Abstract
Na2CO3/NaHCO3 mixtures with different oxidants were used to leach uranium in the lignite which was obtained from Lincang, Yunnan province. The experimental results showed that the optimal solid/liquid ratio and / ratio for uranium leaching were 1 : 20 (g/mL) and 2 : 1, respectively. With the increase of carbon concentration from 0.1 mol/L to 1.1 mol/L, the leaching efficiency of uranium increased from 14.64% to 42.39% after 6 h leaching. The oxidants could significantly enhance the uranium leaching efficiency, which was up to 72.23% by injecting O2 at 1.5 L/min after 12 h leaching. The oxidative leaching process of uranium from the lignite was better fitted to the pseudo-second-order reaction model. The sequential extraction results illustrated that the oxidants could effectively enhance the leaching of organic matter bound uranium in the lignite, which was decreased from 76.86 mg/kg to 9.00 mg/kg by injecting O2. The infrared spectrum analysis demonstrated that the corresponding transmittance at about 3197 cm−1 was prominently reduced after the oxidative leaching, which intimated that the phenolic and alcoholic hydroxyl might be the main functional groups combined with uranium in the lignite.
1. Introduction
The radiation pollution caused by the consumption of the uranium-bearing coals has gradually aroused great public concern [1]. The average uranium content of coals in China was 2.31 mg/kg based on analyses of 1535 coal samples [2]. Though the content of uranium in coals is relatively low, high abundance of uranium in coals was found in some regions of China. The concentration of uranium in the Late Permian coals was 111 mg/kg from the Heshan coalfield of southern China [3], and it was 153 mg/kg in the lignite from the Yanshan coalfield of Yunnan province [4]. Moreover, the uranium content of coals could be extremely high in individual regions that could be used as uranium resources. For instance, the uranium concentration in coals from the Yili Basin of Xinjiang province was up to 7207 mg/kg [5].
In order to reduce the environment pollution, some study tried to leach the uranium from the combustion products of the uranium-bearing coals in recent years [6, 7]. However, the uranium leaching efficiency generally decreased with the increase of combustion temperature and time [8]. To prevent the reduction of the leaching efficiency, the combustion process needs to be strictly controlled while leaching uranium from the combustion products. In addition, although the uranium tends to be concentrated in the bottom ash and fly ash during the coal combustion [9], some of the fine particles containing uranium (especially the submicron particles) were difficult to be captured by the air pollution control devices, and those fine particles usually diffused into the air and led to uranium pollution in the environment [10].
Considering the environmental pollution caused by uranium-bearing coals and the potential economic value of the uranium in the coals, leaching uranium from the uranium-bearing coals such as lignite can efficiently reduce the radiation pollution and brings economic benefits. The study of the leaching of uranium from coals has been started since the 1950s [8] and the leaching reagents were mainly sulfuric acid, nitric acid, and other inorganic acids. However, these techniques could bring new problems. As the organic matter in coals partially dissolves in the leaching reagent during the acid leaching process, it is difficult to separate the liquid phase from the solid leaching residue. Moreover, the coals are difficult to be reused after leaching with the inorganic acids.
Otherwise, the effect of the oxidants on the uranium leaching was studied as well. It has been proved that the oxidants could enhance the leaching efficiency of uranium. U(IV) in the tailing could be oxidized to U(VI) by air, H2O2, O3, NaClO3, H2SO5, MnO2, and KMnO4 [11–13]. The study of Shlewit and Alibrahim [14] revealed that the removing ratio of uranium in the phosphate polluted soil increased from 30% to 60% by adding H2O2. Francis et al. [12] found that the uranium leaching efficiency in the soil increased by about 10% when adding 20 mg/g KMnO4.
