溶液性質對碳酸鈣晶體成長之影響

Effects of Solution Properties on Crystal Growth of Calcium Carbonate

林彥志(Yen-Chih Lin)

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(2005年)

https://doi.org/10.6342/NTU.2005.01181

摘要

近年來，在化學工業的程序上，流化床結晶槽(pellet reactor, crystalactor)技術佔有相當重要的地位，其應用在許多方面，例如：硬水軟化、重金屬去除、除磷、除氟等。就硬水軟化而言，為了利用流體化床來處理硬水，因此必須要找出流體化床中，碳酸鈣晶體的成長動力學模式；早期的文獻多為經驗式，如：總晶體成長模式，雖然此模式相當簡單，但是因為此模式的適用範圍受到限制，所以雙重阻力模式逐漸被採用來詮釋流體化結晶槽內的晶體成長模式。
因此為了探討溶液性質對於碳酸鈣晶體成長的影響，本研究以流體化床為實驗裝置，採用定組成法進行晶體成長實驗，接著利用雙重阻力模式的分析方式，將實驗結果的相對過飽和度與成長速率帶入計算，求得動力學參數之表面反應速率係數、質傳擴散速率係數。並且改變可能會影響晶體成長的變數：pH值、過飽和度、離子強度、離子活性比等，以探討各個變數對於晶體成長動力學參數的影響。另一方面以天然的文石晶體作為晶種，改變溶液的pH值，觀察pH值對於文石成長速率。
本實驗將以下條件固定不變：晶種平均粒徑774 μm、水流表面速度為0.047 m/s、溫度為298 K。為了探討溶液性質，『pH、I與R』，的影響，因此在特定的pH、離子強度與離子活性比下，藉由量測不同相對過飽和度所對應的成長速率，可以探討溶液性質對於成長速率、Kd與Kr的影響。
結果發現pH=8.5~9.5，I=0.005~0.035 M，R=0.4~5.54時，成長速率隨著pH和I值的增加而上升，但是當R約等於1時，成長速率有一最大值。
利用表面反應階數為2的雙重阻力模式分析成長數據時，所得到的質傳擴散速率係數不受pH和離子活性比的影響，但是會隨著離子強度的增加而提高；而表面反應速率係數則是隨著pH值和離子活性比的增加而上升，但是卻不受離子強度的影響。
然而在文石晶種的成長實驗中，pH值的操作範圍在8.3~10之間，隨著pH的上升，成長速率遞減，但是pH值介於9.5~10時，對於成長速率的影響並不顯著。

關鍵字

方解石 ； 定組成 ； 動力學 ； 雙重阻力模式 ； 文石

並列摘要

Recently a pellet reactor, which plays an important role in chemical engineering process, has been developed for water softening, fluoride and phosphate removal, and heavy-metal recovery. In the design of a pellet reactor for water softening, we need to know the kinetics of CaCO3 in a fluidized bed. Most of the reported growth-rate model of calcite is rather empirical, for example, an overall growth-rate model. Although the over-all growth model is convenient to use, the applicability of it was limited to a certain range of operating variables. Therefore, the two-step growth model was used to modify the growth kinetics of calcite in a pellet reactor.
In this research, the measurement of crystal growth of calcite in a fluidized-bed was designed by using a constant-composition technique, which can be used to study the effects of solution properties. The calcite growth-rate data was analyzed by the two-step growth model to obtain the kinetic parameters of surface-reaction coefficient and mass-transfer coefficient. The effects of operating variables, such as pH, supersaturation, ionic activity ratio, and ionic strength on the coefficients were studied. In addition, an experiment was conducted to study the influence of pH on aragonite growth rate.
In the calcite growth experiment, several operation variables were fixed, such as L = 774 μm, v = 0.047 m/s, and T=298 K, then the crystal growth rate data were measured at various supersaturation under specified pH, activity ratio and ionic strength. In the range of the pH value between 8.5 and 9.5, the ionic strength (I) between 0.005 and 0.034 M, and the activity ratio between 0.4 and 5.54, the calcite growth rate increase with an increase in supersaturation, pH value, and ionic strength, however, the growth rate data shows a maximum at an activity ratio around 1.
Finally the growth-rate data were analyzed by the two-step crystal growth model by assuming r = 2. The calculated mass-transfer coefficient is rather independent of pH and activity ratio, but varies with ionic strength, the surface-reaction coefficient increase with pH and activity ratio but rather independent of ionic strength.
In the experiments of aragonite crystallization, the growth rate decreases with an increase in the pH range between 8.3 and 10, however, the effect of pH is not significant for pH being between 9.5 and 10.

並列關鍵字

calcite ； constant composition ； kinetic ； two-step growth model ； aragonite

參考文獻

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