Page 196 - Materials Chemistry, Second Edition

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Sustainability of Municipal Solid Waste Management
A consideration in glass recycling is color separation. Permanent dyes
are used to make different colored glass containers. The most common col-
ors are green, brown, and clear (or colorless). In the industry, green glass is
called emerald, brown glass is amber, and clear glass is flint. For bottles and
jars to meet strict manufacturing specifications, only emerald or amber cul-
let (crushed glass) can be used for green and brown bottles, respectively. The
glass is color sorted and sent to a glass crusher or ball mill.
Glass can be recycled indefinitely as part of a simple but hugely benefi-
cial process, as its structure does not deteriorate when reprocessed. In the case
of bottles and jars, up to 80% of the total mixture can be made from reclaimed
scrap glass, called “cullet”. Cullet from a factory has a known composition and
is recognized as domestic cullet. From bottle banks it is known as foreign and
its actual properties will not be known.
The cullet is then mixed with the raw material used in the production
of glass. After the batch is mixed, it is melted in a furnace at temperatures
ranging from 1,200°C to 1,400°C. The mix can burn at low temperature if
more cullets are used. The melted glass is dropped into a forming machine
where it is blown or pressed, drawn, rolled, or floated depending on the final
products. The newly formed glass containers are slowly cooled in an anneal-
ing furnace.
The manufacture of glass uses energy in the extraction and transporta-
tion of the raw materials, and during processing as materials have to be
heated together to a very high temperature. Large amounts of fuel are used
and the combustion of these fossil fuels produces a lot of emissions. An effi-
cient furnace will require 4 GJ of energy for each ton of glass melted.
The most important fuels for glass-melting furnaces are natural gas, light
or heavy fuel oil, or liquefied petroleum gas. Electricity (frequently installed
as supplementary heating) is also used in some special cases with low pro-
duction capacity because energy requirements range from 3.7 to 6.0 GJ/t glass
produced.
If recycled glass is used to make new bottles and jars, the energy needed
in the furnace is greatly reduced. In addition recycling reduces the demand
for raw materials. There is no shortage of the materials used, but they do
have to be quarried from the landscape, so from this point of view, there are
environmental advantages to recovering and recycling glass.

5.7 Foam Glass
Another technology for glass recycling is foam glass. There are numerous
patents on foam glass production dating back to the 1930s. Even though
there are numerous patents there are only a few that have been adopted on a
commercial basis. Foam glass, also referred to as cellular glass, was origi-
nally manufactured from a specially formulated glass composition using vir-
gin glass only. Currently, there are a number of foam glass production plants