Green and Efficient Process for Extracting Chromium from Vanadium

Jun 8, 2017 - Engineering Research Institute, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China ...
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Research Article pubs.acs.org/journal/ascecg

Green and Efficient Process for Extracting Chromium from Vanadium Slag by an Innovative Three-Phase Roasting Reaction Yilong Ji,†,‡ Shaobo Shen,*,†,§ Jianhua Liu,‡ Shiyu Yan,†,§ and Zhitao Zhang†,§ †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China ‡ Engineering Research Institute, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China § School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China S Supporting Information *

ABSTRACT: The traditional industrial practice for extracting vanadium from vanadium slag involves a Na2CO3−Na2SO4−NaCladded pellet roasting at 800 °C followed by water leaching. About 5% of chromium and 85% of vanadium can be extracted in this case. The disposal of this leaching residue containing high contents of chromium and vanadium is still an unsolved problem and has seriously impeded the sustainable production of vanadium in China. A complete extraction of chromium and vanadium from vanadium slag might be a solution. To extract efficiently chromium from vanadium slag, some NaOH-added pellets were used to replace traditional pellets here. It was found that the volume of a NaOH-added pellet increased by 144% and many cavities were formed spontaneously throughout the pellet during roasting and a three-phase reaction of solid (slag)-liquid (NaOH)-gas (O2) occurred in the porous pellet. The Cr extraction was thus increased by 43% when a NaOH-added pellet sample was used to replace the corresponding powder sample. The Cr extraction increased with increasing temperature and reached up to 97.5% and 99.1% at 700 and 800 °C, respectively, with a molar ratio of NaOH to V of 18.5. This process avoids the emission of toxic gases such as chlorine and hydrogen chloride. KEYWORDS: Efficient chromium extraction, Vanadium slag, Three-phase reaction, Sodium hydroxide, Optimal extraction conditions, Mechanisms

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INTRODUCTION Most of vanadium in titaniferous magnetites is first reduced into hot metal by coke in a blast furnace and then oxidized in the LD converter and enriched into steel slag, which is usually called as vanadium slag.1−8 About 5−8 wt % of vanadium and 2−5 wt % of chromium are contained in the vanadium slags.1,2,4,5,9−13 The current traditional industrial practice for extracting vanadium from vanadium slag involves a Na2CO3− Na2SO4−NaCl-added oxidation roasting in air at about 800 °C followed by water leaching.4,7,12,14,15 The melting points of Na2CO3, Na2SO4 and NaCl are 851, 884 and 801 °C, respectively. Therefore, the traditional process is basically regarded as a two-phase reaction between the solid pellet (vanadium slag solid plus sticky liquid mixture of Na2CO3− Na2SO4−NaCl with a high viscosity) and the gas (O2). It is usually thought that insoluble Cr3+ from V slag was converted to soluble Cr6+ during the oxidation roasting and the extracted chromium exists in the leaching solution.6,16 The chromium extraction for above industrial process is about 5% and less than 85% of vanadium is also simultaneously extracted.6 The © 2017 American Chemical Society

disposal of the leaching residue containing high contents of both chromium and vanadium constituted a large environmental problem unsolved so far in China. In addition, some toxic gases such as chlorine and some chlorides are also produced during the roasting.4 These have constituted serious impediment to the sustainable production of vanadium in China. An efficient extraction of chromium and vanadium from the vanadium slag might be a solution.17 In this work, a small amount of NaOH solid was used to replace the salts of Na2CO3, Na2SO4 and NaCl, which are used in the traditional industrial process. To the best of our knowledge, this is a novel process for recovering chromium from vanadium slag. The optimal conditions for recovering chromium from the vanadium slag by this process were studied. The mechanisms involved in this reaction process were also investigated. Received: March 20, 2017 Revised: April 28, 2017 Published: June 8, 2017 6008

DOI: 10.1021/acssuschemeng.7b00836 ACS Sustainable Chem. Eng. 2017, 5, 6008−6015

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ACS Sustainable Chemistry & Engineering Table 1. Main Chemical Composition of the Original Vanadium Slaga in wt %

a

Elements

V2O3

Cr2O3

FeO

SiO2

TiO2

MnO

Al2O3

MgO

CaO

wt %

12.22

5.14

38.48

19.54

10.82

5.72

2.22

2.95

2.23

Particle size of the original vanadium slag ranged from 49 to 74 μm.

