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Guotao Zhou, Yilin Wang, Tiangui Qi, Qiusheng Zhou, Guihua Liu, Zhihong Peng, and Xiaobin Li, Comparison of the effects of Ti- and Si-containing minerals on goethite transformation in the Bayer digestion of goethitic bauxite, Int. J. Miner. Metall. Mater., 30(2023), No. 9, pp.1705-1715. https://dx.doi.org/10.1007/s12613-023-2628-3
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In 2021, the global scale of the alumina industry reached 135 million tons [1], of which over 30wt% was manufactured through the Bayer method with goethitic bauxite (goethite content exceeding 50wt% of iron minerals) as raw materials. Some of the iron in goethite could be replaced by aluminum, with substitution rates ranging from 0 to 33mol% [2]. In a typical low-temperature Bayer digestion process (digesting gibbsite with caustic alkali at 135–150°C) [3], the inertness of associated goethite (including that replaced by aluminum) affects red mud sedimentation and reduces alumina recovery [4–5]. Adding lime or intensifying digestion conditions (temperature and alkali concentration) favors goethite transformation [6–8]. However, lime addition increases alumina loss due to the newly formed hydrogarnets [9–11]. Meanwhile, escalating digestion conditions significantly increases the reactivity of Ti-containing minerals (anatase and rutile) and Si-containing minerals (kaolinite and quartz), thus aggravating caustic alkali loss [12–14]. Therefore, a reductive Bayer method (adding reductants in the Bayer digestion process) was proposed to solve the goethite transformation. Moreover, the reductive Bayer method has the advantage of treating goethitic bauxite with low SiO2 (<3wt%) and high Fe2O3 (>15wt%) and allows the convenient acquisition of iron-rich red mud, which could be further utilized by the steel industry.
Davis et al. [15] presented a method for treating goethitic bauxite by performing digestion in the presence of reductants to achieve goethite transformation and enhance alumina recovery. The settlement of red mud slurry can also be enhanced by the transformation of goethite with adding the mixture of lime and reductants (sugar) [16]. For example, the additions including glycerol, Mg–Al–Fe metal powder, hydrogen, and ferrous sulfate could enhance the transformation of goethite into hematite and/or magnetite during the reductive Bayer process [17–21]. Notably, these studies presented a large variation in the minimal amounts of reductants, which was obviously related to the content of impurity minerals in bauxite.
The influence of Ti-containing minerals on goethite transformation has received considerable attention. Anatase has a significant retarding effect on goethite conversion at high-temperature (250–260°C) Bayer digestion [22–23], resulting from the formation of a sodium titanium layer on goethite particles [24]. Wu et al. [25] suggested that the adsorption of the generated titanate on goethite retards goethite transformation. Nevertheless, both points of view for the retarding effect on goethite transformation lack sufficient evidence. Our previous work [26] found that Ti-containing minerals form dense sodium titanate layers on the hematite surface during high temperature Bayer digestion, hindering the further reaction between hematite and the sodium aluminate solution. Goethite transformation is a more complex process than hematite transformation because it involves dehydroxylation and is free from crystal lattice aluminum and iron mineral reconstruction [27]. Hence, the interaction mechanism of goethite and anatase needs to be studied in detail.
During the industrial-scale test of reductant-assisted Bayer digestion, we found that the amounts of reductants required to convert goethite completely were different from those required for the conversion of bauxite with various silicon contents, implying that Si-containing minerals also affect goethite transformation. In high temperature Bayer digestion, almost all Si-containing minerals dissolve in the sodium aluminate solution to form silicate ions and then react with sodium aluminate ions to form the sodium aluminosilicate hydrate [28–29]. Kaolinite and quartz in diasporic bauxite hinder the transformation of hematite into magnetite during reductive Bayer digestion due to the strong interaction of desilication products and iron powder [30]. In the presence of silicates, the transformation of ferrihydrite into goethite or hematite could be retarded likely because silicate species link to each other into an immobile network that adsorbs onto ferrihydrite particles [31]. Hence, we speculated that Si-containing minerals could still have a retarding effect on goethite transformation in Bayer digestion.
