ã€China Aluminum Industry Net】 Abstract: This paper introduces the basic situation of spray paint, expounds the construction method of spray paint and the matters needing attention.
Keywords: spray coating, bauxite, dry, semi-dry, wet, flame spray
Foreword:
With the development of the refractory industry and the progress of the society, some of the refractory materials with large working strength and slow construction speed have gradually been replaced, and the amount of unshaped refractory materials in the metallurgical industry is increasing. The larger amount of unshaped refractories is the castable, followed by the spray coating. Spray paints are widely used in kilns and other thermal equipment. They can be used to spray new liners, and can also be used to repair linings. It can be used for constructing and repairing linings and coating in cold state, and is more suitable for repairing linings in hot state.
The spray coating developed and produced by Hebi Zhongsheng Kiln and Kiln Engineering Co., Ltd. solved the problem that ordinary construction methods of refractory materials could not be operated in complex or heterogeneous parts. In addition, the spray coating construction does not require a supporting template, and an anchor may be directly set on the sprayed surface for construction or spraying on the surface of the refractory material. From the above, it can be seen that spray coating is an effective technical measure for accelerating the construction schedule, shortening the repair time, prolonging the service life of the kiln, and reducing the consumption of refractory materials. It is an excellent material with promising development.
1 Basic overview of spray coating
Spray coating is an indefinite refractory material that is constructed by mechanical injection using a pneumatic tool. The fire-retardant paint is applied to the pipeline by means of compressed air or mechanical pressure in order to obtain a sufficient speed, and it can be sprayed onto the sprayed surface through the nozzle to form a solid spray coating. The spraying method can be divided into four categories: wet method, dry method, semi-dry method and flame method. According to the state of receiving the material from the sprayed surface, it can be divided into cold material spraying method and molten material spraying method.
Fire-resistant spray coating is basically similar to refractory castables of the same kind. The difference is that the critical grain size of fire-resistant aggregates is relatively small, generally 3 to 5 mm, and the total amount of fire-resistant powders, ultrafine powders and binders is more, generally 35%. 45%. Due to the similar composition of materials, the coagulation hardening mechanism of spraying paint and the physical and chemical changes at high temperature are basically the same. Among the key technologies are adhesion, adhesion, strength and sinterability. These characteristics are not only closely related to the material itself, but more importantly, they are restricted by jet machines and other mechanical equipment and construction process parameters, and are also affected by factors such as the state of the sprayed body and the conditions of use [1].
Spray paint must have the nature of:
(1) It has a certain degree of particle gradation to ensure that the material has a certain degree of fluidity;
(2) The spray coating must have certain plasticity and solidification, so that the material can be well adsorbed on the sprayed layer, and can quickly solidify and have a certain strength;
(3) Control the amount of water added to ensure wetting of the material without causing run-off.
Pay attention to the construction:
(1) Spray pressure and air volume to avoid rebound and shedding;
(2) The distance and angle between the spray gun mouth and the sprayed body to prevent the material from being sprayed onto the sprayed surface with too much force or too little to ensure uniform spraying;
(3) thickness control when spraying, too thick and easy to peel off.
Specific considerations will be discussed in detail later.
2 Factors Affecting the Performance of Bauxite Base Spray Coatings
The bauxite spray coating is made of bauxite as the main raw material, calcium aluminate cement and silicon micropowder as the binding system. Kyanite and andalusite are added according to the situation, and mullite at high temperature is used to offset sintering shrinkage. In order to improve its performance, some additives have been added. In order to promote coagulation hardening, some coagulants and the like are added. In the formula of ZS-CM spray coating of Hebi Zhongsheng Kiln and Kiln Engineering Co., Ltd., the effect of a small amount of additives on the spray coating is analyzed.
2.1 The influence of silicon powder [2]
Studies have found that increasing the fineness of the fine powder in the refractory material can promote the sintering of the product, resulting in a series of excellent properties. Because of its high specific surface area and high surface activity, silicon micropowders bring a series of excellent performances to refractory products and thus attract much attention.
2.1.1 Line shrinkage
Fig.1 Effect of content of micronized powder and heat treatment temperature on the rate of change of sample line
Silicon micropowders have many defects on the surface, and the activation and disorder of the surface particles are more. They have the characteristics of high energy state and large activity, which can promote the sintering process. With the increase of the heat treatment temperature, the silicon micropowder gradually transforms into a liquid phase, which is beneficial to the filling of the pores. Under the effect of surface tension, the distance between the sample particles is drawn closer, so the shrinkage rate of the material increases. After the sample was heat-treated at 1500°C, the linear shrinkage of the sample gradually decreased with the increase of the content of silicon micropowder until the expansion occurred. This is because the silicon micro-powder reacts with corundum to form mullite, and at the same time it will generate volumetric expansion, thus causing the line shrinkage of the sample to decrease until expansion occurs. When the mass fraction of silicon micropowder is 5%, the linear shrinkage of the material after different heat treatment temperatures is not very different, and the shrinkage of the material is very small. If the material shrinkage ratio is too large, it will cause the spray coating to be used during use. Shrinkage caused by shrinkage, so it will reduce the life of the material.
