FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
1 TITLE OF THE INVENTION :
A METHOD FOR STEEL MANUFACTURE INVOLVING THE STEP OF DEPHOSPHORIZING OF THE HOT METAL.
2 APPLICANT (S)
Name : JSW STEEL LIMITED.
Nationality : An Indian Company.
Address : Jindal Mansion, 5-A, Dr. G. Deshmukh Marg, Mumbai - 400 026,
State of Maharastra, India.


3 PREAMBLE TO THE DESCRIPTION
COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.


FIELD OF THE INVENTION
The present invention relates to a method for refining of hot metals during manufacture of steel and in particular to the method of pre-treatment of hot metal to achieve dephosphorisation in transfer ladles in steel making process. More particularly, the present invention is directed to a method for dephosphorisation to reduce the phosphorous input at desired low level before the processing of steel in LD converter. A Hot Metal Pre-Treatment facility is proposed in the steel making shop to carry out said dephosphorisation and resultantly reduce the phosphorus content in input liquid metal in LD converter to reduce total reaction/processing time, slag generation/volumes, reduced lancing/re-blow requirements at converter, reduced lime consumption and thus improving productivity and yield with consistent quality of steel at reduced input costs. More advantageously, the invention is directed to bring down the phosphorous content from existing level of 0.15 wt% to about 0.06 wt %, and a commensurate reduction in average phosphorous content in steel in converter and substantial reduction in the quantum of oxygen re-blow. The slag produced is suitable for reutilization for other purposes. The present method of dephosphorisation involving Hot Metal Pre-Treatment process is thus provided for wide industrial use with significant economically advantageous application particularly in steel plants for improved productivity and quality of steel with less volume of reusable slag generation in an energy efficient manner.
BACKGROUND ART
Conventional steel making process comprising de-sulphurisation followed by primary steel making in LD converters, secondary steel making in steel ladles and casting into slabs at continuous casting plant. The hot metal impurities like the sulphur is removed in de-sulphurisation station, whereas silicon, carbon and phosphorous is removed in LD converter in the sequenced processing order for steel production. Silicon is removed as Si02 and phosphorous as P2O5 into the slag and carbon is removed in the form of CO and C02, as the products of oxidation after reacting with blown oxygen. It is well known in the art of steel making that increased presence of Si and P in hot metal, tends to produce higher volumes

of Si02 and P2O5 transferred to slag and hence higher is the slag generated. This slag is usually dumped into pits after the process.
The existing steel making facilities are having limited refining capabilities. The demand driven high production rates associated with customer/application driven stringent quality criteria, often makes it difficult to achieve the customers' satisfaction. The problem is further compounded because of deteriorating input raw material quality. Conventionally, total phosphorous removal is carried out during steel making in converter using lime-saturated slag. It essentially requires higher lime consumption. Any increase in the level of phosphorous in hot metal results in increased slag generation and adversely affects process control, prolong the processing time and more so increase the chances of phosphorous reversal. Additionally this complex slag is unsuitable for re-utilization.
There has thus been a need in the art to developing a method for dephosphorisation in steel which would on one hand limit the phosphorous content in input metal to LD converter for further processing and also would minimize the processing time and the slag generation thereby minimizing the overall total production batch cycle time enhancing the productivity and yield and on the other hand would ensure reliable and consistently good finished steel quality free of undesired phosphorous content to the detriment of customers' end requirements. This necessitated the requirement of additional pre-refining units in the present route to provide de-phosphorisation functions.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for a method for refining of hot metals during manufacture of steel and in particular to the method of pre-treatment of hot metal to achieve dephosphorisation in transfer ladles in steel making process
Another object of the present invention is directed to selective dephosphorisation of hot metal by Sinter fines and CaF2 and oxygen additions in selective proportions in transfer

ladles in steel making process, to reduce the reaction time and slag generation in subsequent processing of steel in converter and the like, improving productivity and quality of steel product.
Another object of the present invention is directed to developing a method for dephosphorisation of hot metal in transfer ladles wherein the method involves Hot Metal Pre-Treatment (HMPT) facility for the first time for desired removal of phosphorous by injection of reagents and fluxes in the transfer ladle.
A further object of the present invention is directed to developing a method for dephosphorisation of hot metal by sinter fines in transfer ladles directed to improve de¬phosphorisation by increasing the dissolution of lime, resulting in a highly basic and fluid slag.
A further object of the present invention is directed to developing a method for dephosphorisation of hot metal by sinter fines in transfer ladles with selective concentration in order to achieve desired higher phosphorous partition ratio.
A further object of the present invention is directed to developing a method for dephosphorisation of hot metal in transfer ladles such as to improve the slag fluidity by lowering the melting point.
A further object of the present invention is directed to developing a method for dephosphorisation of hot metal by sinter fines in transfer ladles whereby phosphorous level can be brought down from 0.15 to 0.07 wt %.
A further object of the present invention is directed to developing a method for dephosphorisation of hot metal by sinter fines in transfer ladles whereby said method would enable bringing down the average phosphorous levels in converter hot metal by 0.06%.
A further object of the present invention is directed to developing a method for dephosphorisation of hot metal by sinter fines in transfer ladles whereby the number of re-blows could be reduced from 10% to 6% and the consumption of lime and slag generation can also be reduced in the LD converter while using said pretreated molten metal from transfer ladles.

SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a method for steel manufacture the step of pre-refining of the hot metal comprising:
hot metal pre-treatment involving selectively de-phosphorisation of the hot metal in transfer ladle such as to maintain low phosphorous input in the molten metal in the LD converter and onward processing of the molten metal.
Another aspect of the present invention is directed to a method wherein said step of dephosphorization of the hot metal in the ladle comprises addition of CaF2 (fluorspar) in the transfer ladle.
A further aspect of the present invention is directed to a method comprising:
said hot metal pre-treatment in the ladle involving said de-phosphorizing and also de-siliconizing the hot metal in transfer ladle such as to maintain low phosphorous and silicon input in the molten metal in the LD converter and onward processing of the molten metal.
A still further aspect of the present invention is directed to said method comprising injection of reagents and fluxes in the transfer ladle such as to favor generation of fluid slag (CaO+CaF2+Si02+FeO) having higher phosphate capacity and increased kinetics for removal of phosphorous.
A still further aspect of the present invention is directed to said method wherein said de-siliconisation comprises oxidizing the hot metal along with added lime and sinter fines (iron oxide flux) along with gaseous oxygen.
A still further aspect of the present invention is directed to said method wherein the sinter fines used as iron oxide flux comprise sinter fines of less than 5 mm size.
5

A still further aspect of the present invention is directed to said method wherein the sinter fines are added onto the surface of the hot metal from the bunkers though the addition chute.
A still further aspect of the present invention is directed to said method wherein after 75% of the blowing time, sinter fines addition is stopped and replaced by gaseous oxygen to maintain the kinetics of the reaction.
A still further aspect of the present invention is directed to said method wherein the exothermic reaction of silicon to silica raises the temperature of the bath which gets compensated for the temperature drop due to said additions.
A still further aspect of the present invention is directed to said method comprising providing lance heights adapted to keep the FeO content in the slag after the treatment optimum.
A still further aspect of the present invention is directed to said method wherein the ladles are de-slagged after every treatment.
A still further aspect of the present invention is directed to said method comprising selective blowing and addition pattern to selectively dephosphorize the hot metal involving said sinter fines,CaF2 and oxygen in transfer ladle and maintaining desired sinter fines :oxygen ratio index for process control.
A still further aspect of the present invention is directed to said method wherein
said selective blowing and addition pattern comprises optimized sequential addition and
said desired sinter fines :oxygen ratio index comprises range of 1-3.

The present invention and its objects and advantages are described in greater details with reference to the following accompanying non limiting illustrative figure and example.
BRIEF DESCRIPTION OF THE ACOMPANYING FIGURE
Figure 1: is the schematic illustration of the blowing and addition pattern developed according to the method of the present invention to selectively de-phosphorise the hot metal using sinter fines, CaF2 and oxygen in transfer ladle.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURE
The present invention relates to a method for dephosphorisation of hot metal by use of sinter fine in transfer ladle and more particularly, the present method is directed to selective dephosphorisation comprising controlled additions of sinter fines as iron oxide and CaF2 (fluorspar) along with oxygen blowing in hot metal in transfer ladles in order to reduce the phosphorous input at desired low level, prior to processing of steel in LD converter. A Hot Metal Pre-Treatment facility having said transfer ladle installed and used in the steel making shop to carry out said dephosphorisation and resultantly reduce the phosphorus content in input liquid metal in LD converter to reduce total reaction/processing time, slag generation/volumes, reduced lancing/re-blow requirements at converter, reduced lime consumption and thus improving productivity and yield with consistent quality of steel at reduced input costs. The phosphorous content has been brought down from existing level of 0.15 wt% to about 0.06 wt %, and a commensurate reduction in average phosphorous content in steel in converter and substantial reduction in the quantum of oxygen re-blow results.
Hot Metal Pre-Treatment (HMPT) facility for removal of phosphorous by injection of reagents and fluxes in the transfer ladle has been implemented at plant level. Sinter fines are used as iron oxide flux in the present method having less than 5 mm size. Sinter fines are added

