Process Mineralogy of Vanadium Titanomagnetite Ore in Panzhihua, China

Abstract

After decades of development of the vanadium titanomagnetite (VTM) ore in the Panzhihua district changes have taken place to the ore characteristics. In addition, preliminary research carried out in this area mainly focused on the separation and extraction of iron resources and lacked a systematic research focus on the process mineralogy of ilmenite, resulting in a low grade and yield of the separated ilmenite. Therefore, the present study investigates the compositions, textures, element distributions, and particle size characteristics of VTM ores in Panzhihua were studied via mineral dissociation analysis (MLA), electron microscope–microprobe analysis (EPMA), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that the chemical and mineral compositions of the VTM ore samples were basically stable. However, the size of the embedded particles of ilmenite and titanomagnetite was smaller than that reported previously. In addition, the olivine content was found to have significantly increased, and a large number of altered minerals (such as chlorite and sphene) were also present, as a result of which the degrees of monomer dissociation of ilmenite and titanomagnetite reached 85% when the fraction of particles with size in the range of 0.04–0.15 mm was 75%.

1. Introduction

Vanadium titanomagnetite (VTM) is a valuable multi-element symbiotic ore mainly composed of iron, vanadium, and titanium, as well as small amounts of chromium, nickel and cobalt [1,2,3]. The global resource reserves of the VTM ore are extremely rich (over 40 billion tons) and are mainly distributed in Russia, South Africa, China, the United States, Canada, Norway, Finland, India, and Sweden [4,5]. However, the distribution of total iron (TFe), TiO₂, and V₂O₅ in the ore varies with geographic location. Consequently, the utilization of Fe, V, and Ti resources in the ore also varies. To date, China and Russia have emerged as world leaders in the comprehensive utilization of VTM [6,7].

In China, VTM reserves are highly concentrated in the Panzhihua-Xichang region, and they are mainly found in the layered basic and ultrabasic rock mass in the middle section of the Sichuan-Yunnan SN tectonic belt (Anning River tectonic belt), which is a belt complex with a length of about 300 km from north to south and a width of 10–50 km from east to west [8]. Due to the control of the deep faults, most of the ore-bearing rocks are concentrated in the long, narrow zone between the west side of the Anning River tectonic belt and the Xigeda fault belt, forming the Taihe, Badong, Baima, Anning village, Maanshan, Zhonggangou, and other large- and medium-sized deposits from north to south [9,10,11]. In addition, a small number of deposits in the Panzhihua region appear at the junction of the Yalong River fault belt, Yongsheng, and Ninglang in the Yunnan Province [12]. The Panzhihua mining area consists of typical magmatic differentiation basic rock deposits [13]. The rock mass transitions from basic to ultrabasic rock from top to bottom, and the ore also transitions from light gabbro to dark gabbro and olivine [14]. Furthermore, the grain sizes of the ore-bearing rock mass gradually transition from fine to medium-coarse [15]. The Panzhihua deposit is a monoclinal layered immersion body with a length and width of about 19 and 2.5 km, which is roughly immersed in the dolomitic limestone of the Dengying Formation of the Sinian system, with a northeast strike and a northwest dip. The SN translational fault divides the whole mining area into six ore blocks: Zhujiabaobao, Lanjiahuoshan, Jianshan, Daomakan, Gongshan, and Nalaqing [16,17,18].

The Panzhihua VTM was the first mining area to be developed in China [19]. After more than 50 years of development, the mining area has entered the stage of medium-deep mining, and the ore characteristics have changed [20]. In addition, as previous research has mainly focused on the separation and extraction of titanomagnetite, the treatment of ilmenite has been neglected, resulting in a low grade and recovery of the ilmenite concentrate after mining [21]. The ilmenite grade obtained by beneficiation is about 47%, and the total amount of the CaO and MgO impurities in the concentrate is more than 5%, which makes it difficult to meet the production requirements of titanium dioxide by chlorination and sponge titanium; thus, the concentrate is currently mainly used for the production of TiO₂ using the sulfuric acid process [22]. In addition, the TiO₂ recovery of the whole process is less than 22%, and a large amount of the titanium resources in the tailings and titanomagnetite is not effectively utilized [23]. Therefore, it is necessary to study the mineralogy process of VTM ores in the Panzhihua mining area. The present study characterizes the compositions, ore textures, minerals, and particle size of the VTM in the Panzhihua region using a variety of techniques, including MLA, EPMA, SEM, and XRD. The results from this study are expected to provide guidance for the efficient separation of ilmenite and titanomagnetite.

2. Ore Characteristics

2.1. Ore Selection

To provide suitable guidance for production, ore samples were taken from Pangang Group Co., Ltd.; these samples were subjected to 3D dynamic ore blending, three-stage closed-circuit crushing, and dry throwing. The ore blending and tailing operations have been reported in previous studies [24,25]. The samples were extracted every 3 days over a period of 27 days. The changes in the TiO₂ and TFe contents of the ore samples are shown in

Table 1. Contents of TiO₂ and TFe in the different ore samples.

