No abstract

No abstract

No abstract

No abstract

No abstract

The crystalline microstructure of ground sepiolite has been investigated. A reference sample of sepiolite and products of its comminution by dry grinding were studied through X-ray diffraction pattern analysis, specific surface measurements by nitrogen adsorption and complementary analysis of field emission scanning electron microscope images. A statistical model of polycrystals was applied to describe and determine the crystalline microstructure of the studied specimens. The model parameters characterizing the microstructure were prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice strain distribution, and they were determined for each sample by modelling a selected part of the X-ray diffraction pattern and fitting the simulated pattern to a measured one. A strict correlation of microstructure parameters with grinding time and with specific surface magnitudes was observed. A parallelepiped with edge-length ratios almost independent of grinding time (for longer times) was found to be the predominant crystallite shape. The crystallite size distributions were found to be close to logarithmic normal ones, with the mean values decreasing with increasing grinding time and the standard-deviation-to-mean-value ratios approximately constant. The second-order crystalline lattice strain distributions were found to be close to some simple function with the mean value equal to zero, the mean deviation increasing with increasing grinding time and the standard-to-mean-deviation ratios approximately constant. It was demonstrated that the specific surface can be calculated on the basis of the microstructure characteristics. Some details of the relation between crystallites and crystalline grains were explained by comparing the results of analyses via X-ray diffraction and scanning electron microscopy.

No abstract

This article addresses the kinetics of the dissolution of olivine for nano-silica production at extreme conditions. The extreme conditions are pH values between -0.7 and 1, temperature between 50 and 90 °C, solid content around 250 g/l and percentage dissolved between 80 and 99%. This work is structured in 3 parts: 1) chemical and mineralogical characterization of the dunites employed; 2) mechanism of the olivine dissolution focusing on the possible resistances to the transport; and 3) determination of the kinetic parameters k T and n.The results shown here demonstrate that: 1) the limiting step of this process is not the diffusion through a silica layer but the surface reaction; and 2) the dissolution of olivine under the olivine nano-silica production conditions is well described by:where r is the dissolution rate (mol⋅ cm -2 ⋅s -1 ), A the pre-exponential factor (mol⋅cm -2 ⋅s -1 ), E a the activation energy (kJ/mol), R the gas constant (8.314⋅10 -3 kJ⋅mol -1 ⋅K -1 ), T the temperature (K), a H + the hydrogen ion activity (mol/L) and n the reaction order. The average error of the reaction rate calculated using these parameters is 5.5% for dunite CRS-US. In addition, this model is successfully applied to the dissolution of other commercial dunites and for bigger reactor volumes. Therefore, this model can be considered to be robust, and it can be used in the industrial production of olivine nano-silica.