Environ. Sci. Technol. 2002, 36, 1636-1639
Advanced Mercury Removal from Gold Leachate Solutions Prior to Gold and Silver Extraction: A Field Study from an Active Gold Mine in Peru MATTHEW M. MATLOCK, BROCK S. HOWERTON, MIKE A. VAN AELSTYN, FREDRIK L. NORDSTROM, AND DAVID A. ATWOOD* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
Mercury contamination in the Gold-Cyanide Process (GCP) is a serious health and environmental problem. Following the heap leaching of gold and silver ores with NaCN solutions, portions of the mercury-cyano complexes often adhere to the activated carbon (AC) used to extract the gold. During the electrowinning and retorting steps, mercury can be (and often is) emitted to the air as a vapor. This poses a severe health hazard to plant workers and the local environment. Additional concerns relate to the safety of workers when handling the mercuryladen AC. Currently, mercury treatment from the heap leach solution is nonexistent. This is due to the fact that chelating ligands which can effectively work under the adverse pH conditions (as present in the heap leachate solutions) do not exist. In an effort to economically and effectively treat the leachate solution prior to passing over the AC, a dipotassium salt of 1,3-benzenediamidoethanethiol (BDET2-) has been developed to irreversibly bind and precipitate the mercury. The ligand has proven to be highly effective by selectively reducing mercury levels from average initial concentrations of 34.5 ppm (parts per million) to 0.014 ppm within 10 min and to 0.008 ppm within 15 min. X-ray powder diffraction (XRD), proton nuclear magnetic resonance (1H NMR), Raman, and infrared (IR) spectroscopy demonstrate the formation of a mercury-ligand compound, which remains insoluble over pH ranges of 0.0-14.0. Leachate samples from an active gold mine in Peru have been analyzed using cold vapor atomic fluorescence (CVAF) and inductively coupled plasma optical emission spectroscopy (ICP-OES) for metal concentrations before and after treatment with the BDET2- ligand.
Introduction In its 2001 Mineral Commodity Summaries, the U.S. Geological Survey Department and the U.S. Department of the Interior reported that in the year 2000 the world production of gold reached 2445 metric tons (1). Of the 2445 metric tons of mined gold, South Africa accounted for nearly 18%, the United States for 13%, and Peru for nearly 6% (1). Within the * Corresponding author phone: (859) 257-7304; fax: (859) 3231069; email:
[email protected]. 1636
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U.S., nearly 80% of the 330 metric tons of gold mined is derived from California and Nevada (1). For over a century, extraction with sodium cyanide has been the dominant and universally utilized process for the isolation of gold from free milling gold ores. During the Gold-Cyanide Process (GCP), gold is leached from the ores utilizing an alkaline cyanide solution as depicted in eq 1.
4Au + 8CN- + O2 + 2H2O h 4Au(CN)2- + 4OH- (1) During this process, a linear two-coordinate Au(I) complex, [Au(CN2)]-, predominates. The square-planar Au(III) complex, [Au(CN)4]-, has also been identified but is not typically formed during the standard cyanidation process (2). The chemistry of gold-anion coordination is an area that has been explored thoroughly, and there have been several papers published in the area (3-5). In addition to readily forming complexes with gold, cyanide also coordinates other metals such as iron, zinc, copper, and mercury (6). For gold ores rich in heavy metals, soluble metal-cyano complexes in the leachate solution are a serious problem, which can ultimately have devastating effects both on the recovery process and on the quality of downstream waters. To remove heavy metals such as Hg, Cu, and Cd from the waste leachate solutions, various techniques have been investigated such as precipitation-flocculation, solvent extraction, filtration using microemulsion liquid membranes, and sorption by activated carbon or ion-exchange resins (7). The use of thiocarbamates (such as diethyl dithiocarbamates) has also been suggested (and in limited cases are utilized) as a means of precipitating metal contaminants such as mercury (7, 8). Significant problems with thiocarbonate and thiocarbamate ligands have recently been noted due in part to poor and indiscriminate metal bonding (9). This leads to unstable metal-ligand complexes that tend to decompose and release the