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Physical Structure and Chemical and Mineralogical Composition of the Mazidagı (Turkey) Phosphate Rock A. K. O 2 zer,*,† M. Gu 1 labogˇ lu,† and S. Bayrakc¸ eken‡ Departments of Chemistry Engineering and of Chemistry, Atatu¨ rk University, 25240 Erzurum, Turkey
The physical structure and chemical and mineralogical composition of Mazidagı (Turkey) phosphate rock have been investigated by using chemical analysis, X-ray powder diffraction, scanning electron microscopy (SEM), thermal analysis, mercury porosimetry, and BET surface area methods. The results of X-ray powder diffraction analysis showed that the major minerals of the rock are calcite, fluorapatite, and carbonate fluorapatite. It was also found that the phosphate rock consists of two different phases which are dispersed in the rock heterogeneously in the SEM photographs, namely, a dark gray calcite-rich phase and a light gray phosphorusrich phase. Qualitative analysis based on the SEM photograph revealed that fluorapatite and carbonate fluorapatite were only concentrated in the phosphorus-rich phase. Calcite having particle sizes smaller than approximately 2 µm were agglomerated between compact phosphorusrich phases having particle sizes between 100 and 300 µm. The rock has a large pore-size distribution with a range of 3-1000 nm. The total porosity in the rock is 18.70%, and porous parts in the rock have been generally accumulated in the calcite-rich phase. The BET surface area of the rock is 15.68 m2/g. 1. Introduction Most of the known world phosphate rock reserves are found in sedimentary marine deposits of the Upper Cretaceous and Eocene ages of the Mediterranean phosphogenic province which exists in Morocco, Spain, Sahara, Algeria, Tunisia, Egypt, Israel, Jordan, Syria, Saudi Arabia, Turkey, and Iraq. They are made of deposits laid down in the ancient Tethy Sea of the Mesozoic and Tertiary ages. The importance of the phosphate rock deposits in the Upper Cretaceous and Eocene ages is that they form more than 70% of the total world phosphate reserves (El-Jallad et al., 1980). The Mazidagı (Turkey) phosphate rock used in this investigation is situated near the border with Syria and was deposited during the Turonian and Senonian (Cretaceous) ages.The phosphatic formations in Mazidagı were investigated by Lucas et al. (1981). They found that these phosporite deposits resulted from erusion of more and less calcitized previous deposits and redeposition in a phosphatizing environment, phosphatization of this detrital material, and weathering causing an enrichment of the ores. The main minerals found in the phosphate rock are fluorapatite (9CaO‚3P2O5‚CaF2), hydroxyapatite (9CaO‚ 3P2O5‚Ca(OH)2), collophane (9CaO‚3P2O5‚CaCO3‚H2O + H2O), francolite or carbonate flourapatite (9CaO‚3P2O5‚ Ca(F,CO3)‚H2O), and dahllite (9CaO‚3P2O5‚CaCO3‚ H2O). The phosphate rocks can be found in a form associated with one or several of these minerals. The gangue minerals associated with the phosphate rocks are carbonates, clays, and quartz (Wazer, 1961). Phosphate ores show a wide diversity in the composition of their gangue minerals but generally fall into one of three categories, based on the major associated gangues: * To whom correspondence should be addressed. † Department of Chemistry Engineering. ‡ Department of Chemistry.
