Taphole Clay for Blast Furnace: Selection of Raw Materials, Binders, Additives & Production Process
Clay gun mud (taphole clay) is a refractory plastic material composed of aggregates, fine powders, binders, and additives, which forms a carbon-bonded structure after firing. It is specifically designed for plugging the tapholes of blast furnaces.
During tapping, molten iron and slag flow out of the furnace through a channel drilled in the center of the taphole clay within the taphole. Slag generally begins to appear about 20 minutes after tapping starts and continues until all the molten iron has been discharged. This requires the taphole clay to maintain a uniform taphole diameter and ensure a stable tapping flow.
After tapping, the taphole clay is injected into the taphole channel by a mud gun to fill the taphole, thereby preventing the outflow of molten iron and slag. This helps maintain sufficient taphole depth and forms a mud pad inside the hearth, protecting the furnace wall and bottom in the taphole area.
The purity and types of raw materials, binders, additives, and production processes are the main factors affecting the quality of taphole clay. The base raw materials directly determine its performance, and the chemical composition of taphole clay is evolving toward higher purity and increased carbon content.
01. Basic Raw Materials of Taphole Clay
Clay: It mainly acts as a binder and lubricant. There are many types of clay with varying contents of Al₂O₃ and SiO₂, which can result in poor adhesion, unstable sintering behavior, and difficulty in ensuring the early strength of water-based taphole clay. The selection and dosage of clay mainly affect the plasticity of the taphole clay. Xia Xinpeng et al. analyzed the microstructures of different kaolins in taphole clay components and found that the particle size distribution and chemical composition of kaolin have a significant influence on the properties of taphole clay.
Corundum: With a bulk density of 3.95–4.0 g/cm³ and a melting point of 2050 ℃, corundum features stable chemical properties, as well as excellent slag resistance and corrosion resistance. It is used as both an aggregate and a fine powder in taphole clay.
Coke: As an amorphous carbon material, coke has high porosity, which facilitates the volatilization of moisture and volatile components and improves the air permeability of taphole clay. It has an extremely high melting point and excellent slag resistance, thermal shock resistance, thermal conductivity, and electrical conductivity. Coke helps maintain a reducing atmosphere at the taphole and protects carbon materials such as SiC.
SiC (Silicon Carbide): Industrial SiC is generally synthesized artificially, with a density of 3.17–3.37 g/cm³. Pure SiC is colorless and transparent, while industrial SiC appears yellow, green, blue, or black due to varying types and contents of impurities. SiC used in China is mainly black and green, both of which are hexagonal crystals. SiC remains stable below 2600 ℃ in a reducing atmosphere. It has the advantages of high thermal conductivity, good wear resistance, high high-temperature strength, a low thermal expansion coefficient, and excellent thermal shock stability, all of which improve the slag corrosion resistance and scouring resistance of taphole clay.
Asphalt: As shown in Table 1, asphalt is classified into low-, medium-, and high-temperature grades according to different softening points. Asphalt is a carbon-containing organic binder. It undergoes polycondensation and coking as the temperature increases, forming a carbonized network that increases the wetting angle and high-temperature strength of taphole clay.
02. Binders for Taphole Clay
At present, the main binders used for taphole clay are coal tar–asphalt or resin. Taphole clay bonded with tar–asphalt features excellent plasticity, making it easy to fill the taphole and maintain taphole depth. However, coal tar has a complex composition, high viscosity at room temperature, a non-uniform liquid state, and poor dispersibility, which can result in structural defects after sintering. The moisture content in tar has a significant influence on the quality of taphole clay. In addition, tar hardens slowly during application, leading to long taphole plugging times, low high-temperature strength after sintering, large volume shrinkage, and poor erosion resistance. At present, tar is rarely used as a binder for taphole clay in Europe.
Phenolic resin is a commonly used binder for carbon-containing refractories and is characterized by strong adhesion, high green strength, and high carbon yield. Replacing tar with phenolic resin helps improve the bonding strength of taphole clay. Furthermore, reducing harmful solvents improves the operating environment in front of the furnace and enhances environmental performance. Resin is relatively viscous in use and needs to be adjusted by adding tar, ethanol, and other agents; however, tar is not easily soluble in resin. The use of a tar–resin composite binder in taphole clay production provides good plasticity, air permeability, and slight volume expansion. Studies have shown that a binder system mainly composed of powdered resin, asphalt, and inorganic phosphate can be used for taphole clay, with tar acting as a plasticizer to delay the condensation and premature hardening of the resin.
