Michael J. Strelbisky, General Manager, James Manning, Project Consultant, A.H. Tallman Bronze Co. Ltd.
Slag splashing has become a powerful tool to be used by the steelmaker to increase furnace life, maximize production from existing equipment and reduce refractory and gunning costs. Slag splashing will be discussed in this article within the context of an overall refractory improvement program. Emphasis will be on current slag splashing practices in selected mills.
LASERING OF THE FURNACE
An essential element in an overall refractory improvement program is the ability to monitor furnace wear so that an optimum furnace wear pattern can be developed when the furnace is producing its highest yields. Modern laser equipment provides a complete measurement scan of the vessel in about 12 minutes and scans of specific wear areas are completed much faster still. Laser measurement programs are used to monitor furnace wear and to identify specific areas in the furnace in need of repair. Laser measurement scans are used to determine when slag splashing and or slag coating is required.
During the last decade slag coating methods have been developed to extend vessel life. Slag coating is a process whereby a portion of the slag is retained in the furnace after tapping and the slag is used to repair or build up worn areas on the refractory surface. These areas are reached by rocking the furnace and allowing slag to flow into worn areas of the refractory material. The refractory qualities of the slag are equal to or better than the original brick used in the furnace.
Slag coating has become a common method to repair the tap belly, the charge pad area and the furnace bottom. By rocking the furnace over the area in need of repair, a layer of slag is built up to cover the low spots.
Very low bottoms, as an example, can be repaired by leaving slag in the bottom of the vessel and allowing it to solidify. A similar procedure may be used to repair a low spot in the charge pad and tap bell area. Although this method of repair is successful, it takes the vessel out of production for at least the time solidification of the slag takes place. Regular coating practice of rocking the vessel back and forth over the repair area takes much less time and is thus more economically desirable.
Slag quality is important to ensure the success of slag coating and slag splashing procedures. A low FeO (iron oxide) and a high MgO (magnesia) slag which has a creamy to gummy consistency is desirable for both slag coating and slag splashing. Slag magnesia levels of 8% to 14% have been suggested by mill operators as an acceptable range for both slag coating and slag splashing. Slag magnesia levels of 8% or more can lead to poor metallurgical results. Poor phosphorus removal is the most common product of high magnesia levels.
Flux charges must also be changed so as to reach the magnesia saturation point of the slag. A high magnesia charge ensures that the thermal qualities of the slag are similar to those of the original refractory material. Such improvement in slag condition is achieved through the addition of dolomitic lime after tapping. Alternately, an increase in the addition of dolomitic lime and a reduction in the burnt lime in the flux charge, produces a high magnesia slag. Optimum slag conditions can be achieved and maintained with closely monitored magnesia levels.
Slag coating is not an effective solution for repairs to the sides of the furnace. Prior to the invention of slag splashing, gunning (spraying of refractory material with a telescoping boom) was the preferred method of repairing the sides of the vessel. The gunning material wears very quickly and must be reapplied frequently.
The charge pad and tap pad can also be repaired by pouring a refractory slurry patch on to the affected areas. This repair method must be done when the furnace is off line and can take up from 5 to 48 hours to dry.
Some success for side cone repairs has been seen when the slag was splashed onto these areas with the refractory gun. The furnace is placed in a position where a pool of slag can be reached with a gunning pipe. Using the refractory gunning system, air and water is injected into the slag, splashing it up onto the cone. This is a hot and slow process. In several cases the air/water mixture does not have sufficient velocity to carry the slag onto all areas to be repaired.
Whereas slag coating coats only the areas that can be reached by rocking the furnace, slag splashing methods can be used to coat all areas of the furnace.
Slag splashing is accomplished by injecting nitrogen into a conditioned slag at a given flow rate and lance height. The existing oxygen lancing equipment is used. Varying lance height and nitrogen flow rates slag can be selectively targeted and blown into particular areas of the furnace. Slag splashing is faster than slag coating. The process time for slag splashing is between 1 to 4 minutes.
Large repairs to the bottom, tap belly and charge pad must still be carried out by slag coating and refractory gunning.
A well designed nitrogen slag splashing program can extend furnace life to 8000 heats. Records are being set with some steelmakers exceeding 10000 heats per campaign. Other cost savings are evident.
Slag splashing usually begins when the laser readings show a defined wear area in the refractory. Typically, slag splashing is initiated when the refractory thickness is reduced to 4 or 5 inches. The wear areas usually express themselves in the barrel trunnion area.
Once slag splashing is started it should be done on a regular basis. Indications are that most mills practice slag splashing after every heat. Regular laser readings are taken to make sure that the desired furnace refractory profile is being maintained. Successful slag splashing requires close monitoring of slag composition, viscosity and temperature. Consistency in operating procedures, persistent employee training, employee involvement in design and program implementation and good communications are also part of the formula for success.
