Dry triboelectrostatic beneficiation of fly ash.pdf

8/11/2019 Dry triboelectrostatic beneficiation of fly ash.pdf

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E L S E V I E R

PIh S0016-2361 97)00045-8

uel

Vol, 76, No. 8, pp. 801-805, 1997

1997 E lsev ier Science L td . A l l r igh ts r eserved

Printed in Great Britain

0016-2361/97 17.00+0.00

r y t r i b o e le c t r o s t a t ic b e n e f ic i a tio n

of f ly ash

He n g Ba n T i a n X . Li J a m e s C . Ho we r J o h n L . S c h a e f e r a n d

J o h n M. S t e n c e l

Center for App l ied E nergy R esearch, Universi ty o f Kentucky, 3572 Iron W orks Pike, Lexington,

KY 40571-8433, USA

Received 1 February 1996; revised 10 Jan uary 1997)

A laboratory-scale triboelectrostatic separation system in conjunction w ith analytical techniques was used to study

fly ash beneficiation. Fly ash samples were characterized by size analysis and carbon content an d then subjected to

dry triboelectrostatic separation. Due to differences in the surface physical and chemical properties of the carbon

and ash, particles of unburnt carbon and fly ash were triboelectrically charged to opposite polarity and then

separated by passing them through a static electric field. Ash fractions deposited on the positive and negative

electrodes were collected, analysed for carbon content and subjected to SEM and petrographic analyses. The

results indicate that the physical and chemical properties of the ash dictate the m aximu m carb on-a sh separation

that would be possible. In addition, the potential of dry separation technology for removing unburnt carbon from

coal ash was demonstrated.

1 9 9 7 E l s e v i e r S c i e n c e L t d .

( K e y w o r d s : f ly a s h ; b e n e f i c i a t io n ; t r i b o e l e c t r o s t a t i c s e p a r a t i o n )

M o s t c o a l c o m b u s t i o n a s h p r o d u c e d i n t h e U S A i s

lan d f i l l ed , th e co s t o f w h ich in so me s ta tes h as r i s en to

m o r e th an 3 0 / to n . Th is f in an c ia l b u r d en , in c r eased in te r es t

i n i m p r o v i n g t h e e n v i r o n m e n t a l a n d s o c i a l i m p a c t s o f c o a l

u t i l i za t io n , an d th e p o ss ib i l i ty o f reco v er in g co n s t i tu en ts o f

f ly ash an d /o r u s in g i t f o r in d u s t r ia l ap p l ica t io n s ( e . g .

cemen t , s t r u c tu r a l an d p a in t /p las t i c f i l l e r s , cen o sp h er es ,

mag n e t i t e an d sp ec ia l ty ca r b o n s ) p o in t to i t s f u tu r e p o ten t ia l

as an imp o r tan t in d u s t r ia l c o m m o d i t y ~.

A n im p o r tan t p r o p er ty o f f ly ash w h ich l imi t s i t s u se is th e

c a r b o n c o n t e n t . H o w e v e r , r e d u c i n g t h e c a r b o n c o n t e n t t o

m e e t C l a s s F o f t h e A S T M C 6 1 8 - 8 9 a a n d C 3 1 1 - 9 0

sp ec i f ica t io n s 2 f o r u se o f a sh as a min er a l ad m ix tu r e in

P o r t l a n d c e m e n t c o n c r e t e i s n o t a n e a s y t a s k . F o r e x a m p l e ,

t h e i n f o r m a t i o n g i v e n i n

T a b l e 1

sh o w s th a t , ev en f o r h ig h

c a r b o n c o n v e r s i o n e f f i c ie n c i e s d u r i n g c o a l c o m b u s t i o n , t h e

ca r b o n co n ten t o f f ly ash i s d i f f i cu l t to r ed u ce b e lo w th e

6 w t% LO I ( lo s s o n ig n i t io n ) C6 1 8 c las s i f i ca t io n s tan d ar d . I t

m a y b e p o s s i b l e t o r e d u c e t h e c a r b o n c o n t e n t e f f i ci e n t l y b y

u s e o f b e n e f i c i at i o n t e c h n o l o g i e s. B e c a u s e > 9 0 % o f t h e f l y

ash in th e U SA i s h an d led in th e d r y s ta te , d r y b en ef ic ia -

t i o n - - o r t r ib o e le c t ro s ta t i c s e p a r a t i o n - - h a s b e e n t h e re c e n t

f o c u s o f r e se a r c h a t C A E R , s o m e o f w h i c h i s d i s c u ss e d i n

th i s p ap er .

