ui Io Un

As(V)StabilityPorphyry copper mineTailingSpent ore

nouby Se toper

2+ 2+

incorporated in schwertmannite can retard or signicantly inhibit Fe -catalyzed transformation of

ncipalhn etated d

Among the contaminants, arsenic (As) represents a specic con-cern because of its high toxicity at low concentrations, and someAs-bearing minerals are highly soluble in a wide range of pH con-ditions (Coussy et al., 2012). The high concentrations of As found inmining environments are mainly due to the oxidation of As-bear-ing sulde minerals such as arsenopyrite (FeAsS) and arsenianpyrite (Antelo et al., 2013; Burton et al., 2009). Generally,

r, mg L , levels

nvironmenawareness

toxicity of As and better understanding of its impact on thronment (Long et al., 2012). Therefore, As control is an impissue in mining operations at porphyry copper deposits. Reof As by schwertmannite has attracted much attention in recentyears due to its strong binding afnity to toxic As species(Bigham et al., 1996, 1994; Fukushi et al., 2003; Regenspurg andPeiffer, 2005).

Schwertmannite is a poorly crystalline Fe(III)-oxyhydroxysul-fate mineral (Fe8O8(OH)82x(SO4)x with x typically 11.75) com-monly formed in acidic (pH 3.04.0) iron- and sulfate-rich

Corresponding author. Tel.: +81 11 706 6315.E-mail address: h.sengpasith@gmail.com (S. HoungAloune).

Minerals Engineering 74 (2015) 5159

Contents lists availab

n

elsings ponds; and (3) spent ore piles where the leaching operationhas ceased in the case of heap leach operations. These wastesmay be exposed to the environment, presenting the potential forcontaminant transport (Dold and Fontbote, 2001).

acid-sulfate soil (ASS) groundwater at lowe(Burton et al., 2009).

Recently, the regulations for As release to the ebecome more stringent with increasing publichttp://dx.doi.org/10.1016/j.mineng.2015.01.0030892-6875/ 2015 Elsevier Ltd. All rights reserved.t haveof thee envi-ortanttentionmonly mined by open-pit methods (Berger et al., 2008;Khorasanipour et al., 2011). Extraction and beneciation of copperore produces large amounts of rock waste and mine tailings(Antelo et al., 2013; Nordstrom, 2011). The waste may be dividedinto three major categories: (1) waste rock piles or dumps; (2) tail-

and As in the drainage waters (Smedley and Kinniburgh, 2002).This is termed Acid Mine Drainage (AMD) when the pH is acidicand Contaminated Neutral Drainage (CND) when the pH remainscircum-neutral (Coussy et al., 2012). For these reasons, As canoccur in Acid-Mine Drainage (AMD) at hundreds of mg L1 and in

11. Introduction

Porphyry copper deposits, the priwide (Khorasanipour et al., 2011; Jolow-grade, epigenetic, intrusion-relschwertmannite to goethite under acidic conditions (pH 34). The Sch-As aged at different pHs from 3to 11 at 25 C exhibits no mineralogical phase changes even after ageing for 120-days; however the con-centration of As released from the solid phase appears to be strongly pH-dependent even after ageing foronly 24 h. The release of As was negligible at pHs from 2 to 7, and there was considerable release of As atextremely acidic and alkaline conditions. This indicates that the release of As from Sch-As was controlledby environmental factors such as pH, Cu2+, and Fe2+ rather than time.

2015 Elsevier Ltd. All rights reserved.

source of copper world-al., 2010) are relativelyeposits that are com-

As-bearing minerals are liberated and disposed as waste rock ortailings after mining and mineral processing due to the low eco-nomic value (Coussy et al., 2012). When disposed unprotectedfrom the weather, the tailings react with atmospheric oxygenand meteoric water, leading to the release of sulfates, metals,Keywords:Schwertmannite

gates the effect of Cu , Fe , pH, and ageing time on the stability of As(V)-sorbed schwertmannite (Sch-As). The results indicate that Cu2+ has no signicant effect on the stability of Sch-As and that the As(V)