Figure 1. XRD patterns of (a) original vanadium slag, (b) pellet roasted at 600 °C for 5 min with a R(Na/Cr) of 18.5, (c) pellet roasted at 700 °C for 2 h with a R(Na/Cr) of 18.5, and (d) pellet roasted at 800 °C for 2 h with a R(Na/Cr) of 18.5.

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METHODS

Cr extraction (%) = [Cr]V × 100/WCr

Materials. About 30 kg of vanadium slag from one steel-making plant of China was used in this work. The slag was crushed, dried and sieved. The dried slag samples were kept in a desiccator before use. Unless otherwise specified, the slag sample with a particle size of 49− 74 μm was used in the experiments. The chemical analysis of the vanadium slag is shown in Table 1. The content of Cr2O3 from the slag was 5.14 wt %. Experimental Procedure. To make the pellets used in this work, about 0.20 g of deionized water was added into a powder mixture composed of 2.20 g of the vanadium slag (49−74 μm) and 1.10 g of solid NaOH in most of cases unless otherwise specified. The molar ratio of NaOH to Cr, which was simply denoted as R(Na/V), was 18.5 in this case. The mixture thus obtained was pressed into a cylindric pellet with a diameter of 15 mm and a height of 8 mm under a pressure of 0.5 MPa. This pellet was placed in a corundum boat of 30 mL. The boat was then put in a muffle oven already set at a temperature of 400−800 °C and kept in it for a preset time. After that, the boat was taken out from the muffle oven and then cooled to the room temperature. The boat with the content was placed in a glass beaker of 500 mL. About 250 mL of deionized water was added to the beaker to immerse the boat. The water in the beaker was raised to 90 °C for 5 min by a heating plate. The mixture was stirred with a Teflon agitator at 200 rpm for 10 min during the water leaching. In this way, the content of the boat could be completely rinsed and transferred into water in the beaker. After the solution was cooled to room temperature, it was filtered through a Whatman GF-A membrane with a pore diameter of 0.4 μm. The filtrate was analyzed with ICPAES (SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH) to measure the concentration of chromium. The solid residue and the membrane were dried at 105 °C for 4 h. Then the solid leaching residue was removed from the membrane and weighed. The percentage of extracted chromium was calculated on the basis of the following:

(1)

Where WCr is the weight of chromium in milligrams from the sample, [Cr] the concentration of chromium from the filtrate in milligrams per liter and V the volume of the filtrate in liters. Chemical Analysis and Characterization. A dried slag sample or leaching residue sample of 0.3000 g was put in a platinum crucible and mixed with alkaline lithium metaborate (LiBO2), lithium fluoride (LiF) and lithium bromide (LiBr) solution. Then the crucible was put in a muffle oven at 1200 °C for about 25 min. After that, the hot content of the crucible was transferred to a glass beaker containing a solution of HNO3 and HCl. Finally, a clear digestion solution was obtained and the metal concentrations in the solution were analyzed using ICP-AES. The X-ray diffraction (XRD) patterns of the fine sample powders with particle size less than 58−74 μm were recorded with a Rigaku TTRIII X-ray diffractometer equipped with a Cu Kα radiation source (λ = 0.154 05 nm). The diffraction patterns of XRD were analyzed using the software of X’Pert HighScore Plus. The SEM sample was normally prepared by pouring a liquid composed of epoxy, thylenediamine and dibutyl phthalate on the sample powder placed at the bottom of a plastic mold. After standing for 12 h, a solidified sample was obtained. The solid sample was grinded and polished. The polished surface was sprayed with fine gold powder. Then the sample linked with double-sided carbon conductive tape on the surface was mounted on a SEM. The SEM observation was performed with the SEM (Zeiss Ultra 55).

