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Extraction of vanadium from stone coal by roasting in a fluidized bed reactor
Fuel 142 (2015) 180–188
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Extraction of vanadium from stone coal by roasting in a fluidized bed reactor Xi Zeng a, Fang Wang b, Huifeng Zhang a,c, Lijie Cui c, Jian Yu a,⇑, Guangwen Xu a,⇑ a
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, China c University of Chinese Academy of Sciences, Beijing 100039, China b
h i g h l i g h t s The new roasting process consists of two fluidized bed reactors. Experimental conditions were optimized on a laboratory fluidized bed reactor. Fluidized roasting and impregnating additive on stone coal had better results.
a r t i c l e
i n f o
Article history: Received 19 July 2014 Received in revised form 1 October 2014 Accepted 22 October 2014 Available online 10 November 2014 Keywords: Stone coal Vanadium leaching Fluidized bed roasting Additive Chlorine
a b s t r a c t In order to improve leaching efficiency of vanadium pentoxide (V2O5) from stone coal and to reduce its connected environmental pollution, this article investigated the oxidizing roasting of stone coal in a laboratory fluidized bed reactor to optimize the roasting method and conditions. The examined parameters included reaction temperature, reaction time and amount and mixing method of additive. The removal of Cl from the generated effluent gas was implemented using CaO as the adsorbent, and comparison was made between the fluidized bed roasting and static roasting in a muffle oven. The results show that the fluidized roasting is more favorable to leach V2O5 from stone coal, which can shorten roasting time for reaching the maximal leaching rate, while the realized leaching rate is also higher. Comparing the mixing method applied to stone coal and additive, the impregnation method of additive on stone coal not only increased the leaching rate and shortened the roasting time for reaching the maximal leaching rate, but also reduced the amount of additive required. Adding CaO in stone coal roasting sharply decreased the content of Cl-containing gases in flue gas, which can greatly alleviate the possible environmental pollution. The optimal conditions for fluidized bed roasting were found to be impregnating 6 wt.% additive on coal, adding 3 wt.% CaO and performing roasting at 800 °C for 0.75 h. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Vanadium and its compounds are important strategic resources and are playing important roles in many industries such as ferrous and non-ferrous metallurgy, chemical production, and electron and battery manufacture [1,2]. Apart from vanadium titano-magnetite, stone coal is another important vanadium-bearing resource. In China, the gross reserve of vanadium in stone coal is estimated to be about 1.18 million tons. In terms of V2O5 amount, it takes more than 87% of China’s total domestic reserve of vanadium. In recent years, the V2O5 extracted from stone coal has reached 40% of the totally produced V2O5 in China [3]. With the increasing market ⇑ Corresponding authors. Tel.: +86 10 82544885. E-mail addresses: [email protected] (J. Yu), [email protected] (G. Xu). http://dx.doi.org/10.1016/j.fuel.2014.10.068 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
demand for vanadium products and the shortage of high-grade ores, it becomes more and more necessary and urgent to exploit and utilize the low-grade stone coal resource for V2O5 production. In stone coal, vanadium exists dominantly as trivalent vanadium (V(III)) in addition to very little as quadrivalent (V(IV)) and quinquevalent vanadium (V(V)). The presence ratio of these different valent states is closely related to the reduction atmosphere for forming the stone coal [4]. Since V(III) and V(IV) have the ionic radius, electronegativity and coordination number similar to those of Al(III), they usually replace Al(III) in dioctahedral structure to form isomorphism in mica and clay minerals [5]. This sort of vanadium is most prevalent and representative in stone coal but it is difficult to be extracted. Besides, the ore containing vanadium in stone coal is always complicatedly combined with carbon, which further increases the difficulty for extracting vanadium form stone coal.
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China has started to extract vanadium from stone coal since 1960s, whose fundamental principle is oxidizing the acid- and water-insoluble V(III) to V(IV) and/or V(V), and then precipitating and separating V(IV) from the leaching solvent [6]. Up to now, dozens of technologies have been proposed according to the properties of raw stone coal ore in different regions, and some of them have been applied to industrial productions. These technologies can be divided into two categories, direct acid leaching and acid leaching after roasting [7–10]. The direct acid leaching is adopted only by limited companies because of its inherent disadvantages such as poor feedstock adaptability, low vanadium recovery ratio, large consumption of H2SO4, difficulty in separating and concentrating vanadium from many impurities (Fe, Al, Mg, K, Na, etc.), and serious environmental pollution [11,12]. Now, most of the companies producing vanadium from stone coal adopt first roasting and then acid leaching. The representative process is roasting with sodium chloride under oxidizing conditions to convert vanadium in stone coal to acid-soluble salt [13]. In this process, the roasting plays a critical role in oxidizing V(III) to V(IV) and V(V), which is determinative to the available total vanadium recovery ratio and is thus the rate-limiting step. The roasting reactors popularly used include flat kiln, shaft kiln and rotary kiln. All of them, however, have the limitations of low treatment capacity, long roasting time, insufficient heat and mass transfer efficiency in reactors and difficulty in scale-up [6]. The common additive used in the traditional roasting technologies is NaCl, thus bringing about serious emission of poisonous gases of HCl and Cl2 [14]. In recent years, the Chinese government has banned several hundreds of production lines that adopted the direct acid leaching technology. It is consequently urgent to develop new roasting technology with high recovery of vanadium and having as well controlled pollution to environment for extracting vanadium from stone coal. A new fluidized bed two-stage roasting process consisting of a fluidized bed (FB) reactor and a transport bed reactor has been proposed in Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS). As shown in Fig. 1, stone coal is first autothermally roasted in the fluidized bed reactor for partial decarbonization, and its products are in turn forwarded to the transport bed reactor to implement complete decarbonization and deep oxidation of vanadium through adding an additive. In the transport bed, the produced poisonous gases, such as HCl and Cl2 are removed through injecting an adsorbent that is likely CaO. The big particles of stone coal that are not adequately roasted in the transport bed reactor is collected by a cyclone and then returned to the fluidized bed reactor for further roasting. While large particles, usually stone, are withdrawn from the bottom of the transport bed, the roasted fine particles carried with hot flue gas are collected by another cyclone and further cooled and collected for acid leaching. By integrating the advantages of fluidized bed and transport bed reactors [15], the new twostage roasting process is not only applicable to powder feedstock (e.g., powder stone coal), but enables also large treatment capacity in comparison with the kiln reactors. By simulating the practically possible conditions for the process illustrated in Fig. 1, a small laboratory batch fluidized bed reactor was used to test how the roasting process affects the downstream acid leaching. The results were compared with the traditional fixed bed roasting to demonstrate the technical feasibility and advantages of the new proposal. The effects on leaching rate for V2O5 were systematically examined for the parameters including roasting temperature and time, additive type and amount, mixing method between additive and stone coal. Moreover, the chlorine removal from the generated flue gas by CaO was also investigated. As a consequence of this study, a roasting process is finally proposed to ensure the high-efficiency recovery of vanadium from stone coal.
