7. Prospects for the application of bioadsorbents to actual cyanide processes

As mentioned in the preceding section, gold(I) can be quantitatively recovered by means of adsorption using bioadsorbents under acidic conditions similar to gold (III) after oxidizing gold(I) into gold(III) by the oxidation treatment using sodium hypochlorite, for example.

Also as mentioned earlier, the main hydrometallurgical process for gold and silver ores is cyanide leaching followed by gold recovery by means of adsorption on strongly basic anion exchange resins and activated carbon or by means of cementation using zinc powder. In the adsorption process, the adsorbed gold is recovered by incinerating these loaded adsorbents because the elution of gold adsorbed on these adsorbents is difficult. On the other hand, the cementation using zinc powder also suffers from some problems, one of which is severe control of oxygen or air, except for which it would consume too much amount of zinc powder and cause redisolution of the resulted elemental gold powders. The major gold plating process is also that using cyanide plating solution, in which gold is recovered by the same processes. In these processes, after the recovery of gold, spent cyanide solutions are discharged into environment after the decomposition of cyanide using sodium hypochlorite, for example, as mentioned in the preceding section.

However, by means of the adsorption using bioadsorbents as mentioned above, more economical and more environmental benign process can be proposed as schematically depicted in Figure 20.

In the new process shown in Figure 20, trace concentration of gold(I) contained in cyanide solution will be able to be quantitatively recovered using bioadsorbents, which are easy to be incinerated at comparatively low temperature consuming less amount of energy leaving only gold powder as shown in Figure 9, for example.

A number of processes for recovering cyanide from gold plant barren solutions or pulps also have been developed [32]. For example, the acidification, volatilization and reneutralization (AVR) process as schematically depicted in Figure 21 was practiced at Pachuca silver mine in Mexico and at Flin Flon mine in Canada more than 60 years ago and still now is under operation. Further, it has been recently installed at several other mines around the world.

environmentally benign process using bioadsorbents can be proposed as shown in

Flow sheet of AVR process for the recovery of gold by cyanide leaching followed by recycling of cyanide.

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

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

Further, a more recent advancement is the sulfidization, acidification, recycling, and thickening of precipitate (SART) process schematically shown in Figure 23 developed for ores containing high content of copper, which consumes large

Figure 22 only by changing the order of the step of acidification.

Figure 21.

Figure 22.

121

Modification of AVR process using bioadsorbents.

In this process, by adding acid to the barren solution after recovering gold, cyanide is converted into HCN gas, which is scrubbed using caustic solution, returning into cyanide for reuse again. For this process, more economical and more

Figure 20.

New recovery process of gold from cyanide solutions using bioadsorbents.

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

Figure 21.

(III) after oxidizing gold(I) into gold(III) by the oxidation treatment using sodium

Also as mentioned earlier, the main hydrometallurgical process for gold and silver ores is cyanide leaching followed by gold recovery by means of adsorption on strongly basic anion exchange resins and activated carbon or by means of cementation using zinc powder. In the adsorption process, the adsorbed gold is recovered by incinerating these loaded adsorbents because the elution of gold adsorbed on these adsorbents is difficult. On the other hand, the cementation using zinc powder also suffers from some problems, one of which is severe control of oxygen or air, except

redisolution of the resulted elemental gold powders. The major gold plating process is also that using cyanide plating solution, in which gold is recovered by the same processes. In these processes, after the recovery of gold, spent cyanide solutions are discharged into environment after the decomposition of cyanide using sodium

However, by means of the adsorption using bioadsorbents as mentioned above, more economical and more environmental benign process can be proposed as sche-

In the new process shown in Figure 20, trace concentration of gold(I) contained in cyanide solution will be able to be quantitatively recovered using bioadsorbents, which are easy to be incinerated at comparatively low temperature consuming less amount of energy leaving only gold powder as shown in Figure 9, for example. A number of processes for recovering cyanide from gold plant barren solutions or pulps also have been developed [32]. For example, the acidification, volatilization and reneutralization (AVR) process as schematically depicted in Figure 21 was practiced at Pachuca silver mine in Mexico and at Flin Flon mine in Canada more than 60 years ago and still now is under operation. Further, it has been recently

In this process, by adding acid to the barren solution after recovering gold, cyanide is converted into HCN gas, which is scrubbed using caustic solution, returning into cyanide for reuse again. For this process, more economical and more

for which it would consume too much amount of zinc powder and cause

hypochlorite, for example, as mentioned in the preceding section.

hypochlorite, for example.

Elements of Bioeconomy

matically depicted in Figure 20.

Figure 20.

120

installed at several other mines around the world.

New recovery process of gold from cyanide solutions using bioadsorbents.

Flow sheet of AVR process for the recovery of gold by cyanide leaching followed by recycling of cyanide.

environmentally benign process using bioadsorbents can be proposed as shown in Figure 22 only by changing the order of the step of acidification.

