Development of a Laboratory-Scale Leaching Plant for Metal Extraction from Fly Ash by Thiobacillus Strains

Abstract

Semicontinuous biohydrometallurgical processing of fly ash from municipal waste incineration was performed in a laboratory-scale leaching plant (LSLP) by using a mixed culture of Thiobacillus thiooxidans and Thiobacillus ferrooxidans. The LSLP consisted of three serially connected reaction vessels, reservoirs for a fly ash suspension and a bacterial stock culture, and a vacuum filter unit. The LSLP was operated with an ash concentration of 50 g liter⁻¹, and the mean residence time was 6 days (2 days in each reaction vessel). The leaching efficiencies (expressed as percentages of the amounts applied) obtained for the economically most interesting metal, Zn, were up to 81%, and the leaching efficiencies for Al were up to 52%. Highly toxic Cd was completely solubilized (100%), and the leaching efficiencies for Cu, Ni, and Cr were 89, 64, and 12%, respectively. The role of T. ferrooxidans in metal mobilization was examined in a series of shake flask experiments. The release of copper present in the fly ash as chalcocite (Cu₂S) or cuprite (Cu₂O) was dependent on the metabolic activity of T. ferrooxidans, whereas other metals, such as Al, Cd, Cr, Ni, and Zn, were solubilized by biotically formed sulfuric acid. Chemical leaching with 5 N H₂SO₄ resulted in significantly increased solubilization only for Zn. The LSLP developed in this study is a promising first step toward a pilot plant with a high capacity to detoxify fly ash for reuse for construction purposes and economical recovery of valuable metals.

Biohydrometallurgy, an interdisciplinary field involving geomicrobiology, microbial ecology, microbial biochemistry, and hydrometallurgy (23), is a novel promising technology for recovering valuable metals from industrial waste materials (e.g., bottom and fly ash, galvanic sludge, and filter dust) and for detoxifying these materials for environmentally safe deposition. Biohydrometallurgical processing of solid waste allows recycling of metals, similar to natural biogeochemical metal cycles, and diminishes the demand for resources, such as ores, energy, and landfill space. Fly ash from municipal waste incineration (MWI) is a concentrate containing a wide variety of toxic heavy metals (e.g., Cd, Cr, Cu, and Ni). The zinc concentrations in fly ash (3%, wt/wt) can be similar to the concentrations in ores subjected to conventional mining (16), which makes MWI fly ash a suitable candidate for economical zinc recovery. The low acute and chronic toxicity of fly or bottom ash for a variety of microorganisms (8) and the low mutagenic effect (17) of this material have been demonstrated previously. However, the deposition of materials containing heavy metals results in a severe risk that spontaneous metal leaching may occur due to natural weathering processes and uncontrolled bacterial activities (18, 21, 23). Agenda 21 adopted at the 1992 Earth Summit in Rio de Janeiro, Brazil, established that there is a strong requirement to support sustainable development, including ecological treatment of wastes and safe disposal of wastes. Biological metal leaching of fly ash is a step in this direction.

Biohydrometallurgy is a technology that is cleaner and consumes less energy than technologies used in the pyro- and hydrometallurgical industries. The latter technologies are well-established, and many of them are patented, whereas patents for biohydrometallurgical processing of industrial wastes are rarely published (7). The first effort to develop biohydrometallurgical treatment of industrial waste was made 20 years ago, and greater efforts are necessary now. This is an important subject of research and should result in a wide range of investigations and applications in the future. However, the previous data on biohydrometallurgical treatment of fly ash or other industrial waste obtained with bacteria or fungi included residence times for leaching of up to 50 days (4–6, 11, 25). Most of these experiments were performed on a small scale with low amounts of heavy-metal-containing material.

In this paper, a semicontinuous laboratory-scale leaching plant (LSLP) is described; this LSLP achieved high leaching efficiencies, which resulted in an elevated load of elements in the effluent. Treatment times were found to be less than treatment times obtained with batch extraction procedures. A mixture of Thiobacillus thiooxidans producing sulfuric acid and Thiobacillus ferrooxidans oxidizing reduced metal compounds (19) was used to perform the leaching experiments. The results were compared to chemical (abiotic) leaching efficiencies. In addition, we investigated whether T. ferrooxidans was needed for leaching of fly ash. Metals can be biotically released from fly ash by mechanisms such as direct enzymatic reduction, indirect action resulting from extracellular metabolic products, or acid formation (nonenzymatic dissolution), as previously shown in an anaerobic system (10). It was possible to differentiate among these release mechanisms in an oxic acidic fly ash-Thiobacillus system.