onto the surface of the hot metal from the bunkers though the addition chute. The process of de-phosphorisation is distinctively different by making use of CaF2 (fluorspar) to improve de-phosphorisation by increasing the dissolution of lime, resulting in a highly basic and fluid slag. Phosphorous partition ratios were increased with increase in CaF2 concentration. Addition of CaF2 at the beginning of the process improves the slag fluidity by lowering the melting point. This fluid slag (CaO + CaF2 + Si02 + FeO) has higher phosphate capacity and increased kinetics for removal of phosphorous.
According to an important aspect of the said addition and blowing pattern, the sinter fines addition is stopped after about 75% of the blowing/treatment time in transfer ladle since the starting of the pre-treatment process; sinter fines addition is then replaced by gaseous oxygen blown at desired rate, preferably at 40Nm3/nnin rate to maintain the kinetics of the reaction. Lance heights are optimized in order to keeping the FeO content in the slag after the treatment optimum. The ladles are de-slagged after every treatment. Importantly, also the method of dephosphorisation of liquid metal in transfer ladle makes advantageous use of the sinter fines to Oxygen ratio index for process control.
Reference is now invited to the accompanying Figure 1 that schematically illustrates the blowing and addition pattern developed to selectively de-phosphorise the hot metal using sinter fines, CaF2 and oxygen in transfer ladle. It is clearly apparent from the accompanying Figure 1 that the lime fines are continually added for the entire pre-treatment process of molten metal that continues for 25 to 30 minutes in transfer ladle at the rate of 40 kg/min. the CaF2 addition at the rate of 50 kg/min for shorter duration of 2 to 4 minutes after 5 minutes from start of the process. Immediately after the CaF2 addition starts, sinter fines are added at the rate of 160 kg/min that continued for 15 to 17 minutes. Oxygen blowing starts towards end of the treatment process before the sinter fine addition is stopped. Oxygen is blown at a rate of 40 Nm3/min with incremental flow at the end of process. The pre-treatment of molten metal following the above pattern, prior to processing in LD converter according to the present method thus favorably control the phosphorous content in finished steel from 0.15 to 0.07 wt %. The requirements of number of re-blows have been reduced from 10% to 6 % in LD converter coupled with reduction in lime consumption and slag generation.
It is thus possible by way of the present invention to developing a method for selective dephosphorisation of molten metal by sinter fines, CaF2 and oxygen addition in transfer ladle, in steel production process wherein the selective addition of sinter fines, CaF2 and

oxygen in a predetermined pattern on one hand ensure containing the phosphorous content in converter input within desired limit and thus reducing processing time, number of oxygen re-blows, slag volume in converter process of steel making and also favour desired control on FeO content by optimizing the lance height during oxygen blowing. Hot Metal Pre-Treatment (HMPT) facility according to the method of the invention for removal of phosphorous by injection of reagents and fluxes in the transfer ladle thus ensure producing steel with low 'P' content to suit customers' requirement and favor wide industrial application.

WE CLAIM:
1. In a method for steel manufacture the step of pre-refining of the hot metal comprising:
hot metal pre-treatment involving selectively de-phosphorisation of the hot metal in transfer ladle such as to maintain low phosphorous input in the molten metal in the LD converter and onward processing of the molten metal.
2.A method as claimed in claim 1 wherein said step of dephosphorization of the hot metal in the ladle comprises addition of CaF2 (fluorspar) in the transfer ladle.
3. A method as claimed in claim 1 anyone of claims 1 or 2 comprising:
said hot metal pre-treatment in the ladle involving said de-phosphorizing and also de-siliconizing the hot metal in transfer ladle such as to maintain low phosphorous and silicon input in the molten metal in the LD converter and onward processing of the molten metal.
4. A method as claimed in anyone of claims 1 to 3 comprising injection of reagents and fluxes in the transfer ladle such as to favour generation of fluid slag (CaO+CaF2+Si02+FeO) having higher phosphate capacity and increased kinetics for removal of phosphorous.
5. A method as claimed in anyone of claims 1 or 2 wherein said de-siliconisation comprises oxidizing the hot metal along with added lime and sinter fines (iron oxide flux) along with gaseous oxygen.
6. A method as claimed in claim 5 wherein the sinter fines used as iron oxide flux comprise sinter fines of less than 5 mm size.
7. A method as claimed in anyone of claims 5 or 6 wherein the sinter fines are added onto the surface of the hot metal from the bunkers though the addition chute.
8. A method as claimed in anyone of claims 1 to 7 wherein after 75% of the blowing time, sinter fines addition is stopped and replaced by gaseous oxygen to maintain the kinetics of the reaction.

9. A method as claimed in anyone of claims 3 to 8 wherein the exothermic reaction of
silicon to silica raises the temperature of the bath which gets compensated for the
temperature drop due to said additions.
10. A method as claimed in anyone of claims 1 to 9 comprising providing lance heights adapted to keep the FeO content in the slag after the treatment optimum.
11. A method as claimed in anyone of claims 1 to 10 wherein the ladles are de-slagged after every treatment.
12. A method as claimed in anyone of claims 1 to 11 comprising selective blowing and addition pattern to selectively dephosphorize the hot metal involving said sinter fines, CaF2 and oxygen in transfer ladle and maintaining desired sinter fines: oxygen ratio index for process control.
13. A method as claimed in claim 12 wherein
said selective blowing and addition pattern comprises optimized sequential addition and said desired sinter fines :oxygen ratio index comprises range of 1-3.
14. In a method for steel manufacture the step of dephosphorizing of the hot metal
substantially as here in described and illustrated with reference to the accompanying
figures.