1. Siliceous ores: These contain quartz, chalcedony, or different forms of silica. 2. Clayey ores: These mainly contain clays and hydrous iron and aluminum silicates or oxides as gangue minerals. 3. Calcareous ores: These contain calcite and dolomite as the major impurities with small amounts of silica. The phosphate ores are mainly used to produce the phosphoric acid. The gangue minerals found in the phosphate ores cause various problems during production of phosphoric acid. To decrease these gangue minerals, methods and techniques of varying sophistication are used in the processing of the ore depending on its grade, type, and quantity of gangues associated with the phosphate mineral. The majority of techniques, however, depend on physical methods of separation such as screening, flotation, scrubbing and desliming, heavy media separation, etc. However, in some cases these physical processes are ineffective in upgrading the phosphate ore, and calcination is required (Becker, 1983). Besides, fluidized-bed combustion technology continues to be a focus of coal-fired electric utilities because of the ability to control emissions to within acceptable limits. By using a suitable sorbent as the bed material, high sulfur coal can be burned while maintaining low SO2 emissions without substantial penalty in combustion efficiency. The most commonly used sorbents are calcium-based materials, in particular, natural limestone and dolomites which capture released sulfur oxides in the form of calcium sulfate. Such a choice of sorbent is technically sound because of the thermodynamic stability of calcium sulfate under fluidized-bed operating conditions (Borgwardt, 1970; Dam-Johansen and Qstergaard, 1991; Marsh and Ulrichson, 1985; Roy and Weisweiler, 1882; Haji-Sulaiman and Scaroni, 1991). Naturally occurring limestone and dolomites generally contain impurities, varying from less than 1% to
10.1021/ie990441v CCC: $19.00 © 2000 American Chemical Society Published on Web 01/29/2000
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Figure 1. Results of TG analysis of the phosphate rock.
as high as 20%. Most of the impurities are in the form of quartz and clay. It was presented in the most research that the inert materials such as MgO existing in the dolomite and Fe2O3 added to dolomite or to limestone indicated beneficial effects on sulfation conversion. In addition, because these inert materials delayed the closure of pores, higher conversions of sulfate were obtained (Alvfors and Svedberg, 1988; Stein, 1988; Dhupe et al., 1987). In our previous study, we investigated the possibility of using the Mazidagı (Turkey) phosphate ore as an alternative for basic materials such as limestone and dolomite in the flue gas desulfurization. The results of the experiments have shown that the conversation of sulfate obtained with Mazidagı phosphate ore is higher than that obtained with limestone and dolomite (O ¨ zer, et al., 1995). In the present study, the physical structure and chemical and mineralogical composition of the Mazidagı phosphate ore were investigated by using experimental techniques such as chemical analysis, thermogravimetry, X-ray powder diffraction, scanning electron microscopy (SEM), mercury porosimetry, and BET surface area. The aim of this study was to identify the structure of the raw phosphate rock. The results dealing with the behavior of the phosphate rock under calcination at higher temperatures have been given in another study (O ¨ zer et al., 2000). In a future study, we will compare the results of X-ray powder diffraction, SEM, mercury porosimetry, and BET analysis of flue gas desulfurization products with the results obtained in this study. In light of these results, the ways to increase the effectiveness of the phosphate rock in the flue gas desulfurization and the effects of the inert materials in the phosphate rock on the sulfate conversion will be investigated.
Table 1. Chemical Analysis of Phosphate Rock component
% wt
CaO P2O5 MgO Fe2O3 Al2O3 SiO2 F2 loss on ignition CO2 others
50.32 21.37 1.61 0.80 1.13 0.82 2.87 20.19 (16.60) 0.89
2. Experimental Section Material and Method. Phosphate ore used in this investigated was provided from the phosphate deposits in Mazidagı (Turkey). The sample was crushed, ground, and then sieved to obtain a fraction of 500-710 µm, which is the particle size used at the flue gas desulfurization. The chemical analysis of the phosphate rock was carried out by standard gravimetric, volumetric, and spectrometric methods, and the results of chemical analysis are given in Table 1. Thermogravimetric analysis (TGA) was used to determine the decrease in the sample weight with increasing temperature. Investigation on a -200 mesh sample was carried out with a Shimadzu TGA 50 thermal analyzer. The results of TGA are shown in Figure 1. X-ray powder diffractometry was used to identify the initial mineralogical constituents of the phosphate samples using the conventional X-ray technique and X-ray tables. The intensities were estimated from the heights of the emission peaks. The interpretation was made by reference to the ASTM index. The results of X-ray analysis carried out with a Siemens D-5000 X-ray diffraction machine are shown in Figure 2.
