Experimental Study on Electric Separation of Ti/Zr-Bearing Minerals in Gravity Separation Concentrate After Thermal Activation Roasting

Abstract

To solve the problem of purifying concentrates of rutile and zircon, a new method of electric separation after thermal activation roasting at 800 °C was proposed to strengthen the separation of Ti/Zr-bearing minerals. The results showed that the grade of TiO₂ in the conductor increased by 2.55~6.45% and the content of ZrO₂ decreased by 0.83~2.60% after thermal activation roasting and electronic separation, in contrast with electronic separation without roasting. To further explore the mechanism of activation roasting, the electrical conductivity, the phase evolution, and the microstructure of the gravity separation concentrate (GSC), pure rutile and pure zircon before and after roasting were investigated. The results of conductivity testing showed that the roasting pretreatment significantly improved the conductive difference between rutile and zircon, thus strengthening their separation performance. The XRD results revealed that the thermal activation roasting made the anatase in the GSC transform into rutile, thus enhancing the conductivity. Meanwhile, the crystallinity of both of the pure minerals was improved. The SEM results showed that the GSC particles formed loose and porous sinters, suggesting the reconstruction of the unstable anatase into rutile. Small amounts of cracks and protrusions occurred on the surface of both pure minerals, ascribed to the dehydration and deoxygenation at a high temperature.

1. Introduction

Titanium, a pivotal strategic metal, is utilized in the manufacture of high-strength, low-density, high-temperature-resistant and corrosion-resistant alloys. These alloys exhibit considerable utilization value in emerging domains such as aerospace, marine engineering, recreational sports, and medicine [1,2]. Moreover, its versatility extends to traditional sectors, including chemical industry, electric power production, and salt production. Rutile and ilmenite are the primary raw materials in the extraction of titanium. Rutile is distinguished by its high grade and low impurities, in contrast with ilmenite. The global rutile reserves are estimated to be approximately 45 million tons, and the majority of the resources are located in Australia, South Africa, and India [3]. Rutile ore is usually categorized into primary rutile ore and rutile sand ore. China possesses a substantial abundance of rutile ore resources. However, the majority of these resources are characterized by low grade, fine grain size, and a complex mineral composition, which restrict their large-scale utilization [4].

The separation performance of titanium and zirconium polymetallic minerals is not satisfactory when separately using gravity concentration, magnetic separation, electric separation (ES), and flotation processes, due to their complex mineral composition [5]. Thus, a waste of resources occurs. To solve these problems, a variety of combined technologies were developed to increase the metal recovery. Rejith [6] applied a combination of unit operations like gravity, magnetic, and electrostatic separation to separate strategic heavy minerals from bulk samples taken from Kappil-Varkala and Shanghumugham-Kovalam in Thiruvananthapuram, India. The results showed that the rough concentrate contained above 65.0% total heavy minerals, including ilmenite, sillimanite, zircon, rutile, and leucoxene, and was further purified using physical methods. Premaratne [7] optimized the application of various separators, namely magnetic, electrostatic, and gravity separators, to concentrate Ti-bearing minerals from the beach sand deposits of Sri Lanka. A commercial grade ilmenite concentrate assaying 63.7% TiO₂ and a rutile concentrate assaying 93.4% TiO₂ were obtained. Routray [8] reported the recovery of zircon from beach sand and red sediments of bad land topography by spiral concentrator, high intensity magnetic separator, table concentrator, and flotation. Zircon concentrates containing 98.1% zircon with a ZrO₂ recovery of 80% were obtained. Okoli [9] adopted dry a high-intensity magnetic separator (DHIMS) and an air-floating separator (AFS) to upgrade the low-grade zircon from Arikya, Nasarawa State. The results showed that the DHIMS produced a ZrO₂ grade of 52.48% with a recovery of 57.99% and an enrichment ratio of 0.78, and the AFS generated a ZrO₂ grade of 65.52% with a recovery of 70.81% and an enrichment ratio of 1.25.

