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Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
Minerals Engineering, Vol. 13, No. 10-1, pp. 1161-1175, 2000
Pergamon
0892-6875(00)00099-6
© 2000 Elsevier Science Ltd All rights reserved 0892-6875/00/$ - see front matter
INHIBITION OF ACID GENERATION FROM SULPHIDIC WASTES BY THE ADDITION OF SMALL AMOUNTS OF LIMESTONE
E. MYLONA, A. XENIDIS and I. PASPALIARIS Laboratory of Metallurgy, Dept. of Mining and Metallurgical Engineering, National Technical University of Athens, GR-15780 Zografos, Greece. E-mail: [email protected] (Received 20 December 1999; accepted 13 June 2000)
ABSTRACT Limestone addition is a commonly applied technique to prevent acid generation from sulphidic wastes containing 1-10% S. In the present paper, the effectiveness of small amounts, lower than the stoichiometric requirement, of this alkaline additive in inhibiting acid generation from a pyrite concentrate material is studied. Long term laboratory column tests were conducted on a partially oxidised pyrite concentrate, where limestone was added by thoroughly mixing. The amount of alkaline additive ranged from 6.4 to 29% wt. corresponding to 5-30% of the stoichiometric quantity. The performance of the pyrite-limestone mixtures was evaluated by monitoring the drainage quality of the columns. Furthermore, a detailed geochemical characterisation of the column solid residues was performed after a monitoring period of 270 days. The effect of secondary oxidation-neutralisation products on the hydraulic conductivity of material was also examined. Dissolution of previously formed oxidation products occurred in the control column during the monitoring period, resulting in the release of a significant amount of Fe, Zn, Mn, Cd, As and S04 and to a lesser extent Pb. However, due to the presence of secondary products, further oxidation of pyrite particles was delayed. The experimental results showed that homogeneous mixing of pyrite with limestone amounting to only a fraction of the contained acidity inhibited the generation of acidic drainage and significantly reduced the dissolved amount of metals and sulphates for a test period of 270 days. Under the alkaline conditions prevailing in the limestone amended columns, secondary precipitation of ferric hydroxides and gypsum occurred. A ten-fold decrease of hydraulic conductivity was observed for the material amended with 207 kg CaCOJt, corresponding to 15% of the contained acidity. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords Acid rock drainage; oxidation; environmental; reclamation
INTRODUCTION Acid generation from sulphidic wastes constitutes a significant environmental problem in coal, lignite and polymetallic sulphide mining. The sulphide minerals, particularly pyrite, FeS2, and pyrrhotite, Fel-xS, contained in wastes are oxidised in the presence of air, water and bacteria, resulting in the production of low-pH waters loaded with dissolved SO42-, iron and other heavy metals and toxic compounds.
1161
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E. Mylonaet al.
Pyrite oxidation resulting in the generation of acidic drainage can be described by the following reactions, Singer and Stumm (1970): 2FeS2 + 702 + 2H20 + 2Fe 2+ + 4S042 + 4H + 4Fe 2+ + Oz + 4H +--~ 4Fe 3+ + 2H20 FeS2 + 14Fe3++ 8H20 --~ 15Fe 2+ + 2SO42 + 16 H +
(1) (2) (3)
Under acidic conditions, pH < 3.5, the ferrous to ferric iron reaction (2) is slow and the contribution of iron-oxidising bacteria such as the Thiobacillusferrooxidans species becomes important. These bacteria act as electron acceptors during the Fe(II) oxidation, increasing the reaction rate by several orders of magnitude. The Fe(III) produced is capable of rapidly oxidising pyrite by an abiotic pathway thus regenerating Fe(II) and propagating the cycle, Singer and Stumm (1970). Pyrite oxidation theory and control technologies have been well documented in the literature, Evangelou (1995). Addition of alkaline materials aiming at the control of pH is a commonly applied technique to prevent acid generation from sulphidic wastes. Materials like lime, Rose and Daub (1994), Heltz et al. (1987), limestone, Day (1994), Payant et al. (1995), Lapakko et al. (1997), sodium carbonate, He]tz et al. (1987), as well as industrial by-products including phosphatic slimes, Watkin and Watkin (1983), Renton et al. (1988) and alkaline siliceous materials such as fly ash, Farah et al. (1997) have been reported to effectively prevent acidic drainage. The potential of limestone to prevent acid generation has been well recognised from the fact that in sulphide and coal mines containing abundant natural limestone or other carbonate minerals, limited if any acidity is formed. In sulphidic waste stockpiles when calcite is present, the overall oxidation-neutralisation reaction depends on pH, Morin and Hutt (1994): pH<6.4 FeS2(s) + 2CaCO3(s) + 3.7502(g) + 1.5H20 + Fe(OH)3(s) + 2SO42 + 2Ca 2+ + 2CO2(g)
(4)
pH>6.4 FeS2(s) + 4CaCO3(s) + 3.7502(g) + 3.5H~O + Fe(OH)3(s) + 2SO42 + 4Ca 2++ 4HCO3
(5)
Based on the stoichiometry of reaction (4), under acidic conditions, 1 mole of pyrite is neutralised by 2 moles of calcite. At pHs above 6.4, the carbonate product will be HCO3 rather than CO2. In this case, twice as much CaCO3 is required to neutralise the same amount of acidity. However, the addition of limestone to sulphidic wastes does not only neutralise the generated acidity but also can prevent the pyrite oxidation process by the activation of the appropriate physical and chemical mechanisms. In this case the cost of the method will be significantly reduced because the amount of additive required would be significantly less than the stoichiometric one (reaction 4). There are four mechanisms by which limestone may control pyrite oxidation. The first mechanism involves precipitation of ferric iron in the hydroxide form, thus its further participation as an oxidising agent in the dissolution of pyrites is inhibited, Kelley and Tuovinen (1988). The second mechanism involves the raising of the pH of pore water to high values (pH: 6.1-8.4), thus the activity of the oxidising bacteria Thiobacillus ferrooxidans, is significantly impaired, Nicholson et al. (1988). Another mechanism involves the precipitation of oxidised compounds on the sulphides surface. It is reported that when carbonate minerals are present and available to neutralise the acid produced during the oxidation of pyrite, a protective ferric oxy-hydroxide layer will accumulate around the pyrite grains, impairing its further dissolution, Nicholson et al. (1990). Finally, according to field studies the presence of carbonate material in sulphide tailings may enhance the formation of cemented layers (hardpan) on the stockpile surface. The hardpan consists of the oxidation-neutralisation products such as ferric oxy-hydroxide and gypsum that cement the tailings together forming a low permeability mass that acts as an oxygen and water diffusion barrier, Blowes et al. (1991), Tasse et al. (1997), Lin (1997). Based on previous studies, homogeneous mixing of sulphidic waste rock, containing 9.7% S, with limestone in amounts corresponding to 3 and 10% of the stoichiometric quantity reduced the acid
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1163
production rate by 83 and 98% respectively after a 3 years test period, Payant et al. (1995). Furthermore, laboratory kinetic studies over a period of 397 weeks showed that a 84% decrease in the rate of sulphide oxidation was effected by mixing waste rock containing 2.1% S with limestone at an amount corresponding to 16% of the stoichiometric requirement, Lapakko et al. (1997). The addition of greater amounts of limestone (30% of the contained acidity or higher) further reduced the acid generation rate so that the host rock silicate mineral dissolution would adequately neutralise the resultant acid production, ensuring the long term prevention of acidic drainage, Lapakko et aL (1997). Published work on the effectiveness of limestone addition to prevent acid generation from highly sulphidic wastes is limited. In a previous study, it was shown that thorough mixing of ground limestone with a pyrite concentrate at an amount corresponding to 5% of the stoichiometric requirement reduced the rate of pyrite oxidation by 53%, Mylona et al. (1996). This was mainly attributed to the activation of chemical mechanisms including pH increase, ferric hydroxide precipitation and diminution of bacterial activity. No effect on the water permeability of material was observed. The present study aimed at a thorough evaluation of the effect of limestone content on the pyrite oxidation process. A series of long term column kinetic tests were conducted on a partially oxidised pyrite concentrate, which was homogeneously mixed with various amounts of limestone corresponding to 5 up to 30 % of the stoichiometric quantity. For the evaluation of results, an integrated methodology was followed that combined the monitoring of the column drainage quality and the examination of physical, chemical and mineralogical characteristics of the solid residues.
MATERIALS AND METHODS Materials The material used in this study was a pyrite concentrate assaying 42.6% S. 5% of total sulphur content was in the sulphate form indicating the presence of past oxidation products in the concentrate that has been stockpiled for more than 15 years. The material was finer than 1.4 mm and had an average particle size, d50 of 70 lam. Quarry quality limestone consisting of calcite and dolomite (11% wt.) was used as alkaline additive. The material was ground to a particle size similar to that of the sulphidic concentrate. Chemical assays of the pyrite and limestone are given in Table 1.
TABLE 1 Chemical composition of pyrite concentrate and limestone (analyses %) Element
Pb
Pyrite 0.34 concentrate Limestone <0.02 ND: Not determined
Zn
Mn
Fe
0.34
0.027
38.50
Cd ppm 28.0
0.004
0.002
2.25
1.2
As
Stota!
S
Ca
Mg
2.21
0.45
0.10
ND
37.0
1.46
(504) 0.99
42.74
<0.05
<0.20
Methods K i n e t i c tests
To determine the effect of limestone on the pyrite oxidation process, long term laboratory kinetic tests were carried out in plexi-glass columns with a diameter of 160 mm and a height of 100 cm in total. Five (5) columns were set up; one column was loaded with pyrite material without any limestone addition and was used as reference (control column) and the other four contained homogeneous mixtures of pyrite with limestone. Limestone content of the mixtures ranged from 6.4 to 29% wt., as given in Table 2. Neutralisation Potential (NP), and Maximum Potential Acidity (MPA) of pyrite and limestone were determined with the Acid Base Accounting (ABA) method and expressed in kg CaCO3/t, Sobek et al. (1978), Adam et al. (1997). For the limestone amended systems these values were calculated according to equation (6), Day (1994):
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E. Mylonaetal.
(6)
P=(PLML + PpMp)/(ML+Mp)
where P refers to NP or MAP, and ME and Mp are the weight of limestone and pyrite in the columns. The results are given in Table 3.
