Recovery of Gold with AgNO3 Pretreatment by Cyanidation at Heap Leaching Cijiwa Gold Ore Processing

This research was undertaken to study the effect of adding silver nitrate (AgNO3) during leaching of gold ore for Au recovery. Its focus is to obtain the weight of Au in feed and concentrate, the effect of AgNO3 on Au recovery, theconcentration of AgNO3 that led to optimum recovery, and the weight of bullion and Au content in bullion. This research was conducted using quantitative descriptive method with experimental technique and the research steps include the following: 1) rock and treatment plant preparation, 2) tests with variable AgNO3 concentrations, 3) analysis feed and concentrate samples using atomic absorption spectroscopy (AAS), 4) measurement of concentrate density, 5) burning of activated carbon to get the bullion, and 6) weighing bullion and Au content. There was increase Au recovery from 12.57% to 36.15%. On addition of 0 to 4 gram of AgNO3, whereby highest recovery was obtained on the addition of 4 g of AgNO3 concentration to 150 kg of feed.

Cyanidation is one process that can be used to extract gold (Au) from its ore and is safer for environmental health than amalgamation [1]. It can be performed via heap leaching, vat leaching, or agitated leaching [2,3]; however, heap leaching is mostly adopted by local people because of its simple design, low operating costs, and small investment [4]. Leaching can be done with cyanide solution or microorganisms (bioleaching) [5]. The Au recovery during cyanidation can be increased by optimizing operating conditions [6], increasing the amount of dissolved oxygen in the concentrate, and reducing mineral impurities. Oxygen in the concentrate can be increased by adding hydrogen peroxide (H 2 O 2 ) [7]. The Au content of low ore and the presence of mineral impurities hinder the gold solubility, resulting in the need of more cyanide solution for processing. In gold deposits containing pyrite and arsenopyrite, the gold tends to be chemically bound to arsenic in pyrite and arsenopyrite, which would lead to a small Au recovery. This condition indicates that gold atoms replace arsenic or iron atoms in sulfide lattice [8].
Generally, in cyanidation, impurities of metallic minerals would dissolve in the cyanide solution, whereas quartz impurity is insoluble. Several studies have been conducted to remove mineral impurities from gold ores prior to processing using cyanidation methods; this would prevent the impurities from interfering with the dissolution of gold ions in the cyanide solution. There are different treatment methods for reducing different mineral impurities. For example, gold ore containing high manganese (Mn) can be pretreated by adding FeS 2 and H 2 SO 4 during leaching to reduce Mn [9], while arsenic impurities can be reduced by adding Fe 3 O 4 @ SiO 2 @ TiO 2 nanosorbent during leaching [10].
Research has shown that mineral impurities may interfere with the binding of Au with the cyanide solution [9,10]. If a gold ore contains sulfide minerals, Pb salts can be added, such as Pb (NO 3 ) 2 and Pb (CH 3 COOH) 2 ; they would help oxidize the metals contained in rocks, and can increase the degree of liberation and the contact of granules with NaCN solution. In addition, Pb 2+ ions can precipitate sulfide ions from mineral impurities. When added during leaching, Pb(NO 3 ) 2 has proven to adequtely increase Au recovery [11]. However, the Pb elements contained in the tailings and wastewater may be harmful to the environment since Pb is a non-biodegradable heavy metal. If Pb sinks into groundwater or is absorbed into plants around the tailings, it would potentially harm the health of humans and other living things that consume them because of its accumulation as it moves along the food cycle [12]. This accumulation can interfere with activity, growth, metabolism, or reproduction [13].
Several research on reducing mineral impurities has been widely conducted, and has led to reducing Mn and arsenic impurities [9,10]. Cijiwa gold ore contains sulfur (S) as high as 5.18% and an Au content, of only 4.05 ppm; therefore, a research involves pretreatment of sulfur removal is of much interest. In such research, Pb salts were used to crush pyrite, oxidize the metal contained therein, and precipitate the S 2ions, which resulted in a favorable Au recovery of over 90% [11]. However, the Pb 2+ ions from PbNO 3 would enlarge the content of minerals in the tailings, such as copper, lead, zinc, and sulfide [14].
This study refined previous research by using silver nitrate (AgNO 3 ) salt as oxidizing agents instead of Pb salts. The former can destroy pyrite, expand the contact surface of the ore with cyanide solution, oxidize metals contained in rocks, and precipitate sulfur ions to avoid the formation of thiocyanate. Moreover, Ag + ions that may be included inwaste are non-toxic and do not endanger human health, and the salt oxidizing properties of Ag are stronger than those of Pb salts. The addition of AgNO 3 in the leaching process improved Au recovery.
Comminution. Rocks as much as 750 kg were smoothed with a jaw crusher to a size of about 0.5 cm. This study consists of five experiments, so the rocks were split into five parts, each weighing 150 kg, and the Au content in rocks was analyzed using AAS analysis. The comminution process is to loosen Au from the mineral impurities.
Leaching. The experiment was conducted five times with AgNO 3 concentrations of 0, 1, 2, 3, and 4 grams. Rocks containing gold were destroyed to a size of about 0.5 cm. Leaching at each experiment was done by mixing 150 kg of gold rocks/feed, AgNO 3 , 400 grams of NaCN, 50 grams of chalk (CaO), and 70 liters of water into a heap leaching treatment tub, which had a waterproof material of high density polyethylene as its base. The bottom of the tub is sloped 3⁰ ̶ 6⁰ to facilitate the flow of the circulating concentrate.
The chalk was dissolved in water before it was introduced into the treatment basin. The pH of the Aurich solution was maintained around 10 ̶ 11 to optimize Au recovery. After chalking, AgNO 3 was added to destroy sulfide compounds such as pyrite, chalcopyrite, and other impurities and oxidize the metals contained therein. Essentially, Au is expected to oxidize into Au + June 2018 Vol. 22  No. 2 ion and bind with CNions. A solution of NaCN was then sprayed into the treatment basin using a sprinkler. The leaching solution prior to passing the activated carbon was a rich solution (concentrate).
During leaching, the metallic minerals would dissolve in the NaCN solution. The dissolution reaction of Au is shown in equation (1) [15], while those of mineral impurities are shown in (2) The S 2ions formed in equations (2) and (3) would react with the oxygen shown in (5). In addition, sulfide ions can also react with cyanide ions and oxygen to form thiocyanates, as shown in equation (6).
Heap Leaching Processing. After leaching for about 30 minutes, the rich solution was passed through activated carbon in a column (Carbon in Column / CIC), and the process of circulation lasted for 24 hours for each test. The image of the concentrated circulation is shown in Figure 1.

