Experimental study on triboelectric charging of mineral particles

Journal of Electrostatics, 23 (1989) 157-168 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

157

EXPERIMENTAL STUDY ON TRIBOELECTRIC CHARGING OF MINERAL PARTICLES

R. CICCU, R. PERETTI, A. SERCI, M. TAMANINI and A. ZUCCA Dipartimento di Ingegneria Mineraria e Mineralurgica - Centro Studi Geominerari e Mineralurgici del CNR Universita' degli Studi di Cagliari 09123 CAGLIARI, I t a l y -

SUMMARY The relevant aspects of tribocharging as a mineral processing technique are dealt with in the paper. The experimental set up and the instrumentation used in the basic research are described in d e t a i l . Charging results are represented as a function of mineral-related properties, rubbing surface c h a r a c t e r i s t i c s and experimental conditions. Results so f a r achieved are discussed with the aim of developing a suitable model f o r i n t e r p r e t i n g tribocharging phenomena. INTRODUCTION Selective separation of tribocharged mineral p a r t i c l e s fed into a f r e e - f a l l chamber provided means of

concentrating a worth component

objectionable pilot are

with f i e l d electrodes is gaining increasing i n t e r e s t

plant

impurities

from

a crude

from an already upgraded

ore

product.

or

in the near f u t u r e ,

beneficiation is concerned. However the

basic

especially as far as coal

a

removing

Laboratory

r e s u l t s are encouraging and prospects of i n d u s t r i a l

promising

as

and

application

and

phosphate

(refs. 7 - 8)

charge

transfer

phenomena is not yet f u l l y understood,

mechanism underlying

tribocharging

probably because of the fact that many

processes are taking place concurrently. In

fact,

carriers

either

across the

electrons interface

or ions or both can betweentwo

solio

be

substances in

depending on whether moisture coating films are present. charging may occur as the result

of

stresses,

abrasion,

on.

gas or vapour adsorption,

These may overlap

involved

charge

contact,

Moreover secondary

thermoelectric effects,

the main contact

as

mechanical

emission of electrons, and so

e l e c t r i f i c a t i o n mechanism, thus

rendering the overall charging process quite complex. (ref. 6) Natural

minerals have distinct semiconductor properties in that

characterized

by

0304-3886/89/$03.50

a band structure typical

of

they

are

semiconducting solids:

the

© 1989 Elsevier Science Publishers B.V.

158

conduction band,

normally empty, is separated from the valence band by a more

or less wide forbidden gap where localized levels are always present. Minerals

can be

either

n-type

when localized

p-type in the case of predominant acceptor states. the

threshold

of the i n t r i n s i c f i e l d ,

holes, respectively.

charging

prevail or

At temperatures well below

conduction is mainly by electrons

or

(ref. 4)

The electrophysical contact

donor states

properties of mineral crystals of major

phenomena

are

measured with

interest

suitable

in

laboratory

instrumentation. Contact (usually

potential difference with respect to a stable reference

electrode

gold) gives an i n d i r e c t evaluation of the work function which

on the findings of e a r l i e r i n v e s t i g a t i o n is thought to affect charge

based

polarity

in tribocharging phenomena. Electric intrinsic which

conductivity field

as

a function of absolute temperature

allows the determination of the forbidden gap

up

to

amplitude

the on

the overall possible extent of Fermi level s h i f t i n g by means of thermal

treatments depends. Finally

at s u f f i c i e n t l y low temperatures the Seebeck c o e f f i c i e n t

provides

information on the kind of p r e v a i l i n g conduction and enables the electrons-toholes r a t i o to be roughly estimated. All

these

different

(ref. 5)

data are being collected f o r a number of mineral

nature

and o r i g i n in order to find a r e l i a b l e

specimens of

model f or

contact

charging phenomena. The

results so fa r obtained suggest that t r i b o e l e c t r i c charge p o l a r i t y

in

most cases is in f a i r l y good agreement with the work function of the

mineral

substance

are n i l

r e l a t i v e to the contact surface provided that disturbances

or at least minimal. The charge f i n a l l y retained is strongly influenced by leakage phenomenadue to e i t h e r conductance, properties,

in

corona emission or both,

particular

surface

depending on mineral-related

and bulk conductivity,

as

well

as

on

environmental conditions, namely temperature and humidity. Moreover contact conditions (kind,

energy and time) play a very

important

role too, as already demonstrated in previous research work. Of mineral

course

the

particle.

