EAF REFRACTORY PERFORMANCE AT PACIFIC STEEL NEW ZEALAND

Transcription

1 EAF REFRACTORY PERFORMANCE AT PACIFIC STEEL NEW ZEALAND Don Sanford Product Manager Refractories Chemiplas, New Zealand Ltd Level 2, 42 Upper Queen Street Auckland, New Zealand Brian Garside Steel Plant Manager Pacific Steel 259 James Fletcher Drive Otahuhu, New Zealand Chris Schonewille Steel Plant Operations Manager Pacific Steel 259 James Fletcher Drive Otahuhu, New Zealand and Tom Miller, Asia Technical Manager LWB Refractories, Australia 232 East Market Street York, Pennsylvania U.S.A Keywords: Foaming, Slag, Models ABSTRACT This paper deals with the practical application of Isothermal Stability Diagrams to foaming slag practice in the Electric Arc Furnace at Fletcher Industries, Pacific Steel, Auckland, New Zealand. Refractory life in the furnace increased from the range of 800 heats to more than 2500 heats as a direct result of applying the new foaming slag practice. With further optimization of the practice, associated operational cost savings in increased yield and lower power consumption, was realized. 1

2 INTRODUCTION The Steel Plant at Pacific Steel (Fletcher Building New Zealand) has increased their EAF refractory sidewall campaigns life from 800 heats to 2614 heats within 18 months, by adopting a foamy slag practice. This was achieved by a joint effort between: Pacific Steel Chemiplas (installation/management of refractories at Pacific Steel) LWB Refractories Applied Technology (formerly Baker Refractories). PLANT BACKGROUND Pacific Steel was founded in 1962, its purpose initially to develop Ironsand processing and to recycle the scrap iron and steel to provide finished steel products for the New Zealand economy. The plant consisted of a low power EAF, which cast into ingots. Pacific Steel upgraded their facilities significantly over the next 10 years by upgrading their EAF transformers, installing water-cooled panels on the EAF sidewalls, the installation of a Ladle Furnace and an in-line H&K Bar Mill. The steel plant is equipped with 1 (one) 35 MVA, 50 ton Krupp Electric Arc Furnace with one (1) side mounted Moore oxygen and carbon injection lance. The typical oxygen consumption is 23Nm³ per tonne. Tap to tap time is 74 minutes average. The EAF has a side mounted slidegate system. Secondary refining is via a Krupp Ladle Furnace. The ladle furnace feeds a three strand mm Rokop Billet Caster, which typically casts at a speed of 1.5m/min. The alloy and flux additions are added during tap. Argon for ladle stirring is via a porous plug, which is used at tap and at the ladle furnace. The ladle furnace is equipped to add alloys and fluxes from a hopper system above the refining station and with an Obermath wire injection feeder to feed Ca wire to the heat during refining and before sending it to the caster. Application include reinforcing bar, rebar coil, rounds, bar, angles, un-equal angles, flat, channels and wire rod. FURNACE HISTORY As mentioned previously, the electric arc furnace has had several enhancements to improve its performance, such as: Water-cooled panels above the sidewall slag line. Modified roof design to include water-cooled panels, leaving only a refractory delta section. Oxy-fuel burner technology to enhance the melting process. Foaming slag coupled with a hot heel practice that speeds up the melting phase of the operation. Side mounted slide gate system for virtually slag free tapping. The EAF refractory lining has not changed much over the past 6 years (Figure 1). The bricks for the entire furnace are supplied by LWB Refractories. The sidewall brick is a magnesia-carbon with 96% MgO with 13 % retained carbon. The mast wall has a higher carbon content of 17%. Sidewall campaign life was consistently around heats, until the foaming slag practice was adopted from LWB Applied Technology.

