Phase-Relationships in the system Fe-Na-O
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I.
INTRODUCTION
II.
THE system Fe-Na-O has received increasing attention in recent years in view of its practical implications in metallurgical processes. Several operating problems in the blast furnace process have been reported ~,2 due to alkali input. It is now established that the scaffolds and scabs, which result in hanging and slipping, have a high alkali content. The presence of alkali also causes refractory damage and disintegration of pellets and coke. In the cooling system of some nuclear reactors, the oxygen present in liquid sodium coolant leads to the corrosion of the stainless steel walls 3 due to the formation of a stable compound Na4FeO3. The beginning of the melting process in the blast furnace has been attributed to the formation of a low temperature melt in the Fe-O-S system. 4 Staffansson,s however, points out that it is equally possible that the melting process could be started by an FeO-Na:O melt. The drastic reduction in the melting point of FeO by small additions of Na20 has earlier been observed by Bengtsson. 6 The Fe-Na-O system is significant even from a theoretical point of view. The solution of foreign ions in the wustite lattice may greatly alter its defect concentration, which will affect the behavior of the compounds in reduction and oxidation processes. Till the inception of the present work, no systematic attempt has been made to determine the phase diagram of the system Fe-Na-O. A study of the system FeO-Na20 may be complicated by the disproportionation reactions
LITERATURE REVIEW
A review of the earlier work carried out on the system Fe-Na-O is given in Table I. Special mention must be made of the measurements of Shaiu et a1.18 These authors have measured the free energies of formation of the compounds Na2Fe204 and Na2FeO2 by means of the following galvanic cells involving two different types of solid electrolytes: Ca, CaF2IICaF211NaF,FeO, Fe, Na2FeO2 FeO, Na2Fe204, Na2FeO2llZrO2-CaOl[Cu20, Cu in the temperature range 700 to 1100 K. The formation of the compound Na2Fe204 from the electrode components according to the reaction [2] would alter the composition of the electrodes. 2~ III.
EXPERIMENTAL
A. Materials
3FeO + Na20 = Na2Fe204 + Fe
[1]
'FeO' was prepared by mixing Fe203 (Fischer certified reagent) and carbonyl iron so that the composition of 'FeO' was near the wustite-iron boundary. The mixture was then compacted and sintered at lO00 ~ for 12 hours in oxygenfree argon. The compounds in the Fe-Na-O system were prepared by mixing appropriate amounts of 'FeO' and NazO (synthetic grade, E. Merck) and melting the mixture in sealed platinum crucibles. In some cases NazFe204 was prepared from Fe203 and Na2CO3 (pro analysi, E. Merck) according to the reaction
2FeO + NazFeO2 = NazFe204 + Fe
[2]
Fe203/s) + Na2CO3(sl = Na2Fe204is) + CO2(g)
resulting in the formation of a ferric compound and the precipitation of metallic iron. Differential thermal analysis and emf measurements involving ZrO2-CaO solid electrolytes were the main techniques used in the present work. In some
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