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.     

Effects of discharging vibration conditions and coating structures on improving discharge particle flowability in a smaller particle admixing system

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the effects of coating structures on the improvement of flowability are not yet fully understood. In this study, we focused on vibrating discharge particle flowability and investigated the effect of discharging vibration conditions and coating structures on improving the flowability. Main and admixed particles of 60.8 μm and 8 nm in diameter, respectively, were mixed in various mass ratios, and the discharge particle flow rates of the mixed particles were measured. Scanning electron microscopy and scanning probe microscopy images were used to analyze the coverage diameter, surface coverage ratio, and coverage height of the admixed particles on the main particle surfaces. As a result, the admixing mass ratio that gave maximum flowability was found to depend on the maximum value of the vibration acceleration. This could be explained by the relationship between the coating structures of admixed particles and the coated average surface distances due to the vibration acceleration.     

Influence of surface roughness created by admixing smaller particles on improving discharge particle flowability of main particles

Particle flowability can be improved by admixing with particles smaller than the main particles. However, the mechanism by which this technique improves flowability is not yet fully understood. In this study, we focused on vibrating discharge particle flowability as one type of flowability and investigated the influence of the main particle roughness created by the adhesion of the admixed particles on improving the flowability. The diameters of the main and admixed particles (MPs and APs) were 41.4 or 60.8?μm and 8 or 104?nm, respectively. The main and admixed particles were mixed in various mass ratios, and the discharge particle flow rates of the mixed particles were measured. Scanning electron microscopy images were acquired from two different angles to determine the three-dimensional surface roughness using image analysis software. We then calculated the coating structure parameters from the obtained three-dimensional surface roughness. The observed trends for improving the vibrating discharge particle flowability were found to differ from those reported for compression particle flowability. Furthermore, the main particle roughness conditions that led to the greatest improvement involved the presence of several admixed particle agglomerations between the main particles.     

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.     

DEM simulation analysis of the improvement in particle discharge flowability using adhesive force distribution models based on admixed particle coating

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the details of the mechanism of this improvement are not yet fully understood. In this study, we used a discrete element method simulation to investigate the effects on the particle flowability of the adhesive force distribution at each contact point based on the admixed particle coating. We used the non-uniform, random, and uniform surface adhesive force distribution models and calculated the discharge flow rates. The non-uniform models had a larger discharge flow rate compared with the other models because the non-uniform adhesive force distribution destabilized the force balance in the bed, and thus destabilized the particle arching structure, which generated discontinuous layers more frequently and improved the flowability. Consequently, in a smaller particle admixing system, the adhesive force distribution at each contact point would help to improve the flowability.     

A concise treatise on model-based enhancements of cohesive powder properties via dry particle coating

This paper presents a review of our key advances in model-guided dry coating-based enhancements of poor flow and packing of fine cohesive powders. The existing van der Waals force-based particle-contact models are reviewed to elucidate the main mechanism of flow enhancement through silica dry coating. Our multi-asperity model explains the effect of the amount of silica, insufficient flowability enhancements through conventional blending, and the predominant effect of particle surface roughness on cohesion reduction. Models are presented for the determination of the amount and type of guest particles, and estimation of the granular Bond number, used for cohesion nondimensionalization, based on particle size, particle density, asperity size, surface area coverage, and dispersive surface energy. Selection of the processing conditions for LabRAM, a benchmarking device, is presented followed by key examples of enhancements of flow, packing, agglomeration, and dissolution through the dry coating. Powder agglomeration is shown as a screening indicator of powder flowability. The mixing synergy is identified as a cause for enhanced blend flowability with a minor dry coated constituent at silica < 0.01%. The analysis and outcomes presented in this paper are intended to demonstrate the importance of dry coating as an essential tool for industry practitioners.     

