Thermal strength,deformation, and thermal fatigue characteristics of dinas products before and after service in the high-temperature zones of the hot-blast stoves

Conclusions The deformation and strength characteristics of the dinas specimens (drawn from the industrial walling and checkerwork products before and after service) were studied under uniaxial tension and compression at a temperature up to 1600°C.It was established that their ultimate strength decreases with increasing test temperature and that the load-bearing capacity is insignificant at 1570–1600°C; when subjected to cyclic heating (1250 1450°C) in the laboratory tests as well as during service in a hotblast stove operating under alternating oxidizing and reducing environmental conditions, dinas bricks exhibit embrittlement and softening (their tensile and compressive strength characteristics differ by almost 10 times).The obtained data showed that the limiting temperature at which the strength characteristics of dinas abruptly decrease under a constant stress and multiple cyclic action of alternating atmospheres amounts to 1600°C; this fact must be taken into account when designing and operating hot-blast stoves.Translated from Ogneupory, No. 6, pp. 19–22, June, 1986.     

Effect of the firing medium and other technological parameters on the properties and phase composition of dinas articles on dry mineralizers

The effect of the gaseous firing medium and other technological factors (grain composition of the charge, pressing pressure, temperature and time of the hold at the final firing temperature) on the properties of dinas and the degree of transformation of silica in it is investigated. On the basis of results of a comparative analysis of the phase compositions and properties of specimens the following parameters are recommended as optimum technological parameters for fabricating dinas on dry mineralizers that provide high operating characteristics for the articles: the maximum grain size of quartzite is 2 mm, the pressing pressure is at least 50 MPa, the firing temperature is 1420°C, the duration of the hold is 44 h. Petrographic and x-ray analyses are used to show the preferableness of firing dinas in a reducing medium, which promotes a fuller transformation of quartz in the dinas and provides high operating properties for the articles.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 6, pp. 14–17, June, 1996.The work has been done in collaboration with L. M. Khvostishkova. The physicochemical properties of the specimens were investigated in specialized laboratories of the Ukrainian Research Institute of Refractories.     

Dependence of the properties of chamotte on the firing regime of the briquette

Conclusions The physicomechanical properties and phase composition of chamotte depends on the composition of the gas medium, the firing temperature, and the cooling regime. In the briquettes fired at 1500°C in an oxidizing medium with the cooling temperature lowered from 1500 to 1200°C, the apparent density increases and the porosity decreases. In the briquettes fired in a reducing medium with the cooling temperature lowered from 1500 to 1300°C, there is a reduction in the apparent density and an increase in the porosity but with a further reduction of the cooling temperature to 1200°C, these properties change in the opposite direction.The apparent density of chamotte and its concentration of mullite is increased with a reduction in the cooling rate. The apparent density of the chamotte from the Polozhsk kaolin under analogous cooling conditions increases more rapidly than that from the Novoselitsk kaolin chamotte.The temperature interval in which the cooling rate effectively increases the apparent density of the chamotte fired in an oxidizing medium is 1500–1300°C. In a reducing medium this interval is shifted 100°C toward lower temperatures.The experiments have shown that an increase in the cooling rate of the fired briquettes at temperatures below 1200°C has only a very slight effect on the apparent density and porosity of the Novoselitsk kaolin chamotte. When the chamotte has been obtained from Polozhsk kaolin, a reduction in the cooling rate at temperatures below 1200°C, and particularly in a reducing medium, has a quite perceptible effect on the increase in apparent density and reduction of porosity of the material.Translated from Ogneupory, No. 3, pp. 22–26, March, 1982.     

High-temperature abrasion of refractories

Conclusions The most rapid abrasion in dinas refractories occurs in the 700–900°C range; periclase 800–1000°C; mullite-corundum articles exhibit a smooth increase in abrasion with rise in temperature. The abrasion of refractory concretes is much higher than for bricks.It is possible by determining the abrasion resistance to carry out quality control of the firing process, and also to check the quality of linings made from concretes.Translated from Ogneupory, No. 1, pp. 13–14, January, 1991.     

