Due to the secondary mullite reaction in the firing process of high-alumina products, the product size is difficult to control and the product quality is unstable. This article discusses the theoretical mechanism of high-aluminum blast furnace brick firing expansion through production examples. Four ratios are designed for testing, and each link of the production process is controlled. Through the comparison and analysis of external dimensions and physical and chemical indicators of the finished product, the theoretical calculation is carried out. , Finally determined a reasonable production process plan. Change the ratio of the amount of primary bauxite clinker added to aggregate and fine powder to meet the requirements of the thermal system, so as to ensure that the product dimensions and physical and chemical indicators can meet the requirements of enterprise standards.
GL-55 high-alumina blast furnace bricks are made of special-grade calcined coke YNS-44, first-grade bauxite clinker GAI-80 and high-quality clay, which are fired under three thermal systems of tunnel kilns. The thermal system is 1430℃×10 h, 1365℃×8 h, 1330℃×8h, and the dimensional expansion rate of the product is 2%~3.5%, 1.0%~2%, 0.5%~1%, respectively. It can be seen that: under the thermal system of 1430℃ ×10h, the expansion momentum of the fired product is the largest; the expansion momentum is the second at 1365℃× 8h. In these two cases, the overall dimensions of the finished product exceed YB/T 5015-1993. The dimensional tolerance range specified by the standard. In the third thermal system, the sintering expansion momentum is weakened, and the expansion rate is between 0.5% and 1%. Although the overall dimensions fully meet the requirements of the standard, it is measured from the physical and chemical indicators of the finished product. Most of the porosity exceeds the porosity index required by GL-55 and YB/T 5015-1993, which shows that the sinterability of the product is greatly lacking when sintered under this thermal system. The GL-55 high-aluminum blast furnace bricks with the above formula fired by the above three thermal systems cannot meet the requirements of external dimensions and physical and chemical indicators. Therefore, it is considered to re-adjust the formula and change the ratio of the amount of first-grade bauxite clinker added to the aggregate and fine powder to meet the requirements of the thermal system and ensure that the product's overall dimensions and physical and chemical indicators meet the standard requirements.
1 The conception and design of the experiment
1.1 Preliminary analysis of expansion
According to the correlation diagram of Al2O3-SiO2 binary system, at the composition point of mullite A3S2, the content of Al2O3 is 71.8%, that is to say, when the content of Al2O3 reaches or approaches this value, the tendency of mullite petrochemical will be great. , And the secondary mullite petrochemical is accompanied by a volume effect of 10%. According to the formula, calculate the content of Al2O3 in the matrix and aggregate:
A= (35% • M+10% • N)/45% = (0. 35×83% + 0. 10×30. 18%)/45% = 71.2%
(1) B=(10% • M+45% • P)/55%=(0. 1×83% + 0.45×45%)/55% = 51.9%
(2) Among them, A——the content of Al2O3 in the matrix, %;
B——The content of Al2O3 in the aggregate, %;
M——Al2O3 content in grade 1 bauxite clinker, %;
N——The content of combined clay Al2O3, %;
P——Al2O3 content of super calcined coke, %.
From the above calculation, it can be seen that the Al2O3 content in the matrix is as high as 71.2%, which is close to the composition point of mullite A3S2. Because the particles of the fine powder are very small, the specific surface area of the particles is S = 3/R, which is significant as the radius decreases. Therefore, the larger the specific surface area, the higher the activity of the particles. The larger the contact area, the more complete the reaction. Therefore, during the firing process, the secondary mullite reaction is mainly concentrated in the matrix, and the secondary mullite petrochemical starts at 1200 ℃ and ends at 1400-1500 ℃. Therefore, the higher the firing temperature, the secondary mullite petrochemical The more thorough the process is in the matrix; the lower the temperature, the less complete the process. Due to the 10% volume effect of the reaction, different volume expansion effects are exhibited during the firing of the above three thermal systems.
