Alumina trihydrate crystallization. Part 1. Secondary nucleation and growth rate kinetics

The kinetics of crystallization of alumina trihydroxide (hydrargillite) from caustic aluminate solutions of industrial composition (second stage BAYER process) were studied in a laboratory batch isothermal crystallizer. The yield and crystal size distribution of precipitate with time were obtained by varying the initial seed concentration. In this paper, the mass balance was used to determine nucleation and growth rate kinetics. First, from among the laws available in the literature, those which applied to alumina trihydrate crystallization were defined (model A). Kinetic constants of this model were calculated from experimental results. Then, a model based on a secondary nucleation mechanism was proposed (model B) that fits experimental data, and explains the induction periods found in the decomposition of aluminate solutions, even when seeds are present.     

Smoke reduction for flexible PVC-high surface area: Alumina trihydrate vs. magnesium hydroxide

This paper reviews three flexible PVC formulations using various types of alumina trihydrate (ATH) and magnesium hydroxide [Mg(OH)₂]. Smoke values are compared. In two of the formulations, Mg(OH)₂ has better smoke values than ATH; in a third formulation, ATH has better smoke values than Mg(OH)₂. However, the third formulation uses an ATH with a very high surface area, which may indicate that smoke suppression may be a function of not only the type of flame retardants used but also surface area.     

General treatment of polymer crystallization kinetics—Part 2. The kinetics of nonisothermal crystallization

The isothermal macrokinetic equation of polymer crystallization has been generalized to cover nonisothermal situations as well. The process of nonisothermal crystallization of poly(caproamide) has been studied experimentally. It is shown that the macrokinetic equation of the autocatalytic type can be used to solve both direct and inverse problems of nonisothermal polymer crystallization. Use of the inverse approach has made it possible to establish the value of the constants of the new kinetic equation. The general statement of the problem of nonisothermal crystallization of polymers in an inhomogeneous temperature field is considered. Inhomogeneous temperature and conversion fields have been calculated for model products of different sizes and configurations.     

Modelling of multiple-mechanism agglomeration in a crystallization process

A three-level agglomeration model coupled with crystal growth is developed. It accounts for Brownian, laminar, and turbulent agglomeration. The desupersaturation profiles, the particle size distributions, the average sizes, and variances (or standard deviations), as well as the instantaneous agglomeration degrees for each mechanism, can be calculated as functions of time. The model is applied to the crystallization of an amorphous solid into a crystalline polymorph in a batch crystallizer. A runaway phenomenon is detected for agglomeration when crystals are switching over from the Brownian regime to the laminar one: this switchover significantly affects the desupersaturation curve and the crystal shapes.     

The coarsening of Bayer alumina trihydrate by means of crystallization modifiers

Modern aluminium smelting practice requires that Bayer plants produce coarse, sandy non-dusting alumina. However, the coarseness of Bayer products is normally achieved at the expense of lower liquor productivities. In the course of work carried out on the so-called ‘poisons” of alumina trihydrate seed, it was discovered that minute additions of fine calcium carbonate to Bayer seed slurries at the time of seeding can considerably decrease the fine fraction in the resulting final products without affecting the yield of alumina. The action of calcite in the precipitation is that of a crystal-growth modifier resulting in an improved particle agglomeration and making the agglomerated particles in the products more resistant to breakage in mixing and pumping operations.     

Alumina trihydrate in flexible PVC—effects of alumina particle morphology

Recent increased use of ultrafine mechanically ground alumina trehydrate (ATH) as a flame retardant and smoke suppressant has occurred. This growth has, to some extent, displaced the traditional fine precipitated forms of ATH in a variety of polymer applications. In general, the morphology of the more expensive precipitated ATH is different from ground ATH. In this study there appears to be little effect of ATH particle morphology on the PVC performance properties tested.     

Aluminum Oxide and Alumina Ceramics (Review). Part 2. Foreign Manufacturers of Alumina Ceramics. Technologies and Research in the Field of Alumina Ceramics1

Refractories and Industrial Ceramics - This paper contains information on foreign manufacturers of alumina ceramics, and mainly, fine technical ceramics. The data concerning mechanical, thermal,...     

