Characterization of Mine Waste and Acid Mine Drainage Prediction by Simple Testing Methods in Terms of the Effects of Sulfate-Sulfur and Carbonate Minerals

minerals

Article Characterization of Mine Waste and Acid Mine Drainage Prediction by Simple Testing Methods in Terms of the Effects of Sulfate-Sulfur and Carbonate Minerals

Shinji Matsumoto 1,*, Hirotaka Ishimatsu 2, Hideki Shimada 2, Takashi Sasaoka 2 and Ginting Jalu Kusuma 3

1 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Ibaraki, Tsukuba 305-8567, Japan 2 Department of Earth Resources Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan; [email protected] (H.I.); [email protected] (H.S.); [email protected] (T.S.) 3 Department of Mining Engineering, Institut Teknologi Bandung, Jl. Ganesha 10, Bandung 40132, Indonesia; [email protected] * Correspondence: [email protected]; Tel.: +81-29-861-3347

Received: 13 July 2018; Accepted: 10 September 2018; Published: 13 September 2018

Abstract: Characterization of mine waste rocks and prediction of acid mine drainage (AMD) play an important role in preventing AMD. Although high-tech analytical methods have been highlighted for mineral characterization and quantification, simple testing methods are still practical ways to perform in a field laboratory in mines. Thus, this study applied some simple testing methods to the characterization of mine wastes and AMD prediction in addition to a leaching test and the sequential extraction test with HCl, HF, and HNO3, which have not been applied for these purposes, focusing on the form of sulfur and the neutralization effects of carbonates. The results of the Acid Buffering Characteristic Curve test supported the changing trend of the pH attributing carbonates only during the first 10 leaching cycles in the leaching test. The change in the Net Acid Generating (NAG) pH in the sequential NAG test reflected the solubility of sulfur in the rocks, providing information on the form of sulfur in the rocks and the acid-producing potential over time. Consequently, the sequential NAG test and sequential extraction with the acids in combination with the current standards tests (Acid Base Accounting and NAG tests) provided important information for preventing AMD.

Keywords: open-cast mining; acid mine drainage (AMD); cover system; leaching test; sequential extraction test; sulfate-sulfur; carbonate minerals

1. Introduction Acid mine drainage (AMD), i.e., polluted acidic water, is formed from exposure of sulfide minerals from waste rocks that are excavated during mining operations to oxygen and water. Several previous studies reported that AMD is a serious, global environmental issue that occurs during and/or after mining activity [1,2]. Several options currently exist to treat AMD consequences, such as pH control via neutralization and wetland treatment [3,4]. However, these treatment processes are often expensive in terms of both capital and operating costs [5]. AMD preventive measures (e.g., sulfide mineral isolation and source rock desulfurization prior to backfilling [6,7]) play a more important role than post-treatment of acidic water because prevention reduces operating costs involved in the treatment activities as well as the amount of treated wastewater. Therefore, preventive action is an important

Minerals 2018, 8, 403; doi:10.3390/min8090403 www.mdpi.com/journal/minerals Minerals 2018, 8, 403 2 of 14 option when dealing with AMD issues in addition to treatment after mining activities, which leads to resource development with smaller AMD impacts and more environmentally friendly mining. Cover systems, as a preventive method widely used in industry for AMD treatment, involve the use of a cover layer that prevents water from interacting with sulfide minerals. A cover system is an example of an isolation method, i.e., the use of an impermeable sheet to create a water shield with cement [8,9]. In this system, mining waste rocks, classified as non-source rocks for AMD, are commonly used as cover materials due to their low cost and easy use at mine sites compared to the abovementioned materials [10–12]. A cover layer comprising non-source rocks inhibits the supply of oxygen to sulfide minerals throughout the waste dump [13]. Waste rocks are classified as AMD source or non-source rocks on the basis of their potential to produce acid and their neutralizing capacity, analyzed via an acid–base accounting (ABA) test, in order to obtain the net acid producing potential (NAPP) and the net acid generating (NAG) pH. Source rocks are commonly known as potentially acid forming and non-source rocks are known as non-acid forming [14,15]. These simple tests are used by industry and frequently conducted in field laboratories at mining sites. However, current standards used to classify waste rocks as AMD source or non-source rocks have several drawbacks. For example, the current standard tests (i.e., the ABA and NAG tests) do not account for the effects of sulfur and iron carbonate mineralogy [16–20]. Stewart et al. [17] observed that acid consumption via iron carbonate in samples prevents the Fe2+ oxidation process in solution during the ANC test, which results in misinterpretations of ANC results. Furthermore, Dold stated that the NAG test does not account for the Fe(III) hydroxide and Fe(III) hydroxide sulfate groups (e.g., the jarosite–alunite group, schwertmannite) as well as sulfates [18]. To overcome these problems, mineral characterization and quantification methods have been developed as precise analytical techniques to predict AMD occurrence. The use of quantitative mineralogical methods (e.g., quantitative evaluation of minerals by scanning electron microscopy and mineral liberation analysis) for predicting AMD occurrence are expected to produce reliable results [18,21]. Although these high-tech analytical methods have gained popularity in recent years, combined with problems posed by simple geochemical tests (e.g., ABA and NAG tests) [16–20], simpler testing methods are still widely used and are more practical in field mining laboratories with limited resources [22]. Improvement in these simple testing techniques must consider the effects of non-sulfide sulfur and carbonate minerals that cause the misinterpretation of AMD predictions and rock types [18]. To overcome this problem, several methods have been developed: the chromium reducible sulfur test [23,24], a leaching method with a hot NH4-oxalate leachate [25], sequential NAG tests [14,17], a modified NP test [26], and the acid buffering characteristic curve (ABCC) test [14,17]. However, an alternative and/or additional simple method is still being discussed worldwide in terms of a practical method performed in a field in mines. The present study attempts to apply some simple testing methods to the characterization of mine wastes and AMD prediction in addition to a leaching test and the sequential extraction test with HCl, HF, and HNO3, which has not been applied for the purposes. To elucidate the effects of the form of sulfur and the neutralization obtained by carbonate minerals on the formation of AMD over time, the simple tests are performed in combination with the current standards test (ABA and NAG tests). Furthermore, on the basis of the results, the application of the simple testing methods to the classification of waste rocks for cover systems preventing AMD is reviewed in the context of sustainable mining.

