Knowing the bulk density of mineral and mineral products are important for a Geologist who is engaged in mineral administration. The royalty of the minerals are computed based on the weight of the mineral. The bulk density of the mineral when it is in-situ is different from when it extracted and stored. Likewise, when a mineral is crushed into different sized mineral products, the bulk density varies. Usually, the Geologists have to measure the weight of the mineral stocked in a stock yard of dealer’s license holder or may have to find out the weight of the mineral or its products while transporting the same in vehicles.
At present, the royalty of the mineral is given as per
weight of mineral in tonnes. However, in mining plans, the mineable reserves
are mentioned as volume. In order to compute the royalty amount, the weight of
the mineral is to be computed. Another scenario may be like one person has
stocked different sized granite aggregates (rubble, metal, M-Sand etc.)
illegally and the geologist has to compute the royalty of the mineral. Since it
is not practical to weigh the mineral, usually volume of the mineral/mineral
product is computed using measuring tape and then convert the same to weight of
the mineral.
In this report, two simple methods are suggested for
calculating bulk density and specific gravity using apparatus/tools found at
home
Method for determining the bulk density
The items required for measurement of bulk density are
given below:
1.
A container having regular shape
2.
A weighing machine
3.
A measuring scale
1.
Measure the volume of container ( in cubic
meter)
2.
Measure the weight of empty container (in kg)
3.
Measure the weight of the mineral or mineral
products (in kg)
Knowing the weight of the mineral/mineral products for a
known volume calculate the weight of the mineral for 1 cubic meter
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Method for determining the specific gravity
However, we have to find out the specific gravity of the
mineral to know the in-situ bulk density. For example, if someone say, the
mineable mineral resource is 2 lakhs cubic meters of Charnockite, then royalty
has to be computed for the same. The royalty is computed as per the weight of
mineral in tonnes.
The simplest way of finding out specific gravity of
mineral is by using Pycnometer Method. A pycnometer allows measuring the volume
and the density of solid objects in a non-destructive manner.
Method:
First, the following weights are measured using a balance
as shown below:
G = [M2-M1]
/ [ (M2-M1) –
(M3-M4)]
Where,
G = Specific Gravity
M1 = Mass of empty container
M2 = Mass of Container + Dry Soil
M3 = Mass of Container + Dry Soil + Water
M4 = Mass of Container + Water
If a pycnometer is not available, an ordinary glass jar can be used.
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| Glass jar used in lieu of pycnometer |
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| Measuring the SG of soil sample |
Measurements for different minerals were taken and the
results are given below:
Discussion
The methods used in this study are very crude but are based
on proven scientific methods. The accuracy of the values arrived may not be
that good. But it will give a clear idea about how one has to take specific
gravity and bulk density data while computing the royalty of minerals which
occur in-situ as well as stocked in the form of different aggregates. These
methods can be replicated in laboratories and the accuracy of such data will be
very high.
The results obtained in the above method was compared the
published data. For building stone, the specific gravity ranges from 2.6 to
2.9. The results obtained are also on par with the published data. It can be
seen that the specific gravity of laterite comes to 2.045 and that of ordinary
earth comes to 1.978. There are different types of laterite (ferruginous
laterite, aluminous laterite etc.) and the specific gravity varies as the
composition changes. Similarly, for ordinary earth also specific gravity
changes with the change in chemical composition and due to the presence of
organic materials. Therefore, it is not possible to generalize the specific
gravity using one or two values.
Importance of specific gravity and bulk
density in mineral administration
In mineral administration, one has to keep in mind that
when permit or lease is granted, the weight of the mineral in metric tonnes
(based on which royalty is computed) should be computed by multiplying specific
gravity of the mineral multiplied with the volume of mineral. Once a mineral is extracted and stored, the weight of
the mineral so stored shall be computed by multiplying the bulk density with
volume of the mineral stored.
In the above experiment, both the specific gravity and
bulk density of same mineral has been computed and the results are discussed
below.
Depending on the size, packing of grains, water content
and void space, the bulk density varies. So when an aggregate heap is measured,
the weight of the aggregate has to be computed using bulk density and not by
using specific gravity. Similarly, when a mining plan is approved, the weight
of the mineable mineral reserve shall be computed using the actual specific
gravity value.
The specific gravity value of ordinary earth/soil is to
be used with utmost care. The specific gravity of soil is the average of
specific gravity sand, silt, clay and organic material present in the soil.
Even though the specific gravity of soil is 1.978, the
weight of one cubic meter in-situ soil (compact) after a rain is about 1.58
tonnes and during summer season, with lowering of moisture content, the weight
reduces to 1.355 tonnes. Similarly the weight of loose soil that is loaded into
a lorry also shows different weight per cubic meter depending on the moisture
content.
Conclusion
A geologist who is engaged in mineral administration
should know the difference between specific gravity and bulk density. When it
comes to ordinary earth, there is no way to get the correct weight per cubic
meter as the weight varies on different factors (some factor not even connected
to the property of minerals found in soil). For computing actual weight of
crystalline rocks, a database of specific gravity of rocks occurring in the
state can be made by making scientific measurements. In case of laterite, the
specific gravity varies from place to place depending on the mineral content.
Specific gravity values of ordinary earth/soil are of no use to compute the
weight of the in-situ mineral. Moreover, the mineral administrators can not
keep on changing values as per the whims and fancies and hence a consensus has
to be reached to fix an average value. At present, the specific gravity value
of 2 is taken for soil, but in actual scenario, the value may be somewhere
between 1.5 to 1.7. An average value of
1.6 would be a safe bet.
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