Microwave‐sintered mullite structural ceramics based on low‐grade bauxite applied for fracturing proppants

Abstract

This study aimed to assess the feasibility of manufacturing fracturing proppants by microwave sintering and using low‐grade bauxite as raw material. The effects of microwave hotspot SiC and sintering additive MnO₂ content on the performance of the mullite‐based structural materials were studied, respectively. The optimum sintering condition was determined by single‐factor experiments. The sintering process and mechanism were explored based on the analysis of physicochemical properties, phase transitions, and microstructure. The results showed that (1) mullite ceramic composites could be successfully prepared only with SiC added and with poor interparticle bonding microstructure. (2) With the addition of MnO₂ and CaO, the granular‐shaped mullite crystals transformed into rod‐like mullite crystals, forming a net structure. (3) As the input power increased, the overfast sintering rate would reduce proppants' mechanical properties, and it was also necessary to select a reasonable sintering time to avoid overburning. (4) When the mass ratio of MnO₂:CaO:SiC:bauxite was 2:1.5:12:84.5 and under the sintering condition of 1000 W, 2 h, the performance (breakage ratio of 8.5% under 28 MPa closed pressure, apparent density of 2.58 g/cm³, turbidity of 52 FTU, and acid solubility of 6.77%) could meet the requirements of the Chinese Petroleum and Gas Industry Standard (SY/T 5108–2014). This study provides a powerful way for reducing the fracturing cost, which not only improves the low‐grade bauxite utilization scale within the ceramic industry, but also expands the application of microwave sintering technology in mullite structural materials for the petroleum and gas industry.

INTRODUCTION

Increasing energy consumption has made unconventional resources the exploitation hotspot in various countries. The unconventional reservoirs to be addressed here have very low permeability and mainly include tight gas reservoirs, tight sandstone oil, and gas and oil shale reservoirs. Therefore, hydraulic fracturing needs to be applied to generate high-conductivity channels so as to achieve commercial hydrocarbon recovery.¹ Proppant is an essential consumable during hydraulic fracturing treatment and is used to prevent artificial fracture closure. With the global large-scale application of multistage fracturing in horizontal wells, proppant consumption has doubled, accounting for approximately 20% of the total well construction.²

Commonly used proppants today are quartz sand, ceramic proppants, and resin-coated proppants (RCPs). Quartz sand can be naturally mined and has the greatest economic advantage over the other two. Since sand cannot support fractures well at higher closure stresses up to 6000 psi because it gets crushed, ceramic proppants with more crush resistance are introduced to the market that can withstand high closure stresses, especially where closure stresses exceed 8000–10,000 psi.³ Since frac proppants mentioned above create fines when they are overstressed, the coating technology has been applied to enhance the conductivity of sand-packed fractures. RCP can also be used to prevent proppant flowback to the wellbore and to prevent sand production.^4,5 However, the main disadvantage of the RCP is that it is more costly in preparation and usage, which limits its wide application as a fracturing material. Therefore, ceramic proppants are widely used due to their outstanding advantages through their combination of beneficial properties.

High-grade bauxite with an alumina content of more than 75% is preferred as a raw material to prepare ceramic proppants. In recent years, low-grade bauxite with an alumina content of 40%–60% has been used to develop ceramic proppants with high strength.^6–8 On the one hand, China's low-grade bauxite reserves are abundant and cheap,⁹ the preparation of proppant based on this type of raw material is conducive to reducing the fracturing cost. On the other hand, the rapid development of the domestic alumina industry has greatly accelerated the consumption of high-alumina–silica-ratio bauxite resources in China, and the dependence on foreign bauxite in China has exceeded 55% at this stage.^10,11 Therefore, manufacturing ceramic proppants based on low-grade bauxite is not only a powerful way for low-cost fracturing technology, but also a requirement for the sustainable development of China's aluminum industry and resource protection.

