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Ind. Eng. Chem. Res. 1996, 35, 1747-1752
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Pyrophoric Nature of Iron Sulfides Robert Walker* Department of Materials Science and Engineering, University of Surrey, Guildford GU2 5XH, U.K.
Alan D. Steele and David T. B. Morgan Shell Research Centre Ltd., Thornton Research Centre, P.O. Box 1, Chester CH1 3SH, U.K.
Hydrogen sulfide, often present in crude oil tankers, can react with rust to form various sulfides including mackinawite (FeS), greigite (Fe3S4), and pyrite (FeS2). The tendency for these compounds to react with oxygen in air to form potentially explosive mixtures depends upon their morphology and the environmental conditions. The experimentally determined heat of oxidation of finely divided mackinawite was -7.45 kJ g-1. For samples with a larger particle size and smaller surface area the values measured were lower due to incomplete oxidation of the sulfide. All the sulfides produced, whether from magnetite or acicular, prismatic or spherical geothite, were approximately spherical in form. The heat of oxidation of greigite was found to be approximately -2100 kJ mol-1, and the heat of formation of greigite is approximately -320 kJ mol-1. 1. Introduction This work on iron sulfide powder was originally initiated after two explosions occurred in the 1970s during discharge of crude oil from vessels in Thailand that were not fitted with inert gas equipment. The reactions of rust with hydrogen sulfide in the tanks containing Qatar land crude oil have been reported [Walker et al. (1987)] to give iron sulfide in the form of mackinawite, FeS, at low temperatures and a mixture of greigite, Fe3S4, and pyrite, FeS2, at higher temperatures. The oxidation of dry mackinawite has been shown [Walker et al. (1988)] to be pyrophoric in air with a relative humidity above about 50% but slow and controlled below 50%. Moist mackinawite, however, oxidized in a two-stage process, and the resulting product was pyrophoric. The pyrophoric nature of iron sulfide has been known for a long time, and this particular form of powder may be even more dangerous because it can form as a fine powder and the reactivity of a substance normally increases with the surface area. The particle size of the iron sulfide which forms in the holds of tankers carrying crude oil is important because it may affect the safety of the tanker. The aim of this paper is to determine the nature of the processes which can occur during the oxidation of iron sulfides produced from different forms of goethite and magnetite. Experiments have been designed to identify the products and hence the reactions taking place. An attempt is made to quantify the effects of the shape and size of the particles on the heat of oxidation. The results in this paper should provide a better understanding of the pyrophoric nature of iron sulfides. 2. Literature Review The corrosion of steel usually produces goethite {RFeO(OH)} and, if hydrogen sulfide is present, iron sulfides. Matsuo and Tominaga (1982) found that mackinawite (FeS) was formed when steel corroded in an atmosphere of nitrogen and 0.86 vol % hydrogen sulfide. This product gradually transformed to greigite (Fe3S4). In a similar atmosphere containing 20 vol % oxygen at 100% relative humidity, the production of mackinawite was faster and it was oxidized to iron
oxyhydroxide. Popa et al. (1992) studied the Girdler sulfide process and reported that the reaction of hydrogen sulfide with alloy steels gave mackinawite which then transformed easily into other phases. They wanted to increase the corrosion resistance of the carbon steel so they preconditioned it to convert the mackinawite into pyrite (FeS2). In very different environments similar reactions have been observed. Schoonen and Barnes (1991) used iron monosulfide precursors below 100 °C and found that the mackinawite produced became greigite which changed to pyrite; a similar distribution pattern has occurred in modern marine sediments. According to Krs et al. (1992) in brown coals there were finely dispersed forms of greigite, and this gradually altered on heating to pyrite and marcasite (also FeS2). Ge et al. (1992) found that the pyrite present in coal transformed to magnetite (Fe3O4) on heating, and Gracia et al. (1993) reported that pyrite naturally weathered to give geothite. Electrochemical studies by Wang et al. (1992) demonstrated