An agglomeration efficiency model for gibbsite precipitation in a turbulently stirred vessel

Gibbsite crystal agglomeration is a critical size and morphology control operation in the commercially important Bayer process for producing alumina. There has long been a need for a mathematical gibbsite agglomeration model that is both capable of describing the process and is easy to implement. In this paper, a simple model is presented for the agglomeration efficiency of gibbsite crystals during precipitation from seeded sodium aluminate solutions in a turbulently stirred vessel. The model has been developed by viewing agglomeration as a combination of reversible and irreversible pseudo-reaction steps, involving the sub-processes of particle transport, collisions, capture, aggregate rupture, and aggregate cementation. A range of models for these sub-processes were trialled. The model was tuned using gibbsite precipitation data from an extensive experimental program covering a wide range of temperatures, supersaturations, particle sizes, shear rates and solids concentrations relevant to the Bayer process. The resultant agglomeration efficiency model is a function of liquor viscosity, gibbsite crystal growth rate, particles size, impeller tip speed and fluid shear rate. The effects of the key process variables of liquor composition, operating temperature and hydrodynamics are incorporated via these terms. The agglomeration efficiency model was validated by incorporating it into a population balance model of a laboratory precipitator and demonstrating good agreement between the simulated dynamic particle size distributions and the experimental values.     

A NEW FORM OF NONPLASTIC MATERIAL1

This article indicates the results obtained by introducing into a porcelain mixture an unstable nonplastic aggregate, consisting chemically of alumina, silica, and a fluxing oxide, which latter has been heat-treated in such a manner that its crystallization has been arrested. Upon subsequent kiln Wing, crystallization takes place and the arrested crystals grow, orienting themselves in such a manner as to form a network of interwoven crystals. The effect of this crystallization improves the physical properties of the porcelain.     

Crystallization of aluminium hydroxide from the reactive NaAl(OH)4–NaHCO3 solution: Experiment and modeling

The crystallization kinetics of aluminium hydroxide from the sodium aluminate solution reacted with sodium bicarbonate were systematically investigated in a steady-state MSMPR (mixed-suspension mixed-product removal) crystallizer for the first time, and the expressions of the nucleation rate, growth rate and the agglomeration kernel of aluminium hydroxide were successfully regressed. The aluminium hydroxide particles precipitated from the reactive system are identified as gibbsite by XRD and SEM examinations. The volume growth rate order of gibbsite with respect to the relative supersaturation of the solution is above the linear growth rate order, and the spiral growth mechanism for the growth of the basal face of gibbsite in the reactive system was further identified by the growth rate, morphology analysis as well as the calculated surface entropy factor. The secondary nucleation rate of gibbsite from the reactive system is three to four orders of magnitude larger than that from seeded process reported in the only available literature reference. The agglomeration kernel of gibbsite in the reactive system increases linearly with growth rate and residence time, and the positive order about 0.55 of magma density is thoroughly different from the negative order of magma density for gibbsite agglomeration in seeded process presented in the literature.     

Cristallisation du trihydrate d'alumine: Effet des conditions d'agitation

Crystallization of alumina trihydrate in an isothermal CSTR has been studied at different mixing levels. Experimental work allows prediction of kinetic constants for three phenomena which take place during the crystallization of alumina. New models for agglomeration and attrition are introduced to simulate industrial crystallizers with greatly improved accuracy.     

超细ZSM-5分子筛的制备及其形貌表征

对比考察了模板剂的种类、晶化温度以及铝含量等影响因素对超细ZSM-5分子筛晶化行为的影响,采用扫描电镜（SEM）、透射电镜（TEM）和X射线衍射光谱（XRD）技术对所制备的超细产物的结构及形貌进行了表征。结果表明,模板剂的种类对分子筛的晶化过程有重要影响,四丙基氢氧化铵是优良的制备超细ZSM-5分子筛的模板剂;以四丙基氢氧化铵为模板剂,适宜采用静态法制备超细ZSM-5分子筛的晶化温度为70～ 120 ℃;凝胶体系中铝的存在抑制ZSM-5分子筛晶核的生长,无铝条件下,分子筛的晶化速度最快;随着铝含量的增加,分子筛的晶化速度变慢,在相同的晶化时间内,生成的分子筛的粒径更小。     

