Potential use of lignite fly ash for the control of acid generation from sulphidic wastes

Waste Management 22 (2002) 631–641 www.elsevier.com/locate/wasman

Potential use of lignite fly ash for the control of acid generation from sulphidic wastes Anthimos Xenidis*, Evangelia Mylona, Ioannis Paspaliaris Laboratory of Metallurgy, Department of Mining and Metallurgical Engineering, National Technical University of Athens, 157 80 Zografos, Athens, Greece Accepted 30 July 2001

Abstract In the present paper, the potential use of lignite fly ash in the control of acid generation from sulphidic tailings disposed of at Lavrion, Greece was studied. Long-term laboratory column kinetic tests were performed on tailings containing 27% S, which were homogeneously mixed with various amounts of fly ash, ranging from 10 to 63% w/w. The drainage quality of the columns was monitored over a test period of 600 days. Chemical and mineralogical characterisation of column solid residues was performed after a 270-day test period. The hydraulic conductivity of the mixtures was also measured to evaluate the potential of fly ash to form a low permeability layer. Based on the results, the addition of fly ash to sulphidic tailings, even at the lower amount, increased the pH of the drainage at values of 8.6–10.0 and decreased the dissolved concentrations of contaminants, mainly Zn and Mn, to values that meet the European regulatory limits for potable water. Higher fly ash addition to tailings, at amounts of 31 and 63% w/w also reduced the water permeability of material from 1.2
105 cm/sec to 3
107 and 2.5
108 m/s, respectively. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction Past mining activities for the exploitation of mixed sulphide ores (argentiferous galena, sphalerite and pyrite) in Lavrion, Greece have resulted in the production of significant quantities of sulphidic tailings, covering a surface area of 94,000 m2 [1]. The sulphide minerals, mainly pyrite (FeS2), contained in these wastes are oxidised in the presence of air, water and bacteria, resulting in the formation of acidic solutions with increased concentrations of sulphate anions and dissolved metals. The overall pyrite oxidation process can be described by Eq. (1) [2]. þ FeS2 þ 3:75O2 þ 3:5H2 O ! FeðOHÞ3 þ 2SO2 4 þ 4H

ð1Þ This phenomenon, namely Acid Rock Drainage, represents the major environmental problem associated with base metal, gold and uranium mining operations as well as the coal and lignite mining industry. * Corresponding author. Tel:. +30-1-0-7722043; fax: +30-1-07722168. E-mail address: [email protected] (A. Xenidis).

A variety of techniques have been developed to prevent the acid generation process from sulphidic wastes. These techniques aim at restricting the principal components of the acid generation process, i.e. sulphides, oxygen and water and include: 1. Segregation of sulphides. 2. Underwater storage, e.g. in constructed surface impoundments, flooded pits, underground workings and natural water bodies. 3. Application of covers constructed from low permeability soils, synthetic materials, organic substances and composites. Moreover, preventive methods aim at controlling the factors that largely affect the nature and extent of acid formation and include addition of alkaline minerals to control water pH and dissolved ferric iron and application of bactericides to inhibit the bacterial action [2,3]. Subaqueous disposal in surface constructed impoundments has limited applicability in regions with long dry periods such as Greece. Alternative techniques including dry covers and alkaline additives may be preferably applied for the inhibition of acid generation from existing stockpiles of sulphidic wastes. The effectiveness

