SHS TECHNOLOGY FOR DEVELOPMENT OF HIGH-TEMPERATURE THERMAL INSULATION COMPOSITE

Vladimir Alexandrovich Grachev ^1*, Andrey Yevgenievich Rozen ²,Chir Gen Pak ² and Victor Mikhailovich Batrashov ²

¹ A.N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, 119071, Moscow, Leninsky prospect, 31, Bldg 4, Russia

² Penza State University, 440026, Penza, Krasnaya St., 40, Russia

*Corresponding Author:

Vladimir Alexandrovich Grachev
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, 119071, Moscow, Leninsky prospect, 31, Bldg 4, Russia
E-mail: vagrachev@gmail.com

Received Date: 06 April, 2017; Accepted Date: 08 April, 2017

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Abstract

The article describes self-propagating high-temperature synthesis method for obtaining hightemperature thermal insulation composite materials. The authors set a task to reduce the energy costs of obtaining high-temperature thermal insulation materials and the density to improve the energy efficiency of such materials’ application. The article has represented the research, as a result of which an ultra-lightweight high-temperature porous material based on aluminium phosphate was obtained in the mode of self-propagating exothermic synthesis. Such material is suitable for use as a high-temperature heat insulation of industrial and power units, with a purpose of minimizing operating costs by increasing the efficiency of energy resources use.

Key words

High-temperature insulation, Composite materials, Self-propagating high-temperature synthesis, SHS, Solid-phase combustion, Exothermic reaction

Introduction

As The modern economy’s requirements to the energy efficiency of industrial units are high. Hightemperature units used in metallurgy, chemical industry, energetics, and other industries consume large amounts of energy, much of which is heat loss. Therefore, the use of high-temperature thermal insulation helps to reduce production costs, reduce the prime cost of the product and improve the operational characteristics of thermal units (Mamayev and Morev, 2010).

The main criterion for the effectiveness of heat insulation is its heat conductivity. This value is at the lowest for high porosity materials with low density. Therefore, development of new high porosity thermal insulating composites is an important technical challenge.

It should be noted that currently lightweight (density <500 kg/m³), fired thermal insulation materials find widespread use (Khoroshavin, et al., 2000). The need for an extended hard firing is their main drawback leading to an increase in their prime cost and to environmental problems caused by the burning of large amounts of fuel. The need for thermal treatment causes the rigid standardization of the product range, which makes it difficult to install them in an insulated unit.

Therefore, the reduction of energy costs of obtaining high-temperature thermal insulation materials and the reduction of density to improve the energy efficiency of their use are important tasks.

Method

The chemistry of the SHS reaction in hightemperature porous materials (obtaining the matrix of the material)

The obtainment of lightweight and ultra-lightweight materials using the method of self-propagating exothermic synthesis is possible in the case of formation in the course of the reaction of a large volume of gas, which swells up the reaction mixture. The most promising exothermic reaction for producing lightweight and ultralight-weight heat-resistant thermal insulation materials is the interaction between aluminum and orthophosphoric acid (OPA). The hydrogen gas formed during the reaction swells the reaction mixture, and the released heat contributes to water evaporation and solidification of the mixture (Pak and Abyzov, 2009).

The main products of the OPA reaction with aluminum are partially hydrated aluminum phosphates. According to the X-ray phase research, residual unreacted aluminum can also be observed in the reaction products. During the first heating of the obtained material to a desired operating temperature (over 1,500°C), a series of processes, including structural and phase transitions, can be observed (Batrashov, 2013).

When heated to 220°C, crystallization water is released and monosubstituted aluminum orthophosphate is formed, which is evidenced by the appearance and enhancement of reflections of AlPO₄ (cristobalite type) on roentgenograms in this temperature range. The process proceeds according to the following equation:

(1)

With further heating from 270°C to 500°C, the amorphous phase is observed on roentgenograms. Presumably, at this stage there happens the dehydration of monosubstituted orthophosphate to trimetaphosphate through tripolyphosphate, described by the equation:

The greatest amount of aluminium metaphosphate is observed at a temperature of 900°C to 1,000°C. Also, in the range of 560°C to 700°C aluminium phosphide AlP is formed, which decays before reaching 900°C. Also, in the range of about 1,000°C, the residual aluminium is oxidized.

