The effect of magnetic field and operating parameters on cathodic copper winning in electrowinning process

Chemical Engineering Science 199 (2019) 1–19

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The effect of magnetic field and operating parameters on cathodic copper winning in electrowinning process Mahjabin Najminoori a, Ali Mohebbi a,⇑, Kambiz Afrooz b, Babak Ghadami Arabi c a

Department of Chemical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Department of Electrical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran c Sarcheshmeh Copper Complex, Iran b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Copper electrowinning cell was

studied under magnetic field and operating parameters.
A continuous process in presence of an industrial Fe3+ ion in electrolyte was used.
Magnetic field decreased current efficiency in presence of Fe3+ ions as rival ions. 3+
Increasing Fe concentration decreased CE and formed FT microstructure. 2+
All factors except Cu concentration had overall positive effect on thickness uniformity.

a r t i c l e

i n f o

Article history: Received 10 August 2018 Received in revised form 4 December 2018 Accepted 31 December 2018 Available online 23 January 2019 Keywords: Magnetic field Operating parameter Copper Electrowinning Cathodic copper morphology

⇑ Corresponding author. Tel./ Fax: +983432118298 E-mail address: [email protected] (A. Mohebbi). https://doi.org/10.1016/j.ces.2018.12.061 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t Current efficiency and quality of deposited copper are important parameters in copper electrowinning industries. The effects of imposing an external magnetic field, Fe3+ and Cu2+ mass concentrations and electrical current density on the current efficiency, the thickness of the cathodic copper sheet and its morphology in copper electrowinning process were studied at different experimental operating conditions in a cuboid cell. A full factorial design of experiment is applied here to relate current efficiency and the standard deviation (STDEV) of the thickness with and without magnetic field, Fe3+ concentration (2.5 and 3.5 g/L), Cu2+ concentration (35 and 40 g/L) and electrical current density (150 and 220 A/m2) with constant acid concentration of 175 g/L at temperature of 40 °C for a synthetic electrolyte. Scanning electron microscope (SEM) is applied in studying the surface morphology of the deposited cathodic copper in all tests. The obtained results indicated that the presence of the magnetic field and increasing Fe3+ concentration could decrease the current efficiency up to 17 and 7%, respectively. While increasing Cu2+ concentration and electrical current density increased the current efficiency. Presence of magnetic field, increasing Fe3+ concentration and electrical current density increased the thickness uniformity of copper sheets. Investigation of SEM surface images revealed variations in these main parameters had different effects on deposited cathodic copper morphology. Ó 2019 Elsevier Ltd. All rights reserved.

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Nomenclature B c CE CD cref Ctd d F i iL k R T

magnitude of the imposed magnetic field (Tesla),
 copper concentration mol=m3 current efficiency (%) drag coefficient reference concentration (initial concentration of copper) (kg=m3 ) turbulent dispersion coefficient gas bubble diameter (m) Faraday’s constantðA:s=molÞ electrical current density (A=m2 ) limiting current density (A=m2 ) turbulence kinetic energy (j=kg) general gas constantðj=mol:KÞ absolute temperatureðKÞ

1. Introduction The quality of cathodic copper in copper electrowinning is one of the most important parameters of cell performance. Copper deposited on the cathode must be uniform and firm. Some parameters, which affect the quality of deposition structure and crystal shape consist of: incoming metal ions rate, the surface finishing of the base metal and inhibition intensity. Moreover, these parameters are influenced by metal ion concentration, additives, electrical current density, temperature, agitation, and polarization. In simple terms, it can be stated that more mixing and uniformity in electrolyte solution lead to a more uniform distribution of copper ions on the cathode surface. Consequently, these ions are placed inside electron sites in proper form and increase the uniformity and quality of the deposited copper. For the first time, the structure of deposited copper from a sulphate solution was studied by Barnes et al. (1960); Pick et al. (1960), where it was discovered that changing the current density changed the structure. Their observations revealed that an increase in the current density changed the structure growth from fibers shape ? plates ? blocks ? fine crystals. They observed that using impurities, which affect the potential of the solution, and increasing the temperature both increase the current density required for the formation of the same structure in a solution. At low current densities, the structures of the ridge, plate, or platelet are observed. Authors in Winand (1994) have shown that the structure formed in the electrodeposition processes, is the outcome of a competition between the new grain and the growth of the existing seeds, either by lateral growth or by longitudinal growth. It is also known that the parameters which increase the cathode polarization, tend to reduce crystal size. Polarization increases with an increase in current density; which leads to a decrease in crystal size. Of course, it should be noted that polarization alone is not enough to characterize the observed structures. Winand (1994) suggested the two parameters below to describe the structures of electrodeposited metal: 1. ðic =iL Þ or ðic =C b Þ: the current density to the diffusion limiting current density or to the bulk concentration ratio. 2. Inhibition intensity: the extent of inhibition due to the existence of ions in the electrolyte which are different from the ones being deposited on the cathode surface and is also related to the exchange current density. According to Doesburg and Ivey (2000) metals with low exchange current density are known as high inhibitors (e.g., nickel and iron), while metals with high exchange current density as weak inhibitors (e.g., silver and tin).

! ! u!2  u!1 slip velocity (m=s)  u 2  u 1  size of slip velocity V voltage (v) z valency of ion (-) ai volume fraction of phase i (i = 1 and 2 indicate liquid and gas respectively) (-) b copper concentration-related coefficient of expan
 sion m3 =kg d diffusion layer thichness (m) l magnetic permeability (H=m) 3 qref reference density (taken as electrolyte
density) (kg=m ) vm molar magnetic susceptibility m3 =mol x angular velocity (rad=s)

The five types of microstructures devised through the singleelectrolytic electroplating process were first categorized by Fischer (1954), briefed as: (1) Field-oriented isolated crystals (FI): these crystals are observed in conditions with low inhibition intensity and by increasing the current density strings, prismatic crystals, dendrites and powders are formed, respectively. (2) Basis-oriented reproduction (BR): in these microstructures, there is more lateral growth than that of the FI; of course, the crystals are large enough to hold the electrolyte in the deposit. This type has a rough column structure observed at medium current density and inhibition intensity, which provides lateral growth. (3) Twinning intermediate (Z): this type of microstructure is an intermediate between the second (BR) and the fourth (FT) types, on which there exists little information. (4) Field-oriented texture (FT): this microstructure is observed at relatively high inhibition and/or high-current density and appears in large granules form, extended perpendicular to the surface of the cathode. (5) Unoriented dispersion (UD): this type is observed at very high inhibition intensity and/or high current density and is formed by a large number of small crystals in the structure. Experimental observations of scientists have shown that an increase in current density increases the thickness of the growth layers which decrease in presence of inhibitors; therefore, in presence of a constant inhibitor the lateral growth rate decreases by an increase in the current density, which forms isolated crystals or even dendrites. It is observed that when the inhibition intensity increases at a constant current density, the lateral growth rate increases, leading to coherence deposits with high-density. In general, at high inhibition intensity and/or high ðic =C b Þ, the new crystals are formed by a three-dimensional germination (Winand, 1994). The introduction of another parameter like a magnetic field can provide a technique to control the microstructure of deposited copper. Recently researches have revealed that an increase in convection in electrolyte solution, increases electrolyte stirring, which is also important to keep the copper concentration in the electrolyte at a constant value and causes a decrease in limiting diffusion boundary layer thickness, increases cathodic mass transfer rate, consequently, an increase in uniformity of the deposited cathodic copper.

