Sintered tabular alumina has high sintering activity, which can promote the combination of substrates and particles. By adopting Sintered tabular alumina into the production of high-purity alumina bricks and then observe the effect of different sintered corundum on the performance of alumina bricks, engineers found that the sintered tabular alumina particles is small and full of pores. In the sintering process, that characteristic helps spreading substrates sintering, which can also improves the sintering strength and the permeability resistance of the alumina brick by combining substrate and particles more tightly.
Alumina bricks are refractory products with corundum as the main crystalline phase. They have good chemical stability and strong resistance to acid & alkaline slag, metal and molten glass. Mainly used in iron making blast furnaces, blast furnace hot blast furnaces, refining furnaces outside steelmaking furnaces, glass melting furnaces and petrochemical industrial furnaces. At present, the high-purity alumina bricks on the market are mainly produced with fused alumina raw materials. The production of fused alumina consumes much energy with large loss which is not environmental friends. The use of fused corundum raw materials to produce high-purity alumina bricks is difficult to sinter and with low slag resistance ability. In recent years, as a high-grade refractory material, the technology and output of sintered tabular alumina have been improved by leaps and bounds. Let’s see the advantage of making alumina bricks with sintered tabular alumina.
1 Test
1.1 Material
We use sintered tabular alumina as material to do the trial production. The Tabular alumina we use is with appearance porosity rate 5.7%, water absorption rate 1.6%, bulk density is 3.48g/cm3. The rival material is fused alumina with 8.8% appearance porosity rate,2.4% water absorption rate,3.61g/cm3 bulk density. The indexes are as below:
| Item | ω% | ||||
| C1 | C2 | C3 | C4 | C5 | |
| Tabular alulmina | 90 | 70 | 50 | 25 | 0 |
| Fused alumina | 0 | 20 | 40 | 65 | 90 |
| Activeα-Al2O3 powder | 10 | 10 | 10 | 10 | 10 |
| Binder(added) | 3 | 3 | 3 | 3 | 3 |
1.2 Trial manufacturing
Using a 15Kg roller mixer, adding grits for pre-mixing for 3 minutes, then add 3% binder and knead for 1 min, and finally add fine powder and knead for 15 minutes, and shape on a 100t hydraulic press with a molding pressure of 280 MPa. The molded samples are cylinder brick withφ50mm×50mm cuboid brick with 150mm×25mm×25mm and crucible with outer dimension φ50mm×50mm and inner hole size φ25mm×25mm. The brick samples are made in a ultra-high temperature electric furnace heated at 1750℃ for 3 hours after keeping 110℃ for 3 hours and drying.
1.3 Performance Test
Test the heating permanent line change, the volume density and apparent porosity, compressive strength and flexural strength at normal temperature, flexural strength at high-temperature (at 1400°C for 0.5h) of samples by national standards. Test slag resistance ability by static crucible method and observe microstructure of the sample by SEM scanning electron microscope.
2 Result and conclusion
2.1 Microstructure of material
The image 1 below shows the microstructure of the raw material particles. It is found the sintered tabular alumina is composed of oi-Al2O3 crystals with a particle size of 40~120μm, and there are a certain amount of closed spherical pores. The structure of fused alumina is denser, there are some open pores with bigger size.
(a)Sintered tabular alumina grain (b) fused tabular alumina grain
Image 1.
2.2 Reheating linear change
The image 2 shows the reheating linear change curve of samples made from different raw materials. The experimental results shows that all samples has trend of firing shrinkage. However, as the content of sintered tabular alumina increased, firing shrinkage increased in the meantime. Comparing the raw material indexes, we found that the sintered tabular alumina particles contain much more pores. If the true density of α-Al2O3 is 3.99g/cm3 and the bulk density is 3.48g/cm3, then the total amount of porosity is about 13%. Furthermore, with very small crystal size of sintered tabular alumina, it make spreading and mass transfer sintering easily in sintering process. Thus get volume shrinkage because some pores are removed from the crystal boundary along with the movement of substances. The bulk density of fused alumina particles is 3.61g/cm3, and the percentage of all pores is about 9%. Since fused alumina is produced by melting and condensing in a high-temperature electric arc furnace, the raw material has a large crystal size and few crystal boundary channels. Therefore, the sintering shrinkage is smaller than that of sintered tabular alumina particles.
Image 2 Reheating linear change on different samples.
2.3 Apparent porosity and bulk density
In Image 3 shows generally the samples with higher sintered tabular alumina content have lower apparent porosity and higher bulk density. This is because the apparent porosity of the sintered tabular alumina is much small about 5.7%, while the apparent porosity of the fused alumina is 8.8%. In addition, compared with the fused alumina, the pores in the sintered tabular alumina are easier to remove from the crystal, which reduces the porosity and get a larger volume shrinkage, and further increases the bulk density of the sample. Therefore, the apparent porosity of the fired sample decreases with the increase of the sintered tabular alumina percentage.
