The Surface Structure Change of Columbite-(Fe) Dissolution in H₂SO₄

Abstract

The mineral surface structure and ions’ interaction were of significant interest to understanding mineral dissolution and reaction. In this study, X-ray photoemission spectroscopy combined with ICP emission spectrometer was used to investigate the influence of the leaching reaction conditions of 8 M dilute sulfuric acid and 12 M concentrated sulfuric acid on the surface chemical composition, chemical (valence) state and ion distribution of Columbite-(Fe) (FeNb₂O₆). The binding energy of the cations (Fe, Nb) bonding with different anions (O²⁻, SO₄²⁻) and the ratio of Fe³⁺/Fe²⁺ oxidation–reduction provided direct understanding of Fe and Nb releasing from the mineral surface during leaching. The results showed that the binding energy of the Nb⁵⁺-O bond was much smaller than that of Nb⁵⁺-SO₄, and the binding energy decreased in sequence as Nb⁵⁺-O < Fe²⁺-O < Fe³⁺-O and increased in sequence as Fe³⁺-SO₄ < Fe²⁺-SO₄ < Nb⁵⁺-SO₄. The mineral surface reaction during the leaching could be expressed with the formula: Fe-O + H₂SO₄ → Fe-SO₄ + H₂O, Nb-O + H₂SO₄ → Nb-SO₄ + H₂O. The results also revealed that Nb dissolution from Columbite-(Fe) occurred more easily compared to Fe. Nb dissolution from the mineral was owed to the content of H⁺ in solution, and increasing the H⁺ concentration could promote the dissolution. For Fe dissolution from the mineral, the oxidation potential could play an effective role in enhancement dissolution.

1. Introduction

Nb and Ta were often labeled as strategic raw materials which were considered critical materials utilized widely in the steel, electronic, high-tech industries [1] and the defense, nuclear and chemical industries [2,3]. The columbite minerals ((Fe, Mn) (Ta, Nb)₂O₆) [4] group was the most common Nb-Ta species compound with other metal elements, which mainly included the members of columbite-(Fe) (FeNb₂O₆) and columbite-(Mn) (MnNb₂O₆) [5].

In industry, the main method for the extraction of niobium from columbite-(Fe) was the concentrated hydrofluoric acid (HF) leaching process; associated with this technology was the evaporation loss of HF. Rodriguez et al. [6] studied the extraction of niobium and tantalum from ferrocolumbite by hydrofluoric acid pressure leaching, which showed that, at the maximum extraction of Nb 90%, it achieved a HF concentration of 9% v/v, reaction temperature of 220 °C. Majima et al. [7] studied the dissolution of columbite and tantalite in acidic fluoride media; the results showed that the presence of both H⁺ and F in the leaching solution is necessary for the fast dissolution of columbite and tantalite. This method was also widely used for the decomposition of columbite–tantalite minerals and brought the environmental and operation problems [8]. The sulfuric acid process [9,10] was mainly used for composite ore which was easy to decompose. It could comprehensively recover valuable metal components in ore and had a high metal leaching conversion rate. Adding sulfuric acid on the basis of hydrofluoric acid could effectively inhibit the harmful effects of HF. Studies by He et al. [11] showed that sulfuric acid could reduce the partial pressure of HF, reduce volatilization loss, and in this way reduce the total acid consumption. Zhu et al. [12] pointed out that the sulfate in the extracts could be removed together with fluoride ions by precipitation with alkaline reagents (such as lime). In addition, the mixture of HF-H₂SO₄ also promoted the dissolution of Ta/Nb better than the HF acid leaching alone [13]. Sulfuric acid was attracted as an economic method to leach the low-grade columbite-tantalum bearing mineral; sulfuric acid (60%) was used to decompose manganotantalite at 200 °C and 1.5 MPa for 2 h, and only 8.5% and 9.9% tantalum and niobium were leached, respectively [14]. Nitric acid was added to increase the oxidation potential during leaching which could improve the leaching extent obviously [15]. In our previous work, concentrated sulfuric acid was used to decompose columbite minerals, and the result indicated that there was nearly 10% niobium still in residue [16]. How to improve the niobium dissolving from the mineral was a key mission for metallurgical researchers. The information about columbite-(Fe) mineral surface structure, reaction and dissolution occurring in sulfuric acid leaching systems was essential to promote niobium leaching efficiency.

