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APPLIED CHEMISTRY Dissolution Kinetics of Phosphate Ore in H2SO4 Solutions F. Sevim,* H. Sarac¸ , M. M. Kocakerim, and A. Yartas¸ ı Department of Chemical Engineering, Ataturk University, 25240 Erzurum, Turkey
The kinetics of the dissolution of phosphate rock and the crystallization of calcium sulfates, which occurs at the end of the reaction, were investigated in sulfuric acid solutions. The samples used in experiments were obtained from Mazıdagˇı, Mardin, Turkey. The effects of the particle size, solid-to-liquid ratio, gas flow rate, and reaction temperature on the dissolution process were determined. A mathematical model in the following form, including the effects of parameters was established to express the dissolution kinetics of phosphate ore: -ln(1 - X) ) ktm. The activation energy for the process was found to be 29.66 kJ‚mol-1. 1. Introduction Phosphate fertilizers, which have a important place in the fertilizer industry, are produced from phosphate rocks. Recently, the worldwide consumption of phosphate fertilizers has increased 8% on average per year. Acidic dissolution of natural phosphate rocks is a basic process in the production of inorganic phosphorus fertilizers, such as superphosphate (mono and double), wet-process phosphoric acid (the main semiproduct for subsequent fabrication of various kinds of inorganic fertilizers, e.g., ammonium phosphate or amorphous), nitroamorphous, and other complex and mixed fertilizers.1-4 The process of phosphate rock dissolution occurring inside an industrial reactor is not yet quite clear. The most important use of phosphate rock is the production of fertilizers. The treatment of phosphate ores is achieved through two basic processes: 1. Wet-Process Phosphoric Acid. Digesting raw phosphate rock or occasionally calcined rock with sulfuric acid produces the phosphoric acid. In this process, HCl is used to acidify (in slight excess to prevent the formation of monocalcium phosphate) an organic solvent (C4 or C4 alcohol) to extract the phosphoric acid, and water is used to strip out the phosphoric acid (with a small amount of solvent and hydrochloric acid). 2. Electric-Furnace Phosphorus and Phosphoric Acid. This process permits the use of lower-grade rock than the wet-process phosphoric acid process, because the slag carries off impurities. The principal requirement is cheap electricity. Pure, strong phosphoric acid is manufactured from elemental phosphorus by oxidation and hydration.5 Domestic phosphate rocks are essentially fluorapatite containing various proportions of other compounds of calcium, fluorine, iron, aluminum, and silicon. Apatite is widely distributed phosphate mineral with a hexagonal crystal structure and is named according to the F-, Cl-, OH-, and CO3-2 groups it contains, including * To whom correspondence should be addressed. E-mail:
[email protected].
carbonate apatite [Ca10(PO4)6‚CO3‚H2O] and fluorapatite [Ca10(PO4)6‚F2]. Phosphate compounds are not simple inorganic compounds but represent a complex branch of chemistry. Phosphate compounds play an important role in many biochemical processes forming polyphosphates with metal cations and producing various organic and inorganic polymers.1 Many attempts to elucidate the mechanism of acidic phosphate dissolution have been reported in the literature. McCollough et al.6 studied the production of dicalcium phosphate from phosphate rock in water containing SO2 and carbonyl compounds. Drechsel7 worked on the production of dicalcium phosphate and phosphoric acid from various-grade phosphate rocks using H2SO4 as the leaching agent. Sardisco et al.8 investigated the production of phosphoric acid and some other phosphate compounds from low-grade phosphate ores using phosphoric acid. Phosphoric acid has also been produced by the wet method from phosphate rock containing flourapatite.9 In another study, the dissolution kinetics of crystals of fluorapatite in HCl solution was investigated; the activation energy of the dissolution process was calculated as 41.82 kJ‚mol-1.10 The kinetics of the dissolution of Egyptian phosphate ore in nitric acid was examined, and the effect of the particle diameter on the dissolution rate was found to be insignificant.11 Several studies have been carried out on the dissolution of Mazıdagˇı phosphate ore to produce phosphate compounds.12,13 In addition to the above studies, the dissolution of phosphate rocks for phosphoric acid and some phosphate compounds production has been investigated in different acidic solutions.14,15 The aim of this work was to study the effects of particle size, solid-to-liquid ratio, gas flow rate, and reaction temperature on the dissolution kinetics of phosphate rock in sulfuric acid solutions. A mathematical model including the effects of parameters was established to express the dissolution kinetics of phosphate ore.
