Two integrated steel plants (Phase 1 and 2), both

Transcription

1 STEELMAKING AND CASTING Capacity enhancement at Emirates Steel through improvement in EAF performance with hot DRI charge The two integrated DRI-EAF plants built for Emirates Steel by Danieli in 2009 and 2011 have consistently outperformed design specifications. To further increase productivity, plant number 2 was enhanced in 2013/14 by a combination of plant and process improvements, while maintaining the basic 150t tap weight and 90:10 hot:cold DRI charge. All key performance indicators were exceeded, for instance, resulting in a new record of 40 heats/day (247tls/h) and electric energy consumption of 378kWh/t, with 34Nm 3 /t oxygen. Authors: D Patrizio, P Razza and A Pesamosca Danieli & C. Officine Meccaniche and Emirates Steel Two integrated steel plants (Phase 1 and 2), both comprising direct reduction plant (DRP), EAF, LF, caster and rolling mill, were supplied by Danieli to Emirates Steel, Abu Dhabi, in 2009 and Both plants produce steel for rebar and structural applications (including sheet piles), starting from iron ore as the feed material. They are based on a DRP (ENERGIRON technology) that directly feeds the melt shops by means of a pneumatic transport (HYTEMP). Both EAFs work by keeping a hot heel of 50t and are designed for a rated tap size of 150t, producing 1.4Mt/yr. Some EAF key features are reported in Table 1. The nominal charging practice is 10 cold DRI and 90 hot DRI, continuously fed from the fi fth hole and coming from the DRI plant through the HYTEMP tower (see Figures 1 and 2). The EAF transformer has a rated apparent power of MVA and allows a selection of 18 tap positions to obtain the best combination of arc voltage, arc current and power factor during the various process stages. The secondary circuit is designed for a maximum current of 80kA. The main electrical data are summarised in Table 2. The performances achieved from EAF Phase 1 have been described previously [1]. Table 3 and Figure 3 describe the injectors. A comparison between the EAF design parameters for plant number 2 and excellent results achieved a few months after start-up (2011) are summarised in Table 4. Item Value Tap weight, t 150 Lower shell diameter, m 7.0 Electrode diameter, mm 710 Pitch circle, mm 1,400 r Table 1 Main geometrical data of EAFs installed in Phase 1 and 2 Electrical data Value EAF transformer rated power 130MVA Overload 20 Frequency 50Hz Primary voltage 33kV Secondary voltage range 1, V Secondary voltage at full power 1,250-1,120 V SVC rated power 170MVAr Series reactor rated reactance 1.3ohm r Table 2 Main electrical data NEW PRODUCTIVITY TARGET Although the performance of the two plants has exceeded expectations, Emirates Steel requested even more productivity, aiming for 1.68Mt/yr for both plants and starting from Phase 2. This was to be achieved by keeping the original heat size a r Fig 1 EAF and HYTEMP tower at ESI plant 45

2 Module Oxygen flow rate Nm3hr 2, Oxygen jet Carbon jet Carbon injector Material feed rate kg/min r Table 3 EAF injection system Parameter Units DRI hot/cold Tap to tap Power on time Tapped steel Productivity Electrical energy Oxygen Charge yield Electrode consumption Average power Charged carbon Injected carbon Tap temperature 90/10 min 46.0 min 37.0 tls 150 tls/hr 196 kwh/tls 420 Nm3/tls kg/tls 1.4 MW 102 kg/tls 0 kg/tls 16 C 1,630 DRI PLANT Total Iron Metallisation Carbon Design target 2011 Results June 6th 2011 (15 heat sequence) 92/ ,643 r Fig 2 Plant sectional view of 150t and the charge mix at 10:90, cold DRI:hot DRI, coupled with plant changes as follows: r Table 4 EAF Phase 2: Results achieved in 2011 ` Revamping of DRP and oxygen plants to provide the additional raw materials. ` Maintaining the high quality of the DRI, but with r Fig 3 Layout of oxygen and carbon injectors 46 the possibility to increase carbon content from 2.0 to 2.5. ` Increase hot DRI maximum feeding capacity to 6.5 t/min ` Reduce the power-on time from 37 to 32 minutes via use of increased chemical energy input ` Reduce power-off time from 9 to 6 minutes by increasing the taphole (EBT) diameter from 180 to 200mm and by using a gunning robot, to allow significant improvement in refractory repair operations. ` Engineer the EAF slag to improve foaming consistency and increase the life of shell refractory, monitored with a 3D laser scan ` Increase the size of the oxygen jets to 2,500Nm3/h in order to sustain the higher DRI feed rate and also to decrease electrical energy consumption by higher chemical energy input. As a consequence the new nominal productivity target increased from 196 to 236.8tls/hr.

