Glass melting furnaces are thermal equipment for melting glass made of refractory materials. When selecting refractory materials for glass furnaces, the type of furnace, the use position, the damage mechanism, the type of glass to be melted, and the type of pigment should be considered. Refractory materials for glass furnaces should first be able to resist the erosion of glass melt and gaseous agents, and should have a sufficiently high load softening temperature and thermal stability.
It is usually hoped that the refractory materials in various parts of the furnace will not have local premature corrosion to ensure that the entire furnace has a sufficiently long service life. However, due to the different mechanical, physical and chemical conditions in various parts of the glass furnace, this requires the performance of the refractory materials used to be adapted to them, and at the same time requires that they do not have an adverse effect on other adjacent types of bricks.
The following analyzes the considerations for selecting refractory bricks for glass furnaces from multiple aspects.
1. Clay bricks
Clay products are acidic refractory products, and their acidity increases with the increase of SiO2 content. It has a certain resistance to acidic erosion, but poor resistance to alkaline erosion. There are different types of clay bricks, such as low-porosity clay bricks, ultra-low-porosity clay bricks, low creep, low porosity, low iron clay bricks, etc. Therefore, clay products are suitable for use as refractory materials for acid kilns. Large clay bricks for glass kilns are clay refractory bricks with a unit weight of not less than 50kg used for building glass kilns.
The use conditions of clay bricks are different due to different molding methods and different aggregates and binders of clay bricks. According to these characteristics, different clay bricks can be selected in different parts of the pool furnace due to different erosion conditions. The lower grate arch and the lower checker bricks of the regenerator are less eroded by the dust and volatiles of the batch, the temperature is also lower, and the load is heavy, so this type of brick requires high mechanical strength. Therefore, the binder in the brick can contain an appropriate amount of SiO2 and Fe2O3, but the clay bricks used in high-temperature parts mainly require high refractoriness and less impurities in the binder.
2. Silica bricks
Silica refractory materials are acidic materials. They have strong resistance to acidic slag or solution erosion and good high temperature performance: such as refractoriness of 1690-1730℃, load softening start temperature of 1640-1680℃. The volume expansion rate is large. When the temperature is 1450℃, the total volume expansion is about, which is conducive to ensuring the structural strength and air tightness of the masonry. Therefore, silica bricks are the main material of glass kilns and are widely used in large arches, breast walls, heat storage chamber tops and other parts.
Precautions for silica bricks when baking in kilns: When silica bricks are at 200-300℃ and 573℃, the volume suddenly expands due to crystal transformation. Therefore, when baking, do not heat up too quickly below 600℃. When cooling to below 600℃, avoid drastic temperature changes and try not to contact with alkaline substances.
3. Fused zirconium corundum bricks
ZA series oxidation fused zirconium corundum products are made of high-quality purified raw materials, using a special arc furnace, and a long arc melting and oxidation treatment casting process. This electric melting casting process has almost no carbon pollution from the electrode.
Due to the use of this casting process and the use of high-purity raw materials, the ZA series oxidation fused zirconium corundum products have special, high resistance to glass liquid erosion, and the pollution to the glass is also minimal.
AZS-33 products are particularly superior in preventing contamination of glass liquid. It has a very small tendency to cause stones, bubbles and basic glass phase precipitation in the glass. It is suitable for the upper structure of the melting pool, the pool wall bricks and paving bricks of the working pool, the material channel, etc.
AZS-36 is a standard oxidation fused zirconium corundum brick, which has the characteristics of special high resistance to glass liquid erosion and low pollution, and the performance in these two aspects is balanced. It is suitable for the parts of the glass melting furnace that are directly connected to the glass liquid, such as the melting pool wall bricks, paving bricks, and the feeding port.
AZS-41 is the highest-grade oxidation-process fused zirconium corundum brick, which has the highest resistance to glass liquid erosion and outstanding low pollution to glass liquid. It is suitable for parts of glass furnaces that require particularly high erosion resistance, such as all-electric melting furnaces, flow holes, kiln sills, bubbles, bricks at the corners of charging ports, etc.
Properties and performance indicators of fused zirconium corundum bricks - the baddeleyite phase can resist the erosion of glass liquid, and the glass phase can buffer the stress caused by volume changes, which can keep the product airtight. However, the more its quantity, the lower the seepage temperature, the worse the quality of the brick. The seepage of the glass phase will destroy the structure of the brick and affect its life. Bubbles will be released at the same time as the seepage, and stones that are insoluble in glass may be produced, which seriously affects the quality of the glass. The seepage temperature is usually expressed by the foaming index.
4. Fused corundum bricks
The main crystal phase of corundum bricks is corundum, which has excellent properties, such as high melting point, high hardness, and is a neutral mineral. Therefore, the product has high hardness, anti-scouring, wear resistance, and anti-erosion. Fused α-Al2O3 bricks are mainly composed of α-corundum, which is a fine-grained solid structure. It is mainly used in glass furnaces. It is suitable for melting borosilicate glass and opal glass. However, its high-temperature erosion resistance is poor. At a high temperature of 1600℃, its resistance to glass liquid erosion will decrease rapidly, so some borosilicate glass furnaces tend to use fused quartz bricks. The fused α-Al2O3 brick products have a bulk density of 3.15-3.30g/cm3, contain a small amount of glass phase (about 2%), fill the gaps between crystals, and have high purity. Because it does not pollute the glass liquid, and has good resistance to glass liquid erosion and excellent high-temperature wear resistance below 1350℃, it is an ideal refractory material for the clarification part, cooling part, working pool and other parts of the glass melting furnace.
