Research Article Kinetic Study of Spent Nickel Catalyst Dissolution in HCl and Aqua Regia Medium The leaching kinetics of spent nickel oxide catalysts in HCl and aqua regia solu- tion is described. The considered parameters affecting nickel recovery are particle size, reaction time, acid concentration, and reaction temperature. Nickel could be extracted with HCl by ∼87.41 % and with aqua regia by ∼81.01 % using a spent catalyst with 200 mesh particle size at a reaction temperature of 100 °C. The reac- tion time was 360 min and the concentrations of HCl and aqua regia were 37 % and 80 %, respectively. The leaching kinetics indicates that shrinking sphere and diffusion were the rate-controlling processes during the reactions with HCl and aqua regia, respectively. The activation energies for HCl and aqua regia were determined as 24.04 and 19.03 kJ mol –1 , respectively. Keywords: Leaching kinetics, Nickel recovery, Spent catalyst, Waste materials Received: December 09, 2010; revised: October 16, 2011; accepted: November 21, 2011 DOI: 10.1002/ceat.201000536 1 Introduction For many years, nickel has been considered the most suitable metal in steam reforming of hydrocarbons. It is cheap, suffi- ciently active, and allows suitable catalysts to be economically produced. Nickel is widely used as a catalyst in several techno- logical processes: hydrogenation, hydrodesulfurization, hydro- refining including fat-hardening processes (Ni, Mo/Al 2 O 3 , NiO/Al 2 O 3 , Raney nickel alloy), refinery hydrocracking (NiS, WS 3 /SiO 2 Al 2 O 3 ), methanation of carbon oxide from hydrogen and ammonia synthesis gas (NiO/Al 2 O 3 , NiSiO 2 ), and also, e.g., in metal steel and vegetarian oil industries [1, 2]. The initial composition of a nickel catalyst depends on the reaction involved during industrial operation. For example, a catalyst for hydroalkylation reaction contains other metals such as tungsten in the presence of nickel. Initially, these cata- lysts perform at high activity but with time they become less active and reach a point where they are no more sufficiently effective to be used in a commercial process. Recently, great attention has been paid to research considering recovery of nickel from secondary resources. Therefore, there is continu- ous effort necessary to develop methods of recovering nickel in usable form [2]. Several methods have been described in literature regarding nickel recovery not only from spent catalysts but also from other solid and liquid wastes such as electrowinning, fluidized beds, and burning processes [3–5]. As a result of kinetic stud- ies on nickel recovery from spent catalysts it was found that the activation energy to control the chemical reaction at the catalyst particle surface is more than 41.87 kJ mol –1 and even values of up to 83.74 kJ mol –1 have been reported. For the reac- tion rate at diffusion control, activation energies between 1 and 25.12 kJ mol –1 have been stated [6, 7]. In Iran, so far oil and gas industries released the spent cata- lysts in the environment, but now the Iran Environment Orga- nization has noted environmental problems due to the dis- charge of these waste materials. Therefore, our research team started a program for studying and researching this issue, focusing mainly on nickel extraction. All studies on kinetic evaluations were related to nickel recovery in H 2 SO 4. Thus, here the kinetics of leaching nickel from spent catalysts (NiO/Al 2 O 3 ) is investigated in two different solutions, HCl and aqua regia, and the results are compared to other meth- ods. The process parameters studied include particle size of the spent catalyst, acid concentration, temperature, and time of reaction. 2 Experimental 2.1 Materials and Apparatus Spent nickel catalysts used in this study were provided by Tab- riz Oil Refinery (Iran). The fresh nickel catalysts (Johnson Matthey Catalysts Company; purity of 16 wt % NiO) have been used up to five years in the steam reforming unit of the Tabriz Refinery Co. and unloaded as spent catalysts. The grayish black rings of these catalysts were crushed and powdered. The elemental compositions of the spent catalysts have been Chem. Eng. Technol. 2012, 35, No. 4, 729–734 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Seyed A. Hosseini 1 Mohammad A. Khalilzadeh 2 Ahmad Jamshidi 2 1 Department of Chemical Engineering, Qaemshar Branch, Islamic Azad University, Qaemshar, Iran. 2 Department of Chemistry, Qaemshar Branch, Islamic Azad University, Qaemshar, Iran. – Correspondence: Dr. S. A. Hosseini (sahmad_h@iust.ac.ir), Depart- ment of Chemical Engineering, Qaemshar Branch, Islamic Azad University, Qaemshar 163, Iran. Leaching kinetics 729