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Characterization of activated carbons utilized in the gold industry: Physical and chemical properties, and kinetic study
Minerals Engineering, Vol. 6, No. 6, pp. 585-596, 1993
0892-6875/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
CHARACTERIZATION OF ACTIVATED CARBONS UTILIZED IN THE GOLD INDUSTRY: PHYSICAL AND CHEMICAL PROPERTIES, AND KINETIC STUDY A.C.Q. LADEIRA§, M.E.M. FIGUEIRA) and V.S.T. CIMINELLI) § Research Group, Morro Velho Mining S.A., Brazil t Dept. of Chemical Engineering, Federal University of Minas Gerais, Brazil Dept. of Metallurgical Engineering, Federal University of Minas Gerais, Rua Espirito Santo, 35, Belo Horizonte-MG, 30160.030, Brazil (Received 11 December 1992; accepted 18 December 1992)
ABSTRACT This work presents a comparative characterization of five samples of activated carbon and a kinetic evaluation o f gold adsorption from diluted cyanide solutions. The characterization was aimed at determining and quantifying some physical and chemical properties considered important for adequate performance of carbon in gold plants. The kinetic study analyzed the influence o f gold concentration in solution, temperature, pH, agitation and time on the rate o f gold adsorption on carbon. The data obtained were compared with those reported in the literature. A discussion of these results based on the suggested mechanism is also presented.
Keywords Activated Carbon, Gold Extraction, Kinetics, Gold Adsorption INTRODUCTION The process of gold and silver adsorption on activated carbon was introduced in large scale operations in the beginning of the 70's. Since then, it has rapidly spread as an alternative to the traditional process of gold recovery through cementation with zinc powder (MerrillCrowe process), becoming today the preferred route for new projects. This is due, in part, to the development of efficient methods for gold elution and carbon regeneration. Furthermore, the process of gold adsorption on activated carbon offers some advantages over the Merrill-Crowe process, such as adsorption being less affected by impurities of the liquor, clarification and filtration stages not being needed and the loss of soluble gold being significantly decreased. Therefore, the preference for the activated carbon approach instead of the traditional Merrill-Crowe approach is due to the improved efficiency in the recovery of precious metals in low grade solutions and also to the lower operational and capital costs of the former [1]. The techniques most used for gold adsorption on activated carbon may be classified as carbon-in-pulp (CIP), carbon-in-leach and carbon-in-column (CIC). The main features of the CIP and CIL processes are discussed by Fleming[l] in a recent paper. For optimum performance in the circuit, the activated carbon must have some important characteristics, such as high rate of gold extraction, high gold loading capacity and high resistance to abrasion [2]. This last feature is important in reducing gold losses associated with carbon fines, generated particularly in the CIP and CIL circuits. For a CIC operation, less resistant carbons are required, since the abrasive effect in this system is not so pronounced. 585
586
A . C . Q . LADEIRA et al.
Prior to its utilization in an adsorption circuit, activated carbons must be evaluated with respect to their physical and chemical properties. To do so, standard tests are performed at laboratory scale aimed at characterizing the adsorbent and assessing the feasibility of its usage at industrial scale. However, the tests usually emphasize physical and chemical properties, disregarding the kinetic aspects. It is important to highlight that in practice, for kinetic reasons (long time necessary to completely utilize the maximum loading capacity of the carbon) and also economic reasons (minimize fines losses), the work is done under conditions far from the equilibrium, thus resulting in low loading rates, and for this reason, in circumstances in which the kinetic factors are predominant. Furthermore, the standard tests utilize concentrated gold solutions, not compatible with the range normally used in industrial circuits. Considering the aforementioned aspects, a study was conducted in order to: (i) characterize five activated carbon samples encompassing the determination of their physical and chemical properties, (ii) analyze the kinetics of gold adsorption from dilute solutions to establish the influence of the process variables on the reaction rates of the different samples. The results were compared with the data available in the literature. METHODOLOGY The analyzed samples are identified by the letters A, B, C, D and E (Table 1). The nominal particle sizes of the samples were of 8x16 Mesh Tyler (2.36 x 1.00 mm), with the exception of sample A, having particle size of 6x16 Mesh Tyler (3.35 x 1.00mm). The iodine number was determined by the ASTM- 1510/60 standard, in which the activated carbon is placed in contact with a iodine solution of 0.1 N. After a given period of time, the residual iodine concentration in solution is analysed. The determination of the specific surface area of the samples was based on nitrogen adsorption, according to the BET model. A Quantasorb equipment, manufactured by Quantachrome Corp. was used. In determining the attrition resistance, the AWWA-B604-74 standard was applied. The method evaluates activated carbon resistance to attrition, comparing the results of the particle size analysis before and after inserting the sample in a closed recipient, together with steel balls, under a condition of intense vibration provided by a RO-TAP shaker. Visual characteristics of the samples such as grain shape, and the presence of cracks and impurities were analysed with the aid of an optical and a scanning electron microscope (SEM). The kinetics studies were carried out in a stirred glass reactor of 2000 cm 3 capacity, immersed in a thermocontrolled bath. 1.0g of carbon was added to the reactor containing 1000 cm 3 of a synthetic solution with a gold concentration of 2 #g/cm 3. In samples periodically taken, gold concentration was analysed by means of atomic adsorption. RESULTS AND DISCUSSION Characterization of the Activated Carbon
The results obtained in the characterization of the various types of carbon are presented in Table 1. Specific Surface and Iodine Number
The results shown in Table 1 show a direct relationship between specific surface areas and iodine numbers. In other words, high values for the specific surface are related to high values for the iodine number. Recommended values are 1050 to 1200 m2/g and 1000 to
Characterization of activated carbons
587
l l50mg/g for BET surface area and iodine number, respectively. The adsorption of different molecules by activated carbons can be used to evaluate the pore size distribution. For instance, since the iodine molecule can be adsorbed in pores down to 10 A, the iodine number indicates the pore fraction above this size [2]. TABLE 1 Characterization of the Activated Carbons Carbon A B
Raw material
Cracks (% vol)
Specific surface (m2/g) 814
Abrasion resistance (%) 80.40
Iodine amber (mg/g) 840
Fixed carbon (%) 76.25
Volatile matter (%7 4.42
1210
81.45
1035
78.41
4.11
11.46 I,D
0.9 1132 (charcoal)
90.90
1040
80.44
4.68
14.93 I
1.7
1000
88.35
819
75.66
4.80
(siP2, 1215
94.80
1127
84.60
2.96
Coconut 20.13 I Coconut 2 7 . 4 5 I,D
lmpmities (%vol) 1.0(SiO2) slag 2.7(SIO2, Fe203, slag,
charcoal) C
D
E
Coconut and Babassu palm Peach seed
Coconut 4.55 I
I= intermediatelayer
furnace coating) 0.2
D= deep layer
The data obtained in the kinetic study, described later, will show that carbons having a high index of specific surface area and iodine number adsorb gold more rapidly. These results disagree with those of Davidson et al [3], who did not find direct relationship between (i) kinetic constant and specific surface area, (ii) kinetic constant and iodine number, and (iii) specific surface area and iodine number.
