Abstract:
Disclosed is a system for electrolytic processing or recovery of a metal from an electrolyte solution. The system may comprise electrolysis cells and a rectifier. The cells may comprise interleaved anodes and cathodes. The anodes or the cathodes of a first cell may have an electrical connection to a terminal of the rectifier, respectively, via a first electrical path having a first resistance. The anodes or the cathodes of a second cell may have an electrical connection to a terminal of the rectifier, respectively, via a second electrical path having a second resistance. The second resistance is configured to be higher than the first resistance. The system may further comprise a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a national stage application filed under 35 USC 371 based on International Application No. PCT/FI2011/050385 filed Apr. 28, 2011, and claims priority under 35 USC 119 of Finnish Patent Application No. 20100184 filed Apr. 30, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the electrolytic processing of metals. Examples of electrolytic recovery of metals are electrorefining and electrowinning. The invention relates particularly to control and distribution of electrical power within a single electrolytic cell or distribution of electrical power between multiple electrolytic cells. 
         [0004]    2. Background of the Invention 
         [0005]    In electrolytic recovery of metals an electric current is passed between two electrodes immersed in an electrolyte which is a solution containing the metal in a dissolved ionic form. The electrical current causes the metal to be deposited on the cathode. 
         [0006]    Electrorefining (ER) is an electrolytic process for purifying a metal. An impure metal anode is dissolved and the pure metal is deposited at the cathode. The anode is made electrically positive and the cathode is made negative by application of an external voltage, so that an electrical current passes through the electrolyte between the anodes and cathodes. For example, in the electrorefining of copper, the anode is made of impure copper, the copper enters the electrolyte as the anode dissolves anodically giving copper (II) ions (Cu2+(aq)) historically referred to as “cupric” ions. Typically, the electrolyte contains copper as copper sulfate with sulfuric acid as a supporting electrolyte. The copper (II) ions are transported through the electrolyte and reduced at the cathode, where pure copper is deposited. Impurity elements from the anode can remain as solids and deposit in the anode slimes in the cell, or they can dissolve in the electrolyte. Impurity elements comprise, for example, nickel or arsenic. 
         [0007]    In copper electrorefining plants, the concentration of impurity elements such as nickel and arsenic typically build up with time, and liberator cells are typically used for purifying the tankhouse electrolyte from the main production process. Electrowinning (EW) is an electrolytic process for recovering dissolved metals from an electrolyte. A number of metals can be won from solution using electrolytic methods. These metals include but are not limited to copper, nickel, gold, silver, cobalt, zinc, chromium and manganese. 
         [0008]    In industrial electrowinning of copper, the cell voltage is usually approximately 2.0V, the current density can be in the range 200 to 400 Amperes per square meter and the area of each electrode face is usually about 1 square meter. The term cell is used to describe a tank in which at least one anode and one cathode are immersed in an electrolyte solution which is usually aqueous. In the following description the term tankhouse means an arrangement, wherein at least one cell (or tank) and power source are present in a building or enclosed structure, that is, a house. In a typical configuration a tankhouse comprises a plurality of cells. 
         [0009]    When using a plurality of cells in conventional electrowinning, it is usual to have several cells powered by the same rectifier, giving the same current density at each cathode. 
         [0010]    Liberator cells are similar to standard electrowinning cells. During the process copper is plated out on the cathodes and copper ion concentration in the electrolyte decreases. 
         [0011]    In certain technical solutions of the electrolytic recovery of metals, the state of the electrolyte changes during the process. For example, in a purification process of the electrolyte from copper electrorefining, it is desirable to remove most of the copper ions along with harmful impurities from the electrolyte. The properties of the electrolyte—especially the copper concentration—change with time during the process. 
         [0012]    In later stages of electrolytic recovery of metals in liberator cells, where copper concentration is low, if the current density is too high, the result can be the undesired production of a powdery copper deposit (or copper sludge) at the cathode. There is also a risk of toxic arsine gas production at the cathode surface. 
         [0013]    In terms of electrode materials, liberator EW cells are similar to standard electrowinning cells, the anodes are insoluble. For copper liberators the anodes are usually lead-based alloys; rolled lead-calcium-tin alloy, or antimonial lead. Mixed metal oxide (MMO) coated titanium anodes—also known by the trademark name of Dimensionally Stable Anode™ (DSA) may also be used. 
