The lining of a calcium carbide furnace can be divided into two main systems: the furnace cover and the furnace wall. The furnace cover is subjected to high-temperature electric arc radiation, high-speed airflow containing CO and dust, and thermal shock from frequent start-ups and shutdowns. The furnace wall leveling layer is located behind the already laid high-alumina bricks, mainly serving to seal brick joints, ensure the furnace shell and brick lining are properly bonded, and improve airtightness. Mechanical impact and temperature gradients are relatively mild there. These differences in working conditions determine that the performance requirements for refractory castables are not the same.
I.Selection of Furnace Cover Castables
Furnaces with furnace covers generally use "high-alumina, low-cement castables". The low-cement formula (CaO≤2.5%) offers three advantages:
1. Low water consumption (generally 4%～5%), fewer drainage channels during baking, and lower risk of cracking;
2. Post-firing strength can reach over 60 MPa, capable of withstanding airflow erosion and mechanical vibration;
3. Small thermal variation at high temperatures, excellent thermal shock resistance, and extended furnace cover maintenance cycle.
Therefore, high-alumina, low-cement castables are the preferred choice for the furnace cover, regardless of whether it is a large or small furnace.
II. Selection of Castables for Furnace Wall Leveling Layer
The furnace wall leveling layer is located behind the high-alumina bricks, with an operating temperature 100～150℃ lower than the furnace cover, and is not directly subjected to airflow erosion, allowing for a relatively wide range of material selection. In practice, there are three approaches:
1. Conventional high alumina refractory castables – The aggregate to powder ratio is approximately 7:3. Water is added on-site and stirred until it reaches a self-flowing state. This mixture is then poured into the gaps between the brick joints and the furnace shell. After curing, the strength is 15–20 MPa, sufficient to meet sealing and support requirements, and it exhibits good compatibility with the expansion joints of the brick lining.
2. Dry filler – Composed only of aggregate and fine powder, without adding water, it is poured directly into the gaps. It sintersties on its own during furnace heating to form a dense layer. This method offers the fastest construction, but its strength is lower than that of wet casting. It is mostly used in small submerged arc furnaces.
3.Usable material approach – To reduce on-site material changes and simplify management, some companies use high-alumina, low-cement castable for the furnace cover and also for furnace wall leveling. Although the cost is slightly higher, it avoids material confusion, reduces construction error rates, and no significant negative cases have been observed with large-capacity calcium carbide furnaces.
III Selection and Construction Recommendations
1. Principle Differentiation:The furnace cover must be erosion-resistant and thermal shock-resistant, with high-alumina, low-cement castable preferred. Furnace wall leveling should focus on sealing and filling, using ordinary high-alumina castable or dry filler.
2. Construction Coordination:If the furnace wall is to use the same material as the furnace cover, a "general" clause should be clearly stated in the technical agreement to prevent disputes due to differences in specifications during acceptance.
3. Expansion Control:The furnace wall leveling layer thickness is generally 30–50 mm. Transverse expansion joints should be installed at 2/3 of the brick layer height, filled with ceramic fiber blankets to prevent compression of the brick lining after heating.
4. Baking Schedule:For wet-cast sections, a three-stage slow baking process is recommended: "room temperature → 150℃ × 8 h → 350℃ × 12 h → 600℃ × 8 h" to ensure sufficient removal of free water and crystal water. 5. Material Identification: When stacking materials on-site, different grades of castable refractory must be clearly labeled and covered with a waterproof membrane to prevent misuse due to similar appearance.
The performance requirements for refractory castables differ between the furnace cover and the furnace wall leveling layer of a calcium carbide furnace. In principle, materials should be selected and applied separately. However, in enterprises with high levels of management sophistication and low cost sensitivity, using the same material for all applications is feasible. The key is to assess the differences in operating conditions in advance, develop corresponding construction and baking plans, and strictly implement them on-site to ensure a long furnace lining life, reliable furnace shell sealing, and safe and efficient operation.