2. Experimental Methods Six types of waste rocks were collected from three open-cast coal mines in Indonesia in addition to a crystalline pyrite sample obtained in Japan. The waste rocks were collected in the pits from each layer prior to excavation, packed into plastic bags to prevent oxidation, and then transported to the ﬁeld laboratory. The samples were labeled as A, B, C, D, E, F, and G. Sample E was a crystalline pyrite collected in Japan. Sample A and B, C and D, and F and G were taken in three mines, respectively. Minerals 2018, 8, 403 3 of 14

Sample A, B, C, D, and F were categorized as argillaceous rock, and sample G as sandy silt according to the geological data in the study area. The samples were pulverized to a 75 µm grain size for geochemical analysis and 1.0–2.0 mm for leaching tests. The pulverized samples were used for the ABA test, as well as to measure the acid potential (AP) and ANC and to perform the NAG, the paste pH [14], sequential acid extraction [27], the Bernard calcimeter [28,29], the acid buffering characteristic curve (ABCC) [14,17], the sequence NAG [14,17], the X-ray photoelectron spectroscopy (XPS) analyses, and the leaching tests. Sequential acid extraction [27] and the sequential NAG test [14,17] were performed to understand the effects of the sulfur structure on AMD occurrence over time due to varying mineral dissolution rates. Previous studies have shown that sequential extraction is a simple technique to improve AMD prediction precision [18,30,31]. In this study, mineral extraction was performed with HCl, HF, and HNO3 according to the methods described in [27], where sulﬁdes in abandoned mine tailings were extracted. We expected sulﬁde and sulfate separation to occur through the application of this extraction process to waste rocks found at active mining sites in order to predict AMD occurrence based on the samples used in this study. Sequential NAG tests consider the net effects of the presence of non-acid forming S and neutralizing species not measured by the ANC test [17]. In addition, the Bernard calcimeter [28,29] and ABCC tests [14,17] were used to examine the effects of neutralization on changes in water quality during the leaching tests. The ABCC test, developed in 1995, involves sample titration with an acid while continuously monitoring the pH. The ABCC test results can complement the ANC test results by providing an indication of the relative ANC reactivity during short time frames [17]. Furthermore, X-ray diffraction (XRD) and XPS analyses were conducted to observe different mineral form compositions in the seven samples. The pH, electrical conductivity (EC), and electric potential of the normal hydrogen electrode (Eh) throughout the analyses and leaching tests were measured using WM-32EP (DKK-Toa, Tokyo, Japan), GST-2729C DKK-Toa, Tokyo, Japan), and PST-2739C (DKK-Toa, Tokyo, Japan) with a 3.3 mol/L Ag/AgCl electrode, as well as CT-27112B (DKK-Toa, Tokyo, Japan). GST-2729C was calibrated with standard C6H4(COOK)(COOH) and KH2PO4 + Na2HPO4 buffer solutions with pH value of 4.01 and 6.86, respectively. The measured oxidation reduction potential (ORP) was converted to Eh based on the reference showing single electrode potentials in a solution at 20 ◦C[32].

2.1. Geochemical Characteristics The ABA, NAG, and paste pH tests were conducted based on the regulations described in the Australian Mineral Industries Research Association Ltd. (AMIRA) standard in addition to XRD and X-ray fluorescence (XRF) analyses [14]. The total sulfur content in each sample was quantified via XRF analysis. XRD analyses were performed after the samples were dried at 80 ◦C for 24 h in an oven equipped with a wide-angle goniometer, RINT 2100 XRD (Rigaku, Tokyo, Japan), operated under the following conditions: continuous scanning with 2Theta/Theta, Cu Kα radiation, 0.050◦ step scanning, a scan speed of 2000◦/min, and a scan range of 2000◦–65.000◦. During the XPS analysis, X-ray photoelectron spectra were collected for samples A and D based on the results of the sequential acid extractions over a binding energy range of 0 to 1400 eV. XPS analyses used a Kratos AXIS-165 (Shimadzu, Tokyo, Japan) operating in the Al Kα X-ray radiation mode. Spectral sets were processed using the Casa XPS software (Ver. 2.3.12). Background corrections were made using the Shirley method for S 2p and peak shapes were defined using a Gaussian–Lorentzian function [33]. The C–C bond has a well-defined position at EB (binding energy) [C 1s] = 284.6 eV and was used as a reference peak to correct for the charging effects. The XP-spectra were collected under the same conditions as those used for samples A and D.

2.2. Sequential Acid Extraction Sequential acid extractions were performed according to the methods described by Sasaki et al. [27]. A total of 2.50 g of pulverized, dried sample (<75 µm) was dissolved with 20 mL of 1 mol/L HCl, 60 mL of 10 mol/L HF, and 10 mL of concentrated HNO3. The shaking time of the Minerals 2018, 8, 403 4 of 14

mixture in the HNO3 extraction stage was changed from 2 h, previously used [27], to 6 h to completely dissolve the sulfide minerals [34]. Sulfur concentration in the extracted solutions was measured using an ICP–OES (VISTA-MPX ICP-OES, Seiko Instruments, Chiba, Japan). The proportion of each sulfur form was calculated based on the amount of sulfur that was extracted (mg/g) from the rock samples. According to previous studies, HCl extracts readily soluble minerals, HF extracts silicate minerals, and HNO3 extracts the refractory minerals such as sulfide minerals [27]. We assumed that the amount of sulfur produced in the HCl extraction stage indicated the amount of sulfate based on the sulfate dissolution process with HCl [35]. In this study, the amount produced in the HNO3 extraction stage was assumed to be the amount of sulfides. We did not measure the amount of sulfur extracted with HF because the amount of sulfur does not significantly change the water quality according to the XRD analysis results.

2.3. ABCC Test The ABCC test was conducted to understand rocks’ capacity to neutralize various species similar to the ANC test [14,17]. In this test, the samples were titrated with HCl while continuously monitoring the pH. Incremental HCl concentrations were determined according to the AMIRA procedure [14]. Even though the neutralizing capacity is not accurately quantiﬁed in the ANC test, due to Fe carbonate (e.g., siderite) dissolution effects [17], it can be quantiﬁed by accounting for the effects of siderite during the ABCC test using sequential titration [14]. In this test, we calculated the amount of H2SO4 to be added based on the volume of HCl already added and the equation proposed in the AMIRA procedure [14]. A neutralization curve was drawn by plotting kg H2SO4/ton on the X-axis and pH on the Y-axis.

2.4. Sequential NAG Test

In the NAG test, the sulfide content is commonly determined using H2O2 extractions. However, sulfides do not completely dissolve during the NAG test extraction process due to elevated sulfur concentrations. This results in misinterpretations of the NAG pH [17]. In the sequential NAG test, sulfides dissolve completely with H2O2 in several extraction stages [14]. The experimental procedure is identical to the NAG test to the point at which 15% H2O2 is added to 2.5 g pulverized dried sample (<75 µm) during a 2 h period to accelerate the reaction [14]. After 2 h, the beaker is placed on a hot plate at 150–200 ◦C and gently heated until effervescence stops. After returning to room temperature, the sample’s pH is measured as the NAG pH. The residue is reused after filtering procedures by adding 15% H2O2 until no further reaction is visually observed.

