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Biooxidation of thiocyanate-containing refractory gold tailings from Minacalpa, Peru
Hydrometallurgy 81 (2006) 159 – 166 www.elsevier.com/locate/hydromet
Biooxidation of thiocyanate-containing refractory gold tailings from Minacalpa, Peru Gregory J. Olson a,⁎, Corale L. Brierley b , Andrew P. Briggs c , Ernesto Calmet d a b
Little Bear Laboratories, Inc., 5906 McIntyre St., Bldg. 2, Golden, CO 80403, USA Brierley Consultancy LLC, P.O. Box 260012, Highlands Ranch, CO 80126-0012, USA c Fluor Chile, S.A., Reyes Lavalle 3340—7th Floor, Los Condes, Santiago, Chile d Minera Aurifera Calpa, S.A., Av. Arequipa 330 OF 101, Lima 1, Peru
Received 28 June 2005; received in revised form 12 October 2005; accepted 23 November 2005
Abstract Refractory, sulfidic tailings from gold mining operations may be biooxidized to increase the amount of gold recoverable by cyanide extraction. Tailings containing thiocyanate must be pretreated to remove thiocyanate prior to biooxidation due to its toxicity to microorganisms that oxidize sulfide minerals. Thiocyanate in tailings at Minacalpa, Peru, was only partially extractable with water or with dilute sulfuric acid. The thiocyanate also was not completely biodegradable at neutral pH. Consequently, tailings remained inhibitory toward biooxidation after these treatments, releasing thiocyanate into bioleach solutions. The non-extractable form of thiocyanate in Minacalpa tailings was probably copper thiocyanate. A novel approach was developed to remove this thiocyanate using ferric sulfate solution. Tailings were detoxified by washing with either reagent ferric sulfate or solutions derived from biooxidation of pyrite. Detoxified tailings, i.e., those thoroughly washed with ferric sulfate solution, were biooxidized, exhibiting a fairly linear relationship between sulfide-S oxidation and cyanide gold recovery. A process for biooxidation of Minacalpa tailings would involve an initial washing step with biooxidation solutions followed by stirred tank biooxidation or agglomeration of detoxified tailings onto support rock and biooxidation in heaps. © 2006 Elsevier B.V. All rights reserved. Keywords: Biooxidation of tailings; Refractory gold; Thiocyanate
1. Introduction Biooxidation of refractory gold ores and concentrates is practiced commercially in stirred tank reactors and in constructed heaps (Olson et al., 2003a; van Aswegen and Van Niekerk, 2004). There also has been interest in using biooxidation pretreatment for recovery of gold from refractory, sulfidic mill tailings discarded in tailings ponds. Such tailings may still contain significant gold values that could be made amenable to cyanide extraction fol⁎ Corresponding author. Tel.: +1 303 273 5697; fax: +1 303 273 0494. E-mail address: [email protected] (G.J. Olson). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2005.11.003
lowing biooxidation (Livesey-Goldblatt, 1986; Lawson et al., 1990). Minera Aurifera Calpa S.A. (Minacalpa) operates an underground gold mine and treatment plant in the Department of Arequipa in southern Peru. The ore contains sulfides or mixed oxides and sulfides. Processing comprises crushing, ball milling, flotation, discard of flotation tailings, regrinding of concentrates, and cyanide leaching of the reground concentrates for gold recovery. The plant is currently (June, 2005) operating at 30% capacity treating 250 to 300 t/d of ore grading 4.80 to 5.25 g/t gold, giving a concentrate treatment rate of 80 to 90 t/d at a grade of 18.25 to 33 g/t gold.
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Tailings from the Minacalpa gold plant contain appreciable concentrations of refractory gold. These tailings have been deposited in dams over several years and now form a resource that may be economically exploited. Included at this site are 1.2 million tons of old flotation tails assaying 3.2 g/t of gold and 380,000 t of concentrate leach tails assaying 7.3 g/t of gold. This amounts to 205,000 ounces of contained gold. Gold in the tailings is intimately associated with pyrite and is highly refractory to cyanidation. Gold extraction from unoxidized material is only about 40%. Biooxidation is a potential treatment option for these tailings for oxidizing pyrite and making the gold available for subsequent extraction with cyanide. Because the material has been previously cyanided, significant thiocyanate contamination is present and must be removed prior to biooxidation. Thiocyanate is formed by the reaction of reduced sulfur compounds with cyanide and is a primary mechanism of cyanide consumption during cyanidation of sulfidic gold ores (van Aswegen and Van Niekerk, 2004). Although microorganisms that oxidize refractory gold ores are robust, growing under highly acidic conditions (pH b 2.0) and in the presence of high concentrations of dissolved metals, they are extremely sensitive to the thiocyanate anion, SCN−. A thiocyanate concentration of a few parts-permillion (mg/L) in solution inhibits these microorganisms (Cox et al., 1979; Suzuki et al., 1999). Thiocyanate contamination of process water has been reported to result in problems with bacterial activity at stirred tank biooxidation plants (Miller, 1997). However, thiocyanate is readily biodegraded by certain bacteria at neutral pH under aerobic conditions. For example, Gold Fields' ASTER™ process uses biodegradation to remove thiocyanate from process and waste solutions (Muhlbauer and Broadhurst, 1997; van Aswegen and Van Niekerk, 2004). This paper describes the occurrence of thiocyanate in Minacalpa tailings in a form not fully removed by washing with water or with dilute sulfuric acid or by biodegradation. This laboratory study was done to determine how thiocyanate may be removed from Minacalpa tailings and the amenability of the “detoxified” tailings to biooxidation. Preliminary engineering work was undertaken to develop a conceptual process design.
gray-colored and contained 21.9% sulfide-S and 4.95 ppm gold. Most of the work was done with a blend of concentrate leach tails and flotation leach tails, referred to as “B” tailings (Table 1). The particle size of “B” tailings was 4.2% N 425 μm; 20.0% 425 × 150 μm; 21.2% 150 × 75 μm; 10.2% 53 × 75 μm and 44.4% b 53 μm. Pyrite was the dominant sulfide mineral in the tailings. 2.2. Thiocyanate extraction from tailings Ten g of tailings were added to 50 mL plastic centrifuge tubes along with various amounts of deionized water, 1 N sulfuric acid, or 0.2 M ferric sulfate in 1 N sulfuric acid. Tubes were gently shaken at room temperature (20 to 25 °C) for periods of time ranging from 1 min to 3 h. After shaking the tubes were centrifuged at 2000 r.p.m. for 5 min. Thiocyanate in the supernatant solution was analyzed colorimetrically with ferric nitrate reagent (American Public Health Association, 1992). Samples were generally diluted significantly prior to analysis due to the high extinction coefficient of the Fe(III)– SCN complexes. Although extraction solutions containing ferric sulfate spontaneously formed the red color characteristic of Fe(III)–SCN complexes, addition of the ferric nitrate–nitric acid reagent to the diluted samples greatly intensified the red color. Addition of the reagent imparted the oxidizing conditions and high ferric concentrations resulting in the most efficient detection of thiocyanate. 2.3. Biodegradation of thiocyanate in tailings A bacterial isolate tentatively identified as Thiobacillus thioparus was maintained in a neutral pH culture medium with thiocyanate as a growth substrate (S7 medium, Hutchinson et al., 1965). The organism was inoculated into slurries of “A” or “B” tailings in S0 salts solution (S7 medium less thiocyanate) in flasks shaken at 180 r.p.m. at room temperature. Tray tests of thiocyanate biodegradation were performed by adding the culture (106 cells/g, final concentration) to 2.0 kg “A” tailings brought to near saturation with 0.6 mM phosphate buffer in a plastic tray (15 × 30 × 9 cm deep).
