Long-term acid generation containing heavy metals from the tailings of a closed mine and its countermeasures

Transcription

1 Long-term acid generation containing heavy metals from the tailings of a closed mine and its countermeasures Kimleang KHOEURN Candidate for the Degree of Doctorate in Engineering Supervisor : Professor Toshifumi IGARASHI Division of Sustainable Resources Engineering Introduction Acid mine drainage (AMD) from abandoned mines is a well known problem all over the world. The AMD containing heavy metals is mainly produced by the oxidation of pyrite and metal sulfides, which could affect the surrounding environment as well as human health when they are ingested through food, crop, and contaminated water. Numerous researches have been conducted to solve these problem by neutralizing the AMD (e.g., calcium hydroxide (Ca(OH) 2 ), sodium hydroxide (NaOH), etc.). However, it remains unclear when the treatment would stop. To answer this question, an in depth study of the mechanisms controlling the long-term leaching of heavy metals (i.e., Zn, Cu, and Fe) from the tailings was conducted. A clearly understanding on a long-term acid generation, chemical form and leaching behaviors of heavy metals from the tailings could provide information on improvement of tailings management systems. Specifically, this study aims to achieve the following: - To characterize the tailings dam (i.e., batch experiments) - To evaluate long-term acid generation and heavy metals leaching from the unweathered tailings (e.g., column experiments), and - To evaluate the effectiveness of eggshell for removal of metals and neutralization of AMD Distribution of zinc, copper, iron in the tailings dam of an abandoned mine in Shimokawa, Hokkaido, Japan Mine wastes can generally be classified into two major categories, waste rock and tailings (Younger et al., 22). Tailings are produced when usable ore are separated from unusable materials, before smelting (Zhang et al., 2). Many closed mines are located in Hokkaido, Japan, most of which have been continuously generating AMD after their closure (Ito et al., 2). To understand mechanisms of heavy metal release from the tailings when contacted to the environment, Chapter 2 focuses on the characterization of the tailings, metals speciation in response to mobilization of metals, and the AMD prediction. Material and methods Sampling, chemical and mineralogical analyses The sample were collected from a tailings dam as shown in Figure, from boreholes B2, B3, B, and B. Geological formation of the samples are represented by different colors as shown in the box. The depths of boreholes are m for the boreholes B2 and B3, and m for B and B. The chemical composition and mineral constituents of the samples were determined using X-ray fluorescence spectrometry (XRF; XEPOS, Rigaku Corp., Japan) and X-ray diffraction spectrometry (XRD; MULTIFLEX, Rigaku Corp., Japan), respectively. Figure Cross-sectional view of the tailing dam Batch leaching experiments Samples less than 2 mm in diameter were provided for the experiments. Fifteen grams of samples were mixed with ml of deionized water (8 cm) in a 2 ml flask and the suspensions were mixed using a lateral-reciprocating shaker at a speed of 2 rpm for h at room temperature. After shaking,, electrical conductivity (EC), temperature, and oxidation-reduction potential (Eh) of the suspensions were measured, followed by filtration of the leachates through. μm Millex filters (Merck Millipore, USA). The leachates were analyzed using an inductively-coupled plasma atomic emission spectrometer (ICP-AES; ICPE-9, Shimadzu Corp., Japan). Selective sequential chemical extractions are often used to determine the distribution of Zn, Cu, and Fe with different sorptive phases and their mobilization in soils and mine wastes (Dold and Fontboté, 2). This method operationally partitions Zn, Cu, and Fe into six fractions: () water-soluble, (2) exchangeable, (3) carbonate, () poorly crystalline, () crystalline, and () sulfide/organic matter fraction. One gram of the tailings was mixed with an extractant (Table 2.2 in Chapter 2 of dissertation) in a ml centrifuge tube. The solid-liquid separation was achieved by centrifugation at, rpm for min. The supernatant was then separated by a pipette and placed in a clean ml volumetric flask. The residue was washed with 8 ml of deionized water before the next step was carried out. The mixture of the supernatant and washed water was

