3.1. Red Mud Samples Characterization
Table 1 shows the chemical composition of two red mud samples. It can be seen that the Fe
2O
3 content in the sample B is almost 50%, which is promising for iron extraction, but high percentage of Na
2O is one of the adverse factors for this purpose by conventional industrial methods, e.g., blast furnace smelting. The sample U, which was leached by lime milk, has insignificant sodium content, but due to the calcium enrichment by the treatment, a percentage of valuable components such as iron, aluminum and scandium is less. Accordingly, the basicity (CaO + MgO)/(SiO
2 + Al
2O
3) of the samples B and U differs considerably and is 0.46 and 1.21, respectively.
Figure 2 compares the diffractograms of the samples B and U with phase signs. The main minerals of both samples are hematite (Fe
2O
3) and katoite (Ca
3Al
2Si
3(OH)
12), but the minor phases are different. The sample B contains few hydrated aluminum phases, namely kaolinite (Al
2Si
2O
5(OH)
4), hydrogarnet (Al
2Ca
3(OH)
12), diaspore (AlOOH), while they are undetected in the sample U. Probably, the sample B can contain some of these phases in lower contents, but due to high peaks of CaCO
3 and Ca(OH)
2 transferred from leaching solution, they are imperceptible.
Figure 3 and
Table 2 compare the Mössbauer spectra of the red mud samples obtained at room temperature and at low temperature.
It can be seen that the spectra of both samples are similar to each other and consist of a sextet distorted into the internal part of the spectra, two additional lines (asymmetric in intensity) in the internal part of the spectra and a broad resonance line in the middle of the spectra. The sample U spectrum differs from the sample B spectrum by the presence of additional absorption between the resonance lines 1–2–3 and 4–5–6 of the sextet in the room-temperature spectrum, by the presence of shoulders in the lines 1 and 6 of the sextet in the low-temperature spectrum, by high intensity, small width and low asymmetry of two additional lines in the internal part of the spectra, and by low intensity of the middle line of the spectra. A decrease of temperature results in an insignificant decrease of the intensity of the broad middle line of the U spectrum, while the intensity of paramagnetic part (asymmetric doublet and broad middle line) of the B spectrum is practically independent of temperature. It can be concluded that asymmetry of the resonance lines of the sextets at room temperature cannot be caused only by superparamagnetism (characteristic for nano- or highly dispersed iron particles) [
66], but can be resulted from locally inhomogeneous environment of Fe atoms, at least in one phase present in the samples. On the other hand, the shoulders in the low-temperature spectrum of the U sample and the resonance lines out of the base line indicate superposition of the spectra of at least two magnetically ordered phases, one of which is responsible for distortion of the background line in the middle part of the spectrum at room temperature.
The spectra obtained at room temperature can satisfactory be described as superposition of four (for the sample B) or six (for the sample U) sextets and two doublets (
Table 2). Six sextets for the sample U include of four subspectra that are analogous to those for the sample B and two additional subspectra. The low-temperature spectra of both samples are satisfactory described as superposition of three sextets and two doublets.
The detailed description of the spectra for the sample U is presented in [
67]. The subspectra parameters of the sample B is similar to those of the sample U. The main difference of the B sample spectra is the absence of subspectra 5, 6, and 12. This group of sextets, which is present only in the sample U spectra, lowers the background in the middle part of the room-temperature spectrum, and brings about the shoulder in the resonance lines 1 and 6 of the sextets in the low-temperature spectrum. This group of sextets corresponds to alumogoethite, i.e., goethite [
68,
69,
70], in which part of Fe atoms is isomorphically substituted by Al [
71].
The first four sextets in the room-temperature spectra (1–4,
Table 2) are common for both samples. The parameters of the sextet 1 are quite similar to the parameters characteristic for hematite [
68,
72]. The internal sextets 2–4 that are broader, have lower intensity and lower magnetic splitting than the sextet 1 correspond to defect hematite structures different by a degree of isomorphic substitution of Fe atoms by Al atoms [
73].
In the low-temperature spectra, these “phases” are represented by two or three sextets (9–11,
Table 2) that differ by the width of resonance lines. The presence of Al atoms in iron sublattice leads to a lower value of the magnetic splitting for the external sextet than that characteristic for hematite [
68] and to the absence of the Morin transition [
74] in the investigated temperature interval. It should be noted that a decrease of the Morin transition temperature can be caused by an increase of the hydroxyl content in the oxygen sites of alumohematite [
75]. Lower magnetic splitting and broadening of the resonance lines in the subspectra of the B sample compared to those of the U sample indicate lower magnetic ordering of the B sample.
