Ind. Eng. Chem. Res. 1990, 29, 14-21
14
for the formation of N2 in the Fen(L)NO + Srvsystem also.
Acknowledgment This work was supported by the Assistant Secretary for Fossil Energy, Office of Coal Utilization Systems, U.S. Department of Energy under Contract DE-ACOS76SF00098 through the Pittsburgh Energy Technology Center, Pittsburg, PA.
Literature Cited Ackermann, M. N.; Powell, R. E. Air Oxidation of HydroxylamineN-Sulfonate. Inorg. Chem. 1967, 6 , 1718-1720. Bonner, F. T.; Pearsall, K. A. Aqueous Nitrosyliron(I1) Chemistry. 1. Reduction of Nitrite and Nitric Oxide by Iron(I1) and (Trioxodinitrato)iron(II) in Acetate Buffer. Intermediacy of Nitrosyl Hydride. Inorg. Chem. 1982, 21, 1973-1978. Buchholz, J. R.; Powell, R. E. The Decomposition of Hyponitrous Acid. I. The Non-chain Reaction. J . Am. Chem. Soc. 1963, 85. 509-5 11. Chang, S. G.; Littlejohn, D.; Lin, N. H. Flue Gas Desulfurization; Hudson. J. L.; Rochelle, G. T., Eds.; ACS Symposium Series 188; American Chemical Society: Washington, DC, 1982; pp 127-152. Chang, S. G.; Littlejohn, D.; Lynn, S. Effects of Metal Chelates on Wet Flue Gas Scrubbing Chemistry. Enuiron. Sci. Technol. 1983. 17, 649-653. Griffiths, E. A,; Chang, S. G. Effect of Citrate Buffer Additive on the Absorption of NO by Solutions of Ferrous Chelates. Ind. Eng. Chem. Fundam. 1986, 25, 356-359. Huie, R. E.; Peterson, N. C. Trace Atmospheric Constituents; Schwartz, S.E., Ed. Advances in Environmental Science Techfiology 12; Wiley: New York, 1982; pp 117-146. Latimer, W. M.; Hildebrand, J. H. Reference Book of Inorganic Chemistry; Macmillan Co.: New York, 1959; pp 204-205. Littlejohn, D.; Chang, S. G. Kinetic Study of Ferrous Nitrosyl Complexes. J . Phys. Chem. 1982,86, 537-540. Littlejohn, D.; Chang, S. G. Identification of Species in a Wet Flue Gas Desulfurization and Denitrification System by Laser Raman
Spectroscopy. Enuiron. Sci. Technol. 1984, 18, 305-310. Miyadera, T.; Hiramine, S.; Shimada, Y.; Sugimoto, Y.; Teranishi, H. Absorption of Dilute Nitric Monoxide in Aqueous Solutions of Fe(I1)-EDTA and Mixed Solutions of Fe(I1)-EDTA and Na2SOR. J . Chem. Eng. Jpn. 1978, 11, 450-457. Narita, E.; Sato, T.; Shioya, T.; Ikari, M.; Okabe, T. Formation of Hydroxylamidobis(su1fate) Ion by the Absorption of NO into Aqueous Solutions of Na2S03Containing F e h d t a Complex. Ind. Eng. Chem. Prod. Res. Deu. 1984,23, 262-265. Nunes. T. L.: Powell. R. E. Kinetics of the Reaction of Nitric Oxide with Sulfite. Inorg. Chem. 1970, 9, 1916-1917. Oblath, S. B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. Kinetics of the Initial Reaction of Nitrite Ion in Bisulfite Solutions. J . Phys. Chem. 1982,86, 4853-4857. Sada, E.; Kumazawa, H.; Kudo, I.; Kondo, T. Individual and Sim&aneous Absorption of Dilute NO and SO2 in Aqueous Slurries of MgS03 with Fe"-EDTA. Ind. Eng. Chem. Process Des. Deu. 1980, 19, 377-382. Sada, E.; Kumazawa, H.; Takada, Y. Chemical Reactions Accompanying Absorption of NO into Aqueous Mixed Solutions of Fe'L edta and Na2S03. Ind. Eng. Chem. Fundam. 1984, 23, 60-64. Sada, E.; Kumazawa, H.; Hikosaka, H. A Kinetic Study of Absorption of NO into Aqueous Solutions of Na2S03with Added Fe'L edta Chelate. Ind. Eng. Chem. Fundam. 1987a, 26, 386-390. Sada, E.; Kumazawa, H.; Machida, H. Absorption of Dilute NO into Aqueous Solutions of Na2S03with Added FeI'NTA and Reduction Kinetics of FeII'NTA by Na2S03. Ind. Eng. Chem. Fundam. 