An EffEctIvE ProcESS for SIMultAnEouS ExtrActIon of vAluABlE MEtAlS (v, cr, ti, fe, Mn) froM vAnAdIuM SlAg uSIng AcIdIc SodIuM chlorAtE SolutIon undEr WAtEr BAth condItIonS

In the present work, a novel process for effective extraction of valuable metals (V, Cr, Ti, Mn, Fe, Al, and Mg) from the vanadium slag is proposed. The leaching parameters for valuable metals extraction from the vanadium slag by acidic sodium chlorate solution were studied to obtain conditions for vanadium to be leached without using a pyro metallurgical step since the roasting process requires high energy consumption. The optimal H 2 SO 4 concentration, weight ratio of NaClO 3 to sample, leaching time, leaching temperature, agitation speed and ratio of liquid volume to solid mass were 6 M, 0.2, 6 h, 95 ℃ , 200 rpm and 10, respectively. Under these optimal conditions, the extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Al, and Mg) were 85.8% and 91.2%, respectively. Compared with salt roasting, this process could simultaneously extract valuable metals from vanadium slag. More importantly, the experimental temperature was low underwater bath conditions.


Introduction
Vanadium titanomagnetite ore is scattered in Australia, China, Russia, and South Africa [1]. The main pyrometallurgical processes for producing vanadium slag include shaking ladle process in South Africa [2], hot metal ladle process in New Zealand, and converter process in China and Russia [3]. In China, large amounts of vanadium slag with many valuable elements (V, Cr, Ti, Fe, and Mn) are generated annually [4]. Titanium is widely used in aircraft due to its particular properties of being lightweight, strong, and corrosion-resistant [5]. Vanadium and chromium are important alloying elements used largely in manufacturing microalloyed steel [6,7]. Manganese and iron are important elements used largely in synthesizing ferrites [8,9]. Thus, efficient utilization of such resources is of great importance.
Nowadays, the salt roasting is the main solution for vanadium extraction from vanadium slag, which includes the following procedure: salt (Na 2 CO 3 , NaCl, NaOH, Na 2 SO 4 , CaCO 3 or CaO one or more mixed) roasting of vanadium slag under oxidation conditions [10][11][12], then water leaching of the roasted products, vanadium isolation from the leachate and vanadium precipitation [13,14], calcination precipitation, reduction of V 2 O 5 . The objective of salt roasting is to convert insoluble vanadium (Ⅲ) from vanadium slag to water-soluble sodium vanadate (Ⅴ) [15], meanwhile, Cr 3+ converts to Cr 6+ under oxidizing conditions. Although the slag roasting process increases the extraction ratio of vanadium, it consumes a large amount of energy. In today's society, energy and the environment are major problems faced by human beings. In order to decrease energy consumption, roasting stage should be removed. Haoran et al. [16] used manganese dioxide as an oxidant to increase the acid leaching efficiency. Mirazimi [17] reported that direct acid leaching process was used to recover vanadium from LD converter slag. Li et al. [18] studied extraction of iron from vanadium slag using pressure acid leaching. Although the acid leaching process is recently researched widely, there are some problems: Vanadium slag usually contains a large amount of valuable metals (Ti, Cr, Mn, Fe, Mg, and Al) except V, which are not investigated and effectively utilized. Recently, based on principles of simultaneous extraction of iron, vanadium, manganese, chromium, and titanium from vanadium slag, novel processes, named the molten salt electrolysis, have been developed [4]. Although the molten salt electrolysis process can achieve simultaneous extraction of To utilize the vanadium slag in China efficiently, an effective process for simultaneous extraction of vanadium, iron, manganese, chromium, titanium, aluminum, and magnesium from vanadium slag, using acidic sodium chlorate solution underwater bath conditions, is proposed by authors. In this process, roasting procedure was removed. The extraction ratio of valuable metals at optimal condition is high.

Materials
Vanadium slag used in this study was obtained from Chengde Iron and Steel Company. It was crushed, dried, ground and sieved, and the sample with particle size fraction of 49-74 μm was used in this work. The sample was dried at 105 ℃ for 2 h and kept in a desiccator. The chemical compositions of the vanadium slag were 10.05% V 2 O 3 , 5.84% Cr 2 O 3 , 37% FeO, 20.88% SiO 2 , 11.38% TiO 2 , 5.93% MnO, 3.39% Al 2 O 3 , 3.15% MgO, 2.38% CaO. An industrial solid sodium chlorate (NaClO 3 ) with a purity of 99.0% was used in this work. Sulfuric acid (H 2 SO 4 ) with analytical grades, as well as deionized water were used in this work.

