GEOENVIRONMENTAL CHARACTERIZATION OF MINE WASTES FROM CARBONATITE-HOSTED NIOBIUM DEPOSITS: FERRONIOBIUM SLAG

Niobium (Nb) is a critical metal mined principally from carbonatite deposits. It occurs in the mineral pyrochlore, which also hosts uranium (U) and thorium (Th). The aluminothermic reduction of pyrochlore concentrate yields ferroniobium (FeNb) as a commercial product and waste slag enriched in U, Th and other trace elements. This study presents results from mineralogical, geochemical, and radiological investigations of slag found at the former St-Lawrence Columbium mine in Oka, Quebec. Hibonite, grossite, perovskite and a Na-F bearing aluminate glass are the main phases present in the slag. The bulk composition of samples (n=3) is dominated by Al₂O₃ (50%), CaO (15%), TiO₂ (5%), and variable amounts of SiO₂ mainly from foundry sand. Significant trace elements include Ce (5%), Zr (1%), and variable amounts of unrecovered Nb. The median U and Th concentrations observed are 1188 and 1574 ppm, respectively. Median calculated specific activities for U and Th decay series are 213 and 111 Bq/g, respectively. Based on a field radiometric survey of the slag disposal area, median (n=37) concentrations of U and Th equivalents are 1069 and 2101 ppm, respectively. Corresponding median specific activities for U and Th decay series are 192 and 94 Bq/g, respectively, indicating a reasonable agreement with laboratory results. A field-based leach experiment was conducted over an 18-month period on three 220 L barrels filled with gravel to cobble-sized pieces of weathered slag and open to atmospheric conditions. The pH of the leachate varies between 8.8 and 9.4. Concentrations of most dissolved constituents, including Na, the dominant cation, rise initially, then decrease and level off before increasing again at the end of the experiment. This behavior is attributed to ongoing mechanical (freeze-thaw) weathering of the slag. Concentrations of Al vary between 2.5 and 8.5 mg/L while those of U range between 15 and 65 µg/L. Concentrations of Al and U increase concomitantly with F, which varies between 18 and 46 mg/L. Concentrations of Nb increase with pH and reach 230 µg/L. Slag from the production of FeNb is readily weathered and represents a technologically enhanced naturally occurring radioactive material (TENORM) requiring environmental management because of its radioactivity and the poor quality of its leachate.