1987b, 26, 2016-2019. Sato, T.; Simizu, T.; Okabe, T. Studies of the Formation and Decomposition of Dithionate. 11. The Formation of Dithionate by the Reaction of Iron(II1)-edta with Sodium Sulfite. Nippon Kagaku K a ~ h 1978, i 361-366. Teramoto, M.; Hiramine, S.; Shimada, Y.; Sugimoto, Y.; Teranishi, H. Absorption of Dilute Nitric Monoxide in Aqueous Solutions of Fe(I1)-EDTA and Mixed Solutions of Fe(I1)-EDTA and Na2S03. J . Chem. Eng. Jpn. 1978, 11, 450-457. Receiued for review April 5, 1989 Accepted September 6, 1989
Physicochemical Aspects of the Leaching of Molybdenum from Co-Mo/y-A120s Hydrodesulfurization Catalyst Waste Using DMSO-SO2 Mixed Solvent Prafulla R. Raisoni and Sharad G. Dixit* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
Molybdenum can be selectively leached from a spent HDS catalyst using a solution of SOz in DMSO. The spent catalyst contains CoMoO,, CoA1204,Cogs8,and MoSz, and Mo(IV), MOW),and Mo(V1) are present after calcination. The solution contains S032-and S2052-.SEM reveals the formation of a layer of product on the surface of reacting particles. The rate measurements are consistent with control by diffusion through this layer. The Co-Mo/y-Al20, catalyst consisting of molybdenum deposited on alumina together with a promoter, cobalt, is currently being used for the hydrodesulfurization (HDS) process in the petroleum refining. A large quantity of catalyst waste is generated. The spent Co-Mo/Al,O, HDS catalyst is a rich secondary source of cobalt and molybdenum. Therefore, several efforts have been made to recover the valuable metals from the waste catalyst (Berkesi et al., 1985; Georgescu et al., 1980; Haehn et al., 1985; Vicol et al., 1986). In recent times, solutions containing sulfur dioxide have attracted the attention of researchers as lixiviants to extract metallic values. Dimethyl sulfoxidesulfur dioxde (DMSO-SOZ), dimethylformamide-sulfur dioxide (DMF-S02), and acetonitrile-sulfur dioxide (CH3CN-S02) have been shown to be very good leaching *To whom all correspondence should be addressed.
0888-5885/90/2629-0014$02.50/0
agents (Gill et ai., 1984). Also, aqueous solutions of sulfur dioxide have been studied as leachants (Miller and Wan, 1983; Khalafalla and Pohlman, 1981). We have investigated the leaching of cobalt and molybdenum from calcined spent catalyst using aqueous SOz solutions (Raisoni and Dixit, 1988a,b). Both cobalt and molybdenum could be easily leached out, and no selectivity was observed. The DMSO-SO, solvent system has also been studied with respect to Co-Mo/Al,03 spent catalyst, since it offers the possibility of selective leaching. Furthermore, DMSO is not an expensive solvent, and it can be easily recovered and reused. In the present case, the use of DMSO-SOZ may be economically justified since high value products are obtained. 'IJntil now, the dissolution chemistry, reaction mechanism, and kinetic factors affecting the rate of leaching with the DMSO-SO, mixed solvent have not been studied. C 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 15
0.61I-
u
a
a W
0.60-
0
I 0-562
0
: I-
0.52-
a LL
200
100
600
800
IOOC 0.21
rPm
1
0
I
20
Figure 1. Effect of speed of rotations on Mo dissolution.
These have been investigated by employing various techniques such as SEM, IR, and XPS along with the actual leaching experiments, and the results are presented in this paper.
