Environmental Aspects of Phosphate and Potash Mining United ...

The Environmental Approach of the Phosphate Rock and Potash Mining Industry
The waste slimes and brine are transferred underground
through a slurry pipeline and discharged to the
excavated salt rooms. Generally, two or three salt rooms
are filled simultaneously. Deposition and settling of the
slimes occurs in one room, while the clarified brine
overflows into the second and/or third.
The clarified brine is recovered from the salt rooms and
returned to the processing plant where it is evaporated
to produce both a fresh water condensate, and a concentrated
brine. The fresh water condensate is used
largely as an input to the process with excess released
through the holding pond. The concentrated brine is
returned to the crystallization circuit for recovery of
residual potassium chloride.
The water balance of the operation is essentially
closed. Water released from the site is related to either
on-site rain or snowfall or the excess fresh water condensate
from the processing facility. Both of these
sources are retained in a continuously monitored holding
pond prior to release to the local river.
The adoption of the closed circuit waste loop, demonstrates
some of the benefits that arise from an effective
pollution prevention program. The potential sources of
pollution in the form of the three major waste streams
have been integrated into the mining process. The salt
tailings have become an essential component of the cut
and fill mining method, recycling of the brines has
increased product recovery and eliminated the need for
contaminated discharges, and the slimes and some
brine are effectively disposed of in the salt rooms.
The major environmental benefit of the approach is the
small surface impact of the operation. The lack of any
surface salt tailings stack, slime and brine ponds has
reduced the potential for surface environmental
impacts to negligible levels.
The approach does incur an additional cost in comparison
to conventional surface waste disposal methods. In
the case of the New Brunswick operation, this has been
partially offset by greater recovery of the potash ore
resource, lower product transportation costs due to the
operations location with respect to markets and a
potentially a lower future rehabilitation cost during
operation closure.
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Treatment of the Clay Fines to Remove Salts
The IMC Potash Colonsay operation in Saskatchewan,
Canada is developing a process to remove salt and
brines from the clay slimes waste. Removal of the salt
allows the insolubles to be disposed of without special
precautions.
The process involves a number of steps:
Separation of the slime component by thickening to
remove brine;
Washing of the slimes with water to dissolve any
residual salt;
Cycloning to remove brine;
Filtering with the addition of fresh water to produce a
relatively dry, desalinated product.
The filtered slurry forms small, stable, agglomerated
particles, facilitating handling and reducing the likelihood
of dust problems.
Results from earlier trials are being used to improve the
process.
This method removes the need for large areas to be set
aside for clay slimes settling ponds. The slimes are
instead converted into an environmentally benign, easily
disposed product that may possibly be sold as a clay
substitute for brick and pottery or a water retention
amendment for sandy soils.
Figure 3.6.5
Flowsheet of the slimes desalination and
dewatering process. IMC Potash, Canada.
Fresh
water
Clay, salt fines
and brine
Cold slimes
thickener
Filter
Flocculant
Brackish water
Desalinated and dewatered clay
(75 - 80% solids)
Washing
Cyclone
Clay
and
brine