Mullite–Silicate Proppants Based on High-Iron Bauxite and Waste from Metallurgical Industry in Kazakhstan

Abstract

The suitability of microsilica as a raw material for the production of ceramic mullite–silicate proppants was assessed. The chemical and mineralogical compositions of the initial materials were studied. The mineral composition of bauxite is mainly represented by gibbsite Al(OH)₃ and, to a lesser extent, kaolinite Al₄[Si₄O₁₀](OH)₈, with impurities of hematite and quartz. It is established that, in order to obtain mullite–silicate proppants, compositions containing 10–20% microsilica are optimal. The sintering of these compositions occurs at 1350–1380 °C. A lightweight ceramic proppant was obtained with a bulk density of 1.21–1.41 g/cm³, breaking ratio at 51.7 MPa of 19.1–20.3%, and sphericity and roundness of 0.7–0.9, and the optimal roasting temperature was determined as 1370–1380 °C.

1. Introduction

The growth of hard-to-recover hydrocarbon reserves in the modern world has increased the relevance of and demand for the use of hydraulic fracturing in the oil and gas industry. During hydraulic fracturing, proppants (i.e., granular particles of 0.2–2.0 mm in size) are injected under pressure into the well to prevent fracture closure and maintain the permeability of the well after pressure reduction [1,2,3,4].

In this regard, the need for proppants is expected to increase and, so, their production technologies must be constantly improved. Commonly used propping agents include frac sand, resin-coated sand, and ceramic proppants. Despite the large selection of propping agents, ceramic proppants with a wide range of compositions and a range of functional properties remain highly popular.

The raw materials for obtaining ceramic proppants include various natural and synthetic alumina- and silica-containing materials. During the firing process, mullite is formed on their basis, which is the main phase in the structure of granules, determining the main functional properties of proppants, such as their strength (breaking ratio), density, chemical resistance, and so on.

One of the most important problems faced by modern manufacturers of functional materials is the creation of competitive products, which are based on traditional raw materials as well as secondary and production waste materials. The advantage of using recycled and production waste materials is that there is no need to extract them. Furthermore, waste disposal will reduce the consumption of primary natural resources, solve the issues of raw material security, reduce production costs, and, at the same time, contribute to improvement of the environmental situation in the region.

An analysis of the available scientific studies in the literature revealed that proppants with aluminosilicate compositions are still the most popular. The works [5,6,7,8] intended to produce aluminosilicate proppants through the use of natural clay materials and bauxite with various compositions have been presented. For example, researchers [5] have shown the possibility of obtaining ceramic proppants based on natural aluminosilicate raw materials in Kazakhstan. It was shown that Arkalyk refractory clays and Krasnooktyabrskiy bauxites are basic aluminosilicate raw materials with Al₂O₃ contents of 48.3 and 62.0% (on a calcined substance), respectively. Compositions and technological parameters for the production of lightweight ceramic proppants were also presented, and the efficiency of heat treatment of raw materials at 1000 °C to increase the density and strength of ceramic samples was assessed.

Researchers have come to pay more and more attention to the use of waste materials in the production of proppant materials for the oil industry. In [9], the authors proposed to obtain aluminosilicate proppants based on man-made raw materials (i.e., corundum dust obtained during the crushing of electro-fused corundum and a sintering additive, as well as granodiorite obtained from screenings of crushed stone production). The samples were obtained by pressing a charge containing corundum dust (55 wt%) and granodiorite (45 wt%) at a specific pressing pressure of 20 MPa. Samples of aluminosilicate proppants based on this charge with an apparent density of 2.38–2.76 g/cm³ and water absorption of 1.4–10.9% were obtained. The strength of the obtained samples increased with an increase in the roasting temperature, ranging from 17 to 32 MPa at a roasting temperature of 1180–1200 °C. The disadvantage of this work was the analysis of proppant quality based on the testing of ceramic samples, rather than the proppants themselves.

In [10], the authors obtained proppants using drill cuttings and modifying additives; in particular, on a wt% basis: aluminum oxide—5.0; cullet glass—20; and NaF (over 100 wt%)—4. The optimal roasting temperature was 1100 °C with a rate of 6 °C/min. However, this composition produced proppants with low sphericity and roundness, which affects their permeability during hydraulic fracturing.