When heated over 1,300°C, stable aluminium orthophosphate of cristobalite type AlPO₄ becomes the dominant component of the system, and aluminium oxide formed previously undergoes transformation into a stable high-temperature modification -α-Al₂O₃ (corundum).

Aluminium orthophosphate of cristobalite type is a high-temperature compound that decomposes into aluminium and phosphorus oxides Al₂O₃ and P2O5 at temperatures exceeding 1,700°C, which allows to continuously use the materials on its base at 1,500°C. Unlike many exothermic synthesis reactions, the reaction between OPA and aluminium occurs at room temperature, which eliminates the need for external initiation. However, ambient temperature plays an important role: heating of system components to 30°C reduces the time before the start of the reaction to uncontrollable values.

Moreover, the reaction product-a porous nonmetallic material-is already a finished product. This means that by laying the mixture in an appropriate form, high-temperature thermal insulation articles of any geometric shape and size can be obtained.

Precursor materials for SHS of porous materials

The precursor components for obtaining exothermically synthesized porous phosphate composite material are particulate aluminum, OPA and OPA-based compositions, heat-resistant fillers and modifying additives.

Particulate aluminum of various brands may be used for the synthesis of phosphate composite material: ASD, POS-15, PA-4 powders, PAP-1 and PAP-2 powders, and etc. Dispersion, morphological features of the aluminum powder and its amount are among the parameters determining the proceeding of the reaction along with the volume and composition of the liquid phase. When reducing the particle size of aluminum, its surface area increases. This leads to an increase in the contact area of the reactants and to a significant acceleration of the solid-phase reaction. At the same time, the swelling and evaporation of water intensifies. An excess in fine particulate aluminum makes the reaction uncontrollable, and the resulting material has an uneven porosity and low physico-mechanical characteristics. Aluminum is the most expensive component of the mixture, and therefore it is impractical to increase the porosity by increasing the aluminum content in the reaction mixture (Batrashov and Pak, 2012).

Another essential component of the reacting system is OPA, which forms the basis of the liquid phase. Various concentrations of OPA are used for the synthesis. The 70% OPA solution with a density of 1.5 g/cm³ has the best properties, which ensure uniform mixing. Phosphate high-temperature materials based only on the OPA solution have poor mechanical properties, so modified OPA (Phosphate binders) is usually introduced into the composition of the liquid phase (Abyzov, 1979).

Phosphate binders include a large group of compositions that represent dispersed systems based on OPA with hydrogen substituted by cations of predominantly two- and trivalent elements. The most widely used and commercially available are the phosphate binders based on aluminum, chromium, boron and magnesium, the most important of which are the alumochromophosphate (ACPB) and alumoborophosphate (ABPB) binders. The introduction of the phosphate binders into the reaction mixture allows to increase the amount of time during which the mixture may be homogenized, contributes to strengthening of the connection between the phases and the structural elements, resulting in increased physical and mechanical characteristics. Upon heating the articles made from porous heatresistant phosphate material produced with the use of phosphate binders, the phase transitions of the components stabilize, which leads to an increase in heat resistance and the high-temperature strength (Batrashov and Pak, 2011).

The parameters of OPA and phosphate binders’ introduction allow to regulate the process of mixing and swelling, as well as the properties of the resulting material in a wide range. The main parameters of the liquid phase are the type of the introduced binder and the mass ratio of the binder and the OPA solution. Also, the volume ratio of liquid phase to the solid mass (L/S) is of a great importance. This value depends on the grain-size composition of the dry components. With a constant grain-size composition, the increase in the liquid phase content leads to the reaction intensification and onset acceleration, which hinders the homogenization and formation of the mixture.