The objective of the study is to enhance the uranium leaching efficiency based on the premise that the coals after leaching are suitable for reuse. Therefore, the / systems with different oxidants were used to leach uranium in the lignite, which was obtained from Lincang, Yunnan province. Specifically, we sought to determine the following:(1)The effect of the solid/liquid ratio (S/L ratio), / ratio, the leaching reagent concentration, and oxidants on the uranium leaching efficiency of the lignite(2)The kinetics and the mechanism of the uranium leaching process in the lignite.
2. Methods and Techniques
2.1. Materials
The lignite from Lincang City, southwest of Yunnan province, China, which contained 102.10 mg/kg uranium, was used in this study. The lignite samples were ground, using a Hubei Geological Research Laboratory GSXX-4 planetary ball mill, to a homogeneous dry powder of less than 200 mesh. The chemical compositions of the lignite were analyzed by a PANalytical Axios mAX X-ray fluorescence (XRF) spectrometry (Table 1).
30% aqueous H2O2, NaClO solution with 5% available chlorine, KMnO4, K2S2O8, NaHCO3, and Na2CO3 were purchased from the Sinopharm Chemical Reagent Company. All the chemicals used in the experiments were of analytical purity grade. High purity O2 (99.5%) was purchased from the Wuhan Iron and Steel (Group) Corporation.
2.2. Analytical Method
The total uranium content in the lignite was determined by the method from Dai et al. [15] and the modified sequential extraction procedure proposed by Xiong et al. [16] was used to analyze the speciation of uranium in the lignite. The concentration of uranium was analyzed using a PerkinElmer Sciex ELAN DRC-II inductively coupled plasma-mass spectrometer (ICP-MS). Fourier transform infrared (FTIR) spectra of the samples were recorded between 400 and 4000 cm−1 at a constant temperature (20°C) on a Nicolet 6700 FTIR spectrometer. pH was measured using a Thermo Orion model 868 pH-meter.
2.3. Carbonate/Bicarbonate Leaching Experiment
Various S/L ratios, / ratios, and the concentrations of the leaching reagent were designed to determine the optimal conditions for the alkaline leaching of uranium from the lignite. The temperature and the shaking speed in the leaching experiments were 25°C and 150 r/min, respectively. The leachates were centrifuged at 5000 r/min for 5 min and the supernatants were collected for the uranium content analysis. The uranium leaching efficiency was defined as the ratio of the uranium content in the leaching solution to the total amount of uranium in the lignite, expressed as a percentage.
2.4. Oxidative Leaching Experiment
Under the optimal S/L ratio and / ratio, four oxidants, which were 30% aqueous H2O2, NaClO solution with 5% available chlorine, KMnO4, and K2S2O8, were selected to assess the effect of oxidation on the leaching efficiency of uranium from the lignite. In the experiment, the concentration of the leaching agent was 0.5 mol/L. Besides, the dosage of H2O2 and NaClO solution was defined as the volume ratio of the oxidant to the leaching agent, while the dosage of KMnO4 and K2S2O8 was defined as the mass ratio of the oxidant to the lignite sample.
2.5. Kinetic Experiment
Kinetic experiments were conducted using 0.5 mol/L of the leaching agent with the optimal S/L ratio, / ratio, and the oxidant dosage for a period of 24 h. In addition, high purity oxygen (99.5%) was injected to the leaching agent at a constant rate of 1.5 L/min in order to compare the leaching efficiency. Samples were collected at different predetermined time intervals to evaluate the kinetic performance of the uranium leaching process.
The leaching of uranium can be interpreted as the uranium desorption from the lignite. The pseudo-first-order equation and the pseudo-second-order equation were applied to investigate the leaching kinetic [17].
The pseudo-first-order equation can be formulated as follows:where is the rate constant and and are the solid-phase concentrations of uranium in the lignite at equilibrium and any time . With the initial condition of at , (1) is integrated and yields the following:
The value of is calculated according to
where is the initial concentration of uranium in the leaching solution (mg/L), is the concentration of uranium in the leaching solution at (mg/L), is the volume of the leaching solution, and is the mass of the lignite.
