Basalt fiber is a high-performance, green, and environmentally friendly inorganic mineral fiber made of basalt ore as a raw material with high-temperature melting and drawing [1]. Based on the natural inorganic oxide chemical composition of basalt fiber, it exhibits stable chemical performance and outstanding mechanical properties. It is widely used in many fields such as national defense, military, construction, fire protection, environmental protection, and with increasing attention, there is a broader prospect in the field of fiber research and application [2–4]. Because basalt is a natural ore, its chemical composition and crystal distribution are quite different. Therefore, the preparation of basalt fibers with excellent performance not only requires mature technology and equipment, but also needs to clarify the distribution and function of the chemical composition of the raw materials [5,6].
Basalt fiber is rich in silica and aluminum, which allows it to act as a skeleton in the spatial network structure of the fiber, and other elements are supplemented inside and outside the skeleton in different ways, thus determining the performance of the fiber [7]. As the main component element of basalt fiber, iron both affects the crystalline structure and appearance color of the fiber. However, the total iron content of basalt fiber fluctuates over a wide range from different growing areas, and the total iron content of some fibers even exceeds 15% [8].
Ying and Zhou [9] proposed that the presence of basalt iron component is beneficial to enhance the thermal stability of fibers, which is mainly due to the formation of Fe3O4 as a natural nucleating agent during the melting process of basalt; when the fiber is heated below 300°C, the fiber structure will recombine around the nucleating agent. Zhao et al. [10] systematically studied the effect of iron content on the crystallization behavior of CaO–MgO–Al2O3–SiO2–Fe2O3 system. With the increase of iron content, free iron gradually aggregated into clusters, which affected the crystallization stability and crystallization potential. The aggregation of iron is mainly due to a smaller bond energy of the Fe–O bond than Si–O bond, and the bonding between iron and oxygen ions destroys the structure of silicon–oxygen tetrahedron, which provides the possibility for the formation of iron-rich aggregates [11]. Huo et al. [12] investigated the crystallization behavior of basalt glass. Crystal formation can be inhibited effectively by reducing the total iron content, and the result shows that the iron component has an important influence on the thermal stability of basalt fiber and its corrosion resistance. Eick et al. [13] studied the acid erosion of basalt under different pH conditions, and the results indicated that the acid etching of cations in basalts follows the order of Fe ≈ Mg > Si > Al ≈ Ca at low pH and in citrate and oxalate environments, which is consistent with the solubility of minerals (olivine > pyroxene > feldspar > ilmenite). When the basalt fiber surface is corroded by hydrochloric acid, fibrous Fe2O3 and other substances easily react to form a smooth surface [14]. However, in the study of Li et al. [15], it was suggested that the surface smoothness of basalt fibers in hydrochloric acid solution has a direct relationship with its concentration, and hydrochloric acid has the dual function of “cleaning” and corrosion. In the process of sulfuric acid corrosion, the gloss of the fiber surface and the iron content are gradually reduced with the phenomenon of layer shedding and cratering [16]. The experimental results showed that after adding basalt, the iron-containing substances in the raw materials could release Fe3+ under acidic conditions, which affected the photosynthesis of photosynthetic bacteria, thus improving the hydrogen production capacity [17]. Combined with the aforementioned analysis, iron has a prominent effect on the structure and properties in basalt fiber, but the research on the occurrence form of iron components and the acid etching mechanism of acid etching process is not adequate in the past, and there is a lack of systematic research data supporting the research and development of basalt fiber and products. Thus, the main objective of this study is to investigate the occurrence form of iron components in basalt fiber, and the acid etching behavior and mechanism in sulfuric acid solution system in order to provide theoretical and technical support for basalt fiber performance research and post-product development.
The basalt fiber raw silk used in this experiment is produced by the Basalt Fiber Technology Co. Ltd. (Sichuan, China), and the color of the raw silk is brown. Analytical sulfuric acid (98%) was mainly used in the experiment and purchased from Kelong Chemical Industry (Chengdu, China).
A certain amount of basalt fiber raw silk was weighed into a 100 mL glass beaker, and sulfuric acid was added in different amounts to form a suspension. The suspension was then continuously stirred (200 rpm) for the acid etching reaction at ambient temperature controlled by an oil bath, and the temperature control accuracy was set at ±1°C. After the reaction, the suspension was sampled and analyzed, and the residual solid samples were analyzed for structural morphology and chemical composition. During the experiment, the single-factor experiment method was mainly used to study the effects of sulfuric acid concentration, acid etching time, acid etching temperature, and liquid–solid ratio on the acid etching of iron components. The experimental scheme is presented in Table 1. All experiments were performed in triplicate, and the results were averaged.
