Journal of Environmental Protection, 2010, 1, 293-301
doi:10.4236/jep.2010.13035 Published Online September 2010 (
Copyright © 2010 SciRes. JEP
Complex Processing of Pulverized Fly Ash by Dry
Separation Methods
Vladimir Vasilievich Zyryanov1, Dmitry Vladimirovich Zyryanov2
1Institute of Solid State Chemistry and Mechanochemistry, Russian Academy of Sciences, Siberian Branch, Novosibirsk, Russia;
2Nanopowder Technology Ltd, Novosibirsk, Russia.
Received May 18th, 2010; revised June 16th, 2010; accepted June 19th, 2010.
Pulverized fly ash (PFA) is produced about 500 billions tons every year in the world in a result of coals combustion.
Most of the fly ash collected in power plan ts is dispos ed by deposition in landfills , situated as a ru le near big cities with
well developed infrastructure and high cost of land. Moreover, the pollution of environmental by fine solid wastes is
inevitable and takes place in area of residing of a basic part of the population. The only solution is a complex process-
ing of fine wastes with a production of value added materials. New conception of complex processing of PFA is pro-
posed on the base of facilities of Electro-mass-classifier (EMC) and other techniques. The characterization of separated
fractions was carried out by SEM and optic microscopy, XRD, laser diffraction, Mössbauer spectroscopy and other
methods. A fine fraction of glass microspheres presents the main interest as filler in various materials.
Keywords: Pulverized Fly Ash (PFA), Processing, Solid Wastes, Utilization, Dry Separation, Fly Ash Components,
Glass Microspheres, Magnetospheres, Fillers, Electro-Mass-Classifier
1. Introduction
Coal fly ash is the particulate matter remaining after
combustion of the carbonaceous component of coal. The
residual, accessory minerals, predominantly clay, sili-
ceous and iron minerals generally comprise the bulk of
the ash. Fly ash was considered to be the sixth most
abandoned mineral in the USA [1]. Usually less than
20% is reused commercially all over the world, pre-
dominantly in cement or as a fill material in constru ction
[2]. The main cause of low utilization of PFA is a
polydisperse inhomogeneous mix containing remnants of
unburnt carbon—coke or char. Other problem for proc-
essing of PFA is related to high fineness of this waste
complicating the separation of carbon by dry physical
methods. Such problem may be solved by the use a green
engineering—Electro-mass-classifier (EMC) technique
operating with charged aerosols in closed volume in a
wide range of particle sizes. Acid fly ash of type F ac-
cording to ASTM prevails among different coal combus-
tion products. In addition, commercial potential of this
PFA after separation is maximal [2]. So, the p erspectives
of PFA complex processing technology based on the dry
separation methods for environmental protection and
profitable production of value added materials are better
to show in such class of wa ste. The possible solutions for
utilization with profit for other types of PFA—fly ash C
according to ASTM classification and ash from low
temperature boilers with fluidized bed were found as
well. Based on the mechanical activation in EMC, these
solutions are presented in our bo ok [2].
2. Materials and Methods
Various specimens of PFA were used for investigations
produced in the largest power plants with high tempera-
ture boilers of the former USSR – Reftinskaya GRES
(specimen No 1, coal from Ekibastuz deposit), Troitz-
kaya TEC (specimen No 2, coals from Ekibastuz and
Kuznetsk deposits, both plants from Ural region), No-
vosibirskaya TEC 5 (specimen No 3, coal from Kuznetsk
deposit, Siberian region). The fineness of PFA specimens
was close one to another due to use of similar electro-
static precipitators. Ash particles were ranged from 200
nm to 500 m with mean particles size about 40 m.
Different techniques were used for dry physical sepa-
ration of PFA including Electro-mass-classifier (EMC)
[3,4], magnetic separator with permanent magnet from
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
FeNdB-alloy (MS), triboadhesion separator (TAS), elec-
trostatic separator with electric field 6 kV/cm (ES) and
their combinations. All these devices are able to operate
in discrete and continuous modes that are important for
laboratory studies and industrial processing respectively.
