Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C1/00—Electrolytic production, recovery or refining of metals by electrolysis of solutions
  • C25C1/12—Electrolytic production, recovery or refining of metals by electrolysis of solutions of copper
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/06—Operating or servicing

Definitions

• the invention pertains to a new method for copper electrorefining using electrical potential control, which has application in the copper industry.
• anode made of impure copper obtained during a fire refining process or from other sources such as recycling, scrap etc. is subjected to electrorefining.
• the anodes are hung in electrolytic tanks filled with an electrolyte consisting of copper ions, sulphuric acid, organic additives and chloride ions.
• An electrolyte composition is presented in Table 1 .
• Concrete tanks covered with lead, as well as newer ones made of resin concrete reinforced with glass fiber bars, are used to contain the electrolyte.
• the resin ones are resistant to sulphuric acid. They are also dielectrics and good heat insulators.
• Cathodic "pads" in the form of sheets are hung between the anodes and connected to the current source. There are from thirty to sixty pairs of anodes and cathodes connected in parallel in each tank.
• a continuous, laminar electrolyte flow through the tanks (about 0.02 m 3 /min) at constant temperature and flow pressure is the condition required to conduct a proper electrorefining process.
• the electrolyte flow speed is usually in the range from 0.01 -0.03 m 3 /min which enables a full electrolyte replacement every 4 to 6 hours.
• acid-resistant pumps, heaters, polyethylene tissue covering the tanks In order to do this specialised equipment is used: acid-resistant pumps, heaters, polyethylene tissue covering the tanks. Maintaining an appropriately high temperature (60- 65°C) is also highly significant in the electrorefining process.
• ions of such impurities as As, Bi, Co, Fe, Ni and Sb constantly dissolve into the solution from the anode.
• the concentration of these elements in the post-refining electrolyte should not exceed the following values: As - 20 g/dm 3 , Bi - 0.6 g/ dm 3 , Fe - 2 g/dm 3 , Ni - 25 g/dm 3 and Sb - 0.7 g/dm 3 .
• impure refining electrolyte should be removed and replaced with sulphuric acid.
• the ISA SYSTEM was introduced at a number of locations (Townsville - Australia, Copper Range Co.
• Cathode current density is the most important economic parameter of the copper electrorefining process.
• Most research work devoted to electrorefining processes concerns the improvement of cathodic deposition quality and purity. It concentrates especially on how to avoid the formation of dendrites on the cathode which may cause short-circuits between the anode and cathode thereby preserving the highest possible cathode current density. Research on how to avoid passivation and corrosion pits has also been undertaken. For reasons of economy, copper electrorefining processes should proceed at the highest current density whilst maintaining an appropriate (fine-crystalline) structure and chemical composition of the cathode.
• a method of industrial copper electrorefining comprising, arranging at least one anode of copper material to be refined in contact with an electrolyte solution; arranging at least one cathode in contact with the electrolyte solution; electrically connecting the anode and cathode to an electrical source, and operating the electrical source under electrical potential controlled conditions such that during at least part of the application of the said conditions, the electrical potential at the cathode is -0.30 V to -0.55 V with respect to the copper material at the anode, thereby causing the deposition of electrorefined copper at the cathode.
• an industrial copper electrorefining system comprising:
• At least one first electrode formed from copper material to be refined and being positioned within use in contact with the industrial electrolyte with the container; at least one second electrode positioned within use in contact within the industrial electrolyte within the container; and,
• a power supply operable under electrical potential controlled conditions and connected electrically when in use to each of the said at least one first and at least one second electrodes, such that during at least part of the application of the said conditions, the electrical potential at the at least one second electrode is -0.30 V to -0.55 V with respect to the copper material at the at least one first electrode, thereby causing the deposition of electrorefined copper at the at least one second electrode.
• the apparatus according to the second aspect is adapted to perform the method according to the first aspect of the invention.
