FI123559B

FI123559B - Power control system in cells for electrolytic recycling of a metal

Info

Publication number: FI123559B
Application number: FI20125143A
Authority: FI
Inventors: Duncan Grant; Michael H Barker; Henri Kalervo Virtanen
Original assignee: Outotec Oyj
Priority date: 2012-02-10
Filing date: 2012-02-10
Publication date: 2013-07-15
Also published as: CN104220646A; EP2812465A1; CA2860813A1; EA025799B1; FI20125143A; WO2013117805A1; AU2013217827A1; MX2014009506A; EA201491434A1; EP2812465A4; CL2014002109A1; AU2013217827B2; PE20141695A1; US20160010233A1; CA2860813C

Abstract

According to an aspect of the invention, the invention is a system for electro¬ lytic processing or recovery of a metal from an electrolyte solution (233). The system comprises electrolysis cells (210,220) and a rectifier (240). The cells comprise interleaved anodes (216) and cathodes (214).The anodes or the cathodes of a first cell have an elec¬ trical connection to a positive (204) or a negative terminal (202) of the recti¬ fier (240), respectively, via a first electrical path having a first re¬ sistance (215).The anodes or the cath¬ odes 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 (225). The second re¬ sistance is configured to be higher than the first resistance. The system further comprises a channel (232) for electro¬ lyte 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

TITLE OF THE INVENTION

SYSTEM FOR POWER CONTROL IN CELLS FOR ELECTROLYTIC RECOVERY OF A METAL

5 BACKGROUND OF THE INVENTION

Field of the invention:

The invention relates to the electrolytic processing of metals. Examples of electrolytic recovery of metals are electrorefining and electrowinning.

10 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.

15 BACKGROUND OF THE INVENTION

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 cur- 20 rent causes the metal to be deposited on the cathode.

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 25 cathode is made negative by application of an external co voltage, so that an electrical current passes through ° the electrolyte between the anodes and cathodes. For £ example, in the electrorefining of copper, the anode o is made of impure copper, the copper enters the elec-

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30 trolyte as the anode dissolves anodically giving coper per (II) ions (Cu2+(aq) ) historically referred to as ^ "cupric" ions. Typically, the electrolyte contains lo copper as copper sulfate with sulfuric acid as a ευρό porting electrolyte. The copper (II) ions are trans- c\i 35 ported through the electrolyte and reduced at the 2 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 5 example, nickel or arsenic.

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 elec-10 trolyte from the main production process. Elec trowinning (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 15 copper, nickel, gold, silver, cobalt, zinc, chromium and manganese.

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 20 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 25 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 $2 typical configuration a tankhouse comprises a plurali- o ^ ty of cells.

o 30 When using a plurality of cells in convened tional electrowinning, it is usual to have several x cells powered by the same rectifier, giving the same Q.

current density at each cathode, co

Liberator cells are similar to standard elects 35 trowinning cells. During the process copper is plated ° out on the cathodes and copper ion concentration in the electrolyte decreases.

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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 5 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.

10 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 15 also a risk of toxic arsine gas production at the cathode surface.

In terms of electrode materials, liberator EW cells are similar to standard electrowinning cells, the anodes are insoluble. For copper liberators the 20 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.

25 The cathodes in liberator cells are usually spent anodes from the electrorefining tankhouse, but can also be permanent cathodes with stainless steel co ^ blades. Older refineries may still use copper starter ^ sheet technology.

cp 30 Copper cathodes deposited in the liberator co cells which contain impurities (such as arsenic and g antimony) are returned to the smelter to be melted and

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cast into anodes for electrorefining, co ^ Decopperised electrolyte can be sent to the

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cm 35 electrorefining cells, or further processed, for exam- ^ pie, for nickel removal.

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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 de-5 sired to remove all copper together with harmful impurities from the electrolyte. This means that properties of the electrolyte change with time during the process .

Liberator cells are similar to normal elec-10 trolytic 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 15 as antimony are returned to the smelter to be melted and cast into anodes. Purified electrolyte is recycled to the electrolytic cells.