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2. Experimental section 2.1. Roasting apparatus and test As shown in Fig. 2, the fluidized bed roasting apparatus consists of a fluidized bed reactor (FB), an electric furnace, a gas supply section, a feeding hopper, a cyclone and a gas cleaning system. The FB reactor was made of high purity quartz glass and was 50 mm in inner diameter (I.D.). Its reaction zone was 300 mm long between a bottom gas preheating section of 300 mm long and an expanded freeboard of 100 mm in inner diameter and 150 mm in length. Experiment was started by heating the reactor to a desired temperature, such as 800 °C, and then high-purity air at a specified rate was fed into the reactor to form the roasting atmosphere. The roasting was initiated by feeding exactly 30 g stone coal or its mixture with the adopted additive into the reactor through a hopperlike feeder. The generated gas, with fly ash as well, passed through a cyclone and several washing bottles of sodium hydroxide solution immersed in an ice-water bath to remove the poisonous gas components, mainly HCl and Cl2. After experiment, the reactor was taken out from the electric furnace to quickly cool it to the room temperature. The roasted stone coal collected from the cyclone and reactor was taken to analyze its content of V2O5 according to the measurement method depicted in the Section 2.2. The static roasting of stone coal was conducted in a muffle furnace, and the result was compared with that from the fluidized bed roasting in terms of the available extraction efficiency of vanadium. At a desired temperature, the mixture of stone coal and additive was loaded into a ceramic crucible, and this crucible was in turn put into the muffle furnace to implement the roasting in air atmosphere. After reaching the roasting time, the ceramic crucible was taken out and cooled quickly to the room temperature, and the roasted sample in the crucible was similarly analyzed as for that from the fluidized bed roasting.
2.2. Material and measurement Raw stone coal used in this study was taken from Sansui county, Guizhou Province of China. Prior to experiment, the stone coal was crushed and sieved to the sizes of 0.1–0.15 mm, and then dried in an oven at 105 °C for 2 h. Table 1 shows the results of industrial analysis and ultimate analysis for the stone coal and the contents of major components in its ash. The content of vanadium in terms of V2O5 is up to 1.26 wt.%, a prospective value for application development of vanadium leaching. On the other hand, the fixed carbon in stone coal is about 15%, which can seriously hinder the release of vanadium from stone coal due to its strong surface energy and ability of adsorption. The roasting tests used a commercial additive with its effective components being Na2CO3, NaCl and some other species. Two methods were adopted to mix stone coal and additive, physical mixing method by mechanical agitation and ‘‘impregnation’’ method by blending stone coal into a solution of additive containing 15 wt.% water and then aging the coal for about 5 min. All percents cited for additive were against the mass of dry stone coal. Measurement of V2O5 in the roasted stone coal was performed via the following procedure. A certain amount of the roasted stone coal (via either fluidized bed or static roasting) was dissolved into a sulfuric acid in a conical flask, and the resulting solution was in turn heated with a water-bath preset at 40 °C under 2-h continuous stirring and further filtered. The vanadium concentration in the filtering solution was determined according to the method of Chinese National Standard of YB/T 5328-2006 based on ferrous ammonium sulfate titration using N-phenylanthranilic acid as an indicator [16].
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Flue gas 3 3 6 8 CaO 5
4
7 5 5
2
1
Air Stone coal
Particle flow Gas flow
5 Air Roasted stone coal
1-Fluidized bed; 2-Transport bed; 3-Cyclone; 4-Distribution valve; 5-Receiver; 6-Heat exchanger; 7-Separator; 8- Induced draft fan. Fig. 1. Two-stage fluidized bed roasting process with additive for vanadium extraction.
Hopper
T Cyclone
T
Condenser
T T FB Gas
Air
Tank
Gas washing equipment
Fig. 2. A schematic diagram of the used fluidized bed roasting apparatus.
Table 1 Characterization of the tested stone coal and its ash.a Proximate analysis (wt.%) Mad Coal analysis
Ash XRF analysis (wt.%) a
Aad
Ultimate analysis (wt.%) Vad
FCad
Cad
Had
LHV (KJ/kg) Sad
Oad
4.4
74.7
6.5
15
3.86
4.2
0.3
17.6
4458
SiO2
Al2O3
Fe2O3
MgO
V2O5
BaO
P2O5
K2O
SO3
CaO
58.13
16.88
4.92
4.12
1.58
3.91
3.77
3.52
1.93
1.24
ad: Air-dried basis; daf: Dry ash free basis.
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The leaching efficiency of vanadium was defined as the ratio of the V2O5 amount in the filtering solution to that in the treated stone coal sample. The chloride ion adsorbed by the solution of NaOH was measured according to the ammonium thiocyanate volumetric method using ammonium thiocyanate and silver nitrate as its indicators. The absorbing rate for Cl was defined as the ratio of Cl adsorbed by NaOH to that in the additive added into the experiment. A JEOL JSM-840 scanning electron microscopy (SEM) was used to observe the surface morphology of the raw and roasted stone coal (accelerating voltage being 20 kV and working distance being 20 mm). The samples were also analyzed with X-Ray diffraction (XRD) to characterize the transformation of carbonaceous structure in roasting. The XRD analyzer had a Philips PW 1710 X-ray generator fitted with a copper radiation source (k1 = 1.54060 Å, k2 = 1.54439 Å) and worked at 40 kV and 20 mA with a scanning rate of 4°/min.
