In January 2008, the modernized finger-shaft electric arc furnace (EAF) of Stahl Gerafingen AG (part of the Beltrame Group) was restarted following a project duration of ten months and a furnace downtime of slightly more than three weeks for disassembly, erection and commissioning activities.
The objective of this modernization, carried out by Siemens Metals Technologies, was to increase productivity and at the same time to reduce the emissions of the steel works which is located in the midst of the town of Gerlafingen. The EAF was equipped with a new shaft which included scrap-retaining fingers for the scrap preheating as well as an improved offgas exhaust system and an optimized charging process. The contractual performance parameters were met within a week. The capacity of the modernized furnace was increased considerably and the availability of the plant exceeded 98 percent within just a few weeks after commissioning.
The Siemens project scope of the modernization comprised the engineering, supply and installation of a new shaft and furnace equipment. This included the roof of the EAF vessel, the shaft-support structure, the shaft itself, the scrap-retaining fingers within the shaft and a shaft-roof slide gate. A tighter offgas exhaust system equipped with a movable collar for better dilution-air infiltration, and a newly designed scrap-basket-guidance and -positioning system to improve scrap charging into the shaft to minimize spillage was also part of the project. The hydraulic, automation and visualization systems were also upgraded. The finger-shaft EAF had been originally supplied by Siemens Metals Technologies in 1996.
The furnace has a tapping weight of 80 tons of steel. Following furnace modernization, the tap-to-tap times were shortened by more than two minutes and with a corresponding increase in productivity. This is primarily due to the more efficient and faster charging of scrap into the shaft, improved scrap preheating in addition to the optimized offgas flow within the shaft. The natural gas consumption for postcombustion could be reduced by more than six percent due to the enhanced offgas control system and reduced dilution-air intake. Emissions to the surrounding steel bay were also significantly lowered thanks to the much tighter shaft design and the faster scrap-charging system. Finally, the design of the shaft top and funnel together with exact scrap-bucket positioning for charging means less scrap spillage and potential damage outside the shaft, reducing downtime and maintenance.
The shaft furnace is a highly innovative, energy-saving solution of Siemens Metals Technologies employed in electric steelmaking. Through the preheating of scrap in a shaft installed above the EAF by utilizing the latent as well as chemical heat of the EAF offgas, the electrical energy consumption and tap-to-tap times can be significantly reduced. At Stahl Gerlafingen, special scrap-retaining "fingers" suspend the scrap within the shaft where it is preheated for the next charge. After the liquid steel has been tapped, the fingers open downwards and the first charge of preheated scrap drops into the EAF shell. The second scrap basket is then immediately charged into the emptied shaft. As the scrap inside the EAF shell melts, the scrap within the shaft gradually descends. Once there is enough room in the shaft, the fingers are closed again and the first scrap charge for the next heat is loaded. In this way, 100 percent of the entire scrap charge can be efficiently preheated.
Stahl Gerlafingen, the leading Swiss supplier of reinforcing steel products, produces steel for the construction industry as well as for the machine and vehicle construction industry from scrap which is sold as structural and reinforcing steel, steel mats and accessories, flat steel bars, universal plate and rolled wire.
Posted April 19th,2008