Kinetics of oxidation of MoS 2 by CaO in the presence of water vapor
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[1]
CaO(s) + H2S(g) = CaS(s) + n20(g)
[2]
Overall: MexS(s) + CaO(s) = MexO(s) + CaS(s)
[3]
It is seen that, although gaseous species HzO and H2S are involved in the reaction, the overall reaction is between the metal sulfide and lime, producing the corresponding oxide and calcium sulfide. Since there is no net consumption or generation of gaseous species, it is not necessary to maintain a flow of gas into or out of the reactor. This eliminates the possibility of emitting sulfur-containing gaseous pollutants into the surroundings. Although the overall reaction is between two solids, very little reaction takes place without the presence of the gaseous intermediates. [3] When the solid species are in their pure states, reaction [3] will take place only if the Gibbs free energy of reaction is negative. This is the case only for certain metal sulfides including ZnS, MoS2, and FeS. The kinetics of the reaction of ZnS according to this scheme have been reported in a previous paper.I41 In this communication we present the results for the reaction of MoSs. The experimental apparatus and procedure were the same as described elsewhere. [2'3'4]The M o S 2 particles used in this work were supplied by Ventron Alfa Products and had a purity of +98 pct. Lime was obtained by calcining reagentgrade Ca(OH) 2 particles, supplied by Spectrum Chemical Corp., at 923 K for 3 hours before each experimental run. The MoS2 and Ca(OH)2 particles were ground and screened to minus 1 lzm before use. When molybdenum sulfide and lime powders were intimately mixed and reacted in steam, calcium molybdate was formed, contrary to the proposed reaction mechanism discussed above, according to the following: MoSs + 3CaO + H 2 0 = CaMoO4 + 2CaS + H:
89
+ H20 = 89
+ HsS
CaO + HsS = CaS + H20 Overall:
89
2 +
CaO = 89
+ CaS
[5] [2] [6]
Rate measurements were conducted in the region where diffusion of gaseous species in the MoS2 pellet does not affect the rate. This condition was attained using pellets of small thicknesses and high porosities. To prepare such a porous pellet, MoS2 particles were uniformly mixed with ammonium carbonate powder and compacted in a cylindrical die. The disk was then heated in a nitrogen flow at 353 K for 24 hours. The pellet thus prepared was placed in a crucible surrounded by loosely packed lime powder. A very thin porous alumina-silica fiber separated the pellet from the surrounding lime powder to prevent direct contact, as shown in Figure 1. The amount of ammonium carbonate was increased until the MoS2 pellets thus prepared reacted in the absence of diffusional effects, 12'31as mentioned above. Rate Equations. The kinetics of reactions between fine particles of metal sulfides and water vapor were determined to follow the following rate equation: [3'4 '6] [-ln(1 - XB)]l'" = bk,(C,z o - CHas/K1)t
[7]
POWDER
CaO //
REACT ION TUBE
// //
OO OOOO O,//~//j I
OOOOOOO//~
MoS 2 P E L L E T
/ /
oo
o o
ooooo1
POROUS
~ ALUMINA-SILICA FIBER
[4]
It is known that calcium molybdate can be prepared either by adding
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