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Recovery of gold from gold slag by wood shaving fly ash
Journal of Colloid and Interface Science 287 (2005) 394–400 www.elsevier.com/locate/jcis
Recovery of gold from gold slag by wood shaving fly ash Amphol Aworn a , Paitip Thiravetyan b,∗ , Woranan Nakbanpote c a School of Energy and Materials, King Mongkut’s University of Technology Thonburi, 91 Prachautit, Bangmod, Thungkru, Bangkok 10140, Thailand b Division of Biotechnology, School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, 83 Moo. 8 Thakham,
Bangkhuntien, Bangkok 10150, Thailand c Pilot Plant Development and Training Institute, King Mongkut’s University of Technology Thonburi, 83 Moo. 8 Thakham, Bangkhuntien,
Bangkok 10150, Thailand Received 9 November 2004; accepted 3 February 2005 Available online 9 April 2005
Abstract Wood shaving fly ash was used as an alternative adsorbent for gold preconcentration from gold slag. The maximum gold adsorption capacity of wood shaving fly ash washed with tap water (WSFW) at 20, 30, 40, and 60 ◦ C was 8.68, 7.79, 7.44, and 7.25 mgAu /gadsorbent , respectively, while of activated carbon it was 76.78, 60.95, 56.13, and 51.90 mgAu /gadsorbent , respectively. Deionized water at 100 ◦ C could elute gold adsorbed onto WSFW to 71%. The effect of the increasing temperature of water, 30, 60, and 100 ◦ C, implied that the adsorption mechanism was mainly physical adsorption. The negative values of enthalpy change (H ) and free energy change (G) indicated an exothermic and spontaneous process, respectively. The positive values of entropy change (S) indicated increasing disorder of the system. The advantages of wood shaving fly ash are the purification of gold and the easier recycling of gold from the gold-adsorbed adsorbent. 2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; Gold cyanide; Gold slag; Wood shaving; Fly ash
1. Introduction According to the World Gold Council, the demand for gold has shown an increasing trend during the last decade [1]. This increase is due not only to the jewelry market, but also to the increasing uses of gold in industry, as well as medical applications [2]. Therefore, gold recycling from scraps and leach residues has also raised much interest [3]. Gold slag is wasted in the jewelry industry. It is known that industrial gold extraction commonly uses cyanide as a leaching agent [4]. The cyanidation process has been used by the mining industry for over 100 yr in the extraction of precious metals [5,6]. The recovery of gold by the adsorption of auro-cyanide complexes onto activated carbon is a well-established process and more recently adsorption by ion-exchange resins has been used [7,8]. Furthermore, other means of recovering gold from aqueous solution, such as ion * Corresponding author. Fax: +662 4523455.
E-mail address: [email protected] (P. Thiravetyan). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.02.048
exchange resins, have been studied. However, these types of systems are expensive and substances that are detrimental to the environment are produced during their fabrication [9,10]. A feasible alternative could be the use of biological materials for the biosorption of gold ions [11,12]. Activated carbon has proven to be an efficient and economical material for adsorption in systems that handle large amounts of highly concentrated aqueous solutions of gold. Activated carbon has also found increased application in the field of hydrometallurgy, especially in the recovery of gold and silver from cyanide solutions [13,14]. Gold recovery by activated carbon adsorption requires an activated carbon with the capability to adsorb gold that will be released easily in a later step. To be a suitable adsorbent for use in the gold recovery processes, an activated carbon must, therefore, have (a) a high mechanical strength, (b) a high adsorption capacity (loading capacity), and (c) a high adsorption rate [15]. The aim of this research is to recover gold from gold slag derived from the jewelry industry. Usually, the gold slag is extracted by a cyanidation process and refined
A. Aworn et al. / Journal of Colloid and Interface Science 287 (2005) 394–400
by electro-winning processes. The electro-winning process found that the gold-cyanide solution was at a low concentration (1–4 mg/l), which could not be refined. Therefore, gold adsorption by wood shaving fly ash is investigated. The effects of contact time, dosage, system pH, the adsorption isotherm, and thermodynamic and elution tests are examined to determine the optimum conditions and probable mechanisms of the gold adsorption.
2. Materials and methods 2.1. Adsorbent preparation Wood shaving fly ash (WSF) is a waste material from the textile industry. This industry uses wood shavings as a fuel in boilers in thermal power plants. It consists of 64.39% carbon, 31.09% oxygen, 2.50% potassium, and a little sodium, magnesium, calcium, phosphorus, chlorine, sulfur, and silicon. The WSF used in these experiments was divided into three types; (1) nontreated (WSFN), (2) washed with water for 1 h and then washed with an excess of double distilled water until the pH of the washing solution was constant (WSFW), and (3) soaked with 0.1 N H2 SO4 for 1 h and then washed with an excess of double distilled water until the pH of the washing solution was constant (WSFA). A commercial activated carbon (AC) of coconut shells was obtained from Mazuma (Thailand) Co., Ltd. All adsorbents were dried at 105 ◦ C and sieved to a size of <75 µm. 2.2. Extraction of gold slag Gold slag was obtained from jewelry waste. It was leached by sodium cyanide and then adjusted to pH 10.5. The slag leaching process used the ratio gold slag:NaCN: H2 O = 250 g:5 g:1000 ml, with feeding oxygen gas (30 mg/l) as an oxidizing agent [16], as shown in Eq. (1), at an equilibrium time of 2 h at 30 ◦ C. The mixture was then filtered through Whatman no. 5 filter paper and the supernatant was used for gold preconcentration by wood shaving fly ash: 4Au + 8NaCN + 2H2 O + O2 4Na[Au(CN)2 ] + 4NaOH.
