32nd International Conference of SSCHE
May 23–27, 2005, Tatransk´e Matliare, Slovakia
Po-We-4, 054p.pdf
RECYCLING OF WASTE FGD GYPSUM
Slobodanka Marinković, Aleksandra Kostić-Pulek, Vesna Logar and Prvoslav Trifunović
University of Belgrade, Faculty of Mining and Geology, Djusina 7, 11000 Belgrade, Serbia and
Montenegro, Tel: (38111)3219-108, Fax: (38111)3235-539, [email protected]
Key words: recycling, FGD gypsum, sulphogypsum, β-calcium sulphate hemihydrate, construction
industry
FGD gypsum (sulphogypsum) is a principal residue from coal-fired power plant fitted with flue
gas desulphurization (FGD) equipment. This is the product of the reaction between SO2 in the flue
gases and limestone used to separate it from the flue gases.
FGD gypsum can be used to make a range of materials employed in the building works and
construction industry (floors screeds, plaster boards, gypsum blocks). Before FGD gypsum can be
used for that purpose, it must be converted into hemihydrate or anhydrite – binder products that
harden after water has been added.
In this study, the possibility of recycling FGD gypsum from lignite Bohemian power plantHvaletice for obtaining β-calcium sulphate hemihydrate was investigated. There are many lignite
power plants in Serbia which have not an FGD system installed yet. Nevertheless, it will be
obligatory to install this system soon. Therefore, the results of these examinations would be a
contribution to the future solution of environmental and economic problems in connection with
waste FGD gypsum in Serbian power plants.
In order to obtain β-calcium sulphate hemihydrate (β-CaSO4⋅0.5H2O) three kinds of starting
materials were heated in a dryer at 130 oC: 1. raw FGD gypsum, 2. washed in water FGD gypsum
(to eliminate soluble in water impurities) and 3. treated in H2SO4 solution (to eliminate carbonates)
and than washed in water FGD gypsum. The three obtained products were investigated by
qualitative IR analysis. These results have shown that β-CaSO4⋅0.5H2O obtained from raw FGD
gypsum (product 1.) and washed in water FGD gypsum (product 2.) contained carbonates
(impurities), contrary to β-CaSO4⋅0.5H2O obtained from treated in acid solution FGD gypsum
(product 3.).
When three products (β-CaSO4⋅0.5H2O) were mixed with water at a standard Water/Powder
ratio (W/P of 0.8), pastes, that got hardened by storage, were formed. The compressive strength of
the hardened samples was measured 7 days after and their composition was determined by
qualitative IR analysis. The experimental results show that β-calcium sulphate hemihydrate (βCaSO4⋅0.5H2O) completely recrystallize to dihydrate (CaSO4⋅2H2O) in all the hardened samples
and that all the hardened samples are sufficiently resistant to compression for usage in the building,
sculpturing and modeling. Because of the unsuitable color (light – gray), the presence of impurities
and the lowest compressive strength value (2.58 MPa) β -hemihydrate from raw FGD gypsum
could have very limited application. Contrary, β -hemihydrates from previous prepared FGD
gypsum (washed in water-product 2., as well as treated in acid solution and than washed in water
FGD gypsum-product 3.) could have a great practical application, because of their white color, the
amount of impurities (small amount only in product 2) and the great compressive strength (4.33 and
7,79 MPa of products 2.and 3., respectively).
Therefore the best scheme for production of β-calcium sulphate hemihydrate from FGD gypsum
is:
raw FGD gypsum → treating in 0.05 M H2SO4 → filtration → washing in water →
filtration→ heating in drier,
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32nd International Conference of SSCHE
May 23–27, 2005, Tatransk´e Matliare, Slovakia
Po-We-4, 054p.pdf
RECYCLING OF WASTE FGD GYPSUM
Slobodanka Marinković, Aleksandra Kostić-Pulek, Vesna Logar and Prvoslav Trifunović
University of Belgrade, Faculty of Mining and Geology, Djusina 7, 11000 Belgrade, Serbia and
Montenegro, Tel: (38111)3219-108, Fax: (38111)3235-539, [email protected]
Key words: recycling, FGD gypsum, sulphogypsum, β-calcium sulphate hemihydrate, construction
industry
INTRODUCTION
The burning of coal in electric power plants produces sulphur dioxide (SO2) gas emission. The
1990 Clain Air Act Amendments (in U. S. A) and the regulations in the European Union (the low
No.218/1992 in Czech Republic, for example) mandated the reduction of power plant SO2 emission
by so called flue gas desulphurization (FGD) processes (1, 2). These processes are based on SO2
absorption by an alkaline sorbent consisting of lime or limestone. The final consequence of the use
of FGD processes is the formation of large quantities of FGD gypsum (sulphogypsum) – waste
material.