The mineral/aqueous interfaces’ structure and ions’ interaction were of significant interest to understanding the mineral dissolution and reaction. Researchers made great efforts to modify the columbite-(Fe) mineral surface structure which could improve the utilization of the limited niobium resources and niobium leaching from the mineral [17,18]. Yang et al. [19] studied the interfacial structure change and selective dissolution of the columbite-(Fe) mineral during HF acid leaching; the results showed that under this acid leaching condition, the leaching rate of Fe and Nb elements increased with the increase in the HF concentration, and Nb atoms were easier than Fe atoms to dissolve. However, there was less knowledge about dissolution based on the surface structure and surface reaction during leaching, and on the bond breakage and metals dissolution sequence occurring at the columbite surface. The aim of this paper was to study the columbite-(Fe) mineral surface structure and structural control reaction and dissolution in sulfuric acid leaching, and to improve the knowledge of governing columbite/fluid interface leaching. The X-ray spectroscopic method was carried out to investigate the columbite-(Fe) mineral surface structure and surface charge state, and to explore the mineral surface atomic binding bond. The bond breakage order and bond binding sequence were estimated selectively to the leaching reactivity. The surface structure and structure-sensitive reaction in highly acid systems were explored. The mechanism for the leaching reaction sequence and selective dissolution was investigated.

2. Materials and Methods

2.1. Material Preparation

Columbite-(Fe) samples were prepared by solid-state reaction with stoichiometric amounts of Nb₂O₅ and FeC₂O₄•2H₂O of high purity (>99.9%). FeC₂O₄•2H₂O could decompose to form FeO to provide divalent iron ions. The reaction equation was expressed as FeC₂O₄•2H₂O + Nb₂O₅ → FeNb₂O₆ + CO↑ + CO₂↑ + H₂O↑. The particle size of Nb₂O₅ and FeC₂O₄•2H₂O was uniform and mixed according to the molar ratio of columbite-(Fe). The mixtures were pressed, and a solid-state reaction at a vertical high-temperature tube furnace was conducted. The process was divided into three steps: firstly, the mixed blocks were heated up to 175 °C; the ferrous oxalate dihydrate was dehydrated, then heated up to 235 °C and kept at this temperature to make sure that FeC₂O₄ decomposed into FeO completely; finally, the sample was heated to 1100 °C and kept for 15 h and cooled with an Ar atmosphere to room temperature. The synthesized niobium iron phase is the FeNb₂O₆ phase, and the lattice parameters of the synthesized mineral phase of FeNb₂O₆ are consistent with those of the natural mineral phase [20].

2.2. Leaching Experiment and Surface Structure Characterization

According to the concentration limit of sulfuric acid, 8 M dilute sulfuric acid and 12 M concentrated sulfuric acid were selected for acid leaching of Ferrocolumbite. The leaching experiment conducted at 350 °C for 3 h in an air atmosphere. The minerals after the experiment (also called residues) were collected through a filter and cleaned with deionized water. The ferrocolumbite mineral and residues were analyzed with X-ray photoelectronic spectroscopy (XPS) and XRD. The leaching contents of Fe and Nb in the leaching filtrate were ascertained by ICP.

