EP3452640A1 - Equipment for decopperising an electrorefining process and way of operating the process - Google Patents

Abstract

The invention relates to an electrorefining cell and its use in a method of electrorefining of copper. The cell comprises: one or more electrorefining anodes comprising impure copper; one or more cathodes; an electrolyte containing copper sulfate and sulfuric acid; one or more electrowinning anodes; and one or more point-of-use power converters configured to supply current to the or each electrowinning anode whereby to control the copper concentration of the electrolyte when said cell is in use.

Description

EQUIPMENT FOR DECOPPERISING AN ELECTROREFINING PROCESS AND WAY

OF OPERATING THE PROCESS

INVENTION SUMMARY

The method relates to electrowinning of a metal from an electrolyte. The method covers examples of electrowinning (EW) of copper inside a copper electrorefining (ER) cell for in situ liberation of copper.

Prior art 1 discloses the use of point of use (POU) power converters to drive current through the anode-cathode gaps of electrolytic cells locally rather than the conventional practice of using a central rectifier to produce current flow through a parallel-series arrangement of anodes and cathodes in a multiplicity of cells.

The inventors have realised that by employing POU converters to drive at least one anode - cathode EW pair in a nominally copper ER cell, improved control can be gained of the copper concentration in the electrolyte circulating in the electrorefining tankhouse, decreasing the requirement for the removal of copper in the copper ER electrolyte purification section, decreasing the amount of copper returned to the smelter and improving the process, efficiency, energy efficiency and materials handling efficiency of the liberator process.

The productivity of the ER tankhouse can be increased by the electrowinning of one or more additional cathode plates in each ER cell, with all the process improvements and cost benefits that are thereby enabled.

The description covers examples of electrowinning anodes and ancillary equipment: power electronics and method of delivering electrical power to the electrowinning anode, and apparatus for the control of the acid mist generated to ensure safe operation when used in an ER tankhouse.

As will be understood, the term "electrolyte" as used herein is intended to refer to an aqueous electrolyte. Any reference to electrowinning is thus intended to refer to electrowinning of a metal from an aqueous electrolyte which contains the metal in ionic form.

As will also be understood, the terms "insulating" or "insulated" as herein described are used in the context of an electrical insulator. BACKGROUND OF THE INVENTION

For this method it is necessary to understand three variations of electrolytic processes used in the production of metals:

(i) Electrorefining (ER) - an electrolytic process for purifying an impure metal - most commonly used for the refining of copper (Cu ER). The simplest form of an ER cell is an arrangement of one anode and one cathode immersed in an electrolyte.

(ii) Electrowinning (EW) - an electrolytic process for plating a metal from an electrolyte solution, where the solution concentration of the metal is maintained at a constant level by continuously replenishment. This is used for the production of copper, zinc, nickel, cobalt and several other metals. Here we consider copper electrowinning (Cu EW).

(iii) Liberation - an electrolytic process that is a variation on electrowinning, where the metal is removed in order to decrease the metal ion concentration in a given electrolyte solution.

Figure 1 shows a simple schematic of the electrolyte purification process in a copper electrorefining tankhouse.

Figure 2 shows a sketch of a conventional electrorefining tank or "cell" 100 which contains many anodes 101 connected in parallel and many cathodes 102 connected in parallel. The standard practice in electrorefining is to use n cathodes and n+1 anodes. In an ER cell there is usually an anode in the first and last positions in the electrode row. For illustrative purposes Figure 2 shows the arrangement of electrodes in a Cu ER cell containing forty-nine cast anodes 101 and forty-eight permanent cathodes 102. The actual number of electrodes per cell in a given Cu ER plant can be a greater number than or fewer than shown in Figure 2.

In Cu ER the electrodes are immersed in an electrolyte 103 and an electrical current is passed between electrically positive electrodes or anodes 101 and electrically negative electrodes or cathodes 102.

In Cu ER the electrolyte 103 typically contains copper as copper sulfate with sulfuric acid as a supporting electrolyte. The electrolyte is circulated around the ER plant. The electrolyte is fed into the cells via a feed-pipe 104 and exits the cell via the electrolyte overflow 105.

The impure copper anode to be refined 101 is usually a cast anode and contains approximately 99% copper. The impure copper anodes are dissolved and more pure copper plates are deposited at the cathodes 102. The cast Cu ER anodes 101 have protrusions at the top on each side. These are commonly called lugs 106. The anode lugs support the weight of the anode in the cell and are the conductors by which electric current passes into the anode. At the ER anode surface metallic copper dissolves anodically, copper (II) ions enter the electrolyte and electrons pass into the external circuit.

Most Cu ER plants today use stainless steel permanent cathodes 102 with stainless steel blades. Some older refineries may still use copper starter-sheet technology. This is well described elsewhere, for example in Prior art 1.

At the ER cathode surface, copper (II) ions are reduced by electrons from the anode process and high purity copper metal is deposited.

Figure 3a is a plan view sketch of three ER cells with attention focussed on the middle cell, 100. In this sketch the first three anodes 101 and first two cathodes 102 are shown in each cell, immersed in electrolyte 103. 106 represents the lugs of the anode 101. Figure 3b shows a side view of the cell 100, where 103a represents the electrolyte-air interface. The permanent cathodes 102 are suspended in the cells by hanger bars 107.

In a Cu ER tankhouse, a plurality of cells are connected in series and an electrical current, derived from a central rectifier, is passed through the whole arrangement sometimes called a cell section.

Electrically conducting inter-cell busbars 108a and 108b rest on the lateral walls of the cells and perform two main roles:

- They serve to connect cells in electrical series. For example, inter-cell busbar 108a connects the first cell to the second cell 100 and inter-cell busbar 108b connects the second cell 100 to the third cell.

In any given cell, inter-cell busbars provide the means for connecting all the anodes in that cell in parallel and for connecting all the cathodes in that cell in parallel. For example, in the cell 100, the inter cell busbar 108a connects all anodes in parallel and busbar 108b connects all cathodes in parallel.

Hence a parallel-series connection of anodes and cathodes is produced.

In each cell electric current flows from the anodes to the cathodes. Insulators 109 prevent inappropriate connections between anodes and any given busbar and between cathodes and any given busbar. These insulators 109 take the form of capping blocks or capping boards and will be described later.

The voltage of the inter-cell busbars changes progressively with the voltage of busbar 108a being approximately 0.3 Volts greater than the voltage of busbar 108b. The central rectifier (not shown) is connected to the first and last cells in the series.

Although Figures 3a and 3b illustrate a single-contact system, the method is also applicable to double contact systems (where secondary "equaliser" bars are employed). A skilled person will readily be able to make the adaptations required for a double contact system.