In a liberator tankhouse, the aim is to remove copper from the solution as a solid copper cath-20 ode 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.

25 Reference is now made to Figure 1 which il lustrates a system for electrolytic recovery of metals in prior art. In Figure 1 there is a voltage source

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^ 100 which provides a negative voltage to conductor 102 ^ which is further connected to a cathode busbar 118.

o 30 Voltage source 100 provides a positive voltage to con- co ductor 104 which is further connected to anode busbar g 129 of a cell 120. There are two electrolytic cells,

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namely a cell 110 and cell 120. Cell 110 comprises a co ^ cathode 114 and an anode 116. Cell 110 contains an LT3 c\j 35 electrolyte solution. Cell 120 comprises a cathode 124 ^ and an anode 126. Cell 120 contains an electrolyte so lution. Cathode busbar 118 is connected to cathodes in 5 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 5 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.

During the liberation process the electrolyte 10 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 15 electrolyte, since all the cells are connected elec trically in series and carry approximately the same current.

In prior art a cell contains a plurality of anodes, all connected electrically in parallel and a 20 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 25 1.7 to 2.8 Volts in the case of the electrowinning of copper, depending on the current density employed.

It is difficult to convert electrical power

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^ from mains voltage to a dc voltage of this magnitude ^ efficiently. For this reason it is common practice to o 30 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.

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By this means the voltage rating of the central dc co ^ current source, commonly called a rectifier, is ele in c\j 35 vated and high efficiency can be obtained.

^ The difficulty with this arrangement is that the same current density is used in all cells. Cells 6 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 5 current densities. Thus, a liberator tank house uses more electrical power than in an optimum case.

SUMMARY OF THE INVENTION:

According to an aspect of the invention, the 10 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 15 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 20 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 25 from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, m metal concentration in the first cell being higher δ than in the second cell.

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-A According to another aspect of the invention, o q 30 the invention is a system for electrolytic processing 00 of a metal comprising at least two electrolysis cells x £ for the metal and a rectifier, wherein a first cell co comprises a plurality of cathodes interleaved between

Vj lo a plurality of anodes and a second cell comprises a C\j 5 35 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 7 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 elec-5 trical 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 10 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.

15 In one embodiment of the invention, the con figuration 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 20 may be rectangular, for example, 1 meter by 1 meter. 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 25 the distances. By substantially parallel may be meant at most an angle of 10 degrees between plates.

In one embodiment of the invention, the at

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least two cells comprise the at least three anodes and o ^ the at least two interleaved cathodes, the cathodes 0 30 being interleaved between the anodes. Thus, between two anodes there is a cathode.

1 In one embodiment of the invention, the ana- odes of a first cell have an electrical connection to co ^ a positive terminal of the rectifier, respectively,

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c\j 35 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, respec 8 tively, via a second electrical path which has the second resistance.

In one embodiment of the invention, the cathodes of a first cell have an electrical connection to 5 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 10 second resistance.

In one embodiment of the invention, the first electrical path and the second electrical path comprise metal conductors.

In one embodiment of the invention, the first 15 electrical path consists of conducting material and the second electrical path comprises at least one resistor device in addition to at least one conductor.

In one embodiment of the invention, the first electrical path comprises conducting material and the 20 second electrical path comprises at least one resistor device .

In one embodiment of the invention, the se cond electrical path comprises a resistor and an anode or a cathode bar in series, the anode or the cathode 25 bar being connected to each anode or cathode, respectively, of the second cell.

In one embodiment of the invention, the se-

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^ cond electrical path comprises a resistor and an anode ^ bar in series, the anode bar being connected to each o 30 anode of the second cell.

m In one embodiment of the invention, the se- cond electrical path comprises a resistor and a cath- Q_ ode bar in series, the cathode bar being connected to co ^ each cathode of the second cell, m oj 35 In one embodiment of the invention, the se- ^ cond electrical path comprises an anode bar to which are connected electrical paths for each of the at 9 least three anodes of the second cell, the electrical paths for each of the at least three anodes having respective resistors.

