Kinetics of chloridization of nickel oxide with gaseous hydrogen chloride
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I.
INTRODUCTION
CHLORINATION and chloridization are the two important methods used for the extraction of several metals such as Ti, Zr, Hf, Nb, Sn, Ni, etc.[1–4] Chlorination is also used in the extraction of a very pure form of Si.[2] These two methods are used for the extraction of those metals whose chlorides are thermodynamically more stable than their oxides. However, for those metals that possess a high affinity for oxygen, chlorination may be achieved either by introducing a strong reducing agent such as carbon or hydrogen, which can remove oxygen continuously from the system, or by using a high flow rate of chlorine so that equilibrium shifts to the right. Since gaseous HCl contains hydrogen, the reaction between it and metal oxides is thermodynamically more favorable than the solid-state reaction between metal chloride and metal oxide. Although the kinetics of chlorination of metal oxide is well documented, it may be noted that no general consensus on the mechanism has been reached among the different investigators.[5–9] Each problem has been tackled in a way that suited the authors best to explain their experimental data. As far as the kinetics of chloridization are concerned, the situation is rather worse as the information available is practically insignificant. This is possibly because of the problem associated with the evolution of H2O vapor as one of the products of chloridization. The H2O (v) tends to combine with anhydrous metal chloride, which, in many instances, is hygroscopic in nature. Further, some hydrated chlorides such as CaCl2[10] may undergo swelling either due to melting or due to an increase in molar volume of the solid product, thereby blocking the pores of the reactant oxide. Therefore, most of the chloridization reactions with gaseous HCl have been carried out at higher temperatures when the metal chloride is removed by volatilization from the reaction interface.[11,12] In these two articles, an attempt has therefore been made to study the kinetics of chloridization of pure nickel oxide as well as nickel-bearing latS.B. KANUNGO, Scientist-F, and S.K. MISHRA, Research Associate, are with the Regional Research Laboratory, Bhubaneswar 751013, India. Manuscript submitted September 20, 1995.
METALLURGICAL AND MATERIALS TRANSACTIONS B
eritic iron ore at relatively lower temperatures to understand the mechanism of this important gas-solid reaction.
II.
EXPERIMENTAL
A. Materials and Methods for Their Characterization Laboratory reagent grade nickel oxide powder (Lobachemie, Bombay, India), blackish in color and having (in wt pct) 89.5NiO, 1.7CaO, 2.0 alkali metal oxides, and 7.5 loss on ignition at 950 7C, was used in the present work. The sample was characterized for (a) particle size distribution by a MALVERN (Malvern, England, United Kingdom) particle size analyzer (model 3600) and (b) crystallinity by a PHILIPS* Diffractometer (model PW 1700) using *PHILIPS is a trademark of Philips Electronic Corp., Mahwah, NJ.
Cu Ka radiation. Besides the polysize powdered sample, chloridizatio
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