3 STEELMAKING AND CASTING The modifi cations for Phase 2 started in December 2013 and were completed in March 2014, culminating in a successful performance test. In April 2014, only one month after commissioning completion, more than 181,500t high quality DRI had been produced, strongly contributing to the extraordinary records achieved at the EAF plant. PHASE 2 RESULTS After the modifi cations, the Energiron plant was in operation at an average rate higher than the design, of more than 250t/h. The EAF melting practice was based on the continuous feeding of cold and hot DRI during the heat, reaching a maximum feed rate of 5.8t/min. The new revamp targets were defi ned as reported in Table 5: the goal for productivity was 236.8tls/hr. During the fi rst weeks of operation after DRP and oxygen plant revamp, some tuning was necessary to defi ne the optimum DRI characteristics. Eventually the best results were achieved and are reported in Table 5 where it can be appreciated how the initial targets of the project were reached with a different DRI composition: With the aim of maximising yield, it was decided to change the chemistry of the DRI by lowering the carbon content from 2.5 to 2 and increasing the metallisation from 94 to At the same time, tuning of process parameters allowed maintenance of DRI temperature at 510 C at EAF inlet. The lower oxygen input (-6Nm 3 /tls compared to the new design value) would result in a potential loss of useful energy in the range of 18-21kWh/tls, but the improved DRI metallisation gives a benefi t of 24kWh/tls. Considering the lower tapping temperature, the change of strategy did not modify the electrical consumption, but improved the yield. The productivity exceeded the initial revamp target of 236.8tls/h reaching 247.3tls/h on 12 April 2014, when 40 heats were tapped (power-off 4.8 min and average power 111MW). Figure 4 shows target and the best daily performances before and after the revamp. Even before the enhancement, the original target productivity had already been exceeded, and a record of 36 heats/day was obtained in 2013, based on an average power of 107.6MW and 5.8 minutes power-off. Both indicators exceed the original nominal values, and the application of higher power input was a result of fi ne process tuning with special attention to the control of slag chemistry, while the reduced power-off was made possible by operational excellence and utilisation of gunning robot for refractory monitoring and repair. The productivity record after the capacity upgrade was achieved by several factors. The increased capacity of oxygen injection system allowed higher chemical energy Parameter Units New design Results: 14 April Targets 2014 (21heat sequence) DRI hot/cold 90/10 90/10 Tap to tap min Power on time min Tapped steel tls Productivity tls/h Electrical energy kwh/tls Oxygen Nm 3 /tls Charge yield Electrode consumption kg/tls Average power MW Charged carbon kg/tls 6.2 Injected carbon kg/tls 10.0 Slag builders kg/tls 36.2 Tap temperature C 1,617 DRI PLANT Total iron 92.4 Metallisation Carbon r Table 5 EAF Phase 2: capacity enhancement targets and results r Fig 4 Best daily performances, EAF 2 utilisation and the higher decarburisation rate needed to match the higher DRI input. The optimisation of melting resulted in an average active power of 111MW (+4.1 MW higher than design target). Power-off was further decreased to 4.8 minutes, thus improving previous record and demonstrating the great effort and professionalism of ESI personnel that made it possible to achieve the performance guarantee tests after only two months from completion of revamping. Figure 5 shows the power-off frequency distribution showing the repeatability of EAF operation and the improvements achieved. a 47