10.1021/ie020168o CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003
Ind. Eng. Chem. Res., Vol. 42, No. 10, 2003 2053 Table 1. Analytical and X-ray Diffraction Analysis of Phosphate Ore component
content (wt %)
X-ray diffraction
P2O5 CaO SiO2 SO3 Al2O3 MgO Fe2O3 F ignition loss other total
16.58 44.38 19.02 1.05 0.18 0.20 0.05 2.05 15.01 1.48 100
17.63 46.43 19.87 a 0.15 0.16 0.08 a 13.31 2.37 100
a
Not analyzed.
desired temperature, a given amount of the ore sample was added to the solution while the stirring was started. At the end of the chosen reaction time, the stirring was stopped, and all of the reactor contents were then filtered off. The phosphate ion content of the filtrate was determined using a Shimadzu UV spectrophotometer by the vanadium molybdate phosphoric acid method.17 3. Results and Discussion Dissolution Reactions. A large number of schemes have been given for the acidic dissolution process indicating that hydroxyapatite forms first. The scheme given by Dorozhkin14 for the dissolution of phosphate rock in phosphoric acid solutions is based on five successive chemical reactions, with hydroxyapatite forming at the first stage and Ca5(PO4)3+, Ca3(PO4)2, CaHPO4, and H3PO4, respectively, forming at the successive stages. The dissolution process here was carried out using various concentrations of sulfuric acid solutions. When H2SO4 is in contact with water, the following equilibria are established
10H2SO4(aq) + 10H2O S 10H3O+(aq) + 10HSO4-(aq) (1) 10HSO4-(aq) + 10H2O S 10H3O+(aq) + 10SO42-(aq) (2) Figure 1. X-ray diffractogram of phosphate rock used in the study.
The dissolution reaction of phosphate ore in water can be written as follows
Table 2. Parameters Used in the Experiments and Their Ranges
Ca10(PO4)6‚F2(s) S
parameter particle size (µm) acid concentration (wt %) solid-liquid ratio (g‚mL-1) reaction temperature (°C)
values 1180-1000, 860-710, 600-500, 180-150, 150-125 40, 60, 75, 90, 98 0.01, 0.025, 0.05, 0.1 14, 20, 30, 50, 70
2. Experimental Procedure The phosphate rock used in the study was provided from the Mardin-Mazıdagˇı phosphate plant, which is based in southeastern Anatolia of Turkey. The visible impurities were removed by hand, and the ore was ground and then sieved using ASTM standard sieves, giving size fractions of 1180-1000, 860-710, 600-500, 180-150, and +150-125 µm. The ore was analyzed by gravimetric and volumetric methods.16 The analytical and X-ray diffraction characteristics of the ore are reported in Table 1. It was also determined, by X-ray diffraction, that the ore contains flourapatite, carbonateapatite, calcite, and quartz (Figure 1). The parameters and the ranges used in the experiments are listed in Table 2. The dissolution process was carried out in a 250-cm3 spherical glass vessel at atmospheric pressure. A mechanical stirrer was used to stir the reactor contents, and a thermostat was employed to maintain the reaction medium at a given temperature. To prevent loss of the reactants and products by evaporation, a cooler was attached to the reactor. The particle characteristics and the experimental conditions are summarized here. In the dissolution experiments, 100 cm3 of H2SO4 solution was first put into the reactor, and after it reached the
10Ca2+(aq) + 6PO4-3(aq) + 2F-(aq) (3) The presence of H3O+ ions from the H2SO4 dissolved in the solution results in the following equilibria, increasing the dissolution of the ore
6PO4-3(aq) + 18H3O+(aq) S 6H3PO4(aq) + 18H2O (4) 2F-(aq) + 2H3O+(aq) S 2HF(g) + 2H2O
(5)
10Ca2+(aq) +10SO42-(aq) + 20H2O S 10CaSO4‚2H2O(s) (6) Thus, the overall reaction can be written as
Ca10(PO4)6‚F2(s) + 10H2SO4(aq) + 20 H2O S 10CaSO4‚2H2O(s) + 6H3PO4(aq) + 2HF(g) (7) In the case of carbonate apatite, the same reactions take place in the system, except that 2F- is replaced by CO32- in reaction 3 and 2HF by CO2 in reactions 5 and 7. The overall reaction for carbonate apatite can be written as follows
Ca10(PO4)6‚CO3‚H2O(s) + 10H2SO4 + 18H2O S 10CaSO4‚2H2O(s) + 6H3PO4(l) + CO2(g) (8) On the other hand, the CaSO4-P2O5-H2O system has been investigated by Gurbuz et al.,18 who stated that the number of waters of crystallization of the calcium
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Figure 4. Electron microscope image of filtrated phosphate rock resude (75% w/w acid concentration, 10 min, and X ) 0.25). Figure 2. CaSO4-P2O5-H2O system.
