Evaluation of the Mechanism of Greek Calcareous Phosphate Ore

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Ind. Eng. Chem. Res. 1998, 37, 4306-4313

Evaluation of the Mechanism of Greek Calcareous Phosphate Ore Dissolution by Acetic Acid Solutions by X-ray Powder Diffraction and Thermal Analyses Tiberius C. Vaimakis* and Evangelos D. Economou Department of Chemistry, University of Ioannina, P.O. BOX 1186, 45110 Ioannina, Greece

The dissolution process of the low-grade natural phosphate ores from the Epirus Area (Greece) using dilute acetic was studied by X-ray diffraction (XRD) and thermal analyses of remained solid after dissolution for an appropriate time from 1 to 80 min. There were three kinds of carbonates present in the phosphorite. The first one was the laminas of pure calcite, which primarily dissolved from acetic acid solutions. The second kind was the calcite in phosphopelloids particles, which was protected from a phosphorite cover. The third type was the carbonates in the francolite lattice. The two last kinds did not react with acetic acid solutions. A rate equation of the general form ln[1/(1-X)] ) kt m (where X is the conversion fraction, k is a rate constant, t is times, and m is a constant) was found to represent these data. 1. Introduction Phosphate ores are usually complex raw materials, with the predominant constituent mineral being apatite. The phosphate ores are mainly used in the manufacture of phosphate fertilizers. Calcareous phosphate ores usually are sedimentation ores that were found in large amounts in several areas around the world, including a phosphorite deposit in Epirus Area (Greece). The results of many studies on the enrichment and benefication of Greek calcareous phosphate ores by conventional methods (Gremillion et al., 1979; Tsailas et al., 1980; Anastasakis, 1990, 1992) have shown that, because of the intimate mixture of the minerals (francolite and calcite) and its well-hardened nature, there is no feasible method to produce suitable feed for acidulation processes. The enrichment of phosphorite either by selective extraction (leaching) of the contained calcite with HCl and NH4Cl solutions or by calcination of the phosphorite and water-leaching of the produced CaO were successful but uneconomical (Tsailas et al., 1980). An acidulation process (Sdoukos and Economou, 1985a, b; Economou, 1984) with diluted H3PO4 as the leaching agent of the calcite was successful and was based on the relatively higher dissolution rates of calcite than of the francolite that is present in phosphorites. Sadeddin, Abu-Eishah, and their co-workers (Sadeddin and Abu-Eishah, 1990; Abu-Eishah et al., 1991) used dilute acetic acid solution as a leaching agent for calcite. By using this technique on Jordanian phosphorites, the amount of calcium carbonate is reduced to a possible minimum without affecting the phosphates. In this paper we studied the mechanism of the selective dissolution of calcite from the low-grade natural phosphate ores (Epirus Area, Greece) using dilute acetic acid by thermal analysis and powder X-ray diffraction analysis of the remaining solids after the dissolution (condensates). This study was part of a research project with a goal of examming the potential * Author to whom correspondence should be sent. Telephone: +30-651-98352. Fax: +30-651-44836. E-mail: [email protected].

benefit of low-grade Greek raw phosphate by conventional and innovatire techniques, such as acid thermal treatment (Vaimakis, 1985, Vaimakis and Sdoukos, 1987) and mechanochemistry (Karagiannis et al., 1995). 2. Theoretical Background The dissolution of calcite is an interface reaction in which the kinetic behavior is sensitive to reaction conditions, which may be varied to produce changes in rate and mechanism characteristics. It is important to note that concentrated acetic acid would not react with calcite due to the large polarity of the acetic acid O-H bond. Therefore, acetic acid may adsorb on the surface of the particles and decompose apatite. On the other hand, in dilute solution, water molecules tend to decrease the effect of the polarity of the acetic acid O-H bond and increase the degree of dissociation, which, in turn, makes the acetic acid capable of attacking the carbonates. The reaction between the dilute solutions of acetic acid and calcium carbonates may be written as follows:

