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Nitric acid leaching of silver sulphide precipitates
Hydrometallurgy 74 (2004) 213 – 220 www.elsevier.com/locate/hydromet
Nitric acid leaching of silver sulphide precipitates P.C. Holloway a,*, K.P. Merriam b, T.H. Etsell a a
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 b Hudson Bay Mining and Smelting Co., P.O. Box 1500, Flin Flon, Manitoba, Canada R8A 1S8 Received 23 June 2003; received in revised form 8 April 2004; accepted 5 May 2004
Abstract Leaching of silver sulphide precipitates with nitric acid was performed at a variety of temperatures, pressures, acid concentrations and solids concentrations, with a maximum extraction of 96.1% achieved by leaching at 150jC, 1100 kPa(g), and 9.6% solids with twice the stoichiometric amount of nitric acid. Further improvement in the silver extractions would be expected with further optimization of the leaching conditions and this process could represent a hydrometallurgical alternative for the recovery of silver from silver sulphide precipitated from photographic solutions. Hydrogen reduction from nitric acid leach solutions resulted in near quantitative precipitation of silver (99.8%) in 1 h at 150jC and 4000 kPa(g) hydrogen pressure. High precipitation efficiencies with hydrogen reduction were achieved even at silver concentrations well in excess of the solubility limit for silver sulphate at room temperature. High purity silver with a particle size of 96.2% passing 106 Am was produced. D 2004 Elsevier B.V. All rights reserved. Keywords: Silver sulphide; Nitric acid; Pressure leaching; Secondary recovery; Hydrogen reduction; Silver; Photographic solutions
1. Introduction Silver sulphide (Ag2S) is a very stable silver compound, which is not readily dissolved using mineral acids under atmospheric conditions. Because of its low solubility, several processes for the recovery of silver from photographic solutions use sulphidizing agents, such as sodium sulphide (Na2S) or sodium hydrosulphide (NaHS), to precipitate silver have been proposed (Kunda and Etsell, 1988, 1991, 1992, 1996). Pyrometallurgical methods have also been described for producing silver from silver sulphide (Kunda, 1981, 1983), but these methods produce sponge silver
* Corresponding author. Fax: +1-780-492-2881. E-mail address: [email protected] (P.C. Holloway). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.05.003
as the product, which would have to be remelted or reformed before it could be reused. The majority of the work done on nitric acid leaching of sulphide materials has focused on the treatment of refractory (i.e., pyrite, arsenopyrite or chalcopyrite) ores containing precious metals. Several processes have been reported in the literature for nitric or nitrous acid leaching of these complex sulphide ores (Daugherty et al., 1973; Kennecott, 1979; Kunda, 1982, 1984; Posel, 1972, 1976a,b; Raudsepp and Beattie, 1989) but, to date, the direct leaching of silver sulphide with nitric acid has not been documented in the literature. Therefore, the objective of this research was to determine whether metallic silver could be produced hydrometallurgically from silver sulphide precipitated from photographic solutions, by pressure leaching with nitric acid to dissolve the silver followed by
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hydrogen reduction to produce a high purity silver powder from the nitric acid leach solutions.
2. Experimental
analyzed gravimetrically for silver at the University of Alberta and were analyzed for sulphur with LECO combustion by International Plasma Laboratory, of Vancouver, BC, Canada. Metallic silver produced in the hydrogen reduction tests was dissolved in nitric acid and analyzed with atomic absorption.
2.1. Feed materials 2.3. Leaching tests Silver sulphide was generated by precipitation either from solution prepared from reagent grade AgNO3 (Tests 1 to 7), or from solutions generated by dissolving high purity silver (99.9999%) in dilute nitric acid (Test 8). Technical grade Na2S was used as the precipitating agent. The black precipitate formed was filtered, washed and dried overnight at 80jC. The two batches of silver sulphide produced analyzed 77.5% and 75.2% Ag and 12.1% and 14.7% S, respectively. Silver sulphide (Ag2S) was identified in the feeds by X-ray diffraction (XRD) with all major peaks identified (Fig. 1). Based on the chemical analysis of the feed and X-ray diffraction of residue samples, elemental sulphur (Sj) is likely also present in the feed in minor quantities below the XRD detection limit. 2.2. Analysis Solutions were analyzed for silver using atomic absorption spectroscopy (AA). Leach residues were
Leaching and hydrogen reduction tests were performed in a stainless steel Parr autoclave (250 mL) fitted with a Teflon liner and single axial agitator. In the leaching tests, the solids and solution were charged to the autoclave, heated over 0.5 to 1 h to the desired temperature and allowed to react for 1 h at temperature. As nitric acid fumes at the reaction temperatures tested, offgases from the autoclave were constantly vented to maintain the desired operating pressure (1100 or 1800 kPa(g)). After 1 h, the autoclave was cooled to 95 to 100jC and allowed to react for 15 min to allow any elemental sulphur in the leach slurry to solidify. The autoclave was then cooled to 60 to 70jC. The slurry was then filtered and the solids washed with hot water to remove any entrained leach solution from the solids and allowed to dry in an 80jC oven overnight. The leach solution and wash water were combined and collected.