Furthermore, frac ceramic proppants are essentially an alumino-silicate material, and their industrial preparation methods are mainly based on flue gas heating with high energy consumption and a long sintering time.¹² Meanwhile, microwave heating has been employed in ceramic manufacturing as an innovative energy-efficient alternative to conventional heating technologies, while also attaining substantial energy and cost savings on future applications of possible commercial interest. In addition, the feasibility of preparing mullite thermal insulation material by microwave heating has been studied.^13–15 In summary, compared with traditional sintering, ceramic proppants preparation by microwave sintering might be an effective green manufacturing method from technical, environmental, and economic perspectives, but few studies have focused on this.

With this motivation and background, this study aimed to investigate the feasibility of manufacturing ceramic proppants from low-grade bauxite by microwave sintering. This goal was achieved through four specific steps. First, the effect of SiC contents on the microwave heating process was studied. Second, the optimal raw material formula with the best performance of the block material was obtained. Third, low-grade bauxite-based proppants with the best performance were obtained under the optimum sintering condition determined by single-factor experiments. The effects of the four main process parameters were discussed to ensure the strength properties of the proppants. Fourth, the sintering process and mechanism were explored based on the analysis of physicochemical properties, phase transitions, and microstructure.

Besides, this study provides a powerful way for reducing the fracturing cost, which not only improves the low-grade bauxite utilization scale within the ceramic industry but also expands the application of microwave sintering technology in mullite structural materials in the petroleum and gas industry.

MATERIALS AND METHODS

Raw materials

The bauxite was provided by a ceramic proppant production factory in Panzhihua, Sichuan Province, China. The high-purity SiC, MnO₂, and CaO used in this study were purchased from Chengdu Huachun Technology Co. Ltd.

Experimental procedure

The formulations of the raw material mixtures are shown in Table 1. All raw materials were ground in a planetary ball mill for 8 h and then dried at 120°C and sieved through a 300-mesh (aperture size of 48 μm) sieve. Then 50 g was weighed and loaded into the mold, followed by pressing into columnar specimens with a diameter of 50 mm and a height of 45 mm. When preparing proppant particles, the raw material was shaped into spherical green bodies with the addition of water, dried at 100°C for 2 h, and passed through a 20/40 mesh (aperture size of 425–850 μm) sieve.

Table 1 Different experiment formulas (wt%).

Samples	Low-grade bauxite	SiC	MnO2	CaO
1	97	3	0	0
2	94	6	0	0
3	91	9	0	0
4	88	12	0	0
5	85.5	12	1	1.5
6	84.5	12	2	1.5
7	83.5	12	3	1.5
8	82.5	12	4	1.5

Samples were placed in corundum crucibles and sintered in a microwave chamber; the microwave input power, sintering temperature, and sintering time were set according to the experimental design schemes. To determine the optimum raw material ratio, samples with different experiment formulas were prepared (Table 1) and sintered at 800 W for 2.5 h. In determining the effect of SiC contents on the performance of ceramic composites and identifying the optimum addition amount, four sets of repetitive experiments were designed to ensure the accuracy of the results. The microwave frequency was 2.45 GHz with a maximum input power of 1.6 kW. An armored thermocouple is used to measure the temperature of samples, and once the temperature reaches the set point, it would remain unchanged and fulfilled by the controlling system in the apparatus. Figure 1 illustrates the flowchart for the preparation of ceramic materials.

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Test methods

Test method of physical and chemical properties

The chemical compositions of raw materials were analyzed by an X-ray fluorescence spectrometer (PANalytical Axios). Major mineralogical phases of low-grade bauxite were measured by an X-ray diffractometer (made by Panacore), with Cu-Kα radiation (40 kV and 40 mA). A step scan was performed from 5° to 80° at 2θ, and a scanning rate of 10°C/min. The diffraction pattern was analyzed using MDI Jade Version 6. The microstructures of the samples were observed using a Quanta 650 environmental scanning electron microscope (SEM) from FEI. The SEM is equipped with an OCTANE PRO energy spectrum energy-dispersive X-ray spectroscopy (EDS) signal receiver from AMETEK, with electronic cooling and a detection count of >4000.

Thermogravimetry (TG)–differential thermal gravity (DTG) was performed using DTG-60 SHIMADZU at a heating rate of 5 K/min in an air atmosphere from 30°C to 1000°C.