2.3. XPS Surface Analysis Technique

XPS was intrinsically surface sensitive which provided a 5-nm-thick surface layer structure and the chemical condition of its atoms [21], and enabled the study of mechanisms of the chemical and structural transformation of material surfaces under energy treatments [22]. The X-ray emission spectrum was recorded in a constant radiation transmission mode of 10 eV, and the basic elements in columbite mineral such as Nb, Fe, O, S, etc. were recorded before and after the leaching reaction. The internal electronic energy spectrum of the basic elements Nb 3d, Fe 2p, O 1s, S 2p, C 1s was recorded at 40 eV. The bond energy scale (Ebond) was corrected by the basic level Au 4f_5/2 (Ebond = 83.96 eV), Ag 3d_5/2 (Ebond = 368.21 eV) and the main standard sample (gold, silver, copper) with a precision accuracy of ±0.03 eV. The charge of elements was estimated by the spectrum of C 1s (284.8 eV). Overall, elements Nb 3d, Fe 2p, O 1s, S 2p, C 1s had photoelectron intensity and the signal originating from columbite-(Fe) and the residue, and both were well fit using the Lorentz-Gaussian peak with sub-peak regions. The background parameters were optimized, and the number of bands for the experimental spectral description was minimized. The peak position error was ±0.01 eV. The element content and ratio on the surface was determined with the expression as follows:

I = n × S

(1)

where I—the peak intensity, n—the concentration of the element to be tested in the sample, and S—the sensitivity factor.

3. Results and Discussion

During the Columbite-(Fe) leaching process, FeNb₂O₆ reacted with the concentrated sulfuric acid solution. In the presence of H⁺, the bond Fe-O, Nb-O in the ferrocolumbite surface could break, and the -SO₄ bond would replace O in the bond Fe-O, Nb-O and form the bond of Fe-SO₄, Nb-SO₄. The bind energy of every binding bond in the surface mineral could be defined with the photoelectron spectra. The content of elements with different atomic charge states could be determined by combining the atomic chemical state spectra peak with its surface phase composition. The change in the surface layer depends on the different acid leaching conditions.

3.1. Ion Spatial Distributions in Columbite Surface

The columbite family of compounds was more representative of the AB₂O₆ series in which the A position was occupied mostly by Fe²⁺ and other bivalent ions such as Mn²⁺, Ti²⁺, and the B position mostly by Nb⁵⁺ and Ta⁵⁺. The cell unit of columbite-(Fe) (FeNb₂O₆) was with A = Fe and B = Nb, and the chemical bond in the mineral was basically Fe-O and Nb-O. Ferrocolumbite for XPS measurements was that the close proximity of the Fe 2p and Nb 3d orbitals meant that they were collected simultaneously in the same kinetic energy window using a single photon energy at the same depth in the solution. There was, therefore, no need to normalize to photon flux or to the transmission function of the hemispherical energy analyzer. The XPS experiment focused on ion spatial distributions. XPS photoelectron spectra of Fe 2p, O 1s and Nb 3d were collected from the surface of original FeNb₂O₆, and C 1s was standard sample. The Fe 2p spectra are composed of a narrow low-spin Fe²⁺ peak at about 709.8 eV (above 40% of total iron), and the Fe³⁺ peak at about 711.0 eV. The Nb 3d spectra was 207.0 eV. The H₂SO₄ leaching residues’ XPS spectra showed that the spectra peak positions of Fe 2p, O 1s, S 2p and Nb 3d were as a function of kinetic energy and provided further information about the amount of oxygen that decreased and the amount of Fe and Nb that increased.

Figure 1 showed Fe 2p photoelectron spectra collected from columbite-(Fe) and leaching residues. The spectra showed that the ions’ distribution and the ions’ interaction binding energy changes were also provided from the spectra. Each spectrum was fit with two peaks from low to high energy. The Fe 2p photoelectron spectra recorded two peaks which meant that Fe 2p_1/2 and Fe 2p_3/2 orbitals exhibited two double-peaks, respectively, and the integrated peak area of Fe 2p_3/2 orbitals contributed to more than that of Fe 2p_1/2. Fe 2p photoelectron spectra were fitted with two components, the Fe³⁺ and Fe²⁺. The Fe 2p_3/2 orbitals appeared with an electron binding energy of 709.8 eV which corresponded to the maximum Fe²⁺-O. The Fe 2p_3/2 orbitals appeared with an electron binding energy of 711.0 eV which corresponded to the maximum Fe³⁺-O. Every element ion state in the columbite-(Fe) surface was estimated with this element chemical state spectrum peak intensity and its corresponding sensitivity factor. The sensitivity factor of the Fe 2p orbital was 2.957.