Figure 1. Concentrate Circulation
Atomic absorption spectroscopy analysis. From the AAS, the ppm feed (feed) and concentrate for each test was obtained. The AAS test is conducted at LIPI Geotek laboratory, Bandung.
Activated carbon burning. The activated carbon containing gold metal is burned to obtain the bullion, which is a mixture of gold, silver, and other metals.
Recovery. In cyanidation, the law of conservation of the weight of the metal is applied. The weight of the input metal must be equal to the weight of the output metal plus the weight of the accumulated metal. Au recovery is obtained from equation (7).  Table 1.

Results and Discussion
To calculate the weight of Au contained in the concentrate, the concentrate density must be calculated first as presented in Table 2. The concentrate density was measured using pycnometer.
Based on Table 1 and Table 2, for feeds weighing 150 kg and concentrate of 70 liters per test, Au weight contained in the feed and concentrate were obtained as presented in Table 3.  (7), Table 1, and Table  3, the Au recovery obtained is presented in Table 4. Figure 2. Figure 2 shows how addition of AgNO 3 to 150 kg of feed affects Au recovery. Recovery increased with addition of AgNO 3 , and was highest at the addition of Besides Au, Ag is very present in gold ore adding AgNO 3 during leaching would amount of Au + ions in the concentrate. Furthermore, (from ore pretreatment and AgNO 3 ) would ion (from impurities mineral) to form [20], thereby minimizing the possibility and (6). This would increase the amount of Au binds to CNion, especially since S 2ion oxygen and CNion to form [S 2 O 3 ] 2and CNS (5) and (6), respectively. If the reactions depicted by and (6) occur, the oxygen supply in equation (1) would reduce. In cleaning and leaching processes, liquid

Figure 2. Effect of AgNO 3 Concentration
June Based on equation (7), Table 1, and Table  3, the Au recovery obtained is presented in Table 4.
The effect of AgNO 3 concentration on Au recovery is presented in Figure 2.
to 150 kg of feed Recovery increased with addition of the addition of 4 g of AgNO 3 . gold ore; therefore, during leaching would increase the Furthermore, Ag + would react with S 2-) to form Ag 2 S precipitate of equations (5) would increase the amount of Au + ion which would react with and CNSion as in If the reactions depicted by (5) equation (1)

