3186
Ind. Eng. Chem. Res. 2008, 47, 3186-3191
Hydrometallurgical Recovery of Germanium from Coal Gasification Fly Ash. Solvent Extraction Method Fa´ tima Arroyo and Constantino Ferna´ ndez-Pereira* E.S. Ingenieros, Dpto. Ingenierı´a Quı´mica y Ambiental, UniVersidad de SeVilla, Camino de los Descubrimientos S/N, E-41092, SeVilla, Spain
This article is concerned with a simple hydrometallurgical method for the selective recovery of germanium from fly ash (FA) generated in an integrated gasification with combined cycle (IGCC) process. The method is based on the leaching of FA with water and a subsequent concentration and selective separation of germanium by a solvent method. Regarding the leaching step, the different operational conditions studied were liquid/ solid (L/S) ratio and time of contact. The solvent extraction method was based on germanium complexation with catechol (CAT) in an aqueous solution followed by the extraction of the Ge-CAT complex with an extracting organic reagent diluted in an organic solvent. The main factors examined during the extraction tests were aqueous phase/organic phase (AP/OP) volumetric ratio, aqueous phase pH, amounts of reagents, and time of contact. Germanium extraction yields were higher than 90%. Alkaline and acid stripping of organic extracts were studied obtaining the best results with 1M NaOH (85%). A high-purity germanium solution was obtained. Experimental data presented in this work show that the extraction of germanium by the solvent method designed can be selective toward germanium, and this element can be effectively separated from arsenic, molybdenum, nickel, antimony, vanadium, and zinc. 1. Introduction Germanium is very widely distributed in nature. The amount of germanium in the earth’s crust is 1.5 ppm on average.1 Most of the germanium in the lithosphere is in association with zinc and its minerals. The zinc ores have large and recoverable quantities of germanium, so nowadays germanium is mainly obtained as a byproduct of zinc ore processing. Besides, around 30% germanium used in the world is obtained from recycled materials, particularly electronical devices.2 Sometimes germanium is concentrated in certain coals,3 basically joined to the organic matter. If coal is burned under proper conditions, germanium is concentrated in combustion4-7 and gasification8 byproducts, especially in fly ash, that can reachc a germanium content 10 times higher than the germanium content in coal. The uses for germanium have been largely reviewed.2 Electrical and optical properties of germanium promote the use of this element in novel and high technological industrial applications that has increased its price in the last years. In 2006, germanium metal and germanium dioxide reported prices were 720 and 690 $/kg, respectively.9 Germanium is used in the manufacture of advanced electronical and optical devices, fiber optics, as a polymerization catalyst in PET (polyethylene terephthalate) plastics, in the field of photovoltaic cells, and in the manufacture of thermal solar cells.2,10 Although germanium is currently recovered as a byproduct of zinc industries, another interesting source of germanium comes from the leaching of coal fly ash.8,11-14 Many studies have focused on the recovery of germanium from FA using several methods. The first step of most of these processes is the leaching of fly ash, followed by a process to separate germanium from other elements contained in leachates, such as arsenic, molybdenum, nickel, antimony, vanadium, or zinc. Conventional processes are precipitation with tannin,8 distillation of GeCl4,12,13 and solvent extraction (SX).15 SX appears to be * To whom correspondence should be addressed. E-mail: pereira@ esi.us.es.
