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1/6

Dissolution kinetics of nickel from spent catalyst in nitric acid medium

A.R. Sheik *, M.K. Ghosh, K. Sanjay, T. Subbaiah, B.K. Mishra

Institute of Minerals and Materials Technology, Bhubaneswar 751013, India

1. Introduction

Supported metal catalysts play a significant role in many

chemical processing industries and nearly 75% of the all industrial

chemical processes are based on catalysis [1]. Thus spent catalysts

are a potential source of the contained critical metals [2]. Nickel

bearing catalysts are used in various industrial processes such as

hydrogenation, hydrodesulphurization, hydrorefining, steam

reforming, etc. In a fertilizer industry, nickel catalysts are used

in steam reforming method (SRM) for the generation of hydrogen

which in turn is required for ammonia production. These catalysts

have an average lifetime of 67 years after which it cannot be used

for the process anymore [3,4]. Typically these catalysts contain

about 2.520% nickel in the form of metallic nickel or nickel oxide

on an inert support like alumina/silica [5]. Spent catalyst falls

under the category of hazardous industrial waste and their

disposal is a problem. The treatment of spent catalysts has gained

importance recently owing to two reasons the metal values

present and the need for safe disposal to avoid environmental

pollution.Hydrometallurgical processing is a preferred route for metal

recovery from industrial wastes due to energy saving, environ-

mentally friendly and easy operating methods. Acid leaching of

nickel spent catalyst is a widely used method as a first step to

recover the metal value. Amongst the mineral acids, sulphuric acid

is the most sought after leaching agent [610]. Al-Mansi and Abdel

Monem [6] reported the results of sulphuric acid leaching of

Egyptian spent catalyst. More than 99% Ni extraction could be

achieved in 5 h under the conditions: 50% H2SO4 concentration,

100 8C reaction temperature, S/L ratio of 1:12 and


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sulphuric acid media. Most researchers have found that the

mechanism of leaching follows a diffusion controlled reaction

mechanism. Table 1 summarizes the observations of some of the

recently reported kinetic studies on nickel spent catalyst leaching.

It can be observed from Table 1 that in majority of the cases thedissolution rate was controlled by product layer diffusion. The

present study is focused on the evaluation of the kinetic

mechanism involved in the nitric acid leaching of nickel spent

catalyst.

2. Experimental

The spent nickel catalyst samples used in this study were

obtained from a fertilizer industry. The spent catalyst samples

were ground and sieved into four size fractions (90 + 63) mm,

(180 + 125) mm, (425 + 250) mm and (850 + 425) mm. The

geometric average particle sizes of the four size fractions are

75 mm, 150 mm, 326 mm and 601 mm, respectively. The chemical

composition of the catalyst is shown in Table 2. Fig. 1(a) shows theXRD analysis of the spent catalyst. No other phases except NiO and

Al2O3 were detected in the XRD patterns. The catalyst primarily

contains NiO in an inert substrate of alumina.

Leaching was carried out in a 250 ml capacity, double-walled

cylindrical glass reactor in which hot water was circulated from a

constant temperature water bath. The reactor lid contained port

for sampling and reflux condenser. The circulating water from hot

bath maintained the temperature constant. The reflux condenser

prevented the evaporation loss of solution. The stirring was

accomplished using a magnetic paddle by placing the reactor over

a magnetic stirrer plate. The solid:liquid (S/L) ratio was kept

constant at 1:10 and the reaction was carried out for 120 min.

Samples were taken out at regular intervals and analyzed for Ni

using Perkin-ElmerAA200Atomic Absorption Spectrophotometer.

3. Results and discussion

The dissolution of nickel oxide from the spent catalyst follows

the following reaction:

NiO 2HNO3 ! NiNO32H2O (1)

The dissolution of a-Al2O3 in nitric acid under the present

conditions is negligible. This was confirmed by leach liquor

analysis of aluminum by inductively coupled plasma-optical

emission spectrophotometer (ICP-OES). The XRD pattern of the

leached spent catalyst shown in Fig. 1(b) clearly indicates the

disappearance of nickel oxide phases after leaching.

3.1. Effect of HNO3 concentration

The effect of nitric acid concentration on the leaching of nickel

from the spent catalyst was studied by varying the initial

concentration of HNO3 from 1.5 to 5.0 M while keeping tempera-

ture, particle size and S/L ratio constant at 80 8C, 75 mm and 1:10,

respectively. The experimental results shown in Fig. 2 indicate that

extraction of nickel is strongly affected by nitric acid concentration

under the above leaching conditions. The fraction of nickeldissolved increases with concentration of HNO3. While only 26%

Ni extraction was observed with 1.5 M HNO3 extraction increased

to 97.5% at 5.0 M HNO3.

