Effect of grinding media on the surface property and ...

Powder Technology 322 (2017) 386–392

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Effect of grinding media on the surface property and flotation behavior of scheelite particles Chengwei Li, Zhiyong Gao ⁎ School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 25 March 2017 Received in revised form 21 August 2017 Accepted 26 August 2017 Available online 12 September 2017 Keywords: Scheelite Flotation Grinding Wettability Oleate Shape index

a b s t r a c t Grinding for mineral liberation is a prerequisite for a successful flotation separation. Different grinding media produce mineral particles with different surface properties and flotability. In this study, the surface properties and flotation behavior of scheelite particles having a size of −74 + 38 μm produced by ball and rod mills were studied through single mineral flotation experiment, scanning electron microscopy (SEM) observation, wettability measurement, and X-ray diffraction (XRD) test. The wettability and flotation results showed that, compared to the ball mill particles, the rod mill ones have a lower critical surface tension and thus a greater hydrophobicity when treated with the collector solution, and accordingly perform a better flotation recovery using oleate as the collector. In addition, the rod mill particles have a smaller specific surface area, so the full monolayer adsorption of the collector on their surfaces is achieved at a lower oleate concentration. The SEM analysis further confirmed that mineral grains obtained from the rod mill possess larger elongation and flatness values, which are essentially required for their attachment with air bubbles. The XRD observations revealed that mineral particles from both mills (i.e. ball and rod) have similar exposure intensity of abundant {112} surface. However, the rod mill particles have more {101} surface exposed, while the ball mill particles have more {001} surface exposed, leading to a stronger interaction of the collector with the rod mill particles. Keeping in view the stronger interaction with the collector and the easier attachment to air bubbles, the rod mill scheelite particles are deemed to be more hydrophobic and have a higher flotation recovery. These details studied will help establish the relation between the particle surface properties and the grinding media, and provide guidance for optimizing flotation separation. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In ore deposits, the valuable minerals always coexist with the gangue minerals. Therefore, an adequate liberation of those minerals from each other is essential to enrich the valuable minerals. There are two consecutive stages to achieve the adequate liberation [1]. First, the mined ores are needed to be crushed to achieve the preliminary liberation, and are then ground for further desired liberation. The most commonly used grinding methods are ball and rod mills. For the ball mill, the media filling rate can be easily changed to control the size distribution of the product. Although the rod mill is less popular than the ball mill, it has been used by plants for the advantage of producing more uniform particle products and avoiding over-grinding. Especially those plants involve the gravity separation processes [2]. The influence of the grinding media on the surface properties has been reported. The research group led by Yekeler from Cumhuriyet University has performed much significant work in this area [3–7]. They systematically studied the surface properties of the ball, rod and autogenous milled particles of different minerals (including calcite, quartz, ⁎ Corresponding author. E-mail address: [email protected] (Z. Gao).

https://doi.org/10.1016/j.powtec.2017.08.066 0032-5910/© 2017 Elsevier B.V. All rights reserved.

talc, barite and coal). From their studies it was concluded that the rod mill particles have the most elongated and smoothest surfaces, and thus possess the highest hydrophobicity and flotability, which is ascribed to the stronger adhesion force between both prismatic and elongated particles with air bubbles [8,9]. Their studies were further confirmed by Guven [10] and Dehghani [11] in their latest work. Moreover, recent publications have indicated that different (dry or wet) grinding environments also have a considerable influence on the surface properties and flotation behavior of the ground particles. Spodumene samples were ground with dry and wet mills by Zhu et al. [12] and Xu et al [13]. The separate research groups independently found that the wet mill particles expose more {110} and {100} surfaces while more {010} surfaces are exposed for the dry mill samples. In addition, the former are more hydrophobic and accordingly have a better flotation recovery. Prestidge and Tsatouhas [14] reported that the wettability of sulfate powders could be controlled by adjusting the relative proportion of different exposed crystal surfaces. Scheelite CaWO4, the most prominent mineral source for tungsten, often coexists with other calcium-containing minerals such as calcite (CaCO3) and fluorite (CaF2) [15–17]. The selective flotation separation of scheelite from calcite and fluorite is predominantly based on the difference in the surface properties of these minerals [15,18]. Therefore,

C. Li, Z. Gao / Powder Technology 322 (2017) 386–392

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research on the surface properties and flotation behavior of scheelite particles ground by different mills is of great significance for the selective separation.

that the special surface areas of the ball and rod mill particles were 0.040 and 0.027 m2/g, respectively.

