FIELD OF THE INVENTION

The invention generally refers to a process for recovering metals, specifically germanium, gallium, vanadium and nickel, from fly ash generated in an Integrated Gasification Combined Cycle (IGCC) type thermal power plant.

BACKGROUND OF THE INVENTION

Steel mills and thermal power plants generally produce a relatively significant amount of fly ash containing variable concentrations of valuable metals, for example germanium (Ge), gallium (Ga), vanadium (V), nickel (Ni), etc., that justify the recovery thereof. Different processes are known for recovering said metals from said ash. As an illustrative example, US patent 4,244,735 can be mentioned, which discloses a process for recovering Ge from fly ash originating from a steel mill upon lixiviation with chlorides; US patent 4,915,730 discloses the recovery of Ga from fly ash by means of a process that comprises mixing said fly ash with sodium chloride, burning the mixture in the presence of oxygen, solubilizing with cyanide and the subsequent precipitation; or French patent FR 2,187,878 discloses the recovery of V from fly ash by means of lixiviation with an aqueous alkaline solution. Murase K. et al. [J. Alloys Compd., 264 (1-2), 151-156 (1998)] describe the recovery of V, Ni and Mg from fly ash from a bitumen-in-water emulsion by chlorination and chemical transport.

The use of sulphuric acid or caustic soda for leaching germanium from fly ash generated in power plants is disclosed by A.I. Zouboulis et al., " Possibility of Germanium Recovery from fly ash," Chimika Chronika, New Series, 18, 85-97 (1989).

Integrated Gasification Combined Cycle (IGCC) type thermal power plants produce electric energy by means of a technology differing from that of conventional thermal power plants, which consists of a process of gasification of a fuel at a high temperature to generate a synthetic gas that is subsequently burned in a combined cycle to produce electric energy. This process generates solid residues in the form of fly ash and vitreous slag that is insoluble in water. The chemical composition of the fly ash generated in an IGCC type power plant depends on the fuel used, generally, however, they are essentially composed of silica and aluminum oxide, with much smaller ratios of different metallic oxides, for example, of sodium, potassium, magnesium, calcium, manganese, germanium, gallium, vanadium, nickel, in addition to iron, lead, zinc sulfides and other metals.

The normal destination of the fly ash generated in an IGCC type power plant consists of its deposit in controlled dumps. This destination has several drawbacks since, on one hand, there is a risk that the metals present in the ash will be lixiviated and passed to the aquifers, thus generating a serious environmental problem, and on the other hand, as it is expected that the amount of this type of ash will gradually increase due to a progressive implantation of IGCC technology, a larger number of dumps or larger dumps are going to be required, provided with the suitable emissions sealing and control conditions. In order to get an approximate idea of the magnitude of the problem, it is enough to mention that approximately 16,000 tons of fly ash are generated annually in the ELCOGAS IGCC type power plant in Puertollano (Ciudad Real).

SUMMARY OF THE INVENTION

The invention faces the problem of developing an alternative treatment to dumping in dumps that is currently carried out with large amounts of fly ash generated as a residue in an IGCC type thermal power plant.

The solution provided by this invention is based on the recovery of Ge from said fly ash by means of lixiviation in water the Ge is thereby solubilized and a solid phase is separated containing other valuable metals, for example Ga, V, Ni, etc., that can be recovered by means of suitable treatments.

The solution proposed by the invention prevents dumping said residue with the contamination problems it involves, and, furthermore, permits recovering valuable metals and valorization of said residues.

BRIEF DESCRIPTION OF THE DRAWINGS

• Figure 1 shows the result of the mineralogical analysis by means of X-ray diffraction of two IGCC ash samples.
• Figure 2 shows 2 graphs representing the granulometric analysis results of 2 representative IGCC ash samples.
• Figure 3 shows 2 graphs representing the effect of temperature (25°C and 50°C) on Ge content lixiviation with water in an IGCC ash sample, changing the lixiviation time (or residence) and solid/liquid ratio.

