In the glass melting furnace, in addition to the contact with the flame as we mentioned before, the entire kiln pool is in contact with the high-temperature glass liquid from the melting part, cooling part to the forming part. The temperature of the glass liquid ranges from 1200 to 1600℃, is in a convection state, presents a variety of flow modes, and the flow rate of each part is different. The combustion airflow flows above the glass liquid surface. Therefore, it has strong erosion performance. This part of the contact brick body can be divided into the pool wall, pool bottom, feeding pool, cooling part (or working part), flow channel, etc.
Damage characteristics of each part and commonly used materials
01 Pool wall brick
At present, the pool wall brick mainly uses fused zirconium corundum brick. The damage mechanism of this brick is the melting process of multi-phase materials. The damage process is roughly divided into three steps:
① High-temperature glass phase seepage The melting of fused AZS bricks depends on the high-temperature glass phase seepage temperature (at 1320~1420℃). The glass phase seepage in the brick forms a high-viscosity glass liquid with the nearby glass liquid, which prevents the brick from being further eroded. But holes are left in the fused brick. Since the surface tension of fused bricks is smaller than that of glass liquid, fused bricks are always wetted by glass liquid. The melting rate depends on the glass phase seepage temperature, the amount of glass phase and the viscosity of the melt.
② Due to the flow and flushing of glass liquid, the high viscosity glass liquid layer on the surface of the fused brick leaves its original position and enters the glass liquid. The remaining holes are exposed.
③ After the glass liquid penetrates the fused layer, the new glass liquid penetrates into the exposed new surface and holes, reacts with the phases around the holes, and continues to melt the fused brick. First, the mullite in the structure is decomposed to form aluminum oxide and new glass phase, and then the baddeleyite and corundum fine particles in the structure are dissolved, and finally the brick structure is completely disintegrated. The penetration rate depends on the diffusion rate and the organizational structure of the brick (such as the size, shape, number and distribution of pores).
The above three-step process is repeated continuously, so that the fused brick is continuously corroded. When the temperature of the glass liquid increases, the corrosion will accelerate. When the outer surface of the pool wall brick is cooled by air blowing, the corrosion will slow down. The corrosion conditions vary with different glass components. For example, when using Glauber's salt, there will be "salt water" on the liquid surface, which will aggravate the corrosion of the pool wall bricks. When containing fluoride, the corrosion is very fast.
The brick joints of the pool wall bricks are very easy to be corroded, which is due to the flow of glass liquid and the upward drilling. This is also the gas-liquid-solid three-phase interface. The bubbles on the interface promote the stirring of the liquid phase, accelerate the dissolution of the solid phase, and the reaction layer is easily washed away by the glass liquid. As the material of the pool wall brick, it is required to be resistant to corrosion, not cause any defects to the glass liquid (such as streaks, stones, bubbles), not color the glass liquid, and have a certain degree of thermal shock resistance and low thermal conductivity. The ideal pool wall brick is fused zirconium corundum brick. The melting part of the pool wall usually uses fused AZS bricks containing 41% zirconium oxide, and the cooling part uses fused AZS bricks containing 33% zirconium oxide. For fused AZS products, pay attention to the abnormal volume change accompanied by crystal transformation at 1050~1150℃ when baking the kiln.
Some enterprises also choose to use α-β-fused corundum products, which are composed of tightly staggered α-alumina (53%) and β-alumina (45%) fine particles, and the space between the crystals is filled with glass phase (2%). It can be used as large bricks for vertical laying of cooling pool walls, cooling bottom bricks and seam covering bricks, etc. Among the materials of the upper structure, a β-fused corundum product is also launched. It is composed of large β-alumina grains. The brick body contains a certain amount of pores, but there is almost no glass phase. Therefore, this brick has excellent resistance to thermal shock and alkali vapor corrosion.
02 Pool bottom bricks
The conditions of the pool bottom bricks are that the glass liquid temperature is low and the flow is weak. It has the following three damage characteristics.
