Production technology for refractories using sinter-reactive chromium oxide bodies and their glass resistance

Abstract

The working term of glass furnaces and the quality of the glass they turn out largely depend on the choice of refractories. The corrosiveness of molten aluminoborosilic ate glasses make it impossible to use traditional materials in the tank for freaking glass fiber. The resistance of fused quartz blocks even with the use of evaporative cooling is too low. The only known material capable of standing up to this type of melt for long periods at temperatures above 1500~C is chromium oxide. Chromium oxide refractories in the form of ground blocks are used for the bottom and walls of the tank in the melting and working zones. The Ukrainian Institute has developed a method of making chromium oxide blocks designed for building the tanks of glass furnaces used to melt colorless aluminoborosilic ate glasses. The technology was mastered at the experimental factory of the Institute. It is known that dense uniform products made from finely dispersed sinter-reactive powders are very resistant to the action of corrosive media and in this regard are better than articles obtained from ordinary granular powders. The new method is based on the common principles of obtaining articles from sinter-reactive materials [1]. Technical chromiun oxide of metallurgical grade OKhM with 99~ Cr203 (GOST 2912-73) was used; it consisted of a powder with a predominant grain size of 2-30/za. In order to accelerate sintering with the aim of getting dense and strong articles from chromium oxide, we used a combination of three methods of activating the materials: fine milling, additionof sintering additives, and firing in a reducing atmosphere with a partial pressure of oxygen Po2=l.013x 102-1.013 10 -2 Pa (10-3-10 -7 kgf/cm2). The chromium oxidewas ball-miUedby the dry method to obtain a predominant grain size of less than 10 pro. After 10-12 h grinding, the sintering additive was put in the mill (4% preparation T). The total milling time is 18 h. The ratio of material :bails in the millwas 1 : 1. Mixing was controlled with a microscope. The milled chromium oxide with the sinter additive is moistened in a z-shaped mixer with organic combustible additions to a body water content of 6-6.5%. After being moistened, the body passes through an attrition screen with apertures of 3-5 ram, and is left in a closed container for aging over 1-2 days. The blocks weighing 90 kg or more are pressed on 630-ton screw friction presses to a green density of 3.36-3.38 g/cm 3. Directly after pressing and drying, thegoods are free of repressing cracks, and possess a strength that is sufficient for transportation to the firing site. The compressive strength of the green brick immediately after pressing is (59-64) x 104 Pa (6-6.5 kgf/cm2), and after drying (26-32) x 105 Pa. The green brick is dried in the workshop (in natural conditions) to a residual moisture content of about 1%. The dried brick is fired in periodic kilns heated with natural gas at 1650~ in closed muffles. These are placed parallel to the flow of gases at a distance of 120 mm from each other in order to form gas channels and ensure uniform heating and cooling of the goods. A reducing atmosphere with PO 2-1.013  10 -2 Pa is maintained during firing. The firing time was increased so as to provide uniform removal of the residual moisture and uniform heating of the green brick: firing and cooling together take 250 h.