Kinetics of sulfidization of iron oxide with SO 2 -CO mixtures of high sulfur potential
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THE
sulfidization of iron oxides is a reaction of int e r e s t for the selective r e m o v a l of iron from certain m i n e r a l s such as, for example, ferruginous bauxites. Fink and Marchi ' were the f i r s t t o p r o p o s e this procedure as they successfully sulfidized iron oxide contained in various bauxites by reaction with sulfur. A patent t o A. W e l c h2 considers the selective sulfidization of the iron contained in ilmenite by reaction with S O J C O mixtures and the resulting separation of iron and titanium. More recently Holliday and Milne3 have shown the feasibility of purifying bauxites of t h e i r iron oxide content by reaction with S O J C O mixtures. Also the sulfidization of nickel oxide with sulfur dioxide has been used t o extract nickel from its ores by a gassolid reaction.4 The sulfidization of m e t a l oxides with CO//SO2 has not been studied extensively and very little data is available on the kinetics of such reactions. More gene r a l l y sulfidization has been done with H2S/H2 mixt u r e s for which the mechanism and kinetics would, of course, be substantially different. On exploring different processes for reacting s e l e c tively the iron oxide contained in various bauxitic o r e s , it was found that the reaction with CO/SO2 mixtures is not only attractive from the point of view of efficiency, but also offers some interesting features of kinetics and product morphology. This study reports results obtained on the kinetics of sulfidization of iron oxides, specifically hematite, by a mixture of CO and SO2 g a s e s . The hematite samples used in the present experiments were prepared from fine powders in the form of a sintered porous m a t e r i a l of specific surface area of about 0.41 m 2 / g , Gas compositions above and below a CO:SO2 r a t i o of 2:1 were u s e d . Near this r a tio the partial pressures of the two reactive species, $2 and COS, a r e near maximum, which is expected to produce a maximum reaction r a t e . A temperature
r a n g e between 500 and 900°C was selected on the b a s i s of preliminary experiments indicating that in this r a n g e the reaction rate increases from zero t o a nearly constant value. THERMODYNAMIC ANALYSIS The carbosulfidizing gas mixtures used t o r e a c t with the iron oxide produce, an equilibration, s e v e r a l gaseous molecular species active in the reduction a n d / o r sulfidization reactions. Based on available thermodynamic data the following gas phase equilibria were deemed pertinent under the experimental conditions t o be used:
3C0+S02 ~COS+2COz
[1]
4CO + 2SO2 ~-~$2
[2]
+ 4COz
2CO2
~ 2CO + 02
[3[
3S2
~- 2S3
[4a]
2Sz
-~ S~
[4b]
5S2
-~ 2S~
[4c]
3S2
~ $6
[4d]
7S2
~ 2S7
[4el
4S2
~ S8
[4f]
Consequently, the reacting gas mixture is assumed t o contain, in addition t o the SO2 and CO, the various sulfur molecules ~($2 t o $8), CO2, COS, and O2. The equilibrium pressure of each of the constituents for a given temperature and total gas pressure can be calculated by solving the equilibrium constant equati
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