High efficiency extractions of V, Cr, Ti, Fe and Mn from vanadium slag by microwave heating

The vanadium slag (V-slag) is generated from smelting vanadium
 titanomagnetite ore, which contains valuable elements, such as V, Ti, Cr, Fe
 and Mn. The traditional methods were mainly focused on the extractions of V
 and Cr by oxidation or reduction processes. In the present work,
 chlorination method was adopted to keep the valence state of each elements
 as original state. In order to speed up the diffusion of elements and reduce
 volatility of molten salt, microwave heating has been examined in the
 current paper. The results indicated that it only took 30 min to chlorinate
 V-slag at 800 ?C, and the chlorination ratios of V, Cr, Mn, Fe and Ti could
 reach to 82.67%, 75.82%, 92.96%, 91.66% and 63.14%, respectively. Compared
 with the results by conventional heating for 8 h, this extraction rate by
 microwave heating shows greater advantages. In addition, microwave heating
 can reduce effectively volatilization of AlCl3 by shortening the reaction
 time. The volatilization ratio of AlCl3 in this microwave heating was 3.92%
 instead of 8.97% in conventional heating (1h). The mechanism of efficient
 chlorination can be summarized as the enhancement of ions diffusion process
 and enhanced chemical reaction due to local high temperature.


Introduction
Vanadium is a very important metal element widely used in steel, catalysts and aerospace [1]. Vanadium can be extracted from V-slag. A large amount of V-slag can be obtained from vanadium-containing molten iron by converter process. With the increase of global vanadium product, production of V-slag in China was about 400,000 tons in 2013 [2]. The V-slag contains a large amount of valuable metals, such as V, Cr, Mn, Fe and Ti. The traditional method for extracting valuable metals in V-slag mainly include oxidation, reduction and molten salt chlorination [3][4][5][6][7][8][9]. The oxidation methods mainly include that V 3+ was oxidized to V 5+ , Cr 3+ converts to Cr 6+ [3][4][5][6]. In reduction process, the pulverized V-slag was reduced by carbon in molten iron to form a vanadium-iron alloy [8]. V and Cr in V-slag can be utilized by oxidation and reduction method, while the molten salt chlorination process [10] can simultaneously recover V, Cr, Mn, Fe and Ti from V-slag [9]. V, Cr, Mn, Fe and Ti in V-slag are chlorinated to water-soluble chlorides of the original valence metal ions at 900 °C for 8 h. The molten salt chlorination method has its limitations, such as high volatilization of anhydrous AlCl3 [11], long chlorination time and high energy consumption. Thus, a new method for molten salt chlorinating V-slag, which was high efficiency and saved energy, should be developed. The advantages of microwave heating include rapid heating [12], selective heating [13], instant heating [12] and non-thermal effect [14]. Microwave heating can save energy [12], short processing time [15], reduce the temperature [16,17] and catalysis reaction [18]. Microwave heating technology were widely used in biology, chemistry, materials and metallurgy. In the microwave field, microwave selective heating of different microwave absorption capacity mineral components resulted in thermal stress cracking [19][20][21]. Microwave can improve the surface properties of materials, which reduce the molten salt synthesis temperature and shorten the time [16,17]. The microwave roasting can alter the spinel structure of the V-slag, enhance the reactivity of the V-slag, and decrease sensitivity to temperature of the V leaching from the V-slag [22]. The purpose of this paper was to chlorinate V-slag with microwave heating. Effects of chlorination temperature and time on microwave heating chlorination V-slag were investigated. The results of microwave heating chlorination V-slag were compared with conventional heating. In addition, the mechanisms of strengthening chlorination V-slag by microwave field are elucidated.

Raw Materials
The V-slag was provided by PanGang Group Co., LTD in Panzhihua, China. All the samples were crushed, milled and screened with particle size < 0.074 mm. V-slag was dried at 105 °C for 4 h. XRD pattern of V-slag was shown in Figure 1, the main crystalline phases of the V-slag were vuorelainenite ((Mn,Fe)(V,Cr)2O4), fayalite, manganoan ((Fe,Mn)2SiO4) and titanomagnetite (Fe2.5Ti0.5O4). The composition of Vslag was determined by inductively couple plasma optical emission spectroscopy (ICP-OES), which was shown in Table 1.

