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Optimization and kinetics of the cementation of lead with
aluminum powder
Fariba Farahmand a, Davood Moradkhani b,c,d,, Mohammad Sadegh
Safarzadeh c,d, Fereshteh Rashchi a
a Department of Metallurgy and Materials Engineering, University
of Tehran, P.O. Box 11155-4563, Tehran, Iranb Faculty of
Engineering, Zanjan University, Zanjan, Iranc Laboratory for
Leaching and Purication Processes, R&D Center, Iranian Zinc
Mines Development Company (IZMDC), P.O. Box 45195-1445, Zanjan,
Irand Research and Engineering Company for Non-ferrous Metals
(RECO), P.O. Box 45195-1445, Zanjan, Iran
a b s t r a c ta r t i c l e i n f o
Article history:
Received 15 February 2009Received in revised form 1 April
2009Accepted 1 April 2009Available online 7 April 2009
Keywords:
Lead recoveryAluminum
powderCementationKineticsDifusion-controlled process
The cementation of lead with aluminum powder from the
brine-leaching ltrate of lead-bearing zinc plantresidues (ZPRs) has
been investigated. The inuence of several parameters on the course
of the reaction suchas aluminum powder quantity, time, aluminum
powder particle size and temperature was examined. Theoptimum
cementation conditionswere foundto be as aluminum powder quantity=
1.5 timesof stoichiometricvalue, time= 90 min, aluminum powder
fraction with a particle size125+88 m and temperature=70
C.Cementation of lead was shownto be a feasible process to achieve
a high degreeof lead removal (~95%)withina fairlyreasonable contact
time. Also, thekinetics of thecementationwas studied over a range
of experimentalparameters. The cementation was shown to be a
diffusion-controlled reaction in the range of 50 70 C
withactivation energy of 23.6 kJ/mol. However, the process was
found to be highly temperature sensitive in therange 4050 C,
suggesting a chemical reaction control model for that range. This
nding was in concordancewith a severe slope change in Arrhenius
curve in the range 4050 C.
2009 Elsevier B.V. All rights reserved.
1. Introduction
Hydrometallurgical methods canbe used forthe production of
zincand lead from secondary materials. Zinc plant residues (ZPRs)
areconsidered as hazardous wastes and expensive secondary
materialsdue to their signicant zinc, lead and cadmium content.
Also, it hasbeen shown that heavy metal solubilization in ZPR damps
and itstransferring by natural phenomena is a very strict
environmental risk(Altundoan et al., 1998). In some plants such as
inkur, Kayseri,Turkey (Altundoan et al., 1998) and NILZ, Zanjan,
Iran (Safarzadehet al., 2005), these residues are stockpiled until
the recovery of val-uable metals in the residues become economic
and/or the grade ofzinc ores decreases.
There are several methods that can remove lead ions from
solu-tions such as chemical precipitation, ion exchange adsorption,
reverse
osmosis and electrodialysis. All of these methods have
drawbacks, e.g.chemical precipitation requires extremely long
settling times; bothion exchange and carbon adsorption are very
expensive and may re-quire frequentregenerations for adequate
performance. Reverse osmosisand electrodialysis require expensive
equipment, and have high operat-ing costs (Slapiet al., 1982).
The advantages of the cementation process include: (i) high
pro-cess efciency, permitting practically complete removal or
detoxica-
tion of heavy metals; (ii) highprocessrate; (iii) simplicity of
treatment
facilities; (iv) recovery of most metals in pure metallic form;
and (v) arelativeabsenceof sludge. Since inexpensive scrapmetals
canbe used,process operational costs can be kept low. Most of the
industrialcementation processes use metal powder within stirred
tank reactors.Their major interests are a low-energy requirement,
an easy controland a frequent removal of the metallic species under
its metal form(Khudenko, 1987).
The removal of lead ions from different kinds of solutions
bycementation has been studied by a number of researchers; and
someof themhave investigated the kinetics of cementation as well.
Schwartzand Etsell (1998) invented a process for the recovery of
lead from usedbattery pastes. The process consisted of a leaching
step by ammoniacalammonium sulfate (AAS) solution to dissolve the
lead sulfate foundin used battery pastes and then recovering of
metallic lead from AAS
solution via cementation, using Ni powder as the reductant. The
leadconcentration was set at 40 g/L for all test runs. By using
11.1 g/L Nipowder, about 86% of lead was recovered at 100 C in 180
min. It wasfound that below 135 C, reaction kinetics was
rate-controlling (activa-tion energy was approximately 50 kJ/mol)
and above 135 C, diffusionwas rate-controlling.Raghavan et al.