Moreover, all these installations are compatible and can
operate in closed volume without aerosol contamination
of air atmosphere. Flow sheet for pro cessing of PFA may
be realized for one run in production module with capac-
ity 0.5-1 TPH. Operating in the range of particles size
~50 nm-2 mm, EMC technique allows to use with high
efficiency other more selective separators after removing
from PFA of the dust with particles size < 20 m. Pr inc i-
ple of operation of multifunctional green EMC engineer-
ing is based on the generation of charged aerosols and
their separation to unlimited number of fractions under
action of centrifugal and other forth’s [3,4].
The combination of instrumental and chemical ap-
proaches was used for characterization of separated ash
fractions: XRD (DRON-3M, Russia, Bruker D8 Advance,
Germany, Cu K radiation), SEM + EDS (Hitachi
TM-1000, JSM-6700F), optical microscopy Neophot 21,
Germany, particle size analyzer (Laska, Russia), based
on a laser diffraction method, Mössbauer spectroscopy
(NZ-640, Hungary). Losses on ignition (LOI) were de-
termined at 850, 1 hour. Chemical compositions were
determined in spectrometer ARL-9900XP, Swiss.
3. Separation
In a result of investigation of dozen various PFA speci-
mens of F type, the optimal scheme for complex proc-
essing was found, Figure 1 [2]. After removing of fine
and partly medium fractions in EMC, the rest of PFA can
be divided by custom separators. Additionally four frac-
tions can be obtained [2]. From practical point of view,
fractions were called as potential products: Super-alpha
(fine fraction No 1 in Figure 1), Alpha (medium fraction,
No 2 + No 3), Delta (large fraction, No 4), coarse by-
product (No 5), magnetic byproduct (magnetic fraction,
No 6). Particle size distributions for obtained products
Figure 1. Flow sheet for complex dry processing of F type PFA.
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
are shown in Figu re 2 for specimen No 1 (No 2 and 3 are
very similar). Magnetic fraction has a particles size dis-
tribution close to Delta. After refinement of magnetic
byproduct, magnetospheres with promising catalytic
properties in methane conversion process can be received
[5]. Magnetospheres present spherical particles of nano-
composites consisted from spinel and hematite in
iron-rich silica glass matrix [5,6]. Coarse byproduct is a
concentrate of relatively expensive materials—coke and
cenospheres (hollow glass microspheres with density < 1
g/cm3). Dry physical methods are not efficient for more
deep separation of coarse fraction. So, the yield of this
byproduct must be minimized to decrease the role of wet
technologies in complete processing of PFA [2]. Frac-
tions No 2 and 3 have close fineness and properties. So,
for simplification of flow sheet these fractions are rea-
sonable for uniting in a product Alpha. The yields of
products and their LOI are presented in Table 1. For en-
richment of coarse fraction by coke, relatively low elec-
tric field with E = 6 kV/cm was applied in ES.
The carbon content in fine fraction hardly is higher
than in initial PFA due to milling of porous char particles
(Figure 3) during treatment in EMC in discrete mode of
operation. The decrease of velocity of rotor rotation in
EMC results to lowering of LOI in fine fraction. In EMC
with continuous mode of operation, the life time of ash
particles in camera of charged aerosol generation sh arply
drops resulting to a little milling of coke and lowered
contents of carbon in fine and medium fractions—Super-
alpha and Alpha respectively.
