Can chemists come up with better uses of
mineral resources to make catalysts that are more sustainable? For
a growing
number of
researchers, the answer is yes, and the key is taking advantage of
materials that are
already out of the ground. Red mud, the noxious by-product of the Bayer
process for
extracting aluminum from
bauxite ore,
makes a good case study.
The majority
of material processed
in mining
operations ultimately goes to waste. For every ton of alumina extracted
from bauxite,
more than a ton of red mud is produced; aluminum mining leaves behind
some 120
million metric tons per year of the salty, highly alkaline, heavy-metal-laden
material,
according to the International
Aluminum Institute.
Some 4 billion metric
tons of the material is lying about globally,
much of it
held in retention ponds.
Mining
companies have long tried to find ways to recycle the environmentally
problematic red
mud. It
is a classic problem
in search
of a solution. One approach
is neutralizing
red mud with
seawater or treating it
with CO2 or sulfur compounds. The modified materials have been tried as
fill for
mining and construction, as pigment and filler
for bricks and cement, and as a sorbent for water treatment. Others have
looked at
extracting more aluminum from
red mud,
or obtaining other useful
metals such as
sodium, copper, and nickel. But so far there have been few safe and
economical large-scale applications.
On a
new front, some chemists are trying to go catalytic, focusing on iron oxide, the
chief component
of
red mud. But given the
purity and
properties of red mud, researchers
have found
it typically is not an active
enough catalyst
to compete against existing
commercial catalysts.
That’s because the
mineral composition,
particle size, and surface
properties are
important in developing heterogeneous
catalysts. With red mud,
finding the
right combination is a work in progress.
One
early sign of success comes from Foster A. Agblevor of
Utah State University’s
USTA Bioenergy Center and coworkers in conjunction
with Pacific Northwest
National Laboratory
researchers. They have
been testing red mud as a bulk catalyst to replace
zeolites in a fluidized-bed
reactor to pyrolyze
biomass
to make crude
oil.
The
team processes
the biocrude
oil using a traditional catalytic
hydrotreating process
to make a gasoline-
type fuel
and has tested it on a lawn
mower or
lawn trimmer. “We are able to run an engine on the fuel without difficulty,” Agblevor says. The
Utah
State researchers have applied
for a
patent for their process. They are working with
catalyst company Nexceris to scale
up catalyst production
and with
Wildland Forestry
& Environmental to
harvest wood
from pinyon-juniper
range lands in
the western U.S. to scale up biofuel production.
The team
is also expanding the
scope of
using red mud beyond biomass pyrolysis, Agblevor says. The researchers have
applied the
catalyst to coal gasification, he notes, as well as to a process for catalytic
pyrolysis of
waste tires for fuel production. Despite raw red mud’s ultimate utility as a
catalyst, its
story points to other possibilities
for recovering
metals that have already
been extracted
and
used. For example,
industrial processing,
the use of consumer
goods and
medicines, and even the wearing
away of jewelry
leads to measurable
amounts of
catalyst metals such as gold, silver,
and platinum accumulating at wastewater treatment plants.