Although the benefits outweigh the risks, slag splashing presents some operating challenges. Some of the disadvantages of slag splashing are lance skull buildup, bottom buildup due to the high magnesia slag and mouth and cone buildup.
By trying new practices with respect to blowing height and nitrogen flow during slag splashing most of these problems can be solved. Some mills remove the build up on the lance on a regular basis and find that lance skull buildup can be reduced by making sure that the slag is free from steel. Some mills use an alternative lance to slag splash.
A program of regular bottom washing is critical once slag splashing begins. Slag used to cover other areas of the furnace also has a tendency to accumulate on the furnace bottom. The object of bottom washing is to produce a hot high FeO slag and have it melt away the slag bottom buildup. Care must be taken not to wash the knuckles out of the furnace. Bottom washing practices must be developed to meet each mill's need. A bottom maintained at the proper height returns good yields and cleaner operation by reducing slop.
The mouth and upper cone buildup can be overcome by using a properly designed deskulling lance. Extreme care must be taken when using a deskulling lance as it can damage furnace refractory very quickly. Mechanical means of removing the mouth and cone build up have proven effective.
SLAG SPLASHING PRACTICES
In North America 10 mills are currently either experimenting with or using slag splashing as a standard practice. In 1996 at least 5 additional steel mills will be instituting a slag splashing program. Figure 1 outlines actual practices a 4 North American mills. As Figure 1 shows , some mills have elaborate slag splashing practices. In some mills slag splashing is used as required and determined by laser readings, in other mills a single slag splashing practice is preferred.
Since all shops are different in furnace and oxygen supply systems, each nitrogen system requires matching to the individual plant. Each practice and procedure must be custom designed to meet each mill's objectives for slag splashing. Mills develop slag splashing practices by trial and error.
Another interesting observation is that Mills 1 and 2 slag splash every heat while Mills 3 and 4 slag splash only as required. Mills 1 and 2 report that slag splashing is practiced every heat to maintain a consistent practice and to take the decision to slag splash or not to slag splash out of the hands of the operators. Mills 3 and 4 on the other hand, rely on laser measurement results to determine when and what location in the furnace slag splashing is required. Figure 1 also indicates the slag addition practices of each mill. Mill 4 has adjusted their flux calculation to produce the desired slag consistency for slag splashing whereas the other mills check the condition of the slag after tapping to determine if additions to the slag are required.
Most, if not all the slag splashing practices indicated in Figure 1 take into account furnace design limitations. Some mills have a limited nitrogen supply while others can only lower their lance so far into the furnace.
LANCE TIP DESIGN
Most mills use the same lance tip for both oxygen injection and for nitrogen slag splashing. These lance tips where originally designed to match the BOF shape and melting needs. If the lance tip port angle is too light slag splashing may cause buildup at the cone and mouth, it may also cause large buildup on the lance. If the lance tip port angle is too wide slag will only be splashed onto the lower part of the furnace.
Most mills do not have the luxury of having a lance that is dedicated solely to slag splashing and use the same lance for blowing oxygen. In most cases the existing oxygen injection lance tip appear to work adequately for slag splashing. Work is currently being done to develop an optimum lance tip design for slag splashing.
Another avenue of lance tip redesign can also be explored. Most lance tip designs do not take into account new operating conditions and opportunities brought about by nitrogen slag splashing. Refractory wear can now be controlled by slag splashing and the lance tip designer can focus on optimizing turn down conditions while addressing the slag splashing requirements of the lance tip.
As mentioned above, slag splashing can reduce gunning costs by over 75% while reducing brick cost due to the reduction in the required number of relines. Furnace utilization is also significantly increased. The cost to purchase and install the required equipment to slag splash varies from mill to mill. Cost can range from as little as $400,000.00 to as much as $1,400,000.00 or more. These costs do not take into account the additional costs of upgrading the nitrogen gas vendor's equipment. These upgrade costs are usually passed on from the gas supplier through increased nitrogen costs. The payback for slag splashing comes in the form of furnace availability, reduced gunning and fewer relines. Payback justification is usually less than 12 months.
SUMMARY AND CONCLUSIONS
The steel industry has shown increased interest in slag splashing. Interest in slag splashing will continue to increase because of the cost saving involved. Slag splashing is a proven cost effective technology for increasing furnace life thereby maximizing production from existing equipment and reducing refractory costs. The condition of the slag is critical to a successful slag splashing program. As can be seen in Figure 1 steel mills use a variety of slag splashing practices to reach their operating and cost reduction goals. These practices take into account each mill's unique furnace design constraints. Slag splashing practices are developed through experimentation and close communications between the operators, supervisory staff, and vendors is critical. Work is currently being done to respond to the steel industry's needs for optimized slag splashing and improved turn down performance through lance tip redesign and re-engineering.
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