Resistance to Abrasion The samples submitted to the abrasion resistance test were previously composed in compliance with the same granulometric proportion. The abrasion resistance correlates with the percentage of cracks in the grains, with the type of raw material and the presence of impurities in the sample. As shown in Table 1, sample E has the greatest index of abrasion resistance, that is, 94.8%. This means that after the test, 5.2% of the material was below the initial average particle size. Together with sample E, the activated carbons C and D proved more suitable to the CIP and CIL circuits, as a result of their high indexes, (90.9 and 88.4%) respectively. The presence of impurities, such as quartz, hematite, slag and furnace coating, may increase the abrasion resistance and lead to an inaccurate assessment of this index. Among the activated carbons produced from the same raw material, sample E had the smallest percentage of cracks (4.6% in volume) and the highest index of abrasion resistance (94.8%). Sample B, although having a high percentage of cracks (27.5% in volume), showed a relatively high abrasion resistance (81.55%). This can be partially explained by its high impurity content, especially of quartz.
Optical and Scanning Electron Microscopy Observation under optical and scanning electron microscopes is important for determining the textural and morphological characteristics of the sample, as well as for the identification of contaminants. As has been shown previously, these characteristics can be directly related with carbon properties such as abrasion resistance. Together with the presence of cracks, irregular shapes may accentuate the fines generation in industrial circuits. Figure 1 shows
588
A . C . Q . LADEIRAet al.
the highly porous nature of an activated carbon surface, as well as the differences in pore sizes. The occurrence of cracks is shown in Figure 2.
Fig.1 Porous Surface of an Activated Carbon (Magnification Factor 200 X)
Fig.2 Cracks Present in Activated Carbon Grains (Magnification Factor 100 X)
Fixed Carbon, Volatile Matter and Ash The contents of fixed carbon and volatile substances are a measurement of the efficiency of the activation process. An efficient activation generates carbon with high fixed carbon and a low volatile matter content. According to Table 1, sample E has a fixed carbon
Characterization of activated carbons
589
content of 84.60%, while sample C contains 80.44%. The other samples show values that are much inferior to these (Table 1). Recent studies [4] have shown that the adsorption of the gold cyanide complex increases with the aromaticity, meaning graphitic structure, of the adsorbent. Aromaticity, on the other hand, increases sharply with a carbon content above 80% [5]. These findings may "signal" a new approach to activated carbon evaluation. The presence of mineral impurities, quantified by the ash content, may be a result of technical failures or mistakes in handling the raw material and/or the product. The organic contaminants (for instance, charcoal) contribute to the increase in the fixed carbon content. Table 1 shows the variation in the impurities content of the different samples. K i n e ti c Evaluation
The kinetic parameters were calculated statistically from the functions defined by the least squares method. E f f e c t of G o l d C o n c e n t r a t i o n
Figure 3 shows the effect of gold concentration in gold extraction rate for carbon A. As observed for the other carbons, this variation did not influence significantly the final levels of extraction. For solutions with gold concentration of 1 #g/cm 3, the extraction after 4 hours of testing was slightly higher. These results do not confirm that there is a direct relationship between the increase in the final levels of extraction and the decrease in the gold concentration in solution. DO o4
8O _
O.
113"
A2
SAMPLE C
/ 2O
pH : 10,7
/ !
298 K 9 0 0 rain-t
I I
I 2
I 3
I 4
TIME: (h)
Fig.3 Effect of Concentration in Gold Extraction Table 2 illustrates the effect of variation in gold concentration in the initial extraction rate. As can be observed, a correlation between the concentration of metal in solution and initial rate is not evident. The results obtained agree with those of Fleming et al [6], who also observed that in diluted solutions the adsorption process is not influenced by the variation in gold concentration. It can be concluded that, under the experimental conditions adopted in the present work, t h e initial rate (Table 2) and the gold extraction (Figure 3) are not affected by the initial concentration of the metal in solution.