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RESULTS AND DISCUSSION Theoretical Analysis of Chromium Extraction. Based on XRD patterns shown in Figure 1a, the main Cr-containing phase from the vanadium slag was chromium-containing spinel phase (Mn, Fe)(V, Cr, Ti)2O4 (PDF 00-035-0550). The current traditional method for extracting V from vanadium slag involves an oxidation roasting of Na2CO3-added pellet in air at 6009

DOI: 10.1021/acssuschemeng.7b00836 ACS Sustainable Chem. Eng. 2017, 5, 6008−6015

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ACS Sustainable Chemistry & Engineering 800 °C. This process accompanies a simultaneous Cr extraction, which might be expressed by eq 2. It is usually thought that a water-soluble Cr6+-containing Na2CrO4 is formed. (4/7)FeCr2O4 + (8/7)Na 2CO3 + O2 (g) = (8/7)Na 2CrO4 + (2/7)Fe2O3 + (8/7)CO2 (g)

(2)

In the current work, NaOH was used to replace Na2CO3. The corresponding reaction might be expressed by eq 3. (4/7)FeCr2O4 + (16/7)NaOH + O2 (g) = (8/7)Na 2CrO4 + (2/7)Fe2O3 + (8/7)H 2O(g)

(3)

According to the thermodynamic calculations based on HSC 6.0 software, the reaction in eq 3 proceeds more readily than that in eq 2 (Figure 2). Thus, NaOH is preferred to Na2CO3 for the oxidation roasting of FeCr2O4. Thus, NaOH was used instead of Na2CO3 for the chromium extraction in the current work. Figure 3. Variation of Cr extraction with (a) time at 600 °C and (b) temperature for 2 h for both powder sample and pellet sample with a same R(Na/Cr) of 18.5.

sample or pellet sample increased dramatically with time in the first 30 min and then increased slowly with time from 30 to 240 min (Figure 3a). The Cr extractions of both powder sample and pellet sample were 73% and 87%, respectively, at 240 min (Figure 3a). A comparison of Cr extraction with increasing temperature between pellet and powder samples for 2 h is presented in Figure 3b. The Cr extraction of either powder sample or pellet sample increased with increasing temperature in the temperature range of 400 to 700 °C. The Cr extractions of both powder sample and pellet sample at 700 °C were 68% and 97%, respectively (Figure 3b). The Cr extractions of both powder sample and pellet sample at 800 °C were 86% and 99%, respectively (Figure 3b). In general, the Cr extraction was increased when a pellet sample was used instead of a powder sample under the same roasting conditions. There was no cavity on the smooth surface of the black fresh pellet (Figure 4a). There were many dispersed exposed white NaOH particles on the surface of the fresh pellet (Figure 4a). After the roasting, many cavities with an average pore diameter of 0.5 mm were formed on the surface of the brown roasted pellet sample (Figure 4c). Moreover, the diameter of the pellet increased by about 25% and the volume thus increased by 144% after the roasting (Figure 4a,c). It indicated that the pellet expanded during the roasting. Thus, some cavities were probably formed inside the roasted pellet. Generally speaking, the polished section surface of one particle from the original vanadium slag was smooth and there was no cavity on it (Figure 4b). However, the section surface of the roasted particle was loose and porous (Figure 4d) and many cavities (dark spots filled with the resin) with an average pore diameter of 5 μm were observed. It indicated that the cavities were indeed formed throughout the whole pellet during the roasting. In addition, the roasted powder samples sintered and adhered on the bottom of corundum boat (Figure 4e). Instead, the roasted pellet sample did not adhere on the corundum boat at all and could be taken out easily from the boat (Figure 4c). The melting point of NaOH is 318 °C. At the reaction temperature of 700 °C (Figure 4), the particles of NaOH on the pellet

Figure 2. Variation of standard Gibbs free energy ΔG° of several reactions with temperature obtained by thermodynamic calculations using HSC 6.0 software.

