SonobioleachingBlack shaleMicroorganismMinerals dissolution

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. . . .2.1. Shale . . . . . . . . . . . . . .

Hydrometallurgy 117118 (2012) 112

Contents lists available at SciVerse ScienceDirect

Hydrometallurgy2.1.3. Metallogeny of black shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.4. Weathering of black shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Techniques for extraction of metals: extraction metallurgy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.1. Pyrometallurgy techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2. Hydrometallurgy techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.3. Biohydrometallurgy techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.4. Microbial weathering/bioleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.4.1. Autotrophic (chemolithotrophic) leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.4.2. Heterotrophic (chemoorganotrophic) leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.4.3. Mechanism of bioleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.4.4. Bioleaching of metals by autotrophs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.4.5. Bioleaching of metals by h3.4.6. Bioleaching of metals from

3.5. Sonobioleaching . . . . . . . . .3.5.1. Sonobioleaching of black

Corresponding author. Tel.: +92 3346575057; fax:E-mail addresses: foziaanjum2008@yahoo.com (F. A

0304-386X/$ see front matter 2012 Elsevier B.V. Aldoi:10.1016/j.hydromet.2012.01.007. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3try of black shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.1. Types of shales . . . . .2.1.2. Evolution and geochemisContents

1. Introduction . . . . . . . . . .2. Types of ore . . . . . . . . . .most promising and revolutionary biotechnologies. The products of such processes are dissolved in aqueous so-lution, thereby rendering them more amenable to containment, treatment and recovery. On top of this, biohy-drometallurgy can be conducted under mild conditions, usually without the use of any toxic chemicals.Consequently, the application of biohydrometallurgy in the recovery of metals from lean grade ores and wasteshas made it an eco-friendly technology for enhanced metal production. This paper reviews the current status ofbiohydrometallurgy of low grade ores around the world. Particular attention is focused on the bioleaching ofblack shale ore and its metallogenic diversity in the world. The review assesses the status of bioprocesssing ofmetals to evaluate promising developments. Bioleaching of metals is comprehensively reviewed with the em-phasis on the contribution ofmicrobial community, especially fungal bioleaching coupled with ultrasound treat-ment. In this manuscript, the principles of bioleaching, their mechanisms, and commercial applications arepresented. The case studies and future technology directions are also reviewed.

2012 Elsevier B.V. All rights reserved.

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Keywords:BioleachingBiohydrometallurgy, which exploits microbiological processes to recover metal ions, is regarded as one of the

secondary environmental poAccepted 28 January 2012Available online 15 February 2012

tional techniques such as pyReview

Biohydrometallurgy techniques of low grade ores: A review on black shale

Fozia Anjum a,, Muhammad Shahid b, Ata Akcil c

a Department of Chemistry, Government College University, Faisalabad-38000, Pakistanb Department of Chemistry and Biochemistry, University of Agriculture, Faisalabad 38040, Pakistanc Department of Mining Engineering, Mineral Processing Division (MineralMetal Recovery and Recycling Research Group), Suleyman Demirel University, TR32260, Isparta, Turkey

a b s t r a c ta r t i c l e i n f o

Article history:Received 12 April 2011Received in revised form 27 January 2012

The demand for metals is ever increasing with the advancement of the industrialized world. On the other hand,worldwide reserves of high grade ores are close to depletion. However, there exists a large reserve of metals inlow and lean grade ores and other secondary sources.Metal recovery from low and lean grade ores using conven-

rometallurgy, etc. requires high energy and capital inputs which often result in thellution. Thus, there is a need to utilize more efcient technologies to recover metals.

j ourna l homepage: www.e lsev ie r .com/ locate /hydrometeterotrophs: a literature survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6black shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

+92 41 9200764.njum), ataakcil@sdu.edu.tr (A. Akcil).

l rights reserved.

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There is growing concern throughout the world that the heavymetal contents of soils are increasing as a result of industrial, mining,

kidneys, etc. (Singh et al., 2010). Therefore, reliable remediation tech-niques are required to clean up such sites.

Major compounds Average Major Average

2 F. Anjum et al. / Hydrometallurgy 117118 (2012) 112agricultural and domestic activities. Unlike many other pollutants,heavy metals are difcult to remove from the environment. Thesemetals cannot be chemically or biologically degraded and are ulti-mately indestructible (Anjum et al., 2010a). Besides, their signicantrole in technology progression, their natural abundance in the envi-ronment sometimes causes damage to the environment. The poison-ous effects of heavy metals result mainly from the interaction ofmetals with proteins in the form of enzymes and inhibition of meta-bolic processes. When heavy metals accumulated in the soil, theyare toxic for plants, animals, humans and even to aquatic life. For in-

(%) elements (mg/kg)

SiO2 55.2 S 260.0Al2O3 15.3 Cr 100.0Fe2O3 2.8 Mn 950.0FeO 5.8 Cu 55.0MnO 0.2 Zn 70.0MgO 5.2 As 1.8CaO 8.8 Ag 0.07Na2O 2.9 Cd 0.2K2O 1.9 Ba 425.0TiO2 1.6 Hg 0.08P2O5 0.3 Pb 13.03.6. Leaching techniques . . . . . . . . . . . . . . . . . . . .3.6.1. Laboratory-scale (010 dm3) . . . . . . . . . . . .3.6.2. Pilot-plant (b10 m3) . . . . . . . . . . . . . . .3.6.3. Commercial-scale (>10 m3) . . . . . . . . . . . .

3.7. Commercial applications of bioleaching . . . . . . . . . . .3.8. Future prospects . . . . . . . . . . . . . . . . . . . . . .

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Global ore deposits are fast being depleted due to rapid industriali-zation and demand. However, this difculty can be overcome by utiliz-ing huge reserves of low and lean grade ores (Rawlings, 2004). Therecovery of metals from these ores, shale and schist, etc., using conven-tional technologies, is not only very expensive due to capital invest-ments and less favorable economics, but also increasing theenvironmental costs due to high levels of pollution (Mishra et al.,2004; Pradhan et al., 2006). Biohydrometallurgical methods for proces-sing and remediation are less energy intensive and eco-friendly and noemission of any harmful gas or chemical in the environment than anyother conventional chemical methods (Akcil, 2004; Anjum et al.,2010a; Brierley, 2010; D'Hugues and Spolaore, 2008; Rawlings, 2004).

Bioleaching is a specialized biohydrometallurgical process. In thisprocess either metabolic activities or products of microorganismsare involved. This process is based on the ability of micro-organisms(bacteria and fungi) to transform solid compounds into soluble andextractable elements, which can be recovered (Akcil and Deveci,2010; Ehrlich, 2004). Most biohydrometallurgical innovations havebeen commercially implemented during times of low metal pricessince the 1950s with the advent of copper bioleaching at the Kenne-cott Copper Bingham Mine (Brierley and Brierley, 2001).

Metal prices, of course, are not the only motivating factor foremploying biohydrometallurgical processes (Brierley, 2008, 2010)but also of cost production (Gericke et al., 2009; Moore, 2008) anddifculty of exploitation of ore deposits by conventional processes(Brierley, 2010). Ore deposits with complex mineralogy are difcultto treat and biohydrometallurgical techniques may be a viable alter-native. The bioleaching allows the recycling of metals from lowgrade ores, industrial wastes like electronic scrap, and used catalystsand clean up of sites by a process closely mimicking natural biochem-ical cycles. This approach reduces the demand for resources such asores, energy or landll space (Bayraktar, 2005; Xu and Ting, 2009).As a result, metal production can be met more often from lowergrade or complex ores, mines and industrial wastes (Rawlings, 2004).

