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Journal of Alloys and Compounds 465 (2008) 255–260

Electrolytic magnesium production and its hydrodynamicsby using an Mg–Pb alloy cathode

Gokhan Demirci, Ishak Karakaya ∗Department of Metallurgical and Materials Engineering, Middle East Technical University,

Inonu Bulvari, 06531 Ankara, Turkey

Received 21 March 2007; received in revised form 10 October 2007; accepted 13 October 2007Available online 24 October 2007

bstract

Physical interaction of magnesium and chlorine was minimized by collecting magnesium in a molten Pb cathode at the bottom of the electrolytend placing anode at the top where the chlorine gas was evolved. Thus the magnesium losses associated with the formation of suspending dropletsnd fine magnesium particles were eliminated and current losses were mainly due to the recombination reaction of dissolved magnesium andhlorine. Current yield changed by changing the tip angle of the conical anode. It was due to the fact that the amount of chlorine diffused into theelt was proportional to the chlorine bubble area in contact with the electrolyte per unit time. Therefore, correlation of experimentally measured

lectrolysis data requires the knowledge of the size and the total residence time of the chlorine bubbles in inter-electrode region. Average diameternd total residence time of the bubbles were determined for anode tip angles that were used in electrolysis experiments by a room temperatureydrodynamic model. Amount of magnesium that was lost as a result of reaction with the dissolved chlorine was calculated by assuming the

issolution of chlorine gas as the rate determining step. Theoretical magnesium losses calculated by using the data from the room temperatureydrodynamic model were in good agreement with the electrolysis experiments. Furthermore, calculated cell voltages that use the sum of theoreticalecomposition potential and IR drop obtained from the composite resistance due to the electrolyte and chlorine bubbles were also in agreementith the experimental data.2007 Elsevier B.V. All rights reserved.

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eywords: Magnesium; Magnesium alloy; Molten salt; Magnesium chloride; H

. Introduction

The main reason of current losses in electrolytic magnesiumroduction is the reaction between chlorine and magnesium pro-uced at the anode and the cathode, respectively. The interactionetween the electrode products in their elemental state is highlyependent on cell geometry and could be decreased by increas-ng inter-electrode distance or employing a separation wall asn early cell designs [1]. Larger inter-electrode distance, on thether hand, increases the cell voltage and energy consumption.he residence time of the magnesium metal and chlorine gas in

he inter-electrode region was shortened by increased electrolyte

elocity due to lifting action of chlorine gas at smaller inter-lectrode distances in recent industrial magnesium cell designs2,3]. However the drastic decrease in energy consumption that

∗ Corresponding author. Tel.: +90 312 210 2533; fax: +90 312 210 1330.E-mail address: kkaya@metu.edu.tr (I. Karakaya).

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925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2007.10.070

dynamics

as to be achieved to have a considerable decrease in produc-ion costs has not been experienced with newer cell designs yet.he recombination reaction still remains as the main reason forurrent inefficiency.

It was reported that the dissolution of magnesium into theelt was the decisive factor that controls the back reaction

4,5] as a result of the calculations based on systems that mightnvolve fine magnesium particles. This assumption is not quiten accord with higher current efficiencies obtained from cellesigns involving fast removal of chlorine from electrolysisells [6,7]. More accurate results could be obtained with a cellesign where the fine magnesium particle formation was elim-nated. Moreover the bubble characteristics that were obtainedor the actual cell design and electrolysis conditions, as in thease of present study, could help to increase the viability of the

ssessments.

The physical interaction between the chlorine gas and mag-esium metal was minimized with the present cell design [8,9].agnesium was collected as an Mg–Pb alloy at the bottom of

2 lloys and Compounds 465 (2008) 255–260

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Fig. 1. Schematic diagram of the hydrodynamic model. A: graphite rod modelaee

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56 G. Demirci, I. Karakaya / Journal of A

he cell and the chlorine gas was evolved from the graphite anodet the top. Magnesium losses associated with the formation ofuspending droplets and fine magnesium particles were alsoliminated by molten pool cathode design. Therefore, the backeaction was limited to the interaction of dissolved magnesiumnd chlorine. That feature provides advantages to investi-ate the back reaction mechanism that remains still partiallyevealed.

