Quantifying the Modes of Occurrence of Trace Elements in Coal Using Sequential Laboratory Leaching Techniques
Poster

By Curtis A. Palmer¹, Sharon S. Crowley¹, Stanley J. Mroczkowski¹, Robert B. Finkelman¹, and John H. Bullock, Jr.²

¹U.S. Geological Survey, 956 National Center, Reston, VA 20192
²U.S. Geological Survey, Denver Federal Center, M.S. 973, Denver CO, 80225

SUMMARY

Sequential leaching can rapidly and accurately quantify modes of occurrence of inorganic elements in coal. Nine as-mined coals and seven commercially cleaned coals were leached using U.S. Geological Survey (USGS) procedures. All samples were taken from commercially important coal seams. Ten of these samples were from the northern Appalachian Basin, including six from the Pittsburgh seam (four from Pennsylvania and two from West Virginia), two samples from the Lower Kittanning seam (Pennsylvania), and two samples from the Upper Freeport seam (Pennsylvania). The one sample from the central Appalachian Basin was a blend of coals from the Elkorn and Hazard seams in Kentucky. A sample from the Warrior Basin in Alabama was from the southern Appalachian Basin. Two samples from the Illinois basin included one each from Illinois and Indiana, and two samples from the Powder River Basin included one sample each from Wyoming and Montana. All coals were bituminous except the two subbituminous coals from the Powder River Basin.

The reproducibility of the sequential leaching procedures is about ±5 to 10 percent absolute. The resulting mass balances from the multitechnique analysis of both the leachates and the residual solids provide a high degree of confidence in the procedure. Modes of occurrence were quantified for 14 environmentally sensitive elements. On average, about 50 percent of the As and Fe are leached by HNO₃, indicating an association with pyrite. Pb and Zn are leached by both HCl and HNO₃ and are associated with both monosulfides and disulfides. With few exceptions, more than 50 percent of the Be, 40 percent of the Cr, up to 50 percent of the Zn, and up to 40 percent of the U are leached by HF, an indication that an association with silicates exists. Se and Hg are associated with pyrite in most coals, but the fact that as much as 95 percent of the Se and 100 percent of the Hg remain unleached in the final residual indicates an organic association. Generally, more than 50 percent of the Sb and significant amounts of U, Th, and Pb are organically associated. Mn is primarily associated with the carbonates. Co and Ni have multiple modes of occurrence with no dominant mode. Large differences in the leachability of Cr, Co, U, and Th between the as-mined coal and cleaned coal indicate that physical coal cleaning is preferentially removing the mineral-associated portion of these elements, preferentially leaving behind the organically associated fraction.

DISCUSSION

Each of the samples in table 1 was leached with four solvents (fig. 1). The samples were analyzed using the procedures shown in table 2. The concentration of the elements in the solutions and the residuals were compared with the original concentrations in order to determine the percent leached by each of the solvents.

Figure 1. Diagram showing the four steps of the leaching procedure used in this experiment.

Pyrite

Pyrite (FeS₂) is dissolved in the leaching procedure by HNO₃. Nitric acid mobilizes significant percentages of Fe (fig. 2), As (fig. 3), Hg (fig. 4), and Se (fig. 5), suggesting that these elements are often associated with pyrite. In cases where the total pyrite is low (as determined by low pyritic sulfur) (samples R2 and EH) or the total ash content is very high (sample SA, 57 percent ash) (table 1), pyrite is not the major mode of occurrence of Fe (fig. 2). Pyrite is also not the major mode of occurrence of Fe in sample K1, which has 36 percent ash, but is the major mode of occurrence of sample E1 with the same ash content. It is also possible that some pyrite may have been oxidized in these samples. All four samples where pyrite is not the major mode of occurrence have elevated percentages of HCl soluble Fe. Oxidized pyrite would form iron oxides and sulfates, which are HCl soluble. The major mode of occurrence of As (fig. 3) is pyrite, with the exception of EH, R1, and R2. The concentrations of As in R1 and R2 are less than those in the other samples (fig. 3). Figure 4 shows that the amount of unleached Hg is larger than the unleached amount of Fe or As. In one sample, the fact that no Hg was leached indicates more organically bound Hg in these samples. Significant percentages of Se (fig. 5) are associated with pyrite, but there are also large percentages of organically associated Se in some samples.

Figure 2. Fe leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Fe in the original samples is given below the graph.

Figure 3. As leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of As in the original samples is given below the graph.