2. Experimental procedure

2.2.2. Micro-flotation test Micro-flotation tests were carried out in an XFG flotation machine with a 40 mL plexiglass cell, at an impeller speed of 1500 rpm. The flotation machine was depicted in our previous publications [19,20]. The mineral suspension was prepared by adding 2.0 g of minerals and 36 mL DI water into the flotation cell and agitated for 2 min. The pH of the mineral suspensions was adjusted by adding NaOH or HCl for 2 min. Then the sodium oleate with a certain concentration was added and the suspension was kept conditioned for 3 min. The stable pH value was recorded before the flotation. The flotation process lasted for 3 min before the flotation products were collected, filtered, dried and weighted. The flotation recovery was calculated using the weight of the dry products.

2.1. Material and reagents Representative samples of pure scheelite crystals were obtained from the Huili mine, in Sichuan, China. The chemical analysis confirmed that the scheelite samples were over 98% pure. Chemically pure sodium oleate (NaOL, C18H33O2Na) reagent was supplied by Baisaiqin Chemical Technology co., Ltd., Shanghai, China. The pH of solution was adjusted with sodium hydroxide (NaOH) or hydrochloric acid (HCl) stock solutions. Deionized (DI) water with a resistivity of over 18 MΩ × cm was used throughout the experiments. 2.2. Methods 2.2.1. Grinding tests Representative samples of pure scheelite crystal samples (Fig. 1a) were first crushed from a diameter of 40 mm to 0.5 mm using the JC6 jaw crusher (Beijing Grinder Instrument Equipment Co., Ltd.). Then, the crushed samples were ground by ball and rod mills. The grinding tests were operated at the 146 mm × 200 mm laboratory size mill (Fig. 1b). For the ball mill, corundum balls with diameters of 21, 16 and 12 mm were used as grinding media. For the rod mill, corundum rods of 15 and 11 mm in diameter and 15 cm in length were used. Scheelite samples with a total mass of 200 g were fed into the mill and ground for 30 s each run to prevent over-grinding. After a run, the ground products were sieved by a standard screen with a pore size of 74 μm (200 mesh). The oversize particles were returned to the mill for the next run, while the products under the screen were collected for further sieving to obtain the particles in the size fraction of −74 + 38 μm. The samples with required size were rinsed with DI water and dried at the temperature of 60 °C. The surface area measurement (BET) indicated

2.2.3. X-ray diffraction (XRD) measurement X-ray diffractometer (D8-ADVANCE Bruker-AKS) was run in the reflection mode with Cu Kα radiation (λ = 1.5406, tube potential of 40 mV, and tube current of 40 mA), and a goniometer speed of 4 (°)/min. It is important that the samples were randomly oriented during the sample preparation process.

2.2.4. Particle shape characterization by SEM The shape characterization of milled particles was imaged by the JSM-6490LV SEM instrument. And assuming that the projection of the particles had the ellipse-like shape [21], the imaged micrographs were analyzed using CorelDraw × 4 software to measure the length (L) and width (W) of the particles. As shown in Fig. 2, the particles with no overlap and no border out of the picture were chosen. For each particle, the mean value of the five liner lengths and widths were taken as the real length (L) and width (W), respectively [4]. Then more than 200 particles were measured and the L and W of the particle groups were calculated by averaging all values of the chosen particles.

Fig. 1. Pure scheelite crystal sample (a), grinder (b), corundum rod (c) and ball (d) media used in the grinding tests.

388


Fig. 2. Measurements of the length (L) and width (W) of a particle from a SEM micrograph on CorelDraw software.

Based on the ellipse-like shape assumption, Eqs. (1) and (2) were used to calculate the area (A) and perimeter (P), respectively. A≈

P≈

πLW 4

ð1Þ

1 3 π ðL þ WÞ−ðLWÞ1=2 2 2

ð2Þ

F¼

4πA p2

ð3Þ

P2 4πA

ð4Þ

L W

ð5Þ

ER ¼

W L

ð6Þ

It should be noted that the maximum value of the roundness is 1.0 for a circle, while the flatness has a minimum of 1.0 for a circle.