         [0014]    The cathodes in liberator cells are usually spent anodes from the electrorefining tankhouse, but can also be permanent cathodes with stainless steel blades. Older refineries may still use copper starter sheet technology. 
         [0015]    Copper cathodes deposited in the liberator cells which contain impurities (such as arsenic and antimony) are returned to the smelter to be melted and cast into anodes for electrorefining. 
         [0016]    Decopperised electrolyte can be sent to the electrorefining cells, or further processed, for example, for nickel removal. 
         [0017]    In certain technical solutions of electrolytic recovery of metals, the state of the electrolyte inevitably changes during the process. For example, in a purification process of the electrolyte it is desired to remove all copper together with harmful impurities from the electrolyte. This means that properties of the electrolyte change with time during the process. 
         [0018]    Liberator cells are similar to normal electrolytic cells, but they have lead anodes in place of copper anodes. Copper in the solution is deposited on copper starting sheets. As the copper in the solution is depleted, the quality of the copper deposit is degraded. Liberator cathodes containing impurities such as antimony are returned to the smelter to be melted and cast into anodes. Purified electrolyte is recycled to the electrolytic cells. 
         [0019]    In a liberator tankhouse, the aim is to remove copper from the solution as a solid copper cathode deposit. The current densities employed are typically lower than in standard copper electrowinning or Electrorefining. As the process proceeds, the copper concentration of the electrolyte gets down to lower cation concentrations. 
         [0020]    Reference is now made to  FIG. 1  which illustrates a system for electrolytic recovery of metals in prior art. In  FIG. 1  there is a voltage source  100  which provides a negative voltage to conductor  102  which is further connected to a cathode busbar  118 . Voltage source  100  provides a positive voltage to conductor  104  which is further connected to anode busbar  129  of a cell  120 . There are two electrolytic cells, namely a cell  110  and cell  120 . Cell  110  comprises a cathode  114  and an anode  116 . Cell  110  contains an electrolyte solution. Cell  120  comprises a cathode  124  and an anode  126 . Cell  120  contains an electrolyte solution. Cathode busbar  118  is connected to cathodes in cell  110  such as cathode  114 . Cathode busbar  128  is connected to cathodes in cell  120  such as cathode  124 . Anode busbar  119  is connected to anodes in cell  110  such as anode  116 . Anode busbar  129  is connected to anodes in cell  120  such as anode  126 . Cell  110  and cell  120  are connected electrically in series so that anode busbar  119  is connected along its length to cathode busbar  128 . 
         [0021]    During the liberation process the electrolyte has initially high copper concentration. As the electrolyte is processed the copper concentration decreases and acid concentration increases. In prior art solutions, the maximum current density which can be used is dictated by the copper concentration in the lean electrolyte, since all the cells are connected electrically in series and carry approximately the same current. 
         [0022]    In prior art a cell contains a plurality of anodes, all connected electrically in parallel and a plurality of cathodes also connected electrically in parallel. The voltage across a cell is therefore approximately equal to the voltage that would be experienced between a single anode and a single cathode. By way of example, this voltage would be approximately 1.7 to 2.8 Volts in the case of the electrowinning of copper, depending on the current density employed. 
         [0023]    It is difficult to convert electrical power from mains voltage to a dc voltage of this magnitude efficiently. For this reason it is common practice to connect cells in series so that they all conduct the same current, but the voltage across the series chain of cells is equal to the sum of all the cell voltages. By this means the voltage rating of the central dc current source, commonly called a rectifier, is elevated and high efficiency can be obtained. 
         [0024]    The difficulty with this arrangement is that the same current density is used in all cells. Cells may operate at a much lower current density than the copper concentration in the electrolyte would permit. This causes that there are more cells compared to a situation where the process would be run using optimum current densities. Thus, a liberator tank house uses more electrical power than in an optimum case. 
