Title of Invention

AN IMPROVED METHOD FOR DE-PHOSPHORISATION OF STEEL IN AND LD STEEL MAKING CONVERTER

Abstract

The main object of the present invention is to enhance the mixing for improved mass transfer during blowing through the elements mounted in the base of the converter, called tuyeres, without increasing the gas flow rate. This and other objects of the present invention can be achieved by distributing the total gas flow through the bottom tuyeres differentially among the tuyeres. As explained in the last section, conventionally the flow through the bottom tuyeres are distributed uniformly among the tuyeres. In contrast to this uniform flow rate, the differential flow rate used in the present invention increases the mixing and mass transfer by as high as 35%. Thus the present invention provides an improved method for dephosphorisation of steel in an LD steel making converter comprising the steps of: blowing oxygen using a top oxygen lance for refinina steel; combined blowing of inert gases from a plurality of elements arranged at the base of said converter for enhancing mixing for improved mass transfer; and regulating the flow of said inert gases through said elements for distributing the total gas flow differentially among said elements.

Full Text

FIELD OF APPLICATION The present invention generally relates to an improved method for de-phosphorisation of steel in an LD steel making converter. In particular it relates to a method for enhancing the mixing of molten metal for improved mass transfer by injecting the gas in differential flow pattern among the tuyeres. BACKGROUND OF THE INVENTION Since the inception of basic oxygen steel making process into operation, this has been subjected to continuous development. One of the most important developments has been the introduction of combined blowing, where the refining achieved with the top oxygen lance augmented by the injection of inert gases such as argon and nitrogen from elements mounted in the base of the converter called tuyeres. The advantages of combined blowing can be fundamentally attributed to improved hydrodynamics within the bath as a result of far better mixing and improved agitation in the converter. The chronological change in number of the bottom tuyeres is found to enhance mixing starting from a single concentric bottom tuyeres to as high as eight number of bottom tuyeres. In conventional practice, the flow rate among the tuyeres is kept uniform besides optimizing the radial placement of the set of tuyeres and an increase in total gas flow rate through them during the course of a heat in steel making. Submerged gas injection through bottom of a metallurgical reactor containing molten metal such as LD steel making converter has been an established practice for enhanced mass transfer rates, improved mixing and closer approach to chemical equilibrium. The improvement with respect to the primitive LD process, with only top blowing, is attributed to the better homogenization of the temperature and chemical composition of the bath and improving the reaction rates by bringing reactants together and moving products from the reaction site. This additional bath stirring helps in attaining metallurgical advantages in terms of improved metallic yield, better process control and meeting stringent quality requirements by achieving controlled oxygen, carbon, phosphorous and nitrogen contents in the steel. Various benefits obtained out of combined blowing were such as increased steel yield, improved slag metal interaction, better ferroalloy recovery etc. This is attributed to the fact that in steelmaking process the metallurgical characteristics of the process is linked to the attainment of the slag metal equilibrium at any instant during the blow and it is mainly dependent upon the efficiency of the bath mixing and mass transfer rate. The variables such as radial placement of the tuyeres, tuyere configuration (circular, linear etc.), and total bottom gas flow rate have been studied vividly to improvise mixing and mass transfer in the steel making converter vessel. Numerous studies had been carried out keeping the improvement in mixing and mass transfer rates of slag-metal in LD steel making converter in perspective through changing different tuyere arrangements, the bottom injection parameters such as the gas injection rate and the PCD (pitch circle diameter - the ratio of radial distance of tuyere to that of the radius of the vessel) of the placement of the bottom tuyeres, the number of tuyeres. The thermodynamic behavior of the different refining reactions is well established. During the decarburization period of the blow, a vigorous CO evolution helps in getting the aided mixing, an inherent advantage of the process. Hence the bottom gas flow through the bottom injection elements