Morphology of Hydrophobic Agglomerates of Molybdenite Fines in Aqueous Suspensions

In this work the morphology of agglomerates formed in the hydrophobic agglomeration of molybdenite fines in aqueous suspensions has been studied by measuring fractal dimension with image analysis method. The results have shown that the morphology closely correlated with the mechanical conditioning in the hydrophobic agglomeration. The higher the stirring speed and the longer the stirring, the more spherical and more compact the agglomerates. Also, kerosene addition could make the agglomerates to be more spherical and compact.     

Einfluß der Agglomeration auf die Filtrierbarkeit von Suspensionen

Influence of agglomeration on the filtering properties of suspensions. In various kinds of filter processes, agglomeration of the particles to be separated is essential for economic operation. Important parameters are the size, porosity, and consistency of the agglomerates, for which a special measuring technique has been developed. The influence of these parameters in various filter processes is described. The physicochemical fundamentals and the chemical engineering of agglomeration in suspension are presented, and the influence of flocculants discussed.     

Migration and agglomeration of adhesive microparticle suspensions in a pressure-driven duct flow

Transport of suspensions consisting of monodisperse adhesive microparticles in a pressure-driven duct flow with the Reynolds number in the range of 5.4–108 is analyzed by means of a coupled lattice Boltzmann method with the discrete element method. The influence of the adhesion on migration and agglomeration of particles is investigated. The results show that the suspensions undergo a transition from purely lateral focusing to completely agglomeration as the surface energy is increased. A dimensionless adhesion number, defined as the ratio of the adhesive force to the drag force, is employed to characterize the degree of agglomeration. It is found that all the particles come into agglomerates as the adhesion number reaches a critical value. Moreover, a detailed examination of size evolution and velocity illustrates that the agglomerates mainly grow spherically, and their velocities depend on the lateral positions rather than their sizes, as the Stokes number is far below one.     

Combustion Synthesis, Powder Characteristics, and Shrinkage Behavior of a Gadolinia–Ceria System

Pure and gadolinia-doped nanocrystalline ceria powders were prepared through the gel combustion route. Analysis of crystallite size, surface area, and particle size indicates that the powders are agglomerates of nanosize primary particles. The degree of agglomeration increases with increase in the gadolinia content. The shrinkage behavior on firing of the powders was studied using a dilatometer. Although the shrinkage starts at a lower temperature, the densification completes at a higher temperature as the gadolinia content increases. The wider temperature range required for shrinkage in high gadolinia content ceria is attributed to the wider distribution of interparticle pore size in the green compact.     

Consolidation of Alumina—Zirconia Mixtures by a Colloidal Process

The relationship between the dispersion of colloidal powder particles in Al₂O₃–ZrO₂ suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al₂O₃ powder and ZrO₂ sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al₂O₃ and ZrO₂ layers were obtained from well-dispersed suspensions with pH values below about 4 and salt concentration of 0.0527 M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al₂O₃ and ZrO₂ agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO₂ agglomerates were distributed in an Al₂O₃ matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single-component suspensions. The ZrO₂ particles formed three-dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation.     

Calcium carbonate precipitation in a continuous stirred tank reactor

The precipitation of calcium carbonate, brought about by mixing aqueous solutions of calcium nitrate and sodium carbonate in a continuous reactor, has been studied. The pH (8.5-10.0), reagent concentration (0.25-1.7 N), temperature (15-60°C), residence time (1-3 min), and mixing speed (100-500 rpm) were chosen so that the effects of agglomeration could be examined The presence of agglomerates was confirmed by microscopy. Precipitate size distributions were log normal, and the geometric mean diameters, ranging from 5 to 13 p.m, were extremely sensitive to pH. Large precipitates were obtained under conditions favouring a high agglomeration rate; namely high temperatures, concentrations, and mixing speeds. The effect of residence time was minimal. The geometric standard deviation increased with the mean size of the precipitate. The solids recovery decreased with increasing temperature, but was not significantly affected by the other parameters     

