1. Introduction
Activated carbon (AC) has excellent adsorbent properties owing to its large specific surface area (1000–1500 m
2/g), which indicates a large number of active sites [
1]. AC is extensively used in several industries like pharmaceutical, chemical, metallurgical, and petroleum, among others. Additionally, AC has important applications in water treatment, both prior to distribution and before effluent discharge from industries, as well as treatment of flue gases [
1,
2,
3]. Specific industrial applications of AC are removal of dyes, adsorption, and concentration of heavy metals [
1,
4,
5,
6,
7]. The removal of dyes from industrial waters avoids contamination of water bodies, complying with environmental regulations. In addition to the removal of contaminants such as heavy metals, AC in the industry allows the concentration of precious metals like gold and silver. Hence, AC is widely employed in the mining and metallurgical industry.
In Ecuador, gold mining has greatly increased during the last decade owing to government stimulus and global demand [
8,
9]. The gold reserves of the country are estimated at 8.5 million ounces [
8], and there is a high demand for activated carbon because gold recovery techniques are migrating from traditional amalgamation to a more technical process, like cyanidation leaching and AC adsorption process [
8]. The typical gold extraction process has multiple stages: ore comminution process (crushing and milling), leaching, AC adsorption, AC elution, electro-winning or zinc precipitation, and gold refining [
10,
11]. Leaching is achieved typically with a solution of NaCN; the
ion complexes gold to form an aurocyanide complex, which remains dissolved in the solution. Subsequently, AC in the carbon in pulp process (CIP) absorbs and concentrates the aurocyanide complex. The gold is desorbed from the loaded AC by an elution process, and it is recovered as metal via electrolysis. Once the elution is over, AC goes to a reactivation process that regenerates its adsorptive capabilities, and this regenerated AC is reused in the CIP process [
1,
10,
11].
The gold extraction industry is a very profitable business worldwide. It involves the use of large amounts of activated carbon, especially in medium and large capacity operations. AC is used owing to the high gold recovery reached during the CIP process. However, the gold adsorption capacity of AC decreases after several batch cycles owing to the blockage of the carbon pores caused by the progressive deposition of clays, oxides, and hydroxides of calcium and magnesium on the AC surface. Consequently, the reactivation process is not only essential to restore the adsorption capacities of the carbon, but also it could improve the economy of the process by reducing the amount of virgin carbon needed [
10,
12,
13,
14].
Some of the most used reactivation procedures include the following: acid-wash, treatment with supercritical fluids, microwave reactivation, electrochemical process, and thermal reactivation. Previous studies concluded that, in order to achieve a better reactivation degree, it is critical to combine acid-wash with thermal reactivation of the spent activated carbon (SAC). Thus, whereas acid-wash removes inorganic pollutants by dissolving them, thermal reactivation eliminates organic waste by thermal decomposition [
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27].
The variation in some properties of activated carbon after reactivation has been investigated in previous studies. Compared with SAC, the moisture and the ash content of AC subjected to a reactivation process decreased [
28,
29]. In order to be reused in the CIP process, reactivated carbon should have a value of ash content between 2% and 4% [
29]. After reactivation, the iodine number and the Brunauer–Emmett–Teller (BET) specific surface area of AC increased significantly [
13,
15,
28,
29]. For instance, a carbon subjected to a chemical-thermal reactivation increased the iodine number from 534 mg I
2/g CA before the treatment to 1070 mg I
2/g CA after the reactivation [
15]. If the activated carbon is subjected to a thermal reactivation, the iodine number increased with the rise in temperature [
15,
25,
30]. Another important property of an activated carbon is the gold adsorption value
(k), which should be between 15 and 25 mg Au/g AC in order to use the carbon satisfactorily in the CIP process [
15,
26,
30].
The cost of the virgin carbon (Calgon GCR-20) used in this study is $3.5/kg. According to our estimations, the cost of reactivation rises up to $0.5 per kg of SAC, based on the quantity of acid required for the acid wash and the energetic cost associated to maintain the temperature needed in the furnace during the thermal reactivation. Therefore, reactivated carbon is preferred over virgin carbon because reactivation constitutes a viable alternative to significantly reduce operative costs in the CIP process.
