RH Vacuum Degassing Technology
RH Vacuum Degassing Technology
Vacuum degassing is an important secondary steel making process. This process was originally used for hydrogen removal from the liquid steel but presently it is also being used for secondary refining and has become increasingly important process of secondary steel making. Pressure dependent reactions are the reason for the treatment of liquid steel in this process. There are many vacuum degassing systems but RH type degassers are very popular. The RH process has been named after Ruhrstahl and Heraeus where this process was initially developed. The RH degassing technology was first introduced in the late 1950s and presently it has become an important tool of secondary metallurgy in most of the modern steel plants.
The RH circulation degassing process has proved its vast suitability in large number of shops worldwide, for operation with short tap to tap times covering heat sizes up to 400 tons. The vacuum treatment in RH plants produces steel which fulfills the demand of high steel qualities. To achieve this, the liquid steel is allowed to circulate in a vacuum chamber where a considerable drop in pressure causes it to disintegrate into the smallest of the parts. The increase in the surface area allows the liquid steel to degas to the best possible extent. The process needs reliable vacuum units since it should be able to suck off very large flow rates under very difficult conditions of dusty atmosphere and high temperatures. The mechanism of the vacuum treatment of liquid steel in RH process is shown in Fig 1.
Fig 1 Mechanism of the vacuum treatment of liquid steel in the RH process
RH degassing process
The process mainly consists of a refractory lined cylindrical reaction vessel with two steel pipes attached to the bottom of this vessel. The reaction vessel is lined with fire clay bricks in the upper portion and alumina bricks in the lower portion. The two steel pipes are the inlet and the outlet snorkels. Both of them are completely refractory lined with alumina refractories on the inside but only the lower part is refractory coated on the outside. The inlet snorkel is equipped with a number of gas injection pipes arranged in the lower section in one or two levels and equally distributed around the circumference. The reaction vessel is designed such that the liquid steel is raised through the inlet snorkel and falls back into the steel ladle after degassing through the outlet snorkel. Top side of the reaction vessel is provided with exhaust, facilities for alloy additions along with observation and control windows.
The degassing process is started after both snorkels are sufficiently immersed into the liquid steel. Before snorkel immersion the injection of inert gas, usually argon, is started in the gas pipe of the inlet snorkel. Argon acts as a lifter gas to increase the molten steel velocity which is entering into the inlet snorkel. After achieving the required immersion depth of the snorkel, the reaction vessel is evacuated by means of a vacuum pump system which is connected to the reaction vessel through off take duct (exhaust). The liquid steel density at 1600 deg C is assumed at 6.94 tons per cubic meters. The atmospheric pressure exerted on the ladle surface causes the steel in the snorkels to rise to a barometric height of approximately 1.45 m under deep vacuum conditions.
Since the RH process is based on the exchange of liquid steel between the steel ladle and the RH vessel, the rate of steel recirculation determines the velocity of metallurgical reactions and the duration of the process assuming a defined metallurgical target. Liquid steel circulation depends on the geometry of the equipment such as snorkel diameter, the radius of the equipment, and the position and number of lift gas tuyeres.
Process characteristics and development
When the RH process was introduced, the primary objective was to reduce hydrogen content in the liquid steel. The first result was not as successful as expected due to insufficient vacuum in the vessel. The application of steam ejector vacuum pumps in the early 1960s enabled sufficiently low pressure to be reached, leading to hydrogen contents of less than 1 ppm. Since then, RH process is continuously being developed with respect to vacuum condition, reaction vessel design and geometry (size and shape), cross section of snorkels and capacities of RH units.
Application of the RH process for decarburization was introduced at the end of the 1970s. Today extremely low final carbon contents of less than 20 ppm can be obtained, as needed for the production of automotive sheets. The addition of alloying elements during degassing has the advantages of achieving higher yields for ferro alloys and high accuracy of chemical analysis of steel due to the absence of air and the avoidance of metal slag reactions.
Further developments were the use of gaseous oxygen during RH treatment in RHO, RH-OB, RH-KTB, RH-MESID, and MFB processes. The aim of these processes were to accelerate the decarburization reaction, to reheat liquid steel by alumino-thermic reaction, to remelt skulls, to keep vessel at high temperature by converting generated CO gas to CO2 gas during the decarburization period and to heat the refractory lined vessel between the treatments. Recently, some RH TOP lances have been used for blowing powder onto the liquid steel for reducing sulphur or carbon contents to the lowest contents. Today all these processes, except RH-OB, are called RH TOP.
RH vacuum processes generally donot reach equilibrium and the amount of hydrogen, carbon and nitrogen removal are governed by kinetic considerations. The decarburization mechanism is fairly complex as the reaction kinetics depends both on the circulation rate and the rate of decarburization. The bath mixing has also effect on the decarburization.
The following are the steps in the operation of RH process.
- Reaction vessel is preheated to the desired temperature which usually varies in the range of 900 deg C to 1500 deg C as per the plant requirements.
- Reaction vessel is lowered into the liquid steel to the desired level.
- Reaction vessel is evacuated so that the liquid steel begins to rise in the vessel. Lifter gas is introduced. This gas expands and creates buoyant force to increase the speed of the liquid steel rising into the inlet snorkel.
- Liquid steel in the reaction vessel is degassed and flows back through the outlet snorkel into the steel ladle. The degassed steel is slightly cooler than the liquid steel in the steel ladle. Buoyancy force created by density difference (density of cooler degassed liquid steel being greater than the hot liquid steel in the ladle) stirs the bath.
- Rate of circulation of liquid steel in the reaction vessel controls the degassing. Circulation rate depends upon the amount of lifter argon gas and the degree of vacuum.
- Alloy additions can be made at the end of degassing depending on the superheat of the liquid steel.
Schematic diagram of RH installation
The schematic diagram of a 145 ton RH unit with auxiliary equipment as installed in steel melting shop of Voest Alpine Stahl, Linz, Austria is shown at Fig 2.
Fig 2 Schematics of 145 ton RH installation at Voest Alpine, Linz, Austria