FIELD OF INVENTION
This invention relates to the production of liquid steel using the process of basic oxygen steelmaking, using hot metal as the primary raw material. It specifically deals with conversion of high phosphorus hot metal to low phosphorus steel
through single-slag basic oxygen processing, in an economically and ecologically sustainable way.
BACKGROUND OF THE INVENTION
Partition of phosphorus between (quid steel and stag has been of Importance to steel makers round the globe, ever since the begmnhg of pneumatic stee! making. The adaptations of production of steel with low P content have varied from region to region, depending on the local raw materials and economic considerations. Several steel plants resorted to injecting part of the oxygen through tuyeres located at the bottom or the side-wall of the Basic oxygen Furance (BOF), along with a liquid or gaseous protective agent. 30-60 % of the total lime requrement was also added pneumatically through the bottom tuyere,, along with the oxygen gas. This adaptation of submerged injection of oxygen and lime was reported to work successfully for hot metals containing upto 0.5 mass % phosphorus and 2.0 mass % silicon.
External refining of hot metal, prior to commencement of the basic oxygen steelmaking process, has been another successful method of production of tow phosphorus steel. This practice, commonly termed as hot metal pre-treatment.
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has been adapted largely by the Japanese steel manufacturers. Under this process, hot metal is subjected to removal of one or more of silicon, phosphorus and sulphur, by addition of synthetic fluxes. The partly refined hot metal, sufficiently depleted of phosphorus, silicon and sulphur, is then processed though the 60F to produce high purity steel. Indian Iron ore and coking coal deposits are characterized by high phosphorus content. As a result, the hot metal produced in most Indian steel plants suffers from high concentration of phosphorus, unless low-P imported raw materials are used. Significant work has been carried out towards developing methods of production of low phosphorus steel from high phosphorus hot metal experienced in typical Indian steel plants.
In addition to pre-treatment of hot metal, specially designed synthetic fluxes can also be added during the process of steeknaking in the BOF for enhanced removal of phosphorus from the steel. However, this process is reported to be less efficient than external dephosphorisatfon of hot metal because the increase fn temperature during steetmaking in the BOF renders the reaction(s) thermodynamlcalry less favourable.
The most common practice for production of low phosphorus steel, so far, has been to maintain a very high basicity (between 3.0 and 4.0) of the BOF slag. The high concentration of CaO ensures sufficiently low activity of P2O5 and much of the phosphorus coming in from hot metal is thus transferred to the slag. It has been reported that P2O5 reacts with lime to form phosphate and phosphate-silicate of calcium, which are stable phases in steefmaking slag and minimise the risk of phosphorus reversion from slag to liquid steel.
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It is noteworthy that [P] of 0.040 mass % or less was considered "tow phosphorus' upto the eighties and during good part of the nineties. Hence the early workers were satisfied when the phosphorus content in steel could be restricted to 0.050 mass%. The requirement has changed drastically over the years and a grade of steel, at the onset of the 21st century, would hardly be termed as "low phosphorus' unless the [P] content was maintained below 0.010 mass%. Thus, the demand on the steelmaJcing process, for removal of phosphorus, has Increased manifold.
Difficalties In sustalnable dephoophorfaction with the prior practices:
Oxygen injection through bottom/side -walls - Maintenance of the equipment was a major disadvantage of this process. The submerged tuyeres were frequently choked with ingress of liquid metal and slag. Overheating of the tuyeres due to improper cooling by the protective gas/liquid was another significant problem.
High basicity BOF operation - This practice consumed excessive quantity of calcined lime, thus adding to the cost of production. Further, it was very common for the steelmaking slag to contain some amount of undissoived or precipitated lime; commonly designated as "free lime". The presence of this'free lime" made the slag less suitable for further use as cement binder or road construction material. In addition to the direct demerits, production of large quantities of calcined lime required consumption of commensurate amount of energy and release of C02 into the atmosphere during calcinations of limestone.
Pulverised lime addition through oxygen fance - This method was capable of producing low phosphorus steel from hot metal containing as high as 0.5 mass% [P]. However, the lance suffered from frequent choking due to the hygroscopic nature of calcined lime. This led to increased cost of maintenance and eroded the viability of the process.