The paramagnetic part of the spectra of both samples includes two doublets that correspond to atoms Fe
2+ (7 and 13,
Table 2) and Fe
3+ (8 and 14,
Table 2) in high-spin state in octahedral oxygen surrounding [
76]. The width of the resonance lines for the Fe
2+ doublet is significantly lower than the width of the second doublet and does not change with temperature. On the contrary, the quadrupole splitting of this doublet significantly increases with temperature decreasing. This indicates that this subspectrum corresponds to a unified well-crystallised phase, containing Fe
2+ ions. Judging by the value of the quadrupole splitting and its temperature dependence, it can assume that this subspectum corresponds to one of alumosilicate minerals [
77].
The relative intensity of the Fe
3+ doublet (nos. 8 and 14) of the sample B, different from the U sample, is twice higher than that of the Fe
2+ doublet. Moreover, this doublet has much broader lines, which indicates that this subspectrum is a superposition of several sub-subspectra with closer parameters, corresponding to several phases, and also that these phases have lower degree of crystallinity. The intensity of this doublet for the sample U changes insignificantly, since it contains a paramagnetic part of alumogeothite in the room-temperature spectrum. The sample B, which does not contain this phase, has almost no temperature dependence of the intensity and quadrupole splitting, which excludes the description of this doublet as from superparamagnetic particles, similar to [
78].
Thus, the main iron-containing phase of the sample B is hematite, while in the sample U besides hematite/alumohematite there is a considerable content of alumogoethite.
3.2. Thermodynamic Calculations
The thermodynamic calculations by HSC Chemistry 9.9 software were carried out to compare a behavior of different components in the samples B and U during carbothermic roasting in the presence of Na2SO4. Based on the results of elemental and phase analyses and Mössbauer spectroscopy, the model compositions of the red mud samples were constructed as follows:
Red mud B, wt.%: 49.81—Fe2O3; 11.27—Ca3Al2Si3O12; 6.72—Al2O3∙2SiO2∙2H2O; 2.10—3CaO∙Al2O3∙2H2O; 7.95—CaTiO3; 4.17—CaCO3; 8.24—AlOOH; 2.04—CaSO4; 0.65—MgO; 3.30—Na2O; 1.07—SiO2; 0.85—P2O5.
Red mud U, wt.%: 29.89—Fe2O3; 7.80—FeOOH; 14.97—Ca3Al2Si3O12; 3.54—Al2O3∙2SiO2∙2H2O; 8.90—Ca(OH)2; 6.02—CaTiO3; 15.61—CaCO3; 8.26—AlOOH; 0.59—CaSO4; 1.01—MgO; 0.27—Na2O; 1.07—SiO2; 0.97—P2O5.
All the species were set as pure with the activity coefficient of 1. Due to difficulty of iron carburizing process, iron carbides were excluded from the calculation.
Figure 4 and
Figure 5 show the results of the calculation for the red mud samples B and U, respectively.
It can be seen that full iron metallization is possible both in the absence and in the presence of Na
2SO
4. Goethite contained in the sample U thermally decomposes into hematite at 210–350 °C [
79]. It is well-known that the carbothermic reduction of hematite, which is the main phase of the red mud samples, occurs gradually as follows: Fe
2O
3 → Fe
3O
4 → FeO → Fe [
80]. In addition, it is notable that sulfur, which is harmful impurity for steelmaking, generates iron-free compounds, so it can be favorable to obtain a good-quality concentrate for ferrous metallurgy by carbothermic roasting of red mud with sodium sulfate followed by magnetic separation.
Apparently, the following general reactions take place in the presence of sodium sulfate:
The Reaction (3) always occurs in the presence of sodium sulfate and carbon at reducing conditions [
81]. A behavior of the Reactions (4) and (5) depends on chemical composition of the raw materials. In the system with red mud sample B (
Figure 4) the Reactions (4) and (5) generate Na
2O∙Al
2O
3 and NaAlSiO
4, respectively, that lead to decreasing of gehlenite (2CaO∙Al
2O
3∙SiO
2) amount with increasing of sodium sulfate addition. It should be noted that both 2CaO∙Al
2O
3∙SiO
2 and NaAlSiO
4 are hardly soluble phases [
59,
60] that can complicate following extraction of aluminum from the tailings. In the system with red mud sample U (
Figure 5) high content of calcium inhibits the Reaction (5) by calcium silicate 3CaO∙2SiO
2 generation. As a result, Na
2O∙Al
2O
3 represents a single aluminum-containing phase, which is water-soluble [
82], in the system with red mud sample B with addition over 23.4% Na
2SO
4, so it seems to be more encouraging to use the raw materials with high basicity not only for iron recovery, but for the facilitation of a following aluminum extraction from the tailings.