• CFC complex form current
• CFP complex form potential
• the electrical potential controlled conditions include the application of complex form potential.
• the electrode potential When current is controlled the electrode potential cannot be controlled but changes with time (and at points in space such as at the electrode/electrolyte interface) according to the particular electrochemical processes mechanism and kinetics (e.g. charge transfer, chemical reactions of electroactive species, diffusion of electroactive species).
• the electrical potential When the electrical potential is applied in a controlled manner the current is not controlled but changes with time according to the electrochemical processes mechanism and kinetics.
• Such electrical potential is applied by a power supply which ensures that the potential applied is substantially independent of the current drawn from the power supply (within normal operational limits of the apparatus).
• the rate of the industrial copper electrorefining is an activation (charge transfer) controlled process.
• the activation control is often cited as a required condition for the industrial electrorefining.
• the reason for using such low cathodic current densities in the galvanostatic (or more general current control) industrial electrorefining as cited above is that in the currently used refineries the increase in current density results in creation of nodular and dendritic structures at the cathode and finally, at current densities close to the limiting current densities, a copper powder is produced.
• potential controlled electrorefining allows the application of more negative cathodic potentials than are found in known refineries (under current control). Whilst the potentials applied may lie in the range of -0.30V to -0.55V, preferably the range used is - 0.35V to -0.55V, more preferably -0.40V to -0.55 V. In contrast, present refineries use potentials of around -0.3V.
• the studies have been carried out on the copper cathode, with the anode formed from a titanium net covered with platinum.
• the results of the studies have shown that copper can be deposited in the electrolytic process in the form of impure cathodes.
• the main impurity of copper deposited on the cathode is arsenic which reacts with copper and creates copper arsenide as well as bismuth and antimony.
• the process parameters used in implementing the invention are very close to those currently used in industrial electrorefining, especially the same basic substrates i.e. electrolytes and anodes are used in a new potential-controlled electrorefining process.
• the advantage of the new process is in that by controlling the cathode potential the limiting current of the process can be reached and, according to the above given exemplary limiting current densities, the cathodic current density can be approximately 3 to 5 times higher than in the current controlled (e.g. galvanostatic) electrorefining process.
• the cathodic potential is very precisely controlled in the electrorefining process.
• a very high overpotential and complexity of the anodic processes make potential control in the electrowinning process much more difficult to implement on an industrial scale.
• a constant electrical potential may be applied during the electrorefining, it is also contemplated that one or more of the magnitude and polarity of the electrical potential are modulated. Such modulation provides control over the resultant structure of the deposited copper.
• the electrical potential may be modulated as a rectangular waveform having a magnitude of the electrical potential at the cathode of between -0.30 V and -0.55 V.
• potentiostatic pulse electrolysis (PPE) conditions may be applied in which, for example, a number of cathodic pulses in the range 3 to 300 are applied, each having a substantially constant potential in the range -0.30V to -0.55V with reference to the copper material at the anode, and each having a duration of between 5 and 18000 seconds, wherein the pulses are separated in time by open circuit breaks, each having a duration in the range 0.1 to 100 seconds.
• periodic potential reversal (PPR) conditions are applied in which a cathodic pulse having a potential in the range -0.30 V to - 0.55 V, with reference to copper material anode is applied for a duration in the range 5 to 18 000 seconds, the cathodic pulse being followed by an anodic pulse in the range of +0.05 V to +0.60 V, with reference to the copper material anode, whereby the duration of the anodic pulse is shorter than the cathodic pulse by at least 50% and wherein the sequence formed from the cathodic pulse and anodic pulse is repeated from 3 to 30 times.
• multiple pulses may be applied before any later potential reversal during any particular sequence.
• PPR periodic potential reversal
• a cathodic pulse having a potential in the range -0.30 V to -0.55 V, with reference to copper material anode is applied for a duration in the range 5 to 18 000 seconds, the cathodic pulse being followed by an anodic pulse in the range of +0.05 V to +0.60 V, with reference to the copper material anode, whereby the duration of the anodic pulse is shorter than the cathodic pulse and wherein open circuit conditions are applied for a period between the cathodic and anodic pulses and the sequence formed from the cathodic pulse and anodic pulse is repeated from 3 to 30 times.
