**4.2. Germanium from coal combustion and gasification fly ashes**

Coal plays an essential role in our global energy scheme for power generation. There are 1,139,331 million tons of proven coal reserves worldwide, sufficient to meet 153 years of global production which makes coal a reliable source [59, 60]. However, coal is currently a target to accomplish with the Paris climate agreement for both countries and companies which has caused a decline in coal production and consumption. Pulverized coal combustion (PCC) is the most widely used technology for coal power generation and, in a lesser extent, integrated gasification combined cycle (IGCC). In both processes, coal with a proximate Ge content <100 ppm can either vaporize totally and then be easily adsorbed on the finest coal fly ash (FA) particles during flue gas cooling or vaporize partially and enrich in both the coal FAs and, in a lesser extent, bottom ashes or slags (**Figure 3**). FAs are normally captured in particulate control devices with a high efficiency (>90%), but a small fraction of them may reach the flue-gas desulfurization (FGD).

Although part of the FA components may dissolve in the aqueous phase of the sorbent slurry when the flue gas passes through the sprayers, remaining in the FGD, the content of Ge in the FGD by-products (water effluent and FGD-gypsum) is not significant [60, 61]. Coal FAs are regarded as the main output stream of Ge.

The current annual production of coal FA worldwide is estimated to be around 750 million tons, and this is anticipated to increase in the near future [62]. The average content of Ge in coal FA is approximately 18 mg/kg, but as some research showed, it can reach 420 mg/kg [26, 63]. Therefore, one attractive source of Ge comes from coal FAs [64–66]. Coal combustion FA is a fine powder made up of spherical high Si-Al-Ca-K-Fe-Ti-Mg vitreous particles with Fe oxides and Al-Si species and irregular unburned coal and ash particles (**Figure 4**). It is generally accepted that vitreous FA particles consist of a relatively pure Al-Si-Ca-K-Fe glass within on which mullite crystals form a network [67, 68]. Furthermore, other types of particles such as calcite (CaCO3 ), lime (CaO), quartz (SiO<sup>2</sup> ), and gypsum (CaSO<sup>4</sup> .2H<sup>2</sup> O) are formed.

characterized by a predominant alumina-silicate glass matrix and a wide variety of crystal-

**Figure 4.** Scanning electron microscope photomicrographs of (a) FAs from coal and petroleum coke combustion and (b)

Germanium production from coal FAs usually consists of two stages. The first step creates a concentrate and the second is the actual recovery. The first published studies on Ge recovery from coal FAs were those based on pyro-metallurgical practices [70] but at the present

during coal gasification.

Germanium: Current and Novel Recovery Processes http://dx.doi.org/10.5772/intechopen.77997 19

line-reduced species, mainly sulfides, because of the low levels of O<sup>2</sup>

FAs from PCC [61].

**Figure 3.** Configuration of a PCC power plant and partitioning of Ge.

**Figure 5** shows the typical round morphology of IGCC FA particles [26].

moment are not applied due to the high economic and environmental cost [1].

During coal gasification, most of the mineral matter of the coal is transformed and melted into slag. As opposed to PCC, coal gasification produce very little FA (10–15%), which is

**Figure 3.** Configuration of a PCC power plant and partitioning of Ge.

For the separation of Ge4+ from Zn2+, Ga3+, and Fe3+ in solutions with a high acidity (80 g/L

A single contact system consisting of 10% LIX 63 and 2% Ionquest 801 to recover Ge from a synthetic leach solution of Zn refinery cementation residues was used [43]. Over 68% Ge4+ was extracted at a low pH at an A/O ratio of 1:1 and 40°C. Almost 73% Ge4+ was stripped with

A common disadvantage of the above SX or SSX processes for Ge4+ extraction is the use, for instance, of strong NaOH for stripping and of some reagents that are not commercially available. Therefore, more efficient and effective SX or SSX systems for the recovery of Ge are

Coal plays an essential role in our global energy scheme for power generation. There are 1,139,331 million tons of proven coal reserves worldwide, sufficient to meet 153 years of global production which makes coal a reliable source [59, 60]. However, coal is currently a target to accomplish with the Paris climate agreement for both countries and companies which has caused a decline in coal production and consumption. Pulverized coal combustion (PCC) is the most widely used technology for coal power generation and, in a lesser extent, integrated gasification combined cycle (IGCC). In both processes, coal with a proximate Ge content <100 ppm can either vaporize totally and then be easily adsorbed on the finest coal fly ash (FA) particles during flue gas cooling or vaporize partially and enrich in both the coal FAs and, in a lesser extent, bottom ashes or slags (**Figure 3**). FAs are normally captured in particulate control devices with a high efficiency (>90%), but a small fraction of them may