(1)
395
fly ash. Batch studies were carried out by adding 0.1000 g of wood shaving fly ash to 10 ml of 3–4 mgAu /l goldcyanide solution and shaken at 150 rpm, at 30 ◦ C for 2 h. Adsorption isotherms were studied by varying the dosage in the range of 0.10–5.00% (w/v) of wood shaving fly ash and 0.006–0.500% (w/v) for activated carbon (from coconut shells). A gold adsorption isotherm was carried out at various temperatures of 20, 30, 40, and 60 ◦ C. The sample was centrifuged at 4500 rpm for 10 min. The supernatant was analyzed for gold concentration and changing pH by inductively coupled plasma spectroscopy (ICP) (Jobin Yvon-JY 124, France) and pH meter (Mettler Delta 340, USA), respectively. The amount of gold adsorbed onto the adsorbent and the adsorption percentage were calculated respectively by [17] V , W C0 − Cf × 100, Adsorption (%) = C0 Qe = (C0 − Ce )
(2) (3)
where C0 is the initial concentration of gold in the solution (mgAu /l), Ce is the residual concentration of gold in the solution at equilibrium (mgAu /l), Cf is the final concentration of gold in the solution (mgAu /l), Qe is the gold adsorption capacity at equilibrium (mgAu /g), V is the volume of the solution (l), and W is the weight of the adsorbent used (g). 2.5. Elution experiments A 40.00-mgAu /l gold-cyanide solution was adsorbed onto 5% (w/v) of wood shaving fly ash and shaken at 150 rpm at 30 ◦ C for 2 h. The gold adsorbed sample was filtered and air-dried. The elution test was carried out by agitating 0.500 g of gold adsorbed wood shaving fly ash with 10 ml of deionized water at 30, 60, and 100 ◦ C and shaken at 150 rpm for 2 h. The sample was centrifuged and then the supernatant was used to determine the percentage of gold elution.
3. Results and discussion
2.3. Appearance and characteristic
3.1. Appearance and porosity of wood shaving fly ash
Wood shaving fly ash and activated carbon (coconut shells) (<75 µm) were dried at 105 ◦ C for 2 h. The appearance and surface area of the adsorbents were determined by scanning electron microscopy (SEM) (JSM-6400, Japan) and the BET method (surface area analyzer) (Quantachrom, Autosorb-1, USA), respectively.
Physical and chemical characterizations of the three types of wood shaving fly ash are given in Table 1. Scanning electron microscope (SEM) photographs of various types of wood shaving fly ash and activated carbon (from coconut shells) are shown in Fig. 1. Wood shaving fly ash treated by water and by H2 SO4 caused increased BET surface areas and total pore volumes because water and H2 SO4 were able to leach some tars and contaminants from the adsorbent (Table 2). The dissolution of potassium and calcium causes hydrogen gas and results in basidic conditions as shown in
2.4. Adsorption experiments The effects of contact time, dosage, and system pH were investigated for gold cyanide adsorption by wood shaving
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(a)
(b)
(c)
(d)
Fig. 1. SEM photographs of (a) wood shaving fly ash (WSF), (b) wood shaving fly ash washed with tap water (WSFW), (c) wood shaving fly ash soaked in 0.1 N H2 SO4 (WSFA), and (d) activated carbon (from coconut shells).
Table 1 Physical and chemical characteristics of wood shaving fly ash Bulk density (g/cm3 )
0.57
Moisture (%)
4.37
Ash (%)
28.70
pH
12.27
Size distribution (%) >250 µm 180–250 µm 150–179 µm 75–149 µm <75 µm
41.10 7.90 4.10 10.20 36.70
Element (%) C O Na Mg Si P S Cl K Ca
64.39 31.09 0.70 0.63 0.03 0.13 0.06 0.12 2.50 0.36
Table 2 BET surface area of various types of wood shaving fly ash and activated carbon Adsorbent
BET surface areaa (m2 /g)
Total pore volume (cm3 /g)
Average pore diameter (Å)
WSFN WSFW WSFA AC (coconut shells)
104.4 395.7 295.3 1113.0
0.0846 0.2564 0.2044 0.6236
32.42 25.92 27.68 22.41
a Specific surface area.
2K(s) + 2H2 O → 2KOH(aq) + H2(g) ,
(4)
Ca(s) + 2H2 O → Ca(OH)2(aq) + H2(g) .
(5)
The pHs of the system after WSF treatment by water and sulfuric acid were 12 and 10, respectively. Tar is easily removed under basidic conditions. Therefore, the BET surface area of WSFW was higher than of WSFA. Activated carbon from coconut shells has the highest surface area and total pore volume because this material passed both through carbonization and activation processes.
A. Aworn et al. / Journal of Colloid and Interface Science 287 (2005) 394–400
3.2. Optimum conditions for gold-cyanide adsorption 3.2.1. Comparison of gold-cyanide adsorption by three types of WSF The efficiency of gold adsorption by WSFW was higher than by WSFA and WSFN, respectively (Table 3). This may explained by the surface area and total pore volume of WSFW being the highest. Activated carbon could adsorb more gold than the three types of WSF because activated carbon had a higher surface area and total pore volume than the three types of WSF [18–20]. The surface basicity and the surface acidity of the adsorbents seems not to influence the gold adsorption because activated carbon had the lowest basicity, while gold adsorption was the highest (Table 3). However, data on the gold adsorption efficiency of the three types of wood shaving fly ash were subjected to analysis of variance and means separation was performed using the new Duncan multiple range test (DMRT). The results were significantly different at P 0.05. The WSFW was a better adsorbent than the WSFA and WSFN, respectively. Therefore, the WSFW adsorbent was selected to study the gold adsorption experiments. 3.2.2. Effect of contact time The effect of contact time on gold adsorption by WSFW is shown in Fig. 2. The results indicate that gold was rapidly adsorbed within 15 min (73%), and then the adsorption rate increased slightly with time [20]. The data subjected
397
to analysis of variance and means separation using the new Duncan multiple range test (DMRT) were not significantly different at P 0.05, indicating that the gold-cyanide adsorption by WSFW from 120 to 300 min was not different. This implies that the gold-cyanide adsorption reached equilibrium at 120 min. 3.2.3. Effect of dosage Fig. 3 shows that the efficiency of gold adsorption increased to 85% at 1.5% (w/v) dosage of adsorbent. The data were subjected to analysis of variance and means separation was performed using the new Duncan multiple range test (DMRT). The results were not significantly different at P 0.05, indicating that the gold-cyanide adsorption by WSFW from 4.5 to 15% (w/v) was not different. This indicates that the optimum dosage for gold-cyanide adsorption by WSFW was 4.5% (w/v). 3.2.4. Effect of system pH The effect of system pH values of 10 and 11 on gold adsorption was studied (see Fig. 4). This indicated an efficiency of gold adsorption of 84–86% at system pH 10–11. However, data subjected to analysis of variance and means separation performed using the new Duncan multiple range test (DMRT) were not significantly different at P 0.05, indicating that the gold-cyanide adsorption by WSFW was not different at pH values between 10 and 11.