Prior art 10 (Hoffmann) tells that in order to produce a dimensionally consistent, smooth- surfaced cast anode, some oxygen (0.1 to 0.3%) must be present in the molten anode copper. The oxygen is present mostly as cuprous oxide (Cu_∑0). During electrorefining the Cu_∑0 is partially dissolved into the electrolyte by sulfuric acid, resulting in the reaction Eqn. (1):

Cu 2 O (s .) + H, 2SCv 4(aq .) CuSO 4„(aq .) + H 2 O (I) + Cu, (s _Λ) Eqn. ( v1) ^/

The dissolution of cuprous oxide is chemical rather than electrochemical. It results in a continuous increase in ionic copper concentration in the circulating ER electrolyte 103 with time. To remove copper from a copper sulfate based electrolyte by electrowinning, the overall electrochemical reaction is given by Eqn. (2)

CuSCv 4(aq) + H 2 O - Cu, (s) + (2) '

The concentration of soluble impurity elements in the electrolyte - originating from the copper anode - also increases with time. The impurity elements which enter the electrolyte are those which are less noble than copper and include nickel, arsenic, antimony, bismuth and iron (see e.g. Prior art 11).

Impurity elements in the anode which are more noble than copper (e.g. silver, gold, selenium and the platinum group metals) remain as solids and report in the anode slimes at the base of the cell.

Cu EW is an electrolytic process similar to Cu ER in that an applied electrical current causes copper to be deposited on the cathode surface.

Cu EW uses insoluble anodes that evolve oxygen in sulfate-based electrolytes. The active part of EW anodes are usually lead-based alloys (e.g. a rolled lead-calcium-tin alloy) or mixed metal oxide (MMO) coated titanium anodes as described in Prior art 9. Such anodes are abbreviated to MMOA or CTA.

By using an MMO coated titanium anode instead of a lead-based anode, the overpotential for the oxygen evolution reaction can be decreased by between 0.2 and 0.3 Volts - representing an electrical energy saving of between 10 and 15%.

The permanent cathodes in Cu EW usually have stainless steel blades. Some older refineries may still use copper starter sheet technology, and some EW technologies use titanium starter sheets (see Prior art 5).

In ER tankhouses the electrodes are suspended from the top of the cell, on top of "cell-top furniture". Cell top furniture includes the inter-cell busbars 108 and insulators which can be either:

(i) insulating capping blocks 109 with fixed inter-electrode spacings or

(ii) insulating capping boards 109b, which will be described later in Figures 7a and 7b.

With capping boards 109b the inter-electrode spacing in the cells can be adjusted. When using capping boards the electrode positioning and inter-electrode spacing is set by the spacing in the crane-bale. The crane-bale is the equipment which loads and unloads electrodes in and out of the cell (not shown).

Figure 2 shows a simple flow-diagram of the electrolyte in a typical ER tankhouse. The exact detail of each individual plant's ER tankhouse electrolyte purification process varies. Most are however based on that outlined in Figure 2. The usual method used to control the concentration of copper and the concentration of impurities in the ER electrolyte 103, is to divert a bleed stream of electrolyte to an electrolyte purification circuit. Detailed examples of a number of ER tankhouse electrolyte purification processes are given in Prior art 4 to 8.

The electrolyte purification section usually has two smaller, secondary electrolytic "liberator" sections to remove copper from the bled ER electrolyte by copper electrowinning (Cu EW), as described in Prior art 2, 10 and 11.

The liberator cells can be understood as performing two roles in removing copper:

(i) Liberators remove the excess copper in the electrolyte which is present due to the

chemical reaction described by Eqn. (1); keeping copper concentration in the electrolyte under control. (ii) After the copper concentration in the bled electrolyte has been sufficiently decreased, it is further de-copperised to permit the removal of arsenic as copper arsenate (Cu₃As).

Following arsenic removal other impurities such as nickel can be accessed and recovered.

The resulting sulfuric acid, containing degraded additives (for example decomposed bone glue), is known as black acid and is further purified in an acid purification unit (APU). Purified sulfuric acid is returned back to the Cu ER tankhouse electrolyte circulation.

In a standard copper liberator EW section, the target is to remove copper from the electrolyte solution as a coherent cathode deposit - meaning a solid panel of copper which can be harvested from the cathode as a single sheet - as opposed to an undesired copper powder which would fall to the floor of the cell and require regular cell cleaning.

Some copper cathodes produced in a liberator EW circuit may satisfy the LME purity grade A (Quality: BS EN 1978:1998 Copper Cathodes. Cathode grade designation Cu-CATH-1) and so be sold directly. Usually however, the liberator EW cells produce copper cathodes which are deposited under non-optimum conditions, and contain unacceptably high levels of impurities such as arsenic. For this reason impure copper cathodes from the liberator cells are usually returned to the smelter to be melted and re-cast into new anodes to be refined in the copper ER tankhouse.

The cathodes used in liberator EW cells are either permanent cathodes with stainless steel blades, or copper starter-sheets. If the liberator cathode copper is intended to be returned to the smelter, then spent (scrap) anodes from the ER tankhouse can be used as cathodes in the liberator circuit. The anode materials for copper liberator cells are similar to standard copper EW electrowinning cells as described earlier.

In a liberator section the EW cells can be arranged so that the electrolyte passes through one or more cells in cascade. In the case of a copper ER electrolyte bleed stream, the initial

3

concentration of copper is usually in the range of 40 to 60 g/dm . As the electrolyte passes through the liberator cells copper is removed from the electrolyte; copper concentration decreases and acid concentration increases, giving a final copper concentration typically below

10 g/dm³. The decrease in copper concentration in the electrolyte from before the liberator circuit and after the liberator circuit is called "delta copper" or "ACu" or "the bite". Delta copper can be in the range of 30 to 50 g/dm³ depending on the operating conditions of the individual ER plant.

In copper electrowinning the cathodic current density ( , units A/m²) is usually in the range 100 to 400 A/m².

Copper metal is deposited on the active part of each cathode face, the area of which is usually in the range of between 0.9 to 1.2 m², giving a total active area of 1.8 to 2.4 m² per cathode.

Cathodes with smaller or larger dimensions may also be employed.

The voltage across a liberator EW cell is approximately equal to the voltage that would be experienced between a single EW anode and a single cathode.

The cell voltage of copper EW is usually in the range of 1.7 to 2.3 Volts and depends on:

• the anodes used (lead based or mixed metal oxide coated titanium),

• the applied current density, j (units A/m²),

• the electrolyte composition (copper, nickel, arsenic, iron and sulfuric acid

concentrations),

• the electrolyte temperature.

The mass in grammes of a metal (m) deposited during electrolysis can be written as Eqn. (3).

m = (I t M_r η) / (z F) Eqn. (3)

Where

• I is the current in Amps,

• t is time in seconds,

• M_r is the molar mass in grammes, for copper 63.55 g/mole,

• η is cathodic current efficiency with a value in the range 0 to 1 - though usually

discussed as a percentage value. In copper ER, the value is typically 0.97 to 0.98 (or 97 to 98%),

• z is the number of electrons involved in the reduction of the metal ions (for Cu²⁺ in sulfate based electrolytes, z = 2),

• F is Faraday's constant = 96485 Coulombs/mole (1 Coulomb = 1 ampere second).

From Eqn. (3) the "electrochemical equivalent" of copper can then be calculated as 1.1855 g/Ah (or 1.1855 kg/kAh or 1.1855 tonnes/MAh), giving Eqn. (4), where time t is now measured in hours. m = 1.1855 I t η Eqn. (4)

Delta copper (ACu) with units of kg/m³ (equivalent to g/dm³) is a value of mass per unit volume; when substituted into Eqn. (4) and using the electrochemical equivalent 1.1855 kg/kAh we obtain Eqn. (5), where V is the electrolyte volume in m³ needed for a given ACu, current and time.

