INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 465 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). 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.
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INVESTIGATION OF GRANULAR ACTIVATED CARBON FROM PEACH STONES 465
Introduction
The ability of activated carbon to adsorb dissolved metal species has been known for aconsiderable time. It has found increasing application, especially in the mining industry, as anadsorbent 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 coconutshell origin, has been studied extensively and is well known (Yapu et al., 1994; Van Deventerand Van der Merwe, 1993; Marsden and House, 1992; Adams and Fleming, 1989). Activatedcarbon 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 acidicthiourea solutions (Syna and Valix, 2003; Zouboulis et al., 1994).
It is of both technological and economic interests to examine the potential of activated peachstones for the adsorption of gold species from acidic thiourea solutions. Peach stones areagricultural by-products that are currently of no economic value, and have a hardlignocellulosic material shell that gives them the potential to be used as raw materials forproduction of granular activated carbon. In Zimbabwe more than 90% of the activated carbonused by the mineral industry is imported at a very high cost (Thixton, 1998).
MASIYA, T.T. and GUDYANGA, F.P. Investigation of granular activated carbon from peach stones for goldadsorption in acidic thiourea. Hydrometallurgy Conference 2009, The Southern African Institute of Mining andMetallurgy, 2009.
Investigation of granular activated carbon frompeach 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 usinggranular activated carbon produced from peach stones, an agriculturalwaste, for the adsorption of gold from acidified thiourea solutions. Inthis work the activated carbon was produced by chemical activationof the peach stones with either phosphoric acid or zinc chloride. Theeffects of several parameters on the adsorption kinetics and equilibrawere examined. Gold adsorption was found to depend significantlyon: solution pH, rate of stirring, activated carbon dosage, carbonparticle size, and the initial concentration of gold and thiourea. Theadsorption equilibrium experimental data fitted well both theLangmuir and Freundlich isotherm models with high correlationcoefficient. The adsorption capacity calculated from the Langmuirisotherm was 31.2 mg Au/g for phosphoric activated peach stones and69.0 mg Au/g for zinc chloride activated peach stones.
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The peach stones used for this study were activated chemically by phosphoric acid and zincchloride. 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 wasinvestigated to ascertain the mechanism for adsorption.
Experimental
The activated carbon particles used in this study were manufactured from peach stones bychemical activation with either phosphoric acid or zinc chloride. Except where statedotherwise, the activated carbons in the size range -1.0 +0.5 mm were used for theinvestigations.
ZnCl2 activation
3.3 g of ZnCl2 was dissolved in 100 ml distilled water. 100 g of the raw peach stones werethen soaked in this solution for 24 hours. At the end of this time, the solution was filtered andthe impregnated peach stones were dried in an oven overnight at 105°C after which they wereactivated at 700°C for 2 hours in a muffle furnace. The sample was then removed from thefurnace, allowed to cool and then washed in hot HCl solution and rinsed in distilled wateruntil the wash water was almost neutral.
H3PO4 activation
The raw peach stones were impregnated with 43 wt% H3PO4 for 24 hours in the weight ratio 1:1 (peach stones: H3PO4), followed by drying in an oven overnight at 105°C. The driedsample was then activated in a muffle furnace at 300°C for 1 hour. The sample was allowed tocool and subsequently washed first in hot NaOH solution followed by a series of soaking anddecanting in hot distilled water until there were no traces of phosphates (identified by addinga few drops of Pb(NO3)2 into the wash water, which turns to a white precipitate in thepresence of phosphates).
