- Email: [email protected]
Effects of ultrasound on the electrochemical cementation of cadmium by zinc powder
Ultrasonics Sonochemistry 11 (2004) 23–26 www.elsevier.com/locate/ultsonch
Effects of ultrasound on the electrochemical cementation of cadmium by zinc powder q M. Aurousseau *, N.T. Pham, P. Ozil Laboratoire d’Electrochimie et de Physico-chimie des Mat eriaux et des Interfaces, UMR 5631 CNRS-INPG-UJF, BP 75, Saint Martin d’H eres Cedex 38402, France Accepted 31 March 2003
Abstract This work is devoted to a kinetics study of cadmium electrochemical cementation on zinc powder under ultrasonic low-frequency field (20 kHz). Compared to mechanical stirring with a Rushton turbine and for the same suspension quality, ultrasound lead to a lower kinetics during the major part of the reaction but to final conversion rate near 100%. Pointing out a thermal modification in the deposit morphology due to acoustic cavitation, gives explanation to these processes changes. Besides several acting parameter effects, such as temperature, metallic ion concentrations or ultrasonic power have been observed and analysed. 2003 Elsevier B.V. All rights reserved. Keywords: Cementation; Cadmium; Zinc powder; 20 kHz ultrasound; Kinetics
1. Introduction Electrochemical cementation is known from a long time to remove noble or toxic metallic ions from solutions and is still used in hydrometallurgy, surface and industrial waste treatments or electrolyte purification. It consists in the spontaneous heterogeneous reduction of a metallic ion present in solution by a more electropositive sacrificial metal. Most of the industrial cementation processes use metal powder within stirred-tank reactors. Their major interests are a low-energy requirement, an easy control and a frequent removal of the metallic species under its metal form. The present study concerns the cadmium–zinc system, selected because of the great interest of cadmium, toxic and heavy metal largely used for industrial applications.
q
This paper was originally presented at Applications of Power Ultrasound in Physical and Chemical Processing (Usound3) Paris December 2001. * Corresponding author. Present address: Laboratoire de Genie des Procedes Papetiers, UMR 5518 CNRS-EFPG-INPG-CTP, BP 65, Saint Martin dÕ Heres Cedex 38402, France. E-mail address: [email protected] (M. Aurousseau). 1350-4177/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1350-4177(03)00130-5
The corresponding global cementation reaction: CdðIIÞ þ Zn () Cd þ ZnðIIÞ
ð1Þ
may be considered as thermodynamically complete (equilibrium constant ¼ 1:5 1012 at 25 C [1]). Whatever the configuration (powder in stirred-tank reactor or rotating-disk electrode (RDE)), reaction kinetics seems to be controlled by the mass transport of cementing species [2,3]. Because of this limitation, using low-frequency ultrasound (US) seemed interesting to improve the cementation efficiency [4]. In fact literature pointed out a US effect on solid–liquid transport for other sonoelectrochemical reactions, as the result of acoustic cavitation and of induced turbulences [5]. Such an effect was previously confirmed for Cd(II)/Zn cementation on a RDE through the observed increase of the mass transfer coefficient k, 4–10 times larger than that without US [4,6]. Nevertheless the direct improvement expected on the cementation kinetics rate is reduced by an other effect: the regeneration of the reaction surface S by US hinders the dendritic deposit morphology obtained without US which had both a specific area increase and a mass transfer enhancement effect [4,6]. As a consequence, the final contribution of US thus remains rather limited for a RDE.
24
M. Aurousseau et al. / Ultrasonics Sonochemistry 11 (2004) 23–26
Nevertheless an improvement of cementation on powders with ultrasound can be still expected at this stage, first because of a lower impact on the surface renewal, and secondly due to a new US effect: the reduction of the agglomeration phenomenon for the zinc powder that occurs in a reactor stirred by a mechanical way as observed by Blaser and OÕKeefe [7].
Besides standard conditions were defined for this study as: T ¼ 25 C, C0 ¼ 4:45 mol m3 , mZn ¼ 0:153 g and Pth ¼ Pacoustic ¼ 83 W or X ¼ 1150 rpm.