The ratio of milled carbon particles and capacity of
EMC installations in separation process are linked pa-
rameters. From the other side, the pozzolana properties
of Super-alpha product don’t depend on the LOI. More-
over, after coke combustion at 850, activity of Super-
alpha even decreases a little. There is a simple explana-
tion of such effects. Fine carbon particles with a size ~n
m after milling get in pore space of cement stone with a
little influence on the strength. In quenched ash glass
particles mechanical strains results to high chemical ac-
tivity in basic solution in mortar (so called mechano-
chemical activity). After thermal annealing, chemical
activity decreases due to relaxation of mechanical strains
in glass particles. Weak effects of carbon and strain re-
laxation on the strength of mortar with additives of Su-
per-alpha show counterly signs and compensate each
4. Chemical Composition
Chemical compositions of obtained products are pre-
sented in Table 2. These results are very similar to other
known data [2]. The iron content in Delta is lowered
050100 150 200 250 300
12 % in
Coarse byproduct
Initial PFA
particle size, m
Figure 2. Particle size distributions for obtained products
from PFA No 1.
Figure 3. Porous coke particle from PFA No 1.
relatively Alpha due to magnetic separation of large frac-
tion only. Nevertheless, in EMC there is a facility to re-
move magnetic particles from fine fractions as well, if
this operation has practical sense for better commerciali-
5. Phase Composition
Phase compositions of all studied PFA specimens are
very similar—small crystallites of mullite (m) and quartz
(q) in silica glass, Figure 4. The content of mullite de-
pends on the particle size. Fine fractions are enriched by
quartz, and coarse fractions by mullite that correlates
with chemical composition. Cenospheres are usually en-
riched by silica and Na/K, so the mullite content is low-
ered relatively main part of microspheres with density >
1 g/cm3. Phase compositions of magnetic fractions differ
slightly as well, Figure 5. The main crystal phase is fer-
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
Table 1. Yields and LOI (%) of products obtained in flow sheet in Figure 1.
Super-alpha Alpha Delta Coarse byproduct
1 2.8 9 3.6 45 2.1 36 2.7 7 4.9 3 1.0
2 3.5 8 4.4 39 2.4 39 3.0 12 8.4 2 1.0
3 3.9 7 4.5 35 2.9 45 3.2 10 10.9 3 1.2
Table 2. Chemical composition of obtained products (for magnetospheres after refinement).
PFA Product SiO2 Al2O3 Fe2O3 CaO MgOTiO2 K
2O Na2O P2O5 MnO
S-alpha 61.3 24.4 3.72 1.97 0.96 1.0 0.81 0.58 0.62 0.08
Alpha 60.5 26.1 5.64 2.39 1.09 1.18 0.68 0.39 0.40 0.11
Delta 58.9 29.5 2.18 2.54 0.99 1.29 0.61 0.27 0.41 0.06
Magnetospheres 26.8 10.9 55.1 2.35 1.14 0.91 0.17 0.43 0.10 1.93
S-alpha 62.7 24.0 4.0 1.55 0.7 1.06 0.8 0.4 0.41 0.07
Alpha 60.1 26.1 5.1 1.74 0.76 1.23 0.59 0.36 0.48 0.14
Delta 59.2 28.5 3.5 1.87 0.69 1.42 0.45 0.27 0.44 0.1
Magnetospheres 24.9 9.69 57.1 1.66 1.59 0.95 0.26 0.34 0.25 1.95
S-alpha 62.0 20.2 4.22 2.05 1.52 0.75 2.9 1.59 0.5 0.05
Alpha 59.8 22.3 4.96 2.82 1.88 0.86 2.81 1.52 0.45 0.10
Delta 57.9 23.8 4.15 3.49 1.69 0.99 2.67 1.33 0.36 0.06
Magnetospheres 22.3 7.1 61.1 2.91 2.90 0.10 0.41 0.79 0.45 2.4
20 30 40 50 60
2 theta
Figure 4. XRD pattern for PFA No 1.
rospinel, and minor phases are hematite (h) and quartz
(q). The content of ferrospinel depends on the magneto-
sphere size with maximum content in medium fractions.
10 20 30 40 50 60 70 80
No 3
No 1
2 theta
Figure 5. XRD patterns of magnetospheres.
6. Morphology
Morphology of the most ash particles presents hollow
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
glass microspheres, Figures 6-8 (marks in m). In large
fractions some glass particles have irregular forms. In
coarse fraction most particles are presented by agglomer-
ates, including plerospheres and composites, Figures 9-10.