0956-053X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(01)00053-8

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of limestone addition and application of dry covers (clay, geomembrane) in controlling acid generation from Lavrion sulphidic tailings has been the subject of previous studies [4,5]. Based on these studies a cost effective rehabilitation scheme was also developed and applied for a tailings dam in Lavrion [6,7]. An alternative alkaline material, which may be used beneficially in the control of acid generation from sulphidic tailings, is fly ash, i.e. the material trapped by electrostatic precipitators in coal and lignite burning power-generating plants. Fly ash is produced in large quantities annually, i.e. world production is estimated at 200 million tonnes. In the European Union approximately 44 million tonnes of fly ash were produced in 1998 [8]. Forty one percent of this amount was utilised in a wide range of applications including the cement and concrete industry, structural fills and road construction. The annual production of fly ash in Greece is estimated at 8 million tonnes of which the major part is used in the backfilling of the open pits. Twelve percent is used in the cement industry [9]. Fly ash is composed of predominantly silt-sized, spherical, amorphous ferro-aluminosilicate minerals [10] and it is generally characterised as having low permeability, low bulk density and high specific surface area [11]. The physical, chemical and mineralogical characteristics of fly ash depend on the parent coal source, the method of combustion and the efficiency and type of emission control device. Two major classes of fly ash are specified in ASTM C618 on the basis of their chemical composition resulting from the type of coal burned. These are designated Class F and Class C. Class F is fly ash normally produced from the burning of anthracite or bituminous coal [SiO2+Al2O3+Fe2O3)570%]. Class C is normally produced from the burning of subbituminous coal and lignite [SiO2+Al2O3+Fe2O3)550%]. Class C fly ash has cementitious properties in addition to pozzolanic properties due to free lime, whereas Class F is rarely cementitious when mixed with water alone. Class C fly ash due to its increased Ca content, mainly as CaO, Ca(OH)2, CaCO3, has significant neutralisation capacity and may be beneficially employed to counteract the acid potential of the mine wastes [12–15]. Fly ash has been added as an alkaline amendment to coal mine spoils and refuse bank to permit their reclamation for plant growth [16]. Amendment of acid soils with up to 1% weight. fly ash, containing 30–40% Ca, increased pH and reduced the water solubility and diethylenetriamine-pentacetic acid (DTPA) extractability of Fe, Mn, Ni, Co and Pb [17]. It was demonstrated that the alkalinity of fly ash plays a significant role in regulating the availability of trace elements in the amended soils. On the other hand, given the pozzolanic and cementitious (for Class C) properties of fly ash, this material may be beneficially used to modify the physical proper-

ties of the wastes by decreasing their permeability to water and air, thus hindering oxidation of the contained reactive minerals. Bowders et al. [18] concluded that fly ash amended with clay and sand would be beneficially used in the construction of a surface hydraulic barrier to control acid generation from surface mined sites. Taha and Pradeep [19] evaluated the potential use of fly ash– stabilised sand mixtures as cover materials for sanitary landfills. It was seen that sand mixtures stabilised with 20% fly ash had a low permeability in the order of 106 cm/s, increased unconfined compressive strength and increased resistance to freeze-thaw and wet–dry cycles. The hydraulic conductivity of mixtures of Class F fly ash and materials like Class C fly ash, sand and bottom ash was studied in laboratory and field scale tests by Palmer et al. [20]. It was seen that mixtures of Class F and Class C fly ashes combined with a coarse aggregate can be compacted to achieve hydraulic conductivity close or below the value specified for landfill liners, e.g. 109 m/s. Based on the above it is deduced that Class C fly ash addition to sulphidic wastes may control the acid generation by the activation of two mechanisms. The first mechanism involves the addition of alkalinity to the system and the neutralisation of acidity produced. The second refers to the reduction of hydraulic conductivity, which results in the inhibition of water penetration to wastes, thus further oxidation of sulphides is prevented. Previous studies refer to either the neutralising effect or the cementation effect of the fly ash. In the present paper Greek lignite fly ash was applied in Lavrion sulphidic tailings and the effectiveness of both mechanisms in controlling acid generation from the waste material was studied. Long-term column kinetic tests were performed on tailings homogeneously mixed with various amounts of fly ash, ranging from 10 to 63% w/w. The drainage quality of columns was monitored over a period of 600 days. Furthermore, after a monitoring period of 270 days, the hydraulic conductivity of the mixtures was determined and complete chemical and mineralogical analysis of the column solid residues was performed.

2. Materials and methods Sulphidic tailings used in the tests originated from Lavrion, Greece and had total sulphur content of 27% weight. Fly ash originated from the plant of Kardia in Ptolemais, Greece. Before the execution of kinetic tests, a detailed characterisation of the materials was performed. The methodology followed is summarised in Table 1. Kinetic tests were carried out in plexi-glass columns with a diameter of 160 mm and a height of 100 cm. A total of five columns were set up (Table 2). One column was filled with sulphidic tailings and was used as the

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control test. Homogeneous mixtures of tailings and fly ash, which was added in amounts ranging from 10 to 63% w/w, were placed in four columns to evaluate the effectiveness of additive in controlling acid generation from Lavrion tailings. The execution of kinetic tests involved the application of wet–dry cycles on a weekly basis. Each cycle involved the addition of 2 l of deionised water. After the collection of leachates, parameters monitored include leachate volume, pH, oxidation-reduction potential, conductivity and metals concentration, i.e. Fe, Pb, Zn, Mn, Cd, As, Ca, Mg and SO4. Determination of metals in solution was carried out with atomic absorption spectrometry (AAS; Perkin Elmer 2100). Sulphate concentration was determined gravimetrically [21]. To determine the effectiveness of fly ash in reducing the hydraulic conductivity of tailings, in situ falling head permeability tests [22] were conducted in the control column and in columns coded FA10 and FA18 after a test period of 270 days. Furthermore, a core sample, 3 cm in diameter, was collected from the control column for the execution of

