close

Вход

Забыли?

вход по аккаунту

?

mcmbaaa.28227.0033

код для вставкиСкачать
A TECHNOLOGY OF PRODUCTION OF SULFOGYPSUMBASED BINDING AGENTS F R O M
THERMAL POWER PLANT WASTE
O S Popova
State University of Railway Communications
Russia
S P Vanninen
Tampere University of Technology
Finland
ABSTRACT. Low-temperature technology of processing the wastes into binding agents
with a high content of the a-modification of calcium sulfate is proposed on the basis of
studies of properties of thermal power plant sulphur - containing wastes obtained by
collecting sulphur from flue gases with the help of lime mortar. Structural characteristics and
other properties of binding agents obtained by this method were investigated. The obtained
binding agents are suitable for the manufacturing of items of higher strength and as
component of gypsum-concrete binding agents.
Keywords: Chemical gypsum, Sulfogypsum, Absorbent, Calcium sulfate a- and pmodification of calcium sulfate, Hemihydrate and dihydrate of calcium sulfate, Waste
processing, Sulfogypsum-based binding agent.
Professor O S Popova is a Doctor of Technical Sciences, currently at the Department of
Construction Materials and Products of State University of Railway Communications, St.
Petersburg, Russia. She specialises in polymeric materials, chemical additives, and hydraulic
and aerial binding agents.
Dr S P Vanninen is a Candidate of Technical Sciences, currently at the Department of
Geology of Tampere University of Technology, Tampere, Finland. She specialises in
chemical additives, solutions and aerial binding agents.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
324
Popova, Vanninen
INTRODUCTION
Sulfur-containing wastes are obtained by collecting flue gases of thermal power plants
according to the following scheme [I]:
S 0 + CaC0 + 0.5 H 0 -> CaHSO .0.5H O + C 0 t
CaHSO .0.5H O + 0 + 3H 0 -> 2CaS0 .2H 0
2
3
3
2
2
2
3
2
2
4
2
2
The size of sulfogypsum crystals varies within 5-100 u.M. The characteristics of chemical
gypsum so obtained match those of the natural gypsum, and may even be superior with
respect to gypsum purity [2].
Some estimates [3] forecast that the amount of collected sulfogypsum in the world will reach
220-280 million tons by the year 2000. The utilization of chemical gypsum may solve
economical problems (obtaining new gypsum sources and relieving the land from heaps)
from one side, and ecological (environmental pollution : soil, water, atmosphere) from the
other side.
This paper describes a technology for sulphoxide-containing waste utilization that yields
binding agents suitable for construction material industry.
EXPERIMENTAL DETAILS
Materials
Samples of sulfogypsum from thermal power plants in Finland were used. Plant quality
certificate for the sulfogypsum.
Chemical composition:
Si02
0 , 6 1 % - 1,05%
0 , 2 3 % - 0,46%
A1203
25,8% - 28,4%
CaO
19,7%- 20,2%
S03
10%
Humidity
Experimental
All experiments were conducted in compliance with the standards ASTM C472, SFS 3167,
andGOST 125-79.
Based on preliminary studies that revealed differences in the structure of natural and
chemically obtained gypsum we developed a new technology of the production of calcium
sulfate hemihydrate from sulfogypsum.
At the first stage of the technological process, sulfogypsum is treated with a water adsorbing
agent. Sulfogypsum is mixed with the sorbent at a 22-24°C room temperature in a 1:1
proportion by weight.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Production of Sulfogypsum-Based Binders 325
The mixture is kept in a closed vessel for 24 h. At this stage, free and a part of bound
adsorbed water is removed from sulfogypsum under «mild» conditions. Upon mixing
sulfogypsum with the sorbent, the temperature of the mixture increases up to 42°C. It is
known for example according to Florke [4] that even below 45°C the formation of the
orthorhombic modification of calcium subhydrates is already possible. According to Van
Hoff [5], anhydride formation is possible below 42°C, and a rhombic syngony modification
of calcium sulfate, dimorphen, was found below 45°C [6]. In our case, it is reasonable to
expect that the process of mild water removal by adsorbent is endothermic. At this stage,
intermediate modifications of sulfogypsum hemihydrates must already be formed. Such
pretreatment of sulfogypsum permits to avoid the evaporation of excess water at the second
stage of dehydration, while drying in an oven at 175°C, and so to decrease the formation of
small crystals of the p-modification of calcium sulfate hemihydrate, as is generally known.
The duration of the second stage is 3 h. However, further studies have shown that this time
period may be shortened to 0,5 h. At this technological stage, the final dehydration of