Fig. 1. X-ray diffraction analyses of batch 2 feed sample.
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215
Table 1 Summary of conditions and results of nitric acid leaching tests Test number Conditions Feed solids batch HNO3 concentration, g/L Temperature, jC Pressure, kPa(g) Solids concentration, % Solids weight g Initial Final Residue analysis, % Ag S Extraction, % Ag S Ag in leach solution, g/La a
1
2
3
4
5
6
7
8
1 213 125 1100 17.3
1 426 125 1100 18.0
1 426 125 1100 14.1
1 426 125 1800 14.1
1 426 150 1100 14.1
1 426 175 1100 14.1
1 426 150 1800 14.1
2 284 150 1100 9.6
20.0 15.7
20.0 12.4
15.0 5.7
15.0 4.4
15.0 5.0
15.0 4.4
15.0 3.6
10.0 0.5
75.1 11.8
68.3 13.0
65.5 16.3
66.7 13.0
68.0 11.2
69.1 10.3
68.7 10.8
63.7 15.2
24.2 24.2 38
45.6 33.9 71
67.7 48.7 79
74.8 68.7 87
70.8 69.4 82
74.0 75.3 86
78.9 78.9 92
96.1 93.7 73
Calculated based on silver extractions and the solids concentration used in leaching.
A white precipitate formed during the cooling of the leach solution, indicating supersaturation of the leach solution with silver sulphate (Ag2SO4); as a result, solids analyses and weights were used for extraction calculations.
temperature, pressure and solids concentration. In all cases, nitric acid was used as the sole oxidant for oxidizing the silver sulphide. Table 1 shows the results of these tests. 3.2. Hydrogen reduction tests
2.4. Hydrogen reduction tests In the hydrogen reduction tests, the pH of the leach/wash solution from the specific leach tests was adjusted to between 5.5 and 6.0 using ammonium hydroxide. This solution, plus any solids that had precipitated from the leach solution, was added to the autoclave. Prior to heating, the autoclave was pressurized with hydrogen and vented three times to try to remove oxygen from the autoclave. The autoclave was then pressurized to the desired pressure and heated to the desired temperature and allowed to react for 1 h. The autoclave was then cooled and its contents filtered. The solution was collected and analyzed for residual silver concentration.
3. Results and analyses 3.1. Leaching Several different variables were investigated in the leaching tests, including nitric acid concentration,
Two preliminary hydrogen reduction tests were completed to determine whether hydrogen reduction of the nitric acid leach solution was feasible. Table 2 summarizes the results of these tests. (Solutions from Test 5 and Test 4 were used in Tests I and II, respectively.). The particle size of the silver powder produced in Test I was analyzed and the powder was 96.2% passing 106 Am (140 mesh).
Table 2 Hydrogen reduction test conditions and results
Temperature, jC Pressure, kPa(g) Time, h Silver weight, g In Out Silver analysis, % Precipitation efficiency, %
Test I
Test II
150 3800 to 4150 1
150 1700 to 1800 1
6.84 0.02 >99.8 99.8
7.67 1.39 97.4 81.9
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4. Discussion 4.1. Effect of acid addition In Tests 1 to 3, the acid additions used correspond to 75%, 150% and 200% of the amount required stoichiometrically based on the following reaction. 3 Ag2 S þ 8 HNO3 R3 Ag2 SO4 þ 8 NO þ 4 H2 O Fig. 2 shows the effect of acid concentration on the silver and sulphur extractions. The silver extraction increased close to linearly from 24% to 68% as the acid concentration increased. As a result, an acid addition of 204% of stoichiometric was used for the remaining tests. 4.2. Effect of temperature and pressure In Tests 3, 5 and 6, the temperature was varied from 125 to 175jC with the pressure kept at 1100 kPa(g). As shown in Fig. 3, each increase of 25jC resulted in an increase in the silver extraction of 3% to 4%, with a maximum extraction of 74% at 175jC. Pressure had a slightly more pronounced effect on the silver extraction than temperature (Fig. 4), as an increase of 600 kPa in the total pressure increased the silver extraction by 7% to 8% at both 125 and 150jC.
