Adsorption of Mercury from a Cyanide Leaching Solution Using Various Activation Rates of Granular Activated Carbon: A Laboratory- and Industrial-Scale Study

: The use of granular activated carbon (GAC) is a typical and sustainable technique for recovering precious metals from a cyanide leaching solution (CLS). The level of GAC activity is a fundamental factor in assessing the rate of precious metal adsorption; thus, it is essential to determine the e ﬃ ciency of carbon elution for reproducing GACs. Since mercury (Hg) adsorption plays a critical role, economically and environmentally, in GAC e ﬃ ciency, we conducted various laboratory and industrial experiments to explore the e ﬀ ect of di ﬀ erent rates of GAC activation (10%, 35%, 70% and 100%) on Hg adsorption from CLS. Assessments of laboratory test results showed a direct relationship between the Hg adsorption and GAC activity; by increasing the GAC activity from 10% to 100%, the recovery of Hg was increased from 20% to 41%. Kinetic modeling results indicated that the Hg adsorption for all GAC activities followed chemisorption mechanisms. There was good agreement between the laboratory test results and the results of experiments on the industrial scale (that used a continuous circuit). These outcomes indicate that by increasing the frequency of carbon reactivation and using GAC with a high level of activity in the ﬁrst tank, Hg desorption was meaningfully decreased and recovery was improved (for 10% GAC activity vs. 35% GAC activity, recovery was 40% vs. 90%, respectively). removing 5 g of carbon, transferring it to fresh pulp, and mixing it with 250 mL of a standard solution that contained 10 mg / L Au for 15 minutes. After this step, GAC was ﬁltered and separated from the solution. The Au adsorption was calculated based on Equation (1) and the di ﬀ erence in the Au concentration in the solution was measured before and after the process. The ratio of calculated Au adsorptions onto the GAC was recorded every day as an index for GAC activity (throughout 20 days). Au was selected to evaluate GAC’s quality as Au was the main product of the plant and other metals (Ag and mercury) were the by-products.


