Agglomeration in suspension of salicylic acid fine particles: Analysis of the wetting period and effect of the binder injection mode on the final agglomerate size

Agglomeration in suspension is a size enlargement method which facilitates the operations of solid processing such as filtration, transport and galenic while preserving the solubilisation properties of fine particles. A small quantity of binding liquid is added into a suspension of microparticles, directly in the stirred vessel where the precipitation or crystallisation took place. With optimised quantity of binder injected and optimised injection mode, spherical and dense agglomerates can be obtained. This paper analyses the first step of the agglomeration process, i.e. the wetting period, which corresponds to the injection of the binder liquid and its dispersion within the particle suspension. The system studied is the agglomeration of salicylic acid microparticles using chloroform as binder. A visualisation cell was developed in order to observe under optical microscope the interactions between a liquid binder droplet and the particles to be agglomerated. Clearly, an immersion mechanism was observed. Experiments were also carried out in a stirred vessel to visualise the wetting phase within the reactor using an image acquisition probe. The effect of the injection mode and quantity of binder on the agglomerate size was analysed. A fast binder injection under high stirring and an optimum quantity of binder favour the formation of small agglomerates.     

Flow induced phase inversion agglomeration: Fundamentals and batch processing

A novel agglomeration technique, based on flow induced phase inversion (FIPI) is described and applied to the batch preparation of polyethylene-bound abrasive calcite agglomerates. Water soluble polymers are used to agglomerate the needle-like crystals of tetraacetylethylene diamine and also sodium chloride crystals. In a typical isothermal FIPI agglomeration process primary particles are dispersed in the molten binder, which is subsequently inverted by the addition of sufficient amount of primary particles, which also defines the critical filler concentration at phase inversion, C_c. Agglomerate particle size is primarily a function of C_p – C_c where C_p is the mean concentration of filler. C_c decreases with increasing binder molecular weight and primary particle surface area. Agglomerate size distribution is affected by processing, mainly by the mixing time after phase inversion. For the non-isothermal FIPI agglomeration process, phase inversion is induced locally, by the addition of fine particles in the molten binder. Phase inversion is then propagated by cooling the dispersion during mixing. Agglomerate characteristics such as particle size, particle size distribution, binder concentration distribution in each agglomerate size range, agglomerate topology, binder morphology in the agglomerates, agglomerate strength, and agglomerate dissolution rate in water were evaluated. These agglomerate characteristics are related to the binder and filler properties as well as to the processing conditions.     

Effect of agglomerate properties on agglomerate stability in fluidized beds

Fluidized bed agglomeration is used to stabilize particulate mixtures and reduce dust emissions. This technology is applied to a variety of production processes for the pharmaceutical, chemical, fertilizer and food industries. In most of these applications, agglomerate stability is an essential criterion. Agglomerates and granules that do not conform to size and shape specifications may create problems in downstream processes, such as tableting, thus compromising process efficiency and product quality. When an agglomerate is formed in a fluidized bed, it can grow by incorporating other bed particles, split into smaller fragments, or be eroded by fluidized bed solids. The objective of the present study is to determine the critical agglomerate liquid content at which the rates of agglomerate growth and shrinkage are balanced when artificial agglomerates made from glass beads and water are introduced into a fluidized bed. This study examined the effects of agglomerate size, agglomerate density, liquid viscosity, binder concentration, and fluidizing gas velocity on the critical initial liquid content. This study found that small agglomerates and low density agglomerates displayed higher critical initial moisture contents. When the viscosity was increased by using sugar solutions, agglomerates were very stable and had very low critical initial moisture contents. The study also found that as the superficial gas velocity increased, the agglomerates started to fragment, rather than erode.     

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.     

Stochastic simulation of agglomerate formation in fluidized bed spray drying: A micro-scale approach

A stochastic model that describes agglomerate growth during fluidized bed spray agglomeration is presented and numerically solved by constant volume Monte Carlo method. The methodology overcomes the difficulties of solving multivariate population balance equations and includes continuous binder addition and drying. Agglomerate formation is treated as a complex combination of consecutive and parallel micro-mechanisms. Due to the discrete nature of the approach, the individual role of the micro-mechanisms on the agglomeration behavior can be analyzed.The results suggest that the droplet capture mechanism governs the agglomeration speed while the maximum agglomerate diameter is ruled by the equilibrium reached between coalescence, rebound and breakage. The mechanism of deposited binder drying is found to play a negligible role on agglomerate formation because of an extremely rapid droplet consumption. The main process variables affecting each micro-mechanism have been identified showing that the liquid spraying rate affects directly the droplet capture mechanism whereas binder properties influence mainly the agglomeration and rebound interactions.The model presented in this study is able to predict qualitatively the experimentally observed response of the system as well as the general shape of the agglomerate size distribution under the variation of several process parameters, demonstrating the potential of the discrete micro-level approach.     

