Mineralogical investigations of Jamaican hematite-rich and goethite-rich bauxites using XRD and solid state 27Al and 31P MAS NMR spectroscopy
Graphical abstract
Introduction
Bauxite, the primary ore of aluminum, is the principal raw material used in producing alumina by the wet-chemical caustic digestion method known as the Bayer process (Smith, 2009; Schlesinger, 2013). Bauxites consist of the aluminum minerals (gibbsite [Al(OH)3], böehmite [AlO(OH)] and/or diaspore [AlO(OH)]), iron minerals (mainly hematite [Fe2O3] and goethite [FeO(OH)]) along with smaller amounts of silicates, titanates, phosphates, carbonates and several metal species at trace concentrations (Gow and Lozej, 1993). All components other than the alumina minerals are classified as impurities by the industry as they may negatively impact the process even if they remain insoluble. Initially, bauxites characterized as being economically mineable were those that contained >45% Al2O3, <20% Fe2O3 and <5% SiO2 however technology now permits the processing of ores with much wider mineral compositions (Gow and Lozej, 1993; Smith, 2009; Songqing, 2011).
Bayer plants extract alumina from bauxites by digestion with concentrated sodium hydroxide solutions (~3.5–6 M) under either low (135–150 °C) or high temperature (230–250 °C) conditions depending on the nature of the aluminum minerals. The supersaturated sodium aluminate liquor that exits digestion is separated from the insoluble residues (red mud), is filtered and is then seeded to produce gibbsite crystals (Hind et al., 1999). After extensive washing, the product-size crystals are calcined (1000–1200 °C) to produce smelter grade alumina (SGA) which must meet a very rigid set of quality parameters (Lindsay, 2005). Approximately 90% of the alumina produced worldwide is smelted via the Hall-Heroult electrolysis process to manufacture aluminum (Plunkert, 2012).
Jamaica is a small Caribbean island (10,990 km2) that accounts for approximately 7.1% of the world's bauxite reserves. The major deposits are in the central parishes of Trelawny, St. Ann, Manchester and St. Elizabeth; smaller deposits in Clarendon and St Catherine are either unavailable because of location or are almost completely mined out (Fig. 1). Most of the bauxite deposits in Trelawny fall into a protected zone that is important for the island's water resources and biodiversity and will not be mined. Optimal use of the available bauxite supplies is therefore crucial.
The bauxites resources available in south-central Jamaica are traditionally classified as either hematite-rich catchment deposits (‘red bauxites’) or goethite-rich hillside deposits (‘yellow bauxites’). The less weathered hematite-rich catchment ores are readily processed in the existing low temperature Bayer plants however they are quickly becoming depleted and soon bauxite companies must resort to processing the more extensively weathered goethitic hillside ores. Goethitic ores originate from the weathering of hematite-rich bauxite deposits, usually under climatic conditions that allow exposure to sufficient warmth and moisture to allow transformation of the initially deposited gibbsite, hematite and apatite minerals into böehmite, alumina-substituted goethite and a range of secondary phosphates, respectively (Kirwan et al., 2009). Studies have shown that goethitic bauxites have significantly higher impurity concentrations than the traditionally mined hematite-rich ores (Malito, 2002; Powell et al., 2009; Greenaway et al., 2013; Henry et al., 2018). These goethitic bauxites typically have total phosphorus concentrations that are 10–100 times higher than the catchment ores and depending on their mineralogy, processing could result in extremely high soluble phosphorus concentrations in the Bayer plant liquors (Lawson et al., 2008; Greenaway et al., 2013; Henry et al., 2018). Mineral impurities containing trace metals such as Cr, Mn or Zn typically occur at relatively high concentrations in these ores as well.
Low temperature processing of high-goethite bauxites may cause several refinery challenges. These include enhanced caustic and alumina losses, higher red mud loads, poor settling characteristics and increased concentrations of dissolved phosphate and other liquor contaminants (Lawson et al., 2008). Owing to the cyclic nature of the Bayer process, soluble phosphorus concentrations will accumulate over time and unless precipitated through use of lime (CaO) or other expensive additives, concentrations may reach levels that reduce alumina productivity, alter red mud surface properties and the P species may become incorporated into the alumina product (Chin, 1971; Lawson et al., 2008). During smelting, the presence of phosphorus species in calcined alumina significantly decreases current efficiency in the electrolytic bath and reduces the effectiveness with which alumina is converted to aluminum; it also lowers aluminum corrosion resistance and causes increased brittleness (Lindsay, 2005; Al-Mejali, 2015). As a consequence, current alumina production standards limit the concentration of phosphorus in smelter grade alumina for use in the Hall-Heroult process to <10 mg/kg; over time, significantly lower limits are anticipated (Lindsay, 2005).
