ARTICLE pubs.acs.org/IECR
Experimental Studies on Phase Transformation during Pyrite Concentrate Oxidation under Circulating Fluidized Bed (CFB) Roasting Conditions Tao Wang, Hai Zhang,* Qing Liu, and Hairui Yang Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: The phase transformation during pyrite concentrate oxidation under the conditions of the circulating fluidized bed (CFB) roasting process was experimentally studied at temperatures of 600900 °C, O2 concentrations of 1%21%, and particle residence times of ∼5.0 s in a drop tube furnace (DTF). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the phase composition and microstructure of the product samples. For the roasting process with a short residence time, the products of pyrite concentrate oxidation are in a nonequilibrium phase. At higher O2 concentration and temperature, Fe3O4 is the dominant product phase. When O2 concentration increases, the micromorphology of sample products tends to be perfectly spherical particles with smooth surface, difficult for the formed Fe3O4 to be oxidized to Fe2O3. Experimental results indicated that, in the CFB roasting, relatively high temperature and low O2 concentration are needed to control the phase transformation into Fe2O3.
1. INTRODUCTION Pyrite concentrates often are the byproducts in the processes of nonferrous metal production, such as copper, zinc, and lead, and coal treatment.1 These products contain rich pyrite (FeS2) and are the raw materials for sulfuric acid production with the roasting process.2 In order to recycle the pyrite concentrates effectively and efficiently, the iron (Fe) phase in the oxidation products should be well-controlled, and, in many cases, it is required to be in the phase of hematite (Fe2O3) but not Fe3O4 (magnetite).3 To reach such a goal, it is important to understand the phase transformation during the pyrite oxidation process. So far, many studies on the phase transformation during the pyrite oxidation process have been carried out.1,418 As noted by the cited studies, Fe2O3 and Fe3O4 are generally the main final oxidation products, as represented by the following reactions:4,19 2FeS2 þ 5:5O2 f Fe2 O3 þ 4SO2 ðgÞ
ð1Þ
3FeS2 þ 8O2 f Fe3 O4 þ 6SO2 ðgÞ
ð2Þ
However, which phase is dominant—Fe2O3 or Fe3O4—is dependent on the experimental conditions. The conditions include reaction temperature, particle residence time, heating rate, and reactant supply method.1,418 Besides, oxygen (O2) concentration (denoted hereafter as [O2]) was determined to be another important influencing factor, which could strongly couple with other factors. When the reaction temperature is >1000 °C and the particle residence time is on the order of several seconds, and the volumetric O2 concentration is given as [O2] = 2%21%, Fe3O4, instead of Fe2O3, is the dominant phase in the product of pyrite oxidation.1,7,8 For example, when [O2] = 5%, the amount of Fe3O4 increased with a residence time at 1316 °C, and no Fe2O3 was present at 1454 °C as long as the particle r 2011 American Chemical Society
residence time was longer than 0.14 s.1 The experiments that were conducted when [O2] = 3% and at a temperature of ∼1230 °C confirmed that Fe3O4 was the dominant phase, while a small quantity of Fe2O3 was observed.13 However, when the reaction temperature is