Test method of mechanical properties of ceramic composite

• (1)

  The uniaxial compressive strength

  The uniaxial compressive strength of the sample was calculated according to Equation (1). According to GB/T 4740-1999 “Standard test method for compressive resistance of ceramic materials,” the tests were performed on YAW-300B microcomputer-controlled equipment, to obtain the uniaxial compressive strength. 1$${R}_{c}=\frac{4F}{\pi {d}^{2}}\times 1000,$$where $${R}_{c}$$ is the compressive strength of the sample (MPa), $$F$$ is the pressure of the sample at destruction (KN), and $$d$$ is the average diameter of the compressed surface (mm).

• (2)

  The properties of proppants

  The properties of ceramic proppant (breakage ratio, bulk density, apparent density, acid solubility, turbidity, sphericity, and roundness) were examined according to the Chinese Petroleum and Gas Industry Standard “Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations” (SY/T 5108-2014).¹⁶

  The breakage ratio was calculated according to Equation (2). 2$$BR=\frac{{w}_{c}}{{w}_{0}}\times 100 \% ,$$where $$BR$$ is the breakage ratio of the proppants, $${w}_{c}$$ is the weight of the crushed proppants after testing (g), and $${w}_{0}$$ is the weight of the proppants before testing (g).

  The apparent density was measured using Archimedes' method and calculated using Equation (3). 3$$\rho =\frac{{m}_{1}{\rho }_{{{\rm{H}}}_{2}{\rm{O}}}}{{m}_{3}-{m}_{2}},$$where $$\rho $$ is the apparent density of sample (g/cm³), $${m}_{1}$$ is the weight of the sample (g), $${m}_{2}$$ is the weight of density bottle, water (g), $${m}_{3}$$ is the total weight of density bottle, water, and sample (g), and $${\rho }_{{{\rm{H}}}_{2}{\rm{O}}}$$ is the density of water at room temperature (g/cm³).

• (3)

  The mechanical strength of individual proppant

The breakage ratio essentially reflects the proppants being subjected to external pressure in the form of a specific stack (24.7 cm³), which is affected by the combination of single-particle performance and multiparticle stacking form. It has been shown that the packing pattern has a large influence on the strength test results,^17,18 therefore, the single-particle compression resistance experiment was introduced in this study to provide a better quantitative characterization of the proppant particle strength (Figure 2). The specific experimental steps are:

• (1)

  Select 30 proppant particles with similar particle size, 20/40 mesh with an average particle size about 0.7 mm).

• (2)

  Load the radial pressure on the single proppant using a particle compressive strength testing machine, and record the maximum pressure before cracking.

• (3)

  Record the pressure resistance value of 30 single proppant particles and take the average.

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RESULTS AND DISCUSSION

Characteristics of raw materials

Physical and chemical properties

The chemical components of bauxite were analyzed, and the results are shown in Table 2, the composition mainly includes SiO₂ and Al₂O₃, and the content of Al₂O₃ is 49.60%, which belongs to low-grade bauxite, and the molar ratio (Al₂O₃/SiO₂) is about 1:1. Figure 3 displays the results of the main mineralogical phases of bauxite. The main phases of bauxite are kaolinite (Al₂(Si₂O₅)(OH)₄) and diaspore (AlO(OH)).

Table 2 The chemical compositions of bauxite (wt%).

Chemical composition	Al2O3	SiO2	Fe2O3	TiO2	CaO	P2O5	K2O	Na2O	ZrO2
Content (%)	49.60	44.97	0.96	2.98	0.29	0.46	0.10	0.11	0.15

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Thermal analysis of raw materials

