Are View Of Graphite Benef I Ciati On Techniques

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A Review of Graphite Beneficiation Techniques S. Chehreh Chelgani, M. Rudolph, R. Kratzsch, D. Sandmann & J. Gutzmer To cite this article: S. Chehreh Chelgani, M. Rudolph, R. Kratzsch, D. Sandmann & J. Gutzmer (2016) A Review of Graphite Beneficiation Techniques, Mineral Processing and Extractive Metallurgy Review, 37:1, 58-68, DOI: 10.1080/08827508.2015.1115992 To link to this article: http://dx.doi.org/10.1080/08827508.2015.1115992

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Date: 22 February 2016, At: 02:51

MINERAL PROCESSING AND EXTRACTIVE METALLURGY REVIEW 2016, VOL. 37, NO. 1, 58–68 http://dx.doi.org/10.1080/08827508.2015.1115992

A Review of Graphite Beneficiation Techniques S. Chehreh Chelgania,b,c, M. Rudolpha, R. Kratzscha,d, D. Sandmann,e and J. Gutzmera Helmholtz-Zentrum Dresden – Rossendorf, Helmholtz Institute Freiberg for Resource Technology, Freiberg, Germany; bDepartment of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, USA; cDepartment of Geology and Mining Engineering, Faculty of Engineering, University of Liberia, Monrovia, Liberia; dDepartment of Mechanical Process Engineering and Minerals Processing, TU Bergakademie Freiberg, Freiberg, Germany; eDepartment of Mineralogy, TU Bergakademie Freiberg, Freiberg, Germany

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a

ABSTRACT

KEYWORDS

Graphite as the most common polymorph of naturally occurring crystalline carbon is required for many different applications such as batteries, refractories, electrical products, and pencils. Graphite resources are currently being subjected to intensive exploration to help meet rapidly growing global demand – and graphite has made it onto the list of critical raw materials as issued by the European Union. Graphite ore is mostly beneficiated using flotation separation techniques. The increasing demand for high-grade graphite products with up to 99.99% carbon has resulted in the development of various approaches to remove impurities even to parts per million range. This paper considers separation and purification techniques that are currently employed for graphite mineral beneficiation, and identifies areas in need of further research.

Graphite; liberation; flotation; leaching; roasting; microwave

1. Introduction Graphite is the most common form of naturally occurring polymorphs of crystalline carbon – another one being diamond. Natural graphite resources are classified based on a multitude of properties, including not only grade but also crystallinity as well as grain (flake) size and shape. According to these attributes, graphite ores are classified to crystalline lump (vein), flake, and microcrystalline (Graffin 1982; Sutphin and Bliss 1990; Zheng et al. 1996; Li, Zhu, and Wang 2013). In addition, graphite resources occur in several geological environments that are used to distinguish different deposit types (Cameron and Weis 1960; Graffin 1982; Mitchell 1993; Kwiecińska and Petersen 2004). Graphite is soft, greasy to the touch, and soils fingers and paper (due to small van der Waals energy between sheets of carbon; Schabel and Martins 1992). It has many unique physical and chemical properties such as refractoriness, high-heat and electrical conductivity, greasiness, high-thermal resistance, inertness, and readily soluble in iron. Graphite has both metallic and nonmetallic properties. These unique properties give rise to its major uses in welding rods, desulfurizing agents, facings, lubrication, refractories, marking instruments, electrical products, batteries, brake linings, bearings, conductive coatings, crucibles, electrodes, and paints (Figure 1) (Wakamatsu and Numata 1991; Andrews 1992; Mitchell 1993; Didolkar et al. 1997; Crossley 2000; Kalyoncu and Taylor 2000; Chang 2002; Dumont 2005; Gredelj et al. 2009; Kaya and Canbazoğlu 2009). The formation of graphite is a result of contact or regional metamorphism of sedimentary organic matter (e.g. organic

CONTACT S. Chehreh Chelgani [email protected] Halsbrücker Straße 34, Freiberg 09599, Germany. © 2016 Taylor & Francis