Thus, thermodynamic calculations have shown that carbothermic roasting of both red mud samples with sodium sulfate leads to full iron metallization, but sample U appears to be more favorable for aluminum extraction than sample B.
3.4. Experimental Results of the Carbothermic Roasting Followed by Magnetic Separation
Table 4 summarizes the possibility and particularities of the roasting and magnetic separation of the red mud samples at various temperatures and sodium sulfate addition.
It can be seen that the results are quite different for the samples B and U. Melting without or with any addition of sodium sulfate in the samples B and U occurred above 1300 °C and 1400 °C, respectively, that led to obtaining of an almost molten and uniform sample, so these conditions are undesirable and unrelated to solid-phase carbothermic reduction process. The separation of the samples without addition is possible at 1250–1300 °C and 1400 °C, while the addition of 13.65% Na2SO4 and over expands the separation range to 1150–1300 °C and 1350–1400 °C for the samples B and U, respectively. Below these temperatures it is impossible to extract a significant amount of non-magnetic tailings.
To elucidate the obtained results, the iron grain growth process was studied.
Figure 7 compares the influence of addition of 13.65% Na
2SO
4 on iron grain growth at constant holding time in the range of 1000–1300 °C to red mud B and U.
The histograms show that at the range of 1000–1100 °C iron particles are mainly distributed between fractions of 1–20 μm in size (
Figure 7a,b). There is a small amount of iron grains larger than 40 μm in the samples of roasted red mud U in contrast with red mud B obtained in the similar conditions. An increase of the roasting temperature to 1150–1200 °C (
Figure 7c,d) led to an increase of the grain size in the sample B with addition of 13.65% Na
2SO
4, where the relative area of iron grains with a size larger 40 μm was more than 50%. It should be noted that in the sample U grains of the fraction larger 40 μm have the relative area about 10–20% under the same conditions. Further rise of the roasting temperature to 1250–1300 °C (
Figure 7e,f) caused a significant growth of iron grains for the sample B both with and without sodium sulfate addition, where the relative area of iron grains with a size larger 40 μm is more than 85%. For the sample U roasted at these temperatures the addition of 13.65% Na
2SO
4 considerably increased the relative area of iron grains with a size larger 40 μm, but it is still half as much as in the sample B.
To find out optimal conditions for the roasting of red mud U, we raised the roasting temperature additionally.
Figure 8 demonstrates the distribution of the iron grain fractions at 1350–1400 °C with and without Na
2SO
4.
It can be seen that, at 1350 °C, the relative area of iron grains larger than 40 μm is more than 60% and 25% in the samples with and without addition of sodium sulfate, respectively, while at 1400 °C it is in the range of 73–84%.
Thus, iron grain size calculations (
Figure 7 and
Figure 8) correlate with the general magnetic separation results (
Table 4). Based on the data comparison, it can be inferred that the main part of iron grains larger than 40 μm predetermines successful magnetic separation of the roasted samples.
Figure 9 demonstrates the magnetic separation indexes of the roasted sample B with addition of 13.65–27.3% Na
2SO
4 at 1150–1300 °C;
Figure 10 illustrates the microstructure of the roasted sample B obtained at 1150–1200 °C with varied amount of the additive.
Figure 9a,b demonstrate that both iron recovery and iron grade dependences are rather sigmoid. The micrographs (
Figure 10) clarify the process of iron grain growth at 1150–1200 °C, where the Na
2SO
4 addition contributed to increase the grain size. The iron particles up to 13.65% Na
2SO
4 addition have a small size and strongly attached to the gangue phase, so no separation occurred in the samples obtained at 1150–1200 °C, while the separation of the samples over 13.65% Na
2SO
4 addition at these temperature range leads to high iron recovery rates, but low grades of the magnetic concentrates. Besides the majority of iron grains, which are larger than 40 μm, the samples roasted at 1150–1200 °C contain a significant percentage of the grains 5–40 μm in size (
Figure 7c,d). Probably, the aggregates of iron grains of such size with gangue are unable to split apart during grinding process that decrease the iron concentrate grade. Apparently, decreasing of iron recovery and increasing of iron grade at the range of 1150–1250 °C are due to a growing proportion of the molten phase during the roasting. On the one hand, the liquid phase promotes diffusion and aggregation of iron metallic particles [
92] that enhances gangue–grain release during grinding and accordingly increases iron grade of the concentrate after magnetic separation. On the other hand, it is complicated to separate magnetically besides large iron particles also the grains of small and middle size attached to gangue from difficult to grind semi-molten material after solidification that results to decreasing of iron recovery. It can be seen from
Figure 9a that the temperature dependence of iron recovery has a minimum at 1250 °C with the addition of 18.2–27.3% Na
2SO
4 that can be explained by attaching of iron grains with size less than 40 μm (see
Figure 7e) to the gangue phase in the presence of a substantial amount of liquid phase (see
Table 4). An increase in iron recovery at 1300 °C is likely due to an increase of the iron grain growth rate and decrease in the proportion of the small grains.