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Figure 2. X-ray powder diffraction spectrum of the phosphate rock. Table 2. Physical Properties of Phosphate Rock Vp (cm3‚g-1) SBET (m2‚g-1) F (g‚cm-1)
0.0867 15.68 1.93
The physical texture of the phosphate rock was examined in a Camebax SX50 SEM. The SEM photographs obtained, which show qualitative analysis of phosphate rock and the boundary between the parts with phosphate and the parts with carbonate, are given in Figures 3 and 4, respectively. In addition, the specific surface area of the phosphate rock was measured by nitrogen adsorption at -195 °C using the BET method. The result of the BET surface area carried out with a Quantachrome QS-17 model apparatus is given in Table 2. The pore-size distribution and pore volume in the rock was investigated using a Micromeritics 9310 mercury porosimeter. The result of pore volume and pore-size distribution are shown in Table 2 and Figure 5, respectively. The density of the phosphate rock was measured by a phycnometer, and the result is given in Table 2. 3. Results and Discussion As can be seen from chemical analysis results given in Table 1, the main components in the rock are CaO, P2O5, and CO2. Except for the main components, flour in the rock has the highest amount, which is 2.88%. Although the amount of SiO2 in the structure of most ores is the high rate, the amount of SiO2 found in these ores is quite low. The thermal analysis curve of the phosphate rock in a N2 atmosphere is shown in Figure 2. In the thermal analysis experiment, a flow rate of 30 cm3/min and a heating rate of 5 °C/min were used. The weight loss occurred from room temperature to 617 °C is due to separation of the water of constitution and volatile
components such as organic matter which can be found in the ore. The weight loss which began at 617 °C and continued until 767 °C is due to the endothermic decomposition of calcium carbonate as follows:
CaCO3(s) f CaO(s) + CO2(g) The weight loss obtained at the range of 617-767 °C is in good agreement with the amount of CO2 found at chemical analysis. In the X-ray powder diffraction analysis results given in Figure 2, it was seen that the main minerals in the ore structure are calcite (CaCO3; ASTM Card Number 5586), fluorapatite (9CaO‚3P2O5‚ CaF2; ASTM Card Number 3411), and carbonate fluorapatite (9CaO‚3P2O5‚Ca(F,CO3)‚H2O; ASTM Card Number 21 141). SEM photographs of raw phosphate rock are given in Figures 3 and 4. In Figure 3, it is shown that the phosphate rock consists of two different phases. In the results of qualitative analysis obtained from the SEM, it was determined that the parts with light gray are formed from phosphorus-rich components and the parts with dark gray are formed from calcium-rich components. The parts with dark gray can be defined as CaCO3, based on chemical analysis. It was observed that there is no phosphorus in the parts of carbonate. In that case, carbonate fluorapatite existing in the ore has only been found in the parts with phosphorus-rich components. In the enlarged photograph (Figure 4) containing the boundary between the part with phosphate and the part with carbonate, it is clearly shown that the parts with carbonate among the big phosphate particles occurred by agglomeration of the little particles. The surfaces of the parts with phosphate have a compact structure which does not contain a big porosity. To explain the pore-size distribution in the ore, it was thought that investigation of the pore structure in the ore by using mercury porosimetry was necessary. In the
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Figure 3. SEM photograph of the phosphate rock and the results of qualitative analysis.
Figure 4. SEM photograph containing the boundary between the part with phosphate and the part with carbonate.
pore-size distribution given in Figure 5, it is probable that the pores (mezopores) smaller than 20 nm are primary pores; that is, the pores did not occur as a result of agglomeration of particles found in the ore naturally. It can be said that the pores between 20 and 1000 nm
Figure 5. Pore-size distribution diagram obtained from mercury porosimetry of the phosphate rock.
are secondary pores which occurred as a result of agglomeration of the little particles. These results are in agreement with SEM photographs.