material is transformed into graphite in response to increasing temperature and/or pressure in the Earth’s crust) (Mitchell 1993; Kwiecińska and Petersen 2004). With the gradual increase of metamorphism, sedimentary carbonaceous material transforms first into an amorphous form of carbon (Landis 1971; Wakamatsu and Numata 1991). For certain industrial applications (see above) this amorphous carbon may substitute for crystalline graphite. Amorphous carbon resources are hosted by moderately metamorphosed marine sedimentary rocks, such as quartzites or phyllites, but also represent former coal beds. The “grain” size of this amorphous graphite is typically between +40–70 µm in diameter. Natural graphite may occur in metamorphosed siliceous rocks (typically quartzite) as well as metamorphosed carbonate rocks. Associated minerals with graphite occur as quartz, rock-forming silicates such as amphiboles and feldspars, also calcite, sulfides, or magnetite (Mitchell 1993; Bulatovic 2014). Flake graphite is commercially classified into coarse (+150– 850 µm in diameter) and fine flakes (+45–150 µm in diameter: which may be furthermore subdivided into +100–150 µm, +75–100 µm, and –75 µm; Mitchell 1993). Partial volatilization and subsequent recrystallization during regional metamorphism are assumed to be responsible for the formation of graphite in veins that crosscut meta-sedimentary host rocks (gneiss, schist, quartzite, and marble). Vein graphite is highly crystalline. Generally, the quality of graphite from these deposits is dependent on the characteristics of the original carbonaceous sediments and on the extent of metamorphic graphitization (Mitchell 1993; Gredelj et al. 2009; Bulatovic 2014).

Helmholtz-Zentrum Dresden – Rossendorf, Helmholtz Institute Freiberg for Resource Technology,

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Figure 1. Common applications of graphite (Moores 2012; DNI Metals Inc. 2014).

According to the variety of applications, graphite demand has markedly increased in the last decade. Graphite is one of the main resources of graphene. Graphene obtained from high-grade graphite usually has good quality and is highly conductive (Jagiello et al. 2014). Global consumption of natural graphite has increased from ~600,000 tons in 2000 to 1.6 M tons in 2015 (Cowie 2012; Graphite One Resources 2015). Around 45% of all graphite demand generated between 2013 and 2018 will be attributable to China alone, the world’s leading consumer of these products (Salwan 2015). Graphite is also fundamental to Europe’s economy, and one of the 21 critical raw materials of the EU (European Commision and Ad-Hoc Working Group on Defining Critical Raw Materials 2014). China is the top producer of graphite in the world (Figure 2). Flake graphite makes up 49% of the total graphite products (mainly extracted in China, Brazil, India, North Korea, and Canada). Amorphous graphite production is 50% of the world graphite output (the biggest producers are China

Figure 2. World’s leading graphite producing countries in 2013 (Olson 2015).

59

and Mexico). Sri Lanka is the only excellent producer of vein graphite with less than 1% of the total production of graphite (Moores 2012; Asbury Carbons 2013; Shaw 2013; Bulatovic 2014; DNI Metals Inc., 2014; Graphite One Resources 2015; Salwan 2015). The main properties to determine the graphite product price are flake size, size distribution, and grade (remaining level of impurities in the final graphite product) (Mitchell 1993; Gredelj et al. 2009). Graphite products with particle sizes coarser than 200 μm and with at least 98 wt% or higher carbon content have the best quality for industry (Bulatovic 2014). As the demand for high-purity natural and synthetic graphite has increased (forecast to rise by 10–12% per year), the development of various techniques to produce high-grade graphite has boosted (Shaw 2013). Graphite is naturally hydrophobic, and the most used technique for graphite beneficiation is froth flotation (Arbiter 1985; Crossley 2000). Generally, graphite production from ores is achieved by a combination of careful crushing, milling, screening, tabling, flotation, magnetic separation, electrostatic separation, and leaching (Jean 1927; Andrews 1992; Kaya and Canbazoğlu 2009; SGS Minerals Services 2012). This paper explores the beneficiation of natural graphite with regard to the processing methods. It demonstrates how these techniques are applied to increase graphite value (grade): comminution (with focus on various types of impurities and liberation, effect of scrubbing attrition), flotation (various reagents, conventional and column flotation), leaching (chemical leaching, roasting and leaching, and microwave), and other methods (such as gravity separation) for graphite beneficiation.

2. Comminution As the size and grade of graphite products are important parameters in their commercial considerations, it is in the best interest to maximize the amount of large particles (flakes), and minimize any processing (crushing–milling) that will reduce flake sizes (flakes in the size range of 250 µm to 1 mm will demand the highest prices) (Mitchell 1993; Asbury Carbons 2013; Bulatovic 2014). Liberated graphite is naturally hydrophobic and floatable, and it is well understood that to increase the recovery and grade, liberation has the critical effect (Subramanian and Laskowski 1993; Bulatovic 2014). These facts demonstrate the essential role to study liberation characteristics and impurities of graphite. Graphite has a layer structure with pronounced cleavage parallel to the layers. Properties parallel to the layers are very different from those normal to the layers. Within the monoatomic layers, carbon atoms share strong covalent bond but the layers are held together by very weak bonds (van der Waals bonds) only. These layers are easily disrupted mechanically, giving graphite its softness (Putnis 1992; Pierson 1993). In the graphite structure, gangue minerals are deposited between layers, stacks, or clusters. Such impurities can be associated with the graphite flake in two ways: mechanically attached to the surface of flakes, or trapped between adjacent flakes, which can be called “intercalated” (Kim et al. 2003; Asbury Carbons 2013). Based on scanning electron microscopy

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S. C. CHELGANI ET AL.