chemical and mineralogical analyses and the test was continued up to 600 days. The addition of fly ash to Lavrion tailings at amounts 31 (column FA31) and 63% w/w (column FA63) resulted in the significant expansion of material that caused the cracking of columns after a testing period of 80 and 100 days, respectively. These columns, although not operational were left in place for an additional period of 170 days. After that period, column solid residues were subjected to chemical and mineralogical analysis. Hydraulic conductivity measurements on representative samples were also performed by the constant and falling head methods (ASTM D5084, D5856).

3. Results 3.1. Characterisation of materials 3.1.1. Sulphidic tailings Based on the chemical and mineralogical analysis, the major minerals content of the Lavrion sulphidic tailings

Table 1 Methodology for the characterisation of tailings and fly ash used in the tests Parameter

Method

Objective/analyses Sulphide tailings

Fly ash

Chemical analysis

AAS/Flame, graphite furnace

Fe, Pb, Zn, Cd, As, Mn, Ca, Mg

Major elements: Fe, Al, Ca, Mg, K, Na, Ti, SiTrace elements: Pb, Zn, Cd, As, Mn, Ni, Cr, Co, Mo, Se

Analysis of Stotal

Leco

SO4 measurement

Gravimetrically

Determine acid generation potential Oxidation degree

Determine any acid generation potential Sulphates content

Mineralogical examination

XRD/ Optical microscopy

Determine sulphides, alkaline minerals, oxidation products

Determine alkaline minerals

Static tests Acid Base Accounting

Standard Sobek method [23]

Determine net neutralisation potential

Determine net neutralisation potential

Toxicity of fly ash Leachability of metals under acidic conditions

TCLP, US EPA, Method 1311 [24]

NDa

Analysis of leachates for Pb, Cd, As, Cr

Metals leachability in deionised water

MWEP, US EPA [25]

ND

Analysis of leachates for Pb, Cd, As, Ni, Cr

AAS, atomic absorption spectrometry; TCLP, toxicity characterisation leaching procedure; MWEP, monofill waste extraction procedure. a ND, not determined.

Table 2 Test conditions in the Lavrion tailings–fly ash columns Column

Code

Weight of tailings, T (g)

Weight of fly ash, F(g)

Fly ash addition ratio F/T (%)

Thickness (cm)

Sulphidic tailings

Control

15,155

–

–

41

Homogeneous mixtures of tailings with fly ash

FA10 FA18 FA31 FA63

15,155 15,155 15,155 13,517

1546 2771 4683 8489

10 18 31 63

56 56 70 88

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Table 3 Mineralogical composition of Lavrion sulphidic tailings Name of mineral

Formula

Content

Sulphides Pyrite, marcasite Sphalerite Galena

FeS2 ZnS PbS

48–50 1–2 2–4

Arsenopyrite Pyrrhotite Chalcopyrite Covellite

FeAsS Fe(1-x)S CuFeS2 CuS

Carbonates Siderite Dolomite Magnesian kutnohorite

FeCO3 CaMg(CO3)2 Ca(Mg,Mn)(CO3)2

10–12

FeO(OH) nH2O CaSO4 2H2O

13–15 3–4

Secondary products Limonite Gypsum Others Quartz Albite Ilmenite

<1

6–8

SiO2 NaAlSi3O8 FeTiO3

10

Table 4 Chemical analyses of fly ash used in the laboratory tests Component

Value (%)

Minor elements

Value (mg/kg)