ACu V = 1.1855 I t η (Eqn. 5)

Prior art 10 (Hoffmann, page 30) tells that the quantity of copper to be removed is fixed by the anode oxide concentration and the impurities (impurity concentration) in the anodes and it is these which define the volume of electrolyte which must be removed as a bleed-stream.

Prior art 1 introduced a new approach to the supply of current to the interelectrode gap between the individual anode and cathode faces. A point of use (POU) power converter is a power supply which provides a desired current or a desired voltage close to the point of usage.

Typically this type of converter is used when power is required at a relatively low voltage.

Voltage drop in cables is avoided if the required current is produced close to the point of use. The POU converter typically draws its power from a relatively high voltage power source.

Typically a POU power converter has a DC output and a DC or AC input. Typically a POU power converter will employ switched mode technology. The input and output may or may not be galvanically isolated from each other. Cost and application requirements will determine which arrangement is used.

As described in prior art 1 to 3 there are a number of advantages in using POU power converters compared with the conventional practice of using a single central rectifier. All the anticipated benefits of using POU power converters are applicable to the present method including:

• The ability to limit or entirely switch off the current which flows between anodes and

cathodes when a short circuit occurs between them. Short circuits are typically formed when a metal dendrite is formed between the cathode and the anode. Such short circuits can reduce the current efficiency of the ER or EW process and result in burning of the anode or cathode. Operators attempt to identify and remove such short circuits as early as possible in their formation. The use of POU power converters facilitates early detection of short circuits and through current control prevents damage to the electrodes.

• If a Cu EW system is left without applied power then chemical reactions can occur

causing corrosion of the anode. In EW tankhouses this risk is mitigated by having an emergency power supply on standby. Such a back-up supply can pass a small current in the order of a few Amps per electrode, which is sufficient to preserve the anodes in the case that the main power should fail. With the apparatus described here, possible damage to the anodes can be avoided by continuing to provide a small current between the anode and neighbouring electrode(s) so that the arrangement remains forward biased and protected. Independently powered POU converters driving individual anode cathode pairs facilitate this form of protection.

PRIOR ART

The following prior art is disclosed:

(1) Grant D: Apparatus for use in electrorefining and eiectrowinning. WO 2012020243 (A1), published Feb, 16, 2012.

(2) Barker MH, Virtanen HK, Grant D: System for power control in cells for electrolytic recovery of a metal: World application: WO2013117805 (A1), published Aug. 15, 2013.

(3) Grant D, Nordlund L, Rantala A, Barker MH, Virtanen HK and Schmachtel S: Self protected anodes and cathodes in electrolytic cell arrangements. Fl 124587 (B), published Oct. 31. 2014.

(4) Riekkola-Vanhanen M: Finnish expert report on best available techniques in copper

production and by-production of precious metals: The Finnish Environment Institute, Helsinki: Number 316, June 1999, 72 pages. ISBN 952-11-0506-2.

(5) Siegmund A, Gadia S, Leuprecht G and Stantke P: Replacement of liberator cells - pilot test study at Aurubis Hamburg using the EMEW technology: Proceedings of Copper 2013: Santiago, Chile, 1-4 Dec. 2013: Volume V, pgs. 275-283.

(6) Kamath BP, Mitra AK, Radhakrishnan S and Shetty KP: Electrolyte impurity control at

Chinchpada Refinery of Sterlite Industries (India) Limited: in Proceedings of Copper 2003, Vol. V, Copper Electrorefining and Eiectrowinning, CIM, Canada, 2003, pgs. 137-150.

(7) Sheedy M, Pajunen P and Wesstrom B: Control Of Copper Electrolyte Impurities - Overview Of The Short Bed Ion Exchange Technique And Phelps Dodge El Paso Case Study: Proceedings of the Sixth International Copper-Cobre Conference: Volume V Copper Electrorefining and Eiectrowinning, (Montreal, Quebec: CIM, Canada, 2007, pgs. 345-357.

(8) Wesstrom BC and Araujo O: Optimizing a Cascading Liberator: Electrometallurgy 2012, 141^st annual TMS meeting, Orlando, Florida 1 1-15 March 2012, pgs. 151-156.

(9) Sandoval SP, Clayton CJ, Robinson TG, Zanotti CJ, Zanotti MK: Structure For Copper Eiectrowinning: US20 3126341 (A1), Published May 23, 2013.

(10) Hoffmann JE: The Purification of Copper Refinery Electrolyte: JOM, 2004, 56(7), 30-33.

DOI: 10.1007/s 11837-004-0088-4

(11) Wang S: Recovering Copper Using a Combination of Electrolytic Cells: JOM, 2002, 54(6), 51-54. DOM0.1007/BF02701852 Wang S: Impurity Control and Removal in Copper Tankhouse Operations: JOM, 2004, 56(7), 34-37. DOI: 10.1007/s 1 1837-004-0089-3.

(12) Mathew RJ: Increasing Oxygen Charge Transfer Resistance on the Anode in Copper

Electrowinning: Electrometallurgy 2012, 141^st annual TMS meeting, Orlando, Florida 1 1-15 March 2012, pgs. 41-48.

(13) Nordlund L, Vanhatalo H, Nieminen V, Virtanen HK, Luoma R, Kaakkolampi N, Aaltonen H, Unkuri J: Frame and electrolysis system: World application: WO2013079788 (A1), published June 6, 2013.

(14) Radford CM and Gregory C: An Electrode Frame: South African patent application

ZA9810968 (A), Published June 1 , 1999.

(15) lacopetti L, Fiorucci A, Calderara A, Perrone P, Faita G, Brown CW, DeMasi B and Barker MH: The De Nora solution - Part II, Acid Mist Abatement: Proceedings of Copper 2013. Santiago, Chile, December 2013. Vol: vol. 5, pgs. 183-194. DOI:

10.13140/RG.2.1.4150.5444

PROBLEMS WITH CONVENTIONAL PRACTICE

Problem 1 - Impure quality liberated cathodes

As outlined in Prior art 5, electrowinning of copper in traditional liberator circuits produces impure copper cathodes or highly impure copper powder. Most of the copper electrowon in the liberator cells is returned to the smelter. Circulation of copper back to the smelting stage of the copper refining process causes extra handling costs and is not energy efficient, it wastes the electrical energy consumed in the liberation process (in the order of 2 MWh per tonne of copper).

It is advantageous for the overall Cu ER process (less handling of copper and more energy efficient) to produce additional saleable LME grade A copper cathodes from liberating copper, instead of sending impure cathodes back to the smelter.

Problem 2 - Non-optimum current density in liberator circuits

In liberator circuits, as outlined in Prior art 2, the standard practice is that the liberator EW cells operate at a current density which can be tolerated by the weakest cells in the circuit.

This requires more liberator cathodes than if each liberator anode-cathode pair (or inter- electrode gap) would operate at its optimum current density.

