Firing dinas goods in an oxidizing atmosphere in a tunnel kiln

Abstract

The features and difficulties of firing dinas products are due to the complexity of the silica polymorphism and the relatively low thermal conductivity of dinas. Higher rates of heating and cooling the products in firing, especially in the intervals of polymorphic inversions of silica with volume changes, lead to e~treme stresses, causing embrittlement of the green brick [1]. Therefore, during the firing of dinas goods it is necessary to observe strictly the stated firing curve and soaking and cooling cycles, and also to ensure a uniform distribution of temperature across the volume of the setting.The most dangerous range is the soaking, since even small increases in temperature in the kiln channel in this section lead to overfiring and loss. The maximum firing temperature is determined by the original physicochemical properties of the quartzites, and also the production parameters, depending on the range of dinas articles, and equals 1380-1450~The stated soaking temperature can be realized both in the reducing and in the oxidizing atmosphere. Figure 1 shows the relationship between the theoretical combustion temperature of natural gas tg and the coefficient of air consumption ~ obtained on the basis of calculation data [1]. It is of interest to examine two regions, in one of which ~ < i (reducing medium), and in the other ~ > 1 (oxidizing). In the reducing medium the change in the coefficient of air consumption in a wide range (0.5 < ~ < 1) causes a smooth change in the temperature of the gas flow. When ~ > 1, the temperature of the gas flow changes rather sharply. Hence, in a reducing atmosphere in the kiln channel it is easier to maintain the stated temperature than in an oxidizing atmosphere. Therefore, in existing periodic and gas-chamber furnaces, in which the conditions for accurate regulation of the coefficient of air consumption are absent, in order to attain a smooth rise in temperatures with low variations in the process of regulation, especially in the soaking process, it is desirable to use a reducing atmosphere.The main working feature of tunnel kilns, in contrast to periodic and gas-chamber, consists in the mutual influence of temperature, hydraulic and aerodynamic regimes in the zones of preheat, soaking, and cooling with respect to each other.The existence of a reducing atmosphere in the soaking zone leads to the formation of conditions in which, the hot air advances to the cooling zone in the latter positions of the soaking area, causing precombustion of the gas. This causes a sharp increase in the temperature of the goods, their overfiring, and hence massive loss.Studies carried out in the 183-m tunnel kiln of the Pervoural' sk Dinas Factory showed that in a reducing atmosphere it is impossible to obtain a stable temperature regime in the soaking zone. In the latter positions of this zone the temperature of the goods increased to 1500~ and the loss in certain articles reached 6@/0.In 1972, as a result of reordering the working of the burners in the firing zone, an oxidizing atmosphere was estab...