MELTING PROCESS INTENSIFICATION AT 20 t EAF - ŽĎAS, A.S.

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1 MELTING PROCESS INTENSIFICATION AT 20 t EAF - ŽĎAS, A.S. Pavel Fila, Oldřich Suchý, Martin Balcar, Ludvík Martínek a, Jaroslav Brhel, Aleš Kosek b a) ŽĎAS, a.s. Strojírenská 6, Žďár nad Sázavou b) PTI Europe s.r.o. Abstract In January 2012, PTI Europe s.r.o. commissioned multifunctional lance system (MFL) for 20 t EAF in ŽĎAS electric steelmaking shop. The system is used to enhance melting process, generate and control slag foaming and improve dephosphoration. The paper describes the system equipment including key parameters and summarised first practical experience and results. 1. INTRODUCTION ŽĎAS, a.s. made significant investments in the melt shop area in past several years. Those investments were primarily focused on extending range of produced steel grades and on improved quality (new secondary metallurgy equipment LF + VD/VOD etc.). Recently the plant focused on improving efficiency and productivity if it s melting equipment while reducing conversion cost. Therefore it was decided to upgrade the plant s largest EAF #2 with liquid steel capacity up to 22t and 7.5 MVA transformer. 2. STATUS BEFORE REVAMPING As part of earlier improvements the furnace used Argon / Nitrogen bottom stirring technology (DPP) via two eccentrically located tuyeres. In addition the furnace used oxygen blowing via manually operated consumable calorised DAIWA lance pipe in the furnace slag door. Oxygen was used during melting to cut the scrap and clean slag door area and later on for decarburization during flat bath stage. 3. NEW CHEMICAL ENERGY PACKAGE Based on plant s document Innovation of scrap melting process in EAF which summarised required process improvement expectations PTI Europe analyzed several possibilities of chemical energy / slag foaming application to the specific furnace. Due to limited available space and furnace type it was impossible to apply conventional slag door or sidewall lance manipulator specifically due to the following reasons: lower shell moves out from melting to charging position which makes it impossible to install any equipment in front of the furnace The floor in front of the furnace moves out when furnace travels to charging position The space around the furnace is limited and not large enough to install any manipulator swinging arm Slag door is relatively small which dictates relatively shallow angle of any slag door lance Based on these limitations it was decided to attach hydraulic lance carrier to the furnace tilting structure and insert lance to the furnace via roof. This concept is shown below at the figure # 1.

2 Fig. 1 Equipment proposed location EAF # 2 - ŽĎAS, a.s. Supplies piping to the lance manipulator with the combined coherent lance include Oxygen natural gas Carbon powder Lime powder Hydraulic fluid Compressed air These lines are connected to the furnace tilting platform via flexible hoses and further on piped to the manipulator attached to the furnace structure Flow control system and hydraulic pressure unit for lance movements are located in safe location on the platform aside from the furnace. 4. ADDITIONAL FURNACE MODIFICATIONS Furnace roof The furnace roof was modified in several areas

3 Relocation of roof lifting mechanism to create necessary room for flue gas extraction via 4 th hole Installation of water cooled chimney attached the roof for fumes exhaust Installation of 5 th hole with water cooled port for lances entry point to the furnace Furnace tilting platform The tilting platform was extended to install lance manipulator with all media connections and new water distribution header. Slag pot and trolley Slag foaming technology required larger volume of slag pot to capture larger slag volume. New slag pots were design to double the available volume. Due to increased slag pot weight new slag pot trolley was installed. Civil works Civil works included foundations for carbon and lime injection machines and storage silos and increased depth of slag pot area to accommodate new trolley and larger slag pot volume. Cooling water distribution system The existing cooling water system was insufficient and it was necessary to completely revamp this part to bring larger cooling water volume and to control water distribution and temperature of the individual circuits. 5. EQUIPMENT DESCRIPTION The equipment consist of one water cooled supersonic coherent jet lance combined with oxy-fuel burner, which is attached to the lance manipulator (carrier) driven by hydraulic motor. Second water cooled lance for carbon and lime injection is attached to the manipulator parallel to the oxygen lance. The hydraulic motor controls lances movement inside the furnace. The lance positions are guarded by limit switches and by continuous position monitoring system. The lance equipment layout on the extended furnace platform is shown at figure 2. Fig. 2 Lances location at the furnace tilting structure

4 Oxygen and gas flows are control by dedicated valve train with the range of up to 2.5 MW power in burner mode and up to 700 Nm3/h supersonic oxygen flow in lance mode. This rating is fully sufficient for given furnace size and transformer rating. Carbon injection equipment is rated for capacity from kg/min and carob particles size mm. Lime injection system is similar principle but with larger injection capacity with the range from 30 80kg/min and particles size 2 12 mm. Fig. 3 Lime and carbon storage silos + main HMI screen Control system is based on PLC Siemens S7-300 and all key process parameters are monitored and data logged on HMI screens created in InTouch Wonderware environment. The HMI also includes trending of all key process variables such as flows, power, temperatures, consumptions etc. 6. OP ERATION DESCRIPTION During the initial stages of scrap melting the multi functional lance (MFL) is used in the burner mode for scrap preheating and melting. As the melting progress the MFL is inserted deeper to the furnace. As soon as the liquid bath is created the lance moves to its low position close to liquid bath ~ 200 mm above liquid steel level and is turned to supersonic coherent lance mode with outlet speed of Mach 2.3 to introduce oxygen to the steel bath. Lime is blown earlier in the heat and carbon blowing controls slag roaming at this stage. The foaming slag is first tapped out to remove phosphorus and later on for final deslaging. The slag rating out used before is eliminated. The entire equipment operation is controlled remotely from the operators screen. 7. EXPECTED BENEFITS The benefits are in several key areas Increased productivity (reduced tap to tap time) Lower electrical energy consumption Lower refractory consumption walls and roof Improved dephosphoration during melting period

5 The important principle of this technology is electrical arc covering by continuously foaming slag. The foaming slag reduce arc radiation the furnace walls, reduce electrodes wear and maximizes heat flux to the charge. Another important principle is enhanced dephosphoration due to oxygen and lime blowing during melting period. As the result, phosphorus content after meltdown is substantially lower. The steel qualities produced in ZDAS call for P max 0,005% in final product, therefore this benefit is of material importance 8. INITIAL EXPERIENCE The equipment installation started on December 17, 2011 and first heat after revamping was tapped 4 weeks later on January 17, The initial operation in January and February included 188 heats where 44 heats were completely made by using MFL to verify equipment functions, set up automatic melting programs for different steel grades to minimize human influence and to train the operators. Currently the whole equipment runs in automatic mode and operator s actions are limited to select specific program for given steel grade. Table #1 shows comparison of the results reached during initial test operation compared to average results of last 6 months of Table. 1 Initial operating results Item Time zone VI. - XII. / 2011 I. - II. / 2012 Number of heats LF/VD Tap weight t El. Energy - meltdown kwh/t El. Energy EAF total kwh/t Average P after meltdown % Average power on time min The operating of MFL was tested only at the heats subsequently treated at LF and / or VD. Tap weight was very similar to the reference period. The electrical energy consumption has significantly dropped (by 57kWh/t for meltdown and 66kWh/t total). The initial P content after meltdown was reduced by 50% and power on time reduced by 17 minutes. 9. CONCLUSION The initial results of this unique applications for such furnace type confirm expected benefits. Another results such as impact on refractory and electrodes consumption, yield etc. require longer term evaluation period and will be presented in future including financial quantification of the benefits.