Figure 5. X-ray diffractogram of product.
Figure 3. Electron microscope picture of filtered phosphate rock residue (60% w/w acid concentration, 3 min, and X ) 0.20).
sulfate that forms depends on the process conditions and can take values of 0, 0.5, or 2. The most important factors affecting number of waters of crystallization are the P2O5 concentration and the temperature of the reaction mixture (Figure 2). After dissolution, the residue from the filtration was analyzed by X-ray diffraction and SEM (Figures 3-5. It was found that the residue contains gypsum (CaSO4‚ 2H2O) and anhydrite (CaSO4). The formations of gypsum and anhydrite in the residue are confirmed by the X-ray micrograph in Figure 5 and the SEM image in Figure 6. Effects of Parameters. Effect of Particle Size. The effect of the particle size (D) on the reaction rate was investigated for the particle size fractions 11801000, 860-710, 600-500, 180-150, and 150-125 µm at a fixed reaction temperature of 70 °C, solid-to-liquid ratio of 0.025 g‚mL-1, and a stirring speed 41.89 s-1. The reacted fraction increases as the particle size decreases, as shown in Figure 7, as a result of the increase in the specific surface area of the solid. Effect of Solid/Liquid Ratio. The effect of the solidto-liquid ratio (S/L) on the dissolution rate was studied for ratios of 0.01, 0.025, 0.05, and 0.1 g‚cm-3 at a reaction temperature of 70 °C, a stirring speed of 41.89
Figure 6. Electron microscope image of product.
s-1, a particle size of 860-710 µm, and a H2SO4 concentration 75% (by weight). The experimental results are exhibited in Figure 8. It can clearly be seen from this figure that decreasing solid-to-liquid ratios favor the dissolution rate, which can be explained by the decrease in the amount of solid per amount of the reagent in the suspension. Effect of Acid Concentration. The effect of the H2SO4 concentration (C) on the dissolution rate was studied for concentrations of 40, 60, 75, 90, and 98% (by weight) at a reaction temperature of 70 °C, a stirring
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Figure 7. Effect of particle size on the dissolution of phosphate ore. (1) 150-125 × 10-6 m, (2) 180-150 × 10-6 m, (3) 600-500 × 10-6 m, (4) 860-710 × 10-6 m, (5) 1180-1000 × 10-6 m.
Figure 8. Effect of solid-to-liquid ratio on the dissolution of phosphate ore. (1) 1/100 g‚cm-3, (2) 2.5/100 g‚cm-3, (3) 5/100 g‚cm-3, (4) 10/100 g‚cm-3.
Figure 9. Effect of acid concentration on the dissolution of phosphate ore. (1) 98% (w/w), (2) 90% (w/w), (3) 75% (w/w), (4) 60% (w/w), (5) 40% (w/w).
speed of 41.89 s-1, a particle size of 860-710 µm, and a solid-to-liquid ratio of 0.025 g‚cm-3. According to the results shown in Figure 9, the dissolution rate increases as the acid concentration increases in the range of values studied. Effect of Reaction Temperature. The effect of the reaction temperature (T) on the reaction rate was investigated at 14, 20, 30, 50, and 70 °C for a particle size of 860-710 µm, a solid-to-liquid ratio of 0.025
Figure 10. Effect of temperature on the dissolution of phosphate ore. (1) 70 °C, (2) 50 °C, (3) 30 °C, (4) 20 °C, (5) 14 °C.