D r y t r i b o e l e c t r o s t a t i c s e p a r a t i o n t e c h n o l o g y i s j u s t

b e g i n n i n g t o b e a p p l i e d t o r e c o v e r p u r i f i e d a s h f r o m f l y

a s h s t r e a m s w h i c h c o n t a i n h i g h c o n c e n t r a t i o n s o f c a r b o n .

D u e t o d i f f e r e n c e s i n t h e s u r f a c e p h y s i c a l a n d c h e m i c a l

p r o p e r t i e s o f t h e c a r b o n a n d a s h , t h e y c a n b e e l e c t r i c a ll y

c h a r g e d t o o p p o s i t e p o l a r i t y b y p a r t i c l e - t o - p a r t i c l e o r b y

p a r t i c l e - t o - s u r f a c e c o n t a c t . B y m a n i p u l a t i n g t h e p o l a r i t y

a n d m a g n i t u d e o f t h i s c h a r g e , t h e c a r b o n a n d a s h c a n b e

s e p a r a t e d b y p a s s i n g t h e m t h r o u g h a n e x t e r n a l e l e c t r i c

field: see F i g u r e 1 . T h e s u c c e s s f u l a p p l ic a t i o n o f d r y

sep ar a t io n tech n o lo g y to ash p u r i f i ca t io n w o u ld b e s ig -

n i f ic a n t b e c a u s e i t w o u l d e n a b l e e x p a n d e d u t i li z a t io n o f a

min e r a l r e so u r ce w h ich o th e r w ise i s lan d f i l l ed o r n o t u sed to

i ts fu l l po ten tial .

A n u m b e r o f d r y t r i b o e l e c t r o s t a t i c s e p a r a t i o n s y s t e m s

h a v e b e e n d e s i g n e d f o r a p p l i c a t i o n t o p a r ti c l e s e g r e g a t i o n ,

o n e o f th em su b m i t ted f o r p a ten t in g as ea r ly as 1 9 04 3 ,4

5 9 10 1 1 12 14

C i c c u e t a l . - I n cu le t , F in se th e t a l . a n d B a n e t a l . -

e x a m i n e d t h e u s e o f t ri b o e l e c tr o s t a ti c s e p a r a t i o n t o p u r i f y

m i n e r a l s a n d t o s e p a r a t e m i n e r a l m a t t e r f r o m c o a l . T h e s e

1 5

e f f o rt s a n d o t h e r s h a v e p r o v i d e d i m p o r t a n t f u n d a m e n t a l

in s ig h t in to p a r t i c le su r f ace e lec t r ica l p r o p er t i e s an d

e n g i n e e r i n g p a r a m e t e r s w h i c h a l l o w c h a r g i n g o f p a r ti c l e s

to o p p o s i te p o la r i t i e s an d th e i r su b seq u en t sep ar a t io n u n d er

a s ta t i c e lec t r ic f i e ld . Co a l f ly ash co u ld b e co n s id e r ed a

d i r ec t an a lo g u e to co a l , b ecau se in b o th cases th e g o a l i s to

s e p a r a t e a m i n o r c o n s t i t u e n t f r o m t h e m a j o r o n e , t h e

d i f f e re n c e b e i n g t h a t i n c o a l t h e m i n e r a l m a t t e r i s t h e m i n o r

co n s t i tu en t an d ca r b o n th e majo r , w h er eas in f ly ash th e

r e v e r s e i s t r u e . A s a c o n s e q u e n c e o f t h e g r o u n d b r e a k i n g

w o r k b y C i c c u

e t a l . 5 - 9

an d F in se th

e t a l .

11 there is r ene we d

in te r es t in th e ap p l ica t io n o f t r ib o e lec t r o s ta t i c s ep ar a t io n

s y s t e m s . A l r e a d y a r e l a t i v e l y c o m p l e x e l e c t r o s t a t ic s e p a r a -

16 17

to r h as b een co n s t r u c ted an d o p er a te d ' a t a s ca le o f

- 1 8 t h - ~. T h e o p e r a t i n g d i f f e r e n c e s b e t w e e n s u c h u n i t s

p r i m a r i l y r e l a t e t o t h e m e t h o d b y w h i c h f i n e p a r t i c l e s a r e

ch ar g ed an d t r an sp o r ted th r o u g h a sep ar a t io n zo n e . F in se th

e t a l . 1 1

a n d C i c c u

e t a l . 5 - 9

h a v e u s e d p n e u m a t i c t r a n s -

p o r t a n d c h a r g i n g m e t h o d s , w h e r e a s t h e c o m m e r c i a l

u n i t 1 6'1 7 u ses mec h an ica l ag i ta t io n f o r t r an sp o r t an d

F u e l 1 9 97 V o l u m e 7 6 N u m b e r 8 8 1


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Dry triboelectrostatic beneficiation of f/y ash: H. Ban

et al.