Polyamide-based composite membranes: Part 2. Interaction, crystallization and morphology

In this work, the interaction, crystallization and morphology of membranes derived from polyamides/poly (vinylalcohol) (PA/PVA, PA: PA66, PA69, PA610 and PA612) blend materials are studied at various weight fractions and various crystallization temperatures. The experimental work includes differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), polarized optical microscopy (POM) and scanning clectron microscopy (SEM). The equilibrium melting temperatures of polyamides in the blends are obtained using Hoffman-Weeks plots, and the interaction parameters are calculated using the Nishi-Wang equation, which is based on the Flory-Huggins theory. The values of the Flory-Huggins interaction parameters χ₁₂ are negative. Significant upward shifts of ν_NH are observed with increasing volume fraction of PVA. The morphology of membranes is impressively complex, diverse, and irregular. All compositions that show nodular morphology are in the μm-scale and the size of the nodules increases with increasing PA6 content. Large void cavities are observed in the substructure for the lower polymer concentrations.     

超声波对三水醋酸钠相分离及结晶的影响

The effects of ultrasonic radiation on phase separation as well as the crystallization of sodium acetate trihydrate,a typical salt hydrate phase change material (PCM),were studied experimentally.It is shown that ultrasonic radiation may inhibit phase separation to a certain degree.The influence of ultrasonic radiation on crystallization is also found.The addition of PCM grain can inhibit supercooling effectively.The crystallization is related to the supercooling degree.Furthermore,the effects of ultrasonic radiation on inhibiting phase separation of salt hydrates are discussed based on theoretical analysis.     

A mathematical model for diffusion with induced crystallization: 2

This article extends the analysis of a mathematical model for solvent induced crystallization. By evaluating the model's parameters for polymer/penetrant systems studied previously we find the predicted behaviours to be consistent with those determined experimentally. Fitting a limiting solution to solvent transport data in poly(ethylene terephthalate) films gives the threshold concentrations for crystallization and the penetrant diffusivity in the amorphous component of the highly swollen polymer. Numerical solutions determine the model's behaviour in intermediate regions where analytical solutions do not apply. These predict negative curvature in plots of weight gain versus √t and a distinct crystallization front behind a swelling boundary; the former results from the rapid crystallization of swollen surface layers, while the latter indicates partial decoupling between solvent transport and polymer crystallization. We also analyse briefly the desorption process following solvent induced crystallization; the relative magnitude of initial sorption and desorption rates depends on the induced crystallinity and the crystallization rate.     

聚集结晶动力学研究方法的改进

在Hounslow工作的基础上,推导了新的聚集机理,改进了聚集模型。通过粒数衡算和质量衡算,结合改进的聚集模型建立了聚集结晶动力学辨识的新方法。文中以头孢哌酮钠结晶过程为例,应用新方法确定了该物系的晶体生长、聚集以及成核动力学。结果表明新方法研究聚集结晶动力学具有一定的可靠性。     

Oxide and Alumina Ceramics (Review). Part 3. Russian Manufacturers of Alumina Ceramics1

Refractories and Industrial Ceramics - This review provides the analysis of aluminum oxide, including the properties associated with its use, types of commercial products, production methods...     

Simulation and analysis of industrial crystallization processes through multidimensional population balance equations. Part 2: a study of semi-batch crystallization

A bi-dimensional population balance model was presented in the previous part of this series of papers to simulate the time variations of two characteristic sizes of hydroquinone particles during crystallization. The multidimensional population balance equations combined with kinetic models and mass balance equations were shown to allow the simulation of the solution crystallization of hydroquinone characterized by a rod-like habit. Semi-continuous isothermal operations were performed at the lab-scale in the presence of various additive concentrations. Both the experimental solute concentration trajectory and the final bi-dimensional crystals size distribution were correctly predicted by the model. The simulated elongation shape factor characterizing the crystal shape was therefore in agreement with the experimental one. Due to the use of tailor-made additive, inhibition effects were observed to affect both primary nucleation and growth kinetics in the length direction. For secondary nucleation, indirect effects were assumed to occur which allowed satisfactory predictions of the final number of fine particles. The representation of the kinetics involved required the evaluation of a set of nine parameters. As a result it was observed that the elongation ratio characterizing the shape of the rod-like particles increases with the length in a nonlinear way. A major interest of the two-dimensional model lies in its ability to relate the time variations of the crystal habit: the particles lengthen in the first moments of their growth and then progressively get thicker until the end of the process.     

