5
Chemical Technology • Septemb
er 2013
special report
and have four faces, in a square, often with inner
dams. The retaining walls must be able to cope with
the water shed by the slime, channelling it for collection
and recycling in the mine.
Everything is a function of cost and efficiency. The
components of a slimes dam are the tailings them-
selves. Coarse tailings create controlled channels for
water. Fine tailings must be contained. A mechanism,
known as cycloning, separates coarse from fine tailings.
Tailings under pressure enter a mechanical cyclone and
the heavier tailings fall to the bottom while water and
finer tailings rise to the top.
The fine fraction is known as the overflow, while the
coarse fraction is called the underflow. Tailings are con-
veyed from the mine into the dams via various inlets,
with a fraction being sent for cycloning, and the under-
flow being used to build and maintain the dam wall.
Correct deposition of the underflow, and monitoring
of the tailings throughout the process, ensure proper
construction and maintenance. Water seepage will hap-
pen. Much of that water is toxic and must be contained
and recycled anyway. Maintaining a safe flow to prevent
piping or overflow or liquefaction is critical.
What happens, though, if the water flow is radioac-
tive? Worse, what happens if water seeping downwards
meets a subterranean river flowing slowly along the
bedrock? That is the situation currently facing the
nuclear power plant at Fukushima in Japan.
In 2011, a tsunami destroyed the plant’s contain-
ment shielding and a steady flow of radioactive water
has exited the site for the ocean. Some 300 tons of
water are flowing into the Pacific daily from the plant,
destroying sea life and endangering the area’s recovery.
Engineers on site have attempted numerous
containment strategies, from attempting to divert
the groundwater flow around the site, to building clay
containment walls, to building a steel barrier. Nothing
has worked.
The difficulty of using approaches from tailings
engineering is that tailings dams always assume that
water will seep out and then be captured for recycling.
In Fukushima, they need to stop water flow entirely.
The situation there is not unlike the acid mine drain-
age experienced around Gauteng. Water flow must be
stopped or processed if it is to be rendered safe.
In Fukushima’s case, radioactive water can’t be
processed. It must be stopped.
Fortunately, we’ve had some experience containing
radioactive waste. In 1962, the Atomic Energy commis-
sion disposed of 6 800 kg of radioactively contaminat-
ed material at a site in North-western Alaska selected
because the ground is permanently frozen. In 1995,
the site was inspected and virtually no radionuclides
had entered the permafrost.
Neither do we have to wait for global freezing to
achieve this. After all, we’ve invented refrigeration and
freezing systems. In “Operation and Maintenance of
the Frozen Barrier at the HRE Pond”, Edward Yarmak
and Elizabeth Phillips describe the use of thermosy-
phons – passive heat removal devices moving heat
against gravity – to freeze the earth and so build
containment walls.
The process is quite straightforward. Whenever the
top of the thermosyphon is at a lower temperature
than at the bottom, condensate flows via gravity to the
bottom and heat is released at the top through vapour.
This flow continues as long at the temperature at the
top is colder. This can happen without moving parts or,
more effectively, with a standard refrigeration compres-
sor. The liquid at the bottom of the syphon just needs
to be something that won’t freeze at the same tem-
perature as water. Brine is useful. A freezer manifold
at the top ensures that the temperature gradient is
maintained.
Creating such an ice-wall has been in use since
the 1960s. Thermosyphons are sunk to the bed-rock
at about one-metre intervals. They have to be evenly
spaced and straight to ensure good convection flow.
The manifold is placed at the top and linked to power
generation, the passive refrigerant pool is added to the
bottom and cooling begins.
The process, which takes about three months, starts
with ice forming around the syphon and gradually ex-
tending in a ring around the column. The walls can be
up to three metres thick.
Neither are these particularly expensive to run. The
frozen barrier, described by Yarmak and Phillips, oper-
ates at -32
o
C and is about 91 m long, and 3,66 m thick.
It takes about 200 000 kWh of electricity consumption
to run annually, or about $14,50 per day.
The wall around the Fukushima plant will be about
1,4 km long and to a depth of about 27 m.
Joe Sopko is civil engineering firm Moretrench's di-
rector of ground freezing and he estimates that power-
ing the wall will take about 4,5 megawatts of electricity
generation. The Japanese government has declared a
budget of $473 million to set up and maintain the wall.
The sadness in all this is that, once again, the bril-
liance of human ingenuity is on display only after a ma-
jor disaster. The achievement of stopping the Deepwater
Horizon spill in the Gulf of Mexico was astonishing as
engineers drilled a three-kilometre drill-string to inter-
sect with the original leaking pipe and stop the flow.
At Fukushima, engineers are once more showing
the technology available to us. Would that such genius
were used to prevent the disasters in the first place.
