CommunicationOn the Electrochemical ReductionMechanism of CaWO4 to W Powder

METEHAN ERDOGAN and _ISHAK KARAKAYA

The reduction mechanism of CaWO4 to W in molteneutectic CaCl2-NaCl electrolyte has been studied. Cyclicvoltammetry and constant potential electrolysis testswere performed to determine the reversible cell poten-tial. Analyses of the experimental results revealed that atleast 2.2 V was required to compensate the potentialsfor the accompanying cell reaction and the electrodepolarizations. A cell reaction was proposed by associ-ating the experimental results and the Gibbs energychanges of the possible reactions.

DOI: 10.1007/s11663-012-9689-4� The Minerals, Metals & Materials Society and ASMInternational 2012

Recently, an electrochemical method to producetungsten powder was reported.[1,2] Similar to oxidereductions of the Fray-Farthing-Chen (FFC) process,[3]

it was based on the results of successful laboratory testsinvolving the reduction of CaWO4 powders in CaCl2containing molten salt electrolytes. Metallic tungstenpowder was produced with significant concentrationsof CaCO3 and/or Ca(OH)2, which could then beremoved by treating the reduced product with diluteHCl solutions.[1] The possible reactions, which mayoccur during electrochemical reduction, suggested basedon constitutions of the reduced products and the lowestreversible decomposition potentials were[4,5]

CaWO4 sð Þ þ 3/2C sð Þ ¼ W sð Þ þ CaO sð Þ þ 3/2CO2 gð ÞE o ¼ �0:31 Vð Þ ½1�

CaWO4 sð Þ þ 2C sð Þ ¼ W sð Þ þ Ca sð Þ þ 2CO2 gð ÞE o ¼ �0:68 Vð Þ ½2�

The detection of chlorine gas during the experiments,especially with larger samples, cast some doubt on aboveequations as cell reactions. Since kinetic factors play animportant role in the mechanisms of cell reactionsduring operation; results from constant voltage electrol-ysis complemented with cyclic voltammetry were used togain an insight into the reduction mechanism of calciumtungstate in CaCl2 containing molten salt electrolytes.

A detailed description of the experimental setup andprocedure for constant voltage electrochemical reduc-tion experiments can be found elsewhere.[1] The CaWO4

pellet (248665; Sigma-Aldrich, St. Louis, MO) waspositioned onto a stainless steel spoon electrode, whichwas welded to the end of a stainless steel wire currentcollector. The pellets used in constant voltage electro-chemical reduction experiments were prepared via thefollowing route: First, the CaWO4 powder was impreg-nated with 2 pct by mass of a binder polyethylene glycol(PEG). Secondly, the components were mixed for40 minutes and then left to dry in air at room temper-ature for 24 hours. Following the sieving stage, thepellets were formed by die pressing the powder and PEGwas removed by holding the pellets at 873 K (600 �C)for 150 minutes. Because the melting point of pureCaWO4 is approximately 1873 K (1600 �C),[4,5] thistemperature was too low for the CaWO4 powder tosinter. The pellets had 15.6 mm diameter and 3 mmheight. A graphite rod (A10134; Alfa Aesar, Ward Hill,MA) having 13 mm diameter was used as the anode.The eutectic (48 pct mole NaCl) NaCl-CaCl2 saltsolution, used as the electrolyte, was molten at theselected operation temperature of 873 K (600 �C).[6] Theexperiment durations varied between 200 to 500minutes, depending on the applied potential.The schematic view of the experimental setup used in

cyclic voltammetry is given in Figure 1. The CaWO4

pellet for the cyclic voltammetry experiment wasprepared according to the same powder processingprocedure as described in the previous paragraph up tothe heating stage. However, after die pressing theCaWO4 powder, the green body was sintered for2 hours at 1473 K (1200 �C). After sintering, a smallhole was drilled through the pellet to attach the pellet toa Kanthal (N 80) wire to form the assembled cathode.The electrolyte and the temperature of the cyclicvoltammetry experiments were the same as the previ-ously mentioned constant voltage electrochemicalreduction experiments. In this electrochemical cell,depending on the electrode reaction under consider-ation, either the CaWO4 pellet cathode or the graphiteanode was made the working electrode (W). When onewas used as the working electrode, the other was used asthe counter electrode (C). A tungsten wire, which wasfound to be noble to the electrolyte during the exper-iments, was used as the reference (R).In the case of cyclic voltammetry performed with a

two electrode cell, the working electrode (W) pole wasconnected to the CaWO4 pellet, whereas the counterelectrode pole (C) and the reference electrode pole (R)were connected to graphite. The experiments wereconducted by a Reference 3000 type potentiostat (GamryInstruments, Warminster, PA). Both the constant volt-age electrochemical reduction and cyclic voltammetryexperiments were conducted under continuous flow ofargon gas at a rate of 100 mL/min.The electrochemical reduction experiments were per-

formed at 873 K (600 �C) by applying four differentconstant potential differences between theCaWO4 cathodeand the graphite anode in molten CaCl2-NaCl eutectic saltsolution. The current values were recorded as a function of

METEHANERDOGAN,Ph.D. Student, and _ISHAKKARAKAYA,Professor, are with the Department of Metallurgical and MaterialsEngineering, Middle East Technical University, 06800 Ankara,Turkiye. Contact e-mail: kkaya@metu.edu.tr

Manuscript submitted December 22, 2011.Article published online June 29, 2012.

