Abdel-aal 2004 - Kinetic Study On The Leaching Of Spent Nickel Oxide Catalyst With Sulfuric Acid

Hydrometallurgy 74 (2004) 189 – 194 www.elsevier.com/locate/hydromet

Kinetic study on the leaching of spent nickel oxide catalyst with sulfuric acid E.A. Abdel-Aal *, M.M. Rashad Central Metallurgical Research and Development Institute, P.O. Box 87 Helwan, Cairo, Egypt Received 14 July 2003; received in revised form 5 January 2004; accepted 23 March 2004

Abstract The results of a leaching kinetics study of spent nickel oxide catalyst with sulfuric acid are presented. The effects of spent catalyst particle size, sulfuric acid concentration, and reaction temperature on Ni extraction rate were determined. The results obtained show that extraction of about 94% is achieved using
200 + 270 mesh spent catalyst particle size at a reaction temperature of 85 jC for 150 min reaction time with 50% sulfuric acid concentration. The solid/liquid ratio was maintained constant at 1:20 g/ml. The leaching kinetics indicate that chemical reaction at the surface of the particles is the rate-controlling process during the reaction. The activation energy was determined as about 9.8 kcal/mol, which is characteristic for a surfacecontrolled process. D 2004 Elsevier B.V. All rights reserved. Keywords: Spent catalyst; Nickel oxide; Nickel recovery; Leaching; Sulfuric acid

1. Introduction The increasing demand for metals in the world has required intensive studies for the extraction of metals from low-grade ores and/or secondary resources. Extraction of nickel can be performed from secondary resources like spent catalysts, fly ash, and boiler ash. There are many papers reporting on the extraction of nickel from spent catalyst using different reagents. However, the kinetics of leaching have not been sufficiently studied. The applied conditions for kinetics studies are different from those used for industrial * Corresponding author. Particle Engineering Research Center, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400, USA. Fax: +1-352-846-1196. E-mail address: [email protected] (E.A. Abdel-Aal). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.03.005

extraction of Ni. In industry, leaching of Ni is carried out at high solids content, high nickel sulfate product concentration and stoichiometric amounts of sulfuric acid using coarse spent catalyst. Ivascanu and Roman (1975) studied extraction of nickel from a spent nickel-catalyst-based-alumina (NiO/Al2O3) in an ammonia plant by leaching with sulfuric acid solution. Ninety-nine percent of the nickel was recovered as nickel sulfate under the following conditions: spent catalyst particle size: 0.09 mm; sulfuric acid concentration: 80%; reaction time: 50 min; reaction temperature: 70 jC. Loboiko et al. (1983) studied leaching of nickel oxide with 60 –70% nitric acid concentration at 120 jC for 2 –3 h. Chandhary et al. (1993) studied leaching of the low-grade spent catalyst with hydrochloric acid. They obtained low Ni extraction efficiency (only

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about 18%). Vicol et al. (1986) studied leaching of spent catalyst with an aqueous solution of 15 –23% ammonia at 60– 90 jC and at pH 7.5 –9. Floarea et al. (1991) studied leaching of
600 Am spent catalyst using ammonium carbonate at 80 jC. Tsai and Tsai (1998) reported extraction of nickel and vanadium from oil-fired fly ash from Taiwan. They indicated that leaching of oil-fired fly ash in 0.5 N sulfuric acid led to an extraction of 65% vanadium, 60% nickel, and 42% iron. Ni extraction increased with an increase in the concentration of sulfuric acid. When leached in 2 N sodium hydroxide solution, the extraction of vanadium was 80%, and the extraction of nickel was negligible. They also reported that if oil-fired fly ash was leached in ammonia water, the extraction of nickel increased with an increase in the concentration of ammonia in water. When leached with 4 N ammonia, the extraction of nickel was 60%, the extraction of vanadium was less than that obtainable from leaching in sulfuric acid solution or in sodium hydroxide solution. They proposed a flowsheet in which fly ash was first leached in an ammoniacal solution containing ammonium sulfate to recover nickel. The leached residues were then taken to recover vanadium. Traditional methods of recovering nickel and vanadium from oil-fired fly ash were to burn and concentrate the ashes to raise the grade of nickel and vanadium then treat the concentrate together with the oil slag in a process of sodium carbonate roastleaching (Tsukagoshi, 1986). Subsequently, ammonium salts were added to the filtrate at pH 8– 9 to precipitate and recover the ammonium metavanadate. The leaching residues were used as source material for extraction of nickel (Tsukagoshi, 1986). A process for the recovery of nickel and vanadium in Japan is reported in which water is used to dissolve the soluble metal salts in fly ash (Committee of Nippon Industrial