• the said open circuit conditions are applied twice during the sequence as the potential is reversed, that is between a transition from cathodic to anodic conditions and from anodic to cathodic conditions.
• the electrolyte used in the electrorefining process typically comprises 90 g/dm 3 to 200 g/dm 3 H 2 S0 4 and 1 g/dm 3 to 50 g/dm 3 Cu as well as other typical components of such solutions.
• a very important advantage of the potential controlled process is the possibility of carrying out the electrorefining at a very wide range of copper ion concentrations, including less than 40 g/dm 3 .
• current industrial processes require copper (II) ion concentrations of not less than around 40 g/dm 3 . It is important to note that a cathode potential controlled copper electrorefining process enables the best exploitation of natural convection.
• the preferred arrangement of the electrodes is such that their spatial separation is 5cm or less in an industrial cell.
• the electrodes are provided as substantially planar structures (such as sheets) arranged in parallel with the above given separation distance.
• the process of potentiostatic copper electrorefining is carried out at temperatures ranging from 18°C to 65°C, advantageously from 18°C to 30°C. Therefore there is no need to heat the electrolyte additionally as in the currently used methods. This is another important advantage since current electrorefining technology does not permit the use of the process at temperatures lower than approximately 50°C.
• the new potentiostatic process can be carried out in industrial electrolytes at temperatures as low as 20°C with cathodic current densities comparable with the present industrial electrorefining process at 60°C. Consequently, the new potential controlled electrorefining can be carried out using simplified installations and with huge energy savings in comparison with current processes.
• the process of potentiostatic electrorefining is conducted using a cathode made of stainless steel or copper.
• the copper material of the anode may be formed from a fire-refined, scrap or recycled copper material.
• An electrolyte management system may perform one or more of filtering, removing impurities, adding other agents (such as sulphuric acid), agitating/circulating/stirring and temperature control of the electrolyte.
• the present invention has a great advantage over the above-described prior art methods because the method of cathode potential controlled copper electrorefining allows the achievement of significantly higher cathode current densities (increasing the production volume) while maintaining high (commercial level) copper purity and a fine- crystalline structure.
• the process of cathode potential controlled electrorefining according to the invention has a number of advantageous characteristics, including:
• the electrolyte may have a similar (although not identical) ionic composition as used currently in the galvanostatic process;
• the process can be carried out at high current densities up to about 2000 A m 2 at the temperature of 60°C and 500 A m 2 at room temperature (about 20°C).
• current densities of such magnitude in a galvanostatic electrorefining process causes a drastic deterioration in cathode copper quality
• the purity of cathodic copper obtained in the process of potential controlled electrolysis may be greater than 99.99%; - the "current efficiency" of the potentiostatic electrorefining process may be higher than 97%.
• Figure 1 is a schematic representation of apparatus used in association with the examples;
• Figure 2 is a flow diagram providing a general overview of the method;
• Figure 3a shows a potential pulse electrolysis waveform having pulses each of which have a constant magnitude
• Figure 3b shows potential pulse electrolysis waveforms having pulses of constant magnitude other than an initial pulse
• Figure 3c shows an applied potential waveform including periodic potential reversal
• Figure 3d shows an applied potential waveform including periodic potential reversal and intervening periods of open circuit conditions.
• FIG. 1 A schematic view of industrial apparatus suitable for performing the present invention is illustrated in Figure 1 .
• a tank 1 is provided, for simplicity this being illustrated as a single container.
• this is formed from a number of individual cells formed from a polymer material which exhibits good long term resistance to the electrolyte.
• the electrolyte is illustrated at 2 and has a composition described in more detail in association with the examples below.
