High-Temperature SHS Heat Insulators Based on Pre-Activated Mineral Raw Materials

Abstract

In this paper, the results of the technological combustion of SHS heat insulators based on mineral origins are presented. It is shown that after mechanochemical treatment of minerals—diatomite—the kinetic characteristics of the combustion process change, providing targeted formation of the phase composition, structure, and properties of the SHS composite. A positive effect of using various modifiers during the MCT of diatomite—the activation of the combustion process—was established. The selection of modifiers provides an increase in the strength of the synthesized SHS composites as a result of the formation of aluminate compounds in the synthesis products, and a decrease in thermal conductivity to 0.157 W/m*K due to the formation of the ultraporous structure of the samples.

1. Introduction

In recent years, heat-insulating concretes have become widely used—both gas-filled (foam concrete, cellular concrete, aerated concrete) and those based on lightweight fillers (expanded clay concrete, perlite concrete, polystyrene concrete, etc.). Due to their high fire resistance, heat-insulating concretes are also used for fire protection barriers. Depending on the type of binder used and the variation in the composition of the filler, for which marshalite, diatomite, diatomaceous earth, feldspar, and quartz are used, the operating temperature of foam concrete is 800–1000 °C. The shape and nature of the surface of the grains of siliceous materials that act as fillers in cellular concrete masses largely determine the degree and quality of the mechanical adhesion between the binder component and the filler in cellular concrete products, and, consequently, the overall quality of the product [1].

Refractory heat insulators are ceramic materials with an operating temperature above 1500 °C. These include fireclay, semi-acid, kaolin, high-alumina and dinas products, as well as chromite, mullite-siliceous and mullite-corundum. Their density does not exceed 1.4 g/cm³. Such heat-insulating products are used in the working (unprotected) lining of furnaces that are not exposed to melts, abrasive forces and mechanical impacts, or in intermediate (protected) insulation.

The quality and functional properties of any material, including composite systems, are determined by their phase composition and structural characteristics. Phase formation—as well as the homogeneity of the structure, the presence of pores and their distribution by volume, and the formation of highly dispersed phases that ensure the strengthening of the system—are largely determined by the thermokinetic characteristics of the synthesis process. One of the effective methods of influencing and regulating the level of activity of the system is mechanochemical treatment (MCT) of powder systems, which allows for changing their degree of dispersion, defectiveness and the formation of highly active formations on the surface of particles [2]. The role of surface structures is extremely important in the creation of modified powder materials with a given set of properties. In order to effectively control the structure, phase and pore formation during the synthesis of SHS composite materials, it is necessary to establish the relationship between the structural changes during MCT of powder materials and the thermokinetic characteristics of the subsequent combustion process.

The modification of mineral powder particles directly during the grinding process is one of the areas of mechanochemical processing of inorganic materials [3,4,5]. This process allows for simultaneously bringing the material to the required degree of dispersion, modifying the surface of the dispersed particles and obtaining plastic, well-formed masses. Mechanochemical processing will allow for purposefully changing the state and chemical activity of the mineral components of the charge mixtures. The tasks are to find the most effective processing modes in relation to each of the raw materials, establish patterns in changing their structure and properties and determine the selection of charge components when obtaining products for a specific purpose. Such information will allow for predicting the choice of the charge composition and its preparation when obtaining a material with specified properties.

Among the wide range of high-temperature heat-protective systems, much attention is paid to aluminosilicate (AS) refractory and heat-insulating SHS materials (AS materials), the main components of which are aluminum and silicon dioxide, which are of considerable interest due to their low cost and widespread use [6]. At present, similar materials and products based on them have already been developed and are used in mechanical engineering and metallurgy. A previous study [7] shows the relationship between the mechanisms of the focal combustion mode and the formation of anisotropic macrostructures of the interaction products during the production of an Al₂O₃-based alloy. SiO₂–Al₂O₃ systems for synthesis in the SHS regime are also of great interest, since they allow for the obtaining of composites containing wollastonite, anorthite and gehlenite. These structural elements provide for increased strength and thermal stability of the material.

