藉由三水鋁石熱行為探討主導動力學曲線模型對於複雜反應之適用性

A Study on the Applicability of Master Kinetics Curve Model to the Complex Reactions of the Thermal Behaviors of Gibbsite

莊人豪(Ren-Hao Chuang)

指導教授 ： 鄧茂華

國立臺灣大學/理學院/地質科學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202104275

摘要

本研究利用主導動力學曲線模型研究三水鋁石複雜的熱行為，進而探討如何提升主導動力學曲線模型的分析能力。三水鋁石在地質上為鋁土礦的組成礦物之一，會因區域變質作用而轉變為軟水鋁石、一水硬鋁石、剛玉等其他含鋁礦物，過去曾有文獻藉此探討區域變質帶的地體構造演化。而在工業領域，三水鋁石熱分解脫水後的產物為氧化鋁，因此具有相當廣泛的應用，如製備鋁金屬、耐火陶瓷、催化劑載體等。然而，三水鋁石在應用上亦有其難處，即粒徑較大者(> 1 μm)會同時形成軟水鋁石與χ-氧化鋁，且兩者為不同的反應途徑。前人研究認為是由於粒徑較大的三水鋁石熱分解形成之水蒸氣難以逸散，因而累積在晶體內部形成有利軟水鋁石生長的環境，但對於確切的熱分解機制與動力學參數等研究結果仍有分歧。對此，本研究藉由主導動力學曲線模型嘗試描述三水鋁石的熱行為，此模型具有使用方便、預測準確等優點，不須針對理想條件作出假設與限制，亦無須事先得知反應機制。本研究團隊已確認主導動力學曲線模型可應用於燒結、相變、氧化、黏土礦物脫水、熱分解等多種領域，然而此模型目前仍無法用於分析複雜反應，導致其應用性受到侷限。故本研究以三水鋁石在238-263℃持溫的熱分解數據為基礎，比較粗粒(90 μm)與細粒(1.2 μm)樣本的熱行為與動力學分析結果，探討導致模型擬合不良的原因並嘗試加以改善。總反應的擬合結果如預期般不佳，因此首先藉由數值模擬的方式將兩種反應分離，求得粗粒三水鋁石熱分解形成軟水鋁石之視活化能值為121.4 kJ/mol。針對三水鋁石熱分解形成χ-氧化鋁之反應數據的擬合不良，本研究利用等轉換法發現視活化能值會隨反應的進行而逐漸降低，其中粗粒樣本約為115-130 kJ/mol，細粒樣本則為135-147 kJ/mol。對於三水鋁石熱分解形成χ-氧化鋁時視活化能值為何會隨反應比率而變化，本研究計算三水鋁石熱分解產物的組成，得知該χ-氧化鋁產物仍帶有部分水分子，且藉由脫水實驗發現在350-450℃持溫時，最大脫水量與持溫溫度呈現正相關。因此推測經三水鋁石熱分解形成之χ-氧化鋁所帶有的水分子並非均勻分布，水分子會以不同形式和強度的鍵結與χ-氧化鋁結合，導致總反應可能由無數個具有不同動力學參數的平行反應所組成，並生成帶有不同比例水分子的χ-氧化鋁。

關鍵字

三水鋁石 ； 熱行為 ； 反應機制 ； 主導動力學曲線模型 ； 可變活化能

並列摘要

This study analyzes the complex thermal behaviors of gibbsite by Master Kinetics Curve model (MKC), and in turn by using the results to improve and broaden the applicability of the MKC model. Gibbsite is one of the common constituent minerals of bauxite; it can easily be transformed into boehmite, diaspore, and corundum through regional metamorphism. In industry, gibbsite has a wide range of applications in producing many kinds of alumina-related materials. Previous studies suggested that the water vapor, formed by the thermal decomposition of gibbsite and trapped in larger gibbsite particles (about > 1 μm) during heating, is the major cause of the production of boehmite. Therefore, there could be two parallel reaction paths to produce boehmite and χ-alumina separately. In our earlier studies, we demonstrated that the MKC model can be applied to the analysis of many reactions, such as sintering, phase transformation, oxidation, and dehydration reactions, and showed that the model has several advantages such as easy to use, accurate prediction, and universal applicability. In this study, we use the MKC model, not surprisingly, to derive a poorly fitted model of the complicated thermal decomposition of gibbsite. After comparing the results of coarse-grained (90 μm) and fine-grained (1.2 μm) gibbsite samples in 238-263℃, we first separate the two different reactions by numerical simulation, and calculate the apparent activation energy of gibbsite→boehmite reaction, i.e., 121.4 kJ/mol. Then we use the isoconversion method to calculate the variation of apparent activation energy during the gibbsite→χ-alumina reaction, i.e., about 115-130 kJ/mol for the coarse-grained samples, and 135-147 kJ/mol for the fine-grained samples. Though the χ-alumina should not contain any water, through the weight loss data we found that there were still some water molecules in the χ-alumina product. Dehydration experiments by heating the thermal decomposition products of gibbsite in 350-450℃ show that the max dehydration is positively related to temperature, thus implying that water molecules may bind to χ-alumina with different forms and strengths. We hypothesize that the total gibbsite→χ-alumina reaction may consist of several parallel reaction paths where the gibbsite decomposes into different χ-alumina with various proportions of water molecules.

並列關鍵字

gibbsite ； thermal behaviors ； reaction mechanism ； Master Kinetics Curve model (MKC) ； variable activation energy

參考文獻

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