sulfogypsum is achieved yielding target binding agent. The baking is performed in a
calorifier with a rapid (within 3-8 min) temperature elevation to the required level.
RESULTS AND DISCUSSION
The sulfogypsum binding agent obtained via the technological process suggested contains
larger amount of particles of the a-form of calcium sulfate hemihydrate than that obtained by
generally employed technology. Figures 1 and 2 show plots that represent the specific
surface area of crystals, obtained (A) by direct process of dehydration in a furnace and (B) by
dehydration preceded by pretreatment, as suggested here. The dispersion degree of
sulfogypsum crystals is 2.5 - 3 times higher in case B than in case A. Besides, a general
tendency toward the increase of crystal particles in evident in case B.
Volume distribution
A • sv_25
x-microns
B • sv_16
DATE: 05.1997
Figure 1 The specific surface area of a sulfogypsum binding agent (sulfogypsum is from
Naantali). A - without dehydrating pretreatment. B - with dehydrating pretreatment
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
326 Popova, Vanninen
Volume distribution
A • sv_17
x-microns
B • sv_28
DATE: 05.1997
Figure 2 The specific surface area of a sulfogypsum binding agent (sulfogypsum is from
Pori). A - without dehydrating pretreatment. B - with dehydrating pretreatment
Figures 3 and 4 show plots that reflect the state of water in sulfogypsum structure obtained by
the method of optical combination scattering [6]. The study is fulfilled using a DFS-24
device with the neon-helium laser. Sulfogypsum transmission spectra are significantly
different in cases A and B. Within the range of main transitions, total transmission area is
greater in case B than in case A. This indicates that, in case B, the microscopic crystal
structure is altered [7] and the mean size of sulfogypsum crystal particles is greater.
1000
"800
1000
cm"
1200
1400
FREQUENCY, cnr
1
1
•/454/4-12
DATE: 02.1998
Figure 3 Transmission spectra (Ne-He) of CaSO .0,5H O, (sulfogypsum is from Naantari).
A - without dehydrating pretreatment. B - with dehydrating pretreatment
4
2
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
P r o d u c t i o n of Sulfogypsum-Based B i n d e r s
327
1000
800
1000
1200
1400
FREQUENCY, c m
1600
1
cm"'
a/454/4-13
DATE: 0 2 . 1 9 9 8
Figure 4 Transmission spectra (Ne-He) of CaSO .0.5H O, (sulfogypsum is from Pori).
A - without dehydrating pretreatment. B - with dehydrating pretreatment
4
2
Figure 5 shows IR spectra of water in the structure of sulfogypsum crystals.
DATE: 10.1996
Figure 5 IR-spectrogram of CaS0 •0.5H O (sulfogypsum is from Naantari).
A - without dehydrating pretreatment. B - with dehydrating pretreatment
4
2
1
The spectra were obtained using a Perkin-Elmer instrument with a 7900-320 cm' working
range. It is known that the vibration spectrum of calcium sulfate always contains components
contributed by water and sulfate groups in addition to components contributed by water
bound by crystallization.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
328 P o p o v a , V a n n i n e n
The frequencies characteristic for intramolecular structures depend on their nearest
environment. Thus, the vibration spectra of molecules reflect characteristics of local crystal
structure [8]. In case B, sulfogypsum spectrum displays characteristic absorption bands at
1600 cm" , an increase in multiplicity at 3600 cm' , and the disappearance of the band of
completely symmetrical vibration at 100 cm" , which is characteristic for the ions of the (5modification of calcium sulfate. Thus, the sulfogypsum binding agent obtained by the twostage processing contains less amount of p-modification crystals and larger crystals with the
size increased 2.5- 3 times, which confirms the convenience of this technology for
sulfogypsum utilization.
1
1
1
PRACTICAL IMPLICATIONS
The technology of sulfogypsum binding agent processing that includes a pretreatment stage
of «mild» dehydration with water sorbents followed by baking the product in a furnace yields
binding agent with a reduced content of the p-modification of calcium sulfate hemihydrate.
The process of gypsum dehydration which is necessary for endowing it with binding
properties is a phase transition without diffusion. As water is being removed from the crystal
lattice of the dihydrate, energy conditions are being built up that cause the uncoupling of Ca
and S0 " from water molecules. Crystallographic rearrangement occurs by shifting Ca-S0 Ca layers. In the elementary crystal cells of the hemihyrated gypsum crystallochemical bonds
are coordinated in much the same, yet somewhat distorted, way as they are in the dihydrated
gypsum, the energy of the crystal lattice of the hemihydrate increasing with dehydration [9].
2+
2
4
4