The innovative point of this research lies in the combination of two traditional reactivation processes (chemical and thermal reactivation). These reactivation methods are already known, but combining them brings interesting new insights into the advantages and limitations of each reactivation process. Additionally, there is already some empirical knowledge about the effectiveness of chemical reactivation using inorganic acids (HCl, HNO3, H2SO4). However, not all the acids would reach the same efficiency, limiting its application at a bigger scale. Thus, presenting the efficiency of these three acids would contribute to better understanding some of the empirical results concerning the chemical activation of activated carbon.
Activated carbon loses its efficiency by the progressive deposition of calcium salts on the pores during the CIP process. In industrial plants having the CIP process, SAC is firstly removed and sieved in order to eliminate fine carbon. Then, there is a next stage of reactivation, mainly chemical or thermal. At the industrial scale in Ecuador, sometimes, the choice of one of these treatments is done using mainly empirical criteria. In the case that chemical reactivation is chosen, the cheaper acid available is selected, but it could not always be the more efficient one. Therefore, in this study, the influence of each inorganic acid (HCl, HNO3, H2SO4) is tested, as an original approach to validate the empirical findings currently extended in the chemical reactivation that follows the CIP process at the industrial scale. Another aspect to highlight in this study corresponds to the hardness of AC. In fact, during the CIP process, there is a fine fraction of carbon lost in the tails owing to the abrasion caused by the agitation. So, as a part of the reactivation process proposed in our study, it is also necessary to quantify the hardness of SAC. Like this, only a minimum decrease in the hardness value of SAC is tolerated to occur in order to make the reactivation process technically viable.
The aim of this work is to assess the effects of chemical and thermal reactivation on the AC adsorptive and mechanical properties in order to suggest a reactivation procedure that allows the reuse of SAC in the CIP process.
3. Materials and Methods
3.1. Acid Wash (Chemical Reactivation) and Thermal Reactivation
For the acid wash, 100 g of the sample of carbon was stirred during 0.5 h with 100 mL of acid solution, and then the carbon was washed with Na(OH) (1% v/v) and rinsed with distillated water until the pH of washing water reached a value of 7. The procedure was performed at 18 °C (room temperature) and 50 °C, with HCl, H2SO4, and HNO3, modifying the concentration of the solution between 5% and 30% v/v. The iodine number was determined after each wash. The first temperature of 18 °C corresponded to the room temperature. For the choice of the second temperature, two phenomena were taken into account. The second temperature should be high enough to facilitate the solubilization of salts on the carbon surface, but not too high so as to provoke a considerable evaporation of the acid. Thus, a good compromise between these two conditions was fulfilled with a temperature of 50 °C for the chemical reactivation.
Regarding thermal reactivation, 15 g of sample of carbon in a closed crucible was heated in an electrical muffle at temperatures between 650 °C and 950 °C, during 0.5 and 1 h. The iodine number was determined after each reactivation.
3.2. Characterization of Carbon Without Reactivation (CIP), Virgin Carbon, and Reactivated Carbon
Each parameter was determined by the corresponding norm:
Moisture content (ASTM-D3173), volatile content (ASTM-D3175), fixed carbon and ashes (ASTM-D3174), sugar discoloration (NMX-F-299-1980), iodine number (AWWA B604-74/4.7), methylene blue index (ASTM C837), ball-pan hardness (ASTM 3802), gold adsorption capacity (AWWA B604-74/2.6), and specific surface area Brunauer–Emmett–Teller (BET).
The BET surface area was measured with a Quantachrome Instruments Nova4200e (Quantachrome Instruments, Boynton Beach, FL, USA). The specific surface area of carbon is best approached by inert gas (N
2) adsorption using the Brunauer, Edward, and Teller method (or BET method) [
37], as described in the standard ISO 9277:2010. The total surface area BET of activated carbons was calculated by a multi-point analysis of the BET isotherms. As microporosity is characteristic of activated carbons, pore volume and pore size distribution were determined using the HR (Horvath, Kawazoe) and BJH (Barrett, Joyner, Halenda) models [
38,
39].
