Tailings Disposal - Mining and the Environment: From Ore to Metal - page 723

There remains considerable controversy over the economics, feasibility, practicability
and environmental trade-offs with dry tailings disposal, especially in areas of high precipi-
tation and runoff. One signii cant disadvantage of dry stacked tailings is the cost, which
ranges from US$ 1.00/tonne to US$ 10.00/tonne, with average cost between US$ 1.50/tonne
and US$ 3.00/tonne. This is much higher than for conventional tailings disposal. However,
it should be recognized that tailings storages created by dry stacking are inherently far
more stable and can usually be rehabilitated more rapidly and at lower cost.
Dry stacking of tailings has not yet been attempted in the earthquake-prone, mountain-
ous, wet tropics, where conventional tailings disposal has proved so difi cult. However,
prima facie , it provides a solution to the problems experienced in these areas. In particular,
the prospect of co-disposal of tailings i lter cake and waste rock appears particularly attrac-
tive. In many cases, the tailings may be suitable to create permeability barriers or encapsu-
lating layers within the storage, assisting in control of acid rock drainage.
Tailings i lter cake is well suited for back-i lling of underground mine openings - to
improve stability and to enable pillars to be safely extracted. Another promising application
is for back-i lling of shallow lateritic Pits, in which i lter cake is placed, spread and compacted
within each pit, prior to replacement of overburden and topsoil. Potential economies involve
back-loading of ore haulage trucks with i lter cake or the use of Innovative Conveyor Systems
( www.innovativeconveying.com ) which enable ore to be conveyed in one direction and solid
waste in the other simultaneously. In such cases, disposal of tailings may be achieved without
increasing the overall footprint of the project. Cementitious substances such as Portland cement
have been mixed with tailings i lter cake to add strength in backi ll situations. Judicious use of
such additives may also be used to provide erosion resistance for drystacked tailings.
Since dewatering technologies such as i ltration are advancing at a rapid rate (Jewell
and Fourie 2006), it is likely that costs will be reduced in comparison to alternatives, and
that the use of dry stacking will become much more common in the future.

Since dewatering technologies
such as fi ltration are advancing
at a rapid rate (Jewell and
Fourie 2006). The use of dry
stacking will become much more
common in the future.

In-pit Storage and Backfi lling
As for waste rock disposal, the best sites for tailings storage may be voids remaining at the
conclusion of mining, the preferred choice for i nal tailings placement for many regulatory
authorities worldwide. Apart from the low costs involved, this means that the tailings are
stored below grade, so that failure by collapse or erosion is physically impossible (USEPA
1994). Potential adverse impacts of in-pit storage of tailings may include:
1. Ore extending below the Pit becomes more difi cult to extract;
2. Water discharged with the tailings, if it contains toxic or hazardous constituents, could
contaminate groundwater in the vicinity of the storage;
3. Similarly, if the tailings are subject to oxidation causing acid generation, local ground-
water could potentially become contaminated. On the other hand, if the storage Pit
extends below the water table, this provides an ideal location for storage of sulphidic
wastes so as to keep them in a saturated condition, protected from oxidation, and
4. The deep, steep-sided coni guration of most mine Pits means that tailings solids will
usually deposit sub-aqueously, with no scope for beach development and consequent
solar drying. Tailings deposited in this way usually drain very slowly, so that decades
may be required before a i rm surface can be developed, suitable for rehabilitation.

While relocation of tailings into the mine Pit after completion of mining is too expensive
for most mining situations, there are situations where it may be used. Three examples of
in-pit tailings storage come from Australia's Northern Territory: Woodcutters Copper
Project and Nabarlek Uranium Project, both of which have been closed and Ranger
Uranium Project, which continues to operate. In all these cases, conventional surface