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Surface chemical and flotation behaviour of chalcopyrite and pyrite

in the presence of Acidithiobacillus thiooxidans

M.N. Chandraprabha, K.A. Natarajan

Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India

Available online 5 May 2006

Abstract

Extraction of valuable metals and removal of sulfide minerals from abandoned mines holds the key for environmental

protection. This paper discusses the utility ofAcidithiobacillus thiooxidansfor the selective removal of pyrite from chalcopyrite for

the economic extraction of valuable copper. Interaction of bacterial cells with the sulfide minerals altered the surface chemistry of

both the minerals and cells. The isoelectric point of both pyrite and chalcopyrite shifted to higher pH after interaction with cells.

Adhesion kinetics of the bacterial cells to both the minerals was similar, however, the adsorption density on pyrite was higher

compared to that on chalcopyrite. Interaction with cells rendered both the minerals hydrophilic. Flotation of minerals,

preconditioned with cells, with potassium isopropyl xanthate as collector resulted in depression of pyrite and good flotation of

chalcopyrite. The observed behaviour is discussed in detail. Poor selectivity achieved when the minerals were floated together was

overcome by conditioning the collector interacted minerals with the bacterial cells prior to flotation. Thus it was possible to

selectively depress pyrite from chalcopyrite at both acidic and neutral pH conditions.

2006 Elsevier B.V. All rights reserved.

Keywords: Acidithiobacillus thiooxidans; Pyrite; Chalcopyrite; Adhesion; Flotation

1. Introduction

Recent years have witnessed depletion of rich grade

ores resulting in the processing of more and more lean

grade ores. Economical extraction of valuable metals

from such ores requires selective separation of minerals

having high metal values from other associated miner-als. Chalcopyrite is often associated with pyrite and

economical extraction of copper demands selective

depression of pyrite from chalcopyrite. Conventionally

sodium cyanide is used as a depressant for the selective

depression of pyrite from chalcopyrite during the froth

flotation process, leading to potentially disastrous

environmental consequences [1]. Use of microbes or

bioreagents, on the other hand, would prove to be both

economically viable and an environmentally benign

process.

Thiobacillus sp., which are chemolithoautotrophs

and which thrives on either iron or sulfur, have been

successfully utilized for the bioleaching of sulfide

minerals and biooxidation of refractory gold ores[2,3]. Acidithiobacillus ferrooxidans are known to

adhere very tenaciously to sulfide mineral surfaces.

Such bacterial adhesion alters the surface chemistry of

the interacted minerals conferring either hydrophobicity

or hydrophilicity [4]. The role of A. ferrooxidans in

influencing the flotation behaviour of pyrite, chalcopy-

rite, sphalerite, galena, etc., has been studied in the past

[5,6]. The chemolithotrophic acidophilic thiobacilli

Acidithiobacillus thiooxidans and A. ferrooxidans both

grow on elemental sulfur, but only the latter uses ferrous

Hydrometallurgy 83 (2006) 146 152

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Corresponding author. Tel.: +9180 23600120;fax: +9180 23600472.

E-mail address:[email protected](K.A. Natarajan).

0304-386X/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2006.03.021
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iron as an energy source. Thus A. thiooxidans has

similar applications asA. ferrooxidansin the processing

of sulfide minerals.

The oxidation of sulfur to sulfuric acid by A.

thiooxidans with sulfite as the key intermediate was

first proposed on the basis of the sulfur-oxidisingenzyme and sulfite-oxidising enzyme systems: S0 + O2+

H2OH2SO3 and H2SO3+(1/2)O2H2SO4 [79].

The mechanism of cell-sulfur adhesion process and

kinetics of sulfur oxidation is also well-established

[10].

However, the utility of this organism for bioflotation

is very limited. Recently Santhiya et al. [11] have

studied the surface chemical changes on galena and

sphalerite brought about by this organism and have

achieved selective separation of galena from sphalerite.

In the present study we investigate the surfacechemical changes on pyrite and chalcopyrite after

interaction with A. thiooxidans in view of selective

separation of pyrite from a mixture of pyrite and

chalcopyrite.