590
A. C. Q. LADEmA et al.
T A B L E 2 E f f e c t of Gold (Au) Concentration on the Initial Extraction Rate (Vi)
SAMPLES Au (pg/cm3)
A Vi
1 2 3 4
B Vi
C Vi
(h -1)
(h -1)
(h -1)
0.4044 0.5254 0.3552 0.3900
0.7090 0.7624 0.9373 0.9146
1.2435 1.4292 1.3795 1.2618
D Vi
E Vi
(h -1)
(h -1)
0.8769 0.6601 0.7428 0.5742
0.7932 1.2681 1.0733
E f f e c t of Temperature
The effect of temperature in the adsorption process is illustrated in Figure 4. Within the range between 298 and 308K, the final extractions are similar for all activated carbons, diminishing at higher temperatures, particularly between 338 and 353K. At 353K the final extraction shows a significant decrease. I00
T(K)
Z98 80
n 323 338
I,l,I O~ 0 (n
~ _ . - - - - ' ~
"
..-"7m"-
60
-j
40
177/
p H = Io7
-900
20
I I
I 2
I 3
rain"
•
I 4
TIME (h)
Fig.4 Effect of Temperature in Gold Extraction With increase in temperature, an increase in the initial rate combined with a decrease in the final loading was to be expected. However, the relationship of increase in initial speed with temperature was found to be irregular. This behavior may be related to the opposite trends of increasing the initial speed and of decreasing the final conversions. This fact is reflected in the curve of gold extraction and, for the same reason, in the calculated value of initial rate. A mean value of apparent activation energy equal to 2.4 kcal/mol was estimated. This result agrees with that of Fuerstenau et al [7] and Fleming et al [6], who reported values of 2.0 and 2.6 kcal/mol, respectively. Low activation energies indicate a reaction controlled by diffusion in the boundary layer or by diffusion within the pores of the carbon particles. The apparent activation energy encompasses kinetic and equilibrium constants. Therefore, the calculated values reflect these opposing trends.
Characterization of activated carbons
591
Effect of pH The study was restricted to high oH (10 to 12), since these are the conditions adopted in industrial practice, and also in view of to the formation of HCN gas. This variable was adjusted with either NaOH or HCI solutions. As shown in Figure 5, pH does not significantly influence gold extraction. This fact is in agreement with the literature which states that there is a great influence in adsorption of gold only when pH is reduced to values below 6, whilst for values greater than 6 there is only a marginal variation in adsorption [8]. In confirmation of the aforementioned, there was no significant change in the values found for the initial of reaction rate or for the final percentage of gold extraction in the pH range between 10 and 12. This behavior was observed for all the samples studied.
I00[ 80 I
<
0 rl A 0
pH :10,0 pH = 10,5 pH = iO,T pH = I1,0
,ov7 It'
SAMPLE A
298 : ,o
I
I
.,.,
I I 2 3 TIME (h)
I
4
Fig.5 Effect of pH in Gold Extraction
Effect of Stirring Rate The stirring rate was varied from 150 to 1800 min "1. Between 150 and 900 min" 1 an increase in the stirring rate increases the final extraction and also the initial rate; above 900 rain "1 this effect is less pronounced (Figure 6). The increase in the initial rate with stirring rate may be correlated to the decrease in the boundary layer thickness, thus indicating a reaction under diffusion control. The minor influence of the stirring rate over the initial rate, at levels above 900 rain "1, suggests the onset of the change in the controlling mechanism, from diffusion in the external boundary layer to chemical control or control by diffusion within the particle pores.
Effect of Time Figure 7 compares the behavior of the various carbons submitted to a 10 hour test. Carbon E gave the best performance. This sample adsorbs about 83% of the gold after 2 hours and reaches a final extraction of 97%. Similar extraction levels are attained by carbon C after 4 hours of contact with the gold solution; a final extraction equal to 98% is observed. Carbons A and B show lower extraction levels than those of carbon E and reach a final extraction of 93%. The least satisfactory results are those obtained with sample D: the lowest ME 6/6:-C
592
A . C . Q . LADEIRAet
al.
initial rate and a final extraction around 85%. It is important to emphasize that despite the different magnitude of the adsorption rates displayed by the five carbon samples, the effects of variables such as pH, temperature, and others, in the kinetics of adsorption of all these samples kinetics are quite similar. I00
o v
80
/
/ V
.9
~~ ' - - ~-
~
o
C3
60
nO (n 0 <
/ 40
<[
o/ )~/
,,(' /v
o v 0
/,4
/ y
zoi
leoo mi. -* 900 . i . - , 300 rain q
29a K ~ ] -- 2~/cmS p H = 10,7
// I I
I Z
I 3
I 4
TIME (h)
Fig.6 Effect of Stirring Rate in Gold Extraction 100
V----OV'-'O V o g &vO
o
800 w
= ~-
0
&
~
o
o
9
o
9
I0
<
60-
40
SAMPLES 0 A
20
V C
I
2
3
4
5
6
7
8
TIME (h)
Fig.7 Effect of Time in Gold Extraction. Temperature 298K, Stirring rate 900 rain "1, Gold Concentration --- 2 (#g/cm3), pH = 10.7 Study of the Kinetic Models
In order to study the influence of the process variables on the initial reaction rate many equations have been tested. The best fit was presented by the following equation [9]:
Characterization of activated carbons
593
X= a.t/(b+t)
(l)
where, a,b t X
= constants (a dimensionless; b (h)) = time (h) = reacted fraction
Coefficients of correlation above 0.99, are obtained by plotting the experimental data shown in Figure 7, according to the linearized form of Eq. (1) (Figure 8).
9,S n 7,2 IK
4,8
m
v SAMPLE C 2,4
I
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
I0
TIME (h)
Fig.8 Adjustment of experimental data to Eq. (1) linear form The literature [6,7,10,11] proposes a first order kinetic equation to describe the gold adsorption phenomena in activated carbons: In([AU]s o / [AU]s ) = k.t
(2)
where, [AU]s o = initial gold concentration in solution [AU]s = gold concentration in solution in time t k = kinetic constant However, the adjustment of the experimental points to Eq. (2) is satisfactory only for reaction times less than to four hours. Fleming and Nicol [6] and Fuerstenau et al [7] have calculated the values of the kinetic constant (k) based on the initial 60 minutes of the reaction. For this short period of time the first order equation fits the experimental data relatively well. This narrow range of application, confirmed in the literature, limits its practical usage. For longer reaction times Eq. (1) proposed in this paper seems to describe the process more adequately. Considering a general rate equation of reaction of order n: d[Au] / dt = k.(A / V).[Au]sn
(3)
