Trans. Nonferrous Met. Soc. China 29(2019) 849-858

Improving vanadium extraction from stone coal via combination of blank roasting and bioleaching by ARTP-mutated Bacillus mucilaginosus

Ying-bo DONG^1,2, Yue LIU¹, Hai LIN^1,2, Chen-jing LIU¹

1. School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China;

2. Beijing Key Laboratory on Resource-oriented Treatment of Industrial Pollutants, Beijing 100083, China

Received 7 May 2018; accepted 24 July 2018

Abstract: In order to improve leaching efficiency of vanadium from stone coal, the combination of blank roasting and bioleaching using Bacillus mucilaginosus (B. mucilaginosus) mutants was evaluated. The atmospheric and room temperature plasma (ARTP) technique was used to generate B. mucilaginosus mutants. The results showed that a mutant B. mucilaginosus BM-50, after ARTP irradiation for 50 s, had the highest acid production. The total content of the organic acid produced by B. mucilaginosus BM-50 was nearly doubled compared with the wild strain after 2 days. After 20 days, vanadium leaching rate with B. mucilaginosus BM-50 reached 18.2%, which was improved compared with the original bacteria (15.3%). A pretreatment via blank roasting for stone coal further improved the vanadium dissolution by bioleaching, namely, 68.3% vanadium was extracted, which was much higher than that without blank roasting. It is shown that bioleaching by bacterial mutants by ARTP irradiation combined with blank roasting has great potential for improving vanadium recovery from low-grade vanadium bearing stone coal.

Key words: atmospheric and room temperature plasma (ARTP); Bacillus mucilaginosus; vanadium-bearing stone coal; blank roasting; bioleaching

1 Introduction

Vanadium (V) is an important rare element and is extensively used in many fields, including the steel industry, titanium-aluminum alloys, vanadium redox battery and catalysts [1]. Stone coal is recognized as a specific vanadium-bearing resource in China. The gross reserve of vanadium (V₂O₅) in stone coal is 118 million tons, which accounts for over 87% of the domestic reserve of vanadium [2]. Various vanadium extraction techniques for stone coal have been investigated, like roasting, acid leaching, ion purification, precipitation, and calcination [3]. Extracting vanadium from stone coal is commonly confronted with the issues of the enormous ore handling quantity, the high acid consumption, and the high production cost [4]. In order to solve these problems, bioleaching is considered as one of the strategic and perfect processing technologies for utilizing mineral resources, where extensive studies have been conducted to extend its application within metallic minerals.

There are some significant advantages associated with bioleaching, such as low investment and energy consumption, with the benefits to the environment [5]. In the past several decades, bioleaching has been successfully applied in metallurgical industries, such as copper, uranium, gold, and cobalt extractions [6,7]. There is less research regarding extracting V in vanadium-bearing stone coal using microbial leaching technology. Previous studies primarily used Thiobacillus, which demonstrated a lower leaching rate [8]. The low leaching rate could be due to the fact that most vanadium within stone coal exists in the crystal lattice of the aluminosilicate minerals and isomorphically replaces Al(III) in muscovite [9], where the Thiobacillus cannot destroy the lattice structure.

Previous studies have shown that silicate bacteria could influence the decomposition and transformation of silicate minerals [10]. For example, after a 20-day interaction with B. mucilaginosus D4B1, the expansion ratios of montmorillonite samples decreased from 83.2% to 76.7% [11]. The actions of quartz and indigenous Bacillus spp. have been tested by STYRIAKOVA et al [12], which showed that the bacteria assisted the release of Fe via the dissolution of quartz particles. B. mucilaginosus, known as a silicate bacterium, is often used as a model strain for studying the role that bacteria play in the weathering processes of silicates [13].

ARTP is a powerful physical microbial mutagenesis tool used for microorganism breeding. The ARTP mutation system has been successfully applied for mutating S. platensis, which enables the generation of mutant 3-A10 with 40.3% higher carbohydrate productivity than the wild-type strain [14]. LI et al [15] also applied ARTP to the mutagenesis of the oleaginous fungus, Mortierella Alpina. The results showed that the arachidonic acid production of mutant strain D20 (5.09 g/L) was increased by 40.61%, more than the original strain (3.62 g/L).

