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SEPARATIONS Gold Ore Column Studies with a New Mercury Precipitant Matthew M. Matlock,† Brock S. Howerton,† John D. Robertson,‡ and David A. Atwood*,† Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, and Department of Chemistry, University of MissourisColumbia, Columbia, Missouri 65211
The 1,3-benzenediamidoethanethiol ligand (BDET) completely precipitates soft heavy metals from aqueous solutions under a wide range of conditions. In the present study, the sodium salt of this ligand was applied during the cyanide leaching of ore samples collected from an active gold-mining site in South America. The ligand irreversibly binds mercury from solutions containing cyanide, similarly to those present in the gold-cyanide process (GCP). The present work tests the industrial viability of BDET in the GCP through a column leaching study on ores from South America. The average mercury concentration in the ore samples was 5.22 ppm (milligrams of Hg per kilogram of ore), leached by 19.1% after cyanide extraction. After addition of BDET to the extractant solution, leached mercury from the columns decreased to 0.001 ppm, whereas the leached gold and silver concentrations remained unaffected. Introduction In 1997, the World Gold Council reported that, within the past 6000 years, over 125 000 tons of gold had been mined worldwide.1 Historically, the discovery of gold at Sutter’s Mill on the American River in 1848 set in motion the Great California Gold Rush. It has been reported that nearly 90% of the 125 000 tons has been mined since 1848.1 During the early era of gold mining, miners’ took advantage of elemental sources or utilized mercury/gold amalgams as a means of freeing gold from raw ores. As technology advanced, so did the methods for extraction. By 1896, gold leaching with alkaline sodium cyanide solutions was becoming widespread.2 The use of cyanide as a leaching solution then enabled the mining of ores that previously would have been considered uneconomical. For over 100 years, extraction with sodium cyanide has been the dominant and universally utilized process for the isolation of gold from free milling gold ores. In the gold-cyanide process (GCP), gold is leached from the ores with an alkaline cyanide solution as depicted in eq 1
4Au + 8CN- + O2 + 2H2O h 4Au(CN)2- + 4OHA linear two-coordinate soluble Au(I) species, [Au(CN2)]-, is the predominant species produced under the GCP.3 Square-planar Au(III), [Au(CN)4]-, has also been identified but is not typically formed under these conditions.3 In addition to readily forming complexes with gold, cyanide also forms coordination compounds with other metals such as iron, nickel, zinc, copper, and * To whom correspondence should be addressed. † University of Kentucky. ‡ University of MissourisColumbia.
mercury.4 For gold ores rich in these heavy metals, soluble metal-cyano complexes in the leachate solution have posed a serious and costly problem. The leaching process is initiated when heap gold ore is stacked in “lifts”, large cylindrical columns with typical dimensions of 20 × 50 ft (Figure 1). Once stacked in the lifts, the heap is repeatedly washed with a concentrated NaCN solution. The same solution can be passed through the columns several times. Following the NaCN wash, the solution (now rich in metal-cyano complexes) is passed over activated carbon (AC), which adsorbs the gold, silver, and mercury cyanide complexes. The metal-loaded AC then undergoes electrowinning and retorting steps to recover the precious metals. The filtrate solution is recycled for gold and silver and finally diluted and sent to holding ponds for later treatment of the soluble heavy metal contaminants. A major problem in this process is that mercurycyano complexes adhere to the AC alongside the precious metals. During the electrowinning and retorting step, mercury is then 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 the workers who handle the mercury-laden AC. Currently, there is not an effective method for removing the mercury from the heap leach solution. This is due largely to the fact that chelating ligands, or other precipitating agents, that can be used under the adverse pH conditions present in the leachate solutions do not exist. The only treatment comes in the final steps of the extraction process where a final retort of mercury is performed. It has recently been demonstrated that 1,3-benzendiamidoethanethiol (BDET) is capable of precipitating a wide range of metals, including mercury, from aqueous solutions (Figure 2). The BDET ligand, the similar 2,6pyridinediamidoethanethiol ligand (PyDET), and sev-
10.1021/ie020006s CCC: $22.00 © 2002 American Chemical Society Published on Web 09/14/2002
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Figure 1. Scheme of the gold-cyanide heap leach process.