Table 3 Comparison of gold adsorption by various types of wood shaving fly ash and activated carbon Adsorbent
Acidity Basicity Efficiency of BET gold adsorption surface areaa (meqHCl /g) (meqNaOH /g) (m2 /g) (%)
WSFN WSFW WSFA AC (coconut shells)
52.28b 86.05b 78.00b 97.10
104.40 395.70 295.30 1113.00
6.78 5.25 6.72 2.25
3.77 3.92 3.83 3.40
a Specific surface area. b The results were significantly different at P 0.05.
Fig. 2. Effect of contact time on gold adsorption by wood shaving fly ash washed with tap water (WSFW).
Fig. 3. Effect of dosage on gold adsorption by wood shaving fly ash washed with tap water (WSFW).
Fig. 4. Effect of system pH on gold adsorption by wood shaving fly ash washed with tap water (WSFW).
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3.3. Maximum gold-cyanide adsorption Adsorption isotherms were analyzed with Langmuir and Freundlich models, [17,21], Ce 1 Ce + = , (6) Qe Qmax b Qmax Qmax Ce Qe = (7) , 1/b + Ce 1/n
Qe = Kf Ce ,
(8)
log Qe = log Kf + (1/n) log Ce ,
(9)
where Ce is the residual concentration of gold in the solution at equilibrium (mgAu /l), Qe is the gold adsorption capacity at equilibrium (mgAu /g), Qmax is the maximum adsorption capacity of gold (mgAu /g), b is the affinity coefficient (l/mg), Kf is the Freundlich constant related to the capacity, and (1/n) is the Freundlich constant related to the intensity (0 < 1/n < 1). The Langmuir parameters Qmax and b were determined from the data using a nonlinear leastsquares program. The Freundlich parameters Kf and 1/n were calculated from a linear least-squares analysis of the data transformed according to Eq. (9). The calculated parameters are summarized in Table 4. Figs. 5 and 6 show the Langmuir isotherm and Freundlich isotherm of gold-cyanide adsorption by WSFW at temperatures of 20, 30, 40, and 60 ◦ C, respectively. The Langmuir
Fig. 5. Langmuir nonlinear isotherm plots related to the adsorption of gold by wood shaving fly ash washed with tap water (WSFW) at temperatures of 20, 30, 40, and 60 ◦ C.
isotherm and Freundlich isotherm of gold-cyanide adsorption by activated carbon from coconut shells are shown in Figs. 7 and 8, respectively. Table 4 shows the maximum gold adsorption capacities of WSFW at 20, 30, 40, and 60 ◦ C were 8.68, 7.79, 7.44, and 7.25 mgAu /g, respectively, while the maximum gold adsorption capacities of activated carbon from coconut shells at 20, 30, 40, and 60 ◦ C were 76.78, 60.95, 56.13, and 51.90 mgAu /g, respectively. The adsorption capacities of both adsorbents increased with decreasing temperature. This implied that the adsorption mechanism was physical adsorption [22,23]. The adsorption isotherm shape was used to determine whether adsorption is favorable in terms of RL , a dimensionless constant referred to as the separation factor. RL was calculated using [24] RL =
1 , 1 + bC0
(10)
where C0 is the initial gold concentration in the solution (mgAu /l). If RL > 1, adsorption is unfavorable; RL = 1, adsorption is linear; 0 < RL < 1, adsorption is favorable; RL = 0, adsorption is irreversible. Table 4 shows that the values of RL were found to be 0.187–0.259 for WSFW and 0.076– 0.102 for activated carbon from coconut shells, respectively, which indicates favorable adsorption (0 < RL < 1) [24].
Fig. 6. Freundlich linear isotherm plots related to the adsorption of gold by wood shaving fly ash washed with tap water (WSFW) at temperatures of 20, 30, 40, and 60 ◦ C.
Table 4 Values of Langmuir and Freundlich constants of gold adsorption by wood shaving fly ash washed with tap water (WSFW) and activated carbon (from coconut shells) at various temperatures Adsorbent
Temperature (◦ C)
Langmuir
Freundlich
Qmax (mg/g)
b (l/mg)
R2
RL
Kf
1/n
R2
WSFW
20 30 40 60
8.68 7.79 7.44 7.25
0.112 0.102 0.099 0.074
0.996 0.990 0.986 0.986
0.187 0.203 0.219 0.259
1.101 0.962 0.928 0.722
0.563 0.555 0.538 0.586
0.994 0.995 0.997 0.986
Coconut shells
20 30 40 60
76.78 60.95 56.13 51.90
0.316 0.307 0.259 0.230
0.995 0.990 0.995 0.989
0.076 0.078 0.092 0.102
16.556 14.455 12.545 11.507
0.449 0.412 0.421 0.414
0.977 0.983 0.984 0.990
A. Aworn et al. / Journal of Colloid and Interface Science 287 (2005) 394–400
Fig. 7. Langmuir nonlinear isotherm plots related to the adsorption of gold by activated carbon (from coconut shells) at temperatures of 20, 30, 40, and 60 ◦ C.