CaCO3 + 2CH3COOH f Ca(CH3COO)2 + H2O + CO2(g) (1) The solubility of calcium acetate at higher temperature may decrease. In reaction in eq 1, the mechanism may include the formation of unstable carbonic acid, which decomposes into CO2 and H2O; reaction for the other impurities depends on the nature and composition of the sample. The main reaction in eq 1 represents, in fact, a lumping of a larger number of steps. The simplest detailed mechanism necessary for the rational understanding of this system may be given as follows (Economou et al., 1998): (a) Ionization of CH3COOH, which is a relatively fast process described by the equation:

2CH3COOH + 2H2O S 2CH3COO- + 2H3+O

(2)

(b) Diffusion of H3+O ions through the liquid and/or exposed surface of the particle;

10.1021/ie9709087 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998

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(c) H3+O ions attack the particles of the calcite in the sample

CaCO3(s) + H3+O f Ca2+ + HCO3- + H2O (3) HCO3- + H3+O S H2CO3 + H2O

(4)

(d) dissociation of formed carbonic acid

H2CO3 f CO2(g) + H2O

(5)

(e) diffusion of the products from the reaction sites to the main bulk; (f) reaction between Ca2+ and CH3COO-

Ca2+ + 2CH3COO- f Ca(CH3COO)2

(6)

The formation of calcium acetate will depend on various parameters, such as the concentration of the acid, reaction time, temperature, and nature and grain of the size of the raw sample used for the dissolution process. Sjo¨berg and Rickard (1984) have shown that calcite dissolution can be discussed in terms of three regimes. At the low pH regime (5.5, the dissolution mechanism is independed of [H3+O] and involves transport of produced calcium and carbonate ions out to the diffusion boundary layer interface, where rapid protonation reactions occur. For pH values between 4.0 and 5.5, there is a transition regime. In previous papers (Economou et al., 1997, 1998), we studied the kinetics of the dissolution of low-grade phosphate ores from the Epirus Area (Greece) using dilute acetic acid. The unaltered phosphorite deposits in the Epirus Area are sedimentation phosphate ores with predominant constituent minerals of francolite (carbonate fluoroapatite) and calcite, with an intimate mixture of them that has a well-hardened nature. The dissolution process took place mainly by dissolving the calcite spaces, while almost all of the francolite remained in the solid phase. The dissolution of calcite spaces took place in two steps. The first step was attributed at the dissolution of liberated calcite particles or/and spaces on the particles, and the second one at the dissolution of calcite in the interspaces between phospho-pelloids. Kinetic data showed that the controlling step of the heterogeneous reaction (calcite dissolution) was the chemical change. The detailed mechanism study of the initial period (up to 30 s) showed that the rate-controlling step of the calcite dissolution was the transformation of H2CO3 to CO2(gas), whereas the overall process, for the two aforementioned steps, can be represented with a rate equation of the general form ln[1/(1 - X)] ) ktm, where X is the conversion fraction, k is a rate constant, t is times, and m is a constant. The more effective conditions of the dissolution were stirring speed of 200 rpm, 1 M acetic acid concentration, 25 °C, 250-500 µm particle size fraction and 25% excess of acetic acid of the amount which was required to dissolve all the carbonates content.