MATERIALS AND METHODS

Bacterial strains, medium, and culture conditions.

T. thiooxidans DSM622 and T. ferrooxidans DSM2391 were cultivated in a medium containing (per liter) 0.1 g of K₂HPO₄, 0.25 g of MgSO₄ · 7H₂O, 2.0 g of (NH₄)₂SO₄, 0.1 g of KCl, and 8.0 g of FeSO₄ · 7H₂O. Elemental sulfur (1%, wt/vol) was added, and the pH was adjusted with sulfuric acid to 2.5 to 2.7. The mixed culture was grown under nonsterile conditions either in 250-ml baffled Erlenmeyer flasks on a rotary shaker (140 rpm) or in an aerated and stirred 1,000-ml beaker. Growth was monitored by monitoring the pH (with a Hamilton single-pore electrode), the cell counts (with a Neubauer counting chamber), and the Fe(II) concentration (12).

Samples and sample preparation.

A 500-kg portion of fly ash retained by electric filters at the MWI plant in Hinwil, Switzerland, was collected by workers at Sulzer Chemtech (Winterthur, Switzerland) at different times on one day and was homogenized in a cement mixer to obtain representative homogeneous samples. The ash was washed with water to remove water-soluble compounds and dried on a vacuum filter. For experiments the ash was ground and dried at 80°C for 48 h. The concentrations of selected elements are listed in Table 1. The value for loss of combustion at 950°C represents the inorganic carbon content.

TABLE 1.

Concentrations of selected elements in fly ash from the MWI plant in Hinwil, Switzerland^a

Element	Detection method(s)b	Concn in fly ash (g kg−1)	SE (%)
Al	A, B, C	70	5.5
Ca	A, B, C	132	5.5
Cd	B	0.49	15.0
Cl	B	4.7	15.0
Cr	A, B	0.7	7.9
Cu	B	1.1	15.0
F	B	8.0	15.0
Fe	A, B, C	28	5.5
Hg	B	<0.01	15.0
K	A, B, C	12	5.5
Mg	A, B, C	14	5.5
Mn	A, B, C	0.77	5.5
Na	A, B, C	11	5.5
Ni	A, B	0.14	7.9
Pb	B, C	8.9	11.1
S	B, D	30	7.9
Si	A, B	100	7.9
Sn	B	9.3	15.0
Ti	A, B	12	7.9
V	B	0.13	15.0
Zn	B, C	31	7.9
Loss of combustion (1 h, 950°C)		126	NDc

^aData reproduced from reference 27 with permission. The measured concentrations allowed us to determine approximate leaching efficiencies. 

^bA, X ray fluorescence with a glass specimen (standard error, ±5%); B, X ray fluorescence with a compacted powder specimen (standard error, ±15%); C, inductively coupled plasma atomic absorption spectroscopy (standard error, ±5%); D, infrared detection after oxidation at 2,000°C (standard error, ±5%). 

^cND, not determined. 

Semicontinuous LSLP.

The LSLP consisted of three serially connected reaction vessels (designated RV-A, RV-B, and RV-C), each having a volume of 1 dm³ (Fig. 1). Pulp from the fly ash storage solution (100 g liter⁻¹) and the bacterial stock culture (10⁹ cells/ml) were mixed in equal amounts and fed every 12 h semicontinuously into the first reaction vessel (RV-A) with a peristaltic pump at an overall dilution rate of 0.021 h⁻¹ (0.5 day⁻¹). RV-A and RV-B were connected by an overflow connector. The pulp was pumped from RV-B to RV-C with a peristaltic pump at the same rate to flush settled fly ash particles. This resulted in a pulp concentration in the LSLP of 50 g liter⁻¹ and a mean residence time of 6 days (2 days in each reaction vessel). The bacterial stock culture and the three reaction vessels were aerated at a rate of about 2 volumes of air per volume of reactor per min. After RV-C, the pulp was transported by gravity flow into a 2-liter vacuum glass filter unit, and the particle-free, metal-rich solution was collected in a 5-liter collecting vessel.

FIG. 1.