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time during each experiment, and they are given inFigure 2. The data for 2.65 and 2.8 V applied constantvoltages were repeated from another publication.[1]

The potential that should be applied for an electro-chemical reaction to take place is always greater thanthe reversible potential caused by the overvoltages andthe resistances. Accordingly, the total applied potentialEapp. for the current electrochemical cell can be given as

Eapp: ¼ Erxn þ Econn: þ Eelectrolyte þ Epellet þ g ½3�

where Erxn is the theoretical reversible reduction poten-tial, Econn. is the voltage drop along the electrical con-nections, Eelectrolyte and Epellet are the so-called IR drop

because of the resistance of the electrolyte and pellet,and g is the overvoltages that can be considered to bemainly because of the CO2 evolution at the anode in thisstudy. The removal of oxygen from the cathode at thesolid state may impose some cathodic overvoltage.Additionally, the steep decrease of the current valuesduring constant potential experiments, observed innearly all of the electrochemical reduction studies ofsolid metal oxide pellets,[1,7–10] was probably because ofthe increase in the resistance of the pellet, which becameimportant after the direct contact between the currentcollector and the pellet was lost. However, at the initialstages of the experiments, these terms can be neglectedbecause the route for oxygen removal is shorter and thepellet resistance is less significant as evident from the re-corded high currents.The voltage drops along the electrical connections were

measured by short circuiting the electrodes in amolten Pbpool at 873 K (600 �C) for the currents recorded in thisstudy. Accordingly, net cell voltages Enet, which excludeEconn., from Eq. [3], were calculated by subtracting thevoltage drops along the electrical connections from theapplied potentials. From the measured data, currentscorresponding to the passage of 0.0052 Faradays(502.6 Coulombs) of electrical charge were determinedand given in Table I. Passage of 0.0052 Faradays corre-sponds to the initial periods of the experiments (10 pcttheoretical reduction) where higher currents were mea-sured. From the list of the calculated values, the Enet vsI graph was plotted as shown in Figure 3.From the preceding discussion, it is apparent that

extrapolation of the regression line of the Enet vs I plot

Fig. 1—The schematic drawing of the experimental setup for thecyclic voltammetry experiments.

Fig. 2—Electrochemical reduction experiments of the CaWO4 pelletsat different applied constant voltages in eutectic CaCl2-NaCl electro-lyte at 873 K (600 �C). The data for 2.65 and 2.8 V applied constantvoltages were repeated from Ref. [1].

Table I. Econn., Enet, and I Values at Different Applied

Potentials Eapp

Eapp. (V) Econn. (V) Enet (V) I (A)

2.5 0.16 2.34 0.392.65[1] 0.24 2.41 0.552.80[1] 0.31 2.49 0.712.95 0.41 2.54 0.97

Fig. 3—Enet vs I plot for the initial periods of the electrolysisexperiments.

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to zero current gives the reversible reduction potentialplus the anodic overvoltage (Erxn+g). It should benoted that the current dependence of the anodicovervoltage is logarithmic, and this dependence can beneglected for the current density values used in thecurrent study. Likewise, the slope of the mentionedregression line gives the resistance of the electrolyte. Theintercept and the slope were determined as 2.22 V and0.35 ohm, respectively. The use of this resistance for1.91 cm2 cathode area and 1 cm anode-cathode distanceleads to computation of 0.67 ohm.cm specific resistancefor the electrolyte. Despite the presence of smallpolarization during reduction, this value is in very closeagreement with specific resistance of CaCl2-NaCleutectic salt solution, given in the range 0.68 to0.81 ohm.cm.[11]

Similar computations performed for later stages of theelectrochemical reduction experiments yielded similarintercepts corresponding to Erxn+g values but higherslopes as expected. For example, the intercept and theslope were 2.25 V and 0.92 X, respectively, at currentvalues corresponding to the passage of 0.035 Faradays(3377.5 Coulombs) where nearly 68 pct of the startingCaWO4 was theoretically reduced. The results at laterstages deviate from above values because of the absenceof steady state especially at higher applied potentialsand approach of current to zero for all constantvoltages.