3.2. Effect of temperature

The effect of leaching temperature variation on Ni extraction is

shown in Fig. 3 under the standard experimental conditions of

Table 1

Literature reported kinetic studies on nickel spent catalyst leaching.

Leaching agent Conditions Rate controlling step Activation energy Ref.

H2SO4 4.05.0mm, 15M H2SO4, 3070 8C Product layer diffusion 16.6kJ/mol Mulak et al. [7]

H2SO4 1040% H2SO4, 4080 8C, (200+100)mm to (74+43)mm Product layer diffusion 15.8kJ/mol Feng et al. [8]

H2SO4 550% H2SO4, 3585 8C, (177+88)mm to (74+53)mm Surface reaction 41.1kJ/mol Abdel-Aal and Rashad [9]

H2SO4 152mm, 610% (v/v) H2SO4, 5090 8C Product layer diffusion 62.8kJ/mol Sahu et al. [10]

(NH4)2SO4 2595 8C, 1.33.3M (NH4)2SO4, (300+216) mm to (106+75)mm Product layer diffusion 16.2kJ/mol Yoo et al. [12]

Table 2

Chemical composition of the spent catalyst.

Constituent Percentage

Ni 13.2

Al 43.15

Co 0.37

Fe 0.15

Mg 1.1

Fig.

1.

XRD

patterns

of

(a)

spent

catalyst

and

(b)

leached

spent

catalyst.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 30 60 90 120 150

5.0M

4.0M

2.5M

1.5M

Time (min)

Conversion(

x)

Fig.

2.

Effect

of

[HNO3]

on

Ni

extraction.

Conditions:

80 8

C,75

mm

size,

S/L

ratio

1:10.

A.R. Sheik et al./Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 3439 35


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75 mm particle size, 5.0 M [HNO3] and S/L ratio 1:10. It is evident

from Fig. 3 that temperature has a significant effect on the Ni

extraction. The dissolution rates at lower temperatures were

significantly low and increase with increase in leaching tempera-

ture. At 60 8C only 17% Ni extraction was obtained in 120 min

which increased to 98.6% by increasing the temperature to 90 8C.

3.3. Effect of particle size

In order to investigate the effect of catalyst particle size on

nickel extraction, four different particle sizes 75, 150, 326 and

601 mm were employed while keeping the HNO3 concentration,

leaching

temperature

and

S/L

ratio

constant

at

5

M,

80 8C

and

1:10,respectively. The results obtained are shown in Fig. 4. These results

indicate that finer the particle size higher is the dissolution rate. In

the higher particle sizes, i.e. 150, 326 and 601 mm, conversion did

not change significantly with the particle size.

3.4. Kinetic analysis

Leaching is a fluidsolid heterogeneous reaction and can be

represented as:

Afluid bBsolid!products (2)

For the above reaction system the following steps are considered to

occur

in

succession

during

the

reaction:

1. Diffusion of fluid reactants from bulk liquid to fluid film

2. Diffusion of reactants across the fluid film to the particle surface

3. Diffusion of reactants across the product layer to the unreacted

core

4. Reaction on the unreacted core surface between fluid reactant

and solid

Each of the above steps offers a resistance to the overall

reaction. The step with the largest resistance, i.e. the slowest step

becomes the rate controlling step. Steps 1 and 2 are dependent on

the hydrodynamics or mixing effects inside the reactor. If any of

these steps is slow then it can be increased by increasing the speed

of rotation of the impeller or by improving the reactor design to

promote better mixing. It can be assumed that the bulk diffusion is

not rate controllingwhen the mixing is high enough tomaintain all

the particles in suspension. In case of a system which has an inert

substrate or an insoluble, adherent reaction product, it forms a

product layer around the reacting core. Diffusion across the

product layer is mainly dependent on the thickness and porosity of

the layer. For a particle reacting under shrinking core mode the

integrated rate equations for three different rate control mecha-

nisms can be written as follows [19,20].

