The dependence of the oxidation state of vanadium on the oxygen pressure in melts of VO x , Na 2 O-VO x , and CaO-SiO 2
- PDF / 825,124 Bytes
- 10 Pages / 603.28 x 783.28 pts Page_size
- 61 Downloads / 161 Views
po]/4,
I.
INTRODUCTION
MOST of
the vanadium used is produced from vanadium oxide containing slags, which originate in the refining of vanadium containing hot metal. For the thermodynamic treatment of the oxidation and reduction reactions, it is necessary to know the oxidation state of vanadium in the liquid slags. It is well established that, in the solid state, the oxides VO, V203, VO2, and V205 can be stable phases and that, in aqueous solutions, vanadium can exist as divalent, trivalent, quadrivalent, and quinquevalent cations. With regard to oxide melts, several investigations have been carried out to deduce the relationship between composition and oxygen pressure for pure VOx, tl 4] but for ternary and m u l t i c o m p o n e n t melts, only few and conflicting data are available on this matter. [5-8] In the present work, the oxidation state of vanadium has been determined as a function of oxygen pressure in pure VOx melts in the temperature range of 808 ~ to 1000 ~ in Na20-VOx melts at 1000 ~ and in CaOSiO2-VO~ melts at 1600 ~
II.
GENERAL EXPERIMENTAL PROCEDURE
The samples of 2 to 10 g were contained in platinum crucibles and were equilibrated with gas phases of controlled oxygen pressure. Depending on the system, different techniques were used to determine the melt composition. In the first technique, the crucible was suspended on a sensitive thermal balance (Cahn balance) and the mass changes, which occur at the change in oxygen pressure, were recorded. In the second technique, the sample was quenched after equilibration. It was then RAINER M I T T E L S T A D T , Scientist, and K L A U S S C H W E R D T FEGER, Professor, are with the Institut fiir Allgemeine Metallurgie, Technische Universit/it Clausthal, Robert Koch Str. 42, D-3392 Clausthal-Zellerfeld, Federal Republic of Germany. Manuscript submitted August 11, 1988. METALLURGICAL TRANSACTIONS B
reoxidized in air at a lower temperature, at which it is in the solid state. Again, the mass change was measured. In the third technique, the quenched sample was analyzed chemically. The furnace used was a T a m m a n n furnace. A gas-tight alumina tube was inserted into its carbon tube, which was either connected at its upper end to the Cahn balance, if the thermogravimetric technique was applied, or to a water-cooled brass head for quenching, if the samples were to be quenched. Hence, quenching was performed in the same gas by pulling the sample into the brass head. The temperature of the furnace was controlled electronically within ---4 ~ via a thermocouple placed close to the crucible. The gas mixtures were composed with capillary flow meters. Starting gases were argon, oxygen, analyzed argon oxygen mixtures with about 2 pct oxygen, carbon dioxide, and carbon monoxide. All of the gases were of high purity. Starting materials for preparing the oxide samples were V205, Na2CO3, CaCO3, and SIO2, all being of reagent grade quality. These materials were dried at 120 ~ before weighing, then carefully mixed by grinding in an agate mortar, heated to 500 ~ (Naz
Data Loading...
