Steel mill thermal energy recovery for calcination

#The present invention relates to the recovery of the thermal energy from steel mill waste gases for calcination.

#In electric arc melting of steel, the energy efficiency and overall cost is greatly improved by the creation of a basic foamy slag within the furnace. Foaming of the slag in order to engulf the electric arc is well known in the art, and is described, for example, in U.S. Pat. No. 4,447,265 and in U.S. Pat. No. 4,528,035. The procedures described involve the injection of carbonaceous material, such as lime, into the furnace slag to cause carbon monoxide gas evolution, and the escaping gas causes the slag to effervesce or foam. As a result of the effervescence or foaming, the slag is increased in its volume, engulfing the electric arcs and improving electrical and heat transfer efficiency. Also described in the prior art, calcium oxide (lime) is sometimes injected along with the carbon and the oxygen in order to raise the slag basicity and stabilize the foam that is created. In addition, other benefits are derived such as reduced refractory lining wear, higher productivity, reduced graphite electrode consumption, and increased yield of iron units from the melted scrap. Foamy slag practice is arguably one of the greatest improvements to electric arc steel melting technology of the 20th century.

#Several steel mill operations within a single plant may result in offgas streams which contain large quantities of thermal energy. For the most part, steelmaking operations are intermittent and as a result, many of these gas streams are not continuous. In addition, the thermal load contained in many of the offgas streams fluctuates throughout the steelmaking cycle. As a result, it has generally not been possible to attempt recovery of the thermal energy contained in the offgases. One notable exception is the plethora of scrap heating processes developed over the past 30 years. The offgases produced in the electric arc furnace are passed through scrap in order to pre-heat the scrap prior to charging it to the furnace. The scrap may be held in a scrap bucket, a pre-heating tunnel or a shaft sitting above the furnace. (For example, U.S. Pat. No. 4,328,388, U.S. Pat. No. 4,543,124, U.S. Pat. No. 5,153,894, U.S. Pat. No. 5,264,020 and U.S. Pat. No. 6,024,912). For preheating scrap offgas temperature fluctuations are acceptable and a continuous off gas stream is not necessary.

#While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the same can be better understood from the following description, taken in conjunction with the accompanying drawing, in which:

#FIG. 1 illustrates a schematic view of an exemplary embodiment of a steel mill recovering thermal energy of waste gases in accordance with the present invention.

#Reference can now be made in detail to various exemplary embodiments of the invention, some of which are also illustrated in the accompanying drawing. Throughout the specification and claims, all parts and percentages are by weight unless otherwise specified.

#In one embodiment and as shown in FIG. 1, a steel mill 10 recovers some of the thermal energy from the steel making process for the calcination of lime. In this embodiment, the steel mill 10 has a bag house 20, a melt shop 30, a caster 40, and a rolling mill 50. This steel mill 10 also includes a kiln 70.

#The rolling mill 50 has a tunnel furnace 60 for heating or maintaining heat in slabs supplied by the caster 40. Hot waste gases, usually about 2000° F., from the tunnel furnace 60 are exhausted into the exhaust system 62. In the prior art, the exhaust system typically exhausts all of the waste gas to an exhaust stack. However, in this embodiment of the present invention, at least some of the exhaust gas is fed to the kiln duct 72. The kiln duct 72 delivers the hot waste gases to the kiln 70.