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WATER + WASTEWATER TREATMENT
Novel method for rapid
removal of water from
phosphatic clay slurry
BY STEVE ORLANDO, SENIOR DIRECTOR, MEDIA RELATIONS, UNIVERSITY OF FLORIDA
Cleaning up the water left over from mining operations can literally take
generations – 25 to 50 years on average – leaving billions of gallons of this
precious resource locked up and useless.
AUniversity of Florida researcher has recently figured out
how to trim the time taken to clean up water left over from
mining operations dramatically, from up to 50 years, to just
two to three hours. This is a potential boon to mining companies, the
environment and the global regions where water is scarce.
‘I think that the ability to save water is going to be really big,
especially when you’re talking about China and other parts of the
world,’ says Mark Orazem, a distinguished professor of chemical
engineering at the University of Florida’s Herbert Wertheim
College of Engineering.
Orazem’s team’s idea has attracted the attention of the Minnesota-
based Mosaic Company, which produces phosphate and potash
for fertiliser, and operates four phosphate mines in central Florida.
‘We value the longstanding partnership that we have with the
University of Florida. It’s important for us to continually seek
out research that improves the efficiencies of our processes,’
says Paul Kucera, Senior Engineer Adviser in Research and
Development with Mosaic.
Mining operations use water for mineral processing, dust
suppression and slurry transport. When they’re finished with
it, the water holds particles of mineral by-products, known in
the phosphate mining business as clay effluent. In the case of
phosphate mines that are so common in Florida, the clay effluent
has the consistency of milk.
‘It looks like a solid, but if you throw a stone into it, it’ll splash,’
That water is pumped into enormous settling ponds – some are as
large as a square mile with a depth of about 40 feet – where the
particles can sink to the bottom. Florida alone is home to more than
150 square miles of such ponds – an area that would cover about
half of New York City.
But it’s a lengthy process, as the slurry (a mixture of sand, clay and
phosphate) particles are extremely small, and, more importantly,
the clays are electrically charged – like charges repel and opposite
charges attract. The clays’ strong charges cause them to repel
each other, which keeps them suspended in the water instead of
sticking together and sinking to the bottom.
That means that mining companies can re-use only a bit of the
water at a time – the part skimmed off the top of the settling
areas. Not only is the remaining particle-filled water useless, but
the land that those settling ponds occupy is also a valuable asset
that could be used for other purposes.
Ideas for speeding up this process go back centuries. In 1807, an
early application of the battery invented by Volta in 1800 showed
that clay particles moved in response to an electric field. In the
1990s, an electric field was used to separate clay and water in
batches, but that concept was deemed uneconomical.
Over the years, the industry has spent millions of dollars on ways to
speed up slurry settlement; the most common is by the addition of a
flocculating agent (such as anionic polyacrylamide). While this does
substantially improve the rate of settling, there are consequences
to this practice. Because of the substantial quantities of waste
slurry produced, cost becomes a major factor. Other considerations
regarding flocculation are how the polymer residue in the recycled
water affects the beneficiation, as well as the knowledge that, at
some point in the future, these waste clay areas may be re-mined
to obtain the phosphate particles still present within the clay matrix.
Whether the flocculated material can then be separated using the
standard procedures is open for debate.
Orazem’s design is different, because it allows for a continuous
feed of clay effluent into a separation system. There, upper-
and lower-charged plates are used as electrodes. An electrical
potential is applied across the plates, creating an electric field,
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