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Biologically produced sulphide for purification of process streams, effluent treatment and recovery of metals in the metal and mining industry
Hydrometallurgy 83 (2006) 106 – 113 www.elsevier.com/locate/hydromet
Biologically produced sulphide for purification of process streams, effluent treatment and recovery of metals in the metal and mining industry Jacco L. Huisman ⁎, Gerard Schouten, Carl Schultz Paques B.V., P.O. Box 52, 8560 AB, Balk, The Netherlands Available online 15 May 2006
Abstract One of the best available technologies for the removal of metals from water is in the form of metal sulphides. Metal removal by sulphide precipitation is a well-known process that is characterised by compact residues and very high removal efficiencies. Compared to neutralisation alone the sludge volume is 6 to 10 times lower and the toxic metals are removed to a 0.01–1 ppm level. Furthermore, selective metal precipitation is possible, allowing for separate recovery of valuable metals like copper, nickel, cobalt and zinc from nuisance metals like arsenic and antimony. However, the cost of reagent (NaHS or H2S gas) and safety aspects are often prohibitive. This paper describes a novel biological process for safe and cost effective production of sulphide from elemental sulphur, waste sulphuric acid or sulphate present in effluents. With this technology, gaseous or dissolved H2S is produced on-site and on-demand in an engineered, high rate bioreactor. Experience with industrial applications at metal processing plants will be presented. The technology can serve to selectively recover metals from e.g. bleed streams, leach liquor, effluent streams and acid mine drainage. Lower overall costs and increased safety (no transport or storage of sulphide, production on-demand and at ambient pressure) are the main advantages of this new process compared to its alternatives. © 2006 Published by Elsevier B.V. Keywords: Biogenic sulphide production; Metal sulphide precipitation
1. Introduction 1.1. Origin and problems related to wastes in metal and mining industry The mining and metallurgical industry generate large volumes of waste- and process water that are polluted with dissolved heavy metals or sulphate, or both. For ⁎ Corresponding author. Tel.: +31 514 608 500; fax: +31 514 603 342. E-mail address: [email protected] (J.L. Huisman). 0304-386X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.hydromet.2006.03.017
instance, acid mine and rock drainage is currently one of the most widespread forms of pollution worldwide. In Canada alone, it is estimated that 12,500 ha of tailings and 750 million tonnes of waste rock are present that can release acid and toxic compounds [1]. Acid mine drainage can have a moderate (0.35–0.55 g/l [2]) to high (1.5–7.2 g/l [3]) sulphate concentration. Metals of particular interest in acid mine drainage and industrial wastewaters include copper, zinc, cadmium, arsenic, manganese, aluminium, lead, nickel, silver, mercury, chromium and iron, in a concentration that can range from 10−6 to 102 g/l. The composition of such a
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
wastewater reflects the particular combination of heavy metals originating from a metallurgical operations. This toxic leachate can cause severe aquatic habitat degradation downstream of the mine when discharged untreated. Slag from metallurgical processes poses a similar problem. Even though it often contains high concentrations of (potentially valuable) heavy metals, it is dumped in landfills or tailings ponds. Depending on the composition of the slag, these metals can be released as a result of weathering of the slag. Presently, removal by precipitation as metal hydroxide is the most widely used treatment method for water contaminated with heavy metals. This is because of the simplicity and the low costs of this method. For the same reasons, sulphate removal is mostly accomplished by precipitation with Ca2+, added as lime. However, more stringent legislation in future and an increasing scarcity of resources creates a need for heavy metal and sulphate removal technologies with a better performance. Thus, treatment processes should aim to recover valuable metals and other possible resources from waste streams such as sulphur compounds and process water. 1.2. Removal and recovery of metals as metal sulphides When it comes to sludge volume, reusability of the sludge and effluent quality, precipitation of metals with sulphide is superior to precipitation as hydroxides. It has many advantages over lime precipitation: ➢ High reactivity of sulphides with heavy metal ions and very low solubility of the resulting metal sulphides over a broad pH range resulting in lower effluent concentrations (see Fig. 1). ➢ Sulphide precipitation, unlike hydroxide precipitation, is relatively insensitive to the presence of complexes and most chelating agents. ➢ Sulphide removes chromates and dichromates without preliminary reduction of the chromium to the trivalent state. ➢ A high degree of selective metal precipitation is possible with sulphide, as opposed to hydroxide precipitation. ➢ Metal sulphide sludges generally are more dense and stable than metal hydroxide sludges [4], exhibiting better thickening and dewatering characteristics than the corresponding metal hydroxide sludge, which facilitates further processing. Previous objections against the use of sulphide, i.e. that it is toxic and corrosive, do not hold anymore because of the application of adequate safety measures
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Cu(OH)2 Pb(OH)2
As2S3 Zn(OH)2 NiS ZnS PbS
Cd(OH)2
CdS Ni(OH)2 ZnS
NiS Ni(OH)2
CuS
PbS
Fig. 1. Comparison between the equilibrium concentrations of metal hydroxides and metal sulphides [5].