Previous studies showed that the application of different enhanced treatments, such as supplement of sucrose, adding surfactant, and roasting pretreatment, improved the metals recovery in the bioleaching  process [16]. Research by LIU et al [17] showed that the cobalt leaching efficiency could be boosted by more than 21% when 0.1 g/L of surfactant (Tween-20) was added. ZHAO et al [18] found that vanadium was liberated from the crystal structure of mica through roasting, which improved the recovery of vanadium.

Bioleaching technology was chosen to recover vanadium from stone coal in this study. B. mucilaginosus was selected as the experimental bacterium because it could destroy the silicate mineral structure. ARTP mutagenesis and various enhanced treatments were used to improve the V leaching and to achieve the reutilization of V from a low-grade vanadium-bearing stone coal.

2 Experimental

2.1 Microorganisms and culture media

The Bacillus mucilaginosus (the CGMCC number: 1.0910) was purchased from China General Microbiological Culture Collection Center (CGMCC). The bacterium was aerobically cultured in a silicate bacterial culture medium, which comprised of sucrose  (5 g/L), Na₂HPO₄·12H₂O (3.0 g/L), CaCO₃ (0.1 g/L), (NH₄)₂SO₄ (2 g/L), MgSO₄ (0.5 g/L), KCl (0.1 g/L) and FeCl₃ (0.005 g/L). The composition of the agar medium was silicate bacterial culture medium and agar (15 g/L). The initial pH was adjusted to 7.0 with 10% H₂SO₄. The bacteria were cultured in flasks which were shaken in an air thermostat shaker at 180 r/min and 30 °C.

2.2 Mineral samples

The vanadium-bearing stone coal was sourced from Yunxi, Hubei province, China. The samples were prepared by crushing and screening the material to particle sizes below 74 μm. The particles were analyzed for their chemical contents via X-ray fluorescence (XRF). The chemical composition of the stone coal is given in Table 1. The main component of the stone coal was SiO₂. The V₂O₅ content was 1.19%. X-ray diffraction (XRD) analysis with the scanning rate of 6 (°)/min from 10° to 100° was used to detect mineral phases. The X-ray diffraction pattern (Fig. 1) showed that the main mineral phase was quartz, followed by muscovite.

Table 1 Chemical composition of vanadium-bearing stone coal (wt.%)

Fig. 1 XRD pattern of stone coal

2.3 ARTP mutagenic treatment

The ARTP mutation breeding system used in the experiment was a multifunctional plasma mutagenesis instrument (MANDELA, Beijing Wei Nesi Technology Co., Ltd., China). ARTP mutation was performed when  B. mucilaginosus cell growth was in the logarithmic phase, with a cell density of 10⁴-10⁵ CFU. Pure helium was used as the plasma working gas during the ARTP mutation. The operating parameters were as follows: (1) the gas flow was 10 L/min, (2) the distance between the plasma torch nozzle exit and the sample plate was 2 mm, and (3) the temperature of the plasma jet was 23.0-35.0 °C. At first, 20 μL of fresh bacterium suspension was uniformly coated on the sterilized steel plates. Then, they were exposed to the ARTP jet for 10, 20, 30, 40, 50, 60, 70, 80 and 90 s, respectively. An exposure for 0 s was used as the control (wild strain). After each treatment, 980 μL of sterile water was added to the steel plate for dilution. 100 μL of diluted solution was used to coat the agar medium for 48 h at 30 °C. The lethality rate was determined by counting the individual colonies formed by wild strain and each mutant.

2.4 Bioleaching tests

All bioleaching tests were performed in 250 mL Erlenmeyer flasks with 100 mL of sterilized culture medium. The initial pH was adjusted to 7.0 with 10% H₂SO₄. The 1% (vol.%) original bacteria or the mutagenic bacteria in a logarithmic phase were added to each flask. The pulp density was 1% (wt.%). All flasks were shaken in an air thermostat shaker at 180 r/min and 30 °C. During the experiments, leachate was periodically collected from each flask to determine the pH and the heavy metals concentrations.