In this work, we adopted anatase, kaolinite, and goethite to simulate the related minerals in goethitic bauxite. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were utilized to demonstrate the effect of Ti- and Si-containing minerals on goethite transformation. Furthermore, experiments on goethitic bauxite treatment were conducted through reductive Bayer digestion. The results contribute to a better comprehension of the reaction between Ti- or Si-containing minerals and goethite in Bayer digestion and provide theoretical guidance for achieving goethite transformation.
Goethite was synthesized through a hydrothermal process [32]. The specific procedures were as follows: 100 g of Fe(NO3)3·9H2O (analytical grade) was dissolved in a beaker containing 400 mL of deionized water. Then, 5 and 0.5 mol·L–1 potassium hydroxide solutions were poured into the beaker sequentially under stirring to ensure that the pH was 12.9. The beaker was sealed and placed in an oven at 70°C for 14 d. Finally, the goethite-containing slurry in the beaker was filtered, and the filter cake was washed with deionized water and dried in an oven at 100°C for 6 h.
Anatase (TiO2, >99wt%) was bought from Shanghai Macklin Biochemical Co. Ltd., China. Kaolinite (Al2O3·2SiO2·2H2O) was purchased from Qingdao Yuzhou Chemical Co. Ltd., China. The sodium aluminate solution was prepared by dissolving sodium hydroxide and aluminum hydroxide (NaOH and Al(OH)3, >99wt%) in boiled water. Analytical-grade hydrazine hydrate (N2H4·H2O, >98wt%) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd., China.
The XRD patterns and SEM images of the synthetic goethite, anatase, and kaolinite are displayed in Figs. S1–S2. Fig. S1(a) and (b) shows that the characteristic peaks of the goethite and anatase samples matched well with the standard cards of goethite [ICSD (29-0713)] and anatase [ICSD (73-1764)], respectively. There was kaolinite as the main phase in initial material, as shown in Fig. S1(c). Fig. S2 demonstrates that the microstructure of synthetic goethite, anatase, and kaolinite were rod-shaped particles, sphere-like particles, and flakes, respectively.
The goethitic bauxite used in the experiments was from the Boke Province, Republic of Guinea, and was crushed into particles with size of <96 μm (>70wt%). The main composition of bauxite is displayed in Table 1, which shows that the chemical composition of goethitic bauxite was 44.01wt% Al2O3, 2.60wt% SiO2, 2.38wt% TiO2, and 25.63wt% Fe2O3. The XRD pattern (Fig. S3) depicts that goethitic bauxite consisted of gibbsite, boehmite, kaolinite, anatase, hematite, goethite, and rutile, and the semiquantitative results are presented in Table 2. The contents of goethite and hematite were 18.7wt% and 10.6wt%, respectively.
| Al2O3 | Fe2O3 | SiO2 | TiO2 |
| 44.01 | 25.63 | 2.60 | 2.38 |
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| Goethite | Hematite | Gibbsite | Boehmite | Kaolinite | Anatase |
| 18.7 | 10.6 | 60.3 | 2.5 | 1.4 | 1.6 |
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Synthetic goethite digestion experiments were conducted in a molten mixed nitrate salt cell (YYL-150ML/6, Dingda Chemical Machinery Co. Ltd., China). 1 g of goethite, preset amount of hydrazine hydrate, and anatase or kaolinite were added to stainless steel reactors containing 100 mL of sodium aluminate solution (αk = 2.05, ρNa2Ok = 210 g·L–1). Anatase or kaolinite was added at 0wt%, 3wt%, 6%wt, 12wt%, 24wt%, and 48wt% of the goethite dosages. The amount of hydrazine hydrate was 10wt%, 25wt%, 50wt%, 100wt%, and 150wt% of the goethite dosages depending on the experimental conditions. Na2Ok denotes caustic alkali in Na2O, and αk is the molar ratio of Na2Ok and Al2O3 in the sodium aluminate solution. Then, the reactors were sealed and immersed in the cell at 260°C for 10, 20, 30, and 40 min. After a designated duration, the reactor was removed from the cell and immediately cooled with tap water. Then, the resulting slurry was filtered, and the filter cake was washed with hot water and dried at 100°C for 6 h.
