Leaching potential of coal waste organic compounds; a simulation study with fresh and saline water (Upper Silesia Coal Basin, Poland)

The vast amount of waste rocks excavated during coal mining and stored in dumps presents a significant environmental problem as they readily undergo spontaneous heating. The process produces large amounts of organic compounds, relatively well-soluble in water. The problem is severe because self-heating has been registered in many coal mining regions. The brine application as an anti-pyrogenic agent to prevent self-heating is a method that is still being developed. Its advantages are the ignition temperature increase, rock pores sealing by crystallizing salt, and permeability decrease, preventing contact with oxygen. Since there is no knowledge about the brine potential to leach organic compounds from coal wastes, this project aims to characterize components leached by rainwater and brine applied as an anti-pyrogen.

The Starzykowiec dump has stored coal wastes since the beginning of the 20th century. They consist of mudstones, siltstones, claystones, and arcosic sandstones. Most self-heating occurred from the 1940-60s. For the procedure, samples representing six rock types were selected, all thermally affected to different degree. Maceral composition and mineral content were determined at 500 points, and random reflectance was measured on vitrinite particles at 5-100 points. Two rock portions (ca 60g) powdered to <0.2 mm grain size were leached with 200 ml of distilled water and brine (25%, wt. NaCl), respectively (24h). Then, mixtures were stirred (30 min.) and set aside to settle. Leached organic compounds were isolated using solid phase extraction (SPE) on C18 PolarPlus columns. The composition of coal waste and SPE extracts were analyzed with a gas chromatograph-mass spectrometer (full scan mode, DB-5 column).

The content of organic matter was 35.0-88.2%. It was mostly unaltered or weakly weathered; only one sample was strongly altered. The unaltered and oxidized samples were composed mostly of vitrinite while strongly altered sample was dominated by masive coke. Since the Starzykowiec coal wastes were deposited for a few ten years in the dump, exposed to the air and rainwater, they have been weathered prior to the experiments. Despite that they still show leaching potential, particularly fine-grained coal muds; leaching yield 0.0003-0.0017% (wt.) for brine extracts and 0.0005 to 0.0029% (wt.) for water. There is a distinctive difference in composition between water and brine leachates. Water leachate n-alkanes show distributions similar to that in source coal waste, corresponding to mature kerogen type III. Biomarkers and PAHs are still present. The organic compounds in the brine show features of organic evaporates - a Gaussian n-alkanes outline in a short distribution, no heavier biomarkers, and only lighter PAHs occurring. These compounds were expelled from a kerogen macromolecule elsewhere within the dump and precipitated on colder fragments. They are not chemically bonded to organic matter and occur on the grains' surface; thus, the brine leached them, whereas the compounds present within the rock fragments were not removed.

Since leaching yields in the case of brine are significantly lower and less hazardous compounds such as n-alkanes are the main compounds removed, sodium chloride can also act as an anti-leaching agent as well as an anti-pyrogen.