(SEM) studies, Kim et al. (2003) distinguished four types (type I–IV) of solid inclusions in graphite particles. For type I, a particular impurity is captured between spheroidal graphite clusters. In type II, a small impurity is lying between clusters, but it has a micro-textured structure, which is strongly bound to graphite surfaces. Micro-textured solids showing a strong adhesion and tangle onto cluster surfaces are classified as type III. Type IV shows marked flat solids being sandwiched between the stacks within a graphite cluster. The lowered melting temperature of impurities during the formation of graphite is regarded as the main reason of their micro-textured structure (Kim et al. 2003). Careful assessment of liberation and distribution of remaining impurities during beneficiation of graphite ore is needed to avoid overgrinding and to maximize flake size, product grade, and recovery. Therefore, graphite beneficiation requires a combination of careful grinding and screening to recover coarse graphite particles. Comminution flow sheets depend highly on the type of ore to be treated, and liberation characteristics can be rather complex and variable. During milling with balls (ball mill), graphite particles are influenced mainly by the type of impulsive stress, which can be either of compression or shear type (Chen et al. 1999; Ong and Yang 2000; Janot and Guérard 2002). Through processing by planetary ball mills, where the shear component of the applied stress is dominant, highly anisotropic particles are obtained, while amorphous carbon often results from milling with highintensity compression (Antisari et al. 2006) (through ball milling in water, shearing force created by the grinding media does not effectively transfer to the graphite particle surface; Kim et al. 2002). The main problem in this procedure is producing very fine graphite particles. To overcome this problem, a slow speed tumbling mill equipped with flint pebbles can be used (Bulatovic 2014). In addition, jet milling of graphite can reduce the often large particle sizes by highintensity impact stress while retaining the flake anisotropic particle shape (Herstedt et al. 2001). Moreover, for the last cleaning step when impurities are attached to the graphite particles and presented in graphite concentrate, attrition mills have been used in some plants (Bulatovic 2014). In an attrition mill the graphite particles are liberated by selectively shearing off the gangue, not stressing the graphite grains, and thus maintaining the crystallinity of graphite (Kim et al. 2002). It was reported that surface cleaning with attrition scrubbing could also be an effective way to improve the selective separation of fine graphite (Grondin and St-Hilaire 1996; Lu and Forssberg 2001a, 2002; Kaya and Canbazoğlu 2009). The scrubbing process might also be applied before flotation (Kaya and Canbazoğlu 2007) or after rougher flotation in a wet medium (Grondin and St-Hilaire 1996; Lu and Forssberg 2001a, 2002; Kim et al. 2003; Aslan et al. 2008). During attrition, fine graphite is cleaned by rubbing of individual particles against each other and/or an attrition medium (Grondin and St-Hilaire 1996; Lu and Forssberg 2001a). To have the suitable effect (improving the flotation selectivity), the scrubbing time should be maintained at a moderate level (45 min) (Lu and Forssberg 2001a; Kim et al. 2003). The scrubbing process is mostly considered an effective

way to remove clay minerals, or impurity types I to III (see above) as these can be removed from the expanded particles by mechanical impact (Grondin and St-Hilaire 1996; Kim et al. 2003; Kaya and Canbazoğlu 2007). Scrubbing of the rougher flotation feed has been found to have a less positive effect on removing impurities of graphite in comparison to the scrubbing of rougher concentrate (feed of cleaners) (Lu and Forssberg 2001a; Kim et al. 2003; Kaya and Canbazoğlu 2007; Aslan et al. 2008). Experiments indicated that attrition scrubbing can increase the grade of final leaching–roasting product by 7–9% (Lu and Forssberg 2001a; Kim et al. 2003). To estimate liberation, various techniques, such as grain counting or light microscopic examination, X-ray intensity size (XRD analysis), the mesh of grind, QUEMSCAN technique, and SEM, can be used. Based on established methods for assessing liberation, light microscopic studies commonly can reveal the first stage of liberation of graphite mineral grains. Estimations of the liberation by different methods indicate the approximate size with various accuracies (under 120–150 µm) (Klein 1992; Mitchell 1993; Falutsu 1994; Patil et al. 1997; Kaya and Canbazoğlu 2009; Stinton et al. 2013). Mineral liberation analysis (MLA) by automated SEM-based image analysis can be used effectively to characterize graphite raw materials, and can provide information relevant to understand the success of graphite beneficiation. In addition, results of this technique can provide access to valuable data on graphite and impurities, grain size distribution, and composition of mineral association, which cannot be obtained by other analytical tools currently available (Sandmann et al. 2014). Various investigations indicated that the beneficiation of graphite by different separation techniques (such as flotation) is strongly related to the liberation information (Jean 1927; Mitchell 1993; Acharya et al. 1996; Didolkar et al. 1997; Patil et al. 1997; Kaya and Canbazoğlu 2009; Asbury Carbons 2013; Spahr et al. 2013; Bulatovic 2014).

