Machine vision based particle size and size distribution determination of airborne dust particles of wood and bark pellets

Dust management strategies in industrial environment, especially of airborne dust, require quantification and measurement of size and size distribution of the particles. Advanced specialized instruments that measure airborne particle size and size distribution apply indirect methods that involve light scattering, acoustic spectroscopy, and laser diffraction. In this research, we propose a simple and direct method of airborne dust particle dimensional measurement and size distribution analysis using machine vision. The method involves development of a user-coded ImageJ plugin that measures particle length and width and analyzes size distribution of particles based on particle length from high resolution scan images. Test materials were airborne dust from soft pine wood sawdust pellets and ground pine tree bark pellets. Subsamples prepared by dividing the original dust using 230 mesh (63 μm) sieve were analyzed as well. A flatbed document scanner acquired the digital images of the dust particles. Proper sampling, layout of dust particles in singulated arrangement, good contrast smooth background, high resolution images, and accurate algorithm are essential for reliable analysis. A “halo effect” around grey-scale images ensured correct threshold limits. The measurement algorithm used Feret's diameter for particle length and “pixel-march” technique for particle width. Particle size distribution was analyzed in a sieveless manner after grouping particles according to their distinct lengths, and several significant dimensions and parameters of particle size distribution were evaluated. Results of the measurement and analysis were presented in textual and graphical formats. The developed plugin was evaluated to have a dimension measurement accuracy in excess of 98.9% and a computer speed of analysis of < 8 s/image. Arithmetic mean length of original wood and bark pellets airborne dust particles were 0.1138 ± 0.0123 and 0.1181 ± 0.0149 mm, respectively. The airborne dust particles of wood and bark pellets can be described as non-uniform, finer particles dominated, very finely skewed with positive skewness, leptokurtic, and very well sorted category. Experimental mechanical sieving and machine vision methods produced comparable particle size distribution. The limitations and merits of using the machine vision technique for the measurement of size and size distribution of fine particles such as airborne dust were discussed.     

An improved estimation of size distribution from particle profile measurements

Several methods are available to measure particle size. The majority of them, such as sieving, are off-stream techniques where samples must first be separated from the main stream for analysis.Therefore, the search for on-line particle size analysis systems has provided the impetus for the introduction of image-based particle size analysers to the mineral industry in the past three decades. Generally, the estimation of particle size distribution on the basis of image analysis depends on measuring a single parameter of particle profile. For example the equivalent area diameter (d_A) or mean Feret's diameter (d_F) distributions, then transforming this data to the equivalent size distribution. However, due to the irregularity of particles being analysed, it is believed that this kind of analysis may increase the error in estimation of particle size distribution since profiles of irregular particles carry more information than can be represented by a single parameter.In this paper, a proposed technique which measures two parameters, equivalent area diameter (d_A) and mean Feret's diameter (d_F), for each particle profile has been developed. The accuracy of the technique has then been investigated in the laboratory by successfully estimating (unfolding) the size distribution, where size refers to sieve size, of three samples of different particle shapes with known size distribution.     

Influence of particle size distribution on rheology and particle packing of silica-based suspensions

The effect of particle size, particle size distribution and milling time on the rheological behaviour and particle packing of silica suspensions was investigated using slurries containing total solids loading of 46 vol.%. Three silica powders with different average particle sizes (2.2, 6.5 and 19 μm), derived from dry milling of sand, and a colloidal fumed silica powder with 0.07 μm were used. Different proportions of colloidal fumed silica powder were added to each of the coarser silica powders and the mixtures were ball-milled for different time periods. The influence of these factors and of the particle size ratio on the rheological behaviour of the suspensions and densities of green slip cast bodies was studied.The results show that the flow properties of slips are strongly influenced by the particle size distribution. The viscosity of suspensions increases with the addition of fine particles, imposing some practical limitations in terms of volume fraction of fines that can be added. On the other hand, increasing the size ratio enhanced the shear thinning character of the suspensions, while decreasing the size ratio led to an accentuation of the shear thickening behaviour. For all mixed suspensions, green densities increased with increasing milling time, due to size reduction of silica powders and a more efficient deagglomeration of fumed silica. Increasing amounts of fumed silica led to a first increase of particle packing up to a maximum, followed by a decreasing trend for further additions. Good relationships could be observed between rheological results and packing densities.     

