Chronopotentiometric study of aluminium deposition from MClAlCl3 melts (MNa, K and Cs)

£kcrr°cAbnirp Ann . Vol. 29, No . 3 . pp . 397-401 . 1987 . Printed in Great arilvn .

W I3-46t6/a4 53.Oe+ 0 .00 © 1944 . Pergamon Press Ltd .

CHRONOPOTENTIOMETRIC STUDY OF ALUMINIUM DEPOSITION FROM MCI-AICI 3 MELTS (M = Na, K AND Cs) M . GABtO, P . FELLNER and L. LunvovA The Institute of Inorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Czechoslovakia (Received 26 April 1983)

Abstract-The kinetics of aluminium deposition from NaCI-AICI 3 , KCI-AICI 3 and CsCI-AICI 3 melts (C,rlc1 . = 3 x 10 - '-1 x 10 -3 mol em -3 ) was studied using the chronopotentiometric technique . It was found that in the system NaCI-AIC 3 the reduction of AI(III) species is controlled by diffusion only, while in the system KCI-AIC1 3 and CsCI-AICI 3 , a chemical reaction precedes the reduction step . In the two tatter systems the AI(III) in the complex anion AICI can be reduced also directly, if current density is sufficiently high .

INTRODUCTION The electrowinning of aluminium from chloride melts is a process which can be considered as a potential rival of the classical Hall-Heroult aluminium smelting[I] . The laboratory scale electrolysis of the aluminium chloride-alkali chloride melts showed[2], in agreement with the literature reports[3] . that the efficiency of the process can be rather high . The current efficiency can be close to 100% and in comparison with the classical process, the electric energy consumption is lower by about 30%_ In order to understand the nature of the cathode process in those melts we studied this system using different experimental techniques. In this paper we present the results of chronopotentiometric study of the molten mixtures NaCI-AIC1 3 , KCl -AlCl3 and CsCl-AICI„

EXPERIMENTAL All chemicals used were of reagent grade quality . NaCl, KCI and CsCI (Merck) were dried at 500"C . These chlorides were mixed in a dry-box with altrrninium chloride (Fluka). MAICI, was prepared in a sealed Pyrex tube by heating slowly the mixture of MCI and AIC1 3 . Chemical analysis of each product was done. A sketch of the cell is shown in Fig . 1 . The crucible material was sintered alumina . One electrode made of tungsten wire was sealed in Simax glass, the other one made of vitreous carbon was well fitted in boron nitride . Surface of electrodes was 0.1-0.7 cm' . The exact surface of each electrode was calculated from its actual geometry observed under microscope The surface was also checked by chronopotentiometric measurements in the system NaCI-PbCI,,, in which the diffusion coefficient of lead is well known[4] . The counter electrode was a graphite rod (10 mm 0). A slightly modified Ag/AgCI reference electrode was used 5] . The details of its construction are described in[6 . The assembling of the cell and the weighing of the salts were performed in a dry-box under nitrogen atmosphere . The cell was placed in the isothermal space of a Kanthal wire-wound resistance furnace . In all experiments the furnace was preheated to 200°C

Fig . I . A sketch of the experimental cell .

(1) carbon cover; (2) alumina crucible; (3) alumina tube ; (4) thermocouple; (5) cathode - tungsten wire; (6) reference Ag/AgCI electrode ; (7) anode-graphite rod ; and (8) melt .

and purified argon was passed through it at a slight overpressure. The chemical analysis had to be repeated after each series of experiments, because of decreasing AIC1 3 concentration due to evaporation. The temperature of the cell was measured with a calibrated Pt-PtlORh thermocouple . All measurements were carried out at 830±2 .5°C . As a current source we used a potentiostat/galvanostat VPZ B6chovice which was controlled by a programmable sawtooth generator[7] . The long transition times were recorded on an X-Yplotter HP 70004 B and the short responses on a storage oscilloscope Tektronix 434 .

RESULTS AND DISCUSSION The experimental results arc given in Figs 2, 3 and 4 . The term irtn vs i is plotted for different concentrations of aluminium chloride in the corresponding 397



M. GAato.

398

P. FELLNER AND 2. LunyovA

.25

125

1 .00 075

f~,~rf~+`r+ 4

rv

4

0 75

050

4 _ 0.50

0.25

0 .25

3

# 0

5

I 0 .5

0 /A Cm'

Fig . 2 . Variation of it't 2 with I at various concentration AICI, in NaCl-AICI, melts . Curve(1) 3.50 x 10 -5 mol cm - ', _3, Mail cm -3 , (4)3+70 (2) 9 .32 x 10 - ' mol cm -3 , (3) 1 .86 x l0-4 x 10 - ' mot cm and (5) 5.04 x to -4 mot cm -3.