• First electrodes 3 (shown as solid lines) are provided, formed from copper material to be refined and are arranged to form the anodes within the cells. These take the form of planar sheets and are spaced at regular intervals, hanging vertically within the electrolyte 2.
• Second electrodes 4 are provided taking a similar form to the first electrodes, again hung vertically, although in this case being formed from either previously electrorefined copper or stainless steel.
• the second electrodes form the cathodes within each cell and are positioned equally spaced between the anodes, for example at a distance of a few centimetres from the anodes.
• the anodes and adjacent cathodes may be thought of as "pairs" for gaining an understanding of the apparatus.
• a potential controlled power supply 5 is provided to drive the electrorefining process.
• Each anode is connected electrically to the power supply via a supply line 6; similarly each cathode is also connected electrically by a supply line 7.
• An electrolyte system 8 is illustrated.
• This performs a number of functions including filtering the electrolyte, controlling its composition (by the addition and removal of impurities/agents), maintaining the electrolyte at a predetermined temperature and ensuring the circulation of the electrolyte within the cells.
• the apparatus is controlled by a controller 9 which is in communication with the electrolyte system 8 and power supply 5.
• FIG. 2 illustrates a general overview of the process.
• the anodes 3 are manufactured from a material which it is desired to be refined.
• cathodes 4 are obtained (these may have been used in a previous electrorefining cycle).
• the anodes and cathodes are arranged in their cells within the tank 1 and are connected electrically to the power supply 5.
• the electrolyte 2 is then introduced into the tank and the electrolyte system 8 is operated by the controller 9 so as to establish a flow of electrolyte within the cells at the appropriate temperature, which may be room temperature.
• the controller 9 operates the power supply 5 so as to deliver electrical potential controlled conditions. Monitoring of the process conditions (including the current and potential in each cell) is performed throughout the process by the controller 9.
• step 600 This may involve the application of a constant potential, although optionally a pulsed electrorefining and/or periodic potential reversal may be applied (to be described in association with the examples below).
• a pulsed electrorefining and/or periodic potential reversal may be applied (to be described in association with the examples below).
• This process continues for an extensive period (which may be hours or days) until a sufficient amount of anode material has been refined.
• step 700 the electrical power supply is terminated, the eroded anodes are removed (unless they contain sufficient material for reuse) and the cathodes (containing the refined copper) are washed.
• step 800 the cleaned cathodes are then subjected to mechanical removal of the high purity copper which has been deposited.
• a pair of electrodes is provided in an electrochemical tank made from polyvinylchloride.
• the cathode is made from stainless steel sheet the thickness of which is 0.1 mm and 2 cm 2 of surface area.
• the anode (reference electrode) is made from 0.25 mm thick copper sheet, the surface of which has an area of 100 cm 2 .
• the process is conducted at room temperature (about 20°C).
• the tank is filled with an electrolyte of the following composition: 46 g/dm 3 Cu, 180 g/dm 3 H 2 S0 4 and 0.1 g/dm 3 Fe, 0.3 g/dm 3 Sb, 0.03 g/dm 3 Bi, 5 g/dm 3 Ni, 10 g/dm 3 As, 0.00015 g/dm 3 Ag, 0.001 g/dm 3 Ba, 0.4 g/dm 3 Ca, 0.001 g/dm 3 Cd, 0.03 g/dm 3 Co, 0.02 g/dm 3 Mg, 0.0004 g/dm 3 Mn, 0.007 g/dm 3 Pb and 0.001 g/dm 3 Pd.
• an electrolyte of the following composition: 46 g/dm 3 Cu, 180 g/dm 3 H 2 S0 4 and 0.1 g/dm 3 Fe, 0.3 g/dm 3 Sb, 0.03 g/dm 3 Bi, 5
• the electrolyte composition resembles a typical industrial electrorefining electrolyte as for example used in a prior art copper electrorefining process at the KGHM PM copper works (discussed earlier).
• organic additives are not included within the electrolyte.