Fig. 8. Freundlich linear isotherm plots related to the adsorption of gold by activated carbon (from coconut shells) at various temperatures of 20, 30, 40, and 60 ◦ C.
399
Fig. 9. Plot of ln b vs 1/T for gold adsorption by wood shaving fly ash washed with tap water (WSFW).
Fig. 10. Plot of ln b vs 1/T for gold adsorption by activated carbon (from coconut shells). Table 5 Thermodynamic parameters of gold adsorption by wood shaving fly ash washed with tap water (WSFW) and activated carbon (from coconut shells) at various temperatures
3.4. Thermodynamic study The parameters of the Gibbs free energy change (G), enthalpy change (H ), and entropy change (S) were calculated according to G = −RT ln(b),
(11)
G = H − T S, H ln b = − + constant, RT
(12) (13)
where R is the gas universal constant (8.314 J/K mol) and T is temperature (K) [24–27]. These parameters were estimated to evaluate the feasibility and exothermic nature of the adsorption process. The values of enthalpy change (H ) were calculated from the slope and intercept of the plot of ln b versus 1/T as shown in Figs. 9 and 10. These thermodynamic parameters for gold-cyanide adsorption on WSFW and activated carbon are summarized in Table 5. The negative G values confirm the feasibility of the adsorption process and the spontaneous nature of adsorption. Negative values of H indicated the exothermic nature of the process. This also confirmed the possibility of physical adsorption, as increasing temperature resulted in decreasing gold adsorption [26]. Positive S values corresponded to an increase in the degree of freedom of the adsorbed species [25].
Adsorbent
Temperature (◦ C)
G (kJ/mol)
H (kJ/mol)
S (J/(mol K))
WSFW
20 30 40 60
−11.50 −11.64 −11.77 −11.91
−8.48
10.32 10.43 10.53 10.53
Activated carbon (from coconut shells)
20 30 40 60
−14.02 −14.42 −14.46 −15.05
−6.96
24.11 24.64 23.98 24.30
Table 6 Elution of gold adsorbed onto wood shaving fly ash washed with tap water (WSFW) by deionized water as an eluent at various temperatures Eluent Deionized water
% Desorption 30 ◦ C 60 ◦ C 100 ◦ C
10.05 21.45 71.15
3.5. Elution study Deionized water was used as an eluent to elute gold from the gold adsorbed adsorbent. Deionized water at 30, 60, and 100 ◦ C could elute gold to 10.05, 21.45, and 71.15%, respectively (Table 6). This implied that increasing temperature
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higher than 100 ◦ C could elute gold more than 70%. The result indicated that temperature has an effect for the elution, because the gold adsorption by wood shaving fly ash is an exothermic process. Therefore, the desorption increases with increasing temperature. This confirmed that gold adsorbed onto WSFW mainly by physical adsorption.
4. Conclusions Gold adsorption by WSFW was higher than that by WSFA and by WSF because WSFW had a higher surface area and total pore volume. The gold adsorption by WSFW reached equilibrium at 120 min and optimum dosage 4.5% (w/v). There was no difference in gold adsorption at a system pH of 10–11. The maximum gold adsorption capacity by WSFW at 20, 30, 40, and 60 ◦ C was 8.68, 7.79, 7.44, and 7.25 mgAu /gadsorbent , respectively, while by activated carbon it was 76.78, 60.95, 56.13, and 51.90 mgAu /gadsorbent , respectively. Increasing temperature caused decreasing gold adsorption capacity. This implied that the mechanism of adsorption was mainly physical adsorption. Thermodynamic parameters indicate the process was exothermic, spontaneous, and favorable. However, WSFW is feasible for use as an alternative adsorbent for gold preconcentration and purification before electro-winning techniques were used. Although this adsorbent showed low adsorption capacity, the advantage lies in the easier elution of gold adsorbed adsorbent compared to activated carbon. This will also increase the value of waste wood shaving fly ash. An alternative way of increasing the adsorption capacity is to activate the wood shaving fly ash in order to increase the surface area of this adsorbent.
Acknowledgment The authors thank the Shell Centennial Educational Foundation for financial assistance.