Numerical study on compression processes of cohesive bimodal particles and their packing structure

In this study, the compression characteristics of bimodal cohesive particles were investigated using a discrete element method (DEM) simulation. The compression and packing processes were simulated under different conditions of size ratios of 1–4 and fine particle mixing ratios of 0–0.5. The cohesive force was expressed using the surface energy proposed by the Johnson-Kendall-Roberts (JKR) cohesion model having a surface energy of 0–0.2 J/m². The calculated results demonstrated that even in the case of cohesive particles, an increase in the particle size ratio reduced the void fraction of the powder bed during the packing and compression processes. In addition, it was found that the cohesive force decreased the contact number, especially the coarse-coarse contacts, although it had little impact on the void fraction. Our DEM simulations suggested that it is necessary to evaluate the contact numbers even under similar void fractions, which will be essential in the case of different material mixtures, such as all-solid-state batteries.     

尖棱状两组分颗粒堆积密度的研究

为了探讨耐火材料制品的密堆积结构特性,从而获得较好的颗粒堆积密度,研究了圆锥破碎机制得的粗、细两种离散颗粒体系的自由堆积密度变化．研究结果表明：当粗、细两种颗粒组分的拉径比大于5、且粗颗粒的质量分数在60%～70%之间时,体系的自由堆积密度较大；当粗、细两种颗粒的粒径比小于5、且无细粉存在时,离散颗粒体系的自由堆积密度变化不大。     

Flow improvement of fine oxidizer using nano-additives

Ammonium perchlorate (AP) is an oxidizer and major component for any solid composite propellant. The handling and processing of AP powder plays an important role in the performance of rocket fuel. AP being hygroscopic in nature; especially particle size<50 μm is highly cohesive in nature, which results in extensive agglomeration and poor flowability. This causes inherent handling problems in the propellant processing plant. In this work, the flowability of 8 μm and 50 μm AP particles was improved through the dry coating method using nano-additives such as TiO₂ and Fe₂O₃. In addition, the influence of hydrophobic nano-silica (R972 type) coating on AP particles under high humidity was investigated. The results showed that coating of AP with nano-additive reduced the inter-particle forces hence cohesivity and improves the packing and flowability of fine AP powder. Hydrophobic coating was found to have added advantage at higher humidity. Moreover, coating with these nano-additives also improved the exothermicity of AP.     

Impact of powder composition on processing-relevant properties of pharmaceutical materials: An experimental study

In this work, we studied the influence of powder composition on packing density and other processing-relevant properties of binary mixtures, including powder flowability. Binary mixtures of pharmaceutical powders with different particle size ratios, α and varying fractions of large and small particles were analyzed systematically. Mixtures of three excipients and one API with different composition (2, 5, 10, 30, 50, 70, 90, 95 and 98 wt%) were prepared in a Turbula mixer. Powders with different properties and particle size distribution were chosen, in order to obtain three binary mixtures with different size ratios. Then, macroscopic powder properties including bulk (poured) and tapped density (BD and TD) were measured. A powder rheometer was used to measure the flow function coefficient (ffc), cohesion, compressibility and permeability of the binary mixtures. We considered experimentally three classes of binary mixtures, which are characterized by two critical ratios of particle diameter: the critical size ratio of entrance (α_c) and the critical size ratio of replacement (α_r), where α_c = 0.154 and α_r = 0.741. Below the critical size ratio of entrance (α_c), the particle asymmetry (ratio between large and small particle diameters) is high and small particles can fill the voids between larger ones. Between α_c and the critical size ratio of replacement (α_r), the smaller particles are too large to fit in the voids between larger particles (packing structure changes). Above α_r, the particles are more or less symmetric in size and overall packing structure does not change by mixing the particles. Our experiments show that there is a non-linear and non-monotonic dependence of all relevant properties on composition for powder mixtures that have an α < α_r. This non-linear behavior is even more significant for strongly asymmetric binary mixtures with α < α_c. We argue that this behavior is related to the composition dependence of random packing of particulate systems. Our results have relevance to pharmaceutical particle processing operations where constant powder mixture properties are needed to ensure quality standards are met; such operations include capsule or die filling during tableting, and the continuous feeding of powders via screw feeders. Our results suggest that for pharmaceutical particle processing operations, where constant powder mixture properties are a prerequisite for process robustness, the size ratio of API and excipient particles, α should not be smaller than α_r = 0.741.     