(1998) cemented out lead andsilveras highsilverleadspongefrom brine
leach solutionwith the helpof aluminum scrap and then melted the
sponge to obtain a high leadsilver alloy which could be processed
through the conventional pyro-rening technique. The brine solution
was obtained from brine leach-ing of zinc leach residue. For a lead
content of brine solution equal to7.79 g/L, the lead extractionwas
92.7%. Makhlou et al. (2000) studiedlead ions removal from acidic
aqueous solutions by cementation on
Hydrometallurgy 98 (2009) 8185
Corresponding author. Faculty of Engineering, Zanjan University,
Zanjan, Iran.Tel.: +98 241 4221027; fax: +98 241 4221027.
E-mail addresses:[email protected],[email protected](D.
Moradkhani).
0304-386X/$ see front matter 2009 Elsevier B.V. All rights
reserved.
doi:10.1016/j.hydromet.2009.04.001
Contents lists available at ScienceDirect
Hydrometallurgy
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l
o c a t e / h yd r o m e t
mailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.hydromet.2009.04.001http://www.sciencedirect.com/science/journal/0304386Xhttp://www.sciencedirect.com/science/journal/0304386Xhttp://dx.doi.org/10.1016/j.hydromet.2009.04.001mailto:[email protected]:[email protected]
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rotating iron disc. The lead-bearing solution was prepared
usingreagent-grade lead nitrate and its pH value (pH=2) was
controlledbyaddingH2SO4 andHCl.The cementation wasshownto be a
diffusion-controlled reaction with activation energy of 9.6 kJ/mol
(2580 C).The cementation efciency was 73% at 70 mg/L initial
concentrationof lead ions after 180 min of cementation.Shin et al.
(2000)recoveredlead as lead sponge from a 1.0 wt.% HCl solution
containing Pb+2 andCl by cementation using a pure aluminum or a
magnesium rod as
the reductant.Raghavan et al. (2000)described
hydrometallurgicalprocessing of lead-bearing materials generated at
zinc plants. In theirapproach,lead sulfate residuewas treated
withbrine solutionand thenlead was cemented-out from resultant
ltrate using aluminum scrap.With initial lead ion concentration of
4.5 g/L, a 97.1% lead extractionwas achieved. Abdollahi et al.
(2006) extracted zinc and lead fromIranian zincplant residuesusing
brine-leaching method. Theyblendedthe residue with H2SO4at 70 C and
then for lead recovery, after zincextraction, the residual solid
was subjected to brine leaching usingNaCl. Lead content of
brine-leaching ltrate was cemented by metallicaluminum. Cementation
efciency for lead content of 16.36 g/L wasabout 9798%.
The brine leaching of lead-bearing ZPR has been reported in
theprevious work (Farahmand et al., 2009). However, the scope of
thepresent study is the recovery of lead from the ltrate of brine
leach-ing of ZPR by cementation with aluminum powder. In view of
that,the effect of experimental parameters such as aluminum
powderquantity, time, temperature and particle size of aluminum
powder onthe cementation of lead was examined and the kinetics of
the processwas investigated according to shrinking core model
(SCM).
1.1. Precipitation of lead by aluminum
The overall cell reaction for the cementation of lead by
aluminumcan be described as:
3Pb2
2Al 2Al3
3Pb 1
The standard cell potential for reaction (1) is 1.534 V. Since
only in
those cases where the cell potential is less than 0.3 V is there
anypossibility of signicant back-reaction; for the Pb2+/Al (Al3+)
system,the reaction may be considered irreversible. It must be
emphasizedthat standard potential data provide only a general guide
to the valueof the cell potential existing in practice. This is
particularly the casein reactions where aluminum is the
precipitating agent. Its high ten-dency to form a surface oxide lm
in many environments confers amore noble potential on the Al3+/Al
system and this results in de-creased cell potential value
(Jackson, 1986).
2. Experimental
The stock solution was obtained from the brine leaching of
lead-bearing ZPR under the optimum conditions (Farahmand et al.,
2009).
The leach liquor was analyzed using an atomic absorption
spectro-meter (AAS) (AA300 Perkin Elmer model). The chemical
compositionof the solution used in the experiments was as follows
(g/L): Pb-2.99;Zn-0.02 and Fe-Trace.
The cementation reactions were conducted in a glass beaker of 2
Lvolume equipped with a mechanical stirrer submerged in a
thermo-static bath. The mechanical stirrer (Heidolf RZR 2020) had a
digitalcontroller unit and its agitator shaft was made of glass.
Firstly, 1 L ofsolution was put into the beaker and when the
desired temperatureof the beaker content (40, 50, 60 and 70 C) was
reached, a predeter-mined amount of aluminum powder with dened size
distributionwas added into the solution while the content of the
beaker was beingstirred at a rate of 110 rpm. This was marked as
the beginning of theexperiment. The progress of the cementation
reaction was followed
by measuring the amounts of unprecipitated lead in the solution.