3 Sidewall bricks: mag-carbon Taphole surround blocks: mag-carbon Taphole sleeve gunning: magnesite Sub-hearth bricks: mag-carbon Hearth ramming: magnesite Taphole sleeve: mag-carbon Steel Shell Hearth safety bricks: magnesite Fig. 1. EAF design at Pacific Steel ISOTHERMAL STABILITY DIAGRAMS The Isothermal stability diagram (ISD) shown in Figure 2 is an example of the primary tool used by LWB Refractories to characterize EAF slags. As can be seen, the figure shows the phases present in EAF slags by plotting %MgO versus %FeO+%MnO at 1600 C and a specific basicity ratio (B 3 of 1.6 in the example). B 3 is defined by the ratio of lime over alumina plus silica in the slag. Indications on the diagram are the MgO and CaO saturation levels of the slag as well as the point of dual CaO/MgO saturation. These charts are generated according to a specific temperature and basicity for each operation as well as for specific grades of steel being made. The dotted lines in Figure 2 are the inferred areas for optimum foaming. Slags with chemistries falling in the MW+L (Magnesia Wustite) phase area and close to the dotted line will not only foam well with carbon injection but will also be fully compatible with Magnesia-Carbon refractories. Good foaming is also possible in the region of the dotted line in the C2S + L area but excellent control of the FeO levels in the slag would be required to maintain the foaming. Slags falling in the C2S + L liquid area are vulnerable to increases in FeO content since it could move the slag composition from a good foamy condition to the all-liquid region for only a small increase in FeO content. Slags falling into the all-liquid area of the chart will not have the correct viscosity to form a good sustained foamy slag. Targeting slags in the MW + L area not only ensures refractory compatibility but it also create slags that can absorb significant amounts of FeO while maintaining good foaming properties. 3

4 Dual Saturation MgO Saturation CaO Saturation Fig. 2: Isothermal Stability Diagram (ISD) at a constant basicity of 1.6 and a temperature of 1600 C The theory behind the construction of the ISD diagrams is fully discussed in various references (1,2,3). This paper deals with the application of the technology specifically to the EAF at Fletcher's Pacific Steel in Auckland, New Zealand. With close cooperation between Chemiplas, LWB and Pacific Steel, huge gains were made in refractory performance as well as specific operational advantages. FOAMY SLAG MODEL A Foamy Slag Model developed by LWB Refractories was used to evaluate the EAF slags at Pacific Steel. Figure 3 is a captured screen shot of program and it shows that the model is very flexible in the number of options available for building EAF slags. It allows for the use of either Dolomite or Magnesia as a source of MgO. The typical chemistries of the flux materials for the slags are also input. The horizontal sloped line calculated in the program and plotted in the ISD, represents the evolution of the slag over increasing amounts of FeO. The program will then make a recommendation for a new slag chemistry based on the original B 3 basicity ratio by adjusting the amounts of lime and MgO, as shown in the bottom right of the program. The program also makes it possible to change the basicity of the recommended slag or the level of MgO super saturation and then calculates where it would fall on the diagram. The timing of slag foaming, either later or earlier in the heat, may be controlled by the basicity of the slag. A higher basicity slag will foam later in the heat (higher FeO) than a lower basicity slag. This would prove to be very useful in heats making low carbon steels where high amounts of oxygen are blown and larger amounts of FeO are generated. 4

5 In the above example, the final slag falls into the liquid area of the chart and therefore will not be foaming at the end of the heat. From the evolution of the slag, there was a period of good foaming when the total FeO reached about 18% and continued till the FeO exceeded 23%. To have this slag in the good foamy area during the final stages of the heat, it is recommended to add sufficient MgO, 405kg, to bring the MgO level to 1.5% above saturation, or 11.9%. Figure 3. Screen capture of the interface of the ISD Foamy Slag Program The ISD in Figure 3 also shows an almost ideal situation with the recommended 405kg of MgO added to the slag (dashed line). The slag will start foaming early in the heat, once the FeO levels exceed 10% and will continue foaming till around 30%, when the heat is finished. This type of slag will also allow for good foaming beyond the 30% FeO, till nearly 38% FeO. ACTUAL EXPERIENCE AT PACIFIC STEEL Table 1 is a collection of slag samples taken during the "Old Practice" before any changes were made. One noteworthy feature in this table is the wide range of FeO levels in the slag ranging from 19 to 48% FeO as well as the variation in the B 3 basicity of the slags (1.5 to 3.0) 5

