Kinetic model based on initial rates of alkali hydrothermal dissolution of Verdete rock

Abstract

This work aimed to obtain the kinetic parameters of the hydrothermal dissolution of a Brazilian glauconitic siltstone (known as Verdete) with calcium hydroxide. Heat treatments were carried out in stainless steel autoclaves internally coated with Teflon in conventional and rotary (84 rpm) furnaces. The aqueous fraction of hydrothermal products was analyzed by flame photometry and pH-metry. The product was also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). After an initial inertia of 30–45 min, the concentration of OH− ions decreased as K+ ions were extracted from the K-feldspar structure. K recovery was proportional to the Ca(OH)2/Verdete ratio and gradually increased over time, reaching 65% after 24 h. Ca(OH)2 and Verdete agglomerated when high temperatures (453 K) and Ca(OH)2/Verdete ratios were employed simultaneously, causing loss of reaction surface area and interruption of the dissolution process. Agglomerations were avoided when rotating the furnace. However, rotation did not seem to increase the K+ extraction rate, which demonstrated to be proportional to OH− concentration. The dissolution reaction was half-order in respect to OH− concentration, and presented a 26.9 kJ mol−1 activation energy.