Sampling representativeness for metallurgical testing 

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Geological background

The Cornubian granite province is well known for its historically important deposits of tin and base metals (Manning and Hill, 1990), and for its current international importance as a major producer of china clay (kaolin).

The Cornubian Sn-W province

Tin is distributed on the continents in relatively narrow continent-sized belts (Schuil-ing, 1967). For instance, most of tin deposits across Europe are related to Variscan granites (Figure 1.5). The major tin belt passes from the Cornubian massif (Redruth, Camborne, etc.) in UK through the French Massif Central (Echassières) and con-tinues towards Italy with tertiary tin deposits (Elbe, Tuscany and Scilly). Other tin belts are also related to varsican granites, like the one which pass from Northen Por-tugal (Panasqueira) through Spain or the one around the Bohemian massif (Cinovec-Altenberg). Indeed most of these tin deposits are associated with Sn-W cupolas located at the top of tardi- to post-orogenic plutons emplaced at shallow depths (Jébrak and Marcoux, 2008).
(b) Outline geologic map of south west England peninsula showing the location of the major plutons of the Cornubian batholith, the kaolinised areas and the active kaolin pits, modified from Černý et al. (2005). (c) Distribution of the felsite dykes (elvans) and Sn-W mineral lodes around and within the St Austell rare-metal granite after Bray and Spooner (1983).
The batholith of SW England was formed during late Variscan orogeny in the Late Carboniferous to Early Permian (270-300 Ma) intruding deformed Devonian-Carboniferous terrestrial to marine sediments. The batholith has a 200-km-long ex-tension (Willis-Richards and Jackson, 1989), comprising six major and several minors granite bodies. The major bodies, from east to west, are Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land’s End and the Scilly Isles (Figure 1.5b).
The outcrops are dominated by biotite-bearing monzogranite, with minor intrusions of tourmaline-bearing aplites, pegmatites and local fine-grained facies (Manning et al., 1996). In addition there is some occurrences of other granite types, resulting from multiple intrusive episodes, such as the lithium-mica granites or the topaz-bearing granite in the Tregonning-Godolphin Granite and the St Austell Granite (Manning and Exley, 1984; Manning and Hill, 1990; Manning et al., 1996). The batholith was affected by several episodes of alteration including quartz-tourmaline veining associated with greisening, intrusion of rhyolite dykes, quartz-hematite veining. Kaolinisation is the last alteration event, which is believed to have a meteoric origin (Psyrillos et al., 1998), and was relatively extensive in the western part of the St Austell pluton where the majority of the active kaolin pits are located (Figure 1.5b).
According to Willis-Richards and Jackson (1989) there is a close spatial association between the batholith and the rich Sn, Cu mineralization of the Cornubian Peninsula. Most of the ores of Sn, Cu, W, Zn and As came from vein deposit type minerali-sation, along or parallel to the axis of the batholith (Moon, 2010). Within the St Austell granite zones of intense kaolinisation are spatially related to swarms of steeply dipping, quartz-tourmaline cassiterite wolframite sheeted veins with greisen (quartz-muscovite-tourmaline topaz wolframite) alteration selvages (layered alteration zones), observable at Goonbarrow pit (Bray and Spooner, 1983). These mineral lodes are lo-cated South (and North) of the St Austell granite, but some of them actually occurred within the St Austell granite kaolinised area (Figure 1.5b and Figure 1.6). Figure 1.5 illustrate the multi-scale spatial relationship between Variscan granites and Sn-W de-posits from the Variscan belt to the St Austell district. This shared relationship is confirmed by the numerous similarities between the St Austell rare-metal granite and several rare-metal granites worldwide, that will be discuss further in Section 1.2.3.

