Mulak 2005 - Kinetics Of Nickel Leaching From Spent Catalyst In Sulphuric Acid Solution

Int. J. Miner. Process. 77 (2005) 231 – 235 www.elsevier.com/locate/ijminpro

Kinetics of nickel leaching from spent catalyst in sulphuric acid solution Wyadysyawa Mulak *, Beata Miazga, Anna Szymczycha Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wrocy aw University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-350 Wrocy aw, Poland Received 20 December 2004; received in revised form 22 June 2005; accepted 29 June 2005 Available online 1 August 2005

Abstract The kinetics of spent nickel oxide catalyst (NiO/Al2O3) leaching in sulphuric acid solutions was investigated. The effects of sulphuric acid concentration, temperature, stirring speed, and particle size on the rate of nickel leaching were studied. In addition, the reaction residues at various levels of nickel extraction were examined by SEM, X-ray diffraction, electron microprobe, and chemical analysis. The results of the kinetic analysis of the leaching data for various experimental conditions indicated that the reaction is controlled by diffusion through the catalyst network with the activation energy of 16.6 F 0.9 kJ/ mol. A linear relationship between the rate constant and the inverse square of the initial particle diameter is also characteristic for a diffusion-controlled process. D 2005 Elsevier B.V. All rights reserved. Keywords: Spent catalyst; Nickel; Leaching; Sulphuric acid

1. Introduction Recently, relatively great attention has been paid to the research connected with the recovery of nickel from secondary resources. Recycling of spent catalysts became an unavoidable task not only for lowering the catalyst cost but also for reducing the catalyst waste to prevent the environmental pollution. Nickel is widely used as a catalyst in several technological processes: in hydrogenation, hydrodesulphurisation, hydrorefining including fat * Corresponding author. Tel./fax: +48 71 32843 30. E-mail address: [email protected] (W. Mulak). 0301-7516/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2005.06.005

hardening process (Ni, Mo/Al2O3, Nio/Al2O3, Raney nickel alloy); in refinery hydrocracking (NiS, WS3/ SiO2Al2O3); in methanation of carbon oxide from hydrogen and ammonia synthesis gas (NiO/Al2O3, Ni SiO2) (Thomas, 1970). Typically, nickel spent catalysts contain metallic nickel and nickel oxide, although nickel sulphides may occasionally occur as well as admixtures of coke, hydrocarbons or fat. Ivascan and Roman (1975) published their studies on the recovery of nickel from a spent catalyst ammonia plant by leaching in sulphuric acid. The nickel was recovered as NiSO4 with 99% yield. Al-Mansi and Abdel Monem (2002) studied the sulphuric acid

W. Mulak et al. / Int. J. Miner. Process. 77 (2005) 231–235

Table 1 Chemical analysis of spent catalyst Element

Ni

Al

Ca

Mg

C

H

Weight percentage %; (the oxygen content is not included)

13.5

40.2

0.33

0.05

1.08

0.31

leaching process for the recovery of nickel as a sulphate from a spent catalyst in the steam reforming industry. It was also shown that high recovery of 99% nickel as nickel sulphate was achieved. Chaudhary et al. (1993) reported hydrochloric acid leaching process for the recovery of nickel as nickel oxide from a spent catalyst containing 17.7% Ni. They found that maximum of nickel extraction 73% could be achieved by carrying out the leaching process with 28.8% HCl at 80 8C. In an attempt to improve the nickel extraction the application of chlorine gas was investigated but no appreciable improvement was observed. Several other methods have been also reported for the leaching of nickel from a spent refinery catalysts (Furimsky, 1996). In this paper the effects of sulphuric acid concentration, temperature, stirring speed, and particle size on the leaching rate of nickel from a spent catalyst have been examined.

2. Experimental 2.1. Materials Spent nickel oxide catalyst used in this study was obtained from Fertilizers Research Institute in Pulawy, Poland. The spent catalyst was in the form of granules with a diameter of 3.0–8.0 mm. The chemical analysis of the spent catalyst is shown in Table 1. The existence of nickel oxide in the spent catalyst was confirmed by X-ray diffraction and a comparative infrared spectra methods. 2.2. Equipment and procedures All the experiments were performed with grains of the size 4.0–5.0 mm, except those relating to the effect of particles size on the reaction kinetics. In each experiment a flask containing 200 ml of sulphuric acid solution was submerged in a tank, the tempera-

ture of which was kept constant to within 0.1 8C. When the required temperature was reached, a 0.25 g of the spent catalyst was added and the stirring was started. A mechanical glass agitator of L shape with 25 mm impeller was applied. Its tip speed converted from 600 rpm equals 0.785 m/s. The leaching was carried out for 60 min during which five 1 ml samples of the solution were taken for determination of the nickel concentration by atomic absorption method. The aluminium concentration in the final solution was determined complexometrically.