A. C. Q. LADEIRA et al.
594
where, A = surface area V --- fluid volume and modifying it in terms of the reacted fraction (X): [AUls = [Aulso.(1-X)
(4)
the following expression is obtained: d X / d t = k.(A / V).[AUlson-l.(1-X)
(5)
The initial rate (Vi) is calculated by derivating Eq. (5), for t = 0, and n = 1, i.e., assuming first order reaction: Vi --- [dx/dt]t= 0 = k . A / V
(6)
Equation (6) shows the non dependence of the initial rate with gold concentration in solution, in accordance with the experimental data (Figure 3), thus, indicating a diffusion controlled mechanism. There is some controversy in the literature concerning the reaction controlling mechanism. In one of the papers [6], the kinetic constant is defined by the following equation: k = ks.kc.Av / (kc.K + ks)
(7)
where ks and kc are mass-transfer coefficients, K is an equilibrium constant and Av is a parameter expressed in area per volume units. From this definition, and assuming that the overall mass transfer is controlled by diffusion in the boundary layer, i.e., ks << kc, and spherical particles, then: k= 6.ks / K.d The dependence of been considered an [6]. Nevertheless, it of the particle size,
(8) the kinetic constant, k, with the inverse of the particle diameter has indication of a reaction controlled by diffusion in the boundary layer is known that the mass-transfer coefficient is expressed as a function d, as shown by Eq. (9):
ks = 2.0.De / d
(9)
Equation (9) is derived from Eq. (10) below, for systems in a laminar flow regime ( low Reynolds number) [12]: d.ks / De = 2.0 + 0.6 Rel/Z.(Sc) 1/3
(10)
where, De = diffusity coefficient Re = Reynolds' number Sc = Schmidt's number B~ substituting Eq. (9) in Eq. (8), one may verify that k becomes inversely proportional to k --- 12.De / k.d 2
(11)
When the mass transfer resistance in the pores is high, it can be demonstrated that the kinetic constant for diffusion in the pores is inversely proportional to the particle diameter
Characterization of activated carbons
595
[12]. The experimental result obtained by Fleming and Nicol [6] (k proportional to d "1) is, therefore, an indication of pore diffusion control. This assumption is supported by the high gold concentration (around 30/Jg/cm 3) utilized in the experiments. Considering the mass transfer coefficient, (ks), inversely proportional to the particle diameter (equation (10)), Fuerstenau et al [7] found the following expression: k = K' / d 2
(12)
where K' is a constant that depends on the diffusion coefficient. The fitting of the experimental data to Eq. (12), supported the hypothesis of a reaction controlled by diffusion in the external boundary layer [7]. In agreement with the findings of Fuerstenau et al [7], the increase in the initial rate with stirring rate observed in the present investigation also indicates a reaction controlled by film diffusion. The small magnitude of the activation energy is an additional evidence of the proposed mechanism. CONCLUSION Based on the results obtained with five activated carbon samples, a discussion of physical and chemical properties considered important for an adequate performance of the adsorbent in gold plants has been presented. Among the studied variables that may affect gold adsorption from dilute cyanide solutions, only the stirring rate and the reaction time have shown marked influence on the rate and on the final gold extraction. Therefore, the evaluation of these parameters is fundamental in order to work with optimum conditions in practice. The present work proposes a non-phenomenological equation to describe gold adsorption on activated carbon for long reaction times, Through the evaluation of the obtained results it is suggested that the controlling reaction mechanism in dilute gold cyanide solutions is diffusion in the external boundary layer. REFERENCES °
2. 3.
.
5. 6.
.
8.
Fleming, C.A., Hydrometallurgy of precious metals recovery. Hydrometallurgy 30, 127 (1992). McDougall, G.J., The physical nature and manufacture of activated carbon. J. o/ South African Institute of Mining and Metallurgy, 91 (4), 109 (1991). Davidson, R.J., Douglas, W.D. & Tumilty, J.A., Provisional specification of activated carbon suitable for use in CIP recovery circuit. In: Aus. I. M. M. Carbon-in-Pulp Seminar, July 1982. Proceedings .. s.n.t. (1982). Ibrado, A.S. & Fuerstenau, D.W., Effect of the structure of carbon adsorbents on the adsorption of gold cyanide. Hydrometallurgy, 30, 243 (1992). Blayden, H.E., Gilson, J. & Riley, H.L., Proceedings of the conference for ultrafine structure of carbons and cokes, B.C.U.R.A., London, 176 (1944), as cited in [4]. Fleming, C.A.& Nicol, M.J., The adsorption of gold cyanide onto activated carbon. III. Factors influencing the rate of loading and the equilibrium capacity. J. o/South African Institute of Mining and Metallurgy, 84 (4) 85 (1984). Fuerstenau, M.C., Nebo, C.O., Kelso, J.R., & Zaragoza, M. Rate of adsorption of gold cyanide on activated carbon. Minerals and Metallurgical Processing 177 (1987). Davidson, R.J., Properties of carbon for gold adsorption and their measurement. Randburg: CIP School, SAIMM 24 (1986).
596
. 10. 11. 12.
A . C . Q . LADEIRAet al.
Ciminelli, V.S.T., Silva, R.M. & Rodrigues, S.A., The influence of mineralogical variation on the reactivity of phosphate rocks from the Arax~ district, Brazil. Technical Paper. TMS, paper n. A89-24, 21p. (1989). Nicol, M.J., Fleming, C.A. & Cromberge, G., The adsorption of gold cyanide onto activated carbon. I. The kinetics of adsorption from pulps. J. of South African Institute of Mining and Metallurgy, 84 (2), 50 (1984). Stange, W., Woollacott, L.C. & King R.P., Towards more effective simulation of CIP and CIL processes. 3. Validation and use of a new simulator. J. of South African Institute of Mining and Metallurgy, 90 (12), 323 (1990). Levenspiel. O., Engenharia das rea~Ses quirnicas. Edgar Bli~cher Ltda., Sao Paulo (1974).