3. Flotation Pure graphite is naturally hydrophobic on the cleavage planes due to the low surface energy. It is thus not surprising that graphite was the first ore mineral to be concentrated by flotation (Bessel Bros., 1877; Mitchell 1993). A German patent by Bessel brothers was issued in Berlin, and presented a first true flotation process for the concentration of graphite. In 1877, this patent outlined a process for the flotation of graphite from ores using 1 to 10% of oil and listing more than 16 sources for the oil. The Bessel Brothers produced a concentrate containing more than 90% graphite. To reduce costs, in 1886, the Bessel Brothers patented another gas-generating method for the process by adding acid with carbonates (Bessel Bros., 1886). Contact angle (hydrophobicity factor) on the surface of graphite is relatively large, although it depends on changes in pH (77° and 88° in the pH range of 2–9) (Arbiter et al. 1975; Solaris et al. 1986; Grabowski and Drzymała 2008; Li et al. 2014). The iso-electric point for graphite particles was reported at pH between 2.2 and 4.5. The presence of impurities (gangue minerals and substitution of other elements into the lattice of graphite) can be the main reason for variation of

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iso-electric point of graphite (Wark and Cox 1935; Wakamatsu and Numata 1991; Kim et al. 1995; Patil et al. 2000; Miettinen 2007; Tran et al. 2010). In spite of graphite’s natural floatability, the separation from gangue minerals (feldspar, quartz, mica, and carbonate) is normally improved by the addition of surfactants and depressants (Raghavan et al. 1992; Mangaiah et al. 2005; Gredelj et al. 2009; Vasumathi et al. 2015). In graphite flotation, frothers play an essential role. Several studies indicate that methyl isobutyl carbinol (MIBC) is the most efficient frother in graphite flotation (Fentaw et al. 2000; SGS Minerals Services, 2012; Veras et al. 2014). In comparison with the other most widely used types of frothers, such as pine oil and polyglycolic ethers, MIBC can obtain higher surface tension lowering and the smallest bubble sizes, and as a result is the most efficient frother in preventing critical coalescence concentration (CCC) (Baltar 2010; Veras et al. 2014). However, it was demonstrated that polyoxypropylene glycol butyl ether could be a more effective frother than MIBC for graphite particles in laboratory and on plant scale (Pugh 2000). In graphite flotation, non-ionic hydrocarbons such as kerosene, fuel oil, paraffin, and diesel oil are generally used as collectors. Kerosene was observed in several studies to be the best collector for graphite flotation (Mitchell 1993; Deo and Rao 1995; Didolkar et al. 1997; Kaya and Canbazoğlu 2007; SGS Minerals Services 2012; Bulatovic 2014). Also, a new ecofriendly single reagent “Sokem 705C” (ether-alcohol), which is

61

superior to an oil base system, can be considered as an economical reagent for graphite flotation (Vasumathi et al. 2013). Sodium silicate, quebracho, gelatin, tannic acid, and starch may be used as depressants. Studies have indicated that sodium silicate is the most effective depressant – especially for silicates. With increasing its concentration through the flotation process, the total carbon content (CT) of graphite did not change significantly (Park and Dodd 1994; Didolkar et al. 1997; Kaya and Canbazoğlu 2007; Ravichandran et al. 2012). Moreover, sodium cyanide was considered as a pyritedepressant (Gandrud et al. 1934). By adjusting the pH, selectivity can increase through flotation and optimum pH was reported between 7 and 10 (Arbiter 1985; Crozier 1992; Didolkar et al. 1997; Kaya and Canbazoğlu 2007; Ravichandran et al. 2012). Sodium carbonate and lime are common pH regulators in graphite flotation (Salgado 2001; Ravichandran et al. 2012). Generally, graphite flotation is carried out by combining a simple rougher flotation step with several stages of cleaning (Acharya et al. 1996; Tarnekar and Ravindranath 1997; Abd El-Rahiem 2004; Charbonneau and Lauzier 2014). Conventional and column flotation have been used to float graphite particles (Figure 3). Experimental studies indicate that column flotation can give superior performance compared with conventional cell flotation. This is not only because the number of stages of operation can be reduced, but also by improving grade and recovery (Table 1). By

Figure 3. A simplified flow sheet of graphite processing.

Table 1. Comparison between conventional and column cell in flotation of graphite. [spanname=“2to4”]Cell Sample Chotanagarpur –144 µm (India) Tumudibandha (India) Ganjaudar (India) Rajunagfena (India) Um Qureia –45 µm (Egypt) Um Qureia +45 µm (Egypt) Orissa -210µm (India)

[spanname=“5to7”]Column

FC% of Con.