Table Carbon content of fly ash versus carbon burnout and coal

mineral matter content

Mineral matter in coal Carbon burnout

Carbon in ash (wt )

(wt )

(wt )

10

99

8.3

10

97

21.2

20

99 3.8

20 97 10.7

Feeder

,? /

Carrier Gas

Charger

IIcII

Copper

Plates

- -15 kV

Figure The principle of dry triboelectrostatic separation of

carbon and fly ash

charging. The research reported in this paper used

pneumatic transport processing.

Very little information has been published in which

physical and chemical properties of fly ash are related to the

separation purities that can be attained using dry triboelec-

trostatic separation. Those properties expected to play an

important role include particle size and size distribution, ash

mineralogy, extent of liberation between the carbon and ash

forms, the form of the carbon, and surface segregation of

elemental species. Hower et ~1. have presented an

overview of potential forms of carbon and minerals,

including the glassy phases which dominate as-received

fly ashes, thi spine1 minerals, magnetite, and the carbons.

The carbon phases were shown to include inertinite, a coal

maceral which has been observed to pass through a boiler

unburnt,

and the forms which were recognized as isotropic

and anisotropic coke.

There is a need to optimize dry fly ash separation

technologies because of the vast amount of coal ash

produced in the USA and the growing interest in applying

superior technologies because of their economic and

environmental performance. In this paper, the results from

triboelectrostatic separation of coal fly ashes using a

laboratory-scale system are presented. The design of the

triboelectrostatic separation system and the data obtained on

the ashes in this study will be used in future work to

optimize ash separation using feed rates typical of industrial

and utility systems.

EXPERIMENTAL

A laboratory-scale

triboelectrostatic separation system,

shown in

Figure 2

was used in the fly ash beneficiation

study. The fly ashes were fed to the tribocharging unit by a

vibratory feeder which was contained in a sealed environ-

ment tank. Each ash was metered into a pneumatic transport

tube where it was entrained in N2 carrier gas. The gas-

particle mixture was then passed through the Cu tribochar-

ger loop where the fly ash was charged by particle-wall

(and particle-particle) contacts. The exit of the charger was

connected to a separation chamber which contained a

parallel Cu plate configuration, across which was estab-

lished a high-intensity electric field. A filter was placed at

the bottom of the separation chamber to retain any particles

not deflected to the Cu plates. The exit of the separation

chamber was connected to an induced-draught fan.

A sample of -10 g of each ash was weighed and loaded

into the vibratory feeder. The average carrier gas flow

velocity in the Cu tribocharger was - 15 m s-, The electric

field strength was maintained at 200 kV m-l.

Ash samples were acquired from either electrostatic

precipitator (ESP) hoppers or storage silos at commercial

pulverized coal boilers. Prior to separation tests, the samples

were evaluated for particle size and carbon content. After

the triboelectrostatic separation, samples were collected

from the separation chamber (see below) and their weight

and carbon content determined. Representative sample

fractions were also examined using scanning electron

microscopy (SEM) and energy-dispersive spectrometry

(EDS).

Fly ash fractions were prepared for petrographic analysis

by mixing 1 g, or less in some cases, of fly ash with an epoxy

Custom

Gas Blend

Izced raft

Filter

Figure 2 Schematic of the experimental setup

802

Fuel 1997 Volume 76 Number 8


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Dry tr iboelectrostatic beneficiation o f fly ash: H Ban et al.

l a c ed w i t h S u d a n B l a c k d y e . T h e 2 . 5 4 c m d i a m e t e r e p o x y -

f l y a s h p e l l e ts w e r e p o l i s h e d t o a f in a l p o l i s h o f 0 . 0 5 m .

P e t r o g r ap h i c e x a m i n a t i o n w a s c o n d u c t e d u s i n g o i l im m e r -

s i o n o b j e c t i v e s a t 4 0 0 m a g n i f i c a t i o n .