2. Experimental
Table 1 Analyses of Minacalpa “B” tailings before and after ferric sulfate extraction
2.1. Tailings samples
Sample
Minacalpa provided two samples of dry tailings. Concentrate leach tails, referred to as “A” tailings, were
“B” tailings 13.8 0.17 0.07 11.4 Detoxified “B” tailings 14.2 0.069 0.06 13.0
Fe (%)
Cu (%)
As (%)
S=–S Total S Au (%) (%) (ppm) 13.3 14.4
4.66 4.11
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Uninoculated, thymol-treated controls were employed in both tests.
Table 2 Thiocyanate extraction from Minacalpa tailings by various solutions Tailings
SCN by water extraction (mg/kg)
SCN by 1 N H2SO4 extraction (mg/kg)
SCN by 0.2 M ferric sulfate extraction (mg/kg)
“A” “B”
1020 198
1765 381
2064 595
2.4. Shake flask biooxidation of detoxified tailings Biooxidation of detoxified tailings was done in 15 wt.% slurries in MKM culture medium in Erlenmeyer flasks. MKM medium contained 0.4 g (NH4)2SO4, 0.4 g MgSO4 heptahydrate, and 0.04 g KH2PO4 in 1000 mL deionized water adjusted to pH 1.8 with sulfuric acid. Flasks were inoculated with an active culture of mixed mesophilic microorganisms (grown on pyrite in MKM medium) and shaken at 180 r.p.m. at room temperature. A control flask was not inoculated and was treated with thymol in methanol to prevent microbial growth. At intervals, flasks were sacrificed and the solids recovered for sulfide-S analysis and cyanidation. 2.5. Biooxidation of tailings in columns Column tests were done with “B” tailings (400 g) agglomerated with 1) 100 mL of 5 N sulfuric acid and 2000 g coarse Minacalpa ore (6.35 × 12.7 mm), or 2) 100 mL biooxidation solution (18 g/L Fe, pH 1.45) and 2000 g andesite river rock (6.35 × 19.1 mm) taken from near the mine area. Agglomerates were loaded in 10 cm diameter columns. Thiocyanate was extracted from the agglomerates by recirculating biooxidation solutions through the columns for several days at room temperature. Following detoxification (described in results) columns were leached in recycle fashion at a rate of 12 L/h/m2. 2.6. Gold extraction Gold extraction from recovered tailings was carried out for 24 h at 40% solids in agitated Erlenmeyer flasks at room temperature. NaCN concentrations were maintained at 1 to 2 g/L and the pH was maintained at 10.5 to 11.0 with lime. 3. Results 3.1. Thiocyanate extraction from tailings The measured thiocyanate concentration of the tailings depended on the method of extraction (Table 2). More thiocyanate was extracted with 1 N sulfuric acid than with water (1 h shaking). Ferric sulfate in 1 N sulfuric acid extracted more thiocyanate than 1 N sulfuric acid alone. Little or no additional thiocyanate was extracted if samples were shaken for 3 h rather than 1 h. Additional tests with “B” tailings showed similar amounts of thio-
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cyanate were extracted over a range of ferric sulfate solution to solids ratios ranging from 1 : 1 to 50 : 1. The concentration of sulfuric acid in the ferric sulfate solution (0.01 to 1.0 N) did not affect the extent of thiocyanate extraction. These results indicate washing with water or dilute acid would be inadequate to detoxify these tailings; biooxidation solution, essentially acidic ferric sulfate solution, would dissolve additional thiocyanate and poison the microorganisms. This was confirmed by extraction tests showing that tailings could be detoxified after extensive washing with 0.2 M ferric sulfate in 0.1 N sulfuric acid. The washed tailings were added to MKM medium (5% w/v), inoculated with pyrite-oxidizing bacteria, and supported subsequent microbial growth and pyrite oxidation. Tailings washed less extensively or with only water or sulfuric acid did not support biooxidation. In these latter cases, thiocyanate was detected in solution at concentrations (3.6 to 13.5 mg/L) sufficient to inhibit biooxidation. 3.2. Form of thiocyanate in Minacalpa tailings The water-insoluble form of thiocyanate in Minacalpa tailings was not conclusively identified but is probably copper thiocyanate (CuSCN). This assumption is supported by several observations. Firstly, reagent CuSCN was virtually insoluble in 0.1% w/v suspensions prepared in deionized water or in 0.02 N sulfuric acid. Conversely, CuSCN was completely soluble in ferric sulfate solution. These results, together with the thiocyanate solubility characteristics of the tailings (Table 2), suggest 50% to 67% of thiocyanate may occur as CuSCN in “A” tailings and “B” tailings, respectively. The remaining thiocyanate is a water soluble form. Secondly, dissolved copper and thiocyanate in a ferric sulfate extract (0.2 M ferric sulfate in 0.1 N sulfuric acid) of “B” tailings (tailings contained 0.17% Cu) were precipitated when the extract was brought to a pH of 4 to 5 with lime. The dried orange-colored precipitate consisted of 70% gypsum and 30% amorphous solids as determined by Xray diffraction and contained significant concentrations of Fe, S, Ca, along with 0.22% copper, as determined by X-ray fluorescence. Thiocyanate was extracted from the
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precipitate similarly to the tailings: the highest amount of thiocyanate (747 mg/kg) was extracted by ferric sulfate in sulfuric acid. Water extracted only 39% of this amount and dilute sulfuric acid extracted 72% of this amount. Thirdly, thiocyanate did not precipitate significantly when a solution containing 0.2 M ferric sulfate and 0.01 M KSCN was treated with lime, but over 80% of thiocyanate was precipitated with lime from a solution containing 0.1 M ferric sulfate and 0.01 M CuSCN. Thus, neutralization of ferric sulfate extracts containing Cu and SCN resulted in precipitation of iron, copper and thiocyanate.
no evidence of biodegradation; thiocyanate concentrations in the tailings determined by ferric sulfate extraction remained unchanged. It is possible that poor oxygen transfer into the wet, paste-like tailings retarded aerobic thiocyanate biodegradation. Results of the biodegradation tests indicate neither slurry nor in-place biodegradation processes are feasible for completely removing thiocyanate from Minacalpa tailings. The water insoluble form of thiocyanate was not biodegraded at neutral pH and was subsequently mobilized when the tailings were placed under conditions for the growth of pyrite-oxidizing microorganisms.
