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WATER + WASTEWATER TREATMENT
which pulls the particles downward, thereby increasing the density
of the material as entrapped water is released. This densified
matrix, termed a ‘cake’, can then be used to fill the excavated
areas that are created by the mining operations. Solids contents
as high as 35–40 per cent by weight were achieved when
starting at a 10 per cent by weight solids content. Multiple tests
were performed by varying both the conveyor speed and/or the
voltage potential. The above test results were produced using
a residence time of one to two hours (for instance, a conveyor
speed of 0.5 miles per hour), and an applied electric field of four
volts per centimetre. A secondary benefit of this method is that
the solids and supernatant water are separated while the cake is
being formed. Thus, it can be removed at various locations along
its travel path. The water is now clear enough to be re-used for
processing additional mined phosphate ore.
‘Instead of having the water tied up in these clay settling areas,
water is sent back through the process, and then re-used and re-
used and re-used,’ Orazem says.
Orazem’s team has created a lab-sized prototype. The next step,
he says, is to determine how to scale it up to a point where it can
work in a full-scale mine.
The economics of scaling in terms of power and energy were
extrapolated from laboratory test results. It is estimated that the
power requirement for batch-mode dewatering of a 10 per cent by
weight slurry in a one-square-mile (2.6 square kilometres),
40-foot (12 metres) deep settling pond to 25 per cent by weight
would require 44,000 megawatts.
In stark contrast, for continuous dewatering of the slurry as it is
discharged from Mosaic’s Four Corners Mine plant, the required
power was estimated to be as low as 29 megawatts. Moreover,
this reduction in power requirement was accompanied by a
40 per cent increase in solids content, or 35 per cent by weight,
which is very close to the targeted 40 per cent by weight figure.
Another perspective on the efficiency improvement is seen in
the reduction of energy consumption from 41.3 watt hours per
kilogram of reclaimed water for the batch procedure to 6.6 watt
hours per kilogram for the continuous process. These figures may
be expressed in terms of dry solids produced as 248 watt hours
per kilogram versus 47 watt hours per kilogram, respectively.
It should be noted that these estimates are based on the
performance of a laboratory-bench scale instrument; however,
the improvement in performance metrics as compared to batch
operations suggests that a fully continuous electrokinetic
dewatering system may be a cost-effective method of reducing or
eliminating clay-settling areas while enhancing the availability of
water for recycling.
The efficiency of the current configuration can be further improved
by optimising both cake formation and cake dewatering zones.
The team is exploring system designs that will allow separation of
the cake build-up and cake dewatering zones, thus eliminating the
remixing of water and solids, which represents a major inefficiency
in the current design. In tandem, mathematical models are being
developed that may enhance the separator configuration.
While Orazem’s concept was designed for Florida phosphate
mines, he says that it could be used anywhere, and that it would be
especially useful in arid North Africa. In Morocco and the Western
Sahara, with 85 per cent of the world’s phosphate reserves, water
is in especially short supply.
‘Recycling water is going to be critically important,’ Orazem says. ‘So,
in Florida, it’s an issue. In the desert, it’s going to be a major issue.’
Steve Orlando can be contacted on:
Office: 352 846 3903 or 352 273 3819
Cell: 352 215 4232
These photographs show the results of an initial nine per cent by weight clay suspension from the Four Corners Mine in Bartow, Florida. Mesh electrodes
at the top and bottom were spaced 20 centimetres apart. A potential of 80 volts (providing an electric field of four volts per centimetre) was applied. After a
period of nine hours, a semi-solid mass of clay formed with a solids content of 33.5 per cent by weight. Photos courtesy Patrick McKinney.
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