Figure 11. Agreement of experimental data with the Avrami kinetic model for different temperatures. (1) 70 °C, (2) 50 °C, (3) 30 °C, (4) 20 °C, (5) 14 °C.
g‚cm-3, a stirring speed of 41.89 s-1, and an acid concentration of 75% (by weight). The dissolution rate increases as the reaction temperature increases because of the exponential dependence of the rate constant in the Arrhenius form, as shown in Figure 10. Reaction Kinetics. The experimental results were examined using the conversion fraction P2O5 in the solid [X ) (amount of P2O5 dissolved)/(total amount of P2O5 in the sample)] versus time. Using the data in Figures 7-11, the results were assembled in the form of graphs of X plotted as a function of time. The kinetic data of the present study were analyzed using graphical and statistical methods. As the experimental results were analyzed using fluid-solid heterogeneous reaction models by graphical and statistical methods, it was found that the data fit none of these kinetic models.19,20 However, further attempts to fit the results to a kinetic model led to the following expression
-ln(1 - X) ) ktm
(9)
which is called the Avrami model.21 A regression analysis with 150 experimental data gave
-ln(1 - X) ) kt0.7
(10)
Experiments were conducted at 287, 293, 303, 323, and 343 K to calculate the apparent activation energy of the reaction. Thus, the agreement of the experimental data with the Avrami kinetic model for different tem-
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Figure 12. Arrhenius plot of experimental data.
peratures is shown graphically in Figure 11. The Arrhenius plot is given in Figure 12. The apparent activation energy, Ea, is 29.66 kJ‚mol-1 where
k ) f(D,C,S/L,T)
(11)
Multiple regression analysis, including the effects of all parameters, gave the following kinetic expression, with a regression coefficient of 0.98
-ln(1 - X) ) [koD-0.52C1.93(S/L)-0.27e-3567.52/T]t0.70 (12) where D is the initial particle size, µm; C is the initial acid concentration, mol dm-3; S/L is the initial solidto-liquid ratio, g dm-3; t is the reaction period, s; and T is the reaction temperature, K. As seen from the kinetic model for the dissolution process in eq 12, the most effective parameter is the acid concentration, with the particle size and then the reaction temperature following. The significant effect of the acid concentration and the low activation energy of 29.66 kJ‚mol-1 confirm that the dissolution is controlled by solid film diffusion. Also, covering of fluorapatite by gypsum and anhydride crystals occurs in the solid-solution reactions, causing the diffusion to decrease, and the stirring speed is not an effective parameter. Therefore, it could be said that the dissolution might be controlled by solid film diffusion. To test the agreement between the experimental conversion values and the values calculated from the empirical eq 12, a plot of Xexp versus Xtheo was constructed, as seen in Figure 13. The agreement between the experimental and calculated values is very good. For Figure 13, r2 ) 0.98. The Avrami model21 is applied to explain the diffusion process for systems in which crystallization on the sample surface occurs. As seen from the X-ray analysis, CaSO4‚2H2O and CaSO4 solid forms are produced at the end of the dissolution reaction. The study can be extended to a pilot-scale work with higher solid-to-liquid ratios and to the production of phosphoric acid from the phosphate-containing solution. The effect of the stirring speed on the dissolution rate was ignored because of the experimental results. Stirring speed was also not considered in the overall correlation equations.
Figure 13. Agreement between experimental conversion values and theoretical values from one semiempirical expression.
