Waste rock drainage prediction methods compared to actual seepage water quality
Teemu Karlsson1, Päivi M. Kauppila1 & Marja Lehtonen2, 1Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, Finland, 2Geological Survey of Finland, P.O. BOX 96, FI-02151 Espoo, Finland, e-mail: teemu.karlsson(at)gtk.fi
The wastes and waste facilities are the most prominent sources of pollution after mine closure. To predict the long term behavior of disposed material and to minimize the amount of waste itself trough efficient utilization, a proper characterization of the waste material is crucial. Also the methods available to successfully close a waste facility and to remediate the impacted areas are dependent on the geochemical processes derived from the properties of disposed material.
For ARD prediction, acid base accounting (ABA) tests are the most commonly used static test methods. They are designed to measure waste materials capacity to produce and neutralize acidity, but not to provide information on reaction rates of mineral weathering. These simple tests characterize if the waste is either safe for disposal (NP exceeds AP) or potentially acid generating (AP exceeds NP). (Sobek et al. 1978, White et al. 1999, Jambor 2003)
The net acid generation (NAG) test is used as a standalone prediction tool (Miller et al. 1997) and as a supplement to another static test e.g. the ABA tests (Jambor 2003). During the NAG test acid generation and acid neutralization occur simultaneously, the results presenting a direct measurement of the net amount of acid generated by the sample.
The leachability of potentially harmful elements varies for different waste types. Therefore, leaching tests are needed to evaluate the mobility of these elements from the mine waste. One of the most common selective extractions used for mine waste characterisation is the hot Aqua Regia leach (Niskavaara 1995), which is widely used e.g. in Finland (Luodes et al. 2011). The element concentrations measured from the acid leach solution reflects the element concentrations that are bound to mica and clay minerals, salt minerals and sulphide minerals (Doležal et al. 1968, Räisänen et al. 1992).
Also the extract contents produced by the NAG test can be used in assessing contaminant mobility during long-term acid generating reactions (Räisänen et al. 2010). During the basic NAG procedure a portion of NAG extract is separated before titration, and element concentrations are determined from the extract. Furthermore, the leachability of elements can be assessed using two-stage batch leaching test/shake-flask test SFS-EN 12457-3, which complies with the waste disposal related Decrees 202/2006 and 403/2009 of the Finnish Government (Finnish Government Decree 202/2006, 403/2009).
The objective of this study was to review ore deposit geology, waste rock geochemistry and mineralogy and their relation to actual seepage water geochemistry. The suitability of a modified ABA test and corresponding NAG test for predicting ARD generated by waste rocks was evaluated. As the largest discrepancies in static tests and their interpretation are related in the assessment of the neutralization potential (Lawrence & Scheske 1997, Jambor 2003), in this study the NP was determined with two methods: by static NP test presented in prEN15875, and based on total carbonate NP. As the NAG test does not estimate neutralisation potential, e.g. the AMIRA guidebook (AMIRA 2002) recommends to use the net acid production potential (NAPP) together with NAG for more detailed classification of acid generation, which was also applied for this study. Furthermore, the dissolution of metals and metalloids during the NAG test, Aqua Regia extraction and shake-flask test were evaluated to assess mobility of contaminants during long term waste rock storage.
Study areas and analytical methods
Seepage water and waste rock samples were collected from seven operating or closed mine sites, in total from nine target sites of varying commodities and ages. The target sites are presented in Table 1. The deposit types were obtained from the Fennoscandian Ore Deposit Database (FODD 2015).
Table 1. Data on the study sites
Seepage water quality
Seepage waters were collected from the points in the edges of the waste rock piles where the water was running out from the pile. Samples were collected during late spring to early summer (14th May to 1st July). One to two seepage water samples were collected from each mine site. Field measurements were made in the field using portable multi-parameter meter for pH, EC, O2 (mg/l), O2 (%) and redox potential. Alkalinity was titrated with a HACH digital titrator with 1.6N H2SO4 to an end point of pH 4.5.
Unfiltered samples were collected for measurements of anions, pH, EC, alkalinity and suspended solids. Filtered (0.45 µm), HNO3-acidified samples were collected for dissolved cations and trace metal analyses, and unfiltered, HNO3-acidified samples for the measurements of total element concentrations of cations and trace metals. Filtered (0.45 µm), HCl-acidified samples were taken for Fe2+ analysis, and filtered (0.45 µm) and unfiltered H2PO4-acidified samples to measure dissolved organic carbon and total carbon, respectively.