2+Stability of As(V)-sorbed schwertmanniteconditions

Sengpasith HoungAloune , Naoki Hiroyoshi, MayumDivision of Sustainable Resources Engineering, Graduate School of Engineering, Hokkaid

a r t i c l e i n f o

Article history:Received 5 September 2014Accepted 7 January 2015

a b s t r a c t

Arsenic (As), a very poisodeposits. Retention of Asattention in recent years duschwertmannite under cop

Minerals E

journal homepage: www.nder porphyry copper mine

toiversity, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan

s inorganic pollutant is a major toxicant in tailings of porphyry copperchwertmannite (a ferric-oxyhydroxysulfate mineral) has attracted muchits strong binding afnity to toxic As species. The stability of As(V)-sorbedmine waste conditions is not fully understood. The present study investi-

le at ScienceDirect

gineering

evier .com/locate /mineng

of schwertmannite. Laboratory research recently reported by our

ls Egroup has indicated that it is possible to synthesize schwertman-nite from copper heap leach solutions with high efciency enablingAs(V) removal (HoungAloune et al., 2014). Schwertmannite can beexpected to nd application in As immobilization of copper minewaste in tailings and spent ore of either dump or heap leach piles.The particulars of the copper mine tailings may be very varied, forexample they may contain signicant amounts of iron ions due tooxidation of the rejected pyrite (FeS2) and other sulde minerals(Davidson et al., 2008; Lottermoser, 2003), and some amount ofcopper ions could also be present in the tailings as 100% copperrecovery is not possible in the extraction processes. Further, coppermine waste may present a wide range of pH-values, particularly intailing ponds. This makes it of interest to evaluate the stability ofAs(V)-sorbed schwertmannite in copper mine tailing conditions.The present study investigates the effect of Cu2+, Fe2+, pH, and age-ing time on the stability of As(V)-sorbed schwertmannite.

2. Materials and methods

2.1. Solution preparation

Solutions containing known concentrations of H2SO4, Na2CO3,Fe3+, Fe2+, Cu2+, and As(V) were prepared by dissolving reagent-grade H2SO4, Na2CO3, FeSO47H2O, Fe2(SO4)3nH2O, CuSO45H2O,and Na2HAsO47H2O (Wako Pure Chemical Industries, Ltd., Japan)in distilled/ion-exchanged water. The results for the solid-phaseanalyses are presented on a dry weight basis, unless notedotherwise.

2.2. Sample preparation

Schwertmannite was prepared using the procedure describedby HoungAloune et al. (2014). Briey, 500 mL of solution contain-ing 10 mM H2SO4 and 50 mM Fe3+ was prepared and heated to65 C in a beaker under magnetic stirring at 350 rpm. After discon-tinuing the magnetic stirring 1 M Na2CO3 was titrated to the solu-tion to adjust the nal pH to 3.04.0 (titration rate, 1500 lL/min).The resulting suspension was left to settle, and the supernatantwas subsequently replaced with deionised water. This rinsing pro-(10003000 mg/L) environments (Bigham et al., 1996, 1994;Blgham et al., 1990; Bigham and Nordstrom, 2000). Schwertman-nite is also known as a metastable compound that is transformedto stable crystalline goethite via dissolution and re-precipitation.The overall reaction of the phase transformation can be expressedas (Bigham et al., 1996; Knorr and Blodau, 2007; Paikaray andPeiffer, 2012; Regenspurg et al., 2004):

Fe8O8OH82xSO4x 2xH2O! 8FeOOH xSO24 2xH 1The transformation process may be delayed or accelerated

depending on the pH of the surrounding environment and thepresence of other ions in solution or in the mineral structure(Antelo et al., 2013). It has been found that As(V) partiallyexchanged with sulfate without signicant structural disruptioncan stabilize schwertmannite and retard or signicantly inhibitthe transformation into goethite (Fukushi et al., 2003; Burtonet al., 2010, 2009; Raghav et al., 2013; Regenspurg and Peiffer,2005).