A New Method for Prediction of Bulk Particle Packing Behavior for Arbitrary-Shaped Particles inContainers of Any Shape

This article describes the recent developments in the computer modeling of packing of complex-shaped particles and prediction of physical properties of the structures represented by the packing. The computer model DigiPac is capable of packing particles of any shapes and sizes in a container of arbitrary geometry. The ability to predict the packing structure of real particle shapes and to compute directly some structure-dependent physical properties such as liquid permeability, mechanical strength/stability, compaction and sintering, and dissolution and leaching is obviously highly desirable and has significant potential in industrial applications. Examples are presented relating to the packing of bulk and granular materials.     

A New Method for Prediction of Bulk Particle Packing Behavior for Arbitrary-Shaped Particles inContainers of Any Shape

This article describes the recent developments in the computer modeling of packing of complex-shaped particles and prediction of physical properties of the structures represented by the packing. The computer model DigiPac is capable of packing particles of any shapes and sizes in a container of arbitrary geometry. The ability to predict the packing structure of real particle shapes and to compute directly some structure-dependent physical properties such as liquid permeability, mechanical strength/stability, compaction and sintering, and dissolution and leaching is obviously highly desirable and has significant potential in industrial applications. Examples are presented relating to the packing of bulk and granular materials.     

Effect of the particle size ratio on the structural properties of granular mixtures with discrete particle size distribution

In this study, the discrete element method was used to examine the structural properties and geometric anisotropy of polydisperse granular packings with discrete uniform particle size distributions. Confined uniaxial compression was applied to granular mixtures with different particle size fractions. The particle size fraction (class) was defined as the fraction of the sample composed of particles with a certain size. The threshold value of number of particle size fractions (i.e., the value above which structural properties of assemblies remain constant) was determined. The effect of heterogeneity in particle size on the critical value of number of particle size fractions was investigated for packings with different ratios between diameters of the largest and smallest grains. The threshold number of particle size classes decreased from five to three as the diameter ratio between the largest and smallest grains increased. Regardless of the diameter ratio, the critical number of particle size fractions (above which the packing density and coordination number of the granular mixtures remained constant) was determined to be five. The study has also shown an increase in packing density of binary mixtures with particle size ratio increasing up to 2.5, which was followed by decrease in density of mixtures with larger particle size ratios, which has not so far been reported in the literature.     

Powder characteristics and coating conditions of fresh and reused polyester resins for electrostatic powder coating: powder recycling and loss prevention

Powder characteristics and coating conditions are significant factors in electrostatic powder coating. In this work, powder characteristics of the reused polyester resin or recycled powder particles in terms of shape, size, particle size distribution, moisture content, density, flowability, fluidity and chargeability were compared with those of fresh resin or as-received powder to consider powder recycling. The coating conditions for a metal lantern electrostatic powder coating were studied with fresh polyester resin on a work-piece of 70 × 150 mm using a manual corona spray gun by varying the spray gun voltage, the distance between the spray gun and work-piece, and the gun output. For full utilization of powder particles, the effects of different weight ratios of fresh and reused polyester resins on film thickness were observed using the obtained coating conditions accordingly. The suitable weight ratio of fresh and reused polyester resins was 3:1. In addition, to prevent loss of powder particles in automatic electrostatic powder coating due to over-spraying, the relations between the displacements of the spray guns and conveyor distance were simulated by Microsoft Excel XP on a model metal lantern of 600 × 1216 × 120 mm. No overlapping of the displacements of the spray guns must be considered.     

石墨微粉的流动性及其影响因素

针对石墨微粉在运输过程中易振实,严重影响施放烟幕质量的问题,根据Carr指数法原理测试多个样品的流动性能,并通过模拟汽车运输振动试验获得石墨微粉的具体振实情况,研究粉体压缩度、粒径分布、流散剂包覆效果等因素对流动性能的影响规律。结果表明:当粒径在4.0~13.0μm的石墨微粒的体积分数大于60%、中值粒径为5.5μm左右,且流散剂混合与包覆效果较均匀时,石墨微粉的流动性和抗振实性能较好。     

Effect of particle agglomeration on the magnetization reversal of fine powders

The magnetization reversal of loose low-coercivity fine powders of magnetite or iron is independent of the value of the particle packing fraction. However, when these powders are compacted under pressure in a solid matrix of Perspex, the coercivity of the samples increases logarithmically with the increasing powder density. We assume that in these powders small superparamagnetic particles are agglomerated into clusters. Supposing the magnetization process to be due to a nucleation mechanism, the critical field at which this nucleation starts appears to be a function of the number of particles agglomerated in a cluster. This number varies with the density of the powder compacted in the matrix. By the suggested theoretical model, we have estimated the diameter of the superparamagnetic particles of the experimentally investigated fine iron powder to be approximately 46 Å. Valuable information about the magnetic structure of these powders has been obtained from the studies of the hyperfine splitting in their Mössbauer patterns and from those of the changes in the line intensities caused by the application of an external magnetic field.     