In this study, ES of the gravity separation concentrate (GSC) after drying and dry magnetic separation was carried out. However, the separation performances of Ti/Zr-bearing minerals were poor due to the feeding containing considerable amounts of leucoxene. Leucoxene mainly consists of rutile, anatase, and brookite, and the latter two minerals present poor electrical conductivity (EC) [10,11]. To further improve the separation effect of ES for the Ti/Zr-bearing minerals, an innovative method of thermal activation roasting, (TAR)-ES, was proposed. Moreover, three kinds of feedings, including the GSC [12], pure rutile, and pure zircon, were investigated before and after roasting using conductivity measurements, X-ray diffraction (XRD) analyses, and scanning electron microscope (SEM) testing to reveal the mechanisms of roasting pretreatment. The goal was to provide a new method for the purification of refractory coastal titanium placer ore.

2. Materials and Methods

2.1. Materials

The GSC containing 60.0–62.0% TiO₂ and 6.0–8.0% ZrO₂ was provided from Guangdong Ubridge New Material Technology Co., Ltd., Maoming, China [12]. The company is dedicated to the production of rutile and zircon products. The previous research results showed that the titanium mainly existed in the form of rutile and leucoxene, and the Zr-bearing mineral was mainly zircon. Pure rutile mineral assaying 94.5% TiO₂ at a particle size of −37 μm and pure zircon mineral assaying 64.7% ZrO₂ at a particle size of −37 μm were used to carry out the investigation of the mechanism.

2.2. Methods

The EC of the samples before and after thermal activation was measured by using a four-probe resistivity tester (ST2742B, Suzhou Jingge Electronics Co., Ltd., Suzhou, China). The device has a resistivity measurement range of 15.0 × 10⁻⁶–200.0 × 10³ Ω·cm, a current output range of 0.1 mA–1000 mA, and an applied pressure range of 0–30.0 MPa. The crystalline structure of samples was characterized by using an XRD analyzer (D/MAX 2200, Rigaku, Kyoto, Japan) operated at a voltage of 40 kV and a tube current of 40 mA. The scanning range was set between 10° and 90°, with a scanning speed of 5°/min. The surface morphology was analyzed using a Philips XL30 ESEM-TM scanning electron microscope (Philips, Amsterdam, Netherlands) equipped with an ETD detector. The operating conditions were an accelerating voltage of 10.00 kV and a probe current of 30 PA. The physical signals generated by the interaction of the focused electron beam with the sample surface were used to form the image contrast, enabling the acquisition of SEM images for the three samples before and after roasting.

3. Results and Discussion

3.1. TAR Enhancing the ES

Figure 1 shows the flow sheets of dry magnetic separation-ES and TAR-ES for the gravity separation concentrate. The primary valuable minerals in the GSC include rutile, leucoxene, Fe-bearing rutile, zircon, and ilmenite [12]. Among these minerals, ilmenite and rutile with a high Fe content, which exhibited magnetic properties, were removed by dry magnetic separation, thus improving the purity of Ti/Zr-bearing intermediate products. ES was subsequently applied to separate rutile from the zircon on the basis of the notable difference in the EC between both minerals, as indicated by the solid line.

To improve the separation efficiency of Ti/Zr-bearing minerals, the concentrate originating from two-stage sieving plate ES and conductor II underwent TAR before the ES, as indicated by the dotted line. The roasting was conducted at a temperature of 800 °C for 2 h [13,14]. Both samples were placed in a covered ceramic crucible, guaranteeing a neutral atmosphere. The experiment results are presented in

Table 1. Results of ES before and after TAR for the GSC after dry magnetic separation.

Conditions	Products	Yield/%	Grade/%		Recovery/%
			TiO2	ZrO2	TiO2	ZrO2
Before roasting	Magnetic concentrate I	32.48	59.28	0.65	31.50	3.09
	Magnetic concentrate II	4.13	35.41	0.74	2.39	0.45
	Conductor I	14.57	87.50	1.67	20.85	3.56
	Midding I	6.10	75.11	7.60	7.50	6.79
	Midding II	4.86	25.65	33.10	2.04	23.51
	Midding III	7.28	76.10	7.10	9.07	7.57
	Tailing II	9.06	15.50	27.10	2.30	35.90
	Conductor II	18.44	75.75	4.00	22.85	10.79
	Midding IV	3.08	29.70	18.50	1.50	8.35
	Feeding	100.00	61.12	6.84	100.00	100.00
After roasting	Magnetic concentrate I	32.48	59.28	0.65	31.64	2.89
	Magnetic concentrate II	4.13	35.41	0.74	2.41	0.42
	Conductor I′	20.08	90.05	0.84	29.71	2.31
	Midding I′	3.08	75.50	5.25	3.83	2.21
	Midding II′	6.23	15.10	41.30	1.55	35.19
	Midding III′	3.41	73.50	8.70	4.12	4.06
	Midding IV	3.08	29.70	18.50	1.51	7.80
	Conductor II′	13.45	82.20	1.40	18.17	2.57
	Tailing I	4.99	55.15	13.40	4.52	9.13
	Tailing II	9.06	17.15	27.00	2.55	33.42
	Feeding	100.00	60.85	7.32	100.00	100.00