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Magnetoelectrolysis, electrolysis through imposed magnetic field (Fahidy, 1983), which can increase convection in the electrolyte solution, has been and is being investigated. The magnetic field can influence mass transport, deposit morphology and electrode kinetics of electrochemical reactions (Hinds et al., 2001). In 1972, Mohanta and Fahidy (1972) investigated the effect of coupling of the electric and uniform magnetic fields on the mass transfer rates of copper at the electrode from acidified copper sulphate solution. Their results revealed that the diffusion layer thickness decreases and mass transfer rate increases in the presence of magnetic field, which are due to the Lorentz force produced by the coupling of two electric and magnetic fields. Presence of magnetic field as great as 0.7 T increases the cathodic limiting current density by 30% in a 0.05 M solution. The influence of an imposed magnetic field on electrochemical processes are those run mostly by Fahidy (1983) and Tacken and Janssen (1995). In 1997, Shannon et al. (1997) studied the effect of magnetic field on deposited nickel morphology in two vertical and horizontal orientations, where confocal scanning laser microscopy is applied. Both the imposed magnetic fields increase the uniformity of cathodic deposition in relation to the case where the magnetic field is absent. By applying vertical magnetic field, the uniformity increases with an increase in magnetic flux density, while the most uniform deposit is obtained at the lowest magnetic flux density of the horizontal field. In 2003, Alfantazi and Valic (2003) investigated the effects of copper electrowinning parameters (current density, temperature and copper concentration) on the current efficiency and the quality of the deposited cathode. Current efficiency increases by an increase in all these parameters with the least effect from current density. Increasing current density and copper concentration lead to deposition of larger copper crystal size. Here temperature is of little effect on crystal size and morphology. Their results indicated that the current efficiency alone could not be applied as a predictor of copper cathode quality. Bund et al. (2003) studied the effect of magnetic field on the electrochemical behaviour of the Cu and Ni electrodeposition. The four (two parallels and two perpendiculars) orientations of B field in relation to electric current lines are investigated. The results indicate that by natural convection and the Lorentz force interaction, limiting current density for Cu deposition increases in the presence of the perpendicular magnetic field. Magnetohydrodynamic (MHD) convection effect is influential in Ni deposition grain size and more fine-grained deposition is formed in presence of magnetic field. In 2005 and 2007, Krause et al. (2005); All factors except Cu2+ concentration had overall positive effect on thickness uniformity. studied the effect of magnetic field on current density, deposition rate and current efficiency during the electrodeposition of Co, Ni and Cu for two parallel and perpendicular orientations in relation to electric field. Their experiments are run by applying corresponding metal sulphate solution where sodium sulphate is added as supporting electrolyte. (containing ions are not considered as rival ion for corresponding metal ions and its conductivity is much larger than that of electroactive species). They observed the micro vortexes in the diffusion layer, which caused magnetoconvection. This phenomenon is known as micro-magneto- convection (MMC). The effects of MHD convection and MMC are investigated and the results indicate that an increase in magnetic field decreases the deposition rate in parallel orientation due to the dominant effect of MMC. For perpendicular orientation, a reduction in deposition rate is observed at the beginning due to the MMC effect followed by an increase in it due to MHD effect. Matsushima et al. (2007), investigated crystal orientation and surface morphology of electrodeposited copper in a sulphuric acid solution in presence of magnetic field (0–5 T). An increase in magnetic field intensity decreases the grain size and suppress the columnar dendritic electrodeposits growth in a significant manner.

MHD convection increases the surface concentration of Cu2+, which decreases the i/iL ratio that ends the dendrites growth. Yue et al. (2009), investigated the effects of current density and temperature on crystal orientations and surface microstructure of electrodeposited aluminium of mild steel in an ionic liquid. Their results indicate that the deposit morphology is independent of current density at low temperature, while at high temperatures, the type of deposit microstructure changes as a function of current density in a sense that an increase in current density, makes deposition of smaller particles more dense. At highest current density, non-spherical particles are formed as small clusters on the surface. In 2013, Mühlenhoff (2013) studied the effect of MHD convection on copper deposition thickness uniformity in a cubic cell with vertical electrodes. The Lorentz force is induced through two pairs of NdFeB permanent magnets and cause a vertical motion in the electrolyte. The two antiparallel and parallel orientations between the Lorentz and buoyancy forces are applied and the results indicate that in the first, the profile of deposit layer thickness is similar to the case of without magnetic field and in the second, the thickness distribution is more uniform than that of without the magnetic field. It is notable that in previous studies (Krause et al., 2005; Krause et al., 2007) where batch process is run the metal deposition takes place only when the corresponding metal sulphate solution in acid sulphuric is consumed or a supporting electrolyte is added to the solution. In their studies only copper ion is reduced at cathode and if a positive ion (e.g., Na2SO4 as a supporting electrolyte) is in electrolyte, the standard reduction potential of it was less than that of the corresponding metal (Cu2+) and it did not act as a rival ion for consumption of electrons at the cathode. In this study due to having the same conditions as an industrial process, a continuous process is run and Fe3+ ion is present as a rival ion in the electrolyte solution. 2. Theory In electrochemical processes, a static magnetic field, B, can be contributive in inducing convection in an aqueous solution by the means of Lorentz force, F L . In brief, the main body forces acting on the electrolyte solution, whether in the absence or presence of magnetic field, are tabulated in Table 1 (Hinds et al., 2001). The first seven forces of this table do not rely on magnetic field, while the rest are subject to the magnetic field. The driving force for

Table 1 Forces acting on electrolyte in copper electrowinning process. Nos

Force

Expression

1

!  Diffusion F D

2

Electromigration

3

Natural convection (concentration-related !  buoyancy) A i

! RT r c ! zFc r V h i ! ! ! A i ¼ ai qi g bðc  cref Þ ; A 2 ¼ 0

4

phase- related buoyancy !  Drag F i

! ! ! ! ! ! F 2 ¼  F 1 ¼  34 CdD q1 a2  u 2  u 1 ð u 2  u 1 Þ

qðrxÞ2 =2d0

5

ai ðqi  qref Þ! g

6

Forced convection

7

Turbulent

! ! T 2 ¼  T 1 ¼ Ctd qi kra2

8

Paramagnetic !  force F P

! B2 vm r c=2l0

9

Field gradient !  force F B !  Lorentz force F L

vm cBr B=l0

10

!  dispersion T i

!

! ! i  B

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diffusion only apply in the boundary layer near the electrode where, there exists ions concentration gradient and in the electrolyte bulk, this force is negligible (Hinds et al., 2001). Applying an electric field causes ionic species movement in electrolyte toward the electrode of opposite charge. This movement is the driving force for electromigration (Loong Hey, 2007). Driving force for natural convection is a consequence of density gradients (qbulk > qN) which arise due to electrolyte in the front of the cathode (Nernst layer, density qN) depleted from the cation species and causes an upward flow in this thin layer (Bund et al., Dec. 2003). In addition to the natural convection, known as concentration-related buoyancy force, another type of buoyancy force known as phaserelated buoyancy force acts on gas bubbles; accelerates their motion upward and drag electrolyte upward (Najminoori et al., 2015). Due to the stationary electrode and low Reynolds number, the forced convection that appears in the case of a rotating electrode and turbulent dispersion forces are considered zero in this study. Applying magnetic field during electrodeposition of ionic species can generate the last three forces in Table 1. When a magnetic field is applied on the electrolyte with a non-uniform concentration of ions in the Nernst layer, a variation in the paramagnetic susceptibility (v of the Nernst layer is lower than that of bulk because it is depleted from the paramagnetic ions Cu2+) generates the para! magnetic gradient force ( F P ). This force which acts in the direction ! of the concentration gradient ( r c) is parallel to the driving force for diffusion, counteracts with this force and consequently reduces the mass deposition rate. If deposition is controlled by diffusion, diffusion force can be one of the strongest force acting close to electrode (Alfantazi and Valic, 2003; Krause et al., 2005). The driv! ing force of paramagnetic force ðB2 vm r c=2l0 Þ to that of diffusion ! force ðRT r cÞ ratio is ðB2 vm =2l0 RTÞ with a small magnitude order, hence, according to Hinds et al. (2001) the effect of this force is negligible. Non- uniformity magnetic field and existence of field gradient ! ! ( r B) generate the field gradient force ( F B ), which can be caused by the source of the magnetic field or by a magnetic electrode (deposited magnetic layers or ferromagnetic electrodes) located in a homogeneous magnetic field. In this study, it is assumed that magnetic field is uniform over the whole the electrochemical cell ! ! ! volume and F B is zero. Both F P and F B depend on magnetic properties of the electrolyte and are independent of magnetic field direction (Krause et al., 2007). The interaction between homogeneous electric and static magnetic fields generates Lorentz force, which acts on the moving ions, and is calculated through Eq. (1):