charge is d i r e c t l y related to the size and

shape of

Generally coarser p a r t i c l e s hold greater charges

not proportionately on account of leakage phenomena. (ref. 6)

the

although

159

Charges

on metals

substances owing

like

to

are

glass,

substantially

smaller

than

p l a s t i c and ceramic materials,

on

non-conductor

but more consistent

the fact that highly r e s i s t i v e surfaces hold s t a t i c charges

up

to

saturation thus modifying t h e i r a b i l i t y to charge transfer. Regarding histograms

charge

distribution,

f o r most

monometric

minerals

frequency

can be s a t i s f a c t o r i l y interpreted by the Gauss model or less often

by a log-normal law. Minerals of the other c r y s t a l l o g r a p h i c groups are instead characterized

by

superimposition

complex

distributions

apparently

resulting

from

the

of two or more simple d i s t r i b u t i o n s probably one for each

of

the d i f f e r e n t crystal faces. Typical charge frequency histograms for single grains of b a r i t e ,

fluorspar

and c a l c i t e upon contact on a nickel chute are reported in a previour

paper.

Whereas simple d i s t r i b u t i o n models hold f o r fluorspar (monometric system) and calcite,

for

barite

two

superimposing

experimental data better.

log-normal

distributions

fit

the

(ref.6)

LABORATORY SET UP In order to f u r t h e r elucidate mineral grain tribocharging against a rubbing surface a research programme has been undertaken with the aim of assessing the influence of the relevant variables under varying experimental conditions. For t h i s purpose a suitable device f o r studying well

defined

vibrating

experimental

plate

interchangeable ceramic

or

with rubbing

plastic

conditions has been set

controlled

frequency

surfaces are clamped;

material) can be heated to

Grains characterized by mineral nature, fed

through

frictional well

a chute

the charging process under

and

It

consists

amplitude,

of

a

on

which

the surface (made of

metal,

predetermined

temperatures.

mass, size and shape are i n d i v i d u a l l y

from a revolving dispenser;

contact on the surface,

up.

after

a

given

each grain is discharged into

time a

of

Faraday

connected to a high input impedence d i g i t a l electrometer interfaced with

a computer for automatic data storage and s t a t i s t i c a l c a l c u l a t i o n s . The system in

housed

in a vacuum-tight box where environment

conditions

(temperature,

humidity and gas pressure) can be c a r e f u l l y controlled. The set up is shown in Figure I. The v i b r a t i n g plate is driven by means of two pairs of electromagnets: first the

the

controls the frequency and amplitude of transversal v i b r a t i o n generating relative frictional

action while the second,

after t i l t i n g

the plate

in

160

| . . . . .

1

-

2 3 4 5 6 7 8 9 I0

-

Revolving dispenser Photocell Chute t i l t i n g device Feeding duct Disk d r i v i n g motor Clamped chute Electromagnets Vibrating plate Faraday well Shielded feedthrough pin

FIGURE I . D e t a i l of the charging device.

the

d i r e c t i o n of the discharge s l o t ,

superlmposes a

longitudinal

vibration

d i r e c t e d to dump the g r a i n i n t o the Faraday w e l l . Both

frequency and amplitude can be v a r i e d independently from 1.4 to

23.8

Hz and from 2 to 8 mm r e s p e c t i v e l y , by d r i v i n g the p l a t e w i t h a wave generator and a d j u s t i n g a b u i l t - i n conditions

leverage system.

can be reproduced,

impingement-and-rebound

In t h i s way a wide range of rubbing

from g e n t l e f r i c t i o n

contacts.

