Lightweight castables are mainly used in the safety lining of various melting furnaces or the lining of atmosphere furnaces in metallurgical and petrochemical industries. Traditional lightweight castables have the problems of high-temperature shrinkage, low strength, insufficient heat-insulating properties, and low service temperature, which limit their use and make them even more difficult to meet the requirements of high-temperature kiln working lining. Due to the abundant resources of aluminum and silica in our country, the price is relatively low, and the lightweight refractories with high aluminum quality are widely used in high temperature industry at present. Therefore, the development of high-performance lightweight castables with high cost-effectiveness to meet the use of temperature in the range of 1200～1500℃, mainly based on the raw materials of Al2O3-SiO2 system, is the focus of this project.
1.Choice of lightweight aggregate
In recent years, researchers have studied a variety of lightweight aggregates, such as vermiculite, perlite, ceramsite, refractory lightweight aggregate, refractory fiber, hollow sphere, etc. The lightweight refractory castables made from these lightweight aggregates have their own applicable scopes. Vermiculite and perlite can produce castables with very small bulk density and excellent thermal insulation performance, but the use temperature is low; ceramsite castables not only have a low use temperature but also a large bulk density, but have higher strength; refractory fiber can produce castables with good thermal insulation performance, but poor strength and low use temperature; refractory lightweight aggregate can be used to make castables suitable for use at various temperatures, and has high strength, but large bulk density and slightly poor thermal insulation performance. Most natural lightweight aggregates, due to the uncontrollable factors in their natural formation process, have a certain degree of defects in their own structure, unstable composition and high impurity content, resulting in uneven properties of each part, and the use temperature is limited, generally not higher than 1200℃.
In view of the disadvantages of natural lightweight aggregates, this paper develops high-performance lightweight castables based on the use of artificially synthesized lightweight aggregates. Artificially synthesized lightweight aggregates have high purity, uniform composition, and superior high-temperature performance. Such as electromelted alumina hollow spheres, multiphase hollow spheres, microporous mullite, etc. For the target operating temperature range of 1200-1500°C, alumina hollow sphere lightweight aggregates have problems such as excess Al2O3 content and high prices. Therefore, the choice of lightweight aggregates in this work is mainly based on synthetic microporous lightweight mullite aggregates and multiphase hollow spheres.
2.Experimental protocol
2.1 Main raw materials The raw materials used in this work include synthetic lightweight mullite and complex hollow spheres, as well as high alumina clinker, floating beads, kyanite, silica powder, alumina powder, pure calcium aluminate cement, bauxite powder, andalusite, etc.
2.2 Experimental formula. In the optimization of this part of lightweight aggregate, the lightweight mullite is gradually replaced by multiphase hollow spheres, and the replacement amount increases from 0 to 40%.
2.3 Sample preparation process Accurately weigh various raw materials according to the proportion in Table 3, add them into a mixer and dry mix for 2 minutes, then add water and stir evenly, vibrate and cast into 40mm×40mm×160mm strip samples, cure for 24 hours, dry at 110℃×24 hours, and heat treat at 1350℃×3 hours for use. Test the bulk density, heating permanent line change, room temperature flexural strength, room temperature compressive strength, thermal conductivity and other indicators according to relevant standards.
3.Results and Discussion
3.1 Effect of aggregate type on flexural strength of lightweight castables
In lightweight castables, the main part of the castable is supported by lightweight aggregates. Therefore, if the strength of the aggregate is low, the strength of the castable will inevitably be low. Figure 1 shows the room temperature strength of Q1 and Q5 after different temperature treatments. Among them, CMOR is the room temperature flexural strength, and CCS is the room temperature compressive strength.
It can be seen that since the strength of the composite hollow spheres is significantly higher than that of the lightweight mullite aggregate, the strength of the sample Q5 using the composite hollow spheres as aggregate is significantly higher than that of the sample Q1 using the lightweight mullite aggregate. Therefore, in order to ensure sufficient room temperature and high temperature strength of the lightweight castable, the aggregates in this experiment are lightweight mullite and composite hollow spheres.
3.2 Effect of the proportion of composite hollow spheres on the bulk density of lightweight castables
The change in bulk density after replacing lightweight mullite aggregate with composite hollow spheres. Since the bulk density of lightweight mullite aggregate is 0.80g/cm3, and the bulk density of composite hollow spheres is 0.60g/cm3, the bulk density of the castable shows a downward trend as the amount of composite hollow spheres added increases. When 40% of lightweight mullite is replaced, the bulk density of the sample after drying at 110℃×24h decreases from 1.86g/cm3 to 1.68g/cm3. The composite hollow spheres have a relatively obvious effect on reducing the bulk density of castables.
3.3 Effect of the ratio of composite hollow spheres on the compressive strength of lightweight castables at room temperature
Figure 3 shows the change trend of the compressive strength of castables after adding composite hollow spheres in different ratios. Since the strength of composite hollow spheres is significantly higher than that of lightweight mullite aggregates, and the composite hollow spheres are relatively dense and have a lower water absorption rate than lightweight mullite aggregates, the amount of water added to the castables is decreasing. Therefore, the compressive strength of the castables after drying and high-temperature sintering is increasing. The compressive strength after heat treatment increases from 25.2MPa of Q1 to 52.6MPa of Q4. Therefore, the introduction of composite hollow spheres can significantly improve the strength of the castable.
3.4 Effect of the ratio of composite hollow spheres on the thermal conductivity of lightweight castables
The thermal conductivity of the samples was tested after heat treatment at 1350℃×3h, and the effect of different composite hollow sphere contents on the thermal conductivity of castables was studied. As shown in Figure 4, the composite hollow spheres have a composite hollow structure and the closed pores inside have good thermal insulation properties. Therefore, the thermal conductivity of the castable is reduced after adding the composite hollow spheres. The thermal conductivity of samples Q1 and Q5 are 0.852W/(m·K) and 0.682W/(m·K), respectively. However, due to the large pore diameter of the composite hollow spheres, the effect of reducing the thermal conductivity of the material is limited according to the principle of heat transfer.
Final Conclusion
(1) The use of composite hollow balls to replace lightweight mullite aggregates reduces the amount of water added to the castable and significantly improves the room temperature strength of lightweight high-aluminum castables.
(2) Since the composite hollow balls contain a composite hollow structure, the closed pores inside have good thermal insulation properties. Therefore, the thermal conductivity of the castable is reduced after adding the composite hollow balls. However, due to the large pore diameter of the composite hollow balls, the effect of reducing the thermal conductivity of the material is limited.
(3) The high-performance lightweight high-aluminum castable developed using composite aggregates of composite hollow balls and lightweight mullite microporous aggregates has a lower thermal conductivity, higher high-temperature strength and volume stability. It is used in a variety of high-temperature furnaces at 1200-1500℃ in industrial furnaces, with significant energy-saving effects, good use results, and significant economic and social benefits.