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 Fe
3+ and Fe
2+. The Fe 2p
3/2 orbitals appeared with an electron binding energy of 709.8 eV which corresponded to the maximum Fe
2+-O. The Fe 2p
3/2 orbitals appeared with an electron binding energy of 711.0 eV which corresponded to the maximum Fe
3+-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 2p3/2 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 Fe3/2 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 Fe3/2 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 Fe
3+/Fe
2+ 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 Fe
2+ and Fe
3+.
Figure 2 shows the change in Fe
2+/Fe
3+ ratios at different minerals and residues. The contents of the Fe
2+-O and Fe
3+-O in the columbite-(Fe) surface were 45.4% and 54.6%, respectively, depending on the corresponding peak intensity of Fe
2+-O and Fe
3+-O. Fe-SO
4 was formed on the surface of residues after columbite-(Fe) leaching. The peak of Fe
3+-SO
4 and Fe
3+-O was split from spectra of Fe
3+, and the peak of Fe
2+-SO
4 and Fe
2+-O was split from spectra of Fe
2+. The XPS-derived amount of Fe
2+ and Fe
3+ on the residues’ surface for both the 8 and 12 M H
2SO
4 acid leaching medium is shown in
Figure 3. The results of XPS analysis revealed that the amounts of Fe
3+-O in the 8 M leaching residue surface was 32.8%, and Fe
2+-O content was 14.2%. The contents of Fe
2+-SO
4 in the 8 M leaching residue surface was 21.1%, and that of Fe
3+-SO
4 on the residue surface was 31.9%. The amounts of Fe
3+-O on the residue surface was increased from 32.8% to 43.2% after 12 M H
2SO
4 acid leaching, and Fe
2+-O content was 16.9%. The contents of Fe
2+-SO
4 on the 12 M residue surface was 16.6%, and that of Fe
3+-SO
4 on the residue surface was 23.3%.
Changes in the Fe
2+/Fe
3+ 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 H
2SO
4 acid leaching medium. The Fe
2+/Fe
3+ 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 Fe
2+/Fe
3+ 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 Fe
2+-O/Fe
3+-O ratio and Fe
2+-SO
4/Fe
3+-SO
4 ratio of columbite-(Fe) and residues. The ratio of Fe
2+-O/Fe
3+-O on the 8 M H
2SO
4 acid leaching residue surface was 0.55 which was 0.28 lower than the ratio of the initial mineral. The Fe
2+-O on the residue surface decreasing meant Fe
2+-O was broken and dissolved into the acid medium. The ratio of Fe
2+-SO
4/Fe
3+-SO
4 on the 8 M H
2SO
4 acid leaching residue surface was 0.66. The ratio of Fe
2+-O/Fe
3+-O on the 12 M H
2SO
4 acid leaching residue was 0.50, which was 0.33 lower than that of the initial mineral, and the ratio of Fe
2+-SO
4/Fe
3+-SO
4 on the surface was 0.71. The decreasing Fe
2+-O/Fe
3+-O ratio deduced that this increase in Fe
2+ content meant the improvement in mineral surface dissolution. It could be due to the ability of the Fe
2+ ions to react with the H
2SO
4 acid more effectively than with the Fe
3+ 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 H
2SO
4 acid leaching residue. The Nb 3d
5/2 orbitals corresponding to the peak binding energy of Nb
5+-SO
4 was 207.4 eV, which was 0.2 eV larger than that of Nb
5+-O with 207.2 eV.
Figure 4c shows the Nb 3d orbital photoelectron spectrum of the 12 M H
2SO
4 acid leaching residue, and the binding energy was 207.2 eV and 207.6 eV corresponding to Nb
5+-O and Nb
5+-SO
4, respectively. Nb
5+-SO
4 was formed during the H
2SO
4 acid leaching process; the main peak of Nb
5+-SO
4 on the residue surface was shifted from 207.4 eV for 8 M H
2SO
4 to 207.6 eV for the 12 M H
2SO
4 acid content, which meant O in Nb
-O was easily replaceable with the -SO
4 bond with the concentrated H
2SO
4 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, Fe
3+-O, Fe
2+-O and Nb
5+-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 Fe
2+-O on the 8 M residue was 533.4 eV, which was shifted 0.2 eV compared to columbite-(Fe). Fe
3+-O and Nb
5+-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 Fe
2+-O, Fe
3+-O and Nb
5+-O on the 12 M H
2SO
4 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 H
2SO
4 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 Fe
3+-O was the most stable, which meant Fe
3+ was the most difficult to dissolve from the columbite-(Fe) mineral. The chemical leaching reaction and dissolution with H
2SO
4 were under the sequence Nb
5+-O < Fe
2+-O < Fe
3+-O.
Figure 7a,b show the S 2p orbital photoelectron spectra of the 8 M H
2SO
4 and 12 M H
2SO
4 leaching residue, respectively. Naked Fe
3+, Fe
2+ and Nb
5+ in the mineral surface contacted with SO
42− and formed bonds. The binding energy of Fe
3+-SO
4, Fe
2+-SO
4 and Nb
5+-SO
4 on the 8 M H
2SO
4 leaching residue surface was 168.1 eV, 168.5 eV and 169.3 eV, respectively, and that on the surface of the 12 M H
2SO
4 leaching residue was 168.1 eV, 168.6 eV and 169.4 eV, respectively. The electron binding energy of Nb
5+-SO
4 was higher than that of Fe
3+-SO
4 and Fe
2+-SO
4, which meant the structure Nb
5+-SO
4 was the most stable. The formation of Nb
5+-SO
4 would promote the breakdown of Nb
5+-O on the mineral surface during H
2SO
4 leaching. The binding energy of Fe
3+-SO
4 was 0.4 eV larger than that of Fe
2+-SO
4. The metal ion pairing SO
42− in sequence was Fe
3+-SO
4 < Fe
2+-SO
4 < Nb
5+-SO
4 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 shows the leaching results of the ICP detection columbite-(Fe) mineral leached in different H
2SO
4 acid concentration solutions. The dissolution content of Nb and Fe improved as the H
2SO
4 acid concentration increased. The ratio of Nb/Fe in 8 M H
2SO
4 leaching liquor (dissolving ratio of Nb/Fe) was 2.29, and the ratio of Nb/Fe in the 12 M H
2SO
4 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 H
2SO
4 acid concentrations; there was a large amount of Fe
2+-SO
4 existing on the residue surface. The dissolution of Fe from the mineral into the leaching liquor meant that Fe
2+-O broke and released Fe
2+ in the solution at first and then was oxidized to Fe
3+. The redox reaction mechanism shows as follows:
The results of the Nb dissolution from the mineral indicated that Nb
5+-O was directly digested by H
+, and the released Nb
5+ complex with SO
42− ion formed cation−anion pairs. The ratio of Nb
5+-SO
4 was 55.1% with Nb
5+-O accounting for 44.9% on the 8 M H
2SO
4 acid leaching residue surface. The ratio of Nb
5+-SO
4 was 36.6% and Nb
5+-O of 63.4% on the 12 M H
2SO
4 acid leaching residue surface. It was because the quantity of molecular H
2SO
4 in the bulk with concentrations of above 12 M lessened the ionization species of SO
42− and HSO
4−. 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
+, SO
42−, which meant more Nb
5+-SO4 was formed. The leaching reaction mechanism shows as follows:
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 H
2SO
4 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 Fe
2+ ions on the columbite-(Fe) surface converted to the Fe
3+ ones through the concentrated sulfuric acid with oxidization. The greater tendency of Fe
3+ ions was on the surface; less content of Fe dissolved in the acid medium.