S with Metal Sulfides or Metals - American Chemical Society

18 Thermochemical Decomposition of H2S with Metal Sulfides or Metals

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: March 26, 1980 | doi: 10.1021/bk-1980-0116.ch018

HIROMICHI KIUCHI, TETSUO IWASAKI, ISAO NAKAMURA, and TOKIAKI TANAKA

Department of Metallurgical Engineering, Faculty of Engineering, Hokkaido University Sapporo 060, Japan

Hydrogen plays an important role as a reducing agent in extractive metallurgy. The trend toward energy system improvement

based on hydrogen will therefore have great influence on the field of metallurgy and not only with respect to energy source but also to the appearance of new metallurgical processes.

The authors are now studying the simultaneous recovery of metal and sulfur by a combination of the hydrogen reduction of

sulfide ore and the decomposition of H2S to H2 and S as follows. MSX + xH2 = M + xH2S xH2S = xH2 + xS MSX = M + xS Here the decomposition of H2S is an extremely important reaction.

Although H2S is at present a by-product of the desulfuriza-

tion of fossil fuels on a large scale, only the recovery of free sulfur is carried out by the Claus treatment. On the other hand, the application of H2S decomposition as an H2 evolution method is proposed for use in the thermochemical process undertaken for water splitting. Thus, the thermochemical decomposition of H2S has wide-spread applications in various field. Methods with metal sulfides

Hydrogen sulfide can fracture into hydrogen and sulfur merely

by thermal decomposition, but the equilibrium H2 concentrations are at best those shown in Table I.

In this study, the experiment based on a combination of two reactions illustrated in Table II was carried out. The equilibrium H2 concentration generally becomes higher with a decrease in tem-

perature in the sulfurization of the metal sulfides by H2S, while 0-8412-0522-l/80/47-116-349$05.00 © 1980 American Chemical Society In Hydrogen: Production and Marketing; Smith, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

350

HYDROGEN: PRODUCTION AND MARKETING

Table I. The equilibrium H2 concentrations for H2S thermal decomposition

Temperature (°C) H2 concentration (vol-%)

400 0.1

500 0.4

600 1.3

700 2.8

1000 13.4


Table II. Thermochemical decomposition of H2S

M-^Sy + zH2S = M^Sy+z + zH2

(1)

MxSy+z = MxSy + zS

(2)

zH2S = zH2 + zS the thermal decomposition of the sulfide proceeds faster at higher

temperatures. Therefore, the decomposition efficiency of H2S will

be increased by a combination of reaction (1) at low temperatures

and reaction (2) at high temperatures. The following conditions are necessary for the sulfide used in this cycle. First, reaction (1) has a high equilibrium H2 concentration at temperatures above 500°C.

This

is a solid-gas

heterogeneous reaction so that the rate may markedly decrease below 500°C. Second, the sulfide formed in reaction (1) should not preferably be a higher sulfide such as polysulfide. A high sulfur activity in the higher sulfide is suitalbe for reaction (2) , but a

high H2 concentration can not be expected in reaction (1). Third,

reaction (2) can proceed at temperatures below 900°C. This limiting temperature was considered on the assumption of using of an

H.T.G.R. (High Temperature Gas Reactor). Under the above conditions, the use of the non-stoichiometric composition peculiar to sulfides and the use of a monosulf ide which

can form lower sulfides by the thermal decomposition were considered.

Figure 1 shows the outline of the experimental apparatus used.

The sulfide was packed in No. 3 in this figure. The H2 recovery

under H2S flow and the sulfur recovery under argon flow were alternately repeated many times. The thermal decomposition for sulfur was carried out under normal or reduced pressure. In this study, the repeat of the experiment associated with the former was

called the normal pressure cycle and that associated with the latter was called the reduced pressure cycle, respectively. Moreover,

the H2 concentration of off-gas was analyzed by gas chromatography

and the behavior of the H2 formation was investigated during the

H2 recovery experiment. These results in Figure 2 were obtained with iron sulfide used as an example of a non-stoichiometric composition and were obtained repeatedly in both cycles.

In Hydrogen: Production and Marketing; Smith, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


18. kiuchi et al.

Thermal Decomposition of Hydrogen Sulfide

jij Ar,S2 Mini

M

351

a T h2S 1 7||

IQ

"-

ate12

SV

UJ r

Figure 1. Schematic of experimental apparatus for metal sulfide: (1) reaction

tube; (2) electric furnace; (3) metal sulfide; (4) quartz wool; (5) trap; (6) cold bath; (7) sampling tube; (8) flow meter; (9) trap; (10) vacuum detector; (11) trap; (12) vacuum pump.

50 I

FeS

1

1

1

40

U W

0 ö

0

^^ i 20 40 60 TIME (minute)

Figure 2. Hydrogen evolution curves for normal and reduced pressure cycles 80

with pyrrhotite (FeS): (\J), normal pres-

sure cycie at 600°C; (O), reduced pressure cycle at 550° C.


