| dc.description.abstract | The objective of this work was the study of the sulphuric leaching of roasted zinc concentrate, from Votorantim Metais - Zinco, that involved three great stages: a complete characterization of the raw material before and after the roasting step, leaching kinetic determination of the roasted concentrate in sulphuric acid and the use of the population balance description with the residence time distribution as analysis tools ofthe behaviour of zinc extraction in real reactors. In the characterization stage, the SEM/EDS analysis, with the X-ray diffraction technique, allowed to identify significant presence of remaining sphalerite (ZnS) and the presence of the zinc ferrite (ZnFe2O4) in the roasted concentrate samples, indicating possible problems in the roasting process.The micrographs analyses of the polishing sections of the roasted concentrate corroborate the oxidation mechanism of the iron bearing sphalerite - (Zn, Fe)S proposed by Graydon and Kirk (1988), which is constituted by a stage of iron diffusion into the particles surface, by a stage of simultaneous oxidation of zinc and iron sulphides and bya separation stage of ZnO from ZnFe2O4. The results of the kinetic study had revealed that, even in mild leaching conditions (Cácid = 4 g.L-1 and T = 30ºC), the particles conversion of the roasted concentrate reaches its maximum value among the first five minutes of reaction, indicating a high zinc extraction rate. The kinetic modelling of the of the conversion curves data, using the Shrinking Core and Progressive Conversion models, shows mathematically that both the diffusion in the product layer and thechemical reaction are stages that describe the leaching assays. However, the physical analysis of the process points that the stage of chemical reaction describes better the leaching in question, because does not have the formation of an inert layer of products due to reduction of the particles size in function of time (Shrinking Core Model). Thethermodynamic and kinetic parameters that represent the leaching of the roasted concentrate in sulphuric acid have values of the order of: 13,45 kJ.mol-1 for the activation energy, 11,07 kJ.mol-1 e -256,20 J.mol-1.K-1 for the apparent enthalpy and entropy, respectively, and 3,00 x 10-2 cm.s-1 for the chemical reaction constant rate. The roasted concentrate is described by RRB model (Rosin-Rammler-Bennet) of size distribution. The results of this description with the population balance equations andwith the residence time distribution show that the adimensional time to reach the maximum conversion of particles is equal the 6 and based in this time three continuous reactors of mixture (CSTR) are necessary to reach the same level of conversion of a plug flow reactor. | |
