Kinetics of phase transitions of gibbsite during heat treatment in air and in water vapor

Abstract

Results are provided for a study of the kinetics and mechanism of phase transitions (PT) of gibbsite during heat treatment in air and in water vapor (in the range 170 -550°C) by iso-and non-isothermal thermogravimetry, x-ray-phase analysis (XPA), transmission electron microscopy (TEM), nuclear magnetic resonance in 27 Al nuclei (NMR 27 Al), and chemical analysis. It is shown that PT of gibbsite in air there are two interconnected transitions that occur in parallel: gibbsite ® boehmite (g-AlOOH) and gibbsite ® x-ray amorphous Al 2 O 3 that are realized by a topochemical mechanism. In water vapor gibbsite only experiences transition to boehmite (PT-3) that occurs by a dissolution -precipitation mechanism. Apparent activation energies for the PT that occur are determined.Finely crystalline boehmite (g-AlOOH) is used extensively for preparing refractories, absorbents, catalysts, abrasive micropowders, low porosity ceramics, etc. It forms during phase transitions (PT) of gibbsite (g-Al(OH) 3 ) during heat treatment in air or within a closed volume, including under hydrothermal conditions [1]. Published data for PT of gibbsite are incomplete and mainly contradictory. For example according to [1] boehmite forms during calcination of gibbsite in air at 180°C, but in publication [2]it appears that in parallel with boehmite there is formation of an x-ray amorphous aluminum oxide. In [3] single-phase specimens of g-AlOOH were only obtained with heat treatment of gibbsite in the range 200 -375°C within a closed volume, i.e. under conditions of increased partial pressure of water vapor released during dehydration of gibbsite. Until now there is also a question about the mechanics and limiting stages of chemical transitions of g-Al(OH) 3 . The aim of this work is to study the kinetics and clarify the mechanism of PT for gibbsite during calcination in air in water vapor. EXPERIMENTAL PROCEDUREAn original specimen of gibbsite was prepared from technical grade aluminum hydroxide (AH) of the following composition, wt.%: Al 2 O 3 64.2±0.5, H 2 O 35.5±0.5, Me 2 O (Me + -K + , Na + ) 0.4±0.1. For this AH was subjected to wet screening on a screen No. 0045, carefully washed from alkali metal hydroxides, first with a 0.1-normal solution of sulphuric acid, and then with distilled water, dried at 110°C to a constant weight and ground. According to XPA, TEM and chemical analysis an air dried specimen of gibbsite was single-phase, i.e. g-Al(OH) 3 , and it consisted of flat crystals of irregular shape with sizes from 5 to 30 mm; its composition Al 2 O 3 ·3.05H 2 O is close to stoichiometric.Heat treatment of gibbsite in air was carried out in alundum crucibles, and in water vapor, i.e. in autoclaves with a volume of 3 cm 3 . In order to study PT kinetics a heating unit was created making it possible to exclude to a maximum the effect of heating kinetics and a temperature gradient in the furnace on the PT of gibbsite. A holder of twelve crucibles (autoclaves) was first placed in the first furnace in which for time t 1 , not exceeding 15 min,...

Thermally Activated Al(OH)3 Part II—Effect of Different Thermal Treatments

“…In contrast, the GDAH-05 sample had a narrow particle size distribution when compared with the other products, with the median diameter D50 sharply reduced from around 70 µm before calcination to around 55 µm after calcination at 400 °C. The same phenomenon was identified also for the other characteristic diameters D10 and D90, in accordance with similar studies [64]. The thermal treatment triggered therefore a decrease in both the coarse and fine fractions of samples containing mostly large particles.…”

“…Literature studies consider the loss in crystallinity to be a result of the porous surface layer that forms when gibbsite is calcined in air. In these cases, the pore diameters shift with the temperature from 5 (170 • C) to 20 (500 • C) nm [64]. Moreover, from Table 2 it can be seen that up to 400 • C, neither of the two phase transitions mentioned above are fully finished, in accordance with the purpose of the experiment-to promote amorphous phase growth.…”

An Unreacted Shrinking Core Model for Calcination and Similar Solid-to-Gas Reactions

“…Candela and Perlmutter (1986) reported E = 260±20 kJ/mol as the activation energy for gibbsite decomposition, and this is reasonably consistent with the value presented by Stacey (1987), 272±12 kJ/mol. The E value estimated by Lopushan et al (2007) was 150±12 kJ/mol, obviously not in agreement with either previous study. These studies were mainly conducted at low temperatures.…”

“…The performance of the proposed model was examined against experimental gibbsite conversion versus time data reported by Lopushan et al (2007) and Wang et al (2006) over a wide range of temperatures. Since there is uncertainty over the dependency of the reaction rate on water vapour pressure and temperature as outlined in the previous section, we adopted a parameter estimation approach and explored two cases.…”

“…530K) for two reaction orders, n, as well as the experimental data published by Lopushan et al (2007). This temperature range is primarily of interest in experimental studies into kinetics as the reaction times are relatively long, simplifying measurement; industrialscale alumina production is always operated at higher temperatures.…”