2 Depth (m) Depth (m) Depth (m) provided for analysis of Zn, Cu, and Fe using ICP-AES. For the residue, the extractant was changed to proceed to the next fraction. Results and discussion Mineralogical and chemical composition of the samples Among all samples, the tailings, including oxidized tailings, contained greater levels of Zn (37 9, mg/kg), Cu (27,9 mg/kg), and Pb (8 8 mg/kg) than the lapilli tuff samples. In the oxidized tailings, the contents of these elements (37 32 mg/kg for Zn, 27 mg/kg for Cu, and 2 3 mg/kg for Pb) were generally less than those of the deeper tailings (3,8 9, mg/kg for Zn,,3,9 mg/kg for Cu, and mg/kg for Pb). Sulfur (S) content was also less in the oxidized tailings (.3. wt%) than in the deeper tailings (2.. wt%). These results indicate that Zn, Cu, Fe, and S had leached out of the weathered tailings in the top. m of the tailings during the past years. The XRD results showed that the major minerals in the tailings were quartz and albite while pyrite and sphalerite were detected as trace components. Batch leaching results Figure 2 shows the leaching concentration of heavy metals and SO 2- of the samples at different depths of borehole B3. Lower values and higher concentration of heavy metals and SO 2- were observed in the deeper tailings compared to other samples, such as weathered tailings and lapilli tuff. The results show that the tailings are the main sources of heavy metals leaching Cu Fe Zn SO Concentration (mg/l) 2- Figure 2 Concentration of heavy metals and SO at different depths of borehole B3 results Figure 3 shows the results of sequential extraction of the tailings samples of borehole B3. The water-soluble fractions of Zn and Cu were greater in the middle part of the tailings, and lower at the surface and bottom of the tailings These results reflect that the water-soluble fractions of these elements were almost flushed out from the oxidized tailings and the bottom of the tailings. The amounts of Zn, Cu, and Fe in the water-soluble fraction were likely related to the degree of oxidation. Notably, 2 higher in poorly crystalline and crystalline fractions of Cu, Zn and Fe were observed in the weathered tailings, indicating that precipitation of secondary minerals of Fe(III) oxyhydroxides likely occurred after years of being disposed to the environment. On the other hand, the deeper tailings have not yet been oxidized because of the lower concentrations of electron donor. The contaminants in the middle parts of the tailings could be mobilized when they contact air and water. a) b) c) Cu % 2 8 Fe% 2 8 Water-soluble Carbonate Crystalline Zn % Exchangeable Poorly crystalline Sulfide and Organic Figure 3 Results of sequential extraction of the tailings samples from borehole B3 Table Principle component analyses Parameters PC PC2 PC3 Al Ca Cd Cu Fe.3..8 K Na Zn Si...28 Cl SO EC Eh Eigenvalues Percentage of variance (%) 9 Cumulative (%) 79

3 Eh (mv) Principle component analysis Table shows the PCA results of the leaching experiments. The first three components accounted for 79% of the total variation. The loadings of the first component were larger for Zn, Cu, Fe, SO 2-,, EC, and Eh, which accounted for % of the total variance. This simply reflects the fact that the sulfide minerals (e.g. pyrite, sphalerite, and chalcopyrite) produce Zn, Cu, Fe, and SO 2-. These minerals also contribute to the contamination from the tailings. The loadings of the second component, which was 9%, were attributed to K, Na, and Si, reflecting the fact that minerals like feldspar influenced K, Na, and Si leaching concentrations. However, the third component accounted for % of total variance, which was dominated by Ca. This likely reflects the addition of a neutralizer during tailings deposition, as pointed out by the sequential extraction results. Long-term acid generation and heavy metal leaching from the tailings: Colum study under natural conditions Actually, studying about tailings are numerous especially in the context of characterization and short-term behavior of heavy metals. However, long-term leaching behavior of heavy metals and release mechanisms from the tailings when contacted with the environment are still lacking. In Chapter 3, column study was conducted to investigate the mineralogy and geochemistry of the tailings before and after experiments and to evaluate long-term leaching behavior of heavy metals under different irrigated rainfalls. Material and methods Three columns were used in the experiments. The columns were packed with 38 g of the tailings to a thickness of cm for a bulk density