Metacaulim: investigação da reatividade e da cinética de dissolução do alumínio em ácido sulfúrico

Abstract

A research about reactivity and dissolution kinetics of metakaolin was carried out using kaolin for paper coating derived from the Rio Capim region (Pará State, Northern Brazil). The kaolin was calcined at 600, 700, 800, 900 and 1000ºC during 2 hours in order to obtain metakaolin, followed by leaching with sulfuric acid at constant temperatures within the range of 50-95ºC ± 3ºC. In the leachings, amounts of sulfuric acid corresponding to 10% above the stoichiometric values were used with respect to aluminum present in the material. Samples were collected in predetermined intervals every 15 minutes until a total time of 3 hours, which were subjected to aluminum analysis by the EDTA titrimetric method. The kaolin and products of calcinations and leachings were characterized by XRD, XRF, DTA-TG, SEM, BET analysis as well as granulometric and real density analysis. The results showed that the initial material was a mesoporous solid, in which the calcination process at 700ºC increased the number of mesopores in the material and sulfuric leaching process at 95ºC promoted the formation of a microporous solid with a high specific area (297.13 m²/g) and consisting essentially of amorphous silica. The calcination at temperatures above 800ºC indicated the beginning of the structure consolidation process. The reactivity of metakaolin was reduced with the increase of calcination temperatures in the range studied. The dissolution kinetics of aluminum in the metakaolin, obtained by calcination at 700ºC, followed the methods of homogeneous (Integral Method, Method of Half-Lives and Method of Initial Rates) and heterogeneous (Shrinking Core Model) reaction used. 96.85 kJ/mol of activation energy was found as well as leaching reaction of the first order with respect to aluminum from metakaolin and the first order with respect to acid concentration. Researches carried out previously, using one of the reagents with excess above 90%, are consistent with the data found in this study developed with only 10% excess.