PROCESSING OF REFRACTORY PRODUCTS
A very significant technological redistribution affecting the formation of ICS, as well as other materials, is the special processing of molded and compacted products with one, two or more external influences on the material in some sequential or combined order. The treatment can be thermal, heat and moisture, chemical, electrophysical, autoclave, vacuum-impregnating, radiation (often combined with vacuum-impregnating), etc. The main goal of the treatments is to ensure the development of micro- and macrostructural processes with the most complete transfer of systems from metastable and mutable states to thermodynamically stable. And although the corresponding processes can continue even after the processing, including during the operational period of the design, however their avalanche share proceeds at the stage of processing, less often at the stage of keeping the products in ordinary, "normal" conditions.
The efficiency of processing is characterized by a gradual or rapid hardening of the structure of freshly made products with the transition into a solid or solid state. The curing agent basically cures, because the other is a filling part - a part of the conglomerate consists of a mixture of already solid components. In the knitting part, either one, a new phase is formed, or there may be several. A new phase in the form of chemical compounds that arise under the influence of chemisorption reactions on the surface of solid particles or in solution (melt), first appears as a cluster of micro-embryos; In the subsequent period, the kinetic development of the reaction centers proceeds. The products of chemical reactions are released into an independent phase, the concentration of which increases with time.
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In complex reactions involving two or more simple reactions, each of the subsequent reactions proceeds independently and to each of them the equations of the kinetics of simple reactions are applicable.
The rate of chemical reactions increases rapidly with increasing temperature according to the Arrhenius equation:
InA: = AIT + B, (2.6)
where A and B are the individual constants for the given reaction; T - the absolute temperature on the Kelvin scale; k is the reaction rate constant, in magnitude is the reverse reaction rate. The quantity A is proportional to the activation energy according to the physical meaning: A - E */A, where E * is the activation energy, which means excess energy (in comparison with its average value), which the molecule possesses at the moment of effective collision with the other during the formation of chemical interaction; R is the gas constant).
It follows from the equation that the reaction rate constant (and, consequently, the reaction rate) varies with temperature fluctuations more strongly in those reactions that have an increased activation energy. At a low activation energy, the reaction rate changes slightly with increasing or decreasing temperature. To increase the energy of the reacting molecules, i.e., to activate them, various methods are used, taking into account: the nature of the substances, the kinetic energy; increase the energy of mutual vibration of atoms in the molecule; increasing the energy of the motion of electrons in atoms, for example, as a result of rupture of valence bonds (in particular, in the dissociation of molecules into atoms, absorption of electromagnetic oscillations, etc.); activation of molecules by electric discharge; effects of ultrasonic vibrations and radiation, for example high-energy light fluxes - X-ray, gamma-radiation, etc.
Fig. 2.9. Dependence of the concentration C of the values of H and A on time
The quantitative dependence of the reaction rate on temperature is also expressed by the approximate Van't Hoff rule, according to which the rate of chemical reactions increases by 2-4 times when the temperature is raised by 10 ° C. The number, showing how many times, the speed increases, is called the temperature coefficient of reaction speed. This rule can be expressed by the formula
where vi and ry are the reaction rates at temperatures respectively 1 1 and/ 2 ; the temperature coefficient of the reaction rate.
The rate of chemical reactions depends only on the temperature, but also on the concentration of the reacting substances. In the simplest cases, when homogeneous reactions occur and they occur in highly dilute solutions (or in an ideal gas medium), the law of mass action functions: at a constant temperature, the rate of chemical reaction is proportional to the product of the concentrations (c) of reacting substances raised to the degree of their stoichiometric coefficients. For example, 2Hr + O 2 = 2HhO, a v - c Hi with 0 . This well-known in chemistry law is the basis of chemical kinetics, but in complex reactions its effect becomes less reliable.
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Not always by adjusting the concentrations of reactants and raising the temperature, the desired acceleration of the manufacturing process is achieved. A great effect can be achieved by acting on the reaction system by a catalyst of radiation and other factors.
Various substances are used as catalysts, but the specificity of their action is that each reaction is accelerated by some specific catalyst that is not capable of accelerating other reactions, ie, the reaction catalyst participates in such a way that it leaves at the final stage in its initial form and quantity. The effect of the catalyst leads to the formation of an additional number of chemical compounds per unit time with increasing volume of the new phase, most often crystalline.
