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Ind. Eng. Chem. Res. 2003, 42, 4290-4295
Decomposition and Oxidation of Pyrite in a Fixed-Bed Reactor Jens Peter Hansen,*,†,‡ Lars Skaarup Jensen,‡ Stig Wedel,† and Kim Dam-Johansen† Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark, and Research & Development, F. L. Smidth A/S, Vigerslev Alle´ 77, 2500 Valby, Denmark
The oxidation of pyrite (FeS2) has been investigated in a fixed-bed laboratory reactor to gain knowledge about the SO2 formation mechanisms and kinetics at conditions relevant to the upper stages of a cyclone preheater tower in a modern dry kiln system for cement production. Experiments were carried out with a high sulfide containing shale (a mass-average diameter equal to 21 µm) and with pure FeS2 particles (between 32 and 64 µm). Measurable SO2 formation started at about 350 °C for the shale and at 400 °C for the FeS2 particles and increased with temperature for both materials. Experiments showed that the conversion of pyritic sulfide to SO2 was independent of the inlet SO2 concentration up to at least 925 ppmv and that the conversion decreased as the O2 concentration was increased from 5% to 20% (v/v). A shrinkingcore reaction mechanism with FeS2 as the core and with porous FeS as the intermediate product layer is proposed to account for the experimental observations. According to this mechanism, the oxidation of the FeS2 core to FeS is relatively fast at moderate O2 levels (e.g., 5%) because it is easy for O2 and SO2 to diffuse through the porous product layer of almost non-oxidized FeS. At increased O2 levels (e.g., 20%), we believe that the FeS layer also starts to oxidize so its porous structure is disrupted whereby the resistance to diffusion through it increases and thus further oxidation of the FeS2 core is inhibited, leading to lower overall conversions. Introduction SO2 emissions from cement production are mainly caused by oxidation of pyrite (FeS2) in the raw materials during the preheating process.1 The raw materials for a Portland cement clinker are typically a mixture of two or more components of limestone, marl, shale, and clay, and impurities of pyrite in these materials are often unavoidable because of its frequent occurrence in nature. It is difficult to generalize about the concentration of pyrite in the raw materials, both from a regional and a local point of view. Great variations are seen between raw materials from different countries and different layers within a single quarry where small color variations often are associated with small variations in chemical composition and pyrite content. As an example, the average sulfide content in the raw mixture from 24 different cement plants (15 plants in USA, 2 in Canada, and 7 in other countries) is 0.16% S (w/w) with a standard deviation of (0.11% S.2 For a world production of cement of about 1500 million tons/year and an assumed conversion of sulfide to stack SO2 of 20%, the estimated world emission from cement production is 1.0 million tons of SO2/year. A so-called dry process kiln system with a cyclone preheater tower is the most used cement clinker production facility because of its low specific energy consumption compared to older wet processes. A preheater tower typically consists of four to six cyclone stages, which are used to exchange heat between the hot combustion gases and pulverized raw mix by successively mixing and separating the raw mix and gases. It has been known for more than a decade that the pyrite oxidizes at about 300-600 °C in the upper stages of the * To whom correspondence should be addressed. Fax: +45 36182647. E-mail:
[email protected]. † Technical University of Denmark. ‡ F. L. Smidth A/S.
preheater tower,1,3 but not much is known about the chemical mechanisms and kinetics at relevant conditions. Thus, there is a need for data on pyrite oxidation at conditions simulating preheater operation. Literature Survey Several researchers have studied the thermal decomposition of pure pyrite in an inert atmosphere.4 Though discordant results for the rate and activation energies are reported, there seems to be good agreement on the mechanism. Hong and Fegley5 have shown that pyrite decomposes to a porous structure of pyrrhotite (Fe1-xS) and gaseous sulfur in an inert atmosphere. For simplicity, this can be written as
FeS2(s) f porous FeS(s) + 1/2S2(g)
(1)
The reaction is more complicated and difficult to study in the presence of oxygen, and only sporadic information can be found in the literature. Hong and Fegley5 studied the oxidation of pyrite by hanging thin and regular slices of FeS2 on a wire in a 100 ppmv O2 in CO2 gas atmosphere. They found two different types of behavior: at higher temperatures (484-538 °C), no hematite (Fe2O3) but only pyrrhotite was observed, and the rate constant as well as the apparent activation energy was about the same as that in inert gases. At lower temperatures (392-460 °C), both hematite and pyrrhotite formed and the rate constant was appreciably higher than that extrapolated from higher temperatures. The apparent activation energies in the high and low temperature ranges were 293 and 82 kJ/mol, respectively. McCarty et al.6 studied the oxidation of FeS2 in air by in situ Raman spectroscopy. A sample was heated from 250 to 600 °C in 50 °C increments and held 30 min at each temperature. No oxidation was observed below 400 °C, while transformation to hematite (Fe2O3) was ob-
10.1021/ie030195u CCC: $25.00 © 2003 American Chemical Society Published on Web 08/14/2003
