3.1. Study of Raw Materials
The main mineral of the refractory clay from the Arkalyk deposit is kaolinite. The sample contained a small amount of gibbsite, quartz, and anatase (
Figure 2).
The DTA curve of the refractory clay sample shows endothermic effects at 119.2, 310.7, 560, and 575.2 °C. At the same time, they correspond to minima on the DTG curve at 95, 306, 559, and 566 °C. All of them are developed against the background of a decrease in sample weight. This means the gradual dehydration of kaolinite. The exothermic effect with a peak at 975.6 °C, developed against the background of a stable mass, can be interpreted as a manifestation of a restructuring of the intermediate metastable phase of metakaolinite (
Figure 3). The endothermic effect with maximum development at 575.2 °C refers to the enantiotropic polymorphic transformation of quartz. The combination of endothermic effects with extremes at 310.7 °C and 575.2 °C reflects the presence of gibbsite. The fire resistance of Arkalyk clay was 1650–1700 °C.
Microsilica is a secondary product. It is formed during the high-temperature processing of silica-containing raw materials at MK KazSilikon LLP, associated with the sublimation process of silicon oxides. A finely dispersed colloidal, primarily amorphous material is formed when the sublimate condenses during the cooling process. The predominant particle size of microsilica is from 1 to 0.01 microns or less, which determines its high chemical activity (
Figure 4).
According to X-ray phase analysis data, the microsilica sample is presented at (wt.%) 83.8 for the amorphous phase and 16.2 for the crystalline phase. The crystalline part is represented by mass.%: moissanite (SiC—64.5), cristobalite (SiO2—20.2), and tridymite (15.3), which means that the total content in the sample is 10.4 moissanite, 3.3 cristobalite, and 2.5 tridymite. The amorphous phase of the product was represented by high-silica glass.
Based on the microsilica, samples were molded and fired at 1000–1500 °C in order to assess the behavior of this material during heat treatment. It was noted that with increasing firing temperature, amorphous silica microparticles sintered, forming aggregates. The silicon carbide and metallic silicon particles present remained unchanged. The structure of the samples fired at 1400 °C is porous (
Figure 5a). In the structure of the fired microsilica samples, it was noted that the highly dispersed phase of amorphous silica was almost completely transformed into cristobalite and tridymite, and the spheroidal particles were sintered, forming aggregates.
The presence of inclusions of silicon carbide and metallic silicon (light inclusions) was also noted in the firing products (
Figure 5b). The fire resistance of microsilica was 1600–1630 °C.
3.2. Selection of the Composition of the Refractory Binder
Traditionally, the mixture for producing refractory products consists of a filler and a binding matrix. The filler is a set of large fractions, and the binding part is made of a finely ground component containing a fraction of less than 0.063 mm.
The binding part of the refractories is most susceptible to aggressive action during the use of refractory materials in metallurgical units. Therefore, the best option is to create a refractory material where the phase composition of the binder consists predominantly of corrosion-resistant mullite.
In this regard, the work was devoted to selecting high-alumina compositions using microsilica for application as a binder for mullite–silica refractory products. The use of highly dispersed silica (microsilica) for the synthesis of a mullite matrix can be effective due to its developed surface and high chemical activity.
Refractory compositions have been formulated with varying ratios of refractory clay, alumina and microsilica. We calculated the chemical compositions (for a calcined substance) related to mullite–silica compositions to select compositions.
Calculations have shown that all compositions, except those based on refractory clay with the addition of 10 microsilica, meet the GOST requirements for Al
2O
3 content to produce mullite–silica products (
Table 2).
Table 2 shows that compositions R5 and R6 have the highest aluminum oxide content and are more effective for obtaining mullite-containing compositions. Based on these compositions, samples were molded, followed by firing in the range of 1400–1600 °C to identify the patterns of structural and phase transformations that occur during heat treatment.
During heat treatment of compositions, structural and phase transformations occur associated with changes in the initial minerals and the formation of new phases based on them.
First, structural and phase changes occur in the clay component, which is the main one. At temperatures up to 1000 °C, dehydration of kaolinite is observed, with the restructuring of its structure into metakaolinite and the subsequent formation of primary mullite and quartz:
The presence of these phases is established by the presence in the XRD pattern of lines of mullite (d = 3.38; 2.88; 2.69; 2.54; 2.20) and quartz (d = 4.26; 3.33; 1.83) (
Figure 6). Studying the samples of fired clay at a temperature of 1100–1200 °C with a microscope (in transmitted light) revealed the formation of a colorless, weakly anisotropic, uniaxial, negative phase in kaolinite with a refractive index of 1.55–1.58. The newly formed mineral with an imperfect structure is probably metakaolinite—a phase preceding mullite. A colorless, weakly anisotropic mineral was also noted in samples fired at 1400 °C, but with a higher refractive index equal to Ng—1.681, Np—1.661, biaxial, negative. This phase is the main one and, according to the X-ray phase analysis of the sample, is identified as mullite (d = 5.37; 3.38; 2.20) (
Figure 6). In samples fired at 1400–1500 °C, recrystallization of mullite is observed, the crystals of which acquire a plate-like shape. The experimental results (
Table 3) showed that samples based on refractory clay are sintered in the range of 1400–1500 °C. Increasing the firing temperature to 1600 °C leads to the appearance of some signs of burnout (deformation of samples, partial melting, decrease in density, and increase in open porosity). The optimal firing temperature for samples made from Arkalyk clay is in the range of 1400–1500 °C. A more detailed study of the structural-phase transformations that occur during heat treatment of Arkalyk clay is presented in works [
27,
28].