Batch Number	TiO2 (%)	TFe (%)
No. 1	11.10	27.71
No. 2	11.18	28.07
No. 3	11.19	27.90
No. 4	11.16	27.88
No. 5	11.15	27.06
No. 6	11.18	27.33
No. 7	10.90	27.96
No. 8	10.82	27.43
No. 9	11.34	27.65
Average	11.02 ± 0.16	27.67 ± 0.33

, whence it can be seen that the TiO₂ and TFe contents were basically stable, with values of 11.02% ± 0.16% and 27.67% ± 0.33%, respectively. To further analyze the stability of the samples, XRD (X’pert PRO, PANalytical, Almelo, Netherlands) measurements were carried out on sample Nos. 1, 3, 6, and 9 (as defined in Table 1). Figure 1 shows that all the samples basically contained the same main phases, namely ilmenite, titanomagnetite, augite, chlorite, and plagioclase, with the main differences between the samples being in the content of certain gangue minerals. Figure 2 shows the appearances and particle size distributions of the ore samples. The samples were dark gray in color and had poor reflectance, which indicates that the overall grade of the ore was low. Further, the particle size distribution of the samples was uneven, with most (79%) of the particles being larger than 0.45 mm, indicating a relatively coarse overall particle size.

2.2. Ore Composition

To better understand the differences between the samples, their specific chemical compositions at different particle sizes are listed in

Table 2. Chemical composition of the ore for different particle size ranges.

Chemical Composition	>4.75 mm (%)	4.75–0.30 mm (%)	0.30–0.15 mm (%)	0.15–0.074 mm (%)	<0.074 mm (%)	Mixed Sample (%)
TiO2	10.86	11.09	11.33	11.53	9.04	11.10
TFe	27	28.7	29.2	29.5	21.7	27.7
V2O5	0.255	0.268	0.261	0.243	0.172	0.223
Cr2O3	—	—	—	—	—	0.024
FeO	21.44	22.51	23.18	24.66	20.37	21.04
CaO	7.7	7.03	6.92	6.83	7.73	7.26
MgO	6.92	6.46	6.64	6.66	9.11	6.89
SiO2	24.79	22.94	22.36	21.9	27.8	23.57
Al2O3	8.89	9.02	8.55	8.36	9.89	8.91
MnO	0.288	0.281	0.293	0.303	0.236	0.288
S	0.528	0.568	0.623	0.664	0.871	0.592
P2O5	0.046	0.046	0.048	0.045	0.098	0.056
Na2O	0.841	0.788	0.825	0.949	0.857	0.853
K2O	0.072	0.061	0.082	0.086	0.087	0.075
Co	—	—	—	—	—	0.023
Ni	—	—	—	—	—	0.010

, which reveals that the contents of TiO₂, TFe, and FeO are maximum in the particle size range of 0.074–0.15 mm. In addition, the TiO₂ content in the ore was positively correlated with TFe but negatively correlated with the total content of gangue minerals. Furthermore, the ore contained small amounts of V₂O₅, Cr₂O₃, Co, and Ni, with V₂O₅ being the most abundant and also the most valuable of these. It is also worth noting that small amounts of S and P₂O₅ impurities were present, which need to be removed during beneficiation to avoid lowering the concentrate grade of the ore. Indeed, according to YB/T 4031-2015 standard, the content of the S and P impurities in Ti concentrate should be less than 0.35% and 0.10%, respectively [26].

2.3. Ore Textures

The ore texture plays an important role in the in-depth analysis of the mineral particle size and distribution characteristics [27]. Therefore, coarse-grained ore particles which were visibly mixed with light-colored or white gangue (labeled as A-2 in Figure 3) could be separated from the particles, which were uniform in color and did not contain any visible gangue (labeled as A-1 in Figure 3). The differences between these two particle types were further analyzed using reflected light microscopy (BX53M, Olympus Corporation, Tokyo, Japan) and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) (SIGMA 500, Carl Zeiss AG, Oberkochen, Germany) (Figure 4 and Figure 5). The results show that the main minerals in the two ore types (A-1 and A-2) were ilmenite and titanomagnetite and that the gangue minerals were composed of diopside, plagioclase, anorthosite, chlorite, forsterite, amphibole, magnesium–aluminum spinel, enstatite, and pyrrhotite. Furthermore, the grains of ilmenite and titanomagnetite were closely inlaid, and the gaps between the aggregates were filled with gangue minerals. However, there were certain differences between the two ore types, namely:

(1) In type A-1, the ilmenite and titanomagnetite (Fe-Ti oxide) aggregates exhibited mainly euhedral/semi-euhedral and semi-euhedral/other crystalline structures, corresponding to gangue minerals being scattered in the gaps between the Fe-Ti oxide aggregates (Figure 4(a₁,a₂)). In addition, local Fe-Ti oxide aggregates were also present and exhibited spongy meteorite-like and other granular structures (Figure 4(a₃,a₄) and Figure 5(a₁–a₃)).