Recovery (%)

No. of stages

FC% of Con.

Recovery (%)

No. of stages

Reference

88.3 88.4 88.4 75.1 79.6 45.2 79.0

95.6 99.1 98.7 95.0 86.6 89.2 80.1

5 4 4 4 4 4 5

88.0 87.5 88.0 80.7 78.6 46.1 80.1

86.0 99.0 96.0 80.1 98.3 89.3 80.1

2 1 1 2 2 2 2

Misra 2003 Misra 2003 Misra 2003 Misra 2003 Abd El-Rahiem 2004 Abd El-Rahiem 2004 Acharya et al. 1996

Note: FC% of Con = % Fixed carbon of concentrate.

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S. C. CHELGANI ET AL.

adopting flotation column in the cleaning circuit, the capacity of the graphite beneficiation plants has been increased, the processing cost of the product decreased, and the process control has reportedly been easier (Andrews 1992; Acharya et al. 1996; Didolkar et al. 1997; Didolkar et al. 2000; Biswal et al. 2002; Misra 2003; Abd El-Rahiem 2004; Lyons et al. 2014). Fine graphite can be upgraded to about 95% carbon by flotation, but further upgrading by physical methods is challenging. After flotation, the main impurities in the concentrate are minute silicate mineral grains and chemical compounds of S, K, Na, Ca, Mg, Fe, and Al disseminated in graphite scales.

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4.1. Acid leaching Chemical purification by means of leaching is the most common technique to produce high-purity graphite after flotation (Goldberger, Carney, and Reed 1992; Sun and Cheng 1995; Watanabe et al. 1995; Lu and Forssberg 2002; Kim et al. 2003; Kaya 2006; Asbury Carbons 2013). Based on the impurities remaining, different acids, such as HCl, HF, H2SO4, and HNO3, or a mixture of these, can be used (Table 2) (Canbazoğlu 1981; Rivera-Utrilla et al. 1996; Bolat, Sağlam, and Pişkin 1998; Linares-Solano et al. 2000; Sarici-Özdemir, Önal, and Akmil-Başar 2006). Various investigations have shown that although finely powdered graphite was attacked by these acids, only a very limited amount of carbon was released into the leach solution. Bolat et al. (1998) reported that unlike leaching with HCl, HNO3, and H2SO4, treatment with HF simultaneously dissolved impurities and organic parts and caused a high weight loss. Reaction of impurities in samples with various acids can take place as follows (Woodruff and Pausch 1921; Ferris 1967; Letowski 1994; Patnaik, Patil, and Bhima Rao 1997; Takahashi et al. 2001; Bhima Rao and Patnaik 2004; Kaya and Canbazoğlu 2009): FeS2 þ 2Hþ ! Fe2þ þ H2 S þ

FeS þ HNO3 þ 3H ! Fe

3þ

(1)

þ NO þ 2H2 O þ S

(2)

þ FeS2 þ 5HNO3 ! Fe3þ þ 2SO2 4 þ H þ 2H2 O þ 5NO CaSO4  2H2 O þ 2HCl ! CaCl2 þ H2 SO4 þ H2 O CaCO3 þ 2HCl ! CaCl2 þ CO2 þ H2 O 4HF þ SiO2 ! SiF4 þ 2H2 O SiF4 þ 2HF ! H2 SiF6 SiO2 þ 4HF ! 2H2 O þ SiF4 Al2 O3 2SiO2 H2 O þ 14HF ! 2AlF3 þ 2SiF4 þ 9H2 O CaF2 þ H2SO4 ! 2HF þ CaSO4 Al2 Si2 O5 ðOHÞ4 þ 6Hþ ! 2Alþ3 þ 2H4 SiO4 þ H2 O:

(3) (4) (5) (6) (7) (8) (9) (10) (11)

Leaching of graphite as a pretreatment prior to flotation has been reported before (Subramanian and Laskowski 1993; Patnaik et al. 1997), as well as combined with rougher flotation (Letowski 1994). Leaching reduces the content of impurities and increases graphite hydrophobicity. Flotation test results on leached and un-leached samples indicated that the grade and recoveries of leached samples improved measurably (Subramanian and Laskowski 1993; Patnaik et al. 1997). Leaching in froth can be carried out separating the nonfloatable fraction of particles from the floatable fraction (Letowski 1994). Acid leaching mostly is an effective way to remove part of silicate impurities, while the products usually contain too much sulfidic sulfur (Letowski 1994; Ge et al. 2010). Examinations indicated that H2SO4 and HCl are not as effective as HF in leaching clay minerals. However, HF does not remove pyrite, provides insoluble fluoride compounds (CaF2), and has environmental problems (Bhima Rao and Patnaik 2004; Kaya and Canbazoğlu 2009). Thus, an additional purification stage would be required to diminish sulfidic sulfur and rest of impurities before the leaching process.

