Effects of Magnetic Field and Solution Properties on the Crystal Growth of Calcium Carbonate Polymorphs
Date Issued
2011
Date
2011
Author(s)
Lin, Meng-Chieh
Abstract
Cooling water has always accounted for the largest proportion of industrial process water. So if we could properly and efficiently use cooling water, it would not only lower the operating cost but also alleviate the problem of water scarcity. In the industrial processes, the heat exchangers and boilers are usually operated at high temperature, so those sparingly soluble salts, calcium carbonate (CaCO3) in the majority, easily precipitate on the wall of units, which is so-called “scale.” The scale may decrease heat-transfer efficiency and block the piping system. To maintain the Ca2+ concentration below a specific level for preventing scale formation, water discharge and fresh water make-up measures are taken to result in a great loss of water. Magnetic water treatment device (MWTD) for anti-scaling 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 numerous variables which would influence the scale formation, the lack of precise research approach, and the variety of CaCO3 crystal structure. In this research, a constant-composition technique, which could fix the specific operating variables, such as Ca2+/CO32- activity ratio, pH, relative supersaturation, ionic strength and temperature. This approach was used to investigate the single variable and multi-variables effects on crystal growth of CaCO3. To confirm the mechanism of CaCO3 phase transformation, the EDTA was added in the solution to adsorbed the metal ions, which might be the medium of magnetic effect. Besides, an external-clamping magnetic device was tested to compare the efficiency with that of a commercial MWTD. The experiments were carried out in a stirred-tank crystallizer.
According to the experimental results, different seed sources were used and curing time were varied to see the effects on CaCO3 crystal growth. The crystal growth rate was greatly affected by the surface structure and the surface composition on the seed. Thus, the curing step is important to obtain reliable data.
At room temperature and in the absence of magnetic field, the CaCO3 crystal growth was significantly influenced by the Ca2+/CO32- activity ratio (R). No matter calcite or aragonite seed was used, the maximum value of crystal growth rate was obtained at the activity ratio of 1.0, but the calcite crystal growth rate is much higher than the other. When the solution temperature increased, the crystal growth rate of calcite was reduced, however; the overall trend of growth rate remained unchanged.
For the interactive effects of activity ratio and pH value on the calcite growth, the maximum calcite crystal growth rate occurred at the activity ratio of 1.0 for the pH value of 8.5 and 9.0, and the maximum calcite crystal growth rate shifted to the activity ratio of 2.0 for the pH of 9.5 and 10.0.
For the interactive effects of solution temperature and pH value on the calcite crystal growth, the crystal growth rate increased with the increase in the pH value at room temperature, and the maximum crystal growth rate occurred at the pH value of 9.5. As the solution temperature was increased to 35 ℃, the crystal growth rate still increased with the increase in the pH value, but the maximum growth rate disappeared. Besides, the pH effect on calcite crystal growth was weak at the higher temperature.
In the presence of magnetic field, the magnetic field caused the inhibition of calcite crystal growth, and the accelaration of aragonite crystal growth. Besides, the maximum value of calcite growth rate and the minimum value of aragonite growth rate occurred at the activity ratio of 1.0. Thus the magnetic field caused an opposite effect on the calcite and aragonite growth.
For the interactive effects of activity ratio, magnetic field and pH on the aragonite crystal growth, the crystal growth rate increased with the decrease in the pH value, and the trend was not influenced by the activity ratio and the magnetic field. When the magnetic field was absent, the aragonite growth rate at activity ratio of 1.0 was the highest, however; when the magnetic field was present, the crystal growth rate at activity ratio of 1.0 was slower than that the others. Thus the magnetic field effect was the weakest at the activity ratio of 1.0.
In the EDTA-adding experiment, the aragonite crystal growth rate decreased with increase in the concentration of EDTA, but the growth rate didn’t become zero. The experimental result means that the CaCO3 phase transformation occurred in the solution of none metal ions, and could confirm that the CaCO3 phase transformation caused by magnetic field, instead of rare metal ions in the solution.
Using the external-clamping magnetic device, the efficiency of the device increased when the gap width in the magnetic tube was narrow, the permanent magnets were inversely arranged, and the stainless was used as the tube material. As the fluid velocity increased, the efficiency was also improved, but there was an optimum velocity existing. The efficiency of external-clamping magnetic device is lower than the commercial magnetic device (Descal-A-Matic DC 1). However, these two types of magnetic device have similar effects on crystal growth of CaCO3.
Based on the ion clustering in the classical crystallization theory and the cluster transformation hypothesis, a novel concept related to the driving force of crystal growth is proposed. In the classical crystallization theory, the supersaturation of solution calculated by the activities of Ca2+ and CO32- is the driving force of crystal growth. Judged from the experimental results of this research, the driving force of crystal growth would be the concentration of cluster in the solution. The concentration of cluster might be a function of Ca2+/CO32- activity ratio (R), and increased with the increase in the limiting ionic concentration.
Subjects
crystal growth
calcium carbonate
calcite
aragonite
activity ratio effect
magnetic field effect
cluster
SDGs
Type
thesis
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