Economic Optimization of BOF Raw Materials Consumption in a Volatile Scrap Market and With Limited Hot Metal Availability

摘要

All the stretching tools tried performed well in the EOF without major difficulty. Table 1 summarizes the recent Mittal Steel USA -Sparrows Point results and other methods used in the past (including the Bethlehem plant experience). The table shows that there are nearly 20 ways to "skin the hot metal stretching cat." Sparrows Point and other shops use some other techniques in addition to those tried recently. For example, many shops overblow heats to lower carbons, often purposely to stretch metal. Mittal Steel Riverdale taps to lower-than-usual temperatures to stretch and recover the energy and chemistry at the ladle furnace. (Sparrows Point does not have a ladle furnace.) The old Bethlehem plant recovered significant energy by using insulated sub covers and fewer subs. Riverdale also uses the sub covers plus rice hull insulation to mitigate the effects of a seven- to 10-hour, 25- to 30-mile-long trip from the blast furnace. Many shops would love to have HBI or DRI to avoid the need for multibox scrap charge -- if the material is available. Most shops use some amount of cold pig iron. Scrap preheating schemes, while proven effective in the 1960s, '70s and '80s, are not being considered because of high capital cost and/or major extensions of process time. Plastic and other waste material-based fuels do not work well due to low density and potential environmental problems. Aluminum bulk addition trials have been unsuccessful in the past. The Sparrows Point trials confirmed the Bethlehem plant experiences with anthracite coal and postcombustion lances (PCL) and the early trials performed by Wheeling-Pittsburgh Steel using calcium carbide. However, handling and loading are safety issues that must be addressed. The materials' potency and cost are attractive. However, it must be weighed against shop logistics and safe handling issues. During the trial, most of the stretching effects of the various tools were linearly additive, and there was only one interaction effect between SiC and CaC_2. There were no apparent environmental effects with any of the methods tried. Concerning scrap issues, scrap yard loading capability is influenced by the desired scrap mix and by meeting the scheduled BOF charging times. Thus, when stretching hot metal, all these scrap issues must be in sync with sustained high scrap charge box weights. The higher the box weights, the more dense scrap must be available in the scrap yard to stretch hot metal and still maintain a high production rate from the BOF shop. Finally, as a result of the prior work at the Bethlehem plant, and recent Riverdale and Sparrows Point trials, Mittal Steel USA has learned a number of practical techniques to deal with hot metal shortages for both long and short terms. Hot metal percents can quickly and easily be reduced from 77 to 66 percent with a judicious combination of stretching techniques. There is significant activity under way at the remaining Mittal Steel BOFs (Burns Harbor, Cleveland and Indiana Harbor) to develop experience in many of these techniques.