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 . Some estimates  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  that even below 45°C the formation of the orthorhombic modification of calcium subhydrates is already possible. According to Van Hoff , anhydride formation is possible below 42°C, and a rhombic syngony modification of calcium sulfate, dimorphen, was found below 45°C . 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 . 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  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 . 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 . 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.