Based on a later mechanism investigation, Na3CrO4 instead of Na2CrO4 was formed during the NaOH-added pellet roasting at 700 °C. Thus, the possible chemical reaction involved in the chromium extraction for the current work is as follows. 2FeCr2O4 + 12NaOH + 2.5O2 (g) = 4Na3CrO4 + Fe2O3 + 6H 2O(g)

(4)

Stoichiometrically, 12 mol of NaOH and 2.5 mol of O2 are required to react with 2 mol of FeCr2O4 based on eq 4. The molar ratio of Na to Cr (R(Na/Cr)) is 3.0 for the reaction in eq 4. Comparison of Cr Extraction between Pellet and Powder Samples. Initially, we did the chromium extraction using the powder sample. However, it was found that the roasted powder sample adhered on the corundum boat seriously. That process was not practicable in industry. Then we did the experiment using a pellet sample. To our surprise, the roasted pellet sample did not adhere the corundum boat at all. To illustrate the role of NaOH-added pellet, a comparison of Cr extraction between pellet and powder samples is presented in Figure 3. The Cr extractions of either powder 6010

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at 700 °C with a R(Na/Cr) value of 18.5, good transport kinetics of both liquid NaOH and external O2 and effective utilization of NaOH, a higher Cr extraction was thus obtained for the pellet sample compared to the powder sample (Figure 3). For the original powder sample, there were large voids among stacked mineral particles. The capillary forces could not be formed by these large voids. Thus, there was no capillary force to suck the molten liquid NaOH to flow toward the interior of the sample. The small molten liquid drops of NaOH passed through the large voids and converged gradually. The converged liquid NaOH drops were pooled together and reacted with mineral particles at the large interface and led to the sintering of the samples (Figure 4e). As a result, only a part of minerals in the powder sample reacted with liquid NaOH and resulted in a lower Cr extraction (Figure 3). In addition, there were fewer cavities on the smooth surface of sintered sample during the roasting (Figure 4e). This prevented the external O2 from entering the interior of the powder sample, which thus limited the oxidation of unreacted Cr3+. In addition, the molten small liquid drops of NaOH passed through the voids of stacked particles and flowed toward the bottom of corundum boat due to the gravity and converged there. The converged liquid NaOH reacted with both the corundum boat and the closed mineral particles and caused the adhesion between the reacted mineral particles and the reacted corundum boat (Figure 4e). The reaction of liquid NaOH with the corundum boat at the roasting temperature can be illustrated by eq 5.

Figure 4. (a) Photo of original pellet; (b) SEM of one particle section from the original vanadium slag; (c) photo of the pellet roasted at 700 °C for 2 h with a R(Na/Cr) of 18.5; (d) SEM of one particle section from the pellet roasted at 700 °C for 2 h with a R(Na/Cr) of 18.5; (e) photo of the powder sample roasted at 700 °C for 2 h with a R(Na/ Cr) of 18.5.

2NaOH + Al 2O3 = 2NaAlO2 + H 2O(g)

(5)

Part of NaOH was thus wasted to react with the corundum boat. Because of the difficult transport of external O2 inside the powder sample caused by powder sample sintering, local reaction of NaOH with the minerals and the unnecessary consumption of NaOH by the corundum boat, a lower Cr extraction was thus attained for the powder samples (Figure 3). In addition, a serious adhesion between the reacted mineral particles and the reacted corundum boat also occurred. Therefore, only some pellet samples were used in the following experiments. Effects of Roasting Temperature and Molar Ratio of Na to Cr. The effect of roasting temperature on Cr extraction is presented in Figure 5a. The Cr extraction increased with increasing temperature in the temperature range of 400 to 700 °C. The Cr extraction was almost the same in the temperature range of 700 to 800 °C for each R(Na/Cr) value of 7.40, 11.1, 14.8, 18.5 and 22.2 (Figure 5a). The maximum Cr extraction, which was 99.1%, was obtained at 800 °C with a R(Na/Cr) value of 18.5. Thus, the Cr extraction was increased from traditional 5% to the current 99% at 800 °C. The Cr extraction was thus increased by 1880% ((99%−5%)/5%). In addition, the Cr extraction was 97.5% at 700 °C with a R(Na/Cr) value of 18.5, which was very close to the maximum Cr extraction. Therefore, the optimal temperature for the Cr extraction was 700 °C, based on the viewpoint of reducing the consumption of energy and refractory. The Cr extraction was increased by 43% ((97%−68%)/68%) in this case when a pellet sample was used instead of a powder sample (Figure 3b). This optimal temperature was 100 °C lower than that of the current industrial application temperature of 800 °C, whereas the Cr extraction for the current method was increased by 1840%

