3. Adsorption behaviors of bioadsorbents for metal ions

All of the bioadsorbents prepared by the method mentioned above exhibited extraordinary high selectivity only to gold(III) in the adsorption from hydrochloric acid solutions. For example, Figure 2 shows the % adsorption of some metal ions onto bioadsorbent prepared from orange waste (orange juice residue) from various concentrations of hydrochloric acid solution [21], where the % adsorption denotes the percentage of metal ion adsorbed on the adsorbent from aqueous solution and defined by the following equation.

%Adsorption ¼ ðMass of metal ion adsorbed on the adsorbent= Mass of metal ion initially present in the aqueous solutionÞ � 100 ¼ fðinitial concentration of the metal ion � concentration of the metal ion after adsorptionÞ=initial concentration of the metal iong � 100 (1)

As seen in this figure, only gold(III) is quantitatively adsorbed over the whole concentration range of hydrochloric acid tested, while other metal ions, not only precious metals such as palladium(II) and platinum(IV) but also base metals such as

Figure 2.

and adsorption have been employed. Of these processes, precipitation and solvent extraction are suitable for the recovery from solutions of high concentration, while adsorption and ion-exchange are suitable from those of low or trace concentration. During long operation, solvent extraction reagents, adsorbents, and ionexchangers gradually deteriorate and finally they are discarded. For example, in the cases of ion-exchange resins, they deteriorate through the formation of many cracks and clogging of micropores of the resins by fine particles present in actual solutions,

For the effective separation and concentration in hydrometallurgical processes, high selectivity and high loading capacity for targeted metals are strongly required for solvent extraction reagents and adsorbents. However, the selectivity exhibited by a majority of commercially available ion-exchange resins including chelating

Ion-exchange resins are plastic beads produced from petroleum. In recent years, environmental pollutions by microplastics have been deeply worried all over the world and big expectations are placed on biodegradable plastics. However, their high production costs prevent their actual employments in various fields.

In our recent studies, we found that adsorption gels prepared from various kinds of biomass materials including various biomass wastes, i.e., bioadsorbents, exhibit high selectivity and high loading capacity for targeted metals such as hazardous heavy metals and valuable metals. These are prepared from waste wood [1–4] and straws of rice and wheat [5], spent papers [6–10], cotton [11], waste seaweeds [12, 13], persimmon tannin [14–16] or wastes of persimmon [17, 18] and grape [19, 20] rich in tannin compounds, wastes of citrus such as orange [21] and lemon

In the present chapter, we introduce the adsorptive recovery of gold from printed circuit boards (PCBs) of spent mobile phones, a typical e-waste, and actual gold ore, a primary resource of gold, as well as that of trace concentration of gold from simulated spent cyanide solutions using some of these bioadsorbents.

The bioadsorbents for gold recovery can be easily prepared in a simple manner as schematically shown in Figure 1. Pieces of feed materials of biomass are stirred in

both of which impede smooth operation using packed columns.

[22], and residue of microalgae after extracting biofuel [23, 24].

2. Preparation of bioadsorbents for gold recovery

Figure 1.

104

Flow sheet of the preparation of bioadsorbents.

resins has not been always satisfactory.

Elements of Bioeconomy

Percentage adsorptions of some metal ions on bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid [21].

Figure 3.

Percentage adsorptions of some metal ions on bioadsorbent prepared from orange waste by means of carbonization at 800°C.

copper(II) and zinc(II), are not practically adsorbed. Similar phenomena were also observed also for all bioadsorbents prepared by the method using boiling concentrated sulfuric acid.