2.5. Column Leaching Test Figure1 shows a schematic view of the leaching test. A column, with an inner diameter of 40.5 mm and a height of 57 mm, was filled with 1.0–2.0 mm grain size pulverized samples (A–G) to a height of 20 mm. At the bottom of each column, to a height of 10.0 mm, 1.0 mm of glass beads, a filter paper, and a wired net were placed to prevent the samples from leaking. Column porosity was set at approximately 60% based on the calculations using the sample density and the height to which each column was filled. A total of 100 mL of deionized water (pH 6.71, EC < 1 µS/cm) was gently poured into the column. The column was allowed to saturate for one minute by closing the cock on the tube at the bottom of the column, followed by leaching by opening the cock. After measuring the pH, EC, and Eh of the leachate, the leachate was subjected to ion-chromatography (ICS-90, Dionex, Sunnyvale, CA, USA) coupled with an auto-sampler (AS50, Dionex, Sunnyvale, CA, USA) and a suppressor 2− 2+ (CSRS 300, 4 mm and ASRS 300, 4 mm, Dionex, Sunnyvale, CA, USA) to measure SO4 and Ca concentrations after the addition of 100 µL of HNO3. The samples in the column were dried using an artificial light for 6 h after leaching. The drying process continued until the point at which the column’s weight, with the sample, was identical to the weight prior to leaching. We repeated this wetting and drying cycle 40 times by assuming that these conditions were similar to the rainwater supply to rocks Minerals 2018, 8, 403 5 of 14 and drying conditions in the field. In addition, the total amount of water that was poured over the column was converted into the amount of annual Indonesian rainfall based on the column’s cross section,Minerals 2018 the, amount8, x FOR PEER of water REVIEW that was poured, and the annual Indonesian rainfall calculated using5 of 14 Equations (1)–(4) below [36]. According to these calculations, the total amount of water that we poured calculated using Equations (1)–(4) below [36]. According to these calculations, the total amount of over a period of 40 cycles corresponds to the amount of annual rainfall observed in this region. water that we poured over a period of 40 cycles corresponds to the amount of annual rainfall observed in this region. A = R2 × 3.14 = (2.025)2 × 3.14 = 12.88 (cm2) (1) A = R2 × 3.14 = (2.025)2 × 3.14 = 12.88 (cm2) (1) 3 VA = V × A = 3037 × 0.1 × 12.88 = 3912 (cm ) (2) VA = V × A = 3037 × 0.1 × 12.88 = 3912 (cm3) (2) 3 × 3 VtotalVtotal = 100= 100× 40 = 404000 = 4000(cm (cm) )(3) (3)

Y =Y V=totalV/totalVA /=V 1.02A = (year) 1.02 (year) (4) (4)

2 2 wherewhereA Ais is the the sectional sectional area area of of the the column column (cm (cm),),R Ris is the the internal internal diameter diameter (cm), (cm),V VAAis is the the volume volume of of thethe water water in in the the column column calculated calculated based based on on regional, regional, annual annual rainfall rainfall (cm (cm3),3),V Vis is the the annual annual rainfall rainfall (mm),(mm),V Vtotaltotal isis thethe totaltotal volumevolume ofof waterwater pouredpoured duringduring thethe leachingleaching test,test, and andY Yis is the the ratio ratio of of the the total total amountamount ofof waterwater pouredpoured intointo thethe columncolumn toto thatthatrequired required to to be be equivalent equivalent to to the the total total Indonesian Indonesian annualannual rainfall. rainfall.

FigureFigure 1. 1.Schematic Schematic of of the the leaching leaching test. test.

3.3. Results Results and and Discussion Discussion 3.1. Geochemical Characteristics 3.1. Geochemical Characteristics Figure2 shows the sample XRD patterns, and Table1 summarizes the results of the ABA, NAG, Figure 2 shows the sample XRD patterns, and Table 1 summarizes the results of the ABA, NAG, paste pH, and Bernard calcimeter tests. Quartz and kaolinite (Al4Si4O10(OH)8) peaks were obtained paste pH, and Bernard calcimeter tests. Quartz and kaolinite (Al4Si4O10(OH)8) peaks were obtained for all samples. Samples C and E show clear pyrite (FeS2) peaks, which are not observed in the other for all samples. Samples C and E show clear pyrite (FeS2) peaks, which are not observed in the other samples. Illite [(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)], muscovite (KAl2AlSi3O10(OH)2), albite samples. Illite [(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)], muscovite (KAl2AlSi3O10(OH)2), albite (NaAlSi3O8), and siderite (FeCO3) are also observed in the samples, and samples B and G showed clear (NaAlSi3O8), and siderite (FeCO3+ 3) are also observed in the samples, and samples B and G showed siderite peaks. Jarosite (KFe 3(OH)6(SO4)2) is only observed in samples D and F. In Table1, samples C clear siderite peaks. Jarosite (KFe3+3(OH)6(SO4)2) is only observed in samples D and F. In Table 1, and E are characterized by high NAPP values (383.9 and 776.6 kg H2SO4/ton, respectively), which are samples C and E are characterized by high NAPP values (383.9 and 776.6 kg H2SO4/ton, respectively), supported by clear pyrite peaks in Figure2. Although we do not observe pyrite, except for samples C which are supported by clear pyrite peaks in Figure 2. Although we do not observe pyrite, except for and E, all samples have positive NAPP values which causes AMD formation. All samples are classiﬁed samples C and E, all samples have positive NAPP values which causes AMD formation. All samples as AMD source rocks according to the classiﬁcation scheme described in Miller et al. [14,37]. Clear are classified as AMD source rocks according to the classification scheme described in Miller et al. siderite peaks in samples B and G support the presence of elevated ANC values (20.3 and 9.2 kg [14,37]. Clear siderite peaks in samples B and G support the presence of elevated ANC values (20.3 and 9.2 kg H2SO4/ton, respectively). In addition, the elevated carbonate content in sample B is consistent with the presence of a siderite peak (Figure 2). Although previous studies have reported that siderite acts as both a neutralizer and an acid producer under certain conditions [18], based on the experimental conditions used in this study, we expect siderite to act as a neutralizer according to the acidic conditions associated with AMD formation (pH 1–5).

Minerals 2018, 8, 403 6 of 14

H2SO4/ton, respectively). In addition, the elevated carbonate content in sample B is consistent with the presence of a siderite peak (Figure2). Although previous studies have reported that siderite acts as both a neutralizer and an acid producer under certain conditions [18], based on the experimental conditions used in this study, we expect siderite to act as a neutralizer according to the acidic conditions Mineralsassociated 2018, with8, x FOR AMD PEER formation REVIEW (pH 1–5). 6 of 14

FigureFigure 2. 2. SampleSample X X-ray-ray diffraction diffraction (XRD) (XRD) patterns. patterns. Ill: Ill: Illite Illite,, Ka: Ka: Kaolinite, Kaolinite, Qz:Qz: Quartz, Quartz, Ja: Jarosite, Ja: Jarosite, Py: Pyrite,Py: Pyrite, Al: Albite, Al: Albite, Si: Siderite, Si: Siderite, and andMu: Mu:Muscovite. Muscovite.