3. Results and discussion 3.1. Comparison between mechanical and low-frequency ultrasonic stirrings
2. Experiments
XCdðIIÞ ¼
C0 C C0
Two trails were performed ensuring the same conditions of particles suspension according to the criteria by Zwietering [9]. The first one under mechanical stirring by a suitable 6 straight-blade Rushton turbine (diameter 0.025 m) [10,11] and the second one with US. Fig. 1 shows the characteristic cementation curves XCdðIIÞ ¼ f ðtÞ obtained on one hand for Pth ¼ 83 W and on the other hand for X ¼ 1150 rpm. Each curve involves two main reaction steps: an initial fast kinetics (10 min) and an asymptotic zone at the approach of the reaction end (after 50 min), with a progressive transition area between. Cementation without US appears to be faster for durations lower than 20 min: as an example, the value of XCdðIIÞ at t ¼ 10 min is 69% without US and 54% with US. However this period corresponds without US to the end of the reaction with a final value of XCdðIIÞ not exceeding 85%, so proving the existence of another limiting or blocking step in this configuration. On the other hand, the cementation with US goes on to complete conversion (95.3% at t ¼ 50 min). Moreover important differences are observed concerning the morphology of both the deposit and the zinc particles with or without US as shown by Fig. 2. Reaction (1) leads to replace each mole of dissolved zinc by one mole of deposited cadmium. Cadmium being more voluminous, the apparent size of zinc particles covered by cadmium should increase. Nevertheless without US, the mean particle size decreases from 56 to 20 lm after 10 min showing a particles fragmentation. This phenome1 0.8 X Cd (II)
The reactor was equipped with a water-jacket ensuring the temperature control. It offered a 0.350 L reaction volume for a 0.075 m internal diameter. The ultrasonic field was applied to the solution by a titanium horn (0.035 m diameter) introduced by the top of the reactor and located at 0.015 m from the bottom to respect the criteria of a complete and homogeneous particles suspension. It was connected to a 20 kHz generator (Branson type 450). The global calorimetric method [8] allowed estimating the energetic yield (ratio of the thermal and electrical powers Pth =Pe ) to the constant value 0.65. The solutions were prepared from cadmium and zinc sulfate salts and their initial ionic force was adjusted to unity by adding a suitable mass of sodium sulfate [6]. The range of their natural pH (5.7–6) leads both to a complete suspension of zinc particles and to a negligible role of the parasitic acid attack of zinc by protons; Cd(II) and Zn(II) species are present into the solutions under Cd(SO4 )6 and Zn(SO4 )6 sulfate complexed 4 4 forms respectively. Finally the solutions were de-aerated by nitrogen bubbling during 30 min. The mass of zinc powder mZn (purity 99.99%, size range: 50–63 lm) corresponded for all the trials to a molar excess of 1.5 referred to the reaction stoichiometry. The concentrations of metallic ions were measured vs. time on 0.001 L samples by spectrophotometry of atomic absorption (SAA). The powder surface was observed by scanning electronic microscopy (SEM), most often coupled to energy dispersive spectrometry (EDS) to analyze the corresponding solid composition [6]. The classical description of cementation kinetics uses the logarithmic evolution of the concentration for the cementing species with time, that is only pertinent if admitting a constant apparent rate constant K (¼ kS). But in fact, during cementation on powders, deposition and dissolution induce a continuous change of the particle size with time, thus leading to variations of S and k. Therefore kinetics are described here in a more realistic way by the evolution of the Cd(II) conversion rate defined as:
0.6 US stirring
0.4
Rushton stirring 0.2 0
ð2Þ
from the Cd(II) concentrations, C0 at t ¼ 0 and C at time t.
0
10
20
30 40 time (min)
50
60
70
Fig. 1. Cd(II) conversion rate vs. time with US (Pth ¼ 83 W) or Rushton turbine (X ¼ 1150 rpm) stirring for standard conditions.
M. Aurousseau et al. / Ultrasonics Sonochemistry 11 (2004) 23–26
25
Fig. 2. Solid fraction after 10 min reaction time with Rushton stirring (left) and US stirring (right) for standard conditions.