The acidic solution generated in this study was used to synthesis calcium sulfate whiskers.
Acid etching experimental scheme of iron components in basalt fiber
| Leaching time (h) | Sulfuric acid concentration (mol/L) | Temperature (°C) | Liquid–solid ratio (mL/g) | |
|---|---|---|---|---|
| Scheme1 | 2, 4, 6, 8, 10 | 1 | 80 | 20:1 |
| Scheme2 | 4 | 1, 2, 3, 4, 5 | 80 | 20:1 |
| Scheme3 | 4 | 1 | 50, 60, 70, 90, 100 | 20:1 |
| Scheme4 | 4 | 1 | 90 | 10:1, 15:1, 20:1, 25:1, 30:1 |
For residue analysis, the basalt fiber etched in the acid solution was washed with deionized water three times to ensure the removal of residual acid and reaction products. Then, it was dried in an oven at 105°C for 2 h.
The iron content in the solution obtained by the acid etching reaction was analyzed by UV-vis spectrophotometry [18]. For the analysis of iron component content in basalt fiber raw silk, it was first placed in an agate crucible for full grinding, and 1.00 g of basalt fiber precursor powder was prepared into 500 mL solution after digestion, then an appropriate amount of solution was taken to analyze the total iron content (510 nm) and ferrous content (364 nm) by UV-vis spectrophotometry. XRF was performed using an Axios-X-ray fluorescence spectrometer with the melting method to observe the chemical composition of the basalt fiber raw silk. XPS spectra of the iron form in basalt fiber were recorded by an XSAM800 X-ray photoelectron spectroscopy analyzer with Al Kα radiation (1486.6 eV); the operating voltage and current were 12 kV and 15 mA, respectively, by using FAT mode, the background vacuum of the analysis chamber was 2 × 10−7 Pa, and the binding energies were corrected by referencing the C 1s (284.6 eV) at the peak. Scanning electron microscopy combined with energy spectrum analysis (SEM-EDS) was used to observe the structural morphology and main element distribution of basalt fiber raw silk before and after acid etching using a UItra55 high-resolution cold field emission scanning microscope. The sample was coated with gold film for detection, and its resolution was 0.8 nm at 15 kV, 1.6 nm at 1 kV, and 4.0 nm at 0.1 kV. The magnification was 12−9 × 105 × (SE), 100−9 × 105 × (BSE), the maximum range index of sample movement was less than 120 mm (X, Y direction) and 50 mm (Z direction), and the inclination and rotation angle were −4 to 70° and 360°, respectively.
The dissolution ratio (α) was calculated according to equation (1)
m 1 – total iron content in acid etching solution (mg).
Appropriate amounts of raw basalt fiber and silk were thoroughly ground and pressed in an agate mortar. The XRF analysis results of the samples are listed in Table 2. It can be seen that the total iron content (calculated as Fe2O3) in fiber raw basalt is 11.20%, which is one of the main components.
Main chemical composition of basalt fiber
| Composition | SiO2 | CaO | Al2O3 | Fe2O3 | MgO | TiO2 | Na2O | K2O |
|---|---|---|---|---|---|---|---|---|
| Content/% | 50.63 | 9.32 | 14.38 | 11.20 | 8.32 | 1.33 | 3.74 | 0.61 |
According to the analysis, the total iron content in the basalt fiber is 11.13%, which is consistent with the results of the XRF analysis. The ferrous content is 4.46%, accounting for 40.07% of the total iron content.
Different volumes of iron standard solution (0.00, 2.00, 4.00, 6.00, 8.00, 10.00 mL) with the Fe3+ content of 10 mg/L were added to six 50 mL volumetric flasks. The sample preparation test was carried out according to the standard methods [18]. The standard curve was plotted as shown in Figure 1, using the iron content as the horizontal coordinate and absorbance as the vertical coordinate. The standard curve equation was y = 0.2384x + 0.0011, and R 2 = 0.9998.
Standard curve of iron.
Figure 2 shows the effect of acid etching time on the dissolution behavior of iron components in basalt fiber raw silk. It can be seen from Figure 2 that with the extension of the acid etching time, the mass loss of the raw silk increased continuously, but the dissolution rate of the iron component first increased and then decreased, finally tending to balance. In addition to iron, basalt fiber also contains a large amount of aluminum, magnesium, titanium, and other components that can react and dissolve under acidic conditions. With the extension of reaction time, the reaction develops from the surface to the deep layer, destroys the lattice, and releases the embedded metal elements, leading to the mass loss of basalt fiber raw silk [19]. The iron component in basalt fibers is dominated by the presence of Fe3+ and Fe2+. In the sulfuric acid solution system with pH < 3.0 under higher concentrations of Fe3+, FeOOH can be formed in the solution after a long time at high temperatures. At the same time, trace Ti(IV) fractions in basalt fibers can hinder the dissolution of FeOOH [20,21]. To suppress FeOOH formation, control of Fe2+ oxidation requires both low pH and reduced temperature which is necessary. Therefore, the iron component in basalt fiber is continuously dissolved in dilute sulfuric acid at high temperatures, and then the dissolved iron component is precipitated and dissolved, and finally reaches equilibrium [22].
