With the gradual deepening of national environmental governance and greater efforts, green chromium-free environmentally friendly alkaline refractories have shown more advantages. Magnesia-aluminum spinel bricks are the leading products used in the transition zone of large and medium-sized cement rotary kilns because of their The advantages of high strength, high temperature resistance, thermal shock resistance, and strong thermal stress resistance have been universally recognized by users for a long time. At this stage, it is still the first choice for refractory materials for transition zone. In this work, the effect of pre-synthesized fused magnesia-aluminum spinel on its performance was studied.
1 test
1.1 Raw materials
This experiment uses sintered magnesia, fused magnesia, and fused magnesia aluminum spinel as the main raw materials.
1.2 Contrast test of adding amount and particle size of different magnesium aluminum spinel
Accurately weigh the materials according to the proportioning requirements. First, add the pellets to the wet mill for dry mixing for 2 to 3 minutes, add 3% (w) lignin solution binder and mix for 3 to 5 minutes, then add 0.088 mm fine powder and mix for 8 to 10 minutes. Even, the fine powder is fully wrapped on the pellets, no raw material, no mud, and the hand feels even and soft, and the material can be discharged. It is formed by a 630 t electric screw press. After the green body is dried at 110℃×24 h, it is loaded into a high-temperature tunnel kiln for firing. After a total of 5 high-temperature points are kept for 8 hours, it is cooled and taken out of the kiln.
1.2 Performance testing
Test the volume density and apparent porosity according to GB/T5998-2000, test the compressive strength at room temperature according to GB/T 5072-2008, and test the thermal shock resistance according to YB/T376.1-1995.
2 Result analysis
2.1 The influence of magnesium-aluminum spinel addition on material properties
2.1.1 Influence on apparent porosity and bulk density
The influence of the amount of magnesium-aluminum spinel added on the apparent porosity and bulk density of the sample.
2.1.2 Influence on the compressive strength of products at room temperature after firing
It can be seen that with the increase in the amount of magnesium-aluminum spinel, the compressive strength of the sample shows a decreasing trend. Although the decrease is not large, it gradually decreases. When the addition amount (w) is greater than 20%, the strength decreases more obviously .
2.1.3 The impact of anti-thermal shock performance
It can be seen that as the amount of magnesium-aluminum spinel added increases, the thermal shock resistance of the sample gradually increases. When the amount of magnesium-aluminum spinel (w) is greater than 24%, the thermal shock resistance improves slowly. Almost no longer rises.
2.2 The influence of magnesia-aluminum spinel with different particle sizes on the properties of products after firing
2.2.1 Influence on bulk density and apparent porosity
It can be seen that the particle size of the magnesium-aluminum spinel affects the bulk density and apparent porosity of the product. Too large or too small particle size is not conducive to reducing the apparent porosity and increasing the bulk density. The best condition is reached only when the particle size is within the appropriate interval of 3.5～1 mm. The measured bulk densities of samples B-1, B-2, B-3, and B-4 are 2.94 g·cm-3 and 2.96 g·cm, respectively -3, 2.95 g·cm-3, 2.95 g·cm-3, the apparent porosity was 16.7%, 16.2%, 16.4%, 16.5%, respectively.
2.2.2 Influence on compressive strength at room temperature
The particle size of the magnesium-aluminum spinel affects the compressive strength at room temperature, and the appropriate particle size is beneficial to improve the compressive strength at room temperature, and the larger or the smaller ones are not conducive to the improvement of the compressive strength at room temperature. The average room temperature compressive strength of samples B-1, B-2, B-3, and B-4 are 61.3 MPa, 68.5 MPa, 65.4 MPa, and 63.7 MPa, respectively.
2.2.3 The impact of anti-thermal shock performance
With the increase of the particle size of the magnesium-aluminum spinel, the thermal shock stability of the sample shows a trend of first increasing and then decreasing. The thermal shock resistance of samples B-1, B-2, B-3, and B-4 were 14 times, 16 times, 12 times, and 9 times, respectively.
2.3 Analysis
Since the volume density of the added fused magnesia-aluminum spinel (3.72 g·cm-1) is higher than that of high-purity magnesia (3.25 g·cm-1), the volume density of the added magnesia-aluminum spinel increases With the increase, the apparent porosity showed a downward trend. When the magnesia-aluminum spinel is added to more than 20%, the product will form secondary spinel during the firing process, and the brick body will expand and microcracks will increase, resulting in a decrease in volume density and an increase in apparent porosity. Because spinel and periclase are the same equiaxed crystal system, the thermal expansion coefficient of magnesia-aluminum spinel is 7.6×10-6, and that of periclase is 13.5×10-6. M-MA bricks mainly take advantage of the large difference in thermal expansion coefficient between the two. A certain number of micro-cracks are formed during the firing and cooling process. The generation of micro-cracks improves the thermal shock resistance of the material. A proper amount of micro-cracks can be used in use. Buffer the thermal stress caused by the temperature change of the kiln and reduce the peeling of the product. However, too many micro-cracks will adversely affect the strength of the material. Therefore, as the number of magnesium aluminum spinel increases, the thermal shock resistance of the material improves. The compressive strength at room temperature is reduced.
3 Conclusion
(1) With the increase in the amount of magnesia-aluminum spinel, the compressive strength of magnesia-aluminum bricks at room temperature will gradually decrease, and the thermal shock performance will gradually improve. The overall volume density, apparent porosity, compressive strength at room temperature, thermal shock stability, etc. Factors, the reasonable addition amount (w) is 20%, and the number of thermal shock resistance hardly increases after the addition amount exceeds 24%;
(2) Adding magnesia-aluminum spinel to form secondary spinel with magnesia (M-MA) during the firing process, resulting in an appropriate amount of microcracks, which is beneficial to improve the thermal shock performance, but the strength is reduced;
(3) Appropriately increasing the particle size of the magnesium-aluminum spinel is beneficial to improve the thermal shock resistance. The test result is that the volume density of the product when the particle size is 3.5-1 mm is added, the apparent porosity is the best, the strength is moderate, and the thermal shock stability is good. .