Introduction
Recovery of precious metals from cyanide leaching solution (CLS) by granular activated carbon (GAC) is a typical process in gold processing plants [1,2]. Compared with other methods, using GAC has such advantages as high selectivity for the adsorption of precious metals, ease of elution and reactivation, high adsorption capacity, and fast filtration due to the large GAC particle size [3][4][5]. Adsorption from CLS has been found to be affected by various conditions (pH, Cynide (CN) and target metal concentration [6]), carbon properties (such as particle size, free surface area, ash content and moisture [7][8][9]) and operation conditions (such as contacting time, agitation rate and the carbon concentration in the CLS [10][11][12]). Sustainability 2020, 12, 3287 3 of 10 the pulp and added to the 250 mL standard solution with 10 mg/L Au. Following this, the adsorption reaction took 15 minutes. The adsorption capacity was calculated based on the following reaction [19]: where C i (mg/L) and C e (mg/L) are the concentrations of metals in the solution before and after adsorption, respectively. V is the volume of solution (L) and M is the mass of sorbent (g). The ratio of calculated adsorption capacity for each day to the adsorption capacity for fresh carbon (100%) shows the level of GAC activity. Figure 1 shows the level of GAC activity on different days. The GAC activity was calculated according to the following equation: where A GAC is the activity of the granular activated carbon, q x is the gold adsorption and q f is the gold adsorption with fresh activated carbon. The Hg adsorption from CLS was carried out with various GAC activities: 10%, 35%, 70% and 100%. Laboratory tests were performed in a 10-liter reactor with a mechanical mixer, which contained 5 (l) pulp with a pulp density of 40% and 1.5 g of GAC. The pH and CN concentration were adjusted to 10.5 and 120 g/t, respectively. Each test continued for 24 hours. An amount of 50 mL of pulp (without carbon) was withdrawn from the output of the tank in the selected intervals and, after the measurement of the mercury in the solution, the Hg adsorption was calculated based on Equation (1) at different reaction times. The metal concentrations were analyzed using atomic absorption spectroscopy (Varian 220). Chemical analysis of the pregnant solution showed that the concentration of Au, Ag and Hg was 0.8, 1.1 and 18 (mg/L), respectively ( Table 1). The Hg recovery was calculated based on the difference between primary Hg concentration in the solution and its concentration at the end of the process. In the plant, carbon activities were determined daily by removing 5 g of carbon, transferring it to fresh pulp, and mixing it with 250 mL of a standard solution that contained 10 mg/L Au for 15 minutes. After this step, GAC was filtered and separated from the solution. The Au adsorption was calculated based on Equation (1) and the difference in the Au concentration in the solution was measured before and after the process. The ratio of calculated Au adsorptions onto the GAC was recorded every day as an index for GAC activity (throughout 20 days). Au was selected to evaluate the GAC's quality as Au was the main product of the plant and other metals (Ag and mercury) were the by-products.
3 from the pulp and added to the 250 mL standard solution with 10 mg/L Au. Following this, the adsorption reaction took 15 minutes. The adsorption capacity was calculated based on the following reaction [19]: where (mg/L) and (mg/L) are the concentrations of metals in the solution before and after adsorption, respectively. is the volume of solution (L) and is the mass of sorbent (g). The ratio of calculated adsorption capacity for each day to the adsorption capacity for fresh carbon (100%) shows the level of GAC activity. Figure 1 shows the level of GAC activity on different days. The GAC activity was calculated according to the following equation: where AGAC is the activity of the granular activated carbon, qx is the gold adsorption and qf is the gold adsorption with fresh activated carbon. The Hg adsorption from CLS was carried out with various GAC activities: 10%, 35%, 70% and 100%. Laboratory tests were performed in a 10-liter reactor with a mechanical mixer, which contained 5 (l) pulp with a pulp density of 40% and 1.5 g of GAC. The pH and CN concentration were adjusted to 10.5 and 120 g/t, respectively. Each test continued for 24 hours. An amount of 50 mL of pulp (without carbon) was withdrawn from the output of the tank in the selected intervals and, after the measurement of the mercury in the solution, the Hg adsorption was calculated based on Equation (1) at different reaction times. The metal concentrations were analyzed using atomic absorption spectroscopy (Varian 220). Chemical analysis of the pregnant solution showed that the concentration of Au, Ag and Hg was 0.8, 1.1 and 18 (mg/L), respectively ( Table 1). The Hg recovery was calculated based on the difference between primary Hg concentration in the solution and its concentration at the end of the process. In the plant, carbon activities were determined daily by removing 5 g of carbon, transferring it to fresh pulp, and mixing it with 250 mL of a standard solution that contained 10 mg/L Au for 15 minutes. After this step, GAC was filtered and separated from the solution. The Au adsorption was calculated based on Equation (1) and the difference in the Au concentration in the solution was measured before and after the process. The ratio of calculated Au adsorptions onto the GAC was recorded every day as an index for GAC activity (throughout 20 days). Au was selected to evaluate the GAC's quality as Au was the main product of the plant and other metals (Ag and mercury) were the by-products. *The concentrations of Cr, Cd, As, Cl and SO4 2-were below the detection limit.

Modeling
Kinetic modeling can be used to explain adsorption mechanisms [20,21]. According to the pseudo-first-order model (Equation (3)), the adsorption rate was dependent on the number of effective adsorption sites [22].
where q t is the amount of Hg adsorbed at time t (mg/g), q e is the adsorption capacity at equilibrium (mg/g), k f is the pseudo-first-order rate constant (min −1 ) and t is the contact time (min).
The pseudo-second-order model is described based on the following equation [23]: where q e is adsorption capacity at equilibrium (mg/g) and k s is the pseudo-first-order rate constant (g/mg.min). The Elovich equation, which describes the chemisorption mechanism, is [24]: where α is the initial adsorption rate (mg/g.min), β is the desorption constant (g/mg) and t is time (min). Finally, the intra-particle diffusion model, which describes mass transfer in an amorphous and homogeneous sphere, is defined based on the following equation [25]: where C is the intercept and k id (mg/g.min −1/2 ) is the intra-particle diffusion rate constant.