Figure 5 shows the image of optical microscope of the bioadsorbent prepared from residue of microalgae after biofuel extraction after adsorption of gold(III). In this photograph, aggregates of elemental gold particles are observed as brilliant yellow lumps, while black particles are bioadsorbents of microalgae. The formation of elemental gold was confirmed also from the observation by X-ray diffraction (XRD) analysis. Similar phenomena were observed also for other bioadsorbents we prepared. From these results, it can be concluded that the adsorbed gold(III) was reduced into elemental gold on the surface of the bioadsorbent and that the extraordinary high adsorption capacity for gold(III)

Adsorption isotherm of gold(III) on bioadsorbent prepared from orange waste [21], where q and Ce denote the amount of adsorbed gold(III) and concentration gold(III) present in the aqueous solution at equilibrium,

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

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

is attributable to the formation of elemental gold particles on these

Zn(II): 0.763 V.

107

Figure 4.

respectively.

reduced into elemental gold as follows.

membered metal chelates.

bioadsorbents. Furthermore, it can be concluded that the high selectivity for gold(III) over other metal ions is attributed to the higher oxidation-reduction potential (ORP) for gold(III) than other metal ions; e.g., those of some metal ions are as follows. Au(III): 1.52 V, Pd(II): 0.915 V, Cu(II): 0.340V, Ni(II): 0.257 V,

The mechanism of adsorptive reduction of gold(III) is shown in Figure 6. Gold (III) present in aqueous solution is adsorbed on the surface of the bioadsorbents and

1. Interaction of positively charged gold(III) ion with oxygen atoms of hydroxyl groups and ether oxygen atoms of polysaccharide molecules or tannin

compounds contained in bioadsorbents followed by adsorption forming stable five-membered chelate rings. Here, by the cross-linking reactions using boiling concentrated sulfuric acid, structures of polymer chains of polysaccharide and tannin molecules are transformed into those suitable for forming stable five-

Figure 3 shows the similar plots in the case of the adsorption on the bioadsorbent of orange waste prepared by means of carbonization at 800°C, for comparison. Although gold(III) is quantitatively adsorbed over the whole concentration range of hydrochloric acid also on this bioadsorbent, considerable amount of platinum(IV) and palladium(II) is also adsorbed at low concentration range in particular; i.e., the selectivity of the carbonized bioadsorbent to gold(III) is inferior to that prepared by using boiling concentrated sulfuric acid.

Figure 4 shows the adsorption isotherm of gold(III), i.e., the relationship between the amount of adsorption of gold(III) and its concentration present in the aqueous solution (0.1 mol/L hydrochloric acid solution) at equilibrium at 30°C, on the adsorbent prepared from orange waste. The amount of adsorption increases with increasing concentration of gold(III) at low concentration range, while it tends to approach a constant value at high concentration range, suggesting the typical Langmuir-type adsorption isotherm. From the constant value, the maximum adsorption capacity for gold(III) on this adsorbent was evaluated as 10.5 mol/kg ( = 2.07 kg gold(III)/kg adsorbent), which is an extraordinarily high value, greater than the weight of the adsorbent. Similarly, very high values of adsorption capacity for gold(III) were observed also for other adsorbent prepared from different kinds of biomass materials. Table 1 shows the maximum adsorption capacities for gold (III) on the adsorbent prepared from various biomass materials and those on other adsorbents reported in some literatures, for comparison.

As seen in this table, some of bioadsorbents exhibit much higher adsorption capacity for gold(III) than commercially available adsorbents such as activated carbon and chelating resins.

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

Figure 4.

copper(II) and zinc(II), are not practically adsorbed. Similar phenomena were also observed also for all bioadsorbents prepared by the method using boiling concen-

Figure 3 shows the similar plots in the case of the adsorption on the bioadsorbent of orange waste prepared by means of carbonization at 800°C, for comparison. Although gold(III) is quantitatively adsorbed over the whole concentration range of hydrochloric acid also on this bioadsorbent, considerable amount of platinum(IV) and palladium(II) is also adsorbed at low concentration range in particular; i.e., the selectivity of the carbonized bioadsorbent to gold(III)

Percentage adsorptions of some metal ions on bioadsorbent prepared from orange waste by means of

is inferior to that prepared by using boiling concentrated sulfuric acid.

adsorbents reported in some literatures, for comparison.

carbon and chelating resins.

