Iraqi Journal of Chemical and Petroleum Engineering Vol.12 No.4 (December 2011) 21-35 ISSN: 1997-4884 Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi University of Baghdad, Colloge of Engineering, Chemical Engineering Department Abstract Spent hydrodesulfurization (Co-Mo/γ-Al2O3) catalyst generally contains valuable metals like molybdenum (Mo), cobalt (Co), aluminium (Al) on a supporting material, such as γ-Al2O3. In the present study, a two stages alkali/acid leaching process was conducted to study leaching of cobalt, molybdenum and aluminium from Co-Mo/γ- Al2O3 catalyst. The acid leaching of spent catalyst, previously treated by alkali solution to remove molybdenum, yielded a solution rich in cobalt and aluminium. Keywords: spent HDS catalyst; sodium carbonate; sulfuric acid; leaching; valuable metals Introduction The petroleum refining industry makes extensive use of catalysts for desulphurization of various fractions. The most common hydrodesulphurization (HDS) catalysts are the Ni,Mo and the Co,Mo both on the γ-alumina support [1]. During hydrodesulphurization reactions, the catalysts are deactivated by compounds of S, C, V, Fe, Ni, Si and traces of As and P [2-4]. As a result, the spent catalysts are classified as hazardous materials [5]. Due to their toxic nature, the disposal of spent catalysts can pollute the environment since heavy metals are leached out. To avoid pollution in land disposal as well as to minimize landfill space, the spent catalysts are subjected to metal extraction by various solubilization processes and reused in a variety of applications [6-8]. However, such waste materials containing high metal concentrations may be considered as “artificial ores” since they can serve as secondary raw materials with a consequent reduction in the demand for primary mineral resources. Recycling of spent catalysts became an unavoidable task not only for lowering catalysts costs but also for reducing their waste to prevent the environmental pollution [1]. Spent HDS catalysts generally consist of 10– 30% molybdenum,1–12% vanadium, 0.5–6% nickel,1–6% cobalt, 8–12% sulphur, 10–12% carbon and the balance is alumina, which makes it economically viable for recovery of valuable metals [9]. During the past decade, more research work was conducted to recover molybdenum, vanadium and other valuable metals from spent HDS catalysts [10]. A number of processes and technologies for the recycling of valuable metals from spent catalysts have been University of Baghdad College of Engineering Iraqi Journal of Chemical and Petroleum Engineering Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 22 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net developed and reported [11-15]. Basically, there are two main processes, hydrometallurgical and hydropyrometallurgical processes to recover valuable metals from spent catalysts [16]. Hydrometallurgical processes involves leaching with alkaline or acidic solutions. Many reagents, such as NaOH, H2SO4, NH3, (NH4)2SO4 and oxalic acid with H2O2 and Fe(NO3)2 have been tested [17,8] Hydropyrometallurgical processes such as calcination to remove the hydrocarbons, C and S followed by leaching with suitable reagent [18,19] or roasting of spent catalyst with Na2CO3, NaCl and NaOH at high temperatures followed by leaching [20,21,15] have been studied extensively for recovery of metals from spent HDS catalyst. The objective of the work presented in this study is intended to investigate the best operating conditions of metal removal (i.e. Mo, Co and Al) from spent Co- Mo/γ-Al2O3 catalyst by using two stages of extraction which contain alkali stage (i.e. sodium carbonate and hydrogen peroxide solution) and acid stage (i.e. sulfuric acid and hydrogen peroxide solution) [22]. Experimental Materials The following chemical agents were used in the experiments: 1. Spent Co-Mo/γ-Al2O3 catalyst. The spent Co-Mo/γ-Al2O3 catalyst supplied from Al-Daura refinery was used. The shape of the catalyst was cylindrical (2.5mm x 