Service of nonpowderable dinas in high-temperature stoves of blast furnaces

Conclusions Experience with the use of low-powdered (low embrittlement) dinas in the high-temperature zones (walls, cupolas, combustion chamber, and checkers) of blast furnace stoves showed that at subcupola temperatures of 1350–1450°C after 3 years service the original physicochemical properties of the product were not impaired, and the structure had preserved its satisfactory condition.A study of the properties of low-powdered dinas after service showed that despite the reduction in the density as a result of additional transition in the quartz, the structure of the dinas is not embrittled, but is densified.Considering the relatively good deformation properties of the low-embrittlement dinas, its use should be tested in the operation of stoves with subcupola temperatures of 1500–1550°C.Translated from Ogneupory, No. 6, pp.28–31, June, 1972.     

Using dinas in the top zone of checkers for blast furnace stoves

Conclusions Experience with the use of dinas in the upper zone of blast furnace stove checkers showed that, at a hot blast temperature of 900–1000°C and with a temperature of up to 1200°C in the gases under the cupola, the dinas practically preserved its refractoriness and strength in four years operation; cracking was slight and the checker channels were completely preserved.When these cupola conditions prevailed, the structure of the top 60–65 rows of checker (9–9.5 m), made from dinas is quite adequate, and the underlying rows of firebrick were in an excellent state.Fluxing oxides falling into the stoves from the blast gas and air used for combustion, and also from moist blast gas, reduce the refractoriness of the changed zones of the dinas, but only by 30–40°C.It is necessary to study carefully the behavior of dinas in these conditions at higher subcupola gas temperatures, say at up to 1400–1500°C.Translated from Ogneupory, No. 4, pp. 31–35, April, 1968.     

Some strength characteristics of ceramics made from vitreous silica

Conclusions The form of the original material and the production parameters during its treatment have an important effect on the properties of quartz ceramic.The strength of the ceramic increases with an increase in the fineness to which the original material is milled and with a decrease in the moisture content of the suspension.Ceramic made from vitreous silica shows no significant shrinkage or increase in strength after drying and firing at up to 800°C. The maximum strength (on the whole) and best thermal-shock resistance are characteristic of specimens fired at 1200°C while maximum shrinkage and minimum water absorption is found in specimens fired at 1300–1350°C. This difference in temperatures is probably because under the chosen experimental firing regimes, the beginning of cristobalite-formation slightly outstrips the sintering of the material.A real possibility of using cullet from previously fired articles made of vitreous silica, particularly of synthetic silica, has been established.Translated from Ogneupory, No. 8, pp. 55–60, August, 1980.     

Changes in the properties of aluminosilicate refractories under prolonged reducing conditions

Conclusions Studies of corundum and aluminosilicate refractories of dense and granular structures in an atmosphere of hydrogen and dissociated ammonia at 1200, 1500, and 1700°C in periods of 175 and 50 h showed that the resistance of the products increases with an increase in the alumina concentration and density. The maximum resistance is exhibited by corundum products. In the aluminosilicate refractories there is some additional sintering of the material with the separation of mullite and glass. Simultaneously on the surface of the specimens we detected deeper mineralogical changes, accompanied by the decomposition of the mullite, with the formation of corundum, silicon monoxide, and glass.The changes in the phase composition are accompanied by a change in the structure, and an increase in the creep. Considering that a reduction in the temperature of 100°C causes a reduction in the creep by approximately a half [26], it can be recommended that corundum refractories should be used (under a load of 2 kg/cm²) in a reducing atmosphere at temperatures of up to 1550–1600°C, sillimanite up to 1450–1500°C, kaolin and chamotte (high-grog) up to 1300°C, with a reduction in the load and an increase in the density, the temperature of application for the products examined, especially corundum, can be increased.Translated from Ogneupory, No.5, pp.26–32, May, 1972.     

