Ind. Eng. Chem. Res. 2009, 48, 3573–3579
3573
Hydrometallurgical Recovery of Germanium from Coal Gasification Fly Ash: Pilot Plant Scale Evaluation Fa´tima Arroyo,† Constantino Ferna´ndez-Pereira,*,† Joaquı´n Olivares,† and Pilar Coca‡ E.S. Ingenieros, Departamento de Ingenierı´a Quı´mica y Ambiental, UniVersidad de SeVilla, Camino de los Descubrimientos S/N, E-41092, SeVilla, Spain, and ELCOGAS, Carretera de Calzada, km 27, 13500, Puertollano, Ciudad Real, Spain
In this article, a hydrometallurgical method for the selective recovery of germanium from fly ash (FA) has been tested at pilot plant scale. The pilot plant flowsheet comprised a first stage of water leaching of FA, and a subsequent selective recovery of the germanium from the leachate by solvent extraction method. The solvent extraction method was based on Ge complexation with catechol in an aqueous solution followed by the extraction of the Ge-catechol complex (Ge(C6H4O2)32-) with an extracting organic reagent (trioctylamine) diluted in an organic solvent (kerosene), the relevant reaction probably being 2(C8H17)3N(org) + Ge(OH)o4(aq) + 3(C6H6O2)(aq) T ((C8H17)3NH)2Ge(C6H4O2)3(org) + 4H2O(aq) followed by the subsequent stripping of the organic extract. The process has been tested on a FA generated in an integrated gasification with combined cycle (IGCC) process. The paper describes the designed 5 kg/h pilot plant and the tests performed on it. Under the operational conditions tested, approximately 50% of germanium could be recovered from FA after a water extraction at room temperature. Regarding the solvent extraction method, the best operational conditions for obtaining a concentrated germanium-bearing solution practically free of impurities were as follows: extraction time equal to 20 min; aqueous phase/organic phase volumetric ratio equal to 5; stripping with 1 M NaOH, stripping time equal to 30 min, and stripping phase/organic phase volumetric ratio equal to 5. 95% of germanium were recovered from water leachates using those conditions. 1. Introduction Worldwide, about 30% of the total germanium consumed comes from recycled materials, particularly metals used in the manufacture of electronic and optical devices. Regarding the “new” germanium production,1 the main sources are the zinc and copper industries, from which germanium is obtained as a byproduct. Nevertheless, due to the uses for germanium in novel and highly technological industrial applications, germanium metal and oxide have increased in price ($1300/kg of germanium metal in December 2007),2 and other sources have started to emerge, mainly combustion3-6 and gasification7 of coal fly ashes. When the coal is burned/gasified under proper conditions, the fly ash can reach germanium contents 10 times higher than the germanium content in the original coal. Conventional processes for Ge separation/recovery are precipitation with tannin,8 distillation of GeCl4,9-11 flotation,12 adsorption onto activated carbon,13 and solvent extraction (SX).14-16 The first step of most of the methods for recovering germanium from coal fly ash (FA) is the leaching of fly ash, followed by a process to separate Ge from other elements present in leachate, such as As, Mo, Ni, Sb, V, or Zn. In a previous paper, we described a SX method for the recovery of germanium from an integrated gasification with combined cycle (IGCC) fly ash based on the germanium-catechol (C6O2H6) chelate and the subsequent extraction of the complex using tri-n-octylamine ((C8H17)3N).17,18 Two stripping possibilities are possible because the chelate is unstable both in strong acidic solutions and in alkaline solutions. The equilibrium extraction and the acidic and basic strippings are respectively given by the following equations: * To whom correspondence should be addressed. Phone: +34 954481379. Fax: +34954461775. E-mail:
[email protected]. † Universidad de Sevilla. ‡ ELCOGAS.
o + 2(C8H17)3N(org) + Ge(OH)4(aq) 3(C6H6O2)(aq) T ((C8H17)3NH)2Ge(C6H4O2)3(org) + 4H2O(aq) (1)
((C8H17)3NH)2Ge(C6H4O2)3(org) T 2(C8H17)3NH+(org) + Ge4+(aq) + 3(C6H6O2)2-(aq) (2) ((C8H17)3NH)2Ge(C6H4O2)3(org) + 2OH-(aq) T 2(C8H17)3N(org) + Ge(C6H6O2)3(aq) + 2H2O(aq) (3) All the studies carried out on the solvent extraction of germanium prior to complexation with catechol (CAT) were limited to laboratory tests with artificial solutions, with limited practical applications. The present work focuses on the design, engineering, and operation of a facility plant for germanium recovery from the fly ash produced in the 335 MW IGCC power plant of ELCOGAS in Puertollano (Spain), which burns a 50: 50 blend of a local metal-rich bituminous coal19 and petroleum coke and produces annually approximately 12000 tonnes of FA. Pilot plant operation is usually needed to generate information about the behavior of the system to be used in the design of larger facilities; therefore, a 5 kg/h of FA pilot plant was designed and built in the laboratories of the Engineering School of the University of Seville (Spain) to check this hydrometallurgical method and to find out possible operational problems in a future semi-industrial plant at the ELCOGAS facilities in Puertollano. The experimentation covers operational parameters such as leaching time, liquid/solid ratio in the leaching stage, volumetric ratio between phases in SX, agitation speed, and residence time. In addition, the reutilization of some streams was tested. The main points related to equipment design and assembly of the pilot plant were continuity and versatility, taking into account the scaling up from the pilot scale to an industrial level.