of.37 g/cm 3 and porosity equal to %. Distilled water (, 2 and cm3) was introduced into the columns (cases, 2, and 3, respectively) once a week and allowed to flow down by gravity. Addition of distilled water started simultaneously for all columns until 8 weeks. After 2 days, the effluents were collected then, EC, ORP and temperature were immediately measured. After 8 weeks of the experiments, the tailings sample were divided into segments, air dried, conducted batch leaching and sequential extraction. Results and discussion Chemical composition of the tailings before and after experiments Table 2 shows the chemical composition of the tailings before and after experiments. Prior to the experiment, the original tailings contained 3.3wt% of SiO 2 and 9.8wt% of Al 2 O 3. The Si and Al indicate the presence of silicates and alumino-silicates in the gangue minerals. The tailings also contained.3wt% of CaO,.wt% of MnO, and 3 3.8wt% of MgO. It was highly enriched in Cu of 3,73 mg/kg and Zn of 7, mg/kg, respectively. The high contents of S and Fe 2 O 3 were also found in the tailings of.2wt% and 2.9wt%, respectively. After finishing column experiments, the contents of Cu, Zn, and S in the tailings were depleted, but Fe 2 O 3 were similar for all cases. Overall, the total contents of Cu, Zn, and S in case was higher than those of cases 2 and 3 (case > case 2 > case 3) (Table 2). These differences may be due to the influence of the irrigation rate. Table 2 Chemical compositions of the tailings before and after experiments Elements Before After experiments experiments Case Case 2 Case 3 SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O.3 <..2. K 2 O....7 P 2 O S Cu 3,73 3,2 3,27 2,79 Zn 7,8 7,9 7,39,9 Unit of all elements are in wt. %, except Cu, and Zn in mg/kg. Results of column experiments Figure shows the and Eh values changes with time. The values were lower at the beginning of the experiments for all cases then increased and almost stable with time. Notably, the value of case 3, highest irrigated rainfall, had highest values followed by cases 2 and. However, the of the effluents were acidic ( ) throughout the experiments for all cases. The results revealed that the different rainfall did not have a significant effect on acidic condition of effluents. The Eh values were in oxic condition ranging -8 mv throughout the experiments. This condition is favorable to the oxidation of residual pyrite and other sulfide minerals Case Case 2 Case Time (weeks) Figure and Eh values changes with time

4 Cumulative release of Fe (%) Cumulative release of Zn (%) Cumulative release of Cu (%) Fe (mg/l) Zn (mg/l) Cu (mg/l).. (b) Case Case 2 Case 3 release of Cu, Zn, and Fe from columns and fractions of Cu, Zn, and Fe by sequential extraction. The highest release of heavy metals at the beginning of the column experiments likely corresponded to ion exchangeable fraction. The continuous release of Cu and Zn as time elapse are considered to be from the sulfide fraction of the sequential extraction, whereas that of Fe is likely from Fe-Mn oxides fraction and sulfide fraction. Focusing on cumulative releases of heavy metals with time (Fig. 7), highest releases of heavy metals were observed in Zn followed by Cu and Fe (Zn > Cu > Fe), which means that Zn was more mobilized than Cu and Fe in the tailings. -3 (c) Before experiment 2 3 Content of Cu ( 3 mg/kg) 2 Content of Fe ( mg/kg) Time (Weeks) Figure Concentrations of heavy metals changes with time Figure shows the changes in concentration of heavy metals and sulfate with time. The leaching concentration of heavy metals and sulfate were similar for all cases. The highest leaching concentrations were observed at the beginning of the experiments, whereas the slowly decreased in concentrations were observed as time elapse. The results suggested that two-step leaching occurred during the experiments: () easy dissolution of oxidation product of heavy metals and (2) slower oxidation of sulfide minerals. Results of sequential extraction before and after experiments Figure shows the results of sequential extraction of Cu, Zn, and Fe before and after experiments. Prior to the experiment, Cu and Zn fractions were dominated by ion exchangeable of 7 and,27 mg/kg, sulfide fractions of and,3 mg/kg, and the residual fraction of and 2 mg/kg, respectively. The large amount of Fe was mainly associated with the residual fraction of 3,8 mg/kg. Additionally, the Fe fraction associated with ion exchangeable, oxide, and sulfide fractions were,, 9, and,7 