TABLE 2 Test conditions in the pyrite and limestone amended columns a/a
1 2 3 4 5
Column
Control, Pyrite Amended with limestone mixture
Total weight
Limestone content
(g)
(% w.t)
13,590 14,003 15,379 16,310 19,103
6.4 12.0 17.0 29.0
Thickness of material (cm) 29.5 36.5 42.0 43.5 56.5
TABLE 3 ABA parameters of the pyrite and limestone amended material Code
Column
MAP I NP I NNP kg CaCO3/t
Control Control, Pyrite 6.4% Amended with limestone CaCO3 mixture 12% C a C O 3 17% CaCO3 29% CaCO3
Alkalinity added kg % of stoichiometric CaCO3/t quantity
1,335.6 1,249.6
-41.9 25.2
-1,335.6 - 1,224.3
69
5
1,173.9 1,106.9 945.1
84.2 136.5 262.8
- 1,089.7 -970.4 -682.3
138 207 413
10 15 30
The pyrite concentrate material had a Net Neutralisation Potential (NNP: NP-MAP) o f - 1 , 3 3 5 . 6 kg CaCO3/t indicating that a limestone addition of 1.3 t/t material is stoichiometrically required to balance its total potential acidity. The amount of limestone added in the present tests ranged from 69 to 413 kg CaCO3/t corresponding to 5-30% of the stoichiometric quantity. Wet/dry cycles of 7 days were imposed in the column tests. Each cycle involved the addition of 2 litres of deionised water; the water was allowed to infiltrate and depending on the hydraulic conductivity of material, the leachate was collected at the second to the forth day of the cycle. The columns were left to dry till the seventh day and then the next cycle started. The cycles continued for a total of 270 days. Parameters monitored include leachate volume, pH, oxidation-reduction potential, conductivity, acidity/alkalinity and metals concentration, i.e. Fe, Pb, Zn, Mn, Cd, As, Ca, Mg and SO4. Determination of metals in solution was carried out with AAS (Perkin Elmer 2100). Sulphates concentration was determined gravimetrically. Permeability measurements
To determine any effect of limestone on the hydraulic conductivity of pyrite material due to the precipitation of secondary reaction products, falling head permeability tests were conducted in the columns after the completion of the tests. Flooding the material from the base of each column initially saturated the columns. Then for the permeability tests, a water head (H1) of 50 to 75 cm was applied to avoid piping of the samples. Water was allowed to flow through the sample and the time, t, required for the hydraulic head to reach the value of H 2 w a s recorded. The permeability coefficient was calculated from the following equation, Fetter (1994): k = 2.3
L.f Hl log-F.t Ho
(7)
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1165
where k: coefficient of hydraulic conductivity, L: thickness of the sample, f, F: cross section area of the tube and the sample respectively, HI, H2: hydraulic head at the start and the end of the test respectively. Characterisation of the solid residues To evaluate the overall effect of limestone on the pyrite oxidation process, columns were dismantled after a monitoring period of 270 days, and solid residues were subjected to chemical, mineralogical analyses and non-sequential extraction tests. For two columns (control, mixture--17% wt. CaCO3), core samples were collected for the execution of analyses and the tests were continued. The mineralogical composition of the solid residues was determined with X-ray diffraction (XRD) and optical microscopy on thin and polished sections. The non-sequential extraction tests, described in Table 4, were based on the work performed by McGregor et al. (1995) and aimed at determining the amount and composition of secondary oxidationneutralisation products. TABLE 4 Non-sequential extraction tests applied on the column solid residues
Extraction Deionised water
HC!
Hydroxylamine hydrochloride (NH2OH.HCl) acetic acid (CH3COOH)
Target phases - - W a t e r soluble fraction Soluble salts, e.g. gypsum (CaSO4.2H20), ferrous sulphates (FeSO4.nHzO), metals desorbed during extraction •Acid leachable fraction Water soluble fraction plus water insoluble phases e.g. jarosite, poorly crystalline iron (oxy)hydroxides --Reducible fraction Water soluble and acid leachable fractions plus crystalline iron, manganese oxy(hydroxides).
Method 100 g of de-ionised water added to 2 g dried sample, constant agitation for 24 h at room temperature 30 ml 20% (v/v) HC1 added to 2 g dried sample, 20 min at room temperature 30 ml 2M NH2OH.HC1 and 25% v/v CH3COOH added to 2 g dried sample, 24 h at 95 °C
Following each extraction, the leachate was analysed for Fetot~l, Fe 2+, Pb, Zn, Cd, As, Mn, Ca, Mg and 504; the weight loss was also measured. To allow comparison of the amount and type of oxidation products between the feed material and the solid residues, the non-sequential extraction tests were performed on the feed pyrite material as well.
RESULTS AND DISCUSSION Characterisation of the feed pyrite material
Based on the mineralogical analysis, pyrite, which consists of fairly liberated grains, was the major sulphide mineral of the concentrate used in the tests. Traces of galena, sphalerite, arsenopyrite, chalcopyrite, magnetopyrite and quartz were also identified. XRD diffraction patterns combined with optical microscopy indicated that szomolnokite (FeSO4.H20), gypsum (CaSO4.2H20) and anglesite (PbSO4) are the major oxidation products of material. Based on the non-sequential extraction test results, the water soluble, acid leachable and reducible fractions of metals and sulphate for the feed pyrite material, expressed as a percentage of the total content, are shown in Figure 1 (a). The pyrite material used in the kinetic tests contained a significant amount of water soluble oxidation products. The percentage of total sulphur content reported in the water extract was 4%. The respective figures for metals amounted to 4% for Fe, 9% for Pb, 77% for Zn, 50% for Cd and 91% for Mn.
1166
E. Mylonaet al. 100
I I
10
l
i
i
Fe
Pb
i
Zn
i
Cd
As
S
Ca
Mn
(S04) Water solubie [] Acidleachable II Reducible 100
z
z
2 ;
~iz
7z
z i
m s
10 Z
~
-i
2 il
iX
i
Fe
Pb
Zn
Cd
As
S
Ca
Mn
(S04) [IN Drainage [] water soluble r-1Acid ILchable IReducible 100 t~
10
©
1
,
Fe
Pb
Zn
,
Cd
As
,
S (S04)
Ca
Mn
Drainage [] Waier soluble r-lAcid [eachable IIReducibie
Fig.1
Non sequential extraction test results of (a) the feed pyrite material, (b) the solid residue in the control column and (c) solid residue in the column amended with 207 kg CaCO3/t. The results are compared with the amount of metals dissolved in the drainage.
The water soluble fraction of Fe was found in the ferrous state and was associated with the dissolution of szomolnokite. Its solubility product as given in equation (8) indicates the high solubility of monohydrate ferrous sulphate. The solubility product was calculated based on the relation developed by Reardon and Beckie (1987).
Inhibition of acid generation fromsulphidic wastes by the addition of small amounts of limestone FeSO4.H20 --* Fe 2+ + 5042- --l-H20
logKsp: -0.90 (T: 25 °C)
1167 (8)
The acid leachable and reducible fractions of Fe represented 5.5 and 5.7% of total metal mass. Ferric iron accounted for 40% of these fractions, however, any ferric iron precipitates contained in the pyrite material were not detected by XRD and optical microscopy. The high water soluble fraction of Zn, equal to 77% of total metal content indicates that the major portion of ZnS encountered in the pyrite was oxidised. The acid leachable fraction of metal was equal to the water soluble one suggesting that no oxidation of ZnS occurred after leaching with 20% v/v HCI. Lead water soluble fraction is attributed to the dissolution of anglesite. PbSO4 is a common secondary mineral of PbS weathering exhibiting, however, a very low solubility in water, Chatzioannou (1984): PbSO4 --> Pb 2+ + 5042-
logKsp=- 7.9
(9)
The amount of As solubilised after washing of the pyrite sample with de-ionised water was below detection limit, i.e. < 150 mg/kg, .whereas the acid leachable and reducible fraction of this element amounted to 60%. Finally, the total mass of Ca contained in the pyrite concentrate was reported in the water extract suggesting that it solely occurred in the form of gypsum. The amount of gypsum in the feed material was estimated to 2% wt.
Drainage quality Control
The pyrite concentrate material, as expected, produced acidic drainage with pH averaging 2.0_+0.7 for a test period of 270 days (Figure 2).
6 ~Z ca, 4
0 0
I
I
100
200
300
Time (Days) ~_~_ Control --0-6.4% CaCO3 --0- 12% CaCO3 17% CaCO3 --~29% CaCO3 Fig.2 pH variation vs. time for limestone amended columns as compared with control. The oxidation-reduction potential of the leachates averaged 418+23 mV throughout the test period; a conductivity value of 50 mS/cm was measured in the initial leachates whereas during the next cycles the conductivity ranged from 2.5 to 15.1 mS/cm. Along with the above parameters, the leachates produced from the control column were heavily contaminated with metals and sulphates, i.e. Fe: 15-85,300, Zn: 0.414,700, Pb: 0.6-14.6, As: 2.5-2,900, SO4:1,100-192,070 mg/l. The dissolution rate of 5 0 4 , FetotaI and Ca, expressed in mg/kg of pyrite, vs. L/S is shown in Figure 3. L/S is defined as the ratio of the volume of ieachant (L), which at any given time has been in contact with the amount of material tested, to the dry mass (S) of material prior to testing, Van der Sloot et al. (1997). Expressing the results in the above terms rather than those commonly applied, e.g. dissolution rate (mg/kg material) vs. time (week), also shown in Figure 3, is preferable given that it facilitates the comparison of results obtained from different types of leaching tests.