Modelling of agglomeration in suspension: Application to salicylic acid microparticles

The agglomeration in suspension technique consists of adding directly into the suspension a small amount of a second liquid which acts as an interparticle bonding agent. The system (salicylic acid particles, aqueous solution, chloroform) is studied experimentally by in situ image analysis. After a brief period of wetting of the particles by the binding liquid, the agglomerates grow by a coalescence-like process until they reach a maximum size. Porosity measurements reveal that the agglomerates are then getting more compact. Eventually, the agglomeration mechanism is likely governed by the agglomerate deformability, as it is often suggested in granulation. A population balance modelling is proposed to describe the growth period. The agglomeration kernel is built according to the experimental observations. It is expressed as the product of two factors which relate the meeting probability and the sticking efficiency, respectively. The probability of encounter is governed by the hydrodynamics. The sticking efficiency compares the sticking force, directly linked to the deformation induced by the agglomerate-agglomerate impact under consideration, with the shear-induced disruptive force. This phenomenological model fits well the experimental results obtained for the salicylic acid particles.     

Kinetics of fluidized bed spray agglomeration for compact and porous particles

The effect of the primary particle porosity during the formation of agglomerates in spray fluidized beds is presented in this study. The method is based on the single micro-interactions occurring within the fluidized bed such as inter-particle collisions, droplet spread on the particle surface, aging of the deposited droplets and particle coalescence. The porous character of the particles is expected to directly affect the aging process of the deposited binder layer by penetration into the pores of the substrate. The droplet penetration process is experimentally analyzed by single droplet deposition on spherical, porous alumina particles. The results indicate that the penetration process is mainly governed by the viscosity of the liquid and that at relatively low viscosities, droplet penetration is fast. For highly viscous liquids, the penetration velocity slows down and an additional mechanism, namely drying becomes important. A combined imbibition–drying model is developed and included into a comprehensive stochastic agglomeration model that allows the simulation of agglomerate formation in a batch process. Lab-scale agglomeration experiments with porous and non-porous particles are carried out in an attempt to validate the general tendencies predicted by the main agglomeration model. The results show that the agglomeration rate for porous particles is significantly reduced due to the losses of deposited droplets into the pores of the primary particles; this tendency is much more pronounced at low binder viscosities.     

Hardness of moist agglomerates in relation to interparticle friction, capillary and viscous forces

Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. Recent micro-scale studies have identified the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles and on the saturation of the agglomerate mass.On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not accounted for, the model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology.     

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.     

Compaction behaviour of agglomerated alumina powders

The effects of agglomerate properties, such as the binder type, binder content, moisture level, and agglomerate size, on a model compaction process was investigated by using green density-pressure interrelationships for a range of agglomerated alumina powders. The model compaction process involved single ended nominal uniaxial stress transmission in a cylindrical die. The influences of the sample aspect ratio, die wall lubrication, and compaction rate were also investigated. Two types of water soluble polymeric agents, a poly(vinyl alcohol) (PVA) and a poly(ethylene glycol) (PEG), were used. It was shown that certain agglomerate properties have a strong influence upon the compaction behaviour of these ceramic powders. The extent of the compaction is enhanced by using agglomerates with a low agglomerate yield point. In the PVA system, the agglomerate yield points decreased with increasing moisture content. The compaction behaviour of the agglomerates showed a rate dependency, that is, the compaction is retarded with increased pressing rate. The green densities of the compacts prepared in the unlubricated die were lower than those of the compacts prepared in the lubricated die due to the higher wall frictional forces operating in the unlubricated die.     

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.     

Ceramic Composites with Three-Dimensional Architectures Designed to Produce a Threshold Strength—I. Processing

A processing method was developed to produce a composite architecture that consisted of polyhedra of one material separated from one another by thin layers of a second material. The materials used for this study were selected so that the material separating the polyhedra would develop large compressive stresses during cooling because of differential thermal contraction. This architecture was developed to determine if it could be used to produce a ceramic composite that exhibited an isotropic threshold strength; a threshold strength has been previously demonstrated for periodic laminates containing thin layers in residual compression. To produce the current architecture, spherical alumina agglomerates were produced by suspending aqueous slurry droplets in an upward flow of a hygroscopic liquid; during the suspension period, the water was absorbed from the droplets, thereby consolidating the particles within an agglomerate. The agglomerates were then coated with thin layers of mullite–alumina using methods commonly employed in the pharmaceutical processing industry. The coated agglomerates were then consolidated into compacts by a two-step process in which the agglomerates were first uniaxially pressed at low pressure and then isopressed at high pressure. The uniaxial consolidation introduced a small degree of anisotropy into the composite architecture. Edge-cracking was observed for compressive layers containing 55 vol% mullite, thereby confirming that the appropriate compressive stresses were developed within the architecture.     

Fractal Dimension of Alumina Aggregates Grown in Two Dimensions

The concepts of fractal geometry are applied to the analysis of 0.4-μm alumina constrained to agglomerate in two dimensions. Particles were trapped at the bottom surface of a drop of a dilute suspension, and the agglomeration process was directly observed, using an inverted optical microscope. Photographs were digitized and analyzed, using three distinct approaches. The results indicate that the agglomerates are fractal, having a dimension of approximately 1.5, which agrees well with the predictions of the diffusion-limited cluster–cluster aggregation model.     

