Thermodynamic stability of metallurgical coke relative to graphite
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(I)
Since CaF2 and CaC2 do not have significant mutual solid solubility at the experimental temperatures, their activities at each electrode are unity. The electrochemical reaction at the right electrode is 2C (coke) + CaF2 + 2e = CaC2 + 2F
[1]
At the left electrode, the corresponding reaction is: 2F- + CaC2 = 2C (graphite) + CaF2 + 2e
[2]
The virtual cell reaction is C (coke) = C (graphite)
[3]
for which AG~ = - F E , where E is the electromotive force (EMF) of the cell in volts and F is the Faraday constant. The electrodes are designed to convert the carbon potential into an equivalent chemical potential of fluorine, which can be measured by the CaF2 solid electrolyte. At the chemical potentials of fluorine prevailing at the electrodes, CaF 2 is predominantly an ionic conductor. [4,5,61 Since coke contains several impurities, it is possible that the chemical potential of fluorine at the right electrode is influenced by spurious reactions such as [7,8] Stoke + CaF2 + 2e- = CaS + 2F-
[4]
Qcoke + CaF2 + 2e- = CaO + 2F-
[5]
K.T. JACOB, is Professor, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India, and Guest Professor, Department of Theoretical Metallurgy, Royal Institute of Technology, Stockholm, Sweden. S. SEETHARAMAN, Professor, is with the Department of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted April 1, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS B
Sulfur is present in coke partly in dissolved form and partly as sulfides. The solubility of oxygen in coke is negligible, especially at the low-partial pressures of oxygen prevailing at the electrodes. Most of the oxygen in coke is present as inert inorganic oxides. Therefore, the possibility of Reaction [4] is significantly higher than that of Reaction [5]. The coke used in this study was prepared from highgrade coal, which was washed to remove ash and then subjected to normal coking. When the coke was used for EMF measurements, the volatile matter released at high temperature was found to interfere with the measurement. The EMF decreased with time at constant temperature probably due to the transport of material between the electrodes via the gas phase. The removal of volatile matter from the coke was therefore attempted. The coke was heated under vacuum (--10 Pa) at 1273 K for - 1 2 ks. The ultimate analysis of the coke after vacuum treatment gave the following contents: carbon, 95.2 pct; hydrogen, 0.4 pct; oxygen, 0.6 pct; nitrogen, 0.3 pct; sulphur, 0.4 pct; and ash, 2.9 pct by weight. Coke contains plane polyaromatic units. There are molecular orientation domains, with local parallel orientation, distributed in an amorphous matrix. The average size of these domains determined by transmission electron microscopy was - 1 0 rim. The use of vacuumdegassed coke resulted in reproducible and constant EMF at each temperature. To check for the possible effect of the sulfur content of coke on EMF, a batch of coke after degassing was treated with carbon disulfide. Th
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