1 0.8 X Cd(II) (-)
non is not due to the only mechanical stirring but to the zinc dissolution on the anodic sites which induces the formation of cavities getting the particles more brittle. A comparative trial with the only supporting electrolyte in the absence of cadmium sulfate did not show any change in the zinc particle size, so confirming this interpretation. When using US, the mean particle size increased up to 60 lm at the end of the reaction. The cadmium deposit appears as amorphous globules (white zones) located on the zinc surface (grey zones). These globules should result from the cadmium fusion due to the very high-local energy during the bubbles implosion (estimated temperature close to several thousands of Kelvins [12]). The complementary information provided by the SEM observations and the kinetics curves allow explaining the behavior differences between cementation without and with US. In the presence of mechanical stirring, zinc particles fragmentation leads to a continuous increase of the reaction surface vs. time, so increasing the kinetics rate. However the zinc fragments are progressively covered by a rather dense cadmium deposit, able to finally block the reaction. In fact a mass transfer limitation by the Zn(II) transport from anodic sites towards solution could then occur [13]. On the other hand, using US keeps unchanged the particle size, insuring a free access of the reacting species through the amorphous cadmium deposit and so allowing a complete cementation reaction.
57 83
0.4
120 0.2 0 0
10
20
30
40
50
60
70
time (min)
Fig. 3. Ultrasonic power Pth (W) effect on Cd(II) cementation for standard conditions.
plosion effect and should tend to compensate the positive effects of acoustic agitation on mass transfer. 3.3. Influence of temperature T A temperature increase from 15 to 60 C lead to an expected increase of the reaction rate (Fig. 4), but in a lower extent beyond 45 C. This attenuation of the positive effect of temperature on the mass transfer induced by an increase with the temperature of the diffusion 1
3.2. Influence of the ultrasonic power Pth
0.8 XCd(II) (-)
Fig. 3 shows that increasing the ultrasonic power enhances the reaction rate, for conditions insuring a complete and uniform suspension of particles and leading to a quite complete cementation reaction. However the benefit corresponding to a power change from 83 to 120 W appears to be rather weak in comparison with the added power. In fact at high power, both the size and the number of bubbles are largely increased as observed experimentally at 120 W. The resulting cushion effect induced by the bubbles leads to a reduction of the im-
0.6
0.6
15˚C 25˚C 45˚C 60˚C
0.4 0.2 0 0
10
20
30
40
50
60
70
time (min)
Fig. 4. Temperature T (C) effect on Cd(II) cementation for standard conditions.
26
M. Aurousseau et al. / Ultrasonics Sonochemistry 11 (2004) 23–26
quency ultrasonic field (20 kHz) allowed to point out the following main results which have been interpreted through the physical action of acoustic cavitation and the morphological modification of cadmium deposit:
1
XCd(II) (-)
0.8 0.6 17.8 4.45
0.4
0.89 0.2 0 0
10
20
30 40 time (min)
50
60
70
Fig. 5. [Cd(II)]0 (mol m3 ) effect on Cd(II) cementation for standard conditions.
coefficient in the layer near the zinc particle, could be explained by a decrease of the bubble implosion intensity at the highest temperature studied as classically observed. 3.4. Influence of initial Cd(II) and Zn(II) concentrations Increasing the initial Zn(II) concentration within the range 0–76.5 mol m3 has no effect on the reaction kinetics. On the other hand, increasing the initial Cd(II) concentration [Cd(II)]0 within the range of industrial interest, 0.89–17.8 mol m3 , improves both the reaction rate constant and the final conversion rate (Fig. 5). This later result could appear quite surprising if considering a first-order reaction rate as classically admitted. According to this assumption, only the reaction rate should be increased while the evolution of the conversion rate should be unchanged, because of the constant molar zinc excess 1.5 for all the trials. The size and the morphology of final particles (t ¼ 60 min) do not allow explaining this positive effect of the initial Cd(II) concentration. In fact, for a 17.8 mol m3 concentration, SEM observations allowed to estimate the mean particle diameter about 50–60 lm and showed a cadmium deposit having a morphology very close to that obtained for 4.45 mol m3 ; this last result being consistent with close final conversion rates. Only small needleshaped crystals (1 lm) made of cadmium (as shown by EDS analysis) are observed. Their origin could be explained by a higher deposition rate and the associated local microturbulences. However the only presence of these crystals is not able to justify the important increase of the conversion rate (þ0:4 at t ¼ 10 min). The real reason of this increase should be a deposit structure differing when the reaction begins but no more visible at the end.
4. Conclusions Studying the cadmium cementation on zinc powder in a stirred-tank reactor under the action of a low-fre-
• compared to a Rushton turbine, using ultrasound in the same other conditions leads to a kinetics rate which is lower at the reaction beginning but higher at the end; such phenomena are correlated to both existence or not of particles fragmentation and of a probable limiting mass transfer step by oxidized zinc species. SEM photos supplied with EDS analysis suggest a local thermal effect due to US, inducing a partial melting of the deposit and therefore changes in its morphology. • temperature increase has a positive effect between 15 and 45 C and in a lower extent up to 60 C; • a higher initial Cd(II) concentration also enhances kinetics while Zn(II) concentration is of no effect; • increasing ultrasonic power in the range 50–120 W is favorable but from 80 W it remains a secondary effect.