0892-6875/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
CHARACTERIZATION OF ACTIVATED CARBONS UTILIZED IN THE GOLD INDUSTRY: PHYSICAL AND CHEMICAL PROPERTIES, AND KINETIC STUDY A.C.Q. LADEIRA§, M.E.M. FIGUEIRA) and V.S.T. CIMINELLI) § Research Group, Morro Velho Mining S.A., Brazil t Dept. of Chemical Engineering, Federal University of Minas Gerais, Brazil Dept. of Metallurgical Engineering, Federal University of Minas Gerais, Rua Espirito Santo, 35, Belo Horizonte-MG, 30160.030, Brazil (Received 11 December 1992; accepted 18 December 1992)
ABSTRACT This work presents a comparative characterization of five samples of activated carbon and a kinetic evaluation o f gold adsorption from diluted cyanide solutions. The characterization was aimed at determining and quantifying some physical and chemical properties considered important for adequate performance of carbon in gold plants. The kinetic study analyzed the influence o f gold concentration in solution, temperature, pH, agitation and time on the rate o f gold adsorption on carbon. The data obtained were compared with those reported in the literature. A discussion of these results based on the suggested mechanism is also presented.
Keywords Activated Carbon, Gold Extraction, Kinetics, Gold Adsorption INTRODUCTION The process of gold and silver adsorption on activated carbon was introduced in large scale operations in the beginning of the 70's. Since then, it has rapidly spread as an alternative to the traditional process of gold recovery through cementation with zinc powder (MerrillCrowe process), becoming today the preferred route for new projects. This is due, in part, to the development of efficient methods for gold elution and carbon regeneration. Furthermore, the process of gold adsorption on activated carbon offers some advantages over the Merrill-Crowe process, such as adsorption being less affected by impurities of the liquor, clarification and filtration stages not being needed and the loss of soluble gold being significantly decreased. Therefore, the preference for the activated carbon approach instead of the traditional Merrill-Crowe approach is due to the improved efficiency in the recovery of precious metals in low grade solutions and also to the lower operational and capital costs of the former [1]. The techniques most used for gold adsorption on activated carbon may be classified as carbon-in-pulp (CIP), carbon-in-leach and carbon-in-column (CIC). The main features of the CIP and CIL processes are discussed by Fleming[l] in a recent paper. For optimum performance in the circuit, the activated carbon must have some important characteristics, such as high rate of gold extraction, high gold loading capacity and high resistance to abrasion [2]. This last feature is important in reducing gold losses associated with carbon fines, generated particularly in the CIP and CIL circuits. For a CIC operation, less resistant carbons are required, since the abrasive effect in this system is not so pronounced. 585
586
A . C . Q . LADEIRA et al.
Prior to its utilization in an adsorption circuit, activated carbons must be evaluated with respect to their physical and chemical properties. To do so, standard tests are performed at laboratory scale aimed at characterizing the adsorbent and assessing the feasibility of its usage at industrial scale. However, the tests usually emphasize physical and chemical properties, disregarding the kinetic aspects. It is important to highlight that in practice, for kinetic reasons (long time necessary to completely utilize the maximum loading capacity of the carbon) and also economic reasons (minimize fines losses), the work is done under conditions far from the equilibrium, thus resulting in low loading rates, and for this reason, in circumstances in which the kinetic factors are predominant. Furthermore, the standard tests utilize concentrated gold solutions, not compatible with the range normally used in industrial circuits. Considering the aforementioned aspects, a study was conducted in order to: (i) characterize five activated carbon samples encompassing the determination of their physical and chemical properties, (ii) analyze the kinetics of gold adsorption from dilute solutions to establish the influence of the process variables on the reaction rates of the different samples. The results were compared with the data available in the literature. METHODOLOGY The analyzed samples are identified by the letters A, B, C, D and E (Table 1). The nominal particle sizes of the samples were of 8x16 Mesh Tyler (2.36 x 1.00 mm), with the exception of sample A, having particle size of 6x16 Mesh Tyler (3.35 x 1.00mm). The iodine number was determined by the ASTM- 1510/60 standard, in which the activated carbon is placed in contact with a iodine solution of 0.1 N. After a given period of time, the residual iodine concentration in solution is analysed. The determination of the specific surface area of the samples was based on nitrogen adsorption, according to the BET model. A Quantasorb equipment, manufactured by Quantachrome Corp. was used. In determining the attrition resistance, the AWWA-B604-74 standard was applied. The method evaluates activated carbon resistance to attrition, comparing the results of the particle size analysis before and after inserting the sample in a closed recipient, together with steel balls, under a condition of intense vibration provided by a RO-TAP shaker. Visual characteristics of the samples such as grain shape, and the presence of cracks and impurities were analysed with the aid of an optical and a scanning electron microscope (SEM). The kinetics studies were carried out in a stirred glass reactor of 2000 cm 3 capacity, immersed in a thermocontrolled bath. 1.0g of carbon was added to the reactor containing 1000 cm 3 of a synthetic solution with a gold concentration of 2 #g/cm 3. In samples periodically taken, gold concentration was analysed by means of atomic adsorption. RESULTS AND DISCUSSION Characterization of the Activated Carbon
The results obtained in the characterization of the various types of carbon are presented in Table 1. Specific Surface and Iodine Number
The results shown in Table 1 show a direct relationship between specific surface areas and iodine numbers. In other words, high values for the specific surface are related to high values for the iodine number. Recommended values are 1050 to 1200 m2/g and 1000 to
Characterization of activated carbons
587
l l50mg/g for BET surface area and iodine number, respectively. The adsorption of different molecules by activated carbons can be used to evaluate the pore size distribution. For instance, since the iodine molecule can be adsorbed in pores down to 10 A, the iodine number indicates the pore fraction above this size [2]. TABLE 1 Characterization of the Activated Carbons Carbon A B
Raw material
Cracks (% vol)
Specific surface (m2/g) 814
Abrasion resistance (%) 80.40
Iodine amber (mg/g) 840
Fixed carbon (%) 76.25
Volatile matter (%7 4.42
1210
81.45
1035
78.41
4.11
11.46 I,D
0.9 1132 (charcoal)
90.90
1040
80.44
4.68
14.93 I
1.7
1000
88.35
819
75.66
4.80
(siP2, 1215
94.80
1127
84.60
2.96
Coconut 20.13 I Coconut 2 7 . 4 5 I,D
lmpmities (%vol) 1.0(SiO2) slag 2.7(SIO2, Fe203, slag,
charcoal) C
D
E
Coconut and Babassu palm Peach seed
Coconut 4.55 I
I= intermediatelayer
furnace coating) 0.2
D= deep layer
The data obtained in the kinetic study, described later, will show that carbons having a high index of specific surface area and iodine number adsorb gold more rapidly. These results disagree with those of Davidson et al [3], who did not find direct relationship between (i) kinetic constant and specific surface area, (ii) kinetic constant and iodine number, and (iii) specific surface area and iodine number.
