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CHROMITE: FROM THE MINERAL TO THE COMMODITY
Full text linkChromite belongs to the spinel group with the general chemical formula XY2O4, where X and Y represent divalent and trivalent metal ions, respectively. Four types of chromite ore deposits occur as either lode or secondary deposits. Lode chromite ore deposits comprise stratiform and podiform deposits, whereas secondary chromite ore deposits comprise laterite and placer deposits.
Chromite is an important industrial mineral used in the refractory industry. Moreover it is the only industrial source of chromium for chemical and metallurgical industries and to be marketed as a valuable commodity it requires very demanding quality parameters that differ according to the kind of application. Chromite ore cannot be usually sold as it is and, therefore, some kind of ore beneficiation is required in order to separate chromite from gangue minerals. The most commonly used beneficiation methods for chromite ores are the gravity methods, such as the shaking table, jig, spiral and Reichert cone methods.
The present work deals with the geologic studies that, coming after prospection, lead to the evaluation of the chromite ore quality and to planning and/or improvement of beneficiation plants.
The quality of chromite ore deposits, in order to rank them for possible exploitation, was studied in five deposits of Madagascar (Andriamena, Antanimbary, North Befandriana, North Belobaka and North Toamasina), in collaboration with UT Group s.r.l., for the development of chromite mining in the country. The basic geological, mineralogical and geochemical study of the deposits led to the reconstruction of genetic models for each of them. This was the starting point for a more detailed study on the quality of the chromite ore. The five most important chromite ore localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of probable Neoproterozoic to Cambrian age. Metamorphism and alteration affected, at different degrees, all chromitites, but never completely obliterate their primary characters. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, while primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblage comprises serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data are compatible with an ophiolite origin for North Befandriana chromitites and a layered intrusion origin for Andriamena, North Toamasina, North Belobaka and Antanimbary chromitites. These latter show differences that can be related to a different position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. North Befandriana is a high quality deposit that could be exploited without any beneficiation of the ore, Andriamena needs beneficiation to reach market standard, Antanimbary and North Belobaka is low quality for metallurgical or chemical use but could be a good prospect for refractory market. Finally North Toamasina chromite ore is not suitable for any market even after beneficiation.
An innovative study of geologic processes that can affect chromite ore beneficiation was applied to Vavdos chromite deposit (Greece) hosted in the Vavdos ultramafic massif belonging to the Halkidiki ophiolite of the Circum-Rhodope orogenic belt. Here metamorphic modification of chromite led to redistribution of Cr2O3 from chromite to silicates. The effect of the redistribution is to lower the efficiency of gravity plants as Cr2O3 contained in silicate phases will be preferentially discharged into the tailing during enrichment. The influence of this process that is widespread in chromite ores, on chromite enrichment was evaluated quantitatively. Generally accepted assumption that chromite ores do host Cr only in chromite is misleading as metamorphosed chromite ores host significative amounts of Cr in gangue phases and especially in Cr-chlorite. This study, of a completely metasomatized chromite ore, shows that about 3 wt% of total Cr2O3 in the rock is hosted in Cr-chlorite; while only about 0.2 wt% of total Cr2O3 is hosted in serpentine. As Cr-chlorite can host even more Cr2O3 than at Vavdos, and as the deepest alteration of chromite due to metasomatism occurs for ores containing about 34% chromite, the amount of Cr2O3 redistributed within the gangue can be even higher than at Vavdos, especially in low grade disseminated ores, where probably about 5-6% of Cr2O3 can be hosted in the gangue, a value that could rise to 7-8 wt% for high Cr2O3 Cr-chlorite. The effect of this wrong assumption is a mistake in the calculation of plant efficiency that will be overestimated. Mistakes due to redistribution of Cr2O3 during metamorphism can be easily avoided through mineralogical analysis that can detect the presence of Cr-chlorite in the ore. Planning of beneficiation in Cr-chlorite-bearing chromite ores requires additional investigation, concerning Cr2O3 content in Cr-chlorite and Cr-chlorite amount in the ore.
A procedure to evaluate efficiency and results of gravity chromite enrichment plants was tested on Brieville enrichment plant (Madagascar), which belongs to Kraomita Malagasy mining company. Here a detailed study of chromite sand quality parameters at each step within the plant together with the measuring of all sand flow rates led to the reconstruction of separation efficiency at each step of chromite processing. The results of grain size, XRD, XRF, EMP and grain counting analyses together with separation efficiency (SE) and liberation degree (LD) evaluation allow to conclude that Brieville plant does not properly work due to the low sorting of sands feeding shaking tables that negatively affects their separation efficiency. Moreover the low degree of liberation of chromite, especially in the coarsest grain sizes, negatively affects the re-cycling process of mix materials. Plant efficiency and quality of final product can be improved by: moving the Cr2O3-enriched mixes to the concentrate and the Cr2O3-depleted mixes to the waste; grinding to finer grain size the overall feed of shaking tables and the mixes that will be re-cycled and substituting hydrosizers with screens. The first change, that does not involve any additional operational cost, has been effectively applied to the plant just after publication of the present study. The other changes involve additional operational costs and require a detailed economic analysis before being applied to the plant.