and the use of modern corrosion-resistant construction materials (plastics) eliminate these disadvantages. Sulphide precipitation would be the method of choice, if the high cost of transporting, storing or producing sulphide on-site (as NaSH or H2S) would not have hampered its widespread application in the metal and mining industry. In present applications for heavy metal removal with sulphide, the sulphide needed for precipitation is mostly obtained from chemical sources such as Na2S, NaHS, CaS, FeS and H2S. In the remainder of this paper, technology will be described to produce sulphide on-site and on-demand using biotechnology. This eliminates the hazards and costs that accompany the transport, handling and storage of chemical sulphides. 2. Biotechnological sulphate reduction The first clue that respiration could be an anaerobic way of life was obtained in 1895 by Beijerinck [6], who showed that sulphate could be reduced to sulphide in sediments. Although many bacteria can produce sulphide, only a few do so at a sufficient rate for application in high-rate processes. These rapid sulphide-generating bacteria are able to conserve energy by the reduction of sulphur oxyanions like sulphate, sulphite and thiosulphate [7], and they are generally termed sulphate-
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reducing bacteria (SRB). A typical overall conversion equation is (neglecting the small amount of organic material required to produce biomass): þ − − SO2− 4 þ CH3 COOH þ 2H →HS þ 2HCO3 þ þ 3H
ð1Þ
Eight electrons are transferred from acetic acid to sulphate in order to produce sulphide. Especially the last decade, the use of SRB for treatment of process- and wastewaters containing sulphur compounds and metals in mining and metallurgical industries has become a topic of scientific and commercial interest. Biological treatment of waste- and process water from the mining and metallurgical industries offers an interesting alternative to conventional technologies. Biological treatment essentially consists of biological reduction of sulphur oxyanions to sulphide followed by chemical precipitation of metal sulphides. In this way dissolved metals and sulphate are concentrated into a solid. Compared to conventional chemical treatment with lime and hydroxide, the biological treatment can achieve much lower effluent sulphate (< 250 mg/l vs. ∼ 1500 mg/l) and metal (ppbvs. ppm-level) concentrations. This high rate technology has a constant and predictable effluent quality, This in contrast to for example constructed wetlands where the removal of metals can range between 0% and 99% [8]. The company Paques BV (www.paques.nl) has developed full-scale applications for effluent treatment with SRB: the Sulfateq® technology, which is part of the Thiopaq technologies. Paques BV is a company that specialises in the development, design and realisation of high rate biotechnological applications for industry. Technologies for recovery of metals and for conversion of a broad range of organic and inorganic compounds from water and gas streams are available. Worldwide,
over 500 industrial installations using Paques technologies have been constructed since 1982. Recovery of valuable base metals using biogenic H2S, combined removal of sulphate, nitrate and heavy metals, selenium removal and fluoride removal are examples of applications in the metal and mining industry. For several applications metal sulphide precipitation is the best solution, but sometimes the effluent or process water characteristics are such that biological sulphide production from sulphate present in the effluent is not possible. For example when the water properties like pH, temperature, salinity and overall composition are not compatible with the working range of the bacteria. An example is electrolyte bleed in copper refining. For these applications the Thioteq technology has been developed by Paques BV. 3. Thioteq technology: off line biogenic sulphide production The Paques Thioteq process consists of two stages: a chemical and a biological. A schematic flowsheet for the recovery of one metal as its metal sulphide is shown in Fig. 2. The water to be treated only passes through the chemical stage. Sulphide is produced in the biological stage and transported to the chemical (precipitation) stage with a carrier gas. 3.1. Biological stage Within the biological stage there are two major differences between the in-line application and an offline application. In the latter process: 1) The liquid and solids residence time are equally long, 2) the sulphur source is preferably elemental sulphur (or otherwise concentrated waste sulphuric acid). Lean gas Alkali source
Sulphur Reductant
Clarifier Bioreactor
Effluent
Contaminated water Contactor
Metal sulphide product Fig. 2. Thioteq water treatment and recovery of one metal product.
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In the case that elemental sulphur is used as a sulphur source and acetic acid is used as electron donor, the conversion can be written as: 4S0 þ CH3 COOH þ 2H2 O→4H2 S þ 2CO2
ð2Þ
A typical bacterium that can be found in such a system is Desulphuromonas acetoxidans, which grows using the energy released by linking the oxidation of acetate to the reduction of elemental sulphur [9]. Accumulations of such cells have a characteristic pink colour. This bacterium is true sulphur-reducing, strictly anaerobic, gram negative, flagellated, and rod shaped. It acquires its energy from sulphur respiration and completely oxidizes acetate with S0 via the citric acid cycle to carbon dioxide and hydrogen sulphide [10]. The mechanism was partly clarified by [11]. The bioreactor is fed with powdered sulphur. For biological sulphur reduction the bacteria need not to be attached to elemental sulphur, in contrast with the biological sulphur oxidation. The mechanism occurs via polysulphides, which are formed when sulphide reacts with the S8-ring of elemental sulphur. The chemistry of elemental sulphur in aqueous sodium polysulphide solutions is well documented [12]. The initial ring-opening reaction:
S8 +
HS–
↔
S8S2–
+
H+
ð3Þ
is followed by rapid chain degradation and establishment of an equilibrium among polysulphide ions of different sizes, sulphide, and hydroxyl ions according to the following reaction [12]:
↔
ð4Þ
The polysulphide SnS2− is the reactive species for sulphur reduction. Acetic acid, ethanol or hydrogen are typical electron donors. But other organic compound can be used as long as they are available in a concentrated form and when in addition other start-up cultures are used. The electron donor requirement for sulphur reduction is only 25% of that of sulphate reduction, as can be seen by comparing Eq. (2) with Eq. (1). The off-line process of Fig. 2 can also be operated successfully with concentrated sulphuric acid. Clearly, the electron donor consumption is equal to that in Eq. (1). Concentrated sulphuric acid can be attractive when a waste stream with such acid must be processed. The biological process is carried out under ambient conditions, which makes it safe to operate. In addition the process is self-controlling; high sulphide concentra-
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tions become inhibitory to the bacteria that subsequently stop producing but are not killed. Therefore, a dangerous runaway situation is impossible. The bacteria become active again when the sulphide is withdrawn from the bioreactor by the precipitation process. Finally, the sulphide inventory in the bioreactor is mostly maintained as ionic–and therefore dissolved–HS− . Little H2S is therefore released in case of a leak. The sulphide is available on-demand within the capabilities of the bacteria as long as the sulphide demand is balanced by the addition of electron donor. 3.2. Chemical stage—contactor The flowsheet in Fig. 2 is a standalone plant with one metal precipitation circuit, although multiple contactor– clarifier units can be provided if more than one metal product is to be recovered as will be shown below. The chemical stage consists of a gas–liquid contactor. The sulphide is transported to the contactor with the help of a gas recycle with a carrier gas (a mixture of CO2 and N2) and metal-loaded water (e.g. acid mine drainage) is fed to the contactor. Metals like copper precipitate as a sulphide according to the following reaction: Cu2þ þ S2− →CuS
ð5Þ
