Ferroalloy Submerged Arc Furnace: A Solution to Furnace Bottom Overheating and Redness

Ferroalloy Submerged Arc Furnace: A Solution to Furnace Bottom Overheating and Redness

In traditional ferroalloy submerged arc furnaces, the furnace lining is typically constructed using pre-baked carbon blocks with wide, coarse joints. The furnace load (or electrode immersion depth) is controlled by adjusting the primary current. This case study examines the primary equipment of a company's two 25.5 MVA submerged arc furnaces. Although a refined semi-graphitic carbon brick fine-joint masonry technology was initially employed, furnace bottom overheating and redness appeared just three months after commissioning. A detailed post-mortem analysis was conducted to identify the root cause and develop a solution.

Operational Timeline and Incident Description:

   Furnace No. 1 was commissioned and began its power-on preheating ("electric oven") phase. Initial charging occurred on day 6, with the first heat tapped on day 7.

   As furnace temperature increased, the secondary voltage was gradually raised, and Furnace No. 2 was powered up on day 12.

   On day 43 of Furnace No. 1's operation, a hard object was found obstructing the taphole, a condition which improved after the taphole was replaced.

   By day 83, operational issues emerged, characterized by excessive slag volume, reduced metal yield, and frequent low metal output.

   By day 90, the furnace bottom temperature exceeded 600°C, reaching 1050°C by day 95. At this point, the bottom steel shell showed localized reddening in five areas near the centerline.

   Furnace No. 2 subsequently exhibited a similar pattern, with its bottom temperature rising from 425°C to over 550°C, necessitating a shutdown.

Cause Analysis

Premature furnace bottom redness or wear in submerged arc furnaces typically results from a combination of factors related to lining materials, construction quality, daily operation, and furnace design. For this incident, the furnace lining was identified as the key factor.

1. Furnace Lining Analysis (Primary Factor)

In a silicomanganese alloy furnace, the melting zone forms a high-temperature cavity ("crucible") around the tips of the three electrodes, where solid charge is melted and gasified. Temperatures within this cavity reach 2000–3000°C. The crucible wall's hot face is approximately 1800–2000°C, its cold face 1500–1800°C, and the solid charge adjacent to the furnace lining's inner wall reaches 1500–1700°C.

The failed fine-joint masonry used the following materials (actual installed thicknesses in parentheses):

   N42 Fireclay Brick (0.6345m)

   L75 High-Alumina Brick (0.335m)

   Pre-baked (Semi-Graphitic) Carbon Brick (1.206m)

   Asbestos Fiberboard Insulation (0.02m)

   Furnace Shell Steel Plate (0.03m)

   Joint materials: Anhydrous Carbon Paste, Phosphate Slurry, Hard High-Alumina Fine Powder, Low-Temperature Coarse-Joint Electrode Paste.

   Semi-Graphitic Silicon Carbide Brick was also used.

A critical issue was identified in the thermal interface. The load softening point of high-alumina brick generally does not exceed 1200°C. If the operating environment exceeds this, a layer of lightweight carbon brick must be added between the carbon brick and the high-alumina brick to reduce the interface temperature below 1200°C. This buffer was absent.

Furthermore, masonry quality is crucial. Different refractories have varying thermal expansion coefficients, necessitating an appropriately designed and sized elastic buffer zone. A zone that is too thin cannot accommodate expansion, while one that is too thick fails to provide adequate restraint from the furnace shell, potentially widening brick joints.

Proper lining preheating ("baking") according to a defined heating curve is essential, considering the lining's composition (base vs. heat storage layers), refractory properties, thickness, location, and heating method. Other operational factors, such as controller settings, arc characteristics, and current distribution, also significantly impact lining life.

Furnace Post-Mortem and Conclusions

After a one-week shutdown, Furnace No. 1 was emptied and inspected:

1.  A coke layer approximately 1.3 meters from the furnace top was found to be 60% thicker than normal.

2.  At about 2.4 meters depth (in the three-phase electrode zone), the carbon brick structure had risen. The normal furnace depth was 3.6 meters.

The root cause of the bottom redness was determined to be thermally induced stress within the brick lining during heat-up. This stress caused bulging, top cracking, and subsequent molten metal infiltration. The triangular configuration of the carbon bricks concentrated this thermal stress, leading to localized "bulging" failure.

Recommended Solution:

While traditional wide-joint furnace construction can ensure a lining life of over one year, a more robust solution is required for extended campaigns. The recommended approach is to replace the pre-baked carbon block wide-joint masonry with a cold-rammed monolithic lining. Alternatively, employing a fine-joint masonry technique using self-baking carbon blocks can reliably guarantee a furnace lining life of at least three years. The success of these methods hinges critically on the proper design and selection of furnace lining materials, particularly ensuring adequate thermal buffering at material interfaces and accommodating thermal expansion.

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