Sedimentation is a classic particle size technique based on the Navier-Stokes settling equation. During a Sedimentation analysis, particles of known density are allowed to settle through a liquid with a known density and viscosity.   Evaluating the rate at which the particles settle generates an equivalent spherical diameter or “settling diameter”.  In x-ray sedimentation, an x-ray beam is directed at a cell containing the particles in a fluid.  The x-ray intensity is monitored as the particles settle.  An increase in x-ray intensity correlates with a lower concentration of particles as they pass through the detection zone.  Evaluation of the relative intensity at a specific time and cell location allows the instrument to determine the percentage of particles at or above an expected size, as calculated by Stokes Law. The analytical detection range for the instrumentation utilized by PTL is approximately 0.2µm up to 150µm, but can vary slightly based on the density of the particles.  The “true” density of the sample can be determined by helium pycnometery (also available at PTL).  Viscosity measurements of the suspension media can also be measured in-house using a viscometer.

Particles of different sizes settle at different rates.  The larger particles will settle more rapidly than the fines assuming a similar density and shape.  Flat platy particles have a different settling rate than spherical particles and are a potential area for bias under this particle sizing technique.   In addition, particles which are transparent to x-rays cannot be detected and result in errors in the measurement.  Organic samples, for example vegetation or detritus in sediment samples as well as most pharmaceuticals are not appropriate for this technique.  Conversely, materials with a high atomic mass are optimal for x-ray sedimentation.

Sedimentation particle size analysis has found applications in various industries over the years and is still used extensively in the cement, inorganic pigment and ceramic sectors.  Clays and soils are other sample types which are commonly analyzed by this technique.   Also, concentrated effluent samples can be analyzed by this technique if only the inorganic fraction’s particle size is of interest.

It is PTL’s goal to provide the widest variety of analytical techniques in the field of particle size characterization.  Sedimentation is just one of the proven, classic analytical methodologies available at PTL.  Please give us a call to find out if x-ray sedimentation can provide insight into your sample’s characterization question.

Sedigraph Model 5110
Click here for Sedigraph typical sample report

Statistical analysis of inorganic particulate sedimenting throught an x-ray beam in an appropriate fluid (Volume weighted distribution standard)

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