Refractories for the outer verticals of coke ovens

Conclusions The mechanism of the destruction of dinas in the structure of the outer verticals of coke ovens was examined during the cooling of the structure deep down and during reversible low-temperature inversions in the silica modifications.Destruction of the dinas in the structure of the outer verticals is caused by its low spalling (thermal-shock) resistance because of the reversible low-temperature changes of the silica, and also the action of moisture brought in with the carbon batch. The moisture migrates into the cooler sections of the structure, and the steam resulting from this weakens the structure of the dinas.An increase in the service life of the dinas in the outer verticals is possible not only as a result of the use of articles with a high spalling resistance but with the use of unfired carbon batches.In laboratory and industrial conditions we established means of improving the spalling resistance of the dinas by adding fireclay together with lime-iron slip.The deformation properties of the dinas with the additives (fireclay) are preserved at a high enough level permitting its use in the outer verticals of coke ovens.It is necessary to prepare an experimental industrial batch of dinas using the new method in order to carry out service tests. During industrial tests we should ensure a reduction in the moisture content of the carbon batches employed.Translated from Ogneupory, No. 6, pp. 14–19, June, 1982.     

Thermal transformations during the interaction of chamotte and orthophosphoric acid

Conclusions Up to 300°C chamotte does not react with orthophosphoric acid. In the range 300–800°C crystallization of the type SiO₂·P₂O₅ occurs. With further increase in temperature the silicophosphate dissolves, and at 1030°C is converted from the low- to the high-temperature form, and at 1200°C it changes completely into the melt.Starting from 500°C the mullite decomposes. The intensity of the mullite line at 1400°C is slight.In the range 700–1000°C a large quantity of liquid phase, formed during decomposition of the silicophosphates and mullite, sharply depresses the refractoriness-under-load and increases their shrinkage.At 1200°C AlPO₄ crystallizes in the cristobalite form. An increase in the quantity of AlPO₄ increases the refractoriness-under-load of the specimens.Mixtures of chamotte and orthophosphoric acid after firing at 1400°C contain (in reducing order) AlPO₄ (cristobalite form), cristobalite, mullite, and quartz; they may be recommended as low sintering mortars.Translated from Ogneupory, No.2, pp. 39–43, February, 1970.     

Use of magnesite refractories in the chequerwork of the air heaters of blast furnaces

Conclusions Magnesite refractories are a promising material for use in the upper rows of the chequerwork, particularly where it is necessary to increase the thermal capacity of the air heaters without increasing their dimensions.When the magnesite refractories are used, the mass of the chequerwork and the unit load on the subchequerwork grid and the lower rows of the chequerwork are increased.The improved efficiency of magnesite refractories in the chequework of air-heaters by comparison with other types of refractory is becouse of their high storage capacity for heat as a result of the increased apparent density and better thermophysical characteristics.The magnesite refractory used in the chequerwork should contain a minimal amount of impurities, particularly of Fe₂O₃.When magnesite refractories are used to replace dinas, the cost per ton of chequerwork increases 50% while for the replacement of mullite by magnesite, the cost per ton of chequerwork is reduced by 16%.Contact between the magnesite refractories and the dinas (without a load) is permissible at a temperature not higher than 1450°C; with sillimanite, at not more than 1350°C.In order to prevent the action of finely dispersed particles of dust and alkali oxides accidentally introduced into the air heaters it is sensible for the upper row of chequerwork to be made from sillimanite or mullite refractories.The theoretical extreme temperature for the use of magnesite refractories in the chequerwork of the air heaters (allowing for creep and the interaction with other types of refractories) is 1650°C for M-I type blocks and for M-II, not more than 1450°C. For greater safety it is recommended that the ultimate working temperatures be reduced by 150°C.Taking into account the favorable experience abroad in using magnesite blocks in the chequerwork of air heaters, it is necessary to carry out industrial tests of the magnesite products in the upper zones of the chequerwork.Translated from Ogneupory, No. 7, pp. 32–36, July, 1982.     

Effect of preliminary heat treatment of alumina on the thermal-shock resistance and strength of corundum ceramics

Conclusions The firing temperature of the starting alumina is one of the important factors influencing the formation of the microstructure of corundum materials and determines the properties of the ceramics. At low alumina firing temperatures (1400°C) the structure of the specimens is finely crystalline. An increase in the firing temperature (to 1600–1750°C) causes an increase in the crystal sizes.The finely crystalline structure ensures a high strength in corundum materials, while the coarsely crystalline structure reduces it.The high thermal-shock resistance of specimens is determined by the formation of regular, random orientation, and close contacting crystals of corundum. An important role in this case, apparently, is played by the structure of the crystals. The thermal-shock resistance of the materials is favored by a combination of starting materials having different firing temperatures, namely, aluminas fired at 1750 and 1600°C.Translated from Ogneupory, No. 2, pp. 44–49, February, 1971.     