New Insights into Additive Structure Effect on Crystal Agglomeration of L‐Valine

Specific small amounts of amino acids caused agglomeration of L‐valine (L‐Val) crystals during evaporative crystallization from aqueous solutions. The agglomeration of L‐Val occurred only under acidic condition when guest amino acids satisfied several conditions. Only L‐form amino acids that have carboxylic acid groups and sufficiently long alkyl chain in the side‐chains could induce agglomeration of L‐Val. The length of alkyl chain in the side‐chains controls the degree of agglomeration. Data indicated only 0.5 wt % of L‐2‐aminoadipic acid, which has a similar chemical structure to L‐glutamic acid (L‐Glu), produced the large agglomerates > 1000 μm. The particle size was ～ 500 μm when using the same amount of L‐Glu. Based on the results from previous tests and this paper, the whole mechanism for the L‐Val agglomeration in the presence of specific guest amino acids has been revealed.     

Charging Behavior at the Alumina–Water Interface and Implications for Ceramic Processing

The interaction of water and the alumina surface is comprehensively reviewed. Water can be incorporated in the alumina crystal structure resulting in the formation of aluminum hydroxides such as gibbsite. Alumina dissolves into water to an extent that depends primarily upon the solution pH and temperature. The soluble Al (III)_aq species (hydrolysis products) likewise depend upon the solution pH, temperature, aluminum, and other salt concentrations. The development of charge on the surface of alumina is controlled by amphoteric surface ionization reactions. The charging behavior of both alumina powders and single crystal faces is compared. The differences can be explained by the reactivities of different types of surface hydroxyl groups. The substantial difference in surface charging behavior of single crystal sapphire and alumina powders indicates that experiments and modeling conducted on single crystals is of limited use in predicting suspension behavior. The atomic scale structure of the hydroxylated sapphire (0001) basal plane is nearly identical to the gibbsite (001) basal plane. The observed surface structures are consistent with the charging behavior of the surfaces. The role of surface charge on the adsorption of processing additives is briefly discussed. How surface charge and processing additives at the alumina aqueous solution interface influence surface forces between particles is reviewed. The influence of these forces on suspension properties such as rheological behavior is outlined. The importance of controlling these behaviors to improve colloidal ceramic powder processing is stressed.     

A urea crystal templating method for the preparation of porous alumina ceramics with the aligned pores

A novel process for the preparation of porous alumina ceramics with an aligned pore structure has been reported. The urea dissolved in the aqueous alumina slurry at higher temperature formed rod shaped crystals aligned in the direction of gravity when cooled to room temperature. The gelatin used to set the slurry, controls the crystallization of urea such that the urea crystals produced in the suspension containing the gelatin had much lower width and thickness compared to that produced in the suspension without the gelatin. The alumina powder catalyzed the thermal decomposition of urea that enabled the removal of the majority of the urea crystals from the alumina green body by isothermal heating at temperature much lower than the melting point of urea. The void space created by the removal of the urea crystals remained as pores in the sintered ceramics.     

Hydrothermal synthesis of submicrometer crystals of boehmite

The effect of solvents, mechanically ground gibbsites and amorphous hydrated alumina gels on the formation of well-crystallized and submicrometer-sized crystals of boehmite was studied. Nearly monodispersed boehmite particles of 0.2–0.3 μm with a parallelogram shape were hydrothermally synthesized from ball-milled gibbsite in water at 200°C for 6 h. The 100 nm-sized crystals of boehmite were prepared from amorphous hydrated alumina gels at 255°C for 24 h.     

磷酸氢钙结晶聚结过程及主要影响杂质的探究

聚结现象常常伴随着反应结晶过程发生,是影响产品粒度、形貌等性状的主要因素之一。围绕磷酸氢钙的聚结现象分析了聚结发生的过程步骤,并探究了工业生产中阻碍聚结现象的主要杂质元素。结果表明,以纯净稀磷酸为原料,可制得粗大、均匀、稳定性好的大颗粒磷酸氢钙产品,这种大尺寸聚结体经历了单晶团聚、小尺寸簇状晶聚结、大尺寸簇状球形晶形形成等3个关键生长步骤。结晶母液中含氟元素胶体微粒隔断聚结第二步的发生,成为阻碍聚结现象的最关键因素。探索合理利用聚结现象的工艺方法,将为饲料级磷酸氢钙工业生产中解决过滤难、干燥成本高等问题开辟新思路。     

Particle distribution model of gibbsite precipitation process in alumina production

The precipitation is a process involving complicated crystallization and mass transfer. As a key step in the Bayer process in alumina production, it has great influences on both outputs and quality of alumina products. Thus laboratory experiments, industrial tests and numerical simulations have been carried out to study the precipitation. Based on the dynamics law and particle population balance, a combined model is developed with MATLAB/Simulink to study the process of crystal nucleation, growth and agglomeration, the model also take the variation characteristics of the particle size of the MSMPR (mixed-suspension, mixed product removal) gibbsite precipitation process into account. The particle size distribution is predicted through the method of moments at different conditions. The deviation between the model prediction and the test data is less than 9%, which is within the acceptable accuracy range for the process control.     