DETAILED DESCRIPTION OF THE INVENTION

The invention refers to a process for recovering valuable metals, for example germanium, gallium, vanadium and/or nickel, from fly ash generated in an Integrated Gasification Combined Cycle (IGCC) type thermal power plant, heretofore IGCC ash. Said process comprises solubilizing Ge after subjecting said IGCC ash to lixiviation in an aqueous medium and, if desired, recovering one or more valuable metals present in the solid phase resulting after lixiviation, for example Ga, V or Ni, by means of suitable treatments.

I. IGCC ash

IGCC ash generated in fuel combustion in an IGCC type power plant constitutes the starting material for recovering valuable metals. In a particular embodiment, said fuel is constituted of a mixture of coal and petroleum coke, in different ratios between them.

The physical-chemical and mineralogical characterization of IGCC ash clearly shows that said IGCC ash has specific properties depending on the fuel used in the IGCC type power plant, in other words, on the ratio of coal and coke and on the relative composition thereof. In a particular embodiment, the fuel used in the IGCC type thermal power plant is a mixture of coal and petroleum coke (1:1), generating IGCC ash that has the typical composition included in Table 1 and Figure 1 of Example 1, which also describe the mineralogical characterization of typical IGCC ash.

As can be seen, said IGCC ash is essentially composed of silica and aluminum oxide, with much smaller ratios of different metal oxides, for example, of sodium, potassium, magnesium, calcium, manganese, germanium, gallium, vanadium, nickel, in addition to calcium, iron, lead, zinc sulfides and other metals.

II. Ge recovery

The invention provides a process for recovering Ge from IGCC ash, which comprises:

• a) lixiviating said IGCC ash with water to solubilize all or part of the Ge content in said IGCC ash;
• b) separating the solid and liquid phases existing in the solution or suspension resulting from step a) into a solid phase (S1) and into a liquid phase (L1) containing Ge; and
• c) recovering the Ge from said liquid phase (L1).

Generally, the state of the art clearly shows that in order to extract Ge from fly ash generated in steel mills or conventional thermal power plants, it is necessary to carry out complex distillations with different solvents due to the fact that germanium oxide (GeO₂), which normally appears in these materials, is a water insoluble clinorhombic form. Surprisingly, however, in IGCC ash (obtained in a gasification process in determined pressure and temperature conditions), GeO₂ seems to be a water soluble, hexagonal shape, which permits the extraction thereof from the IGCC ash by means of using water.

Lixiviation can be carried out in varying conditions within broad intervals. In a particular embodiment, the lixiviation of the IGCC ash can be carried out at a temperature comprised between 0°C and 400°C, for a time period comprised between 6 and 24 hours, with a liquid/solid ratio comprised between 1 and 10 (1 to 101 of the extracting agent per kg of IGCC ash to be lixiviated), depending on the Ge content of the IGCC ash and on the concentration thereof that is to be reached in the lixiviate [liquid phase (L1)]. Lixiviation can be carried out in any suitable container, for example, a closed or optionally open reactor provided with stirring means. By operating in the aforementioned conditions, it is possible to extract an amount of Ge that is equal to or greater than 85% of the Ge initially present in the treated IGCC ash. After lixiviating the IGCC ash, a suspension is obtained containing a solid phase and a liquid phase that can be separated into a solid phase (S1), containing valuable metals such as Ga, V or Ni, and into a liquid phase (L1) containing Ge together with other water soluble metals, by means of conventional methods and techniques, for example by means of using a filter press or centrifugation, and optionally washing the solid phase S1 to increase the extraction yield.

The recovery of the Ge content in the liquid phase (L1) can be carried out by conventional methods, for example by means of using ion exchange resins [Sahoo S.K., Bull. Chem. Soc. Japan, 68(8), 2484-2487 (1991)], extraction with solvents [US 4,915,919; US 4,432,952; Cote G & Bauer D., Hydrometallurgy, 5, 149-160 (1980); Li X. Et al., Source Xiandai Huagong, 20(8), 34-36, (2000)], or by means of precipitation with sulfides [Kirk-Othmer Encyclopedia of Chemical Technology, 12, 545-547; Ullmann's Encyclopedia of Industrial Chemistry, 12, 354-358].

