Novel application of talc nanoparticles as collector in flotation

Abstract

The objective of this work is to study the application of natural hydrophobic talc nanoparticle as a new class of solid flotation collector. Quartz was used as the model mineral, and XRF, XRD, DLS and SEM as well as modified large scale Hallimond flotation cell were applied. Zeta potential measurements identified pH 1.5 as the best condition for adsorption of talc on the quartz particles. Talc nanoparticles size and time of flotation were found to affect the required amount of collector. Reducing the talc nanoparticles size from 567 nm to 235 nm caused the collector dosage to be significantly decreased from 45 kg t⁻¹ to 2 kg t⁻¹ within 30 min of flotation. The effect of flotation time on the collector dosage was found to be more significant for coarser talc nanoparticles. The results of this investigation introduce a new concept of natural hydrophobic talc nanoparticles acting as collector in flotation process.

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1. Introduction

The growing global demand for minerals and extracting higher grade ores in the past have caused mineral processing industries to encounter new mining, environmental and economic challenges. The low grades of minerals together with complex textures as well as fine mineral composition require very fine grinding to achieve sufficient liberation of the valuable minerals. Most researchers have reported various deleterious roles of clay minerals such as slime coating in froth flotation.^1–4 The behaviour of such fine particles, on the other hand, may make them suitable for some other useful applications. In this work, the effect of the adsorbed talc on quartz particles on the hydrophobicity of the quartz is discussed.

Flotation is a comprehensive method for separating mineral particles based on differences in their physico-chemical properties of surface hydrophobicity. Collectors are often used to improve the attachment of particles and bubbles in flotation. In general, there are two types of water-insoluble collectors: oily hydrocarbons and long-chain amphipathic compounds. All these materials are widely used as flotation collectors for beneficiation of minerals.^5,6

The industrial collectors are different types of liquid with an extended molecular length of about 1 nm. In contrast, nanoparticles collectors based on polystyrene with diameters between 46 and 120 nm, as well as graphene nanoparticles can be also used.^7,8 For example, application of inorganic and polymeric soft and hard nanoparticles in flotation of glass beads and preparing thin films of nickel sulphide has been reported.^7,9–11 Commercial grades of precipitated calcium carbonate and colloidal silica have been employed as collectors for flotation of glass beads.⁹ The ability of cationic polystyrene nanoparticles to induce flotation has been also demonstrated by floating hydrophilic, negatively charged glass beads.^7,10,12 In the past few years, polymeric latexes have been also developed as flotation collectors.^13,14

Electrostatic attraction promotes the spontaneous deposition of the nanoparticles on the glass surfaces, raising the effective contact angle to facilitate the attachment of beads to the air bubbles. As little as 10% coverage of the bead surfaces with effective nanoparticles could promote high flotation efficiencies, whereas conventional molecular collector requires 25% or more coverage for a good recovery.^7,15

Pazokifard et al. have treated commercial TiO₂ nanoparticles surface with various silane precursors at different pH to make the nanoparticle surfaces more hydrophobic.^16,17 This can also enhance the selectivity in flotation recovery. Therefore, it could be possible to introduce larger hydrophobic nanoparticles based on their natural hydrophobicity as potential collectors in flotation. As mentioned earlier, the current work presents application of natural hydrophobic talc nanoparticles (50 to 600 nm) as collector in flotation of quartz as a model hydrophilic mineral. Application of talc nanoparticles is novel in this field. Although researchers have used some other inorganic particles (such as styrene/N-butyl acrylate copolymers, polystyrene copolymer latexes, cationic polystyrene-core–poly(n-butyl methacrylate)-shell, precipitated calcium carbonate and colloidal silica) as solid collector, but results are very limited.^11,13,14

In this paper, application of talc nanoparticles as natural hydrophobic collector in quartz flotation is discussed. The effect of several parameters such as talc nanoparticle size and dosage, quartz particle size, flotation time and pH on the quartz flotation performance will be discussed. The heart of this research is to answer this fundamental question “is it possible to recover coarse hydrophilic mineral particles (e.g. quartz) by using natural talc hydrophobic nanoparticle?”

2. Materials and methods

2.1. Minerals characterization studies

2.1.1. Quartz. The quartz sample was purchased from Tekno Silica Ltd., Iran and it was analysed by X-Ray Fluorescence (XRF, Philips X Unique II) (Table 1). XRD was used to identify the main mineral phases. Afterwards, the sample was crushed, ground and screened to different size fractions.

Table 1 The chemical composition and various size fractions of the quartz sample

a d50: the cumulative 50% point of diameter (the average particle size or median diameter).
XRF (head sample)	SiO2 (98.51%)	Al2O3 (0.85%)	Fe2O3 (0.24%)	MgO (0.15%)	CaO (0.05%)	Na2O (0.01%)	K2O (0.03%)
Size range	−500 + 38 μm		−250 + 38 μm		−125 + 38 μm
d50^a	295 μm		150 μm		85 μm

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2.1.2. Talc. Talc sample was obtained from Tanin Arad Yekta Ltd., Iran, and it was analysed by XRF (Table 2). XRD analysis showed talc as the main mineral. The talc sample was crushed, ground and finally reground using Planetary Ball Mill to classify different sizes. The average size of the talc samples were analysed using dynamic light scattering (DLS, Nano-ZS, UK) (Table 2 and Fig. 1A). Interestingly, the presence of talc nanoparticles on the quartz surface can be visualized by SEM as demonstrated in Fig. 1B.

