Malvern Laser diffraction Technology

Malvern Instruments is a global leader in the application of laser diffraction technology. A proven and robust particle measurement technique, laser diffraction has been pioneered by Malvern Instruments for more than 30 years and is the standard in most particulate processing industries. It has many benefits in withstanding the demands of the process environment.

• Rigorous Mie light scattering theory allows determination of the complete particle size distribution
• No calibration required
• No long data acquisition intervals that can mask process behavior
• Hi gh concentration measurement
• Unique to Insitec systems is a patented multiple scattering algorithm

A significant challenge for on-line analysis using laser diffraction is to accommodate the multiple scattering that takes place at the high particle concentrations encountered in a process line.

Where particle concentrations are low enough, turbidity is linear with concentration. However, this is not the situation at higher concentrations. Particles are so close together that the scattered radiation is re-scattered by other particles.

This is not an issue in laboratory instruments because the user has control over the amount of sample measured. However, in a process instrument using continuous sampling techniques, measurement must be accurate even when higher loadings are present in the process stream. This is especially significant during plant start-up, shutdown and process changes.

Use of the patented multiple scattering algorithm ensures accurate particle size measurement using Insitec systems, regardless of instantaneous process loading.

Laser diffraction/Mie theory

Light from a laser is shone into a cloud of particles e.g. cement, which are suspended in a transparent gas e.g. air. The particles scatter the light, smaller particles scattering the light at larger angles than bigger particles. The scattered light can be measured by a series of photodetectors placed at different angles. This is known as the diffraction pattern for the sample. The diffraction pattern can be used to measure the size of the particles using light scattering theory that was developed in the early 20th century by Mie (pronounced "me"). As the instrument measures clouds of particles rather than individual ones, it is known as an "ensemble" technique, with the advantage that at smaller sizes e.g. 10 microns, the system is measuring literally millions of particles which gives some statistical significance to the measured results.

Although several correlations and theories exist on light scattering, the most comprehensive and rigorous theory is that of Mie's which is based on Maxwell's electromagnetic field equations. There are two assumptions made in this theory that are pertinent to the result obtained:

• The particle is assumed to be spherical
  This is important as few particles are actually spherical. Laser diffraction is sensitive to the volume of the particle. For this reason, particle diameters are calculated from the measured volume of the particle, but assume a sphere of equivalent volume.
  
• The suspension is dilute
  The particle concentration is assumed to be so low that scattered radiation is directly measured by the detector (i.e. single scattering) and not rescattered by other particles before reaching the detector (ie multiple scattering).

Multiple scattering / high concentration particle sizing

If the particle concentration is low enough, then the instrument will follow Beer-Lambert's law i.e. the turbidity (i.e. log of the inverse of the transmission, or a measure of the incident light lost due to scattering) is linear with the concentration. However, there will come a point when Beer's law no longer holds, when the particles are so close to one another that scattered radiation is rescattered by other particles. Insitec uses a patented technique that can correct for the onset of multiple scattering, which effectively increases the concentration dynamic range of the instrument.

In laboratory instruments, this is never an issue, as the technician has total control over the amount of sample measured. However, in a process instrument using continuous sampling techniques, the instrument must be able to continue measuring accurately even when higher loadings are present in the process stream. Thus as is illustrated in the following graph, the size reported by the instrument remains stable at much higher transmission values than would be the case with normal laser diffraction instrumentation:

Particle sizing

The particles themselves are rarely spherical, but as laser diffraction is sensitive to the volume of the particle, the size reported back assumes that the particle is spherical and the diameter is therefore calculated from the measured volume. The instrument calculates the particle size distribution of a powder. As opposed to a population of particles that are all the same size (monodisperse), most powders have particles which are of several sizes (polydisperse), forming a distribution. Some instruments can only measure the distribution by assuming some characteristic such as Gaussian or Rosin-Rammler. Insitec can calculate the distribution with no a priori assumptions about it, meaning that irregular distributions, such as those with two peaks (bimodal) or more, can be measured:

Example of bimodal distribution

The instrument is also capable of measuring a variety of derived parameters eg the average particle size, the estimated specific surface area or common parameters such as the D[4,3] diameter.