352


The influence of the sulfurization temperature was investigated in each cycle. The behavior of each optimum sulfuriza-

tion temperature is compared in this figure. The sulfur composition of FeS ranges from FeS^+o to FeSl+0.2

at 600°C, and the equilibrium H2 concentration in that range

varies from about 100 % to about 4 %. On the other hand, the compo-


sition of the thermally decomposed product was FeS^#H under normal pressure and FeSi.06 under reduced pressure. The sulfurized

products were both approximately FeS^#2. Therefore, the H2 forma-

tion behavior in the figure was explained as showing a concentra-

tion corresponding closely to the composition-variation of FeS. The iron sulfide used in the experiment was obtained from the thermal decomposition of pyrite (FeS 2). The particles were extremely porous, with pores sizes of several tens of microns. In contrast,

the reaction behavior obtained with natural pyrrhotite or synthetic FeS composed of fine particles, gave much worse results . Thus , the influence of the specific surface area of a solid on the formation behavior was thought to be important. In order to understand the characteristics of chalcopyrite,

which has a wide range of non-stoichiometric compositions similar to iron sulfide and is a double sulfide, an exeriment with a copper concentrate was carried out.

The concentrate was composed of fine chalcopyrite particles

of ca. 50 micron. The maximum H2 concentration in both the normal and reduced pressure cycles was larger than the value obtained

with iron sulfide. The results are shown in Figure 3.

Based on the identification by X-ray diffraction and observation by micrography, the variation was found to be within the

non-stoichiometric composition of chalcopyrite in the normal pres-

sure cycle. Despite the decomposition into borniteû^FeS^) and

pyrrhotite during the reduced pressure cycle, the chalcopyrite was

found to be completely restored to its original chalcopyrite form by the succeeding sulfurization. The sulfur composition of the pyrrhotite was very low, and

FeS^#oi* Since a favorable H2 formation behavior was not obtained

from the experiment using synthetic bornite, the excellent results obtained during the reduced pressure cycle were thought to be due to the pyrrhotite.

M3S2 is a known lower sulfide as compared to NiS and shows a high equilibrium H2 concentration over a wide range of sulfur com-

positions.

The sulfurization of Ni3S2 to NiS proceeded easily, though the thermal decomposition of NiS into Ni3S2 was found to be diffi-

cult under the reduced pressure. The repeated results for the re-

duced pressure cycle are shown in Figure 4. It shows that consistent behavior is difficult to obtain.

The melting point of Ni3S2 is approximately 800°C and that of Ni3S2_x is 645°C. Accordingly, the melting or sintering of the sulfide in a packed bed

occured, and the surface area of the


18. kiuchi et al.


353

solid was thought to be reduced by each repetition of the thermal decomposition . The cycle combined with the thermal decomposition at a lower

temperature and higher vacuum degree may be suitable for this.


Methods with metals

Generally, the sulfurization of metal exhibits a remarkably high equilibrium H2 concentration, compared to the sulfurization of metal sulfide as shown in Table III.

Table III, The equilibrium H2 concentrations for metal sulfurizations

H2 (vol-%) Temp. (°C)

Bi

Cu

Pb

400 500 600 700

47.3 27.3 16.2 10.2

99.9 99.9 99.9 99.9

99.9 99.8 99.6 99.0

However, few sulfides are capable of being thermally decom-

posed into metal and sulfur. Noting the decomposibility of Bi2S«,

Soliman et al(l) proposed a cycle using Bi. The authors found that Ag2S decomposed at 800°C, under a reduced pressure of a few mm Hg, to form Ag. The equilibrium H2 concentrations for sulfurizations are, however, small for both Bi and Ag. The reaction is hindered by the sulfide film formed on the surface which occurs

during the

sulfurization of solid metal.

In this study, the use of liquid metal was examined. A smelting reaction was considered for the recovery of metal from the

sulfide. Since sulfur changed to SO2 in this case, the reaction

of SO2 and H2S by the Claus reaction was assumed. The reaction equations of the cycle using liquid Pb are shown in Table IV. As a means of preparing Pb from PbS, the roast-reaction or air-reduction method is well-known in nonferrous extrac-

tive metallurgy. The reactions for this method can be presented as follows

:

2PbS + 302 = 2PbO + 2S02 2PbO + PbS = 3Pb + S02 The authors have already found the direct production of Pb from PbS by oxidation under low oxygen partial pressure. (2) Accordingly, the experiment of H2 formation with lead was carried out.