For economic and environmental reasons, it is always preferable to utilize a waste rather than
dispose of it. The utilization of waste gypsum as raw material for production of commercial binders
(hemihydrate and anhydrite), as well as other new binders (fly ash-lime-gypsum binders, for
example) has been considered widely (3-9). These binders harden after water has been added and
have application in the building works and in construction industry for the production of
prefabricated products (floor screeds, plaster boards, blocks, bricks). The construction (building)
uses large tonnages of hemihydrate plaster and anhydrite, that is outlet for large quantities of FGD
gypsum.
The acceptability of FGD gypsum for the industrial reutilization depends of its characteristics,
primary of the presence of the impurities in it. The impurities may be constituted by the coal ash
(which negatively influence the colour), by the impure substances found in the limestone or lime at
origin, by the non oxidized calcium sulphite and by mineral salts (principally chlorides) deriving
from the burnt coal. It is necessary to keep the coal ash quantities in the FGD gypsum under control
as these can alter the gradation of white in the manufactures. Also, the chloride contents must be
controlled as these may produce deterioration in the adhesion in the lining used in plasterboard
manufactures. The specification limits for the characteristics of FGD gypsum, according to the
Italian and German recommendations (3), are reported in Table 1.
Table 1 Characteristics of FGD gypsum+ required for industrial reutilization
CaSO4⋅2H2O CaCO3
in
≥95 %
<1.5 %
Italy
in
≥95 %
Germany
=
for humidity absent product
SO2
MgO
Na2O
Cl-
Colour
pH
<0.25 %
<0.01 %
<0.06 %
<0.01 %
white
5-8
≤0.25 %
≤0,1 %
≤0.06 %
≤0.01 %
white
5-8
Serbian plants (mainly lignite power plants) have not a FGD system installed yet, but it will be
obligatory to install this system soon. This will generate great amounts of waste FGD gypsum. The
aim of this work was to investigate the possibility of recycling FGD gypsum. The results of these
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May 23–27, 2005, Tatransk´e Matliare, Slovakia
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examinations would be a contribution to the future solution of environmental and economic
problems in connection with waste FGD gypsum in Serbian power plants. Because of that, the
application of FGD gypsum of a similar lignite Bohemian power plant-Hvaletice, as a raw material,
for obtaining β-calcium sulphate hemihydrate was investigated in this study.
The transformation of gypsum (CaSO4⋅2H2O) into β-calcium sulphate hemihydrate (βCaSO4⋅0.5H2O) takes place by the dry method, e g. by controlled heating of the dihydrate in the
temperature range 110-130 o C in open kettles (atmospheric pressure) (10) in accordance with the
following reaction equation:
CaSO4⋅2H2O (s) = β-CaSO4⋅0.5H2O (s) + 1.5 H2O (g)
EXPERIMENTAL
FGD gypsum from lignite Bohemian power plant-Hvaletice-Czech (no Serbian plants have a
FGD system installed yet) was used in this study. This material was examined in order to determine
its chemical composition and some physical properties. The chemical composition of FGD gypsum
was determined by quantitative chemical analysis (Table 2). Also, it was subjected to qualitative IR
analysis in the range from 4000 to 400 cm-1(Table.3). The IR spectrum was recorded on a Perkin
Elmer Spectrophotometer 782 using the pressed KBr technique (the same spectrophotometer and
the same technique were also used to record all the other IR spectra in this study). The physical
properties of FGD gypsum: density by pycnometer and granulometric composition by wet sieving
through Tyler sieves were determined (Table 4).
Table 2 Chemical analysis of FGD gypsum
CaO
SO3
MgO CO2 SiO2 Al2O3 Fe2O3
mas. 32.80 45.72 2.18
%
1.16 0.63
0.14
0.38
Water
soluble Cl0,02
Crystal
water
17.18
pH
7,5
Table 3 IR analysis of FGD gypsum
Wave number of absorption bands (cm-1)
3560
3420
1680
1625
1450
1160
1120
670
600
Table 4 Physical properties of FGD gypsum
Granulometric composition
size fraction (mm)
mass (%)
+0.104
0.36
0.104+0.074
2.98
0.074+0.000
96.66
Density (g/cm3)
2.34
Colour
light - gray
On the basis of the results of the chemical analysis of FGD gypsum (Table 2) it is apparent that it
is essentially a pure substance (about 5 % of impurities).
