Chapter 1 General Introduction -...

Chapter 1

General Introduction

Chapter: 1 General Introduction

1.1 Preamble

Metals have been in service of mankind since the early days of

civilization. The history of metallurgy probably began with the

accidental discovery that certain stones could be melted by fire, thus

releasing those materials known as "metals". The history of mankind

from time immemorial shows intimate and inalieanable relationship

with minerals and metals. Mineral-metals play significant role for the

economy and industrial growth of any developing nation. The birth of

hydrometallurgy resulted from the discovery that the water dripping

from the roof of underground mine working or flowing out of ore

dumps often contain metallic compounds and that these compound

could be readily recovered by simply letting the liquor dry up in

suitable pond. The earliest records of the wide spread practice of

hydrometallurgy go back to ancient mining around 166 A.D., in the

island of Cyprus, famous for its copper mine (1).

Metals In the minerals are generally present as carbonates,

oxides and sulphides. The minerals in which the metals are

sufficiently concentrated and form commercial source of desired

metals are called ore (2). A process applied to obtain pure metals after

excavation of ore from the mine is referred to as mineral processing

(3). The con\entional mineral processing or metal recovery from the

high-grade ores and concentrates can be categorized as

pyrometallurgy and hydrometallurgy. Pyrohydrometallurgy includes

the combination of both the above processes (4-6).

Bioleaching refers to the conversion of insoluble solid metals

values into their water-soluble forms using microorganisms and/ or

their products (7). Biooxidation describe the microbiological oxidation

of host mineral, which contain metal compound of interest. As a result

metal values remain in the residues in a more concentrate form. In

1


gold mining, biooxidation operation is used as a pre-treatment process

to remove (partially) pyrite of arsenopyrite. This process is also called 11 biobeneficiationll where solid materials are refined and unwanted

impurities are removed (8). The term llbiomining11,

11 bioextractionll or 11 biorecoveryll are applied to describe the mobilization of elements from

solid materials mediated by bacteria and fungi. Biomining concerns

mostly applications of microbial metal mobilization process in large

scale operation of mining industry for an economical metal recovery

(9, 1 0). 11Biohydrometallurgyll covers biomining or bioleaching process.

Biohydrometallurgy represents an interdisciplinary field where

principles of microbiology, geochemistry, biotechnology,

hydrometallurgy, mineralogy, geology, chemical engineering and

mining engineering are combined (11). The term II biogeotechnology 11

is also used instead of 11 biohydrometallurgy11• Since the advent of

biohydrometallurgical processing many ma.Jor breakthroughs have

been achieved and this economically profitable and sustainable

technology that now finds wide applications in various field ranging

from metal extraction to environmental clean up. So, more attentions

are needed for rationalization and optimisation of

biohydrometallurgical processes (12). Three main areas of application

of biohydrometallurgy are identified;

fi] Metal extraction from minerals and rocks.

fii] Pre-treatment of minerals to make them amenable to further

processing.

(iii] Environmental protection.

Recent advancement in the use of biotechnological principles

for the successful extraction and recovery of metals from low and high

grade ore, selected concentrates and industrial waste are expected to

make a significant contribution to fulfil the future demand of many

precious metals.

2


Bioleaching is a simple effective, clean and economically viable

supplementary technology to the conventional process (13).

Biohydrometallurgical processes have been applied for the extraction

of copper, gold, uranium, cobalt, zinc, nickel, gallium, molybdenum,

cadmium, manganese, antimony, lead, and iridium from their metal

sulphide and oxides. Sulphide mineral occluding platinum grade

metals viz. platinum, rhenium, rubidium, palladium, osmium and

iridium can also microbiologically pretreated. Academic and

commercial applications of biohydrometallurgy are extensively

increasing in laboratory, pilot and commercial scale operations

(14,15).

1.1.1 The advantages of biomining processing

Microorganisms are the backbone for the biomining activity. A

variety of microorganisms are found in leaching environment and have

been isolated from leachate and acidic mine drainage. Very rich

microbial diversity is found in the bioleaching environment, which

include bacteria, fungi, algae and some of the yeast (16). The Rio Tinto

mines in south western Spain are usually considered the cradle of

biohydrometallurgy. The copper mine of Rio Tinto was probably the

first large-scale operation in which the major role is played by

microorganisms. The role that microorganisms plays in biomining

process was demonstrated in 194 7, when Colmer and Hinkel isolated

bacteria belonging to the Thiobacillus genus from acid mine water ( 1 7).

Later Thiobacillus ferrooxidans and Thiobacillus thiooxidans were

isolated and characterized. (18, 19) Thiobacillus ferrooxidans and

Thiobacillus thiooxidans have been recently renamed as

Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans

respectively (20). There are many advantages of biomining operation

as compared to the conventional mining processes (15), which are

listed here;

3


Use of naturally occurring key components: microorganisms, water

and air

Work at ambient pressure and temperature

Simple stepwise expandability by a single reactor or in modules of

reactors

Process is simple to operate and maintain

Low energy input and capital cost

Dust and S02 free process

Applicable for bulk complex concentrate

Low to moderate capital investment

Provide essential pre-treatment for refractory ores for selective

leaching

Economically viable for extraction of metal from low grade ores and

major mine waste

Flexible for the treatment for mineral resources with a variety of

metals and in variable concentrates

Environment friendly process.

Recent detail investigation, based on molecular methods such

as DNA-DNA hybridization, 16S rDNA sequencing, PCR based

methods with pnmers derived from rRNA sequencing and

immunological techniques revealed that microbial bioleaching

communities are composed of a vast variety of microorganisms

resulting in the complex microbial interaction (synergism, mutualism,

competition and predation) and nutrient flow (21). Selected

microorganisms of these communities are given in Table 1.1.1 (22).

4

Table 1.1.1. Microbial diversity of acidic bioleaching environments and acidic mine d tage (22).

Domain Organism Nutrition Type Main p tange pH Temperature Leaching Agent Opt. (oC)

chaea Acidianus ambivalens facult.heterotrophic sulfuric acid Acidianus brierleyi facult.heterotrophic sulfuric acid ac Jhilic 1.5-3.0 45-75 Ferroplasma acidiphilum chemolithoautotrophic ferric iron 1.: .2 1.7 15-45 Metallosphaera sedula chemolithoautotrophic ferric iron, ac Jhilic extr.

sulfuric acid thermophilic Sulfolobus acidocaldarius chemolithoautotrophic ferric iron, 0.~ .8 2.0-3.0 55-85

sulfuric acid Sulfolobus brierleyi chemolithoautotrophic ferric iron, extr.

sulfuric acid thermophilic Sulfolobus metallicus chemolithoautotrophic Sulfolobus yellowstonii chemolithoautotrophic ferric iron, extr.

sulfuric acid thermophilic Bacteria Acidiphilium cryptum heterotrophic organic acids 2.( 0 mesophilic

Acidobacterium capsulatum chemoorganotrophic 3.( 0 mesophilic Bacillus coagulans heterotrophic 5.£ 0 22 Bacillus megaterium heterotrophic citrate Gallionella sp. autotrophic ferric iron 6.£ 8 6-25 Leptospirillum ferrooxidans chemolithoautotrophic ferric iron 2.5-3.0 30 Q Leptospirillum chemolithoautotrophic ferric iron 1.7-1.9 45-50

$::) ""'::t

thermoferrooxidans !i .. Leptothrix discophora facult.autotrophic ferric iron, sulfuric 5.f 8 5-40 .....

acid C'l !!

