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The adverse effect of disrupted water-borne bacteria cells on flotation

Wenying Liu, C.J. Moran, Sue Vink

PII: S0301-7516(16)30230-7DOI: doi: 10.1016/j.minpro.2016.10.007Reference: MINPRO 2970

To appear in: International Journal of Mineral Processing

Received date: 6 May 2016Revised date: 10 October 2016Accepted date: 26 October 2016

Please cite this article as: Liu, Wenying, Moran, C.J., Vink, Sue, The adverse effect ofdisrupted water-borne bacteria cells on flotation, International Journal of Mineral Process-ing (2016), doi: 10.1016/j.minpro.2016.10.007

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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ACCEPTED MANUSCRIPTThe adverse effect of disrupted water-borne bacteria

cells on flotation

Wenying Liu*1, C. J. Moran, and Sue Vink

Sustainable Minerals Institute, The University of Queensland, Brisbane, Queensland, 4072, Australia

Abstract

Existing studies have demonstrated the potential threat of bacteria to copper and gold flotation when

mineral processing operations use bacteria-laden water, such as treated sewage effluent, as an

alternative water source. Once being introduced into a mine water system, water-borne bacteria may

experience cell disruption causing the release of constituent organic molecules into process water.

However, little is known about the potential impact of these organic molecules released from disrupted

bacteria cells on flotation performance. In this study, using the same representative system as previously

used with intact cells, we investigated the effect of disrupted cells on flotation. This representative

system consists of E. coli cells disrupted by sonication (also called lysed cells) as the model bacterium

and three copper-containing mineral systems of increasing complexity as the ore model. It was found

that the disrupted cells had a negative effect on copper flotation, which tended to weaken as ore

composition became more complex. The negative effect of the disrupted cells on copper flotation was

more pronounced than that of the intact cells. Flotation of gold and pyrite was found to be depressed as

well possibly due to the preferential adhesion of the lysed cells on pyrite. Findings in this study enhance

understanding and management of the risk posed by bacteria-laden water which is being increasingly

used as an alternative water source for mineral processing.

Keyword: Bacteria-laden water; Treated effluent; Lysed bacteria cells; Mineral processing

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ACCEPTED MANUSCRIPT1. Introduction

Water sources that contain a variety of bacteria at varying concentrations, such as treated effluent, are

being increasingly accessed by the mining industry as alternative water sources to minimize freshwater

withdrawal (Brereton et al., 2008; Schumann et al., 2003; Slatter et al., 2009). Bacteria can exist in these

water sources as intact cells which maintain their structural intactness, either viable or dead. Bacterial

growth is sensitive to a range of physical, chemical and biological factors in the surrounding

environment, with different optimal conditions for different types of bacteria (Adamberg et al., 2003;

Leroi et al., 2012). When the surrounding conditions are beyond the range that a bacterial cell can grow,

intact cells undergo cell disruption or cellular breakdown, a process called bacterial lysis (Geciova et al.,

2002; Lewis, 2000). Examples of physical factors that can induce cell disruption are the exposure to

temperatures beyond the growth temperature range (Hagen et al., 1964; Weiser and Osterud, 1945), and

to mechanical forces, such as grinding with kieselguhr and sand (Kelemen and Sharpe, 1979). Cell

disruption by high concentrations of monovalent cations, detergents, or chelating agents are examples of

chemically induced cell lysis (Tsuchido et al., 1995). UV radiation applied to disinfect water and

wastewater is an example of cell disruption by photobiological factor (Chang et al., 1985). Cell

disruption by any means would result in the release of a range of organic molecules contained in

bacteria cells, such as polysaccharides, proteins, and DNA (Madigan and Martinko, 2006).

Under the context of using bacteria-laden water for flotation, bacteria cells may be disrupted in different

components of a mine water system due to physical, chemical, and biological factors associated with

these components. According to a published framework (Liu et al., 2013c), these factors can be

considered from within a concentrator (also called internal factors) and outside a concentrator (also

called external factors). Within a concentrator, cell disruption may occur in grinding mills due to the

mechanical forces exerted by grinding media and ore on bacterial cells; in the flotation circuits, cell

disruption may occur due to exposure of the bacterial cells to the harsh conditions beyond its tolerance

limits, such as high pH and the addition of chemicals. The disrupted cells may be immediately brought

back to the concentrator by water internal reuse, where water is recovered and reused without passing

through tailings storage facilities. Cell disruption may also occur outside a concentrator due to exposure

to chemicals and UV in water and tailings storage facilities. These disrupted cells can be brought back

to the concentrator through water external reuse, where water is recovered from tailings storage

facilities and then reused in the concentrator.

