Leaching of nickel from supported nickel waste catalyst using aqueous sulfur dioxide solution
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I.
INTRODUCTION
S U P P O R T E D nickel catalysts are widely used in petrochemicals, hydrogenation, and other chemical industries. Such catalysts are prepared either by impregnation or coprecipitation processes where NiO is supported on alumina and silica. In some cases, it may be further reduced to get active catalyst. After use and deactivation, these catalysts are considered as secondary sources of this metal. Therefore, recovery of nickel from such catalysts has attracted the attention of investigators. Depending upon the nature of the catalyst, i.e., nickel content, extent of nickel support interaction, heat treatment of the catalyst during preparation and use, and impurities picked up during the use of the catalyst, various methods have been developed for the extraction of nickel.[ 1-10] Recently, aqueous solution of sulfur dioxide has been investigated as an efficient leaching agent. It offers the advantages of rapid and selective extraction of metal values at ambient temperature and pressure. Acting both as an effective reducing agent and an acid, it can be used commercially for leaching of industrial wastes as well as complex oxidized ores and sea nodules. Pahlman and Khalafalla t11] have selectively extracted Ni, Co, and Mn from sea nodules by aqueous SO2 solution within 10 minutes. Warren and Hay [121have studied the dissolution of iron ores in aqueous solution of SO2. Miller and Wan [13] have investigated the leaching kinetics of MnO2 by SO2. Raisoni and Dixit [14] have extracted Co and Mo from a Co-Mo/A1203 hydrodesulfirization (HDS) catalyst using aqueous SO2 solution. In this work, we have studied leaching behavior of nickel in a NiO/A1203 waste catalyst using aqueous solution of SOa. The effects of various parameters on the rate of dissolution have been investigated, and a dissolution mechanism has been proposed. Surface investigation of the waste catalyst as received and leached samples has also been carried out to elucidate the nature of the dissolution using X-ray photoelectron spectroscopy (XPS).
II.
EXPERIMENTAL
A. Materials The waste catalyst was procured from Electroalloys Pvt. Ltd., Ahmedabad, India. The sample was ground, and various particle size fractions were obtained. The waste catalyst contained 25 pct Ni (31.8 pct NiO), 18.5 pct A1 (35 pct A1203), 8.5 pct MgO, 6.8 pct Fe203, 0.7 pct SIO2, 2 pct loss on ignition, and about 14.8 pct acid-insoluble materials. The SO2 gas was supplied as commercially available bottled gas by M / S Star Freeze Industries, Bombay, India, and was used as such. The chemicals used were of analytical grade. B. Methods The leaching experiments were carried out in a 1-L cylindrical glass reactor placed in a thermostatic water bath. A four-bladed glass stirrer, a thermometer, a gas sparger, and a solution sampler could be introduced through the lid of the reactor. The waste catalyst was ground and sieved to get particle size fractions in the range of 177 to 500/xm. The SO2 gas was passed into the distilled water to obtain solutions of various SO2 concentratio
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