Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02642181 2008-10-21
SPECIFICATION
Introduction
Tailing dams continue to be an important component of the mining supporting
infrastructures. However, they pose
significant environmental and engineering challenges. Dewatering the tailing
slurries was always one of the biggest
challenges facing the mining industry. Some attempts such as Thickened
Tailings Disposal proposed to dewater the
slurries at the source prior to discharge to the dam. The proposed method will
dewater the slurries within the basin.
Proposed Technique
There are two main steps during dewatering process. First is sedimentation
where there is no effective stress between
the solid particles. Second process is the consolidation. Both steps may be
combined into so-called large strain
consolidation. In both steps, the time required to achieve every process will
depend on how far the water has to travel to
dissipate the excess pore water pressure which is the length of the drainage
path.
This invention propose the use of embedded subdrains in the tailing slurries
to decrease the drainage path. This
invention also propose some practical methods to construct such subdrains.
The consolidation theory known in Soil Mechanics is somewhat applicable to the
tailing slurry behavior. It indicates that
the time needed to achieve field consolidation at the same degree as the
laboratory consolidation equals the product of
laboratory time at that degree of consolidation times the square of the
relative thicknesses as shown in the following
formula:
(Hdr)field
tfield - Z X tiab
(Hdr)lab
Where:
tiab : the time needed to achieve certain consolidation degree in the lab (50,
80, 90%, ..etc)
tfield : the time needed to achieve the same degree in the field
Hdr : the length of drainage path (if drained from top and bottom then it is
half of the thickness)
Similarly, the consolidation time needed for tailing slurry of different
thicknesses is proportional to the square of the ratio
of their lengths of drainage paths. Therefore, by embedding subdrains which
will reduce the drainage path, the time
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needed to achieve stable slurries will be decreased. In addition, other
benefits will achieved such as increase the
dams capacity and reduces its potential risks. These benefits will be
discussed in details later.
Generally, the drainage path in typical tailing dam is as illustrated in
Figure 1 enclosed. The drainage path will equal to
the depth of the tailings in a given dam. In the proposed method, a series of
subdrain system will reduce drainage path.
Figure 2 illustrates the drainage path when one subdrain is installed.
Consequently, the drainage path is reduced to one
third, and the consolidation time by almost one ninth. For thirty meter deep
tailing dam, if subdrains were installed every
three meters (drainage path of 1.5 m), the consolidation time will be reduced
by 400 times.
This concept seems simple, but probably the big challenge is how to
effectively construct such subdrain in tailing dams.
Some practical techniques are discussed later in the text.
Practical Construction Techniques
Large scale permeable layer may be constructed by using geotextile to separate
the tailing away from the proposed
permeable draining layer. Figure 3 illustrates this technique. Two geotextiles
separated by coarse gravel such as rip rap
will allow the water to drain and excess pore water pressure to dissipate. The
gravel layer could be connected to outlet
which collect the water to be used in mining again.
One challenge to this technique is to be able to place gravel on the top of
the slurries without being sunk. One might use
plastic-type gravel which is light enough to float on the slurries, coarse
enough to maintain high permeability, and strong
enough to transfer the stresses between the top and the bottom layers.
Further, to reduce the cost associated with the
installation, the two layers of geotextiles and the middle permeable layer in
between may be manufactured off-site in a
form of thick mat then brought to the site. The bulk density of such mat could
be adjusted (by choosing different
permeable materials) to accommodate the density of the slurries.
Another method is the use of perforated network of pipes sunk within the
tailing dam. Once in place, negative water
pressure may be applied, i.e., suction which will reduce the drainage path.
Any other method by embedding drains in the tailing body will be useful in
reducing the drainage path will also reduce
the consolidation time. The drains may be installed vertically or horizontally
or in an angle before, during, or after mining
operation, i.e., accumulation of tailing slurries.
Advantages of This Technique
The implementation of the new technique will have some practical advantages as
discussed below.
1. The main advantage is to reduce the time that these tailing will
consolidate, and the surface land can be used for
development at an earlier time.
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2. Increase the capacity of the dams. Along with the consolidation process,
the void ratio of the tailing slurry
decreases. Therefore, a given dam may be able to accommodate more tailing than
ever before. This will
increase the live expectations of tailing dams and reduce the need for new
dams.
3. Reduce the environmental and general risk associated with these dams. Risks
are function of the hazard
(severity) multiplied by the exposure which is in turn function of time. For
example, the risk for an individual to get
into a car accident increases if the time spent driving increases. Insurance
companies apply similar rules while
providing car insurance. Similarly, the risk associated with tailing slurries
will be depended on how long the
slurries need to become stable.
4. The draining by interlayer will also decrease the pore water pressure in
the deep layers remarkably as illustrated
in Figure 4. This will increase the effective stresses and lead to over
loading. Which will also accelerate the
consolidation process even further.
5. If negative pore water pressure (suction) applied to the permeable draining
layer, the effective stresses will be
increased. This is similar to the overloading condition which will accelerate
the consolidation process. In addition,
it will help increasing the tailing shear strength and might be useful to
absorb any excess pore water pressure that
might generate as a result of liquefaction.
6. After the tailing has been stabilized, air might be pumped into the
permeable layer. The air will increase the
liquefaction resistance significantly because the soil will be in a state of
unsaturated conditions. Yang et al (2004)
have indicated that the liquefaction resistance increases (CRR) by at least
two time when the saturation degree
decrease to 90 %. For near dry conditions, the resistance is practically
infinite.
Limitations
The assumption that the consolidation time is proportional to the square of
the relative length of the drainage path is
based on the applicability of the consolidation theory on the overall drainage
process in tailing slurries.
Reference
Yang, J., Savidis, S. and Roemer, M. (2004). Evaluating liquefaction. strength
of partially saturated sand. J. Geotech.
Geoenviron. Engng, ASCE 130 (9), pp. 975-979.
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