Minerals Engineering, Vol. 13, No. 4, pp. 391-400, 2000
© 2000 Elsevier Science Ltd All rights reserved 0892-6875/00/$ - see front matter
PRECONCENTRATION OF GOLD BY RICE HUSK ASH* W. NAKBANPOTE §, P. THIRAVETYAN § and C. KALAMBAHETF § School of Bioresources and Technology, King Mongkut's University of Technology Thonburi, Prachauthit Road, Thungkru, Bangkok, 10140, Thailand. E-mail [email protected]
i PTT Research and Technology Institute Petroleum Authority of Thailand, 71 Phahonyothin Road, Wangnoi, Ayuthaya, 13170, Thailand (Received 2 November 1999; accepted 5 January 2000)
ABSTRACT The goal of this research was to develop a new, efficient adsorbent of gold-thiourea complex, [Au(CS(NHz)J +. In this experiment, rice husk was heated at three different temperature: 300°C, 400°C and 500°C Rice husk ash heated at 300°C adsorbed gold thiourea complex, whereas rice husk ash heated at 400°C and 500°C did not. The structure of rice husk ash heated at 300°C had specific silanol groups and oxygen functional groups of carbon, while rice husk ash heated at 400°C and 500°C concained siloxane groups. Maximum gold adsorption of rice husk ash heated at 300°C and activated carbon, was 21.12 and 33.27 mg Au/g adsorbent, respectively. However, rice husk ash absorbed 0.072 mg thiourea/g adsorbent, which was less than activated carbon adsorbed (0.377 mg thiourea/g adsorben O. In addition, the adsorbed gold could be eluted from this rice husk ash by sodiumthiosulf~!te more easily than from activated carbon. The results revealed that rice husk ash heated at 300°C can be used as a new adsorbent for gold thiourea complex. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords Activated carbc,n; hydrometallurgy; ion exchange; biotechnology; mineral processing INTRODUCTION Industrial gold extraction commonly uses cyanide as a leaching agent, but due to environmental concerns, thiourea solution is increasingly being used instead of cyanide (Lee et al., 1997; Deschenes, 1987; Murthy, 1990). In addition, thiourea is also used to extract gold from refractory ores, such as arsenopyrite, pyrite, carbonacous sulphide and chalcopyrite, higher than cyanide. However, the utilisation of thiourea still requires development in the recovery of gold from solution in order to minimise thiourea consumption and thus production costs (Lee et al., 1997; Deschenes, 1988; Deschenes and Ritcey, 1990; Deschenes et aL, 1994). In the case of a gold solution lower than 8 mg/1, a pre-concentration process is also essential before the gold separation process can be conducted (Lee et al., 1997; Deschenes and Ritcey, 1990; Deschenes et al., 1994). Activated carbon has been used as an adsorbent to concentrate gold and has the ability to adsorb gold-thiourea complex, but it also adsorbs thiourea and the elution of adsorbed gold is difficult (Deschenes, 1987; Deschenes and Ritcey, 1990; Deschenes et al., 1994; Hisshion and Waller, 1984; Yen et al., 1994). Cation exchange resin is an alternative adsorbent, providing a good recovery of gold-thiourea complex with lower thiourea consumption and easier elution than activated carbon (Deschenes, 1987; Deschenes *Presented at Minerals Engineering '99, Falmouth, Cornwall, England, September 1999 391
w. Nakbanpoteet al.
