Ind. Eng. Chem. Res. 2006, 45, 1307-1311
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Leaching Kinetics of Calcined Magnesite in Citric Acid Solutions Fatih Demir, Oral Lac¸ in,* and Bu1 nyamin Do1 nmez Department of Chemical Engineering, Atatu¨rk U ¨ niVersity, 25240 Erzurum, Turkey
The kinetics of leaching of calcined magnesite by citric acid solution was studied. The effects of reaction temperature, particle size, and acid concentration were investigated. The leaching data showed that the reaction is under chemical kinetics control with an activation energy of 39.1 kJ mol-1. Additionally, the fact that leaching was controlled by the chemical reaction was also supported by the relationship between the rate constant and the particle radius. Introduction
Table 1. Chemical Analysis of the Calcined Magnesite
Magnesite ore has played an important role in the production of magnesium and its compounds.1,2 Turkey has extensive magnesite ores. Magnesite can be used in three main forms: (1) crude magnesite, primarily for use in chemicals, in the pharmaceutical industry as an antacid, in agriculture, in lime, and in the paper and sugar industries; (2) dead-burned magnesia, a durable refractory material for use in cement, in the pharmaceutical industry, in glass, in steel, and in metallurgical industries; and (3) caustic calcined magnesia, for use in making oxychloride and oxysulfade cements and in various environmental and chemical applications. Both magnesite ore and calcined magnesite can also contain some impurities such as silisium, iron, and calcium, and these impurities affect the quality of products. As inorganic acids such as H2SO4, HNO3, or HCI are used, some undesired impurities, particularly Fe, can be appreciably dissolved in the leaching process. Therefore, organic reagents are usually more selective than inorganic reagents. For example, to dissolve nickel and iron from Greek laterite ores, several organic acids have been tested and compared with inorganic acids. Consequently, it is seen that citric, acetic, and oxalic acids display remarkable selectivities according to the inorganic acids in iron and nickel extraction.3 With this purpose, the use of organic reagents has provided many advantages.4 On the other hand, organic acids cannot be used as leaching agents for hard dissolving compounds. Also, the use of organic acids at high temperatures can be limited because of their low boiling temperatures and decomposition. Organic acids and bases are among the most frequently used chemicals in industry. Also, citric acid is one of the world’s largest-tonnage fermentation products. It is most widely used as an organic acidulant and pH-control agent in foods, beverages, pharmaceuticals (e.g., in laxatives and cathartics), in soft drinks, and in technical applications. Its salts have many uses in making blueprint paper as ferric ammonium citrate.5,6 The rate of reaction strongly depends on the properties of the solid as well as the process conditions.7 The two basic factors affecting the specific surface area are the time and the temperature of calcination. The reactivity of MgO is directly related to its specific surface area.8 In general, calcined magnesite is a porous multiphase solid, and its porosity and heterogeneity can the allow use of traditional models of liquidsolid reactions. When magnesium oxide is chemically reacted with citric acid, magnesium citrate and water are formed. The magnesium citrate * To whom correspondence should be addressed. Tel.: + (442) 2314554. Fax:+ (442) 2360957. E-mail:
[email protected].
component
wt %
MgO CaO Fe2O3 SiO2
92.14 2.82 1.05 3.99
formed has rather wide usage fields such as medicine and dietary supplements.9 Therefore, it is thought that the kinetic data for the reaction of magnesia by citric acid are very important for industrial applications. In the literature, although different works on the dissolution mechanism of magnesite by various reagents have been presented,1,10-12 studies concerning the leaching kinetics of calcined magnesite by organic acids have not been reported. Therefore, the aim of this work was to study the leaching behavior of calcined magnesite by citric acid solutions as a different approach. Experimental Section In a previous work, natural magnesite was comparatively calcined for a kinetic analysis by the Coats-Redfern and Suzuki methods using TGA data.13 From this study, kinetic parameters of calcination were extracted. That is, the calcined magnesite used in the work was obtained by the calcination of natural magnesite obtained from Erzurum-Oltu in Turkey in a furnace under nitrogen atmosphere at 700 °C for 2 h. The calcined magnesite was ground, and its chemical composition was analyzed by standard gravimetric and volumetric methods.14 The results are presented in Table 1. An X-ray diffractogram of calcined MgO is presented in Figure 1. In addition, the samples were sieved using ASTM standard sieves, giving particle size fractions of 215, 478, 855, and 1590 µm. Citric acid (C6H8O7, 2-hydroxy propane-1,2,3-tricarboxylic acid) used as the reactant and EDTA (ethylenediamine tetraacetic acid) used for analysis were of reagent grade. Citric acid is a naturally occurring fruit acid, produced commercially by microbial fermentation of a carbohydrate substrate, and it decomposes at higher temperatures. Experimental Procedure. Leaching experiments were carried out in a well-mixed spherical glass batch reactor (500 mL) heated by a constant-temperature bath and equipped with a mechanical stirrer having a digital controller unit, a thermometer, and a back cooler. After 250 mL of citric acid solution had been added to the reaction vessel and the temperature had been set at the desired value, a charge of approximately 2.0 g of calcined magnesite was added to the reactor while the contents of the reactor were stirred at a certain speed. After each test, an
10.1021/ie0507629 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/11/2006
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Figure 1. XRD pattern of the calcined magnesite. Table 2. The Parameter Values for Dissolution of Calcined Magnesite in Citric Acid Solutions parameter temperature (°C) acid concentration (mol‚L-1) particle size (µm) a
values 23,a
15, 30, 37, 45 0.5, 0.75, 1.0,a 1.5, 2, 3 1590, 855,a 478, 215
Constant value used to investigate the effect of other parameters.
