Henk G. Merkus

New developments in particle characterization by laser diffraction: size and shape

Abstract Laser diffraction has become a popular technique in many fields for measuring particle size distributions (PSDs). It is not only one of the standard techniques for laboratory measurements but is also gaining importance for on-line process monitoring and process control. This article reports some developments of this technique while recognizing the potential that still exists. In this report both particle size and shape are taken into account. For optimum results, both effective sensing and analysis of the diffraction pattern are required. This is the goal of our present study with a special interest in on-line application. In this paper, a statistical approach — principal component analysis (PCA) — to the scattered light signals from a conventional detector array is shown to improve the sensitivity to a few large particles relative to the main size distribution. For determining particle shape, a wedge-type photo-detector for sensing the azimuthal light intensity has been applied together with the procedures of cross-correlation, sweep selection for single particle presence and Fourier analysis. Finally, a novel light scattering sensor is presented, which enables the measurement of both size and shape and which offers even more promising applications for on-line process control.

On‐line Measurement of Particle Size and Shape using Laser Diffraction

Particle size and shape play an important role during particle formation and processing as they affect directly the quality of the intermediate and final products. Therefore, appropriate monitoring and control of these processes, preferably using in- or on-line instruments, are much needed in the modern powder industries. Compared with other particle sizing techniques, laser diffraction has the advantage of high speed, good reliability and high reproducibility, it is increasingly applied for particle sizing in many powder processes. Since shape information can be subtracted from azimuthal light intensity distributions, laser diffraction can also be applied for shape measurement. This paper describes a newly developed technique in laser diffraction using a CMOS pixel array. The advantages to use this pixel array are–direct measurement of scattering pattern is possible with its high dynamic range;–the detection areas are easily adjustable to give optimum or high-resolution results for different particle sizes and shapes, and offers more flexibility for the application;–the instrument is robust in operation as no alignment is required. This technique has been applied to measure the crystal particle size and shape in crystallization processes and improve operation control in a jet mill grinding system. The overview of the results is presented to address the feature using this technique.

Deconvolution of light-scattering patterns by observing intensity fluctuations

Results are presented on the statistical fluctuations occurring in a forward-light-scattering experiment to determine the particle size distribution. A sample of glass beads was measured using a Malvern 2600D instrument and analyzed with a proposed deconvolution procedure that incorporates the observed intensity fluctuations. This procedure yields a qualitative improvement of the solution, provides error intervals, and offers a better means for model discrimination.

Control of industrial crystallizers

The particle size distribution produced in particulate processes generally has a large impact on process economics, and the crystal size distribution produced in industrial crystallizers is no exception. Crystal Size Distribution (CSD) control is therefore desirable. Its establishment is the main objective of a joint research program between three research groups within the Delft University of Technology and several industrial participants. This paper presents a review of this project and intends (1) to illustrate the development of an on-line crystal size measuring technique, (2) to illustrate how information on CSD can be used to derive a dynamic process model and (3) to emphasize the need for effective process inputs and to make suitable suggestions in that direction.

Measurement of cluster formation in aqueous citric acid solutions by photon correlation spectroscopy

Abstract Aqueous solutions of citric acid at volume fractions of 27 to 54% have been investigated by photon correlation spectroscopy. From the time dependence of the scattered light the collective diffusion coefficient was obtained. Interactions between the solute molecules resulted in a concentration dependence of the diffusion coefficients. From this, the interaction parameters have been estimated. It appears that strong attractive forces between the molecules play a dominant role in these solutions. Even below the saturation concentration clusters of about 5–10 molecules develop. This is much smaller than the critical nucleus size of crystalline citric acid. The clusters are presumed to be aggregates of citric acid molecules which are not precursors of crystalline citric acid, but form a different phase.