! ! ! FL ¼ i  B

ð1Þ

! ! where i (Am2 ) is the electrical current density and B (T) is the magnitude of the magnetic field. If the lines of magnetic field are perpendicular to the electric field lines, the Lorentz force gets the maximum value, and in the parallel direction, the effect of the Lorentz force becomes negligible (Ispas and Bund, 2005). The Lorentz force causes additional convection in the hydrodynamic layer near the surface of electrode (Krause et al., 2005). As observed in ! ! Fig. 1, the Lorentz force is in directed perpendicular to i and B plane (right-hand rule), consequently, perpendicular to rc and recent studies indicate that efficient convective mass transport results in this condition (Coey et al., 2007). Furthermore, there exists a component FCL, which is the result of acting of the Lorentz force on the convectional flow (Bund et al., 2003). In this study, both convectional flow and magnetic field lines are in the positive y-direction, Fig. 1, subsequently, according to the right-hand rule, the FCL is negligible. Body force vectors that

Fig. 1. Schematic representation of fields and force vectors that act on electrolyte (FL is perpendicular to the electric and magnetic fields plane).

act on the electrolyte solution in the presence of magnetic field are shown in Fig. 1, where the Lorentz force appears in the XZ plane and causes a movement of electrolyte in this direction. By pouring a few pepper powder on the electrolyte surface, the pathway of the electrolyte on the cell surface is detected. 3. Materials and methods 3.1. Electrolyte solution This solution is a mixture of copper sulphate, Fe (III) sulphate, sulphuric acid, double distilled water, cobalt(II) sulphate and govar. All of analytical grade chemical materials consumed in this study were purchased from Merck, Germany. No other additional agent was used. Constant values of parameters in all tests are tabulated in Table 2. 3.2. Methods The method adopted in this study is analytic–descriptive. A schematic of the experimental setup for copper electrowinning process is shown in Fig. 2. The tests are run subject to different operating conditions. All tests are run inside a cubic cell made of PMMA (polymethyl methacrylate) with inner dimensions, Fig. 3b. As observed in Fig. 2, the electrolyte is entered into the cell by a Watson Marlow 505 Du peristaltic pump. The electrowinning cell consists of two electrodes, one cathode and one anode. In order to provide similar industrial conditions, the cathode and anode were made of stainless steel 316L and Pb-Sn-Ca alloy, respectively. The reason of using Pb-Sn-Ca alloy anode (rather than Cu) is that in copper electrowinning process, the anode transfers copper from the electrolyte to the cathode surface as a mediator. If Cu anode is used instead, the anode is corroded into the electrolyte, leads to an increase in the copper ions of the electrolyte and causes some problems in electrowinning process. The purpose of copper electrowinning process is to extract the copper from the electrolyte rather than anode and the use of copper anodes is common in the electrorefining process. Dissolving Pb-Sn-Ca alloy anode followed by PbO2 formation is also an issue in copper electrowinning process, which could contaminate the electrolyte. The corrosion rate is reported about 2.48 mm/year and formation of a stable PbO2 layer takes as long as 30 to 60 days in practice (Msindo, 2010; Tunnicliffe, 2011). In this study, because of low lead corrosion rate and experimental test time (2 h), anodic dissolution could be neglected.

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M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19 Table 2 Constant values of parameters in all of the tests. Parameter

Temperature

Volumetric flow rate of electrolyte

Time of process

Cobalt(II) ion concentration

Govar concentration

Sulphuric acid concentration

value

40 °C

2ðcc=minÞ

2h

0.12 ðg=LÞ

0.0001ðg=LÞ

175ðg=LÞ

Fig. 2. The schematic of used copper electrowinning process setup.

Fig. 3. Details of the experimental setup: (a) cathode and anode dimensions, (b) electrowinning cell dimensions and (c) magnets in the electrowinning cell.

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M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19

The electrodes are cut to laboratory scale from industrial cathode and anode. Dimensions of cathode and anode are (L 5.5 cm  W 2.7 cm  H 0.3 cm) and (L 5.5 cm  W 2.7 cm  H 0.6 cm), respectively, Fig. 3a. The dimensions of cathode and anode hanger bars are: (L 11.5 cm  W 1.5 cm  H 0.3 cm) and (L 11.5 cm  W 1.5 cm  H 0.6 cm), respectively and are of the same quality of that of the electrodes. To have a better electrical conductivity, these hanger bars are covered with a thin copper foil. Before each test, the electrodes are polished with sandpaper and rinsed in double distilled water and acetone. The side of the cathode not facing the anode side is coated with colourless lacquer to prevent backside plating. A GPS -3303 laboratory DC power supply, which converts AC into DC, is applied in this process. To keep temperature constant, here, the electrowinning cell is placed in a Thermo Haake Phoenix water bath. The spent exits through a hose located on the cell top. The electrodes are placed parallel to each other on the cell edge within 25 mm distance in all tests, Fig. 3b. As observed in Fig. 3c, two Neodymium- iron- boron (NdFeB) grade 42 permanent magnets with dimensions of (L 100 mm  W 20 mm  H 20 mm) are placed at the top and bottom of the cell within 33 mm distance. The surface magnetization of each magnet is about 1.3 T. This pair of magnets are arranged in a manner that their magnetic field lines are perpendicular to the electric field direction. There exist many parameters that affect copper electrowinning cell performance and morphology of deposited copper like electrical current density, temperature, the mass concentrations of Cu2+, Fe3+, cobalt(II) ion and govar, process time, sulphuric acid concentration, the volumetric flow rate of electrolyte etc. If magnetoelectrolysis is applied, magnitude and direction of the magnetic field become the very important parameters affecting morphology and current efficiency of deposited cathodic copper. In this study, more important parameters, like magnetic field, mass concentrations of Cu2+ and Fe3+and electrical current density are selected. At the beginning of this study, the tests order is determined through the statistical software package, Minitab. Each parameter is of two upper and lower levels, which are close to common maximum and minimum levels of operating parameters applied in industrial copper electrowinning cell in Sarcheshmeh Copper Complex, Iran. In this study, a full factorial method is adopted for the tests. Based on this method, 2k tests are required in examining the effects of parameters at two levels, where k is the number of parameters. However, a higher design level, i.e. three-levels produces more information about the process while experience reveals that, this approach is not necessary in this study, which involves a synthetic solution. It is notable that for a real solution other important variables like the substrate, the process time and major impurities should be introduced (Alfantazi and Valic, 2003). Here, 16 tests must be run in two replications, that is 32 tests to make the analysis reliable. These many tests are to specify the significance of each parameter and determine the existence of any considerable interaction among the selected parameters. After each test, high-resolution SEM images with different magnifications of (50, 200, 1k, 5k, 10k) are obtained from the cathode surface. The SEM images with 50, 100 and 1k zoom expose the surface uniformity (macroscopic), while 5k and 10k detect surface microstructure. The thickness of the deposited layer is measured at different points on three parts named: bottom, middle and upper, which are averaged over each part. 4. Results and discussion Copper current efficiency is the amount of electrical current consumed in depositing copper and is calculated through Eq. (2):