Actually metals are not usually encountered in elementary form.The earth's crust is composed entirely of oxides of metals and non-metals (Table 1.1). This Table 1.1 shows the average abundance ofthe elements present in the earth's crust that are most importantfrom an environmental point of view (Peralta, 1997).stance, cadmium causes lungs cancer whereas lead causes cancer inIn contrast to organic pollutants, metals are not mineralized bymicro-organisms but can be oxidized or reduced, i.e. transformed todifferent redox stages or complexed by organic metabolites (Ren etal., 2009). Owing to the ever increasing demand for metals from lim-ited sources, the sustainable development approach seems a feasiblesolution. To understand sustainable development for metals produc-tion, a brief background of metal resources is necessary.

Generally, metal resources can be classied into two main groups:

a) Naturally occurring resources, providing the majority of metals forindustries

b) Second hand resources e.g. industrial wastes and used materials.

An ore contains metals in the form of minerals or an aggregate ofminerals, as well as gangue minerals. Gangue is undesired mineralswhich are associated with ore and are mostly non-metallic in nature(Kojima, 2002).

2. Types of ore

Ores are divided into:

i) High grade ores, i.e. the ones in which metal concentration is rela-tively high.

ii) Low grade ores, are those with a low concentration of metals, likeshale and schist etc. (Chow et al., 2010).

2.1. Shale

Shales are sedimentary rocks formed during the latter part of theCambrian era and up to the rst part of the Ordovician era, approxi-mately 540480 Ma ago (Falk et al., 2006). Pakistan has huge reservesof shale deposits formed by the consolidation of beds of mud, clay orsilt (Yu et al., 2009). Their color is commonly some tone of gray,brown, green or black due to the presence of some organic pigmentsin microbes inhabiting the shales. These are composed chiey of kao-lin, mica etc. but are too ne grained to permit the cognition of theirmineral constituents by the eye alone. The introduction of quartz

Table 1.1Mineral composition of earth crust.

and dolomites with varying contents of organic matter as high as

3F. Anjum et al. / Hydrometallurgy 117118 (2012) 11250% in some very high grade deposits. In most cases the organicmatter varies between 5 and 25%. Organic matter is present incombination with high contents of oil and other volatile compo-nents with no free hydrocarbons which indicated that oil shalesare immature sources of oil (Dyni, 2006).

ii) Black shale: These shales contain relatively lower amounts of or-ganic material than the oil shale. Its black color is due to organicmatter of algae, bacteria and other life forms that lived in the seaat that time. It can be considered as discarded ore used for build-ing purposes for the manufacturing of cement, fertilizer and as aplant stimulant (Zhao et al., 2009). The black shale ores varyfrom others in mineralogical as well as chemical properties andin recovery of metals. These are mainly portrayed by copper con-tents not more than 5.5% and other metals like silver (0.01%).Roughly 3 to10 time greater metals contents are present in bitu-minous shale ore than carbonate and sandstone forms(Luszczkiewicz, 2000). The former is therefore considered as anatural polymetallic concentrate. Some metals in the shale oreare present as bituminous organometallic compounds, like por-phyrins in shales, and hence it reduces the metal recovery byusing classical methods of ore enrichment. Afnity betweenmetal ions and organic substances like kerogen could also xmetals in to black shale sediments (Hai et al., 2005; Steiner etal., 2001). Black shale is sometimes known as alum shale, whichis mainly composed of clay minerals such as illite and montmoril-lonite, smectite and chlorite in combination with ne particles ofquartz and mica (Bustin, 2005). On a global scale, there are wide-spread occurrence of black shales and interbedded cherts. Theseshales can play an important role in the economy of several coun-tries (Anjum et al., 2009a; Mao et al., 2002).

2.1.2. Evolution and geochemistry of black shaleThe composition of the earth's atmosphere is maintained by bio-

logical activities operating on an enormous scale. This results in theproduction of several atmospheric gasses, which are linked directlyto the interaction of biological as well as chemical processes actingworldwide (Arzani, 2004). Biodegradation of organic matter accumu-lated in the earliest sedimentary rocks, results in the release of oxy-gen to the atmosphere. Many lacustrine, deltaic and projected typesof sediment contain considerable concentrations of organic carbon(Perkins et al., 2008). After lithication under increased pressureand temperature, these become the organic carbon rich shales,mudrocks and marls that can be a signicant source of fossil fuel.Among these, black shale is a ne grained laminated sedimentaryrock rich in organic matter that also contain chemically distinctredox sensitive metals (Piper et al., 2007) and sulde minerals(FeS2). Other heavy metals such as Cd, Cu, Pb and Zn may also precip-caused an increase in the size of their grains and transform them intosandstones. Shale deposits containing the elements Fe, K, Si and Al aremost susceptible to microbial transformation, making a study of thesedeposits both economically and environmentally important (Harris,2005; Piper and Calvert, 2009).

2.1.1. Types of shalesThere are mainly two types of shales on the basis of organic matter:

i) Oil shale: Sedimentary rocks containing up to 50% organic matteralong with a considerable amount of oil, can be processed to pro-duce oil and other chemicals and minerals. Oil shale contains sig-nicant amounts of petroleum like oil and rened products likegasoline, fuel oil and many other products due to the presence oforganic material like kerogen (Ketris and Yudovich, 2009). Oilshales include organic rich shales, marls, and clayey limestonesitate as solid sulde in the sediments (Falk et al., 2006). Moaccumulation was noted under less than anoxic conditions(McManus et al., 2006; Tribovillard et al., 2008).

Minerals present in black shale form enormous sulde units posi-tioned very close to the stratographic contact between middle toupper Devonian strata made up of silicious shale, mudstone, phos-phatic chert, and spheroidal limestone. Black shale is mainly com-posed of loams, acidic minerals, organic compounds and detritalminerals (Luszczkiewicz, 2000), whereas some organic matter is pre-sent in the form of laminas, small inclusions, loamy interlayers andorganic residues. Organic matter contents vary from 1% to 30% withan average of 6%. However, the contents of bitumen range from 1%to 11% (Ketris and Yudovich, 2009). These shales can be consideredas natural polymetallic ore deposits in the form of bituminous organ-ometallic compounds such as porphyrins that makes the availabilityof metals from these ores a laborious assignment when by using clas-sical methods of ore enrichment (Hai et al., 2005).

