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http://slidepdf.com/reader/full/paper81-56d9dba010fed 1/14

TRENDS ON FINE PARTICLES FLOTATION: CHEMICAL SPECIFICITY AND

ENGINEERING CONCEPTS LINKED TO OPTIMIZE ELECTROFLOTATION

PROCESS

G. Montes-Atenas1 and R. Mermillod-Blondin

2

1 Laboratoire environnement et Mineralurgie INPL-ENSG, UMR 7569 du CNRS. BP40. 54501. Vandoeuvre-les-Nancy.

France.

Email : [email protected] .2 Unité de Recherche et de Service en Technologie Minérale, Université du Québec en

Abitibi-Témiscamingue, 445, boul. de l’Université, Rouyn-Noranda (Qc), J9X 5E4

Canada.

Email : [email protected] .

I. Introduction

Froth flotation process constitutes one of the most versatile procedures to performselective mineral separation at large scale. In consequence it is not surprising that the

optimization of such process be fruitful through scientific community [King, 2001;

Somasundaran, 1980]. Classical flotation process involves three major phases

interacting at same time: solid (mineral), liquid (aqueous solution) and gas.

The pulp phase is determined in terms of solid-liquid ratio. There, when the mineral has

gained enough hydrophobicity [Ralston et al., 2001; Montes and Montes Atenas, 2005;

Ata et al., 2004; Yuhua and Jianwei, 2005] achieved often by collector adsorption, solid

particles are ready to interact with the arising front of bubbles. It is also important to

state that every mineral phase has its own self-flotation behaviour. This latter fact is

conceived through flotation tests without any further mineral surface modification but

its immersion into aqueous solution.

The effective collisions between hydrophobic mineral and bubbles, which travel across

the pulp phase or particle suspension, derive towards a reversible attachment.

Subsequently, the mineralized bubbles go upwards and mineral phase is recovered at the

top of the aqueous system.

In order to get a flotation system working properly there are several conditions and

physical constraints that must be accomplished simultaneously. A first attempt to

understand such type of systems has been performed studying each sub-system

separately. For example, froth dynamics [Grassia et al., 2002; Neethling and Cilliers,

2001], particle-bubble attachment [Wang et al., 2005; Nguyen, 2004], collectoradsorption on surface mineral [López Valdivieso et al., 2004], bubble stability [Bhakta

and Ruckenstein, 1996; Duan et al., 2004; Magny et al., 1992], etc. This orientation will

be briefly addressed in section II.

More specifically, when flotation is set facing fine particles problem the overall process

becomes utterly inefficient [Yekeler and Sonmez, 1997]. This evidence, in research, has

led to split both classical flotation procedures and fine particles separation. Therefore,

new technologies such as dissolved-air flotation or water electrolysis to produce the

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gases which will act as mineral carriers to perform the separation, among others have

been extensively studied [Ketkar et al., 1991; Venkatachalam S., 1992; Burns et al.,

1997]. Strikingly electroflotation development has been focused on wastewater

treatment [Khelifa et al., 2005; Gao et al., 2005] and not fine mineral particles recovery.

One aspect maybe not considered is that it is quite easy to identify several aspects

between the experimental conditions associated to the new trends assessed and classical

flotation pathways and configuration [Fuerstenau et al., 1957]. This work aims to try tovisualize how the chemistry and the system configuration aspects might make

improvements on fine particles flotation. Particularly, the process to be addressed in the

present work will be the electroflotation. The aimed orientation will be the construction

of a link between the fundamental basis of flotation process and engineering issues.

II. Brief description of the interacting elements present in flotation process.

This section does not pretend to be an authoritative description of how flotation carries

out. Instead, it is going to be an attempt to state a set of issues related to the mentioned

process leading to a final statement exposing a pathway to follow in its scientific

research.