Correspondingly,

w i t h o u t apparent break to the

frictional

energy

involved can be also v a r i e d . A the

mica-insulated electric

r e s i s t a n c e i s sandwiched between the

plate

and

bottom of the rubbing surface whose temperature can be c o n t i n o u s l y v a r i e d

up to 250 %. The

mineral

consisting

of

grains a

are

fed

to the

charging

device

metal r e v o l v i n g disk having a sequence

via of

a

dispenser

through

holes,

161

drilled

along the outer border,

measurement the

where single grains are housed.

After

disk is rotated one step while s l i d i n g t i g h t l y on

a

each steady

disk of the same diameter provided with a c o l l i m a t i n g hole thus bringing a new grain ready f o r t e s t i n g . Step by step hole matching allowing the grain to f a l l onto the surface is c o n t r o l l e d by a photocell. The

vacuum system comprises a removable bell-shaped box laying on

plate

equipped

with a set of feedthrough pins for a l l the functions

device as well as with the a i r o u t l e t and gas i n l e t valves. down to

0.01Pa a booster pump is s u f f i c i e n t

a

base

of

the

For medium vacuum

while f o r high vacuun down to

0.0001 Pa a second stage d i f f u s i o n pump is actuated. The

instrumentation

capable computer

consists of a Keithley 617 programmable

of detecting charges in the femto-Coulomb range, for

interfaced

data storage and s t a t i s t i c a l c a l c u l a t i o n s ;

potentiometer for thermocouple temperature control; for heating to preset temperatures; multi-purpose

tester;

electrometer

a

with

a

vacuum gage;

a

a variable voltage supply

a frequency adjustable wave generator;

a DC voltage source f o r studying the e f f e c t of

a

charge

density on the rubbing surface.

EXPERIMENTAL PROGRAMHE The

study is scheduled over a long period in order to c o l l e c t enough

data

for constructing and v e r i f y i n g a r e l i a b l e charging model. The general research programme includes: -

characterization of a s u f f i c i e n t l y broad range of mineral species of various composition (oxides, by

measuring

the

sulphides,

carbonates, s i l i c a t e s , sulphates, halides)

electrophysical

properties

tribocharging such as the work function,

of

relevant

interest

the e l e c t r i c c o n d u c t i v i t y and

for the

Seebeck c o e f f i c i e n t as functions of absolute temperature; - measurement

of

the

frictional

e l e c t r i f i c a t i o n charges of

under varying experimental conditions (temperature, upon contact

against d i f f e r e n t

substances

mineral

grains

humidity, gas pressure)

(metals,

glass,

plastic

and

ceramic materials) possibly having known electrophysical properties; -

attempt to f i n d a comprehensive model capable of explaining contact charging phenomena, as a useful tool f o r commercial separator design. The research, started some years ago, has recently been resumed with the aim

of

gaining

improve

the

a deeper i n s i g h t into the s c i e n t i f i c fundamentals needed to mineral processing techniques based on

triboelectric

help

charging.

162

Actually

the main reason why e l e c t r o s t a t i c separation has not yet found

i n d u s t r i a l application l i e s in the scarce understanding of physical

wide

processes

underlying the experimental f i n d i n g s . Experience

gained

h i t h e r t o indicates that charge

polarity

substantially

depends on the nature of c o l l i d i n g substances. ~efs. I-3) In t y p i c a l e l e c t r o s t a t i c separators fed with a stream of dry ground mineral p a r t i c l e s two kinds of contacts can take place: themselves

and/or

successfully i.e.

against

solving

a

target

surface.

impingement between p a r t i c l e s A necessary

requisite

any separation problem is that charging be

consistent,

e a c h mineral component takes the same p o l a r i t y in e i t h e r of

cases.

This

for

the

above

condition is met provided that the properties of the surface l i e

between those of the species to be separated.

