Acid and metalliferous release occurring when sulfide (principally pyrite)-containing rock from mining activities and from natural environments is exposed to the elements is acknowledged as a major environmental problem. Acid rock drainage (ARD) management is both challenging and costly for operating and legacy mine sites. Current technological solutions are expensive and focused on treating ARD on release rather than preventing it at source. We describe here a viable, practical mechanism for reduced ARD through the formation of silicate-stabilized iron oxyhydroxide surface layers. Without silicate, oxidized pyrite particles form an overlayer of crystalline goethite or lepidocrocite with porous structure. With silicate addition, a smooth, continuous, coherent and apparently amorphous iron oxyhydroxide surface layer is observed, with consequent pyrite dissolution rates reduced by more than 90% at neutral pH. Silicate is structurally incorporated within this layer and inhibits the phase transformation from amorphous iron (oxy)hydroxide to goethite, resulting in pyrite surface passivation. This is confirmed by computational simulation, suggesting that silicate-doping of a pseudoamorphous iron oxyhydroxide (ferrihydrite structure) is thermodynamically more stable than the equivalent undoped structure. This mechanism and its controlling factors are described. As a consequence of the greatly reduced acid generation rate, neutralization from on-site available reactive silicate minerals may be used to maintain neutral pH, after initial limestone addition to achieve neutral pH, thus maintaining the integrity of these layers for effective ARD management.

No abstract

No abstract

No abstract

Although the acid generating properties of pyrite (FeS) have been studied extensively, the impact of galvanic interaction on pyrite oxidation, and the implications for acid and metalliferous drainage, remain largely unexplored. The relative galvanic effects on pyrite dissolution were found to be consistent with relative sulfide mineral surface area ratios with sphalerite (ZnS) having greater negative impact in batch leach tests (sulfide minerals only, controlled pH) and galena (PbS) having greater negative impact in kinetic leach column tests (KLCs, uncontrolled pH, >85 wt% silicate minerals). In contrast the presence of pyrite resulted consistently in greater increase in galena than sphalerite leaching suggesting that increased anodic leaching is dependent on the difference in anodic and cathodic sulfide mineral rest potentials. Acidity increases occurred after 44, 20, and 12 weeks in the pyrite-galena, pyrite-sphalerite, and the pyrite containing KLCs. Thereafter acid generation rates were similar with the Eh consistently above the rest potential of pyrite (660 mV, SHE). This suggests that treatment of waste rocks or tailings, to establish and maintain low Eh conditions, may help to sustain protective galvanic interactions and that monitoring of Eh of leachates is potentially a useful indicator for predicting changes in acid generation behavior.

No abstract

No abstract

To better understand chalcopyrite dissolution in hydrogeochemical processes and the related environmental issue of acid and metalliferous drainage (AMD), the kinetics as well as the influence of solution composition and the nature of surface species formed during chalcopyrite dissolution have been examined under the controlled conditions of E h 650 mV (SHE), pH 1.0-2.0 and 75°C, with/without 4 mM Fe 2+ addition. SEM and XPS analyses indicate that the surface products, both at micro-and nano-scales, did not passivate dissolution under the conditions examined. Extensive S 0 was formed mostly as discrete particles rather than coatings on the chalcopyrite surface. Jarosite was only observed for dissolution at pH 2.0 with 4 mM added Fe 2+ . Without Fe 2+ addition, the initial dissolution rate was observed to be only correlated to H + activity as a 0:12ðAE0:01Þ H þ , indicating chalcopyrite dissolution was controlled via chemical oxidation of chalcopyrite by H + /O 2 . When reaction between chalcopyrite and Fe 3+ predominated chalcopyrite dissolution (i.e. later stage of dissolution without Fe 2+ addition), the dissolution rate was found to be positively correlated to the activities of Fe 3+ , as a 1:54ðAE0:07Þ Fe 3þ, and H + , as a 0:13ðAE0:05Þ H þ . When 4 mM Fe 2+ was added, no clear correlation was observed between the dissolution rate and the activities of either Fe 3+ or H + . It is proposed that the relative reactive surface area may not be proportional to that predicted by a shrinking sphere model as was assumed for derivation of the rate laws for the systems without added Fe 2+ , with the predicted rate being greater than the measured rate. Irrespective, it is clear that the addition of a relatively low Fe 2+ concentration plays an important role in accelerating the copper dissolution rate at this E h .