X-ray photoelectron spectra were collected with Fe 2p_3/2 spectra at photoelectron kinetic energies (pKEs) of 709.6, 710.8, 714.6 and 712.2 eV for the 8 M H₂SO₄ acid dissolution residues. The Fe_3/2 spectra of 8 M H₂SO₄ acid residue recorded two Fe-O electron binding energy states which corresponded to the Fe³⁺-O peak maximum value of 710.8 eV and Fe²⁺-O peak value of 709.6 eV. The binding energy of Fe³⁺-O was 1.2 eV larger than that of Fe²⁺-O which meant ferric iron in the columbite-(Fe) mineral was more difficult to break and for dissolution. The Fe_3/2 spectra peaks were also assigned to the surface species Fe²⁺-SO₄ (714.6.1 eV) and Fe³⁺-SO₄ (712.2 eV). The binding energy of Fe²⁺-SO₄ was 2.4 eV larger than that of Fe³⁺-SO₄ which meant the Fe²⁺ more easily formed the structure of Fe²⁺-SO₄. We could deduce that the columbite-(Fe) mineral with more ferrous was a much easier structure to break and leach. The Fe 2p orbital spectrum recorded from the 12 M H₂SO₄ leaching residue surface photoelectron kinetic energies of 711.1, 709.2, 715.7 and 713.2 eV. The spectra were fitted with four ions’ bond components, Fe³⁺-O, Fe²⁺-O, Fe²⁺-SO₄ and Fe³⁺-SO₄. The binding energy of Fe³⁺-O was 1.9 eV larger than that of Fe²⁺-O which meant ferric iron in the columbite-(Fe) was more difficult to break and for dissolution. The binding energy of Fe²⁺-SO₄ was 2.5 eV larger than that of Fe³⁺-SO₄ which meant the Fe²⁺ more easily formed the structure of Fe²⁺-SO₄.

The Fe³⁺/Fe²⁺ ratio across the columbite-(Fe) surface was significant for understanding how the oxide-ion current couples with the pronounced oxidation state changes at the surface, providing insight into the behavior of the columbite-(Fe) solution. The area under the peaks indicates the relative contents of Fe²⁺ and Fe³⁺. Figure 2 shows the change in Fe²⁺/Fe³⁺ ratios at different minerals and residues. The contents of the Fe²⁺-O and Fe³⁺-O in the columbite-(Fe) surface were 45.4% and 54.6%, respectively, depending on the corresponding peak intensity of Fe²⁺-O and Fe³⁺-O. Fe-SO₄ was formed on the surface of residues after columbite-(Fe) leaching. The peak of Fe³⁺-SO₄ and Fe³⁺-O was split from spectra of Fe³⁺, and the peak of Fe²⁺-SO₄ and Fe²⁺-O was split from spectra of Fe²⁺. The XPS-derived amount of Fe²⁺ and Fe³⁺ on the residues’ surface for both the 8 and 12 M H₂SO₄ acid leaching medium is shown in Figure 3. The results of XPS analysis revealed that the amounts of Fe³⁺-O in the 8 M leaching residue surface was 32.8%, and Fe²⁺-O content was 14.2%. The contents of Fe²⁺-SO₄ in the 8 M leaching residue surface was 21.1%, and that of Fe³⁺-SO₄ on the residue surface was 31.9%. The amounts of Fe³⁺-O on the residue surface was increased from 32.8% to 43.2% after 12 M H₂SO₄ acid leaching, and Fe²⁺-O content was 16.9%. The contents of Fe²⁺-SO₄ on the 12 M residue surface was 16.6%, and that of Fe³⁺-SO₄ on the residue surface was 23.3%.

