what is boehmite
γ-Alooh was first discovered in 1925 by the German chemist Johann Bohm (1895-1952), who was thought to be a major component of bauxite. This was confirmed in 1927 by DE La Parante's analysis of bauxite mines in Lebo, Provence, and the ore formed from -alooh was named boehmite.
Effect of temperature on the structure of hydrolyzed products and Boehmite.
A series of sols with different rheological properties and physicochemical properties were obtained by changing the hydrolysis temperature without changing other preparation conditions. The hydrolysis temperature was lower than 70℃, and -Al(OH)3 appeared in the product, which could not be dissolved by gel solvent. Although sols can form at 75℃, their stability is poor. The sol formed at the hydrolysis temperature of 85 ~ 95℃ is clear, transparent and stable. After 20 ~ 95℃ water heat treatment, if pH<9, high dispersion, transparent sol products can be obtained. The reaction temperature increased from 45℃ to 90℃, and the grain size increased from 2.6nm to 4.2nm, nearly 1 times. After crystallization at 200℃, the grain size was 28.3nm, 10.9 times as much as that at 45℃ and 6.7 times as much as that at 90℃. Hydrothermal synthesis provides a high temperature and high pressure liquid environment in which the solubility of particles increases, thus promoting continuous dissolution and crystallization, grain size increase, and crystallinity increase.
With the growth of grains and the improvement of crystallinity, the specific surface of particles decreases rapidly, from 644m2/g at 45℃ to 391m2/g at 90℃. After being crystallized at 200℃, the specific surface of particles decreases significantly. The continuous dissolution of small particles and the continuous growth of large particles are the main reasons for the surface drop. With the increase of the reaction temperature, the pore volume decreases gradually. After crystallization at 200℃, the pore volume decreases significantly, and the average pore diameter increases significantly.
Effect of water heat treatment time on the structure of Boehmite
With the extension of crystallization time, the specific surface decreases continuously, and the pore volume gradually decreases. The crystallization time varies greatly from 6h to 12h, indicating that there is a mutation during this period, and the pore volume changes after 12h are not obvious. The average aperture first rose and then fell. After 12h reaction, the average aperture did not change significantly. The pore diameter distribution curve of the products with different crystallization time has a small peak at about 2.7nm. In addition, there was a peak at 18nm, 20nm, 30nm and 30nm at 3h, 6h, 12h and 24h, respectively. In contrast, 6h, 12h and 24h apertures are more unevenly distributed.
In the early stage of crystallization, the system is in the state of supersaturation, crystal growth plays a leading role, and the boomerite grains gradually increase until the crystal dissolution and growth process reach dynamic equilibrium. At this time, the so-called aging process occurs, "The solubility of small grains is large, but the change is small; The solubility of large grain is small, but the change is great. Therefore, too long crystallization time is not conducive to the preparation of uniformly dispersed boehmite powder.
Effect of water addition on reaction process
The amount of water has an important effect on hydrolysis and heat treatment of water. The larger the amount of water added, the smaller the particles and the higher the dispersion index of the hydrolyzed products. Water heat treatment process is to use the pressure of aqueous solution to promote the formation of crystal nucleus and grain growth, the higher the filling degree, the more conducive to the formation of uniform stable sol.
Summary
With the increase of hydrolysis temperature, the crystallinity of the product increased obviously, and the specific surface and pore volume decreased significantly. Controlling a certain pH value, increasing the process of water heat treatment and increasing the amount of water added can obviously improve the dispersibility of Boomerstone sol. Prolonged hydrothermal time will result in the decrease of product ratio surface and pore volume. The results showed that the optimal hydrolysis temperature was 85 ~ 90℃, the heat treatment temperature was 200℃, and the treatment time was 3h.