R E S U L T S A N D D I S C U S SI O N

D u r i n g o p e r a t i o n o f t h e t r i b o e l e c t r o s ta t i c s e p a r a t o r s y s t e m ,

t h e c a r b o n - e n r i c h e d c o m p o n e n t w a s d e p o s i t e d o n t h e

n e g a t i v e e l e c t r o d e a n d t h e a s h - e n r i c h e d c o m p o n e n t w a s

d e p o s i t e d o n t h e p o s i t i v e e l e ct r o d e . T h e d e p o s i t s w e r e i n t h e

f o r m o f l o n g , n a r r o w r i b b o n s o f m a t e r i a l , s t ar t i n g f r o m n e a r

t h e e x i t o f th e t r a n s p o r t i n g t u b e a n d e x t e n d i n g t o t h e e n d o f

t h e c o p p e r p l a t e s . A n a l y s i s o f u p t o f o u r s e q u e n t i a l a x i a l

s e c t i o n s o f t h e d e p o s i t e d s a m p l e s s h o w e d t h e c a r b o n c o n t e n t

t o b e h i g h e s t a t t h e t o p o f t h e n e g a t i v e e l e c t r o d e a n d l o w e s t

a t th e t o p o f t h e p o s i t i v e e l e c tr o d e . T h e c a r b o n c o n t e n t o n

t h e p o s i t i v e e l e c t r o d e i n c r e a s e d w i t h d i s t a n c e f r o m t h e i n l e t

o f t h e e l e c tr o s t a t i c s e p a r a t o r , w h i l e t h e c a r b o n c o n t e n t o n

t h e n e g a t i v e e l e c t r o d e d e c r e a s e d w i t h d i s t a n c e f r o m t h e

i n l e t . S i n c e t h e c a r b o n a n d a s h c o n t e n t s o n t h e e l e c t r o d e s

c o u l d b e r e p r e s e n t e d b y a c o n t i n u o u s d i s t r i b u t i o n , i t w a s

p o s s i b l e t o m a k e a n a r b i t r a r y s p l i t o f t h e s e p a r a t e d p r o d u c t s

t h a t s a t i s f i e d d e s i r e d p u r i t y r e q u i r e m e n t s . H o w e v e r , a s i n

a n y p h y s i c a l s e p a r a t i o n p r o c e s s , h i g h e r - p u r i t y p r o d u c t s a r e

a c h i e v e d a t t h e e x p e n s e o f l o w e r y i e l d .

A p r o c e d u r e w a s e s t a b l i s h e d f o r s a m p l e c o l l e c t i o n a n d

a n a l y s i s . F o r e a c h t e s t , a t o ta l o f 1 2 s a m p l e f r a c t i o n s w e r e

c o l l e c te d : t e n w e r e f r o m f i v e a x i a l re g i o n s o n e a c h o f th e

t w o e l e c t r o d e s , o n e w a s f r o m t h e c e n t r e f i l te r a t t h e b o t t o m

o f t h e s e p a r a t o r , a n d o n e w a s r e m o v e d f r o m t h e P l e x i g l a s s

w i n d o w s a t th e e d g e s o f th e C u e l e c t r o d e s . T h e s e f r a c t io n s

a s w e l l a s th e f e e d w e r e w e i g h e d a n d a n a l y s e d f o r c a r b o n

c o n t e n t , a n d t h e d a t a w e r e p l o t t e d i n a m a n n e r s i m i l a r t o a

w a s h a b i l i t y o r r e l e a s e a n a l y s i s c u r v e 1 9 , 2 0 , u s i n g t h e a n a l o g y

of each f r ac t ion a s e i the r a f loa t o r s ink p rodu c t .

C o n s e q u e n t l y t h e s e d a t a i n c lu d e a n a s s e s s m e n t o f m a s s

b a l a n c e s , b e c a u s e a s h n o t c o l l e c t e d b y t h e g a s f i lt e r s y s t e m

w a s a l s o t a k e n i n t o a c c o u n t .

C a r b o n a n d a s h r e c o v e r y d a t a , a n d p a r t i c l e s i z e a n d

c a r b o n d i s t r i b u t i o n s , f o r f l y a s h s a m p l e A a r e s h o w n i n

Figures 3 5. S a m p l e A w a s o b t a i n e d f r o m a u t i l i t y b o i l e r

b u r n i n g b i t u m i n o u s c o a l h a v i n g a s u l fu r c o n t e n t o f ~ 2 w t % .