The thermal stability of raw materials was analyzed with TG and DTG, while offering a reference for the determination of the sintering temperature of ceramic composites. As we can see from Figure 4, two weight loss peaks on the DTG curve occurred at 250°C and 515°C, allowing the heating process to be divided into two stages: (1) When raw material was heated from 30°C to 350°C, the TG curve showed a slight weight loss of about 0.8% due to the evaporation of water. (2) As the temperature went from 400°C to 730°C and there was a clear peak on the DTG curve (515°C), the mass loss of around 6.2% might be mainly attributed to the decomposition of diaspore and the loss of crystalline water from kaolinite.¹⁹ Kaolinite was transformed to metakaolinite by dehydroxylation at 400–600°C.²⁰ At an elevated temperature of 700–1000°C, the metakaolinite segregated into the Al–Si spinel phase and the amorphous SiO₂ phase.²¹ The following chemical reactions may occur during the heating process of the sample: 4$$2\mathrm{AlO}(\mathrm{OH})\to {\mathrm{Al}}_{2}{{\rm{O}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}({\rm{g}}),$$ 5$${\mathrm{Al}}_{2}({\mathrm{Si}}_{2}{{\rm{O}}}_{5}){(\mathrm{OH})}_{4}\mathop{\to }\limits^{400 \mbox{-} 600^\circ {\rm{C}}}{\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 2{\mathrm{SiO}}_{2}+2{{\rm{H}}}_{2}{\rm{O}}({\rm{g}})$$ 6$$2({\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 2{\mathrm{SiO}}_{2})\mathop{\to }\limits^{900 \mbox{-} 1000^\circ {\rm{C}}}2{\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 3{\mathrm{SiO}}_{2}+{\mathrm{SiO}}_{2}(\mathrm{amorphous}\unicode{x02007}\mathrm{phase}).$$

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Effect of SiC contents on the performance of ceramic composites

Microwave heating process

Due to the characteristic of microwave selective heating,²² the main mineral components of bauxite are difficult to couple with microwave to heat up, therefore, SiC is chosen as the local hot spot for microwave sintering preparation because it is easy to generate heat by coupling with the microwave at a low temperature.^23–25 The effects of SiC contents on the highest sintering temperature and heating rate are shown in Figure 5A, four sets of repetitive experiments were designed to ensure the accuracy of the results. The practical microwave heating profile under different SiC contents in experiment 4 was further plotted, as shown in Figure 5B. They all showed three stages: slow temperature increase, rapid temperature rise, and temperature maintenance:

• (1)

  The temperature rise was slow in the first 20 min of heating, and the maximum temperature was near 150°C. At the initial heating stage, only SiC particles absorbed the microwave energy, forming isolated hot spots within the samples and increasing the local temperature.²⁶

• (2)

  The temperature rapidly went up to 150–900°C within 20-120 min. On the one hand, as SiC continued to generate heat by coupling with microwaves, the dielectric loss of SiC also increased with the rising temperature, and thus its absorption efficiency of microwaves gradually enhanced. On the other hand, it can be seen from Figure 4, the decomposition of Al₂O₃ in bauxite around 515°C could become a new hot spot in samples, and the dielectric loss of Al₂O₃ gradually grew with the raising of the temperature,^27,28 as well as the capacity to absorb microwaves, which further promoted the acceleration of the heating rate, and so it was manifested as a larger slope in the sintering curves.

• (3)

  The temperature remained stable during the subsequent heating process, indicating that the heat generated by the absorption of microwaves was balanced by the heat dissipated, this may be due to the oxidation of SiC to SiO₂ and the excess SiC.

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Phase compositions and microstructure

Figure 6 illustrates the X-ray diffraction (XRD) patterns of ceramic composites containing various weight fractions (3, 6, 9, and 12 wt%) of SiC additives. It can be obvious observed that the phases of mullite, SiO₂, and SiC were detected in all samples, and the amount of SiC addition had a significant effect on the phase compositions at the same time.