The present cell design [9] was used successfully in collect-ng electrolysis data and as a model to correlate cell voltagend current efficiencies to the theoretical decomposition poten-ial of MgCl2, electrolyte resistance, physiochemical propertiesf electrolyte and cell dynamics. Since cell geometry remainedonstant in the previous study [9], current efficiency computa-ions were limited to only one geometrical arrangement. Anodeurface orientation with respect to the horizontal level, on thether hand, affects both the size and the residence time ofhlorine bubbles in inter-electrode region. Therefore differenthlorine dissolution rates can be obtained from the chlorine bub-les into the melt by changing the tip angle of the conical anode.urthermore, a hydrodynamic model can be used to calculate

heoretical magnesium losses as a result of the recombinationeaction after obtaining the size and residence time of the bub-les. The required data about the size and the residence time ofhe bubbles were acquired via a room temperature hydrodynamic

odel to avoid the difficulties associated with the measurementsn actual electrolysis conditions.

. Experimental

Details of the experimental set-up for the electrolysis experiments and cal-ulations were given elsewhere [9]. Schematic diagram of the hydrodynamicodel is given in Fig. 1.

Gas evolution at the anode surface was simulated by passing argon gashrough a 1.75 cm diameter cylindrical porous graphite rod for the hydrody-amic model. Graphite rod was of the same origin as the anodes used for thelectrolysis experiments. All graphite surfaces other than the lower end, whereas liberation occurred, and the upper end, where argon gas connection wasade, were covered with oil-paint to prevent any gas leak.

The model anode was immersed into 10 wt% NaNO3 containing water solu-ion at room temperature that represents magnesium cell electrolyte at 700 ◦C.he required similarity between the chloride melts and the modeling solutionas satisfied by using a strong ionic solution which has comparable density,iscosity, and surface tension to chloride melts. The physical factors that controlhe flow pattern within the electrolyte have been established as [10]:

cell = Qmodel (1)

cell = νmodel (2)

γ

ρ

)cell

=(

γ

ρ

)model

(3)

here Q is volumetric rate of gas evolution, ν is kinematic viscosity of the elec-rolyte, γ is surface tension of the electrolyte and ρ is density of the electrolyte.

t

aF

able 1omparison of physical properties of the model liquid with the actual electrolyte

arameter 10 wt% NaNO3–H2O at 20 ◦C

(m2 s−1) 1.00 × 10−6

/ρ (m3 s−2) 69.5 × 10−6

node, B: 10 wt% NaNO3 solution, C: digital camera and video camera, D:xtruded foam support, E: reflective aluminum sheet, F: spotlights, G: transpar-nt polycarbonate container, H: argon gas, I: ratemeter.

he density and viscosity of the 20 wt% MgCl2, 1:1 NaCl:KCl electrolyte is.544 g cm−3 and 1.95 cP at 700 ◦C respectively [1]. The surface tension oflectrolyte was taken as 94.5 dyne cm−1 [1]. Table 1 shows that these conditionsere approximately satisfied when 10 wt% NaNO3–H2O solution was used as

he model liquid. Gas flow rate per unit area of model anode was determined as2 ml cm−2 min−1 for argon gas at room temperature corresponding to chlorineroduction rate at 0.48 A cm−2 applied current density and around 700 ◦C usedor the electrolysis experiments.

Hydrodynamic model set-up was tilted to the required inclination that corre-ponds to the anode tip angle used in electrolysis experiments. Bubble formationn the anode surface was recorded by a Panasonic NV-GS75 video camera thatas placed under the model cell as shown in Fig. 1. Video camera was replacedy a Panasonic DMC-FZ5 digital camera to take pictures of bubbles on thenode surface that simulate chlorine bubbles during electrolysis. Higher resolu-ion pictures obtained by the digital camera were used to determine the averageubble size.