(2) The Fe-Ti oxide aggregates in A-2 ore were mainly composed of spongy meteorite-like iron and other crystalline structures, which were embedded around gangue particles or scattered along the gaps between gangue minerals (Figure 4(b₂–b₄) and Figure 5(b₂,b₃)). However, these aggregates showed automorphic/semi-automorphic crystal or semi-automorphic/other crystal structures. Both the ilmenite and titanomagnetite particles were relatively coarse, which facilitates monomer dissociation (Figure 4(b₁) and Figure 5(b₁)).

(3) Differentiation structures of solid solution were also widespread, with the most typical being titanomagnetite, where ilmenite grew regularly along the (100) crystal plane parallel to the magnesia–alumina spinel lamellar direction, forming micro-lattice or box structures [28]. The guest ilmenite was evenly distributed in two or three groups along the (111) crystal plane of magnetite, forming lattice, leaf, and triangular structures [29]. As a result, the titanomagnetite exhibited sieve, lattice, or worm-like structures (Figure 5(b₂,b₃)). In addition, the titanomagnetite with a sand filter and a shimmering structure could be separated from the diopside by solid solution by forming a solid solution [30]. However, its particle size was fine, making monomer dissociation difficult (Figure 5(b₃)).

(4) Both types of ore contained metasomatic residues and reaction edge structures. The minerals crystallized in the early stages were metasomatized by the minerals generated in the later stage along the edge, dissociation, and crack positions. In some places, the titanomagnetite was metasomatized by chlorite (Figure 4(a₃)), and the pyrrhotite, chlorite, and limonite were also metasomatized to generate (Fe, Mg)SO₄, which filled the gaps of the Fe-Ti oxide aggregate (Figure 5(a₁,a₂)).

(5) Inclusion structures were present in both ore types. Ilmenite (or titanomagnetite) with euhedral (or rounded granular) structures was wrapped in gangue minerals, such as coarse-grained diopside (Figure 4(a₄)), and fine-grained pyrrhotite was often wrapped in gangue and titanomagnetite, making monomer dissociation more difficult (Figure 4(b₁,b₂,b₄) and Figure 5(b₁)).

2.4. Mineral Compositions and Element Distributions

MLA (MLA650F, FEI Company, Hillsboro, OR, USA) was used to characterize the mineral contents of sample Nos. 1–6, with the results shown in

Table 3. Mineral contents of the different ore samples.

Batch Number	Ilmenite (%)	Titanomagnetite (%)	Gangue Minerals (%)	Sulfides (%)
No. 1	10.27	34.71	53.53	1.49
No. 2	10.32	33.45	54.67	1.56
No. 3	9.79	33.56	55.14	1.51
No. 4	10.65	32.65	55.05	2.04
No. 5	10.54	33.14	54.81	1.51
No. 6	11.33	34.72	51.86	2.09
Average	10.48 ± 0.51	33.71 ± 0.84	54.18 ± 1.27	1.70 ± 0.28
Ref. [31]	11.12	40.26	47.10	1.52

. It is evident that the mineral content of the different samples did not vary greatly. Furthermore, the average ilmenite content (10.48%) was significantly lower than that of titanomagnetite (33.71%), while gangue minerals accounted for the largest proportion (54.18%). The proportion of gangue minerals is higher than values reported previously (47.10%) due to the decline in the ore grade caused by continuous mining [31]. However, the average sulfide content (1.70%) was basically the same as that found in an earlier study (1.5–2.0%). The distribution of these elements in the different minerals found in the ore is given in

Table 4. Elemental composition of the different minerals present in the ore.

Elements	Ilmenite	Titanomagnetite	Gangue Minerals	Sulfides	Apatite
Ti (%)	46.09 ± 3.60	44.34 ± 4.46	balance	0.1 ± 0.04	—
Fe (%)	12.63 ± 2.41	69.44 ± 3.71	balance	1.37 ± 0.44	—
V (%)	5.43 ± 1.61	91.9 ± 1.70	balance	—	—
Mg (%)	7.73 ± 3.30	3.21 ± 1.95	balance	0.63 ± 0.28	—
Al (%)	—	4.72 ± 1.55	balance	0.49 ± 0.12	—
Si (%)	0.04 ± 0.07	2.09 ± 0.92	balance	0.07 ± 0.03	—
Ca (%)	0.12 ± 0.06	1.67 ± 0.90	balance	—	0.35 ± 0.17
Na (%)	—	—	100.00	—	0
Mn (%)	44.63 ± 3.96	43.87 ± 4.47	balance	—	0
P (%)	—	—	—	—	100.00
S (%)	—	1.47 ± 0.38	balance	95.72 ± 0.65	—

, and the main observations are as follows:

(1) Ilmenite and titanomagnetite contained basically the same proportion of Ti, namely 46.09% ± 3.60% and 44.34% ± 4.46%, respectively. However, after smelting, the Ti in titanomagnetite enters the blast furnace slag, meaning that this titanium resource is not fully utilized [32,33]. In addition, ~10% of the Ti element was distributed in gangue minerals, which cannot be easily recycled. Finally, because of the low recovery of ilmenite in the beneficiation process, <22% of the titanium present in the VTM is utilized [34].