I Ic i/Acm 2

Fig .4 . Variation of it" with tat various Aid 3 concentration in CsC1-A1Cl, melts . Curve (1) 3.40 x 10- ' mot cm -3 , (2) 2 .65 x 10 -4 mot cm ', and (3) 6.37 x 10 ' mot cm ' .

t12

Sand equation is valid[81 and the product ir is constant . Adecreaseinir 1 t vs i indicates that chemical reaction precedes the electrochemical reaction . While in the former case (Fig . 2) the reduction of aluminium chloride was found to be diffusion controlled, in the latter two systems (Figs 3 and 4) the observed dependence of transition time on current density can be explained by a chemical reaction preceding the discharge of Al(III) species . Since in equilibrium practically all aluminium chloride in these systems is bound in the complex anion AICIn [9, 10], the chemical reaction can be written schematically as

15 #

MAIC14 r l AICI 3 +MCJ .

(1)

Since in the melts we are working with, there is an excess of alkali metal chloride, we can use the relationship derived by Delahay and Berzins[1l] for evaluation of the kinetic constants k„ k-, and of the diffusion coefficient . . Fig 3 . Variation of ir't' with i at various AICI 3 concen-3 tration in KCI-AIC1 3 melts . Curve (1) 1 .76 x 10 -4 mot cm (2)1.20 x 10 -4 mol cm -3 , ( 3) 4 .82 x 10 -4 mol cm - ', (4) 6.51 x 10 -4 mel cm -3 , and (5) 9 .48 x 10 - ' mot cm -3 . alkali metal chloride melt . From these figures it is evident that there is a remarkable difference between behaviour of the system NaCl-AICI 3 (Fig . 2) on the one hand, and the system KCI-AICI 3 (Fig-3) and CsCI-AICI 3 (Fig . 4) on the other hand . For an electrochemical reaction without kinetic complications the

it r, 2 = } rz t t2 nFC°D 1 t2 -(a 112 i/2K (k, +k-1) 112 ) tl2 ) ( 2) xerf((kl+k_1)'"r C° corresponds to the initial concentration of AICI n determined analytically. The kinetic parameters characterizing the reduction of Al(III) species in alkali metal chloride melts calculated from experimental data according to Equation (2) are listed in Table 1 . The values of the

Table 1 . Kinetic parameters of aluminium deposition from MCI-AICI, melt (M = Na, K and Cs) System NaCl- AICI, KCI-AICI, CsCI-AICI 3

No . of experiment

D x I0 3/cm2 s - '

k 1 /s - '

k . 1 /s - '

6 7 3

6.7±0.6 4.9±0.7 3.5±0A

33x109 6x104

31x10' 2 2x108



Chronopotentiometric study of aluminium deposition obtained diffusion coefficients seem to be correct, when compared with other diffusion coefficients in molten salt chlorides . A discrepancy about ± 10 % is due to errors involved in extrapolation of the experimental curves ir' 11 vs i to zero current and to the uncertainty in establishing the transition time. As we did not know value of the equilibrium constant K for system CsCI-AICI3 and it is not possible to determine it from our experiments, the rate constant k, and k-, for KCI-AICI 3 only were calculated . The values of the k, and k_, for NaCI-AICI 3 system introduced in Table 1 were obtained by comparison of experimental values Irrlz/C with those calculated from the Equation (2) for different values of k_, . The calculations were performed with a HP-9821 A calculator . The value of K(k,+k-,)u2 >500 for this system indicates that the kinetic effect of the preceding chemical reaction is experimentally not detectable by chronopotentiometric method[12, 13]. The chemical reaction prior to the electrochemical step influenced the shape of the logarithmic function E vs In (r i i 2 - 1 1 /2 ) markedly . The logarithmic plot was linear only in small range of the values (ti 2 - ( 1 j 2 ) . The values of n, calculated from the slope of the linear part of the logarithmic plot, varied between 1 .7 and 4.9 for KCI-AICI 3 and CsCI-AICI, systems . This deviation from the expected theoretical value n = 3 is due to the absence of the explicit relationship E=f(t) in electrochemical systems in which first a chemical reaction takes place[14] . The calculated value of n for NaCl-AICI, melt varied between 1 .5 and 3 .1 . A logarithmic analysis for the system with a high concentration of the electroactive species in the basic melt is not possible because the nucleation phenomena can disturb the chronopotentiometric curve substantially, as it is shown in Fig . 5 . Before we compare our results with the literature data, let us present a qualitative confirmation of our

