Material: Recycled
Glass
Issue: Understanding the chemical
composition of container glass is critical to recycling for a number
of reasons. First, the chemistry
has significant implications for managing the behavior of molten glass
in any remelt application, from large-scale container manufacturing
down to cottage-industry pressed glass.
Second, the composition has important implications for evaluating
health concerns related to glass handling, such as free silica content
(see Analysis of Glass Dusts
Best Practice). In addition, product manufacturers
are required to produce Material Safety Data Sheets (MSDS) which describe
chemical composition and special handling precautions. Finally, the chemistry is important in determining
the compatibility of glass with new and novel secondary uses.
Best
Practice: More
than 95% of all manufactured glass is made from sodium oxide, calcium
oxide, and silicon dioxide, commonly referred to as a soda-lime-silica
composition. Due to universal
similarities in manufacturing techniques for glass containers, particularly
the viscosity requirements of high-speed “press and blow” production
equipment, the chemical formulations of glass food and beverage containers
have relative uniformity and compatibility.
Container glass is formulated to set up quickly and to hold
its shape when mechanically blown into molds.
A random mix of container glasses is anticipated to be relatively
homogenous, provided there are no significant contamination levels.
The principle element of container glass, silica, refers
to a naturally occurring mineral consisting of silicon dioxide (SiO2). The majority of silica occurs as quartz, a
crystalline form of SiO2.
Crystalline forms of silica, also known as “free” silica can
contribute to causing certain lung diseases under conditions of prolonged
exposure to airborne particles.
While SiO2 is a primary ingredient in the manufacturing
of bottle glass, when glass is formed, the crystalline structure is
changed to an amorphous structure and the SiO2 is no longer
considered crystalline.
The function of soda ash content in container glass
formulations is to lower the fusion point of the silica sand and improve
workability of the glass at moderate furnace temperatures. Lime is included in an amount sufficient to
adjust the viscosity and increase durability of the finished glass. With the development of increasingly rapid
manufacturing machinery, soda content has been lowered
and lime increased, as shorter working times are required. There has also been economic motivation to
reduce soda contents, as it is the most expensive major ingredient
in glass.
The addition of post-consumer cullet to raw materials
promotes rapid melting of the batch, lowering energy requirements.
However, after one or more reheatings, the dissolved gases
and the volatile alkalis in the cullet are driven off, making it necessary
to add chemicals to the mix in order to reinstate lower viscosity
and improve working characteristics.
Coloring agents are a minor constituent in container
glass, but can effect chemical compatibility and furnace performance
during remelting. Amber bottle
glass is produced with a sulfur-iron colloidal solution, under reducing
conditions. Green and clear glass are produced in oxidizing
reactions with small amounts of Cr2O3 dissolved
in the glass. These are general
guidelines. Of domestic container production, 63% is clear, 23% is
amber and 13% is green, but varies significantly geographically and
seasonally.
Implementation: While major variations in the
chemical formulations of container glass are not anticipated, the
composition of specific post-consumer sources of container glass should
be periodically tested to confirm the actual chemical composition. It is especially important to understand the
chemical nature of soda-lime container glass during the development
of alternative uses.
Benefits:
For
many uses of recycled glass, including remelt applications and abrasive
blasting grit, knowledge of the chemical composition of the post-consumer
glass cullet is critical to accessing end-use markets. In remelt applications, this information provides the basis for
evaluating color modification potential (see Color Modification
of Post-Consumer Recycled Glass Best Practice). In industrial mineral applications, this information
is important to assess glass’s compatibility with the chemistry of
the industrial process.
Application
Sites: Glass
beneficiation facilities, glass container and fiberglass insulation
manufacturing facilities, studio glass shops.
Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org.
References: Methods for Sampling and Testing
Glass Cullet, Clean Washington Center, 1996. Cullet Remelting Process Assessment,
Clean Washington Center, 1996.
Issue
Date / Update: November 1996 |