and Ritcey, 1990)]. Moreover, cation exchange resin has the ability to concentrate gold and reduced unwanted metals from the pregnant solution (Deschenes, 1987; Nakahiro, 1992). However, the cation exchange resin is expensive. Rice husk is an agricultural waste. Thailand produces 20-26 million tonnes of rice each year. From this, an estimated 4-7 million tonnes office husk are available. The composition office husk is : 32.24% cellulose, 21.34% hemiceUulose, 21.44% lignin, 1.82% extractives, 8.11% water and 15.05% mineral ash (Rahman and Ismail, 1993; Rahman et al., 1997). Typical chemical composition of the mineral ash is: 96.34% SiO2, 2.31% K20, 0.45% MgO, 0.2% Fe203, 0.41% A1203, 0.41% CaO and 0.08% K20 (Darnel, 1976). Rice husk biomass can be exploited for many uses, including : energy production (Beagle, 1978), a mulch (Beagle, 1977), production of activated carbon (Usmani et al., 1993) silicon carbide (Moustafa et al., 1997) and tetramethoxysilane (Akiyama et al., 1993) and as silica for ceramics (James and Rao, 1986) and portland cement (Barkakati et aL, 1994). Since the main components of rice husk are carbon and silica, it has the potential to be used as an adsorbent. For example, rice husk ash heated at 500°C has been used as an adsorbent in vegetable oil refining (Proctor and Palaniappan, 1989; Proctor and Palaniappan, 1990; Palaniappan and Proctor, 1990; Proctor et al., 1995). The aim of this research is to produce a new adsorbent from rice husk used in stead of activated carbon to adsorb gold-thiourea complex with a low thiourea consumption and also easily in elution. A new adsorbent must have cation exchange properties but must also be less expensive. The paper accessed the ability of rice husk ash adsorb gold-thiourea complex. Physical and chemical characteristics of rice husk ash heated at three different temperatures were investigated. The comparative efficiency of rice husk ash and activated carbon to adsorb thiourea and elute gold was also conducted.
MATERIALS AND METHODS
Sample preparation Rice husk was obtained from Royal Mill, Bangkok, Thailand, and dried at 105°C for 2 hours, before heating to 300°C, 400°C or 500°C (RHA 300, R.HA 400 and RHA 500, respectively) for 2 hours in a muffle furnace (Thermolyne 48000, USA). The rate of heating was 25-35°C/min. The heated rice husk and activated carbon (Sigma C-5261, USA) were crushed and sieved to obtain an approximate diameter size of < 75 ~tm. The pH of the rice husk ash and of the activated carbon, was determined following the method of Menon and Dave (1993).
Shake flask experiments Gold-thiourea adsorption by rice husk ash and activated carbon was compared. The gold-thiourea complex was obtained from tetra-chloroaurate by dilution with thiourea solution. Gold-thiourea solution (pH 3.0) was used in this experiment. This experiment was performed in a 250 ml Erlenmeyer flask with 0.5 g of adsorbent and 100 ml of gold-thiourea, at a concentration of 100 mg/1, and shaken at 250 rpm. A sample was taken in an eppendroff tube and centrifuged at 10,000 rpm for 15 minutes. The supernatant was then analysed by Inductive Couple Plasma Spectroscopy (ICP) (JOBIN YVON-JY24, France). In the control experiment (without added gold solution), thiourea adsorption by RHA300 and activated carbon was also examined. Thiourea concentration was determined by Rapid oxidation method (Winlder, 1995) The elution test was carried out by using 0.05 M and 0.5 M sodiumthiosulf'ate in a carbonic acid buffer of pH 9.0 as an eluent. This was performed in a 250 rnl Erlenmeyer flask with 0.5 g of the gold-adsorbed material (10 mg Au/g adsorbent) and 100 ml of 0.05 M or 0.5 M sodiurnthiosulfate, while shaking at 250 rpm at 30°C. A sample was taken in an eppendroff tube and centrifuged at 10,000 rpm for 15 minutes. The gold concentration in the supematant was determined by ICP.
Column experiments A disposable syringe, 0.5 cm in diameter and 10 cm in height (Nissho Nipro, Thailand), was used as a column. Each column contained either 0.05 g of RHA 400, RHA 500 or silicic acid. These were pre-treated with 10 ml of 2 M sulphuric acid and excess de-ionised water to both remove other metal oxides and adjust the pH of the adsorbent from 10.0 to 4.0 (near the pH of RHA 300). The column was conditioned with 10 ml of thiourea 20 g/1 and then 50 ml of 1 rag/1 of gold thiourea solution (pH 3.0) was loaded at a flow rate of 0.5 ml/min by vacuum manifold. The concentration of gold in the effluent was determined by ICP.