Although part of the FA components may dissolve in the aqueous phase of the sorbent slurry when the flue gas passes through the sprayers, remaining in the FGD, the content of Ge in the FGD by-products (water effluent and FGD-gypsum) is not significant [60, 61]. Coal FAs are

The current annual production of coal FA worldwide is estimated to be around 750 million tons, and this is anticipated to increase in the near future [62]. The average content of Ge in coal FA is approximately 18 mg/kg, but as some research showed, it can reach 420 mg/kg [26, 63]. Therefore, one attractive source of Ge comes from coal FAs [64–66]. Coal combustion FA is a fine powder made up of spherical high Si-Al-Ca-K-Fe-Ti-Mg vitreous particles with Fe oxides and Al-Si species and irregular unburned coal and ash particles (**Figure 4**). It is generally accepted that vitreous FA particles consist of a relatively pure Al-Si-Ca-K-Fe glass within on which mullite crystals form a network [67, 68]. Furthermore, other types of particles such as calcite (CaCO3

.2H<sup>2</sup>

During coal gasification, most of the mineral matter of the coal is transformed and melted into slag. As opposed to PCC, coal gasification produce very little FA (10–15%), which is

O) are formed.

), and gypsum (CaSO<sup>4</sup>

),

almost 100% using 250 g/L NaOH at an A/O phase ratio of 1:2.

SO<sup>4</sup> .

**4.2. Germanium from coal combustion and gasification fly ashes**

required using commercially available reagents.

18 Advanced Material and Device Applications with Germanium

reach the flue-gas desulfurization (FGD).

regarded as the main output stream of Ge.

lime (CaO), quartz (SiO<sup>2</sup>

), a SSX system was also used [57]. The process consisted of 30% (v/v) D2EHPA and 15% (v/v) TBP. The extraction efficiency was 94.3% in two stages, and the strip efficiency was

H2 SO<sup>4</sup>

0.5 M NaOH and 1.0 M Na<sup>2</sup>

**Figure 4.** Scanning electron microscope photomicrographs of (a) FAs from coal and petroleum coke combustion and (b) FAs from PCC [61].

characterized by a predominant alumina-silicate glass matrix and a wide variety of crystalline-reduced species, mainly sulfides, because of the low levels of O<sup>2</sup> during coal gasification. **Figure 5** shows the typical round morphology of IGCC FA particles [26].

Germanium production from coal FAs usually consists of two stages. The first step creates a concentrate and the second is the actual recovery. The first published studies on Ge recovery from coal FAs were those based on pyro-metallurgical practices [70] but at the present moment are not applied due to the high economic and environmental cost [1].

optimized Ge and Ga recovery from coal FA using different extractants in a wide range of extraction conditions. High extraction yields of Ge (up to 90%) and Ga (up to 82%) were obtained using weak oxalic acid and sulfuric acid solution, respectively, within 1–2 h extrac-

Germanium: Current and Novel Recovery Processes http://dx.doi.org/10.5772/intechopen.77997 21

Recently, the recovery of Ge from coal FA using vacuum reduction metallurgical process was investigated [38]. Its principle is that the saturation vapor pressure of metal under the vacuum condition is lower than normal pressure to separate metals. These authors achieved a 94.6% recovery of Ge from the coal FA at a temperature of 900°C, 10 Pa, and heating time of 40 min. These studies confirm that coal FA can be explored as an alternative source of Ge and other valuable elements and minerals. The recovery of Ge from coal FA is suitable, reducing disposal costs of coal FA. However, further research is needed to develop adequate results in

Flue dust from Cu smelting has also been suggested as a potential source of Ge, since relatively high contents of this element may be present in Cu-sulfide ores [27]. Copper is extracted from the ore through hydrometallurgical and/or pyrometallurgical processes. The selection of the process is determined by the Cu minerals bound to ores, being Cu sulfides predominantly treated by pyrometallurgical process and Cu oxides by hydrometallurgical process. However, the pyrometallurgical process is the most commonly employed technology for Cu [79].

