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PSD measurement of a crystalline population

Sifting

A series of superimposed sieves with decreasing downward apertures is stirred electromechanically in order to encourage the decreasing flow pass through each sieve. The rejected material remains on the sieve mesh.

d: the sieve opening is standardized.

The dimension that determines whether or not a particle crosses the sieve mesh is the intermediate dimension.

The Coulter particle counter

A suspension of particles in the electrolyte is sucked in through a cylindrical orifice. Then, for each particle, we detect the resistance variation between two electrodes placed on either side of the orifice. The calculation is based on two hypotheses:

  • - 1st hypothesis: the particle has a length of x and a cross-section of kx2;
  • - 2nd hypothesis: current transmission in the orifice is not altered by the particle’s presence. Ro = pL/S is the resistance of the liquid channel without the particle.
Principle of the Coulter counter

Figure 1.3. Principle of the Coulter counter

p: resistivity of the liquid (much lower than that of the crystal).

With the solid’s presence, resistance becomes:

A pulse of resistance, and thereby voltage, corresponding to the passage of a particle, is proportional to the particle’s volume.

We see that if we calibrate the device by comparison with measurements performed by microscopy or sifting, we can immediately obtain the volume distribution of a particle population.

The minimum size detectable is in the order of 10 qm.

Sedimentation

In order to apply Stokes’ law:

There must be no influence of particles on one another. Accordingly, the solid volume fraction must be:

We can then, taking the maximum fall speed V, deduce the particle size dp. This method is applicable for:

Image analysis

Under a microscope, we sweep over the sample in the light of a laser. The image is then projected onto a television screen on which light areas represent the crystals. Granulometry is provided by means of a software. The laser can be used up to 32% of crystals [WIT 92].

 
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