3. Results and discussion 3.1. Effect of stirring speed The effect of stirring speed on the nickel extraction from a spent catalyst was investigated in a solution of 2.0M H2SO4 at 50 8C in the range of 300 to 1200 min
1. The results presented in Fig. 1 show that the leaching rate of nickel is independent of the stirring speed. This indicates that the diffusion of the reactants from the solution towards the surface of the particle, and the products away from the surface of the particle were fast, and hence did not control the leaching rate within the range of stirring speeds tested. All subsequent experiments were carried out at a stirring speed of 600 min
1 to assure invariance of this parameter. 0.8

Fraction of nickel extracted (α)

232

0.7 0.6 0.5 0.4

300 rpm 600 rpm 900 rpm 1200 rpm

0.3 0.2 0.1 0.0 0

10

20

30

40

50

60

Time, min Fig. 1. Effect of stirring speed in 2.0M. H2SO4 at 50 8C.

W. Mulak et al. / Int. J. Miner. Process. 77 (2005) 231–235

3.2. Effect of sulphuric acid concentration

233

The influence of H2SO4 concentration on the leaching of nickel and aluminium from a spent catalyst was determined by varying the initial concentration of H2SO4 from 1.0 to 5.0 M. The experimental results in Fig. 2 show that extraction of nickel and aluminium from a spent catalyst was not affected by H2SO4 concentration for the above leaching conditions. To exclude the lixivant composition changes a high excess of the acid (H2SO4/Ni molar ratio c 340 was applied).

Fraction of nickel extracted (α)

0.8 0.7 0.6 0.5 30oC

0.4

40oC 0.3

50oC

0.2

60oC 70oC

0.1 0.0 0

10

20

3.3. Effect of temperature The leaching was carried out in the temperature range 30–70 8C with 2.0 M sulphuric acid solution at constant stirring speed of 600 min
1. The leaching results for the temperature effects on nickel extraction from a spent catalyst are presented in Fig. 3. These results show that temperature only mildly affected the nickel leaching. The experimental data have been analysed in terms of several models connecting the fraction reacted (a) with time (t) (Wadsworth, 1979). The diffusion model (Ginstling and Brounshtein, 1950) for the relationship a (t) has been applied: 1
2=3a
ð1
aÞ2=3 ¼ kt
1

ð1Þ

) = const / d o2

where k (min is the rate constant and d o denotes the initial particle diameter. 70

40

50

60

Fig. 3. Effect of temperature on the fraction of nickel extracted in 2.0 M. H2SO4.

The plot of the data from Fig. 3 for temperature range 30–70 8C drawn according to Eq. (1) is depicted in Fig. 4. The apparent rate constant, k, obtained from the slopes of the straight lines in Fig. 4 was used to determine the activation energy of 16.6 F 0.9 kJ/mol with the correlation coefficient R 2 = 0.995, as is shown on Arrhenius plot in Fig. 5. The magnitude of the activation energy may support the theory that the leaching of nickel from a spent catalyst is controlled by a solid state diffusion process. This activation energy is close to the values of the activation energy of 13.4 kJ/mol calculated for the diffusion controlled reactions of low-grade zinc silicate ore with sulphuric acid (Abdel-Aal, 2000) and 18.4 kJ/mol calculated for R2=0.991

0.12

Ni Al

60

30oC

50 40 30 20

R2=0.998

40oC

0.10

1- 2/3 α - (1- α )2/3

Ni, Al extraction (%)

30

Time, min

50oC

R2=0.999

60oC

0.08

70oC

R2=0.995

0.06 R2=0.978 0.04

10 0.02 0 1

2

3

4

5

Concentration H2SO4, M

0.00 0

10

20

30

40

50

60

70

80

Time , mi n Fig. 2. Effect of H2SO4 concentration on nickel and aluminium extraction at 50 8C.

Fig. 4. A plot of the data in Fig. 3 according to Eq. (1).