Acid leaching
10.0g of vanadium slag sample was usually used in this work, unless otherwise mentioned. The vanadium slag and 100 mL of 6 M sulfuric acid were added to 500 ml cylindrical plastic bottles. Then 500 mL cylindrical plastic bottle was transferred to shockbox under a gentle shock keeping at 200 rpm to obtain a mixture. The solution temperature was then raised to 40 ℃ for 24h. The cylindrical plastic bottle was taken out of the shockbox.

Water Bath
The solution in cylindrical plastic bottle was transferred to flask as shown in Figure 1. The solution temperature was then raised to 95℃ using a hot water bath. Solution was agitated gently. Air was passed inside the flask by air pump. The 2.0g NaClO 3 was added to the flask at a quick rate. Meanwhile, the magnetic agitation speed was kept at 200 rpm for 6h, and then the flask reactor was taken out of the water bath. The solution was immediately filtered through a Whatman GF-A membrane with a pore diameter of 0.4 μm. The filtrate was analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH) to measure the concentration of V. The solid residue and the membrane were dried at 105℃ for 4 h, then the solid residue was removed from the membrane and weighted.
The extraction ratio of vanadium and valuable metals (Fe, Mn, V, Cr, Ti, Mg, and Al) were calculated based on equation (1-2): where W v is the mass of vanadium in milligrams from the original vanadium slag; [V] is the concentration of element V from the filtrate in mg/L; V is the volume of the filtrate in liter. (2) where W O and W R are the weights of dried sample before and after leaching, respectively. The sum of mass percentage of oxide (FeO, V 2 O 3 , Cr 2 O 3 , MnO, TiO 2 , Al 2 O 3 , and MgO) is 76.74.

The possibility of reaction of various elements using acidic sodium chlorate solution underwater bath conditions
The XRD pattern in Figure 2 shows that the main crystalline phase of the original vanadium slag are fayalite, maganoan ((Fe,Mn) 2 SiO 4 ), titanomagnetite (Fe 2.5 Ti 0.5 O 4 ) and vuorelainite ((Mn,Fe)(V,Cr) 2 O 4 ). Figure 3 are a backscatter electron image of an original vanadium particle and the elemental distribution image of red section by EDS, and four obvious phases can be observed. Because isomorphism widely exists in vanadium slag, the four phase can be written as ((Fe,Mn) 2 SiO 4 ), ((Mn,Fe)(V,Cr,Ti,Mn) 2 O 4 ), SiO 2 and CaSiO 3 . Moreover, the spinel phase is randomly dispersed in the fayalite phase. Thus, the fayalite phase has to be decomposed in order to extract vanadium, chromium, and titanium. Acid leaching and oxidation by sodium (3)

Effect of NaClO 3 addition
The effect of NaClO 3 addition on extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) is presented in Figure 4. The vanadium extraction ratio increased from 80.7% to 92.3%, with the increase in NaClO 3 mass from 1.0 g to 4.0 g. The extraction ratio of valuable metals (V, Cr, Ti, Fe, Mn, Mg and Al) was maintained steadily in 88.3% to 92.8 % across the NaClO 3 mass range from 1.0 g to 6.0 g. Assuming V 2 O 3 and FeO in 10.0 g of vanadium slag completely reacted with NaClO 3 to form the corresponding compound ((VO 2 ) 2 SO 4 and Fe 2 (SO 4 ) 3 ), the theoretical mass of NaClO 3 is 1.4 g. However, due to the MnO and Cr 2 O 3 , an excess amount of NaClO 3 should be supplied. XRD patterns of residue at 2.0 g of NaClO 3 are given in Figure 2. It was observed that the Fe 2 SiO 4 disappeared and SiO 2 was produced, which could explain the fact that the extraction ratio of valuable metals was high. The scanning micrograph and EDS analysis of the residue are shown in Figure 5 and Table 1, respectively. It can be seen that the average particle size of residues are less than 20 μm. The EDS analysis of residues indicates that the content of valuable metals (V, Cr, Fe, Mn, Ti, Mg, and Al) are low and the content of Si is high. Thus, valuable metals are effectively extracted. The vanadium extraction ratio decreased from 92.3% to 84.5% with the increase in NaClO 3 mass from 4.0 g to 6.0 g. The yellow sodium jarosite may wrap the vanadium slag and hinder the vanadium leaching. The chemical reaction is as following: The element of S existed in solid leaching residue in Figure 5 and table 1, which may be attributed to form CaSO 4 and yellow sodium jarosite. Thus, decomposition of the mineral structure by sulfuric acid only is not enough, which need to join to oxidizer to promote the extraction of valuable metals.