the most attractive procedure because of the easy scale-up and operation control. The utilization of carbon tetrachloride to recover germanium from HCl solutions was described in 1950.16 The only reported interference was arsenic (III), but the method presented the problem of the high volatility of the tetrachloride of germanium, which could provoke losses. The germanium can be recovered for this route using other solvents, such as chloroform, benzene, ether, tributhylphosphate, or methyl isobuthyl ketone, but these solvents extract other metals besides the germanium, so their utilization is not widespread. Other authors have proposed different procedures to separate germanium from aqueous solutions using complexation with solvent extraction. The complexants often are organic substances, usually with oxygenated groups, and sometimes with oxygen and nitrogen groups, such as hydroxyquinoline and derivatives17,18 and hydroxyoximes.19-23 Other procedures are based on the usage of amines (secondary or tertiary) together with certain polyhydroxy complexants of germanium.15 The Ge-catechol (CAT) complex was described first by Bevillard,24 and its structure was confirmed later by other authors.25,26 The structure (Ge/CAT molar ratio 1:3) is shown in Figure 1. The formation of the complex at pH > 4 is shown27 in eq 1:
Ge(OH)4o + 3C6H4(OH)2 T Ge(C6H4O2)32- + 2H+ + 4H2O (1) In 1967, Andrianov and Avlasovich described the extraction of germanium and catechol complex with trioctylamine (TOA) diluted in kerosene at room temperature.28 The same authors29 also studied the extraction of germanium with trioctylamine in kerosene from sulfuric solutions that contained catechol and/or sodium oxalate. The equilibrium extraction of Ge-CAT complex by TOA in kerosene is given by the following scheme reactions:
10.1021/ie7016948 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/22/2008
Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3187
(C8H17)3N(org) + H+(ac) T (C8H17)3NH+(org)
(2)
2(C8H17)3NH+(org) + Ge(C6H4O2)32-(ac) T ((C8H17)3NH)2Ge(C6H4O2)3(org) (3) Therefore, the overall process (eq 1-3) can be described as follows:
2(C8H17)3N(org) + Ge(OH)4(ac)o + 3(C6H6O2)(ac) T ((C8H17)3NH)2Ge(C6H4O2)3(org) + 4H2O(ac) (4) The equilibrium constant of the above reaction can be expressed:
K)
[(C8H17)3Ge(C6H4O2)3]org [(C8H17)3N]org2‚[Ge(OH)4o]ac‚[C6H6O2]ac3
Figure 1. Germanium-catechol structure. Table 1. Chemical Analysis of IGCC Fly Ash major components (% w/w)
(5)
Once the germanium is in the organic phase, two stripping possibilities are apparently possible because the quelate is not stable in strong acidic or alkaline solutions. Acid stripping reaction can be described as,
((C8H17)3NH)2Ge(C6H4O2)3(org) T 2(C8H17)3NH+(org) + Ge4+(ac) + 3(C6H4O2)2-(ac) (6) On the other hand, when an organic phase containing the Ge-CAT-TOA complex is put in contact with an alkaline aqueous solution, the [TOA - H]+ bond breaks (pK ) 3,5),30 and the overall stripping reaction may be described as,
((C8H17)3NH)2Ge(C6H4O2)3(org) + 2OH-(ac) T 2(C8H17)3N(org) + Ge(C6H4O2)32-(ac) + 2H2O(ac) (7) Studies on the SX of Ge-CAT complexes have been limited to solutions generated in the laboratory, without any practical application. The present work focuses on the development of a process for germanium recovery from fly ash produced in an Integrated Gasification Combined Cycle (IGCC) power plant. The 335 MW IGCC power plant of ELCOGAS in Puertollano (Spain) gasifies a 50:50 blend of a local metal-rich bituminous coal31 and petroleum coke. The annual production of Puertollano IGCC plant fly ash is approximately 12 000 tonnes. 2. Materials and Methods 2.1. Materials. All of the reagents used in this study were analytical grade reagents. 2.2. Characterization of Fly Ash. Fly ash was produced in Puertollano IGCC power plant under the following conditions: 50:50 coal/pet coke blend and 2.5% limestone addition as fluxing agent. An exhaustive characterization of IGCC fly ash has been carried out, and the results have been published elsewhere.32 Main chemical characteristics of the fly ash used in the present study (extracted from other publications32) are shown in Table 1. 2.3. Leaching Procedure. The leaching procedure was performed in 1000 mL agitated flasks at different temperatures using an AGIMATIC-N shaker (agitation speed 700-900 rpm). The liquid/solid ratio and contact time were varied throughout the study. The leaching agent was distilled water, and the operating conditions were performed according to the results of Font.8,32 After the required contact time, the flasks were left to settle overnight; the leachates were
