6. Recovery of gold from simulated spent cyanide solutions using bioadsorbents

As mentioned earlier, cyanide solution has been extensively employed for a long time in gold mining and also in plating applications because of its special complexing capabilities in aqueous solutions, creating the soluble Au(CN)2 � complex. Also as mentioned in the preceding section, such Au(CN)2 � complex is recovered 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]:

1. Oxidative decomposition using sulfur dioxide (INCO process)

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 to the following reaction:

$$\rm{CN}^- + \rm{SO}\_2 + \rm{O}\_2 + \rm{H}\_2\rm{O} = \rm{OCN}^- + \rm{SO}\_4^{2-} + 2\rm{H}^+ \tag{4}$$

where OCN� ion is unstable and easily hydrolyzed into ammonium bicarbonate. The sulfur dioxide gas can be replaced by sulfurous acid or sodium pyrosulfite (Na2S2O5).

2. Oxidative decomposition using hydrogen peroxide

Cyanide ion is decomposed by the aid of hydrogen peroxide also catalyzed by cupric sulfate according to the following reaction:

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

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

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.

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

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

ore sample.

116

Mongolian gold ore.

Figure 16.

Figure 15.

Elements of Bioeconomy

adsorption of gold(III) from this leach liquor.

$$\rm{CNV}^{-} + \rm{H\_2O\_2} = \rm{OCN}^{-} + \rm{SO\_4^{2-}} + 2\rm{H^+} \tag{5}$$

#### 3. Decomposition using Caro's acid

Cyanide is decomposed using Caro's acid, which is formed by interacting hydrogen peroxide with sulfuric acid according to the following reaction:

$$\text{CN}^- + \text{H}\_2\text{SO}\_5 = \text{OCN}^- + \text{SO}\_4^{2-} + 2\text{H}^+ \tag{6}$$

4.Decomposition by the aid of microorganisms

Cyanide is decomposed by microorganisms according to the following reaction:

$$\rm{CN}^- + 2\rm{H}\_2\rm{O} + 1/2\rm{O}\_2 + \rm{Ba} = \rm{HCO}\_3^- + \rm{NH}\_3\tag{7}$$

been currently employed in noncyanide gold plating. The trace amount of Au(I) is

Effect of adsorbent dose on the concentration of gold(I) remained in the aqueous solution after the adsorption on bioadsorbent of cellulose at pH = 3 where the test aqueous solution initially contained 0.3 mmol/L gold(I) in

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

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

In the adsorption of gold(I) in the absence of hypochlorite, only negligible adsorption of gold(I) was observed regardless of pH values. However, by adding sodium hypochlorite to the gold(I) solution in hydrochloric acidic media, the adsorption was drastically improved as shown in Figure 18, suggesting that the addition of sodium hypochlorite provides suitable chemical changes for gold(I). Additionally, a high selectivity to gold(I) was also observed over other metals similar to the case of the adsorption of gold(III) from hydrochloric acid solution as

It is considered that gold(I) was oxidized into gold(III) by the aid of sodium

Here, the sample solution of sodium gold(I) sulfite, Na3[Au(SO3)2], was colorless. But, after the addition of excess amount of sodium hypochlorite in the presence of hydrochloric acid, the color was changed to pale yellow, the color of AuCl4

Figure 19 shows the effect of solid/liquid ratio, the ratio of added amount (dry weight) of bioadsorbent of cellulose prepared by treating in boiling concentrated sulfuric acid to unit volume of the test solution, on the concentration of gold(I) remained in the aqueous solution after the adsorption from the aqueous solution initially contained 60 mg/L gold(I). As seen from this figure, gold(I) can be quantitatively recovered from the solution at the solid/liquid ratio = around 1 g/dm<sup>3</sup>

7. Prospects for the application of bioadsorbents to actual cyanide

As mentioned in the preceding section, gold(I) can be quantitatively recovered by means of adsorption using bioadsorbents under acidic conditions similar to gold

NaClO þ HCl ➔ HOCl þ NaCl (9)

�, i.e.,

.

2Au<sup>þ</sup> þ 3HOCl þ 3H<sup>þ</sup> þ 5Cl� ➔ 2AuCl4̄þ 3H2O (10)

hypochlorite according to the following reaction and adsorbed onto the

Au(III) solution, which visually evidence the oxidation reaction.

also exhausted from such gold sulfite-based plating baths.

shown in Figure 2, for example.

0.1 mol/L sodium hypochlorite [35].

Figure 19.

bioadsorbent of pure cellulose.

processes

119

5. Decomposition by the aid of sodium hypochlorite

Cyanide is decomposed by the aid of sodium hypochlorite in alkaline media according to the following reaction [15]:

$$\text{2NaCN} + \text{5NaClO} + \text{H}\_2\text{O} = 2\text{NaHCO}\_3 + \text{N}\_2 + \text{5NaCl} \tag{8}$$

The recovery of trace concentrations of gold remaining in spent cyanide solutions has been difficult due to relatively high processing costs as well as other various technical problems. However, the recovery of such trace concentration of gold has become highly attractive from an economical point of view due to the high price of gold in recent years. Consequently, we attempted to recover such trace concentration of gold(I) from waste cyanide solutions.