In addition to chemical reactions, the crystallization of the dissolved substance from the supersaturated solution leads to the formation of a new phase. Supersaturation occurs for various reasons: removal of a part of the liquid dispersion medium, for example, by evaporation or evaporation of water, alcohol, ethers or other solvents; change in the temperature of a saturated solution (usually when it is lowered); change in external pressure; chemical interaction of the initial components in a complex solution, etc. The strongest effect is due to a decrease in temperature, since the solubility of many substances then decreases, although some substances are still characterized by a negative solubility coefficient, that is, their solubility decreases with increasing solution temperature. In complex solutions, supersaturation is often associated with the formation of new chemical compounds.
The supersaturated solution has a relatively small thermodynamic stability. Its transition to a relatively stable state can be caused by extraneous factors: the introduction of "seeding" in the form of a fine crystal, enlarged molecules, micelles or with a special combination of ions of a dissolved substance, suspended dust particles of a foreign body, and also under the influence of mechanical action, especially impact. Without initiating factors, the metastable state of the system can persist for a long time; a large activation energy, a large thermal effect, and other factors make it difficult for the system to transition to a stable saturated state. Then a high degree of supersaturation of the solution is possible before the "embryos new phase.
According to modern views, first appear nuclei of a new phase in the form of a cluster of a small number of atoms, the formation of associations of particles in collisions in the solution of individual ions (molecules) of the solute. In the subsequent stage, the growth of embryos leads to the appearance of discrete particles of submicroscopic dimensions. But the embryo particles continue to be in a mobile equilibrium for the time being and no visible crystallization occurs. This period is characterized as hidden, induction. The rate of formation of submicroscopic embryos increases with increasing temperature, with mixing of the solution, under mechanical external influences (shaking, vibration, stress, friction, etc.), in the presence of solid inclusions with a large surface (grain, filament, tape, etc.). The crystallizing system often introduces so-called "crystals" - crystallization components-additives that perform the functions of micro-embryos. The rolls contribute to the intensification of hardening (for example, a cement test) and to the improvement of the quality of the microconglomerate.
At a certain stage, the embryo particles reach a critical size, in which each has sufficient surface energy to cause additional adsorption of the solute particles. The tiny particles of other substances in the system, including particles of tumors, are also entrained. The embryos thus become the centers of crystallization. The latter are first isolated in the form of amorphous particles, which usually transfer to a crystalline state at a high rate with coarsening due to the deposition of matter on the faces of the crystals. By measures of increasing the size of crystals
the surrounding phase becomes supersaturated and, therefore, unstable with respect to large crystals, but unsaturated for small and small crystals.
According to the Gibbs-Curie principle, each crystal assumes its form at its growth, at which the total value of its surface energy has the smallest value, i.e. S = oi - Si + 02 ■ S2 + ... - * min for v = const, where v is the volume of the crystal; S - the surface of various faces of the crystal; o is their surface tension. The closer the surface energy of the faces to each other, the closer the polyhedron to the spherical shape, and in very rare cases, when the surface energy of the faces is the same, that is, 01 • Si = 02 S =, ..., o "• S n , the crystal takes the form of a sphere. With other dependences of the growth rate on the actual conditions of crystallization, it is possible to form other forms and structures of crystals-polyhedral, lamellar, acicular, dendritic (tree-like), etc.
The solubility of a crystalline substance does not remain constant, increasing with an increase in its dispersity. Upon contact with a saturated solution, the large crystals are further consolidated by dissolving small crystals. As a result of this continuous process, large crystals can relatively quickly reach a size at which the surface forces of this part of the crystals (crystalline aggregates and intergrowths) practically cease to affect the overall equilibrium of the system. Such a state of large crystals (intergrowths) is adequate, as it were, to exit the phase from the system. As a result, conditions appear for a new supersaturation of the solution and the appearance of nuclei of a new phase, the formation of new crystals increasing in size until the next yield of crystalline intergrowths from this metastable system. Repeating this kind of periodic cycles leads to a complete transfer of the solution to the crystalline state. The rate of spontaneous supersaturation before the nucleation of the new phase and crystallization decreases with decreasing temperature, and with sufficiently high supercooling of the solution, the velocity becomes negligible. Then some liquids pass into a glassy state with a little ordered arrangement of particles and an excess supply of internal energy.