2. Materials and methods

2.1. Minerals

Pure handpicked mineral samples of pyrite were

obtained from Alminrock Indser Fabricks, Bangalore

and chalcopyrite from Gregory, Bottley and Lloyd, UK.The mineral samples were dry-ground with a porcelain

ball mill and dry-sieved to obtain different size fractions.

The 106+75 m fraction was used for flotation

studies. The 37 m fraction was further ground in a

Retsch mortar grinder. The particle size analysis of this

sample was done using a Malvern Mastersizer 3000-

model and the mean size was found to be 5 m. This

fraction was used for adsorption and electrokinetic

studies. The minerals were stored in a desiccator under

nitrogen atmosphere. The surface area was estimated by

BET nitrogen specific surface area method and wasfound to be 1.26 m2/g for pyrite and 1.61 m2/g for

chalcopyrite respectively. The purity of the mineral

samples were ascertained by mineralogical studies and

X-ray diffraction using a JDX-8030 X-ray diffractom-

eter system.

2.2. Microorganism

The bacterial culture used was a strain of A.

thiooxidans (MCM B41) obtained from MACS, Pune.

The bacteria were cultured in sterile modified 9K basal

medium containing 2 g/l ammonium sulfate, 0.25 g/

l dipotassium hydrogen phosphate, 0.5 g/l magnesium

sulfate and 10 g/l sulfur powder.

2.3. Growth of bacteria and preparation of cell pellet

The sterilized medium was inoculated with 10% (v/v) of active inoculum in 250 ml standard Erlenmeyer

flasks. The flasks were incubated at 30 C on a rotary

shaker at 200 rpm. The bacterial count was monitored

by a direct count method using a Petroff Hausser counter

viewed with a Leitz phase contrast microscope (Labro-

lux K Wild MPS12). The solution pH was also regularly

monitored using a Systronics digital pH meter.

The grown culture was initially filtered through

Whatman 42 filter paper to remove the precipitates. The

filtrate was then centrifuged at 12,000gfor 20 min in a

Sorvall RC-5B refrigerated high-speed centrifuge at5 C. The pellet obtained was resuspended in pH 2

H2SO4solution and then centrifuged as before to obtain

metabolite free cells.

2.4. Adsorption studies

Culture containing cells in the exponential growth

phase was centrifuged to obtain the cell pellet. This

pellet was washed twice with pH 2 H2SO4solution and

suspended in 100 ml 103 M KCl solution at the desired

pH in 250 ml standard Erlenmeyer flask. One gram of

the mineral sample was pulped in the cell suspensionand the resulting slurry was agitated on a rotary shaker at

200 rpm for 30 min for equilibration. After equilibra-

tion, the slurry was vortex mixed for 1 min to remove

loosely held cells, centrifuged at 500g for 5 min to

settle the mineral particles and the cell number in the

supernatant was recorded. For experiments on adhesion

kinetics, the above procedure was repeated at regular

intervals and the cell data with respect to time recorded.

2.5. Electrokinetic studies

The electrophoretic mobilities of the mineral samples

before and after interaction with the bacterial cells was

determined using a Malvern Zetasizer 3000 instrument.

KCl solution (103 M) was used as the base electrolyte

in all the experiments. One gram of the mineral sample

was interacted with the desired cell concentration and

pH for the required time. The slurry after interaction was

centrifuged at 500gfor 3 min to settle only the mineral

particles. The mineral sample was resuspended in KCl

solution, vortexed and washed two to three times to

remove loosely held cells. The mineral sample so

obtained was equilibrated in KCl solution that was

147M.N. Chandraprabha, K.A. Natarajan / Hydrometallurgy 83 (2006) 146152


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preadjusted to the desired pH before taking the

measurements. Electrophoretic mobility of cells after

interaction with the minerals was also recorded by

equilibrating the interacted cells in 103 M KCl solution

at different pH. In all the experiments 5 readings were

recorded and the results reported represent the averagevalue. The standard deviation was


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shifted to around pH 3 and 3.4 respectively. The

electronegative character of the mineral also reduced

after interaction with cells. The shift in iep and also the

reduction in the electrophoretic mobility for chalcopy-

rite was lower compared to that of pyrite.

