Modeling of the solubilities of NiO/NiAl 2 O 4 and FeO/FeAl 2 O 4 in cryolite melts at 1300 K
- PDF / 358,748 Bytes
- 9 Pages / 606.24 x 786 pts Page_size
- 51 Downloads / 173 Views
1/17/04
12:47 PM
Page 133
Modeling of the Solubilities of NiO/NiAl2O4 and FeO/FeAl2O4 in Cryolite Melts at 1300 K YUNSHU ZHANG, XIAOXIA WU, and ROBERT A. RAPP Experiments to measure the solubilities of NiO/NiAl2O4 and FeO/FeAl2O4 were performed, and the results confirmed existing literature values. The solubilities of NiAl2O4 and FeAl2O4 in Al2O3-saturated cryolite melts at 1300 K were modeled thermodynamically in terms of the Ni-containing complexes Na2NiF4 and Na4NiF6, and the Fe-containing solutes FeF2, Na2FeF4, and Na4FeF6. The experimental solubility data were fitted to multiple simultaneous equilibria. Equilibrium constants and G0f values for the formation reactions of the these solutes were thereby estimated. The solubilities of NiO/NiAl2O4 and FeO/FeAl2O4 and solute distributions in Al2O3-undersaturated cryolite melts were calculated for a number of melt compositions from the present model. The existence of several competitive solute species is inherent to highly buffered ionic cryolite solutions where the traditional log-log methodology had previously failed to identify dominant single solutes. In such solutions, individual solutes of oxides are not likely to dominate over a wide composition range so that a more global modeling is required. The principal solute species identified in the present study exhibit reasonable three-dimensional (3-D) anion geometries.
I. INTRODUCTION
IRON is a common impurity in primary aluminum produced by the Hall–Heroult process. Iron compounds dissolved in cryolite may have a negative influence on the current efficiency and on the resulting metal quality. To replace the conventional consumable carbon anodes, composite inert anodes have been an active subject for research and development in the aluminum industry, and the oxides of iron and nickel are considered as constituents for inert anode materials.[1,2,3] One of the most important requirements for such an inert anode is an acceptably low solubility in cryolite melts, because their higher electrochemical nobility would result in electrodeposition of the impurities into the aluminum. Therefore, a knowledge of solubilities in cryolite melts of NiO/NiAl2O4 and FeO/FeAl2O4 is of practical interest. NiAl2O4 and FeAl2O4 are stable phases in the NaF-AlF3-Al2O3 system above certain Al2O3 contents. Among others,[4,5] Jentoftsen et al.[6,7] measured the solubilities of NiO and NiAl2O4 in cryolite melts at 1293 K in an oxygen atmosphere; the solubilities of FeO and FeAl2O4 were measured in a reducing atmosphere. They determined experimentally: (1) the solubilities of NiAl2O4 and FeAl2O4 in Al2O3saturated cryolite melts for 1293 K over wide melt composition ranges of 2 cryolite ratio r 12 and 2 r 8, respectively (cryolite ratio r is defined as the ratio of moles NaF to moles AlF3); (2) the temperature dependence of the solubility of NiAl2O4 and FeAl2O4 in the Al2O3-saturated “stoichiometric” cryolite melt (r 3); and (3) the solubilities of NiO/NiAl2O4 and FeO/FeAl2O4 in Al2O3-undersaturated “stoichiometric” cryolite mel
Data Loading...
