Environmental Engineering Reference
In-Depth Information
Table 19.5.
Typical tailings permeability (adapted from Vick, 1983).
Type
Average permeability (m/sec)
10
4
to 10
5
Clean coarse, or cycloned sands with less than 15% fines
10
5
to 5
10
6
Peripheral-discharged beach sands with up to 30% fines
10
7
to 5
10
9
Non plastic or low-plasticity slimes
10
6
to 10
10
High plasticity slimes
Note: These are likely to be representative of average vertical permeabilities.
19.2.3.6
Properties of water in tailings
The water (or liquor) accompanying tailings often contains dissolved salts, heavy metals
and other residual chemicals from the mineralogical process.
The water may be highly acidic or alkaline, have high salts content, a heavy metals con-
tent or have arsenic or cyanide present. Each may have important implications on the
impact of the tailings disposal area on the environment. Some common problems are:
- Gold tailings - cyanide;
- Coal tailings - high salts content, sometimes sulphides;
- Copper, lead-zinc, tin tailings - heavy metals, sulphides (which oxidise and lead to
leaching of heavy metals and yield acid water);
- Alumina tailings - caustic soda (NaOH), unless the tailings are neutralized before dis-
charge;
- Uranium tailings - heavy metals, radon;
- Other tailings may have quite acceptable water quality, e.g. some bauxite and iron ore
washeries.
The science of predicting the chemistry of the tailings liquor and the fate of contami-
nants in the groundwater has advanced considerably over the last 10 years. It is a special-
ist area and the advice of a suitably qualified person should be sought.
19.3
METHODS OF TAILINGS DISCHARGE AND WATER RECOVERY
19.3.1
Tailings discharge
Most tailings are pumped to the tailings disposal area after thickening as a high water
content (e.g. 35% to 40% solids content) slurry. In thickened tailings the solids content may
be as high as 50% to 60%. The tailings are discharged into the storage from a single
or several discharge points, or from spigots, as shown in
Figures 19.6
and
19.7
. A single
fixed discharge point gives little flexibility in management of the tailings and is usually
undesirable.
Spigotting facilitates the uniform spreading of the tailings and encourages desiccation.
For conventionally non-thickened tailings spigotting encourages the segregation shown in
Figure 19.6 with the sand and coarse silt settling near the embankment.
For some fine grained high clay content tailings, a “spray bar” is attached to each spigot
to further distribute the tailings and allow uniform beaching. The “spray bar” consists of, for
example, a 100 mm diameter pipe with 20 mm diameter holes at 500 mm spacing. The objec-
tive is to give a uniform deposition of tailings, allowing the greatest degree of sorting (or
“classifying”) of the tailings away from the discharge points and to optimise desiccation.