In a particular embodiment, Ge recovery comprises concentrating the Ge solubilized in the liquid phase (L1) into a germanate form by means of using an ion exchange resin of the anionic type in order to extract Ge, optionally together with other elements that may be forming anions, for example arsenic (As), which could be found in the arsenate form and which would solubilize together with Ge during lixiviation of the IGCC ash. Ge can be separated as from the liquid phase resulting from the regeneration of the anion exchange resin by means of distillation thereof in the form of tetrachloride after adding chlorine to said liquid phase. Adding chlorine permits oxidizing the possible As present in the liquid phase resulting from the regeneration of the anion exchange resin to As⁵⁺. The germanium tetrachloride obtained in the liquid phase is a commercial product. Alternatively, the germanium tetrachloride could be hydrolyzed in an aqueous medium to obtain germanic acid and from it, by means of drying, obtain germanium oxide.

In another particular embodiment, the recovery of Ge present in the liquid phase (L1) is carried out by means of selective extraction of Ge with a suitable solvent.

In another particular embodiment, the recovery of Ge present in the liquid phase (L1) is carried out by means of precipitation in the form of sulfide after adding, for example, sodium sulfide.

III. Recovery of other valuable metals (Ga, V, Ni)

The invention also contemplates the possibility to recover valuable metals present in the solid phase S1. Therefore, in a particular embodiment, the invention provides a process for obtaining Ga and/or V as from the solid phase (S1) resulting from the lixiviation of the IGCC ash with water comprising:

• a) lixiviating said solid phase (S1) with an extracting agent comprising an aqueous base solution to solubilize all or part of the Ga and/or V content in said solid phase (S1);
• b) separating the solid and liquid phases existing in the suspension resulting from step a) into a solid phase (S2) and into a liquid phase (L2) containing Ga and/or V; and
• c) recovering the Ga and/or V from said liquid phase (L2).

The extracting agent is an aqueous base solution, for example an aqueous sodium or potassium hydroxide solution, with a concentration comprised between 0.1 and 5 M, preferably between 2 and 3 M. Lixiviation is carried out at a basic pH, preferably comprised between 12 and 14; in heat, at a temperature comprised between 50°C and 250°C, preferably between 150°C and 200°C; and with a solid/liquid ratio comprised between 1/2 and 1/20, preferably 1/5 or 1/10. After lixiviating the solid phase (S1) with an alkaline solution, a suspension is obtained by means of conventional methods and techniques that contains a solid phase and a liquid phase that can be separated into a solid phase (S2) and into a liquid phase (L2) containing Ga and /or V, together with other metals that are soluble in an alkaline medium.

The recovery of Ga and/or V present in the liquid phase (L2) can be performed by means of conventional methods, for example by means of extraction with the suitable solvents, as from an alkaline solution [Gutiérrez B. Et al., Waste Management Research, 15(4), 371-381 (1997)] or from an acid solution [Gutiérrez B. Et al., Solvent Extraction and Ion Exchange, 11(5), 769-782 (1993); Gutiérrez B. Et al., J. Chem. Tech. Biotechnol., 61, 241-245 (1994)] or by means of precipitation by neutralizing the liquid phase (L2) [Ullmann's Encyclopedia of Industrial Chemistry, 12, 164-166].

In another particular embodiment, the invention provides a process for obtaining Ni as from the solid phase (S1) resulting from the lixiviation of the IGCC ash with water, comprising:

• a) lixiviating said solid phase (S1) with an extracting agent comprising an aqueous acid solution to solubilize all or part of the Ni content in said solid phase (S1);
• b) separating the solid and liquid phases existing in the suspension resulting from step a) into a solid phase (S3) and into a liquid phase (L3) containing Ni; and
• c) recovering the Ni from said liquid phase (L2).

The extracting agent is an aqueous acid solution, such as an aqueous solution of an organic acid, for example acetic acid, etc., or of an inorganic acid, for example nitric, hydrochloric, hydrofluoric, perchloric acid, etc., with a concentration comprised between 0.1 and 5 M, preferably between 2 and 3 M. Lixiviation is carried out in heat at a temperature comprised between 50°C and 250°C. After lixiviating the solid phase (S1) with an acid solution, a suspension is obtained by means of conventional methods and techniques that contains a solid phase and a liquid phase that can be separated into a solid phase (S3) and into a liquid phase (L3) containing Ni, together with other metals that are soluble in an acid medium. The recovery of Ni present in the liquid phase (L3) can be carried out by conventional methods, for example by means of selective extraction with a suitable solvent, or by means of precipitation by neutralizing the liquid phase (L3) [Khanra S. et al., Water, Air and Soil Pollution, 107, 251-275 (1998); Tsai S-L & Tsai M-S, Kuangye (Taipei), 42(3), 35-46 (1998)].