Metallogenium sp. heterotrophic ferric iron 3.~ 8 4.1 ~ Pseudomonas putida heterotrophic citrate, gluconate -5<

"'to a ~ !"\ "'to ... c ::::

Table 1.1.1 Continue ...

Domain Organism Nutrition Type Main Leaching Agent

Thermothrix thiopara chemolithoautotrophic sulfuric acid Thiobacillus acidophilus mixotrophic sulfuric acid Thiobacillus albertis chemolithoautotrophic sulfuric acid

chemolithoautotrophic sulfuric acid Thiobacillus capsulatus chemolithoautotrophic sulfuric acid Thiobacillus concretivorus chemolithoautotrophic sulfuric acid

Thiobacillus delicatus mixotrophic sulfuric acid Thiobacillus ferrooxidans chemolithoautotrophic ferric iron, sulfuric

acid Thiobacillus intermedius facult.heterotrophic sulfuric acid Thiobacillus kabobis mixotrophic sulfuric acid Thiobacillus neapolitanus chemolithoautotrophic sulfuric acid Thiobacillus novellus chemolithoautotrophic sulfuric acid Thiobacillus rubellus chemolithoautotrophic sulfuric acid Thiobacillus thiooxidans chemolithoautotrophic sulfuric acid Thiobacillus thioparus chemolithoautotrophic sulfuric acid

Eukarya Actinomucor sp. heterotrophic succinate Fungi Alternaria sp. heterotrophic citrate, oxalate

Aspergillus niger heterotrophic oxalate, citrate, gluconate, malate, tartrate, succinate

Aspergillus ochraceus heterotrophic citrate Fusarium sp. heterotrophic oxalate, pyruvate

malate, oxalacetate Paecilomyces variotii heterotrophic citrate, oxalate Penicillium ch so enum heterotro hie

p ~ange pH Opt.

ne· u 1.~ 0 3.0 2.( 5 3.5-4.0

0.~ 0

5.0-7.0 1.-' 0 2.4

1.~ 0 6.8 l.f 0 3.0 3.( 5 6.2-7.0 5.( 0 7.8-9.0

5.0-7.0 0.~ 0 2.0-3.5 4.~ ).0 6.6-7.2

Temperature (oC)

60-75 25-30 28-30

45

25-30 28-35

30 28 28 30

25-30 10-37 11-25

27 32 30

28

28

(j

! &" ..

Table 1.1.1 Continue ...

easts

Organism

Rhizopus japonicus Trichoderma lignontm Trichoderma viride Candida lipolytica Rhodotontla sp. Saccharomyces cerevisiae Tontlopsis sp. Trichosporon

Nutrition Type

heterotrophic heterotrophic heterotrophic heterotrophic heterotrophic heterotrophic heterotrophic heterotrophic

Algae, Protozoa and Amoebae are not identified

Main Leachin ent

pH 1ge pH 0 t.

Temperature (oC)

24-26 32 30

28


The bioleaching microorganisms obtain a large portion of their energy

from the oxidation of inorganic substance includes oxidation of

sulphidic minerals to soluble sulphates and ferrous iron to ferric form.

The mesophilic chemolithotrophic iron and/ or sulphur oxidizing

bacteria notably Acidithiobacillus ferrooxidans, Acidithiobacillus

thiooxidans, and Leptospirillum ferrooxidans are the most extensively

used microorganism in the biomining and metallurgical industries for

the oxidation of sulphidic minerals (23).

The organism studied the most is Acidithiobacillus ferrooxidans.

Although this is the best-known organism from acidic habitats, one

may not conclude that this organism is dominant in this ecosystem.

Recently in 2002, it has been found that under specific environmental

condition Leptospirillum spp. 1s even more abundant than

Acidithiobacillus ferrooxidans suggesting an important ecological role

in the microbial community structure of bioleaching habitat (24,25).

Thermoacidophilic archaeobacteria have been known for bioleaching

process since many years. All thermophiles belong to the Sulfolobales

a group of extremely thermophilic sulphur and ferrous oxidizers

including genera such as Sulfolobus, Acidianus, Metallosphaera and

Sulphurisphaera (26).

1.1.2 Why extremophilic bioleaching consortium?

Microorganisms that are able to develop under extreme

conditions have recently attracted considerable attention because of

their peculiar physiology and ecology. These extremophiles also have

important biotechnological applications. Acidic environment 1s

especially interesting because, in general, the low pH of the habitat is

the consequence of microbial metabolism and not a condition imposed

by the system as is the case in many other extreme environment such

as, ionic strength, high ferrous and ferric as substrate and product

respectively, temperature, radiation, pressure, etc. (22).

8

Chapter: 1 Getteral Introduction

Ferrous oxidation ability of acidophilic bacteria is exploited for

bioextraction of metals from sulphidic minerals. In non-contact

bioleaching mechanism, acidophilic iron oxidizers play a key role in

production of ferric iron as a lixivient. The ferrous-ferric ratio has a

dominating effect on the solution redox potential in biooxidation

systems. The acidophilic iron oxidizing microorganisms mesophilic or

haemophilic that increase the redox potential resulting in metal

sulphide biooxidation. The iron biooxidation rate and extent of

regeneration of ferric iron by the microorganisms play an important

role in heap, percolation leach system and even in the agitation

leaching. In two stages of indirect bioleaching with effective

separation process, the first stage of ferrous biooxidation should be

improved with fast kinetics of ferric iron production, this can be

achieved by developing iron oxidizing consortium at extreme high

concentration of ferrous, ferric and H+ ion and ionic strength. During

bioleaching process, large amount of jarosite precipitation is

unwanted phenomenon, which can be minimized by lower pH value.

High ferric iron concentration and extreme acidic pH are the most

favourable conditions for high yielding metal extraction process. Many

researchers have proved that mixed cultures (consortium) of

bioleaching bacteria are more promising than pure cultures (22,25).

So, it is essential to develop bacterial consortium in extreme

conditions, such as high concentration of ferrous, ferric, H+ ion and

ionic strength for sulphidic mineral bioprocessing. The commercial

use of thermophilic bacteria with their ability to operate at

temperature exceeding 45 oc has great potentials for improving the

kinetics of metal extraction from sulphidic minerals. Several

commercial bioleaching plants are using thermophilic bacteria for

extraction of base metals (Cu, Zn and Ni) from metal sulphides and

their concentrates (26).