References [1] World Gold Council, Gold Demand Trends (February 23, 2001) 34. [2] A.V. Pethkar, K.M. Paknikar, J. Biotechnol. 63 (1998) 121–136. [3] M. Iglesias, E. Anticó, V. Salvadó, Anal. Chim. Acta 381 (1999) 61– 67. [4] D.S.R. Murthy, Hydrometallurgy 25 (1) (1990) 51–60. [5] C.A. Fleming, Hydrometallurgy 30 (1992) 127–162. [6] G.A. Kordosky, J.M. Sierakosky, M.J. Virnig, P.L. Mattison, Hydrometallurgy 30 (1992) 291–305. [7] J. Marsden, I. House, The Chemistry of Gold Extraction, Horwood, New York, 1992. [8] J.D. Le Roux, A.W. Bryson, B.D. Young, J. S. Afr. Inst. Min. Metall. 91 (3) (1991) 95–103. [9] R. Oleshuk, A. Chow, Talanta 44 (1997) 1371–1378. [10] W. Lin, Y. Ly, H. Zeng, J. Appl. Polym. Sci. 49 (1993) 1635–1638. [11] M. Hosea, B. Greene, R. McPherson, M. Henzl, M.D. Alexander, D.W. Darnall, Inorg. Chem. Acta 123 (1986) 161–165. [12] A. Nakajima, T. Sagaguchi, J. Chem. Technol. Biotechnol. 57 (1993) 321–326. [13] M.D. Adums, C.A. Fleming, Metall. Trans. 208 (1989) 315–325. [14] R.J. Davidson, J. S. Afr. Inst. Min. Metall. 74 (1974) 67–79. [15] J.E. Urbanic, R.J. Jula, W.D. Faulkner, Miner. Metall. Process. 2 (1985) 193–198. [16] O. Bayat, H. Vapur, F. Akloy, C. Poole, Miner. Eng. 16 (2003) 395– 398. [17] G. McKay, Use of Adsorbents for the Removal of Pollutants from Wastewater, CRC Press, New York, 1996. [18] W. Nakbanpote, P. Thiravetyan, C. Kalambaheti, Miner. Eng. 13 (2000) 391–400. [19] N. Syna, M. Valix, Miner. Eng. 16 (2003) 421–427. [20] Y. Mustafa, I.A. Ali, Hydrometallurgy 63 (2002) 201–206. [21] D.M. Ruthven, Principles of Adsorption and Adsorption Process, Wiley, New York, 1984, pp. 29–61. [22] W.J. Weber, Physicochemical Process for Water Quality Control, Wiley, New York, 1972. [23] W. Nakbanpote, P. Thiravetyan, C. Kalambaheti, Miner. Eng. 15 (2002) 549–552. [24] W.S. Wan Ngah, K.H. Liang, Ind. Eng. Chem. Res. 38 (1999) 1411– 1414. [25] C. Raymon, Chemistry: Thermodynamic, McGraw–Hill, Boston, 1998, pp. 737–749. [26] S.R. Logan, Chemisorption and the Chemisorbed State, Fundamentals of Chemical Kinetics, Langman Group Limited, London, 1996, pp. 146–151. [27] A.K. Jain, Suhas, A. Bhatnagar, J. Colloid Interfac. Sci. 251 (2002) 39–45.
Recovery of gold from gold slag by wood shaving fly ash Amphol Aworn a , Paitip Thiravetyan b,∗ , Woranan Nakbanpote c a School of Energy and Materials, King Mongkut’s University of Technology Thonburi, 91 Prachautit, Bangmod, Thungkru, Bangkok 10140, Thailand b Division of Biotechnology, School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, 83 Moo. 8 Thakham,
Bangkhuntien, Bangkok 10150, Thailand c Pilot Plant Development and Training Institute, King Mongkut’s University of Technology Thonburi, 83 Moo. 8 Thakham, Bangkhuntien,
Bangkok 10150, Thailand Received 9 November 2004; accepted 3 February 2005 Available online 9 April 2005
Abstract Wood shaving fly ash was used as an alternative adsorbent for gold preconcentration from gold slag. The maximum gold adsorption capacity of wood shaving fly ash washed with tap water (WSFW) at 20, 30, 40, and 60 ◦ C was 8.68, 7.79, 7.44, and 7.25 mgAu /gadsorbent , respectively, while of activated carbon it was 76.78, 60.95, 56.13, and 51.90 mgAu /gadsorbent , respectively. Deionized water at 100 ◦ C could elute gold adsorbed onto WSFW to 71%. The effect of the increasing temperature of water, 30, 60, and 100 ◦ C, implied that the adsorption mechanism was mainly physical adsorption. The negative values of enthalpy change (H ) and free energy change (G) indicated an exothermic and spontaneous process, respectively. The positive values of entropy change (S) indicated increasing disorder of the system. The advantages of wood shaving fly ash are the purification of gold and the easier recycling of gold from the gold-adsorbed adsorbent. 2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; Gold cyanide; Gold slag; Wood shaving; Fly ash
1. Introduction According to the World Gold Council, the demand for gold has shown an increasing trend during the last decade [1]. This increase is due not only to the jewelry market, but also to the increasing uses of gold in industry, as well as medical applications [2]. Therefore, gold recycling from scraps and leach residues has also raised much interest [3]. Gold slag is wasted in the jewelry industry. It is known that industrial gold extraction commonly uses cyanide as a leaching agent [4]. The cyanidation process has been used by the mining industry for over 100 yr in the extraction of precious metals [5,6]. The recovery of gold by the adsorption of auro-cyanide complexes onto activated carbon is a well-established process and more recently adsorption by ion-exchange resins has been used [7,8]. Furthermore, other means of recovering gold from aqueous solution, such as ion * Corresponding author. Fax: +662 4523455.
E-mail address: [email protected] (P. Thiravetyan). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.02.048
exchange resins, have been studied. However, these types of systems are expensive and substances that are detrimental to the environment are produced during their fabrication [9,10]. A feasible alternative could be the use of biological materials for the biosorption of gold ions [11,12]. Activated carbon has proven to be an efficient and economical material for adsorption in systems that handle large amounts of highly concentrated aqueous solutions of gold. Activated carbon has also found increased application in the field of hydrometallurgy, especially in the recovery of gold and silver from cyanide solutions [13,14]. Gold recovery by activated carbon adsorption requires an activated carbon with the capability to adsorb gold that will be released easily in a later step. To be a suitable adsorbent for use in the gold recovery processes, an activated carbon must, therefore, have (a) a high mechanical strength, (b) a high adsorption capacity (loading capacity), and (c) a high adsorption rate [15]. The aim of this research is to recover gold from gold slag derived from the jewelry industry. Usually, the gold slag is extracted by a cyanidation process and refined
A. Aworn et al. / Journal of Colloid and Interface Science 287 (2005) 394–400
by electro-winning processes. The electro-winning process found that the gold-cyanide solution was at a low concentration (1–4 mg/l), which could not be refined. Therefore, gold adsorption by wood shaving fly ash is investigated. The effects of contact time, dosage, system pH, the adsorption isotherm, and thermodynamic and elution tests are examined to determine the optimum conditions and probable mechanisms of the gold adsorption.
