Lignite Quality : Under Ground Mining Options

The areas down dip of the 7:1 stripping ratio
line defined that part of the basin that could be
potentially mined by underground methods.
Within this area it was possible to calculate the
average in situ lignite quality and the mineable
lignite quality values.


13.7 LIGNITE RESERVE ESTIMATES :-
           The lignite reserves were calculated by assessing
the data provided from borehole logs
(Whateley 1992) obtained by drilling,
within the limits of the open pit. These
limits were established between the outcrop
line and the 7:1 stripping ratio line. On the
northwest and the east sides of the pit, the
limit is determined at the depositional edge of
the seam. This limit was determined by using
the basement and basal KM2 structure contour
maps. The pit outline was
placed on the isopach map of the total vertical
thickness of the KM2 seam, and the area between
each isopach was measured with the aid
of a planimeter. The areas
were multiplied by the average vertical thickness
between the isopach lines (the thickness
value at the midpoint) to obtain the in situ
volume. The volume was multiplied by the
average specific gravity to obtain the in situ
tonnage.
The mineable lignite tonnage was calculated
from the in situ tonnage by applying a recovery
factor, examples of which are given in the final
row of Table 13.4. The weighted average recovery
for the open pit is 91%, but this varies from
as low as 45% in borehole 208 to 100% in many
of the remaining holes. The recoveries of mineable
lignite from the total lignite will change
during mining depending on the local geology.
Development drilling immediately in advance
of production will determine these recoveries.
The reserve figures were calculated as follows:
in situ reserves 49.4 Mt, mineable reserves
40.6 Mt, and kriged estimate of in situ reserves
46.3 Mt (Lebrun 1987).