9

Chapter: 1 General bttroduction

1.1.3 Microbial metal leaching mechanism

There are three types of mechanism involved in the microbial

mobilization of metals. The bacterial cell can affect metal sulphide

dissolution by contact and non-contact mechanism, which are also

known as direct and indirect mechanism respectively, and third is

galvanic conversion. The non-contact mechanism assumes that the

bacteria oxidize only dissolved ferrous to ferric ions. The later can

attack metal sulphides and be reduced to ferrous, which in turns, can

be again microbially oxidised. The contact mechanism requires

attachment of bacteria to the sulphide surface. The pnmary

mechanism for attachment to pyrite is electrostatic in nature. In case

of Acidithiobacillus ferrooxidans, bacterial exopolymers contains ferric

iron each complex by two-uronic acid residue. The resulting positive

charge allows attachment to the negative charge pyrite. Thus, the first

function of complex ferric iron in the contact mechanism is mediation

of cell attachment while second function is oxidative dissolution of the

metal sulphide, similar to the. role of ferric ions in non-contact

mechanism. In both cases, the electrons extracted from the metal

sulphide reduce molecular oxygen via a complex redox chain located

below the outer membrane, the periplasmic space and the cytoplasmic

membrane of leaching bacteria. The dominance of either

Acidithiobacillus ferrooxidans or Leptospiri.llum ferrooxidans in

mesophilic leaching habitats is highly likely to result from differences

in their biochemical ferrous iron oxidation pathway, especially the

involvement of rusticyanin (27). Several biomolecules are involved in

the aerobic respiration on reduced sulphur and iron compounds. It

has been found that up to 5% of soluble protein of Acidithiobacillus

ferrooxidans is made of an acid stable blue copper protein called

rusticyanin. Additionally the ferrous iron respiratory system contains

(putatively) a green copper protein, two types of cytochrome c, one or

more type of cytochrome a, a porin and an ferrous sulphate chelate.

10


The acid stability of rusticyanin suggests that it is located in the

peri plasmic space (22).

The following equations describe the Contact and Non-contact

mechanism for the oxidation of pyrite (28,29).

Contact mechanism:

Iron and/or sulfur oxidizer

Non-contact mechanism:

Iron oxidizers

Chemical 3FeS04+ 2S

......... ( 1)

.......... (3)

The third mechanism of sulphidic mineral oxidation is known as

"Galvanic interaction". It is inherent phenomenon in mix sulphidic

minerals that operate automatically in a heterogeneous system or

wherever two or more different environments coexist (30).

The role of galvanic interaction in the bioleaching of mixed

sulphidic minerals has been reported extensively. Mineral with a

comparative lower rest potential value behave anodically and undergo

dissolution (oxidation) whereas the mineral with the higher rest

potential value acts as a cathode at which reduction of oxygen occurs

(31). The electro chemical reactions involved are as below (32),

Anodic reaction on active sulphide sites:

MS -----lllo•M+ + so +2e- (Where 'M' is bivalent metal ....... (4)

11


Cathode oxygen reduction on noble mineral:

02 + 4H+ +4e- ............. (5)

The mru.n two mechanisms are "Contact" and "Non-contact"

bioleaching. One step bioleaching process have slow kinetics resulting

in the residence time of several day to few weeks, which restricts its

commercial application for the metal recovery from the concentrate by

the stirred tank process. With respect to the bioleaching process with

indirect mechanism the possibilities for the faster kinetics are more

promising. It can be achieved by performing the bioleaching process

into two separate stages;

(i) The chemical oxidation of sulphide by ferric Iron (chemical

stage)

(ii) The biological oxidation of the ferrous iron produced (biological

stage)

By applying the Indirect Bioleaching with Effective Separation

(IBES) process each stage can be separately optimised. The biological

stage can be improved by performing the ferrous iron oxidation in

separate reactor and chemical stage can be improved by raising the

operation temperature, by the use of catalysts (for Cu extraction) and

control of the pH (33,34).

1.1.3.1 Integral model of indirect bioleaching of metal sulphide

Metal sulphides are degraded by mainly ferric irons and/ or

protons on the crystal lattice. The primary ferric irons are supplied by

bacterial extracellular polymeric substances (EPS), complex with

glucuronic acid. The mechanism and chemistry of attack is

predominantly determined by the mineral structure solubility product

12


of the metal sulphide and EPS glucuronic acid - ferric iron formation

around the minerals, an important prerequisite for bacterial leaching

to proceed. The role of bacteria is to regenerate the ferric iron and/ or

protons and concentrate them at the interface mineral/bacterial cell

wall, enhancing the metal dissolution. Extra cellular polymeric

substances (EPS) layer with a thickness of nanometer surrounding the

cell forms the site of chemical reaction.

Based on electronic configuration and solubility of metal

sulphide mechanism of indirect bioleaching of metal sulphide is

classified into two major pathways.

a) Thiosulphate pathway with ferric attack on the mineral

surface (FeS2, MoS2, WS2).

b) Polysulphide pathway with ferric and/ or proton attack on

the mineral surface (ZnS, CuFeS2, PbS, MnS2).

The term "Contact" is insignificant and that "Non contact"

mechanism is predominantly involved in leaching of metal sulphide.

The role of bacteria in metal sulphide leaching is to supply the

oxidizing agent namely ferric by oxidizing the ferrous iron formed

during chemical attack of the mineral. Hence, it has been also

concluded that predominantly most of the mineral leaching are

through chemical process and the role of bacteria is purely as

catalyst. Pathways of leaching of minerals based on their electronic

and molecular structure are shown in Table 1.1.2 (35).

13


Table 1.1.2 Pathways of leaching of minerals based on their

electronic and molecular structure (35).

Minerals Formula/ Nature Oxidizing Products Mechanism structure of the agents formed ofdegrada-

minerals due to tion the

attack Pyrite FeS2/ Acid Ferric Thio- TP

disulphide Insoluble iron Sulphate a

Molybdenite MoS2/ Acid Ferric Thio- TP layered Insoluble iron Sulphate a

Tungstenite WS2/ Acid Ferric Thio- TP disulphide Insoluble iron Sulphate a

Sphalerite ZnS/ Acid Ferric Poly- PM sphalerite Soluble and sulphides~

proton (H2Sn) Chalco- CuFeS2/ Acid Ferric Poly- PM pyrite sphalerite Soluble and sulphides~

proton (H2Sn) Galena PbS/halite Acid Ferric Poly- PM

Soluble and sulphidesf3 proton (H2Sn)

Hauerite MnS2/ Acid Ferric Poly- PM disulphide Soluble and sulphides~

proton (H2Sn) Orpiment As2S3/ Acid Ferric Poly- PM

layered Soluble and sulphidesf3 proton (H2Sn)

TP=Thiosulphate Pathway; PM=Polysulphide mechanism

Bacteria effective

in leaching

(I), (II)

(I), (II)

(I), (II)

(I),(II),(III)

(I), (II), (III)

(I), (II), (III)

(I), (II), (III)

(I), (II), (III)

a Thiosulphate 1s oxidized via tetrathionate, disulphane

monosulphonic acid and trithionate to sulphate in a cyclic manner.