News, 1974). After solid and liquid were separated, the double salt of nickel could be obtained from the filtrate by crystallization. Addition of oxidants to the residual liquid of the nickel would yield the precipitation of vanadium pentoxide. Parton et al. (1993) used the method of leaching in sulfuric acid and selective precipitation to recover nickel, vanadium, iron, magnesium hydroxide, and carbon material in fly ash. Amer (2002) optimized the conditions of extraction of nickel and vanadium by hydrometallurgical processing of Egyptian boiler ash using aqueous sulfuric acid under atmospheric and oxygen pressure to produce leach liquor of sulfates of both nickel and vanadium free from iron. Al-Mansi and Abdel Monem (2002) investigated the possibility of extraction of nickel from Egyptian spent catalyst. The optimum conditions for 99% nickel extraction were 50% sulfuric acid concentration, solid/liquid ratio of 1:12, less than 500 micron particle size for contact time higher than 5 h and 800 rev/min stirring rate at 100 jC reaction temperature. This study investigates the kinetics of leaching nickel from spent catalyst (NiO/Al2O3) with sulfuric acid. The process conditions studied include particle size of spent catalyst, sulfuric acid concentration, temperature, and time.

2. Experimental 2.1. Materials and apparatus Spent nickel oxide catalyst used in this study was kindly provided by El-Nasr Fertilizers and Chemicals Company in Talkha (Egypt). The spent catalyst was ground and sieved. The elemental compositions of the different size fractions are given in Table 1. Commercial sulfuric acid from Abu-Zabaal Fertilizers and

Table 1 Chemical analysis of the studied spent catalyst fractions Particle size of ore fractiona Mesh

Am


80 + 170
170+ 200
200 + 270


177 + 88
88 + 74
74 + 53

a

ASTM standard.

Ni%

Al%

Na%

Mg%

Ca%

Co%

Cd%

Cu%

11.9 11.9 12.1

42.9 42.8 42.6

1.1 1.2 1.0

0.75 0.72 0.77

0.28 0.29 0.27

0.013 0.011 0.010

0.0061 0.0058 0.0051

0.0051 0.0048 0.0046

E.A. Abdel-Aal, M.M. Rashad / Hydrometallurgy 74 (2004) 189–194

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Chemicals Company was also used in this study. It had a concentration of 98% H2SO4 and a density of 1.84 g/mL. The reaction between spent catalyst and sulfuric acid was performed in a 500-mL roundbottom flask placed in a thermostatically controlled water bath. 2.2. Procedure Twenty grams of spent catalyst was added at one time to the agitated sulfuric acid solution (400 mL) of the required concentration at the required temperature. The reaction mixture was agitated at a rate of 500 rev/ min. At selected time intervals, 2-mL solution samples were taken using a syringe filter of 1 Am pore size and the solids corresponding to that volume were discarded. Consequently, the solid/liquid ratio was maintained constant at 1: 20 g/mL. The cumulative volume removed by sampling was not significant compared to the original solution (about 6%). The samples were chemically analyzed for determination of nickel content using atomic absorption spectroscopy (AAS). After that, the percentage extraction of nickel was calculated.

Fig. 2. Relation between Ni extraction and time using various sulfuric acid concentrations (
200 + 270 mesh particles at 75 jC).

initial sulfuric acid concentration and leaching temperature were kept constant at 30% and 75 jC, respectively. The solid/liquid ratio was 1: 20 g/mL. The results are given in Fig. 1. These results show that particle size has a significant effect on the dissolution of nickel oxide. After 3 min leaching time, 2.5 –3.2% Ni was extracted, depending on the spent catalyst particle size. In addition, the results show that about 71% of the Ni present in the fine fraction (
200 + 270 mesh) of the spent catalyst was extracted after 150 min.

3. Results and discussion

3.2. Effect of sulfuric acid concentration

3.1. Effect of particle size

A plot of Ni extraction against time is presented in Fig. 2 for fine spent catalyst of
200 + 270 mesh particle size and sulfuric acid concentration in the range of 5 – 50% at a constant temperature of 75 jC. The solid/liquid ratio was kept constant at 1: 20 g/mL. The sulfuric acid concentration also has a pronounced effect on the dissolution of NiO. About 86% of the Ni present in fine fraction of the spent catalyst was extracted using 50% sulfuric acid solution after 150 min.

The effect of particle size on leaching of spent nickel oxide catalyst was studied using different size fractions (
80 + 170 mesh,
170 + 200 mesh and
200 + 270 mesh). Within the series of tests, the

3.3. Effect of reaction temperature

Fig. 1. Relation between Ni extraction and time using different spent catalyst particle sizes (at 75 jC, 30% H2SO4).