Interaction of bacterial cells with minerals not onlyalters the surface properties of the minerals but also has

effect on the surface properties of the bacterial cells as

shown in Fig. 1(c). The iep of cells before interaction

with minerals is located around pH 2.9 and consequent

to interaction with the minerals the iep is shifted to

higher pH values. The magnitude of shift is higher for

pyrite-interacted cells compared to chalcopyrite-inter-

acted cells. The electrophoretic mobility of the cells is

also reduced consequent to interaction with the

minerals with the magnitude in reduction being higher

for the pyrite-interacted cells. This reduction in theelectrophoretic mobility can be attributed to the

proteinaceous secretion resulting from microbeminer-

al interaction and indicated specific adsorption of cells

on minerals due to chemical interaction [4]. The

observed difference in the electrokinetic behaviour of

the minerals and cells consequent to their mutual

interaction confirms that the cells exhibit selectivity

towards the minerals.

3.2. Adhesion studies

The adhesion kinetics ofA. thiooxidans at an initialcell concentration of about 6.25 108 cells/ml is

depicted inFig. 2. The kinetics of adsorption is similar

on both the minerals and the adsorption equilibrium is

attained in about 80 min. Presence of copper ions in the

chalcopyrite lattice does not seem to affect the kinetics

of adhesion of cells on its surface. However, the cell

density on the surface of chalcopyrite was lesser when

compared to that on pyrite, confirming that the bacteria

shows selectivity towards pyrite. This could be due to

the presence of toxic copper ions in the lattice of

chalcopyrite [12,13]. This explains the observed

difference in the electrokinetic behaviour of these

minerals consequent to their interaction with the

bacterial cells.

The adsorption isotherm ofA. thiooxidans for pyrite

and chalcopyrite at pH 2.5 is depicted in Fig. 3. The

adsorption density increases with increase in the

equilibrium number of cells up to about 2.2 1011

cells/100 ml and 71010 cells/100 ml respectively forpyrite and chalcopyrite and thereafter a plateau is

reached for both the minerals. The adsorption density is

higher on pyrite compared to chalcopyrite. The structure

of chalcopyrite is built up of copper, iron and sulfide

ions and presence of toxic copper ions may result in

lower adsorption density of A. thiooxidans cells on

chalcopyrite compared to pyrite.

Both the isotherms exhibit Langmuirian behaviour

confirming monolayer coverage of cells. Fig. 4 shows

the scanning electron micrograph ofA. thiooxidanscells

adhering onto pyrite after 1 h interaction. It is clear fromthe photograph that the cells form monolayer coverage

on the mineral surface.

3.3. Microflotation tests

Flotation recovery of pyrite and chalcopyrite as a

function of pH at different collector (potassium

isopropyl xanthate, PIPX) concentrations are shown in

Fig. 5a. Pyrite was most floatable under acidic

conditions and the recovery dropped with increase in

alkalinity. This is due to the changes in solubility of iron

hydroxides on the pyrite surface as the pH changes[14].

0 40 80 120 160

1010

1011

6.125x10

8

cells/ml

6.25x108cells/ml

Cellsadsorbed(cells/m

2)

Time (min)

PyriteChalcopyrite

Fig. 2. Adsorption kinetics of A. thiooxidans cells on pyrite andchalcopyrite.

107 108 109

1010

1011

pH 2.5

Cellsadsorbe

d(cells/m2)

Equilibrium concentration (cells/ml)

PyriteChalcopyrite

Fig. 3. Adsorption isotherm of A. thiooxidans cells on pyrite and

chalcopyrite.



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By contrast, the recovery of chalcopyrite remained

almost constant at all the pH studied.The flotation recovery of pyrite and chalcopyrite

after interaction with bacterial cells and further

conditioning with the collector are shown in Fig. 5b.