E. Mylona et al.
1168
Time (days) 0
50
100
150
200
250
100
so41
10
1 0.1 0.01
t...,
0 0
0.001 0.0001 0.25
1.25
2.25
3.25
4.25
Cumulative volume of leachate, L, per mass of material, S (1/kg) Fig.3 Dissolution rate of $04, Fe and Ca, in g/kg of material vs. L/S in the control. Due to the oxidation history of pyrite material used in the present work, the above results indicate both the dissolution of pre-existing oxidation products and the generation of oxidation products during the test period. It is seen that after the initial cycles the shape of the sulphate curve is continuous and there is a decrease in Fe concentration with time. Given that the pyrite material has no neutralisation potential, soluble calcium and corresponding sulphate derive from existing gypsum dissolution suggesting that after 270 days there is still dissolution of oxidation products in the control column. Geochemical modelling of the drainage chemistry using the computer geochemical equilibrium speciation model MINTEQA2, Allison et al. (1990), suggested that the solutions were at or near saturation with respect to gypsum. This fact complicates the interpretation of results and the estimation of actual pyrite oxidation rate. Limestone a m e n d e d colunms
During the initial flush, limestone amended columns produced acidic drainage due to the washing of existing oxidation products. Then, the leachates pH of all the colunms remained alkaline for a test period of 250 days. The values o f p H ranged from 7.1 to 8.6, averaging 7.8 (6.4% CaCO3), 7.9 (17% CaCO3) and 8.0 (12 and 29% CaCO3) with a standard deviation of 0.4, Figure 2. The oxidation-reduction potential of the leachates produced from the pyrite - limestone mixtures ranged from 128 (29% CaCO3) to 183 mV (6.4% CaCO3). Referring to the conductivity, after the initial cycles, the values measured in the leachates of the amended columns averaged 2.4 _+0.5 mS/cm. The cumulative percentage of metals and sulphates dissolved in the limestone amended columns as compared with the control are given in Table 5. In agreement with previous findings, Mylona et al. (1996), the addition of limestone to the pyrite concentrate material significantly impaired the solubilisation of metals and sulphate. It is seen that the major portion of the total amount of contaminants reported in the drainage of the pyrite-limestone mixtures was dissolved during the initial cycles. At the next stages, in the neutral leachates generated from the limestone amended columns, Fe, Pb and As levels were below the detection limit, i.e. <0.l, 0.5 and 1 mg/l respectively. Referring to Zn, the cumulative percentage of metal reported in the leachates of amended columns was reduced by 44 to 72% as compared with the control; the respective figures for Mn were lower, i.e. 7 to 53%.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1169
TABLE 5 Cumulative percentage of metals and sulphate dissolved in the limestone amended columns as compared with the control Column
Period
Fe
] Pb
I Zn
] Cd
I Mn
IS(SO4)
(%) Control
6.4% CaCO3
12% CaCO3
17% CaCO3
29% CaCO 3
0-20 days (0.5 1/ks) 0-270 days (~5 I/ks) 0-20 days (0.5 l&g) 0-270 days (~5 l&g) % reduction 0-20 days (0.5 l&~) 0-270 days (~5 I/k~) % reduction 0--20 days (0.5 1/kg) 0-270 days (~5 l&g) % reduction 0-20 days (0.5 l/kg) 0-270 days (~5 1/kg) % reduction
4.8
0.1
87.8
46.1
90.7
2.9
5.5
0.3
93.0
49.3
94.6
4.1
0.9
<0.01
50.4
15.2
83.1
0'.6
0.9
<0.01
52.0
16.7
88.0
1.2
83.4 2.4
98.5 0.02
44.1 49.7
66.1 20.7
7.0 69.1
70.2 2.0
2.4
0.02
49.8
20.8
70.1
2.4
56.0 1.4
93.9 0.02
46.5 33.1
57.8 17.6
25.9 41.3
41.1 1.6
1.4
0.02
33.5
18.0
44.3
2.2
73.5 1.1
93.9 0.02
63.9 25.9
63.5 16.9
53.2 41.4
46.1 1.0
1.1
0.02
26.1
17.3
44.1
1.5
79.6
93.9
72.0
64.9
53.2
61.8
Sulphates along with Ca were the major ions in solution of the limestone amended columns. After the initial flush, sulphate and calcium dissolved concentrations amounted to 340-370 and 1400-1500 mg/1 respectively and were mainly controlled by gypsum solubility as predicted by MINTEQA2. A higher reduction in the cumulative amount of iron and sulphate dissolved was observed for the column containing the lower amount of limestone, equal to 69 kg CaCO3/t. This is associated with the reduced quantity of metals released during the initial washing of this column and may be attributed to the high homogeneity of the mixture as Well as channelling effects. On the contrary, when the performance of the remaining columns is compared, higher limestone addition resulted in higher reductions in the cumulative amount of Fe and SO4 dissolved. Solid residues
The data obtained on the drainage quality are not considered to be sufficient lor the assessment of the effect of limestone on the pyrite oxidation process. This is because the metals and sulphate release in the drainage was affected by a) the dissolution of previously formed oxidation products and b) the precipitation of secondary products of the neutralisation reactions which occurred in the pyrite-limestone columns. To evaluate the performance of the systems examined, it is necessary to identify and quantify the precipitates in the solid residues and for this reason a detailed geochemical characterisation of the solid residues was performed. Control column
Based on the results of the extraction tests performed for the feed and the residue as well as the drainage composition of the control column, the weight loss of pyrite during the kinetic test was 8.2 %. The amount
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E. Mylonaetal.
of metals and sulphate reported in the three chemical extracts of the solid residue as compared to that measured in the drainage, expressed as percentage of total content in the feed, is shown in Figure l(b). Based on the solid residue analysis after 270 days of kinetic testing, past pyrite oxidation products were washed out and no significant precipitation of such products occurred under the test conditions. The amount of Fe in the water extract represented less than 0.1% of total metal mass suggesting the absence of soluble ferrous sulphates in the residue; the respective figures in the acid and hydroxylamine hydrochloride extracts were also low, 0.9 and 1.5%. Considering that the acid leachable fraction of Fe is mainly derived from the dissolution of jarosite and any poorly crystalline ferric (oxy)hydroxides, McGregor et al. (1995), it is deduced that under the low pH conditions met in the control column no precipitation of Fe 3+ occurred. The acid leachable and reducible fractions of Zn accounted for 1 and 3% of its total mass respectively. A significant amount of Pb total mass, i.e. 6% was still associated with the water soluble fraction. A low fraction of sulphur, i.e. 0.8% occurred in the sulphate form. Sulphate ions reported in the water extract of the residue were mainly associated with the dissolution of gypsum and anglesite. Given that most of the oxidation products accumulate at the grain boundaries of sulphide minerals, oxidation would begin only after these products were sufficiently depleted to allow oxygen and water diffusion to the sulphide grains. Furthermore, it is suggested that dissolution of existing sulphate minerals, i.e. szomolnokite and gypsum and subsequent sulphate yields may have suppressed sulphide oxidation to sulphate due to the common ion effect. Limestone amended columns
The weight loss of the pyrite-limestone columns after 270 days of testing ranged from 2.5 to 3.5%. As opposed to the control, extraction tests performed on the solid residues of the amended columns resulted in the dissolution of a considerable amount of metals suggesting the presence of secondary neutralisation products. The distribution of metals and sulphates in the drainage and the three extracts of the solid residues for the column amended with 207 kg CaCO3/t is shown in Figure 1 (c). The results are expressed as a percentage of the total content in the feed. Calcium and 8 0 4 w e r e the major ions reported in the water extract of the solid residues, and they were associated with the dissolution of gypsum. The presence of gypsum was confirmed by optical microscopy combined with XRD. Based on the water extraction test results, the amount of gypsum in the residues of the limestone amended columns ranged from 6 to 6.9% wt. This quantity was higher as compared to that measured in the feed material, i.e. 2% wt., indicating that dissolution of carbonates (calcite and dolomite) followed by the formation and precipitation of gypsum was a principal process in the pyrite-limestone mixtures during the tests. Based on the pH of the drainage of the limestone amended columns, i.e. pH: 8.0, the carbonate dissolved would be mainly in the form of bicarbonate, Evangelou (1995), thus the neutralisation of acidity and subsequent formation of gypsum can be described by the following reactions: CaCO3 + H + + 8042- + 2H20 ~ CaSO4.2H20 + HCO3 "
(10)
CaMg(CO3)2 + 2H + + 2SO42- + 2H20 --* CaSO4.2H20 + MgSO4 + 2HCO3
(ll)
A microscopic view in transmitted light of gypsum in the residue of the mixture containing 17% wt. limestone (207 kg CaCO3/t) is shown in Figure 4.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
Fig.4
1171
Microscopic view in transmitted light of the solid residue in the column amended with 207 kg CaCO3/t, X 160 (Py: Pyrite, Cal: Calcite, Fe: Ferric hydroxides, Gp: Gypsum)
Based on mass balance calculations, the amount of limestone consumed during the tests (dissolved in the drainage and precipitated as gypsum), expressed in g CaCO3/kg pyrite, and the amount of additive remained in the column solid residues are given in Table 6.
TABLE 6 CaCO3 consumed in the amended columns during the test
Column
Limestone content Amount of limestone in the residue consumed during the test (%) (g CaCO3/kg pyrite) 6.4% CaCO3 3.4 33* 12% CaCO3 9.0 30 17% CaCO3 13.5 37 29% CaCO3 25.0 48 *Dissolution of dolomite accounts 8 g/kg material Consumption of limestone is represented by the dissolution of calcite except for the column containing the lower amount of limestone, i.e. 6.4% wt. where the total Mg mass in the solid residue was reported in the water extract, suggesting the complete consumption of dolomite, CaMg(CO3)2. Higher consumption rates were observed for the columns containing increased amount of limestone. This was associated with the reduced amount of metals and sulphates measured in the drainage of these columns (see Table 5). Based on the results of the acid extraction test, the acid leachable Fe in the residues of the limestoneamended columns ranged from 3 to 5% of total metal mass, and was solely in the form of Fe 3+. Macroscopic observations of the solids indicated the presence of XRD amorphous ferric hydroxides (Figure 4). Based on the findings of this study, it is suggested that Fe z+ released to the pore water of mixtures during the test was oxidised to Fe 3+ and precipitated as a ferric hydroxide. The type(s) of ferric hydroxides that may be formed as a result of sulphides oxidation-neutralisation reactions, depend on parameters including pH and concentration of anions such as SO4 and CO3 and metals. They include the hydrous amorphous ferric hydroxides, i.e. ferrihydrite (FesOs.4H20), Carlson and Schwertmann (1981), limonite (FezO3.H20), goethite (ccFeOOH), lepidocrosite (TFeOOH) as well as the stable ferric oxides, e.g. hematite (ctFe203) and maghemite (7Fe203). According to Bigham et aL (1996) ferric precipitates formed at pH: 6.5 or higher were composed of ferrihydrite or a mixture of ferrihydrite and goethite whereas schwertmannite [Fe808(OH)6(SO4)2] with trace amounts of goethite were precipitated from waters having pH: 2.8-4.5. Previous studies have shown that when Fe z+ solutions are oxidised in the
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E. Mylonaet al.
presence of HCO3 , condition met in the present experiments, formation of goethite instead of lepidocrocite is enhanced, Lin (1997). The reactions illustrating the formation of amorphous ferric hydroxide [nomimally Fe(OH)3] and goethite are given below: Fe 3÷ + 3H20 --+ Fe(OH)3 + 3H +
(12)
Fe 3+ + 2H?O ~ c~FeOOH + 3H +
(13)
Based on the microscopic observations, ferric hydroxides along with gypsum were found in the contact of pyrite-limestone particles (Figure 4). Furthermore, replacement of the pyrite and limestone grains by these secondary neutralisation products was observed. Precipitation of ferric hydroxides around the periphery of pyrite particles forming a protective rim was also an active mechanism in the pyrite-limestone mixtures, as shown in Figure 5.
Fig.5
Microscopic view in transmitted light of the solid residue in the column amended with 207 kg CaCO3/t, X 160 (Py: Pyrite, Fe: Ferric hydroxides).
