FIELD OF INVENTION
The invention relates to an improved method of flux addition during the process
of refining liquid iron, commonly called hot metal, to produce liquid steel by
adopting basic oxygen (BOF) process having combined blowing facility.
BACKGROUND OF INVENTION
In the BOF process the refractory lined container is used to carry out high
temperature reactions, which is known as 'converter' or' vessel'. The BOF
refinement process consists of removing silicon, phosphorous and carbon from
the hot metal received from the Blast furnace. Hot metal handled in different
converters is not the same and illustrative variations are given in table 1.
The refinement process consists of (i) charging the converter Vessel with hot
metal or liquid iron and solid scrap (ii) addition of fluxes like burnt lime, dolomite
etc (iii) blowing oxygen by using a specially designed lance at a supersonic speed
gives thermal energy to carry out pyro-metallurgical reactions. The distance
between the lance tip and level of hot metal, popularly known as bath height, is
very critical and vertical as per requirement to enable speedy refinement.
Significant metallurgical changes during the blow are for example, formation of
slag on reaction of added flues like lime (CaO) with the generated iron oxide
(FeO) and SiO2 (SiO2 formed on the oxidation of Si of hot metal), removal of
phosphorus and carbon. The rate of P removal and holding capacity in the slag
depends critically on the adopted blow profile, flux addition practices, which
together ultimately control the temperature and chemistry of slag.

Simultaneous injection of inert gases, from the bottom of the converter, into the
charge while blowing oxygen from the top into the charge is known as combined
blowing. Combined blowing helps controlling generation of lower amounts of iron
oxide which is critical to achieve desired slag chemistry throughout the
refinement process. Normally the blow period is about 15-20 minutes depending
on the oxygen flow rate and quantity of input hot metal. After the blow is
complete the refined hot metal, called liquid steel, is transferred or trapped into
a container called steel ladle.

In the first stage of hot metal refinement, also known as the early part of the
blow, oxygen is blown into the charge by keeping the blowing lance at the
specified distance from the bath. During this period the temperature of the
charge goes upto - 1500 C, generates more iron oxide which dissolves lime to
generate slag having constituents Cao-SiO2-FeO. With more lime going into
solution the formal slag is conductive to P removal. Subsequently the blow profile
is changed by lowering the lance.

In the second stage of refinement the blow is suitably changed to create
conditions suitable for carbon removal. In the final stage, to ensure desired
carbon removal oxygen blow is stopped and the lance is lifted up from the bath
slowly which is likely to generate more iron oxide. During the entire period of
refinement inert gas is blown from the bottom of the vessel to contain excessive
generation of iron and to ensure homogeneity of the bath.
In BOF steel making process, refinement of hot metal by high speed oxygen
blowing is done to remove primarily metalloids like silicon phosphorous, and
carbon. Thus during BOF refinement process, silicon, manganese, phosphorous
(P), and carbon are removed from the hot metal by blowing oxygen while
adhering to a suitable slag practice. The higher amounts of FeO generated
during refining, which can be as high as 30%, can corrode vessel lining To
achieve higher campaign life to the tune of 4000 heats, for increasing vessel
availability and other economic considerations, the popular slag practice has
been to maintain MgO content in the range of 6-8% which is the process
thermodynamic equilibrium requirement. Also such high MgO containing slags
made to coat on the hot face of the working lining by a technique known as 'slag
splashing', which effectively lowers lining wear. Also in plants having higher
phosphorous exceeding 0.2% in the hot metal a two step process of first
removing phosphorous in the hot metal and subsequently carbon removal by
BOF are popularly adopted. Such two step processes are although popular, but
time consuming and require substantial capital investment.

Further, maintaining higher 'MgO' in the BOF end slag to the tune of 6-8% is
conducive to converter lining life, but known to be unfavourable for effective de-
phosphorization of hot metal. Normally in plant having low phosphorous loads in
the hot metal, which is typically less than 0.07%, it is not a problem to maintain
6-8% MgO in the slag to protect lining wear. But for plants having phosphorus
loads (i.e. above 0.2%) and having the need to make quality steels of less than
150 ppm (0.015%) of phosphorus in the finished steel, maintaining 6-8% MgO is
unacceptable. The conditions favouring lining life are in contradiction to that of
de-phosphorization. The known disadvantage of higher MgO to the tune of 6-8%
in the slag while handling high phosphorous hot metal are : (i) reduced slag
fluidity which adversely effect kinetics of dephophorization (ii) impeding
formation and growth of phosphorous absorbing mineral phases like di-calcium
silicate (2CaO. Si02).
As a result plants intending to refine high phosphorous hot metal by BOF route
could neither enhance slag MgO to equilibrium requirement nor opt for slag
splashing resulting in low campaign life of 2000-3000 heats.
In refining, by BOF route, high phosphorous containing hot metal whose P
content typically varies in the range of >0.1% -0.2%, a complete absence of
MgO contributing flux additions has been practiced in the prior art to achieve
high degree of de-phosphorizzation. This NON-MgO flux practice however,
generates process slag very corrosive to the converter lining which is made of
mag-carbon refractory, and thus limits converter campaign life to as low as 2000
heats. Low converter life is not only non-economical, but reduces productivity of
the shop. Plants refining low phosphorous containing hot metals having P