Relationship between acid-etching time and dissolution rate of iron components.
Figure 3 shows the effect of sulfuric acid concentration on the dissolution of iron components in the basalt fibers. With the increase of sulfuric acid concentration, the mass loss of basalt fiber increased slightly and then decreased, while the dissolution rate of the iron component decreased continuously. The main reason for this experimental phenomenon lies in two aspects: one is that a high concentration of sulfuric acid has a passivation effect [23]. When the acid-etching temperature is high, iron, aluminum, and other elements can form an oxide film in a sulfuric acid environment, and Fe2+ and Fe3+ in the solution can form precipitates such as ferrous sulfate monohydrate, iron oxide, and jarosite. The resulting film or precipitate will adhere to the surface of the precursor, thereby preventing the reaction from deepening internally [24,25]. Second, a larger concentration of sulfuric acid results in greater viscosity of the system, blocking molecular movement and weakening ion exchange diffusion, which is unfavorable for the dissolution of components in raw silk [26].
Relationship between sulfuric acid concentration and dissolution rate of iron components.
Figure 4 shows the effect of different temperatures on the dissolution behavior of the iron components in the basalt fibers. It can be seen from Figure 4 that with the reaction temperature between 50 and 90°C, the mass loss of basalt fiber and the dissolution rate of iron component both show an upward trend. When higher than 80°C, the increasing trend slows down and tends to be stable, which may be attributed to the molecular movement in the system. At high temperatures, faster molecular motion is conducive to breaking the reaction barrier, accelerating diffusion, and effectively promoting the acidolysis reaction and dissolution of iron components [27]. However, when exceeding a certain temperature, the dissolution rate of the iron component is no longer significantly increased, mainly because the iron component on the surface of the precursor is completely dissolved, reducing the effective collision probability of the internal iron element and H+.
Relationship between temperature and dissolution rate of iron components.
Figure 5 shows the effect of the liquid–solid ratio on the dissolution behavior of iron components in basalt fiber. As shown in Figure 5, when the quality of the basalt precursor was fixed, the mass loss rate and iron dissolution rate of the basalt precursor increased continuously with increasing dilute sulfuric acid dosage. When other conditions are fixed, more sulfuric acid means more H+ content in the system, and it can effectively increase the differential concentration at the solid–liquid interface during dissolution, leading to lower viscosity and molecular diffusion resistance, thereby promoting mass transfer between the interfaces [28,29].
Relationship between liquid–solid ratio and dissolution rate of iron components.
Figure 6 shows the phase analysis results of basalt fiber raw silk before and after acid etching in a sulfuric acid solution. An inconspicuous diffraction peak is observed in Figure 6, which corresponds to the amorphous material. After acid etching, the position of the main peak shifted to a smaller angle, indicating that iron atoms were released from the network gap, which can induce a low lattice distortion rate and crystallite size [30].
XRD patterns of basalt fibers before and after acid etching.
Figure 7 shows the morphological changes and micro-analysis results of the basalt fiber before (Figure 7a) and after (Figure 7b) acid etching in a dilute sulfuric acid solution. As shown in Figure 7, basalt fiber raw silk with the presence of Si, Fe, Ca, Al, Mg, Na, Ti, and other elements is uniformly dispersed and has a smooth surface before acid etching, of which the iron component content reaches 16.2%. After acid etching, the basalt fiber precursor still maintained a uniform fraction, but the surface was obviously chapped with fine particles attached to the surface. Microzone analysis shows that the main composition of the surface changed significantly compared with that before acid etching, with the percentage of S and O content increasing and the percentage of Fe and Al content decreasing.
Morphological changes and micro-analysis results of basalt fiber before and after acid etching before acid etching, (b) after acid etching, (c) micro-analysis result of basalt fiber before acid etching, (d) micro-analysis result of basalt fiber after acid etching.
