Effects of the agglomerated states and the gap of coverage for admixed particles on particle-bed packing fractions

One of the techniques used to decrease the cohesive force between particles is the admixing of nano-particles. However, the optimal conditions that will produce a minimum amount of force have not been established. In this study, we investigated the effects of the agglomerated state and the gap of coverage for admixed particles on particle-bed packing fractions in uni-axial compression. The main particles were made up of 397 nm silica particles. The admixed particles included 8, 21, 62 and 104 nm silica particles. The main and admixed particles were mixed using a mortar and pestle for 5 min for various mass ratios. SEM images were used to analyse the coverage diameter and the surface coverage ratio. As a result, the packing fractions with admixed particles of 8 and 21 nm were larger than when admixed particles were not used, and these admixed particles adhered onto the surface of the main particles as agglomerates. However, packing fractions of 62 and 104 nm were almost constant and were independent of the coverage states of admixed particles. Furthermore, these admixed particles with relatively larger diameters were adhered onto the surface as single particles. From the coverage diameter and actual surface coverage ratio obtained by the SEM image, the average gaps between agglomerates of 8 and 21 nm on the main particle were calculated. When the gap approached twice the size of the coverage diameter, packing fractions of 8 and 21 nm proved to be the maximum values. However, when the gap was less than the coverage diameter, the packing fractions deteriorated.     

Aggregation of nanoparticles in high ionic strength suspensions: Effect of Hamaker constant and particle concentration

The kinetics of aggregation of nanocolloidal particles in suspensions has been studied using computer simulations based on Discrete Element Method. This study presents the analysis of the influence of Hamaker constant, Brownian forces and particle concentration on the aggregation time of nanoparticles in high ionic strength suspensions. Particle adhesion and cohesion were simulated using the van der Waals force equation. Half the particles were assigned a Hamaker constant of 9.0 × 10^?20 J and the other half of the particles had the Hamaker constant varied from case to case with values between 1.0 × 10^?20 and 9.0 × 10^?20 J. Aggregation times obtained from analysing the number of interparticle contacts and number of singlets in the suspensions have been used to characterise the speed of the aggregation process. The simulation results show that when the strength of the van der Waals interaction increases the aggregation time decreases following a power law. In addition, the presence of Brownian forces speeds up the aggregation process. Finally, the relationship between packing fraction and aggregation times for singlets and contacts has been very well expressed by power laws.     

Particle size distribution control and related properties improvements of tungsten powders by fluidized bed jet milling

Fine grinding process of different particle size tungsten powders was carried out by fluidized bed jet milling. The results showed that the jet milling treatment caused deagglomeration of tungsten powders, which led to particles sufficient dispersion and narrow particle size distribution. Grinding gas pressure of 0.70 Mpa made the particles achieve high speed which promoted the particles collision contributing to particle dispersion and shape modification. For tungsten powder with particle size of 3 μm FSSS, a higher packing density with 5.54 g/cm³ was obtained, compared with the 3.71 g/cm³ of the original powder. For tungsten powder with particle size of 1 μm FSSS, the big agglomerates disappeared and the particle size distribution become narrower, while small aggregates about 2–3 μm still exist after the jet milling process. For tungsten powder with particle size of 5 μm and 10 μm FSSS, different medium diameter particle size and narrow particle size distribution of monodisperse tungsten powders can be produced by the optimized jet milling parameters. More important, the effective dispersion, favorable shape modification and precise classification have been achieved in the simple process.     

Dynamic shape factor for particles of various shapes in the intermediate settling regime

There are a number of shape factors available in the literature to describe the deviation of non-spherical particles to spheres. Dynamic shape factor (χ) is the most suitable parameter for flow applications as it takes into account the dynamic properties of the non-spherical particles. In this study, dynamic shape factors for spheres, cubes, cuboids and cylinders settling in the transition regime (2 < Re_t < 500) are determined experimentally. Wall effects on the terminal settling velocity are accounted based on literature models. χ for spheres and cubes are found to be constant with particle size when conventional wall effect models are followed. χ for cuboids and cylinders, on the other hand, are found to be a function of the particle characteristic length-to-column diameter ratio (l/D) and the particle aspect ratio (l/d). An empirical correlation is developed to estimate χ for both cuboids and cylinders settling in the intermediate regime. The correlation can provide a low average error of 3% for all the cuboids and cylinders used in this study.     