typically around 0.07% thus, adopted MgO containing flux practices and have
been maintaining 6-8% MgO in vessel slag for satisfying thermo-chemical
equilibrium requirements and effectively performing 'slag splashing' to achieve
vessel campaign life exceeding 4000 heats.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of flux
addition in a BOF slag making process.
Another object of the invention is to propose an improved method of flux
addition in a BOF slag making process, which is capable of achieving effective
de-phosphorization.
A still another object of the invention is to propose an improved method of flux
addition in a BOF slag making process, which allows an extended converter
campaign life.
Yet another object of the invention is to propose an improved method of flux
addition in a BOF slag making process, which results in enhancement of slag
MgO.
A further object of the invention is to propose an improved method of flux
addition in a BOF slag making process, which enables producing hot metal
having low phosphorous and high phosphorous to the tune of 0.3%.

A still further object of the invention is to propose an improved method of flux
addition in a BOF slag making process, which has the potential to lower
refractory wear resulting in enhancing converter campaign life to 4000 heats and
higher without resorting to slag splashing.
BRIEF DESCRIPTION OF THE ACOMPANYING DRAWINGS
Fig. 1 - graphically shows effect of MgO on viscosity and melting point of slag in
middle point of slag in middle stage of blow (slag basicity: 1.75)
Fig. 2. - effect of MgO on viscosity and melting point of slag bath in the stage of
blow (slag basicity : 3.8)
Fig. 3 - frequency distribution of dolo addition and average MgO in end blow
slag during corroborating experimentation.
Fig. 4 - Average hot metal P distribution and average and average turn down P
observed during corroborating experimentation.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved method of flux addition in a hot metal
refining process to produce liquid steel, the refining process, adapting basic
oxygen process (BOF) using at least one refractory lined converter having
combined blowing facility and consisting of the steps of charging the converter
with hot metal including solid scrap, addition of known fluxes, blowing oxygen by

a lance at a supersonic speed at a specified bath height, and concurrent injection
of inert gases from the bottom of the converter into the charge, the entire
refinement process being implemented in at least three stages, each stage
maintaining different blow profiles, blow periods, temperature and bath height
for removal of different impurities such as silicon, phosphorous, and carbon, the
method comprising the steps of: determining the fluidity of slag produced during
the refining process being dependent on the basicity or CaO/SiO2 or V ratio
content of the slag; and adding Mgo or Mgo containing fluxes to maintain Mgo in
the range varying 0-2% towards the end of blow.
DETAIL DESCRIPTION OF INVENTION
During development of the invention, the inventors observed that the vessel slag
containing MgO in the range of 6-8% is although good for minimizing lining wear
while such slag is undesirable for P removal. Thus, a method to provide optimum
MgO at relevant time of blow was thought of. The possibilities to optimize MgO
including the pressure of MgO on the slag when there was no MgO input as flux
were evaluated.
The available phase diagrams and published literatures to evaluate the role of
MgO or MgO supplying fluxes on the critical properties of BOF slag has been
studied. One of the critical properties that is supposed to influence the
dephosphorization activity is identified to be the fluidity of slag which changes
depending on the basicity or CaO/SiO2 or V ratio content of slag. In fact in the
BOF steelmaking during the progress of blow period, V ratio of slag increases

from >1 to >3 as the more quantum of lime melts with increase of temperature
and completion of fluxing reactions.
In the early blow period when the slag basicity is between >1-1.5, a moderate
quantum of MgO, say 0-4%, is expected to form compounds like Cao-Mgo-SiO2
(monticilite) and 3Cao-MgO-2Si02 (Merwinite) which are liquids under
steelmaking conditions. Formation of low melting compounds containing CaO-
MgO-SiO2 help to dissolve lime and help to increase fluidity. Towards the end of
the blow, the V ratio reaches beyond 3, and attempts to increase MgO upto 0-
4%, which helps to control the fluidity of slag by reacting with FeO. Thus,
maintaining a lower fluidity in the end blow, the slags has been found to be
useful for both holding of phosphorous by the slag and lowering corrosion of
refractory. Some of the published data corroborating the effect of marginal
increase in slag MgO on viscosity are presented in Figs 1 & 2.
A study of plant slag data according to the present method, shows, in those
campaigns without any additional of MgO or MgO bearing fluxes, that the
average MgO content was typically around 0.75%. This indicates that the actual
dissolution from refractory lining into the slag has been much lower than what
equilibrium recommends. This type of anomaly can be explained based or non-
equilibrium conditions prevailing during BOF refining and also to the improved
corrosion resistance of fused periclase containing refractory that is currently used
as lining material.

