Reactivity as one of the criteria for the selection of raw materials for the production of active fine mineral powders

Introduction. The simplest and most common method of obtaining fine mineral powder is mechanical dispersion. In the process of grinding the material went through some work (energy), which is spent on the formation of a new surface. Therefore, it is not always clear whether the destruction of the crystal lattice of a solid lead to the activation of the resulting crushed material. Thus, the key characteristics of finely dispersed mineral powders, by which activation processes can be judged, are the value of the specific surface area (Ssp) and the specific free surface energy, numerically equal to the surface tension (σ) of the solid. Therefore, a number of models based on the determination of these characteristics have been proposed to estimate the mechanical activation process of the raw material. So, one of the most correct methods, in our opinion, is the method based on an energy approach to the evaluation of mechanical activation processes. This model determines the relative change in the free surface energy (ΔES/ES0) of the material when obtaining a dispersed system. At the same time, the choice of the most effective raw materials for obtaining composite binders can be carried out on the basis of the surface activity criterion (ks), which is used as a criterion characterizing the reactivity of fine mineral powders after their mechanical disintegration. Therefore, the aim of this study was to calculate the relative change in the surface energy of fine mineral powders of various raw materials and to identify possible functional relationship between the parameter ΔES/ES0 and the amount of surface activity for the studied rock systems. Methods and Materials. Sedimentary rocks formations the Arkhangelsk region were selected as materials for the research: polymineral sand and saponite-containing material (a representative of bentonite clays). Before conducting experiments, rock samples were brought to a constant mass at a temperature of 105оС. The chemical composition of the samples was determined on an X-ray fluorescence analyzer “Metexpert”. Highly dispersed rock fractions were obtained by dry grinding on a planetary ball mill Retsch PM100. The dimensional characteristics were determined using the DelsaNano submicron particle size analyzer by photon correlation spectroscopy. The specific surface area was determined by the gas permeability method on the PSX-10 device. To calculate the surface tension, the edge angle was measured on the “Easy Drop” installation. The surface tension for highly dispersed samples was calculated by the OWRK method. Results and Discussion. The calculated macro-energy characteristics of the studied samples have showed that the atomization energy for polymineral sand was 1910.72 kJ/mol, and for saponite–containing material was 1826.94 kJ/mol. At the same time, the mass specific atomization energy for sand and SCM are 30.41●103 kJ/kg and 26.94●103 kJ/kg, respectively. In the process of dispersion, several fractions of highly dispersed rock powders were obtained, which are characterized by an average particle size and specific surface area. The surface tension (and its components) calculated by the OWRK method showed that for all the samples studied, the polarization effect (σS P) prevails over the dispersion interaction (σS D). At the same time, as Ssp increases, the numerical value of the ratio σS P/σS D increases. This indicates an increase in the number of active surface centers associated with the redistribution of the energy potential of the system. The calculated values of free surface energy (ЕS), surface activity (ks) and the relative change in free surface energy showed, that ks and ΔES/ES0 increase as the duration of powder dispersion increases.The obtained functional dependences ks = f(ΔES/ES0) for the studied samples of polymineral sand and saponitecontaining material are linear in nature and obey the general equation у = a●x + b. In this case, the coefficient “a” characterizes the dynamics of changes in the reactivity of the material with an increase in the duration of mechanical grinding, and the parameter “b” – reactivity in the microstate. Comparison of the coefficients “a” of the studied dispersed systems showed that, unlike polymineral sand, the reactivity of saponitecontaining material increases 1.5 times faster as the grinding duration increases. The obtained functional relationship between the criteria used for evaluating the process of mechanical activation of mineral raw materials of various natures shows the correctness of the models used. Conclusion. The calculated macro-energy characteristics of rock samples showed that the atomization energy for polymineral sand and saponitecontaining material has similar values. To assess the effectiveness of the process of mechanical activation for mineral raw materials, it is proposed to use the relative change in free surface energy or surface activity.