Natural Resources Extraction: Stressors and Impact Management - Sustainable Practices in Geoenvironmental Engineering - page 170

options available for management of slurry tailings ponds. The irst option is to leave the
ponds in their disposal state. This is not a generally acceptable option, but nevertheless must
be considered, in the event of negligent abandonment of such ponds. Health and safety con-
cerns require implementation of (a) monitoring procedures for these ponds and (b) health
and safety measures to protect human and wildlife population. Figure 5.16 also shows three
distinct categories of sustainable land-use options for the slurry tailings ponds are available:
(a) return of land to ab initio physical condition, (b) reclamation of the ponds in the keep-it-wet
state, and (c) reclamation of the ponds in the take-it-to-dry state. Returning the land to its ab
initio landscape condition fulils the landscape portion of sustainability requirements.
Two basic options are available for the keep-it-wet state: (1) freshwater pond, and (2) wetlands.
In the take-it-to-dry category, the options available depend to a large extent on (a) quality or
competency of dry land obtained from the reclamation process and (b) regulatory and commu-
nity requirements. The basic element of all the schemes for reclamation of slurry tailings ponds
and other types of containment facilities must deal with the question of “what to do with the
stagnant layer.” Liquid-solids separation and treatment of the released water are basic require-
ments for any of the pond reclamation options. Physical methods for liquid-solids separation
include surcharging the top of the stagnant layer to achieve consolidation—a geotechnical
process that provides compression of the solids skeletal matrix through the applied surcharge
and drainage of the water in the skeletal structure. Other physical methods include removal of
the stagnant layer for treatment, and illing of the emptied pond with new ill material.
Chemical and physicochemical methods for increasing the sedimentation rate of the
suspended solids in the stagnant layer include the use of polyacrylamides and polyelec-
trolytes. The basic intent of these kinds of locculants is to overcome the domination of
interparticle forces typical of colloidal interaction. Various kinds of locculants and loc-
culating agents have been developed for such types of slimes and sludges (Yong and Sethi,
1982, 1983, 1989). They all have the aim of promoting aggregation of the particles into loccs,
thus increasing the mass of individual groups of particles and hence allowing for gravi-
tational forces to dominate and sedimentation to occur. Calculations performed by Yong
and Wagh (1985) using two-particle collision theory to study the stability of the suspended
solids in the stagnant zone have shown the effect of aggregation on the settling velocities
of the suspended solids. Conirmation of their calculations has been obtained from exper-
iments on a pure clay mineral suspension (kaolinite) and the red mud discharge from
bauxite processing (Figure 5.17). Aggregation of the red mud particles with increasing
solids concentration caused the increase in settling velocity. For the kaolinite soil suspen-
sion, increasing solids concentration in the soil suspension served to decrease the settling
velocity—probably due to the hindrance effect posed by the proximal particles.
Another method for increasing the settling velocity of the suspended particles is to
increase the zeta potential (ξ) of the particles. This is the potential that represents the charge
at the shear layer between the suspended particle surface and the suspending luid. We
recall from Equation 5.5 that the potential ψ provides us with a means for determining the
electric charge distribution at a distance from the particle surface. This potential has two
basic components: (1) the potential at the surface of the particle, represented by ψ o , and (2) the
potential ψ s at the Stern layer boundary (double-layer boundary) where the shear action with
the liquid medium occurs. This potential, which is commonly identiied as the zeta potential
(ξ) is a function of the nature of the surface charge possessed by the suspended particle, the
ions in the suspending luid, and the ions in the double layer. By changing the zeta potential,
aggregation of the suspended particles in the stagnant zone can be obtained, with resultant
increased settling velocities. Figure 5.18 shows the relationship between zeta potential (ξ)
and the dispersion stability for clay soils reported by Yong (2001b).