Estudio de las reacciones anódica y catódica en el proceso de electro-recuperación de zinc
Despite the electrowinning of zinc has been in operation for more than 100 years [Van Arsdale, 1965], unsolved problems still exist due to technological lags as well as a lack of knowledge of interfacial phenomena related with the electrodic reactions. All these factors impact on the energy consumpt...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2009 |
| País: | México |
| Institución: | Universidad Autónoma Metropolitana |
| Repositorio: | Repositorio Institucional de la UAM Iztapalapa |
| Idioma: | español |
| OAI Identifier: | oai:bindani.izt.uam.mx:dj52w487g |
| Acceso en línea: | https://doi.org/10.24275/uami.dj52w487g |
| Access Level: | acceso abierto |
| Palabra clave: | info:eu-repo/classification/LEM/Zinc -- Anodic oxidation info:eu-repo/classification/LEM/Chemical reaction info:eu-repo/classification/LEM/Reacción oxidación-reducción info:eu-repo/classification/LEM/Zinc -- Oxidación anódica info:eu-repo/classification/cti/2 |
| Sumario: | Despite the electrowinning of zinc has been in operation for more than 100 years [Van Arsdale, 1965], unsolved problems still exist due to technological lags as well as a lack of knowledge of interfacial phenomena related with the electrodic reactions. All these factors impact on the energy consumption in zinc electrowinning. In this investigation anodic and cathodic interfaces were studied under the traditional zinc electrowinning conditions. Current efficiency and energy consumption were used as merit figures to evaluate the influence of additives such as sodium lauryl sulfate (SLS), cetyl trimethyl ammonium bromide (CTABr) and arabic gum (AG), that are used in the Zn(II)/Zn(0) process on the aluminum electrode. The electrodes used for testing were those commercially employed, aluminum and silver-lead, in an electrolyte containing principally Zn(II), H2SO4 and Mn(II), which simulated that used in industry. The current efficiency, evaluated at constant current with a rotating cylinder electrode (RCE), assumed values of 95, 96 and 99% for SLS, CTABr and AG, respectively. These values are higher than those found without the additives (84%) and those registered industrially (88 – 93%). The study of the anodic layer formation and passivation mechanisms, on the Pb-Ag electrode, revealed that this is modified by the presence of Mn(II) in the electrolyte. This modification can be divided into four stages. The first, which takes place in the initial 5 minutes, consists of Pb3O4 formation on the metallic lead surface, which favors α-MnO2 buildup, increasing the current generated. The second stage occurs at 5 minutes, when α-MnO2 completely covers the surface, generating high current densities and showing properties that are electroactive for the formation of other manganese species. The third step takes place between 5 < t < 60 minutes, when the anodic layer loses its electroactive properties as the α-MnO2 layer, due to the formation of MnO4 - and δ-MnO2, compounds that posses less electroactivity. The fourth stage begins at t > 60 minutes, when δ-MnO2 formation predominates, transforming the crystalline surface into an amorphous and non-electroactive layer. On the other hand, the anodic layer fractures, liberating Pb2+ ions and exposing the metallic lead, which is oxidized to PbO2. The recently released Pb2+ ions form PbMn8O16 on the electrodic surface and PbSO4 in the bulk electrolyte. The study presented in this thesis is a first approximation in the development of a stable, electroactive MnO2 layer for oxygen evolution. A commercial zinc electrolysis cell was scaled-down (1:15). This cell, in the absence of current, showed a well mixed reactor behavior because the electrodes acted as deflectors, which favor concentration homogeneity in the entire cell. Electrolysis experiments, performed on the scale-down cell at a constant current density of 30 mA cm-2 with aluminum and silver-lead electrodes, showed potential distributions, caused by zinc concentration heterogeneity, that negatively impacted on the current efficiency of the Zn(II)/Zn(0) process. The current efficiencies were optimized through the control of the cathodic potential, by progressively dosing gelatin. This procedure achieved efficiencies greater than 90%, while in the absence of this control, efficiencies were approximately 50%. On the other hand, the hydrodynamics, modified by the optimization of interelectrodic spacing and electrolyte velocity, favored the rapid dispersion of the oxygen produced at the anode, decreasing the electrolyte potential drop, cell potential and electrolytic energy consumption. |
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