that the initial surface oxidation of pyrite involved the chemisorption of hydroxyl groups from water, and this process is transport controlled. Also pyrite surfaces in contact with water and oxygen underwent rapid oxidation. In crude oil refineries there are many different products resulting from a wide range of reactions. These are of particular interest in this work. The scales present in the zones of intense corrosion of unalloyed steel in refineries contained pyrite and oxyhydroxides of iron [Peev et al. (1992)]. Other work with petroleum mixtures showed that steel reacted with 500-1800 mg L-1 hydrogen sulfide to give mackinawite and kansite [Getmanski et al. (1982)], and elsewhere pyrite, pyrrhotite, troilite, and kansite were reported [Obukhova et al. (1982)]. Hughes et al. (1976) studied the reactions in the hatch areas on oil tankers and postulated that hydrogen sulfide in the gas inside the tanks reacted with rust to give iron sulfide which combined with air on discharge of the oil to give pyrophoric species which caused explosions when in contact with the flammable petroleum vapors. They have suggested that the formation of iron sulfide and its subsequent oxidation may be represented as
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1748 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996
F2O3 + 3H2S f 2FeS + S + 3H2O
(1)
4FeS + 7O2 f 2Fe2O3 + 4SO2
(2)
and
For the sulfidation reaction (1), the enthalpy, ∆H, has a value of -165.1 kJ mol-1 and the free energy, ∆G, is -66.8 kJ mol-1; the corresponding values for the oxidation reaction (2) are -1226 and -1147 kJ mol-1. Hence, both of these reactions are exothermic, and the temperature rise observed for the oxidation of the sulfide (80 °C) was always greater than that of the sulfidation of the oxide (15 °C). Provided that little dissipation of the energy occurred, the reactions could produce high temperatures. The reactivity of the iron sulfide was found to depend upon the type of iron oxide from which it was derived as well as upon the particle geometry. It was also shown that these reactions could be recycled, and this resulted in a reduction in the reactivity which was considered to be due to the screening or diluting effect of the sulfur. It is interesting to note that a correlation between a high reactivity, as indicated by pyrophoric oxidation, and a low density of packing was reported [Hughes et al. (1976)]. The compression of the powder had the effect of lowering the apparent surface area and the tendency to react. Oxygen also played an important role. The ratio of hydrogen sulfide to oxygen was significant, and a value less than unity tended to favor the formation of a relatively inactive product while greater than unity gave an increase in the possibility of forming a pyrophoric sulfide. When there was less than 10% oxygen in the nitrogen atmosphere the sulfide samples oxidized in a controlled, slow manner while above 10% oxygen favored pyrophoric oxidation. Goethite and elemental sulfur were always found in the scale/rust deposits on the surfaces of the oil tanks in the ullage and/or hatch areas [Hughes et al. (1976)]. Goethite has been shown [Walker et al. (1987)] to react with the hydrogen sulfide, which has been found to be present in concentrations of up to 5 vol % in the crude oil. The composition of the compounds formed depended upon the conditions and the time of storage of the products. Under a slow rate of flow of hydrogen sulfide gas at lower temperatures (0 and 20-25 °C), mackinawite was formed and the reaction in an environment was
2FeO(OH) + 3H2S f 2FeS + S° + 4H2O
(3)
At a faster flow rate greigite was formed with mackinawite. Higher temperatures of 50, 75, and 100 °C gave the product greigite, with concentrations of pyrite 12, 28, and 38%, respectively, at the slower flow rate. Hence increasing the temperature and the flow rate brought about product mixtures along the route
mackinawite f greigite f pyrite During storage of the sulfide mixture formed at 50 °C, which was the approximate temperature of the deck and ullage in Thailand, aging occurred and brought about an increase in the pyrite content [Walker et al. (1988)]. Possible reactions are
FeS + S° f FeS2
(4)
Fe3S4 + 2S° f 3FeS2
(5)
This observation is significant because it confirms the likelihood of substantial amounts of potentially dangerous pyrite being formed in the cargo tanks of vessels in tropical climates.