• the usual additives such as thiourea and animal glue undergo hydrolysis, so after only a few days only their hydrolysis products are present in the solution.
• the new method should be tested in the electrolyte containing non-electroactive components since their presence influences the rate of the mass transport of copper (II) ions to the cathode and consequently the value of the limiting current.
• Electrolysis time t 1 h
• Stationary current density of approximately 300 A/m 2 at the cathode is achieved after a constant potential of -0.300 V is applied to the electrodes for about 25 seconds.
• the cathode deposit After having deposited copper on the stainless steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using an EDS/EDX method. It is found that the obtained cathode deposit has a fine crystalline structure without dendrites. Oxygen makes up about 0.05% of the weight and is the only impurity present in the obtained cathode copper. So the obtained cathode copper has a purity higher than 99.95%. Having compared the deposited copper mass with the theoretical mass of copper that should be deposited (using Faraday's law), it is found that the current efficiency of the process is higher than 97%. This examples uses a similar potential magnitude as found in many known industrial (current controlled) prior art refining processes.
• Example 2 In this second example the experimental set-up and electrolysis conditions are similar to those in Example 1 except that a different cathode potential is used, causing a higher current.
• Electrolysis time t 1 h
• Electrolysis time t 1 h
• Stationary current density of approximately 1400 A m 2 at the cathode is achieved after constant potential -0.450 V had been applied to the electrodes for about 25 seconds.
• the cathode deposit After having deposited copper on the steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using an EDS/EDX method. It is found that the obtained cathode deposit has a fine-crystalline structure without dendrites. Oxygen makes up about 0.05% of weight and is the only/sole impurity present in the obtained cathode copper. So the obtained cathode copper has the purity higher than 99.95%. Having compared the deposited copper mass with the theoretical mass of copper that is deposited using Faraday's law, it is found that the current efficiency of the process is higher than 97%.
• Example 3 The experimental set-up and electrolysis conditions are the same as in Example 3 (including a process temperature of 60°C) although here the solution is stirred with a frequency of 50 rotations per minute. A shorter electrolysis period is used also.
• Stationary current density of approximately 1600 A m 2 at the cathode is achieved after constant potential -0.450 V has been applied to the electrodes for about 25 seconds.
• the cathode deposit After having deposited copper on the stainless steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using an EDS/EDX method. It is found that the obtained cathode deposit has fine-crystalline structure without dendrites. Oxygen makes up about 0.05% of the weight and is the only/sole impurity present in the obtained cathode copper. So the obtained cathode copper has a purity higher than 99.95%. Having compared the deposited copper mass with the theoretical mass of copper that should have been deposited (calculated using Faraday's law), it is found that the current efficiency of the process is higher than 97%. Thus it is observed that the agitation of the electrolyte using stirring allows even higher current densities to be achieved under potential controlled conditions than in Example 3.
• Example 5 In this case the physical experimental arrangement is modified in comparison with the earlier examples to more closely represent an industrial refinery.
• the cathodes are made of stainless steel sheet the thickness of which is 0.3 mm and cathode surface area is 0. 2 m 2
• the anode (reference electrode) is made of 0.25 mm thick copper sheet, the surface of which is 0.22 m 2 .
• the distances between the cathodes and each of the anodes is 5 cm.
• the new potential control electrorefining method should be tested in different geometries since according to the theory macro-geometry of the electrolytic cell may influence considerably the limiting current established in natural convection conditions.
• the process is conducted at room temperature (about 20°C).
• the vessel is filled with an electrolyte of the same composition as presented in Example 1 although this is diluted 2.6 times with sulphuric acid of concentration of 180 g/dm 3 . Consequently, each of the electrolyte's component concentrations given in Example 1 , except of H 2 S0 4 , should be divided by 2.6 and so, for instance, the copper concentration is equal to 17.5 g/dm 3 .