2. Materials and methods 2.1. Adsorbent preparation Wood shaving fly ash (WSF) is a waste material from the textile industry. This industry uses wood shavings as a fuel in boilers in thermal power plants. It consists of 64.39% carbon, 31.09% oxygen, 2.50% potassium, and a little sodium, magnesium, calcium, phosphorus, chlorine, sulfur, and silicon. The WSF used in these experiments was divided into three types; (1) nontreated (WSFN), (2) washed with water for 1 h and then washed with an excess of double distilled water until the pH of the washing solution was constant (WSFW), and (3) soaked with 0.1 N H2 SO4 for 1 h and then washed with an excess of double distilled water until the pH of the washing solution was constant (WSFA). A commercial activated carbon (AC) of coconut shells was obtained from Mazuma (Thailand) Co., Ltd. All adsorbents were dried at 105 ◦ C and sieved to a size of <75 µm. 2.2. Extraction of gold slag Gold slag was obtained from jewelry waste. It was leached by sodium cyanide and then adjusted to pH 10.5. The slag leaching process used the ratio gold slag:NaCN: H2 O = 250 g:5 g:1000 ml, with feeding oxygen gas (30 mg/l) as an oxidizing agent [16], as shown in Eq. (1), at an equilibrium time of 2 h at 30 ◦ C. The mixture was then filtered through Whatman no. 5 filter paper and the supernatant was used for gold preconcentration by wood shaving fly ash: 4Au + 8NaCN + 2H2 O + O2 4Na[Au(CN)2 ] + 4NaOH.
(1)
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fly ash. Batch studies were carried out by adding 0.1000 g of wood shaving fly ash to 10 ml of 3–4 mgAu /l goldcyanide solution and shaken at 150 rpm, at 30 ◦ C for 2 h. Adsorption isotherms were studied by varying the dosage in the range of 0.10–5.00% (w/v) of wood shaving fly ash and 0.006–0.500% (w/v) for activated carbon (from coconut shells). A gold adsorption isotherm was carried out at various temperatures of 20, 30, 40, and 60 ◦ C. The sample was centrifuged at 4500 rpm for 10 min. The supernatant was analyzed for gold concentration and changing pH by inductively coupled plasma spectroscopy (ICP) (Jobin Yvon-JY 124, France) and pH meter (Mettler Delta 340, USA), respectively. The amount of gold adsorbed onto the adsorbent and the adsorption percentage were calculated respectively by [17] V , W C0 − Cf × 100, Adsorption (%) = C0 Qe = (C0 − Ce )
(2) (3)
where C0 is the initial concentration of gold in the solution (mgAu /l), Ce is the residual concentration of gold in the solution at equilibrium (mgAu /l), Cf is the final concentration of gold in the solution (mgAu /l), Qe is the gold adsorption capacity at equilibrium (mgAu /g), V is the volume of the solution (l), and W is the weight of the adsorbent used (g). 2.5. Elution experiments A 40.00-mgAu /l gold-cyanide solution was adsorbed onto 5% (w/v) of wood shaving fly ash and shaken at 150 rpm at 30 ◦ C for 2 h. The gold adsorbed sample was filtered and air-dried. The elution test was carried out by agitating 0.500 g of gold adsorbed wood shaving fly ash with 10 ml of deionized water at 30, 60, and 100 ◦ C and shaken at 150 rpm for 2 h. The sample was centrifuged and then the supernatant was used to determine the percentage of gold elution.
3. Results and discussion
2.3. Appearance and characteristic
3.1. Appearance and porosity of wood shaving fly ash
Wood shaving fly ash and activated carbon (coconut shells) (<75 µm) were dried at 105 ◦ C for 2 h. The appearance and surface area of the adsorbents were determined by scanning electron microscopy (SEM) (JSM-6400, Japan) and the BET method (surface area analyzer) (Quantachrom, Autosorb-1, USA), respectively.
Physical and chemical characterizations of the three types of wood shaving fly ash are given in Table 1. Scanning electron microscope (SEM) photographs of various types of wood shaving fly ash and activated carbon (from coconut shells) are shown in Fig. 1. Wood shaving fly ash treated by water and by H2 SO4 caused increased BET surface areas and total pore volumes because water and H2 SO4 were able to leach some tars and contaminants from the adsorbent (Table 2). The dissolution of potassium and calcium causes hydrogen gas and results in basidic conditions as shown in
2.4. Adsorption experiments The effects of contact time, dosage, and system pH were investigated for gold cyanide adsorption by wood shaving
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(a)
(b)
(c)
(d)
Fig. 1. SEM photographs of (a) wood shaving fly ash (WSF), (b) wood shaving fly ash washed with tap water (WSFW), (c) wood shaving fly ash soaked in 0.1 N H2 SO4 (WSFA), and (d) activated carbon (from coconut shells).
Table 1 Physical and chemical characteristics of wood shaving fly ash Bulk density (g/cm3 )
0.57
Moisture (%)
4.37
Ash (%)
28.70
pH
12.27
Size distribution (%) >250 µm 180–250 µm 150–179 µm 75–149 µm <75 µm
41.10 7.90 4.10 10.20 36.70
Element (%) C O Na Mg Si P S Cl K Ca
64.39 31.09 0.70 0.63 0.03 0.13 0.06 0.12 2.50 0.36
Table 2 BET surface area of various types of wood shaving fly ash and activated carbon Adsorbent
BET surface areaa (m2 /g)
Total pore volume (cm3 /g)
Average pore diameter (Å)
WSFN WSFW WSFA AC (coconut shells)
104.4 395.7 295.3 1113.0
0.0846 0.2564 0.2044 0.6236
32.42 25.92 27.68 22.41
a Specific surface area.
2K(s) + 2H2 O → 2KOH(aq) + H2(g) ,
(4)
Ca(s) + 2H2 O → Ca(OH)2(aq) + H2(g) .