The reaction (3) of goethite with hydrogen sulfide gives water as well as mackinawite and sulfur. In air these react with oxygen and follow a two-stage process with a temperature plateau at 70-80 °C during which the water evaporated: only after evaporation did the second stage of oxidation occur and this was pyrophoric. Water in the atmosphere was also important. At room temperature with a relative humidity below 50% the reaction between mackinawite and sulfur was slow and controlled and resulted in the formation of magnetite, Fe3O4, and some R-Fe2O3. Under the same conditions but with a relative humidity of above 50%, the reaction was pyrophoric and a one-stage process giving goethite, R-FeO(OH), and R-Fe2O3. 3. Experimental Procedure A variety of different iron oxides was used to study their reaction with hydrogen sulfide gas to form iron sulfides. There were four main categories, viz. (i) black magnetite, Fe3O4, (ii) yellow acicular goethite, (iii) spherical goethite which was usually red, and (iv) prisimatic goethite. The different morphologies are shown in the SEM photographs (Figure 1). Approximately 30 g of the oxide powder was compressed into a cylindrical plug and placed in a U-tube of diameter 2.5 cm (cross-sectioned area 5 cm2) fitted with glass wool plugs and taps at both ends. This was fitted into an apparatus and flushed out with pure nitrogen to remove the air and then filled with hydrogen sulfide gas. The flow of hydrogen sulfide through the U tube and oxide plug was controlled at 0.5 cm3 s-1. The sulfides formed were used for the following: (1) chemical analysis for the total sulfur to iron ratio, (2) sampling for X-ray diffraction to find out if mackinawite were the only species present and validate reaction (3), (3) differential thermal analysis to determine the apparent heat of sulfidation, (4) measuring the average particle size, and (5) measuring the surface area. Extreme care was taken in all the experiments because of the high reactivity of the powders, and nitrogen atmospheres were normally used. The sulfur/iron ratio was calculated from the concentrations of iron and sulfur. The analytical method for iron was adapted from ASTM Designation E277-69 “Total Iron in Iron Ores by Stannous Chloride Reduction and Dichromate Titration” and the sulfur was that of Vogel described in Quantitative Inorganic Analysis as “The Determination of Sulfur from Iron Ores”. X-ray diffraction using the Debye-Scherrer powder technique was employed to prove that only mackinawite was produced. All the sampling was carried out in a glove box under a nitrogen atmosphere to prevent any sample oxidation reaction. Initially a glass capillary contained the powder but a better method was developed with a glass fiber, diameter 0.25 mm, and Collodion (a solution of cellulose nitrate). The fiber was dipped into the Collodion, then into the powder while still wet, and again into the Collodion. This coating was found to stop any oxidation of even the most reactive sulfide for several days. The sample was then allowed to dry, installed into the camera within 10 min, and exposed to X-rays. The mackinawite was poorly crystalline which would account for the fact that the lines on the X-ray diffraction photographs were not sharp. Fortunately, the patterns for the greigite and pyrite were better (Figure 2) so that identification was possible. Differential thermal analysis was employed to measure the heat of sulfidation. The sample crucible was loaded in a glove box under nitrogen, sealed, and
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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1749
Figure 1. SEM photos of goethite prior to sulfiding.
method was based on the quantity of nitrogen required to form a monolayer on a specific weight of sample. The advantages of this technique are that it is relatively quick and quite accurate with errors of only 5-10%. Unfortunately it was not possible to measure the particle size or surface area of the sulfides prepared at 100 °C because they were so reactive that oxidation took place during the transfer of weighed samples to the Fisher or BET apparatus. Hence the observed heats of oxidation were recorded as a function of the particle size of the oxide precursor. Results and Discussion Figure 2. X-ray powder diffraction photographs: (A) mackinawite; (B) greigite; (C) pyrite.