The mechanism and kinetics of calcium sulfate dihydrate dehydration process depends upon
the dehydration technology employed. When water is removed by evaporation, the thermal
processing of the surface of dihydrate particles occurs. This results in the loosening of
gypsum crystal lattice and the generation of the p-modification of calcium sulfate
hemihydrate. p- hemihydrate looks like highly dispersed fine filaments and its specific area is
2,5-5 times higher than that of the a-modification. The crystals of the p-modification are
small, poorly formed, and defective. When dehydration is performed under an elevated
pressure, water is removed in drop-liquid state. This is not associated with crystal lattice
loosening and crystal destruction, and the dihydrate forms prismatic crystals of the a-form of
calcium sulfate. The crystals of ct-hemihydrate are big, dense, and less defective, which
provides for the strength of products manufactured.
In the suggested technology of sulfogypsum binding agent production, partial dehydration is
performed by water adsorption under normal conditions, instead of using an elevated
pressure. Therefore, only the remainders of adsorbed water are removed by the subsequent
baking which provides for the formation of bigger crystals of the a-modification of calcium
sulfate hemihydrate.
In Table 1 the values are presented of the specific surface area of sulfogypsum binding agents
obtained by the suggested and generally employed technological processes. From the data it
is clearly seen that the «mild» dehydrating pretreatment results in yielding bigger crystals of
calcium sulfate hemihydrate.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Production of Sulfogypsum-Based Binders 329
2
Table 1 Sulfogypsum specific surface area (cm /g)
(A - sulfogypsum was not pretreated; B - sulfogypsum was dehydrated by pretreatment)
SULFOGYPSUM
SOURCE
SPECIFIC SURFACE AREA
OF DIHYDRATE
CaS0 .2H 0
4
S. Kipso oy
Naantari
Pori
A
1100
1100
1000
SPECIFIC SURFACE AREA OF
HEMIHYDRATE CaSO .0,5H O
4
2
2
B
1800
1500
1900
A
4300
6200
5000
B
2000
1500
1900
CONCLUSIONS
1.
A technology is suggested for the processing of sulfoxide-containing waste into the
binding agent suitable for using as a raw material for manufacture and as a component
for gypsum-concrete binding agents. The technology includes the pretreatment of
sulfogypsum with dehydrating absorbents followed by baking it in a furnace.
2.
The sulfogypsum binding agent produced as a result of two-stage processing contains
significantly less amount of p-modification of calcium sulfate hemihydrate. The
specific surface area of sulfogypsum crystals so obtained is 2,5 - 5 times less than that
of sulfogypsum obtained by the generally used method.
3.
Crystal structure and water state in the crystals of calcium sulfate hemihydrate were
investigated by IR-spectroscopy and combinational light scattering. A direct effect of
dehydrating pretreatment on the state of bound water and crystal structure was found.
The crystals of sulfogypsum binding agent become bigger virtually devoid of the phemihydrate of calcium sulfate.
REFERENCES
1.
WAHLSTROM, M. Hilivoimalaitoiden rikinpoistojatteiden hyottokaytto.
1988, Kemia-Kemi, Vol. 15.
2.
WIRSCHUNG, F, HULLER, R, AND OLEJINIK, R. Gypsum from flue gas
desulphurisation plants: definition and legislation in the European Communities, in
OECI, and in Germany. Zement Kalk Gips, 1994, 2, pp 66-69.
3.
UTILIZATION OF THE PHOSFOGYPSUM PRODUCED IN THE FERTILIZERS
INDUSTRY. UNIDO/4s, 533.23 Mags., 1985, pp 2, 7, 20.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Espoo,
330 P o p o v a , V a n n i n e n
4.
FLORKE, O, W. Kristallographischen und rontgenographishen Untersuchungen in
System CaS0 -CaS0 -2H20. N. Jb. Min. Abh., Stuttgart, 1952, Vol. 84, SS. pp 189240.
4
4
5.
HAND, R, J. Calcium sulfate hydrates: a review. British Ceramic Transactions,
1997, Vol. 96, N 3.
6.
KUMARESON, P, AND DEVANARAYANAN, S. J. Mater. Sci (UK), 1989, Vol.
24, No. I,pp288.
7.
Bo6poB B . C , jKuryu YL. T., PoMauiKOB A. B . , KuceAeBa A. B. H3BecTHfl
AH CCCP, c e p . HeopraHHH. M a T e p . 1 9 8 7 , t . 2 3 , Nq 8, c. 1 3 6 4 .
(BOBROV, V, S, ZHIGUN, I, G, ROMASHKOV, A, V, KISELEVA, L, V.
Izvestiya, AN SSSR, ser. Neorganich. mater., 1987, Vol. 23, No. 8, pp 1364 in Russ.).
8.
HASS, M, AND SUTHERLAND, G, B. Proc. Roy. Soc. (London), Ser. A, 1956,
Vol. 236, pp 427.
9.
MHeAAOB-IIeTpOCHH
O.
II.
XHMHfl
M a T e p H a A O B . M.: CTpoHHSAaT,
1971.
HeopraHHHeCKHX
CTpOHTeAbHblX
(MCHEDLOV-PETROSIAN, O, P.
Chemistry of inorganic construction materials. Stroyizdat, Moscow, 1971.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Документ
Категория
Без категории
Просмотров
2
Размер файла
311 Кб
Теги
0033, mcmbaaa, 28227
1/--страниц
Пожаловаться на содержимое документа