2.5 Enhanced treatments

To further improve the vanadium removal after ARTP mutagenesis, three types of enhanced treatments were selected: supplement of sucrose, adding surfactant, and roasting. The leaching system without any enhanced treatments was set as the blank control.

(1) Supplement of sucrose

Two methods were set to supplement the carbon source. Plan A: adding 5 g/L of sucrose to the flask every four days during the bacterial leaching process. Plan B: increasing initial concentrations of sucrose from 5 to 25 g/L.

(2) Adding surfactant

Two types of surfactants were selected to add to the bioleaching system in accordance with the classification of surfactants: Tween-20 and sodium dodecyl sulfate (SDS). The concentration of the surfactants were 0.05, 0.1 and 0.2 g/L.

(3) Roasting

Stone coal samples were placed in a porcelain dish to roast without any additives as a pretreatment.

2.6 Analysis methods

The growth curve of each mutant was measured via a Bioscreen automatic growth curve analyzer. The concentrations of Si, Al, and V in the leachate and the pH values were measured using an inductively coupled plasma emission spectrometer (ICP-OES) and an S20seveneasy pH meter, respectively. The content of organic acid in the medium solution was determined by an LC-2030 high-performance liquid chromatography (HPLC), manufactured by Japan Shimadzu Corporation. The surface morphology changes of the stone coal before and after leaching were explored with scanning electron microscopy (SEM) equipped with energy-dispersive microanalysis facilities (EDS). Fourier transform infrared (FTIR) spectra in the mid-IR (MIR) region (4000-400 cm^-1) were obtained by using a Nicolet Nexus 670 FT-IR spectrometer.

2.7 Statistical analysis

All experiments were performed in triplicate. Analysis of means and standard deviations were done by using Microsoft Excel Software, version 2016.

3 Results and discussion

3.1 Effect of ARTP

In this study, the ARTP dose applied to the bacterial sample was dependent on treatment time. The effect of various treatment time on the lethality rates of Bacillus mucilaginosus is shown in Fig. 2.

Fig. 2 Effect of treatment time on lethality rates of Bacillus mucilaginosus

Figure 2 shows that when the treatment time was 50, 60 and 70 s, the lethality rates reached 82%, 87%, and 96%, respectively. When the mutation treatment time was over 80 s, all bacteria were killed. It was found in previous studies that exposure time and lethality rate is proportional to a certain range, where longer exposure time led to higher lethality, which affected the mutation rate [19]. During the mutation process, the particles produced by ARTP could alter the physicochemical properties of the cell wall and the cell membrane, causing tissue damage. The cells were forced to start the SOS repair mechanism with high fault tolerance level, which produced a variety of the mismatch sites in the repair process. This finally allowed the genetic stability of the mutant strains [20]. The modern theory of breeding states that: when the mortality rate of microorganism mutagenesis reached 80%-90%, the positive mutation rate was the highest and the mutation effect was considered optimum [21].

The growth curves of B. mucilaginosus and mutagenic bacteria are shown in Fig. 3. The growth rate of B. mucilaginosus rapidly increased from 4 to 16 h, which was obviously in the logarithmic phase. The growth rates during 16-20 h were relatively stable, and the growth rates started to decrease at 20 h. Figure 4 presents the pH curve of the B. mucilaginosus and the mutagenic bacterial strains. The pH value in the solutions decreased from 7 to 4 during the initial 12 h, likely because of the acidic metabolites produced by the bacteria [22]. The pH stabilized at <4. The pH of the mutant ARTP with a treatment time of 50 s (B. mucilaginosus BM-50) dropped rapidly and stabilized at 3.58, which was lower than that of the original bacterial culture (pH 4.00). The results inferred that the growth curves of all mutants were better than those of the original strain, but the difference was not significant, and the B. mucilaginosus BM-50 showed the best acid- production performance.