3.3. Biodegradation of thiocyanate in tailings
3.4. Batch detoxification of thiocyanate from tailings by ferric sulfate extraction
Thiocyanate Concentration, mg/L
Thiocyanate was biodegraded in slurries of “A” tailings at pH 6.5 in shake flasks inoculated with a culture of neutral pH thiocyanate-degrading bacteria (Fig. 1). Uninoculated control flasks showed no thiocyanate degradation. Similar results were obtained with “B” tailings. However, the solids recovered from these tests after all measurable thiocyanate in solution had disappeared still contained 385 mg/kg (“B”) to 838 mg/kg (“A”) thiocyanate as determined by ferric sulfate extraction. These tailings were not able to be biooxidized in a 20% slurry at pH 2.0 inoculated with pyrite-oxidizing microorganisms. In summary, water soluble thiocyanate leached from the tailings was biodegradable at neutral pH but the water-insoluble form of thiocyanate in the tailings was not biodegraded and subsequently prevented biooxidation of sulfides in the tailings. Tests also were performed with wet tailings in trays to determine if in-place biodegradation of thiocyanate in tailings heaps may be feasible. After 33 days there was
“B” tailings (600 g) were washed twice with reagent ferric sulfate solution (0.2 M ferric sulfate in 0.1 N sulfuric acid) to reduce the thiocyanate concentration in the tailings from 641 to 5.3 mg/kg. Recovery of tailings was 563 g representing 6.2% weight loss. The washed tailings contained significantly less copper than the starting material (Table 1). This copper was accounted for (94%) in the pooled extraction solutions. The gold content of the washed tailings was also somewhat lower, perhaps due to partial extraction by the oxidizing thiocyanate solution which may form soluble complexes with gold (Barbosa-Filho and Monhemius, 1994a; Wan et al., 2003). The sulfur content of the washed tailings was somewhat higher. This may be due to analytical error or the sulfur may have become concentrated if weight loss from washing was due to extraction of nonsulfur containing material. These detoxified tailings were used in shake flask biooxidation tests described below.
250 200
150
100
50
0 0
3
6
9
12
15
18
Days cells 5% solids
cells 22% solids
control 5% solids
control 22% solids
Fig. 1. Biodegradation of the water soluble fraction of thiocyanate in slurries of Minacalpa “A” tailings at 5% solids and at 22% solids at pH 6.5.
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163
100
3.5. Biooxidation and gold extraction from detoxified tailings
90
3.5.1. Shake flask tests These tests were done to establish the relationship between sulfide biooxidation and cyanide gold extraction in Minacalpa tailings. There was a lag of about 20 days before significant microbial growth and iron oxidation began in shake flasks containing the detoxified tailings. This lag is attributed to the small amount of residual thiocyanate present in the tailings. Thereafter, sulfide-S biooxidation proceeded at about 1.1%/d (Fig. 2). The lag time may have been shortened by another ferric sulfate wash of the tailings. Gold extraction was about 40% from unoxidized tailings and increased to a maximum of 87% after 85% sulfide-S biooxidation (Fig. 3). This extent of biooxidation took a total of 107 days (Fig. 2). Only 4.2% sulfide oxidation had occurred in the uninoculated control flask after 107 days. The slow rate of biooxidation was due to the comparatively coarse particle size of the tailings (24.2% N 150 μm). Tailings that were ground to 98% b 53 μm were biooxidized nearly twice as fast and more extensively in duplicate flasks. Nearly all sulfide-S was removed in these tests (95% to 98%) after 78 to 87 days of biooxidation at 20 to 25 °C, resulting in 98% to 99% gold extraction (Fig. 3). A limited number of shake flask biooxidation tests also were done with detoxified “A” tailings. The results were similar to “B” tailings, showing a fairly linear relationship between sulfide-S biooxidation and cyanide gold extraction (Fig. 3).
100
Sulfide-S Biooxidized, %
90 80 70 60 50 40 30 20 10 control 0 0
20
40
60
80
100
120
Days of Biooxidation Fig. 2. Rate of sulfide biooxidation of detoxified Minacalpa “B” tailings in shake flask slurries (15 wt.%).
Au Extraction, %
80 70 60 50 40 control
30 20 10 0 0
10
20
30
40
50
60
70
80
90
100
Sulfide-S Biooxidized, % "B"
"A"
reground B
columns
Fig. 3. Relationship between sulfide-S biooxidation and cyanide gold extraction from “B” tailings, “A” tailings, and reground “B” tailings (the two highest gold extraction data points) biooxidized in shake flasks and from “B” tailings biooxidized in columns.
The results of the shake flask tests indicate that detoxified Minacalpa tailings can be nearly completely biooxidized resulting in nearly complete liberation of gold. Biooxidation time could be shortened by regrinding the tailings and by operation at higher temperature. Additional tests with continuous stirred tank reactors (CSTRs) are required in order to more accurately predict biooxidation kinetics by such a process. 3.5.2. Column tests Minacalpa tailings also might be biooxidized in a heap configuration following agglomeration of tailings onto coarse ore or support rock (Whitlock, 1997). In this case, tailings might be detoxified by rinsing the stacked heap with biooxidation solutions. Two possible support substrates were evaluated: 1) coarse ore and 2) local river rock (andesite). 3.5.2.1. Biooxidation of “B” tailings agglomerated onto coarse ore. Thiocyanate was extracted from the agglomerates by circulation of 125 mL of biooxidation solution through the column for 20 h. The biooxidation solution was produced by a culture of iron- and sulfuroxidizing microorganisms grown on pyrite. The solution had a pH 1.2, an Eh of 770 mV (SHE), and contained 60 g/L dissolved Fe. This treatment extracted 91% of thiocyanate in the tailings based on the removal of 595 mg/kg being 100% (Table 2). The column was drained and rinsed with 500 mL additional biooxidation solution. After draining, the column was inoculated with 100 mL of biooxidation solution along with 200 mL of MKM medium.
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30
900
25
800
20
700
15
600
10
500
5
400
Thiocyanate, mg/L
0
Eh, mV (SHE)
3.5.2.2. Biooxidation of “B” tailings agglomerated onto andesite. Although coarse ore is a potential support material for agglomeration with tailings, biooxidation of
sulfides in the ore would generate considerable additional acid and dissolved iron that must be treated prior to disposal. Agglomeration of tailings with inexpensive and abundant river rock located near the mine would reduce neutralization costs. Thiocyanate was removed from the agglomerates by recirculating aliquots of acidic ferric sulfate solution totaling 1.15 L through the columns over a time period of several days. The leachate was passed through 2.0 g activated carbon particles in small plastic columns (1.5 × 5 cm) to adsorb gold that may have been extracted by thiocyanate. About 11% of gold was lost from the tailings during thiocyanate extraction, similar to the results with tailings that were detoxified prior to shake flask tests. This gold, based on fire assay of the activated carbon, amounted to a loss of 0.52 to 0.54 ppm from the original 4.66 ppm. Following thiocyanate extraction, columns were inoculated with 100 mL of biooxidation solution along with 200 mL of MKM medium. Initially, leachates contained 8 to 10 mg/L thiocyanate. A lag period of about 10 days ensued before leachate redox potentials and dissolved iron concentrations began to increase indicating biooxidation of the sulfides. The solution Eh increased to 850 mV after 30 days and remained between 850 and 900 mV for the duration of the tests. Dissolved iron increased to about 40 g/L after 70 days and remained fairly stable for the remainder of the tests. Agglomerates in the columns were allowed to biooxidize at room temperature for 84 days (column 1) to 127 days (column 2). Following biooxidation, the entire content of the columns was shaken in water to separate the oxidized tailings from support rock. Tailings were
Thiocyanate extraction from the tailings was not quite complete; a concentration of 24 mg/L thiocyanate was present in the initial recirculating leach solution. This concentration was inhibitory to sulfide biooxidation. However, the concentration of thiocyanate declined slowly over the following weeks (Fig. 4). The initial solution redox potential slowly declined from 770 to about 635 mV where it stabilized for several days. After solution thiocyanate concentrations declined to near zero, the redox potential increased, reflecting microbial iron oxidation in the column (Fig. 4). The slow decline in thiocyanate concentrations is attributed to chemical oxidation of thiocyanate via an autoreduction reaction that occurs in solutions containing ferric ions (Barbosa-Filho and Monhemius, 1994b). Biodegradation of thiocyanate at low pH does not occur (Olson et al., 2003b). Two additional column tests were set up with a lower ratio of tailings to ore (1 : 10). Larger quantities of biooxidation solution (2.0 and 3.5 L) were used to extract thiocyanate. This resulted in lower concentrations of thiocyanate in starting leach solution and shorter lag periods before an increase in solution redox potential was observed. These column tests showed biooxidation solution could be used to remove thiocyanate from tailings agglomerated onto coarse ore. Additional test work is required to optimize the thiocyanate removal process in this configuration.