( r e f . 4)

Often i n t e r - p a r t i c l e charging is the most important as can be observed with the

'Turbocharger',

the l a t e s t separator being developed at the

Mining

and

[viineral Engineering Department of the University of C a g l i a r i . For instance, i t has

been found that coal p a r t i c l e s are p o s i t i v e l y charged in the presence

of

carbonate gangue and negatively with s i l i c a t e . Separation can be successful in each case but becomes unfeasible when both kinds of gangue are present at the same time. Therefore this

end

shaped will

so

charging by mutual f r i c t i o n

rubbing

is a key point

to

investigate.

surfaces cut from natural mineral specimens

as to be clamped onto the v i b r a t i n g plate of the

be systematically tested.

and

suitably

charging

device

Minerals w i l l then be ranked according

more complete and accurate t r i b o e l e c t r i c series,

To

to

since that proposed in

a the

l i t e r a t u r e is often u n r e l i a b l e . As

for

the operating variables and environmental

conditions,

the

broad

experience already gained proves that the temperature of the mineral grains or of

the

contact

surface

or of both is often a decisive

factor,

like

for

instance in the case of phosphate b e n e f i c i a t i o n . Air

humidity

leakage rate;

also plays a very important role since

it

controls

charge

in most cases a dry environment is highly beneficial for strong

and selective charging. As for rubbing energy,

the greater the better,

as c l e a r l y demonstrated by

the results of charging experiments with a i r microcyclones at increasing i n l e t pressures.

163

TEFLON

-12 10 C

CERAMIC

150

PERSPEX

I00,

50

ALUMINUM ,,A,NLE"

35

N'C,EL

I00 200

35

100 20°

CO''E"

35

100 200

35

IOO 200

CAOM,UM

35

IO0 200

35

65

35

I00 200

35

100 200

FIGURE 2. Average charge c, individual grains of fluorspar upon contact d i f f e r e n t metals and.insulating materials. Grain size range: -2.0 +I.7 mm. o Plain oars: rooin atmosphere (20-25 C; 40-60% R.H.) Hatched bars: under vacuo (0.01Pa)

on

RESULTS The more recent tests with the above described charging device dealt

with

fluorspar and quartz on d i f f e r e n t metals at varying temperature and humidity. Tests have been also extended to i n s u l a t i n g materials l i k e perspex,

teflon

and a ceramic obtained by f i r i n g soapstone at 1200°C. (see figures 2 and 3) Mineral grains, screening, water

recovered from selected samples of ores by comminution and

were c l a s s i f i e d according to shape and weight, washed in d i s t i l l e d

in an ultrasound vessel,

dried at moderate temperature and stored in a

vacuum desiccator. For

e a c h combination

statistical

about 50 measurements were

representativity;

during

deemed necessary

each test the kind of contact

for

( e it h e r

r o l l i n g , s l i d i n g or jumping) was observed and correlated to charge. Re p r o d u c i b i l i t y

was

always very good provided that

c a r e f u l l y c o n t r o l l e d and the surface cleaned.

the

conditions

were

164

CERAMIC TEFLON 12

C

I0

90

50

35

100 200

35

STAINLESS STEEL

IO0 200

35

~O0 200

35

100 200

35

100 2010

~

L U l l ~l-llJ ~ - i i

NICKEL

ALUMINUM

COPPER

35

65 35

35

100 200

100 20Q

CADMIUM

PERSPEX

FIGURE 3. Average charge of individual grains of quartz upon contact on d i f f e r e n t metals and i n s u l a t i n g materials. Grain size range: -2.0 +1.7 mm. ¢ l a ~ udrS: ro0,~ ~,~osphere (20-25°C; 40-60% R.H.) Hatched bars: under vacuo (0.01 Pa) DISCUSSION AS expected,

the greatest charges are achieved in the case of contact with

i n s u l a t i n g materials fo r which leakage phenomena by both conductance or corona emission charges

are

minimal.