. Results and discussion

.1. Electrolysis results

Full details of electrolysis experiments and the resultsbtained are given elsewhere [11]. They are summarized inable 2. The variation of net cell voltage, En, is given in Fig. 2.n is found by subtracting the short circuit voltage from thebserved cell voltage. The full line given in Fig. 2 was calcu-ated by adding theoretical decomposition potential [12] and IRrops. IR drops used in computations include resistance due to

he electrolyte and chlorine bubbles [9].

Current efficiency had a marked tendency to drop at largernode tip angles as can be seen in Fig. 3. The full line given inig. 3 was calculated from the hydrodynamic model that will

20 wt% MgCl2, 1:1 NaCl:KCl at 700 ◦C

1.26 × 10−6

61.2 × 10−6

G. Demirci, I. Karakaya / Journal of Alloys and Compounds 465 (2008) 255–260 257

Table 2Results and details of the electrolysis experiments

Experiment no.

1 [8] 2 3 4

Anode tip angle (◦) 98 118 147 180Temperature, T (◦C) 690 690 705 690Electrolysis duration, (min) 60 25 25 31Mg collected, WMg (g) 0.81 0.29 0.27 0.31Theoretical Mg collected, Wth (g) 0.91 0.37 0.37 0.47Applied current, I (A) 2.0 2.0 2.0 2.0Electrode surface area, A (cm2) 4.16 4.16 4.16 4.16Current density, j (A cm−2) 0.48 0.48 0.48 0.48Average net cell voltage, En (V) 3.72 3.43 3.16 3.57Percent current efficiency (%CE) 89.3 77.9 72.9 65.4Percent energy efficiency (%EE) 63.9 59.3 60.2 47.8Energy for electrolysis (kWh kg−1 Mg) 9.2 9.7 9.6 12.0

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ig. 2. The effect of anode geometry on the net cell voltage. Circles are exper-mental results. The full line is calculated by adding theoretical decompositionotential and IR drops.

e discussed later. Energy consumption was nearly constant at

bout 9.5 kWh for producing 1 kg of magnesium at lower anodeip angles and it increased at higher anode tip angles as can beeen in Fig. 4.

ig. 3. The effect of anode geometry on current efficiency. Circles are experi-ental results. The full line is calculated from the hydrodynamic model.

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ig. 4. The effect of anode geometry on energy consumption. Circles are exper-mental results. The full line is calculated from the hydrodynamic model.

.2. Hydrodynamic model

The correlation of the experimentally measured net cell volt-ge and current efficiency to the hydrodynamic model used inhis study requires the knowledge of the average size and totalesidence time of bubbles in the electrolyte. The average bubbleize for each geometrical arrangement of anode was determinedy analyzing 2560 × 1920 pixel photographs of bubbles takenrom the model anode. The pictures of bubbles for indicatednode tip angles are given in Fig. 5.

The largest 20 bubbles at the upper half of the anode were usedo determine the average bubble size just before detachment,xcept for the 180◦ anode tip angle where mature bubbles movedandomly. The bubble size increased as the angle between thenode surface and the horizontal level decreased as given inable 3.

Total residence time of bubbles in the electrolyte is the sum ofhe time elapsed during nucleation and the time during the escapef bubbles. The time it takes for nucleation and the escape veloc-ties of bubbles were determined by analyzing 50 consecutive

40 × 480 pixel photographs stored per second by the videoamera. The time it takes for bubbles to raise inside the elec-rolyte and escape from the top was calculated for the actual

able 3ummary of the data collected from hydrodynamic model

Anode tip angle (◦)98 118 147 180

ubble size, d (cm) 0.12 0.13 0.16 0.22esidence time for nucleation (s) 0.06 0.10 0.22 0.44esidence time for bubble raise (s) 0.04 0.04 0.04 0.07otal residence time, t (s) 0.10 0.14 0.26 0.51ubble production rate (bubbles s−1) 913 718 385 148lectrolysis duration (min) 60 25 25 31hlorine area in contact with electrolyte,Ab (cm2 s−1)

4.13 5.34 8.05 11.49

hlorine flux (10−3 mol m−2 s−1) 3.49 3.49 3.49 3.49g loss rate (10−6 mol s−1) 1.44 1.86 2.81 4.01alculated Mg loss (g) 0.126 0.068 0.103 0.181alculated current efficiency (%) 89.3 82.0 72.9 61.3

258 G. Demirci, I. Karakaya / Journal of Alloys and Compounds 465 (2008) 255–260

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Fig. 5. Photographs of bubbles that were formed on model anode. Anode

node geometry by using the bubble raise velocities. The dis-ance through which the bubbles moved was taken as the distancerom the circle that corresponds to half of the anode area to thedge along the conical anode surface.

