When discussing the common defects in the glass manufacturing process, the defects caused by refractory materials cannot be ignored, and the defects caused by the interaction between refractory materials and glass melt are more typical. Due to space limitations, this article only discusses the relationship between glass defects and fused cast AZS blocks from a chemical perspective to explain the chemical mechanism of certain defects.
1. Overview of glass defects
2. AZS refractory materials
3. AZS refractory materials and bubbles
4. AZS refractory materials and nodules
5. Summary
1. Overview of glass defects
Glass defects, such as bubbles, stones, nodules, ribs, tin sticking, tin drops, iridescence, scratches, etc.,
will affect the appearance quality of the glass. In severe cases, the defective rate of glass may increase. The causes of glass defects are relatively complex, that is, they may occur at various stages of glass production. For example, raw material contamination, wrong weighing during batching or uneven mixing of batches, refractory material alteration during the melting process and the presence of unmelted minerals or lumps, tin drops and tin sticking during the float process, scratches during the annealing stage, iridescence during the tempering stage, etc.
♦ The main causes of nodules are the erosion of refractory materials such as fused cast AZS blocks, the melting of hot alkali vapor and material dust, etc., which fall into the glass melt in the form of molten droplets or peeled blocks.
♦ The main cause of ribs is uneven batch materials, or problems with the clarification and homogenization of glass, or the erosion and scouring of refractory materials by glass liquid, and they are constantly carried away by the flowing glass liquid, forming a narrow and long ribbon-shaped inclusion phase.
♦ The main cause of bubbles is the generation of gas by the high-temperature reaction of raw materials, insufficient clarification time, gas products of chemical reactions between refractory materials and glass melt, and gas entrained during the operation process.
To accurately determine the source of glass defects, we need to start from the following aspects:
• first, observe the appearance, shape, position on the glass plate, mineralogical characteristics, i.e. microstructure, chemical composition analysis and determination of defects.
• In addition, we need to consider the following factors from the perspective of process: first, the furnace melting system, actual melting temperature, whether the working conditions of the furnace fluctuate, the type of refractory materials of the furnace, and whether there is erosion or alteration.
AZS refractory materials are A1203, ZrO2, SiO2 refractory materials. They are mainly used as fused cast AZS blocks, sintered zircon mullite bricks, sintered zircon corundum bricks and zircon ramming materials for the furnace side wall and breast wall of the glass furnace. Fused cast AZS blocks are made by melting the raw materials (mainly zircon, recycled AZS blocks, silicon oxide, zirconium oxide, aluminum oxide and soda ash, etc.) at a high temperature of 2200-2400℃ in an electric melting furnace, pouring them into a mold (sand box) made of special sand, and then slowly annealing to eliminate thermal stress. The main phases of fused cast AZS blocks are baddeleyite, corundum and glass phases, and there are also many voids in the products.
Baddeleyite has a high melting point, good chemical stability and strong resistance to glass erosion.
Corundum has a high melting point and is not as good as baddeleyite in resisting glass erosion.
The glass phase is the glass filling between the grains, which mainly plays the role of bonding the two physical phases of baddeleyite and corundum together. The softening temperature of this glass phase is low, and the liquid phase will appear at high temperature and seep out of the block.
⇒ The seepage of the glass phase will destroy the structure of the block and shorten the service life of the block. Bubbles will be released at the same time, and defects such as glass stones, nodules or stripes may occur. The uniformity of fused cast AZS blocks is poor, that is, the size, chemical composition and porosity of the crystals are not evenly distributed in the block body.
3. AZS refractory materials and bubbles
3.1 Physical mechanism of AZS refractory materials and bubble formation
The fused cast AZS block contains a lot of pores inside, and the inside of these pores is filled with gas. The composition of the gas is roughly the same as that of air, that is, mainly nitrogen, oxygen, and a small amount of carbon dioxide gas. As the contact time between fused cast AZS blocks and glass liquid increases, the internal voids will be exposed due to the erosion of the block surface, and the gas filled inside will be released at the same time, forming bubbles.