1. Film diffusion:

x kft (3)

kf 3bkCCArsr0

(4)

2. Product layer diffusion:

1 2

3x 1 x2=3 kdt (5)

kd 2bDeCArSr

20

(6)

3. Surface reaction:

1 1x1=3 krt (7)

krbkSC

nA

rSr0(8)

where b is the stoichiometric coefficient in Eq. (2), CA is the

concentration of fluid reactant (mol/m3), De is the effective

diffusivity (m2/s), kc is the liquidsolid mass transfer coefficient

(m/s), kf is the apparent rate constant for film diffusion (s1), kd is

the apparent rate constant forproduct layerdiffusion (s1), kris the

apparent rate constant for surface chemical reaction (s1), ks is the

intrinsic reaction rate constant, n is the reaction order, r0 is the

initial particle radius (m), t is the reaction time (s),x is the fraction

of

conversion,

rs is

the

molar

density

of

solid

(mol/m3

).To determine the rate control mechanism and kinetic param-

eters the experimental conversion data were analyzed on the basis

of shrinking coremodel. From the shape of time vs. conversiondata

observed in Figs. 24 it appears that film diffusion is not a rate

controlling step in the present investigation. The experimental

time vs. conversion data were tested with the model equations (5)

and (7). The different apparent rate constants (kr and kd) and

correlation coefficient values obtained by fitting the shrinking core

model equations under different experimental conditions are

summarized in Table 3.

For 75 mm particle size, best fit plots were obtained with the

shrinking core model for surface reaction controlled mechanism at

all concentrations and temperatures up to about 90% conversion

(Fig.

5).

From

the

plot

of

ln

krvs

ln[HNO3]

the

order

of

the

reaction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 30 60 90 120 150

60 C

70 C

80 C

90 C

C

onversion(

x)

Time (min)

Fig. 3. Effect of temperature on Ni extraction. Conditions: [HNO3] 5 M,75 mm size,

S/L ratio 1:10.

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100 120 140

75 um

150 um

326 um

601 um

Time (min)

Conversion(x)

Fig. 4. Effect of particle size onNi extraction. Conditions: [HNO3] 5 M,80 8C, S/L ratio

1:10.

A.R. Sheik et al./Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 343936


4/6

was found to be 1.92 (Fig. 6) which is close to the stoichiometric

coefficient value of fluid reactant in Eq. (1). This also supports that

the mechanism is surface reaction controlled as for diffusion

controlled mechanism the order of reaction with respect to fluidreactant is 1. Apparent reaction rate constants calculated from the

slopes of Eq. (7) plots at different temperatures (Fig. 5b) were used

in the Arrhenius plot, i.e. ln kr vs. 1/T plot (Fig. 7). The estimated

activation energy from the slope was 83.44 kJ/mol. The obtained

high activation energy value further supports that the rate control

step is surface reaction because diffusion controlled reaction has

activation energy in the range of 13 kcal/mol, i.e. 4.2012.50 kJ/

mol [20].

However, beyond 90% conversion the data were well repre-

sented by product layer diffusion controlled mechanism. With the

progress of reaction the thickness of the product layer increases

which in turn increases the resistance to diffusion. This causes the

diffusion rate to become slower than the surface reaction rate and

hence the reaction mechanism shifts [19].

Experimental conversion data for different particle sizes werefitted to reaction control and product layer diffusion control model

equations (Fig. 8). It is evident that kinetics has a complex

relationship with the particle size. Unlike 75 mm particles, the

larger particlesdo not obey surface reaction controlledmechanism

even at lower conversions. They obey diffusion controlled

mechanism from the beginning of the reaction.

Deviation from the reaction controlled mechanism for more

than 90% conversion and particle size greater than 75 mm can be

explained through simple theoretical analysis [19]. The rate of

reaction of a solid reactant at any time t is given by the following

equations:

Table 3

Rate constants values under different kinetic model equations.

Conditions Reaction controlled Product layer diffusion controlled

kr (103min1) R2 kd (10

3min1) R2

1.5M 0.88 0.992 0.06 0.839

2.5M 3.81 0.998 1.05 0.897

4M 6.24 0.997 1.85 0.966

5M 8.70 0.998 2.81 0.932

60 8C 1.04 0.962 0.10 0.923

70 8C 3.06 0.983 0.69 0.95980 8C 8.71 0.998 2.47 0.963

90 8C 11.01 0.995 2.81 0.932

75mm 8.70 0.998 2.0 0.963

150mm 2.67 0.832 0.53 0.999

326mm 2.33 0.816 0.45 0.993

601mm 2.21 0.908 0.42 0.991

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 30 60 90 120 150

5.0M

4.0M

2.5M

1.5M

Time (min)

1-

(1-x

)1/3

A

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150

1-

(1-

x)1/3

Time (min)

60 C

70 C

80 C

90 C

B

Fig. 5. Plots of 1 (1 x)1/3 vs time at various (A) HNO3 concentrations and (B) temperatures.