heavy metals back into the environment over varying, but usually short, periods of time, especially under adverse pH conditions such as those present in the GCP (9-11). In addition to high dosage ratios, many of the thiocarbonate and thiocarbamate ligands themselves decompose to hazardous materials (9, 12). Previously, we have designed an economical dipotassium salt of 1,3-benzendiamidoethanethiol (BDET2-) based on hard-soft acid-base (HSAB) interactions (13-15). In general, the HSAB principle predicts that a soft base, such as BDET2-, should preferably interact and bond with soft metals, such as Hg2+ and Cd2+ (14). This is indeed the case. Reactions between the BDET2- ligand and Hg2+ and Cd2+ produce stable and insoluble metal-ligand precipitates at pH ranges of 0.014.0 (13, 16). The BDET2- ligand is an aromatic compound which utilizes two chains at the 1,3 positions consisting of three carbons, one nitrogen, and two sulfur end groups. The theoretical reaction, producing the insoluble compounds, is represented in eq 2: H2O
K2C12H14N2O2S2(aq) + M2+(aq) 98
C12H14N2O2S2M(s) + 2K+(aq) (2)
where M2+ ) Cd, Pb, Hg, Fe, Zn, etc. Bonding through the terminal sulfurs with secondary interactions between the amide groups in the ligand backbone should allow for more efficient binding of heavy metals 10.1021/es0112285 CCC: $22.00
2002 American Chemical Society Published on Web 02/09/2002
FIGURE 1. Computer model of BDET-Hg with a calculated bond S-Hg-S angle of 179° and Hg-S bond lengths of 2.3 Å. through use of a multidentate bonding arrangement around the central metal atom such as mercury (Figure 1) (7). We have shown previously that BDET2- can reduce divalent metal concentrations in water and sediments to below EPA limits (13, 16, 18). This paper outlines the effectiveness of the BDET2- ligand on leachate waters deriving from the GCP. The goals of this experiment were to selectively reduce mercury concentrations to below the EPA discharge limit of 0.2 ppm (18) without reducing the gold or silver concentrations.
Experimental Section Sampling Sites. To demonstrate the utility of the BDET2ligand, an active gold mining site in Peru was selected. Field samples were collected with the cooperation of a local environmental firm, which (in conjunction with various mining companies) is currently exploring new technology for the treatment of heap leachate solutions prior to the recovery of Au and Ag. Upon the request of the mining company, precise geographical locations are being held confidential. The gold mining site in Peru is currently generating a daily flow of 5.76 million gallons of leachate water at an average flow rate of 4000 gallons/min. The estimated gold reserve for this site is 8 years. Mercury leaching from the gold extraction process is estimated to be an average of 16 037 kg annually. During this process, heap gold ore (which is also rich in heavy-metal contaminants) is stacked in lifts, which are large cylindrical columns with dimensions of 20 by 50 feet. Once stacked in the lifts, the heap is repeatedly washed with a concentrated NaCN solution. Following the NaCN wash, the solution (now rich in metal-cyano complexes) is passed over activated carbon (AC) to capture the gold and silver cyanide complexes. The AC ultimately undergoes electrowinning and retorting steps to isolate the precious metals. The filtrate solution is recycled for Au and Ag, and finally diluted and sent to holding ponds for later treatment of the soluble heavy-metal contaminants. In addition to contamination in the final waste solutions, problems occur when portions of the mercury-cyano complexes (19) adhere to the AC. During the electrowinning and retorting steps, mercury can be (and often is) emitted to the air as a vapor. This poses a severe health hazard to plant workers and the local environment. Additionally, there
