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Casting process of zirconium corundum brick for glass kiln
A melting process
1.AZS raw materials
AZS with five kinds of raw materials: alumina, zircon sand, zirconium-rich sand, soda ash, borax.
1) the introduction of alumina generally use industrial alumina, the production requirements of its water content should be less than 0.3%, the loss of the best less than 0.15%, one tonne of AZS-33 fused about 0.62 tons of alumina.
2) Zircon sand is mainly introduced and, but it also contains a small amount of, can make AZS product dyeing, resulting in coarse grain trachoma, but also affect the cracking, so its content should be less than 0.2%, affecting the casting in the cooling process cracking Will also affect the oxidation process of the product, so its content should be limited to less than 0.2%, tons of AZS-33 fused tricks about 0.65 zircon sand.
3) Zirconia, also known as deboned zirconium, AZS in the silica all derived from zircon, the lack of part, supplemented by zirconium-rich sand.
4) soda ash into the sodium oxide, as AZS brick flux, generally used two products, one ton of AZS-33 fused brick about 20kg soda ash.
5) borax is also AZS brick flux, a ton of AZS-33 fused brick about 20kg borax.
The working principle of electric arc furnace
The electric arc furnace is composed of three parts: the bottom of the furnace, the furnace wall, and the lid. Three graphite electrodes are inserted into the furnace lid, and the triangular shape is placed in the center of the three electrodes, and the coke is taken. So that three-phase electrode synchronization down, a coke, triggering three poles at the same time from the arc, after the arc, slowly feeding, to not break the arc principle, and the current does not exceed the rated value is appropriate, the general arc half an hour later, The current will be stable, then enter the control period. The material resistance is very large, the arc reaches the batch material will produce huge heat, in order to bring the material melt.
3 .AZS melting principle
AZS melting process for the purpose of four: the alumina in the batch melting and re-crystallization of corundum; zircon is the decomposition of zirconia after melting and re-crystallization of oblique zircon; with sodium boron as a denaturant, To produce and produce glass products with silica to produce products; the preparation of chemical composition of uniform, low carbon content, with minimal bubbles, and has a certain temperature and good casting performance of the melt in order to cast high-quality products.
3.1 Physical and chemical reactions of the melting process
The liquid crystallizes as corundum when cooled
Which mainly contains zirconium silicate, according to the following decomposition:
The melting point of zircon is 2430 degrees, but in the AZS flux under the action does not need such a high temperature, in short, zirconia from low temperature crystal (hexagonal) to high temperature crystal (tetragonal) into oblique Zircon.
3) soda ash and borax
Soda ash decomposition, borax decomposition, and then with zircon silica and impurities such as iron and titanium flux to generate silicate glass.
4) insufficient sodium oxide
Part of the silicon oxide in the melt produces mullite, which is undesirable for the presence of crystals in the product.
5) Melting process
As the electrode in the continuous combustion, the following reaction occurs:
Part of the silica is reduced by carbon:
In the cold part of the upper part of the furnace, silicon is oxidized to produce silica, precipitated as white cotton flakes.
6) raw materials moisture or mix moisture absorption
The water in the furnace under the action of high temperature decomposition, the resulting hydrogen melting in the melt, so that the melt bubble or not easy to shrink, resulting in casting shrinkage or dense part of a large number of pinholes, many people think that the water will be in the furnace Drying, but the actual observation found that the material layer in the arc directly into the high temperature melt, no drying stage, the electric arc furnace temperature is very high, the furnace immediately sintered, no drying time, so the charge must be dry and anhydrous to melt Making a dense product.
4. Melting process
Zirconia corundum brick melting process is divided into two kinds of reduction and oxidation
4.1 reduction method
Reduction method, also known as submerged arc method, is the graphite electrode buried in the charge, the electrode in the absence of oxygen in the atmosphere of combustion, a series of reduction reactions occur, so that some of the high-priced oxide in the low-cost Unstable state, while the carbon also penetrate the melt, which will eventually make the melt color darker, if the arc is very short or the electrode in a semi-submerged state, part of the arc exposed to reduce the degree of melt, but also belong to The scope of the reduction method.
4.2 Oxidation method
The charge in the circuit is not carburized or fused carburized and decarburized before casting. This process, which ultimately makes the melt free of carbon, is called oxidation. Because the reduction method makes the presence of carbon in the zirconia corundum brick, when used in the glass kiln, the high temperature carbon combustion produces the gas to soften the glass phase to accelerate the erosion of the glass to the brick, so the presence of carbon on the zirconia corundum brick Of the corrosion resistance is unfavorable, and now more use of oxidation instead of reduction method, zirconium corundum brick in the main source of carbon graphite is not burning in the discharge and with the arc into the furnace, so to find ways to prevent the furnace into the furnace The The following are several commonly used methods of carbon production:
4.2.1 long arc method:
That is, the electrode is not immersed in the furnace melt, thereby eliminating the transfer of carbon between the electrode and the melt. This method is to make the length of the arc enough to make the carbon before the melt reaches the total oxidation of combustion, in the form of carbon dioxide or carbon monoxide volatilization, the arc length of about 50 mm is long arc melting. The melting method using the argon protection electrode also belongs to the category of long arcs because the arc itself becomes an ionized argon to inhibit the oxidation of carbon or immersion in the melt. So the electrode consumption is also very low.