surface became liquid and entered into the interior of the pellet probably due to the spontaneous imbibition of liquid NaOH drops in capillary channels of the pellet and also due to its diffusion inside slag particles via some chemical reactions with some minerals at 700 °C. In general, the NaOH particles initially located at the pellet surface became liquid drops and then were sucked into the interior of the pellet due to capillary forces and disappeared completely during the roasting (Figure 4c). Many cavities were thus formed spontaneously. Therefore, the roasted pellets did not adhere on the bottom of corundum boat at all. The slag particles (49−64 μm) and internal closed capillary channels of the pellet isolated the small liquid NaOH drops transformed from small NaOH particles. This prevented the small liquid NaOH drops from aggregating together and flowing toward the bottom of corundum boat due to the gravity. In fact, these liquid NaOH drops in the pellet had been gradually disappeared by reacting with various minerals surrounding them before they flowed to the bottom of corundum boat. Therefore, both slag particles (49−64 μm) and internal capillary channels of the pellet acted as a skeleton just as the coke does in a blast furnace of making iron in pyrometallurgy. Several solid phases (dry slag minerals), some liquid phases (many small isolated liquid drops of NaOH) and a gas phase (O2) were involved in this process. Thus, this was a three-phase reaction. In addition, the cavities spontaneously formed improved the transport kinetics of liquid NaOH drops and gaseous O2 inside a partially roasted pellet and accelerated the oxidation of the unreacted chromium based on the reactions of eq 4. Because of the absence of pellet sintering 6011

DOI: 10.1021/acssuschemeng.7b00836 ACS Sustainable Chem. Eng. 2017, 5, 6008−6015

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Figure 5. (a) Variation of both Cr extraction and viscosity of NaOH with temperature for 2 h and with a R(Na/Cr) of 18.5; (b) variation of Cr extraction with time with a R(Na/Cr) of 18.5.

((97%−5%)/5%) compared to the traditional industrial method. The Cr extraction significantly increased with increasing R(Na/Cr) value in the R(Na/Cr) value range of 7.4 to 18.5 (Figure 5a). It is noteworthy that the Cr extractions at the R(Na/Cr) values of both 18.5 and 22.2 were almost the same. However, the Cr extraction decreased significantly when the R(Na/Cr) value was increased from 22.2 to 29.6. Thus, the optimal R(Na/Cr) value was 18.5 based on the viewpoint of reducing NaOH consumption. When the R(Na/Cr) value was 18.5, the weight ratio of NaOH to vanadium slag was 0.5, which was much less than the corresponding value of 4.0 required by the molten salt method.6 Effect of Roasting Time. The effect of roasting time on Cr extraction is shown in Figure 5b. The Cr extraction dramatically increased with time within the first 30 min for all the temperatures. After that, the Cr extractions at various temperatures slowly increased with time. The Cr extractions at both 700 and 800 °C reached the maximum at 120 min. Thus, the optimal time for the Cr extraction was 120 min and the Cr extractions in this case were 97.5% and 99.1% at 700 and 800 °C, respectively. Compared with 6 h required for Cr extraction by the molten salt method,18 a 4 h reduction in the required time was achieved for the current method. Main Phases and Cr Occurrence in Vanadium Slag. Based on XRD patterns shown in Figure 1a and SEM displayed in Figure 6, the main Cr-containing phase from the original vanadium slag was a Mn2+-, Fe2+-, V3+-, Cr3+-, Ti3+-containing spinel phase (Mn, Fe)(V, Cr, Ti)2O4 (PDF 00-035-0550). In addition, another Ti3+-containing spinel phase Fe2.5Ti0.5O4 phase (PDF 01-040-496) and a silicate phase (Fe, Mn)2SiO4 (PDF 01-085-1347) were also observed. To learn the structure relationships among these phases in micrometer scale, the element distribution of one particle section from one original vanadium slag was analyzed with SEM. The particle section was