Industrial Scale Test
GAC activity at the industrial scale can be controlled by changing the frequency of carbon elution and steam-activation in the kiln. The adsorption unit in the "Pouya Zarcan Agh-Darreh" plant (with 7 tanks) works in a countercurrent format, which means that the last tank in the circuit has the highest GAC activity level and that the first tank has the lowest level. The first tank was selected to study the effect of GAC activity on an industrial scale because, as mentioned, the plant works in a countercurrent format, and the concentrations of metals in this tank are significantly higher than those in other tanks. Regular sampling (a 5 g sample every day) from this tank was conducted to calculate the GAC activity. To take samples, a bottle roll was turned off every day for a short time, and the carbon was withdrawn using a screen. The size of the screen was 800 µm. An amount of 5 grams of carbon was taken and the rest was returned to the bottle. Investigation of the plant indicated that by increasing the frequency of elution from 12 to 30 times and the number of steam-activations from 6 to 15 per month, the activity increased from 10% to 35% in the tank. In an elution event, 6.3 t of GAC is removed from this tank (the volume of the elution cell equals 6.3 t of GAC) and sent to the kiln after elution (more than 30 hours). In order to study the effect of GAC activities, the concentration of Hg was measured in the samples before and after increasing the activity.

Laboratory-Scale Studies
As was expected, analyses indicated that fresh GAC (100% activity) has the highest Au and Ag adsorption. While after 48 hours GAC with 10% activity adsorbed 1.33 mg/g Au, fresh GAC adsorbed 1.75 mg/g Au (a 24% increase). This increase was 43.6% for Ag adsorption (0.972 and 1.724 mg/g for 10% and 100% GAC activity, respectively). Figure 2 shows the Hg adsorption (based on Equation (1)) onto the surface of GAC with different activities at various times. These results indicate that increasing the activity can increase Hg adsorption. The Hg loading on the GAC with 10% activity is 5.33 mg/g, while it is 17.29 mg/g for the GAC with 100% activity. These variations show that Hg adsorption was increased by 224% by increasing the GAC activity from 10% to 100% and that the Hg recovery significantly increased ( Figure 3). Generally, the laboratory results illustrated that the kinetics of adsorption are very fast during the initial minutes while they dropped significantly after a few minutes. This phenomenon occurs due to a large number of sites on GAC surfaces that are available for adsorption at the initial stage of the process [26].
recovery significantly increased (Figure 3). Generally, the laboratory results illustrated that the kinetics of adsorption are very fast during the initial minutes while they dropped significantly after a few minutes. This phenomenon occurs due to a large number of sites on GAC surfaces that are available for adsorption at the initial stage of the process [26].

Modeling
Four different models, including pseudo-first-order, pseudo-second-order, chemisorption, and intra-particle diffusion models, were fitted to the laboratory data ( Figure 4). The calculated recovery significantly increased ( Figure 3). Generally, the laboratory results illustrated that the kinetics of adsorption are very fast during the initial minutes while they dropped significantly after a few minutes. This phenomenon occurs due to a large number of sites on GAC surfaces that are available for adsorption at the initial stage of the process [26].

Modeling
Four different models, including pseudo-first-order, pseudo-second-order, chemisorption, and intra-particle diffusion models, were fitted to the laboratory data ( Figure 4). The calculated parameters for each model are listed in Table 2.