R = 0.94054

-8

-7

-6

-5

-4

-3

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8ln [HNO3]

ln

kr

Fig. 6. Plot of ln kr vs ln [HNO3].

R = 0.95045

-8

-7

-6

-5

-4

-3

2.7 2.75 2.8 2.85 2.9 2.95 3 3.051/T x103 (K-1)

ln

kr

Fig. 7. Arrhenius plot.



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Surface reaction controlled:

dN

dt 4pksC

nAr

2c (9)

Diffusion controlled:

dN

dt 4pr2De

dC

dr(10)

where rc is the radius of shrinking core, r is the radius of product

layer at any instant and N is the number of moles of solid reactant.

Integrating the right side of Eq. (10) across the product layer

from r0 to rc and CA to 0, we obtain

dN

dt4pDeCArCro

r0 rc(11)

The effective diffusivity (De) and the intrinsic reaction rate

constant

(ks)

were

evaluated

using

Eqs.

(6)

and

(8), respectively,from the apparent rate constants (kd and kr) obtained from the

respective shrinking core model plots at different concentrations

and particle sizes (Figs. 5A and 8B). A plot of calculated rate of

reaction vs. radius of shrinking core for the particle radius of

37.5 mm (i.e. 75 mm average particle size) shows that the rate of

diffusion decreases with thickness of product layer and at one

point becomes lesser than the reaction rate (Fig. 9). This explains

the observed shift in rate controlling step from reaction control to

diffusion control at higher conversions. In order to explain the shift

to product layer diffusion with higher particle sizes, a plot of (dN/

dt) vs. r0 for rc= 0.7r0 (i.e. when the radius of the shrinking core is

70% of the initial particle radius) is drawn (Fig. 10). It can be seen

from Fig. 10 that the rate of diffusion is much lesser than the

surface

reaction

rate

at

for

high

particle

sizes

but

as

the

particlesize is reduced the rate of reaction reduces appreciably and

becomes comparable to diffusion rate. At a certain particle size the

reaction rate becomes lesser than the diffusion rate and hence the

observed shift in reaction with particle size.

4. Conclusions

Dissolution kinetics of nickel from the spent catalyst was

studied. Dissolution rate was strongly influenced by the leaching

temperature and nitric acid concentration. In the lower range of

particle size extraction was comparatively high but for sizes

>150 mm dissolution rate was not significantly influence by

particle size. The reaction rate is controlled by surface reaction in

the studied range of temperature and acid concentration as well as

for particle sizes 75 mm. In the higher particle size product layer

diffusion controls the overall kinetics. The rate controlling step

shifts from surface reaction to product layer diffusion for

conversion >90%. High activation energy of 83.44 kJ/mol coupled

with the empirical reaction order of 2 with respect to HNO3concentration supports the surface reaction rate controlling

mechanism.

References

[1] Oza R, Shah N, Patel S. Recovery of nickel from spent catalysts using ultra-sonication-assisted leaching. J Chem Technol Biotechnol 2011;86(10):1276.

[2] Siemens RE, Jong BW, Russel JH. Potential of spent catalysts as a source ofcritical metals. Conserv Recycl 1986;9(2):189.

[3] Trimm DL. The regeneration or disposal of deactivated heterogeneous cata-

lysts.

Appl

Catal

A

2001;212(12):153.

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100 120 140

75 m

150 m

326 m

601 m

1-2/3X-(1-X)2/3

B

Time (min)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100 120 140

75 m

150 m

326 m

601 m

Time (min)

A

1-(1-X)1/3

Fig. 8. Diffusion controlled (A) and reaction controlled (B) plots at various particle sizes.

0

0 5 10 15 20 25 30 35 40

Diffusion rate

Reacon rate

-dN/dt(mo

l/m3s)

Radius of shrinking core (m)

Fig. 9. Plot of dN/dt vs. radius of shrinking core (rc) for37.5 mmparticle radius (r0).

Conditions: [HNO3] 5 M, 80 8C, S/L ratio 1:10.

0

0 50 100 150 200 250

Diffusion

Reacon

Radius of parcle (m)

-dN/dt(mol/m3s)

Fig. 10. Plot of dN/dt vs. particle radius (r0) for shrinking core radius (rc) = 0.7 r0.

Conditions:

[HNO3]

5

M,

80 8

C,

S/L

ratio

1:10.

A.R. Sheik et al./Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 343938


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