With the values of L, W, A and P, and the four shape factors, including the roundness (Ro), flatness (F), elongation (ER) and relative width (RW), were calculated using Eqs. (3)–(6) [21,22]. Ro ¼

RW ¼

2.2.5. Wettability measurement To measure the wettability of the nubby samples, the sessile drop technique [23] and the captive bubble method [24] have been widely used, while for powder samples, the washburn method [25] and the flotation method [26] were applied. In this study, the flotation method was employed to investigate the wettability of the milled particles treated by the oleate solution. This method is based on the assumption that the flotation recovery of the minerals decreases with the decrease in the surface tension (γLA) of the solution [27]. Therefore, methanol solution with different concentrations were used as the flotation media and 2.0 g of samples was conditioned in 7.5 × 10−6 mol/L sodium oleate for 3 min. After 3 min of flotation, the concentrates and tailings were filtered, dried and weighted to calculate the flotation recovery.

100

pH=6.3-6.5 80 60 40 Ball mill products Rod mill products

20 0 0.0

-6

5.0x10

-5

1.0x10

-5

1.5x10

-5

2.0x10

-5

2.5x10

-5

3.0x10

Sodium oleate concentration (mol/L)

Scheelite flotation recovery (%)


100 Ball mill products Rod mill products

80 60 40

-6

20

NaOL: 7.5 10 mol/L

0 6

7

8

9

10

11

pH Fig. 3. Flotation recoveries of the ball and rod mill products as a function of the NaOL concentration.

Fig. 4. Flotation recoveries of the ball and rod mill products as a function of the solution pH.


Surface tension (mN/m)

100 80 60 40 20 0 0.0

20.0

40.0

60.0

80.0

100.0

Methanol concentration (v/v%)


Fig. 5. Variation of the surface tension (γLA) of the methanol solutions with different concentration [3].

70 60 50

Ball mill products Rod mill products pH=9 -6 NaOL: 7.5 10 mol/L

40 30

c2 c1

20 10 0 20

25

30

35

40

45

50

Surface tension (mN/m) Fig. 6. Flotation recoveries of the ball and rod mill products as a function of the methanol surface tension.

3. Results and discussions 3.1. Micro-flotation experiment results Micro-flotation experiments were conducted to study the flotation behavior of the ball and rod mill particles in the fraction of −74 + 38 μm. The flotation results, using different concentrations of NaOL solution at a pH of 6.3–6.5, are provided in Fig. 3. Both the ball and rod mill particles have a similar trend of flotation recovery. The flotation recovery increases steadily with the increasing of NaOL concentration till up to 1.5 × 10−5 mol/L for the rod mill particles and 2.5 × 10−5 mol/L for the ball mill ones, where the maximum recovery for the two samples are obtained at given concentrations of NaOL, respectively. The maximum recovery may be caused by the monolayer adsorption of collector on the scheelite surface [28], and the results for the ball mill particles are in good agreement with the previous report [29]. In case of rod mill samples, it is easier to reach the monolayer adsorption because of the smaller specific surface area of 0.027 m2/g compared with that of