Figure 6. (a) SEM of one particle section from original vanadium slag; Elements of (b) Si, (c) Ca, (d) Fe, (e) Ti, (f) V, (g) Cr, (h) Mn distributed in the particle section (a).

observed with one SEM image (Figure 6a). The element distribution of the section was analyzed with EDS of the SEM and presented in Figure 6b−h. The distributions of V, Cr and Ti were very similar. It implied that these three elements were enriched in the spinel phase (Mn, Fe)(V, Cr, Ti)2O4 (Figure 1a). The spinel phase (Mn, Fe)(V, Cr, Ti)2O4 did not contain Si because there was a complementary element distribution between Si and V, Cr, Ti. Thus, the spinel phase was detached from a silicate phases of (Mn, Fe)2SiO4 (Figure 1a). Mn and Fe were distributed in both the silicate phases of (Mn, Fe)2SiO4 and the spinel phase (Mn, Fe)(V, Cr, Ti)2O4. The distributions of Ca and Si were similar. It implied that Ca was only presented in the silicate phases of (Mn, Fe)2SiO4. The spinel phases were partially encompassed by the silicate phases (Figure 7b−h). V3+ was mainly distributed in the spinel phase. It means that NaOH probably had to destroy the silicate phases surrounding the Vcontaining spinel phases before attacking the spinel phase. Because NaOH is a stronger base than Na2CO3, NaOH had larger capacity to attack the silicate phases. This partially explained why the Cr extraction was higher if NaOH was used instead of Na2CO3 in the vanadium slag roasting. Phase Transformation during the Pellet Roasting. To investigate the mechanisms involved in the NaOH roasting, the roasting slags with and without water leaching were studied. After the roasting at 600 °C for 5 min, the roasted pellet was leached with hot water. The leaching residue was observed with SEM and analyzed with EDS. The result is presented in Figure 7. The Cr extraction was 14% in this case (Figure 5b). It 6012

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phase was one rhombohedral phase Fe9TiO15 (PDF 00-0541267), which was transformed from one cubic spinel phase (Mn, Fe)(V, Cr, Ti)2O4. It implied that most of V and small amount of Cr was detached from the spinel phase (Mn, Fe)(V, Cr, Ti)2O4 after the roasting at 600 °C for 5 min and regrouped into a new green phase (Cr0.15V0.85)2O3, because V from the spinel phase (Mn, Fe)(V, Cr, Ti)2O4 was more readily to be oxidized than Cr as mentioned previously. The not detached part of the spinel phase was transformed into a new blue phase Fe9TiO15 as mentioned previously. This phase conversion process might be illustrated by eq 8. 600 ° C

(Mn, Fe)(V, Cr, Ti)2 O4 ⎯⎯⎯⎯⎯⎯→ Fe9TiO15 + (Cr0.15V0.85)2 O3

(8)

Where there existed (Cr0.15V0.85)2O3 phase marked with green color line (Figure 7b−h), Na was almost not there. It implied that the (Cr0.15V0.85)2O3 phase was less attacked by NaOH in that case. However, the Na-containing yellow region closely surrounded the (Cr0.01V0.99)2O3 phase (Figure 7d). Si, Fe and O were also present in the yellow Na-containing region (Figure 7c,g,h). Moreover, the distribution of Si, Fe and O in the yellow line region were similar. It implied that NaAlSiO4 phase and NaFeO2 phase were present in the yellow line region according to XRD patterns (Figure 1b). It indicated that the silicate phase surrounding the central (Cr0.01V0.99)2O3 phase had been attacked by NaOH. The formation of NaAlSiO4 phase and NaFeO2 phase might be illustrated by eq 9. O2 ,NaOH,600 ° C