Experimental Section Materials and Procedures. The spent Co-Mo/y A1203(HDS) catalyst was procured from M/s Metals and Molybdates, Ltd., Bombay. The SO2 gas was supplied from M/s STAR FREEZ IND., Bombay, and used as such without further purification. The spent catalyst nominally contained 6.12% Co304, 10.83% Moo3, 65.15% A1203,17.7% Si02, 0.1% S, and 0.35% The spent HDS catalyst was calcined at 573-923 K ( 2 h) and 923 K (4 and 6 h) in a thermostatically controlled furnace using fused silica crucibles. The leaching experiments were carried out in a 0.5-L cylindrical glass reactor immersed in a thermostatically controlled water bath. The upper lid had three openings through which a stirrer, a fritted solution sampler, and a gas sparger were introduced into the glass reactor. Generally, the leaching experiments were carried out at 0.2% solid, 426 pm, 850 rpm, pH = 1, and 310 K (unless reported otherwise). The total solvent volume was 250 mL. SO2gas was bubbled through the solvent, and the total SO2 concentration was in the range 6.4-7.2 g/L. Finally, the pH adjustment was done using AR-grade H2S04. The dissolution reaction was initiated by adding the required amount of the solid (0.2%) to the reactor. The sample was withdrawn at regular intervals for the estimation of the dissolved metals. The total Mo and Co contained was estimated by the thiocynate method and the EDTA method, respectively. Instrumental Conditions. IR spectra were recorded by using KBr pellet techniques in the range 4000-400 cm-' on a Perkin-Elmer spectrometer, while XPS spectra were recorded on a V. G. Scientific-I Mark-3 (UK). Nonchromatized A1 ( K a ) radiation was employed as the excitation source. The electron spectrometer was operated at 10 kV, 10 mA, and a pressure of lo4 Torr. An Al(2p) line of 74.5 eV of the support was used as an internal reference for determination of BE (binding energy).
c.
Results and Discussion The selective leaching of Mo from a spent HDS catalyst was studied in order to study the effect of various param-
1
I
I
100
80
60
40
T I M E , min
Figure 2. Effect of SO2 concentration on Mo dissolution.
0.9-
n W
I-
V
a
K
0.6W
$ LL 0
z
O_
0.3-
I-
I
0
30
I
I
I
120
150
I
60 90 TIME , m i n s
Figure 3. Effect of particle size on Mo dissolution.
eters such as stirring speed, pH, particle size, solid concentration, temperature, and calcination temperature. Effect of Stirring Speed. The speed of rotation was varied from 250 to 1050 rpm. The dissolution of Co-Mo bearing spent catalyst was investigated at 0.2% solid, 426 pm, and 310 K. The results are shown in Figure 1. It will be seen from the figure that the dissolution rate of Mo (in 100 min) increases with an increase in the stirring speed up to 850 rpm. A further increase in rpm did not have any effect on the extraction. Therefore, a speed of 850 rpm was fixed for the rest of the experiments. Effect of SO2 Concentration. The effect of SO2 concentration on Mo dissolution was studied at 0.2% solid, 426 pm, 850 rpm, and 310 K. The concentration was varied from 12.80 to 16.00 g/L. The rate of dissolution increases with concentration as shown in Figure 2. Effect of Particle Size. The effect of particle size was studied by taking the following size fractions: 855, 605, 426, 301, and 187 pm. In Figure 3 is depicted the fraction of Mo extracted against time. It will be seen from the figure that as the particle size decreases the rate of dissolution increases. However, beyond a particle size of 426 pm, the particle size had no
16 Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990
c 0
Q.
a
LL. 0
0.4
z IO
Q.
a
0.2
1
LL
I
I
0
-300
K
-305
K
-310K
1
-31SK I
1
I
(b)
Figure 6. SEM micrographs of (a) spent HDS catalyst (X75) (b) partially reacted catalyst with DMSO-S02 (50 min) and (c) partially reacted catalyst with DMSO-SO2 (100 min) (X75).
,t 5
573
0
673
773
0
873
923 (6h)
CALCINING TEMPERATURE ( K
Figure 5. Effect of calcining temperature on Mo dissolution.
