| The solution chemistry and statistical modelling of the O{dollar}sb2{dollar}-CaCl{dollar}sb2{dollar}-HCl leaching of a high-grade arseniferrous uranium ore (Key Lake, N. Saskatchewan) were studied with the objective of optimizing the co-extraction of uranium and radium. To this end, two types of experiments were carried out: (a) those designed to study arsenic speciation under simulated U-leaching conditions, with the use of model arsenide minerals, namely niccolite (NiAs), rammelsbergite (NiAs{dollar}sb2{dollar}) and gersdorffite (NiAsS); and (b) statistically designed tests involving actual leaching of samples of the Key Lake ore.; Niccolite and rammelsbergite were found to yield H{dollar}sb3{dollar}AsO{dollar}sb3{dollar} in solution as oppossed to gersdorffite, which was found to yield H{dollar}sb3{dollar}AsO{dollar}sb4{dollar}. H{dollar}sb3{dollar}AsO{dollar}sb3{dollar} was found to be metastable in the Fe{dollar}sb2{dollar}(SO{dollar}sb4)sb3{dollar} leaching system, while in the FeCl{dollar}sb3{dollar} and pressure (O{dollar}sb2{dollar}) leaching systems, further oxidation to H{dollar}sb3{dollar}AsO{dollar}sb4{dollar} was observed to take place at relatively slow rates. A mechanism has been advanced to account for the latter slow oxidation of H{dollar}sb3{dollar}AsO{dollar}sb3{dollar} to H{dollar}sb3{dollar}AsO{dollar}sb4{dollar}. Finally, it was concluded that the formation of As(III)/As(V) in oxidative acid leaching is independent of the prevailing suspension redox potential which, in turn, renders impractical the possible stabilization of As(III) via potential control.; The solubility of RaSO{dollar}sb4{dollar} in HCl-MeCl{dollar}sb{lcub}rm x{rcub}{dollar} leaching media was quantitatively established by employing the "reduced activity coefficient" method. The estimated solubility profiles clearly demonstrated the effectiveness of CaCl{dollar}sb2{dollar} in solubilizing radium as oppossed to other monochloride salt additions.; Finally, optimization of the co-extraction of U and Ra was attained with the use of a computer-aided response surface methodology. An incomplete three-level, four-variable factorial design (Box-Behnken) was applied in the region: 0.33 kg/L {dollar}leq{dollar} d{dollar}sb{lcub}rm s{rcub}{dollar} {dollar}leq{dollar} 0.99 kg/L; 0.5N {dollar}leq{dollar} (HCl) {dollar}sb{lcub}rm o{rcub}{dollar} {dollar}leq{dollar} 1.5 N; 1.5 M {dollar}leq{dollar} (CaCl{dollar}sb2rbracksb{lcub}rm o{rcub}{dollar} {dollar}leq{dollar} 3.5 M; and 2 hrs {dollar}leq{dollar} Time{dollar}leq{dollar} 6 hrs. Temperature and oxygen pressure were kept constant at 68{dollar}spcirc{dollar}C and 510 kPa (75 psi), respectively. Uranium and radium were found to dissolve simultaneously and rapidly (99% co-extraction in 2 hrs). Effective reduction of radioactivity levels from 10,000 pCi/g Ra{dollar}sp{lcub}226{rcub}{dollar} down to 160 pCi/g has been achieved in a single-stage batch process, thus making possible the production of environmentally innocuous tailings. |