2.1.3. Metallogeny of black shaleDuring evolutionary decay, organic-rich mud (black shale) be-

comes a reservoir for number of trace elements and sulfurous com-pounds like arsenopyrite (FeAsS), pyrite (FeS2), chalcopyrite(CuFeS2), (Cu2S, CuS, CuS2), Ni-sulde (NiFe)xS, (NiFe)S2, galena(PbS), sphelrite (ZnFe)S, and pyrrhotite (Fe1-xS). The mineralogy ofblack shales changes accordingly to metamorphic grades of the sedi-ments (Riekkola-Vanhanen, 2005; Tuttle and Breit, 2009). Theseshales are differentiated by the presence of Fe, Ni, As, Mo, Cu, andZn suldes, and consist of pyrite (46%), vaesite (NiS210%), amor-phous type pyrite (2%), sphalerite and wurtzite (2%), NiAsFe-sul-des (gersdorfte), CuSb (tetrahedrite) and Mo-sulde (jordisite),amorphous silica, intergrown bitumen (1%) and abundant organicmatter. Black shale hosted deposits contained base, trace and noblemetals of interest. Base metals occur in sulde minerals, while noblemetals (Au, Ag and platinum group elements (PGE)) are considerablyenriched in the organic matter fraction. Galindo et al. (2007) reportedthe presence of calcite, dolomite, quartz, pyrite and clays along withradionuclides of U and Th in Moroccan black shale. Paja Formationshales consist of quartz, mica, illite, chlorite, mixed layer illite, smec-tite, rectorite, pyrophyllite and brushite-like minerals (Alvarez andRoser, 2007; Anjum et al., 2010a). Zhi-Yong (2006) detected mica, ka-olin, free oxide, taltalite and garnet in black shale. Worldwide de-posits of black shale ores generally contain a considerable amountof base, heavy, rare and precious metals (Anjum et al., 2010a;Galindo et al., 2007; Li et al., 2009, 2010a, 2010b; Orberger et al.,2003). Black shale ores are typically multimetal ores with a differentproportion of suldic components. Metalliferous black shales are or-ganic rich rocks that are commonly enriched in metallic elementslike Mo, Ni, Si, P, S, Al, Fe, Co, Cu, Pb, U, Mn and As, localized in specichorizons (Mooiman et al., 2005; Polgary et al., 2006; Zhao et al.,2009). Some black shales contain economically signicant concentra-tions of metals such as Mo, Ni, PGE, Cu, Co, Pb, Zn, and Se, As, Hg, Sb,Ag, Au and U (Jiang et al., 2007; Kribek et al., 2007; Orberger et al.,2005; Pan et al., 2005). Utica black shale contains considerableamounts of rare earths and precious metals (Alvarez and Roser,2007; Chakrabarti et al., 2007). Average concentrations of rare earthsand precious metals in the black shales are much higher than thosefor typical shales and mudstones in the crust (Yu et al., 2009). Someenriched elements in the shales, such as Re, Mo, Cd, and U, areredox-sensitive (Nameroff et al., 2002).

2.1.4. Weathering of black shaleDuring weathering of black shales, a highly reducing environment

rich in organic carbon and sulde minerals is brought in close contactwith the strongly oxygenated conditions of the earth's surface. Organ-ic matter and sulde minerals in the rock become oxidized, consum-ing oxygen and liberating inorganic carbon and sulfates and generates

acidity, lowering the pH of water contained within the weathered

in their experiments as reported by Ehrlich (2004). Kennecott CopperCorporation patented copper bioleaching for the rst time in the1950s (Brierley and Brierley, 2001). Today, biohydrometallurgy is an in-terdisciplinary promising technology used for obtaining valuable metalcompounds from low grade ores and for detoxication of industrialwaste products. Moreover, there has been an increase in companies ap-plying bioleaching techniques for metal recovery. Many reports de-scribed the recovery of precious metals from industrial wastes,sewage sludge, soil, coal as well as fossil fuels (Akcil, 2004; Garcia-Balboa et al., 2010; Rawlings, 2004).

3.4. Microbial weathering/bioleaching

4 F. Anjum et al. / Hydrometallurgy 117118 (2012) 112shale, dissolving and mobilizing some mineral phases and contribut-ing to increased rock permeability (Tuttle et al., 2003, 2009). If micro-organisms are present during black shale weathering, their activitymay hold signicant implications for understanding control on chem-ical reactions of organic matter occurring during shale weathering(Burford et al., 2003). In natural systems, degradation of organic mat-ter (OM) from coal or shales results in the release of inorganic com-pounds like heavy metals. These heavy metals enter theenvironment either as inorganic salts or organic heavy metal com-pounds, thus increasing the bioavailability of metals (Burba et al.,2000). Specic chemicals and the microbiological role in weatheringof black shales have been evaluated in recent years (Petsch et al.,2003). Recent laboratory, bench and pilot scale experiments haveshown the amenability of the black shale to microbial weatheringwith high metal recoveries (Riekkola-Vanhanen, 2005). These metalsare extracted from ore by a variety of techniques known as extractionmetallurgy.

3. Techniques for extraction of metals: extraction metallurgy

Extractive metallurgy or mining is the extraction of metals fromraw materials by physical and chemical processes involving manipu-lation of their properties in bulk as well as at the atomic level. Gener-ally, the rst step is the separation of undesired minerals or ganguefrom the ore, followed by ore dressing which involves disintegra-tion to small sizes, which makes separation of different kinds of min-erals possible. The different techniques that can be utilized during theextraction process are given below (Ojeda et al., 2009).

3.1. Pyrometallurgy techniques

Pyrometallurgy techniques involve high temperature heating pro-cesses. The ore is heated in the presence of a heating agent in a fur-nace, which results in the production of molten metal and slagalong with impurities which is separated from the metal at a selectedtemperature. Pyrometallurgical techniques involve preliminary treat-ment such as roasting, calcinations, liquidication and smelting.Smelting is a pyrometallurgical process of electrorenning of suldeconcentrates and it one of the biggest pyrometallurgical applications(Lam, et al., 2010; Ojeda et al., 2009; Pesic, 2007).

3.2. Hydrometallurgy techniques

Hydrometallurgy techniques involve aqueous solutions as well asinorganic solvents to achieve the desired outcome by electrowinningoperations. These techniques are utilized when a high purity of me-tallic product is required with less environmental hazards (Gerickeet al., 2009; Mooiman et al., 2005).

3.3. Biohydrometallurgy techniques

Biohydrometallurgy techniques involve microbes in metal extractionby electrowinning operations e.g. bioleaching, biosorption, biobeneca-tion etc. Biohydrometallurgy has originated about 2000 years prior todetection of microorganisms (Rossi, 1990). Hallberg and Rickard(1973) rst reported the microbial recovery of copper from the copperdeposit of the Falun Mine in central Sweden since 1687. Rossi (1990)cited in his book Biohydrometallurgy that copper was microbially re-covered from copper deposits in Rio Tinto in Spain in 1752 on a com-mercial scale. Reports of bacterial leaching of metals from acid minedrainage appeared in the 1950's (Colmer and Hinkle, 1950). Its iron ox-idizing physiology was studied in more detail by Temple and Colmer(1951), who named the organism Thiobacillus (later renamed Acidithio-bacillus by Kelly andWood (2000)). Leathen et al. (1956) isolated an ac-idophilic, autotrophic iron oxidizer, which they named Ferrobacillus

ferrooxidans. Unlike T. ferrooxidans, it did not oxidize elemental sulfurMicrobial leaching is the mining of metals from their ores usingmicrobes. Microbial technology offers an economic substitute forthe mining industry, at a time when high grade mineral resourcesare being exhausted (Akcil, 2004; Mulligan et al., 2004). In general,bioleaching is a process described as dissolution of metals fromtheir mineral sources by certain naturally inhabiting microbes orthe use of microorganisms to transform elements so that the ele-ments can be extracted from a material when water is lteredthrough it (Ndlovu, 2008; Olson et al., 2003; Rawlings et al., 2003).There are two types of microbial leaching on the basis of microbeswhich are known: chemolithoautotrophic leaching and heterotrophicleaching (Gadd, 2004).

3.4.1. Autotrophic (chemolithotrophic) leachingMost autotrophic leaching is carried out by chemolithotrophic, ac-

idophilic bacteriawhich x carbon dioxide and obtain energy from theoxidation of ferrous iron or reduced sulfur compounds. These meta-bolic processes yield Fe(III) or H2SO4 as the respective endproducts.