The properties of the sulphide gas from the bioreactor and the contactor design results in metal sulphides with good settleability and filterability. The contactor off-gas, containing primarily N2, CO2 and water vapour is recycled to the bioreactor again. There is no significant gaseous discharge during normal operation. However, a typical plant is equipped with an absorber to clean the gas bleed. Copper can be precipitated as a sulphide usually without pH adjustment and without significant precipitation of other heavy metals present in the water. The result is a product with a high copper sulphide content usually greater than 90%. Other metals such as zinc and nickel can be recovered as separate high-grade sulphide products when the number of precipitation stages is increased. These metal sulphides can be shipped to smelters as high quality concentrates. A pH control using an alkali source might be required to meet the optimum precipitation conditions. Toxic metals, such as arsenic, antimony, cadmium, and lead, that typically occur in smaller amounts in mine drainage and other industrial effluents, are also removed and report to one of the sulphide products. The precipitated metal concentrates are recovered in a clarifier and then dewatered using a filter press to
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meet smelter feed moisture requirements. In some cases, plant effluent from the clarifier overflow can be directly discharged to the environment as shown in the flowsheet in Fig. 2. In other cases, the water can be recycled for process use, or might require further treatment before discharge, although secondary treatment requirements will be significantly reduced as will be explained in the examples further down. 3.3. Overall process It can be noted that the bioreactor is operated independently of the chemical stage, with none of the feed water passing through the bioreactor. The size of the bioreactor is based on the metal load that has to be precipitated and can therefore be significantly smaller than the components of the chemical (metal recovery) stage, which is sized based on the hydraulic loading. Sulphide produced by the Thioteq process has a significantly lower cost than chemical reagent sulphide such as NaHS, Na2S or H2S. The technology also has the advantage that the reagent is produced on-demand only. This results in more efficient utilisation and eliminates any safety concerns related to transportation and on-site storage. In addition, the turn-down ratio is down to 10%. At any time, the sulphide inventory at the site is very small and limited to the amount in the headspaces, solutions of the bioreactor and contents of the contactor. Therefore, the Thioteq technology can be regarded as a safe and economic alternative to common sources of hydrogen sulphide. 3.4. Integration of Thioteq technology and lime plants The Thioteq technology can be particularly effective when integrated with an existing lime treatment, as
shown in Fig. 3. The Thioteq process is situated upstream of the lime plant. In this example, a 2-stage metal precipitation and recovery circuit is used to produce separate concentrate products. In the example shown, copper and zinc products are recovered. If, however, metal recovery is a prime factor in project economics, metal loadings in the feed water and metal prices must be taken into account in determining the optimum flowsheet. In some cases it might be more economic to recover only one metal. In any case, the plant discharge from the final clarifier passes to the lime plant for final acid neutralisation and precipitation of iron and aluminium, if required. Removal of metals upstream of a lime plant can not only offset treatment costs through metal product sales but can also result in savings in lime and a reduction in both volume and toxicity of the lime plant sludge thereby reducing long-term liability of sludge storage. Reagent savings can be particularly significant if zinc or nickel are present in the mine water. Both these metals require a high pH for removal in a lime plant but can be recovered at neutral or slightly acidic pH as sulphides. This not only saves lime but also eliminates the need for acidification prior to discharge to meet the discharge standard for pH. Significant Mg concentrations in the water can also result in high reagent consumption in plants operated at high pH. Even if metal concentrations are too low to justify their recovery as saleable products, integration of the two treatment technologies can be potentially beneficial for environmental control by improving discharge water quality, reducing lime consumption and sludge volume, and improving the settling, filtration and chemical/physical characteristics of the sludge for disposal. Lime (reduced consumption)
Alkali source
Lean gas Clarifier
Clarifier
Clarifier
Contaminated Water
Contactor
Contactor H2S-rich gas Cu sulfide Product
Lime reactor Zn sulfide product
Sludge (Reduced volume and toxicity)
Sulphur Reductant
Bioreactor
Fig. 3. Thioteq process upstream of lime water treatment plant.
Effluent (high quality)
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
3.5. Integration of Thioteq technology with hydro metallurgical processes In addition to environmental benefits, removing metals from metallurgical process streams could have significant benefit for the efficiency (rates and recoveries) of the metallurgical process. Fig. 4 shows an example of how the Thioteq technology could be applied for this purpose. A conventional copper heap leach–solvent extraction–electrowinning circuit is shown, with the Thioteq process being used to treat a raffinate bleed stream. In addition to reducing the inventory of both economic and undesirable metals such as As, Cd, Pb, Zn, Ni, and Mn circulating in the leach solution, there is the potential for improved leach kinetics and metal recovery from the heap. Furthermore, acidity resulting from metal precipitation would be added back to the heap leach and reduce acid requirements from external sources. The same flowsheet principals could also be applied for heap decommissioning purposes by providing a means of recovering metals from solutions at the conclusion of heap and dump leach operations, if not for revenue then for the environmental benefit of removing metals from the site and avoiding long-term sludge storage and maintenance. Other potential applications include TDS reduction and solution control in metals processing, steel making, automobile manufacture and in power plants. In some cases, integration with the biological sulphate reduction process might be preferable due to the production of alkalinity in the process.
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4. Case study A zinc mine in North America stopped operating several years ago. However, the mine continues to operate a lime treatment plant to treat underground mine drainage that has an average flow of 700 m3/day. The mine also has a sizeable deposit of old tailings, stored separately by the previous operator. These tailings contain significant quantities of pyrite, zinc and copper. Over the years these tailings have become a source of acidity and soluble metals due to oxidation. A solution is required to retreat and/or reclaim the tailings. A treatment plant to remove metals from the mine drainage upstream of the existing lime plant was designed (see Fig. 5). This plant was constructed on schedule and budget and was based on the process flowsheet shown in Fig. 3 but with only one contactor, and incorporated a bioreactor, contactor and settling/ dewatering. The plant was started up in November 2001 and within 3 months the plant had reached the steady state operating capacity that was required to meet water treatment needs. The plant operated throughout 2002 recovering a saleable zinc concentrate, including the removal of copper, cadmium and lead from the wastewater prior to entering the existing lime treatment plant for iron and aluminium removal. The treated water was discharged to local receiving waters within the guidelines of existing permits. A planned shutdown of minewater collection and treatment took place during the winter. This was a one-time change in operating practice that resulted in simplified collection of the mine water for
Paques Thioteq Process Rich gas
Sulphur Nutrients
Lean gas
H+
Product 1
Product 2
Product 3
Bleed
Heap Leach Cu Cathodes
EW
SX
Fig. 4. Integration of biological sulphur reduction with copper heap leaching.
