The Mixing of Particulate Solids of Different Particle Size
Khan, Mohammad I.
Khan, Mohammad I.
Publication Date
End of Embargo
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The University of Bradford theses are licenced under a Creative Commons Licence.
Peer-Reviewed
Open Access status
Accepted for publication
Institution
University of Bradford
Department
Postgraduate School of Powder Technology. Chemical Engineering Department
Awarded
1971
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Additional title
Mixing of particles of different size has been studied and a method devised for predicting the quality of mixtures from a measurement of the segregating tendency of the components
Abstract
This thesis is an account of research into the mixing and segregation of particulate solids of different particle size.
The main aim of the work was to develop a simple laboratory test ifor the measurement of the segregation tendency of the mix components, the results of which could be used in the selection of a suitable mixer for a particular duty. Binary test mixtures of different diameter ratios prepared from narrowly sieved fractions of the same material were used in segregation tests to establish a standard test procedure for the measurement of the segregation tendency of the components. Segregation in test mixtures was measured by three different methods and a segregation index i.e. coefficient of segregation, which is based on the difference in the composition in the two halves of the sample was used to measure the degree of segregation. The inclined rotating drum was found to be most suitable and simple apparatus and a standard testing procedure was established.
The effects of the diameter ratio and mean diameter of the components and the effect of moisture content in sand mixtures, on the coefficient of segregation were studied and it was found that the segregation increases with the diameter ratio between particles up to a value of 4.3 but for higher diameter ratios, the coefficient of segregation is not affected. At a constant diameter ratio the coefficient of segregation was found to decrease when the mean diameter of the components was reduced below 500 microns. Segregation in sand mixtures of various diameter ratios was found to decrease markedly as water was added, falling to a very small value with the addition of 1% of water by weight, and an equation was developed to calculate the amount of water required for adhesion between equally sized particles.
The ’thief' probe sampling technique was critically examined and a new sampling technique was developed which causes very little disturbance in mixtures in comparison with the ’thief' probe sampling which is very widely used.
The performance of three different types of mixers was investigated by measuring the degree of mixing of different sized components having different segregation tendency. It was found that the degree of mixing in a Nauta mixer was much better than that in a tumbling mixer. For higher diameter ratio mixtures the performance of the Nauta mixer was unchanged whereas the performance of a Ribbon blender was slightly reduced. In tumbling V-mixers the degree of mixing was very markedly reduced with an increase in the diameter ratio between components up to a value of about 2.4 ; at higher diameter ratios the quality of mixing was slightly improved.
The effect of the change of sample volume on the quality of mixing in a tumbling V-mixer was studied and it was found that the standard deviation is not affected when the volume of sample is changed from 1.42 cubic cm to 6.28 cubic cm.
From the mixing and segregation tests it was found that the segregation tendency of the mix components determined in the inclined rotating drum can be used to evaluate the ultimate degree of mixing in a mixer in which the segregation is predominantly due to the percolation as the segregation mechanisms prevailing in the inclined rotating drum is similar to that occurring in the mixer.
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Type
Thesis
Qualification name
PhD