The St Austell rare-metal granite

Rare-metal granites are so-called because of their mineralisations being dissemi-nated through the granites rather than concentrated in layers, lodes, etc. The St Austell rare-metal granite share numerous characteristics with other granites of SW England, summarised in many reference papers and textbooks (Floyd et al., 1993; Manning and Hill, 1990; Manning et al., 1996). These granites were originally de-scribed by four different granite types, all observed in distinct SW England granites: biotite granite, lithium mica granite, tourmaline granite and topaz granite (Hill and Manning, 1987). Additional granite types were added later based on field and textural observations within the St Austell granite, indicating complex late-stage magmatic and hydrothermal processes (Manning et al., 1996). The St Austell rare-metal granite is composed of six major granite types, each unit being characterised by a specific mineral assemblage and textures (Manning et al., 1996), see Figure 1.6.
The biotite granite (BG) is the main lithology, which represent up to 70% of the outcrop of the St Austell granite, exposed in the Western Area, at the eastern part of the pluton and intermittently within the Central Area. It corresponds to the megacrys-tic biotite granite observed at Land’s End and Dartmoor (Manning and Exley, 1984; Manning et al., 1996). It is coarse grained, mainly composed of quartz, K-feldspar and micas with tourmaline and topaz as minor phases. Biotite is the main mica, but mus-covite is also present. The main accessory phases are rutile, topaz, apatite, monazite, cassiterite, zircon and uraninite (Manning et al., 1996). The main host for the LREE differs depending on the authors. Works of Jefferies (1985) on Carnmenellis biotite granite describe monazite as the main LREE-bearing mineral accounting for approx-imately 75% of the total LREE content. In contrast microprobe analysis on samples from Cornubian granites by Alderton et al. (1980) show significant concentrations (up to 0.5%) of LREE in other phases, particularly apatite, but also zircon and sphene.
The lithium mica granite (LMG) has a similar grain size and texture than the bi-otite granite (Manning et al., 1996). It is characterized by the presence of lithium mica (zinnwaldite) and plagioclase (albite component). The accessory minerals are present as inclusion in zinnwaldite (apatite, monazite, zircon and rutile) or in plagio-clase (apatite, fluorite and secondary micas). Additional alteration products include fine-grained topaz aggregates, and replacements of zinnwaldite by tourmaline (Manning et al., 1996).
The tourmaline granites were subdivided Manning et al. (1996) in a tourmaline granite (TG), it-self regrouping a globular quartz and medium coarse-grained equigran-ular facies, and in a fine-grained tourmaline granite (FGTG) with fine-grained (<0.5 mm) equigranular facies. The globular quartz facies is characterized by a consider-able textural variation. It contains large quartz grains, phenocrysts of K-feldspar, zinnwaldite with a fine grained groundmass of quartz, plagioclase, zinnwaldite, tour-maline and topaz. The accessory phases are limited to apatite and rutile. On the contrary, the fine-grained facies is an equigranular rock with euhedral albite, lithium mica (zinnwaldite). Accessory minerals include monazite, apatite, zircon and rutile with some occurrences of arsenopyrite and cassiterite (Manning et al., 1996).
The topaz granite (TZG) is medium grained, characterized by euhedral-subhedral fluorine-rich topaz, interstitial lithium mica, albite, plagioclase feldspar with perthitic orthoclase, and subhedral quartz (Manning and Hill, 1990). It contains a smaller proportion of accessory phase but a wider variety including sub-economic minerals (Manning et al., 1996). The accessory minerals are apatite, amblygonite, zircon, Nb-Ta oxides (columbite-tantalite and ilmenorutile) and uraninite. According to (Manning and Exley, 1984), the field relations indicates a separate origin of the topaz granite. The mineralogy of the six granites is summarized in Table 1.2.

GEOLOGICAL BACKGROUND

St Austell granite (Bray and Spooner, 1983). Whereas two different types of miner-alisations, a Sn breccia-stockwork in the Treliver area, generally unenriched in base metals, and a E-W veins complex enriched in base metals have been identified north of the St Austell pluton (Camm and Dominy, 1999; Camm and Moon, 2001). In addition, Müller and Halls (2005) describe an intrusive tourmaline breccia in biotite granite at Wheal Remfry where zoned rutile is the principal mineral hosting Sn (up to 1.88%) and also includes high W (up to 1.95%) and Nb (up to 2.05%) domains.