Numerical simulation for the transport of solid particles with a vortex ring

The objective of this study is to search for the possibility to transport or deliver small solid particles by a vortex ring. The numerical simulation for the motion of a vortex ring and glass particles is performed. At the launch of a vortex ring into quiescent air, spherical particles are arranged on the cross-section of the vortex ring. The cases of the Stokes number St of 0.01 and 1 are simulated by the vortex method. The simulation for St = 0.01 highlights that the vortex ring involves the particles at the launch and that it can transport the particles at a distance of 5.5 times longer than the initial diameter of the vortex ring. The simulation also clarifies the effect of St on the behavior of the vortex ring and the particle motion.     

Convection induced by vibrating rod in fine-powder bed

In this study, vibration-induced convection was studied experimentally using a fine powder with a mass median particle diameter of 8 μm. A cylindrical rod arranged vertically in a powder container was vibrated horizontally with simple harmonic motion at a frequency of 300 Hz using a piezoelectric vibrator. For a vibration amplitude of 10 μm, particles around the cylindrical rod were consolidated to a certain extent due to gravity; however, for a vibration amplitude of 70 μm or more, a pair of convection rolls formed on both sides of the vibrating rod. The strength of the convection was quantified from the particle velocity distribution in the convection rolls, and the relationship between the convection strength and vibration amplitude was elucidated. In addition, the air-pressure distribution in the powder bed was measured showing that the convection strength correlates with the characteristic positive pressure, i.e., the average value of positive pressure measurements. Elliptical motion and circular motion as well as simple harmonic motion were applied to the cylindrical rod by adding two harmonic motions in directions orthogonally crossing each other with a phase difference of π/2 rad. The convection of the particles varied according to the Lissajous trajectory of the cylindrical rod. Even for simple harmonic motion, heaps of a pair of convection cells overlapped each other. In the case of elliptical motion, the overlapping portion of the heaps became larger. In the case of circular motion, the two heaps were integrated into one circular heap, and there were no effects of the circumferential angle on the particle velocity and the characteristic positive pressure.     

Wavelet packet analysis of particle response to turbulent fluctuation

The fluid–particle synchronous measurements in a boundary layer wind tunnel were conducted to determine the particle concentration response to turbulent velocity fluctuation. Three groups of natural sand samples (diameter of 300–500, 100–125 and 63–80 μm) were employed in the experiments. Consecutive instants of saltating particles were recorded by using a high-speed digital camera at 2000 frames per second and a constant-temperature hot-wire anemometer was used to measure the turbulent fluctuation simultaneously. The particle concentration in the saltation layer was calculated by the dynamic-threshold binarization algorithm. The results confirm that the concentration fluctuation is a fairly typical stochastic process, and the low-frequency variation of particle concentration is closely related to the turbulent fluctuation. Moreover, a method was developed to apply wavelet packet transform to two-phase data analysis from the viewpoint of frequency-domain energy structure. Further analysis shows that the concentration fluctuation is predominant in the low frequency band less than 250 Hz. In addition, the particle concentration response to the turbulent fluctuation is significantly correlated with the particle diameter. For the fine particles (63–80 μm), medium particles (100–125 μm) and coarse particles (300–500 μm), the highest response frequencies of particle concentration variation to the turbulent fluctuation are 60, 40 and 30 Hz, respectively, which demonstrates that an appropriate sampling rate is crucial in saltation measurement. These qualitative and quantitative results are beneficial to understand the fluid–particle interaction mechanism.     

Effects of main particle diameter on improving particle flowability for compressed packing fraction in a smaller particle admixing system

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the mechanisms by which this technique improves flowability are not yet fully understood. In this study, we examined compressed packing in a particle bed, which is affected by particle flowability. To estimate the mechanism of improvement, we investigated the effects of the main particle diameter on the improvement of compressed packing fractions experimentally.The main particles were 397 and 1460 nm in diameter and the admixed particles were 8, 21, 62, and 104 nm in diameter. The main and admixed particles were mixed in various mass ratios, and the compressed packing fractions of the mixtures were measured. SEM images were used to analyze the coverage diameter and the surface coverage ratio of the admixed particles on the main particles. The main particle packing fraction was improved as the diameter ratio (=main particles/admixed particles) increased. This was explained by a linked rigid-3-bodies model with leverage. Furthermore, the actual surface coverage ratio at which the most improved packing fraction was obtained decreased with increasing main particle diameter. This was explained by the difference in the curvature of the main particle surface.     