Investigation of kinetics of agglomerate growth in oil agglomeration process

In this study, the kinetics of agglomerate growth in a batch oil agglomeration process has been studied using bituminous coal. The effect of operating variables such as kerosene concentration, pulp density and speed of agitation on the agglomeration process was investigated. It has been found that the second-order kinetic equation describes the growth of agglomerates adequately. The growth of the agglomerates in the oil agglomeration process shows a self-preserving growth. Using this, a characteristic curve has been developed. For the prediction of the size distribution of the agglomerates, the d₅₀ values of the agglomerates must be known. Therefore, a model has been developed by using the kinetic equation for estimation of d₅₀ values of agglomerates for this coal. It was shown that the size distribution of the agglomerates for any levels of the process variables studied can be predicted using the equation of characteristic curve and d₅₀ values. Knowledge obtained from this study will be helpful for technological advancement of this kind of study.     

Prediction of the size distribution of agglomerates in the coal—oil agglomeration process

A summary has been developed to predict the size distribution of the agglomerates at different periods of agglomeration using the second-order kinetics equation and the equation for the characteristic curve of the coal—oil agglomeration process. Experiments have been conducted to estimate the size distribution of the agglomerates and to verify the model. It has been found that the predicted size distribution of the agglomerates agrees closely with the experimental results.     

Experimental study of droplet vaporization in a fluidized bed

The injection of liquid in a hot fluidized bed is used in many industrial processes. The larger droplets are likely to engulf particles and create agglomerates that should be avoided when a fast vaporization is expected. A new model is proposed to estimate the chances of agglomeration based on a critical bed temperature below which agglomerates form. This critical temperature for safe operating conditions is based on heat transfer between the particles and the liquid and takes the porosity of the solid into account. The model could be applied to pure and multi‐component mixtures and has been validated experimentally.     

Stabilization of Ethanol-Based Alumina Suspensions

Al₂O₃ powders have been successfully dispersed in ethanol by varying the suspension acidity. An operational pH (O.pH) was defined to measure the acidity of these ethanol-based suspensions. The isoelectric point of Al₂O₃ in ethanol was at an O.pH of 8. According to Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, suspensions between an O.pH of 3.5–10.5 possessed only attractive inter-particle potential. Suspensions below 3.5 had high ζ potential, fine particle size, and were Newtonian. However, suspensions at high pH were shear thinning and consisted of agglomerates, despite their high ζ potential. The use of citric acid as a dispersant has also been investigated. At an O.pH of 3, optimum additions of citric acid between 0.6 and 1.0 wt% decreased the particle size, resulted in repulsive inter-particle potentials and increased the solid loading capacity to 15 vol% from 2 vol% while maintaining Newtonian behavior and similar viscosity to suspensions at O.pH 2. Addition of citric acid created agglomerated suspensions that were negatively charged at O.pH 10.5 (obtained by adding NH₄OH), but positively charged suspensions at O.pH 13.6 (obtained by adding tetramethylammonium hydroxide).     

Experimental and theoretical study on the agglomeration arising from fluidization of cohesive particles—effects of mechanical vibration

A novel technique that can prevent the disruption of agglomerates when sampling the agglomerates from a fluidized bed has been developed and has been applied to the investigation of the agglomeration behaviour of cohesive particles during fluidization with and without mechanical vibration. A new model for the prediction of agglomerate size has also been established on the basis of the energy balance between the agglomerate collision energy, the energy due to cohesive forces and the energy generated by vibration. The accuracy of the model is tested by comparing the theoretical results with the experimental data obtained both in the present work and in the literature. Effects of gas velocity and mechanical vibration on agglomeration for two cohesive (Geldart group C) powders in fluidization are examined experimentally and theoretically. The experimental results prove that mechanical vibration can significantly reduce both the average size and the degree of the size-segregation of the agglomerates throughout the whole bed. However, the experiments also reveal that the mean agglomerate size decreases initially with the vibration intensity, but increases gradually as the vibration intensity exceeds a critical value. This suggests that the vibration cannot only facilitate breaking the agglomerates due to the increased agglomerate collision energy but can also favour the growth of the agglomerates due to the enhanced contacting probability between particles and/or agglomerates. Both the experimental and theoretical results show that a higher gas velocity leads to a smaller agglomerate size.     