Measurement of the floating particle size distribution by a buoyancy weighing-bar method

The particle size distribution of cylinder-shaped solid particles was measured using a buoyancy weighing-bar method where the liquid phase density was adjusted to settle or float the particles. The particle size distribution obtained in our experiment agrees with the particle size measured by other method. The present study demonstrates that a buoyancy weighing-bar method, a novel method for measuring the particle size distribution, is suitable for measuring the particle size distribution of a floating solid. The precision of the resulting particle size distribution is comparable to that of a laser diffraction/scattering method as well as a direct measurement with a micrometer. Moreover, this buoyancy weighing-bar method can measure the particle size distribution even in a mixture of two particles with different sizes.     

Control of particle size distribution for the synthesis of small particle size high solids content latexes

A polymerization process to synthesize bimodal latexes with maximum particle diameters below 350 nm and solids content above 65 wt% has been developed.The process is based on an iterative strategy to determine the optimal particle size distribution that gives the maximum packing factor for a given range of particle sizes and at a given solids content. The calculated optimal bimodal PSD was experimentally obtained in a seeded semi-continuous emulsion polymerization reaction as follows: in the first step, a polymer seed latex was loaded in the reactor and grown, under monomer starved conditions, until a given particle size. At this point a fraction of the same seed was added to the reactor and the feed was continued until the desired particle size distribution and solids content were achieved. The point at which the seed was added again to the reactor and the amount of seed required were determined by the iterative strategy and depended on the competitive growth rate ratio of large and small particles that is an input for the iterative strategy.Implementation of the solution obtained from the iterative strategy, and for the first time in the open literature, led to the production of a coagulum free and stable bimodal latex with 70 wt% of solids content and particle sizes below 350 nm.     

Variation of particle size and its distribution during the synthesis of silicalite-1 nanocrystals

Recent studies imply that the external surface area of the nanozeolite product may, at least in some cases, be related to the average size of the particle population participating in aggregative nucleation, a population which itself is a product of aggregation of even smaller primary nanoparticles. This possibility puts more importance on our understanding of the variation of particle size and its distribution during the crystallization of zeolite nanoparticles. Variation of the particle size and PSD during nanoparticle silicalite-1 crystallization was followed with respect to time by a laser light scattering device with a scattering angle of 173°, for several starting synthesis compositions. Effects of varying TPAOH and water contents in the starting synthesis mixtures on the variation with time of the particle sizes and PSDs, especially across the two distinct aggregation events, were investigated. The products were also analyzed by XRD and AFM. Parallel to the decrease in the average particle size of the final product population with increasing alkalinity and organic template content, its PSD was observed to become narrower too. A reversal in the dependence on TPAOH content, of the average size of the population formed by aggregation, with respect to that of the population participating in aggregation, was observed across both aggregation events, implying that smaller particles aggregated to form larger particles, while larger particles aggregated to form smaller particles during these processes, and this was also seen from the AFM images, to be reflected to the surface features of the final product particles.     

Alkali-activated waste ceramics: Importance of precursor particle size distribution

The waste ceramics belongs to wide range of aluminosilicate materials which can be alkaline-activated to geopolymer cement – possible “green” alternative to conventional Portland cement. The studied ceramic material is generated during the size adjustment of ceramic building blocks by means of grinding. It means that most of the material is very fine, but it contains also some larger shards. This ceramic powder was used as geopolymer precursor “as received” and after removal of particles retained on 1, 0.5 and 0.125 mm sieves. These four types of precursor were activated by sodium silicate (SiO₂/Na₂O = 1) solution. The prepared mortars were tested for strength, basic physical properties, transport parameters and characterized by help of XRD and thermal analysis. It was found that the best mechanical performance provided the precursor after removal of particles retained on 0.5 mm sieve thanks to the highest geopolymerization rate. The presence of coarser particles in precursor gave rise to porosity, what consequently influenced transport parameter of geopolymers towards the lower thermal conductivity and faster moisture transport.     