1.5mm). Catalyst contained residue of oil and washed by acetone then by distilled water and dried overnight in an oven maintained at 85 ᵒ C. Finally, it was ground and sieved. For this study, two different size fractions of spent catalyst were prepared: (less than 75μm and greater than 75μm) and mixed thoroughly to obtain uniform sample for chemical analysis and leaching experiments. Chemical analysis of the examined samples are shown in Table (1). The analysis was done in Ibn Sina company by using atomic absorption analysis. Table 1, Chemical analysis of selected samples Catalyst Composition %Co %Mo %Al Co-Mo/γ-Al2O3 2.2 10.12 41.16 2. Hydrogen peroxide (H2O2) supplied by Applichem GmbH Ollowg with assay of 50%. 3. Sulfuric acid (H2SO4) supplied by Sd fine-chem limited with a concentration of 98%. 4. Sodium carbonate (Na2CO3) Analar. Apparatus The apparatus used in the experiments was: 1. Water bath equipped with temperature controller (Baird and Tatlaock Unitemp water bath). 2. Mechanical mixer, which was calibrated using a digital tachometer (Victor DM6234P+). 3. Three neck round – bottom flask (500ml) in volume. The general arrangement of equipments used for extraction are shown in Fig. (1). Fig. 1, Schematic diagram of the apparatus unit 1 4 2 5 3 Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi Available online at: www.iasj.net IJCPE Vol.12 No.4 (December 2011) 23 Experimental design Two stage leaching process was studied: I- Study of Alkali leaching stage In this study, sodium carbonate and hydrogen peroxide as extractant agents were used to extract Mo from spent catalyst. Different parameters for this leaching stage were studied as listed below. 1. Agitation time (30, 60, 90 and 120 minutes). 2. Agitation speed (100, 300 and 500 rpm). 3. Extractant concentration (sodium carbonate concentration of 40, 60, 80 and 120 g/l and hydrogen peroxide concentration of 5, 7, 10 and 12 vol.%). 4. Pulp density (5, 10, 20 and 30 w/v%). 5. Operating temperature (30, 50, 70 and 90 ° C) 6. Particle size (greater than 75 μm and less than 75 μm). II- Study of Acid Leaching Stage In this study, sulfuric acid and hydrogen peroxide as extractant agents were used to extract Co and Al. Different parameters for this leaching stage were studied. 1. Agitation time (30, 60, 90 and 120 minutes). 2. Agitation speed (100, 300 and 500 rpm). 3. Extractant concentration (sulfuric acid concentration of 10, 20 and 30wt.% and hydrogen peroxide concentration of 0, 0.05, 0.1 and 0.15 M) . 4. Pulp density (5, 10, 20 and 30 w/v%). 5. Operating temperature (30, 50, 70 and 90 ° C). Experimental Procedure Alkali Leaching Stage In alkali leaching, the spent catalyst was ground and sieved. For this study, two different size fractions of spent catalyst were prepared: (less than 75μm and greater than 75μm) and mixed thoroughly to obtain uniform samples for leaching. The solution was prepared by dissolving a certain amount of sodium carbonate in deionized water to get different concentration of solution. Leaching experiments were carried out by taking 200ml of leachant in 500ml round bottom flask fitted with a seal to avoid evaporation loss. The contents were stirred with mechanical mixer. After the temperature of the solution reached the desired value, a certain amount of catalyst with different ratios (S/L) was added to the flask and then hydrogen peroxide was added. The addition of H2O2 exothermic started the reaction. 10 ml samples were withdrawn at the end of each run and were filtered from residue by vacuum filtration using filtration paper and analyzed for Mo, Co and Al. The analysis was made in Ibn Sina company by using atomic absorption technique. The recovery percentage was calculated by using the following relationship: …(1) Acid Leaching Stage In acid leaching, the residue of alkali leaching was dried and analysis for Mo, Co and Al. The solution was prepared by adding a certain amount of sulfuric acid to deionized water to get a different concentration of solution. leaching experiments were carried out by taking 200ml of leachant in 500ml round bottom flask fitted with a seal to avoid evaporation loss. The contents were stirred with mechanical mixer. After the temperature of the solution reached the desired value, a certain amount of catalyst with different ratios (S/L) was added to the flask then … 3.1 Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 24 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net hydrogen peroxide was added. 