Recirculation degassing

In the recirculation degassing technology, molten steel is allowed to circulate in the vacuum chamber continuously by special arrangement.

In RH degassing technology (developed by Rheinstahl Heinrich – Shutte at Hattingen, Germany),a cylindrical refractory lined shell with two legs (also called snorkel) is designed such that steel is raised in one leg and falls back into the ladle after degassing through the second leg. Top side of the cylindrical shell is provided with exhaust, alloy additions, observation and control window. Cylindrical

Shell is lined with fire bricks in the upper portion, and alumina bricks in the lower portion in order to sustain high temperature. The legs are lined with alumina refractory.  A lifter gas argon is injected at the inlet snorkel in order to increase the molten steel velocity entering into inlet snorkel. Figure26.3 shows a schematic sketch of a RH degassing unit.

The operation of RH degasser is as follows:

i)  Cylindrical chamber is heated to the desired temperature (varies in between  to  in      different plants).

ii)  The chamber is lowered into molten steel up to a desired level.

iii)  The chamber is evacuated so that molten steel begins to rise in the chamber. Lifter gas is introduced.       This gas expands and creates a buoyant force to increase the speed of molten steel rising into the        inlet snorkel.

iv)  Molten steel in the chamber is degassed and flows back through the other snorkel into the ladle. This       degassed steel is slightly cooler than steel in the ladle. Buoyancy force created by density difference       density of cooler liquid steel is > hot steel) stirs the bath

v)  Rate of circulation of molten steel in cylindrical chamber controls the degassing. Circulation rate           depends upon amount of lifter gas and the degree of vacuum. A 110 T steel can be degassed in 20      spminutes by circulating molten steel at 12 tons/min., amount of argon is around                             .

vi)  Alloy additions can be made at the end of degassing depending on the superheat.

vii)  Process has several advantages like

• Heat losses are relatively low.
• Alloy additions can be adjusted more closely
• Small vacuum pumping capacity is adequate since smaller volume is to be evacuated as compared with ladle to ladle or stream degassing.