Flotation Behavior of Malachite Using Hydrophobic Talc Nanoparticles as Collectors

Abstract

In this study, the flotation behavior of malachite was investigated using hydrophobic talc nanoparticles (TNs) as collectors. To improve the floatability of TN-deposited malachite, various experimental parameters were systematically investigated. We found that the floatability sharply increased as the size of the TNs decreased. The floatability of malachite was enhanced in the presence of smaller TNs, since higher amounts of smaller TNs were deposited on the surface of the malachite, thus rendering the surface more hydrophobic. Moreover, the floatability of the TN-deposited malachite increased as the pH decreased, likely due to the more favorable interaction between TNs and malachite by means of electrostatic attraction. Furthermore, the floatability became more enhanced as the TN concentration increased, likely associated with increases in the amount of TNs deposited on the surface of the malachite, thus enhancing the floatability by altering the hydrophobicity of the surface. Our findings suggest that the application of natural hydrophobic TNs as collectors in malachite flotation should be introduced as a new concept.

1. Introduction

Due to its high separation performance, low capital expenditure, and operating cost, flotation is one of the most widely used techniques to recover useful resources in mineral processing. Despite the many advantages of flotation, however, some hazardous chemicals used in flotation limit further development and more widespread usage of the process. For example, the most commonly used flotation process for Cu oxides is still sulphidizing the surface of oxide minerals in advance, followed by flotation using sulfhydryl collectors such as xanthate, because this process is well known to be effective with respective to the selectivity of Cu-bearing minerals [1,2,3,4]. However, this method has critical disadvantages; specifically, sulphidization and sulfhydryl reagents are rather toxic and are not environmentally friendly [5,6]. Accordingly, new alternative flotation methods are required to expand knowledge and to develop flotation technology to recover Cu oxide minerals.

Recently, nanoparticle flotation was suggested as an alternative to the conventional process. In nanoparticle flotation, nanoparticles are use as collectors and their deposition onto the target particles is governed by physicochemical interactions [7,8]. Hence, unlike the typical Cu oxide mineral flotation process mentioned above, a sulphidization step is not required. Moreover, in the literature regarding deposition of polystyrene nanoparticles on glass beads, as little as 10% coverage by nanoparticles on the glass bead surface can promote high flotation performance, whereas conventional molecular collectors require 25% or greater coverage for good flotation performance [9,10]. Considering these phenomena, researchers have exerted efforts to clarify the feasibility of nanoparticles as collectors in the flotation process. Yang et al. reported the application of polystyrene nanoparticles as collectors in the flotation of glass beads and pentlandite [11,12]. The floatability of the glass beads and pentlandite was improved using softer polystyrene nanoparticles, because they were more firmly deposited onto the surface of said glass beads and pentlandite due to their greater contact area. Hajati et al. analyzed the flotation behavior of silica using talc nanoparticles (TNs) as collectors [13]. By adjusting the pH of a solution, talc and silica can become oppositely charged, and thus, electrostatic attraction between silica and talc is generated. More recently, our group was the first to confirm the feasibility of malachite flotation using nano-sized carbon black as a collector [14]. We found that excessive levels of suspended carbon black nanoparticles reduced the kinetic rate of the attachment of malachite to bubbles. Likewise, although researchers have made substantial efforts to identify the feasibility of nanoparticles as collectors in the flotation process, relevant studies on the flotation of Cu oxide minerals are scarce.

In this article, to further expand the understanding of the roles of TNs as collectors in malachite flotation, we systematically studied the effects of the size of the TNs, the concentration of the TNs, and the pH of the solution used to gain useful information on the critical experimental factors for malachite floatability.