Resistance to Abrasion The samples submitted to the abrasion resistance test were previously composed in compliance with the same granulometric proportion. The abrasion resistance correlates with the percentage of cracks in the grains, with the type of raw material and the presence of impurities in the sample. As shown in Table 1, sample E has the greatest index of abrasion resistance, that is, 94.8%. This means that after the test, 5.2% of the material was below the initial average particle size. Together with sample E, the activated carbons C and D proved more suitable to the CIP and CIL circuits, as a result of their high indexes, (90.9 and 88.4%) respectively. The presence of impurities, such as quartz, hematite, slag and furnace coating, may increase the abrasion resistance and lead to an inaccurate assessment of this index. Among the activated carbons produced from the same raw material, sample E had the smallest percentage of cracks (4.6% in volume) and the highest index of abrasion resistance (94.8%). Sample B, although having a high percentage of cracks (27.5% in volume), showed a relatively high abrasion resistance (81.55%). This can be partially explained by its high impurity content, especially of quartz.
Optical and Scanning Electron Microscopy Observation under optical and scanning electron microscopes is important for determining the textural and morphological characteristics of the sample, as well as for the identification of contaminants. As has been shown previously, these characteristics can be directly related with carbon properties such as abrasion resistance. Together with the presence of cracks, irregular shapes may accentuate the fines generation in industrial circuits. Figure 1 shows
588
A . C . Q . LADEIRAet al.
the highly porous nature of an activated carbon surface, as well as the differences in pore sizes. The occurrence of cracks is shown in Figure 2.
Fig.1 Porous Surface of an Activated Carbon (Magnification Factor 200 X)
Fig.2 Cracks Present in Activated Carbon Grains (Magnification Factor 100 X)
Fixed Carbon, Volatile Matter and Ash The contents of fixed carbon and volatile substances are a measurement of the efficiency of the activation process. An efficient activation generates carbon with high fixed carbon and a low volatile matter content. According to Table 1, sample E has a fixed carbon
Characterization of activated carbons
589
content of 84.60%, while sample C contains 80.44%. The other samples show values that are much inferior to these (Table 1). Recent studies [4] have shown that the adsorption of the gold cyanide complex increases with the aromaticity, meaning graphitic structure, of the adsorbent. Aromaticity, on the other hand, increases sharply with a carbon content above 80% [5]. These findings may "signal" a new approach to activated carbon evaluation. The presence of mineral impurities, quantified by the ash content, may be a result of technical failures or mistakes in handling the raw material and/or the product. The organic contaminants (for instance, charcoal) contribute to the increase in the fixed carbon content. Table 1 shows the variation in the impurities content of the different samples. K i n e ti c Evaluation
The kinetic parameters were calculated statistically from the functions defined by the least squares method. E f f e c t of G o l d C o n c e n t r a t i o n
Figure 3 shows the effect of gold concentration in gold extraction rate for carbon A. As observed for the other carbons, this variation did not influence significantly the final levels of extraction. For solutions with gold concentration of 1 #g/cm 3, the extraction after 4 hours of testing was slightly higher. These results do not confirm that there is a direct relationship between the increase in the final levels of extraction and the decrease in the gold concentration in solution. DO o4
8O _
O.
113"
A2
SAMPLE C
/ 2O
pH : 10,7
/ !
298 K 9 0 0 rain-t
I I
I 2
I 3
I 4
TIME: (h)
Fig.3 Effect of Concentration in Gold Extraction Table 2 illustrates the effect of variation in gold concentration in the initial extraction rate. As can be observed, a correlation between the concentration of metal in solution and initial rate is not evident. The results obtained agree with those of Fleming et al [6], who also observed that in diluted solutions the adsorption process is not influenced by the variation in gold concentration. It can be concluded that, under the experimental conditions adopted in the present work, t h e initial rate (Table 2) and the gold extraction (Figure 3) are not affected by the initial concentration of the metal in solution.
