A Novel Process for Transforming Selected Metal Sulfides to Oxides without Emitting

Sulfur-Containing Gaseous Pollutants

JOURNAL OF METALS· January 1984

H. Y. Sohn and Daesoo Kim

SUMMARY

A novel reaction scheme for transforming metal sulfides to the corresponding oxides has been developed. In this process, metal sulfides are reacted with lime with an initial addition of steam. Steam oxidizes the sulfide to the oxide, and the hydrogen sulfide produced from this reaction further reacts with lime to form calcium sulfide and regenerate steam. There is no net consumption or generation of gaseous species. Thus the overall reaction can be carried out in a closed system as far as the gas phase is concerned. This eliminates the possibility of emitting sulfur-containing gaseous pollutants.

Only certain metal sulfides are thermodynamically amenable to this treatment. The sulfides of molybdenum, zinc, and iron are major examples. Chalcopyrite treated by this process is transformed into bornite and magnetite.

A further potential application of this scheme may be to the selective oxidation of certain sulfides in mixed sulfides ores as a treatment prior to the separation of minerals in complex ores.

INTRODUCTION

Conventional roasting operations often suffer from the problem of S02 emissions into the atmosphere. As an alternative to conventional roasting, Haver and Wong1 and Bartlett and Haung2 investigated the use of lime to remove sulfur dioxide in the roasting of copper concentrates by air. In this process, copper sulfate and calcium are formed and the former is leached selectively. These investigators reported some difficulties associated with this process, notably the likelihood of extreme local temperatures and ferrite formation.

In this article we describe a novel process for transforming certain metal sulfides to the corresponding oxides, recently developed in this laboratory.3 The process uses water vapor instead of oxygen as the oxidizing gas. However, the reaction of steam with most metal sulfides has very small equilibrium constants. The partial pressure of hydrogen sulfide must be maintained at an extremely low level to carry out the reaction. This makes the reaction very unattractive from a commercial standpoint. Lime is an excellent scavenging agent for various sulfur-containing gases includ­ing hydrogen sulfide.4-8 By placing lime in the vicinity, the partial pres­sure of hydrogen sulfide can effectively be lowered. Furthermore, the reac­tion of lime and hydrogen sulfide rElgenerates a stoichiometric amount of water vapor and fixes sulfur as solid calcium sulfide. The overall process can be expressed in the following general form:

Overall reaction:

MexS(s) + H20(g) = MexO(s) + H2S(g)

CaO(s) + H2S(g) = CaS(s) + H20(g)

MexS(s) + CaO(s) = MexO(s) + CaS(s)

(1)

(2)

(3)

As can be seen, although water vapor is the oxidizing gas, the overall net reaction is between the metal sulfide and lime. There is no net con­sumption or generation of gaseous species. Therefore, it is not necessary to maintain a continuous flow of gases into or out of the reactor, and the possibility of emitting hydrogen sulfide is eliminated.

The gaseous species H20 and H2S act as carriers of oxygen and sulfur between the metal sulfide and lime without themselves being consumed or generated. In this respect the overall reaction between the metal sulfide and lime can be considered as being "catalyzed" by the gaseous species. Without these gaseous intermediates the overall reaction does not proceed

67

68

Table I: F .... Energy end Enthalpy Changes for Reactions of Various Sulfides·

T (K) 9OO0K l0000K lloooK dGo Ow)

CaD(s) + H2S(g) =

CaS(s) + HzD(g) - 7504 - 7004 - 66.4

60° Ow) Me.S(s) + HzD(g) =

Me.D(s) + H2S(g) 1I2Mo~ 56.2 55.1 54.1 ZnS 65 .0 64. 64.2

Me.S NiS 75.6 77 .6 79.5 CU2S 150.4 155.2 159.1 FeS 50.2 51.0 51.6 PbS 107 .2 108.0 109.0