The microorganisms involved in autotrotrophic leaching includesulfur-oxidizing bacteria, e.g. Thiobacillus thiooxidans, iron- and sul-fur-oxidizing bacteria, e.g. Thiobacillus ferrooxidans and iron-oxidizing bacteria, e.g. Leptospirillum ferrooxidans. As a result of sulfurand iron-oxidation by these bacteria, metal suldes are solubilizedand the pH of their immediate environment is decreased, which en-hances the solubilization of other metal compounds. The autotrophicleaching of metal suldes by Thiobacillus species and other acidophilicbacteria is well established for use in industrial scale biomining pro-cesses of fossil fuels that originated from ores, shales and schist etc.(Anjum et al., 2009a; Gadd, 2001; Mishra et al., 2008) Table 3.4I.

3.4.2. Heterotrophic (chemoorganotrophic) leachingHeterotrophic leaching is a different form of microbial leaching in

which the organisms need organic carbon sources to survive the leach-ing processes. The metabolites excreted are the result of metabolism ofthe organic carbon source, in which the ore takes no part. The metabo-lites happen to consist of organic acids which can leach certain ores, i.e.a completely indirect mechanism with no interaction between organ-isms and ore. This type of leaching is suitable for minerals that are lowin sulfur and suldes, and which therefore do not offer an adequate

Table 3.4ISome chemolithotrophic bacteria with biohydrometallurgy potential.

Organism Ore minerals References

Thiobacillus ferrooxidans Chalcopyrite concentrate Nakazawa et al. (1998)Pyrite concentrate Fowler et al. (2001)Covellite concentrate(CuS)

Curutchet and Donati(2000)

Heazelwoodite (Ni3S2) Giaveno and Donati (2001)T. thiooxidans Pyrrhotite Veglio et al. (1998)T. caldus Arsenopyrite Dopson and Lindstrom (1999)L. ferrooxidans Pyrite concentrate Boon and Heijnen (1998)Sulfobacillusthermosuldooxidans

Pyrite and Arsenopyriteconcentrate

Konishi et al. (1999)

sulfur source for sulfuric acid production by autotrophic acidophiles.Heterotrophic organisms can produce organic acids which are very

plexes and chelates. The most important species of fungi are Aspergillussp. and Penicillium sp. because of their ability to excrete abundant con-

I. Contact microbial leaching: Microorganisms have direct contact witha specic part of the mineral surface, but not to the whole mineralsurface. Thus, metal solubilization is due to electrochemical interac-tion. The adsorption of cells to suspended mineral particles takesplace within some minutes or hours (Akcil and Deveci, 2010). Inthisway,microorganismdevelopment and the heavymetal leachingtakes place together. This technique is easy to perform, but the mi-crobial metabolism and growth can be negatively affected by dis-solved metal ions which then limit the bioleaching processefciency. (Brocht et al., 2004; Conner, 2005; Leahy et al., 2005;Sadowski, 2005).

II. Non contact microbial leaching: This mechanism involves the gen-eration of organic and inorganic acids by fungal and bacterialstrains. Bioleaching is carried out in two stages. In the rst one,

Table 3.4IISome heterotrophic microbes with biohydrometallurgy potential.

Organism Ore minerals Refs

Aspergillussp.+Penicillium sp.

Low-grade nickelcobaltoxide ores

Agatzini and Tzeferis (1997)

Low-grade Laterite ores Valix et al. (2001)Aluminosilicate(95% spodumene)

Rezza et al. (2001)

Aspergillus niger Zinc and nickel silicates Castro et al. (2000)Rhodotorula rubra Aluminosilicate

(95% spodumene)Rezza et al. (2001)

5F. Anjum et al. / Hydrometallurgy 117118 (2012) 112centration of organic acids for bioleaching of metals from ores (Anjumet al., 2009b; Rezza et al., 2001). Generally, fungi canworkwell betweenpH range of 28, and in a temperature range of 2040 C. Fungi have rel-atively high tolerability to heavy metals, therefore they have been uti-lized in the past for the leaching of carbonaceous low grade ores andmining wastes. Degradation of persistent carbon sources, such as char-coal and black shale, is accelerated by fungal activity which results inthe liberation of inorganic minerals and metals (Bhatti et al., 2011;Wengel et al., 2006) (Table 3.4II).

3.4.3. Mechanism of bioleaching

3.4.3.1. Bacterial leaching. A generalized reaction can be used to de-scribe the biological oxidation of a mineral sulde involved in leaching:

MS 2O2MSO4 1

where M is the bivalent metal that get solubilized from the system bythe action ofmicrobialmetabolites. Until now, the bioleaching/biobene-ciation of ores includes the following two types of mechanism in mi-crobial metal solubilization of sulde minerals (Gadd, 2004).mildly acidic which could be suitable for solubilizingmetals of econom-ic value in the pH range between 4 and 6, while keeping ferric iron pre-cipitated and can therefore yield a leach solution essentially free of iron.They could also produce non-acidic complexing agents as metabolitesfrom the catabolism of proteins which could be suitable use in alkalineleaching systems like ammonia leaching (Anjum et al., 2010a; Gadd,2004; Johnson, 1998). Heterotrophic microorganisms with leaching ac-tivity are mostly lamentous fungi and bacteria. Metal leaching gener-ally involves an indirect as well as direct process with microbialproduction of organic acids, amino acids and other metabolites. Thesemetabolites dissolve metals by displacement of metal ions from theore matrix by hydrogen ions or by the formation of soluble metal com-Fig. 3.4.3.1. Contact and non-contact mthe microorganisms are allowed to grow in an adequate mediumunder appropriate cultural conditions to produce active metabo-lites for the leaching process. Afterwards, the spent culture medi-um under aggressive leaching conditions (low pH, hightemperature, etc.) is used in the second stage as a leaching agentfor the mineral in the absence of growing microorganisms. There-fore it increases the metal's removal. In many cases the directmechanism is favored over the indirect mechanism due to the di-rect physical contact of microbes with the mineral's surface(Mulligan et al., 2004; Sadowski, 2005) (Fig. 3.4.1a).

3.4.3.2. Fungal leaching. Fungal leaching of metals from minerals ismainly based on:

I. Acidolysis: Protonation of oxygen atoms occurs around the sur-face of metallic compounds. The protons and oxygen associat-ed with water displace the metal from the surface (Johnson,2006; Mulligan et al., 2004; Xu and Ting, 2009).

II. Complexolysis: Metal complex formation results in the solubili-zation of the metal ion e.g. complex of oxalic acid and iron. Fur-thermore, this process often reduces the toxicity of heavymetals toward fungi (Johnson, 2006).

III. Redoxolysis: Reduction of metal ions in an acidic environment,such as reduction of ferric iron and manganese under the inu-ence of oxalic acid (Brombacher et al., 1997). A series of organicacids are formed by fungal metabolism resulting in organic acid-olysis, complex and chelate formation (Mulligan et al., 2004).