At porphyry copper mines, generation of schwertmannite fromthe efuent or rafnate of copper heap leaching is of economic andenvironmental interest for As control. Such leach solutions gener-ally contain high concentrations of Fe3+ and SO42, the components

52 S. HoungAloune et al. /Mineracedure was repeated 5 times to remove soluble ions. The precipi-tate in the beaker was then ltrated, dried at 40 C in a vacuumoven for 24 h, and ground using an agate mortar.The schwertmannite loaded with As(V) was prepared by adding0.1 g of dried schwertmannite to 200-mL asks each containing100 mL solution consisting of 1.5 mM As(V). This suspension wasadjusted to pH 34 by drop-wise addition of HCl and shaken for24 h in a water bath at 25 C at 120 rpm. The solid phase was sep-arated by ltration, oven dried at 40 C and stored at room temper-ature. In the following, pure schwertmannite and schwertmanniteloaded with As(V) will be denoted as Sch and Sch-As, respectively.

Both dried Sch and Sch-As products were next subjected to X-ray diffractometry (XRD). The composition of Sch and Sch-As wasdetermined after digestion of the dried solids in 1 M HCl (1 g L1)and the Fe and S contents were measured by inductively coupledplasma atomic emission spectroscopy (ICP-AES).

2.3. Effect of Cu2+ on Sch-As stability

The prepared Sch-As samples were suspended in solution con-taining various concentrations of Cu2+. Experiments were carriedout as follows: 0.1 g of dry Sch-As and predetermined amountsof CuSO45H2O and Na2SO4 powers were added to 100-mL of pre-heated (65 C) distilled water in a glass bottle. Here, the totalSO42 concentrations were set to be the same in all solutions bythe addition of Na2SO4. Therefore, when comparing the results ofthe experiments, the effect of the SO42 concentration on the stabil-ity of the Sch-As can be disregarded. The suspensions wereadjusted to pH 34 by drop-wise addition of HCl or NaOH and stir-red at 350 rpm at 65 C. After a 24 h reaction period, the solidphase was separated by centrifugation at 3500 rpm for 5 min andthe supernatant was subjected to arsenic analysis. The recoveredsolid was subjected to XRD analysis after drying overnight in a vac-uum dryer at 40 C.

2.4. Effect of Fe2+ on Sch-As stability

For the determination of Fe2+effects, 0.1 g of dry Sch-As and pre-determined amounts of FeSO47H2O and Na2SO4 were added to100 mL of preheated (65 C) distilled water in a glass bottle. Here,the total SO42 concentrations were set to be the same in all solu-tions by the addition of Na2SO4. Therefore, when comparing theresults of the experiments, the effect of the SO42 concentrationon the stability can be disregarded. The suspensions were adjustedto pH 34 by drop-wise addition of HCl or NaOH and stirred at350 rpm at 65 C under a nitrogen atmosphere. After 24 h, the solidphase in the ask was separated by centrifugation at 3500 rpm for5 min, the supernatant was subjected to arsenic analysis, and afterdrying overnight in a vacuum dryer at 40 C, the recovered solidwas subjected to XRD analysis.

2.5. Effect of pH on Sch-As stability

To investigate the effect of pH on Sch-As stability, 0.1 g of driedSch-As was added to 100 mL of distilled water in a 200-mL ask.The initial pH was adjusted to pH ranges between 1 and 12 withNaOH or HCl prior to addition of the Sch-As. The ask was subse-quently shaken in a water bath at 25 C at 120 rpm for 24 h. Thenthe nal pH was measured and the suspended solid was harvestedby centrifugation at 3500 rpm for 5 min. The supernatant was sub-jected to arsenic and sulfur analysis. The recovered solid was sub-jected to XRD analysis after drying overnight in a vacuum dryer at40 C.