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J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113 Lime (reduced consumption: 495 instead of 650 ton/yr)
Lean gas Clarifier
Contaminated Water 700 pH 450 30 150 3,800
m3/day 3.7 mg/L Zn mg/L Cu mg/L Fe mg/L SO4
Contactor
Clarifier
Contactor
H2S-rich gas
Lime reactor
Zn/Cu sulfide Product
Sulphur Reductant
Sludge w. reduced volume and toxicity (31000 instead of 52000 m3/yr)
Effluent (high quality)
Bioreactor
Fig. 5. The flowsheet for the North American zinc mine with typical flows and concentrations.
delivery to the treatment plant and overall cost savings for mine water treatment in the longer term. 50 days prior to the shutdown in October, the plant feed averaged 660 mg/l Zn, 19 mg/l Cu and 337 Fe3+, with some peak concentrations well above the average for the period. These concentrations can be compared with the design values of 450 mg/l Zn, 30 mg/l Cu and 15 mg/l Fe3+. The plant, however, coped with the unpredictable and highly variable metal content in the acid water. Nevertheless, the metal recovery exceeded design expectations for copper, with consistent copper recovery to < 0.01 parts per million in the discharge. Zinc recovery was within design expectations from water that has consistently exceeded design parameters with respect to copper, zinc and iron content. Since the metal concentrations in the plant feed ranged from 1.5 to 2 times the design quantity, the plant was not able to treat the entire mine water flow at all times. At the same time, however, the rate of sulphide
generation in the bioreactor per unit volume of mine drainage exceeded design expectations, ranging from 0.26 to 0.43 kg of sulphide per cubic meter of acid drainage, compared to 0.24 kg/m3 design capacity. As a consequence overall metal treatment exceeded design capacity per unit volume of the mine drainage. During the 50-day period, the overall plant availability averaged 94.5%, with the primary source of plant downtime being electrical outages caused by aging electrical switchgear. The availability of the Thiopaq plant was 98% in the same period. During its operation in 2002, the Stage 1 plant recovered nearly 35 tonnes of zinc concentrate containing copper, cadmium and lead. The concentrate was delivered and accepted for sale. Operation of the Thioteq plant upstream of the lime plant has resulted in significant lime savings and sludge production as well as concentrate sales to reduce water treatment costs. Sludge volumes produced in the lime plant were also reduced and, of long-term environmental significance, sludges contained none or only trace amounts of heavy metals. Another example of an installation under construction for effluent treatment is shown in Fig. 6. 5. Conclusions
Fig. 6. Thioteq installation under construction.
The Thioteq technology is a new alternative for hydroxide precipitation based on safe and on-site hydrogen sulphide production. Biological reduction of elemental sulphur or sulphate produces low cost sulphide reagent that can be used effectively in water treatment for both environmental control and metal recovery. Commercial applications in industrial plants have shown the technology to be safe and robust. High rate, engineered bioreactor systems offer many possibilities for application in mining and metallurgy.
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
Current and potential applications for the technology include the treatment of, and metal recovery from: ➢ Acid mine/rock drainage (AMD or ARD), ➢ Lime plant feed, ➢ Metallurgical bleed and process streams like those in smelters, refineries and metals processing plants and, ➢ PLS from (bio)leaching. The advantages of the technology that has been proven in several full-scale installations as an alternative to conventional treatment include the following: ➢ Better environmental control, producing lower heavy metal concentrations in plant discharge (metals can be removed below 0.1 mg/l), ➢ Metals recovered selectively for sale, offsetting treatment costs or allowing treatment at a profit, ➢ Metal contaminants recovered for removal offsite, eliminating sludge storage requirements and the associated long term liability, ➢ Compared to neutralisation, the metal sulphide volume is 6 to 10 times lower, ➢ Selective metal precipitation is possible, allowing for separate recovery of valuable metals like copper, nickel, cobalt and zinc from nuisance metals like arsenic and antimony.
113
References [1] Mulligan, C., in: Hartmans, S., Lens, P. (Eds.), 4th Int. Symp. on Environm. Biotechn., Noordwijkerhout, The Netherlands (2000), 201–204. [2] Farmer, G.H., Updegraff, D.M., Radehaus, P.M., Bates, E.R., Metal removal and sulfate reduction in low-sulfate mine drainage. in: Hinchee, R.E., Means, J.L., Burris, D.R. (Eds.), Bioremediation of inorganics, Batelle Press, Columbus, Ohio. ISBN 1-57477-011-X (1995), 17–24. [3] Maree, J.P., du Plessis, P., Water Sci. Technol., 29 (1994), 285–296. [4] Peters, R.W., Ku, Y., AIChE Symposium Series Separation of Heavy Metals, vol. 81 (1985), 9–27. [5] Bhattacharyya, D., Jumawan, A.B., Sun, G., Sund-Hagelberg, C., Schwitzgebel, K., in: Bennett, G.F. (Ed.), Water-1980, AIChE Symposium Series, American Institute of Chemical Engineering, New York, vol. 77 (1981), 31–38. [6] Beijerinck, M.W., Zentralbl. Bakteriol. Abt., 2(1) (1895), 49–59. [7] Widdel, F., Hansen, T.A., in: Balows, A., Trüper, H.G., Dworkin, M., Harder, W., Schleifer, K.-H. (Eds.), The Prokaryotes, 2nd. edn., Springer-Verlag, New York (1992), 583–624. [8] O'Sullivan, Aisling D., Murray, Declan A., Otte, Marinus L., Mine Water Environ., 23(2) (2004), 58–65. [9] Pfennig, N., Biebl, H., Arch. Microbiol., 110 (1976), 3–12. [10] Brugna, M., Nitschke, W., Toci, R., Bruschi, M., Giudiciorticoni, M.-T., J. Bacteriol., 181 (1999), 5505–5508. [11] Blumenthals, I.I., Itoh, M., Olson, G.J., Kelley, R.M., Appl. Environ. Microbiol., 56 (1990), 1225–1262. [12] Teder, A., Acta Chem. Scand., 25 (1971), 1722–1728.