Equilibrium adsorption studies
Equilibrium data was collected by taking 200 ml solution of known gold concentration into aseries of 500 ml containers. To each container, different activated carbon dosages (0.05 to 2.0g) were added. Prior to their use activated carbon samples were washed thoroughly with wateruntil most of the fines were removed, and then dried at 105°C in an oven for 24 hours. Thecontainers were sealed and bottle rolled for 72 hours until equilibrium was assumed to havebeen reached. The solutions were filtered and the filtrates were analysed for residual goldconcentration 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 ofactivated carbon with 300 ml acidified aqueous gold-thiourea solution of desired initialconcentration in a series of 800 ml Duran beakers on a magnetic stirrer (HeIdolph type) withan adjustable stirring speed. Temperature was maintained at room temperature during theinvestigations. Aliquots of 10 ml were withdrawn regularly (after 15, 30, 45, 60, 90, 120, 180and 240 minutes) and filtered through a filter paper. Filtrates were analysed for residual goldconcentration using an atomic absorption spectrometer (AAS).
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Results and discussion
Effect of pH on gold adsorption
Gold adsorption on peach granular activated carbon and commercial granular activated carbonas a function of initial pH was studied for the pH range 1 to 12, and the results are shown inFigure 1. In the pH range 1.4 to 4 the per cent gold recovery initially decreases for bothactivated carbons as the initial pH is increased. As solution pH is increased above pH 4.1 asharp increase in gold recovery is observed, especially with activated peach stones. This isattributed to the fact that beyond pH 5, thiourea is known to decompose to sulphur andcyanamide through an intermediate product, formamidine disulphide.
[1]
Zouboulis et al. (1994) attributed this observed increase in gold recovery in the alkalinerange 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 fromthiourea solutions. The gold adsorption kinetic is faster at lower initial pH than at higherinitial pH values. The adsorption of metal ions depends on solution pH, which influenceselectrostatic binding of ions on the activated carbon to corresponding metal groups insolutions (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 afunction of contact time in comparison with that of the fraction -1.0 +0.5 mm of the sameactivated carbon. As is expected, the smaller the size of granules, the faster the rate ofadsorption of gold by the activated carbon. Smaller particle size, for the same mass ofactivated carbon, enhances gold adsorption (Yannopoulos, 1990). This is attributed to increasein surface area for adsorption and reduced mean pore length through which gold species travelwithin the activated carbon particles as particle size decreases. The ultimate carbon loading
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; initialgold content = 5.0 mg/L; pH = 1.7; carbon dosage = 0.5 g)
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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-pulpplants 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 thioureasolutions was investigated for cases where agitation resulted in activated carbon being fullysuspended in the gold solutions (i.e. between 200 and 400 rpm). Below 200 rpm it wasobserved that some of the carbon remained on the base of the beaker and at 500 rpm themagnetic follower became unstable. The observed increase in rate of adsorption of gold withstirring speed in the range 200 to 400 rpm is attributed to the improvement in contact betweenthe gold species in solution and the active sites on the carbons, thereby promoting effectivetransfer 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 ofthe 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 theavailable surface area is low and subsequently the fractional adsorption becomes independentof initial concentration. However, at higher concentration the available sites for adsorptionbecome fewer compared to the moles of gold species present and hence the percentageremoval of gold is dependent upon the initial gold concentration, i.e. there is increasedcompetition among gold species for the available active sites on the carbon surface. For thesame initial gold concentration there is an increase in per cent gold removal as the carbondosage is increased from 0.25 g to 0.5 g. This is because there was an increase in number ofavailable active adsorption sites for the same number of moles of gold species as carbondosage was increased
Figure 6 shows the variation of the actual amount of gold adsorbed per gramme of activatedcarbon with time. An increase in the initial gold concentration leads to an increase in theadsorption capacity of gold on activated carbon. This indicates that the initial goldconcentration 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 peachcarbons concentrations (bottle rolled; solution volume = 200 ml; contact time = 5 hours; pH = 1.7)
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Effect of initial thiourea concentration on gold recovery
The effect of the amount of free excess thiourea concentration on equilibrium gold-thioureacomplex adsorption on activated peach stones was examined and the results are presented inFigure 7.
