X-ray diffraction and scanning electron microscopy quantitative mineralogical evaluation of the karst bauxite deposits in Campania, southern Italy

Bauxite resources in the world are estimated to be between 55 to 75 billion tons, occurring in Africa (32%), Oceania (23%), South America and the Caribbean (21%), Asia (18%) and elsewhere (6%) (United States Geological Survey Minerals Yearbook, 2011). These resources are mostly used to produce alumina for metallurgical purposes. Approximately 70 to 80% of the world’s bauxite production is processed first into alumina (Bayer process) (Hind et al., 1999), and then into aluminium by smelting and electrolysis (Hall– Herault process). The mineralogical variation in the composition of bauxite can affect the Bayer process, e.g. the effect of reactive silica, which contaminates the aluminium extraction process, and titanium minerals, which reduce the efficiency of the Bayer Process (Dudek et al., 2012; Smith, 2009). In this study, mineralogical characterization of some bauxite occurrences in Southern Italy has been carried out, including a comparison of results obtained from the quantitative analysis of the ore using the classic XRD-QPA (X-ray diffraction-quantitative phase analysis) method, and QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy) automated technology, which represents a mineralogical investigation tool common-used in the bauxite industry (Goodall et al., 2005). The bauxite of Southern Italy that is presently uneconomic can be considered as a model analogue for economic karst bauxite ores. Several bauxite occurrences from two former mining districts in Southern Apennines (Matese Mts. and Caserta province) have been analyzed. XRD-QPA allowed an exact evaluation of the amounts of minerals occurring in the bauxite. The main Al-rich mineral is boehmite, whereas the most common silica-bearing phase is kaolinite. Hematite is the main Fe-bearing mineral. Goethite also occurs, as well as sporadic lepidocrocite. Anatase and rutile are quite ubiquitous. QEMSCAN was used in the fieldscan mode and allowed: (1) the identification and Abstracts classification of the mineralogical phases on the basis of their chemical composition, and (2) the abundance of these phases. Moreover, this technique has provided evidence of the textural characteristics of the ore that can be important in the Bayer process. In particular, it has been possible to determine that boehmite, which tends to be more concentrated in the ooids than in the matrix, contains variable amounts of Fe. Hematite occurs solely as detrital elements, kaolinite occurs mainly in the bauxite matrix and, Ti-oxides are generally associated with goethite and hematite. Comparison between the XRD quantitative mineralogical analyses and the corresponding analyses determined by QEMSCAN shows convergent results (Boni et al.). The XRD quantitative evaluation of bauxite phases gives useful constraints for the interpretation of QEMSCAN raw data, which may be very detailed but cannot separate polymorphous minerals that occur in bauxite. Furthermore, QEMSCAN quickly highlights the main textural characteristics of the bauxite, which can affect the Bayer process, but there are also some problems in identifying the composition of phases that have a grain size lower than the provided scanning resolution. However, by using both methods, and with the support of manual SEM (scanning electron microprobe), all the issues were isolated and resolved. For this reason the XRD-QPA and QEMSCAN techniques should be considered complementary for bauxite ore evaluation, and can provide a very powerful tool for their exploitation and mineral processing.