Effect of Pd, Cu, and Ni additions on the kinetics of NiCl 2 reduction by hydrogen
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INTRODUCTION
A dominant role in the production of nickel powders is the reduction of NiO by hydrogen at high temperatures.[1–4] Rao[5] reported an increase in nickel yield, obtained by reduction, as a result of increasing the reaction temperature or by use of additives. The effect of additives on nickel production has been mostly studied with NiO reduction by hydrogen; Delmon[6] and Konchakovskaya et al.[7] found that adding Pd, Cu, and Ni had a positive effect on the kinetics of NiO reduction by hydrogen. Kinetic and thermodynamic data[6,8,9] reveal NiCl2 reduction at a lower temperature, below 550 7C, with a lower activation energy, compared to the NiO reduction by hydrogen. Recently, there has been a growing interest in the production of nickel from aqueous solutions of NiCl2, because of its good solubility in water and its role in low-temperature processes of chlorination of Cu and Ni sulfide concentrates.[10] According to Yentis and co-workers,[11,12] addition of Pt to NiCl2, in the 300 7C to 500 7C temperature range and hydrogen atmosphere, causes its complete transformation at considerably lower temperatures (300 7C), by dissociation and spillover of hydrogen.[13] This can be used in the formation of bimetal phases of catalysts of strictly controlled compositions. Also, reduction of chloride solutions has been widely applied to recover nickel from spent catalysts;[14] a polycrystalline nickel powder, which normally absorbs hydrogen at low pressures, will become a S.R. STOPIC´ and I.B. ILIC´, Professors, are with the Department of Nonferrous Metallurgy at Faculty of Technology and Metallurgy, 11000 Belgrade, Yugoslavia. D.P. USKOKOVIC´, Professor and Head, is with the Division for Advanced Materials and Processes, Institute of Technical Sciences, the Serbian Academy of Sciences and Arts, 11000 Belgrade, Yugoslavia. Manuscript submitted September 9, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
significant hydrogen absorbent if palladized (0.1 mass pct of Pd or less).[15] Nickel powder particles of relatively spherical shape were obtained by reduction of NiCl2 gas by hydrogen at 1000 7C,[16] and by ultrasonic spray pyrolysis of aqueous solutions of the same compound above 900 7C,[17,18] but because of the high process rate and temperature, the kinetics of the reduction were not investigated. Williams et al.[8] have studied NiCl2 reduction by hydrogen in a temperature interval of 260 7C to 515 7C, and concluded that the NiCl2 reduction mechanism includes formation of nickel nuclei and their growth, the reduction being limited by the chemical reaction rate in the 260 7C to 380 7C temperature range. The induction period, considered as a reduction initiation, precedes the nuclei formation. Nuclei growth corresponds to the autocatalytic period, autocatalysis being a consequence of the autocatalytic behavior of nickel. The aim of our investigation was to study the effect of Pd, Cu, and Ni on the kinetics and the mechanism of NiCl2 reduction by hydrogen, on which no literature data are available, and to determi
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