effect on the rate of dissolution. Effect of Temperature. The dissolution of Mo in DMSO-SO2 solvent was studied a t 0.2% solid, 850 rpm, 426 pm, and 300-315 K. It is clear from Figure 4 that as the temperature increases the dissolution rate of Mo also increases up to 310 K. However, a further rise in temperature (315 K) resulted in a slight decrease in the percent extraction of Mo. This may be due to a decrease in solubility of SO2 in DMSO. Effect of Calcining Temperature. The spent HDS catalyst was calcined at 573-923 K (2 h) and 923 K (4 and 6 h). The dissolution of Mo was studied a t 850 rpm, 0.2% solid, 426 pm, and 310 K. It can be seen from Figure 5 that the rate of molybdenum dissolution increases up to 96% with an increase in calcination temperature. Selectivity is achieved a t the leaching stage itself, and thus, further separation and purification stages can be avoided. About 72% molybdenum can be selectively leached from spent HDS catalyst a t 850 rpm, 0.2% solid, 426 pm, and 310 K as opposed to 96% molybdenum extracted from a calcined spent HDS catalyst (923 K, 6 h). Cobalt remained unleached in both of the above cases. Nature of Dissolution. SEM, IR, and XPS techniques were employed to understand the structure of the spent catalyst, chemistry of the dissolution process, and kinetic
factors affecting the reaction rate. Scanning Electron Microscopy Investigations. The SEM micrographs of spent HDS catalysts and partially reacted catalysts with DMSO-SO2 solvent for 50 and 100 min have been shown in Figure 6. The SEM photograph of the original spent catalyst clearly shows macroporosity as dark portions. A white superficial layer is also seen, which may be the sulfide layer formed on the catalyst surface. The SEM photographs of the partially reacted catalysts clearly show the formation of a product layer on the surface of the reacting solid. This product layer is likely to affect the rate of dissolution, as diffusion through this product layer may be rate controlling. Characterization of Spent Co-Mo/A1203 (HDS) Catalysts: An Infrared Investigation. In order to understand the dissolution behavior, it is necessary to know the structural aspects of the spent catalyst. Several studies have been reported with regard to the fresh catalysts, but there are almost no investigations on the spent catalyst. In the present case, effort has been made to know the species formed on the surface of the solid, the structure, and the valency changes taking place during the calcination prior to leaching and also during the leaching with DMSO-S02. Infrared spectra of the spent Co-Mo/r-A1203 (HDS) catalyst and calcined catalyst (673 K, 2 h; 923 K, 6 h) were recorded in the range 4000-400 cm-' on a Perkin-Elmer spectrophotometer. The following observations may be made by reading the IR spectra as shown in Figure 7. A characteristic peak observed a t 1390-1420 cm-l is invariably present in the spectra of the spent HDS catalyst.
Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 17
80
60
40
20
0 1800
1600
1400
la00
1000
600
600
400
WAVENUMEER ( C M-’)
Figure 7. IR spectra of (a) spent HDS catalyst, (b) calcined catalyst (673 K, 2 h), and (c) calcined catalyst (923 K, 6 h).
This peak becomes sharper and more intense with an increase in calcining temperature and time. It may be assigned to the formation of CoA1204(Nyquist and Kagel, 1971). The increase in peak intensity in the calcined sample is due to most of the cobalt diffusing into the alumina lattice. It is observed that spent HDS catalyst does not contain any individual oxides of Co or Mo, e.g., Co304,Moo3, and MOO,. This may be inferred from the absence of any characteristic peaks of these oxides in the spectrum of the spent catalyst (Nyquist and Kagel, 1971). A broad and strong peak is observed at 1620-1650 cm-’ in the spent catalyst spectrum. This may be attributed to the formation of CoMo04 in the spent catalyst (Nyquist and Kagel, 1971). This characteristic peak becomes sharper in the calcined catalyst spectra, which may be taken as an indication of the dispersion of Co and Mo into the alumina (or silica) lattice. The number of octahedrally coordinated molybdenum species increases due to the increase in calcination temperature (Mohan Ram and Gopalkrishnan, 1986). In the IR spectra of the spent (HDS) catalyst, a peak observed at 900 cm-’ (Figure 7a) becomes small in the calcined catalysts (Figure 7b). Also, this peak splits in the spectra of the calcined catalyst (Figure 74, and a doublet occurs at 890-925 cm-’. In general, this peak may be due to the presence of silicate species, probably aluminosilicate. The splitting of the peak and the occurrence of the peak at 890 cm-’ may be due to the formation of a small amount of CoSi04. Infrared spectra of DMSO, DMSO + SO2, and DMSO + S02-spent Co-Mo/r-AlzO, completely dissolved (Figure 8) and partially reacted with DMSO + SOz for 50 and 100 min (Figure 9) were recorded in the range 4000-400 cm-’.