Fe2O3 Al2O3 CaO MgO K2O Na2O TiO2 SiO2 SO3 LOIa

5.56 13.04 33.89 4.48 0.76 0.29 0.71 31.85 6.83 2.67

Ni Cr Pb Zn Cd Mn Co Se

461 425 138 87 14 339 68 3

As

25

a

is given in Table 3. The tailings contain 26.2% weight. sulphide-S, mainly as pyrite with lesser amounts of sphalerite and galena. Marcasite, pyrrhotite, chalcopyrite, arsenopyrite and covellite were also identified. The carbonate content of the tailings is approximately 13% weight as CO3, predominantly as siderite (FeCO3) and dolomite [CaMg(CO3)2]. Energy dispersive X-ray spectroscopic (EDS) analysis of dolomite-mineral grains indicated the presence of Mn, thus magnesian kutnohorite [Ca(Mg,Mn)(CO3)2] can also be contained in the tailings. Quartz was the main gangue mineral detected in the XRD patterns of Lavrion tailings sample. Minor gangue minerals include albite and ilmenite. The tailings also contained 0.8% weight. S(SO4) indicating the presence of sulphides oxidation products. Gypsum and limonite (XRD amorphous) were the major weathering products identified. Based on the microscopic observations of sulphidic material, large crystals of gypsum have been formed in the contact of pyrite and carbonate grains. Formation of ferric hydroxide rims in the periphery of pyrite and dolomite grains was also observed. Based on the Acid Base Accounting test results [23], Lavrion sulphidic tailings had a neutralisation potential (NP) value of 133.7 kg CaCO3/t. Given the high S (sulphide) content of tailings, acid potential (AP) amounted to 817.2 kg CaCO3/t, resulting in a negative net neutralisation potential (NNP) value, equal to 683.5 kg CaCO3/t, thus the material was classified as potentially net acid generating [2]. 3.1.2. Greek lignite fly ash The chemical analysis of fly ash used in the tests is given in Table 4. The material had (SiO2+Al2O3+Fe2O3) content higher than 50% weight and according to the classification of the American Society for Testing and Materials (ASTM C618) is classified as Class C. The mineralogical phases identified in the XRD pattern of the fly ash sample included anhydrite (CaSO4), lime (CaO), quartz (SiO2), melilite (Ca8Al2Mg3Si7O28),

LOI, Loss of ignition.

Table 5 Leachability test results for fly ash used in the tests (analyses in ppb) Test

pH

Pb

Cd

As

Cr

Ni NAa

TCLP

12.19

5.8

<0.2

<1.0

865

MWEP, 1st leaching step, 18 h MWEP, 2nd leaching step, 36 h MWEP, 3rd leaching step, 54 h MWEP, 4th leaching step, 72 h

11.27 12.12 11.45 11.36

9.3 9.9 7.6 6.0

<0.2 <0.2 <0.2 <0.2

<1.0 <1.0 <1.0 <1.0

1100 700 78.7 28.4

4.2 <1.0 1.9 1.7

5000 50

NS 50

Toxicity limits [24] Potable water limits [26]

NSb 6.5–8.5

5000 50

1000 5

TCLP, toxicity characterisation leaching procedure; MWEP, monofill waste extraction procedure. a NA, not analysed. b NS, not specified.

5000 50

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portlandite [Ca(OH)2], anorthite [CaAl2(SiO4)2], calcite (CaCO3) and gismondine (CaAl2Si2O8.4H2O). Microscopic observation of material indicated the presence of iron oxides mainly magnetite and hematite. Given the high Fe content of fly ash (Fe2O3: 5.56%), the presence of amorphous Fe bearing alumino-silicates phases is also possible. Based on the Acid Base Accounting test results, the Net Neutralisation Potential of fly ash was significant, i.e. 544.4 kg CaCO3/t and is attributed to the CaO, Ca(OH)2 and CaCO3 phases contained in the material. To assess the potential environmental risk from fly ash application, it was critical that the leachability of trace metals contained in the material is determined. The results of the standard leachability tests, i.e. toxicity characterisation leaching procedure (TCLP) [24] and monofill waste extraction procedure (MWEP) [25] performed on the fly ash samples are given in Table 5. Based on the TCLP test results, the leachability of elements examined was well below the toxicity limits specified by US EPA [24]. The levels of Pb, Cd and As in the TCLP leachates were also in agreement with the potable water limits [26]. Based on the MWEP method, the solubility of these elements in deionised water was also minimal. Previous studies aiming at evaluating the leaching behaviour of Greek lignite fly ash [27] have shown that the release of both heavy metals and polynuclear aromatic hydrocarbons from fly ash was minimal indicating a low risk for the environment.

635

Zinc and manganese were the major contaminants of the drainage produced. After the initial washing of material, the dissolved levels of these metals ranged from 6.6 to 88.9 mg/l for Zn and 35 to 280 mg/l for Mn, well above the limits for industrial effluents discharge, equal to 1 and 2 mg/l respectively. The dissolution rate of Zn and Mn, expressed in mg/kg material/week vs. time is shown in Fig. 3. In this figure, the variation of

Fig. 1. pH of the leachates of Lavrion tailings–fly ash mixtures vs. time as compared with the control.

