Hydrothermal precipitation of nickel and cobalt oxides

The aim of this study was to investigate the possibility of nickel and cobalt oxide precipitation by hydrothermal methods from synthetic and industrial metallurgical plant chloride solutions. The effects of temperature (120–190 °C), pH (6–10) and oxygen partial pressure (5–15 bar) on precipitation products and their purity were studied in a laboratory-scale autoclave. Results showed that direct hydrothermal cobalt oxide precipitation could be conducted from both synthetic and industrial solutions and the precipitation of cobalt oxide (purity ≥99 %) spinel occurred directly under the investigated conditions. Furthermore, it was observed that more aggressive experimental conditions were required to precipitate cobalt oxide from industrial cobalt solutions (T = 190 °C, pH = 10, and pO2 = 10 bar) compared to synthetic cobalt solutions (T = 150 °C, pH = 8, and pO2 = 5 bar). This method can provide an alternative cobalt oxide precipitation route compared to the state-of-art method through crystallization and annealing. Opposite to cobalt, direct hydrothermal precipitation of nickel oxide was not successful with the experimental parameters investigated, as only nickel hydroxide with impurities such as calcium sulfate, magnesium sulfates and sodium chloride, was precipitated. Consequently, these precipitates were subjected for conventional annealing in a furnace in order to convert the hydroxides to oxides. XRD analysis confirmed nickel oxide phase formation
that was achieved at 400 °C with an annealing time of 2 h.