 A cell as claimed in claim 12 further comprising lifting means that is capable of being engaged with a collector electrode to lift it relative to the holder. 14 A cell as claimed in any one of the preceding claims wherein the first electrode comprises a meshed good electrical conductivity member.

   A cell as claimed in claim 14 wherein the meshed good electrical conductivity member comprises a Woven metal. 16 A cell as claimed in claim 14 wherein the meshed good electrical conductivity member comprises an expanded metal. 17 A cell as claimed in claim 15 or 16 wherein the metal comprises titanium. 18 A cell as claimed in claim 15, 16 or 17 wherein the metal is protected by a protective coating. . 19 A cell as claimed in claim 18 wherein the protective coating is selected from the group containing the oxides of ruthenium, iridium or similar materials such as tantalum ) oxide, or of platinum metals.

   PCT/ZA2003/000145

   . 20 A cell as claimed in any onc of the preceding claims wherein the first electrode comprises a "pored” high surface area material which is selected from the group comprising carbon felt, graphite felt, reticulated vitreous carbon and oxide coated titanium foam or mesh. 21 A cell as claimed in claim in any one of claims 14 to 19 and 20 comprising both a meshed good electrical conductivity member and a "pored” high surface area member. 22 A cell as claimed in claim 21 wherein the meshed good electrical conductivity member and the "pored” high surface area members are in close electrical and physical contact. 23 A cell as claimed in claim 77 wherein the said members are arranged in the form of two spirals one within the other. 24 A cell as claimed in claim 14 or any claim dependant thereon wherein the second electrode comprises the same material as the meshed good electrical conductivity member. A cell as claimed in any one of the preceding claims and being arranged so that solution can circulate therethrough, the cell including a housing defining a chamber with a base and having an outlet from the base and an inlet into the chamber . 4-3 Ereme manne Incated between the inlet and the outlet.

   PCT/ZA2003/000143

   . 26 A cell as claimed in claim 25 wherein the filter means comprises porous polypropylene. 27 A cell as claimed in claim 25 of 26 wherein the filter means comprises a cylinder surrounding the outlet pipe. 28 A cell as claimed in claim 27 wherein the first electrode fits closely around the cylinder.

   26. A cell as claimed in claim 13 or any onc of the proceeding claims dependant thereon wherein the cell comprises a housing having a top plate in which the collector electrodes are mounted and wherein the top plate is movable so that the collector electrodes can be moved into proximity to the lifting means. 29 The combination of a cell as claimed in any one of the preceding claims with primary electrowinning cell means having a drain pipe wherein the drain pipe is connected to the inlet to the cell.

   . 30 The combination of claim 79 wherein the cell has a drain which is connected to recirculate drained solution to the primary electrowinning cell means. + 8 tam 20 including pump means to recirculate the drain

   PCT/ZA2003/000145 i 32 The combination of a cell as claimed in any one of claims 1 to 29 with an electrowinning solution circuit the arrangement being such that an electrowinning solution can be circulated through cell and the electrowinning solution circuit. 33 The combination of a cell as claimed in any One of claims 1 to 29 with a source of current supply. 34 The combination of claim 33 wherein the source of current supply 1s capable of supplying a direct current supply.

   The combination of claim 33 or 34 wherein the source of current supply iS capable of supplying a pulsed direct current supply. 36 A method of electrowinning metal, comprising passing a solution loaded with metal into a cell in the combination as claim in claim 33, 34 or 35; connecting the first electrode to the negative terminal of the source of electrical power so that it acts as a cathode, connecting the electrode means to the positive terminal of the source of electrical power so that it acts as an anode treating the solution to load the first electrode with metal and . thereafter connecting the first electrode to the positive terminal of the electric power source and the electrode means to the negative terminal so that the metal loaded on to the first electrode is transferred to the collector electrode means.

   ” PCT/ZA2003/000145

   } 37 A method of electrowinning metal as claimed in claim 36 using a cell as claimed in claim 2 ’ wherein when the first electrode is connected tO the negative terminal the second electrode is connected to the positive terminal and the collector electrode is isolated and when the first electrode is connected to the positive terminal of the power source the collector electrode is connected to the negative terminal and the second electrode is isolated. 38 A method as claimed in claim 36 or 37 when dependant upon claim 35 wherein when the collector electrode means 1S connected the negative terminal, the electrodes are supplied with pulsed current {0 transfer the metal to the collector electrode means. 39 A method as claimed in claim 36, 37 or 38 comprising the steps of removing the remnants of the metal loaded solution from the housing after the first electrode has been loaded with metal filling the housing with an electro winning solution connecting the first electrode to the positive terminal and the collector electrode means is connected to the negative terminal. 40 A method as claimed in any one of claims 36, 37, 38 or 39 wherein the metal } is transferred to the collector electrode means to plate the collector electrode means. 41 A method as claimed in claim 40 comprising transferring the plated collector ’ er tanorion where the metal can be recovered from the plated collector

   R4305.1 ® | 00

   42 A method as claimed in any one of claims 36, 37, 38 or 39 wherein the metal is transferred to the collector electrode means to produce a removable form thereon. 43 A method as claimed in any one of claims 36 to 42 wherein the metal is gold. 44 A method as claimed in claim 43 wherein the solution loaded with metal is a cyanide solution. 45 A method as claimed in claims 39 and 43 or 44 wherein the electro winning solution is a chloride solution. 46 A method as claimed in claims 39, 43, 44 or 45 wherein the electrowinning solution is a sulphate solution. 46 An electro winning cell having parts substantially as hereinbefore described with reference to and as illustrated in Figures 2, 3, 4 and 5; Figure 6 or Figures 7 and 8 of the accompanying drawings. 47 An electro winning circuit substantially as hereinbefore described with reference to and as illustrated in Figure 1 and in Figures 2, 3, 4 and 5; Figure 6 or Figures 7 and 8 of the accompanying drawings. : 48 A method of electro winning a metal substantially as hereinbefore described with . reference to any one of the embodiments and the examples.