6Go (kJ) Me,S( ) + CaD(s) =

Me.D(s} + CaS(s) 1I2MoS2 - 19.2 15.3 12.3 ZnS - lOA - 5.6 - 2.2

Me,S is 0.2 7.2 13.1 CuzS 75.0 4.8 92.7 FeS - 25.2 - 1904 - 14.8 PbS 31.8 37.6 42.6

d Ho (kJ) Me,S(s} + CaD(s) =

Me,D( ) + caScs) 1/2 MOS2 0.70 0.74 0.88

Me,S ZnS 10.9 10.7 10.5 Fe 24.6 24 . 24.9

• All the data were obtained from Elliott and Clei..,r.9 e, cept the free energy 10 and the enthalpyll of formal Ion of Zn .

with any measurable rate, whereas in their presence it proceeds at signifi­cant rates, as will be discussed subsequently.

If the overall reaction is to occur as described by Equation 3 involving only pure solid phases, it must have a negative standard free energy of reaction. The following section will briefly examine the thermodynamics of the reactions involved.

THERMODYNAMIC CONSIDERATIONS

The free energies of Equations 1-3 are summarized in Table I for a number of metal sulfides. Of the sulfides listed, the reaction of MoS2, ZnS, and FeS according to Equation 3 shows negative overall free energy changes indicating thermodynamic feasibility . The standard free energies of reac­tion of other sulfides are positive. For these sulfides, reaction according to Equation 3 is not feasible unless the activity of the product solid could be lowered by forming a solution. It is also seen that the feasible reactions are very slightly endothermic. This suggests that there would be no likeli­hood of extreme local temperature.

We selected MoS2 and ZnS to test the proposed reaction scheme. Chalcopy­rite was also tested. The reaction of chalcopyrite produced bornite and magnetite as the products, as subsequently discussed. When molybdenum disulfide is exposed to a continuous flow of steam, the reaction yields molybdenum trioxide as a final product, contrary to the proposed scheme. However, this reaction cannot proceed in the presence of lime in a closed sys­tem due to the positive ilGo of the overall reaction. For example, ilGo of the reaction between molybdenum sulfide and steam producing molybdenum trioxide is 72 kJ/mol of H20 at 1000oK, whereas that of the lime-hydrogen sulfide reaction is -66 kJ/mol at the same temperature, resulting in overall ilGo of 6.0 kJ/mol.

EXPERIMENTAL

The apparatus in which experiments were carried out is illustrated in Figure 1.

The system consisted of a 1.2-m (4-ft) silica tube with 5.1-cm (2-in.) inside diameter, located within a cylindrical furnace, and a steam generator. Both ends of the tube protruding out of the furnace were wound with a nichrome heating element embedded in a refractory cement to ensure that steam would not condense. The steam generator contained a round-bottomed flask surrounded by a 300-W heating mantle. Inert gas was bubbled through the water to ensure an even rate of steam production. The steam partial pressure was determined by the temperature of the water. Steam composi-

JOURNAL OF METALS· January 1984

It is known that calcium molybdate can be prepared either by adding calcium chloride to molybdenum-containing solutions or by reacting lime and molybdic oxides above 673°K. Equation 4 may represent an alternative method of producing calcium molybdate without producing sulfur-containing gases. Furthermore, it is noted that hydrogen is produced as a by-product from this reaction. It was, however, beyond the scope of this work to carry out a detailed study of this rather interesting reaction.

To prevent calcium molybdate formation, lime and molybdenum sulfide were physically separated as shown in Figure 2. In this arrangement, the reaction between molybdenum sulfide and lime proceeding through steam is represented by the following:

112 MOS2 + H20 = 112 M002 + H2S

CaO + H2S = CaS + H20

(5)

(6)

Overall reaction: 112 MOS2 + CaO = 112 M002 + CaS (7)

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 thickness and high porosities. To pre­pare such a porous pellet, molybdenum sulfide particles were thoroughly mixed with ammonium carbonate powder and compacted in a cylindrical die. The disk was subsequently heated in a nitrogen flow at 353°K for 24 h. 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 2.