By using liberator anodes and cathodes at optimum current density the overall number of electrodes required is lower. Problem 3 - ER Tankhouse expansions

Today the trend is towards larger ER tankhouses. The result is that existing tankhouses are being expanded. Prior art 6 reports an example of an ER plant in Silvassa, India that was commissioned in 1997 with a production capacity of 60000 tonnes per annum (TPA) of cathode copper. By 2003, after aggressive expansion, the production had been increased three-fold to 180000 TPA of copper.

An increase in the size of the ER plant means that a greater mass of extra copper enters the electrolyte due to Eqn. (1). This extra copper needs to be removed in the liberator circuit of the electrolyte purification section of the plant.

In a conventional ER plant, when the main ER tankhouse is expanded, the liberator circuit capacity must then also be increased accordingly. This normally involves the addition of extra liberator EW cells, extra rectifier capacity for those cells and in some plants, the construction of another building.

The Inventors have realised that by using in-situ liberation in the Cu ER cells to control the ER tankhouse copper concentration, in the case where a Cu ER plant is expanded, the need for increased liberator capacity in the electrolyte purification will no longer be automatic, but will instead be decided by the need for impurity control rather than control of copper concentration. In the case of a newly constructed plant - the size of the required liberator section can be less than is conventionally required, leading to a smaller plant footprint.

Problem 4 - Handling of acid mist

A consequence of placing electrowinning anodes into electrorefining cells is that there is oxygen evolution in the ER cells. This is in contrast to ER in which there is no oxygen evolution. The evolution of the gas oxygen does not in itself represent a hazard since oxygen is a major component of air. As oxygen bubbles rise to the surface and burst micro-droplets of electrolyte are ejected into the tankhouse atmosphere forming an "acid mist" which if uncontrolled, is hazardous to the health of plant workers and can cause corrosion of plant equipment.

In Cu EW, as per Eqn. (2), for each mole of copper electrowon at the cathode, half of one mole of oxygen gas is evolved at the anode. As per Eqn. (4) the electrochemical equivalent for copper is 1.1855 g/Ah.

Taking for example a cathodic current density of 300 A/m², assuming 100% current efficiency, in 1 hour at a single cathode face approximately 356 g (or 5.6 moles) of copper would be deposited. At the corresponding anode this equates to the evolution of 2.8 moles of oxygen gas. According to the Ideal Gas Law shown in Eqn. (6):

PV = nRT Eqn. (6)

Where:

• P is the pressure of the gas in pascals (Pa) and is 101325 Pa at sea level,

• V is the volume of the gas in m³,

• n is the number of moles of the gas,

• R is the ideal gas constant (8.314 J K^"1 mor¹),

• T is the temperature of the gas in Kelvin.

For 2.8 moles of oxygen at sea level and a Cu ER electrolyte temperature of 50°C (323.15 K) an estimated 74 litres of oxygen gas will be evolved at a single anode face per hour.

Many copper refineries are located in mountainous areas and at altitudes of up to 4000m where the air pressure is lower (~ 61500 Pa). This is important to note - at higher altitude with lower air pressure, the volume of gas evolved, and volume of acid mist to be handled may be larger. The technologies for control of acid mist in EW tankhouses include anti-mist spheres, chemical foaming agents and mist capture hoods. Mist-capture hoods are used in modern Cu EW tankhouses in order to control the evolved acid mist above the Cu EW cells. A vacuum extracts the air and acid-mist mixture from above the EW electrodes. The acid mist laden air is sent to a central scrubber for treatment.

Electrowinning from a typical Cu ER process electrolyte would present another, greater risk - it will typically contain both arsenic and nickel not usually present in Cu EW. This would make any acid mist generated by using EW equipment in a Cu ER electrolyte more hazardous than the acid mist found in a typical Cu EW tankhouse.

Hence whilst the concept of using one or more EW anodes in an ER cell does not require acid mist control for its function, acid mist control is however, generally required to maintain a safe working environment. Arrangements for acid mist control are therefore included herein.

STATEMENT OF THE INVENTION

The application describes a method and equipment for control of copper concentration in an ER tankhouse by liberation of copper. The electrolyte purification circuit for a Cu ER plant is described earlier and in the flow chart in Figure 1. Conventional liberator cells are dual-purpose electrowinning cells - they perform two functions in the overall refining process:

(i) removing copper to keep the ER tankhouse electrolyte copper concentration under control,

(ii) removing copper to permit access to other impurities such as arsenic and nickel.

This method transfers the first of those two functions from the liberator cells in the electrolyte purification circuit to the main Cu ER tankhouse cells.

In this method the insertion of one or more EW anodes into a Cu ER cell allows control of the copper concentration directly in the ER tankhouse electrolyte. By removing the excess copper at the point in the process where that excess is generated - in the ER tankhouse electrolyte due to the reaction in Eqn. (1) - the method can be described as an in situ liberation.

The method gives the advantage of a more efficient liberation of copper than is possible when using conventional dual-purpose copper liberator cells. Higher production efficiency is obtained for in situ copper removal using a small ACu and using the optimum applied current density, producing saleable grade A cathodes from the copper removed for control of copper concentration.

When the tankhouse arrangement allows, by electrowinning one (or more) additional, grade A cathode copper plates in each cell, the method may give the benefit of increasing the production of copper in the overall process, thereby increasing the overall productivity of the plant. This is in contrast to conventional liberator cells which use a large ACu and a non-optimum applied current density. In conventional liberator practice most of the copper cathodes produce are impure and need to be returned to the smelter and refined a second time.

Point of use (POU) power converters, as employed in Prior art 1 , permit individual, insoluble, EW-style anodes to be inserted into the ER cells. These auxiliary EW anodes are positively polarised (with respect to the adjacent electrode) by POU converters (power supplies) which provide the current required for copper deposition in a manner independent of the central rectifier.

The method also gives an advantage in the case that the main ER tankhouse is expanded. This traditionally requires an increase in the capacity of the liberator circuit. An additional benefit of moving the function of control of copper concentration away from the EW liberators and into the main ER cells, is a decrease of the load on the liberator cells in the electrolyte purification section. This frees up capacity in the copper liberator cells.

With the method described in this application the main ER tankhouse copper production capacity may be then expanded, without automatically requiring an expansion of the existing liberator sections. The freed up liberator capacity can then be used to treat the impurities in the electrolyte of an expanded Cu ER tankhouse.

The acid-mist above the electrolyte - generated by the in situ liberator (EW) anode - is contained within an anode bag. Whilst acid mist control is not the main concept of this application, it must generally be carried out to ensure safe operation of the equipment. It is therefore an important part of the method that acid-mist be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the apparatus and method and constitute a part of this specification. The drawings are not to scale. The drawings illustrate arrangements of the apparatus and together with the description help to explain the principles of how the apparatus may be operated. In the drawings:

Figures 1-3 are examples from a standard Cu ER tankhouse.

• Figure 1 is a block diagram of a Cu ER process including an electrolyte solution purification section.

• Figure 2 is a 3D sketch of an electrorefining cell containing 48 cathodes and 49 anodes.

• Figure 3a is a plan view sketch of the first three anodes and two cathodes of the same electrorefining cell as 2. Figure 3b is a side-elevation sketch of the same equipment layout as 3a.