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The present invention is aimed to remove the above narrated prior art difficulties by reducing calcium oxide addition in steel making slag and allowing phosphorus removal with reduced average basicity of slag
The main objection of this men ton is to develop viable means of reducing the average basicity of oxygen steelmaklng slag, retailing Its phosphorus holding capacity.
Another objective of the Invention Is to prevent dissolution of MgO from the refractory lining during maintenance of reduced basicity of liquid slag.
Accordhg to this invention there Is provided a method of producing low phosphorus steel containing below 0.010 % phosphorus from high phosphorus hot metal charged in a single slag basic oxygen furnace comprtshg the steps of maffitahing a low slag basicity in Bquid steel in the range of 2.5 to 2.7 at 1600°C to 1650°C with FeO% h the slag in the range of 10 to 30% mass through additions of measured quantities of calcined lime in pulverized and/or brtquetted forms, continuously or in batches upto the completion of meltng, maintaining equilibrium phosphorus partton ratio in the range of 280-340 and lower P2Q5 activity in the slag in the range of -20.2 to -19.6 in log scale which ensures complete phosphorus removal to the slag upto 99.99% and also prevent any MgO dissolution from the refractory ining in the slag.
The present mention will be better understood from the following description with reference to the accompanying drawings to which
Figure 1 relates to variation of Ca) content and basicity of slag in typical existhg BOF heats during use of high phosphorus hot metal.
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Figure 2 relates to variation of equilibrium phosphorus partition ratio with basicity (at constant temperature of 1650°C)
Figure 3 relates to effect of basicity on solubility of MgO in steelmaking slag (at constant temperature of 1650°C)
Figure 4 relates to effect of basicity on activity coefficient of P2O5 In slag (at constant temperature of 1650°C)
Effect of basicity on equilibrium phosphorus partition ratie:
In the existing BOF heats for low phosphorus steels as shown in Figure 1, it is observed that % Cao in slag is kept within the range of 45% to 55% and basicity (% Cao/SiO2) is maintained with the range of 3 to 5. This episode clearly indicates low activity of P2O5, increased amount of slag volume and requirements of large quantity of calcined lime and thus increasing amount of energy requirement and cost of production for calcinations; of limestone.
In the production of Low Phosphorus steel according to the invention FeO in slag is maintained within 10-30% mass.
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Figure 2 shows the effect of slag basicity on the equilibrium phosphorus partition ratio, at temperature of 1650°C. The equilibrium phosphorus partition ratio, commonly represented by the symbol Lp is calculated from the ratio between the phosphorus concentrations in the slag and the metal (steel), held at equilibrium.
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The three curves in Figure 2 correspond to three different levels of iron oxide concentration in the slag - 10 mass%, 18 mass% and 20 mass%. The equilirium phosphorus partition ratio, at any given value of basicity, first increases as the iron oxide concentration is increased from 10 mass% to 18 mass%, and then decreases when the iron oxide concentration is increased further to 20 mass%.
It can be seen from this figure that the phosphorus partition ratio reaches a maximum at basicity of 2.5-2.6 and further increase in basicity causes negligible increase in the phosphorus partition ratio. Therefore, maintaining an operating basicity greater than 3.0 provides no additional benefit in terms of phosphorus partition.
Therefore, maintaining a basicity of 2.5 - 2.6 in the BOF slag is sufficient for obtaining the desired level of dephosphorisation. Increase in basicity rather leads to enhanced consumption of calcined lime:, thus increasing the overall flux consumption and specific slag volume.
Removal of phosphorus from steel is governed by the reaction 2[P] + 5[O] = (P2O5). The equilibrium constant for the reaction can be written as
K is constant at a given temperature.
Therefore, lowering of the P2O5 activity, a(P2O5) would necessarily cause reduction in the activities of phosphorus and oxygen in the steel (h[p] and h[o]), at any given temperature. For dilute solution of [P] and [0] in liquid steel, the numerical values of h[p] and h[o] may be taken same as their respective mass% and hence, decrease
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of P2O5 activity in the slag indicate;; a higher ratio of equilibrium phosphorus partition. Figure 4 shows the effect of basicity on the activity of P2O5 in the slag. The two curves correspond to iron oxide concentrations of 15 mass% and 20 ma$s% tot the slag. It can be seen clearly that the activity value decreases to a minimum when the basicity reaches 2.5 - 2.6. Further increase in basicity has negligible effect on the activity of P2O5. Thus it is concluded that increase of basicity beyond 2.7 is unlikely to cause any further increase in the equilibrium phosphorus partition ratio.