Finally a completely new technology for high performance beneficiation of chromite sand that leads to the production of chromite refractory sands was tested. For this work chromite concentrate sand from Krasta enrichment plant (Albania) was used. The innovative beneficiation plant of Omega Foundry Machinery LTD. comprises a drum magnet and the new Inclined Fluidised Separator that uses an air cushion as the fluidizing agent during gravity driven grain separation. Refractory chromite sand chemical and technical requirements are the most demanding in chromite market and no chromite ore can attain them by simple crushing and grinding. On the other hand usual enrichment methodologies either cannot meet the required parameters or have a very low refractory sand recovery. The combination of drum magnet and Inclined Fluidised Separator in the Omega Foundry Machinery LTD. pilot plant not only produces a good quality refractory sand, but the result is reached with a high recovery, making of this plant an optimal solution for the production of refractory chromite sand. The Inclined Fluidised Separator is particularly performing as it combines a very high recovery of silica in the waste with an increase of the grain size of concentrate
A quantitative mineralogical study of chromite ores from Pozanti-Karsanti ophiolite and its effect on chromite sand beneficiation
No full textMetamorphic redistribution of Cr within chromitites and its influence on chromite ore enrichment
No full textMineralogical and chemical characters of ore strongly affect the efficiency of chromite sand physical concentration. Usually chromite ore enrichment is based on the assumption that all Cr in the rock is hosted within chromite, but alteration in metamorphic conditions can lead to partial redistribution of Cr from chromite to silicates. Such redistribution was studied on the Vavdos chromite deposit (Greece) where more than 3% of whole rock Cr(2)O(3) can be found in silicates. The effect of Cr(2)O(3) redistribution is to lower the efficiency of gravity plants as Cr(2)O(3) contained in silicate phases will be preferentially discharged into the tailing during enrichment. Tests confirm the effect of Cr redistribution on enrichment and show that preferential separation of different silicates does not occur or is negligible. As the redistribution of Cr within the rock is not indicated by any change in whole-rock chemistry only the study of mineralogy, texture and mineral chemistry of the ore can provide a reliable basis for final product quality and Cr(2)O(3) recovery prediction
Projects for the extraction of pyrite in Albania and Kosovo
No full textPyrite (FeS2), is an iron sulfide and one of the most ubiquitous minerals of earth crust. It is found in igneous, metamorphic and sedimentary rocks and crystallizes at both high and low temperatures.
Pyrite was widely used in the past for the production of sulfuric acid, but due to environmental pollution nowadays this use is limited to China and pyrite lost its value as an industrial mineral. In the last decade anyway new industrial applications of pyrite have opened new markets for this mineral. Such applications comprise: stainless steel production (60% of total pyrite market), abrasives (15%), dyes, pigments for glass (20%), brakes (3%), rechargeable batteries (2%). These new applications need small amounts of pyrite but require very demanding quality parameters.
The present work deals with projects for the extraction of pyrite in Albania and Kosovo for these new industrial applications.
For the projects three different ways to recover pyrite were considered: a) as a by-product of pyrite-bearing active mines (Trepça, Kossovo; Fusharrez, Albania); b) re-opening of abandoned pyrite mines used in past for sulfuric acid production (Spaç, Albania); c) exploitation of a new pyrite deposit (Lunik, Albania).
Pyrite is an important sulfide phase in the active lead and zinc Trepça Mine, Kosovo. Trepça Belt belongs to the Kosovo sector of the Serbo-Kosovo-Macedonian-Rhodope Metallogenic belt of Oligocene-Miocene age, which includes base and precious-metal districts in Kosovo, southern and western Serbia, variscan structures marginal to the Serbo-Kosovo-Macedonia, northern Greece and southern Bulgaria (Heinrich and Neubauer, 2002). At Stanterg (Trepça mine), massive sulfide ore of economic importance forms continuous, columnar shaped ore bodies of carbonate replacement type (skarn) related to the emplacement of tertiary magmas (granodiorite and dacite-andesite). These are located along the carbonate-schist contact and dip parallel to the plunge of the anticline. The ore bodies extend along a strike length of 1200 m, and have been explored to a depth of 925 m below the surface (11 levels). Pyrite can be also recovered from tailing dumps.
In northern Albania pyrite can be recovered within the Mirdita ophiolite belt, in similar geological settings, as a by-product and from the tailings of the copper Fusharrez mine and from the abandoned pyrite Spaç mine.
Mirdita is located in the Jurassic age Mirdita-Pindos ophiolite belt of Albania-Greece that ranges from ultramafic to mafic rocks with a number of andesitic and felsic volcanic domes in the central portion. The volcanic rocks are overlain by a sedimentary melange.
Finally in eastern Albania the never exploited pyrite Lunik deposit is placed inside volcanic rocks. It was formed underwater together with pillow basalts and at low temperature hydrothermal conditions.