Figures 4 to 6 relate to ER tankhouses with cell top furniture using capping blocks with fixed inter-electrode spacing.

• Figure 4a is a plan-view sketch of the end of an ER cell with the addition (next to the end wall of the ER cell) of an asymmetric, EW anode 110 and an accompanying POU power converter 115. Figure 4b is a side-elevation view sketch of the same equipment layout as shown in Figure 4a. • Figure 5a is a plan-view sketch of an ER cell where the first anode position in the row is occupied by an asymmetric EW anode 110 with an accompanying POU power converter 115. Figure 5b is a side-elevation sketch of the same equipment layout as shown in Figure 5a.

• Figure 6a is a plan view sketch of an ER cell where the second anode position in the row is occupied by a symmetric EW anode 110 with an accompanying POU power converter 115. Figure 6b is a side-elevation sketch of the same equipment layout as Figure 6a.

Figure 7 is an arrangement to be applied in an ER tankhouse where the cell top furniture includes capping boards 109b (without fixed inter-electrode spacing).

• Figure 7a an EW anode is positioned similarly to Figure 4a, but differs by the addition of one more cathode to the cell. Figure 7b is a side-elevation sketch of the same equipment layout as Figure 7a.

Figure 8 shows the arrangement of equipment for handling of acid mist generated at the EW anode

• Figure 8a is a front elevation sketch of an EW anode 110.

• Figure 8b is a front elevation sketch of the same EW anode seen in Fig 8a. The EW anode is housed inside an "anode bag" 120 and an anode frame 121. The anode arrangement includes an integrated pumping system 123-125 for control of acid mist.

Figure 9 shows an EW anode with an arrangement for control of acid mist, similar to Figure 8, but differing in that the pump 124 is separate from the EW anode, and is now positioned outside of the cell.

DETAILED DESCRIPTION OF THE INVENTION

There are a number of possible arrangements for implementing the method. The choice of arrangement will depend on whether the apparatus is to be:

(i) installed in a newly constructed plant designed with this method in consideration, or

(ii) retro-fitted to an existing ER tankhouse.

Whilst the second scenario is likely to be the more common one, and is therefore summarised in Table 1 , the scenarios mentioned in Table 1 will also be appropriate in for use in a new plant design. And so the method is not limited to retro-fitting of apparatus in existing ER cells, nor is it limited to the examples given. It is also to be understood that the aspects and embodiments of the method described below may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the method. Table 1 - Summary of options for placement of an EW anode in an ER cell

We summarise a number of example arrangements for in situ copper liberation in a Cu ER cell using EW anodes 110. The method is not limited to the example anode structures 110 described, other designs may also be used.

The arrangements will be further described using Figures 4 to 7. Figures 4 to 6 relate to ER tankhouses with cell-top furniture using insulating capping blocks 109a with fixed inter-electrode spacings. Figures 7a and 7b show an ER cell design that uses capping boards 109b such that the inter-electrode spacing in the ER cells can be adjusted. When using capping boards 109b the electrode positioning and spacing is set by the spacing in the crane-bale - the device which loads and unloads electrodes in and out of the cell.

The amount of copper which needs to be removed from the ER cell electrolyte - i.e. the ACu value - will determine the amount of charge (current) that must be passed through the EW anodes. This in turn, determines the number of EW anodes to be installed in a tankhouse. The ACu value for in situ liberation of copper in an ER cell may be lower or higher than used in a standard copper EW plant where the target value for ACu is around 2g/dm³. There are several possible strategies which can be used:

• For a low ACu value for in situ liberation:

o The preferred option is the use of EW anodes in selected ER cells. The anodes are operated at "standard" EW current density (in the order of 300A/m²).

o As a less preferred option - the use of an EW anode in every ER cell, with the anode operating at lower current densities.

o Another, less preferred option is the use of smaller sized EW anodes in all cells operated at a "standard" anodic current density. Whilst this configuration is possible, those skilled in the art will appreciate the associated technical challenges.

• For a high ACu value for in situ liberation, the use of more than one EW anode in a single cell is the likely arrangement.

The first example arrangement is shown as a plan view in Figure 4a and as a side view in Figure 4b. Figure 4a represents one end of an electrorefining cell 100. For ease of acid mist handling (which will be described later in Figures 8a, 8b and 9), the most appropriate place to install the apparatus is the end of the cell 100 next to the electrolyte overflow (indicated by 105 in Figure 2). This does not exclude the use of the apparatus at the end of the cell 100 where the electrolyte is fed into the cell (indicated by 104 in Figure 2).

For clarity and simplicity of drawings, neither 104 nor 105 are shown in Figures 3 to 9, but it is to be understood that they are both present in the cell 100.

In Figure 4a an asymmetric EW anode 110 is positioned next to the end wall of the ER cell 100. The EW anode 110 has a hanger bar 11 1 seen in Figure 4b which rests on the side walls of the cell 100. Alternatively the EW anode can be hung inside the cell (supported by the end wall). The EW anode 110 (Figure 4a) may be accurately lowered into the cell and reproducibly positioned in the cell 100 by means of brackets (or guides) 119 as shown in Figure 4b. The guides 1 19 are attached to the inside of the end wall of the cell and keep the EW anode in a fixed position whilst the cell is in operation.

In this arrangement, the face of the cast copper ER anode 101 shown in Figure 4a as adjacent to the EW anode 110 acts as a cathode surface (copper is deposited onto it). The second face of the ER anode 101 - that which is opposite the cathode 102 - acts as an ER anode as normal. The copper deposited on the anode 101 will be returned to the smelter along with the other scrap anodes. In this arrangement there is zero net change in production of grade A cathode copper in the tankhouse.

To force an EW process to take place between the EW anode 110 and the cast copper ER anode 101 an electric current must be forced to flow from the anode 110 through the electrolyte to the copper anode 101. To make this happen the EW anode 110 must be at a voltage approximately 1.7 Volts above that of the copper anode 101 when using a MMO coated Ti-mesh based anode (or 2.0V when using a lead based anode). This is achieved by connecting a power supply 115 between the EW anode 110 and the inter-cell busbar 108a upon which the lug 106 of the cast ER copper anode 101 rests. This is achieved by cable 117 as shown in Figure 4a and described later.

The power supply may be used in the voltage-controlled mode or current-controlled mode - most likely in the current-controlled mode - so that the current density at the faces of the electrodes can be held at a value which results in copper deposition of the desired rate to maintain the desired copper concentration in the circulating ER electrolyte.

As Figure 4b shows, the active part of the EW anode 110 may be a mixed metal oxide (MMO) coated titanium mesh 112. The mesh is electrically connected to titanium clad (Ti-clad) copper bars 113 through a conducting spacer 114 which is welded along its length in between the mesh 112 and the Ti-clad copper bars 113. The tops of the Ti-clad bars 113 are connected to and suspended from a conducting hanger bar 111. The design of the EW anode is not limited to this design. Other EW anode designs may also be used for example the traditional rolled lead alloy sheets.

In Figure 4a the positive terminal of the POU power converter 115 is connected to the EW anode 110 by a cable or cables 1 16.

