Papers - Mining Engineering Education - The Sedimentation Balance for Measurement of Size Distribution of Fine Materials (Mining Technology, Nov. 1939) (with discussion)

There is acute need for a method that will measure the size distribution of finely divided materials, particularly when the particle sizes are smaller than the openings of the finest screen cloth regularly available for testing. Several methods and types of apparatus are in common use, but none are entirely satisfactory for all purposes. The commonest methods are microscopic grain counts, elutriation by liquids or air, and sedimentation. Valuable results have been secured by air elutriating devices, typified by the Haultain Infra-Sizer and the Pearson Air Analyzer. These are especially useful in obtaining sized fractions that can be examined and analyzed separately. They are particularly advantageous because the sizes of the samples produced are adequate for a considerable amount of subsequent manipulation and testing. The sedimentation methods, such as the Wagner and Klein turbidimeters, and the sedimentation balance, do not deliver sized fractions, but they do provide a means for reasonably rapid comparative determina-tions of fineness and surface area, which are important in many industries. The sedimentation balance consists essentially of a balance pan suspended in a liquid that contains falling particles. These are weighed as they settle onto the pan, and the time rate of settling is determined. The size distribution is computed from the settling rate. The balance appears to be inherently more accurate than any of the other methods, and it has served a useful purpose in the study of numerous problems, but there have been limitations which have restricted its scope of application. Because of the essential simplicity of the method, it was felt that intensive investigation might lead to modifications that would make it a valuable tool [or the operators in industries that require precise information regarding the results secured from grinding equipment. A large number of size-distribution determinations below 200 mesh were required in connection with a particular research problem in fine grinding. The sedimentation balance was used in this investigation, and a technique was gradually developed that ultimately gave good results. A serious difficulty appeared early in the work. It was discovered that less material always settled on the sedimentation pan and more on the bottom of the containing jar than could be accounted for by calculation, and also that the material that settled to the bottom of the jar outside the edges of the pan was considerably finer than that on the pan. This segregation was so great that the distribution obtained for the finer sizes was obviously very much in error; material ground to about the degree of comminution of cement might have only 60 per cent of the calculated weight actually settling on the pan, and the error increased with the fine-