4 ELECTRICAL POWER PROFILE The electrical profi le is one of the key factors that allowed the plant to reach the above-mentioned results. Figure 6 shows a typical melting profi le recorded during the capacity enhancement. The key characteristics can be summarised as follows: r Fig 5 Power-off frequency distribution (October 2011, April 2014) r Fig 6 Typical electrical melting profile post enhancement ` Aggressive power ramp up: the maximum power is applied in less than 20 seconds. ` Very stable active power throughout the heat. The average active power applied is 111.5MW, with a maximum power of 114.3MW. The stability exceeded the original expectations achieving a ratio Pavg/Pmax = 97.5 and a relative standard deviation of 1.1. ` The maximum power input is practically utilised for the total duration of the power-on time. The working point corresponds to an active power of 114.3MW, secondary current of 76.3kA, and arc voltage of 470V. The resulting refactory wear index (RWI) is in the range of kWV/cm 2 and it can be considered the average RWI. Good management of the hot heel and the ability to foam the slag in the early stages of the process enables maximum power to be applied. The stability of the active power was helped by controlling the DRI feed rate via level 2: the average feed rate being 5.7t/min, with an average specifi c feed rate of 52kg min/mw. Among other AC EAFs that operate with DRI, the case of ESI represents the plant with the highest value of specifi c power, defi ned as active power /(tap weight * bath surface) and expressed in kw/(t*m 2 ). Data from three other Danieli installations are as follows: Plant A 100 Hot DRI Plant B 20 scrap + 80 Cold DRI Plant C 35 scrap + 65 Cold DRI r Fig 7 Specific power applied in DRI-based EAF These are shown in Figure 7 where it can be seen that ESI has the highest value of specifi c power, (22.8kW/t/ m 2 ), and the smallest difference between the maximum value and the average value along the heat (0.5kW/t/ m 2 ), indicating the high level of optimisation of the process. If now we relate the productivity reached, 24.3tls/h, with the utilised power, 111.5MW, the phase 2 EAF has reached the remarkable value of 2.21tls/h/MW. Despite melting hot DRI, the specifi c productivity is comparable with some of the most effi cient scrap based EAFs. In Figure 8, the specifi c productivity of Emirates Steel EAF is compared to the values obtained in other furnaces supplied by Danieli. 48

5 STEELMAKING AND CASTING SLAG PRACTICE AND REFRACTORY LIFE In a DRI-based process, fl at bath conditions with variable slag level throughout the heat result in severe stress on the whole lining. For this reason it is necessary to check internals quite frequently and act promptly to avoid local wear leading to bricks collapsing and, in the worst case, a furnace breakout. In particular, major wear is observed for the slag door area, and hot spots in front of electrode 1 (on the left of slag door) and phase 2 (transformer side). The utilisation of the gunning robot signifi cantly improves repair operations as it reduces personnel exposure, increasing operational safety, and allows for fast and precise detection of damaged areas, which in turn results in decreased power-off time and also in lower consumption of gunning material. The possibility of performing a complete laser scan of internal surfaces with 3D visualisation of actual refractory status (residual thickness) is a further tool to monitor the results of the repair operations. Due to 100 fl at bath operation, the control of slag chemistry is a key factor in allowing high power input and avoiding excessive refractory wear. The electrical power profi le adopted is very aggressive and, in order to sustain this, adequate foaming is needed from the very beginning of power-on. During the fi rst stages of DRI feeding good foaming is observed even without oxygen injection. Apart from slag basicity, this can be explained considering that the initial slag is more oxidised compared to the average slag produced during the melt. Also, the very short poweroff time between end of tapping and start of power-on minimises bath cooling. These factors allow fast kinetics of reaction between iron oxide and DRI carbon. It is therefore possible to reach maximum power quickly due to complete arc coverage. The presence of stable foaming slag during the heat is ensured by continuous feeding of slag builders at a feed rate of kg/min, with a total consumption of 18kg/t lime and 18kg/t dololime. In Figure 9, the slag isothermal solubility diagram at 1,600 C is reported, considering the average slag composition of samples taken towards the end of the heat. The IB2 and IB3 basicity indexes are 2.46 and 1.9 respectively (37 CaO, 15 SiO 2, 4.5 Al 2 O 3 ). IB2 is CaO/ SiO 2 and IB3 is CaO/( SiO 2 + Al 2 O 3 ). In theory, optimal foaming should be achieved by reaching saturation in MgO-FeO or 2CaO-SiO 2, due to increased slag viscosity. In practice, a major role is played by the high CO generation rate (average 660Nm 3 / m 2 hr) allowing good foaming even if slag does not reach saturation point. In principle, some improvement could be made to maximise iron recovery, but it should be considered that the fast process with intensive oxygen blowing makes it EAF campaign No Total heats Hot repair (kg/tls) Gunning Fettling Slag door Cold repair (kg/tls) Brick Ramming Total refractory consumption (kg/tls) r Table 6 Refractory consumption MgO source kg/tls DRI Dololime Refractory r Table 7 Estimated origin of MgO in slag Location April May June July Settling chamber Cyclone Bag fi lter Total r Table 8 Dust generation in 2014, kg/t r Fig 8 Specific productivity in Danieli-supplied EAFs more diffi cult to obtain equilibrium conditions inside the furnace. Since slag chemistry is not far from the MgO saturation curve, the driving force for MgO-based brick dissolution into the slag is somewhat reduced. The relevant results of refractory consumption and bottom shell life are reported in Table 6 for some consecutive campaigns in Considering that slag generated is around 165kg/t with 7.2 MgO, the total quantity of MgO in slag is 11.9kg/t with the MgO sources as shown in Table 7. The result is quite consistent with the total amount of a 49