The flavor problem of soybean oil. IV. Structure of compounds counteracting the effect of prooxidant metals

Summary A study has been made of the effectiveness of various polycarboxylic acids and polyhydric alcohols in improving the stability of soybean oil. Certain observations have been made regarding the structural groups required and the possible mechanism of reaction. Since salts and esters of organic acids are inactive, free carboxyl groups are required. Among the four carbon atom dicarboxylic acids activity increases with the number of hydroxyl groups. Within the polyalcohols activity increases with the increase in number of hydroxyl groups. Steric immobility and loss of hydroxyl groups by dehydration reduces activity. Evidence is presented which attributes to citric acid and certain polyhydric alcohols the role of metal scavenger. For example, it has been demonstrated that the addition of citric acid and sorbitol to soybean oil containing prooxidant metallic salts effectively increases the oxidative and flavor stability of the oil. By using a sample of treated soybean oil with no detectable tocopherols, in order to eliminate synergistic effects of citric acid, it has been shown that the prooxidant effect of iron stearate is counteracted by the presence of citric acid. The demonstration that polyhydric alcohols increase the flavor and oxidative stability is compatible with their known metal complexing properties. Evidence is presented which indicates a relationship between flavor stability and oxidative stability. Presented at the 39th Annual Meeting of the American Oil Chemists' Society, May 4–6, 1948, in New Orleans, Louisiana. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

表面活性剂对丁二酸冷却结晶中聚结的抑制

借助聚焦光束反射测量仪(FBRM)和颗粒录影显微镜(PVM)对丁二酸冷却结晶过程中的聚结现象进行研究,通过加晶种实验和不加晶种实验初步推断大小颗粒的共存及颗粒间的相互结合力是形成聚结需要考虑的两个因素。在此基础上,将3种类型的表面活性剂十六烷基三甲基溴化铵(CTAB)、十二烷基苯磺酸钠(SDBS)和吐温80(Tween80)加入到丁二酸冷却结晶系统,考察了表面活性剂对结晶过程中颗粒聚结的抑制作用。晶体形貌变化表明,所选的表面活性剂均能吸附于晶体表面,且可大大降低聚结,可推测表面吸附产生的机械隔离能够降低晶体间的相互结合力,并促使结晶产品的粒度更为均匀。这一研究证明添加表面活性剂可降低结晶过程的聚结程度。     

Parameters determining the agglomeration behavior of anhydrous L-ornithine-L-aspartate (LOLA) crystals prepared by drowning out crystallization

Crystallization of L-ornithine-L-aspartate (LOLA) by drowning out was performed for the production of the anhydrous form of LOLA. The needle-like LOLA crystals were formed and spherically agglomerated during precipitation in a semibatch crystallizer. The primary crystal size in the agglomerate remains unchanged after completion of the crystallization. Therefore, the agglomeration process of primary crystals played an important role for controlling LOLA crystal size. The agglomeration of LOLA crystals was governed by not only the physico-chemical parameters such as the temperature and feed concentration, but also the hydrodynamic parameters such as agitation speed and feeding rate. The crystal size and the shape have been shown to be important factors in product impurity and flowability. Thus, the optimum condition of LOLA crystallization process by drowning-out could be obtained.     

Tailor-made additives to influence the habit of barite (BaSO4) obtained by precipitation

In order to influence the habit of barite (BaSO₄) single crystals obtained by precipitation, the tailor-made additives 2-aminomethanol, 3-aminoproponal, 5-aminopentanol, 4-aminobutane sulfonic acid, and several other growth inhibitors have been tested. The crystallization experiments have been carried out in a fully automated crystallizer with reproducible educt feed rates. The kinetics of the reaction has been measured by means of the conductivity. Scanning electron microscopic analysis of the obtained crystal habits led to the raesult that most of the additives tested did not influence the growth form of the crystals, but the predicted, tailor-made additives resulted in a more isometric crystal form.     