Strengthening freshly pressed dinas green compacts for the purpose of automating their transportation

Conclusions Automation of removal of freshly pressed dinas compacts from presses and stacking of them on furnace carts may be done only with a significant increase in the strength of the corners and edges of the compacts.Sufficient strengthening of the angles and edges of freshly pressed dinas compacts may be provided with a short heat treatment for a moisture content of not more than 3%, even with the addition to the mixture of a normal production lime-iron mixture consisting of milk of lime (2.5% by CaO), iron-containing additions (0.8% by Fe₂O₃), and sulfite-yeast mash (0.8% by dry residue) and pressing of the compact to an apparent density of not less than 2.24 g/ cm³.A short heat treatment of freshly pressed dinas compacts at a heat carrier temperature of not less than 150°C for strengtening of them is promising for mechanization and automation of transportation of the compacts from the presses and loading of them on the furnace carts.Consideration of the question of the design and construction of an automated production line for the transportation of freshly pressed dinas compacts from the presses to the tunnel furnaces must be advanced from the stage of laboratory experiments to the stage of production testing.The development and testing of the proposed variation of automation of transportation of freshly pressed dinas compacts from the presses and their heat treatment, stacking in the elements of the charge (columns), and placing on the furnace carts would, in addition to the reduction in labor costs, significantly reduce dust production in the press-dryer-tunnel furnace area.Translated from Ogneupory, No. 5, pp. 20–24, May, 1982.     

Slag and hot-slag resistance of basic refractories

Conclusions Using the instrument described, which has been improved in design for testing refractory materials for slag and hot slag resistance, we investigated the properties of magnesite, magnesite-chromite, and chrome-magnesite specimens. It was established that their life is determined by the resistance of the refractory bond. The best resistance is possessed by periclase-spinel specimens. An increase in the content of calcium oxide in the slags reduces the slag resistance of all periclase-spinel refractories that were tested. Impregnating the magnesite products with a saturated solution of chromium acetate and firing them after impregnation at 400–500°C markedly increases their slag resistance.Translated from Ogneupory, No. 7, pp. 47–52, July, 1968.     

Features of the production technology and the properties of the components made from Al2O3 and SiO2 of different particle-size distributions

Conclusions It is recommended that the mullite-silica products made from polydispersed mixtures of alumina and silica in the presence of oxide additives be fired at a temperature of 1380–1410°C according to the firing regime of the dinas refractories. We obtained products containing a mixture of iron and calcium oxides (maintaining their total content not exceeding 3% and a 21 ratio). Oxide additions exceeding 3.5% have a negative effect on the technological effectivenesss and the properties of the products.In the absence of additives, mullite-silica products must be fired at 1550–1600°C maintaining the heating and cooling rates below 30°C/h. The mullite-silica products based on the polydispersed mixtures of corundum and crystalline quartz are recommended for the linings of the units used in the production of steel and cast iron and for lining coke-ovens and other thermal units (furnaces).Translated from Ogneupory, No. 7, pp. 9–12, July, 1988.     

Firing dinas in a tunnel kiln with natural gas

Conclusions The firing of products for open-hearth, cooking, and glass furnaces and electric-steel furnaces was developed in a tunnel kiln 180 m long. Firing can be done on a level with other types of fuel with natural gas, the combustion of which should be done with double duct burners.For the firing of dinas products it is necessary to prolong the firing zone and this permits maximum temperatures to operate over a large section. With an annual output of 55,000 tons of products, the goods in the kiln are well cooled. The temperature of the goods upon discharge from the kiln does not exceed 40–50°C.During a four-year working period the factory staff substantially improved the working factors of the kiln. The specific fuel consumption for firing products in a tunnel kiln of this type is very low.Translated from Ogneupory, No. 8, pp. 22–25, August, 1969.     

