磁場、溫度與雜質對於文石及方解石晶體成長之影響

Effects of Magnetic Field, Temperature and Impurity on Crystal Growth of Aragonite and Calcite

張煥杰(Huan-Chieh Chang)

指導教授 ： 戴怡德

國立臺灣大學/工學院/化學工程學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.03514

摘要

在工業上利用磁場抑制熱交換管及鍋爐內壁的結垢已有多年的歷史，學界亦對磁能防垢的作用機制提出各種看法。但由於在研究上可探討的變因眾多，且研究方向各不相同，因此造成實驗結果頗多分歧。結垢的主要成份為碳酸鈣，而碳酸鈣的兩種主要晶型，文石與方解石，皆會隨著溫度的提升而降低溶解度，因此通常結垢的問題多發生在高溫的熱交換器或鍋爐中。此外，在冷卻水中多有不同的雜質存在，可能會對於結垢的形成有影響。故本研究將磁場有無、溫度高低以及亞鐵離子/鍶離子濃度視為實驗變因，探討單變因或多變因對於碳酸鈣成長的影響。
文石的晶體成長實驗發現，在未受磁的情況下，成長速率隨溫度增加而提升。在常溫下(25 ℃)，添加微量的亞鐵離子，成長速率隨濃度增加而提升，而添加微量的鍶離子則會使文石晶種表面成核出方解石，晶種成長速率隨鍶離子濃度的增加而提升。提高溶液溫度至35 ℃，成長速率仍隨亞鐵離子濃度增加而提升，且發現高溫與亞鐵離子濃度效應具有加成性；在鍶離子效應方面，發現低濃度區高溫成長速率大於常溫，而高濃度區趨勢則相反。
磁場作用下，文石成長速率隨著溫度的升高而增加。常溫下的磁場-亞鐵離子效應，成長速率隨著亞鐵離子濃度提升而下降，且晶種表面由針狀轉為平滑表面；常溫下的磁場-鍶離子效應，晶種成長速率亦隨著鍶離子濃度提升而下降，且晶種表面由針狀轉為珊瑚狀。提高溶液溫度至35 ℃，發現無論在亞鐵離子或鍶離子方面，皆會造成晶種成長速率下降，但當溶液中存在亞鐵離子時，高溫的成長速率會大於低溫；而當鍶離子存在時趨勢則相反。
方解石的晶體成長實驗發現，在常溫且磁場作用下，添加亞鐵離子，成長速率在低濃度區為提升，但高濃度區則下降；在鍶離子效應方面，無論在低濃度區或高濃度區，成長速率皆隨鍶離子濃度增加而提升；添加亞鐵離子/鍶離子皆不會改變晶種表面晶貌，仍維持方解石之菱面體。而提升溫度後，會使方解石的成長速率下降，但不會改變整體成長速率隨雜質濃度變化的趨勢。
綜觀而言，有無磁場作用下，雜質效應對於介穩晶型的文石成長速率及晶貌的影響，有很大的差異；但對於方解石而言，表面晶貌則不易受到雜質的影響而改變，多變因影響下的成長速率亦可由單獨變因對方解石成長速率的影響來預測。

關鍵字

晶體成長 ； 文石 ； 方解石 ； 磁場效應 ； 溫度效應 ； 雜質效應 ； 亞鐵離子 ； 鍶離子

並列摘要

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.

並列關鍵字

Crystal growth ； Aragonite ； Calcite ； Magnetic field effect ； Temperature effect ； Impurity effect ； iron(Ⅱ) ion ； strontium ion

參考文獻

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