A significant reduction in energy consumption in metal electrowinning from sulphate solutions can be achieved by the use

of catalyti: anodes in combination with methanol addition to the electrolyte. The effect of current density on methanol oxidation

has knst udied on different types of catalytic anodes like Ru4f ilm electrode and sponge electrode. The effect of periodic

current reversal on the anodic process has also been investigate

Meyyappan, R.M.

Ramachandran, P.

Srinivasan, R.

Venkateswaran, K.V.

IR@CECRI

B. Electrochem 4(6) June 1988, PP 593-595 CATALYTIC ANODES FOR ELECTROWINNING - PART I: EFFECT OF METHANOL ADDITION P RAMACHANDRAN, RM MEYYAPPAN, K V VENKATESWARAN and R SRINIVASAN Central Electrochemical Research Institute, Karaikudi - 623 006, INDIA [Received: 1988 March; Accepted: 1988 April] A significant reduction in energy consumption in metal electrowinning from sulphate solutions can be achieved by the use of catalyti: anodes in combination with methanol addition to the electrolyte. The effect of current density on methanol oxida- tion has k n  studied on different types of catalytic anodes like R u 4  film electrode and sponge electrode. The effect of periodic current reversal on the anodic process has also been investigated. Key words: Catalytic anode, electrowinning, methanol oxidation INTRODUCTION E lectrolysis energy in electrowinning of metals like zinc accounts for nearly 80% of the total energy. Reduction of this energy component can offer rich rewards. Many years of development and experimentation with different electrolytes, different current densities, smaller anode-cathode distances and different anode materials have not significantly reduced this energy requirement. The most promising approach to this problem in electrowinning operation appears to be the use of other oxidation processes in place of oxygen evolution. An anodic reaction suitable for a fuel cell can be considered for electrowinning. Among soluble fuels, methanol is most practical to be used in an electrowinning cell. The use of methanol in zinc electrowinning has been reported by several workers [ 1.21. The catalytic activity of the precious metal coated titanium anode has been proved by carrying out the electrolysis under periodically reversed current conditions [3 7 .  The use of catalytic anodes based on catalysts like Ru02 and Ir02 is one of the recent trends in metal electrowinning processes. Here titanium or lead can be substrate material. The anodic stability of these electrocatalysts under oxygen evolution ~onditions poses problems in the development of usable electrowinning anodes. At- tempts to develop an electrocatalyst on a substrate stable in H2S04 have not been so successful as to encourage commercial adoption. In this paper a new approach where a combination of methanol addition and catalytic anodes is employed has been investigated and results reported. EXPERIMENTAL Ru02 film electrodes were prepared by forming a thin coating of Ru02 onto titanium strip by adopting the method of thermal decomposition of ruthenium trichloride solution. Similarly Ru02 sponge electrodes were prepared using titanium sponge of 3 mm size and embedding the activated sponge particles onto lead iubstrate. These electrodes were used in the polarisation studies. Potentiodynamic studies were carried out with Ru02 film elec- rode using a Wenking Potentioscan (POS 73) and an X-Y recorder ,Model Digitronic 2000 series). Platinum foil was used as the aux- liary electrode. An H-type glass cell of 200 c m 3  capacity was used. Steady state galvanostatic studies were carried out using a galvanostat of 1 ampere capacity. 1M H2SO4 was used as elec- trolyte in all the experiments. Methanol addition was varied from 0 to 5 wt%. Galvanostatic studies with periodic current reversal was also investigated. All the solutions were prepared from the AR grade samples and double distilled water. All the measurements were carried out at 302 2 1K. The current densities employed were based on the apparent area of the working electrodes. A saturated calomel electrode was used as the reference elec- trode and all potentials are quoted with respect to this reference unless otherwise stated. RESULTS AND DISCUSSION Fig.1 shows the effect of methanol addition on the potentiodynamic behaviour of the Ru02 film electrode in H2S04 electrolyte. In absence of methanol in the electrolyte in the anode reaction is ox- ygen evolution as indicated by a steep current rise at higher anodic potential. H 2 0  \02 + 4Hf + 4e ; EO = l.23V (NHE) Addition of methanol results essentially in the evolution of C02; evolution occurs relatively at lower anodic potential. The anode reaction involving methanol oxidation has a lower positive potential than the deposition of water to give oxygen oc- curring in conventional electrowinning, with the result that cell potential is reduced. It is evident from these curves that the ad- vantage of anode potential reduction is more pronounced at lower current densities. At a current density of 2.5 rnA cm-2 the anode potential reduction is nearly 150 mV. As the current density rises this trend is progressively reduced. Thus at a current density of 45 mA cm-2 the potentiodynamic curves recorded in presence and absence of methanol are found to merge indicating that there is no effect due to methanol addition. Fig. 2 shows the effect of methanol addition on anode potential reduction for the Ru02 film electrode in steady state galvanostatic Ramachandran et al - Catalytic anodes for electrowinning - Part I: effect of methanol addition E, Vrr  SCE Fig. 1: Potentiodynamic curves of Ru02 film electrode, A, in 1M Y2S04 and 6, in 1M HzS04 + 3 wt% CH3 OH at 10 mVs-1 neasurements. Comparison of the polarization curves obtained n presence and absence of methanol indicates that the anode poten- ial reduction is more prominent at lower current densities. 