Traditional ferroalloy submerged arc furnaces typically employ pre-baked carbon blocks and wide, coarse joints for constructing the furnace lining, with adjustments to the primary current to control electrode insertion depth (i.e., the electric furnace's load). Taking the main equipment of a company's 2*25.5MVA submerged arc furnace as an example, its lining utilized semi-graphite charcoal brick fine-seam masonry technology. However, redness at the furnace bottom emerged just three months after startup. Through detailed furnace dissection and cause analysis, we identified solutions to this issue.
Furnace No. 1 was placed in the electric oven, and six days later, the initial charge was introduced, with the first furnace iron produced on the seventh day. As furnace temperature increased, the secondary voltage was gradually raised on the twelfth day, and Furnace No.2 began transmitting electricity.
On the 43rd day of Furnace No.1's operation, a hard object obstructed steel braze drainage during tapping, which improved after replacing the tap hole. By the 83rd day, there was more slag than iron, often resulting in iron shortages. By the 90th day, the furnace bottom temperature exceeded 600°C, reaching 1050°C on the 95th day, with the bottom steel plate nearing redness at five points near the centerline. A similar situation occurred in Furnace No.2, with the bottom temperature rising from 425°C to over 550°C, necessitating a shutdown.
When the bottom of a ferroalloy submerged arc furnace exhibits redness or wear, the primary causes are the lining material, furnace quality, daily operations, and furnace structure.
The smelting zone of a silico-manganese alloy electric furnace features gas at the ends of the three electrodes, with surrounding solid materials melting and gasifying to form a cavity-shaped crucible. Temperatures within the cavity can reach 2000–3000°C, with the crucible wall's hot surface at about 1800–2000°C, the cold surface at 1500–1800°C, and the solid material outside the crucible reaching 1500–1700°C on the inner wall of the furnace lining.
The previously employed fine-seam masonry technology used N42 clay bricks, L75 high alumina bricks, pre-baked (semi-graphite) carbon bricks, anhydrous carbon mortar, phosphate slurry, hard high alumina fine powder, low-temperature coarse seam electrode paste, and semi-graphite-silicon carbide bricks. During construction, the actual thicknesses were 0.6345m for clay bricks, 0.335m for high alumina bricks, 1.206m for semi-graphite carbon bricks, 0.02m for asbestos fiberboard for heat insulation, and 0.03m for the furnace shell steel plate.
We then analyzed the thermal conductivity and interface temperature of the lining material. The primary issue in furnace wear accidents in ferroalloy electric furnaces lies in the refractory lining. The load softening point of high alumina bricks generally does not exceed 1200°C. If the working environment temperature exceeds this, a layer of light carbon bricks should be added to the contact surface between carbon and high alumina bricks to reduce the temperature to below 1200°C. Simultaneously, the masonry quality of the furnace lining also affects its lifespan. Different refractory materials exhibit varying thermal expansions, necessitating an appropriately thick elastic buffer belt. If too thin, it cannot compensate for refractory compliance; if too thick, the furnace shell cannot restrain refractory expansion, enlarging brick gaps.
The lining baking process must adhere to a heating curve, dividing the lining into a base layer and a heat storage layer based on thermal conductivity. The oven curve should be formulated according to lining characteristics, refractory material, thickness, construction location, and baking method.
Other factors affecting furnace lining lifespan include the operation of the combined controller electric furnace, arc characteristics, and current distribution.
After a week-long shutdown, Furnace No.1 was cleaned, revealing a coke layer about 1.3 meters from the furnace mouth, 60% thicker than normal. At 2.4 meters from the furnace mouth, in the three-phase electrode area, the charcoal brick body had risen, with the normal furnace depth being 3.6 meters.
The cause of furnace bottom redness was temperature stress in the brick lining during heating, leading to bulging, top cracking, and iron infiltration. The triangular area of carbon bricks concentratedly released heat, forming "bulging."
Traditional wide-slit furnace construction technology can guarantee a furnace lining lifespan of over one year. To achieve a longer overall furnace lining lifespan, cold ramming can replace pre-baked carbon block wide-slit masonry. Building with small joints using self-baking carbon blocks can guarantee at least three years of furnace lining life. The key to this method lies in the design and selection of furnace lining materials.
We are a professional electric furnace manufacturer. For further inquiries, or if you require submerged arc furnaces, electric arc furnaces, ladle refining furnaces, or other melting equipment, please do not hesitate to contact us at susie@aeaxa.com
