Dec. 2007
Journal of China University of Mining & Technology
J China Univ Mining & Technol
Vol.17
No.4
2007, 17(4): 0498 – 0502
Cementing Properties of Oil Shale Ash FENG Xiang-peng1, NIU Xue-lian2, BAI Xue2, LIU Xiao-ming2, SUN Heng-hu1 1
2
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China School of Resource and Safety Engineering, China University of Mining & Technology, Beijing 100083, China
Abstract: The oil crisis has prompted renewed interest in direct burning of oil shale as an alternative energy source. A major problem in this process is the large portion of ash produced. The cementing properties of this ash were investigated to determine its applicability as a building material. By means of XRD, IR, NMR and ICP, we have studied the effects of burning temperature on the reactivity of ash. Maximum reactivity was obtained with ash samples produced at 700 °C to 900 °C. In this range, the strength of oil-shale-based material, with properties similar to cement, which is composed of oil shale and several other kinds of solid wastes, can achieve the standard of 42.5# cement. Our study has provided an experimental foundation and theoretical base for a massive utilization of oil shale. Key words: oil shale; thermal activation; cementitious activity CLC number: TG 31
1
Introduction
Oil shale is a kind of marlite containing combustible organisms, which is formed by the simultaneous sedimentation of granule mineral fragments and the rotting organisms of low ranked animals and plants. The abundant reserve of oil shale is about 32.989 Gt in China, mainly distributed in Jilin Province (17.427 Gt), Guangdong Province (5.515 Gt) and Liaoning Province (4.505 Gt). As an important potential energy source, the amount of heat that could be converted from oil shale in fossil fuel is second only less than that of coal[1]. The amount of oil contained in oil shale is about 475 Gt, equal to 5.4 times of the amount of crude oil in the world. Experience confirms that the fluidized bed combustion process with temperatures ranging from 850 °C to 900 °C is sufficient to burn oil shale. Fine grains of ground oil shale, that contain 20%–25% very fine particles of sizes <160 µm, stay in the combustion chamber for a relatively short period (about 2–4 min). During this time, organic compounds burn out and the generation of new crystalline phases takes place in the mineral constituent of the combustible material. This combustion process produces a waste product, called oil shale fly ash (OSFA). It was found that OSFA is a kind of high-calcium
fly ash. It contains anhydrite, calcite, lime (CaOfree), quartz, apatite, as well as clinker minerals (β-C2S, C3A, C4AF) and has hydraulic activity[2–4]. Because OSFA has latent cementitious performance, it can be used as the raw material of cement. The purpose of the present work is to study the thermal behavior of the oil shale, and to get an efficient way to enhance the reactivity of heated oil shale.
2
Raw Materials and Experimental Methods
2.1 Raw materials The oil shale used in this experiment is taken from Jilin Province. Oil shale is a low-calorie fuel, with a low content of organic matter. Its inorganic part consists mainly of quartz, calcite, fluorapatite [Ca10 (PO4)6F2] and gypsum as well as small amounts of amorphous substances. The chemical composition of oil shale is listed in Table 1. Table 1 Oil shale
Chemical composition of oil shale
(%)
SiO2
Al2O3
CaO
MgO
Fe2O3
K2O
61.60
25.76
0.70
0.52
10.00
1.11
2.2 Experimental methods The chemical composition of the raw material can be analyzed by using X-ray fluorescence analysis
Received 02 February 2007; accepted 25 May 2007 Projects 50674062 supported by the National Natural Science Foundation of China and 2006BAC21B03 by the National Key Technologies R&D Program Corresponding author. Tel: +86-10-62794738; E-mail address:
[email protected]
FENG Xiang-peng et al
Cementing Properties of Oil Shale Ash
