There are three extraction methods currently used by Imerys to extract china clay, which are hydraulic mining, cut & carry and dry mining.
This is the historical kaolin mining extraction methods in Cornwall and Devon. This method uses pressurised water system within the pit to provide high pressure (14-20 bar) water at the nozzle of washing hoses called “monitors” (Figure 1.9a). These can be compared to fire-fighters hoses projecting high volumes of water at high pressure. Clay matrix is either blasted or pushed by bulldozers (depending on rock hardness) from the quarry face towards the monitor to facilitate the hydraulic mining process. High pressure monitor water blasts the kaolin from the clay matrix which is then held in suspension and runs in a clay stream, leaving behind the less well decomposed ma-terial in the form of sand and rock, locally called “stent”. The resulting “clay stream” flows into the pit sink (Figure 1.9b) and is then pumped to the in-pit processing loop (Figure 1.9c). Pit processing removes finer sand and residue waste before the clay slurry is pumped to settling tanks to increase in density before it is sent to the re-finer for further processing. This extraction method is not perfect and raises many problems in terms of selective mining and product traceability. Indeed the material in front of the monitor must either be washed or tipped as waste and contamination may occur while the “clay stream” flows into the pit. Therefore a small amount of stained clay present in the clay matrix can contaminate a large amount of high quality clay. This can cause a problem if a pit is operating a small number of monitors, all of which have poor kaolin quality at one time. In addition there is no traceability or the material pumped into the processing route because of the mixing of the clay streams from diﬀerent monitors into the pit sink and the above-mentioned contamination issues.
Cut and carry
Cut and Carry is a modern variation of the traditional hydraulic method. Cut and carry adds flexibility to the kaolin extraction process as the clay matrix is transported from an excavation site and tipped at a monitor for washing. Thus an area in a quarry can be selectively mined for the desired quality of clay and possible sources of contam-ination can be removed. The kaolin and stent waste is processed in the same manner as highlighted in hydraulic mining method (Figure 1.9d).
Dry mining is the most modern kaolin extraction method used by Imerys. This method oﬀers many advantages as it do not use in pit pressurised water system and removes the need for pumping water and clay slurry over large distances as for the historical hydraulic mining method. A centralised processing plant, localised in the Western Area is fed with clay matrix, which is transported from Wheal Remfry and Melbur-Virginia pits to the processing plant by a dedicated clay matrix team consisting of a wheeled loader and fleet of dump trucks. This is the most interesting kaolin extraction method as it avoids contamination issues and allows traceability of the kaolin ore processed in the dry mining plant. About 45% of Imerys’ output is now produced using this method-ology (Highley et al., 2009). In addition Western Area kaolin operations, produce in excess of 600 000 t of kaolin per annum, making it one of the largest mining operations within the Imerys group. The present work focused on this kaolin processing plant.
KAOLIN FROM ST AUSTELL
Kaolin dry mining processing
Designed to process several million tonnes of matrix a year, the Western Area Dry Mining (WADM) plant operates six days a week and is shut down one day a week for routine maintenance (Imerys Ltd, 2008). This centralised processing plant is fed with clay matrix, which is transported directly from the pit by cut and carry operations without any washing (dry mining). Two static grizzlies within the pits are used for primary segregation of large boulders and the segregated material is then delivered to storage bays at the dry mining plant. A wheeled loader selects the desired grade of material from the designated storage bay, and tips this material into the feed hopper. Because there is no washing the material fed into the plant is much coarser and more classification stages a required to remove the coarsest materials.
The first stages of kaolin ore processing consist in removing the coarsest fractions of the clay matrix which are not valuable for kaolin recovery. Thus the WADM plant can be seen as a classification plant producing diﬀerent size classified wastes and a fine kaolin product (Figure 1.10). The first sized-classified wastes i.e. crushed stones (21-11 cm) travels on the top of the grizzly cassettes and fall into a jaw crusher, then the conveyors transport the material to the crushed stone stockpile. Because the material has not been washed yet, a certain amount of clays is loss with this material as it is visible in Figure 1.10a. The underflow from the grizzly screen is fed into a rotary washing barrel where the majority of the kaolin is released from the matrix. The washed material is then separated onto a screen where +8 mm material is washed on the wash screen to remove traces of clay and sand (Figure 1.10b) before it is conveyed onto the gravel (110-8 mm) stockpile. The underflow flows into a sand separation system consisting of two bucket wheels de-sander (Figure 1.10c) and de-watering screens which remove the sand material which is then conveyed to the sand (8-0.5 mm) stockpile. Sand is the single largest waste stream derived from the original matrix and can be generated at plus 300 t/h (Imerys Ltd, 2008). The last classification stage is realised by series of hydrocyclone loops. The finer (-53 m) particles go to the product stream (hydrocyclone overflow, Figure 1.10e) and are pumped to the refiner for further classification and processing whereas the coarser ( 500-53 m) particles are reported to the micaceous residue (hydrocyclone underflow, Figure 1.10d) where they are held in suspension and pumped to a tailing dam.