300
0
1
2
5
7
9 12 14 19 21 22 26 35 42 50 56
Days thiocyanate
Eh
Fig. 4. Correlation between solution Eh and thiocyanate concentrations in column with “B” tailings agglomerated on coarse ore.
G.J. Olson et al. / Hydrometallurgy 81 (2006) 159–166 Table 3 Sulfide-S biooxidation and gold recovery from “B” tailings agglomerated on river rock and biooxidized in small columns Column Duration (d)
S=–S oxidation Au extraction from oxidized (%) tailings (%)
1 2
53 62
84 127
66.5 76.0
Gold extraction from unoxidized tailings was 41.5%.
53% to 62% biooxidized and cyanide gold extraction was 66.5% to 76% (Table 3). These values of gold extraction versus sulfide biooxidation compare fairly closely with the shake flask data (Fig. 3). 4. Overview of a proposed process for treating Minacalpa tailings Steps in a process for treatment of Minacalpa tailings for gold recovery may first involve feed reclamation from existing tailings dams using high pressure water sprays consisting of biooxidation solution. The tailings slurry would be sent to an attrition tank and would be agitated with the biooxidation solution to remove thiocyanate. Dewatering would be used to separate the solution phase and the washed tailings. Because some gold would be dissolved during this process, the solution phase would be sent through carbon adsorption, followed by neutralization and either disposal or water recycle to the process. Biooxidation of detoxified tailings could be done in stirred tank reactors or in heaps. The latter would be performed by agglomeration of tailings onto support rock. In either case, regrinding of the tailings may be desirable to improve biooxidation kinetics. Following biooxidation, fines would be separated from biooxidation solution or support rock, neutralized with lime slurry, and transferred to the existing gold plant for leaching in cyanide. Biooxidation solution would be used to reclaim additional tailings. 5. Summary Minacalpa tailings are amenable to biooxidation if first treated to remove thiocyanate. Thiocyanate cannot be completely extracted with water or acid; ferric sulfate is required for near complete extraction and detoxification. Reagent ferric sulfate or biooxidation solutions both were effective in detoxifying tailings. Once detoxified, tailings were biooxidized, and exhibited a roughly 1 : 1 relationship between extent of sulfide oxidation and cyanide gold extraction. Nearly complete gold extraction (98% to 99%) was obtained from reground tailings bio-
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oxidized to near completion (95% to 98% sulfide-S oxidation). This approach may be useful in treating thiocyanate-contaminated refractory gold tailings at other mines as a detoxification step prior to biooxidation. However not all tailings behave as those at Minacalpa. Tailings samples from two other gold mines showed water extracted as much thiocyanate as ferric sulfate solution. It is believed the difference is due to the presence of copper, producing a water insoluble copper thiocyanate. Nonetheless, if a biooxidation process is contemplated for tailings containing thiocyanate, it is important to determine if a simple water or acid wash will remove all the thiocyanate or if additional thiocyanate is mobilized by biooxidation solutions. Acknowledgements We thank Doug Halbe for his valuable discussions and encouragement. This work was presented at the 6th International Gold Symposium in Lima, Peru in May, 2004 and is dedicated to the memory of Ernesto Calmet. References American Public Health Association, 1992. Standard Methods for the Examination of Water and Wastewater, 18th edition. APHA, Washington, DC. Barbosa-Filho, O., Monhemius, A.J., 1994a. Leaching of gold in thiocyanate solutions. Part 3: rates and mechanism of gold dissolution. Trans. Inst. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.) 103, C117–C125. Barbosa-Filho, O., Monhemius, A.J., 1994b. Leaching of gold in thiocyanate solutions. Part 2: redox processes in iron(III)–thiocyanate solutions. Trans. Inst. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.) 103, C111–C116. Cox, J.C., Nicholls, D.G., Ingledew, W.J., 1979. Transmembrane electrical potential and transmembrane pH gradient in the acidophile Thiobacillus ferrooxidans. Biochem. J. 178, 195–200. Hutchinson, M., Johnstone, K.I., White, D., 1965. The taxonomy of certain thiobacilli. J. Gen. Microbiol. 41, 357–366. Lawson, E.N., Taylor, J.L., Hulse, G.A., 1990. Biological pre-treatment for the recovery of gold from slimes dams. J. S. Afr. Inst. Min. Metall. 90, 45–49. Livesey-Goldblatt, E., 1986. Bacterial leaching of gold, uranium, pyrite bearing compacted mine tailing slimes. In: Lawrence, R.W., Branion, R.M.R., Ebner, H.G. (Eds.), Fundamental and Applied Biohydrometallurgy. Elsevier, Amsterdam, pp. 89–96. Miller, P.C., 1997. The design and operating practice of bacterial oxidation plant using moderate thermophiles (the BacTech process). In: Rawlings, D.E. (Ed.), Biomining. Springer Verlag, Berlin, pp. 81–102. Muhlbauer, R.M., Broadhurst, J.L., 1997. Biodegradation of thiocyanate and cyanide contained in biooxidation cyanidations tailings. IBS '97 Biomine 97. Australian Mineral Foundation, Glenside, South Australia, pp. CT3.1–CT3.6.
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Olson, G.J., Brierley, J.A., Brierley, C.L., 2003a. Bioleaching review part B: progress in bioleaching: applications of microbial processes by the minerals industries. Appl. Microbiol. Biotechnol. 63, 249–257. Olson, G.J., Clark, T.R., Mudder, T.I., Logsdon, M., 2003b. Thiocyanate inhibition of microbial sulfide oxidation for the control and prevention of acid rock drainage. Tailings and Mine Waste '03. Swets and Zeitlinger, Lisse, the Netherlands, pp. 403–411. Suzuki, I., Lee, D., Mackay, B., Harahuc, L., Oh, J.K., 1999. Effect of various ions, pH and osmotic pressure on oxidation of elemental sulphur by Thiobacillus thiooxidans. Appl. Environ. Microbiol. 65, 5163–5168. van Aswegen, P.C., Van Niekerk, J., 2004. New developments in the bacterial oxidation technology to enhance the efficiency of the
BIOX process. Proc. Bac-Min Conference, Bendigo, Victoria, Australia, Nov. 8–10, 2004, pp. 181–189. Wan, R.Y., Brierley, J.A., Acar, S., LeVier, K.M., 2003. Using thiocyanate as lixiviant for gold recovery in acidic environment. In: Young, A.A., Alfantazi, A.M., Anderson, C.G., Dreisinger, D.B., Marris, B., James, A. (Eds.), Hydrometallurgy 2003. Leaching and Solution Purification, vol. 1. The Minerals, Metals and Materials Society, Warrendale, PA, pp. 105–121. Whitlock, J.S., 1997. Biooxidation of refractory gold ores (the Geobiotics process). In: Rawlings, D.E. (Ed.), Biomining: Theory, Microbes and Industrial Processes. Springer-Verlag and Landes Bioscience, Berlin, pp. 117–127.