2.1.2 Bulk density
In the low temperature, high temperature, with the increase of the content of silicon fine powder is conducive to the filling of the pores inside the sample, the bulk density has increased. However, when the sample is heat-treated at 1500 °C, the bulk density of the sample exhibits a decreasing variation with the increase of the content of silicon micro-powder. This is because the silicon micro-powder reacts with corundum to form mullite, which also generates volume. Swelling causes loose internal structure of the specimen. With the gradual increase of the content of silicon micro-powder, the amount of mullite formed increases accordingly, and the volume expansion also becomes more and more obvious. As a result, the bulk density of the sample gradually decreases.
2.1.3 Flexural strength and compressive strength
At lower temperatures, the Si-OH bonds formed after hydration of the surface of the silicon microparticles dehydrate and polymerize to form a strong micro-powder network chain structure bonded by Si—O—Si bonds. The long-chain reaction of the micropowder is as follows:
SiO2-Si-OH+HO-Si-SiO2→SiO2-Si-O-Si-SiO2+H2O.
With the increase of the mesh chain structure, the binding performance of the silica gel is stronger, so the bending strength and compressive strength of the sample increase correspondingly with the increase of the content of the silicon micropowder at a low temperature of 110°C. In the high temperature of 1000, 1300 °C, the silicon micro-powder reacted with corundum to form mullite, and the strength increased. Therefore, as the content of silicon fine powder increases, the flexural strength and compressive strength of the sample also increase accordingly. After the specimens were heat-treated at 1500 °C, the flexural strength and compressive strength of the specimens first decreased and then increased as the content of silicon micro-powder increased. This is because there is a liquid phase that causes the material to melt, so its strength is relatively large. After that, as the content of silicon micropowder increases, the amount of mullite conversion also increases accordingly, so the flexural strength and compressive strength of the sample also increase accordingly.
2.1.4 Thermal expansion coefficient
Under the same temperature condition, the coefficient of thermal expansion of the bauxite-based spray coating decreases with the increase of the content of silicon micropowder.
2.1.5 Conclusion
(1) In this experiment, the mass fraction of the preferred silicon micropowder for preparing the bauxite-based spray coating is 5%;
(2) After the drying at 110°C, the flexural strength and compressive strength of the bauxite-based sprayed coating increase with the increase of the content of silicon micropowder after heat treatment at 1000 and 1300°C;
(3) The coefficient of thermal expansion of the bauxite-based spray coating decreases with the increase of the content of silicon micropowder.
2.2 Effect of sodium aluminate [3]
Spray paint construction requires better working time and faster hardening. If spray paint does not harden for a long time after it is sprayed on the lining, spray paint will fall down. In this case, Requires that the spray paint be hardened within a relatively short period of time after construction to prevent the material from falling due to hardening.
In the experiment, samples with addition of 0.1% and 0.2% sodium aluminate were compared with those without sodium aluminate.
2.2.1 Coagulation hardening
Through the experimental results of Hebi Zhongsheng Kiln and Kiln Engineering Co., Ltd., it has been proved that at room temperature, the sprayed paint without sodium aluminate has a longer hardening time, and the coagulation hardening time of the sample added with sodium aluminate is significantly shortened. Therefore, it is added in the spray coating. Sodium aluminate can play a role in accelerating the hardening of the spray coating. The shortening of the hardening time leads to a corresponding shortening of the working time, which is detrimental to the finishing work after the spray paint is applied, and the addition of excess sodium aluminate also reduces the compressive strength of the material. Therefore, considering the effect of different sodium aluminate additions on spray coating working time, hardening time, and compressive strength of health, it can be seen that the preferred addition amount of sodium aluminate under this experimental condition is 0.1%. Its mechanism of action is:
CaO·Al2O3+ H2O→CaO·Al2O3·10 H2O (hexagonal) (below 20°C～22°C),
CaO·Al2O3+ H2O→2CaO·Al2O3·8 H2O (hexagonal) + Al2O3·3 H2O (>25°C) →
3CaO·Al2O3·6 H2O (hexagonal) + Al2O3·3H2O (35°C～45°C).