10.1021/ie800730h CCC: $40.75 2009 American Chemical Society Published on Web 02/25/2009
3574 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 1. Chemical Analysis of IGCC Fly Ash7 Major Components (% w/w) SiO2 Al2O3 CaO
59.3 20.6 3.2
Fe2O3 K2O SO3
4.2 3.5 2.4
MgO Na2O TiO2
0.7 0.5 0.6
P2O5 MnO
0.5 0.04
Minor Components (mg/kg) As Ba Cd Co
955 433 24 53
Cr Cu Ga Ge
155 392 320 420
Mo Ni Sb Se
135 2296 381 19
Sn V Zn LOI
67 6256 7230 4.0
2. Experimental Section 2.1. Materials. All the reagents used in this study were analytical-grade reagents. Catechol was supplied by EMD Chemicals S.L., analytical grade; trioctylamine was supplied by Merck Chemicals; petroleum ether 200-250 °C, supplied by Panreac Quimica S.A.U., was used as a diluent. 2.2. Characterization of Fly Ash. Fly ash was produced at the Puertollano IGCC power plant under the following conditions: 50:50 coal/pet coke blend and 2.5% limestone addition as fluxing agent. Analysis of several FA samples from the Puertollano IGCC plant under different operating conditions has been carried out and the results have been published elsewhere.7 The main chemical characteristics of the FA samples used in the present study are shown in Table 1. 2.3. Pilot Plant. Pilot plant tests were performed over a period of 5 months at the Engineering School of the University of Seville (Spain). The main objective was to determine the feasibility of the hydrometallurgical method for germanium recovery from IGCC fly ash. The pilot plant design was based on the results obtained at laboratory scale.14 A schematic arrangement of the flow configuration is illustrated in Figure 1. The pilot plant comprised two main stages: germanium extraction from FA (leaching with water) and selective extraction of germanium from other elements dissolved in the leachate (interferences) and germanium concentration using catechol and trioctylamine by a single SX step. The pregnant solution from the leached fly ash is mixed in a single-step mixer-settler unit with an organic phase carrying an extractant for the germanium complex. After solvent extraction the aqueous and organic phases are allowed to separate in the settler. The organic solution from the extraction stage feeds the single-step stripping mixersettler where it is mixed with an acidic or alkaline stripping solution that reverses the extraction process. The pilot plant consisted of a leaching reactor unit (100 L, Table 2), a complexation tank (30 L), two mixer-settler units for extraction (Table 2) and stripping (Table 2) and three auxiliary reactors for raffinate discharge and storage (30 L), organic phase preparation (25 L), and germanium-rich final solution storage (10 L), all of them constructed of stainless steel. Auxiliary dosifiers, tanks, pumps, valves, and control devices were also integrated in the process. Due to the experimental character of the facility, it was designed with operational flexibility, allowing modifications and improvements in the equipment and assembly. The pilot plant mass balance (Table 3) was performed with a feedstock of 5 kg/h of FA and a theoretical recovery of germanium of 1.03 g/h (as GeO2). I. Leaching Stage. One fundamental stage of the process was the fly ash germanium extraction carried out by leaching with water and performed in a reactor with water and FA continuous additions. After the leaching, wet ash and leachate were separated by a vacuum rotary filter.
II. Solvent Extraction Method. After germanium complexation by catechol, the Ge-CAT complex is separated from the other metals in solution through the formation of an ion pair with TOA, which is extracted in kerosene. After that, the Ge is stripped from the kerosene organic phase using different aqueous (acid or base) stripping solutions. The structure of the SX process is shown in Figure 1. Optimum operational parameters established in laboratory tests were as follows:14 L/S ratio ) 5 L of water/kg of fly ash in the leaching stage; CAT/Ge molar ratio of 15 and TOA/Ge molar ratio of 9 from an acid solution (pH ) 2 - 3), using a high aqueous phase to organic phase (A/O) volumetric ratio to maximize the germanium yield and to minimize impurity extraction and cost of reagents in the SX stage. The molar excesses of CAT (5 times the stoichiometric amount) and TOA (3 times the stoichiometric amount) were established in a previous work14 to optimize the germanium yield and to minimize impurities extraction. The extraction step has very good kinetics (time 97) and is selective toward germanium. Different tests were carried out using NaOH, HCl, and H2SO4 in the stripping step, with a high organic phase to stripping phase (O/S) volumetric ratio for germanium concentration in the aqueous extract. Very high A/O and O/S ratios are sometimes undesirable because they may lead to high solvent losses, so A/O and O/S of 10 as maximum were used for design and experimentation in this study. 3. Results and Discussion 3.1. Leaching Stage. In relation to the leaching stage, the main goals were to determine possible operational problems when this kind of fine fly ash was added continuously and when different water/fly ash ratios were used. Some leaching tests with three different L/S ratios (3, 5, and 7) were carried out varying the water flow rate within 15-35 L/h. At leaching times
