Determination of Particle Size Distribution by Examining Gravitational and Centrifugal Sedimentation According to the Pipet Method and with Divers
Authors: Berg S 
Published: March 2009

The aim of the fineness analysis is to determine the particle-size distribution. Methods by which only a numerical value is found for a quality of the product depending on the fineness are of slight value except in cases where the particlesize distribution is already known with the exception of a constant.

With the pipet apparatus for examining coarse products, the sampling from the sedimenting suspension is effected from a horizontal tube in the side wall of the sedimentation vessel by means of an automatic pipet. With ethylene glycol as suspending medium the particle-size distribution can be determined from a particle size of about 500μ when calculating the particle size by Stokes' law. With water as suspending medium the particle-size distribution can be determined from a particle size of about 150 μ, when calculating the particlesize by Allen's formula.

A correction is introduced for the retarding influence of the walls of the container on the sedimentation, and it is shown that the time which elapses until the particle has obtained the constant sedimenting velocity may be neglected even for the largest particles examined.

When the particle size is below about 20 μ, measurements can, in addition to the pipet method, be effected by means of divers. With the diver method the distribution of concentration in the suspension is determined by measuring the depth below the surface at which a body of known specific gravity, a socalled diver, completely submerged below the surface of the suspension during the measurements, comes to a floating equilibrium.

When a diver is in equilibrium in a sedimenting suspension it will move downward with the same speed as the largest particles found on the level of its geometrical center of gravity, and thus the depositing of particles which occurs in the hydrometer method is avoided or subsidiarily minimized. By using globular divers having a diameter of about 7 mm only, measurements can be effected at a slight distance below the surface of the fluid and very fine products can thus be examined.

By numerical calculations it is shown that the influence of diffusion may often be neglected for heterodisperse products down to a particle size of 0.035 μ, but that the influence is pronounced in the examination of monodisperse products when the particle size is less than 0.2 μ.

The range of measurements is 20 μ to 0.2 μ even for monodisperse products.

For the examination of centrifugal sedimentation a beaker centrifuge is used with conoidal sedimentation vessels and the centrifuge housing is arranged as a thermostat.

The measuring of the distributions of concentration can be effected by the pipet method with an automatic pipet and also by means of globular divers of a diameter of 7 mm only.

It is shown that the influence of diffusion by centrifugal sedimentation, even for monodisperse products, will first become noticeable at a practicle size of 0.005 μ.

Further it is pointed out that when new materials are to be examined peptizing experiments compose the major part of the fineness analysis, both by gravitational and by centrifugal sedimentation.