389

0.040 m2/g for the ball mill ones. Then the recovery slightly decreases as the concentration increases, which could be ascribed to the bilayer adsorption of the oleate species onto the monolayer and/or the precipitation of calcium oleate on the mineral surfaces [20]. The flotation results, obtained at a pH of 6–11 and at a NaOL concentration of 7.5 × 10−6 mol/L, are presented in Fig. 4. It shows that the recoveries of both the ball and rod mill particles follow a similar trend, but a higher recovery for the rod mill particles was observed. It was also observed that the flotation recovery increases with increasing of solution pH to a value of 9, and it is due to the significant increasing of the oleate ion concentration in the solution [30]. When the pH is above 9, the recovery begins to decrease, which may be caused by the competition for the adsorption Ca sites on the mineral surfaces between the oleate ions and the abundant OH– [31]. Therefore, it was concluded from the micro-flotation results that the flotation behavior of the ball and rod mill particles is different, and the rod mill ones have a better flotation performance than the ball mill ones. 3.2. Wettability test results The flotation method was used to investigate the wettability of both the ball and rod mill particles. The experiments were conducted at a pH of 9 using 7.5 × 10−6 mol/L NaOL as the collector, and at different concentration of methanol solution. The critical surface tension (γc) of wetting was determined by plotting the percent recovery (R%) versus the surface tension (γLA) of the methanol solution, which gives a recovery R% = 0 at γc = γLA. If γLA b γc, the particles are wetted and cannot float, indicating that the mineral with a lower γc is more hydrophobic. The γLA of the methanol solution and the flotation results are shown in Fig. 5 and Fig. 6, respectively. Fig. 6 shows that the γc of the ball and rod mill particles are approximately 37.5 and 34 mN/m, respectively, indicating that the ball mill ones are more likely to be wetted while the rod mill ones are more hydrophobic, which agrees well with the flotation results in Section 3.1. The differences in wettability and flotation behavior may be caused by the differences in surface properties of the two different mill-type particles. 3.3. Shape indexes of scheelite particles by different mills The shape characterizations of both the ball and rod mill particles were determined using 2D shape analysis (SEM method), and the mean values of the main shape indexes calculated according to Eqs. (1)–(6) are listed in Table 1. In addition the distribution of the roundness (flatness = 1/roundness) and the elongation (relative width = 1/elongation) were calculated and are shown in Fig. 7. Furthermore, the software SPSS and the “two-sample t-test” were applied to study the differences of the shape indexes (flatness, roundness, elongation and relative width) of the products produced by the ball and rod mills. The values of the shape indexes were compared pair-wise for any statistically significant differences at the 95% confidence interval with the results listed in Table 2. As shown in Table 1, the rod mill particles have higher values of elongation (E) and flatness (F), while the ball mill particles possess larger values of roundness (R) and relative width (RW). Since all values at the column of “Sig. (2-side)” listed in Table 2 are less than 0.05, a significant difference between the ball and rod mill scheelite products with a confidence interval of 95% is inferred [32]. It was noted from the literature that the 2D shape data can be used to reproduce the crucial shape

Table 1 Mean values of the shape indexes of both the ball and rod mill products of the scheelite mineral. Milled products

Particle number

L(μm)

W(μm)

A(μm2)

P(μm)

E

F

R

RW

Ball Rod

240 240

96.140 108.867

63.830 67.412

4817.244 5796.279

253.741 281.199

1.505 1.615

1.069 1.097

0.935 0.916

0.664 0.623

390


35

30

a

30

Frequency counts of ball mill products Distribution of roundness

b

25

20

Frequency (%)

Frequency (%)

25

Frequency counts of rod mill product Distribution of roundness

15 10

20 15 10

5

5

0 0.87 0.88 0.89 0.90 0.91 0.92 0.93 0.94 0.95

0 0.90

0.91

Roundness 35

25

c Frequency (%)

Frequency (%)

25

0.95

0.96

30

Frequency counts of rod mill products Distribution of elongation

30

0.92 0.93 0.94 Roundness

20 15 10

d

Frequen counts of ball mill products Distribution of elongation

20 15 10 5

5 0 1.45

1.50

1.55

1.60

1.65

0

1.70

1.40

1.44

Elongation

1.48

1.52

1.56

Elongation

Fig. 7. The distribution of roundness index of the rod (a) and ball (b) mill products, and elongation index of the rod (c) and ball (d) mill products.

characteristics of the real 3D shape indexes when the measured particles number is greater than 200 [6,33–37]. It is worth mentioning here that the shape indexes obtained in this study are reliable. Those differences in the shape indexes between the ball and rod mill particles could be explained by the different breakage mechanism of the two grinding media. The ball mill exerts point loads on the particles [3], and the predominant breaking forces are abrasion and chipping, making particles more rounded. While the rod mill exercises line loads on the particles, so that the main force is the impact, causing the particle to be more elongated [38]. As shown in Fig. 8, the SEM images of the milled products confirm the above statements. Considering the differences in the flotation behavior and the shape indexes of the two milled products, it can be concluded that the rod mill particles, with a lower specific area, larger elongation and increased flatness, exhibit a higher hydrophobicity and better flotability, indicating that the attachment of air bubbles to the flat and elongated particle is easier than to the rounded ones. This can be attributed to the sharp

edges of elongated particles produced by the rod mill which are beneficial for rupturing the water film at the mineral/solution interface and help shorten the attachment time and improve the collision efficiency [39,40]. 3.4. XRD measurement results Recent reports showed that the relative exposure proportion of different crystal surfaces changes when the grinding environment varies [12,13]. In this work, the exposure of different crystal surfaces of the two mill products was investigated, and the XRD spectrums are shown in Fig. 9. It is apparent that the major cleavage surfaces of scheelite are {112}, {101} and {001}, which is in good agreement with the recent reports [41–44]. The relative intensity of a maxima (or peak) in the spectrum is believed to represent the relative exposure proportion of the crystal surfaces in the particulate samples [45,46]. Fig. 9 shows that the {112} surface is