CE ¼

mactual  100 mtheoretical

ð2Þ

where m actual and m theoretical are the actual mass of plated copper and theoretical mass of copper that should have been deposited based on the Faraday’s Law, respectively.

mtheoretical ¼

i:A:t:Mw n:F

ð3Þ

where A (m2), t (s), Mw (g/mol) and n (-) are the effective cathode area, process time, the molecular weight of Cu and the number of transmitted electrons per mole of deposited copper (2 for Cu), respectively (Joy et al., 2010). As observed in Fig. 3, the height of the cathode is 2.7 cm, but because of solution exit location, only 2.3 cm of it is immersed in the electrolyte, making the effective surface of the cathode is 12:65  104 m2. Because the cathode is partly immersed in the electrolyte solution, it is far from ideal and causes corrosive water-line effects on the electrodes which enhances in the presence of an organic (from solvent extraction circuits) matter in the electrolyte and the effect is more important on the anode rather than the cathode. Because the synthetic electrolyte solution is free of organic matter and the corrosion effect is not an issue in this study, this effect is not investigated. The parameter i (Am2 ) in Eq. (3) is the total electrical current density. In an industrial electrowinning process other Faradic reactions other than copper reduction contribute to the consumption of electrical current and this causes a decrease in the cathodic current efficiency. Possible electrode reactions during the electrowinning of copper consist of: at cathode:

Cu2þ þ 2e ! Cu ðmain reactionÞ ðE ¼ 0:34 VÞ

ð4Þ

Fe3þ þ e ! Fe2þ

ð5Þ

2Hþ þ 2e ! H2

ðsecondary reactionÞ ðE ¼ 0:773 VÞ ðsecondary reactionÞ ðE ¼ 0:00 VÞ

ð6Þ

ðmain reactionÞ ðE ¼ 0:773 VÞ

ð7Þ

and at anode:

Fe2þ ! Fe3þ þ e

H2 O ! 0:5O2 þ 2Hþ þ 2e

ðsecondaryreactionÞ ðE ¼ 1:23VÞ ð8Þ

All experimental conditions of this study including the mass of deposited copper and current efficiency in replications 1 and 2 are tabulated in Table 3. In this table, numbers 0 and 1 indicate absence and the presence of magnetic field, respectively. In order to have high precise calculations, the deposited copper mass is measured with a Sartorius BP121S analytical balance at 0.0001 gr or 0.1 mg accuracy. The relations between current efficiency as a dependent variable and the magnetic field, electrical current density and electrolyte mass concentrations of Cu2+ and Fe3+ as independent variables are obtained through the full factorial experiment design. The analysis of variance results is run through the statistical software package, Minitab which applies the F-value to calculate the p-value. The F-value is a value on the F distribution (a theoretical distribution) which is used in the analysis of variance (ANOVA) and the p-value is a probability that measures the evidence versus the null hypothesis. These two values are applied to make a determination about the statistical significance of the experiment. Lower probabilities (usually less than 0.05) provide stronger evidence against the null hypothesis and a high P-value indicates that there exists no association between the variables and results. As observed in Table 4, the P-values for the independent variables and their two-, three- and four-way interactions are less than 0.05, thus, the main and interactive parameters are statistically

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M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19 Table 3 Mass of deposited copper and current efficiency in different conditions * 0 without magnetic field, 1 with magnetic field. Test number

Current density A/ m2

Cu2+ conc. g/L

Fe3+ conc. g/L

Presence of magnetic field*

Mass of deposited copper (mg) 0.1 replication# 1

Mass of deposited copper (mg)  0.1 replication# 2

Current efficiency (CE %), replication# 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

150 220 150 220 150 220 150 220 150 220 150 220 150 220 150 220

35 35 35 35 40 40 40 40 35 35 35 35 40 40 40 40

2.5 2.5 3.5 3.5 2.5 2.5 3.5 3.5 2.5 2.5 3.5 3.5 2.5 2.5 3.5 3.5

0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

425.7 617.2 395.8 610.5 426.1 621.9 420:9 617.5 357.9 579.2 329.6 563.9 383.7 609.1 378.3 603.0

424.4 618.6 396.4 608.9 426.2 622.5 419.3 615.3 359.6 578.3 328.9 562.6 384.4 608.5 379.0 604.4

94.70 93.59 88.05 92.57 94.77 94.31 93.62 93.64 79.61 87.82 73.33 85.51 85.34 92.36 84.16 91.44

Table 4 Analysis of variance for current efficiency of all tests. Variables (parameters)

F-Value

PValue

Model Linear Magnetic field Fe3+ concentration Cu2+ concentration Current density 2-Way interactions Magnetic field * Fe3+ concentration Magnetic field * Cu2+ concentration Magnetic field * Current density Fe3+ concentration * Cu2+ concentration Fe3+ concentration * Current density Cu2+ concentration * Current density 3-Way interactions Magnetic field * Fe3+ concentration * Cu2+ concentration Magnetic field * Fe3+ concentration * Current density Magnetic field * Cu2+ concentration * Current density Fe3+ concentration * Cu2+ concentration * Current density 4-Way interactions Magnetic field * Fe3+ concentration * Cu2+ concentration * Current density

3118.10 9354.26 21850.31 2222.67 6150.68 7193.35 1490.01 4.78 2073.31 5030.28 791.78 544.28 495.64 102.46 11.02 5.49 21.58 371.77 4.56 4.56

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.044 0.000 0.000 0.000 0.000 0.000 0.000 0.004 0.032 0.000 0.000 0.049 0.049

0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01

Current efficiency (CE %), replication# 2 94.41 93.81 88.18 92.33 94.81 94.39 93.28 93.31 79.98 87.69 73.16 85.31 85.52 92.28 84.31 91.65

0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01

parts of the cathode surface, which in turn can be due to the two main sources of: 1) depletion of electrolyte from copper ions ? natural convection and upward flow near the cathode ? the electrolyte with lower Cu2+ concentration to move towards the higher points of the cathode surface and 2) as to the location of the inlet and outlet, a fresh electrolyte at a constant concentration enters from the lower part of the cell and exits at the higher part. Therefore, the electrolyte is of a lower Cu2+ concentration at the upper zone of the cathode surface, thus, less thickness. The analysis of variance is run for the thickness of deposited copper STDEV, Table 6. The independent variables and their interactions cause all probability values (P-values) to become zero. Therefore, the main and interactive parameters are statistically independent in a significant sense. Statistical analysis of the STDEV of the thickness data leads to a linear relation between this parameter and independent parameters. The main and interactive parameter effects on the STDEV of thickness are shown in Figs. 6 and 7. The effect of each parameter on the STDEV is fully explained in its corresponding section.

