Investigation of granular activated carbon from peach stones for gold adsorption in acidic thiourea

MASIYA, T.T. and GUDYANGA, F.P. Investigation of granular activated carbon from peach stones for gold adsorption in acidic thiourea. Hydrometallurgy Conference 2009, The Southern African Institute of Mining and Metallurgy, 2009. Investigation of granular activated carbon from peach stones for gold adsorption in acidic thiourea T.T. MASIYA* and F.P. GUDYANGA *Institute of Mining Research, University of Zimbabwe, Zimbabwe Department of Metallurgy, University of Zimbabwe, Zimbabwe This paper reports on an investigation into the effectiveness of using granular activated carbon produced from peach stones, an agricultural waste, for the adsorption of gold from acidified thiourea solutions. In this work the activated carbon was produced by chemical activation of the peach stones with either phosphoric acid or zinc chloride. The effects of several parameters on the adsorption kinetics and equilibra were examined. Gold adsorption was found to depend significantly on: solution ph, rate of stirring, activated carbon dosage, carbon particle size, and the initial concentration of gold and thiourea. The adsorption equilibrium experimental data fitted well both the Langmuir and Freundlich isotherm models with high correlation coefficient. The adsorption capacity calculated from the Langmuir isotherm was 31.2 mg Au/g for phosphoric activated peach stones and 69.0 mg Au/g for zinc chloride activated peach stones. Introduction The ability of activated carbon to adsorb dissolved metal species has been known for a considerable time. It has found increasing application, especially in the mining industry, as an adsorbent for the extraction of gold from leached solutions (Petersen and van Deventer, 1994; Hurter, 1986). The adsorption of gold cyanide onto activated carbon, especially of coconut shell origin, has been studied extensively and is well known (Yapu et al., 1994; Van Deventer and Van der Merwe, 1993; Marsden and House, 1992; Adams and Fleming, 1989). Activated carbon has also been used for gold recovery from non-cyanide solutions (Haque, 1989). Lignite and activated bagasse were successfully assessed for adsorption of gold from acidic thiourea solutions (Syna and Valix, 2003; Zouboulis et al., 1994). It is of both technological and economic interests to examine the potential of activated peach stones for the adsorption of gold species from acidic thiourea solutions. Peach stones are agricultural by-products that are currently of no economic value, and have a hard lignocellulosic material shell that gives them the potential to be used as raw materials for production of granular activated carbon. In Zimbabwe more than 90% of the activated carbon used by the mineral industry is imported at a very high cost (Thixton, 1998). INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 465

The peach stones used for this study were activated chemically by phosphoric acid and zinc chloride. The effect of different process conditions such as ph, particle size, carbon dosage, stirring speed, initial gold and thiourea concentration on per cent gold recovery was investigated to ascertain the mechanism for adsorption. Experimental The activated carbon particles used in this study were manufactured from peach stones by chemical activation with either phosphoric acid or zinc chloride. Except where stated otherwise, the activated carbons in the size range -1.0 +0.5 mm were used for the investigations. ZnCl 2 activation 3.3 g of ZnCl 2 was dissolved in 100 ml distilled water. 100 g of the raw peach stones were then soaked in this solution for 24 hours. At the end of this time, the solution was filtered and the impregnated peach stones were dried in an oven overnight at 105 C after which they were activated at 700 C for 2 hours in a muffle furnace. The sample was then removed from the furnace, allowed to cool and then washed in hot HCl solution and rinsed in distilled water until the wash water was almost neutral. H 3 PO 4 activation The raw peach stones were impregnated with 43 wt% H 3 PO 4 for 24 hours in the weight ratio 1:1 (peach stones: H 3 PO 4 ), followed by drying in an oven overnight at 105 C. The dried sample was then activated in a muffle furnace at 300 C for 1 hour. The sample was allowed to cool and subsequently washed first in hot NaOH solution followed by a series of soaking and decanting in hot distilled water until there were no traces of phosphates (identified by adding a few drops of Pb(NO 3 ) 2 into the wash water, which turns to a white precipitate in the presence of phosphates). Equilibrium adsorption studies Equilibrium data was collected by taking 200 ml solution of known gold concentration into a series of 500 ml containers. To each container, different activated carbon dosages (0.05 to 2.0 g) were added. Prior to their use activated carbon samples were washed thoroughly with water until most of the fines were removed, and then dried at 105 C in an oven for 24 hours. The containers were sealed and bottle rolled for 72 hours until equilibrium was assumed to have been reached. The solutions were filtered and the filtrates were analysed for residual gold concentration using the Spectra Atomic Absorption Spectrometer (AAS). Batch kinetic adsorption studies The kinetic adsorption studies were carried out by stirring between 0.25 g and 1.0 g of activated carbon with 300 ml acidified aqueous gold-thiourea solution of desired initial concentration in a series of 800 ml Duran beakers on a magnetic stirrer (HeIdolph type) with an adjustable stirring speed. Temperature was maintained at room temperature during the investigations. Aliquots of 10 ml were withdrawn regularly (after 15, 30, 45, 60, 90, 120, 180 and 240 minutes) and filtered through a filter paper. Filtrates were analysed for residual gold concentration using an atomic absorption spectrometer (AAS). 466 HYDROMETALLURGY CONFERENCE 2009