Additional techniques are used to improve the brightness (whiteness) and particle size of specific grades of clay. These include blending, fine grinding, chemical reductive bleaching and/or the removal of iron-bearing impurities using superconducting mag-nets. Some kaolin products are also calcined at specific temperatures to give diﬀerent products.
Kaolin extraction in Cornwall is a moderate waste-generating process which typ-ically produces for each tonne of kaolin product, 9 tonnes of sized-classified wastes including crushed stones and gravels, both accounting for 4 tonnes, sands and mica-ceous residues accounting for 4 tonnes and 1 tonne respectively (Highley et al., 2009). The annual waste production of the whole industry is about 10 million tonnes (High-ley et al., 2009) which are in part sold as construction material locally for the coarsest fractions but the majority is engineered and disposed in large waste piles. However fine.
wastes such as the micaceous residue are not valorized. These wastes are transported via pipelines for storage under water to prevent wind dispersal in mica lagoons or mica dams. The dams are not homogeneous and comprise layers which contribute to the stability of the structure. In a normal circumstance the backfilled areas have layers of micaceous residue, clay and sand. Most of the tailing dams were fed with material from diﬀerent pits covering distinct granite type. The major tailing dams of the St Austell area are (for location of the tailing dams see Figure 1.6):
• Kernick dam, backfilled by micaceous residue from Western Area biotite granite as well as lithium-micas and tourmaline granite (Scott and Bristow, 2002),
• Dubbers, an active china clay waste disposal currently fed by micaceous residue,
• Maggi Pie dam, an historic dam, now de-watered which was mostly fed by ma-terial from Western Area biotite granite,
• Innis Moor and West Carclaze, backfilled by micaceous residue from historic workings of Carclaze, Goonbarrow and Rocks pits,
• Great Treverbyn, a historic tailing dam backfilled by micaceous residue from Western Area biotite granite located adjacent to Carclaze Pit.
CRMs (LREE, Nb-Ta, W) and Sn as by-products
A by-product is by definition, a secondary or additional product (usually a metal) which is secondarily recovered in an industrial extraction process, in addition to a pri-mary product. This notion is independent of the respective selling prices of the main products and by-products.
REEs are recovered as a co-products/by-products of certain other minerals in many mining operations (Kumari et al., 2015). The only active mine in the western world op-erated exclusively for the recovery of rare earths is the Mountain Pass (United States) deposit, where bastnäsite is the main REE-mineral (Chen, 2011). However there is a possibility of recovering REE as a primary product in at least three deposits in Aus-tralia, especially Mt. Weld and to a lesser extent Yangibana, and John Galt deposits (Chen, 2011; Gupta and Krishnamurthy, 2004; Kingsnorth, 2014a).
Apart from these few examples the commercial mine production of REE is feasible in most cases only as a by-product of some other mineral commodity. For instance in 2009 about 49% of the Total Rare Earths Oxides (TREO) production were ensured by by-products (Table 1.4). This is due to the fact that most of REE occurrences are considered as low grade ores and thus the extraction of REE alone is not economical and cannot support entirely the mining and production costs involved in such an operation (Golev et al., 2014). However, when rare earths are recovered as a by-product, the recovery and sale of rare earths is not a necessary condition for making the recovery and the sale of the main product economically viable (Gupta and Krishnamurthy, 2004).
For instance in placer deposits or beach sands, REE minerals occurs as a minor phase while the major minerals are ilmenite, rutile, zircon, and quartz. In these de-posits, the REE minerals have been invariably recovered as a by-product of ilmenite in most of the cases, cassiterite in southeast Asia, and in some cases as a by-product of placer gold (Gupta and Krishnamurthy, 2004). Other examples are recovery of REE as by-product of uranium extraction at Denison Mines (Canada) or (formerly) as by-product of iron ore mining at Bayan Oba (China) (Gupta and Krishnamurthy, 2004).
Niobium and tantalum mineral concentrates can be produced as by-products of mining operations worldwide, but the majority of the world’s niobium concentrate is produced as primary concentrates by three mines in Araxá and Catalão (Brazil) and Niobec or St. Honoré mine (Canada) whereas the mines in the Congo geographic area (Burundi, Congo, and Rwanda), account for about 53% of world’s tantalum production (US Geological Survey, 2015).
