Environmental Engineering Reference
In-Depth Information
5.4.2 Nature of Contained Slurry Tailings
Studies on the nature of the solids in suspensions in slurry tailings facilities (ponds, etc.)
where solids remain in suspension for some considerable length of time show that the
principal factors responsible for the dispersion stability of the suspended solids include
(a) colloidal nature of the solid ines, (b) reactive surfaces on the ines, and (c) chemistry of
the suspending luid. The theoretical basis for the dispersion stability of the colloidal-type
solid ines has been well developed and reported (Kruyt, 1952). The diffuse double layer
(DDL) model is a good it with the types of suspended ines (e.g., montmorillonite, kaolin-
ite, amorphous materials) found in many slurry tailings. It essentially provides one with a
basis for determining (theoretically) the maximum volume of water or luid in the diffuse
ion layer that surrounds individual reactive suspended particles.
The intensity of the interaction forces between the two particles resulting from the inter-
penetration (or overlapping) of the contiguous diffuse ion layers is a function of (a) the
extent of the overlapping or inter-penetration of the adjoining diffuse ion layers, (b) the
nature of the reactive surfaces of the particles, and (c) the chemical composition of the sus-
pending luid. The electrostatic interactions of the ions in the diffuse ion layer and their
relation to the surfaces of the reactive suspended particles are expressed as an electric
potential ψ that decreases in an exponential manner as one departs further from a particle
surface. The DDL model provides one with the basis for computing the average electric
potential ψ as a function of distance from the surface of the particle as follows (Yong and
Warkentin, 1975):
22
2
κ
T
x
8
π
εκ
z en
T
ψ
=−
ln coth
i
i
(5.5)
e
2
where the negative sign on the right-hand side indicates that the potential ψ decreases as
one departs further away from each particle surface and where κ is the Boltzmann con-
stant, T is the temperature, e is the electronic charge, n i and z i are the concentration and
valence of the i th species of ions in the bulk solution, respectively, and ε is the dielectric
constant. A detailed treatment of the DDL theory and models can be found in Kruyt (1952).
The development and application of the DDL models to soil mineral particles such as those
found in slurry tailings can be found in Yong and Warkentin (1975) and Yong (2001a).
Calculations of the volume of water associated with a gram of soil particle in equilib-
rium in an aqueous phase, based on type of soil fraction and DDL interactions, can be
made using the DDL models. These can be compared with measurements of equilibrium
solids concentrations obtained in soil suspension experiments. The results of solids sus-
pension tests reported by Yong (1984) are shown in Table 5.3 for some typical soil solids
found in slurry tailings. These results are expressed as the equilibrium volume of water
per unit weight of suspended solids, and the units are given as cc/g of soil. The void ratios
shown in the third column of 5.3 have been calculated from the measured equilibrium
volumes.
Yong (1984) has shown good correlation between calculated and measured equilibrium
solids concentration for the stagnant region of a slime pond using the equilibrium volumes
shown in Table 5.3. In the actual cases examined, predicted solids concentrations were
compared with actual solids concentrations obtained from samples in the stagnant zone
(Figures 5.7 and 5.8) for phosphatic slimes, aggregate slimes, tin mining slimes, benei-
ciation slurry (slimes), tar sand sludges, etc. Table 5.4 shows that except for the aggregate
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