3.2. Kinetic tests 3.2.1. Control During the initial cycles, Lavrion tailings produced drainage with acidic pH, i.e. pH: 4.0, due to the washing of past oxidation products. Thereafter, the pH of leachates has remained circumneutral averaging pH: 7.1  0.7 for a test period of 600 days, as shown in Fig. 1. The oxidation–reduction potential (Eh) of solutions ranged from 139 to 350 mV. The conductivity of leachates ranged from 3.0 to 6.2 mS/cm, averaging 4.4  0.7 mS/cm, Fig. 2. Given the pH–Eh values measured in the drainage collected from the control column, ferric iron is expected to precipitate in the form of hydroxide. Actually, iron dissolved concentrations in the neutral pH leachates of the control column were low, ranging from < 0.1 to 34 mg/l. In 25 out of 73 measurements, Fe level was found below the prevailing limit for industrial effluents discharge, i.e. 2 mg/l [28]. After the initial cycles, Pb concentration in the drainage of the control column was below detection limit, < 0.5 mg/l. A slight increase in the metal level, up to values of 1.2 mg/l, was observed after 450 days of testing. Arsenic level was below detection limit, equal to 1.3 mg/l, throughout the test period.

Fig. 2. Conductivity of the leachates of Lavrion tailings–fly ash mixtures vs. time as compared with the control.

Fig. 3. Zinc and Mn dissolution rate (mg/kg/week) vs. the cumulative volume of leachates produced per mass of material (l/kg) for the control column.

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dissolution rate with the cumulative volume of leachate, expressed in L(l) per mass of material, S (kg) is also presented. Zinc and Mn exhibited a similar dissolution pattern; during the period 0–115 days, the dissolved amount of Zn and Mn was significantly reduced from 99.5 and 275 to 0.08 and 3.8 mg/kg/week respectively. Thereafter, the metals dissolution rate increased reaching the values of 8.7 and 21.3 mg/kg/week for Zn and Mn, respectively after a test period of 344 days. A reduction in the metals dissolution rate was observed for the last test period, corresponding to an L/S ratio higher than 5 l/kg, during which, the dissolved amount of Zn and Mn averaged 1.8 and 7.3 mg/kg/week, respectively. Referring to sulphate, after the initial cycles, its concentration in the solution produced from the control column ranged from 1800 to 4720 mg/l. The dissolution rate of SO4 averaged 311 79 mg/kg whereas a total amount of 25.7 g/kg was produced after the test period of 600 days (Fig. 4). During the monitoring period, a significant amount of Ca and Mg was also dissolved from the control column. After the initial cycles, Ca and Mg concentrations in the leachates ranged from 270 to 590 and 153 to 790 mg/l, respectively. 3.2.2. Fly ash amended columns 3.2.2.1. Drainage quality. pH and conductivity of the leachates produced from the fly ash amended columns vs. time, as compared with the control are shown in

Figs. 1 and 2, respectively. The addition of fly ash to sulphidic tailings, even at the lower amount, equal to 10% w/w, resulted in the production of highly alkaline leachates with pH averaging 9.5  0.4. For FA18 column containing 18% w/w fly ash, pH of the drainage averaged 10 0.7. The conductivity of the leachates produced, shown in Fig. 2, was also reduced as compared with the control averaging 2.8  0.6 and 1.9  0.3 for FA10 and FA18 columns, respectively. Under the highly alkaline conditions met in the fly ash amended columns, Fe, Cd, As, Zn and Mn levels were close to or below detection limit, i.e. < 0.1, 0.05, 1.0, 0.1 and 0.1 mg/l, respectively. Thus, the cumulative amount of above metals dissolved in the fly ash amended columns was significantly reduced as compared to the control (Table 6). Regarding Pb, increased dissolution of metal was observed during the initial cycles, as shown in Fig. 5. This may be attributed to the high pH of the leachates produced initially from the fly ash amended columns, i.e. pH: 12.0 and the formation of hydroxo lead–ion complexes. Studies have shown that Pb presents minimum leachability at pH: 9–10 [29]. However, after the initial washing, Pb concentration remained below 0.5 mg/l throughout the test period. The major ions in the solutions produced from Lavrion tailings–fly ash mixtures were Ca and SO4. Calcium dissolution in the drainage of FA10 column showed a decreasing trend for a test period of 100 days. Then, the level of metal increased averaging 568  80 mg/l. Higher

Table 6 Cumulative amount of metals and sulphate dissolved in the fly ash amended columns as compared to the control (% of the total content) Code

Fe

Pb

Zn

Cd

Mn

S(SO4)

Control (0–600 days, 8 l/kg) FA10 (0–600 days, 8 l/kg) % Reduction FA18 (0–600 days, 7 l/kg) % Reduction FA31 (0–80 days, 1l/kg) FA63 (0–100 days, 1l/kg)

0.023 0.0002 99.13 0.0002 99.13 <0.0001 <0.0001

0.027 0.29 – 0.022 18.52 0.03 0.003

4.48 0.02 99.60 0.005 99.89 0.003 0.001

0.57 0.53 7.02 0.21 63.16 0.08 0.002

34.38 0.002 99.99 0.002 99.99 0.0005 <0.0001

3.1 1.58 49.03 0.86 72.26 0.17 0.11

Fig. 4. Cumulative amount of SO4 dissolved in the leachates of fly ash amended columns as compared with the control.

