soil covers for controlling acid generation in mine tailings: a laboratory evaluation of the physics...

SOIL COVERS FOR CONTROLLING ACID GENERATION IN MINETAILINGS: A LABORATORY EVALUATION OF THE PHYSICS AND

GEOCHEMISTRY

ERNEST K. YANFUL1∗, PAUL H. SIMMS1 and SERGE C. PAYANT21 Geotechnical Research Centre, Department of Civil and Environmental Engineering, The

University of Western Ontario, London, Ontario, N6A 5B9 Canada;2 Noranda Technology Centre,240 Hymus Boulevard, Pointe-Claire, Québec, H9R 1G5 Canada

(∗ author for correspondence, e-mail: [email protected])

(Received 11 November 1997; accepted 24 September 1998)

Abstract. To evaluate the effectiveness of soil covers, column experiments were conducted ontailings protected by a three-layer soil cover and tailings directly exposed in the open laboratoryfor a period of 760 days. Periodic rain application was performed to simulate field conditions, andat four times during the experiments the pore water was completely flushed out of each column foranalysis. Profiles of oxygen, temperature, and volumetric water content were measured throughoutthe experiment, and the post-testing pore water quality was also characterized. A one-dimensionalsemi-analytic diffusion model was used to simulate oxygen profiles in the uncovered tailings. Mod-elling performed using the geochemical code MINTEQ showed that in the laboratory, aluminiumconcentrations in the tailings pore water were controlled by Al(OH)SO4, sulphate by gypsum andAl(OH)SO4 and iron by lepidocrocite in the upper half and by ferrihydrite in the lower half. In thefield, however, the iron oxyhydroxide minerals formed in the oxidized zone appear to be dissolving.It was found that the cover was effective in preventing significant desaturation of the clay, evenover a 150-day drying period. The covered tailings did not oxidize much during the experiments. Inthe uncovered tailings, oxygen modelling and examination of the geochemistry show that the rateof gross oxidation and the advancement of the oxidation front decreases with time. However, porewater quality is controlled by geochemical processes other than oxidation, as reported in the field.

Keywords: diffusion coefficient, gaseous oxygen, mine tailings, oxidation, water content

1. Introduction

Acid generation from reactive sulphide-bearing mine tailings is a major environ-mental problem facing the mining industry. There are more than 200 million tonnesof potentially acid generating tailings covering more than 15 000 hectares of landacross Canada (Wheeland and Feasby, 1991). Several field studies have shown thatreactive tailings have contributed to the degradation of water quality in both surfaceand subsurface environments (Boorman and Watson, 1976; Blowes and Jambor,1990; Yanful and St-Arnaud, 1992). Canada has tackled the acid drainage problemthrough a joint industry-government research consortium known as MEND (MineEnvironment Neutral Drainage) program. Since the inception of the MEND pro-gram in 1988, there has been a resurgence of interest in the use of engineered soil

Water, Air, and Soil Pollution114: 347–375, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

348 E. K. YANFUL ET AL.

covers for controlling acid generation in reactive tailings. In 1989 MEND initiateda project to develop a laboratory methodology for evaluating the effectiveness ofsoil covers for mitigating acid drainage in reactive mine waste. The project wasfollowed by the establishment of test plots at the decommissioned Waite Amulettailings site, near Rouyn-Noranda, Québec, Canada, to evaluate the field perfor-mance of soil covers. The field study was supplemented with a set of laboratoryexperiments designed to investigate the effectiveness of a three-layer soil coverin reducing acid generation in the Waite Amulet tailings. The cover comprised acompacted, nearly saturated clay placed between two capillary barriers to minimizegravity drainage and surface evaporation (Yanfulet al., 1994).

The purpose of this paper is to present, analyze and discuss results obtainedfrom the laboratory investigation of the three-layer soil cover. The use of watercontent and oxygen data to evaluate cover performance is emphasized. Oxygentransport and consumption in the uncovered tailings are simulated using a one-dimensional diffusion model. Finally, the geochemistry of waters draining the cov-ered and uncovered tailings is discussed and compared with field geochemistry. Adetailed description of the experimental set up and the philosophy behind the coverdesign is presented by Yanful (1993) and is only briefly summarized here.

2. Materials and Methods

2.1. COLUMN EXPERIMENTS

The laboratory experiments were conducted in six Plexiglas square columns, eachof which had an area of 28 cm2 and a length of 105 cm. The top 15 cm of eachcolumn had a larger area (39 cm2) and was provided with perimeter holes for col-lecting runoff. Four of the columns (the Tests) were filled with 45 cm of compactedunoxidized tailings and overlain sequentially with 15 cm of coarse sand, 30 cmof compacted clay and 15 cm of an uppermost fine sand layer. The tailings wereobtained from the saturated zone of the Waite Amulet tailings impoundment. Thetailings consist of reactive sulphides, namely pyrite (20% by weight), pyrrhotite(6% by weight), and small amounts of chalcopyrite, sphalerite, and galena andabout 70% gangue minerals (quartz, plagioclase, chlorite, hornblende and mica).

