Natural graphite is found in three commercial varieties: crystalline flake, microcrystalline, and crystalline vein (or lump) [1
]. Graphite formed in contact metamorphism of coaly materials as a result of igneous intrusion is referred to as microcrystal graphite (or earthy graphite) [2
]. The fixed carbon content on the air dried basis (FCad
) of some European microcrystalline graphite deposits is about 55% [3
], while the FCad
of parts of deposits in China is 50%–70%. In order to expand its application and get a higher price, it should be upgraded above 80% (FCad
), or even 90% (FCad
). For commercial microcrystalline graphite ore with an FCad
of 60%–70% can only be upgraded to 80%–84% (FCad
) by flotation for its poor beneficiability [4
Graphite is naturally floatable due to its inherent hydrophobic property. Despite the natural floatability, the separation of gangue is normally improved by the addition of a small amount of kerosene as collector and sodium silicate as gangue depressant, and floated with pine oil or methyl isobutyl carbinol (MIBC) as the frother [7
]. Besides of quartz/feldspar, graphite is usually associated with layered minerals such as biotite, sericite and kaolinite [8
]. Due to the natural interleaving of mica with the flake graphite, it is not possible to produce grades of over 95% by flotation. The entrainment of sericite also leads to a poor flotation selectivity of the microcrystalline graphite ore [9
]. Depressants such as starch and carboxylethyl cellulose have been reported to depress clay minerals thereby increase the grade of concentrate [11
]. An investigation of reverse flotation separation of sericite from graphite was also conducted using a surfactant: formaldehyde condensate of methyl naphthalene sulfonic sodium salt (MF) [12
]. Not only in the graphite flotation, entrainment of gangue also obviously affected the grade of concentrate in coal, Au/Cu sulphide ore [13
], ultrafine sphalerite flotation [14
], and the flotation of base metal ores [15
Many research projects have been carried out to reduce the entrainment of gangues. Some researchers tried to reduce water recovery (thus entrainment) by increasing the flotation rate of hydrophobic particles [16
]. In order to encourage the drainage of gangues through the froth zone, technical solutions such as water spray to the froth layer [17
], vibration of the froth zone, and the use of a centrifugal force field flotation cell have been tested [13
]. Mulleneers modified mechanical flotation cells by adding a counter current sedimentation zone to prevent the entrainment [18
]. Cao and Liu proposed that the entrainment of fine and ultrafine hydrophilic particles can be reduced by enlarging their particle size using either inorganic depressants or high molecular weight polymers [19
]. Apart from the methods mentioned above, optimizing the grinding fineness and flow-sheet may be an effective way to reduce the entrainment of gangue.
This study aims at the reduction of gangue entrainment and the improvement of the separation selectivity in purification of microcrystalline graphite ore. Mineralogy and laboratory grind-flotation tests were employed to determine a reasonable fineness of grind and the number of grinding stages; the mechanism of how multi-stage grinding facilitates the improvement of flotation selectivity was also investigated using the size distribution analysis. In the end, the optimized three-stage grinding-flotation flow-sheet was applied to the purification of the low grade microcrystalline graphite ore.
3. Results and Discussion
3.1. Mineralogical Characteristics
The proximate analysis results of the raw ore are presented in Table 3
. The ash on air dried basis (Aad
) of this sample was 43.45%, which revealed that it was a high ash content sample; the FCad
was 49.26%, indicating that it belonged to a low-grade ore.
Phase composition of the raw ore was investigated by powder X-ray diffraction and Rietveld quantitative phase analysis. Figure 3
displays the XRD pattern of the raw ore. The content of those minerals is given in Table 4
. The content of the clay minerals reached up to 18%, with the clay minerals including sericite and kaolinite with a small grain size of often less than five microns, which suffers from sliming [20
]. It was expected that clay minerals were easily entrained into the froth layers and caused high gangue entrainment in the froth flotation as a result of the fine size [10
The mineralogical knowledge, particularly the ore texture, is required if efficient processing is to be carried out. The texture refers to the grain size, dissemination, association and shape of minerals within the ore. Optical microscopy research can provide ore texture information as shown in Figure 4
. Grind size can be estimated with the aggregate size of graphite. The aggregate size of graphite was heterogeneous, which varied greatly from 150 μm to 1 μm, usually ranging from 60 μm to 20 μm (Figure 4
a,b). In Figure 4
b, irregular granular microcrystalline graphite (M) was disseminated in the gangue (G), and the association of minerals was very complicated.
3.2. Determination of the Ultimate Grinding Fineness
As the entrainment of gangue minerals was closely dependent on particle size, the ore should not be ground any finer than its liberation fineness. A single stage of flotation was performed on the samples ground to various degrees. The flotation tests were repeated three times. The average results and error bars are given in Figure 5
The flotation selectivity was efficiently improved with the grind size decreasing from −74 μm 63% to −74 μm 95%. Thus, the FCad of flotation concentrate was increased to 76.04% from 65.14%. The recovery of graphite was sharply reduced from 92.28% to 45.06% likely due to insufficient collectors. Because the decrease of the grind size led to a significant increase of the surface area, more collector was required to make the surface of fine graphite particles hydrophobic.