Zinc and Mn reported in the acid extracts of the solid residues accounted for 20-40% and 15-30% of their total metals mass respectively. According to Blowes and Jambor (1990) the major removal mechanism for Zn in oxidised tailings is sorption and coprecipitation with ferric hydroxide and sulphate compounds. Regarding Pb, 50-60% of total mass was reported in the acid extracts. This fraction, apart from the dissolution of anglesite, may be also associated with the dissolution of galena. Similar figures were obtained for As. Based on previous studies, ferric hydroxides are considered to be efficient scavengers for this element, Sun and Doner (1996). The above results, indicate that thorough mixing of pyrite with ground limestone at an amount corresponding to a low fraction of the contained acidity results in the significant reduction of the amount of iron and sulphate dissolved in the drainage, mainly due to the precipitation of ferric hydroxides and gypsum. A 53% reduction in the pyrite oxidation rate was previously observed for the mixture containing limestone at an amount corresponding to 5% of the stoichiometric quantity, Mylona e t al. (1996). Estimation of this figure for the mixtures examined in the present study could not be made. This is attributed to the delay observed in the pyrite oxidation process mainly due to the increased amount of oxidation products contained in the pyrite material. Another factor to be considered includes the different configuration used (columns with a material thickness of 40 cm as compared to lysimeter, 30 cm wide with a material thickness of 4 cm used in the previous study). The results of this study suggest that increased limestone addition would improve the drainage quality and may also have a beneficial effect in the reduction of the hydraulic conductivity of material, as described in the following paragraphs.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1173
Hydraulic conductivity Based on the results of the falling head permeability tests, the coefficient of hydraulic conductivity, k, of pyrite material was found k: 5x10 -5 cm/sec. Mixing of the pyrite concentrate material with limestone resulted in the increase of permeability by up to 25 times. After a monitoring period of 270 days, the coefficient of hydraulic conductivity for the mixtures containing 17 and 29% wt. limestone was reduced by one order of magnitude as compared with that measured in the beginning of the tests. A value of k: 2x 10-4cm/sec was obtained for the mixture containing 17% wt. limestone, being, however, higher than that found for the pyrite material. The reduction in the permeability of pyrite-limestone mixtures may be attributed to the filling of pores by the secondary neutralisation products, i.e. gypsum and ferric hydroxides. This process occurring in nature has resulted in the formation of hardpan layers at the surface of sulphidic tailings disposal areas. Hardpan layers inhibit the downward migration of rainfall waters and restrict the oxygen diffusion through the tailings, thus further oxidation of sulphides is prevented. Based on field observations, hardpans formed by sulphate minerals, e.g. gypsum, are more persistent laterally as compared to those consisting of ferric hydroxides which occur as discontinuous mineral segregations with a limited lateral extension, Blowes et al. (1991), Tase et al. (1997), Lin (1997). The formation of a hardpan was also observed in the laboratory, Chermak and Runnells (1997). Addition of limestone and/or lime to the surface layer of acid generating waste rock (NNP: - 5 0 kg CaCO3/t) resulted in the formation of a low permeability layer (k: 2.8 x l 0 7 cm/sec), consisting of gypsum and amorphous ferric (oxy) hydroxides. In this case, however, the drainage produced from the amended waste rock was acidic. Based on other studies, when an acidic ferric solution flows through calcareous sand, the reduction in the permeability (up to four orders of magnitude) results primarily from CO2 exsolution. The reduction in the permeability due to the precipitation of ferric hydroxides alone does not exceed two orders of magnitude, Fryar and Schwartz (1998). The reduction in the permeability of the pyrite-limestone mixtures examined in the present study may indicate a primitive stage of hardpan formation. The phenomenon was observed in the mixtures containing higher amount of limestone suggesting that the relative proportion of pyrite and limestone materials play a major role in the hardpan formation. Another factor to be considered includes the application of wet/dry cycles in the columns. A higher reduction in the permeability of the limestone amended columns may be achieved by decreasing the frequency of water flushing in order to inhibit the solubilisation of secondary phases and also promote the conversion of iron hydroxides into stable iron oxides.
CONCLUSIONS The effect of lower than the stoichiometrically required amounts of limestone on the pyrite oxidation process was studied. Long-term kinetic column tests were performed on a partially oxidised pyrite concentrate homogeneously mixed with various amounts of limestone corresponding to 5-30% of the stoichiometric quantity. Based on the drainage quality monitoring results, the analyses of the solid residues and the hydraulic conductivity measurements, the following conclusions can be drawn: The pyrite material used in the tests contained a significant amount of water soluble oxidation products. Column kinetic testing, involving the application of weekly wet/dry cycles, performed on the material over a period of 270 days, resulted mainly in the dissolution of these products and further oxidation of pyrite particles was delayed. Homogenous mixing of the pyrite concentrate with limestone, at amounts lower than the stoichiometric requirements, prevented the generation of acidic drainage and significantly reduced the amount of metals and sulphates dissolved in the drainage. Under the alkaline conditions prevailing in the limestone amended columns, i.e. pH: 8.0, Fe, Pb and As levels in the drainage were below detection limit, i.e. <0.1, 0.5 and 1.0 mg/1 respectively. The reduction effected in the dissolved amount of Zn, Cd and Mn was estimated up to 72, 66 and 53% respectively as compared to the control.
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E. Mylonaetal.
Neutralisation reactions in the limestone-amended columns resulted in the precipitation of secondary products, mainly gypsum and ferric hydroxides. The amount of these products in the pyrite-limestone mixtures was estimated 6-7 and 3-5% wt. respectively. Formation of a protective layer consisting of ferric precipitates around the pyrite particles was observed. Furthermore, precipitation of ferric hydroxides and gypsum in the contact of pyrite-limestone particles was an active mechanism in the systems examined. This resulted in the 10-fold reduction in the permeability for material amended with 207 kg CaCO3/t, corresponding to 15% of the stoichiometric quantity. The amount of limestone consumed during the test period of 270 days ranged from 30 to 48 g/kg pyrite. Higher consumption rates were observed for the mixtures containing higher amount of limestone. Assuming stable conditions, the experimental time required for the complete consumption of the alkaline additive in the amended columns was estimated to range from 8 months to 6 years. Due to the oxidation history of pyrite material used in the tests, the test duration, although long, i.e. 270 days, may not be considered sufficient to reliably assess the actual oxidation rate of pyrite and evaluate the inhibition effect of limestone addition. Additional data to be obtained from the continuation of the tests and sampling of the solid residues and/ or geochemical modelling, would allow the estimation of above parameters. However, based on the results of this study, it is seen that limestone may be beneficially used, at least in the short term, for the acid generation control from oxidised sulphidic wastes.
REFERENCES
Adam, K., Kourtis, A., Gazea, B. and Kontopoulos, A., Prediction of Acid Rock Drainage in Polymetallic Sulphide Mines. Transactions oflMM, 1997, Section A, January-April, A1-A8. Allison, J. D., Brown, D. S. and Novo-Gradac, K. J., MINTEQA2/PRODEFA2, a geochemical assessment model for environmental systems. Version 3.0 User's Manual. EPA/600/3-91/021, Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA, 1990. Bigham J.M., Schwertmann U., Traina S.J., Winland R.L. and Wolf M., Schwertmannite and the chemical modelling of iron in acid sulfate waters. Geochimica et Cosmochimica Acta, 1996, 12, 2111-2121. Blowes, D.W. and Jambor, J.L., The pore water geochemistry and the mineralogy of the vadose zone of sulphide tailings, Waite Amulet, Quebec. Applied Geochemistry, 1990, g, 327-346. Blowes, D.W., Reardon E.J., Jambor, J.L., and Cherry, A., The formation and potential importance of cemented layers in inactive sulphide mine railings. Geochimica et Cosmochimica Acta, 1991, 55, 965978. Carlson, L. and Schwertmann, U., Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochimica et Cosmochimica Acta, 1981, 45, 421~429. Chatzioannou, Th. P., Qualitative analysis and chemical equilibrium, 8th edn. 1984, Grafikes Tehnes, Athens. Chermak, J. A. and Runnells, D.D., Development of chemical caps in acid rock drainage environments. Mining Engineering, 1997, 49(6), 93-97. Day, S. J., Evaluation of Acid Generating Rock and Acid Consuming Rock Mixing to Prevent Acid Rock Drainage. In Proc. International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage. U.S Department of the Interior, Bureau of Mines Special Publication SP 06B-94, Pittsburgh, 1994, Vol. 2, pp. 77-86. Evangelou, V. P., Pyrite oxidation and its' control, 1995, CRC Press. Farah, A., Hmidi, N., Moskalyk, R., Amaratunga, L. M. and Tombalakian, A. S., Numerical modeling of the effectiveness of sealants in retarding acid mine drainage from mine waste rock. Canadian Metallurgical Quarterly, 1997, 36(4), 241-250. Fetter, C. W., Applied Hydrogeology, 3rd edn. 1994, Prentice Hall, Englewood Cliffs. Fryar, A. E. and Schwartz, F. W., Hydraulic conductivity reduction, reaction-front propagation, and preferential flow within a model reactive barrier. Journal of Contaminant Hydrology, 1998, 32, 333351. Heltz, G.R., Dai, J.H., Kijak, P.J., Fendinger, N.J. and Radway, J.C., Processes controlling the composition of acid sulphate solutions evolved from coal. Applied Geochemistry, 1987, 2, 427-436.