Results and discussion Effect of ph on gold adsorption Gold adsorption on peach granular activated carbon and commercial granular activated carbon as a function of initial ph was studied for the ph range 1 to 12, and the results are shown in Figure 1. In the ph range 1.4 to 4 the per cent gold recovery initially decreases for both activated carbons as the initial ph is increased. As solution ph is increased above ph 4.1 a sharp increase in gold recovery is observed, especially with activated peach stones. This is attributed to the fact that beyond ph 5, thiourea is known to decompose to sulphur and cyanamide through an intermediate product, formamidine disulphide. [1] Zouboulis et al. (1994) attributed this observed increase in gold recovery in the alkaline range to increased adsorption capacity by the finely divided elemental sulphur formed. Figure 2 presents the effect of initial ph value on the kinetics of gold adsorption from thiourea solutions. The gold adsorption kinetic is faster at lower initial ph than at higher initial ph values. The adsorption of metal ions depends on solution ph, which influences electrostatic binding of ions on the activated carbon to corresponding metal groups in solutions (Ahalya et al., 2005). Effect of activated carbon particle size on gold adsorption Figure 3 presents gold adsorption behavior of -0.150 mm peach activated carbon particles as a function of contact time in comparison with that of the fraction -1.0 +0.5 mm of the same activated carbon. As is expected, the smaller the size of granules, the faster the rate of adsorption of gold by the activated carbon. Smaller particle size, for the same mass of activated carbon, enhances gold adsorption (Yannopoulos, 1990). This is attributed to increase in surface area for adsorption and reduced mean pore length through which gold species travel within the activated carbon particles as particle size decreases. The ultimate carbon loading Figure 1. Effect of initial solution ph on equilibrium gold adsorption on peach granular activated carbon (PGAC) and commercial granular activated carbon (CGAC) from thiourea solutions (initial solution concentration = 7.5 mg Au/L; solution volume = 200 ml; carbon dosage = 0.2 g) INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 467

Figure 2. Effect of initial ph on the rate of gold adsorption from acidic thiourea solutions (initial solution volume = 300 ml, carbon dosage = 0.5g, initial gold content = 10.5 mg/l) Figure 3. The effect of activated carbon size on rate of gold adsorption from thiourea solutions Figure 4. Variation of gold recovery with time for different stirring speeds (initial solution volume = 300 ml; initial gold content = 5.0 mg/l; ph = 1.7; carbon dosage = 0.5 g) 468 HYDROMETALLURGY CONFERENCE 2009

