The thermal behavior of mechanically activated galena by thermogravimetry analysis
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INTRODUCTION
GALENA is a typical semiconductor, with properties dependent on the number and type of defects present.[1] One of the methods of influencing the defect concentration of a mineral consists of activation of the mineral by irradiation with gamma rays.[2] Another way is mechanical grinding: a process known as “mechanical activation” in specially designed equipment, such as a planetary ball mill. Recently, this method has successfully been applied to improve oxidative leaching of chalcopyrite and sphalerite, etc.[3–6] Since chemical handling can obscure the true nature of the initial products, the best way to study mechanochemical transformation by mechanical activation for ores is to analyze in situ the milled mixture using appropriate spectroscopic methods. The most common tools are infrared and X-ray diffraction (XRD) analysis techniques, which normally allow the identification of the products.[7,8] Other techniques like high-resolution electron microscopy, extensive X-ray fine-absorbent microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray cyclotron resonance, etc., have been used to study the new surfaces of the ground products.[9] For example, P. Baláz˘ et al.[10] investigated the changes produced in cinnabar by its mechanical activation in a planetary mill through XPS, the Brunauer–Emmett–Teller method, XRD analysis, and differential scanning calorimetry (DSC). The DSC curves of mechanically activated cinnabars and nonactivated cinnabar represent an association of endothermic effects, which differ from each other in shape and value of the extreme temperatures. P. Balá˘z and Brian˘cin[11] also found that the disordering in the structure of galena HUIPING HU, Associate Professor, QIYUAN CHEN, ZHOULAN YIN, and PINGMIN ZHANG, Professors, and LUSHENG YE, Undergraduate Student, are with the Institute of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted March 21, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
and sphalerite produced by intensive grinding results in the decrease of experimental activation energy during direct reduction of mechanically activated galena and sphalerite with hydrogen. Except for our previous work,[12,13] few investigations have been carried out on the thermal stability of mechanically activated galenas with TGA. In this article, mechanically activated galenas were obtained after grinding the natural galena for 20, 40, 120, 180, and 260 minutes, respectively. Nonisothermal decomposition processes of these galenas under a dynamic argon atmosphere have been studied using the TGA–differential TGA technique. The changes of the specific granulometric surface area (SG), the structural disorders, and the elemental sulfur contents of different samples were studied. The differences in reactivity between nonactivated and mechanically activated galenas are also discussed.
II. EXPERIMENTAL DETAILS Natural pure hand-sorted galena and sphalerite ore were purchased f
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