Generally, fungi need a large amount of energy for their microbialactivities. Fungal leaching is somewhat slower than bacterial leach-ing, but in spite of this, fungal leaching is better when the materialcontains a large amount of organic matter, such as in black shale(Anjum et al., 2009b, 2010a, 2010b). Fungal leaching results in theformation of soluble complexes with metal ions in neutral environ-ments or is less toxic, which is another advantage of the presence offungal leaching agents (Deepatana et al., 2006). The metabolic pro-cess of fungi is parallel to those of higher plants with the exceptionechanisms (Grundwell et al., 2000).

also developed a more feasible and economical method than chemicalleaching for the biorecovery of Cu, Zn, Ni and Fe up to 68, 46, 34 and

6 F. Anjum et al. / Hydrometallurgy 117118 (2012) 112of carbohydrate synthesis. Organic acids are produced from glucose ina glycolytic pathway by fungi (Nalini and Sharma, 2004). Most activeleaching fungi are from the genera Penicillium and Aspergillus (Anjumet al., 2010a, 2010b). Aspergillus niger exhibits good potential in gen-erating organic acids like oxalic and citric, malic and tartaric acids,which are effective for metal solubilization (Anjum et al., 2009b;Mulligan et al., 2004; Murad et al., 2003).

3.4.4. Bioleaching of metals by autotrophsSome recent research demonstrated the microbial recovery of

metal ions like Al, Mo, V, U and radionuclides by chemolithotrophicbacteria in acidic environments (Acidithiobacillus ferrooxidans, Acid-ithiobacillus thiooxidans, sulfolobus caldarius) (Anjum et al., 2009a).These bacteria utilized sulde minerals as a source of energy presentin shale and schist as impurities. These practices are now used for thesolubilization of number of metal ions from their sulde mineralssuch as suldic copper and refractory gold ores (Pina et al., 2005),as well as manganese and iron ores (Abdelouas et al., 2000). A. thioox-idanswere isolated by Waksman and Joffe to oxidize elemental sulfurrapidly as early as 1922 (Rossi, 1990). When T. ferrooxidans wasadded to a mixture of sulde minerals during the leaching process,the cells adhered preferentially to pyrite and suppressed its oatabil-ity (Nagaoka et al., 1999). Thus, by using the microbes, pyrite could beseparated from raw ores (Hackl, 1997). In this way, different metalslike Zn, Pb and Cu were microbially recovered from mines (Konopkaet al., 1993). Microbial recovery of metals is more efcient thanchemical leaching, as bacterial strains leached metals to its maximumextent. Scanning electron microscopic (SEM) analysis of the solid ma-terial after leaching showed considerably more surface erosion of thebiologically leached material than observed in the control chemicalleaching experiments (Willscher and Bosecker, 2003).

During the direct method of bioleaching, bacterial strains can alsohave a high tolerance to heavy metals as reported in a study of landcontamination by oil (Jenifer et al., 2004). Thus, bacterial leachingtechnique can be an economical alternative in the mining industries.Different metals can also be recovered microbially from differentsources, e.g. mine drainage, industrial efuents, used catalysts,waste sludge, river sediments and electronic scrap etc. (Akcil, 2004;Mishra et al., 2004). Cuban serpentines are assumed to be one ofthe richest deposits of Ni and Co in the world. A. thiooxidans wereused to produce inorganic acids as metabolites using elemental sulfuras energy source to leach these ores. A high percentage of metal sol-ubilization, Co (100%) and Ni (80%) respectively, was achieved justafter 15 days of incubation. In indirect bioleaching experimentsusing sulfuric acid metabolites, signicant recovery of Ni (79%) andCo (55%) was also obtained (Orquidea et al., 2008). Flotation by-products of mines also contain signicant level of metals like Co, Cuand As. These metals were extracted by microbes isolated fromwater. Bioleaching is a somewhat slower process than chemicalleaching, but has more advantages like wastages consumption by mi-crobes as substrates during the process of oxidation. Energy releasedduring this process is utilized by microbes and producing organic andinorganic acids as metabolites that can be utilized in commercial foodproducts as ingredients (Uryga et al., 2004).

Pradhan et al. (2006) reported the effective recovery of aluminafrom ore after just 6 days by Bacillus circulans in-situ bioleachingand found bacterial leaching more efcient in alumina recoverythan fungal bioleaching. Retrieval of valuable metals in the form ofsuldes has also been made from bioleaching solution by sulfate re-ducing bacteria (Junya et al., 2009). These bacteria can extract Fefrom pyrite and Cu from chalcopyrite up to 95% within 30 days of in-cubation (Zhang et al., 2010). Wen-Tang et al. (2011) extracted P andFe from high-phosphorus iron ores. Signicant recoveries of Cu, Cr, Ni,Zn and Pb was also reported by Liang et al. (2010) and Akinci andGuven (2011), using A. thiooxidans, in an analogous way to the recov-

ery of these metals by A. ferrooxidans after 24 days of cultivation. The7%, respectively, by using agricultural wastes as substrates by A. niger.Mandal and Banerjee (2004) reported that a sufcient amount of

Fe could be recovered from China clay using the same microbe.Santhiya and Ting (2005) reported oxalic acid production by A. nigerin the presence of spent catalyst, which yielded an increase in the ex-traction of metals. Aung and Ting (2005) reported increased mobili-zation of Ni, Fe, Al, V and Sb from ore by A. niger compared tomixed culture of two acidophiles gives better results than a single onedid. Xiang et al. (2010) reported the recovery of metals from industri-al wastes in a time of less than 24 days. Important metals are not onlyextracted microbially from mines and other deposits of low gradeores, but also from waste incinerator bottom ash as reported byMoura et al. (2005). This indicates that these wastes can be a signi-cant alternative source of metals in future.

3.4.5. Bioleaching of metals by heterotrophs: a literature surveySeveral species of fungi can be used for bioleaching process. An enor-

mous amount of literature has been published concerning the ability offungi to extract metals from different resources. Wenberg et al. (1971)extracted metals from carbonaceous low grade ores using fungal strains,whereas Silverman and Munoz (1971) detected extracted Ti from rockafter fungal activity. Golab and Orlowska (1988) reported zinc extractionthrough the production of tartaric and citric acid by A. niger. Groudev(1990) carried out bioleaching of Au from ores and gold dust using Peni-cillium spp. As Penicillium spp. can produce some organic acids such ascitric, tartaric, lactic and malic acids, it can be used for bioleaching of dif-ferent ores (Anjum et al., 2009b). The use ofmicroorganisms in ore leach-ing to extract metals has long been a commercial business. Earlierinvestigations have revealed that bacteria and fungi could be effectivelyused to extract Fe and Si from clays, sands and low grade ores(Ogurtsova et al., 1990). Cameselle et al. (2003) investigated improved re-covery of Fe from kaolin with oxalic acid produced by A. niger. Bousshardet al. (1996) conducted leaching experiments in shaken asks for the re-covery of differentmetals from y ash using A. niger. Agatzini and Tzeferis(1997) leached Ni and Co from nonsuldic nickel ores using Aspergillusand Penicillium spp. They conrmed the presence of citric, oxalic andother organic acids in the leach liquors, indicating their role in the bio-leaching process. Bosecker (2001) extracted Cu, U and Au from lowgrade ore as a result of productionof organic acids and chelating and com-plexing compounds excreted into the environment bymicrobial activitiesof fungi and bacteria whereas Shanableh and Ginige (1999) reported therecovery of Cr, Ni, Zn, Cu, Cd and Pb from ore through fungal bioleaching.Bioleaching of important elements like Al, Fe, Co, Cu, Zn, Sn, Pb and Nifrom their ores using Penicillium as well as Aspergillus spp. havealso been examined earlier (Brandl et al., 2001; Castro et al., 2000;Gadd, 2004; Rezza et al., 2001; Sayer and Gadd, 2001; Tang, 2004; Valixet al., 2001). A set of experiments performed by Mulligan et al. (2004)have also shown that these two fungal strains were able to mobilizemetals by the production of some organic acids such as citric, tartaric, lac-tic and malic acids.