10 ml samples were withdrawn at the end of each run and were filtered from residue by vacuum filtration using filtration paper and analyzed for Co and Al. The analysis was made in Ibn Sina company using atomic absorption technique. The recovery percentage was calculated according to Eq. (1). Factors Influencing the Experiments Alkali Leaching Stage Effect of Agitation Time In this set of experiments, the effect of agitation time on Mo extraction was investigated. A sample of solution was taken at the end of each period (30, 60, 90 and 120 min.). Operating conditions (temperature=30 ᵒ C, agitation speed =300rpm, PD=20%, P.S.=less than 75μm, sodium carbonate concentration of 60g/l and 10vol.% hydrogen peroxide concentration) were kept constant. Effect of Agitation Speed In this set of experiments, the effect of agitation speed on Mo extraction was investigated. A sample of solution was taken at the end of each run of different speeds (100, 300 and 500 rpm) to determine the metals concentration. Agitation time was fixed from the previous experiments. Other variables such as temperature=30 ᵒ C, PD=20%, P.S.=less than 75μm, sodium carbonate concentration of 60g/l and 10vol.% hydrogen peroxide concentration were kept constant. Effect of Extractant Concentration In this set of experiments, the effect of extractant concentration on Mo extraction was investigated. A sample of solution was taken at the end of each run of a different concentration of sodium carbonate (40, 60, 80 and 120 g/l) at fixed hydrogen peroxide concentration (5, 7, 10 and 12 vol.%). From previous sections the agitation time and agitation speed were fixed. Other variables such as operating temperature =30 ᵒ C, PD=20%,and P.S.=less than 75μm were kept constant. Effect of Pulp Density In this set of experiments, the effect of pulp density on extraction of Mo was investigated. Different values of pulp density were tried (5, 10, 20 and 30 w/v%). The agitation time, agitation speed, sodium carbonate and hydrogen peroxide concentration were fixed from previous sections and other variables such as temperature =30 ᵒ C and P.S.= -75μm were kept constant. Effect of Temperature The effect of temperature on extraction of Mo was investigated. The solution was heated to different temperatures (30, 50, 70 and 90 ᵒ C). The agitation time, agitation speed, pulp density, sodium carbonate and hydrogen peroxide concentration were fixed from previous sections while particle size was kept constant less than 75 μm. Effect of Particle Size The effect of particle size on extraction of Mo was investigated. Particle size of catalyst less than 75μm or (>75μm) was used. The agitation speed, pulp density, temperature, sodium carbonate and hydrogen peroxide concentration were fixed from previous sections. Acid Leaching Stage Effect of Agitation Time In this set of experiments, the effect of agitation time on Co and Al extraction was investigated. A sample of solution was taken at the end of each period (30, 60, 90 and 120 min.). Operating conditions (temperature=30 ᵒ C, agitation speed Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi Available online at: www.iasj.net IJCPE Vol.12 No.4 (December 2011) 25 =300rpm, PD=20%, sulfuric acid concentration of 10 wt.% and without hydrogen peroxide) were kept constant. Effect of Agitation Speed In this set of experiments, the effect of agitation speed on Co and Al extraction was investigated. A sample of solution was taken at end of each run of different speeds (100, 300 and 500 rpm) to determine the metals concentration. Agitation time was fixed from the previous experiments. Other variables such as temperature=30 ᵒ C, PD=20%, sulfuric acid concentration of 10wt.