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served at temperatures of 450 °C and above. Pelovski and Petkova7 made simultaneous thermogravimetry, differential thermogravimetry, and differential thermal analysis (DTA) curves of pyrite heated 5 °C/min in air and then investigated the samples heated to 560, 660, and 1000 °C by X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy. A measurable mass loss was found to start at 410 °C and to reach a maximum intensity at 688 °C. γ-Fe2O3, R-Fe2O3, FeS, and FeS2 were found in the sample heated to 560 °C, while all of FeS2 had disappeared in the sample heated to 660 °C. Hausen8 made DTA measurements of pyrite in air and at different O2 and SO2 concentrations. In air, an exotherm started at about 390 °C and reached a maximum intensity at about 560 °C. Fe2O3 and FeSO4 were identified in samples heated to 640 °C in the oxidizing atmospheres. To summarize, there seems to be agreement that the oxidation of pyrite starts at about 400 °C and that pyrrhotite and hematite are formed as primary products. Seidler and Hoenig9 have recently investigated the release of SO2 from cement raw meal in a fixed-bed reactor. Parameters such as the raw meal composition and partial pressure of H2O(g), SO2(g), and O2(g) were studied. The experiments differ from those performed in the present work in two main aspects: (1) In the experiments by Seidler and Hoenig,9 the samples were heated simultaneously with the chemical reactions taking place. Under these nonisothermal conditions, it was not possible to obtain reliable kinetic information. In this work, the samples are first heated to a constant temperature in an inert atmosphere, after which the reaction is initiated by adding O2 to the inlet gas (within a second); i.e., the SO2 release is measured under isothermal conditions. (2) Typical raw meals with a low sulfide content and thus a relatively high content of SO2 absorbing components, in particular CaCO3 and MgCO3, were used in the experiments by Seidler and Hoenig.9 As a consequence, it is difficult to separate the SO2 formation from the SO2 absorption because only the net SO2 release was measured. In this work, it is the intention to study the pyrite reaction independently of any SO2 absorption by making experiments with pure pyrite particles and a shale with a relatively high content of pyrite and a low content of CaCO3 and MgCO3. Experimental Setup The experimental setup is sketched in Figure 1 and consists of two gas cylinders for O2 and N2, two mass flow controllers, a quartz reactor placed in an electrically heated oven, and an UV analyzer to measure SO2 concentrations between 0 and 2000 ppm. The O2 concentration, adjusted by the mass flow controllers for O2 and N2, was checked with an electrochemical sensor connected after the SO2 analyzer. A thermocouple was placed immediately under the porous plate as shown on the figure and used to check if the reactor and sample were heated to the set-point temperature of the oven. Valve 4 is a three-way valve and was used to switch between two modes of O2 admission: Inert operation when O2 was added after the bed via the thin tube surrounding the thermocouple or oxidative operation when O2 was added before the bed via the solid feeding tube. A PC was used for data acquisition (10 Hz).
Figure 1. Illustration of the fixed-bed reactor: MFC, mass flow controller; TC, thermocouple; (1) feeding tap for sample; (2) open/ close valve; (3) lid; (4) three-way valve.
Experimental Procedure A total of 1.00 g of coarse sand (ASTM, C109) was first distributed on the porous plate to protect it from being plugged by smaller particles. The weighed sample (between 0.100 and 0.400 g) was then diluted with coarse sand (ASTM, C109) to a total of 1.00 g and stored in the sample feeder. The coarse sand was added to improve quantitative transfer of the sample to the reactor and to favor a more homogeneous distribution of the sample in the bed. It also acted as a heat buffer for the exothermic reactions. The reactor was preheated to a constant temperature with valve 2 closed so N2 entered through the main inlet and passed upward through the outer annulus of the quartz reactor, then downward through the sample resting on the porous plate, and finally through the SO2 analyzer placed just after the reactor. The sample with added sand was stored in the sample feeder, which was flushed with N2 to eliminate any O2 in the chamber by opening valve 2 and lid 3 for 30 s. Valve 2 and lid 3 were then closed again, and the sample was dropped down on the coarse sand on the porous plate by opening feeding tap 1 for a short period. Inert heating of the sample was performed for 240 s before the oxidation was initiated by turning valve 4 so O2 admission was switched from below to above the sample. The total flow out of the reactor was 2 NL/min with 5% O2 as the standard. All gas concentrations are on a volume basis. Materials. The first series of experiments were made with a shale from a quarry in Northern Ireland close to Sean Quinn Cement Plant in Ballyconnell, Ireland. The chemical composition of the shale is given in Table 1 and its size distribution in Figure 2. The detailed sulfur analyses reveal that the shale does not contain any sulfite (e.g., CaSO3) or monosulfides (e.g., FeS) but 2.73% S in the form of disulfide. On the basis of an XRD spectrum of the powdered shale, which revealed a relatively strong pyrite (FeS2) peak, but no peaks of marcasite (FeS2), pyrrhotite (Fe1-xS), or troilite (FeS), the 2.73% S is assumed to be bound as FeS2-sulfide. For better control of the composition and particle size, additional experiments with pure pyrite particles in the size range of 32-64 µm were also carried out. The pyrite
4292 Ind. Eng. Chem. Res., Vol. 42, No. 19, 2003 Table 1. Chemical Composition of Shale [% (w/w)] SiO2 Al2O3 Fe2O3 CaO MgO Mn2O3 TiO2 P2O5 K2O Na2O SrO SO3a loss of ignition total
47.20% 15.40% 6.05% 8.81% 2.01% 0.19% 0.88% 0.09% 1.86% 0.78% 0.01% 0.94% )0.38% S 13.90% 98.12%
carbon total sulfur (Leco CS-300) sulfide + sulfiteb monosulfideb (FeS) sulfiteb disulfide (FeS2)
0.93% C 3.58% S 2.73% S 0.00% S