2.6. Effect of time on Sch-As stability

ngineering 74 (2015) 5159Experiments were carried out for 120 days with 0.1 g of Sch orSch-As in 100 mL deionized water in 200-mL asks. The experi-ments were performed at various pH conditions. The initial pH

was adjusted to pH ranges between 3.5 and 12 with NaOH or HClprior to the addition of the Sch and Sch-As. The asks were shakenreciprocally in a water bath at 25 C at 120 rpm under normalatmospheric conditions. At a preset ageing time, the suspensionwas sampled with a syringe and this sample was ltrated througha 0.2-lm membrane. The supernatant was analyzed for sulfur,arsenic, and pH. After 120 days, the suspended solid was harvestedby centrifugation at 3500 rpm for 5 min. The recovered solid wassubjected to XRD analysis after drying overnight at 40 C.

2.7. Analytical methods

The aqueous phase was analyzed for iron, sulfur, arsenic, andpH. The concentrations of arsenic, iron, and sulfur were measuredby inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Seiko Instruments SPS7800. In the ICP-AES analyses,

has the potential to affect the stability of Sch-As, and to establish

pH 34.

3.3. Effect of Fe2+ on Sch-As stability

The effect of Fe2+ on the stability of schwertmannite has beenreported by several researchers. Burton et al. (2008) found that

Sch-As 8.48 1.03 1.35 8.2 0.16

S. HoungAloune et al. /Minerals Ecalibration with sets of standards was performed and the regres-sion coefcients exceeded 0.999; dilutions from 1:10 to 1:1000were used to ensure that the concentrations of the elements inthe samples were within the concentration range of the standards,the error was estimated to be below 5%. A pH-meter and combinedelectrode with temperature compensation was used to measurethe pH and was calibrated regularly with standard buffer solutionsof pH 1.68, 4.01, and 6.86. The solid phase was ground to a homo-geneous powder in a mortar and subsequently analyzed by X-raydiffraction (XRD) using a RIGAKU powder diffractometer with CuKa radiation (40 kV and 30 mA) to identify the minerals presentin the samples. Peak maxima were read from the diffractogramand minerals were identied by comparison with a schwertman-nite standard from an XRD pattern identical to that of the schwert-mannite described by Blgham et al. (1990).

3. Results and discussion

3.1. Characterization of Sch and Sch-As

The XRD patterns (Fig. 1) conrm that schwertmannite is theonly mineral detectable in both the Sch and Sch-As samples. Theelemental composition of Sch and Sch-As are shown in Table 1.The molar Fe/S ratio, determined from the Fe and S molar propor-tions, is 5.3 and 8.2 for Sch and Sch-As, respectively. The Sch-Ascontained As/Fe at a ratio of 0.16. Based on the Fe, S, and As con-tents, the average stoichiometry of the Sch and Sch-As productscan be represented as Fe8O8(OH)4.98(SO4)1.51 and Fe8O8(OH)4.98(SO4)0.93(AsO4)1.28, respectively. The chemical formula shows thatSch represents a product with intermediate sulfate content. Low(1.021.30) and high (1.602.05) values of the relative sulfate con-Fig. 1. XRD patterns for (a) Sch and (b) Sch-As samples. Schwertmannite peaks aredenoted as Sh.details of this the effect of 1, 10, 50, and 100 mM Cu2+ on the sta-bility of Sch-As was investigated in the present study. The experi-ments were carried out at 65 C in order to accelerate thetransformation.

Fig. 2a shows XRD patterns of recovered solids after suspensionof Sch-As in solutions containing various concentrations of Cu2+.The results show schwertmannite peaks and there is no indicationof other phases for any of the samples suspended in these solutionswith Cu2+. This indicates that the Sch-As here is stable and that nosolid phase transformation has occurred. The concentration of Asreleased from the Sch-As solid suspended in the solutions contain-ing various concentrations of Cu2+ was also measured. As sug-gested by Fig. 3a, only a small fraction of released As ( 6 and Fe2+P 5 mmol L1.

ncen

ls EFig. 2. XRD patterns for Sch-As samples suspended in solutions containing various coare denoted as Sh.54 S. HoungAloune et al. /MineraHoungAloune et al. (2014) reported that transformation of schw-ertmannite to goethite occurred within 1 h in the presence ofFe2+ at lower pH conditions (pH 34). To elucidate this further,the effect of 1, 10, 50, and 100 mM Fe2+ on the stability of Sch-Aswas investigated in the present study. The experiments were con-ducted at 65 C to accelerate the transformation.