Synthesis of ZrO2 Nanoparticles by Freeze Drying

The interest in zirconia nanoparticles for either structural or functional applications depending on the stabilized phases has promoted the search for different synthesis methods. One of these methods is freeze drying. The present work deals with the preparation of zirconia nanoparticles by freeze drying using ZrOCl₂ solutions. The freeze-dried powders consist of nonspherical granules made up of soft agglomerates of ZrO₂ with a d _BET down to 40 nm and specific surface area values of 2–27 m²/depending on the thermal treatment. Granules have a low remaining porosity formed as a consequence of the sublimation of ice. The thermal treatment determined the crystalline phase formed, and so a study of the phase transformations with temperature has been carried out by X-ray diffraction and transmission electron microscopy. The results indicate that at temperatures ≤400°C and a short duration of treatment, t -ZrO₂ is obtained with an acicular shape, while higher temperatures lead to the transformation into a monoclinic phase, which is accomplished by grain size growth, larger aspect ratio, and increased agglomeration. After sonication of the diluted suspensions, monomodal particle size distributions are obtained, with average sizes of ≅100 nm, thus demonstrating that agglomerates are soft and can be easily broken down.     

Effect of dispersion states (electrostatic/electrosteric stabilization) on particles arrangement in the yttria-stabilized zirconia sediments

In the present work, particle arrangement and their packing in the sediment layer of zirconia suspension were studied. To evaluate the particle settling, aqueous suspensions of zirconia nanoparticles were prepared in different dispersion states. In one state, Dolapix CE64 was used as a dispersant to provide electrosteric mechanism. In another state, pH of the suspension was adjusted at 4 to provide electrostatic mechanism. The other state was the combination of dispersant and pH adjustment which resulted in the most stable suspension. First of all, the stability of all dispersion states was evaluated by zeta potential, sediment volume (SV) and height, viscosity, and packing density (PD). Then, the sediment layers of all suspensions were characterized. Incorporation of electrostatic mechanism was resulted in a main decrease in viscosity with high surface charges, while electrosteric mechanism caused lower sedimentation of particles. Fall velocities of particles/agglomerates were estimated, and the influences of dispersion states on the particles fall velocities were characterized. The microstructural observation revealed homogeneous packing of particles in the sediment layer of the stable suspension demonstrating the proper dispersion of particles. Dolapix CE64 and pH adjustment resulted in a uniform arrangement of particles without agglomeration and spherical and regular granules with a uniform shape.     

Agglomeration behavior of anhydrous L-ornithine-L-aspatate crystals during semi-batch drowning-out crystallization

Crystallization of L-ornithine-L-aspartate (LOLA) by drowning out was carried out to produce the anhydrous form of agglomerates. The primary crystal size in the agglomerate remained unchanged after completion of the crystallization. The LOLA aqueous solution introduced into the system was immediately dispersed and cluster coagulated on the surface of the crystals. On the surface of the crystals, a cluster reached critical nuclei size, nucleated and intergrowth to form agglomerates. It was proposed that a spherical agglomeration occurred during secondary nucleation by coagulation model and intergrowth. The agglomerates size and size distribution were varied with the process parameters. The agglomerate sizes of LOLA crystals appeared to be ruled not only by secondary nucleation rate but also by the mass of suspended agglomerates. Moreover, the agglomeration rates of fine particles were higher than the agglomeration rates of large agglomerates. Using these properties, the uniform agglomerates size distribution could be obtained.     