Acknowledgements This work has been possible with the loan of ultrasonic materials by Pr. C. Petrier, Univ. de Savoie-ESIGEC––Lab. de Chimie Moleculaire and Env.––Le bourget du Lac––France.
References [1] D.R. Lide, Handbook of Chemistry and Physics, 71st ed., CRC Press, New York, USA, 1990–1991. [2] T.R. Ingraham, R. Kerby, Transactions of the Metallurgical Society of AIME 245 (1969) 17–21. [3] E.C. Lee, F. Lawson, K.N. Han, Institution of Mining and Metallurgy 84 (1975) C87–C91. [4] N.T. Pham, Ph.D. Thesis of INPG, Grenoble, France, 1999. [5] D.J. Walton, S.S. Phull, Advances in Sonochemistry 4 (1996) 205– 284. [6] N.T. Pham, M. Aurousseau, J.-C. Delachaume, P. Ozil, C. Petrier, in: First Conference on Applications of Power Ultrasound in Physical and Chemical Processing, Toulouse, France, 1997, pp. 219–224. [7] M.S. Blaser, T.J. OÕKeefe, in: Proceedings of 3rd International Symposium on Hydrometallurgy, 112th AIME Annual Meeting, Atlanta, USA, 1983. [8] V. Renaudin, Ph.D. Thesis of Savoy University, Chambery, France, 1994. [9] T.N. Zwietering, Chemical Engineering Science 8 (1958) 244–253. [10] J.H. Rushton, E.W. Costich, H.J. Everett, Chemical Engineering Progress 46/8 (1950) 395–404. [11] J.H. Rushton, E.W. Costich, H.J. Everett, Chemical Engineering Progress 46/9 (1950) 467–476. [12] M.A. Margulis, Sonochemistry and Cavitation, Gordon and Breach Publishers, Luxembourg, 1993. [13] P. Kruus, D.A. Robertson, L.A. McMillen, Ultrasonics 29/9 (1991) 370–375.
Effects of ultrasound on the electrochemical cementation of cadmium by zinc powder q M. Aurousseau *, N.T. Pham, P. Ozil Laboratoire d’Electrochimie et de Physico-chimie des Mat eriaux et des Interfaces, UMR 5631 CNRS-INPG-UJF, BP 75, Saint Martin d’H eres Cedex 38402, France Accepted 31 March 2003
Abstract This work is devoted to a kinetics study of cadmium electrochemical cementation on zinc powder under ultrasonic low-frequency field (20 kHz). Compared to mechanical stirring with a Rushton turbine and for the same suspension quality, ultrasound lead to a lower kinetics during the major part of the reaction but to final conversion rate near 100%. Pointing out a thermal modification in the deposit morphology due to acoustic cavitation, gives explanation to these processes changes. Besides several acting parameter effects, such as temperature, metallic ion concentrations or ultrasonic power have been observed and analysed. 2003 Elsevier B.V. All rights reserved. Keywords: Cementation; Cadmium; Zinc powder; 20 kHz ultrasound; Kinetics
1. Introduction Electrochemical cementation is known from a long time to remove noble or toxic metallic ions from solutions and is still used in hydrometallurgy, surface and industrial waste treatments or electrolyte purification. It consists in the spontaneous heterogeneous reduction of a metallic ion present in solution by a more electropositive sacrificial metal. Most of the industrial cementation processes use metal powder within stirred-tank reactors. Their major interests are a low-energy requirement, an easy control and a frequent removal of the metallic species under its metal form. The present study concerns the cadmium–zinc system, selected because of the great interest of cadmium, toxic and heavy metal largely used for industrial applications.