The results show that schwertmannite appears as the dominantphase for all Sch-As products suspended in solutions containingvarious concentrations of Fe2+ (Fig. 2b), suggesting that no solidphase transformation occurs in the presence of Fe2+. Small frac-tions of released arsenic (

These As(V) species are expected to desorb at high pH because athigh pH, schwertmannite has a very large negative charge

Carlson (2005) observed the complete transformation of schwert-

(a) (b)

pH pH

SO42

- rel

ease

(mm

ol g

-1)

As r

elea

se (

mg

L-1 )

Fig. 4. Amount of (a) SO42 and (b) As released from Sch-As samples suspended at different pH at 25 C for 24 h. The dashed lines correspond to the total amounts of (a) sulfateand (b) arsenic in the initial Sch-As.

S. HoungAloune et al. /Minerals Engineering 74 (2015) 5159 55tion (Jnsson et al., 2005; Paikaray and Peiffer, 2010). The release ofSO42 at pH above pH 7 indicates a contribution of SO42 releaseform both the internal structure and the surface of Sch-As. Here,transformation of schwertmannite to goethite is expected to occurat pH above 7. However, the XRD results (Fig. 5) show that schw-ertmannite appears as the dominant phase and there is no forma-tion of any new phase at any of the pH-values (nal pH), indicatingthat no transformation of schwertmannite to goethite occurred.The loss of schwertmannite-bound SO42 without simultaneousmineralogical changes has also been reported as a result of pH-dependent SO42 desorption (Burton et al., 2010; Jnsson et al.,2005).

The amount of As released from the Sch-As after suspension insolutions with different pH values (nal pH) was also measured.The results show that the release of As was strongly pH-dependent(Fig. 4b). At extremely acidic conditions (below about pH 2), a highrelease of As was observed, may be due to the dissolution of Sch-As. It has been reported that iron oxides dissolve under stronglyacidic conditions and minor elements including As will also tendto be released during this dissolution (Smedley and Kinniburgh,2002). The release of As was almost negligible from above aboutpH 2 to neutral pH conditions and increased at alkaline conditions(above pH 7). This can be explained by considering aqueous As(V)speciation and the surface charge of schwertmannite. The aqueousspeciation of As(V) comprises H3AsO40, H2AsO4, HAsO42, and AsO43

(Burton et al., 2009; Smedley and Kinniburgh, 2002). AqueousAs(V) is predominantly present as H2AsO4 at low pH (below aboutpH 6.9) and as HAsO42 at higher pH (H3AsO40 and AsO43 may bepresent in extremely acidic and alkaline conditions, respectively).Fig. 5. XRD patterns for Sch-As products aged at different pH for 24 h, plotted withthe original pattern (initial Sch-As). Schwertmannite peaks are denoted as Sh.mannite to goethite in deionized water at pH 7.2 after 100 days.Paikaray and Peiffer (2012) reported the transformation of schw-ertmannite to goethite after a 120-day ageing period at pH 7 and8. Antelo et al. (2013) found that conversion of schwertmanniteto goethite was almost complete within 94 days even at pH 3.These results suggest that pH is an important factor affecting thelong-term stability of schwertmannite, therefore long-term stabil-ity of Sch-As at various pH conditions were carried out and will bediscussed in detail in Section 3.5.

3.5. Effect of time on Sch-As stability

For the determination of time effects, experiments were carriedout with Sch or Sch-As samples at different pH-values at 25 C for120 days. The SO42 release from the Sch and Sch-As structure tothe solution at 120-day is shown in Fig. 6. The results show a clearpH dependence of the released SO42. The released SO42 after(PZC = 7.2, Jnsson et al., 2005). These factors facilitate signicantdesorption of As(V) at high pH.

