Geoenvironmental characteristics of ferroniobium slag from the saint Lawrence columbium mine, Oka, Québec, Canada

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Niobium (Nb) is a critical metal usually hosted in pyrochlore and mined from carbonatite deposits. The aluminothermic reduction of pyrochlore concentrate yields ferroniobium (FeNb) alloy and a slag waste. This study presents results from mineralogical, geochemical, and radiochemical investigations of FeNb slag found at the former Saint Lawrence Columbium mine in Oka, Québec, Canada. Slag casts exhibit a vertical zonation resulting from fractional crystallization and differentiation in the melt. Sampling of the slag was designed to characterize resulting chemical and mineralogical variations within casts. The median bulk composition of samples (n = 14) is dominated by Al2O3 (47%) and CaO (16%) from process reagents. The slag also contains significant TiO2 (4%) and trace elements including Zr (6763 ppm), U (601 ppm), and Th (1371 ppm) from pyrochlore that partition preferentially from concentrate to the slag. Based on radiochemical analyses of daughter products and assuming secular equilibrium, median (n = 3) activity concentrations for U and Th decay series are 213 and 111 Bq/g, respectively. A field spectral radiometric survey of the slag disposal area found median (n = 37) equivalent U and Th concentrations of 1069 and 2101 ppm, respectively. The primary mineralogy of the top and middle of slag casts is dominated by hibonite, grossite, and perovskite. Most of the U and Th is hosted by perovskite. The bottom of slag casts is an assemblage of mainly gehlenite, ferri-gehlenite, hercynite, grossite, and anorthite. The casts also contain a (Na,F,Ce) calcium-aluminate glass decreasing in abundance from top to bottom, and a (Na,F,Ce,Ti,Zr) calcium-aluminosilicate glass in the cast bottoms. The (Na,F,Ce) calcium-aluminate glass, grossite, and hibonite are the phases most susceptible to weathering. Perovskite, gehlenite, and hercynite show little or no signs of alteration. A leach test was performed on three columns filled with material from the top, middle, and bottom of casts. In the field, a parallel leach experiment was conducted with three barrels filled with undifferentiated slag material and open to atmospheric conditions. Column leachate was found to be highly alkaline, with pH in the top column reaching 13.2. Median pH values in column and barrel leachate were 10.4 and 9.2, respectively. The maximum U concentration in column leachate was 13 μg/L whereas in barrel leachate it was 65 μg/L. Maximum Th concentrations in column and barrel leachates were 7.65 and 0.2 μg/L, respectively. Median Al concentrations in column and barrel leachates were 74.9 and 3.9 mg/L, respectively. 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 alkaline nature of its leachate.