q
This paper was originally presented at Applications of Power Ultrasound in Physical and Chemical Processing (Usound3) Paris December 2001. * Corresponding author. Present address: Laboratoire de Genie des Procedes Papetiers, UMR 5518 CNRS-EFPG-INPG-CTP, BP 65, Saint Martin dÕ Heres Cedex 38402, France. E-mail address: [email protected] (M. Aurousseau). 1350-4177/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1350-4177(03)00130-5
The corresponding global cementation reaction: CdðIIÞ þ Zn () Cd þ ZnðIIÞ
ð1Þ
may be considered as thermodynamically complete (equilibrium constant ¼ 1:5 1012 at 25 C [1]). Whatever the configuration (powder in stirred-tank reactor or rotating-disk electrode (RDE)), reaction kinetics seems to be controlled by the mass transport of cementing species [2,3]. Because of this limitation, using low-frequency ultrasound (US) seemed interesting to improve the cementation efficiency [4]. In fact literature pointed out a US effect on solid–liquid transport for other sonoelectrochemical reactions, as the result of acoustic cavitation and of induced turbulences [5]. Such an effect was previously confirmed for Cd(II)/Zn cementation on a RDE through the observed increase of the mass transfer coefficient k, 4–10 times larger than that without US [4,6]. Nevertheless the direct improvement expected on the cementation kinetics rate is reduced by an other effect: the regeneration of the reaction surface S by US hinders the dendritic deposit morphology obtained without US which had both a specific area increase and a mass transfer enhancement effect [4,6]. As a consequence, the final contribution of US thus remains rather limited for a RDE.
24
M. Aurousseau et al. / Ultrasonics Sonochemistry 11 (2004) 23–26
Nevertheless an improvement of cementation on powders with ultrasound can be still expected at this stage, first because of a lower impact on the surface renewal, and secondly due to a new US effect: the reduction of the agglomeration phenomenon for the zinc powder that occurs in a reactor stirred by a mechanical way as observed by Blaser and OÕKeefe [7].
Besides standard conditions were defined for this study as: T ¼ 25 C, C0 ¼ 4:45 mol m3 , mZn ¼ 0:153 g and Pth ¼ Pacoustic ¼ 83 W or X ¼ 1150 rpm.
3. Results and discussion 3.1. Comparison between mechanical and low-frequency ultrasonic stirrings
2. Experiments
XCdðIIÞ ¼
C0 C C0
Two trails were performed ensuring the same conditions of particles suspension according to the criteria by Zwietering [9]. The first one under mechanical stirring by a suitable 6 straight-blade Rushton turbine (diameter 0.025 m) [10,11] and the second one with US. Fig. 1 shows the characteristic cementation curves XCdðIIÞ ¼ f ðtÞ obtained on one hand for Pth ¼ 83 W and on the other hand for X ¼ 1150 rpm. Each curve involves two main reaction steps: an initial fast kinetics (10 min) and an asymptotic zone at the approach of the reaction end (after 50 min), with a progressive transition area between. Cementation without US appears to be faster for durations lower than 20 min: as an example, the value of XCdðIIÞ at t ¼ 10 min is 69% without US and 54% with US. However this period corresponds without US to the end of the reaction with a final value of XCdðIIÞ not exceeding 85%, so proving the existence of another limiting or blocking step in this configuration. On the other hand, the cementation with US goes on to complete conversion (95.3% at t ¼ 50 min). Moreover important differences are observed concerning the morphology of both the deposit and the zinc particles with or without US as shown by Fig. 2. Reaction (1) leads to replace each mole of dissolved zinc by one mole of deposited cadmium. Cadmium being more voluminous, the apparent size of zinc particles covered by cadmium should increase. Nevertheless without US, the mean particle size decreases from 56 to 20 lm after 10 min showing a particles fragmentation. This phenome1 0.8 X Cd (II)
The reactor was equipped with a water-jacket ensuring the temperature control. It offered a 0.350 L reaction volume for a 0.075 m internal diameter. The ultrasonic field was applied to the solution by a titanium horn (0.035 m diameter) introduced by the top of the reactor and located at 0.015 m from the bottom to respect the criteria of a complete and homogeneous particles suspension. It was connected to a 20 kHz generator (Branson type 450). The global calorimetric method [8] allowed estimating the energetic yield (ratio of the thermal and electrical powers Pth =Pe ) to the constant value 0.65. The solutions were prepared from cadmium and zinc sulfate salts and their initial ionic force was adjusted to unity by adding a suitable mass of sodium sulfate [6]. The range of their natural pH (5.7–6) leads both to a complete suspension of zinc particles and to a negligible role of the parasitic acid attack of zinc by protons; Cd(II) and Zn(II) species are present into the solutions under Cd(SO4 )6 and Zn(SO4 )6 sulfate complexed 4 4 forms respectively. Finally the solutions were de-aerated by nitrogen bubbling during 30 min. The mass of zinc powder mZn (purity 99.99%, size range: 50–63 lm) corresponded for all the trials to a molar excess of 1.5 referred to the reaction stoichiometry. The concentrations of metallic ions were measured vs. time on 0.001 L samples by spectrophotometry of atomic absorption (SAA). The powder surface was observed by scanning electronic microscopy (SEM), most often coupled to energy dispersive spectrometry (EDS) to analyze the corresponding solid composition [6]. The classical description of cementation kinetics uses the logarithmic evolution of the concentration for the cementing species with time, that is only pertinent if admitting a constant apparent rate constant K (¼ kS). But in fact, during cementation on powders, deposition and dissolution induce a continuous change of the particle size with time, thus leading to variations of S and k. Therefore kinetics are described here in a more realistic way by the evolution of the Cd(II) conversion rate defined as:
0.6 US stirring
0.4
Rushton stirring 0.2 0
ð2Þ
from the Cd(II) concentrations, C0 at t ¼ 0 and C at time t.