106

Figure 4 shows the adsorption isotherm of gold(III), i.e., the relationship between the amount of adsorption of gold(III) and its concentration present in the aqueous solution (0.1 mol/L hydrochloric acid solution) at equilibrium at 30°C, on the adsorbent prepared from orange waste. The amount of adsorption increases with increasing concentration of gold(III) at low concentration range, while it tends to approach a constant value at high concentration range, suggesting the typical Langmuir-type adsorption isotherm. From the constant value, the maximum adsorption capacity for gold(III) on this adsorbent was evaluated as 10.5 mol/kg ( = 2.07 kg gold(III)/kg adsorbent), which is an extraordinarily high value, greater than the weight of the adsorbent. Similarly, very high values of adsorption capacity for gold(III) were observed also for other adsorbent prepared from different kinds of biomass materials. Table 1 shows the maximum adsorption capacities for gold (III) on the adsorbent prepared from various biomass materials and those on other

As seen in this table, some of bioadsorbents exhibit much higher adsorption capacity for gold(III) than commercially available adsorbents such as activated

trated sulfuric acid.

carbonization at 800°C.

Elements of Bioeconomy

Figure 3.

Adsorption isotherm of gold(III) on bioadsorbent prepared from orange waste [21], where q and Ce denote the amount of adsorbed gold(III) and concentration gold(III) present in the aqueous solution at equilibrium, respectively.

Figure 5 shows the image of optical microscope of the bioadsorbent prepared from residue of microalgae after biofuel extraction after adsorption of gold(III). In this photograph, aggregates of elemental gold particles are observed as brilliant yellow lumps, while black particles are bioadsorbents of microalgae. The formation of elemental gold was confirmed also from the observation by X-ray diffraction (XRD) analysis. Similar phenomena were observed also for other bioadsorbents we prepared. From these results, it can be concluded that the adsorbed gold(III) was reduced into elemental gold on the surface of the bioadsorbent and that the extraordinary high adsorption capacity for gold(III) is attributable to the formation of elemental gold particles on these bioadsorbents. Furthermore, it can be concluded that the high selectivity for gold(III) over other metal ions is attributed to the higher oxidation-reduction potential (ORP) for gold(III) than other metal ions; e.g., those of some metal ions are as follows. Au(III): 1.52 V, Pd(II): 0.915 V, Cu(II): 0.340V, Ni(II): 0.257 V, Zn(II): 0.763 V.

The mechanism of adsorptive reduction of gold(III) is shown in Figure 6. Gold (III) present in aqueous solution is adsorbed on the surface of the bioadsorbents and reduced into elemental gold as follows.

1. Interaction of positively charged gold(III) ion with oxygen atoms of hydroxyl groups and ether oxygen atoms of polysaccharide molecules or tannin compounds contained in bioadsorbents followed by adsorption forming stable five-membered chelate rings. Here, by the cross-linking reactions using boiling concentrated sulfuric acid, structures of polymer chains of polysaccharide and tannin molecules are transformed into those suitable for forming stable fivemembered metal chelates.


#### Table 1.

Maximum adsorption capacities for gold(III) on bioadsorbents we prepared and those reported in some literatures.


The surface of polysaccharides and tannin compounds cross-linked by the aid of boiling concentrated sulfuric acid functions as catalysts for the reduction reaction of

Mechanism of the cross-linking between polymer chains of cellulose molecules by the aid of concentrated sulfuric

Image of optical microscope of the bioadsorbent prepared from residue of microalgae after biofuel extraction

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

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

gold(III) ions into elemental gold(0) under acidic conditions.

acid and that of reductive adsorption of gold(III) on the cross-linked cellulose [25].

Figure 5.

Figure 6.

109

after adsorption of gold(III).