TableTable 1. SummarySummary of of the the sulfur sulfur content, content, and and results results of of the the acid acid–base–base accounting (ABA) (ABA),, n netet a acidcid ggeneratingenerating (NAG) (NAG),, paste paste pH, pH, and and Bernard Bernard calcimeter calcimeter tests. tests. AP AP:: acid acid potential, potential, ANC: ANC: acid acid neutralizing neutralizing capacity,capacity, NAPP: net acid producing potential, NAG pH: net acid generating pH. AP ANC NAPP Sample SulfurSulfur (%) AP ANC NAPPPaste pHPaste NAG pHNAG CarbonateCarbonate (%) Sample (kg H2SO4/ton) (kg H2SO4/ton) (kg H2SO4/ton) (%) (kg H2SO4/ton) (kg H2SO4/ton) (kg H2SO4/ton) pH pH (%) A 0.89 27.4 0.0 27.4 3.10 2.80 0.50 AB 0.89 0.77 23.527.4 20.30.0 3.227.4 2.823.10 2.28 2.80 1.580.50 BC 0.77 12.55 383.923.5 0.020.3 383.93.2 1.212.82 1.04 2.28 0.231.58 D 4.99 152.8 0.0 152.8 1.74 1.47 0.30 CE 12.55 25.46 779.1383.9 2.50.0 776.6383.9 1.241.21 1.08 1.04 0.380.23 DF 4.99 5.67 173.5152.8 3.70.0 169.8152.8 4.801.74 2.90 1.47 0.240.30 EG 25.46 0.93 779.1 28.7 9.22.5 19.6776.6 6.161.24 4.37 1.08 0.290.38 F 5.67 173.5 3.7 169.8 4.80 2.90 0.24 GThere is a0.93 large difference28.7 between the paste9.2 and NAG pH for19.6 samples B,6.16 F, and4.37 G. This difference0.29 results from the neutralization effects of carbonate dissolution, as indicated by the high ANC values (20.3,There 3.7, andis a 9.2large kg difference H2SO4/ton, between respectively). the paste Carbonate and NAG minerals pH for aresamples characterized B, F, and by G. higher This differencevalues of solubilityresults from relative the neutralization to sulfide minerals. effects of Therefore, carbonate nearly dissolution, all carbonate as indicated minerals by the dissolve high ANCin the values first water (20.3, flush 3.7, and without 9.2 kg completely H2SO4/ton, dissolving respectively) the.sulfide Carbona mineralste minerals during are thecharacterized paste pH test. by higherThis results values in of an solubility increase inrelative the paste to sulfide pH. On minerals. the other Therefore, hand, in thenearly NAG all pH carbonate test, decreases minerals in dissolvethe NAG in pH, the associatedfirst water withflush sulfide without mineral complete dissolutionly dissolving using the H sulfide2O2, surpass minerals the during carbonate the bufferpaste pHeffects. test. InThis addition, results in although an increase sample in the B exhibitspaste pH. the On highest the other values hand, for in both the NAG ANC pH and test, carbonate decrease (%)s invia the the NAG Bernard pH, calcimeterassociated testwith (20.3 sulfide kg Hmineral2SO4/ton dissolution and 1.58%), using positive H2O2, ANCsurpass values the carbonate for samples buffer E, F, effects.and G areIn addition, not correlated although with thesample carbonate B exhibit contents the (%).highest The values Bernard for calcimeter both ANC test and analyzes carbonate the total(%) viacarbonate the Bernard in samples calcimeter via CO test2 (20.3volumetric kg H2SO analysis,4/ton and which 1.58%) is liberated, positive viaANC the values addition for sample of HCls to E, the F, andsamples G are [28 not,29 ].correlated While the with Bernard the carbonate calcimeter content test is a(%). simple The and Bernard inexpensive calcimeter method, test analyzes the errors the of totalless thancarbonate 5% in in the samples test results via CO should2 volumetric be taken analysis into account, which [38 is]. liberated via the addition of HCl to the samples [28,29]. While the Bernard calcimeter test is a simple and inexpensive method, the errors of less than 5% in the test results should be taken into account [38]. Therefore, the mineral form, within a rock, contributes significantly to pH changes, and the effect on changes in water quality over time are not identifiable solely using the ABA and NAG tests.

Minerals 2018, 8, 403 7 of 14

Therefore, the mineral form, within a rock, contributes signiﬁcantly to pH changes, and the effect on changes in water quality over time are not identiﬁable solely using the ABA and NAG tests.

3.2. Changes in Water Quality During the Leaching Test Figure3 plots the changes in the pH during the leaching tests, and Figure4 shows the change in EC and Eh for 40 leaching cycles. Changes in pH are divided into three patterns: a change at a highMinerals pH 2018 in, sample, 8,, xx FORFOR GPEERPEER (approximately REVIEWREVIEW pH 7.4), a gradual increase in samples A and D, and a change7 of 14 at a low pH in samples B, C, E, and F (approximately pH 3.0). Comparing the results in Table1 with changeslowlow pHpH in inin pH samplessamples during B,B, the C,C, leaching E,E, andand FF test, (approximately we observe that pH the 3.0) change.. ComparingComparing at a high thethe pH resultsresults in sample inin TableTable G correlates 11 withwith changes inin pHpH during thethe leachingleaching ttest, we observe that thethe change at a high pH in sample G with decreased AP, elevated ANC, and a pH of 6.16 for the paste pH. However, samples A and B are correlates with decreased AP, elevated ANC, and a pH of 6.16 for the paste pH. However, samples characterized by a lower pH than sample G even though their AP values are lower than that of sample A and B are characterized by a lowerlower pHpH thanthan sample G even though their AP values are lowerlower thanthan G. AP values for samples A and B are 27.4 and 23.5 kg H2SO4/ton, respectively, and that for sample G thatthat ofof sample G. AP values for samples A and B are 27.4 and 23.5 kg H22SO44/ton,/ton, respectively,respectively, and is 28.7 kg H2SO4/ton. This difference is due to the neutralization effects indicated by their respective thatthat forfor sample G isis 28.7 kg H22SO44/ton./ton. ThisThis difference isis duedue toto thethe neutralization effects indicated ANC values. Furthermore, samples A and B exhibit an opposite trend in pH changes relative to their by theirir respectiverespective ANC values. Furthermore,, samplessamples AA andand BB exhibit an opposite trend in pH NAPP values. Although samples C, E, and F, with NAPP values of more than 100 kg H SO /ton, yield changes relative to theirir NAPP values. Although samples C, E, and F,, with NAPP values2 4 of more a steady change at a pH of 3 as well as high EC values (more than 7.0 mS/cm) during early leaching thanthan 100100 kg H22SO44/ton/ton,, yield a steady change at a pH of 3 as well as high EC values (more than 7.0 stages, as shown in Figures3 and4, sample D shows changes similar to sample A despite a NAPP mS/cm) during early leachingleaching stages,, asas shownshown inin Figures 3 and 4, sample D shows changes similar value of more than 100 kg H2SO4/ton. Based on these results, changes in pH associated with AMD toto sample A despite a NAPP value of more than 100 kg H22SO44/ton./ton. Based on thethese results, changes inin occurrence over time are difﬁcult to estimate based solely on pH values from the NAPP and NAG pH associated with AMD occurrence over time are difficult to estimate based solely on pH values tests, which are used in current schemes to classify waste rocks. With regard to cover materials, for the from thethe NAPP and NAG teststests,, whichwhich areare usedused inin currentcurrent schemes toto classify waste rocks.. WithWith scenario with an absence of non-source rocks in the mining area, samples A and D, whose acidity is regard to cover materials, for the scenario with an absence of non-source rocks inin thethe mining area,, expectedsamples toA and decrease D, whose in arelatively acidity is shortexpected period to decrease with a gradual in a relatively increase short in pH, period are preferable with a gradual cover systemincreaseincrease rocks inin pH,pH, due areare to preferable their low potentialcover system to produce rocks due acid. to theirtheir lowlow potential to produce acid..

(a)(a) (b)(b)

FigureFigure 3. 3.( a((a))) Changes Changes in inin pH pHpH overoverover thethethe 404040 leachingleachingleaching cyclescycles during thethe leachingleaching testtest test;;; (( (bb))) change changess inin in pHpH pH withoutwithout sample sample G. G..