3. Results and discussion 3.1. Superiority of fluidized bed roasting The realized vanadium leaching rate via stone coal roasting is mainly affected by the factors of reactor type, reaction temperature and time, additive amount and its blending method [5]. Figs. 3 and 4 show the variations of vanadium leaching rate with reaction temperature and time in static and fluidized bed roasting without (Fig. 3) and with (Fig. 4) an additive, respectively. For the static roasting without an additive in Fig. 3(a), the vanadium leaching rate increased quickly in the reaction time of 2–4 h and then slowly in 4–5 h at temperatures of 800 °C and 850 °C. At 900 °C, the realized vanadium leaching rate decreased gradually in the entire roasting time. Generally, at lower reaction temperature and shorter reaction time, the removal of carbon from stone coal was not complete, which significantly affected the oxidation of V(III) to V(IV) and V(V) [5]. At the higher reaction temperature and longer reaction time, however, some secondary reactions would occur [17]. For example, the soluble vanadium would react with elements Ca and Fe to form the insoluble FeVO4, NaV8O15, Ca(VO4)2 and others. On the other hand, at high temperatures SiO2 would occur reactions to form insoluble silicate, which would partially encapsulate vanadium and further decrease the vanadium leaching rate. The fluidized bed roasting without any additive shown in Fig. 3(b) displays obviously different variation trends in the vanadium leaching rate. In the tested temperature range of 800–900 °C, the vanadium leaching rate sharply increased with elevating the reaction time from 0.5 h to 0.75 h and reached its maximum at 0.75 h. Further extending the reaction time, the vanadium
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leaching rate significantly decreased. Thus, 850 °C was the optimal temperature for fluidized bed roasting of stone coal without additive, and the realized vanadium leaching rate was 32.2%. Considering the realized low leaching rate at 900 °C in Fig. 3 for both static and fluidized bed roasting without an additive, the roasting experiments will be carried out at temperatures of 800 °C and 850 °C. Comparing Figs. 3 and 4, one can see differences obviously in the realized vanadium leaching rates for the cases without additive and with 10 wt.% physically-mixed additive in the stone coal roasting. In Fig. 4(a), for static roasting the vanadium leaching rate increased sharply with varying the roasting time from 2 h to 3 h and from 2 h to 4 h at 800 °C and 850 °C, respectively. Then, the vanadium leaching rate was stably kept when further extending the roasting time. At 800 °C and 850 °C, the available maximal leaching rates were about 76% and 80% (at 4 h), respectively. For the fluidized bed roasting with 10 wt.% physically-mixed additive, its realized vanadium leaching rate raised quickly with extending the reaction time, and reached the maximum at the tested 1 h. The roasting at 800 °C allowed an obviously higher vanadium leaching rate than at 850 °C, and their enabled highest vanadium leaching rates were 84% (at 1 h) and 73% (at 1 h), respectively. Fig. 5(a) and (b) compares further the typical vanadium leaching rates realized for the static and fluidized bed roasting at 800 °C and 850 °C for the cases without and with additive, respectively. The use of additive greatly increased the available maximal vanadium leaching rate (above 75% against about 30%), and for reaching the similar leaching rate, the fluidized bed roasting required much shorter reaction time (0.75–1 h) than the static roasting did (4 h). Consequently, the fluidized bed roasting not only shortened the reaction time but enabled also high vanadium leaching rate. The favorable temperature for fluidized bed roasting with 10 wt.% physically-mixed additive appears to be 800 °C, and the suitable reaction time is 1 h. Generally, pure sodium chloride is very stable and not easy to decompose. At higher temperatures it can catalytically oxidize vanadium in the tested stone coal. The essential chemical reactions are as follows [18]:
4NaCl þ O2 ¼ Na2 O þ 2Cl2 "
ð1Þ
2NaCl þ H2 OðgÞ ¼ Na2 O þ 2HClðgÞ "
ð2Þ
2V2 O3 þ O2 ¼ 2V2 O4
ð3Þ
3Cl2 þ 3V2 O4 ¼ 2VOCl3 þ 2V2 O5
ð4Þ
4VOCl3 þ 3O2 ¼ 2V2 O5 þ 6Cl2 "
ð5Þ
Fig. 3. Variation of vanadium leaching rate with roasting temperature and time in (a) static and (b) fluidized bed roasting without additive.
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Fig. 4. Variation of vanadium leaching rate with roasting temperature and time in (a) static and (b) fluidized bed roasting with 10 wt.% additive.
Fig. 5. Comparison of vanadium leaching rate between static and fluidized bed (FB) roasting with 10 wt.% impregnated additive.
Na2 O þ V2 O5 ¼ NaVO3
ð6Þ
to high valence (Reactions (3)–(5)). The produced V2O5 and Na2O would occur Reaction (6) further to produce sodium metavanadate (NaVO3), which is soluble in water and acid.
Following these reactions, one can see that when roasting stone coal with an additive in the oxygen- and steam-containing atmospheres at temperatures above 500 °C, NaCl can readily decompose to form Cl2 and Na2O (Reactions (1) and (2)), especially when coal contacted/interacted with many metal oxides in stone coal (V, Fe, Mn, Mg, Al, etc.). The generated Cl2, Na2O and the supplied O2 have very strong oxidizability, which can destroy the vanadiumcontaining illite structure and oxidize vanadium from low valence
The physical mixing of additive and stone coal is hard to obtain good dispersion of the additive on the vanadium species in stone coal. On the other hand, the additive is highly water-soluble, and
Fig. 6. Vanadium leaching rates for different additive addition methods of impregnation or physical mixing.
Fig. 7. Vanadium leaching rate varying with additive amount for impregnation and physical mixing methods at 800 °C for 0.75 h.
3.2. Effect of additive mixing method
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Fig. 8. Vanadium leaching rate varying with reaction time for fluidized bed roasting with impregnated additive.
fully using the moisture in newly mined stone coal, which is generally 20 wt.% in southwest region of China, is possible to get the higher dispersion of the additive. We call this method of adding the additive into the high-moisture content stone coal as the
(a)
(c)
(e)
185
‘‘impregnation’’ method (see experimental section for method details). Fig. 6 compares the vanadium leaching rates for the additive addition methods of impregnation and physical mixing (10% additive) realized at the same roasting temperature of 800 °C in fluidized bed. At the same roasting time, the impregnation method resulted in obviously higher vanadium leaching rate. While the maximal leaching rate of about 88% occurred at 0.75 h for the impregnation method, this value was 83% and reached at 1 h for the physical mixing method. These fully demonstrated that the impregnation method not only increased the leaching rate but shortened also the roasting time for its better contact or interaction between the additive and stone coal. Fig. 7 compares the effect of additive amount added with impregnation and physical mixing method on the vanadium leaching rate at 800 °C for roasting time of 0.75 h. For both methods, increasing the additive amount first sharply increased and then stably kept the vanadium leaching rate. In terms of reaching the possibly highest leaching rate, the suitable additive amount was about at 6 wt.% for the impregnation method but 9% for the physical mixing method. At the same additive amount, the vanadium leaching rate was always higher for the impregnation method than for the physical mixing method, implying that the impregnation method allowed the higher maximal vanadium leaching rate
(b)
(d)
(a) Raw stone coal (b) Residue at 800 °C for 0.5 h without additive (c) Residue at 800 °C for 1.5 h without additive (d) Residue at 800 °C for 0.75 h with physically-mixed additive (f) Residue at 800 °C for 0.75 h with impregnated additive
Fig. 9. SEM images of raw stone coal and roasted solid residue in fluidized bed under different conditions.
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Fig. 10. XRD spectra of raw stone coal and solid residue from fluidized bed roasting: (a) raw stone coal, (b) roasting without additive at 800 °C for 0.75 h, (c) roasting with 10 wt.% additive at 800 °C for 1 h.