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

#### Figure 5.

Image of optical microscope of the bioadsorbent prepared from residue of microalgae after biofuel extraction after adsorption of gold(III).

#### Figure 6.

2. Reduction of the adsorbed gold(III) ions into elemental or metallic gold

Maximum adsorption capacities for gold(III) on bioadsorbents we prepared and those reported in some

Adsorbent Maximum adsorption

Cross-linked lignophenol prepared from sawdust of cedar 374 [3] Cross-linked lignocatechol prepared from sawdust of cedar 472 [3] Cross-linked lignopyrogallol prepared from sawdust of cedar 374 [3] Cross-linked lignophenol prepared from rice straw 552 [5] Cross-linked lignophenol prepared from wheat straw 217 [5] Cross-linked cellulose 1491 [25] Cross-linked dextran 1418 [25] Cross-linked alginic acid 1111 [25] Cross-linked pectic acid 946 [25] Cross-linked paper 1005 [10] Cross-linked cotton 1221 [11]

Cross-linked persimmon tannin (CPT) 1517 [14] Cross-linked persimmon peel waste (PP) 985 [17] Cross-linked orange juice residue (OJR) 1970 [21] Cross-linked lemon peel 1300 [22] Cross-linked chestnut pellicle 2100 [26] Cross-linked grape waste 1962 [19] Cross-linked microalgal residue (CMA) 650 [23] Microalgal residue, feed material of CMA 79 [23] Commercially available wood-based activated carbon 493 [3] Rice husk carbon 150 [27] Barley straw carbon 290 [27] Wattle tannin cross-linked using formaldehyde 8000 [28] Chitosan cross-linked using glutaraldehyde 566 [29]

capacity (g/kg)

1162 [14]

114 [30]

Reference

gold(III) ions, releasing hydrogen ions, where the hydroxyl groups are

3. Protonation of the carbonyl groups followed by returning back to hydroxyl

4.Aggregation of elemental gold particles into bigger lumps followed by isolation

groups which function again as the adsorption sites.

oxidized into carbonyl groups.

Commercially available chelating resin containing thiol

functional groups (Duolite GT-73)

Persimmon extract powder (PT powder, feed material of

CPT)

Elements of Bioeconomy

Table 1.

literatures.

108

from surface of the bioadsorbents.

particles by the aid of hydroxyl groups that take part in the interaction with the

Mechanism of the cross-linking between polymer chains of cellulose molecules by the aid of concentrated sulfuric acid and that of reductive adsorption of gold(III) on the cross-linked cellulose [25].

The surface of polysaccharides and tannin compounds cross-linked by the aid of boiling concentrated sulfuric acid functions as catalysts for the reduction reaction of gold(III) ions into elemental gold(0) under acidic conditions.

#### Figure 7.

Effect of equilibrium pH (pHe) on the % adsorption of gold(III) on the bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid where chloride concentration was maintained constant at 0.1 mol/L.

Figure 8 shows the effect of solid/liquid ratio, the ratio of dry weight of the added adsorbent to volume of aqueous solution, on the concentration of gold(III) remained in the aqueous solution after the adsorption from 0.25 mol/L hydrochloric acid solution containing 1.05 mg/L gold(IIII) on bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid. As seen in this figure, the concentration of gold(III) is lowered down to as low as 0.02 mg/L (20 ppb) by this bioadsorbent; i.e., about 98% recovery was achieved from such trace concentration

Thermogravimetric curves of the bioadsorbents prepared from microalgae residue after biofuel extraction before

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

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

(blue line) and after (red line) the adsorption of gold(III) [23].

