Effects of Weathering on Leaching Properties of Slag from a Niobium Mine, Oka, Quebec

Abstract

Between 1961 and 1976 niobium ore was mined by the St. Lawrence Columbium Group in the community of Oka, located approximately 50 km W of Montreal, Quebec. Ferroniobium was formed through aluminothermic reduction of pyrochlore-group minerals. This process produced circular slag casts that were discarded in a small dump at the mine site. This work is designed to provide knowledge with respect to the long-term stability of Oka slag and the reaction processes that control drainage water chemistry in slag disposal areas. The slag casts were sampled at the top, middle, and bottom based on the expectation that fractional crystallization had influenced the mineralogy and geochemistry during the cooling process. The slag mineralogy is dominated by hibonite, β-alumina, perovskite, calcium zirconate and F-Na-bearing calcium aluminate glass. Secondary minerals occur in close proximity to aluminum-calcium rich phases (hibonite, perovskite, gehlenite) and they were investigated in detail at the nanoscale using (S)TEM analyses and the results indicate that the secondary coatings are composed of a number of discrete phases. Whole-rock geochemical characterization demonstrates that Oka slag contains significant quantities of REE, U and Th. Bench scale leaching experiments were conducted using columns filled with crushed slag from the top, middle and bottom of the casts to investigate the mobility of U, Th, and REE in water draining from the slag. In general, the column effluent is characterized by high pH (maximum 13) and elevated Al (maximum 4906 mg/L) and F (704 mg/L). In the top sample, weathering of alumino-silicates and Al-rich glass resulted in high pH and elevated F- concentrations, which rendered the slag from the top of the casts to be highly reactive, releasing high concentrations of Al, Ca, Na, and K and leading to the formation of mixed secondary amorphous Al/Ca hydroxide and barite (BaSO4). The leaching and secondary-mineral properties of slag from the middle of the casts were similar, although aqueous concentrations in the leachate were slightly lower and secondary-minerals included amorphous Si oxy-hydroxides. In general, leachate from the bottom of the slag casts displayed the lowest concentrations of major cations and the lowest reactivity. Secondary minerals were lesser abundant, but similar in composition to the top and middle of the casts except for the occurrence of Fe oxy-hydroxides resulting from weathering of ferri-gehlenite.