(2) Fe was mainly concentrated in titanomagnetite (69.44% ± 3.71%) and ilmenite (12.63% ± 2.41%). This can be recycled through beneficiation and subsequent separation processes; however, the remainder, which is mainly distributed in the gangue minerals, cannot be used. Additionally, ~92% of V was distributed in the form of a homogeneous phase in titanomagnetite, which enters the molten iron for utilization after smelting in a blast furnace [35]. The rest was distributed in ilmenite and the gangue minerals, of which only the former can be partially utilized [36,37]. Mn accounts for similar proportions in ilmenite and titanomagnetite, most of which enter the molten iron after smelting and can be utilized [38,39]. However, the remaining ~11.5% of the Mn distributed in the gangue minerals is difficult to utilize.

(3) Gangue minerals (such as diopside, plagioclase, olivine, hornblende, and chlorite) were mainly composed of Mg, Ca, Al, and Si, and some Mg was also found in ilmenite, resulting in the ilmenite showing “high magnesium” characteristics, which is not conducive to its separation during mineral processing [40]. Similarly, some of the Al, Mg, and Si present were distributed in titanomagnetite, increasing the difficulty of subsequent smelting and separation processes [41]. Finally, the P present mainly originates from apatite, while S mainly originates from pyrrhotite, pyrite, and (Fe, Mg)SO₄.

3. Ore Mineral Characteristics

3.1. Ilmenite

Ilmenite is the main mineral from which titanium raw materials are extracted at present. It often coexists closely with titanomagnetite and is embedded in the gaps between gangue minerals in euhedral/semi-euhedral and semi-euhedral/allohedral structures. To more deeply analyze the mineral characteristics of ilmenite, its element distribution was characterized using electron probe microanalysis (EPMA) (JXA-IHP200F, JEOL, Shizuoka, Japan). Figure 6 shows that the edges of the ilmenite grains were irregularly embedded with other minerals and that cracks were present in the grains. Therefore, fine-grained ilmenite is readily produced in the grinding process, which is not conducive to its flotation separation [42,43]. Furthermore, Ti was mainly concentrated in ilmenite and, to a lesser extent, titanomagnetite, while the content of Fe in titanomagnetite and pyrrhotite was significantly higher than that in ilmenite. Since Mg and Mn can partially replace Fe²⁺ ions in ilmenite in an isomorphic pattern (particularly Mg, which can form MgTiO₃ guest crystals), this results in the distributions of Mg and Mn in ilmenite being basically uniform. A large number of gangue minerals with different shapes were also distributed at the edges and in the internal cracks of ilmenite, which hinders monomer dissociation and affects the grade of the ilmenite concentrates due to the fine particle size. However, no S impurities were observed in ilmenite, as these were mainly distributed in sulfide minerals (and can therefore be separated out using flotation methods) [44]. Finally, the content of V in the ilmenite was basically uniform, and V can thus be separated and recovered in subsequent processing [45].

Theoretically, the contents of TiO₂ and FeO in ilmenite (FeTiO₃) are 52.65% and 47.35%, respectively. However, it can be seen from

Table 5. Chemical composition of ilmenite in the different ore samples.

Batch Number	TiO2 (%)	FeO (%)	V2O5 (%)	MgO (%)	MnO (%)	Al2O3 (%)	SiO2 (%)	Na2O (%)	K2O (%)	Cr2O3 (%)	Co (%)	Ni (%)	S (%)	P2O5 (%)
No. 1	52.684	41.464	0.446	5.219	0.681	0.082	—	—	0.005	—	0.082	—	—	—
No. 2	52.954	42.499	0.475	5.403	0.562	0.092	0.003	—	—	0.008	0.059	0.011	0.013	0.017
No. 3	53.344	40.854	0.53	5.407	0.628	0.086	—	—	—	—	0.010	—	—	—
No. 4	54.137	41.364	0.552	5.745	0.687	0.005	—	—	—	0.005	0.054	—	—	—
No. 5	53.405	40.895	0.461	5.573	0.600	0.060	—	—	—	—	0.075	—	—	0.002
No. 6	53.481	40.137	0.442	5.549	0.585	0.062	0.017	—	0.004	—	0.024	—	—	—
No. 7	54.303	40.032	0.563	5.612	0.666	0.065	0.007	0.033	0.014	0.021	0.087	—	—	—
No. 8	53.903	40.683	0.529	5.621	0.549	0.088	0.005	—	0.007	—	0.038	—	—	—
No. 9	53.328	41.716	0.536	5.737	0.562	0.007	0.024	0.004	0.001	—	0.050	—	—	0.008
No. 10	53.733	41.205	0.466	5.526	0.599	0.031	0.024	—	—	0.051	0.069	—	—	—
No. 11	53.632	41.238	0.526	5.448	0.648	0.069	—	—	0.013	—	0.116	—	0.012	—
No. 12	51.458	43.512	0.570	3.334	0.892	0.070	0.028	0.017	—	0.038	0.096	—	—	—
No. 13	51.173	44.447	0.580	3.311	0.786	0.077	—	0.004	0.016	0.021	0.080	—	—	0.010
No. 14	50.668	45.828	0.486	3.137	0.819	0.086	0.044	0.017	0.004	0.026	0.081	0.026	—	0.023
Average	53.015	41.848	0.512	5.044	0.662	0.063	0.011	0.005	0.005	0.012	0.066	0.003	0.002	0.004