transferred to the DTA head. The initial atmosphere was nitrogen which was passed through the furnace. The seal on the crucible was then removed, the heating rate set at 0.17 °C s-1, the nitrogen flow switched off, and the oxygen supply switched on. The exotherm produced was then quantified by determination of the area under the peak: in earlier work this had been shown to give a scatter of less than 1% over three experiments. Each experiment was repeated three times. The average particle size was measured with a Fisher subsieve sizer. The sulfide samples were stored in a vacuum desiccator containing phosphorous pentoxide to maintain dryness, and this was situated in a glove box with a nitrogen atmosphere. By applying the vacuum overnight it was proved that at least 99 wt % of the water was removed. The instrument operates on the air permeability principle because particles in the path of a regulated air flow affect the air flow in relationship to their size. The sample tube contains 1 cm3 of powder, and this must be weighed, in a balance cabinet flooded with nitrogen, in order to calculate the density. The average particle size was in good agreement with measurements taken on the scanning electron microscope. The surface area of the sulfide particles was measured with a Micromeritics MIC-2200 analyzer. This BET
The results of the work carried out at ambient temperature (20-25 °C) are summarized in Table 1, and the graphs of the heat of oxidation of the sulfides vs average sulfide particle size are given in Figure 3 and vs specific surface area in Figure 4. The geometry of the sulfide product was rather surprisingly always approximately spherical irrespective of the shape of the oxide precursor. This is illustrated in Figure 5 for three different sulfides formed from acicular, prismatic, and spherical powders. The heat of oxidation for each sulfide is given as per gram and not per mole as the value consists of two components which could not be separated in the isotherm. These were the fraction due to the oxidation of the iron sulfide and that due to the oxidation of the elemental sulfur to sulfur dioxide in the reaction
4FeS + 2H2O + 7O2 f 4FeO(OH) + 4SO2
(6)
Therefore the values cannot be expressed in units of moles of one particular species. The observed heat of oxidation was generally highest for the sulfides of the smallest particle size (Figure 3) and vice versa. The measured values of the specific surface area of the iron sulfides were consistent with the respective average particle size measurements. This confirms that the observed heat of oxidation was dependent, at least in part, upon the surface area of the iron oxide (Figure 4). Heat of Oxidation of Sulfide Made from Bayer 920 Goethite. All the sulfides prepared from goethite
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1750 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996
Figure 3. Effect of the particle size of sulfide on the heat of oxidation. Table 1. Effect of Sulfide Particle Size and Surface Area on the Observed Heat of Oxidation at Ambient Temperature oxide precursor Bayer 920 R-FeO(OH) Ramsden R-FeO(OH) Deanox R-FeO(OH) Kodak R-FeO(OH) Bayer 3910 R-FeO(OH) Bayer 420 R-FeO(OH) Bayer 600 R-FeO(OH) Riedel de Haen R-FeO(OH) Fisons Fe3O4 May and Baker Fe3O4 Ramsden Fe3O4 Ramsden Fe3O4
geometry
ave oxide particle size (µm)
ave sulfide particle size (µm)
sulfide specific surface area (m2 g-1)
total S/Fe ratio
sulfide identity (XRD)
sulfide reactivity in air (3 g)
∆H oxidation (kJ g-1) dry sulfide
acicular
0.3
0.5
14.1
1.53
mack
pyro
-7.45
acicular
0.35
0.6
10.7
1.54
mack
pyro
-6.33
acicular
0.4
0.55
12.4
1.55
mack
pyro
-6.81
acicular
0.45
0.75
6.9
1.55
mack
pyro
-5.66
prismatic
0.3
0.5
13.9
1.53
mack
pyro
-7.18
prismatic
0.4
0.65
8.7
1.53
mack
pyro
-5.91
irregular
0.55
0.80
6.1
1.53
mack
pyro
-4.85
spherical
0.95
0.90
4.8
1.56
mack
pyro
-5.37
spherical spherical
0.45 0.65
0.95 1.10
3.9 3.0
1.37 1.34
mack mack
pyro pyro
-4.90 -4.50
angular irregular
6.3 2.2
oxide would not sulfide oxide would not sulfide
at ambient temperature had an experimentally determined total sulfur/iron ratio of approximately 1.5 which suggests that the reaction took the following route:
2FeO(OH) + 3H2S f 2FeS + S° + 4H2O
(3)
Hence, 1 g of dried product should contain approximately 0.846 g of mackinawite (the molecular fraction). For the mackinawite oxidation reaction (6), the value of ∆H is -640.3 kJ mol-1 FeS at 298 K. Hence, 0.846 g of mackinawite, of molecular weight 87.9, when completely oxidized should give an approximate increase in enthalpy ∆H of -6.16 kJ. The highest observed value of ∆H oxidation, however, is much higher, -7.45 kJ g-1, than this theoretical value. This difference may be explained by the simultaneous oxidation of 0.154 g of amorphous elemental sulfur to sulfur dioxide for which ∆H ) -296.9 kJ mol-1 at 298 K; this must be considered as it is necessarily produced during the preparation of the sulfide and it represents -1.43 kJ. Hence, the total value for the