6 Table I. Slag analyses of samples taken during the "Old Practice" Heat # FeO CaO SiO 2 TiO 2 Al 2 O 3 MgO P 2 O 5 V 2 O 3 MnO Cr 2 O 3 CaO Added MgO Added B3 Ratio Average The ISD in Figure 4 is based on slags taken during heats made with the old practice, prior to good foaming. The average chemistry slag input is based on samples taken during 1998 when the practice was to add 1200kg of lime and 160kg of MgO per heat (Table I). As can be seen in the ISD, the slag crossed the MgO saturation line around 25% FeO and therefore tended to go liquid during the refining period. Although some foaming occurred early in the process, it was not an ideal situation. Also, because of the relatively high refractory consumption in the EAF, a portion of the MgO in the slag was being contributed by the monolithic hearth, gunning material and refractory brick lining in the furnace. Figure 4. ISD of the averaged slag composition for the "Old Practice" 6

7 INITIAL FOAMY SLAG PRACTICE After evaluating the slags with the Foamy Slag Program it was clear that the slags were MgO-deficient and it was decided to be fairly conservative and change the flux practice to add 1400kg of lime and 400kg of MgO per heat. The slag analyses in Table II are a small example of this initial foaming slag practice. This practice produced a very good and consistent foaming slag, as shown on the ISD in Figure 5. Table II. Slag compositions of early trials with 1400 kg lime 400 kg MgO additions Heat No FeO CaO SiO 2 TiO 2 Al 2 O 3 MgO P 2 O 5 V 2 O 3 MnO Cr 2 O 3 B3 Ratio Average Figure 5. ISD of the average slag composition of early trials with 1400 kg lime 400 kg MgO additions OPTIMIZATION OF FOAMY SLAGS In July of 2001, it was realized further gains could be made in the efficiency of the EAF by optimization of the foamy slag practice. It was observed during the process that large volumes of slag were lost over the sill. This lost slag represented unnecessary energy used to melt it and a lost yield in scrap due to the lost FeO. In order to decrease the amount of slag in the furnace, the basicity of the slag was decreased by adding less lime, but still sufficient MgO to ensure MgO saturation. Through trials, it was found that 800 kg of lime produced a volume of slag that could be retained in the furnace. Through the use of the Foamy Slag model, 300 kg of MgO was found to be optimal for this charge of lime. While the basicity of the slag of the "Optimized Foamy Practice" (B 3 = 1.6) was lower than the initial trials (B 3 = 2.2) and the "Old Practice" (B 3 = 2.0), it did not constitute in a phosphorous problem. For the grades of steel produced, the basicity of the slag was adequate to remove sufficient phosphorous to be well within the phosphorous specifications of the grades of steel produced. 7

8 Table III. Slag compositions representative of the optimized foamy slag practice H Heat FeO CaO SiO 2 TiO 2 Al 2 O 3 MgO P 2 O 5 V 2 O 3 MnO Cr 2 O 3 CaO MgO B 3 Ratio Added Added Average The above table is an example of typical slags run during the optimization period of foaming slag practice. As can be seen, the level of MgO was decreased during this time from 400kg to 300 kg on average and the CaO was decreased from 1400 kg to 800 kg on average. This resulted in excellent foaming slags throughout all the subsequent heats, even though there were variations in scrap composition. Figure 6 is the ISD plot of the average slag typical for the optimization period of foaming. As can be seen, the evolution of slag during the heat stays well above the 1% super saturation of MgO and allows for a wide range of FeO levels. In fact, with this slag, foaming is possible from the beginning till the end of the heat. This practice was started in July of 2001 and is continued till present. Figure 6. ISD of the average slag composition of the optimized practice 8