Biologically produced sulphide for purification of process streams, effluent treatment and recovery of metals in the metal and mining industry Jacco L. Huisman ⁎, Gerard Schouten, Carl Schultz Paques B.V., P.O. Box 52, 8560 AB, Balk, The Netherlands Available online 15 May 2006
Abstract One of the best available technologies for the removal of metals from water is in the form of metal sulphides. Metal removal by sulphide precipitation is a well-known process that is characterised by compact residues and very high removal efficiencies. Compared to neutralisation alone the sludge volume is 6 to 10 times lower and the toxic metals are removed to a 0.01–1 ppm level. Furthermore, selective metal precipitation is possible, allowing for separate recovery of valuable metals like copper, nickel, cobalt and zinc from nuisance metals like arsenic and antimony. However, the cost of reagent (NaHS or H2S gas) and safety aspects are often prohibitive. This paper describes a novel biological process for safe and cost effective production of sulphide from elemental sulphur, waste sulphuric acid or sulphate present in effluents. With this technology, gaseous or dissolved H2S is produced on-site and on-demand in an engineered, high rate bioreactor. Experience with industrial applications at metal processing plants will be presented. The technology can serve to selectively recover metals from e.g. bleed streams, leach liquor, effluent streams and acid mine drainage. Lower overall costs and increased safety (no transport or storage of sulphide, production on-demand and at ambient pressure) are the main advantages of this new process compared to its alternatives. © 2006 Published by Elsevier B.V. Keywords: Biogenic sulphide production; Metal sulphide precipitation
1. Introduction 1.1. Origin and problems related to wastes in metal and mining industry The mining and metallurgical industry generate large volumes of waste- and process water that are polluted with dissolved heavy metals or sulphate, or both. For ⁎ Corresponding author. Tel.: +31 514 608 500; fax: +31 514 603 342. E-mail address: [email protected] (J.L. Huisman). 0304-386X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.hydromet.2006.03.017
instance, acid mine and rock drainage is currently one of the most widespread forms of pollution worldwide. In Canada alone, it is estimated that 12,500 ha of tailings and 750 million tonnes of waste rock are present that can release acid and toxic compounds [1]. Acid mine drainage can have a moderate (0.35–0.55 g/l [2]) to high (1.5–7.2 g/l [3]) sulphate concentration. Metals of particular interest in acid mine drainage and industrial wastewaters include copper, zinc, cadmium, arsenic, manganese, aluminium, lead, nickel, silver, mercury, chromium and iron, in a concentration that can range from 10−6 to 102 g/l. The composition of such a
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
wastewater reflects the particular combination of heavy metals originating from a metallurgical operations. This toxic leachate can cause severe aquatic habitat degradation downstream of the mine when discharged untreated. Slag from metallurgical processes poses a similar problem. Even though it often contains high concentrations of (potentially valuable) heavy metals, it is dumped in landfills or tailings ponds. Depending on the composition of the slag, these metals can be released as a result of weathering of the slag. Presently, removal by precipitation as metal hydroxide is the most widely used treatment method for water contaminated with heavy metals. This is because of the simplicity and the low costs of this method. For the same reasons, sulphate removal is mostly accomplished by precipitation with Ca2+, added as lime. However, more stringent legislation in future and an increasing scarcity of resources creates a need for heavy metal and sulphate removal technologies with a better performance. Thus, treatment processes should aim to recover valuable metals and other possible resources from waste streams such as sulphur compounds and process water. 1.2. Removal and recovery of metals as metal sulphides When it comes to sludge volume, reusability of the sludge and effluent quality, precipitation of metals with sulphide is superior to precipitation as hydroxides. It has many advantages over lime precipitation: ➢ High reactivity of sulphides with heavy metal ions and very low solubility of the resulting metal sulphides over a broad pH range resulting in lower effluent concentrations (see Fig. 1). ➢ Sulphide precipitation, unlike hydroxide precipitation, is relatively insensitive to the presence of complexes and most chelating agents. ➢ Sulphide removes chromates and dichromates without preliminary reduction of the chromium to the trivalent state. ➢ A high degree of selective metal precipitation is possible with sulphide, as opposed to hydroxide precipitation. ➢ Metal sulphide sludges generally are more dense and stable than metal hydroxide sludges [4], exhibiting better thickening and dewatering characteristics than the corresponding metal hydroxide sludge, which facilitates further processing. Previous objections against the use of sulphide, i.e. that it is toxic and corrosive, do not hold anymore because of the application of adequate safety measures
107
Cu(OH)2 Pb(OH)2
As2S3 Zn(OH)2 NiS ZnS PbS
Cd(OH)2
CdS Ni(OH)2 ZnS
NiS Ni(OH)2
CuS
PbS
Fig. 1. Comparison between the equilibrium concentrations of metal hydroxides and metal sulphides [5].