Both the minerals were completely depressed upon

interaction with cells. When pyrite, interacted with cells,

was further conditioned with collector, at higher

collector concentrations the mineral exhibited some

floatability in the acidic pH region. The maximum

recovery at 0.5 mM PIPX was about 30% and it

increased to 41% when the collector concentration was

increased to 1 mM. By contrast, conditioning with cells

did not significantly reduce the flotation recovery of

chalcopyrite. As seen from the figure, when the mineralinteracted with cells was further conditioned with

0.5 mM and 1 mM collector concentration, the recovery

obtained was 68% and 76% approximately at all pH

studied.

The alkylxanthate species formed when pyrite is

treated with an aqueous solution of potassium alkyl-

xanthate is dialkyl dixanthogen, which is hydrophobic,

and physisorbed to the surface in aqueous systems. In

case of chalcopyrite, the alkylxanthate ions are coordi-

nated to specific Cu ion sites on the mineral surface

forming solid copper (I) alkylxanthate, which is highlyhydrophobic and strongly chemisorbed to the surface

[15]. This explains the observed difference in the

floatability of the two minerals after interaction with

the cells and collector.

3.4. Differential flotation tests

Microflotation studies suggest possibility of selective

separation of pyrite from chalcopyrite consequent to

their interaction with A. ferrooxidans cells. Results of

the differential flotation studies of a 1:1 synthetic

mixture of pyrite and chalcopyrite is tabulated inTable1. Two conditions were tested. In the first case the

minerals were interacted with the cells and/or reagents

separately, and the interacted minerals were mixed

together and floated. In the second case, the minerals

were interacted together with the cells and/or collector

and floated together.

In the first condition, when the minerals were

conditioned individually and floated, consequent to

interaction with 4 108 cells/ml bacterial cells and

further conditioning with 0.5 mM collector, the recovery

of pyrite and chalcopyrite were 23.7% and 63.4% at pH

20

30

40

50

60

70

80

90

100 (a)

0.5 mM 1 mM PIPXFlotationrecovery(%

)

Pyrite Chalcopyrite

4 5 6 7 8 9 10

20

30

40

50

60

70

80 (b)

Pyrite Chalcopyrite

Flotationrecovery

(%)

pH

Cells (4x108 cells/ml)

Cells + 5x10-4M PIPX

Cells + 1x10-3M PIPX

Fig. 5. (a) Flotation behaviour of pyrite and chalcopyrite with respect

to pH at different collector (PIPX) concentration. (b) Effect of cells onxanthate-induced flotation of pyrite and chalcopyrite.

Table 1

Differential flotation of pyritechalcopyrite (1:1) mixture after

conditioning with cells followed by collector

Flotation

pH

Experimental

condition

Weight % in floated fraction

Pyrite Chalcopyrite

4.5 Individually 23.7 63.4

Together 41.3 69.1


Together 32.3 72.3

Conditions: PIPX: 0.5 mM (5 min).Cells (pH 4.5): 4 108 cells/ml (5 min).

Fig. 4. Scanning electron micrograph showing attachment of A.

thiooxidans cells on pyrite.

150 M.N. Chandraprabha, K.A. Natarajan / Hydrometallurgy 83 (2006) 146152


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4.5 respectively. However, when the minerals were

conditioned together the recovery of pyrite increased to

41.3% and that of chalcopyrite increased to 69.1%.

Similar behaviour was observed at pH 6.5.

Galvanic interactions between a noble and an active

mineral reduce the floatability of the noble mineral, the

effect on the active mineral being minimal[16]. Blank

flotation tests carried out on pyrite

chalcopyrite mixtureafter preconditioning the minerals in distilled water

showed that the flotation of pyrite was slightly increased

when conditioned in the presence of chalcopyrite while

that of chalcopyrite remained almost the same [17].

Thus, the observed increase in the flotation of

chalcopyrite in Table 1 could be attributed to the

preferential adsorption of the cells onto pyrite surface,

i.e., due to the decrease in cell adhesion on the

chalcopyrite surface when both the minerals are present

together. However, the increase in the floatability of

pyrite could not be explained with this.