% and without hydrogen peroxide were kept constant. Effect of Extractant Concentration In this set of experiments, the effect of extractant concentration on extraction of Co and Al was investigated. A sample of solution was taken at the end of each run of a different concentration of sulfuric acid (10, 20 and 30 wt.%) at fixed hydrogen peroxide concentration (0, 0.05, 0.1and 0.15 M). From previous sections the agitation time and agitation speed were fixed. Other variables such as operating temperature =30 ᵒ C and PD=20% were kept constant. Effect of Pulp Density In this set of experiments, the effect of pulp density on extraction of Co and Al was investigated. Different values of pulp density were tried (5, 10, 20 and 30 w/v%). The agitation time, agitation speed, sulfuric acid and hydrogen peroxide concentration were fixed from previous sections and temperature of 30 ᵒ C was kept constant. Effect of Temperature The effect of temperature on extraction of Co and Al was investigated. The solution was heated to different temperatures (30, 50, 70 and 90 ᵒ C). The agitation time, agitation speed, pulp density, sulfuric acid and hydrogen peroxide concentration were fixed from previous sections. Results and Discussion Catalyst Characterizations Powder XRD analysis was performed on the deoiled sample in order to detect metal foulants such as sulfur, argon, titanium, iron and zinc that block the pore mouths and retarding the access of the reactants to the active sites of the catalyst with pores and identify different crystalline phases present in the catalyst. XRD pattern of spent CoMo/γ-Al2O3 catalyst is given in Fig. (2). The analysis was done in the Ministry of Science and Technology. Fig. 2, X-ray parttern of spent CoMo/γ- Al2O3 catalyst Alkali Leaching Stage Effect of Agitation Time The obtained results are plotted in Figs. (3-6) and Fig. 6 illustrate the effect of operating conditions on Mo, Co and Al. It is obvious that the extraction efficiency of Mo increased as the agitation time was increased to a certain point where it remained Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 26 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net constant. The highest extraction efficiency of 56.37% for Mo was obtained after 90 minutes because extraction processes proceed until equilibrium is reached. Therefore, further increase in the time of experiment had no significant influence on the extraction. The results of Angelidis et al (1995) [6] had also the same trend in selective dissolution of Mo, Co and Ni from diesel and naphtha spent hydrodesulfurization catalyst and in agreement with Shariat et al (2001) [19] in technical note optimizing conditions for hydrometallurgical production of purified molybdenum trioxide from roasted molybdenite of Sarcheshmeh. Co and Al extraction was less than 1% at these operating conditions due to acidity. Fig. 3, Effect of agitation time on Mo extraction Fig. 4, Effect of agitation time on Co extraction Fig. 5, Effect of agitation time on Al extraction Fig. 6, Effect of agitation time on metals extraction Effect of Agitation Speed As seen from Figs. (7-10), the stirring speed shows a small effect on the dissolution of metals within the range used. When the leaching time is 90 min, the stirring speed of 300 rpm gave the best extraction efficiency of Mo (Mo=56.37%). This indicates that the diffusion of the reactants from the solution towards the surface of a catalyst particle and the products away from the surface of the particle was fast and hence increasing agitation speed did not control the leaching rate within the range of the stirring speeds tested. All subsequent experiments were carried out at a stirring speed of 300 rpm. Similar results were observed by Mulak et al (2006) [1]; David (2007) [23] and Qi-ming et al (2009) [24]. 