In one embodiment of the invention, the se-5 cond electrical path comprises an anode or a cathode bar, respectively, of the first cell.

In one embodiment of the invention, the metal is copper.

In one embodiment of the invention, the first 10 cell and the second cell are liberator cells.

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 15 cell. There may be local converters for each cell in the system. The local converters may be connected to a number of cells.

In one embodiment of the invention, the electrolytic process is electrowinning or electrorefining.

2 0 In one embodiment of the invention, high ca thodic 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.

25 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

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^ current density m each individual cell, each individ- ^ ual cathode, group of cells or sections of rectifiers o 30 much easier.

co In one embodiment of the invention an exter- nal resistance is used to control the distribution of

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current between at least two cells connected in paral-co ^ lei, in that way each cell would have a cathode with a

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c\J 35 different current density.

° In one embodiment of the invention, there is a power management system for a tank house for elec- 10 trorefining 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 5 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 10 a plurality of cells.

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 15 or a portion of a cell.

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 20 converters.

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. 25 A system or an apparatus to which the invention is related may comprise at least one of the embodiments of the invention described hereinbefore, co ^ It is to be understood that any of the above ^ embodiments or modifications can be applied singly or o 30 in combination to the respective aspects to which they refer, unless they are explicitly stated as excluding ^ alternatives.

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The benefit of the present invention is that co ^ it is possible to use the best possible current densi- LT3 cvj 35 ty 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 11 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 5 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 10 best possible results at the given conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention 15 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:

Fig. 1 is a block diagram of a system for 20 electrolytic recovery of metals in prior art;

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 embodi-25 ment of the invention;

Fig. 3A illustrates a resistor in one embodi- ^ ment of the invention; co o Fig. 3B illustrates a parallel combination of

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d- a resistor and a transistor in one embodiment of the o ' 30 invention; o 00 Fig. 3C illustrates a transistor in one em- £ bodiment of the invention; co Fig. 4 is a block diagram of a system for electrolytic recovery of metals where anode and cath-

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£ 35 ode busbars for two cells are connected in parallel, 00 the anode busbar having a shared resistor, in one em bodiment of the invention; 12

Fig. 5 is a block diagram illustrating an alternative system for resistive control in a liberator arrangement, in one embodiment of the invention;

Fig. 6 is a block diagram showing a system 5 employing a central rectifier to produce two current paths, in one embodiment of the invention;

Fig. 7 illustrates mounting of linear regulators on anode hanger bars, in one embodiment of the invention; 10 Fig. 8 shows a method of connecting cell sec tions such that several current densities are provided, in one embodiment of the invention;

Fig. 9 is a block diagram showing a system where different current densities are provided for up-15 stream and downstream cells with respect to the electrolyte flow using serial connection of cells, in one embodiment of the invention; and

Fig 10 is a block diagram showing an alternative method of connecting the cells or cell sections 20 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

25 Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

o Figure 2 is a block diagram of a system for

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1 electrolytic recovery of metals where anode and cath- ° 30 ode busbars for two cells are connected in parallel, o 00 the anodes having individual resistors, in one embodi- £ ment of the invention.

In Figure 2 there is a power supply 240, the >- negative terminal of which is connected to conductor ^ 35 202 which is further connected to a cathode busbar 218 o ^ of a cell 210 and a cathode busbar 228 of cell 220.

Thus, cathode busbars 218 and 228 are connected in 13 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 5 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 re-10 sistor 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 15 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 20 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 25 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 con- o ^ nected to anodes in cell 210 such as anode 216 via re- 0 30 sistors such as resistor 215 and resistor 217. The re- ^ sistance in resistors between anode busbar 219 and an- 1 odes in cell 210 may be low or the resistors are op- Q_ tional. Anode busbar 229 is connected to anodes in co cell 220 such as anode 226 via resistors such as re- <N 35 sistor 225 and resistor 227. The resistances in resis- tors 215 and 217, which are connected to the anodes at the ends of cell 210, have a higher resistance com- 14 pared 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 5 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 10 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 15 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 20 Figure 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 Q1 and Q2. Transistors Q1 25 and Q2 are acting synchronously in anti phase. Power supply 240 may incorporate a switching regulator in order to be highly efficient in the conversion of co ^ electrical power. The switching regulator may be a ^ buck circuit. Power supply 240 may also be a central o 30 rectifier.