However, even at the best conditions in Tests 1 to 7, the extractions indicate that 20 to 30% of the silver sulphide remained unleached. Since silver sulphate (Ag2SO4) precipitate formed on cooling of the leach solutions and similar crystals were present in the leach solids, it was proposed that the precipitation of silver sulphate during leaching prevented the leaching reaction from going to completion. The solubility of silver sulphate in nitric acid at 25jC is about 64 g/L for the nitric acid concentration in the starting leach solution, but decreases as the nitric acid concentration in solution decreases (Linke, 1958). From the calculated leach solution analysis reported in Table 1, the silver concentrations in all, but one test, were greater than the initial solubility of Ag2SO4 at 25jC, and, thus, would be much greater than the solubility of Ag2SO4 in the leach solutions after taking into account the decrease in the solubility of Ag2SO4 as nitric acid is consumed in the leaching reactions. Though, from the results of the leaching tests, the solubility of Ag2SO4 increases significantly with temperature, precipitation of Ag2SO4 would be expected on cooling of the leach solution, given the silver concentrations in solution, and would be possible during leaching with the high solids concentrations used in Tests 1 to 7. X-ray diffraction analysis (Fig. 5) confirms the presence of Ag2SO4 in the leach residues. This
Fig. 2. Effect of nitric acid addition on silver and sulphur extractions.
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217
Fig. 3. Effect of temperature on silver and sulphur extractions.
analysis also indicates that the leach residues, depending on the reaction conditions and extent of leaching, contain a combination of silver sulphide sulphate (Ag6S3O4), silver sulphate (Ag2SO4) and elemental sulphur (Sj). The distinct colors of the various phases involved in this system (black (Ag2S), brown (Ag6S3O4), white to light yellow (Ag2SO4) and bright yellow (Sj)) are a further indicator of this behaviour; the leach went from brown (Ag6S3O4) to white (Ag2SO4) with increased pressure and temperature (Table 3) and some residue samples showed evidence of the presence of elemen-
tal sulphur. This transition from Ag2S to Ag6S3O4 to Ag2SO4 shows the progressive oxidation of sulphur in the leach solids as the leaching reaction goes to completion. To determine the amount of silver sulphate in the leach residues, the solids from Tests 4 and 5 were mixed with hot water at 100jC for 1 h to dissolve any remaining silver sulphate and the solids residues were analyzed to determine the increase in silver extraction. The water leach increased the overall extractions of silver to 79.1% and 80.9%, respectively, for Tests 4 and 5, corresponding to
Fig. 4. Effect of temperature and pressure on silver and sulphur extractions.
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Fig. 5. Observed X-ray diffraction patterns for selected leach residue samples (A—Ag2SO4, B—Ag6S3O4, S—Sulphur (Sj)).
increases of 4% to 10% in the extraction, and indicating that 80% to 81% was ultimately the highest extraction possible for these leaching conditions. Table 3 Description of the leach solids Test number
Description
Feed 1 2 3 4 5 6 7 8
Black solids Very dark brown with yellow flecks Reddish brown with light yellow flecks Lighter brown than Test 2 with yellow flecks Very pale brown with yellow flecks Light yellow solids Light yellow solids Light yellow to white solids Light to medium brown solids with a few yellow flecks
However, the amount of unleached silver sulphide in the water leach residue indicates that the low extractions are not just a result of unleached silver sulphate and further point to the precipitation of silver sulphate inhibiting the leaching of the remaining silver sulphide. 4.3. Effect of solids concentration One test was performed at a lower solids concentration (9.6%) and 150jC and 1100 kPa to determine whether lowering the solids concentration could improve the silver extraction (Test 8). The silver extraction from this test was 96.1%, indicating that near complete extraction of the silver from silver sulphide could be achieved with a lower solids concentration.