Modeling
Four different models, including pseudo-first-order, pseudo-second-order, chemisorption, and intra-particle diffusion models, were fitted to the laboratory data ( Figure 4). The calculated parameters for each model are listed in Table 2 Outcomes of the modeling demonstrated that the pseudo-second-order model has the highest correlation of determination (R 2 ) for Hg adsorption onto GAC surfaces for 10% and 35% activities while the chemisorption model was fitted to the data at 70% and 100% activities. These results suggest that the rate-limiting step may be chemical adsorption involving valent forces, i.e., through sharing or the exchange of electrons between the adsorbent and divalent metal ions [27] at 10% and 35% activities. Additionally, for 70% and 100% activities, the chemisorption mechanism (formation of a chemical reaction between the surface and the adsorbate) leads to Hg adsorption onto the GAC surface. This phenomenon could be due to the fact that most free sites were occupied by contaminant components in the low-activity GAC. The initial sorption rate (h0, expressed in mg/g×min) can be obtained when t approaches zero, based on Equation (7): ℎ = (7) ℎ can be increased by increasing the GAC activity, which shows the direct proportion of Hg adsorption during the initial minutes of reaction. In fact, the Hg adsorption rate is higher for GAC with higher activity at the first stage of the reaction. It should be noted that the low R 2 for the intraparticle diffusion model indicates that Hg diffusion to the GAC surface cannot be controlled by carbon activity (Table 2). Table 2. Pseudo-first-order, pseudo-second-order, chemisorption, and intra-particle diffusion model constants and correlation coefficients for different activities.  Outcomes of the modeling demonstrated that the pseudo-second-order model has the highest correlation of determination (R 2 ) for Hg adsorption onto GAC surfaces for 10% and 35% activities while the chemisorption model was fitted to the data at 70% and 100% activities. These results suggest that the rate-limiting step may be chemical adsorption involving valent forces, i.e., through sharing or the exchange of electrons between the adsorbent and divalent metal ions [27] at 10% and 35% activities. Additionally, for 70% and 100% activities, the chemisorption mechanism (formation of a chemical reaction between the surface and the adsorbate) leads to Hg adsorption onto the GAC surface. This phenomenon could be due to the fact that most free sites were occupied by contaminant components in the low-activity GAC. The initial sorption rate (h 0 , expressed in mg/g×min) can be obtained when t approaches zero, based on Equation (7): h 0 can be increased by increasing the GAC activity, which shows the direct proportion of Hg adsorption during the initial minutes of reaction. In fact, the Hg adsorption rate is higher for GAC with higher activity at the first stage of the reaction. It should be noted that the low R 2 for the intra-particle diffusion model indicates that Hg diffusion to the GAC surface cannot be controlled by carbon activity (Table 2). Table 2. Pseudo-first-order, pseudo-second-order, chemisorption, and intra-particle diffusion model constants and correlation coefficients for different activities.

Industrial-Scale Studies
To study the effect of these activities on the industrial scale by 12 and 30 elutions, the GAC activity in the first tank was set to~10% and~35%, respectively. The Hg concentrations in the CIL feed and liquid residual were analyzed ( Table 3). The Au and Ag concentrations in the CIL process feed (hydrocyclone overflow) and in the residual (the feed of the second tank) are presented in Table 4. In agreement with the laboratory test results, there was a direct relationship between Hg concentration and GAC activities; upon increasing the frequency of elution, the Hg adsorption on the GAC significantly increased ( Figure 5). The results of the industrial experiments show that a quite high amount of dissolved Hg remains unrecovered when GAC with 10% activity is used (40% recovery). Increasing the activity to 35% significantly promoted the Hg recovery (90% recovery). In fact, increasing the frequency of elution from 12 to 30 times increased the Hg adsorption from the solution by 4.68 mg/g (from 4.32 to 9 mg/g). These results indicate that by using GAC with a high level of activity in the first tank (and increasing the number of elutions), the kinetics of absorption can be improved and the desorption can be decreased significantly.  level of activity in the first tank (and increasing the number of elutions), the kinetics of absorption can be improved and the desorption can be decreased significantly.

Conclusions
The effect of GAC activity on Hg adsorption from the cyanide leach solution was investigated.

Conclusions
The effect of GAC activity on Hg adsorption from the cyanide leach solution was investigated. The laboratory experiments were carried out for GAC with 10%, 35%, 70% and 100% activity. The results indicated that increasing the activity of GAC increases the Hg adsorption. In fact, increasing the GAC activity from 10% to 100% increased the Hg adsorption by more than 224%. The kinetic modeling with laboratory data showed that Hg adsorption follows the pseudo-second-order model at the 10% and 35% activities and the Elovich equation (the chemisorption mechanism) at the 70% and 100% activities. Both of these models showed that Hg adsorption follows the chemisorption mechanism; however, the adsorption changes are sharper at high levels of activity due to the logarithmic format of the Elovich equation. The GAC activity can be controlled by changing the frequency of reactivation and steam-activation. The industrial experiments indicated that the GAC activity increases from 10%