2. Materials and Methods

2.1. Materials

A lump of malachite ore sized below 20 mm was obtained (Daye Mining, Hubei, China). The ore was firstly crushed using a laboratory table-top jaw crusher to pass 2.36 mm, and then further crushed in a laboratory stamp mill for size reduction. Two sieves (0.212 mm and 0.074 mm) were used to obtain a fraction size ranging from 74 to 212 μm, which was used as the sample in this study. Talc powder was supplied by Woojin Powder (Jecheon, Korea). To confirm the purity of the samples used in this study, X-ray fluorescence (XRF; Sequential XRF-1800, Shimadzu, Kyoto, Japan) and inductively coupled plasma (ICP; Optima 5300PV, PerkinElmer, Inc., Middlesex, MA, USA) analysis were carried out. From the analyses, it was confirmed that the malachite sample contained 55.26% Cu and that the talc sample contained 56.37% SiO₂ and 36.08% MgO, indicating the high purity of the samples. The malachite and talc samples were further analyzed by X-ray diffraction (XRD; D/Max-2200/PC, Rigaku, Tokyo, Japan), showing their mineralogical purity (Figure 1).

The talc powder was subject to grinding using a laboratory-scale attrition mill equipped with a zirconia ball and pot (Φ = 2 mm, 1 L). The grinding was conducted under wet conditions and the grinding time varied from 0.5 to 20 h. The particle size of talc after a certain duration of grinding for 0.5, 2.5, 8, and 20 h (denoted as TN-1, TN-2, TN-3, and TN-4) was analyzed using the dynamic light scattering (DLS) method (ELS-Z, Otsuka Electronic Co., Osaka, Japan). The median diameter (d₅₀) of the TNs (1–4) ranging from 178 to 1462 nm was obtained by grinding (Figure 2).

2.2. Zeta Potential and Contact Angle Measurements

In order to determine the zeta potential of the malachite and talc, samples smaller than 5 μm were added into a 1 mM NaCl solution and the electrophoretic mobility of the samples was measured at different pH conditions using a zeta potential analyzer (ELS-Z, Otsuka Electronic Co., Osaka, Japan). The measured mobility values were converted to zeta potentials based on the Smoluchowski equation [17,18]. The pH of the solution was adjusted using 1 M HCl and NaOH (Fisher Scientific, Rockingham, NH, USA). In addition, to investigate the degree of hydrophobicity of the malachite and talc samples, the hand-picked pure malachite and talc samples were cut to a proper size, polished, and finally washed in deionized (DI) water.

The contact angles of the cleansed samples were measured using a goniometer (Phoenix150, SEO, Suwon, Korea). The drop shape was recorded with a camera, and the images were processed by a computer and then stored. The contact angles of the malachite and silica were determined to be approximately 26.12° and 73.48° (Figure 3a,b), indicating that the surface properties of the malachite and talc were hydrophilic and hydrophobic, respectively. Furthermore, to identify the degree of hydrophobicity of the surface of TN-deposited malachite, the cleansed malachite samples were immersed in the desired TN suspension for 30 min. Then, they were air-dried at room temperature. The treated samples were also used to conduct the contact angle measurements. The zeta potential and contact angle measurements were carried out at least in duplicate for each condition.

2.3. Deposition of TNs onto the Surface of the Malachite

To identify the concentration of TNs deposited onto the surface of the malachite, absorbance measurements were carried out. To prepare sample suspensions, 1 g of malachite with a desired TN concentration was distributed into 50 mL plastic conical tubes. Then, the suspensions were rotated at 50 rpm for 30 min. After settling the suspension of TN-deposited malachite via gravity for 1 min, the remaining amount of TNs in the supernatant was obtained. The extent of TN deposition on the surface of the malachite was determined by measuring the absorbance of the TN suspension in the supernatant before and after the deposition process at 600 nm using a UV-Vis spectrometer (DU800, Beckman Coulter, Brea, CA, USA). The quantity of deposited TNs was calculated using a calibration curve of absorbance versus the concentration of TN. These tests were performed in at least duplicate for each condition. In addition, to visually identify TN deposition onto the surface of the malachite, a field emission scanning electron microscope (FE-SEM) with an energy dispersive X-ray spectrometer (EDS) (INCA x-act, Oxford Instruments, Abingdon-on-Thames, Oxfordshire, UK) was employed. Figure 4 shows the TN deposition on the surface of the malachite, the patterns of which likely involve a partial patch as a multi-layer.

