ABSTRACT: Initially started in 1987 on one refuse area, the use of alkaline amendments for preventing Acid Mine Drainage (AMD) in coal refuse was instituted in earnest by the West Virginia DEP Office of Mining and Reclamation, Oak Hill office, in 1993. An overview of the data required to assess the potential of the refuse to produce AMD, the types and application of different types of amendment used and the results to date on the water quality of the discharges are discussed. This method has, to date, allowed the recovery of coal resources that might otherwise be too toxic to mine and refuse in an environmentally acceptable manner.
DISCUSSION: The first objective of evaluating what amendment may be required in neutralizing coal refuse, is to quantify the acidic potential of the material in question. Several different techniques have been used to quantify the nature of the refuse and the amount of alkaline material that would be required to neutralize the AMD (Acid mine drainage). These techniques are (1) acid/base accounting (see Attachment 1) (2) column leaching (see Attachment 2a) (3) accelerated Soxhlet leachate testing (4) sulfur forms and quantifying the pyritic content of the refuse. All of these techniques attempt to predict the acidic production of the pyritic sulfur oxidation products, either empirically through leaching tests or, by predictive analysis of the theoretical total acid potential of the pyrite oxidation. In addition, other empirical historical and water data from adjacent correlatable geologic strata is used to predict the amount of AMD possible.
If the coal refuse facilities include an impoundment where the fine slurry refuse is segregated from the coarse refuse, then each component needs to be analyzed separately. This is because the size of the pyrite contained in the refuse could dictate significant differences in the pyrite content of the slurry and the coarse refuse (see Attachment 4). The amount of pyrite and its size is site specific to the coal seam and its depositional model. It has been observed that some coal seams contain more pyrite in the coarse size fraction, and some seams more pyrite content in the fine fraction.
The samples that are needed for the various tests can either be direct samples from an operating coal preparation plant or they can be constituted from exploration cores in the following manner (1) The company must provide a preparation plant flow sheet and the proposed specific gravity at which they intend to operate the coal preparation. It must also be determined whether or not a slurry impoundment will be used because it will require sampling the coarse and fine refuse (2) At the proposed specific gravity, the entire mineable seam section should be float and sunk in the coal laboratory. The sink portion of this test represents the refuse portion of each coal seam. Each coal seam that is mined and prepared at the plant should be separated in this manner. It is important that the seam section analyzed be representative of the seam section to be mined, including all the coal partings, roof and floor that may be taken in the mining process and delivered to the preparation plant for washing (3) After the sink portion is separated in the analytical procedure, it should be tested for sulfur forms and acid/base accounting. From the sulfur form data we can determine the pyritic sulfur content which theoretically determines the total acid potential of a material and from the acid/base accounting we can evaluate the empirical data for the acidic/alkaline potential of the material.
The following criteria for evaluating the sample analyses (see Attachment 1), based on our experience and literature on the subject of AMD potential, suggests that:
Initially we required the companies to do Soxhlet or longer term leachate testing at varying amendment percentages by weight, by seam and by type of amendment. After some trial and error it was decided that the Soxhlet leachate studies be conducted for at least 10 weekly cycles to stabilize the pH curve. Column leachate studies would probably be of even longer duration. In the interest of obtaining analogous results to the leachate studies in a more expeditious and cost effective manner, we compared the amount of amendment required for neutralization as indicated by the leachate studies, and the amount of amendment required by the formula %S(pyritic) x 3.125 x .75(NP)/%CaCO3 equivalent = Amendment % by weight. This formula is a composite of previously proposed ways to calculate the amount of neutralization quantity needed based on stoichiometric equations for neutralization. The results of the calculation by formula and by leachate studies indicated a close correlation of the final amendment percentage estimated by both methods. The formula method was selected as the currently suggested method of calculating the amendment percentage required in the refuse. This method was not only cheaper and more expeditious than leachate studies, it was also selected for other important reasons (1) the laboratory test for sulfur forms is an ASTM Standard test that has been used for around a century while the Acid/Base analyses and the Soxhlet leachate tests are not performed to ASTM Standards and are not repeatable to the extent required (2) Because the amount of pyritic sulfur in a coal seam is a standard analysis for (a) preparation plant design (b) coal quality parameters for sales (c) used to evaluate prep plant efficiency in sulfur removal, the amount of data, even from older core holes, etc. is considerable and would be statistically much more significant than the leachate studies which use very small and spatially insignificant samples that are not indicative of the coal seam to be mined and refused (3) the amount of pyritic sulfur in the coal is a parameter that is, in most cases, an analysis that is already available from operations and can be used as a monitoring parameter to make sure the amendment percentage being used is commensurate with what percentage is required.