assumption that AI(III) .can be reduced in KCI-AIC1 3 and CsCI-AICI 3 melts from two different species. At a concentration of AIC1 3 lower than about 5 x 10 - ' mol cm - ' in the melt, only a single potential wave is observed . In Fig. 6 the chronopotentiometric curve obtained from the KCI-AICI 3 melt can be seen. At higher concentrations, and therefore at higher current densities, (r was in the range 5-0.02 s), a second wave appears as it is shown in Fig . 7 . Figure 3, curve 5, shows a change of the term it 112 caused by the existence of the second wave . We assume that this wave corresponds to direct reduction of AI(HI) from the complex anion AICI. . Potential of this process is about 150 mV more negative than that of the Al(III) reduction from species formed by the chemical reaction. However, no such phenomenon was observed in the system NaCI-AICI3 . It can be explained by greater stability of AICI; complex in KCI and CsCI melts[9 . 10] . We believe that the mechanism of AI(III) deposition is the same in all MCI-AICI, (M = Na, K and Cs) systems, but in NaCI-AICI, melt a kinetic effect is not detected under given experimental conditions . It should be noted that all works devoted to the study of electrode processes in molten system of alkali metal chlorides-aluminium chloride claim that the reduction process of Al(Ill) is controlled only by diffusion[ 15-18] .Odegard et al . [17] reported also the rate constant k° for AICI, reduction . Differences with regard to results obtained in this study can be explained either by use of different experimental technique which was not sensitive enough to detect the described phenomenon or by different experimental conditions. In[1618] linear sweep voltametry or potential step amperometry were used and the obtained experimental data could be explained by simple diffusion control of the process . ln[15], the same method as here, ie chronopotentiometry, was used . The authors observed phenomena similar to those

Fig. 5. Chronopotentiogram of 9 .48 x 10 -4 molar - ' AICI, in KCI-AICI 3 melt on vitreous carbon electrode, with a vertical sensitivity of 0 .5 V div"', a horizontal sensitivity of 1 s div - ' . 8A 29t3-4

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M . GABZO, P. FELLNER AND

1.

LUBYOVA

Fig. 6. Chronopotentiogram of 3 .20x 10 - ° mol cm - ' AICI 3 in KCI-AICI, melt on tungsten wire electrode, with a vertical sensitivity of 0.5 V div - ' . a horizontal sensitivity of 0.2 s div" .

Fig . 7 . Chronopotentiogram of 9 .48 X10 ' mol em ' AIC1, in KCI-AICI, melt on vitreous carbon electrode with a vertical sensitivity of 0 .2 V div - ', a horizontal sensitivity of 0 .05 s div - ' .

discussed in the present paper . They found that the term ir'/2/CO was not stable as it should be if the electrode process was governed by simple diffusion, but they proposed to explain their experimental data in terms of "concentration dependent diffusion coefficient" . A quantitative description of the second wave on chronopotentiometric curves will be studied next .

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chemistry. Interscience, New York (1954). 13, D . D . MacDonald, Transient Techniques in Electrochemistry . Plenum Press, New York (1977) . 14 . M. Skyllas-Kazacos, B . J . Welch and H . C. Gaur, Trans. Indian Inst. Metals 34, 242 (1981). 15 . V . I . Sal'nikov, V . P. Butorov, B. V. Me1'nikov, V. A . Lebedev and I . F, Nichkov, Soviet Electrochim . 10, 1199 (1974). 16 . S. I_ Goldstein, S . P . Raspopin and V. A . Fedorov, Soviet Electrochem . 13, 1791 (1977). 17 . R . Odegard, A . Rjq,rgun, A . Sterten, I . Thonstad and R . Tunold, Electrochim . Acta 27, 1595 (1982) . 18 . Yu .K .Delimarskii,V.F.Makogon,G.I.Dybkovaand0 . P . Gritscnko, Ckr . khlm . Zh . 48. 923 (1982) .