Sensitivity of raw materials to microwave heating
The vertical microwave furnace was provided by China Tangshan Microwave Nayuan Co., Ltd. The frequency of the microwave furnace is 2.45 GHz, and its power can be adjusted between 0 and 4000 W. The structure was shown in Figure 2. 10 g V-slag, 10 g V-slag and 15 g AlCl3, 10 g V-slag and 40 g AlCl3-(NaCl-KCl) mixture samples were placed in a quartz crucible, respectively. Then samples were put in the middle position of the microwave furnace with insulation material (no microwave absorption capacity). K-type thermocouple wrapped in stainless steel was inserted into the center of sample to measure the temperature with a precision of ±5 °C. In the experiment, the argon (99.9%) was used as a protective gas. At a given microwave power of 2000 W, the temperature of sample rose to 800 °C after a period of time. After turning off the microwave, the product was obtained and cooled it to room temperature in air.

Chlorination V-slag by microwave heating
In microwave heating chlorination V-slag process, 10g of V-slag was used in each experiment. The mass ratio of AlCl3/V-slag and (NaCl-KCl)/AlCl3 were 1.5:1 and 1.66:1, respectively. The sample was hold for a varied duration (10-60 min) once the temperature reached the desired value. The products of the experiment were immersed in 500 mL of deionized water for 1 h. Then, the products were separated by a suction filter. The experiment was repeated three times and the average value was reported. The chlorination ratio of V, Cr, Mn and Fe was calculated by Equation 1, and the volatilization ratio of Ti was calculated by Equation where ρTi was the volatilization ratio of Ti, mTi was the Ti content in solid product, MTi was the initial content of Ti in V-slag.

Characterization of the microwave heating chlorination V-slag
The patterns of chlorinated V-slag were measured by XRD (Rigaku TTRIII, Rigaku, Japan). ICP-OES (Varian715-ES, Varian, USA) was used to analyze the concentration of Fe, Mn, V, Cr and Al in chlorinated products. XRF (AxiosmAX, Axios, Netherlands) was used to analyze concentrations of valuable metals remaining in chlorination products. The morphology of the chlorination products were analyzed by SEM (Quanta250, FEI, Netherlands).

Thermodynamics Chlorination V-slag by AlCl3
The main crystalline phases of V-slag were FeCr2O4, FeV2O4, Fe2SiO4 and Fe2TiO4. The thermodynamic equations for the reactions of V-slag with AlCl3 were shown in Equations 3-10. The FactSage 7.0 program was used to calculate the standard Gibbs free energies for these reactions. The results at different temperature are presented in Figure 3. The standard Gibbs free energies of reactions 3-10 was negative at 300 °C-900 °C, indicating that the chemical reaction can happen. According to the relevant nature of Gibbs free energy, the smaller standard Gibbs free energy value of the equation, the easier the reaction will occur. The standard Gibbs free energies at the same temperature . Thus, chlorination of Fe2SiO4 was easier than chlorination of FeV2O4 and Fe2TiO4. After Fe2SiO4, FeV2O4, Fe2TiO4 and MnO chlorination, Cr2O3, TiO2, and V2O3 will react with AlCl3. Therefore, thermodynamic calculations show that AlCl3 chlorination V-slag can be used in a range of 300 °C -900 °C.

Microwave heating performance of molten salt chlorination V-slag
At a given microwave power of 2000 W, microwave heating temperature characteristic curve of V-slag, V-slag-AlCl3 mixture, V-slag-(NaCl-KC)l-AlCl3 mixture are shown in Figure 4. The microwave absorbing performance of material depends on the nature of the composition and structure [23], which can be evaluated by the temperature characteristic curve. The average heating rate of V-slag reached up to 134 °C/min and temperature of 800 °C was achieved within 6 min. The average heating rate of V-slag-AlCl3 mixture reached up to 120 °C/min and temperature of 800 °C was achieved within 7 min. The curve shows that the average heating rate of V-slag-(NaCl-KCl)-AlCl3 mixture reached up to 67 °C/min and heating for 12 min to reach 800 °C. The microwave absorbing capacity was V-slag V-slag-AlCl3 mixture V-slag-NaCl-KCl-AlCl3 mixture. The spinel and olivine phases in the V-slag have a face-centered cubic octahedral defects structure [24]. Structural defects can cause materials to absorb microwave [25]. In addition, titanomagnetite have good microwave absorption capacity. For V-slag, the rapid rise of temperature was mainly ascribed to the presence of the titanomagnetite, spinel and olivine.
In the V-slag temperature characteristic curve, it was shown that the temperature started to appear unstable after 6 min at 800 °C. The difference between highest temperature and lowest temperature approximately reached 80 °C. It was attributed to the good microwave absorption capacity of V-slag, which led the temperature to rise too fast. In the curve of V-slag-AlCl3 mixture, temperature start to decrease after rising to 800 °C. Compared with the curve of V-slag-AlCl3 mixture, the heat preservation process of Vslag-(NaCl-KCl)-AlCl3 mixture curve was relatively stable. The result indicated that the molten salt has a positive effect on the stabilizing sample temperature. Furthermore, molten salt have great potential as thermal energy storage [26]. Introducing a molten salt medium into the microwave heating chlorination V-slag can stabilize the chlorination temperature and provide a liquid phase reaction environment.