For many applications, spherical form of glass parti-
cles is very attracting, especially as filler in cement, con-
crete mixtures, polymer composites, and so on. So, the
removing from PFA of coarse byproduct, consist of coke
and nonspherical glass particles with irregular form and
porous microstructure sharply enhances the quality of
obtained products as fillers.
Part of carbon is in aggregates with silica glass micro-
spheres, Figure 10. Because of this, the efficient remov-
ing of coke from PFA is limited by individual coke parti-
cles only. EDX analysis of glass particles in Figure 9
displays a different chemical composition: for particle 1 -
Figure 6. Product Alpha. (a) optical image (PFA No 2); (b)
SEM image (PFA No 1).
Figure 7. SEM image of Super-alpha No 1.
Figure 8. Optical image of Delta No 2.
Figure 9. SEM image of coarse byproduct No 1.
Si/Al = 1.1, for particle 2 - Si/Al= 2.1, dark particles –
coke aggregates.
Magnetospheres display various morphological types
in every kinds of PFA, Figures 11-13. Magnetospheres
from different power plants were studied by combination
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
Figure 10. Aggregate from coke and glass microspheres in
coarse byproduct No 1.
Figure 11. Optical (a) and SEM (b) images of magneto-
spheres No 1.
Figure 12. Optical (a) and SEM (b) images of magneto-
spheres No 2.
of structural methods—XRD with Rietveld analysis and
Mössbauer spectroscopy [6]. Structural formulas of ferro-
spinels were determined that allows to search a correla-
tions with catalytic properties of magnetospheres with
different origin [5]. Such correlations help to select op-
timal magnetospheres and modify them for promising
applications in catalysis.
Cenospheres separated from coarse byproducts by wet
technology are shown in Figure 14. Cenospheres are
well known product with a relatively high cost and large
area for applications [2]. The content of cenospheres in
coarse byproduct is about 2-4% (up to 30% of total con-
tent in PFA).
The iron contents in nonmagnetic fractions of glass
microspheres and cenospheres in studied specimens of
PFA are in the range 3-5%. As a result, the color of glass
microspheres usually grey, that complicates their appli-
cations in white and colored polymer compositions. Ac-
cording to studies of glasses by Mössbauer spectroscopy
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
Figure 13. Optical (a) and SEM (b) images of magneto-
spheres No 3.
[2], Fe2+ state have coordination number 4.5 and Fe3+ - 6
(usual octahedral position). Parameters of iron in glass
are presented in Table 3. After thermal treatment in air,
the content of Fe2+ decreases with narrowing of lines that
corresponds to annealing of mechanical strains in quen-
ched glass. The color of powders acquires poorly red
Other valuable product which can be separated from
coarse byproduct is a coke with LOI > 90%. Th e content
of extracted coke in coarse byproducts was about 2-4%.
A coke represents a practical interest not only as a clean
solid fuel, but mostly a source of rare volatile elements—
V, In, Co, U (Figure 16), and as absorbent [2].
The obtaining of cenospheres and coke makes possible
a profitable wet chemical processing of coarse byproduct.
In such a case the waste after separation coke and ceno-
spheres presents the source of rare earth elements, sec-
ondary products are silica, alumina and raw for zeolite
synthesis [2]. Coarse byproduct may be used without
Figure 14. Optical images of cenospheres divided from PFA
No 1 (a), 2 (b) and 3 (c).
processing in fabrication of low dense building ceramics.
However, the capacity of region market usually is less
than quantity of produced coarse byproduct. Diversifica-
tion of value added products allows to realize with profit
a complete utilization of produced PFA.
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
Table 3. Parameters of Mössbauer spectra of nonmagnetic cenospheres separated from PFA No 3.
