In reply to David Jonsson's message of Mon, 4 Oct 2010 18:49:04 +0200: Hi, [snip] >Zone melting is a technique to separate impurities in a material. >http://en.wikipedia.org/wiki/Zone_melting > >There is no public theory on the purification effect of zone melting. It is >an empirical subject. Wikipedia says just blankly that the impurities >diffuse more towards the center of the melt. Why would they diffuse more >than the fluid? Elsewhere it is described as if the impurities "prefer" to >stay in the melt: >http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/backbone/r6_1_2.html#_1, >That article is linked two clicks away from the Wikipedia article on >zone >melting. >Typical for the lack of theory is to have animistic ideas and attribute the >phenomena with properties of live beings having preferences. > >I have an idea where the heat gradients in zone melting causes different >thermal expansion and thus different pressure so that matters move >differently towards or away in the gradient. The force on foreign matter in >the fluid in the melted zone are determined by the derivative of pressure in >regard to temperature dP/dT = ßK (ß = coefficient of thermal expansion and K >= bulk modulus) as I have shown on two Iwone conferences. The gradient has >opposite directions around the heated zone so both type of impurities are >forced away. If ßK is higher for the impurity compared to ßK for the fluid >it will be move against the gradient and thus flow ahead of the moving >melted zone. Likewise if ßK is lower for the impurity than for the >surrounding melted fluid it will move towards the gradient and thus gather >just behind the moving melted zone. An impurity with similar ßK as the fluid >would not be possible to purify with zone melting.
I suspect that it's not so much about the melting as the recrystalization at the boundary of the zone. Impurities tend to disrupt the crystal lattice making it less ordered. Consequently the energy release when the crystal forms is greater if the crystal is pure. The same force tends to cause sea salt to separate from sea ice as it forms. By moving the melt zone through the mass what is really happening is that the recrystalization boundary moves through the mass, pushing the impurities ahead of it. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