Materials obtained on the basis of the SiO₂–Al₂O₃ system, which are characterized by a wide range of structural modifications, are an interesting and promising subject in the creation of composite materials using preliminary mechanochemical treatment aimed at forming various phase structures. By adjusting the parameters of the MCT and the state of the activated reagents, it is possible to control the reaction paths during subsequent SHS, purposefully changing the thermal effect of reactions and obtaining products with specified morphologies and sizes of structural components. Along with other well-known methods [8,9,10], this approach—the integrated use of mechanochemical treatment and SH synthesis—will expand the possibilities of obtaining materials with a variable set of properties.

2. Materials and Methods

This study involved research on obtaining SHS heat insulators with pre-activated raw materials. Experimental work was carried out using natural mineral raw materials—namely, grade “A” calcined diatomite crumb (fraction 0–0.2 mm). During the preliminary mechanical activation, graphite was used as a modifier in the amounts of 10% and 20%. Aluminum grade APV was used as a reducing agent. Sodium liquid glass served as a binder.

Electron microscopic analysis was carried out on a Jem-100CX U-100kv (JEOL, Tokyo, Japan) electron transmission microscope. Samples were prepared via suspension in distilled water with subsequent ultrasonic dispersion. During the study, it was found that water interacts with the alcohol residues remaining during quartz abrasion, dissolving not only the surface layer but also penetrating into the volume of particles with the formation of silicic acid. Therefore, further preparation of objects for study was carried out using the dry preparation method.

To determine the particle size of the samples, the dry measurement method of the Scirocco-Malvern Mastersizer 2000 was used. The main modules were the following: the optical unit, the unit for dry and liquid samples and the computer system. The optical unit was used to collect the raw data, which was used to measure the particle size of the sample. The computer system used is a stand-alone computer that runs the Malvern 2000 software. The software controls the optical unit and the powder collection units, and analyzes the raw data from the optical unit to determine the particle size.

The bulk density was determined according to GOST 2211-94 and IUS 992. The bulk density of the powder material is calculated using the formula:

⍴_bulk = m/V

(1)

where m is the powder weight, g, and V is the volume of the cubic vessel, cm³.

The test sample, dried to a constant mass, was poured without compaction into a previously weighed cubic vessel and weighed. The mass of the vessel was subtracted from this result, and the resulting value was divided by the volume of the vessel.

X-ray phase analysis was carried out on a DRON-4M diffractometer (Burevestnik, St. Petersburg, Russia) using cobalt K_α-radiation, as well as copper K_α-radiation in the range of 2θ = 10°–70°. To determine the broadening of X-ray lines, the value of which is associated with the size of the crystallites of the substance and the deformation of the crystal lattice, additional X-ray diffraction images were taken at a low speed of 1/2 deg/min in the range from 40 to 80° q. Its values are defined as the width of the X-ray line at half maximum (FWHM), measured in degrees.

Mechanochemical treatment (MCT) of powders was carried out in centrifugal planetary mills (CPM) “NXQM-2A (NANBEI, Zhengzhou, Henan, China)” with a working chamber volume of 500 mm³, platform rotation speed of 650 rpm, acceleration of grinding balls of 40 g and energy consumption of 0.75 kW/h. During grinding, the grinding time, the ratio of the mass of the material to the mass of the grinding balls (M_p/M_b) and the amount of modifying additives introduced were varied.

The activated powders were pressed into square samples of 100 × 100 mm with a height of 30 to 50 mm, respectively. The samples were formed on a hydraulic press of the HJ0802 (Rossvik, Astrakhan, Russia) brand with a force of 8 tons. Subsequent technological combustion was carried out in a muffle furnace SNOL 7.2/1100 (Thermvac, Hwaseong-si, Republic of Korea) with a set temperature of 1000 °C.

The combustion temperature was measured using pyrometric thermometers of the DT-8869H brand (Cenkay, Umraniye, Istanbul). The temperature of the sample was measured throughout the combustion process. After SHS, the phase composition of the synthesized materials was determined, and the density, strength and thermal conductivity of the samples were measured.

3. Results

The physicochemical properties and morphology of both the original and activated diatomite powders were studied. According to X-ray phase analysis, diatomite contains 88.5% SiO₂, 5.8% NaAl₃Si₃O₁₁ and 2.8% (Fe₄Si)₀.₄ (Figure 1). Thus, the natural mineral raw materials used mainly consist of silicon dioxide and sodium aluminosilicates.

