GRAIN SIZE CHARACTERISTICS OF SILICA-RICH MINE WASTE FROM A KYANITE MINE IN DILLWYN, VIRGINIA

Mine waste can pollute the environment and occupy large areas of land, creating an economic burden. One approach to address this problem is to recycle mine waste for environmental applications. Critical to this is screening waste to assure that waste is environmentally benign and identifying economically viable uses. Several tens of tons of silica-rich mine waste is produced daily at a kyanite mine in central Virginia. Provided that the necessary grain characteristics are met, this waste could potentially be recycled as a medium for constructed wetlands in northern Virginia. Fifteen samples of waste were investigated to determine the grain characteristics of the material to assess if the material may be suitable for constructed wetland applications.

Grain size was found to be very consistent in both distribution and mode, and therefore permits further modeling of flow and geotechnical properties. Grain size typically varied from 1.00 mm to 0.075 mm. Modes of 0.425 mm and 0.250 mm were most common. Coefficient of uniformity values varied from 2.00 to 2.33 and a coefficient of uniformity value <4.0 is desired to prevent clogging in constructed wetlands. SEM investigation indicates the mine waste is dominated by quartz, muscovite, kyanite, and hematite. Grains were found to be highly angular with comparatively high surface areas. Kyanite often occurred as inclusions in muscovite grains. Hematite often occurred as platy growths on muscovite surfaces. No environmentally harmful minerals were detected, suggesting that the material may be safe for constructed wetland applications. Grain properties may be conducive to biofilm development, a necessary attribute of wetland material.

The state of Virginia prohibits any net loss of wetland acreage. Compensatory mitigation options available for unavoidable wetland impacts include wetland creation and restoration. Recycling the mine waste for in-state constructed wetlands is not only environmentally attractive, but it may enable lower costs of construction and generate additional revenues for central Virginia, a region in need of economic development.