tary deposits, with francolite and calcite as the main minerals. The phosphorite had a chemical composition of 13.92% P2O5, 51.62% CaO, 0.39% MgO, 0.76% Al2O3, 0.57% Fe2O3, 0.53% Na2O, and 23.25% CO2 (Economou et al., 1997). Francolite is the name of carbonate fluorapatite with syntactic type [empirical formula: Ca9.51Na0.35Mg0.14(PO4)4.74(CO3)1.26F2.50]. From the chemical analysis and syntactic type we calculated the approximate mineralogical composition of the phosphorite as 40.0% francolite and 52.9% calcite. The texture of phosphorite was a well-hardened, massive phosphatic and cherty limestone with veinlets of crystalline calcite. The francolite and calcite were so intimately associated that liberation of discrete minerals by grinding did not appear (Gremillion et al., 1979; Tsailas et al., 1980). The microfacies analysis indicated that phosphate ore consisted of thin lamina of phosphorite in intercalation with thin laminated limestone. The phosphorite lamina consisted of sand-size phosphate particles resembling pellets (phospho-peloids), which were surrounded by fine crystalline francolite and in interspaces there was crystalline calcite (Varti-Matalaga et al., 1988). The acetic acid solutions were prepared by dissolution with distilled water of glacial acetic acid (100 Gew %, 100% w/w; FERAK LABORAT GMBH, Berlin). 4. Experimental Section Natural phosphate material (phosphorite) from the Kosmira-Epirus Area (Greece) was crushed with a jaw crusher and then sieved to obtain a fraction of 500250 µm. The dissolution process was carried out in a 500-mL open glass reactor. the reactor contents was stirred at a rate of 200 rpm with a mechanical stirrer, and a thermostat was used to control the reaction temperature. For each experiment, 5 g of phosphorite sample was transferred with 41.4 mL of 1 M acetic acid solution into the reactor. The dissolution process was stopped at the proper time (1-80 min) by addition of 500 mL of cold water and a quickly filtration. The remaining solids (condensates) were dried at room temperature. Each experiment was replicated three times, and the collected condensates were weighed and mixed. The relative standard deviation of the condansate weight was 1.2%. The X-ray diffraction (XRD) patterns of the dissolution condensates and also the raw material were obtained with a Siemens D-500 diffractometer with Cu KR radiation (λ ) 1.54060 Å). The experimental operation conditions were step size of 0.02° and time per step of 1 s, and the tube operation conditions were 30 mA and 40 kV. The thermal analysis of the dissolution condensates as well as the raw material was carried out using a Chyo-TRDA3H derivatograph with simultaneous recording of temperature (T), thermogravimetry (TG), differential thermogravimetry (DTG), and differential thermal analysis (DTA). In all cases, the sample size was 100 mg and R-Al2O3 was used as a blank. Analyses took place at a heating rate of 5 °C min-1 under air flow of 50 cm3 min-1.

3. Materials

5. Results

The material used was unaltered phosphate ore from the Kosmira-Epirus Area, in Northwest Greece. Mineralogical study (Gremillion et al., 1979; Tsailas et al., 1980) indicated that the phosphorites were synsedimen-

The XRD patterns of the dissolution condensates as well as the raw material are illustrated in Figure 1. In Figure 2 are the XRD patterns of the pure minerals (francolite and calcite). Results of quantitative analysis

4308 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998

Figure 2. X-ray diffraction patterns of calcite and francolite. Table 1. Rock Quantitative Phase Analysis of Phosporite and Conversion Degree of Pure Calcite (X) phase composition, %

Figure 1. X-ray diffraction patterns of raw phosphorite (0) and condensates. The dissolution time of condensates are indicated.

of the XRD patterns, evaluated by the full-pattern fitting method using the DIFFRAC-AT V3.3 (SOCABIM) software, are shown in Table 1. The thermal analysis results are illustrated in Figure 3 for TG curves, Figure 4 for DTG curves, and Figure 5 for DTA curves. The curves after each experiment were reduced by multiplied with proper coefficient (c) to corresponded to 100 mg of the initial weight of raw mineral.

c ) (100Wp)/(5Ws)

