The difference between ordinary magnesia bricks and waterproof and seismic magnesia bricks Jiaxin Refractory

The flue of a metallurgical furnace in a steel plant was changed from two-way to one-way, and the flue gas temperature in the smoke exhaust system (this article refers to the ascending wall, supporting wall arch, transition zone, vertical flue top, etc.) increased by 300-500 °C, which seriously affected The work of the rear system. The method of spraying water mist to reduce the temperature of the flue gas causes serious hydration of the brick lining of the smoke exhaust system, and during the smelting and tapping periods, the temperature fluctuates greatly, causing the brick lining to break and fall off seriously, and even the top of the roof collapses and the wall surface collapses. Collapse accident. For this reason, the lining bricks used in the flue are developed and used in practice. The details are as follows.

2 Trial analysis

The test formula properties of ordinary magnesia alumina bricks and magnesia chrome bricks developed are listed in Table 1. Figure 1 shows the residual bricks after using 1 to 2 furnaces.

Table 1 Properties of the test formulations

Note: A, B, C are for the development of magnesia-chrome bricks, and D is for ordinary magnesia-alumina bricks.

Figure 1 Brick residue after use of 1 to 2 furnaces

The main performance typical values of magnesia-chrome bricks produced by special process based on formula A and their comparison with ordinary magnesia-alumina bricks are listed in Table 2. It can be seen that the high temperature load softening performance of magnesia-chrome bricks produced by special technology is significantly improved, and the waterproofing and thermal shock resistance is more than 3 times higher.

Table 2 Comparison of main properties of brick lining for smoke exhaust system

It can be seen from Figure 1 that the residual thickness of ordinary magnesia-alumina bricks is very small after 1 to 2 furnaces, and can no longer be used; while magnesia-chrome bricks are only in the most severe conditions after using 2 furnaces, that is The defect of 0-40 mm on the rotating brick working surface of the lowest layer within the width of 1 m in the middle of the supporting wall arch (Fig. 2), the residual bricks are still quite intact even after 4-5 furnaces (Fig. 3). It can be seen that the service life of magnesia-chrome bricks still has great potential for improvement.

Figure 2 The situation after removing the supporting wall arch 1 m in the middle after 2 furnaces

Figure 3 The residual bricks after 4 to 5 furnaces

Obviously, the use of waterproof and thermal shock-resistant magnesia-chrome bricks can be calculated according to the standard of "no need for repairs for minor repairs, and repairs for medium repairs, that is, 1 set of 3 medium repairs (5 furnaces), and two repairs". Reduce the original furnace repair rate from 60% to 30% (4#, 9# furnace), reduce brick consumption by 50%, and reduce the furnace repair rate from 85.2% to 33.3% (6#, 7# furnace), The brick consumption is reduced by 60.9%, and the average brick consumption is reduced by 55.5%. Only supporting wall arches and transition zones can save millions of dollars per year. If it is extended to the entire smoke exhaust system, the benefits are even more considerable. Now, the waterproof and thermal shock-resistant magnesia-chrome brick has been promoted and used.

3 Microstructure of waterproof and thermal shock-resistant magnesia-chrome bricks

The original ordinary magnesia-alumina bricks and ordinary burnt magnesia bricks are mainly damaged by pulverization and peeling, and the magnesia-chrome bricks are developed with the focus on improving the performance of high temperature resistance, waterproofing and thermal shock resistance of the products. Sand and selected chrome ore are the base materials, which are made by crushing and screening, fully mixing, high-pressure molding and ultra-high temperature sintering. The comparison between its main properties and the original ordinary magnesia-alumina bricks is listed in Table 1 and Table 2. It can be seen that the performance of the three developed bricks is obviously better than that of ordinary magnesia-alumina bricks, and A is the best.

The AHMT3-513NU universal professional research microscope and the BHS-753P advanced polarizing microscope produced by 0LYMPUS in Japan were used to examine the microstructure of ordinary magnesia alumina bricks, ordinary burned magnesia bricks and high temperature resistant, waterproof, thermal shock resistant magnesia chrome bricks for smoke exhaust systems. Conduct observational studies.