354



Table IV. Two methods with Pb(l)

Pb(l) + H2S = PbS + H2

(3)

PbS + 02 = Pb(l) + S02

(4)

SO2 + 2H2S = 2H20 + 3S

(5)

(3)+(4) : H2S + 02 = H2 + S02 (3)+(4)+(5) : 3H2S + 02 = H2 + 2H20 + 2S Figure 5 shows an outline of the experimental appratus. The fused lead was placed in a reaction tube made of quartz. The reac-

tion was studied by the bubbling method in which H2S was bubbled into the fused lead and by a soft blowing method in which H2S was

blown onto the surface of the lead. The H2S gas stored in vessel

No. 1 was circulated by pump No. 2. Since the lead content was in excess of the H2S in order to maintain the liquid state, the reaction behavior was examined in this experiment until the gas reached the equilibrium composition.

The equilibrium H2 concentration was 99.8 % at 500°C and 97.9 % at 800°C. The value is very high, in spite of the temperature being high.

As a result, the higher the reaction temperature, the

more favorable the H2 formation behavior in this reaction system.

In addition, the sulfurization of Pb is an exothermic reaction and a slight rise in temperature was observed during the experiment. However, the acceleration of the reaction at low temperature was also examined.

Figure 6 shows the result obtained by the bubbling method.

Since the conversion of the ordinate are in proportion of formed H2

by the reaction, it should reach the same value as

the equilibrium H2 concentration. The value is approximately 99 %

at this temperature. Very small amounts of various metals were added to the lead to accelerate the reaction rate. As a result, Ni was found to be an effective metal. The critical amount of the effective Ni was

about 1 wt-%, which corresponds to the solubility of Ni in fused Pb. Therefore, the acceleration of the reaction was obviously dependant to the Ni dissolved into the lead.

The effect was also confirmed as being maintained after the completion of Ni sulfurization, even if a simultaneous sulfuriza-

tion of Ni was assumed. Consequently, it was thought that the dissolved Ni did not merely participate in the prior sulfurization but acted catalytically.

The results obtained by a gas sweeping method were rather poor compared to those obtained by the bubbling method. In particular, the reaction almost stagnated after a time of about 30


18. kiuchi et al.

50 i

1


1

355

1

w-/A «

\


z O

/ w \\

I

^ ir~W^2^ I jrntînlet ^W' >5r, —2—

Figure 5. Schematic of experimental apparatus for molten lead: (1) gas chamêr> C^ circulation pump; (3) flow meter;

(4) pressure gauge; (5) sampling tube; (6) blowing nozzle; (7) quartz tube; (8) reaction tube; (9) electric furnace; (10) electric furnace; (11) water jacket; (12) thermocouple; (13) thermocouple; (14) silicon stopper.


356



100 |

1

80

1

P^^

~b^

5 60 - PA z

hX-f

/

S

2 40 - / z

Figure 6. Effect of Ni addition to

/H

/

/

cp

molten lead on the conversion of H2S to

SL.

H 2 by the bubbling method at 550° C:

I

o

7n

100 i

1

(0),Pb;(n)lu>t%Ni-Pb.

, ^_\

I

fin

UJnute,

80

I

on

i?n

-^&=ertr-&

f

5 60 — X-l

z co Q_

/

A

7^

> 40 / Figure 7. Effect of Cu addition on the

conversion of H2S to H2 by the softblowing method at 600°C: (A), Pb-Cu;

(U)> Pb-Ni; (O), Pb.

«£>!

0 E— 0

I 30

I

I 60

TIME (minute)


I

I 90

120

18. kiuchi et al.


357

minutes had elapsed. From observation of the liquid surface during the reaction, the surface at that time was found to be covered with a film of the lead sulfide formed. The formation of this

film was also seen in lead containing added Ni, and no accelerating effect could be found after this time with agitation.


The addition of Cu, however, resulted in an outstanding effect as shown in Figure 7. The amount of Cu addition was limited to that corresponding to the solubility of about 1 wt-%. In this case, too, the behavior was considered to involve catalytic action, instead of the prior or simultaneous sulfurization. In addition, the surface observation showed that the sulfide formed with Cu addition did not cover the surface but accumulated

by swelling from the surface. The sulfide accumulated was extremly porous. Consequently, the dissolved Cu was considered to have effect on the growth of PbS crystals. Summary

The decomposition efficiency of H2S was increased by a reduc-

ed pressure cycle of a few mm Hg for metal sulfides. . The utilization of a metal may be more promising as a H2

recovery method rather than the decomposition of H2S.

As a primary approach toward "Hydrogen Economy" , the H2S

by-product obtained from fossil fuels or by extractive metallurgy of sulfide ores should be considered as more important for H2 recovery than for the sulfur which can be recovered.

References

1. Soliman, M.A., Calty, R.H., Conger, W.L., Funk, J.E., Can.J.Chem. Eng., 1975, 53, 164-169. 2. Kiuchi, H., Tanaka. T., Trans.Soc.Min.Eng.AIME, 1977, 262, 248254.

Received July 12, 1979.


S with Metal Sulfides or Metals - American Chemical Society

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