Similarities with granites-based kaolin deposits worldwide

From all the magmatic units of the St Austell pluton, the topaz granite is the youngest, and the most geochemically different, relatively enriched in Li, Rb and with a much higher Nb/Zr ratio (Manning and Hill, 1990). Nonetheless, there are close similarities between the mineralogical assemblages described above and the accessory minerals assemblage of evolved granite elsewhere described as peraluminous high to intermediate phosphorus granites (Linnen and Cuney, 2005). All the St Austell granite types are felsic and weakly peraluminous (1.16>A/CNK>1) granites. However if Li2O is included, the molecular ratio A/CNKL is reduced, to less than 1 in the case of topaz granite (Manning et al., 1996). The comparison of the P2O5=SiO2 ratio of the St Austell granites with other rare-metal granites shows that the St Austell granites belong to the intermediate phosphorous granite (IHP) type (Figure 1.5a). The relatively low Zr and Th contents of St Austell granites, visible in Figure 1.7b are characteristic of non-peralkaline rare-metal granites.
two-micas granites, corresponding to the S-type, rich in hydroxylated minerals (mus-covite) and enriched in lithophile elements such as Nb, Ta, Li, Be, P , F (Jébrak and Marcoux, 2008). Among these granites some display similarities with the St Austell granite and are associated with Sn, W, and Ta-Nb disseminated mineralisations (Černý et al., 2005). Some of this granites are associated to kaolin deposits which are also known to display similarities with Cornish kaolin deposits (Wilson and Jiranek, 1995; Wilson et al., 1997).
According to Scott et al. (1998), the Yichun granite in China is one of the closest rare-metal granite, in terms of lithology. It comprises also several magmatic units in-cluding a biotite granite and a Li-mica (lepidolite) granite such as the St Austell granite. The Li-micas granite is the most evolved magmatic unit (cf. St Austell topaz granite) and display a similar accessory assemblage of amblygonite/montrebasite, apatite, flu-orite, Mn-tantalite, microlite, Ta-cassiterite, ilmenite, monazite, zircon, sphalerite and topaz (Belkasmi et al., 2000; Yin et al., 1995). The Beauvoir topaz-lepidolite albite granite (Massif Central, France) is also very similar to the St Austell topaz granite and display a Sn-Li-Ta-Nb-Be disseminated mineralisation with an accessory minerals assemblage composed of topaz, apatite, amblygonite, columbite-tantalite, microlite, zircon, uraninite and sphalerite (Cuney et al., 1992; Raimbault et al., 1995). Further similarities exist with the Podlesí granite in Czech Republic which is composed of three magmatic units, including biotite granite, and two Li-mica granites. All gran-ite types contain disseminated Ta-Nb-Ti-W-Sn minerals including rutile, cassiterite, Fe-columbite, ixiolite and ferberite (Breiter et al., 2007).
All the above-cited deposits are considered as potential source for Nb, Ta, Sn or W and some of them are actually mined for these metals. For instance the Yichun granite is mined for Ta and Li (Schwartz, 1992) and the Beauvoir granite is known to produce cassiterite (800 g/t Sn) and columbite (190 g/t Ta plus 120 g/t Nb) as by-product of the kaolin production (Pohl, 2011).
By analogy with the Beauvoir granite it is possible to estimate the most prospective Nb-Ta-Sn and REE target. Rare-element chemistry of the Beauvoir granite shows that the concentration of most metallic elements including Nb, Ta, Sn and W is very high and increase with the Li contents from the bottom to the upper part of the granite body (Cuney et al., 1992). The outcrop surface dimensions of the Beauvoir Granite (around 14 k m2) are considerably less than even the smallest occurrence of topaz granite, but it correspond to the top of the cupola, which is the most enriched part of the orebody in terms of rare-metals, due to a combination of magmatic process and interaction with meteoric water (Raimbault et al., 1995). It is therefore the upper parts of the topaz granite that should have the greatest potential for disseminated magmatic mineralization. The exact morphology and the intern organisation of the topaz granite is poorly constrained but Manning and Hill (1990) suggested that the individual out-crops of topaz granite may be connected at depth. The field relations indicate that the Nanpean stock represents a position well below the roof of the granite body and that the Hensbarrow stock represents the roof. Thus, from a pure metallogenic perspective, the Hensbarrow stock, which represents the upper part of the St Austell topaz granite, is the most prospective target zone for Ta, Nb and Sn within the topaz granite.

