Aluminum
has long been the poster child of recycling. About half of all aluminum used in
the United States
is now recycled, and this recycling has clear and dramatic benefits: Pound for
pound, it takes anywhere from nine to 18 times as much energy to produce
aluminum from raw ore as from recycled material.
Because
it saves so much energy—and therefore money—aluminum recycling continues to
expand. But a new Massachusetts Institute of Technology (MIT) analysis finds
that this expansion could run into problems unless measures are taken to reduce
impurities that can build up as aluminum is recycled over and over again: Everything
from paint and labels on cans to other metals that are accidentally mixed in.
Such impurities will continue to add up, the MIT researchers say, but can be
managed so as to keep the accumulation to acceptable levels if extra steps are
taken while the recycled goods are sorted, or during their molten processing.
MIT
researchers Randolph Kirchain and Elsa Olivetti, of the Materials Systems Lab,
along with Gabrielle Gaustad of the Rochester Institute of Technology, published
their findings in Resources, Conservation and Recycling.
A
major aluminum producer requested this analysis to help decide whether to
install improved separation systems to prepare for impurities that could become
more serious over time. “They couldn’t make the business case based on what’s happening
today,” Kirchain says—but his team’s analysis showed that it would indeed make
sense to install such systems in anticipation of future changes.
For
now, the problem remains manageable, Kirchain says, because different uses
require different grades of aluminum. For example, aluminum engine blocks, one
major market for recycled material, can be made from metal with relatively high
levels of impurities without suffering any loss of performance or durability.
But more specialized applications, such as electronic circuits or aerospace
materials, require much higher purity.
“There
is a huge range of impurity tolerance,” Olivetti says. “The question is, how
will the balance of such markets over time compare with the kinds of materials
coming through the recycling stream?”
The
study found many techniques available to reduce impurities in recycled
aluminum. In some cases, these technologies are simply extensions of those
already used in the initial separation of aluminum from raw ore; others are
extensions of processes used to separate different materials in the recycling
stream. Most of these systems are difficult to add as retrofits to existing
plants, the study found, so it makes more economic sense to add them as new
plants are built, even if they are not yet needed.
“We’re
continuing to collect more and more scrap,” Kirchain says, which suggests that “we’re likely to have more and more problems” with accumulating impurities. So
far, the operators of aluminum-smelting plants have been able to accommodate
variations in quality. “If material comes in that’s more contaminated, they’ll
divert that toward more forgiving applications,” he says. The cleanest material
is reserved for the most specialized applications, such as airplane parts.
Kirchain
says his team’s analysis—although directed specifically at aluminum—is also an
attempt to develop methods for analyzing the life cycle of other materials that
are becoming more significant parts of the recycling stream. And it includes
analysis of the social factors governing people’s decisions on disposal of
materials, which can affect how much contaminating material ends up in a given
waste stream—or whether potentially useful material ends up in a landfill
instead of being reused.
In
order to maximize the utility of recycled aluminum, as well as other recycled materials,
there is a need for more research on reducing accumulated contaminants,
Kirchain says. “This is a technological area that has been underinvested in,”
he says. “Technology for dealing with garbage is not an exciting, high-profile
field, but there is real value in investing in this.”
David
Leon, an engineer at Alcoa Technology’s Casting Technology Division, who was
not involved in this research, says, “Developing methodologies to increase the
use of ever-decreasing quality scrap is of major importance to the industry.
Even as important is the development of tools to make the right decisions
regarding implementation of these technologies.”