(88% vs. 73%). Thus, via impregnation of additive, it not only increased the extraction rate of V2O5 from stone coal and shortened the reaction time for realizing the maximal leaching rate, but also reduced the necessarily required additive amount by about 40%. Fig. 8 compares the effect of reaction time and temperature for fluidized bed roasting with 6 wt.% impregnated additive. At 800 °C, the vanadium leaching rate increased sharply and reached a ratio of about 88% at 0.75 h and then kept stable. The roasting at 850 °C resulted in the peak leaching rate of about 73% also at 0.75 h and then the rate decreased, further stabilized at about 64% after roasting for 1 h. The two curves in Fig. 8 clarify that the roasting temperature 800 °C was more beneficial to vanadium leaching. It is possibly because that the higher temperature of 850 °C incurred some side reactions of the extracted vanadium due to the better contact and interaction between the additive and stone coal. Thus, the suitable operating parameters for fluidized bed roasting of stone coal with impregnated additive are at 800 °C for 0.75 h and with 6 wt.% additive against the dry-base coal mass. 3.3. Morphology variation and Cl removal Fig. 9 shows the SEM images of raw stone coal and roasted residue from fluidized roasting under different conditions. The surface of raw stone coal was very rough with very few pores, and the particles themselves were well separated from each other (Fig. 9(a)). After fluidized bed roasting (without additive) at 800 °C for 0.5 h, the combustion of its carbon would fracture the surface of the residual particles to form lots of particulate matters on the particle surface (Fig. 9(b)). For rather longer roasting time, such as 1.5 h, sintering even fusion among particles occurred to form an irregular porous polymeride, as shown in Fig. 9(c). This was mainly caused by the local high temperatures due to the full combustion of carbon and its consequent reactions between SiO2 and other mineral matters to capsulate the vanadium-containing mineral and hinder the release of vanadium [19]. After physically mixing additive and stone coal, the roasting (for 0.75 h) destroyed the basic crystal structure of stone coal to form the porous structure that can
promote the release of vanadium from the mineral (Fig. 9(d)). Impregnating additive on the surface of stone coal caused the pore structure of the particles more developed, even with more uniform pore sizes (Fig. 9(e)) to further improve the roasting behavior and vanadium extraction rate consequently. Fig. 10 shows the results of XRD analysis for raw stone coal and the roasting residue with and without additive. There are four crystalline mineral phases, quartz (SiO2), sanidine ((K,Na)(Si3Al)O8), olivine ((Mg,Fe)2(SiO4)) and illite (K(Al,V)2(Si,Al)4O10(OH)2) in the raw stone coal (Fig. 10(a)). Among them, the illite is the only vanadium-bearing mineral phase [11]. Comparing with XRD patterns of raw stone coal, after roasted at 800 °C in fluidized bed for 0.75 h (without additive), the peak intensities of quartz, sanidine, olivine did not have obvious change (Fig. 10(b)), indicating that this roasting did not destroy the crystal lattice structure of such mineral phases. Nonetheless, the peak intensity of illite much reduced, indicating that illite mineral in stone coal was partially destroyed. Due to the strong stability of illite in stone coal, its complete destruction would require rather higher temperature or need to add additive. The fluidized bed roasting with physically-mixed additive greatly changed the XRD patterns and intensities of the minerals in stone coal, as is shown in Fig. 10(c). The peak intensities of quartz, illite and sanidine sharply decreased, and it is even unable to detect the illite phase via XRD to indicate the complete destruction of this mineral in the roasting. Meanwhile, a new mineral phase of sanidine was formed. The decrease in the peak intensity of quartz shows that SiO2 reacted with other matters, as illustrated below for its reaction with the vanadium-bearing minerals:
KðAl; VÞ2 ðSi; AlÞ4 O10 ðOHÞ2 þ NaCl þ SiO2 þ O2 ¼ ðK; NaÞðSi3 AlO8 Þ þ NaVO3 þ HClðgÞ þ Cl2ðgÞ :
ð7Þ
Calcium oxide (CaO) was added to the fluidized bed roasting reactor as a sorbent to remove HCl and Cl2 from the formed flue gas through fixing them into the solid residue. Fig. 11 shows the variations of vanadium leaching rate and Cl removal with increasing the CaO amount from 0 to 6 wt.% against the mass of dried stone coal in fluidized bed roasting. The roasting was at 800 °C for 0.75 h and
X. Zeng et al. / Fuel 142 (2015) 180–188
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coal in additive impregnation and the added CaO sorbent to reduce the emission of poisonous gas. 4. Conclusions
Fig. 11. Effect of added CaO amount on Cl removal and vanadium leaching rate in fluidized bed roasting.
Stone coal
Crushing
V2O5
Additive
Extracting
Crushing to fine particle
Impregnating
In order to develop a clean and high-efficiency roasting technology for extracting vanadium from stone coal, this article investigated the vanadium leaching behavior after static (in muffle oven) and fluidized bed roasting to optimize the roasting conditions and meanwhile to test the removal of chlorine-containing species from the roasting-generated flue gas using CaO additive. Fluidized bed roasting provided the favorable conditions for roasting and V2O5 leaching, which greatly shortened the roasting time and increased the available maximal leaching rate of V2O5. Impregnating additive onto stone coal by taking advantage of the moisture in stone coal not only increased the vanadium leaching rate and shortened the roasting time but also reduced the necessarily required amount of additive. Adding CaO as a sorbent into the roasting reactor sharply decreased the content of Cl-containing gaseous species in the effluent gas of roasting and thus alleviated the environmental pollution. The optimal fluidized bed roasting conditions are shown to be adding 6 wt.% additive (against dry stone coal mass) into stone coal by impregnation, roasting at about 800 °C for 0.75 h and with 3 wt.% of CaO (against dry coal mass) as sorbent to remove Cl from flue gas. Under these conditions the available maximal vanadium leaching rate reached 91%, and Cl removal was about 80% against the formed Cl gaseous components in the roasting-generated flue gas. Acknowledgements
Aging
CaO
Fixing Cl
The authors are grateful to the financial supports of National Natural Science Foundation of China (21306209, U1302273). Partial roasting in FB
Complete roasting in TFB
Fig. 12. Conceptual flow chart of the newly proposed process for extracting V2O5 from stone coal.
with 6 wt.% additive based on impregnation. The realized Cl removal sharply increased with raising the added CaO amount until 3 wt.%, and it became then stable (varied little). The vanadium leaching rate gradually increased with raising the CaO amount. The CaO addition ratio of 3 wt.% resulted in the maximal Cl removal of about 80% and the vanadium leaching rate of about 91%. Thus, for fluidized bed roasting adding about 3 wt.% CaO can lead to the desired good roasting performance. In summary of the preceding results of fundamental studies, we can propose a process shown in Fig. 12 to roast stone coal and leach vanadium. The mined raw stone coal is crushed into particle sizes below 2 mm and further adjusted to moisture content of about 15 wt.%. Then, the stone coal is impregnated with a given amount of additive using the moisture in stone coal through mixing the coal and additive and further aging the blended material for a certain time about 0.5 h. The obtained stone coal is in turn roasted in the reaction system consisting of a fluidized bed and a transport bed (see Fig. 1) for full decarburization and also possible high-degree oxidation of the vanadium species in the stone coal. In the deep roasting step, CaO is added to remove Cl2 and HCl from the flue gas, and the roasted stone coal is finally crushed into fine particles to implement the vanadium extraction via acid leaching. The clean and high-efficiency vanadium leaching is hopefully ensured with the proposed process because it fully utilizes the moisture of fresh raw stone
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[16] Li W, Zhang YM, Liu T, Huang J, Wang Y. Comparison of ion exchange and solvent extraction in recovering vanadium from sulfuric acid leach solutions of stone coal. Hydrometallurgy 2013;131:1–7. [17] Li J, Li ZJ, Wu XW, Zhong XL, Wang HH, Liu SQ, et al. Technology of roasting process on extraction of vanadium from stone coal and mechanism discussion. Hunan Nonferr Metal 2007;23:7–11 (in Chinese).