(Mn, Fe)2 SiO4 + Al 2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NaAlSiO4 + NaFeO2

The yellow silicate phase attacked by liquid NaOH was close to the cavity “1” of the particle (Figure 7a). It indicated that liquid NaOH passed through the cavity “1” and attacked the silicate phase (Fe, Mn)2SiO4. Compared to the yellow silicate phase, the green (Cr0.01V0.99)2O3 phase was farther from the cavity “1” and thus not attacked by NaOH at 600 °C for 5 min. In addition to the yellow region, Na was also present in another region, which is marked by brown line and on the left side of the particle (Figure 1b). In the brown region, Fe, Ti and O were also presented. It indicated that a NaFeTiO4 phase was formed in the brown region according to the XRD (Figure 1b). This formation of this phase might be illustrated by eq 10.

Figure 7. (a) SEM of one particle section from the pellet roasted at 600 °C for 5 min with a R(Na/Cr) of 18.5; elements of (b) Na, (c) Si, (d) V, (e) Cr, (f) Ti, (g) Fe and (h) O distributed in the particle section (a).

indicated that only small amount of Cr3+ in the pellet was oxidized during the roasting at 600 °C for 5 min. The V extraction was 45% in this case, which will be reported elsewhere. It implied that V3+ from the spinel phase (Mn, Fe)(V, Cr, Ti)2O4 was more readily to be oxidized than Cr3+, which was possible according to the thermodynamic calculations (Figure 1). The reactions involved in the oxidation of V3+ and Cr3+ from the spinel phase (Mn, Fe)(V, Cr, Ti)2O4 can be approximately represented by eqs 6 and 7.

O2 ,NaOH,600 ° C

Fe9TiO15 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NaFeTiO4

V2O3 + 6NaOH + O2 (g) = 2Na3VO4 + 3H 2O(g)

(10)

It implied that NaOH also attacked the Cr-containing spinel phase (Mn, Fe)(V, Cr)2O4 according to eqs 8 and 10. The brown NaFeTiO4 phase was close to the cavities of “2” and “3” (Figure 7a). It indicated that liquid NaOH probably passed through the cavities of “2” and “3” to attack the spinel phase close to them. Above two situations indicated that NaOH attacked the slag particle from the exterior to the interior gradually (Figure 7). It implied that the internal diffusion of liquid NaOH drops inside the particle was probably the limiting step for the oxidization roasting of Cr3+. After the roasting at 700 °C for 2 h, the roasted pellet without water leaching was studied with XRD and presented in Figure 1c. To our surprise, the main Cr-containing phase in the roasted pellet was Na3CrO4 (PDF 00-029-1199) rather than Na2CrO4. The chromium valence state in the Na3CrO4 was +5. Thus, the possible reaction involving the oxidation roasting of chromium at 700 °C for 2 h is presented in eq 11.

(2/3)Cr2O3 + (8/3)NaOH + O2 (g) = (4/3)Na 2CrO4 + (4/3)H 2O(g)

(9)

(6) (7)

A polished surface of one particle section from the leaching residue was observed with SEM (Figure 7a). The element distribution in the section was analyzed with EDS and presented in Figure 7b−h. The distributions of V and Cr in the region, which is marked with green color line, are similar (Figure 7d,e). It implied that V and Cr were in the same phase for this region. This green phase was (Cr0.15V0.85)2O3 (PDF 01089-8205) according to the XRD (Figure 1b). In addition, Cr was also distributed in another region, which is marked with blue color line and located at the lower position of the particle (Figure 7e). In this blue region, Ti, Fe and O were also present (Figure 7f−h) and thus in the same phase as Cr. This blue 6013

DOI: 10.1021/acssuschemeng.7b00836 ACS Sustainable Chem. Eng. 2017, 5, 6008−6015

ACS Sustainable Chemistry & Engineering O2 ,NaOH,7000C

(Cr0.15V0.85)2 O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Na3CrO4

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(11)

*Shaobo Shen. E-mail: [email protected]. Tel/ Fax: 86-10-62332525. ORCID

Shaobo Shen: 0000-0002-4775-629X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for the project of 51374023 from the National Natural Science Foundation of China is gratefully acknowledged.

