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Laser diffraction vs. Sieving comparison

8 February 2011 No Comment

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Compare laser diffraction with sieving for particle size analysis

There is no doubt that your choice of particle characterization tool is going to have a great deal to do with the overall task at hand. Horses for courses, as they say!

Manufacturers, tasked with the quality assurance of a product line, are likely to wish for an easy to use, easy to maintain workhorse to produce consistent and reliable results over and over again, every day, as quickly and efficiently as possible. Sieving is a traditional choice for particle size measurement, one that has the attraction of mechanical and operational simplicity.

However, in these times of high competition and tough economics, it bodes well to look beyond the obvious and establish which technique is truly optimal for the application. Laser diffraction is increasingly widespread, with many people now taking advantage of the rapid efficiency it delivers, on and off-line, to nose ahead of the competition.

The pros of sieving compared with laser diffraction

So what exactly are we discussing here? Well, sieving is a fractionation technique that measures the mass of material retained as sample is passed through a series of fine mesh sieves stacked one on top of the other. The particle size distribution is calculated by measuring the mass of particles retained on each sieve. In doing this it is assumed that shaking the sieve stack induces the particles to fall into the correct distribution. This requires shaking the stack for a well-defined, often significantly long, time.

Some of the advantages though are that it:

  • provides mass distributions over a wide range of sizes
  • sieving offers relatively low instrument cost
  • requires little calibration

However, contrary to popular opinion, sieving does not actually produce a true weight distribution!

Consider a round peg and a square hole…

“In considering the results of a sieve analysis it must be remembered that the particles passing through the sieve are not necessarily round or symmetrical in shape, but may often be needle-like in form, and though they pass through the sieves they are apt to be larger in bulk than particles retained which approach the more ideal spherical shape”.

The above quotation, taken from a 1904 text relating to geological sample analysis, was quoted by my colleague Alan Rawle in a recent webinar, and highlights an important aspect of sieve analysis when working with non-spherical particles. Consider a sieve screen of nominal dimension 63 mm. A cylinder of 50 mm diameter and 50 mm height will obviously traverse this screen. However, a particle of 50 mm in diameter and 100 mm in height will also fall through the screen, even though it is clearly twice the mass of the first particle we considered. Also consider particles of the same size but different densities. These too will pass the same screen aperture although differing in mass. So particles are not sorted on the basis of mass alone – there is a mixing of size, shape and density generating the stated weight distribution.

The pros of laser diffraction vs. sieving

Laser diffraction exploits our understanding of the behaviour of light. It involves measuring light scattering from a group of particles, and reports the size distribution of spheres that would produce the recorded pattern.

Some of its advantages are that it:

  • is far more rapid than fractionation techniques
  • can be used with a very wide range of particle sizes and types
  • reports results as a “volume” distribution – the most appropriate description for bulk material properties
  • can be applied on-line for real-time measurement
  • is highly reproducible and repeatable

Laser diffraction and sieving can provide similar results when characterising spherical or semi-spherical particles. However, it is not unusual to observe significant differences for non-spherical particles because each technique measures different particle properties. Visitors to the Malvern website can explore how to effectively compare laser diffraction and sieve results in more detail.

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Shire horse or thoroughbred?

So, sieving is one of the oldest and simplest techniques and remains a useful method of separating particles based on their size. However, the time it takes to obtain accurate results, the poor resolution and problems associated with particle agglomeration and sieve blockage, have seen sieving being replaced in most industries. Offering integration, automation and real-time on-line measurement, with excellent reproducibility and repeatability, laser diffraction is often its natural successor.