The appearance and growth of crystals and crystalline aggregates can also occur from the gaseous state of the substance, bypassing the liquid phase, for example, with a sharp drop in temperature or a sudden increase in pressure. This peculiar kind of crystallization due to sublimation and subsequent cooling and condensation of the gas (desublimation) is characteristic of some ICS in which gases or vapors, for example, naphthalene, water, magnesium chloride, ammonium salts, etc. are present. But the process of desublimation is often is absent, and then the crystalline phase is removed from the building conglomerate (for example, based on the tar cement) by irreversible sublimation with a partial loss of the positive properties of the material, for example, density, .
The properties of crystals are affected not only by the shape, size or nature of the crystallizing material. So, for example, an admixture of surfactants in solution in very small quantities can stop the growth of crystals even at high degrees of supersaturation of the solution or, conversely, promote their growth. The formation of large crystals is facilitated by their slow growth and small degrees of supersaturation. Stirring during crystallization favors diffusion transfer of matter to the crystal faces and their growth, but at the same time causes the formation of nuclei and, consequently, the accumulation of small crystals. Different vibration modes in the technology make their significant adjustments to the crystallization process.
In firing conglomerates, melt as a kind of variety of chemical solutions caused by certain external factors serves as an astringent part. The transition from the liquid to the solid state upon cooling also occurs under the influence of crystallization of the components. In more rare cases, there is a gradual transition from the liquid state to solid, amorphous, vitreous, vol. the liquid becomes supercooled. The supercooling of the liquid is easier if there are no suspended suspended solids, dissolved gases or air bubbles in it, and also at complete rest. The introduction of a solid particle of the same substance, or even more so a crystalline, leads to an immediate process of crystallization of the supercooled melt with the conversion of it completely into a crystalline state. In this case, the molecules are rearranged from the chaotic disorder into an ordered one, caused by the crystal lattice.
The process of crystallization from melts begins and ends at certain temperatures, which depends on the components entering into the melt. In the melt, spontaneous crystallization can occur first at individual points, creating an anisotropic structure, and then gradually filling the entire volume. For each substance there is an optimum temperature at which the largest number of crystallization centers is created. At a higher temperature, there is a disorientation of molecules randomly receiving an organized crystalline state, and at lower temperatures, viscosity increases, preventing the molecules from moving and their correct orientation in the system with the formation of crystals. The optimal temperatures for the formation of the largest number of crystallization centers during cooling of the melt do not coincide, as a rule, with the highest rate of crystallization, shifting relative to each other.
To the majority of melts, which as a binder participate in the formation of ICS, we apply the universal law of the eutectic. Its essence consists in the striving of the melt to such a mixture of components that ensures the transition of the melt when solidified to the solid state at the lowest eutectic temperature.
The melts begin to crystallize from the component that is present in the melt in excess, spontaneously dropping the excess of this component in the form of a crystalline phase and approximating the melt residue to the eutectic composition, which is achieved when the temperature is lowered. The phase composition of the crystalline substances formed is determined by the phase diagrams. When the components of the melt are insoluble, the temperatures of the onset of crystallization follow along lines intersecting at a point. It is characterized by eutectic temperature and eutectic composition. At this point, with a strictly defined phase composition, there can exist both a liquid and a solid eutectic, that is, the solidification of the melt occurs at a constant temperature. For those alloys in which the components have a certain mutual solubility, solid state formation along the solidus line with crystals as pure components and containing small amounts of other solid component dissolved in them is characteristic. In systems, new compounds may appear that arise as a result of the chemical interaction of the original components. Some new crystalline compounds may have a melting point higher than the melting point of the pure components. For such complex systems, two eutectic points appear on the general state diagrams, since in addition to the two usual pure components, an intermediate intermediate corresponding to a new chemical compound with a higher melting point appears. Similar compounds between the components in different systems are formed relatively frequently, and when melting they become a liquid melt of the same composition (congruent melting) or the crystalline compound during melting reversibly decomposes to form a liquid of a different composition and a new solid phase (incongruent melting) If in the physical and chemical system there are not two, as noted above, but three components or more, the process of crystallization of the melt with a gradual decrease in temperature becomes even more complicated. In this system, a eutectic consisting of three or more components appears. New chemical compounds can arise with their characteristic melting points, solid solutions, etc. A variety of systems arise, the composition of which is most thoroughly studied by differential thermal analysis and other physical methods.As noted, the melt passes into a solid state gradually or rapidly, which depends on the external temperature, secreting crystals of variable composition, and only in the eutectic there is an instantly solid state of the melt, i.e., an alloy. It is possible, however, that in the initial state the melt already had crystals that did not pass at a given temperature of heating the system to a liquid state. Then the total mass cools down in the presence of these crystals, passing into the solidified material in the form of inclusions, which are very characteristic of so-called porphyry rocks. If the melt cools down quickly, the crystallization processes do not fully pass, and the glassy part (glass) in one or another quantity remains in the resulting solid. In melts, as in ordinary solutions, a gas phase is often present, which is formed either in the course of chemical reactions as a basic or intermediate product, or under the influence of the evaporation of individual ingredients, being in a mixture in the form of vapor, or as a result of sublimation, , bypassing the transition to a liquid state. As a result, solid systems appear with the inclusion of a gas phase in their pores.