To understand the potential risk of water-borne bacteria to flotation performance, we have previously

studied the effect of intact cells on flotation (Liu et al., 2013a). That study was carried out in a

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ACCEPTED MANUSCRIPTrepresentative system consisting of E. coli in intact form as the model bacterium and three copper-

containing mineral systems of increasing complexity as the ore model, i.e., high-purity chalcopyrite,

simulated ore with controlled gangue minerals, and porphyry copper-gold ore. The intact cells were

found to have a negative effect on copper and gold flotation (Liu et al., 2013b). Compared with the case

of intact cells, there is a lack of knowledge on the potential impact on flotation associated with organics

released from disrupted cells. To understand this, we investigated the effect of the lysed cells on

flotation using the same representative system. Flotation tests were done in two ways: by addition of

deliberately disrupted cells by sonication to flotation; and by addition of the intact cells to the grinding

step prior to flotation. The latter was done because grinding is an integral step in mineral processing,

where cell disruption is likely to occur.

2. Experimental

2.1. Materials

2.1.1. Bacterial strain

The model bacterial strain used was the same as previous experiments (Liu et al., 2013a). A pure culture

of E. coli strain K12 was cultured in LB medium. The cell pellets were washed and suspended in a PBS

solution (phosphate buffer saline). The bacterial cell concentration was measured by optical density

(OD600) using a Nanodrop ND-1000 spectrophotometer (Thermo Scientific).

2.1.2. Mineral samples and reagents

The three types of ore used, i.e., high-purity chalcopyrite, simulated ore with controlled gangue, and

porphyry copper-gold ore, were prepared according to the same procedures as outlined in the previous

work (Liu et al., 2013a). High-purity chalcopyrite (90%) and pyrite (95%) were crushed with a roll

crusher. The crushed samples were used to prepare the simulated ore, which was a mixture of 24%

chalcopyrite, 33% quartz, and 43% pyrite. The copper-gold ore, referred to as the real ore in this study,

was from a copper-gold porphyry deposit (0.35% copper, 0.74 ppm gold). Just prior to flotation, the

high-purity chalcopyrite and the simulated ore were dry-ground with a pulverizer to a particle size of

D80 of 150 μm. The real ore was wet-ground using a rod mill to a particle size of D80 of 106 μm. All

chemicals used were of analytical grade.

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ACCEPTED MANUSCRIPT2.2. Flotation test procedure

Flotation tests with the three types of ore were conducted as per the procedure described previously (Liu

et al., 2013a). Baseline tests were carried out using distilled water without the addition of the lysed cells.

To test the effect of the lysed cells on flotation, different concentrations of E. coli cells were added

either in the flotation step as lysed cells or in the grinding step as intact cells.

For the flotation of the high-purity chalcopyrite and the simulated ore, 100 g of ore samples was mixed

with 500 mL of distilled water to form slurries. The slurry was transferred to a 1.5 L flotation cell with

the impeller speed set at 750 rpm. The pulp pH was the natural pH of the ground sample, which was

approximately at pH 8.0. Sodium ethyl xanthate was added as the collector to a concentration of 200 g/t

for the high-purity chalcopyrite and 50 g/t for the simulated ore. The frother used was MIBC (methyl

isobutyl carbinol), with a concentration of 200 g/t for both samples. Air flow rate was maintained at 5

L/min for the high-purity chalcopyrite and 3 L/min for the simulated ore. Concentrates were collected at

1, 2, 4, and 8 min.

For the real ore flotation, 1 kg of the ore sample was ground with 500 mL of distilled water to a particle

size of D80 of 106 μm. The slurry was transferred to a 2.5 L flotation cell with the impeller speed set at

1000 rpm. The pulp pH was the natural pH of the ground ore, which was approximately pH 8.3.

AEROPHINE® 3418A and PAX (potassium amyl xanthate) were added as collectors to a concentration

of 10 g/t and 20 g/t, respectively. MIBC was added as the frother to a concentration of 35 g/t. Air flow

rate was maintained at 3 L/min. Concentrates were collected at 1, 3, 5, and 10 min. To test whether cell

disruption could occur in the grinding step, the intact cells were added into the grinding mill, the slurry

from which was used for the subsequent flotation process.