(7)

where Wp is the weight of remained solid after dissolution (g) and Ws is the weight of sample in thermobalance (mg). From the TG curves we calculated the percentage weight loss for various stages of decomposition in the temperature range 400-1000 °C. The results are shown in Figure 6 for the dehydroxylation (open triangle) and the carbonate decomposition steps. The carbonate decomposition steps were two up to 20 min of dissolution time (open and solid circles), and after that, a new step was observed (open square). Also shown in Figure 6 (*) are the sum of the first step of

time of dissolution, min

calcite

fluorapatite

pure calcite

X

0 1 2.5 5 10 20 40 80

73 ( 4 60 ( 4 39 ( 4 29 ( 4 24 ( 4 16 ( 4 10 ( 4 7(4

27 ( 4 40 ( 4 61 ( 4 71 ( 4 76 ( 4 84 ( 4 90 ( 4 93 ( 4

66 53 32 22 17 9 3 0

0 0.20 0.52 0.67 0.74 0.86 0.95 1

the carbonate decomposition (open circles) and the new peak (open squares). From the DTG curves we calculated the area of the peaks in the temperature range 610-1000 °C with SPECTRA CALC (Galactic Industries Corp.) sofware. The correlation of determinations (R2) of the peak area calculations varied from 0.99941 to 0.99840, and the results are showed in Figure 7. 6. Discussion On the basis of the XRD patterns, it can be said that there is a continuous decrease of the calcite diffraction lines and a corresponding increase of the francolite one. Quantitative analysis of calcite indicates an amount of 7% in the remaining solid after a dissolution time of 80 min. Because the other calculated phase was the fluorapatite and in our previous paper (Economou et al., 1997) we found that almost the calcite dissolved by this time, we could assume that the amount of calcite that remained in the solid phase after a dissolution time of 80 min was a part of francolite or calcite with special construction. We corrected the calcite content, that is, we calculated the “pure calcite”, by subtract 7 points from each calcite content (Table 1). In our previous paper (Economou et al., 1997) we found that the solids, before and after dissolution, were nonporous materials with a shrunk grain size. For this


Figure 3. TG curves of raw phosphorite (0) and condensates with heating rate 5 °C min-1. The dissolution time of condensates are indicated.

Figure 5. DTA curves of raw phosphorite (0) and condensates with heating rate 5 °C min-1. The dissolution time of condensates are indicated.

which described more successful the dissolution kinetics by two steps.

Figure 4. DTG curves of raw phosphorite (0) and condensates with heating rate 5 °C min-1. The dissolution time of condensates are indicated.

reason, we examined the two-dimensional diffusion model for reaction of a cylindrical particle (eq 8) and the three-dimensional diffusion model for reaction proceeding in a spherical particle (eq 9, which is usually called Jander equation; Bamford and Tipper, 1980, p 69) and also, the contracting volume geometrical model (eq 10), which describes the dissolution of Morocco phosphate rock by sulfuric acid (Lowrison, 1989, p 295). After unsuccessful fitting of the diffusion model just mentioned, we examined the pseudohomogeneous firstorder reaction model (eq 11; Hulbert and Huff, 1970),

(1 - X) ln(1 - X) + X ) kt

(8)

[1 - (1 - X)1/3]2 ) kt

(9)

1 - (1 - X)1/3 ) (k/Fr)t

(10)

-ln(1 - X) ) ktm

(11)

where X is the conversion fraction (Table 1), k is the rate constant, F is the density of particle, r is the equivalent radius of particle, and m is a constant, and t is time. The quantitative phase analysis results of calcite, in both cases, were tested using the plots of [(1 - X) ln(1 - X) + X] versus t (Figure 8), [1 - (1 - X)1/3]2 (Figure 9) and of [1 - (1 - X)1/3] (Figure 10) versus t. The plots were not linear, indicating that the diffusion control and also the contracting volume geometrical mechanisms were not the predominant ones. These results are in accordance with our previous studies (Economou et al., 1997). Figure 11 shows the plot of the eq 11. It can clearly be seen from the figure that the pseudohomogeneous first-order reaction model successfully fit the experimental data of the dissolution process and given eqs 12 and 13 for “total calcite” and “pure calcite”, correspondingly.

-ln(1 - X) ) 0.0996t0.3795

(12)

-ln(1 - X) ) 0.0606t0.4993

(13)

In our previous paper, the calculated constants for the first step of the pseudohomogeneous first-order reaction model were k ) 0.0736 and m ) 0.447, which are very close to the calculated constants in this paper.


