Process Technology Manufacture of lightweight refractories using fly ash drift beads Li Dezhou 1 Miao Wenming 1 Yin Yanhui 2 (1. Pingdingshan New Type Refractories Co., Ltd.; 2. Henan Fly Ash Research Center, 467000) Description of fly ash beads The process of producing refractory materials aims to increase the promotion and application process of fly ash production refractories.
1 Basic principles Refractory materials are known as lightweight refractory materials with high porosity and low bulk density. They have a porous structure and high thermal insulation properties. Therefore, they are also called insulating refractory materials. Refractory materials consist of a main crystal phase and a matrix. Due to different material structures (such as particle size and distribution, porosity, pore size, and distribution), the thermal conductivity is also different, and thus directly affects the thermal conductivity of the material. Heat is conducted in the main crystal phase and the matrix in a conductive manner, while heat in the pores is conducted in a radiative and convective manner. Because the thermal conductivity of gas is only 1/10 of the solid material, it can effectively prevent the heat transfer of the gas in the pores. The lightweight structure of the lightweight refractory material has a porous structure and a low bulk density, so that it has a heat insulation and energy saving effect.
1.1 Physicochemical properties of fly ash float beads Lightweight refractories produced using fly ash floating beads can effectively overcome the large proportion of refractory materials produced by traditional methods (combustible addition method, foam method, chemical method, etc.) The advantages of low mechanical strength, large change in shape, long production cycle, low production efficiency, and poor thermal insulation performance have good prospects for popularization and application. Hollow drift beads and fly ash are homologous substances, with excellent heat resistance, heat insulation, insulation and other properties, is a multi-purpose material, its interior is full of C2 and the content is 58~85% 15~41% Its physical and chemical properties vary with the type of coal, coal-fired boiler structure, combustion conditions, and combustion methods.
The physical and chemical properties of some fly ash floating beads at home and abroad are shown in Table 1. 1.2 Mineral Composition of Floating Beads In the composition of floating bead minerals, silicate glass phase accounts for 80 Table 1 Physical and chemical properties of fly ash floating beads at home and abroad Composition China, United Kingdom, United Kingdom In Shandong, Guiyang, Shandong Province, Pingdingshan Mountain burning mullite accounted for 10 ~ 15% of other minerals (including quartz variants) accounted for about 1.3 float beads of thermal phase changes from room temperature to 1000 Â°C, float ball changes; 1000 began to appear cristobalite crystals Spheroid shrinkage; crystallization at 1200Â°C is significant; mullite content formed by AI2O3 and Si2 in the 1200 to 1250Â°C 0-inch material tends to be stable; the cristobalite content reaches a high value, and the spheres deform significantly and adhere to each other; Above 1300C, the melting degree of the silicate glass phase in the float bead increases and the body burns too much.
2 Fly Ash Floating Beads The brick floating bricks are made of floating beads and fly ash as the main raw materials, combined with clay, high alumina bauxite, clinker and other raw materials. Its mechanical strength, high temperature resistance, thermal conductivity and performance are all superior to mid-range thermal insulation materials, and even comparable to aluminum silicate fibers. Therefore, it is a new type of good insulation and fireproof material with wide application.
2.1 Production process The production of floating bead bricks adopts semi-dry molding, and the process flow is as follows: Bead drying (iron removal) - Ingredient additive - Mixing - Sleeping material - Forming drying â€» Burning â€» Picking packaging 2.2 Drift beads and The selection of binders 2.2.1 The choice of beads The physical properties of hollow beads have great fluctuations due to different types of coal and methods of operation, which can be used in the classification. Generally speaking, it is more appropriate to use a bulk density of less than 0.4 g/cm3, a smooth surface of the beads, a complete ball ratio of 98% or more, and a low iron content. When used in production, it is also necessary to pay attention to the particle size composition of the float beads, which can affect the molding performance, firing process and performance of the products.
2.2.2 The choice of binders Beads are non-plastic materials, small particles, large surface area, a reasonable choice of binder and determine the amount of one of the key to the production of floating beads.