The
duration of experiments was up to 90 min and 10 mL samples
werewithdrawn at 10, 20, 40, 60 and 90 min. The samples were
immedi-atelyltered using lter paper circles and then diluted and
analyzedfor lead. Lead recovery was calculated according to
difference betweenthe initial and nal lead concentrations of the
solution.
Aluminum powder (purity N 99.5%, from Khorasan Powder
Metal-lurgy Inc.) was used as the sacricing metal for Pb2+ ions
removal. Inorder to determine the effects of different particle
size distributions onthe efciency of cementation, 500 g of aluminum
powder was sieved.Four aluminum powder fractions were prepared. The
experimentalparameters and values that have been studied are
summarized inTable 1.
3. Results and discussion
3.1. Effect of aluminum powder quantity
In order to optimize the quantity of aluminum powder
needed,cementation experiments were carried out at the temperature
of50 C, stirring speed of 110 rpm and particle size of aluminum
powder149 +125 m.Fig. 1shows the cementation efciency against
time,where 1, 1.2,1.5 and 2 times of the stoichiometrically
required amountof aluminum powder was utilized. It can be seen that
the efciency ofcementation at a certain time increased with
increasing of aluminumadded to the solution. As quantity of
aluminum powder increases,there will be greater surface area of
aluminum powder per lead ionwhich results in increasing cementation
efciency. Also it was ob-served that as the time increased within
the range of 10 to 90 min, the
cementation efciency increased for all aluminum powder
quantities.According toFig. 1, the cementation efciency reached 81%
for molarratio of Al:Pb=1.5 and 91.8% for molar ratio of Al:Pb=2
after 90 min.However, molar ratio of Al:Pb=1.5 was used in further
experiments.
3.2. Effect of aluminum powder size
This effect was studied using four particle sizes of
aluminumpowder as dened inTable 1.In these series of experiments,
molarratio of Al:Pb, temperature and stirring speed were maintained
at 1.5,50 C and 110 rpm, respectively. Fig. 2shows that as the
aluminumpowder size decreased within the range from 177+149 m
to125+88 m, the cementation efciency increased at a certain timeand
was determined to be 88.9% after 90 min for 125+88 m
particle size. With decreasing aluminum powder size to88+53
m,the cementation efciency decreased from 88.9% to about 76.3%
after90 min. As the aluminum powder size decreases to 125+ 88
m,surface area forcementation reactionwill be greater and therefore
thenumber of lead nucleation sites increases; thus the cementation
ef-ciencyof lead increases. However, by more decreasing
theparticlesize(88+ 53 m); probably aluminum powder particles are
oxidizedbefore contributing in cementation reaction that causes to
decreasethe active reaction sites and therefore cementation
efciency.
3.3 Effect of temperature
The effect of temperature on the cementation of lead was
inves-tigated in the range of 4070 C. Molar ratio of Al:Pb,
particle size of
aluminum powder and stirring speed werexed at 1.5,125+88 m
Table 1
Experimental parameters and their values.
Parameters Chosen values
Aluminum/lead molar ratio 1, 1.2, 1.5, 2Time (min) 10, 20, 40,
60, 90Aluminum powder particle size
fractions (m)177+149, 149+125, 125+88, 88+53
Temperature (C) 40, 50, 60, 70
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and 110 rpm, respectively. As shown inFig. 3, the cementation
ef-ciency increased at a certain time as the reaction temperature
in-creased. This may be attributed to the increased temperature in
whichthe deposit structure reveals that the oxide lm has been
destroyedand hence, highercementation efciency has been obtained
(Dnmezet al.,1999). According to theresults, a maximum efciency of
94%wasobtained after 90 min at 70 C.
Thus, the optimum conditions for cementation of lead from
brineleach solutions were found to be as follows: molar ratio of
Al:Pb= 1.5,particle size of aluminum powder=125+88 m, temperature
of70 C andstirringspeedof 110 rpmfor 90 min. Under these
conditions,about 95% of lead was recovered.
3.4. Kinetics of cementation
The shrinking core model (SCM) was used to describe the
cemen-tation reaction (Levenspiel, 1999). Therefore, three
different modelscanbe consideredas reaction mechanismwith their
constant rate equa-
tions as is discussed in the following section.
3.4.1. Film diffusion control for spherical particles
Thefraction of lead reacted at any time, t, i n a lm diffusion
controlsituation can be calculated from the following equation
(Levenspiel,1999):
XPb= kt 2
where XPb stands for the fraction of lead reacted and k (min1)
is
reactionrate constant. Based on the experimental data shown in
Fig.3,
theleft-hand side of Eq. (2) is plotted against time in Fig.4.
