Incorporating TiO 2 additives markedly improves the sintering of CaO-based refractories [ 1, 2]. It reduces the rate of atmospheric hydrolysis of the sintered material, not only because of the improvement in its porosity, but also because of the formation of nonhydrolyzable compounds in the system CaO-TiO 2. Thus, TiO~_ as a sintering agent for CaO is suitably distinguished from other additives such as A1203, B203, SiO 2, etc. which are recommended in the literature [i].To improve the process for making lime refractories it is important to stress the firing cycle, since various calcium titanates are formed during firing, whose region of formation has not been precisely established.Investigations of the binary system CaO-TiO 2 suggest the existence of three double compounds: Ca2Ti207, Ca4Ti3Olo, and CaTiO 3. The authors of [3] detected only two compounds in this system -CaTiO 3 and Ca3TiaO 7. By studying the kinetics of their development, they showed that the rate of formation in the solid phase of perovskite CaTiO 3 may be 1.5-2 times greater than the rate of formation of Ca3Ti207. According to [4] the compounds Ca3TiO 7 and Ca4Ti30 94 melt incongruently at 1740 and 1755~ respectively, with the formation of CaTiO 3 and liquid. In [5] the authors established the presence of solid solutions between Ca3Ti207 and Ca4Ti3OlO.From the crystallochemical viewpoint the structure of perovskite CaTiQ is most conveniently considered to be a volume-centered cubic lattice, deformed along the axes [100] and [001], with the densest packing formed by atoms of calcium and oxygen [6], in which the titanium atoms occupy the octahedral spaces formed by the oxygen atoms. According to Goldschmidt, the formation of perovskite type structures ABO 3 requires a certain relationship to hold between the polarized radii A and B (r A and rg) and the radius of the oxygen ro:where t is a numerical multiplier, assuming values from 0.8 to 1.0.Compounds of the ABO 3 type with the perovskite structure can be divided into two main groups. The first covers those with a cubic structure, ideal or slightly deformed, and this deformation consists in a change in one or several lattice parameters, which leads to the formation of a tetragonal, rhombic, or rhombohedral structure (BaTiO3). The second group contains compounds in which the deformation causes an increase in the elementary cell (CaTiO3). Deformation of the cubic lattice CaTiO 3 leads to a reduction in the symmetry of this compound to the rhombic form.In the structure of Ca3Ti207 the double layers of perovskite are separated by CaO layers. The ratio of the axes c/a in Ca3Ti207 is 5:1 [6]. The authors of [7] confirmed the possibility of the existence of such compounds with ternary and quaternary layers ofperovskite, separated by CaO layers. In particular, the compound Ca4Ti3Olo has the ratio c:a = 7:1. The main parameters of the crystalline structure of calcium titanate are shown in Table 1.In order to study the solid phase sintering processes in the CaO-TiO 2 system we prepared mixtures of CO and T...
In the manufacture of lime and lime-containing refractories the kinetics of the hydroxylation reaction CaO + H20 --~ Ca(OH)2 -or as it is not quite accurately called, hydration -plays an important role in determining the preservation conditions and subsequent use of the materials. This applies especially to clinkers formulated for making sintered refractories (with a ceramic bond). This article examines the influence of TiO 2 impurity on the hydroxylation of CaO and the evaluation of this process.Kinetie Models of Hydroxylation. The degree of hydroxylation ~ can be assessed from the volume proportion of CaO reacting with water [calculated as Ca(OH)2]: c~ = (V 0 -V)/V o, where V 0 is the volume of CaO particles with an initial radius Ro; V is the volume of particles of unreacted CaO with average size R. If the hydroxylation mechanism is defined as the diffusion of water through a layer of Ca(OH) 2 and the surface of the CaO particles, then the rate of increase in the thickness of the layer of Ca(OH) 2 is inversely proportional to it:
Silicon carbide and boron carbide are used in various fields of industry owing to their high levels of physicochemical properties and hardness.In the absence of special additives, these materials are usually sintered at elevated (2000~ and above) temperatures.The materials based on silicon carbide in which binding agents form at relatively low temperatures (1500~ with the participation of a gaseous medium (atmosphere) are finding wide application.In view of this, it was of interest to explore the possibility of using a similar technological route for boron carbide also. For this purpose, we studied the behavior of boron carbide during heat treatment in a carbonaceous particulate charge consisting of a mixture of coke and quartz sand.It is known that the gaseous phase in the C--Si02 system consists of CO, C02, and SiO [i]; besides this, the gaseous phase of the aforementioned charge contains nitrogen and traces of oxygen.The information available on the behavior of boron carbide in various gaseous media is quite limited.In the atmosphere of pure oxygen [2], oxidation of boron carbide starts at 600~ and the oxidation rate increases at 800-i000~ thereafter, it decreases and at 1200-1300~ it abruptly increases once again due to the formation of liquid B20s and its vaporization. Besides this, the formation of volatile lower oxides of boron is also possible.
Calcium oxide has a temperature coefficient of expansion and thermal-conductivity values that are lower than those of magnesium oxide; it also has a higher elasticity modulus, and therefore a better thermal-shock resistance and heat-insulating properties. Since it is a strong base, calcium oxide is capable of absorbing practically all impurities contained in molten metals, which, in respect to CaO, should exhibit the properties of acids [I].However, attempts to set up a production scheme for lime refractories (Fig. I) cannot be considered as entirely successful. For example, scheme I is highly energy consuming, and involves large capital investment. For a sufficiently large production volume, scheme II is less energy-consuming, but the choice and use of special additives still require consideration, since in the optimum case, within the CaO-additive system, the liquid phase should develop at a temperature higher than the operating temperature of the refractory, i.e., the eutectic should, firstly, be situated in the region close in composition to the additive, and secondly it should not be below 1970 K.Moreover, the compounds of CaO and additives should not be subject to atmospheric hydrolysis, and the calcium oxide grains should be protected from it.Taking only these requirements into consideration, the number of possible additives for activating the densification of CaO during sintering is sharply reduced compared with those recommended in [2-4]*; and moreover, some of them, e.g. TiH 4, TiN, V20 s, HgF 2, and BeO are expensive and toxic. Analysis shows that TiO 2 and its components are most suitable as the basis of sintering additives.The use of CaO as a refractory with a low apparent density (2.1 g/cm 3) in the production of steel-casting batchers, deoxidized with aluminum,% although it gives positive results, is hardly appropriate, since the rate of atmospheric hydrolysis of the CaO increases with increase in the volume proportion of the pores P, proportionately to [P/(I -p)]2/3It is clear that to achieve uniform mixing of a small amount of additive with CaO or calcium carbonate, to provide the same quantity of particles in unit mass, their particle size should satisfy the following conditionwhere Sg is the specific surface of the particles of additive, S c is the specific surface of the particles of original CaO or CaC03, Pc and pg are respectively the true density of CaO or CaCO 3 and additive; x is the mass proportion of additive in the mixture.Thus, for an average particle size of CaCO~ equal to 80-100 ~m and a mass proportion of 98%, the specific surface of the additive particles, e.g. TiOz, should be 0.8-1.0 m2/g, in order that the numbers of particles of additive and the original CaCO 3 powder in the mix will be identical.The choice of the amount of TiO 2 sintering additive was determined by the fact that the eutectic temperature of CaO-TiO 2 equals 2010 K [5] and the calcium titanates should not undergo atmospheric hydrolysis. The optimum amount of additive was determined empirically *Licenses 56-78470,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.