4.1. The effect of magnetic field presence independent. Analyzing the data is run to determine the coefficients of the linear relation between the parameters. Statistical analysis of these 32 data should result in a relationship between current efficiency and independent variables by considering the interactive effects of these parameters. In factorial experimental design method, this model is a linear relation. The type and extent of studied main and interactive parameters effects can be determined by applying Figs. 4 and 5 content, respectively. That the presence of the magnetic field and Fe3+ concentration have negative effects on current efficiency while Cu2+ concentration and electrical current density have positive effects, is shown in Fig. 4. Further details on the effect of each parameter would be presented in their appropriate section. Another parameter that changed significantly is the deposited copper layer thickness on the cathode surface. Details of the thickness of deposited copper are tabulated in Table 5. The thicknesses at the current density of 150 A/m2, 35 g/L Cu2+ and 2.5 g/L Fe3+ concentrations in the absence of the magnetic field are 113.236, 79.376 and 42.51 lm at the bottom, the middle and the upper of the sample, respectively. Layer thickness reduction is due to difference in the concentration of copper ion at different

The main objective of this study is the investigation of the effect of magnetic field presence on electrowinning cell performance. As observed in Figs. 4 and 5 and Table 3, unlike previous studies, the applied magnetic field decreases the deposited copper mass and current efficiency. The reason for this reduction could be due to an increase in hydrogen evolution reaction (HER) as discussed by Krause et al. (2005). In general, the following two phenomena affect the efficiency due to the presence of magnetic field: (1) MHD and (2) MMC. Although MMC is generated by the interaction between concentration gradient force and paramagnetic force, its difference with MHD is that the later causes an increase in efficiency while MMC decreases efficiency. The effect of these phenomena depends on the direction of magnetic and electric fields relative to each other. As mentioned before if the magnetic field is perpendicular to the electric field, MHD is dominant and if parallel, the MHD is negligible and MMC is dominant. In this study, the relation orientation between fields is perpendicular and based on the available findings it is expected that the current efficiency would increase by the presence of magnetic field due to Lorentz force (MHD) but as stated, previous researches did not consider the effect of any rival ion. As observed in Eqs. (4) and (5), Fe3+ ions with higher standard reduction potential are the rival ions for Cu2+

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Fig. 4. Main parameter effects on current efficiency (dashed line: average value of current efficiency in all tests).

Fig. 5. Interactive parameter effects on current efficiency.

Table 5 The thickness of deposited copper at three zones on the cathode surface. Test number

Current density A/ m2

Cu2+ conc. g/L

Fe3+ conc. g/L

Presence of magnetic field

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

150 220 150 220 150 220 150 220 150 220 150 220 150 220 150 220

35 35 35 35 40 40 40 40 35 35 35 35 40 40 40 40

2.5 2.5 3.5 3.5 2.5 2.5 3.5 3.5 2.5 2.5 3.5 3.5 2.5 2.5 3.5 3.5

0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

Thickness of sample at different zones (lm) Upper

middle

bottom

42.510 71.476 35.176 81.823 85.020 67.150 56.053 75.990 41.066 118.033 82.763 145.146 58.310 138.876 152.987 121.946

79.376 109.473 35.173 66.206 105.523 86.146 31.410 85.773 61.756 152.570 57.683 136.056 134.803 168.976 66.146 205.023

113.236 186.780 85.985 98.455 154.803 101.950 56.806 106.900 155.180 172.110 67.400 139.503 145.460 125.396 164.273 248.286

Average thickness of sample (lm)

78.374 122.576 52.111 82.161 115.115 85.082 48.089 89.554 86.000 147.571 69.282 140.235 112.857 144.416 127.802 191.751

M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19 Table 6 Analysis of variance for STDEV of sample thickness at all tests. Variables (parameters)

FValue

PValue

Model Linear Magnetic field Fe3+ concentration Cu2+ concentration Current density 2-Way interactions Magnetic field * Fe3+ concentration Magnetic field * Cu2+ concentration Magnetic field * Current density Fe3+ concentration * Cu2+ concentration Fe3+ concentration * Current density Cu2+ concentration * Current density 3-Way interactions Magnetic field * Fe3+ concentration * Cu2+ concentration Magnetic field * Fe3+ concentration * Current density Magnetic field * Cu2+ concentration * Current density Fe3+ concentration * Cu2+ concentration * Current density 4-Way interactions Magnetic field * Fe3+ concentration * Cu2+ concentration * Current density

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

9

ions and are thermodynamically favoured over Cu2+ ions reduction. Because Fe3+ charge is more than that of Cu2+ and this ion is lighter with smaller ionic radius in comparison with Cu2+, Table 7. Here, the exerted electric force on Fe3+, its velocity, the Lorentz force and their stirring in the electrolyte are greater than those of Cu2+. The effects of intensity of the main parameters and their interaction on current efficiency are shown in Fig. 8, where, as observed the main important parameters in this study consist of: presence of magnetic field, electrical current density, Cu2+ and Fe3+ concentrations, respectively. As to interactive effects, the magnetic fieldelectrical current density is the most effective and interaction of magnetic field- Fe3+ concentration- Cu2+ concentration is of the least effect on current efficiency. As observed in Fig. 5, the magnetic field interaction with all other parameters has a negative effect on current efficiency. As observed in Table 8 and Fig. 5, decreasing in the current efficiency due to the presence of the magnetic field is more in high values of Fe3+ concentration. As mentioned before, this is due to the effect of Lorentz force on Fe3+ ions which is more than that of Cu2+ ions. A decrease in CE due to magnetic field presence is more at low levels of electrical current density, because in this case, the electron sites on the cathode surface decrease, while, in

Fig. 6. Main parameter effects on STDEV of thickness (dashed line: average value of STDEV of thickness in all tests).

Fig. 7. Interactive parameter effects on STDEV of thickness.

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Table 7 Density and ionic radius of Fe3+ and Cu2+ (Joy et al., 2010; Rao, 1986). Parameter 3

Density (kg/m ) Ionic radius (pm)

Fe3+

Cu2+

7870 69

8960 87

the presence of magnetic field, Fe3+ ions are more likely to consume electrons. It is expected that a decrease in CE because of magnetic field presence is more at low levels of Cu2+ concentration, Table 8. The SEM images of experimental tests with 35 g/L Cu2+ and 2.5 g/L Fe3+ concentrations and 150 A/m2 current density as operating conditions in absence and presence of magnetic field is shown in Fig. 9, where in section a, there exist clearing cavities in the absence of magnetic field, with 100 lm size at some points. These cavities or pores might have developed by trapping hydrogen gas bubbles generated at the cathode according to Eq. (6). The number of these cavities decrease extremely in presence of magnetic field. According to Fig. 10, in microscopic studies of the samples, it is observed that in presence of magnetic field the fibers are formed coarser and thicker. This could be due to the force applied on the

ions in the solution and make them to sit in the lateral locations of the fibers. As observed in Fig. 6, the presence of magnetic field has a little effect on thickness uniformity of the deposited layer and slightly reduces STDEV of the thickness, while as observed in Fig. 7, its effect is different at low and high levels of Fe3+ and Cu2+ concentrations. This indicates that applying magnetic field at low levels of ion concentrations causes more uniformity in thickness and reduce the STDEV, with an inverse effect on high levels of the ion concentrations. In Fig. 7 the effect of current density on STDEV of thickness with and without magnetic field is the same. In brief, the presence of magnetic field has a positive effect on thickness uniformity. The effects of magnetic field on thickness variation for four tests are shown through cross-section images in Fig. 11. In Table 5, it is found that the average thickness of samples in presence of magnetic field is higher than that of without magnetic field in the same conditions, while according to Table 3, these samples have less mass, thus, less CE. Therefore, the magnetic field reduces the deposited layer densification or make it puffy. This result can be achieved by comparing SEM images (a) and (b) or (c) and (d) in Fig. 11.

Fig. 8. Intensity of main and interactive parameters on the current efficiency.