capacity, however, is virtually independent of particle size (Marsden and House, 1992). However, the use of fine activated carbon is not recommended, especially in carbon-in-pulp plants as it will result in higher gold losses via the carbon fines. Effect of stirring speed on kinetic adsorption of gold from thiourea solutions The effect of stirring (Figure 4) of the adsorbent/adsorbate system in acidic gold thiourea solutions was investigated for cases where agitation resulted in activated carbon being fully suspended in the gold solutions (i.e. between 200 and 400 rpm). Below 200 rpm it was observed that some of the carbon remained on the base of the beaker and at 500 rpm the magnetic follower became unstable. The observed increase in rate of adsorption of gold with stirring speed in the range 200 to 400 rpm is attributed to the improvement in contact between the gold species in solution and the active sites on the carbons, thereby promoting effective transfer of adsorbate ions to the adsorbent site. Effect of initial gold concentration on equilibrium adsorption The equilibrium removal of gold from solution decreases as the initial gold concentration of the solution is increased for both carbon dosages investigated (see Figure 5). This is because at lower concentration, the ratio of the initial moles of gold species to the available surface area is low and subsequently the fractional adsorption becomes independent of initial concentration. However, at higher concentration the available sites for adsorption become fewer compared to the moles of gold species present and hence the percentage removal of gold is dependent upon the initial gold concentration, i.e. there is increased competition among gold species for the available active sites on the carbon surface. For the same initial gold concentration there is an increase in per cent gold removal as the carbon dosage is increased from 0.25 g to 0.5 g. This is because there was an increase in number of available active adsorption sites for the same number of moles of gold species as carbon dosage was increased Figure 6 shows the variation of the actual amount of gold adsorbed per gramme of activated carbon with time. An increase in the initial gold concentration leads to an increase in the adsorption capacity of gold on activated carbon. This indicates that the initial gold concentration plays an important role in the adsorption capacity of gold on activated carbon. Figure 5. Effect of initial gold concentration on equilibrium gold recovery for two different initial activated peach carbons concentrations (bottle rolled; solution volume = 200 ml; contact time = 5 hours; ph = 1.7) INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 469

Figure 6. Variation of amount of gold adsorbed as a function of time for two different initial gold concentrations (initial solution volume = 300 ml; carbon dosage = 0.5 g; ph = 1.7) Figure 7. Effect of initial thiourea concentration on gold recovery (bottle rolled: initial gold concentration = 4 mg /L; solution volume = 200 ml; carbon dosage = 0.25 g and 0.5 g; contact time = 5 hours; ph = 1.8) Effect of initial thiourea concentration on gold recovery The effect of the amount of free excess thiourea concentration on equilibrium gold-thiourea complex adsorption on activated peach stones was examined and the results are presented in Figure 7. The loading capacity for gold on activated carbon was found to decrease as the amount of free excess thiourea concentration was increased. Thiourea concentration is an important parameter when considering the adsorption of gold from thiourea solutions on activated carbon. Thiourea is an organic molecule that has a high tendency to adsorb on activated carbon. When there is not enough carbon, or the number of surface active sites is not sufficient for adsorbing both gold-thiourea complexes and free thiourea, then thiourea will be loaded preferentially over gold, and gold adsorption will be suppressed (Petersen and van Deventer, 1994; Zouboulis et. al., 1994). 470 HYDROMETALLURGY CONFERENCE 2009