Effect of Initial Porosity. The conversion of molybdenum sulfide was measured varying the initial porosity of the pellet at the highest temperature (l173°K) in the temperature range used in this work. The results are given in Figure 3. As the initial porosity increased, the conversion increased until the weight ratio of ammonium carbonate to molybdenum sulfide reached 3.5, beyond which porosity ceased to have any effect. This indicated that the effect of diffusion was eliminated. The initial porosity of the pellet, which was fabricated with this ratio, was determined to be 0.943 according to the following equation:

Er = I-Papp/Pt (8)

where apparent and true densities of the solid, Papp and Pt, respectively, were 0.287 g/cm3 and 5.06 g/cm3.

Generally, at lower temperatures the relative importance of the diffusional resistance is less than at higher temperatures due to the lower reactivity of the solid. In view of this, one can be sure that a pellet of 94.3% porosity will react in the chemically controlled regime in experiments carried out at lower temperatures.

Effect of Reaction Temperature. The reaction temperature was varied in the range 993-1173°K maintaining the total pressure at the atmospheric level PHz.Q +PH2S = 85.3 kPa. The conversion-versus-time relationships are shown in l"igures 4a and b. It is seen that at temperatures between 1200-11000K the reaction is completed in 30-60 min. A detailed quantitative analysis of the kinetics can be found elsewhere. 12 Here only the salient results of the analysis will be given.

The conversion-versus-time relationship for the reaction between MOS2 and water vapor obeys the following nucleation and growth kinetics:

[-In (I_X)]lIn = bk(CH2o - CH2S/Ke)t (9)

where X is the fractional conversion of the sulfide and b is the number of moles of MOS2 consumed by 1 mole of H20 which from Equation 5 is 112. The gas concentrations are in mollm3. The analysis12 of the experimental data gave the following results:

n = 2.6 (10)

and

k = 162 exp (-18,118/T) m3/(mol·s) (11)

Here, T is in K. The relationship between X and t for the overall reaction in the presence

of lime will be given below after the reaction of zinc sulfide is discussed.

JOURNAL OF METALS • January 1984

C.O POWDER

MoS 2 PELLET

POROUS d?i~~~~~~===i~~~ ALUMINA-SILICA

FIBER

MoS 2 PELLET

-.L O.04cm

T r-- 1.29cm --1

Figure 2. Configuration of MOS2 pellet In the presence of lime.

o /0-

0.8

/ ... "" ... ,," /

AmmonlumC.,bonata to

~ o 0 4.0

/ I:J. 3.5

~ 0.15

e ~ 0.4 a 2.0

0.2

00':-0 -~--:~~-~'~0-~--:~~--='80 TUI£(mln)

Figure 3. Conversion-versus-tlme relation­ships for various ratios of ammonium car­bonate to MOS2.

69

Figure 1. Experimental apparatus.

70

HEATING TAPE

'1>

~ I

THERMOCOU~

.,CONDENSER

SAMPLE

ICONTROLLERI I I

Lnn_~ POw'ER SUPPLyl

tions were verified by measuring the rate of water condensation. The temperature of water was varied by the voltage applied to the heating mantle.

The molybdenum disulfide and zinc sulfide particles used in this work were supplied by Ventron Alfa Products, Ca(OH)z by Spectrum Chemical Corporation, and chalcopyrite concentrate by Kennecott. The MoS2 sample had a purity of +98%, and the ZnS sample was of analytical grade. These solid samples were ultrafinely ground to 0-1 fLm size. For the reaction of sulfide-lime mixture with steam, lime powder was obtained by calcining reagent-grade calcium hydroxide powder at 923°K for 3 h and preserved in a desiccator to prevent moisture absorption. This procedure was carried out before each experiment. Won and Sohns reported that the lime samples with a lower initial moisture content showed much slower reaction rates than those with a higher moisture content due to the increased surface area of the lime crystallites resulting from dehydration. Also it is reported that the pore structure of lime depends on the time-temperature history of the lime calcination in the reaction with sulfur dioxide. By calcining the calcium hydroxide particles, which are stable crystals, under the same conditions in terms of temperature and time before each experiment, a consistent reactivity of the lime could be assured.

