Reduction kinetics of Ni(OH) 2 to nickel powder preparation under hydrothermal conditions
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I.
INTRODUCTION
THE production of metal powders from aqueous solution by pressure reduction with hydrogen has been carried out on a commercial scale, t~-61 Derry and Whittemore reported at the 2nd International Symposium of Hydrometallurgy in 1973 about production of Ni powder in a pressure reduction system using slurries of Ni(OH)2 generated from sulfate solution and using H2 at 170 ~ to 250 ~ and discussed the reduction rate of zero order, tTl Sista and Sliepcervich reported the hydrogen reduction kinetics of metal sulfate solutions tS~ in 1981: however, studies of the slurry system are very few because of the complex heterogeneous phase of solid, liquid, and gas. Kutty et al. tgJ in 1981 studied the hydrothermal reduction of metal hydroxide slurry. The slurry was first dissolved to solution under hydrothermal conditions, and then metal ion was reduced to metal powder. Matsuda and Majima reported the kinetics of hydrothermal reduction of nickel sulfate in 1981 .[lot They showed three distinct stages of the reaction process, dissolution of hydrogen, reduction in solution, and deposition of metal. It was then suggested that this reduction could not proceed from an insoluble hydroxide state such as occurs in alkaline solution. However, Kening e t al.t~ u succeeded in the preparation of metal nickel from insoluble nickel hydroxide slurry under alkaline conditions using PdC12 as an activater. The aim of this work is to examine the reduction kinetics of Ni(OH)2 using anthraquinon as an activator and attempting the preparation of fine metal powder. NAKAMICHI YAMASAKI, Professor and Director, is with the Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi-shi 780, Japan. LIANG HUANZHEN, Associate Professor, is with the Institute of Chemical Metallurgy, Academia Sinica, Beijing, People's Republic of China. Manuscript submitted August 4, 1992. METALLURGICAL TRANSACTIONS B
II.
EXPERIMENTAL
The reduction was carried out using a microautoclave of 10 c m 3 inner volume, as shown in Figure 1. The inside of the autoclave was lined with INCONEL*-600 and *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
the outside with carbon steel. The induction heating system (Figure 2) using commercial current could tolerate a rapid temperature rise (100 ~ with agitation by rocking motion. The starting mixed powder of Ni(OH)2 and the activater was added with water into autoclave. Hydrogen gas was introduced through a pressure valve after flashing a few times. After sealing, the autoclave was placed in an induction furnace, rapidly rised to planned temperature, and maintained for a fixed time with stirring by a rocking motion of the furnace. The autoclave was then cooled by air fan. The resulting sample was washed out with water and the solid sample collected by filtration. The powder product was dried in a vacuum desicator at room temperature to prevent the oxidation of metal. The powder shape was observed by a scanning electron microscope (Hitachi, S-530), and the structure
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