are regulated in such a way that it should be operated at desired flow rates as per the thermodynamic state of the refining reactions. It is operated from a flow rate barely minimum to avoid clogging to a highest during the end of the blow so as to take care of mixing and mass transfer in the bath. But whatever the state of the flow rate at which it is operated at, one of the typical features of the bottom blowing has been a uniform flow through all the bottom tuyeres. However, it is worth noting that all the investigations that have been carried out till date relate to the enhancement of mixing and mass transfer through modifying the parameters such as increased gas flow rate through bottom, increasing number of bottom tuyeres, changing the radial position of the tuyeres, or changing the total flow rate of through the tuyeres during course of the blow. But increased gas flow rate also means higher mechanical wear of the tuyere wall and hence resulting in lower refractory lining life. A need therefore exists for achieving optimum mixing and mass transfer without increasing the gas flow rate or changing the tuyere position. SUMMARY OF THE INVENTION The main object of the present invention is to enhance the mixing for improved mass transfer during blowing through the elements mounted in the base of the converter, called tuyeres, without increasing the gas flow rate. This and other objects of the present invention can be achieved by distributing the total gas flow through the bottom tuyeres differentially among the tuyeres. As explained in the last section, conventionally the flow through the bottom tuyeres are distributed uniformly among the tuyeres. In contrast to this uniform flow rate, the differential flow rate used in the present invention increases the mixing and mass transfer by as high as 35%. Thus the present invention provides an improved method for dephosphorisation of steel in an LD steel making converter comprising the steps of: blowing oxygen using a top oxygen lance for refinina steel; combined blowing of inert gases from a plurality of elements arranged at the base of said converter for enhancing mixing for improved mass transfer; and regulating the flow of said inert gases through said elements for distributing the total gas flow differentially among said elements. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The invention can now be explained with the help of the accompanying drawings where Figure la is a schematic sketch showing uniform flow rate through the bottom tuyeres. Figure lb shows differential flow rate through the tuyeres of the present invention. Figure 2 is a schematic sketch showing the radial position of the bottom tuyeres in the model used for study and the arrows shows the position of the trunion. Figure 3 shows a schematic sketch of the experimental set up. Figure 4 shows a typical mixing time plot. Figure 5 shows a differential flow rate scheme with flow rate distribution. Figure 6 shows a comparison of mixing time of existing uniform flow with differential flow condition at tuyeres position in existing LD shop vessel. Figure 7 shows a comparison of mixing time of uniform flow rate with differential flow condition at S(l) tuyere configuration. Figuere 8 shows a comparison of mixing time of uniform flow with differential flow condition at S(2) tuyere configuration. Figure 9 shows the change in concentration of benzoic acid in the water with time for differential flow condition for existing S(0) configuration. Figure 10 shows a comparison of mass transfer rate constant of uniform and differential flow for existing S(0) configuration. Figure 11 shows a comparison of mass transfer rate constant of uniform and differential flow for S(l) configuration. DETAILED DESCRIPTION A schematic illustration of the flow uniformity through the tuyeres is shown in Figure 1A. In contrast to this uniform flow rate, the differential flow rate of the present invention is shown in Figure 1B. Mixing time and mass transfer experiments were carried out on a physical model geometrically and dynamically similar to an existing LD steel making vessel. The Modified Froude Number was used to make the model dynamically similar to the prototype. The details of the prototype and the model are enlisted in Table 1. For the mixing time experiment, slag phase was not simulated as the overall mixing in a converter is exclusively governed by fluid in the bulk metal phase. However, during mass transfer experiments a separate phase simulating slag was used along with water simulating liquid steel. Mixing time experiments were carried out by monitoring the conductivity of the bath by means of conductivity probe and kCI solution as tracer. However, during the study the location of the bottom tuyere at different PCD had been tried out. The details