O v e r 6 5 w t % o f th e a s h w a s r e c o v e r e d w i t h a c a r b o n c o n t e n t

8

o o I

o . I

i

3 0 :

20 t---

1 0 - -

> 1 5 0 1 5 0 - 7 5 7 5 - 4 5 4 5 - 3 8 3 8 - 2 5 < 2 5

S iz e m ic r o n s )

Fig ure 3 Fly ash mass, carbon concentration and carbon mass

distributions in each size fraction of fly ash samp le A

>

~

-

/1

8 6 o

o 2 0

o

0 1 0 2 0 3 0 4 0 5 0

C a r b o n %

F i g u r e 5 Carbon recovery curve of fly ash sample A, showing C

concentration in carbon-enriched product

t - -

/ )

3 5 w t % . T h e p a r t ic l e

s i z e d is t r ib u t i o n d a t a s h o w e d t h a t a s i g n i f i c a n t a m o u n t o f t h e

a s h w a s o f s i z e > 1 5 0 / ~ m a n d < 2 5 / ~ m . T h i s w i d e

d i s t r ibu t ion o f pa r t i c le s i ze p resen t s a s ign i f i can t cha l l enge

t o d r y s e p a r a t i o n s y s t e m s , d u e t o a n o r d e r - o f - m a g n i t u d e

r a n g e o f a e r o d y n a m i c d r a g a n d g r a v i t a ti o n a l f o r c e s .

S e p a r a t i o n r e s u l t s f o r f l y a s h s a m p l e B a r e p r e s e n t e d i n

Figures 6 a n d 7 . T h i s s a m p l e w a s o b t a i n e d f r o m a u t i l i t y

F u el 1 99 7 V o l u m e 7 6 N u m b e r 8

8 0 3


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Dry riboelectrostatic beneficiation of fly ash: H Ban

et al.

10 20 30 40 50

Carbon

Figure 7 Carbon recovery curve of fly ash sample B, showing C

concentration in carbon-enriched product

burning bituminous coal having a sulfur content of

-3.5 wt . The ash recovery data are plotted for one-stage

and two-stage processing schemes. A one-stage processing

scheme would not include recycle of material passing

through the electrostatic chamber, whereas a two-stage

scheme would recycle such material, i.e. particles not

reporting to either positive or negative plates. The data show

that the second-stage separation increased the ash recovery

by - 15 wt , and hence it may be important to recycle ash

that is not influenced by the electric field. Overall for sample

B, nearly 55 wt of the ash was recovered with a carbon

content of < 3 wt , while > 60 of the carbon was

recovered with a carbon purity > 40 wt .

The partitioning of glass, mullite, spinel, quartz and the

carbon forms for three additional Class F fly ashes2 is shown

in TabEe 2 All these samples originated at industrial sites

and represent a broad range of carbon concentrations. In all

cases, samples collected on the positive electrode had a

greater concentration of glass than those collected on the

negative plate; samples collected on the negative plate had a

greater concentration of the various carbon forms.

Table 2 Partitioning (vol. ) of mineral and carbon forms for three type F fly ashes

Plant Producta

Ash Carbon Glass Mullite

Spine1 Quartz

Isotropic C Anisotropic C

Inertinite

C feed

92.2 7.6 86.6

0.8 3.2

0.6 3.8 4.6

0.4

negative

74.6 26.8 45.0

4.0 3.0

1.0 22.0 21.0

4.0

positive

99.6 0.6 97.6

0.6 1.0

0.2 0.4 0.0

0.2

centre

90.4 7.2 72.4

2.4 3.8

2.0 8.0 8.4

3.0

D feed

77.0 23.4 69.8

0.8 1.0

0.4 10.4 15.2

2.8

negative

49.8 50.2 28.0

1.0 0.0

0.0 24.0 43.0

4.0

positive

97.2 3.0 96.6

0.0 0.0

0.0 1.2 1.5

0.6

centre

66.2 39.2 40.5

0.5 0.5

1.0 24.0 31.0

2.5

E feed 55.4 44.4 38.4 2.0 0.4 18.4 11.2 20.0 9.6

negative

36.3 60.6 17.0

1.0 1.0

2.0 37.0 35.0

7.0

positive

78.8 17.4 86.8

0.0 0.0

0.8 6.0 4.4

2.0

centre

60.8 41.2 57.0

0.5 1.0

4.0 15.0 20.5

2.0

Negative, positive and centre denote material collected from the negative and positive electrodes and bypass filter respectively