Goethitic bauxite digestion experiments were carried out as follows: 29 g of goethitic bauxite, preset amount of hydrazine hydrate, and anatase or kaolinite were added to stainless steel reactors containing 100 mL of sodium aluminate solution (αk = 3.01, ρNa2Ok = 210 g·L–1). The amount of hydrazine hydrate was 0wt%, 0.25wt%, 0.50wt%, 1.00wt%, and 1.25wt% of the goethitic bauxite dosage, depending on the experimental conditions. The addition amounts of anatase or kaolinite were 0wt%, 1wt%, 3wt%, 5wt%, and 7wt% of the goethitic bauxite dosage. The reactors were sealed and immersed in the cell at 260°C for 80 min. The operation after the reaction was the same as the synthetic goethite digestion experiments.
The mineral phases of the samples were determined by XRD (TTR III, Rigaku, Japan) using Cu Kα radiation. The morphology and microcomposition analyses of samples were conducted with SEM (MIRA3-LMH, TESCAN, Czech Republic) and TEM (TecnaiG2 F20, FEI, USA) with EDS (EDX-MAX20, Oxford, England), respectively. XPS was performed (ESCALAB Xi+, ThermoFisher, USA) with a step size of 0.05 eV. The content of SiO2, Al2O3, and TiO2 in solid samples were measured through the alkaline fusion method. The specific operations were as follows: 0.25 g of the solid sample and 3 g of sodium hydroxide were mixed uniformly in a silver crucible, which was subsequently roasted in a muffle furnace at 750°C for 15 min. After the reaction, the silver crucible was removed from the muffle furnace. Then, boiling deionized water was poured into the silver crucible to dissolve the reaction products completely. The resulting solution was poured into the 250 mL volumetric flask containing 40 mL of hydrochloric acid solution (6 mol·L–1), and the volume was fixed. The fixed volume solution was diluted 20 times and analyzed by ICP (ICAP7400 Radial, Thermo Fisher Scientific, USA). The Fe2O3 content in solid samples was measured using Cr6+ titration [33]. The Na2O content in solid samples was determined with a flame photometer (AP1302, Aopu, China).
The peaks of goethite (110), hematite (110), and magnetite (311) in the XRD patterns were used to calculate the transformation rate of goethite. Intensities were calculated from the areas under the peaks. The transformation of synthetic goethite and goethite into goethitic bauxite was calculated in accordance with Eqs. (3–4), respectively [17].
| MBauxite=I1,Goethite,110I1,Goethite,110+I1,Hematite,110 | (1) |
| MRedmud=I2,Goethite,110I2,Goethite,110+I2,Hematite,110 | (2) |
| ηGo=(1−IGoethite,110IGoethite,110+IHematite,110+IMagnetite,311)×100% | (3) |
| ηAG=MBauxite−MRedmudMBauxite×100% | (4) |
where MBauxite and MRed mud refer to the proportion of goethite in the total iron oxides in bauxite and red mud, respectively; ηGo and ηAG denote the transformation rates of synthetic goethite and goethite in goethitic bauxite, respectively, after Bayer digestion; I1,Goethite,110 and I1,Hematite,110 indicate the intensity of goethite and hematite at the crystal plane (110) in bauxite; I2,Goethite,110 and I2,Hematite,110 are the intensity of goethite and hematite at the crystal plane (110) in red mud; IGoethite,110 and IHematite,110 are the intensity of the corresponding goethite and hematite peaks at crystal planes (110) in the reaction products of synthetic goethite; IMagnetite,311 is the intensity of the magnetite peak at crystal planes (311) in the reaction products of synthetic goethite.
The relative alumina digestion rate during Bayer digestion was calculated with Eq. (5):
| ηAl2O3=ω1−ω2ω1−1×100% | (5) |
where ηAl2O3 is the relative alumina digestion rate; ω1 and ω2 are the mass ratios of alumina and silica in goethitic bauxite and red mud, respectively.