Two Control columns (without soil cover) were packed with 90 cm of unox-idized tailings to serve as a reference for the Test columns (with soil cover). Allcolumns were instrumented to collect drainage water and also measure gaseousoxygen concentration, volumetric water content and temperature. Gaseous oxygenwas measured with a percent oxygen analyzer (Teledyne Model 340 FBS) thatrequired a sample volume of 2 mL and registered a stable reading within 20 s. Watercontent was measured by time domain reflectometry (Toppet al., 1980). Two newTest columns were added to the experiments later (approximately 200 days afterthe initial set up) when it was discovered that atmospheric oxygen had by-passed

SOIL COVERS FOR CONTROLLING ACID GENERATION IN MINE TAILINGS 349

Figure 1.Photographs of two Test columns with soil cover (left) and two Control columns with cover(right).

the cover and entered the tailings through corners, instrumentation holes and gassampling ports. Additional entry of oxygen through these avenues could lead tooxidation and give unreliable assessment of cover performance. It was observedthat the high ionic strength of oxidized tailings pore water in the Control columnsadversely affected TDR waves and prevented reliable analysis. The new columnswere, therefore, packed more rigorously to minimize tailings pre-oxidation andwere also provided with fewer sampling ports and instrumentation holes (Yanfuland Payant, 1991; Yanfulet al., 1994). Elimination of these test artefacts gavemore reliable water content data from which cover performance could be assessed.Figure 1 presents a photograph of the Control and Test columns.

Water was added to the packed columns at a pre-determined rate to simulaterainfall and snow melt. Plastic vessels connected to the base of the columns wereused to collect water draining through the columns. The Test (covered tailings)columns did not produce any drainage water as the clay layer essentially preventedwater percolation. To allow a comparison of the drainage water quality from thecovered and uncovered tailings, the covered tailings were intentionally washed orflushed to remove any accumulated oxidation products and to provide drainagewater for physico-chemical analysis. To achieve this, plastic tubing was connectedto the covered tailings through a hole drilled in the side of each Test column. Deion-ized distilled water was then supplied to the tailings by means of the plastic tubing.


Both the covered and uncovered tailings were flushed four times in the course ofthe experiments and the drainage water recovered was analyzed for acidity, pHand metals. Acidity was measured by titration to a pH of 8.3 (Standard Methods,1975). Metals were analyzed by inductively coupled plasma spectroscopy, and pHwas measured using a combination electrode and appropriate buffers. Oxidationproducts accumulated in each column were considered completely removed orflushed when the measured acidity was less than 0.1 g L−1 CaCO3. At the endof testing (760 days), both covered and uncovered tailings were sampled and ana-lyzed for density, pore water quality (pH, sulphate and metal concentrations), andbacteria population. Details of the measurement are presented elsewhere (Yanfuland Payant, 1991; Yanful, 1993; Yanfulet al., 1994).

2.2. MODELLING OF OXYGEN TRANSPORT AND CONSUMPTION IN THE

CONTROL COLUMNS

Further analysis of the oxygen data from the Control Columns was performed usingone-dimensional diffusion theory. The computer program POLLUTE (Roweet al.,1994) was employed. POLLUTE can model pollutant transport in porous mediaunder different boundary conditions. It uses a semi-analytic solution to the equationfor one-dimensional diffusive transport with species removal:

∂C

∂t= De

∂2C

∂z2−KC (1)

whereDe is the effective diffusion coefficient for gaseous oxygen diffusion, whichin a partially saturated medium is a function of volumetric water content.KC

represents the mass of oxygen removed per unit volume per unit time, whereC

is the concentration, andK is a first order reaction rate constant. Oxygen transportin the upper 50 cm of the tailings was modelled using the following boundaryconditions:

C(z = 0, t) = C0 (2)

C(z = ∞, t) = 0 (3)

The use of this program implicitly ignores intraparticle diffusion, that is, diffusionwithin a tailings particle. Other models, such as the Shrinking Core Model usedby Davis and Ritchie (1986) assumes that oxygen transport by vertical and intra-particle diffusion is the rate limiting step for pyrite oxidation in a sulphide wasteimpoundment. This model was, however, developed to handle oxidation in wasterock, which has considerably larger particles than tailings. Intraparticle diffusionmay not be significant in tailings as the particle size is much smaller.

As oxidation progresses in a tailings impoundment, the tailings near the surfacebecome completely oxidized, and no longer consume oxygen. With time, this zone


extends down into the tailings. The depth above which no oxygen consumptionoccurs is termed the oxidation front. In our modelling, the oxidation front is definedby a depth above whichK is set to zero.K is held constant in the remaining tailingsprofile.

Using TDR, the volumetric water content in the columns was measured dailyat depths of 6, 19, 36, and 50 cm throughout the experiment. As a result of theflushing and subsequent drainage, dissolved ions and salts accumulated in the lowerpart of the tailings and produced erroneous TDR readings in this zone, as alreadynoted. However, the effect of these errors on the diffusion modelling was foundto be negligible, since significant amounts of oxygen were observed only in thetop 30 cm of the column. We used empirical equations for gas diffusion proposedby Millington and Shearer (1971) to calculate oxygen diffusion coefficients fromwater contents measured at 6, 19 and 36 cm.

