Modes of occurrence and removal of toxic elements from high‐uranium coals of Rongyang Mine by stepped release flotation

The Rongyang coals from Guizhou, southwestern China, are significantly enriched with uranium (U) and other toxic elements, such as vanadium (V), chromium (Cr), molybdenum (Mo), cobalt (Co), copper (Cu), and selenium (Se). Therefore, it is necessary to remove these toxic elements from high‐U coals before combustion. Previous studies have shown that U cannot be effectively removed by flotation and gravity separation. In this study, the stepped release flotation and acid leaching tests were conducted for coals of the Longtan Formation from the Rongyang Mine. The results from these analyses are as follows: (a) Stepped release analyses revealed that a multistage cleaning process can help to separate toxic elements from cleaned coals; (b) The removal of toxic elements by stepped release and acid leaching is closely related to their mode of occurrence. Vanadium, Cr, Mo, and U were difficult to be removed by stepped release flotation, which due to a significant proportion of them occur in organic matter and fine‐grained minerals; (c) Compared to flotation, most elements were better to be removed by stepped release flotation. In comparison with gravity separation, U had a higher removability by stepped release flotation, while the removal effect of thiophilic element by gravity separation was greater than by flotation; (d) After stepped release flotation, toxic elements Sc, V, Cu, Se, Mo, Hg, and U are still enriched in cleaned coals, suggesting that these elements cannot be fully removed through stepped release flotation.


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DUAN et Al. characterized by their high S and U contents. Our previous work has shown that Rongyang coals are characterized specifically by high pyritic S contents and elevated concentrations of the trace elements U, vanadium (V), chromium (Cr), cobalt (Co), copper (Cu), selenium (Se), and molybdenum (Mo) due to marine influence and exposure to hydrothermal fluids during syngenetic or early diagenetic stages. 23 Because U, V, Cr, and Mo primarily occur in organic matter and in fine-grained minerals, these elements cannot be removed from coal by gravity separation and flotation. 23,24 In order to effectively remove these toxic elements from the Rongyang high-U coals, stepped release flotation and acid leaching were conducted in this study. Furthermore, the removal of toxic elements from high-U coal is of great significance to environmental protection.

| GEOLOGICAL SETTING
Rongyang Mine is located in the southwestern part of Guizhou Province, southwestern China ( Figure 1). The coal-bearing stratum in the mine field is the Upper Permian Longtan Formation, which has a thickness of 230 m. The Longtan Formation has a disconformable contact with the underlying Middle Permian Maokou Formation and is conformably overlain by the Late Permian Changxing Formation. 23,25 The Longtan Formation is mainly composed of siltstone, chert, limestone, mudstone, shale, and coal seams and contains fossils including brachiopods, pteridophyte, and ferns. 23 The Kangdian Upland was the dominant sediment-source region for the Late Permian coals from western Guizhou, eastern Yunnan, south Sichuan, and Chongqing. 25, 26 The Late Permian Longtan Formation coal was formed in a tidal flat environment on an open carbonate platform. 27

| Sample background
Samples were collected from the coal-bearing Upper Permian Longtan Formation. The volatile content of Rongyang coals is 8.90%, and the ash yield is 19%. The total S content is 5.44%, which is dominated by pyritic S (4.19% total or 77.02% of the S content), followed by organic S and sulfate (SO 4 ), indicating that the Rongyang coal is a medium-ash and high-S anthracite. 23

| Stepped release flotation
Stepped release flotation testing was performed according to the Chinese Ministry of Coal Industry standard MT 144-86. 28 The 1.5-L single flotation cell (XFD-63) was used for stepped release flotation. In the stepped release analysis, coal slime was separated by one-stage roughing and multistage cleaning, after which flotation products with different ash contents were obtained. Samples were initially crushed to <0.5 mm for testing. Flotation conditions for Rongyang coal were an impeller speed of 1800 revolutions/min, a pulp concentration of 100 g/L, and an aeration quantity of 0.25 m 3 /m 2 / min. Diesel was used as a collector with a content of 800 g/t, F I G U R E 1 Paleoenvironment and location of the Rongyang Mine in southwestern China 23 and octanol was used as a frother with a content of 400 g/t. The stepped release flotation test followed the procedures shown in Figure 2.