The bulk of dry matter in the mixture are refractory fillers. The main requirement for refractory fillers is their heat resistance, comparable to the heat resistance of aluminum phosphate. Also, they should not have a significant effect on the chemical reaction of the composite material synthesis. Another important parameter of fillers is their grainsize composition: there is an over-expenditure of the liquid phase when the dispersity is high. The problem of over-expenditure of the liquid phase can be solved by preliminary aggregation of fine particles by introducing pure phosphate binder into the dry filler. Since the mechanical properties of the porous heat-resistant phosphate materials are provided by the aluminum phosphate matrix, the requirements for the physical and mechanical properties of the fillers are not significant. However, the materials whose introduction in the filler leads to an increase in the performance of the material are of greatest value.

Oxides of aluminum, boron, chromium (III), and silicon are particularly suitable as filler materials, which get incorporated into the aluminum phosphate matrix and increase the high-temperature strength and the heat resistance of the material. Pure graphite, silicon and other heat-resistant substances and materials may also be used. However, the use of the pure components specifically synthesized for the use as fillers is economically unsound.

Low requirements for the properties of the fillers stipulate the possibility of using various waste products of the metallurgical, ceramic and chemical production. Use of industrial waste allows to reduce the prime cost of the produced material, as well as to partially solve the problem of disposing of unclaimed resources without significant deterioration of the material’s properties. Another source of fillers could be the natural mineral resources with a low cost, such as diatomaceous earth and etc.

Substances, whose introduction enhances the critical characteristics of phosphate material, are used as modifying additives. They can be introduced both into filler and into the liquid phase. The main modifying additives for phosphate materials are oxides of boron and chromium (Abyzov, et al., 2011).

Chromium (III) oxide improves the high-temperature strength and heat resistance of porous phosphate material, bringing the maximum temperature of its long-term operation up to 1,550°C to 1,600°C. A disadvantage of its introduction into the liquid phase in the ACPB form is the high cost of ACPB and toxicity of its production process due to the use of chromium (VI) compounds and formalin in the manufacturing process. Therefore, the most environmentally sound option is the introduction of chromium (III) oxide into the composition of the filler.

Boron oxide also has a stabilizing effect on the phase transformation of the phosphate material when it is heated, without increasing the threshold temperature of its application. A major advantage of boron oxide is its low cost and the possibility of introducing it into the liquid phase as ABPB, the production process of which does not require the use of toxic substances.

Development of structures of composite porous materials of different density

The aim of this work was to obtain porous phosphate high-temperature composite material with a density less than 400 kg/m³. To obtain a material of such density, the intensification of the process of swelling and polarization is required. The reaction rate is affected by the amount of OPA, as well as by the amount and dispersity of aluminum.

Aluminum powder PAP-2, which has a large specific surface area up to 10,000 cm²/g, was used as the particulate aluminum. The diagram of aluminum powder particle distribution is shown in Figure 1.

Figure 1: PAP-2 Aluminium powder particle distribution

The distinguishing feature of aluminum powder is the presence of a paraffin protective layer on the surface of the particles, which prevents the oxidation of aluminum in air. Due to this, the onset of reaction with OPA is delayed, which allows to homogenize the mixture, thus providing a more uniform polarization. Based on preliminary experiments on PAP-2 powder, 6% above the filler mass was taken, which provided that heating of the mixture up to 120°C during the course of the reaction.

Thermal 70% OPA with a density of 1.5 g/cm³ was used as the basis of the liquid phase. ABPB was introduced into the composition of the liquid phase as a modifier. The reason for this was an adequate cost price of ABPB. The density of the resulting liquid phase was 1.6 g/cm³. The ratio of the volume of the liquid phase to the weight of the solid phase was the main variable parameter, which varied in the range of 0.32 l/kg to 0.47 l/kg.

Only industrial waste was introduced into the filler: chamotte dust, silica-graphite waste of different origin, alumochrome waste. The chemical composition and properties of the fillers are shown in Tables 1 and 2.

