Soil Science

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Prochnow, L. I.1; Clemente, C. A.1; Dillard, E. F.2; Melfi, A.1; Kauwenbergh, S.2

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Scanning electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray techniques were used to obtain data about the mineralogical composition of single superphosphates (SSP) produced from apatite concentrates originating from Araxá, Jacupiranga, Catalão, and Tapira igneous phosphate rock deposits. The SSP samples were investigated both in their original form and after leaching with water to remove water-soluble compounds. Results of chemical analysis and instrumental techniques showed that P compounds were still present in the water-leached material. Anhydrite (CaSO4 with no hydration water) was the main CaS component in the SSP samples. Monocalcium phosphate monohydrate [Ca(H2PO4)2.H2O] was identified in the original but not in the water-leached samples. Results of SEM, EDX, and X-ray confirmed the presence of crystals of triiron (III) potassium octahydrogen hexaphosphate hexahydrate [Fe3KH8(PO4)6.6H2O] in the SSP. The results of SEM and EDX analyses suggested that the compounds triiron (III) nonahydrogen hexaphosphate hexahydrate [Fe3H9(PO4)6.6H2O], triiron (III) 15-hydrogen octaphosphate tetrahydrate [Fe3H15(PO4)8.4H2O], triiron (III) potassium 14-hydrogen octaphosphate tetrahydrate [Fe3KH14(PO4)8.4H2O], and barium sulfate (BaSO4) were present in some of the SSP samples. Use of SEM, EDX, and X-ray techniques was shown to be adequate for precise characterization of compounds present in the SSP studied, which will help to understand further the plant availability of the phosphorus contained (especially) in the water-insoluble P fraction of this fertilizer.

In Brazil, one of the most important fertilizers used to increase the amounts of plant-available soil phosphorus (P) to commercial crops is the single superphosphate (SSP). This P source is also produced in other countries in the world, especially in Australia and New Zealand (Young and Davis, 1980). SSP is produced by attacking the phosphate rock (PR) or apatite concentrate with sulfuric acid in order to convert the apatite to more soluble compounds, which can then provide available P to the plants once the product is applied to the soil. When high-grade or premium PR concentrates, i.e., those with at least 13.5% P (Lehr, 1980), are used, the major chemical compounds obtained are calcium sulfate (CaSO4), with varying degrees of hydration water (0, 0.5, or 2 mol), and monocalcium phosphate monohydrate [(MCP, Ca(H2PO4)2.H2O)] (Hatfield, 1964), which is highly water-soluble. Low-grade or marginal-grade PRs vary in composition and contain high contents of cationic impurities, especially iron (Fe) and aluminum (Al) (Lehr, 1980). As a result of using low-grade PR to produce SSP, compounds in addition to CaSO4 and MCP, primarily Fe and Al phosphates, are formed in the final product (Gilkes and Lim-Nunez, 1980; Lehr, 1980; White, 1976). These compounds are generally water-insoluble (AOAC, 1999) and are usually represented as generic FeAl-P compounds. Premium-grade PR sources are limited, and availability and costs will force industries to rely on lower-grade quality ores. Better knowledge of the mineralogical composition of SSP and the plant availability of P in the impurities formed will provide information useful in the management of these P sources.

Scanning electron microscope (SEM) and qualitative element identification through energy dispersive X-ray (EDX) are techniques that can help to characterize adequately the composition of P fertilizers. Frazier et al. (1989) used these techniques to identify the compounds formed in a pure phase system of Fe2O3-K2O-P2O5-H2O and to determine which compounds may occur as components of acidulated phosphate fertilizers, especially single and triple superphosphates.

The main objective of this study was to obtain information about the mineralogical composition of five single superphosphates produced from marginal-grade Brazilian PR utilizing SEM, EDX, and X-ray powder diffraction techniques.

© 2001 Lippincott Williams & Wilkins, Inc.




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