KAOLIN FROM ST AUSTELL

China clay, also called kaolin, is a commercial clay material mainly composed of kaolinite (Al2Si2O5(OH)4), an hydrated aluminosilicate clay mineral. Its main use is for coating and filling in paper industry (75% of world’s production), but it has a wide variety of uses for plastics, ceramics, ink, paint, rubber, and pharmaceutical industry. The commercial value of a kaolin product is based on its whiteness and its fine particle size. Particle size affects fluidity, strength, plasticity, colour, abrasiveness and ease of dispersion. Other important properties include its flat particle shape, which increases opacity or hiding power, its soft and non-abrasive texture, due to the absence of coarser impurities, and its chemical inertness (Highley et al., 2009).

Table of contents :

1 Introduction 
1.1 Scope of the study
1.1.1 The European Raw Material Initiative
1.1.1.1 Raw materials supply in Europe
1.1.1.2 Defining Critical Raw Materials
1.1.1.3 Applying the methodology : list of critical raw materials
1.1.2 CRMs (LREE, Nb-Ta, W) and Sn consumption
1.1.3 The STOICISM project
1.1.3.1 Project summary
1.1.3.2 Project partners and consortium structures
1.1.3.3 Task 2.6 CRM Recovery
1.2 Geological background
1.2.1 The Cornubian Sn-W province
1.2.2 The St Austell rare-metal granite
1.2.3 Similarities with granites-based kaolin deposits worldwide
1.3 Kaolin from St Austell
1.3.1 St Austell Kaolin deposits
1.3.2 Kaolin extraction
1.3.3 Kaolin dry mining processing
1.3.4 Waste management
1.4 Literature review
1.4.1 CRMs (LREE, Nb-Ta, W) and Sn as by-products
1.4.2 CRM-bearing minerals processing overview
1.4.2.1 Monazite processing
1.4.2.2 Cassiterite processing
1.4.2.3 Columbite-tantalite processing
1.4.2.4 Wolframite processing
1.4.3 Gravity concentration
1.4.3.1 Principle
1.4.3.2 The unit processes of gravity concentration/choice of equipment
1.4.3.3 Gravity processing of fine particles
1.4.4 By-product recovery of CRMs and Sn from kaolin production
1.4.4.1 Beneficiation of Sn as by product of Beauvoir kaolins
1.4.4.2 Previous work at St Austell
1.5 Study objectives
1.5.1 Scientific objectives
1.5.1.1 Process development for CRMs recovery from kaolin residue
1.5.1.2 Evaluate representativeness of process samples
1.5.1.3 Geometallurgy and by-product resource estimation
1.5.2 Industrial challenge
2 Materials and methods 
2.1 Materials sampling and sample preparation
2.1.1 Waste streams sampling for waste selection and characterisation
2.1.1.1 Sampling of waste streams
2.1.1.2 Sub-sampling and sample preparation
2.1.2 Micaceous residue sampling for metallurgical testing and variographic analysis
2.2 Chemical analysis
2.2.1 Inductively Coupled Plasma (ICP) analysis
2.2.2 X-Ray Fluorescence (XRF) analysis
2.3 Material Characterisation
2.3.1 Particle size analysis
2.3.2 Heavy medium separation
2.4 Mineral Characterisation
2.4.1 X-Ray diffraction (XRD) analysis
2.4.2 Zeta potential
2.4.3 Scanning electron microscopy
2.4.4 Electron microprobe analysis
2.5 Mineral processing
2.5.1 Sample pre-treatment
2.5.2 Gravity processing
2.5.2.1 Spiral concentrator
2.5.2.2 Shaking table
2.5.2.3 Falcon concentrator
2.5.3 Jar-tests
2.5.4 Froth flotation