The elution or desorption of the adsorbed gold(III) is difficult or nearly impossible using usual elution agents. In such cases, as will be mentioned in the latter section, gold-loaded adsorbents are incinerated leaving solid gold particles in the incineration residues. The bioadsorbents prepared from biomass materials are easy to be incinerated at relatively low temperature compared with commercially available ion-exchange resins, plastic beads produced from petroleum, which is another

Figure 9 shows the thermogravimetric curves (relationship between percentage decrease in the weight of materials and temperature) of bioadsorbent of microalgae residue after extracting biofuel before and after gold adsorption. As seen from this figure, both samples are completely decomposed at the temperature between 500 and 600°C. In this figure, the difference between red and blue lines at the temperature higher than 600°C corresponds the weight of gold loaded on this bioadsorbent

4. Recovery of gold from printed circuit boards of spent mobile phones

As an example of the use of bioadsorbents we prepared, recovery of gold from printed circuit boards (PCBs) of spent mobile phones is introduced in this section.

of gold(III) solution.

Figure 9.

advantage of bioadsorbents.

sample.

111

#### Figure 8.

Relationship between concentration of gold(III) remained in the aqueous solution after the adsorption on bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid and solid/liquid ratio, the ratio of dry weight of the added adsorbent to volume of aqueous solution.

Figure 7 shows the effect of pH on the adsorption of gold(III) on the bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid. As seen from this figure, although gold(III) is quantitatively adsorbed at pH less than 6 (acidic condition), no adsorption of gold(III) takes place at pH higher than 8 (basic condition) in accordance with the mechanism mentioned above.

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

Figure 9.

Thermogravimetric curves of the bioadsorbents prepared from microalgae residue after biofuel extraction before (blue line) and after (red line) the adsorption of gold(III) [23].

Figure 8 shows the effect of solid/liquid ratio, the ratio of dry weight of the added adsorbent to volume of aqueous solution, on the concentration of gold(III) remained in the aqueous solution after the adsorption from 0.25 mol/L hydrochloric acid solution containing 1.05 mg/L gold(IIII) on bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid. As seen in this figure, the concentration of gold(III) is lowered down to as low as 0.02 mg/L (20 ppb) by this bioadsorbent; i.e., about 98% recovery was achieved from such trace concentration of gold(III) solution.

The elution or desorption of the adsorbed gold(III) is difficult or nearly impossible using usual elution agents. In such cases, as will be mentioned in the latter section, gold-loaded adsorbents are incinerated leaving solid gold particles in the incineration residues. The bioadsorbents prepared from biomass materials are easy to be incinerated at relatively low temperature compared with commercially available ion-exchange resins, plastic beads produced from petroleum, which is another advantage of bioadsorbents.

Figure 9 shows the thermogravimetric curves (relationship between percentage decrease in the weight of materials and temperature) of bioadsorbent of microalgae residue after extracting biofuel before and after gold adsorption. As seen from this figure, both samples are completely decomposed at the temperature between 500 and 600°C. In this figure, the difference between red and blue lines at the temperature higher than 600°C corresponds the weight of gold loaded on this bioadsorbent sample.

### 4. Recovery of gold from printed circuit boards of spent mobile phones

As an example of the use of bioadsorbents we prepared, recovery of gold from printed circuit boards (PCBs) of spent mobile phones is introduced in this section.

Figure 7 shows the effect of pH on the adsorption of gold(III) on the bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid. As seen from this figure, although gold(III) is quantitatively adsorbed at pH less than 6 (acidic condition), no adsorption of gold(III) takes place at pH higher than 8 (basic condition) in accordance with the mechanism mentioned above.

ratio, the ratio of dry weight of the added adsorbent to volume of aqueous solution.

Relationship between concentration of gold(III) remained in the aqueous solution after the adsorption on bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid and solid/liquid

Effect of equilibrium pH (pHe) on the % adsorption of gold(III) on the bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid where chloride concentration was maintained constant at

Figure 7.

Elements of Bioeconomy

0.1 mol/L.

Figure 8.

110

Figure 10. Flow sheet of the dismantling of spent mobile phones.

Spent home appliances such as mobile phones are dismantled by hand work into various parts to recover various valuables for their reuses as shown in Figure 10.