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Kelley, B. C. and Tuoniven, O. H., Microbiological oxidations of minerals in mine tailings. In Chemistry and Biology of Solid Waste: Dredged Material and Mine Tailings, ed. W. Salomons and U. Forstner. Springer-Verlag, Berlin, FRG, 1988, pp. 33-53. Lapakko, K., Antonson, D. A, and Wagner, J. R., Mixing of limestone with finely crushed acid producing rock. In Proc. Fourth International Conference on Acid Rock Drainage. Vancouver, B.C. Canada, 1997, Vol. III, pp. 1345-1360. Lin, Z., Mobilization and retention of heavy metals in mill-tailings from Garpenberg sulfide mines, Sweden. The Science of the Total Environment, 1997, 198, 13-31. Mc Gregor, R. G., Biowes, D. W. and Robertson, W. D., The application of chemical extractions to sulphide tailings at the copper Cliff tailings area, Sudbury, Ontario. In Proc. Conference of Sudbury '95 I Mining and the Environment, ed. T. P. Hynes, and M. C. Blanchette. CANMET, Ottawa, 1995, Vol. 3, pp. 1133-1142. Morin, K.A. and Hutt, N.M., Observed preferential depletion of neutralization potential over sulphide minerals in kinetic tests. Site specific criteria. In Proc. International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage. U.S Department of the Interior, Bureau of Mines Special Publication SP 06A-94, Pittsburgh, 1994, Vol. 1, pp. 148-156. Mylona, E., Adam, K., and Kontopoulos, A., Mechanisms involved in the control of acid generation from sulphide wastes with limestone addition. In Proc. III International Conference on Protection and Restoration of the Environment, ed. E. Diamantopoulos and G. Korfiatis. Technical University of Crete, Hania, Greece, 1996, pp. 474-483. Nicholson, R.V., Gillham, R.W., and Reardon, E.J., Pyrite oxidation in carbonate - buffered solutions: 1. Experimental kinetics. Geochimica et Cosmochimica Acta, 1988, 52, 1077-1085. Nicholson, R.V., Gillham, R.W., and Reardon, E.J., Pyrite oxidation in carbonate - buffered solutions: 2. Rate control by oxide coatings. Geochimica et Cosmochimica Acta, 1990, 54, 395-402. Payant, S., St-Arnaud, L. C., and Yanful, E. Evaluation of techniques for preventing acidic rock drainage. In Proc. Conference of Sudbury '95 - - Mining and the Environment, ed. T. P. Hynes, and M. C. Blanchette. CANMET, Ottawa, 1995, Vol. 1, pp. 485-493. Reardon, E. J. and Beckie, R. D., Modelling chemical equilibria of acid mine-drainage: The FeSO4HzSO4-H20 system. Geochimica et Cosmochimica Acta, 1987, 51, 2355-2368. Renton, J.J., Stiller, A.H., and Rymmer, T.E., The use of phosphate materials as ameliorants for acid mine drainage. In Proc. Conference on Mine Drainage and Surface Mine Reclamation. USBM Information Circular IC 9183, 1988, Vol. 1, pp. 67-75. Rose, A. W and Daub, G. A., Simulated Weathering of Pyritic Shale with Added Limestone and Lime. In Proc. Conference on International Land Reclamation and Mine Drainage and the Abatement of Acidic Drainage. U.S Department of the Interior, Bureau of Mines Special Publication SP 06B-94, Pittsburgh, 1994, pp. 334-340. Singer, P.C. and Stumm, W., Acid mine drainage: The rate-determining step. Science, 1970, 167, 11211123. Sobek, A. A., Schuller, W. A., Freeman, J. R., and Smith, R. M., Field and laboratory methods applicable to overburden and minesoils. US EPA 600/2-78/054, Industrial Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Cincinnati, Ohio, 1978. Sun H. and Doner, H. E., An investigation of arsenate and arsenite bonding structures on goethite by FTIR. Soil Science, 1996, 161(12), 865-872. Tasse, N., Germain, D., Dufour, C. and Tremblay, R., Hard-pan formation in the Canadian Malartic mine tailings: Implication for the reclamation of the abandoned impoundment. In Proc. Fourth International Conference on Acid Rock Drainage. Vancouver, B.C, Canada, 1997, Vol. III, pp. 1797-1812. Van Der Sloot, H. A., Heasman, L., Quevauviller, Ph. and Va Berg, M. J. A., Harmonisation of leaching/ extractions tests (Studies in Environmental Science, 70), 1997, Elsevier Science Ltd., Amsterdam. Watkin, E. M and Watkin, J., Inhibiting Pyrite oxidation Can Lower Reclamation Costs. Canadian Mining Journal, 1983, December, 29-31.
C o r r e s p o n d e n c e o n p a p e r s p u b l i s h e d in M i n e r a l s E n g i n e e r i n g bwills @ m i n - e n g . c o m
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INHIBITION OF ACID GENERATION FROM SULPHIDIC WASTES BY THE ADDITION OF SMALL AMOUNTS OF LIMESTONE
E. MYLONA, A. XENIDIS and I. PASPALIARIS Laboratory of Metallurgy, Dept. of Mining and Metallurgical Engineering, National Technical University of Athens, GR-15780 Zografos, Greece. E-mail: [email protected] (Received 20 December 1999; accepted 13 June 2000)
ABSTRACT Limestone addition is a commonly applied technique to prevent acid generation from sulphidic wastes containing 1-10% S. In the present paper, the effectiveness of small amounts, lower than the stoichiometric requirement, of this alkaline additive in inhibiting acid generation from a pyrite concentrate material is studied. Long term laboratory column tests were conducted on a partially oxidised pyrite concentrate, where limestone was added by thoroughly mixing. The amount of alkaline additive ranged from 6.4 to 29% wt. corresponding to 5-30% of the stoichiometric quantity. The performance of the pyrite-limestone mixtures was evaluated by monitoring the drainage quality of the columns. Furthermore, a detailed geochemical characterisation of the column solid residues was performed after a monitoring period of 270 days. The effect of secondary oxidation-neutralisation products on the hydraulic conductivity of material was also examined. Dissolution of previously formed oxidation products occurred in the control column during the monitoring period, resulting in the release of a significant amount of Fe, Zn, Mn, Cd, As and S04 and to a lesser extent Pb. However, due to the presence of secondary products, further oxidation of pyrite particles was delayed. The experimental results showed that homogeneous mixing of pyrite with limestone amounting to only a fraction of the contained acidity inhibited the generation of acidic drainage and significantly reduced the dissolved amount of metals and sulphates for a test period of 270 days. Under the alkaline conditions prevailing in the limestone amended columns, secondary precipitation of ferric hydroxides and gypsum occurred. A ten-fold decrease of hydraulic conductivity was observed for the material amended with 207 kg CaCOJt, corresponding to 15% of the contained acidity. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords Acid rock drainage; oxidation; environmental; reclamation
INTRODUCTION Acid generation from sulphidic wastes constitutes a significant environmental problem in coal, lignite and polymetallic sulphide mining. The sulphide minerals, particularly pyrite, FeS2, and pyrrhotite, Fel-xS, contained in wastes are oxidised in the presence of air, water and bacteria, resulting in the production of low-pH waters loaded with dissolved SO42-, iron and other heavy metals and toxic compounds.
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Pyrite oxidation resulting in the generation of acidic drainage can be described by the following reactions, Singer and Stumm (1970): 2FeS2 + 702 + 2H20 + 2Fe 2+ + 4S042 + 4H + 4Fe 2+ + Oz + 4H +--~ 4Fe 3+ + 2H20 FeS2 + 14Fe3++ 8H20 --~ 15Fe 2+ + 2SO42 + 16 H +
(1) (2) (3)
Under acidic conditions, pH < 3.5, the ferrous to ferric iron reaction (2) is slow and the contribution of iron-oxidising bacteria such as the Thiobacillusferrooxidans species becomes important. These bacteria act as electron acceptors during the Fe(II) oxidation, increasing the reaction rate by several orders of magnitude. The Fe(III) produced is capable of rapidly oxidising pyrite by an abiotic pathway thus regenerating Fe(II) and propagating the cycle, Singer and Stumm (1970). Pyrite oxidation theory and control technologies have been well documented in the literature, Evangelou (1995). Addition of alkaline materials aiming at the control of pH is a commonly applied technique to prevent acid generation from sulphidic wastes. Materials like lime, Rose and Daub (1994), Heltz et al. (1987), limestone, Day (1994), Payant et al. (1995), Lapakko et al. (1997), sodium carbonate, He]tz et al. (1987), as well as industrial by-products including phosphatic slimes, Watkin and Watkin (1983), Renton et al. (1988) and alkaline siliceous materials such as fly ash, Farah et al. (1997) have been reported to effectively prevent acidic drainage. The potential of limestone to prevent acid generation has been well recognised from the fact that in sulphide and coal mines containing abundant natural limestone or other carbonate minerals, limited if any acidity is formed. In sulphidic waste stockpiles when calcite is present, the overall oxidation-neutralisation reaction depends on pH, Morin and Hutt (1994): pH<6.4 FeS2(s) + 2CaCO3(s) + 3.7502(g) + 1.5H20 + Fe(OH)3(s) + 2SO42 + 2Ca 2+ + 2CO2(g)
(4)
pH>6.4 FeS2(s) + 4CaCO3(s) + 3.7502(g) + 3.5H~O + Fe(OH)3(s) + 2SO42 + 4Ca 2++ 4HCO3
(5)
Based on the stoichiometry of reaction (4), under acidic conditions, 1 mole of pyrite is neutralised by 2 moles of calcite. At pHs above 6.4, the carbonate product will be HCO3 rather than CO2. In this case, twice as much CaCO3 is required to neutralise the same amount of acidity. However, the addition of limestone to sulphidic wastes does not only neutralise the generated acidity but also can prevent the pyrite oxidation process by the activation of the appropriate physical and chemical mechanisms. In this case the cost of the method will be significantly reduced because the amount of additive required would be significantly less than the stoichiometric one (reaction 4). There are four mechanisms by which limestone may control pyrite oxidation. The first mechanism involves precipitation of ferric iron in the hydroxide form, thus its further participation as an oxidising agent in the dissolution of pyrites is inhibited, Kelley and Tuovinen (1988). The second mechanism involves the raising of the pH of pore water to high values (pH: 6.1-8.4), thus the activity of the oxidising bacteria Thiobacillus ferrooxidans, is significantly impaired, Nicholson et al. (1988). Another mechanism involves the precipitation of oxidised compounds on the sulphides surface. It is reported that when carbonate minerals are present and available to neutralise the acid produced during the oxidation of pyrite, a protective ferric oxy-hydroxide layer will accumulate around the pyrite grains, impairing its further dissolution, Nicholson et al. (1990). Finally, according to field studies the presence of carbonate material in sulphide tailings may enhance the formation of cemented layers (hardpan) on the stockpile surface. The hardpan consists of the oxidation-neutralisation products such as ferric oxy-hydroxide and gypsum that cement the tailings together forming a low permeability mass that acts as an oxygen and water diffusion barrier, Blowes et al. (1991), Tasse et al. (1997), Lin (1997). Based on previous studies, homogeneous mixing of sulphidic waste rock, containing 9.7% S, with limestone in amounts corresponding to 3 and 10% of the stoichiometric quantity reduced the acid
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1163
production rate by 83 and 98% respectively after a 3 years test period, Payant et al. (1995). Furthermore, laboratory kinetic studies over a period of 397 weeks showed that a 84% decrease in the rate of sulphide oxidation was effected by mixing waste rock containing 2.1% S with limestone at an amount corresponding to 16% of the stoichiometric requirement, Lapakko et al. (1997). The addition of greater amounts of limestone (30% of the contained acidity or higher) further reduced the acid generation rate so that the host rock silicate mineral dissolution would adequately neutralise the resultant acid production, ensuring the long term prevention of acidic drainage, Lapakko et aL (1997). Published work on the effectiveness of limestone addition to prevent acid generation from highly sulphidic wastes is limited. In a previous study, it was shown that thorough mixing of ground limestone with a pyrite concentrate at an amount corresponding to 5% of the stoichiometric requirement reduced the rate of pyrite oxidation by 53%, Mylona et al. (1996). This was mainly attributed to the activation of chemical mechanisms including pH increase, ferric hydroxide precipitation and diminution of bacterial activity. No effect on the water permeability of material was observed. The present study aimed at a thorough evaluation of the effect of limestone content on the pyrite oxidation process. A series of long term column kinetic tests were conducted on a partially oxidised pyrite concentrate, which was homogeneously mixed with various amounts of limestone corresponding to 5 up to 30 % of the stoichiometric quantity. For the evaluation of results, an integrated methodology was followed that combined the monitoring of the column drainage quality and the examination of physical, chemical and mineralogical characteristics of the solid residues.
MATERIALS AND METHODS Materials The material used in this study was a pyrite concentrate assaying 42.6% S. 5% of total sulphur content was in the sulphate form indicating the presence of past oxidation products in the concentrate that has been stockpiled for more than 15 years. The material was finer than 1.4 mm and had an average particle size, d50 of 70 lam. Quarry quality limestone consisting of calcite and dolomite (11% wt.) was used as alkaline additive. The material was ground to a particle size similar to that of the sulphidic concentrate. Chemical assays of the pyrite and limestone are given in Table 1.