The IR spectrum of FGD gypsum (Table 3) contains the absorption bands (3560, 3420, 1680,
1625, 1160, 1120, 670 and 600 cm-1), which are characteristic for CaSO4·2H2O, i.e. gypsum (11,
12). Beside these bands, the IR spectrum contains weak band at 1450 cm-1, which confirms the
presence of some carbonates in FGD gypsum (13). This result is in accordance with the chemical
analysis of FGD gypsum (1,16 % CO2, Table 2).
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32nd International Conference of SSCHE
May 23–27, 2005, Tatransk´e Matliare, Slovakia
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The results of the granulometric composition determination (Table 4) show that FGD gypsum
has a suitable particle size distribution, because the smallest fraction (particle size smaller than
0.074 mm, e.g. 74 µm) is the major fraction in it (96.66 %). Consequently, it could be used without
preliminary grinding, for β-calcium sulphate hemihydrate production.
Because of the unsuitable colour (light – gray, Table 4) and greater contents of carbonates and
Cl – (compare Table 2 and Table 1) it is evident that FGD gypsum from Bohemian power plantHvaletice-Czech is not suitable for direct use in construction industry. To purify FGD gypsum,
before its heating and transformation into β-hemihydrate, this material is washed in water (to
remove any traces of Cl – ions an to attain white colour of material) and treated in 0,05M solution of
H2SO4 (to eliminate carbonates). Also, raw FGD gypsum (unwashed and untreated in 0,05M
solution of H2SO4) is heated and transformed into β-calcium sulphate hemihydrate and such
obtained product (β-CaSO4⋅0.5H2O) is compared with those obtained from purified FGD gypsum.
Three ways of β-CaSO4⋅0.5H2O obtaining, used in this study, are shown by following schemes:
1. raw FGD gypsum → heating in drier
2. raw FGD gypsum → washing in water → filtration → heating in drier
3. raw FGD gypsum → treating in 0.05 M H2SO4 → filtration →washing in water →
filtration→ heating in drier.
Raw FGD gypsum , FGD gypsum washed in water and FGD gypsum treated in H2SO4 and than
washed in water were heated in a drier at 130 oC and. the dehydration was monitored by qualitative
IR analysis in order to confirm the complete transformation of CaSO4⋅2H2O into β-CaSO4⋅0.5H2O.
The disappearance of bands corresponding to gypsum crystallization water (at 3420 and 1680 cm-1)
and the appearance of bands characteristic for hemihydrate (at about 3615 and 1005 cm-1) in IR
spectra of gypsum heated products indicated that the transformation occurred.
In order to investigate the resistance to compression of the products, standard pastes were
prepared by mixing β-calcium sulphate hemihydrate (obtained by three mentioned ways) with
water at a W/P ratio (water volume/mass of β-CaSO4⋅0.5H2O power) of 0.8 cm3/g. The compressive
strength of the hardened standard pastes was tested after 7 days. The preparation of standard pastes
and the testing of hardened standard pastes were carried out by procedures reported in the literature
(14) (in accordance with Yugoslav Standard requirements). The compressive strength determined
for hardened standard pastes was compared with those which are required for different kinds of
gypsum according to Yugoslav standard (14) (Table 5.).
Table 5. Required compressive strength for different kinds of gypsum according to Yugoslav
Standard
Kind of gypsum Stucco gypsum Plaster Alabaster gypsum Modeling gypsum
compressive
2.5
2.2
2.5
2.5
strength after 7
days (MPa)
The stucco gypsum has application in building, alabaster gypsum in building and sculpturing and
modeling gypsum for manufacture of different models.
Finally, all the hardened pastes were pulverized after 7 days and examined by qualitative IR
analysis for their chemical composition.
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May 23–27, 2005, Tatransk´e Matliare, Slovakia
Po-We-4, 054p.pdf
RESULT AND DISSCUTION
The results of IR analysis of the products obtained on the mentioned three different ways are
presented in Table 6.