Analytical techniques Determination of gold concentration Inductive Couple Plasma Spectroscopy (ICP) was used for the determination of gold concentration in aqueous solution. The instrument employed was Jobin Yvon-JY24, France. Gold was analysed at a wavelength of 242.8 nm.
The percentage of carbon in adsorbents Carbon analysis (ttoriba, EMIA-8200H, Japan)was used to obtain the total carbon of each adsorbent. Spectroscopic graphite powder (C > 99.99%) was used as a standard.
The percentage of silicon dioxide in adsorbents X-ray fluorescence spectrometry (Horiba, MESA-500, Japan) was used for the determination of silicon dioxide in the adsorbents. Typically, 1 g of dried sample (< 75 ktm) was mixed with 5 g of cassava powder. The mixed powder was pressed into a 32 mm diameter disc in a steel disc mould at a force of 250 kN for 1 minute. All measurements were carried out against a standard disc prepared by mixing known amounts of silicic acid with cassava powder and pressing into a disc as described above.
The structure of adsorbents The rice husk ash and activated carbon were ground with an agate pestle and mortar and then sieved to obtain an approximate size of < 75 ktm, before being analysed by X-ray diffractometer (Philips PWl710, USA). Copper Ko: radiation was generated by a Philips PW1710 X-ray diffractometer utilising an acceleration voltage of 40 kV and a current of 30 mA. A diffraction angle of 10-40 ° 20 was scanned at a rate of 2 deg/min.
The study of functional groups on adsorbents Infrared spectra were obtained from a Fourier transform infrared spectrophotometer (Bio-Rad FTS 185, USA). The spectra were recorded using a diffuse reflectance accessory. Typically, 0.001 g o f d r i e d s a m p l e (<75~tm) was m i x e d with 0.2 g o f p o t a s s i u m b r o m i d e p o w d e r (IR Spectroscopy grade, Fluka Chemical) in an agate pestle and mortar. Each specmma was recorded with 16 scans.
The porosity of adsorbents The porosity of the adsorbent was determined by N2 adsorption at 77 K in conventional volumetric equipment of the Micromeritics (ASAP 2000, USA). The N2 apparent surface area was calculated by using the BET equation (]3runauer et al., 1938). Total pore volume and average pore diameter were calculated by using BJH-method (Barrett et al., 1951).
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RESULTS AND DISCUSSION Physical and chemical characterisation
The physical and chemical characteristics of the adsorbents are shown in Table 1. The result indicated that heating of rice husk at different temperatures produced ashes containing different content of carbon and silicon dioxide. The colour of RHA 300, RHA 400 and RHA 500 were black-brown, grey and white, respectively. The rice husk ash heated at higher temperatures had reduced percentages of carbon but an increased proportion of silicon dioxide. Almost all of the carbon was lost when heated at _>400°C. This was also the primary reason for the large difference between RHA 300 and the other two samples. RHA 400 and RHA 500 had similar physical and chemical properties, in which the higher pH was caused by an increased percentage of metal oxide. Pore diameters of the adsorbents were micropore (Brewer, 1964), but total pore volume and the BET surface area of rice husk ash was less than activated carbon. The activated carbon had a higher total pore volume and BET surface area. This was because it was heated at 700°C to 800°C in the absence of oxygen and also activated by steam. TABLE
1 Physical and Chemical characteristics temperatures and activated carbon
Property Colour Carbon, % Silicondioxide, % PH Average pore diameter, nm Total pore volume, ml/g BET surface area, m~/g
300°C Black-brow n 38.00 32.02 4.40 4.20 0.042 20.26
of rice husk ash heated at three different
Rice husk ash 400°C
1.88 79.27 9.92 14.49 0.182 50.14
0.20 81.04 10.10 15.16 0.155 40.93
91.60 0.00 9.74 3.35 0.489 583.74
However, the X-ray diffraction patterns of RHA 300, RHA 400 and RHA 500 show a broad 'hump' between 15 and 35 ° 20 diffraction angle, whilst lacking any def'med peaks (Figure 1). These patterns are similar to X-ray diffraction patterns of silicic acid (Figure 2) and is characteristic of amorphous substances. The results indicated that heating rice husk ash at 500°C did not change the structure of silicon dioxide from an amorphous structure to a crystalline structure, such as quartz, critobalite or tridrimite. 120
RHA 300 RHA 400 and RHA 500
B . . ! . I. . ! ! I I 1 I 1 1 1 1 1 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 |
Fig. 1 X-ray diffraction pattern of RHA 300, RttA 400 and RHA 500.