Table 1: Chemical composition of the fillers

Table 2: Physical and mechanical properties of fillers

Chamotte dust is a byproduct of firing fire clay in the production of chamotte. The main disadvantage of chamotte dust is its excess dispersity: the particle size is less than 50 microns, resulting in an over expenditure of the liquid phase. ABPB that was inscribed in an amount of 10 wt% was used for larger units. The mixture was then stirred mechanically. The result was a powder with a particle size of no less than 250 microns. Aggregate chamotte dust was 40 wt% of the filler.

Fly ash of a thermal power plant, graphite crucible’s pieces and spent graphite electrodes were the silicongraphite waste. The amount of silicon-graphite waste in the filler was 40 wt%.

The main portion of silicon-graphite waste after grinding is represented by particles of size in the range of 50 to 315 microns (Figure 2).

Figure 2: Distribution of waste particles from silicongraphite crucibles of the Novocherkassk Electrode Plant

To identify the phase composition of the silicongraphite waste powder, X-ray diffraction analysis was conducted. The findings are shown in the Figure 3.

Figure 3: The X-ray diffraction pattern of silicon-graphite waste.

The main components of the silicon-graphite waste are graphite, silicon dioxide, silicon carbide, aluminum oxide and mullite, which provides high refractoriness -1,400°Ð¡. In the event of a temperature gradient (during heating or cooling), due to its low strength graphite ensures absorption of the stress occurring in the material, which increases resistance of the material against thermal cycling. The presence of solid inclusion of aluminum and silicon oxides, silicon carbide and mullite in the composition contributes to a significant increase in the durability of the material.

To assess the morphological features of the silicongraphite crucible waste powder’s surface, powders were analyzed using a scanning (raster) electron microscope LEO 14XX (VP) (Figure 4).

Figure 4: Electron microscope images of silicon-graphite crucible waste powder

Silicon-graphite waste powder particles are represented by irregular shape polyhedrons, and therefore the role of the modifying agent is enhanced, as it increases the grain boundary between the modifying agent and matrix of the system. According to preliminary studies, the introduction of 40 wt% silicon-graphite waste into the filler increases the strength and quantity of refractory material’s thermal cycling (material density 370 kg/m³) by a factor of 2 in comparison with the pure aggregated chamotte dust material (Batrashov, et al., 2011).

Alumochrome waste is the spent catalyst IM-2201, consisting of corundum and 10...20 wt% of chromium (III) oxide. Cr₂O₃ is an important modifying agent comprehensively improving the thermomechanical properties of heat-resistant materials based on aluminum phosphate. 20 wt% alumochrome waste was added into the filler. With further increase of the spent catalyst’s content, the growth rate of hightemperature strength and heat resistance remained practically unchanged.

Based on the results of sieve analysis, as can be seen in Figure 5, the bulk of alumochrome waste consists of grains 0.2...0.315 mm (≈ 57%).

Figure 5: Distribution of the spent alumochrome IM-2201catalyst particles

The melting point of alumochrome waste depends mainly on the content of flux and varies in the range 2,050°C to 2,100°C. Mineral (phase) composition of alumochrome waste, determined by X-ray and petrographic analysis (Figure 6), is represented mainly by corundum α-Al₂O₃ and gamma-modification of alumina γ-Al₂O₃, as well as by chromium oxide Cr₂O₃, which is a structural and chemical artificial analogue of a natural mineral eskolaite. A small amount of amorphous silicon dioxide SiO₂ is present.

Figure 6: The X-ray diffraction pattern of IM-2201 powder.

To evaluate the morphological characteristics of the surface of the alumochrome waste powder, powders were examined using scanning (raster) electron microscope LEO 14XX (VP) (Figure 7). Alumochrome waste particles have a rounded shape with minor deviations on the average size. In the process of the exothermic reaction of the precursor components, water vapor and hydrogen are released, which swell and porizate the mixture. The shape and dimensions of the fillers directly affect the chemical reaction, as well as the formation of porous composition’s structure. Rounded shape of the filler and the slight variations in size reduce the resistance exerted by the released gas that passes through the mixture, and thus a more uniform structure of porous heatresistant concrete is formed.