X-ray phase analysis of the fired clay (1400 °C) showed that the content of mullite is (approximately) 57, quartz is 26, and corundum is 10%, the rest being impurity minerals (
Figure 6).
Introducing 10% microsilica into the composition can slightly change the phase formation process during the firing of samples.
Also, dehydration of the gibbsite occurs during heat-treating refractory clay. Gibbsite is also a source of aluminum oxide for the subsequent formation of mullite. First, primary mullite with an imperfect structure is formed from metakaolinite during the firing process. Aluminum oxide formed during the decomposition of gibbsite interacts with both silicon oxide of the starting material and microsilica introduced into the charge, forming secondary mullite. Due to the high activity of microsilica, the formation of secondary mullite with its participation is more likely. Moreover, microsilica interacts with raw material impurities, such as oxides of iron, titanium, magnesium, and others, with the formation of low-temperature compounds that form a eutectic melt of complex composition. The silicon carbide in microsilica does not change at the indicated temperatures.
The microstructure and phase composition of samples R9, R6, and R5 fired at 1500–1600 °C were studied using X-ray diffraction and microscopic analysis (
Table 4,
Figure 7,
Figure 8 and
Figure 9).
Introducing alumina and 10% microsilica into the mixture increases the open porosity of the samples (
Table 3, compositions R9 and R8).
The results showed that with an increase in the alumina content in the compositions, the values of open porosity and water absorption of refractory compositions increase, and loosening of the structure of the samples is observed. It indicates the intensive formation of mullite in the temperature range of 1500–1600 °C. Adding alumina to the charge increases the fire resistance of the compositions. For complete sintering and phase formation, increasing the firing temperature up to 1600 °C and increasing the soaking time are necessary.
The results of the X-ray phase analysis (
Table 4) of fired samples showed that the mullite content increases with an increase in alumina additions. The most considerable amount of mullite was found in compositions containing 30–40% alumina and fired at 1500 °C. The mullite content in these compositions is about 86–98% according to X-ray phase analysis. However, the structure of the samples remains quite porous. It is required to increase the duration of the firing process at this temperature to compact the structure of the samples.
Increasing the firing temperature to 1600 °C affected the polymorphic transformation of quartz and microsilica into cristobalite, the content of which in the fired samples is approximately 1.2% (
Table 4,
Figure 9).
The structure of the sample R6 is porous and consists of mullite intergrowths. In the intervals, a silicate phase can be noted (
Figure 10).
3.3. Obtaining Mullite–Silica Refractory
Mullite–silica refractory samples were obtained using traditional fireclay technology. The required filler fraction (less than 3 mm) was obtained as follows: refractory clay was briquette, dried, and fired at 1400 °C to produce fireclay. Preliminary firing of refractory clay into fireclay reduces shrinkage and improves the finished products’ performance properties. The fired fireclay briquettes were crushed, ground, and fractionated into the required fractions (
Table 5). The composition R6 and finely ground fireclay are used as a binding mass.
Lignosulfonate solution (1.23 g/cm3) was used as a temporary binder. The molding humidity was 8–9%. Refractory samples were obtained at four specific pressures—40, 60, 80 and 100 MPa—on a hydraulic press to determine the optimal pressing pressure. The obtained samples were dried and then fired.
Molded refractory samples were dried at 100–105 °C and fired at 1500 °C. Fired refractory samples were tested to determine properties. The results are shown in
Table 6.
Test results showed that the optimal pressing pressure is 60 MPa. The thermal deformation temperature under a load of 0.2 MPa was determined to be 1350 °C. X-ray phase analysis showed that the main phase of the refractory is mullite, the approximate content of which is 86% (
Figure 11). Impurities include corundum, quartz, and cristobalite. The structure of the sample is quite porous. Pores of various shapes are round, channel-shaped, and oblong (
Figure 12).
The research results showed that the main phase of refractories is mullite, which determines the basic performance properties of products, such as strength, temperature at which deformation begins, corrosion, and thermal resistance.