Tailings Disposal - Mining and the Environment: From Ore to Metal - page 713

general, the higher proportion of 'i nes' (silt and clay fractions), the lower the permeabil-
ity and hence slower drainage rates. This of course also has its benei ts. Consider bauxite
residues, commonly known as 'red mud'. As illustrated in Figure 18.2 red mud predomi-
nantly consists of silt and clay-sized particles. These small particles, once they have settled,
form a soil mass of very low permeability with a high moisture retention capacity. As a
result of these properties, the remnants of caustic solution from the Bayer process, used to
dissolve alumina from unwanted host materials, remain locked in the matrix of the mud,
and therefore cannot be readily leached. These properties also mean that the mud can be
used as a capping or sealing layer to prevent ini ltration or drainage.
If tailings contain a sufi cient percentage of coarse materials (sand), the coarser frac-
tion can be separated and used to construct embankments to impound the much weaker
i ne tailings materials. Coarse tailings are also used, with or without added cement, for
back-i lling of underground mine openings. If coarse tailings materials are absent, all con-
struction materials for tailings embankment construction must obtained elsewhere, with
the result that additional environmental impacts due to borrow pits and material move-
ment may occur. In most mining situations, selected waste rock from the mine excavation
is incorporated in tailings embankments.

Tailings deposition occurs over
days while consolidation requires
years or tens of years.

Pulp Density
Pulp density refers to the water content of the solid-water mixture that forms tailings. Once
the minerals of interest have been extracted from milled and processed ore, i ne solids that
remain are mixed in suspension with water that may also contain remnants of dissolved salts
and reagents used in the process. Typical tailings slurries have solids contents of 20 to 40% by
weight ( Table 18.1 ) . Once discharged to a tailings storage facility, water is removed by decan-
tation, drainage and evaporation. Safe on-land storage of tailings slurry is difi cult to achieve
and reclamation of wet tailings storages requires long-term planning and i nancing. A recent
trend is to reduce the water content of tailings prior to i nal disposal by using paste and thick-
ened tailings technologies (Jewell and Fourie - 2005). Tailings with reduced water contents
involve lower environmental risks. However, such dewatering technologies are relatively
expensive and are used mainly in cases of severe water shortage, to recover expensive process
reagents, or in situations where secure conventional tailings systems cannot be constructed.
When discussing thickened tailings or paste, it is also important to recognize that no
two tailings will necessarily have the same solids concentration when formed in these
states, since their particle size distributions, clay content, particle shape, mineralogy, elec-
trostatic forces and l occulants dosing vary tremendously. Table 18.1 gives some typical
slurry and paste solids concentrations for a range of tailings types (Williams 2004). The
percentage of solids in the thickened state is between the slurry and paste values.

Safe on-land storage of tailings
slurry is diffi cult to achieve
and reclamation of wet tailings
storages requires long-term
planning and fi nancing. Tailings
with reduced water contents
involve lower environmental risks.

Tailings Chemistry
Tailings chemistry, not surprisingly, rel ects the host rock geochemistry. Many tailings con-
tain pyrite which may oxidize on exposure to the atmosphere, producing acid in the same
way that waste rock can oxidize to give acid rock drainage. Depending on the process, rea-
gents may be introduced that may be harmful to the environment. Since all water enter-
ing the tailings impoundment is potentially released to the environment either as decant
water, run off, seepage, leachate, or by evaporation, understanding of long-term chemistry
of tailings water is essential. It is important, however, to recognize that while some tailings
are toxic, many tailings are not. More often than not, tailings solids are inert rock particles
and their direct environmental impacts are of a physical, not chemical nature. Similarly,
while some tailings liquids contain dissolved substances that may be toxic or hazardous,
others contain water with no deleterious constituents.

More often than not, tailings
solids are inert rock particles
and their direct environmental
impacts are of a physical, not
chemical nature.