of the different configurations are listed in Table 2 and are shown in Figure 2. An integrated experimental set up used during the study is shown in Figure 3. The mixing time for each experiment was calculated from the conductivity versus time plots for a degree of mixing of 95 % (+ 5%). Sufficient care was taken to ensure that the variation in the mixing time for any identical sets of experiments did not exceed three seconds. A typical mixing time experiment is depicted graphically in Figure 4. In mass transfer experimentation, a new system (engine oil-water) had been used for this study. Phase 1 simulates the steel whereas Phase II simulates the siag phase. The following equation was used to calculate the mass transfer rate constant (k). where k is the mass transfer rate constant of exchange material from Phase I to Phase II; Ce is the equilibrium concentration of the exchange material in Phase I; Ct is the concentration of the exchange material in Phase I after time t; C0 is the initial concentration of exchange material in Phase I. The exchange material used for the study was benzoic acid. Hence all the mass transfer experiments were carried out with this system. In the mass transfer experiments, the transfer of benzoic acid from the water to the engine oil grade-68 was studied, A typical mass transfer experiment was carried out by first filling up the model with 90 litres of benzoic acid solution (about 2 gms/lit), then 18 litres of engine oil was gently poured on it. This amount was similar to the proportion of the slag with respect to liquid steel volume in the steel making vessel in an existing LD shop. Samples were collected simultaneously from centre and periphery of the vessel at an interval of 2 minutes. These samples were subsequently analyzed for their benzoic acid content volumetrically by titrating against 0.05 N potassium hydroxide (KOH) solution using phenolphthalein as the indicator. For determining the impact of differential flow rate on mixing in the LD converter, various schemes were formulated and mixing time experiments were carried out by varying the bottom gas flow rate and keeping other parameters like top lance flow rate, lance height, bath height constant as enlisted in Table 1. Figure 5 shows a scheme of differential flow rate among the bottom tuyeres by clubbing together all the 8 tuyeres into three groups of tuyeres. The flow rates among them were varied from low, intermediate and highest keeping the total bottom gas flow rate constant. As shown in Figure IB the flow rates are in the ratio of 2:3:5 in the three groups. The best location for putting the probe in the case of the differential flow conditions was optimized. As explained in the previous section, the flow rate was varied so as to cover up the minimum flow rate through tuyeres corresponding to the start of the blow to a maximum flow rate at the end of the blow. A consistent improvement in mixing time of the order of 33-41 % was observed across a wide range of bottom gas flow rate of as low as 10 Ipm to 50 lpm which covering the minimum and the maximum flow rates corresponding to that under practice in LD steel making process. But in the flow regime at which an LD shop operates an improvement in mixing time of more than 30 % was achieved with differential flow compared to the existing practice of uniform flow through the tuyeres. The comparison of mixing time between the uniform flow conditions with the differential flow condition for existing tuyeres position is shown in Figure 6. A similar improvement in terms of mixing time has been obtained with the tuyeres configurations such as S(l), S(2) as shown in Figures 7 and 8 respectively. The mixing results from a combination of many phenomena such as molecular diffusion, convective flow and turbulent diffusion. Any one of these phenomena is not sufficient for perfect mixing. What happens in the vessel is that when the bath is stirred through bottom purging and top lance injection, turbulent motion is generated in the bath which drives the convective flow or bulk flow. Such bulk flow causes disintegration of the larger size pockets of fluid into smaller pockets called eddies and gets dispersed to improve mixing process which is termed as eddy diffusion. This type of mixing in bulk of the vessel is termed as macro mixing. When the flow rate through all the tuyeres is uniform, the circulation cells formed in the liquid metal pool will have uniform strength. This causes little interaction amongst the cells. But in the case of differential flow rates, the cells formed will have different strengths which aids in more and more bulk convection as there is clearly a gradient of energy in three different regions of the bath and thereby enhancing the intermixing amongst the cells. To determine the impact of