However niobium and tantalum can also be extracted as a by-product of tin smelter waste that arises from the smelting of cassiterite ore concentrates, i.e. tin slags (Fields and Bally, 1967). More than 56% of the western supply of niobium-tantalum came as a by-products from tin smelting from cassiterite in Thailand and Malaysia (Suri et al., 1992). But more recent data reported by the Tantalum-Niobium International Study Center suggest that this proportion tends to be much lower for tantalum, around 20% of total global production (Tantalum-Niobium International Study Center, 2013), see Table 1.5. Proportion of niobium produced in this way is far less significant as it ac-counts for less than 2% of total global niobium production (Shaw and Goodenough, 2011). Some tin smelter waste contains 8 to 10% T a2O5, although exceptionally this may rise to 30% (Shaw and Goodenough, 2011). Low-grade smelter wastes can be upgraded by electrothermic reduction, yielding a synthetic concentrate with up to 50% tantalum and niobium oxides (Roethe, 1989). Tantalum is also recovered as by-product of cassiterite in the middlings of some placer deposits using shaking tables, and mag-netic/electrostatic separation methods (Shaw and Goodenough, 2011; Bose and Gupta, 2002).
There are many occurrences of Sn-W mineralisations, especially in the Variscan belt (see Figure 1.5) such as the Panasqueira, Borralha and Montesinho Sn-W deposits in Spain or the tin deposits of the Cornubian orefield such as Camborne, Redruth, Saint-Just, deposits (Jébrak and Marcoux, 2008). In these deposits tungsten and tin have often been mined as co-/by-products. For instance, there are many deposits in the Cornubian orefield where tungsten has been extracted as a by-product of tin and copper mining (Scrivener et al., 1997). In some mines tin and tungsten are by-product of other metals, including zinc, silver and tantalum but this only represent a small share of world tin and tungsten production.
However one atypical example where CRMs are produced as a by-product, is the Beauvoir-Echassières kaolin deposit (Massif Central, France) which is known to produce cassiterite (800 g/t Sn) and columbite (190 g/t Ta plus 120 g/t Nb) as by-product of kaolin production (Pohl, 2011). This deposit is of primary importance for this work as it displays similarities with the St Austell mineralisations (see Section 1.2.3) and Sn by-product recovery operations at the Beauvoir-Echassières kaolin deposit will be further discussed in Section 18.104.22.168.
CRM-bearing minerals processing overview
The CRM-bearing minerals are numerous for the CRMs of interest, i.e. LREE, Nb-Ta, W and Sn. However in the following section focus will be made on the major potential CRM-bearing minerals occurring within the St Austell area. Table 1.6 lists the major potential CRM-bearing minerals within the St Austell rare-metal granite reported in the literature, and their main physico-chemical properties, including chem-ical formulas, specific gravities (i.e. density), magnetic properties and CRM contents (when data is available). As it can be seen, the overall densities of the potential CRM-bearing minerals are relatively high (>3 g:cm3 ) in comparison with densities of typical gangue minerals from granitic rocks which are usually around 2.6 g:cm3 . This shared property is one of the most fundamental characteristic of the CRM-bearing minerals as it allows their common separation by gravity concentration techniques. However the magnetic properties diﬀer from one mineral to another which gives potential insights to separate these minerals.
The highlighted minerals in Table 1.6, monazite (LREE), cassiterite (Sn), columbite-tantalite (Nb-Ta) and wolframite (W) correspond to the most important CRM-bearing minerals within the St Austell granite described in the literature (Manning and Hill, 1990; Manning et al., 1996; Scott et al., 1998; Shail et al., 2009) and also to the most commonly extracted CRM-bearing minerals (Angadi et al., 2015; Bulatovic, 2010; Jor-dens et al., 2013).
Table of contents :
1.1 Scope of the study
1.1.1 The European Raw Material Initiative
1.1.2 CRMs (LREE, Nb-Ta, W) and Sn consumption
1.1.3 The STOICISM project
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
1.4.3 Gravity concentration
1.4.4 By-product recovery of CRMs and Sn from kaolin production
1.5 Study objectives
1.5.1 Scientific objectives
1.5.2 Industrial challenge
2.1 Materials sampling and sample preparation
2.1.1 Waste streams sampling for waste selection and characterisation
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
2.5.4 Froth flotation
3.2 Selection and characterisation of the valuable stream from WADM plant
3.3 Comparison with other locations
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
4.3.1 Experimental individual variograms
4.3.2 Variograms on PCA scores
5.1 Gravity processing of the micaceous residue
5.1.2 Materials and methods
5.2 Modelling heavy and gangue mineral size recovery curves in spiral concentration
5.2.2 Materials and methods