. 82 (2007) 80–85www.elsevier.com/locate/ijminpro

Int. J. Miner. Process

A kinetic study of copper cementation with zinc in aqueous solutions

Nizamettin Demirkıran, Ahmet Ekmekyapar, Asım Künkül, Ahmet Baysar ⁎

Department of Chemical Engineering, Faculty of Engineering, Inonu University, Malatya 44280, Turkey

Received 5 June 2006; received in revised form 13 October 2006; accepted 13 October 2006Available online 28 November 2006

Abstract

Cementation of copper from zinc containing copper solutions using metallic zinc was studied in this work. The effect of copper,zinc and ammonium chloride concentration, stirring speed, pH and temperature on the cementation of copper was determined.Cementation rate increased with initial copper concentration, stirring speed and temperature. pH variation from 1 to 4 increased thecementation rate but at higher pH, the rate was not significantly effected. The cementation rate of copper increased with Zn2+ ionconcentration. However, the rate of this rise was slightly less compared to the rise that occurred in the Zn2+ ions free coppersolution.

The cementation reaction followed first order kinetics. It was observed that the reaction progressed with consecutive surfacereaction and diffusion controlling steps. The activation energy and pre exponential factors for each step were calculated and amodel describing the process was proposed.© 2006 Elsevier B.V. All rights reserved.

Keywords: Copper; Zinc; Cementation; Kinetics

1. Introduction

Today, high-grade copper reserves have been dimin-ished and the remaining reserves contain low-gradecopper. As a result, pyrometallurgical copper productionmethods are replaced by hydrometallurgical productionprocesses.

Copper ores are generally found in sulfide and ox-idized form in nature. Copper may be directly transferredto the aqueous phase by leaching oxidized ores butleaching of sulfide copper ores is more difficult. Thecomposition and purity of the resulting leaching solu-tion depend on the structure of the ore and the type ofreactants used for leaching. Leaching solutions obtained

⁎ Corresponding author. Tel.: +90 505 310 2213; fax: +90 422 3410690.

E-mail address: abaysar@inonu.edu.tr (A. Baysar).

0301-7516/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.minpro.2006.10.005

with strong acids may contain more impurities. Lowimpurity containing leaching solutions may be obtainedwith weak acids, bases and salts. These reactants arehighly selective for copper dissolution (Kordosky, 1981;Chase, 1980; Künkül et al., 1994).

It is reported that when oxidized copper ores areleached with ammonium salts, iron dissolved from the orematrix precipitates as iron III hydroxide (Ekmekyaparet al., 1988, 2003). As a result, ions left in the leachingsolution as impurities are mainly zinc and aluminum.Cementation is the process of precipitating ametal ion in asolution with a more active metal. This process is widelyapplied in mineral industry. Generally, iron is used forcementation of copper (Nadkarni and Wadsworth, 1967).However, during cementation of copper with iron, therecovery of economically valuedmetals such as zinc fromthe solution is difficult since the solution contains highiron concentration. Therefore, the recovery of zinc as a

Table 1Experimental parameter variation intervals for the copper cementationreaction

Parameter Value

Copper chlorideconcentration (mol/L)

0.01 0.025 0.05 0.1

Stirring speed (rpm) 200 300 400 500pH 1 2 3 4 (free) 4.5Temperature (K) 308 313 318 323 328NH4Cl concentration

(mol/L)0.25 0.5 1

Zinc chloride concentration(mol/L)

0.01 0.025 0.05 0.1

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side product from zinc rich oxidized copper ores withweak character leaching reactants such as ammoniumsalts may be important.

The objective of this work was to precipitate copperfrom zinc containing copper solutions using metallic zincand investigate the kinetics of this cementation process.

Copper cementation has been extensively studiedand this subject is still widely investigated (Stricklandand Lawson, 1970; MacKinnon and Ingraham, 1970;Rickard and Fuerstenau, 1968; Nadkarni et al., 1967;Stefanowicz et al., 1997; Dönmez et al., 1999; Gamboaet al., 2005; Karavasteva, 2005). Nadkarni et al. (1967)and Nadkarni and Wadsworth (1967) studied thekinetics of copper cementation on iron from cupricsulfate solutions at various concentrations and stirringspeeds. The cementation was reported to follow firstorder kinetics. Depending on the stirring speed precip-itated copper formed spongy layer to colloidal mass. Thereaction was reported to reach a limiting value and atheoretical model based on diffusion through a limitingboundary film was proposed.