Fig. 3 Growth curves of Bacillus mucilaginosus and mutagenic bacteria

Fig. 4 pH curves of Bacillus mucilaginosus and mutagenic bacteria

The results of the HPLC showed that oxalic acid, tartaric acid, citric acid and malic acid were detected in the fermentation broth of both the original strain and the B. mucilaginosus BM-50 during cultivation (Fig. 5). Previous studies have reported that Bacillus mucilaginosus produces organic acid, and this could be the cause for the decreased pH value in the culture medium [23]. YANG et al [11] studied the effect of B. mucilaginosus D4B1 on the montmorillonite structure. The results suggested that the alteration in the mineral structure of montmorillonite could be triggered by the organic acids produced by bacteria. The content of organic acid increased with the incubation time, during the initial phase (1-2 days). As time progressed, the organic acid content (except tartaric acid) declined. The results indicated that when the bacteria grew and were reproduced, the organic acids that the cell synthesized and secreted into the fermentation broth were used as nutrients by bacteria. This is caused by the continuous depletion of nutrients in the medium. This finding agrees with the previous report. XIAO et al [24] found that the decrease of organic acids in solution was due to the use of silicate bacteria as nutrients when nutrients were scarce. The yield of the organic acids in the fermentation broth of B. mucilaginosus BM-50 was greater than that in the original bacterium. The total content of the organic acid produced by the original strain was 82.6 mg/L on day 2, while that of the B. mucilaginosus BM-50 was 162.6 mg/L.

Fig. 5 Organic acid content of original bacteria (a) and B. mucilaginosus BM-50 (b) with different incubation time

3.2 Bioleaching effect of ARTP mutants

The pH curves and leaching efficiency of the vanadium-containing stone coal by B. mucilaginosus are shown in Fig. 6. The pH values of all cultures showed a similar trend (Fig. 6(a)), which initially decreased and then increased. The mutants showed different extents of change during the various treatment time. The B. mucilaginosus BM-50 showed a large decline. The pH dropped during the initial period, due to the organic acid that was produced by the bacterial metabolism. As the bioleaching time increased, the pH value increased. This indicated that the medium had poor nutrition, which caused the bacteria to use the organic acids and the extracellular polysaccharide as nutrients. Figure 6(b) shows that the V leaching rates of all strains were similar, and the highest leaching rate of B. mucilaginosus BM-50 was 18.2%, while that of original bacterial was 15.3%.

Fig. 6 pH curves (a) and V leaching rates (b) via original bacteria and mutant bacteria

3.3 SEM-EDS analysis of vanadium-bearing stone coal

3.3.1 Morphological changes of mineral surface

The SEM-EDS images of the raw stone coal and the bioleaching residue are shown in Fig. 7. The flat and undamaged mineral surface of the raw samples is shown in Fig. 7(a). The samples shown in Fig. 7(b) that reacted with bacteria were fragmentary and contained etch pits and had fuzzier surfaces than the raw samples. The leaching residues of the mutagenesis bacteria (Fig. 7(c)) showed more etch pits than the original bacteria, and the surface attachments were thicker and closer. The EDS results showed that the Si, Al and V contents in the residue decreased compared to the raw coal, and the reduction of the element contents in the residue after acting with B. mucilaginosus BM-50 was more significant. It was found in previous literature that during leaching, the bacteria attached to the surface of the mineral particles and their metabolites, such as organic acid and polysaccharides, were used to acidify and chelate the mineral particles. This damaged the weak chemical bonds of the particles and released metal elements [25].

3.3.2 Element distribution analysis on mineral surface

The element distributions on the raw stone coal surface and the bioleaching residue are shown in Figs. 8-10. The area fractions of the elements O, Si, O and Al on the samples (Fig. 9) that reacted with the bacteria were less than those of the raw material (Fig. 8). The decrease extent of these elements shown in Fig. 10 was more obvious. The results showed that SiO₂, V and Al gradually dissolved, due to the interactions between bacteria and their metabolites with mineral particles, where the mutagenesis bacteria played a significant role.