1

I

I

90

t l

Isotropic Coke

B Anisotropic Coke

20

10

0

Cf C- C+ CO Df D- D+ DO Ef E- E+ EO

Feed &Separated Fractions

Figure 8 Distribution of carbon forms based on petrographic

analysis. Symbols following C, D, E have the following

connotation: f, original feed ash; +, product collected from posi-

tive plate; -, product collected from negative plate; 0, product not

reporting to plates but collected in bypass filter

The distribution of the carbon forms obtained by

petrographic analyses is shown in Figure 8 and Table 2

These data suggest that the efficiency with which carbon

could be removed may be related to the amount of carbon in

the feed; however, no mass balances were performed for

these petrographic samples. Sample C, with 8.8 vol.

carbon in the feed, had 0.6 vol. carbon in the glass-rich

positive-plate fraction, while sample E, with > 40 vol.

carbon in the feed, had > 10 vol. carbon in the glass-rich

fraction. The results for sample C are similar to the carbon

removal efficiencies obtained from idealized samples

containing physical mixtures of spherical glassy carbon

and silica particles12, from which > 95 removal of carbon

could be attained. If samples C and E were strictly physical

mixtures of carbon and ash, and if the forms of the mineral

and carbon phases did not influence separability, it could be

expected that the carbon content of the ash-enriched product

on the positive plate for sample C would be as low as 0.4 ,

whereas the carbon content of the ash-enriched product for

sample E would be

-2 . That these values were not

attained attests to the possible importance of ash properties

influencing maximum separation efficiency.

No clear trends are evident in

Figure 8

for mullite, spine1

and quartz beneficiation. This is possibly a consequence of

the low quantities of these components and/or the broad size

804 Fuel 1997 Volume 76 Number 8


5/5

D ry t r i b o e le c t ro s ta t i c b e n e f i c i a t i o n o f f l y a s h : H B a n et al

d i s t r ib u t i o n o f th e c o m p o n e n t s i n th e f e e d . S t u d i e s o f t h e

s i z e d i st r i b u ti o n o f o t h e r f l y a s h e s h a v e s h o w n t h a t a

u n i f o r m d i s t ri b u t i o n o f p e t r o g r a p h i c c o m p o n e n t s i s n o t

c o m m o n f o r a f ly a sh o r f o r s i m i l a r s i z e r a n g e s i n d i f fe r e n t

f l y a s h e s 18. I s o t r o p i c a n d a n i s o t r o p i c c a r b o n s t e n d t o b e

c o n c e n t r a t e d i n t h e l a r g e s t s iz e f ra c t i o n s ( > 7 5 m ) a s

c o m p a r e d w i t h th e m i n e r a l a s h , w h i l e s o m e f ly a s h e s h a v e

s i g n i fi c a n t a m o u n t s o f c a r b o n e v e n i n t he f i n e s t si z e

f r a c ti o n s . I n m a n y a s h e s , t h e < 2 5 m f r a c t i o n c a n f o r m

7 0 - 8 0 w t % o f t h e t o t a l 18.

I n al l c a s e s , t h e f ly a s h w h i c h r e p o r t e d t o n e i t h e r a

p o s i t i v e n o r a n e g a t i v e e l e c t r o d e , b u t r a t h e r t o t h e f i l t e r a t

t h e b o t t o m o f th e s e p a r a t o r , c o n t a i n e d l a r g e r p a r t ic l e s t h a n

t h a t w h i c h r e p o r t e d t o t h e p o s i t iv e o r n e g a t i v e p l a t e s . T h e s e

l a r g e r p a r t i c l e s c o m p r i s e d g l a s s , c a r b o n a n d s p i n e l s , t h e

l a r g e s t b e i n g p r e d o m i n a n t l y s p i n e ls . L a r g e , m a s s i v e p a r ti -

c l e s te n d t o h a v e s m a l l e r c h a r g e / m a s s r a t io s t h a n s m a l l , li g h t

p a r t i c l e s b e c a u s e t r i b o c h a r g i n g p r o d u c e s p a r t i c l e s h a v i n g

n e a r l y c o n s t a n t v a l u e s o f c h a r g e p e r u n i t s u r f a c e a r e a 14.

