The transfer of molten metal is responsible for consuming more tons of refractory than any other operation in the steel industry.
In order to extend refractory lining life and reduce production cost per ton, modern steelmakers have specified high-performance refractories, including magnesia-carbon or alumina-carbon bricks, for the working linings of transfer ladles. These sophisticated refractory products typically have high thermal conductivity, which allows heat to escape from the melt, raising ladle shell temperatures and reducing pouring time. Recently, high-performance structural insulating products have been developed to reduce heat loss and maximize the service life of ladle refractory.
The steelmaking industry is constantly evolving in order to increase productivity, control costs and meet the needs of its customers. As the demand for new alloys and greater operating efficiency grows, the industry is expecting improved performance from refractory linings in a variety of heat-processing vessels. It is estimated that 50% of refractory purchased by the steel industry is consumed as ladle lining materials for the transfer of molten iron and steel. This high usage makes the transfer of molten ferrous metals the focus of development for steelmakers and refractory suppliers alike.
The traditional refractory-lining cross section for molten-metal transfer ladles will consist of a refractory-brick working lining backed up by a safety lining consisting of either a brick or cast refractory. The working-lining refractory wears, or loses thickness, as the transfer ladle completes multiple pours. Depending on the practice of the mill, the working lining is eventually replaced based upon any of these three criteria: after a set number of pours, when the working lining reaches a minimum thickness or when the ladle shell temperature reaches a maximum value.
The use of an effective backup insulation behind the safety lining may extend the working-lining life or decrease the ladle shell cold-face temperature, both of which yield savings to the user.
|Fig. 2. Silplate® 1308 being installed in a steel transfer ladle|
Extending Working Lining Life
To meet the steel industry’s requirements, the refractory makers have developed high-performance materials for direct contact with molten steel. In order to balance extended service life against refractory cost per ton of metal processed, the refractory industry has introduced new working-lining refractory compositions, including:
- Magnesia-carbon bricks
- Alumina-magnesia-carbon bricks
- Alumina-carbon and ASC bricks
- Fired magnesia and magnesia-chrome bricks
- Burned, direct-bonded dolomite brick
- Resin-bonded dolomite brick
The main characteristic of these new working-lining refractory compositions is their resistance to molten metal and slag. This product feature increases the number of heats provided by the ladle working lining. One performance trade-off encountered with the use of these dense bricks is that these materials usually have a very high thermal conductivity. To optimize safety and performance, operators must consider a significant improvement in the safety lining and backup insulation layer of the ladle.
|Fig. 3. Silplate® 1308 with safety-lining brick being installed|
Backup Insulation Improvement
For the most part, the development of new insulation materials has not kept pace with the evolution of refractory linings for ferrous ladles. Insulation suppliers have continued to offer materials that were designed for less-demanding applications. Most of the material choices available for backup insulation offer either low compression resistance with low refractoriness under loading or high compression resistance with high thermal conductivity.
The AISI Technical Report No. 9 has standardized some extremely important guidelines for molten-metal processing. One important parameter that is recommended to determine the length of the ladle campaign is cold-face temperature. The AISI Technical Report No. 9 recommends that the cold face for ladles constructed with a carbon-steel shell should not exceed more than 750°F (400°C). At cold-face temperatures above this limit, permanent ladle shell deformation and significant fatigue stress may occur. Temperatures above 400°C may result in the steel shell losing its mechanical characteristics, exposing the ladle system to collapse. The main risk associated with high ladle shell temperatures occurs when transporting a loaded ladle at the end of the ladle refractory campaign. At this point, due to the decreased lining thickness, the cold face is near to or exceeds 400°C. A permanent deformation of the shell can occur.
Standard transfer-ladle refractory practice in most steel plants is based on a working-lining reduction to one-third of the original thickness during the campaign. At the end of the working-lining campaign, the interface temperature between the working and safety lining increases considerably.
Optimal ladle-refractory designs must feature material with high refractoriness and low thermal conductivity as the backup insulation. An improper material employed as backup insulation can result in serious damage in the ladle. If the backup insulation does not have a proper compression resistance or an adequate refractoriness, it can shrink and deform. This may result in a gap between the lining and the steel shell. With the molten-metal pressure bearing on the refractory lining, the bricks will tend to move back and occupy the room left by the shrinking insulation. As this takes place, the tight joint between the brick in each ring of the working lining is damaged and a joint attack takes place. This molten-metal attack on the brick joints can reduce the useful life of the lining.
Another common design error is the use of material with high organic content as a backup insulation. In molten iron or steel ladles, the material used as a backup does not achieve a high enough temperature to burn out the organic binder. This partial oxidation of the binder results in carbonization. This leads to an increase in thermal conductivity for the lining system, raising the cold-face temperature instead of reducing it.
|Fig. 4. Completed transfer ladle with working lining, safety lining and backup lining refractories installed|
By understanding the market requirements for this demanding application, a structural insulating board product called Silplate® was developed by Unifrax.
Silplate is a unique structural insulating board developed to support high temperature while providing high compression strength and maintaining low thermal conductivity. The product is designed to keep its physical properties even at maximum working temperature limits. These features stabilize the entire lining system, reducing the joint attack in the working lining and saving energy.
Silplate boards present excellent chemical stability, resisting the attack of most acids and corrosive agents (except hydrofluoride, phosphoric, hydrochloride and sulphuric acids and concentrated alkalis). Made with high-purity materials, the product has a very low Fe2O3 content. Silplate board was specially developed as a backup lining for molten-metal transportation.
The main performance advantages of Silplate structural insulating boards include:
- Higher temperature capability
- Low thermal conductivity
- Excellent mechanical strength
- Dimensional stability
Typical applications include:
- Backup linings in general for ladle, torpedo cars and caster tundish
- Runners troughs – blast furnace cast-houses
- Insulation of electric-arc furnaces
Other insulating materials commonly considered for this application are magnesium silicate, calcium silicate, microporous silica or ceramic-fiber-based board products. Each of these product forms has merit for specific applications. However, for use in molten-metal transfer applications where the working-lining refractory will lose thickness, the Silplate family of board products provides insulating value, crushing strength and refractoriness/temperature limit. Also provided is a balance of properties to maximize ladle refractory performance, safely expand capacity and extend lining life. IH
For more information: Contact Brian Bradley, senior engineering manager, Furnace Related Products; Unifrax I LLC, Corporate Headquarters, 2351 Whirlpool Street, Niagara Falls, NY 14305; tel: 716-278-3806; e-mail: firstname.lastname@example.org