The Effect of Chromium on the Roasting Process of Vanadium Extraction

Vanadium is an important metal used almost exclusively in ferrous and non-ferrous alloys due to its physicochemical and mechanical properties such as grain refinement, high tensile strength, hardness and fatigue resistance; it has a wide range of uses in the metallurgical and chemical sectors [1–3]. There are mainly three kinds of raw materials, bone coal, vanadium slag and waste vanadium catalyst, used to produce vanadium pentoxide in China. In addition, few foreign countries extract vanadium from petroleum residue, including the United States, Japan and others [4]. Vanadium slag is one of the most important sources of vanadium, which accounts for 58% of the production of vanadium [5]. The vanadium slag mixed with salt roasts in an oxidizing atmosphere, vanadium precipitates from the mineral structure and oxidizes to pentavalent vanadium oxide, and then vanadium pentoxide reacts with sodium oxide decomposed from sodium to form soluble sodium vanadate [6–8]. There is a lot of literature on the extraction of vanadium pentoxide from vanadium-containing material using NaCl, Na2SO4, and Na2CO3 as additives [9–13]. The representative vanadium extraction route mainly consists of the following procedures: sodium salt roasting, water leaching, solution purification and V precipitation [5]. However, vanadium slag usually contains 3%–6% chromium (III) oxide in the form of chromium spinel, which is thermodynamically more stable than vanadium spinel and cannot be recovered at vanadium roasting temperatures [14]. Chromium is an important strategic resource, but hexavalent chromium is a highly toxic and carcinogenic substance, and it can corrode and damage the organism [14,15]. Hexavalent chromium is one of the key pollutants [14] that creates air, soil, surface and groundwater pollution [16], causes geological disasters, and is a long-term environmental pollution [17]. Chromium slag is one of the important sources of chromium pollution. Vanadium slag is a source of chromium slag, so it is a difficult problem to extract chromium from vanadium slag. However, there are only a few studies of elements’ effects on the salt roast vanadium process with the addition of calcium (every 1% increase of CaO in slag would cause a 4.7%–9.0% V2O5 loss [18]). This study will investigate the effect of chromium on salt roasting to provide a theoretical basis for the separation of chromium and vanadium in the future. The change in the mineralogy of vanadium slag


Introduction
Vanadium is an important metal used almost exclusively in ferrous and non-ferrous alloys due to its physicochemical and mechanical properties such as grain refinement, high tensile strength, hardness and fatigue resistance; it has a wide range of uses in the metallurgical and chemical sectors [1][2][3].There are mainly three kinds of raw materials, bone coal, vanadium slag and waste vanadium catalyst, used to produce vanadium pentoxide in China.In addition, few foreign countries extract vanadium from petroleum residue, including the United States, Japan and others [4].Vanadium slag is one of the most important sources of vanadium, which accounts for 58% of the production of vanadium [5].
The vanadium slag mixed with salt roasts in an oxidizing atmosphere, vanadium precipitates from the mineral structure and oxidizes to pentavalent vanadium oxide, and then vanadium pentoxide reacts with sodium oxide decomposed from sodium to form soluble sodium vanadate [6][7][8].There is a lot of literature on the extraction of vanadium pentoxide from vanadium-containing material using NaCl, Na 2 SO 4 , and Na 2 CO 3 as additives [9][10][11][12][13].The representative vanadium extraction route mainly consists of the following procedures: sodium salt roasting, water leaching, solution purification and V precipitation [5].
However, vanadium slag usually contains 3%-6% chromium (III) oxide in the form of chromium spinel, which is thermodynamically more stable than vanadium spinel and cannot be recovered at vanadium roasting temperatures [14].
Chromium is an important strategic resource, but hexavalent chromium is a highly toxic and carcinogenic substance, and it can corrode and damage the organism [14,15].Hexavalent chromium is one of the key pollutants [14] that creates air, soil, surface and groundwater pollution [16], causes geological disasters, and is a long-term environmental pollution [17].Chromium slag is one of the important sources of chromium pollution.Vanadium slag is a source of chromium slag, so it is a difficult problem to extract chromium from vanadium slag.
However, there are only a few studies of elements' effects on the salt roast vanadium process with the addition of calcium (every 1% increase of CaO in slag would cause a 4.7%-9.0%V 2 O 5 loss [18]).This study will investigate the effect of chromium on salt roasting to provide a theoretical basis for the separation of chromium and vanadium in the future.The change in the mineralogy of vanadium slag was studied by XRD, SEM/EDS and TG-DSC techniques; the effect of the roasting temperature on vanadium extraction and the characterization of leach residues are also discussed.