The mineral raw material—diatomite—was ground in a planetary centrifugal mill for 10, 20, 30 and 40 min at different ratios of powder mass (Mp) and ball mass (Mb), namely at Mp/Mb = 1/2, 1/3 and 1/4.

The structure and dispersion of diatomite particles were studied depending on the conditions of mechanochemical treatment. The bulk density of the powder was measured, which does not change monotonically, reflecting the change in the ratio of particles of different sizes, and, consequently, the change in its packing density. This characteristic is the first visual indicator of both structural (dimensional) and morphological changes in the powder during MCT.

Table 1. Bulk density of diatomite depending on the parameters of the MCT.

Mp/Mb	Bulk Density, g/cm3
	Activation Time, min
	10	20	30	40
1/2	1.14 ± 0.091	1.20 ± 0.091	1.11 ± 0.091	1.13 ± 0.091
1/3	1.15 ± 0.091	1.21 ± 0.091	1.13 ± 0.091	1.11 ± 0.091
1/4	1.19 ± 0.091	1.26 ± 0.091	1.18 ± 0.091	1.05 ± 0.091

shows data on the bulk density of diatomite depending on the MCT parameters (time and Mp/Mb ratio).

Diatomite powder after treatment for 20 min is characterized by a higher bulk density of the powder with a tendency to decrease with an increase in the number of grinding balls. After determining the bulk density, the 20 min timeframe and a powder mass to ball mass ratio of 1/4 were selected for further research.

The activity of modified powders also depends on the morphological features of the mechanochemically and ultrasonically processed particles. Electron microscopic studies of the morphology [11] of particles depending on the MCT regimes of the modifier used were carried out in stages as the complexity of the structure of the processed systems increased. After processing, not only was diatomite ground, but also the surface layer of the particles of the material under study was more deeply destroyed (Figure 2).

The results of the electron microscopy study showed that the original diatomite contains a large number of meso- and micropores, which reduce thermal conductivity; such materials have high heat resistance and chemical stability. But the strength indicators of such materials are very low (Figure 2a). The results of the distribution of the particles of the original diatomite showed that the particle sizes are up to 50 microns. Figure 2b shows that the effect of mechanical activation of diatomite after 20 min of treatment led to the destruction of the particles themselves and a decrease in size to 20 microns. This indicates that new types of active centers and stresses are formed on the surface of the material. After 20 min of activation, the particle size is reduced to 5 microns and accounts for 50% of the total mass. This leads to an increase in bulk density and packing density during their further use in obtaining thermal insulation materials. After MCT of diatomite with 20% graphite, it is clear that the surface of the material under study is destroyed and the particle sizes have decreased to 10 microns (Figure 2c).

Figure 3 shows the combustion thermograms of the SH samples of the initial diatomite and diatomite after MCT in the presence of 10% and 20% graphite. The results show that diatomite in the initial state has a low induction period of ignition and a maximum temperature of up to 1273 °C. But after 20 min of MCT of diatomite, the effect of the induction period of ignition is more pronounced and the maximum combustion temperature increases to 1400 °C. This is due to the formation of active centers on the surface of the particles, which motivate the oxidation–reduction reaction between diatomite and aluminum.

After MCT of diatomite with 10% graphite and the introduction of the obtained powder into the batch with aluminum, there is a significant reduction in the induction period of ignition, an increase in temperature at all stages of the combustion process compared to non-activated fuel. The development of the combustion process clearly reflects the efficiency of using an activated mixture of diatomite with graphite in the batch during SHS synthesis of systems. The maximum combustion temperature is 1442 °C and is recorded with the content of activated diatomite with 10% graphite. Samples after SHS samples are porous. A distinctive feature of the combustion of such systems is a more stable development of the process, especially at the stage of post-processes.

Table 2. Obtained characteristics of SHS samples after technological combustion.

