Journal of Chemical Engineering of Japan, Vol. 37, No. 9, pp. 1129–1134, 2004 Research Paper Copyright © 2004 The Society of Chemical Engineers, Japan 1129 Leaching of Nickel from a Hydrodesulphurization Spent Catalyst with Ammonium Sulfate Jae-Min YOO 1 , Jae-Chun LEE 2 , Byung-Su KIM 2 , Hi Sun LEE 3 and Jin-Ki JEONG 2 1 Department of Metallurgical Engineering, Chonbuk National University, Jeonju 561-756, Korea 2 Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350, Korea 3 Research Division, Korea Environment Institute, Eunpyung-gu, Seoul 122-040, Korea Keywords: Recycling, Spent Catalysts, Leaching of Catalysts, Nickel Leaching Leaching of nickel from a hydrodesulphurization spent catalyst generated from the sulfur removal process of petroleum refineries was investigated using ammonium sulfate ((NH 4 ) 2 SO 4 ) solution. The spent catalyst used in the study was composed of vanadium and nickel as sulfides, supported in an alumina matrix. Experiments were carried out in a stirred batch reactor under the temperature range of 298–368 K and the (NH 4 ) 2 SO 4 solution of 1.3–3.3 mol/dm 3 . As a result, about 94.0% of nickel was leached from the 2.6 mol/dm 3 (NH 4 ) 2 SO 4 solution in 90 min at the temperature of 368 K. A shrinking core model was found to be useful in describing the rate of nickel leaching. The rate of nickel leaching is limited by ash layer diffusion. The nickel leaching is of the first order with respect to the (NH 4 ) 2 SO 4 concentration and has an activation energy of 16.2 kJ/mol. Introduction Hydrodesulphurization catalysts have been usu- ally used in the oil refining process to produce light oil by removing sulfur from heavy oil. Most of the cata- lysts are small spheres (diameter: 3 mm) composed of nickel–molybdenum or cobalt–molybdenum, supported on an aluminum inert substrate. In general, hydrodesulphurization spent catalysts can be regenerated two or three times, but eventually, they become poisoned and must be discarded because these catalysts lose activity or selectivity after being used for a period of time. Large amounts of the spent catalysts have thus been generated from oil refineries. The spent catalysts contain significant amounts of valu- able metals like nickel or cobalt, molybdenum, and va- nadium as sulfides in an alumina matrix as well as some organic contaminants. The vanadium and organic com- pounds were deposited from the heavy oil in the sulfur removal process. So, the disposal of the spent cata- lysts is environmentally dangerous if improperly man- aged (Habermehl, 1988; Berrebi et al., 1993). A number of hydrometallurgical processes were developed and commercialized to minimize the envi- ronmental problems with recovering valuable metals Received on December 12, 2003. Correspondence concerning this article should be addressed to J.-C. Lee(E-mail address: jclee@kigam.re.kr). from hydrodesulphurization spent catalysts (Zhang et al ., 1995). In general, the recovery of the valuable metals from the spent catalysts has been carried out by conventional methods (Toda and Matsuda, 1988; Toda, 1989): soda roasting, hot water leaching, and precipi- tation as NH 4 VO 3 and CaMoO 4 by adding NH 4 Cl and CaCl 2 , respectively. However, the recovery of nickel using the conventional methods is very difficult be- cause during roasting at the high temperature over 873 K with adding a soda salt like Na 2 CO 3 , Na 2 SO 4 or NaOH the compound of NiAl 2 O 4 is formed, which is insoluble in water (Toda and Matsuda, 1988; Toda, 1989). Thus, an autoclave process was additionally used to recover nickel, by which the operating coast would be expensive (Zhang et al., 1995). Moreover, the price of nickel, which is second abundant element in the spent catalysts, keeps increasing recently. Therefore, it would be highly desirable to find an al- ternative process for recovering nickel from hydrodesulphurization spent catalysts. Keeping this point in mind, the present research is concerned with investigating the kinetics for nickel leaching from a hydrodesulphurization spent catalyst generated from an oil company in Korea using an am- monium sulfate ((NH 4 ) 2 SO 4 ) solution. The effects of stirring speed, (NH 4 ) 2 SO 4 concentration, particle size, and leaching temperature were investigated. It is thus expected that this work will help to develop and