It has been extensively reported that within durations of a fewmonths schwertmannite transforms to more stable iron oxides atvarious pH conditions (Antelo et al., 2013; Knorr and Blodau,2007; Paikaray and Peiffer, 2012; Regenspurg et al., 2004;Schwertmann and Carlson, 2005). Here, Schwertmann andSO42

- rel

ease

(mm

ol g

-1)

3 4 6 8 11

pH

Fig. 6. Release of SO42 from Sch (black) and Sch-As (white) after suspension insolutions with different pH for 120 days. The total amounts of sulfate in the initialSch (dashed line) and Sch-As (dotted line) are shown for comparison. The indicatedpH values are the nal pH after 120-day of ageing.

120 days increased with increasing pH. At pH 3, the amount ofSO42 released from solid phase SO42 was 46.6% for Sch and 24.3%for Sch-As. At pH 11, the amount of released SO42 increased to99.1% for Sch and 90.4% for Sch-As. The released SO42 observedhere might be due to the transformation of schwertmannite to goe-thite according to Eq. (1). If this is the case, the result in Fig. 6 indi-cates that the transformation is more signicant at higher pHs.

Fig. 7 shows the XRD patterns of (a) Sch and (b) Sch-As productsafter 120 days of ageing at different nal pH-values. The resultsindicate that schwertmannite remains the dominant mineral withthe appearance of few characteristic goethite peaks for the Schsample aged at pH 3 and pH 4 (Fig. 7a). Some goethite peaks appearat pH 6, and the goethite contribution continues to increase withthe pH value. At pH 11, goethite peaks are clearly observed inthe XRD patterns, indicating that the transformation rate isstrongly pH-dependent for Sch. Schwertmannite appears as thedominant phase without formation of other phases for the Sch-As products aged 120 days at all pH-values (Fig. 7b), indicating that

However, further investigation is required to establish details ofthese phenomena.

The data plotted in Fig. 8(a and b) is re-plotted as pH versus theamount of released arsenic (Fig. 9) in an attempt to describe theeffect of pH on the release of arsenic from Sch-As solid phases dur-ing the 120-day ageing period. As suggested by Fig. 9, Sch-As prod-ucts released a very low amount of arsenic into the solution withpH less than about 7; and the release were signicant at higherpH throughout the ageing period. This indicates that the As releasefrom Sch-As was mainly controlled by outside environmental fac-tors such as pH rather than time. The curve in Fig. 9 could be well

Initial Sch-As

pH 3

pH 4

pH 6

pH 8

pH 11

(b)

As r

elea

se (%

); pH

8, 1

1

days

As r

elea

se (%

); pH

3, 4

, 6

(a)

(b)

Fig. 8. (a) Release of arsenic and (b) pH values after ageing for different times atdifferent pH within 120-day of Sch-As stability experiments. Note the two y-axis inFig. 8(a) and the indicated pH values are the nal pH after 120-day of ageing.

56 S. HoungAloune et al. /Minerals Engineering 74 (2015) 5159Sch-As products here may be stable and that no solid transforma-tion occurs. As(V) incorporated into schwertmannite may retard orsignicantly inhibit the transformation into more stable iron oxi-des (Fukushi et al., 2003; Burton et al., 2010, 2009; Raghav et al.,2013; Regenspurg and Peiffer, 2005).

As suggested by Fig. 8a, a fraction of the arsenic was releasedfrom the Sch-As solid phase throughout the 120-day ageing periodat all pH-values. After the maximum release of As from the Sch-Assolid at all pH-values, the amount of As measured in the aqueousphase was subsequently dropped as the ageing period increased.These gradual declines in released As may represent a re-sorptionof arsenic onto Sch-As solid phases. As shown in Fig. 8b, all pH-val-ues were gradually decreased throughout the ageing period,enabling the anionic As(V) species to re-sorp onto Sch-As at lowerpH because at lower pH, schwertmannite processes more posi-tively charged sorption sites.

Re-sorption of As(V) may have caused blocking of the crystalgrowth of new minerals as reported for other anions such as As(III)(Paikaray and Peiffer, 2012), SO42 (Davidson et al., 2008), and fortrace metals (Lin et al., 2003; Sun et al., 1996). Paige et al. (1996)demonstrated that 1 mol% As(V) restricted the transformation offerrihydrite to goethite due to surface adsorption and inhibitionof growth of a new mineral. It was also found that the incorpora-tion of As can prevent the crystal growth of schwertmannite(Maillot et al., 2013; Vithana et al., 2014; Waychunas et al.,1995). This may be the reason why no solid transformationoccurred for the Sch-As aged for 120 days at any pH-value.