       SUMMARY OF THE INVENTION 
       [0025]    According to an aspect of the invention, the invention is a system for electrolytic processing of a metal comprising at least two electrolysis cells for the metal and a rectifier, wherein the at least two cells comprise at least three anodes and at least two interleaved cathodes. For the system is characteristic that the anodes or the cathodes of a first cell have an electrical connection to a positive or a negative terminal of the rectifier, respectively, via a first electrical path having a first resistance; the anodes or the cathodes of a second cell have an electrical connection to a positive or a negative terminal of the rectifier, respectively, via a second electrical path having a second resistance; the second resistance is configured to be higher than the first resistance; and the system further comprises a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell. 
         [0026]    According to another aspect of the invention, the invention is a system for electrolytic processing of a metal comprising at least two electrolysis cells for the metal and a rectifier, wherein a first cell comprises a plurality of cathodes interleaved between a plurality of anodes and a second cell comprises a plurality of cathodes interleaved between a plurality of anodes. For the system is characteristic that the anodes of the first cell have an electrical connection to a positive terminal of the rectifier via a first electrical path having a first resistance; the anodes of the second cell have an electrical connection to a positive terminal of the rectifier via a second electrical path having a second resistance; the number of anodes and cathodes in the second cell is configured to be higher than the number of anodes and cathodes in the first cell to diminish a difference between the first resistance and the second resistance; and the system further comprises a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell. 
         [0027]    In one embodiment of the invention, the configuration of anodes and cathodes in the first and second cells is such that a cathode plate is placed between two anode plates. The cathode and anode plates may be substantially parallel in a cell. The plates may be rectangular, for example,  1  meter by  1  meter. 
         [0028]    The distances from cathodes to neighboring anodes, between which the cathodes are arranged, may be substantially same. By substantially the same distances may be meant a difference of less than 10 centimeters in the distances. By substantially parallel may be meant at most an angle of 10 degrees between plates. 
         [0029]    In one embodiment of the invention, the at least two cells comprise the at least three anodes and the at least two interleaved cathodes, the cathodes being interleaved between the anodes. Thus, between two anodes there is a cathode. 
         [0030]    In one embodiment of the invention, the anodes of a first cell have an electrical connection to a positive terminal of the rectifier, respectively, via a first electrical path having a first resistance, the anodes of a second cell have an electrical connection to a positive terminal of the rectifier, respectively, via a second electrical path which has the second resistance. 
         [0031]    In one embodiment of the invention, the cathodes of a first cell have an electrical connection to a negative terminal of the rectifier, respectively, via a first electrical path having a first resistance, and the cathodes of a second cell have an electrical connection to a negative terminal of the rectifier, respectively, via a second electrical path which has a second resistance. 
         [0032]    In one embodiment of the invention, the first electrical path and the second electrical path comprise metal conductors. 
         [0033]    In one embodiment of the invention, the first electrical path consists of conducting material and the second electrical path comprises at least one resistor device in addition to at least one conductor. 
         [0034]    In one embodiment of the invention, the first electrical path comprises conducting material and the second electrical path comprises at least one resistor device. 
         [0035]    In one embodiment of the invention, the second electrical path comprises a resistor and an anode or a cathode bar in series, the anode or the cathode bar being connected to each anode or cathode, respectively, of the second cell. 
         [0036]    In one embodiment of the invention, the second electrical path comprises a resistor and an anode bar in series, the anode bar being connected to each anode of the second cell. 
         [0037]    In one embodiment of the invention, the second electrical path comprises a resistor and a cathode bar in series, the cathode bar being connected to each cathode of the second cell. In one embodiment of the invention, the second electrical path comprises an anode bar to which are connected electrical paths for each of the at least three anodes of the second cell, the electrical paths for each of the at least three anodes having respective resistors. 
         [0038]    In one embodiment of the invention, the second electrical path comprises an anode or a cathode bar, respectively, of the first cell. 
         [0039]    In one embodiment of the invention, the metal is copper. 
         [0040]    In one embodiment of the invention, the first cell and the second cell are liberator cells. 
         [0041]    In one embodiment of the invention, the system further comprises an intermediate voltage supply configured to supply local converters. The local converters are connected to anode or cathode busbars of a cell. There may be local converters for each cell in the system. The local converters may be connected to a number of cells. 
