磁場對碳酸鈣各種晶型結晶之影響

Effects of Magnetic Field on the Crystallization of Calcium Carbonate Polymorphs

張孟淳(Meng-Chun 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/博士(2009年)

https://doi.org/10.6342/NTU.2009.02319

摘要

冷卻用水向來為工業用水之最大宗，故若能降低冷卻水使用量，將有助於舒解水資源匱乏之問題。冷卻水於熱交換系統內流動經冷卻塔時，會有部分水分蒸發損失，加以溫度及酸鹼值變化，導致微溶物系結晶於壁上形成水垢，其中以碳酸鈣最為常見，水垢會降低熱交換效率並引起管路堵塞，因此熱交換系統需採排水及補水措施，使水中各離子濃度維持在某一限度以下，如此即需消耗大量水資源。工業上以磁場抑制碳酸鈣結垢的方法已出現百年以上，但效果並不穩定，學術上由於缺乏精密的研究方法，且碳酸鈣有多種晶型，故文獻中磁場對碳酸鈣結晶影響之結論亦眾說紛紜。本研究採定組成法，於流體化床結晶槽內進行實驗，有效固定可能影響碳酸鈣結晶之各變數：pH值、離子強度(I)、相對過飽和度(σ)、離子活性比(R)等，探討二種不同磁場－帝斯卡磁能鈍水器（Descal-A-Matic DC-3，有效強度1800.0 Gauss）及永久磁鐵（MagneGen Model 100，有效強度212.6 Gauss），對碳酸鈣二種主要晶型－方解石及文石結晶所產生之效應，並比較作用方式及時間對磁場效能之影響。
晶體成長實驗結果發現，在未加入磁場作用時，方解石晶體可在碳酸鈣過飽和溶液中正常成長，而文石晶體則否；然而在磁場存在之條件下，方解石晶體成長會被抑制，過飽和度σcal < 2.0之溶液，方解石成長終將完全停止，然而文石晶體受磁後則逐漸開始成長，隨著時間成長速率加快，終於達一穩定值。與永久磁鐵MagneGen Model 100相比，帝斯卡磁能鈍水器Descal-A-Matic DC-3之效能可在較短時間內發揮，文石晶體在經Descal-A-Matic DC-3磁化90分鐘後之過飽和溶液中之成長速率，甚至高於在經MagneGen Model 100磁化20小時後之過飽和溶液中。對於同一磁場而言，作用方式不同也會產生不同之效應，MagneGen Model 100直接作用於文石晶種，其效果會比單純作用於溶液發揮得更迅速。在過飽和溶液充分受磁後，成長速率隨過飽和度上升而增快，此趨勢與文獻中方解石之成長實驗結果相同，但pH值及R值部分，文石之趨勢則與方解石相反，文石在pH低時晶體成長較快，而在R = 1.0處為最小成長速率。由成核實驗結果則可看出，無論磁場是作用於溶液成核前或是成核後，磁場均可提高晶核中文石所佔比例，Descal-A-Matic DC-3磁能鈍水器可使已形成之晶核由球霰石轉為文石，而未成核之過飽和溶液(σcal < 2.23)以MagneGen Model 100作用二天後，可使引發之初成核全數為文石。而在CaCO3粒子/過飽和溶液界面電位量測上，界面電位可定性地反應磁場對過飽和溶液產生的改變，但在定量上並非決定晶體成長速率最重要的因素。
由結晶理論中分子聚集體觀點出發，本研究提出一磁場作用之可能機制，可合理解釋多數文獻及本研究之實驗結果。另根據此機制而設計出一套嶄新之防垢程序，目前已進入先導型設備測試階段，與現行之防垢方法相比，預期可大幅降低排水、補水頻率。

關鍵字

冷卻水 ； 碳酸鈣 ； 文石 ； 方解石 ； 成核 ； 成長 ； 分子聚集體

並列摘要

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.
In 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.
From 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.

並列關鍵字

cooling water ； calcium carbonate ； aragonite ； calcite ； nucleation ； crystal growth ； cluster

參考文獻

施雅齡，磁場與雜質對文石成長速率之競爭作用，碩士學位論文，台大化工所，2007年

張名惠，在超重力系統中製備氫氧化鎂與氧化鎂奈米粉體，碩士學位論文，台大化工所，2004年

林彥志，溶液性質對碳酸鈣晶體成長之影響，碩士學位論文，台大化工所，2005年

劉鎮宗，以酒廠泥移除水中鉻銅鋅鎳之硏究，博士學位論文，台大農化所，2008年

Sugihara H., Anan T., Adachi K., Baba A., Egashira N., Nishiguchi H., Ishihara T., and Takita Y., “Synthesis of disk-like calcium carbonate (Part 2),” Journal of the Ceramic Society of Japan, 105, 953-958 (1997)