0 20 40 60 0 50 100 150 % M o E x tr a ct io n Time, min. Mo 0 0.1 0.2 0.3 0.4 0.5 0 50 100 150 % C o E x tr a ct io n Time, min. Co 0 0.05 0.1 0.15 0.2 0.25 0 50 100 150 % A l E x tr a ct io n Time, min. Al 0 10 20 30 40 50 60 0 50 100 150 % M e ta l E x tr a ct io n Time, min. Mo Co AL Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi Available online at: www.iasj.net IJCPE Vol.12 No.4 (December 2011) 27 Fig. 7, Effect of agitation speed on Mo extraction Fig. 8, Effect of agitation speed on Co extraction Fig. 9, Effect of agitation speed on Al extraction Fig. 10, Effect of agitation speed on metals extraction Effect of Extractant Concentration As shown in Fig. (11), a maximum efficiency of Mo extraction was achieved around 60g/L Na2CO3 for 90 minutes and decreased gradually thereafter. Where as, the percentage extraction of Co and Al was not significant when compared with Mo extraction as shown in Figs. (12-14). The effect of H2O2 addition varied in values of (5, 7, 10 and 12vol.%) at a constant concentration of Na2CO3 (60 g/L). The results are shown in Figs. (15-18). The results showed a gradual decrease of Mo leaching efficiency above 10 vol.% H2O2. Whereas, the percentage extraction of Co and Al increased gradually, which is due to the acidic nature of the leach solution (decrease of pH). It was observed that, a particular concentration of Na2CO3 and H2O2 had a significant effect on maximum Mo solubilization. At higher concentrations of either Na2CO3 or H2O2, a reaction with each other probably occurs forming sodium percarbonate and thereby decreases the availability of reactants to react with the catalyst, which causes a decrease in Mo leaching efficiency as mentioned by Park et al (2006b) [25]. Leaching of spent catalyst with Na2CO3 in the presence of oxidizing agent such as H2O2 involves oxidation of sulphides to sulphate, which is exothermic, and the reaction of Mo with sodium carbonate forming sodium molybdate. The role of oxidant is to break the MoS2 complex and convert metal and S to the maximum oxidized state and oxidize the oil and free carbon to CO2 and H2O. Whereas, the role of Na2CO3 is to react with the liberated Mo forming soluble sodium molybdate, neutralization of SO3 produced during oxidation to soluble sodium sulphate, and thereby avoiding environmental problems. The reaction of molybdenum content of the spent catalyst in presence of Na2CO3 and 0 20 40 60 0 200 400 600 % M o E x tr a ct io n Agitation speed, rpm Mo 0 0.1 0.2 0.3 0.4 0.5 0 200 400 600 % C o E x tr a ct io n Agitation speed, rpm Co 0 0.05 0.1 0.15 0.2 0.25 0 200 400 600 % A l E x tr a ct io n Agitation speed, rpm Al 0 10 20 30 40 50 60 0 200 400 600 % M e ta l e x tr a ct io n Agitation speed, rpm Mo Co AL Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 28 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net H2O2 can be represented by Equation [25]: MoS2+3Na2CO3+H2O2+4O2 →Na2MoO4+2Na2SO4 +H2O + 3CO2 …. (2) Fig. 11, Effect of sodium carbonate on Mo extraction Fig. 12, Effect of sodium carbonate on Co extraction Fig. 13, Effect of sodium carbonate on Al extraction Fig. 14, Effect of sodium carbonate on metals extraction Fig. 15, Effect of hydrogen peroxide on Mo extraction Fig. 16, Effect of hydrogen peroxide on Co extraction Fig. 17, Effect of hydrogen peroxide on Al extraction 0 20 40 60 0 50 100 150 % M o E x tr a ct io n [Na2CO3], g/l Mo 0 0.2 0.4 0.6 0 50 100 150 % C o E x tr a ct io n [Na2CO3], g/l Co 0 0.05 0.1 0.15 0.2 0.25 0 50 100 150 % A l E x tr a ct io n [Na2CO3], g/l Al 0 10 20 30 40 50 60 0 50 100 150 % M e ta ls E x tr a ct io n [Na2CO3], g/l Mo Co Al 0 20 40 60 0 5 10 15 % M o E x tr a ct io n [H2O2], vol.% Mo 0 0.1 0.2 0.3 0.4 0.5 0 10 20 % C o E x tr a ct io n [H2O2], vol.% Co 0 0.1 0.2 0.3 0 10 20 % A l E x tr a ct io n [H2O2], vol.