Zinc sulfide particles were reacted by mixing them uniformly with lime particles. Molybdenum disulfide particles were pressed into pellets and separated from lime particles with a layer of alumina-silica wool. This was because if they were in direct contact, calcium molybdate, instead of molyb­denum dioxide, was formed, as discussed later. To make a porous molybde­num sulfide disk, ammonium carbonate powder was thoroughly mixed with sulfide powder and compacted into a cylindrical steel die, and the disk was heated in a nitrogen atmosphere at 353°K for 24 h. These disks were 1.29 cm (0.509 in.) in diameter and 0.04 cm thick.

The crucible containing a pellet of metal sulfide surrounded by lime powder or a mixture of fine particles of sulfide and lime was placed at the center of the reactor. The system was purged with nitrogen until the sample was brought to the reaction temperature. Then the nitrogen flow was shut off, and a flow of steam-nitrogen mixture was introduced into the reactor for about 5 min, and both ends of the reactor were sealed. After the reaction the sample was cooled in a stream of nitrogen.

Reacted samples containing molybdenum sulfide pellets were physically separated from the surrounding lime powder, and conversion was deter­mined from weight change between the unreacted and reacted sample. Reacted samples of lime-zinc sulfide mixture were chemically analyzed to determine the extent of conversion. (Details of the chemical analysis are given in Reference 12.) The reacted samples were also analyzed using x-ray diffraction to verify that the solid products were metal oxides and calcium sulfide and that no other solid product was formed.

RESULTS Reaction of Molybdenum Disulfide

When molybdenum disulfide-lime mixture was heated in oxygen-free nitrogen for more than 3 h at 11700K no solid-solid reaction products were obtained. If molybdenum disulfide and lime powders were intimately mixed and reacted in steam, calcium molybdate was formed contrary to the proposed reaction mechanism represented by Equations 1-3. The reaction occurs according to the following:

MoS2 + 3CaO + H20 = CaMo04 + 2CaS + H2 (4)

JOURNAL OF METALS· January 1984

Reaction of Zinc Sulfide

Unlike molybdenum disulfide, zinc sulfide particles proceed to react ac­cording to Equations 1-3 even in direct contact with lime particles. Thus the overall reaction is as follows:

Overall reaction:

ZnS + H20 = ZnO + H2S

CaO + H2S = CaS + H20

ZnS + CaO = ZnO + CaS

(12)

(13)

(14)

Figure 5 shows plots of typical results. As can be seen, the reaction rate increases as the relative amount of lime increases. These indicate that under these conditions the reaction between lime and hydrogen sulfide is not fast enough to be in equilibrium. Examination of the figure also reveals that the reaction of zinc sulfide is slower than that of molybdenum disulfide at the same temperature. This is partly due to the fact that with the relative amount of CaO used, the CaO-H2S reaction is not fast enough to keep the partial pressure of H2S at the equilibrium level and also in part due to the fact that the ZnS-H20 reaction has a lower value of equilibrium constant. These factors reduce the concentration driving force for the reaction. Temperature had considerably less effect on the rate of this reaction. 12 From a detailed analysis of these experimental data the following rate parameters were obtained. 12

Rate Expression for ZnS-H20 Reaction. This rate expression is given also by Equation 9 with

n = 1.28 (15)

b = 1 (16)

and

k = 45.3 exp(-14,608/T) m3/(mol·s) (17)

Rate Expression for CaO-H2S Reaction. As mentioned earlier the kinetics of this reaction depend on the nature of lime particles. For the lime freshly produced by decomposing Ca(OH)2 particles as was done in this work, the rate expression is as follows:

with

d=l kCao = 408 exp(-99211T) m3/(mol·s)

and X. is given as a function of temperature in Figure 6.