equipment (XRF-1700, Daojin). The physical composition of material can be analyzed by X-ray diffraction spectrometer (Rigaku D/max-RB) using Cu-Kα radiation (40 kV, 100 mA) with a scanning rate of 4° per minute from 5° to 70°. The infrared spectroscopy is done by using the Spectrum GX FTIR spectrophotometer (PE U.S.A). All MAS solid state NMR spectra are acquired using a BRUKER-AM300 Solid State NMR. The resonance frequency of 27Al and 29Si at a magnetic field strength of 7.05 T are 78.20 and 59.62 MHz respectively. After being heated at different temperatures, the samples were ground into grain sizes less than 50 µm. Samples of one gram which were heated at different temperatures and ground into fine particles, respectively into 100 mL, 1 mol/L NaOH solution, and then sealed up and put in the 20 °C maintaining room for 7 days. The samples were then filtrated and the filtrate sealed in a plastic bottle. The dissolving quantity of Si and Al ions was measured by ICP PROFILE equipment produced by Leeman. The oil shale ash used in this experiment with 0.080-mm sieve residue 3.0% and Blaine specific area 563 m2/kg. Granulated slag used in this experiment is taken from Tangshan steel plant, with 0.080-mm sieve residue 5.0% and Blaine specific area 456 m2/kg. Fly ash is also taken from Tangshan, with 0.080-mm sieve residue 6.0% and Blaine specific area 463 m2/kg. In the cement mortar case, cement was mixed with water in a water/cement ratio of 0.5 and then cured in fog room at (20±3) °C for 24 h, followed by curing in water at (20±3) °C.
3 3.1
499
From Fig. 1, we can find that when the heating temperature is lower than 500 °C, the main mineral composition of heated oil shale consists of kaolin, quartz and siderite. At this temperature, the eliminating hydroxyl reaction of kaolinite is inconspicuous and siderite had not been decomposed, so the crystalline structure and mineral composition of oil shale had not evidently changed with the same XRD spectra. When the heating temperature was 500 °C, several peaks of siderite in the XRD spectrum had disappeared, which indicated that most siderite had been decomposed. With the increasing of temperature, kaolinite decomposes and nearly all of the kaolinite in the oil shale had been decomposed at 600 °C. The diffraction peaks of kaolinite disappeared in the XRD spectrum at 700 °C. Because metakaolin is a kind of non-crystalline substance, it only has diffuse diffraction characteristic. As a result, there was no diffraction peak of metakaolin in the XRD spectrum, but only an obviously steamed bread diffuse peak between 10° and 30°. When the heating temperature was higher than 600 °C, the diffraction peaks of quartz became clearly stronger with an increase in temperature, which was the result of the decomposition of metakaolin. 3.2 IR spectra analysis of heated oil shale Fig. 2 shows the IR spectra of oil shale heated at different temperatures.
Results and Discussion Effect of heating temperature on mineral composition of oil shale
To study the effect of heating temperature on the mineral composition of oil shale, X-ray diffraction was carried out on the oil shale treated at different temperatures. The results are shown in Fig. 1. Quartz Kaolin Siderite Illite Rutile Hematite 1000ć 900ć 800ć 700ć 600ć 500ć 400ć 300ć 100ć
10
20
30
40 50 2-Theta (°)
60
Fig. 1 XRD patterns of oil shale heated at different temperatures
70
Fig. 2
IR spectra of oil shale heated at different temperatures
Three bands, whose sharp peaks are lying at 3698 cm–1, 3655 cm–1, and 3622 cm–1 respectively, token the stretching vibration of -OH in the crystallization water. Each of these three bands has integrated, sharp and clear shapes, which shows that the crystallization of kaolinite is in a good condition. The different conditions of -OH induced various bond lengths and different peak values occurred in the spectra. Two strong bands at 3698 cm–1 and 3622 cm–1, individually, belonged to the vibration absorption of exogenous hydroxyl and endogenous hydroxyl. Because the -OH axis of the endogenous hydroxyl is nearly parallel with the layer, the vibration frequency of this hydroxyl is low. The peak at 3655 cm–1 is the result of the combination of two medium strength hydroxyl
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Journal of China University of Mining & Technology
