3. Particle Size Measurement Techniques
There are different analytical methods for particle size classification: Sieve, Microscopic, Laser Diffraction, and others.
Sieve Analysis Methods
This is the most broadly used technique for analysis and fragmentation of the size of minerals (Perry & Chilton, 1973). It is also the fastest and most conventional method for particles larger than 50 micra (Figure 3). A few micromesh sieves were developed for the analysis of particles smaller than 1 micron. Mesh is defined as the opening number per linear inch. The sieves used in laboratories present an average 20 cm diameter with uniform openings (orifice).
Figura 3: a) Agitador eletro-magnético e peneiras redondas para análise
granulométrica. b) Distribuição das partículas nas peneiras.
The series of sieves usually employed is of the standardized Tyler type ranging between 3.5 (5.6 mm) and 635 mesh (20mm). The sieve analysis can be performed either in the dry or moist mode. The humid sieving analysis is preferably used when particles tend to agglomerate minimizing the dust emissions and errors resulting from size analysis. On the other hand, the dry sieve analysis is reasonably precise for larger particles. The sieve analysis efficiency can be defined as the weight ratio of the material that passes through a given sieve by the total weight of the material that is smaller than its opening. The sieve analysis efficiency is very sensitive to operational variables such as size distribution and particle surface properties, sieve opening, vibration method, and particle charge.
The microscope is considered to be the main tool for solid size analysis owing to its precision and also because of its use in the calibration of other instruments. The microscope is not only used for size measurement, but also for a complete particle characterization, such as its shape, morphology (surface), color, and mineralogy.
The microscopes can be divided into two classes: electronic and optical. Electronic microscopes (Figure 3) are able to measure the particle size smaller than 0.001micra though usage is limited by measurement costs. Optical microscopes are used to characterize particles larger than 1 micron.
Figures 4 and 5 display images of a few materials as well as information about solid features by image analysis.
Figure 4: Micrographies obtained through SEM:
a) Glass Spheres; b) Sand; c) Alumina (Santos, 1997)
Laser Diffraction Method
Laser diffraction (Figures 6 and 7) is able to operate at a measurement range from 0.05 to 3,000µm. It operates at an optical system and with one or more beams of light, which allows measuring the precise particle size distribution. The measurements can occur either at a dry or moist mode. It is considered to be the best analyzer for several areas, such as: metal, mining, pharmaceuticals, chemistry, environment, construction, agriculture, soil, and the ones that require the measurement of high and different densities.
Figure 6: Particle Analyzers through Laser Diffraction
Figure 7: Size distribution of corn starch (dry measurement)