Kinetics of the leaching of alumina-containing slag for alumina recovery
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The leaching kinetics of an alumina-containing slag produced from the smelting-reduction of low-grade bauxite, which contained a significant amount of iron and low levels of SiO2 and TiO2, is studied. The leaching rates were evaluated by sampling the pregnant liquid solution at different periods using Inductively Coupled Plasma-High Resolution-Mass Spectrometer (ICP-HR-MS). In addition, the effect of temperature to the leaching extent was also studied at several temperatures. The leaching experiments were performed in a mixing reactor that was heated inside of an oven at atmospheric pressure. The conditions for the solvent concentration, liquid to solid ratio, and mixing speed were kept at 60 g/L Na2O, 20, and 50 swings per minute, respectively. The leaching mechanism was studied by investigating the cross-section of the slag particles after the leaching with Scanning Electron Microscope (SEM). It was found that the particles in residue have agglomeration tendency during the leaching. Moreover, the CaCO3 and Ca3Al2(OH)12 produced from the leaching reactions may create a passive layer on the surface of the slag so that it makes the attack of fluid reactant to the unreacted slag become more delicate. It is shown that the slag leaching reactions could be defined as a solid layer diffusion rate limiting, as a good correlation between the fraction conversion and time and dependent on the temperature was observed. Although the solid-fluid reaction in the system is complex, it is indicated that both ash diffusion and “Ginstling and Brounshtein” models for shrinking core model show good correlation with results.