that while the TiO₂ content in ilmenite was significantly higher than the theoretical value, the FeO content was lower. In addition, ~5% MgO was present, which indicates that the ilmenite contained a large amount of MgTiO₃ because its theoretical TiO₂ content is as high as 63.77%. In addition, the TiO₂ and MgO contents of sample Nos. 12–14 were lower than those of other samples, while their FeO and MnO contents were higher, indicating that the TiO₂ content in ilmenite was negatively correlated with that of FeO and that the similarity between Mg²⁺ and Fe²⁺ is stronger than that between Mn²⁺ and Fe²⁺ [46]. Finally, Table 5 shows that ilmenite also contained V, Mn, Al, Si, Na, K, Cr, Co, Ni, S, and P, of which only V, Mn, Al, and Co were relatively evenly distributed, indicating that these elements can be separated through beneficiation.

3.2. Titanomagnetite

Titanomagnetite was mainly formed in the late magmatic period and naturally occurs in granular aggregates with ilmenite. As the internal structure of titanomagnetite is complex and is rich in host and guest minerals, it was characterized using SEM-EDS (Figure 7). The results show that the Ti content at positions 1, 3, 5, and 6 was low and that position 1 was rich in Mg and Al; this is indicative of typical magnesia–alumina spinel ((Mg, Fe)(Al, Fe)₂O₄) guest crystals, which often occur in the form of flakes, grains, and dots in the titanomagnetite [47]. The thickness of the lamellae was ~1–5 μm, which requires fine grinding to achieve monomer dissociation. The Mg and Al contents at positions 3 and 5 were relatively low, as these were mainly occupied by crystalline magnetite (FeO·Fe₂O₃) and guest crystalline uivospinel (2FeO·TiO₂); this is due to the fact that the Fe content at this position did not reach the theoretical value of magnetite (72.4%). Owing to the formation of a solid solution between magnetite and ilmenite and the extremely fine grains of the latter, it was impossible to achieve complete separation between them. Positions 2 and 4 were occupied by the guest crystalline ilmenite (FeO·TiO₂), as their Ti contents were below the theoretical value (31.6%). They also contained some amount of Mg and Al due to the presence of the magnesia–alumina spinel. Ilmenite was dominated by regular and uniform plate-like lamellae with wafer thicknesses of less than 1 μm. Due to the extremely thin nature of the guest minerals (uivospinel, ilmenite, and magnesia–alumina spinel) in the titanomagnetite concentrate, as well as of some continuous silicate minerals (position 6), it is difficult to directly separate the titanium-bearing minerals from the titanomagnetite.

EPMA was also used to measure the elemental composition of titanomagnetite (Figure 8). The results show that titanomagnetite had a significantly lower Ti content and a considerably higher Fe content than ilmenite. Furthermore, a large amount of magnesia–alumina spinels were distributed in a grid or lattice shape in titanomagnetite and very fine gangue flakes filled the edges and the internal cracks. Some of them have also altered (Fe, Mg)SO₄, which increases the difficulty of titanomagnetite separation.

Table 6. Chemical composition of titanomagnetite in the different ore samples.

Batch Number	TiO2 (%)	TFe (%)	V2O5 (%)	MgO (%)	Al2O3 (%)	MnO (%)	SiO2 (%)	Cr2O3 (%)	Co (%)	Na2O (%)	Ni (%)	S (%)	P2O5 (%)
No. 1	12.916	58.948	0.724	2.282	4.458	0.363	—	0.067	0.178	—	0.000	0.005	0.011
No. 2	12.986	60.061	0.776	1.859	3.893	0.314	0.018	0.018	0.115	—	0.040	—	—
No. 3	12.320	59.507	0.707	1.869	3.756	0.360	0.034	0.034	0.131	0.034	—	0.006	—
No. 4	12.614	56.488	0.567	2.634	4.292	0.252	0.015	0.045	0.139	0.062	—	0.000	0.017
No. 5	13.491	55.533	0.664	3.800	5.022	0.146	0.012	0.046	0.063	0.047	0.052	0.004	0.009
No. 6	12.252	57.283	0.648	1.685	5.768	0.263	—	0.058	0.105	0.038	—	—	—
No. 7	12.694	56.977	0.657	1.546	5.078	0.408	0.033	0.038	0.064	0.037	—	—	0.022
No. 8	12.443	56.723	0.794	1.213	4.010	0.343	0.436	0.000	0.127	0.009	—	0.036	0.032
No. 9	14.986	59.304	0.779	1.473	3.321	0.466	0.041	0.084	0.096	—	0.005	—	—
No. 10	15.519	57.805	0.920	0.525	3.004	0.401	0.207	0.087	0.109	0.005	—	0.015	—
Average	13.222	57.863	0.724	1.889	4.260	0.332	0.080	0.048	0.113	0.023	0.010	0.007	0.009