The loading capacity for gold on activated carbon was found to decrease as the amount offree excess thiourea concentration was increased. Thiourea concentration is an importantparameter when considering the adsorption of gold from thiourea solutions on activatedcarbon. Thiourea is an organic molecule that has a high tendency to adsorb on activatedcarbon. When there is not enough carbon, or the number of surface active sites is notsufficient for adsorbing both gold-thiourea complexes and free thiourea, then thiourea will beloaded preferentially over gold, and gold adsorption will be suppressed (Petersen and vanDeventer, 1994; Zouboulis et. al., 1994).
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)
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However, when there is excess activated carbon, or the number of surface active sitesexceeds the total number of molecules to be adsorbed, then free thiourea does not interferewith adsorption of gold-thiourea complexes.
Deschenes and Ghali (1988) explained this decline in gold adsorbed differently. Theyattributed it to the fact that thiourea is less stable at high concentrations and decomposes tocyanamide 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 thecarbon, 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 acidicthiourea solutions, the adsorption isotherm data was evaluated using the Langmuir andFreundlich adsorption isotherms.
The basic assumption of the Langmuir adsorption process is the formation of a monolayer ofadsorbate 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 Qe is the equilibrium quantity of gold adsorbed on activated carbon (mg/g), Qm themaximum monolayer adsorption, KL the langmuir equilibrium constant for the adsorptionreaction (L/mg) and Ce the equilibrium gold concentration in the solution (mg/L).
A linear plot of 1/Qe versus 1/Ce was employed to give the values of Qm and KL from theintercept and slope of the plot (Figure 8).
The Freundlich adsorption isotherm, on the other hand, is an indicator of the extent ofheterogeneity of the adsorbent surface. A linear form of the isotherm is given by:
Figure 8. Comparison of the Langmuir isotherms for ZnCl2 and H3PO4 granular activated peach carbons (GAC) andcommercial activated carbons (CGAC) (bottle rolled for 72 hours; pH = 1.8; initial gold solution = 11.5 mg/L)
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[3]
where KF and n are the Freundlich constants and represent the significance of adsorptioncapacity and intensity of adsorption, respectively.
Values of KF and n are calculated from the intercept and slope of the plot log Qe versus logCe (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 byactivated peach stones is dependent on several parameters, which include: solution pH, rate ofstirring, 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 highcorrelation coefficient (R2 = 0.9938), while the phosphoric acid and commercial activatedcarbons produced good fits with the Langmuir isotherm, with high correlation coefficients (R2 = 0.9802 and 0.9758 respectively).
Figure 9. Comparison of the Freundlich isotherms for ZnCl2 and H3PO4 granular activated peach carbons (GAC)and commercial activated carbons (CGAC) (bottle rolled for 72hours; pH = 1.8; initial gold solution = 11.5 mg/L)
Adsorption isotherm constants for chemically activated peach and commercially activated carbons
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Francis GudyangaSecretary for Science and Technology Development, Zimbabwe
Professor Francis Gudyanga is currently the Permanent Secretary ofthe Ministry of Science and Technology Development in Zimbabwewhile at the same being on the academic staff on a part-time basis inthe Department of Metallurgical Engineering at the University ofZimbabwe (UZ). He obtained (1988) a PhD in Minerals Technologyand a DIC in Electrochemical Engineering from the Royal School ofMines, Imperial College, after working on the electrohydro-
metallurgical reduction of cassiterite associated with sulphide minerals. In 1989 he joined heteaching staff in the Department of Metallurgy, UZ, carrying out research in hydrometallurgyprincipally 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 NickelCorporation’s refinery on the production of Ni, Cu and Co by the Outokumptu process. Hespent a year’s sabbatical (1996) at Mintek, Randburg, South Africa, working on the bacterialleaching of sulphide ores. In 2000 he was appointed Deputy Director General (Technical) ofthe Scientific and Industrial Research and Development Centre (SIRDC) in Harare. He wasChairman of the Research Council of Zimbabwe (2000–2007). He has been on the board ofthe Zimbabwe Mining and Development Corporation and/or its subsidiaries since 1991. Hewas 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. Heserves on several other board and committees.
