Methods of Particle Size Measurement

There are many technologies available to determine particle size distribution of

materials. Of all the technologies available, laser diffraction has become one of the most

widely used and preferred methods. This article will look at some of the more widely

accepted techniques used in different industries today.

What is Particle Size?

Particles are three-dimensional objects. In order to provide a complete

description of a particle, three parameters are required — length, breadth and height.

Thus, it is impossible to describe a particle using a single number that equates to

particle size. Therefore, most sizing techniques assume that the material being

measured is spherical because a sphere is the only shape that can be described by a

single number, its diameter, thus simplifying the way particle size distributions are

represented.

Widely Accepted Particle and Surface Measurement Techniques

Adopting different measurement techniques can produce different results

when measuring non-spherical particles. That said, any instrument or technique used

for particle size analysis needs to generate data in a form that is relevant to the process.

The method also needs to be reliable, simple to use and able to generate reproducible

data.

1. Laser Diffraction

Laser diffraction is the one of the most widely used particle sizing

techniques and has become the standard method in many industries for

characterization and control. This type of particle size analyzer relies on the fact

that particles passing through a laser beam will scatter light at an angle that is

directly related to their size. When particle size decreases, the observed scattering

angle increases logarithmically. Scattering intensity is also subject to particle size,

diminishing with particle volume. What this means is that large particles scatter

light at narrow angles with high intensity while small particles scatter at wider

angles with low intensity.

Laser diffraction has a wide dynamic range, from 0.2 to 2000 microns and is very

fast and reliable. It is also very flexible as it can be applied to dry powders,

aerosols and emulsions. In addition, laser diffraction does not require calibration

but can be easily verified.

2-Sedimentation

This is a traditional method widely used in the paint and ceramics

industries. Equipment as simple as the Andreason pipette or as complex as

centrifuges and X-rays can be used in this method. The main advantage of this

technique is that it determines particle size as a function of settling viscosity.

However, as the density of the material is needed, this method is no good for

emulsions where the material does not settle or for dense material that settles too

quickly. It is also based on spherical particles, so can give large errors for particles

large aspect ratio.

3-Sieve analysis

This continues to be used for many measurements because of its simplicity, cheapness,

and ease of interpretation. Methods may be simple shaking of the sample in sieves until

the amount retained becomes more or less constant. Alternatively, the sample may be

washed through with a non-reacting liquid (usually water) or blown through with an air

current. Advantages: this technique is well-adapted for bulk materials. A large amount of

materials can be readily loaded into 8-inch-diameter (200 mm) sieve trays. Two

common uses in the power industry are wet-sieving of milled limestone and dry-sieving

of milled coal. Disadvantages: many PSDs are concerned with particles too small for

separation by sieving to be practical. A very fine sieve, such as 37 μm sieve, is

exceedingly fragile, and it is very difficult to get material to pass through it. Another

disadvantage is that the amount of energy used to sieve the sample is arbitrarily

determined. Over-energetic sieving causes attrition of the particles and thus changes

the PSD, while insufficient energy fails to break down loose agglomerates. Although

manual sieving procedures can be ineffective, automated sieving technologies using

image fragmentation software are available. These technologies can sieve material by

capturing and analyzing a photo of material.

4-Electrical sensing zone method – Coulter Counter

Instrument measures particle volume which can be expressed as dv : the

diameter of a sphere that has the same volume as the particle. The number and size of

particles suspended in an electrolyte is determined by causing them to pass through an

orifice an either side of which is immersed an electrode. The changes in electric

impedance (resistance) as particles pass through the orifice generate voltage pulses

whose amplitude is proportional to the volumes of the particles.

5-Microscopy

Optical microscopy (1-150µm), Electron microscopy (0.001µ-)

Being able to examine each particle individually has led to microscopy being considered

as an absolute measurement of particle size.

• Can distinguish aggregates from single particles

• When coupled to image analysis computers each field can be examined, and a

distribution obtained.

• Number distribution

• Most severe limitation of optical microscopy is the depth of focus being about

10µm at x100 and only 0.5µm at x1000.

• With small particles, diffraction effects increase causing blurring at the edges -

determination of particles < 3µm is less and less certain.

Manual Optical Microscopy

Advantages

• Relatively inexpensive

• Each particle individually examined - detect aggregates, 2D shape, colour,

melting point etc.

• Permanent record - photograph

• Small sample sizes required

Disadvantages

• Time consuming - high operator fatigue - few particles examined

• Very low throughput

• No information on 3D shape

• Certain amount of subjectivity associated with sizing - operator bias.

Conclusion

Fine particles play essential roles in determining the characteristics of both

natural and manmade materials and have considerable influence on processes such as

dissolution, adsorption and reaction rate. In the majority of cases, these effects are a

function of the size, shape, surface area or porosity of the individual particles or of an

agglomeration of particles. These particle-related characteristics must be controlled in

order to optimize the desired effects, and efficient control requires measurement. These

same particle characteristics are either the causes of, the results of, or a determining

factor in natural phenomena. In this category, understanding or exploitation rather than

control is more likely the objective and, again, measurements provide fundamental

information used in achieving the objective. As this article has illustrated, there likely are

multiple techniques for determining the same particle dimension and each has its

advantages and disadvantages. Selecting a technique that is inappropriate for the

application can have a profound impact on the quality of the measurement you obtain.