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(12)

2Ti 2O3 + 8NaOH + O2 (g) = 4Na 2O·TiO2 + 4H 2O (13) 3+

In summary, the oxidization roasting of Cr from the V slag probably underwent the following procedures:

O2 ,NaOH,7000C

(Cr0.15V0.85)2 O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Na3CrO4

(14) (15)

In addition, the following side reactions probably also occurred. O2 ,NaOH,600 ° C

Fe9TiO15 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NaFeTiO4 O2 ,NaOH,600 ° C

(Mn, Fe)2 SiO4 + Al 2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NaAlSiO4 + NaFeO2

(16) (17)

The current process only involves an oxidation roasting followed by water leaching, which is very similar to the traditional industrial practice for extracting vanadium from vanadium slag. At present, some large multiple hearth furnaces and rotary kilns are being used in metallurgical industry for the vanadium slag roasting in order to extract vanadium from the vanadium slags. These roasting furnaces might be applied directly in the present process to extract chromium simultaneously. This will avoid the investment of new equipment and additional processes. Thus, this novel method is probably practicable.

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REFERENCES

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4FeO + 4NaOH + O2 (g) = 2Na 2O ·Fe2O3 + 2H 2O

600 ° C

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Corresponding Author

Some thermodynamic calculations using FactSage software indicated that a higher roasting temperature favors the formation of a lower valence state of chromium compounds such as Na2Cr2O4 (Figure S1). The solubility of Na3CrO4 is unknown so far. The Na3CrO4 from the roasting products was probably transformed to the very soluble Na2CrO4 during the hot water leaching, because the Cr extraction was 97% at 700 °C for 2 h (Figure 5a). After the roasting at 800 °C for 2 h, all Cr-containing crystalline phases from the original V slag disappeared and converted to some amorphous substances. The Cr extraction was 99% in this case. It indicated that these amorphous Crcontaining substances were probably water-soluble. Fe2+ and Ti3+ were mainly presented in the original spinel phase (Mn, Fe)(V, Cr, Ti)2O4. After the roasting at 600 °C for 5 min, part of Fe2+ and Ti3+ in the spinel phase were oxidized to Fe3+ and Ti4+ (NaFeTiO4), respectively, which is possible based on the thermodynamic calculations (Figure 2). The oxidation of Fe2+ and Ti3+ in the presence of NaOH can be represented by the following reactions of eqs 12 and 13, respectively (Figure 2).

(Mn, Fe)(V, Cr, Ti)2 O4 ⎯⎯⎯⎯⎯⎯→ Fe9TiO15 + (Cr0.15V0.85)2 O3

Research Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00836. Predominance diagrams of Fe−Cr−Na−O system (PDF) 6014

DOI: 10.1021/acssuschemeng.7b00836 ACS Sustainable Chem. Eng. 2017, 5, 6008−6015

Research Article

ACS Sustainable Chemistry & Engineering chromium from vanadium slag by stepwise sodium roasting-water leaching. Hydrometallurgy 2015, 156, 124−135. (17) Yu, K. P.; Chen, B.; Zhang, H. L.; Zhu, G. J.; Xu, H. B.; Zhang, Y. An efficient method of chromium extraction from chromiumcontaining slag with a high silicon content. Hydrometallurgy 2016, 162, 86−93. (18) Liu, B.; Du, H.; Wang, S. N.; Zhang, Y.; Zheng, S. L.; Li, L. J.; Chen, D. H. A Novel Method to Extract Vanadium and Chromium from Vanadium Slag using Molten NaOH-NaNO3 Binary System. AIChE J. 2013, 59 (2), 541−552.

6015

DOI: 10.1021/acssuschemeng.7b00836 ACS Sustainable Chem. Eng. 2017, 5, 6008−6015