Schematic diagram of the LSLP. A, B, and C, serially connected RV-A, RV-B, and RV-C, respectively; 1, fly ash reservoir; 2, bacterial stock culture; 3, peristaltic pump; 4, filter unit; 5, collecting vessel; 6, to vacuum pump; solid lines, liquid flow; dashed lines, air flow.

Determination of mobilization mechanisms.

Forty milliliters of an 8% (wt/vol) fly ash suspension (acidified with sulfuric acid to pH 5.4) was diluted with 40 ml of a Thiobacillus culture (10⁹ cells/ml) after different treatments (see below) and incubated for 8 days on a rotary shaker (150 rpm) at room temperature (23 to 24°C). All samples were incubated in triplicate. Several mechanisms of metal mobilization were distinguished, as described below.

The direct enzymatic effect on the release of metals was determined by diluting the ash suspension with bacterial stock cultures (pH 1.1). The cells were in direct contact with the fly ash. Growth of T. ferrooxidans might have been stimulated by increased energy available from oxidation of reduced solid particles.

Leaching with cell-free spent medium revealed that indirect solubilization by extracellular metabolic products occurred. The stock culture was centrifuged at 23,700 × g, and the supernatant was filtered through a 0.22-μm-pore-size Teflon filter to obtain cell-free spent medium. The cell-free spent medium was checked for viable cells by incubating a 5-ml sample in 80 ml of fresh medium.

Cell-free spent medium (see above) was autoclaved (12 min, 121°C) to obtain a sterile leaching solution without enzymatic activities to evaluate the leaching ability of the acid formed. The solution was checked for precipitates and for changes in redox state after the heat treatment.

Forty milliliters of fresh uninoculated medium was added to the fly ash suspension and used as a control.

Elements such as Cd or Zn might have been chemically mobilized during preparation of the ash suspension due to the acidification to pH 5.4.

Chemical leaching with sulfuric acid.

Eighty milliliters of the fly ash suspension was leached at a concentration of 5% (wt/vol) with a maximum of 11 ml of 5 N sulfuric acid (final pH, pH 2) at the following pH values: at an initial pH of 10, at pH 8 (corresponding to the carbonate buffer pH), at pH 4 (corresponding to the potassium-aluminum buffer pH) (2), and at pH 2 (a possible end point of a leaching experiment). The concentrations of the solubilized metals and the acid consumption were measured. The pH at each value was controlled with a pH-Stat (Metrom Impulsomat model 614). Fly ash was suspended in distilled water and stirred at a constant pH for 24 h. A new suspension was prepared for each pH step.

Analytical procedures.

Metal analyses were performed by using inductively coupled plasma atomic absorption spectroscopy (ICP-AES; Spectro Analytical Instruments, Kleve, Germany) and standard addition methods at the following wavelengths: Al, 396.2 nm; Cd, 228.8 nm; Cr, 267.7 nm; Cu, 324.8 nm; Fe, 261.2 nm; Mn, 294.9 nm; Ni, 352.5 nm; and Zn, 206.2 nm. Prior to the inductively coupled plasma analysis, the samples were centrifuged at 23,700 × g for 15 min, acidified with 5 drops of concentrated HNO₃ per 30 ml of aqueous solution, passed through a glass fiber filter (Whatman type GF/C) to guarantee particle-free suspensions, and stored at 4°C.

RESULTS AND DISCUSSION

LSLP.

A semicontinuous three-stage leaching plant for extraction of heavy metals from MWI fly ash was developed. Biohydrometallurgical processing of fly ash from MWI poses, especially at high pulp densities, severe problems due to the high content of toxic metals in the fly ash and the saline and strongly alkaline environment. It is necessary to obtain reduced treatment times for high pulp concentrations without additional acidification; this is important for reducing the capital and maintenance costs of a pilot plant.