To determine the effect of Ti- and Si-containing minerals on goethite transformation during the typical Bayer digestion process, the reaction behavior of anatase or kaolinite with goethite was investigated by varying the amount of raw materials (0wt%, 3wt%, 6wt%, 12wt%, 24wt%, and 48wt%). The XRD patterns of the products are presented in Fig. 1. The ηGo calculated in accordance with diffraction peak intensity is shown in Fig. 2.
Fig. 1 shows that in the absence of the addition of anatase or kaolinite, only the diffraction peaks of hematite were presented in the reaction product, indicating that goethite completely transformed into hematite when reacted with the sodium aluminate solution at 260°C for 40 min. Fig. 1(a) illustrates only the diffraction peaks of goethite were presented in the products with 3wt% anatase, implying that a very low amount of anatase could inflict a strongly retarding effect on goethite transformation. Notably, the peak diffraction of sodium titanate (Na2O·3TiO2) appeared as the anatase dose was increased to 48wt%, indicating the retarding effect of anatase on goethite transformation was resulted from the sodium titanate generated during the reaction. Fig. 1(b) shows that adding 3wt% kaolinite resulted in the partial conversion of goethite into hematite, given that the diffraction peaks of hematite and sodium aluminosilicate hydrate could be observed. When the kaolinite dose was increased from 6wt% to 24wt%, the goethite and sodium aluminosilicate hydrate diffraction peaks intensified, whereas the intensity of the hematite diffraction peaks weakened, and hematite finally disappeared as the kaolinite dose reached 24wt%. This result implies that kaolinite also has a retarding effect on the transformation of goethite, which is related to the generation of sodium aluminosilicate hydrate. As noted in Fig. 2, ηGo was 0 when added the anatase. However, when the addition of kaolinite reached 24wt%, ηGo was 0. This finding illustrates that the retarding effect of anatase on goethite transformation is greater than that of kaolinite. This difference may be related to the varying mechanisms of the retardation of goethite transformation.
The SEM and EDS results of the products with different amounts of anatase or kaolinite are displayed in Fig. 3 and Table 3. Fig. 3(a) shows the products were flaky hematite particles in the absence of anatase or kaolinite. With the addition of 3wt% anatase, rod-shaped goethite and some spherical particles were observed, as shown in Fig. 3(b). These findings, combined with the EDS data, indicate that the surface of rod-shaped goethite contained Na, Ti, and O elements, presumably generating amorphous Na–Ti–O compounds (amorphous sodium titanate) on goethite. Anatase reacted with free alkali (except for the alkali bound to aluminate ion) to form amorphous sodium titanate (Na2O·3TiO2) in the sodium aluminate solution at elevated temperature. This reaction could be represented by Eq. (6). With the increase in anatase dosage to 12wt%, the spherical particles disappeared, and the Na and Ti contents on the surface of the goethite continued to increase (Fig. 3(c)). As the amount of anatase increased to 48wt%, elongated fibrous sodium titanate particles intertwined with rod-shaped goethite particles appeared, as shown in Fig. 3(d). This phenomenon demonstrates that the Na–Ti–O compound that had attached to the surfaces of the goethite and fibrous sodium titanate retarded goethite transformation.
| No. | Additive | Dosages / wt% | Chemical composition / at% | |||||
| O | Fe | Ti | Na | Al | Si | |||
| (1) | Anatase | 0 | 21.01 | 78.37 | — | 0.25 | 0.37 | — |
| (2) | 3 | 75.83 | 19.33 | 2.69 | 1.72 | 0.43 | — | |
| (3) | 12 | 65.17 | 27.59 | 3.30 | 3.58 | 0.36 | ||
| (4) | 48 | 72.65 | 5.24 | 14.60 | 7.00 | 0.51 | — | |
| (5) | Kaolinite | 0 | 18.83 | 80.71 | — | 0.17 | 0.29 | — |
| (6) | 3 | 66.63 | 31.37 | — | 0.81 | 0.83 | 0.36 | |
| (7) | 12 | 68.90 | 22.70 | — | 4.33 | 2.45 | 1.62 | |
| (8) | 48 | 60.58 | 5.02 | — | 13.95 | 11.31 | 9.14 | |
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| 3TiO2(Anatase)+2NaOH=Na2O·3TiO2(Amorphous)+H2O | (6) |
Upon the addition of 3wt% kaolinite, a small amount of rod-shaped goethite and numerous flaky hematite particles appeared (Fig. 3(f)). With the increase in the added amount of kaolinite, the flaky hematite particles disappeared, as shown in Fig. 3(g–h). However, a large amount of rod-shaped goethite appeared. The EDS results in Table 3 show that Fe, Na, Al, Si, and O were presented on the goethite surface, implying that the generated sodium aluminosilicate hydrate was attached to the goethite surface. As the amount of kaolinite was increased to 48wt%, the spherical-like particles disappeared, and large sodium aluminosilicate hydrate particles appeared to cross-link with goethite. The results indicate that the retardation of goethite transformation by kaolinite may be caused by the generation of sodium aluminosilicate hydrate on its surface.