[18] Wang MY, Wang XW, Shen JF, Wu RN. Extraction of vanadium from stone coal by modified salt-roasting process. J Cent South Univ Technol 2011;18:1940–4. [19] Zhu XB, Zhang YM, Liu T. Experimental and mechanism of vanadium extraction from stone coal by roasting with activators. Chinese J Rare Metals 2013;37:283–8.
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Extraction of vanadium from stone coal by roasting in a fluidized bed reactor Xi Zeng a, Fang Wang b, Huifeng Zhang a,c, Lijie Cui c, Jian Yu a,⇑, Guangwen Xu a,⇑ a
State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, China c University of Chinese Academy of Sciences, Beijing 100039, China b
h i g h l i g h t s The new roasting process consists of two fluidized bed reactors. Experimental conditions were optimized on a laboratory fluidized bed reactor. Fluidized roasting and impregnating additive on stone coal had better results.
a r t i c l e
i n f o
Article history: Received 19 July 2014 Received in revised form 1 October 2014 Accepted 22 October 2014 Available online 10 November 2014 Keywords: Stone coal Vanadium leaching Fluidized bed roasting Additive Chlorine
a b s t r a c t In order to improve leaching efficiency of vanadium pentoxide (V2O5) from stone coal and to reduce its connected environmental pollution, this article investigated the oxidizing roasting of stone coal in a laboratory fluidized bed reactor to optimize the roasting method and conditions. The examined parameters included reaction temperature, reaction time and amount and mixing method of additive. The removal of Cl from the generated effluent gas was implemented using CaO as the adsorbent, and comparison was made between the fluidized bed roasting and static roasting in a muffle oven. The results show that the fluidized roasting is more favorable to leach V2O5 from stone coal, which can shorten roasting time for reaching the maximal leaching rate, while the realized leaching rate is also higher. Comparing the mixing method applied to stone coal and additive, the impregnation method of additive on stone coal not only increased the leaching rate and shortened the roasting time for reaching the maximal leaching rate, but also reduced the amount of additive required. Adding CaO in stone coal roasting sharply decreased the content of Cl-containing gases in flue gas, which can greatly alleviate the possible environmental pollution. The optimal conditions for fluidized bed roasting were found to be impregnating 6 wt.% additive on coal, adding 3 wt.% CaO and performing roasting at 800 °C for 0.75 h. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Vanadium and its compounds are important strategic resources and are playing important roles in many industries such as ferrous and non-ferrous metallurgy, chemical production, and electron and battery manufacture [1,2]. Apart from vanadium titano-magnetite, stone coal is another important vanadium-bearing resource. In China, the gross reserve of vanadium in stone coal is estimated to be about 1.18 million tons. In terms of V2O5 amount, it takes more than 87% of China’s total domestic reserve of vanadium. In recent years, the V2O5 extracted from stone coal has reached 40% of the totally produced V2O5 in China [3]. With the increasing market ⇑ Corresponding authors. Tel.: +86 10 82544885. E-mail addresses: [email protected] (J. Yu), [email protected] (G. Xu). http://dx.doi.org/10.1016/j.fuel.2014.10.068 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
demand for vanadium products and the shortage of high-grade ores, it becomes more and more necessary and urgent to exploit and utilize the low-grade stone coal resource for V2O5 production. In stone coal, vanadium exists dominantly as trivalent vanadium (V(III)) in addition to very little as quadrivalent (V(IV)) and quinquevalent vanadium (V(V)). The presence ratio of these different valent states is closely related to the reduction atmosphere for forming the stone coal [4]. Since V(III) and V(IV) have the ionic radius, electronegativity and coordination number similar to those of Al(III), they usually replace Al(III) in dioctahedral structure to form isomorphism in mica and clay minerals [5]. This sort of vanadium is most prevalent and representative in stone coal but it is difficult to be extracted. Besides, the ore containing vanadium in stone coal is always complicatedly combined with carbon, which further increases the difficulty for extracting vanadium form stone coal.
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China has started to extract vanadium from stone coal since 1960s, whose fundamental principle is oxidizing the acid- and water-insoluble V(III) to V(IV) and/or V(V), and then precipitating and separating V(IV) from the leaching solvent [6]. Up to now, dozens of technologies have been proposed according to the properties of raw stone coal ore in different regions, and some of them have been applied to industrial productions. These technologies can be divided into two categories, direct acid leaching and acid leaching after roasting [7–10]. The direct acid leaching is adopted only by limited companies because of its inherent disadvantages such as poor feedstock adaptability, low vanadium recovery ratio, large consumption of H2SO4, difficulty in separating and concentrating vanadium from many impurities (Fe, Al, Mg, K, Na, etc.), and serious environmental pollution [11,12]. Now, most of the companies producing vanadium from stone coal adopt first roasting and then acid leaching. The representative process is roasting with sodium chloride under oxidizing conditions to convert vanadium in stone coal to acid-soluble salt [13]. In this process, the roasting plays a critical role in oxidizing V(III) to V(IV) and V(V), which is determinative to the available total vanadium recovery ratio and is thus the rate-limiting step. The roasting reactors popularly used include flat kiln, shaft kiln and rotary kiln. All of them, however, have the limitations of low treatment capacity, long roasting time, insufficient heat and mass transfer efficiency in reactors and difficulty in scale-up [6]. The common additive used in the traditional roasting technologies is NaCl, thus bringing about serious emission of poisonous gases of HCl and Cl2 [14]. In recent years, the Chinese government has banned several hundreds of production lines that adopted the direct acid leaching technology. It is consequently urgent to develop new roasting technology with high recovery of vanadium and having as well controlled pollution to environment for extracting vanadium from stone coal. A new fluidized bed two-stage roasting process consisting of a fluidized bed (FB) reactor and a transport bed reactor has been proposed in Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS). As shown in Fig. 1, stone coal is first autothermally roasted in the fluidized bed reactor for partial decarbonization, and its products are in turn forwarded to the transport bed reactor to implement complete decarbonization and deep oxidation of vanadium through adding an additive. In the transport bed, the produced poisonous gases, such as HCl and Cl2 are removed through injecting an adsorbent that is likely CaO. The big particles of stone coal that are not adequately roasted in the transport bed reactor is collected by a cyclone and then returned to the fluidized bed reactor for further roasting. While large particles, usually stone, are withdrawn from the bottom of the transport bed, the roasted fine particles carried with hot flue gas are collected by another cyclone and further cooled and collected for acid leaching. By integrating the advantages of fluidized bed and transport bed reactors [15], the new twostage roasting process is not only applicable to powder feedstock (e.g., powder stone coal), but enables also large treatment capacity in comparison with the kiln reactors. By simulating the practically possible conditions for the process illustrated in Fig. 1, a small laboratory batch fluidized bed reactor was used to test how the roasting process affects the downstream acid leaching. The results were compared with the traditional fixed bed roasting to demonstrate the technical feasibility and advantages of the new proposal. The effects on leaching rate for V2O5 were systematically examined for the parameters including roasting temperature and time, additive type and amount, mixing method between additive and stone coal. Moreover, the chlorine removal from the generated flue gas by CaO was also investigated. As a consequence of this study, a roasting process is finally proposed to ensure the high-efficiency recovery of vanadium from stone coal.