Table 8 Decrease in CE due to the presence of the magnetic field. Test number        

1 9 2 10 3 11 4 12 5 13 6 14 7 15 8 16

Test parameters (current density A/m2, Cu2+ conc. g/L, Fe3+ conc. g/L, presence of magnetic field)  150; 35; 2:5; 0 150; 35; 2:5; 1  220; 35; 2:5; 0 220; 35; 2:5; 1  150; 35; 3:5; 0 150; 35; 3:5; 1  220; 35; 3:5; 0 220; 35; 3:5; 1  150; 40; 2:5; 0 150; 40; 2:5; 1  220; 40; 2:5; 0 220; 40; 2:5; 1  150; 40; 3:5; 0 150; 40; 3:5; 1  220; 40; 3:5; 0 220; 40; 3:5; 1

Current efficiency (CE %), replication# 1  94:70  0:02 79:61  0:02  93:59  0:01 87:82  0:01  88:05  0:02 73:33  0:02  92:57  0:01 85:51  0:01  94:77  0:02 85:34  0:02  94:31  0:01 92:36  0:01  93:62  0:02 84:16  0:02  93:64  0:01 91:44  0:01

Current efficiency (CE %), replication# 2  94:41  0:02 79:98  0:02  93:81  0:01 87:69  0:01  88:18  0:02 73:16  0:02  92:33  0:01 85:31  0:01  94:81  0:02 85:52  0:02  94:39  0:01 92:28  0:01  93:28  0:02 84:31  0:02  93:31  0:01 91:65  0:01

Decrease in CE due to presence of magnetic field (%), replication# 1

Decrease in CE due to presence of magnetic field (%), replication# 2

15.94

15.28

6.15

6.52

16.72

17.04

7.63

7.60

9.95

9.80

2.07

2.24

10.11

9.61

2.36

1.78

M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19

11

Fig. 9. SEM images with 50 magnification: (a) test No. 1 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (b) test No. 9 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, with magnetic field).

Fig. 10. SEM images with 10k magnification: (a) test No. 1 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (b) test No. 9 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, with magnetic field).

4.2. The effect of increasing electrical current density in the presence and absence of the magnetic field Recent findings indicate that there are different results on the effect of current density on current efficiency. Some of them report direct effect (Alfantazi and Valic, 2003) or inverse effect (Yue et al., 2009) and others (Bao et al., 2018; Das and Krishna, 1996) state that the mentioned effect depends on current density range. A small decrease in CE can be seen in Table 9 only for two current density increases. But as shown in Fig. 4 increasing the current density totally results in a significant improvement in current efficiency and this effect is observed in interaction with other parameters as well, Fig. 5. It is revealed that an increase in current density from 150 to 220 A/m2 would weaken the effect of magnetic field and Fe3+ concentration. An increase in electrical current density, in addition to an increase in current efficiency, affects copper structure, Fig. 12. The SEM images with 200 magnification in Fig. 12 show that an increase in current density from 150 to 220 A/m2, forms fine and spherical crystals on cathode surface. By comparing Fig. 12b and c, it is observed that when the magnetic field is applied to the process at the current density of 220 A/m2, more

uniform deposition takes place and there is no trace of spherical shape of the grains. Fine crystals observed at 1kx and 5k magnifications are shown in Fig. 13a and b, where crystals do not appear to be separate particles and are located in some parts of the structure with high growth rates. By comparing Figs. 10a and 13c, it is found that an increase in current density provides an appropriate potential for increasing growth rate, which is a solid growth at a particular direction in sample structure. Moreover, by comparing Fig. 13a and d the effect of the magnetic field on the uniformity is clearly observed. Another effect of the magnetic field on the microstructure of the deposited cathode with 220 A/m2 current density, is achieved by comparing Figs. 14 and 13b, where the deposited cathode structure is close to BR type with compressed strings, which is of a Zstructure, but after applying the magnetic field, this structure becomes more regular and interconnected. This finding is in accordance with Doesburg and Ivey (2000). It can be argued that an increase in current density increases growth rate and formation of coarse-grained structures, and by applying the magnetic field, faster movement of the ions is yielded; therefore, it is possible to develop structures that grow in all directions.

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Fig. 11. Cross section SEM images of samples: (a) test No. 6 (Current density: 220 A/m2, Cu2+ concentration: 40 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (b) test No. 14 (Current density: 220 A/m2, Cu2+ concentration: 40 g/L, Fe3+ concentration: 2.5 g/L, with magnetic field); (c) test No. 8 (Current density: 220 A/m2, Cu2+ concentration: 40 g/L, Fe3+ concentration: 3.5 g/L, without magnetic field); (d) test No. 16 (Current density: 220 A/m2, Cu2+ concentration: 40 g/L, Fe3+ concentration: 3.5 g/L, with magnetic field).

Table 9 Variations in CE due to increasing electrical current density. Test number        

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Test parameters (current density A/m2, Cu2+ conc. g/L, Fe3+ conc. g/L, presence of magnetic field)  150; 35; 2:5; 0 220; 35; 2:5; 0  150; 35; 3:5; 0 220; 35; 3:5; 0  150; 40; 2:5; 0 220; 40; 2:5; 0  150; 40; 3:5; 0 220; 40; 3:5; 0  150; 35; 2:5; 1 220; 35; 2:5; 1  150; 35; 3:5; 1 220; 35; 3:5; 1  150; 40; 2:5; 1 220; 40; 2:5; 1  150; 40; 3:5; 1 220; 40; 3:5; 1

Current efficiency (CE %), replication# 1  94:70  0:02 93:59  0:01  88:05  0:02 92:57  0:01  94:77  0:02 94:31  0:01  93:62  0:02 93:64  0:01  79:61  0:02 87:82  0:01  73:33  0:02 85:51  0:01  85:34  0:02 92:36  0:01  84:16  0:02 91:44  0:01

Current efficiency (CE %), replication# 2  94:41  0:02 93:81  0:01  88:18  0:02 92:33  0:01  94:81  0:02 94:39  0:01  93:28  0:02 93:31  0:01  79:98  0:02 87:69  0:01  73:16  0:02 85:31  0:01  85:52  0:02 92:28  0:01  84:31  0:02 91:65  0:01

Variation in CE due to increasing current density (%), replication# 1

Variation in CE due to increasing current density (%), replication # 2

1.19

0.64

4.88

4.49

0.49

0.44

0.03

0.03

9.36

8.79

14.24

14.24

7.59

7.33

7.96

8.01

M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19

13

Fig. 12. SEM images with 200 magnification: (a) test No. 1 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (b) test No. 2 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (c) test No. 10 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, with magnetic field).

As expected, an increase in current density increases the deposition rate of the solution, thus, an increase in deposited layer thickness. As to effect of current density on thickness STDEV, Fig. 6, an increase in current density decreases this STDEV and forms a more uniform deposited layer. The interaction of current density with other parameters has a positive effect on the uniformity thickness, Fig. 7. 4.3. The effect of increasing Fe3+ concentration in the presence and absence of the magnetic field Based on the obtained results, it is observed that under constant current density, an increase in Fe3+ concentration decreases the current efficiency, and this is quite clear in lower current densities. When two or more reactions occur in a simultaneous manner at the cathode, the number of total consumable electrons corresponds to the sum of the number of electrons that each reaction consumes. As explained before, Fe3+ reduction is thermodynamically favoured over copper deposition because of its more positive standard reduction potential and the rate of this reaction is limited by its mass transfer towards the cathode, consequently, it occurs at its limiting current density expressed through Eq. (9), (Joy et al., 2010):

iL ¼ nFC

DFe3þ d

ð9Þ

where C (mol/m3) and DFe3þ ðm2 =sÞ are the Fe3+ ion bulk concentration and its diffusivity, respectively. According to Eq. (5), Fe3+ ions in electrolyte consume some electrons, and are reduced at the cathode to Fe2+ ions. Fe2+ ions migrate to the anode and subsequently reoxidize into Fe3+ ions, see Eq. (7). This process occurs in a continuous manner. As Eq. (9), Fig. 4 and Table 10 indicate, an increase in Fe3+ concentration decreases CE due to an increase in the amount of limiting current density of Fe3+ reduction. This finding corresponds with that of Das and Krishna (1996). Fe3+ concentration interaction with other parameters is negative for current efficiency. It can be claimed that for the same increase in Fe3+ concentration, a decrease in CE is less for the cases where the Cu2+ concentration or current density are higher, Fig. 5 and Table 10. As a result of acting the Lorentz force on Fe3+ ions, the stirring of these ions in the electrolyte is more than that of Cu2+ ions; therefore, it is expected that in the presence of magnetic field, an increase in Fe3+ mobility is more than that in Cu2+. Consequently, a decrease in CE due to an increase in Fe3+ concentration is more in the presence of magnetic field. This effect is not clearly shown in Fig. 5, but by comparing the data of the first four rows in Table 10 with those of the second four rows of the same table in replication 1, leads to this result. The electrical charges of both Cu2+ and Fe3+ ions are positive, and a repulsive force is generated between them. This force near