amount of sample taken from the leach slurry was filtered immediately, and the Mg2+ content in the leach solution was determined by EDTA in the medium of buffer solution (about pH 10).15 The dissolution tests were performed as a function of several parameters, the ranges and values of which were listed in Table 2. Results and Discussion
Figure 2. Effect of particle size on the leaching of MgO.
The reaction steps between calcined magnesite and citric acid can be written as follows
C6H8O7(s) + H2O(l) T H3O+(aq) + C6H7O7-(aq) (1) C6H7O7-(aq) + H2O(l) T H3O+(aq) + C6H6O72-(aq) (2) C6H6O72-(aq) + H2O(l) T H3O+(aq) + C6H5O73-(aq) (3) 3MgO(s) + 2C6H8O7(aq) f 3Mg2+(aq) + 2C6H5O73-(aq) + 3H2O(l) (4) The pKa values presented in the literature are pK1 ) 3.128, pK2 ) 4.761, and pK3 ) 6.396.16 Because the pH of the reaction medium was less than 4, a basic reaction was carried out with respect to eq 1. Effect of Particle Size. The experiments were performed for four different particle sizes (215, 478, 855, and 1590 µm) in solutions containing 1.0 M citric acid at a stirring speed of 600 rpm. The effect of particle size was studied at 23 °C. As seen in Figure 2, the leaching rate increases with decreasing particle size of magnesium oxide, which can be attributed to the increase in contact surface that accompanies the decrease of the particle size per unit weight of the solid. In addition, values of the rate constants are plotted versus the reciprocal of the particle radii, yielding a linear relationship with a correlation coefficient of 0.999 (Figure 3).
Figure 3. Plot of k versus 1/R.
Effect of Reaction Temperature. The experiments were carried out in the temperature range of 17-45 °C in 1.0 M citric acid at a stirring speed of 600 rpm for 855-µm particles. Typical rate curves are shown in Figure 4. From this figure, it is observed that the dissolution rate is highly sensitive to the reaction temperature. Effect of Acid Concentration. To observe the effect of the acid concentration, in the range of 0.5-3.0 M, experiments were performed at 23 °C with an agitation speed of 600 rpm for 855µm particles. From Figure 5a and b, for the concentration range
Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006 1309
Figure 4. Effect of temperature on the leaching of MgO.
Figure 6. Effect of acid concentration on the leaching of MgO (for 60 and 120 s).
tion range of 0.5-1.0 M. Additionally, the constant concentration value is selected as 1.0 M in the experiments. Kinetics Analysis. Fluid-solid heterogeneous reaction systems have many applications in chemical and hydrometallurgical processes. A successful reactor design for these processes depends strongly on kinetics data. In such systems, the reaction rate can be generally controlled by one of the following steps: diffusion through the fluid film, diffusion through the ash, or chemical reaction at the surface of the core of unreacted materials.18 To determine the kinetic parameters and rate-controlling step in the leaching of calcined magnesite in citric acid solutions, the experimental data are tested on the basis of the shrinkingcore model. This reaction model between a fluid and a solid can be represented by
A(fluid) + bB(solid) f products
(5)
If no ash layer covers the unreacted core as the reaction proceeds, there could be only two controlling steps, namely, fluid film diffusion or chemical reaction. If the process is controlled by resistance of the fluid layer, then eq 6 is used
t)
RFB X 3bkLCA B
(6)
If the process is controlled by the resistance of chemical surface reactions, then eq 7 is used Figure 5. Effect of acid concentration on the leaching of MgO for (a) 0.5-1.0 and (b) 1.0-3.0 M.
