Natural Resources Extraction: Stressors and Impact Management - Sustainable Practices in Geoenvironmental Engineering - page 158

(a) Simple Stokesian settling of liquid − solids mixture

Clear supernatant

Liquid−solids
mixture in holding
pond at discharge

Pond

Sedimented
solids

Initial discharge Final state

(b)

Holding ponds with recalcitrant suspended solids (sludge, slime, etc.)

Ground surface

Time t = discharge time

Liquid−solids
mixture in holding
pond at discharge

Supernatant

Time t = some time period
after discharge

Increasing concentration of
solids due to some
Stokesian settling

Suspended solids with
concentration of solids
increasing slightly with
depth

Supernatant

Time t = many years after
discharge

FIGURE 5.9
Two types of behavior of liquid-solid discharge in slurry tailings containment structures. The examples shown
are containment ponds. (a) Simple Stokesian settling of the solids in the liquid-solids mixture. (b) Solids behav-
ior over some period, ending up with recalcitrant performance of suspended solids.

2. Containment structures that contain solids' suspensions. The solids in these sus-
pensions may or may not inally settle to the bottom of the containment structure
to form sediments. Using containment ponds as an example, the bottom diagrams
in Figure 5.9 show the characteristics of these kinds of ponds. The dispersion sta-
bility of these types of slurry tailings will be discussed in the next subsection.

For suspended ines in slurry tailings that do not exhibit liquid-solid separation behav-
ior, some common features can be identiied. If one determines the solids concentration
with depth of the slurry tailings, one will obtain at least four distinct zones. These are
(1) clear supernatant liquid, (2) a transition zone where the solids concentration begins to
register some small value that increases as one progresses in depth, (3) a stagnant zone
where the solids concentration remains relatively constant or increases imperceptibly with
depth, and (4) a sediment zone that contains the solids that have inally settled to the
bottom of the containment structure. Figure 5.10 shows a typical solids concentration (sc)
proile in the four zones that are typical of various kinds of sludges, slimes, slurries, etc.
The sc values refer to the weight ratio of the suspended solids to aqueous phase that con-
stitutes the suspension luid. Figure 5.11 shows the solids concentrations in the stagnant
zones for various types of slurry tailings. Leaving the suspended ines in the containment
structures is not an acceptable land-use option. Strategies have been developed to render
the material in the containment structures to a state that would not pose a threat to the
immediate environment and biota (Yong, 1983a, b).