The pseudo-second-order equation is expressed as follows:where is the rate constant. With the initial condition of at , (4) is integrated and yields
According to the kinetic equations, the rate constants ( and ) and the theoretical capacity of the uranium leaching efficiency were computed using the nonlinear fitting.
2.6. Leaching Mechanism Analysis
The leachates collected from the kinetic experiments were centrifuged at 5000 r/min for 5 min and the residues washed with deionized water were taken for the sequential extraction and FTIR analysis. The mechanism of uranium leaching from the lignite was analyzed based on the comparisons of the sequential extraction for the uranium speciation in the lignite before and after leaching. Furthermore, FTIR spectra were used to investigate the leaching process.
3. Experimental Results
3.1. The Effect of S/L Ratios on the Uranium Leaching of the Lignite
The leaching efficiency of uranium in 0.5 mol/L Na2CO3 solution under various S/L ratios is shown in Figure 1. After 6 h reaction, the leaching process tends to be in equilibrium. The results indicate that, with the decrease of S/L ratio, the leaching efficiency of uranium increases gradually and is up to % at the optimal S/L ratio of 1 : 20 g/mL. However, a further decrease of S/L ratio does not lead to an observable increase of the uranium leaching efficiency.
3.2. The Effect of the Concentration of Leaching Agents and the Ratio on the Uranium Leaching Efficiency
Figure 2 shows the leaching efficiency of uranium by different concentration of the leaching agents and / ratio at the optimal S/L ratio after 6 h leaching. It can be observed that the leaching efficiency of uranium increases with the concentration of the leaching agent. The maximum uranium leaching efficiency of the 0.1 mol/L, 0.3 mol/L, 0.5 mol/L, 0.7 mol/L, 0.9 mol/L, and 1.1 mol/L leaching agent is 15.21%, 19.28%, 23.74%, 26.30%, 40.69%, and 42.39%, respectively. The main reaction in the solution can be expressed as follows [11]:
With the increase of NaHCO3 in the leaching agent, the ultimate pH of the leaching agent gradually declines (as shown in Figure 3). As the concentration of the leaching agent ranges from 0.5 mol/L to 1.1 mol/L, the maximum leaching efficiency of uranium is achieved when the / ratio is 2 : 1, which can be the optimal / ratio for the leaching of uranium in the study. Furthermore, the ultimate pH in the leaching experiment under the optimal / ratio is around 9.70. Some studies have shown that could combine with to produce sodium diuranate precipitation at pH values above 11 (see (7)). Santos and Ladeira [18] found out that, in the pH range of 9~10.2, the precipitation of sodium diuranate could be reduced by using NaCO3/NaHCO3 mixture, which is similar to the result obtained in our study
3.3. The Effect of the Oxidant on the Uranium Leaching Efficiency
Figure 4(a) compares the uranium leaching efficiencies at different dosages of oxidants after 6 h leaching. In the experiment, the concentration of the leaching agent was 0.5 mol/L and the / ratio was 2 : 1. The results indicate that the oxidants can enhance the uranium leaching efficiency. It is concluded that the optimal dosage of KMnO4 and K2S2O8 is 2 wt.% while the corresponding leaching efficiency of uranium is 34.71% and 50.45%, respectively. Besides, the optimal dosage of H2O2 (30% aqueous solution) and NaClO solution (5% available chlorine) is 1 vol.% with the corresponding leaching efficiencies 39.57% and 41.26%, respectively.
(a)
(b)
Moreover, as the dosage of NaClO solution increases from 4 vol.% to 16 vol.%, the ultimate pH in the solution continually increases from 9.97 to 11.65 (Figure 4(b)) as the hydrolysis of , which leads to the occurrence of (7) and reduces the uranium leaching efficiency from 41.98% to 29.88%. Besides, the oxidative leaching experiment with a high dosage of H2O2 cannot be carried out, as the leaching agent reacts violently with H2O2 (which is actually a weak acid) at a dosage of 16 vol.%. Furthermore, the ultimate pH of the leaching solutions containing KMnO4 and K2S2O8 is reduced very slightly.