Photographs obtained from the video camera recordings forubble velocity and residence time determinations are given inig. 6. The corresponding anode tip angles are indicated at the

op of each column in this figure. The 10 observable nucleationites are shown by cross signs at the same sites on subsequentictures. The time for bubble nucleation was taken as the averagef the measurements for the 10 nucleation sites for 98◦, 118◦ and47◦ anode tip angles. Only one nucleation site was marked for80◦ anode tip angle because there was a constant movementf the bubbles in the recorded pictures. The times for bubbleso nucleate and raise inside the electrolyte for anode tip anglessed in electrolysis experiments are given in Table 3.

The reaction between magnesium and chlorine is fast. Dis-olution of either magnesium or chlorine is the rate determiningtep and they cannot coexist inside the melt in dissolved form.

agnesium and chlorine solubilities are 2.0 and 0.8 mol m−3

13,14] respectively in the technical magnesium electrolysis.herefore, it is reasonable to assume the dissolution of chlo-

ine as the rate determining step of MgCl2 formation due to itsower solubility.

From the knowledge of average size and total residence timeor bubbles in the electrolyte together with the kinetic proper-

0cfi

gles: 98◦, 118◦, 147◦ and 180◦ are indicated on corresponding pictures.

ies, magnesium that was theoretically consumed as a result ofhe recombination reaction could be calculated from the chlo-ine dissolution into the melt. Using 0.413 cm3 A−1 s−1 chlorineroduction rate, total chlorine surface area in contact with elec-rolyte per unit time, Ab, is calculated as:

b = 0.413I

4/3πr3 4πr2t (4)

here I is applied current, r is average bubble radius and t ishe total residence time of the bubbles in inter-electrode region.hen the chlorine flux from bubble surface into the melt inol m−2 s−1, NCl2 , was calculated from the Fick’s Law:

Cl2 = −DCl2�CCl2

δ(5)

he diffusion coefficients of molecular chlorine in molten alkalihlorides and magnesium chloride are of the order of 0.5 to 2.0 ×0−7 m2 s−1 [1]. Diffusivity of Cl2, DCl2 , is approximately cal-ulated as 0.96 × 10−7 m2 s−1 for the melt composition used inhis study. Electrolyte was assumed to be saturated with Cl2 athe bubble surface. Therefore concentration gradient of Cl2 athe bubble surface, �CCl2 , is taken as its solubility in the melt,

.8 mol m−3 [13]. The thickness of the boundary layer betweenhlorine bubbles and the electrolyte, δ, controls the chlorine dif-usion into the melt. From the current losses of the present studyt was determined as 0.022 mm [11].

G. Demirci, I. Karakaya / Journal of Alloys and Compounds 465 (2008) 255–260 259

F ence ◦ ◦ ◦1 as inc

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ig. 6. Photographs of bubbles obtained from video camera recordings for resid80◦ are indicated on the corresponding picture columns. Corresponding times

The kinematic properties of the electrolyte at electrolysisonditions are similar to those of 10 wt% NaNO3–H2O solu-ion at room temperature. It was reported that the boundaryayer thickness value normally ranges from about 0.5 mmn an unstirred system down to about 0.01 mm in vigorouslygitated aqueous solutions [15]. In addition to this, mass trans-er limiting current was observed in molten carbonate fuelells across around 0.005 mm electrolyte film thickness [16].herefore the boundary layer thickness value of 0.1 mm used

n the previous calculations [4,5] seems overestimated. Fur-hermore, the larger diffusion layer thickness increases therobability of chlorine dissolution being the rate determiningtep.