P.C. Holloway et al. / Hydrometallurgy 74 (2004) 213–220
4.4. Sulphur extractions Sulphur extractions were generally similar to or, in a few cases, slightly lower than the silver extractions in these tests. Silver and sulphur extractions should, in theory, follow each other as, to dissolve silver, the sulphur would have to be oxidized to sulphate in solution to maintain the charge balance. The analyses indicate an excess of sulphur in the feed solids which is quite possible from sulphide precipitation from acidic solutions (i.e., by the oxidation of S2 to elemental sulphur or polysulphides). This excess is reflected by the presence of elemental sulphur in the leach residue, which are visible as bright yellow flecks are detected by X-ray diffraction analyses (Fig. 5) and could explain why the sulphur extractions are slightly lower than the silver extractions in some of these tests. One test was attempted with the addition of sulphuric acid to the leach solution to try to lower the amount of oxidation of sulphur to sulphate and, thus, lower the amount of nitric acid consumed. The test was performed at 150jC, 1100 kPa(g) and with 107 g/L nitric acid (204% of stoichiometric) and an H2SO4 concentration of 220 g/L. This test resulted in a lower silver extraction (16%) than was achieved at similar conditions with only nitric acid (Test 5). As well, with a sulphur extraction of 35%, the ratio of silver to sulphur extractions was less than 0.5, which, compared with ratios of greater than 1 for all the other tests indicates that more sulphur was oxidized to sulphate in this test per mole of silver dissolved than in the tests using only nitric acid.
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Previous researchers all precipitated silver by hydrogen reduction from solutions below the room temperature solubility of silver sulphate in their leach solutions (Kunda and Hitesman, 1978, 1980; Kunda and Etsell, 1989). However, the silver concentrations of the leach solutions in these tests were considerably higher than the room temperature solubility of silver sulphate, as noted previously, as excess silver sulphate precipitated on cooling of these solutions. Thus, it is evident from this research that, with the right set of hydrogen reduction conditions, silver can be quickly precipitated from silver sulphate solutions which would be supersaturated at room temperature. 5. Conclusions This research indicates that nitric acid leaching can be effective in leaching silver sulphide precipitates with a maximum extraction of 96.1% achieved by leaching at 150jC, 1100 kPa(g), and 9.6% solids with twice the stoichiometric amount of nitric acid. Further optimization of the leaching conditions would be required to further improve on the silver extractions achieved. Hydrogen reduction from nitric acid leach solutions was also successful with nearly quantitative precipitation of silver in 1 h at 150jC and 4000 kPa(g) hydrogen pressure. In these tests, high precipitation efficiencies were achieved at silver concentrations well in excess of the solubility limit for silver sulphate at room temperature. Silver of greater than 99.8% purity with 96.2% passing 106 Am was produced in these hydrogen reduction tests.
4.5. Hydrogen reduction
References
With the best hydrogen reduction conditions tested, near quantitative precipitation (99.8%) of silver from the leach solution was possible in 1 h. The silver powder produced was of high purity (>99.8%) and quite fine (96.2% passing 106 Am), thus, making it an excellent material for powder metallurgy applications. (The lower silver grade of the Test II precipitate was likely a result of contamination with silver sulphate on cooling because of the lower precipitation efficiency in that test.) Further testing of hydrogen reduction conditions would be necessary to determine the optimum conditions for this process.
Daugherty, E.W., Erhard, A.E., Drobnick, J.L., 1973. Oxidation of metal sulfides. German Offenlegungsschrift 2,232,570, February 8. Kennecott Copper, 1979. Extraction of metals from sulfide ores with nitric acid. Japan Kokai Tokkyo Koho 79, 47,804, April 14. Kunda, W., 1981. Process for the production of elemental silver from silver chloride or silver sulphate. U.S Patent 4,306,902, December 22. Kunda, W., 1982. Process for the recovery of silver values from silver-containing material which also contains iron and arsenic. U.S. Patent 4,331,469, May 25. Kunda, W., 1983. Process for the production of silver powder from silver chloride, sulphate or sulphide. U.S. Patent 4,388,109, June 14.
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Kunda, W., 1984. Hydrometallurgical processing of silver concentrates. Precious Metals: Mining, Extraction, and Processing. Metallurgical Society of AIME, Warrendale, PA, pp. 397 – 423. Kunda, W., Etsell, T.H., 1988. Method for recovering silver from waste solutions containing thiosulfate compounds. U.S. Patent 4,755,453, July 5. Kunda, W., Etsell, T.H., 1989. Recovery of silver from the silver nitrate system by hydrogen reduction. In: Jha, M.C., Hill, S.D., El Guindy, M. (Eds.), Precious Metals ’89. TMS/AIME, Warrendale, PA, pp. 431 – 442. Kunda, W., Etsell, T.H., 1991. Silver recovery from spent photographic solutions. Canadian Patent 2,000,527, Apr. 12. Kunda, W., Etsell, T.H., 1992. Photographic fixer regeneration. U.S. Patent 5,162,106, November 10. Kunda, W., Etsell, T.H., 1996. Silver recovery from spent photographic solutions. Canadian Patent 2,033,788, Dec. 10. Kunda, W., Hitesman, R.H., 1978. Recovery of silver from silver containing solutions. U.S. Patent 4,129,441, December 12.