1.2 Experimental design
In order to obtain good dimensions and meet the physical and chemical index requirements of the product, the raw materials were adjusted for testing, aiming to reduce the content of Al2O3 in the matrix, making it far below the composition point of mullite, and balancing the aggregate and matrix content. The content of Al2O3 prevents the reaction from occurring in a large amount in the matrix. After adding part of the alumina to particles, the content of Al2O3 in the aggregate will increase, and the content of Al2O3 in the matrix will decrease. Due to the small specific surface area and poor activity of the aggregate, it rarely participates in the reaction with free SiO2, even if the reaction is limited to the thin layer on the surface, it will not significantly affect the expansion. At the same time, considering the high-temperature performance requirements of GL-55 high-aluminum blast furnace bricks, it is necessary to add part of the first-grade bauxite clinker to the matrix to optimize the matrix and stabilize the product performance. Therefore, the following four ratios are designed for testing.
2 Test process and test data analysis
2.1 Raw materials
It adopts super calcined coke (YNS-44 grade), Shanxi Yangquan first grade bauxite clinker (GAI-80 grade) and bonded clay. See Table 1 for the physical and chemical indicators of raw materials.
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Table 1. Physical and chemical index detection of raw materials
2.2 Mud preparation
The weighed materials are added to the wet mill in the order of adding the aggregate first, then adding the binder, and then adding the fine powder to the wet mill. All water is brought in by paper pulp, and the effective mixing time of the mud is 8 min.
2.3 Granularity analysis of mud
The mixed mud is analyzed for particle size, and the analysis results are shown in Table 2.
Table 2. Particle size composition and moisture of mud
2.4 Molding
It is formed on a 300T friction brick press, and the number of presses is controlled 7 times to control the bulk density and porosity of the wet billet.
2.5 Firing
A 1.8 m×1.8 m kiln car is used for loading, mounted on the second floor and above, and fired in a tunnel kiln. The thermal system for firing is 1360℃×8 h. The dimensional change rate (average value) of the finished product is shown in Table 3.
Table 3 .Finished product size change rate (average value)
From the data in Table 3, it can be seen that the changes of 1# and 4# formulations are basically the same, the direction of loading pressure is slightly shrinking, and the 2# formulation shows an obvious shrinking trend due to the small amount of first-grade alumina added to the matrix. #The formula shows a slight swelling momentum due to the high amount of primary bauxite clinker added in the matrix. The rate of change of the four formulas is much smaller than that of the original production formula, and the size of the finished product fully meets the requirements of YB/T 5015-1993.
2.6 Inspection of physical and chemical indicators of finished products
Take 4 formulations for index testing. The physical and chemical index testing of the finished product is shown in Table 4. Among them, 4-1 samples have low bulk density, and the finished product porosity and pressure resistance are unqualified. The production process can be adjusted to increase the bulk density of the wet billet. The finished product porosity and pressure resistance index requirements. Except for the 4-1 sample, the physical and chemical indexes of the other samples all meet the requirements of YB/T 5015-1993.
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Table 4. Physical and Chemical Index Testing of Finished Products
3 Analysis and calculation
3.1 Theoretical analysis
In the process of making bricks and firing, due to the occurrence of secondary mullite reaction, the product swells, and there are two reasons for the secondary mullite reaction of the product: First, the first grade bauxite (DK type) is calcined in the raw material. When the sintering is insufficient, there is a potential secondary mullite reaction in the firing of the product; second, the free SiO2 brought in by the combined clay and the pseudo-corundum in the bauxite clinker undergo a secondary mullite reaction, causing swelling . This test was fired under the thermal system of 1360℃×8h, and the first-grade bauxite clinker rot temperature was as high as 1450℃. Therefore, the secondary mulliteization reaction has not been completed in the first-grade bauxite clinker, and the reaction will not proceed anymore when the brick is fired. Therefore, the secondary mullite reaction between the free SiO2 brought in by the clay and the pseudo-corundum in the alumina clinker is the main reason for the expansion of the product. Therefore, if an excessive amount of primary bauxite clinker is added to the matrix, a large amount of secondary mullite will be carried out in the matrix, causing the size of the finished product to exceed the standard. If the amount of primary bauxite clinker added to the matrix is reduced, then after the free SiO2 in the combined clay is digested by the corundum phase in the insufficient bauxite clinker, there is still some remaining, then this part of the free SiO2 will be in the bauxite The secondary mulliteization reaction occurs on the surface of the clinker particles, causing the particles to expand locally, and the particles and particles spread out each other, which enlarges the gap between the particles. This part of the gap is difficult to combine by liquid phase sintering. Based on this point, it was discussed how much bauxite clinker was added to the matrix to digest the free SiO2 brought in by the bound clay, so as not to cause secondary mulliteization reactions on the surface of the particles.