2.4. Microflotation Tests

To identify the flotation behavior of TN-deposited malachite, a Hallimond tube was used to perform the flotation tests. For all tests, the following identical conditions were used: 1 g of the sample used with nitrogen gas (purity < 99%) with an injection rate of 20 mL/min and an agitation speed of 200 rpm by a magnetic stirrer bar, and the flotation time was 30 min. First, to identify the flotation behavior of the TN-deposited malachite with different median TN diameters, the flotation experiments were performed using TN (1–4) at pH 6. The concentration of TNs was 1500 g/ton. Second, to examine the flotation behavior of the TN-deposited malachite using solutions of different pH levels, the flotation tests were performed using TN-2 at pH 6, 9, and 11. The concentration of TNs was 1000 g/ton. Lastly, to identify the flotation behavior of the TN-deposited malachite with different concentrations of TN, the flotation experiments were performed with 50–3000 g/ton TN using TN-2 at pH 6. Both the floated and unfloated samples resulting from the flotation tests were dried at 60 °C in a dryer for 3 h. The flotation tests were carried out at least in duplicate for each condition.

3. Results and Discussion

3.1. Effects of the Size of the TNs

To investigate the influence of the size of the TNs on the floatability of the malachite, flotation experiments were performed using TN (1–4) ranging from 178 to 1462 nm in size. Figure 5 shows the floatability of the TN-deposited malachite with TNs of different sizes in the presence of same concentration of TNs at pH 6. Overall, the floatability gradually increased as the size of the TNs decreased. Specifically, the floatability was determined to be approximately 44.8%, 70.4%, 81.1%, and 89.8% using TN-1, TN-2, TN-3, and TN-4, respectively. The difference in the floatability of malachite was likely associated with differences in concentrations of the TN deposited on the surface of the malachite, which altered the extent of the hydrophobicity that induces interactions with bubbles. To confirm the results, the amount of TNs deposited on the surface of the malachite was measured (

Table 1. The amounts of TNs deposited on the surface of malachite using TN-1, TN-2, TN-3, and TN-4, respectively. The absorbance tests were carried out with 1500 g/ton TN concentration at pH 6.

Sample	Amounts of TNs Deposited on the Malachite Surface (mg/g)
TN-1	0.863 ± 0.16
TN-2	1.671 ± 0.17
TN-3	1.809 ± 0.14
TN-4	1.998 ± 0.11

). In agreement with the gradual increase in the floatability of the malachite, the amount of TNs deposited on the surface of the malachite increased as the size of TNs decreased. This is also well supported by the previous literature that reports that the floatability of silica increased as the size of TNs decreased [13].

3.2. Effects of pH

Figure 6 shows the electrokinetic behaviors of malachite and TNs as a function of pH. A distinct difference in zeta potential behavior was observed. The isoelectric points (IEPs) of the malachite and TNs were approximately pH 8–9 and pH < 3, respectively. These results are in good agreement with previous works [18,19]. Based on this, the maximum deposition of TNs on the surface of malachite could occur at pH 6 by electrostatic attraction between the malachite and TNs.

The floatability of the TN-deposited malachite was investigated as a function of pH at the same concentration of TNs using TN-2, and the results are presented in Figure 7. As expected, the floatability increased as the pH decreased. The floatabilities of the malachite were determined to be 70.4%, 41.1%, and 26.6% at pH 6, 9, and 11, respectively.

Table 2. The amounts of TNs deposited on the surface of malachite according to changes in pH. The absorbance tests were carried out with 1000 g/ton TN concentration using TN-2.

Sample	Amounts of TNs Deposited on the Malachite Surface (mg/g)
6	1.671 ± 0.17
9	0.954 ± 0.19
11	0.799 ± 0.15

shows that deposited amount of TNs on the surface of the malachite increased as the pH decreased. The maximum floatability of the malachite was observed at pH 6 due to a large difference in surface charges of the malachite and TNs.