It is also observed from the figure that the P2O5 activity, for basicity £ 2.7, is lower for slag containing 15 mass% iron oxide, than for slag containing 20 mase% iron oxide. This trend is in agreement with the observation in Figure 2, where the value of b was seen to decrease when the Iron oxide content In slag was increased from 18to20mass%.
Maintaining the BOF slag basicity within 2.5 - 2.7 raises apprehensions about the possibility of MgO dissolution from the refractory lining. It has been reported earlier that the BOF slag has a solubility for MgO and hence there is risk of adverse impact on the safe operating life of the BOF refractory.
Fig. 3 shows the equilibrium solubility of MgO in BOF slag, for different levels of FeO concentration in the slag. It can be seen clearly that the solubility of MgO in the slag reaches a minimum, and then maintains near-saturation. The value of basicity, where this saturation is attained, ranges typically from 2.5 - 2.7. Further the solubility of MgO, at any given level of basicity, increases with increasing FeO concentration. This trend is to be expected, since FeO acts as a solvent for MgO.
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The equilibrium phosphorus partition ration is determined by the following laboratory equipments and procedures.
A horizontal tube furnace, with electrical heating, was used for performing the experiments. Steel and slag samples, in granulated / powdered form were put in dense-sintered magnesia crucible and placed in the high-temperature furnace. The sample was heated up to temperature of 1600 - 1650°C and held for 8 hours so that the molten steel and slag could attain equilibrium. After the requisite duration of soaking, the crucible containing the molten slag and steel was withdrawn from the furnace and cooled rapidly. The solidified slag and steel were then separated and analysed for chemical composition .
Arrangements in the furnace were made for entry and outlet of gas from opposite ends. The thermocouple was inserted through the stopper on the same side as gas inlet. The samples were inserted and withdrawn from the gas exit end of the
furnace.
All the experiments were carried out under a stream of purified argon for maintaining inert atmosphere.
The invention is illustrated with a typical mass balance for 1000 kg of liquid steel production with a comparative table 1 distinguishing improvement as defined and described in the present invention over prior art.
Typical mass balance of the present invention
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Average metallic charge in 8QF is
Hot metal + scrap input 1150 kg per TLS, Hot metal temperature: 1200°C-1400°C
Average scrap proportion in hot metallic charge = 10-15%
Phosphorus content in hot metal: 0.20 - 0.25 mass%
Target temperature of liquid steel: 1650°C
Target phosphorus content in steel: 0.010 mass% (max.)
Target phosphorus partition ratio: 130 - 160 corresponding to 280-340 equilibrium
phosphorus partition ratio.
Target percentage of input phosphorus removed to slag is 95.5-96.5 in hot metal
Sensible heat of liquid slag (with respect to 25°C): 2.1 - 2.2 MJ / kg
CaO content in limestone: 48 - 50 ma$s% Purity of limestone: 96 mass%
Efficiency of lime utilisation: 94-95 mass%
The average phosphorus partition ratio achieved in a BOF ranges from 40% to 60%
of the equilibrium partition ratio.
The average phosphorus partition ratio achieved in a BOF under prior art is 90-120.
In the illustration as stated herein hot metal is maintained at 1200-1400°C and liquid steel is maintained at 1600° - 1650°C.
In the illustration as above considering the average compositions, percent of input [P] removed to slag of hot metal is given by
Mass of [P] input through Hot Metal and scrap - mass of [P] in steel = 95.5-96.5 Mass of [P] input through Hot Metal + scrap
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/

TABLE!

