Knowledge Management System Of Institute of process engineering,CAS
|Place of Conferral||北京|
|Keyword||铝氧水合物 反应结晶 介稳区 介稳中间体 结晶机理|
高纯氧化铝作为一种重要的高附加值化学品，在光学、新型发光材料、医疗、化工以及材料等领域用途十分广泛。本文主要研究铝-有机碱溶液反应结晶-诱导调控制备高纯水合氧化铝新工艺中铝-水反应、产物晶型、粒度调控等技术问题，以介稳区和介稳中间体调控以及反应结晶过程机理为科学问题，通过在线、离线手段对过程进行解耦分析，研究了铝-水反应机理与铝氧水合物结晶机理。主要取得如下结果：（1）首先研究了铝水反应动力学与有机碱界面作用机理，确定铝-水反应速率关于碱浓度、铝箔表面的反应级数为1，活化能为45.92 kJ/mol，此表观活化能低于其他活化方法表观活化能平均值（52.55 kJ/mol）。深入研究有机碱在活化铝水反应中的界面作用机理，发现有机碱的解离平衡在碱消耗和碱再生过程中起桥梁作用。（2）采用FBRM等在线监测设备研究了铝水反应结晶介稳区宽度（MSZW）的变化规律：碱浓度越高，MSZW越大；温度越高，MSZW越小。并且通过添加晶种等改变反应结晶介稳区宽度，制备出单一晶型的三水铝石型水合氧化铝，在60 oC下以二次成核为主，而在90 oC下以生长为主。（3）利用糖醇添加剂调控介稳中间体结构，通过宏观实验确定糖醇主要通过影响有机铝酸根的分解起到调控作用。并且通过软硬酸碱理论分析糖醇作用机制，确定糖醇具有弱酸的作用，而介稳中间体具有弱碱的作用，并且糖醇酸度越大，对三水铝石结晶抑制作用越强，更有利于一水软铝石的生成。（4）对由FBRM获取的在线弦长分布进行矩阵变化，将其转化为体系在不同时刻的粒度分布，确定了新的成核、生长速率测定方法。确定铝水反应结晶初级成核速率关于过饱和度的级数为2.86，活化能为95.31 kJ/mol。通过晶种诱导介稳中间体，使其仅发生生长过程，确定三水铝石生长速率关于过饱和度的级数为0.24，活化能为45.34 kJ/mol。（5）利用晶种等调控手段，使介稳中间体更倾向于在活化后的晶种表面上生长，从而能够获得大粒度（d4,3=30.905 μm）、窄分布的产物晶体。采用平推流反应结晶器将铝水反应结晶过程进行时空分隔控制，增加对反应结晶体系控制的自由度，改变反应结晶体系在不同时刻所处的热力学状态，强化成核过程动力学，获得小粒度（d4,3=4.712 μm）、窄分布的产物晶体。
High-purity alumina, as one of the important high-value-added chemicals, is widely used in optics, advanced luminescent materials, medical treatment, chemical engineering and materials etc. This dissertation concentrates on the technical problems about the Al-H2O reaction, product polymorph and crystal size control in the high-purity alumina preparation process by induced adjust and control of reaction crystallization of aluminum and organic alkali solution and takes the manipulation of metastable zone and metastable intermediates and the reaction crystallization mechanism as the scientific problems. The mechanisms of Al-H2O reaction and crystallization of alumina hydrates were investigated by decoupling the process through the on-line and off-line equipments. The following results were obtained:(1) The kinetics of Al-H2O reaction and mechanism of organic alkali interface interaction were investigated, with the reaction order of Al-H2O reaction rate with respect to alkali concentration and aluminum surface area to be 1 and the activation energy to be 45.92 kJ/mol which is lower than the average apparent activation energy of other activation methods being 52.55 kJ/mol. By studying the interface interaction of organic alkali in the activation of Al-H2O reaction, it is found that the dissociation equilibrium of organic alkali plays a role of bridge between the depletion and regeneration of alkali.(2) The metastable zone width (MSZW) of Al-H2O reaction crystallization is investigated by the application of on-line equipments such as FBRM and it is found that the higher alkali concentration can lead to the wider MSZW and the higher temperature can cause narrower MSZW. The single-phase gibbsite product was obtained by seeding to change the MSZW of reaction crystallization and it is concluded that the secondary nucleation dominates under 60 oC and the growth dominates under 90 oC.(3) The structure of metastable intermediates was adjusted by the addition of alditols. It is determined that the alditols adjust the process by affecting the decomposition of aluminate ions based on the macroscopic experiments. As for the interaction between the alditols and intermediates, the alditols act as the acid (electrophile) and the metastable intermediates act as the base (nucleophile) by analyzing the mechanism through Hard and Soft, Acids and Bases (HSAB) theory. The larger the softness of alditols, the stronger the inhabitation effect on the aluminum hydroxide crystallization and more favorable for the boehmite crystallization. (4) New methods of determining the nucleation and growth kinetics were developed by applying matrix transformation to the chord length distribution acquired by FBRM and converting them into the crystal size distribution at different time. The reaction order of primary nucleation rate with respect to supersaturation is determined to be 2.86, with the activation energy being 95.31 kJ/mol. The rate order of gibbsite growth rate with respect to the supersaturation is 0.24, with the activation energy being 45.34 kJ/mol by seeding to induce the metastable intermediates to conduct growth process only.(5) The crystal products of large size (d4,3=30.905 μm) and narrow distribution can be obtained by seeding to make the metastable intermediates more inclined to grow on the activated seeds. The plug flow reaction crystallizer was applied to control the Al-H2O reaction crystallization with the separation of time and space, increase the freedom degrees of controlling the reaction crystallization system, change the thermodynamic state of the system at different time, intensify the nucleation process and make it easier to acquire the crystal products of small size (d4,3=4.712 μm) and narrow distribution.
|王鸿奇. 铝氧水合物反应结晶介稳区调控与结晶机理[D]. 北京. 中国科学院研究生院,2016.|
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