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  • 學位論文

磁場與溶液性質對不同晶相碳酸鈣晶體成長之影響

Effects of Magnetic Field and Solution Properties on the Crystal Growth of Calcium Carbonate Polymorphs

指導教授 : 戴怡德

摘要


冷卻水向來佔工業用水的最大宗,故若能夠適當且有效地利用冷卻水,除了能降低成本以外,將有助於紓解水資源匱乏的問題。在工業程序中,較高溫度的熱交換器及鍋爐容易造成微溶物系結晶於壁上形成水垢,其中以碳酸鈣(CaCO3)最為常見,水垢會降低熱交換器的效率並引起管線堵塞,因此需要經常排水及補水以維持水中鈣離子濃度在某一限度以下,因此需要消耗大量水資源。工業上以磁場方法抑制碳酸鈣結垢已出現百年以上,磁能防垢方法雖然有低成本、容易操作及對環境友善等優點,但其效果時有時無,學術上由於影響結垢的變數眾多,且研究的方法各不相同,再加上碳酸鈣擁有多種晶相,導致文獻中磁場對碳酸鈣結晶影響之結論眾說紛紜。本研究採用定組成方法,於攪拌結晶槽內進行碳酸鈣晶體的成長實驗,有效控制影響碳酸鈣晶體成長之各變數,包括Ca2+/CO32-離子活性比(R)、pH值、相對過飽和度(σ)、離子強度(I)與溫度。在實驗中探討單一變數之影響或多變數之交互作用對晶體成長之影響,並以添加金屬螯合劑EDTA之實驗確認碳酸鈣的磁場轉相機制;另外,再測試外夾型磁能裝置對碳酸鈣晶體成長之影響,並與商業用磁管(帝斯卡磁能鈍水器)之效率做比較。 由不同來源的晶種成長實驗以及不同晶種預處理時間之晶種的成長實驗,發現晶體成長速率會隨晶種表面的結構及組成而變,故為獲得可信度高的成長速率數據,晶種預處理步驟很重要。 在常溫且無磁場作用之下,溶液的Ca2+/CO32-離子活性比(R)對碳酸鈣晶體成長有很大的影響,方解石及霰石晶體成長速率均於離子活性比為1.0時出現極大值,但方解石成長速率遠大於霰石成長速率;若提升溶液溫度,方解石晶體成長速率隨離子活性比變化的趨勢不變,但成長速率較慢;在離子活性比與溶液pH值間之交互作用方面,發現當pH值為8.5及9.0時,方解石晶體成長速率的極大值出現於離子活性比為1.0,而隨著pH值提升至9.5及10.0時,成長速率的極大值則位移至離子活性比為2.0;而在溶液溫度與pH值之交互作用方面,則發現在常溫時方解石晶體成長速率隨pH值增加而提升,並且pH值為9.5時出現一極大值,當溫度提升至35 ℃時成長速率亦隨pH值增加而提升,但提升的幅度不大,且未見極大值存在。 在磁場作用下,磁場會抑制方解石晶體成長與促進霰石晶體成長,且當離子活性比為1.0時,方解石晶體成長速率依然出現極大值,而霰石則出現極小值,由此可見磁場作用會對方解石及霰石晶體成長造成完全相反的效應;在離子活性比、磁場及pH值間的交互作用對霰石晶體成長之影響方面,發現霰石晶體成長速率均隨pH值下降而提升,不受離子活性比及磁場有無的影響,即低pH值條件為霰石晶體成長的有利條件;另外,發現無磁場作用時,R值為1.0之晶體成長速率較大;但當磁場作用加入時,則是R值為1.0之晶體成長速率較小,磁場提升霰石晶體成長的效果較差,即磁場作用的效果在趨近R值為1.0時無法發揮。在磁場作用下,添加金屬螯合劑EDTA對霰石晶體成長的實驗中,其晶體成長速率隨EDTA濃度上升而下降,但未完全降至零,代表在溶液中完全排除金屬離子的條件下,碳酸鈣仍在轉相,可確認造成碳酸鈣轉相的原因為磁場作用,而非受到溶液中微量金屬離子影響而轉相。 在測試外夾型磁能裝置方面,發現磁管內部縫隙寬愈窄、永久磁鐵以交替式排列且管線採用不銹鋼材質,其磁場效率較好;當流經過磁場的流速愈快,磁場效率亦愈好,但當流速大於某一特定值之後,即不再影響磁管效率。外夾型磁能裝置的效率較帝斯卡磁能鈍水器差,但二種磁場對碳酸鈣晶體成長的影響相同。 以結晶理論中分子聚集體觀點與分子聚集體轉相假說為基礎,本研究提出一嶄新的觀念,即以往觀念中,晶體成長之驅動力是以溶液中Ca2+及CO32-的活性表示之過飽和度,但實際之驅動力可能是來自溶液中各別型態之分子聚集體濃度,而分子聚集體濃度可能隨Ca2+/CO32-離子活性比變化,即驅動力可能隨溶液中限量離子濃度增加而提升。

並列摘要


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.

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