A comparison of the two spectra of DMSO and DMSO
+ SO2clearly indicates that the pure DMSO has broad and
very strong peaks between 1010 and 1060 cm-l. The peak is due to the S-0 stretching frequency (Harrison et al., 1979). In the case of DMSO + SOz, a rather sharp peak is observed at 1010-1025 cm-’. Thus, the peak is shifted
1800
1600
1200 1000 WAVE NU M BER ( CM-’)
1400
800
600
Figure 8. IR spectra of (a) DMSO, (b) DMSO + SOz, and (c) DMSO-S02 + spent Co-Mo/Al,O, HDS catalyst (dissolved).
in the DMSO + SOz spectra, and this may be taken as an indication of an adduct formation as suggested by Gill et al. (1984). S-0 stretching frequency data for different metal cations are available (Harrison et al., 1979). When the data are plotted against IEM- E0I2, where EM is the electronegativity of the metal cation and Eo is the electronegativity of oxygen, an excellent straight line relationship is obtained as shown in Figure 10. This supports the observation that coordination of the metals takes place through oxygen. Molybdenum is selectively leached from a Co-Mo/A1203 waste catalyst by a DMSO-SO, mixed nonaqueous solvent. Molybdenum disulfate is the final product formed (as evidenced by the observation of SZOy2-peaks at 1010-1060, 690, and 665 cm-’) (Nyquist and Kagel, 1971). The IR spectra of the partially reacted catalyst show the characteristic bands of SO3,- and SzO,” (1065,1040-1130 cm-’)
18 Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 1001
0
I
I
1
I
I
I
0
Table I. Observed BE Values for Spent HDS Catalyst spent (HDS) catalyst CoMoO, CoA1,O;
Mo(3P3jz)
BE, eV Mo(3dsp) 232.5
Mo(3d3p) 235.4
CO&
MoSZ Mo(V1) MOW) Mo(IVI
231.8 236.4 236.4 233.1
401 401 40 1
O(W 531.9 531.9
Cd2P3/2) 781.3 782.5 778.3
S(2P) 161.7 161.7
228.5 233.1 232.0 230.0
(Nyquist and Kagel, 1971), which indicate that the product layer formed on the surface of the catalyst is mixed and contains sulfite (S032-)and disulfite (S20h2-)apart from the disulfate, S2O7’-. These observations generally support the mechanism suggested by Gill et al. (1984): 0
M’
-
/s--0
0
-
0 [M-O] sullite
3%
[
1 \/
M ‘ 0 ’ O
\s/ O
‘ 0O 1
disulfite
M W 7
disulfate
X-ray Photoelectron Spectroscopy Investigations. XPS is really a true surface-sensitive technique. Here, we have used spent HDS catalysts; however, there are almost no XPS investigations made on spent HDS catalyst. Therefore, the present investigation has been undertaken to study the Co and Mo species present on the surface of .the catalyst. The nature of the surface after partially reacting the solid (catalyst) with DMSO-S02 solvent, as well as the effect of calcining temperature (673 K, 2 h; 923 K, 6 h), has been investigated. Studies by Kasztelan (1987) confirm the description of the supported species as small oxymolybdenum entities, well dispersed and occupying only a small oxymolybdenum fraction of the support surface. Mohan Ram and Gopalkrishnan (1986) have studied the high-temperature phase transition of metal molybdates of the type AMo04 where A is Fe, Co, or Ni. In the case of CoMo04, the transition takes place a t 780 K. The main structural change asso-
I 4
1 3
2
-
IE: EoZl Figure 10. Plot of S-0 stretching frequency, against JEM2 - Eo21.
ciated with the phase transition appears to be a change in valency:
+
Co2+ Mo6+ = Co3+ + Mob+ The high-temperature phase is of mixed valency. Hence, the compound could be described as C O ~ - ~ ~ + C O , ~ + Mo1->+M0>+04. The observed XPS BE values for the spent catalyst have been given in Table I.
Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 19 Table 11. Observed XPS BE Values for Different Used Catalyst Samples catalyst sample spent HDS catalvst partially reacted-catalyst (50 min) calcined catalyst (673 K, 2 h) calcined catalyst (923 K, 6 h) reported values
Al(2p)
BE, eV C(lS)
O(lS)
74.5 77.7 74.5 75.7 74.5 (Chin and Hercules, 1982)
284.9 288.0 284.9 286.0 284.5 (Patterson et al., 1976)
531.9 534.3 531.9 533.0 531.9 (Chin and Hercules, 1982)
1
3
283
28 6 BE
, cV
289
292
Figure 12. XPS BE peaks for C(ls) lines. (a) Spent HDS catalyst, (b) calcined catalyst (673 K, 2 h), and (c) calcined catalyst (923 K, 6 h).