         [0042]    In one embodiment of the invention, the electrolytic process is electrowinning or electrorefining. In one embodiment of the invention, high cathodic current density is used in the first cell in the process, where copper concentration is high and lower cathodic current densities are used in the second cell where copper concentration is lower. In one embodiment of the invention, there is used a separate voltage supply, such as a local converter, on every cathode that would make control of current density in each individual cell, each individual cathode, group of cells or sections of rectifiers much easier. 
         [0043]    In one embodiment of the invention an external resistance is used to control the distribution of current between at least two cells connected in parallel, in that way each cell would have a cathode with a different current density. 
         [0044]    In one embodiment of the invention, there is a power management system for a tank house for electrorefining and electrowinning. The system comprises a plurality of cathode and anode pairs arranged into at least one cell and a plurality of voltage supplies coupled to each of the cells. The plurality of voltage supplies are configured to supply voltage to said cells as a response to the properties of the electrolyte at each cell. The properties include, for example, copper concentration and acid concentration and the properties may vary within a tankhouse comprising a plurality of cells. 
         [0045]    In one embodiment the voltage supplies are local converters. Voltage supplies mentioned above are configured to supply each of the pairs individually or they may also be configured to supply a group of pairs or a portion of a cell. 
         [0046]    In one embodiment of the invention cells are liberator cells. In one embodiment of the invention the power management system further comprises an intermediate voltage supply configured to supply local converters. 
         [0047]    The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. A system or an apparatus to which the invention is related may comprise at least one of the embodiments of the invention described hereinbefore. 
         [0048]    It is to be understood that any of the above embodiments or modifications can be applied singly or in combination to the respective aspects to which they refer, unless they are explicitly stated as excluding alternatives. 
         [0049]    The benefit of the present invention is that it is possible to use the best possible current density in electrowinning and electrorefining when done in a liberator cell or similar cell in which it is not beneficial to maintain the same current density at each of the cells. For example, when the concentration of copper is low, high current densities cannot be used because of the risk of producing a copper powder deposit or arsine gas. The present invention achieves different current densities in different cells in the same tankhouse. A further benefit of the present invention is that it provides better control of electrowinning and electrorefining processes when the current density can be chosen such that it will provide best possible results at the given conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]    The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings: 
           [0051]      FIG. 1  is a block diagram of a system for electrolytic recovery of metals in prior art; 
           [0052]      FIG. 2  is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anodes having individual resistors, in one embodiment of the invention; 
           [0053]      FIG. 3A  illustrates a resistor in one embodiment of the invention; 
           [0054]      FIG. 3B  illustrates a parallel combination of a resistor and a transistor in one embodiment of the invention; 
           [0055]      FIG. 3C  illustrates a transistor in one embodiment of the invention; 
           [0056]      FIG. 4  is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anode busbar having a shared resistor, in one embodiment of the invention; 
           [0057]      FIG. 5  is a block diagram illustrating an alternative system for resistive control in a liberator arrangement, in one embodiment of the invention; 
           [0058]      FIG. 6  is a block diagram showing a system employing a central rectifier to produce two current paths, in one embodiment of the invention; 
           [0059]      FIG. 7  illustrates mounting of linear regulators on anode hanger bars, in one embodiment of the invention; 
           [0060]      FIG. 8  shows a method of connecting cell sections such that several current densities are provided, in one embodiment of the invention; 
           [0061]      FIG. 9  is a block diagram showing a system where different current densities are provided for upstream and downstream cells with respect to the electrolyte flow using serial connection of cells, in one embodiment of the invention; and 
           [0062]      FIG. 10  is a block diagram showing an alternative method of connecting the cells or cell sections in which the polarity of the current bars on either side of the cells are swapped over, in one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0063]    Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
         [0064]      FIG. 2  is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anodes having individual resistors, in one embodiment of the invention. 