234

W. Mulak et al. / Int. J. Miner. Process. 77 (2005) 231–235 -6.2

Fraction of nickel extracted (α)

0.7

-6.4

ln k, min-1

Ea= 16.6 +/- 0.9 kJ/mol -6.6

-6.8

-7.0

0.6 0.5 0.4 0.3

4.0-5.0 mm 5.0-6.3 mm 6.3-8.0 mm

0.2 0.1 0.0

-7.2 2.9

3.0

3.1

3.2

0

3.3

10

20

vanadium leaching from spent sulphuric acid catalysts by H2SO4 solution (Lozano and Juan, 2001). 3.4. Characterization of reaction products Residues from several leaching experiments at various levels of nickel extraction were examined using a scanning electron microscope (SEM), X-ray diffraction (XRD), microprobe and chemical analysis. The most interesting microscopic evidence found in this study is provided in the cross-section shown in Fig. 6. Reacted particle (a = 0.69) were mounted in a low viscosity embedding media and then polished to reveal the product layer which rims the unreacted core of a particle. As seen in Fig. 6 the cross-section

40

50

60

Time, min

1000 / T, K-1 Fig. 5. Arrhenius plot for leaching of nickel in 2.0 M. H2SO4.

30

Fig. 7. Effect of particle size on the fraction of nickel extracted in 2.0 M. H2SO4 at 50 8C.

of the particle shows two phases. Electron microprobe analysis of the particle cross-section indicated by a cross in Fig. 6 shows high content of aluminium (47.6%) but very small amount of nickel (2.0%). Unreacted core of a particle (marked by a circle in Fig. 6) shows higher content of nickel (14.0%) and 39.6% of aluminium. The aluminium-rich layer surrounds the unreacted part of the particle and grows inward as particle reacts. The reacted particle retains the same geometrical shape and dimension of the particle before leaching even when nickel is almost completely extracted. The X-ray diffraction patterns of this reaction residue (4 h leaching, 60 8C, a = 0.99) confirmed that it is composed mainly of a-Al2O3 which is the catalyst network. This confirmed that the reaction rate of nickel leaching from a spent

1-2/3 α - (1-α)2/3

0.08

R2=0.999

4.0-5.0 mm 5.0-6.3 mm 6.3-8.0 mm

0.06

R2=0.996 R2=0.998

0.04

0.02

0.00 0

Fig. 6. SEM micrograph of the particle cross-section of spent catalyst after 1 h leaching in 2.0M H2SO4 at 50 8C, a = 0.69, + and 8 = two different phases subjected to the electron microprobe analysis.

10

20

30

40

50

60

70

80

Time, min Fig. 8. A plot of the data presented in Fig. 7 according to Eq. (1).

W. Mulak et al. / Int. J. Miner. Process. 77 (2005) 231–235 1.4 R2=0.999

k*10-3, min-1

1.2

1.0

0.8 0.02

0.03

0.04

0.05

235

speed, which indicates that the reaction is not controlled by the diffusion in the liquid phase. (2) The change in the concentration of sulphuric acid within the range 1.0–5.0 M has no significant effect on the nickel leaching. (3) The leaching rate at temperatures between 30– 70 8C is controlled by the diffusion through the catalyst network with an activation energy of 16.6 F 0.9 kJ/mol. (4) A linear relationship between the rate constant and the inverse square of the initial particle diameter was established.

1/do2, mm-2 Fig. 9. A plot of the rate constant versus square of the initial particle diameter of a spent catalyst.

Acknowledgements

catalyst is controlled by diffusion through this porous solid state layer.

The authors wish to thank the Polish Committee for Scientific Research for financial support (grant No 4T09B 13725).

3.5. Effect of particle size The experiments were carried out with the three particle sizes (4.0–5.0, 5.0–6.3, 6.3–8.0 mm) in 2.0 M H2SO4 solution at 50 8C. The results are shown in Fig. 7. The data from Fig. 7 were analysed according to Eq. (1), the plot of these data is demonstrated in Fig. 8. The calculated apparent rate constants are plotted vs. the inverse square of the initial particle diameter d o (Fig. 9). The linear relationship between the rate constant and the inverse square of d o indicates that the product layer diffusion is, indeed, the rate limiting step of the leaching process. The d o was estimated as the arithmetic average for the 4.0–5.0 mm grain fraction after the sieve separation.

4. Conclusions From the results of these studies the following conclusions can be drawn: (1) The leaching rate of nickel from a spent catalyst in sulphuric acid is independent of the stirring

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