3 Selection and characterisation of the most valuable stream
3.1 Introduction
3.2 Selection and characterisation of the valuable stream from WADM plant
3.3 Comparison with other locations
3.4 Conclusion
4 Sampling representativeness for metallurgical testing 
4.1 Introduction
4.1.1 Theory of Sampling
4.1.2 Classical variographic approach
4.1.3 On the multivariate aspects of heterogeneity
4.1.4 Application of multivariate variograms to process sampling
4.2 Materials and methods
4.2.1 Material sampling
4.2.2 Sample preparation
4.2.3 Chemical analysis
4.2.4 Particle size analysis
4.3 Results
4.3.1 Experimental individual variograms
4.3.2 Variograms on PCA scores
4.3.3 Multivariogram
4.3.3.1 Multivariogram applied to heterogeneity contributions
4.3.3.2 Multivariogram applied to PCA scores
4.4 Discussion
4.5 Conclusion
5 Gravity processing of the selected residue 
5.1 Gravity processing of the micaceous residue
5.1.1 Introduction
5.1.1.1 Gravity processing of low grade ores
5.1.1.2 Response surface method (RSM)
5.1.2 Materials and methods
5.1.2.1 Material
5.1.2.2 Chemical analysis
5.1.2.3 X-Ray Diffraction (XRD)
5.1.2.4 Gravity concentration set-ups
5.1.2.5 Experimental designs
5.1.3 Results
5.1.3.1 Spiral pre-concentration
5.1.3.2 Table testing
5.1.3.3 Overall performance of the tested flowsheet for metal recovery
5.1.4 Conclusion
5.2 Modelling heavy and gangue mineral size recovery curves in spiral concentration
5.2.1 Introduction
5.2.2 Materials and methods
5.2.2.1 Materials
5.2.2.2 Spiral set-up
5.2.2.3 Particle size analysis and modelling
5.2.2.4 Partition curve calculation
5.2.2.5 Design of experiments
5.2.3 Results
5.2.3.1 Size recovery curve modelling
5.2.4 Discussion
5.2.5 Conclusion
6 Processing of fines
6.1 Introduction
6.1.1 Froth flotation of monazite
6.1.2 Falcon UF concentrator
6.1.3 Effect of clay slimes on mineral processing
6.2 Materials and methods
6.2.1 Material
6.2.2 Jar tests
6.2.3 Chemical analysis
6.2.4 Zeta potential
6.2.5 Flotation
6.2.6 Falcon concentrator
6.3 Results
6.3.1 Selection of the dispersing agent
6.3.2 Flotation
6.3.2.1 Comparing flotation performance with different reagents
6.3.2.2 Enhancing flotation performance with dispersion
6.3.3 Falcon UF results
6.3.3.1 Saturation tests
6.3.3.2 Effect of rotation speed
6.4 Discussion
6.5 Conclusion
7 Towards a geometallurgical model 
7.1 Introduction
7.2 Materials and methods
7.2.1 Sampling
7.2.1.1 Sample processing protocol
7.2.1.2 Sub-sampling of core samples for calibration
7.2.2 Pilot-scale gravity concentration testing
7.2.3 Multivariate calibration/PLS regression
7.2.4 Methodology
7.3 Chemical database correction
7.3.1 Metal grade calibration
7.3.2 Multivariate LREE grade calibration
7.3.3 Relationship between some elements and oxides
7.4 Prediction of process performance
7.4.1 Effect of feed grade
7.4.2 Effect of particle size
7.5 Potential application to core sample data
7.6 Conclusion
8 General discussion 
8.1 CRM recovery process proposal
8.2 Evaluating project profitability
8.2.1 Capital costs estimation
8.2.2 Revenue
8.3 On the micaceous residue commercial potential
9 Conclusions and Perspectives
9.1 General Conclusion
9.2 Perspectives

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