mackinawite and sulfur reactions is -7.59 kJ g-1 which is close to the observed value of ∆H ) -7.45 kJ g-1 for the sulfide prepared from Bayer 920 goethite. The values of ∆H observed for sulfides produced from other oxides, however, were significantly less. A possible explanation for this is that the smaller surface area available for oxidation prevented the complete reaction so that these samples could contain some unoxidized iron sulfide. This was investigated using the sulfide product prepared from the Riedel de Haen goethite for which the heat of oxidation, ∆H, was -5.37 kJ g-1. Ten differential thermal analysis oxidation runs were carried out using a total of 0.629 g of sulfide product. Any remaining sulfur was oxidized in a mixture of bromine and nitric acid and the total sulfur content determined by chemical analysis and found to be 0.090 g. The total sulfur weight in 0.629 g of the mixture should be 0.291 g. The extent of the reaction of the Riedel de Haen product may be calculated from the ratio of its values
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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1751
Figure 4. Effect of the surface area of sulfide on the heat of oxidation. Table 2. Effect of Particle Size on Observed Heat of Oxidation at 100 °C oxide precursor Riedel de Haen R-FeO(OH) Bayer 420 R-FeO(OH) Bayer 920 R-FeO(OH) May and Baker Fe3O4
geometry
ave oxide particle size (µm)
∆H oxidation (kJ g-1)
spherical
0.95
-7.03
prismatic
0.4
-6.61
acicular
0.3
-7.01
spherical
0.65
-2.20
of ∆H -5.37 kJ g-1 compared with that of -7.59 kJ g-1, for the complete reaction: the completion can be assumed to be 70.8%. This corresponds to a value of the weight of unreacted sulfur of 0.085 g which is in good agreement with the experimentally determined value of 0.09 g. This result confirms the fact that mackinawite samples of relatively small surface area tended to undergo incomplete oxidation under the conditions encountered in the DTA apparatus. Iron Sulfides Prepared at 100 °C. The iron sulfide products of the reaction of goethite and hydrogen sulfide, flow rate 0.5 cm3 s-1, at 100 °C are greigite and (35-39 wt %) pyrite [Walker et al. (1987)]. The aging of this mixture in a slow stream of hydrogen sulfide has been shown to yield up to 50 wt % pyrite. The experimentally determined heats of oxidation of four different oxides, covering a cross section of particle size/geometry are given in Table 2. It was not possible to measure either the average particle size or specific surface area because of the high oxidative reactivity of the powders. The heats of oxidation cannot be compared with any theoretical values because the products contained greigite and approximately 37 wt % pyrite. The only thermodynamic data known for greigite is a value for the standard free energy of formation, ∆G ) -290 kJ mol-1 at 298 K. The heat of oxidation of the 37 wt % pyrite and 63 wt % greigite mixture, prepared from the Riedel de Haen goethite at 100 °C, was determined experimentally and found to be -7.03 kJ g-1. For pyrite the oxidation
Figure 5. SEM photographs of iron sulfides produced from goethite of different morphology.
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1752 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996
process can be considered to be
4FeS2 + 1102 f 2Fe2O3 + 8SO2 and the heat of oxidation, ∆H, is -827 kJ mol-1 at 298 K. Hence, the value of ∆H for the complete oxidation of 1 g of pyrite is -6.89 kJ. The contribution of 37% pyrite to a 1 g mixture of pyrite and greigite is -2.55 kJ. The value due to the oxidation of 0.63 g of greigite would therefore be the difference between -7.03 and -2.55, i.e., -4.48 kJ. Hence for greigite the value of
∆H ) -7.11 kJ g-1 or -2100 kJ mol-1 It is possible to determine an approximate value for the heat of formation for greigite assuming that the oxidation follows the route
2Fe3S4 +
25 O f 3Fe2O3 + 8SO2 2 2
Heat of formation of greigite ) 1/2(∆Hproducts - ∆Hreactants) ) 1/2(3 × -822 + 8 × -297) - 1/2(2 × -2100) ) -321 kJ mol-1 This value for greigite can only be regarded as approximate because it ignores the fact that there is a small (5%) amount of elemental sulfur present. It also assumes that the heat of oxidation of the pyrite component of the mixture is the same as the standard heat of oxidation for pyrite. It is interesting to note that the reactivity of greigite is utilized in the manufacture of Chinese export fire crackers which contain a mixture of chemicals containing 5-25% greigite [Chen (1989)]. Conclusions It has been established that, under the conditions used in this work, the particle size and