Fig. 5. Dissolved concentration of Pb in the leachates of fly ash amended columns as compared with the control. DL=detection limit.

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fly ash addition resulted in lower Ca dissolved concentrations. For FA18 column, Ca dissolved level averaged 255 80 mg/l for the test period 100–300 days. Then, metal level gradually increased, averaging 43,260 mg/l. The dissolution of Ca in the drainage of FA31 and FA63 columns was minimal after 80 and 100 days of testing, respectively. Sulphate concentrations in the drainage of fly ash amended columns ranged from 920 to 1830 mg/l for FA10 and 330 to 1560 mg/l for FA18. The cumulative amount of sulphate dissolved in the drainage of FA10 and FA18 columns after 600 days of testing was reduced

637

by 49 and 72%, respectively as compared with the control (Fig. 4, Table 6). Referring to Mg, its dissolved level in the drainage of fly ash amended columns FA18, FA31 and FA63 was below detection limit, i.e. < 1 mg/l, throughout the test period. For column containing the lower amount of fly ash (FA10), an increase to Mg dissolved concentration up to 40 mg/l was observed after a test period of 300 days. Finally, periodic measurements of Al indicated that the dissolution of this metal was minimal, i.e. < 1.2 mg/l. 3.2.2.2. Hydraulic conductivity. The hydraulic conductivity coefficient of the column solid samples after 270 days of testing vs. the amount of fly ash added in Lavrion tailings is shown in Fig. 6. It is seen that the control sample had increased hydraulic conductivity, equal to k: 1.2
105 m/s. The hydraulic conductivity coefficient of the mixtures of tailings and fly ash having 10–63% w/w of additive was 3–500 times lower than the control sample. The addition of fly ash at the higher amount reduced the water permeability to 2.5
108 m/s.

Fig. 6. Effect of fly ash addition on the hydraulic conductivity coefficient of Lavrion sulphidic tailings.

3.2.2.3. Solid residues. The addition of fly ash to Lavrion sulphidic tailings at amounts of 31 and 63% w/w

Fig. 7. Macroscopic view of FA63 column solid residue.

Fig. 8. XRD patterns of the solid residues of columns FA31 (B) and FA63 (C) as compared with the control (A). Ab, Albite; Dol, Dolomite; Cal, Calcite; Etr, Ettringite; Gn, Galena; Gp, Gypsum; Mel, Melilite; Py, Pyrite; Qtz, Quartz; Sd, Siderite.

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FeS2 þ 3:75O2 þ ð1:5 þ 2x þ 2yÞH2 O þð1 þ xÞCaMgðCO3 Þ2 ! FeðOHÞ3 þ ð2-yÞSO2 4 þð1 þ x  yÞCa2þ þ ð1 þ xÞMg2þ þ yCaSO4 :2H2 O þ4xHCO 3 þ ð2  2xÞCO2 ð2Þ

resulted in the production of monolithic samples, as shown in Fig. 7. The XRD patterns of corresponding FA31 and FA63 column solid residues as compared with the control are shown in Fig. 8. It is seen that phases including lime, portlandite and anhydrite identified in the fly ash sample were not detected in the residues of tailings-fly ash mixtures, whereas both FA31 and FA63 samples contained a new phase, namely ettringite [Ca6[Al(OH)6]2(SO4)3 26H2O]. Both gypsum and ettringite were detected in the XRD patterns of FA31 column residue. The absence of gypsum peaks at the diffraction pattern of FA63 column residue indicates that the higher amount of fly ash added to tailings favoured the formation of ettringite. A microscopic view of the sample showing the replacement of an idiomorphic gypsum crystal by ettringite is shown in Fig. 9.