Mineralogical analysis indicates that the predominant sulphide mineral presentin the unoxidized tailings used for the experiments is pyrite [FeS2] (Yanful et al.,1994). Pyrrhotite is also present but in a smaller amount (6% compared to 20% forpyrite) and would oxidize much more rapidly. Pyrite oxidation would therefore ac-count for the consumption of oxygen over the long-term. The following equationsdescribe pyrite oxidation (Singer and Stumm, 1970):

FeS2 + 7

2O2 + H2O→ Fe2+ + 2SO2−

4 + 2H+ (4)

Fe2+ + 1

4O2 + H+ → Fe3+ + 1

2H2O (5)

Fe3+ + 3H2O→ Fe(OH)3 + 3H+ (6)

FeS2 + 7Fe2(SO4)3 + 8H2O→ 15FeSO4 + 8H2SO4 (7)

Equation (4) describes the direct oxidation of pyrite by oxygen, while Equations (5)and (6) describe the oxidation of ferrous to ferric iron and the subsequent precipita-tion of ferric iron as ferric hydroxide. Equation (7) describes the oxidation of pyriteby ferric iron. Ferric iron has been shown to oxidize pyrite at a much faster ratethan oxygen. Recent research by Moses and Herman (1991) suggests that the roleof oxygen is primarily to regenerate ferric iron by oxidizing ferrous iron producedin Equation (4), at neutral and at low pH.

The rate of oxidation is dependent on several factors, including oxygen supply,temperature, pH, bacterial activity, and surface area of both the tailings particlesand pyrite grains. While the first three factors influence the rate of reaction, thesurface area determines how much reactive mineral is exposed to oxygen andtherefore available for oxidation. Initially, at neutral pH, the rate of the reactionis limited by the chemical oxidation of Fe(II) to Fe(III) by oxygen, which is slow.However, at lower pH, microbes such asThiobacillus ferrooxidanscan catalyze this


TABLE I

Parameters used in POLLUTE modelling of oxygen transport

Depth at which Modelled Oxygen diffusion coefficient

water content diffusion (m2 s−1)

was measured layer 0–100 days 100–211 days 450–602 days

0.06 m 0.00–0.10 m 0.285 0.285 0.285

0.19 m 0.10–0.25 m 0.0864 0.121 0.176

0.36 m 0.25–0.50 m 0.025 0.049 0.064

reaction, and may also directly attack pyrite. The optimal environment for bioticpyrite oxidation is low pH and high temperature. Biotic rates in this range are re-ported to be from 20 to 100 times abiotic rates under similar conditions (Nicholson,1994).

Numerous authors (Mcibben and Barnes, 1986; Nicholsonet al., 1988; Mosesand Herman, 1991; Moseset al., 1987) have shown that rates of pyrite consumptionare proportional to surface area and have reported rates normalized with respect toa specific surface area. The specific surface area,Ss , is defined as the surface areaper unit mass of pyrite (sphere of diameterd and densityρ):

Ss = 6

ρd(8)

The rate of pyrite consumption also varies with oxygen concentration. Experimen-tal results (Nicholsonet al., 1988) suggest that the abiotic reaction rate approacheszero-order behaviour with respect to oxygen as concentration increases. Severalauthors (Moses and Herman 1991; Luther, 1987) explain this behaviour by notingthat the pyrite surface becomes ‘saturated’ with oxidants, possibly oxygen andferric iron. Thus, while the supply of oxidants determines the reaction rate for‘unsaturated pyrite’, it does not for a ‘saturated’ reaction surface. Therefore, manyempirically derived expressions model the dependence of reaction rate on oxygenas somewhere between zero and first orders.

In our analysis, we assumed a linear relationship between oxygen and reactionrate, that is, the oxygen consumed at a point was proportional to the concentrationof oxygen at that point. The constant of proportionality was the first order reactionrate constantK.

Oxygen diffusion in the control columns was modelled during four drying cy-cles: days 0–137, 137–267, 267–490 and 490–760. The beginning of the modelledperiod was set at the time when the measured oxygen concentration in the upperpart of the tailings dropped to near zero, following flushing with water. Figure 2illustrates the idealization of the laboratory column in the model. Layers 1 through


Figure 2.Schematic of conceptual Control (tailings) columns used in modelling.

3 are each considered to have a uniform water content and hence diffusion coef-ficient. Diffusion coefficients were averaged with time for each drying cycle (seeTable I). The position of the oxidation front is assumed to be constant for eachmodelled period. The position of the oxidation front in the last time period couldbe estimated from geochemical analyses of the column performed at the comple-tion of the experiment. The modelling therefore involved estimatingK, which wasassumed to be constant for all time periods, and the average depth of the oxidationfront for each period.


Figure 3.Volumetric water contents measured directly in soil cover and tailings (Test column #1).

