Efforts to improve the processing ratio of the BOF type furnace for hot metal dephosphorization at the Kakogawa Works

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1 Efforts to improve the processing ratio of the BOF type furnace for hot metal dephosphorization at the Kakogawa Works KAZUMASA ADACHI* YOSHIO SUZUKI* KOSUKE SAITO* ATSUHIKO YOSHIDA* * Steelmaking Department, Kakogawa Works, Iron and Steel Business - KOBE STEEL, LTD, JAPAN -1-

2 Outline 1. Process flow of steel making process at Kakogawa Works 2. Change due to new facility operation 3. Issues after starting up the BOF type furnace for hot metal dephosphorization 4. Improvement of adhered metal melting method in furnace 1) Investigation of metal melting method 2) Optimization of top-blown oxygen condition 3) Results of actual application 5. Conclusion -2-

3 1. Process flow of steel making process at Kakogawa Works Construction in 2014 Treatmen Plant No.2,3 Blast Furnuce No. 1, 2 KR Furnace Hot Metal pretreatment No.(1),2 Dephosphorization Furnace Steel Making Plant 1LF 2LF No.1, 2, 3 LD-OTB No.3&4 Continuous Casting Shop 2CAS 1RH 4RH 1CAS 2RH 3RH- MRP 2CC (Bloom) 6CC (Bloom) Ingot 3CC (Slab) 4CC- 1STR (Slab) 4CC- 2STR (Slab) Blooming Mill Hot Strip Mill Plate Mill Fig.1 Process flow of steel making process at Kakogawa Works -3-

4 2. Change due to new facility operation Change due to new facility operation De-S : 100% of the TP After improvement KR 98% TP 2% De-P : 24% of the TP After improvement DP 58% TP 10% -4-

5 3. Issues after starting up the BOF type furnace for hot metal dephosphorization Cycle time( min/ch) In order to improve the processing ratio by shortening the cycle time at the beginning of the operation of the BOF type furnace for hot metal dephosphorization, the processing ratio was improved to 58%. subsequent to October 2014 the BOF type furnace for hot metal dephosphorization processing ratio fell to 48%. Effort to improve Effort increase of the metal melting time : 3% processing ratio start up Target processing ratio58% 9% Target cycle time 26min/ch After improvement this time Processing ratio (%) 24.0 Cycle time Processing ratio A M J J A S O N D J F M

6 4. Improvement of adhered metal melting method in furnace It adheres to the entire circumference from the furnace entrance to the slag line. Adhesion of the metal was promoted by slopping due to the decrease in internal volume of the furnace, and yield reduction occurred. Bare metal -6-

7 4. Improvement of adhered metal melting method in furnace Current status Kobe works) H-furnace After improvement Method Metal melting lance Secondary combustion Secondary combustion Conceptual diagram Extended processing time 10min 0min 0min De-Si De-P -7-

8 4. Improvement of adhered metal melting method in furnace 1) Investigation of metal melting method Method of dissolving the extensively adhered metal There is a method of melting the metal using slag which has attained a high temperature. This is done by secondary combustion of CO gas in the forming slag. < Condition comparison > Impact pressure : within a range of somewhat higher Oxygen accumulation : equivalent I/R(melting range ) : larger than the range of the H-furnace Impact pressure (Pa) Oxygen accumulation (Nm3/t) Kobe works)h-furnace Slag-forming stage 1600~ ~ ~ ~ [Si]+1.9 ~8.3 [Si]+4.1 Metal melting stage Kakogawa works) The BOF type furnace for hot metal De-Si stage De-P stage 3.3~7.1Nm 3 /t 7.9 [Si] ~6.9Nm 3 /t I/R ~ ~0.47 Evaluation Optimization necessary < Supplement > R= Distance from lance center axis to furnace wall(mm) I= Distance from the point farthest from the center of the gas jet contact surface on the bath surface to the furnace wall refractory(mm) -8-

9 4. Improvement of adhered metal melting method in furnace 1) Investigation of metal melting method Formula for calculating I I was calculated by the formula (2) using the hard core length Z C of the gas jet described in formula (1). Z (4.12 P ) O d C I H tan (θα) ZC cos θ tan (θα) ZC sin θ D R (1) (2) < Explanation of symbols > I= Distance from the point farthest from the center of the gas jet contact surface on the bath surface to the furnace wall refractory(mm) R= Distance from lance center axis to furnace wall(mm) H= Distance from the static bath surface to the lower end of the lance(mm) D= Distance from lance center axis to nozzle exit outermost periphery(mm) ZC = Gas jet hardcore length(mm) Θ= Nozzle inclination angle( ) α=ejection spread angle after nozzle ejection( ) Fig. 7 Conceptual diagram of I / R Po=Nozzle front pressure(kgf/cm2) D=Nozzle throat diameter(mm) I R -9-