(5)
The pHs of the system after WSF treatment by water and sulfuric acid were 12 and 10, respectively. Tar is easily removed under basidic conditions. Therefore, the BET surface area of WSFW was higher than of WSFA. Activated carbon from coconut shells has the highest surface area and total pore volume because this material passed both through carbonization and activation processes.
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3.2. Optimum conditions for gold-cyanide adsorption 3.2.1. Comparison of gold-cyanide adsorption by three types of WSF The efficiency of gold adsorption by WSFW was higher than by WSFA and WSFN, respectively (Table 3). This may explained by the surface area and total pore volume of WSFW being the highest. Activated carbon could adsorb more gold than the three types of WSF because activated carbon had a higher surface area and total pore volume than the three types of WSF [18–20]. The surface basicity and the surface acidity of the adsorbents seems not to influence the gold adsorption because activated carbon had the lowest basicity, while gold adsorption was the highest (Table 3). However, data on the gold adsorption efficiency of the three types of wood shaving fly ash were subjected to analysis of variance and means separation was performed using the new Duncan multiple range test (DMRT). The results were significantly different at P 0.05. The WSFW was a better adsorbent than the WSFA and WSFN, respectively. Therefore, the WSFW adsorbent was selected to study the gold adsorption experiments. 3.2.2. Effect of contact time The effect of contact time on gold adsorption by WSFW is shown in Fig. 2. The results indicate that gold was rapidly adsorbed within 15 min (73%), and then the adsorption rate increased slightly with time [20]. The data subjected
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to analysis of variance and means separation using the new Duncan multiple range test (DMRT) were not significantly different at P 0.05, indicating that the gold-cyanide adsorption by WSFW from 120 to 300 min was not different. This implies that the gold-cyanide adsorption reached equilibrium at 120 min. 3.2.3. Effect of dosage Fig. 3 shows that the efficiency of gold adsorption increased to 85% at 1.5% (w/v) dosage of adsorbent. The data were subjected to analysis of variance and means separation was performed using the new Duncan multiple range test (DMRT). The results were not significantly different at P 0.05, indicating that the gold-cyanide adsorption by WSFW from 4.5 to 15% (w/v) was not different. This indicates that the optimum dosage for gold-cyanide adsorption by WSFW was 4.5% (w/v). 3.2.4. Effect of system pH The effect of system pH values of 10 and 11 on gold adsorption was studied (see Fig. 4). This indicated an efficiency of gold adsorption of 84–86% at system pH 10–11. However, data subjected to analysis of variance and means separation performed using the new Duncan multiple range test (DMRT) were not significantly different at P 0.05, indicating that the gold-cyanide adsorption by WSFW was not different at pH values between 10 and 11.
Table 3 Comparison of gold adsorption by various types of wood shaving fly ash and activated carbon Adsorbent
Acidity Basicity Efficiency of BET gold adsorption surface areaa (meqHCl /g) (meqNaOH /g) (m2 /g) (%)
WSFN WSFW WSFA AC (coconut shells)
52.28b 86.05b 78.00b 97.10
104.40 395.70 295.30 1113.00
6.78 5.25 6.72 2.25
3.77 3.92 3.83 3.40
a Specific surface area. b The results were significantly different at P 0.05.
Fig. 2. Effect of contact time on gold adsorption by wood shaving fly ash washed with tap water (WSFW).
Fig. 3. Effect of dosage on gold adsorption by wood shaving fly ash washed with tap water (WSFW).
Fig. 4. Effect of system pH on gold adsorption by wood shaving fly ash washed with tap water (WSFW).
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3.3. Maximum gold-cyanide adsorption Adsorption isotherms were analyzed with Langmuir and Freundlich models, [17,21], Ce 1 Ce + = , (6) Qe Qmax b Qmax Qmax Ce Qe = (7) , 1/b + Ce 1/n
Qe = Kf Ce ,
(8)
log Qe = log Kf + (1/n) log Ce ,
(9)
where Ce is the residual concentration of gold in the solution at equilibrium (mgAu /l), Qe is the gold adsorption capacity at equilibrium (mgAu /g), Qmax is the maximum adsorption capacity of gold (mgAu /g), b is the affinity coefficient (l/mg), Kf is the Freundlich constant related to the capacity, and (1/n) is the Freundlich constant related to the intensity (0 < 1/n < 1). The Langmuir parameters Qmax and b were determined from the data using a nonlinear leastsquares program. The Freundlich parameters Kf and 1/n were calculated from a linear least-squares analysis of the data transformed according to Eq. (9). The calculated parameters are summarized in Table 4. Figs. 5 and 6 show the Langmuir isotherm and Freundlich isotherm of gold-cyanide adsorption by WSFW at temperatures of 20, 30, 40, and 60 ◦ C, respectively. The Langmuir
Fig. 5. Langmuir nonlinear isotherm plots related to the adsorption of gold by wood shaving fly ash washed with tap water (WSFW) at temperatures of 20, 30, 40, and 60 ◦ C.
isotherm and Freundlich isotherm of gold-cyanide adsorption by activated carbon from coconut shells are shown in Figs. 7 and 8, respectively. Table 4 shows the maximum gold adsorption capacities of WSFW at 20, 30, 40, and 60 ◦ C were 8.68, 7.79, 7.44, and 7.25 mgAu /g, respectively, while the maximum gold adsorption capacities of activated carbon from coconut shells at 20, 30, 40, and 60 ◦ C were 76.78, 60.95, 56.13, and 51.90 mgAu /g, respectively. The adsorption capacities of both adsorbents increased with decreasing temperature. This implied that the adsorption mechanism was physical adsorption [22,23]. The adsorption isotherm shape was used to determine whether adsorption is favorable in terms of RL , a dimensionless constant referred to as the separation factor. RL was calculated using [24] RL =
1 , 1 + bC0
(10)
where C0 is the initial gold concentration in the solution (mgAu /l). If RL > 1, adsorption is unfavorable; RL = 1, adsorption is linear; 0 < RL < 1, adsorption is favorable; RL = 0, adsorption is irreversible. Table 4 shows that the values of RL were found to be 0.187–0.259 for WSFW and 0.076– 0.102 for activated carbon from coconut shells, respectively, which indicates favorable adsorption (0 < RL < 1) [24].