shows the chemical compositions of titanomagnetite in the different ore samples, revealing that titanomagnetite contained 13.222% TiO₂ on average. However, these titanium resources cannot be separated by grinding. In addition, due to the presence of Mg, Al, Mn, and Si impurities, the TFe content of titanomagnetite is only ~58%, which increases the difficulty of Fe recovery.

3.3. Gangue and Sulfide Minerals

In addition to ilmenite and titanomagnetite, a large number of gangues and small amounts of sulfide minerals were found in the VTM ores.

Table 7. Mineral contents of the gangue and sulfide minerals in the different ore samples.

Classification	Minerals	No. 1 (%)	No. 2 (%)	No. 3 (%)	No. 4 (%)	Average (%)
Pyroxenes	Diopside	18.87	20.53	17.65	26.14	20.80 ± 3.75
	Hornblende	7.28	6.69	8.07	3.09	6.28 ± 2.20
	Chlorite	5.56	6.05	5.29	3.75	5.16 ± 0.99
	Others	0.85	1.15	1.05	1.10	1.04 ± 0.12
Feldspars	Plagioclase	14.23	12.54	16.47	13.58	14.21 ± 1.66
	Anorthosite	3.70	4.22	3.24	4.92	4.02 ± 0.72
	Albite	0.50	0.65	0.48	0.42	0.51 ± 0.10
Olivines	Chrysolite	1.62	1.89	1.10	1.67	1.57 ± 0.33
	Forsterite	0.92	0.95	0.79	0.37	0.76 ± 0.27
Sulfides	Pyrrhotite	1.16	1.10	1.05	1.21	1.13 ± 0.07
	Pyrite	0.17	0.23	0.22	0.35	0.24 ± 0.08
	(Fe,Mg)SO4	0.16	0.23	0.24	0.48	0.28 ± 0.14

shows the contents of the main gangues and sulfide minerals in sample Nos. 1–4, where minerals other than feldspar, olivine, and sulfide minerals are classified as pyroxenes. The results show that the diopside content in the pyroxene minerals was the highest (20.8% on average), followed by that of hornblende (6.3%) and that of chlorite (5.2%). As chlorite is an altered product of the diopside, it is suitable to be slimed during grinding, which seriously affects the beneficiation separation. Additionally, plagioclase (14.21%) accounted for the highest proportion of the feldspar minerals, followed by anorthosite and albite. As feldspars are non-magnetic minerals, they can be separated after monomer dissociation using strong magnets [48]. Olivine, which was mainly composed of forsterite and fayalite, was present in a considerably higher amount compared with the earlier study (2.26%), which may be due in part to the underlying peridotite-gabbro-type surrounding rock [49]. As olivine and ilmenite have similar flotation properties, this seriously affects the flotation separation of these minerals [50]. Finally, pyrrhotite, which was the main sulfide mineral, has two crystalline structures, hexagonal (paramagnetic) and monoclinic (ferromagnetic), which are contiguous to each other and are difficult to separate [51]. Since sulfide affects the concentrate grade, separation is required during beneficiation. In addition, the sulfide minerals also contained a small amount of pyrites and (Fe,Mg)SO₄, both of which are non-magnetic substances and can therefore be removed by high-intensity magnetic separation.

EPMA was also used to characterize the distributions of the main elements in the gangue and sulfide minerals (Figure 9). The results show that the diopside was rich in Si, Mg, Al, Ca, Fe, and Ti. As it contains iron and titanium oxides with unstable compositions, the separation between diopside and ilmenite is difficult and requires the use of flotation and other techniques. Plagioclase was mainly composed of Na, Al, Si, and Ca, which were often closely inlaid with a diopside to form gangue aggregates. However, there were few iron titanium oxides and sulfides in these aggregates, making them easy to separate from ilmenite and titanomagnetite. Olivine, which was rich in Mg, Fe, and Si, is very difficult to separate from ilmenite owing to its fine particle size and the fact that it has beneficiation properties similar to those of ilmenite [52]. Finally, pyrrhotite was mainly rich in Fe and S, its particle size was fine, and it was dispersed in the ore, which also increases the difficulty of beneficiation and separation.

Table 8. Chemical composition of the different gangues present in the ore.