Table 2 The statistical shape differences between the ball and rod mill products determined by the two-sample t-test. Parametera

Pair differences Mean

Flatness Roundness Elongation Relative width a

Fd Rd Ed RWd

0.02648 −0.02291 0.10053 −0.04127

Standard deviation

0.02363 0.01986 0.02021 0.00750

Standard error of mean

0.00673 0.00464 0.00483 0.00222

t

df

Sig.(2-side)

17.359 −17.555 76.444 −85.232

239 239 239 239

0.000 0.000 0.000 0.005

95% Confidence interval of the difference Lower

Upper

0.01614 −0.0301 0.05160 −0.04910

0.03873 −0.0118 0.14945 −0.00886

Fd, Rd, Ed and RWd represent the shape differences of the scheelite particles produced by rod and ball mills. df represents the degrees of freedom.


391

4000

a {101}

3000 2500

{001}

Intensity (counts)

3500

{112}

Fig. 8. SEM pictures of the ball (a, c) and rod (b, d) mill products of scheelite minerals.

2000 1500 1000 500 0 0

10

20

30

40

50

60

4000

3000

4. Conclusions

2500

In this study, the surface properties and flotation behavior of scheelite particles with a size of −74 + 38 μm produced by ball and rod mills were studied through single mineral flotation experiment, SEM observation, wettability measurement, and XRD test. The wettability measurements showed that the values of the critical surface tension (γc, a measure of surface tension) are 34.0 and 37.5 mN/m for the scheelite particles treated by the oleate solution produced by the rod and ball mills, respectively, indicating that the rod mill particles possess a higher hydrophobicity. The flotation results showed that the rod mill particles have a higher flotation recovery and are much easier to achieve a monolayer adsorption of collector compare to the ball mill ones. The SEM observations and distribution calculation demonstrated that, compared with the rounder ball mill particles, the rod mill ones have larger values

2000 {001}

Intensity (counts)

{101}

b

3500

{112}

Two-Theta (deg)

the most abundant plane, and the intensity of the {112} surface is nearly the same for both the mill products. In this case, the exposure proportion of {112} surface in the mill products is considered as 100%. Therefore, the relative proportions of the {101} plane in the ball and rod mill samples are 71.9% and 87.7%, respectively, while for the {001} plane are 44.4% and 33.8% respectively. The ball mill samples expose more {001} surfaces while the rod mill samples show more {101} surfaces. It has been reported that every Ca atom on the scheelite {101} surface is fivefold-coordinated, while Ca on the {001} surface is sixfoldcoordinated. The Ca atom in bulk scheelite crystal is eightfold-coordinated to oxygens. Therefore the Ca atom on the {101} surface is more active than that on the {001}, based on the conclusion that the atom with more dangling bonds is more active [44,47,48]. The atomic simulation results showed that a larger adsorption energy (− 102 kJ/mol) of the oleate on the {101} surface was calculated while the adsortpion energy was only −93 kJ/mol for the oleate on the {001} surface [49]. Thus, it is tenable to infer that the rod mill products with relatively more {101} surface have a stronger interaction with the collector oleate and a higher flotation recovery.

1500 1000 500 0 0

10

20 30 40 Two-Theta (deg)

50

60

Fig. 9. XRD spectra of the ball (a) and rod (b) mill products of scheelite minerals.

392


of elongation and flatness, which is beneficial for the attachment of collectors and air bubbles. The differences between the shape indexes were found to be statistically significant with a 95% confidence interval by applying the SPSS and the two-sample t-test. The XRD spectrums showed that both mill particles have similar expose intensity of abundant {112} surface. However, the rod mill particles have more {101} surface exposed while more {001} surface is exposed for the ball mill particles, leading to a stronger interaction of the oleate with the rod mill particles. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (51774328, 51404300), the Innovation-driven Program of Central South University of China (2017CX007), the Key Program for International S & T Cooperation Projects of China (2016YFE0101300), and the National 111 Project (B14034).

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