In [11], kaolin from the Borovichsko–Lyubytinskoye deposit was used as the main raw material for the production of ceramic proppants. The authors proposed increasing the pre-calcination temperature of kaolin from 850 to 980 °C. This ensured increased activity in the subsequent sintering of the granulated material, improving the strength properties of proppants after firing both at 1450 °C and 1500 °C. Proppants with a bulk density of up to 1.83–1.90 g/cm³ were obtained, withstanding destructive pressures up to 70 MPa. However, proppants based on the proposed raw materials require increased energy consumption (firing temperatures up to 1500 °C).

In [12], the authors used kaolinite clay and waste coal to produce aluminosilicate proppants. It was noted that the proppant was obtained at a roasting temperature of 1400 °C. The bulk density was 1.27 g/cm³, the apparent density was 2.79 g/cm³, and the breaking ratio was 3.27% at 35 MPa and 8.36% at 52 MPa. The authors suggested that the addition of solid waste could significantly reduce the cost of a ceramic proppant.

A low-density and high-strength ceramic proppant was obtained by the authors of [13], based on high-iron bauxite at 1300–1360 °C. The bulk density of the sample was 1.42 g/cm³, the apparent density was 2.67 g/cm³, and the breaking ratio at 52 MPa was 5.1%. The main phase of obtained samples was needle-shaped mullite.

Another study [14] focused on the orthogonal optimization of the design of low-density and high-strength ceramic proppants made of low-grade bauxite and feldspar. The authors studied the effects of the raw material grinding degree and the proppant roasting mode on the properties of the finished product. They found that the optimal parameters for the production of proppants that met the properties of the considered standard were the grinding of bauxite within 4 h, feldspar within 8 h, and a sintering temperature of 1280 °C. Proppants obtained using the optimal combination had a bulk density of 1.48 g/cm³, an apparent density of 2.7 g/cm³, a breaking ratio of 4.07% at 52 MPa, and a solubility in acid of 2.15%.

In [15], corundum–mullite ceramic proppants were successfully synthesized using natural bauxite, solid waste coal, and calcium carbonate additives as raw materials. The effects of calcium carbonate supplementation on the phase composition, microstructure, and mechanical characteristics were studied. The results showed that the addition of CaCO₃ promotes the formation of a liquid phase at a lower sintering temperature, contributing to the compaction of the samples. Ceramic proppants with an addition rate of 5% sintered at 1350 °C had a breaking ratio of 8.41% at 52 MPa, due to the appropriate amount of liquid phase and grinding of mullite grains.

The authors of [16] noted the advantage of using ultra-lightweight (ULW) proppants with high strength in hydraulic fracturing technologies. Traditional proppants are prone to settling in fracture operations, which can seriously affect the effectiveness of the operation. The article provided a comprehensive review of more than 50 articles published over the past few decades on ultra-lightweight proppants. The purpose of this study was to provide an overview of the current state of development of ultralight proppants concerning the raw materials, manufacturing process, performance characteristics, hydrophobic and lipophilic properties, and practical applications, in order to facilitate research on novel ultralight proppants.

An analysis of the available information reveals that aluminosilicate ceramic proppants can be produced using various alumina- and silicate-containing natural materials, as well as man-made waste. The improvement of technological parameters for the production of aluminosilicate proppants depends on the characteristics of the raw materials used.

Special attention has been paid in recent years to the use of lightweight proppants in the implementation of hydraulic fracturing technology. In this regard, there is an increased interest in the field to create lightweight proppants using various natural types of raw materials, as well as man-made waste.

During the production of metallurgical silicon at KazSilikon LLP (Almaty, Kazakhstan), a huge amount of waste is generated, namely, microsilica, which settles in dust collectors. Using this microsilica in the production of ceramic materials would make it possible to obtain value-added products. The SiO₂ content in this product varies widely, from 80 to 99%, and the average particle size ranges from 0.01 to 1.0 mm.

Microsilica is a microscopic form of amorphous silicon dioxide. Amorphous SiO₂ is essentially not found in nature, and this is even more so the case for its ultrafine form, microsilica. At the same time, the demand for microsilica in the industry is steadily growing [17,18,19,20,21]. It should be noted that, while microsilica is widely used for the production of various building materials, there is almost no information on its application in the field of ceramic proppants.