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4. Conclusions Because the calcite content in the rock is high (37%) and the silica content (0.82%) is lower, the Mazidagı phosphate rock falls in the calcareous ores category, as explained in the Introduction section. The results of X-ray powder diffraction analysis showed that the main minerals of the rock are calcite, fluorapatite, and carbonate fluorapatite. It was found that the rock consists of two different phases which are dispersed in the rock heterogeneously, namely, the dark gray calcite-rich phase and the light gray phosphorusrich phase in the SEM photographs. Qualitative analysis based on the SEM photographs revealed that fluorapatite and carbonate fluorapatite were only concentrated in the phosphorus-rich phase. Calcite having particle sizes smaller than approximately 2 µm was agglomerated among compact phosphorus-rich phases having particle sizes between 100 and 300 µm. The rock has a large pore-size distribution with a range of 3-1000 nm. The total porosity in the rock is 18.70%, and porous parts in the rock have generally been accumulated in the calcite-rich phase. The BET surface area of the rock is 15.68 m2/g. In a previous study, it was determined that the parts with only calcite in the phosphate ore are reacted with SO2 and that the parts with phosphorus are unreacted. For this reason, it can be thought of as a suitable raw material in the phosphoric acid production, because the product obtained at the end of desulfurization spends much less acid for the parts with calcite during the phosphoric acid production comparison with the raw ore. As a result, in light of these observations, it can be said that Mazidagı phosphate rock, because the apatites have low reactivity and the ore is quite porous and calcite-rich, is a convenient sorbent for flue gas desulfurization. Literature Cited Alvfors, P.; Svedberg, G. Modelling of the sulphation of calcined limestone and dolomitesa gas-solid reaction with structural changes in the presence of inert solids. Chem. Eng. Sci. 1988, 43 (5), 1183-1193.
Becker, P. Phosphorous and Phosphoric Acid; Fertilizer Science and Technology Series; Marcel Dekker: New York, 1983. Borgwardt, R. H. Kinetics of the reaction of SO2 with calcined limestone. Environ. Sci. Technol. 1970, 6 (4), 59-63. Dam-Johansen, K.; Qstergaard, K. High temperature reaction between sulfur dioxide and limestonesI. Comparison of limestone in two laboratory reactors and a pilot plant. Chem. Eng. Sci. 1991, 46 (3), 827-837. Dhupe, A. P.; Jayaraman, V. K.; Gokarn, A. N.; Doraiswamy, L. K. An experimental study of the effect of inerts on gas-solid reactions. Chem. Eng. Sci. 1987, 42 (10), 2285-2290. El-Jallad, I. S.; Abouzeid, A. Z. M.; El-Sinbawy, H. A. Calcination of phosphates: reactivity of calcined phosphate. Powder Technol. 1980, 26, 187-197. Haji-Sulaiman, M. Z.; Scaroni, A. W. The calcination and sulfation behaviour of sorbents in fluidized-bed combustion. Fuel 1991, 70, 169-175. Lucas, J.; Prevot, L.; Ataman, G.; Gundogdu, N. Mineralogical and geochemical studies of the phosphatic formations in southeastern Turkey (Mazidagi-Mardin). Spec. Publ.sSoc. Econ. Paleontol. Miner. 1981, 29, 149-152; Chem. Abstr. 1981, 95, 223146q. Marsh, D. W.; Ulrichson, D. L. Rate and Diffusional study of the reaction of calcium oxide with sulfur dioxide. Chem. Eng. Sci. 1985, 40 (3), 423-433. O ¨ zer, A.; Gu¨labolu, M.; Bayrakc¸ eken, S.; Weisweiler, W. Flue gas desulfurization by Mazidagı (Turkey) phosphate rock. Yanma ve Hava Kirlilii Kontrolu III. Ulusal Sempozyumu, Ankara 1995, 51-58. O ¨ zer, A.; Gu¨labogˇlu, M.; Bayrakc¸ eken, S. The Calcination of The Mazidagı (Turkey) Phosphate Ore in the fluidized and Fixed Bed. Ind. Eng. Chem. Res. 2000, submitted for publication. Roy, G. K.; Weisweiler, W. Absorption of sulfur dioxide by limestone in a high temperature fluidized-bed. Chem. Eng. Div. 1982, 62 (2), 33-36. Stein, R. M. Trockene schwefeldioxid sorption mit dolomit, dolomitkalk und dolomitkalkhydrat im wirbelschicht-reactor. Dissertation, University of Karlsruhe, Karlsruhe, Germany, 1988. Wazer, U. Phosphorus and its Compounds; Interscience: New York, 1961.
Received for review June 17, 1999 Revised manuscript received November 23, 1999 Accepted November 30, 1999 IE990441V