% Al Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi Available online at: www.iasj.net IJCPE Vol.12 No.4 (December 2011) 29 Fig. 18, Effect of hydrogen peroxide on metals extraction Effect of Pulp Density By examining Figs. (19-22), it can be seen that a maximum efficiency of Mo extraction was achieved around 20%PD and decreased gradually thereafter with an increase in the pulp density. Similar behavior was observed by Park et al. (2006a) [26] in hydrometallurgical processing and recovery of molybdenum trioxide from spent catalyst. A large solid to liquid ratio increases the resistance for diffusion of fresh extractant to the surface of catalyst. Therefore, requires longer time and requires higher extractant concentration as concluded by Shariat et al (2001) [19]. Fig. 19, Effect of pulp density on Mo extraction Fig. 20, Effect of pulp density on Co extraction Fig. 21, Effect of pulp density on Al extraction Fig. 22, Effect of pulp density on metals extraction Effect of Temperature As temperature increases from 30 to 90 ᵒ C, the Mo extraction efficiency after 90 minutes leaching was improved from 56.37% to 75%. This is obvious in Figs. (23-26). An increase in the extraction efficiency was observed with increasing the temperature. This may be attributed to the following reasons: 0 10 20 30 40 50 60 0 5 10 15 % M e ta ls E x tr a ct io n [H2O2], vol.% Mo Co Al 0 20 40 60 0 20 40 % M o E x tr a ct io n Pulp density, w/v% Mo 0 0.1 0.2 0.3 0.4 0.5 0 20 40 % C o E x tr a ct io n Pulp density, w/v% Co 0 0.1 0.2 0.3 0.4 0 20 40 % A l E x tr a ct io n Pulp density, w/v% Al 0 10 20 30 40 50 60 0 20 40 % M e ta ls E x tr a ct io n Pulp density, w/v% Mo Co Al Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 30 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net  Increasing leaching temperature will increase the rate of reaction between the solvent and value mineral. It was found that the relation between the rate of reaction and the temperature at which the reaction takes place can be represented by the Arrhenius equation [27]. ( ) ... (3) Where: K is the reaction rate constant.  Increasing the temperature will increase the diffusion coefficient, since it also follows an Arrhenius relationship [28]. ( ) … (4) Where : D is the diffusion coefficient. Similar results were observed by Angelidis et al (1995) [6] in selective disolution of critical metals from diesel and naphtha hydrodesulfurization catalyst. Fig. 23, Effect of temperature on Mo extraction Fig. 24, Effect of temperature on Co extraction Fig. 25, Effect of temperature on Al extraction Fig. 26, Effect of temperature on metals extraction Effect of Particle Size It can be seen from Figs. (27-29) that the particle size has a significant effect on dissolution of metals. It was found that the highest extraction efficiency was achieved at a catalyst particle size of less than 75μm. The Mo extraction efficiency for (<75) and (>75) μm particle size reaches 75% and 56.46%, for Co efficiency reaches 1.34% and 1.05% and for Al reaches 0.98% and 0.77% after 90 minutes leaching, respectively. By decreasing the particle size, the contact area between the catalyst particle and the fluid is increased, therefore the reaction rate is increased which is also concluded by Souza et al (2007) [29] and Qi-ming et al (2009) [24]. Since the acid leaching involved leaching of residue from alkali leaching, particle size was the same. 0 20 40 60 80 0 50 100 % M o E x tr a ti o n Temperature, ᵒC Mo 0 0.4 0.8 1.2 1.6 0 50 100 % C o E x tr a ct io n Temperature, ᵒC Co 0 0.5 1 1.5 0 50 100 % A l E x tr a ct io n Temperature, ᵒC Al 0 20 40 60 80 0 50 100 % M e ta ls E x tr a ct io n Temperature, ᵒC Mo Co Al Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi Available online at: www.iasj.net IJCPE Vol.12 No.4 (December 2011) 31 Fig. 27, Effect of particle size on Mo extraction Fig. 28, Effect of particle size on Co extraction Fig. 29, Effect of particle size on Al extraction Acid Leaching Stage Effect of Agitation Time Fig. (30) shows increasing of extraction efficiency with increasing the agitation time which was also observed by Anglidis et al (1995) [6], Mulak et al (2006) [1], David (2007) [23] and Qi-ming et al (2009) [24]. Experiments showed that 90 minutes contact time is sufficient enough to reach equilibrium between solid phase and aqueous phase. Only Co and Al seem to dissolve considerably, and small quantities of remaining Mo were detected in leachate. The quantity of dissolved Al was considerably higher than the one achieved during alkali leaching experiments, since alumina is readily soluble in acids as mentioned by Anglidis et al (1995) [6]. Fig. 30, Effect of agitation time on metals extraction Effect of Agitation Speed The results presented in Fig. (31), show that the leaching of cobalt and aluminum is independent of the stirring speed. In the study on a spent HDS catalyst leaching with sulfuric acid, Mulak et al (2005) [30] also observed similar behavior of stirring speed on the leaching kinetics. This indicates that the external diffusion of H2SO4 and CoSO4 between the fluid and the surface of particles is fast due to small particle size, and hence does not control the dissolution of metals within the investigated range of stirring speed. All subsequent experiments were carried out at a stirring speed of 300 rpm in order to assure the invariance of this parameter. Fig. 31, Effect of agitation speed on metals extraction 0 50 100 0 20 40 60 80 Time , min. % M o E x tr a ct io n Mo extraction at P.S.=-75 Mo extraction at P.S.=+75 Mo μm μm 0 0.5 1 1.5 0 50 100 % C o E x tr a ct io n Time , min. Co extraction at P.S.=+75μm Co extraction at P.S.=- 75μm Co 0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 % A l E x tr a ct io n Time, min. Al extraction at P.S.=- 75μm Al extraction at P.S. =+75μm Al 0 5 10 15 20 25 30 35 40 0 50 100 150 % M e ta l e x tr a ct io n Time , min. Co Al 0 5 10 15 20 25 30 35 40 0 200 400 600 % M e ta l e x tr a ct io n Agitation speed, rpm Co Al Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 32 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net Effect of Extractant Concentration As shown in Fig. (32), the effect of sulfuric acid concentration on cobalt and aluminum extraction efficiency increases with increasing the sulfuric acid concentration. For 10 wt.% concentration, there is virtually an acid deficiency effect. Nevertheless, there is no beneficial effect of increasing acid concentration more than 20%. However, after elapsing a certain time, namely, 90 minutes, the metals extraction efficiency practically remains constant. The increase in the extraction of Co and Al may be due to the following reasons:  The high penetrative of acid through the solid particle.  The high reaction rate between the acid and metals. These results are in agreement with Angelidies et al (1995) [6] and Qi- ming et al (2009) [24]. The cobalt extraction efficiencies after 90 minutes extraction with 10%, 20% and 30% sulfuric acid are found to be 35.93%, 40.43% and 40.43%, respectively and for aluminum extraction efficiencies are found to be 6.53%, 8.84% and 8.86%, respectively. The influence of H2O2 concentration on the extraction of cobalt and aluminum from the spent CoMo/γ- Al2O3 catalyst was determined by varying the initial concentration of H2O2 from 0.0 to 0.15M at 30 ᵒ C in 20wt.% H2SO4 solution. Fig. (33) shows summarized metals extraction after 90 minutes extraction. Generally, the extraction of Co and Al gradually increased up to 0.1M H2O2 and then remained constant. This behavior may be explained by the fact that as the acid concentration in the medium increases, the apparent rate of solid product (CoSO4) increases, and the internal diffusion resistance of H2SO4 through the product layer is enhanced. Consequently, the dissolution process slows down. The extraction of Al was practically not affected by the concentration of H2O2 within the whole range. The highest extraction of cobalt (48%) and aluminum (8.8%) was observed in 20wt.