Figure 2. Structure of BDET.
eral associated metal derivatives have now been fully characterized.5,6 On the basis of Raman spectroscopy and MAS solid-state NMR spectroscopy for 13C and 199Hg, it is thought the ligand binds through the sulfur atoms, forming a linear geometry with the trapped mercury atoms (Figure 3).5,6 In earlier studies, BDET was found to remove mercury nearly completely from laboratory stock solutions.5 More recently, the ligand has been shown to be effective in removing metals from actual water samples for acid mine drainage, lead from batteries in lead-battery recycling facilities, mercury from soils, and mercury from cyanide leachate solutions.6-9 The present work explores the use of BDET under conditions matching those of the real-world GCP. Specifically, the ligand is used as an additive in NaCN leachate solutions to suppress mercury concentrations in the final heap leachate solutions. The goal of this experiment is to selectively reduce mercury concentrations in the cyanide leachate solutions under industrial
Figure 3. Binding of BDET with mercury as predicted from computer modeling.
gold extraction conditions. In doing so, the resulting heap leachate solution is ideally made mercury-free.
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Experimental Section General. The synthesis and characterization (including spectroscopic and computational support) of the BDET ligand and BDET-Hg compound followed the previously reported literature.5-7,10 Sampling Sites. For this study, samples from an active gold mine in South America were obtained. The mining site is currently generating a daily flow of nearly 5 million gallons of leachate water at an average flow rate of several thousand gallons per minute. In 2000, production from this site was several million ounces of gold, with an estimated future reserve of many times that amount million ounces. The amount of mercury liberated during the gold extraction process is estimated to be greater than 14 000 kg annually. A 50-lb ore sample was collected from one of four total open-pit mines with the cooperation of a local environmental firm, which, in conjunction with various mining companies, is currently exploring new technologies for removing mercury from the heap leachate solutions. Metal Analyses. For the determination of the total gold content in the ore samples, fire assay fusion (FAF) for sample decomposition was used, followed by atomic absorption spectroscopy (AAS) on a Perkin-Elmer atomic absorption spectrometer. For this procedure, the samples were fused with a mixture of lead oxide, sodium carbonate, Borax, and silica and coupled to yield a precious metal bead. The bead was digested for 30 min in a dilute nitric acid solution, followed by the addition of hydrochloric acid for an additional hour. The solution was cooled, diluted to 7.5 mL with DI water, homogenized, and analyzed by AAS. The concentrations of other metals (excluding Au and Hg) were determined using X-ray fluorescence (XRF) and a digestion procedure followed by analyses with an inductively coupled plasma optical emission spectrometer (ICP-OES). For the XRF analyses, the energydispersive X-ray fluorescence measurements were performed with a Spectro X-lab 2000 instrument that is equipped with a Bragg-polarized excitation source. The full width at half-maximum energy resolution of the detector used in these measurements is 135 eV for the 5.90-keV Mn KR X-ray. The samples were prepared for analysis by mixing ca. 1 g of finely ground mineral sample with 2 g of X-ray mix (Chemplex) in a fresh polycarbonate vial for 15 min on a mixer mill. The resulting mixture was then pressed into a 32-mmdiameter pellet. The XRF measurements were made using a combination of three excitation targets with a Pd anode: molybdenum for Cr-Y and Hf-Th (35 kV, 4.4 mA), aluminum oxide for Zr-Nd (52 kV, 5.7 mA), and highly oriented pyrolytic graphite for Na-V (15 kV, 13 mA). The concentrations were determined using a combination of the Compton and fundamental parameters models. The method was calibrated with over 70 pressed pellets of standard reference materials. Comparative analyses of the total metal concentration (excluding Hg and Au) were performed using ICP-OES on a 1999 Thermo Jarrell Ash Duo HR Iris Advanced ICP-OES instrument. For the ICP-OES analyses, a 0.500-g sample of the solid ore was digested with a mixture of perchloric, nitric, and hydrofluoric acids and analyzed for total metals. Determination of total mercury content in the ore samples was performed by sample digestion followed by analysis with cold vapor atomic fluorescence spectrometry (CVAFS). For the CVAF analyses a 1994, model
Table 1. Total Metal Concentrations (ppm) of Gold Ore Samples before GCP metal