10 Iron yield difference between the normal de-p treatment(%) 4. Improvement of adhered metal melting method in furnace 2) Optimization of top-blown oxygen condition I / R and the amount of dissolved metal ( yield difference between normal processing ) Conditions with a large degree of metal melting are in good agreement with the proper range of H furnace (I / R = 0.08 to 0.25) The lance height increased, the collision pressure was 200 ~ 800 Pa(H-furnace 200 ~ 1200 Pa) Appropriate range of H-furnace (1.0) (2.0) Error bar is 1σ (3.0) I/R -10-

11 De-C amount(%) Temperature difference after treatment( ) (Calculated - Performance) 4. Improvement of adhered metal melting method in furnace 2) Optimization of top-blown oxygen condition Amount of de-c reduced by % Change in post-treatment temperature post-treatment temperature Si=0.25~0.45% Temperature Reduction of de-c amount by % Unit of De-si outside oxygen consumption(nm3/t) -15 Conventional Error bar is 1σ After improvement From this result, the of secondary combustion heat was estimated. -11-

12 4. Improvement of adhered metal melting method in furnace 2) Optimization of top-blown oxygen condition The secondary heat of combustion is generated by the surplus oxygen generated by the decrease in the reaction amount of de-c and the CO gas in the slag Calculate the rate at which secondary combustion heat contributed to melting and raising the temperature of the metal from thermal balance <Concept of thermal balance> Metal melting 地金溶解処理 treatment 1Decrease in de-c reaction volume( C=-0.09%) CO ランス上昇 gas bubble A surplus blowing oxygen of 0.09% occurs 2Secondary combustion with CO gas by excess oxygen 3Base metal melting and raising temperature due to secondary combustion 3metal melting and rising temperature 2Secondary combustion <Heat balance> 1) 1+3 : Temperature drop( ) 2) 2 : Temperature rise(+) (1+3)+2 γ = 5 Excess oxygen generation 1Reduced amount of de-c -12-

13 Hot iron temperature change ( ) Change in hot metal temperature( ) 4. Improvement of adhered metal melting method in furnace 2) Optimization of top-blown oxygen condition Calculation of heat efficiency of secondary combustion heat Definition formula < Supplemental formula > De-C reaction heat : Q CO = C W q CO CO secondary combustion heat : Q CO2 = C W q CO2 Fe melting and rising heat : Q Fe = (q Fe + t m Fe ) w Slight change in hot metal temperature : T = (Q CO +Q Fe +γ Q CO2 ) / (W m Fe ) 5 Heat efficiency 60% C (%) 50% 40% 30% 20% 10% 0% Symbol Item Unit Precondition Remarks q CO CO reaction heat kj/kg 9,240 C + 1/2O 2 (g) = CO(g) q CO2 CO CO 2 reaction heat kj/kg 23,700 CO(g)+1/2O 2 (g) = CO 2 (g) q Fe FeHeat of dissolution MJ/t -277 Fe(s)=Fe(l) m Fe Specific heat of iron(1300 ) kj/kg T Hot iron temperature change De-C temperature rise -9 Melting Temperature rising Total-5 De-C reaction Metal melting Other Secondary combustion The heat efficiency is 38% on average metal temperature 1000 After melting temperature1351 w Metal melting amount t/ch 1.86 Fe melting amount+0.7% 265t=1.86t W Amount of hot metal t/ch 260 Actual amount of hot metal molten metal Other Hot metal heat insulation -13-

14 Yield improvement effect index(-) Number of times of use of metal melting lance (Times / day) Metal melting dissolution treatment ratio (%) 4. Improvement of adhered metal melting method in furnace 3) Results of actual application The frequency of use of the bare metal melting lance 3.0 times/day 0.9 times / day The metal melting processing ratio 0 % 40 % < Consistent yield > Iron loss reduction 0.0 Conventional After improvement : +0.17% Processing ratio : +0.05% Conventional After improvement Processing 0.28 ratio 62% Conventional Iron loss reduction +0.28% After improvement The BOF type furnace for hot metal dephosphorization alone Iron loss reduction +0.17% Increase processing ratio +0.05% - Conventional After improvement - Consistent yield -14-

15 5. Conclusion Improvement of metal melting method By improving the metal melting process, the metal melting processing ratio was applied up to 40%, and the usage frequency of the metal melting lance was reduced, from 3.0 times / day to 0.9 times / day. As a result, in addition to a consistent yield % improvement by processing ratio + 4%, integrated yield improved by % due to iron loss reduction % due to melting of the metal.