         [0065]    In  FIG. 2  there is a power supply  240 , the negative terminal of which is connected to conductor  202  which is further connected to a cathode busbar  218  of a cell  210  and a cathode busbar  228  of cell  220 . Thus, cathode busbars  218  and  228  are connected in parallel. The positive terminal of power supply  240  is connected to conductor  204 , which is further connected to an anode busbar  219  of cell  210  and an anode busbar  229  of cell  220 . Thus, anode busbars  219  and  229  are connected in parallel. A potential difference or voltage is applied between conductor  202  and conductor  204 , such that there is a potential difference between resistor  217  and cathode busbar  218 . The same potential difference is applied, in parallel, between resistor  227  and cathode busbar  228 . In this way electrodes connected to resistor  217  and resistor  227  are held at an anodic potential. Cathode busbar  218  and cathode busbar  228  are held at a cathodic potential. Cell  210  comprises a number of cathodes such as a cathode  214  and a number of anodes such as an anode  216 . The number of anodes in cell  210  is one larger than the number of cathodes. The anodes and cathodes may be plates. Cell  210  contains electrolyte  212 , when the cell is used for electrolysis. Cell  220  comprises a number of cathodes such as a cathode  224  and a number of anodes such as an anode  226 . Cell  220  may contain electrolyte  222 . In cell  220  the number of anodes is one larger than the number of cathodes. The anodes and cathodes may be plates, for example, 1 meter by 1 meter plates, in cells  210  and  220 . Cathode busbar  218  is connected to cathodes in cell  210  such as cathode  214 . Cathode busbar  228  is connected to cathodes in cell  220  such as cathode  224 . Anode busbar  219  is connected to anodes in cell  210  such as anode  216  via resistors such as resistor  215  and resistor  217 . The resistance in resistors between anode busbar  219  and anodes in cell  210  may be low or the resistors are optional. Anode busbar  229  is connected to anodes in cell  220  such as anode  226  via resistors such as resistor  225  and resistor  227 . The resistances in resistors  215  and  217 , which are connected to the anodes at the ends of cell  210 , have a higher resistance compared to the resistance in other resistors between anode busbar  219  and the respective anodes. Similarly, resistances in resistors  225  and  227 , which are connected to the anodes at the ends of cell  220 , have a higher resistance compared to the resistance in other resistors between anode busbar  229  and the respective anodes. This is due to the fact that the current to anodes at the ends of cells  210  and  220  is lower since these anodes face only one cathode, on one side. The difference in the resistance between a resistor connected to an anode at a cell end and a resistor connected to an anode located between two cathodes is proportional to the difference in current caused by an anode plate facing only a single cathode plate instead of two cathode plates. A pipe  230  provides electrolyte to cell  210 . Pipe  232  provides the electrolyte to from cell  210  to cell  220 . The processed solution exits from pipe  234 . The arrows  231  and  233  indicate the direction of the electrolyte flow. As may be seen from  FIG. 2 , the solution flows in series through the cells. Power supply  240  may be a step down Direct Current (DC) to DC converter. Power supply  240  may comprise a direct current source  242 , an inductor  244 , a capacitor  246 , transistor Q 1  and Q 2 . Transistors Q 1  and Q 2  are acting synchronously in anti phase. Power supply  240  may incorporate a switching regulator in order to be highly efficient in the conversion of electrical power. The switching regulator may be a buck circuit. Power supply  240  may also be a central rectifier. 
         [0066]    Electrically, cell  210  and  220  are not connected in series but in parallel. In general, all anode busbars are connected to the positive terminal of the central rectifier, that is, power supply  240 . All the cathode busbars, namely, busbar  218  and  228  are connected to the negative terminal of central rectifier  240 . Cathodes of cell  220  operate at a lower current than those in the first cell  210  as a result of having a resistance connected in series with each cathode or anode. The current in the cathodes decreases progressively in the cell chain, for example, from 600 Amps in the first tank to 200 Amps in the last tank. 
         [0067]    In one embodiment of the invention, cell  210  and cell  220  are liberator cells. Cell  210  represents a first stage of a liberator cell circuit and cell  220  represents a second stage of a liberator cell circuit. 
         [0068]    In the first stage the electrolyte is distributed in cascade through at least cell  210 . There may also be at least one other cell in the first stage. In the second stage the electrolyte is distributed in cascade through at least cell  220 . There may also be at least one other cell in the second stage. By the use of two stages may be achieved a decrease in the copper concentration from an initial value of 40-60 g/dm3 in the feed solution down to 10-15 g/dm3. In the second stage copper is removed from 10-15 g/dm3 down to ca. 1 g/dm3. In the first stage copper may be removed from the solution as solid copper, which is deposited on the cathodes. The electrolyte is cascaded through the liberator (EW) cells, and an electrical current is applied. The current densities employed are set to be lower than in standard copper electrowinning or electrorefining. 