(18)

(19)

(20)

Rate Expression for the Overall Reaction between ZnS (and also MOS2) and CaO in the Presence of Steam. The conversion-versus-time relationship for the overall reaction can be obtained by combining Equations 9 and 18 to give

II bk(l + liKe) .dX (1 + lIKCao)Hn(1 - X)] n + dkCao/X. [exp(bMX.) - 1]

where

CT = CH20 + CH2S

M = mol CaO per unit volume/atom S per unit volume

Reaction of Chalcopyrite

(21)

(22)

(23)

When chalcopyrite (CuFeS2) was reacted according to the scheme de­scribed above, it was transformed to bornite (Cu5FeS4) forming magnetite (Fe304), according to the following reaction:

15 3 1 1 - CuFeS2 + H20 = - CU5FeS4 + - Fe304 + -S + H2S (24) 16 16 4 8

together with Equation 2.

JOURNAL OF METALS· January 1984

1.0 ,--.---,---,----.----r----r---, t/,/ 0/ 7"' ..--p-

0.8 (1/ ,,/ /~ "o( ./ 0 j / / /' TEMPEAATUAE( K)

0.8 0 0 0 ;is '173

1/ j / /' ~ ~~~!

HI' jV / ~ ~~~~ 0.4 11 /

··1/ 0.00 40 80 120

TtME(min)

0.'

z o o.

~ ~ "0<

TIME(mln)

Figure 4. Conversion of MOS2 in the pres­ence of lime at various temperatures; (a) 1173-1 093°K, . and (b) 1073-993°K.

z o

i o

0.'

(.l 0.2

TlME(min)

Figure 5. Conversion of zinc sulfide during the reaction of ZnS-CaO mixture with steam at 1093°K.

71

'"~ o~

800

0'_________ 0 __ 0_

1000

TEMPERATURE (K)

1100

Figure 6. Pore-blocking rate constant ver­sus temperature.S

10

0.8

~/ /0,:/0-fir! /,0//

0.0 '; J'/, ~ ~;~61. .. 61S3K,2

0.' /~~ ~ :~~~ : :.::~ 0._

d ~ 0 773K I 2.330

~ 0.0

0.0 0.0 1.0 U " TIME(,"ln)

Figure 7. Converslon-versus-tlme relation­ships for the reaction between CuFeS2 and steam at various temperatures.

72

Bornite is much more readily leached than chalcopyrite, which will facilitate copper extraction by hydrometallurgical means. Detailed kinetics for this system were not determined. Based on the typical conversion curves for the reaction between chalcopyrite and steam shown in Figure 7, the rate of the reaction between chalcopyrite and lime in the presence of steam can be expected to be quite rapid.

The kinetics parameters for Equation 24, which can also be represented by Equation 9, were determined to be as follow: 12

and

n = 1.7

15 b=-

16

k = 1.135 exp (-46711T) m3/(mol·s)

CONCLUSIONS

(25)

(26)

(27)

A novel reaction scheme, by which certain sulfide minerals such as molybdenum disulfide and zinc sulfide can be transformed into the corresponding oxides without emitting sulfur-containing pollutants, was investigated. Chalcopyrite treated by this process is transformed into bornite, which is more readily leachable and has less sulfur content, and magnetite. The reaction of an intimate mixture of lime and molybdenite with steam yielded calcium molybdate as a solid product.

The intrinsic kinetics of the reactions between sulfides and steam and those between chalcopyrite and steam have been determined and analyzed. The overall rates of the reaction of sulfide-lime mixtures have also been determined and analyzed based on the kinetic theory for reactions between solids proceeding through gaseous intermediates. When a lime-sulfide mix­ture containing MoS2, ZnS, or chalcopyrite was heated in an inert atmosphere, no solid-solid reaction products were obtained.