Major anions were determined with ion chromatography, pH and EC potentiometrically (pH SFS 3021:1979, EC SFS-En 27888:94), alkalinity using titrimetric method (SFS-3005:1980), and suspended solids by gravimetric analysis (SFS-EN 872:2005). Major cations and trace metals were measured with ICP-OES/MS, Fe2+ using spectrophotometry, and total and dissolved organic carbon pyrolytically following standard SFS-EN 1484:97. Duplicate samples and field blanks were used for quality control. All the laboratory analyses were carried out at the FINAS-accredited testing laboratory of Labtium and in its’ subcontractors.
Characterization of the waste rocks
Waste rock samples were collected as 10-15 kg composite samples of fist sized subsamples taken from the waste rock pile surface above the seepage points.
Mineralogical characterization of waste rock samples was carried out measuring mineralogical composition using Jeol JSM-7100F SEM equipped with EDS. Prior to analysis the samples were dried (< 40°C), crushed and ground. Total concentrations of elements were measured with XRF, and total carbon, carbonate carbon, and total sulphur and sulphidic sulphur using pyrolytic methods. Aqua regia was used to dissolve the samples according to the modified ISO-11466 standard (see Selective extractions). Acid production potential of the tailings was studied with ABA (CEN-EN 15875) and NAG tests and leachability of elements using shake-flask test/batch leaching test (SFS-EN 12457-3). Leaching of elements was further studied by analyzing major cation, trace metal and anion concentrations from the leachates of the NAG test.
The neutralization potential (NP) was calculated also based on carbonate content by multiplying the carbonate carbon concentration by 83.34 to yield the NP in units of kg CaCO3/t (“Carbonate NP”).
Results and discussion
Mineralogical composition of the waste rocks
The mineralogical composition of the samples varied from low to high sulphide and carbonate contents. The majority (6/9) of the rock samples had a high (1% or higher) sulphide concentration. Three samples contained sulphides 0.16 % or below. The majority (7/9) of the samples contained also high amounts (2% and higher) of carbonates, while two samples contained only few individual grains. The mineralogical compositions including main minerals (total amount > 5%), sulphides and carbonates of the waste rock samples are presented in Table 2.
Table 2. Mineralogical composition of the waste rock samples.
|Target site||Commodity||Main minerals||Sulphides||Carbonates|
|Mine 1||Au||Albite, K-feldspar, Mg-biotite, quartz||Very few single grains of sulphides (tot 0.07 %)||Dolomite (4%), siderite, calcite|
|Mine 2||Cu, Co, Zn, Ni||Quartz, Mg-biotite, plagioclase||Pyrrhotite (1.7%), pyrite (0.4%), tot 2.2%||Calcite ( 4.5%), dolomite|
|Mine 3 (old)||Talc||Quartz, chlorite, magnesite||Pyrite (1%), pyrrhotite (1%), tot 2.0%||Magnesite (8%), dolomite (2.6%), calcite (0.4%)|
|Mine 3 (fresh)||Talc||Quartz, Mg-biotite, magnesite||Pyrrhotite (2.9%), pyrite (0.3%), tot 3.3%||Magnesite (9%), dolomite (3%), calcite|
|Mine 4||Cu-Co-Ni-Zn-Au||Plagioclase, quartz, Mg-biotite||Pyrite (4.0%), pyrrhotite (1.6%), tot 5.9%||Calcite 2%, dolomite|
|Mine 5||Ni, Co||Quartz, Mg-biotite, chrysotile||Pyrrhotite (5.2%), pyrite (0.6%), tot 6.0%||Few grains of calcite and dolomite|
|Mine 6 (old)||Apatite||Mg-biotite, calcite, Fe-hornblende, dolomite||Very few single grains of sulphides (tot 0.14%)||Calcite (21%), dolomite (8%)|
|Mine 6 (fresh)||Apatite||Albite, Mg-biotite, aegerine-augite||Very few single grains of sulphides (tot 0.16%)||Calcite (4.5%)|
|Mine 7||Cu, Zn||Quartz, chlorite, albite||Pyrrhotite (0.5%), chalcopyrite (0.3%), pyrite (0.2%), tot 1%||Few grains of calcite|
Waste rock chemistry versus seepage chemistry
Acid Production Potential
The acid production potentials of the rock samples measured with several methods are presented in Table 3, as well as the measured pH values in the actual seepage waters. According to the ABA test, six samples were likely to produce acid drainage (NP/AP < 1) and three samples were non-acid generating. The NP/AP ratio based on the carbonate NP yielded different results than ABA test for the samples obtained from the Mine 3. The acid production potential of the sample Mine 3 (old) was “Likely” in ABA test but “Low” based on carbonate NP (NP/AP between 2 and 4), and the acid production potential of the sample Mine 3 (fresh) was respectively “Likely” and “Possibly” acid producing (NP/AP between 1 and 2).
