Date of Award
Doctor of Philosophy
Environmental Resources & Policy
The potential for mobilization of SO4 and Cl from coal stockpiles and coal processing waste and refuse (waste rock) disposal areas to the receiving streams and groundwater is an environmental concern and proper management practices are necessary to minimize the impact of these discharges. In an effort to characterize the long-term environmental impact of weathering in from both a typical coal stockpile and coal waste disposal areas a series of laboratory- scale and field-scale kinetic tests were performed with the ultimate goal of improving both coal and coal waste management at coal mines in a manner that minimizes the discharge of sulfate (SO4) and chloride (Cl). Laboratory experiments demonstrated that kinetic testing is a productive method for understanding the chemistry of surface water discharges from coal stock piles. However, these tests proved to be problematic in simulating the weathering of coal refuse. In an effort to improve the kinetic tests, field test columns were constructed that eliminated this deficiency. Unfortunately, field-scale test columns were found to be difficult to construct and, due to extremes weather events, difficult to maintain for an adequate test period. In the course of the experiment elemental and mineralogical data were collected both before and after weathering of fresh, run-of-mine coal from the Springfield (No.5) coal seam and coal refuse samples from processing the Springfield (No. 5) and Herrin (No. 6) coal seams. Duplicate columns were constructed in 2008 to conduct kinetic testing of the fresh run-of-mine No. 5 coal collected at an active underground mine in Southeastern Illinois. These columns measured 15.4 cm (6-inch) diameter by 61 cm (2-ft.) tall and were leached in batch mode for 32 months (27 leach cycles) using locally-collected rainfall water at a rate consistent with climatic data. Similarly, triplicate columns were constructed in 2009 to conduct kinetic testing of fine and coarse coal collected at the cooperative mine. The coal refuse test columns also measured 15.4 cm (6-inch) diameter by 61 cm (2-ft.) tall and were leached in batch mode for 41 months (31 leach cycles). Coal refuse was emplaced into the columns using a measured amount of compaction and a controlled moisture content (15.3%) based on data from previous laboratory engineering tests (Proctor testing). Locally-collected rainfall water was again used for leaching at a rate consistent with climatic data. Three columns investigated the leaching of compacted coarse refuse (the control) and three columns investigated the leaching of compacted 80:20 blend of coarse and fine refuse. To verify the results of the laboratory-scale, kinetic tests on coal refuse six field-scale (208 L or 55 gal.) columns were constructed in 2011 which measured 57.2 cm (22.5-in.) diameter by 85.1 cm (33.5-in.) tall and were leached in batch mode for 18 months (17 leach cycles). Two columns were again investigated the leaching of compacted coarse refuse (the control), while two additional columns leached compacted 85:15 blend of fine and coarse refuse and two columns tested a 80:10:10 blend of coarse refuse, fine refuse, and ground limestone. These field-scale tests allowed the use of full-sized refuse particles and were subject to natural precipitation events. An improved geochemical data set was obtained by these experiments due to an extension of the laboratory kinetic tests from 12 to 32 months to better simulate a mine-site stockpile conditions. Similarly, kinetic tests on coal refuse were extended from 12 to 41 months to better simulate SO4 and Cl release from a coal refuse facility. Three coal refuse disposal options were investigated in these experiments: 1) compacted coarse refuse (the control), 2) a compacted blend of fine and coarse refuse and 3) a compacted blend of coarse refuse, fine refuse, and ground limestone. Trends observed during the course of this research in leachate chemistry, as well as, comparisons of refuse placement options provide important insights necessary for development of management practices which minimize SO4 and Cl in coal mine discharge. The observed temporal changes were largely due to the presence of carbonate and aluminum mineral buffering of pH; three stages were observed during the kinetic testing of the Springfield (No.5) coal (Stages 1 through 3). Conversely, only Stage 1 and Stage 2 were observed in leaching tests of coal refuse due to the greater amount of compaction, which reduced the hydraulic conductivity and slowed the weathering rate. The identification of these three stages is important because of the improvement in coal and coal refuse management and water quality treatment realized by an understanding of these geochemical trends. The stages observed in the coal column leachate are: Stage 1: Laboratory coal column leachate collected during the first 7 months of simulated weathering of the No.5 coal maintained a favorable pH (> 6.4) maintained by an excess in bicarbonate alkalinity which minimized discharge of SO4 and common coal mine drainage metal Fe. The concentrations of Na and Cl in the leachate were elevated in early leach cycles, but declined rapidly as water soluble salts were flushed from the coal columns, which was an indicator that a portion of the Cl was water soluble forms such as salts and dissolved Cl- ions in pore water and not bound to the organic structure. Stage 2: A transitional period (Stage 2) occurred during the next 3 months of simulated coal stockpile weathering (7 to 10 months). This stage marked the exhaustion of the carbonate mineral buffer and an acceleration of coal weathering. Stage 2 leachate was characterized by a rapid decrease in the leachate pH to 4.0 and an increase in concentration of SO4 and dissolved Fe. Both Na and Cl in the leachate continued to decline in Stage 2, but the release related to flushing rate and not pH. Stage 3: After 10 months of simulated coal stockpile weathering, the leachate pH continued