As can be seen, the invention provides a process for upgrading IGCC ash, which comprises recovering Ge and possibly other valuable metals such as Ga, V and/or Ni. Furthermore, other valuable metals could be recovered that are present in the IGCC ash in concentrations that justify the recovery thereof, for example lead, zinc, antimony, etc.

A feature of the process of recovering valuable metals provided by this invention is based on its modular character, such that it can be applied for recovering only those valuable metals Ge and (Ga, V and/or Ni) present in a concentration justifying the recovery thereof, or alternatively, it can be applied completely so as to carry out an integral treatment of said IGCC ash and recover said valuable metals (Ge, Ga, V and/or Ni).

The following examples serve to illustrate the invention and must not be considered limiting of the scope thereof.

EXAMPLE 1

Obtaining IGCC ash

The gasification of fuels composed of coal and petroleum coke in an IGCC type thermal power plant gave way to IGCC ash. The main features of said IGCC ash are included below.

Chemical composition

The representative and typical chemical composition of generated IGCC ash is included in Table 1.

Representative and typical composition of IGCC ash expressed in % for larger oxides and elements and in mg/kg for trace elements

Representative

Typical

%

SiO₂

50.0-65.0

62.6

Al₂O₃

15.0-25.0

17.8

Ca

2.0-5.0

2.3

Fe

2.0-5.0

3.4

K

2.5-4.0

3.0

S

0.8-1.8

1.1

Pb

0.2-0.8

0.4

Zn

0.2-0.8

0.4

Mg

0.3-0.7

0.5

TiO₂

0.3-0.7

0.6

P₂O₅

0.4-0.7

0.4

V

0.3-0.6

0.5

Na

0.3-0.5

0.4

Ni

0.1-0.3

0.2

mg/kg

As

450-850

564

Ga

100-350

149

Ge

100-350

194

Mn

200-600

472

Mineralogical composition

The mineralogical characterization of IGCC ash was carried out buy means of X ray diffraction. The results of the mineralogical characterization of a typical IGCC ash sample are shown in Figure 1, where the presence of the phases corresponding to galena (PbS), sphalerite (ZnS), wurtzite-2H (ZnS) and pyrrhotite (Fe_x-1S_x) can be seen. Furthermore, the acid pH and the high correlation between acidity and calcium content imply the presence of oldhamite (CaS).

Granulometric analysis

Figure 2 shows the results of the granulometric analysis of 2 representative IGCC ash samples, indicating that the ash has a very fine granulometry. The typical samples have a median granulometry of less than 3 microns.

EXAMPLE 2

Ge recovery [Lixiviation with water]

Different representative IGCC ash samples were subjected to lixiviation with water at 25°C during different lixiviation times (6, 12 and 24 hours) and different ash/liquid ratios (1/2, 1/5 and 1/10), in a closed reactor provided with stirring mechanisms. The mean yield obtained, expressed as an extraction percentage of the Ge content originally present in the ash, is included in Table 2.

Extraction yields (%) of Ge from a typical IGCC ash with water at 25°C at different times and ash/water ratios (S/L in kg/l)

S/L

Ge extraction %

6 h

12 h

24 h

1/2

54

57

60

1/5

59

70

75

1/10

75

78

84

To analyze the influence of the temperature on the Ge lixiviation with water (extracting agent), several lixiviation trials were carried out, varying the temperature (from 25°C to 50°C), lixiviation time (6, 12 and 24 hours) and the S/L ratio (1/2, 1/5 and 1/10). The obtained results are shown in Figure 3, where it is clearly shown that the Ge extraction yield increases with temperature and is practically 100% after 6 hours, at 50°C using water as an extracting agent and an ash/water ratio of 1 kg per 10 liters. Very high yields (>95%) were also obtained with lower doses of water (1 kg per 5 liters).