Image 3 Apparent porosity and Bulk density for different samples
Image 4 shows that the normal temperature compressive strength (CCS) of the pure sintered tabular alumina material C1 brick is much larger than that of the pure fused alumina material C5 brick. There are two main reasons for it. Firstly, from the aspect of the raw material strength, the crystal size of the sintered tabular alumina material is small, and the fracture strength (σ) of the material and the crystal size (G) have the following functional relationship:
σ=f(G-1/2)
Therefore, the strength of sintered tabular alumina material is relatively high, while the fused alumina material is brittle and easy to peel off (as shown in Image 5(a)), and there is also a small amount of β-Al2O3 phase in it, which reduces the strength of the material.
Secondly, from the aspect of the bonding state of the material, the bonding between the sintered tabular alumina particles and the substrate is good, almost sintered into a whole. The fused alumina particles are not well bonded with the substrate and ring-shaped cracks are easily formed around the particles (Image 5) (b)). Due to the above two reasons, the mechanical strength of pure sintered tabular material C1 brick is better than that of pure fused alumina material C5 brick.
Image 4 Normal temperature compressive strength and bending resistance strength for different samples
Image 5 the microstructure of samples made from fused alumina
After adding 20g of gasification slag into the crucible (see Table 2 for slag composition), heating the crucible to 1550℃ in the test electric furnace at a heating rate of 100℃/h and keep for 3h, and then cutting the crucible along the axial direction after cooling to room temperature, observe the microstructure changes in the longitudinal section.
Chemical composition of furnace slag shows as below:
| Chemical | SiO2 | Al2O2 | Fe2O3 | TiO2 | CaO | MgO | K2O | Na2O |
| Contentω% | 40.8 | 23.6 | 5.1 | 1.1 | 20.9 | 3.8 | 1.1 | 3.6 |
Image 6 Static crucible anti-slag profile
After the corrosion test of coal-water slurry gasification furnace slag, observe the microstructure by electron microscope. The slag of coal water slurry gasification is of fishbone shape, mainly of anorthite phase (as shown in Image 7(a)); the slag reacted with the alumina in the test bricks and get a magnesium-aluminum-iron composite spinel phase. Energy spectrum analysis shows the composition of the composite spinel phase is (x/%): MgO 40.43%, Al2O 347.61%, Fe2O3 11.96%. The magnesium-aluminum-iron composite spinel phase formed by the reaction forms a ring around the alumina particles (as shown in Image7(b)). The thickness of the ring around the sintered tabular alumina particles is 60~90μm, and the thickness of the ring around the fused alumina particles is 50~ 70μm, it can be seen that the slag is easier to react with the sintered tabular because the sintered alumina has big sintering activity, smaller crystals, more closed pores, and more crystal boundaries. The slag is easy to penetrate along the crystal boundaries and chemically react with the sintered tabular alumina.
(a)Slag (b)C2 Working surface
Image 7 Microstructure of alumina brick sample after slag and corrosion resistance test
C1、C2、C3、C4和C5的侵蚀深度没有明显差别，都约为1mm，图8分别为C1砖和C5砖侵蚀后的显微结构照片，炉渣首先与砖基质反应，使刚玉颗粒呈孤岛状，然后与颗粒反应，将颗粒蚕食掉。
All erosion depth of C1, C2, C3, C4 and C5 are around 1mm, has no obvious difference. Image 8 shows the microstructure photos of C1 bricks and C5 bricks respectively after erosion. The slag first reacts with the brick substrate to make the alumina particles become island shapes and then react with the particles to eat away the particles.
Image 8 Microstructure of alumina brick sample after slag resistance test
Image 9 shows the penetration ways of test bricks with different formulations are similar. The slag penetrates into the bricks along the pores, exist in the intergranular and pores as the glass phase and anorthite phases.
Image 9 Microstructure of C5 permeable layer of alumina brick sample after slag resistance test
But different samples show different anti-permeability properties: the bellowing table shows the penetration depth of SiO2 in different samples. As the content of sintered tabular alumina in the brick decreases, the penetration depth of slag shows an increasing trend.
| Distance from working surface | SiO2 content(ω%) | ||||
| 0.2mm | 4mm | 8mm | 12mm | 16mm | |
| C1 | 5.64 | 5.78 | 3.73 | 1.1 | 0 |
| C2 | 6.99 | 5.12 | 3.32 | 3.14 | 0 |
| C3 | 7.08 | 4.42 | 4.73 | 3.57 | 0 |
| C4 | 6.38 | 5.95 | 6.34 | 4.12 | 3.3 |
| C5 | 6.47 | 6.7 | 5.21 | 5.46 | 2.74 |
There are two reasons for this result:
- The sample with high sintered tabular alumina content has a lower apparent porosity;
- The sintered tabular material particles are better bonded with the substrate, which prevents the penetration of slag into the bricks.
3 Conclusion
Due to the small crystal size of tabular alumina, there are a large number of pores existing in the particles, which is helpful to make mass transfer sintering. Some pores are removed from the crystal along the crystal boundary with the movement of substances, get a volume shrinkage. That results in shrinkage rate increasing and apparent porosity decreasing in the sintering by increasing the sintered tabular alumina content.
The pure sintered tabular alumina has a fine-grained structure wiht high strength and high sintering activity. The sintered tabular alumina particles in the brick have a good bond with the substrates, so the mechanical strength performance increases as the content of sintered corundum increases.
Since the tabular alumina have two significant advantages : Low apparent porosity and excellent bonding ability with substrate, it shows that sintered tabular alumina can slow down the penetration of slag into the brick.
