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April 1, 2009

Recycling Cathode Ray Tubes

As CRTs become an obsolete form of video display technology, recycling their lead-filled glass is increasingly important

Kate Torzewski

Earlier this decade, liquid crystal display (LCD) and plasma screens surpassed cathode ray tubes (CRTs) as the visual display of choice in computers and televisions. While the production of CRTs has dwindled to nearly nothing, we will still deal with end-of-life (EoL) units for years to come....

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Comments (2) for Recycling Cathode Ray Tubes
I read your article Recycling Cathode Ray Tubes (CRTs; April, pp. 24 – 27) on how CRTs are being reprocessed and the technology associated therewith. Well done and well written; however, as a metallurgist, I recognize that glass is a very stable material when exposed to normal environments. The windshield on your and my automobile is unaffected by rain, sunshine, air pollution, an so on. I expect that glass buried in landfills 100 years ago is still there, no different than it was when it was buried. High-end glass crystal used to be (and still may be) made from PbO. Glasses that you put into the dishwasher will etch after hundreds of washings, but that alkaline and hot water environment is a far cry from that of a landfill. To the best of my knowledge, nobody has ever been stupefied by lead that was part of properly vitrified glass.

While we are very concerned about lead in landfills, I question the need for such concern when the lead is a chemically bound part of the stuff that makes up glass. I think this concern falls under the same technically incompetent concern that "environmentalists" have for free-machining brass (which contains 5 – 7 wt% lead to improve machining) in potable water systems: to contaminate the water passing through lead-containing castings, the lead would have to be extracted as part of the general corrosion of the castings, and if that happened at a rate high enough to measure the amount of lead in the water, brass castings would have a service life of only a few years. As we know, brass castings typically last the lifetime of the plumbing system in which they are installed.

How about a story explaining why leaded glass is a concern when it is put into a landfill. Please explain the mechanism by which lead is leached from glass, contaminating the ground around it — and the water supply.

Or maybe I’ve just missed something. If so, please send me a reference by a technically competent organization that explains why otherwise stable glass suddenly becomes soluble when placed in a landfill.

Posted by Walter J. Sperko, P. on Tuesday, June 9, 2009 @ 07:23 PM
Editor replies:

The EPA has a toxicity threshold of 5 mg/L for the leaching of lead, and crushed CRT glass has been found to exceed this threshold, leaching 18.5 mg/L (in a study by the University of Florida). Though CRTs are often disposed of as whole units (from which lead does not readily leach, as you point out), it is the more extreme case of a finely broken down CRT that must be considered.

In any case, with regulations in place for the handling and recycling of CRTs, the focus of this article was to highlight the technology currently in place for the efficient processing of CRT glass.

The University of Florida report can be found here:
Posted by Kate Torzewski, Assi on Tuesday, June 9, 2009 @ 07:32 PM

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