The choice of binding agent should follow three principles: 1 At room temperature, the binding agent should have good physical adsorption and chemical affinity with the bead surface, and can be evenly dispersed on the surface of the beading body and form a uniform adhesive film; And after drying, the floating beads can still be well bonded together, so that the green body has a certain strength; 2 in the middle temperature range, the binding agent can still be better bonded to the floating beads, so that the strength of the green body is not reduced or there is Increased; or binder can be physicochemical reaction with the surface of the bead, the product still has the ability to bond beads; 3 binding agent is stable at high temperatures, better than the intermediate temperature can be bonded beads, or at high temperatures It can react with the bead surface to produce stable mullite phase with higher strength, which is beneficial to the improvement of product strength or other properties.
Commonly used binding agents include inorganic, organic and composite types: Inorganic binders include fire-resistant clay, water glass, aluminum sulfate, aluminum dihydrogen phosphate, aluminum chloride, etc.; organic binders are paste Fine, carboxymethyl cellulose process technology, phenolic resin, urine aldehyde resin, etc.; composite refers to the binding agent is the use of two or more binders mixed binder.
It needs to be pointed out here that the use of a binding agent generally requires a pretreatment. When the refractory clay is used as the binder, those with high plasticity and high dispersity should be selected and then finely crushed before use. When the binder is used as the binder, dilution should be performed in advance, like dextrin and carboxymethyl. Cellulose and the like are preliminarily hydrolyzed. In addition, according to the technical requirements of lightweight heat-insulating bricks and the amount of beads used in the formulation, different binders should be used in the production and the optimal dosage should be determined.
2.3 Production Process 2.3.1 Formulation When formulating a floating bead brick formula, the amount of bead and binder should be determined according to the technical specifications of the beading, and the proportion of raw meal and clinker in the formula should be noted. When the physical and chemical properties of the beads change, they should be adjusted in time and type. In the case of ingredients, burned or other lightweight aggregates may also be introduced to adjust the bulk density and improve the performance of the product. For example, the introduction of globular burned material in the formulation not only can reduce the specific gravity of the product, but also can improve the reburning line change and the thermal conductivity of the product. Table 2 lists the dosing amount of various beads.
Table 2 The amount of beads to be used for each type of marking product Item Beads 2.3.2 When mixing and semi-dry forming, forced blender should be used. In the order of addition, under normal conditions, the binder and water should be added after the beads and other materials are mixed to ensure the homogeneity of the mixture.
For semi-dry molding, the dry mixing time of the material should be controlled within 3~4min, and the wet mixing time should be 5min. After mixing, the moisture content of the material should be between 13 and 20%. During the production process, it must also be adjusted appropriately depending on the seasonal changes and product specifications.
2.3.3 Trapped material Usually, after the semi-dry mixing, it is difficult for raw materials to reach a uniform moisture content within a short period of time. If formed immediately, the body is easily deformed, affecting the quality of the product. The trapping time is generally 8 hours. The mud surface should be avoided as much as possible during the trapping time. The trapped material can also make the clay in the mud material fully hydrolyzed, and the plasticity of the mud material is improved, which facilitates molding.
2.3.4 The molding float beads are glassy and have a smooth surface. The material can easily move during molding. The required pressure is not too large. Static pressure of 15~30t is appropriate. Defects often occur during molding. In this case, immediate treatment is required to prevent the defective product from entering the next process. For example, some alumina fine powder may be coated on the mold when a sticking die occurs, and two pressurizations or static additions may be used to solve the layering. In addition, the common defects include burning and loss, and the phenomenon of spalling due to elastic after-effects. At this time, the moisture content of the material technology can be appropriately increased.
2.3.5 The body of dried hollow bead bricks has a fine-grained structure, water is relatively easy to discharge, and the semi-dry method is adopted. The moisture content in the body is not high, so the quick-drying method can be adopted to improve the production efficiency. The drying of the body can be done in a drying chamber or a tunnel dryer. The heat required for the dryer is provided by the exhaust gas from the kiln. When a tunnel dryer is used, the air and the green body should be moved countercurrently to allow the temperature of the green body to rise steadily to maintain the dense structure of the green body. If the drying chamber is used for drying, the blanks should be placed on the blanks with gaps between them so that the drying medium can flow smoothly and the moisture can be quickly discharged. In general experience, the temperature of the drying medium does not exceed 150Â°C. When the moisture content of the dried green body does not exceed 4%, it can be burned into the kiln. However, it is also necessary to prevent the green body from being excessively dry because the excessively dried green body is brittle and is not conducive to handling.