• the electrodes are connected with the aid of a special cable to the commercially available rectifier which can be used to programme the duration of the potentiostatic electrolysis process from 1 minute to several days and which enables to conduct the studies at the current of up to 500 A flowing between the rectifier and the electrodes. The current changes depending on the duration of the electrolysis are measured during the process. The solution is not stirred in this example. Parameters of potentiostatic electrolysis:
• Stationary current density of approximately 100 A m 2 at the cathode is achieved after a constant potential -0.350 V has been applied to the electrodes for about 25 seconds.
• the cathode deposit After having deposited copper on the stainless steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using EDS/EDX and ASTM copper elemental analysis methods. According to ASTM copper elemental analysis, the copper deposited copper has a purity >99.999%. The refined material has a smooth surface without nodules and dendrites.
• the obtained cathode deposit has a fine-crystalline structure. Having compared the deposited copper mass as well as the theoretical mass of copper that should have been deposited using Faraday's law, it is found that the current efficiency of the process is higher than 96%.
• the experimental set-up and electrolysis conditions are the same as in Example 5 except that one cathode and 2 anodes are used.
• the anodes are placed at an equal distance of 25 cm from each side of the cathode.
• Example 6 The experimental set-up and electrolysis conditions are the same as in Example 6 except that, instead of a stainless steel cathode, a copper cathode made of 0.25 mm thick copper sheet is used, this having a surface of 0.22 m 2 . Again, anodes are used. The anodes are placed, equally spaced from the cathode at distances of 5 cm on each side of the cathode. Additionally, the electrolyte composition is the same as in Example 1 except the copper content is equal to 41 g/dm 3 .
• Copper sheet cathode potential with reference to copper anode E -0.550V;
• the cathode deposit After having deposited copper on the copper cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using EDS/EDX and XRD methods. According to the EDS/EDX and XRD analysis the deposited copper has a purity >99.95%. Again it is found that the deposited material has a smooth surface without nodules and dendrites. Having compared the deposited copper mass with the theoretical mass of copper that should have been deposited using Faraday's law, it is found that the current efficiency of the process is higher than 99%.
• PCR periodic current reversal
• PCR is being employed in at least 1 1 copper refineries (under current controlled conditions) to increase the rate of cathode production by increasing the applied current density.
• PCR is a method by which a forward current is applied for a length of time followed by a quick current reversal.
• the forward to reverse period ratio is typically between 20/1 to 30/1.
• W.G. Davenport, M. King & M. Schlesinger entitled Extractive Metallurgy of Copper p.
• the method may therefore comprise a process of potentiostatic pulse electrolysis (PPE) or periodic potential reversal (PPR) copper deposition or a combination of PPR and PPE.
• PPE potentiostatic pulse electrolysis
• PPR periodic potential reversal
• Examples of potential pulse electrolysis (PPE) and periodic potential reversal (PPR) pulses applied to the cathode are presented in Figures 1 a to 1 d where: E c is cathode potential, t c is the length of cathodic pulse, E a is the reverse pulse (anodic) potential applied to the cathode, t a is the length of the potential reverse pulse (anodic) applied to the cathode.
• Advantageous implementations of PPE and PPR potentiostatic electrolysis processes are illustrated in Figure 3a) to 3d) in which:
• Fig. 3a shows a PPE process with cathodic potential pulses £ k in the range from -0.3V to -0.55V, in reference to the copper electrode, with a duration time f k from 5 s to 18 000 s, and potential breaks between pulses (open circuit) with a duration time from 0.1 s to 100 s.
• the number of potential pulses and potential breaks is from 3 to 30.
• Fig. 3b shows a PPE process with different values of cathodic potential pulses E c in the range from -0.3V to -0.55V, in reference to copper electrode, with a duration time t c from 5 s to 18 000 s, and potential breaks (open circuit) between pulses from 0.1 s to 100s.
• the numbers of potential pulses and potential breaks is from 3 to 30.