The concentrates and tails were filtered, dried in an oven at 70 ºC overnight and weighed for the

analysis. Samples were analyzed for copper by Inductively Coupled Plasma-Atomic Emission

Spectroscopy (ICP-AES), and for gold by Atomic Absorption Spectroscopy (AAS).

The standard errors were calculated and presented as error bars in all charts.

3. Results and Discussion

3.1. Flotation of the high-purity chalcopyrite

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ACCEPTED MANUSCRIPT

consist of 15% protein, 7% nucleic acids, 3% polysaccharides, and 2% lipids and

metabolites, with the remaining 73% being water (Watson, 2003). These organics could have absorbed

onto mineral particle surfaces and

3.2. Flotation of the simulated ore

.

the percentage

decline in final recovery based on the recovery of the baseline test. The higher the absolute value of the

relative recovery decrease, the stronger the depression effect. Fig. 2 (B) shows that t

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The galvanic reaction between chalcopyrite and pyrite leads to a decrease in chalcopyrite

surface negative charges and an increase in the surface hydrophobicity (Buckley and Riley, 1991;

Fairthorne et al., 1997; Zachwieja et al., 1989). This causes the electrostatic repulsion between the

negatively charged lysed cells and chalcopyrite to reduce and therefore the attachment of the lysed cells

to chalcopyrite surfaces to increase (Han and Lee, 2006), leading to an intensification of the depression

effect on the simulated ore. Similar to the case of high-purity chalcopyrite, fitting the cumulative

recovery data to the classic first order kinetic model showed that

suggested that the disrupted bacteria cells negatively affected flotation kinetics of chalcopyrite in the

simulated ore.

dividing chalcopyrite recovery by pyrite recovery)

3.3. Flotation of the real ore

In almost all cases, ore deposit that a mine site deals with is more complex that the simulated ore. Real

ore generally contains only a small proportion of valuable minerals and a large proportion of gangue

minerals. This could lead to more complex interactions of the lysed cells with different valuable and

gangue minerals and multiple mineral-mineral interactions. To gain some insights into these

interactions, flotation tests were carried out using copper-gold ore (referred to as the “real ore”) from a

copper-gold porphyry deposit.

Fig. 4 shows the effect of the lysed cells on the flotation recovery and kinetics of copper and gold in the

real ore. The negative effect of the lysed cells on copper flotation recovery was present in the real ore

(Fig. 4 (A)). As the concentration of the lysed cells was increased, the copper recovery decreased and

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ACCEPTED MANUSCRIPTcopper final grade did not seem to be affected even though there was an initial drop in copper grade

(Fig. 4 (C)). Similarly, flotation of gold was also negatively affected by the lysed cells. Gold recovery

and grade decreased with increasing concentration of the lysed cells (Fig. 4 (B) and (D)). The presence

of the lysed cells also negatively affected flotation kinetics (Fig. 4 (E) and (F)) with decreasing rate

constant k and maximum recovery Rmax at increasing concentration of the lysed cells.

The effect of the lysed cells on copper and gold flotation recovery was different. Fig. 5 shows that the

relative recovery decrease of gold was more remarkable than that of copper, especially at higher

concentrations of the lysed cells. This corresponded to an increase in the selectivity index between

copper and gold dividing copper recovery by gold recovery. This result suggested an

affinity of the lysed cells for gold-bearing particles. Given that gold is frequently associated with pyrite

(Möller and Kersten, 1994), the possible preferential adhesion of the lysed cells to gold-bearing particles

might be related with the preferential adhesion of the lysed cells to pyrite, as discussed in the case of the

simulated ore.

The relative recovery decrease of copper in the presence of the lysed cells was compared among the

three mineral systems, the result of which was shown in Fig. 6. Given that the pulp density of the three

mineral systems was different, the ratio of the number of the lysed cells to the weight of the solid

particles was used for the comparison instead of the initial bacterial concentration. The relative recovery

decrease of copper was the least significant with the real ore, the mineral system that had the most

complex mineralogy. It seemed that the negative effect of the lysed cells weakened as ore mineralogy

became more complex, possibly due to more complex interactions between valuable and gangue

minerals and between minerals and the organic molecules. Nonetheless, this seemingly minor decrease

could have a significant economical implication for mine operations.