3. Results

3.1. WATER CONTENT

Figures 3 and 4 present the volumetric water content in the two test columnsmeasured using TDR (time domain reflectometry). Since the data from the TestColumns are similar, only the Test #1 data are discussed in detail. The water contentof the uppermost fine sand layer (depth of 7 cm) was highly dependent on waterapplication or rainfall events. The peaks shown in Figure 3 represent water contentsobserved at times when water was applied to the cover. The water content of theclay remained constant at near saturation (≥95%) throughout the experiments. Thecoarse sand below the clay layer stayed dry except when the columns were flushedto measure acidity (peaks near 90, 110 and 240 days). The water content data arealso summarized in Figure 5 at four different times during the experiments (t =0, 51, 112 and 465 days). The coarse sand was packed at a water content corre-sponding to its residual saturation of about∼5%, which was close to the averagewater content observed during dry periods. Application of a higher suction thanthat corresponding to residual saturation (hr ) in the coarse sand would produce


Figure 4.Volumetric water contents measured directly in soil cover and tailings (Test column #2).

little or no change in water content. The role of hr in the performance of the claylayer and, therefore, the entire cover system will be discussed later.

The water content of the tailings below the coarse sand varied from 30 to 38%until the first column flush at 90 days when the tailings became saturated andremained so to the end of the experiments. Under field conditions, saturation ofcovered tailings may occur following heavy rainfall and snow melt, but may notpersist for long periods because of drainage. The observed high and nearly constantwater content of the clay layer suggests that the upper fine sand performed well asan evaporation barrier and prevented the clay from losing water.

The accuracy of the TDR data from the Test Columns (Figures 3 and 5) was ex-amined by comparing them to gravimetric water contents obtained from weighingsoil samples before and after oven drying at 105◦C for 24 hr. Mass-based watercontent,w, was converted to volumetric water content,θ by assuming a waterdensity of 1 Mg m−3 and by using the following equation:

θ = ρdw (9)

whereρd is the density of the material. The dry density of each material wasobtained from the bulk density measured during packing.


Figure 5.Typical average volumetric water content versus depth in Test column.

After a long dry period (150 days), water content was measured at differentdepths using TDR; holes were then drilled through the column to sample the coversoils and underlying tailings. Gravimetric water content was measured on the re-covered samples at 23 locations along the length of the column. Water was thenapplied to the column to simulate rain and water content was immediately mea-sured by both TDR and gravimetry. Figure 6 compares volumetric water contentsbefore and after rain application. As expected, the upper sand was very sensitiveto rain application with both TDR and gravimetric water contents increasing by as


Figure 6. Comparison of water contents obtained by TDR (time domain reflectometry) andgravimetry before and after rain application.

much as 90%, following rain. The data also confirm that the clay layer essentiallyretained its water, even over a long dry period (150 days). The observed waterlosses were mainly due to capillary rise in the fine sand and evaporation from thesurface. The small differences in water contents, before and after rain, observedin the clay, coarse sand and tailings are probably within acceptable experimentalerrors. The results also indicate the TDR method is sensitive enough to detectchanges in water content following evaporation or rainfall.

Figure 6 also shows that TDR water contents were generally higher than thoseobtained gravimetrically, except in the clay. The higher gravimetric water contentobserved in the clay could be due to the release of adsorbed water during oven dry-ing at 106◦C. Adsorbed water tends to be rigidly bonded to clay particle surfacesand may not be measurable by TDR.

The higher TDR water contents in the tailings can be attributed to an increasein ionic strength of the tailings pore water following prolonged drying. TDR wavesobtained in oxidized tailings with highly acidic drainage water were poorly definedand difficult to interpret. Subsequent removal of accumulated oxidation productsthrough flushing yielded much better TDR waves and hence more reliable watercontents.

TDR and gravimetric water content data are compared further in Table II, whichindicate that, with the exception of the clay, TDR water contents were about 0.5 to


TABLE II

Water contents measured by TDR and gravimetry

Material type Dry Volumetric water content Difference

density by TDRa by gravimetry

(mg m−3) (%) (%) (p-value = 0.05)

Fine sand 1.35 1.9 1.4 –0.5

21.6 19.3 –2.3

Clay 1.39 42.5 47.4 4.9

44.0 45.3 1.3

43.1 48.7 5.6

43.3 47.8 4.5

Coarse sand 1.86 4.5 2.8 –1.7

6.6 2.6 –4.0

Tailings 1.65 45.5 38.4 –7.1

49.1 34.2 –14.9

48.4 40.9 –7.5

49.7 40.3 –9.4

a TDR = Time Domain Reflectometry.

15% higher than those obtained by gravimetry at the 5% level of significance. Inthe clay layer, however, gravimetric water contents exceeded TDR values by 1.3 to5.6%, which represents an overall maximum agreement of 11%.

3.2. TEMPERATURE

Pyrite (FeS2) oxidation is an exothermic reaction that produces about 1440 kcalof heat for each mole of pyrite oxidized. The temperature of both the covered anduncovered tailings during the first 240 days was, therefore monitored to discern ifpyrite oxidation would produce enough heat to warm the columns. It was foundthat temperatures in the cover and underlying tailings averaged 24◦C and weresimilar to the laboratory temperature. Evaporation of water from the surface of thefine sand, following cessation of water application, was found to produce coolingthat reflected in about 2◦C drop in temperature, as illustrated in Figure 7 for twoportions of the monitoring period. Cooling resulting from evaporation is confirmedby the water content data for the fine sand, shown in the lower part of Figures 3 and4. Cooling or decrease in temperature around days 23–28 and 210–211 (Figure 7)corresponds to a decrease in water content for the same period (Figures 3 and


Figure 7.Typical temperature in Test column.