120 100 t
60 40 20
. . . . . . . . . . . |
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 020 Fig.2 X-ray diffraction pattern of Silicic acid.
The gold-thiourear adsorption Gold adsorption by RHA 300 and activated carbon reached equilibrium after six hours (Figure 3). The pH of the gold-thiourea solution of activated carbon decreased from 5.4 to 4.6, while the pH of the gold solutions of RHA 300, RHA 400 and RHA 500 were relatively constant (Figure 4). RHA 300 was able to adsorb gold thiouzea complex, whereas RHA 400 and RHA 500 could not. Even in the column experiments, the pre-treated RHA 400 and RHA 500 could still not adsorb the gold complex.
40 2O ---'-¢-
r . . . . .
T i m e (hours) Fig.3 Kinetics of adsorption of gold-thiourea complexes on RHA 300 (A), RHA 400 (o), RHA 500 (X) and activated c,arbon ([2).
W. Nakbanpote et al.
6.0 5.0 4.0 .ALL..:..,. i
..% ~ _
1.0 0.0 0
Time (hours) Fig.4 The pH profile of gold-thiourea solution during gold adsorption on RHA 300 (A), RHA 400 (o), RHA 500 (×) and activated carbon (). The FTIR spectra of RHA 300, RHA 400 and RHA 500 revealed that heating rice husk ash at temperatures greater than 300°C increased the siloxane groups, but decreased the silanol groups, hydroxyl groups and oxygen functional groups (of carbon) (Figure 5). RHA 300 contained silanol groups, hydroxyl groups and oxygen functional groups, such as carboxylic and phenolic groups, that may support gold adsorption as explained in the following section. The main functional groups of RHA 400 and RHA 500, were the siloxane groups that can not adsorb the gold complex. However, the FTIR spectra of RHA 400 and RHA 500 indicated the presence of silanol groups, but these silanol groups were in the form of silicon dioxide structure (-Si-O-Si-OH). The structure is similar to the silanol groups of silicic acid, which also can not adsorb the complex (Figure 6).
0.5 0.4 L O A3
< 0.2 0.1 Si-OH, -OH
2200 1600 2800 Wave numbe r ( c m "1)
Fig.5 The Fourier transform infrared spectra of RHA 300 ( - ) RHA 400 (...) and RHA 500 (-).
Preconcentration of gold
L4 1.2 ¢g
.< 0.6 CI.4
_ A J I
2800 2200 1600 Wavenumbe r (cm"s)
Fig.6 The FTIR spectrum of silicic acid.
Adsorption isotherm of goid-thiourea complex
The Langrnuir equilibrium adsorption isotherm, as shown in Equation (1), was originally developed to represent chemisorption on a set of distinct localised adsorption sites (Ruthven, 1984). At equilibrium, the rates of adsorption and desorption are equal :
q = l+bC
q C qmd2¢ b
is the concentration of gold concentration in the adsorbent at equilibrium (mg Au/g adsorbent), is the concentration of gold in the solution at equilibrium (mg Au/rnl), is the maximum gold adsorption (mg Au/g adsorbent), and is a constant (b= ka/kd; ka is the adsorption rate constant and kd is the desorption rate constant).