The effect of copper ions on the flotation behaviourof pyrite was studied earlier and it was observed that the

recovery of pyrite in the acidic pH region was increased

in the presence of copper [17]. The copper ions were

able to activate pyrite surface and increase its flotation.

Dissolution behaviour of chalcopyrite under different

conditions was tested and the results obtained are

tabulated in Table 2. The concentration of copper

recorded in the solution when chalcopyrite and pyrite

are conditioned together was less than that recorded

when chalcopyrite was conditioned alone. This indi-

cates that the copper released to the solution from thechalcopyrite surface might be migrating to the pyrite

surface. To test this, pyrite and chalcopyrite of two

different size fractions were conditioned together. The

conditioned minerals were separated by sieving and

acid digested for copper analysis. Presence of copper in

the pyrite-digested sample indicated that the copper

released from chalcopyrite surface was migrating to

pyrite surface (data not shown). As seen from the table,

there was dissolution of copper ions from the chal-

copyrite surface even in the presence of cells. Hence,

the observed increase in the recovery of pyrite was due

to copper dissolution from chalcopyrite resulting inpyrite activation [18]. Similar behaviour was observed

at pH 6.5.

When conditioned with the collector, copper disso-

lution from chalcopyrite was very low indicating that the

interaction of collector with the surface copper ions to

form stable Copper(I) alkylxanthate species was

Table 2

Dissolution behaviour of chalcopyrite under different conditions

(15 min conditioning)

Conditions Dissolved Cu (ppm)

Chalcopyrite (pH 4.5) 2.14

Chalcopyrite + pyrite (pH 4.5) 1.53Chalcopyrite + cells (pH 4.5) 1.73

Chalcopyrite+103 M xanthate (pH 6.5) 0.16

0

20

40

60

80

100

PIPX

Cells

Cells

+PIPX

PIPX+

cells

PIPX

Cells

Cells

+PIPX

PIPX+

cells

FlotationRecovery(%) Pyrite

Chalcopyrite

Fig. 6. Effect of sequence of conditioning with cells and collector on the flotation recovery of pyrite and chalcopyrite.

Table 3

Differential flotation of pyritechalcopyrite (1:1) mixture after

conditioning with collector followed by cells

Flotation

pH

Experimental

condition

Weight % in floated fraction

Pyrite Chalcopyrite

4.5 Individually 20.3 81.4Together 21.3 86.2


Together 19.3 84.6

Conditions: PIPX: 0.5 mM (5 min).

Cells (pH 4.5): 4 108 cells/ml (5 min).



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instantaneous thereby reducing the dissolution of copper

from the surface. This observation suggests that the

initial conditioning of the minerals with the collector

would suppress the activation of pyrite.

The effect of bacterial cells on the minerals

conditioned with the collector was further studied toascertain the flotation behaviour of the minerals. Fig. 6

shows the effect of the sequence of conditioning with

collector and cells on the flotation behaviour of pyrite

and chalcopyrite at pH 4.5. It was observed that the

bacterial cells were able to effectively depress xanthate-

interacted pyrite while having negligible effect on the

xanthate-induced floatability of chalcopyrite. It was also

interesting to note that the initial interaction with

collector followed by conditioning with cells improved

the flotation of chalcopyrite compared to the earlier

case. Thus, with this sequence of interaction betterseparation of the minerals can be obtained. Differential

flotation results of the 1:1 pyritechalcopyrite mixture

after conditioning with the collector followed by

interaction with cells are given inTable 3. Under these

conditions the selectivity was markedly improved, i.e.,

pyrite recovery was significantly reduced while chalco-

pyrite recovery was above 80%. The bacterial cells were

able to effectively depress collector interacted pyrite

even when the minerals were conditioned together.

Similar behaviour was observed when the minerals were

conditioned together with cells and collector simulta-

neously (data not shown). Thus it was possible toselectively float chalcopyrite with significant pyrite

depression at both acidic and alkaline pH.

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