Moose River Basin: geology and mineral potential - Geology Ontario

Lignite and Industrial Mineral Resources
nite forming up to 60 weight percent of the substance (Edwards 1953, p.29),
contain from 62 to 66.5 percent carbon. The material is described by Edwards
(1953, p.29) as follows:
The freshly precipitated and filtered humic acid is a swollen jelly like substance, that shrinks
strongly and hardens in drying, when it can be crushed to a black, glistening powder. Dried in air it
retains 20 percent moisture; completely dried out it is hygroscopic, indicating a relationship be
tween the drying behaviour of brown coals, and their content of humic acid. The brown colour of the
coal derives from these compounds.
In an analysis of lignite the moisture content is determined and values are
given for ash, volatiles and fixed carbon. The sources of ash are detrital mineral
matter (quartz, clay), salts (calcium, magnesium and iron sulphate solutions in
pore water) and plant ashes. Detrital mineral matter tends to predominate
near the edges and top and base of lignite seams while the centre is character
ized by chemically derived ashes. Sulphur in lignite occurs in sulphates, (e.g.
gypsum) in sulphides (pyrite, marcasite) and as elemental sulphur. The total
sulphur content is variable and up to 9 percent has been recorded (Edwards
1953, p.56). The use of lignite ash for production of synthetic aggregate or as re
placement of cement in concrete is being studied by several organizations in
Canada and the U.S.A. Manz (1973,p.213) reports that in the near future up to
25 percent of the cement is expected to be replaceable by these ashes.
The physical characteristics of lignite partly determine the preferred
method of mining. Lignite with more than 45 percent moisture can generally
be excavated without the use of explosives. Upon exposure to dry air moisture
content may be reduced to about 20 percent causing cracking and shrinkage at
the surface of the coal. The drying process rarely advances more than a few
inches from surface, however, and part of the lost moisture will be replaced in
humid weather.
Exposure of lignite to air causes oxidation. Where large surface areas are
involved, as in dust, oxidation may lead to spontaneous combustion. On the
other hand fine coal dust is sometimes applied to a stockpile to prevent air cir
culation and consequently stop, for lack of oxygen, the internal heat buildup
and spontaneous combustion. Paradoxically stockpile fires are sometimes initi
ated by rain showers because of the excess heat developed when internal capil
lary surfaces of lignite are wetted.
The capillary structure of brown coal affects the drying characteristics of
the material. The fineness of the capillaries determines the strength of bonding
of the capillary water. Conversely this knowledge is used to derive the capillary
size distribution from the relationship of vapour pressure and moisture content
of the lignite. The information is essential in understanding the resistance or
amenability of the material to briquetting, which allows shipping of the partly
dried lignite as an industrial or domestic fuel. Weaker briquettes, according to
Edwards (1953, p.33,34), are thought to result from increasing fineness of the
capillaries since resistance to compaction of the coal particles increases. In a
study by Gauger (1932) referred to by Edwards (1953, p.34), it was found that
in going from wood to peat the relative volume of large capillaries increases
while further coalification results in a progressive decrease of this volume.
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