of 0.5-1.0 M, the increase in acid concentration increases the dissolution rate of calcined magnesite, but for 1.0-3.0 M, the increase in concentration decreases the dissolution rate. For both situations, the effect of concentration is clearly shown in Figure 6 at two different times. The inverse effect for high acid concentrations can be attributed to the fact that, after a certain value of acid concentration, the intensity of the negative effect of a decreasing water content is more dominant than that of the positive effect of an increasing acid concentration. Additionally, this behavior can be explained by the fact that, as the acid concentration in the medium increases, the rate of appearance of product increases, and as the product reaches the saturation value near the solid particle, it forms a sparingly soluble product film layer around the particle. Consequently, the dissolution rate decreases above an acid concentration of 1.0 M.17 However, the leaching kinetics is basically investigated for the concentra-
t)
RFB [1 - (1 - XB)1/3] bkSCA
(7)
The fit of all the experimental data into the integral rate is analyzed by using a computer program, and multiple regression coefficients obtained for the integral rate expression are calculated. In the calculations, it is seen that the best value of the regression coefficient correcting the rate expression is for surface reaction control. To confirm the results of these statistical analyses, the experimental data for each parameter were also tested by graphical methods. From the results of the statistical analyses, it was found that the leaching of calcined magnesite in citric acid solutions is controlled by chemical reaction. Also, it was determined that the integral rate expression obeyed the equation
1 - (1 - x)1/3 ) kt
(8)
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was found to be 39.1 kJ mol-1. Such a model agrees with the values of similar works concerning the dissolution of magnesium oxide.7,19 (ii) In the leaching process, it was observed that the reaction rate is rather sensitive to both particle size and temperature. Thus, the solubility increases with increasing reaction temperature and with decreasing particle size. (iii) Increasing citric acid concentration in the range 0.51.0 M and decreasing concentration in the range 1.0-3.0 M accelerate the magnesium oxide dissolution. (iv) It can be said that the process can be more advantageous than dissolution of raw magnesite because of fast dissolution under more moderate conditions and less acid consumption per unit amount of magnesium citrate. (v) In terms of human health and environmental pollution, this nontoxic technique using inorganic acids and some organic acids can be a major reason for the research and development of such processes. (vi) After leaching of MgO in citric acid solutions, magnesium citrate can be produced from the solutions, which is an important medical material used in fields such as the soft drink and medicine industries (e.g., laxatives, cathartics) and as an additive for fertilizers.
Figure 7. 1 - (1 - x)1/3 vs time at various reaction temperatures.
Explanation of Symbols
Figure 8. Arrhenius plot for the leaching of MgO.
Equation 8 yields the best straight lines in comparison with the other equations tested. The coefficient value is calculated as 0.9986. From the Arrhenius equation, the k term is known as
k ) k0e-E/RT
(9)
The plots of 1 - (1 - x)1/3 versus t for the reaction temperature are shown in Figure 7. From the slopes of the straight lines in Figure 7, the apparent rate constants were evaluated. Additionally, in Figure 8, ln k versus ln(1/T) is plotted for the each value of the temperature, and the following values were obtained
E ) 39.1 kJ‚mol-1,
ko ) 8986
Consequently, eq 8 can be rewritten as
1 - (1 - x)1/3 ) 8986e-39.1/RTt
(10)