Figure 6. Weight loss of thermal decomposition of the condensates for various stages: (4) dehydroxylation, (O) calcite decomposition, (b) francolite decomposition, (0) new endothermic peak; and (*) the sum of calcite and new endothermic peaks.

Figure 7. DTG peak area of thermal decomposition of the condensates for various stages: (O) calcite decomposition, (b) francolite decomposition, (0) new endothermic peak; and (*) the sum of calcite and new endothermic peaks.

The thermal analysis results showed that the thermal decomposition of samples took place at three endothermic steps up to dissolution time of 20 min. The first one, which occurs in the temperature range 500-610 °C, was attributed to the dehydroxylation and the other two, at the temperature range 610-900 °C, were attributed to the decomposition of calcite and the carbonate group of francolite, respectively. In the samples in which the dissolution process took place for >20 min, a

Figure 8. Analysis of two-dimensional diffusion model of the rate law for the quantitative phase analysis of “total calcite” (O) and “pure calcite” (black points).

Figure 9. Analysis of three-dimensional diffusion model of the rate law for the quantitative phase analysis of “total calcite” (O) and “pure calcite” (b).

new endothermic peak appeared at temperature between 750 and 800 °C. The percentage weight loss (based on initial untreated phosporite) for the dehydroxylation and carbonate decomposition of francolite stages (Figure 6, open triangles and solid circles, respectively) remained almost constant for all condensates. The calculated content, from syntactic type of francolite and mineralogical composition of phosphorite, of the francolite carbon dioxide is about ≈2.30%, whereas the value observed from TG curve of thermal analysis of raw phosphorite was 1.51%. The


Figure 10. Analysis of contracting volume geometrical model of the rate law for the quantitative phase analysis of “total calcite” (O) and “pure calcite” (b).

constant whereas a rupture at dissolution time of 20 min was observed. The weight loss of the new endothermic peak (Figure 6, open squares), which appeared after a dissolution time of 20 min, for all condensates was also constant and varied between 0.82 and 1.24%, with a mean value of 1.06 and standard deviation of 0.16. On the other hand, when we added the aforementioned curve with this one of calcite decomposition, a new more normal curve for kinetic data was obtained (Figure 6, open squares). From these observations, two assumptions could be made: (a) This peak is also in the thermograms of the dissolution solid products with dissolution time up to 20 min as well as the raw material, but because of the high area of the main calcite peak, it was overlapped. (b) The new peak was calcite decomposition that could not reacted with acetic acid, had a special construction from the other calcite, and was attributed to calcite spaces that were protected by francolite covers. This construction was observed by microfacies and mineralogical analyses, which indicated that the unaltered phosphate ore from the Kosmira-Epirus Area consisted of thin lamina of phosphorite in intercalation with thin laminated limestone. The phosphorite lamina consisted of sand-size phosphate particles resembling pellets (phospho-peloids) that were surrounded by fine crystalline francolite and in interspaces there was crystalline calcite (Varti-Mataragka et al., 1988). The aforementioned conclusions, from TG curves, are confirmed by the quantitative DTG peak area analysis of the thermograms (Figure 7). The results from DTG peak area were more accurate than TG weight loss and therefore were selected for further kinetic analysis. To calculate the conversion fraction X, we corrected the calcite peak area by the equation

Ac ) Ai - 0.996

(14)

X ) Ac/(21.102 - 0.996)

(15)

and

Figure 11. Analysis of the pseudomogeneous first-order reaction kinetics models for the quantitative phase analysis of “total calcite” (O) and “pure calcite” (b).