Therefore, the purpose of this work was to evaluate the utility of microsilica for the production of mullite–silica proppants.

2. Materials and Methods

The raw materials used in the work were high-iron bauxite from the Krasnooktyabrskiy deposit and microsilica (a by-product of KazSilikon LLP, Almaty, Kazakhstan).

The chemical and mineralogical compositions of the starting materials, as well as the structural and phase transformations occurring during the heat treatment of refractory compositions based on these materials, were studied through the use of X-ray fluorescence, X-ray diffraction, thermal, chemical, and microscopic analysis methods.

Samples with particular ceramic compositions were obtained using methods commonly adopted for the production of ceramics and refractories, including the grinding and fine grinding of raw materials, the calculation and composition of charges, the preparation of molding compound, and the pressing of samples from semi-dry masses on a hydraulic press. The granulation of ceramic mixtures for the production of proppant granules was performed using an EL1 Eirich mixer–granulator (Maschinenfabrik Gustav Eirich GmbH & Co KG, Hardheim, Germany).

The determination of the technical properties of ceramics was performed according to the national standards GOST 2409-2014 [22], GOST 4071.1-2021 [23] and GOST 51761-2013 [24].

The structure and phase composition were also studied through electron probe microanalysis (EPMA) using a JEOL JXA-8230 (JEOL, Ltd., Tokyo, Japan) with combined WDS/EDS.

The XRD patterns of the samples were recorded using a Bruker D8 Advance XRD (Bruker Corporation, Billerica, MA, USA) diffractometer (Cu Kα, λ = 1.5406 Å). A thermal analysis of ceramic samples was performed using a simultaneous thermal analyzer (NETZSCH STA 449 F3 Jupiter (NETZSCH group, Selb, Germany).

3. Results and Discussion

3.1. Study of Raw Materials

The chemical composition of bauxite is determined and given in

Table 1. Chemical composition of raw materials.

Raw Material	Content of Oxides, Mass. %
	SiO2	MgO	CaO	Al2O3	Fe2O3	K2O	Na2O	TiO2	LOI
Bauxite	7.6	0.1	1.14	47.8	17.3	0.03	0.03	3.5	22.5
Microsilica	95.20	-	-	-	-	-	-	-	4.80

. Microsilica composition was provided by KazSilikon LLP.

The mineral composition of the bauxite was represented mainly by gibbsite Al(OH)₃ and, to a lesser extent, kaolinite Al₄[Si₄O₁₀](OH)₈ with characteristic X-ray reflexes (Figure 1). The sample contained a significant amount of iron impurities (in the form of hematite) and, to a lesser extent, quartz.

The thermal analysis of the bauxite sample established the endothermic effects of varying intensity on the DTA curve (Figure 2), the maximum development of which occurred at 338.40 °C and 553.60 °C. There was also a weak exothermic effect on the DTA curve, with a peak at 961 °C. Additional thermal effects were recorded on the dDTA curve. It is possible to note endothermic effects with extremes at 311.3 °C, 321.5 °C, and 575.4 °C. Two exothermic effects with peaks at 380.90 °C and 570.0 °C appeared.

The endothermic effect (338.4 °C) on the DTA curve may be a manifestation of the dehydration of gibbsite and iron hydroxides. Two endothermic effects (311.3 °C, 321.5 °C) were recorded on the dDTA curve in the area of the development of the above-mentioned effect. This provides evidence for the dehydration processes of gibbsite and iron hydroxides in the temperature range of 150–400 °C.

The presence of an exothermic effect of 380.9 °C on the dDTA curve may indicate the presence of amorphous iron hydroxide in the sample; in particular, this peak indicates formation of the hematite crystal lattice.

The manifestation of kaolinite shows an endothermic effect with maximum development at 553.6 °C and an exothermic effect with a peak at 961 °C on the DTA curve. The exothermic effect recorded on the dDTA curve may be a reflection of the transition of maghemite γ-Fe₂O₃ to the α modification. Endothermic effects indicate the dehydration of aluminum and iron hydroxides, while the exothermic effects indicate the formation of aluminohematite (570 °C dDTA) and the formation of corundum (961 °C DTA). The very weak endothermic effect with an extreme at 575.40 °C on the dDTA curve is a manifestation of quartz impurity inversion.