Biooxidation of thiocyanate-containing refractory gold tailings from Minacalpa, Peru Gregory J. Olson a,⁎, Corale L. Brierley b , Andrew P. Briggs c , Ernesto Calmet d a b
Little Bear Laboratories, Inc., 5906 McIntyre St., Bldg. 2, Golden, CO 80403, USA Brierley Consultancy LLC, P.O. Box 260012, Highlands Ranch, CO 80126-0012, USA c Fluor Chile, S.A., Reyes Lavalle 3340—7th Floor, Los Condes, Santiago, Chile d Minera Aurifera Calpa, S.A., Av. Arequipa 330 OF 101, Lima 1, Peru
Received 28 June 2005; received in revised form 12 October 2005; accepted 23 November 2005
Abstract Refractory, sulfidic tailings from gold mining operations may be biooxidized to increase the amount of gold recoverable by cyanide extraction. Tailings containing thiocyanate must be pretreated to remove thiocyanate prior to biooxidation due to its toxicity to microorganisms that oxidize sulfide minerals. Thiocyanate in tailings at Minacalpa, Peru, was only partially extractable with water or with dilute sulfuric acid. The thiocyanate also was not completely biodegradable at neutral pH. Consequently, tailings remained inhibitory toward biooxidation after these treatments, releasing thiocyanate into bioleach solutions. The non-extractable form of thiocyanate in Minacalpa tailings was probably copper thiocyanate. A novel approach was developed to remove this thiocyanate using ferric sulfate solution. Tailings were detoxified by washing with either reagent ferric sulfate or solutions derived from biooxidation of pyrite. Detoxified tailings, i.e., those thoroughly washed with ferric sulfate solution, were biooxidized, exhibiting a fairly linear relationship between sulfide-S oxidation and cyanide gold recovery. A process for biooxidation of Minacalpa tailings would involve an initial washing step with biooxidation solutions followed by stirred tank biooxidation or agglomeration of detoxified tailings onto support rock and biooxidation in heaps. © 2006 Elsevier B.V. All rights reserved. Keywords: Biooxidation of tailings; Refractory gold; Thiocyanate
1. Introduction Biooxidation of refractory gold ores and concentrates is practiced commercially in stirred tank reactors and in constructed heaps (Olson et al., 2003a; van Aswegen and Van Niekerk, 2004). There also has been interest in using biooxidation pretreatment for recovery of gold from refractory, sulfidic mill tailings discarded in tailings ponds. Such tailings may still contain significant gold values that could be made amenable to cyanide extraction fol⁎ Corresponding author. Tel.: +1 303 273 5697; fax: +1 303 273 0494. E-mail address: [email protected] (G.J. Olson). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2005.11.003
lowing biooxidation (Livesey-Goldblatt, 1986; Lawson et al., 1990). Minera Aurifera Calpa S.A. (Minacalpa) operates an underground gold mine and treatment plant in the Department of Arequipa in southern Peru. The ore contains sulfides or mixed oxides and sulfides. Processing comprises crushing, ball milling, flotation, discard of flotation tailings, regrinding of concentrates, and cyanide leaching of the reground concentrates for gold recovery. The plant is currently (June, 2005) operating at 30% capacity treating 250 to 300 t/d of ore grading 4.80 to 5.25 g/t gold, giving a concentrate treatment rate of 80 to 90 t/d at a grade of 18.25 to 33 g/t gold.
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Tailings from the Minacalpa gold plant contain appreciable concentrations of refractory gold. These tailings have been deposited in dams over several years and now form a resource that may be economically exploited. Included at this site are 1.2 million tons of old flotation tails assaying 3.2 g/t of gold and 380,000 t of concentrate leach tails assaying 7.3 g/t of gold. This amounts to 205,000 ounces of contained gold. Gold in the tailings is intimately associated with pyrite and is highly refractory to cyanidation. Gold extraction from unoxidized material is only about 40%. Biooxidation is a potential treatment option for these tailings for oxidizing pyrite and making the gold available for subsequent extraction with cyanide. Because the material has been previously cyanided, significant thiocyanate contamination is present and must be removed prior to biooxidation. Thiocyanate is formed by the reaction of reduced sulfur compounds with cyanide and is a primary mechanism of cyanide consumption during cyanidation of sulfidic gold ores (van Aswegen and Van Niekerk, 2004). Although microorganisms that oxidize refractory gold ores are robust, growing under highly acidic conditions (pH b 2.0) and in the presence of high concentrations of dissolved metals, they are extremely sensitive to the thiocyanate anion, SCN−. A thiocyanate concentration of a few parts-permillion (mg/L) in solution inhibits these microorganisms (Cox et al., 1979; Suzuki et al., 1999). Thiocyanate contamination of process water has been reported to result in problems with bacterial activity at stirred tank biooxidation plants (Miller, 1997). However, thiocyanate is readily biodegraded by certain bacteria at neutral pH under aerobic conditions. For example, Gold Fields' ASTER™ process uses biodegradation to remove thiocyanate from process and waste solutions (Muhlbauer and Broadhurst, 1997; van Aswegen and Van Niekerk, 2004). This paper describes the occurrence of thiocyanate in Minacalpa tailings in a form not fully removed by washing with water or with dilute sulfuric acid or by biodegradation. This laboratory study was done to determine how thiocyanate may be removed from Minacalpa tailings and the amenability of the “detoxified” tailings to biooxidation. Preliminary engineering work was undertaken to develop a conceptual process design.
gray-colored and contained 21.9% sulfide-S and 4.95 ppm gold. Most of the work was done with a blend of concentrate leach tails and flotation leach tails, referred to as “B” tailings (Table 1). The particle size of “B” tailings was 4.2% N 425 μm; 20.0% 425 × 150 μm; 21.2% 150 × 75 μm; 10.2% 53 × 75 μm and 44.4% b 53 μm. Pyrite was the dominant sulfide mineral in the tailings. 2.2. Thiocyanate extraction from tailings Ten g of tailings were added to 50 mL plastic centrifuge tubes along with various amounts of deionized water, 1 N sulfuric acid, or 0.2 M ferric sulfate in 1 N sulfuric acid. Tubes were gently shaken at room temperature (20 to 25 °C) for periods of time ranging from 1 min to 3 h. After shaking the tubes were centrifuged at 2000 r.p.m. for 5 min. Thiocyanate in the supernatant solution was analyzed colorimetrically with ferric nitrate reagent (American Public Health Association, 1992). Samples were generally diluted significantly prior to analysis due to the high extinction coefficient of the Fe(III)– SCN complexes. Although extraction solutions containing ferric sulfate spontaneously formed the red color characteristic of Fe(III)–SCN complexes, addition of the ferric nitrate–nitric acid reagent to the diluted samples greatly intensified the red color. Addition of the reagent imparted the oxidizing conditions and high ferric concentrations resulting in the most efficient detection of thiocyanate. 2.3. Biodegradation of thiocyanate in tailings A bacterial isolate tentatively identified as Thiobacillus thioparus was maintained in a neutral pH culture medium with thiocyanate as a growth substrate (S7 medium, Hutchinson et al., 1965). The organism was inoculated into slurries of “A” or “B” tailings in S0 salts solution (S7 medium less thiocyanate) in flasks shaken at 180 r.p.m. at room temperature. Tray tests of thiocyanate biodegradation were performed by adding the culture (106 cells/g, final concentration) to 2.0 kg “A” tailings brought to near saturation with 0.6 mM phosphate buffer in a plastic tray (15 × 30 × 9 cm deep).