Figure 4. Hg leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Hg in the original samples is given below the graph

Figure 5. Se leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Se in the original samples is given below the graph

Phosphates

Some Th (fig. 6) and U (fig. 7) are leached by nitric acid, which might be interpreted as an indication of sulfide association. A separate experiment in which the nitric acid leach was replaced by a second HCl leach removed Th and U but did not remove Fe and As (which are pyrite associated). We interpret this finding to indicate that Th and U are associated with HCl-soluble phosphates that have been encapsulated in silicates. Scanning electron microscopy (SEM) identified phosphates in some of the whole coal samples in this study, but, because of their low abundance, it was not possible to prove that the Th- and U-bearing phosphates were encapsulated in silicates.

Figure 6. Th leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Th in the original samples is given below the graph.

Figure 7. U leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of U in the original samples is given below the graph.

Carbonates

Calcite is soluble to a large extent in ammonium acetate at pH7 under the conditions of our experiment. Thus, the HCl-soluble Mn (fig. 8) and probably most of the ammonium acetate-soluble Mn results from the solubility of carbonates. We have detected Mn in siderites by SEM in some of these samples.

Figure 8. Mn leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Mn in the original samples is given below the graph.

Monosulfides

The monosulfides -- sphalerite (ZnS) and galena (PbS) -- are HCl soluble. Although sphalerite and galena are important modes of occurrence for Pb (fig. 9) and Zn (fig. 10), they are clearly not the only modes of occurrence for these elements. Significant percentages of Zn and some Pb are associated with the silicates and probably with clays. Significant percentages of Pb and some Zn are HNO₃ soluble, suggesting that these elements are also associated with pyrite. Substantial quantities of Pb are not leached in several samples, indicating an organic association.

Figure 9. Pb leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Pb in the original samples is given below the graph.

Figure 10. Zn leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Zn in the original samples is given below the graph.

Organic Associations

Elements that are organically associated will generally not be leached in this procedure. About 50 percent of the Sb (fig. 11) is unleached. Twenty to 65 percent of the Be in samples from the northern Appalachian and central Appalachian Basins (fig. 12) is unleached, but there is little unleached Be in the high-ash southern Appalachian Basin sample, the Illinois Basin samples and the one Powder River Basin samples where Be was in high enough concentrations to be measured in the leachate. In addition, significant percentages of Cr (fig. 13), Se (fig. 5), Hg (fig. 4), Co (fig. 14), Ni (fig. 15), Pb (fig. 9), Th (fig. 6), and U (fig. 7) are unleached. Relatively small percentages of Fe (fig. 2), As (fig. 3), Zn (fig. 10), and Mn (fig. 8) remain unleached.

Figure 11. Sb leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Sb in the original samples is given below the graph.

Silicates

Be (fig. 12) and Cr (fig. 13) are mostly leached with HF and are primarily associated with the silicates. We have detected Cr in illite by microprobe analysis at about 200 ppm. Leaching results indicate that smaller percentages of many other elements are associated with silicates, especially Zn (fig. 10), Sb (fig. 11), Ni (fig. 15), Mn (fig. 8), and U (fig. 7). In three samples, 30 to 40 percent of the Fe (fig. 2), which is generally associated with pyrite in the other samples, is mobilized with HF. In only one of these samples (K1) was the percentage of HF leached larger than the percent leached by HCl or HNO₃.

Figure 12. Be leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Be in the original samples is given below the graph.

Figure 13. Cr leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Cr in the original samples is given below the graph.

Mixed Modes

On average, Co (fig. 14) and Ni (fig. 15) have subequal amounts of HCl-, HF-, and HNO₃-soluble material. In addition, both of these elements have significant but variable amounts of unleached, organically associated material. Most other elements also have several modes of occurrence, but, in most other cases, one form is dominant.

Figure 14. Co leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Co in the original samples is given below the graph.

Figure 15. Ni leached by the four solvents is shown as stacked bar graphs. The total height of the bar represents the total leached by all solvents. The concentration of Ni in the original samples is given below the graph.

Differences in Total Amount Leached of Commercially Cleaned Coals Compared to As-Mined Coals

Large differences in the total amount of Cr (fig. 13), Co (fig. 14), U (fig. 15), and Th (fig. 6) leached between the as-mined coal and cleaned coal indicate that physical coal cleaning is preferentially removing the mineral-associated portion of these elements and preferentially leaving behind the organically associated fraction.