TABLE 1 Chemical composition of pyrite concentrate and limestone (analyses %) Element
Pb
Pyrite 0.34 concentrate Limestone <0.02 ND: Not determined
Zn
Mn
Fe
0.34
0.027
38.50
Cd ppm 28.0
0.004
0.002
2.25
1.2
As
Stota!
S
Ca
Mg
2.21
0.45
0.10
ND
37.0
1.46
(504) 0.99
42.74
<0.05
<0.20
Methods K i n e t i c tests
To determine the effect of limestone on the pyrite oxidation process, long term laboratory kinetic tests were carried out in plexi-glass columns with a diameter of 160 mm and a height of 100 cm in total. Five (5) columns were set up; one column was loaded with pyrite material without any limestone addition and was used as reference (control column) and the other four contained homogeneous mixtures of pyrite with limestone. Limestone content of the mixtures ranged from 6.4 to 29% wt., as given in Table 2. Neutralisation Potential (NP), and Maximum Potential Acidity (MPA) of pyrite and limestone were determined with the Acid Base Accounting (ABA) method and expressed in kg CaCO3/t, Sobek et al. (1978), Adam et al. (1997). For the limestone amended systems these values were calculated according to equation (6), Day (1994):
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E. Mylonaetal.
(6)
P=(PLML + PpMp)/(ML+Mp)
where P refers to NP or MAP, and ME and Mp are the weight of limestone and pyrite in the columns. The results are given in Table 3.
TABLE 2 Test conditions in the pyrite and limestone amended columns a/a
1 2 3 4 5
Column
Control, Pyrite Amended with limestone mixture
Total weight
Limestone content
(g)
(% w.t)
13,590 14,003 15,379 16,310 19,103
6.4 12.0 17.0 29.0
Thickness of material (cm) 29.5 36.5 42.0 43.5 56.5
TABLE 3 ABA parameters of the pyrite and limestone amended material Code
Column
MAP I NP I NNP kg CaCO3/t
Control Control, Pyrite 6.4% Amended with limestone CaCO3 mixture 12% C a C O 3 17% CaCO3 29% CaCO3
Alkalinity added kg % of stoichiometric CaCO3/t quantity
1,335.6 1,249.6
-41.9 25.2
-1,335.6 - 1,224.3
69
5
1,173.9 1,106.9 945.1
84.2 136.5 262.8
- 1,089.7 -970.4 -682.3
138 207 413
10 15 30
The pyrite concentrate material had a Net Neutralisation Potential (NNP: NP-MAP) o f - 1 , 3 3 5 . 6 kg CaCO3/t indicating that a limestone addition of 1.3 t/t material is stoichiometrically required to balance its total potential acidity. The amount of limestone added in the present tests ranged from 69 to 413 kg CaCO3/t corresponding to 5-30% of the stoichiometric quantity. Wet/dry cycles of 7 days were imposed in the column tests. Each cycle involved the addition of 2 litres of deionised water; the water was allowed to infiltrate and depending on the hydraulic conductivity of material, the leachate was collected at the second to the forth day of the cycle. The columns were left to dry till the seventh day and then the next cycle started. The cycles continued for a total of 270 days. Parameters monitored include leachate volume, pH, oxidation-reduction potential, conductivity, acidity/alkalinity and metals concentration, i.e. Fe, Pb, Zn, Mn, Cd, As, Ca, Mg and SO4. Determination of metals in solution was carried out with AAS (Perkin Elmer 2100). Sulphates concentration was determined gravimetrically. Permeability measurements
To determine any effect of limestone on the hydraulic conductivity of pyrite material due to the precipitation of secondary reaction products, falling head permeability tests were conducted in the columns after the completion of the tests. Flooding the material from the base of each column initially saturated the columns. Then for the permeability tests, a water head (H1) of 50 to 75 cm was applied to avoid piping of the samples. Water was allowed to flow through the sample and the time, t, required for the hydraulic head to reach the value of H 2 w a s recorded. The permeability coefficient was calculated from the following equation, Fetter (1994): k = 2.3
L.f Hl log-F.t Ho
(7)
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1165
where k: coefficient of hydraulic conductivity, L: thickness of the sample, f, F: cross section area of the tube and the sample respectively, HI, H2: hydraulic head at the start and the end of the test respectively. Characterisation of the solid residues To evaluate the overall effect of limestone on the pyrite oxidation process, columns were dismantled after a monitoring period of 270 days, and solid residues were subjected to chemical, mineralogical analyses and non-sequential extraction tests. For two columns (control, mixture--17% wt. CaCO3), core samples were collected for the execution of analyses and the tests were continued. The mineralogical composition of the solid residues was determined with X-ray diffraction (XRD) and optical microscopy on thin and polished sections. The non-sequential extraction tests, described in Table 4, were based on the work performed by McGregor et al. (1995) and aimed at determining the amount and composition of secondary oxidationneutralisation products. TABLE 4 Non-sequential extraction tests applied on the column solid residues
Extraction Deionised water
HC!
Hydroxylamine hydrochloride (NH2OH.HCl) acetic acid (CH3COOH)
Target phases - - W a t e r soluble fraction Soluble salts, e.g. gypsum (CaSO4.2H20), ferrous sulphates (FeSO4.nHzO), metals desorbed during extraction •Acid leachable fraction Water soluble fraction plus water insoluble phases e.g. jarosite, poorly crystalline iron (oxy)hydroxides --Reducible fraction Water soluble and acid leachable fractions plus crystalline iron, manganese oxy(hydroxides).
Method 100 g of de-ionised water added to 2 g dried sample, constant agitation for 24 h at room temperature 30 ml 20% (v/v) HC1 added to 2 g dried sample, 20 min at room temperature 30 ml 2M NH2OH.HC1 and 25% v/v CH3COOH added to 2 g dried sample, 24 h at 95 °C
Following each extraction, the leachate was analysed for Fetot~l, Fe 2+, Pb, Zn, Cd, As, Mn, Ca, Mg and 504; the weight loss was also measured. To allow comparison of the amount and type of oxidation products between the feed material and the solid residues, the non-sequential extraction tests were performed on the feed pyrite material as well.
RESULTS AND DISCUSSION Characterisation of the feed pyrite material
Based on the mineralogical analysis, pyrite, which consists of fairly liberated grains, was the major sulphide mineral of the concentrate used in the tests. Traces of galena, sphalerite, arsenopyrite, chalcopyrite, magnetopyrite and quartz were also identified. XRD diffraction patterns combined with optical microscopy indicated that szomolnokite (FeSO4.H20), gypsum (CaSO4.2H20) and anglesite (PbSO4) are the major oxidation products of material. Based on the non-sequential extraction test results, the water soluble, acid leachable and reducible fractions of metals and sulphate for the feed pyrite material, expressed as a percentage of the total content, are shown in Figure 1 (a). The pyrite material used in the kinetic tests contained a significant amount of water soluble oxidation products. The percentage of total sulphur content reported in the water extract was 4%. The respective figures for metals amounted to 4% for Fe, 9% for Pb, 77% for Zn, 50% for Cd and 91% for Mn.
1166
E. Mylonaet al. 100
I I
10
l
i
i
Fe
Pb
i
Zn
i
Cd
As
S
Ca
Mn
(S04) Water solubie [] Acidleachable II Reducible 100
z
z
2 ;
~iz
7z
z i
m s
10 Z
~
-i
2 il
iX
i
Fe
Pb
Zn
Cd
As
S
Ca
Mn
(S04) [IN Drainage [] water soluble r-1Acid ILchable IReducible 100 t~
10
©
1
,
Fe
Pb
Zn
,
Cd
As
,
S (S04)
Ca
Mn
Drainage [] Waier soluble r-lAcid [eachable IIReducibie
Fig.1
Non sequential extraction test results of (a) the feed pyrite material, (b) the solid residue in the control column and (c) solid residue in the column amended with 207 kg CaCO3/t. The results are compared with the amount of metals dissolved in the drainage.
The water soluble fraction of Fe was found in the ferrous state and was associated with the dissolution of szomolnokite. Its solubility product as given in equation (8) indicates the high solubility of monohydrate ferrous sulphate. The solubility product was calculated based on the relation developed by Reardon and Beckie (1987).
Inhibition of acid generation fromsulphidic wastes by the addition of small amounts of limestone FeSO4.H20 --* Fe 2+ + 5042- --l-H20
logKsp: -0.90 (T: 25 °C)
1167 (8)
The acid leachable and reducible fractions of Fe represented 5.5 and 5.7% of total metal mass. Ferric iron accounted for 40% of these fractions, however, any ferric iron precipitates contained in the pyrite material were not detected by XRD and optical microscopy. The high water soluble fraction of Zn, equal to 77% of total metal content indicates that the major portion of ZnS encountered in the pyrite was oxidised. The acid leachable fraction of metal was equal to the water soluble one suggesting that no oxidation of ZnS occurred after leaching with 20% v/v HCI. Lead water soluble fraction is attributed to the dissolution of anglesite. PbSO4 is a common secondary mineral of PbS weathering exhibiting, however, a very low solubility in water, Chatzioannou (1984): PbSO4 --> Pb 2+ + 5042-
logKsp=- 7.9
(9)
The amount of As solubilised after washing of the pyrite sample with de-ionised water was below detection limit, i.e. < 150 mg/kg, .whereas the acid leachable and reducible fraction of this element amounted to 60%. Finally, the total mass of Ca contained in the pyrite concentrate was reported in the water extract suggesting that it solely occurred in the form of gypsum. The amount of gypsum in the feed material was estimated to 2% wt.
Drainage quality Control
The pyrite concentrate material, as expected, produced acidic drainage with pH averaging 2.0_+0.7 for a test period of 270 days (Figure 2).
6 ~Z ca, 4
0 0
I
I
100
200
300
Time (Days) ~_~_ Control --0-6.4% CaCO3 --0- 12% CaCO3 17% CaCO3 --~29% CaCO3 Fig.2 pH variation vs. time for limestone amended columns as compared with control. The oxidation-reduction potential of the leachates averaged 418+23 mV throughout the test period; a conductivity value of 50 mS/cm was measured in the initial leachates whereas during the next cycles the conductivity ranged from 2.5 to 15.1 mS/cm. Along with the above parameters, the leachates produced from the control column were heavily contaminated with metals and sulphates, i.e. Fe: 15-85,300, Zn: 0.414,700, Pb: 0.6-14.6, As: 2.5-2,900, SO4:1,100-192,070 mg/l. The dissolution rate of 5 0 4 , FetotaI and Ca, expressed in mg/kg of pyrite, vs. L/S is shown in Figure 3. L/S is defined as the ratio of the volume of ieachant (L), which at any given time has been in contact with the amount of material tested, to the dry mass (S) of material prior to testing, Van der Sloot et al. (1997). Expressing the results in the above terms rather than those commonly applied, e.g. dissolution rate (mg/kg material) vs. time (week), also shown in Figure 3, is preferable given that it facilitates the comparison of results obtained from different types of leaching tests.