3.2 Theoretical calculation
The main mineral component of the selected bonded clay is kaolinite (Al2O3 • 2SiO2 • 2H2O), the content of Al2O3 is 30.18%, and the content of SiO2 is 52.86%. A series of physical and chemical reactions occur during the heating process. When the temperature rises As high as 1200℃, A3S2 and free SiO2 are precipitated. Due to excess SiO2 and insufficient Al2O3, the free content is calculated as follows:
3 Al2O3 • 2 SiO2
3×102 2×60 X——means the consumption of SiO2%
30. 18% X
X=(2×60×30.18%)/(3×102)=11.84%
(3) The content of remaining SiO2: 52. 86% — 11. 84% = 41.02%
(4) According to Wang Jinxiang's research, the calcined phase content of the alumina material of diaspore-kaolinite (D-K type) is calculated as follows:
Mullite content M=343.8-3.76A=343.8-3.76×83. 2 = 31.0%
(5) Corundum content C=3.6A-239 = 3.6×83.2-239 = 60.5%
(6) Glass phase content G = -4.21 + 0.1523A = -4.21 + 0.1523×83. 2 = 8.5%
(7) If the bound clay is added by 8%, after its own mulliteization, the free content of the remaining SiO2 in the whole ingredients is: 8%×41.02%×100% = 3.28%, this part of the free SiO2 the amount of bauxite clinker consumed in the secondary mullite petrochemical process in the matrix is represented by Y
3α- Al2O3 + 2SiO2 A3S2
3×102 2×60
60. 5% Y 3. 28%
Y = (3×102×3.28%)/(2×60×60.5%) = 13.8%
(8) Therefore, 13.8% of primary alumina clinker is added to the matrix, which is enough to combine the excess free SiO2 in the clay. All are consumed without secondary mulliteization reaction on the surface of bauxite clinker particles.
4 Control technology of GL-55 high alumina blast furnace brick
Based on the above test results combined with theoretical calculations, select high-quality raw materials, optimize the particle ratio, change the ratio of the amount of first-grade bauxite clinker added to the aggregate and fine powder, strictly control the process operation and process parameters, and produce qualified GL-55 high alumina brick.
4.1 Selection of raw materials
Special-grade calcined coke (YNS-44 grade) and first-grade bauxite clinker (GAI-80 grade) with good calcining quality are selected from high-quality bonded clay.
4.2 Adjustment of raw clinker ratio
Reduce the amount of bonding clay added, increase the amount of clinker, and reduce the amount of secondary mullite reaction.
4.3 Balance the Al2O3 content in aggregate and matrix
Reduce the Al2O3 content in the matrix and increase the Al2O3 content in the aggregate so that the Al2O3 content in the matrix is much lower than the composition point of mullite. However, it is necessary to ensure that the Al2O3 content of the matrix is slightly higher than the Al2O3 content of the aggregate.
4.4 Determination of the amount of primary bauxite clinker added in the matrix
According to the above calculation, adding 13.8% of primary alumina clinker to the matrix, and adding particles to the rest of the alumina clinker can ensure that the free SiO2 brought in by the combined clay is completely digested in the matrix, and will not be in the matrix. The surface of bauxite particles reacts, and the shortage of fine powder can be supplemented with super-grade coking as fine powder.
4.5 Molding
Ensure that the number of pressings is not less than 8 times, control the bulk density of the wet billet, and ensure the shape quality of the wet billet.
4. 6 Firing
Adopt the thermal system of 1360℃±10℃×8 h to ensure the good sintering of the products.
5 Conclusion
Using super calcined coke (YNST4), first-grade bauxite clinker (GAI-80) and high-quality bonded clay, optimize the particle ratio, change the ratio of the amount of first-grade bauxite clinker added to aggregate and fine powder, in the matrix add 13.8% of the first grade bauxite clinker, and the remaining bauxite clinker is added with granules. The production process parameters are strictly controlled. Under the thermal system of 1360 ℃ (±10 ℃) ×8h, the micro-expansion GL- 55 high-aluminum blast furnace bricks have good dimensions and good physical and chemical indicators, and all meet corporate standards. At present, this process has been mass-produced on a large scale.
