The effect of organic compounds on nickel electrowinning and product quality

Abstract

The present work describes the effects of some organic compounds on the current efficiency, cell voltage and specific energy consumption during nickel electrowinning, and on the deposit features (morphology, crystal orientation, microstructure, and hardness). The organics tested here (extractant, diluent, flocculant, lubricating oil and acid mist suppressant) are currently used in industrial plants. The results indicated that only the flocculant and acid mist suppressant exhibit relevant negative impacts on the current efficiency and energy consumption of the electrowinning process. The current efficiency dropped from 93.6 % (organic-free solutions) to 86.0 % and to 92.8% in electrolytes containing 100 mgL -1 of acid mist suppressant and flocculant, respectively. At this concentration, their addition to the electrolyte led to increases of the specific energy consumption in the order of 18 % (acid mist suppressant) and 2 % (flocculant). Measurements by cyclic voltammetry showed that Ni reduction starts at more negative potentials at 50 and 100 mgL -1 of flocculant or suppressant in the electrolyte, thus indicating an inhibition of the deposition process. The deposit quality was affected at the higher concentration of all organics (except in the presence of the diluent), as more hydrogen pitting was observed. The addition of acid mist suppressant to the electrolyte caused delamination of the Ni deposit at 50 and 100 mgL -1 , probably due to a significant reduction of the crystallite size, as confirmed by microstructural analysis. The addition of flocculant levelled off the deposit surface with the growth of elongated Ni crystals, while the addition of the lubricating oil did not affect the crystallite size and shape, but the deposits produced from electrolytes containing this impurity exhibited more pits and also areas depleted of metal deposition. The addition of flocculant and acid mist suppressant to the electrolyte also caused an increase of the nanohardness of the Ni deposit. At 100 mgL -1 of flocculant or suppressant, it increased about 31 and 72%, respectively. Such increase may lead to a decrease in the material ductility, which may affect the stripping and cutting of Ni sheets. Our findings may be helpful to the industry since strong adverse effects of some organics that may be present in the electrolyte were identified, thus indicating that the organic´s dosage should be controlled and optimized in the corresponding operations.