03. Additives for Taphole Clay
Additives are important components of taphole clay. They are used to compensate for deficiencies in the service performance of the main raw materials. Although added in small amounts, they have a significant influence on workability and application performance. According to their functions, additives can be divided into the following categories.
(1) Plasticizers and Lubricants
These improve the plasticity and lubricity of taphole clay. Common plasticizers used in plastic refractories, such as sericite and clay, can be applied. Typical lubricants include graphite and pyrophyllite powder.
(2) Expansive Agents
Expansion is generated through phase reactions during heating to offset shrinkage caused by drying and sintering, thereby reducing apparent porosity and increasing bulk density. Common expansive agents include quartz, kyanite, and andalusite.
(3) Sintering Aids
Theoretically, approaches to improving the sintering properties of materials include solid-phase sintering, liquid-phase sintering, and reaction sintering. Solid-phase sintering and reaction sintering are ideal methods, as they generally do not degrade the high-temperature performance of the material and can strengthen the matrix. In contrast, liquid-phase sintering may reduce hot strength and resistance to slag and iron corrosion due to the formation of liquid phases at high temperatures. However, if a liquid phase forms in the early stage of sintering to promote densification and then disappears to form high-melting-point phases, such "indirect liquid-phase sintering" can also be beneficial.
There are many types of sintering aids. Those used for monolithic refractories include α-Al₂O₃, boric acid, silica fume, clay, feldspar, metallic Si powder, SiO₂ micropowder, and rare earth oxides. Most sintering aids are low-melting-point materials and may impair the corrosion resistance of taphole clay. In recent years, silicon nitride or ferrosilicon nitride has often been added to taphole clay to promote sintering, refine the microstructure, and enhance corrosion resistance. The microstructure and corrosion resistance of taphole clay are improved through the following reactions:
Through the above reactions, SiC, AlN, and other phases are formed in the matrix, which improves the microstructure. The generated Fe acts as a catalyst for SiC formation and promotes the development of SiC bonding phases. In addition, since these reactions release N₂ and CO, the penetration of slag is inhibited, thereby improving corrosion resistance. However, the addition amount of ferrosilicon nitride is not simply a case of "the more, the better"; some studies indicate that the addition should not exceed 10%.
(4) Antioxidants and Resin Powder for Improving Taphole Clay Strength
The addition of resin powder increases the residual carbon bonding phase, forming both homogeneous and heterogeneous carbon bonding, with a residual carbon ratio of up to 85%. When heated, the resin powder softens and migrates into the pores, ensuring the final strength of the taphole clay. In addition, it can generate a reducing atmosphere, inhibit the oxidation of other carbonaceous raw materials, and provide good oxidation resistance.
04. Production Process of Taphole Clay
The formulation design, production process, and pre-service storage of taphole clay all affect its quality. To ensure qualified taphole clay, the proportioning scheme must be accurately measured first. The milling time depends on the charging sequence and the type of mixer. The main current challenges include improving the milling efficiency of mixers, adopting computerized batching, and upgrading the working environment.
Various raw materials for taphole clay are crushed and ground by manufacturers to achieve qualified particle sizes and fine powders. Coal tar is supplied by coking plants. All raw materials are mixed in a mulling mixer according to different charging sequences. The ambient temperature during mixing is preferably controlled within 10–30 ℃.
Controlling the storage temperature of water-based taphole clay within 10–25 ℃ is critical to maintaining stable performance. The storage of mixed taphole clay also significantly influences its service performance; therefore, attention must be paid to changes in ambient temperature. In hot summer conditions, protective measures should be taken for water-based taphole clay to prevent moisture evaporation during aging. In cold winter conditions, anhydrous taphole clay must be stored under heating to prevent condensation and hardening.
Taphole clay can be stored separately according to its type: water-based taphole clay at room temperature and anhydrous taphole clay in a heated room. The general aging time is 24 hours.