Basalt rocks of Lunik are placed over gabbro through gabbrodiabase or over the ultramafic sequence of the western ophiolites through ocean metamorfics (metabasalt), and covered normally by the upper-medium Jurassic siliceous radiolarite, or transgressively by heterogeneous ophiolitic melange of upper Jurassic (Titonian). Vulcanites of this series in many sectors underwent low grade metamorphism in zeolite to greenschist facies and are affected by low temperature hydrothermal alteration that was responsible for the precipitation of pyrite.
CONCLUSIONS
Kosovo Trepçe technical problems to export pyrite, Fusharrez, Spaç and Lunik Albania better possibilities to export rock and floated pyrite. According with Ut group the most important Pyrite analyses will be done in Spain, and the rest of the analyses chemical method is the best and easiest solution.
References:
Heinrich C.A and Neubauer. 2000. F, Cu-Au-Pb-Zn-Ag Metallogenic of the Alpine - Balkan - Carpathian - Dinaride geodynamic province. Mineralium Deposita. 7: 533-540
Magnetic enrichment of braunite-rich manganese ore at different grain sizes
No full textMetamorphism of primary manganese deposits changes the oxi-hydroxide assemblage into a
braunite-rich paragenesis. Braunite magnetic properties, together with the increase in grain
size during metamorphism, can allow magnetic enrichment at gravel-sand grain size. Samples
were collected from a manganese deposit in the Eskis ̧ehir region of Turkey, where a
metamorphic event changed mineralogy from an original Manganese (Mn) oxi-hydroxide
and chert assemblage to a braunite-rich quartzite and also showed an increase in grain size
as a result of metamorphism. These samples were first crushed to a fine grain size and braunite
was collected using a magnetic separator. Results show that metamorphic braunite-rich
manganese deposits can be successfully enriched at high Mn recovery with the best results
in the þ1–10mm grain size range
Application of an innovative beneficiation technique to Krasta chromite ore (Albania) for the production of high grade – low silica chromite sand
No full textThe Mirdita ophiolite is located in the northern ophiolite belt of Albania. Based on differences in the internal stratigraphy and chemical composition of the crustal units, two types of ophiolites have been recognized in the Mirdita ophiolite, namely the Western Mirdita Ophiolite (WMO) and the Eastern Mirdita Ophiolite (EMO) (Dilek et al., 2008). Boninitic dikes and lavas crosscut and/or overlie earlier extrusive rocks in the EMO (Beccaluva et al., 1994). The crustal section of the WMO has MORB affinities, whereas that of the EMO predominantly shows SSZ geochemical affinities. The extrusive sequence in the EMO consists of pillowed to massive flows ranging in composition from basalt and basaltic andesite in the lower section to andesite, dacite, and rhyodacite in the upper part (Bortolotti et al., 1996). Large peridotite massifs are exposed at the western and eastern ends of the Mirdita ophiolite. Plagioclase-bearing peridotites are frequently observed in the WMO, whereas harzburgite is dominant in the EMO (Beqiraj et al., 2000).
In this work, we focus on Bulqiza peridotite massif located in the EMO, because it has economically important chromite ores. Chromite is an important mineral used in the metallurgy, chemistry and refractory industries and often requires enrichment processes to achieve the chemical parameters for different markets.
This work deals with disseminated chromite ore samples collected at Krasta Mine, located in the central southern part of the Bulqiza Massif. First of all the samples, having an average Cr2O3 content of 23.66 wt%, were enriched using spirals and shaking tables at Krasta plant. The first chromite sand concentrate has 46.58 wt% Cr2O3 and 10.35 wt% SiO2. In order to meet the very demanding chemical parameter requirements for refractory market chromite first concentrate sand was re-enriched using a combination of dry magnetic and gravity separation at the pilot plant of Omega Foundry Machinery LTD. in Peterborough (UK). In a second step sand was enriched using a drum magnet. New concentrate was then enriched in a third step by means of an Inclined Fluidised Separator (IFS) that works in dry conditions using an air cushion as fluidisation agent.
Preliminary results show that the pilot plant is able to strongly re-enrich the primary concentrate sand, producing a final concentrate sand with up to 60.01 wt% Cr2O3 and 2.43 wt% SiO2 with a tail that is still suitable for the steel market (Fig. 1).
Fig. 1. Three steps Krasta chromite ore enrichment: spirals and shaking tables at Krasta plant (white symbols), drum magnet (grey symbols) & IFS (black symbols) at Peterborough pilot plant. Square = feed, triangles = concentrates and circles = tails. First and second concentrates are the feeds of the following steps.
References
BECCALUVA, L., COLTORTI, M., PREMTI, I., SACCANI, E., SIENA, F., ZEDA, O. (1994): Ofioliti, 19, 77–96.
BEQIRAJ, A., MASI, U., VIOLO, M. (2000): Exploration and Mining Geology, 9, 149–156.
BORTOLOTTI, V., KODRA, A., MARRONI, M., MUSTAFA, F., PANDOLFI, L., PRINCIPI, G., SACCANI, E. (1996): Ofioliti, 21, 3–20.
DILEK, Y., FUMES, H., SHALLO, M. (2008): Lithos, 100, 174–209
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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