Elucidation of the Nature of Structural Heterogeneity During Alkali Leaching of Non-activated and Mechanically Activated
- PDF / 3,134,587 Bytes
- 18 Pages / 593.972 x 792 pts Page_size
- 47 Downloads / 240 Views
s an aluminum oxyhydroxide (cAlOOH) (orthorhombic; a = 0.36936 nm, b = 1.2214 nm, c = 0.28679 nm; space group Cmcm) consisting of double layer of AlO4(OH)2 octahedra which are linked by H-bond between hydroxyl groups.[1,2] Alkali leaching of boehmite is of interest in the Bayer process of alumina production[3] and processing of nuclear wastes.[4] Several investigations have focused on alkali leaching of boehmite.[2–19] The kinetics of alkali leaching is described in terms of ‘shrinking core model with surface reaction’[4,13,19] and ‘first-order chemical reaction.’[15–17] Most studies have focused on the effect of alkali concentration and temperature on boehmite solubility.[6–14] Widely different solubility of boehmite is reported in literature. Gre´nman et al.[14] observed only 4 pct dissolution after 3 hours of leaching in 6 M NaOH solution at 358 K (85 C). High solubility of boehmite (up to 60 pct) has been reported for nano-size (0.07 to 0.1 lm) boehmite by Packter[9] and high surface area RAKESH KUMAR, Chief Scientist and Head of Division, and THOMAS C. ALEX, Principal Scientist, are with the Metal Extraction and Forming Division, National Metallurgical Laboratory, Jamshedpur 831 007, India. Contact e-mail: [email protected] Manuscript submitted August 13, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
(264 m2/g) boehmite by Alex et al.[18,19] under ambient pressure leaching. In spite of different reported solubilities, the effect of nature of solid phase, e.g., purity (chemical and structural), specific surface area, porosity, crystallite size, etc.,[20,21] have not been adequately addressed. Mechanical activation, the term introduced by Sme´kal[23, p. 7572] to describe change in reaction ability of solids due to physico-chemical changes during milling, is used to enhance the leachability of Al-oxyhydroxide minerals, including boehmite.[2,18,22–31] After 120 minutes of mechanical activation in a planetary mill, an increase in leachability from 58 to 88 pct and 12 to 92 pct is observed during alkali leaching (temperature—363 K (90 C), alkali concentration—180 g/l Na2O, leaching time—30 minutes) for boehmite prepared by thermal and hydrothermal transformation of gibbsite, respectively.[18,30] Kinetics of alkali leaching of mechanically activated boehmite has been explained in terms of ‘second-order reaction model.’[31] The nature of solid phase may change continuously during leaching. For example, higher solubility of edge, corner, and grain boundary regions vis-a`-vis crystal faces is observed during initial stage of leaching of phase pure boehmite.[19] Structural heterogeneity is an essential feature of mechanically activated solids.[32–35] The nature of solid changes continuously during chemical
reactions as high energy regions (i.e., regions of solid phase characterized by greater structural disorder) are preferentially attacked.[34,35] Alex et al.[19,31] have used model-fitting methods to analyze the kinetics of boehmite leaching. The model-fitting methods are inadequate to provide a complete and reali
Data Loading...
