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Brazilian Journal of Chemical Engineering

Print version ISSN 0104-6632On-line version ISSN 1678-4383

Braz. J. Chem. Eng. vol.17 n.1 São Paulo Mar. 2000

http://dx.doi.org/10.1590/S0104-66322000000100011 

Effect of magnetic field on the crystallization of zinc sulfate

 

 

A. M. B. Freitas1, F. J. G. Landgraf 3, J. Nývlt4 and M. Giulietti1, 2
1 Chemical Engineering Department of the Federal University of São Carlos, P.O. Box 676,
CEP 13565-905, São Carlos - SP, Brazil, E-mail: freitas@ipt.br
2 Chemistry Division of the Institute of Technology Research, IPT, P.O. Box 0141,
CEP 01064 - 970, São Paulo - SP, Brazil, E-mail: giu@ipt.br
3 Metallurgy Division of the Institute of Technology Research, IPT, E-mail: landgraf@ipt.br
4 Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic,
E-mail: NYVLT@uach.icc.cas.cz

 

(Received: December 10, 1998; Accepted:August 20, 1999)

 

 

Abstract – The effect of magnetic field on the crystallization of diamagnetic zinc sulfate was investigated in a series of controlled batch cooling experiments. Zinc sulfate solutions were exposed to magnetic fields of different intensities, up to a maximum of 0.7T. A clear influence of magnetic field on the following zinc sulfate crystallization parameters was found: an increase in saturation temperature, a decrease in metastable zone width, and an increase in growth rate and average crystal size. These effects were observed for the diamagnetic zinc sulfate, but not in similar, previously reported experiments for paramagnetic copper sulfate.
Keywords: batch crystallization, magnetic field, zinc sulfate, diamagnetism

 

 

INTRODUCTION

The crystallization process, a tool for product recovery and purification, is one of the oldest unit operations used by mankind, and is exemplified by the production of sodium chloride and calcium carbonate. On the other hand, its development as a science and technology only gathered momentum during the decade of the 1960’s.

Since 1970, there has been a growing body of literature on the investigation of the influence of magnetic field on crystallization and precipitation, however, most of the experiments reported so far are rather qualitative and the results are sometimes contradictory. Nývlt and Kricková (1976) found that exposure to magnetic field increases the nucleation kinetics during crystallization of MgSO4 from aqueous solutions. Mitrovic et al. (1988) showed that magnetic fields changed the growth rate distribution and slows the growth rate of the majority of small Rochelle salt crystals from aqueous solutions. Higashitani et al. (1993) studied the effects of magnetic field on formation of CaCO3 crystals from the reaction between solutions of CaCl2 and Na2CO3, having found that (1) the nucleation frequency of CaCO3 crystals is suppressed but the growth of crystals is accelerated, if the exposed magnetic flux density is greater than about 0.3T and the exposure time is larger than 10 min; (2) the magnetic effect on the formation of CaCO3 is caused mainly by the effect on Na2CO3 solutions rather than CaCl2; (3) solutions exposed to a magnetic field maintain the magnetic effect on the formation of CaCO3 for at least 120 h; (4) the formation of aragonite structure of CaCO3 crystals is accelerated by the magnetic exposure. Recently, Barrett and Parsons (1998) published a report reproducing many of the results obtained on the precipitation of the diamagnetic calcium carbonate by Higashitani et al. (1993).

Berton et al. (1993) studied the effects of low-intensity, low-frequency electromagnetic fields on the nucleation and crystal growth of BaC2O4 from aqueous solutions of Ba(NO3)2, having found a decrease in nucleation rate, and an increase in crystal growth rate. Lundager Madsen (1995) investigated the influence of magnetic field on the precipitation of paramagnetic and diamagnetic inorganic salts, concluding that only phosphates and carbonates with diamagnetic metal ion are affected, through increased nucleation and growth rates.

Aiming at investigating the influence of intensity of the magnetic field on some crystallization parameters of the zinc sulfate-water system, batch crystallization experiments under controlled cooling were performed, and their results are described and discussed in this paper.

 

THEORY

Determination of the kinetic parameters of crystallization in batch crystallizers is very attractive, owing to the ease with which experiments can be conducted and simplicity of the experimental apparatus. On the other hand, evaluation of experimental results is complex when compared to the simple mathematical description of the continuous crystallizer experiments (constant supersaturation, constant suspension concentration and constant crystal surface area, in the steady state).