FigureFigure 4. 4.Changes Changesin inin ( ((aa))) electrical electrical conductivityconductivity (EC) and (b)) electric electric potential potential of of the the normal normal hydrogen hydrogen electrodeelectrode (Eh) (Eh) over over the the 40 40 leaching leaching cyclescycles duringduring thethethe leachingleachingleaching test.test.

3.3. Neutralization Effects Figure 3 shows a small peak in samples B and F over the first 10 cycles of leaching. This results from thethe neutralization effects caused by carbonate dissolution, as indicated by high ANC values. Meanwhile, sample E also has a positive ANC value, whose pH changes at low pH (less than pH 3.0) due to a high AP value (779.1 kg H22SO44/ton/ton),, whichwhich exceeds the neutralizing capacity effects.. TheThe appearance of a small peak in samples B and F isis consistent with a decrease inin Eh during thethe firstfirst 1010 leachingleaching cyclescycles inin Figure 4,, which suggests the presence of a neutralization effect. The Eh value for

Minerals 2018, 8, 403 8 of 14

3.3. Neutralization Effects Figure3 shows a small peak in samples B and F over the first 10 cycles of leaching. This results from the neutralization effects caused by carbonate dissolution, as indicated by high ANC values. Meanwhile, sample E also has a positive ANC value, whose pH changes at low pH (less than pH 3.0) due to a high AP value (779.1 kg H2SO4/ton), which exceeds the neutralizing capacity Mineralseffects. 2018 The, 8 appearance, x FOR PEER REVIEW of a small peak in samples B and F is consistent with a decrease in Eh during8 of 14 the first 10 leaching cycles in Figure4, which suggests the presence of a neutralization effect. The Eh samplevalue for E, samplewith a positive E, with ANC a positive value, ANC also decrease value, alsos slightly decreases at the slightly beginning at the of beginningleaching but of increases leaching immediatelybut increases afterward immediately due afterward to its high due potential to its high to potentialproduce toacid. produce Moreover, acid. Moreover,high ANC highvalues ANC in samplevalues ins B samples and F only B and affect F only the pH affect for the the pH first for 10 the leaching first 10 cycles, leaching shown cycles, by the shown appearance by the appearance of a small peakof a small, which peak, suggest whichs that suggests estimat thating estimating long-term long-term pH change pHs changesis difficult is difficult when using when only using the only ANC the valueANC.value. FigureFigure 55 showsshows thethe changechange inin pHpH withwith thethe additionaddition ofof HClHCl duringduring thethe ABCCABCC test.test. SignificantSignificant neutralizationneutralization effects effects are are observed observed in in sample sample B, B, which which exhibits exhibits a a small small peak peak in in Figure Figure 33,, asas wellwell asas samplesample G,G, which which is characterizedis characterized by a by steady a steady change change in pH (near in pH pH =(near 7). Both pH observed = 7). Both neutralization observed neutralizationeffects are caused effects by siderite are caused dissolution by sideri [18].te Although dissolution neutralization [18]. Although effects neutralization in sample B appear effects as in a samplesmall peak B appear during as the a small first 10 peak leaching during cycles the (Figurefirst 103 leaching), a steady cycles change (Figure near pH 3), =a 3steady is observed change due near to pHa higher = 3 is value observed of AP due than to ANC. a higher However, value sampleof AP than G exhibits ANC. aHowever, steady change sample in theG exhibit pH (nears a pHsteady = 7) changedespite in a higherthe pH value(near ofpH AP = 7 compared) despite ato higher ANC. value Based of on AP the compared paste pH to for ANC. sample Based G, on this the value paste is pHthe for highest sample among G, th allis value samples is the (pH highest 6.16). Thus,among the all ABCC samples test (pH results 6.16) should. Thus, bethe combined ABCC test with results the shouldpaste pH be testcombined results with to predict the paste AMD. pH Althoughtest results sample to predict F shows AMD. a Although small peak sample (Figure F shows3), we a do small not peakclearly (Figure observe 3), we a neutralization do not clearly effect observe during a neutralization the ABCC test.effect The during effects the of ABCC high test. AP valuesThe effects exceed of highany occurrenceAP values ofexceed neutralization any occurrence effects of in neutralization sample F during effects the in ABCC sample test, F whichduring leads the ABCC to a steady test, whichchange leads near to a pHa steady of 3. In change the ANC near and a pH Bernard of 3. In calcimeter the ANC tests,and Bernard the solution calcimeter is titrated test tos, the quantify solution the isrock titrated neutralizing to quantify capacity the afterrock sampleneutralizing dissolution capacity with after the sample required dissolution concentration with and the amount required of concentrationacids, which yieldsand amount undissolved of acids, minerals which duringyields undissolved the tests. In mine addition,rals during since the the fizz tests. rate In inaddition, the fizz sincetest (included the fizz rate in thein the ANC fizz test) test is(included determined in the via ANC visual test observation,) is determined the via test visual results observation, depend on the the testexperience results depend and skill on of the the experience observers. and Conversely, skill of the during observers the ABCC. Conversely, test, we during obtain the the ABCC generation test, weof acidobtain production the generation and neutralization of acid production over time and via neutralization titration with over HCl time in several via titration stages with until HCl the pH in severalreaches stages 2.5. The until concentration the pH reaches and 2.5. amount The concentration of HCl added and during amount titration of HCl are added accounted during for titration when arecalculating accounted the for values when plotted calculatin in Figureg the values5. Therefore, plotted thein Figure ABCC 5. test, Therefore, with less the human ABCC error,test, with is more less humanreliable error when, is evaluating more reliable the rock when neutralization evaluating the effects rock compared neutralization to the effects ANC andcompared Bernard to calcimeter the ANC andtests. Bernard The appearance calcimeter of tests. a small The peak appearance during the of firsta small 10 leaching peak during cycles the indicates first 10 that leaching it is difficult cycles indicatesto evaluate that long-term it is difficult pH changesto evaluate that long result-term from pH the change AMDs occurrencethat result whenfrom the relying AMD solely occurrence on the whenABCC relying test. The solely results on the of the ABCC ABCC test. test The are results useful of to the evaluate ABCC pHtest changesare useful that to occurevaluate only pH for change a shorts thatperiod occur during only the for leaching a short tests period conducted during in this the study leaching (i.e., approximately tests conducted 3 months in this in thestudy field (i.e., area approximatelybased on these 3 calculations). months in the field area based on these calculations).

FigureFigure 5. ChangeChangess in in pH pH with with the the addition addition of of HCl HCl during the acid buffering characteristic curve (ABCC)(ABCC) test. test.