β-Al2O3 is a white product composed of coarse and bright crystals, containing 92%-95% Al2O3, only less than 1% glass phase, and the product structure strength is low due to the loose lattice. The Na2O contained in β-Al2O3 reacts with SiO2 to produce a large volume shrinkage, which may cause cracks in the brick and cause damage. Therefore, it is only suitable for superstructures that do not contain SiO2 dust, such as the superstructure of the working pool, the breast wall near the combustion port, the small furnace mouth and the hanging wall. Because this brick does not react with volatile alkali metal oxides, there is no need to worry about the molten material dripping from the brick surface and contaminating the glass.
5. Magnesium refractory materials
(1) Magnesium materials for glass kiln regenerator
Generally, the top layer of the regenerator grid body uses high-grade magnesia bricks (w(MgO) > 97%); the upper layer uses medium-grade magnesia bricks (w(MgO) is about 95% ~ 96%); the middle layer uses direct-bonded magnesia chrome bricks; the lower layer uses low-porosity clay bricks. The important properties of the grid body are corrosion resistance and creep resistance. To improve creep resistance, it is necessary to use raw materials with high purity, low iron, low pores and large grains, and use high temperature for sufficient sintering.
(2) Configuration of regenerator when using alternative fuels
After using petroleum coke, the refractory materials in the regenerator of the glass kiln are easily corroded, among which the 95 magnesium bricks are the most seriously damaged. A large amount of SiO2 and CaO invade the brick body, destroying the original MgO-M2S bond and forming a continuous CMS-M2S low-melting bond phase. On the other hand, petroleum coke is difficult to ignite and burns slowly. After using petroleum coke, the unburned components enter the regenerator and continue to burn in the regenerator, increasing the temperature of the regenerator. In this way, under the action of high temperature and erosion, the lattice softens and then collapses. Based on the cause of the damage, the glass company increased the amount of 97 magnesium bricks and direct bonded magnesium chrome bricks in the regenerator, replacing the poorer 95 magnesium bricks, and the life of the regenerator was extended from 1 year to 3 years. The experiment using petroleum coke has achieved initial success. In order to meet the demand for alternative fuels, it is also necessary to develop high-purity magnesium-aluminum spinel materials with good corrosion resistance and adaptability to oxidation-reduction atmosphere changes. High-purity magnesium-aluminum spinel bricks have good creep resistance and alkali vapor corrosion resistance, and can also be used in all-oxygen combustion glass melting furnaces.
6. Other refractory materials
Zircon decomposes in large quantities when heated at 1680℃, and zircon products also have good corrosion resistance. Zircon bricks have excellent properties such as high temperature resistance, good thermal shock stability and resistance to glass liquid corrosion. Aluminum-zirconium-silicon (AZS) cast bricks and fired bricks have good resistance to glass liquid corrosion and can be used for the pool wall and upper structure of glass melting tank furnaces.
7. Insulating refractory materials
In order to save energy, more and more glass furnaces are equipped with insulation structures. These insulation materials include clay insulation refractory bricks, silica insulation refractory bricks, diatomaceous earth bricks and insulation ramming materials.
So what are the performance indicators that should be considered first when selecting and purchasing refractory materials for glass melting furnaces?
(1) Organization structure
Refractory materials are heterogeneous bodies composed of two parts: solid phase (including crystal and glass phase) and pores. The macroscopic organizational structure characteristics of the product are the main factors affecting its high-temperature performance.
① Porosity
In refractory materials, there are often many pores of different sizes and shapes. Those that are connected to the atmosphere are called open pores (also called visible pores), those that penetrate each other are called connected pores, and those that are not connected to the atmosphere are called closed pores. Porosity is usually used to reflect the density of refractory materials, that is, the percentage of pore volume in the brick to the total volume.
Porosity is a basic technical indicator of refractory materials, which affects almost all properties of refractory products. The smaller the porosity, the better the corrosion resistance of the refractory material and the higher the structural strength. The higher the porosity, the lower the thermal conductivity of the material.
② Bulk density
Bulk density is the mass of a refractory material per unit volume (total volume including pores). It directly reflects the density of refractory products and is an important indicator for measuring the quality level of refractory materials and dense refractory products. Generally speaking, the higher the bulk density, the smaller the porosity, and the better the series of properties such as strength and high temperature load softening temperature.
③ True density
True density refers to the ratio of the mass of a porous material to its true volume (excluding pores). True volume refers to the volume of solid material in a porous body. Its value has nothing to do with density and porosity, but is related to the chemical mineral composition of the refractory material.
(2) Thermal properties
① Thermal expansion
When a refractory material is heated, the property that its volume expands with the increase in temperature is called thermal expansion. It is usually expressed by the linear expansion coefficient or volume expansion coefficient.
When using refractory materials, their thermal expansion must be taken very seriously. Expansion joints should be reserved in the melting furnace structure according to the characteristics of the thermal expansion of the refractory materials. When igniting the kiln, a reasonable heating curve should be formulated according to the thermal expansion of the refractory materials and necessary measures such as adjusting the tie rods should be adopted.
② Thermal conductivity
Thermal conductivity indicates the thermal conductivity of the refractory material, which is expressed by thermal conductivity. The so-called thermal conductivity refers to the heat flow rate through the unit area of the material under the unit temperature gradient. It mainly depends on the chemical composition and organizational structure of the material.
③ Heat capacity
Thermal capacity, also known as specific heat capacity, refers to the heat required to heat 1kg of refractory material to raise its temperature by 1°C under normal pressure. It is of great significance in designing and controlling the heating and heat storage capacity of the melting furnace.
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