• Fig. 3c shows a PPR process with the cathodic pulses in cathodic potential £ c in the range from -0.3 V to -0.55 V, in reference to copper electrode, with a duration time t c from 5 s to 18 000 s, and then the anodic pulses in anodic potential E a i in the range from +0.050 V to +0.6 V, in reference to copper electrode, with duration time f a i at least 50% shorter than time t c .
• the number of potential pulses and potential breaks is from 3 to 30.
• Fig. 3d shows a combination of PPE and PPR processes with the cathodic potential pulses E c in the range from -0.3 V to -0.55 V, in reference to copper electrode, with duration time t c from 5s to 18 000s, then the potential breaks between anodic and cathodic pulses (open circuit) with a duration time from 0.1 s to 100 s and anodic potential pulses Eao in the range from +0.050 V to +0.6 V, in reference to copper electrode, with duration time fao ⁇ t c .
• the number of potential pulses and potential breaks is from 3 to 30.
• a pair of electrodes is provided in an electrochemical tank made of polyvinylchloride.
• the cathode is made of stainless steel sheet having a thickness of 0.3 mm.
• the anode (reference electrode) is made of 0.25 mm thick copper sheet the surface of which is 0.22 m 2 .
• the process is conducted at room temperature (about 20°C).
• the vessel is filled with an electrolyte of the same composition as presented in Example 1.
• Each of the electrodes is connected with the aid of a special cable to a commercially available rectifier which can be used to programme the duration of a periodic potential reversal (PPR) electrolysis process.
• PPR periodic potential reversal
• the duration of the applied potential may be controlled to be from 1 ms to several days, using a current of up to 500 A flowing between the rectifier and the electrodes.
• the current changes depending on the duration of the electrolysis are measured during the process.
• the solution is not stirred. Parameters of PPR electrolysis:
• the cathode deposit After having deposited copper on the steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using an EDS/EDX method and an X-ray diffraction (XRD) technique. It is found that the obtained cathode deposit has a fine crystalline structure without dendrites.
• XRD X-ray diffraction
• Oxygen making up about 0.05% of the weight is the only/sole impurity present in the obtained cathode copper. So the obtained cathode copper has a purity higher than 99.95%. Having compared the deposited copper mass with the theoretical mass of copper that should have been deposited using Faraday's law, it is found that the current efficiency of the process is higher than 98%.
• Example 9 The experimental set-up and electrolysis conditions are the same as in Example 8. Parameters of PPR electrolysis:
• the cathode deposit After having deposited copper on the steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using an EDS/EDX method and an X-ray diffraction (XRD) technique. It is found that the obtained cathode deposit has coarse-crystalline structure without dendrites. During the anodic pulses the electrodeposited copper undergoes a pit corrosion and consequently the copper sheet surface roughness/porosity is much higher than in the case of potentiostatic electrolysis presented in Examples 1 to 7. Oxygen makes up about 0.05% of the weight and is the only/sole impurity present in the obtained cathode copper. So the obtained cathode copper has a purity higher than 99.95%.
• This example uses a PPE process, that is the application of cathodic pulses, interspersed with short zero potential breaks and without anodic pulses.
• the experimental set-up and electrolysis conditions are the same as in Example 8. Parameters of PPE electrolysis:
• the cathode deposit After having deposited copper on the steel cathode, the cathode deposit is removed from the cathode mechanically, washed with water, air-dried and the composition of the obtained copper is studied using an EDS/EDX method and an X-ray diffraction (XRD) technique. It is found that the obtained cathode deposit has a columnar-crystalline structure without dendrites and consequently the copper sheet surface roughness is higher than in the case of potentiostatic electrolysis presented in Examples 1 to 7. Oxygen makes up about 0.05% of the weight and is the only/sole impurity present in the obtained cathode copper. So the obtained cathode copper has a purity higher than 99.95%. Having compared the deposited copper with the theoretical mass of copper that should have been deposited using Faraday's law, it is found that the current efficiency of the process is higher than 98%.

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