Figure 6 shows the changes in the total amount of SO42− and Ca2+ over 20 leaching cycles. The change in gradient from a steep to gentle slope at 3–5 leaching cycles in all samples represents the decrease in the amount of released SO42− and Ca2+ with each progressive leaching cycle. This indicates that minerals related to SO42− and Ca2+, in particular, dissolve during the early leaching stages. Sample B exhibits the highest concentration of Ca2+ and the lowest concentration of SO42− (Figure 6), resulting in a small peak during the early leaching stages (Figure 3). This is consistent with its high ANC value (Table 1). However, samples C and E, which have no small peaks in Figure 3, are characterized by high Ca2+ concentrations compared to sample F. This suggests that an increase in pH is not simply associated with Ca2+ dissolution. Since samples C and E have high SO42− concentrations and high

Minerals 2018, 8, 403 9 of 14

2− 2+ Figure6 shows the changes in the total amount of SO 4 and Ca over 20 leaching cycles. The change in gradient from a steep to gentle slope at 3–5 leaching cycles in all samples represents the 2− 2+ decrease in the amount of released SO4 and Ca with each progressive leaching cycle. This indicates 2− 2+ that minerals related to SO4 and Ca , in particular, dissolve during the early leaching stages. Sample 2+ 2− B exhibits the highest concentration of Ca and the lowest concentration of SO4 (Figure6), resulting in a small peak during the early leaching stages (Figure3). This is consistent with its high ANC value (Table1). However, samples C and E, which have no small peaks in Figure3, are characterized by Minerals 2018, 8, x FOR PEER REVIEW 9 of 14 high Ca2+ concentrations compared to sample F. This suggests that an increase in pH is not simply 2+ 2− associatedNAPP values with, small Ca peaksdissolution. do not Sinceoccur samples (Figure C3) anddue Eto have significant high SO acid4 producconcentrationsed by the and sample. high NAPPFurthermore, values, sample small peaks F displays do not a small occur peak (Figure in Figure3) due 3 to and signiﬁcant generates acid high produced Ca2+ concentration by the sample.s and 2+ Furthermore,low SO42− concentrations sample F displays compared a small to peaksamples in Figure B, E,3 and generatesC (Figure high 6). CaSampleconcentrations G, which is 2− andcharacterized low SO4 byconcentrations a steady change compared in pH (pH to samples= 7; Figure B, 3 E,), andalso Chas (Figure the lowest6). Sample SO42− concentration G, which is 2− characterized(Figure 6). Consequently, by a steady change mineral in dissolution pH (pH = 7; related Figure 3to), alsothe hasrelease the lowestof SO42 SO− and4 Caconcentration2+ influences 2− 2+ (Figurechange6s). in Consequently,water quality associated mineral dissolution with AMD. related However, to the since release ABCC of test SO results4 and are Ca consistentinﬂuences with changesthe change in water in pH quality for only associated 10 leaching with cycles, AMD. shown However, by sincethe appearance ABCC test of results a small are peak consistent such withas in thesample change B, inestimating pH for only changes 10 leaching in water cycles, quality shown associated by the appearance with AMD of a smallover peaktime suchrequires as in another sample B,parameter. estimating changes in water quality associated with AMD over time requires another parameter.

2−2− 2+ FigureFigure 6. 6.Integrated Integrated concentrations concentrations of of dissolved dissolved (a ()a SO) SO4 4 and (b) Ca overover 20 20 leaching leaching cycles. cycles.

3.4.3.4. EffectsEffects ofof thethe FormForm ofof SulfurSulfur FigureFigure7 shows7 shows the the proportion proportion of sulfide of sulfide and sulfate and sulfate in all samples, in all calculatedsamples, calculated based on sequential based on acidsequential extraction acid results. extraction We results. calculate We the calculate ratio of the amountratio of the of sulfuramount in of the sulfur HCl andin the HNO HCl3 andextraction HNO3 stagesextraction to the stages total to amount the total of amount sulfur. The of sulfur. samples The are samples categorized are categori into twozed groups, into two samples groups A, samples and D, inA whichand D, more in which than more 90% ofthan the 90% sulfur of isthe sulfate, sulfur andis sulfate, samples and B, samples C, E, F, andB, C, G, E, in F, which and G, more in which than 50%more of than the sulfur50% of is the sulfide. sulfur For is sulfide. samples For D sample and F, thes D sulfateand F, the content sulfate is derivedcontent fromis derive jarosited from (Figure jarosite2). Since(Figure clear 2). jarositeSince clear peaks jarosite are not peaks observed are not in observed samples Ain andsample G dues A toand low G sulfurdue to content low sulfur (0.89% content and 0.93%,(0.89% respectively), and 0.93%, respectively) compared to, samplescompared D andto sample F (4.99%s D andand 5.67%, F (4.99% respectively), and 5.67%, the respectively) sulfate content, the insulfate samples content A and in sample G is alsos A derivedand G is fromalso derive jarosite.d from Sulfate jarosite. dissolution Sulfate dissol in samplesution in A samples and D during A and theD during early leaching the early stages leaching due stages to high due sample to high solubility sample maysolubility have ledmay to have gradual led to increases gradual in increase the pHs (Figurein the pH3). Since(Figure there 3). Since is a difference there is a ofdifference more than of 100more kg than H 2SO 1004/ton kg H for2SO the4/ton NAPP for the values NAPP between values samplesbetween Asamples and D, A the and sulfur D, the form sulfur appears form to appears be a more to be valuable a more factorvaluable totrack factor changes to track in change pH than,s in forpH example, than, for theexample, NAPP the and NAPP NAG and pH. NAG Although pH. Although sample B sample shows aB smallshows peak a small in pH peak during in pH the during first 10the leaching first 10 cycles,leaching the cycles, high sulfide the high proportion sulfide (moreproportion than 90%(more of than the sulfur) 90% of causes the sulfur) pH changes causes near pH achange pH ofs 2.5–3.0near a afterpH of 10 2.5 leaching–3.0 after cycles. 10 leaching Conversely, cycles. significant Conversely, neutralization significant neutralization effects and a highereffects proportionand a higher of sulfateproportion in sample of sulfate G compared in sample with G compared sample B leadwith tosample changes B le atad high to change pH in thes at leachinghigh pH testin the (approximately leaching test (approximately pH 7.4), although pH sample 7.4), although B has similar sample NAPP B has values.similar ChangesNAPP values. in water Change qualitys in shouldwater quality be estimated should from be estimated the proportion from the of differentproportion sulfur of different forms combined sulfur forms with combin the NAPPed with value, the calculatedNAPP value, from calculated the absolute from value the absolute of the sulfur value content. of the sulfur However, content. although However, a good although relation a exists good betweenrelation sequentialexists between extraction sequential and leaching extraction test and results leaching obtained test in results this study, obtained extraction in this with study, HCl extraction with HCl may dissolve acid-soluble sulfide or Fe(III) hydroxide sulfate minerals [18], thereby requiring further experimentation with this testing method.

Minerals 2018, 8, 403 10 of 14 may dissolve acid-soluble sulﬁde or Fe(III) hydroxide sulfate minerals [18], thereby requiring further MineralsexperimentationMinerals 2018 2018, ,8 8, ,x x FOR FOR with PEER PEER this REVIEW REVIEW testing method. 1010 ofof 1414

FigureFigure 7.7.7. Proportion PProportionroportion of ofof sulﬁde sulfidesulfide and andand sulfate sulfatesulfate in all inin samples allall samplessamples calculated calculatedcalculated based on basedbased sequential onon sequentialsequential acid extraction acidacid extractionresults:extraction calculated results:results: calculatedcalculated via the ratio viavia of thethe the ratioratio amount ofof thethe of amountamount sulfur in ofof the sulfursulfur HCl inin and thethe HNO HClHCl and3andextraction HNOHNO33 extractionextraction stages to stagestotalstages sulfur. to to total total sulfur. sulfur.