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The mullite phase is closely related to the physical–mechanical properties of the material. As the SiC content increased, the diffraction peaks of mullite steadily enhanced, which implied a better development of the crystalline phase. It might be mainly due to two reasons: on the one hand, SiO₂ generated from the oxidation of SiC continued to react with bauxite to form mullite; on the other hand, the highest firing temperature rose with the increasing SiC content, which promoted the decomposition of metakaolinite into mullite and amorphous SiO₂. It is generally believed that the transformation of amorphous SiO₂ into crystalline SiO₂ occurs above 1200°C, but the complex components of the raw material encouraged the transformation process,²⁹ as evidenced by the enhancement of the intensity of the SiO₂ diffraction peaks. Moreover, the enhanced intensity of the SiC diffraction peaks implied that the added SiC was not completely consumed. The chemical reactions that may occur during the heating process of the sample are Equations (4)–(7). 4′$$\mathrm{AlO}(\mathrm{OH})\to {\mathrm{Al}}_{2}{{\rm{O}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}({\rm{g}}),$$ 5′$${\mathrm{Al}}_{2}({\mathrm{Si}}_{2}{{\rm{O}}}_{5}){(\mathrm{OH})}_{4}\to {\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 2{\mathrm{SiO}}_{2}+2{{\rm{H}}}_{2}{\rm{O}}({\rm{g}}),$$ 6′$$2({\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 2{\mathrm{SiO}}_{2})\to 2{\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 3{\mathrm{SiO}}_{2}+{\mathrm{SiO}}_{2}(\mathrm{amorphous}\,\mathrm{phase}),$$ 7$$3(2{\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 3{\mathrm{SiO}}_{2})\to 2(3{\mathrm{Al}}_{2}{{\rm{O}}}_{3}\cdot 2{\mathrm{SiO}}_{2})+5{\mathrm{SiO}}_{2}.$$

Figure 7 shows the SEM images of mullite composites with various contents (3, 6, 9, and 12 wt%) of SiC additives. According to Figure 7, pores and cracks existed in all samples with poor interparticle bonding, which characterized the insufficient generation of liquid phase during the sintering process.

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The EDS spectrum of point A and area B in Figure 8 shows the generation of granular mullite. The energy spectrum indicates that the main elements in point A were O, Al, Si, and C, and the corresponding atomic percentages of the elements were 50.056%, 32.343%, 11.527%, and 3.168%, respectively. The atomic ratio of Al and Si was 3:1, which was close to the corresponding atomic percentage of mullite, and in combination with the XRD patterns (Figure 6), it can be inferred that granular mullite was formed in all samples. The atomic percentages of the four major elements, O, Al, Si, and C, in the B region were 38.804%, 16.309%, 31.393%, and 13.493%, respectively, and it was assumed that the mullite grains at this location were encapsulated by molten phases. The main reason for samples that merely had granular mullite might be the insufficient molten phases and the low interdiffusion rate between Al₂O₃ and SiO₂, resulting in inefficient conversion of the mullite grains as well as a loose ceramic structure.

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In summary, the optimum degree of mullite crystallization was achieved with 12 wt% SiC, and the final firing temperature was only slightly higher compared with 9 wt% SiC, while a significant amount of SiC remained. Meanwhile, limited by the low alumina content of the raw material, the preferred content of SiC additives was 12% for economic reasons. Furthermore, the microstructure of mullite ceramics prepared directly with SiC additives clearly exhibited an insufficient molten phase and only formed granular mullite grains, which may cause difficulties in obtaining excellent physical–mechanical properties.

Effect of MnO₂ on the performance of ceramic composites

Since improving the microstructure is essential to enhancing the mechanical performance of ceramic proppants, sintering additives have been used as an effective strategy for densification. Considering the low alumina content and the low Al–Si ratio in bauxite, MnO₂ was introduced to promote the mullite generation, besides, 1.5 wt% CaO was added to lower sinter temperatures and to improve experimental efficiency.

Physical and chemical properties

To recognize the contents of MnO₂ effect on the physical–mechanical performance of microwave-sintered ceramic composites, as well as to determine the optimum raw material ratio, samples with varying MnO₂ contents were prepared (Table 1) and sintered at 800 W for 2.5 h. Figure 9 shows the results of samples' testing properties, including the uniaxial compressive strength and apparent density.

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The uniaxial compressive strength of samples increased from 77.27 to 86.48 MPa when the addition of MnO₂ from 1 to 4 wt%. The max value of 86.48 MPa was 36.27% higher than the peak strength of the sample with only 12 wt% SiC added, which was obtained at 2 wt% MnO₂ content.

Phase compositions and microstructure

The XRD patterns of the material with different MnO₂ contents are shown in Figure 10. The intensity of the mullite diffraction peaks increased significantly compared with the samples without adding CaO–MnO₂ (Figure 6), implying that the additive effectively promoted the development of the crystals.