The effect of reaction temperature on Ni extraction at different reaction times is plotted in Fig. 3 for spent catalyst of
200 + 270 mesh particle size and sulfuric acid concentration of 50% at temperatures in the range of 35 – 85 jC. The solid/liquid ratio was kept constant at 1: 20 g/mL. The obtained results show that the studied reaction temperatures have a noticeable

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Fig. 3. Relation between Ni extraction and time at various reaction temperatures (
200 + 270 mesh particles in 50% H2SO4).

effect on dissolution of NiO. About 94% of the Ni present in fine fraction of the spent catalyst is extracted after 150 min at a reaction temperature of 85 jC. 3.4. Kinetic aspects Nickel oxide present in the spent catalyst reacts with sulfuric acid according to the following reaction equation (Al-Mansi and Abdel Monem, 2002): NiO þ H2 SO4 ! NiSO4 þ H2 O

ð1Þ

The matrix (a-alumina) is not reacted with sulfuric acid (Treybal, 1980). The reaction of nickel oxide can proceed in a topochemical manner, in which the inner core of the unreacted particle decreases with time. It is clear in Fig. 1 that the rate of reaction decreases with time. This is due to the decrease of the reactant surface

Fig. 4. Plots of 1
(1
x)1/3 versus time at various reaction temperatures (
200 + 270 mesh particles in 50% H2SO4).

Fig. 5. Plots of 1
2/3x
(1
x)2/3 versus time at various reaction temperatures (
200 + 270 mesh particles in 50% H2SO4).

area. The rate of reaction is given for models based on control by (a) chemical reaction at the particle surface; (b) diffusion through the product layer; and (c) a combination of both. (a) Rate control by chemical reaction at the particle surface (Habashi, 1969 and Levenspiel, 1972): Kc t ¼ 1
ð1
xÞ1=3

ð2Þ

where Kc = reaction rate constant (min
1); t = time in minutes; x = fraction reacted of NiO. x ¼ %Ni extraction=100

ð3Þ

Based on the experimental data in Fig. 3, a plot of the right-hand side of Eq. (2) versus time is given in Fig. 4. During the whole reaction time, the data in this figure are linear, which indicates that the rate of reaction is controlled by chemical reaction at the surface of nickel oxide particles. The results correlated

Fig. 6. Arrhenius plot for the leaching of spent nickel oxide catalyst.

E.A. Abdel-Aal, M.M. Rashad / Hydrometallurgy 74 (2004) 189–194

by this model yield straight lines and nearly zero point intercepts were obtained. In the case of straight lines, the slope equals the rate constant Kc: Kc ¼ MbKcc C=qr min
1

ð4Þ

where Kcc = chemical rate constant (cm min
1); b = stoichiometric coefficient (dimensionless); M = molecular weight of NiO (the major dissolved compound in spent catalyst); C = concentration of sulfuric acid (mol/m 3 ); r = radius of unreacted particle (m); q = density of spent catalyst (kg/m3), (pycnometer method = 4380 kg/m3). (b) Rate control by diffusion through the product layer (Habashi, 1969; Levenspiel, 1972). When diffusion through the product layer is ratecontrolling, the kinetics may be correlated graphically using the Valensi equation: Kp t ¼ 1
2=3x
ð1
xÞ2=3

ð5Þ

where Kp =rate constant (cm min
1). Again, based on the experimental data in Fig. 3, a plot of the right-hand side of Eq. (5) against time is given in Fig. 5. The data cannot be correlated by this model as neither straight lines nor zero point intercepts were obtained. (c) Calculation of the activation energy. The activation energy of a diffusion-controlled process is characterized as being 1 to 3 kcal/mol (Habashi, 1969), 2 to 5 kcal/mol (Anand and Das, 1988) or 3 to 6 kcal/mol (Romankiw and De Bruyn, 1963). In addition, the activation energy for a chemically controlled process is usually greater than 10 kcal/mol (Habashi, 1969) or more specifically falls between 10 to 20 kcal/mol (Anand and Das, 1988). To calculate the activation energy, the values of ln Kc were plotted against 1/T in Fig. 6. The activation energy of the overall reaction is calculated as about 9.8 kcal/mol (41.1 kJ/mol). This activation energy is near the values of activation energy of 45.9 kJ/mol calculated for sodium hydroxide leaching of a gibbsitic bauxite (Abdel-Aal, submmitted for publication), 48.15 kJ/mol calculated for oxidative ammonia leaching of sphalerite (Ghosh et al., in press), and 34 kJ/mol calculated for oxidative sodium hydroxide leaching of mechanically activated low-grade wolframite concentrate (Amer, 2000).

193

4. Conclusions Leaching of spent nickel oxide catalyst with sulfuric acid was studied. Extraction efficiency of about 94% of the NiO present in the spent catalyst was achieved under the following conditions: particle size
200 + 270 mesh, temperature 85 jC, reaction time 150 min, sulfuric acid concentration 50% and solid/ liquid ratio 1:20 g/ml. The kinetic study indicates that the leaching of NiO is a surface chemically controlled process. The activation energy was calculated as about 9.8 kcal/mol (41.1 kJ/mol) which is consistent with values of activation energies reported for surfacecontrolled reactions.

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