Since steam oxidizes the sulfide and the hydrogen sulfide produced is absorbed by lime to yield CaS and regenerate steam, there is no net con­sumption or generation of gaseous species in the sulfide-lime mixture reaction. Therefore, the reaction can be carried out in a closed system as far as gas phase is concerned. This eliminates the emission of sulfur­containing pollutants.

Various options for the further treatment of CaS produced in the use of lime for the treatment of sulfide minerals have been discussed in the literature.7

The overall reaction, as represented by Equation 3, involves changes only in the solid species. In order for such a reaction to be thermodynamically feasible, the standard free energy of reaction must be negative (assuming the solid phases to be pure). Only certain metal sulfides satisfy this re­quirement as discussed in the text. This leads to the possibility of applying this reaction scheme to selectively oxidizing certain sulfides in mixed sulfide ores as a means of separating the minerals in complex ores.

ACKNOWLEDGMENTS

This work was supported in part by National Science Foundation under Grant No. ENG 75-13085 and by a Camille and Henry Dreyfus Foundation Teacher-Scholar Award to H. Y. Sohn. During the course of this work, D. Kim was supported by a Mining and Mineral Resources Research Institute Fellowship provided under U.S. Department of Interior Grant No. G 1186018.

References

1. F. P. Haver and M. M. Wong, Mining Eng., 24 (6) (1972), p. 52. 2. R. W. Bartlett and H. H. Haung, J. Metals, 25 (12) (1973), p. 28. 3. H. Y. Sohn, Process for Treating Sulfide-Bearing Ores, U.S. Patent No. 4,376,647, March 15, 1983. 4. H. Kay, in High Temperature Refractory Metals, edited by W. A. Krivsky, Gordon and Breach, New York, 1968, p. 33. 5. R. E. eech and T. D. Tiemann, Trans. Met. Soc., AIME, 245 (1969), p. 1727. 6. F. Habashi and R. Dugdale, Met. Trans., 4 (1973), p. 1865. 7. F. Habashi and B. I. Yotos, J. Metals, 29 (7) (1977), p. 11. 8. S. Won and H. Y. Sohn, "The Hydrogen Reduction of Metal Sulfides in the Presence of Lime," Met. Trans. B, submitted. 9. J. F. Elliott and M. Gleiser, Thermochemistry of Steel Making, Addison-Wesley, London, Vol. 1, 1960. 10. T. B. Reed, Free Energy of Formation of Binary Compounds, The MIT Press, 1971. 11. R. A. Robie, B. S. He;"ingway, and J. R. Fisher, "Thermodynamic Properties of Minerals and Related Substances at 298.15 K and 1 Bar (10. Pascals) Pressure and at Higher Temperatures," U.S. Geological Suroey Bull. 1452, 1979. 12. D. Kim, The Oxidation of Metal Sulfides with Lime in the Presence of Steam, Ph.D. Dissertation, University of Utah, 1980.

JOURNAL OF METALS· January 1984

ABOUT THE AUTHORS

Hong Yong Sohn, Pro­fessor, Department of Metallurgy and Metallur­gical Engineering, Univer­sity of Utah, Salt Lake City, Utah 84112-1183.

Dr. Sohn received his BS from Seoul National University, Korea, in

1962, MS from the University of New Brunswick, Canada, in 1966, and PhD from the University of California, Berkely, in 1970, all in chemical engineering. He joined the Uni­versity of Utah in 1974, where he is also adjunct professor of fuels engineering. He has coauthored a book entitled Gas-Solid Reac­tions, coedited 5 books, and written some 100 papers. He is a member of The Metallur­gical Society of AIME and a member of its Board of Directors.

New!

Oaesoo Kim, Section Chief, Pyrometallurgy Laboratory, Korea Insti­tute of Energy and Re­sources, Seoul, Korea, 150-06.

Dr. Kim received his BS and MS in physics from Seoul National Uni­

versity, Korea, in 1971 and 1975, respectively, and PhD in metallurgy from the University of Utah in 1980. His current work involves proc­ess development for rare metal production, including fused-salt electrowinning.

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