° Electrically, cell 210 and 220 are not con- nected in series but in parallel. In general, all an-

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ode busbars are connected to the positive terminal of co ^ the central rectifier, that is, power supply 240. All

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ru 35 the cathode busbars, namely, busbar 218 and 228 are S connected to the negative terminal of central rectifi er 240. Cathodes of cell 220 operate at a lower cur- 15 rent 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 5 600 Amps in the first tank to 200 Amps in the last tank.

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 10 represents a second stage of a liberator cell circuit. 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 15 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 20 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 ap-25 plied. The current densities employed are set to be lower than in standard copper electrowinning or electrorefining .

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£ Thus, when the electrolyte is cascaded ^ through a plurality of anode-cathode pairs the current o 30 density is preferably controlled in order to get the m best possible result.

As metallic copper is deposited on the cath-

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ode surface, the copper concentration in the electro-co ^ lyte solution is depleted and the quality of the cop-

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c\i 35 per deposited at the cathode can decrease.

° In one embodiment of the invention, the re sistors may be in series with the cathodes instead of 16 the anodes. For example, so that each of cathodes in cell 220 are connected to cathode busbar 228 via their own resistors.

The different currents are obtained by using 5 different values of series resistor. For better cur

rent 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. Al-10 ternatively the resistor can be replaced entirely by a transistor to give complete current control. These options are illustrated in Figure 3. Figure 3A shows a resistor alone. Figure 3B shows a parallel combination of resistor and transistor (power MOSFET, Metal-Oxide-15 Semiconductor Field-Effect Transistor). Figure 3C

shows a transistor (power MOSFET) alone.

In the embodiment of the invention, an external resistance is used to control the distribution of current between two or more cells connected in paral-20 lei. 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 25 the process. The resistors in Figure 2 are just an example of means for providing desired current to each of the cells. A person skilled in the art understands co ^ that this may be provided also by using different ^ means, such as local convertors. The external re- o 30 sistances would be of differing values and electrical- co iy connected before the anodes in the process in order to control the distribution of current between cells

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connected in parallel. The external resistance may al-co ^ so be adjustable. In that way each cell (or section of m c\J 35 cells) would have cathodes with a current density oj which is a function of the external resistance.

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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 5 high current density (e.g. 300A/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. 100A/m2). In intermediate cells 200 A/m2 might be used. In this way it will be 10 possible to gain good current efficiency in each set of cells, so that use of electrical power is optimized.

Figure 4 is a block diagram of a system for electrolytic recovery of metals where anode and cath-15 ode busbars for two cells are connected in parallel, the anode busbar having a shared resistor, in one embodiment of the invention.

In Figure 4 there is a power supply 240 as disclosed in Figure 2. The power supply provides a 20 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 25 410 and an anode busbar 429 of cell 420. Cell 410 com prises 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 o ^ of cathodes such as a cathode 424 and a number of an- o 30 odes such as an anode 426. Cell 420 may contain elec- ° trolyte 422. In both cells the number of anodes is one g larger than the number of cathodes. Cathode busbar 418 Q.

is connected to cathodes in cell 410 such as cathode co 414. Cathode busbar 428 is connected to cathodes in

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c\i 35 cell 420 such as cathode 424. Anode busbar 419 is con- ° nected to anodes in cell 410 such as anode 416. Anode busbar 429 is connected to anodes in cell 420 such as 18 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 ex-5 its from pipe 434. As may be seen from Figure 4, the solution flows in series through the cells.

Figure 5 illustrates an alternative method for incorporating resistive control in a liberator arrangement, in one embodiment of the invention.