To further specify the retarding effect of anatase and kaolinite on goethite, the products after the addition of 48wt% anatase and kaolinite were analyzed using TEM and EDS, as shown in Fig. 4. After the addition of anatase, the products had crystal faces with d-spacing of 0.2505 and 0.5077 nm, which could be attributed to the (110) and (020) face of hematite and goethite, respectively, as shown in the enlarged Fig. 4(a). Notably, a dense layer was found on the outermost part of the hematite. The EDS data demonstrated that the goethite crystal contained a small amount of Na, Ti, and O, indicating that dense Na–Ti–O compound layers were presented on the surface of goethite.
Fig. 4(b) reveals that after the addition of kaolinite, the crystalline surface spacing on the surface of the rod particles reached 0.2505 nm, indicating the existence of a hematite layer on the surfaces of the rod-like goethite. EDS analysis found small amounts of Na, Al, Si, and O in addition to Fe. The sodium aluminosilicate hydrate attached to the surface of the goethite, as shown in Fig. 3. This finding indicates that kaolinite retarded the transformation of goethite mainly by generating a loose sodium aluminosilicate hydrate layer on the surface. Compared with the dense amorphous layer of Na–Ti–O, the loose sodium aluminosilicate hydrate layer had weaker effect on the reaction of goethite with the sodium aluminate solution. Therefore, the retarding effect of anatase on goethite transformation is greater than that of kaolinite.
XPS analysis was conducted to evaluate the surface chemistry of the reaction products of goethite with the addition of different amounts of anatase. Fig. 5(a) shows the characteristic peaks at the binding energy of 284.8, 458.3, 529.7, 714.48, and 1071.7 eV corresponded to C 1s, Ti 2p, O 1s, Fe 2p, and Na 1s, where the C 1s signal peak originated from the sample during transfer due to the absorption of CO2 in the air. Therefore, in addition to Fe and O elements, a certain amount of Na and Ti was also presented on the surface of the products. This finding proved that a layer of Na–Ti–O was generated on the surface of the goethite and confirmed the TEM results. With the increase in anatase dosage, the intensity of the Na 1s and Ti 2p peaks gradually increased, whereas that of the Fe 2p peaks gradually decreased, indicating that the greater the anatase dosages, the thicker the titanate layer on the surface of goethite. When 3wt% anatase was used, three binding energies of O 1s (529.7, 531.1, and 532.8 eV) were observed in Fig. 5(b), belonging to the O2− species, OH− group, and adsorbed oxygen species, respectively [34]. As the amount of anatase increased, the intensity of the O2− peak increased, whereas that of the OH− peak and adsorbed oxygen peak decreased, proving the existence of additional oxides on the surface of the goethite. The above results show that the Na–Ti–O nanolayer can be loaded on the goethite, thus hindering its reaction with the sodium aluminate solution. Fig. 5(c) shows that the Fe 2p spectra of the sample prepared with 3wt% anatase exhibited two main peaks near 711.4 and 724.6 eV that belonged to Fe 2p3/2 and Fe 2p1/2, respectively [35]. The Fe 2p3/2 peaks were deconvoluted into two peaks near 711.2 and 714.5 eV that belonged to Fe(III)–O and Fe(III)–OH, indicating that the iron on the surface of the particles was in the form of Fe(III). Fig. 5(d) shows that Ti 2p spectra did not significantly change with the increase in anatase amount.