Venkateshwara et al. (2002) examined 20 isolates of fungal strainsof genera Aspergillus, Penicillium and Rhizopus for Cu extraction fromlow grade chalcopyrite, whereas Valix et al. (2001) found Penicilliumand Aspergillus spp. the most efcient microorganisms not only in or-ganic acids production, but also in leaching of Ni, Co and Fe from lat-erite in a direct leaching process that was comparable with chemicalleaching recovery of these metals. In another report, Valix and Loon(2003) described the tolerance behavior of different fungal strainsfor these metals from laterite ores and found that certain metals areessential for microbial activity. Bioleaching is also a more feasibletechnique for the extraction of Mn up to 64.6% from ore after30 days of incubation (Acharya et al., 2004). Mulligan et al. (2004)chemical leaching.

7F. Anjum et al. / Hydrometallurgy 117118 (2012) 112Heterotrophic fungi are capable of producing metabolic complex-ing agents (citrate, malate and lactate) that can play an importantrole in the metals extraction (Deepatana et al., 2006; Thangavelu etal., 2006). Pradhan et al. (2006) detected Al from low grade ore justafter 15 days of in situ leaching by A. niger. However, Le et al.(2006) found that greater concentrations of metals (Al, Co, Cr, Cu,Fe, Mg, Mn, Ni and Zn) can inactivate the cellular function critical inmaintaining organic acid production and leaching of minerals. Metalslike Al, Mn, Zn, Cu and Pd has been extracted from MSW incineratory ash by A. niger. Gluconic acid was detected as the main lixiviantin the bioleaching of metals (Wu and Ting, 2006). Tang and Valix(2006) utilized citric, lactic and malic acids for leaching of Ni and Cofrom limonite and nontronite ores. The effect of leaching on themetallogenic nature of the ore was investigated by optical microscopyand by synchrotron based X-ray diffraction. It was found that Ni andCo dissolution were dependent on acid activity, oxygen reduction po-tential, ore density and mineralogy. The extent of metal dissolutionwas less affected by the type of acids. The trend of selectivity in limo-nite and nontronite is Co>Mn>Mg. Baldi et al. (2007) reported theleaching of Pb, Cu, Zn, Ni, Cd and Cr from soil by organic acid metab-olites of fungi. Willscher et al. (2007) demonstrated that a microbialconsortia able to mobilize considerable amounts of heavy metals.Hosseini et al. (2007) elucidated the recovery of Fe from kaolin claymineral using the A. niger strain isolated from pistachio shell whereasGhorbani et al. (2007) reported the leaching of Al by isolated A. nigerand P. notatum from low grade bauxite in shaked ask leaching ex-periments. In another report, Ghorbani et al. (2008) demonstratedthe extraction of Al by fungi isolated from red mud along with a sig-nicant concentration of Ti. This demonstrated that some Iranian fun-gal isolates have potential applications in the extraction of metals.Tzeferis et al. (2008), Orquidea et al. (2008) and McDonald andWhittington (2008) all reported the production of signicantamounts of citric, oxalic and other organic acids by Aspergillus aswell as Penicillium spp. that enhanced the recovery of Ni and Cofrom non sulde nickel ore and laterite tailings.

Mishra et al. (2008) and Zhou et al. (2008) found maximum re-covery of U using two Aspergillus sp. fungi isolated from water sam-ples collected from uranium mines. Cu and Fe have been extractedin shaking as well as submerged bioleaching techniques, with theshaking technique showing better recovery. It was found that themajor organic acids involved in bioleaching were citric and oxalicacid. Ore cracks caused by mechanical action generated from myceliagrowth also contributed. Lian et al. (2008) found that Aspergillus spp.promoted K solubilization by means of at least three likely routes, onethrough the complexation of soluble organic ligands, another to im-mobilize biopolymers such as the insoluble components of secretion,and the third by the mechanical forces acting in association with thedirect physical contact between cells and mineral particles.

Xu and Ting (2009) also reported that A. niger is known to be ca-pable of bioleaching heavy metal ions from municipal solid waste in-cineration y ash. They concluded that citric acid productioncorrelated with and was responsible for the leaching of Al, Fe andZn. Metals can also be recovered from contaminated soil in an indus-trial area using organic acid metabolites (Ren et al., 2009). Yang et al.(2009) reported the toleration of A. niger for heavy metals and indi-cated that Al and Fe inhibited the growth of A. niger signicantly.Wang et al. (2009) veried that bioleaching of useful metals fromsolid waste incinerator y ash is an environment friendly processwith an extraction efciency of K, Na, Ca and Cr. Mohapatra et al.(2009) predicted maximum Ni extraction after 37.5 days from laterit-ic nickel ore of Sukinda mines, Orissa, through microbial leachingusing A. niger. Same strain, isolated from pistachio shell, has been uti-lized by Aghaiea et al. (2009) for the removal of Fe impurities from anIranian kaolin sample. Yang et al. (2011) reported the recovery of Ni,Cu and Co from low grade Ni, Cu and Co bearing sulde ore after just

60 days of bioleaching in a shaking ask, whereas the recovery ofthese metals was achieved after 120 days in column leaching. Amiriet al. (2011) extracted up to 100% of W, Fe, Mo, Ni, and Al fromspent catalyst by organic acids.

3.4.6. Bioleaching of metals from black shaleIn the past, carbonaceous low-grade ores as well as mining wastes

underwent leaching processes as a result of decomposition of carbonsources, such as charcoal and black shale and these were acceleratedby fungal activities (Wengel et al., 2006). The release of heavy metalsi.e. Fe, Mn and Ni were accelerated by the attack of fungus on the or-ganic matter (OM). In the case of Mn leaching, metal solubilizationmight be the result of enzymatic degradation of highly oxidizedmetal compounds (Ehrlich, 2001) and by compounds with at leasttwo hydrophilic reactive groups (e.g., phenol derivatives) which areexcreted into the culture medium and dissolve heavy metals by directdisplacement of metal ions from the ore matrix by hydrogen ions andby the formation of soluble metal complexes of organic acids(Bosecker, 2001). Generally bioleaching is the microbial transforma-tion of solid metal values into their water soluble forms. In the caseof black shales, microbial activities results in the dissolution ofmetal ions in the form of sulfurous compounds from the ore, thusmetals are present in the aqueous phase. The ability to predictmetal release from black shale materials is an important aspect in pol-lution abatement (Rawlings, 2004). Bioleaching of metals from blackshale follow the following leaching processes: column and shakingask leaching (on laboratory scale) and heap leaching (on industrialscale) (Lizama, 2004; Lizama et al., 2005). The leachability of suldeminerals in black shale is in the order of Pyrrhotite (Fe1-xS)>Sphal-erite (ZnFe) S>Galena (PbS)>Ni-Sulde (NiFe) Xs>(NiFe) S2>Cu-Sulde (Cu2S, CuS, CuS2)>Chalcopyrite (CuFeS2)>Pyrite (FeS2)>Ar-senopyrite (FeAsS) (Farbiszewska-Kiczma et al., 2004).

Potential release of heavymetals fromblack shale depends upon theinteraction of abiotic and biotic factors or biological degradation. Mi-crobes like bacteria and fungi therefore can play an important role inheavy metal mobilization, depending on the physicochemical natureof the heavy metals as well as that of the site (Gadd, 2004). It wasreported that leaching of heavy metals from Polish black shale was en-hanced by bacteria over a period of time of 7 months (Tasa et al., 1997).