4.2. Roasting Roasting is an effective method to eliminate both silicates and sulfides from graphite concentrates (Letowski 1994; Lu and Forssberg 2001b; Ge et al. 2010). A roasting process comprises roasting, water washing, and then acid leaching (Lu and Forssberg 2002; Ge et al. 2010; Charbonneau and Lauzier 2014). Direct roasting only at high temperatures (over 500

Table 2. Graphite leaching tests through various conditions. Sample Inebolu (Turkey)

Size (µm) 150

Acids HCl+HF

Akdagmadeni (Turkey)

150

Coraklidere (Turkey)

4

Temperature (° C) 85

Feed FC % 23.5

Product FC % 40.1

HCl+HF

4

85

37.9

60.5

150

HCl+HF

4

85

42.3

59.5

Birnin Gwari (Nigeria) Tamil Nadu (India) Liaoning (China)

75 100 200

1 20 min 50 min

120 40 100

90.0 11.0 94.2

98.0 13.2 99.5

Kangwon (South Korea) Neimeng (China) Natural graphite (Brazil) Bogala (Sri Lanka)

200

H2SO4 HCl H2SO4 +HNO3 H2SO4 +HNO3 HCl+HF NHxFy +H2SO4 HCl

Reference Kaya and Canbazoğlu 2009 Calcite, feldspar, clay, Kaya and Canbazoğlu quartz 2009 Gypsum, pyrite, clay, quartz Kaya and Canbazoğlu 2009 Apatite, pyrite, clay Nwoke et al. 1997 Calcite, quartz Patnaik et al. 1997 Quartz, clay Kim et al. 2003

50 min

100

92.9

98.8

Quartz, clay

Kim et al. 2003

4.5 4

100 90

88.9 98.5

99.9 99.9

Quartz, pyrite, clay Fe, Al

Ma et al. 1996 Zaghib et al. 2003

1.15

65

98.5

99.4

Fe, Cu, Mg, Ca

Amaraweera et al. 2013

100 20 53

Time (h)

Impurities Pyrite, clay, quartz

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produced by the caustic roasting–leaching method (Sun and Cheng 1995; Kaya 2006). Although this process has many advantages (lower amount of acid consumption compared with leaching only and mostly carried out at moderate temperatures (below 500°C); Bhima Rao and Patnaik 2004), during this process large amount of caustic soda is consumed. Based on literature reports, treatment of 1-ton graphite flotation concentrate needs 450–500-kg NaOH and it entails numerous steps of digestion, washing, and drying of graphite (Goldberger et al. 1992; Niu et al. 2007). This huge amount of alkalis leads to environmental issues (especially in wastewater; Niu et al. 2007). Therefore, it would be necessary to study other alternative techniques that have the same effect on removing impurities.

to 900°C) would be effective to remove sulfides (thermal oxidation to SO2) (Sun and Cheng 1995; Liu et al. 2000), which is costly and restricted in many countries by environmental legislation (Letowski 1994). However, if the material was treated with an alkali reagent (caustic or alkali roasting), it has been reported that the process would be effective even at roasting temperatures much lower than 500°C (Lu and Forssberg 2002; Charbonneau and Lauzier 2014; Zunairoh and Santy 2014). Due to the insolubility of calcium silicate, calcium compounds are not acceptable alkali additives, and sodium hydroxide (NaOH) has been preferred in the early application of the process (Dorenfeld 1957). Under high temperature (mostly below 500°C), impurities react with NaOH, and a part of them dissolve in water. Silicates, for example, form water-soluble alkali silicates, which then can be eliminated by a simple water-leach (Sun and Cheng 1995). The solubility of minerals in NaOH takes place in the following reaction processes (Lu and Forssberg 2002; Bhima Rao and Patnaik 2004): FeS þ 2NaOH þ 2O2 ! FeO þ Na2 SO4 þ H2 O SiO2 þ 2NaOH ! Na2 SiO3 þ H2 O Al2 O3 þ 2NaOH ! 2NaAlO2 þ H2 O Fe2 O3 þ NaOH ! FeðOHÞ3 þ NaOH þ H2 O P2 O5 þ 6NaOH ! 2Na3 PO4 þ 3H2 O TiO2 þ 2NaOH ! Na2 TiO3 þ H2 O V2 O5 þ 6NaOH ! 2Na3 VO4 þ 3H2 O:

