In-situ chromic-acid regeneration and purification process: Laboratory-scale feasibility study and mathematical modeling.

摘要

Acid chromate (Cr₂O₇2–) plating baths become contaminated by Ni2+, Fe2+, Cu 2+ ions through corrosion of metal accessories, and by Cr3+ ions as a result of Cr₂O₇2– reduction. These contaminants at mM level degrade the quality of hard-chrome deposit. It is, therefore, desirable to remove them continuously from the plating bath to avoid the high cost and liability associated with the disposal process. A recent technique used a “porous pot” in conjunction with a set of auxiliary electrodes serve to collect the contaminants as hydroxides.; In this work, we explore a novel membrane separation concept, that is, combining an ion-conducting polymer membrane with a fuel cell cathode to concentrate Cu, Fe, and Ni contaminants inside the membrane cell, while oxidizing Cr 3+ to Cr₂O₇2– in the plating bath. The electrolysis cell consists of a rectangular tank divided into two compartments via an ion exchange membrane (Nafion-117), and uses a lead anode and a gas diffusion electrode as the cathode. The laboratory scale cell was used as a simulated plating bath containing Cu2+, Fe 2+, Ni2+, and Cr3+ as contaminants. The performance of the process was assessed by operating the cell under constant current conditions, different initial concentrations, and catholyte-to-anolyte volume ratios.; Electrochemical Impedance Spectroscopy (EIS) and stationary polarization curves were used to characterize the performance of a fuel cell cathode in the regeneration cell. The configuration of the MEA, current collector, and flow distributing backing plate may, during long-term operation lead to excessive ohmic resistance, which necessitates a special design of the cathode assembly. X-ray diffraction indicated the deposition of Cu, Fe, Ni, and Cr on the electrode matrix, leading to deactivation of the Pt-catalyst. The deactivation causes a rising cell voltage during electrolysis. Nevertheless, the energy consumption of the regeneration cell is at least one volt less than that of a comparable cell with hydrogen evolving cathode.; A mathematical model was developed to estimate contaminant fluxes due to migration, diffusion and convection in the laboratory-scale batch electrolysis cell. The mathematical model was used to estimate process parameters from experimental results, assuming quasi-stationary operation. Ionic mobilities of Cu, Fe, and Ni through the Nafion-117 membrane were found to be 5.4, 1.7, 5.2*10–10 cm2/V.s for Cu, Fe, Ni, respectively.