SiO2 Al2O3 CaO
59.3 20.6 3.2
Fe2O3 K2O SO3
As Ba Cd Co Cr
955 433 24 53 155
Cu Ga Ge Mo Ni
4.2 3.5 2.4
MgO Na2O TiO2
0.7 0.5 0.6
minor components (mg/kg) 392 Pb 4796 320 Sb 381 420 Se 19 135 Ge 420 2296 Sn 67
P2O5 MnO
0.5 0.04
V Zn V
6256 7230 6256
LOI
4.0
then filtered and the filtrates were analyzed for element contents by inductively coupled plasma-mass spectrometry (ICPMS), model VG PLASMAQUAD PQ2, with detection limits typically less than 0.2 ppb (except for zinc with 0.5 ppb) for nondiluted samples and interference effects usually minimal. 2.4. Solvent Extraction Procedure. Solvent extraction was performed by mixing 1000 mL of the aqueous phase (AP) from a fly ash leachate with the required volumes of the organic phase (OP) at room temperature. The volumes of both phases were adjusted to different AP/OP ratios. The aqueous phase was prepared adding catechol to the germanium-bearing leachate solution and H2SO4 or NaOH solutions to adjust the pH. The organic phase was prepared by dilution of the extractant trioctylamine in a diluent (kerosene). The extractant concentration was adjusted to the selected values. The separation funnels were filled with the mixtures and were shaken for 5 min; the shaking intensity was set to provide a uniform dispersion of both phases. After that, both phases were left to settle until a good separation was achieved, then raffinates were stored for measuring by ICP-MS. Finally, germanium-loaded organic phases were mixed with the required volumes of stripping solutions (NaOH, HCl, and H2SO4), and the separation process was repeated as in the extraction stage to obtain a residual organic phase and a germanium-loaded final solution. To evaluate the extraction and stripping processes the following terms have been used: The distribution ratio (D) of metal (M), which is defined by:
DM )
∑ [M]org ∑
(8) [M]aq
The distribution ratio is related to the extraction yield and the relation between D and the extraction yield of a metal (M) is given by,
EM(%) )
100DM DM + AP/OP
(9)
3188
Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008
Figure 2. Leaching of fly ash using distilled water for different leaching times, temperatures, and L/S ratios.
where [M]org and [M]aq are the equilibrium germanium concentrations in the organic and aqueous phases, respectively. [M]aq was determined by ICP-MS and [M]org was calculated by the difference between the initial concentration of metal in the aqueous phase and the equilibrium concentration in the aqueous phase. The distribution ratio, also referred to as the partition coefficient, is often expressed as a logarithm. In the stripping stage, an analogous distribution factor (S) has been used,
SM )
∑ [M]str ∑
(10) [M]org
where [M]org and [M]str are the equilibrium germanium concentrations in the organic and stripping phases, respectively. [M]str was determined by ICP-MS and [M]org was obtained by difference as mentioned above. The separation factor (R) is a relative distribution ratio. It is a measure of an extracting system to separate two solutes in the extraction phase, i.e., two metals (M and Me).
R)
DM DMe
(11)
3. Results and Discussion 3.1. Leaching Experiments. Several leaching treatments were carried out with different L/S ratios (3, 4, and 5), temperatures (room and 50 °C), and four different leaching times (3, 6, 9, and 24 h). Germanium extraction yields can be seen in Figure 2. When the relation L/S increased, the extraction yield increased too, reaching EGe(%) values over 80% for L/S ) 5. In addition, it can be verified that the temperature favored the germanium solubility in water. Regarding the results at different times, one can see that leaching time increased the performance of extraction, but the improvement between 9 and 24 h was not significant. The presence of several metals in the leachates could complicate the germanium separation in the subsequent solvent extraction method. For this reason it was important to study the simultaneous leaching of some of those metals, especially arsenic, gallium, nickel, antimony, and vanadium. In relation to the interferences (Table 2), the element showing a higher extraction yield was antimony, followed by arsenic and nickel. Gallium and vanadium practically were not extracted. With regard to the influence of the L/S ratio, the trend was the same observed in the case of germanium: the higher the L/S ratio, the higher was the extraction yield.
