Due to unstable factors in converter operation, premature damage may occur in certain areas of the furnace lining. At this point, repairs to the converter lining are required and must be maintained until the lining reaches the end of its service life. Repairing the lining helps balance wear distribution, reduce production costs, and extend its operational lifespan. During the later stages of refractory lining service, repair volume increases continuously, and repair time extends progressively. If this begins to impact the stable operation of the converter, the refractory lining’s service life is considered exhausted. Within the total refractory material consumption for the lining, the optimal usage of repair refractories ranges from one-third to one-half. Refractory lining repair methods can be categorized into two types: patching and spray repair.
1.Refractory materials for casting and lining
Pour the repair material into the furnace through the furnace opening and shake the furnace body; residual heat inside the furnace gives the repair material a certain degree of fluidity; the repair material spreads over the damaged areas of the furnace lining. The refractory materials used for repair possess the following characteristics:
(1) Refractory materials used for lining repairs at the residual heat temperature of the converter lining (800–1200°C) must exhibit excellent spreadability;
(2) After spreading, the repair refractory material should solidify rapidly;
(3) The solidified repair material must demonstrate good bonding properties with the original lining material;
(4) The repair material itself must possess excellent resistance to erosion.
The primary raw materials for ramming and repair refractories are magnesia sand and magnesia-dolomite sand, with their performance primarily determined by the binder. Common binders include resins, bitumen, and their mixtures. Bitumen is favored by China’s steel enterprises due to its low cost and ease of use in ramming and repair refractories.
2.Refractory materials for spray lining
When localized damage occurs to the converter lining (such as at the trunnion), refractory materials can be concentrated and sprayed onto the affected area, sintering the refractory material to the existing lining bricks to achieve lining repair. Spray repair methods are primarily categorized into three types: wet, semi-dry, and flame. Currently, semi-dry spray repair technology is widely adopted by steel enterprises both domestically and internationally.
(1) Wet Method: The wet spraying repair primarily uses magnesia as the refractory material. It is loaded into a spraying tank, mixed with an appropriate amount of water, and sprayed onto the eroded areas of the furnace lining. The repair layer reaches 20–30 mm. This method is convenient to use, and one batch of material can be used for three spraying operations.
(2) Semi-dry Method: The spray machine for semi-dry repair consists of a storage tank, compressed air delivery mechanism, and nozzle. Refractory material from the storage tank is conveyed by compressed air to the nozzle, where it mixes with 10%–18% moisture. Propelled by air pressure at a specific velocity onto the furnace lining surface, the sprayed refractory material bonds and cures. Factors affecting repair effectiveness include:
- 1) Temperature. Refractory lining repair is performed under hot conditions, where the residual temperature of the working surface significantly impacts repair effectiveness. The optimal temperature range is 800–1000°C.
- 2) Particle size distribution, binder type, water content, and air pressure of the sprayed refractory material. The raw material consists of magnesia or magnesia-dolomite sand with particle sizes under 0.1 mm. Binders include powdered sodium silicate, sodium phosphate, calcium and potassium phosphates, and chromates. Adding a binder enhances the spray material’s adhesion, enabling it to effectively bond to the furnace lining surface. Simultaneously, the binder forms high-temperature mineral phases at elevated temperatures, not only firmly sintering the spray material to the lining surface but also imparting excellent corrosion resistance to the material itself.
The adhesion rate of sprayed refractories generally exceeds 85%, with a service life of 3 to 5 applications.
Semi-dry spraying is a simple and feasible method for repairing converter linings. However, during the spraying process, residual heat causes the added moisture to generate significant steam upon contact with the repair surface, accumulating steam pressure that impedes the bonding between the sprayed refractory material and the working surface.
(3) Flame Method: Flame lining does not involve adding water but instead incorporates combustible materials (such as coke powder or coal powder) and combustible gases (such as propane, methane, or oxygen). The refractory material burns and generates heat during spraying, with some components reaching a molten state. Upon contact with the high-temperature working surface, it rapidly melts and sinter together. Flame spraying is typically performed during operational breaks after ladle tapping in converters. The spraying duration is brief, the residual temperature of the furnace lining is high, adhesion is excellent, and service life is relatively long, generally lasting for 10 to 20 applications.