Decomposition and Oxidation of Pyrite in a Fixed-Bed Reactor

Abstract

The oxidation of pyrite (FeS 2 ) has been investigated in a fixed-bed laboratory reactor to gain knowledge about the SO 2 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 FeS 2 particles (between 32 and 64 µm). Measurable SO 2 formation started at about 350 °C for the shale and at 400 °C for the FeS 2 particles and increased with temperature for both materials. Experiments showed that the conversion of pyritic sulfide to SO 2 was independent of the inlet SO 2 concentration up to at least 925 ppmv and that the conversion decreased as the O 2 concentration was increased from 5% to 20% (v/v). A shrinkingcore reaction mechanism with FeS 2 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 FeS 2 core to FeS is relatively fast at moderate O 2 levels (e.g., 5%) because it is easy for O 2 and SO 2 to diffuse through the porous product layer of almost non-oxidized FeS. At increased O 2 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 FeS 2 core is inhibited, leading to lower overall conversions.

A Thermodynamic Analysis on the Roasting of Pyrite

“…In an oxidative roasting process, the formation of pyrrhotite likely conforms to a shrinking-core reaction model with pyrite as the core and pyrrhotite as the shell [22]. In addition, the rate of pyrrhotite formation from the pyrite oxidation by O 2 is two orders of magnitude larger than that from the pyrite pyrolysis [22]. This is possibly due to the fact that in an O 2 -containing atmosphere, once the intermediate S 2 makes contact with O 2 , it is easily oxidized as volatile SO 2 , which will rapidly decrease the S 2 concentration in the reaction interface of pyrite and thus improve the formation of pyrrhotite.…”

“…Not surprisingly, with the formation of SO 2 , pyrite is readily oxidized by O 2 to FeS (Equation 4) or Fe 0.875 S (Equation 5). Research also found that the oxidation rate of pyrite core to pyrrhotite (FeS) was relatively fast at moderate oxygen concentration levels (e.g., 5 vol % of O 2 ) [22]. It can thus be considered that, in the presence of O 2 , pyrite firstly undergoes partial desulphurization to produce pyrrhotite and S 2 (Equations (1) and (2)), and then the easy oxidation of S 2 (Equation (3)) occurs with S 2 acting as an intermediate in Equations 4and (5).…”

“…(iii) During the roasting of pyrite particles, the S 2 desorption from the pyrite surface has been suggested to be the rate-controlling step for pyrite pyrolysis [20]. In an oxidative roasting process, the formation of pyrrhotite likely conforms to a shrinking-core reaction model with pyrite as the core and pyrrhotite as the shell [22]. In addition, the rate of pyrrhotite formation from the pyrite oxidation by O 2 is two orders of magnitude larger than that from the pyrite pyrolysis [22].…”

Linking the SO2 emission of cement plants to the sulfur characteristics of their limestones: A study of 80 NSP cement lines in China

“…As a result, the concentration of SO 2 in the precalciner is very low as shown in Fig. 4 (Rasmussen, 2012;Hansen et al, 2003). The sulfur from the raw meal, however, is split, some exist within clinker and the rest in stack emission.…”

“…Thus, the SO 2 emission in stack gas is originated mainly from sulfides in raw meal. Table 1 The SO 2 release and capture in a typical NSP cement line (Gossman, 2011;Horkoss, 2008) The SO 2 concentration of flue gas at different locations of a typical NSP cement line (Hansen et al, 2003) 1.3 The sulfur-containing minerals in fuel and raw materials Sulfur in raw meal and fuel can be classified into organic and inorganic sulfurs, and the later mainly refers to sulfides (such as pyrite) and sulfates (in terms of gypsum). More than 80% sulfur in coal is organic sulfur, and the rest is mainly pyrite (FeS 2 ), gypsum (CaSO 4 ), and few ferric sulfates (Oliveira et al, 2011).…”