absorption peaks, which were induced by the stretching vibration absorption of the exogenous hydroxyl[5]. The peak at 1614 cm–1 represents the curving vibration of -OH and the sharp peak at 913 cm–1 at a medium energy level represents the swing absorption of exogenous hydroxyl. With the heating temperature increasing from 100 °C to 700 °C, the absorption spectra of hydroxyl decreased gradually, which indicates that the moisture in oil shale had been reduced to the point of disappearance as the temperature increased. The peaks at 1090 cm–1, 1034 cm–1 and 1008 cm–1 represent the asymmetric stretching vibration of Si-O bond. The peaks at 798 cm–1 and 779 cm–1 belong to the symmetric stretching vibration of Si-O-Si bond and the peaks at 694 cm–1 and 648 cm–1 belong to the asymmetric stretching vibration of Si-O-Al bond. The peak at 537 cm–1 is mainly caused by the stretching vibration of Si-O-Al bond. Although the bands at less than 500 cm–1 are the result of the overlap of Si-O bond curving vibrations, the Al-O bond stretching vibration and the -OH bond parallel shift, they were mainly induced by the curving vibration of Si-O bond[6]. The key bands of SiO2 are at 1094 cm–1, 798 cm–1, 779 cm–1, 694 cm–1 and 470 cm–1, whose meta structure is shown in Fig. 1. Correspondingly, the key bands of kaolin are at 1034 cm–1, 1009 cm–1, 940 cm–1, 913 cm–1, 737 cm–1, 470 cm–1 and 431 cm–1. Combining Figs. 1 and 2, we see that the IR spectra of oil shale heated at 100 °C, 300 °C and 400 °C show little change as temperature increased. But when the temperature increased to 500 °C, the key band at 1434 cm–1 representing CO32– had disappeared, which indicates the siderite had decomposed. At this point, the reduction of the kaolinite band in the high frequency zone illustrated that the kaolinite had began to lose the hydroxyl in its structure. But the key bands of SiO2 at 1094 cm–1, 798 cm–1, 779 cm–1, 694 cm–1 and 470 cm–1 still exist. As the temperature increased continually, the rate of hydroxyl elimination in the kaolinite accelerated and the strength of the IR absorption bands induced by the stretching vibration, the curving vibration, the swing vibration and parallel vibration of hydroxyl, became gradually weaker and finally disappeared. As Fig. 2 shows, when the temperature was higher than 500 °C, the bands of 6-coordianted AlVI-OH at 913 cm–1, 6-coordianted AlVI-O-Si at 537 cm–1 and 6-coordianted AlVI-O at 431 cm–1 had disappeared. Meanwhile, the band at 568 cm–1, tokening the double rings structure formed by AlIV-O and SiIV-O had appeared. Each of these features are consistent with the Tonpsion Rule that 4-coordinated AlIV keeps steady at high temperatures and 6-coordinated AlVI keeps steady at low temperatures. In the IR spectrum of oil shale heated at 600 °C, a new band at 568 cm–1 indicates the appearance of
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metakaolin. Moreover, a peak at 516 cm–1 toking the illite had appeared, but it disappeared when the temperature rose to 900 °C. In the IR spectra of oil shale heated above 800 °C, the absorption band representing planar water had appeared again at 3434 cm–1 and 1627 cm–1, as the result of water re-absorbed by the finely ground samples. The kaolinite of oil shale heated at 500 °C had begun to lose hydroxyl in its own structure, but this reaction was still weak. Compared with the IR spectra of oil shale heated at low temperatures, both the position and strength of kaolinite absorption bands had changed little. Besides the disappearance of the hydroxyl absorption band in the IR spectrum of oil shale heated at 700 °C, there were some other changes. The bands induced by Si-O vibration and Al-O vibration merged to four main bands at 1088 cm–1, 797 cm–1, 779 cm–1 and 464 cm–1. The band at 1088 cm–1 still belonged to the stretching vibration of Si-O bond and the band at 468 cm–1 was caused by the curving vibration of Si-O bonds. These changes show that, in the course of eliminating hydroxyl of oil shale, the Al atom of aluminum-oxygen octahedron was changing from AlVI into AlIV. Although the number of main absorption bands in the IR spectra was the same as that at 700 °C, when the temperature reached 1000 °C, the position of Si-O vibration absorption peaks tended to shift to a high frequency zone. The strength of the absorption band also increased. From the shape of this absorption peak, we see that kaolinite treated at 1000 °C still retains broad amorphous peaks. However, the shapes had become symmetrical, which is obviously different form metakaolinite. 3.3 27