remain

interfaces

This

localized

is due to the fact that on

at the contact points

insulator

whereas

at

an e l e c t r i c double layer with high charge density

surfaces

particle-metal is

established

generating an intense e l e c t r o s t a t i c f i e l d . However as

time

elapses charge density on in s ulat or

surfaces

gradually

increases up to saturation with the consequence that the conditions f or charge transfer

across

the interface are continuously

modified;

on

the

contrary

grounded

metals maintain a steady density due to charge m o b i l i t y so that

the

contact e l e c t r i f i c a t i o n process is e s s e n t i a l l y independent of time. This e f f e c t is susceptible to contact energy: i f i t is too low, charging is generally achieve

affected consistent

to

a greater extent so that high energy is

charging over a long time as in the

case

required of

to

industrial

165

separation

processes. Actually the most e f f i c i e n t separators are those where

contacts

are

achieved by high speed impingements in turbulent medium or

by

sliding on bent surfaces in a centrifugal f i e l d . Concerning the contradictory. gradients

kind

In

of

contact,

the

results

obtained

are

the absence of thermoelectric effects due to

feeble

conductor minerals on metals take the highest

continuous

sliding

motion and the

smallest

when r o l l i n g ;

occasional

separation gives intermediate charges.

temperature charges for sliding

On the contrary

minerals

on metal surfaces attain the highest charges on r o l l i n g

smallest

on

sliding.

somewhat

with

conductor and the

This can be explained by the charge leakage mechanism

mentioned previously. The opposite inside

the

majority

can be observed in the presence of

a temperature gradient

mineral grain for instance when i t is fed onto a heated surface:

carriers

towards the

(electrons

in n-type or holes in

p-type

minerals)

cool zone away from the particle-metal interface

thus

move

possibly

impairing the charge exchange process which can be restarted by bringing contact

into

a new zone with a r o l l i n g movement. Of course this aspect deserves

more attention and must be further checked with suitable experiments. Heating effect

I t has the

dual

of gradually removing the moisture coating on the mineral surface

shifting i.e.

is often a decisive factor in contact charging.

the Fermi level upwards in p-type and downwards in n-type

f r o m the

average level

of localized states to the

and

minerals,

midpoint

of

the

forbidden gap in the temperature range beyond the i n t r i n s i c f i e l d threshold. Therefore can

the energy structure of mineral substances in the surface

be modified

in

selective charging.

order to achieve the

most favourable

layer

conditions

for

An interesting example is offered by phosphate ores with

carbonate gangue: t r i b o e l e c t r i c separation is satisfactory i f carried out at temperatures

higher than 120 - 130 °C and deteriorates

rapidly

below this

c r i t i c a l point. As the diagram of figure 2 shows, charges acquired by fluorspar grains on metals decrease with temperature at room conditions and increase

in

vacuo

s t i l l remaining positive in both cases. This Moisture

is

due to

many effects which however are

difficult

may account in part for the different behaviour since

reasonably

to it

isolate. can

be

assumed that under vacuum conditions humidity is desorbed even at

low temperatures whereas in the open air drying is

gradually

produced by

166

heating. A second factor is the metal surface oxidation which is p r a c t i c a l l y n i l in vacuo

at

any temperature but is enhanced by heating

in

air.

Actually

the

greatest differences in behaviour have been observed with copper which is more susceptible to oxidation and the least s i g n i f i c a n t with stainless steel. Therefore

tests

in

vacuo where both oxidation and moisture

coating

are

avoided or at least s u b s t a n t i a l l y reduced, can be assumed as representative of the

e f f e c t of Fermi level s h i f t i n g at increasing temperature.

selected

was

a p-type mineral,

so that i t s a t t i t u d e

to

The fluorspar

release

electrons

increases with temperature as apparently confirmed by charging experiments. Finally

the

experimental

thermoelectric

procedures

e f f e c t must not

be

underestimated.