4. Conclusions In this study, the dissolution kinetics of phosphate ore obtained from Mazıdagˇı, Mardin, Turkey, was investigated in sulfuric acid solutions. In the experiments, the particle size, solid-to-liquid ratio, and reaction temperature were chosen as parameters. It was observed that the dissolution rate increased with increasing reaction temperature, acid concentration, and stirring speed, whereas it decreased with increasing particle size and solid-to-liquid ratio. It was also determined that the precipitations of CaSO4‚2H2O and CaSO4 on the mineral surface created difficulty for H3O+ to diffuse to the unreacted mineral surface. Two processes are involved: dissolution of fluorapatite and precipitation of calcium sulfates. The formation of gypsum and anhydrite depends strongly on the concentration of sulfuric acid. A mathematical model in the following form, including the effects of parameters was established to express the dissolution kinetics of phosphate ore: -ln(1 - X) ) ktm. The activation energy for the process was found to be 29.66 kJ‚mol-1. Literature Cited (1) Hamilton, W. R.; Wooley, A. R.; Bishop, A. C. Minerals, Rocks and Fossils, 4th ed.; Country Life Books: Twickenham, Middlesex, England, 1987; p 86. (2) Yartas¸ ı, A.; Kocakerim, M. M.; Yapıcı, S.; O ¨ zmetin, C. Dissolution kinetics of phosphate ore in SO2-saturated water. Ind. Eng. Res. 1994, 33, 2220-2225. (3) Sevim, F. Mazıdagˇi fosfat cevherinin su¨lfu¨rik asitde c¸ o¨zu¨ndu¨ru¨lmesi. Doctoral Thesis, Fen Bilimleri Enstitu¨su¨ (Graduate School of Natural and Applied Sciences), Atatu¨rk University, Erzurum, Turkey, 1996. (4) Seckler, M. M.; Bruinsma, O. S. L.; Van Rosmalen, G. M. Phosphate Removal in a Fluidized Bed. 1. Identification of Physical Processes. Water Res. 1996, 30 (7), 1585-1588. (5) Austin, G. T. Shreve’s Chemical Process Industries, 5th ed.; McGraw-Hill Book Company: New York, 1984; Chapter 16. (6) McCullough, J.; Phillips, J. F.; Tate, L. R. Preparation of dicalcium phosphate rock by the use of sulphur dioxide, water, and carbonyl compounds. U.S. Patent 4,113,842, 1978. (7) Drecshel, E. K. Monocalcium phosphate and phosphoric acid by acidulation of natural phosphate with phosphoric acid. Belgian Patent 876,325, 1979. (8) Sardisco, J. B.; Holcomb, D. E.; Drecshel, E. K. Phosphoric acid and additional products from phosphate ore. U.S. Patent 4,479,923, 1984.
Ind. Eng. Chem. Res., Vol. 42, No. 10, 2003 2057 (9) Weston, W. C.; Wen, J. W.; Mandel, F. S. Process phosphoric acid from phosphate rocks containing fluorapatite and related minerals. U.S. Patent 4,485,078, 1984. (10) Tarantsova, M. I.; Kulikov, B. A.; Chaikina, M. V.; Kolasov, A. S.; Boldyre, V. V. Kinetics of dissolution of fluorapatite crystals in solutions of hydrochloric acid. Ser. Khim. Nauk. 1980, 4, 5561. (11) Hussein, M.; Seif, S. Extraction of Egyptian rock phosphate with nitric acid. Chem. Econ. Eng. Rev. 1980, 12, 39-46. (12) Bayramogˇlu, M.; Demirciogˇlu, N.; Tekin, T. Dissolution kinetics of Mazıdagˇı phosphate rock in HNO3 solution. Int. J. Miner. Process. 1992, 36, 259-271. (13) Sevim, F..; Kocakerim, M. M.; Yartas¸ ı, A.; Donmez, B. Mazıdagı fosfatlarının sulfurik asit cozeltilerindeki cozunurlugu. In II. Ulusal Kimya Muhendisligi Kongresi, Istanbul, Turkey, July 9-13, 1996; Tiryaki, A., Ed.; pp 148-154. (14) Dorozhkin, V. S. Fundamentals of the wet-process phosphoric acid production. 1. Kinetics and mechanism of the phosphate rock dissolution. Ind. Eng. Chem. Res. 1996, 35, 4329-4335. (15) Dombolov, I.; Isiali, R.; Mohamed Ali, B.; Gruncharov, I.; Pelovski, I. Decomposotion of Jordanian phosphorite with phosphoric acid. God. Sofii. Tekhnol. Univ. 1991, 31 (1), 176-183.
(16) Skoog, D.; West, D. M. Fundamentals of Analytical Chemistry, 3rd ed.; Holt, Rinehart, and Winston: New York, 1976. (17) Greenberg, A. E., Trussell, R. R., Clesceri, L. S., Eds. Standard Methods, 16th ed.; American Public Health Association: Washington, DC, 1985. (18) Gu¨rbu¨z, H. Yas yo¨ntem fosforik asitin mono ve dikalsiyum fosfat kristalizasyonlari ile saflastirilmasi. Ph.D. Thesis, Istanbul Technical University Institute of Sciences, I˙ stanbul, Turkey, 1991. (19) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley and Sons: New York, 1999; pp 566-586. (20) Hulbert, S. F.; Huff, D. H. Kinetics of alumina removal from a calcined kaolin with nitric, sulphuric and hydrochloric acid. Clay Miner. 1970, 8 (337) 340-345. (21) Avrami, M. Kinetics of phase change III. J. Chem. Phys. 1941, 9, 177.
Received for review March 1, 2002 Revised manuscript received January 16, 2003 Accepted February 14, 2003 IE020168O