~ : Polysulphides formed are primarily converted in to elemental

sulphur, which are further oxidized to sulphate by bacteria.

(I) At. ferrooxidans (II) L. ferrooxidans (III) At. thiooxidans

14

Clzapter: 1 Generallntroductiotz

(a) Thiosulphate pathway (FeS2, MoS2, W~)

Molecular orbital and valence bond theory proposes, that orbital of

single atom or molecule forms electron bond with highest energy

level is referred as valence bonds. In case of molybdenite (Mo82),

pyrite (Fe82), Tungstenite (W82), the valence bonds are derived from

orbital of metal atoms only, consequently the valence bonds do not

contribute to the metal-sulphide moiety (M-82-2) and hence these

minerals are acid insoluble. Hence M---82-2 bond is broken by

powerful oxidizing agent, ferric iron.

The ferric hexahydrate ions cleave the chemical bonding between

iron and disulphide (Fe---82-2) in pyrite lattice (Eq.6), after the

oxidation of disulphide group to thiosulphate and ferrous ion are

released in the medium. Then the ferrous iron is oxidized by the

bacteria to ferric iron, which in turn attack the mineral surface and

aiding in metal dissolution. The thiosulphate formed during the

attack, is converted to sulphate by chemical attack of ferric ion

(Eq. 7), via tetrathionate, disulphane-monosulphonic acid and

trithionate.

(b) Polysulphide pathway (ZnS; CuFeS2; PbS; MnS2)

In these groups of metal sulphides, the valence bonds of the

molecule are derived from both metal and sulphur orbital. Hence

the electrons can be removed easily either by ferric iron and/ or

protons from valence bonds and thereby breaking the bonding

between metal and the sulphur moiety of the disulfide group

(M---82-2). Hence, these metal sulphides are acid soluble.

15


__ __,..., .. M+2 + H ......... (8)

----+..., 0.125 Ss+ Fe+2 ......... (9)

........ (10)

Unlike pyrite or molybdenite in this group of minerals, metal

sulphide bond is cleaved first (M---82-2) by proton, before the

oxidation of disulfide to sulphate. The hydrogen sulphide released

(Eq. 8), is reduced chemically to neutral sulphur via tetrasulphide by

ferric ions (Eq. 9). The neutral sulphur is oxidized to sulphate by the

bacteria (Eq. 10).

This model of bioleaching indicates that predominantly metals

are leached by indirect mechanism and the degradation of minerals by

ferric irons operates through 'Thiosulphate' or 'Polysulphide' pathway

depending on the mineral lattice. It also shows that 'direct' leaching

as 'contact' leaching, where the extra polysaccharide substance (EPS)

forms contact with the mineral surface, forming junction where all the

above process of 'indirect' leaching proceeds (35). Schematic

mechanisms of thiosulphate and polysulphide pathway in bioleaching

of metal sulphide are depicted in Fig 1.1.1 (35) and the schematic

mechanistic bioleaching model is shown in Fig 1.1.2 (22).

16


Fig. 1.1.1 Scheme of thiosulphate (MoS2, FeS2) and polysulphide pathway (ZnS, CuFeS2) in bioleaching of metal sulphide

Thiosulphate

Mechanism

Fe +3

r Atf, Lf

' Fe +2

MS: Metal sulphide M +2: Metal ions S203-2: Thiosulphate

(Atf, Att)

Sn-2: Polysulphide with chain length (n) SsO: Elemental sulphur

Poly sulphide

Mechanism

Fe +3

T Atf , Lf

'\.

(Atf, Att)

Ss0

(Atf, Att)

MS- ZnS, CuFeS2, PbS

17

H+

Chapter: 1 Ge~teral Introduction

Fig. 1.1.2 Schematic mechanistic bioleaching model (22)

I

I

4··· -......... -.~ -... -~,

·-·"' -....... .

.. . . -~, .

, __ ~ ....... ~ ... i ·~.;:

:.!.::·.::--·.

·.~: ;__ ,'_: ...

. _,.._ -~- ~ . "':. . : -~ ......

, ...... ,.,_ __ ..... . .

.. ',- y >""' f'''"' ··-•• -

C: cytoplasm; CM: cellmembrane;

• PS

PS: periplasmaticspace; OM: outer membrane; EP: exopolymers; Cyt: cytochrome; RC: rusticyanin; MeS: metal sulphide

. .. ~ ... ' ..

18


1.1.4 Bioleaching techniques

Different laboratory, pilot and commercial scale bioleaching

techniques under practice are listed in Table 1.1.3

Table 1.1.3 Bioleaching techniques (1)

Laboratory Scale

Manometric techniques

-Constant volume manometry

- Constant pressure manometry

Stationary flask techniques

Shake flask technique

Percolator .

Air sparger

Bioreactors -Mechanically

agitated

-Air-lift

Pressure leaching

Pilot Scale

Columns

Agitated tanks and reactors

Commercial Scale

In situ leaching

Dump leaching

Heap leaching

Vat leaching

Agitation leaching

Reactor leaching

-Mechanically stirred reactors

- Air-lift reactors

19


The shake flask technique 1s suitable for assessment of the

amenability and also for the acquisition of some preliminary, basic

information on factors influencing process kinetics that is quite

inadequate as a faithful simulator of the pilot-scale or the commercial

operation. This inadequacy derives from the difficulty of adjusting and

controlling gas mass transfer effectiveness independently of agitation

speed and of carrying out a continuous operation. 'Scale-up' means

the study of the problems associated with transforming data obtained

in laboratory and pilot plant equipment to industrial production,

which requires a certain degree of physical similarity between the

laboratory or pilot scale and commercial equipment.

Bacterial leaching is usually performed by heap of ground ore

or by dumps of waste or spent material. Heaps and dumps are

irrigated in close circuit with an acidic liquor that contains a fraction

of the bacterial population, the rest of the population being attached

to mineral. When the desired metal concentrations attained the rich

liquor is pumped to the solvent extraction (SE) section and then sent

to the electrowinning (EW) where the fine material is recovered. The

SE section is recycled to the heap or dump and then spent liquor of

the EW section is recycled to theSE operation (36-38).

Heaps and dumps leaching presents a number of advantages

such as simple equipment and operation, low investment and

operation cost and acceptable yields. However, these operations suffer

some severe limitations viz, the piled material is very heterogeneous

and practically no process control can be exerted, except for

intermittent pH adjustment and the addition of some nutrients.

Moreover, the rates of oxygen and carbon dioxide transfer are low and

extended periods of operation are required in order to achieve

sufficient conversion (39). Some of these limitations can be solved by

the use of reactor in bioleaching. The use of reactor would allow good

controls of the pertinent variable resulting in a better performance.