For the analysis of carbon texture, a scattering electron microscopy analysis was performed with a Vega TESCAN microscope (TESCAN, Brno, Czech Republic). Qualitative characterization of activated carbons was performed by infrared analysis with a Perkin Elmer equipment Spectrum One (Perkin Elmer, Shelton, CT, USA). All tests were performed using KBr pills prepared with 0.15 mg of AC-O or CM mixed with 300 mg of KBr and dried overnight at 110 °C.
The content in ashes: Ca, Mg, Na, K, Fe, Cu, and Zn was determined by atomic absorption spectrometry, after calcination at 950 °C and acid digestion. These measurements were done with a Perkin Elmer A-Analyst 300 spectrometer (Perkin Elmer, Shelton, CT, USA).
3.3. Normal Abrasion in Pulp Resistance
This test was performed in order to determine the mechanical strength of activated carbon (−2.4 + 1.4 mm) in normal operation conditions. First, a pulp of 24% solids was prepared using an Ecuadorian auriferous ore (see
Appendix C,
Table A2). This pulp (2 L) was stirred with activated carbon (10 g per litter of pulp) during 24 h. Then, the carbon was recovered by sieving (+1.4 mm), and dried during 12 h at 110 °C. The final weight of the dried carbon was registered. Finally, by weight difference, the percentage of carbon lost was determined.
3.4. Gold Adsorption Isotherms
For gold adsorption isotherms, 1 L of cyanide solution was prepared with a content of NaCN of 1 g/L. Then, 10 mL of standard solution of gold with a concentration of 1000 mg/L was added to the solution, in order to attain a final gold concentration of 10 mg/L. Next, 0.01 g of carbon was added to 100 mL of the gold-cyanide solution and stirred during 24 h. This procedure was repeated, but incorporating 0.1, 1.0, and 10 g of carbon each time. After the adsorption time was reached, samples were taken in order to determine the gold content remaining in the solution by atomic absorption spectrometry with a Perkin Elmer A-Analyst 300 spectrometer (Perkin Elmer, Shelton, CT, USA).
3.5. Gold Adsorption Kinetics
The same gold-cyanide solution prepared for the isotherms test (
Section 3.4) was used in the adsorption kinetics test. A sample of 0.1 g of carbon was stirred with 100 mL of gold-cyanide solution during 0.25, 0.5, 1, 2, and 3 h. After each time, samples of the solution were taken, in order to determine the gold content by atomic absorption spectrometry with a Perkin Elmer A-Analyst 300 spectrometer (Perkin Elmer, Shelton, CT, USA).
3.6. CIP Process
Firstly, the cyanidation of the auriferous mineral was conducted. The NaCN concentration and the content of gold in solution were verified at the following times: 0.5, 1, 2, 4, 6, 8, and 24 h. Once the cyanidation time concluded, 10 g of activated carbon per litter of pulp was added and the adsorption process started. The adsorption process finished after 4 h. The content of gold in solution was verified at 0.25, 0.5, 1, 2, 3, and 4 h.
4. Conclusions
The best experimental conditions for the reactivation of spent activated carbon (SAC) were achieved via acid washing with HNO3 at 20% v/v and 50 °C during 30 min and a subsequent thermal reactivation at 850 °C during 1 h. This reactivation process did not have much influence on the mechanical properties of carbon, as the ball-pan hardness of reactivated carbon compared with virgin carbon remained at 98%. Besides, the reactivated carbon produced in this study had good macro, meso, and micro porosity. In fact, the values of the iodine number (1199 mg I2/g AC), methylene blue index (27.9 mg/100g AC), and sugar discoloration index (34 basic units of reference) of the activated carbon were similar to those reported for the virgin carbon. The final reactivated carbon had a k value of 14.9 mg Au/g AC and a BET surface area of 1079 m²/g. Therefore, reactivated carbon obtained with the reactivation procedure proposed in this study can be used successfully in the carbon in pulp process (CIP).
The type of acid employed for the chemical reactivation plays an important role in the reactivation process. Chemical reactivation was performed using three acids (HCl, HNO3, and H2SO4). From these acids, the more appropriate acid for this process turned out to be HNO3 (20% v/v at 50 °C), because it reached the highest removal of Ca (77%) from the carbon surface, owing to the high solubility of the Ca(NO3)2 formed during the acid wash.