Thehighestvalue of the correlation coefcientR2 for linearity of the
plot is ob-tainedfor thelowest temperature data (i.e.,40 C). At
highertempera-tures, the experimental data deviates further from a
linear form. Noneof the straight lines can be considered close
enough to t the experi-mental data to suggest a lm diffusion model
for the cementationprocess.
3.4.2. Ash diffusion control for spherical particles
Diffusion of Pb2+ through the blanket of ash at any time, t, can
becalculated from the following equation (Levenspiel, 1999):
1 3 1XPb 2=3 + 2 1 XPb = kt: 3
In order to test the possibility of diffusion through the ash,
the left-hand side of Eq.(3)was plotted against time (using the
experimentaldata shown inFig. 3) and is shown inFig. 5.It is
obvious that the dataarewell tted to the corresponding lines.
Therefore, the diffusion through
the ash could be rate determining step in the cementation
process.
3.4.3. Chemical reaction control for spherical particles
The fraction of lead reacted at any time, t, in a chemical
reactioncontrol process can be calculated from the following
equation(Levenspiel, 1999):
1 1XPb 1=3 =kt: 4
Based on the experimental data plotted inFig. 3, the left-hand
sideof Eq. (4)is plotted against reaction time in Fig. 6. By
comparing
Fig. 3.The effect of temperature on the cementation of lead
(molar ratio of Al:Pb=1.5,particle size of aluminum powder=149+125
m,R =110 rpm).
Fig. 4.Plot ofXPbversus time at various temperatures.
Fig.1. The effect of aluminum powder quantity on the cementation
of lead (particlesizeof aluminum powder=149+125 m,T=50 C,R =110
rpm).
Fig. 2.The effect of aluminum powder size on the cementation of
lead (molar ratio of
Al:Pb=1.5,T=50 C andR =110 rpm).
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Figs. 46, it is revealed that the R2 values forFig. 5are closer
to 1 thanthose ofFigs. 4 and 6. Therefore the data can be
correlated to an ashdiffusion control model and diffusion of Pb2+
through the blanket ofash controls the reaction rate.
3.4.4. Activation energy
The temperature dependence of the reaction rate constant (k)
canbe calculated by the Arrhenius equation (Levenspiel, 1999):
k= A exp Q
RT
5
whereA represents the frequency factor; Q(kJ/mol) is the
activationenergy of the reaction; R is the universal gas constant
and T is theabsolute temperature. The values ofkat different
temperatures can becalculated from the slope of the lines shown
inFig. 5. As seen inFig. 7,two straight lines are obtained. The rst
region shows the variationbetween 40 and 50 C and the second region
is for 5070 C. A shift inthe activation energy at 50 C probably was
a result of the change in
thereactionmechanism(Habashi,1969; Demirkranetal.,2007).Itmaybe
assumed that two consecutive processes are controlling the rate.
Adiffusion-controlled process is characterized by being slightly
depen-dent on temperature, while the chemically controlled process
isstrongly dependent on temperature (Habashi, 1969). This can
bereadily observed inFig. 3. With increasing the temperature from
40 to50 C in constant time, the cementation efciency increased
stronglywhile for temperature between 50 and 70 C the variation
ofcementation efciency was small. As a matter of fact, the overall
reac-tion controlling step for region between 40 and 50 C is
chemical reac-
tion that is highly sensitive to temperature. For the second
region (5070 C) the activation energy was calculated as 23.6 kJ/mol
and as seeninSection 3.4.3, diffusion of Pb2+ through the blanket
of ash controlsthe rate of cementation reaction in this region.
3.5. Characterization of the precipitated lead
Scanning electron microscopy (SEM) was used for a
morphologicalstudy of cemented deposits at optimum conditions. SEM
micrographof the cemented lead sample is presented inFig. 8. As
shown inFig. 8,cementation of lead from brine leach solutions on
aluminum powderproduces a porous product with a dendritic
structure. The chemicalcomposition of the precipitated lead at
optimum conditions is as fol-lows (wt.%): Pb-98.43; Zn-0.01;
Fe-0.01 and Al-0.6.
4. Conclusions
The cementation of lead with aluminum powder from the
brine-leaching ltrate of lead-bearing zinc plant residue was
investigated.Accordingto results, stoichiometric ratio Al:Pb of
1.5, aluminum powderfractionwith a
particlesize125+88m,temperatureof70C,stirring
speed of 110 rpm and 90 min reaction time were found to be
optimumFig. 6.Plot of 1(1XPb)1/3
versus time for various temperatures.
Fig. 7.Arrhenius plot for the cementation of lead by aluminum
powder.
Fig. 8.SEM micrograph of the precipitated lead sample.
Fig. 5.Plot of 13(1XPb)2/3+2(1XPb) versus time at various
temperatures.
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