Kunda, W., Hitesman, R.H., 1980. Recovery of silver from silver containing solutions. Canadian Patent 1,082,926, Aug. 5. Linke, W.F., 1958. Solubilities, inorganic and metal – organic compounds; a compilation of solubility data from the periodical literature, fourth edition, American Chemical Society, Washington, DC. Posel, J.G., 1972. Method for the recovery of copper, nickel, and silver values. German Offenlegungsschrift 2,307,297, February 22. Posel, J.G., 1976a. Recovery of nitric acid soluble transition metals from sulfur and iron containing ores of the same. U.S. Patent 3,965,239, June 22. Posel, John G., 1976b. Nitric acid leaching process for the recovery of copper, nickel, and silver from polysulfide ores of the same. Canadian Patent 992,749, July 7. Raudsepp, R., Beattie, M.J.V., 1989. Recovery of precious metals from pyrite or arsenopyrite via nitrate route. Australian Patent 582,961, April 13.
Nitric acid leaching of silver sulphide precipitates P.C. Holloway a,*, K.P. Merriam b, T.H. Etsell a a
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 b Hudson Bay Mining and Smelting Co., P.O. Box 1500, Flin Flon, Manitoba, Canada R8A 1S8 Received 23 June 2003; received in revised form 8 April 2004; accepted 5 May 2004
Abstract Leaching of silver sulphide precipitates with nitric acid was performed at a variety of temperatures, pressures, acid concentrations and solids concentrations, with a maximum extraction of 96.1% achieved by leaching at 150jC, 1100 kPa(g), and 9.6% solids with twice the stoichiometric amount of nitric acid. Further improvement in the silver extractions would be expected with further optimization of the leaching conditions and this process could represent a hydrometallurgical alternative for the recovery of silver from silver sulphide precipitated from photographic solutions. Hydrogen reduction from nitric acid leach solutions resulted in near quantitative precipitation of silver (99.8%) in 1 h at 150jC and 4000 kPa(g) hydrogen pressure. High precipitation efficiencies with hydrogen reduction were achieved even at silver concentrations well in excess of the solubility limit for silver sulphate at room temperature. High purity silver with a particle size of 96.2% passing 106 Am was produced. D 2004 Elsevier B.V. All rights reserved. Keywords: Silver sulphide; Nitric acid; Pressure leaching; Secondary recovery; Hydrogen reduction; Silver; Photographic solutions
1. Introduction Silver sulphide (Ag2S) is a very stable silver compound, which is not readily dissolved using mineral acids under atmospheric conditions. Because of its low solubility, several processes for the recovery of silver from photographic solutions use sulphidizing agents, such as sodium sulphide (Na2S) or sodium hydrosulphide (NaHS), to precipitate silver have been proposed (Kunda and Etsell, 1988, 1991, 1992, 1996). Pyrometallurgical methods have also been described for producing silver from silver sulphide (Kunda, 1981, 1983), but these methods produce sponge silver
* Corresponding author. Fax: +1-780-492-2881. E-mail address: [email protected] (P.C. Holloway). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.05.003
as the product, which would have to be remelted or reformed before it could be reused. The majority of the work done on nitric acid leaching of sulphide materials has focused on the treatment of refractory (i.e., pyrite, arsenopyrite or chalcopyrite) ores containing precious metals. Several processes have been reported in the literature for nitric or nitrous acid leaching of these complex sulphide ores (Daugherty et al., 1973; Kennecott, 1979; Kunda, 1982, 1984; Posel, 1972, 1976a,b; Raudsepp and Beattie, 1989) but, to date, the direct leaching of silver sulphide with nitric acid has not been documented in the literature. Therefore, the objective of this research was to determine whether metallic silver could be produced hydrometallurgically from silver sulphide precipitated from photographic solutions, by pressure leaching with nitric acid to dissolve the silver followed by
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hydrogen reduction to produce a high purity silver powder from the nitric acid leach solutions.