Agglomeration of magnesium particles in magnesium-sodium nitrate combustion

Agglomeration phenomenon of magnesium particles during combustion of Mg NaNO₃ propellant has been studied. High speed photographs of combustion zones and the burning surface temperature data indicate that the metal particles form agglomerates on the burning surface in varying degree depending on the mass fraction of NaNO₃. It is found that the increase of oxidizer content increases the metal agglomeration and the agglomerate size depends on the initial particle size of the ingredients. An attempt has been made to predict the size of the agglomerates based on the consideration that the agglomerate size depends on the thickness of the molten oxidizer layer enveloping the metal particles in the condensed phase and surface heat flux providing local temperature environment to agglomerate the metal particles and to eject from the burning surface for the vapour phase combustion. The results were compared with the experimental data. The prediction describes fairly well the observed effects of the concentration and particle size.     

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.     

Influence of Powder Agglomerates on the Structure and Rheological Behavior of Injection-Molded Zirconia–Wax Suspensions

Surface-modified zirconia powders with 20% to 100% of the particle surface area covered with stearic acid was blended with 50 vol% of an organic wax vehicle for rheological study. These fractions of surface coverage gave rise to different degrees of powder agglomeration. A further adsorption of the wax on partially modified particle surfaces was examined, which was likely to exhibit different degrees of solid surface-wax affinity depending upon whether the particle surfaces were "bare" or "premodified." The rheological behavior of the suspensions revealed that the shear viscosity as well as the yield stress increased appreciably with decreased fractions (or surface coverage) of the pre-adsorption. The observed suspension rheology due to incomplete surface modification can be accounted for by the formation of agglomerates which suppress suspension flowability to a significant extent. The formation of the agglomerates alters the suspension structure by reduction of the maximum solid concentration (φ_max) that is attainable for a given powder. This change in suspension structure (decrease in φ_max) leads to a restriction of particle mobility, reflected as a linear function of the yield stress of the suspensions.     

Enhanced Densification of In2O3 Ceramics by Presintering with Low Pressure (5 MPa)

Sintering of In₂O₃ has been carried out in air to full density. Because of the difference in densification between the agglomerates and the matrix, large interagglomerate pores were observed to form at the initial stage of sintering. Such pore formation could be prevented by applying a small external pressure which resulted in the beneficial rearrangement of agglomerates.     

Simultaneous particulate design of primary and agglomerated crystals of steroid by spherical agglomeration in liquid for dry powder inhalation

Spherical agglomerates of steroid KSR-592, consisting of fine primary drug crystals suitable for dry powder inhalation (DPI), were prepared by the spherical agglomeration method in liquid with a bridging liquid. It was found that the particle size of primary crystals increased until the dispersing medium was saturated with the bridging liquid, whereas the spherical agglomeration of primary crystals was continued even after the saturation of medium with the bridging liquid. The growth rates of primary crystals and agglomerates increased with an increase in the temperature and/or a reduction in the agitation speed of the system. The agglomerates were easily disintegrated into the primary crystals depositing ideally on carrier lactose particles for DPI by mixing. The in vitro efficiency of the mixed system of lactose and disintegrated primary crystals of drug was 2 to 3 times higher than that of crystals prepared conventionally. Furthermore, the soft agglomerates disintegrated easily into respirable particles in air stream when emitted from the inhalation device were prepared by reducing the agitation speed after the dispersing medium was saturated with the bridging liquid.     

Agglomeration during the Drying of Fine Silica Powders, Part II: The Role of Particle Solubility

The effect of particle solubility and the dissolution rate on agglomeration was studied by drying silica and titania particles from aqueous slurries with pH values in the range of 2–12. The agglomerate strength and strength distribution were measured by a calibrated ultrasonic force, and the strength increased as the solubility and dissolution rate increased. Two silica powders of different particle size (60 nm and 500 nm) were studied, and smaller-sized particles formed stronger agglomerates. The drying rate of the powders was varied by using spray drying and tray drying, and slower drying was shown to lead to higher agglomerate strength. The agglomerate strength of titania powder (insoluble in water) was independent of pH, whereas the agglomerate strength of silica was dependent on pH. It was concluded that the solubility and dissolution rate are important parameters that govern the strength of agglomerates.