Materials
Vanadium slag before roasting was obtained from a steel production site in Chengde, China.Vanadium slag after roasting was obtained after extracting vanadium from vanadium slag.
Figure 1 shows that the main mineral phases in vanadium slag are mainly vanadium spinel FeV 2 O 4 , chromium spinel FeCr 2 O 4 , olivine Fe 2 SiO 4 , quartz SiO 2 and so on.Figure 2 depicts the XRD pattern of vanadium slag after roasting; the main phases are iron oxide Fe 2 O 3 , magnetic iron oxide Fe 3 O 4 , quartz SiO 2 , diopside (Ca, Mn)(Mg, Fe, Mn)Si 2 O 6 , and sodium salt NaCrSi 2 O 6 , NaFeO 2 .The chemical compositions of vanadium slag before roasting and vanadium slag after roasting are given in Tables 1 and 2.
for the separation of chromium and vanadium in the future.The change in the mineralogy of vanadium slag was studied by XRD, SEM/EDS and TG-DSC techniques; the effect of the roasting temperature on vanadium extraction and the characterization of leach residues are also discussed.

Materials
Vanadium slag before roasting was obtained from a steel production site in Chengde, China.Vanadium slag after roasting was obtained after extracting vanadium from vanadium slag.
Figure 1 shows that the main mineral phases in vanadium slag are mainly vanadium spinel FeV2O4, chromium spinel FeCr2O4, olivine Fe2SiO4, quartz SiO2 and so on.Figure 2 depicts the XRD pattern of vanadium slag after roasting; the main phases are iron oxide Fe2O3, magnetic iron oxide Fe3O4, quartz SiO2, diopside (Ca, Mn)(Mg, Fe, Mn)Si2O6, and sodium salt NaCrSi2O6, NaFeO2.The chemical compositions of vanadium slag before roasting and vanadium slag after roasting are given in Tables 1 and 2.
The XRD pattern of vanadium slag before roasting.
The XRD pattern of vanadium slag after roasting.for the separation of chromium and vanadium in the future.The change in the mineralogy of vanadium slag was studied by XRD, SEM/EDS and TG-DSC techniques; the effect of the roasting temperature on vanadium extraction and the characterization of leach residues are also discussed.

Materials
Vanadium slag before roasting was obtained from a steel production site in Chengde, China.Vanadium slag after roasting was obtained after extracting vanadium from vanadium slag.
Figure 1 shows that the main mineral phases in vanadium slag are mainly vanadium spinel FeV2O4, chromium spinel FeCr2O4, olivine Fe2SiO4, quartz SiO2 and so on.Figure 2 depicts the XRD pattern of vanadium slag after roasting; the main phases are iron oxide Fe2O3, magnetic iron oxide Fe3O4, quartz SiO2, diopside (Ca, Mn)(Mg, Fe, Mn)Si2O6, and sodium salt NaCrSi2O6, NaFeO2.The chemical compositions of vanadium slag before roasting and vanadium slag after roasting are given in Tables 1 and 2.
Oxidative decomposition of spinel Vanadium pentoxide reacts with the sodium salt to form water-soluble sodium vanadate The reactions of chromium or vanadium in slag are:

Experimentation
Sample pretreatment: With the raw materials in proportion (vanadium slag before roasting, vanadium slag after roasting, anhydrous sodium carbonate, sodium chloride), mixed ingredients into the blender for 3 h, roasted into the vertical muffle furnace, stirred raw when roasting.A temperature controller was used to maintain the predetermined furnace temperature, with the furnace door kept open to maintain an oxidizing atmosphere.After roasting was completed, cooled the sample in water according to the ratio of liquid poured.The liquid was heated at 80 ˝C, and stirred 40 min at the rate of 200 revolutions/min.Then the liquid was collected by filtration.The slurry was separated via vacuum filtration, and the residue was washed several times with distilled water.Ammonium ferrous sulfate titration was used to determine the concentration of vanadium in the solution.
Conversion of vanadium in sodium roasting stage was calculated with: %Conversion " rVss rVs 0 ˆ100% where [V] 0 is the mass of M in the V-Cr slag for the stage before roasting; [V]s is the mass of vanadium after roasting.The conversions were calculated with concentration data in the leaching liquor instead of content data in the leaching residue due to the fact that it is difficult to determine the mass of all residues accurately and estimation of the mass will significantly increase the error.Parallel experiments were conducted three times and average extractions were obtained to ensure the validity of results.

Effect of Cr 2 O 3 on Vanadium Conversion Rate
The controlled vanadium content in the mixture was 4.7% in this experiment, the alkali/vanadium ratio was 1.2, and the alkali salt/vanadium ratio was 3.5/1; it was insulated for 100 min at 780 ˝C as shown in Table 3.Each set of experiments was repeated three times, average value was obtained to ensure the validity of results.As the Figure 3 shows, when the chromium oxide content was about 3.2%, the conversion rate of V reached the maximum value of 95%.When the chromium content was less than 3.2%, Cr and spinel would react to form Cr-Mn spinel, which increased the conversion rate of vanadium, but when the chromium content was more than 3.2%, the Cr-Mn spinel content would not increase, and the excess chromium would compete with the vanadium to react with the sodium carbonate, resulting in a reduced vanadium conversion rate.When the chromium content was more than 5.24%, the conversion rate of vanadium was gradually increased, and the reaction of Cr further decomposed the silicate and then increased the conversion rate of the vanadium.When the content of Cr 2 O 3 in the mixture was between 3.229%-5.246%,the conversion rate decreased 0.74% for each increase of 0.1% Cr 2 O 3 .The maximum conversion rate is higher than that of Li [27]  Parallel experiments were conducted three times and average extractions were obtained to ensure the validity of results.

Effect of Cr2O3 on Vanadium Conversion Rate
The controlled vanadium content in the mixture was 4.7% in this experiment, the alkali/vanadium ratio was 1.2, and the alkali salt/vanadium ratio was 3.5/1; it was insulated for 100 min at 780 °C as shown in Table 3.Each set of experiments was repeated three times, average value was obtained to ensure the validity of results.As the Figure 3 shows, when the chromium oxide content was about 3.2%, the conversion rate of V reached the maximum value of 95%.When the chromium content was less than 3.2%, Cr and spinel would react to form Cr-Mn spinel, which increased the conversion rate of vanadium, but when the chromium content was more than 3.2%, the Cr-Mn spinel content would not increase, and the excess chromium would compete with the vanadium to react with the sodium carbonate, resulting in a reduced vanadium conversion rate.When the chromium content was more than 5.24%, the conversion rate of vanadium was gradually increased, and the reaction of Cr further decomposed the silicate and then increased the conversion rate of the vanadium.When the content of Cr2O3 in the mixture was between 3.229%-5.246%,the conversion rate decreased 0.74% for each increase of 0.1% Cr2O3.The maximum conversion rate is higher than that of Li [27] (V2O5 = 7.8%, Cr2O3 = 2%) at 90% and Li [28] (V2O5 = 8.93%, Cr2O3 = 8.67%) at 88.6%.Literature [29] indicates that the main components of the spinel central portion are Cr2O3, V2O3, MgO, and FeO, of which Cr2O3 and V2O3 are the nucleation of monomer spinel.The chemical composition of the chromium-rich vanadium spinel central portion can be approximated considering (Fe 2+ Mg 2+ )(Cr 3+ V 3+ )O4.The optimum vanadium extraction temperature for transforming Cr2O3 to CrO4 2− was around 1200 °C by chromite FeCr2O4, and the first roasting conversion rate of Literature [29]  usually less than 20% in slag at the optimum calcination temperature (<850 ˝C).Thus, the chemical stability of the chromium-rich vanadium spinel is higher, which makes the crystal structure of the central portion of the vanadium-containing spinels remain more complete under calcination conditions, and a relatively complete crystal structure will restrict the soluble transforming of part of V 2 O 3 in the crystal.With greater content of Cr 2 O 3 and a larger size of the vanadium slag, this phenomenon is more obvious.Therefore, it is very unfavorable for the realization of disposable roasting vanadium when there is a great deal of Cr 2 O 3 in the vanadium slag.