The final amendment evaluation should include the following:
OBSERVATIONS: Over sixteen (16) new and existing refuse areas now have a refuse amendment program in place. They are presently using an alkaline refuse amendment in combined refuse areas, impoundments where both the coarse and fine fractions are amended at different rates and with slurries that are injected underground. The alkaline products used for the amendment range from 78% CaCO3 limestone sand to Calcium oxide with a CaCO3 equivalency in excess of 110%. The main determination on selection has been made on price and availability. The companies have to balance the amount of CaCO3 equivalency in the product with its price. A rule of thumb cost for the refuse amendment is approximately $0.25/ % amendment/clean ton of coal produced (at approximately a 50% recovery rate). The sizing of the material and its surface area available for reaction is also a determining factor in the selection of the amendment. Very fine material such as kiln dust and calcium oxide have high neutralization effects and are disseminated very quickly into the refuse environment, however they are much more difficult to handle in all weather conditions and the calcium oxide in particular presents some worker hazards in handling because of it’s caustic nature. The finer material would also be consumed much more rapidly in the neutralization process, while a material such as limestone sand would react slower but be more effective over a longer period. Some companies have used a combination of a stronger, finer neutralizer on the front end and then used limestone sand after the first few months of refuse placement. It is critical that the alkaline amendment in new refuse areas be started at the very beginning, because the lower parts of the refuse pile will be the areas that will be below the phreatic level once the water table regenerates in the pile. An additional benefit of an alkaline amendment is the better ground stabilization that occurs from the pozzolanic effect of the alkaline amendments, particularly in the combined refuse piles which contain a large moisture percentage from the fine coal fraction.
All of the refuse areas that have been treated with an alkaline amendment from initial refuse placement, have exhibited pH’s in the +6 to 8 range without any additional treatment (see Attachments 10a and 10d). Four other refuse areas that started using refuse amendments late in their history have also seen water quality improvements. We have one mine complex in Nicholas County, WV that has two refuse areas that are adjacent to each other that have refused the same coal seams, with the older area only amended in it’s last year of life and the new area amended from the first refuse placement (see Attachment 11). Whereas the older refuse area needed heavy treatment with caustic soda and other neutralizers to meet the +6 pH limit, the new area exhibits +- 7 pH with relatively low metals. The differences in the Iron and Manganese concentrations are dramatic between the two refuse areas and the pH is consistent in the new refuse pile. The consistency is important for the treatment of the older pile, because the company used so much caustic soda at times to neutralize the pH and get the metals (ie FE & MN) in compliance, that the pH upper limit of 9 was periodically exceeded and the amount of sludge became a significant problem in pond capacity and the sludge’s disposal. The added benefit of metals reduction in the new pre-treated refuse area has replaced the costs from caustic soda or other treatment at the outlet that was performed on the old refuse area, and has produced a significant cost benefit for the pre-treatment method versus the post treatment traditionally used to increase pH and reduce metals. In addition to monitoring the water from the amended refuse areas, a number of samples per lift (ie. 2’ compacted) were taken and run for NP and pH (see Attachment 12). Although the results over a large data set could be correlated to an average .75 NP, the individual analyses of the refuse with the amendment (as placed) was very inconsistent, which is attributable to the size distribution of the samples, and the virtual impossibility of the sampling technician to obtain consistent samples.
All of the refuse areas that have been pre-treated from their inception with an alkaline amendment have exhibited consistently acceptable water quality, some for several years. The older refuse areas, that have experienced alkaline pre-treatment in their later years, have have seen improving discharge water quality from these piles.
CONCLUSIONS: The following conclusions and implications can be drawn from the observations made on alkaline pre-treated refuse:
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