3.4. Kinetics of the Leaching of Uranium from the Lignite
Figure 5 depicts the time variation of the uranium leaching efficiency from the lignite. The results show that the significant increases in the leaching efficiency of uranium are correlated with the addition of oxidants. In the experiment without adding any oxidant, the leaching process approaches equilibrium within 4 h and the corresponding uranium leaching efficiency is only 29.10%. On the contrast, the leaching process reaches equilibrium at the shortest time of about 2 h with the addition of 1 vol.% H2O2 (30% aqueous solution) and the leaching efficiency is up to 43.51%. On the other hand, the equilibrium time of about 6 h is needed for the leaching process with the addition of 2 wt.% KMnO4 and K2S2O8, the corresponding leaching efficiency of which is 42.35% and 48.34%, respectively. Furthermore, under the condition of adding 1 vol.% NaClO solution (5% available chlorine) and injecting O2 at a constant rate of 1.5 L/min, it takes about 12 h for the leaching process to attain the equilibrium state and, correspondingly, the uranium leaching efficiency of 52.06% and 72.23% is achieved, respectively.
Figure 6 displays the kinetics of the leaching of uranium from the lignite. The pseudo-first-order and pseudo-second-order equations that are applied to describe the kinetics of the leaching process and the kinetic parameters are listed in Table 2. The correlation coefficients () of the two kinetic equations reveal that the leaching process with the five oxidants is better fitted to the pseudo-second-order model, in which the maximum is 0.989 under the condition of injecting O2. On the contrary, the leaching process without oxidant is better fitted to the pseudo-first-order model and the corresponding is 0.957. The better applicability of the pseudo-first-order model may be due to the strong affinity of the lignite to uranium, which also shows that the leaching efficiency is mainly dependent on the amount of the leachable uranium in the lignite.
(a)
(b)
(c)
(d)
(e)
(f)
Further analysis of the values of shows that when the leaching process reaches the equilibrium state, the solid-phase-concentration of uranium in the lignite samples is much lower in the oxidative leaching process with the five oxidants, which tally with the experimental results. Thus, the use of the oxidant to enhance the leaching of uranium from the lignite is an effective option, not only because the oxidant can oxidize U(IV) to the significantly more soluble U(VI), but also because it can degrade the organic matter in the lignite to reduce the speciation which is difficult to leach, such as the organic matter bound uranium. This is what we are going to discuss in the following sections.
3.5. Sequential Extraction Experiment
Figure 7 shows the contents of the speciation of uranium in the lignite samples before and after leaching experiment by the sequential extraction procedure. The result indicates that the main speciation of uranium in the original lignite is the organic matter bound, which significantly decreased in the oxidative leaching process. At the conditions of adding H2O2, K2S2O8, NaClO, and O2, the organic matter bound uranium is reduced from 76.86 mg/kg to 38.89 mg/kg, 27.27 mg/kg, 25.72 mg/kg, 23.39 mg/kg, and a minimum of 9.00 mg/kg, respectively, while, in the experiment without adding any oxidant, it is decreased to 45.34 mg/kg only.
Otherwise, the reduction of the Fe-Mn oxides bound, sulfide bound, and aluminosilicate bound uranium in the leaching experiments is similar. In addition, a small amount of in the leachate can be adsorbed onto the lignite sample, which will easily be desorbed again during the sequential extraction procedure. Therefore, the contents of the water soluble and carbonates bound uranium in the lignite samples increase slightly after leaching.
The reduction of the organic matter bound uranium shows significant positive relationship with the corresponding uranium leaching efficiency, which illustrates that the leaching efficiency mainly depends on the leaching of the organic matter bound uranium in the lignite.
