A PROCESS FOR REFINING HOT METAL CONTAINING HIGH SILICON
AND PHOSPHORUS IN TWIN HEARTH FURNACE (THF)
INTRODUCTION TO THE FIELD OF INVENTION
The present invention relates to a modified process for refining hot metal with
high Silicon and high Phosphorus in any steel melting shop making steel through Twin
Hearth Furnace (THF) with modifications in charge mix and oxygen blowing pattern.
Adoption of this modified process results in better control of phosphorus in finished steel
resulting in production of special quality steels with higher NSR (Net Sales Realization)
values, more over the modified process also helps in refining hot metal with higher
silicon percentage and thus helps in increase in productivity of steel melting shop.
PRIOR ART AND DRAWBACKS
Steel Melting Shop, ISP, Burnpur is equipped with two twin health furnaces
(replacing OH furnaces) with 110t capacity of each bath producing semi killed steel
through ingot casting. The production capacity is around 0.5MT/year. It produces mainly
semi killed variety of steel. The problem faced by ISP is wide variation in hot metal
chemistry with respect to silicon, phosphorous sulphur and temperature. Analysis of hot
metal is not available prior to charging of fluxes and scrap. Addition of fluxes and scrap
is done on the basis of previous heat tapped. Mixer capacity being low (500 tons), metal
coming from blast furnace is directly poured in the furnace and hence there is wide
fluctuation in hot metal composition from ladle to ladle. Blast furnace slag carry over is
also substantial. The refining time varies over a wide range depending on the input hot
metal composition. The Twin Hearth process of steel making is an oxidizing one. This
means that in THF the impurities of the charge such as C, Si, S, P, etc. are oxidized and
extracted into the slag or the gas phase of the furnace. Fluxes are added to form basic slag
to fix S and P. Hot metal, fuel and combustion product supplies the necessary heat in
THF steel making.