4). This cooling is due to the energy consumption for latent heat of vaporization(Wilson, 1990; Yanful and Choo, 1997).

Figure 8 presents temperature profiles in the Control (uncovered tailings) Col-umn for the first 80 days of the experiments. The data do not show any appreciabletemperature gradient that can be attributed to heat production from pyrite oxidationof sulphide minerals present in the tailings. The temperature of the top 30 cm ofthe tailings (the zone of active oxidation) was found to be similar to the laboratorytemperature. As was the case for the soil cover, evaporation appeared to havecooled the surface of the tailings, as indicated by the temperature data from 20to 30 days (Figure 8). We have no obvious explanation for the lower temperatureobserved at the bottom of the tailings around the 60th and 70th days.

3.3. OXYGEN

Gaseous oxygen concentrations obtained at different depths in the covered tailingsare presented in Figure 9. Apart from the small dip around the 10th day, oxygenconcentrations in the fine sand layer (7 cm depth) were generally atmospheric (20–21%) during the experiments. The dip around the 10th day reflects slightly lowoxygen concentrations (∼18%) measured at the initial stages of the experimentswhen the columns were heavily rained, as confirmed by the very high water content(∼28%) observed in the fine sand around the same period (Figure 3). Oxygen con-

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Figure 8.Typical temperature in Control column.


Figure 9.Gaseous oxygen concentrations in Test column #1.

centrations in the clay at the two depths monitored show the same trends. Oxygenpeaks coincide with the final stages of the drying cycles, as shown on the watercontent graph for the fine sand. The lowest oxygen concentrations in the clay wereobserved shortly after each rainfall event. In addition, the oxygen concentration at21 cm was higher than that at 31 cm, suggesting a downward transport of oxygeninto the clay. In fact, it can be shown from a simple one-dimensional diffusionmodelling that the build up of oxygen concentration during each drying cyclereflects gradual diffusion of gaseous oxygen through the available pores in thenearly saturated clay. Diffusion will continue until either steady state conditionsare attained or the next rain is applied (Yanful, 1993).

3.4. PERFORMANCE OF THE COVER SYSTEM

The cover system was designed to minimize the availability of oxygen and waterto the tailings. One way of implementing an effective oxygen and water barrier isto place a layer of fine-grained soil, such as clay, between two coarse-grained soils.The coarse grained soils have a much higher tendency to lose water than the fine-


Figure 10.Soil water characteristic curves of cover soils and mine tailings showing AEV (air entryvalue) and residual moisture content.

grained soil, as the large pore sizes generate comparatively small capillary forcescompared to those in the fine grained soil. When the water table is low, the bottomcoarse-grained layer will drain quickly, giving it a very low water content and avery low hydraulic conductivity. As the bottom coarse-grained soil, called a cap-illary barrier, is unable to transmit significant quantities of water, very little waterwill drain from the clay. Similarly, during evaporation, the hydraulic conductivityof the top layer will drop, allowing little water to drain from the clay (Simms andYanful, 1997).

The above phenomena can be further illustrated with the Soil Water Characteris-tic Curves (SWCC) of the soils used in the Test columns (Figure 10). These curvesdefine the relationship between suction, or negative pressure, and volumetric watercontent. Two important characteristics of a SWCC are the air-entry value (AEV)and the residual water content (θr ) of a soil. The AEV is the suction at which thesoil-pores begin to desaturate; it is relatively low for coarse-grained soils such assands. The hydraulic conductivity of a soil decreases significantly past its AEV.The residual water content is the water content past where the slope of the SWCCis nearly flat, where large increases in suction are required to drain more waterfrom the soil. In coarse-grained soils this water content represents the point wherethe water phase in the soil becomes discontinuous, and more water can only be


Figure 11.Modelled and measured oxygen profiles in Control columns.

removed from the soil in the form of vapour. The AEV and residual water contentof the coarse sand are indicated in Figure 10.

As mentioned previously, the water content of the coarse sand did not decreasebelowθr . The water content of the fine sand, however, deceased beyond itsθr , to ap-proximately 2%. Under evaporative conditions, this water content would probablycorrespond to the suction where the relative humidity of the pore-space equals therelative humidity of the laboratory environment. Lower suctions and hence lowerwater contents are not possible, since the existence of a lower relative humidity inthe soil pores than in the atmosphere would imply vapour transport into the soil.Wilson et al. (1997) have discussed this phenomenon in more detail.

