Refractory prefabricated parts have the advantages of convenient on-site construction, fast and long service life, and they are more and more widely used in the field of high-temperature industrial furnaces.
Its production process is relatively simple, basically according to the process of batching, stirring, molding, drying and other processes. However, there are often some problems in the production process. This paper mainly discusses the common problems in production and how to deal with them.
1. Pulverization of impurities in bauxite clinker
Bauxite clinker is one of the commonly used refractory raw materials for refractory materials, and its quality has a great impact on the performance of refractory products. Bauxite clinker, that is, bauxite clinker, is made of bauxite through high temperature calcination, and its Al2O3 content should be greater than 50%. The impurity content in the product shall not exceed 2%, and foreign inclusions such as limestone, loess, high calcium and high iron shall not be mixed. Due to the geological distribution characteristics of bauxite raw ore, it is often associated with limestone, loess, etc. If the selection after calcination is not sufficient, limestone and other impurities will be mixed into the bauxite clinker. In the process of drying, firing or use, due to the pulverization of limestone, local pit defects appear in the product, which not only affects the appearance quality of the product, but also affects the internal quality of the product. Therefore, before the bauxite clinker is used, it is necessary to carry out the inspection of the pulverization rate. After drying at ℃ temperature, after passing through a 3 mm sieve, the weight of the particles on the sieve is called M2, and the pulverization rate can be expressed as:
Powdering rate (%)=(M1-M2)/M1×100%
It is advisable that the pulverization rate should not exceed 0.20%. If the measured pulverization rate is too high, in order to ensure the quality of the product, the batch of raw materials needs to be pretreated, which can be impregnated with water, dried and screened before use.
2. Powdering of brown corundum
Among the unshaped refractories, corundum is used as refractory aggregate and powder, which is used more and more, and has achieved remarkable results. Corundum is generally made of industrial alumina or bauxite after sintering or electrofusion, including white corundum, subwhite corundum, tabular corundum, high alumina corundum, brown corundum, etc. Among them, brown corundum is produced by smelting lightly burning high alumina material, coal and iron filings as the main raw materials. The smelting process is divided into two types: shelling furnace and dumping furnace. The crystallization degree of different parts of the material produced by the shelling furnace is quite different, and the distribution of iron is wider at the same time. The brown corundum produced by the dumping furnace has uniform quality and good body density, but due to the uniform quality, the classification is less, and the comprehensive index may be slightly worse. According to the production practice, the brown fused alumina produced by the shelling furnace is much more likely to be pulverized than that produced by the dumping furnace. If brown corundum with high pulverization rate is used to produce prefabricated parts, after high temperature firing, pulverization and cracking occurs locally on the surface of the product, which not only affects the quality of the product, but also greatly reduces the firing pass rate and increases the production cost. Because the use of brown corundum with high pulverization rate has serious quality problems, it is necessary to test the pulverization rate.
At present, there is no detection method and standard for pulverization rate. The methods used in this paper are as follows:
Qualitative testing: that is, for each batch of brown corundum, a product is prefabricated according to a certain formula. After drying, it is fired at a low temperature of 600 ° C or 1 000 ° C to see if it cracks, so as to judge whether the batch of brown corundum is powdery or not. change.
Quantitative detection: Take a certain particle size sample with a weight of M3, generally with a particle size of 3-1 mm, boil it in water for 60 min in a pressure cooker (or treat it in an electric furnace at 1 000 °C × 1 h), and observe its particles after drying. The change of color and particle size, after passing through a 1 mm screen, the weight of the material on the screen is recorded as M4, and the pulverization rate can be expressed as:
Powdering rate (%)=(M3-M4)/M3×100%
The pulverization rate tested is not more than 0.10% as qualified. For different refractory products, the standard of pulverization rate control can be different.