3.2 AZS refractory and chemical mechanism of bubble formation
The fused cast AZS refractory contains impurity elements that can be oxidized to form gases, such as carbon, sulfur, zirconium carbide, nitrides, nitrogen oxides, etc. When the fused cast AZS refractory block is heated to above 1400℃, these impurity elements will undergo oxidation reaction to form corresponding gases such as nitrogen, carbon monoxide, carbon dioxide, sulfur dioxide, etc.
When the fused cast AZS block is cooled and heated again, oxygen bubbles and secondary bubbles are generally generated at the contact point between the glass liquid and the fused cast AZS block. This is because in general, the fused cast AZS block contains a small amount (<0.3% (mass fraction)) of variable valence elements, such as iron (Fe2+, Fe3+) or titanium (Ti3+, Ti4+). These variable valence elements can change their valence state with the change of temperature, thereby completing the function of absorbing or releasing oxygen, as shown in reaction equation (1).
4Fe2++O2=4Fe3++2O2-。一 (1)
When the temperature decreases, the reaction equation (1) proceeds in the positive direction, which is conducive to the formation of Fe3+. When the temperature increases, the reaction equation (1) proceeds in the opposite direction (reverse reaction), which is conducive to the formation of O2.
Another reason for the generation of oxygen is the electrochemical reaction (battery reaction) between the glass melt and the fused cast AZS refractory block, or the “remote” electrochemical reaction when the melted glass and the fused cast AZS refractory block are not in contact. The alkali metal or alkaline earth metal ions are responsible for the electron transfer. In this battery reaction, the oxygen ions (O2-) in the glass melt are oxidized to oxygen (O2), which is the anode reaction:
202(glass melt)=02(gaseous oxygen in the glass melt)+ 4e(diffused into AZS) (2)
The high-valent iron (Fe3+) or titanium (Ti4+) in the refractory is reduced, which is the cathode reaction;
4Fe3+(AZS)+4e(AZS)=4Fe2+(AZS) (3)
Overall chemical reaction:
4Fe3+(AZS)+202-(glass melt)=4Fe2+(AZS)+02(gaseous oxygen in glass melt) (4)
The reaction rate of this electrochemical reaction is proportional to the difference in the content of alkali metal or alkaline earth metal ions in the glass melt and the fused cast AZS refractory. The glass melt is in contact with the fused cast AZS block. The concentration of alkali metal or alkaline earth metal ions in the glass melt is much higher than that in the refractory. In addition, due to the infiltration of the glass phase, these metal ions Mn+ will diffuse into the refractory. As a result, they are neutralized with the electrons diffused into the AZS refractory to maintain electrical neutrality.
4/n Mn+(glass melt)=4/n Mn+(diffused into AZS)(5)
The process of this battery reaction can be described by Figure 1.
Combining equation (4) and equation (5) gives the overall chemical reaction equation for the battery reaction process shown in Figure 1:
4Fe3+(AZS)+202-(glass melt)+4/n Mn+(glass melt)=4Fe2+ (AZS)+O2,(gaseous oxygen in glass melt)+4/n Mn+ (diffused into AZS) (6)
4. AZS refractory materials and nodules
One of the reasons for the formation of nodules is that the glass phase in the AZS refractory material gradually seeps out and eventually enters the glass melt. The Al2O3 content in the glass phase infiltrated by alkali metals and alkaline earth metals is high, and after entering the glass melt, it is easy to form aluminum-rich nodules. This seepage process is driven by the following three forces:
(1) The effect of gravity
The glass phase in AZS refractory materials has high viscosity. Under the action of gravity, it penetrates from the inside of the refractory materials to the surface and enters the glass liquid. At this time, the glass liquid invades along the gaps by capillary action, thereby completing the role of replacing the glass phase position in the refractory materials. In addition, the infiltration of alkali metal and alkaline earth metal ions accelerates the dissolution of corundum alumina in AZS refractory materials, increases the volume of the glass phase in AZS refractory materials, and is more conducive to the outflow of the glass phase.