3.5. MODELLED OXYGEN PROFILES

Oxygen concentrations were also measured in the Control columns (uncoveredtailings) at 100, 211, 323, and 602 days after the start of the experiment, at depthsof 7, 19, 31 cm (Cycles 1–4 in Figure 11). Oxygen concentrations measured atdeeper locations were close to the detection limit of 0.1% for the oxygen analyzer.The columns were flushed with water at 200, 260, and 450 days; at these times,the tailings were saturated with water so that the concentration of oxygen at everydepth was essentially zero. We obtained the best fits to experimental data usinga reaction rate constantK of 50 per day for the unoxidized tailings and average


oxidation fronts at depths of 0, 0.1, 0.2, and 0.3 m for each drying cycle. However,in order to better compare measured acid flux with acidity calculated from oxygenflux using the stoichiometric relationship in Equation (4), we remodelled the firstdrying cycle from the time the alkalinity in the tailings was used up (Day 60).Setting the initial oxygen concentration profile to that observed on day 60 and em-ploying an average oxidation front depth of 5 cm yielded a good fit. The modellingresults are shown in Figure 11. The kinks in the predicted oxygen concentrationprofiles are caused by the change in diffusion coefficient between layers. The cur-vature of the profiles below the oxidation front is a function of the rate of oxygenconsumption. Models that assume an instantaneous reaction between oxygen andpyrite would predict a zero percent oxygen concentration below the oxidation front.The model predictions could be improved if the variation of water content in thetailings with depth and time was more precisely known, which would allow for abetter discretization of the control columns.

Using Equation (4) and an average abiotic reaction rate of 5×10−10 moles ofpyrite m−2 s−1 (Nicholson, 1994), one can calculate a first order rate for oxygenconcentrations in the range of 0 to 20%. This reaction rate was obtained fromexperiments with grains of similar surface area and in oxygen-saturated conditions.In the present study, the calculated rate constant is 0.001 per day, which is 5000times slower than the model prediction of 50 per day. We attribute this differenceto the following:

1. The above reported rates were for abiotic and neutral pH conditions; iron-oxidizing bacteria are known to be present in the tailings (Davéet al., 1986)and pore water samples showed very low pH and high concentrations of ferriciron. Under these conditions, pyrite oxidation may take place up to a hundredtimes faster than reported abiotic rates.

2. Oxygen is consumed by the oxidation of pyrrhotite as well as pyrite; the rate ofpyrrhotite oxidation has a reaction rate up to two orders of magnitude higherthan that of pyrite. The measured oxygen profiles on which the model predic-tion was based represents the effect of both pyrite and pyrrhotite oxidation.

3. Coating of tailings particles with oxidation products may lower the effectivediffusion rate of oxygen downward through the tailings.

3.6. MEASURED AND PREDICTED ACIDITIES

Acidity generated in the tailings during each time period was measured and com-pared with acidity calculated from model predictions of oxygen consumption. Theaverage daily acidity flux was determined by measuring total acidity in the drainagewater obtained by flushing out the tailings at the end of each drying cycle. Oxygenflux predicted by POLLUTE was converted to equivalent acid (H2SO4) flux usingEquation (4). These data are presented in Figure 12.


Figure 12.Measured and predicted acid generation rates in Control columns: Predicted values arederived from modelling of gaseous oxygen.

Both the measured and predicted acidities showed a downward trend with time;the measured acidity, however, decreased at a slower rate than the predicted acidity.Predicted acidity was higher than measured acidity during the first two cycles.Buffering, mineral dissolution and metal precipitation reactions had greater con-trol in the experimental columns during these initial cycles. In later cycles thealkalinity would have been exhausted, resulting in decreased pH values and dis-solution of secondary minerals that had precipitated during the first two cycles,which would influence the measured acidity. Predicted acidity was based only onoxygen consumption and did not consider other geochemical reactions.

3.7. COMPARISON OF LABORATORY AND FIELD GEOCHEMISTRY AND

MINERALOGY

Using pore water data from the final drying period, the saturation indices (SI)of various minerals were calculated using the geochemical equilibrium modelMINTEQ (Felmy et al., 1984). The variation of SI with depth is shown in Fig-ure 13. The saturation indices of all the iron hydroxide minerals increase signif-icantly below 30 cm, which is the approximate position of the oxidation frontdetermined in the oxygen diffusion modelling. Geothite and lepidocrocite are su-persaturated throughout the profile, while ferrihydrite is undersaturated in the top30 cm. These results indicate extensive weathering and oxidation of tailings in


Figure 13.Profiles of saturation index for various secondary minerals (Present Laboratory Study).

the top 30 cm. Oxidation products of iron appear to have accumulated more inthe lower half of the tailings profile resulting in supersaturation of the pore waterwith respect to iron oxyhydroxide minerals. The saturation index of Al(OH)SO4

increases slightly at 30 cm, but subsequently decreases again. Gypsum is slightlysupersaturated at all depths with SI values of approximately 0.2 throughout theprofile. These results suggest aluminium concentrations in the tailings pore waterwere controlled by Al(OH)SO4, sulphate by gypsum and Al(OH)SO4 and iron bylepidocrocite in the upper half and by ferrihydrite in the lower half.