Description	T max	Thermal Conductivity, W/m*K	Strength, MPa
Diatomite initial	1273 ± 10	0.337 ± 0.002	5.6 ± 0.3
Diatomite after MCT	1400 ± 10	0.203 ± 0.002	23.2 ± 0.3
Diatomite + 10% graphite 20 min ¼ MCT	1442 ± 10	0.157 ± 0.002	13.4 ± 0.3
Diatomite + 20% graphite20 min ¼ MCT	1261 ± 10	0.315 ± 0.002	4.9 ± 0.3

shows the results of the SH synthesis of diatomite with different modifiers on 100 × 100 samples. The samples with an increased size show the highest results for the maximum combustion temperature in the presence of 10% graphite (1442 °C) and compressive strength, as well as low thermal conductivity with an index of 0.157 W/m*K. But the initial diatomite after mechanical activation shows the highest compressive strength of 23.2 MPa (1400 °C) and a thermal conductivity coefficient of 0.203 W/m*K. SH samples after MCT of diatomite with stearic acid also have a maximum combustion temperature of 1410 °C. This is possibly due to the participation of bound water in the process, which interacts with aluminum with the release of hydrogen and a large amount of heat, which accelerates other oxidation–reduction reactions. As a result, the combustion process intensifies and the temperature increases to 1296–1410 °C.

Thus, the mechanochemical activation and modification of the surface layers of the dispersed diatomite particle can be used to purposefully influence the development of the combustion process and its thermokinetic characteristics, which should ultimately be realized in the phase composition of the synthesized material.

Table 3. Phase composition in combustion products of Diatomite + 37.5% Al samples depending on the conditions of diatomite MCT.

Description	Phase Composition, %
	Phases
	Al2O3	Si	AlN	FeAl3Si2	SiO2	Al
Diatomite initial	48.4	12.9	-	1.8	24.7	12.2
Diatomite after MCT	59.9	15.4	4.1	1.4	10.7	9.5
Diatomite + 10% graphite 20 min ¼ MCT	62.2	20.5	6.2	1.8	7.0	2.3
Diatomite + 20% graphite 20 min ¼ MCT	58.0	24.5	7.8	1.3	6.7	3.0

shows the results of the X-ray phase analysis of the synthesized samples after MCT and modification.

It follows from the presented data that the modification of diatomite with graphite during MCT promotes the more complete development of oxidation–reduction processes with the formation of the main phase—corundum (Al₂O₃), reduced silicon and the formation of aluminum nitride (AlN). The most complete realization of the initial aluminum and reduction of silicon takes place when the batch contains modified diatomite. Aluminum nitride is formed during combustion by the interaction of aluminum with atmospheric nitrogen. The amount of aluminum nitride in the reaction products increases with increasing combustion temperatures. The presence of graphite in sufficient quantity has a positive effect on increasing the amount of the matrix phase (corundum) and aluminum nitride. With an increase in its amount, the yield of corundum in the synthesis products increases, which should result in, together with the presence of aluminum nitride, an increase in the quality of the material in terms of its strength and heat-insulating properties.

The construction industry offers many types of thermal insulation materials. For example, the thermal conductivity coefficient of sand-lime brick is 0.7–0.8 W/(m*K) and that of clinker brick is 0.56–0.60 W/(m*K). Diatomite, completely replacing quartz sand, reduces the density of thermal insulation bricks—improving their thermal insulation properties to 0.157–0.337 W/(m*K) and their strength—and also reduces the cost by 15–20% [12]. Comparative analyses show that the resulting diatomite-based thermal insulation materials have a lower thermal conductivity coefficient compared to sand-lime bricks, and are also characterized by low cost. The resulting thermal insulation materials are promising for use in the construction industry, providing high cost efficiency and good thermal insulation properties.

Thus, a fine-porous structure with dense partitions is observed in diatomite samples with graphite in samples obtained with fuel in the form of aluminum modified with carbon. Porosity is a consequence of the formation of a gas phase in the synthesis products. This fact indicates the prospects for using such materials to obtain thermal insulation systems, which is confirmed by the results of measuring the properties of the synthesized samples. Thermal conductivity indicators vary depending on the type and amount of modifying additives used.

4. Conclusions

The mechanochemical treatment of diatomite leads to a change in the dispersion and defectiveness of particles. An important role of MCT in changing the structural characteristics of dispersed particles has been established. Based on the results of this study, optimal modes of surface treatment of charge mixture particles have been established, providing an increase in the activity of powders after MCT. This increase is manifested in a change in the kinetic parameters of technological combustion for obtaining heat-insulating materials via the SHS method with a low thermal conductivity coefficient.