The surface elemental composition and chemical states of the basalt fiber before and after acid etching studied by XPS measurements are shown in Figure 8. It can be seen from Figure 8(a) that the iron component in the basalt fiber raw silk mainly exists in the form of Fe2O3. After dilute sulfuric acid etching, the iron component in the precursor is acidified, and the component in the form of Fe2O3 disappears (as shown in Figure 8(b)). However, the microzonation analysis results show that the iron fraction content remained 6.3%, revealing that there are other insoluble components of sulfuric acid co-existing with iron oxide in basalt fiber raw silk, such as iron atoms embedded in silicate lattices [31]. As reported in the reference literature, iron exhibits a dualistic influence on fiber mechanics: Elevated iron content in basalt glass lowers the crystallization activation energy during fiber formation, inducing spontaneous crystal nucleation that leads to surface defects and degrades tensile strength. Conversely, at optimal concentrations, iron serves as both network former and modifier. The resulting [FeO4] tetrahedra cross-link the silicate framework, enhancing structural stability while improving tensile performance [32,33]. Thus, the residual 6.3% iron primarily sustains fiber stability.
XPS analysis results of basalt fiber before (a) and after (b) acid etching.
Table 3 summarizes the dissolution of iron fractions from basalt fibers after leaching in a sulfuric acid solution. The leaching solution is mostly in the presence of Fe3+ and contains a certain amount of Fe2+, which accounts for 13.62% of the total amount of dissolved iron components. These results further confirm that the iron oxide on the surface of basalt fiber is easily decomposed and dissolved by sulfuric acid, and the iron component embedded in the silicate lattice can also be hydrolyzed or replaced under certain conditions.
Dissolution of iron components in basalt fiber
| Number | Dissolution ratio (%) | Total iron content (mg) | Ferrous iron content (mg) |
|---|---|---|---|
| BFSL-Y-01 | 1.72 | 7.3112 | 1.0875 |
| BFSL-Y-02 | 1.59 | 7.3268 | 0.9458 |
| BFSL-Y-03 | 1.67 | 7.3034 | 0.9551 |
| Average value | 1.66 | 7.3138 | 0.9961 |
| R 2 | 2.38% | 0.02% | 7.99% |
Acid etching conditions: H2SO4 concentration,1 mol/L; acid etching temperature, 90°C; liquid–solid ratio, 20 mL/g; reaction time, 4 h.
The basalt fiber is mainly composed of silicate, and there are other structural units such as island [SiO4]4−, chain [Si2O6]4−, laminated [Si2O6]4−, and silica–aluminum substitution in the microfine structure, which forms a “planar-like” structure. Under certain conditions, Fe2O3 with variable coordination numbers (four or six) can be inserted into the silicate network as a network-forming body or a network intermediate to connect the [SiO4] tetrahedron [30]. Combined with the results of the characterization and the test of dissolution, a possible acid etching mechanism is proposed in Figure 9. The process may be that under a sulfuric acid environment, at first, the soluble substance will be dissolved and quickly migrate to form surface defects in the fiber, exposing iron-containing components as H+ corrosion sites (equations (2) and (3)). With the deepening of acid etching, grooves are formed on the fiber surface. After reaching a certain degree, the intermediate atoms (Fe, Mg, Na, K, etc.) bound to the [SiO4] tetrahedron will be leached by acid, which may drive the three-dimensional silicate structure transformed into a chain-like or layered structure until fiber surface delamination occurs, or be cracked due to drying at corrosion sites. In some conditions, the iron component embedded in the silicate lattice can also undergo acid etching, with H+ occupying the Fe2+/Fe3+ position.
Mechanism diagram of the iron component in basalt fiber in sulfuric acid solution.
Basalt fiber raw silk, produced by Basalt Fiber Technology Co. Ltd., in China was selected as the research object. The effects of acid etching time, sulfuric acid concentration, acid etching temperature, liquid–solid ratio, and other factors on the acid etching process of basalt fiber were studied by a single-factor experiment. The chemical composition, phase, structure, and morphology of the basalt fiber before and after acid etching were analyzed by instrumental analysis combined with chemical analysis, and the dissolution mechanism of the iron component was speculated. The main conclusions are as follows:
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(1)
Increasing the acid etching time, sulfuric acid concentration, acid etching temperature, and liquid–solid ratio can promote the dissolution of iron components in basalt fiber, which can accelerate the acid etching of basalt fiber in sulfuric acid solution. Under the conditions of sulfuric acid concentration of 1 mol/L, acid etching temperature of 90°C, liquid–solid ratio of 20 mL/g, and reaction time of 4 h, the dissolution rate of iron component in basalt fiber was 1.66%.
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(2)
During the acid etching process, the phase change of the basalt fiber was not obvious. Due to the dissolution of the iron component, the lattice distortion rate of the material is reduced.
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(3)
During the acid etching process, the iron components exposed at the surface defects of the fiber first began to be etched, and then the acid etching deepened, resulting in a chapped surface of the fiber, forming ravines, and even delamination.