Saturation indices for the above minerals were also calculated by Blowes andJambor (1990) using unsaturated zone pore water data from the Waite Amuletsite, which had similar tailings to those used in our study. A comparison of oursaturation indices to those reported by Blowes and Jambor (1990) indicated the SIvalues follow the same trends with depth (Figure 14), for the minerals mentionedabove. The SI values for goethite and lepidocrocite calculated from the field datawere, however, found to be negative above the water table, and near zero below. Alower saturation index indicates a higher tendency for a mineral to dissolve. Themineral may exist and be dissolving, or may no longer exist. However, the presenceof geothite and lepidocrocite in the oxidized zone is confirmed by minerologicalexamination of field samples (Blowes and Jambor, 1990). The laboratory exper-iments simulated the geochemical environment in the early life of an oxidizing


Figure 14.Profiles of saturation index for various secondary minerals at the Waite Amulet tailingssite near Rouyn-Noranda, Quebec, Canada (Adapted from Blowes and Jambor, 1990).

tailings impoundment when secondary minerals were forming. With time, as pHrises and rain and groundwater flush the tailings and reduce metal concentrations,the secondary minerals would start dissolving.

Figures 15 and 16 show profiles of various chemical species in the uncoveredtailings pore water. The concentrations of Fe, SO4, mg, and Al show a substantialincrease at 25 cm, and then decrease with depth to the base of the tailings. ThepH is quite low (<2) in the 5–25 cm zone. The 25-30 cm depth is the zone ofactive oxidation, which would explain the rise in iron and sulphate concentrations.Dissolution of chlorite by infiltrating acid water may explain the increase in Mg andAl concentrations. X-ray diffraction analysis performed at the end of the columnexperiments gave results that seemed to suggest a decrease in the 0.7 and 1.4 nmchlorite peaks at depths of 5 to 25 cm followed by an increase at greater depths(Figure 17). The small number of sampling points, however, makes it difficult toconclude if the difference in the relative peak heights from one depth to the otheris statistically significant. The x-ray intensity of dolomite, the other mineral withthe potential to contribute magnesium, did not change with depth.

Figure 18 shows the change in the tailings’ specific gravity and percentage ofpyrite with depth in the control column, while Figure 19 shows the alteration indexof pyrite (ratio of altered pyrite to original pyrite) and the percentage of ferric


Figure 15.Post-testing tailings pore water sulphate, iron and pH in Control columns.

hydroxide. These data indicate pyrite was substantially consumed in the top 30 cmof the tailings, the predicted zone of active oxidation. The consumed pyrite wasreplaced by the lighter ferric oxyhydroxides. These results are consistent with thefield observations made by Blowes and Jambor (1990).

3.8. COMPARISON OF PORE WATER IN THE COVERED AND UNCOVERED

COLUMNS

In Table III we compare pH and the concentrations of key chemical species in porewaters extracted from covered and uncovered tailings at the end of the columntests. The concentrations of all species listed in the table are considerably higherin the uncovered tailings. The sulphate concentrations in the covered tailings aresimilar to those found in the field in tailings far below the water table (Yanful andSt-Arnaud, 1992). Figure 20 shows the saturation indices of minerals in the testcolumn calculated using MINTEQ. They are comparable to the saturation indicesfor secondary minerals below the oxidation front in the control columns (Fig-ure 13). The moles of sulphate and the sum of moles of calcium and magnesiumin the test column pore water are equal at each depth, which suggests that sulphate


Figure 16.Post-testing tailings pore water calcium, magnesium, potassium and aluminium in Controlcolumns.

concentrations in the test column are mainly controlled by gypsum and magnesiumsulphate minerals. These minerals were probably formed prior to deposition of thetailings: Blowes and Jambor (1990) reported the presence of gypsum in unalteredfield tailings. They attributed the source of Ca and SO2−

4 to milling agents.

4. Discussion

The oxidation front in the laboratory experiments was at a depth of about 30 cmafter 1.6 yr. At the Waite Amulet tailings impoundment, the depth of the oxidationzone at the centre of the impoundment is restricted by a shallow water table. Wherethe water table is deep, significant oxidation, as evidenced by alteration of thesulphide minerals (Blowes and Jambor, 1990) has progressed down to depths of70 cm after 20 yr. Blowes and Jambor (1990) modelled the position of the oxidationfront using the method of Davis and Ritchie (1986). We applied this model to thelaboratory experiments, and predicted a 0.4 m deep oxidation front after 1.6 yr.In this prediction, we ignored intraparticle oxygen diffusion and used a verticaldiffusion coefficient of 1.0× 10−6 m2 s−1, spatially and temporally averaged forthe whole tailings column. Using the same assumptions, and a field diffusion co-efficient estimated fromin situwater content, we predicted a 0.8 m deep oxidation


Figure 17.Intensities of first and second order reflections of chlorite peaks relative to 0.344 nm quartzpeaks.

front after 20 yr. This analysis suggests that the rate of penetration of the oxidationfront does substantially decrease with time. The rate of oxidation is initially fastdue to the high rate of oxygen diffusion through the relatively dry upper tailings.As time goes on, diffusion through wetter tailings and longer diffusion path foroxygen will reduce the rate of penetration of the oxidation front.