Changes in the Fe²⁺/Fe³⁺ ratios in the active regions are a consequence of oxidation dissolution on columbite-(Fe) surface shifts caused by the interaction between the surface chemical ion and the concentration of the H₂SO₄ acid leaching medium. The Fe²⁺/Fe³⁺ ratio on the mineral surface measured from XPS was 0.83, data which demonstrated the ability of XPS to resolve redox-active regions spatially under operating conditions by oxidation leaching. The ratio Fe²⁺/Fe³⁺ that shifted on the surface suggested that the reacted rates changed with leaching contents by the surface reaction and electron transport. Figure 3 shows the Fe²⁺-O/Fe³⁺-O ratio and Fe²⁺-SO₄/Fe³⁺-SO₄ ratio of columbite-(Fe) and residues. The ratio of Fe²⁺-O/Fe³⁺-O on the 8 M H₂SO₄ acid leaching residue surface was 0.55 which was 0.28 lower than the ratio of the initial mineral. The Fe²⁺-O on the residue surface decreasing meant Fe²⁺-O was broken and dissolved into the acid medium. The ratio of Fe²⁺-SO₄/Fe³⁺-SO₄ on the 8 M H₂SO₄ acid leaching residue surface was 0.66. The ratio of Fe²⁺-O/Fe³⁺-O on the 12 M H₂SO₄ acid leaching residue was 0.50, which was 0.33 lower than that of the initial mineral, and the ratio of Fe²⁺-SO₄/Fe³⁺-SO₄ on the surface was 0.71. The decreasing Fe²⁺-O/Fe³⁺-O ratio deduced that this increase in Fe²⁺ content meant the improvement in mineral surface dissolution. It could be due to the ability of the Fe²⁺ ions to react with the H₂SO₄ acid more effectively than with the Fe³⁺ ions.

Figure 4a shows the Nb 3d orbital photoelectron spectrum collected from the columbite-(Fe) surface. The green curve was the total peak after sub-peak fitting. The Nb 3d orbitals exhibited double peaks corresponding to Nb 3d_3/2 and Nb 3d_5/2 double peaks which appeared at 209.8 eV and 207.0 eV, respectively. Nb 3d_5/2 made an obviously main contribution. Figure 4b shows the Nb 3d orbital photoelectron spectrum of the 8 M H₂SO₄ acid leaching residue. The Nb 3d_5/2 orbitals corresponding to the peak binding energy of Nb⁵⁺-SO₄ was 207.4 eV, which was 0.2 eV larger than that of Nb⁵⁺-O with 207.2 eV. Figure 4c shows the Nb 3d orbital photoelectron spectrum of the 12 M H₂SO₄ acid leaching residue, and the binding energy was 207.2 eV and 207.6 eV corresponding to Nb⁵⁺-O and Nb⁵⁺-SO₄, respectively. Nb⁵⁺-SO₄ was formed during the H₂SO₄ acid leaching process; the main peak of Nb⁵⁺-SO₄ on the residue surface was shifted from 207.4 eV for 8 M H₂SO₄ to 207.6 eV for the 12 M H₂SO₄ acid content, which meant O in Nb_-O was easily replaceable with the -SO₄ bond with the concentrated H₂SO₄ acid increasing.

3.2. Columbite-(Fe) Surface Structure-Sensitive of Selective Reaction

The photoelectron spectrum of columbite-(Fe) fitted O 1s orbital with three components is shown in Figure 5, and the three chemical elements’ state combined with oxygen, namely, Fe³⁺-O, Fe²⁺-O and Nb⁵⁺-O, with the corresponding electron binding energy decreased in proper sequence, namely, 533.2 eV, 531.8 eV and 530.2 eV, respectively. The binding energy of the bond structure of Fe²⁺-O on the 8 M residue was 533.4 eV, which was shifted 0.2 eV compared to columbite-(Fe). Fe³⁺-O and Nb⁵⁺-O and the corresponding electronic binding energy were 532.7 eV and 530.4 eV in sequence. The increased three photoelectron spectra meant the different valence in the 8 M residue. The binding energy of Fe²⁺-O, Fe³⁺-O and Nb⁵⁺-O on the 12 M H₂SO₄ acid leaching residue surface was 533.6 eV, 532.9 eV and 530.8 eV, respectively. Fe-O shifted to 0.2 eV increasing and Nb-O shifted 0.4 eV increasing from the 8 M residue to the 12 M one.

