Presentation on theme: "Results and Discussion"— Presentation transcript:
1Results and Discussion An Experimentally Preconditioning Process for Extended Passivation of Alloyed Lead Anodes in Chemical Industry „Zorka”, ŠabacMiloš B.Rajković 1, Dušan Stanojević 2 and Dragan V.Tošković 21 Faculty of Agriculture, Zemun, University of Belgrade, Serbia & Montenegro2 Faculty of Technology, Zvornik, University of East Saraevo, Republic of Srpska, Bosna&HerzegovinaOn the basis of preconditioning Ag (0.8 wt%.) alloyes lead electrodes for a proper passivation, optimal, chemically balanced and more economical process has been developed for use in Zn electrowinning. A relatively short time anodic polarization of Pb(Ag) electrodes in sulphuric acid containing F- ions (as a stimulating agent for protective and compact layer formation of β-PbO2) is found to give a compact, homogeneous, long lasting passivation layer of β-PbO2. The presence of a small amount of F- ions in the passive layer of β-PbO2, enables the passive oxide deposit to behave as long lasting. Monitoring of Pb2+ ions emission testifies that the preconditioned electrode will have more than ten years of life zinc electrowinning. An additional contribution is the exchanged chemical balance of manganese species in solution: the detrimental MnO2 production is dramatically reduced. The most important echievement of the process represents winning of pure zinc.Material and MethodsThe pilot-plant equipment was so designed to simulate exactly the industrial production in zinc electrowinning. The cell was made made from polyester and glass wool reinforced PVC in a vertical rectangular shape sides of a parallelepiped body with conic bottom. The electrolyte flow was upward with the overflow at the top of cell. Two parallel Al cathodes were verically positioned close to the side plates of the cell body, while Pb (0.8% wt.Ag) anode was in between on equal distance from both cathodes (Fig. 1). The Initial electrolyte for zinc electrowinning has been of the standard composition used in the electrolytic process in the electrolysis: Zn(II) as sulphate g/dm3, H2SO4 150 g/dm3, Cd(II), Co(II) less than 1 mg/dm3, Cl- less than 100 mg/dm3, and the absence or traces of Cu, Sb, As ions, bone glue, glass water, any surfactant. The electrolyte volume flow rate was continuous the whole time of electrolytic proces (Q = 10 dm3/h).The preconditioning passivation procedureThe preconditioning process was carried out under natural convencion. The preparation of the Pb (0.8 wt.% Ag, of originally high purity Pb wt.%) electrode consisted in one day or more conditioning in H2SO4 solution (1:1), followed cleaning. The next step was rinsing under strong water-jet at the end through washing with twice distilled water, followed with inserting into the solution for preconditioning. The composition of electrolyte for preconditioning passivation was as follows: 43 g/dm3 of F- ions calculated on the basis of KF and 39 g/dm3 SO42-, as calculated from pure H2SO4. The anodic current density for the preconditioning process was 600 A/dm3. The optimum operating temperature under such pilot-plant conditions was 30C.Fig. 1 Shematic of cell for electrolytic zinc operations and pre-conditioning anode 0.8% Pb-AgThe quality of product zinc was analysed by using atomic absorption spectroscopy. The morphological quality was studied by cristallographic analysis with electronic microcopy (SEM JOEL, Japan).Results and DiscussionA view of plant equipment in industrial production of zinc electrowinning with preconditioning anodes is shown in Fig. 2.The effect of preconditioning of Ag-alloyed Pb electrodes reflects first of all in the lead content in electrowon zinc. The expectancy is of simple reasoning: the lower Pb(II)-ions emission by anodic dissolution, the lower its concentration in the bulk of electrolyte, and as a consequence, there should be remarkably lower Pb concentration in cathodically produced Zn, was distincly confirmed by the experimental results (Fig. 3). It is clear that the content of Pb in electrodeposited zinc depends on the concentration gradient of the former in vicinity of Al-sulphate cathode. Fig. 3. clearly shows that many months of time are necessary to get some sort of spontaneous passivation upon fresh Pb (Ag) anodes and produce Zn with less than 15 ppm. Contrary to that, precondititioned anode already shows its passivating properties and enables to produce almost chemically pure Zn. In that respect,Fig. 4. Lead in cathode zinc vs. anode using 0.8% Pb(Ag) anodesFig. 3. Effects of anode pre-conditioning on Zn cathode Pb contentZinc electrowon in the cell with preconditioned and well passivated anode, there appears a typical growth of hexagonal close packed crystals of rather large sizes and almost without inclination to the substrate surface, that is one of the best measure of the obtained purity of the product. A perfect and homogenous crystal structure, typical for pure elementary Zn, their grain size, crystal position and crystallographic orientation (002), all point to the high purity of the coating (Fig. 5.).Fig. 4. displays the quality of electrowon Zn in the present pilot-plant conditions with and without preconditioned Ag-alloyed Pb anodes, expressed in the amount of Pb in the final product, that has been analyzed every day during the first month of operation.Fig. 5. SEM photograph of crystalline structure of zinc cathodeFig. 2. A view of plant equipment in industrial production of zinc electrowinning with preconditioning anodesConclusionIt has been shown that preconditioning method described here in the presence of F- ions, substantially decreases the passivation time for silver alloyed lead anodes. There is substential qualitative difference between spontaneous and induced precondtioning: while the former after many months arises based upon the mixture of α- and β-PbO2 passivating layer, the latter, just due to the effect of F- ions forms only β-PbO2, as the protective layer, which continuously reproduces itself and for a rather long time passivates the anode. The protective effect of β-PbO2 exhibits much more advanced passivating properties and remarkably extends the life-time of (Ag)Pb electrodes in zinc electrowinning.The most important conclusion might be that the preconditioning process takes place in the same Zn producting electrolytic cells, and therefore does not need any investment to be introduced in industrial practice. The most remarkable fact is that the preconditioning and zinc producing process both proceed at the same current density, and thereby there is no need for any cell reconstruction. Meanwhile, the preconditioning requires continuos cooling to keep proper temperature for homogeneous growth of the passivating β-PbO2 layer. Thus, the overall expenditure represents the energy spent for preconditionig, ehich is negligible in comparison with lot of a advantages resulting therefrom.Preconditioning Ag (0.8 wt.%) Pb anode could be employed in many other important and interesting industrial electrolytic processes, such as in situ oxidation of starch into dialdehyde of starch via periodate anodically generated from iodate ions.References: 1. M.B.Rajković, G.T.Vladisavljević, N.M.Ristić, M.M.Jakšić, Bulletin of Electrochemistry, 14(3) (1998), pp M.Jakšić, M.B.Rajković, D.Stanojević, Hem.Ind., 41(6-7) (1987), s M.B.Rajković, D.Stanojević, M.Jakšić, Hem.Ind., 41(9) (1987), s M.B.Rajković, D.Stanojević, M.Jakšić, CHISA’93, August 29 – September 3, Praha, Ref. No M.B.Rajković, D.Stanojević, D.Tošković, Č.Lačnjevac, „Interdisciplinarni pristup problematici zaštite konstrukcionih materijala”, Tara, – , Knjiga radova, sAcknowledgements: The work was supported by the Ministry of Science and Environmental Protection of the Republic of Serbia (Grant No. 1941).