Fig. 6. Freundlich linear isotherm plots related to the adsorption of gold by wood shaving fly ash washed with tap water (WSFW) at temperatures of 20, 30, 40, and 60 ◦ C.
Table 4 Values of Langmuir and Freundlich constants of gold adsorption by wood shaving fly ash washed with tap water (WSFW) and activated carbon (from coconut shells) at various temperatures Adsorbent
Temperature (◦ C)
Langmuir
Freundlich
Qmax (mg/g)
b (l/mg)
R2
RL
Kf
1/n
R2
WSFW
20 30 40 60
8.68 7.79 7.44 7.25
0.112 0.102 0.099 0.074
0.996 0.990 0.986 0.986
0.187 0.203 0.219 0.259
1.101 0.962 0.928 0.722
0.563 0.555 0.538 0.586
0.994 0.995 0.997 0.986
Coconut shells
20 30 40 60
76.78 60.95 56.13 51.90
0.316 0.307 0.259 0.230
0.995 0.990 0.995 0.989
0.076 0.078 0.092 0.102
16.556 14.455 12.545 11.507
0.449 0.412 0.421 0.414
0.977 0.983 0.984 0.990
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Fig. 7. Langmuir nonlinear isotherm plots related to the adsorption of gold by activated carbon (from coconut shells) at temperatures of 20, 30, 40, and 60 ◦ C.
Fig. 8. Freundlich linear isotherm plots related to the adsorption of gold by activated carbon (from coconut shells) at various temperatures of 20, 30, 40, and 60 ◦ C.
399
Fig. 9. Plot of ln b vs 1/T for gold adsorption by wood shaving fly ash washed with tap water (WSFW).
Fig. 10. Plot of ln b vs 1/T for gold adsorption by activated carbon (from coconut shells). Table 5 Thermodynamic parameters of gold adsorption by wood shaving fly ash washed with tap water (WSFW) and activated carbon (from coconut shells) at various temperatures
3.4. Thermodynamic study The parameters of the Gibbs free energy change (G), enthalpy change (H ), and entropy change (S) were calculated according to G = −RT ln(b),
(11)
G = H − T S, H ln b = − + constant, RT
(12) (13)
where R is the gas universal constant (8.314 J/K mol) and T is temperature (K) [24–27]. These parameters were estimated to evaluate the feasibility and exothermic nature of the adsorption process. The values of enthalpy change (H ) were calculated from the slope and intercept of the plot of ln b versus 1/T as shown in Figs. 9 and 10. These thermodynamic parameters for gold-cyanide adsorption on WSFW and activated carbon are summarized in Table 5. The negative G values confirm the feasibility of the adsorption process and the spontaneous nature of adsorption. Negative values of H indicated the exothermic nature of the process. This also confirmed the possibility of physical adsorption, as increasing temperature resulted in decreasing gold adsorption [26]. Positive S values corresponded to an increase in the degree of freedom of the adsorbed species [25].
Adsorbent
Temperature (◦ C)
G (kJ/mol)
H (kJ/mol)
S (J/(mol K))
WSFW
20 30 40 60
−11.50 −11.64 −11.77 −11.91
−8.48
10.32 10.43 10.53 10.53
Activated carbon (from coconut shells)
20 30 40 60
−14.02 −14.42 −14.46 −15.05
−6.96
24.11 24.64 23.98 24.30
Table 6 Elution of gold adsorbed onto wood shaving fly ash washed with tap water (WSFW) by deionized water as an eluent at various temperatures Eluent Deionized water
% Desorption 30 ◦ C 60 ◦ C 100 ◦ C
10.05 21.45 71.15
3.5. Elution study Deionized water was used as an eluent to elute gold from the gold adsorbed adsorbent. Deionized water at 30, 60, and 100 ◦ C could elute gold to 10.05, 21.45, and 71.15%, respectively (Table 6). This implied that increasing temperature
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higher than 100 ◦ C could elute gold more than 70%. The result indicated that temperature has an effect for the elution, because the gold adsorption by wood shaving fly ash is an exothermic process. Therefore, the desorption increases with increasing temperature. This confirmed that gold adsorbed onto WSFW mainly by physical adsorption.
4. Conclusions Gold adsorption by WSFW was higher than that by WSFA and by WSF because WSFW had a higher surface area and total pore volume. The gold adsorption by WSFW reached equilibrium at 120 min and optimum dosage 4.5% (w/v). There was no difference in gold adsorption at a system pH of 10–11. The maximum gold adsorption capacity by WSFW at 20, 30, 40, and 60 ◦ C was 8.68, 7.79, 7.44, and 7.25 mgAu /gadsorbent , respectively, while by activated carbon it was 76.78, 60.95, 56.13, and 51.90 mgAu /gadsorbent , respectively. Increasing temperature caused decreasing gold adsorption capacity. This implied that the mechanism of adsorption was mainly physical adsorption. Thermodynamic parameters indicate the process was exothermic, spontaneous, and favorable. However, WSFW is feasible for use as an alternative adsorbent for gold preconcentration and purification before electro-winning techniques were used. Although this adsorbent showed low adsorption capacity, the advantage lies in the easier elution of gold adsorbed adsorbent compared to activated carbon. This will also increase the value of waste wood shaving fly ash. An alternative way of increasing the adsorption capacity is to activate the wood shaving fly ash in order to increase the surface area of this adsorbent.
Acknowledgment The authors thank the Shell Centennial Educational Foundation for financial assistance.
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