The US document 3881917 discloses a method for refining an iron-base metal
such as pig iron which contains minor amounts of elements including silicon, manganese,
phosphorus, carbon, sulfur, and other incidental impurities, characterized by the steps of
containing molten iron-base metal in a metallurgical vessel, heating and oxidizing the
molten metal to cause chemical reactions to remove excessive contents of the said
elements, adding solid lime and a mixture of fluorspar and slag conditioners of relatively
high basicity (lime-to-silica ratios of 2.5 to 4.0) which conditioners consist essentially of
oxides of selected metals including iron, manganese, silicon, magnesium, and calcium,
and which conditioners have a melting point of from about 2,000° to 2,800°F, and
continuing to heat while oxidizing the mixture of the molten metal, slag conditioner,
fluorspar and slag-forming materials until said minor amounts of elements are reduced to
the desired percentages, and are retained in the slag formed. More specifically, the
function of the slag-conditioner is to eliminate the encapsulation of large particles of lime
by dicalcium silicate and thereby more rapidly taking the lime into solution. The function
of the mixture of fluorspar and slag conditioner is to promote more rapid solution of the
solid lime than is obtainable with the use of either fluorspar or slag conditioner alone.
The United States Patent 3904399 discloses a method for refining pig iron
containing usual amounts of impurities and elements in a metallurgical vessel into which
at least one stream of oxygen is introduced into the body of the pig iron to form at least
one reaction zone, and introducing at least one oxide of an element contained in the pig
iron in an amount sufficient to saturate the reaction zone with said oxide and thereby
promote the oxidation of all other elements contained in the pig iron to the exclusion of
the element whose oxide is introduced.
The document JP2001181724 discloses a molten iron refining method, in which
the occurrence of slag can be reduced as far as possible, the manufacturing cost of the
molten steel can be reduced, and the efficient and stable dephosphorizing treatment and
decarburizing treatment can be carried out by using the slag containing substantially no
fluorine.
The document GB1011897 discloses a method of refining iron or steel which has
been obtained by the reduction of iron ore comprises treating the molten iron or steel in a
converter with a chemical refining agent whereby contaminants separate out as slag, the
reaction of the iron or steel with the chemical refining agent being controlled by eccentric
rotation of the converter about a vertical axis. Sulphur may be removed by addition of an
agent such as calcium carbide, burnt lime or soda ash. Treatment of the melt with oxygen
gas may produce preferential oxidation of carbon, silicon, titanium, vanadium, chromium
and/or phosphorus, depending upon the chosen speed of eccentric rotation of the
converter and the pressure of the oxygen gas.
Phosphorous and Sulphur are considered undesirable because they reduce
elevated temperature ductility and hence affect the stress rapture strength and thermal
fatigue life. Sulphur forms MnS/ FeS in steel which during rolling from stringers. These
have low melting point and result into under-bead cracks during welding. Higher content
of Phosphorous increases the strain aging tendency of the steel.
Based on the above considerations objective was to evolve a process, which meets
the basic requirements of furnace for better control of phosphorus in finished steel
resulting in production of special quality steels with higher NSR (Net Sales Realization)
values, more over the modified process also helps in refining hot metal with higher
silicon percentage and thus helps in increase in productivity of steel melting shop.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a modified process for refining
hot metal with high Silicon and high Phosphorus in any steel melting shop making steel
through Twin Hearth Furnace (THF) with modifications in charge mix and oxygen
blowing pattern.
It is another object of the invention to propose a such modified process that results
in better control of phosphorus in finished steel resulting in production of special quality
steels with higher Net Sales Realization values (NSR).
It is further object of the invention to propose a modified process that helps in
refining hot metal with higher silicon percentage and thus helps in increase in
productivity of steel melting shop.
It is further object of the invention to propose a modified charge mix and oxygen
blowing parameters along with improvement in hot metal chemistry in achieving the
desired opening phosphorus level, decrease in the incidences of bath feeding and
decrease in refining time.
DETAILED DESCRIPTION OF THE INVENTION
According to this invention, there is provided a process for refining hot metal with
high Silicon and high Phosphorus in any steel melting shop making steel through Twin
Hearth Furnace (THF) with modifications in charge mix and oxygen blowing pattern.
Adoption of this modified process results in better control of phosphorus in finished steel
resulting in production of special quality steels with higher Net Sales Realization values
(NSR) values, more over the modified process also helps in refining hot metal with
higher silicon percentage and thus helps in increase in productivity of steel melting shop.
Base data study was carried out on Twin Hearth Furnace practice. Wide variation
in hot metal chemistry with respect to quantity of impurities like Si and P was observed.
Si sometimes reaches as high as 2.4% and so does P, which can attain any value from
0.19 to 0.5% over a period of time. This sort of variation makes it difficult for the melter
to choose an optimum charge mix which further results in high opening of phosphorus
leading to multiple bath feedings. Opening bath phosphorus also is very difficult to
decrease in case of high P combined with High Si. The fluctuation in hot metal chemistry
is as follows:
The process of the present invention includes Twin hearth furnace operation for
phosphorus control under high silicon conditions was studied and on the basis of
findings, a modified twin hearth flux practice and certain associated activities was
proposed and tried out. To counter these problems, a nomogram (Fig: 1) has been
developed based on theoretical and collected plant operating data. This correlated the
primary THF operating parameters like Hot Metal wt, Hot Metal analysis, final slag
basicity, limestone, iron ore and oxygen requirement. This helped the furnace operators
in determining the optimum charge mix i.e. the amount of fluxes and iron ore to be
charged for refining to achieve the desired result. To overcome high phosphorus refining
problem, the steel processing has been modified with respect to amount of iron ore in
bulk charge and charging sequence. Also, measures were adopted for proper flushing of
slag at right viscosity and temperature. Preheating of charge mix for at least 15 minutes
with tar burner was introduced for ensuring good flushing of slag and to achieve desired
level of opening phosphorus.
Standard Operating Practice (SOP) was prepared suggesting different charge mix
for different chemistry of hot metal. Trials were carried out in phases with modification
in charging practice and using nomogram. Total 53 heats were monitored. Charging
practice along-with modified oxygen blow rate was adopted to take care of the meltdown
and refining period.
The number of bath feedings (addition of fluxes/ iron ore) was decreased
considerably from 83% during first trial to almost 38% by the time 4th trial ended. This
further lead to a decrease in refining time to 2 hr 31 min. (trial average).
The Standard Operating Practice (SOP) suggested is being followed by the shop.
The implementation of modified charge mix and oxygen blowing parameters along with
improvement in hot metal chemistry has helped in achieving the desired opening
phosphorus level, decrease in the incidences of bath feeding and decrease in refining
time. The salient features of trial result are shown in table 1 and table 2.