Figure 3. Effect of CAT amount in germanium extraction from aqueous acid solutions (TOA/Ge ) 6 molar ratio). Table 2. Composition of Aqueous Leachates (mg/L) at Room Temperature and Different L/S L/S
Ge
Ga
As
Sb
V
Ni
3 4 5
78.9 76.7 71.0
0.0 0.0 0.0
47.6 45.2 46.8
69.7 74.6 80.8
0.6 0.5 0.5
65.4 58.8 54.4
3.2. Solvent Extraction Method. There are different parameters that must be fully controlled to achieve good performance, either during the extraction or the stripping operations. In this study, the pH of the aqueous phase, composition of aqueous and organic phases, aqueous-organic phase ratio, contact time, stripping solutions, and reutilization of the organic phase were studied. Effect of Aqueous Phase Acidity. The extraction of germanium was carried out from different media in the pH range of 1-10 in the presence of catechol, keeping constant the TOA/ Ge molar ratio at 5. pH adjustments were performed with 1M NaOH and 2M H2SO4 solutions. Quantitative extraction of germanium was observed in the pH range of 1.97-3.02. This agreed with the pH interval of TOA protonation in kerosene30 and with the pH interval for germanium-catechol complex existence.25,27 The distribution coefficients obtained for germanium and other elements are shown in Table 3. Also, the separation factors of the main interferences of the germanium: arsenic, molybdenum, nickel, antimony, vanadium, and zinc have been calculated because they were extracted from leachates by factors of distribution of the same order as that of the germanium (though lower in all of the cases). In this point it has to be noticed that the molybdenum concentration in the leachate was 0.3 mg/L, so it cannot be considered a very important interference in spite of DMo values. Separation factors from germanium increased in the cases of nickel and arsenic when the pH of the AP was in the range of 2-3, showing a maximum for pH 2.66. The minimal values of the separation factor (lower values than 0.00001) were reached with the following conditions: AP/OP ) 4 and the pH of the FA is in the range of 2-3. Extraction as a Function of Catechol Concentration. Slightly acid aqueous solutions of catechol of varying concentrations (CAT/Ge molar ratio ) 3-30) were employed. Two different solutions were studied: standard germanium solutions and aqueous leachates from IGCC fly ash (Figure 3). For germanium standards, it was found that a CAT/Ge molar ratio of 10 was sufficient for the quantitative extraction of germanium (>90%) at pH 2.5, but a CAT/Ge molar ratio of 15 was used to ensure complete extraction of the germanium (>98%). For leachates, the CAT/Ge molar ratio required to reach a germanium extraction yield higher than 98% at pH 2.15 was 30. Effect of Foreign Ions. The need to add to leachates a catechol amount double than that added to germanium standard
Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3189 Table 3. Effect of pH on the Extraction of Metals and Selectivity of the Process pH
AP/OP
DGe
DAs
DMo
DNi
DSb
DZn
RAs/Ge
RMo/Ge
RNi/Ge
RSb/Ge
RZn/Ge
3.02 3.02 2.66 2.66 1.97 1.97
2 4 2 4 2 4
46.66 131.14 38.32 109.31 24.25 56.70
0.03
3.57 8.04 4.02 8.04 4.02 8.04
0.03 0.19 0.01 0.01 0.13
1.39 22.09 3.23 11.13 10.04 5.16
2.20
0.00104 0.00001 0.00160 0.00001 0.00827 0.00400
0.00765 0.00613 0.10491 0.00736 0.16577 0.14180
0.00116 0.00001 0.00038 0.00001 0.00674 0.00300
0.05100 0.04000 0.13463 0.10200 0.51125 0.36900
0.08056 0.04500 0.07485
0.04 0.01 0.16 0.24
1.80 7.21 1.57 5.84
0.08002
Table 4. Effect of Contact Time on the Extraction of Metals and Selectivity of the Process from Acid Aqueous Phase (CAT/Ge Molar Ratio: 18, pH ) 3 and TOA/Ge Molar Ratio: 6) contact time (min)
DGe
DAs
DMo
DNi
DSb
DZn
RAs/Ge
RMo/Ge
RNi/Ge
RSb/Ge
RZn/Ge
1 2 3 4 5
87.56 150.50 150.50 150.50 150.50
0.94 0.51 0.68 0.43 0.79
0.00 0.00 0.00 0.00 0.00
2.93 2.42 2.57 2.32 2.69
1.38 1.12 1.36 1.32 1.83
0.48 0.13 0.26 0.08 0.26
0.0108 0.0034 0.0045 0.0028 0.0053
0.0000 0.0000 0.0000 0.0000 0.0000
0.0309 0.0223 0.0237 0.0220 0.0242
0.0158 0.0074 0.0090 0.0087 0.0122
0.0054 0.0009 0.0017 0.0005 0.0017
Table 5. Effect of TOA/Ge Molar Ratio on the Extraction of Metals and Selectivity of the Process from Acid Aqueous Phase (CAT/Ge Molar Ratio: 15, pH ) 3) TOA/Ge
DGe
DAs
DNi
DSb
RAs/Ge
RNi/Ge
RSb/Ge
2 3 4 5
30.5 90.4 124.6 114.1
0.2 0.2 0.2 1.1
0.3 0.5 0.5 1.0
1.9 1.1 0.7 1.7
0.00656 0.00221 0.00161 0.00964
0.00984 0.00553 0.00401 0.00876
0.0623 0.01217 0.00562 0.0149