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The crystalline phase containing manganese was not detected in the XRD pattern, the reason is that the ionic radius of Mn⁴⁺ and Al³⁺ is close to each other, and they occupy similar space in the microstructure, so that Mn⁴⁺ may replace Al³⁺ and enter the mullite crystal lattice to form a solid solution.³⁰

SEM photomicrographs of samples with different MnO₂ contents are shown in Figure 11. With the addition of MnO₂, rod-like crystals appeared, and in combination with the atomic ratio of Al and Si and XRD patterns, it can be inferred that the granular grains of mullite crystals transformed into rod-like mullite crystals. These anisotropic rod-like mullite crystals intertwined with each other to form net structures, consequently enhancing the strength with low density.³¹ MnO₂ promotes the formation of a low-temperature liquid phase, which is conducive to mass transfer and diffusion and facilitates the sintering reaction, while the generation of the liquid phase can effectively promote the anisotropic growth of mullite,³² and the granular grains of primary mullite crystals served as the seeds for the nucleation of rod-like secondary mullite crystals,³³ as shown in Figure 12.

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The amount of liquid phase increased significantly with higher MnO₂ content and was encapsulated around the grains. This can be explained by the fact that MnO₂ reduced the softening temperature of the glass phase,³⁴ and the liquid formation filled the pores and increased densification during sintering, which was consistent with the analysis of the growth apparent density shown in Figure 9.

In summary, when the mass ratio of MnO₂:CaO:SiC:bauxite was 2:1.5:12:84.5, the maximum uniaxial compressive strength of the material can be obtained, so it was set as the appropriate proportion for the manufacture of microwave-sintered proppants.

Parameter selection of sintering conditions on the performance of proppants

Once the mass ratio of bauxite:SiC:MnO₂:CaO was determined to be 84.5:12:2:1.5, the proppants' properties and structure are mainly affected by the sintering conditions, including sintering temperature, sintering time, and sintering rate,³⁵ which are directly correlated with the input power in the microwave sintering process. Therefore, the effects of the above four main process parameters on microwave manufacturing were first discussed in this study to ensure proppants' mechanical properties.

To recognize the relationship between microwave input power and conventional sintering parameters and to provide a reasonable range for parameters design, proppants were sintered at 800–1200 W for 1–2.5 h, setting the highest sintering temperature at 1150°C. The test results of proppants' physical–mechanical properties are shown in Figure 13.

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With the increase of input power, it can effectively improve the highest temperature and heating rate during the sintering process at the same sintering time of 2.5 h, as shown in Table 3.

Table 3 Effect of microwave input power on sintering parameters (total sintering time 2.5 h).

No.	Input power (W)	Highest temperature (°C)	Heating rate (°C/min)
A	800	1050	14.00
B	900	1100	16.92
C	1000	1150	25.56
D	1100	1150	32.86
E	1200	1150	38.33

The sintering temperature is an essential parameter that has a direct impact on the final performance of the ceramic material and needs to be carefully controlled. As input power increased from 800 to 1200 W, sintering temperature rose from 1050°C to 1150°C, the mechanical strength of individual proppant increased from 13.58 to 18.45 N due to crystalline phase development and molten phase filling of interval pores, as can be seen from in Figure 14. Moreover, as the molten phases increased, proppants formed a dense enamel layer, enhancing their apparent density from 2.39 to 2.55 g/cm³ (Equation 3).

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A suitable sintering rate is critical for adequate crystal synthesis, especially because microwave sintering has a fast but uncontrollable heating rate compared with conventional sintering, where the heating rate can be accurately controlled, and the sintering temperature increases steadily. According to Table 3, the mechanical strength of single proppant decreased from 18.45 to 17.33 N, when the sintering rate rose from 25.56°C/min to 38.33°C/min, the decrease in properties may be related to incomplete solid phase reaction due to excessive sintering speed. Meanwhile, the overfast sintering rate reduced the formation of the molten phase, and the apparent density decreased from 2.55 to 2.47 g/cm³.

Considering the sintering temperature, the selection of an appropriate heating rate is also an important factor in enhancing the proppants' performance, so the optimal microwave sintering input power was chosen as 1000 W.