are concerns for the safety of workers when handling the mercury-impregnated AC. Currently, mercury treatment from the heap leach solution is nonexistent. This is due to the fact that commercial or common chelating ligands cannot effectively work under the adverse pH conditions present in the heap leachate solutions. The only treatment comes in the final steps of the extraction process where a final retort of mercury from the AC is performed. Materials. The following reagent-grade materials were used in the synthesis of the dipotassium salt of the BDET2ligand (15): isophthaloyl dichloride (98+% purity), 2-aminoethanethiol hydrochloride (98+% purity), and triethylamine (99+% purity), which were purchased from Aldrich Chemicals. Dry HPLC grade chloroform and potassium hydroxide pellets were purchased from Fisher Scientific. Five homogeneous, 1.0 L unfiltered samples were collected from each site, and delivered to the University of Kentucky. The samples were stored at 4 °C prior to analyses and experimentation. Analytical Methods. Nuclear magnetic resonance (1H and 13C) was performed on a Varian-Gemini 400 instrument. The NMR analyses were run in deuterated water, hydrochloric acid, and dimethyl sulfoxide from Cambridge Isotope Laboratories, Inc. Infrared spectroscopy (IR) data on the BDET2- ligand and the BDET-mercury compounds were collected as potassium bromide pellets using spectroscopic grade potassium bromide from Aldrich Chemicals. Infrared and Raman data were collected on a Nicolet-Avatar 320 FTIR and a FT-Raman 960 Nicolet series spectrometer using a liquid nitrogen cooled germanium detector. For the XRD analyses on the BDET2- ligand and BDET-Hg compound, samples were mounted on glass slides with ethanol and analyzed with a Rigaku unit at 40 kV and 20 mA using Cu KR1 (λ ) 1.540598 Å) radiation. Elemental analyses (CHNS) were determined on a Vario Elementar III. All masses were determined using Sartorius BP2100S or Mettler AE 240 analytical balances. Dosage of the BDET2-. Experiments have shown that the BDET2- ligand thermodynamically precipitates Hg2+ when compared to Fe2+, Pb2+, Cd2+, Cu2+, and Zn2+. The initial dose of the BDET2- was based on a 1:1 stoichiometric dose with mercury. A second series of experiments were conducted with the ligand dosage increased to compensate for the high concentrations of copper. A final series of experiments were conducted with a 100-fold excess of BDET2- to ensure that, in an event of a ligand overdose, Au and Ag concentrations would remain unaffected. Determination of Total Dissolved Mercury (TM) of Leachate Samples. TM levels of the leachate samples were determined using cold vapor atomic fluorescence spectrophotometry (CVAFS) on a Varsal Atomic Fluorescence Spectrometer, model number VI2000, using EPA cold vapor methods (20). Digestion of the leachate samples was performed following EPA methods for mercury detection in liquid waste (20). The EPA method for mercury in liquid waste involves using an aqua regia solution under heat, followed by the addition of potassium permanganate and potassium persulfate under heat. Finally, the excess potassium permanganate is reduced using a sodium chloride-hydroxylamine sulfate solution. The average initial concentrations of mercury in the sampled heap leachate solutions contained 34.5 ppm of TM. Determination of Gold and Total Dissolved Metals of Leachate Samples. Aqueous metal analyses were performed with a 1999 Thermo Jarrell Ash Duo HR Iris Advanced Inductive Coupled Plasma Optical Emission Spectrometer (ICP-OES). The average initial concentrations of gold and silver in the leachate solutions were 94.1 ppm of Au and 39.8 ppm of Ag. The average results of the raw mine samples are outlined in Table 1. VOL. 36, NO. 7, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Initial Metal Concentrations (ppm) from Collected Heap Leachate Solutions metal sample 1 sample 2 sample 3 sample 4 sample 5 average As Cd Co Cu Au Fe Pb Ni Ag Zn Hg
2.80 2.88 2.96 2.88 2.88 2.88 0.227 0.285 0.291 0.283 0.284 0.274 0.692 0.682 0.690 0.668 0.674 0.681 137 135 138 133 134 135 94.1 94.3 94.0 94.2 94.2 94.1 20.4 20.2 20.5 19.9 20.1 20.2 0.119 0.115 0.130 0.125 0.119 0.122 242 243 247 240 241 243 39.7 39.6 40.1 39.5 40.3 39.8 533 531 532 531 532 532 34.5 34.3 34.1 34.9 34.7 34.5
TABLE 2. Average Metal Concentrations (ppm) at 10, 15, 30, and 1440 min and 1 Week following a 1:1 Stoichiometric Dosage of BDET2metal
10 min
15 min
30 min
1440 min
1 week
As Cd Co Cu Au Fe Pb Ni Ag Zn Hg
2.88 0.274 0.682 136 94.0 20.2 0.122 243 40.1 532 5.02
2.87 0.274 0.681 135 93.8 20.4 0.121 244 39.8 532 4.87
2.85 0.271 0.681 135 93.6 20.1 0.122 243 39.8 532 4.07
2.85 0.273 0.681 135 93.6 20.2 0.122 243 40.6 532 4.05
2.85 0.273 0.681 135 93.4 20.2 0.122 243 40.5 532 4.05
TABLE 4. Average Metal Concentrations following a 100-Fold Stoichiometric Increase in BDET2- Dosage (from 1:1 Hg-BDET2-) metal
av initial concn (ppm)
final av concn after 1 week (ppm)
As Cd Co Cu Au Fe Pb Ni Ag Zn Hg
2.88 0.274 0.681 135 94.1 20.2 0.122 243 39.8 532 34.5
2.85