The sodium aluminate, a coagulant, is added to the spray coating to accelerate the precipitation of calcium aluminate, calcium aluminate, and calcium aluminates in the cement component into the solution, thus accelerating the hydration reaction of the cement and allowing the cement to harden faster. Thus, the spray paint shortens the hardening time and also reduces the working time.
2.2.2 Bulk Density and Line Shrinkage
Sodium aluminate is a kind of low-melting-point salt substance. Adding it to the spray coating can promote the sintering of the material at a high temperature, resulting in the continuous reduction of the pores and the increase of densification. As a result, the bulk density of the sample increases. At the same time, during the sintering process, a liquid phase is generated inside the sample. Under the effect of surface tension, the distance between the particles of the sample is drawn. Therefore, as the content of sodium aluminate increases, the linear shrinkage of the sample gradually increases. .
2.2.3 Flexural strength and compressive strength
After the sample was dried at 110 °C, the flexural strength of the sample decreased with the increase of the sodium aluminate mass fraction. After heat treatment at 1000 °C, the flexural strength of the sample increased with the increase of sodium aluminate mass fraction. Large; After heat treatment at 1300°C, the flexural strength of the specimen does not change significantly with the increase of sodium aluminate mass fraction; after heat treatment at 1500°C, the flexural strength of the specimen increases with the increase of sodium aluminate mass fraction. Big. After the sample was dried at 110 °C and heat-treated at 1000 °C and 1300 °C, the compressive strength of the sample decreased with the increase of the sodium aluminate mass fraction. After heat treatment at 1500 °C, the compressive strength of the sample followed. With the increase of sodium aluminate mass fraction. It can be seen that the addition of sodium aluminate in the spray coating will have a certain influence on the flexural strength and compressive strength of the material after low temperature drying, but it will have an increased effect on the flexural strength of the material at medium, high and medium temperatures. At the same time, the compressive strength of medium and high temperature in the material is affected. Although the flexural strength and compressive strength at high temperature of 1500°C have played an increasing role, due to the melting phenomenon of the sample, the surface of the sample has cracks. Therefore, the use temperature of the material is reduced.
2.2.4 Conclusion
(1) The preferred amount of sodium aluminate is W(Na2O·Al2O3)=0.1% under the experimental conditions of Hebi Zhongsheng Kiln Engineering Co., Ltd.
(2) Adding sodium aluminate to the spray coating will reduce the spray paint application temperature.
3 spray coating construction technology [4-5]
In the early days, there were dry and wet methods for spraying. One of the advantages of dry spray has been widely used. However, dry spraying has disadvantages such as large dust, relatively large amount of springback, poor performance of the construction body, and so on. Therefore, a semi-dry spray technique has been developed to obtain a low-moisture and high-filling construction body. Because it has no dust and high coating adhesion, it has been rapidly applied.
One of the purposes of the development of semi-dry spray coating is to avoid wet mixed premixing operations. Due to the obvious advantages of using wet methods, the current interest in technology has turned to wet spraying.
Later, flame spraying was developed. The flame spraying has a very good effect compared with the previous wet spraying. The flame sprayed layer is firmly bonded to the lining of the furnace, dense in structure, high in refractoriness, strong in erosion resistance, and can significantly increase the service life and reduce the cost of building furnace materials.
3.1 Dry spraying
The dry material enters into the rotating fabric bucket from the silo. The cloth bucket of good material is rotated at a certain angle. The upper mouth is connected with the compressed air channel. The material is transported through the pipeline to the nozzle and meets with the water. The material is in the nozzle. Mixed with water and sprayed on the sprayed surface.
Figure II: Structure of wet and dry spray equipment and process
When using dry spray, pay attention to the following matters.
(1) The amount of water should be appropriate. Too little material can't be wetted well, and dry material can easily be rebounded. If the amount of water added is too large, the coating can easily flow, and the amount of adsorption can also be reduced.
(2) The air pressure and air volume to be sprayed should be appropriate. Excessive particles will have excessive impact on the sprayed surface, easily rebound, too small, and the adhesion force will be insufficient and easy to fall off.
(3) The distance between the spray gun mouth and the sprayed surface should be appropriate, so as to prevent the material from being sprayed onto the sprayed surface with too much or too little force. The spray gun moves up and down to ensure uniform thickness.
(4) The thickness of each spray should not be too thick, thick and easy to peel off, not more than 50mm.
(5) Control the plasticity and coagulability of the material, so that the material can be well adsorbed on the sprayed layer, and can be solidified relatively quickly to obtain a certain strength.