The effect of grinding fineness on separation efficiency are presented on Figure 5
b. Obviously, the efficiency index reached a peak at the grind size of 92% passing 74 μm. Thus, the suitable grind size was around 92% passing 74 μm which gives a reasonable concentrate FCad
and recovery in rougher flotation.
If the graphite aggregates were liberated from gangue minerals at the suitable grinding fineness of 92% passing 74 μm, a multiple cleaning concentrate at the grinding fineness of −74 μm 92% was observed by optical microscopy and scanning electron microscope (SEM). The FCad
of this flotation concentrate was 88.5%, suggesting that a relative high grade product could be obtained at the grind size of 92% passing 74 μm. The optical microscope image and secondary electron image of flotation concentrate are shown in Figure 6
. The optical microscope image (Figure 6
a) showed that the liberated gangue particles (the white spots) left in the concentrate were likely recovered by entrainment. The secondary electron images by SEM of the flotation concentrate (Figure 6
b) clearly revealed that the vast majority of graphite aggregates (G) with high purity, apart from the liberated gangue particles (the white spots). It was hard to find gangue mineral disseminated even in the graphite aggregates at the size of approximate 50 μm.
3.3. Comparison of Single-Stage Grinding Circuit and Three-Stage Grinding Circuit
The ultimate grinding fineness of the sample was around 92% passing 74 μm. Two strategies of grinding were proposed for consideration: a single-stage of grinding and a multi-stage grinding flow-sheet. Generally, a single-stage grinding flow-sheet is more convenient for process management and is typically employed in the processing of a homogeneous grained ore; a multi-stage grinding flow-sheet incurs lower electricity consumption because parts of gangue minerals discharged at a coarse particle size.
The flow-sheet and reagent scheme are given in Figure 1
. The grinding fineness is shown in Table 2
. The final grinding fineness of both the single-stage grinding flow circuit and the three-stages grinding circuit was approximately 91% −74 μm. For each circuit, the flotation tests were repeated three times. The average results and error bars are given in Figure 7
As shown in Figure 7
, the recovery and separation selectivity were remarkably improved when the three-stage grinding circuit was employed. In the single-stage grinding circuit, the recovery and FCad
of the flotation concentrate were 27.77% and 86.52% (FCad
). For the three-stage grinding circuit, the FCad
of concentrate was increased by 1.93% to 88.45% (FCad
), meanwhile the recovery was drastically increased by11.38% to 39.15%. In addition, the cumulative recovery (concentrate, CTI, CTII, CTIII, CTIV, CTV, CTVI, CTVII and CTVIII) is 4.94% higher for the grinding circuit with three-stages.
In the three-stage grinding circuit, one possible mechanism which led to the beneficial effects on the separation effect was proposed: reducing the foam entrainment of gangue slimes.
The recovery of gangue minerals by entrainment closely depends on the particle size of both hydrophilic minerals and hydrophobic valuable minerals [21
]. At the first-stage grinding-flotation circuit, a large part of gangue minerals such as sericite and kaolinite, which suffer from sliming, were discharged into the tailing at a coarse particle size, a typical value for coarse particles is often +75 μm [23
]. In the second and third stage of grinding, a small amount of gangue was left and fewer over-ground particles were generated. A comparison of the particle size distribution of the feed in the fifth stage of cleaning was carried out using laser particle size analysis. The results are shown in Figure 8
. It is clearly indicated that the content of slimes (<5 μm) in the three-stage grinding flotation circuit was obviously lower than the single-stage grinding flotation circuit. Therefore, in the three-stage grinding flotation circuit, the amount of gangue slimes recovered by froth entrainment was significantly reduced from the second cleaning to the eighth cleaning. The flotation selectivity was significantly improved as shown in Figure 9
. The FCad
difference between concentrate and tailings from the second cleaning to the eighth cleaning was remarkably higher, as compared to that of single-stage grinding circuit.
The results of these experiments strongly supported that the multi-stage grinding was an effective solution to reducing the foam entrainment of fine gangue minerals.
3.4. Application of the Multi-Stage Grinding Flotation Process
The multi-stage grinding flotation process was proposed in the purification of microcrystalline graphite. The optimal three-stage grinding flotation circuit was used in purification of this sample. The details of the optimal three-stage grinding flotation process are shown in Figure 2
. The grinding fineness of the first, second and third stage were −74 μm 40%, −74 μm 87% and −74 μm 92%, respectively. After the grinding process, multi-stages of cleaning were carried out to remove the liberated gangue particles as much as possible. Entrainment of the gangue increased with the decrease of the particle size, thus three stages of cleaning should be carried out after the third stage of grinding to reject the entrainment gangue, while only single stage of cleaning was conducted after the first stage of grinding. A large number of middles were generated after stages of cleaning. To increase the recovery of graphite, recleaning of middles were necessary, the recleaning concentration with low purity are suitable for different markets. The results are given in Table 5
Three types of concentrate were obtained. The total recovery reached 81.16%. The high grade concentrate (K3) met the standard of WT88 with the fraction recovery of 36.64%, which could be used as the raw material in the production of, for example, anodes, pencils and graphite bearings. The middle grade concentrate (K2) met the standard of W80 with the fraction recovery of 34.86%, which was usually used in the fields of, for example, manufacturing refractory materials, foundry technology and electrode manufacture. The low grade concentrate (K1) with a lower price could also be obtained in the flow-sheet, with a recovery of 9.65%.