differential flow rate on Mass Transfer Rate in the LD converter, mass transfer study was carried out for all the configuration such as S(0), S(1), S(2) with uniform and differential flow. These locations were selected based on the basis of better mixing. As discussed earlier, during the mass transfer experimentation, samples were taken from centre as well from the periphery of the vessel. It was found that there was not much difference in the concentration in the samples obtained from the centre as well periphery of the vessel as shown in Figure 9. As expected, concentration of benzoic acid decreased with time. The mass transfer between the two fluids owing to the mixing of the phases was calculated assuming first order kinetics by the expression as given in equation (1). As in the present investigation interracial area 'A' for mass transfer was not determined, therefore, the mass transfer coefficient could not be determined instead the rate constant 'K' was determined. The value of Ce in the present investigation was found to be 1.246 gms/litre. The value of rate constant was determined by plotting -In[(Ct-Ce)/(C0-Ct)] against time and by fitting straight lines; the slope of each line gave the value of rate constant V when the differential flow rate condition was imparted to the bottom gas flow rate. Figure 10 shows that the rate constant 'K' increased by 28 % when flow pattern was changed from uniform to differential flow at existing tuyere location S(0). Figure 11 shows the improvement in 'K' when a differential flow in place of uniform flow condition was tried out for tuyere configured S(l) and an improvement of magnitude of about 65% was obtained. It is evident that with respect to existing tuyere configuration S(0) with uniform flow rate the improvement in mass transfer rate constant 'K' was 18 % when changed to S(l) with uniform flow rate. WE CLAIM 1. An improved method for dephosphorization of steel in an LD steel making converter comprising the steps of: - blowing oxygen using a top oxygen lance for refining steel; - blowing of inert gases like argon or nitrogen at rate of 2.2-6 N m3/min through a plurality of tuyeres elements placed at the bottom of said converter for enhancing mixing for improved mass transfer with a stirring effect and - regulating the flow of said inert gases through said element for distributing the total gas flow differentially among said elements clubbed together as plurality of groups. 2. The method as claimed in claim 1, wherein said plurality of elements called tuyeres are eight in number and arrange radially at the base of said converter. 3. The method as claimed in claim 1, wherein the differential flow rate among said bottom tuyeres is achieved by clubbing the tuyeres into three groups. 4. The method as claimed in claim 47-wherein the flow rate among said three groups are in the ratio of 2:3:5. 5. An improved method for de-phosphorisation of steel in an LD steel making converter,substantially as herein described and illustrated in the accompanying drawings. The main object of the present invention is to enhance the mixing for improved mass transfer during blowing through the elements mounted in the base of the converter, called tuyeres, without increasing the gas flow rate. This and other objects of the present invention can be achieved by distributing the total gas flow through the bottom tuyeres differentially among the tuyeres. As explained in the last section, conventionally the flow through the bottom tuyeres are distributed uniformly among the tuyeres. In contrast to this uniform flow rate, the differential flow rate used in the present invention increases the mixing and mass transfer by as high as 35%. Thus the present invention provides an improved method for dephosphorisation of steel in an LD steel making converter comprising the steps of: blowing oxygen using a top oxygen lance for refinina steel; combined blowing of inert gases from a plurality of elements arranged at the base of said converter for enhancing mixing for improved mass transfer; and regulating the flow of said inert gases through said elements for distributing the total gas flow differentially among said elements.

Documents:

327-KOL-2005-FORM 27.pdf

327-kol-2005-granted-abstract.pdf

327-kol-2005-granted-claims.pdf

327-kol-2005-granted-correspondence.pdf

327-kol-2005-granted-description (complete).pdf

327-kol-2005-granted-drawings.pdf

327-kol-2005-granted-examination report.pdf

327-kol-2005-granted-form 1.pdf

327-kol-2005-granted-form 13.pdf

327-kol-2005-granted-form 18.pdf

327-kol-2005-granted-form 2.pdf

327-kol-2005-granted-form 3.pdf

327-kol-2005-granted-form 5.pdf

327-kol-2005-granted-gpa.pdf

327-kol-2005-granted-others.pdf

327-kol-2005-granted-reply to examination report.pdf

327-kol-2005-granted-specification.pdf