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Given a polydisperse solid-liquid system, two fundamental principles are operative in the determination of subsieve particle size distributions by sedimentation methods. (1) Stokes' law as applied to gravitational or centrifugal sedimentation gives a continuous scale of measurement for subsieve particle sizes. Inherent in its derivation are a number of assumptions. Therefore, an evaluation is made of the limitations within which sedimentation methods yield valid results. (2) The cumulative settling law applies directly to the distribution of sizes and permits a classification of gravity methods into cumulative, accurate incremental and approximate measurements. The resulting distributions are thus established for the simplest type of sedimentation, which in turn has been extensively investigated.
Analogous in many respects to Odén's tangential intercept method of gravity settling is the Romwalter and Vendl modification for beaker-type centrifugal sedimentation. With this extension of the cumulative settling law, experimental techniques are developed that employ inexpensive and readily available equipment. The requirements for absolute particle size distributions are described. Simplified procedures are given where results of relative fineness suffice and speed of measurement is the main emphasis. Included for purposes of illustration are the determinations of characteristic particle size distributions of aqueous, paint, and rubber dispersions of titanium dioxide.
The literature on gravitational, beaker-type centrifugal, supercentrifugal, and ultracentrifugal sedimentation is reviewed with the object of systematizing available information and pointing out the wide variety of materials that have been submitted to sedimentation analyses.
If the subject of particle size is viewed in its entirety, there is evident a dependence between the method of dispersion and the resultant particle size distribution. A recognition of this fact permits the study and control of milling processes employed in pigment manufacture. Also, in the pigmentation of aqueous or organic vehicles, disperse systems are obtained that are readily amenable to size analysis by sedimentation methods. Because of the close approach to industrial reality, the particle size measurements acquire considerable technical significance.
Martin, S. W.
Research Chemist, Titanium Division, National Lead Co., Sayreville, N. J.