0
10
20
30 40 time (min)
50
60
70
Fig. 1. Cd(II) conversion rate vs. time with US (Pth ¼ 83 W) or Rushton turbine (X ¼ 1150 rpm) stirring for standard conditions.
M. Aurousseau et al. / Ultrasonics Sonochemistry 11 (2004) 23–26
25
Fig. 2. Solid fraction after 10 min reaction time with Rushton stirring (left) and US stirring (right) for standard conditions.
1 0.8 X Cd(II) (-)
non is not due to the only mechanical stirring but to the zinc dissolution on the anodic sites which induces the formation of cavities getting the particles more brittle. A comparative trial with the only supporting electrolyte in the absence of cadmium sulfate did not show any change in the zinc particle size, so confirming this interpretation. When using US, the mean particle size increased up to 60 lm at the end of the reaction. The cadmium deposit appears as amorphous globules (white zones) located on the zinc surface (grey zones). These globules should result from the cadmium fusion due to the very high-local energy during the bubbles implosion (estimated temperature close to several thousands of Kelvins [12]). The complementary information provided by the SEM observations and the kinetics curves allow explaining the behavior differences between cementation without and with US. In the presence of mechanical stirring, zinc particles fragmentation leads to a continuous increase of the reaction surface vs. time, so increasing the kinetics rate. However the zinc fragments are progressively covered by a rather dense cadmium deposit, able to finally block the reaction. In fact a mass transfer limitation by the Zn(II) transport from anodic sites towards solution could then occur [13]. On the other hand, using US keeps unchanged the particle size, insuring a free access of the reacting species through the amorphous cadmium deposit and so allowing a complete cementation reaction.
57 83
0.4
120 0.2 0 0
10
20
30
40
50
60
70
time (min)
Fig. 3. Ultrasonic power Pth (W) effect on Cd(II) cementation for standard conditions.
plosion effect and should tend to compensate the positive effects of acoustic agitation on mass transfer. 3.3. Influence of temperature T A temperature increase from 15 to 60 C lead to an expected increase of the reaction rate (Fig. 4), but in a lower extent beyond 45 C. This attenuation of the positive effect of temperature on the mass transfer induced by an increase with the temperature of the diffusion 1
3.2. Influence of the ultrasonic power Pth
0.8 XCd(II) (-)
Fig. 3 shows that increasing the ultrasonic power enhances the reaction rate, for conditions insuring a complete and uniform suspension of particles and leading to a quite complete cementation reaction. However the benefit corresponding to a power change from 83 to 120 W appears to be rather weak in comparison with the added power. In fact at high power, both the size and the number of bubbles are largely increased as observed experimentally at 120 W. The resulting cushion effect induced by the bubbles leads to a reduction of the im-
0.6
0.6
15˚C 25˚C 45˚C 60˚C
0.4 0.2 0 0
10
20
30
40
50
60
70
time (min)
Fig. 4. Temperature T (C) effect on Cd(II) cementation for standard conditions.