Challenges in particle size distribution measurement past, present and for the 21st century

The field of particle size distribution (PSD) characterization and measurement has experienced a renaissance over the past ten years. This revitalization has been driven by advances in electronics, computer technology and sensor technology in conjunction with the market pull for PSD methods embodied in cost effective user friendly instrumentation. The renaissance can be characterized by at least four activities. (1) End user innovation exemplified by techniques such as hydrodynamic chromatography (HDC), capillary hydrodynamic fractionation (CHDF) and field flow fractionation methods (SdFFF, FlFFF, and ThFFF). (2) Revitalization of older instrumental methods such as gravitational and centrifugal sedimentation; (3) Evolution of research grade instrumentation into low cost, routine, user friendly instrumentation exemplified by dynamic light scattering (DLS). (4) The attempt to meet extremely difficult technical challenges such as: (a) providing a single hybrid instrument with high resolution over a very broad dynamic range (4+ decades in size; e.g., Fraunhofer/Mie; photozone sensing/DLS); (b) PSD measurement of concentrated dispersions (acoustophoretic, dielectric measurements, fiber optic DLS (FOQELS)); (c) in-situ process particle size sensors (in-line or at line, e.g., FOQELS); (d) routine measurement of particle shape and structure (e.g., image analysis). Instrumental methods resulting from these activities are discussed in terms of measurement principles and the strengths and weaknesses of these methods for characterizing PSDs. Business and societal driving forces will impact customer perceived instrumentation and knowledge needs for the 21st century and the ability to meet the specific difficult technical challenges in particle size distribution characterization mentioned above. Anticipated progress toward meeting these technical challenges is discussed in conjunction with the associated anticipated advances in required technologies.     

A particle analyser for measuring particle size distribution and shape,on-line or in the laboratory

Norsk Hydro has developed a Particle Analyser for on-line or laboratory use, which measures particle size, size distribution and the deviation from sphericity (called nonspherical).The principle for this system is that the particles fall in a monolayer curtain in front of a high resolution CCD camera. The computer unit in the analyser measures and calculates the particle size and a sphericity factor for each particle. The data are presented as four real time trend curves, shown simultaneously for the on-line version. These curves show % oversize and fines, d₅₀ and the percentage nonspherical particles. For the laboratory version the data are presented in table and cumulative form.The on-line particle analyser has been installed in two of Norsk Hydro's prilling plants in Norway. The analyser has in both plants improved the product quality during the two years of installation.     

On the role of an unconventional rigid rodlike cationic surfactant on the styrene emulsion polymerization. Kinetics, particle size and particle size distribution

An unconventional amphiphile (1-[ω-(4′-methoxy-4-biphenylyloxy)octyl]pyridinium bromide, PC8) was used as surfactant in the emulsion polymerization of styrene. At low surfactant concentration (6, 12 or 36 mmol l⁻¹), curves of polymerization rate versus conversion obeyed the typical behavior characterized by intervals I, II and III. However, at high concentration (48 or 72 mmol l⁻¹) the interval II was not observed. The particle size distribution curves showed two families of polymer particles, indicating the participation of at least two mechanisms of particle formation, one being the simple micellar nucleation and the other probably the coagulative nucleation of precursor particles. The latter was considered to occur during the nucleation interval.     

Measurement methods of particle size distribution in emulsion polymerization

The particle size distribution of polymer always develops in emulsion polymerization systems, and certain key phenomena/mechanisms as well as properties of the final product are significantly affected by this distribution. This review mainly focuses on the measurement methods of particle size distribution rather than average particle size during the emulsion polymerization process, including the existing off-line, on-line, and in-line measurement methods. Moreover, the principle, resolution, performance, advantages, and drawbacks of various methods for evaluating particle size distribution are contrasted and illustrated. Besides, several possible development directions or solutions of the in-line measurement technology are explored     

Effect of wet screening on particle size distribution and coal properties

Wet screening is one of the methods used to remove fine material from the coal feed to gasification. Sasol Synfuels in South Africa undertook an investigation to quantify fine coal generation in the coal supply to gasification. Coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (−0.5 mm) increased almost five times when the coal was wet screened compared to dry screening. This study was subsequently initiated by Sasol Technology R&D to establish the mechanism of fine coal generation during wet screening, as well as the effect of wet screening on particle size distribution (PSD) and chemical properties of coal. Changes in the PSD and chemical properties of coal from individual coal sources used at Sasol Synfuels were compared. Composite coal samples with a predetermined PSD of all individual coal sources used at Sasol Synfuels were screened under wet and dry conditions. The PSD was again determined after screening, as well as the mineral composition (by X-ray diffraction) of the fines. Results indicated that wet screening caused clay minerals to be removed from the coal structure leading to an increase in the fines. This removal of minerals weakened the coal structure causing further size degradation of coarser fractions.     