2.3.6 Firing and firing is the key to the quality control of floating bead bricks. The firing temperature of the floating bead brick depends on the mineral composition of the bead and other raw materials. If the firing temperature is too low, the brick is not well sintered and the firing temperature is too high, the brick blank shrinks too much, which affects the performance of the product. General firing temperature control ~ 4 hours. In addition to a strict firing system, the firing process should also pay attention to the loading method, otherwise it will lead to a decline in the quality of a large number of products.
3 fly ash floating beads for the manufacture of amorphous refractories 3. Used as thermal insulation fire-resistant mud aggregate fly ash drift beads small particle size, after screening can be used as thermal insulation refractory aggregate slurry, on the one hand can reduce the mud The thermal conductivity, on the other hand, is conducive to slurry construction. The size of light refractory slag is larger than that of heavy refractory slag of the same material. Because the use of fly ash increases the fluidity of the mud, it is easy to spread on the surface of the brick to make the masonry ash full.
3. It is used to make porous clinker aggregate porous clinker. It is made up of a variety of fire-proof powders by adding pore-forming technology into balls or compacts, and then calcined at high temperature. The inside is porous or honeycomb, and the skeleton is strong. . There are many methods for producing porous clinker, and it is a common method to add fly ash and floating beads as a low-melt material. Fly ash floating beads can be directly incorporated to maintain its own hollow spherical shape, and its particle size and amount can be determined according to the requirements of porous clinker performance. For example, in the production of high-strength clay porous clinker, besides refractory powders such as bauxite clinker and soft and hard clay, bead beads with Al23 content greater than 33% should be added, and then water is used for mixing, compacting, and demoulding. Drying and firing. The firing temperature is 1320~1350C for 4~6min, and the flexural strength after firing is about 3.5MPa. The volume density of the finished product is about 1.13~1.17g/cm3. 3.3 It is used for lightweight castables using high-quality aggregates and binders to assist with the float beads. , can be prepared high-quality insulation refractory castables. Fly ash beads are small in size, light in weight, hollow, and hard and thin, which can reduce the bulk density of light-weight fire-resistant castables. Because they have a certain activity, they have a certain contribution to strength. The introduction of fly ash drift beads can improve the workability of lightweight castables and satisfy the construction properties of castables. As a matrix material of refractory castables, fly ash drift beads and lightweight porous aggregates form a relatively uniform gas phase continuous structure, thereby reducing the thermal conductivity of the material. The bulk density of lightweight refractory castables decreased with the increase of beads, but the compressive strength did not change significantly.
4 Factors affecting the performance of the bead product The mechanical strength of the product affects the mechanical strength of the product. The main one is the amount and type of the bead and binder. The compressive strength of the same label product generally increases with the addition of beads and binder.
Changes in the reburning line of the product (the effect of the floating bead on the reburning of the product) The size of the reburn line varies with the performance of the product. The main influencing factors are the burning loss of raw materials and the amount of liquid phase produced under high temperature conditions. The larger the ignition loss, the more the liquid phase, and the greater the shrinkage of the product line. Therefore, it is necessary to select raw materials that have a small burn reduction and low impurity content. In addition, the introduction of appropriate amount of intumescent materials, such as kyanite, quartz, etc., in the formulation can also improve the softness of the product at a certain degree and within a certain range.
Thermal Insulation The lower the thermal conductivity of lightweight insulation, the better the insulation performance. Practice shows that the thermal conductivity is related to the type of raw materials, particle size, porosity and pore size, the lower the bulk density, the lower the thermal conductivity, and the better the thermal insulation performance. The more closed air holes in the product, the lower the thermal conductivity. In the production of low thermal conductivity products, comprehensive consideration should be given to strive for the best.
Conclusion Production of lightweight refractories with fly ash beads, mature technology, production of lightweight refractories using dry ash in the original state is more difficult than production of lightweight refractories using beads, but the basic process is the same, only in the molding, fire-resistant There are special requirements in terms of degree of weight and weight. The use of fly ash floating beads (dry ash) to produce lightweight refractories has outstanding economic and social benefits, and it has a catalytic effect on the technological progress of the kiln industry.
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