The TG curve shows the change in the weight of bauxite during the heat treatment of the sample. In the range up to 300 °C, mechanically bound water is removed, and the mass loss is insignificant. The main change in mass occurs in the temperature range from 300 to 1000 °C. In this temperature range, kaolinite dehydrates with a restructuring of its structure into a metakaolinite structure with the subsequent formation of primary mullite and quartz.

3.2. Obtaining Ceramic Compositions Based on Bauxite and Microsilica

To study the effects of the microsilica content on the phase composition and the properties of aluminosilicate proppants based on Krasnooktyabrskiy bauxite, ceramic samples were obtained according to the compositions specified in

Table 2. Chemical contents of compositions.

Composition	Composition of Mixture, Material, Mass. %	Content of Oxides, Mass. %
		SiO2	MgO	CaO	Al2O3	Fe2O3	K2O	Na2O	TiO2
M5	Bauxite, 95 microsilica, 5	14.25	0.14	1.35	58.71	21.2	0.04	0.04	4.27
M10	Bauxite, 90 microsilica, 10	18.62	0.11	1.28	55.69	20.09	0.04	0.04	4.13
M20	Bauxite, 80 microsilica, 20	27.94	0.096	1.12	49.34	17.84	0.03	0.03	3.6
M50	Bauxite, 50 microsilica, 50	54.30	0.09	0.9	31.2	11.2	0.03	0.03	2.25

.

As shown in the table, the ratio of Al₂O₃/SiO₂ decreases with an increase in the content of microsilica in the composition; that is, the theoretically possible formation of mullite (without taking into account the participation of impurity minerals in the raw material) will also decrease.

Experience from studies of compositions based on Krasnooktyabrskiy bauxite performed earlier [25] indicates that the sintering interval is 1300–1500 °C, depending on the ratio of the initial components. Therefore, molded and dried cylindric samples were fired at 1350 °C. The properties of fired ceramic samples are detailed in

Table 3. Properties of samples of compositions.

Composition	Open Porosity, %	Apparent Density, g/cm3	Compressive Strength, MPa
M5	15.30	2.58	32.8
M10	17.45	2.43	30.1
M20	23.05	2.17	26.7
M50	30.66	1.67	10.2

.

Table 3 shows that the properties of the samples gradually decrease with an increase in the content of microsilica. The structure of the sample becomes more porous with increasing microsilica content, the apparent density and strength under compression decrease, and the appearance of the ceramics suggests under-roasting. This indicates the need to increase the firing temperature with an increase in microsilica in the composition. Obviously, with an increase in the content of microsilica, the formation of low-melting eutectic decreases during the firing process.

As indicated by the X-ray phase analysis results for the samples, the main phases of the compositions were mullite, cristobalite, and spinel phases of the complex composition containing aluminum oxide and impurity oxides of raw materials. The maximum amount of mullite was established in compositions based on bauxite with 10–20% microsilica (Figure 3, Figure 4, Figure 5 and Figure 6,

Table 4. XRD results of compositions.

Composition	Phase	Formula	Quantity, %
M5	α-Al2O3, syn	Al2O3	54.8
	Mullite, syn	Al(Al0.83Si1.08O4.85)	23.5
	Hematite, syn	Fe2O3	12.8
	Cristobalite, syn	SiO2	8.9
M10	Mullite, syn	Al(Al0.83Si1.08O4.85)	69.2%
	Cristobalite, syn	SiO2	25.6%
	Magnetite, magnesian, syn	(Mg0.22Fe0.78)(Al0.40Mg0.78Fe0.82)O4	5.3%
M20	Mullite, syn	Al(Al0.83Si1.08O4.85)	49.9
	Cristobalite, syn	SiO2	45.7
	Magnetite, magnesian, syn	(Mg0.22Fe0.78)(Al0.40Mg0.78Fe0.82)O4	4.4
M50	Mullite, syn	Al(Al0.83Si1.08O4.85)	23.7
	Cristobalite, syn	SiO2	69.2%
	Quartz, syn	SiO2	4.0%
	Magnetite, magnesian, syn	(Mg0.22Fe0.78)(Al0.40Mg0.78Fe0.82)O4	3.2%

).