         [0069]    Thus, when the electrolyte is cascaded through a plurality of anode-cathode pairs the current density is preferably controlled in order to get the best possible result. 
         [0070]    As metallic copper is deposited on the cathode surface, the copper concentration in the electrolyte solution is depleted and the quality of the copper deposited at the cathode can decrease. 
         [0071]    In one embodiment of the invention, the resistors may be in series with the cathodes instead of the anodes. For example, so that each of cathodes in cell  220  are connected to cathode busbar  228  via their own resistors. 
         [0072]    The different currents are obtained by using different values of series resistor. For better current control, the resistor can be replaced by a resistor and transistor, typically a power MOSFET, in parallel with the resistor operating as a controlled resistor and providing fine control of the current. Alternatively the resistor can be replaced entirely by a transistor to give complete current control. These options are illustrated in  FIG. 3 .  FIG. 3A  shows a resistor alone.  FIG. 3B  shows a parallel combination of resistor and transistor (power MOSFET, Metal-Oxide-Semiconductor Field-Effect Transistor).  FIG. 3C  shows a transistor (power MOSFET) alone. 
         [0073]    In the embodiment of the invention, an external resistance is used to control the distribution of current between two or more cells connected in parallel. In that way each cell would have a cathode with a different current density. The concept is to use external resistances to divide the current from a single rectifier, such that different current densities can be obtained in different cells (or cell sections) in the process. The resistors in  FIG. 2  are just an example of means for providing desired current to each of the cells. A person skilled in the art understands that this may be provided also by using different means, such as local convertors. The external resistances would be of differing values and electrically connected before the anodes in the process in order to control the distribution of current between cells connected in parallel. The external resistance may also be adjustable. In that way each cell (or section of cells) would have cathodes with a current density which is a function of the external resistance. 
         [0074]    In a copper liberator cell house, wherein it is desired to remove as much copper as possible from the electrolyte solution, it should be possible to divide current from a single power supply such that a high current density (e.g. 300 A/m2) can be applied in the first cells where Cu concentration is high and a lower current density in the last cells where Cu concentration is low (e.g. 100 A/m2). In intermediate cells 200 A/m2 might be used. In this way it will be possible to gain good current efficiency in each set of cells, so that use of electrical power is optimized. 
         [0075]      FIG. 4  is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anode busbar having a shared resistor, in one embodiment of the invention. 
         [0076]    In  FIG. 4  there is a power supply  240  as disclosed in  FIG. 2 . The power supply provides a negative voltage to conductor  402  which is further in parallel connected to a cathode busbar  418  of a cell  410  and a cathode busbar  428  of cell  420 . Power supply  440  provides a positive voltage to conductor  404  which is further connected to an anode busbar  419  of cell  410  and an anode busbar  429  of cell  420 . Cell  410  comprises a number of cathodes such as a cathode  414  and a number of anodes such as an anode  416 . Cell  410  may contain electrolyte  412 . Cell  420  comprises a number of cathodes such as a cathode  424  and a number of anodes such as an anode  426 . Cell  420  may contain electrolyte  422 . In both cells the number of anodes is one larger than the number of cathodes. Cathode busbar  418  is connected to cathodes in cell  410  such as cathode  414 . Cathode busbar  428  is connected to cathodes in cell  420  such as cathode  424 . Anode busbar  419  is connected to anodes in cell  410  such as anode  416 . Anode busbar  429  is connected to anodes in cell  420  such as anode  426 . Anode busbar  429  is connected to conductor  404  via a resistor  427 . A pipe  430  provides electrolyte to cell  410 . Pipe  432  provides the electrolyte to from cell  410  to cell  420 . The processed solution exits from pipe  434 . As may be seen from  FIG. 4 , the solution flows in series through the cells. 