II.1.- Mineral-collector interaction: Currently one of the most well understandable sub-

systems inside mineral processing. When any solid phase, and particularly, a mineral

sample, is immersed into an aqueous medium, the appearing of a surface charge is

almost unavoidable. This latter fact comes from the electrical nature that all materials

possess. After introducing the so-called surface active agents the value of surface charge

may change. This behavior leads towards the thermodynamical and electrical

equilibrium where charges try to reach a type of compensation. This compensation

where positive and negative ionized charges are attracted tends to equilibrate the final

global charge of particles [Sposito, 1998; Kosmulski, 2003]. The mineral surface turns

out to be more hydrophobic and subsequently closer to the natural hydrophobic

behavior of gases. Those surface active agents to be introduced are the so-called

“collectors”. These reagents can have either organic or inorganic nature. Furthermore,

some of them have both characteristics: an organic tail with a strong hydrophobic nature

and an ionic part which ensures both strong association to other opposite charges in

solution and sometimes solubility increasing. The latter molecular structures are the

basis of the named surfactant specie. Depending on the energy associated to the bonding

between mineral surface and collectors it has been arbitrarily denoted a physisorption or

chemisorption having the last one a higher energy. In order to fix ideas, the first type of

sorption process has been associated to oxides due to their slight capacity of producing

strong bonds between aqueous molecules and oxide surface while the second type are

quietly adequate to describe sulphides behavior mainly due to their high reactivity in

facing electrochemical reactions [Woods, 1988]. Nevertheless, new trends have

demonstrated that pathways in which adsorbed molecules have a much more generalinherent characteristic due to the heterogeneous surface energy and it is required further

kinetics information to clarify the overall adsorption process. What is more or less well

defined and there is consensus among the scientific community is that mineral particles

require of a certain grade of surface hydrophobicity to have more chances to get an

efficient collision with a gas phase and be recovered with the froth. Research studies

have focused on the understanding of collector single and multi-layers orientation over

mineral surface and adsorption isotherms [Mielczarski and Mielczarski, 2005].

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II.2.-Bubble-surfactant interactions: Broadly reported, this subject has enormous

applications in the industrial field. It is well known that the foams stabilization and froth

phase formation come with the introduction of surfactants which decrease the surface

tension [Wanless, 1997]. Specifically, the air-liquid interface where this type of

molecules is fixed allows the decrease, sometimes sharp, of surface tension. This

fixation is not easy to well describe. In fact numerous issues might happen when a

bubble is formed and even several scenarios might take place when two or more bubblesattach generating bi-interfaces [Weaire et al., 2001]. First of all, the surfactant has

intrinsically a tendency to goes towards air-liquid interface. This natural tendency

implies that a fraction of surfactant molecules will be fixed at that interface but not

everyone. Moreover, there will be a kinetics given by molecular diffusion which will be

critical to foam behavior. After molecules are fixed at the interface the next step is to

understand how they become ordered, if that is the case. At this point of discussion the

concept of self-assembled layers comes naturally [Tchoukov et al., 2003]. In fact, that is

the case of proteins and other macromolecules which give strong characteristics of

flexibility to a bi-interface; however, when this self-assembling and the subsequent

liquid drainage take place the film thinning may arise leading to other type of instability

pathways. As a consequence of that, it has been at least identified two major factors

affecting bubbles stability: (i) capacity of surfactant to provide flexibility to films and(ii) surfactant effect on thin films rheology (which is quite different from aqueous bulk

rheological properties [Durand, 2002]).

The flexibility associated to an aqueous bubble structure is the starting point to visualize

how important is the dynamics involved in foam movement. In fact, when a foam film

is deformed the surface area changes and so the structural parameters related to the

surfactant association at that interface. When a foam film is deformed in time the

interface surface area increases and the surfactant population at the interface changes

making the film goes to other more instable state. This is the effect named Marangoni

[Monteux, 2004]. Then, it is clear that not only the lowering of surface tension is

important but also the dynamics involved on that decrease. The next stage is, therefore,

to get the knowledge about the surfactant mobility at the interface and how fast can they

get ordered after some disturbance previously applied.

II.3.-Bubble-mineral interactions: After mineral hydrophobicity is achieved other

important factor to be evaluated is the number of bubble-mineral collisions per unit of

both time and volume. Following the same type of rationalization to ideal gas molecules

collisions, the energetic condition to make a collision effective must overcome some

energy threshold [Nguyen and Schulze, 2004a]. The fluid-dynamics simulates well the

hydrodynamics of a liquid flowing around a solid material. It has been accepted that

fluid streamlines are formed around foams moving upwards which represent an implicit

type of shield for solid particles travelling towards the bubble. If solid particles are very

small, they will flow through those streamlines without interacting efficiently with bubbles. That fact has been usually taken into account to explain the low efficiency of

classic froth flotation process to recover fine particles. Therefore, in order to solve this

problem it should be searched other pathways to produce smaller bubbles. Several

alternative techniques have been evaluated to accomplish this aim, such as dissolved air,

electroflotation, etc. Apparently, not a great success has been obtained since every

further scientific research has been mainly focused on reagents optimization [Schulze,

1983, Batterham and Moodie, 2005].