The appropriate sintering time is also closely related to the densification of the proppant structure. Differing from the conventional sintering process can accurately control the holding time; the higher the microwave power, the shorter the heating time and the longer the holding time of the samples under the same conditions of sintering temperature and sintering time. Therefore, to further optimize the sintering time, the experiments were set for 1, 1.5, 2, and 2.5 h at 1150°C and under the power of 1000 W, respectively. It can be seen from Figure 15 that the maximal strength of a single proppant and apparent density can be attained at the same time under the sintering time of 2 h, which were 21.09 N and 2.58 g/cm³, respectively. When the time was extended to 2.5 h, the proppants were obviously overburning, since the long holding time created excessive amorphous phases, which can destroy the structure of proppant and even cause serious adhesion. Therefore, it is necessary to strictly control the sintering time under different microwave input powers.

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In summary, it was optimized that the input power of 1000 W, the sintering temperature of 1150°C, the sintering time of 2 h, and the proppants obtained the highest single sphere mechanical strength of 21.09 N and the apparent density of 2.58 g/cm³.

According to the standard SY/T 5108–2014, the series properties of proppants were determined, and their performance meets the standard requirements, as listed in Table 4. The breakage ratio was 8.5% under 28 MPa closure pressure, and the apparent density was less than 3.00 g/cm³, which could be categorized as low-density proppants. A low apparent density will reduce the transport cost of fracturing fluid containing proppants, and it is more conducive to delivering the proppant to the distal formation fractures, leading to favorable conductivity.

Table 4 The performance of low-grade bauxite-based proppants.

Performance	Low-grade bauxite-based proppants	Standard value
Roundness	0.7	≥0.7
Sphericity	0.7	≥0.7
Breakage ratio (%)	8.5a	≤9
Apparent density (g/cm3)	2.58b	No
Turbidity (FTU)	52	≤100
Acid solubility (%)	6.77	≤7

CONCLUSION

In summary, a novel study was conducted to manufacture frac proppants based on microwave sintering from low-grade bauxite:

• (1)

  The main phases of low-grade bauxite were kaolinite (Al₂(Si₂O₅)(OH)₄) and diaspore (AlO(OH)), which were difficult to directly couple with microwave. By introducing SiC as microwave hotspot to heat up, mullite ceramic materials can be successfully prepared. Nevertheless, pores and cracks existed in all samples with poor interparticle bonding, and only granular-shaped mullite grains were formed with only SiC added.

• (2)

  When the mass ratio of MnO₂:CaO:SiC:bauxite was 2:1.5:12:84.5 and under the sintering condition of 800 W, 2 h, the material with the maximum uniaxial compressive strength of 86.48 MPa could be obtained, which was 36.27% higher than the peak strength of the sample with only SiC added.

  Meanwhile, the SEM showed that with the addition of MnO₂ and CaO, the granular grains of mullite crystals were transformed into rod-like mullite crystals, which formed net structures and consequently enhanced the uniaxial compressive strength with low density. The amount of liquid phase increased significantly and filled the pores with a higher MnO₂ content, increasing densification during sintering.

• (3)

  With the increase in input power, it can effectively improve the highest temperature and heating rate during the sintering process. When the sintering rate increased from 25.56°C/min to 38.33°C/min, the pressure resistance of a single proppant decreased from 18.45 to 17.33 N. It should also be noted that the overfast sintering rate would lead to an incomplete solid-phase reaction and reduce proppants' mechanical properties, since microwave sintering has a fast but uncontrollable heating rate.

  The selection of a reasonable sintering time under different microwave powers would further enhance the effect of microwave sintering of proppant. When the sintering time was extended to 2.5 h at 1000 W, the mechanical strength of the proppant decreased due to overfiring since too much amorphous phase was produced.

• (4)

  When the mass ratio of MnO₂:CaO:SiC:bauxite was 2:1.5:12:84.5 and under the sintering condition of 1000 W,2 h, the performance (breakage ratio of 8.5% under 28 MPa closed pressure, apparent density of 2.58 g/cm³, turbidity of 52 FTU, and acid solubility of 6.77%) could meet the requirements of the Chinese Petroleum and Gas Industry Standard “Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations” (SY/T 5108–2014).

ACKNOWLEDGMENTS

The authors are grateful for the financial support from the Key Program of National Natural Science Foundation of China (Grant No. 52234003).

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