E. Mylona et al.
1168
Time (days) 0
50
100
150
200
250
100
so41
10
1 0.1 0.01
t...,
0 0
0.001 0.0001 0.25
1.25
2.25
3.25
4.25
Cumulative volume of leachate, L, per mass of material, S (1/kg) Fig.3 Dissolution rate of $04, Fe and Ca, in g/kg of material vs. L/S in the control. Due to the oxidation history of pyrite material used in the present work, the above results indicate both the dissolution of pre-existing oxidation products and the generation of oxidation products during the test period. It is seen that after the initial cycles the shape of the sulphate curve is continuous and there is a decrease in Fe concentration with time. Given that the pyrite material has no neutralisation potential, soluble calcium and corresponding sulphate derive from existing gypsum dissolution suggesting that after 270 days there is still dissolution of oxidation products in the control column. Geochemical modelling of the drainage chemistry using the computer geochemical equilibrium speciation model MINTEQA2, Allison et al. (1990), suggested that the solutions were at or near saturation with respect to gypsum. This fact complicates the interpretation of results and the estimation of actual pyrite oxidation rate. Limestone a m e n d e d colunms
During the initial flush, limestone amended columns produced acidic drainage due to the washing of existing oxidation products. Then, the leachates pH of all the colunms remained alkaline for a test period of 250 days. The values o f p H ranged from 7.1 to 8.6, averaging 7.8 (6.4% CaCO3), 7.9 (17% CaCO3) and 8.0 (12 and 29% CaCO3) with a standard deviation of 0.4, Figure 2. The oxidation-reduction potential of the leachates produced from the pyrite - limestone mixtures ranged from 128 (29% CaCO3) to 183 mV (6.4% CaCO3). Referring to the conductivity, after the initial cycles, the values measured in the leachates of the amended columns averaged 2.4 _+0.5 mS/cm. The cumulative percentage of metals and sulphates dissolved in the limestone amended columns as compared with the control are given in Table 5. In agreement with previous findings, Mylona et al. (1996), the addition of limestone to the pyrite concentrate material significantly impaired the solubilisation of metals and sulphate. It is seen that the major portion of the total amount of contaminants reported in the drainage of the pyrite-limestone mixtures was dissolved during the initial cycles. At the next stages, in the neutral leachates generated from the limestone amended columns, Fe, Pb and As levels were below the detection limit, i.e. <0.l, 0.5 and 1 mg/l respectively. Referring to Zn, the cumulative percentage of metal reported in the leachates of amended columns was reduced by 44 to 72% as compared with the control; the respective figures for Mn were lower, i.e. 7 to 53%.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1169
TABLE 5 Cumulative percentage of metals and sulphate dissolved in the limestone amended columns as compared with the control Column
Period
Fe
] Pb
I Zn
] Cd
I Mn
IS(SO4)
(%) Control
6.4% CaCO3
12% CaCO3
17% CaCO3
29% CaCO 3
0-20 days (0.5 1/ks) 0-270 days (~5 I/ks) 0-20 days (0.5 l&g) 0-270 days (~5 l&g) % reduction 0-20 days (0.5 l&~) 0-270 days (~5 I/k~) % reduction 0--20 days (0.5 1/kg) 0-270 days (~5 l&g) % reduction 0-20 days (0.5 l/kg) 0-270 days (~5 1/kg) % reduction
4.8
0.1
87.8
46.1
90.7
2.9
5.5
0.3
93.0
49.3
94.6
4.1
0.9
<0.01
50.4
15.2
83.1
0'.6
0.9
<0.01
52.0
16.7
88.0
1.2
83.4 2.4
98.5 0.02
44.1 49.7
66.1 20.7
7.0 69.1
70.2 2.0
2.4
0.02
49.8
20.8
70.1
2.4
56.0 1.4
93.9 0.02
46.5 33.1
57.8 17.6
25.9 41.3
41.1 1.6
1.4
0.02
33.5
18.0
44.3
2.2
73.5 1.1
93.9 0.02
63.9 25.9
63.5 16.9
53.2 41.4
46.1 1.0
1.1
0.02
26.1
17.3
44.1
1.5
79.6
93.9
72.0
64.9
53.2
61.8
Sulphates along with Ca were the major ions in solution of the limestone amended columns. After the initial flush, sulphate and calcium dissolved concentrations amounted to 340-370 and 1400-1500 mg/1 respectively and were mainly controlled by gypsum solubility as predicted by MINTEQA2. A higher reduction in the cumulative amount of iron and sulphate dissolved was observed for the column containing the lower amount of limestone, equal to 69 kg CaCO3/t. This is associated with the reduced quantity of metals released during the initial washing of this column and may be attributed to the high homogeneity of the mixture as Well as channelling effects. On the contrary, when the performance of the remaining columns is compared, higher limestone addition resulted in higher reductions in the cumulative amount of Fe and SO4 dissolved. Solid residues
The data obtained on the drainage quality are not considered to be sufficient lor the assessment of the effect of limestone on the pyrite oxidation process. This is because the metals and sulphate release in the drainage was affected by a) the dissolution of previously formed oxidation products and b) the precipitation of secondary products of the neutralisation reactions which occurred in the pyrite-limestone columns. To evaluate the performance of the systems examined, it is necessary to identify and quantify the precipitates in the solid residues and for this reason a detailed geochemical characterisation of the solid residues was performed. Control column
Based on the results of the extraction tests performed for the feed and the residue as well as the drainage composition of the control column, the weight loss of pyrite during the kinetic test was 8.2 %. The amount
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E. Mylonaetal.
of metals and sulphate reported in the three chemical extracts of the solid residue as compared to that measured in the drainage, expressed as percentage of total content in the feed, is shown in Figure l(b). Based on the solid residue analysis after 270 days of kinetic testing, past pyrite oxidation products were washed out and no significant precipitation of such products occurred under the test conditions. The amount of Fe in the water extract represented less than 0.1% of total metal mass suggesting the absence of soluble ferrous sulphates in the residue; the respective figures in the acid and hydroxylamine hydrochloride extracts were also low, 0.9 and 1.5%. Considering that the acid leachable fraction of Fe is mainly derived from the dissolution of jarosite and any poorly crystalline ferric (oxy)hydroxides, McGregor et al. (1995), it is deduced that under the low pH conditions met in the control column no precipitation of Fe 3+ occurred. The acid leachable and reducible fractions of Zn accounted for 1 and 3% of its total mass respectively. A significant amount of Pb total mass, i.e. 6% was still associated with the water soluble fraction. A low fraction of sulphur, i.e. 0.8% occurred in the sulphate form. Sulphate ions reported in the water extract of the residue were mainly associated with the dissolution of gypsum and anglesite. Given that most of the oxidation products accumulate at the grain boundaries of sulphide minerals, oxidation would begin only after these products were sufficiently depleted to allow oxygen and water diffusion to the sulphide grains. Furthermore, it is suggested that dissolution of existing sulphate minerals, i.e. szomolnokite and gypsum and subsequent sulphate yields may have suppressed sulphide oxidation to sulphate due to the common ion effect. Limestone amended columns
The weight loss of the pyrite-limestone columns after 270 days of testing ranged from 2.5 to 3.5%. As opposed to the control, extraction tests performed on the solid residues of the amended columns resulted in the dissolution of a considerable amount of metals suggesting the presence of secondary neutralisation products. The distribution of metals and sulphates in the drainage and the three extracts of the solid residues for the column amended with 207 kg CaCO3/t is shown in Figure 1 (c). The results are expressed as a percentage of the total content in the feed. Calcium and 8 0 4 w e r e the major ions reported in the water extract of the solid residues, and they were associated with the dissolution of gypsum. The presence of gypsum was confirmed by optical microscopy combined with XRD. Based on the water extraction test results, the amount of gypsum in the residues of the limestone amended columns ranged from 6 to 6.9% wt. This quantity was higher as compared to that measured in the feed material, i.e. 2% wt., indicating that dissolution of carbonates (calcite and dolomite) followed by the formation and precipitation of gypsum was a principal process in the pyrite-limestone mixtures during the tests. Based on the pH of the drainage of the limestone amended columns, i.e. pH: 8.0, the carbonate dissolved would be mainly in the form of bicarbonate, Evangelou (1995), thus the neutralisation of acidity and subsequent formation of gypsum can be described by the following reactions: CaCO3 + H + + 8042- + 2H20 ~ CaSO4.2H20 + HCO3 "
(10)
CaMg(CO3)2 + 2H + + 2SO42- + 2H20 --* CaSO4.2H20 + MgSO4 + 2HCO3
(ll)
A microscopic view in transmitted light of gypsum in the residue of the mixture containing 17% wt. limestone (207 kg CaCO3/t) is shown in Figure 4.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
Fig.4
1171
Microscopic view in transmitted light of the solid residue in the column amended with 207 kg CaCO3/t, X 160 (Py: Pyrite, Cal: Calcite, Fe: Ferric hydroxides, Gp: Gypsum)
Based on mass balance calculations, the amount of limestone consumed during the tests (dissolved in the drainage and precipitated as gypsum), expressed in g CaCO3/kg pyrite, and the amount of additive remained in the column solid residues are given in Table 6.
TABLE 6 CaCO3 consumed in the amended columns during the test
Column
Limestone content Amount of limestone in the residue consumed during the test (%) (g CaCO3/kg pyrite) 6.4% CaCO3 3.4 33* 12% CaCO3 9.0 30 17% CaCO3 13.5 37 29% CaCO3 25.0 48 *Dissolution of dolomite accounts 8 g/kg material Consumption of limestone is represented by the dissolution of calcite except for the column containing the lower amount of limestone, i.e. 6.4% wt. where the total Mg mass in the solid residue was reported in the water extract, suggesting the complete consumption of dolomite, CaMg(CO3)2. Higher consumption rates were observed for the columns containing increased amount of limestone. This was associated with the reduced amount of metals and sulphates measured in the drainage of these columns (see Table 5). Based on the results of the acid extraction test, the acid leachable Fe in the residues of the limestoneamended columns ranged from 3 to 5% of total metal mass, and was solely in the form of Fe 3+. Macroscopic observations of the solids indicated the presence of XRD amorphous ferric hydroxides (Figure 4). Based on the findings of this study, it is suggested that Fe z+ released to the pore water of mixtures during the test was oxidised to Fe 3+ and precipitated as a ferric hydroxide. The type(s) of ferric hydroxides that may be formed as a result of sulphides oxidation-neutralisation reactions, depend on parameters including pH and concentration of anions such as SO4 and CO3 and metals. They include the hydrous amorphous ferric hydroxides, i.e. ferrihydrite (FesOs.4H20), Carlson and Schwertmann (1981), limonite (FezO3.H20), goethite (ccFeOOH), lepidocrosite (TFeOOH) as well as the stable ferric oxides, e.g. hematite (ctFe203) and maghemite (7Fe203). According to Bigham et aL (1996) ferric precipitates formed at pH: 6.5 or higher were composed of ferrihydrite or a mixture of ferrihydrite and goethite whereas schwertmannite [Fe808(OH)6(SO4)2] with trace amounts of goethite were precipitated from waters having pH: 2.8-4.5. Previous studies have shown that when Fe z+ solutions are oxidised in the
1172
E. Mylonaet al.
presence of HCO3 , condition met in the present experiments, formation of goethite instead of lepidocrocite is enhanced, Lin (1997). The reactions illustrating the formation of amorphous ferric hydroxide [nomimally Fe(OH)3] and goethite are given below: Fe 3÷ + 3H20 --+ Fe(OH)3 + 3H +
(12)
Fe 3+ + 2H?O ~ c~FeOOH + 3H +
(13)
Based on the microscopic observations, ferric hydroxides along with gypsum were found in the contact of pyrite-limestone particles (Figure 4). Furthermore, replacement of the pyrite and limestone grains by these secondary neutralisation products was observed. Precipitation of ferric hydroxides around the periphery of pyrite particles forming a protective rim was also an active mechanism in the pyrite-limestone mixtures, as shown in Figure 5.