When the parameters of the medium (pressure and temperature) change, the physicochemical system can pass from the steady stable equilibrium corresponding to the Gibbs phase rule to an unstable one with possible separation of new solid phases, a change in the phase composition and structure. In this respect, crystals that are deformed by constrained growth conditions, as well as crystalline bodies in the crushed state in the form of crystal fragments, are less stable. Then the substance has more chemical activity and less chemical resistance, greater ability to phase transformations, greater solubility, etc. The amorphous state is always less stable than the crystalline state. Many substances can exist in various modifications, that is, they tend to polymorphism with increasing or decreasing temperature (calcination, cooling).
The processes of crystallization in the formation of ICS structures are the most typical. Under idealized conditions, the crystals are formed from the microparticles regularly located in them - atoms, ions, molecules. The nature of this pattern of the arrangement of microparticles is due to the composition of the substance, and the bonds between the crystal particles can be diverse, including those combined in different parts of the body.
Crystals with an ionic bond (ionic lattice) have relatively high melting points. The ionic bond is characterized by the fact that the bond ions are due to the electrostatic attraction of oppositely charged ions (cations, anions).
With a covalent bond, the crystals usually have a high hardness, very high melting points. A directional covalent bond is expressed in the fact that closely approximated atoms transmit one or more electrons to the formation of stable outer electronic shells, through which the bond is made between them. The covalent bond can be nonpolar, when the interacting atoms belong to relatively identical elements, and the polar one, when the electron shell (electron pair) connecting the atoms belongs to them not in the same degree, but as if shifted to one of them, being a longer time near it. The polar connection refers to the intermediate between the ionic (in it, too, as it were, an electron pair is formed) and nonpolar bonds.Crystals with molecular crystal lattices have comparatively low melting points, low hardness, significant volatility, especially organic compounds (for example, naphthalene, etc.).
In addition to crystals with typical forms of communication, including metallic ones, there are transitional and mixed forms of communication. The composition of ionic crystals can include, for example, some neutral molecules located between ions or layers of ions (for example, water molecules). There are other deviations in the structure and chemical composition of the crystals with the appearance of defects, foreign atoms and ions in the form of interstitial impurities and substitutional impurities. Under real conditions, inevitably there are reasons for changes in the external form and internal structure of the crystals, which has a strong effect on their properties, especially on mechanical strength. An even stronger influence on the properties is the character of the steady-state contact and bonds between individual crystals, especially when they consist not of one but of two, three or more of their species, for example, in hard alloys or in crystallization products from complex solutions. Significant influence on strength, deformability and other properties is caused by contacting crystals or their fragments through thin interlayers of a foreign substance, often in a vitreous state.
Glassy substances are characterized, firstly, by isotropy (most crystals are characterized by anisotropy, i.e., vectorial properties) and, secondly, the ability to heat gradually to a liquid state. It is known that a crystalline substance completely passes into a liquid state at one constant temperature characteristic of it. A spontaneous transition of matter from the vitreous to the crystalline state is possible, accompanied by the release of small amounts of heat, by overcoming the energy barrier associated with the formation of double adsorption and ionic shells around the particles, and layers of a medium of increased viscosity. In technology, this barrier is often overcome by imposing additional mechanical effects on the hardening system.
In certain types of binder, certain proportions of the volumes of crystalline and amorphous (glassy) phases are established after the solidification of the system, which under the influence of operational factors can undergo deviations both due to additional isolation of tumors, and due to ordering in the arrangement of particles (atoms, ions , molecules) of a glassy phase with a gradual transition to a crystalline, to some extent deformed, state.
The processes of structure formation and the accompanying phenomena include contraction and shrinkage, exothermic and endothermic effects, relaxation and retardation.