Copper cementation on aluminum disks as a functionof copper concentration, temperature, pH and periph-eral velocity was studied by MacKinnon and Ingraham(1970). Depending on the temperature of the cementingsolution, the rate of deposition was either ionic diffusionor surface reaction controlled. Strickland and Lawson(1970) investigated the kinetics of copper cementationon rotating zinc disks. Deposition reaction was first orderand a diffusion boundary layer formed. The degree ofagitation was found to affect the reaction rate.

2. Experimental

Cementation experiments were carried out in a tem-perature controlled and mechanically stirred 400 mlglass reactor. A 45-mm-diameter Teflon stirring bladewas used for stirring. The pH of the cementation solutionwas continuously monitored during the experiments.Process variables were copper concentration, tempera-ture, stirring speed, pH, and zinc concentration. Table 1shows the variation interval for these parameters.Chemical reagents CuCl2.2H2O, ZnCl2 and granularzinc (5±0.5 mm diameter particles) from Merck(Germany) were used for the experiments. Experimentswere carried out as described below. Copper (Cu2+)containing solution was brought to the reaction temper-ature and 1.5 times of stoichiometrically required zincgranules were added to the reactor. The pH of thesolution was not buffered and set free except for theexperiments for which the effect of pH was investigated.At preset times, Cu2+ ions concentration was determined

from the samples and precipitated copper amount wascalculated from the experimental results.

3. Results and discussion

3.1. Effect of parameters

The concentration of copper ions was set at 0.01,0.025, 0.05 and 0.1 mol/l. During the experiments thetemperature and stirring speed were kept constant and at313 K and 500 rpm, respectively. The cemented fractionof copper was plotted versus time at initial copperconcentrations mentioned above. It was observed thatthe precipitation rate of metallic copper increases as theconcentration of copper ions is increased.

The effect of stirring speedwas tested at 200, 300, 400and 500 rpm values. Experiments were conducted at0.05 mol/l Cu2+ concentration and the temperature waskept at 313 K. The cemented fraction of copper increasedwith the stirring speed.

The effect of pH was investigated at pH of 1, 2, 3, 4.5and free pH (pH of 0.05 mol/l Cu2+ solution). Ammoniaand hydrochloric acid solutions were used as pH con-trolling agent. The effect of pH is shown in Fig. 1. It isclear that maximum cementation rate is obtained at pHof 3. The yield obtained at free pH (∼4, this is thenatural pH of the copper chloride solution) is close to theyield at pH of 3. At higher pH values, a decrease in theyield is observed.

The effect of temperature on copper cementation wasinvestigated at 308, 313, 318, 323 and 328 K. Theconcentration of Cu2+ ions and stirring speed for tem-perature effects were 0.05 mol/l and 500 rpm, respec-tively. The results showed that the cemented fraction ofcopper increases with temperature.

The effect of zinc chloride on the cementation ofcopper in zinc containing solutions was investigated.Equimolar concentrations of Zn2+ and Cu2+ mixtures

Fig. 1. The effect of pH on the copper cementation rate.

Fig. 3. The effect of ammonium chloride concentration on the coppercementation rate.

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(0.01, 0.025, 0.05 and 0.1 mol/l) were prepared. For theseexperiments, the temperature was kept at 313 K and thestirring speed was set at 500 rpm. Fig. 2 clearly shows thatthe cementation rate of copper increases with increasedZn2+ ion concentration. However, the rate of coppercementation in Zn2+ ions containing solution was slightlylower compared to the rate that occurred in the Zn2+ ionsfree copper solution.

The dissolution of copper ores with ammonium saltshas been reported in literature (Bingol et al., 2005;Ekmekyapar et al., 2003; Oudenne and Olson, 1983). Todetermine the effect of ammonium chloride on thecementation rate in copper and zinc containing solutions

Fig. 2. The effect of zinc chloride concentration on the copper ce-mentation rate.

was also investigated. The concentration of ammoniumchloride was chosen as 0.25, 0.5 and 1 mol/l. It wasobserved that the ammonium chloride concentration didnot affect the cementation rate at 313 K, 500 rpm stirringspeed and 0.5 mol/l equal concentrations of Cu2+ andZn2+ ions (Fig. 3).