Conclusions The kinetics of the liquid-solid reaction between calcined magnesite and citric acid was investigated at the various parameter levels. The main purpose of this approach was to observe the possible effects of the reaction rate on the dissolution of calcined magnesite with citric acid solutions. The major conclusions derived from the present work are the following: (i) The dissolution kinetics obeys a shrinking-core model with the surface chemical reaction as the rate-controlling step. The linear dependence of the rate constant on the inverse particle radius is more evidence in favor of the proposed surface reaction of shrinking-core model. The activation energy of the process
XB ) converted fraction (XB ) x) T ) temperature (K) E ) activation energy (kJ mol-1) t ) reaction time (s) k ) reaction rate constant (s-1) kS ) rate constant for surface reaction (cm s-1) k0 ) frequency or preexponential factor (s-1) R ) universal gas constant (kJ mol-1 K-1) FB ) molar density of B in the solid (g mL-3) R ) average radius of solid particles (cm) b ) stoichiometric coefficient CA ) bulk concentration of the fluid (mol cm-3) kL ) mass-transfer coefficient for the liquid film [(cm3 of liquid)3 (cm of surface)-2 s-1] Literature Cited (1) Ranjitham, A. M.; Khangaonkar, P. R. Leaching Behaviour of Calcined Magnesite with Ammonium Chloride Solutions. Hydrometallurgy 1990, 23, 177-189. (2) Raschman, P. Leaching of Calcined Magnesite Using Ammonium Chloride at Constant pH. Hydrometallurgy 2000, 56, 109-123. (3) Tzeferis, P. G.; Agatzini-Leonardou, S. Leaching of Nickel and Iron from Greek Nonsulphide Nickeliferous Ores by Organic Acids. Hydrometallurgy 1994, 36, 345-360. (4) Lac¸ in, O.; Do¨nmez, B.; Demir, F. Dissolution Kinetics of Natural Magnesite in Acedic Acid Solutions. Int. J. Min. Process. 2005, 75, 9199. (5) Karaffa, L.; Sandor, E.; Fekete, E.; Szentirmai, A. The Biochemistry of Citric Acid Accumulation by Aspergillus niger. Acta Microbiol. Immunol. Hung. 2001, 48 (3-4), 429-440. (6) Leung, H. W.; Paustenbach, D. J. Organic Acids and Bases: Review of Toxicological Studies. Am. J. Ind. Med. 1990, 18 (6), 717-735. (7) Tkacova, K.; Turcaniova, L.; Hocmanova, I. Thermal and Mechanical Activation as Alternative or Combined Methods of Pre-leach Treatment of Magnesite. In Proceedings of the 2nd World Congress on Nonmetallic Minerals; Beijing, China, 1982; pp 876-879. (8) Ersahan, H.; Ekmekyapar, A.; Sevim, F. Flash Calcination of a Magnesite Ore in a Free-Fall Reactor and Leaching of Magnesia. Int. J. Min. Process. 1994, 42 (1-2), 121-136. (9) Hawley, G. G. The Condensed Chemical Dictionary, 9th ed.; Van Nostrand Reinhold Company: New York, 1977. (10) Fredd, C. N.; Fogler, H. S. The Kinetics of Calcite Dissolution in Acetic Acid Solutions. Chem. Eng. Sci. 1998, 53 (22), 3863-3874.
Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006 1311 (11) Demir, F.; Do¨nmez, B.; C¸ olak, S. Leaching Kinetics of Magnesite in Citric Acid Solutions. J. Chem. Eng. Jpn. 2003, 36 (6), 683-688. (12) Raschman, P.; Fedorocˇkova´, A. Study of Inhibiting Effect of Acid Concentration on the Dissolution Rate of Magnesium Oxide during the Leaching of Dead-burned Magnesite. Hydrometallurgy 2004, 71, 403-412. (13) Demir, F.; Do¨nmez, B.; Okur, H.; Sevim, F. Calcination Kinetic of Magnesite from Thermogravimetric Data. Chem. Eng. Res. Des. 2003, 81 (A6), 618-622. (14) Furmann, N. H. Standard Methods of Chemical Analysis, 6th ed.; Van Nostrand Reinhold: New York, 1963. (15) Gu¨lensoy, H. Kompleksometrik Titrasyonlar Ve Kompleksometrinin Temelleri; Fatih Yayinevi: Istanbul, Turkey, 1974. (16) Marinovic V.; Despic, A. R. Hydrogen Evolution from Solutions of Citric Acids. J. Electroanal. Chem. 1997, 431, 127-132.
(17) Imamutdinova, V. M. Kinetics of Dissolution of Borates in Mineral Acid Solutions. Zh. Prikl. Khim. 1967, 11, 2593-2596. (18) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley and Sons: New York, 1999. (19) Ekmekyapar, A.; Ers¸ ahan, H.; Do¨nmez, B. Calcination of Magnesite and Leaching Kinetics of Magnesia in Aqueous Carbon Dioxide, Doga-Tr. J. 1993, 17, 197-204.
ReceiVed for reView June 27, 2005 ReVised manuscript receiVed December 12, 2005 Accepted December 13, 2005 IE0507629
