Selection of Anti-adhesive Refractories for the Burning Zone of Vertical Kilns in Dead-burned Magnesia Production
Dead-burned magnesia
Dead-burned magnesia accounts for over 40% of magnesia refractory raw materials and occupies a crucial position in magnesite output. Currently, most dead-burned magnesia in China is produced by calcination in vertical kilns. During the calcination process, uneven distribution of fuel and raw materials leads to lower gas resistance around the kiln body, resulting in excessively high temperatures on the kiln wall. This causes magnesite decomposition, and the small particles generated by hot pressing continuously adhere to the surface of the kiln wall refractories under the influence of impurities in the raw materials and ash in the fuel. In severe cases, "kiln bonding" accidents occur. Handling such accidents is time-consuming, disrupts normal production, and resuming operation wastes substantial raw materials and energy. To avoid kiln bonding, manufacturers generally reduce the calcination temperature, which is also the main reason for the current significant decline in the quality of dead-burned magnesia.
To resolve kiln bonding while maintaining the required calcination temperature, this study conducted adhesion tests on four types of basic firing refractories with magnesite at 1700 °C for 3 hours under a pressure of 0.2 MPa. The results showed that, compared with the other three types of refractories, fused rebonded magnesia-chrome bricks exhibited no adhesion even after ten repeated tests with magnesite. The research results have been applied to the design of refractories for the burning zone of large-scale vertical kilns with an annual output of 80,000 tons of dead-burned magnesia.
Part 01 Experiments
1.Magnesite and Basic Firing Refractories for the Experiments
Representative magnesite, high-purity magnesia bricks, magnesia–zirconia bricks, direct-bonded magnesia–chrome bricks, and fused rebonded magnesia–chrome bricks were selected. Their chemical compositions are shown in Table 1.
2. Experimental Process
Four types of basic firing refractories were processed into cylindrical gaskets with dimensions of φ100 mm × 30 mm. Magnesite samples sized φ50 mm × 50 mm were placed on the surfaces of these gaskets, and the assemblies were then placed into a self-made special vertical high-temperature furnace. Under a pressure of 0.2 MPa, the temperature was raised to 1700 °C at a rate of 5 °C/min and held for 3 hours. After natural cooling, the degree of adhesion between the magnesite samples and the refractory gaskets was examined.
The phase composition at the bonded interface was analyzed using an X'Pert Powder X-ray diffractometer (Cu target, λ = 0.15406 nm, voltage = 40 kV, current = 40 mA, scanning speed = 10°/min, step size = 0.01°, scanning range 2θ = 10°–90°) manufactured by PANalytical B.V. (Netherlands). A Quanta Inspect tungsten-filament scanning electron microscope produced by FEI Company was used to observe the microstructures of the magnesite samples and refractories, and surface-scanning analysis was conducted on the bonded area of the high-purity magnesia bricks.
Part 02 Results and Discussion
1.Adhesion Test Results Between Magnesite Samples and Basic Firing Refractories
A self-developed special device was used in the laboratory to simulate the adhesion behavior between refractories in the burning zone of a vertical kiln and calcined magnesite. The results are shown in the figure. Among the four types of refractories, no adhesion occurred on the fused rebonded magnesia–chrome bricks even after ten repeated tests, whereas the other three types all exhibited varying degrees of adhesion in the first test.
2. Microstructures of Magnesite (Magnesia) Samples and Four Types of Basic Firing Refractories After the Experiment
Figure 2 shows the SEM images of the magnesite (magnesia) samples after the experiment at 1700 ℃ for 3 hours. It can be observed that under hot-pressing conditions, large cracks formed inside the samples, and the periclase grains were well developed. A liquid phase existed between the grain boundaries. Based on EDS analysis at point a, the liquid phase was identified as CMS (calcium–magnesium silicate, monticellite).
