5. Recovery of gold from Mongolian gold ore

At present, majority of gold has been recovered from gold and silver ores by means of cyanide process developed at the beginning of twentieth century as schematically shown in Figure 14. In this process, pulverized ores are leached using alkaline cyanide solution to extract gold as gold(I)-cyanide complexes according to the following reaction:

$$4\text{Au} + 8\text{CN}^- + \text{O}\_2 + 2\text{H}\_2\text{O} = 4\text{AuCN}\_2^- + 4\text{OH}^-\tag{3}$$

The extracted gold(I) as anionic species, AuCN2 �, is adsorbed onto activated carbon or strongly basic anion exchange resins, which are termed as CIP and RIP

Figure 14. Flow sheet of conventional gold recovery process from gold ores using alkaline cyanide solutions.

## Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents DOI: http://dx.doi.org/10.5772/intechopen.84770

processes, respectively. Because it is difficult to desorb the gold adsorbed onto these adsorbents, these are incinerated at high temperature to recover metallic gold. This cyanide process has suffered from some problems as follows:


As alternatives to cyanide leaching, some noncyanide leaching processes such as those using hypochlorous acid, bromine, thiosulfate, and thiourea have been proposed. However, these new processes also suffer from their own drawbacks as follows. Thiourea is known as carcinogen and, additionally, it is expensive and chemically unstable compared to cyanide, while it has a big advantage of much faster dissolution rate of gold than cyanide; that is, it was reported that the dissolution rate of gold using the mixture of 1% thiourea in 0.5% sulfuric acid containing 0.1% ferric ion is over 10-folds faster than that using the mixture of 0.5% sodium cyanide and 0.05% calcium oxide [33].

Following the recovery of gold from spent PCBs, a typical secondary resource, we attempted to apply the bioadsorbents we prepared to noncyanide leach liquor of actual gold ore (one example of typical primary resources). The sample of the ore was kindly donated by Western Mongolian Metals Co. Ltd., Ulaanbaatar, Mongolia. It was fine powder, the particle size of which was around 75–150 μm and the metal contents (mg/g) were as follows: gold 0.046, platinum 0.018, aluminum 0.694, iron 64.75, cobalt 0.008, nickel 0.040, copper 0.779, and zinc 0.069.

In the present work, the recovery of gold from the abovementioned gold ore was investigated by means of leaching using acidothiourea solution consisting of 0.1 mol/L thiourea and 0.05 mol/L sulfuric acid followed by adsorption using bioadsorbent of cotton. Figure 15 shows the effect of liquid/solid ratio (ratio of volume of the leach liquor to unit dry weight of the sample of ore powder) on the leached amount of gold and platinum from the ore sample. From this result, 30 mL/g appears to be the most suitable liquid/solid ratio for extracting gold and platinum from the ore sample; i.e., addition of about 0.23 g of thiourea and 0.15 g of sulfuric


#### Table 2.

Comparison of dissolution rates of gold using some lixiviants [32].

5. Recovery of gold from Mongolian gold ore

The extracted gold(I) as anionic species, AuCN2

the following reaction:

Figure 13.

Elements of Bioeconomy

Figure 14.

114

At present, majority of gold has been recovered from gold and silver ores by means of cyanide process developed at the beginning of twentieth century as schematically shown in Figure 14. In this process, pulverized ores are leached using alkaline cyanide solution to extract gold as gold(I)-cyanide complexes according to

Effect of solid/liquid ratio on the % adsorption of various metals from leach liquor of chlorine-containing HCl

using the bioadsorbent of cotton prepared by treating in boiling concentrated sulfuric acid.

� þ 4OH� (3)

�, is adsorbed onto activated

4Au þ 8CN� þ O2 þ 2H2O ¼ 4AuCN2

Flow sheet of conventional gold recovery process from gold ores using alkaline cyanide solutions.

carbon or strongly basic anion exchange resins, which are termed as CIP and RIP

#### Figure 15.

Effect of liquid/solid ratio on the leaching amount of gold and platinum from the Mongolian gold ore sample using acidothiourea consisting of 0.1 mol/L thiourea and 0.05 mol/L sulfuric acid.

elemental gold, suggesting that gold was recovered as elemental gold particles also

As mentioned earlier, cyanide solution has been extensively employed for a long

ered by means of adsorption on activated carbon or strongly basic anion-exchange resin. However, such adsorptive recovery of gold is not always quantitative and trace concentrations of gold still remain in the cyanide solution. Spent cyanide solutions generated after the recovery of gold are treated for cyanide decomposition before discharging in environments according to the following processes [34]:

In this process, cyanide ion is decomposed by the aid of sulfur dioxide and oxygen gasses blown into the cyanide solution catalyzed by cupric sulfate according

where OCN� ion is unstable and easily hydrolyzed into ammonium bicarbonate.

Cyanide ion is decomposed by the aid of hydrogen peroxide also catalyzed by

CN� <sup>þ</sup> SO2 <sup>þ</sup> O2 <sup>þ</sup> H2O <sup>¼</sup> OCN� <sup>þ</sup> SO<sup>2</sup>�

The sulfur dioxide gas can be replaced by sulfurous acid or sodium pyrosulfite

2. Oxidative decomposition using hydrogen peroxide

cupric sulfate according to the following reaction:

� com-

� complex is recov-

<sup>4</sup> þ 2H<sup>þ</sup> (4)

6. Recovery of gold from simulated spent cyanide solutions using

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

DOI: http://dx.doi.org/10.5772/intechopen.84770

time in gold mining and also in plating applications because of its special complexing capabilities in aqueous solutions, creating the soluble Au(CN)2

1. Oxidative decomposition using sulfur dioxide (INCO process)

plex. Also as mentioned in the preceding section, such Au(CN)2

XRD pattern of the cotton adsorbent after adsorption of gold(III).

in this system.

Figure 17.

bioadsorbents

to the following reaction:

(Na2S2O5).

117

Figure 16.

Effect of solid/liquid ratio on the adsorption of gold using cotton adsorbent from the leach liquor of the Mongolian gold ore.

acid is necessary for complete extraction of gold and platinum from unit gram of the ore sample.

Figure 16 shows effect of solid/liquid ratio (ratio of dry weight of the added adsorbent to unit volume of the leach liquor containing gold(III)) on the adsorption of gold using bioadsorbent of cotton prepared by treating in boiling concentrated sulfuric acid from the leach liquor of Mongolian gold ore. This figure indicates that addition of at least 3 g of bioadsorbent of cotton is necessary for quantitative adsorption of gold(III) from this leach liquor.

Figure 17 shows the XRD pattern of the bioadsorbent of cotton after adsorption of gold(III). Four sharp peaks in this figure obviously evidence the presence of solid Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents DOI: http://dx.doi.org/10.5772/intechopen.84770

Figure 17. XRD pattern of the cotton adsorbent after adsorption of gold(III).

elemental gold, suggesting that gold was recovered as elemental gold particles also in this system.
