Effects of Magnetic Field on the Crystallization of Calcium Carbonate Polymorphs
Date Issued
2009
Date
2009
Author(s)
Chang, Meng-Chun
Abstract
Cooling water accounts for a large proportion of industrial process water. It would be beneficial to solve the water-shortage problem if an efficacious cooling water-saving process was brought up. After a certain period of circulation in a cooling system, some of the water flowing through the cooling tower would evaporate and make the Ca2+ concentration higher. The variation in pH and temperature may cause sparingly soluble salts, CaCO3 in the majority, precipitate on the wall of pipes of heat exchangers, so called “scale.” The scale decreases heat-transfer efficiency and plugs up the piping system. To maintain the Ca2+ concentration at a low level, water discharge and water make-up measures are taken. Then, a lot of water is wasted. Magnetic water treatment device for scale prevention has been around for more than a century. It is a low-cost, easy operating, and environment friendly way. However, the performance of magnetic device is not stable. In the academic community, the magnetic effects on CaCO3 crystallization reported in the literature were widely divided, due to the lack of precise research approach and the variety of CaCO3 crystal structure. In this research, a constant-composition technique, which could fix all the operating variables, was first used to investigate the magnetic effects on CaCO3 crystallization. Two kinds of magnetic devices were tested here, i.e., MagneGen Model 100, which is a pair of permanent magnets with an effective intensity of 212.6 Gauss, and Descal-A-Matic DC-3, which is a magnetic water treatment device with an effective intensity of 1800 Gauss. The experiments were carried out in a fluidized-bed crystallizer.n the part of crystal growth experiments, the calcite crystal seeds grew normally without the magnetic field, but the aragonite crystal seeds did not. In the presence of the magnetic field, the calcite growth rates would be completely stopped if the degree of supersaturation (σcal) was lower than 2.0; on the contrast, the aragonite seeds began to grow gradually and finally reached a constant rate. Comparing the two magnetic devices, the Descal-A-Matic DC-3 developed its influence in a shorter time. The growth rate of aragonite seeds was faster in the solution magnetized by Descal-A-Matic DC-3 for 90 min than in the solution magnetized by MagneGen Model 100 for 20 h. For the same magnetic device, different acting position showed different performances. When the MagneGen Model 100 acting on the aragonite crystal seeds directly, it performed better than that acting on the bulk solution. After the supersaturated solution being completely magnetized, the aragonite growth rate increased with increasing level of supersaturation, which showed the same trend as calcite crystals. However, in the effects of pH and activity ratio (R), aragonite and calcite crystals behaved differently. At low pH, the aragonite grew faster but slower for calcite; at R = 1.0, the aragonite showed a minimum but a maximum for calcite. The nucleation experiments also demonstrated that the magnetic field favors the aragonite formation whether the magnetic field was applied before or after nucleation. The magnetic field would transform the precipitated vaterite to aragonite. On the other hand, pure aragonite was produced after the supersaturated solution (σcal < 2.23) had been magnetized by MagneGen Model 100 for two days. In addition, measurements on the zeta potential of CaCO3 particles also qualitatively reflected the magnetic effect on the solution. However, zeta potential was not the critical factor that determines the growth rate.rom the point of clustering in the classical crystallization theory, a working mechanism of the magnetic effect has been drawn to explain the experimental results obtained in this experiment and reported in the literature. In addition, an innovative anti-scale water treatment process is developing which is based on the findings of this experiment.
Subjects
cooling water
calcium carbonate
aragonite
calcite
nucleation
crystal growth
cluster
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-98-F91524073-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):b67c50483b5d7a27577ef8abdf87a89e