| Mineralogical analyses
A Leitz MPVIII optical microscopy was used for mineral observation, and the images were captured using white-light reflectance microscopy.

| Acid leaching
The leaching method used in this study is based on those presented by Fisher et al, 29 Qin et al, 30 and Duan et al. 24 Coal samples were ground to <200 mesh (75 μm). Two coal samples (3 g each) were leached by 40 mL of 2 mol/L HCl and 4 mol/L HCl for 24 hours (interval of ultrasonic shaking), respectively. After extraction, the coal samples were centrifuged for 20 minutes at 5000 r/min to separate solids from the solution. The supernatant was removed and then diluted to determine trace element contents by inductively coupled plasma mass spectrometer (ICP-MS; Thermal Fisher, X Series II). Finally, the rate of extraction of toxic elements by dilute HCl was calculated.

| Trace elements
All samples were crushed and ground to <75 μm. ICP-MS was then used to determine the concentrations of trace elements in the samples, with the exception of mercury (Hg) and fluorine (F). Samples were digested for ICP-MS using an UltraCLAVE microwave high-pressure reactor (Milestone). Digestion reagents were 2 mL of 40% (v/v) hydrogen fluoride (HF) and 5 mL of 65% (v/v) nitric acid (HNO 3 ) for the coal samples, and 5 mL of 40% (v/v) HF and 2 mL of 65% (v/v) HNO 3 for coal-related material samples. The procedures for ICP-MS analysis of coal and coal-related material microwave digestion were based on the methods described by Dai et al. 31 Concentrations of arsenic (As) and Se were determined using an ICP-MS system equipped with automated, third-generation collision-cell technology (CCT ED ), as outlined by Li et al, 32 which effectively diminishes the spectral interference of the argon (Ar)-based polyatomic ions 40 Ar 35 Cl (chlorine) and 40 Ar 38 Ar to 75 As and 78 Se, respectively. 32 Hg concentrations were determined using a Milestone DMA-80 Hg analyzer, following the method outlined by Dai et al. 33 F concentrations were measured by pyrohydrolysis with an ion-selective electrode in accordance with ASTM D5987-96. 34

| Mode of occurrence of fine-grained minerals
The optical microscopy observations found that minerals in Rongyang coals are mainly composed of pyrite and clay minerals. It is consistent with the results obtained from the XRD data. 23 There are a large amount of fine-grained clay minerals and pyrite existing in Rongyang coals ( Figure 3). Wang et al 35 considered that fine-grained pyrite is generally <10 μm in size. The sizes of these fine-grained minerals in Rongyang coals are in the range of 1-10 μm. The fine-grained clay minerals are occurring in collodetrinite ( Figure 3a) and discrete in cavities of fusinite ( Figure 3b). The fine-grained pyrite is partly disseminated (speckles) in collotelinite ( Figure 3c), but another part of fine-grained pyrite occurs in fusinite coexisting with clay minerals (Figure 3d). Duan et al 23,24 found that U occurs in fine-grained pyrite, clay minerals, and anatase, which coexists with fine-grained pyrite or clay minerals. In addition, U is also associated with organic component. 23,24 These fine-grained minerals are difficult to be liberated from coals.