However, when there is excess activated carbon, or the number of surface active sites exceeds the total number of molecules to be adsorbed, then free thiourea does not interfere with adsorption of gold-thiourea complexes. Deschenes and Ghali (1988) explained this decline in gold adsorbed differently. They attributed it to the fact that thiourea is less stable at high concentrations and decomposes to cyanamide and elemental sulphur. Similar observations were also reported by Amer (2002). They believed that the elemental sulphur formed blocks on some of the macropores on the carbon, resulting in gold species being blocked from reaching the micropores. Adsorption isotherm equilibria To quantify the adsorption capacity of activated peach stones for removal of gold from acidic thiourea solutions, the adsorption isotherm data was evaluated using the Langmuir and Freundlich adsorption isotherms. The basic assumption of the Langmuir adsorption process is the formation of a monolayer of adsorbate on the outer surface of the adsorbent and after that no further adsorption takes place. A linear form of the Langmuir equation is given by: [2] where Q e is the equilibrium quantity of gold adsorbed on activated carbon (mg/g), Q m the maximum monolayer adsorption, K L the langmuir equilibrium constant for the adsorption reaction (L/mg) and Ce the equilibrium gold concentration in the solution (mg/l). A linear plot of 1/Q e versus 1/C e was employed to give the values of Q m and K L from the intercept and slope of the plot (Figure 8). The Freundlich adsorption isotherm, on the other hand, is an indicator of the extent of heterogeneity of the adsorbent surface. A linear form of the isotherm is given by: Figure 8. Comparison of the Langmuir isotherms for ZnCl 2 and H 3 PO 4 granular activated peach carbons (GAC) and commercial activated carbons (CGAC) (bottle rolled for 72 hours; ph = 1.8; initial gold solution = 11.5 mg/l) INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 471

Figure 9. Comparison of the Freundlich isotherms for ZnCl 2 and H 3 PO 4 granular activated peach carbons (GAC) and commercial activated carbons (CGAC) (bottle rolled for 72hours; ph = 1.8; initial gold solution = 11.5 mg/l) Table I Adsorption isotherm constants for chemically activated peach and commercially activated carbons Langmuir constants Isotherm Freundlich isotherm constants K L (L/mg) Q m (mg/g) R 2 K F (mg/g) n R 2 H 3 PO 4 GAC 15.3 31.2 0.9802 26.5 3.8 0.9067 ZnCl 2 GAC 48.3 69.0 0.9033 110.6 3.0 0.9938 C.GAC 63.0 158.7 0.9758 203.4 3.9 0.9696 where K F and n are the Freundlich constants and represent the significance of adsorption capacity and intensity of adsorption, respectively. Values of K F and n are calculated from the intercept and slope of the plot log Q e versus log C e (Figure 9), which is a straight line. The constants for the two isotherms, as calculated from the plots in Figure 8 and Figure 9, are shown in Table I, together with their correlations coefficients. Conclusions It can be concluded that the adsorption of gold (I) ions from acidic thiourea solutions by activated peach stones is dependent on several parameters, which include: solution ph, rate of stirring, carbon dosage, gold and thiourea concentration in solution and carbon particle size. Zinc chloride activated peach stones fitted well the Freundlich isotherm, with a very high correlation coefficient (R 2 = 0.9938), while the phosphoric acid and commercial activated carbons produced good fits with the Langmuir isotherm, with high correlation coefficients (R 2 = 0.9802 and 0.9758 respectively). [3] 472 HYDROMETALLURGY CONFERENCE 2009