10 In the arrangements shown in Figures 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 cath-15 odes 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. 20 Figure 5 illustrates such an arrangement. In Figure 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 25 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, co l- cells 510 and 515 share anode busbar 520 and cathode o ^ busbar 521. Similarly, cells 511 and 514 share anode o 30 busbar 522 and cathode busbar 523. Similarly, cells co 512 and 513 share anode busbar 524 and cathode busbar 525. Cathode busbar 521 and anode busbar 522 is con- Q_ nected using conductor 550, whereas cathode busbar 523 co ^ and anode busbar 524 is connected using conductor 551.

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c\J 35 Thus, the barrier separated cell pairs are connected

Si in series. The cell electrical current in cells 510 and 511 may be higher than in cells 512 and 513. Re- 19 spectively, 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 5 flows from positive terminal 204 of rectifier 540 to a negative terminal 202 of rectifier 540.

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 10 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 15 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 appre-20 dated 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 25 that set out with respect to Figures 2 and 4.

In one embodiment of the invention, each tank in Figure 5 is divided into two equal halves by a bar-

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T- rier such as barriers 540, 541 and 542, which separate o ^ the electrolyte in the two halves in the tanks. The o 30 flow of electrolyte is illustrated with arrow 502.

P3 However, the cathode busbars and anode busbars along the side of the cells are continuous.

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In one embodiment of the invention, instead co ^ of two cell sections, two separate cells can be used

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c\i 35 and the cathode bars and anode bars of these can be ^ electrically connected. An equalizer bar type arrange ment can be used to join cells in series. There will 20 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 5 of the cell, for example, the upper halves in Figure 5, and flows in a contrary direction through the other cell half sections, for example, the lower halves in Figure 5. Current flows from a rectifier positive terminal 204 to a rectifier negative terminal 202. The 10 cell voltages are additive.

Figure 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 15 240 of Figure 2 may also be used.

In Figure 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 20 via an electrical conductor. The cells are divided in two equal sections as illustrated in Figure 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, 25 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 divid-

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T- ed. The arrangement therefore may be described as a ^ two series of half-length cells. Resistors 632, 634 o 30 and 636 are employed to produce an exchange of current cTj between the first and the second current paths. As the ^ concentration of the target metal ion (e.g. copper) in

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the electrolyte decreases in the upper series of cells co ^ 610 - 614, the current in this path is decreased by

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c\j 35 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 concen- 21 trations 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 5 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 10 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.

Figure 7 shows how transistors acting as cur-15 rent-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.

The use of on-board linear regulators is par-20 ticularly applicable to the cell arrangement shown in Figures 2 and 5. Cathodes or anodes in which the current is to be regulated (shown shaded in Figures 2 and 5) may be replaced by current regulated electrodes of the design illustrated in Figure 7 in which current 25 passes between the hanger bar 713 and the electrode blade 714 via transistors 715 - 719 (typically power MOSFET transistors). These transistors operate in the co I- linear regime to control current flow between the o hanger bar and the electrode blade, cp 30 Figure 8 shows a method of connecting cell sections such that several current densities are pro-vided as might be advantageous in a liberator EW pro- Q.

cess, in one embodiment of the invention. The numbers co ^ 1-30 in each cell in columns indicate the cathodes.

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cm 35 The accompanying numbers by the side of numbers 1-30 S in columns indicate the current in amperes flowing through each cathode. In this illustration, the series 22 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 5 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 10 810 as are the anode bars and cathode bars. Electro lyte flow 802 takes the electrolyte around these barriers. Anode bars and cathode bars are connected in sequence by busbars or cables.

Figure 9 shows a system where cells are sepa-15 rated into three stages, in one embodiment of the invention. In Figure 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 20 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 25 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

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T- connection of cells draws from a positive terminal 904 o ^ of rectifier 540 a current of, for example, 6000 Amps o 30 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 em- Q.

ployed in that cell, co ^ Figure 10 shows an alternative method of con- LT3 c\J 35 necting the cells or cell sections in which the polar- ° ity of the current bars on either side of the cells are swapped over (anode bars are swapped with cathode 23 bars) to make for easier and shorter connections, in one embodiment of the invention.