2. Experimental
Table 1 Analyses of Minacalpa “B” tailings before and after ferric sulfate extraction
2.1. Tailings samples
Sample
Minacalpa provided two samples of dry tailings. Concentrate leach tails, referred to as “A” tailings, were
“B” tailings 13.8 0.17 0.07 11.4 Detoxified “B” tailings 14.2 0.069 0.06 13.0
Fe (%)
Cu (%)
As (%)
S=–S Total S Au (%) (%) (ppm) 13.3 14.4
4.66 4.11
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Uninoculated, thymol-treated controls were employed in both tests.
Table 2 Thiocyanate extraction from Minacalpa tailings by various solutions Tailings
SCN by water extraction (mg/kg)
SCN by 1 N H2SO4 extraction (mg/kg)
SCN by 0.2 M ferric sulfate extraction (mg/kg)
“A” “B”
1020 198
1765 381
2064 595
2.4. Shake flask biooxidation of detoxified tailings Biooxidation of detoxified tailings was done in 15 wt.% slurries in MKM culture medium in Erlenmeyer flasks. MKM medium contained 0.4 g (NH4)2SO4, 0.4 g MgSO4 heptahydrate, and 0.04 g KH2PO4 in 1000 mL deionized water adjusted to pH 1.8 with sulfuric acid. Flasks were inoculated with an active culture of mixed mesophilic microorganisms (grown on pyrite in MKM medium) and shaken at 180 r.p.m. at room temperature. A control flask was not inoculated and was treated with thymol in methanol to prevent microbial growth. At intervals, flasks were sacrificed and the solids recovered for sulfide-S analysis and cyanidation. 2.5. Biooxidation of tailings in columns Column tests were done with “B” tailings (400 g) agglomerated with 1) 100 mL of 5 N sulfuric acid and 2000 g coarse Minacalpa ore (6.35 × 12.7 mm), or 2) 100 mL biooxidation solution (18 g/L Fe, pH 1.45) and 2000 g andesite river rock (6.35 × 19.1 mm) taken from near the mine area. Agglomerates were loaded in 10 cm diameter columns. Thiocyanate was extracted from the agglomerates by recirculating biooxidation solutions through the columns for several days at room temperature. Following detoxification (described in results) columns were leached in recycle fashion at a rate of 12 L/h/m2. 2.6. Gold extraction Gold extraction from recovered tailings was carried out for 24 h at 40% solids in agitated Erlenmeyer flasks at room temperature. NaCN concentrations were maintained at 1 to 2 g/L and the pH was maintained at 10.5 to 11.0 with lime. 3. Results 3.1. Thiocyanate extraction from tailings The measured thiocyanate concentration of the tailings depended on the method of extraction (Table 2). More thiocyanate was extracted with 1 N sulfuric acid than with water (1 h shaking). Ferric sulfate in 1 N sulfuric acid extracted more thiocyanate than 1 N sulfuric acid alone. Little or no additional thiocyanate was extracted if samples were shaken for 3 h rather than 1 h. Additional tests with “B” tailings showed similar amounts of thio-
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cyanate were extracted over a range of ferric sulfate solution to solids ratios ranging from 1 : 1 to 50 : 1. The concentration of sulfuric acid in the ferric sulfate solution (0.01 to 1.0 N) did not affect the extent of thiocyanate extraction. These results indicate washing with water or dilute acid would be inadequate to detoxify these tailings; biooxidation solution, essentially acidic ferric sulfate solution, would dissolve additional thiocyanate and poison the microorganisms. This was confirmed by extraction tests showing that tailings could be detoxified after extensive washing with 0.2 M ferric sulfate in 0.1 N sulfuric acid. The washed tailings were added to MKM medium (5% w/v), inoculated with pyrite-oxidizing bacteria, and supported subsequent microbial growth and pyrite oxidation. Tailings washed less extensively or with only water or sulfuric acid did not support biooxidation. In these latter cases, thiocyanate was detected in solution at concentrations (3.6 to 13.5 mg/L) sufficient to inhibit biooxidation. 3.2. Form of thiocyanate in Minacalpa tailings The water-insoluble form of thiocyanate in Minacalpa tailings was not conclusively identified but is probably copper thiocyanate (CuSCN). This assumption is supported by several observations. Firstly, reagent CuSCN was virtually insoluble in 0.1% w/v suspensions prepared in deionized water or in 0.02 N sulfuric acid. Conversely, CuSCN was completely soluble in ferric sulfate solution. These results, together with the thiocyanate solubility characteristics of the tailings (Table 2), suggest 50% to 67% of thiocyanate may occur as CuSCN in “A” tailings and “B” tailings, respectively. The remaining thiocyanate is a water soluble form. Secondly, dissolved copper and thiocyanate in a ferric sulfate extract (0.2 M ferric sulfate in 0.1 N sulfuric acid) of “B” tailings (tailings contained 0.17% Cu) were precipitated when the extract was brought to a pH of 4 to 5 with lime. The dried orange-colored precipitate consisted of 70% gypsum and 30% amorphous solids as determined by Xray diffraction and contained significant concentrations of Fe, S, Ca, along with 0.22% copper, as determined by X-ray fluorescence. Thiocyanate was extracted from the
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precipitate similarly to the tailings: the highest amount of thiocyanate (747 mg/kg) was extracted by ferric sulfate in sulfuric acid. Water extracted only 39% of this amount and dilute sulfuric acid extracted 72% of this amount. Thirdly, thiocyanate did not precipitate significantly when a solution containing 0.2 M ferric sulfate and 0.01 M KSCN was treated with lime, but over 80% of thiocyanate was precipitated with lime from a solution containing 0.1 M ferric sulfate and 0.01 M CuSCN. Thus, neutralization of ferric sulfate extracts containing Cu and SCN resulted in precipitation of iron, copper and thiocyanate.
no evidence of biodegradation; thiocyanate concentrations in the tailings determined by ferric sulfate extraction remained unchanged. It is possible that poor oxygen transfer into the wet, paste-like tailings retarded aerobic thiocyanate biodegradation. Results of the biodegradation tests indicate neither slurry nor in-place biodegradation processes are feasible for completely removing thiocyanate from Minacalpa tailings. The water insoluble form of thiocyanate was not biodegraded at neutral pH and was subsequently mobilized when the tailings were placed under conditions for the growth of pyrite-oxidizing microorganisms.
