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Characterization of Oxidized Zinc (Calamine) Ores by Scanning Electron Microscopy and Electron Microprobe Analysis

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Characterization of Oxidized Zinc (Calamine) Ores by Scanning Electron Microscopy and Electron Microprobe Analysis Scanning Microscopy Volume 1 Number 1 Article 8 12-1-1986 Characterization of Oxidized Zinc (Calamine) Ores by Scanning Electron Microscopy and Electron Microprobe Analysis G. Bonifazi Universitá di Roma "La Sapienza" P. Massacci Universitá di Roma "La Sapienza" Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Life Sciences Commons Recommended Citation Bonifazi, G. and Massacci, P. (1986) "Characterization of Oxidized Zinc (Calamine) Ores by Scanning Electron Microscopy and Electron Microprobe Analysis," Scanning Microscopy: Vol. 1 : No. 1 , Article 8. Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss1/8 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 1, No. 1, 1987 (Pages 73-83) 0981-7035/87$3.00+ .00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA CHARACTERIZATION OF OXIDIZED ZINC (CALAMINE) ORES BY SCANNING ELECTRON MICROSCOPYAND ELECTRON MICROPROBEANALYSIS G. Bonifazi and P. Massacci* Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia Universita' di Roma "'La Sapienza"', via Eudossiana 18, 00184 Roma, Italy (Received for publication March 28, 1986, and in revised form December 01, 1986) Abstract Introduction Textural and structural features of Sardinian Approximately 90 percent of all lead and zinc oxidized zinc (calamine) ores have been estab­ marketed today comes from sulfide ores. However lished by scanning electron microscopy and elec­ renewed interest is being shown in utilizing oxi­ tron microprobe analysis, the elements analyzed dized lead and zinc ("'calamine"') ores because of being Zn, Pb, Ca and Fe. The surface composition the discovery of new deposits in recent years. of grains produced by comminution has also been The beneficiation of calamine ores is diffi­ determined, especially with reference to the cult owing to their texture and structure, which presence of smithsonite and cerussite. leads to low metal recoveries (less than 50 per­ The methodological approach adopted has re­ cent in some cases). There are several reasons vealed intimate intergrowth of the mineral for such poor results: a) extensive intergrowth species present and contamination of the main of the two main species - smithsonite (Zn co ) mineral by inclusions of other species, which 3 and cerussite (Pb co3 ) - with one another and explains why it is so difficult to obtain good with other minerals, especially iron oxides; b) recoveries and high-quality concentrates by wide range of variability in texture of the zinc beneficiation processes. and lead phases present in the ores, resulting in dispersion of these grains and associations of particles of diverse minerals, which adversely affects subsequent beneficiation processes (flo­ tation), and makes recovery more difficult); c) presence of clays, oxides and hydroxides of iron, which necessitates complex preliminary desliming to remove these materials that would interfere with flotation, and d) limited selec­ tivity of conventional flotation agents with regard to mineral surfaces present in the pulp. In order to design a flow-sheet for a reasonable ore treatment,it is extremely important to have a complete picture of the chemical, mineralogi­ cal, textural and structural characteristics of the ores. Textural and structural relationship among the various mineral phases can be properly assessed by using SEM and electromicroprobe anal­ ysis, as demonstrated in this study. Such techniques must be adopted to supplement the information which can be acquired by means of optical analysis using reflected and transmitted light. It is notoriously difficult or even impos­ sible to differentiate carbonate minerals by the Key words: Optical microscope, Electron micro­ usual petrographic methods (FRIEDMAN, 1959) (WAR­ scope, Electron microprobe, Calamine ore, Cerus­ NE, 1962) (BELARDI & BONIFAZI, 1985). With trans­ site, Smithsonite. mitted light, zinc carbonate can, of course, be distinguished from carbonate gangue minerals (calcite and dolomite) whose reflectivity is considerably weaker, but confusion among cerus­ t-Address for correspondence: site and other carbonates is always possible For reprints and other information contact (RAMDOHR, 1980). P. Massacci at the address above. Yet even when such techniques permit easy Phone no.: 39-6-4687337 identification of the mineral particles they do 73 G. Bonifazi and P. Massacci not provide sufficient information on coating by the Gonnesa Formation, being the result of synge­ impurities or micro-intergrowths, which play a netic deposition in epicontinental carbonate decisive role in separation processes that in­ platform sedimentary environments; they are con­ volve surface phenomena (flotation, for in­ nected with small lagoon environment carbonate stance). This kind of information has to be col­ facies formed where there was a relative lack of lected at micron scale, which can be done by oxygen and a moderate contribution of detritus. means of SEM techniques and microprobe analysis The mineralizations occupy a relatively constant (REED, 1975) (FISHER, 1964). position in the Gonnesa carbonate sequence, but they have been dislocated, orientated and some­ times concentrated by tectonic action, particu­ Ores studied larly by the N-S Hercynian phases. The original sedimentary structure of these mineralizations The study covered two oxidized lead and zinc has been obliterated in some cases, especially ores (calamine ores) from Sardinia, one consist­ where tectonic action or recirculation has been ing mainly of smithsonite (Zn co ) from the most intense. 