Our previous work [36] found that adding iron powder could eliminate the retarding effect of sodium titanate on the transformation of hematite into magnetite during Bayer digestion due to the generation of loose and porous sodium titanium ferrate. However, the transformation of goethite is very different from that of hematite because it involves a dehydration process. Anatase or kaolinite was used in the experiments to clarify the elimination mechanism of the retarding effect in the reductive Bayer process. Different reaction times were examined under the following conditions: 100 mL of sodium aluminate solution, reaction temperature of 260°C, 1 g of goethite, 50wt% hydrazine hydrate, and 6wt% anatase and 96wt% kaolinite.
Fig. 6 shows that the mineral phase of the products changed significantly after 5 min. The product of anatase addition was goethite. By contrast, the products of kaolinite addition were mainly magnetite and goethite, small amount of unreacted kaolinite and sodium aluminosilicate hydrate. After 20 min of reaction, the former phase was primarily magnetite, whereas the latter was magnetite accompanied by sodium aluminosilicate hydrate. Fig. 7 depicts that with the addition of 50wt% hydrazine hydrate, goethite was completely converted into magnetite in the presence of 6wt% anatase and 96wt% kaolinite after 20 min. Notably, after the addition of 96wt% kaolinite, ηGo was 55.31% and 100% at 5 and 10 min, respectively, and was considerably greater than obtained in the presence of 6wt% anatase (0 and 4.55%) likely because the sodium aluminosilicate hydrate generated on the goethite surface was not as dense as the Na–Ti–O compound and the reductants could react with goethite well.
To further illustrate the interaction of goethite with anatase and kaolinite, Fig. 8 shows the SEM images of the products of goethite, hydrazine hydrate, and anatase and kaolinite after different treatment time in sodium aluminate solution at 260°C.
Under the addition of 6wt% anatase, the reaction product remained goethite with a rod-shaped structure at the former 5 min. After 10 min, small quantities of magnetite with a fine octahedral structure were observed on the goethite surface. With the prolongation of time, the original rod-shaped structure of the goethite particles disappeared, whereas long columnar particles appeared. In the product obtained with 40 min, large long columnar particles were mainly observed. This finding, combined with the results of XRD analysis in Fig. 6, indicated that these long columnar particles were magnetite. The change in magnetite growth orientation might be due to titanium doping during the growth of fine-grained octahedral magnetite. Similar effects have been reported in the literature [37].
The SEM images of the initial transformation products obtained with the addition of 96wt% kaolinite and the treatment time of 5 min revealed the presence of rod-shaped goethite, flaky hematite, and long columnar magnetite. At 10 min of reaction, the rod-shaped goethite particles almost disappeared, the amount of long cylindrical magnetite particles increased, and large lumpy sodium aluminosilicate hydrate particles emerged. As the reaction time was further extended, the rod-shaped goethite completely disappeared, and a large amount of columnar magnetite and large lumpy sodium aluminosilicate hydrate appeared. The addition of hydrazine hydrate induced the transformation of goethite into magnetite, which changed the interfacial properties of goethite and the newly generated sodium aluminosilicate hydrate. The retarding effect was thus eliminated.
To define the elimination mechanism, different dosages of the reductants (hydrazine hydrate), 1 g of goethite, and 6wt% anatase were examined in 100 mL of sodium aluminate solution at a reaction temperature of 260°C for 40 min. Fig. 9 depicts that the diffraction peak of magnetite appeared when the added amount of hydrazine hydrate was 10wt%, and that ηGo was 8.26%. In high-temperature alkaline systems, the reductant hydrazine hydrate reacted with goethite to form magnetite. The reaction formula could be expressed as Eq. (7) [38]. As the amount of hydrazine hydrate increased, the degree of the transformation of goethite into magnetite increased. When the added amount of hydrazine hydrate increased to 50wt%, goethite was completely converted into magnetite.