Concentration on Au Recovery
cyanide, and water are needed. Oxygen was mainly i troduced through a sprinkler. It was also obtained from the air above the leaching basin and the air contained in concentrate, which was continuously circulated for 24 hours. Cyanide ion bonded with Au to form a complex ion Au (CN) 2-. The reaction that occurs during leaching is shown in equation (8)  Although the Au recovery obtained in this study is still below 50%, there is a 23.57% increase in recovery due to pretreatment. Pretreatment was done by adding AgNO 3 to the concentrate so that Au recovery increased from 12.57% to 36.14%. The improvement in Au r covery in this study is higher than those of Qiu and Wei Li, where pretreatment improved recovery by 8.34% for Qiu and 3.45% for Wei [6,16]. Qiu et al were able to increase the leaching of Au from 85% (without pr treatment) to 93.34% (with pretreatment) [6], whi et al increased Au recovery from 85.44% (without pr treatment) to 88.89% (with pretreatment) [19].
In this research, the Au recovery is still below 50% b cause the grain size of ore (ca. 0.5 cm) is too fine for heap leaching. Grain size that is too percolation of concentrate in piercing the collision of ore granules during circulation heap leaching is about 1 ̶ 2.5 cm, while for sizes below 1 cm, vat leaching processing method is best employed [1].
In this study, Au recovery can be increased by roasting pretreatment to reduce sulfide minerals in order to achieve an Au recovery of 60 ̶ 80 shows that there is a good potential for AgNO prove Au recovery. A better recovery of above 50% can also be obtained by changing the size of the ore to about 1-2.5 cm to improve percolation of concentrate

Conclusion
For five tests, Au concentrations contained in the feed (ore) were 3.3259 ppm, 5.5460 ppm, 4.1504 ppm, 5.5135 ppm, and 4.5980 ppm. Similarly, those in the concentrate were 0.9055 ppm, 2.5703 ppm, 2.7699 ppm, 4.1691 ppm, and 3.6693 ppm. It was found that AgNO can increase Au recovery from 12.57% to 36.15%, and the optimal concentration of AgNO recovery was 4 g per 150 kg ore (feed) cyanide, and water are needed. Oxygen was mainly introduced through a sprinkler. It was also obtained from the air above the leaching basin and the air contained in concentrate, which was continuously circulated for 24 yanide ion bonded with Au to form a complex . The reaction that occurs during leaching is shown in equation (8) [17,18].
Au recovery obtained in this study is still below 50%, there is a 23.57% increase in recovery due to pretreatment. Pretreatment was done by adding to the concentrate so that Au recovery increased from 12.57% to 36.14%. The improvement in Au rein this study is higher than those of Qiu and Wei Li, where pretreatment improved recovery by 8.34% for Qiu and 3.45% for Wei [6,16]. Qiu et al were able to increase the leaching of Au from 85% (without pretreatment) to 93.34% (with pretreatment) [6], while Wei et al increased Au recovery from 85.44% (without pretreatment) to 88.89% (with pretreatment) [19].
In this research, the Au recovery is still below 50% because the grain size of ore (ca. 0.5 cm) is too fine for heap leaching. Grain size that is too fine can disrupt percolation of concentrate in piercing the collision of ore granules during circulation. A good grain size for .5 cm, while for sizes below 1 cm, vat leaching processing method is best employed Au recovery can be increased by roasting pretreatment to reduce sulfide minerals in order to 60 ̶ 80% [15]. This study shows that there is a good potential for AgNO 3 to improve Au recovery. A better recovery of above 50% can also be obtained by changing the size of the ore to about 2.5 cm to improve percolation of concentrate.
For five tests, Au concentrations contained in the feed (ore) were 3.3259 ppm, 5.5460 ppm, 4.1504 ppm, ppm. Similarly, those in the concentrate were 0.9055 ppm, 2.5703 ppm, 2.7699 ppm, 4.1691 ppm, and 3.6693 ppm. It was found that AgNO 3 can increase Au recovery from 12.57% to 36.15%, and the optimal concentration of AgNO 3 to increase Au r 150 kg ore (feed).