The adsorption isol~erm of gold-thiourea complex onto activated carbon and RHA 300, fit the Langmuir model (Eq (1)) as :~hown in Figure 7. From the Langmuir model, the maximum gold adsorption (qmJ of RHA 300 and activated carbon were 21.12 and 33.27 mg Au/g adsorbent, respectively. However, commercial practices, such as New England Antimony Mines use activated carbon to recover gold from thiourea leach solution containing antimony, gold, etc. The adsorbed gold material is sold as a carbon concentrate containing only 6-8 mg Au/g carbon (Hisshion and Waller, 1984). Therefore, maximum goldthiourea adsorption on rice husk ash is sufficient for the preconcentration of gold from a low gold-thiourea solution. In addition, activated carbon adsorbed thiourea higher than RHA 300 (Table 2), perhaps because it has many functional groups that can adsorb thiourea, gold and other metals (Deschenes, 1987; Lee et al., 1997) and also a high surface area. TABLE 2 Maximum gold-thiourea adsorption and percentage of thiourea consumption by RHA 300 and activated carbon
Adsorbent RHA 300 Activated carbon
qm,,~ (mg Au/g adsorbent) 21.12 33.27
Thiourea consumption (mg Thiourea/g adsorbent) 0.072 0.377
W. Nakbanpote et al.
lO 5 0 0
C (mg Au/1) Fig.7 The adsorption isotherm of gold-thiourea complexes onto activated carbon ([2) and RHA 300 (A).
Elution test From the results, RHA 300 may contain cation exchange properties. The gold adsorbed material is examined by using sodiumthiosulfate (Na2S203) as an eluent to change from gold-thiourea complex, [Au(CS(NH2)2)] +, to gold-thiosulphate, [Au(S203)]-. The reaction occurred because the overall formation (log [3) of gold-thiosulphate (log 13--26) was higher than gold-thiourea complex (log [3=21.95) (Nakahiro et al., 1992). The adsorbed gold complex was eluted from RHA 300 at a higher rate than activated carbon (Figure 8). This also indicated that rice husk ash had cation exchange properties and eluted gold more easily than activated carbon.
4 6 2
I I 10 12
I 0 14 16
Time (hours) Fig.8 The elution of adsorbed gold from RHA 300 and activated carbon by 0.05 M and 0.5 M sodiumthiosulfate in carbonic acid buffer pH 9.0 at 30°C. (RHA 300:0.05 M sodiumthiosulfate (A), 0.5 M sodiumthiosulfate (A); Activated carbon: 0.05 M sodiumthiosulfate (ll), 0.5 M sodiumthiosulfate (E2)).
The possible mechanism of gold-thiourea adsorption onto activated carbon may be due to the oxygen functional group on the carbon structure. This would account for decreasing pH in the solution as shown in Figure 4. The adsorption mechanism is the covalent bonds that made it difficult to elute with sodiumthiosulphate, (Figure 9(a)). Whereas RHA 300 contains oxygen functional groups and specific arranged silanol groups, both of which adsorbed gold-thiourea complex (Figure 9(a)-9(b)). (a)
"z AuTU + 0 /
C - - O \ AuTU +
Fig.9 "[he mechanism of gold adsorption by (a) activated carbon and (b) RHA 300.
CONCLUSIONS The simple heating office husk to 300°C produced a novel adsorbent for gold thiourea complex. Maximum gold adsorption of KHA 300 was 21.12 mg Au/g adsorbent, which is sufficient to use in the gold industry. But investigation of RHA 300 to adsorb a gold-thiourea solution containing gold and other metals still needs further study. In addition, RHA 300 has an ion-exchange property. However, in order to reuse RHA 300 most effectively, further research is needed regarding optimisation of gold elution from RHA 300.
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