20


Inspite of these, heap leaching will continue to be the choice to treat

low-grade ore and tailing, while tank-leaching technology will probably

increase its application for gold, copper and the other precious base

metal concentration (40). The future of bioreactors in mining appears

promising. Gold biooxidation operation tends to increase in number

and size in several countries of the world.

1.1.5 Factor influencing bioleaching

Metal bioleaching in acidic environment is influenced by a

series of factors, which are listed in Table 1.1.4. Physico-chemical as

well as microbiological factors are affecting rates and efficiencies of

metal extraction. In addition, properties of the solids to be leached are

of major importance (41). The effluence of different parameters such

as activities of the bacteria itself, source of energy, mineralogical

composition, pulp density, temperature and particle size was studied

for the oxidation of sphalerite by Acidithiobacillus ferrooxidans (42).

The best zinc dissolution was obtained at low pulp densities (50 g.l-1),

small particle SJ.Ze (+ 150 to -325 B.S.S#) and temperature

approximately 35 °C. The heavy metals such as copper, nickel, cobalt,

cadmium, uranium and thorium inhibit the iron oxidation rate by iron

oxidizer (43). Concentration of 3 to 7 mg.l-1 carbon dioxide showed

optimum growth rates of At. ferrooxidans (44), whereas more than 10

mgJ-1 was inhibiting the growth of At. ferrooxidans (45). Pulp density

more than 30 gJ-1 decreases the rate of iron oxidation. During

bioleaching processes co-precipitation of metals with minerals phases

such as jarosite can reduce leaching efficiencies (46). In Non-contact

bioleaching the first stage of ferrous biooxidation, the concentration of

ferrous and ferric act as a substrate and product concentration

respectively. This initial substrate and/ or product concentration

inhibits the growth of iron oxidizers that seriously ~feet the iron

oxidation rate. However, both ferrous and ferric concentration above

0.5 M proved to be toxic to Acidithiobacillus ferrooxidans (4 7).

21


Table 1.1.4 Factors and parameters influencing bacterial mineral

oxidation and metal mobilization (22).

Factors Parameters

Physicochemical temperature ~arameters of a bioleaching pH [environment redox potential

water potential oxygen content and availability carbon dioxide content mass transfer nutrient availability iron(III) concentration ionic strength light pressure surface tension .Qresence of inhibitors

Microbiological parameters microbial diversity jof a bioleaching population density !environment microbial activities

spatial distribution of microorganisms metal tolerance ad~tation abilities of microorganisms

!Properties of the minerals mineral type Ito be leached mineral composition

mineral dissemination grain size surface area porosity hydrophobicity galvanic interactions formation of second~ minerals

Processing leaching mode (in situ, heap, dump or tank leaching) pulp density stirring rate (in case of tank leaching operation) heap geometry ( in case of heap leachin_g_ o_Qeration)

22


Recently, Blight, K. R. et al. has proved that the ionic strength is

a significant variable in the growth of iron oxidation with batch

culture of chemolithotrophic bacteria (48).

Organic solvents such as floatation or solvent extraction agents,

which are added for the downstream processing of leachates from

bioleaching, are responsible for inhibition. It has been demonstrated

recently that the addition of small amounts of amino acids (e.g.

cysteine) resulted in increased pyrite corrosion by At ferrooxidans as

compare to control (49). It was reported that Tween 80 increases the

attachment of At ferrooxidans on molybdenite and the oxidation of

molybdenum in the absence of ferrous iron (50).

1.1.6 Why bioreactor in mineral processing?

The ore or mineral concentrates functions as a substrate for the

growth of the microbial population and the bioleaching process.

Contact bioleaching is enzymatic process, which transforms the

mineral and the solubilization rate of the mineral is directly related to

the growth rate of the microbial population.

Nowadays, bioleaching is being applied as the main process in

large-scale copper mining operation and as an important pre

treatment stage in the processing of refractory gold ores. From a

process-engineering standpoint, the complex network of biochemical

reaction encompassed in bioleaching would be well performed in

reactor. The use of reactor would allow a good control of the pertinent

variables, resulting in a better performance. Parameters such as

volumetric productivity and degree of extraction can be significantly

increased (51).

The selection of a suitable reactor for a bioleaching process and

its design should be based on the physical, chemical and biological

23

Chapter: 1 Getzeral ltztroductiou

The selection of a suitable reactor for a bioleaching process and

its design should be based on the physical, chemical and biological

characteristics of the system. Adequate attention should be paid to

the complex nature of the reaction sludge composed by an aqueous

liquid, suspended and attached cells, suspended solids and air

bubbles (52). Because of very large volume of material to be processed,

to obtain optimum bioleaching and biooxidation rate, the best

performed in a continuous mode of operation in which volumetric

productivity is high and reactor volumes can be kept low. Considering

the kinetic characteristics of microbial growth, continuous stirred

tank reactor (CSTR) appears as the first choice. An important

consideration in selecting suitable reactor refers to the autocatalytic

nature of microbial growth.

The bioreactor appears to offer the best potential for leaching

operation since pronouncedly higher reaction rates can be achieved as

compared to conventional processes. Various types of reactor that

have been studied for their application in biomining are the

percolation column, the air-lift column and some special design such

as fixed-film column and rotary disc reactor (53).

1.1.7 Global scenario of commercial scale bioleaching operations

Academics and commercial applications are extensively

increasing in laboratory, pilot and commercial-scale operations of

bioleaching. Several bacterial species are used in many commercial

applications of biohydrometallurgical process, which has gained

acceptance and gaining prominence in several parts of the world, such

as South America, Australia, South Africa, China, Canada, Spain and

even in India. The Rio Tinto mines, Huelva in south western Spain are

considered the cradle of biohydrometallurgy that has been exploited

since pre Roman times for their copper, gold and silver values. Today,

the contribution of bioleaching is estimated to be approximately for

24

Chapter: 1 Ge11eralltrtroductio1l

30, 25 and 13% of the total world production of copper, gold and

uranium respectively (40).

The current situation of commercial size bioleaching operations

and ongoing projects in developing countries are blossoming especially

for copper and gold bioextraction. Major large-scale bioleaching

operations are located in developing countries. This is not purely

accidental, but the necessary result of two important factors, like

many developing countries having significant mineral resources and

mining constitutes one of their main sources of income, the second,

bioleaching is a technique especially suitable for developing countries

because of its simplicity and low capital cost requirement (54, 55).

Literature data shows that biohydrometallurgical technology can

significantly contribute to the economic and social development of a

country. Developing countries like Chile, Indonesia, Mexico, Peru and

Zambia share over 50% of world copper production, which are shown

in Table 1.1.5, and world production of zinc is shown in Table 1.1. 7 .In

case of gold mining Brazil, Chile, Ghana, Indonesia, Papua, New

Gauine, Peru and Uzbekistan rank among 13 top producers countries

with 33% of the world production which are also shown in Table 1.1.8.