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III.- Chemical and engineering configuration compatibility: Reagents specificity

and its connection with the practical conception on fine particles flotation.

Application to electroflotation process.

In the present section it is going to be described a possible research path to follow.

Compatibility among macro- and micro- properties is a subject inherent to every

industrial process. It deals with solving the question: How to make the system do what’srequired? The answer to that question relies in connecting logically all elements set

within the system. For instance, when it is said collector is compatible with a specific

mineral phase, the selectivity may be achieved. The reliability of the separation process

will depends also on the slight affinity to other mineral phases present.

The enhancing of flotation recoveries by mixing two or more collectors does not

represent just selectivity but also compatibility among the reagents to achieve the

separation stage. Nevertheless, compatibility between frothers and collectors or frothers

and depressants are still badly understood mainly due to the diversification of reagents

in nature and behaviour.

III.1.- Overall process of electroflotation.

The basic principle of such process lies on water electrolysis [Koshla et al., 1991].

Subsequently, it is not surprising that it will be determined among other variables on pH

and temperature. To state the reactions taking part, they can be assumed at first instance

as follows,

−+

++ ⎯→ ⎯ eO H OO H 446 322 [1]

O H H eO H 223 222 + ⎯→ ⎯ + −+

[2]

The protonated water shows to be equivalent in both consumption and generation of positively charged specie. Half-reaction [1] is proposed to be achieved in the anode

while the second, at the cathode. Venkatachalam (1992) has performed a rather

complete analysis about the experimental consequences of producing gas through

electrolysis. Some of them are going to be exposed below.

If one of the gases (oxygen or hydrogen) is preferably used to perform the electro-

flotation, the aqueous speciation will change. In fact, this change might be dramatic

nearby the electrode. Similar situation will occur if the other gas is used. As result of

that, the additives ionic state might change as well, given, perhaps further reaction

stages and/or generating undesirable sub-products. Furthermore, it could be take place

the electrode coating process which might stop the gas evolution.

If the current density is significantly low, both convective a diffusive mechanisms lead

to gas to be dissolved and no bubbling will occur. The latter will happen even at high

current densities which represents a waste of energy. Besides, this problem comes in

addition to the inherent chemical irreversibility of both electrode reactions which set the

voltage applied be significant. Among other variables the electrode quality has a key

role on this issue and corresponds to the field named electro-catalysis which goes

beyond the scope of the present work. From the process point of view, during gas

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nucleation, other mechanisms start to work such as bubble coalescence. This

mechanism make bubbles to coarse and produce bubbles of larger size decreasing the

bubbling surface available and inevitably an inefficient global process.

Also, it has been reported that hydrodynamics plays a significant role [Nguyen and

Schulze, 2004b], therefore a coherent stirring might be an adequate way to avoid at

some grade the coalescence effect. The relationship between bubble size and current

density has not been yet correctly explained.

With all facts stated above, further research pathways arise to be taken into account

depending on the particular interface involved, such as:

Gas/liquid interface: Bubble size must be decreased to accomplish a better flotation

process [Diaz-Peñafiel and Dobby, 1994]. However, this element depends on bubble

generation mechanisms and aqueous environment. The extensive literature covers from

how to generate foams even in presence of anti-foaming agents to the establishment of

mathematical conditions that bubbles must attain in order to remains at certain size with

the maximum stability.[Neething et al., 2000] Modelling and the subsequent simulation

have proved to be useful to understand bubbles and froth behaviour [Grassia et al.,

2002].The research, then, may be focused on the development of new surfactants toimprove foaming behaviour and the mixing of active surface agents to enhance a one or

more specific properties [Manev and Pugh, 1993].

