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The Rietveld refinment applied to the bauxite and its claey cover at Juruti,

Para, Brasil

Conference Paper · September 2014

DOI: 10.13140/2.1.2949.3767

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Herbert Pöllmann

Martin Luther University Halle-Wittenberg

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Leonardo Negrão

Martin Luther University Halle-Wittenberg

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Marcondes Lima Da Costa

Federal University of Pará

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Introduction Bauxites are the main raw materials of aluminum and refractory industry. Brazil currently stands as the third largest producer of this ore, also being the country that has its third largest reserves, of which 97% are in the Amazon region. The main restriction of the bauxite’s mining in Amazon region is the up to 20 m thickness overburden made of yellowish clays (Belterra Clay), besides the difficulty and costly of chemical and mineralogical quantification of bauxites, particularly of available alumina and reactive silica. This paper evaluates the use of the Rietveld refinement to address these deficiencies. For this purpose, 17 samples were selected from a bauxite ore and its clay overburden in Juruti, Brazil. After mesoscopic description the samples were analyzed by X-ray diffraction and fluorescence and partly under electronic microscope.

Results The amorphous content was characterized mixing 20% of rutile as standard crystalline phase. Cluster analysis was used to arrange similar x-ray patterns in order to apply different refinement’s strategies for the distinct x-ray patterns types recognized. The bauxite bearing regolith, with 23 m in depth, comprises a mottled horizon at the bottom; a bauxitic horizon consisting mainly of gibbsite; an iron crust; a horizon with ferruginous nodules and clayey matrix; a horizon with bauxitic nodules and clayey matrix, and; clay cover horizon comprising mainly to kaolinite group minerals. Mineral quantification obtained by the Rietveld methods reveled two generations of gibbsite with distinct crystallinity, resulting mainly in asymmetric gibbsite peaks (Fig. 2). The quantified amorphous content of the bauxite was up to 23%, increasing in the clay cover horizon

Conclusions The evaluation of the Rietveld refinement was made by the goodness-of-fit, the R factor values, the fitting of the calculated profile and the reproducibility of the method (Fig 5). The refinement indices and results appear to be satisfactory and proving that the Rietveld method is applicable and feasible to exploit bauxite due to its convenience and speed, crucial to discover new reserves.

1: CAPES Foundation, Ministry of Education of Brazil, 70040-020 Brasilia-DF, Brazil. boiadeiro.negrao@gmail.com; 2: Institut für Geowissenschaften und Geographie, Martin-Luther-Universität Halle-Wittenberg, Von Seckendorffplatz 3 06120 Halle (Saale), Germany. herbert.poellmann@geo.uni-halle.de; 3: e Instituto de Geociências, Universidade Federal do Pará, Av. Augusto Correa 1, Guamá, 66075-900 Belém-PA, Brazil. mlc@ufpa.br

horizon. The use of different kaolinite’s polymorphs structures as dickite, dickite 2m and halloysite resulted in better profile fittings, suggesting the coexistence of these phases (Fig. 3).

Acknowledgements Thanks to the geologist Msc. Gilberto Cruz, to the company ALCOA, to the INCT-GEOCIAM and to the working group friends from GMGA-Universidade Federal do Pará (Brazil) and Mineralogie-Martin-Luther-Universität Halle-Wittenberg (Germany).

Fig. 1: Localization of Juruti-Pará and picture of the bauxite’s mine of ALCOA showing different horizons of the studied profile in the plateau Capiranga. A: Mottled horizon; B: Bauxitic horizon; C: Iron Crust; D: Nodular ferrous horizon; E: Nodular bauxitic horizon; F: Belterra Clay cover.

Fig. 3: A: XRD-Pattern from kaolinite and halloysite mixtures (Brindley et al. 1963). B: Detail of XRD-Pattern from the Clay Cover Horizon, showing similar kaolinite/halloysite peak after Rietveld refinement.

Fig. 4: Profile of the Capiranga plateau with the mainly chemical variations, characterized by x-ray fluorescence.

Fig. 2: Asymmetric peaks (002) of gibbsite due to the presence of two generations of this mineral.

1m- 2m- 3m- 4m- 5m- 6m- 7m- 8m- 9m- 10m- 11m- 12m- 13m- 14m- 15m- 16m- 17m- 18m- 19m- 20m- 21m- 22m- 23m-

Nodular ferrous horizon

Nodular bauxitic horizon

Belterra Clay cover

gibbsite 1 gibbsite 2

kaolinite goethite dickite 2m halloysite

rutile 20,0%

gibbsite 0,7%

anatase 2,7%

kaolinite 20,2%

goethite 7,3%

quartz 0,7%

dickite 2m 19,1%

halloysite 0,1%

amorphous 29,2%

anatase 0,6%

gibbsite 8,2%

goethite 3,4%

hematite 19,7%

rutile 20,0%

kaolinite 3,0%

gibbsite (2) 7,3%

amorphous 37,7%

anatase 1,3%

gibbsite 7,8%

hematite 1,3%

kaolinite 4,0%

rutile 20,0%

dickite 2m 13,4%

halloysite 0,4%

gibbsite (2) 28,7%

amorphous 23,0%

Fig. 8: Refined XRD-Pattern of the bauxitic horizon. A: Lamellar crystal of gibbsite in gibbsitic-kaolinitic matrix; B: meso-crystals of gibbsite in matrix of micro-crystaline gibbsite; C: Bauxite; D: Bauxitic nodules.

Fig. 7: Refined XRD-Pattern of the nodular ferrous horizon. A: Cavernous matrix of the nodular ferrous horizon; B: Spherical aggregates of hematite; C: Subhedral crystals of gibbsite in fe-al cavernous matrix; D: detail of the spherical hematite’s aggregates; E: Iron nodules evolved by clay.

Fig. 6: Refined XRD-Pattern of the Belterra Clay Cover and its sample.

Fig. 5: Mineral content of the bauxite sample quantified three times using the Rietveld refinement.

A B

C D

E

A

B

C

D

Quantification 1 Quantification 2 Quantification 3

Gibbsite Hematite Anatase Clay-minerals Amorphous

Iron Crust

Bauxitic Horizon

Mottled Horizon

GOF: 3,3 Rwp: 6,0

GOF: 5,0 Rwp: 7,2

GOF: 1,4 Rwp: 2,8

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