6 STEELMAKING AND CASTING r Fig 9 Isothermal slag solubility diagram r Fig 10 Refractory scan at the end of campaign refractory material employed for hot and cold repairs. Figure 10 is a refractory scan at the end of campaign 68 (617 heats). Apart from the slag door area, which is particularly subject to erosion due to continuous slag flow, the refractory wear is quite uniform in all zones, residual wall thickness being more than 250mm. Even if the refractory consumption achieved can be considered as a benchmark for 100 DRI use, there is some margin of improvement if we consider that slag is not saturated in MgO. It also has to be considered that, in view of overall process optimisation, increasing the amount of slag would have a negative effect material yield, and higher dololime addition would result in higher energy, consumption mainly because of the quality of material (ignition losses). The iron content in the slag represents the main factor affecting material yield; for instance, dust losses are very low, as shown in Table The target was to increase nominal plant productivity from the original design value of 196tls/h to 237tls/h by a combination of: ` Revamping of DRP and oxygen plants to provide the additional raw materials. ` Increasing hot DRI max feeding capacity to 6.5t/min ` Reducing the tap to tap time from 46 to 38 minutes ` Reducing power-off time from 9 to 6 minutes ` Engineering the EAF slag to improve foaming consistency and increase the life of shell refractory, monitored with a 3D laser scan ` Increasing oxygen injection to sustain the higher DRI feed rate and also to decrease electrical energy consumption by higher chemical energy input ` Use of a gunning robot for the detection and repair of lining wear. Following rapid revamping and rapid commissioning, all key performance indicators have been exceeded, for example, average power 111MW, (97 of maximum power), a new productivity record of 40 heats/ day (247tls/h), and electric energy consumption of 378kWh/t, with 34Nm3/tls oxygen. The minimisation of refractory consumption in a DRIbased process also depends on the strategy adopted to control furnace lining and to perform the necessary repairs when and where needed. In this regard, the utilisation of the gunning robot significantly improved the detection of the areas subject to the erosion, allowing prompt repair and extending the life of refractory lining. A major role is then played by the process stability and repeatability, with extremely reduced power-off and very short melting time during which the arc is always well covered. FUTURE Further actions are planned, including on-line slag composition monitoring to help maximise EAF yield, improve efficiency of operations during power-off, including utilisation of automatic electrode jointer, electrode equalisation platform and EBT automatic sand feeding. Emirates Steel is confident that new performance records will be achieved in MS REFERENCES [1] D Patrizio and P Razza, Operating results with hot DRI charge at ESI (Emirates Steel Industries), in Steel & Metallurgy, September CONCLUSIONS D Patrizio and A Pesamosca are with Danieli & C. Officine Meccaniche, Buttrio, Italy. P Razza is with Emirates Steel Abu Dhabi, UAE. At Emirates Steel s integrated EAF plant number 2, a capacity enhancement was completed at the beginning of CONTACT: info@danieli.com