where 0 < x < 1 depending on the pH of solution Based on Eq. (2), calcium may be removed from solution due to the precipitation of gypsum, which has a solubility of 2.2 g/l [35]. The presence of gypsum in the tailings sample was confirmed by mineralogical analysis. Furthermore, geochemical modelling of the drainage produced from the tailings column, using MINTEQA2 [36], suggested that the solution was near saturation with respect to gypsum throughout the test period. Given the above, the amount of calcium in solution does not correspond to the carbonates dissolution. On the other hand MgSO4 exhibits a high solubility in water, 70,000 mg/l [35], resulting in increased Mg/Ca ratio values in the leachates. This observation, in agreement with previous studies [34] suggests that Mg level in solution could provide information on the dissolution of dolomite. Based on the drainage analysis, the Mg/Ca molar ratio ranged from 1 to 4.6; the variation of molar ratio was associated with the variation in the Mg dissolution rate, whereas Ca level in solution was relatively constant. Based on mass balance calculations, the amount of dolomite dissolved during the test period of 600 days was estimated to 9.5 kg/t. Regarding siderite, also encountered in the tailings, its dissolution under oxic or acidic conditions can be described by the following reaction [37]:

4. Discussion

FeCO3 þ 0:25O2 þ 2:5H2 O ! FeðOHÞ3 þH2 O þ CO2

Fig. 9. Microscopic view of FA63 column solid residue under transmitted and reflected light using semi-crossed nicols, X200. Gp, Gypsum; Etr, Ettringite.

4.1. Control Lavrion tailings examined in the present study, although classified as potentially net acid generating based on Acid Base Accounting criteria, produced drainage with a circumneutral pH during the column kinetic test for a period of 600 days. It is thus deduced that under the test conditions the acidity generated from sulphides oxidation was effectively neutralised by the dissolution of carbonate minerals, mainly dolomite, contained in the tailings. The beneficial effect of carbonates on the inhibition of acid drainage generation from sulphides has been well recognised given the lack of acidity in sulphides and coal mines containing abundant natural limestone or other carbonate minerals. Several authors have studied the sulphides oxidation in the presence of carbonates [22,30–34]. Pyrite oxidation in the presence of dolomite can be described from the following reaction:

ð3Þ

Siderite is initially a neutraliser; however continuous dissolution of mineral results in the production of acidity due to the oxidation of ferrous iron to ferric iron and subsequent precipitation of ferric hydroxides. It is thus deduced that the weathering of siderite has no net neutralising effect [37,38]. Given that the standard Acid Base Accounting (ABA) test [23] used in the present study accounts also for siderite, the NP of Lavrion tailings estimated to 133.7 kg CaCO3/t may be overestimated. Actually, NP determination based on the modified ABA method [39] made at pH:4.3, as suggested by Paktunc [38], gave a value of 76.3 kg CaCO3/t. Based on the above, the amount of alkalinity consumed during the test period accounts for a low percentage, 7%, of the neutralisation potential of tailings. Although, generation of acidic drainage from tailings was delayed, the dissolution of Zn and Mn remains significant; 4.5 and 34.4% of Zn and Mn total mass in the tailings dissolved after 600 days of column kinetic testing.

A. Xenidis et al. / Waste Management 22 (2002) 631–641

Based on MINTEQA2 results, the leachates produced from the control column throughout the test period were supersaturated with respect to rhodochrosite (MnCO3), suggesting that precipitation of this phase may be imposing an upper limit on dissolved Mn concentrations in the drainage of tailings column. Amorphous hydrous metal carbonate such as ZnCO3 H2O might also be a sink for Zn, however solutions were not saturated in respect with this phase throughout the test period. 4.2. Fly ash amended columns The addition of fly ash to the Lavrion tailings increased the NP of the material. Based on the static ABA test results, the quantity of alkalinity introduced to the tailings by their homogeneous mixing with 10–63% w/w/ fly ash amounted to 56–242 kg CaCO3/t, corresponding to 7–41% of the stoichiometrically required quantity. Given the above the leachates pH of the fly ash amended columns was highly alkaline, pH > 9.0, resulting in the minimal dissolution of main contaminants, i.e. Zn and Mn. Calcium and sulphate were the major ions reported in the leachates of fly ash amended systems. Geochemical modelling of leachates chemistry with MINTEQA2 indicated that the solutions were at saturation with respect to gypsum, suggesting that Ca and SO4 concentrations were probably controlled by gypsum solubility. The low magnesium concentrations in the drainage of FA18, FA31 and FA63 fly ash amended columns (41 mg/l), throughout the test period suggested that under the alkaline conditions met in the columns, limited dissolution of dolomite occurred. An increase in the dissolved concentration of Mg in the column containing the lower amount of fly ash, 10% w/w (code FA10) was observed after 300 days of testing; the maximum value measured was equal to, 40 mg/l, being one order of magnitude lower as compared with the control. The addition of 10 and 18% w/w fly ash to Lavrion tailings reduced the water permeability from 105 to 4
106 and 106 m/s, respectively, values significantly higher than that recommended for low permeability layers, 109 m/sec [40]. A major reduction in the hydraulic conductivity of material to values of 3 and 2.5
108 m/s was observed for higher amounts of fly ash addition, i.e. 31 and 63% w/w, respectively. This effect is attributed to the pozzolanic and cementitious properties of fly ash, which favour the following reactions: Formation of hydrated calcium silicates xS þ CH ! CSx H

