Refractory material is the material basis of glass kiln, which has a great influence on the overall efficiency of the kiln, and must be carefully selected. Refractory materials are not only used in large quantities. A 500t/d float glass melting furnace uses about 5000t of refractory materials, and its cost accounts for the main proportion of the entire kiln cost and even the entire factory investment, so it must be selected reasonably. The so-called reasonable, that is, both technical and economic aspects should be “appropriate”.
Damage to refractory materials when used in glass kilns
When refractory materials are used in glass kilns, they will be seriously damaged due to the effects of high temperature, flame, powder, atmosphere, airflow and liquid flow, which greatly affects the service life of the kiln. From the kiln, the use of refractory materials in the kiln starts. Improper operation will also cause great or even serious damage to the refractory materials, so special attention should be paid to it. Several damage scenarios are described below.
The kiln powder, glass liquid and flame gas will erode the refractory material at high temperature. Soda ash, Glauber’s salt, borates, fluorides, and oxides in the batches interact with the surface of the refractory material at high temperatures to form eutectic or loose matter, and continue to refractory by means of the metasomatic reaction of the gap or interface of the refractory material itself. The inside of the brick body penetrates and diffuses, so that the refractory material is gradually dissolved, peeled off, thinned, deteriorated, and recrystallized. The corrosion mechanisms of the above-mentioned various salts and compounds are different, and the corrosion effect of Glauber’s salt is much stronger than that of soda ash.
The erosion effect of the powder on the refractory material is mainly manifested in the erosion of the refractory material by the alkaline vapor evaporated by the powder at high temperature, such as the erosion of the surface of the silica brick, the internal “rat hole”, etc. role, etc. In addition, the fly material of the ultrafine powder in the powder aggregates in the grid of the regenerator, forming nodules and blocking the grid holes. The corrosion effect is intensified with the increase of temperature, and the service life will be shortened by about one year for each increase of the melting temperature by 50~60℃. The front face wall, feeding port, the front space of the melting section, the pool wall, the small furnace, the upper lattice body of the regenerator and other parts will be corroded by the powder.
The corrosion effect of molten glass on refractory material is much smaller than that of powder, and the phase reaction on the interface layer of molten glass and refractory is complicated. The molten glass first dissolves the free SiO2 in the refractory. The dissolution rate of mullite is relatively small, and it aggregates at the interface between the glass liquid and the refractory material. Although the small crystallized mullite is dissolved, the large crystallized mullite even grows during use. After the refractory is eroded, the composition of SiO2 and Al2O3 is added to the melt in contact with it. The melt will diffuse into the rest of the glass. During the diffusion process, the composition of the melt changes, SiO2 and lye increase, and the aggregation of β-Al2O3 crystals occurs at the interface. A stone layer, followed by a β-Al2O3 layer, followed by an uneroded refractory. After the refractory material is dissolved, the viscosity of the glass liquid increases, which promotes the formation of a protective layer that is difficult to move on the surface of the refractory material, and weakens the effect of continued erosion.
The erosion effect of molten glass on refractory materials depends on its physical properties such as viscosity and surface tension. The glass liquid with low viscosity and small surface tension is most likely to infiltrate the refractory material, and it is sucked into the interior from its surface pores, so that the entire refractory material is strongly eroded. High-alkali glass has a lower viscosity, and borosilicate glass has a small surface tension, so their corrosion effect is severe. Increasing the melting temperature reduces the viscosity and surface tension of the molten glass, which also accelerates erosion. The glass liquid containing boric acid, phosphoric acid, fluorine, aluminum and barium compounds has a violent erosive effect on the refractory material. The strong glass liquid convection and unstable liquid level will wash away the protective layer and accelerate the corrosion loss. For the refractory itself, the degree of corrosion is mainly related to its chemical composition, mineral composition and structural state. The structure of general refractory materials is composed of one or more crystal phases, glass phases and gas phases. The pores, especially the open pores, are the channels through which the erosive agent penetrates into the interior of the refractory material and increases the erosion surface. Compared with the crystal phase, the glass phase is the weak link, and its chemical stability is poor. To improve the corrosion resistance of the refractory material, it is necessary to increase the high temperature stable crystal phase, reduce the glass phase content, and have a large softening temperature and viscosity. rate as low as possible. In addition, the crystal phase is also required to be fine and uniformly distributed in the glass phase to form a uniform and dense structure. The uneven surface and cracks of the refractory material will aggravate the erosion. The pool wall bricks at the liquid level and the joints of the pool wall bricks are in places where they are easily damaged by the glass liquid. Smooth, small gun seams, and should be erected in one piece.
The combustion products of gas and heavy oil (containing corrosive gases such as SO2 and V2O5) and the volatiles of individual batching components will also corrode the refractory materials in the flame space, small furnace, and regenerator. Different furnace building materials will react with each other at high temperature, resulting in damage. Such as 1600 ~ 1650 ℃, clay bricks and silica bricks will react violently, high alumina bricks and silica bricks will react moderately, and fused zirconium corundum bricks and silica bricks will react violently and severely eutectic. Fused zirconium corundum bricks have a moderate reaction with quartz bricks and white foam stones, but have contact reactions with corundum bricks. Therefore, corundum brick can be used as a transition material.
The lattice body used in the regenerator is also damaged due to the action of the redox atmosphere. The damage mechanism is mainly that the valence state and coordination state of the variable valence ions in the oxidation and reduction states are different, and the coordination states are different, resulting in volume changes, resulting in reduced product strength and cracking.
Under the action of high temperature for a long time, the refractory material will be damaged by melting (also known as burning flow) or softening and deformation. Some part of the kiln is overheated locally or the refractoriness of the refractory material is not enough. The refractory material is melted. Sometimes, the refractoriness is qualified, but the softening temperature under load is low, and the refractory material will soften and deform during long-term use, which affects the stability and service life of the entire masonry. The severity of burn loss depends on the temperature and the nature of the refractory. The small furnace crater, the small furnace legs, the tongue, the regenerator, the melting kiln and the parapet are the parts that are easily damaged by burning.
Cracking mainly occurs in the kiln stage. When the kiln is baked, a certain temperature difference occurs inside the refractory brick, resulting in corresponding mechanical stress. If the heating rate is too fast and exceeds the allowable ultimate strength of the refractory material, cracks will appear or even break into pieces. Electrofused, highly sintered dense refractories are most vulnerable to breakage. In addition to the stress caused by the temperature difference, the expansion or contraction caused by the change of the crystal form of the refractory material will also cause the stress. When the temperature rises too fast, the crystal form changes rapidly, the volume changes too dramatically, and the stress is too large, which makes the refractory material crack. Therefore, it is necessary to heat up according to the pre-established kiln curve when baking in the kiln. After baking in the kiln, the refractory material is under the action of high temperature for a long time, and the mechanical strength of the refractory material at this operating temperature is much lower than that at room temperature. If the mechanical load acting on the refractory material is too large, the refractory material will undergo inelastic deformation (similar to the flow of a very viscous liquid), resulting in failure.
When the glass liquid flows along the refractory material, it has the effect of dripping water through the stone, grinding the refractory material into grooves, which is mechanical wear. The main wear part is at the glass liquid level. In addition, it can also be clearly seen where the circulating fluid flows (especially where the fluid flow is turbulent). When the liquid level fluctuates and the liquid flow changes (such as affected by temperature fluctuations), the wear is increased.