181
2. Experimental section 2.1. Roasting apparatus and test As shown in Fig. 2, the fluidized bed roasting apparatus consists of a fluidized bed reactor (FB), an electric furnace, a gas supply section, a feeding hopper, a cyclone and a gas cleaning system. The FB reactor was made of high purity quartz glass and was 50 mm in inner diameter (I.D.). Its reaction zone was 300 mm long between a bottom gas preheating section of 300 mm long and an expanded freeboard of 100 mm in inner diameter and 150 mm in length. Experiment was started by heating the reactor to a desired temperature, such as 800 °C, and then high-purity air at a specified rate was fed into the reactor to form the roasting atmosphere. The roasting was initiated by feeding exactly 30 g stone coal or its mixture with the adopted additive into the reactor through a hopperlike feeder. The generated gas, with fly ash as well, passed through a cyclone and several washing bottles of sodium hydroxide solution immersed in an ice-water bath to remove the poisonous gas components, mainly HCl and Cl2. After experiment, the reactor was taken out from the electric furnace to quickly cool it to the room temperature. The roasted stone coal collected from the cyclone and reactor was taken to analyze its content of V2O5 according to the measurement method depicted in the Section 2.2. The static roasting of stone coal was conducted in a muffle furnace, and the result was compared with that from the fluidized bed roasting in terms of the available extraction efficiency of vanadium. At a desired temperature, the mixture of stone coal and additive was loaded into a ceramic crucible, and this crucible was in turn put into the muffle furnace to implement the roasting in air atmosphere. After reaching the roasting time, the ceramic crucible was taken out and cooled quickly to the room temperature, and the roasted sample in the crucible was similarly analyzed as for that from the fluidized bed roasting.
2.2. Material and measurement Raw stone coal used in this study was taken from Sansui county, Guizhou Province of China. Prior to experiment, the stone coal was crushed and sieved to the sizes of 0.1–0.15 mm, and then dried in an oven at 105 °C for 2 h. Table 1 shows the results of industrial analysis and ultimate analysis for the stone coal and the contents of major components in its ash. The content of vanadium in terms of V2O5 is up to 1.26 wt.%, a prospective value for application development of vanadium leaching. On the other hand, the fixed carbon in stone coal is about 15%, which can seriously hinder the release of vanadium from stone coal due to its strong surface energy and ability of adsorption. The roasting tests used a commercial additive with its effective components being Na2CO3, NaCl and some other species. Two methods were adopted to mix stone coal and additive, physical mixing method by mechanical agitation and ‘‘impregnation’’ method by blending stone coal into a solution of additive containing 15 wt.% water and then aging the coal for about 5 min. All percents cited for additive were against the mass of dry stone coal. Measurement of V2O5 in the roasted stone coal was performed via the following procedure. A certain amount of the roasted stone coal (via either fluidized bed or static roasting) was dissolved into a sulfuric acid in a conical flask, and the resulting solution was in turn heated with a water-bath preset at 40 °C under 2-h continuous stirring and further filtered. The vanadium concentration in the filtering solution was determined according to the method of Chinese National Standard of YB/T 5328-2006 based on ferrous ammonium sulfate titration using N-phenylanthranilic acid as an indicator [16].
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Flue gas 3 3 6 8 CaO 5
4
7 5 5
2
1
Air Stone coal
Particle flow Gas flow
5 Air Roasted stone coal
1-Fluidized bed; 2-Transport bed; 3-Cyclone; 4-Distribution valve; 5-Receiver; 6-Heat exchanger; 7-Separator; 8- Induced draft fan. Fig. 1. Two-stage fluidized bed roasting process with additive for vanadium extraction.
Hopper
T Cyclone
T
Condenser
T T FB Gas
Air
Tank
Gas washing equipment
Fig. 2. A schematic diagram of the used fluidized bed roasting apparatus.
Table 1 Characterization of the tested stone coal and its ash.a Proximate analysis (wt.%) Mad Coal analysis
Ash XRF analysis (wt.%) a
Aad
Ultimate analysis (wt.%) Vad
FCad
Cad
Had
LHV (KJ/kg) Sad
Oad
4.4
74.7
6.5
15
3.86
4.2
0.3
17.6
4458
SiO2
Al2O3
Fe2O3
MgO
V2O5
BaO
P2O5
K2O
SO3
CaO
58.13
16.88
4.92
4.12
1.58
3.91
3.77
3.52
1.93
1.24
ad: Air-dried basis; daf: Dry ash free basis.
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The leaching efficiency of vanadium was defined as the ratio of the V2O5 amount in the filtering solution to that in the treated stone coal sample. The chloride ion adsorbed by the solution of NaOH was measured according to the ammonium thiocyanate volumetric method using ammonium thiocyanate and silver nitrate as its indicators. The absorbing rate for Cl was defined as the ratio of Cl adsorbed by NaOH to that in the additive added into the experiment. A JEOL JSM-840 scanning electron microscopy (SEM) was used to observe the surface morphology of the raw and roasted stone coal (accelerating voltage being 20 kV and working distance being 20 mm). The samples were also analyzed with X-Ray diffraction (XRD) to characterize the transformation of carbonaceous structure in roasting. The XRD analyzer had a Philips PW 1710 X-ray generator fitted with a copper radiation source (k1 = 1.54060 Å, k2 = 1.54439 Å) and worked at 40 kV and 20 mA with a scanning rate of 4°/min.