2. Experimental
analyzed gravimetrically for silver at the University of Alberta and were analyzed for sulphur with LECO combustion by International Plasma Laboratory, of Vancouver, BC, Canada. Metallic silver produced in the hydrogen reduction tests was dissolved in nitric acid and analyzed with atomic absorption.
2.1. Feed materials 2.3. Leaching tests Silver sulphide was generated by precipitation either from solution prepared from reagent grade AgNO3 (Tests 1 to 7), or from solutions generated by dissolving high purity silver (99.9999%) in dilute nitric acid (Test 8). Technical grade Na2S was used as the precipitating agent. The black precipitate formed was filtered, washed and dried overnight at 80jC. The two batches of silver sulphide produced analyzed 77.5% and 75.2% Ag and 12.1% and 14.7% S, respectively. Silver sulphide (Ag2S) was identified in the feeds by X-ray diffraction (XRD) with all major peaks identified (Fig. 1). Based on the chemical analysis of the feed and X-ray diffraction of residue samples, elemental sulphur (Sj) is likely also present in the feed in minor quantities below the XRD detection limit. 2.2. Analysis Solutions were analyzed for silver using atomic absorption spectroscopy (AA). Leach residues were
Leaching and hydrogen reduction tests were performed in a stainless steel Parr autoclave (250 mL) fitted with a Teflon liner and single axial agitator. In the leaching tests, the solids and solution were charged to the autoclave, heated over 0.5 to 1 h to the desired temperature and allowed to react for 1 h at temperature. As nitric acid fumes at the reaction temperatures tested, offgases from the autoclave were constantly vented to maintain the desired operating pressure (1100 or 1800 kPa(g)). After 1 h, the autoclave was cooled to 95 to 100jC and allowed to react for 15 min to allow any elemental sulphur in the leach slurry to solidify. The autoclave was then cooled to 60 to 70jC. The slurry was then filtered and the solids washed with hot water to remove any entrained leach solution from the solids and allowed to dry in an 80jC oven overnight. The leach solution and wash water were combined and collected.
Fig. 1. X-ray diffraction analyses of batch 2 feed sample.
P.C. Holloway et al. / Hydrometallurgy 74 (2004) 213–220
215
Table 1 Summary of conditions and results of nitric acid leaching tests Test number Conditions Feed solids batch HNO3 concentration, g/L Temperature, jC Pressure, kPa(g) Solids concentration, % Solids weight g Initial Final Residue analysis, % Ag S Extraction, % Ag S Ag in leach solution, g/La a
1
2
3
4
5
6
7
8
1 213 125 1100 17.3
1 426 125 1100 18.0
1 426 125 1100 14.1
1 426 125 1800 14.1
1 426 150 1100 14.1
1 426 175 1100 14.1
1 426 150 1800 14.1
2 284 150 1100 9.6
20.0 15.7
20.0 12.4
15.0 5.7
15.0 4.4
15.0 5.0
15.0 4.4
15.0 3.6
10.0 0.5
75.1 11.8
68.3 13.0
65.5 16.3
66.7 13.0
68.0 11.2
69.1 10.3
68.7 10.8
63.7 15.2
24.2 24.2 38
45.6 33.9 71
67.7 48.7 79
74.8 68.7 87
70.8 69.4 82
74.0 75.3 86
78.9 78.9 92
96.1 93.7 73
Calculated based on silver extractions and the solids concentration used in leaching.
A white precipitate formed during the cooling of the leach solution, indicating supersaturation of the leach solution with silver sulphate (Ag2SO4); as a result, solids analyses and weights were used for extraction calculations.
temperature, pressure and solids concentration. In all cases, nitric acid was used as the sole oxidant for oxidizing the silver sulphide. Table 1 shows the results of these tests. 3.2. Hydrogen reduction tests
2.4. Hydrogen reduction tests In the hydrogen reduction tests, the pH of the leach/wash solution from the specific leach tests was adjusted to between 5.5 and 6.0 using ammonium hydroxide. This solution, plus any solids that had precipitated from the leach solution, was added to the autoclave. Prior to heating, the autoclave was pressurized with hydrogen and vented three times to try to remove oxygen from the autoclave. The autoclave was then pressurized to the desired pressure and heated to the desired temperature and allowed to react for 1 h. The autoclave was then cooled and its contents filtered. The solution was collected and analyzed for residual silver concentration.
