Energy efficiency of lime brining
Practical Action
Calculating the proportions of calcite and dolomite
Based on the chemical analysis of the limestone in our example (i.e. calcium oxide, CaO =
45.50% and magnesium oxide, MgO = 8.16%), this calculation assumes that all the calcium
and magnesium is present as carbonates - whereas, a small, and probably insignificant,
amount may occur as silicates etc. Firstly, we determine the mount of dolomite,
CaCO3.MgCO3, from the amount of MgO present in our stone compared with the percentage in
100% dolomite.
MgO in pure dolomite =
40.32 x 100 = 21.86%
184.42
An MgO content of 8.16% means the amount of dolomite is calculated thus:
dolomite in stone =
8.16 x 100 = 37.32%
21.86
Now we determine the percentage of the CaO that would be taken up by this amount of
dolomite:
37.32
x 56.08
184.42
= 11.35%
Then we subtract this from the total CaO to obtain the amount that is taken up as CaCO3, i.e.
calcite:
45.50 - 11.35 = 34.15%
The proportion of CaCO3 as calcite in the limestone is thence given by:
34.15 x 100.09 = 60.95%
56.08
Thus the approximate carbonates content of our limestone is 61% calcite and 37% dolomite.
As these amount to a little over 98% and the chemical analysis has other constituents, silica
etc., which amount to 3.42%, then the amount of the total carbonate can be expected to be
somewhat lower, say 96 to 97%
Ls
The available lime content, as CaO and MgO, of quicklime is obtained from the Rapid Sugar
Test(Figure 1). This test should be performed immediately the quicklime is withdrawn from the
kiln. This is because, being very reactive, the quicklime is very prone to carbonation and, if
the test is delayed, a lower value for Ls is likely. A value of Ls could also be obtained by
performing the Rapid Sugar Test on the calcium hydroxide but the value obtained is likely to
be lower, and thus invalidate any calculation of burning efficiency. For the purpose of our
example we will assume Ls = 0.60(Table 4)
Cf
The calorific value of fuel is obtained using a calorimeter. A university department or a
commercial laboratory will probably have the appropriate facilities, i.e. a bomb calorimeter. If
this facility is unavailable, then a general value may taken for a particular fuel(Table 4). For the
purpose of our example, we will assume we have a bituminous coal with a calorific value of
23 MJ/kg(Table 5).
4