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Fig. 13. SEM images (a) 1k magnification; (b) 5k magnification and (c) 10k magnification of test No. 2 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (d) 1k magnification of test No. 10 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, with magnetic field).

Fig. 14. SEM image with 5k magnification for test No. 10 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, with magnetic field).

the cathode surface disperses Cu2+ ions on the surface, thus, it is expected that cathode surface uniformity increases by an increase in Fe3+ concentration. An increase in Fe3+ concentration increases the effect of inhibition intensity and changes the deposition structure form and its orientation. The effect of increasing Fe3+ concentration on surface morphology is investigated here.

The SEM image of a sample with 35 g/L Cu2+ and 3.5 g/L Fe3+ concentrations and 150 A/m2 current density is shown in Fig. 15a. In the primary inspection and comparison of this image with Fig. 9a, it is observed that an increase in Fe3+ concentration increases the uniformity of coating and decreases the number of cavities and comparing Fig. 15a and b show that presence of magnetic field increases this uniformity. By comparing the structure formed from test Nos.1 and 3 in Fig. 16, it is observed that an increase in Fe3+ concentration changes the structure from the case of two-strand structures (BR and Z) to the FT structure, which is more perpendicular to the cathode surface. Different structures are found in samples that are deposited subject to 220 A/m2 current density from the solution containing 35 g/L Cu2+ and 3.5 g/L Fe3+. Comparing Figs. 17a and 13a indicates that an increase in Fe3+ concentration at high current densities and low Cu2+ concentrations leads to stoppage of fine particles and bullets growth on the surface. For more investigation of the effect of an increase in Fe3+ concentration on the morphology of deposited copper, another 5k magnification SEM image from another area of test No. 2 is shown in Fig. 18a and is compared to the image of test No. 4 (Fig. 18b). This comparison shows that an increase in Fe3+ concentration in absence of magnetic field decreases the growth of thin filaments and fine grains on the surface, that is, as observed in Fig. 19, subject to both high Fe3+ concentration and current density conditions, the strands become thicker and merging of these thick strands generate a different morphology.

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M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19 Table 10 Decrease in CE due to increasing Fe3+ concentration. Test number        

1 3 2 4 5 7 6 8 9 11 10 12 13 15 14 16

Test parameter (current density A/m2, Cu2+ conc. g/L, Fe3+ conc. g/L, presence of magnetic field)  150; 35; 2:5; 0 150; 35; 3:5; 0  220; 35; 2:5; 0 220; 35; 3:5; 0  150; 40; 2:5; 0 150; 40; 3:5; 0  220; 40; 2:5; 0 220; 40; 3:5; 0  150; 35; 2:5; 1 150; 35; 3:5; 1  220; 35; 2:5; 1 220; 35; 3:5; 1  150; 40; 2:5; 1 150; 40; 3:5; 1  220; 40; 2:5; 1 220; 40; 3:5; 1

Current efficiency (CE %), replication# 1  94:70  0:02 88:05  0:02  93:59  0:01 92:57  0:01  94:77  0:02 93:62  0:02  94:31  0:01 93:64  0:01  79:61  0:02 73:33  0:02  87:82  0:01 85:51  0:01  85:34  0:02 84:16  0:02  92:36  0:01 91:44  0:01

Current efficiency (CE %), replication# 2  94:41  0:02 88:18  0:02  93:81  0:01 92:33  0:01  94:81  0:02 93:28  0:02  94:39  0:01 93:31  0:01  79:98  0:02 73:16  0:02  87:69  0:01 85:31  0:01  85:52  0:02 84:31  0:02  92:28  0:01 91:65  0:01

Decrease in CE due to increasing Fe3+ concentration (%), replication# 1

Decrease in CE due to increasing Fe3+ concentration (%), replication # 2

7.02

6.60

1.08

1.58

1.22

1.62

0.71

1.15

7.89

8.53

2.64

2.72

1.39

1.41

1.00

0.68

Fig. 15. SEM images with 50 magnification: (a) test No. 3 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, without magnetic field); (b) test No. 11 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, with magnetic field).

Fig. 16. SEM images with 5k magnification: (a) test No. 1 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (b) test No. 3 (Current density: 150 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, without magnetic field).

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Fig. 17. SEM images with 1k magnification: (a) test No. 4 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, without magnetic field); (b) test No. 12 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, with magnetic field).

Fig. 18. SEM images with 5k magnification: (a) test No. 2 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field); (b) test No. 4 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, without magnetic field); (c) test No. 12 (Current density: 220 A/m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, with magnetic field).

The effect of an applied magnetic field on the morphology of surface is observed in Figs. 17 and 18, which proves that the magnetic field caused coarser deposit and more lateral growth of the strands.

The general positive effect of increasing Fe3+ concentration on the thickness uniformity of deposited copper is shown in Fig. 6. By considering that the electrolyte solution entered from the bottom of the cell and according to the results, it could be deduced

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M. Najminoori et al. / Chemical Engineering Science 199 (2019) 1–19

which increases the thickness of the cathodic copper sheet in the upper region. To clarify this deduction, it can be claimed that by increasing Fe3+ concentration, Fe3+ reduction to Fe2+ reaction rate increases, which decreases the copper deposition rate at lower points, and a solution with higher Cu2+ concentration and of course less Fe3+ concentration moves towards the higher points of the cathode and provides conditions for increasing the cathode thickness in upper regions.

4.4. The effect of increasing Cu2+ concentration in the presence and absence of the magnetic field

Fig. 19. SEM image with 10k magnification for test No. 4 (Current density: 220 A/ m2, Cu2+ concentration: 35 g/L, Fe3+ concentration: 3.5 g/L, without magnetic field).

that by increasing the Fe3+ concentration, the solution in contact with the upper part of the cathode has a higher Cu2+ concentration than the case of entering solution with lower Fe3+ concentration,

By increasing Cu2+ concentration, CE increases and this positive effect can be observed in the interaction of this parameter with others in Fig. 5. The percentage of increase in CE is greater for the high levels of Fe3+ concentration in both with and without the magnetic field cases, Fig. 5, Table 11. This CE increases more in presence of magnetic field compared to its absence. This result may be due to the agitation of electrolyte in presence of magnetic field that makes it easy to replace depleted electrolyte with fresh electrolyte, which contains Cu2+ and it causes the efficient consumption of Cu2+ in electrolyte during electrodeposition.