To verify the preceding findings and gain additionalinformation on the reduction mechanism, a two-elec-trode cyclic voltammetry experiment was performed at873 K (600 �C) in molten CaCl2-NaCl salt solution ateutectic composition with a scan rate of 20 mV/sbetween the CaWO4 pellet and the graphite rod in the0 to �3.5 V range. The resulting voltammogram is givenin Figure 4. The CV recorded showed that only non-faradaic currents flow until approximately �1.90 V. Atthis value, the current starts to increase, indicating thatthe applied potential is approaching the reductionpotential, and it forms a reduction peak A1 at approx-imately �2.81 V. Therefore, from the midpoint of theonset and the peak potentials,[12] �2.36 V was calcu-lated as the reduction potential which can be observed inFigure 4. Because the CV was recorded with the two-electrode cell, overvoltages and voltage drops caused byresistances are also included within this value. Theresistance of the electrolyte was previously calculated as0.35 X from Figure 3. For the Kanthal (N 80) wires usedin the cathode and the anode assemblies, the totalresistance was calculated as 0.12 X at 873 K (600 �C) byusing the data given in the literature.[13] Because theresistance of the graphite rod is very small, and thepotentiostat excluded the voltage drops along its con-nections, the overall resistance of the cell was expectedto be approximately 0.47 X. Using 0.34 A current valueat �2.36 V, the voltage drop caused by the resistance ofthe cell can be calculated as 0.16 V. When the voltagedrop caused by the resistance of the cell was excluded, apotential difference of �2.20 V was obtained. Conse-quently, this voltammogram can be considered to be inaccordance with the results obtained from the constantpotential study, which pointed out approximately

2.22 V potential differences at zero current. Afterreversing the potential scan, the CV showed no reactionuntil �2.4 V. At this value, the current started toincrease and formed an oxidation current peak B1 atapproximately �1.2 V.To minimize the influence of overpotentials and

potential drops caused by the cell resistance, cyclicvoltammetry experiments with three electrodes wereperformed. In Figure 5(a), the CV result recorded on anassembled electrode of CaWO4 attached to a Kanthalwire vs tungsten reference electrode between 0 and�1.5 V was presented. The result of CV performedbetween the graphite rod and the tungsten wire referencein the 0 to 1.5 V range is given in Figure 5(b). Again,using the midpoints of the of the onset and peakpotentials,[12] a potential value of �0.3 V from Figure 5(a)and 0.8 V from Figure 5(b) were calculated. Therefore, thedifference between the cathode and the anode electrodepotentials is approximately �1.1 V.Although the applied potentials were lower than the

decomposition potential of the electrolyte, chlorineevolution was observed during the experiments. Inaddition to this, XRD analysis revealed the presenceof calcium compounds in the reduced specimens.[1]

Among all the possible reactions that may occur in thecell, the following reaction was suggested as the overallcell reaction by considering the above constraints andcalculated potentials[4,5]:

CaWO4 sð Þ þ C sð Þ þ CaCl2 lð Þ ¼W sð Þ þ 2CaO sð Þþ CO2 gð Þ þ Cl2 gð Þ

Eo ¼ �0:85 Vð Þ ½4�

The potential requirement of above reaction is 0.25 Vbelow the value obtained from Figure 5. However, itshould be noted that �1.1 V obtained from Figure 5still includes the resistance coming from the workingelectrode and some uncompensated resistance coming

Fig. 4—Cyclic voltammetry result between the CaWO4 pellet and thegraphite rod. Scan rate = 20 mV/s; scan range = 0 to �3.5 V, tem-perature = 873 K (600 �C), and electrolyte = eutectic CaCl2-NaClsolution. The arrows indicate the scan directions.

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from the solution.[12] In addition, potential requirementof Reaction [4] will increase approximately 40 mV whenthe activity of CaCl2 (�0.35) in eutectic NaCl-CaCl2molten salt solution is taken into account.[14] Chlorineevolution decreases the CaCl2 content according toReaction [4], but its contribution to the measuredpotentials over the course of experiments will not bemore than 2 mV.[14] The absence of any other reactionhaving reversible cell potential between �0.89 and�1.1 V and satisfying previously mentioned conditions,adds credibility to the suggested overall cell reaction.

A 1.31 V difference between the measured potential(Figures 3 and 4) and the potential requirement ofReaction [4] could be mainly caused by the anodicoverpotential for CO2 formation. There are not suffi-cient data in the literature about the value of thisoverpotential especially at temperatures at approxi-mately 873 K (600 �C) in CaCl2 containing electrolytes.However, a value well above 1 V for CO2 overpotentialon graphite anode in cryolite melt during aluminumelectrolysis can be calculated by extrapolation of thedata[15] given in the literature to 873 K (600 �C).

The electrochemical reduction mechanism of CaWO4

to W in eutectic CaCl2-NaCl salt solution at 873 K(600 �C) was investigated by means of constant potentialelectrolysis experiments and cyclic voltammetry tech-nique. It has been shown that approximately 2.2 Vmakes up the sum of theoretical reduction voltage andthe overvoltages. The difference between 2.2 V and thereversible reduction potential requirement of the sug-gested reaction 0.89 V was attributed mainly to theovervoltage for CO2 evolution on graphite anode.

The authors acknowledge the financial supportprovided by the Scientific and Technological ResearchCouncil of Turkiye (TUB_ITAK).

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Fig. 5—CV results between (a) the CaWO4 pellet (b) graphite rod and the tungsten wire reference electrode. Scan rate = 20 mv/s, temperature =873 K (600 �C), and electrolyte = eutectic CaCl2-NaCl molten salt solution, scan ranges = (a) 0 to �1.5 V and (b) 0 to 1.5 V. The arrows indicatethe scan directions.

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