The comparison of metal ions’ binding energy on the surface indicated that the structure varied with the H₂SO₄ concentration, and structure-sensitive chemical reaction and dissolution were in sequence. As shown in Figure 6, during acid leaching, Nb-O was preferentially broken and dissolved into leaching liquor, while Fe³⁺-O was the most stable, which meant Fe³⁺ was the most difficult to dissolve from the columbite-(Fe) mineral. The chemical leaching reaction and dissolution with H₂SO₄ were under the sequence Nb⁵⁺-O < Fe²⁺-O < Fe³⁺-O.

Figure 7a,b show the S 2p orbital photoelectron spectra of the 8 M H₂SO₄ and 12 M H₂SO₄ leaching residue, respectively. Naked Fe³⁺, Fe²⁺ and Nb⁵⁺ in the mineral surface contacted with SO₄²⁻ and formed bonds. The binding energy of Fe³⁺-SO₄, Fe²⁺-SO₄ and Nb⁵⁺-SO₄ on the 8 M H₂SO₄ leaching residue surface was 168.1 eV, 168.5 eV and 169.3 eV, respectively, and that on the surface of the 12 M H₂SO₄ leaching residue was 168.1 eV, 168.6 eV and 169.4 eV, respectively. The electron binding energy of Nb⁵⁺-SO₄ was higher than that of Fe³⁺-SO₄ and Fe²⁺-SO₄, which meant the structure Nb⁵⁺-SO₄ was the most stable. The formation of Nb⁵⁺-SO₄ would promote the breakdown of Nb⁵⁺-O on the mineral surface during H₂SO₄ leaching. The binding energy of Fe³⁺-SO₄ was 0.4 eV larger than that of Fe²⁺-SO₄. The metal ion pairing SO₄²⁻ in sequence was Fe³⁺-SO₄ < Fe²⁺-SO₄ < Nb⁵⁺-SO₄ as shown in Figure 7.

The binding energy of Fe-SO₄ increased compared with Fe-O, and the binding energy of Nb-SO₄ compared with Nb-O was a parallel trend. The bond strength was Fe³⁺-O, Fe²⁺-O and Nb⁵⁺-O in sequence which meant the Nb⁵⁺-O bond broke preferentially to the Fe³⁺-O bond. The strength of the corresponding electronic binding energy of Fe²⁺-SO₄, Fe³⁺-SO₄ and Nb⁵⁺-SO₄ increased in sequence, and Fe²⁺-SO₄ was most stable. The contents of the Fe³⁺-SO₄ on the 8 M H₂SO₄ acid leaching residue surface was 31.9% and on the 12 M H₂SO₄ acid leaching residue was a corresponding 23.3%. The contents of the Fe²⁺-SO₄ of 21.1% on the 8 M leaching residue surface declined to 16.6% on the 12 M H₂SO₄ residue. The results indicated that the dissolution of Fe decreased as the acid medium content increased.

The surface of columbite-(Fe) was dissolved with H₂SO₄, and the binding energy of Fe-SO₄ was relatively larger than that of Fe-O, which meant the Fe²⁺-O and the Fe³⁺-O in the surface were broken by H₂SO₄ during the acid leaching process, and a larger binding energy of Fe²⁺-SO₄ and Fe³⁺-SO₄ formed. The bind structure change during the leaching could be expressed with the formula: Fe-O + H₂SO₄ → Fe-SO₄ + H₂O. The binding energy of Nb-SO₄ was relatively larger than that of Nb-O, which meant the more stable Nb-SO₄ formed. The bind structure change during the leaching could be expressed with the formula: Nb-O + H₂SO₄ → Nb-SO₄ + H₂O.

Table 1. ICP detection results of solution elements after different H₂SO₄ acid leaching (mg/L).