Effect of H 2 SO 4 concentration
The effect of H 2 SO 4 concentration on extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) is presented in Figure 6 14 Mole. In our work, 0.6 mole H 2 SO 4 at 6M is used for leaching valuable metals. With the sulfuric acid concentration reaching 6M, probably, the concentration of sulfuric acid is not the limiting aspect of the reaction; thus, further increasing the concentration of sulfuric acid cannot increase the extraction of vanadium. In H 2 SO 4 leaching process, the reaction (3) of H 2 SO 4 and Fe 2 SiO 4 produces colloidal silicic acid. Colloidal silicic acid is down from solid particles with agitation of water bath. However, the viscosity of solution increases with the increase of the H 2 SO 4 concentration. This is most probably due to the higher concentration of H 2 SO 4 , leading to reduce precipitation of colloidal silicic acid. Colloidal silicic acid deposited solid particles and inhibited the ionization of valuable metals [16]. Meanwhile, the colloidal silicic acid can absorb the positive cations of valuable metals [19]. Thus, the extraction ratio of valuable metals decreased beyond 6M. Taking into account the cost of treatment and waste acid, 6M H 2 SO 4 is used for the experiment.  decreases, which can accelerate diffusion rate of ions. Thus, hydrogen ions can more easily enter the spinel and fayalite lattice, leading to an increase in extraction ratio of valuable metals [20]. Judging from the chemical reaction rate, the reaction rate increases significantly with raising the temperature.Thus, the chemical reaction rate is controlled by the chemical reaction, because the impact of the chemical reaction with increasing temperature is far more than the impact of diffusion of ions [21]. Figure 8 presents the effect of time of water bath on extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al). The extraction ratio of vanadium increased remarkably from 50.1% to 92% with increasing time from 2 h to 10 h. The extraction ratio of valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) increased with time up to 6h, and then decreased slowly between 6 h to10 h. When the extraction time is short, the reaction is not complete and the structure of spinel and fayalite cannot be destroyed. Thus, the optimal time is 6 h.

Effect of ratio of liquid to solid
Effect of ratio of liquid to solid (L/S) on extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) are presented in Figure 9. The extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) increased with increasing L/S ratio in range of 3 to 8 and reached a plateau at higher ratio than 8. However, the extraction ratio of vanadium decreases from 89.5% to 70.3% with increasing L/S ratio from 8 to 15. The extraction ratio of valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) decreased slowly with increasing L/S ratio from 8 to 15. The moles of sulfuric acid and the concentration of oxidizing agent significantly affect the extraction rate of vanadium and valuable metals. Assuming V 2 O 3 , FeO, TiO 2 , MnO, MgO, and Al 2 O 3 in 10.0 g of vanadium slag completely reacted with H 2 SO 4 to from the corresponding compound ((VO 2 ) 2 SO 4 , Fe 2 (SO 4 ) 3 ), Ti(SO 4 ) 2 , MnSO 4 , MgSO 4 , and Al 2 (SO 4 ) 3 , the theoretical Mole of H 2 SO 4 is 0.14. In our work, mole of H 2 SO 4 increases from 0.18 to 0.48 with increasing at L/S ratio from 3 to 8. Generally, increasing the mole of H 2 SO 4 would increase the extraction of vanadium and valuable metals. However, this is limited by the concentration (more precisely, activity) of the oxidizing agent [19]. The concentration of the oxidizing agent with increasing ratio of liquid to solid from 8 to 15 is reduced. However, because of the separation of solution and leaching residue, it became very difficult below 10. Thus, the optimal ratio of liquid-solid is 10. (2)

Effect of agitation speed
As shown in Figure 10, the extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Mg, and Al) increased gently with increasing agitation speed from 100 rpm to 400 rpm. The reaction of sulfuric acid leaching of valuable metals is liquidsolid reaction. Stirring can reduce the thickness of the liquid film, improve the diffusion speed, and increase the extraction ratio of valuable metals. In H 2 SO 4 leaching process, the reaction of H 2 SO 4 and Fe 2 SiO 4 produces colloidal silicic acid; meanwhile, colloidal silicic acid is down from solid particles with agitation of water bath. Vanadium slag can be more exposed to sulfuric acid. Accordingly, stirring is required.

conclusions
The extraction of vanadium and valuable metals from vanadium slag using acidic sodium chlorate solution was investigated. The effects of various operation conditions on valuable metals leaching were researched. The optimal leaching temperature, leaching time, H 2 SO 4 concentration, agitation speed of leaching, ratio of liquid volume to solid sample weight, time of water bath, temperature of water bath, agitation speed of water bath, adding mass of NaClO 3 were 30 o C, 24h, 6M, 200 rpm, 10 (ml/g), 6 h, 95 o C, 200 rpm and 2.0 g, respectively. Under these optimal conditions, the extraction ratio of vanadium and valuable metals (V, Cr, Ti, Mn, Fe, Al, and Mg) were 85.8% and 91.2%, respectively. Thus, extraction of valuable metals from vanadium slag was very effective and environmentally friendly with this method.