Figure 11 shows the influence of sodium sulfate addition on the magnetic separation indexes of the sample B roasted at 1250 °C and 1300 °C.
The obtained curves indicate that closely to the melting point at 1250 °C and 1300 °C the mechanism of the iron grain growth and extraction is different probably due to a different liquid phase content. In general, the cumulative evidence suggests that at these conditions sodium sulfate addition leads to approximately equal or lower iron recovery and iron grade.
To study in more detail a behavior of different phases during the roasting and magnetic separation, XRD analysis was carried out.
Figure 12 shows the XRD patterns of non-magnetic fraction of the samples with maximum temperature before full melting and maximum Na
2SO
4 addition.
The XRD patterns points out that the main phases of both samples are melilite CaNaAlSi
2O
7, oldhamite CaS, unextracted metallic iron and sodium sulfide Na
2S. Therefore, there are no the Reactions (4) and (5) at these conditions. Amorphous halo is also present in the both samples due to high content of a liquid phase uncrystallized during the quenching in liquid nitrogen. Furthermore, it is interesting to note that the tailings obtained from the sample B contain insignificant amount of FeS, while it was not detected in the tailings obtained from the sample U. This fact indicates that the roasting of the sample B with a lack of calcium to generate calcium sulfide forms not only Na
2S according to thermodynamic calculations (
Figure 4), but also FeS.
Figure 13 shows the influence of holding time on the magnetic separation indexes (a) and distribution of the iron grain fractions in it (b) at fixed temperature and Na
2SO
4 addition after roasting of the sample B.
As reflected by
Figure 13a, the magnetic separation indexes rise, reach a peak, then drop. Analogous time dependences were obtained in other studies on carbothermic roasting of red mud [
41,
47,
49,
93,
94]. It is clear from the obtained data that iron recovery and iron grade correlate well with the size distribution of iron grains, so the crucial factor of Fe extraction is the iron grain size. The optimal roasting time for 1300 °C with the addition of 13.65% Na
2SO
4 is 60 min, where iron recovery and iron grade were 97.5% and 92.6%, respectively.
Table 5 shows the results of magnetic separation of the roasted sample U.
The data demonstrate that the grade of iron concentrate obtained by roasting of the red mud U at 1350 °C is inferior to even the grade of the roasted red mud B at 1150 °C (
Figure 9b). An increase of the roasting temperature up to 1400 °C improves the grade of iron concentrate, but iron content is still low compared, for example, with the concentrate obtained from the roasted red mud B at 1250 °C.
In general, the temperature range of successful separation of the roasted samples U is narrower than that of the roasted samples B. Thus, obtaining of separated samples only at 1350–1400 °C near melting point indicates that high basicity is undesirable for iron extraction in spite of favorable thermodynamic conditions (
Figure 5).
Table 6 compares the contents of iron, carbon, sulfur and phosphorus in the iron concentrates, obtained by the treatment of the sample B with and without sodium sulfate addition.
It is well known that the conventional content of carbon in direct-reduced iron for steelmaking is 1–2.5% [
95], and the obtained concentrates comply with this range. Phosphorus content in both concentrates is almost similar, but sulfur content differs. A significant sulfur content is in the concentrate obtained with 9.1% Na
2SO
4 addition compared with another one. This fact shows that Na
2SO
4 addition in the amount of 9.1% leads to a considerable increase of sulfur content in the concentrate although the thermodynamic calculation at such amount of sodium sulfate has indicated the presence only non-magnetic CaS (
Figure 4). Moreover, the investigation of iron phase distribution in the roasted samples with addition of 13.65% Na
2SO
4 by Mössbauer spectroscopy (
Figure 6 and
Table 3) detected no iron-sulfur-containing phases. Hence, a small part of gangue including sulfur-containing compounds attaches with iron grains, thereby deteriorating purity and grade of the concentrate.