XRD Analysis
The XRD pattern (Figure 4) shows that in the initial low-temperature sintering conditions (300, 400 ˝C), the sample phases are Na 2 CO 3 and Cr 2 O 3 and sodium chromate does not appear, which indicates that Cr 2 O 3 and Na 2 CO 3 do not react at a low temperature.When the temperature reaches 500 ˝C, there is a small amount of the Na 2 CrO 4 phase in addition to Na 2 CO 3 and Cr 2 O 3 , indicating that the reaction of Cr 2 O 3 and Na 2 CO 3 starts at a low temperature of 400-500 ˝C.
Metals 2016, 6, 157 5 of 9 structure of the central portion of the vanadium-containing spinels remain more complete under calcination conditions, and a relatively complete crystal structure will restrict the soluble transforming of part of V2O3 in the crystal.With greater content of Cr2O3 and a larger size of the vanadium slag, this phenomenon is more obvious.Therefore, it is very unfavorable for the realization of disposable roasting vanadium when there is a great deal of Cr2O3 in the vanadium slag.

XRD Analysis
The XRD pattern (Figure 4) shows that in the initial low-temperature sintering conditions (300, 400 °C), the sample phases are Na2CO3 and Cr2O3 and sodium chromate does not appear, which indicates that Cr2O3 and Na2CO3 do not react at a low temperature.When the temperature reaches 500 °C, there is a small amount of the Na2CrO4 phase in addition to Na2CO3 and Cr2O3, indicating that the reaction of Cr2O3 and Na2CO3 starts at a low temperature of 400-500 °C.At 600 and 700 °C, the sample phase is still Na2CO3, Cr2O3 and Na2CrO4, but the peak intensity of Na2CO3 and Cr2O3 becomes weak, while the phase diffraction peak intensity of Na2CrO4 increases.In the sintering condition of 750 °C, diffraction peaks of Na2CO3 disappear, the peak intensity of Cr2O3 continues to weaken, and the peak intensity of Na2CrO4 increases again while the new phase of Na4CrO4 appears.At 800 °C, the peak intensity of Cr2O3 continues to decrease, while the peak intensity of Na2CrO4 enhances again and becomes the primary phase, and characteristic peaks of Na4CrO4 disappear and a new phase of Na3CrO4 is observed.At 600 and 700 ˝C, the sample phase is still Na 2 CO 3 , Cr 2 O 3 and Na 2 CrO 4 , but the peak intensity of Na 2 CO 3 and Cr 2 O 3 becomes weak, while the phase diffraction peak intensity of Na 2 CrO 4 increases.In the sintering condition of 750 ˝C, diffraction peaks of Na 2 CO 3 disappear, the peak intensity of Cr 2 O 3 continues to weaken, and the peak intensity of Na 2 CrO 4 increases again while the new phase of Na 4 CrO 4 appears.At 800 ˝C, the peak intensity of Cr 2 O 3 continues to decrease, while the peak intensity of Na 2 CrO 4 enhances again and becomes the primary phase, and characteristic peaks of Na 4 CrO 4 disappear and a new phase of Na 3 CrO 4 is observed.

Comprehensive Thermal Analysis
Figure 5 is a TG-DTA curve of a sample of mixed Cr 2 O 3 and Na 2 CO 3 in a 1:1 ratio.Combined with XRD, the figure shows the first weight loss is due to the removal of adsorbed water between room temperature to 200 ˝C; the second weight loss is due to the decomposed Na 2 CO 3 powders and liberated CO 2 gas between 200-700 ˝C; the endothermic peak around 100 ˝C is a removal reaction of the adsorbed water; and endothermic peak around 800 ˝C is due to the reaction of Na 4 CrO 4 by DTA (Differential Thermal Analysis).
Metals 2016, 6, 157 6 of 9 liberated CO2 gas between 200-700 °C; the endothermic peak around 100 °C is a removal reaction of the adsorbed water; and endothermic peak around 800 °C is due to the reaction of Na4CrO4 by DTA (Differential Thermal Analysis).

