MIXING REMOVED COMPONENTS OF MIXTURE
For most technologies, mixing of detached materials is the main operation, which predetermines the quality of the mixture (mass) and the finished molded products. In the mixing apparatus, especially in the production of non-combustible conglomerates, the basic processes of the structure formation of the binding part, in particular, microlayers, arise, and sometimes almost completely complete. But it is possible that mixing is just a usual kind of preparatory work, for example, when making a charge, then heating it to a melt during the production of firing products.
The most widely used method of mixing with the introduction of mechanical energy from the external source, and among the types of mixers - rotary forced action. Mechanical mixing is carried out in two stages: 1) preliminary mixing of dry components; 2) mixing with a liquid accepted as an obligatory component of the manufactured mixture (mass) of non-burning ISK or as arising from low-melting substances in the preparation of firing ISK.
Often the mixing of the detergent components and additives to them is carried out without the preliminary preparation of a dry mixture, i.e. in one stage. With the mixing of dry materials (cold, heated or hot), spontaneous equalization of temperatures occurs with the transition of heat from bodies with a higher temperature to bodies with a lower temperature until the temperature is evenly equalized and to the maximum value of the total entropy of interacting bodies. In this case, the initial bonds between the particles are destroyed, their mobility is ensured, the particles are evenly distributed in the total mixture with the filling of intergranular pores with finer fractions of the filling material. Powder-like materials fill the fine pores of the granular part of the mixture, and some of the finest particles of the powder are mechanically retained or deposited and fixed on the surface of the grains of large material.
When a liquid component is introduced into the mixture, the further process of mechanical mixing is based on the regularity of the flow of dispersed medium (liquid) flowing solid particles of a dispersed phase. Laminar flows or turbulent vortices arise in the dependence of on of the particle velocity of the phase in the medium. In the latter case, the boundary layers of the medium are separated from the surface of the solid particles. In the laminar regime (Reynolds' criterion Re & gt; 30), only the layers that are directly adjacent to the blades and participate in rotation together with them are mixed. Under turbulent conditions (Reynolds' test Re & gt; 10), more intensive mixing of the liquid layers occurs with their separation from the agitator blades. With highly developed turbulence (Reynolds Re & gt; 10), often the cost of additional power to increase the rotational speed of the agitator shaft does not correspond to the resulting stirring effect, for example, in terms of heat and mass transfer coefficients.
The more accurate and rational limits of the Reynolds criteria are due to the design of the mixing apparatuses, their geometric dimensions, but the limit of the effectiveness of the turbulent regime is practically available in all apparatuses. In particular, with turbulence Re & gt; 10, it is possible to achieve a greater mixing and activation effect when using gas, air, steam or steam-air mixture, electric field, gravitational factor, etc. as energy carriers. For this purpose, continuous rotary mixers, ionic device or without it, etc. In some mixers it is ensured not only the rapid production of a homogeneous mixture, but also its heating, which, if necessary, allows the hot forming of the product without special heat treatment at subsequent technological stages.
With mixing in one stage, the liquid medium is fed into the mixing apparatus simultaneously with the solid components of the mixture. The surface of the solid particles is wetted, and the temperature of the mixture is equalized throughout the volume, since the components were received at a different temperature, in addition, and the wetting process is exothermic. The amount of heat generated by wetting can be measured by microcalorimeters or other similar devices in the laboratory. It characterizes the degree of intensity of interaction of components. If, for example, negative hydroxyl ions prevail in the surface layer, then the water wettability is complete, the amount of heat liberated is large, and the surface of the particles is referred to as hydrophilic.
If positive ions of heavy metals prevail, then high wettability is ensured by contact with the oil, and then the surface of the solid particles is referred to as oleophilic. When fully wetted in other liquids, the particles are characterized as lyophilic , regardless of the variety of liquid medium, complete wetting indicates the ability of solids to solubility in it with the formation of true (molecular) solutions as homogeneous (homogeneous) systems. Consequently, lyophilicity is associated with a small interfacial tension, the stability of surfaces to mutual adherence and solubility.
A more characteristic process with mixing of components is the formation of a heterogeneous system that differs from homogeneous (homogeneous) by the presence of two or more phases contacting each other along the interfaces.
Many solids, as components of the common mixture, have a surface with a different combination of positive and negative ions on it. Therefore, they are wetted by both water and oil, although their wettability is worse than hydrophilic or oleophilic.