metal concentration (ppm)
Al Sb As Ba Be Cd Ca Cr Co Cu Au Fe Pb Li Mg Mn Ni K Se Ag Sr Tl Sn V Zn Ti Ga Rb Zr Nb Te La W Bi Th U Ge Hg
5420 41.6 380 471 0.450 0.400 111 95.5 3.95 124 0.820 2100 824 4.90 123 24.5 4.53 909 1.98 4.60 645 1.07 3.83 58.2 24.0 221 14.6 27.4 72.2 6.80 4.08 17.7 9.08 7.26 3.63 1.63 0.100 5.22
VI2000, Varsal atomic fluorescence spectrometer was used in conjunction with the EPA method for cold vapor analyses for mercury.11 The EPA digestion method for mercury in solid or semisolid waste involves using an average 0.200-g solid sample in an aqua regia solution (HCl and HNO3 at a ratio of 3:1 v/v) under heat, followed by the addition of potassium permanganate and potassium persulfate under heat. Finally, the excess potassium permanganate is reduced using a sodium chloridehydroxylamine sulfate solution. Testing Parameters for Column Leaching. For the column leaching experiments, 500-g portions of the ore samples were placed into glass columns (41 × 5 cm). Five hundred grams of alkaline (pH 10.5) 0.5% NaCN leachate solution was then passed through the packed columns and recycled for a total of five passes. For the sixth pass, a 0.5 M (molar) ethanol solution of BDET was introduced into each leachate solution. The volume of BDET introduced was based on a percent volume of 0.5 M BDET relative to the total leachate solution. The volume of BDET was varied for each independent experiment. The BDET cyanide leachate solution was then passed through the columns. Following each pass, aliquots of 10 mL were collected, filtered at 0.2 µm (Nalgene syringe filters, lot 322238), and analyzed for total metal concentration using CVAF for mercury, AAS for gold, and ICP-OES for all other metals. The resulting metal concentrations are reported in Tables 2 and 3.
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Figure 4. Relationship of the BDET dose (as a percent v/v) added to the column leaching solutions to the decline in mercury concentration. Results are based on 500-g ore samples with 500 g of alkaline cyanide leaching solution. Mercury values are derived from the final pass through the column.
Results and Discussion Characterization of BDET and BDET-Hg. Full characterization of the BDET ligand and the related PyDET ligand, as well as the ligand-Hg compounds, has been performed using X-ray diffraction (XRD); CHNS elemental analysis (EA); and 1H NMR, IR, Raman, and MAS solid-state NMR spectroscopies.5,6,8 Results of Metal Leaching before BDET Addition. The ores under investigation contained a variety of metals, including gold (0.820 ppm), silver (4.60 ppm), copper (124 ppm), zinc (24.0 ppm), and mercury (5.22 ppm) (Table 1). Unless stated otherwise, all metal concentrations are reported in units of milligrams of metal per kilogram of solid ore (ppm). Following the column cyanide-leaching experiments, gold recovery was 91%, and silver recovery was 57.8% prior to the addition of BDET (Tables 1 and 2). Other major metal contaminants present in the leachate solution included mercury (0.998 ppm), iron (255 ppm), copper (51.9 ppm), and zinc (2.97 ppm) (Table 2). Mercury Concentrations following BDET Addition. Addition of 0.01 vol % (v/v) 0.5 M BDET to the leaching solution during the final pass through the column reduced the mercury concentration from 0.998 to 0.470 ppm. Increasing the BDET volume within the leachate solution to 0.30% (v/v) significantly reduced the concentration of mercury in the final leachate solution to 0.001 ppm (Tables 2 and 3 and Figure 4). Thus, the need for an increase in BDET volume to precipitate mercury within the leaching columns is likely due to the high concentrations of other metal-cyano complexes present in the solution. Recently published work on mercury removal from heap leach solutions following the column leaching procedure has shown the need for increased ligand dosages from 1:1 stoichiometric values
Table 2. Concentrations (ppm) of Major Metal Constituents Following the GCP Using 500 g of Ore with 500 g of Alkaline Leaching Solution metal concentration (ppm) metal
prior to addition of BDET2-
after addition of 0.30% BDET2-
Al Sb As Cd Co Cu Au Fe Mg Mn Ni Ag Sr V Zn Hg
27.6 0.569 45.1 0.050 0.953 51.9 0.746 255 0.167 6.65 2.19 2.40 0.102 0.142 2.97 0.998
23.6 0.424 44.2 0.044 0.872 52.2 0.738 260 0.121 7.04 1.22 2.37 0.107 0.115 3.10 0.001
Table 3. Final Mercury Concentrations (ppm) from the GCP with Varied Doses of BDET
dosea (% v/v)
initial average Hg concentration (ppm)
treated average Hg concentration (ppm)
% reduction
0.01 0.05 0.10 0.20 0.30
0.998 0.998 0.998 0.998 0.998
0.470 0.073 0.059 0.042 0.001
52.9 92.7 94.1 95.8 99.9
a
Dose-based percent BDET (v/v) in column leaching solution.