Effect of processing methods on physicochemical properties of titania nanoparticles produced from natural rutile sand

Titania (TiO₂) nanoparticles were produced from natural rutile sand using different approaches such as sol–gel, sonication and spray pyrolysis. The inexpensive titanium sulphate precursor was extracted from rutile sand by employing simple chemical method and used for the production of TiO₂ nanoparticles. Particle size, crystalline structure, surface area, morphology and band gap of the produced nanoparticles are discussed and compared with the different production methods such as sol–gel, sonication and spray pyrolysis. Mean size distribution (d₅₀) of obtained particles is 76 ± 3, 68 ± 3 and 38 ± 3 nm, respectively, for sol–gel, sonication and spray pyrolysis techniques. The band gap (3.168 < 3.215 < 3.240 eV) and surface area (36 < 60 < 103 m² g^?1) of particles are increased with decreasing particle size (76 > 68 > 38 nm), when the process methodology is changed from sol–gel to sonication and sonication to the spray pyrolysis. Among the three methods, spray pyrolysis yields high-surface particles with active semiconductor bandgap energy. The effects of concentration of the precursor, pressure and working temperature are less significant for large-scale production of TiO₂ nanoparticles from natural minerals.     

Preparation of spherical silica in emulsion systems using the co-precipitation technique

Silica powders with particles of spherical shape and a low tendency to agglomeration of primary particles have been prepared by precipitation from sodium metasilicate and hydrochloric acid via the emulsion technique. The organic phase of the emulsion system was cyclohexane while a non-ionic surfactant was applied as an emulsifier. In the course of silica precipitation three alternative ways of dispersion induction were applied: the top stirrer, homogenization and ultrasounds. The precipitation was performed at three temperatures (25 °C, 40 °C and 60 °C), using different variants of dosing schemes of the emulsion and the aqueous solutions of substrates. Homogenization of the reactive system yielded silica of spherical particles of the lowest mean particle diameter and the most uniform character. The only appropriate mode of dosing the reagents to the emulsion was the introduction of alkaline emulsion to acidic emulsion. A decreased tendency for silica particles agglomeration could be achieved by increasing the volume of the organic phase in the emulsions prepared. The optimum temperature for the precipitation reaction was 25 °C. The procedure permitted obtaining in SiO₂ particles of the optimum diameter (<1 μm) and particles of spherical shape. Moreover, the silica adsorbents obtained manifested high activity. Their surface area reached values in the 340–390 m² g^?1 range.     

Influence of the hydrophobic force model on the capture of particles by bubbles: A computational study using Discrete Element Method

Discrete Element Method computer simulations have been carried out to analyse the influence of the hydrophobic force model on the capture of particles by a central bubble. Two hundred particles, with diameters ranging between 24 and 66 μm, were randomly positioned within a maximum distance from the surface of a bubble of 2 mm in diameter. Initial particle velocities were random in direction and value and they followed Gaussian distributions with standard deviations between 0.0 and 1.0 m/s. Three possible models, named A, B and C have been used in the simulations. The models correspond to different published relationships of the hydrophobic force with the distance between particle and bubble surfaces, d. Model A corresponds to a hydrophobic force that decays in the form 1/d; the hydrophobic force given by Model B uses a relationship in the form 1/d²; Model C predicts a force that decays in an exponential way in the form exp(?d/λ). These models have also been compared with a base case in which the hydrophobic force only acted when the particles were in contact with the bubble. Therefore, we could better discern between the influence of the initial particle velocities and the long range component of the hydrophobic force. The differences in the capture efficiency of the particles predicted by the three models were drastic. All particles were captured by the bubble in the cases simulated using Model A for any particle–bubble surface distance smaller than 1 mm. However, only 40% and 60% of the particles were captured even for particles located at distances of less than 50 μm from the bubble surface in the cases simulated using Models B and C (λ = 1 μm), respectively. In fact, the capture of particles seems to be more strongly influenced by how the hydrophobic force decays with interparticle distance in the range of tens of micrometres rather than by the differences between the models in the range of micrometres. Therefore, this work should aid in the future determination of a general hydrophobic force model through an experimental comparison of the kinetics of collision of particles against bubbles in flotation cells with the simulation results.     