| Migration characteristics of toxic elements during stepped release flotation
The yield, ash yield (A d ), and content of toxic elements in the products of stepped release flotation are shown in both Table 1 and Figure 4. The yield of Product 1 was too low to determine the content of trace elements. Therefore, the migration patterns of trace elements during stepped release flotation could only be evaluated in general terms. The With an increased number of cleaning stages, the content of toxic elements in the products decreased, indicating that multistage cleaning helped to remove these elements from the cleaned coals. 2. The elements Co, nickel (Ni), Cu, As, Se, antimony (Sb), bismuth (Bi), and Hg exhibited a similar trend in the flotation products. The elements scandium (Sc), Co, Ni, Cu, As, Se, Sb, Bi, and Hg exhibited their lowest contents in cleaned coals. With an increase in the number of cleaning stages, the content of toxic elements in the tailings increased. Unlike Co, Ni, Cu, As, Se, and Hg, the elements zinc (Zn), Sb, lead (Pb), and thorium (Th) exhibited their highest contents in Product 2, but their contents gradually reduced in Product 3, revealing that Zn, Sb, Pb, and Th need more than two stages of cleaning.  The contents of V, Cr, Cu, Se, Mo, Hg, and Pb in cleaned coal are higher than those in feed coal because of the ion exchange between coal and flotation fluids as diesel may contain V, Cr, Cu, Se, Mo, Hg, and Pb.

The distributions of U and Mo in the flotation products
were different from all other elements. The contents of U and Mo in cleaned coals were higher than in most of the other products, of which Product 1 had the lowest content. The content of U in feed coal was higher than in cleaned coal and flotation products, which indicates that a part of U was released into washing water.

| The removability of toxic elements by stepped release flotation
To demonstrate the degree to which toxic elements can be removed through stepped release flotation. Equation 1, proposed by Wang et al, 13 was used to calculate removability.
where R denotes removability, c i denotes the contents of element i in cleaned coal, and Ci refers to the content of this element in feed coal. The removability of toxic elements in cleaned coals via stepped release flotation, flotation, and gravity separation is shown in both Table 2 and Figure 5. Through stepped release flotation, the removability of V, Cr, and Mo is negative, but the removability of U is 25.67%, indicating that a portion of U can be removed, but V, Cr, and Mo cannot be removed by stepped release flotation. Moreover, most of the thiophilic and siderophile elements, such as Co, Ni, Zn, As, Rb, Cd, and Sb, also exhibit a high removability. Compared to simple flotation, and excepting Co, Ni, Cu, and Th, most elements may be better removed by stepped release flotation. When compared with gravity separation, V, Cr, and Mo exhibit a higher removability by flotation, but F, Cu, As, Se, Cd, Sb, Ba, Hg, Tl, and Pb have higher removability by gravity separation. The removability of U (1) by stepped release flotation was higher than that by gravity separation and flotation. The results indicate that thiophilic elements may be better removed by gravity separation than by stepped release flotation.

| Leaching of toxic elements by dilute HCl
As shown in Figure 5, the removability of most of the toxic elements is relatively low. In order to fully remove these elements, the cleaned coals were leached by dilute HCl. The reduction rates of toxic elements from cleaned coals via stepped release flotation using 2 and 4 mol/L dilute HCl are listed in Table 3. As shown in Table 4, the reduction rates of V, Cr, Mo, and U using 4 mol/L HCl were higher than those using 2 mol/L HCl. The highest reduction rate obtained for V was 2.82%, while the reduction rates of Cr, Mo, and U exceeded 15%. The acid leaching of thiophilic and siderophile elements Co, Ni, Zn, As, Cd, and Pb was relatively high. However, the leaching rates of the lithophile elements Sc, Rb, Ba, and Tl were lower, which suggests that the thiophilic and siderophile elements were much easily removed by acid than that of lithophile elements.  The removability of toxic elements in cleaned coal via stepped release floatation. The data are from Duan et al. 24 b The removability of toxic elements in cleaned coal with a size of <0.5 mm via gravity separation, and the data are from Duan et al. 23