T h e r e f o r e , e i t h e r t he e l e c t r i c a l f o r c e ( F = q E , w h e r e q i s t h e

c h a r g e o n a p a r t i c l e a n d E i s t h e e l e c t r i c f i e l d i n t e n s i t y ) w a s

n o t s t r o n g e n o u g h t o p u l l th e h e a v y p a r t i c l e s t o a n e l e c t r o d e ,

o r th e s e p a r t i c l e s w e r e d e f l e c t e d , r e a c h e d a n e l e c t r o d e a n d

t h e n b o u n c e d o f f a s a c o n s e q u e n c e o f c h a r g e a n d / o r

m o m e n t u m e x c h a n g e .

T h e S E M r e s u l ts f o r s a m p l e s c o l l e c t e d o n t h e b y p a s s f i lt e r

s h o w e d n e a r l y s p h e r i c a l p a r ti c l e s w h i c h a p p e a r e d t o b e

p a r t l y o x i d i z e d c o k e . A l s o , p a r t i c l e s c o n s i s t in g o f a m i x t u r e

o f a s h w i t h c a r b o n w e r e o b s e r v e d . T h e c h a r g i n g o f t h e s e

m i x e d c a r b o n - a s h p a r ti c le s w o u l d b e m i n i m a l b e c a u s e n o

d o m i n a n t p o l a r i t y s p e c i e s w a s p r e s e n t . A l t h o u g h t h e

p e t r o g r a p h ic a n a l y s es s h o w e d a s s e m b l a g e s w h i c h c o n s is t e d

o f c a r b o n m i x e d w i t h a s h , i t i s i m p o r t a n t t o m e n t i o n t h a t t h e

a c t u a l c o u n t o f s u c h p a r ti c l e s w a s l o w a n d c o u l d b e b i a s e d

a s a c o n s e q ue n c e o f w h i c h c o m p o n e n t - - a s h o r c a r b o n - -

w a s o b s e r v e d w i t h i n t h e e y e p i e c e c r o s s - h a i r s .

C O N C L U S I O N S

T h i s s t u d y h a s s h o w n t h a t d ry t r ib o e l e c t r o s t a t ic s e p a r a t i o n

o f f ly a sh h a s t h e p o t e n t i a l t o s e p a r a t e u n b u r n t c a r b o n f r o m

f l y a s h . L a b o r a t o r y t e s t s o n a s i m p l e p a r a l l e l - f l o w s e p a r a t o r

s h o w e d t h at 6 0 - 8 0 w t % o f th e a s h c o u l d b e r e c o v e r e d

h a v i n g a c a r b o n c o n t e n t < 5 w t % , a n d u p to 5 0 % o f t h e

c a r b o n c o u l d b e r e c o v e r e d a s m a t e r i a l w i t h a c a r b o n c o n t e n t

> 5 0 w t % . T h e o v e r a l l c o l le c t i o n e f f i c i e n c y a p p e a r s t o b e

r e l a t e d t o th e a m o u n t o f c a r b o n i n t h e fe e d a n d i s p r o b a b l y

i n f l u e n c e d b y t h e s i z e d i s t r ib u t i o n o f t h e c o m p o n e n t s a n d

t h e a m o u n t o f m i x e d p a r t i c le s . A d d i t i o n a l s t u d i e s s h o u l d b e

i n i ti a t e d t o e v a l u a t e t h e e f f e c t s o f a s h p r o p e r t i e s o n

s e p a r a t i o n , w i t h t h e g o a l o f o p t i m i z i n g t h e b e n e f i c ia t i o n

p r o c e s s .

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Book o f AST M S tandar ds, Vo l . 04 .02. P h i l ade lph ia , 1990

p p . 1 8 6 - 1 9 0 , 2 9 8 - 3 0 0 .

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4 Schw e i tze r , E . O. , US Pa ten t No . 2 ,216,254 , 1940 .

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Pe re t t i , R . and Zucca , A. , i n Proceedings of the In terna-

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7 Al fano , G. , Ca rb in i , P ., Ca r t a , M . , C iccu , R . , De l Fa l , C . ,

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8 C iccu , R . , Gh ian i , M. and Fe r ra ra , G. , Pow der a nd Par t ic les ,

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11 F inse th , D. , N ew by , T . and E l s t rod t , R . , i n Process ing and

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12 Ban , H., Sch aefer , J . L., Sai to , K. and Stencel , J . M ., Fuel ,

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