3. Results and discussion 3.1. Superiority of fluidized bed roasting The realized vanadium leaching rate via stone coal roasting is mainly affected by the factors of reactor type, reaction temperature and time, additive amount and its blending method [5]. Figs. 3 and 4 show the variations of vanadium leaching rate with reaction temperature and time in static and fluidized bed roasting without (Fig. 3) and with (Fig. 4) an additive, respectively. For the static roasting without an additive in Fig. 3(a), the vanadium leaching rate increased quickly in the reaction time of 2–4 h and then slowly in 4–5 h at temperatures of 800 °C and 850 °C. At 900 °C, the realized vanadium leaching rate decreased gradually in the entire roasting time. Generally, at lower reaction temperature and shorter reaction time, the removal of carbon from stone coal was not complete, which significantly affected the oxidation of V(III) to V(IV) and V(V) [5]. At the higher reaction temperature and longer reaction time, however, some secondary reactions would occur [17]. For example, the soluble vanadium would react with elements Ca and Fe to form the insoluble FeVO4, NaV8O15, Ca(VO4)2 and others. On the other hand, at high temperatures SiO2 would occur reactions to form insoluble silicate, which would partially encapsulate vanadium and further decrease the vanadium leaching rate. The fluidized bed roasting without any additive shown in Fig. 3(b) displays obviously different variation trends in the vanadium leaching rate. In the tested temperature range of 800–900 °C, the vanadium leaching rate sharply increased with elevating the reaction time from 0.5 h to 0.75 h and reached its maximum at 0.75 h. Further extending the reaction time, the vanadium
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leaching rate significantly decreased. Thus, 850 °C was the optimal temperature for fluidized bed roasting of stone coal without additive, and the realized vanadium leaching rate was 32.2%. Considering the realized low leaching rate at 900 °C in Fig. 3 for both static and fluidized bed roasting without an additive, the roasting experiments will be carried out at temperatures of 800 °C and 850 °C. Comparing Figs. 3 and 4, one can see differences obviously in the realized vanadium leaching rates for the cases without additive and with 10 wt.% physically-mixed additive in the stone coal roasting. In Fig. 4(a), for static roasting the vanadium leaching rate increased sharply with varying the roasting time from 2 h to 3 h and from 2 h to 4 h at 800 °C and 850 °C, respectively. Then, the vanadium leaching rate was stably kept when further extending the roasting time. At 800 °C and 850 °C, the available maximal leaching rates were about 76% and 80% (at 4 h), respectively. For the fluidized bed roasting with 10 wt.% physically-mixed additive, its realized vanadium leaching rate raised quickly with extending the reaction time, and reached the maximum at the tested 1 h. The roasting at 800 °C allowed an obviously higher vanadium leaching rate than at 850 °C, and their enabled highest vanadium leaching rates were 84% (at 1 h) and 73% (at 1 h), respectively. Fig. 5(a) and (b) compares further the typical vanadium leaching rates realized for the static and fluidized bed roasting at 800 °C and 850 °C for the cases without and with additive, respectively. The use of additive greatly increased the available maximal vanadium leaching rate (above 75% against about 30%), and for reaching the similar leaching rate, the fluidized bed roasting required much shorter reaction time (0.75–1 h) than the static roasting did (4 h). Consequently, the fluidized bed roasting not only shortened the reaction time but enabled also high vanadium leaching rate. The favorable temperature for fluidized bed roasting with 10 wt.% physically-mixed additive appears to be 800 °C, and the suitable reaction time is 1 h. Generally, pure sodium chloride is very stable and not easy to decompose. At higher temperatures it can catalytically oxidize vanadium in the tested stone coal. The essential chemical reactions are as follows [18]:
4NaCl þ O2 ¼ Na2 O þ 2Cl2 "
ð1Þ
2NaCl þ H2 OðgÞ ¼ Na2 O þ 2HClðgÞ "
ð2Þ
2V2 O3 þ O2 ¼ 2V2 O4
ð3Þ
3Cl2 þ 3V2 O4 ¼ 2VOCl3 þ 2V2 O5
ð4Þ
4VOCl3 þ 3O2 ¼ 2V2 O5 þ 6Cl2 "
ð5Þ
Fig. 3. Variation of vanadium leaching rate with roasting temperature and time in (a) static and (b) fluidized bed roasting without additive.
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Fig. 4. Variation of vanadium leaching rate with roasting temperature and time in (a) static and (b) fluidized bed roasting with 10 wt.% additive.
Fig. 5. Comparison of vanadium leaching rate between static and fluidized bed (FB) roasting with 10 wt.% impregnated additive.
Na2 O þ V2 O5 ¼ NaVO3
ð6Þ
to high valence (Reactions (3)–(5)). The produced V2O5 and Na2O would occur Reaction (6) further to produce sodium metavanadate (NaVO3), which is soluble in water and acid.
Following these reactions, one can see that when roasting stone coal with an additive in the oxygen- and steam-containing atmospheres at temperatures above 500 °C, NaCl can readily decompose to form Cl2 and Na2O (Reactions (1) and (2)), especially when coal contacted/interacted with many metal oxides in stone coal (V, Fe, Mn, Mg, Al, etc.). The generated Cl2, Na2O and the supplied O2 have very strong oxidizability, which can destroy the vanadiumcontaining illite structure and oxidize vanadium from low valence
The physical mixing of additive and stone coal is hard to obtain good dispersion of the additive on the vanadium species in stone coal. On the other hand, the additive is highly water-soluble, and
Fig. 6. Vanadium leaching rates for different additive addition methods of impregnation or physical mixing.
Fig. 7. Vanadium leaching rate varying with additive amount for impregnation and physical mixing methods at 800 °C for 0.75 h.
3.2. Effect of additive mixing method
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Fig. 8. Vanadium leaching rate varying with reaction time for fluidized bed roasting with impregnated additive.
fully using the moisture in newly mined stone coal, which is generally 20 wt.% in southwest region of China, is possible to get the higher dispersion of the additive. We call this method of adding the additive into the high-moisture content stone coal as the
(a)
(c)
(e)
185
‘‘impregnation’’ method (see experimental section for method details). Fig. 6 compares the vanadium leaching rates for the additive addition methods of impregnation and physical mixing (10% additive) realized at the same roasting temperature of 800 °C in fluidized bed. At the same roasting time, the impregnation method resulted in obviously higher vanadium leaching rate. While the maximal leaching rate of about 88% occurred at 0.75 h for the impregnation method, this value was 83% and reached at 1 h for the physical mixing method. These fully demonstrated that the impregnation method not only increased the leaching rate but shortened also the roasting time for its better contact or interaction between the additive and stone coal. Fig. 7 compares the effect of additive amount added with impregnation and physical mixing method on the vanadium leaching rate at 800 °C for roasting time of 0.75 h. For both methods, increasing the additive amount first sharply increased and then stably kept the vanadium leaching rate. In terms of reaching the possibly highest leaching rate, the suitable additive amount was about at 6 wt.% for the impregnation method but 9% for the physical mixing method. At the same additive amount, the vanadium leaching rate was always higher for the impregnation method than for the physical mixing method, implying that the impregnation method allowed the higher maximal vanadium leaching rate
(b)
(d)
(a) Raw stone coal (b) Residue at 800 °C for 0.5 h without additive (c) Residue at 800 °C for 1.5 h without additive (d) Residue at 800 °C for 0.75 h with physically-mixed additive (f) Residue at 800 °C for 0.75 h with impregnated additive
Fig. 9. SEM images of raw stone coal and roasted solid residue in fluidized bed under different conditions.
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Fig. 10. XRD spectra of raw stone coal and solid residue from fluidized bed roasting: (a) raw stone coal, (b) roasting without additive at 800 °C for 0.75 h, (c) roasting with 10 wt.% additive at 800 °C for 1 h.
