Environmental Engineering Reference
In-Depth Information
concentrations that constitute ore grade sufficient for mining. In effect, the
mining of bauxite ore is the mining of ancient subsoil horizons in which Al has
accumulated.
In terms of classification, the soils in which Australian bauxite has formed
approximate to Red Kandosols (Australian Soil Classification or Ferralsols,
World Reference Base) or Oxisol (Soil Taxonomy). They are ancient and
highly weathered soils (palaeosols) with limited horizon differentiation. They
have little macro-structural development and are micro-aggregated (to
c. 300
m
m) with a significant proportion of piezolites and are thus highly
permeable to water. The fine earth (less than 2-mm fraction) is dominated
by kaolinite and is rich in amorphous Al and Fe oxide minerals which fix
much of the phosphorus, rendering it unavailable to plants. Much of the
cation exchange capacity of the soil is associated with organic matter and is
pH dependent. In the northern mines, a typical profile has soil c. 80 cm over
bauxitic ferricrete; in the western mines, lateritic cap rock may be found even
at the surface and is of substantive nature that requires blasting. In the
northern mines, the landscape is flat and most of the soils are mined out
where as in the west between one third to two thirds of the forest is left
unmined, depending on ore grade.
Soil handling and the restoration process
Although the geomorphology of the landscapes being mined for bauxite in
Australia is somewhat different (hilly in the southwest and flat in the north),
the fundamental processes of forest restoration are similar between regions
and mine sites (e.g., Hinz
1992
; Koch
2007a
). These restoration processes,
particularly with regard to soil handling, are now considered state of the art
(Mulligan et al.
2006
).
Topsoil is a valuable resource for mine site restoration as it is often stripped
and stored separately for use after mining (Bell
2001
). This conventional
method for topsoil handling is shown in
Fig. 15.2
as a generalised model for
bauxite mining in Australia, but applies broadly to other surface- or strip-
mining operations such as coal and mineral sands. The process shown is often
referred to as 'double stripping' where the topsoil and subsoil are removed and
stored separately (Bell
2001
; Mulligan et al.
2006
). The topsoil is of particular
importance as it retains a valuable source of nutrients, micro-oganisms, organic
matter and seeds ( Jasper
2007
;Koch
2007b
;Tibbett
2008
). In order to retain
biological activity in the topsoil, locally provenanced plant species are sometimes
sown on the stockpiles to develop a green cover.
Although the recognition that the original soils provide a valuable
resource to the mine, stripping and storage of soil has some disadvantages.
First, it requires double handling of the soil; that is, the soil needs to be moved