In black shales, refractory organic matter (OM) has the potentialto complex heavy metals (Croue et al., 2003; Ran et al., 2000). Micro-bial degradation of organic matter (kerogen) in black shales results inthe release of energy exploited by microbial activities, followed by therelease of metals in the form of inorganic minerals. Similarly metalswere also recovered from black shale by citric, oxalic, malic and tarta-ric acids produced by A. niger as well as P. notatum as reported byAnjum et al. (2009b, 2010a, 2010b). Farbiszewska-Kiczma et al.(2004) characterized the black shale with high contents of Cu andother metals like Ag and some organometallic compounds like por-phyrins, and concluded that the presence of these organometalliccompounds reduces metals recovery by classical acid bioleachingmethods using bacteria. Shale ore which contained a considerableamount of organic matter accompanying the Cu is more susceptibleto attack by heterotrophic bacteria whereas organometallic Ni ismore resistant to bioleaching than Cu compounds. Jong-Un et al.(2005) studied the effect of several conditional factors on the efcien-cy of U bioleaching using an iron-oxidizer, A. ferrooxidans, from U-bearing black shale. The microbial activity can enhance the rate andextent of U leaching. However, under conditions of enough nutrientsand an energy source (Fe2+) present, microbes in the non-inoculatedblack shale were activated over time and exerted almost the same in-uence on the leaching efciency as the inoculated samples after250 h. Grobelski et al. (2007) studied the bioleaching of metals byheterotrophic pretreatment followed by autotrophic bioleaching ofPolkowice black shale and found it to be the most reliable processingmanner, since the heterotrophic process provides the material's sur-

face area expansion, and therefore strong inuences rate and

8 F. Anjum et al. / Hydrometallurgy 117118 (2012) 112efciency of autotrophic leaching to enhance the metals extraction.The leaching rate of Cu from black shale was also studied bySadowski and Szubert (2007) in a small column and a tank reactor.The rate of bioleaching in the column was comparable to that in theagitated tank reactor.

3.5. Sonobioleaching

The productivity of mineral industries strongly depends on theeconomic use of energy. Under the present circumstances, bioleach-ing is a slow process. The process can be speed up by using a varietyof physical stimuli methods, including ultrasound treatment, whichplays a crucial role to intensify the performance of live biocatalysts.High power sonication is generally associated with damage to cells,but benecial effects can be obtained when controlled sonication ofmetal solubilization is catalyzed by live cells. The use of low power ul-trasound bioleaching of metals is a developing eld in metal miningfrom ore and is becoming increasingly trendy and applied in hydro-metallurgy (Anjum et al., 2010b; Shi-jie et al., 2008).

Ultrasonic waves having frequencies in the range of 16 kHz to500 MHz, inaudible to the human ear, can be transmitted throughany medium with elastic properties, including water, gas-saturatedwater and aqueous particle suspensions. It is transmitted in theform of mechanical energy. The use of ultrasound wave's treatmentin industry has been a subject of research and development formany years for a variety of applications such as in medical imaging,metallic treatment, removing oxide lms, oil, grease and other con-taminants from solid surfaces and plastic welding, and even to re-moving contaminants from soils. Ultrasonic waves work byproviding shear forces to remove the material adhering to a surface.This shear force is developed due to cavitation. Ultrasound causeshigh-energy acoustic cavitation: the formation, growth and collapseof bubbles in a liquid. That is the formation of microscopic vapor bub-bles in the low-pressure (rareed) part of the ultrasonic wave. Thesebubbles collapse in the compression part of the wave creating veryminute, but high-energy movements of the solvent that result in lo-calized high shear forces. During cavitational collapse, intense heatingof the bubble occurs. These localized hot spots have temperatures ofroughly 5000 C, pressures of 500 atm, and a lifetime of a few micro-seconds. Shock waves from cavitation in liquidsolid slurries producehigh-velocity inter-particle collisions, the impact of which is suf-cient to melt most metals (Oncel et al., 2005; Swamy et al., 2005).

Applications of ultrasound to chemical reactions exist in both liq-uids and in liquidsolid systems and have the ability to create clean,highly reactive surfaces on metals. The use of ultrasonics through aprocess of cavitation in the form of shock waves and through microjetformation through a cavitating medium having elastic properties,such as water, gas, saturated water, and aqueous particle slurry sus-pensions, have shown to improve the rate of chemical reactions andhas given better overall conversions. Shock waves from cavitation inliquid solid slurries produce high velocity intraparticle collisions. Ul-trasound also improves the local transport in liquid systems by acous-tic streaming. Acoustic waves are known to produce time-independent vortices in the pores of the suspended solids in aqueoussolution as well as at solidliquid interfaces. Over the past few years, alarge number of researchers working in material processing andmetal extraction have developed an interest in the application ofpower ultrasound for industrial use (Avvaru et al., 2006; Mason,2007). Sonochemical extraction techniques together with classicalmethods gave a faster and selective extraction of metals.

Application of ultrasound in extractive metallurgy has beenreviewed by Mason (2007). The kinetics of the ultrasound assistedleaching of phosphate rock in HNO3 is modeled by Tekin et al. (2001)and that in HCl, by Tekin (2002). Tarasova (1993) and Rao et al.(1997) have used short ultrasonic pulses of higher intensities at the

start of leaching for typical ores such as nickel laterites in Greece andammonical leaching of copper oxide ore. The use of ultrasound in nickelextraction from lateritic nickel ore using a strain of A. nigerwas studiedby Swamy et al. (1995). Ultrasound has also found important uses forinitiation or enhancement of catalytic reactions, in both homogeneousand heterogeneous systems. Low intensity ultrasound has a promisingfuture in pharmacy, medicine and biotechnology, but to develop theseapplications from a rational point of view fundamental studies areneeded on the interaction between ultrasonic waves and cells or en-zymes and on biotechnological processes under thermal conditions(Casadonte et al., 2005; Sangave and Pandit, 2006).

3.5.1. Sonobioleaching of black shaleTechnological and industrial developments are not only correlated

with the progress of any country, but also increase environmentalhazards. Many industrial areas are heavily loaded with heavy metalsand organic compounds, which are recognized as anthropogenicand toxic to any kind of organisms, particularly human beings. There-fore, industry and public ofces are compelled to follow a structuredenvironmental management system. There is a need to nd or devel-op new and more practical technologies to utilize low grade oresmore efciently and reduce environmental concerns (Shi-jie et al.,2008; Swamy et al., 2005). Classical bioleaching methods along withultrasonic waves give more economical (Narayana et al., 1997) aswell as selective (Barrera-Godinez et al., 1992) recovery of metalslike Cu, Zn and Ni from their ores (Anjum et al., 2010b; Swamy etal., 1995). Ultrasonic waves with a frequency of 40 kHz at an intensityof 1.5 W/cm2 give optimum results in 50% less time than convention-al bioleaching (Anjum et al., 2010b). Sukla et al. (1995) extractedthree times more Ni after 14 days with ultrasound treatment and as-cribed it to the enhanced growth of A. niger by ultrasonic waves.Mulligan et al. (1999) recovered 20 times more Cu in a medium con-taining citric and gluconic acid produced by A. niger under ultrasonictreatment just after 15 days than without ultrasonic treatment.Swamy et al. (2005) also reported the enhanced recovery of Ni fromore during sonobioleaching. Ultrasonic waves in combination withleaching techniques can therefore enhance the leaching rates andoutput of the metal recovery (Shi-jie et al., 2008).