MAS NMR analysis of heated oil shale
Al MAS NMR spectra of oil shale heated to 100 °C and 600 °C are shown in Fig. 3. From this figure we see that, the 27Al MAS NMR spectrum of oil shale heated at 100 °C has a single sharp peak at 4.45×106 representing the resonance signal of 6-coordinate aluminum. This peak shows that Al ions in kaolinite exist in the form of [Al-2O(4OH)][7–10]. An obvious change happened in the 27Al MAS NMR spectrum of oil shale heated at 600 °C. The resonance peaks at 9.8×10–6 and 69.11×106 respectively belonged to the resonance signal of 6-coordinate aluminum and 4coordinate aluminum[11]. On the basis of the IR analysis, we discovered that, in the course of the oil shale heating process, the hydroxyl eliminated reaction in kaolinite generated initially; the hydroxyls in the aluminum-oxygen octahedron mutually combined and then eliminated a H2O molecule. As a result, the Al ion coordination number changed from six to five and four, simultaneously; the activity of Al ions had improved. As the hydroxyl elimination reaction continued, the 6-coordinate alu-
FENG Xiang-peng et al
Cementing Properties of Oil Shale Ash
minum signal reduced gradually until it disappeared, while the 4-coordinate aluminum signal increased
slowly.
(a) 100 °C
Fig. 3
3.4
27
The experimental results of He reflect the activity of clay mineral by means of testing the amount of dissolved Si and Al in a 0.5 mol/L NaOH solution[12]. This method can be used to calculate the amounts of active SiO2 and Al2O3 in oil shale heated to different temperatures to reflect the activity of oil shale. The results are shown in Fig. 4. 600 400
(b) 600 °C
Al MAS NMR spectra of oil shale heated to different temperatures
Analytical results of alkali dissolving in heated oil shale
500
Si
silicon-aluminum spinel, as temperatures increase, the amount of dissolving aluminum ions decreased sharply at 1000 °C. 3.5 Strength experiment of oil shale based mortar with cement properties Table 2 presents the results of strength tests of oil shale based mortar with cement properties. The oil shale has been heated to different temperatures. The mass per cent of heated oil shale is 40%; the other components are slag (27%), fly ash (15%), clinker (10%), gangue (6%) and others (2%).
300 200 100 0
Fig. 4
501
Table 2 Al
Heating temperature (°C)
100 300 400 500 600 700 900 1000 1200 Calcined temperature (°C)
Dissolved quantity of silicon and aluminium ions in 1 mol/L NaOH solution
Fig. 4 shows that, with the increasing of heating temperature, the dissolved amounts of activated Al and Si in the heated oil shale at first increased and then decreased. When the temperature was between 500 °C and 900 °C, the number of dissolved Al and Si ions remained, on the whole, constant. When the temperature was 900 °C, the number of dissolved Si and Al ions reached their highest numbers separately at 496.6 mg/L and 399.2 mg/L. Combined with XRD, IR and NMR analyses, we see that, as the temperature increased, the eliminated hydroxyl in kaolin reacted in a minimum level at 500 °C. Nearly all of the kaolin had changed into metakaolin at 700 °C; simultaneously, aluminum-oxygen octahedron changed into aluminum-oxygen tetrahedron and the degree of distortion reached its highest level, but the Si-O tetrahedron still retained its stable structure. At 900 °C, the degree of distortion of the [SiO4] tetrahedron reached its highest point. The number of dissolving Al and Si ions reached a maximum at 700 °C and 900 °C respectively. Because there are many enriched aluminum phase substances formed, such as r-Al2O3 and
Strength test results of oil shale based mortar with cement properties Compressive strength (MPa) 3d
28 d
100
18.5
25.6
600
27.8
47.6
900
27.6
49.2
1000
22.1
37.5
From Table 2, we see that, with an increase in heating temperature, the strength of oil shale based mortar with cement properties has initially a tendency to increase and then to decline. The strength of the oil shale mortar cured for 28 d reached a maximum of 49.2 MPa when the oil shale was heated at 900 °C.