and the use of modern corrosion-resistant construction materials (plastics) eliminate these disadvantages. Sulphide precipitation would be the method of choice, if the high cost of transporting, storing or producing sulphide on-site (as NaSH or H2S) would not have hampered its widespread application in the metal and mining industry. In present applications for heavy metal removal with sulphide, the sulphide needed for precipitation is mostly obtained from chemical sources such as Na2S, NaHS, CaS, FeS and H2S. In the remainder of this paper, technology will be described to produce sulphide on-site and on-demand using biotechnology. This eliminates the hazards and costs that accompany the transport, handling and storage of chemical sulphides. 2. Biotechnological sulphate reduction The first clue that respiration could be an anaerobic way of life was obtained in 1895 by Beijerinck [6], who showed that sulphate could be reduced to sulphide in sediments. Although many bacteria can produce sulphide, only a few do so at a sufficient rate for application in high-rate processes. These rapid sulphide-generating bacteria are able to conserve energy by the reduction of sulphur oxyanions like sulphate, sulphite and thiosulphate [7], and they are generally termed sulphate-
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J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
reducing bacteria (SRB). A typical overall conversion equation is (neglecting the small amount of organic material required to produce biomass): þ − − SO2− 4 þ CH3 COOH þ 2H →HS þ 2HCO3 þ þ 3H
ð1Þ
Eight electrons are transferred from acetic acid to sulphate in order to produce sulphide. Especially the last decade, the use of SRB for treatment of process- and wastewaters containing sulphur compounds and metals in mining and metallurgical industries has become a topic of scientific and commercial interest. Biological treatment of waste- and process water from the mining and metallurgical industries offers an interesting alternative to conventional technologies. Biological treatment essentially consists of biological reduction of sulphur oxyanions to sulphide followed by chemical precipitation of metal sulphides. In this way dissolved metals and sulphate are concentrated into a solid. Compared to conventional chemical treatment with lime and hydroxide, the biological treatment can achieve much lower effluent sulphate (< 250 mg/l vs. ∼ 1500 mg/l) and metal (ppbvs. ppm-level) concentrations. This high rate technology has a constant and predictable effluent quality, This in contrast to for example constructed wetlands where the removal of metals can range between 0% and 99% [8]. The company Paques BV (www.paques.nl) has developed full-scale applications for effluent treatment with SRB: the Sulfateq® technology, which is part of the Thiopaq technologies. Paques BV is a company that specialises in the development, design and realisation of high rate biotechnological applications for industry. Technologies for recovery of metals and for conversion of a broad range of organic and inorganic compounds from water and gas streams are available. Worldwide,
over 500 industrial installations using Paques technologies have been constructed since 1982. Recovery of valuable base metals using biogenic H2S, combined removal of sulphate, nitrate and heavy metals, selenium removal and fluoride removal are examples of applications in the metal and mining industry. For several applications metal sulphide precipitation is the best solution, but sometimes the effluent or process water characteristics are such that biological sulphide production from sulphate present in the effluent is not possible. For example when the water properties like pH, temperature, salinity and overall composition are not compatible with the working range of the bacteria. An example is electrolyte bleed in copper refining. For these applications the Thioteq technology has been developed by Paques BV. 3. Thioteq technology: off line biogenic sulphide production The Paques Thioteq process consists of two stages: a chemical and a biological. A schematic flowsheet for the recovery of one metal as its metal sulphide is shown in Fig. 2. The water to be treated only passes through the chemical stage. Sulphide is produced in the biological stage and transported to the chemical (precipitation) stage with a carrier gas. 3.1. Biological stage Within the biological stage there are two major differences between the in-line application and an offline application. In the latter process: 1) The liquid and solids residence time are equally long, 2) the sulphur source is preferably elemental sulphur (or otherwise concentrated waste sulphuric acid). Lean gas Alkali source
Sulphur Reductant
Clarifier Bioreactor
Effluent
Contaminated water Contactor
Metal sulphide product Fig. 2. Thioteq water treatment and recovery of one metal product.
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
In the case that elemental sulphur is used as a sulphur source and acetic acid is used as electron donor, the conversion can be written as: 4S0 þ CH3 COOH þ 2H2 O→4H2 S þ 2CO2
ð2Þ
A typical bacterium that can be found in such a system is Desulphuromonas acetoxidans, which grows using the energy released by linking the oxidation of acetate to the reduction of elemental sulphur [9]. Accumulations of such cells have a characteristic pink colour. This bacterium is true sulphur-reducing, strictly anaerobic, gram negative, flagellated, and rod shaped. It acquires its energy from sulphur respiration and completely oxidizes acetate with S0 via the citric acid cycle to carbon dioxide and hydrogen sulphide [10]. The mechanism was partly clarified by [11]. The bioreactor is fed with powdered sulphur. For biological sulphur reduction the bacteria need not to be attached to elemental sulphur, in contrast with the biological sulphur oxidation. The mechanism occurs via polysulphides, which are formed when sulphide reacts with the S8-ring of elemental sulphur. The chemistry of elemental sulphur in aqueous sodium polysulphide solutions is well documented [12]. The initial ring-opening reaction:
S8 +
HS–
↔
S8S2–
+
H+
ð3Þ
is followed by rapid chain degradation and establishment of an equilibrium among polysulphide ions of different sizes, sulphide, and hydroxyl ions according to the following reaction [12]:
↔
ð4Þ
The polysulphide SnS2− is the reactive species for sulphur reduction. Acetic acid, ethanol or hydrogen are typical electron donors. But other organic compound can be used as long as they are available in a concentrated form and when in addition other start-up cultures are used. The electron donor requirement for sulphur reduction is only 25% of that of sulphate reduction, as can be seen by comparing Eq. (2) with Eq. (1). The off-line process of Fig. 2 can also be operated successfully with concentrated sulphuric acid. Clearly, the electron donor consumption is equal to that in Eq. (1). Concentrated sulphuric acid can be attractive when a waste stream with such acid must be processed. The biological process is carried out under ambient conditions, which makes it safe to operate. In addition the process is self-controlling; high sulphide concentra-
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tions become inhibitory to the bacteria that subsequently stop producing but are not killed. Therefore, a dangerous runaway situation is impossible. The bacteria become active again when the sulphide is withdrawn from the bioreactor by the precipitation process. Finally, the sulphide inventory in the bioreactor is mostly maintained as ionic–and therefore dissolved–HS− . Little H2S is therefore released in case of a leak. The sulphide is available on-demand within the capabilities of the bacteria as long as the sulphide demand is balanced by the addition of electron donor. 3.2. Chemical stage—contactor The flowsheet in Fig. 2 is a standalone plant with one metal precipitation circuit, although multiple contactor– clarifier units can be provided if more than one metal product is to be recovered as will be shown below. The chemical stage consists of a gas–liquid contactor. The sulphide is transported to the contactor with the help of a gas recycle with a carrier gas (a mixture of CO2 and N2) and metal-loaded water (e.g. acid mine drainage) is fed to the contactor. Metals like copper precipitate as a sulphide according to the following reaction: Cu2þ þ S2− →CuS
ð5Þ