surface area of the sulfide significantly affect the observed heat of oxidation of mackinawite, FeS, prepared by the sulfidation of goethite, R-FeO(OH). This is important because it was the major constituent of the rust found in the cargo tanks of crude oil tankers. The sulfides with smaller particle size and hence larger surface area had an apparently higher heat of oxidation than those with a larger size and smaller area. The highest value of the heat of oxidation, ∆H, was obtained for the sulfide made at ambient temperature from Bayer 920 geothite. This had a measured value of -7.45 kJ g1-1 which compared closely with the theoretical value of -7.59 kJ g-1 based on a calculation that assumed complete oxidation of all the mackinawite and elemental sulfur had occurred. Sulfides with a larger particle size than this had a smaller value of the heat of oxidation. This has been shown to be due to the incomplete oxidation of the sulfide probably due to the reduced surface area of sulfide available for oxidation. All the iron sulfides, prepared from both geothite and magnetite, Fe3O4, tended to consist of approximately spherical particles. The reaction of the geothite can be represented by the equation
2FeO(OH) + 3H2S f 2FeS + S + 4H2O
approximately 1.33 and X-ray diffraction identified mackinawite as the only species present. From the observed values of the heat of oxidation of iron sulfides prepared from goethite at 100 °C it was deduced that the heat of oxidation of greigite, Fe3S4, was approximately -2100 kJ mol-1 for the reaction
2Fe3S4 +
25 O f 3Fe2O3 + 8SO2 2 2
From this value the heat of formation of greigite was determined as approximately -320 kJ mol-1. For pyrite, FeS2, the heat of oxidation is -827 kJ mol-1 and the reaction is
4FeS2 + 11O2 f 2Fe2O3 + 8SO2 Literature Cited Chen, J. Powder formula for the production of export firecrackers Patent CN 1,037,136 (Cl C06 B 29/02), Nov 15, 1989. Appl. 88,102,524, Apr 27, 1988. Ge, Y.; Hsieh, K. C.; Tseng, B. H.; Wert, C. A. Pyrite in coal and organic sulfur in its vicinity. Ranliao Huaxue Xuebao 1992, 20 (1), 9-5. Getmanski, M. D.; Panov, M. K.; Rozhdestvenskii, Yu G.; Nizamov, K. R.; Kalimullin, A. A. Korroz. Zashch. 1982, 1, 5-8. Gracia, M.; Gancedo, J. R.; Barrero, M. L.; Gracia, A. B.; MartinezAlonso, A.; Tascon, J. M. D. Comparative Moessbauer study of the effects of natural weathering and artificial oxidation on iron minerals present in coal. Nucl. Instrum. Methods Phys. Res., Sect. B 1993, B76 (1-4), 191-4. Hughes, R. I.; Morgan, T. D. B.; Wilson, R. W. Is pyrophoric iron sulfide a possible source of ignition? Nature 1974, 248 (5450), 670. Hughes, R. I.; Morgan, T. D. B.; Wilson, R. W. The generation of pyrophoric material in the cargo tanks of crude oil carriers. Trans. Inst. Mar. Eng. 1976, 88, 1-7. Krs, M.; Novak, F.; Krsova, M.; Pruner, P.; Kouklikova, L.; Jansa, J. Magnetic properties and metastability of greigite-smythite mineralization in brown-coal basins of the Krusne Lory Piedmont, Bohemia. Phys. Earth Planet. Inter. 1992, 70 (3-4), 27387. Matsuo, M.; Tominaga, T. Conversion electron Moessbauer study of reactions of iron with hydrogen sulfide and sulfur vapor. Radiochem. Radioanal. Lett. 1982, 52, 163-75. Obukhova, Z. P.; Kulovaya, A. A.; Kiril’chenko, N. E. Determination of the composition of corrosion products. Gazov. Promst. 1982, (4), 35-6. Peev, T.; Taseva, V.; Akala, A. Study of the corrosion processes in primary oil refining by Moessbauer and x-ray investigation, Part 1 Atmospheric distillation columns. Werkst. Korros. 1992, 43 (11), 527-31. Popa, M. V.; Bizadea, G.; Roman, E.; Radovici, O. The iron sulfides and their role in the Girdler sulfide process. Rev. Roum. Chim. 1992, 37 (8), 637-47. Schoonen, M. A. A.; Barnes, H. L. Reactions forming pyrite and marcasite from solution: II Via iron monosulfide precursors below 100 °C. Geochim. Cosmochim. Acta 1991, 55 (6), 150514. Walker, R.; Steele, A. D.; Morgan, T. D. B. The formation of pyrophoric iron sulfide from rust. Surf. Coat. Technol. 1987, 31, 183-97. Walker, R.; Steele, A. D.; Morgan, T. D. B. Pyrophoric oxidation of iron sulfide. Surf. Coat. Technol. 1988, 34, 163-175. Wang, X. H.; Jiang, C. L.; Raichur, A. M.; Parekh, B. K.; Leonard, J. W. Comparative studies of surface properties of pyrite from coal and ore sources. Proc. Electrochem. Soc. 1992, 92-17, 410-32.
Received for review June 28, 1995 Revised manuscript received November 2, 1995 Accepted January 23, 1996X IE950397T
The reaction for the magnetite is considered to be
Fe3O4 + 4H2S f 3FeS + S + 4H2O because the total sulfur/iron molar ratio of sulfides is
X Abstract published in Advance ACS Abstracts, March 15, 1996.