Among the five oxidants, the reduction of the organic matter bound uranium in the lignite by injecting O2 is much more efficient, and the main reasons can be explained as follows: (1) dissolved oxygen (DO) consumed during the leaching process can be continuously supplied by the injection of O2 at a constant rate of 1.5 L/min, which maintains a supersaturated state of DO in the leaching agent; (2) the lignite sample agitated by the injection of O2 is well-mixed in the leaching agent, which increases the contact area between the powdered lignite samples, the leaching agent, and O2 and therefore enhances the oxidative leaching.
3.6. FTIR Analysis
Figure 8 compares the FTIR spectra of the lignite samples before and after oxidative leaching. The main bands in the infrared spectrum of the original lignite and corresponding assignments are as follows: (1) the intense band at about 3197 cm−1 is attributed to the O-H stretching vibrations of alcohols and phenols; (2) the band at about 2919 cm−1 is caused by the C-H stretching vibrations of aliphatic structures; (3) the band at about 1610 cm−1 is assigned to the C=C stretching vibrations of aromatic rings; (4) the band at about 1437 cm−1 is ascribed to the C-H bending vibrations of aliphatic groups; (5) the band at about 1278 cm−1 can be attributed to the C-O stretching of phenolic groups; (6) the band at about 1031 cm−1 is generally attributed to C-O stretching of ethers and phenols [19–21]. According to the spectrum, the original lignite is rich in oxygen-containing functional groups and aromatic rings.
In the infrared spectra of the lignite after oxidative leaching, the peak at about 3197 cm−1 significantly reduces, which indicates that the content of hydroxyl groups in alcohols and phenols decreases by oxidation. The infrared spectra adsorption peaks at 2919 cm−1, 1610 cm−1, 1437 cm−1, 1278 cm−1, and 1031 cm−1 also experience a decrease in the oxidative leaching process with O2, H2O2, and K2S2O8.
The lignite belongs to the low rank coal with a low degree of coalification, which contains a large quantity of humic acid and fulvic acid. It has been studied previously that humic acid provides a lot of functional groups which are able to combine with the metal ions, including the carboxyl [22], phenolic hydroxyl, and alcoholic hydroxyl [23]. In the study, the phenolic hydroxyl and alcoholic hydroxyl groups may be the major functional groups combined with uranium. We conclude that the main reason for the enhancement of the uranium leaching efficiency is the effective degradation of these functional groups by the oxidants.
4. Conclusions
In this study, the effects of the S/L ratio, / ratio, the leaching reagent concentration, and oxidants on the uranium leaching efficiency of the lignite were discussed. Some conclusions can be drawn as follows.
(1) The leaching of uranium from the lignite by Na2CO3/NaHCO3 mixture is efficient under the optimal solid/liquid ratio and / ratio, which are 1 : 20 (g/mL) and 2 : 1, respectively.
(2) The five oxidants, H2O2, NaClO, KMnO4, K2S2O8, and O2, can significantly enhance the leaching of uranium in the lignite. And the use of O2 is the most efficient option which increases the leaching efficiency up to 72.23% after 12 h reaction.
(3) According to the kinetic results, the pseudo-second-order model is suitable to describe the kinetics of oxidative leaching process, and the maximum is 0.989 in the leaching experiment by injecting O2.
(4) The organic matter bound uranium, which is the main speciation of uranium in the lignite, can be effectively reduced from 76.86 mg/kg to 9.00 mg/kg by injecting O2.
(5) The FTIR analyses indicate that the phenolic and alcohol hydroxyl functional groups combined with uranium in the lignite are observably reduced by the oxidants, which may lead to the enhancement of the leaching efficiency of uranium.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.
Acknowledgments
This work was partially supported by the National Natural Science Foundation of China (NSFC, no. 41572233). The authors thank Weiyi Huang, Danqing Liu, and Chen Jing for invaluable contributions to this work.