Impact of freeze-thaw on the behaviour of flotation tailings from a rare earth deposit

Abstract

The Ashram Rare Earth and Fluorspar Deposit, located in northern Quebec (Canada), contains a potential carbonatite-type rock source of rare earth elements (REE). A pilot scale pre-concentration of REE minerals produced flotation tailings. A kinetic leach column study was performed to evaluate the behaviour of these tailings in a northern context (i.e. freeze-thaw cycles). The tailings were exposed to 12 cycles (over 1 year) of leaching with deionized water under two different conditions. The first column, CW (water column), was stored at 5 °C for the duration of the experiment, while the second column, CFT (freeze-thaw column), was subjected to freeze-thaw cycling. A comparison with Quebec environmental guidelines showed that the tailings were not acid generating but deemed leachable for Cd, Pb, Zn and Mn. Leachate analysis for the CW (water column) showed that the behaviour of Ba, Ca, Sr and Mn appeared similar. These elements could originate from the dissolution of calcite [CaCO3], dolomite [CaMg(CO3)2], dolomite-Fe [Ca(Mg,Fe2+)(CO3)2] or ankerite [Ca(Fe2+,Mg,Mn)(CO3)2]. Mg, F and Si could originate from the dissolution of biotite [K2(Mg,Fe2+)6-4(Fe3+,Al,Ti)0-2(Si6-5Al2-3O20)(OH,F)2]. The release of these elements would, however, appear to come from separate minerals. The presence of fluorine in the leachate could come from the dissolution of fluorite [CaF2], apatite [Ca5(PO4)3(O,H,F,Cl)] or bastnaesite [(La,Ce)CO3F]. Mg could come from the dissolution of carbonate minerals such as magnesite [MgCO3]. Despite high Fe content (98,620 mg/kg) in the initial flotation tailings, Fe was not present in the leachate, probably because of potential precipitation in the form of iron oxides/hydroxides. Despite being only slightly soluble (<0.001% mobilized), some heavy REE (HREE), Er, Yb, Lu and Sc, have a distinct leaching behaviour compared to other REE. Overall, the freeze-thaw cycles only decreased the mobility of some elements (e.g. Ca, Ba, Sr and Mn) and, thus, the potential for environmental contamination from the tested REE tailings.