The production of corundum refractories for high-temperature applications is based on the use of various pore-forming additives [i, 2], corundum granules, and porous corundum briquet [3]. Well known is the relationship between the properties of dense corundum ceramics and the type of alumina used, and the various oxide additives [4]. The present paper gives the results of a study of the influence of the type of alumina and certain additives on the properties of corundum heat-insulating refractories.The articles were prepared by slip casting in metal molds and the pore-forming agent consisted of ashless combustible -foamed polystyrene [2] with grain sizes of less than 3 mm in amounts of 60% (by volume) in relation to the alumina. The bond constituent consisted of alumina G-00 ground in a vibromill to a grain size of 6-12 ~m, to which we added three types of filler in order to reduce the shrinkage: unground alumina grade GK and G-00, and also alumina calcined at 1550~ and vibromilled to a grain size of less than 4 #m using the G-00 type.An increase in the amount of bond from 20 to 60-70% regardless of the type of filler leads to an increase in the shrinkage in firing (Fig. i), but an even greater effect on the shrinkage results from the prehistory of the alumina serving as the filler. Thus, the addition of 30% unfired and unmilled alumina G-00 as a filler with the same shrinkage in firing (46%) ensures the production of articles with a much lower apparent density (0.92 g/cm 3) than for specimens to which we added 40% calcined (1550~ and vibromilled alumina G-00, (1.42 g/cm3). This is connected with the different phase compositions of the aluminas. In G-00, not subjected to calcination, the content of ~-AI203 does not exceed 30%, while the remainder consists of transition forms, from hydrates to oxides of aluminum 7-A1203, and 8-A1203, which, changing into corundum, cause increased shrinkage in firing [4]. The use of 40% GK alumina in which there is 99% corundum leads to a reduction in the shrinkage of the specimens in firing (from 42.7% for the corresponding amount of raw unmilled alumina as filler to 34.5%), and in this case there is a simultaneous increase in the apparent density (from 0.82 to 1.17 g/cm3), and to an even greater increase in strength (from 2 to 20.4 N/mm2). The addition of milled G-00 alumina also containing 99% corundum causes an increase in the shrinkage (with the addition of 60% from 34.5 to 46%), in the density (from 1.17 to 1.42 g/cm3), and in the strength (from 20.2 to 33 N/mm2).
The working layers of furnace linings are being increasingly made of refractory fibrous materials [i]. Various types of materials require different methods of attaching the refractories to the support structures.Tiles are attached with cements [2], or on the faces with T-shaped anchors; roll materials are attached to metallic rods that are welded onto the wall or boundary armatures, followed by securement.At temperatures up to 800-I150~ fixing is done with gaskets made of heat-resistant alloys; and at higher temperaturesceramic anchors of different configuration.In our country for the imported furnaces used in mineral fertilizer production, metallurgy and petrochemical industries, they are using ceramic anchors produced by foreign firms, mainly of two types.They consist of hollow, thin-walled nozzles in the form of a truncated cone with poles on the expanded section.The anchors of mullite-silica composition are 60 mm high and are designed for use at temperatures up to 1350~anchors of mullite-corundum composition (two parts) are 90 mm high, porous, and they can be used to 1550-1600~The USSR is not producing any ceramic anchors except for small batches of experimental chamotte units.Their technical development has been undertaken by the Institute. Both types of anchor are of the rather complex, thin-walled variety, and so investigations were done to choose a batch composition and a method of pressing them.Kaolin chamotte was used to obtain mullite-silica anchors together with clay an kaolin. They were pressed by the plastic method, as this is the most convenient in production.After firing at 1350~ the anchors made of chamotte and clay were free of defects and deformations.Their apparent density was 1.96-1.98 g/cm 3, open porosity 18-19%, and tensile strength 0.30-0.44 N/mm 2.The low tensile strength, the high labor content of the task, and the low productivity made it necessary to resort to the dry pressing method.Chamotte grades ShGR and ShKN fractions <2 and <0.5 mm were used, and the bond consisted of DNI clay and KN-I kaolin~ It is known that the reinforcement of materials with refractory fiber enhances their tensile strength [3, 4]. Chamotte anchors with additions of mullite-silica fibers were therefore tested in this work.To obtain uniform distribution of the fiber in the dry body it was milled to produce a material with a predominant size of 0.16-0.25 and 0.3-0.5 wm. The fiber was dry mixed with the chamotte, moistened, and then the clay was added.Cubes were pressed with edges of 50 mm, which were used to determine the compressive strength, since it indirectly characterizes the tensile strength [5]. The composition and properties of the laboratory specimens fired at 1370~ are shown in Table I.The articles made from ShGR chamotte and Druzhkov clay had a refractoriness under load (RUL) at 0.2 N/mm 2 of 1280~ while the maximum service temperature of the anchors is 1350~ Replacing the ShGR chamotte with ShKN boosted the RUL to 1330~Adding milled fiber to the body, regardless of the grain-size composition, did no...
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