Of these dismantled parts, gold and other precious metals such as palladium and platinum are contained in PCBs; i.e., PCBs of spent electronics are typical secondary resources of precious metals. According to the conventional recovery process of precious metals from complex feed materials such as anode slimes of copper and nickel generated in electrorefining processes of these metals which contain many kinds of metals such as gold, silver, palladium, platinum and base metals, they are recovered by repeating dissolution using aqua regia followed by precipitation for many times, which needs tedious long-time operations and high labor costs. In early 1970s, new recovery process was developed and commercialized by INCO [31]. In this process, the feed materials are totally dissolved in hydrochloric acid into that chlorine gas had been blown, abbreviated as chlorine-containing HCl, hereafter. Here, the chlorine gas dissolved in hydrochloric acid solution is converted into hypochlorous acid (HClO) according to the following reaction:

$$\text{Cl}\_2 + \text{H}\_2\text{O} \rightleftharpoons \text{HClO} + \text{HCl} \tag{2}$$

KIKINZOKU KOGYO Co. Ltd., Hiratsuka, Japan. The metal concentrations of this sample solution measured by ICP-AES were as follows (mg/L): Au(100), Pd(8), Pb (342), Fe(314), Cu(250), Ni(411), and Zn(41). The total acid concentration mea-

Effect of solid/liquid ratio on the % adsorption of various metals from leach liquor of chlorine-containing HCl using the bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid.

Figures 12 and 13 show the effect of solid/liquid ratio, the ratio of amount (dry weight) of added bioadsorbent to unit volume of sample leach liquor, on % adsorption of each metal in the case of adsorptive recovery using bioadsorbents of orange waste and cotton prepared by treating in boiling concentrated sulfuric acid, respectively. As seen from these figures, although gold is nearly quantitatively adsorbed,

sured by acid-base titration was around 3.0 mol/L.

Flow sheet for the treatment of spent PCBs in the present work.

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

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

Figure 11.

Figure 12.

113

other metals are not practically adsorbed on these bioadsorbents.

Thus, formed hypochlorous acid functions as a strong oxidation agent, converting solid metals into metal ions, dissolving into hydrochloric acid solution, where the metal ions give rise to stable chloro-complexes interacting with chloride ions; e.g., gold(III) is present as AuCl4 �, anionic species. However, because the hypochlorous acid formed by the abovementioned reaction is unstable and is easily converted into hydrochloric acid, the metal recovery from such solutions is actually the same with that from hydrochloric acid solutions.

In the present work, the sample of spent PCBs was treated in the similar manner using chlorine-containing HCl as schematically shown in Figure 11.

They are incinerated at first at 750°C to extinguish epoxy resin boards on which various parts are placed. Then, the residues are leached using nitric acid solution to remove silver, which impedes the recovery of gold and other precious metals in the latter steps, together with some base metals. The residue of the nitric acid leaching was calcined at 750°C again and leached using chlorine-containing HCl to recover gold and other precious metals. In the present work, the sample of such metalloaded leach liquor was kindly donated by Shonan Factory of TANAKA

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

#### Figure 11.

Spent home appliances such as mobile phones are dismantled by hand work into various parts to recover various valuables for their reuses as shown

hypochlorous acid (HClO) according to the following reaction:

ions; e.g., gold(III) is present as AuCl4

Flow sheet of the dismantling of spent mobile phones.

the same with that from hydrochloric acid solutions.

using chlorine-containing HCl as schematically shown in Figure 11.