The nature of the surface of solids can be significantly altered by the addition of surfactants, when, for example, the hydrophilic surface can become hydrophobic (hydrophobizing process) or hydrophobic surface - hydrophilic (hydrophilic process). The surfactants introduced into the liquid medium are widely used to increase the wettability of the surface of solids, which is based on the decrease in the polarity difference between the surface of solid particles and the liquid. A peculiar process of equalizing the compositions of the surface and inner layers occurs in the mixing apparatus. The speed of this alignment depends on the intensity of mixing, mixer design and other factors. When mechanical stirring is not applied or it was switched off prematurely, the equalization process occurs relatively slowly due to diffusion, a difference in the density of substances or under the influence of thermal convection currents.The most important law of heterogeneous equilibrium, discovered by Gibbs, is the phase rule: F = (К-С) +2, where F is the number of phases of the system; C is the number of degrees of freedom, i.e. the greatest number of conditions (temperature, pressure, concentration of matter) that can be changed without disturbing the equilibrium of the system; it is always greater or equal to zero, so Φ = + + 2, where - is the number of components or chemically individual parts of the system.
With increasing intensity of forced mixing, the thickness of the diffusion layer decreases, within which processes of spontaneous leveling of the concentrations occur, the formation of the heterogeneous system as a whole accelerates. This rate increases with the continuous renewal of the contact surface and with the surface of the solid or liquid phase increasing, for example, with the countercurrent mixing, moreover, often combined with additional dispersion of the solid component (for example, with mixing on runners, disintegrators and some other apparatus) or fluidized layer (the principle of pseudo-cupping).The heterogeneous process is often accompanied by the emergence and accumulation of a new phase in the mixture as a result of the release of the dissolved substance from the supersaturated solution (germinal crystals), the flow of chemical, in particular topochemical, reactions with the formation of the corresponding compounds (neoplasms), the formation of gas or vapor bubbles, and The greatest number of tumors occurs under the influence of catalysts introduced into the mixture. To slow the reaction, non-positive catalysts that accelerate the reaction are used, and negative ones (inhibitors).
Among the heterogeneous processes, physical (sorption) absorption (adsorption and absorption) and chemistry-chemisorption are of considerable importance in the structuring of conglomerates. From the environment are sorbed (absorbed) those substances that are able to reduce the surface energy, which corresponds to the so-called adsorption. An inverse process is also possible-an increase in energy, for example, surface tension, due to partial desorption, which means negative adsorption. An increase in temperature and a decrease in pressure, as well as a decrease in the concentration of the adsorbed substance (adsorbate), contribute to the desorption of the ingredient previously physically absorbed by the adsorbent.
'Structurization with the participation of surface-active substances (surfactants) occurs with the preliminary formation of mono- and polymolecular layers on the surface. These layers have an increased density, and their properties sharply differ (sometimes they become directly opposite) from the properties of the substance of the inner part of the body. On the surface of the adsorbent a field of sorption forces arises - electrostatic and electrokinetic, actively participating in the structure formation. The strength of the fixation of the adsorption layer is due to the amount of surface energy, the nature of the adsorbent, but not to the size of its surface. The last
predetermines the amount of substance adsorbed from the solution. But the surface may not be completely covered by the adsorption layer. The degree of saturation with its adsorbate at a given temperature T depends on the concentration of the adsorbed substance in the surrounding medium. With an increase in the C concentration (Figure 2.5), the amount of the adsorbed substance increases. The dependence Γ = Δc) is called the adsorption isotherm and is expressed by the analytical Langmuir formula
Fig. 2.5. Isotherm of adsorption Г (с) and surface tension а (с) at temperature T = const
where I "- the amount of adsorbed matter at the maximum saturation; 1/a is a constant value characterizing the adsorption activity of the adsorbent; has the same dimension as 1/C. It is proportional to the initial surface activity go for C = 0. The higher this value, the higher the adsorption activity, the steeper the rise of the adsorption isotherm at a given temperature. The value of & pound; o = ~ & lt; 3st/bC, i.e. the substance lowers the surface tension. Then the adsorption is positive (T & gt; 0) and such substances are called surface active. These include many organic substances dissolved in water. In the opposite case, the adsorption is negative: G & lt; 0, and the substance in the solution is surface-inactive, it increases the surface tension.Adsorption layers of various substances contribute to stability (stabilization) of the system the more, the closer adsorption is to its ultimate saturation (G ),
The empirical equation of adsorption from solutions was obtained by Freindlich: a = kt/c, where a is the amount of adsorbed matter; C is the concentration of the dissolved substance in the solution; k and n are empirical parameters that are constant for given adsorbent and solute at a given temperature. In the case of gas adsorption, instead of the concentration C, the gas pressure P is introduced into the formula, and the constant k is the maximum amount of gas that can be adsorbed by the given amount of adsorbent.
