Effects of Magnetic Field, Temperature and Impurity on Crystal Growth of Aragonite and Calcite
Date Issued
2010
Date
2010
Author(s)
Chang, Huan-Chieh
Abstract
The magnetic treatment device has been used to suppress the scale formation on heat-exchanger pipes and boiler walls for a long time. The academia has also brought up various views on the mechanism of scale prevention caused by magnetic field. However, the conclusion of scale prevention becomes a controversial issue because of the numerous control variables and different experimental technique. The main composition of scales is calcium carbonate, both of its two major polymorphs, calcite and aragonite, have a solubility that decrease with rising temperature. Hence, the scale formation usually takes place in the high temperature units, such as heat exchangers and boilers. Besides, there are many different impurities exist in processing water, thus affect the scale formation. In this research, we took magnetic field, temperature levels and different impurities into consideration to observe the influence of one or more variables on the growth of calcium carbonate.
In the series of aragonite growth experiment, we found that the aragonite would not grow at room temperature without magnetic treatment; however, at higher temperatures it started to grow and the growth rate increased with an increase in temperature. At room temperature the addition of iron(Ⅱ) ion could make aragonite grow, and the aragonite growth rate increased with increasing concentration of iron(Ⅱ) ion. On the other hand, the addition of strontium ion would induce calcite nucleate on the aragonite crystal seed surface, and the seed growth rate enhanced with increasing concentration of strontium ion. Rising temperature to 35 ℃, the iron(Ⅱ) ion would further improve the aragonite growth rate. Thus, temperature and iron(Ⅱ) ion had a synergetic effect on the aragonite growth. As far as the effect of strontium ion effect is concerned, the aragonite grew faster at higher temperatures in the low concentration region, but an opposite trend was observed in the high concentration region.
In the presence of magnetic field, rising temperature could improve the aragonite growth rate. When the temperature was held at 25 ℃, the aragonite growth rate decreased with increasing impurity concentration in the presence of magnetic field, either the impurity was iron(Ⅱ) ion or strontium ion. However, the surface structure of aragonite seed crystals transformed from needle-like form to plane form in the presence of iron(Ⅱ) ion, while it transformed to coral-like form in the presence of strontium ion. When the temperature was raised to 35 ℃, the growth rate of aragonite was faster than that at room temperature in the presence of iron(Ⅱ) ion, but the trend of growth rate was opposite in the presence of strontium ion.
When it comes to the growth experiment of calcite crystals in the presence of magnetic field, the calcite growth rate increased in the low iron(Ⅱ) ion concentration region up to 0.1 ppm at room temperature, and then decreased for a further increase in iron(Ⅱ) ion concentration. As to the strontium ion effect, the calcite growth rate would enhance with increasing concentration in both low or high concentration regions. Unlike the aragonite crystals, the surface structure of calcite seed crystal still maintained as their original rhomboidal form even adding either iron(Ⅱ) ion or strontium ion. Rising temperature only made the growth rate of calcite decrease, but would not change the trend of growth rate curve.
Overall, in the absence or presence of magnetic field, the impurities show a great influence on the growth rate and crystal morphology of meta-stable aragonite seed. However, for the stable form of calcium carbonate, calcite, the crystal surface will not be changed by impurities, and the growth rate affected by multiple variables can be predicted qualitatively by the growth rate of single variable effect.
Subjects
Crystal growth
Aragonite
Calcite
Magnetic field effect
Temperature effect
Impurity effect
iron(Ⅱ) ion
strontium ion
Type
thesis
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