If the EW anode 110 (of figure 4a) is to be readily removable, the connection can alternatively be made via a conductive pad, isolated from the inter-cell bar 108 by a layer of insulation in the capping block 109. In this case the hanger bar 111 (of Figure 4b) of the EW anode 110 (of Figure 4a) will have a corresponding conductive region which rests on the conductive pad and completes the connection.

In Figure 4a the negative terminal of the POU power converter 115 is connected by a cable or cables 117 to the inter-cell busbar 108a which in turn contacts the lugs 106 of the anodes 101 in that cell. This connection between the busbar108a and cable 117 is made by a convenient electrical connector or fastener - including but not limited to - cable lugs and bolts or a pressure contact arrangement.

When a double-contact busbar arrangement (previously described) is used, then two POU power converters 115 may be employed per EW anode, one to the left of the tank (as shown in Figure 4a) and a second to the right (not shown) which feeds the second end of the EW anode hanger bar 111 (of Figure 4b) or contact or connection arrangement. This applies also to the arrangements described in Figures 5 to 7.

Power is supplied to the POU power converter 1 15 from an external power source, shown as power feed 118 in Figure 4a. For example a 230V ac supply or a 48V dc supply may be used. Practically any dc or ac supply of adequate power capability can be used. The power supply 115 needs to incorporate galvanic isolation between the input and output sides. The anodes and cathodes in the electrowinning tanks will be at a voltage with respect to earth which can typically be at any value between +200V and -200V depending on the output voltage of the central rectifier and where the electrical circuit of the tankhouse is connected to earth. Often tankhouse electrical circuits are left floating with respect to earth in which case earthing may take place through spills of electrolyte and other such erratic connections. The power supplies 115 must therefore include input-output galvanic isolation which is at least equal to the maximum voltage produced by the central rectifier.

All the benefits of using POU power converters - as described in Prior art 1 to 3 - can be advantageously employed in the present method (for example preventing damage to anodes and cathodes produced by short circuits between them, capture of process data and process optimisation).

If desired, the use of power feed 118 can be avoided by "stealing" power from the main electrorefining circuit. For example, if the voltage difference between inter-cell busbar 108a and inter-cell busbar 108b is 0.25V, this could be used to provide power for the converter 115. However, if the POU power converter 115 is required to deliver 300 Amps at 2.5V, the current draw from the inter-cell busbar would be in the order of 3 kA. It is difficult to make a converter with that specification operate efficiently and the cost is likely to be high. A more practical approach is to "steal" current for the EW anode from the chain of electrorefining tanks (from a suitable inter-cell busbar) at a point where the voltage is appropriate (i .e. about 2.5V higher than the voltage of the copper anode 101 which is facing the EW anode under consideration 110). In its simplest form, this solution only requires a cable to be run along the side of the tank connecting the EW anode to the appropriate inter-cell busbar.

In practice, it will be beneficial to include a regulator (linear or switched-mode) in the circuit (i.e. between the anode and the inter-cell busbar source). This will permit fine control of the current supplied to the EW anode (as well as providing protection for the anode should a short occur between the EW anode 110 and its adjacent copper anode 101).

If the regulator between the anode and the inter-cell busbar source is replaced by a step down converter (non-isolated or isolated), then an inter-cell busbar which is at a higher voltage than 2.5V can be chosen as a power source. This will reduce the current draw from the

electrorefining tank chain.

Ultimately, the power source can be the central rectifier itself. If one input terminal of the power supply 115 is not connected to the inter-cell busbar 108, then an isolated converter will have to be employed. This would be the case if the positive and negative terminals of the central rectifier were used as the power source.

Depending upon the arrangement of electrodes in the cell, and position in the cell where the EW anode 110 is placed, the structure of the anode 1 10 may be based on the following design (as well as others):

- An asymmetrical anode design can be used when the EW anode 110 is placed in the first (or last) electrode position on the cell, as shown in Figures 4a and 4b. An example of an asymmetric anode is a single sheet of MMO coated Ti mesh 112, mounted on the side of the Ti-Clad bars 113 facing towards the neighbouring electrode.

- Or a symmetrical anode design can be used, with two MMO coated titanium meshes 112, one on either side of the Ti-clad bars 113. This is the standard configuration of MMO anodes used in most Cu EW applications (for examples see Prior art 9). This arrangement will be described later in Figure 6.

Appropriate sizing of the EW anode 110 is important to avoid an edge effect on the cathode deposit.

- If the EW anode 1 10 is sized too large with respect to the neighbouring cathode surface then the cathode deposit can have thick ridges around the sides and bottom of the deposit which cause issues - for example, increased susceptibility to dendrite growth and short circuits. - If the EW anode 1 10 is sized too small with respect to the neighbouring cathode surface then the cathode deposit can have feathered edges at the side and bottom of the deposit which cause issues - for example, incomplete stripping of the plate and leaving residual copper which may cause issues in the next plating cycle.

In the arrangement in Figures 4a and 4b, the ER anode 101 acts as a cathode on the side facing the EW anode 110. The anodes 101 in ER are typically sized slightly shorter and less wide than the adjacent cathodes 102 (usually by between 1 and 10 cm in both height and width). For this reason, the anode 110 in the arrangement shown in Figures 4a and 4b must then be sized to be shorter and less wide than the adjacent ER anode 101.

In electrowinning, the resistivity of the EW electrolyte is higher at the top of the anode than at the bottom of the anode due to entrainment of oxygen bubbles. The fundamental issues of electrolyte resistivity versus electrode depth are outlined in Prior art 12.

If an asymmetric EW anode design is used, then the MMO coated mesh 112 (Figure 4b) may be oriented vertically or may, advantageously, be angled towards the opposing electrode to compensate for decreasing electrolyte resistivity with cell depth. Figure 4b shows how the MMO coated Ti-mesh 112 of the EW anode 110 is angled towards the neighbouring electrode by use of the aforementioned titanium spacer 114. The angle of the mesh 112 can for example be in the range of between 0° and 10° from the vertical, but a larger angle may be used if that is beneficial.

By angling the mesh 1 12 towards the neighbouring electrode, the distance between the top of the EW anode 110 and the neighbouring electrode is decreased compared to the distance between the bottom of the EW anode and the neighbouring electrode. Increasing the inter- electrode distance versus the cell depth compensates for the difference in electrolyte resistivity between the top and bottom of the EW anode. This gives the advantage of a more even cathodic current distribution than can be obtained when using the usual vertically oriented anode mesh.

Other methods may be used to angle of the MMO coated Ti-mesh 1 12 for example:

by placing a bend of the required angle in the Ti-Clad bars 113,

by fixing the angle of the anode in the hanger bar 111 ,

or by fixing the angle at the point of suspension from the hanger bar 111. Figures 5a and 5b show an alternative arrangement obtained by replacing the last cast copper ER anode in the row (either at the overflow-end of the cell, or at the feed-end of the cell, or at both ends of the cell) with an asymmetric EW anode 1 10. Current is then driven through the resulting anode-cathode gap with a POU power converter 115, as before.