Table 11 Increase in CE due to increasing Cu2+ concentration. Test number        

1 5 2 6 3 7 4 8 9 13 10 14 11 15 12 16

Test parameter (current density A/m2, Cu2+ conc. g/L, Fe3+ conc. g/L, presence of magnetic field)  150; 35; 2:5; 0 150; 40; 2:5; 0  220; 35; 2:5; 0 220; 40; 2:5; 0  150; 35; 3:5; 0 150; 40; 3:5; 0  220; 35; 3:5; 0 220; 40; 3:5; 0  150; 35; 2:5; 1 150; 40; 2:5; 1  220; 35; 2:5; 1 220; 40; 2:5; 1  150; 35; 3:5; 1 150; 40; 3:5; 1  220; 35; 3:5; 1 220; 40; 3:5; 1

Current efficiency (CE %), replication# 1  94:70  0:02 94:77  0:02  93:59  0:01 94:31  0:01  88:05  0:02 93:62  0:02  92:57  0:01 93:64  0:01  79:61  0:02 85:34  0:02  87:82  0:01 92:36  0:01  73:33  0:02 84:16  0:02  85:51  0:01 91:44  0:01

Current efficiency (CE %), replication# 2  94:41  0:02 94:81  0:02  93:81  0:01 94:39  0:01  88:18  0:02 93:28  0:02  92:33  0:01 93:31  0:01  79:98  0:02 85:52  0:02  87:69  0:01 92:28  0:01  73:16  0:02 84:31  0:02  85:31  0:01 91:65  0:01

Increase in CE due to increasing Cu2+ concentration (%), replication# 1

Increase in CE due to increasing Cu2+ concentration (%), replication # 2

0.07

0.04

0.77

0.62

5.94

5.46

1.15

1.05

6.72

6.47

4.91

4.97

12.87

13.23

6.48

6.92

Fig. 20. SEM images of test No. 5 (Current density: 150 A/m2, Cu2+ concentration: 40 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field), (a) 50 magnification; (b) 200 magnification.

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Fig. 21. SEM image with 200 magnification for test No. 6 (Current density: 220 A/ m2, Cu2+ concentration: 40 g/L, Fe3+ concentration: 2.5 g/L, without magnetic field).

A comparison between the results of test Nos. 1 and 5 in Figs. 9a and 20a show that an increase in Cu2+ concentration decreases the number of pores on the cathode surface. In fact, on some parts of the surface in test No. 1, no deposition takes place and porosity is formed on the surface but in the test No. 5, almost the whole surface is covered with copper deposits. The reason for this is that with an increase in Cu2+ concentration, there is enough Cu2+ ion to be deposited on the entire surface of the cathode and this result is obtained by comparing Figs. 12a and 20b as well. By comparing Figs. 12b and 21 it is revealed that an increase in Cu2+ concentration at high electrical current density improves the quality of cathode surface because of bullet particles formation stoppage. As mentioned before, an increase in Cu2+ concentration at low electrical current densities (test Nos.1 and 5) improves the quality of deposited copper as well because of reducing bullet particles, but at high electrical current densities (test Nos. 2 and 6) a more reduction of the bullet particles is observed. The results indicate that an increase in Cu2+ concentration in presence of magnetic field or at high levels of Fe3+ concentrations has no considerable effect on the microstructure of deposited copper. As observed in Fig. 6, an increase in Cu2+ concentration increases the STDEV of thickness and decreases the uniformity of deposited thickness. But as observed in Fig. 7 the interaction effect between Cu2+ concentration and the magnetic field is vice versa in the presence and absence of magnetic field. The effect of Cu2+ concentration on STDEV of thickness varies at different levels of Fe3+ concentrations. Further studies are necessary to determine the effect of Cu2+ concentration on STDEV of thickness and microstructure of deposited copper. 5. Conclusions In this study, a full factorial experimental design is applied in investigating the effect of Fe3+ and Cu2+ concentrations and electrical current density in presence and absence of the magnetic field on the current efficiency, thickness uniformity and structure morphology of the deposited copper. Because the purpose here is an examination of industrial parameters, all selected values are commonly applied by copper electrowinning in Sarcheshmeh Copper Complex, Iran. More important conclusions in this study consist of:
The independent variables and their interactions are statistically independent in a significant sense Applying magnetic field

increases electrolyte stirring but decreases current efficiency, because the standard reduction potential of Fe3+ ions is higher than that of Cu2+. Moreover, in presence of magnetic field, the thickness and structure uniformity of deposited copper increase and coarser fibers are formed on the surface.
Increasing electrical current density increases CE and decreases the inhibition intensity, caused by Fe3+ ions. Because the growth rate is enhanced by an increase in electrical current density, the STDEV of thickness decreases. In this case, presence of magnetic field causes a change in morphology of the deposited copper and make it more uniform.
An increase in Fe3+ concentration increases inhibition intensity and decreases current efficiency up to 7%. Presence of Fe3+ ions in electrolyte lead to the formation of FT structure on the cathode surface, but after applying the magnetic field the surface becomes smoother.
The obtained results here indicate that an increase in Cu2+ concentration has a positive effect on current efficiency and STDEV of the thickness of deposited copper. The stirring of the electrolyte in presence of magnetic field increases this effect as well. By increasing Cu2+ ions in the electrolyte, the morphology of the cathode surface is improved by decreasing bullet shapes of deposit. Finally, further studies need to be run to understand the influence of all parameters. Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the efforts of Sarcheshmeh Copper Complex, Iran in financial support of this work. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ces.2018.12.061. References Alfantazi, A.M., Valic, D., 2003. A study of copper electrowinning parameters using a statistically designed methodology. J. Appl. Electrochem. 33 (2), 217–225. Bao, S., Chai, D., Shi, Z., Wang, J., Liang, G., Zhang, Y., 2018. Effects of Current Density on Current Efficiency in Low Temperature Electrolysis with Vertical Electrode Structure. Springer, Cham, pp. 611–619. Barnes, S.C., Storey, G.G., Pick, H.J., 1960. The structure of electrodeposited copper— III: the effect of current density and temperature on growth habit. Electrochim. Acta 2 (1–3), 195–204. Bund, A., Koehler, S., Kuehnlein, H.H., Plieth, W., 2003. Magnetic field effects in electrochemical reactions. Electrochim. Acta 49 (1), 147–152. Coey, J.M.D., Rhen, F.M.F., Dunne, P., McMurry, S., 2007. The magnetic concentration gradient force—Is it real? J. Solid State Electrochem. 11 (6), 711–717. Das, S.C., Krishna, P.G., 1996. Effect of Fe (III) during copper electrowinning at higher current density. Miner. Process. 46, 91–105. Doesburg, J., Ivey, D., 2000. Microstructure and preferred orientation of Au–Sn alloy plated deposits. Mater. Sci. Eng. B 78 (1), 44–52. Fahidy, T.Z., 1983. Magnetoelectrolysis. J. Appl. Electrochem. 13 (5), 553–563. Fischer, H., 1954. Elektrolytische abscheidung und elektrokristallisation von metallen, vol. 12. Springer, Berlin Heidelberg, Berlin, Heidelberg. Hinds, G., Coey, J.M.D., Lyons, M.E.G., 2001. Influence of magnetic forces on electrochemical mass transport. Electrochem. Commun. 3 (5), 215–218. Ispas, A., Bund, A., 2005. Influence of a magnetic field on the electrodeposition of nickel and nickel- iron alloys. 15th Riga 6th PAMIR Conf. Fundam. Appl. MHD Magnetoelectrolysis. Joy, S., Staley, A., Perkins, C., Uhrie, J., Moats, M., 2010. Understanding and improvement of electrowinning current efficiency at FMI Bagdad. Proc. Copp. 2010 4, 1379–1392. Krause, A., Uhlemann, M., Gebert, A., Schultz, L., 2005. The effect of magnetic fields on the electrodeposition of Co, Ni and Cu. 15th Riga 6th PAMIR Conf. Fundam. Appl. MHD Magnetoelectrolysis.

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