9 RESULTS OF THE FOAMY SLAG PRACTICE EAF Refractory Performance The following graph (Figure 7) shows the evolution of the performance of the EAF refractory. EAF Sidewall Campaigns (date campaign ended) 12/22/ Number of Heats /05/ /28/ /16/ /25/ /03/ /07/ /09/ /18/ /26/ /07/ /03/ /06/ /25/ /10/ /19/ /16/ /02/ /24/ /21/ /10/ Mar-97 Jun-97 Sep-97 Dec-97 Mar-98 Jun-98 Sep-98 Dec-98 Mar-99 Jun-99 Sep-99 Dec-99 Mar-00 Jun-00 Sep-00 Dec-00 Mar-01 Jun-01 Sep-01 Dec-01 Figure 7. Refractory Performance in the EAF from 1997 to 2001 From the above figure, it is easy to see the increase in the sidewall performance of the EAF. As was stated, the initial foaming slag practice was started just after mid-year The first lining after the foaming slag practice was scheduled for removal just above a typical number of heats at A very significant percentage of the original 400mm thick lining was remaining. As the plant personnel became more confident in the performance of the lining, the total number of heats allowed increased dramatically till the last lining performed 2614 heats. Not only have the refractory lining performed better, but consumption of monolithics and gunning materials has also decreased. These improvements are graphically shown in Figure 8. During the time period of 1999 before slag foaming, the lowest total refractory consumption/cost was 3.59 kg/ton or USD2.40/t. The last campaign refractory consumption/cost was 2.25 kg/ton or USD1.90/t. 9

10 EAF Sidewall Campaign - Material Rate (kgs/mt) Rate (kgs/mt) Campaign total maintenance reline tapholes sill Figure 8. Improvement in the consumption of refractories for the period Jan 1999 through Feb 2001 Cost Benefits of the new foamy practice The cost benefit of the new foamy slag practice is summarized in Table IV. Table IV. Cost benefit of the new foamy slag practice (shown in NZ$) Material "Old Practice" New Practice Change Annual $ Savings Lime (kg) Magnesite (kg) Metallic Yield (%) Electrode (kg/tonne) Electricity (kwh/t) Refractory Brick (kg/tonne) Total Savings

11 The plant was skeptical in the early stages of slag foaming as the MgO usage increased, adding extra dollars to the operating cost. The cost savings are not evident immediately and true results take several months to surface. Pacific Steel have had an increase of yield by 2.9%, electrode consumption and kwh/tonne reduced. The overall savings to the plant is in the excess of NZ$1.5 million and as the plant continues to develop and use the slag models supplied by LWB Refractories, the savings will be far greater. The currently EAF campaign will way exceed the previous campaign of 2614 heats. CONCLUSIONS This paper highlights what can be achieved when applying the thermodynamic and slag engineering principles. The steelmaker has to have a willingness to try new ideas, which may possibility increase their cost on some items but the overall cost savings in adopting these principles will clearly be evident in a short period. ACKNOWLEDGEMENTS The Authors are grateful to Pacific Steel for permission to publish this paper. We would especially like to thank Eugene Pretorius for reviewing the paper. REFERENCES 1. E.B. Pretorius and R.C. Carlisle, "Foamy slag fundamentals and their practical application to electric furnace steelmaking," 56 th Electric Furnace Conference, E.B. Pretorius and R. Marr, "The effect of slag modeling to improve steelmaking processes," 53 rd Electric Furnace Conference, E.B. Pretorius and R. Marr, "Computer modeling of Refractory/Slag/Metal interactions," 5 th International Conference on Molten Slags, Fluxes and Salts. Australia,