1 1 1 content, % 2 2 2 content, %
parameters Dublet Fe3+ Dublet Fe2+
initial 0.868 0.333 1.154 61 0.778 1.010 1.945 39
Т = 850 0.838 0.322 0.991 88 0.560 1.007 2.088 12
- width of lines, - chemical shift, - quadruplet splitting.
Figure 15. Typical SEM images of cenospheres obtained
from PFA No 2.
[U] = 2 + 0.3[C]
R = 0.95
Figure 16. Correlation between U and carbon contents in
fractions of PFA No 1.
7. Pozzolana Properties
The utilization of PFA products is mostly as fillers in
cement and concrete. The profitable processing of PFA is
determined by the capacity of construction market. The
quality of PFA products determines the possible volume
of utilization. The pozzolana properties are shown in
Figure 17 for the fine PFA products.
0,0 0,2 0,4 0,6 0,8 1,0
Figure 17. Compressive strength for mortars cement:sand =
1:3 with substitution of cement by products from PFA No 1
after 3, 7 and 28 days of curing in wet atmosphere.
Partial substitution of cement by Super-alpha drasti-
cally increases the strength of mortars even after short
time of curing. This enhancement of building material is
related to increasing of density of concrete due to filling
of pores between cement particles and absorption of ex-
cessive Ca(OH)2. The substitution of 30% cement by
Alpha results to the same strength properties of concrete.
The fineness of Alpha and custom cement are very simi-
lar, so the effect of substitution is related to pozzolana
properties. A possible use of Delta in mortars or ready
mix is due to partial substitution of sand. In this case a
workability of mortars becomes better thanks to spherical
form of particles, but the compressive strength displays a
weak dependence on the degree of substitution. However,
the density of concrete decreases, so normalized on the
density effect becomes significant, Tabl e 4 [2]. The pos-
sible volume of PFA utilization in construction industry
becomes much more significant after processing. Rela-
tively low quality of mortars with initial PFA is related to
remnants of coke and porous coarse ash particles with
irregular form.
In the case of specially designed coal mix with inor-
ganic additives, prepared with optimal morphology, the
cost of PFA after combustion becomes more significant
than produced electric and heat energy [7]. In other
words, electric and heat energy must become byproduct
Complex Processing of Pulverized Fly Ash by Dry Separation Methods
Copyright © 2010 SciRes. JEP
Table 4. Parameters of 28 day concretes with products separated from PFA No 1, which are equal on the volume to mix ce-
ment:sand = 1:2 (weight ratio).
No Composition Density ρ, g/cm3 Compressive strength σ,
MPa Normalized strength, σ/ρ
1 Cement: 2sand 2.43 36(3) 14.8
2 Cement: sand + Delta 2.09 32(3) 15.3
3 Cement: 2Delta 1.64 22(2) 13.4
4 Cement: 2magnetospheres 2.095 34.5(1.5) 16.5
5 (2/3cement + 1 /3 A lp h a): 2sand 2.37 32(1.5) 13.5
6 (2/3cement + 1 /3 A lp h a): sand + Delta 2.08 30(3) 14.4
7 (2/3cement + 1/3PFA): sand + PFA 2.09 24(1) 11.5
only in chemical reactors for t he production of high tem -
perature materials—glass microspheres and other fillers,
binding materials, and so on.
8. Conclusions
PFA from coal power plants consist of components with
a number of promising applications. One run dry physi-
cal separation in green EMC-based installation of PFA to
3 products and 2 byproducts provides wide possibilities
for utilization with profit of solid wastes. Refinement of
magnetic byproduct and wet processing of coarse by-
product can provide complete processing of solid wastes
to value added products. In such a case, the effect of
elimination environmental pollution is accompanied by
economy of natural reso urces and lowering of CO2 emis-
sion. Obtained results promotes to new eco-friendly
conception to produce a power as a byproduct in rela-
tively small chemical reactors which can provide better
economic perspectives due to local green production of
distributed energy from carbonaceous sources.
9. Acknowledgements
The first author gratefully acknowledges the support of
this research in part through grant Russian Found for
Basic Researches 09-03-00364.
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