3.2. Cementation kinetics

Copper cementation occurs according to the follow-ing reaction:

Cu2þ þ Zn→Zn2þ þ Cu

The progress of the reaction was followed by themeasurement of free Cu2+ ions in the solution. It isreported that cementation reactions mostly follow firstorder kinetics (Nadkarni et al., 1967; Ornales et al.,1998; Sedzimir, 2000; El Batouti, 2003; Strickland andLawson, 1970; Hiskey and Lee, 2003). We also assumedfirst order kinetics for the cementation reactioninvestigated in this work. The model for the first orderreaction is

lnð1� X Þ ¼ –kt ð1Þ

where X is the cemented copper fraction at time t.Cementation reactions are basically heterogeneous andit was thought that parameters such as concentration,temperature and stirring speed may have effect on thereaction rate. Hence the effect of these parameters on therate was investigated. Reaction data were used to obtain

Fig. 4. Plot of ln(1−X) versus time for various initial copperconcentrations.

Fig. 6. Plot of ln(1−X) versus time for various temperatures.

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ln (1−X) versus time graphs to examine the effect ofinitial Cu concentration, temperature and stirring speed.Figs. 4, 5 and 6 indicate linear relationship. Apparentrate constants were determined from the slope of thelines. Experimental results exhibit a good fit to Eq. (1).The following model was envisioned.

lnð1� X Þ ¼ –koCaWbe�E=RTt ð2Þ

where ko is pre exponential factor, C is initial copperchloride concentration, W is the stirring speed, E ac-tivation energy and T is absolute temperature. Theconstants a and b were determined as 0.85 and 1.50,respectively. Assuming Arrhenius type relation, ln kversus 1/T graph should give a straight line. As seen inFig. 7, two straight lines are obtained. First region shows

Fig. 5. Plot of ln(1−X) versus time for various stirring speeds.

the variation between 303–318 K and the second regionbetween 318–328 K.

MacKinnon and Ingraham (1970) and Miller (1973)reported that a shift in the activation energy probablywas a result of the change in the reaction mechanism. Itmay be assumed that two consecutive processes arecontrolling the rate. At low temperatures, the rate con-trolling step is the surface reaction. This is verified bythe value of the activation energy. As a matter of fact, theoverall reaction controlling step is surface reaction forprocesses that are highly sensitive to temperature andfor which the activation energy is above 10 kcal/mol(Habashi, 1969). Above 318 K, rate is controlled by thediffusion. In diffusion controlled processes the activa-tion energy is mostly below 10 kcal/mol.

For the first region (303–318 K) the activationenergy and the pre exponential factor were calculated as12.7 kcal/mol and 3.75×104 min−1, respectively. For

Fig. 7. Arrehenius plot for copper cementation.

Fig. 8. Plot of experimental and predicted cemented copper fractions.

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the second region (318–328 K) these values were cal-culated as 5.4 kcal/mol and 0.37 min−1, respectively.

Eq. (2) may be written for the two regions:

lnð1� X Þ ¼ –3:75� 104C0:85W 1:50e�6400=T t ð3Þ

lnð1� X Þ ¼ –0:37C0:85W 1:50e�2710=T t ð4Þ

To test the models, theoretical and experimentalconversions were plotted in Fig. 8. It is clear that theproposed models appropriately represent a first ordercementation reaction. The reaction rate is proportionalwith the 3/2 order of stirring speed. However, at higherstirring speeds (N500 rpm) the rate did not change. Thisindicates that at lower stirring speeds the reaction rate isdiffusion controlled. At 500 rpm stirring speed andtemperatures below 318 K the reaction is completelycontrolled by surface reaction. When the effect of theother parameters was examined, the stirring speed wasset at 500 rpm. At 500 rpm stirring speed and above318 K, the reaction changed the mechanism and shiftedto diffusion controlled regime.

4. Conclusion

In this work, we have investigated the cementation ofcopper using zinc granules in Cu2+, Zn2+ and Cl−

containing solutions. It was observed that the reactionrate increased with initial copper concentration, stirringspeed and temperature. pH variation (1–4) increased therate but at higher pH the rate was not significantlyeffected. The cementation reaction obeys first orderkinetics and may be represented by the model given inEq. (2) and this relationship is experimentally confirmed.

Many investigators (Karavasteva, 2005) reported thatthe cementation of copper by zinc is more effective thancementation with iron and aluminum. The oxidationpotential of iron is lower then that of zinc. Hence, zinc isa better cementation metal. Although aluminum has ahigher oxidation potential then zinc, an oxidation layerforms on the surface of aluminum during the cementa-tion (Karavasteva, 2005). Thus, the yield with alumi-num is lower.

During the leaching process of zinc containing copperores, the leaching solution contains significant amountof zinc ion. Thus, the cementation of copper from thesolution with zinc will facilitate the recovery of zinc as aside product in the later stages. As a matter of fact, byusing zinc only, the solution will not be contaminatedwith a third ion. Therefore, the cementation of copperwith zinc becomes an important process.

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