3. Rising stratification of magnesium-aluminum preforms containing silicon micropowder
In the production process of magnesium-aluminum preforms containing silicon micropowder, the forming surface often rises, resulting in the phenomenon of product delamination, which will seriously affect the service life and yield of refractory products. There are two kinds of SiO2 micropowder: one is made of high-purity silica, and the other is a by-product of the production of metallic silicon or ferrosilicon. The silicon micropowder usually used in refractory materials refers to the latter. It is hollow spherical, active, non-agglomerated, and has good filling properties. It has pozzolanic reaction at room temperature and forms mullite with Al2O3 at high temperature, which is beneficial to the strength of the castable. improve. But it must have stable physical and chemical properties, otherwise it will affect the performance of the product. In the production process of refractory preforms, there are often fluctuations in the molding properties of the products due to the batch replacement of silicon micropowder in the raw materials. Among them, the most obvious performance is the rise and stratification of the formed products.
The method to deal with the problem of rising stratification is: firstly, sieve the used silicon micropowder to homogenize its composition; secondly, in the process of mixing, increase the amount of retarder added, appropriately increase the amount of water added, and at the same time properly. Extend the wet stirring time, and then shape; finally, appropriately reduce the curing temperature of the product, which can basically solve the problem.
4. Flashing of corundum spinel preforms containing aluminum micropowder
In the production of unshaped refractories, -αAl2O3 micropowder is also one of the commonly used refractory powders. -αAl2O3 ultrafine powder is made by calcining industrial alumina. It is characterized by good dispersion, small particles, easy sintering at high temperature and small volume effect. Corundum spinel preforms containing aluminum powder often appear in production. During the curing process after molding, a layer of milky white liquid and honeycomb pits appear on the molding surface, and bubbles overflow from the pits. After removing the liquid on the molding surface, it is found that the molding surface is basically composed of powder, this phenomenon is called “flooding” phenomenon, and the thickness of the powder layer on the molding surface varies with the degree of flashing.
The flooding problem is more obvious in winter, which brings serious hidden dangers to the quality of refractory prefabricated parts, resulting in uneven structure, low strength, reduced thermal shock and erosion resistance, and low service life. After a lot of investigation and analysis, it is found that the flashing water has a certain relationship with the content of metal oxides K2O and Na2O in the raw material aluminum powder. When the content is above 0.2%, the prefabricated parts are formed with the material mixed with this aluminum powder, and there is basically no flashing phenomenon; when the content is below 0.1%, the mixed material is used for the production of prefabricated parts. Flooding occurs, even quite serious.
For the flooding problem, the following methods can be used to alleviate or solve it. ①On the basis of the normal amount of water added, appropriately reduce the proportion of 0.1~0.3%; ②Adjust the addition ratio of retarder and accelerator, appropriately increase the proportion of coagulant, and reduce the proportion of retarder; ③ Properly increase the molding After the curing temperature; ④ When mixing, add a small amount of fused magnesia fine powder, and the addition amount should not exceed 0.5%.
5. High temperature treatment of prefabricated prefabricated hooks
The high temperature treatment of prefabricated parts with embedded hooks is also one of the problems often encountered in the production of refractory prefabricated parts. The high temperature treatment temperature mentioned here refers to 1100°C or higher. Therefore, it cannot be fired directly as usual, but certain protective measures should be taken to avoid the firing and oxidation of the metal hook.
To this end, firstly, the steel bars with the same thickness as the hooks were used for the test, and three sets of schemes were tested: burying carbon; coating the steel bar with anti-oxidation paint; wrapping the steel bar with refractory cotton, and then using the castable as the external anti-oxidation layer .
Firing in a high temperature furnace, the test results: the rebars fired by buried carbon are intact; the rebars coated with anti-oxidation paint are the most severely broken; the rebar section where the castable is used as an external anti-oxidation layer, because the castable appears during the firing process. Partial oxidation occurs due to microcracks, and the thickness of the oxide layer is 1-2 mm.
It can be seen that the carbon burial treatment is the best choice. During the carbon burial treatment, it should be noted that partial or full carbon burial treatment can be carried out according to the structural characteristics of the prefabricated parts.