3.3. Effects of the Concentration of TNs

Flotation experiments were carried out by varying the concentration of TNs (50–3000 g/ton) at pH 6 using TN-2 to investigate the effects of the concentration on the floatability of the malachite. Figure 8 shows that the concentration of TNs considerably affects the floatability of malachite. The amounts of the floated malachite were 21.4%, 22.8%, 40.7%, 60.1%, 70.4%, 76.2%, 75.2%, 77.3%, and 77.1% in the presence of 50, 100, 200, 500, 1000, 1500, 2000, 2500, and 3000 g/ton, respectively.

To clarify the understanding of the results regarding the flotation behavior of malachite with different concentrations of TNs, the amount and contact angles of the TNs deposited on the surface of the malachite were measured and the results are presented in

Table 3. The amounts of TNs deposited on the surface of malachite according to changes in TN concentrations. The absorbance tests were carried out at pH 6 using TN-2.

TN Concentration (g/ton)	Amounts of TNs Deposited on the Malachite Surface (mg/g)
50	0.023 ± 0.03
100	0.026 ± 0.06
200	0.834 ± 0.11
500	1.189 ± 0.14
1000	1.671 ± 0.17
1500	1.981 ± 0.09
2000	1.983 ± 0.13
2500	1.979 ± 0.11
3000	1.973 ± 0.19

and Figure 9, respectively. Below a concentration of 100 g/ton of TNs, the floatability of the malachite was not significantly improved (Figure 7). Consistent with these results, the TNs were not considerably deposited on the surface of the malachite (Table 3) and the contact angles remained unchanged (Figure 9a,b). These results indicate that the deposition of TNs on the surface of malachite occurs when the concentration of TNs is higher than a certain level (~100 g/ton), which increases the hydrophobicity of the surface of the malachite and thus enhances the floatability of the malachite. The floatability was sharply improved in the presence of a concentration of 200–1500 g/ton of TNs, which increased from approximately 45% to 75% over this range. The amount of TNs deposited on the surface of the malachite increased as the concentration of the TNs increased (Table 3). This observation was further supported by the contact angle of the surface of the malachite after TN deposition (Figure 9c). The value of contact angle was approximately 46.72° for a concentration of 500 g/ton of TNs. The higher contact angle for the concentration of 500 g/ton of TNs indicates that a higher amount of TNs were deposited on the surface of the malachite, thus rendering the surface more hydrophobic. The floatability of the malachite was improved at a concentration of 2000 g/ton of TNs and remained intact at high concentrations (3000 g/ton). To understand the flotation behavior of malachite, we measured the amount of TNs deposited on the surface of the malachite over this range (Table 3). The results reveal that the amount did not significantly improve; it was ~2 mg/g, which is comparable to that of a concentration of 1500 g/ton of TNs. This observation was further supported by the contact angle for the deposition of TNs onto the surface of malachite in the presence of a concentration of 2000 g/ton of TNs (Figure 9d). The contact angle was approximately 62.23° for a concentration of 2000 g/ton of TNs. We conclude that the deposition of TNs on the surface of the malachite and the hydrophobicity peaked at a concentration of 1500 g/ton of TNs. Furthermore, at concentrations exceeding 2000 g/ton of TNs, TNs are no longer deposited on the surface of the malachite, indicating that approximately 2 mg/g is the level of the saturation for a concentration of 1500 g/ton of TNs.

4. Conclusions

In the present study, the effects of TNs as collectors in malachite flotation was systematically investigated. The key experimental findings from the study can be summarized as follows:

• The IEPs of the malachite and TNs were approximately pH 8–9 and pH < 3, respectively. The malachite was positively charged at pH 6, whereas the TNs were negatively charged at this pH value, indicating that the electrostatic interaction between malachite and talc is favorable, resulting in a maximum amount of TNs deposited on the surface of malachite at pH 6. Based on this, the maximum floatability of malachite was observed at pH 6.
• The amount of TNs deposited on the surface of the malachite increased as the concentration of the TNs increased and the size of the TNs decreased. The trends in terms of the amount of TNs deposited on the surface of the malachite were highly consistent with the contact angle measurements. These two experimental results indicate that the higher the amount of TNs deposited on the surface of malachite, the greater the hydrophobicity of the surface of the TN-deposited malachite, resulting in enhanced malachite floatability.