         [0077]      FIG. 5  illustrates an alternative method for incorporating resistive control in a liberator arrangement, in one embodiment of the invention. 
         [0078]    In the arrangements shown in  FIGS. 2 and 4 , all cathodes are in parallel and all anodes are in parallel. This requires a central rectifier of a low-voltage, high-current output. The voltage is approximately that of a single cell. When the number of cathodes and anodes is large, the magnitude of the rectifier current may be inconveniently large. It is then advantageous to use a series arrangement of cells so that the central rectifier voltage becomes larger and its current rating smaller for a given power output.  FIG. 5  illustrates such an arrangement. In  FIG. 5  there is a rectifier  540  and cells  510 ,  511 ,  512 ,  513 ,  514  and  515 . The rectifier has a positive terminal  204  and a negative terminal  202 . The cells have been formed from three larger tanks, such as a combination of cells  510  and  515  would have been. The cells may also be formed by dividing the tanks into two individual cells using barriers  540 ,  541  and  542 . However, cells  510  and  515  share anode busbar  520  and cathode busbar  521 . Similarly, cells  511  and  514  share anode busbar  522  and cathode busbar  523 . Similarly, cells  512  and  513  share anode busbar  524  and cathode busbar  525 . Cathode busbar  521  and anode busbar  522  is connected using conductor  550 , whereas cathode busbar  523  and anode busbar  524  is connected using conductor  551 . Thus, the barrier separated cell pairs are connected in series. The cell electrical current in cells  510  and  511  may be higher than in cells  512  and  513 . Respectively, the cell electrical current in cells  512  and  513  may be higher than in cells  514  and  515 . The flow of electrolyte is illustrated with arrows  530 ,  531 ,  532 ,  533 ,  534 ,  535  and  536 . Electrical current flows from positive terminal  204  of rectifier  540  to a negative terminal  202  of rectifier  540 . 
         [0079]    In cells  515 ,  514  and  513  resistors (not shown) are connected in series with the anodes or cathodes to regulate the flow of current through these cathodes. More accurate control over the current value is obtained by the use of resistors with transistors in parallel or by transistors alone as previously described. The resistor values are chosen so that the total current taken by each cell divides between the upper and lower sections of the cell in the desired ratio for each cell. The resistor values differ for each cell so that there is a gradation of current density experienced by the electrolyte as it flows through the series of cell sections. It will be appreciated that, if required, the cells can be separated electrically and not joined by an equalizer-type arrangement incorporating the cathode and anode bars. In that case a single resistor can be used for the upper and lower sections of the tank in a similar manner to that set out with respect to  FIGS. 2 and 4 . 
         [0080]    In one embodiment of the invention, each tank in  FIG. 5  is divided into two equal halves by a barrier such as barriers  540 ,  541  and  542 , which separate the electrolyte in the two halves in the tanks. The flow of electrolyte is illustrated with arrow  502 . However, the cathode busbars and anode busbars along the side of the cells are continuous. 
         [0081]    In one embodiment of the invention, instead of two cell sections, two separate cells can be used and the cathode bars and anode bars of these can be electrically connected. An equalizer bar type arrangement can be used to join cells in series. There will be more longitudinal current flow along the equalizer bars than is usual in prior art arrangements. Extra anodes will be required at the ends of cell sections. The electrolyte flows through the cells using one half of the cell, for example, the upper halves in  FIG. 5 , and flows in a contrary direction through the other cell half sections, for example, the lower halves in  FIG. 5 . Current flows from a rectifier positive terminal  204  to a rectifier negative terminal  202 . The cell voltages are additive. 
         [0082]      FIG. 6  shows a further embodiment of the invention in which a rectifier  640  is employed to produce two parallel current paths, in one embodiment of the invention. In place of rectifier  640  power source  240  of  FIG. 2  may also be used. 
