A study of the morphological aspects of the indium electrorefining process

Abstract

The cathodic deposition of In was performed under conditions suitable for electrorefining purposes. The main goal of the study was: i) to determine the possibilities for In ultrapurification through multiple electrorefining with controlled parameters and ii) to study the regularities of the morphology of the In deposit as influenced by the electrodeposition conditions. Without application of special high purity techniques, it was possible to obtain an In purity level of 99.997 %, by repeating the refining procedure in a set of modular electrolysis cells with forced electrolyte circulation and applying modified current regimes. Irregular (non planparallel) migration of indium from the anode to the cathode and vice versa, as well as dendrite formation at the edges of the electrode prevented further repetition necessary for the achievment of higher purification levels. This provoked the need to study the morphology of the In-deposits. The phenomena and some regularities of In nucleation and grain growth, as influenced by the applied overpotential and quantity of current passed through the cell, were studied. Electrodeposition was performed onto a stationary Pt-electrode from a mixed sulfate-chloride solution. SEM with magnification up to 1000 × was used to identify the morphology of the deposits. Granular grains were dominant at overpotentials of 85 and 110 mV, while at 160 and 185 mV, needle-like and nonbranched dendrites were visible. The deposit became less compact as its thickness advanced. The apparent thickness of the In layer increased from 20-25 to 320-380 μm when the amount of charge was increased from 1 to 10 mA h cm-2.