(88% vs. 73%). Thus, via impregnation of additive, it not only increased the extraction rate of V2O5 from stone coal and shortened the reaction time for realizing the maximal leaching rate, but also reduced the necessarily required additive amount by about 40%. Fig. 8 compares the effect of reaction time and temperature for fluidized bed roasting with 6 wt.% impregnated additive. At 800 °C, the vanadium leaching rate increased sharply and reached a ratio of about 88% at 0.75 h and then kept stable. The roasting at 850 °C resulted in the peak leaching rate of about 73% also at 0.75 h and then the rate decreased, further stabilized at about 64% after roasting for 1 h. The two curves in Fig. 8 clarify that the roasting temperature 800 °C was more beneficial to vanadium leaching. It is possibly because that the higher temperature of 850 °C incurred some side reactions of the extracted vanadium due to the better contact and interaction between the additive and stone coal. Thus, the suitable operating parameters for fluidized bed roasting of stone coal with impregnated additive are at 800 °C for 0.75 h and with 6 wt.% additive against the dry-base coal mass. 3.3. Morphology variation and Cl removal Fig. 9 shows the SEM images of raw stone coal and roasted residue from fluidized roasting under different conditions. The surface of raw stone coal was very rough with very few pores, and the particles themselves were well separated from each other (Fig. 9(a)). After fluidized bed roasting (without additive) at 800 °C for 0.5 h, the combustion of its carbon would fracture the surface of the residual particles to form lots of particulate matters on the particle surface (Fig. 9(b)). For rather longer roasting time, such as 1.5 h, sintering even fusion among particles occurred to form an irregular porous polymeride, as shown in Fig. 9(c). This was mainly caused by the local high temperatures due to the full combustion of carbon and its consequent reactions between SiO2 and other mineral matters to capsulate the vanadium-containing mineral and hinder the release of vanadium [19]. After physically mixing additive and stone coal, the roasting (for 0.75 h) destroyed the basic crystal structure of stone coal to form the porous structure that can
promote the release of vanadium from the mineral (Fig. 9(d)). Impregnating additive on the surface of stone coal caused the pore structure of the particles more developed, even with more uniform pore sizes (Fig. 9(e)) to further improve the roasting behavior and vanadium extraction rate consequently. Fig. 10 shows the results of XRD analysis for raw stone coal and the roasting residue with and without additive. There are four crystalline mineral phases, quartz (SiO2), sanidine ((K,Na)(Si3Al)O8), olivine ((Mg,Fe)2(SiO4)) and illite (K(Al,V)2(Si,Al)4O10(OH)2) in the raw stone coal (Fig. 10(a)). Among them, the illite is the only vanadium-bearing mineral phase [11]. Comparing with XRD patterns of raw stone coal, after roasted at 800 °C in fluidized bed for 0.75 h (without additive), the peak intensities of quartz, sanidine, olivine did not have obvious change (Fig. 10(b)), indicating that this roasting did not destroy the crystal lattice structure of such mineral phases. Nonetheless, the peak intensity of illite much reduced, indicating that illite mineral in stone coal was partially destroyed. Due to the strong stability of illite in stone coal, its complete destruction would require rather higher temperature or need to add additive. The fluidized bed roasting with physically-mixed additive greatly changed the XRD patterns and intensities of the minerals in stone coal, as is shown in Fig. 10(c). The peak intensities of quartz, illite and sanidine sharply decreased, and it is even unable to detect the illite phase via XRD to indicate the complete destruction of this mineral in the roasting. Meanwhile, a new mineral phase of sanidine was formed. The decrease in the peak intensity of quartz shows that SiO2 reacted with other matters, as illustrated below for its reaction with the vanadium-bearing minerals:
KðAl; VÞ2 ðSi; AlÞ4 O10 ðOHÞ2 þ NaCl þ SiO2 þ O2 ¼ ðK; NaÞðSi3 AlO8 Þ þ NaVO3 þ HClðgÞ þ Cl2ðgÞ :
ð7Þ
Calcium oxide (CaO) was added to the fluidized bed roasting reactor as a sorbent to remove HCl and Cl2 from the formed flue gas through fixing them into the solid residue. Fig. 11 shows the variations of vanadium leaching rate and Cl removal with increasing the CaO amount from 0 to 6 wt.% against the mass of dried stone coal in fluidized bed roasting. The roasting was at 800 °C for 0.75 h and
X. Zeng et al. / Fuel 142 (2015) 180–188
187
coal in additive impregnation and the added CaO sorbent to reduce the emission of poisonous gas. 4. Conclusions
Fig. 11. Effect of added CaO amount on Cl removal and vanadium leaching rate in fluidized bed roasting.
Stone coal
Crushing
V2O5
Additive
Extracting
Crushing to fine particle
Impregnating
In order to develop a clean and high-efficiency roasting technology for extracting vanadium from stone coal, this article investigated the vanadium leaching behavior after static (in muffle oven) and fluidized bed roasting to optimize the roasting conditions and meanwhile to test the removal of chlorine-containing species from the roasting-generated flue gas using CaO additive. Fluidized bed roasting provided the favorable conditions for roasting and V2O5 leaching, which greatly shortened the roasting time and increased the available maximal leaching rate of V2O5. Impregnating additive onto stone coal by taking advantage of the moisture in stone coal not only increased the vanadium leaching rate and shortened the roasting time but also reduced the necessarily required amount of additive. Adding CaO as a sorbent into the roasting reactor sharply decreased the content of Cl-containing gaseous species in the effluent gas of roasting and thus alleviated the environmental pollution. The optimal fluidized bed roasting conditions are shown to be adding 6 wt.% additive (against dry stone coal mass) into stone coal by impregnation, roasting at about 800 °C for 0.75 h and with 3 wt.% of CaO (against dry coal mass) as sorbent to remove Cl from flue gas. Under these conditions the available maximal vanadium leaching rate reached 91%, and Cl removal was about 80% against the formed Cl gaseous components in the roasting-generated flue gas. Acknowledgements
Aging
CaO
Fixing Cl
The authors are grateful to the financial supports of National Natural Science Foundation of China (21306209, U1302273). Partial roasting in FB
Complete roasting in TFB
Fig. 12. Conceptual flow chart of the newly proposed process for extracting V2O5 from stone coal.
with 6 wt.% additive based on impregnation. The realized Cl removal sharply increased with raising the added CaO amount until 3 wt.%, and it became then stable (varied little). The vanadium leaching rate gradually increased with raising the CaO amount. The CaO addition ratio of 3 wt.% resulted in the maximal Cl removal of about 80% and the vanadium leaching rate of about 91%. Thus, for fluidized bed roasting adding about 3 wt.% CaO can lead to the desired good roasting performance. In summary of the preceding results of fundamental studies, we can propose a process shown in Fig. 12 to roast stone coal and leach vanadium. The mined raw stone coal is crushed into particle sizes below 2 mm and further adjusted to moisture content of about 15 wt.%. Then, the stone coal is impregnated with a given amount of additive using the moisture in stone coal through mixing the coal and additive and further aging the blended material for a certain time about 0.5 h. The obtained stone coal is in turn roasted in the reaction system consisting of a fluidized bed and a transport bed (see Fig. 1) for full decarburization and also possible high-degree oxidation of the vanadium species in the stone coal. In the deep roasting step, CaO is added to remove Cl2 and HCl from the flue gas, and the roasted stone coal is finally crushed into fine particles to implement the vanadium extraction via acid leaching. The clean and high-efficiency vanadium leaching is hopefully ensured with the proposed process because it fully utilizes the moisture of fresh raw stone
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