By James Mullins, Mullins Professional Services
Technical Director of DIS
There have been several studies done on this subject, commissioned by AFS and others, which have shed some light on methods to improve melt yield. One study done by W.M. Nicola and V.L. Richards done as AFS Research, included minimizing oxygen (in air) input into the furnace and the effect of rusty steel scrap, boring briquettes and several pig irons on recovery. They measured slag generation as well as yield, and determined optimum melting methods from trends seen. Certainly increasing the amount of rust on charge materials, the surface area to volume ratio of the charge materials and the amount of slag produced from them can be affected by the furnace operating conditions.
The results, from this actual melting study, showed that melting with a sealed furnace was preferred to using an open furnace, thereby increasing metallic recovery and reducing slag production. (Note that sealing the furnace means to cover the open top with a high temperature ceramic blanket, which drastically reduces heat evolution as well as air introduction.) In addition carbon and silicon recovery were improved over using open top melting furnaces. Even the impact of rust on increasing slag generation and reducing carbon recovery appeared to be significantly negated.
Some other observations were that when using thin steel scrap carbon recovery tended to decrease and slag increased with open melting conditions, probably due to oxidation on heating. This slag became worse when using rusty charge materials. Also carbon recovery was improved as the charge was cleaned up. Charging cast iron boring briquettes produced the highest melt losses, some of which was oil and water counted in the material weight.
This study was not totally conclusive, but showed trends and contributed to the knowledge base about melting. Usually everything that we can do to reduce slag helps the bottom line. This includes decreasing slag handling and disposal costs, reducing the effect of rust and slag on refractories, reducing heat losses and improving metallic yield and carbon recovery. All of this can be easily accomplished by using a cover, which seals the top of the furnace well, thus minimizing air input. This cover can be insulating refractory installed on a good metal cover or ceramic fiber blanket, but in either case they must seal properly.
Other work done on this issue of minimizing oxidation losses was a paper done by K. Copi for an AFS Melting Conference. This involved melting in medium frequency furnaces, where he explained several methods to reduce oxidation.
In addition to buying clean scrap and keeping it clean in storage, the operation of a preheater is important. As thin steel scrap is heated above 1200 degrees F, it becomes more susceptible to surface oxidation. Reducing superheating and air input on this scrap can be beneficial to improving melting yield. This thin scrap can present such a large surface area, that it will rapidly absorb heat and oxidize. Using heavier and thicker scrap and/or reducing oxidizing flame impingement and excessive heating of the charge will also help. Note that carbon present in returns and pig iron reduces this oxidizing effect on them somewhat.
The other cost reduction issue comes from modifying the batch melting process, where thin scrap is again heated in an air atmosphere without the protection of carbon in or around it. Keeping a smaller molten heel (15% of capacity) than what was previously used in main frequency melting (up to 1/3 of furnace capacity), where the carbon content is relatively high, has advantages. It allows the steel scrap to be quickly absorbed and recarburized lowering its melting point and reducing any oxidation. This method can allow of greater power input initially as well. Also the carbon additive is more protected from air, oxide and slag thus reducing losses. Adding carbon and alloys directly to the bottom of an empty furnace is not the best practice. They may fuse together or to the bottom and not present the maximum surface area necessary for good absorption. Furthermore when keeping a heel, the amount of one time additions necessary would be somewhat reduced. However, using a preheater is almost always necessary when heel melting, but heating the scrap up to 900 degrees F when done properly can be beneficial to reducing electrical energy input.
If the furnace must be used as a batch melter, a different charge order should be helpful. Instead of heating the steel scrap, which can get to a very high temperature very fast in air where it will be easily oxidized, charge higher carbon, higher density material (pig and dense returns) into the furnace first. They will melt more quickly and at a lower temperature than steel scrap, which must be carburized to lower its melting point. This will again create a high carbon heel as described above, allowing increased power input into the melt and reduce melting time. These steps in combination with covering and sealing the furnace will reduce slag production and melt losses, shorter heat times and lower melt costs. One more benefit may be increased consistency in chemistry results, because of improved recovery, which can take one more variable out of the melting equation.
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