The processing of Cu minerals, associated to sulfides by high temperatures, produces several residues [80]. Among the residues, dust generated from physical process, flue dust and slags from smelting process, and sludge from electrowinning process are those with potential for

Although the qualitative and quantitative phase determination of dust from Cu smelting depends on the compositional characteristics of the fed into smelting furnaces, temperature and oxidative conditions inside the furnace, and equipment, the recovery of rare and precious metals such as Ge from flue dusts has not been a widely studied subject in the literature. Most of the research on flue dust composition from Cu smelting have been focused on As, Zn, and Pb since the main Cu-ores present elevated amounts of As, Zn, Cd, and Pb which are

Font et al. (2011) evaluated for the first time the potential of Cu smelting flue dust (Cu-SFD) as a source of Ge and the possibilities to apply extraction and recovery methods similar to those patented for coal gasification FA [81]. These authors reported Ge concentrations ranging from 417 to 1375 ppm in flue dust samples with Ge extraction yields from 73 to 99%. In 2017, Chilean Cu-SFD was characterized and evaluated for the potential extraction of Li, Rb, and Ge with different chemical leaching agents [27]. The authors found high extraction yields for Ge, Li, and Rb using pure water as extractant at 25°C. Both studies suggest that Ge may occur in the form of highly soluble minerals and that Cu-SFD can be regarded as a promising source

tion period [78].

the Ge recovering.

terms of selective recovery and purification.

**4.3. Germanium recovery from copper smelting flue dust**

potentially hazardous to human health or the environment [27].

of elements with high added value such as Ge.

**Figure 5.** Scanning electron microscope photomicrographs of IGCC FAs [69].

In 1998, a hydrometallurgical procedure for extracting Ge from coal FAs based on the leaching of FA with H<sup>2</sup> SO<sup>4</sup> and NaOH followed by ion flotation separation of Ge was prompted. A recovery of 100 g of Ge per ton of coal FA was achieved between 70 and 200°C [71]. Other studies implemented the ion flotation method using a mixed HCl/HNO3 solution and cetylpyridiniumchloride as a surfactant [72]. The authors obtained 95–100% yield of Ge after 3–5 min of flotation. A research about tannin (poly-hydroxy polyphenols) precipitation to extract Ge was proposed in 2008 [73]. The tannins were capable of forming chelates with Ge ions, resulting in low grade of precipitated tannin-germanium complex. Currently, research is aimed at developing methods for increasing FA utilization focused on reducing the concentrations of heavy metals and at the same time obtaining higher added value products.

The occurrence of Ge as water-soluble species such as GeS<sup>2</sup> , GeS, and hexagonal-GeO<sup>2</sup> , in the FAs generated in an IGCC power plant, prompted the study of Ge recovery from coal FAs using pure water in an attempt to develop an extraction process of a low cost and environmentally able to culminate in a commercial Ge end-product [26]. Results revealed high recovery efficiency (up to 86%) at 90°C, indicating that the extraction temperature was the most important parameter in the process. These results led authors to conduct research toward the study of enrichment and precipitation methods for Ge recovery such as ion flotation, adsorption on activated carbon, and/or SX. Adsorption and SX were the methods that allowed achieving Ge-bearing solutions with 256 and 1623 mg/L, respectively [74, 75].

In 2006, a study evaluated the selectivity of the process developed by [26] for the recovery and purity of Ge by ion flotation tests on the leachates arising from the water extraction of Ge, using pyrogallol, catechol, hydroquinone, and resorb in complexing agents at a pH range of 4–7. Pyrogallol or catechol as complexing agents and dodecylamine as a surfactant showed the isolation of the Ge complex recovering 100% of Ge in 30 min [76].

A complex water leaching process consisting of Ge complexation with catechol followed by SX system, reaching an extraction yield of 95%, was published [77]. In 2014, same authors optimized Ge and Ga recovery from coal FA using different extractants in a wide range of extraction conditions. High extraction yields of Ge (up to 90%) and Ga (up to 82%) were obtained using weak oxalic acid and sulfuric acid solution, respectively, within 1–2 h extraction period [78].

Recently, the recovery of Ge from coal FA using vacuum reduction metallurgical process was investigated [38]. Its principle is that the saturation vapor pressure of metal under the vacuum condition is lower than normal pressure to separate metals. These authors achieved a 94.6% recovery of Ge from the coal FA at a temperature of 900°C, 10 Pa, and heating time of 40 min. These studies confirm that coal FA can be explored as an alternative source of Ge and other valuable elements and minerals. The recovery of Ge from coal FA is suitable, reducing disposal costs of coal FA. However, further research is needed to develop adequate results in terms of selective recovery and purification.