With increasing molecular weight of the adsorbent, its ability to physical adsorption increases. From solutions, substances with a lower solubility in a given medium are better adsorbed. As a rule, the value of adsorption is expressed in micromoles per square meter, and in porous bodies - in micromoles per unit mass (kilogram). The adsorption process is favored by a decrease in temperature. However, adsorption can be significant at high temperatures and is enhanced with further increase. In this case, adsorption is called activated ; it is associated with the occurrence of chemical reactions (chemisorption), an increase in the concentration of reacting molecules.
Adsorption from complex molecular solutions depends on the rate of diffusion or on the time of arrival to the adsorbent surface of free molecules of a surfactant under the influence of diffusion. But the diffusion rate decreases as the diffusion particles become larger, therefore, at first, the solvent molecules may be displaced from the adsorbent surface by a less active substance, and only at a later stage of internal processes the secondary displacement of weakly oriented molecules by a stronger surface-active substance occurs. >
Molecular diffusion is described by Fick's first law: the amount of matter dM, diffusing through the surface F (normal to the diffusion direction), proportionally the concentration gradient dCIdh of this substance:
where D is the diffusion coefficient, which shows the amount of a substance diffused per unit time (1 s) through a surface unit (1 cm) at a concentration gradient of 1
The diffusion rate increases with increasing temperature (by about 1-3% per 1 ° C), especially if the viscosity of the medium is lowered and the particle size r D-RTIN l6nr , where N is the universal constant (Avogadro's number), equal to 6.02 • 10.
Along with the absorption of neutral molecules and, consequently, reversible molecular adsorption, ion adsorption can occur, which is usually accompanied by a phenomenon of ion exchange between the adsorbent and the dissolved substance. Essentially, a chemical (exchange) reaction occurs with the formation of a new compound in the surface layer, and the surface of the particles of the solid is covered with a layer of the reaction product. On the surface of a solid, almost always one of the ions is adsorbed predominantly, t.s. selectively.
Ions selectively adsorbed, which have a common atomic grouping with a given solid surface, but the opposite charge sign. The new compound formed as a result of such chemisorption on the surface has a reduced solubility or complete insolubility in the medium to be contacted. This means that complete adsorption is in most cases irreversible, dilution of the solution does not cause desorption. At the same time, it gives the surface a different nature and properties, while the composition and properties of the inner layers of the solid phase do not change.
The formation of adsorption layers around the particles of the dispersed phase is directly related to the stability of the heterogeneous, or more precisely, micro-sterogenous system. Adsorption layers (films) not only lower (as noted above) the surface tension at the interfaces, but also exhibit considerable tensile strength. Sometimes called solvate shells, they have an increased elasticity, which contributes to the protection of dispersed particles from adhesion or adhesion.
In an increase in the aggregative stability of a disperse system, the electric charges that are redistributed at the phase interface with the formation of a double electric layer are also significant. A definite potential difference is established between the phases, which can change under the influence of selective adsorption of ions from the solution. Within the thickness of the double layer, the concentration of excess ions gradually decreases in the direction from the dense layer at the surface of the solid (particles of the dispersed phase) to the equilibrium concentration in the free part of the medium. Such a decrease in the density or concentration of the electric potential, as well as a similar decrease in the adsorption forces in the molecular adsorption of a surfactant, characterizes the diffuse structure. Within the diffuse layer many properties (for example, strength, viscosity, etc.) can vary in significant sizes (Figure 2.6). In the mixing apparatus, with mixing of the mixture, less frequent processes and phenomena are possible.
Fig. 2.6. Diffuse elephant in the boundary zone AB: a - double electric layer; o is the adsorption-solvate layer; in - ion-adsorption layer; r is the change in mechanical strength (viscosity, shear stress, etc.) in the diffusion layer; 1 is the conditional thickness of the diffusion layer; 2 - the logarithm of the viscosity (strength) of a bulk (free) medium; 3 is the logarithm of viscosity, strength, and so on. in the wall (dense) layer; 4 is the viscosity change curve within the diffusion layer of the medium
Thus, spontaneous condensation of the gel-like part of the binder can occur with the absorption of the syneresis medium by a porous and freeze-dried aggregate. Possible phenomena of contamination are the contamination of the mixture accidentally or deliberately left foreign impurities in the original prepared component materials, for example, the presence of clay, organic or other impurities in construction sand in the manufacture of concrete. Thixotropy phenomena (restoration of the destroyed structure) are also possible, if during the mixing there is a forced stop of the mixing device with the subsequent resumption of interrupted mixing. These and other similarly small-scale phenomena affect the structure formation and, in particular, the formation of the microsloss structure around the particles and grains of the filling material, the structure of the binder, which is distributed in the intergranular pores and voids.