Nevertheless, theoretical studies developed by Mullin and Nývlt (1971) and Nývlt et al. (1973), as well as the model experiments described by Nývlt (1976), have shown that under simplified conditions, the cumulative crystal size distribution produced in a batch crystallizer can be described by the same theoretical relations as those deduced for the continuous crystallizer (MSMPR).

At the end of each batch experiment, the measured cumulative crystal size distribution can be fitted by equation (1) (Nývlt et al., 1985):

a11i01.gif (623 bytes)

where

a11i02.gif (386 bytes)

The distribution can be represented by a straight line in the z-L coordinates system and z values can be calculated for equation (1) from the corresponding value of cumulative weight fraction M(L), solved by a numerical method. It is possible to obtain the values of Lm (z = 3 or M(L) = 64.7% ) and LN (z = 0 or M(L) = 100% ) directly from a plot of M(L) or z versus L. The metastable zone width and growth rate are defined by equations (3) and (4) (Nývlt et al., 1985):

a11i03.gif (284 bytes)

and

a11i04.gif (377 bytes)

 

EXPERIMENTAL PROCEDURES

In accordance with solubility data from Perry and Chilton (1974), 92.84g of reagent grade ZnSO4.7H2O were mixed with 27.16ml of pure water (resistivity » 18Mohm.cm) to produce a solution with a theoretical saturation temperature of 50oC. Since the sulfate is highly hygroscopic, great care is needed in its maintenance and handling in order to obtain a solution with the predicted saturation temperature.

Crystallization was performed in a 100ml glass vessel. The internal temperature of the crystallizer was controlled by a water jacket connected to a thermostatic bath, monitored by a K-type thermocouple immersed in the solution. The suspension was agitated by means of a stirrer with Teflon shaft and blades. The crystallizer was placed between the poles of an electromagnet, as shown in Figure 1. A DC magnetic field was applied and its intensity measured by a Hall probe Gaussmeter.

 

a11f01.gif (8070 bytes)

 

The experimental saturation temperature was determined by the polythermal method proposed by Nývlt et al. (1985), in this case at different magnetic field intensities, to evaluate its effects on solubility.

All experiments were started by heating 120g of the solution up to 58oC and then maintaining it at this temperature for 10 minutes. A given magnetic field intensity was applied and then it was cooled to 30oC at 15oC/h. When the temperature was 2oC below the predetermined experimental saturation temperature (which was found to be dependent on the intensity of the magnetic field), two ZnSO4.7H2O crystal seeds with an average size of 1mm were added to the solution to keep it more stable and induce nucleation.

The nucleation temperature was determined by noting the temperature when the first crystals were optically observed. The metastable zone width for each condition was determined using equation (3). Effects of each intensity of the magnetic field were investigated by repeating the experiment.

At the end of each experiment, the suspension was immediately vacuum filtered and the crystals obtained were held for 12h at ambient atmosphere and temperature for drying. The crystal size distribution was determined by sieving, with mesh apertures of 0.037 to 2.0mm.

The samples from each experiment were examined by X-ray diffraction, in order to determine the crystalline phases present.

 

RESULTS AND DISCUSSION

Table 1 presents the results concerning the effect of intensity of the magnetic field on the experimentally measured saturation temperature, metastable zone width, growth rate, and average crystal size.

 

a11t01.gif (10949 bytes)

 

The exposure of the solution to a magnetic field of 0.3T, when compared to the "no-field" condition, resulted in an increase the experimental saturation temperature above the normal saturation value.

When a magnetic field of 0.3T was applied, the average growth rate increased about 41% and the average crystal size was 38% higher, whereas the metastable zone width decreased by 43% . The crystal size depends on the relation between the nucleation rate and the growth rate. According to Lungader Madsen (1995), based on crystal size measurements, both nucleation and growth rates are increased by the magnetic field leading to a decrease in average crystal size of carbonates and phosphates. Higashitani et al. (1993) and Barrett and Parsons (1998) found an increasing crystal size of calcium carbonate, due to a decrease of nucleation rate and an increase of crystal growth. In zinc sulfate case, of this study, the effect on growth rate prevailed over the effect on nucleation rate (decreasing of metastable zone width), leading to an increase of the average crystal size.