mg/kg, respectively. The sulfide fractions of Cu and Fe in the post-experiment tailings were almost the same as those in the pre-experiment tailings. However, the sulfide fraction of Zn in the post-experiment tailings was less than that in the pre-experiment tailings. Overall, most of Cu, Zn, and Fe fractions in the original tailings were dominated by ion exchangeable, sulfide, and residual fractions while those in the post-experiment tailings were dominated by sulfide and residual fractions. Comparison between cumulative leaching of heavy metals and sequential extraction Figure 7 shows the comparison between the cumulative Before experiment 8 2 Content of Zn ( 3 mg/kg) Figure Solid-phase partitioning of heavy metals before and after experiments Residue Sulfide Fe-Mn oxide Carbonate Exchangeable Case Case 2 Case 3 Ion exchangeable Fe-Mn oxide Residue (b) Carbonate Sulfide/Organic Time (weeks) Figure 7 Cumulative release of heavy metals from the tailings of column experiments with time and sequential extraction

5 Ca + Mg + Cu + Fe + Zn (mole)... Weeks 3-8 Weeks -3 Case Case 2 Case 3 Theoretical line of oxidation Theoretical line of soluble salts E- E-... (mole) Figure 8 Relationship between selected cations and sulfate Dissolution and formation of secondary minerals inside columns Figure 8 shows the relationship between a sum of molar concentrations of Ca, Mg, Cu, Mg, Zn, and Fe in the effluents and that of SO 2-. A green straight line in this figure shows the theoretical relationship by assuming stoichiometric dissolution of soluble salts, which might occur during sampling, transportation or air-dried under room temperature. A red straight line shows the theoretical relationship by assuming stoichiometric dissolution of gypsum, pyrite, sphalerite, and chalcopyrite. The first weeks of the experiments, the plots were likely on the theoretical line of soluble salts. From week to 3 of the experiments, the plots were likely on the theoretical line of dissolution of gypsum, and oxidation of chalcopyrite, sphalerite, and pyrite, which means that slower oxidation process controlled the leaching concentrations of Zn, Cu, Fe, and SO 2- in the effluents. As time elapsed, the sum of the actual molar concentrations was lower than the theoretical line. Thus, at least two phenomena could explain this difference. The first is the precipitation of secondary minerals like iron-oxyhydroxide/ferrihydrite. The second is co-precipitation or adsorption of Cu and Zn onto the surface of these secondary minerals. Neutralization of the acid mine drainage from the tailings by using chicken eggshell Although the AMD from Shimokawa tailings has been treated with Ca(OH) 2, the low-cost absorbent for such AMD should be required to searched. Consequently, to ameliorate the AMD problem, innovative, efficient and low-cost treatment methods should be studied. The eggshell was selected in this study for three reasons. The first reason is its availability in any country, reduction of chicken eggshell wastes and its highly contents in CaCO 3. In Chapter, the aim is to evaluate the eggshell on remediation of the actual AMD from Shimokawa mine tailings. Materials and methods Weeks - SO 2- C for days, ground by motar to get the desired particle size (7- m), stored in airtight containers for further analyses. The AMD were obtained by the batch leaching tests (detailed in Chapter ). The characteristic of the AMD is shown in Table 3. In order to investigate the effect of the particle size of ES on and removal efficiency of heavy metals, g of ES at different particle size (7- m) and 2 ml of the AMD were mixed in a polypropylene conical tube and then shaken horizontally on a mechanical shaker at a speed of 2 rpm at room temperature for 2 h. The effect of ES mass on the removal efficiency of heavy metals was evaluated by the same procedure at the mass ranging from.2 to 2 g. The effect of contact time ( min - 3 h) on the removal of heavy metals was explored by mixing 2 ml of AMD and g of ES of m particle size. The removal efficiency, R (%) was calculated based on the difference between the initial and final concentrations as follows: Ci C f R % () Ci Table 3 Characteristic of the AMD from the tailings Parameters Value Parameters Value Al 8. Zn Ca 32 Fe 28 Mg 7 Fe Mn 9. SO 3, Cd.7 3. Cu 2. OPR 39 All elements are in mg/l except for and ORP (mv) Results and discussion Chemical composition of ES Table shows the chemical composition of ES before and after experiments. The XRF results revealed that the raw ES consisted mainly of CaO (7wt%), which might be CaCO3 in reality. It decreased after batch adsorption (wt.