         [0083]    In  FIG. 6  the first current path goes via cells  610 - 614  and the second current path via cells  620 - 624 . In neighbouring cells, such as cells  610  and  611 , the anode and cathode busbars are connected via an electrical conductor. The cells are divided in two equal sections as illustrated in  FIG. 6  by the separation of cells in first cells  610 - 614  and second cells  620 - 624 . The electrolyte flow is illustrated with arrows  650 ,  652  and  654 . In this case, however, the cathode bars and anode bars are not continuous along the length of two cell sections or two cells, such as cells  610  and  620 , but are also divided. The arrangement therefore may be described as a two series of half-length cells. Resistors  632 ,  634  and  636  are employed to produce an exchange of current between the first and the second current paths. As the concentration of the target metal ion (e.g. copper) in the electrolyte decreases in the upper series of cells  610 - 614 , the current in this path is decreased by diverting part of the current to the lower current path. Similarly, current is added to the lower current path where it meets electrolyte of with higher concentrations of the target metal ions. The cell sections or half cells can be connected by two busbars  660  and  662 . Busbar  662  connected the cathodes of cells  614  and  624  to negative terminal  202  of rectifier  640  while busbar  660  connected the anodes of cells  610  and  620  to the positive terminal  204  of rectifier  640 . There will be more longitudinal current flow than is usual when equaliser bars are used in prior art arrangements. Resistors, transistors or resistors in parallel with transistors may be used as the current diverters. Resistors  632 ,  634  and  636  could be a switched-mode converter in which case the losses could be less that when  632 ,  634  and  636  are resistors. 
         [0084]      FIG. 7  shows how transistors acting as current-mode linear regulators may be mounted on the anode or cathode hanger bars as an alternative to mounting current-controlling elements on the sides of the cells, in one embodiment of the invention. 
         [0085]    The use of on-board linear regulators is particularly applicable to the cell arrangement shown in  FIGS. 2 and 5 . Cathodes or anodes in which the current is to be regulated (shown shaded in  FIGS. 2 and 5 ) may be replaced by current regulated electrodes of the design illustrated in  FIG. 7  in which current passes between the hanger bar  713  and the electrode blade  714  via transistors  715 - 719  (typically power MOSFET transistors). These transistors operate in the linear regime to control current flow between the hanger bar and the electrode blade. 
         [0086]      FIG. 8  shows a method of connecting cell sections such that several current densities are provided as might be advantageous in a liberator EW process, in one embodiment of the invention. The numbers  1 - 30  in each cell in columns indicate the cathodes. The accompanying numbers by the side of numbers  1 - 30  in columns indicate the current in amperes flowing through each cathode. In this illustration, the series connection of cells and cell sections draws from the central rectifier positive terminal  808  a current of 6,000 Amps which is returned via the central rectifier negative terminal  809 . The electrical current paths between cells are illustrated with arrows  811 . The number of cathodes in each section is adjusted according to the current density to be employed in that section. Extra anodes will be required at the ends of the sections. Cells are divided into sections by dividers  810  as are the anode bars and cathode bars. Electrolyte flow  802  takes the electrolyte around these barriers. Anode bars and cathode bars are connected in sequence by busbars or cables. 
         [0087]      FIG. 9  shows a system where cells are separated into three stages, in one embodiment of the invention. In  FIG. 9  there is a rectifier  540 . Rectifier  540  provides a negative voltage via negative terminal  902  to a cathode bar  918  to which a cathode  914  is connected within a cell  910 . Rectifier  540  provides a positive voltage via positive terminal  904  to an anode bar  966  to which anodes are connected within a cell  960 . An anode  912  of cell  910  is connected to an anode bar  916 . Anode bar  916  is connected to a cathode bar  928  of cell  920 . Anodes in cell  920  are connected to anode bar  926 . Electrolyte flows from cell  910  to cell  960  via cell  920 , cell  930 ,  940  and cell  950 . The cells are electrically connected in series. A series connection of cells draws from a positive terminal  904  of rectifier  540  a current of, for example, 6000 Amps which is returned via the negative terminal  902  of rectifier  540 . The number of cathodes in each cell is adjusted according to the current density to be employed in that cell. 
         [0088]      FIG. 10  shows an alternative method of connecting the cells or cell sections in which the polarity of the current bars on either side of the cells are swapped over (anode bars are swapped with cathode bars) to make for easier and shorter connections, in one embodiment of the invention. 
         [0089]    The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. A system or an apparatus to which the invention is related may comprise at least one of the embodiments of the invention described hereinbefore. 
         [0090]    It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.