Effects of ionic strength of eluent on size analysis of submicrometre particles by sedimentation field-flow fractionation

Abstract Sedimentation field-flow fractionation (SdFFF) has a high resolution over a wide range of particle size compared with other methods of sub-micrometre particle size determinations, and has the grant advantage that the fractional collection is sorted by the particle mass. However, the retention behaviour in SdFFF depends strongly on the experimental parameters, especially the ionic strength of the eluent. The sizes calculated from the experimental results of SdFFF are underestimated if an eluent with low ionic strength is used, compared with those obtained by quasi-elastic light scattering spectroscopy, owing to the interparticle repulsion. There is a maximum value of the ionic strength of the eluent for particle size analysis, because rapid flocculation of particles occurs at high electrolyte concentrations. Further, hardly any difference in the retention times was found in SdFFF using different anionic surfactant solutions as the eluent.

Particle Size, Size Distributions and Shape

Only for spheres, the size of a particle can be represented by a single parameter, being e.g., its diameter. For the description of a particle of any other shape more parameters are required. Many different descriptors exist. They can be directly related to visual or microscopic measurement, such as length or breadth, or they are based on the concept of equivalent sphere, yielding the diameter of a sphere that shows the same behavior as the particle or group of particles under consideration. Measurement of particle size by different principles may lead to different results for the same group of non-spherical particles. Examples of equivalent sphere diameters are equivalent sieve (or near-mesh) diameter, Stokes’ diameter and volume diameter. Usually, the particles in a particulate product do not have the same size but a distribution of sizes. Here too, there are different possibilities for quantitative description. One example is a size distribution, based on number, volume or mass of the particles. Other possibilities are a mean size, a single or a small number of descriptors in the middle or at either side of the distribution, or parameters of a model-distribution. The choice of descriptors for particle size and size distribution should be made such that they give the best discrimination for the quality of the particulate product with respect to given properties or for characterization of a production process. If these properties also depend on particle shape, shape and shape distribution should be characterized in addition to size. This can be done in qualitative terms, such as fibers or flakes, or in quantitative terms, such as elongation, roundness, angularity, surface rugosity, percentages of given model shapes, or fractal dimension. Finally, the porosity of particles may play a role in their behavior. This chapter describes some of the many possibilities that exist.

Particle size analysis by sedimentation field flow fractionation. Performance and application

Recently commercial equipment using sedimentation field flow fractionation (SFFF) has become available for analysis of particulate materials in the sub-micron range. This paper describes the DuPont instrument and discusses its performance. A particular study is described on the comparison of the SFFF technique with that of quasi-elastic light scattering (QELS). The paper concludes that the instrument is capable of measuring particle size distributions with high resolution and precision, provided that no particles above the upper size limit — about 1 μm — are present.

Improving the Sensitivity of Forward Light Scattering Technique to Large Particles

The detection of a small number of large particles whose size lies outside the required size distribution is important in the processing of many materials when those particles result in the deterioration of the product quality. Laser diffraction, which is a very popular technique for on-line monitoring and process control, has, however, limited sensitivity to those few large particles. This paper deals with the improvement of this sensitivity. The fluctuations of the signal received by the detectors were analyzed by both experiment and simulation. Two statistical approaches, cross-correlation and principal component analysis (PCA), were applied. A detection procedure is proposed which is based on the combination of sweep selection through PCA and an appropriate deconvolution of the selected sweeps. The preliminary experiments with Al 2 O 3 powder showed an improved sensitivity to the large particles.

Particle-size analysis by laser diffraction with a complementary metal-oxide semiconductor pixel array

Existing laser-diffraction instruments that use photodiode detectors have a limited resolution for particle sizing. We attempt the implementation of a complementary metal-oxide semiconductor pixel sensor for particle-size measurement by laser diffraction. The sensor has unique features: high resolution, no blooming, and a wide dynamic range (i.e., direct measurement of the scattering pattern). The calibration of the sensor is based on each pixel. The signal-processing and the inversion schemes for obtaining the particle-size distribution are described. The results indicate an improved size resolution and an increased flexibility of application.