Eletrorrecuperação de níquel: efeito de variáveis operacionais e características do eletrólito na eficiência energética do processo e na qualidade do produto utilizando catodos de aço inoxidável ou titânio

Abstract

The present work describes the individual and combined effects of key operating variables (i.e. current density, pH and nickel concentration in the electrolyte) and electrolyte's characteristics (i.e. presence of sodium ions, boric acid and sodium lauryl sulphate - SLS) on the current efficiency, cell voltage and energy consumption of the electrowinning ofnickel and on important features of the metal deposit (i.e. morphology, roughness, crystal orientation and microhardness). Two types of substrates (cathodes) were used: 316L SS or Ti. The original approach involved electrowinning experiments, electrochemical measurements and product characterization by means of scanning electron and atomicforce microscopies, X-ray diffraction and microindentation tests. With regard to the effects evaluated on the electrowinning process, only changes of pH and current density and presence of sodium ions in the electrolyte caused relevant effects on the specific energy consumption, regardless of substrate type. While increasing pH and sodium ions addition caused a decrease of this parameter, increasing current density lead to higher energy consumptions. Under conditions typically found in industrial practices (i.e. pH 3, 85g.L-1 of Ni2+ ions, 50g.L-1 of Na+ ions, 8g.L-1 of H3BO3, 60ºC and 300A.m-2), no significant effect of the substrate type was identified on the energy consumption. However, theaverage values obtained when the Ti cathode was used are slightly smaller, which may represent a competitive advantage for this material. With regard to the effects evaluated on the metal product, by adding 10mg.L-1 of SLS to the electrolyte, no relevant effect on energy consumption was identified, but smoother Ni deposits with rare hydrogen pits on the surface and regular edges were produced after 6 h, regardless of substrate type. On the other hand, the presence of sodium ions in the electrolyte increased the average roughness of the deposits produced on both substrates, likely due to the increase of the grain size ofthe material. This effect also resulted in a decrease of the microhardness of the deposit. An opposite effect, increase of microhardness, was observed when boric acid was added to the electrolyte. The findings of this work can be useful to establish optimized experimental conditions that guide the industries to the production of nickel deposits with highercommercial value, which are easily stripped from the cathodes and cut, mainly due to proper surface characteristics and mechanical properties.