(2) Pumping effect caused by volume change accompanying zirconia phase transition
ZrO2 in AZS refractory materials has three crystal forms: monoclinic system at low temperature, with a density of 5.65 g/cm3; tetragonal system at high temperature, with a density of 6.10 g/cm3; and cubic system at higher temperature, with a density of 6.27 g/cm3; the crystal transformation relationship is as follows:
Monoclinic ZrO2⇌Tetragonal ZrO2⇌Cubic ZrO2⇌Liquid
The conversion conditions are 1170℃, 2370℃ and 2715℃ respectively.
It can be seen that during the glass melting stage, the crystal transformation of ZrO2 occurs between monoclinic and tetragonal crystals. The transformation between monoclinic and tetragonal is accompanied by a volume change of 7% to 9%. When heated, the monoclinic crystal transforms into tetragonal crystal, and the volume shrinks; when cooled, the tetragonal crystal transforms into monoclinic crystal, and the volume expands. It is this change between expansion and contraction that forms a pumping effect, pumping the glass phase in the AZS refractory out of the brick.
(3) The role of gas
As mentioned above, the gas formed in the AZS refractory material and on the interface between the refractory and the glass liquid will bring out the glass phase in the refractory due to the discharge of gas. Since the A12O3 content of the glass phase is relatively high, after entering the glass liquid, the viscosity of the glass liquid at the interface of AZS and glass melt increases suddenly. With the convection flow of the glass liquid, this high-viscosity liquid will be mixed into the glass melt. If it does not have time to diffuse before forming, it may eventually produce nodules or ribs.
5. Summary
AZS refractory materials play a key role in the glass production process, but their erosion often leads to glass defects, such as bubbles, nodules, ribs, etc., which seriously affect the quality and production efficiency of glass products. The formation of these defects is closely related to the physical and chemical properties of AZS refractory materials, especially the gas released from its internal pores, the oxidation reaction of impurity elements, the seepage of glass phase, and the electrochemical reaction between glass liquid and glass.
To solve the problem of glass defects caused by erosion of AZS refractory materials, the following measures should be taken:
►Optimize the production process of AZS refractory materials:
by improving the raw material formula, increasing the melting temperature and uniformity, reducing the content of pores and impurity elements, thereby improving the density and chemical stability of AZS blocks.
►Strengthen furnace management:
regularly check the furnace working conditions to ensure the stability of the melting system and avoid excessive temperature fluctuations. At the same time, reasonably control the flow speed and direction of the glass liquid to reduce the scouring and erosion of the refractory materials.
►Select suitable refractory materials:
According to the glass type and furnace conditions, select suitable AZS refractory materials, such as adjusting the ratio of ZrO2, Al2O3 and SiO2 to improve the erosion resistance of refractory materials.
►Use coating protection:
Applying an anti-erosion coating on the surface of AZS refractory materials can effectively reduce its direct contact with glass liquid, thereby extending its service life and reducing the occurrence of defects.
►Regular maintenance and replacement:
Timely maintenance and replacement of severely eroded AZS refractory materials to prevent further detachment and mixing into the glass liquid.
►Strengthen defect analysis and tracking:
By establishing a complete defect analysis system, chemical composition and microstructure analysis of glass defects are carried out, the source of defects is traced, and targeted measures are taken to improve them.
In summary, by comprehensively applying the above measures, the problem of glass defects caused by erosion of AZS refractory materials can be effectively solved, and the quality and production efficiency of glass products can be improved.
Henan SNR Refractory Co., Ltd. has been specializing in the production of fused cast AZS blocks for about 25 years. We use high-quality raw materials and advanced fusion technology to provide customers with high-quality products. From raw material procurement to finished product delivery, every step is strictly quality inspected to ensure that every indicator meets the standards, so you can use it with confidence.
If you have any needs, you can contact me at any time.
Web:www.snr-azs.com
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