A breakthrough in bioleaching practice was the establishment of

the first copper mine exploited solely by bacterial technology, which

was achieved at Minera Pudahuel in Chile, when in the mid-eighties

they switch from a mixed acid and bacterial leaching to full bacterial

heap leaching of an ore containing 1 to 2% copper rendering 14,000

tons of fine copper per year (13,55). The process consisted of crushing,

agglomeration, heap leaching, solvent extraction and electrowinning.

Soon after the start up of the Pudahuel process several other copper

bioleaching operations were established for copper bioleaching in

Chile, which are shown in Table 1.1.6.

25


Table 1.1.5 World copper mine production (thousand of tons)

Country 2000 2001 2002 2003 2004

Australia 829 869 883 830 850 Canada 634 633 600 558 560 Chile 4600 4740 4580 4900 5380 China 590 590 585 610 620 Indonesia 1012 1050 1160 979 860 Kazakhstan 430 470 490 485 485 Mexico 365 367 330 361 400 Peru 554 722 843 831 1000 Poland 456 474 503 495 500 Russia 570 620 695 675 675 United State 1440 1340 1140 1120 1160 Zambia 240 300 330 330 400 Other country 1480 1510 1500 1400 1600 World Total 13200 13700 13600 13600 14500

Table 1.1.6 Copper bioleaching operations in Chile

Mine

Lo Aguirre Quebrada Blanca Cerro Colorado Zaldiwar Ivan-Zar Aldacollo-Cobre

Chunquicamata Los Bronces Zaldivar

Production, tons/Year Heap leaching

Dump leaching

14,000 75,000

1,00,000 1,50,000

12,000 21,000

Over 15,000 11,000 12,500

26

Chapter: 1 Geueral Introduction

Table 1.1.7 World zinc mine production (Metric tons)

Country 2000 2001 2002 2003 2004

Australia 1420000 1519000 1154000 1480000 1492500

Canada 1002242 1012048 894399 1000000 9525000

China 178000 1700000 1550000 1650000 1526000

India 144000 146000 129000 162000 165000

Ireland 262877 225135 252700 250000 255600

Mexico 392791 428828 446104 460000 456800

Peru 910303 1056629 1221830 1250000 1285000

Poland 156900 152700 152200 150000 146500

Russia 136000 124000 130000 125000 125300

Spain 201000 164900 69900 70000 76000

United 852000 842000 780000 738000 780000

States

27


Table 1.1.8 World gold mine production (Kilograms)

Country 2000 2001 2002 2003 2004

Australia 296410 286030 273010 282000 420000

Canada 156207 158875 151504 140559 171000

Chile 54142 42673 38688 40000 42000

India 6200 3700 3800 3100 3500

Indonesia 124596 166091 142238 140000 120000

Mexico 26375 26300 20617 20000 21000

Peru 132585 138022 157013 171551 160125

Poland 367 349 296 300 310

Russia 143000 152500 168411 170068 180400

United 353000 335000 298000 277000 247000

States

Zambia 600 630 635 640 620

Total 2590000 2600000 2580000 2590000 2470000

28

Chapter: 1 General l~ttroductiou

A number of copper bioleaching projects are at this time understudy

and development. Among them the most important one is being

carried out jointly by the Chunquicamata Division of CODELCO

(Chilean National Copper Corporation) and BHP Billiton. The project

aims the large-scale operation of a plant for the bioleaching of copper

concentrate in continuous stirred tank reactors using thermophilic

microorganism. The projects involve the construction of a US $ 60

million large pilot plant able to produce 20,000 tones of copper

cathode per year. Another exciting experience is being carried on in

Mexico; Panoles S.A. in association with Mintek has been able to

produce several tons of copper cathodes in their demonstration plant

in Monterry. The plant is an integrated tank bioleaching, solvent

extraction and electrowinning facility capable of producing 500 kg

copper per day (40).

Bioleaching is also successfully applied in gold m1n1ng, when

the metal is covered with a film of insoluble metal sulphide that

hurdle the extraction of gold with cyanide solutions. In this case, the

sulphide film must be removed in order to obtain satisfactory gold

recoveries and bioleaching is one of the alternative choice for such

pre-treatment step. In 1999, Newmont Mining Corporation

commissioned the first biooxidation heap facility for pre-treatment of

refractory gold ore (56).

The South African company, Gencor (now Billiton) pioneered

commercial tank bioleaching of refractory gold bearing sulphide

concentrates, implementing the world's first such plant at the Fairview

Gold Mine during 1986. The implementation of the other plant was

achieved after approximately ten years of in-house research including

a 75 kg per day pilot plant facility with superior recovery of gold.

Nearly 1000 tones concentrates are processed daily in reactors up to

900m3 in size (57).

29

Chapter: 1 Generallutroductiou

The BIOX® process for pre-treatment of refractory sulphide gold

ores such as pyrite, arsenopyrite and pyrrhotite was developed to

increase gold recovery rates during the metal extraction process. The

gold in these sulphide ores is encapsulated in sulphide minerals,

which prevent the gold from being leached by cyanide. The BIOX®

process solublizing the sulphide minerals and exposes the gold for

subsequent cyanidation and increase recovery rates. Gold recovery

was 30-39% before biooxidation, increasing to 49-60% after

biooxidation. The BIOX® process has many advantages over

conventional refractory process such as roasting, pressure oxidation

and nitric acid leaching. Ashanti, Ghana, is the largest plant in the

world for the treatment of refractory gold ores. Several recent and

ceased large-scale commercial bioleaching operations for gold, copper

and cobalt are shown in Table 1.1.9 (22). The BIOX® stirred tank

leaching also had application 1n base metal, although first

investigation were associated with biologically assisted heap leaching

of low-grade nickel ores termed BioNIC™. It was tested at a

demonstration facility at Billiton Process Research in South Africa in

1996-1997. The copper bioleaching process was termed BioCOP™

CODELCO as the world largest copper producers was approached

regarding the establishment of a pilot plant facility at its

Chunquicamata operation in Chile. BioNIC™, BioCOP™ and BioZINC™

have been successfully demonstrated on a pilot scale (13). The

GEOCOAT process, developed by Geobiotics, Lakewood, Colorado, is a

unique heap leach system for biooxidation pre-treatment of refractory

precious metal concentrates and bioleaching of copper, zinc and

nickel sulphide concentrates (58). The Rio Tinto Mine, Spain

commissioned the Indirect Bioleaching with Effects Separation (IBES)

process for copper-zinc sulphide concentrate, where the capacity of

plant is 105 tones per year and operating cost was about US $ 141

per ton of concentrate. It is a two-stage process and silver is used as

catalyst with 8 to 10 h of retention time. Copper and zinc recovery of

95 and 94% are reported by this process (59,60).