3.3. Biodegradation of thiocyanate in tailings
3.4. Batch detoxification of thiocyanate from tailings by ferric sulfate extraction
Thiocyanate Concentration, mg/L
Thiocyanate was biodegraded in slurries of “A” tailings at pH 6.5 in shake flasks inoculated with a culture of neutral pH thiocyanate-degrading bacteria (Fig. 1). Uninoculated control flasks showed no thiocyanate degradation. Similar results were obtained with “B” tailings. However, the solids recovered from these tests after all measurable thiocyanate in solution had disappeared still contained 385 mg/kg (“B”) to 838 mg/kg (“A”) thiocyanate as determined by ferric sulfate extraction. These tailings were not able to be biooxidized in a 20% slurry at pH 2.0 inoculated with pyrite-oxidizing microorganisms. In summary, water soluble thiocyanate leached from the tailings was biodegradable at neutral pH but the water-insoluble form of thiocyanate in the tailings was not biodegraded and subsequently prevented biooxidation of sulfides in the tailings. Tests also were performed with wet tailings in trays to determine if in-place biodegradation of thiocyanate in tailings heaps may be feasible. After 33 days there was
“B” tailings (600 g) were washed twice with reagent ferric sulfate solution (0.2 M ferric sulfate in 0.1 N sulfuric acid) to reduce the thiocyanate concentration in the tailings from 641 to 5.3 mg/kg. Recovery of tailings was 563 g representing 6.2% weight loss. The washed tailings contained significantly less copper than the starting material (Table 1). This copper was accounted for (94%) in the pooled extraction solutions. The gold content of the washed tailings was also somewhat lower, perhaps due to partial extraction by the oxidizing thiocyanate solution which may form soluble complexes with gold (Barbosa-Filho and Monhemius, 1994a; Wan et al., 2003). The sulfur content of the washed tailings was somewhat higher. This may be due to analytical error or the sulfur may have become concentrated if weight loss from washing was due to extraction of nonsulfur containing material. These detoxified tailings were used in shake flask biooxidation tests described below.
250 200
150
100
50
0 0
3
6
9
12
15
18
Days cells 5% solids
cells 22% solids
control 5% solids
control 22% solids
Fig. 1. Biodegradation of the water soluble fraction of thiocyanate in slurries of Minacalpa “A” tailings at 5% solids and at 22% solids at pH 6.5.
G.J. Olson et al. / Hydrometallurgy 81 (2006) 159–166
163
100
3.5. Biooxidation and gold extraction from detoxified tailings
90
3.5.1. Shake flask tests These tests were done to establish the relationship between sulfide biooxidation and cyanide gold extraction in Minacalpa tailings. There was a lag of about 20 days before significant microbial growth and iron oxidation began in shake flasks containing the detoxified tailings. This lag is attributed to the small amount of residual thiocyanate present in the tailings. Thereafter, sulfide-S biooxidation proceeded at about 1.1%/d (Fig. 2). The lag time may have been shortened by another ferric sulfate wash of the tailings. Gold extraction was about 40% from unoxidized tailings and increased to a maximum of 87% after 85% sulfide-S biooxidation (Fig. 3). This extent of biooxidation took a total of 107 days (Fig. 2). Only 4.2% sulfide oxidation had occurred in the uninoculated control flask after 107 days. The slow rate of biooxidation was due to the comparatively coarse particle size of the tailings (24.2% N 150 μm). Tailings that were ground to 98% b 53 μm were biooxidized nearly twice as fast and more extensively in duplicate flasks. Nearly all sulfide-S was removed in these tests (95% to 98%) after 78 to 87 days of biooxidation at 20 to 25 °C, resulting in 98% to 99% gold extraction (Fig. 3). A limited number of shake flask biooxidation tests also were done with detoxified “A” tailings. The results were similar to “B” tailings, showing a fairly linear relationship between sulfide-S biooxidation and cyanide gold extraction (Fig. 3).
100
Sulfide-S Biooxidized, %
90 80 70 60 50 40 30 20 10 control 0 0
20
40
60
80
100
120
Days of Biooxidation Fig. 2. Rate of sulfide biooxidation of detoxified Minacalpa “B” tailings in shake flask slurries (15 wt.%).
Au Extraction, %
80 70 60 50 40 control
30 20 10 0 0
10
20
30
40
50
60
70
80
90
100
Sulfide-S Biooxidized, % "B"
"A"
reground B
columns
Fig. 3. Relationship between sulfide-S biooxidation and cyanide gold extraction from “B” tailings, “A” tailings, and reground “B” tailings (the two highest gold extraction data points) biooxidized in shake flasks and from “B” tailings biooxidized in columns.
The results of the shake flask tests indicate that detoxified Minacalpa tailings can be nearly completely biooxidized resulting in nearly complete liberation of gold. Biooxidation time could be shortened by regrinding the tailings and by operation at higher temperature. Additional tests with continuous stirred tank reactors (CSTRs) are required in order to more accurately predict biooxidation kinetics by such a process. 3.5.2. Column tests Minacalpa tailings also might be biooxidized in a heap configuration following agglomeration of tailings onto coarse ore or support rock (Whitlock, 1997). In this case, tailings might be detoxified by rinsing the stacked heap with biooxidation solutions. Two possible support substrates were evaluated: 1) coarse ore and 2) local river rock (andesite). 3.5.2.1. Biooxidation of “B” tailings agglomerated onto coarse ore. Thiocyanate was extracted from the agglomerates by circulation of 125 mL of biooxidation solution through the column for 20 h. The biooxidation solution was produced by a culture of iron- and sulfuroxidizing microorganisms grown on pyrite. The solution had a pH 1.2, an Eh of 770 mV (SHE), and contained 60 g/L dissolved Fe. This treatment extracted 91% of thiocyanate in the tailings based on the removal of 595 mg/kg being 100% (Table 2). The column was drained and rinsed with 500 mL additional biooxidation solution. After draining, the column was inoculated with 100 mL of biooxidation solution along with 200 mL of MKM medium.
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30
900
25
800
20
700
15
600
10
500
5
400
Thiocyanate, mg/L
0
Eh, mV (SHE)
3.5.2.2. Biooxidation of “B” tailings agglomerated onto andesite. Although coarse ore is a potential support material for agglomeration with tailings, biooxidation of
sulfides in the ore would generate considerable additional acid and dissolved iron that must be treated prior to disposal. Agglomeration of tailings with inexpensive and abundant river rock located near the mine would reduce neutralization costs. Thiocyanate was removed from the agglomerates by recirculating aliquots of acidic ferric sulfate solution totaling 1.15 L through the columns over a time period of several days. The leachate was passed through 2.0 g activated carbon particles in small plastic columns (1.5 × 5 cm) to adsorb gold that may have been extracted by thiocyanate. About 11% of gold was lost from the tailings during thiocyanate extraction, similar to the results with tailings that were detoxified prior to shake flask tests. This gold, based on fire assay of the activated carbon, amounted to a loss of 0.52 to 0.54 ppm from the original 4.66 ppm. Following thiocyanate extraction, columns were inoculated with 100 mL of biooxidation solution along with 200 mL of MKM medium. Initially, leachates contained 8 to 10 mg/L thiocyanate. A lag period of about 10 days ensued before leachate redox potentials and dissolved iron concentrations began to increase indicating biooxidation of the sulfides. The solution Eh increased to 850 mV after 30 days and remained between 850 and 900 mV for the duration of the tests. Dissolved iron increased to about 40 g/L after 70 days and remained fairly stable for the remainder of the tests. Agglomerates in the columns were allowed to biooxidize at room temperature for 84 days (column 1) to 127 days (column 2). Following biooxidation, the entire content of the columns was shaken in water to separate the oxidized tailings from support rock. Tailings were
Thiocyanate extraction from the tailings was not quite complete; a concentration of 24 mg/L thiocyanate was present in the initial recirculating leach solution. This concentration was inhibitory to sulfide biooxidation. However, the concentration of thiocyanate declined slowly over the following weeks (Fig. 4). The initial solution redox potential slowly declined from 770 to about 635 mV where it stabilized for several days. After solution thiocyanate concentrations declined to near zero, the redox potential increased, reflecting microbial iron oxidation in the column (Fig. 4). The slow decline in thiocyanate concentrations is attributed to chemical oxidation of thiocyanate via an autoreduction reaction that occurs in solutions containing ferric ions (Barbosa-Filho and Monhemius, 1994b). Biodegradation of thiocyanate at low pH does not occur (Olson et al., 2003b). Two additional column tests were set up with a lower ratio of tailings to ore (1 : 10). Larger quantities of biooxidation solution (2.0 and 3.5 L) were used to extract thiocyanate. This resulted in lower concentrations of thiocyanate in starting leach solution and shorter lag periods before an increase in solution redox potential was observed. These column tests showed biooxidation solution could be used to remove thiocyanate from tailings agglomerated onto coarse ore. Additional test work is required to optimize the thiocyanate removal process in this configuration.