590
A. C. Q. LADEmA et al.
T A B L E 2 E f f e c t of Gold (Au) Concentration on the Initial Extraction Rate (Vi)
SAMPLES Au (pg/cm3)
A Vi
1 2 3 4
B Vi
C Vi
(h -1)
(h -1)
(h -1)
0.4044 0.5254 0.3552 0.3900
0.7090 0.7624 0.9373 0.9146
1.2435 1.4292 1.3795 1.2618
D Vi
E Vi
(h -1)
(h -1)
0.8769 0.6601 0.7428 0.5742
0.7932 1.2681 1.0733
E f f e c t of Temperature
The effect of temperature in the adsorption process is illustrated in Figure 4. Within the range between 298 and 308K, the final extractions are similar for all activated carbons, diminishing at higher temperatures, particularly between 338 and 353K. At 353K the final extraction shows a significant decrease. I00
T(K)
Z98 80
n 323 338
I,l,I O~ 0 (n
~ _ . - - - - ' ~
"
..-"7m"-
60
-j
40
177/
p H = Io7
-900
20
I I
I 2
I 3
rain"
•
I 4
TIME (h)
Fig.4 Effect of Temperature in Gold Extraction With increase in temperature, an increase in the initial rate combined with a decrease in the final loading was to be expected. However, the relationship of increase in initial speed with temperature was found to be irregular. This behavior may be related to the opposite trends of increasing the initial speed and of decreasing the final conversions. This fact is reflected in the curve of gold extraction and, for the same reason, in the calculated value of initial rate. A mean value of apparent activation energy equal to 2.4 kcal/mol was estimated. This result agrees with that of Fuerstenau et al [7] and Fleming et al [6], who reported values of 2.0 and 2.6 kcal/mol, respectively. Low activation energies indicate a reaction controlled by diffusion in the boundary layer or by diffusion within the pores of the carbon particles. The apparent activation energy encompasses kinetic and equilibrium constants. Therefore, the calculated values reflect these opposing trends.
Characterization of activated carbons
591
Effect of pH The study was restricted to high oH (10 to 12), since these are the conditions adopted in industrial practice, and also in view of to the formation of HCN gas. This variable was adjusted with either NaOH or HCI solutions. As shown in Figure 5, pH does not significantly influence gold extraction. This fact is in agreement with the literature which states that there is a great influence in adsorption of gold only when pH is reduced to values below 6, whilst for values greater than 6 there is only a marginal variation in adsorption [8]. In confirmation of the aforementioned, there was no significant change in the values found for the initial of reaction rate or for the final percentage of gold extraction in the pH range between 10 and 12. This behavior was observed for all the samples studied.
I00[ 80 I
<
0 rl A 0
pH :10,0 pH = 10,5 pH = iO,T pH = I1,0
,ov7 It'
SAMPLE A
298 : ,o
I
I
.,.,
I I 2 3 TIME (h)
I
4
Fig.5 Effect of pH in Gold Extraction
Effect of Stirring Rate The stirring rate was varied from 150 to 1800 min "1. Between 150 and 900 min" 1 an increase in the stirring rate increases the final extraction and also the initial rate; above 900 rain "1 this effect is less pronounced (Figure 6). The increase in the initial rate with stirring rate may be correlated to the decrease in the boundary layer thickness, thus indicating a reaction under diffusion control. The minor influence of the stirring rate over the initial rate, at levels above 900 rain "1, suggests the onset of the change in the controlling mechanism, from diffusion in the external boundary layer to chemical control or control by diffusion within the particle pores.
Effect of Time Figure 7 compares the behavior of the various carbons submitted to a 10 hour test. Carbon E gave the best performance. This sample adsorbs about 83% of the gold after 2 hours and reaches a final extraction of 97%. Similar extraction levels are attained by carbon C after 4 hours of contact with the gold solution; a final extraction equal to 98% is observed. Carbons A and B show lower extraction levels than those of carbon E and reach a final extraction of 93%. The least satisfactory results are those obtained with sample D: the lowest ME 6/6:-C
592
A . C . Q . LADEIRAet
al.
initial rate and a final extraction around 85%. It is important to emphasize that despite the different magnitude of the adsorption rates displayed by the five carbon samples, the effects of variables such as pH, temperature, and others, in the kinetics of adsorption of all these samples kinetics are quite similar. I00
o v
80
/
/ V
.9
~~ ' - - ~-
~
o
C3
60
nO (n 0 <
/ 40
<[
o/ )~/
,,(' /v
o v 0
/,4
/ y
zoi
leoo mi. -* 900 . i . - , 300 rain q
29a K ~ ] -- 2~/cmS p H = 10,7
// I I
I Z
I 3
I 4
TIME (h)
Fig.6 Effect of Stirring Rate in Gold Extraction 100
V----OV'-'O V o g &vO
o
800 w
= ~-
0
&
~
o
o
9
o
9
I0
<
60-
40
SAMPLES 0 A
20
V C
I
2
3
4
5
6
7
8
TIME (h)
Fig.7 Effect of Time in Gold Extraction. Temperature 298K, Stirring rate 900 rain "1, Gold Concentration --- 2 (#g/cm3), pH = 10.7 Study of the Kinetic Models
In order to study the influence of the process variables on the initial reaction rate many equations have been tested. The best fit was presented by the following equation [9]:
Characterization of activated carbons
593
X= a.t/(b+t)
(l)
where, a,b t X
= constants (a dimensionless; b (h)) = time (h) = reacted fraction
Coefficients of correlation above 0.99, are obtained by plotting the experimental data shown in Figure 7, according to the linearized form of Eq. (1) (Figure 8).
9,S n 7,2 IK
4,8
m
v SAMPLE C 2,4
I
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
I0
TIME (h)
Fig.8 Adjustment of experimental data to Eq. (1) linear form The literature [6,7,10,11] proposes a first order kinetic equation to describe the gold adsorption phenomena in activated carbons: In([AU]s o / [AU]s ) = k.t
(2)
where, [AU]s o = initial gold concentration in solution [AU]s = gold concentration in solution in time t k = kinetic constant However, the adjustment of the experimental points to Eq. (2) is satisfactory only for reaction times less than to four hours. Fleming and Nicol [6] and Fuerstenau et al [7] have calculated the values of the kinetic constant (k) based on the initial 60 minutes of the reaction. For this short period of time the first order equation fits the experimental data relatively well. This narrow range of application, confirmed in the literature, limits its practical usage. For longer reaction times Eq. (1) proposed in this paper seems to describe the process more adequately. Considering a general rate equation of reaction of order n: d[Au] / dt = k.(A / V).[Au]sn
(3)