4.2.2 Oxygenation method:
The same form a lot, but are in the reduction of melting after the end of the oxygen into the melt. Some of the oxygen lance inserted from the top of the furnace, and some of the oxygen lance set at the mouth of the furnace, so that the furnace forward to the melt submerged oxygen lance for oxygen, the melt of carbon in contact with oxygen and combustion escape The
4.2.3 Addition of oxidant method:
In the ingredients to add some oxygen-enriched raw materials, he can release oxygen in the melting process, so that the surface of the molten carbon burning volatilization, but isolated to do so is not enough, usually first with the arc melting, in the refining period with Arc melting, in order to increase the length and stability of the arc can be added to some of the ionization of the material to the furnace air ionization, as the ionization of sodium carbonate and oxidants, together in the late melting. As the melt in the arc under the stirring of the non-stop rotation, so the melting of the furnace will be a certain degree of purification.
Second, the production of sand
1. Raw material for sand preparation
As the AZS brick casting temperature of 1800 degrees or so, so the mold can be used sand casting, sand raw materials used natural sand and quartz sand, different particle size gradation to enhance the strength of sand board, quartz sand with fine sand and surface Sand two kinds of silicon content in more than 99%, each piece of sand casting surface to map a certain thickness of the surface sand, to prevent impurities pollution mold. There are many kinds of sand-type binder, the general choice of water glass, because the water glass source of stable quality, cheap, easy to wash, no smell, and water glass high-purity silicon sand produced by the following technical requirements:
1.1 has a better permeability
The gas precipitated from the zirconia corundum brick melt can penetrate from the gap of the silica sand particles.
1.2 has a good thermal shock resistance
The thermal impact of the zirconia corundum brick during casting can vary the temperature of the inner and outer walls of the sand, but the sand is relatively stable under heating conditions because its refractoriness is not less than 1690 degrees and it is between 20-450 degrees Linear expansion coefficient of about 0.03mm / degrees, so he can withstand high-temperature melt flow of static pressure without rupture.
1.3 has good thermomechanical properties
Sand strength after heating reduced, the soft test that about 320 degrees began to soften, 400 degrees began to destroy.
1.4 zirconium corundum brick surface does not produce chemical sticky sand
AZS melt and sand contact after the rapid cooling, hardening, sand surface melting into a thin layer of sodium silicate or silica sand, and AZS does not react. The amount of water glass added by the experiment proved to be the most suitable for 5.7%, less to reduce the sand of the refractoriness, and more can not be formed.
2. Sand production process
Produced a relatively large wet sand type in the furnace into the process of handling often cracks, in order to make it have a certain intensity, often in the sand on the insert a few holes, and then to the inside filled with carbon dioxide,
Silicic acid solubility is small, the formation of precipitation, so wet sand type in a short time with a certain strength.
Made a good sand to be placed in the resistance furnace drying, so that it has a certain strength to prepare for casting use, drying temperature limit of about 400 degrees.
After the batch has been subjected to the casting standard after the melting and refining stages in the electric arc furnace, the operation of pouring the molten material directly from the furnace into the mold is called casting. Although this process is short, but every step is related to the quality of the final product, is a complex process stage. Here only about China's casting materials commonly used in several ways:
1. The method of casting
1.1 Common casting method (code: China PT, Corning, Asahi, Toshiba RC, Xipu RN): Casting with ordinary riser casting, and in the hot state to remove the riser, the section is divided into two parts, part of the first curing , The crystal fine, the area of casting thickness of 40% -50%, the other part of the curing, there is shrinkage and coarse crystallization. In this way casting the brick price is low, mostly used for the upper structure of the kiln, clarify the wall of the pool.
1.2 tilting casting method (code: China QX, Corning TA, Asahi TC, Toshiba TCL, Xipu RO): tilting casting method is cast before the mold caused by an angle, and the riser into the mold at one end of casting , So that both in the T Department to get a dense area, but also the use of ordinary mold in the T direction to get a higher accuracy. So when using this casting masonry wall, you can use its height.