solutions suggested a competition between germanium and other metals for the complexant reagent. An excess of CAT was necessary for a suitable formation of the complex Ge-CAT (at least 3 times the stoichiometric amount). Effect of Contact Time. The solutions were shaken for varying periods of time, ranging from 1 to 5 min. The study showed that within the first minute, a quantitative extraction of germanium was achieved, so it can be assured that extraction process is very quick (Table 4). As a general rule, a 3 min equilibration time was recommended to ensure the complete extraction of the metal ions. Extraction as a Function of TOA Concentration. Different amounts of this reagent were tested to displace the extraction equilibrium. The germanium extraction was very high in every case (>85%), so it was clear that the influence of this factor on germanium extraction was not very important. Distribution coefficients (Table 5) were much higher in the case of germanium than in other analyzed metals. This confirmed the selectivity of the method. An increase in TOA/Ge ratio increased DGe, but in the case of the main interferences (arsenic, nickel, and antimony), distribution coefficients practically did not change in the whole range studied. The stoichiometric amount of TOA (TOA/Ge molar ) 2) was not enough to obtain a quantitative extraction of germanium from the leachate. A minimum TOA/Ge ratio of 5 was necessary to guarantee the extraction. From the point of view of interferences, a TOA excess permitted a better separation of the germanium (minor value of separation factor, R). Effect of Stripping Agent. Various stripping agents, such as HCl: 1, 3, 6M; H2SO4: 1, 3, 6M; and NaOH: 0.5, 1, 2M were used for the recovery of germanium from the organic extract (Table 6). It was found that recovery of germanium from the organic phase was incomplete with sulfuric acid (43-63%). However, hydrochloric acid (72-80%) and sodium hydroxide (62 - 88%) were more effective stripping agents for the quantitative recovery of germanium from the organic phase. Optimum stripping yields were achieved for 0.5-1 M NaOH solutions (79-88%).
Table 6. Influence of Stripping Agent and the Germanium Stripping from Loaded Organic Phase NaOH 0.5 N
1N
HCl 2N
1M
3M
H2SO4 6M
1M
3M
6M
S (%) 78.59 87.53 62.30 73.8 80.0 73.03 61.5 63 43.5 SGe 36.71 70.19 16.53 28.17 40.00 27.08 15.97 17.03 7.70
The results did not show clearly the effect of acidity on the stripping process. Regarding the interferences, it was shown that the compositions of the three solutions were almost free of metal interferences, except for antimony and aluminum, iron, arsenic, or nickel to a lesser degree. Alkaline stripping was considered the most suitable stripping for this extraction process. Concentration Factor in Extraction (AP/OP Volumetric Ratio). One of the most important aims in the study of the extraction with solvents was obtaining a final solution with a high germanium concentration. One possibility was the utilization of a concentration factor greater than 1. Four factors have been studied (5, 10, 15, and 20). Extraction was feasible using a concentration factor between 5 and 20, with a high germanium extraction (92-95%). Performances of the other metals did not show a strong dependence on this factor. Regarding the interferences, a concentration factor of 5 offered higher values of R, except for vanadium. Differences between the results obtained at AP/OP ) 5 and those obtained at AP/OP ) 10 were very small in most elements. Another aspect to be considered to choose the optimum AP/ OP ratio are: (1) germanium extraction remained constant, or it decreased slightly, with the concentration factor, but the germanium distribution coefficient increased, (2) the extraction process was more selective for FA/FO > 5 but the differences were not very important; (3) phases separation by gravity was easy for AP/OP ) 5; and (4) the higher the AP/OP, the lower is the reagent consumption. Taking into account the considerations above, an AP/OP ratio between 5 and 10 was recommended for the extraction. Concentration Factor in the Stripping Process (OP/SP Volumetric Ratio). Another possibility to obtain a final solution with a high germanium concentration was the utilization of a concentration factor in stripping greater than 1. Four factors were studied (5, 10, 15, and 20) using a TOA/Ge molar ratio of 6 and 2 M NaOH solution in all the cases. It is interesting to notice that the germanium recovery from germanium-loaded organic phases decreases with the volumetric organic/stripping phase ratio (OP/SP) (OP/SP: germanium recovery, 5:90.1%; 10:87.7%; 15:66.4%; 20:31.8%) probably
3190
Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008
Table 7. Compositions of the Germanium-Loaded Final Solutions Composition (mg/L) stripping solution
Al
As
B
Ca
Fe
Ge
K
Mg
Ni
P
Sb
Si
V
Zn
1M NaOH 1M H2SO4 1M HCl