I
I
69
72
I 75 BE c V
,
I 78
I 81
Figure 11. XPS BE peaks for Al(2p) lines. (a) Spent HDS catalyst, (b) calcined catalyst (673 K, 2 h), and (c) calcined catalyst (923 K, 6 h).
The following observation may be made by comparing these values with reference values cited in the literature for fresh unused catalyst. The presence of compounds like CoMo04, CoAlz04, Co9S8, and MoS2 in the spent HDS catalyst (Chin and Hercules, 1982) is indicated. Molybdenum is present in the Mo(VI), MOW), and Mo(1V) states on the surface of the spent HDS catalyst (Patterson et al., 1976). This may be the result of the phase transition a t high temperature, which gives mixed oxidation states. XPS BE values for A1(2p), C(ls), and 1 O(1s) are depicted in Table 11. It is clear from the observed BE values that Al(2p) line intensity decreases with an increase in calcining temperature, as shown in Figure 11. Also, there is a shift of 1.2 eV in the calcined catalyst (923 K, 6 h) and a decrease in the intensity compared to the spent HDS spectra.
Table I11 partially reacted catalyst CoMoO,, CoA1204
Cogs8
BE, eV C O ( ~ P , ~ Mo(3d3j2) ~) Mo(3dSp) O(ls) 781.5 236.4 235.4 534.3 782.3 534.3 778.3
S(2p) 161.3
A decrease in C(1s) intensity peaks (Figure 12) with an increase in calcining temperature indicates that on calcination the combustion of C takes place. XPS values obtained for the partially reacted catalyst with DMSO-SO2 solvent are tabulated in Table 111. It is clear from the observed XPS BE values for the partially reacted spent catalyst with DMSO-SO2 solvent that MoS2 reacts with the DMSO-SOZ solvent while Cogs8 of the spent HDS catalyst remains undissolved (as evidenced by the absence of any characteristic lines for MoS2). Thus, molybdenum is selectively leached from spent HDS catalyst because the cobalt compound present in the spent HDS catalyst, i.e., Cogs8, and chemically inert C0A1204do not react with the mixed DMSO-S02 solvent. According to Gill et al. (1984), cobalt in the +2 state is dissolved in DMSO-SOz solvent but not Co304[Co(III) and Co(II)]. However, this is not true in the present case, as the valence
20 Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990
indicates that the adduct formed between DMSO-S02 breaks when molybdenum is dissolved in DMSO-SOJ and molybdenum sulfite gets converted into molybdenum disulfite:
DMSO participates in the reaction to convert molybdenum disulfite into molybdenum disulfate as a final product (Raisoni and Dixit, 1988): M o ( S ~ O+~ )6 ~0 6DMSO
DMSO
+ S O , SOLVENT
QD
ADSORBED MOLYBDENUM S P E C I E S
0
COBALT CATION
____
SULPHIDE LAYER FORMED DURING H D S TETRAHEDRAL OR OCTAHEDRAL HOLE OF ALUMINA
:a:
F i g u r e 13. Pictorial model of the spent HDS catalyst. (a) Spent HDS catalyst, (b) partially reacted catalyst.
state of cobalt in CoAlz04is +2. Thus, the dissolution characteristic is not dependent upon the oxidation state of the metal cation exclusively, as proposed by Gill et al. (1984). Structure defects and chemical bonding of the metal compounds (oxides, sulfides, etc.) also appear to play a vital role in determining the dissolution characteristics. The following pictorial model may be presented based on these studies. Figure 13a represents the most possible structure of the spent Co-Mo/Alz03 (HDS) catalyst. The molybdenum is deposited on the surface of the catalyst together with the promotor cobalt ion. A white superficial layer formed on the surface. Also, some part of the cobalt is situated into the alumina lattice. During leaching, the formation of a product layer on the solid surface takes place, as shown in Figure 13b. This layer mostly consists of molybdenum species formed as a result of the reaction with DMSO-SOZ. Dissolution Reaction. On the basis of the IR, XPS, and SEM studies, the following sequence of reactions may be presented. In the present studies, a binary nonaqueous solvent (DMSO-SOz) was used, but there is always a small amount of water (