AlthoughAlthough nearly nearlynearly all allall sulfur sulfursulfur in inin samples samplessamples A AA and andand D D i isiss sulfate sulfate,sulfate,, these these samples samples d dodoo not not exhibit exhibit a aa clear clear sulfatesulfate peak peak during during the the XRD XRD analysis analysis ( (Figure(FigureFigure 2 22).)).. Therefore, Therefore,Therefore, the thethe S SS 2p 2p2p XP-spectra XPXP--spectraspectra for forfor samples samplessamples A AA and andand D DD are collected during XPS analysis. The binding energy is corrected using E [C 1 ] from hydrocarbon areare collected collected during during XPS XPS analysis. analysis. The The binding binding energy energy is is corrected corrected using using E EBBB [C [C 1 1SASA]] from from hydrocarbon hydrocarbon = 284.6 eV, as shown in Figure8. According to previous studies, the binding energies of S 2p for iron == 284.6284.6 eV,eV, asas shownshown inin FigureFigure 8.8. AccordingAccording toto previousprevious studies,studies, thethe bindingbinding energiesenergies ofof 3/2 SS 2p2p3/23/2 forfor ironsulfidesiron sulfidessulfides and/or and/orand/or elemental elementalelemental sulfur sulfursulfur are 163–164 areare 163163 eV,––164164 and eV,eV, that andand of thatthat sulfate ofof sulfatesulfate is 168–171 isis 168168 eV––171171 [39 – eVeV41 ]. [[3939 The––4141 sulfate].]. TheThe sulfatevaluessulfate obtainedvaluesvalues obtainedobtained for samples forfor samplesample A andss D AA (Figure andand DD 8 ()(FigureFigure are in 8 good8)) aarere agreement inin goodgood agreementagreement with the bindingwithwith thethe energies bindingbinding energiesreportedenergies reported inreported previous inin previous studies.previous Furthermore, studies.studies. Furthermore,Furthermore, XPS analysis XPSXPS reveal analysisanalysis the revealreveal presence thethe ofpresencepresence sulfide of inof sulfide samplesulfide in Din sampleinsample addition DD inin to addition addition the sulfate toto thethe content sulfatesulfate observed contentcontent observedbasedobserved on sequentialbasedbased on on sequentialsequential acid extraction acidacid extractionextraction results. This resultsresults indicates.. ThisThis indicatesthatindicates the sulfur thatthat tthe inhe sample sulfursulfur inin D samplesample consists DD of consistsconsists a large of amountof aa largelarge of amountamount sulfate of alongof sulfatesulfate with alongalong a small withwith amount aa smallsmall of amountamount sulfide. ofNoof sulfide.sulfide. iron sulfides NoNo ironiron and/or sulfidessulfides elemental and/orand/or elementalelemental sulfur peaks sulfursulfur are peakpeak observedss aarere observedobserved in sample inin sample Asample due toAA due itsdue small toto itsits sulfur smallsmall sulfurconcentrationsulfur concentrationconcentration compared comparedcompared to that to into that samplethat inin samplesample D (Table DD 1 (().TableTable 11))..

FigureFigure 8.8. XX-rayX--rayray photoelectron photoelectronphotoelectron (XP) (XP)-spectra(XP)--spectraspectra of ofof ( ((aaa))) C CC 1s 1s1s and andand ( ((bb))) S SS 2p 2p2p for forfor samples samplessamples A AA and andand D. D.D. The TheThe binding bindingbinding energyenergy is is corrected corrected using using using E E EBB B[C [C[C 1s 1sA 1sA]] Afrom from] from hydrocarbon hydrocarbon hydrocarbon = = 284.6 284.6 = 284.6 eV. eV. Peak eV.Peak Peak a a of of C aC 1sof 1s corresponds Ccorresponds 1s corresponds to to 284.6 284.6 to eV,284.6eV, peak peak eV, b peakb of of S Sb 2p 2pof3/23/2 S corresponds corresponds 2p3/2 corresponds to to 168.5 168.5 to eV, eV, 168.5 and and eV, peak peak and c c peak o off S S 2p c2pof3/23/2 Scorresponds corresponds 2p3/2 corresponds to to 163.2 163.2 to eV. eV. 163.2 eV.

FigureFigure 9 9 9 plots plotsplots the thethe sulfur sulfursulfur dissolution dissolutiondissolution rateraterate overoverover fivefivefive leachingleachingleaching cyclescyclescycles duringduringduring thethethe leachingleachingleaching test,test,test, calculatedcalculated based basedbased on onon the thethe ratio ratioratio of ofof the the amount amount of ofof dissolved dissolveddissolved sulfur sulfursulfur in in the thethe first firstfirst five fivefive cycles cycles to to the the total total amountamount of of dissolveddissolved sulf sulfursulfurur throughout throughout thethe test.test. Sulfur SulfurSulfur dissolution dissolutiondissolution primarily primarilyprimarily occurs occursoccurs during duringduring the thethe early earlyearly leachingleachingleaching stages stagesstages for forfor samples samplessamples A AA and andand D.D. TheThe amountamount ofof dissolveddissolved sulfur sulfursulfur in inin the thethe first firstfirst five fivefive leaching leaching cyclescycles accountsaccounts forforfor more moremore than thanthan 90% 90%90% of ofof the thethe total totaltotal dissolved dissolveddissolved sulfur sulfursulfur during duringduring the leaching thethe leachingleaching test due testtest to duedue the hightoto thethe sulfate highhigh sulfateproportionsulfate proportionproportion in samples inin samples Asamples and D (moreAA andand than DD (more(more 90%of thanthan the sulfur).90%90% ofof Thisthethe sulfur) issulfur) consistent.. ThisThis with isis consistentconsistent the observation withwith thethe of observationaobservation high sulfate ofof proportion aa highhigh sulfatesulfate in samples proportionproportion A and inin samplesD,samples as shown AA andand in FiguresDD,, asas shownshown7 and 8 inin. The FiguresFigures sulfur 77 andand dissolution 8.8. TheThe sulfursulfur rate dissolutionindissolution Figure9 does raterate not inin Figure correlateFigure 99 does withdoes thenotnot NAPP correlatecorrelate value, withwith calculated thethe NAPPNAPP using value,value, the calculated sulfurcalculated content, usingusing but thethe roughly sulfursulfur content,content, but but roughly roughly correlate correlatess with with the the sulfate sulfate proportion proportion in in rocks rocks ( (FigureFigure 7 7)).. N Nonon--sulfidesulfide sulfur sulfur ( (i.e.,i.e., thethe sulfate sulfate sulfur sulfur content) content) is is calculated calculated as as the the acid acid potential potential in in the the NAPP NAPP in in sample sampless A A and and D D in in Table Table 1.1. Although Although the the NAPP NAPP values values a arere higher higher in in sample sample A A than than sample sample B, B, the the paste paste and and NAG NAG pH pH a arere lower lower inin samplesample BB thanthan samplesample AA duedue toto thethe occurrenceoccurrence ofof sulfatesulfate dissolutiondissolution inin thethe pastepaste pHpH andand NAGNAG