26
M. Aurousseau et al. / Ultrasonics Sonochemistry 11 (2004) 23–26
quency ultrasonic field (20 kHz) allowed to point out the following main results which have been interpreted through the physical action of acoustic cavitation and the morphological modification of cadmium deposit:
1
XCd(II) (-)
0.8 0.6 17.8 4.45
0.4
0.89 0.2 0 0
10
20
30 40 time (min)
50
60
70
Fig. 5. [Cd(II)]0 (mol m3 ) effect on Cd(II) cementation for standard conditions.
coefficient in the layer near the zinc particle, could be explained by a decrease of the bubble implosion intensity at the highest temperature studied as classically observed. 3.4. Influence of initial Cd(II) and Zn(II) concentrations Increasing the initial Zn(II) concentration within the range 0–76.5 mol m3 has no effect on the reaction kinetics. On the other hand, increasing the initial Cd(II) concentration [Cd(II)]0 within the range of industrial interest, 0.89–17.8 mol m3 , improves both the reaction rate constant and the final conversion rate (Fig. 5). This later result could appear quite surprising if considering a first-order reaction rate as classically admitted. According to this assumption, only the reaction rate should be increased while the evolution of the conversion rate should be unchanged, because of the constant molar zinc excess 1.5 for all the trials. The size and the morphology of final particles (t ¼ 60 min) do not allow explaining this positive effect of the initial Cd(II) concentration. In fact, for a 17.8 mol m3 concentration, SEM observations allowed to estimate the mean particle diameter about 50–60 lm and showed a cadmium deposit having a morphology very close to that obtained for 4.45 mol m3 ; this last result being consistent with close final conversion rates. Only small needleshaped crystals (1 lm) made of cadmium (as shown by EDS analysis) are observed. Their origin could be explained by a higher deposition rate and the associated local microturbulences. However the only presence of these crystals is not able to justify the important increase of the conversion rate (þ0:4 at t ¼ 10 min). The real reason of this increase should be a deposit structure differing when the reaction begins but no more visible at the end.
4. Conclusions Studying the cadmium cementation on zinc powder in a stirred-tank reactor under the action of a low-fre-
• compared to a Rushton turbine, using ultrasound in the same other conditions leads to a kinetics rate which is lower at the reaction beginning but higher at the end; such phenomena are correlated to both existence or not of particles fragmentation and of a probable limiting mass transfer step by oxidized zinc species. SEM photos supplied with EDS analysis suggest a local thermal effect due to US, inducing a partial melting of the deposit and therefore changes in its morphology. • temperature increase has a positive effect between 15 and 45 C and in a lower extent up to 60 C; • a higher initial Cd(II) concentration also enhances kinetics while Zn(II) concentration is of no effect; • increasing ultrasonic power in the range 50–120 W is favorable but from 80 W it remains a secondary effect.
Acknowledgements This work has been possible with the loan of ultrasonic materials by Pr. C. Petrier, Univ. de Savoie-ESIGEC––Lab. de Chimie Moleculaire and Env.––Le bourget du Lac––France.
References [1] D.R. Lide, Handbook of Chemistry and Physics, 71st ed., CRC Press, New York, USA, 1990–1991. [2] T.R. Ingraham, R. Kerby, Transactions of the Metallurgical Society of AIME 245 (1969) 17–21. [3] E.C. Lee, F. Lawson, K.N. Han, Institution of Mining and Metallurgy 84 (1975) C87–C91. [4] N.T. Pham, Ph.D. Thesis of INPG, Grenoble, France, 1999. [5] D.J. Walton, S.S. Phull, Advances in Sonochemistry 4 (1996) 205– 284. [6] N.T. Pham, M. Aurousseau, J.-C. Delachaume, P. Ozil, C. Petrier, in: First Conference on Applications of Power Ultrasound in Physical and Chemical Processing, Toulouse, France, 1997, pp. 219–224. [7] M.S. Blaser, T.J. OÕKeefe, in: Proceedings of 3rd International Symposium on Hydrometallurgy, 112th AIME Annual Meeting, Atlanta, USA, 1983. [8] V. Renaudin, Ph.D. Thesis of Savoy University, Chambery, France, 1994. [9] T.N. Zwietering, Chemical Engineering Science 8 (1958) 244–253. [10] J.H. Rushton, E.W. Costich, H.J. Everett, Chemical Engineering Progress 46/8 (1950) 395–404. [11] J.H. Rushton, E.W. Costich, H.J. Everett, Chemical Engineering Progress 46/9 (1950) 467–476. [12] M.A. Margulis, Sonochemistry and Cavitation, Gordon and Breach Publishers, Luxembourg, 1993. [13] P. Kruus, D.A. Robertson, L.A. McMillen, Ultrasonics 29/9 (1991) 370–375.