3. Results and analyses 3.1. Leaching Several different variables were investigated in the leaching tests, including nitric acid concentration,
Two preliminary hydrogen reduction tests were completed to determine whether hydrogen reduction of the nitric acid leach solution was feasible. Table 2 summarizes the results of these tests. (Solutions from Test 5 and Test 4 were used in Tests I and II, respectively.). The particle size of the silver powder produced in Test I was analyzed and the powder was 96.2% passing 106 Am (140 mesh).
Table 2 Hydrogen reduction test conditions and results
Temperature, jC Pressure, kPa(g) Time, h Silver weight, g In Out Silver analysis, % Precipitation efficiency, %
Test I
Test II
150 3800 to 4150 1
150 1700 to 1800 1
6.84 0.02 >99.8 99.8
7.67 1.39 97.4 81.9
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4. Discussion 4.1. Effect of acid addition In Tests 1 to 3, the acid additions used correspond to 75%, 150% and 200% of the amount required stoichiometrically based on the following reaction. 3 Ag2 S þ 8 HNO3 R3 Ag2 SO4 þ 8 NO þ 4 H2 O Fig. 2 shows the effect of acid concentration on the silver and sulphur extractions. The silver extraction increased close to linearly from 24% to 68% as the acid concentration increased. As a result, an acid addition of 204% of stoichiometric was used for the remaining tests. 4.2. Effect of temperature and pressure In Tests 3, 5 and 6, the temperature was varied from 125 to 175jC with the pressure kept at 1100 kPa(g). As shown in Fig. 3, each increase of 25jC resulted in an increase in the silver extraction of 3% to 4%, with a maximum extraction of 74% at 175jC. Pressure had a slightly more pronounced effect on the silver extraction than temperature (Fig. 4), as an increase of 600 kPa in the total pressure increased the silver extraction by 7% to 8% at both 125 and 150jC.
However, even at the best conditions in Tests 1 to 7, the extractions indicate that 20 to 30% of the silver sulphide remained unleached. Since silver sulphate (Ag2SO4) precipitate formed on cooling of the leach solutions and similar crystals were present in the leach solids, it was proposed that the precipitation of silver sulphate during leaching prevented the leaching reaction from going to completion. The solubility of silver sulphate in nitric acid at 25jC is about 64 g/L for the nitric acid concentration in the starting leach solution, but decreases as the nitric acid concentration in solution decreases (Linke, 1958). From the calculated leach solution analysis reported in Table 1, the silver concentrations in all, but one test, were greater than the initial solubility of Ag2SO4 at 25jC, and, thus, would be much greater than the solubility of Ag2SO4 in the leach solutions after taking into account the decrease in the solubility of Ag2SO4 as nitric acid is consumed in the leaching reactions. Though, from the results of the leaching tests, the solubility of Ag2SO4 increases significantly with temperature, precipitation of Ag2SO4 would be expected on cooling of the leach solution, given the silver concentrations in solution, and would be possible during leaching with the high solids concentrations used in Tests 1 to 7. X-ray diffraction analysis (Fig. 5) confirms the presence of Ag2SO4 in the leach residues. This
Fig. 2. Effect of nitric acid addition on silver and sulphur extractions.
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Fig. 3. Effect of temperature on silver and sulphur extractions.
analysis also indicates that the leach residues, depending on the reaction conditions and extent of leaching, contain a combination of silver sulphide sulphate (Ag6S3O4), silver sulphate (Ag2SO4) and elemental sulphur (Sj). The distinct colors of the various phases involved in this system (black (Ag2S), brown (Ag6S3O4), white to light yellow (Ag2SO4) and bright yellow (Sj)) are a further indicator of this behaviour; the leach went from brown (Ag6S3O4) to white (Ag2SO4) with increased pressure and temperature (Table 3) and some residue samples showed evidence of the presence of elemen-
tal sulphur. This transition from Ag2S to Ag6S3O4 to Ag2SO4 shows the progressive oxidation of sulphur in the leach solids as the leaching reaction goes to completion. To determine the amount of silver sulphate in the leach residues, the solids from Tests 4 and 5 were mixed with hot water at 100jC for 1 h to dissolve any remaining silver sulphate and the solids residues were analyzed to determine the increase in silver extraction. The water leach increased the overall extractions of silver to 79.1% and 80.9%, respectively, for Tests 4 and 5, corresponding to
Fig. 4. Effect of temperature and pressure on silver and sulphur extractions.
