Full Text (PDF) - Soil Science Society of America

More documents

Recommendations

Info

www.vadosezonejournal.org441Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.neutralize the acid generated from sulfide mineraloxidation. In either case, a cover system may be warrantedto control the fluid and chemical flux at thefacility discharge.The incorporation of sophisticated earthen cover designsinto heap facilities for the purposes of reducing theflux of oxidizing gases into a sulfide heap or waste-rockdump represents an interesting alternative to controllingsulfide mineral oxidation rates (Kim and Benson, 2004).This approach incorporates an earthen cover systemthat reduces the gas-phase flux by maintaining a highwater content in the cover, thereby reducing the airphasecontent and producing a corresponding reductionin the diffusion coefficient for oxygen as given in Eq.[29]. While the concept of reducing the rate of sulfidemineral oxidation to reduce the dissolution rate of metalsbearing minerals is well founded, the impact of thisdesign on the geochemistry of a sulfide bearing heapfacility is not well understood. The coupled relationshipsbetween oxygen concentration, sulfide mineral oxidationrate, redox- and pH-dependent As aqueous complexation,and water–rock sorption reactions producehighly nonlinear geochemical behavior. To demonstratethe efficacy of a gas-flux cover and the nonlinear geochemicalresponse within a heap facility to the installationof a gas-flux cover, this final example is presented.The gas-flux cover is simulated with an 80-cm-thicklayer of clay loam incorporated into the top of the 30-mprofile, while the fluid flux cover is inferred as in theprevious two examples by applying a steady-state rechargeflux of 50 mm yr 21 . The hydraulic and geochemicalproperties of the heap facility rock are identical tothe previous examples (Tables 5 and 6), and the Sips-typeisotherm is used to describe As sorption (Table 7). Theresults are shown in Fig. 4. The oxygen flux rate isdramatically reduced as compared with the previousexamples (Fig. 4g). The partial pressure of oxygen onlyapproaches atmospheric values in the cover, while withinthe profile, the oxidation of arsenian pyrite proceedsmuch faster than the oxygen flux rate, maintaining lowpartial pressures of oxygen within the profile. The reducedoxygen flux rate slows the arsenian pyrite oxidationrate such that the oxidation front has only reached20 m after 200 yr (Fig. 4b). The remaining mineralogythat is dependent on arsenian pyrite oxidation chemistryis similarly affected (Fig. 4a, 4c, and 4d).The reduced gas flux cover significantly reduces theflux of oxygen through the top of the simulated heapprofile such that reducing conditions are maintainedwithin the profile for the entire 200-yr simulation. As aresult, the initial reducing conditions and As(III) speciationare maintained throughout the profile (Fig. 4i),with the exception of the cover, wherein conditions aresuch that As occurs as As(V) (Fig. 4j). The cover preventsthe complete oxidation of the sulfide mineral masswithin the heap profile, and maintains redox conditionsFig. 5. Time series of aqueous As concentrations at the lower boundary discharge point for all examples and both isotherm types.
442 VADOSE ZONE J., VOL. 5, FEBRUARY 2006Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.such that As(III) speciation prevails for the duration ofthe simulation (Fig. 4k).SUMMARYThe three examples are intended to provide a range ofpossible behaviors that could be expected for As oxyaniontransport for the most likely heap facility closureoptions for a carbonate-rich, low-sulfide ore such asthose typical of the Carlin Trend operations. The Asbreakthrough curves at the bottom of the simulationprofile for the three examples are presented in Fig. 5. Thecumulative As mass released from the bottom of the profileis plotted in Fig. 6 for all three examples using theSips-type isotherm. Each of the three examples was runwith both the Sips-type and Keren-type isotherm formulations(Fig. 5). The differences between the isothermformulations are a result of the differences in the mathematicsof each and the overall fit to the experimental data(Decker et al., 2006). The sharp drop in As concentrationin Fig. 5 at 50 and 115 yr is a manifestation of theoxidation front arriving at the bottom of the simulatedheap profile, and As transitioning from As(III) to As(V).The arrival of the oxidation front is similarly the causeof the break in slope at the same times in Fig. 6. Thedifferences in release characteristics for As are a result ofthe coupled effects of variably saturated transport andwater–rock geochemistry.The uncovered example, with a fluid flux rate sixtimes higher than the other examples, produces thelargest cumulative As mass at the simulated outfalllocated at the bottom of the modeled domain. As a resultof the high fluid flux rate, aqueous As concentrationdrops rapidly as the sorbed As is rinsed from rock sorptionsites. The second example, with a simple fluid fluxcover, produces a proportionally smaller cumulative Asmass, and an overall higher As concentration at thebottom of the simulated heap profile. The reduced fluxrate results in a low volumetric water content and aproportionally larger air-phase volumetric content. Thisproduces a larger effective diffusion coefficient for oxygen,and a correspondingly faster sulfide mineral oxidationrate. The arrival of the oxidation front in thesecond example 65 yr earlier than the uncovered simulationis a direct result of modifying the rate of fluid fluxinto the simulated profile.The third example, with a fluid and gas flux cover,produces a larger cumulative As mass at the outfall thanthe simple, fluid-control cover example, despite the factthat both examples are simulated with the same infiltratingfluid flux. Because the gas flux cover prevents theoxidation of As(III) to As(V), and because As(III) issubstantially more mobile than As(V), the gas flux coverallows the faster release of As for a given fluid flux rate.A consequence of maintaining reducing conditions withinthe simulated profile is the persistent high concentrationof As at the outfall (Fig. 5).CONCLUSIONSAn existing variably saturated reactive transport codewas modified so that As transport in heap-leach facilitiescould be simulated. The code modifications included theincorporation of two pH-dependent isotherm formula-Fig. 6. Time series of cumulative aqueous As mass at the lower boundary discharge point for the three examples using the Sips-type isotherm.
Page 4 and 5: www.vadosezonejournal.org433Reprodu

Full Text (PDF) - Soil Science Society of America

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?