adopted at the present stage of

the

mineral

grains were fed cool onto the surface heated at a given

Contact

time

the

grain

gradient

research,

the

temperature.

to the same temperature as the surface;

therefore

bring

a temperature

was established between the contact point and the outer

end

giving

which somewhat affect the charge transfer mechanism. In

case of p-type minerals with work function lower than that of the rubbing

surface since, the to

the

was of the order of 30 seconds which was i n s u f f i c i e n t to

rise to p o l a r i z a t i o n : the

In

and thus p o s i t i v e l y charged, according to G~e band model,

thermoelectric effects

are

favourable

the F.L. of p-type minerals is raised at

hot extremity of the grain (the contact zone) thus increasing i t s a b i l i t y release

electrons.

Similar condition holds in p r i n c i p l e

f or

negatively

charged n-type minerals against a hot surface. On the contrary, f or negatively charged

p-type

minerals

and f o r

positively

charged

n-type

minerals

thermoelectric effects are detrimental due to the formation of a a b a r r i e r

at

the i n t e r f a c e , eventually stopping the charging process. The e f f e c t of heating was even more pronounced in the cases of contact insulator

surfaces.

on

Instead of the monotone trend exhibited by metals here a

peak charge was observed at a temperature around I00 °C, especially in vacuo. In

addition to the effects already mentioned f o r particle-metal

the modification of the substance i t s e l f produced by heating, metals

(apart

mentioned

here.

from

contacts,

negligible

oxidation in the open a i r experiments), should

also

f or be

Charges were always about one order of magnitude higher than

on metals and again p o s i t i v e . As

f a r as quartz is concerned,

obtained,

charges with negative p o l a r i t y

have

been

except fo r contacts on soapstone-fired ceramic and t e f l o n . Moreover

167

charges were comparatively irregular

smaller (about half) maybe because of

the

more

shape characterized by sharp edges favouring corona leakage. The

presence of

peak charge versus temperature for

contact

against

insulator

surfaces is less evident here than in the case of fluorspar and sometimes even absent, although this phenomenon is d i f f i c u l t to explain. From the

above i t

emerges that grains of fluorspar

and quartz

having

properties similar to those of the samples examined can be selectively charged by f r i c t i o n a l

electrification

between themselves and/or

against

a metal

surface as well as against perspex. Actually separator

a separation

made of

cleaning

test carried out with

stainless steel yielded

and two scavenging stages,

the

electrostatic

successful

results:

cyclone

after

one

a concentrate with a CaF2 content higher

than 97% was obtained with an overall recovery s l i g h t l y lower than 90%. In

the

light

separator

are

the finaings of the

has recently

Laboratories. blades,

of

been designed

The charging

basic

research,

and b u i l t

at

a new advanced the

Department's

device consists of a rotor provided with

housed in a coaxial frustum-shaped target surface.

radial

Mineral particles

thrown against the target where both impingement and sliding contacts are

achieved.

Charged particles then enter the free f a l l chamber where they

separated according to charge polarity. Further

improvements are

are

(ref. 9)

anticipated as soon as the

basic

research in

progress allows the tribocharging mechanisms to be better c l a r i f i e d .

CONCLUSIONS The experimental approach i l l u s t r a t e d in the paper seems quite f r u i t f u l that

i t allows to c o l l e c t systematic information capable of

elucidating

in the

physical processes underlying contact e l e c t r i f i c a t i o n phenomena. However the research is far from being concluded owing to the complexity of the

problem where many variables,

especially

when

dealing

with

often d i f f i c u l t natural

to i s o l a t e ,

minerals

having

are

involved

ill-defined

electrophysical properties. Despite

this

drawback,

the results h i t h e r t o obtained have

already

shed

f u r t h e r l i g h t on t h i s subject. Research carried out with the f i n a n c i a l support from the M.P.I. the C.N.R. and EniChem-ANlC

(40 and 60%),

168

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2

3

4

5

6 7

8

9

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