① The metal impurities (mainly iron, the order of metal corrosion intensity is: Sn, Pb, Cu, Al, Zn) brought in by the batch (mainly broken glass) when drilling downwards and the molten metal obtained by glass reduction sink to the bottom of the pool, forming a spherical or flat ring melt. Due to their large surface tension, they form a wedge-shaped opening between the glass liquid and the pool bottom bricks, resulting in downward drilling erosion, especially drilling into the pool bottom brick seams. While drilling, they erode and expand to the non-corrosion-resistant pool bottom brick parts (such as clay bricks, or even lightweight insulation bricks). In severe cases, most of the pool bottom bricks (under the paving bricks) can be hollowed out.
② Upward drilling When the glass liquid and metal liquid melt the brick seam material and penetrate into the cushion layer under the paving bricks, the cushion layer is corroded and damaged. The glass liquid and metal liquid erode the paving bricks vertically upward in a pit-shaped manner, that is, drilling upward. They attack from top to bottom until the paving bricks are melted through.
③ Mechanical wear When the pool bottom is insulated, the bottom glass liquid is in a convection state. The circulating liquid flow wears the pool bottom, especially the feeding area, where the convection is intense and the wear is heavy.
In view of the above damage characteristics, the pool bottom bricks are required to be wear-resistant and have integrity. Nowadays, a multi-layer composite pool bottom structure is mostly used. Generally, a protective layer is built under the bottom bricks (clay bricks), and zircon sand ramming material or fused zirconium corundum ramming material is set on the clay bricks. The latter is better. This layer of ramming material and the pool bottom are sintered into a whole. It is required not to shrink or expand, have no cracks, and be very dense, which can prevent the leakage of glass liquid to the maximum extent and form a solid bottom structure with sufficient strength.
The paving bricks are in direct contact with the glass liquid, which can well protect the ramming material pool bottom. AZS bricks with good corrosion resistance and wear resistance are often used (high-quality sintered AZS bricks can also be used, but the disadvantage is that the size is small and there are many brick joints). The use effect of the composite pool bottom structure is: reducing heat dissipation loss, extending the life of the pool bottom, and improving the quality of glass liquid.
03 Bricks in the charging pool
Bricks in the charging pool are subject to erosion by powder and glass liquid, wear of the material layer, scouring of the liquid flow, and the influence of flames. Therefore, the damage is more serious, especially the corner bricks in the charging pool. The protruding corners are subjected to strong abrasion and scouring. There is a large temperature difference between the inner and outer surfaces of the bricks, which produces large mechanical stress. In addition, there is a certain temperature fluctuation in this part, so it is very easy to be damaged.
It is regarded as one of the weak parts of the tank kiln. High-temperature wear-resistant and corrosion-resistant materials must be used. Commonly used are fused zirconium corundum bricks containing ZrO2: 41% without shrinkage cavities, and the brick thickness is increased and air cooling is strengthened. Except for the corner bricks, other parts can use ordinary cast AZS-33 bricks.
04 Bricks for the cooling part
The cooling part is the melted and clarified glass liquid, which has entered the later stage of the melting process. It must not cause any defects (including stones, streaks, bubbles) to the glass liquid. Since the temperature of the glass liquid here is lower than that of the melting part, the erosion of the bricks is lighter. The cooling part needs to use AZS-33 bricks produced by oxidation method, which has a high glass phase precipitation temperature and will not bring bubbles to the glass liquid.
05 Bricks for runners
The runners use fused AZS-3 bricks or fused α-Al2O3 bricks. These materials have fine-grained and dense structures, with very little glass phase filled between crystals, minimal pollution to the glass liquid, and better corrosion resistance than AZS bricks below 1300℃. Runners and flow trough bricks (1100~1200℃, glass liquid flow rate 0.83~1.67m/min or even higher) are very easy to wear, and high-quality materials must be used. Practice has proved that sillimanite bricks and mullite bricks can also be used for runners, because they have a dense fine-grained structure and are not easy to cause bubbles in the glass liquid.
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