RV-A of the LSLP (Fig. 1) was filled with 500 ml of a bacterial culture (pH 1.5) and 250 ml of a fly ash suspension (10%, wt/vol) for the first adaptation phase. After 36 h, another 250-ml aliquot of the suspension was added to obtain the final 5% (wt/vol) solution. When the pH in RV-A dropped below 2, the LSLP was started. After 48 h (corresponding to four discontinuous feeding cycles) the solution was pumped to RV-C. After 48 h, the microorganisms produced sufficient amounts of sulfuric acid in each reaction vessel to maintain steady-state conditions despite the alkaline pH of the fly ash pulp (pH 9 during the experiment). A distinct pH cascade occurred from one reaction vessel to the following reaction vessel. The pH fluctuated during the steady state depending on the fly ash present; in RV-A the pH fluctuated between 3.7 and 4, in RV-B the pH fluctuated between 2.7 and 3.2, and in RV-C the pH fluctuated between 1.2 and 1.5. Before the bacterial stock was added to RV-A, growth parameters [pH, Fe(II) concentration, cell number] were monitored (Fig. 2). The medium was fully replenished with fresh medium every 72 h (corresponding to an overall dilution rate of 0.01 liter h⁻¹) to avoid aging of the bacterial stock culture.

FIG. 2.

Monitoring of growth and activity of a mixed culture of T. thiooxidans and T. ferrooxidans (maximum growth rate, 0.055 h⁻¹; doubling time, 12.7 h). The experiment was performed in an aerated 2-liter beaker before the culture was used as a stock culture in the LSLP. During the operation of the LSLP the pH remained between 1.0 and 1.3, and the cell count increased slightly to 4 × 10⁹ cells per ml. Symbols: ▵, pH; ◊, Fe(II) concentration; ○, cell count.

Samples used for metal analysis were removed after 168 h from each reaction vessel, from the collection vessel, and (as controls) from the fly ash storage vessel and the bacterial stock culture. For all elements, the concentrations of soluble metals increased continuously with increasing mean residence time in the LSLP (Fig. 3). In RV-C the following amounts of metals were solubilized (per kilogram of fly ash): Al, 37 g; Zn, 25 g; Fe, 3.1 g; Cu, 0.98 g; Mn, 0.53 g; Cd, 0.49 g; Ni, 0.09 g; and Cr, 0.08 g. Ferrous iron added to the bacterial stock culture as an electron source for T. ferrooxidans precipitated in the first two reaction vessels (RV-A and RV-B) in high amounts, resulting in a decrease in the soluble iron level. At higher pH values iron either precipitated as hydroxide or became adsorbed on fly ash particles. In addition, ferric iron coprecipitates with other metals (e.g., As, Cd, Cr, Cu, Pb, and Zn) (10). In RV-A, up to 45% of the iron added to the medium precipitated; in RV-B about 32% of the iron added precipitated. Only at the very low pH in RV-C (pH 1 to 1.3) was 10% net iron leaching observed. Leaching of Pb with sulfur-oxidizing bacteria like members of the genus Thiobacillus is not very effective, because of the low solubility of PbSO₄ in aqueous solutions (15, 20). Sulfate was present in the medium at high levels. Ferrous sulfate was added as an energy source for T. ferrooxidans, and sulfate was produced by T. thiooxidans as a metabolic product of sulfur oxidation. Therefore, mobilized Pb immediately precipitated as PbSO₄ and remained with the leached fly ash in the glass filter unit. To verify this result, ChemEQL (a computer program used to calculate thermodynamic equilibrium concentrations and precipitation conditions) was used. Precipitation was calculated for the following conditions: (i) the maximum expected Pb concentration, 445 ppm (2.15 mM); (ii) the minimum expected sulfate concentration, 1,380 ppm (14.4 mM) (only the sulfate from the medium was considered; the sulfate formed due to bacterial oxidation of S° was not taken into account); and (iii) the maximum Cl⁻ and F⁻ concentrations (Cl⁻ and F⁻ form soluble Pb complexes), 235 ppm (6.63 mM) and 400 ppm (21 mM), respectively. Concentrations were chosen by using the method of Vonmont (27). As shown in Table 2, only 1.2 to 2.6% of the Pb stayed in solution under very acidic conditions. At pH values between 1 and 4, the solubility was <1%. Although Pb concentrations close to 10 g kg⁻¹ can be found in fly ash (Table 1), the leaching efficiencies (expressed as percentages of the amounts applied) were very low (usually <5%).

FIG. 3.

Amounts of solubilized metals obtained from MWI fly ash (50 g liter⁻¹), expressed as percentages of the amounts present in a LSLP with three serially connected reaction vessels (RV-A, RV-B, RV-C). The mean reaction time in each vessel was 2 days. Negative values indicate metal precipitation. ▧, RV-A; ░⃞, RV-B; □, RV-C.

TABLE 2.