A novel fiber-reinforced polyethylene composite with added silicon nitride particles for enhanced thermal conductivity

A novel thermally conductive plastic composite was prepared from a mixture of silicon nitride (Si₃N₄) filler particles and an ultrahigh molecular weight polyethylene–linear low density polyethylene blend. The effects of Si₃N₄ particle sizes, concentration, and dispersion on the thermal conductivity and relevant dielectric properties were investigated. With proper fabrication the Si₃N₄ particles could form a continuously connected dispersion that acted as the dominant thermally conductive pathway through the plastic matrix. By adding 0–20% Si₃N₄ filler particles, the composite thermal conductivity was increased from 0.2 to ～1.0 W m^?1 K^?1. Also, the composite thermal conductivity was further enhanced to 1.8 W m^?1 K^?1 by decreasing the Si₃N₄ particle sizes from 35, 3 and 0.2 μm, and using coupling agent, for the composites with higher filler content. Alumina short fibers were then added to improve the overall composite toughness and strength. Optimum thermal, dielectric and mechanical properties were obtained for a fiber-reinforced polyethylene composite with 20% total alumina–Si₃N₄ (0.2 μm size) filler particles.     

Spectroscopic characterizations of Er doped LaPO4 submicron phosphors prepared by homogeneous precipitation method

Hexagonal shaped LaPO₄ submicron particles doped with various concentrations of Er were successfully prepared by homogenous precipitation method using metal nitrates and ammonium phosphate. Particles of approximate particle size 125 nm and size distribution of 85 nm are obtained with good crystallinity. After heat treatment at 1200 °C for 2 h, the particles are characterized for their various optical properties such as absorption, emission, fluorescence decay and optical band gap. Optical absorption and emission data are numerically analyzed with the help of Judd–Ofelt model to evaluate various radiative spectral properties such as radiative decay rates, radiative quantum yield, emission cross-section and fluorescence branching ratios of various emission transitions. Though the radiative quantum yield of 1554 nm emission approaches the theoretical limit of 100%, the experimentally measured quantum yield is only 11% at 12 W/cm² at 980 nm excitation power density in 2% Er doped LaPO₄.     

Microscopic analysis of saltation of particles on an obliquely oscillating plate

This paper presents a microscopic analysis of the saltation of particles on an obliquely oscillating plate driven by sine waves with an amplitude on the order of tens of micrometers and a frequency on the order of hundreds of hertz. To examine the effect of the diameter of a particle on its motion, the trajectories and velocities of different-sized particles, from 0.5 to 500 μm in mass median diameter, are analyzed using images captured by a high-speed microscope camera. The results show that larger particles bounce higher, whereas smaller particles easily agglomerate and bounce only slightly, owing to the low restitution caused by their loosely packed structure. In addition, larger particles bounce forward and backward repeatedly, while the agglomerated particles always bounce forward, and consequently have the highest transport velocity among these particles. The particle motion and the transport velocity can be explained by a theoretical probability model.     

Image analysis algorithm and verification for on-line molecular sieve size and shape inspection

An online machine vision inspection method is proposed to implement feedback control of molecular sieve growth process in rotary drum granulation. An experimental platform, comprising of a high-resolution digital camera and an image analysis system, has been developed to confirm the validity of the method on particle size distribution (PSD) and sphericity measurements. Experiments were performed with non-uniform molecular sieve particles (1–3 mm diameter) obtained from production line. The particle images are first obtained through digital camera and are then processed by image analysis system. After separating the overlap particles and removing non-target particles of the images, the molecular sieve size and shape are computed in less than 0.9 s. The validity of the size measuring accuracy is confirmed through comparing with the results from micrometer. The experimental results show that the repetitive precision of the proposed inspection system is about ±1%, the diameter measurement error between image method and micrometer is about ±3%, single image inspection speed is around 0.9 s/frame. The proposed method is reliable to provide feedback information for control system in rotary drum granulation.     

Synthesis of nanometer-sized ceria particles by alternating current electrolysis of aqueous solution

Ceria particles in an average size range from 8 to 70 nm were synthesized from cerium nitrate solutions by electrolysis at AC 0.1–10 Hz using platinum wire electrodes at 25–80 °C. The produced ceria particles dissolved in low pH solutions (pH 1.1–2.7) at a longer electrolysis time (>12 h), which caused the decrease of particle size. Increase of the concentration of Ce³⁺ ions and increase of the electrolysis temperature were effective to enhance the particle yield. Small growth of particles (10–20 nm) was measured when the electrolysis temperature was increased to 60–80 °C. When the applied frequency was increased, the particle size decreased. A theoretical equation of particle size as functions of Ce³⁺ ion concentration, electrolysis temperature and applied frequency was derived. The experimental results were in accordance with the prediction from the theoretical model.     