Effect of temperature on chlorination ratio
In this work, V-slag-(NaCl-KCl)-AlCl3 mixture sample was kept at 600 °C -900 °C for 30 min. Effect of temperature on chlorination of valuable metals in V-slag by microwave heating (M) was shown in Figure 5. It can be found that the chlorination ratios of valuable metals V, Cr, Mn, Fe and Ti from V-slag increase significantly with temperature. At 600 °C, the chlorination ratios of V and Cr were only 30.55% and 24.65%, respectively. The chlorination ratios of valuable metals V, Cr, Mn, Fe and Ti for microwave heating chlorination V-slag rise to 82.67%, 75.82%, 92.96%, 91.66% and 63.14% at 800 °C. When the sample was heated by microwave to 800 °C -900 °C, the chlorination ratio gradually reached a steady state. The XRD pattern ( Figure 6) shows that product at 600 °C contains vuorelainenite, titanomagnetite, fayalite and manganoan peak. In addition, microwave heating decreased the intensity of the vuorelainenite, titanomagnetite, fayalite and manganoan peak, suggesting that AlCl3 started to react with V-slag. In Figure 7(a), the void was observed at the phase interface of the spinel (1) and the olivine (2), and the olivine phase (2) was covered by the flocs. The EDS shows that the composition of flocs were Al-Si-O. The chlorination ratios of Fe and Mn were higher than V and Cr at 600 °C, which indicated that AlCl3 firstly reacted with the olivine phase (2). Figure 5 shows that the chlorination ratio of valuable metals at 700 °C was relatively low. It was observed that the Ca(Fe,Mn)Si2O6 in Figure 6 still existed at 700 °C, leading to the relative low chlorination ratio of Fe and Mn. Figure 7(b) shows the SEM morphologies of chlorination product heated to 700 °C for 30 min. The spinel phase (1) and the olivine phase (2) were almost completely wrapped by the flocs, which indicates that the chlorination ratio of valuable metals was relatively low. The Figure 6 suggested that Ca(Fe,Mn)Si2O6 disappeared at 800 °C, which explained that the chlorination ratio of Fe and Mn reached 91.8% and 92.4%, respectively. As shown in Figure 7(c), the morphology of spinel and olivine disappeared in the SEM map, and the residual elements in the surface scan were diffusely distributed. The SEM measurements indicate that V-slag can almost be completely chlorinated by the microwave heating method at 800 °C after 30 min, which coincides with the ICP-OES measurement results.

Effect of holding time on chlorination ratio
The effect of holding time (10 to 60 min) on chlorination of valuable metals Fe, Mn, V and Cr in V-slag was investigated at 800 °C. As shown in Figure 8, the chlorination ratio of Fe and Mn reached 80% within 10 min, while the chlorination ratio of V and Cr reached 75% needs 30 min. Thus, chlorination ratio of V and Cr was more sensitive to holding time than Fe, Mn. The chlorination ratio of V and Cr increased by 30% in 20 min. The chlorination ratio of Fe after treatment for 30 min was 91.66% while it reduced to 89.05% with increasing treatment time to 60 min. The chlorination ratio of Cr has the same change as Fe, which may be related to the volatilization of chloride. The vapor pressures of FeCl2, MnCl2, VCl3 and CrCl3 at various temperature were calculated by Factsage software. The evaporation of FeCl2, CrCl3 and VCl3 with positive vapor pressure was more serious than MnCl2. The decrease of the chlorination ratio for Fe and Cr may be due to the local temperature rise of the sample during the experiment. In this paper, the V-slag-(NaCl-KCl)-AlCl3 mixture sample volatilization at 800 °C for 10 to 60 min was investigated. The volatilization ratio of the sample gradually increased with the chlorination time. The volatilization ratio of the sample after 30 min was 1.96% while it increased to 4.34% with increasing time to 60 min. Thus, the optimal chlorination time was 30 min.