Dissolution equilibrium for Pb from MWI fly ash as determined by ChemEQL at high sulfate concentrations (50 g of fly ash liter⁻¹; maximum total Pb concentration, 445 mg liter⁻¹; minimum H₂SO₄ concentration, 1,380 mg liter⁻¹; maximum Cl⁻ concentration, 235 mg liter⁻¹; maximum F⁻ concentration, 400 mg liter⁻¹)^a

pH	Amt of dissolved total Pb
	ppm	%
0.5	11.51	2.59
1	5.22	1.17
1.5	3.23	0.73
2	2.63	0.59
2.5	2.50	0.56
3	2.57	0.58
3.5	2.76	0.62
4	2.93	0.66

^aThe minimum and maximum concentrations were calculated according to data in Table 1. 

Determination of mobilization mechanisms.

X ray fluorescence analyses carried out by workers at Amt für Gewässerschutz und Wasserbau des Kantons Zürich in 1991 (1) indicated that reduced copper species (chalcocite [Cu₂S] and cuprite [Cu₂O]) were present in MWI fly ash, whereas zinc and other metals were present in their fully oxidized forms. Thus, copper release from fly ash should be directly affected and enhanced by T. ferrooxidans, whereas Zn, Al, Cd, Cr, and Ni are released primarily due to the acidic environment. These different mobilization mechanisms could be distinguished by a series of batch experiments.

In batch cultures with inoculated medium the pH decreased within 8 days from 3.6 to 1.6 as a result of biotically formed sulfuric acid. In freshly filtered cell-free spent medium and autoclaved sterile spent medium the pH remained constant at 3.6; in uninoculated medium the pH remained constant at 5, and the pH remained constant at 5.4 in assays in which distilled water was used instead of medium. Acidification of the fly ash pulp (chemical mobilization) led to significant extraction yields for Cd, Ni, and Zn (Fig. 4), which could be slightly increased by using uninoculated sterile medium as the lixiviant (leaching solution) due to the sulfuric acid present in the culture medium. The level of Al dissolution was low, whereas the Cr and Cu concentrations in both experiments were below the detection limits.

FIG. 4.

Amounts of solubilized metals obtained from MWI fly ash (40 g liter⁻¹), expressed as percentages of the amounts present with different lixiviants within 8 days. All samples were incubated in triplicate. The release of metals due to acidification of the fly ash pulp was defined as chemical mobilization. □, inoculated medium; ░⃞, filtered cell-free spent medium; ░⃞, autoclaved sterile spent medium; ▧, uninoculated medium; ▨, chemical mobilization.

By comparing the amounts of leached copper in filtered cell-free spent medium (0.89 g kg of fly ash⁻¹; standard deviation [s_x] = 0.03 g kg⁻¹; n = 3) and autoclaved sterile spent medium (0.70 g kg⁻¹; s_x = 0.04 g kg⁻¹; n = 3), it was concluded that in contrast to other elements significant amounts of copper (as determined by a paired t test [one sided], P = 0.02) were mobilized by metabolic products of T. ferrooxidans. Leaching with cell-free spent medium, which indicated that a solubilizing mechanism involving extracellular components was present, was significantly more effective than leaching with autoclaved spent medium, in which excreted enzymes were inactivated. It is known that several components involved in the electron transport chain in the genus Thiobacillus (rusticyanin, cytochromes, iron-sulfur proteins) are located in the periplasmic space (3, 24) and might, therefore, also be present in cell-free spent medium and catalyze oxidation of reduced metal compounds. It is possible that heat treatment (autoclaving) of spent medium leads to aggregation of nonenzymatic compounds or changes in the redox state. The two solutions were checked for precipitates (ΔA₆₆₀ = 0.003) and for their redox potentials (E_h) (the values for cell-free spent medium and autoclaved spent medium were 830 and 810 mV, respectively). The results show that aggregation and altered redox conditions due to autoclaving did not occur.

In contrast, for all of the other elements examined (Al, Cd, Cr, Ni, and Zn), the difference between filtered cell-free spent medium and autoclaved cell-free spent medium was not significant. Therefore, it was concluded that the mobilization of these metals from fly ash was caused only by the acidic environment.