Initial Sch

pH 3

pH 4

pH 6

pH 8

pH 11

Sh

Sh Sh Sh

Sh

Sh Sh

G G

G G G G

G G G G

G

G G (a)Fig. 7. XRD patterns for (a) Sch and (b) Sch-As aged at different pH for 120 days, plotteddenoted as Sh, goethite peaks are denoted as G; and the indicated pH values are thdays

pHwith the original pattern (initial Sch and initial Sch-As). Schwertmannite peaks aree nal pH after 120-day of ageing.

tted by Eq. (2), as exhibiting a regression coefcient (R2) of0.8361.

arsenopyrite (FeAsS), which weather when tailings are exposedto the air and oxygen (Antelo et al., 2013; Burton et al., 2009). Thiscan result in the liberation of arsenic and strong acid (i.e. AMD)from the tailings, which may pollute adjacent areas (Lottermoser,2003).

Copper oxide ores are generally recovered by hydrometallurgi-cal processes, including leaching (e.g. dump or heap leaching), sol-vent extraction, and electro-winning (SX/EW) (Peacey et al., 2004).The material remaining in either dump or heap leach piles whenleaching ceases is called spent ore. Leach piles are reported torange in size from 6 m to over 30 m in height and may contain mil-lions of tons of leached ore (EPA, n.d.). The spent ore becomeswaste when active leaching ends.

Elevated arsenic concentrations are usually found in coppermine waste (Lottermoser, 2003). Tailings and spent ore of eitherdump or heap leach piles constitute over 60% of the total wasteat copper mines (EPA, n.d.). It is important to nd a suitablemethod to control the toxic arsenic species that could be releasedfrom such large amounts of waste at copper mines. Among the cur-rent treatment processes for arsenic control at copper mine waste,immobilization of arsenic by schwertmannite may be consideredthe most promising technology and it can be cost-effective as it

As r

elea

se (m

g L-

1 )

pH

Fig. 9. Relationship between pH-values and amount of released arsenic during the120-day of Sch-As stability experiments. Note the total amount of arsenic in theinitial Sch-As is 100 mg L1.

S. HoungAloune et al. /Minerals Engineering 74 (2015) 5159 57Y 0:031exp0:6975X 2where X is the pH value of the solution, and Y (mg L1) is theamount of arsenic release from the Sch-As solid phases during theageing experiments. At a given pH value, the amount of releasedarsenic can be calculated according to Eq. (2), which could be usefulto predict the immobilization of arsenic by schwertmannite undervarious pH conditions at copper mine waste systems.

3.6. Environmental implications

At porphyry copper mines, copper sulde minerals are mainlyrecovered by otation as concentrates that can be converted intocopper metal using pyrometallurgical processes, including smelt-ingconverting and electrolytic rening (Ochromowicz andChmielewski, 2013). The ne-grained fraction of the waste gener-ated during otation is called tailings, which are discharged tothe tailings impoundment structures (Lottermoser, 2003). Tailingscommonly contain sulde minerals, e.g. pyrite (FeS2) andFig. 10. Flowsheet of the proposed treatment process for arsenic iis possible to synthesize schwertmannite from solutions generatedin heap leach operations (i.e. rafnate). It is necessary to investi-gate the stability of arsenic-incorporated schwertmannite (Sch-As) under copper mine waste conditions. The present study hasinvestigated the effect of Cu2+, Fe2+, pH, and ageing time on the sta-bility of Sch-As. The results indicate that Cu2+ and Fe2+ have no sig-nicant effect on the stability of Sch-As; and that arsenicimmobilization by schwertmannite takes place only at pH from 2to 7. Based on the results of this study, schwertmannite may bepractical for arsenic immobilization of copper mine waste as indi-cated in Fig. 10.