3.2 wet coating
Hebi Zhongsheng Furnace Works Co., Ltd. independently developed wet spraying is the use of a pump with good fluidity through the pipe to the nozzle, in the nozzle by the high-pressure air jet to the working lining method. The process flow is basically similar to the dry process. The main difference is that the fire-retardant paint is pre-mixed into a slurry for spraying. The process includes four main phases: mixing, pumping, spraying, and solidification. The mixing and pumping process is not very different from ordinary castables and pump feeds, requiring uniform mixing and good pumping performance. It is characterized by easy pulping, small dust during spraying, and easy operation. However, due to the large amount of water, it is easy to flow and there are more pores in the spray coating. For this purpose, thin-spray and ground-spray methods can be used to make up the furnace and good results can be obtained. Wet spray can be used directly for lining.
Figure 3: Schematic diagram of wet spray equipment
Wet Spraying Notes:
(1) Composition of spray material. First, it should have a reasonable particle size composition, the ratio of aggregate and matrix, and moisture content. With proper adhesion of the matrix portion to the surface of the particles, the adhesion layer should not be too thick and too thin to ensure good plasticity and adhesion to the material layer when the particles are sprayed onto the material layer. Second, additives should be selected, especially the type and amount of flocculants to control the setting time. The commonly used flocculants include sodium aluminate, sodium silicate, polyaluminum chloride, calcium chloride, aluminum sulfate, potassium aluminum sulfate, and the like.
(2) Injection pressure and jet air velocity. When they are too small, the particles do not adhere well to the material. If they are too large, they tend to rebound.
(3) The distance and angle between the spray gun and the body being sprayed. They have a certain influence on the adhesion rate of the material layer.
3.3 Semi-dry coating
The semi-dry method is a method of transporting a powder containing a portion of moisture and adding the remaining moisture to the nozzle portion. The semi-dry method combines the advantages of dry and wet methods, weakening the disadvantages of both. In spite of this, the requirements for the semi-dry spray coating and the precautions for construction are basically the same as the dry method.
3.4 flame spraying
Flame spraying does not add moisture, but is mixed with flammable materials, flammable gases and oxygen, and the burning material burns and heats during the spraying process. Some of the materials will be in a molten state and will come into contact with a working surface with a rather high temperature. Melted and sintered into a whole.
Flame spraying technology has the following features.
(1) The melting powder has strong adhesion to the brick surface;
(2) The spray complement is dense, high in strength, and has good corrosion resistance;
(3) As the spray wall does not cool down significantly, there is no damage to the brick.
The flame spraying technique is characterized in that the flame spraying layer can be firmly combined with the lining bricks in the repaired area, and the physical properties are good, at least not lower than the fire bricks; the construction can be performed in a short time and at a high temperature. The heat source for flame spraying can use three kinds of fuels: gas, liquid, and solid. Among them, gas fuel is easier to control and high-temperature flame can be obtained.
Flame spray material should have the conditions.
(1) It has good conveying performance. In order to be able to perform the gunning operation smoothly, the size of the refractory powder should be able to give the flame spray material a good flowability, so its cohesion cannot be too great.
(2) has a better melting performance. The residence time of the flame spray material in the flame is very short, only 0.02~0.08s. To make the powder mixture into a normal molten state in such a short time, there is a certain requirement for the size of the material.
(3) should have excellent construction performance. Material with high adhesion rate has less loss.
4 Summary
The use of unshaped refractories is getting larger and wider, with a wider range of uses. Spray coating as a type of unshaped refractories is bound to develop very well. A thorough understanding of the influence of various raw materials, binders, and additives of the bauxite spray coating can be used to determine economically reasonable ingredients based on the resources and specific use conditions. With proper construction methods, careful attention to the details of the construction will also increase the service life of spray paint and reduce the amount of refractory materials used.
references:
[1] Han Xinglu. Unshaped refractories (Second Edition) [M]. Beijing: Metallurgical Industry Press. 2005:414~415
[2] Zhang Hao, Dai Wenyong, Li Liang.Effect of silicon micropowder and heat treatment temperature on the performance of bauxite-based spray coating[J]. China Powder Science and Technology, 2010,16(3)
[3] Zhang Hao, Dai Wenyong. Effect of Sodium Aluminate on the Performance of Bauxite-Brown Corundum Spray Coating[J]. Ningxia Engineering Technology, 2010,9(1)
[4] Li Nan, Gu Huazhi, Zhao Huizhong. Refractories [M]. Beijing: Metallurgical Industry Press. 2010: 340~342
[5] Wang Chengxun, Zhang Yixian. Alkaline unshaped refractories [M]. Beijing: Metallurgical Industry Press. 2002: 30~36
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