A hierarchical approach to simulate the packing density of particle mixtures on a computer

In many fields of materials science it is important to know how densely a particle mixture can be packed. The “packing density” is the ratio of the particle volume and the volume of the surrounding container needed for a random close packing of the particles. We present a method for estimating the packing density for spherical particles based on computer simulations only, i.e. without the need for additional experiments. Our method is particularly suited for particle mixtures with an extremely wide range of particle diameters as they occur e.g. in modern concrete mixtures. A single representative sample from such mixtures would be much larger than can be handled on present standard computers. In our hierarchical approach the diameter range is therefore divided into smaller intervals. Samples from these limited diameter intervals are drawn and their packing density is estimated from a simulated packing. The results are used to “fill” the interstices in the sample from the next larger particle interval. To account for the interaction between particles of different sizes we include larger particles into the sample of smaller ones. The larger ones act as part of the boundary during the packing. Thus we obtain more realistic estimates of how dense a fraction of particles can be packed within the whole mixture. The focus of this paper is on the divide-and-conquer approach and on how the simulation results from the fractions can be collected into an overall estimate of the packing density. We do not go into details of the simulation technique for the single packing. We compare our results to some experimental data to show that our method works at least as good as the classical analytical models like CPM without the need for any experiments.     

Monitoring and control of coating and granulation processes in fluidized beds – A review

This review presents a compilation of works of the main techniques for monitoring and control fluidization regimes, particle size and moisture content during coating and granulation processes in the fluidized bed. The development of monitoring and control systems for coating and granulation of particles is highly desirable, not only to allow the operation in a stable bubbling fluidization regime, which intensifies heat and mass transfer, but also to ensure strict quality specifications for products, such as, uniform particle size distribution, low moisture content and good flowability. This paper focuses on the discussion of methods used and results obtained in studies on monitoring and control of granulation and coating process in the fluidized bed reported in the literature in the last decades. Pressure fluctuation signal analysis is widely discussed as a tool of regime monitoring. To monitor particle size, techniques such as, Near Infrared spectroscopy (NIR), Focused Beam Reflectance Measurements (FBRMs), among others are presented in detail. As for moisture content tracking, the methods are reviewed like acoustic signals, capacitance, microwave resonance and spectroscopy. It is evident that although these processes are highly complex, the techniques presented here have evolved mainly due to the efforts of several research groups, showing great potential for applications in industry, emphasizing the importance of this research field.     

Effect of lognormal particle size distributions on particle spreading in additive manufacturing

Additive manufacturing (AM) has attracted much attention worldwide in various applications due to its convenience and flexibility to rapidly fabricate products, which is a key advantage compared to the traditional subtractive manufacturing. This discrete element method (DEM) study focusses on the impact of particle polydispersity during the particle spreading process on parameters that affect the quality of the final product, like packing and bed surface roughness. The particle systems include four lognormal particle size distribution (PSD) widths, which are benchmarked against the monodisperse system with the same mean particle diameter. The results reveal that: (i) the solid volume fraction of the initial packed particle bed in the delivery chamber increases then plateaus as the PSD width increases; (ii) regardless of PSD width, the solid volume fraction of the particle bed increases with spreading layer height before compression, but decreases with layer height after compression; (iii) the bed surface roughness increases with PSD width or layer height both before and after the compression of the spreading layer; (iv) the extent of increase in solid volume fraction during compression is correlated with the extent of decrease in bed surface roughness; and (v) the broader PSDs exhibit larger fluctuations of solid volume fraction of the particle bed and bed surface roughness due to greater variability in the arrangement of particles of different sizes. The results here have important implications on the design and operation of particle-based AM systems.