FIELD OF THE INVENTION
The present invention generally relates to steelmaking in Basic Oxygen Furnace (BOF).
In particular, the invention relates to separation of molten steel and slag at the end of
steelmaking process in Basic Oxygen Furnace (BOF). More particularly, the invention
relates to a slag dart for enhanced separation of slag from molten steel in a steel
making process adapting a Basic Oxygen Furnace (BOF).
BACKGROUND OF THE INVENTION
At the end of the Basic Oxygen Furnace (BOF) process, molten steel and slag needs to
be separated as the slag is an undesirable element leading to impurities and resulting
in subsequent reduction in steel quality. The steel and slag forms two phases with
different density thus making the slag overlie on the molten steel. In conventional
separation method, a refractory shaped dart is used whose specific gravity is less than
that of the steel, but greater than that of the slag, so that the dart tends to "float" at
the interface between the molten steel and slag. The slag dart is inserted mechanically
into a tap hole towards the end of the tapping operation. The dart has a lower stem,
which rests freely into the tap hole. The enlarged head of the dart, when lowered into
its position in the tap hole, the tap hole, at least partially, is closed, and allows
separation the two layers.
However, due to formation of vortex, when the steel is discharged from the Basic
Oxygen Furnace (BOF) vessel, it may still be possible to draw the slag present above
the molten steel layer in the vessel through the tap hole. Conventional shapes of dart
head for example, the dart head disclosed in US Patent 4709903, suffers from the
vortex problem.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a vortex type slag dart for
enhanced separation of slag from molten steel in a steel making process adapting a
Basic Oxygen Furnace (BOF), which eliminates the problem of vortex effect.
Another object of the invention is to propose a vortex type slag dart for enhanced
separation of slag from molten steel in a steel making process adapting a Basic
Oxygen Furnace (BOF), which enables reducing impurities in produced steel thereby
improving the steel quality.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an improved slag dart for
enhanced separation of slag from molten steel in a steel making process adapting a
Basic Oxygen Furnace (BOF)
The slag dart having an enlarged head from which projects downwardly a stem for
fitting into tap hole of the Basic Oxygen Furnace (BOF) vessel. The dart head is
configured to substantially match the shape of a natural vortex resulting from
prevailing physical characteristics of Basic Oxygen Furnace (BOF) during tapping
operation. Thus, a lower surface of the dart head is made to match the shape of a
vortex as generated during tapping operation. The half length of the lower contour of
the dart head is more than twice of the molten steel height at which the vortex just
generates. The upper surface of the dart head is configured as a "bell" shape to
reduce the drag of the overflowing liquid or may be even flat shaped for simplicity of
manufacturing. The thickness of the stem of the dart is selected so as to provide
sufficient strength as per its construction.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Shows an improved slag dart according to the inventor.
DETAILED DESCRIPTION OF THE INVENTION
The dart shown in figure-1 has a vortex shaped head which is cross sectionally
circular. A lower surface (1) of the dart head (5) is derived from experimentally
measured vortex dimensions and thus is enabled to closely follow the slag metal
interface during the end of tapping operation. The lower modified surface (1) as
shown in upright position has its largest diameter at the top (3) of the dart head (5)
such that it is substantially greater than the tap hole in the Basic Oxygen Furnace
(BOF) vessel where it is to function. The dart comprises a steel rod (4) that forms core
of the dart. The rod (4) extends below the dart head (5) to form a dart stem (2). A
refractory material is used for sleeving the lower rod. The stem (2) is located in the
tap hole of the Basic Oxygen Furnace (BOF) vessel when the molten steel is tapped.
The rod (4) extends vertically through the dart head (5) upwardly and outwardly
thereof and provides a portion of the dart to be engaged by a mechanical device used
to position the dart in the tap hole. As the interface of the slag-metal approaches the
tap hole during tapping operation, the lower surface (3) of the dart head (5) closely
follows the interface of the two phases and progressively reduces the top-hole
opening to gradually reduce the flow of steel. The vortex shape of the dart head (5)
always keeps the slag at a distance from tap-hole along the lower surface (1) more
than a critical bath height at which the slag is sucked into the tap hole. The top (3) of
the dart head (5) may additionally be configured in an inflated "bell" shape to reduce
the drag due to overflowing of the fluid or may be flat shaped, depending upon slag
fluidity.

We claim:
1. An improved slag dart for enhanced separation of slag from molten steel in a
steel making process adapting a Basic Oxygen Furnace (BOF), the slag dart
comprising:
- a rod shaped member (4) vertically disposed and downwardly extending to
form a dart stem (2), the stem (2) being disposable over the tap hole;
- a dart head (5) having a lower surface (1) and an upper surface, the rod
shaped member (4) extending vertically upwardly through the dart head (5) to
allow joining of the slag dart with a mechanical device, wherein the lower
surface (1) of the dart head (5) is configured to move in correspondence with
the shifting of an interface of the slag and molten steel and cause to
correspondingly reduce opening of the tap hole, and wherein the lower surface
(1) of the dart head has a length more than twice of the bath height of the
Basic Oxygen Furnace (BOF) at which the vortex starts to appear during
tapping of the molten steel with an overlying slag layer.

2. A dart as claimed in claim 1, wherein the top surface of the dart head (5) is
configured as a "bell shaped" surface.
3. A draft as claimed in claim 1, wherein the top surface of the dart head (5) is
flat.

4. A dart as claimed in claim 1, wherein the molten steel-slag interface is closely
followed by a corresponding movement of the lower surface of the dart head to
allow complete draining out of steel in the vessel without slag being allowed to
pass through the tap hole.
5. A dart as claimed in claim 3, wherein the dart head material is selected as a
refractory material.

ABSTRACT
A vortex type slag dart for enhanced separation of slag and molten steel during
tapping operation in a steel making process adapting a Basic Oxygen Furnace (BOF),
the slag dart comprising a rod shaped member (4) vertically disposed and downwardly
extending to form a dart stem (2), the stem (2) being disposable over the tap hole; a
dart head (5) having a lower surface (1) and an upper surface, the rod shaped
member (4) extending vertically upwardly through the dart head (5) to allow joining of
the slag dart with a mechanical device, wherein the lower surface (1) of the dart head
(5) is configured to move in correspondence with the shifting of an interface of the
slag and molten steel and cause to correspondingly reduce opening of the tap hole,
and wherein the lower surface (1) of the dart head has a length more than twice of
the bath height of the Basic Oxygen Furnace (BOF) at which the vortex starts to
appear during tapping of the molten steel with an overlying slag layer.