Ferroalloy submerged arc furnace: a solution to the temperature rise and redness at the bottom of the furnace

Traditional ferroalloy submerged arc furnaces generally use pre-baked carbon blocks and wide coarse joints to build the furnace lining, and adjust the primary current to control the depth of the electrode inserted into the furnace (that is, the load of the electric furnace). Take the main equipment of a company's 2*25.5MVA submerged arc furnace as an example. Its lining adopts semi-graphite charcoal brick fine-seam masonry technology, but the redness of the furnace bottom appeared three months after the furnace was opened. Through fine furnace anatomy and cause analysis, find out the solution to the problem.

Furnace No. 1 was sent to the electric oven, 6 days later, the initial charge was put in, and the first furnace iron was produced on the 7th day. With the continuous increase of furnace temperature, the secondary voltage was gradually increased on the 12th day, and the No. 2 furnace began to transmit electricity.

On the 43rd day of the operation of No. 1 furnace, a hard object was found to block the drainage of the steel braze during tapping, which improved after the tap hole was replaced. When it was operated to the 83rd day, there was more slag and less iron, and often lack of iron. By the 90th day, the bottom temperature of the furnace was over 600 degrees Celsius, and the bottom temperature reached 1050℃ on the 95th day, and the bottom steel plate was close to Redness in five places near the center line. A similar situation occurred in furnace No. 2 after that, the furnace bottom temperature rose from 425°C to above 550°C and the furnace had to be shut down.

Cause Analysis

When the bottom of the ferroalloy submerged arc furnace is red or worn, the main reasons are the lining material, the quality of the furnace, the daily operation and the furnace structure.

Furnace lining (key factor)

The smelting zone of the silico-manganese alloy electric furnace is gas at the ends of the three electrodes, and the surrounding solid materials are melted and gasified to form a cavity-shaped crucible. The temperature in the cavity is as high as 2000~3000℃, and the hot surface of the crucible wall is about 1800~2000℃. The cold surface is about 1500~1800℃, and the solid material outside the crucible reaches 1500~1700℃ on the inner wall of the furnace lining.

The previously used fine-seam masonry technology used N42 clay bricks, L75 high alumina bricks, pre-baked (semi-graphite) carbon bricks, anhydrous carbon mortar, phosphate slurry, and hard high alumina fine powder. 

Low-temperature coarse seam electrode paste, semi-graphite-silicon carbide brick. When the furnace was built, the actual thickness of these materials 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 furnace shell steel plate.

Then we analyzed the thermal conductivity and interface temperature of the lining material.

The primary problem of furnace wear accidents in ferroalloy electric furnaces is the refractory lining in the furnace. The load softening point of high alumina bricks generally does not exceed 1200℃. If the working environment temperature exceeds 1200, a layer of light carbon bricks should be added to the contact surface of carbon bricks and high alumina bricks to reduce the temperature of the contact surface to 1200. Below ℃. At the same time, the masonry quality of the furnace lining is also one of the factors that affect the life. Various refractory materials will have different thermal expansion. It is necessary to set up an elastic buffer belt, and the thickness should be appropriate. If it is too thin, it will not be able to compensate for the compliance of the refractory. If it is too thick, the furnace shell will not restrain the expansion of the refractory material, and it is easy to enlarge the brick gap.

The baking of the lining must follow the heating curve. The lining is divided into two parts: the base layer and the heat storage layer according to the thermal conductivity. The oven curve should be formulated according to the characteristics of the lining, refractory material, thickness, construction location and baking method.

Other factors that affect the life of the furnace lining are the operation of the combined controller electric furnace, arc characteristics, current distribution and so on.

Furnace analysis and conclusion

After the furnace was shut down for a week, the No. 1 furnace was cleaned, and a coke layer appeared at a position about 1 meter 3 from the furnace mouth, and its thickness was 60% higher than the normal value; 2.4 meters from the furnace mouth, the three-phase electrode area The charcoal brick body appears because the charcoal brick has risen, and the normal furnace depth is 3.6 meters.

The reason for the redness of the furnace bottom: the temperature stress of the brick lining occurred during the heating process, which caused bulging top cracking and iron infiltration. The triangular area of carbon bricks is concentratedly released by heating to form "bulging".

The traditional wide-slit furnace construction technology can guarantee the service life of the furnace lining more than one year. In order to obtain a longer life of the overall furnace lining, cold ramming can be used to replace the pre-baked carbon block wide-slit masonry.

If the self-baking carbon block is built with small joints, at least 3 years of furnace lining life can be guaranteed. The key to this method is the design and selection of furnace lining materials.