Thermal stability of MgO nanoparticles

Thermal stability of nanocrystalline MgO particles with average diameter of 11 nm was investigated by annealing of the cold isostatically pressed compacts between 600 °C and 900 °C for various durations. Sintering time versus grain radius at 800 °C demonstrated a linear line with the slope of ∼ 4 similar to that expected for surface diffusion. High resolution scanning electron microscope images from different specimens showed a porous microstructure of interconnected particles typical for initial sintering. Arrhenius plot of the grain size data revealed the activation energy of 161 ± 11 kJ mol^− 1 for the growth process in agreement with those reported for grain boundary grooving experiments. It was found that MgO particles undergo coarsening already at temperatures as low as 0.31 of the MgO melting point (3125 K). Increase in the particle diameter and decrease in the surface area were associated with surface diffusion mechanism that leads to initial sintering between the particles.     

Dynamics analysis of cavitation disintegration of microparticles during nanopowder preparation in a new Water Jet Mill (WJM) device

A physical analysis of cavitation-based implosive breakage of solid particles focusing on practical application during fine particle disintegration in a liquid suspension is submitted in the present paper. The physical source of the cavitation dynamics phenomena involved is an extreme velocity gradient induced by an ultrahigh-energy liquid jet mixing together with a slow liquid suspension of milled particles. Extreme tensile stresses occurring at velocity gradients over 1000 ms^?1mm^?1 at the operating temperature of 65 °C generates high-intensity pure vapor cavitation in the degassed water dispersion with extreme values of impact pressure in the final of bubble implosions on particle surfaces.Preparation of silicon nanoparticles with median diameter approximately 148 nm using a newly developed “Water Jet Mill” (WJM) device is demonstrated in the present article as an example of application of the aforementioned disintegration method as well as of theoretical analysis of this method. The disintegration method is characterized by a high potential for milling of submicron particles with high efficiency.     

Erosion in the tube entrance region of an air-cooled heat exchanger

Erosion in the tube entrance region of a typical air-cooled heat exchanger is numerically predicted. The erosion rates are obtained for different flow rates and particle sizes assuming low particle concentration. The erosion prediction is based on using a mathematical model for simulating the fluid velocity field and another model for simulating the motion of solid particles. The fluid velocity model is based on the solution of the time-averaged governing equations of 3-D turbulent flow while the particle-tracking model is based on the solution of the governing equation of each particle motion taking into consideration the viscous and gravity forces as well as the effect of particle rebound behavior. The computational model was validated against available experimental data and the comparison resulted in a good agreement. The investigation covered particle sizes from 10 to 350 μm and inlet flow velocities from 0.18 to 4.5 m/s. The results show that the location and number of eroded tubes depend mainly on the particle size and flow velocity at the header inlet. The total rate of erosion was found to increase exponentially with flow velocity. At high flow velocities, the maximum total erosion rate results from large particles and the effect is reversed at low velocities. Similarly, the tube penetration rate was found to increase with the increase of flow velocity for all particle sizes. At the typical velocity of 1.1 m/s, the minimum tube lifetime was caused by the 350 μm particles and the maximum was caused by the 200 μm particles. Based on the obtained results, it is well established that erosion cannot be totally avoided so long as solid particles are present in the fluid. However, the threshold velocity below which erosion is negligible can be accurately defined if an acceptable lifetime (or penetration rate) is defined.     

Encapsulation of a pressure sensitive adhesive by spray-cooling: Optimum formulation and processing conditions

An industrial pressure sensitive adhesive (PSA) was encapsulated by spray-cooling using hydrogenated palm oil. A screening design methodology was used to evaluate the impact of some formulation and process variables on the particle properties. Six operating factors were retained and the results considered were the production yield, the particle volume-surface average diameter D₃₂, the residual humidity, the ratio of the fusion enthalpies of the polymorphs α and β′/β and the normalized peeling force. The statistical analysis of the results showed a negligible impact of the parameters related to the process. The heating temperature, the PSA and surfactant ratios were the most significant factors. It was possible to produce spherical particles with a mean size of 17.7 μm and a normalized peeling force of 0.218 N m²/g. The production yield was 70%. A duplicate test confirmed the results. Mechanical tests on unitary particles showed an increase of the rupture and adhesion forces with particle size.