to slowly decrease to values below 2.0. At the same time the SO4 concentration increased rapidly and Fe concentration increased by over a factor of ten. The decline in pH was believed to have been restricted by iron and possibly aluminum mineral buffering during Stage 3. The release of Na and Cl in the coal increased somewhat during Stage 3 presumably due to slaking of shale contain in the ROM coal and the subsequent increase in the exposure of the soluble portion of the Cl to weathering and flushing. Laboratory leach testing of the Springfield (No.5) Coal from Southeastern Illinois suggests that: (1) SO4 levels in coal stockpile discharge would be relatively low up to ≈7 months. This time period, therefore, should correspond to the operational limit of run-of-mine (ROM) coal storage for this case example; and (2) Elevated discharges Cl- and the Na+ counter ion occurs immediately and may require control by operational measures (dilution and/or periodic blow down from a closed loop water handling system) to minimize Cl. A favorable leachate pH of > 6.4 which typified Stage 1 was also maintained throughout the laboratory-scale experiments for all blended coal refuse and in two of three columns simulating coarse coal refuse. Lower pH conditions similar to Stage 2 in the coal study was observed in leachate from only one of three coarse refuse columns after ≈12 months of kinetic testing. In all laboratory column experiments, the concentrations of Na and Cl in the leachate were elevated in early leach cycles, but declined rapidly as water soluble salts were flushed from the coal refuse columns. However, in the field column experiments favorable pH conditions (> 6.4) were only maintained throughout the 18 month experiment in the two columns which received a relatively high amount or ground limestone addition (10%). Lower pH conditions similar to Stage 2 observed in the coal leachate tests were observed in leachate from two coarse refuse columns and one of two blended refuse columns after ≈12 months. Complementary laboratory and field kinetic testing of coal refuse also suggest that: 1) SO4 levels in simulated coal refuse disposal area can be minimized by systematic compaction and co-disposal of properly dewatered fine and coarse refuse, and 2) elevated Cl (and Na) discharges occur immediately, which may require operational measures such as dilution and/or periodic blow down from the mine’s closed loop system. In most cases all of these measures can be completed using existing facilities at minimal additional costs. This dissertation provides insights into the potential for long-term discharge of SO4 and Cl from coal processing facilities in the Southeastern part of the Illinois and the operational controls that should be considered to minimize these impacts. Additional studies are suggested to confirm the findings with different coal seams and mining regions in the Illinois basin. Some notable findings include: 1) An increased understanding of coal and coal refuse leachate geochemistry can be expected by extending kinetic testing from the normal short-term tests (<12 >months) to longer-term testing (32-41 months). By conducting long-term (>12 months) kinetic tests additional observations were possible regarding limitations on rate of release of Cl and SO4 by both carbonate mineral buffer (pH 6.4) and an mineral ferrihydrite buffer at pH ≈3.4, as well as, increases due to chemical and physical weathering (slaking) of the materials. An understanding of the bicarbonate buffer is necessary to design operational controls during mining and reclamation and to evaluate the impact of alkaline (i.e. limestone) additions. 2) Kinetic testing of coal refuse should simulate real-world placement of these materials in a disposal area. Current coal mining practices require both the placement and compaction of coarse or blended refuse, which is not duplicated in kinetic methods employed by previous investigators that conducted tests using relatively loose-packed materials. Kinetic testing of non-compacted coal refuse is inconsistent with mine safety regulations in the U.S. dictate that compaction will be required for the structural stability of the coal refuse facility. Therefore, this experiment improved on the conventional kinetic testing methodology and provides a more appropriate estimate of the weathering rates as they relate to the release of SO4 and Cl from these materials. However, due the low hydraulic conductivity of the compacted refuse blends the laboratory column leachate volumes were inadequate to conduct key alkalinity analyses when rainfall was applied at physically realistic rates. Larger volume field columns may, therefore, serve as a better alternative. 3) Limitations on the mobility of the powerful oxidant, ferric iron (Fe3+), created by compaction and the presence of alkaline materials that support the presence of a bicarbonate buffer are critical in controlling the release of SO4. 4) Increased compaction in the two coal refuse blends would be expected to restrict the hydraulic conductivity and, therefore, flushing rate of the refuse and, as a result, reduce the corresponding release of Cl. However, the reverse was observed which was due in part to the high release rate of the soluble portion of the Cl from fine-grained materials, which in this case was the <200 mesh>(<0.074mm) fine coal processing waste. Moderate additions (<5%) of fine-grained (< 1 cm) limestone provides both an increased stability of blended coal refuse due to its role as a drying and cementing agent. The environmental benefit of limestone addition to coal refuse blends is the reduction of SO4 release due to: 1) Lower infiltration of H2O and O2(g) as the result of improved compaction, and 2) Increase in the net neutralization potential which results in increased bicarbonate mineral buffering. However, it is recognized that due to the size of these facilities limestone additions at the rate suggested by this research would be costly.
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