To understand whether cell disruption could occur in the grinding step, intact cells were added to the

grinding mill prior to flotation. Fig. 7 (A) shows that at the lowest bacterial concentration, the negative

effect of the lysed cells was the greatest, with the effect of adding the intact cells to the grinding step

being similar to that of the intact cells. This suggested that the intact cells were not disrupted by

grinding at this bacterial concentration. In contrast, the effect of the intact cells became the greatest at

the highest bacterial concentration. This was suspected to be caused by weaker adsorption selectivity of

the lysed cells than that of the intact cells. In other words, the lysed cells could have been consumed by

multiple gangue minerals as opposed to the more dedicated adsorption of the intact cells, leading to the

intact cells eventual strongest effect. At this bacterial concentration, addition of the intact cells to the

grinding step exerted a similar effect as the lysed cells, suggesting the occurrence of cell disruption in

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ACCEPTED MANUSCRIPTthe grinding step. Fig. 7 (B) shows that gold flotation was not affected by the form of the bacteria cells.

It should be noted that the reproducibility of some gold flotation tests was poor.

4. Conclusions

This study assessed the effect of bacteria cell disruption on copper flotation when using bacteria-laden

water for mineral processing. The experimental results show that the disrupted bacterial cells had a

negative effect on copper as well as gold flotation. The magnitude of the negative effect was different

among the three mineral systems used, with the real ore being the least affected. This was possibly

caused by the complex mineral-mineral and mineral-organics interactions in the real ore system. It

seemed that the negative effect weakened as ore mineralogy became more complex. The negative effect

of the lysed cells was stronger than that of the intact cells in the case of the high-purity chalcopyrite and

the simulated ore. In the case of the real ore, the difference depends on the bacterial concentration. At a

lower bacteria concentration, the lysed cells had a stronger effect than the intact cells. This was reversed

at a higher bacterial concentration, which was explained by the possibility that the intact cells had

stronger selectivity than the lysed cells. The disruption of the intact cells was likely to occur in the

grinding step at higher bacterial concentrations given that the intact cells added in the grinding mill

behaved similarly to the lysed cells. From the applied perspective, factors such as ore mineralogy,

concentrations of bacteria in water, and the locations where bacteria-laden water is applied, must be

taken into consideration to ensure the effective use of bacteria-laden water for mineral processing. On

the one hand, any attempt to disinfect process water with bactericides may induce cell disruption leading

to an enhanced negative effect on flotation. One the other hand, possible preferential adsorption of

disrupted bacteria cells to pyrite presents opportunities of using tailings as adsorbent to remove bacteria

cells from process water prior to its use in flotation.

Acknowledgements

Funding for this study was provided by Australian Research Council linkage project (LP0883872)

“Impact of recycled and low quality process water on sustainable mineral beneficiation practices”,

which is part of AMIRA project P260E. All flotation tests were carried out in the lab at Julius

Kruttschnitt Mineral Research Centre (JKMRC).

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ACCEPTED MANUSCRIPTCaptions

Fig. 1. (A) Mass recovery of the high-purity chalcopyrite as a function of time at different concentrations of the lysed

cells (without addition of bacterial cells in the baseline test); (B) Comparison of the effect of the lysed cells with that of

the intact cells on flotation of the high-purity chalcopyrite; (C) Effect of the lysed cells on flotation kinetics

Fig. 2. (A) Copper recovery as a function of time at different concentrations of the lysed cells for the simulated ore

(without addition of bacterial cells in the baseline test); (B) Comparisons between the high-purity chalcopyrite and

the simulated ore and between the lysed cells and the intact cells; (C) Effect of the lysed cells on flotation kinetics

Fig. 3. Relative recovery decrease and selectivity index as a function of the concentration of the lysed cells in the

flotation of chalcopyrite and pyrite in the simulated ore

Fig. 4. Effect of the lysed cells on copper (A) and gold (B) recoveries, copper (C) and gold (D) grades, and copper (E)

and gold (F) flotation kinetics, in the flotation of real ore

Fig. 5. Relative recovery decrease and selectivity index as a function of the concentration of the lysed cells in the

flotation of copper and gold in the real ore

Fig. 6. Comparison of the relative recovery decrease of copper as a function of the ratio of the number of the lysed

cells to the weight of the mineral solid for the three mineral systems

Fig. 7. Comparison of the effect of the intact cells, lysed cells and the addition of the intact cells into the grinding mill

in the flotation of copper (A) and gold (B)

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ACCEPTED MANUSCRIPTHighlights:

The effect of disrupted E. coli cells on copper and gold flotation was studied.

Flotation recovery and kinetics were negatively affected by the disrupted cells.

More complex mineralogy tended to weaken the negative effect of disrupted cells.

The disrupted bacterial cells may preferentially adhere to pyrite surfaces.

The disrupted cells had more pronounced effects on flotation than the intact cells.