.

From Table 1, two products were obtained after ES: conductor I with a TiO₂ grade of 87.50% and a ZrO₂ grade of 1.67% and conductor II with a TiO₂ grade of 75.75% and a ZrO₂ grade of 4.00%. After TAR pretreatment, the TiO₂ grade in conductor I’ increased from 87.50% to 90.05%, and the ZrO₂ content decreased from 1.67% to 0.84%. Meanwhile, The TiO₂ grade in conductor II’ increased from 75.75% to 82.20%, and the ZrO₂ content also decreased from 4.0% to 1.4%. These results confirm that the TAR significantly improved the separation effects of the rutile and zircon, achieving their efficient purification.

Figure 2 presents microscope images of the sample before and after TAR. From the figures, the number of gray-black particles has significantly decreased after the roasting pretreatment, simultaneously transforming into brownish-red particles. This result may be explained by the fact that the TAR induced significant alterations in the crystal structure and mineral phase composition of the titanium-bearing minerals. The reported literature has also confirmed that anatase undergoes a phase transformation into rutile at high temperatures [10]. Thus, the subsequent ES was enhanced.

3.2. EC Measurements

To further investigate the effect of TAR pretreatment on the EC of minerals, EC tests were performed on the GSC, pure rutile mineral, and pure zircon mineral before and after roasting. The results of the tests are presented in Figure 3 and Figure 4.

It is observed from Figure 3 that the EC of the GSC before and after roasting increased with increasing pressure from 2.0 to 18.0 MPa. This phenomenon can be attributed to the fact that increasing pressure enhanced the contact area between particles, while the porosity decreased, thereby minimizing the resistance along the electron transport path and ultimately improving overall conductivity. Notably, the GSC after roasting exhibited higher conductivity compared to the unroasted sample. This result was explained by the fact that the anatase and brookite within the leucoxene particle were transformed into the rutile, which contributed to the improvement in the electrical properties [10,11]. Moreover, the presence of trace elements such as niobium, magnesium, and phosphorus [12] in the raw material further enhanced conductivity when they were incorporated into the mineral lattice [11,15,16]. To further understand the influence of TAR on the EC, pure rutile and pure zircon minerals were separately investigated, as presented in Figure 4a,b.

Combining the data shown in Figure 4a,b, the EC of both the rutile and zircon pure minerals increased after TAR. This may be explained by the fact that the increase in the number of oxygen vacancies after high-temperature processing at a neutral atmosphere enhances the EC of minerals [17,18]. In fact, the EC of mineral powders under low-pressure conditions was very close to that of the mineral itself. At a pressure of 2.0 MPa, the EC of the rutile increased from 0.00229 ns/cm to 0.00335 ns/cm after thermal treatment, while that of the zircon rose from 0.00010 ns/cm to 0.00028 ns/cm. However, the increase in the conductivity for rutile was significantly greater than that for the zircon after thermal activation. The difference in conductivity between both minerals was enhanced, thereby improving the ES efficiency of the Ti/Zr-bearing minerals.

3.3. Phase Evolution Laws of the Samples

To further investigate the effect of TAR on the phase evolution of minerals, XRD analyses were conducted on the GSC, pure rutile mineral, and pure zircon mineral before and after roasting treatment. The results are shown in Figure 5 and Figure 6.

From Figure 5, it can be observed that the position and number of rutile phase diffraction peaks remained largely unchanged, while the intensity of these peaks increased significantly after the TAR of the GSC. This result indicated that the TAR improved the crystallinity of the rutile in the sample, thereby enhancing the structural stability of the crystal phase. In addition, it was also found that the diffraction peaks of a small amount of anatase (101) disappeared, suggesting that TAR caused the crystal phase transformation of the metastable phase anatase TiO₂ into a more stable rutile TiO₂.