Flotation of Fine Gold Particles by the Assistance of Coal-Oil Agglomerates

This paper describes the use of coal-oil agglomerates in flotation to increase the gold recovery from an ore containing fine gold particles. The effects of operating parameters on gold flotation recovery such as oil type, particle size of agglomerating material, agglomerate/ore and oil/ore ratios were investigated. The studies showed that petroleum oils are more effective than vegetable oils in oil agglomeration of Kozlu coal and coal-oil assisted gold flotation. Gold recovery can be increased using a higher amount of agglomerates in the process; however, gold grade of the flotation concentrates is reduced significantly. The use of bridging oil at high concentrations in the agglomeration process provides high-grade gold concentrates, but lower recoveries. The utilization of coarser coal particles in the coal-oil agglomeration stage leads to higher selectivity and recovery values for gold particles.     

A Die Pressing Test for the Estimation of Agglomerate Strength

A die pressing test was developed for quick and inexpensive estimation of the agglomerate strength of ceramic powders. The critical nominal pressure ( p _c) at which contact areas between agglomerates start to increase rapidly was found from the relationship between change in sample height and applied pressure in uniaxial single-ended die pressing. A quantitative microscopic method was used for measuring the area fraction (Ψ) of agglomerates which transmits the force through the assembly. A die pressing agglomerate strength, σ_d, is defined as σ_d= 0.7 p _c/Ψ. This strength was compared with the agglomerate tensile strength obtained from single agglomerate diametral compression tests and found to be 50% higher than the latter because of multipoint loading. A suggested guideline is that the mean agglomerate tensile strength is approximately 52% of p _c determined in a die pressing test for spherical agglomerates. In addition to agglomerate tensile strength, the mean agglomerate size, the interior macropore structure of agglomerates, as well as the packing efficiencies between and inside agglomerates can be estimated by the procedure.     

Agglomeration of fine particles subjected to centripetal compaction

Agglomeration is a common phenomenon in many processes. The mechanical properties of agglomerates strongly depend on their structures. This paper presents a numerical study of the agglomeration of fine particles down to 1 μm in size based on the discrete element method. The agglomerates were formed with particles initially generated randomly in a spherical space and then packed under an assumed centripetal force. Agglomerate structure, packing density, coordination number and tensile strength were analysed with particular reference to the effect of particle size associated with the van der Waals attraction. The results showed that both the packing density and coordination number of the agglomerates decay exponentially to their limits as agglomerate size increases. The tensile strength of the agglomerates was calculated from the simulations and shown to decrease with the increase of particle size. The strength was also estimated from the Rumpf model supported by the empirical equations formulated based on the present simulation results. The good agreement between the results from the simulations and the estimation indicates that the equations are useful to facilitate engineering applications.     

Compression of polymer bound alumina agglomerates at the micro deformation scale

In this paper, the mechanical properties of alumina particle based agglomerates, prepared in a tumble mixer by using a poly (vinyl alcohol) (PVA) binder (ca. 5% by volume) in conjunction with water solvent, are described at a micro deformation level. Uniaxial compressive deformation profiles of these alumina agglomerates, typically 180–200 μm in diameter, are reported and their diverse behaviour during compression have been observed, which vary from a clear brittle rupture to progressive ductile deformation. However, certain common patterns of the agglomerate reaction force response, as a function of the compressive displacement, are identified, such as the similarities in the reloading response after each discrete fracture event. The Hertz Theory and a slope and peak force analysis are applied to establish the common patterns and trends, and generalize the intrinsic deformation characteristics of these agglomerates.     

Small angle light scattering study of improved dispersion of carbon nanofibers in water by plasma treatment

Ultrathin polymer film is deposited on the surfaces of vapor-grown carbon nanofibers by a plasma polymerization using acrylic acid as a monomer. Small angle light scattering is used to investigate the dispersion behavior of the carbon nanofibers suspended in water and provides information on the mechanism by which plasma treatment assists dispersion. Both plasma-treated and untreated nanofibers exhibit a hierarchical morphology consisting of small-scale aggregates that agglomerate to form fractal clusters that eventually precipitate. The time evolution of small-scale aggregation and large-scale agglomeration is studied by fitting the scattering data to a unified model. The morphology of the small-scale aggregates is also studied by extracting the size distribution from the angle-dependence of the scattered intensity, using the maximum entropy method in conjunction with a simplified tube form factor. The aggregates are side-by-side bundles of individual nanofibers or more complex structures. Plasma treatment not only contributes to breaking up of the small-scale aggregates into smaller sizes but also inhibits their agglomeration. For untreated fibers, large agglomerates appear immediately after sonication and their size remains almost unchanged during the precipitation process. For treated fibers, precipitation dominates during the first 8 h, leaving small entities in suspension which form agglomerates after a few days.