A further increase in the intensity of the magnetic field from 0.3 to 0.7T did not cause significant change in any of the measured parameters. Probably the effect of magnetic field reached saturation at some value below 0.3T.

Analysis of the diffraction patterns showed that the crystalline phase formed in the temperature interval investigated was majority ZnSO4.6H2O.

It is interesting to note that no effect of the magnetic field was observed in the crystallization parameters of paramagnetic CuSO4.5H2O (Freitas et al., 1998), while the above results show the clear effect of magnetic field on diamagnetic ZnSO4.6H2O. This difference in behavior is consistent with the findings of Lundager Madsen (1995) for some paramagnetic and diamagnetic salts.

Although some authors try to explain the physics of magnetic field effects, like Lundager Madsen (1995) with the proton transfer model and Berton et al. (1993) proposing an effect on the interfacial energy of the critical cluster of ions, there is no evidence these models agree with all experimental results. A suitable fundamental theory that explains the involved phenomena is still missing.

 

CONCLUSIONS

Based on batch cooling crystallization experiments, we have found the following effects of magnetic field on the ZnSO4.6H2O crystallization parameters: an increase in experimental saturation temperature, an increase in average growth rate about 41% and the average crystal size was 38% higher, whereas the metastable zone width decreased by 43% .

As there is no significant change in these parameters when the intensity of the magnetic field is increased from 0.3 to 0.7T, the effect of magnetic field reaches saturation at some value below 0.3T.

These effects were observed for diamagnetic ZnSO4.6H2O, but not in similar, previously reported experiments for paramagnetic CuSO4.5H2O.

 

NOMENCLATURE

a1 significance level

B intensity of the magnetic field, tesla (T)

DT metastable zone width, (° C)

G average growth rate, (m/s)

L crystal size, (m)

Lm average crystal size, (m)

LN minimum crystal size, (m)

M(L) cumulative weight fraction, (%)

tc batch time, (s)

Tsat experimental saturation temperature, (°C)

Tnuc nucleation temperature, (°C)

z dimensionless crystal size

 

REFERENCES

Barrett, R. A. and Parsons, S. A., The Influence of Magnetic Fields on Calcium Carbonate Precipitation, Wat. Res., vol. 32, pp. 609-612 (1998).

Berton, R. et al., Effect of ELF Electromagnetic Exposure on Precipitation of Barium Oxalate, Bioelectrochemistry and Bioenergertics, vol. 30, pp. 13-25 (1993).

Higashitani, K. et al., Effects of a Magnetic Field on the Formation of CaCO3 Particles, J. Colloid Interface Sci., vol. 156, pp. 90-95 (1993).

Lundager Madsen, H. E., Influence of Magnetic Field on the Precipitation of Some Inorganic Salts, J. Crystal Growth, vol. 152, pp. 94–100 (1995).

Mitrovic, M. M. et al., Influence of Magnetic Field on Growth Rate Dispersion of Small Rochelle Salt Crystals, J. Crystal Growth, vol. 87, pp. 439-445 (1988).

Mullin, J. W. and Nývlt, J., Programmed Cooling of Batch Crystallizers, Chem. Eng. Sci., vol. 26, pp. 369-377 (1971).

Nývlt, J. et al., The Kinetics of Industrial Crystallization. Academia Prague, Prague (1985).

Nývlt, J. et al., Size Distribution of Crystals from a Batch Crystallizer, Collect. Czechosl. Chem. Commun., vol. 38, pp. 3199-3209 (1973).

Nývlt, J., Effect of Kinetic Parameters on the Behaviour and Product Crystal Size Distribution of Batch Crystallizers, AIChE Symp. Ser., vol. 72, pp. 61-63 (1976).

Nývlt, J. and Kricková, J., Der Einflub des Magnetischen Feldes auf die Keimbildung in Wäbrigen Lösungen von Magnesiumsulfat, Chem. Techn., vol. 28, pp. 548-550 (1976).

Perry, R. H. and Chilton, C. H., Chemical Engineer’s Handbook. McGraw-Hill, New York (1974).

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