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