Trial Conception and Design
1. Preliminary analysis of expansion
According to the correlation diagram of Al2O3-SiO2 binary system, it can be seen that at the composition point of mullite A3S2, the content of AI2O3 is 71.8%. Secondary mullitization 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+10P・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)
Where A – the content of A&0s in the matrix, %;
B——the content of A&0s in the aggregate, %;
M——the content of Al2O3 in the primary bauxite clinker, %;
N——the content of combined clay Al2O3, %;
P – Al2O3 content of super calcined coke, %.
It can be seen from the above calculation that the Al2O3 content in the matrix is as high as 71.2%, which is close to the composition point of mullite A3S2. Since the fine powder particles are very small, the specific surface area of the particles is δ=3/R, which increases significantly with the decrease of the radius , so 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 reaction starts at 1200°C and ends at 1400~1500°C. Therefore, the higher the sintering temperature, the more thoroughly the secondary mullitization will be carried out in the matrix; the lower the temperature, the less complete it will be. Due to the 10% volume effect of the reaction, when firing in the above three thermal systems , showing a different volume expansion effect.
2. Experimental design
In order to obtain a good shape and size while meeting the physical and chemical index requirements of the product, the raw materials were adjusted for testing, aiming at reducing the content of Al2O3 in the matrix, making it far below the composition point of mullite, and balancing the content of the aggregate and the matrix. Al2O3 content, thereby curbing the reaction in a large amount in the matrix. After part of the bauxite is added with particles, the content of Al2O3 in the aggregate increases, and the content of Al2O3 in the matrix decreases. 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 have a significant impact on the expansion. At the same time, considering the high-temperature performance requirements of GL-55 high-alumina blast furnace bricks, part of the first-grade bauxite clinker must be added to the matrix to optimize the matrix and stabilize product performance. Therefore, the following four ratios were designed for testing.
Test process and test data analysis
2.1 Raw material
Super calcined coke
2.2 Mud preparation
The weighed materials according to the proportions are added to the wet mill and kneaded in the order of adding aggregate first, then binder, and then fine powder. All the water is brought in by pulp, and the effective mixing time of mud is 8 minutes.
2.3 Particle size analysis of mud
Mixed mud for particle size analysis
2.4 Forming
It is formed on a 300T friction brick press, and the pressing times are controlled for 7 times, and the bulk density and porosity of the wet billet are controlled.
2.5 Firing
The 1.8m×1.8m kiln car is used to load the car, installed above the second floor, and fired in the tunnel kiln. The thermal system of firing is 1360°C×8h.
The changes of 1# and 4# formulas are basically the same, and the loading direction shows a slight shrinking trend. The 2# formula shows an obvious shrinking trend due to the small amount of first-grade bauxite added to the matrix. The amount of soil clinker added is on the high side, showing a tendency of slight expansion. The rate of change of the four formulas is much smaller than the original production formula, and the finished product size fully meets the requirements of YB/T5015-1993.
2.6 Testing of physical and chemical indicators of finished products
Take 4 formulas for index testing respectively. The physical and chemical index testing of the finished product is shown in Table 4. Among them, sample 4-1 has a low bulk density, and the porosity and pressure resistance of the finished product are unqualified. It can be guaranteed by adjusting the production process and increasing the bulk density of the wet billet. Requirements for finished product porosity and pressure index. Except for the 4-1 sample, the physical and chemical indicators of the rest of the samples all meet the requirements of YB/T5015-1993.
3. Theoretical analysis In the process of brick making and firing, due to the secondary mullite reaction, the product expands, and there are two reasons for the secondary mullite reaction of the product: First, the primary bauxite (DK type ) When the raw material is calcined, the sintering is insufficient, and there is a potential secondary mullitization reaction in the product firing; secondly, the free SiO2 brought in by the clay and the pseudocorundum in the bauxite clinker undergo secondary mullitization Reaction, causing expansion This test is fired under the thermal system of 1360℃×8h, and the calcination temperature of the first-grade bauxite clinker is as high as 1450℃. Therefore, the secondary mullite reaction has not been completely carried out inside the primary bauxite clinker, and the reaction will not continue when the brick is fired. Therefore, the free SiO2 brought by the combined clay and the corundum in the bauxite clinker are the main reasons for the anti-primer expansion. 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, after the free SiO2 in the combined clay is digested by the corundum phase in the insufficient amount of bauxite clinker, there is still some remaining, and this part of free SiO2 will be in the bauxite The secondary mullite reaction occurs on the surface of the clinker particles, which causes the particles to expand locally, and the particles spread apart from each other, which increases the gaps between the particles, which are difficult to be combined by liquid phase sintering. How much bauxite clinker is added to the matrix is enough to digest the free SiO2 brought in by the bonded clay, resulting in mullite reaction on the particle surface.