The swift decline in oxygen flux predicted by the oxygen diffusion modellingis not mirrored in the measured acidity for each cycle. Acidity may be controlledby precipitation and/or dissolution of secondary minerals formed as a result ofsulphide oxidation, as well as by adsorption/desorption, solid substitution, and co-precipitation reactions, as observed in the field (Dubrovskyet al., 1980; Blowesand Jambor, 1990; Yanful and St-Arnaud, 1992).

Lepidocrocite (αFeOOH) and goethite (γFeOOH) are common ageing productsof iron weathering. The fact that they have SI values between 0.25 and 1.50 in theoxidized zone (<0.30 m) indicate a high tendency for them to precipitate or form.Extensive weathering and ageing of the oxidation products in the upper 0.30 m ofthe control column would be conducive to their formation (Langmuir and Whit-more, 1971). The field SI values for these minerals suggest they are dissolving inthe oxidized zone and forming in the deeper tailings.


Figure 18.Post-testing specific gravity (Gs ) and percent pyrite in tailings in Control column (AfterYanful et al., 1994).

5. Summary and Conclusions

The preceding results and discussion have shown that under laboratory conditions,a compacted clay in a three-layer cover system was able to essentially maintainits water content over a period of 760 days. The water content of a fine sand layeroverlying the clay fluctuated in response to wetting and drying. During evaporation,the temperature of the fine sand decreased by two 2◦C, as a result of the energyconsumption for the latent heat of vaporization of water.

The cover system was able to prevent oxidation of the tailings duration the 760days of testing. Total iron concentrations in the covered tailings pore water were500 to 1000 times lower than in the uncovered tailings. Significant concentrationsof sulphate observed in the unoxidized covered tailings are most likely derivedfrom milling agents.

A comparison of water content data indicated that gravimetry measured lowerwater contents than TDR (time domain reflectometry) in all layers except the clay,in which TDR predictions were lower. The release of adsorbed water during ovendrying at 106◦C could account for the higher gravimetric water contents observedin the clay.


Figure 19.Pyrite alteration index (ratio of pyrite left to original pyrite) and percent iron oxyhydrox-ides in tailings in Control column.

Modelling of oxygen in the control column was performed using a one-dimen-sional finite-layer transport model (POLLUTE). Published estimation methods wereused to calculate diffusion coefficients from measured water contents. During eachmodelled time period, the tailings were modelled as two zones: an upper zone ofcompletely oxidized tailings, where the rate of oxygen consumption was set to zero,and a lower zone of unoxidized tailings, where the rate of reaction was assumedto be finite. The model predicted a decrease in the overall-rate of oxidation andthe advancement of the oxidation front with time. Lower rate of diffusion throughwetter tailings and longer diffusion path for oxygen will reduce the rate of pene-tration of the oxidation front is reduced by the . Measured acidity, obtained fromanalysis of water flushed through the tailings, did not decrease at the same rate,since mineral precipitation-dissolution reactions and other geochemical processeslimit the solubility and release rate of oxidation products.

The geochemistry of the laboratory tailings is consistent with the early stageof a tailings impoundment, when the gross rate of oxidation is large and extensiveweathering of oxidation products is occurring in the oxidized zone, and precipita-tion in the unoxidized zone. The position of the oxidation front at 30 cm is clearly


TABLE III

Post-testing pore water quality of covered and uncovered tailings

Depth in profile Concentrations (mg L−1)

pH SO2−4 Fe Fe3+ Zn Cu Al Pb

5 cm

Covered tailings 5.30 2670 18.4 2.66 5.84 <0.02 0.76 <0.25

Uncovered tailings 1.68 24420 4500 4500 19.9 83.7 1600 <0.25

15 cm

Covered tailings 5.61 4020 19.2 5.8 18.8 0.75 4.5 0.32

Uncovered tailings 1.63 45900 10400 7220 35.2 163 3000 <0.25

25 cm

Covered tailings 5.04 2640 26.5 4.67 14.9 0.08 1.87 <0.25

Uncovered tailings 3.77 6420 2290 103 20.9 0.09 6740 0.48

36 cm

Covered tailings 4.96 3240 6.35 1.64 7.66 0.05 0.84 <0.25

Uncovered tailings 2.42 47100 12700 5980 662 357 3990 <0.25

defined by the increase in the saturation indices of iron oxyhydroxide minerals anda sharp change in the alteration index of pyrite. Aluminium concentrations in thetailings pore water were found to be controlled by Al(OH)SO4, sulphate by gypsumand Al(OH)SO4 and iron by lepidocrocite in the upper half and by ferrihydrite inthe lower half. An increase in aluminium and magnesium concentrations at thedepth of the oxidation front is attributed to the dissolution of chlorite. The positionof the oxidation front (0.3 m in 1.6 yr) observed in the laboratory is consistent withthat predicted for the Waite Amulet tailings site over the same time frame.

Acknowledgements

The experiments were a part of the Waite Amulet Covers projected initiated underthe Canada’s MEND (Mine Environment Neutral Drainage) program. The mod-elling work was funded with a Strategic Grant No. STR 167477 awarded to thefirst author by the Natural Sciences and Engineering Research Council of Canada.


Figure 20.Profiles of saturation index in Test (covered tailings) columns.

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