The properties of the sulphide gas from the bioreactor and the contactor design results in metal sulphides with good settleability and filterability. The contactor off-gas, containing primarily N2, CO2 and water vapour is recycled to the bioreactor again. There is no significant gaseous discharge during normal operation. However, a typical plant is equipped with an absorber to clean the gas bleed. Copper can be precipitated as a sulphide usually without pH adjustment and without significant precipitation of other heavy metals present in the water. The result is a product with a high copper sulphide content usually greater than 90%. Other metals such as zinc and nickel can be recovered as separate high-grade sulphide products when the number of precipitation stages is increased. These metal sulphides can be shipped to smelters as high quality concentrates. A pH control using an alkali source might be required to meet the optimum precipitation conditions. Toxic metals, such as arsenic, antimony, cadmium, and lead, that typically occur in smaller amounts in mine drainage and other industrial effluents, are also removed and report to one of the sulphide products. The precipitated metal concentrates are recovered in a clarifier and then dewatered using a filter press to
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meet smelter feed moisture requirements. In some cases, plant effluent from the clarifier overflow can be directly discharged to the environment as shown in the flowsheet in Fig. 2. In other cases, the water can be recycled for process use, or might require further treatment before discharge, although secondary treatment requirements will be significantly reduced as will be explained in the examples further down. 3.3. Overall process It can be noted that the bioreactor is operated independently of the chemical stage, with none of the feed water passing through the bioreactor. The size of the bioreactor is based on the metal load that has to be precipitated and can therefore be significantly smaller than the components of the chemical (metal recovery) stage, which is sized based on the hydraulic loading. Sulphide produced by the Thioteq process has a significantly lower cost than chemical reagent sulphide such as NaHS, Na2S or H2S. The technology also has the advantage that the reagent is produced on-demand only. This results in more efficient utilisation and eliminates any safety concerns related to transportation and on-site storage. In addition, the turn-down ratio is down to 10%. At any time, the sulphide inventory at the site is very small and limited to the amount in the headspaces, solutions of the bioreactor and contents of the contactor. Therefore, the Thioteq technology can be regarded as a safe and economic alternative to common sources of hydrogen sulphide. 3.4. Integration of Thioteq technology and lime plants The Thioteq technology can be particularly effective when integrated with an existing lime treatment, as
shown in Fig. 3. The Thioteq process is situated upstream of the lime plant. In this example, a 2-stage metal precipitation and recovery circuit is used to produce separate concentrate products. In the example shown, copper and zinc products are recovered. If, however, metal recovery is a prime factor in project economics, metal loadings in the feed water and metal prices must be taken into account in determining the optimum flowsheet. In some cases it might be more economic to recover only one metal. In any case, the plant discharge from the final clarifier passes to the lime plant for final acid neutralisation and precipitation of iron and aluminium, if required. Removal of metals upstream of a lime plant can not only offset treatment costs through metal product sales but can also result in savings in lime and a reduction in both volume and toxicity of the lime plant sludge thereby reducing long-term liability of sludge storage. Reagent savings can be particularly significant if zinc or nickel are present in the mine water. Both these metals require a high pH for removal in a lime plant but can be recovered at neutral or slightly acidic pH as sulphides. This not only saves lime but also eliminates the need for acidification prior to discharge to meet the discharge standard for pH. Significant Mg concentrations in the water can also result in high reagent consumption in plants operated at high pH. Even if metal concentrations are too low to justify their recovery as saleable products, integration of the two treatment technologies can be potentially beneficial for environmental control by improving discharge water quality, reducing lime consumption and sludge volume, and improving the settling, filtration and chemical/physical characteristics of the sludge for disposal. Lime (reduced consumption)
Alkali source
Lean gas Clarifier
Clarifier
Clarifier
Contaminated Water
Contactor
Contactor H2S-rich gas Cu sulfide Product
Lime reactor Zn sulfide product
Sludge (Reduced volume and toxicity)
Sulphur Reductant
Bioreactor
Fig. 3. Thioteq process upstream of lime water treatment plant.
Effluent (high quality)
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
3.5. Integration of Thioteq technology with hydro metallurgical processes In addition to environmental benefits, removing metals from metallurgical process streams could have significant benefit for the efficiency (rates and recoveries) of the metallurgical process. Fig. 4 shows an example of how the Thioteq technology could be applied for this purpose. A conventional copper heap leach–solvent extraction–electrowinning circuit is shown, with the Thioteq process being used to treat a raffinate bleed stream. In addition to reducing the inventory of both economic and undesirable metals such as As, Cd, Pb, Zn, Ni, and Mn circulating in the leach solution, there is the potential for improved leach kinetics and metal recovery from the heap. Furthermore, acidity resulting from metal precipitation would be added back to the heap leach and reduce acid requirements from external sources. The same flowsheet principals could also be applied for heap decommissioning purposes by providing a means of recovering metals from solutions at the conclusion of heap and dump leach operations, if not for revenue then for the environmental benefit of removing metals from the site and avoiding long-term sludge storage and maintenance. Other potential applications include TDS reduction and solution control in metals processing, steel making, automobile manufacture and in power plants. In some cases, integration with the biological sulphate reduction process might be preferable due to the production of alkalinity in the process.
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4. Case study A zinc mine in North America stopped operating several years ago. However, the mine continues to operate a lime treatment plant to treat underground mine drainage that has an average flow of 700 m3/day. The mine also has a sizeable deposit of old tailings, stored separately by the previous operator. These tailings contain significant quantities of pyrite, zinc and copper. Over the years these tailings have become a source of acidity and soluble metals due to oxidation. A solution is required to retreat and/or reclaim the tailings. A treatment plant to remove metals from the mine drainage upstream of the existing lime plant was designed (see Fig. 5). This plant was constructed on schedule and budget and was based on the process flowsheet shown in Fig. 3 but with only one contactor, and incorporated a bioreactor, contactor and settling/ dewatering. The plant was started up in November 2001 and within 3 months the plant had reached the steady state operating capacity that was required to meet water treatment needs. The plant operated throughout 2002 recovering a saleable zinc concentrate, including the removal of copper, cadmium and lead from the wastewater prior to entering the existing lime treatment plant for iron and aluminium removal. The treated water was discharged to local receiving waters within the guidelines of existing permits. A planned shutdown of minewater collection and treatment took place during the winter. This was a one-time change in operating practice that resulted in simplified collection of the mine water for
Paques Thioteq Process Rich gas
Sulphur Nutrients
Lean gas
H+
Product 1
Product 2
Product 3
Bleed
Heap Leach Cu Cathodes
EW
SX
Fig. 4. Integration of biological sulphur reduction with copper heap leaching.