30

Table 1.1.9. Selection of recent and ceased industrial bioleaching operations for gold, nickel~ >per, and cobalt

Country Locality, Metal Mineral Source Technology :apacity Metal Designation [t d ·1) Yield[t y - 1 ]

Australia Harbour Au Flotation concentrate Tank leaching 40 Lights (160m3) Girilambone Cu Chalcocite Heap leaching 16,000 14,000 Gunpowder Cu Chalcocite, bornite in situ leaching 13,000 Maggie Hays Ni Concentrate Tank leaching 7

(pilot plant) Beaconsfield Au Tank leaching 70 Wiluna Au Flotation concentrate Tank leaching 115

(480m3) Youanmi Au Flotation concentrate Tank leaching 120

(6.480 m3) Brazil Sao Bento Au Flotation concentrate Tank leaching 150

(550m3) Canada Gold bridge Au Pyrite, markasite, Tank leaching 75

arsenopyrite (225m3) Chile Andacollo Cu Chalcocite Heap leaching 10,000

Cerro Cu Chalcocite Heap leaching 16,000 60,000 Colorado

Q Dos Amigos Cu Chalcocite Heap leaching 3,000 ~ Quebrada Cu Chalcocite Heap leaching 17,300 75,000 ~

Blanca "'t .. Zaldivar Cu Chalcocite Heap leaching 20,000 .....

G"}

Ghana Sansu Au Floatation concentrate Tank leaching 1000 ~ ~ -S' :t Q

§-(")

'"" .... w Q ..... ::

Table 1.1.9 Continue •••••

Country Locality, Metal Mineral Source Technology Desi nation

Ghana Ash anti Au Floatation concentrate Tank leaching (6.900 m3

) India Hutti Au Tank leaching (pilot

plant) India Malanjkhand Cu Malachite, chalcocite, Heap leaching 2.5

bornite, covellite Peru Tamborque Au,Ag Arsenopyrite from zinc Tank leaching

flotation (pilot plant 1 m3)

South Fairview Au Flotation concentrate Tank leaching 35 Africa (90m3

) Uganda Kasese Co Flotation concentrate Tank leaching 1,000

(4.1.350 m3)

USA Carlin Au Au - containing Heap leaching 10,000 sulphidic ore

Chino Cu Chalcocite, chrysokolla Heap leaching 55,000 San Manuel Cu Chalcocite in situ leachin 20 000

Q ~ !i .. t-1

C':l ~ !II

~ -S" a §-~ "'to w ...

N §


1.1.8 Indian scenario of metal bioleaching

The mineral wealth of India is a gift of nature and a part of great

inheritance. India is the biggest consumer of gold in the world. Next to

gold, copper, zinc and lead are very essential metals and their demand

is also increasing day by day. The domestic scenario shows the

significant demand supply gap, which is expected to widen in near

future. In other words, more attention has to be paid to the growth of

supplementary metal production and to reduce the import

dependence, which affects the economy of the country. The mineral

resources of India are summarized in Table 1.1.10. As can be seen

from the table, India ranks 2nd in the resources of manganese ore, 3rd

in pyrites and iron ore, 4th in chromites, bauxite and magnetite, 5th in

coal and lignite in the world. A Recent Price Water House report has

identified India as the most promising mining location worldwide

(62,63).

India produces 65 minerals including fuel, metallic, non

metallic and atomic minerals (64). Metals are strategic and the

important role they play in the national economy both during peace

and war can not be underestimated. The yardstick of the per capita of

metal consumption is the measure of the prosperity of the country

and the people. The usage of metal is expected to rise by 8-12 %

annually. Even at present, India is not self sufficient for its demand of

metals (Tablel.l.11). India has a variety of untapped mineral

resources, largely subsurface and hidden due to lack of modem

technology, adequate exploratory efforts and intensive mineral

investigation (Tablel.1.12) (65).

The Demonstration Biooxidation Tank leaching plant was

commissioned in 2002 at Hutti Gold Mines, Karnataka State, India.

The plant was designed for treating gold and silver bearing

arsenopyrite concentrate and can also be used for the bioleaching of

copper, zinc, nickel and other base metal concentrate (40).

33


Table 1.1.10 Mineral resources of India (65)

Metals Ore/Mineral Reserve Geographical distribution (Mt)

Iron Hematite 11950 Bihar, Orissa

Magnetite 4900 Karnataka, Goa, Madhya Pradesh

Manganese Pyrolusite 370 Orissa, Bihar, Madhya Pradesh

Psilomelane Maharashtra, Karnataka

Brunite Gujarat, Goa

Cobalt and Sulphide ores Uttar Pradesh, Rajasthan

Nickel in copper

mines

Copper Chalcopyrite 425 Bihar, Andhra Pradesh Rajasthan

Outer Himalaya

Gold Quartz reefs 30 Karnataka, Andhra Pradesh,

Arsenopyrite Madhya Pradesh, Ladakh

Aluminium Bauxite 279.9 Bihar, MadhyaPradesh,

Maharashtra, Gujarat, Kashmir

Uranium Pegmatitic Bihar, Andhra Pradesh. Rajasthan

Monazite Himalayas, East and West Coast of

India

Zinc Sphalerite 107.1 Rajasthan, Kashmir, Sikkim

Gujarat

Lead Galena -100 Rajasthan, Andhra Pradesh,

Gujarat, Tamilnadu

34


Table 1.1.11 Degree of metal self-sufficiency in India (1998-99) (65)

Thousand tons Order of self-Metals Domestic Domestic supply sufficiency(%)

demand Zinc 232 142 61.2

Copper 273 38 13.9

Lead 129 48 37.2

Aluminium 643 543 84.4

35

Chapter: 1 Getzeral Introduction

Table 1.1.12 Indian scenario of gold, copper, zinc and lead production (65)

Metals 1999 2000 2001 2002 2003

Gold 2468 6082 8356 4486 3176

(Kilogram)

Copper 3316056 3292709 4298892 3252839 2875307

ores

(tons)

Zinc 343328 365730 399804 462095 607677

concent.

(tons)

Lead 62094 57485 51733 57169 68877

concent.

(tons)

36

Chapter: 1 Gmeral Introduction

Thus, the metal sulphide biooxidation technology would be developed

that could significantly contribute to the economic and social

development of India.

1.1.9 Economy of metal bioleaching

Economical factors for commercial scale bioleaching applications

can be divided into capital costs associated with construction, operation

and maintenance cost (66). Operation costs include the running of the

process equipment, the supply of the reagents, services and labour (67).

However, capital costs are generally smaller. According to Barrett et. al

these costs depend on the method of application and it increase in the

order (68),

Dump < Vat ~ Heap < Agitated reactors

Major factors affecting costs of bioreactors are construction

material and equipment. Equipment required for agitated bacterial

oxidation processes includes conventional tanks with impellers to

suspend the solids and disperse the air, compressor for air supply,

thickeners for solid/liquid separation and conventional sluny pumps

for delivering and removing the slurry from the plant. Construction

material needs to be acid resistant, which further more, withstands

temperature of 30-50 °C. In general technical equipments for

bioleaching process is less expensive as compared to physico-:-chemical

processes (22).