Gangues	TiO2 (%)	FeO (%)	MgO (%)	Al2O3 (%)	SiO2 (%)	CaO (%)	Na2O (%)	K2O (%)	S (%)
Diopside	1.891	8.154	14.473	4.758	50.289	19.414	—	—	—
Plagioclase	—	—	—	28.331	51.610	10.920	4.677	—	—
Hornblende	4.599	10.365	13.534	13.941	40.466	10.589	3.752	1.211	—
Chrysolite	—	21.298	38.277	—	39.385	—	—	—	—
Pyrrhotite	—	47.417	—	4.341	—	—	—	—	60.822
Pyrite	—	61.262	—	1.277	—	—	—	—	51.676

shows the compositions of diopside, plagioclase, hornblende, and olivine. It is evident that the chemical compositions of the four silicate minerals were quite different. In particular, the diopside was rich in Si, Ca, Mg, Fe, and Al, and it also contained a small amount of Ti. The Fe and Ti contents in hornblende were significantly higher than those in diopside, and hornblende also contained a certain amount of Na and K. However, during the separation of ilmenite and titanomagnetite, it is not possible to recycle these Ti and Fe resources. Compared with the pyroxene minerals, plagioclase contained large amounts of Si, Al, Ca, and Na, with its Al and Si content in particular being the highest among the four gangue minerals. When the separation was not complete, the concentrate grade was significantly reduced. Chrysolite is an isomorphic intermediate variety of forsterite and fayalite. As its forsterite content is higher than that of fayalite, chrysolite contains much more Mg than Fe [53]. When olivine and ilmenite are not completely separated, the content of the Mg and Si impurities in ilmenite concentrate inevitably increases. Finally, pyrrhotite and pyrite were rich in Fe and S (with the former having a lower proportion of Fe and a higher proportion of S) and contained also a small amount of Al. Since the sulfide content in the ore is low, it has no independent development and utilization value. However, it is partially enriched by the flotation separation process [54].

4. Mineral Grain Sizes

4.1. Particle Size of the Embedded Minerals

MLA was also used to measure the particle sizes of the embedded minerals in the ore (

Table 9. Particle size distribution of the main minerals present in the ore.

Minerals	Classification	>2.0 mm (%)	2.0–1.0 mm (%)	1.0–0.5 mm (%)	0.5–0.2 mm (%)	0.2–0.1 mm (%)	0.1–0.074 mm (%)	<0.074 mm (%)
Ilmenite	Each stage	0.51	7.14	24.33	30.51	18.47	12.32	6.72
	Accumulative total	/	7.65	31.98	62.49	80.96	93.28	100.00
Titanomagnetite	Each stage	1.47	13.89	33.90	28.75	16.38	3.52	2.09
	Accumulative total	/	15.36	49.26	78.01	94.39	97.91	100.00
Pyroxene	Each stage	29.83	54.19	12.37	3.18	0.21	0.10	0.12
	Accumulative total	/	84.02	96.39	99.57	99.78	99.88	100.00
Feldspar	Each stage	/	20.39	34.87	25.92	9.60	3.17	6.06
	Accumulative total	/	/	55.26	81.17	90.77	93.94	100.00
Olivine	Each stage	/	/	16.40	30.91	24.90	9.97	17.82
	Accumulative total	/	/	/	47.31	72.21	82.18	100.00
Sulfides	Each stage	/	/	/	6.72	30.65	22.58	40.05
	Accumulative total	/	/	/	/	37.37	59.95	100.00

). The results show that the pyroxene minerals were the coarsest, followed by feldspar, titanomagnetite, ilmenite, olivine, and sulfide. The specific characteristics of the particle sizes of the various embedded minerals are as follows:

(1) The particle size distribution of ilmenite was mainly concentrated in the range of 0.2–1 mm, accounting for ~55% of the total number of particles, which is significantly lower than that observed in the earlier study (~70%) [31]. The proportion of embedded particles with size <0.074 mm (6.7%) was also higher than that in previous studies (3%–4%), indicating that the size of the embedded particles of ilmenite tends to be smaller. Similarly, the size of the embedded particles of titanomagnetite was also concentrated in the range of 0.2–2 mm, accounting for >76%. However, the overall embedded particle size was coarser than that of ilmenite, with the proportion with size <0.074 mm accounting for only 2%. These results show that the complete dissociation of ilmenite is accompanied by the complete dissociation of titanomagnetite.

(2) Among the different gangue minerals, pyroxene minerals were the coarsest, with particles with a size >1 mm accounting for 84%. Feldspar minerals had smaller particle sizes than pyroxene, with the proportion of particles with size 0.2–2 mm accounting for >90%. However, olivine had the smallest particle size, with >55% distributed in the range of 0.5–0.1 mm, and the proportion of particles with size <0.074 mm was as high as 17.82%. As the size of the embedded particles of olivine is much lower than that of ilmenite, while both minerals have similar flotation characteristics, this increases the difficulty of separation. Additionally, the size of the embedded particles of sulfide was far lower than that of the other minerals, being <0.5 mm overall and concentrated in the range of <0.02 mm. In particular, the range <0.74 mm accounted for > 40%, indicating that complete monomer dissociation of sulfide is extremely difficult to achieve.