Table 3. Acid production potential predicted by several methods.
|Target Site||NAG pH||S (tot) %||NAG||ABA||Carb-NP||Seepage pH|
|Mine 3 (old)||7.46||1.119||NAF||Likely||Low||7.3|
|Mine 3 (fresh)||3.79||1.837||PAF||Likely||Possibly||7.7|
|Mine 6 (old)||10.8||0.074||NAF||None||None||6.5|
|Mine 6 (fresh)||10.5||0.172||NAF||None||None||7.0|
According to the NAG test, only four samples were potentially acid generating (NAG pH < 4.5), and five non-acid forming. Based on the net acid production potential (NAPP, expressed as kg H2SO4 / t) together with the NAG values (Figure 10), the sample of the Mine 4 was placed in the “uncertain” category.
Figure 10. Distributions of the studied rock samples in the NAG pH / NAPP (net acid production potential) diagram. NAF = “non-acid forming”, PAF = “potentially acid forming”, UC = “uncertain”.
The results of the ABA test, carbonate NP and NAG test revealed differences in assessing the acid production potential of the rock samples. Furthermore, contradictions between the test results and the actual measured seepage water pH values were observed; principally the laboratory tests gave too pessimistic results compared to the real situation at the mine sites. Therefore using several different methods, as well as mineralogical data is recommendable in assessing the acid production potential of waste rocks.
Mobility of the potentially harmful elements
When the seepage water samples were plotted on a Ficklin diagram (Plumlee et al. 1999), the samples were classified as low to high-metal waters, most being near neutral (pH 6.3-7.7), except the sample Mine 7, which had a pH value of 3.9 (Fig. 11).
Figure 11. Ficklin diagram (Plumlee et al. 1999) of the waste rock seepage waters showing the sum of dissolved heavy metals (Zn, Cu, Cd, Pb, Co and Ni) in µg/l plotted against pH value.
Figure 12 presents the sums of dissolved heavy metals in extracts produced in the NAG test, Aqua Regia leachate, batch leaching / shake-flask test and in the waste rock seepage waters. The concentrations in the test diagrams should not be considered as exact estimations of seepage water quality, but mere as approximates of potentially elevated metal concentrations.
Figure 12. Sums of dissolved heavy metals (Zn, Cu, Cd, Pb, Co and Ni) in extracts produced in the NAG test (mg/kg), Aqua Regia leachate (mg/kg), batch leaching / shake-flask test (mg/kg) and in the waste rock seepage waters (µg/l).
The Aqua Regia extraction had the best correspondence to the elevated metal concentrations of the actual seepage waters, although it overestimated the load of some inpidual elements, e.g. Cr and Cu. The NAG test leachate underestimated the metal load in some cases (Mine 3 and 4), but was more suitable for Cr and Cu estimations than the AR test.
The batch leaching test was observed to be the most unsuitable for seepage water quality prediction, as the results were mainly under the detection limit of the analytical method. The solvent used in the batch leaching test (water) is probably too weak and the reaction time too short for crystalline waste rock materials. As can be seen, elevated concentrations in any of the tests used in this study indicate a possibility for element load.
Interpretation of the results is complicated by the representativeness and weathering grade of the samples. To properly assess the average waste rock characteristics in a large waste rock pile, more samples would be needed than used in this study. Seepage water qualities should be monitored for a longer time period.
There were differences in the results of the methods predicting acid production potential, and the laboratory tests were principally too pessimistic compared to the real situation at the mine sites. Using several different methods, as well as mineralogical data, is recommendable.
The Aqua Regia extraction had the best correspondence to the actual seepage water quality. The Aqua Regia extraction may overestimate the load of some individual elements, e.g. Cu and Cr. NAG test leachate underestimated the metal load in some cases (Mine 3 and 4).
The batch leaching test was observed to be the most unsuitable for seepage water quality prediction. The solvent used in the method (water) is too weak and the reaction time too short for crystalline waste rock material.
The results obtained from the Aqua Regia extraction and NAG test leachate can be used to predict the elements that will appear as elevated concentrations in the seepage waters. The tests are approximates, not exacts.
Elevated concentrations in any of the tests used in this study indicate a possibility for element load.
Interpretation of the results is complicated by the representativeness and weathering grade of the samples. Mineralogical data should be used side by side with the analytical results.
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