Table 1: Effect of reduced basicity operation on the process parameters
Parameter Existing practice Present Invention
Lime consumption 88 -107 kg 67 - 85 kg
Basicity (%CaO/%SiO2) 3.2 - 3.6 2.5 - 2.7
Slag generated 160-190 kg 130 -160 kg
Limestone to be calcined 176 -214 kg 134-170 kg
CO2 released to atmosphere during calcination 77-94 kg 59-75 kg
Energy consumption for limestone calcination 389.5-473.5 MJ 296.5 - 376.2 MJ
Free lime lost to slag 4-7% < 1%
Net benefit of reduced basicity operation:
All the figures are expressed with respect to I tonne of liquid steel (TLS)
• Reduction in consumption of calcined lime: 20-22 kg/US
• Reduction in limestone requirement: 40 - 42 kg/RS
(limestone is required for production of calcined lime)
• Reduction in CO2 emission to the atmosphere: 18 - 19 kg/TLS
• Reduction in energy consumption during calcinations: 90 - 100 MJ/TLS
• Reduction in slag generation: 30 kg/TLS
• Additional sensible heat available: 60 - 62 MJ/TLS
(This sensible heat, which would have been otherwise carried away by the slag, can be utilised to melt additional scrap and thus increase productivity.)
The invention as described and defined herein should not be read in a restrictive manner as various modifications, alterations, changes and adaptations are possible within the scope and limit of the appended claims.
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We claim:
1. A method of producing tow phosphorus steel containing below O,O10%
phosphorus from high phosphorus hot metal charged fa a stogie slag basic
oxygen furnace comprising the steps of maintaining a low slag basicity In
liquid steel in the range of 2.5 to 2.7 at 1600°C to 16500 Cwith FeO% in the
slag in the range of 10 to 30% mass through additions of measured
quantities of catered lime in pulverized and/or brlquetted forms, continuously
or "m batches upto the completion of melting, maintahing equilbrum
phosphorus parfion ratio in the range of 280-340 and tower P3O5 activity In
the slag In the range of -20.2 to ~19.6 in log scale which ensures complete
phosphorus removal in the slag upto 99.99% and also prevent any MgO
dissolution from the refractory ining In the slag.
2. A method as claimed In claim 1 wherein the equilibrium phosphorus partition
ratio (Lp) is represented as

3. A method as claimed in claim 1 wherein removal of phosphorus from steel is
governed by the reaction 2[P] + 5[O] - P2O5 and the equilibrium constant
for the same reaction is represented as

a (P2O5) represents P2O5 activity,
h[p] represents activity of phosphorus and
h [o] represents activity of oxygen at agiven temperature.
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4. A method as claimed in the preceeding claims wherein maintaining a lower
value of P2O5 activity In the slag provide a higher ratio of equilibrium phosphorus
partition.
5. A method as claimed in the preceeding claims wherein the activity value of
P2O5 in slag decreases to a minimum when the basicity of slag reaches to 2.5 to 2.7.
6. A method as claimed in the preceeding claims wherein mass% of iron oxide
in a slag is 15 to 20.
7. A method is claimed in the preceeding claims wherein P2O5 activity for slag
basicity £ 2.7 is lower for slag containing 15 mass% iron oxide than for slag
containing 20 mass% of iron oxide.
8. A method as claimed in the preceeding claims wherein by maintaining a slag
basicity of 2.5 to 2.7 reduces consumption of calcined lime, limestone thus reducing
overall flux consumption, slag volume, C02 emission, energy consumption during
calcinations and saving free lime lost to slag and saving sensible heat for melting of
the burden.
9. A method as claimed in the preceeding claims wherein the Mgo solubility in
the slag is minimum by maintaining slag basicity at the range of 2.5 to 2.7.
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10. A method of producing low phosphorus steel from high phosphorus hot metal
in single slag basic oxygen steelmaking as herein described and illustrated.

This invention relates to a method of producing low phosphorus steel containing below 0.010% phosphorus from high phosphorus hot metal charged in a single slag basic oxygen furnace comprising the steps of maintaining a low slag basicity in liquid steel In the range of 2.5 to 27 at 1600°C to 1650°C with FeO% in the slag In the range of 10 to 30% mass through additions of measured quantities of calcined lime in pulverized and/or briquetted forms, continuously or in batches upto the completion of melting, maintaining equilibrium phosphorus partion ratio in the range of 280-340 and lower P2O5 activity in the slag in the range of -20.2 to -19.6 in log scale which ensures complete phosphorus removal in the stag upto 99.99% and also prevent any MgO dissolution from the refractory lining in the slag.