Figure 7: Electron microscope images of IM-2201 powder.

Results

Three series of samples were prepared as a result of the experiments. Properties of the samples are represented in Table 3.

Table 3: Properties of samples obtained by varying the ratio of liquid phase to solid phase

The final products of phase transformations (Figure 8) after heating up to 1,400°C are α-Al₂O₃, SiC, SiO₂ (tridymite), Al(PO₃)₃ (A form), CrPO₄, BPO₄ and AlPO₄ (cristobalite type).

Figure 8: The X-ray diffraction pattern of high-temperature insulation composite material based on phosphate binder, chamotte dust, alumochrome waste, and silicon-graphite waste after heating to 1,400°C.

Thus, in the mode of self-propagating exothermic synthesis, an ultra-lightweight high temperature porous material based on aluminum phosphate was obtained. The purpose of this material is hightemperature heat insulation of industrial and power units, with a purpose of minimizing operating costs by increasing the efficiency of energy resources use.

The main advantage of the resulting material is the simplicity of installation of thermal insulation. This is provided by the possibility to obtain products of various geometric shapes and sizes on insulated unit by laying the homogenized reaction mixture into the formwork. However, laying all the necessary amount of the mixture may result in excessive swelling with the formation of a sharply uneven porosity, which can cause destruction of the insulation. Therefore, the reaction mixture is placed in layers.

Layering of phosphate composite material is possible due to the fact that an interface does not form between the layers hardened at different times, which is due to the high adhesion properties of the layers (Batrashov, et al., 2010).

Further improvement of technology of obtaining porous phosphate material is associated with the creation of new compositions with various modifying components, including nanostructured components, allowing to significantly increase the strength without increasing the density of the material.

Another area of development is associated with the creation of materials with variable density. Since the determining factor of density is the ratio between the liquid and solid phases, by using the layering method during manufacturing of articles it is possible to stack layers of mixtures with different amounts of a liquid phase, which will have various mechanical and thermal properties. Therefore, it is possible to obtain products with high density and strength of the surface layer and with a low thermal conductivity of ultra-lightweight inner layer.

Conclusion

With the use of self-propagating exothermic synthesis technology, ultra-lightweight phosphate high temperature composite material with a porous structure has been obtained. Industrial waste was used as the filler in the material. The properties of the material: density less than 400 kg/m³, temperature of continuous operation-1,500°C, heat resistance-up to 35 thermal cyclings, compressive strength after one thermal cycling-1.0 MPa, and heat conductivity of about 0.2 W/m⋅K.

Formation of the structure and the initial properties of the high-temperature heat insulation composite material takes place under normal conditions in the formwork, which allows to produce articles of virtually any geometric shape and size. The final physical, mechanical and thermal properties of the material used as the insulating layer are formed during the start of operation of the high-temperature furnace, in accordance with the technological regime. The final products of the phase transformation after heating to 1,400°C are high-melting compounds of aluminum oxide, silicon carbide, and phosphates of chromium, aluminum and boron.

The developed high-temperature composite material’s thermal properties are not inferior, and its physical and mechanical properties are superior to modern composite materials based on mullitesilica fibers. The material is ready for use as hightemperature insulator in glass, ceramic industries and metallurgy, as well as in mechanical engineering.

Currently, high-temperature composite material is used at JSC “Vasilevsky glass factory” (Russia, Republic of Tatarstan) as the thermoinsulator of the main body of the glass furnace. According to the company, as compared to the previous campaign of melting glass with the same workability, the natural gas consumption has decreased by about 70 m³/h to 80 m³/h and was 220 m³/h to 240 m³/h, and glass quality and furnace productivity have been improved.

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