SEM Analysis
Figure 6 shows the SEM image of a sample of mixed Cr2O3 and Na2CO3 in a 1:1 ratio, at different temperatures for one hour.The sample at 300 °C is composed of the irregular large blocky particles and uniform elliptical small particles, and the elliptical small particles enrich on the surface of the larger block particles.The two kinds of particles are substantially independent of each other.According to XRD analysis, there is no reaction substantially occurring in the sample at 300 °C, so the product is still composed of Cr2O3 and Na2CO3.However, the sample begins to cohere together.The apparent morphology and characteristics of the sample at 400 °C are similar to the sample at 300 °C, but the blocky particle size decreases, while the elliptical-shaped particles appear slightly larger, and small particles show further agglomeration.According to XRD analysis results, the sample has no substantial reaction at 400 °C, and the product is still composed of Cr2O3 and Na2CO3 powder.The figure shows that, compared with the sample at 300 and 400 °C, blocky large particles in the sample at 500 °C basically disappear.According to XRD analysis results, the sample begins to react at 500 °C and is composed of Na2CO3, Cr2O3 and Na2CrO4.The product at 600 °C is similar to the sample at 500 °C and mainly consists of a 1 μm powder composition, approximately elliptical, but some particles start to fuse together.Combined with XRD analysis results, the sample at 600 °C is also composed of Na2CO3, Cr2O3 and Na2CrO4.At the temperature of 700 °C, the sample consists of smaller particles, with obvious agglomeration, but the small particles melt together to form larger particles of different shapes and different sizes.Combined with XRD analysis results, the sample at 700 °C is also composed of Na2CO3, Cr2O3 and Na2CrO4, similar to the sample at 500 and 600 °C.The sample at 750 °C consists of a small number of small particles of 1-2 μm and large particles of different shapes; all the different-sized particles get together, and compared with the sample at 700 °C, more particles fuse together.Combined with XRD analysis results, the sample at 750 °C is composed of Cr2O3, Na2CrO4 and Na4CrO4.The sample at 800 °C basic melts completely, with only a

SEM Analysis
Figure 6 shows the SEM image of a sample of mixed Cr 2 O 3 and Na 2 CO 3 in a 1:1 ratio, at different temperatures for one hour.The sample at 300 ˝C is composed of the irregular large blocky particles and uniform elliptical small particles, and the elliptical small particles enrich on the surface of the larger block particles.The two kinds of particles are substantially independent of each other.According to XRD analysis, there is no reaction substantially occurring in the sample at 300 ˝C, so the product is still composed of Cr 2 O 3 and Na 2 CO 3 .However, the sample begins to cohere together.The apparent morphology and characteristics of the sample at 400 ˝C are similar to the sample at 300 ˝C, but the blocky particle size decreases, while the elliptical-shaped particles appear slightly larger, and small particles show further agglomeration.According to XRD analysis results, the sample has no substantial reaction at 400 ˝C, and the product is still composed of Cr 2 O 3 and Na 2 CO 3 powder.The figure shows that, compared with the sample at 300 and 400 ˝C, blocky large particles in the sample at 500 ˝C basically disappear.According to XRD analysis results, the sample begins to react at 500 ˝C and is composed of Na 2 CO 3 , Cr 2 O 3 and Na 2 CrO 4 .The product at 600 ˝C is similar to the sample at 500 ˝C and mainly consists of a 1 µm powder composition, approximately elliptical, but some particles start to fuse together.Combined with XRD analysis results, the sample at 600 ˝C is also composed of Na 2 CO 3 , Cr 2 O 3 and Na 2 CrO 4 .At the temperature of 700 ˝C, the sample consists of smaller particles, with obvious agglomeration, but the small particles melt together to form larger particles of different shapes and different sizes.Combined with XRD analysis results, the sample at 700 ˝C is also composed of Na 2 CO 3 , Cr 2 O 3 and Na 2 CrO 4 , similar to the sample at 500 and 600 ˝C.The sample at 750 ˝C consists of a small number of small particles of 1-2 µm and large particles of different shapes; all the different-sized particles get together, and compared with the sample at 700 ˝C, more particles fuse together.Combined with XRD analysis results, the sample at 750 ˝C is composed of Cr 2 O 3 , Na 2 CrO 4 and Na 4 CrO 4 .The sample at 800 ˝C basic melts completely, with only a small part of the small particles enriching on the surface of large particles or getting together; the sample mainly consists of melted large particles and adsorbed or aggregated small particles.Combined with XRD analysis results at 800 ˝C, the sample is composed of Cr