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Fig. 5. Observed X-ray diffraction patterns for selected leach residue samples (A—Ag2SO4, B—Ag6S3O4, S—Sulphur (Sj)).
increases of 4% to 10% in the extraction, and indicating that 80% to 81% was ultimately the highest extraction possible for these leaching conditions. Table 3 Description of the leach solids Test number
Description
Feed 1 2 3 4 5 6 7 8
Black solids Very dark brown with yellow flecks Reddish brown with light yellow flecks Lighter brown than Test 2 with yellow flecks Very pale brown with yellow flecks Light yellow solids Light yellow solids Light yellow to white solids Light to medium brown solids with a few yellow flecks
However, the amount of unleached silver sulphide in the water leach residue indicates that the low extractions are not just a result of unleached silver sulphate and further point to the precipitation of silver sulphate inhibiting the leaching of the remaining silver sulphide. 4.3. Effect of solids concentration One test was performed at a lower solids concentration (9.6%) and 150jC and 1100 kPa to determine whether lowering the solids concentration could improve the silver extraction (Test 8). The silver extraction from this test was 96.1%, indicating that near complete extraction of the silver from silver sulphide could be achieved with a lower solids concentration.
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4.4. Sulphur extractions Sulphur extractions were generally similar to or, in a few cases, slightly lower than the silver extractions in these tests. Silver and sulphur extractions should, in theory, follow each other as, to dissolve silver, the sulphur would have to be oxidized to sulphate in solution to maintain the charge balance. The analyses indicate an excess of sulphur in the feed solids which is quite possible from sulphide precipitation from acidic solutions (i.e., by the oxidation of S2 to elemental sulphur or polysulphides). This excess is reflected by the presence of elemental sulphur in the leach residue, which are visible as bright yellow flecks are detected by X-ray diffraction analyses (Fig. 5) and could explain why the sulphur extractions are slightly lower than the silver extractions in some of these tests. One test was attempted with the addition of sulphuric acid to the leach solution to try to lower the amount of oxidation of sulphur to sulphate and, thus, lower the amount of nitric acid consumed. The test was performed at 150jC, 1100 kPa(g) and with 107 g/L nitric acid (204% of stoichiometric) and an H2SO4 concentration of 220 g/L. This test resulted in a lower silver extraction (16%) than was achieved at similar conditions with only nitric acid (Test 5). As well, with a sulphur extraction of 35%, the ratio of silver to sulphur extractions was less than 0.5, which, compared with ratios of greater than 1 for all the other tests indicates that more sulphur was oxidized to sulphate in this test per mole of silver dissolved than in the tests using only nitric acid.
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Previous researchers all precipitated silver by hydrogen reduction from solutions below the room temperature solubility of silver sulphate in their leach solutions (Kunda and Hitesman, 1978, 1980; Kunda and Etsell, 1989). However, the silver concentrations of the leach solutions in these tests were considerably higher than the room temperature solubility of silver sulphate, as noted previously, as excess silver sulphate precipitated on cooling of these solutions. Thus, it is evident from this research that, with the right set of hydrogen reduction conditions, silver can be quickly precipitated from silver sulphate solutions which would be supersaturated at room temperature. 5. Conclusions This research indicates that nitric acid leaching can be effective in leaching silver sulphide precipitates with a maximum extraction of 96.1% achieved by leaching at 150jC, 1100 kPa(g), and 9.6% solids with twice the stoichiometric amount of nitric acid. Further optimization of the leaching conditions would be required to further improve on the silver extractions achieved. Hydrogen reduction from nitric acid leach solutions was also successful with nearly quantitative precipitation of silver in 1 h at 150jC and 4000 kPa(g) hydrogen pressure. In these tests, high precipitation efficiencies were achieved at silver concentrations well in excess of the solubility limit for silver sulphate at room temperature. Silver of greater than 99.8% purity with 96.2% passing 106 Am was produced in these hydrogen reduction tests.
4.5. Hydrogen reduction
References
With the best hydrogen reduction conditions tested, near quantitative precipitation (99.8%) of silver from the leach solution was possible in 1 h. The silver powder produced was of high purity (>99.8%) and quite fine (96.2% passing 106 Am), thus, making it an excellent material for powder metallurgy applications. (The lower silver grade of the Test II precipitate was likely a result of contamination with silver sulphate on cooling because of the lower precipitation efficiency in that test.) Further testing of hydrogen reduction conditions would be necessary to determine the optimum conditions for this process.
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