4.2. Grinding Particle Size

Grinding is an effective method to achieve the dissociation of each mineral monomer. Since the particle sizes of the samples were relatively coarse and the complete monomer dissociation of ilmenite was not achieved, a sealed laboratory prototype (GJ100-1, Hongxing Mineral Processing Equipment Manufacturing Co., Ltd., Ganzhou, China) was used to crush the ore for 15 s (to a particle size <0.38 mm), which was then further crushed using a planetary ball mill (QM3SP01L, Nanjing University Instrument Factory, Nanjing, China). Finally, MLA was used to measure the degree of monomer dissociation, F_i, of the milled mineral for different ball milling times. The value of F_i for a mineral i is calculated as follows [55]:

$$F_{\mathrm{i}}=\frac{f_0}{f_0+\frac{3}{4}A+\frac{1}{2}B+\frac{1}{4}C}\times 100\%$$

(1)

where f₀ is the number of monomer particles of the mineral, and A, B, and C are the particle numbers of the >3/4, 3/4–1/2, and 1/2–1/4 intergrowths of the mineral, respectively.

As the gangue minerals in the ore are relatively coarse, and their cleavage is more complete than that of ilmenite and titanomagnetite, these minerals achieve complete monomer dissociation when ilmenite and titanomagnetite are dissociated. Therefore, the degree of monomer dissociation of the gangue minerals was not measured.

Table 10. Influence of the duration of the grinding process on the particle size distribution of the ore.

Grilling Duration (min)	>0.25 mm (%)	0.25–0.15 mm (%)	0.15–0.10 mm (%)	0.10–0.074 mm (%)	0.074–0.04 mm (%)	<0.04 mm (%)
Without ball milling	38.93	24.10	14.27	9.25	9.83	3.63
30	7.93	30.06	27.90	10.63	14.27	9.21
60	1.08	13.79	45.29	11.70	18.17	9.97
120	0.68	6.64	44.21	12.74	24.34	11.39
240	0.28	4.17	28.15	14.81	35.07	17.52

and

Table 11. Influence of the duration of the grinding process on the dissociation degrees of mineral monomers.

Grilling Duration (min)	Ilmenite (%)	Titanomagnetite (%)	Pyrrhotite (%)
Before ball milling	44.45	47.07	29.35
30	61.92	64.33	46.81
60	86.04	87.26	57.90
120	88.13	91.55	67.04
240	92.65	94.04	85.64

show the effects of different durations of the ball milling process on the particle size distribution of the ore after grinding and the degree of monomer dissociation of the main minerals, respectively. The results indicate that before ball milling, 63% of the ore had a particle size >0.15 mm with a low dissociation of ilmenite, titanomagnetite, and pyrrhotite monomers. However, as the ore particle sizes decreased with increasing grinding time (particularly the proportion of particles with size >0.15 mm), the degrees of monomer dissociation of these three minerals increased. After 240 min of ball milling, the proportion of the particles with size <0.074 mm in the ore reached 52.29%. Although all the minerals had fully dissociated at this time, the fine particle size is not conducive to their separation and enrichment. Therefore, a degree of monomer dissociation above 85% is generally accepted to achieve beneficiation separation [56]. In this work, this was achieved after 60 min of ball milling, with 75% of the ore particles being concentrated in the size range of 0.04–0.15mm.

To further analyze the monomer dissociation of the ore after grinding, MLA chromatogram analysis was carried out on the ore after grinding for 60 min (Figure 10). The results show that the ilmenite, titanomagnetite, and gangue minerals had basically achieved complete monomer dissociation. Furthermore, only a small proportion of ilmenite and titanomagnetite were embedded with chlorite and magnesia–alumina spinel or had edges wrapped with chlorite and sphene, which indicates that complete monomer dissociation of each mineral can be achieved after 60 min of ball milling.

5. Conclusions

The chemical and mineral compositions of the VTM ores from the Panzhihua district were found to be basically stable. The TiO₂ and TFe contents in ores were 11.02% ± 0.16% and 27.67% ± 0.33%, respectively. The main minerals present in the ore samples were ilmenite (10.48%), titanomagnetite (33.71%), pyroxene, chlorite, and plagioclase. The proportions of Ti in ilmenite and titanomagnetite were 46.09% ± 3.60% and 44.34% ± 4.46%, respectively, and the TiO₂ content in the former reached ~53%. However, a large number of Mg and Mn impurities existed in isomorphic ilmenite. The size of the embedded particles of ilmenite and titanomagnetite was lower than that found in earlier studies. In addition, the VTM ores contained a large number of altered minerals (such as chlorite and titanite) and olivine with extremely fine particle sizes, resulting in the degrees of monomer dissociation of ilmenite and titanomagnetite reaching 85% when the proportion of particles with size in the range of 0.04–0.15 mm was 75%.