Liquid/Solid Interface: One of the major advantages of using gases obtained by

electrolysis is the high reactivity of them. In fact, it has been observed that some

minerals may float even in absence of collector [Glembotsky et al., 1975]. The gas can

activate mineral surface in order to make improvements on its recovery. However, it is

also important to mention that gases might also act against recovery depending on

mineral nature.

Further analysis regarding gas characteristics have been pointed out by some authors.

Since bubbles are stabilized by the adsorption of surfactants in the boundary between air

and liquid phases, then it should be expected that bubbles might bear a net charge that in

presence of fine mineral particles may aid to increase the solid recovery.

Gas/solid Interface:

The size of bubbles is pH dependent. In alkali medium, bubbles of oxygen are bigger

than those of hydrogen. The contrary occurs in acid medium at same electrode

conditions where the tendency can be verified [Glembotsky, 1973]. After bubbles are

detached the next step is to collect the fine mineral suspended into the aqueous solution.

There, gas phase superficially covered by surfactants collides with the mineral phase

covered by the so-called collectors. Normally, at the classic froth flotation scenario this

type of interaction is not considered since the particles and bubbles are so large thatother mechanisms more connected with momentum and hydrophobicity govern the

overall process. Instead of that, when particles are very small not only electrostatical

forces become important but also the compatibility between charges and molecular

structures may play a significant role. This fact has been previously stated [ref] in

relation that a frother does not have same behavior in front of collectors from different

nature.

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III.2.- About the engineering design to optimize fine particles flotation.

As shown in the previous part, from the microscopic point of view, the improvement of

the fine particles flotation is mainly reached by increasing the ratio of particle diameter

to bubble size. This increase is accomplished either by increasing particle size by

flocculation or by decreasing bubble size. From the industrial point of view, a kind of

macroscopic point of view, the machines can be designed to favor the collision process between particles and bubbles. These collisions are mainly influenced by the

hydrodynamic conditions into the flotation machines [Dai et al., 2000]. Yoon and

Luttell [1989] claimed that the turbulent hydrodynamic conditions of conventional

mechanical flotation machines constitute a constraint to collisions between fine particles

and bubbles. According to that assumption several flotation machines was developed to

present low turbulent hydrodynamic conditions: flotation column, microbubble

flotation, agitated column. In addition to unfavorable hydrodynamic conditions, Zhou et

al. [1997] declare that the kinetic energy of the fine particles is not sufficient to rupture

the liquid film surrounding bubble. In order to increase the kinetic energy of the fine

particle, some flotation machines have been designed to force bubble – particle collision

in specific devices: Jameson cell with external contractor, flotation cyclone, flotation

centrifuge, turbulent microflotation. The table 1 presents different flotation machinesthat favor the fine particles flotation. The table 1-A present the characteristics of the low

turbulent flotation machine. The table 1-B shows the characteristics of the flotation

machines with forced collision. More details on those types of flotation machines can be

found in the references and in the synthesis of Demers [2005].

Conventional flotation machines generate large bubble size (between 600 to 2000 µm)

[Wills, 1997]. Some spargers can generate mid-bubble size (from 100 to 600 µm)

[Yoon, 1993], but is could be insufficient for the ultrafine particle flotation (< 10 µm)

[Rubio, in preparation]. The generation of microbubble (< 100 µm) is therefore required

and can be achieved either by processes like electroflotation, dissolved air flotation or

electrostatic spraying of air [Burns et al., 1997]. The electroflotation produces the

smallest bubble diameters, sizing between 20 to 40 µm [Ventakachalam, 1992, Burns et

al., 1997].

However, the generation of microbubbles by the electroflotation process has been less

investigated, as a mineral processing technology, during the last decade because of the

high power consumption, the expensive cooling system required because of the

formation of joule heat and the cost of high-quality electrode material [Schulze, 1983].