corresponding weight loss values for the condensates vary between 1.15 and 1.71%, with a mean value 1.36 and standard deviation of 0.18. This fact is in accordance with our previous results (Economou et al., 1997) and indicates that the dissolution process took place mainly by dissolving the calcite spaces, while all of the francolite remained in the solid phase. The weight loss of the calcite decomposition stages (Figure 6, open circles) decreased continuously as the duration of dissolution time was increased. After a dissolution time of 60 min, the calcite decomposition peak remained

where Ac and Ai are the correct and calculated peak areas, respectively, 0.996 is the mean value of the peak area of the peak that appeared after 20 min dissolution time, and 21.102 is the value of the peak area of main calcite peak for the raw sample. Due to definition of kinetic expression we examined, as before, the two-dimensional diffusion model for reaction of a cylindrical particle (eq 8, Figure 12), the three-dimensional diffusion model for reaction proceeding in a spherical particle (eq 9, Figure 13), the contracting volume geometrical model (eq 10, Figure 14), and the pseudohomogeneous first-order reaction model (eq 11, Figure 15). The plots were not linear for the two diffusion-control models and also the contracting volume geometrical mechanisms, which indicating that they were not the predominant ones. The plot of the pseudohomogeneous first-order reaction model shows a successful linear fitting of the experimental data of the dissolution process and given in eq 16

-ln(1 - X) ) 0.0766t0.4420

(16)

The kinetic analysis results from DTG peak area analysis are confirmed by the kinetic analysis by


Figure 12. Analysis of two-dimensional diffusion model of the rate law for the DTG curves.

Figure 14. Analysis of contracting volume geometrical model of the rate law for the DTG curves.

Figure 13. Analysis of three-dimensional diffusion model of the rate law for the DTG curves.

Figure 15. Analysis of the pseudomogeneous first-order reaction kinetics models for the DTG curves.

quantitative XRD analysis and our previous results. The values of calculated m, 0.4993 and 0.3795 for quantitative XRD analysis and 0.4420 for DTG peak area analysis, are out of the empirically established range for diffusion-limited equations (from 0.53 to 0.58), contracting area and volume relations (from 1.08 to 1.04), and for nucleation equations (from 2.00 to 3.00; Bamford and Tipper, 1980, p 78).

(a) The XRD and especially the thermal analysis were useful tools of evaluation of the dissolution process in such complex carbonate systems. The elaboration of the DTG curve of thermal analyses gave detailed information about the construction of carbonate ores. (b) The carbonates present in phosphorite are of three kinds. The first one is the laminae of pure calcite that mainly dissolved from acetic acid solutions. The second is the calcite in the phospho-peloids particle that was protected by a phosphorite cover. The third type is the carbonates in the francolite lattice. The latter two kinds did not react with acetic acid solutions.

7. Conclusions This research led us to the following conclusions:


(c) Kinetics data for two aforementioned techniques can be represented with a rate equation of the general form, ln[1/(1 - X)] ) ktm, where k varies between 0.0606 and 0.0996 and m varies between 0.3795 and 0.4993. Acknowledgment The authors thank Dr. L. Pitsika for useful discussions and Mrs. E. Karagianni, from the Chemical Process Engineering Research Institute, for the quantitative XRD analysis. Literature Cited Abu-Eishah, S. I.; El-Jallad, I. S.; Muthaker, M.; Touqan, M.; Sadeddin, W. Beneficiation of Calcaleous Phosphate Rocks Using Dilute Acetic Acid Solutions: Optimisation of Operating Conditions for Ruseifa (Jordan) Phosphate. Int. J. Miner. Process. 1991, 31, 115-126. Anastasakis, G. N. Influence of Pearl Starch Addition on the Flotation of Phosphate Rock Using Sodium Oleate as Collector. Mineral Wealth 1990, 74, 15-24 (in Greek). Anastasakis, G. N. Influence of Sodium Silicate (as Calcite Depressant) on the Flotation of Phosphate Rock Using Sodium Oleate as Collector. Mineral Wealth 1992, 79, 55-66 (in Greek). Bamford, C. H.; Tipper, C. F. H. Comprehensive Chemical Kinetics, Vol. 22, Reactions in the Solid State; Elsevier Scientific: Amsterdam, The Netherlands, 1980. Economou, E. A Contribution in the Beneficiation of the Phosphate Limestones, Study on the Epirus Phosphate Rocks. Ph.D. Thesis, University of Ioannina, Ioannina (Greece), 1984 (in Greek). Economou, E.; Vaimakis, T. C. Beneficiation of Greek Calcareous Phosphate Ore Using Acetic Acid. Ind. Eng. Chem. Res. 1997, 36, 1491-1497. Economou, E.; Vaimakis, T. C.; Papamichael, E. M. Kinetics of Dissolution of the Carbonate Minerals of Phosphate Ores Using Dilute Acetic Acid. J. Colloid Interface Sci. 1998, 201, 164171. Gremillion, L. R.; McClellan, G. H.; Lehr, J. R. Characterization of two Phosphate Rock Samples (MR-637 and MR-638) from Epirus Area, Greece. I. F. D. C./T. V. A., 1979.

