蒸發岩礦物熱分解反應動力學之研究方法與應用探討

Abstract

中文摘要 礦物的反應動力學在地質學領域中是相當受重視的研究課題，本研究中採用主導曲線模型(MCM)、等反應量法(Ozawa法)以及Avrami Equation等三種動力學方法分析蒸發岩礦物的熱分解反應，並嘗試擬合出各反應之主導曲線以描述其反應過程之溫度、時間與反應歷程。 實驗設計上可分為三部分，首先準備符合石膏、方解石、膽礬、鈉硝石與水菱鎂礦等五種礦物成分之試藥等級粉體，再以XRD確認其礦物相是否正確；第二部分以熱重分析儀進行等升溫與等溫條件熱分解實驗；最後以三種動力學方法分析擬合熱重分析所得到的反應數據。根據實驗分析結果除了可以瞭解反應的特性與溫度、時間對反應變化量的關係以外，還可進一步探討三種動力學方法應用於熱分解反應動力學研究的適用性與其限制。 從研究結果來看，以等升溫條件進行之熱分解實驗可能由於儀器內部的溫度量測誤差而導致根本無法得到正確的分析結果，然而過程中也發現可以透過等溫熱分解實驗克服此問題。從三種動力學方法所得之分析結果除了可比較這些熱分解反應的機制與動力學特性以外，也證實透過各反應的主導曲線可以準確描述蒸發岩類礦物熱分解反應之溫度、時間與反應變化量的函數關係，並預測這些熱分解反應之反應過程。此外，本研究的結果進一步證實除了溫度、時間與成分等三個因素以外，由於表面積效應的影響，礦物顆粒之粒徑也會造成熱分解反應在反應機制上的差異而影響反應速率，粒徑越小則反應速率越慢。本研究最後一部分工作是利用蒸發岩礦物熱分解反應之主導曲線，提出「動力學反應地質溫度計」之概念，此概念說明動力學模型除了可預測各種反應函數的連續變化以外，更可能可以進一步應用於地質研究中，用來推定溫度與時間條件。 總結本研究之成果，主要貢獻可分為三部分：第一部份為透過動力學分析結果表現出蒸發岩礦物的反應特性、熱穩定性以及反應變化量對溫度與時間的函數關係；第二部分則是藉由蒸發岩礦物熱分解反應之分析，可以比較出MCM、等反應量法與Avrami Equation的適用性、應用限制與誤差；最後透過本研究提出動力學反應地質溫度計之概念，期望未來能夠實際應用於地質研究。本研究為一初步研究，未來可針對其它礦物的動力學反應進一步探討這些動力學方法的適用性，並設計新的實驗方法以得到更好的實驗與分析結果。

ABSTRACT Kinetic research of minerals has been a very important area in the research of geology. In this research, three kinetic methods ─ Master Curve Model (MCM), Ozawa method and Avrami Equation method ─ have been used to analyze and predict the thermal decomposition reactions of five evaporite minerals. The research consists of three parts: First is to prepare the five high-purity synthetic evaporite minerals powders, including gypsum, calcite, chalcanthite, nitratine and hydromagnesite, and to verify their phases and purities by XRD. Second is to conduct a series of thermal decomposition experiments of the powders by TGA at various heating rates and isothermal condition. Third is to analyze the weight loss data by the three kinetic methods. The application limitations of MCM, Ozawa method and Avrami Equation on the study of thermal decomposition reactions will be discussed on the basis of the experimental results. From the experimental results, constant heating rate experiments show a significant deviation of the data due to the possible temperature-measuring errors inside the Thermal Gravitometry Analyzer. Fortunately, isothermal experiments can effectively reduce the uncertainty of the data and thus derive adequate results. In this research, MCM demonstrates that it is excellent in the prediction of reaction process. The effect of grain size on reaction rate is shown in this work that smaller grain size samples may lead to a higher reaction rate, as a result of the “surface-area effect.” The final part of this research is to propose a new concept of “geothermometer,” which is based on the modeling of the kinetic reactions, and it may become a powerful tool to find out the possible temperature and time conditions of many geological processes. To sum up, the research contributes in three aspects: First, using three kinetic models to characterize the thermal decomposition reactions and the thermal stability of five evaporate minerals, and most importantly to derive the quantitative relationship of the reaction percentage and the reaction time and temperature. Second, learning the applicability, the limitation and possible error levels of MCM, Ozawa method and Avrami Equation, based on the analysis results. Finally, proposing a new concept of “geothermometer,” and which may be very useful in geologic research. The results of this preliminary study may be extended to other kinetic reactions of various minerals.