Hydrogen reduction of oxidized nickel concentrates
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11/8/03
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Hydrogen Reduction of Oxidized Nickel Concentrates C.J.B. CROWE and T.A. UTIGARD Thermal gravimetric analysis was used to investigate the weight change of Ni/Cu/Co calcines upon heating in an inert as well as hydrogen atmosphere. The two calcines investigated contained approximately 50 wt pct combined of hematite and magnetite in addition to sulfides of Ni, Cu, Co, and Fe. Mass spectrometry was used to analyze the gas species evolved during heating and reduction. The calcine samples are 100 pct less than 100 m with hematite/magnetite rims around a central sulfide core. When heating the calcines at 10 °C/min in hydrogen, reduction starts at around 400 °C and is nearly complete at about 700 °C with all the reducible oxygen removed. Isothermal reduction tests show that at temperatures from 650 °C to 800 °C, half the oxygen is removed in less than 4 min. The TGA results combined with microscopic analysis show that the reduction followed a uniform internal reduction model. The reduced calcines will quickly get re-oxidized if they are allowed to contact air while they remain hot.
I. INTRODUCTION
AT Falconbridge Ltd. (Falconbridge ON, Canada), a nickel-copper-cobalt sulfide concentrate is oxidized in a fluid bed roaster before being sent to an electric furnace where it is smelted to form slag and matte.[1,2] In the smelting process, coke is added in order to reduce the slag, thereby lowering the losses of nickel and cobalt. One disadvantage with adding coke in the electric furnace is that the reduction process is endothermic, possibly limiting the smelting rate in the furnace. Another disadvantage is that large amounts of CO are formed. Therefore, additional air has to be introduced to the freeboard of the furnace to combust the CO formed. This increases the freeboard temperature, the off-gas volume, and the dust carryover. One possible change in the operation is to reduce the roasted calcine in the solid state before it is smelted in the electric furnace. This may be possible since during roasting of pentlandite, pyrrhotite, and chalcopyrite, iron oxide rims form surrounding the sulfide cores.[3] The roasted product is fine with 100 pct less than 100 m,[1] and the reduction kinetics therefore is expected to be fast. One limitation may be the fairly low temperature of the calcine leaving the roasters. Therefore, this investigation was carried out to determine the kinetics of calcine reduction at various temperatures and hydrogen concentrations. The roasted calcine, which is at a temperature of about 500 °C to 600 °C, contains approximately 50 pct of magnetite plus hematite. Since these oxides form a hematite/magnetite rim,[3] it is expected that the reduction kinetics may be somewhat similar to that of hematite and magnetite. Turkdogan et al.[4,5,6] studied the reduction of iron oxides by hydrogen and found that the reduction rate
C.J.B. CROWE, Associate and Head of Pyrometallurgy in Australia, is with Hatch Associates, Perth, Australia. T.A. UTIGARD, Professor, is with the Department of
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