(1) Ordinary magnesia-alumina brick: the particles in this brick are ordinary magnesia [the lower left and lower right corners of Fig. 4(a)], the periclase main crystal phase is small, and a considerable part is caviar-shaped, with secondary crystals The phase is a silicate phase dominated by forsterite; a relatively thick and continuous silicate phase film surrounding the periclase phase is the main feature of the ordinary magnesia microstructure. In the matrix of ordinary magnesia-alumina brick [Fig. 4(b)], except for the periclase phase and silicate phase, the main crystal phase is magnesia-alumina spinel; the magnesia-alumina spinel in this type of ordinary magnesia-alumina brick is often concentrated In the form of a nest, the spinel single crystal is often surrounded by a silicate phase, and there are few cases where a direct bond between spinel and periclase is formed. However, the binding between the particles and the matrix is relatively dense [Fig. 4(a)].

Figure 4 Microstructure of ordinary magnesia-alumina brick

Ordinary calcined magnesia brick: Its mineral phase composition is completely consistent with the particle part of ordinary magnesia-alumina brick. The situation under orthogonal polarized light is shown in Figure 5. It can be seen that the microstructure of ordinary calcined magnesia brick is simple and compact.

Fig.5 Microstructure of ordinary burnt magnesia brick (cross polarized light×600)

Waterproof and thermal shock-resistant magnesia-chrome brick: This brick has more minerals and richer microstructure. The high-quality magnesia grains [Fig. 6(a) up, down, left and right] have few silicate phases, and the periclase phase has a high direct bonding rate and is tightly bonded to the matrix; the selected chrome ore grains [Fig. 6(a) center left and bottom] The inner main crystal phase is FeCrO4, and the secondary crystal phase is a magnesium-containing silicate phase, which is often separated from the matrix by microcracks. The width and extension length of the microcracks vary according to the particle size. The larger the particle size The wider and longer the microcrack is, and vice versa. The purity of the brick matrix was originally high, and the content of silicate phase was low. Part of Fe O and Cr2O3 in the chrome ore diffused into the periclase crystal to promote the growth of the periclase crystal, and some chrome ore reacted with magnesia. The resulting magnesia-chromium spinel exists between periclase, which further improves the direct bonding between solid phases (periclase-periclase, periclase-spinel, spinel-spinel), see Figure 6(b).

Fig.6 Microstructure of magnesia-chrome brick

Through the comparison of the above microstructures, it can be seen that magnesia-chromium bricks have improved high temperature, waterproof, thermal shock resistance than ordinary magnesia alumina bricks and ordinary burned magnesia bricks. The mechanism is as follows:

(1) Using high-quality magnesia and selected chrome ore as the base materials, high-pressure forming and ultra-high temperature firing, so that the direct bonding between the solid phases of magnesia-chrome bricks at high temperatures remains high, with sufficient high-temperature strength and excellent volume stability. and high temperature creep resistance, keep it from deformation and collapse.

(2) Because the hydration resistance of periclase, magnesia-aluminum spinel and magnesia-chromium spinel increases in turn; and the selected chrome ore in magnesia-chromium brick itself is also a kind of spinel, which reacts with magnesia and furthermore Magnesium-chromium spinel is formed, so the waterproofing performance of the product is improved.

(3) The addition of a certain number and grade of chrome ore particles forms an appropriate amount of micro-cracks in the product (caused by inconsistent thermal expansion coefficients). The existence of the micro-cracks can absorb the energy of the propagation and expansion of large exfoliating cracks, making the exfoliation large. Propagation and propagation of cracks are weakened and terminated, thereby improving thermal shock resistance.

4 Practical application comparison 4.1 Ordinary magnesia alumina brick

Ordinary magnesia-alumina bricks (see Table 3 for their performance indicators) are used in supporting wall arches as follows: the first furnace service (minor repair) is repaired (sometimes all replaced), and the second furnace service (medium repair) is completely replaced . For example, the 6# furnace is repaired 9 times a year, all the supporting wall arches are removed and replaced 8 times, and the cover is repaired once, that is, the maintenance rate of this part is 94.4%. The 9# furnace is repaired 10 times a year, and the supporting walls and arches are all dismantled and replaced 6 times, and the maintenance rate is 60%. Therefore, it is urgent to improve the life of the brick lining of the smoke exhaust system and reduce the consumption of bricks.

4.2 Waterproof and thermal shock-resistant magnesia-chrome brick

The situation of waterproofing and thermal shock-resistant magnesia-chrome bricks used for supporting wall arches is as follows: the first furnace service is intact and does not need to be covered; Because the observation during the middle repair is still very complete, there is no need for repairing, but because of the concern that the middle repair will not repair the next minor repair and then the repair progress will be affected, so the repair is carried out within 1 m in the middle, and finally 4 to 5 furnaces will be repaired. It can be replaced during service, so that the service life can be increased by 2 to 3 times.