3.6. Leaching techniques

Leaching techniques can be divided into three main areas depend-ing upon the working volume.

3.6.1. Laboratory-scale (010 dm3)Laboratory-scale leaching techniques can be categorized into two

main groups: the rst type involves a qualitative or semiquantitativeleaching of an ore using microbes. This class includes manometric andstationary ask techniques (Anjum et al., 2010b). The second type, onthe other hand, involves the assessment of quantitative measures inan analytical approach including air-lift percolators and shakingasks, tanks, and pressure bioleaching (Anjum et al., 2010a, 2010b;Mulligan et al., 2004).

3.6.2. Pilot-plant (b10 m3)For developing reliable models for commercial-scale plants, labo-

ratory scale experiments are not considered reliable enough to pro-vide useful information in a relatively short time. Commercial scaletests are carried out under different conditions. Particle size is a crit-ical factor in metal dissolution. A large particle size results in less ex-posed surface area and leads to decreased accessibility of solution andmicroorganisms to valuable metals and consequently results in a de-crease in metal solubilization (Rossi, 1990). Moreover, the controlledenvironmental condition in laboratory-scale tests may not be applica-ble to commercial-scale plants. More reliable commercial scale plantscan be modeled under controlled environmental conditions by fol-

lowing pilot-plant techniques of different types, such as column

along with ultrasonic wave treatment also provide an economical

Ma)

, V

2006 2006 2008Recovery and PEA Resources Recovery and resources

9F. Anjum et al. / Hydrometallurgy 117118 (2012) 112leaching and agitated tanks and reactors (Lizama, 2004; Lizama et al.,2005; Szubert et al., 2006).

3.6.3. Commercial-scale (>10 m3)Commercial scale bioleaching techniques have major applications

in the mining industry. It was rst patented by the copper industry inthe early 1950's and subsequently followed by mining operations forgold (Olson, 1994), uranium (Khalid et al., 1993) and zinc (Agate,1996), from low grade ores requiring extraction on a commerciallevel. Commercial scale leaching has various forms of application,e.g. in-situ leaching (World Nuclear Association and Ian Hore-Lacy,2010), dump leaching (Gericke et al., 2009), heap leaching (Brierley,2008; Finders Resources Limited, 2010; Gericke et al., 2009; Nurmieet al., 2010; Robertson et al., 2006; Watling, 2008), vat leaching andreactor leaching (Rossi, 1990). Complex polymetallic black shale de-posits (Dumont Nickel Inc., 2010), massive sulde deposits (FindersResources Limited, 2010; Scotney et al., 2005) and silicate-boundminerals (Barrick Gold Corporation, 2010) can be exploited by thesetechniques of bioleaching.

3.7. Commercial applications of bioleaching

Biohydrometallurgy is now applied on a commercial scale for theleaching of copper and thepretreatment of refractory gold ores and con-centrates. BIO SHALE Project has been running since 2004 in Finland forthe extraction ofmetals fromblack shale using bioheap leachingprocess(Talvivaara deposits) (PAR BIOSHALE, 1st year report). Copper produc-tion in the world has increased steadily in the period 19842005 from9 Mt to 16 Mt per annum. The Australian Bureau of Agricultural and Re-source Economics (ABARE) has predicted that world copper productionwill reach nearly 18 Mt in 2006. Over 20% of that copper is now pro-duced via hydrometallurgy (Watling, 2006). A utility for bioleachingof uranium has also been demonstrated on a large-scale. There is con-siderable potential for bioleaching and biobeneciation pretreatmentof a wide range of base metal and platinum-group metals (Hoque andPhilip, 2011). Microbial leaching has been shown to be effective at

Table 3.7Black shale projects undertaken on commercial scale.Source: http://www.dnimetals.com/PDF/Secutor_Report_2011-06-21.pdf.

Project Talvivaara Kainuu

Company Talvivaara Mining Co.(Talv:LSE)

Western Areas NL &Minerals (WSA:TSX

Country Finland FinlandMetals of focus Ni, Co, Zn, Cu (Mn, U) Ni, Co, Zn, Cu U, Mo

Started 2003 2010Development stage Producing and expanding ExplorationBase resource* 1.5 B tonnesMining/envisioned mining rate 164,000 tpdMarket cap (C$M) 1613 1034

* includes compliant resource and potential mineral deposits.bench scale for the base metal suldes of Co, Ga, Mo, Ni, Zn and Pb. Sul-deminerals including platinum-groupmetals (Pt, Rh, Ru, Pd, Os and Ir)can bemicrobially pretreated. There are twomain types of processes forcommercial-scale microbially assisted metal recovery. These areirrigation-type and stirred tank-type processes. Irrigation-type leachinginvolves the percolation of a lixiviant through a static bed, whereasstirred tank-type leaching involves ner particle sizes agitated in a lixi-viant (Table 3.7).

3.8. Future prospects

Biohydrometallurgical techniques are longer term future alterna-tives in themining industry than any other processes due to operationalsimplicity, low operating cost, shorter construction times, more eco-nomical and easy technical handling. It is almost certain that some ofyield of onsite acid production for metal extraction which is more envi-ronmentally acceptable than other technologies, and therefore result infaster ofcial approval of operations. In the eld of biotechnology, thereis a strong need to exploit these environment friendly processes fully torecycle minerals in the biosphere. Industry views this as a very promis-ing technology for sustainable development. This technology can beimplemented on a large commercial scale with the help of themicrobialgrowth factor. Complex polymetallic black shale deposits, massive sul-de deposits and silicate-bound minerals are big challenges for bio-leaching technology. On a commercial scale, challenges exist for thepotential use of microorganisms for the extraction of cost effectivemetals like uranium, gold, etc. from low grade ores in a more engi-neered fashion. Environmental pollution problems regarding the acidmine drainage and industrial efuentswould beminimized bymore ex-tensive exploitation of ultrasonically enhanced bioremediation tech-niques. It is now the time to exploit more economical, ecofriendlyprocesses and services. If proven effective, the bioleaching processshould enjoy rapid commercial development, because the technologyoffers a greater range of treatment applications for situations that previ-ously were considered impossible or impractical using standard bio-leaching systems. While the range of ores to be tested will include theclassical bioleachable sulde ores (copper and gold), it is also possiblethat previously leached ores could be re-leached microbially. Geo-Microbial Technologies intends to pursue licensing as a commercializa-tion strategy. New strategic alliances could help to develop the fullrange of applications for the new technology, and direct licensing to tar-geted mining companies would lead to the most rapid commercializa-tion. The most immediate markets targeted for licensing have alreadybeen selected.

Acknowledgmenttoday's researchwill lead to innovative processes for commercial appli-cations in mining and also remedial activities. Bioleaching processes

2.8 B tonnes 1 B tonnes 1.31.5 B tonnes blocked40,000 tpd 100,000500,000 tpd16.30 29.36 16.15Viken Storsjon SBH

gnus Continental PreciousMinerals (CZQ:TSX)

Aura Energy Ltd(ASX:AEE)

DNI Metals Inc. (DNI:TSX)

Sweden Sweden Canada(oil, Ni) U, (Mo, V, Ni) U, (Mo, V, Ni) Mo, Ni, U, V, Zn, Cu, Co,

Cd, Ag (Au, Li)Dr. Fozia Anjum is highly thankful to the Higher Education Com-mission, Islamabad, Government of Pakistan for providing the start-up research grant to establish a research group at Government Col-lege University, Faisalabad, Pakistan.

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