An XRD analysis of fired compositions containing 10% and 20% microsilica (Table 4) showed that the main phase is mullite, while the secondary phase is cristobalite and the spinel phase with complex composition (containing elements of aluminum, iron, and other impurities from the raw materials). The predominant phase is cristobalite in the composition containing 50% of microsilica, with mullite present in a smaller amount.

The structure of the compositions is porous and multiphasic, consisting of mullite intergrowths between which cristobalite, silicates of complex composition, and spinel phases based on aluminum oxides, iron oxides, and impurities present in the raw materials can be observed (Figure 7).

Our studies showed that with an increase in the content of microsilica up to 50%, the formation of mullite decreases and the amount of cristobalite significantly increases. This results in the deterioration of the strength properties of the samples. The maximum possible formation of mullite was observed in formulations containing 10–20% microsilica.

3.3. Proppants

As shown in a previous study [22], during the preliminary heat treatment of bauxite, the dehydration of kaolinite and gibbsite occurs with the formation of intermediate compounds. The formation of metastable intermediate compounds with an imperfect structure after the preliminary heat treatment of raw materials contributes to the activation of the sintering process during the subsequent firing of proppants. Bauxite was pre-heat-treated at 1000 °C and milled to a fraction of less than 0.063 mm.

Charges were prepared based on heat-treated bauxite with a microsilica content of 5–20% (M5, M10 and M20). Methylcellulose 1.5% solution in the amount of 40–45 wt% was used as a binder.

The proppants were obtained using a granulator (Eirich Mixer EL1), which consists of a 1 L steel drum and a turboprop swirl equipped with a blade scraper. The swirl speed can be varied from 300 to 6000 rpm. The drum has two rotation modes—clockwise and counterclockwise—and operates at a speed of 85 or 170 rpm.

The granulation process was performed at the drum rotation speed (170 rpm) and swirl rotation speed (1000–2000 rpm).

The obtained granules of mullite–silicate proppants were dried. The proppant granules were then fired in the range of 1330–1400 °C in order to clarify the sintering temperature of the granulated material. The properties of the experimental ceramic proppants are listed in

Table 5. Properties of the proppants (1330–1380 °C).

Composition	Open Porosity, %	Apparent Density, g/cm3	Water Absorption, %
1330 °C
M5	51.13	1.80	22.08
M10	52.06	1.72	23.26
M20	50.40	1.65	23.43
1350 °C
M5	45.39	1.89	19.34
M10	47.25	1.79	20.87
M20	44.74	1.72	20.71
1370 °C
M5	24.09	2.05	10.50
M10	33.42	1.93	14.75
M20	35.08	1.90	15.60
1380 °C
M5	18.96	2.12	8.20
M10	23.59	1.97	11.10
M20	22.97	1.98	10.40

.

According to the results of the experiments (Figure 8), the proppant granules should be roasted in the range of 1370–1380 °C. When the proppant roasting temperature increases above 1380 °C, the overburning and sticking of granules is observed.

Experimental ceramic proppants (fraction 0.5–2.0 mm) roasted at 1380 °C with a soaking time of 1 h were tested for compliance with the requirements of the GOST 51761-2013 standard [24] (

Table 6. Proppant test results.

Composition	Bulk Density, g/cm3	Breaking Ratio at 51.7 MPa, %	Solubility in HCl Solution, %	Sphericity	Roundness
M5	1.64	26.7	0.40	0.9	0.9
M10	1.41	20.3	0.40	0.9	0.9
M20	1.21	19.1	0.34	0.7	0.9

).

4. Conclusions

The by-product of the production of metallurgical silica by KazSilikon LLP, Kazakhstan—namely, microsilica—was evaluated as a raw material for the production of mullite–silicate proppants.

The effects of the microsilica content in compositions based on high-iron bauxite on the phase composition and properties of ceramic samples were studied. It was found that, during the heat treatments of compositions based on bauxite and microsilica, structural and phase changes leading to the formation of mullite and compounds with complex composition (due to the participation of raw material impurities) occurred.

It was established that the introduction of 10–20% microsilica into the composition based on high-iron bauxite allowed for lightweight proppants with a bulk density of 1.21–1.41 g/cm³, breaking ratio at 51.7 MPa of 19.1–20.3%, and sphericity and roundness of 0.7–0.9 to be obtained, with an optimal roasting temperature of 1370–1380 °C. Furthermore, the obtained proppants met the requirements of the National Standard GOST R 51761-2013.