Conclusions
The mechanism of chromium on the roasting of vanadium slag has been given, and the effect of different contents of chromium on the conversion rate of vanadium in vanadium slag sodium-roasting has been studied.
1. Cr2O3 and Na2CO3 start the reaction at 500 °C and generate Na2CrO4, and the sample generates a new phase of Na4CrO4 at 750 °C; when heating up to 800 °C, Na4CrO4 decomposes to

Conclusions
The mechanism of chromium on the roasting of vanadium slag has been given, and the effect of different contents of chromium on the conversion rate of vanadium in vanadium slag sodium-roasting has been studied.
1. Cr 2 O 3 and Na 2 CO 3 start the reaction at 500 ˝C and generate Na 2 CrO 4 , and the sample generates a new phase of Na 4 CrO 4 at 750 ˝C; when heating up to 800 ˝C, Na 4 CrO 4 decomposes to Na 3 CrO 4 .There are two weight loss intervals and one endothermic peak and one exothermic peak between room temperature to 1000 ˝C.Seen from the SEM, the particles begin to melt at 600 ˝C.
2. When the chromium oxide content is about 3.2%, the conversion rate of V reaches the maximum value of 95%.

Figure 1 .
Figure 1.The XRD pattern of vanadium slag before roasting.

Figure 1 .
Figure 1.The XRD pattern of vanadium slag before roasting.

Figure 2 .
Figure 2. The XRD pattern of vanadium slag after roasting.Figure 2. The XRD pattern of vanadium slag after roasting.

Figure 3 .
Figure 3.Effect of Cr2O3 content on vanadium conversion rate.

Figure 3 .
Figure 3.Effect of Cr 2 O 3 content on vanadium conversion rate.

Figure 4 .
Figure 4.The XRD pattern of a sample of mixed Na2CO3 and Cr2O3 in a 1:1 ratio, at 300-800 °C for 1 h.

Figure 4 .
Figure 4.The XRD pattern of a sample of mixed Na 2 CO 3 and Cr 2 O 3 in a 1:1 ratio, at 300-800 ˝C for 1 h.

2 O 3 , 9 Figure 6 .
Figure 6.The SEM images of a sample of mixed Cr2O3 and Na2CO3 in a 1:1 ratio, at different temperatures for 1 h.

Figure 6 .
Figure 6.The SEM images of a sample of mixed Cr 2 O 3 and Na 2 CO 3 in a 1:1 ratio, at different temperatures for 1 h.

Table 3 .
Scheme of the influence of chrome oxide on the vanadium conversion experiment.

Table 3 .
Scheme of the influence of chrome oxide on the vanadium conversion experiment.

. Cr2O3% Temperature/°C Time/min Alkali Vanadium Ratio Alkali Salt Ratio
indicates that the main components of the spinel central portion are Cr 2 O 3 , V 2 O 3 , MgO, and FeO, of which Cr 2 O 3 and V 2 O 3 are the nucleation of monomer spinel.The chemical composition of the chromium-rich vanadium spinel central portion can be approximated considering (Fe 2+ Mg 2+ )(Cr 3+ V 3+ )O 4 .The optimum vanadium extraction temperature for transforming Cr 2 O 3 to CrO 4 2´w as around 1200 ˝C by chromite FeCr 2 O 4 , and the first roasting conversion rate of Cr 2 O 3 was