1.3 No shrinkage casting (code: China WS, Corning VF, Asahi VF, Toshiba DCL, casting, shrinkage will be concentrated in a certain area, after annealing with a diamond saw he removed, the remaining useful part of the composition of uniform, , The average density of the density close to the theoretical density; the other is cutting the cast leg method: from the reduction of cutting area, casting cast into "L" type, so that the vast majority of shrinkage concentrated in the "L" smaller legs , Which accounts for 60% of the total volume of the casting, and the entire casting is buried in the insulation during the annealing and remains tilted to promote the shrinkage to the legs. This process is expensive due to the high cost of cutting - diamond saws are generally Higher than the price of the casting body, only used in individual cases.
2. Characteristics of the casting process
Casting process, the quality of the casting has a great impact, not only affect the integrity of the casting shape, but also directly affect the quality of the internal casting. The characteristics of the casting process are as follows:
1) During the casting process, the intense heat exchange process and the chemical reaction process between the melt and the mold are carried out. Casting temperature of the melt is very high, and the mold has a large temperature difference between the casting process, the melt is constantly cooling, the temperature is reduced, while the mold is heated, the composition of the material decomposition of gasification, and melt To carry out certain chemical reactions, so that increased pressure within the cavity, the filling negative. Serious when there will be up type, is the casting hole layer or casting defects and other defects.
2) melt casting process is unstable process, casting the impact of the flow of shares and the flow of uneven, etc., will cause a serious casting package, cold separation, casting mouth shell and other defects.
3) The process of filling the melt is similar to the filling of the porous container. Because the sand wall has a certain permeability, if the pressure in the cavity is lower than the pressure in the cavity, the melt will inhale the air caused by the outside air, Conversely, the melt will be pressed into the ply of the wall, causing severe sand.
4) The length of the casting process has a significant effect on the temperature distribution of the casting.
3. Casting process
The casting process includes casting temperature, casting speed, casting time and pouring
3.1 casting temperature
The casting temperature is the temperature at which the melt is poured into the mold, and the temperature of the stream near the mouth of the furnace is generally measured with an optical pyrometer. AZS brick to strengthen the melting, the casting temperature of up to 1820-1840 degrees. The viscosity of the melt depends on the chemical composition and temperature of the melt, and the composition of the melt is determined by the formulation, so the temperature plays an important role, the higher the melt temperature, the lower the viscosity, and thus the better fluidity, The stronger the ability.
However, if the casting temperature is too high, if the casting temperature is too high, the temperature difference between the casting and the model is reduced, and the width of the solidification zone increases from the table to the inside. In the contraction of the stress increases at the same time, the initial grain coarsening, component segregation, the core of the final part of the casting when the final solidification is likely to produce hot cracking, sophomore castings even more so. Therefore, according to the size and shape of castings to specify a casting temperature limit, to prevent cracking, but also to provide a lower limit to prevent filling capacity is insufficient.
3.2 casting speed and casting time
The casting speed determines the casting time. No casting has the best casting time, improper casting time will gradually produce a lot of defects. If the casting speed is too fast, the stream is thick, the flow rate is fast, the impact force on the mold is large, and a part of the mold is broken or melted, and the part is cast. In addition, when the coarse melt is quickly poured into the mold, a portion of the gas is brought into the mold and quickly rises to the top cover of the model. At this time, the melt contacting the cap has been formed into a thin shell, filled with gas under the shell, forming a so-called shell. At the same time, the incoming gas is also easy to form bubbles in the mold. In addition to the gas, the high-speed pouring of the coarse stream may also be the mouth of the raw material into the melt, the formation of inclusions in the melt. On the contrary, if the casting speed is too slow, but also produce such as corners loose, knots, sand and pouring and other defects. When the pouring speed is slow, the flow is very fine, the first pouring into the model of the melt solidified into a small ball, filling to the corners, resulting in corners loose. If the first poured melt has solidified into a thin shell, it shrinks inwardly. The post-pouring melt enters into the gap between the shell and the model to form surface scars. At the same time, if the stream is too thin, the melt has solidified for reaching the corners, resulting in insufficient pouring. Moreover, due to the casting time is too long, mold cover baking time is too long, easy to peel off into the melt caused by sand.
Casting casting is completed, cold shrink after a period of time, in its appear in the shrinkage and then cast the melt, the operation of the process to become pouring. Usually small brick solidification quickly, can not pour, medium-sized brick can be poured, the interval is shorter, only large brick pouring time is longer. Pouring is one of the effective means to reduce the casting shrinkage and increase the bulk density. It is actually equivalent to enlarging the volume of the riser. The key to the operation is to control the optimum time of pouring. Achieving continuous multiple pouring is an important way to increase product bulk density.
4. The relationship between casting and stamping in the casting
It is common for ordinary cast AZS castings to have shrinkage and shrinkage, but it is often found that there are many pores present, and it is clear that the presence of any stomata will directly reduce the quality of the castings.