A. C. Q. LADEIRA et al.
594
where, A = surface area V --- fluid volume and modifying it in terms of the reacted fraction (X): [AUls = [Aulso.(1-X)
(4)
the following expression is obtained: d X / d t = k.(A / V).[AUlson-l.(1-X)
(5)
The initial rate (Vi) is calculated by derivating Eq. (5), for t = 0, and n = 1, i.e., assuming first order reaction: Vi --- [dx/dt]t= 0 = k . A / V
(6)
Equation (6) shows the non dependence of the initial rate with gold concentration in solution, in accordance with the experimental data (Figure 3), thus, indicating a diffusion controlled mechanism. There is some controversy in the literature concerning the reaction controlling mechanism. In one of the papers [6], the kinetic constant is defined by the following equation: k = ks.kc.Av / (kc.K + ks)
(7)
where ks and kc are mass-transfer coefficients, K is an equilibrium constant and Av is a parameter expressed in area per volume units. From this definition, and assuming that the overall mass transfer is controlled by diffusion in the boundary layer, i.e., ks << kc, and spherical particles, then: k= 6.ks / K.d The dependence of been considered an [6]. Nevertheless, it of the particle size,
(8) the kinetic constant, k, with the inverse of the particle diameter has indication of a reaction controlled by diffusion in the boundary layer is known that the mass-transfer coefficient is expressed as a function d, as shown by Eq. (9):
ks = 2.0.De / d
(9)
Equation (9) is derived from Eq. (10) below, for systems in a laminar flow regime ( low Reynolds number) [12]: d.ks / De = 2.0 + 0.6 Rel/Z.(Sc) 1/3
(10)
where, De = diffusity coefficient Re = Reynolds' number Sc = Schmidt's number B~ substituting Eq. (9) in Eq. (8), one may verify that k becomes inversely proportional to k --- 12.De / k.d 2
(11)
When the mass transfer resistance in the pores is high, it can be demonstrated that the kinetic constant for diffusion in the pores is inversely proportional to the particle diameter
Characterization of activated carbons
595
[12]. The experimental result obtained by Fleming and Nicol [6] (k proportional to d "1) is, therefore, an indication of pore diffusion control. This assumption is supported by the high gold concentration (around 30/Jg/cm 3) utilized in the experiments. Considering the mass transfer coefficient, (ks), inversely proportional to the particle diameter (equation (10)), Fuerstenau et al [7] found the following expression: k = K' / d 2
(12)
where K' is a constant that depends on the diffusion coefficient. The fitting of the experimental data to Eq. (12), supported the hypothesis of a reaction controlled by diffusion in the external boundary layer [7]. In agreement with the findings of Fuerstenau et al [7], the increase in the initial rate with stirring rate observed in the present investigation also indicates a reaction controlled by film diffusion. The small magnitude of the activation energy is an additional evidence of the proposed mechanism. CONCLUSION Based on the results obtained with five activated carbon samples, a discussion of physical and chemical properties considered important for an adequate performance of the adsorbent in gold plants has been presented. Among the studied variables that may affect gold adsorption from dilute cyanide solutions, only the stirring rate and the reaction time have shown marked influence on the rate and on the final gold extraction. Therefore, the evaluation of these parameters is fundamental in order to work with optimum conditions in practice. The present work proposes a non-phenomenological equation to describe gold adsorption on activated carbon for long reaction times, Through the evaluation of the obtained results it is suggested that the controlling reaction mechanism in dilute gold cyanide solutions is diffusion in the external boundary layer. REFERENCES °
2. 3.
.
5. 6.
.
8.
Fleming, C.A., Hydrometallurgy of precious metals recovery. Hydrometallurgy 30, 127 (1992). McDougall, G.J., The physical nature and manufacture of activated carbon. J. o/ South African Institute of Mining and Metallurgy, 91 (4), 109 (1991). Davidson, R.J., Douglas, W.D. & Tumilty, J.A., Provisional specification of activated carbon suitable for use in CIP recovery circuit. In: Aus. I. M. M. Carbon-in-Pulp Seminar, July 1982. Proceedings .. s.n.t. (1982). Ibrado, A.S. & Fuerstenau, D.W., Effect of the structure of carbon adsorbents on the adsorption of gold cyanide. Hydrometallurgy, 30, 243 (1992). Blayden, H.E., Gilson, J. & Riley, H.L., Proceedings of the conference for ultrafine structure of carbons and cokes, B.C.U.R.A., London, 176 (1944), as cited in [4]. Fleming, C.A.& Nicol, M.J., The adsorption of gold cyanide onto activated carbon. III. Factors influencing the rate of loading and the equilibrium capacity. J. o/South African Institute of Mining and Metallurgy, 84 (4) 85 (1984). Fuerstenau, M.C., Nebo, C.O., Kelso, J.R., & Zaragoza, M. Rate of adsorption of gold cyanide on activated carbon. Minerals and Metallurgical Processing 177 (1987). Davidson, R.J., Properties of carbon for gold adsorption and their measurement. Randburg: CIP School, SAIMM 24 (1986).
596
. 10. 11. 12.
A . C . Q . LADEIRAet al.
Ciminelli, V.S.T., Silva, R.M. & Rodrigues, S.A., The influence of mineralogical variation on the reactivity of phosphate rocks from the Arax~ district, Brazil. Technical Paper. TMS, paper n. A89-24, 21p. (1989). Nicol, M.J., Fleming, C.A. & Cromberge, G., The adsorption of gold cyanide onto activated carbon. I. The kinetics of adsorption from pulps. J. of South African Institute of Mining and Metallurgy, 84 (2), 50 (1984). Stange, W., Woollacott, L.C. & King R.P., Towards more effective simulation of CIP and CIL processes. 3. Validation and use of a new simulator. J. of South African Institute of Mining and Metallurgy, 90 (12), 323 (1990). Levenspiel. O., Engenharia das rea~Ses quirnicas. Edgar Bli~cher Ltda., Sao Paulo (1974).