3 Buggerru Caitas deposit (Ore A), and the other of At Buggerru, the mineralizations are found in mixed Pb-Zn oxidized species from the San Gio­ the laminated fossiliferous white limestones and, vanni deposit (Massa Pozzo 4, Sublevel +168.00 m to some extent, the grey limestones and the above sea level) (Ore B). "flame" limestones. These lithofacies are between Chemical and mineral composition of the ores 50 and 100 m thick and lie at a somewhat vari­ are given in Table 1 (VIOLO, 1974), (ZUFFARDI, able distance from the contact with the grey 1952, 1968, 1969, 1970). basal dolomites, depending on the positions of the basins. Buggerru-Caitas deposit The mineralized masses as a whole strike N-S and dip almost vertically. They are separated by Most of the rocks in the Buggerru area are of virtually barren zones of limestone. Most miner­ Cambrian or Ordovician age, though there are also alizations are of the oxide type, but the sulfide some sediments attributable to the continental content increases with depth (sphalerite for the phases of the Permo-Triassic, while Quaternary most part and a little galena). The limits of the alluvial and eluvial materials occur, as to vol­ oxidation are not regular, however, unaltered canics of various kinds (BONI, 1978) (COCOZZA et sulfide nuclei being found even at the highest al., 1974). levels. There is also an outcrop of massive sul­ Three characteristic Cambrian formations are fides (sphalerite and pyrite) in the "laminar" found in the lglesiente-Sulcis area: from the limestones, surrounded by belts of oxidized min­ bottom upwards, these are the Nebida, the Gonnesa erals at the highest points. and the Cabitza (BONI & MARINACCI, 1980). The The oxidized mineral have a very vuggy appear­ mineralizations dealt with here are "located" in ance and often occur as concretions and geodes. Table 1: Chemical and mineralogical composition, resulting from normative calculations, with reference to the mineral species identified by X-Ray diffraction analysis. Chemical composition Mineralogical composition Element Content (%) Pure minerals Content (%) ore A ore B ore A ore B 67.38 39. 7 2 Zn 12.83 5 .45 Calcite (Caco 3 ) Pb 1.27 2.60 Dolomite (CaMg(C03)z) 0.68 33.35 24 .61 10.45 Ca 27 .35 23.16 Smithsonite (ZnC0 3 ) 1.64 3.35 Mg 0.09 4.40 Cerussite (PbC0 3 ) Hemimorphite (Zn (0H) (si 0 )Hz0) tr tr Fe 0.35 3.15 4 2 2 7 Quartz (Si0 ) 0.27 3.61 Si 0.27 3.61 2 Al 0.02 0 .21 Sphalerite (ZnS) tr tr s 0.20 0.26 Galena (PbS) tr tr LOI 43.44 39.37 Goethite ((FeO)OH) 0.56 5.00 LOI Loss On Ignition tr trace 74 Characterization of oxidized zinc ores From the mineralogical point of view they consist the resin so as to ensure better coating of the almost exclusively of smithsonite, though there material (this procedure is especially important is some hemimorphite and a very small amount of in the case of granular samples), and b) promote hydrozincite. Cerussite is almost always found polymerization. together with smithsonite, though in smaller The mass was then removed from the teflon cyl­ amounts. Pyrite with various degrees of alter­ inder and ground flat to uncover the surface of ation is present too. the material to be analyzed. In the residual sulfide zones, somewhat light­ It was then machine polished using finer and colored sphalerite is present either impregnating finer emery paper, after which it was hand pol­ the limestone or as a stratiform deposit. ished using a series of diamond pastes starting from 3 \Jm and finishing at 1 µm. After optical San Giovanni deposit study and prior to using the SEM and electron microprobe, the specimen was coated with graph­ This deposit lies 4 km WSW of Iglesias on the ite, by vacuum evaporation, and a silver-collared southern limb of the Iglesias syncline of the specimen holder was then positioned. The analyses Sardinian Cambrian series (BONI & GANDIN, 1979) were run on small fragments of the ores in the formed of shales, limestones, dolomities and "as-mined" condition, various screen-sizes sands tones, the Pb and Zn mineralization being classes of crushed ores, and on polished sections mainly in the limestones (BRUSCA et al., 1968). in zones considered to be of interest. Horizontal zoning is encountered in the miner­ Prior to SEM analysis the fragments of "as­ alized horizons.
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