Suspension of solid mixtures by mechanical agitation

Date of Award


Degree Name

M.S. in Engineering


Department of Chemical and Materials Engineering


Advisor: Kevin J. Myers


Agitation is a critical aspect of many processes, such as food production, mineral processing, and water treatment, with liquid-solid agitators representing a significant portion of all agitation installations. Improper mixing operation in liquid-solid agitators can result in negative financial and environmental issues. Over-mixing may damage solid particles and erode impeller blades as well as waste energy. On the other hand, under-mixing may allow solids to settle down on the base of the tank, which may cause solids to adhere to one another and bring about difficulties of solids removal from the vessel. The purpose of this work is to develop a design guideline that can be used to successfully predict the agitation speed required to suspend a solid mixture in just-suspended condition, in which no solid remains on the base of tank for longer than 1-2 seconds, based on a knowledge of agitation speeds required to suspend the individual components in the solids mixture. The primary design guideline investigated is summing the powers required to suspend the individual solids alone to predict the power required to suspend the solids mixture. The secondary design guideline that is investigated is that the speed required to suspend the solids mixture is equal to the speed required to suspend the more difficult to suspend solid alone. All binary solids mixtures can be categorized into three different groups in this work based on the magnitude of specific gravity of the solids in each system. It is found that speed predicted based on the sum of powers required to suspend the individual solids is normally higher than the actual speed at which a solids mixture is at the just-suspended condition in the case of low-density systems where the specific gravities of both solids are below 1.5 grams per cubic centimeter. In other cases, including mixed-density system, which is a solid with low density (below 1.5 grams per cubic centimeter) plus a solid with high density (above 2.4 grams per cubic centimeter), and high-density system in which both solids have densities above 2.4 grams per cubic centimeter, the prediction speed found by summing the powers required for suspension of each individual component in a solid mixture is approximately equal to that necessary to suspend solids mixture. However, a few systems diverge from these typical behaviors, possibly due to the unusual characteristics of one solid -- olivine sand. Results from those solids mixtures involving olivine sand are not consistent with typical conclusion obtained from the sum of powers hypothesis. Adding olivine sand always reduces mixture suspension speed from the speed predicted by summing of powers estimation. A reasonable explanation of these atypical phenomena should be investigated in future studies. In addition, two systems that consisted of three different solids were tested and it was found that the speed based on summing the powers required for suspension of individual components provided a reasonable prediction of the suspension speed of those ternary systems.


Mixing machinery Design and construction, Machinery Power utilization Testing, Suspensions (Chemistry), Mixtures Density

Rights Statement

Copyright © 2012, author