Minerals 2018, 8, x FOR PEER REVIEW 11 of 14

tests. This also causes discrepancies among the NAPP, paste pH, and NAG pH values in samples A and B in addition to neutralization effects in sample B, as discussed in Section 3.2. Table 2 summarizes the changes in the NAG pH during the sequential NAG test. Although it is difficult to understand AMD formation over time resulting from rock sulfur form based solely on NAG pH (listed in Table 1), the results in Table 2 imply that there is low acid production potential in samples A and G, indicated by a NAG pH of more than 4.50 after five stages of the dissolution process. This is also supported by changes in pH shown in Figure 3 and XPS analyses that show the existence of sulfates (Figure 8). However, the NAG pH of sample D does not exceed 4.50 even after seven stages due to the existence of sulfides (Figure 8). The heterogeneous sulfide distribution in sample D is also similar to pH changes in sample A (Figure 3). These results suggest that the effects

Mineralsof the sulfur2018, 8, form 403 and the heterogeneous sulfur distribution in the rocks are criteria for the evalua11 oftion 14 of rock acid production, which is taken into account by categorizing waste rocks based on their NAG pH using the sequential NAG test in conjunction with the NAPP and NAG pH tests. Changes in NAG correlatespH in the withsequential the sulfate NAG proportion test reflect inthe rocks rock (Figure sulfur 7so).lubility, Non-sulﬁde which sulfur provid (i.e.,es information the sulfate sulfur on the content)form of issulfur calculated contained as the with acidin potentialthe rocks in and the the NAPPir acid in-producing samples A andpotential. D in TableTherefore,1. Although changes the in NAPPNAG pH values during are higherthe sequential in sample NAG Athan test are sample useful B, when the paste estimat anding NAG change pH ares in lowerwater inquality sample over B thantime. sample A more A precise due to theclassification occurrence of of cover sulfate materials dissolution (non in-source), the paste low pH-potential and NAG source tests. rocks, This also and causesAMD discrepanciessource rocks is among achievable the NAPP, using paste the sequential pH, and NAG NAG pH test values in combination in samples Awith and the B inNAPP addition and toNAG neutralization pH tests, which effects take in sample into account B, as discussed the effects in that Section the form3.2. of sulfur has on AMD.

FigureFigure 9. 9.Proportion Proportion ofof thethe totaltotal amountamount ofof dissolveddissolved sulfursulfur forfor thethe firstfirstfive fiveleaching leachingcycles cyclesin in the the leachingleaching test. test. The The proportion proportion is is calculated calculated based based onon the the ratio ratio of of the the total total dissolved dissolved sulfur sulfur during during the the firstfirst five five leaching leaching cycles cycles to to the the amount amount over over 40 40 leaching leaching cycles. cycles.

Table2 summarizesTable the 2. changes Change ins in the NAG NAG pH during pH during the sequential the sequential NAG test. NAG test. Although it is difficult to understand AMD formation over time resulting from rock sulfur form based solely on NAG pH (listedSample/Stage in Table1), the results1 in2 Table 2 imply3 that4 there is low5 acid6 production 7 potential in samples A and G,A indicated2.65 by a NAG3.49 pH of4.20 more than4.35 4.50 after4.65 five stages- of the- dissolution process. This is alsoB supported by2.28 changes 2.81 in pH3.34 shown in3.81 Figure 3 4.12and XPS4.22 analyses 4.31 that show the existence of sulfatesC (Figure 8). However,2.01 2.64 the NAG2.94 pH of sample3.38 D3.71 does not4.00 exceed 4.07 4.50 even after seven stages due toD the existence2.14 of sulfides2.77 (Figure2.79 8). The2.88 heterogeneous 3.28 3.77 sulfide 3.95 distribution in sample D is also similarE to pH changes1.66 in1.87 sample A1.81 (Figure 32.01). These 2.30 results suggest2.83 that3.21 the effects of the sulfur form and theF heterogeneous2.41 sulfur2.84 distribution3.05 in3.66 the rocks4.02 are criteria4.22 for the4.12 evaluation of rock acid production,G which is taken2.81 into3.39 account by3.86 categorizing 4.21 waste4.53 rocks based- on- their NAG pH using the sequential NAG test in conjunction with the NAPP and NAG pH tests. Changes in NAG pH in4. theConclusions sequential NAG test reflect the rock sulfur solubility, which provides information on the form of sulfurThis contained study applied within several the rocks simple and testing their acid-producing methods to the potential. characterization Therefore, of mine changes waste in NAGrocks pHand during AMD theprediction sequential with NAG a focus test on are the useful form whenof sulfur estimating, the neutralization changes in effects water qualityof carbonates over time., and Athe more AMD precise formation classification over time. of C coverhange materialss in pH associated (non-source), with low-potentialAMD occurrence source over rocks, time are and difficult AMD sourceto estimate rocks based is achievable solely on using the theNAPP sequential and NAG NAG pH test tes ints, combinationwhich are currently with the used NAPP to andclassif NAGy waste pH tests,rocks which. ABCC take test into results account are useful the effects for evaluating that the form change of sulfurs in pH has only on AMD. during short periods, such as

Table 2. Changes in NAG pH during the sequential NAG test.

Sample/Stage 1 2 3 4 5 6 7 A 2.65 3.49 4.20 4.35 4.65 - - B 2.28 2.81 3.34 3.81 4.12 4.22 4.31 C 2.01 2.64 2.94 3.38 3.71 4.00 4.07 D 2.14 2.77 2.79 2.88 3.28 3.77 3.95 E 1.66 1.87 1.81 2.01 2.30 2.83 3.21 F 2.41 2.84 3.05 3.66 4.02 4.22 4.12 G 2.81 3.39 3.86 4.21 4.53 - - Minerals 2018, 8, 403 12 of 14

4. Conclusions This study applied several simple testing methods to the characterization of mine waste rocks and AMD prediction with a focus on the form of sulfur, the neutralization effects of carbonates, and the AMD formation over time. Changes in pH associated with AMD occurrence over time are difficult to estimate based solely on the NAPP and NAG pH tests, which are currently used to classify waste rocks. ABCC test results are useful for evaluating changes in pH only during short periods, such as in this study, which corresponds to approximately three months in the research area. The results also indicated that the form of sulfur is crucial for evaluating AMD occurrence over time. Changes in NAG pH during sequential NAG tests reflect sulfur solubility within the rocks, which provides information on the form of sulfur that occurs in the rocks as well as their acid-producing potential. Therefore, changes in NAG pH are useful when estimating changes in water quality over time. A more precise classification of cover materials (non-source), low-potential source rocks, and AMD source rocks is achievable using the sequential NAG test and sequential extraction with HCl, HF, and HNO3 in combination with simple, currently used tests (i.e., the ABA and NAG tests), which take into account effects associated with the form of sulfur and how it affects AMD. In addition, further experimentation with this method combination should be performed in other field sites to validate it, which will contribute to establishing a more precise, simple, and inexpensive method that is feasible in field laboratories at mining sites. This can also lead to a reduction in the cost and labor required to predict AMD, its prevention, and treatment methods, as well as a reduction in the impacts that AMD has on the environment, thus resulting in more sustainable mining practices.

Author Contributions: S.M. designed and performed the experiments and analyzed the data with H.I. On the basis of the data, S.M. wrote the paper. H.S., T.S., and G.J.K. made helpful suggestions for data analysis and supported parts of the experiments. G.J.K. kindly helped conduct a field investigation in Indonesian mines. Funding: This work was partly supported by JSPS KAKENHI Grant Number JP17H07401. Acknowledgments: We are grateful to the mine and researchers at Kyushu University for supporting part of the analysis. Conflicts of Interest: The authors declare no conflict of interest.

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