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J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113 Lime (reduced consumption: 495 instead of 650 ton/yr)
Lean gas Clarifier
Contaminated Water 700 pH 450 30 150 3,800
m3/day 3.7 mg/L Zn mg/L Cu mg/L Fe mg/L SO4
Contactor
Clarifier
Contactor
H2S-rich gas
Lime reactor
Zn/Cu sulfide Product
Sulphur Reductant
Sludge w. reduced volume and toxicity (31000 instead of 52000 m3/yr)
Effluent (high quality)
Bioreactor
Fig. 5. The flowsheet for the North American zinc mine with typical flows and concentrations.
delivery to the treatment plant and overall cost savings for mine water treatment in the longer term. 50 days prior to the shutdown in October, the plant feed averaged 660 mg/l Zn, 19 mg/l Cu and 337 Fe3+, with some peak concentrations well above the average for the period. These concentrations can be compared with the design values of 450 mg/l Zn, 30 mg/l Cu and 15 mg/l Fe3+. The plant, however, coped with the unpredictable and highly variable metal content in the acid water. Nevertheless, the metal recovery exceeded design expectations for copper, with consistent copper recovery to < 0.01 parts per million in the discharge. Zinc recovery was within design expectations from water that has consistently exceeded design parameters with respect to copper, zinc and iron content. Since the metal concentrations in the plant feed ranged from 1.5 to 2 times the design quantity, the plant was not able to treat the entire mine water flow at all times. At the same time, however, the rate of sulphide
generation in the bioreactor per unit volume of mine drainage exceeded design expectations, ranging from 0.26 to 0.43 kg of sulphide per cubic meter of acid drainage, compared to 0.24 kg/m3 design capacity. As a consequence overall metal treatment exceeded design capacity per unit volume of the mine drainage. During the 50-day period, the overall plant availability averaged 94.5%, with the primary source of plant downtime being electrical outages caused by aging electrical switchgear. The availability of the Thiopaq plant was 98% in the same period. During its operation in 2002, the Stage 1 plant recovered nearly 35 tonnes of zinc concentrate containing copper, cadmium and lead. The concentrate was delivered and accepted for sale. Operation of the Thioteq plant upstream of the lime plant has resulted in significant lime savings and sludge production as well as concentrate sales to reduce water treatment costs. Sludge volumes produced in the lime plant were also reduced and, of long-term environmental significance, sludges contained none or only trace amounts of heavy metals. Another example of an installation under construction for effluent treatment is shown in Fig. 6. 5. Conclusions
Fig. 6. Thioteq installation under construction.
The Thioteq technology is a new alternative for hydroxide precipitation based on safe and on-site hydrogen sulphide production. Biological reduction of elemental sulphur or sulphate produces low cost sulphide reagent that can be used effectively in water treatment for both environmental control and metal recovery. Commercial applications in industrial plants have shown the technology to be safe and robust. High rate, engineered bioreactor systems offer many possibilities for application in mining and metallurgy.
J.L. Huisman et al. / Hydrometallurgy 83 (2006) 106–113
Current and potential applications for the technology include the treatment of, and metal recovery from: ➢ Acid mine/rock drainage (AMD or ARD), ➢ Lime plant feed, ➢ Metallurgical bleed and process streams like those in smelters, refineries and metals processing plants and, ➢ PLS from (bio)leaching. The advantages of the technology that has been proven in several full-scale installations as an alternative to conventional treatment include the following: ➢ Better environmental control, producing lower heavy metal concentrations in plant discharge (metals can be removed below 0.1 mg/l), ➢ Metals recovered selectively for sale, offsetting treatment costs or allowing treatment at a profit, ➢ Metal contaminants recovered for removal offsite, eliminating sludge storage requirements and the associated long term liability, ➢ Compared to neutralisation, the metal sulphide volume is 6 to 10 times lower, ➢ Selective metal precipitation is possible, allowing for separate recovery of valuable metals like copper, nickel, cobalt and zinc from nuisance metals like arsenic and antimony.
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References [1] Mulligan, C., in: Hartmans, S., Lens, P. (Eds.), 4th Int. Symp. on Environm. Biotechn., Noordwijkerhout, The Netherlands (2000), 201–204. [2] Farmer, G.H., Updegraff, D.M., Radehaus, P.M., Bates, E.R., Metal removal and sulfate reduction in low-sulfate mine drainage. in: Hinchee, R.E., Means, J.L., Burris, D.R. (Eds.), Bioremediation of inorganics, Batelle Press, Columbus, Ohio. ISBN 1-57477-011-X (1995), 17–24. [3] Maree, J.P., du Plessis, P., Water Sci. Technol., 29 (1994), 285–296. [4] Peters, R.W., Ku, Y., AIChE Symposium Series Separation of Heavy Metals, vol. 81 (1985), 9–27. [5] Bhattacharyya, D., Jumawan, A.B., Sun, G., Sund-Hagelberg, C., Schwitzgebel, K., in: Bennett, G.F. (Ed.), Water-1980, AIChE Symposium Series, American Institute of Chemical Engineering, New York, vol. 77 (1981), 31–38. [6] Beijerinck, M.W., Zentralbl. Bakteriol. Abt., 2(1) (1895), 49–59. [7] Widdel, F., Hansen, T.A., in: Balows, A., Trüper, H.G., Dworkin, M., Harder, W., Schleifer, K.-H. (Eds.), The Prokaryotes, 2nd. edn., Springer-Verlag, New York (1992), 583–624. [8] O'Sullivan, Aisling D., Murray, Declan A., Otte, Marinus L., Mine Water Environ., 23(2) (2004), 58–65. [9] Pfennig, N., Biebl, H., Arch. Microbiol., 110 (1976), 3–12. [10] Brugna, M., Nitschke, W., Toci, R., Bruschi, M., Giudiciorticoni, M.-T., J. Bacteriol., 181 (1999), 5505–5508. [11] Blumenthals, I.I., Itoh, M., Olson, G.J., Kelley, R.M., Appl. Environ. Microbiol., 56 (1990), 1225–1262. [12] Teder, A., Acta Chem. Scand., 25 (1971), 1722–1728.