Under the influence of a complex complex of physicochemical processes and chemical reactions that occur during mixing in mixing apparatus, the prepared and detached raw materials-components lose their (individually or irreversibly) their individual properties, especially in the surface layers, that is, along the boundaries contacting components and neoplasms. At the time of the exit from the mixing device, microstructural processes in some systems are basically completed, in others these processes in mixers are just beginning, but do not reach avalanche development, and therefore with greater or lesser intensity continue at the subsequent stages of the technological cycle. The effectiveness of mixing is often judged by qualitative changes in the main (key) initial components, or by the quantitative yield of neoplastic products, by the strength of the ICS formed from the mixture. However, the increase in strength is often associated with the effect of mechanochemical activation under the influence of additional particle shredding during their mutual collision or impacts on the blade and wall of the mixing apparatus.
The criterion for assessing the quality of a mixture is its homogeneity, determined by statistical methods - by dispersion, root-mean-square deviation, various coefficients. Usually, the calculations are performed on a smaller component, called the key. So, according to A.M. Lastovtsev, the inhomogeneity coefficient is determined by the formula
where Сі is the concentration value of the key component in the samples; Co is the value of the concentration of the key component under ideal mixing; l, is the number of samples with a concentration of C ,; N - the total number of samples; i is the number of sample groups.
Often the mixing process is described by the criterial equation: m = CRe * Fr *, where Re u , Fr a are the centrifugal Reynolds and Froude criteria; t is the mixing time, min; n - the angular speed of rotation of the agitator working part, rpm; C 'is the coefficient of proportionality; A, B are the exponents of the power function. In the region of developed turbulent motion of the mixture in the apparatus, this equation becomes more convenient: m n = const.
The prepared mixture (mass) possesses certain qualitative characteristics, estimated by the properties indicators. The latter express the ability of a substance to react to external and internal factors (mechanical, thermal, gravitational, etc.). The main property of the prepared mixture (mass) is its ability to process - distribution of a layer of a given thickness, compaction, molding with compaction. This ability of the mixture is called labor-saving, formability, mobility and is referred to a group of structural-mechanical or rheological properties. In coarse-grained mixtures, they are measured using conditional methods and instruments, and in fine-dispersed mixtures they are invariant with a test for pure homogeneous shear (in viscosimeters and similar devices). If the mixture has a true (Newtonian) viscosity, then its flow corresponds to the equation/= T | (Di/Ax), where P is the voltage; D and - the difference in flow velocities in two parallel layers; Ax is the distance between layers; Au/Ax is the gradient of flow velocity, σ; ц is the dynamic viscosity (Pa c), which in the equation is proportionality coefficient. It can be seen that in the Newtonian (viscous) flow, even at the lowest stresses, a deformation occurs with a speed directly proportional to the magnitude of the stress or the force applied to the body. However, mixtures with Newtonian viscosity in the practice of building materials are rare.
Mixtures that do not have true viscosity and deformability at a rate proportional to the stress (or applied force) are more often prepared and used. They are plastic, less mobile and structured. The flow of the structured mixture begins only when, under the action of some stress P, c, called the yield point, the structure gradually begins to break down. Such a plastic flow satisfies the equation P = A + iir (An/Ax), where r | n is the Bingham (by the name of Bingham, who studied it) the viscosity of the mixture. As the voltage P increases (or the velocity gradient), the process of destruction of the structure increases (Figure 2.7), and when the voltage Рт the structure is completely destroyed. At the same time, the continuous decrease in the Bingam viscosity also ceases. With a further increase in stress, the viscosity rj remains practically constant, and the structure is completely destroyed. However, the character of the motion can change from a laminar (section A - B) to a turbulent (section above B). On the flow curve (rheological curve), the viscosity is reduced from a voltage equal to P Kl as a conditional static yield point to the point A corresponding to the beginning of a completely destroyed structure with the lowest viscosity rjm. On this curve, there is also a point Pk r - the dynamic yield point and the previously noted point Pk is the yield point, or the ultimate shear stress. The significance of these rheological characteristics is that it is possible to provide allowable stresses without violating the continuity of articles when molded from a mixture (mass).
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