Experimental Conditions	Fe	Nb	Nb/Fe Stoichiometry Ratio
FeNb2O6 mineral	/	/	2
8 M H2SO4	14.01	32.08	2.29
12 M H2SO4	14.44	31.16	2.31

shows the leaching results of the ICP detection columbite-(Fe) mineral leached in different H₂SO₄ acid concentration solutions. The dissolution content of Nb and Fe improved as the H₂SO₄ acid concentration increased. The ratio of Nb/Fe in 8 M H₂SO₄ leaching liquor (dissolving ratio of Nb/Fe) was 2.29, and the ratio of Nb/Fe in the 12 M H₂SO₄ leaching liquor was 2.31, which was 0.29 and 0.31 increased compared with the columbite-(Fe) mineral with the Nb/Fe ratio of 2. The results revealed that Nb in the columbite mineral was more easily leached compared to Fe which was in good agreement with the result of photoelectron spectroscopy experiments.

3.3. Surface Structure Control Reaction and Dissolution

For the dissolution of the Fe element, there are two hypothetical possibilities. One was that Fe²⁺-O in FeNb₂O₆ was first broken under the acidic action of H₂SO₄ to form Fe²⁺ dissolved in the solution, and then Fe²⁺ was oxidized Fe³⁺ by the H₂SO₄ molecule in the solution to promote dissolution. Another was that the H₂SO₄ molecule in the solution oxidized Fe²⁺-O to Fe³⁺-O firstly; then, Fe³⁺-O was broken into Fe³⁺ to dissolve into the solution under the acidic action of H₂SO₄.

The content of Fe²⁺-O on the 8 M H₂SO₄ leaching residue surface decreased by 31.2% and 28.5% on the 12 M leaching residue, compared to the content of Fe³⁺-O which decreased by 21.8% and 11.4%, respectively. The results showed that the mineral structure with ferrous iron was much more easily broken, and relevantly easily leached. The Fe²⁺/Fe³⁺ ratio of leaching residues was smaller than that of the initial mineral, which meant the content of Fe³⁺ on the residues’ surface increased. The reasons were that more Fe²⁺ was soluble in the acid leaching process, or a large amount of Fe³⁺ formed on surface. Therefore, it is necessary to analyze the percentage content of each component of Fe on the mineral surface. Irrespective of whether oxidation of the H₂SO₄ molecule causes a change in the valence state of Fe elements, the decreasing content of Fe²⁺-O and Fe³⁺-O should be equal to the generative Fe²⁺-SO₄ and Fe³⁺-SO₄ correspondingly. The content of Fe²⁺-SO₄ and Fe³⁺-SO₄ on the 8 M H₂SO₄ leaching residue surface increased by 21.9% (Fe²⁺-O decreased by 31.2) and 31.9% (Fe³⁺-O decreased by 21.8%). The content of Fe²⁺-SO₄ and Fe³⁺-SO₄ on the 12 M H₂SO₄ leaching residue surface increased by 16.6% and 23% while the content of Fe²⁺-O and Fe³⁺-O decreased by 16.5% and 11.4%, respectively. The contrast results indicated that there occurred an oxidation reaction during the leaching process. There was 10.1% Fe²⁺ oxidized to Fe³⁺ in 8 M H₂SO₄ and 11.9% in 12 M H₂SO₄.

The columbite-(Fe) mineral was leached with different H₂SO₄ acid concentrations; there was a large amount of Fe²⁺-SO₄ existing on the residue surface. The dissolution of Fe from the mineral into the leaching liquor meant that Fe²⁺-O broke and released Fe²⁺ in the solution at first and then was oxidized to Fe³⁺. The redox reaction mechanism shows as follows:

Fe²⁺-O + H⁺ + SO₄ → Fe²⁺-SO₄ + H₂O

(2)

Fe²⁺-SO₄ − e → Fe²⁺-SO₄

(3)