As Jamaican bauxite plants prepare to begin mining the high phosphate goethitic bauxites, there is strong appreciation of the need for detailed information on the phosphorus mineralogy of the ores at the exploration stage (Malito, 2002). This study investigates the aluminum and phosphorus mineralogies of both the traditionally-mined Jamaican hematite-rich bauxites and the future goethitic deposits using powder x-ray diffraction (XRD) which is the standard industry practice and solid state 27Al and 31P magic angle spinning nuclear magnetic resonance spectroscopies (27Al and 31P MAS NMR). Solid-state NMR spectroscopy is a powerful technique for investigating how elements are bonded together in solid samples. It can give useful information on the mineralogy and structure of materials as it is able to detect elements in minerals where the concentrations are below the detection limits of powder XRD (Feret et al., 1997; Begaudeau et al., 2012). Solid state 31P NMR studies have been used to explore the P environments of peat, soils, compost, manure and other solids while 27Al NMR has been used to examine aluminum species in situ in crystals, minerals, clays and soils (McDowell et al., 2003; Cade-Menun, 2005; Cade-Menun et al., 2005; Eisazadeh et al., 2012). The use of 27Al NMR spectroscopy readily distinguishes between aluminum species in tetrahedral and octahedral coordination environments while 31P NMR can provide useful information to differentiate between the P mineral associations in a sample.
Although solid state 27Al and 31P MAS NMR spectroscopies are not traditionally used to investigate bauxite mineralogy, their use alongside powder XRD allows one to assess the value of NMR as a complementary technique to XRD especially for differentiating between the phosphate and alumina minerals that are encountered in hematite-rich and goethite-rich Jamaican bauxites. The approach seeks to extend the information available from conventional analysis of bauxite exploration samples and opens the potential for local refineries to better predict the processability of goethitic ores and thus avoid the challenges associated with processing bauxites containing phosphate minerals of unusually high solubility.
Section snippets
Sample selection
Prior to mining, bauxite companies conduct detailed exploration studies and systematically collect representative bauxites from across the potential mining areas. In this study five archived exploration bauxites (samples 36, 60, 96, 122 and 162) were selected, one from a traditionally mined hematite-rich area and four from a goethite-rich future mining area (Henry et al., 2012; Greenaway et al., 2013; Henry et al., 2018). All samples were studied using powder XRD as well as 27Al and 31P MAS NMR
Powder X-ray diffraction
The powder XRD patterns for both the traditionally mined red hematite-rich bauxite (162) and the yellow goethitic future ores (bauxite 36) are presented in Fig. 2. The diffractogram for bauxite 36 is typical for the goethitic bauxites with P2O5 concentrations <2.00%. Gibbsite [Al(OH)3] was the dominant mineral identified in both the hematite-rich bauxite 162 and the yellow goethitic bauxite 36 (Fig. 2). Böehmite [AlO(OH)] and anatase [TiO2] were also identified in both types of bauxites however
Conclusion
Solid state 27Al and 31P MAS NMR spectroscopies were used along with powder x-ray diffraction to probe the local Al and P environments for one hematite-rich and four goethitic Jamaican bauxites and the red mud and CBD extraction residues of one of the goethitic ores. Traditional XRD readily identified gibbsite, böehmite, hematite, aluminous goethite and anatase in both the hematite-rich and goethitic Jamaican bauxites however crandallite was only identified in the four goethitic bauxites as the
Abbreviations
- CBD
citrate bicarbonate dithionite
- MAS
magic angle spinning
- NMR
nuclear magnetic resonance spectroscopy
- XRD
X-ray diffraction
- XRF
X-ray fluorescence
- P
phosphorus
- ss
solid state
Author contributions
The manuscript was written through contributions from all authors. All authors have given their approval for the final version of the manuscript.
Funding sources
Funding was received from the Linnaeus Palme Foundation to facilitate travel to Chalmers University of Technology and Gothenburg University, Sweden to conduct solid state 27Al and 31P MAS NMR and XRD studies.
Conflict of interest disclosure
The authors declare no competing financial interest in this work.
Acknowledgements
The authors wish to thank Vratislav Langer (Chalmers University of Technology, Gothenburg) for facilitating the X-ray diffraction studies. We acknowledge the NMR Division, Bruker BioSpin GMBH in Rheinstetten, Germany and the Swedish NMR Centre, Gothenburg, Sweden for assistance with collecting the solid-state NMR data and Robert Lancashire (The University of the West Indies, Mona), for optimizing the quality of the scans. We also thank Anthony Porter (The Jamaica Bauxite Institute) for
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