(Hg to BDET) to compensate for high copper concentrations.6 In this previous work, alkaline NaCN stock solutions containing Cu(I) concentrations exceeding the
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Hg concentration by 3.9 times (Cu to Hg) were treated. To reduce the Hg concentrations to below 0.2 ppm within 15 min in the stock solutions, a stoichiometric dose of BDET to both Hg and Cu was required. ICPOES analyses indicate that the copper concentration remains constant even after filtration at 0.2 µm. The reason for this lies in the probable formation of a soluble complex between [Cu(CN)3]2- and the BDET ligand.6 At this time, the precise thermodynamic data on the interaction of BDET with mercury in the presence of high levels of other metal contaminants has not been established but is currently under investigation. Effect of BDET on Gold and Silver. Even at a BDET volume of 0.30% (v/v), the gold and silver concentrations deriving from the GCP remained statistically unchanged. Prior to the addition of BDET to the leaching column, the leached gold and silver concentrations were 0.746 and 2.40 ppm, respectively. Following the 0.30% (v/v) ligand addition, the gold concentration was found to decrease by 1.1%, and the silver concentration by 1.3% (Table 2). Additionally, previous studies have noted the thermodynamic preference of BDET for mercury and have shown that in the event of the addition of excess volumes of BDET, the gold and silver concentrations still remain unaffected.6 Conclusions Each year, the mining industry generates large volumes of hazardous materials, including acid mine drainage, cyanide waste, and heavy-metal-laced solutions. For most waste materials, reagents exist that can remedy the problem, although often only in the short term. At this time, no single reagent is available to the mining industry that can bind mercury and maintain stability as an insoluble precipitate under the adverse conditions encountered in the GCP. The present study has shown that BDET can be successfully used as an in situ additive to cyanide leaching solutions to control mercury. In addition to the ligand’s ability to reduce the concentration of mercury from the GCP solutions by 99.9%, even at a pH of 10.5, the Hg-BDET precipitates remain stable for the duration of the leaching process.
Because of the extreme stability of the Hg-BDET compound, it can safely remain in the refuse ore and sent to landfills without a future concern for leaching.5,8 Used in situ, the ore slurry within the columns acts to filter out the BDET-Hg precipitates. Thus, the effluent is free of mercury. At this time, the exact economics for the use of BDET salts has not been established, but initial price estimates show that the compound can be produced for less than $200 per pound and can thus meet the financial constraints associated with current mining operations. Literature Cited (1) Hilliard, H. E. Mineral Commodity Summaries: Silver; U.S. Geological Survey: Reston, VA, 2001. (2) Bo¨dlander, G. Z. Angew. Chem. 1869, 9, 583-587. (3) Wadsworth, M. E.; Zhu, X.; Thompson, J. S.; Pereira, C. J. Hydrometallurgy 2000, 57, 1-11. (4) Rees, K. L.; Van Deventer, J. S. Min. Eng. 1999, 12, 877892. (5) Matlock, M. M.; Howerton, B. S.; Atwood, D. A. J. Hazard. Mater. 2001, B84, 73-82. (6) Matlock, M. M.; Howerton, B. S.; Van Aelstyn, M. A.; Nordstrom, N. L.; Atwood, D. A. Environ. Sci. Technol. 2002, 36, 1636-1639. (7) Matlock, M. M.; Howerton, B. S.; Atwood, D. A. Ind. Eng. Chem. Res. 2002, 41, 1579-1582. (8) Matlock, M. M.; Howerton, B. S.; Atwood, D. A. Adv. Environ. Res. 2002, in press. (9) Matlock, M. M.; Howerton, B. S.; Atwood, D. A. Water Res., in press. (10) Atwood, D.A.; Matlock, M. M.; Howerton, B. S. Novel Multidentate Sulfur-Containing Ligands. U.S. Patent pending, Dec 2000. (11) U.S. Environmental Protection Agency (EPA). Method 7470A: Mercury in Liquid Waste. In Test Methods for Evaluating Solid Wastes, Physical/Chemical Methods; Publication SW-846; Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency (EPA): Washington, DC, 1986; Vol. 1, Section A, Chapter 3.
Received for review January 3, 2002 Revised manuscript received July 23, 2002 Accepted July 29, 2002 IE020006S