As shown in Figure 3, except for the fused rebonded magnesia–chrome bricks, the internal grains of the other three types of basic firing refractories are clearly separated by a liquid phase. EDS analyses at points 1, 2, and 3 indicate that the mineral composition of the liquid phase is similar to CMS (calcium–magnesium silicate, monticellite).
Since the fused rebonded magnesia–chrome bricks are made from fused magnesia–chrome sand and fired at a high temperature exceeding 1800 °C, internal minerals such as periclase and magnesiochromite are in a directly bonded state. The crystals are well developed, with large grain sizes and relatively few grain boundaries, and no low-melting mineral phases are found between the grain boundaries.
Among the 4 selected refractory materials, the high-purity magnesia bricks exhibited the most severe adhesion. Surface scanning analysis and XRD phase analysis were conducted on their bonded areas, and the results are shown in Figure 4 and Figure 5.
As shown in the surface scanning images, Ca and Si are relatively concentrated at the joints, and their positions roughly coincide. The XRD pattern indicates that SiO₂ and CaO have reacted with MgO to form impurity minerals such as CMS (calcium–magnesium silicate, monticellite).
Generally, the atomic potential energy of atoms at grain boundaries is higher than that of atoms in the regular crystal lattice, and the activation energy required for their diffusion is lower, resulting in a correspondingly larger diffusion coefficient. Therefore, impurities such as CaO, SiO₂, and CMS at the grain boundaries of magnesite (magnesia) are prone to diffusion. Under the combined effects of high temperature and load:
Temperature effect: According to the formula
D=D0exp(-Q/RT] (where D is the diffusion coefficient and T is the temperature), as the temperature increases, the diffusion coefficient increases while the diffusion activation energy decreases. Thus, higher temperatures facilitate the diffusion of impurities to the joints.
Pressure effect: When magnesite (magnesia) is subjected to pressure, the periclase grains slip due to the presence of a liquid phase between them. The grains are squeezed, and part of the liquid phase is pushed toward the joints. At this time, the liquid phase between the magnesite (magnesia) and the refractory penetrates along cracks and grain boundaries on the surfaces of both the magnesite (magnesia) and the refractory. In this process, the liquid phase acts as an integral "pinning agent," tightly connecting the magnesite (magnesia) to the refractory. For this reason, refractories with higher impurity content-such as high-purity magnesia bricks, direct-bonded magnesia–chrome bricks, and magnesia–zirconia bricks-are more prone to adhesion.
Fused rebonded magnesia–chrome bricks have a dense structure, few grain boundaries, and low impurity content, so they generate relatively little liquid phase at high temperatures. They are widely used in equipment such as RH furnaces, AOD furnaces, VOD furnaces, dead-burned magnesia kilns, and hot metal mixers. Even if impurities in magnesite (magnesia) diffuse to the surface of the bricks, they can hardly penetrate the grain boundaries of the fused rebonded magnesia–chrome bricks. Instead, a liquid film may form on the surface of the bricks, acting as a high-temperature lubricant and facilitating the separation of magnesite (magnesia) from the fused rebonded magnesia–chrome bricks. Therefore, kiln adhesion is less likely to occur with these bricks.
Part 03 Conclusions
1)Under the combined effects of high temperature and load, high-purity magnesia bricks, direct-bonded magnesia–chrome bricks, and magnesia–zirconia bricks contain a large number of grain boundaries, with impurities such as SiO₂ and CaO concentrated at these boundaries. Impurities in magnesite (magnesia) diffuse to the brick surface under high temperature and high pressure and react with the impurities in the bricks, resulting in adhesion.
2)Due to the large grain size and few grain boundaries of fused rebonded magnesia–chrome bricks, it is difficult for impurities in magnesite (magnesia) to penetrate into their interior. Fused rebonded magnesia–chrome bricks have strong anti-adhesion properties, making them an ideal choice for refractories used in the burning zone of vertical kilns for dead-burned magnesia production.