However, since gold(I) cyanide solutions are very toxic and its use is prohibited in our laboratory, a sodium salt of gold(I) sulfite, i.e., sodium gold(I) sulfite, Na3[Au(I)(SO3)2], was employed for the adsorptive recovery test of gold(I) in the present work as a simulated solution of cyanide solutions to obtain the fundamental information for exploring the feasibility for the recovery of gold(I) [35]. The use of the gold(I) sulfite complex for gold plating had been known since 1842 [36] and has

#### Figure 18.

Effect of sodium hypochlorite concentration on the adsorption of some metal ions on bioadsorbent of pure cellulose prepared by treating in boiling concentrated sulfuric acid [35].

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

Figure 19.

CN� <sup>þ</sup> H2O2 <sup>¼</sup> OCN� <sup>þ</sup> SO2�

Cyanide is decomposed using Caro's acid, which is formed by interacting hydrogen peroxide with sulfuric acid according to the following reaction:

CN� <sup>þ</sup> H2SO5 <sup>¼</sup> OCN� <sup>þ</sup> SO2�

CN� þ 2H2O þ 1=2 O2 þ Bac ¼ HCO�

Cyanide is decomposed by microorganisms according to the following reaction:

Cyanide is decomposed by the aid of sodium hypochlorite in alkaline media

The recovery of trace concentrations of gold remaining in spent cyanide solutions has been difficult due to relatively high processing costs as well as other various technical problems. However, the recovery of such trace concentration of gold has become highly attractive from an economical point of view due to the high price of gold in recent years. Consequently, we attempted to recover such trace

However, since gold(I) cyanide solutions are very toxic and its use is prohibited

in our laboratory, a sodium salt of gold(I) sulfite, i.e., sodium gold(I) sulfite, Na3[Au(I)(SO3)2], was employed for the adsorptive recovery test of gold(I) in the present work as a simulated solution of cyanide solutions to obtain the fundamental information for exploring the feasibility for the recovery of gold(I) [35]. The use of the gold(I) sulfite complex for gold plating had been known since 1842 [36] and has

Effect of sodium hypochlorite concentration on the adsorption of some metal ions on bioadsorbent of pure

cellulose prepared by treating in boiling concentrated sulfuric acid [35].

2NaCN þ 5NaClO þ H2O ¼ 2NaHCO3 þ N2 þ 5NaCl (8)

3. Decomposition using Caro's acid

Elements of Bioeconomy

according to the following reaction [15]:

Figure 18.

118

4.Decomposition by the aid of microorganisms

5. Decomposition by the aid of sodium hypochlorite

concentration of gold(I) from waste cyanide solutions.

<sup>4</sup> þ 2H<sup>þ</sup> (5)

<sup>4</sup> þ 2H<sup>þ</sup> (6)

<sup>3</sup> þ NH3 (7)

Effect of adsorbent dose on the concentration of gold(I) remained in the aqueous solution after the adsorption on bioadsorbent of cellulose at pH = 3 where the test aqueous solution initially contained 0.3 mmol/L gold(I) in 0.1 mol/L sodium hypochlorite [35].

been currently employed in noncyanide gold plating. The trace amount of Au(I) is also exhausted from such gold sulfite-based plating baths.

In the adsorption of gold(I) in the absence of hypochlorite, only negligible adsorption of gold(I) was observed regardless of pH values. However, by adding sodium hypochlorite to the gold(I) solution in hydrochloric acidic media, the adsorption was drastically improved as shown in Figure 18, suggesting that the addition of sodium hypochlorite provides suitable chemical changes for gold(I). Additionally, a high selectivity to gold(I) was also observed over other metals similar to the case of the adsorption of gold(III) from hydrochloric acid solution as shown in Figure 2, for example.

It is considered that gold(I) was oxidized into gold(III) by the aid of sodium hypochlorite according to the following reaction and adsorbed onto the bioadsorbent of pure cellulose.

$$\text{NaClO} + \text{HCl} \blackheadrightarrow \text{HOCl} + \text{NaCl} \tag{9}$$

$$2\text{Au}^+ + \text{3HOCl} + \text{3H}^+ + \text{5Cl}^- \rightarrow 2\text{AuCl}\_4^- + \text{3H}\_2\text{O} \tag{10}$$

Here, the sample solution of sodium gold(I) sulfite, Na3[Au(SO3)2], was colorless. But, after the addition of excess amount of sodium hypochlorite in the presence of hydrochloric acid, the color was changed to pale yellow, the color of AuCl4 �, i.e., Au(III) solution, which visually evidence the oxidation reaction.

Figure 19 shows the effect of solid/liquid ratio, the ratio of added amount (dry weight) of bioadsorbent of cellulose prepared by treating in boiling concentrated sulfuric acid to unit volume of the test solution, on the concentration of gold(I) remained in the aqueous solution after the adsorption from the aqueous solution initially contained 60 mg/L gold(I). As seen from this figure, gold(I) can be quantitatively recovered from the solution at the solid/liquid ratio = around 1 g/dm<sup>3</sup> .