F I G U R E 5
The removability of toxic elements in cleaned coal by stepped release flotation, flotation, and gravity separation, respectively T A B L E 3 The reduction rates of toxic elements from stepped-release-flotation cleaned coal using dilute HCl (%)

| Comparison of Chinese coals with world hard coals
As listed in Table 4, the contents of the elements Be, F, Co, Ni, Rb, Ba, Cd, Sb, Cs, W, Tl, Bi, and Th in cleaned coals, which were obtained by stepped release flotation, are lower than the average for both Chinese 26 and world hard coals. 36 However, the contents of Sc, V, Cr, Cu, Zn, As, Se, Mo, Hg, Tl, Pb, and U in cleaned coals are still higher than the average for Chinese and world hard coals. The concentrations of trace elements in cleaned coals by stepped release flotation, in comparison with the average values for world hard coals, are shown in Figure 6. According to the classification scheme for elemental enrichment proposed by Dai et al, 37 Mo and U are significantly enriched (10 < CC < 100), Sc, V, and Se are enriched (5 < CC < 10), Cu and Hg are slightly enriched (2 < CC < 5), and Be, F, Rb, and Sb are depleted (CC < 0.5). The remaining elements (0.5 < CC < 2) are close to the average for the world hard coals reported on by Ketris and Yudovich. 36 The results indicate that Sc, V, Cu, Se, Mo, Hg, and U cannot be fully removed through stepped release flotation.

| The relationship between modes of occurrence and removal of toxic elements
Stepped release flotation uses differences in surface properties to separate minerals from coal; elements Sc, Co, Ni, Cu, Zn, Cd, Sb, Pb, As, Se, Sb, Bi, and Hg mainly occurred in pyrite, 22,38,39,40 while elements Be, F, V, Cr, Ba, W, Cs, and Tl dominantly occur within clay minerals, or other minerals, or organic matter. 23,38,41 Thus, the distributions of Sc, Co, Ni, Cu, Zn, Sb, Pb, As, Se, Sb, Bi, and Hg in the products of stepped release flotation are different from those elements Be, F, V, Cr, Ba, W, Cs, and Tl. Because U and Mo are associated with organic matter and occur in fine-grained minerals, especially clay minerals and pyrite, 23,41 the distributions of U and Mo in the flotation products were different from all other elements. The removability of toxic elements by stepped release flotation and removal rate by acid leaching are closely related to their mode of occurrence. Most of the thiophilic and siderophile elements Co, Ni, Cu, Zn, Cd, Sb, As, Se, Sb, and Bi have the higher removability than those of lithophile elements Be, F, V, Cr, and Tl. The acid leaching results illustrate that small amounts of V, Cr, Mo, and U can be removed, which due to these elements are mainly associated with organic matter or occur in fine-grained T A B L E 4 The contents of toxic elements of cleaned coals by stepped release flotation, Chinese coals, and world hard coals minerals encased by organic matter. 23,24 The removal rate of the thiophilic and siderophile elements Co, Ni, Cu, Zn, As, Cd, and Pb is higher than that of the lithophile elements Sc, Rb, Ba, and Tl. 6 | CONCLUSIONS 1. Stepped release flotation analyses revealed that the multistage cleaning helps to separate toxic elements from cleaned coals. 2. The removal of toxic elements by stepped release and acid leaching is closely related to their mode of occurrence. Vanadium, Cr, Mo, and U were difficult to be removed as they are mainly associated with organic matter or occur in fine-grained minerals. Cobalt, Ni, Cu, Zn, Cd, Sb, As, Se, Sb, and Bi, which occur in pyrite, are much easily removed than Be, F, V, Cr, and Tl which mainly occur in clay minerals and are associated with organic matter. 3. Compared to flotation, most elements are better removed by stepped release flotation. In comparison with gravity separation, U has a higher removability by stepped release flotation, while the removal effect of thiophilic element by gravity separation is greater than by flotation. 4. Compared to world hard coals, toxic elements Sc, V, Cu, Se, Mo, Hg, and U are still enriched in cleaned coals, indicating that Sc, V, Cu, Se, Mo, Hg, and U cannot be fully removed through stepped release flotation.