ð4Þ

where S: SiO2, C: CaO, H: H2O Formation of ettringite 2 6Ca2þ þ 2AlðOHÞ þ 4OH þ 26H2 O 4 þ 3SO4

) Ca6 ½AlðOHÞ6 2 ðSO4 Þ3 :26H2 O

ð5Þ

639

C–S–H consists of the most important constituent of hydrated Portland cement. It has several forms and can be mostly considered as a gel phase. Mineralogical analysis of FA31 and FA63 column solid residues did not indicate the presence of any crystalline C–S–H phases, like tobermorite. On the other hand, the solid residues contained ettringite. XRD combined with microscopic examination of the samples suggested that ettringite grew at the expense of gypsum crystals. Given that gypsum was initially present in the tailings, ettringite could be formed during the early stages of the kinetic test and might be called primary ettringite. According to the study of Myreni et al. [41], ettringite is a stable mineral above a pH of 10.7, a condition met in FA31 and FA63 columns. For lower pH: 10.7–9.5, as in the case of FA10 and FA18 columns, ettringite would undergo incongruent dissolution to gypsum and Alhydroxides, which control Ca2+, Al3+ and SO2 4 activities. At neutral pH, in addition to gypsum and Alhydroxides, Al-hydroxy sulphates would also precipitate. An important feature of ettringite is the accommodation of ion substitution in several lattice sites, for example Fe3+ and Mn3+ could replace Al3+ [42]. The formation of ettringite in the mixtures with 31 and 63% w/w fly ash, however, also resulted in a significant expansion of material. It is thus deduced that the amount of fly ash addition to Lavrion tailings should be lower than 30% w/w. To achieve reduced hydraulic conductivity than that reported in the present study, the mixture could be compacted at its optimum moisture content and adequately cured. A major drawback of fly ash application would be the increase of Pb solubility, observed during the initial cycles, when the pH of solution was 12.0. This adverse effect may be avoided should the fly ash amended material be cured in order to allow time for the formation of cementation products, which would inhibit the dissolution of Pb.

5. Conclusions Based on the results of this study, the following conclusions can be drawn: Sulphidic tailings, originating from Lavrion, Greece examined in the present study, had NNP of 683.5 kg CaCO3/t and were classified as potentially net acid generating according to the static ABA tests. Based on the laboratory column kinetic test, the drainage produced from the tailings over a period of 600 days had neutral pH 7.1. The production of acidity was delayed due to the dissolution of contained dolomite. The amount of carbonate mineral consumed during the test period was estimated to 9.5 kg/t. Zinc and Mn were the major contaminants and their cumulative amount dissolved was estimated to 4.5 and 34.4%, respectively.

640

A. Xenidis et al. / Waste Management 22 (2002) 631–641

The addition of fly ash to tailings at a low amount, i.e. 10% w/w increased the pH of leachates to values of pH: 8.6–10.0 effectively inhibiting the dissolution of Zn and Mn. Calcium and sulphate were the major ions reported in the drainage of fly ash amended columns. Higher fly ash addition rates, i.e. 31 and 63% w/w resulted in the production of monolithic samples with reduced hydraulic conductivity. The greater reduction was achieved for the higher amount added; the mixture of tailings–63% w/w fly ash had a hydraulic conductivity coefficient of 2.5
108 m/sec as compared with 1.2
105 m/s, obtained for the control sample. Mineralogical analysis of the column solid residues indicated that higher fly ash addition favoured the formation of ettringite, which was associated with the volume expansion of material. The results of the present study suggest that the optimum amount of fly ash addition to Lavrion tailings, in terms of alkalinity introduction and permeability reduction, lies between 20 and 30% and further studies have to be conducted in this range. To improve the hydraulic conductivity reduction effect of fly ash, the tailings–fly ash mixtures could be compacted at their optimum moisture and be adequately cured.

Acknowledgements The authors would like to acknowledge the financial support of the European Commission, under the ‘‘BRITE–EURAM’’ Program, Contract. No. BRPRCT96-0297.

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