Iranian Journal of Materials Science and Engineering

The effect of mechanical activation on the kinetics of ammoniacal thiosulfate leaching of a refractory oxide gold ore containing 2.8 ppm Au was investigated. The gold extraction of 99.81% was achieved by 16 h leaching of a sample mechanically activated for 60 minutes. The gold extraction observed for a similar reference sample without mechanical activation was only 55%. Studies revealed that leaching progresses at two different rates depending on the leaching time (0-2 h and 2-16 h). It was observed that diffusion through an ash layer as a dominant mechanism controls the leaching of samples mechanically activated up to 45 minutes during total leaching time, while reaction control and liquid film diffusion are dominant mechanisms for leaching of a sample mechanically activated for 60 minutes during 0-2 h and 2-16 h, respectively. The extraction observed during the ash diffusion step depends significantly upon mechanical activation time.  Mechanical activation of 60 minutes results in high gold extraction in this step which when combined with subsequent chemical reaction gives close to 100% gold extraction in a 16 hour leach.  Mechanical activation for up to 45 minutes leads to a modest improvement in overall gold extraction compared with the reference test without mechanical activation

The main problem of cobalt oxide as a thermochemical heat storage material is its slow re-oxidation kinetics. In addition, redox (reduction and oxidation) behavior of as-received Co₃O₄ is degraded with increasing the number of redox cycles. To overcome this drawback, Al₂O₃ and Y₂O₃ were added to Co₃O₄ and  effect of mechanical activation time (2, 4, 8, and 16 h) on the redox behavior (weight change value/rate, redox reversibility, reduction and re-oxidation values, and particle morphologies) of Co₃O₄-5 wt.% Al₂O₃ and Co₃O₄-5 wt. % Y₂O₃ composites was investigated using thermogravimetry method. The composites were studied by SEM, EDS, and X-ray map analyses before and after redox reactions. Results showed that increasing the mechanical activation time improves the redox kinetics of Co₃O₄-5wt. % Al₂O₃ in comparison with as-received Co₃O₄. Although, the alumina-containing samples, activated in short time showed the better redox kinetics than samples activated in long time. It was found that increasing the activation time to more than 8 h for alumina-containing samples reduces the redox kinetics due to decreasing the positive effect of Al₂O₃ in controlling the particle size growth and sintering. In the case of Co₃O₄-5wt. % Y₂O₃, an increase in activation time generally reduced the redox kinetics. As a result, redox reactions in a 16 h-activated Co₃O₄-5wt.% Y₂O₃ composite was completely stopped. In addition, results showed that weak performance of Co₃O₄-5 wt. % Y₂O₃ is related to intensive sintering and growth of cobalt oxide particles during redox reactions