% H2SO4 solution with 0.1M H2O2 at 30 ᵒ C during the 90 minutes leaching. Hydrogen peroxide was added to H2SO4 solution to help metals dissolution from spent catalyst. These results were in agreement with results of Mulak et al (2006) [1] in preliminary results of Ni, Mo, Al and V leaching from a spent NiMo/γ-Al2O3 catalyst and David (2007) [23] on extraction of valuable metals based on Cu, Ni /γ-Al2O3. Fig. 32, Effect of sulfuric acid concentration on metals extraction Fig. 33, Effect of hydrogen peroxide concentration on metals extraction Effect of Pulp Density Fig. (34) indicates that no significant variations were observed on the dissolution of the valuable metals with 0 5 10 15 20 25 30 35 40 45 0 20 40 % M e ta l e x tr a ct io n [H2SO4], wt.% Co Al 0 5 10 15 20 25 30 35 40 45 50 0 0.05 0.1 0.15 % M e ta l e x tr a ct io n [H2O2], M Co Al Wadood T. Mohammed, Nada S. Ahmedzeki, Mariam F. AbdulNabi Available online at: www.iasj.net IJCPE Vol.12 No.4 (December 2011) 33 the increase in the solid to liquid ratio. Thus 20%PD was the best value for metals extraction. This trend might indicate that equilibrium leaching conditions were reached even by using the lower solid to liquid ratio. These results are in agreement with David (2007) [23]. Fig. 34, Effect of pulp density on metals extraction Effect of Temperature The temperature increas from 30 to 90 ᵒ C caused the cobalt extraction efficiency after 90 minutes leaching to be improved from 48% to 93.2% and for aluminum from 8.8% to 54.3% according to Arrhenius Eq. (3) and Eq. (4) as shown in Fig. (35). These results are in agreement with Angelidis et al (1995) [6] and Qi-ming et al (2009) [24]. Fig. 35, Effect of temperature on metals extraction Conclusions For the present investigation and operating conditions used, it may be concluded that:  For alkali leaching: 1. Alkali leaching for Mo extraction by using Na2CO3 and H2O2, the equilibrium was reached after 90 minutes of contact time. 2. Agitation speed had small effect on the extraction of Mo and 300rpm gave the best result. 3. The rate of Mo extraction increased with increasing in Na2CO3 and H2O2 concentrations to 60 g/l and 10vol.%, respectively and, thereafter, decreased gradually. 4. A maximum efficiency of Mo was achieved around 20% PD and, thereafter, decreased gradually. 5. The results indicate that increasing the temperature from 30 to 90 ° C lead to an increasing in the Mo extraction from 69.12% to 75%. 6. The rate of Mo extraction increased with decreasing the particle size. The particle size which is less than 75µm gave the best extraction than the particle size which greater than 75µm. 7. The results showed that the process controlled by both mass and reaction mechanisum.  For acid leaching: 1. Acid leaching for extraction of Co and Al by using H2SO4 and H2O2, the equilibrium was reached after 90 minutes of contact time. 2. Agitation speed had small effect on Co and Al extraction and 300rpm gave the best result. 3. The rate of Co and Al extraction increased with increasing H2SO4 and H2O2 concentrations up to 40.43% and 48% for cobalt and up to 8.84% and 8.8% for aluminum and remained constant thereafter. 4. It was found that no significant variations on the dissolution of Co and Al with the increase in pulp density. 0 10 20 30 40 50 60 0 20 40 % M e ta l E x tr a ct io n Pulp density, (w/v)% Co Al 0 20 40 60 80 100 0 50 100 % M e ta l e x tr a ct io n Temperature, ᵒC Co Al Extraction of Valuable Metals From Spent Hydrodesulfurization Catalyst By Two Stage Leaching Method 34 IJCPE Vol.12 No.4 (December 2011) Available online at: www.iasj.net 5. The rate of Co and Al extraction increased with increasing the temperature up to 90 ° C. 6. The results showed that the process was controlled by both mass and reaction mechanisum. References 1. Mulak, W., Szymczycha, A., Lesniewicz, A., Żyrnicki, W., (2006), “ Preliminary results of metals leaching from a spent hydrodesulphurization (HDS) catalyst”, Physicochemical Problems of Mineral Processing, 40, 69-76. 2. Trim, D.L., (1989), “Deactivation, regeneration and disposal of hydroprocessing catalyst, in: Catalysts in Petroleum Refining. 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