Coal Refuse. Coarse coal refuse from mining operations is produced at a rate of 120 million tons (109 X 109 kg) per year. Coarse material is banked, while fine coal refuse is put into a silt-sized slurry mix and placed in impoundments. It is estimated that up to 4 billion tons (3.6 X 1012 kg) of coal mining refuse has accumulated in the United States. Concern about spontaneous combustion and leachate of the material (composed of slate and shale with sandstone and clay mixed in) has impeded in-depth studies of the use of coal waste. It is currently being evaluated for use in embankments and as subbase material, two applications that reportedly have been used in the past.
Quarry Wastes. Fairly consistent wastes consisting of fines from stone washing, crushing, and screening and wet, silty clay from washing of sand and gravel are produced from quarrying operations. Most quarry waste is not reusable or sized within standard specifications, are stockpiled in ponds. Reclamation through dewatering and segregating coarse and fine materials would be necessary to use the 175 million tons (159 X 109 kg) of quarry waste produced each year, or any of the approximately 4 billion tons (3.6 X 1012 kg) that have accumulated in the United States. The mineral properties and characteristics of the waste differ from quarry to quarry, limiting the beneficial end use, but quarry wastes have been used as fill and borrow material, flowable fill, and cement-treated subbase.
Mill Tailings. Mill tailings are the remains left after processing ore to concentrate it. Large amounts of mill tailing are generated from copper, iron, lead, zinc, and uranium ores.
They have been used as fill materials, in base courses, and in asphalt mixtures for years in areas where they are abundant and conventional sources are limited. Because of the metal content in the mill tailings, the stockpiles must be carefully analyzed to characterize leachate properties before use would be is deemed appropriate.
Waste Rock. Surface mining operations and subsurface mining operations produce an estimated 1 billion ton of waste rock annually in the United States. Some have been used as construction aggregate and in embankments; however, transportation costs from remote mines to construction areas often render the use of the rock economically infeasible. Where transportation is reasonable, waste rock can be used as stone fill for embankments or as riprap, or crushed for aggregate. These uses have been shown to be successful. Environmental considerations of leachate, low-level radiation, and sulfuric acid content should be investigated before use is deemed appropriate.
