Re: Cullet

Anthony Thompson <thompson@...>

John C. La Rue, Jr. wrote:
I suspect that any coloring agents would have been removed when the cullet was melted in the furnaces, much as carbon and trace additives are removed when scrap iron and steel are melted...the additives unite with the limestone to form slag, which is removed, thus leaving pure iron.
Different process, John. In fact, colored glass is QUITE hard to re-clarify. You can mix brown and green glass, but not put either one into clear glass. And blue glass is a serious contaminant for any other glass color.
There are several reasons, but an important one is that glass melting is a very slow process, mostly due to the viscosity and resulting slow circulation and mixing of the glass--24-hour melts are common, whereas steelmaking can be accomplished in 90 minutes or less. (That's the reason that glass recycling really saves little energy; the process takes about as long, and about as much energy, whether you use raw materials or cullet. But you DO take all those bottle-size voids out of landfills.) Another difference from iron refining is that there is no slag equivalent in glassmaking.

Anthony Thompson
Dept. of Materials Science & Engineering
University of California, Berkeley

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