artículo kiara nieves
TRANSCRIPT
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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.
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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.
A.R. Sheik et al./Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 3439 37
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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.
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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
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0
0.1
0.2
0.3
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0 20 40 60 80 100 120 140
75 m
150 m
326 m
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