Contributions to the Data on Theoretical Metallurgy: [Part] 8. The Thermodynamic Properties of Metal Carbides and Nitrides Page: 56
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Data on Theoretical Metallurgy
Equation (234) gives negative values of AFo at temperatures above
about 1,9000 K. In the case of the corresponding reaction involving
FeO, SiC+3FeO=3Fe(l)+SiO2+CO, the temperature at which AFo
becomes negative is lower. Consequently, a process might be realized
in which chromite is reduced by C to form a ferrochrome containing
high carbon, which in turn is treated with Si to lower the C and most
of the low-carbon ferrochrome tapped off, after which smelting might
be continued, making use of the SiC formed as part of the reducing
material for the following batch of chromite.
Calcium as a decarburizing agent.-The use of Ca as a decarburizing
agent also has been suggested. Calculations similar to those given for
Si will be made in this case, assuming the C present either as Cr5C2
or Cr3C2, the latter being the more probable.
It is estimated that AC, is not far from zero in the reaction
CrsC2+Ca=5Cr+CaC2, and by means of quantities already tabu-
lated AH298.1= -4,890 and AF0298.1= -5,020 are computed. These
figures lead to the following expressions:
Cr5C2+ Ca= 5Cr+ CaC2;
AH= -4,890, (235)
AF = 4,890- 0.44T, (236)
AF 0298.1 = -5,020.
AF01124= -5,385.
The assumptions made before will be used here; that is, that the energy
changes in melting and dissolving Cr5C2 are balanced by the corres-
ponding changes for 5Cr and account is taken only of the heat of fusion
of Ca (35). Consequently, AH1124= -7,120 and AF01124=-5,385 in
the reaction CrsC2(1)+Ca(1)= 5Cr(1)+CaC2, for which AC, is esti-
mated as 3. The following approximations are derived from these
figures:
Cr5C2(1) +Ca(l) -5Cr(l) +CaC2
AH= - 10,490+3T, (237)
AF o = - 10,490- 6.9T log T+ 25.59T, (238)
AF 01700= - 4,880.
As an example, the calculations again are made for 1,7000. In this
case, K17oo0N r =4.24. Considering again a ferrochrome of 6
NCr 5c2* NCa
percent C, 65 percent Cr, and 29 percent Fe, it may be shown that
when the C is reduced to 1 percent, Nor and Ncr5c2 are approximately
0.649 and 0.026, respectively. These figures roughly satisfy the
equation for K700, provided the activity of Ca is taken as unity. In
other words, 1 percent C is about as low as may be obtained by treat-
ment with Ca at 1,7000, provided the C is present as Cr5C2. An
activity of Ca of unity corresponds to having Ca present as a separate
liquid phase.
The corresponding relationships for Cr3C2 will be derived next. It
is estimated also that AC,, is about zero in the reaction Cr3C2+Ca=56
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Kelley, K. K. Contributions to the Data on Theoretical Metallurgy: [Part] 8. The Thermodynamic Properties of Metal Carbides and Nitrides, report, 1935; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc12572/m1/58/: accessed April 27, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.