4.3. Microwave treatment As mentioned above, the commonly applied caustic roasting– acid leaching technology is not only costly but may also cause environmental pollution (Li, Zhu, and Wang 2012). Therefore, production of high-grade graphite by alternative technologies that are both energy-efficient and environmental benign is in great demand. In recent years, there has been a growing interest of microwave heating in mineral beneficiation. Microwave irradiation has many advantages, as it allows for rapid and selective heating, fast switch on and off, flexible modular design, and high-energy efficiency while also being environmentally benign (Matthes, Farrell, and Mackie 1983; Vasilakos and Magalhaes 1984; Xia and Pickles 1997; Li et al. 2008; Chandrasekaran, Basak, and Srinivasan 2013). The organic component of graphite is a relatively weak absorber of microwave energy (Menéndez et al. 2010, 2011), whereas some minerals (such as pyrite) and water in their structures readily heat within an applied electric field. Other minerals appear transparent to the radiation such as quartz (Chen et al. 1984; McGill, Walkiewicz, and Smyres 1988; Walkiewicz, Kazonich, and McGill 1988). By applying various microwave energies, pyrite may be selectively heated and decomposed as pyrrhotite or iron sulfate (Weng 1993). Furthermore, in response to the effects of microwave irradiation, the bonds of sulfur–carbon in organic sulfur compounds are broken and the sulfur is released in gaseous form (Zavitsanos and Bleiler 1978; Chehreh Chelgani and Jorjani 2011). The differences in dielectric properties of the mineral matter content during microwave irradiation increase the amount of defects and cracks in the graphite structure. These cracks will increase the accessible area in the graphite structure for

(12) (13) (14) (15) (16) (17) (18)

Other products such as hydroxides of Fe, Al can be neutralized by HCl /H2SO4 in the consequent acid-leaching process to form chlorides and dissolve in water. This dissolution process removes the surface coating of graphite on impurities as well (Sun and Cheng 1995; Bhima Rao and Patnaik 2004). Various reactions with HCl can take place as follows (Bhima Rao and Patnaik 2004; Niu et al. 2007; Ge et al. 2010): Na2 SiO3 þ 2HCl ! H2SiO3 # þ2NaCl NaAlðOHÞ4 þ HCl ! AlðOHÞ3 # þNaCl þ H2 O CaO þ 2HCl ! CaCl2 þ H2 O MgO þ 2HCl ! MgCl2 þ H2 O

(19) (20) (21) (22)

Fe3þ þ 3HCl ! FeCl3 þ 3Hþ MnO þ 2HCl ! MnCl2 þ H2 O:

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(23) (24)

Roasting–leaching of flotation products is widely used as an industrial technique to refine fine graphite (Table 3). With this technique, the sulfur content at the final product can be reduced to below 0.05 wt% (Lu and Forssberg 2002). At present in China, most high-grade graphite products are Table 3. Various conditions of NaOH caustic roasting. Sample Shandong (China) Hubei Jinchang (China) India Township (Canada) Sukolilo (Indonesia) Woxna (Sweden)

Size (µm) +75– 150 N/A

Roasting temperature (° C) 500

Roasting time (h) 1

1000

20min

N/A +48

500 400

–150 –80

300 250

Acid HCl

Leaching temperature (° Leaching time C) (h) N/A N/A

References Sun and Cheng 1995

HCl

50

30 min

Ge et al. 2010

3 1

HCl H2SO4

20 80

2 30 min

3 1

H2SO4 H2SO4

20 20

N/A N/A

Bhima Rao and Patnaik 2004 Charbonneau and Lauzier 2014 Zunairoh and Santy 2014 Lu and Forssberg 2002

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leach liquors and decrease its strength (Li et al. 2008; Chandrasekaran et al. 2013; Mesroghli et al. 2015). Leaching experiments with HCl and HNO3 and microwave radiation lead to increasing grade of graphite from 95 to 99.43%. SEM and XRD analysis of the products indicated that the morphology (shape) remained unchanged during this process. This is of particular importance to many applications of graphite (see, for example, Zaghib et al. 2003; Shui et al. 2006; Li et al. 2012). These results demonstrate that microwave irradiation of graphite can be considered as an effective pretreatment prior to chemical leaching or comminution of graphite.

5. Other separation technologies

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5.1. Gravity separation The potential capability of gravity separation methods to separate impurities from graphite has been examined in some studies (Laverty, Nicks, and Walters 1994; Tarnekar and Ravindranath 1997; Li et al. 2013; Rugless 2014). For example, a graphite pre-float sample (+50–74 µm) has been subjected to the gravity concentrator of Bartles–Mozley, resulting in the recovery of 97% of liberated graphite (Tarnekar and Ravindranath 1997). Based on these results, a series of tests with hydraulic classification (Whirlsizer classifiers) on graphite samples (–150 µm) have been conducted, and demonstrated that the concentrate derived from a twostage classification process required 34% less acid during acid leaching to remove remaining impurities in comparison with a concentrate without classification (Laverty et al. 1994). Dense media separation (DMS) is the most efficient industrial gravity-based separator used for worldwide coal cleaning (Feng et al. 2003). Generally, the graphite with less gangue minerals but higher carbon content has lower density, which is easier to float in DMS. DMS results using ZnCl2 solutions indicated that microcrystalline graphite (+0.5–25 mm) can be upgraded and provides a flotation product with 92% recovery (Li et al. 2013). These results demonstrate the importance of more studies in this area, and lead to the conclusion that a great deal of research is still required in the pretreatment of graphite prior to leaching.

