Ind. Eng. Chem. Res. 1996, 35, 465-474
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Dissolution Kinetics of CaCO3 in Powder Form and Influence of Particle Size and Pretreatment on the Course of Dissolution Evagelos D. Economou, Nicholaos P. Evmiridis,* and Athanasios G. Vlessidis Laboratories of Analytical Chemistry and Industrial Chemistry, Department of Chemistry, University of Ioannina, 451 10 Ioannina, Greece
A mechanism based on the adsorption of the phosphoric acid on the external surface of the CaCO3 solid phase followed by reaction of surface species with phosphoric acid and further transfer of the formed Ca(H2PO4)2 in the solid-liquid interface for the dissolution of CaCO3 particles in a phosphoric acid solution is assumed. The reaction of surface species with phosphoric acid was found to be the rate-determining step. The dissolution rate of CaCO3 particles is decreased with the increase of the particle size and is affected dramatically by treatment. The dissolution of untreated particles is accompanied by intense breakdown of the particles, and the rate and extent of breakdown depends on the particle size. Adequate pretreatment with steam makes the CaCO3 particle resistant to breakdown, and the dissolution rate becomes very slow. Introduction Apatites are calcium phosphate rocks found in nature in admixture with other components such as calcium carbonates in various compositions. The value of these natural rocks as raw materials for the production of commercial products depends on the kind and quantity of the various components in the rock. Plant fertilizer industry uses the rich phosphate apatites for the production of phosphoric acid by the method of dissolution in mineral acids as described by Becker (1983). The classic method of dissolution of apatites in sulfuric acid forms calcium sulfate precipitate with impurities of cadmium and radium which pollute the environment. A “clean technology” of phosphoric acid was developed by dissolution of apatites in phosphoric acid by van der Sluis et al. (1986), and the kinetics of dissolution were studied by van der Sluis et al. (1987). Previous reports on the kinetics of dissolution of apatites with phosphoric acid were made by Ivanov et al. (1977), who studied the kinetics to apply them to the mathematical modeling of continuous dissoluting processes proposed by Bigdorchic and Sheinin (1970); Serdyuk et al. (1982) studied the effect of strong electrolytes on the rates of dissolution of apatites by phosphoric acid. The kinetics of dissolution of phosphates (apatites) has also been studied in connection with other interests. For example, Huffman et al. (1957) studied the dissolution rates in their effort to understand the processes involved in a soil-plant system when a phosphorus fertilizer is introduced. Also, Chin and Nancollas (1991) studied phosphate dissolution in order to understand the performance of human enamel that contains fluoride ions and hydroxyapatites. Phosphorites are poor phosphate ores that contain significant quantities of CaCO3. The upgrading of phosphorites can be obtained by a chemical process in cases where the phosphate and carbonate crystals are tightly held together. A dissolution of phosphorites with the phosphoric acid process was proposed by Sdoukos and Economou (1985a,b) for upgrading poor phosphorites. The method is based on relatively higher rates of acid dissolution of CaCO3 crystals than the calcium * Author to whom correspondence should be addressed.
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phosphates that are present in phosphorites. However, the acid dissolution kinetics of CaCO3 is important to many other fields of applications, i.e., the maintainance of marble monuments, the uptake of calcium medicines, etc. Kralj and Brecevic (1994) report that there are many metastable calcium carbonate modifications that appear as initial solid phases in many carbonate precipitation processes, and most probably as precursors in carbonate geological sedimentations. The kinetics of CaCO3 dissolution is very sensitive to the surface structure, composition, and physical properties of the crystals. Surface and chemical studies of calcite (CaCO3) published to date occasionally are inconclusive and contradict each other as reported by Huang et al. (1991). This result is partly due to the heterogeneous nature of CaCO3 surfaces which are produced either by grinding materials, as reported by Goujon and Mutaftschiev (1976) and Gammage and Cregg (1972), or by low pH precipitation. Previous experimental works on the investigation of the kinetics of the dissolution process were based on the assumption that the dissolved solid samples are of nearly the same particle size and shape and that no other phenomena were taking place during the dissolution process. However, this is an oversimplification for most cases, and such assumptions may give misleading conclusions about the rate-determining step of the dissolution process. Therefore, different researchers obtain data that vary with conditions not taken into account and lead to different rate equations. On the other hand, adsorption investigations on CaCO3 surfaces give evidence of phosphate-adsorbed species. A fresh look on the kinetics of the CaCO3 is therefore interesting to obtain a more consistent rate equation. Such a rate equation will be able to predict the rates of dissolution at any fixed conditions. Any deviation from the predicted rates will then be used to investigate the occurrence of other events during the dissolution process. Furthermore, the knowledge of the correct rate equation will enable the development of the right technique to enrich the poor phosphorites or to achieve the formulation of the marketed product that meets the performance that is desired in the field of application. In this paper we investigate the kinetics of dissolution of the commercial (AR) CaCO3 powders, of various © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 2, 1996
Table 1. Rate Expressions
1-x ( [ x )] r ) 1-x 1+l ( + l (1 - x) x ) l1SA l2x - l3
1
3
r2 )
r3 )
1 + l4x + l5(1 - x) l1SA[l2x2 - l3(1 - x)] 1 + l4x + l5x
(
l1SAx 1 r1/2 )
r2/3 )
2
)
1 1 + l 2x
l3x + l4(1 - x) 1+ 1 + l 2x
(
l1 ) k-1; l2 ) K1C0; l3 )
(2)
l1 ) k-2; l2 ) K1K2C02; l3
(3)
l1 ) k-3; l2 )
(4)
l1 ) k1C0; l2 )
(5)
l1 ) k3k2K1C02; l2 ) k-2[H2CO3] + k3; l3 ) K1C0; l4 ) K1k2C02
4
l1SA[l2x2 - l3(1 - x)]
l1SAx2
)
l4x2 l2 1 + l3x + l2
particle sizes and after various pretreatments of the largest particles in dilute phosphoric acid. Theoretical Considerations-Principles Mechanistic Model. Recent reports by House and Donaldson (1986) and Suzuki et al. (1986) give evidence of phosphate adsorption on a CaCO3 solid-phase surface. A mechanistic model based on phosphate adsorption is described by the following chemical equations:
step 1
k1
(CaCO3)bS-CO3 + H3PO4 y\ z k -1
HCO (CaCO3)bS