3.4 FTIR analysis of vanadium-bearing stone coal

Infrared spectroscopy was used to examine the possible changes in the minerals after bioleaching. Figure 11 depicts the FTIR spectra of the raw stone coal and the treated stone coal after bioleaching. Previous literature [26] stated that the peak at 3423 cm^-1 was attributed to the H—O—H stretching vibrations of the water molecules between mica layers. The peaks at 1596 and 1625 cm^-1 were attributed to the OH^- bending vibrations. The band at 1440 cm^-1 was attributed to the  vibrations of carbonate minerals. The peak at 1080 cm^-1 was attributed to the Si—O—Si asymmetric stretching vibrations. The peaks at 790 and 690 cm^-1 were attributed to the Si—O—Si symmetrical stretching vibrations. The remaining peaks at 516 and 470 cm^-1 were attributed to the Si—O—M bending vibrations.

Fig. 7 SEM images (a, c, e) and EDS analysis results (b, d, f) of raw stone coal and bioleaching residue

Fig. 8 Element distributions on raw stone coal surface

Fig. 9 Element distributions on bioleaching residue of original bacteria

Fig. 10 Element distributions on bioleaching residue of B. mucilaginosus BM-50

The infrared spectra of minerals changed after bioleaching. First of all, the peak at 3409 cm^-1 shifted to the higher wavenumber of 3453 cm^-1 and a new peak appeared in bioleaching residue of original bacteria at 1623 cm^-1. That is, the hydrogen bonds in the interlayer water have changed, which meant that the cations in the mica structure changed [27]. The vibration peak of  at 1440 cm^-1 in raw stone coal disappeared, which meant that small amounts of carbonate minerals in the raw mineral were decomposed after bioleaching. The peak of Si—O—Si asymmetric stretching vibrations at 1080 cm^-1 was weakened obviously, which indicated that symmetry of the chemical bonds in the lattice became worse. The disappearance of the peak of Si—O—M bending vibrations at 516 cm^-1 might be caused by the dissolution of metal ions, which indicated that the basic elements of mica in the stone coal were distorted and collapsed after leaching by bacteria and their metabolites [28]. The intensity of the band of residue leached by B. mucilaginosus BM-50 was stronger than that by original bacteria, which meant that mutagenic strain played a greater role in bioleaching on stone coal.

Fig. 11 FTIR spectra of raw stone coal and bioleaching residue stone coal

3.5 Effect of enhanced treatments on bioleaching rate of vanadium

The vanadium removal performance and the organic acid production of Bacillus mucilaginosus were improved after ARTP mutagenesis. Various enhanced treatments were selected to further improve the effects of V extraction, where BM-50 was chosen as the experimental bacteria that demonstrated a better bioleaching effect.

3.5.1 Supplement of sucrose

The result of bioleaching by strain B. mucilaginosus BM-50 with the supplementing sucrose is shown in   Fig. 12. As shown in Fig. 12(a), the leaching rate of the vanadium gradually increased when sucrose was added for 4 days. The leaching rate was 24.0% after 20 days of leaching, while that without supplementation was 18.2%. The V leaching rate increased slowly during the initial stage when the supplement was added, which rose rapidly during the late stage reaching 26.4% after 20 days after leaching. The pH curve (Fig. 12(b)) of the system with added supplementation showed that the pH remained stable and did not rebound after it dropped, which indicated that the energy source in the leaching system was sufficient and the metabolites produced by the bacteria were not consumed.

3.5.2 Adding surfactant

Fig. 12 V leaching rate (a) and pH curve (b) of bioleaching system with various supplementation methods

Fig. 13 V leaching rate of bioleaching system with various surfactants

As shown in Fig. 13, the leaching rate initially increased and then decreased as the Tween-20 dosage increased. When the Tween-20 dosage was 0.05, 0.1 and 0.2 g/L, the V leaching rate reached 20.5%, 28.9% and 28.3%, respectively, after 20 days of leaching. The leaching rate gradually decreased as the dosage of SDS increased. When the SDS dosage was 0.05, 0.1 and 0.2 g/L, the leaching rate reached 30.1%, 26.2%, and 21.7%, respectively, after 20 days of leaching. The V leaching rate of the system without a surfactant was 19.1%. The results showed that when the dosage of SDS was 0.05 g/L, the highest V leaching rate of 30.1% was obtained. The leaching efficiencies of the metals decreased when the surfactant concentration was adequate. This could be caused by the bacteria growth becoming inhibited under the higher surfactant concentrations [29]. The effect of the surfactants on the bioleaching of sulfide minerals has been previously investigated, indicating that the addition of surfactants reduces the interfacial tension between the mine and the solution. This is beneficial to the contact of bacteria and mineral contact, which improves the bioleaching [30].

