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In most material-based industries, be it nanomaterials, specialty chemicals, or other materials which need to have specific molecular properties/characteristics, the particles in the product need to be characterized. The characterization of particles within a product can range from the shape and size of the particles to the size distribution of all the particles (dispersity), to the properties of the particles.
The main reason for characterizing particles is to achieve a uniform product (or as uniform as is physically possible) that performs as intended. This is something that extends to the mining industry. In the mining industry, a wide range of materials are dug out of the ground. After extraction, they are made and sold in several forms, and the properties of the material being sold are often dependent on the properties at the particle level.
Moreover, because most materials which are sold from mining activities have had to be extracted from an ore and purified, there are often several impurities present in the material that can affect its performance. So, for the mining industry, being able to not only characterize the material of interest—be it a metal, a mineral or otherwise—but being able to check the levels of impurities is vital to ensuring successful products are borne out of the mining efforts.
The analysis of the particles can occur at many different points along the supply chain. The analysis of the particles can be performed anywhere from the processing stages to when the product is being shipped. Therefore, there are several methods which can be used, ranging from lower-tech (and more physical) methods which provide a rough indication of larger samples, to more high-tech methods which use scientific instruments to analyze smaller product samples.
Lower-Tech Methods
There are a number of lower-tech methods that can be used to determine the size and/or shape of particles and often rely on some kind of physical (or external) input to characterize the sample. These include gravity separation, electrostatic separation, magnetic separation, and sieving methods. Gravity separation enables the differences in particle mass to be separated under gravity alone. There are many sub-methods in this category, such as jigs, sluices, spirals, shaking tables, fine particle separators, and hydrosizers and cyclones, which enable a rough estimate of the size of the particles (and the distribution of particle sizes) to be determined by the heavier particles falling/sinking further than lighter, i.e. smaller particles.
Sieves are another easy method for determining the distribution of particle sizes. As a sample is passed through a series of sieves (with smaller holes the further down the sieve stack the sample goes), the larger particles will be collected first, and the smaller ones last. It can give a rough quantification based on the weight of each layer and can give a visible identification of the size distribution of the particles, as well as their shape. It is a crude, but a low-cost method.
The final methods in this class of techniques for characterizing a mining sample are electrostatic and magnetic separation techniques. These methods enable particles with an electrical charge or inherent magnetism to be manipulated and separated away from the rest of the sample. The amount that they move away from the sample bulk is dependent upon their size (and relative surface area), where smaller particles with a higher relative surface area are moved further away than larger particles.
Scientific Instrument Methods
There are two higher-tech scientific characterization methods which are commonly used on mining samples to determine the size and shape of the particles. These are dynamic image analysis and laser diffraction. Dynamic image analysis is an extension of optical microscopy and offers a way to analyze the size and geometry of sample sizes by taking a photo of each particle and subsequently analyzing it. So, where microscopy is typically a qualitative technique, the particles can also be quantified, and this means that particle distributions can also be quantified when the particles are constantly fed past through the analysis region – as the range of particles in the sample can be compared to each other, giving a distribution/dispersity index.
The other method is laser diffraction. Laser diffraction is best suited for when the particles are too small/fine to be picked up and analyzed by dynamic image analysis. It is a method that can measure particles down to the nanoscale without the need for calibration and can do so in a timely manner. Laser diffraction measures the size of individual particles, and the subsequent distribution of particles in a sample, by firing light beams at them. The particles scatter the light and the analyzers detect the angular variation in the light intensity. Larger particles scatter the light at small angles, whereas small particles scatter it at large angles, which enables the size and distribution of the particles to be backed out by the instrument.
Sources and Further Reading
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