Fig.5
Microscopic view in transmitted light of the solid residue in the column amended with 207 kg CaCO3/t, X 160 (Py: Pyrite, Fe: Ferric hydroxides).
Zinc and Mn reported in the acid extracts of the solid residues accounted for 20-40% and 15-30% of their total metals mass respectively. According to Blowes and Jambor (1990) the major removal mechanism for Zn in oxidised tailings is sorption and coprecipitation with ferric hydroxide and sulphate compounds. Regarding Pb, 50-60% of total mass was reported in the acid extracts. This fraction, apart from the dissolution of anglesite, may be also associated with the dissolution of galena. Similar figures were obtained for As. Based on previous studies, ferric hydroxides are considered to be efficient scavengers for this element, Sun and Doner (1996). The above results, indicate that thorough mixing of pyrite with ground limestone at an amount corresponding to a low fraction of the contained acidity results in the significant reduction of the amount of iron and sulphate dissolved in the drainage, mainly due to the precipitation of ferric hydroxides and gypsum. A 53% reduction in the pyrite oxidation rate was previously observed for the mixture containing limestone at an amount corresponding to 5% of the stoichiometric quantity, Mylona e t al. (1996). Estimation of this figure for the mixtures examined in the present study could not be made. This is attributed to the delay observed in the pyrite oxidation process mainly due to the increased amount of oxidation products contained in the pyrite material. Another factor to be considered includes the different configuration used (columns with a material thickness of 40 cm as compared to lysimeter, 30 cm wide with a material thickness of 4 cm used in the previous study). The results of this study suggest that increased limestone addition would improve the drainage quality and may also have a beneficial effect in the reduction of the hydraulic conductivity of material, as described in the following paragraphs.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
1173
Hydraulic conductivity Based on the results of the falling head permeability tests, the coefficient of hydraulic conductivity, k, of pyrite material was found k: 5x10 -5 cm/sec. Mixing of the pyrite concentrate material with limestone resulted in the increase of permeability by up to 25 times. After a monitoring period of 270 days, the coefficient of hydraulic conductivity for the mixtures containing 17 and 29% wt. limestone was reduced by one order of magnitude as compared with that measured in the beginning of the tests. A value of k: 2x 10-4cm/sec was obtained for the mixture containing 17% wt. limestone, being, however, higher than that found for the pyrite material. The reduction in the permeability of pyrite-limestone mixtures may be attributed to the filling of pores by the secondary neutralisation products, i.e. gypsum and ferric hydroxides. This process occurring in nature has resulted in the formation of hardpan layers at the surface of sulphidic tailings disposal areas. Hardpan layers inhibit the downward migration of rainfall waters and restrict the oxygen diffusion through the tailings, thus further oxidation of sulphides is prevented. Based on field observations, hardpans formed by sulphate minerals, e.g. gypsum, are more persistent laterally as compared to those consisting of ferric hydroxides which occur as discontinuous mineral segregations with a limited lateral extension, Blowes et al. (1991), Tase et al. (1997), Lin (1997). The formation of a hardpan was also observed in the laboratory, Chermak and Runnells (1997). Addition of limestone and/or lime to the surface layer of acid generating waste rock (NNP: - 5 0 kg CaCO3/t) resulted in the formation of a low permeability layer (k: 2.8 x l 0 7 cm/sec), consisting of gypsum and amorphous ferric (oxy) hydroxides. In this case, however, the drainage produced from the amended waste rock was acidic. Based on other studies, when an acidic ferric solution flows through calcareous sand, the reduction in the permeability (up to four orders of magnitude) results primarily from CO2 exsolution. The reduction in the permeability due to the precipitation of ferric hydroxides alone does not exceed two orders of magnitude, Fryar and Schwartz (1998). The reduction in the permeability of the pyrite-limestone mixtures examined in the present study may indicate a primitive stage of hardpan formation. The phenomenon was observed in the mixtures containing higher amount of limestone suggesting that the relative proportion of pyrite and limestone materials play a major role in the hardpan formation. Another factor to be considered includes the application of wet/dry cycles in the columns. A higher reduction in the permeability of the limestone amended columns may be achieved by decreasing the frequency of water flushing in order to inhibit the solubilisation of secondary phases and also promote the conversion of iron hydroxides into stable iron oxides.
CONCLUSIONS The effect of lower than the stoichiometrically required amounts of limestone on the pyrite oxidation process was studied. Long-term kinetic column tests were performed on a partially oxidised pyrite concentrate homogeneously mixed with various amounts of limestone corresponding to 5-30% of the stoichiometric quantity. Based on the drainage quality monitoring results, the analyses of the solid residues and the hydraulic conductivity measurements, the following conclusions can be drawn: The pyrite material used in the tests contained a significant amount of water soluble oxidation products. Column kinetic testing, involving the application of weekly wet/dry cycles, performed on the material over a period of 270 days, resulted mainly in the dissolution of these products and further oxidation of pyrite particles was delayed. Homogenous mixing of the pyrite concentrate with limestone, at amounts lower than the stoichiometric requirements, prevented the generation of acidic drainage and significantly reduced the amount of metals and sulphates dissolved in the drainage. Under the alkaline conditions prevailing in the limestone amended columns, i.e. pH: 8.0, Fe, Pb and As levels in the drainage were below detection limit, i.e. <0.1, 0.5 and 1.0 mg/1 respectively. The reduction effected in the dissolved amount of Zn, Cd and Mn was estimated up to 72, 66 and 53% respectively as compared to the control.
1174
E. Mylonaetal.
Neutralisation reactions in the limestone-amended columns resulted in the precipitation of secondary products, mainly gypsum and ferric hydroxides. The amount of these products in the pyrite-limestone mixtures was estimated 6-7 and 3-5% wt. respectively. Formation of a protective layer consisting of ferric precipitates around the pyrite particles was observed. Furthermore, precipitation of ferric hydroxides and gypsum in the contact of pyrite-limestone particles was an active mechanism in the systems examined. This resulted in the 10-fold reduction in the permeability for material amended with 207 kg CaCO3/t, corresponding to 15% of the stoichiometric quantity. The amount of limestone consumed during the test period of 270 days ranged from 30 to 48 g/kg pyrite. Higher consumption rates were observed for the mixtures containing higher amount of limestone. Assuming stable conditions, the experimental time required for the complete consumption of the alkaline additive in the amended columns was estimated to range from 8 months to 6 years. Due to the oxidation history of pyrite material used in the tests, the test duration, although long, i.e. 270 days, may not be considered sufficient to reliably assess the actual oxidation rate of pyrite and evaluate the inhibition effect of limestone addition. Additional data to be obtained from the continuation of the tests and sampling of the solid residues and/ or geochemical modelling, would allow the estimation of above parameters. However, based on the results of this study, it is seen that limestone may be beneficially used, at least in the short term, for the acid generation control from oxidised sulphidic wastes.
REFERENCES
Adam, K., Kourtis, A., Gazea, B. and Kontopoulos, A., Prediction of Acid Rock Drainage in Polymetallic Sulphide Mines. Transactions oflMM, 1997, Section A, January-April, A1-A8. Allison, J. D., Brown, D. S. and Novo-Gradac, K. J., MINTEQA2/PRODEFA2, a geochemical assessment model for environmental systems. Version 3.0 User's Manual. EPA/600/3-91/021, Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Athens, GA, 1990. Bigham J.M., Schwertmann U., Traina S.J., Winland R.L. and Wolf M., Schwertmannite and the chemical modelling of iron in acid sulfate waters. Geochimica et Cosmochimica Acta, 1996, 12, 2111-2121. Blowes, D.W. and Jambor, J.L., The pore water geochemistry and the mineralogy of the vadose zone of sulphide tailings, Waite Amulet, Quebec. Applied Geochemistry, 1990, g, 327-346. Blowes, D.W., Reardon E.J., Jambor, J.L., and Cherry, A., The formation and potential importance of cemented layers in inactive sulphide mine railings. Geochimica et Cosmochimica Acta, 1991, 55, 965978. Carlson, L. and Schwertmann, U., Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochimica et Cosmochimica Acta, 1981, 45, 421~429. Chatzioannou, Th. P., Qualitative analysis and chemical equilibrium, 8th edn. 1984, Grafikes Tehnes, Athens. Chermak, J. A. and Runnells, D.D., Development of chemical caps in acid rock drainage environments. Mining Engineering, 1997, 49(6), 93-97. Day, S. J., Evaluation of Acid Generating Rock and Acid Consuming Rock Mixing to Prevent Acid Rock Drainage. In Proc. International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage. U.S Department of the Interior, Bureau of Mines Special Publication SP 06B-94, Pittsburgh, 1994, Vol. 2, pp. 77-86. Evangelou, V. P., Pyrite oxidation and its' control, 1995, CRC Press. Farah, A., Hmidi, N., Moskalyk, R., Amaratunga, L. M. and Tombalakian, A. S., Numerical modeling of the effectiveness of sealants in retarding acid mine drainage from mine waste rock. Canadian Metallurgical Quarterly, 1997, 36(4), 241-250. Fetter, C. W., Applied Hydrogeology, 3rd edn. 1994, Prentice Hall, Englewood Cliffs. Fryar, A. E. and Schwartz, F. W., Hydraulic conductivity reduction, reaction-front propagation, and preferential flow within a model reactive barrier. Journal of Contaminant Hydrology, 1998, 32, 333351. Heltz, G.R., Dai, J.H., Kijak, P.J., Fendinger, N.J. and Radway, J.C., Processes controlling the composition of acid sulphate solutions evolved from coal. Applied Geochemistry, 1987, 2, 427-436.
Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone
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