approximately twice the size of the dispersed graphite particles. About 60% of very fine graphite particles can be recovered at the water surface. The study demonstrates that it is possible to achieve good selective ultra-fine graphite separation in mixtures using the film flotation approach for both batch and continuous process systems and even at a larger scale (Tran et al. 2010). 5.3. Two-liquid separation Yet another technology, two-liquid separation methods (twoliquid flotation or particle extraction) has been considered for fine graphite purification (–10 µm). This technology is developed to extract fine particles based on utilizing difference in surface wettability of minerals in water and oil (Glogner 1903; Hayashi 1979; Bulatovic 2014). The thermodynamic basis of the two-liquid separation of fine particles is the same as that encountered in the flushing of pigments (this method in some investigations is called the “flushing method”; Lai and Fuerstenau 1968; Shergold 1982; Hayashi et al. 2000a). Based on the principle of flushing process, it is expected that fine graphite is purified, leaving impurities in the aqueous phase, and graphite particles are transferred from the water suspension into the oil phase or at the oil–water interface (Hayashi et al. 2000a, 2000b). After separation, the graphite within the oil phase is collected and allowed to agglomerate by adding acetone. Reduction of particle size (increasing liberation) will improve the efficiency of this technique. Experimental application of this technique on very fine graphite particles (–2 µm) showed that impurity concentration is reduced from 8 to 2 wt% (Hayashi et al. 2000a). This effect can improve using a commercial oscillating two-liquid continuous separator. Ash content of samples can be reduced from 16 to less than 1.8 wt % (around 90% impurities removed). With this method, after the first step, graphite particles in the oil phase are returned to an aqueous dispersion with the addition of a non-ionic surfactant and sodium carbonate (Hayashi et al. 2000b). The excellent separation performance of two-liquid separation of fine graphite indicates this opportunity to upscale this procedure to process ultrafine graphite particles.

6. Summary 5.2. Film flotation In industry, there is some demand for fine graphite flakes (– 25 µm). However, processing ultra-fine graphite is complicated (optimum particle size for froth flotation lies between +25 and 75 µm). They will coat other minerals, which may be recovered with the graphite concentrate reducing the grade of the product (Mitchell 1993). Recent investigations show that it is possible from a mixture of minerals to separate very fine graphite (–10 µm) selectively by film flotation (Mitchell 1993; Tran et al. 2010). In this method, the graphite particles appear to cluster at the air–water interface. As droplets of the particle suspension (graphite–methanol–water) are continually introduced, the water surface becomes saturated with fine graphite particles forming a film where some areas appear multilayered. As the film breaks on the water surface, cracks are formed. Results show the size of the aggregates to be

Demand of graphite (as a source of graphene) is constantly increasing. This fact has recently motivated the exploration of new graphite deposits. As these graphite deposits are typically of lower grade than those currently in production, there is a strong need for adopting new methods to retain graphite product quality while containing beneficiation process costs. Size of graphite particles as an important commercial parameter is indicating the essential role of comminution. Graphite is an exceptionally soft mineral; therefore, determination of optimum stress intensity ranges for the most efficient grinding process is important, and the techniques which are able to accurately provide information about the structure of impurities in the flakes, estimated size of liberation, and also size distribution would have an essential effect on the whole process design. In this regard, well-established automated mineralogy, e.g. MLA analysis, can provide valuable

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information and effectively guide the optimization of process chains for graphite. Froth flotation of graphite as a physical separation technique is rather well understood. Kerosene as a collector, MIBC as a frother, and sodium silicate as a depressant are common reagents used for graphite flotation. In plants, both conventional and column flotation machines are being used. The flotation column in the cleaning circuit can improve metallurgical performance and reduce processing costs. Although quite a number of studies have been carried out regarding graphite flotation, significant opportunities still remain, especially for the flotation of fine graphite and the effects of attrition scrubbing. Froth flotation is reliant on liberation; however, often impurities are concentrated due to attaching graphite layers leading to reduced grades. Therefore, to upgrade flotation products, chemical (leaching combined with caustic roasting) methods would be required. The caustic roasting (with NaOH)–acid leaching (with HCl/H2SO4) method can produce graphite with 99.99% carbon content, and remove impurities even at parts per million (ppm) range. Although this technique allows producing high-grade products, it is costly and generates a large quantity of acidic and alkaline wastewater. The development of alternative beneficiation techniques that are both cost efficient and environmentally benign will be required. One such alternative could be microwave irradiation.

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