The embodiments of the invention described hereinbefore may be used in any combination with each 5 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.

10 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 15 may vary within the scope of the claims.

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Claims

A system for electrolytically treating metal (212, 222) comprising at least two electrolytic cells (210, 220, 410, 420) for metal and a rectifier (240), wherein at least two cells comprise at least three anodes (216) and at least two cathode (214) arranged alternately with the system, characterized in that: the anodes (216, 416) or the cathodes (214, 414) of the first cell (210, 410) have an electrical connection respectively to positive or negative of the rectifier (240, 440). a hub (202, 204, 402, 404) via a first electrical path that has a first resistance (215, 217, 419) on the first electric path 15; the anodes (226, 426) or cathodes (224, 424) of the second cell (220, 440) having an electrical connection to the positive (204,404) 20 or negative terminal (202,402) of the rectifier (240, 440), respectively, via a second power path has another resistance (227,427); the second resistance (227, 427) is arranged greater than the first resistance (217, 419); and the system further comprises a passageway (232) for the electrolyte from the first cell (210, 410) to the second £ 2 cell (220, 420), wherein the electrolyte contains a metal in dissolved ionic form with a first metal concentration (210, 410) 30 more than in the other cell (220, 420).

The system of claim 1, wherein the first electric path and the second electric path comprise metal conductors (219, S 229). ^ 35

The system of claim 2, wherein the second power path comprises a resistor (427) and an anode (426) or cathode rail (419,429) in series, the anode (426) or cathode rail (419,429) being coupled to a second cell (220, 420), respectively. to each anode (426) or cathode (424).

The system of claim 2, wherein the second power path comprises an anode bus (229) coupled to the electrical path of each of at least three anodes (226) of the second cell (220), each of a path of at least three anodes (226). comprising corresponding resistors (225, 227).

The system of claim 2, wherein the second electricity path comprises a first cell anode (218,219, 418, 419) or cathode rail (228, 428), respectively.

The system of claim 1, wherein the metal is copper.

The system of claim 1, wherein the first cell (210, 410) and the second cell (220, 420) are release cells (Liberator cell).

The system of claim 1, wherein the system further comprises an intermediate voltage supply arranged to supply voltage to local transformers.

The system of claim 1, wherein the electrolytic process is electrolytic metal recovery or electrolytic purification. o

A system for treating metal 4 electrolytically, comprising at least two electrolytic cells (910, 920, 930, 940, 950, 960) for metal and a rectifier (540), wherein the first cell (910) comprises a plurality of cathodes (914). , 918) disposed between a plurality of anodes (912, 916) and a second cell (940, 950, 960) comprising a plurality of cathodes arranged between a plurality of anodes, which the system being characterized in that: the anodes (912, 916) of the first cell (910) have an electrical connection to the positive terminal (904) of the rectifier (540) via a first electrical path (916) having a first resistance; the anodes of the second cell (940, 950, 960) having an electrical connection to the positive terminal (904) of the rectifier via a second electrical path (966) having a second resistance on the second electrical path; The number of anodes and cathodes (940, 950, 960) in the second cell (940, 950, 960) is arranged larger than the number (910) of the anodes (912, 916) and cathodes (914, 918) in the first cell (910) and a second resistance to reduce the difference; and the system further comprises a passage (920, 930, 932) for the electrolyte from the first cell (910) to the second cell (940, 950, 960), wherein the electrolyte contains the metal in dissolved ionic form, the first cell (910) (940, 950, 960).

The system of claim 10, wherein the metal is copper.

The system of claim 10, wherein the first cell (910) and the second cell (940, 950, 960) are release cells. η

The system of claim 10, wherein the system further comprises an intermediate voltage supply 4 arranged to supply a voltage cp q 30 to the local transformers.

The system of claim 10, wherein the electrolytic process is electrolytic recovery or electrolytic purification of the metal co. (M δ (M