The contract consists in the spontaneous compression of the system with a decrease in its initial volume, mainly in connection with the formation of new chemical compounds (chemical shrinkage) with the passage of a certain fraction of the volumetric (free) liquid medium into a chemically bound state. Since the product of the reaction is, as a rule, a new phase of micro- and macrostructure, the resulting porosity (contraction) has a significant effect on the quality of this material system. With high reactivity of the components of the forming complex material, the contraction can be up to 30% or more of the total volume of micro-dispersed pores (smaller than 0.1 μm). With the complication of the composition of the binder and the increase in the fineness of its grinding, the total volume of contractile pores increases.
Shrinkage is a decrease in the volume that occurs under the influence of compressive capillary forces, the transition of solid components to a liquid state, followed by the filling of pores and voids with a liquid medium, evaporation of part of the liquid medium or its syneresis (sweating), cooling (cooling), in including due to the endothermic effect. The general shrinkage consists of physical and chemical shrinkage.
In some material systems, shrinkage is observed instead of shrinkage with an increase in the volume of the conglomerate or astringent part. This phenomenon is due to: swelling, polymorphic transformation, chemical or physico-chemical addition of a large amount of liquid medium with an increase in the volume of amorphous or crystalline neoplasms, expansion of volume with increasing temperature, in particular due to exothermic effects.
As a result of shrinkage and swelling, the more recurring during the technological period of manufacturing the conglomerate or during the operational period, spontaneous stresses in the material and, as a result, micro-pathogenesis with possible deterioration of the physical and mechanical properties of building products often arise. Various methods - regulating the solidification regime, introducing additional components into the mixture, etc. - it is possible to reduce or completely eliminate the influence of shrinkage stresses or strains associated with the decomposition of the structure.
Thermal effects are due to chemical reactions and physical modifications. Endothermic effects occur when the crystal lattice is destroyed or liquid evaporates, polymorphous transformations (inversions) of the substance. Exothermic effects and reactions are caused by the formation of new phases, accompanied by the absorption of the gaseous medium, the transition of an unstable amorphous state to a crystalline state.
Relaxation and retardation are, respectively, the processes of spontaneous stress reduction with fixed deformation and changes in strain at a fixed internal stress. Both occur under the influence of displacements of atoms, ions, molecules, individual links of molecular chains. In the structure formation of ICS, these spontaneous processes have both a positive (removal of excess stresses with the prevention of cracking) and a negative value (weakening the strength of some compounds in structures, weakening the structure due to a drop in previously applied stresses in reinforcing elements, for example, in prestressed reinforced concrete structures). In all cases, these processes and the parameters that characterize them (relaxation time, relaxation coefficient, etc.) are taken into account in calculating the creep and strength of structural elements.
Thus, a complex of complex processes and phenomena that arise and develops during the technological redevelopment to a certain level, and then gradually fading (in burning fires faster than in non-burning ones), allows the product to be obtained from prepared and odorized components. The overwhelming number of processes and phenomena from this complex is characteristic for the knitting part of the conglomerate, since it is its components that are most active at the time of unification, being in a solid, liquid or gaseous state. The formation of an artificial conglomerate can be arbitrarily divided into many simpler processes and phenomena, just as complex chemical reactions represent a definite - parallel or sequential - combination of simple reactions. The predominant role in the structure formation is played by processes that ensure the formation and solidification of the binding part, that is, the microstructure of the conglomerate. At the stage of macrostructuring, a special role belongs to the processes of interaction along the interfaces of structural elements with the solidification of the entire ICS system and the design of the finished product. The structure of such a product now becomes a single, monolithic (see 2.2.5). In the design schemes, it is often, but conditionally, represented as consisting of micro- and macrostructural parts.
To make technological redesigns more efficient and product quality higher, they justify their optimal regimes and parameters at all major stages of production. Usually they are installed by experience, although this method becomes insufficient when the dimensions of the output are increased. Therefore, certain calculation models are introduced in which the state and behavior of real conglomerate mixtures (masses) are simulated. The main rheological characteristic in these calculations and studies is the viscosity of an extremely destroyed or completely undisturbed structure (Newtonian viscosities), as well as partially destroyed structures (structural or Bingham viscosity). The second rheological characteristic - the ultimate shear stress - allows describing the stress state of the mixture (mass) by the equation when optimizing technological regimes and parameters.
Spontaneously proceeding in the technological process of redistribution is the solidification of the conglomerate system, primarily its astringent part. But solidification, being a complex of complex processes, remains little available for visual observations. Therefore, a hypothetical general theory of ICS hardening is proposed below.
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