An arsenic treatment process of tailings is expected to be car-ried out during the active mining. First, schwertmannite is synthe-sized from dump or heap leach solution (rafnate). Thesynthesized schwertmannite (slurry) is then pumped out andmixed with the otation tailing slurries before discharge to tailingsponds. Generally, the pH values here range from 7.5 to 11.5 in mostotation operations (Branson and Ammons, 2004), with high pHvalues in tailing ponds (alkaline) at the initial stage. However,mmobilization by schwertmannite at porphyry copper mines.

ls Ethe pH values generally decrease to acidic pH with time due to theoxidation of the sulde minerals present in the tailings. Heavymetals and arsenic are usually dissolved from the original mineralsin AMD generation. The dissolved arsenic is expected to be sorbedby the schwertmannite in the tailings pond during the pH decline;and at this stage the toxic arsenic is considered to become stabi-lized by schwertmannite.

The AMD water in tailings ponds is commonly treated by addi-tion of chemical-neutralizing agents such as lime. However, sincethe arsenic incorporated with schwertmannite may potentiallybe released back into the mine tailings water due to desorptionof arsenic at high pH (i.e. pH > 7), treatment of AMD water needsto be carried out separately (additionally). The AMD from tailingsponds can then be pumped out and treated in other ponds or tankswhere the chemical-neutralizing agent is added to raise the pH.

As mentioned previously, the spent ore of either dump or heapleach piles becomes waste when active leaching ends, and arsenicimmobilization by schwertmannite is expected to have to be car-ried out during the post-closure stages of dump or heap leach oper-ations. The spent ores may contain a residual of lixiviant (H2SO4)and associated metal ions (iron), which are the main componentsof schwertmannite. A neutralizer (Na2CO3) can be sprayed on thedump or heap piles; and schwertmannite is expected to be formedduring the neutralization. The arsenic released within the pilesmay be subsequently immobilized by the precipitatedschwertmannite.

Control of arsenic contamination in copper mine waste treat-ment are expected to be performed following the methods men-tioned above. However, further study must be carried out tosubstantiate details of these treatment processes.

4. Conclusions

This study provides insight into the stability of As(V)-sorbedschwertmannite (Sch-As) under porphyry copper mining condi-tions by investigating the effect of Cu2+, Fe2+, pH, and ageing timeon the stability of Sch-As. The results indicate that Cu2+ has no sig-nicant effect on the stability of Sch-As and that the As(V) incorpo-rated into schwertmannite can retard or signicantly inhibit theFe2+-catalyzed transformation of schwertmannite to goethiteunder acidic conditions (pH 34). The Sch-As aged at differentpH ranges from 3 to 11 at 25 C exhibits no mineralogical phasechanges even after ageing for 120-days; however the concentrationof arsenic released from the solid phase appeared to be stronglypH-dependent also at ageing for 24 h. The release of As was almostnegligible at pH 2 to 7, and a high release of As was observed atextremely acidic and alkaline conditions. This indicates that therelease of As from Sch-As is controlled by the outside environmen-tal factors such as pH, Cu2+, and Fe2+ rather than time.

Acknowledgements

This work is supported by a collaborative research programbetween Hokkaido University and Sumitomo Metal Mining Co.,LTD.

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S. HoungAloune et al. /Minerals Engineering 74 (2015) 5159 59

Stability of As(V)-sorbed schwertmannite under porphyry copper mine conditions1 Introduction2 Materials and methods2.1 Solution preparation2.2 Sample preparation2.3 Effect of Cu2+ on Sch-As stability2.4 Effect of Fe2+ on Sch-As stability2.5 Effect of pH on Sch-As stability2.6 Effect of time on Sch-As stability2.7 Analytical methods

3 Results and discussion3.1 Characterization of Sch and Sch-As3.2 Effect of Cu2+ on Sch-As stability3.3 Effect of Fe2+ on Sch-As stability3.4 Effect of pH on Sch-As stability3.5 Effect of time on Sch-As stability3.6 Environmental implications

4 ConclusionsAcknowledgementsReferences