Part 2. Quantitative interpretation of model compound sulfonation

The mechanism and kinetics of sulfonation of two PEEK model compounds in concentrated sulfuric acid were studied. The first model compound (monomer) is representative of a PEEK repeat unit. The second one is a PEEK dimer. Because both model compounds are fluoroarylketone‐ended, the sulfonation reactions were followed by ¹⁹F NMR. The sulfonation reactions were treated following a first‐order kinetics with respect to the unsulfonated PEEK repeat unit. The comparison between the monomer and dimer results showed two different types of sulfonation behaviour. If only one apparent kinetic constant is considered for monomer sulfonation, two kinetic constants were taken into account to describe the dimer sulfonation process. This is due to a rate slowing effect of a sulfonic acid group on the sulfonation of the adjacent PEEK repeat unit. Furthermore, the results also confirm the rate slowing effect of the fluorine atom on the sulfonation of the neighbouring PEEK repeat unit. © 2001 Society of Chemical Industry     

球形氯化钾结晶过程与聚结机理研究

针对球形氯化钾结晶过程与聚结机理不明的问题,以冷却结晶法通过过程取样制备与表征了两种不同形貌的氯化钾在不同结晶阶段的晶体形貌与粒度分布特征,从而对比分析了球形氯化钾的结晶过程。并从结晶热力学、经典浸润理论和Lifshitz-Van der Waals酸碱理论出发解释了球形氯化钾形成的主要机理是添加剂的添加减小了结晶体系的介稳区宽度及增大了晶体颗粒在结晶溶剂中的粘附自由能,导致结晶体系更容易以聚结的方式实现球形结晶。介稳区宽度减小和晶体颗粒在结晶溶剂中粘附自由能增大的同步调控有助于实现氯化钾的球形结晶。     

饲料级磷酸氢钙结晶过程中团聚的研究

在反应结晶过程中,一般过饱和度很大,很容易形成大量细小颗粒,这些小颗粒相互作用形成团聚体,会影响最后产品的粒度分布、纯度、晶体形貌、堆密度等参数。饲料级磷酸氢钙生产过程中极易形成团聚体,通过探索团聚体形成的pH范围,对团聚体形成的原因做了分析,并研究了中和条件对团聚形成的影响。结果表明,形成团聚体的pH范围为2.82~3.60。此外,提高反应温度、增加搅拌速度均对团聚体有减缓效果。实验最后建立了磷酸氢钙团聚尺寸模型。     

Dynamic modeling of a batch crystallization process: A stochastic approach for agglomeration and attrition process

This paper deals with the dynamic modeling of a batch crystallizer. A complete model taking into account primary and secondary nucleations, crystal growth, agglomeration and attrition mechanisms is established. The proposed model is not restricted to binary agglomeration and breakage phenomena. From markovian considerations, continuous kernel functions are built and the basic balance equations are then presented. The complete model is solved using a finite difference method for the discretization of the size variable. As to distinguish agglomeration and breakage parameters from the others, on line measurement of the Crystal Size Distribution is necessary, a new on line measurement strategy is proposed. Finally, simulations of the crystal size distribution are compared with experimental results at different times. It appears that simulated curves are in good agreement with the experimental data.     

The influence of organic additives on the crystallization and agglomeration of gibbsite

During the crystallization of gibbsite Al(OH)₃, which leads to the synthesis of alumina in the Bayer process, crystals go through a step of agglomeration. In this work, we present a study concerning the influence of different organic compounds, polycarboxylic acids and polyalcohols on the crystallization and agglomeration of gibbsite crystals. It has been determined that they act as crystal habit modifiers and inhibitors of the agglomeration occurring during the formation of alumina crystallites. Simply by following the kinetics of the reaction, it has been observed that polyalcohols are stronger inhibitors than polycarboxylic acids, which can be linked to the structure of gibbsite crystals. The influence of the characteristics of the additive, such as the number of carboxylic groups and the distance between them for polyacids, the stereochemistry and the length of the carbon chain in the case of polyalcohols, is demonstrated to have a significant effect.     

Simultaneous preferential crystallization in a coupled batch operation mode. Part II: Experimental study and model refinement

In the previous part of this series a parametric study was presented to assess theoretically the performance of a novel crystallizer configuration. Complementary, the present work addresses a comprehensive, systematic experimental examination accompanied by an analysis of some relevant process parameters (mass of seeds, CSD of seeds, starting point for the mother liquor exchange, process duration) of two general concepts for preferential crystallization in which the amino acid threonine dissolved in water was used as a model system. In the first step of investigations, the impact of these process parameters was studied for conventional single batch crystallization. The experimental results are discussed and compared with the theoretical ones. Moreover, a configuration consisting of two coupled crystallization vessels was investigated. The specific manipulation of the concentration profiles of each enantiomer in solution as well as of the temperature profiles was found to be suitable to enhance the process performance. This is reflected by higher achievable process productivities and higher product purities.