300
0
1
2
5
7
9 12 14 19 21 22 26 35 42 50 56
Days thiocyanate
Eh
Fig. 4. Correlation between solution Eh and thiocyanate concentrations in column with “B” tailings agglomerated on coarse ore.
G.J. Olson et al. / Hydrometallurgy 81 (2006) 159–166 Table 3 Sulfide-S biooxidation and gold recovery from “B” tailings agglomerated on river rock and biooxidized in small columns Column Duration (d)
S=–S oxidation Au extraction from oxidized (%) tailings (%)
1 2
53 62
84 127
66.5 76.0
Gold extraction from unoxidized tailings was 41.5%.
53% to 62% biooxidized and cyanide gold extraction was 66.5% to 76% (Table 3). These values of gold extraction versus sulfide biooxidation compare fairly closely with the shake flask data (Fig. 3). 4. Overview of a proposed process for treating Minacalpa tailings Steps in a process for treatment of Minacalpa tailings for gold recovery may first involve feed reclamation from existing tailings dams using high pressure water sprays consisting of biooxidation solution. The tailings slurry would be sent to an attrition tank and would be agitated with the biooxidation solution to remove thiocyanate. Dewatering would be used to separate the solution phase and the washed tailings. Because some gold would be dissolved during this process, the solution phase would be sent through carbon adsorption, followed by neutralization and either disposal or water recycle to the process. Biooxidation of detoxified tailings could be done in stirred tank reactors or in heaps. The latter would be performed by agglomeration of tailings onto support rock. In either case, regrinding of the tailings may be desirable to improve biooxidation kinetics. Following biooxidation, fines would be separated from biooxidation solution or support rock, neutralized with lime slurry, and transferred to the existing gold plant for leaching in cyanide. Biooxidation solution would be used to reclaim additional tailings. 5. Summary Minacalpa tailings are amenable to biooxidation if first treated to remove thiocyanate. Thiocyanate cannot be completely extracted with water or acid; ferric sulfate is required for near complete extraction and detoxification. Reagent ferric sulfate or biooxidation solutions both were effective in detoxifying tailings. Once detoxified, tailings were biooxidized, and exhibited a roughly 1 : 1 relationship between extent of sulfide oxidation and cyanide gold extraction. Nearly complete gold extraction (98% to 99%) was obtained from reground tailings bio-
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oxidized to near completion (95% to 98% sulfide-S oxidation). This approach may be useful in treating thiocyanate-contaminated refractory gold tailings at other mines as a detoxification step prior to biooxidation. However not all tailings behave as those at Minacalpa. Tailings samples from two other gold mines showed water extracted as much thiocyanate as ferric sulfate solution. It is believed the difference is due to the presence of copper, producing a water insoluble copper thiocyanate. Nonetheless, if a biooxidation process is contemplated for tailings containing thiocyanate, it is important to determine if a simple water or acid wash will remove all the thiocyanate or if additional thiocyanate is mobilized by biooxidation solutions. Acknowledgements We thank Doug Halbe for his valuable discussions and encouragement. This work was presented at the 6th International Gold Symposium in Lima, Peru in May, 2004 and is dedicated to the memory of Ernesto Calmet. References American Public Health Association, 1992. Standard Methods for the Examination of Water and Wastewater, 18th edition. APHA, Washington, DC. Barbosa-Filho, O., Monhemius, A.J., 1994a. Leaching of gold in thiocyanate solutions. Part 3: rates and mechanism of gold dissolution. Trans. Inst. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.) 103, C117–C125. Barbosa-Filho, O., Monhemius, A.J., 1994b. Leaching of gold in thiocyanate solutions. Part 2: redox processes in iron(III)–thiocyanate solutions. Trans. Inst. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.) 103, C111–C116. Cox, J.C., Nicholls, D.G., Ingledew, W.J., 1979. Transmembrane electrical potential and transmembrane pH gradient in the acidophile Thiobacillus ferrooxidans. Biochem. J. 178, 195–200. Hutchinson, M., Johnstone, K.I., White, D., 1965. The taxonomy of certain thiobacilli. J. Gen. Microbiol. 41, 357–366. Lawson, E.N., Taylor, J.L., Hulse, G.A., 1990. Biological pre-treatment for the recovery of gold from slimes dams. J. S. Afr. Inst. Min. Metall. 90, 45–49. Livesey-Goldblatt, E., 1986. Bacterial leaching of gold, uranium, pyrite bearing compacted mine tailing slimes. In: Lawrence, R.W., Branion, R.M.R., Ebner, H.G. (Eds.), Fundamental and Applied Biohydrometallurgy. Elsevier, Amsterdam, pp. 89–96. Miller, P.C., 1997. The design and operating practice of bacterial oxidation plant using moderate thermophiles (the BacTech process). In: Rawlings, D.E. (Ed.), Biomining. Springer Verlag, Berlin, pp. 81–102. Muhlbauer, R.M., Broadhurst, J.L., 1997. Biodegradation of thiocyanate and cyanide contained in biooxidation cyanidations tailings. IBS '97 Biomine 97. Australian Mineral Foundation, Glenside, South Australia, pp. CT3.1–CT3.6.
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BIOX process. Proc. Bac-Min Conference, Bendigo, Victoria, Australia, Nov. 8–10, 2004, pp. 181–189. Wan, R.Y., Brierley, J.A., Acar, S., LeVier, K.M., 2003. Using thiocyanate as lixiviant for gold recovery in acidic environment. In: Young, A.A., Alfantazi, A.M., Anderson, C.G., Dreisinger, D.B., Marris, B., James, A. (Eds.), Hydrometallurgy 2003. Leaching and Solution Purification, vol. 1. The Minerals, Metals and Materials Society, Warrendale, PA, pp. 105–121. Whitlock, J.S., 1997. Biooxidation of refractory gold ores (the Geobiotics process). In: Rawlings, D.E. (Ed.), Biomining: Theory, Microbes and Industrial Processes. Springer-Verlag and Landes Bioscience, Berlin, pp. 117–127.