The magnesium losses correspond to the calculated chlorineux multiplied by the total chlorine surface area exposed to theack reaction per unit time, Ab, and the duration of electrolysis.alculations based on the data acquired from the hydrodynamicodel are summarized in Table 3. The calculated current effi-

iencies are also plotted as a full line in Fig. 3.It was reported that the chlorine dissolution was rapid enough

ot to be rate determining and the dissolution of magnesiumetal either from droplets or from a suspension of fine magne-

ium particles would be the decisive factor for the recombinationeaction [4,5]. However, from the good agreement between the

PR

time and bubble velocity determination. Anode tip angles: 98 , 118 , 147 andrements of 1/50 s are given on each picture.

esults obtained from the hydrodynamic model and the electrol-sis experiments without suspended magnesium droplets andne particles, as can be seen in Fig. 3, it can be concluded thathlorine diffusion can be assumed as the rate determining step.urthermore, boundary layer thickness between the chlorineubble and the electrolyte, calculated in the present study, thats in accord with experimental observations for similar systems15,16] supports the assumption above.

. Conclusions

The increase in current losses with an increase in the chlo-ine surface area in contact with the electrolyte per unit timendicates that the recombination reaction in magnesium chlo-ide electrolysis is controlled by the chlorine diffusion into theelt. Therefore, it is important to remove the chlorine gas from

he electrolyte as fast as possible to decrease the extent of recom-ination reaction.

cknowledgments

Authors acknowledge the financial support provided by Statelanning Organization (DPT) and Scientific and Technologicalesearch Council of Turkiye (TUBITAK).

2 lloys

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60 G. Demirci, I. Karakaya / Journal of A

eferences

[1] Kh.L. Strelets, Electrolytic Production of Magnesium, Keter Publishing,Jerusalem, 1977.

[2] K.A. Andreassen, K.B. Stiansen, Method for the molten salt electrolyticproduction of metals from metal chlorides and electrolyzer for carrying outthe method, US3907651 (1975).

[3] O.G. Sivilotti, Electrolytic cell for the production of a metal, US4960501(1990).

[4] J. Wypartowicz, T. Ostvold, H.A. Oye, Electrochim. Acta 25 (1980)151–156.

[5] V.V. Burnakin, P.V. Polyakov, V.M. Shestakov, V.I. Korolev, V.G. Sorokous,

Tsvetnye Metally 1 (1987) 57–60.

[6] V.A. Kolesnikov, K.D. Muzhzhavlev, T.S. Sheka, V.P. Sheka, V.P. Korzun,Tsvetnye Metally 48 (1975) 45–47.

[7] K.D. Muzhzhavlev, V.A. Kolesnikov, T.S. Sheka, V.V. Bashkatov, T.A.Zhuravleva, Tsvetnye Metally 2 (1976) 54–55.

[

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and Compounds 465 (2008) 255–260

[8] M. Guden, I. Karakaya, J. Appl. Electrochem. 24 (1994) 791–797.[9] G. Demirci, I. Karakaya, J. Alloys Compd. 439 (2007) 237–242.10] E.A. Ukshe, G.V. Polyakova, G.A. Medvetskaya, Zh. Priklad. Khim. 33

(1960) 2279–2284.11] G. Demirci, Molten salt electrolysis of magnesium and its hydrodynamics,

M.Sc. Thesis, Middle East Technical University, Ankara, 1998.12] I. Karakaya, W.T. Thompson, J. Electrochem. Soc. 133 (1986) 702–

706.13] T. Ostvold, H.A. Oye, Light Metals 109th AIME Annual Meeting, 1981,

pp. 937–947.14] E. Aarebrot, R.E. Andresen, T. Ostvold, H.A. Oye, Light Metals 106th

AIME Annual Meeting, 1977, pp. 491–512.

15] R.D. Pehlke, Metallurgical Process Engineering, University of Michigan,

1960.16] J.M. Johnson, Chlorine production from anhydrous hydrogen chloride in

a molten salt electrolyte membrane cell, Ph.D. Thesis, Georgia Institute ofTechnology, 2001.