The maximal extraction yields for all elements were obtained with samples incubated with both T. thiooxidans and T. ferrooxidans. The data indicate that there was efficient release of most heavy metals from fly ash due to biotically formed sulfuric acid (the pH decreased within 8 days from 3.6 to 1.6). Copper solubilization increased significantly (1.10 g kg of fly ash⁻¹; s_x = 0.05 g kg⁻¹; n = 3; paired t test [one sided]; P = 0.002), as well did mobilization of Al (n = 3; P = 0.003), Cd (n = 3; P = 0.03), Cr (n = 3; P = 0.0005), and Zn (n = 3; P = 0.03) compared to samples incubated with cell-free spent medium.

Chemical leaching with sulfuric acid.

The value for loss of combustion at 950°C, of 12.6% (Table 1), indicates that considerable amounts of inorganic carbon occurred as carbonates in the fly ash. These carbonates, along with other constituents, caused the pH of the fly ash suspension to be more than 10. Most of the metals were mobilized due to sulfuric acid, the acid necessary to neutralize and acidify MWI fly ash was determined, and the chemical (abiotic) leaching efficiencies of sulfuric acid at different pH values were assessed.

The dissolution of metals in fly ash in alkaline environments (pH 8 and 10) was low. Less than 0.03 g of Al or Zn and less than 0.01 g of Cd, Cr, Cu, Fe, or Ni were solubilized per kg of fly ash. To lower the pH of the fly ash suspension from >10 to 4, 175 ml of H₂SO₄ (95 to 97% pure) per kg of fly ash had to be added. This resulted in release of 15 g of Al kg of fly ash⁻¹ (corresponding to 22% of the total amount present), 20 g of Zn kg⁻¹ (75%), 1.44 g of Fe kg⁻¹ (5%), 0.49 g of Cu kg⁻¹ (53%), 0.38 g of Cd kg⁻¹ (76%), 0.02 g of Ni kg⁻¹ (14%), and 0.01 g of Cr kg⁻¹ (2%). Large amounts of Al (45 g kg⁻¹; 64%) and Zn (32 g kg⁻¹; 100%) were solubilized at pH 2, along with 7.4 g of Fe kg⁻¹ (26%), 0.96 g of Cu kg⁻¹ (88%), 0.44 g of Cd kg⁻¹ (89%), 0.03 g of Ni kg⁻¹ (22%), and 0.08 g of Cr kg⁻¹ (11%), when 311 ml of H₂SO₄ (95 to 97% pure) was added. The leaching efficiencies were comparable to those of the LSLP.

Conclusions.

The ability of microorganisms to leach and mobilize metals from solid materials is based on the following three mechanisms: (i) redox reactions, (ii) formation of inorganic acids, and (iii) excretion of complexing agents (e.g., organic acids). T. ferrooxidans mobilizes metals from solids by redox reactions. Electron transfer from minerals to microorganisms either occurs directly in the case of physical contact between organisms and solids or is based on the biotic oxidation of Fe⁺² to Fe⁺³ when ferric iron catalyzes metal solubilization as an oxidizing agent (9, 14). For solubilization of the reduced copper compounds (chalcocite [Cu₂S] and cuprite [Cu₂O]) present in the fly ash, these release mechanisms are important. The Ni leaching efficiency is also considerably increased by biological activities compared with chemical leaching with sulfuric acid due to the presence of Fe(III) in the leaching solution. The results obtained for bacterial leaching of heavy metals from anaerobically digested sludge also confirmed that the solubilization rates of metals obtained with mixed cultures of T. thiooxidans and T. ferrooxidans were higher than the solubilization rates obtained with single cultures (26).

The three-stage LSLP described here is a promising first step toward the establishment of a semicontinuous or continuous bioleaching leaching plant. The lab work showed the practicability of biotic fly ash leaching despite the presence of saline and strongly alkaline material. When acidophilic autotrophic Thiobacillus strains are used, no aseptic LSLP set-up is required. The possibility of contaminants which interfere with the Thiobacillus strains is minimal, due to the very acidic environment and the absence of organic compounds as carbon sources. Such conditions should reduce the capital and maintenance costs of a pilot plant. A scaled-up version of the LSLP, a pilot plant with three or more reaction vessels for biotic leaching of industrial waste, seems to be technically feasible. Large-scale reactor leaching of this type has been used previously, especially for gold recovery (13, 22). A large-scale bioleaching plant should allow us to detoxify fly ash for reuse in construction, while valuable metals, especially zinc, should be economically recovered for recycling in metal-manufacturing industries.