We placed cloudy cubed dry ice in our -80 Freezer and when we removed it later it had merged into larger crystals that were transparent. How did this happen? Did the dry ice melt and refreeze more purified?
Dry ice will never melt at normal pressure. It will only sublime. However, here is a possible explanation I found in the internet:
At the approach of Hurricane Alicia in 1983 at the local university we had an ultra-cold freezer at -80 degrees that needed protecting when the power went out. We packed it with 20 pound blocks of dry ice, closed it up and evacuated. Coming back 5 days later the blocks of dry ice were gone, but not the carbon dioxide. It had evaporated from the blocks and refrozen on the colder walls of the freezer. It was two inches thick and absolutely clear, like glass. The interesting thing about it is that if you breathed on it, the surface would be instantly crazed with an audible cracking sound. With the door open the surface would soon sublime and you could do it again. Crystallized dry ice is clear.
If I correctly see it, it is what is called grain growth in physical metallurgy. At sufficientily high homologous temperature (usually T/Tmelt) grain boundaries, i.e. the boundaries between the different CO2 crystals get mobile, and the excess energy stored in the grain boundaries gets relieved by reducing the grain-boudary area/grain boundary curvature. Large grains grow at the cost of small grains.
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