From Figure 6a,b, it can be seen that the diffraction peaks of the mineral crystals became sharp, and the full width at half maximum became narrower for the both pure rutile and pure zircon minerals after TAR, indicating that the mineral structure tended to be dense and the crystallinity increased.

3.4. SEM Measurements

To further investigate the morphological changes in the samples before and after TAR, SEM analyses of the three samples including GSC, pure rutile mineral, and pure zircon mineral with and without roasting were carried out. The results are shown in Figure 7 and Figure 8, in which Figure 7c is the magnification of Figure 7a and Figure 7d is the magnification of Figure 7b.

Comparing the GSC without roasting (Figure 7a,c) with the sample after roasting (Figure 7b,d), it can be observed that surface of the GSC particles before roasting was smooth and fine particles exhibited greater dispersion. After TAR, many cracks and protrusions appeared on the surface of the larger particles (Figure 7b). Moreover, loose and porous sinters also formed (Figure 7d). These results can be attributed to the reconstruction of the unstable anatase phase into a rutile phase and the sintering of fine particles induced by TAR.

Figure 8a,c represent pure rutile mineral samples before roasting and Figure 8b,d show the samples after TAR. A comparative analysis indicated that there was no significant structural change in the rutile particles after TAR, except for the formation of small amounts of cracks or protrusions at the surface. These changes can be attributed to surface oxygen vacancies caused by dehydration and deoxidization during high-temperature treatment. The formation of oxygen vacancies enhanced the EC [17,18]. Additionally, it is observed that small particles tended to adhere to the surfaces of larger particles after TAR, ascribed to the large specific surface area, high reactivity, and directional adhesion growth characteristics of the smaller particles [19,20,21]. Thus, the TiO₂ recovery was increased. Figure 8e,g show the pure zircon mineral samples before roasting and Figure 8f,h exhibit the samples after TAR. Comparative observation revealed that the transformation laws of zircon under TAR resembled that of rutile.

In summary, improving the separation performance of Ti/Zr-bearing minerals was contributed to by (1) the anatase and brookite with poor stability being transformed into higher conductive rutile; (2) the difference in conductivity between Ti-bearing minerals and zircon increasing significantly, strengthening the ES performance, although the conductivity of zircon slightly increased because of the surface oxygen vacancy originating from dehydration and deoxidization; and (3) the small particles, which presented a large specific surface area, strong activity, and a large driving force of directional attachment and growth, and adhered to surface of the large particles, thus enhancing the TiO₂ recovery.

4. Conclusions

The ES behavior of Ti/Zr-bearing minerals before and after TAR at 800 °C was investigated. Moreover, the mechanisms of TAR were analyzed by using optical microscopy, conductivity testing, XRD analysis, and SEM observation. The following conclusions were drawn:

(1) After TAR, the TiO₂ grade in the product of conductor I increased from 87.50% to 90.05%, while the ZrO₂ content decreased from 1.67% to 0.84%. Moreover, the TiO₂ grade in conductor II improved from 75.75% to 82.20%, while the ZrO₂ content was reduced from 4.00% to 1.40%. These results confirmed that TAR enhanced the separation and purification performance of Ti/Z-bearing minerals.

(2) The particle colors changed from gray-black to light brownish-red after TAR, indicating that phase evolution occurred. Three samples, including GSC, pure rutile mineral, and pure zircon mineral presented an increase in the EC after roasting. The thermal pretreatment improved the crystallinity of the three samples and facilitated the transformation of metastable anatase TiO₂ into more stable rutile TiO₂. TAR led to the formation of loose and porous sintered layers on the surface of GSC particles, confirming phase reconstruction of the particles. On the other hand, there were small amounts of cracks and protrusions on the surface of pure rutile and pure zircon, caused by oxygen vacancies.

(3) The anatase within the leucoxene with poor stability was transformed into rutile with better stability and higher conductivity after roasting, significantly enhancing the difference in conductivity between leucoxene and zircon, thus strengthening the separation performance. The study will provide a new direction for the separation and purification of refractory Ti/Zr-bearing minerals.