1.01 I I I 0 1 2 3 lop I ( r n ~  .cm-') Fig.2: Anodic polarisation curves of Ru02 film electrode in 1M H2S04 with additions of CH30H I 0 I 0 I a I 1.0 ( a I 3.0 ( 0 l S.Owt% Methanol addition upto 3 wt% results in gradual decrease in anode potential and further addition influences the anode potential very little. The slopes of the polarization curves are more at higher cur- rent densities when compared to those at lower and moderate cur- rent densities. This phenomenon is typical of an organic oxida- tion where partial oxygen evolution at higher current densities makes the process a complex one. It has been reported r f l l  that current efficiency in organic oxidation processes is inversely pro- portional to current density. Various competitive reactions both organic and oxygen evolution are responsible for this. Generally an increase in current density favours oxygen evolution to the detriment of other oxidations. Greater depolarization obtain- ed for methanol at lower current densities indicates better efficiency for methanol oxidation. At higher potentials and current densities the fraction of oxygen evolved increases. Similar studies carried out with sponge electrode give rise to polarization curves given in Fig.3. The sponge electrode, by vir- tue of its higher surface area, results in greater depolarization. Because of higher surface area, the effective current density is lower resulting in effective methanol oxidation. Anode potential rise is investigated in long term polarization studies. The rise is quite low for sponge electrode compared to film electrode at a fixed c.d. The rise at a c.d.of 5mA cm-2 is 62 mA for film electrode and it is 8mA for sponge electrode whereas at a c.d. of 10 mA cm-2 594 B. Electrochem 4(6) June 1988 Ramachandran et al - Catalytic anodes for electrowinning - Part I: effect of methanol additi0.n Fig.3: Anodic polarisation curves of Ru02 sponge electrode in 1M H2SO4 with additions of CH30H. I 0 1 0 ( @ 1 1.0 ( A 1 3.0 I a 1 5.0 wt% it is 120 and 23 mV respectively. Higher current densities as already explained promote oxygen evolution and the reduction in anode potential due to  methanol oxidation is partially lost. Moreover, at higher current densities, the formation of other oxidation pro- ducts apart from C 0 2  is likely. The effect of periodic current reversal (PCR) on the polariza- tion characteristics is illustrated in Table I. It is observed that TABLE-I: Effect of periodic current reversal with sponge electrode at  20 mAcm-2 in 1M with 3 wt% C H 3 0 H  Type of electrolysis Duratiodsecs. Anode potential rise for 6 hrs DC PCR PCR this technique is not useful in arresting the anode potential rise. On the other hand, the polarization is found to be more when com- pared to direct current electrolysis. It has been reported 5 5 1  that PCR technique is useful for methanol oxidation on Pt. Here ox- hibit methanol oxidation and application of periodic current rever. sal techniques may help in the reduction of such surface oxides promoting oxidation of methanol. In the present study the elec. trodes investigated are essentially noble metal oxides for which thir technique is not found to be effective. Anode potential measurements with and without the applica- tion of PCR to  the Ru02  film electrode in methanol-free sulphuric acid solutions indicate a steady potential value of 1.33V in D.C. electrolysis of 6 hr at 20 mA cm-2 in 1M H2SO4, which rises to 4V under PCR conditions. These results indicate the deterioration of active coating in the PCR applications. This is due to  reduction of the oxide catalyst during the current reversal and subsequent shedding or  dissolution of the coating raises the anode potential. CONCLUSION Methanol oxidation associated with lower anode potential is effective only at lower current densities. The R u 0 2  loaded sponge electrode having higher surface area is found to be more useful than the Ru02  film electrode in the present study. The application of periodic current reversal for the effective oxidation of methanol is not found to be useful for R u 0 2  based electrodes in contrast to the reported useful application of the same technique for platinum based electrodes. The economy of the industrial application of this concept has to  be considered from the view points of local conditions like power cost and cost of methanol. REFERENCES 1. J Vereecken, C Capel - Boute and R Winand, ATS Mefallurgie (Mons, Belgium), 10 (1970) 113 2. P F Duby and J A Scott in Energy reducfion fechniques in metal elec~rochemicalprocesses (Eds) R G Bautista and R J Wesely, The Metallurgical Society Inc (1985) p 339 3. P H Vining, J A Scott and P F Duby, U S Pat. No.4 279 71 1 July 21 (1981) 4. K S Udupa, G S Subramanian and H V K Udupa, Pol De Sci, 9 (1961) 541 5. M W Breiter, EIecfrochernicalproce.sses in fuel cells, Springer Verlag, New York (1969) p 32, 112 and 165. rJ f : forward current : reversal current B. Electrochem 4(61 June 1988 

Catalytic anodes for electrowinning-part:I effect of methanol addition

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