Table 6. IR analysis of products obtained by FGD gypsum heating
Product
1. from raw FGD
gypsum
2. from FGD
gypsum washed
in water
3. from FGD
gypsum treated
in H2SO4 and
washed in water
3615
Wave number of absorption bands (cm-1)
3550 1615 1440 1155-1095
1005
655
600
3615
3550
1615
1440
1160-1095
1005
655
600
3610
3550
1615
-
1150-1090
1005
655
600
. In the IR spectrum of product 3. (from FGD gypsum treated in acid solution and than washed in
water) all the present bands are characteristic for calcium sulphate hemihydrate (CaSO4⋅0.5H2O),
according to literature (11,12). Beside the bands which are characteristic of CaSO4⋅0.5H2O, in the
IR spectra of product 2 (from FGD gypsum only washed in water) and product 1 (from raw FGD
gypsum) there are small bands at 1440 cm-1which confirmed that carbonates (the impurities in raw
FGD gypsum) did not disappear during FGD gypsum heating at 130 oC.
The compressive test results and colour for samples prepared with hemihydrate (product 1, 2 and
3.) and water, using the standard W/S ratio of 0.8 (after 7 days) are given in Table 7.
Table 7. Compressive strength and colour of hardened hemihydrate samples prepared using the
standard ratio (W/S = 0.8)
Hardened samples prepared with
product (β-hemihydrate)
1 (from raw FGD gypsum)
2 (from washed in water FGD gypsum)
3 (from treated in acid solution and
washed in water FGD gypsum)
Compressive strength
(MPa)
2.58
4.33
7.79
Colour
light - gray
white
white
The results in Table 7 show that all three samples have compressive strength values higher than
the lowest value according to the Yugoslav Standard (Table 5) necessary for application in the
construction industry, sculpturing and modeling (14).
A comparison of the compressive strength of the samples (Table 7) indicted that the sample
prepared with hemihydrate from raw FGD gypsum was less resistant than those prepared with
hemihydrate from washed in water FGD gypsum (by 40.4 %) and with hemihydrate prepared from
treated in acid solution and washed in water FGD gypsum (by 66.9 %).
A comparison of the compressive strength of the samples 2 and 3 (Table 7) indicted that the
sample prepared with hemihydrate from washed in water FGD gypsum was less resistant than that
prepared with hemihydrate from treated in acid solution and washed in water FGD gypsum (by
44.4%).
Also, it is evident (Table 7) that washing of FGD gypsum in water is necessary to attain the
white colour of products.
The results of IR analysis of hardened samples (powdered after 7 days) are presented in Table 8.
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May 23–27, 2005, Tatransk´e Matliare, Slovakia
Po-We-4, 054p.pdf
Table 8. IR analysis of hardened samples (powdered) after 7 days
Hardened samples based
Wave number of absorption bands (cm-1)
on product (hemihydrate)
1 (from raw FGD gypsum) 3550 3420 1680 1620 1440 1160-1120 670 600
2 (from washed in water
3560 3420 1680 1625 1440 1160-1120 670 600
FGD gypsum)
3 (from treated in acid
3560 3420 1680 1620
1160-1120 670 600
solution and washed in
water FGD gypsum)
All IR spectra (Table 8) of hardened samples obtained by mixing β-hemihydrate (products 1, 2
and 3) and water (W/S = 0.8), after 7 days, have absorption bands which are characteristic of
calcium sulphate dyhidrate (12,13). These results showed the complete recrystallization of
hemyhidrate into dihydrate, according to:
β-CaSO4⋅0.5H2O + 1.5H2O = CaSO4⋅2H2O
The small bands at 1440 cm-1in IR spectra of samples 1 and 2 confirm the presence of carbonates
(the impurities from FGD gypsum) in them.
CONCLUSION
The present study has shown that FGD gypsum, formed by flue gas desulphurozation (FGD)
process in power plant, can be used as raw material for β-calcium sulphate hemihydrate production.
When raw FGD gypsum is used, without any previous preparation, the obtained product
(β-CaSO4⋅0.5H2O) is suitable regarding the compressive strength, but not applicable (or low
applicable) because of its colour (light – gray) and the presence of impurities (chloride, carbonates).
The product (β-CaSO4⋅0.5H2O) from washed in water FGD gypsum is very satisfactory
regarding the compressive strength and colour (white). Small amount of carbonates in this material
should not disturb its application.
Finally, when FGD gypsum is treated with 0.05 M H2SO4 solution (in other to eliminate
carbonates) and washed in water (to eliminate soluble in water impurities), the obtained β-calcium
sulphate hemihydrate has the best quality (the highest compressive strength, white colour).
Therefore, the very best scheme for recycling waste FGD gypsum into β-calcium sulphate
nemihydrate is:
raw FGD gypsum → treating in 0.05 M H2SO4 → filtration → washing in water →
filtration→ heating in drier,
but it requires the highest cost.
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5. A. Jarosinski, Effect of fly ash on the properties of Anhydrite cement obtained from apatite
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