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# Calling Chemical Engineers Watch

1. We would like to separate a mixture of silica and galena by using free settling in water at 293 K. The particle size range goes from 5.21×10?6 m to 2.50×10?5 m. The relative density of silica is 2.65 and that of galena is 7.5. When using differential settling it is possible to obtain fractions with pure silica and pure galena as well as a fraction of mixed particles. Determine the size range of the various fractions obtained in the settling. If the settling is in the laminar region the drag coefficient will be reasonably close to that of spheres.

Is a question I have been given, how do I go about solving?I know its something along the lines of:

You will obtain 3 fractions, the first fraction will be pure galena as its more dense so the large galena particels will sink first. Then at some point there will be a size of galena that will fall at the same rate as the biggest silica particle and this will produce a fraction of mixed partices... and then at the end there will be some silica particle that falls the same rate as the smallest galena particle and everything smaller than this will be pure silica.

Right? So how?
2. I think you'd get better responses in the chemistry forum. http://www.thestudentroom.co.uk/forumdisplay.php?f=130

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Updated: December 10, 2010
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