Suitability of Zavadov deposit dolomites for use as raw material

Conclusions Investigations of raw Zavadov deposit dolomite showed that in strength it belongs to the dense varieties of dolomite with a fine crystalline structure. The weight % of the basic component (dolomite) is 88–98%. The impurities are calcite, quartz, clay particles, and iron oxides.By firing the dolomite in the form of the previously separated 5–25-mm fraction at 1700–1750°C it is possible to provide sintering of practically all of the samples. The use of preliminary grinding and briquetting improves sintering but does not provide significant advantages in comparison with firing in granular form. The addition of scale causes an improvement in sintering and an increase in the resistance to hydration.The resistance of hydration after firing at 1750°C in storage in air of samples Nos. 4–17 was satisfactory while the dolomite of samples Nos. 1–3 hydrated significantly more rapidly. With the addition of 1% scale and firing at 1700°C in storage in air the resistance is significantly greater.As the result of the investigation it was established that the dolomite, with the exception of individual samples, meets the requirements of Industry Standard 14-84-82 for raw metallurgical dolomite for patching and filling the sills of hearth furnaces and also for firing for metallurgical dolomite. A further improvement in dolomite quality is pssible with selective mining and benefication of it.Translated from Ogneupory, No. 2, pp. 36–40, February, 1987.     

Problem of the hydration resistance of calcium oxide

Conclusions It is shown that in specimens prepared from lime with additions of yttrium oxide, during firing at 1950°C, solid solutions are formed on the basis of CaO, contributing to an increase in the hydration resistance.It is established that the specimens prepared from CaO with additions of manganese and chromium oxides, fired at 1750°C, also have a high hydration resistance.Translated from Ogneupory, No. 1, pp. 16–19, January, 1988.     

Electrochemical performance of silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathodes prepared via electroless deposition

Silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathodes for intermediate-temperature solid-oxide fuel cells (IT-SOFCs) were prepared by an electroless deposition process using N₂H₄ as the reducing agent at room temperature. This fabrication technique together with tailored electrode porosity, modified the BSCF electrodes with silver content that varied from 0.3 to 30 wt.% without damaging the electrode microstructure. Both the Ag loading and firing temperatures were found to have a significant impact on the electrode performance, which could facilitate or block the electrochemical processes of the BSCF-based cathodes, processes that include charge-transfer, oxygen adsorption and oxygen electrochemical reduction. At an optimal Ag loading of 3.0 wt.% and firing temperature of 850 °C, an area specific resistance of only 0.042 Ω cm² at 600 °C was achieved for a modified BSCF cathode.     

Damage to stoves and measures to eliminate it

Conclusions As a result of investigations into newly developed refractories and a study of their service conditions in the stoves of blast furnaces, three basic types of failure in the structure and the checkers were determined:the formation of vertical cracks in the dome and walls; deformation of the checkers and the upper part of the combustion chamber; and failure of the lower part of the combustion chamber. The formation of cracks can be eliminated by selecting the optimum joints, and by using a design in the stoves which takes into account the properties of the refractories under variable temperatures.In order to reduce the deformation of the checkers and the combustion chamber it is recommended that these zones be made of refractories possessing the minimum creep, in the first case dinas with low embrittlement factors, and high-alumina products made of kaolin and calcined alumina.The lower part of the combustion chamber should be built from high-alumina and kaolin refractories possessing a high resistance to thermal shock.It is found that the upper rows of the checker become saturated with fusible compounds, formed when batch dust falls on the material, which means a substantial reduction in their thermal-mechanical properties, contributing to an increase in the deformation under load.To increase the blast temperature to 1300–1400°C it is necessary to use, in the high-temperature zones, mullite, mullite-corundum refractories based on kaolin, and low-embrittlement dinas with the compulsory completion of a number of measures: installation of an emission combustion chamber, removal of the dome load from the working walls, the use of perforated checkers, the construction of a special device for putting the furnace on draft, purification of the gases and air to remove dust and fusible substances, a reduction in the heating period, and the organization of paired-parallel operation of the stoves.Translated from Ogneupory, No. 1, pp. 23–32, January, 1972.