Energy-saving of industrial kilns has always been a key issue that needs to be solved urgently by large energy consumers such as metallurgy, machinery, and chemical industries. The use of lightweight heat-insulating materials with low bulk density and low thermal conductivity as furnace lining is one of its effective solutions. Because of its low thermal conductivity, low heat capacity, high temperature resistance, good thermal shock resistance, high dimensional accuracy, and uniform structure, mullite heat-insulating refractory bricks are suitable for various fields such as metallurgy, petrochemical, building materials, ceramics, and machinery. This kind of industrial furnace hot surface lining and backing, because it can be in direct contact with the flame, is an extremely excellent heat insulation refractory material.
Mullite heat-insulating refractory bricks achieve the effect of light weight and heat insulation by making holes inside the material. Therefore, the preparation principle is to introduce pores into the material, mainly including burn-out method, foam method, and chemical method. Common methods such as reaction method, porous material method, gel injection molding method, freeze-drying method, and in-situ decomposition method. Among them, the burnout method can be divided into extrusion method and machine pressing method due to the different molding methods. Different preparation processes have an important influence on the performance of mullite bricks. In order to explore the influence of different processes on mullite bricks, experiments were carried out to prepare mullite bricks by three methods: machine pressing method, extrusion method and foam method. And compared its performance.
Experiment
1.1 Raw materials
The main raw materials for the experiment are as follows: clay, calcined alumina ((ω(Al₂0₃)≥99, D0.5 is 0.043-0.1mm), calcined mullite powder ω(Al₂0₃)≥65, D0.5 is 0.1-0.5mm), Tabular corundum, (ω(Al₂0₃)>199.4, D0.5 is 0.043-0.2mm), kyanite and sillimanite. The foaming agent used in the experiment was sodium dodecyl sulfonate. The burnout materials used were sawdust and polypropylene balls. The binding agent is polyvinyl alcohol (PVA).
1.2 Preparation
Foam method: The experimental raw materials are pre-mixed for 4 hours according to the ratio in the table. Add 30~35wt% of water to mix the powder into a uniform and stable slurry; then add water to the foaming agent and stir at high speed to obtain a stable foam: finally mix the slurry and the foam evenly. Inject it into a 40mmx40mmx160mm mold. And shake it slightly. After removing the large bubbles, place it at room temperature to dry naturally for 8-12 hours. Demould, and bake at 1100°C for 24 hours. After firing at 1550% and keeping it warm for 3 hours, a mullite heat-insulating refractory brick is obtained.
Pressing method: The experimental raw materials were pre-mixed according to the ratio of 2# in Table 1 for 4 hours, then the polyvinyl alcohol was diluted and then added to the uniformly mixed powder. Stirred for 10-15 minutes, and extruded into a 114mm×65mm×230mm billet at a pressure of 5MPa The bricks are baked at 110°C for 24 hours. They are fired at 1550°C and kept for 3 hours to obtain mullite heat-insulating refractory bricks.
Extrusion method: The experimental raw materials were pre-mixed according to the proportion of 3# in Table 1 for 4 hours, and 10-15wt% of water was added and then stirred uniformly. After the process procedures such as material trapping and mud refining, 114mm× was prepared by extrusion. The 65mm×230mm bricks were baked at 1100C for 24h, then fired at 1550℃ and kept for 3h to obtain mullite bricks.
1.3 Characterization
Under the premise that the bulk density of the samples prepared by the three molding methods is 1.0-1.1g/cm3, the performance of each group of samples is tested multiple times, and the average value is taken.
(1) The linear change rate of the sample after burning is determined according to the national standard GB/T5998-2007:
(2) The rate of change of the reburning line shall be determined in accordance with the national standard (GB/T3997.1-1998);
(3) The compressive strength of the sample is determined in accordance with the national standard (GB/T3997.2-1998);
(4) The thermal conductivity of the sample is in accordance with the metallurgical industry standard (YB/T4130-2005). Use a flat thermal conductivity meter (PBD-12-4Y) for measurement;
(5) The high-temperature load softening temperature of the sample is determined in accordance with the national standard (GB/T5989-1998). It is measured by the differential-increasing method.
Results and discussion
2.1 The influence of molding method on line changes
After the mullite brick sample was fired at 1550°C for 3 hours, the linear shrinkage rate of the sample prepared by the foam method was the largest. It reaches 2.4%; the linear shrinkage rate of the sample prepared by the extrusion method is the smallest, only 1.3%. Further reburning the sample at 1620℃ for 12h, the sample prepared by the foam method has the smallest reburning linear shrinkage rate of 0.73%; while the sample prepared by the extrusion molding method has the largest reburning linear shrinkage rate, reaching 1.56%.
The mullite brick prepared by the foam method has the characteristics of large linear shrinkage after firing and low linear shrinkage after re-firing. The main reason is that its structure is more uniform, and the pore size distribution presents a bipolar distribution of micro-nano coexistence, and the sintering is more Fully caused. On the other hand, the linear shrinkage rate and re-fired linear shrinkage rate of the mullite heat-insulating refractory bricks prepared by the machine pressing method are smaller than those prepared by the extrusion method. This is mainly due to the different force directions in the molding process. Caused by. The sample prepared by the machine pressing method will swell to a certain extent during the firing process.
2.2 The influence of molding method on strength
The mullite bricks prepared by the foam method have good compressive strength and flexural strength. The compressive strength reaches 5.6MPa and the flexural strength reaches 3.2MPa; while the samples prepared by the machine pressing method have compressive strength and flexural strength. Both are very low, only 1/4 of the former. The main reason for the lower strength of the latter is the “elastic after-effect” effect of the pore former during the press molding process, which leads to internal cracks in the product.
2.3 Influence of molding method on softening temperature under load
The load softening temperature of the mullite brick prepared by the foam method is 100°C higher than that of the machine pressing method or the extrusion method, while the load softening temperature of the mullite brick prepared by the machine pressing method and the extrusion method is almost the same. The load softening temperature of the insulation material is not only related to the chemical and phase composition of the material, but also inseparable from its pore structure. In the mullite brick prepared by the foam method, round pores are evenly distributed on it, which can effectively disperse the concentration of stress and improve the ability to resist external forces without deformation. At the same time, its micro-nano-level combined pore structure can effectively disperse heat. Stress makes it have better volume stability under high temperature conditions.
2.4 The influence of molding method on thermal conductivity
In the case of the same bulk density, the thermal conductivity of mullite bricks prepared by the foam method is smaller than that of the machine pressing method or the extrusion method. The thermal conductivity is closely related to the porosity of the product, and the porosity increases. The gas-solid phase interface is increased, and the phonon scattering of the solid phase heat conduction is increased, thereby reducing the thermal conductivity of the refractory material. At the same time, the thermal conductivity is also closely related to the pore diameter. Under high temperature conditions, the movement of gas molecules is intensified. The mean free path is reduced due to the increase in collision probability. When the mean free path of gas molecule movement is closer to or even greater than the size of the micropores in this range, the convective heat transfer in the pores weakens and the thermal conductivity of the material decreases. . The pores of the mullite bricks prepared by the foam method are micro-nano pores, the convective heat transfer is greatly reduced, and the heat insulation effect is significantly improved.
in conclusion
By comparing the performance of mullite lightweight insulation bricks prepared by three different molding methods. We can see that the foam method has the advantages of good heat insulation effect, high load softening temperature, good strength, and low reburning linear change rate, so it has obvious advantages.