References ADAMS, M.D. and FLEMING, C.A. (1989), The mechanism of adsorption of aurocyanide onto activated carbon, Metallurgical and Materials Transactions B, vol. 20B, 1989. pp. 315 325 AHALYA, N., KANAMADI, R.D., and RAMACHANDRA, T.V. Biosorption of Chromium (VI) from aqueous solutions by the husk of Bengal gram (Cicer arientinum), Environmetal Biotechnology: Electronic Journal of Biotechnology, vol. 8, no. 3, 2005. AMER, A.M. Processing of copper anode-slimes for extraction of metal values, Physicochemical Problems of Mineral Processing, vol. 36, 2002. pp. 123 134. DESCHENES, G. and GHALI, E. Leaching of gold from chalcopyrite concentrate by thiourea, Hydrometallurgy, vol. 20, no. 2, 1988. pp. 179 202. HAQUE, K.E. Gold Leaching from Refractory Ores-Literature survey, Mineral Processing and Extractive Metallurgy Review, vol. 2, no. 3, 1987. pp. 235 253. HURTER, M.F. A review of advances and established procedures in the carbon in pulp (CIP) gold recovery plant at Western areas Gold Mining Company Limited, Proceeding of Internatioanl Conference on Gold, vol. 2, Extractive Metallurgy of Gold, SAIMM, 1986. pp. 335 351. KADIRVELU, K. and NAMASIVAYAM, C. Agricultural by-products as metal adsorbents: sorption of lead (II) from aqueous solutions onto coir-pith carbon, Environmental Technology, vol. 21, no. 10, 2000. pp. 1091 1097. MARSDEN, J. and HOUSE, C.I. The chemistry of gold extraction, Ellis Horwood, London. 1992. MCKAY, G., BLAIR, H.S., and GARDENER, J.R. Adsorption of dyes on Chitin I: Equilibrium studies, Journal of Applied Polymer Science, vol. 27, no. 2, 1982. pp. 151 155. PETERSEN, F.W. and VAN DEVENTER T.S.J. Comparative performance of porous adsorbents in presence of gold cyanide, organic foulants and solid fines, Hydrometallurgy 94, IMM and SCI, Cambridge, 1994. pp. 501 515. SYNA, N., and VALIX, M. Assessing the potential of activated bagasse as gold adsorbent for gold-thiourea, Minerals Engineering, vol. 16, no. 6, 2003. THIXTON, D.H. Carbon technology for the recovery of gold: A comprehensive guide to current carbon technology for the recovery of gold with special reference to practice in Zimbabwe, Ministry of Mines, Environment and Tourism, Publication No. 21, Dept. of Metallurgy. 1998. VAN DEVENTER, J.S.J. and VAN DER MERWE P.F. The reversibility of adsorption of gold cyanide on activated carbon, Metallurgical and Materials Transactions B, 1993. pp. 433 439. YANNOPOULUS, J.C. The extractive metallurgy of gold, Van Nostrand Reinhold, New York. 1990. YAPU, W., SEGARRA, M., FERNANDEZ M., and ESPIELL, F. Adsorption kinetics of dicyanoaurate and dicyanoargentate ions in activated carbon, Metallurgical and Materials Transactions B, 1994. pp. 185 191. ZOUBOULIS, A.I., KYDOS, K.A., and MATIS, K.A. Adsorption of gold-thiourea complex on Greek lignite, Hydrometallurgy 94, July, 1994. pp. 546 559. INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 473

Francis Gudyanga Secretary for Science and Technology Development, Zimbabwe Professor Francis Gudyanga is currently the Permanent Secretary of the Ministry of Science and Technology Development in Zimbabwe while at the same being on the academic staff on a part-time basis in the Department of Metallurgical Engineering at the University of Zimbabwe (UZ). He obtained (1988) a PhD in Minerals Technology and a DIC in Electrochemical Engineering from the Royal School of Mines, Imperial College, after working on the electrohydrometallurgical reduction of cassiterite associated with sulphide minerals. In 1989 he joined he teaching staff in the Department of Metallurgy, UZ, carrying out research in hydrometallurgy principally in the reductive decomposition of sulphidic mineral ores. He was Deputy Dean (1991 1994) and Dean (1994 1997) of Engineering at UZ. He worked at Bindura Nickel Corporation s refinery on the production of Ni, Cu and Co by the Outokumptu process. He spent a year s sabbatical (1996) at Mintek, Randburg, South Africa, working on the bacterial leaching of sulphide ores. In 2000 he was appointed Deputy Director General (Technical) of the Scientific and Industrial Research and Development Centre (SIRDC) in Harare. He was Chairman of the Research Council of Zimbabwe (2000 2007). He has been on the board of the Zimbabwe Mining and Development Corporation and/or its subsidiaries since 1991. He was a Member of the Executive Board of the International Council for Science (ICSU) (2002 2008) and is a member of the ICSU Regional Committee for Africa since 2004. He serves on several other board and committees. 474 HYDROMETALLURGY CONFERENCE 2009