The results of the Nb dissolution from the mineral indicated that Nb⁵⁺-O was directly digested by H⁺, and the released Nb⁵⁺ complex with SO₄²⁻ ion formed cation−anion pairs. The ratio of Nb⁵⁺-SO₄ was 55.1% with Nb⁵⁺-O accounting for 44.9% on the 8 M H₂SO₄ acid leaching residue surface. The ratio of Nb⁵⁺-SO₄ was 36.6% and Nb⁵⁺-O of 63.4% on the 12 M H₂SO₄ acid leaching residue surface. It was because the quantity of molecular H₂SO₄ in the bulk with concentrations of above 12 M lessened the ionization species of SO₄²⁻ and HSO₄⁻. The result indicated that the dissolution of Nb from the mineral owed to the content of H⁺ in the solution. In the 8 M sulfuric acid solution, there contained more species of H⁺, SO₄²⁻, which meant more Nb⁵⁺-SO4 was formed. The leaching reaction mechanism shows as follows:

Nb²⁺-O + H⁺ + SO₄ → Nb⁵⁺-SO₄ + H₂O

(4)

The extraction of niobium from the columbite-(Fe) mineral with the sulfuric acid solution occurred at the mineral surface. When the mineral surface was exposed to the acid medium ions, the cations could be dissolved and transferred from the mineral surface into the medium. Therefore, the quantity and distribution of ions on the new surface of the mineral was changed. The columbite-(Fe) mineral leached with H₂SO₄ acid was a reaction controlled by the surface structure. Figure 8 shows the surface dissolution of the columbite-(Fe) mechanism schematics. For the dissolution of the Nb element, the experimental results showed that increasing the H⁺ concentration could promote the dissolution. For the dissolution of the Fe element, the oxidation potential could be effective in the enhancement of dissolution. Some of the Fe²⁺ ions on the columbite-(Fe) surface converted to the Fe³⁺ ones through the concentrated sulfuric acid with oxidization. The greater tendency of Fe³⁺ ions was on the surface; less content of Fe dissolved in the acid medium.

4. Conclusions

X-ray photoelectron spectroscopy combined with ICP was used to study the columbite surface chemical state and structure before and after different acid leaching processes. There are several conclusions as follows:

• The columbite-(Fe) surface electron binding energy of Nb⁵⁺-O was lower than that of Fe³⁺-O and Fe²⁺-O. The selective dissolution sequence was Nb⁵⁺-O < Fe²⁺-O < Fe³⁺-O. There formed a structure of Nb⁵⁺-SO₄, Fe³⁺-SO₄ and Fe²⁺-SO₄ on the sulfuric acid residue surface. The electron binding energy of Nb⁵⁺-SO₄ was higher than that of Fe³⁺-SO₄ and Fe²⁺-SO₄, which meant the structure Nb⁵⁺-SO₄ was the most stable. The binding energy of Fe³⁺-SO₄ was larger than that of Fe²⁺-SO₄. The metal ion pairing SO₄ in sequence was Fe³⁺-SO₄ < Fe²⁺-SO₄ < Nb⁵⁺-SO₄. The electron binding energy of Me(Nb⁵⁺, Fe³⁺, Fe²⁺)-SO₄ corresponding to each chemical state element was higher than the electron binding energy of Me(Nb⁵⁺, Fe³⁺, Fe²⁺)-O, which means the Me-O bond was broken, and a more stable Me-SO₄ bond was formed.
• The ratio of Fe²⁺-O/Fe³⁺-O on the 8 M H₂SO₄ acid leaching residue surface was 0.55 which was 0.28 lower than the ratio of the initial mineral. The ratio of Fe²⁺-O/Fe³⁺-O on the 12 M H₂SO₄ acid leaching residue was 0.50, which was 0.33 lower than that of the initial mineral. The decreasing Fe²⁺-O/Fe³⁺-O ratio deduced that this increase in Fe²⁺ content meant the improvement in mineral surface dissolution. It could be due to the ability of the Fe²⁺ ions to react with the H₂SO₄ acid more effectively than with the Fe³⁺ ions.
• The leached and dissolution mechanism of columbite-(Fe) was that Nb⁵⁺-O was directly digested by H⁺, and the released Nb⁵⁺ complex with the SO₄²⁻ ion formed cation−anion pairs. The result indicated that the dissolution of Nb from the mineral owed to the content of H⁺ in the solution, and increasing the H⁺ concentration could promote the dissolution. For the dissolution of the Fe element, the oxidation potential could be effective in the enhancement of dissolution.