| N2H4·H2O+12FeO·OH=4Fe3O4+N2+9H2O | (7) |
The products obtained with different amounts of hydrazine hydrate (10wt%, 25wt%, and 50wt%) were subjected to XPS analysis to clarify the influence of the elemental surface composition and the change in the corresponding chemical state of the reaction products of goethite and anatase in reductive Bayer digestion. Fig. 10(a) illustrates that the intensity of Na 1s peaks in the survey spectra gradually decreased and completely disappeared with the increase in hydrazine hydrate dosage. Meanwhile, the intensity of Fe 2p and Ti 2p peaks gradually weakened, indicating that with the increase in hydrazine hydrate dosage, the Na–Ti–O layer on the goethite surface gradually weakened. As the amount of hydrated hydrazine increased, the intensity of the O2− and OH− peaks increased, as shown in Fig. 10(b), proving the existence of additional oxides and hydroxides on the surface of the goethite. Fig. 10(c) shows that the Fe 2p3/2 peaks were deconvoluted into two peaks about 711.3 and 713.2 eV that belonged to Fe(III). However, as the amount of hydrazine hydrate increased to 25wt%, Fe 2p3/2 was deconvoluted into peaks at 709.6, 710.8, and 713.1 eV, which corresponded to Fe(II)–O, Fe(III)–O, and Fe(III)–OH, respectively. These results demonstrate that Fe(II) and Fe(III) states coexisted on the surface of the particles. Fig. 10(d) shows that Ti 2p spectra did not significantly change with the increase in hydrated hydrazine.
The effect of hydrazine hydrate dosages (0, 0.25wt%, 0.50wt%, 0.75wt%, 1.00wt% and 1.25wt%) on goethite transformation and alumina recovery in goethitic bauxite was investigated. Fig. 11 shows that when the goethitic bauxite was digested at 260°C for 80 min without hydrazine hydrate, goethite continued to exist in the red mud, and ηAG and ηAl2O3 were 14.76% and 92.71%, respectively. With the increase in hydrazine hydrate, the diffraction peak of goethite disappeared, whereas the intensity of the hematite diffraction peak increased, indicating that goethite was transformed into hematite. ηAG increased from 28.88% to 100%, and ηAl2O3 increased from 94.57% to 98.87%, respectively, when the hydrazine hydrate dosage ranged from 0.25wt% to 1.00wt%. The continued increase in hydrazine hydrate dosage did not cause significant change. Therefore, 1.00wt% was selected as the optimal hydrazine hydrate dosage. Table 4 shows that the yield and Fe2O3 percentage of red mud were 39.83wt% and 65.87wt% in the absence of hydrazine hydrate. When the additive dosage was increased to 1.00wt%, the yield and Fe2O3 percentage of red mud reached 35.14wt% and 72.99wt%, respectively, reflecting the reduction of 4.69wt% in red mud yield and an increase of 7.12wt% in the Fe2O3 percentage of red mud.
| Hydrazine hydrate dosages | Red mud yield | Chemical composition | ||||
| Al2O3 | SiO2 | Fe2O3 | TiO2 | Na2O | ||
| 0 | 39.83 | 10.89 | 5.04 | 65.87 | 5.83 | 3.05 |
| 0.25 | 38.86 | 9.75 | 5.23 | 66.94 | 5.96 | 3.17 |
| 0.50 | 37.79 | 9.53 | 5.22 | 67.43 | 6.24 | 3.18 |
| 0.75 | 37.59 | 8.98 | 4.93 | 68.35 | 6.33 | 3.01 |
| 1.00 | 35.14 | 7.00 | 5.93 | 72.99 | 7.07 | 3.62 |
| 1.25 | 34.90 | 6.06 | 5.35 | 73.17 | 7.12 | 3.26 |
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The reaction behavior of anatase and kaolinite with goethitic bauxite when the amount of raw materials (1wt%, 3wt%, 5wt%, and 7wt%) was varied was determined to examine the effect of anatase and kaolinite on goethite transformation in goethitic bauxite during the reductive Bayer process. Experiments were conducted at 260°C for 80 min.