Further, a more recent advancement is the sulfidization, acidification, recycling, and thickening of precipitate (SART) process schematically shown in Figure 23 developed for ores containing high content of copper, which consumes large

Figure 22. Modification of AVR process using bioadsorbents.

hydrochloric acid, not after the recovery of gold but before the gold recovery step. During the acidification, gold(I) and copper(I) are spontaneously oxidized into gold (III) and copper(II) by oxygen in air. From such acidified liquor containing gold (III) and copper(II), gold(III) can be quantitatively and highly selectively recovered over copper(II) using the bioadsorbents as metallic gold in a simple manner, leaving copper(II) in the raffinate, which can be easily recovered by means of solvent extraction using hydroxime reagents or, more simply, by means of precipitation

Gold Recovery Process from Primary and Secondary Resources Using Bioadsorbents

Bioadsorbents for gold recovery were prepared from various biomaterials including biomass wastes such as orange juice residue in a simple manner only by treating in boiling concentrated sulfuric acid. These bioadsorbents exhibited extraordinary high loading capacity and high selectivity for gold in the adsorption from acidic chloride media, which were elucidated to be attributable to the reduction reaction of gold(III) into gold(0), elemental gold, due to the highest oxidationreduction potential of gold(III), catalyzed by the surface of the bioadsorbents pre-

It was confirmed in the recovery tests of gold from printed circuit boards of spent mobile phones, Mongolian gold ore, and simulated spent cyanide solutions containing trace concentration of gold(I) that satisfactory gold recovery was achieved by using these bioadsorbents. Some new gold recovery processes using

By using other types of bioadsorbents, it is possible to recover other precious metals such as palladium and platinum and hazardous materials such as heavy

The authors are deeply indebted to Shonan Factory of Tanaka Kikinzoku Kogyo

Co. Ltd. and Western Mongolian Metals Co. Ltd. for the kind donation of the samples of printed circuit boards of spent mobile phones and Mongolian gold ore, respectively. We also indebted to Miss Kumiko Kajiyama, Miss Miyuki Matsueda, Miss Sayaka Yamada, Mr. Minoru Abe, Jun-ichi Inoue for their assistance in

using sodium sulfide as the precipitates of copper sulfide.

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

pared by condensation reaction using concentrated sulfuric acid.

bioadsorbents were proposed for actual cyanide processes.

8. Conclusion

metals.

123

Acknowledgements

adsorption and recovery tests.

#### Figure 23.

Flow sheet for the recovery of gold by means of cyanide leaching followed by recycling of cyanide by means of SART process.

amount of cyanide, making worse of the economy of gold recovery. In this process, sulfides are added during the acidification by which pH is lowered from about 10 to 4.5. Under such conditions, the copper present as cyanide complex, Cu(CN)4 <sup>3</sup>, is completely converted into the mineral chalcocite, Cu2S, releasing hydrogen cyanide, HCN gas. However, because selectivity of both strongly basic anion exchange resins and activated carbon to gold(I) cyanide are inferior, large amount of copper (I) cyanide are also adsorbed onto these adsorbents together with gold(I) cyanide, which results in tedious posttreatments.

Also for this process, more economical and more environmentally benign new process using biomass adsorbents can be proposed as schematically depicted in Figure 24. In this proposed process, cyanide leach liquor is acidified by adding

Figure 24. Modification of SART process using bioadsorbents.

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

hydrochloric acid, not after the recovery of gold but before the gold recovery step. During the acidification, gold(I) and copper(I) are spontaneously oxidized into gold (III) and copper(II) by oxygen in air. From such acidified liquor containing gold (III) and copper(II), gold(III) can be quantitatively and highly selectively recovered over copper(II) using the bioadsorbents as metallic gold in a simple manner, leaving copper(II) in the raffinate, which can be easily recovered by means of solvent extraction using hydroxime reagents or, more simply, by means of precipitation using sodium sulfide as the precipitates of copper sulfide.

## 8. Conclusion

amount of cyanide, making worse of the economy of gold recovery. In this process, sulfides are added during the acidification by which pH is lowered from about 10 to 4.5. Under such conditions, the copper present as cyanide complex, Cu(CN)4

Flow sheet for the recovery of gold by means of cyanide leaching followed by recycling of cyanide by means of

completely converted into the mineral chalcocite, Cu2S, releasing hydrogen cyanide, HCN gas. However, because selectivity of both strongly basic anion exchange resins and activated carbon to gold(I) cyanide are inferior, large amount of copper (I) cyanide are also adsorbed onto these adsorbents together with gold(I) cyanide,

Also for this process, more economical and more environmentally benign new process using biomass adsorbents can be proposed as schematically depicted in Figure 24. In this proposed process, cyanide leach liquor is acidified by adding

which results in tedious posttreatments.

Modification of SART process using bioadsorbents.

Figure 23.

Figure 24.

122

SART process.

Elements of Bioeconomy

<sup>3</sup>, is

Bioadsorbents for gold recovery were prepared from various biomaterials including biomass wastes such as orange juice residue in a simple manner only by treating in boiling concentrated sulfuric acid. These bioadsorbents exhibited extraordinary high loading capacity and high selectivity for gold in the adsorption from acidic chloride media, which were elucidated to be attributable to the reduction reaction of gold(III) into gold(0), elemental gold, due to the highest oxidationreduction potential of gold(III), catalyzed by the surface of the bioadsorbents prepared by condensation reaction using concentrated sulfuric acid.

It was confirmed in the recovery tests of gold from printed circuit boards of spent mobile phones, Mongolian gold ore, and simulated spent cyanide solutions containing trace concentration of gold(I) that satisfactory gold recovery was achieved by using these bioadsorbents. Some new gold recovery processes using bioadsorbents were proposed for actual cyanide processes.

By using other types of bioadsorbents, it is possible to recover other precious metals such as palladium and platinum and hazardous materials such as heavy metals.
