Bulgarian Chemical Communications, Volume 40, Number 3 (pp. 277-280) 2008

Cast Pb-Ag anodes with silver content of 0.8–1% have been currently used for zinc electroextraction from sulphate electrolyte solutions under industrial conditions. The good characteristics of those anodes have determined their general application. However, the investigations, aiming at improving their tech-nical and economical parameters, namely – corro-sion resistance, electrocatalytic properties, conducti-vity, mechanical indices, price, etc. – as well as the continuous search for alternative anodes, have never ceased. Alloys with a wide range of component concentration with the aim of finding the optimum content of the anode alloy, as well as the electro-chemical catalytic role of the elements and the modifying and the structural factors, are being studied. A great part of the investigations have been connected with the utilisation of Co as an alloying component of the lead anodes, mainly because of its significant catalytic effect on the main anode reaction (2H₂O  4H⁺ + O₂ + 4e). The inclusion of Co in the lead matrix has been carried out in several different ways using various cobalt compounds, namely – metallurgical alloying, metallic-ceramic scorching, detonation deposition of cobalt onto the lead, electrochemical deposition of lead-cobalt-alloy or composite coatings, etc. 1–7.

As compared to the classical Pb-Ag anodes, the Co-containing anodes possess a higher corrosion resistance, a decreased anode polarization as well as a considerably diminished slime formation. At the same time, however, with them a certain unstability of the indices with time has been reported, namely – chemical and electrochemical solubility, mainly, leading to a decrease of the cathode – current effi-ciency as regards zinc, or even, to its reverse disso-lution. Despite the above disadvantages, however, the investigations in developing cobalt-containing anode materials prove to be worthwhile, so that those materials should retain all the qualities of the cobalt catalysts, while at the same time they should stay chemically and electrochemically stable to prevent the harmful effect of the cobalt ions on the cathode process.

The pesent paper has emerged as a continuation of our research on obtaining anode materials on the basis of the composite lead-cobalt coatings (of the type Pb/Co_xMe_yO_z) on lead-alloy substrates with nanodimensions of the disperse particles.

EXPERIMENTAL

The new anodic materials have been obtained by electrodeposition onto lead-calcium (Pb-Ca0.089%) 20×20 mm plates. The pre-treatment of the lead substrates included degreasing and chemical etching. The composite coatings were electrodepo-sited from a lead ammonium sulphamate electrolyte (LASE) containing: 150 g·l^–1 Pb(SO₃NH₂)₂, 100

RESULT AND DISCUSSION

Galvanostatic polarization investigations have been employed with the aim of determining the anodic potential alteration as a function of time during the process of electroextraction of zinc from sulphate electrolytes. A model electrolyte was used, containing 55 g·l^–1 Zn²⁺ (as ZnSO₄.7H₂O), 180 g·l^–1 H₂SO₄ and 5 g·l^–1 Mn²⁺ (as MnSO₄.H₂O). The work electrodes (anodes) were Pb-Co1.22%-Ti2.48% and Pb-Co1.13%-Ti1.72%. Their effective area was 2 cm². The cathodes represented two aluminium plates with a total working area equal to that of the anode. The reference electrode was Hg/Hg₂SO₄ (SSE) with a potential E_SSE = +0.692 V (NHE), while the temperature during the electroextraction was kept constant at 371C.

The alteration of the anodic potential as a function of the time for the composite electrodes is given in Fig. 1. The Pb-Co1.22%-Ti2.48% (curve 1) and the Pb-Co1.13%-Ti1.72% (curve 2) electrodes have exhibited a difference in the anodic potential of approximately 30–40 mV. The Pb-Co1.22%-Ti2.48% electrode, where the Co and Ti content is higher, has a lower potential than that of the Pb-Co1.13%-Ti1.72%-electrode, which is, eventually, due to the larger area of the anode during polarization 8. The composite electrodes have exhibited a negligibly higher anode potential as compared with the cast Pb-Ag1% (curve 3). The slime formation with the composite electrodes is much less and the electro-lyte after the electro-extraction is clearer and more transparent compared with the standard Pb-Ag1% electrodes.

The atomic absorption analysis of the electrolyte after the electroextraction has shown that the Co²⁺-ions content in it does not exceed 1 ppm. This fact indicates that the composite electrodes possess a good corrosion resistance.

Fig. 1. Dependeces of anodic potential on time.

Y. S. Stefanov et al.: Investigations of composite coatings …

Figure 2 has presents the CVA-curves for the

PbSO₄  Pb. After 24 hours of polarization, the cyclic voltammogram has indicated several pecu-liarities (curve 2). The first anodic peak (a) is a bit higher and slightly shifted in the positive direction. In the region of the second anodic peak (b) the curve is shifted in the negative direction, which can be accounted for by the catalytic effect of Co and the roughness of the anodic surface. The reduction peak (c) is well formed. After 48 hours of polarization (curve 3), a well-defined high lead-sulphate peak (a) has emerged. The second anodic peak (b) has over-lapped the same peak in curve 2. The appearance of a new anodic peak (b) at approximately +0.9 V (SSE) has emerged as a characteristic feature. This peak has been described by Yamamoto et al. 10 and is related to the oxidation of Pb to PbSO₄ through the PbO₂-pores, i.e. it appears only in the presence of the formed PbO₂ on scanning the potential in the negative direction.

Figure 3 presents the CVA-dependences for sample Pb-Co1.13%-Ti1.72%. The characteristic anodic and cathodic peaks, registered in the previous figure, are evident here as well. The peaks in this case have a lower current value compared to Fig. 2. Moreover, after 24 hours of polarization a new anodic peak (a) has appeared, which is due to the formation of lead oxisulphate – PbO.PbSO₄ 9.

Fig. 2. CVA for a Pb-Co1.22%-Ti2.48%: 1 - 0 h polarization; 2 - 24 h polarization; 3 - 48 h polarization.

Fig. 3. CVA for a Pb-Co1.13%-Ti1.72%: 1 - 0 h polarization; 2 - 24 h polarization; 3 - 48 h polarization.

Figure 4 presents the morphological structure of the composite electrode Pb-Co1.22%-Ti2.48% before electroextraction. Asymmetrical crystal grains with clearly outlined boundaries between them have been observed. The surface is comparatively hetero-geneous. Individual clusters can be detected, which is probably, due to the Co-Ti particles included.

Fig. 4. Microstructure of composite electrode

Y. S. Stefanov et al.: Investigations of composite coatings …

Fig. 5. Microstructure of composite electrode

CONCLUSION

Y. S. Stefanov et al.: Investigations of composite coatings …

The composite anodes have exhibited a negli-gibly higher anode potential, compared with the cast Pb-Ag-1% alloy, in accordance with the galvano-static polarization investigations. The corrosion resistance of the new anodic materials is very good, while slime-formation, when working with them, is of smaller amount, compared with that of the classical electrodes. The anode oxide film and the cyclic voltammetry curves of the composite anodes, respectively, are very close to those of the cast
Pb-Ag1% electrode. The morphological structure of the lead-cobalt-titanium anodes differs from that of the cast electrode in the emergence of granular clusters, most probably, due to the Co-Ti nano-particles included.

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