4
Conclusions
1) Thermal activation cannot only cause a hydroxyl elimination reaction in the Al-O tetrahedron, but clearly also changes the mineral components and structure of oil shale. As the heating temperature increases, kaolin in the oil shale will change into matekaolin. With the form of Al coordination being changed, the degree of distortion in the silicon-oxygen tetrahedron gradually increases. The amount of dissolved SiO2 reached its maximum at 900 °C. 2) Thermal activation can improve the activity of oil shale remarkably, but the generation of other inert
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Journal of China University of Mining & Technology
compositions will deteriorate the reactivity of oil shale when its heating temperature is higher than 1000 °C. Under our experimental conditions, the strength performance of our material, which has
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properties similar to cement and is mainly produced by oil shale heated to 900 °C, can achieve the strength standard of 42.5# cement.
References [1]
Hou X L. Prospect of oil shale and shale oil industry. Proceedings International Conference on Oil Shale and Shale Oil. Beijing: Chemical Industry Press, 1988: 7–15. [2] Bentur A, Shalom M I, Bassat M B, et al. Cementing properties of oil shale ash: part I. Cement Concrete Research, 1980, (10): 175–182. [3] Bentur A, Shalom M I, Bassat M B, et al. Cementing properties of oil shale ash: part II. Cement Concrete Research, 1981, (11): 645–650. [4] Bentur A, Shalom M I, Bassat M B, et al. Cementing properties of oil shale ash: part III. Cement Concrete Research, 1981, (11): 799–807. [5] Farmer V C. Infrared absorption of hydroxyle groups in kaolinite. Science, 1964, (145): 1189–1190. [6] Farmer V C, Russel J D. The infrared spectra of layer silicates. Spectrochim Acta, 1964, 20: 1149–1173. [7] Dominique M, Pascal D, Jean F A, et al. 27Al and 29Si NMR study of kaolinite thermal decomposition by controlled rate thermal analysis. J Am Ceram Soc, 1995, 11(78): 2940–2944. [8] Mackenzie, Kenneth J D, Hartman J, et al. MAS NMR evidence for the presence of silicon in the alumina spine from thermally transformation kaolinite. J Am Ceram Soc, 1996, 11(79): 2980–2982. [9] Roch G R, Smith M E, Drachman S R. Solid state NMR characterization of the thermal transformation of illite-rich clay. Clay Minerals, 1998, 6(46): 694–704. [10] Amedee D, Etienne B, Guillaume M, et al. Behavior of paramagnetic iron during the thermal transformations of kaolinite. J Am Ceram Soc, 2001, 5(84): 1017–1024. [11] Brown I W M, Mackenzie K J D, Browden M E, et al. Outstanding problems in the kavlinite-mullite reaction sequence investigated by 29Si and 27Al solid-state nuclear magnetic resonance: II, high temperature transformations of metakaolinite. J Am Ceram Soc, 1985, 6(68): 298–301. [12] He C L, Bjarne, Makovicky E P. Reactions of six principal clay minerals: activation, reactivity assessments and technological effects. Cement and Concrete Research, 1995, 25(8): 1691–1702.