In this arrangement the ER cells will now process one less copper cathode plate per cell from the loss of refining of an end position cast ER anode (equivalent to half of a cast ER anode). The decrease in ER cathode production is balanced by the production of one additional EW cathode plate arising from the in situ removal of excess copper in the electrolyte due to Eqn. (1). There is zero net change in production of grade A cathode copper in the tankhouse when using this arrangement.

When using the arrangement in Figures 5a and 5b, the design and the tankhouse operations must ensure that the EW anode 110 in the first anode position in the cell is neither raised nor engaged during standard tankhouse operations (loading of ER anodes into the cell, or harvesting of cathodes).

In the arrangement shown in Figures 5a and 5b, the POU power converter 115 drives current between the electrowinning anode 110 and the adjacent electrode in the tank which in this case is a cathode 102. Copper is then deposited on both sides of the cathode 102. Note that in Figure 5a the connections of the power supply 115 are different from those in Figure 4a. The negative terminal of the power supply is now connected via a cable 1 17 to the inter-cell busbar 108b which contacts the cathode 102 facing the electrowinning anode 110.

In this arrangement the hanger bar 111 of the EW anode 110 sits inside the capping block 109 and will be isolated from the inter-cell busbar 108a by a layer of insulation 109a inserted into the capping block 109 sitting between 108a and 11 1 (Figure 5b). Alternatively an insulating sleeve can be placed around the end of the EW hanger bar 11 1 to prevent electrical contact with 108a. Figure 6a and 6b shows an alternative arrangement where the cast copper ER anode in the second position in the cell is substituted with an insoluble EW anode 110. This arrangement is also valid for substituting the ER anode at any position in the cell other than the first or last. In this arrangement, in the cells where an ER anode 101 has been substituted, the cell will produce two fewer copper cathode plates per cell from the refining of a cast ER anode. This is compensated by electrowinning of two additional cathode plates. There is zero net change in production of grade A cathode copper in the tankhouse when using this arrangement. When using the arrangement in Figures 6a and 6b, the design and the tankhouse operations must ensure that the EW anode 110 in the second anode position in the cell is neither raised nor engaged during standard tankhouse operations (loading of ER anodes into the cell, or harvesting of cathodes).

The main difference between the arrangement in figures 6a and 6b and the earlier arrangements in Figures 4a, 4b, 5a and 5b, is that in placing an EW anode 1 10 between two cathodes 102, the anode must be a symmetrical structure. For example in the case of using an MMO coated Ti- mesh anode, it must have two MMO coated Ti-meshes. When placing an EW anode 1 10 between two cathodes, the anode mesh sheets 112 may be vertical as shown in Figure 6b, or each mesh may be angled towards its opposing cathode 102 as described for the asymmetric anodes in Figures 4a, 4b, 5a and 5b to give an even current distribution on both of the opposing cathodes.

Figures 7a and 7b shows an alternative arrangement for in situ liberation in an ER tankhouse using capping boards 109b in the cell-top furniture (compared to the earlier described capping blocks). Figure 7a is a sketch where the cells use a dog-bone style inter cell busbar 108 and a capping board 109b. As in the earlier capping block examples (Figures 4 to 6), a single-contact hanger bar configuration is shown in Figure 7a for simplicity of the description and drawings. A double-contact hanger bar configuration is also possible when using capping boards.

In the case of retrofitting to an existing tankhouse using capping boards, the inter-electrode spacing set in the crane-bale (not shown) will be decreased by an amount sufficient to allow the insertion of an extra cathode 102 into the cell. The asymmetric EW anode seen in Figure 7b is positioned against the end wall of the cell, in a similar method to the positioning of the EW anode described in the earlier example Figure 4b. The order of the cathodes 102, ER anodes 101 and EW anode 110 in Figure 7a is similar to the arrangement in Figure 5b.

In the configuration shown in Figures 7a and 7b, each cell will produce two extra cathode plates per harvest, one by electrorefining and one by electrowinning. This configuration increases the overall production of grade-A copper in the cell.

The issue of appropriate sizing of the EW anode 1 10 to avoid edge effect on the cathode 102 was discussed earlier. When further comparing Figure 7b and the earlier example Figure 4b:

In Figure 7b the EW anode 1 10 is positioned opposite a permanent cathode 102.

In Figure 4b the EW anode 1 10 is positioned opposite a cast copper ER anode 101 which acts as a cathode on the side facing the EW anode 1 10. In Figure 4b the ER anode 101 is smaller than the permanent cathode 102, and so the EW anode 110 shown in Figure 4b must be sized to be correspondingly smaller than the ER anode 101 in Figure 4b to avoid an edge-effect in the copper deposit on the neighbouring cathode 102. With the exception of size, the designs of the anodes 110 in Figures 4b and 7b can otherwise be the same.

Figure 8a shows a front elevation sketch of an EW anode positioned at the end of a cell 100. In this example the inter-cell busbars (108a, 108b) use a capping board insulator arrangement (109) as described in the examples given in Figures 7a and 7b.

The EW anode arrangement shown in Figure 8a is to be understood as present inside the bag shown in Figure 8b. The arrangements which will be described for acid mist handling during in situ liberation of copper in an ER cell are applicable to all of the previously described

arrangements as described in Figures 4 to 7, as well as to any other arrangement based on the method described in this application.

In the example discussed earlier (on page 12), with an ER electrolyte temperature of 50°C, and at a current density of 300 A/m², a single EW anode face is expected to evolve approximately 74 litres of oxygen gas per hour at sea level. Bursting oxygen bubbles causes acid-mist which must be controlled.

Figure 8b shows an arrangement for controlling the electrowinning acid mist generated by the evolution of oxygen bubbles at the EW anode surface during in situ liberation of copper. The EW anode 1 10 is located inside an "anode bag" 120 - a cloth diaphragm bag for containment of acid mist. The anode bag 120 is suspended from a frame 121. Such an arrangement is used in nickel electrowinning and is described in Prior art 13 and 14.

There is a slot in the frame 121 for receiving an EW anode and a bottom and sides made of a composite material (for example metal reinforced plastics) as described in Prior art 14.

The anode bag 120 is constructed from an acid-resistant diaphragm cloth (as described in Prior art 13). The anode bag 120 is fitted tautly over the frame 121 and held in position by clips, a groove and gasket, a drawstring, a combination of those, or another appropriate securing arrangement 122. Channels extend along the inner surface of the sides and bottom of the anode bag 120 to allow electrolyte to circulate about the immersed part of the EW anode, and for gas evolved at the EW anode - as per Eqn. (2) and Eqn. (6) - to rise within the bag 120 to the top of the frame 121. Gas trapped in the bag 120 and frame 121 is removed by means of an extraction tube 123 which extends through the top of the frame 121. The cloth of the anode bag 120 may be subject to fouling by particulates or solids such as anode slimes entrained in the copper ER electrolyte 103 circulating in the cell. It is therefore important that the bag 120 is easily exchangeable during maintenance. For this reason the appropriate securing arrangement is used to allow easy replacement of said bag 120.

In Figure 8b a sealed pump 124 can be mounted on the anode hanger bar 111. The function of the pump 124 is to remove the acid-mist gas - contained inside the bag 120 and frame 121 - via the extraction tube 123.