Hulbert, S. F.; Huff, D. E. Kinetics of Alumina Removal from a Calcined Kaolin with Nitric, Sulphuric and Hydrochloric Acid. Clay Miner. 1970, 8 (337), 340-345. Karagiannis, G. N.; Vaimakis, T. C.; Sdoukos, A. T. Effect of the Mechanical Activation of Francolite on the Available P2O5. 16th Panhellenic Chemical Congress, 5-8 December: Athens (Greece), 1995 (in Greek). Lowrison, G. C. Fertilizer Tecchology, Ellis Horwood Limited Publishers: Chichester, UK, 1989. Sadeddin, W.; Abu-Eishah, S. I. Minimization of Free Calcium Carbonate in Hard and Medium-Hard Phosphate Rocks Using Dilute Acetic Acid Solution. Int. J. Miner. Process. 1990, 30, 113-125. Sdoukos A. T.; Economou, E. D. Developing a Method for Studying Selectivity in the Ca5(PO4)3-CaCO3-H3PO4 System. Zh. Prikl. Khim. (Leningrad). 1985a, 58, 1937-1943. Sdoukos A. T.; Economou, E. D. Kinetic Study of Selective and Chemical Enrichment for Phosphorites from the Epirus Deposit in Greece. Zh. Prikl. Khim. (Leningrad). 1985b, 58, 1944-1950. Sjo¨berg, E. L.; Rickard, D. T. Calcite Dissolution Kinetics: Surface Speciation and the Origin of the Variable pH Dependence. Chem. Geol. 1984, 42, 119-125. Tsailas, D.; Grossou-Valta, M.; Kalatzis, G. Study on the Possibilities of Beneficiating Epirus Unaltered Phosphate Rocks. Metallurgical Research No 24; Institute of Geological and Mining Research: Athens, Greece, 1980 (in Greek). Vaimakis,T.C.ThermalBehaviouroftheCaHPO4‚2H2O-Ca(H2PO4)2‚ H2O-SiO2 System and Acid Thermal Treatment of Poor Phosphorites (Epirus). Ph.D. Thesis, University of Ioannina, Ioannina, Greece, 1985 (in Greek). Vaimakis, T. C.; Sdoukos, A. T. Acid-Thermal Treatment of the Drimona-Epirus Phosphorites. Chim. Chron., New Ser. 1987, 16, 77-86 (in Greek). Varti-Mataragka, M.; Papastaurou, S.; Perdikatsis, V.; PetridouNazou, V.; Pitsikas, L.; Pomoni-Papaioannou, F.; SkourtiKoronaiou, V. Study of the Genesis Conditions of Phosphatic Formations of the Ionian Zone. Bull. Geol. Soc. Greece. 1988, XX/2, 343-361 (in Greek).

Received for review December 15, 1997 Revised manuscript received June 25, 1998 Accepted July 12, 1998 IE9709087

Evaluation of the Mechanism of Greek Calcareous Phosphate Ore

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