Nowadays, between 25-30% of copper produced yearly is based

on microbiological treatment of mineral resources. Together with other

metals such as cobalt, gold, nickel, uranium, or zinc, biological metal

extraction processes result in a surplus value of over 10 billion US

dollars (22). Recovery of gold from ores could be increased by 2-13% if

biotechnological methods are applied. In comparison to ore roasting

37

Chapter: 1 Geueral Iutroductiou

capital cost for industrial scale treatment plants are reduced by

12-20%, operating cost by 10% (69). Cost of US $ 4.3 per ton ore have

been calculated for bacterial ore treatment compared to US $ 4.1 and

US$ 4.5 with pressure oxidation and roasting respectively (67).

The technical and economical benefits of thermophiles have also

been investigated for nickel, copper-nickel and zinc concentrates. The

increased rates of reactions as well as lower capital and operating cost

are significantly compelling to the conveyance Billiton that thermophiles

at least for stirred tank applications are the key to the future in base

metal bioleaching. The mining industries recognize that biotechnology

offers important tool for economic recovery of metals values (70).

1.1.10 Scope and objectives of the present work

1.1.10.1 The scope of complex sulphidic mineral bioprocessing

Metal recovery from such complex ore and bulk concentrate is

equally important for the economical growth of nation. Recent

advances in bioextractive metallurgy have also opened the door of

metal extraction from complex sulphidic minerals.

The depletion of high-grade mono-mineral ore deposits and

increasing demands for base metals have led mining companies to

consider exploiting multi-metallic and lean ores, which represents

important resources of nonferrous and precious metals. The complex

ore is generally composed of chalcopyrite, galena, sphalerite, pyrite

and/ or pyrrhotite (71). Metal extraction from complex ores and

concentrates can be achieved pyrometallurgically or

biohydrometallurgically. A pyrometallurgical process for complex ores

requires "selective floatation process". To obtain individual

concentrates from complex sulphide ores, a fine grinding step (or ultra

grinding) is required to liberate the mineral species to be separated,

38


which is an energy-consuming, expensive operation, necessitating

skilled labour. So, flotation separation of complex sulphide ores into

individual concentrates has been a difficult task, often due to the size

of liberation and possible interactions among sulphide minerals (32,

72). Moreover, this solution is not satisfactory and economical, in

practice, owing to the high costs of the differential flotation process,

low flotation recovery and to the poor quality of the resulting

concentration, which hinder their access to the market (1,32).

"Bulk flotation" shows significant advantages as compare to

differential flotation in terms of less water and reagents consumption,

higher metal efficiency and less grinding cost. Inspite of these

advantages, at present, the main difficulty lies in selecting an efficient,

inexpensive and flexible leaching process (1,73). This situation

prompted an examination of alternatives in arnving at an

economically sound process to extract metal from finely inter-grown

complex sulphide concentrates. In this respect, Biohydrometallurgical

process is considered as an interesting choice (73).

Biohydrometallurgical methods are currently used In

commercial leaching processes for the recovery of copper, uranium

and gold from sulphidic minerals (74,75). In the future, these

processes will become important for zinc, nickel, cobalt and

molybdenum from the complex ores (76, 77). 'Combination processes'

involving an initial bulk flotation followed by selective leaching of

certain sulphide minerals could prove attractive in the processing of

refractory complex ores (32). It is of tremendous interest to explore if

selectivity could be achieved in the leaching of multi-metal sulphides

(32).

Bacterial leaching, either of the run-of-mine ore or bulk metal

sulphides concentrate appears promising. Production of the later

39


usually involves a relatively coarse grind and is characterised by

satisfactorily high recoveries (78).

Bacterial leaching has been found to be effective in the

dissolution of several sulphide minerals. Possible selectivity for

sulphide mineral oxidation of complex sulphides by bacteria depends

on the following key factors (73),

• Bacterial ability to produce ferric iron for selected sulphide

mineral and indirect microbial attack.

• Chemical interaction between sulphide mineral and

ferric/proton ions.

• Increase the rate of metal extraction.

• Bioregeneration of ferric for the dissolution of metal sulphide.

Application of bioleaching to the processing of complex sulphide

such as the ores containing Pb-Zn-Cu-Fe, Cu-Ni-Mo-Fe and Cu-Ni-Fe

metals is of great importance since flotation-beneficiation of these·

multi-metal ores to yield individual clean concentrates (for

pyrometallurgical processes) is extremely difficult (79). A combination

process would prove to be very efficient in these cases (30). However,

information concerning the bioleaching of multi-metal (complex)

sulphides is limited. To date, experiments have been carried out only

on a laboratory scale on complex sulphide run-of-mine ores, on mixed

metal sulphide flotation concentrates and on synthetic mixtures of

chemical grade base-metal sulphides and sulphites (77).

Two-stage metal bioextraction process was used for copper and

zmc concentrates of Rio Tinto mine, Spain. The maximum

bioextraction of zinc and copper were 85 and 90% respectively (33).

In this context, it is necessary to investigate the amenability and

detailed behaviour of two-stage bioextraction profile for GMDC

40


(Gujarat Mineral Development Corporation) polymetallic concentrate.

This study will help to utilise the national reserves of the mineral

resources, which are not exploited due to the lack of technology. It

also provides some economical benefit by recovering metal value from

the polymetallic concentrates, which are not amenable to conventional

procedure due to obvious reasons.

1.1.10.2 Objectives

The research program was undertaken with the objectives in

mind to develop extremophilic iron oxidizing consortium with special

reference to two-stage bioextraction process to treat copper-lead-zinc

bulk concentrate. The development of extremophilic bioleaching

bacteria and two-stage process is not limited to polymetallic

concentrate but it can be also applied to various sulphidic minerals.

The research programme was designed with following objectives;

• Selection and development of iron oxidizing bacterial consortium

· to improve iron oxidation rate ..

• Development of iron oxidizing consortium in extreme conditions

such as;

-High ferrous sulphate concentration

- High ferric sulphate concentration

- Highly acidic pH

• Metal tolerance study of iron oxidizing bacteria in the presence of

heavy metals such as copper, zinc, cobalt and nickel.

+ Optimization of various physico-chemical parameters for 1ron

oxidation in shake flask study.

• Enhancement of iron oxidation rate by adaptation and selection.

+ Designing of fixed film column bioreactor with developed iron

oxidizing bacteria on different supporting materials such as glass

tubes, glass beads and acrylic pieces for two-stage metal

extraction bioprocess.

41


+ Enhancement of iron oxidation rate by developed fixed film PVC

column/ airlift percolating column bioreactor.

+ Isolation and identification of pure culture of 1ron oxidizing

bacteria from developed fixed film bioreactor.

- 16S rDNA sequencing

- Scanning Electron Microscopy

+ Optimization of chemical oxidation of GMDC polymetallic bulk

concentrate by microbially produced ferric iron.

+ Detail investigation of two-stage microbially produced ferric

mediated cyclic process for bioextraction of copper and zinc.

+ Recycling and bioregeneration of ferric lixivient for two-stage

metal extraction biotechnology.

42


1.2 References

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