The electrolyte mist removed from inside the anode bag 120 and frame 121 leaves the pump 124 through a drain tube 125. Depending on the arrangement used, the tube 125 can be routed to the electrolyte overflow 105 of the cell (not shown in Figure 8, but seen in Figure 2).

The pump 124 can be controlled and powered by electronics housed in the same package as the POU converter 115 or alternatively these functions can be housed in a separate unit on or near to the hanger bar.

As an alternative arrangement, the anode bag 120 can also be open at the base - that is an anode skirt (Prior art 15). In copper electrowinning there is no requirement to maintain a pH gradient across the diaphragm cloth (as is required in nickel electrowinning), and so the cloth skirt can be open to the bulk electrolyte at the bottom. The function of the skirt is then mainly to contain the gas bubbles evolved at the anode surface to prevent acid mist.

The location of the end of the tube 123 (inside the bag 120 and frame 121), and the composition of electrolyte and/or acid mist removed by the pump 124 will depend on whether an anode bag or an open anode skirt is employed as 120.

When using an anode bag 120, a possible mode of operation is to use a negative hydrostatic head inside the bag. The tip of the extraction tube 123 may be positioned inside the anode bag 120 and frame 121 so that the pump 124 removes the oxygen-bubble rich electrolyte at the top of the bag. This will result in an electrolyte level inside the anode bag 120 which is slightly lower (by between 1 and 50mm) than the level of the electrolyte outside of the bag 103a, causing a net electrolyte flow from outside of the bag 120 to inside of the bag. Alternatively, the tip of the extraction tube 123 may also be positioned to be level with the bulk electrolyte surface 103a outside of the anode bag, or positioned above the level of 103a, if either are advantageous.

When using an anode skirt (open to the bulk electrolyte 103 at the bottom) instead of an anode bag 120, then a hydrostatic head is difficult to maintain. The skirt is used mainly to contain acid mist above the electrolyte surface 103a. The function of the tube and pump arrangement (123- 125) is again to remove mist inside the anode frame 121. The tip of the extraction tube 123 is then positioned to be level or slightly above the surface 103a of the bulk electrolyte in the cell.

Figure 9 shows the same arrangement of electrodes as shown earlier in Figures 5a and 5b. The first ER anode in the cell is replaced with an EW anode. The EW anode 1 10 is contained within an anode bag 120 and frame arrangement 121. The anode bag 121 is again held in place with a gasket 122.

In Figure 9 the arrangement of the pump 124 and tubes (123, 125) differs from the arrangement in the earlier Figure 8b. The pump 124 is now separate from the anode and positioned in this example arrangement on a shelf 126 attached to the outside of the end wall of the cell 100. The extraction tube 123 which connects the pump 124 to the inside of the anode bag may be routed over the end wall of the cell 100 (as shown in Figure 9). In an alternative arrangement, the tube 123 may penetrate through the end wall of the cell 100 (not shown).

The pump 124 may also be located in a position other than those examples given in Figure 8b and Figure 9, for example it may also be located on top of the end wall of the cell (this arrangement is not shown). The location of the pump 124 is not limited to the examples given. Nor is the method limited to the use of one pump per anode. In an alternative arrangement a shared pump may be employed serving multiple EW anodes in a single cell. As another alternative arrangement, a central pumping system may be used which serves multiple anodes located in multiple cells, as per the practice in nickel EW (see Prior art 14).

As a further alternative for acid mist control, instead of using an anode bag or skirt, an acid mist capture hood system may be employed. Mist capture hoods are used in some modern copper EW tankhouses and typically require a significant capital investment and this may be best suited to new-build Cu ER tankhouses, rather than retrofitting to an existing Cu ER plant. The use of mist-capture hoods is however, not excluded from any arrangement of apparatus used for this method.

Claims

Claims:

1. An electrorefining cell for the electrolytic refining of copper, said cell comprising:

one or more electrorefining anodes comprising impure copper;

one or more cathodes; and

an electrolyte containing copper sulfate and sulfuric acid;

wherein said electrorefining cell further comprises:

one or more electrowinning anodes; and

one or more point-of-use power converters configured to supply current to the or each electrowinning anode whereby to control the copper concentration of the electrolyte when said cell is in use.

2. An electrorefining cell as claimed in claim 1 , wherein the or each electrowinning anode comprises a lead-based alloy (e.g. a rolled lead-calcium-tin alloy) or is a mixed metal oxide (MMO) coated titanium anode.

3. An electrorefining cell as claimed in claim 2, wherein the or each electrowinning anode is a mixed metal oxide (MMO) coated titanium mesh.

4. An electrorefining cell as claimed in any one of the preceding claims, wherein the or each cathode has a stainless steel blade, or is comprised of copper starter-sheets or titanium starter sheets.

5. An electrorefining cell as claimed in any one of the preceding claims, further comprising brackets or guides configured to maintain the or each electrowinning anode in a fixed position whilst the cell is in use.

6. An electrorefining cell as claimed in any one of the preceding claims which comprises an electrowinning anode positioned adjacent to an end wall of the cell.

7. An electrorefining cell as claimed in claim 6, wherein the electrowinning anode is an

asymmetric anode.

8. An electrorefining cell as claimed in claim 7, wherein the asymmetric anode comprises a single sheet of MMO coated titanium mesh mounted on the side of one or more titanium- clad copper bars, and wherein said anode is positioned such that the MMO coated titanium mesh faces towards the neighbouring cathode.

9. An electrorefining cell as claimed in claim 7 or claim 8, wherein the electrowinning anode is angled towards the neighbouring electrode.

10. An electrorefining cell as claimed in claim 9, wherein the electrowinning anode is

orientated at an angle of up to 10° from the vertical.

11. An electrorefining cell as claimed in any one of claims 1 to 5 which comprises a

symmetrical electrowinning anode positioned between two cathodes.

12. An electrorefining cell as claimed in claim 1 1 , wherein the symmetrical electrowinning anode comprises two MMO coated titanium meshes, one on either side of a titanium-clad bar.

13. An electrorefining cell as claimed in claim 12, wherein each mesh of the electrowinning anode is angled towards an opposing cathode.

14. An electrorefining cell as claimed in any one of the preceding claims which comprises two point-of-use power converters per electrowinning anode.

15. An electrorefining cell as claimed in any one of the preceding claims, further comprising a regulator configured to control the current supplied to the or each electrowinning anode.

16. An electrorefining cell as claimed in any one of the preceding claims, further comprising means for controlling the acid mist generated by oxygen bubbles which evolve at the surface of the or each electrowinning anode when said cell is in use.

17. An electrorefining cell as claimed in claim 16, wherein said means for controlling the acid mist comprises an anode bag or an anode skirt.

18. A method of electrorefining of copper from impure copper, said method comprising the use of an electrorefining cell as claimed in any one of claims 1 to 17. A method of producing an electrorefining cell as claimed in any one of claims 1 to 17, said method comprising the following steps:

providing a conventional electrorefining cell comprising n cathodes and n+1 electrorefining anodes; and

fitting one or more electrowinning anodes to said cell, or

replacing one or more of said electrorefining anodes by one or more electrowinning anodes.

An electrorefining cell produced by a method as claimed in claim 19.