On-line automatic optical inspection system for coarse particle size distribution

A new online automatic optical inspection system (OAOIS) using digital image processing has been developed to measure the coarse particle size distribution. The OAOIS is composed of particle separation module, image acquisition module, image processing and analysis module and PC/PLC-based electric control module. Experiments were performed with non-uniform particles (1-100 mm). The particle size distribution, number of the particles, and accumulated weight percentages of particles are obtained by using the developed system. The experimental results show that the repeated precision of accumulated weight percentages is around ± 1%. To improve the reliability and accuracy of the OAOIS measuring results, the linear regression equation is applied to mapping the result of OAOIS to that of traditional net sieving system (TNSS). It has been shown that the developed system has a high accuracy and precision for coarse particle sizing distribution.     

Effect of particle size distribution of raw powders on pore size distribution and bending strength of Al2O3 microfiltration membrane supports

To lower the sintering temperature of Al₂O₃ microfiltration membrane support, Al₂O₃ powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al₂O₃ grains depends on their agglomeration state: if fine Al₂O₃ grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al₂O₃ grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al₂O₃ powder has less effect on the pore size distribution of the support if only coarse Al₂O₃ grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.     

Effects of binary particle size distribution on minimum pick-up velocity in pneumatic conveying

Minimum pick-up velocities (U_pu) for entrainments of particle mixtures having binary particle size distributions (PSD) are measured in a horizontal pneumatic-conveying line using the weight-loss method. Geldart's groups A, B, and C glass beads having diameters of 400, 170, 40, and 5 μm are used. Variations in U_pu as a function of particle mass fraction (m) are examined. The capability of empirical correlations of monodisperse U_pu in predicting U_pu of binary mixtures is investigated. For group B particle mixtures (i.e. 400 & 170 μm), the particles are entrained separately resulting in linear U_pu variations with m, which is accurately predicted by the monodisperse U_pu correlation. For mixtures involving group A and B particles (i.e. 170 & 40, 400 & 40 μm), the two particles are collectively entrained resulting in U_pu that vary non-linearly with m and that cannot be predicted by the correlation. For mixtures involving group B and C particles (i.e. 400 & 5, 170 & 5 μm), U_pu are comparable to that of the monodisperse group B particles, therefore they are accurately predicted by the correlation. The significant impacts of binary PSD on U_pu found presently indicates that PSD effects on particle entrainment process warrants further investigations.     

Effects of particle size distribution in a three-dimensional micropolar continuum model of granular media

A particle size distribution is incorporated into a three-dimensional homogenisation scheme, devised on the scale of a particle and its immediate (or first ring) of neighbours. Based on this scheme, micropolar continuum models for polydisperse, dry, and densely packed granular assemblies of spherical particles undergoing quasi-static deformation are developed for various particle size distributions. Three different cases are considered: (1) a monodisperse assembly, (2) a defect particle in an otherwise monodisperse assembly, and (3) an assembly of a given particle size distribution. In Case 1, an additional dependence on particle radius is found in 3D systems, compared with previous 2D constitutive laws. In Case 2, it is found that a small (large) particle in an otherwise monodisperse system increases (decreases) the stress compared to a purely monodisperse assembly, but the couple stress may increase or decrease depending on the relative size of the rolling resistance compared with the tangential stiffness coefficients. On the other hand, if the defect particle is substantially smaller or larger than the monodisperse particle size, the stress and couple stress are always increased. In Case 3, three different distributions are examined, i.e. square, normal and a lognormal distribution. For Cases 2 and 3, both the stress and the couple stress increased with the degree of dispersity, from the lower bound value corresponding to the monodisperse system considered in Case 1. Finally, the paper highlights areas that will need to be addressed to enable the future advancement of micromechanical continuum models.     