Comparison of traditional heating and microwave heating
Compared with chlorination ratio of valuable metals in V-slag by microwave and conventional heating at 800 °C, the results are shown in table 2. The chlorination valuable metals behavior of the microwave heating chlorination Vslag exhibits a higher chlorination ratio compared with the conventional heating. The chlorination ratio of Fe, Mn, V, Cr and Ti was 91.66%, 92.96%, 82.67%, 75.82% and 63.14%, respectively. Microwave heating significantly shortens the holding time of chlorination V-slag. Microwave heating has the advantage of rapid heating [12]. In this work, the V-slag-(NaCl-KCl)-AlCl3 mixture sample proved to be a good microwave absorber and rose to 800 °C in 12 min by microwave heating. However, conventional heating required 3.5 h to 800 °C by 4000 W. Microwave heating greatly reduces the energy consumption. According to the calculation of the Fe, Mn, V, Cr and Al ion concentration in the product, the mass balance of AlCl3 in molten salt at 800 °C for 30 min was shown in Table 3.The volatilization ratio of AlCl3 in the microwave heating at 800 °C for 30 min was 3.92%, while conventional heating at 900 °C for 1h was 8.97%. More AlCl3 was fixed in the molten salt. The volatilization ratios of samples in microwave heating and conventional heating at different temperature were shown in Figure 9. At 900 °C, the volatilization ratio of V-slag-(NaCl-KCl)-AlCl3 mixture sample in conventional heating reached 15% for 8 h, while the volatilization ratio of V-slag-(NaCl-KCl)-AlCl3 mixture sample in microwave heating was only 3.12% for 30 min. The volatilization ratio of conventional heating chlorination V-slag sample was significantly higher than that of microwave heating. Shortening the heating time will stabilize more of AlCl3 in the molten salt, which can reduce the volatilization of the AlCl3.  The mechanisms of high-efficiency chlorination V-slag by microwave heating may be explained as follows: (1) Enhanced reaction rate. Microwave heating rapidly fixes AlCl3 in the molten salt, increasing the concentration of AlCl3.The coupling of microwave alternating electric field and molten salt medium enhances the reaction rate; (2) Stirring effects of microwave heating. The microwave furnace operates at frequency of up to 2.45 GHz, and the electric field component of the microwaves causes dipoles to rotate with the alternating field (2450 million times per second) [27]. The diffusion of molten salt ions and metal ions of molten salt media has been strengthened in the microwave field. (3) In the microwave field, the heating ratio of Si was very slow, but the heating ratio of V and Fe was fast [28]. Due to the difference of thermal expansion coefficient in spinel and olivine [29,30], different absorbing capacity minerals generate a stress in the lattice. The stress causes cracks in the V-slag particles and increases the reaction area. (4) Local high temperature effect. V and Fe in V-slag as a good microwave absorber can be selectively heated by microwave. In the microwave heating process, sparks or electric arcs phenomenon was observed. These sparks or electric arcs can be regarded as microplasmas at the microscopic level [27]. This may cause local high temperatures to promote the chlorination of valuable metals [31].

Conclusion
Microwave heating was an efficient method, which can chlorinate simultaneously valuable metals (V, Cr, Mn, Fe and Ti) from V-slag. The following conclusions can be drawn from this study: (1) From the thermodynamic equilibrium, AlCl3 can chlorinate valuable metals Fe, Mn, V, Cr and Ti in V-slag. V-slag, V-slag-AlCl3, and V-slag-(NaCl-KCl)-AlCl3 mixture samples were good microwave absorbers with average heating rate of 134 °C/min, 120 °C/min and 67 °C/min. In addition, the temperature can be stabilized by molten salt during microwave heating. (2) V-slag was treated by microwave heating under an optimal condition at 800 °C for 30 min. The chlorination ratios of valuable metals V, Cr, Mn, Fe and Ti were 82.67%, 75.82%, 92.96%, 91.66% and 63.14%, respectively. The volatilization ratios of AlCl3 and V-slag-(NaCl-KCl)-AlCl3 mixture sample were 3.92% and 1.96%, respectively. Microwave heating method can reduce the volatilization ratio of AlCl3, and its energy consumption was significantly lower than conventional heating method.  Figure 2. Microwave instrument structure schematic. Figure 3. Plots of standard Gibbs free energy as a function of temperature in Equation 3-10 (normalized by 1mol AlCl3). Figure 4. Microwave heating temperature characteristic curve of V-slag; V-slag-AlCl3 mixture; V-slag-(NaCl-KCl)-AlCl3 mixture. Figure 5. Effect of temperature on chlorination of valuable metals V, Cr, Mn, Fe and Ti from microwave chlorinate V-slag for 30 min. Figure 6. XRD patterns of product at different chlorination temperature (product washed with deionized water).     Table 3. AlCl3 participates in the material balance of microwave heating chlorination V-slag