Fig. S4 shows that the diffraction peaks of goethite were presented in the products and that ηAG reached 43.76% with the addition of 1wt% anatase. This finding indicates that a miniscule amount of anatase could also have a strong retarding effect on the transformation of goethite. As the amount of anatase increased, sodium titanate and sodium titanate also appeared in the products. The amount of hydrazine hydrate was insufficient to eliminate the blocking effect of sodium titanate, and goethite could not be converted. However, hematite and sodium aluminosilicate hydrate dominated in the products when 1wt%–7wt% kaolinite was added, implying that goethite had been fully converted with the addition of 1.00wt% hydrazine hydrate. This phenomenon confirms that the blocking effect of anatase on goethite transformation during the reductive Bayer digestion process is greater than that of kaolinite.
(1) Anatase and kaolinite have a retarding effect on the transformation of goethite, especially anatase exerts more significant impact than kaolinite because it reacts with the sodium aluminate solution to produce the dense sodium titanate layer on the goethite surface, whereas the addition of kaolinite produces the loose sodium aluminosilicate hydrate layer on the goethite surface.
(2) Adding hydrazine hydrate as the reductant helps eliminate the retarding effect of anatase and kaolinite on goethite transformation because it promotes magnetite formation during Bayer digestion. During goethite transformation, titanium is embedded into the magnetite lattice to form Ti-containing magnetite. Conversely, the weakening of the interaction between magnetite and sodium aluminosilicate hydrate further reduces the influence of kaolinite.
(3) When goethitic bauxite is used as the raw material, the relative alumina digestion rate reaches 98.87%, and the Fe2O3 percentage in red mud reaches 72.99wt% during reductive Bayer digestion with 1.00wt% hydrazine hydrate at 260°C for 80 min. The obtained iron-rich red mud is expected to be further utilized in the steel industry.
The online version contains supplementary material available at https://doi.org/10.1007/s12613-023-2628-3.
The authors gratefully appreciate the financial support provided by the National Natural Science Foundation of China (No. 52104353) and the National Key Research and Development Program of China (No. 2022YFC3900900).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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| Al2O3 | Fe2O3 | SiO2 | TiO2 |
| 44.01 | 25.63 | 2.60 | 2.38 |
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CSV
| Goethite | Hematite | Gibbsite | Boehmite | Kaolinite | Anatase |
| 18.7 | 10.6 | 60.3 | 2.5 | 1.4 | 1.6 |
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CSV
| No. | Additive | Dosages / wt% | Chemical composition / at% | |||||
| O | Fe | Ti | Na | Al | Si | |||
| (1) | Anatase | 0 | 21.01 | 78.37 | — | 0.25 | 0.37 | — |
| (2) | 3 | 75.83 | 19.33 | 2.69 | 1.72 | 0.43 | — | |
| (3) | 12 | 65.17 | 27.59 | 3.30 | 3.58 | 0.36 | ||
| (4) | 48 | 72.65 | 5.24 | 14.60 | 7.00 | 0.51 | — | |
| (5) | Kaolinite | 0 | 18.83 | 80.71 | — | 0.17 | 0.29 | — |
| (6) | 3 | 66.63 | 31.37 | — | 0.81 | 0.83 | 0.36 | |
| (7) | 12 | 68.90 | 22.70 | — | 4.33 | 2.45 | 1.62 | |
| (8) | 48 | 60.58 | 5.02 | — | 13.95 | 11.31 | 9.14 | |
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| Hydrazine hydrate dosages | Red mud yield | Chemical composition | ||||
| Al2O3 | SiO2 | Fe2O3 | TiO2 | Na2O | ||
| 0 | 39.83 | 10.89 | 5.04 | 65.87 | 5.83 | 3.05 |
| 0.25 | 38.86 | 9.75 | 5.23 | 66.94 | 5.96 | 3.17 |
| 0.50 | 37.79 | 9.53 | 5.22 | 67.43 | 6.24 | 3.18 |
| 0.75 | 37.59 | 8.98 | 4.93 | 68.35 | 6.33 | 3.01 |
| 1.00 | 35.14 | 7.00 | 5.93 | 72.99 | 7.07 | 3.62 |
| 1.25 | 34.90 | 6.06 | 5.35 | 73.17 | 7.12 | 3.26 |
DownLoad:
CSV
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