3.5.3 Roasting

The experiment used blank roasting as a pretreatment process in order to improve the conversion rate of vanadium, to reduce the Cl₂, HCl and other harmful gas produced by traditional vanadium extraction. The result of the blank roasting pretreatment test is shown in Fig. 14. The V leaching rate increased in the prophase and the rate of growth stabilized during the late stage. The V leaching rate of B. mucilaginosus BM-50 reached 68.3% after 20 days of bioleaching, which was much higher than that of the blank control (18.2%).

Fig. 14 V leaching rate of leaching system of stone coal pre- processed by roasting

The essence of roasting was the conversion of vanadium-bearing mica to feldspar because vanadium typically exists in the mica [31]. The crystalline structures of mica and feldspar showed that the phase transition between mica to feldspar belonged to the bond-breaking reconstruction. Most of hydroxy in the crystal lattice of the aluminosilicate minerals was removed during the transformation. The vanadium was liberated from the mica [32]. Most of the vanadium in the stone coal occurred with V³⁺/V⁴⁺. The blank roasting pretreatment oxidized the low-valence vanadium into high-priced vanadium, which was beneficial to the leaching of vanadium via bacteria [33].

4 Conclusions

(1) Bacteria were mutated effectively using atmospheric and room temperature plasma mutation. Under a treatment time of 50 s, the total content of the organic acid in the mutagenic strain medium was 162.6 mg/L after 2 days of cultivating, nearly double that of the original strain. The V leaching rate of the mutant was 18.2% after 20 days of leaching.

(2) The SEM-EDS results showed that the mineral particles were corroded by bacteria and the V element gradually dissolved after interactions with bacteria. The FTIR analysis showed that the Si—O—M bending vibration peak at 516 cm^-1 disappeared, which indicated that the metal ions were dissolved and the basic elements of mica in the stone coal were distorted and collapsed after bacterial leaching.

(3) The results of the enhanced treatments showed that the blank roasting pretreatment had the best effect on the vanadium extraction, which reached 68.3% after 20 days of leaching.

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等离子体诱变菌浸出及石煤的空白焙烧相结合提高石煤中钒浸出效果

董颖博^1,2，刘 悦¹，林 海^1,2，刘陈静¹

1. 北京科技大学 能源与环境工程学院，北京 100083；

2. 工业典型污染物资源化处理北京市重点实验室，北京 100083

摘  要：为了提高石煤中钒的浸出效率，采用空白焙烧及微生物浸出相结合的方法，以胶质芽孢杆菌为原始菌，采用等离子体技术对其进行诱变处理。结果表明，培养2天后，照射时间为50 s时所得诱变菌B. mucilaginosus BM-50代谢产生的总有机酸含量较原始菌株提高近1倍。含钒石煤浸出20天时，诱变菌B. mucilaginosus BM-50的钒浸出率达到18.2%，相比原始菌的钒浸出率(15.3%)有所提高。空白焙烧预处理可以进一步提高钒浸出效率，浸出20天时，诱变菌B. mucilaginosus BM-50的钒浸出率为68.3%，较未预处理的诱变菌浸出体系的钒浸出率大大提高。研究表明，采用等离子体诱变菌浸出与空白焙烧相结合的方法具有提高低品位石煤钒回收率的巨大潜力。

关键词：常温常压等离子体(ARTP)；胶质芽孢杆菌；含钒石煤；空白焙烧；生物浸出

(Edited by Bing YANG)

Foundation item: Project (2015ZX07205003) supported by Major Science and Technology Program for Water Pollution Control and Treatment of China

Corresponding author: Hai LIN; Tel: +86-10-62333603; E-mail: linhai@ces.ustb.edu.cn

DOI: 10.1016/S1003-6326(19)64995-2