Part I: Dynamic evolution of the particle size distribution in particulate processes undergoing combined particle growth and aggregation

The present study provides a comprehensive investigation on the numerical problems arising in the solution of dynamic population balance equations (PBEs) for particulate processes undergoing simultaneous particle growth and aggregation. The general PBE was numerically solved in both the continuous and its equivalent discrete form using the orthogonal collocation on finite elements (OCFE) and the discretized PBE method (DPBE), respectively. A detailed investigation on the effect of different particle growth rate functions on the calculated PSD was carried out over a wide range of variation of dimensionless aggregation and growth times. The performance (i.e., accuracy and stability) of the employed numerical methods was assessed by a direct comparison of predicted PSDs or/and their respective moments to available analytical solutions. It was found that the OCFE method was in general more accurate than the discretized PBE method but was susceptible to numerical instabilities. On the other hand, for growth dominated systems, the discretized PBE method was very robust but suffered from poor accuracy. For both methods, discretization of the volume domain was found to affect significantly the performance of the numerical solution. The optimal discretization of the volume domain was closely related with the satisfactory resolution of the time-varying PSD. Finally, it was shown that, in specific cases, further improvement of the numerical results could be obtained with the addition of an artificial diffusion term or the use of a moment-weighting method to correct the calculated PSD.     

New measurement of particle size distribution by a buoyancy weighing-bar method

The rates which particles from JIS Test Powders 1, Class 2 (silica sand), Class 3 (silica sand), Class 16 (calcium carbonate, heavy), and JIS Test Powder 2, Class GBM-20 (barium titanate glass) settled out of homogenous suspensions were determined using the principles of the buoyancy weighing-bar method as well as a sedimentation balance. Samples were standardized by Japanese Industrial Standard, and the dispersing agent was NaPP or NaHMP, while the viscosity improver was a starch syrup solution. Although constructing a handmade sedimentation balance is difficult, developing a handmade weighing tool capable of measuring particle size distribution is easy. Herein three weighing-bars, which were composed of aluminum, stainless steel, and copper, were used to measure the particle size distribution. However, an aluminum slit-cylinder occasionally replaced the weighing-bars. The weighing data obtained via an analytical balance were controlled by connecting the balance to a personal computer with an RS-232C interface, and the determined particle size agreed well with the data obtained by Andreasen analyses and the sedimentation balance.     

Bimodal particle size distribution polymer/oligomer combinations for printing ink applications

This paper describes a study in which anionically stabilised acrylic latices with a bimodal particle size distribution, produced by blending polymer particles of 50 and 350 nm in different blend ratios, are blended with oligomers. Reversibility, rheology, drying behaviour, film formation and blocking resistance of these systems were studied Reversibility appears to show two regions of linear dependence on the total particle surface area. The lower the surface area, the lower the amount of oligomer needed for good reversibility. The rheological data was fitted by the Krieger–Dougherty equation and it appeared that both in the presence and absence of oligomer an 80/20 large/small blend exhibited the highest maximum packing fraction. Short drying times were obtained with bimodal blends at high solids content and the drying profiles could be explained by the Croll model. The presence of oligomer was shown to exhibit a positive effect on film formation and when the oligomer is hard, the blocking resistance is already very good at low oligomer content, resulting in a very good MFT/blocking resistance balance. The latter can also be obtained when as well the size as the T_g of the polymer particles is varied.     

The effects of particle size distribution and surface area upon cement strength development

Particle size distribution, uniformity of the distribution and specific surface area (SSA) have a great influence on service properties of cement, particularly on strength. In this paper the effects of these physical parameters on strength development were studied using PC 42.5 R.In order to understand the significance of different particle size ranges in a distribution, samples having size distributions such as − 10 µm, − 20 µm, − 30 µm, − 45 µm, − 32 + 3 µm and − 20 + 5 µm were prepared from PC 42.5 R by using a laboratory scale 3rd generation separator. Additionally − 32 + 3 µm and − 20 + 5 µm fractions were added to the original PC 42.5 R in varying amounts to study SSA and uniformity effects. Same strength values were obtained for samples with a narrower size distribution but smaller SSA. Fineness is very important for strength development, particularly in the early stages of hydration.