Particle design Part B: batch quasi-emulsion process and mechanism of grain formation of ketoprofen

This study deals with the spherical crystallization process by the quasi-emulsion mechanism, applied to a pharmaceutical. The objective is to produce spherical agglomerates made of a number of small crystals of the drug, having properties adequate for direct compression when manufacturing tablets. The aim of this work is to make the link between the process and these properties. The different steps occurring in the process are the formation of an emulsion whose droplets are made of the drug dissolved in a solvent, the creation of the supersaturation of the drug in the droplets by mass and heat transfer and the nucleation, growth and agglomeration of drug crystals inside the droplets. The process has been carried out in a batch laboratory scale device. The variation of the operating parameters on the one hand and of the relative proportions of the various components on the other have enabled us to determine the influence on the internal and external structures of the produced agglomerates which influence the ability to be compressed. The identification of the phenomena occurring has led to a proposed mechanism for the formation of the agglomerates.     

Is spherical crystallization without additives possible?

The quasi-emulsion solvent diffusion method of spherical crystallization consists in producing in one step crystallization and agglomeration of small crystals in droplets of an emulsion. Additives are generally used to stabilize the emulsion before crystallization. The aim of this study is to investigate the feasibility of spherical crystallization without surfactant. Experiments were performed in an automated batch laboratory scale crystallization process to study the influence of the process operating conditions on the structure of the particles obtained. The results clearly show that, for the experiments performed two types of particles are formed: primary spherical particles and secondary agglomerates. The pattern of the primary particles, observed under scanning electron microscopy, suggests that these particles results from a spherulitic crystal growth mechanism inside the droplet. The secondary agglomerates results of the agglomeration of the spherical particles. In addition, a set of experiments were performed with carefully selected solvents to study the influence of the crystallization solution/water interfacial tension, at constant hydrodynamic conditions and supersaturation level. The results of these experiments demonstrate that the interfacial tension is not a key parameter for designing such a process.     

氢氧化铝晶体颗粒强度的数值模拟分析

利用有限元方法对氢氧化铝晶体叠合形成的氢氧化铝颗粒进行了静力分析,得到了氢氧化铝晶体叠合角度与叠合形成的氢氧化铝颗粒强度的关系,计算了叠合氢氧化铝晶体尺寸与氢氧化铝颗粒强度变化之间的规律. 研究发现：(1) 当氢氧化铝晶体间叠合角度在60o~80o时,氢氧化铝颗粒所受应力最大,强度最小;(2) 叠合的氢氧化铝晶体尺寸较小(约5 mm)时,氢氧化铝颗粒所受应力较小,强度较大;(3) 叠合的氢氧化铝晶体尺寸较大(20~40 mm)时,叠合晶体尺寸越大,氢氧化铝颗粒所受应力越大,强度越小;(4) 大晶体间镶嵌有细小晶体的叠合方式,形成的氢氧化铝颗粒强度有明显优势.     

Influences of seed crystals on agglomeration phenomena and product purity of m-chloronitrobenzene crystals in batch crystallization

Changes in the fraction of agglomerates, agglomeration kinetics and product purity of m-chloronitrobenzene (CNB) crystals with the number and size of seed crystals were examined in batch crystallization experimentally, and influences of seed types (ground and well-defined) on agglomerate purities were discussed. From the fraction of agglomerates and agglomeration kinetics, it was found that agglomeration occurred more frequently when the number of seed crystals was larger and its size was smaller. The amount of purity decrease by agglomeration was smaller for the smaller number of seeds and for the larger ones. As the number of the elementary crystals constituting agglomerates was smaller, the purity of agglomerates was higher. With ground crystals, more frequent agglomeration occurred and the purity of agglomerates was lower than those for the well-defined crystals.     

Si/F/K/Na杂质对硫酸钙结晶过程的影响

考察了湿法磷酸生产过程中Si, F, K和Na等杂质对CaSO4结晶过程的影响规律. 结果表明,两种碱金属对CaSO4结晶有不同作用,K有利于CaSO4晶体生长,而Na含量增加则会抑制CaSO4团聚;不同形式的K, Si和F对CaSO4结晶的作用不同,SiO2对CaSO4粒径影响不大,但会使其形貌不规则,H2SiF6会造成CaSO4晶体分散,而K2SiF6有利于CaSO4晶体生长.