Ind. Eng. Chem. Res. 1997, 36, 1491-1497
1491
Beneficiation of Greek Calcareous Phosphate Ore Using Acetic Acid Solutions Evangelos D. Economou and Tiberius C. Vaimakis* Department of Chemistry, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece
The enrichment of the low-grade natural phosphate ores from Epirus area (Greece) by selective dissolution of calcite using dilute acetic acid and the optimization of this process studied by kinetic and textural analysis is given. The used conditions of the dissolution were the reaction time, stirring speed, acetic acid concentration, temperature, particle size fraction, and stoichiometry. The specific surface area, pore size distribution, and SEM photographs were used for textural analysis. We found that the more effective experimental conditions led to a concentration of about 31% of P2O5 or 95% of francolite. The dissolution of calcite spaces took place in two steps. The first step was attributed to the dissolution of liberated calcite particles and/or spaces on the particles, and the second one was attributed to the dissolution of calcite in the interspaces between phosphopeloids. A rate equation of the general form ln[1/(1- X)] ) ktm was found to represent these data. 1. Introduction The phosphate ores are usually complex raw materials with a predominant constituent mineral of the apatite which are named from the Greek word RπRτω ´ (to cheat, deceive, defraud, trick, outwit, beguile, or mislead), because it is often mistaken for other minerals. The phosphate ores are mainly used in the manufacture of phosphate fertilizers. Calcareous phosphate ores usually are sedimentation ores which in large amount were found in several areas around the world, including a phosphorite deposit in Epirus area (Greece). Many studies have been carried out for the enrichment and beneficiation of Greek calcareous phosphate ores by conventional methods (Gremillion et al., 1979; Tsailas et al., 1980; Anastasakis, 1990, 1992), which showed that, because of the intimate mixture of the minerals (francolite and calcite) and its well-indurated nature, there is no feasible physical or physicochemical method to produce a suitable feed for acidulation processes. The enrichment of phosphorite by selective extraction (leaching) of the contained calcite with HCl and NH4Cl solutions was successful but uneconomical, while the induction of calcination of the phosphorite and water-leaching of the produced CaO gave better results (Tsailas et al., 1980). Sdoukos and Economou (1985a,b; Economou, 1984) have used diluted H3PO4 as the leaching agent of calcite. The method was successful and was based on the relatively higher dissolution rates of calcite than francolite which are present in phosphorites. A new technique, proposed by Sadeddin, Abu-Eishah (Sadeddin and Abu-Eishah, 1990; Abu-Eishah et al., 1991), and their co-workers, is based on using dilute acetic acid solution as a leaching agent for removal of the calcite from Jordanian phosphorites. By using this technique, the amount of calcium carbonate is reduced to a possible minimum without affecting the phosphate and it is possible to obtain a phosphate concentration of an average tricalcium phosphate content of 70-75%. In this paper we studied the enrichment of the lowgrade natural phosphate ores from Epirus area (Greece) by selective dissolution of calcite using dilute acetic acid * Author to whom correspondence is addressed. Telephone: +30-651-98352. Fax: +30-651-44836. Email:
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and the optimization of this process. This study was part of a research project having as its goal the examination of potential beneficiation of the low-grade Greek raw phosphate by conventional and innovational techniques such as acid thermal treatment (Vaimakis, 1985; Vaimakis and Sdoukos, 1987) and mechanochemistry (Karagiannis et al., 1995). 2. Material The material used was unaltered phosphate ore from Kosmira-Epirus area, in Northwest Greece. Mineralogical study (Gremillion et al., 1979; Tsailas et al., 1980) indicated that the phosphorites were synsedimentary 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 (Karagiannis et al., 1995). Francolite is the name of carbonate fluorapatite with syntactic type (empirical formula) Ca9.51Na0.35Mg0.14(PO4)4.74(CO3)1.26F2.50. The texture of phosphorite was a well-indurated, massive phosphatic and cherty limestone with veinlets of crystalline calcite. 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). 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 acetic acid solutions were prepared by dissolution with distilled water of glacial acetic acid (100 Gew %, FERAK LABORAT GMBH, Berlin). 3. Experimental Section Natural phosphate materials (phosphorite) from Kosmira-Epirus area (Greece) were crushed using a jaw crusher and then sieved to obtain fractions of 20001000, 1000-500, 500-250, 250-125, and