%), which means dissolution of CaCO 3 in AMD was occurred. The increase in weight of Fe 2 O 3 and contents of S, Cu, Cd, and Zn after batch adsorption experiments indicated that these elements were precipitated or sorbed onto ES. The similar results were reported by numerous authors (i.e., Zheng et al., 27; Ahmad et al., 22; Flores-Cano et al., 23; Elabbas et al., 2; Muliwa et al., 28). Functional groups revealed that there is no different peak between ES before and after experiments, which might be due to lower contents of heavy metals than the detection limits of the machines or their precipitates as amorphous form. The effect of ES particle size, ES mass and contact time on the removal efficiency Figures 9 -(c) show the effect of ES particle size, ES mass and contact time on the removal efficiency of heavy metals. The highest removal of Fe (99%), Cu (98%), The ES was collected from the kitchen wastes, dried under

6 Removal efficiency (%) Fe in solution (mg/l) Table Chemical composition of the ES before and after experiments (XRF) Composition of ES Before After MgO (wt%).33. Al 2 O 3 (wt%).27. CaO (wt%) 7.. MnO (wt%).2 9. Fe 2 O 3 (wt%).. S (wt%).7.2 Cu (mg/kg) 3.,9 Cd (mg/kg) <.7 Zn (mg/kg) <.,32 Cd (%), Zn (%), and Mn (2%) were observed in the smallest ES particle size (Fig. 9 ). Their removal efficiency decreased when the particle size of ES was increased. Smaller ES particle size exhibited large specific surface leading to increased dissolution of CaCO 3 in the ES. The removal efficiency of heavy metals increased when ES mass was increased. This means that increase in ES mass resulted in increased sites for precipitation or adsorption, which consequently led to an improved overall performance (Fig. 9 (b)). The results in Fig. 9 (c) show that Fe and Cu were removed rapidly for just 3 and min of contact time, respectively, whereas the removal of Zn, Cd, and Mn were slow and reached the highest removal at the contact time of 2 h. The results revealed that the Fe and Cu were likely co-precipitated, providing surface for Zn, Cd, and Mn to adsorb Paticle size ( m) Mass of ES (g) 8 2 (b) (c) Contact time (min) Figure 9 effect of ES particle size, (b) effect of ES mass, (c) effect of contact time on the removal efficiency Removal mechanisms Cd Cu Fe Mn Zn Figure shows the behavior of Fe and changes during ES react with the AMD. The rapid increase in and decreased in Fe were observed. This means that the concentration of Fe 2+ decreased rapidly and became almost undetectable in min, with color of precipitate changed rapidly from white to yellow. The precipitation of Fe likely act as a scavenger of Cu and Zn by co-precipitation or adsorption. Following are the reaction likely occurred during the experiments: CaCO 3 + H 2 O OH - + HCO Ca 2+ () Fe 2+ + / O 2 + H + Fe 3+ + /2H 2 O (2) Fe 3+ + OH - Fe(OH) 3 (3) Time (min) Figure Removal behavior of iron from AMD Conclusion Fe The tailings dam of Shimokawa mine was acid generation for years ago and weathered tailings was observed at depth of. m. The weathered tailings in the columns was observed at the depth of.2 cm after 8 weeks of experiments, which almost well agreed with the field observation. The columns experiments could mimic the weathering of tailings at the field. The Zn, Cu, and Fe in the tailings were mainly bound to the sulfide and water-soluble fractions. However, weathering transformed these elements from sulfide to exchangeable/water-soluble and poorly crystalline and crystalline forms in which some of Zn and Cu may be adsorbed onto or incorporated into the Fe(III) oxyhydroxide minerals. Zinc is more mobilized than Cu and Fe and significant amounts of Cu, Zn, Fe, and S existed in the columns at the end of the experiments. This suggests that residual minerals continue to produce acidic water and hazardous metals. The ES was able to treat AMD by increasing the in which metals precipiated. Since the ES is abundant and low-cost, further studies on this material should be required. References Dold, B., Fontboté, L., 2. Element cycling and secondary mineralogy in porphyry copper tailings as function of climate, primary mineralogy, and mineral processing. J. Geochem. Explor. 7, 2. Zhang, W., Alakangas, L., Wei, Z., Long, J., 2. Geochemical evaluation of heavy metal migration in Pb-Zn tailings covered by different topsoils, J Geochem Explor, 3-2. (Complete references can be found in the dissertation) 7 3