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Electrochemical processes within the slimes layer of lead anodes during Betts electrorefining González Domínguez, José Alberto


In the Betts process for lead electrorefining the noble impurities originally present in the bullion form a strong and adherent layer of slimes. Within this layer the established ionic concentration gradients can lead to secondary reactions. The following processes were analyzed from a thermodynamic perspective: (A) hydrolysis of the acid (B) precipitation of secondary products (C) reaction of noble compounds. The nature of the concentration gradients within the slimes layer and related secondary processes was studied by using transient electrochemical techniques which include: (A) current interruption, (B) AC impedance, and (C) a variation of SACV (Small Amplitude Cyclic Voltammetry). These studies were complemented by: (A) physico-chemical data on electrolyte properties, (B) "insitu" and "industrially recovered" slimes electrolyte compositions, (C) SEM and X-ray diffraction analysis of the slimes layer. For comparison purposes the electrochemical behaviour of "pure" Pb electrodes was also studied. Upon current interruption the anodic overpotential decays, first abruptly, (as the uncompensated ohmic drop disappears) and then slowly (due to the presence of a back E.M.F. created by ionic concentration gradients that decay slowly). Current interruption measurements showed that: (A) concentration gradients exist across the slimes layer, (B) inner solution potentials within the slimes layer can be larger than those measured from reference electrodes located in the bulk electrolyte, (C) secondary products can shift the inner solution potential to negative values which reverse upon re-dissolution and (D) ionic diffusion is seen upon current interruption but it is complex and difficult to model due to the presence of processes that can support the passage of internal currents. The anodic polarization components were obtained by analyzing the potential and current dependance upon application of a small amplitude sinusoidal waveform. This dependance was found to be linear in the low overpotential region (< 250mV). Thus, upon subtraction of the uncompensated ohmic drop, the remaining polarization is due to the "apparent" ohmic drop of the slimes electrolyte and to liquid junction and concentration overpotentials. These components are directly linked to the electrolysis conditions and to the slimes layer structure. Furthermore, the ratio of these components can be used to obtain the point at which the precipitation of secondary products starts. Changes in this ratio can also be related to the anodic effects caused by the presence of addition agents. AC impedance measurements performed in the presence of a net Faradaic current showed that the impedance increases uniformly as the slimes layer thickens up to the point at which noble impurities start to react. Three electrical analogue models were used to describe the impedance spectra. A steady-state mathematical model that predicts concentration and potential gradients across the slimes layer was developed. Only when a position dependent eddy diffusion term was incorporated in the numerical solution, were reasonable local ionic concentrations and overpotentials obtained.

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