Moreover, the few investigations of electroflotation have been performed in traditional

flotation cells [Schulze et al., 1983, Burns, et al., 1997]. Therefore, there are many

opportunities to investigate the engineering design of the flotation cell according to the

generation of microbubbles by electroflotation. In addition to the optimization of the

pulp conditions (solution pH, solution concentration, temperature, reagents nature andconcentration), the electrode material could be chosen and decrease the electrodes cost

[Ventakachalam, 1992]. The power consumption could be limited by a pulsed system

that allows a bubble-size control [Khosla et al., 1991]. The heat formation and the

electrode degradation, because of the flotation reagent, could be decreased by a fluid

circulation system around the electrode. This fluid circulation would allow the control

of the solution environment during the bubble generation and the transportation of the

microbubbles in the flotation reactor. The electrogeneration of microbubbles seems to

be a relatively low turbulent process and therefore flotation machines as column

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flotation, where particle-bubble collision and separation occurs in the same reactor,

appear to be a suitable choice for electroflotation. Some new column flotation systems

have been recently proposed to target fine particles flotation [Gu and Chiang, 1999,

Rubio, in preparation], but they involved traditional spargers. Those columns flotation

can be adapted to use microbubbles generation by electrodes and therefore improved

fine particles flotation with a high selectivity.

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Table 1: Synthesis of the characteristics of the flotation machines that favor fine

particles flotation

A: Characteristics of the flotation machines with low turbulent hydrodynamic

conditions

Type of flotationmachine

Flotation columnMicrobubble

flotationAgitated column

Main characteristicFeed enter at the top

of the column

Feed enter at the top

of the column

local and slow

agitation in the

column favors

collision

Separation zoneA column (several

meters high)

A column (several

meters high)

A column with

static mixers

Collision zone

The collision is

located in the

separation zone

The collision is

located in the

separation zone

The collision is

located in the

separation zone

Hydrodynamic

condition of the

collision

Countercurrent flow

close to linear flow

Countercurrent flow

close to linear flow

Countercurrent and

low turbulent flow

in the column

Bubbles generation

Spargers at the

bottom of the

column

Spargers

specifically

designed for

microbbubble

generation

Spargers

Bubble sizeDepends on gas

flow rate and

sparger type

Between 100 to 400µm depending of

the sparger

Depends on sparger

type

Other advantage High selectivity High selectivity High selectivity

Development level IndustryLaboratory –

industryLaboratory

References

Castillo et al., 1988;

Finch and Dobby,

1990; Yoon, 1993;

Keyser et al., 1996

Yoon, 1993

Harris et al., 1992;

Ityokumbul et al.,

2000

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B: Characteristics of the flotation machines with a forced collision

Type of flotation

machine

Jameson cell with

an external

contractor

Flotation cyclone or

flotation centrifuge

Turbulent

microflotation

Main characteristicThe collision zoneis located outside

the separation zone

A centrifugal force

field is applied to

the flotation

process

Coagulation or

flocculation of particles using

reagents and

agitation

Separation zoneA column

(few meters high)

The separation zone

is horizontal

following the

centrifugal force

filed

A narrow tube

(pipeline)

Collision zone

The collision is

forced and

controlled in acontactor

The collision

occurs in the

external part of theseparation zone

Heterocoagulation

of particle

aggregates and bubbles

Hydrodynamic

condition of the

collision

Highly turbulent

flow in the

contractor

Highly turbulent

flow with a vortexTurbulent flow

Bubbles generation Spargers

Gas aspiration

nozzle

Air spargers

Bubble sizeDepends on sparger

typeMid-size bubble

Very small bubbles

(≤ 40 µm)

Other advantage

The collision can be

controlled

independently of

the separation

process

Reduction of the

particle – particle

interactions

Fast flotation

High selectivity

Very fine particle

recovery (≤ 1 µm)

Development level Industry Laboratory Pilot

ReferencesTortorelli et al.,

1997

Tils and Tels, 1992

Rubio, in

preparation

Rulyov, 2001

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IV.- Conclusion

According to our present knowledge, there are still possibilities of generating

improvements on fine particles separation. Particularly, on electroflotation field, which

is the finest bubble generation process of the mineral processing industry, several

variables might be considered as opportunity to enhance fine mineral particles selective

separation. Striking behavior at both micro and macroscopically must be taken intoaccount together in order to figure out the essential role of each element inside the

system.

Acknowlegments

R. M.-B. would like to express his gratitude to Unité de Recherche et de Service

en Technologie Minérale, Université du Québec en Abitibi-Témiscamingue and G.M.-

A. thanks to the Polytechnique National Institute. Lorrain. France.

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Zhou, Z.A., Xu, Z., Finch, J.A., Hu, H. and Rao, S.R., Role of hydrodynamic cavitation

in fine particle flotation. International Journal of Mineral Processing, 51:139-149, 1997.

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