Mechanism of skull formation in hot-metal-car ladles

Abstract

At the Magnitogorsk Metallurgical Combine, pig iron is tapped into conventional loo-ton and 450-ton mixer ladles. However, untimely deliveries of the ladles for tapping have led to disruptions in the tapping schedule and reduced the technical-economic indices of the smelting operation. One reason for the late arrival of the ladles is the difficulty encountered in removing the shell of slag that forms inside them. The t00-ton ladles are particularly susceptible to the formation of these "skulls." The following was established as a result of a study of the reasons for skull formation [1] and changes made to the composition and properties of the pig iron and slag during transport [2] in blast-furnace operation at the MMK: -- the slag phase of the skull consists of blast furnace slag, sand, gunite, parts of the refractory lining, and graphite (flakes); -- the skull has high concentrations of SiO 2 and TiO 2 (these concentrations reach 4.5% in some samples, even though the concentrations in the constituent materials are no greater than 1.6%; this shows that titanium compounds accumulate in the skull due to incomplete drainage of the pig iron from the ladle and poor cleaning of the latter); -- skull growth is promoted by accelerated cooling of the pig iron in the ladle (up to a rate of 1.0-1.6~ -- a decrease in the basicity (CaO/SiO 2) of the slag phase of the skull to 0.6-0.7 is accompanied by broadening of the temperature interval in which intensive crystallization begins due to an increase in viscosity; -- massive skull growth in ladies during isolated periods is due to the entry of slag-forming materials into the ladle when ladle turnaround increases. It was also determined that the greatest problem with skull formation is encountered in the blow-in of a blast furnace after a class III overhaul. The reason for this is the low temperature of the pig iron, particularly in the first ladles receiving metal from the first taps. Here, pig temperature may be just 1280-13OO~ Slag temperature is the same as pig temperature in this case and the slag is more viscous, which creates favorable conditions for its crystallization. Thus, the mechanism of skull formation in hot-metal-car ladles can be described as tbllows: -- blast furnace slag, sand, and gunite fall into the ladle along with the pig iron. Erosion of the refractory lining increases the concentration of the slag phase in the skull. During cooling of the pig iron in the course of tapping and transport, titanium accumulates in the slag phase in the form of TiO 2. Carbides and nitrides of titanium may also form under these conditions [3]; -- this slag has a high viscosity due to the high content of SiO 2 and titanium compounds, which makes the slag more likely to crystallize; -- a decrease in pig temperature accelerates solidification of the shell of slag. A further reduction in temperature might also lead to the formation of a salamandar in the ladle; -- to prevent or alleviate skull formation in hot-metal-car ladles, it is necessary to prevent cooling of the pig to the critical temperature by shortening the time it is held in the ladle. An empirical relation was established to determine the critical temperature of pig iron in the ladle (tr) as a function of the temperature of the iron when tapped (t) and the time it is held in the ladle (t): r~ = a-t - b(t - c) 3, where a, b, and c are proportionality factors having the following values: a = 0.95 during blow-in; a = 1.0-1. I during normal furnace operation; b = 1.9-2.1; c = 3.5-4.5. When the temperature of the pig iron in the ladle is below t T, massive skull formation will occur, and even a salamandar may form.