Modified process of the present invention for refining hot metal with high Silicon and
high Phosphorus in any steel melting shop making steel through Twin Hearth Furnace
(THF) with modifications in charge mix and oxygen blowing pattern are as shown
below: -
I. DEVELOPMENT OF STANDARD PRACTICE FOR REFINING HIGH 'Si' &
HIGH 'P' HOT METAL
? A standard practice has been developed for refining hot metal containing high
silicon and phosphorus in twin hearth furnace. Nomogram (Fig 1) has helped the
furnace operators in determining the optimum charge mix i.e the amount of fluxes
and iron ore to be charged for refining to achieve the desired result.
II. THE CHARGING SEQUENCE AS FOLLOWED In THF WAS MODIFIED
SLIGHTLY FROM THE PREVAILING PRACTICE.
? To overcome the problem of phosphorus removal with high levels of silicon
present in hot metal, a 2 stage heat processing was devised:
The charging sequence as followed in THF was modified slightly from the prevailing
practice. On a normal manner 12 ton of scrap followed by whole of limestone (11t)
followed by iron ore (~ 2 t) and finally rest of scrap (12 t) were charged. Slight
modification like 6t of limestone at bottom and rest in sandwich (middle) was done for
protecting and building the bottom. Also 4t of iron ore in the middle and 1t on top
were done in cases of high probable Hot metal silicon. Also, preheating of charge
material with tar burner for 15-20 minutes was adopted to compensate for the heat
losses.
Previous Charging Practice
Scrap (12t) ? Limestone(11t) ? Iron Ore(2t) ?Scrap (12t)? Preheat (1 hr)
with flue gases ? Hot metal (110T)
The Modified Charge Mix and Charging Sequence was advised in view of variable
Hot metal chemistry:

Case -I: Si < 1.2% & P> 0.25
Scrap (12t) ? Limestone(9t) ? Iron ore (2t) ? Scrap (12t)
Case - II: Si > 1.2 < 1.5% & P> 0.25 (Si greater then 1.2% & less then 1.5%)
Scrap (16t) ? Limestone(9t) ? Iron ore (4t) ? Scrap (12t)
Case - III: Si >1.5% & P> 0.25%
Scrap (16t) ? Limestone(11t) ? Iron ore (4t) ? Scrap (12t) ? Iron ore (2t)
III. MODIFICATION IN BLOWING PARAMETERS
? When 4t of ore the optimum oxygen flow rate is 2000 ~2200 Nm3 / hr for flushing
period. When 6t of ore is added, optimum oxygen rate is 1700-1800 Nm3 / hr until
flushing end.
Different embodiments of the invention are possible to achieve the best method of
performance and to obtain the product as stated above to meet the object of the invention.
It will be understood that skilled persons with many modifications, variations and
adaptations may carry out the invention into practice without departing from its spirit or
exceeding the scope of claims in describing the invention for the purpose of illustration.

WE CLAIM:
1. A process for refining hot metal with high silicon and high phosphorus in any
steel melting shop making steel through Twin Hearth Furnace (THF) by
improvement in hot metal chemistry comprising modifications in charge mix and
oxygen blowing pattern.
2. A process for refining hot metal with high silicon and high phosphorus as claimed
in claim 1, wherein the charge mixture consists of from about 12t to 16t scrap,
from about 9t to 16t limestone, from about 2t to 4t iron ore.
3. A process for refining hot metal with high silicon and high phosphorus as claimed
in claim 1, wherein oxygen flow rate is 1800-2000 Nm3/hr during starting period
of blowing.
4. A process for refining hot metal with high silicon and high phosphorus as claimed
in claim 1, wherein oxygen flow rate is 2200-2500 Nm3/hr for flushing period.
5. A process for refining hot metal with high silicon and high phosphorus as claimed
in claim 1, wherein oxygen flow rate is 3200-3500 Nm3/hr for refining period.
6. A process for refining hot metal with high silicon and high phosphorus as claimed
in claim 1, wherein modified charge mix consists of about 1.5-2.0 percent silicon
and of about 0.35-0.45 percent phosphorus.
7. A process, for refining hot metal with high silicon and high phosphorus as claimed
in claim 1, wherein decrease in the incidences of bath feeding and decrease in
refining time is achieved.

8. A process for refining hot metal with high silicon and high phosphorus in any
steel melting shop making steel through Twin Hearth Furnace (THF) by
improvement in hot metal chemistry comprising modifications in charge mix and
oxygen blowing pattern substantially as herein described.

The present invention relates to a modified process for refining hot metal with high
Silicon and high Phosphorus in any steel melting shop making steel through Twin Hearth
Furnace (THF) with modifications in charge mix and oxygen blowing pattern. Adoption
of this modified process results in better control of phosphorus in finished steel resulting
in production of special quality steels with higher NSR values, more over the modified
process also helps in refining hot metal with higher silicon percentage and thus helps in
increase in productivity of steel melting shop.