Thus, formed hypochlorous acid functions as a strong oxidation agent, converting solid metals into metal ions, dissolving into hydrochloric acid solution, where the metal ions give rise to stable chloro-complexes interacting with chloride

hypochlorous acid formed by the abovementioned reaction is unstable and is easily converted into hydrochloric acid, the metal recovery from such solutions is actually

In the present work, the sample of spent PCBs was treated in the similar manner

They are incinerated at first at 750°C to extinguish epoxy resin boards on which various parts are placed. Then, the residues are leached using nitric acid solution to remove silver, which impedes the recovery of gold and other precious metals in the latter steps, together with some base metals. The residue of the nitric acid leaching was calcined at 750°C again and leached using chlorine-containing HCl to recover gold and other precious metals. In the present work, the sample of such metalloaded leach liquor was kindly donated by Shonan Factory of TANAKA

Of these dismantled parts, gold and other precious metals such as palladium and platinum are contained in PCBs; i.e., PCBs of spent electronics are typical secondary resources of precious metals. According to the conventional recovery process of precious metals from complex feed materials such as anode slimes of copper and nickel generated in electrorefining processes of these metals which contain many kinds of metals such as gold, silver, palladium, platinum and base metals, they are recovered by repeating dissolution using aqua regia followed by precipitation for many times, which needs tedious long-time operations and high labor costs. In early 1970s, new recovery process was developed and commercialized by INCO [31]. In this process, the feed materials are totally dissolved in hydrochloric acid into that chlorine gas had been blown, abbreviated as chlorine-containing HCl, hereafter. Here, the chlorine gas dissolved in hydrochloric acid solution is converted into

Cl2 þ H2O ⇌ HClO þ HCl (2)

�, anionic species. However, because the

in Figure 10.

112

Figure 10.

Elements of Bioeconomy

Flow sheet for the treatment of spent PCBs in the present work.

#### Figure 12.

Effect of solid/liquid ratio on the % adsorption of various metals from leach liquor of chlorine-containing HCl using the bioadsorbent prepared from orange waste by treating in boiling concentrated sulfuric acid.

KIKINZOKU KOGYO Co. Ltd., Hiratsuka, Japan. The metal concentrations of this sample solution measured by ICP-AES were as follows (mg/L): Au(100), Pd(8), Pb (342), Fe(314), Cu(250), Ni(411), and Zn(41). The total acid concentration measured by acid-base titration was around 3.0 mol/L.

Figures 12 and 13 show the effect of solid/liquid ratio, the ratio of amount (dry weight) of added bioadsorbent to unit volume of sample leach liquor, on % adsorption of each metal in the case of adsorptive recovery using bioadsorbents of orange waste and cotton prepared by treating in boiling concentrated sulfuric acid, respectively. As seen from these figures, although gold is nearly quantitatively adsorbed, other metals are not practically adsorbed on these bioadsorbents.

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:

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

1. Strong toxicity of cyanide, which causes serious environmental problems and, consequently, needs some costs for safe operation and environmental

2. Interference by other coexisting metals or low selectivity over other metals.

3. Slow dissolution of gold as shown in Table 2 that shows the comparison of

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

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

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

condition

) (aqua regia) 80°C, atm 1800

40°C, atm 180

Dissolution rate (g/cm<sup>2</sup> h)

64.75, cobalt 0.008, nickel 0.040, copper 0.779, and zinc 0.069.

Reagents or mixtures Operating

6 mol/dm<sup>3</sup> HCl + saturated Cl2 (typical chlorine-containing

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

HCl)

Table 2.

115

3 HCl + HNO3 (6 mol/dm<sup>3</sup>

32 wt.% HCl + MnO2(s) 100°C, atm 0.137 32 wt.% HCl + MnO2(s) 90°C, 639 kPa 0.25 6 mmol/dm<sup>3</sup> NaCN + 4 mmol/dm<sup>3</sup> Ca(OH)2 + air 30°C, atm 0.7 0.45 mol/dm<sup>3</sup> NaCN + 0.2 mol/dm<sup>3</sup> NaOH + air 30°C, atm 1.5 6 mol/dm<sup>3</sup> HCl + 0.22 mol/dm<sup>3</sup> H2O2 50°C 4

dissolution rate of gold by some lixiviants.

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

cyanide and 0.05% calcium oxide [33].

protection.

Figure 13.

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.
