磁場與雜質對文石成長速率之競爭作用

The Competing Effects between Magnetic Field and Impurity on Growth Rate of Aragonite

施雅齡(Ya-Ling Shih)

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(2008年)

https://doi.org/10.6342/NTU.2008.02444

摘要

以磁場抑制熱交換管及鍋爐內結垢的研究已有多年，關於磁場有預防結垢的功用，正反都有報導且意見頗為分歧，其原因可能甚複雜。而工業用水中通常含許有雜質，其對碳酸鈣成核及成長之影響相當顯著，所以雜質可能是原因之ㄧ。有鑑於此，本研究的目的在於探討雜質與磁場的競爭作用。
本實驗在改變溶液組成，例如亞鐵離子濃度，鍶離子濃度，活性比及過飽和度等探討磁場對文石成長的影響。研究方法是在攪拌槽中，利用帝斯卡磁能防垢器將溶液受磁二十個小時，確定其完全受磁，之後加入天然文石晶種並利用恆鈣離子定組成法來量測其成長速率。
在無磁場作用下，pH為9.0，離子強度為0.018M，離子活性比為5.54，溫度在25οC的條件下，過飽和度為0.73~1.42，文石成長速率接近零。但在同樣的操作條件下，有磁場時文石則會開始成長，其因推測為磁場使溶液中之分子聚集體(cluster)變為親文石狀態。
無磁場時，溶液中有亞鐵離子存在時，文石亦會開始成長，但在較高過飽和下，文石表面容易轉相為方解石晶型；溶液中有鍶離子則會使文石晶種表面轉相為方解石，且少量的鍶離子濃度就有顯著的影響。
磁場與亞鐵離子同時作用時，會使文石加速成長並在高過飽和度時會抑制表面轉相成方解石。有磁場時，亞鐵離子濃度達到0.5ppm時，即可以完全發揮作用，過量無益，且在此濃度下，雜質效應較磁場效應更大。高過飽和度1.42的操作下，磁場與亞鐵離子共同作用比僅有磁場無亞鐵離子時，成長速率可高至4.6倍，且晶種表面維持針狀之文石。
當溶液中有鍶離子時，在磁場作用下文石晶種之成長速率會比無磁場時緩慢許多。鍶離子存在時，磁場無法抑制晶種表面文石之轉相，而溶液中之分子聚集體為親文石狀態，所以成長緩慢。由SEM圖可知晶種在表面皆為轉相之過渡狀態，此期間所測得的成長速率在定量上亦較無意義。
在探討活性比對磁場作用之影響時，發現在R=1.00文石成長速率皆出現特殊情形，相比於R=5.54及R=0.40時成長速率較緩慢，由實驗結果猜測R=1.00為有利方解石之條件。

關鍵字

雜質 ； 磁場 ； 亞鐵離子 ； 鍶離子 ； 活性比

並列摘要

Magnetic treatment device has been installed to suppress the scale formation in heat exchanger and boiler for many years; however, positive and negative effects have been reported. This treatment technique remains controversial, because the scientific mechanism is unclear and may be complicated. The process water generally contains impurities, which would influence the nucleation and growth of CaCO3. For this reason, the aim of this study was to explore the competing effects between the magnetic field and impurity
This report presents a study of how the solution conditions, including iron concentration, strontium concentration, activity ratio, relative supersaturation influence the effectiveness of magnetic field on the growth of aragonite. The solution was pumped through the Descal-A-Matic DC-1 and then back to the tank for 20h to make sure it was fully magnetized. The aragonite growth rates in the jacketed stirred tank were measured using a constant composition method.
Under the conditions of pH=9.0, I=0.018M, R=5.54, T=25οC without magnetic field, the aragonite seeds growth rate approached to zero over a wide range of supersaturation (σara=0.73 to1.42). Under the same operating conditions, we found that aragonite started growing in the presence of magnetic field. It is conjectured that the structure of CaCO3 clusters in the supersaturation solution became similar to that of aragonite under the influence of magnetic field.
Without magnetic field and with the same operating conditions, we found that aragonite seeds started growing in the presence of ferrous ion (Fe(Ⅱ)) .But the surface of aragonite seed transformed into calcite morphology at high supersaturation and then stopped growing (i.e., σara> 1.33). In the presence of a small quantity strontium (Sr+2), the aragonite seed surface transformed to calcite and its growth rate is similar to that of calcite.
Under the magnetic field, the Fe(Ⅱ) will cause a synergetic effect on the crystal growth of aragonite even at high supersaturation. At σ=1.42, the increasing of aragonite growth rate was about 4.6 fold of that obtained without Fe(Ⅱ). Under the influence of Sr+2, the growth rate of aragonite was significantly lower when the magnetic field was present. It is because the magnetic field were unable to suppress the conversion of aragonite to calcite, and the structure of clusters is similar to aragonite.
In the presence of magnetic field, the aragonite growth rate still exhibited a minimum value at R=1.00 as the case without magnetic field. These experimental results suggest that R=1.00 is a condition that favors the formation of clusters with a structure that is similar to calcite in a supersaturated solution.

並列關鍵字

impurity ； magnetic field ； Fe+2 ； Sr+2 ； activity ratio

參考文獻

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