In order to corroborate the improved effect of the inventive method, field trials
were conducted in one campaign close to 4000 heats. To increase MgO content
of slag, both raw dolomite and MgO bearing cinder was added during the
refinement period. The details of achieved dephophorization and measured slag
MgO are presented in Figs 3 & 4.
Theoratically, there is no adverse impact on the slag viscosity if MgO content in
the vessel slag is maintained up to 4% level, incase the slag V ratio varying from
<1 to >3. Due to prevailing non-equilibrium conditions in BOF process and use of
superior refractory having fused periclalse containing lining materials, MgO
dissolution from lining to the slag, as indicated by chemical analysis of vessel
slags, has been around 0.75%. Hence it is not necessary to adopt slag splashing,
as popularly done by many Plants for maintaining 6-8% MgO in the vessel slag
to minimize refractory wear and to achieve high campaign life. Also for such
plants having the need to refine hot metals having high P exceeding 0.2% by
BOF route having combined blowing facility, addition of MgO or MgO containing
fluxes to maintain MgO in the moderate range of 0-4% in the slag is not adverse
to effective P removal and simultaneous enhancement of converter life.
Based on the knowledge of available literature and study of actual plant data, it
has been found that it is not required to maintain such high level of MgO in the
slag to increase vessel life. Timely addition of MgO containing fluxes to achieve
MgO content in the slag varying from 0-2% is good enough, even while handling
high phosphorus hot metal, in order to create conditions conducive to effect
dephophorization and also to take care of saturation requirements. The results of
the conducted field trials, by maintaining around 2% Mgo in the end blow slag

did not show any adverse impact on effective dephophorization and also the
converter life could be raised to touch 4000 heats level without slag splashing.
Thus, according to the invention, adaptation of the improved flux addition
method, while refining hot metal having P varying from 0.15% - >0.2% content,
in which timely addition of optimum MgO containing fluxes to increase MgO level
in the BOF slag effectively reduces corrosive of the process slag which helps to
enhance vessel campaign life to 4000 heats level. The modified optimum MgO
supply into the slag which varied from 0-2% has no adverse impact no
dephosphorization of handled hot metal of P varying from 0.15% - > 0.2%.
The present invention provides a new and inventive flux making method by
timely introduction of MgO containing fluxes to optimize MgO content of the BOF
slag which significantly reduces corrosiveness of process slag to BOF working
lining made of Magcarbon refractory and also very compatible to effective de-
phophorization of hot metal of high input phosphorous content reaching 0.20%.
Such improved method of flux addition generates less corrosive process slag.
The improved method has the potential to mitigate its corrosive nature to the
vessel lining made of mag-carbon refractory while ensuring no adverse impact no
degree of dephophorization.

ADVANTAGES OF THE INVENTION
The invention is applicable to batch type primary steelmaking by BOF with
combined blowing route having following variations
1. Of input hot metal having carbon : up to 4.5% max, sulphur: 0.030 max,
phosphorous : 0.20% max., Silicon : 0.5-1.2%, Temperature : 1250-
1350°C,
2. Of scrap input varying from : 0-25%, and total hot metal+ scrap
weight/charge varying from : 100-350 tons
3. for production of liquid steel of desirable quality, in terms of all ranges of
carbon needed to suit long product and flat product segments.
4. Using process converters lined with magnesia-carbon refractory having
carbon varying from 5-20% magnesite varying from 80-90) of sintered /
fused or various combinations of both variety and all designs including
stadium type,
5. By adopting a slag making practice capable to generate all types of end
slags having basicity ( CaO/SiO2) varying from 2.8-4 by using input fluxs
like calcined / raw lime, calcined/raw dolomite, iron ore, coke

WE CLAIM
1. An improved method of flux addition in a hot metal refining process to
produce liquid steel, the refining process, adapting basic oxygen process
(BOF) using at least one refractory lined converter having combined
blowing facility and consisting of the steps of charging the converter with
hot metal including solid scrap, addition of known fluxes, blowing oxygen
by a lance at a supersonic speed at a specified bath height, and
concurrent injection of inert gases from the bottom of the converter into
the charge, the entire refinement process being implemented in at least
three stages, each stage maintaining different blow profiles, blow periods,
temperature and bath height for removal of different impurities such as
silicon, phosphorous, and carbon, the method comprising the steps of:
- determining the fluidity of slag produced during the refining
process being dependent on the basicity or CaO/SiO2 or V ratio
content of the slag; and
- adding Mgo or Mgo containing fluxes to maintain Mgo in the range
varying 0-2% towards the end of blow.
2. The method as claimed in claim 1, wherein when the hot mental having
phosphorous, varying from 0.15% - >0.2% content, an optimum Mgo
containing fluxes is added during the refining process.

3. An improved method of flux addition in a hot metal refining process to
produce liquid steel as substantially described and illustrated herein with
reference to the accompanying drawings.

An improved method of flux addition in a hot metal refining process to
produce liquid steel, the refining process, adapting basic oxygen process
(BOF) using at least one refractory lined converter having combined
blowing facility and consisting of the steps of charging the converter with
hot metal including solid scrap, addition of known fluxes, blowing oxygen
by a lance at a supersonic speed at a specified bath height, and
concurrent injection of inert gases from the bottom of the converter into
the charge, the entire refinement process being implemented in at least
three stages, each stage maintaining different blow profiles, blow periods,
temperature and bath height for removal of different impurities such as
silicon, phosphorous, and carbon, the method comprising the steps of: