Deformation properties of synthetic mullite refractories at high temperatures

Abstract

The production of high-alumina refractories on the basis of technical alumina and clay is being extensively developed. The basic versions of the production technology are being perfected as a result of research being done at the D. I. Mendeleev Moscow Institute of Chemical Technology [i], the Ukrainian Scientific-Research Institute of Refractories [2, 3], the Podol'sk refractories factory* [4,5], and in other organizations. However, this technology for high-alumina refractories, being traditional, is complicated by the need to make two firings (briquet and finished goods), and double milling (alumina and chamotte).The Eastern Institute of Refractories has developed a simplified method for making synthetic, high-alumina refractories by direct synthesis from finely milled alumina and granular quartz [6,7].The working proprties of aluminous refractories, including their load resistance at high temperatures, largely depend on the ratio of the crystalline phases making up the brick. Suitable distribution of crystalline phases and a small amount of glassy phase are the distinctive features of high-alumina materials obtained by the direct synthesis of mullite from alumina and silica of different grain sizes, using a method developed by the Eastern Refractories Institute.Synthetic mullite refractories made from coarse-crystalline quartzite and fine corundum, after firing at 1600~ contain (parts by volume) 40-75% mullite, 20-45% corundum, 5-12% metacristobalite, and 5-7% glass. The peculiarity of the microstructure of these materials is that the finely dispersed bond, i.e., the matrix, has a mullite-corundum basis, and the granular framework consists of mullite and cristobalite. Therefore, the technical properties of the synthetic mullite refractories mainly determine the high-temperature subsystem 3AI2Oa'2SiO=--AI2Oa in which the liquid phase appears at 1850~[8].This article gives the results for the deformation properties of synthetic mullite, made from granular quartzite (fractions 0.I-i mm) and fine corundum (particles sizes 7-40 ~m), in relation to the applied loads, test temperature, and soaking time. The behavior of these materials at high temperatures was compared with that for high-alumina brick made with the conventional methods. In addition, in order to predict the behavior of the new materials in actual conditions, we studied their deformation after prolonged heat treatment (aging) at 1600 and 1700~ for 50 and I00 h.For comparison, we chose high-alumina articles MKP-72 and MKS-18 made at the Semiluki Refractory Plant (SOZ) and also mullite and mullite--corundum articles whose production technology was developed by the Ukrainian Institute of Refractories [9]. High-Alumina (mullite) chamotte used in them was obtained from kaolin and technical alumina. Calcined finely milled alumina was also added to the mulllte--corundum batches, in addition to mullite chamotte.In terms of the weight proportions of alumina, the selected high-alumina refractories, compared with the new articles, were the same, or had a higher con...