Table 2 Characteristics of the talc nanoparticles sample

a d50: the cumulative 50% point of diameter (the average particle size or median diameter).
XRF (head sample)	SiO2 (61.95%)	Al2O3 (0.57%)	Fe2O3 (2.03%)	MgO (30.29%)	CaO (0.04%)	L.O.I. (4.82%)

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	Talc-A	Talc-D-0	Talc-D-5
Size range	−2.5 μm + 50 nm	−750 + 65 nm	−500 + 30 nm
d50^a	567 nm	350 nm	235 nm

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Fig. 1 (A) Size distribution of three types of talc nanoparticles detected by dynamic light scattering. (B) High resolution SEM image showing different size of Talc-A nanoparticles (white spots) coated on the quartz mineral surface (dark background) in floatation concentrate.

2.2. Zeta potential measurements

Zeta potential measurements were performed as a function of pH, using a Malvern instrument (Nano-ZS, UK). Initially, 0.05 g of each sample was added to 50 mL of NaCl solution, as background electrolyte (0.02 mol L⁻¹) for different size groups of quartz and talc. The resultant suspensions were conditioned for 15 minutes during which the suspension pH was measured. The pH was adjusted using NaOH or H₂SO₄ solutions. The suspensions were further treated in an ultrasonic bath for one more min and then they were transferred to the rectangular capillary cell of the instrument. The reported results are the average of at least three repeated experiments. The repeated tests showed a measurement error of ±0.01 mV. The temperature was fixed at 25.0 ± 0.1 °C during the measurements.

2.3. Flotation experiments

Flotation experiments were conducted using a 500 mL modified Hallimond tube cell (Fig. 2). This Pyrex flotation cell has a fix diameter of 35 mm and height of 300 mm. It has also two feeding parts for material injection and pH control, as well as air bubble flow rate controller via a fritted glass at the bottom. The upper part of the cell is inclined 50 degrees to collect the floated particles. It is also equipped with a sampler valve to collect floated materials. The level of water in the cell can be controlled by another valve at the top of the cell.¹⁸

Fig. 2 The modified Hallimond tube.¹⁸

In each experiment, 10 g of quartz at different sizes were prepared and conditioned for 2–4 minutes, and pH was adjusted by NaOH or H₂SO₄ solutions. The suspensions were preconditioned by ultrasound when required and were conditioned by different amount of talc nanoparticles with different sizes. The flotation tests were conducted at desired pH and at a constant air flow rate of 1 L min⁻¹.¹⁹ 75 ppm polypropylene glycol (A65, Dow)²⁰ was used as frother. The talc nanoparticles dosage used in flotation tests were 0.5, 2, 8, 10, 20, 45 and 80 kg t⁻¹. The conditioning time for adjusting pH and collector adsorption was 5 minutes for each test. Flotation was investigated at different pH, different size of quartz and talc nanoparticles, as well as different dosage of talc as a function of time. Other factors kept constant. Finally, the concentrates and the tailings were filtered, dried and weighed for further analysis.

2.4. Contact angle measurements

The contact angles of water on the quartz samples were measured using the sessile drop method. These tests were conducted in distilled water in the absence and presence of talc nanoparticles. Hand-picked quartz samples were cut to a proper size, polished and finally washed in distilled water. The cleaned samples were immersed in the talc nanoparticles suspension for 15 min. Then they were washed and air-dried at room temperature. The treated samples were placed in a rectangular glass cell on a flat surface and a drop of water with 2–4 mm diameter was introduced on the sample using a microliter syringe. The needle was maintained in contact with the drop during the measurement. The contact angles were measured using an automated camera (Sony, 18 Mega Pixel resolutions) attached to a computer-controlled apparatus. The contact angle values were determined by fitting a mathematical expression to the shape of the droplet and finally calculating the slope of the liquid–solid–vapour interface line.

3. Results and discussion

3.1. Effects of zeta potential

Zeta potential of quartz and talc nanoparticles as a function of pH is shown in Fig. 3. The pH range at which the zeta potential of quartz and talc are oppositely charged can be used as the best pH that talc nanoparticles can electrostatically attach to the quartz surface. The isoelectric points, i.e.p., of quartz and talc were estimated at around pH 0.8 and 2, respectively, which are within the reported range values (i.e. pH 0.5–3 for quartz^4,21–25 and pH 2–3 for talc^26–32). Therefore, pH between 0.8 and 2 (e.g. 1.5) can be selected for flotation where quartz and talc surfaces are oppositely charged and can electrostatically interact.

Fig. 3 Variation of zeta potential against pH for quartz and talc.

3.2. Flotation of coarse and fine quartz using talc nanoparticles

3.2.1. Effects of entrainment. In flotation, entrainment is transferring of fine particles with water to the concentrate, without any affect from collectors. The relation between water and solid recovery was used to determine the entrainment in flotation of quartz when talc particles were used as collector.³³ It is assumed that at the same water recovery, the amount of entrained solids is the same as the difference between that recovered with and without collector.³⁴ The amount of quartz recovered by true flotation could be calculated from the difference between the mass of solids recovered in the presence and absence of collector at the same water recovery. This was therefore regarded as a suitable method to study the rate of true flotation, and entrainment. Previous researchers have also reported similar results.³⁵ According to Trahar³⁴ the presence of hydrophobic particles should not have an essential effect on the entrainment of hydrophilic minerals at moderate pulp densities.

In each test, 10 g of quartz (−250 + 38 μm) with a mean diameter of 150 μm was used. Fig. 4 shows the quartz recovery with no collector, and with 8 kg t⁻¹ talc nanoparticles (567 nm) as collector. Without using talc as collectors, the recovery of quartz linearly increases until 50%. This phenomenon is due to the hydraulic entrainment and possibly the short height of the flotation cell. By using 8 kg t⁻¹ talc nanoparticles 67% of quartz is recovered during 50 minutes of flotation. Fig. 4 also shows that at the same conditions at constant water recovery during 60 minutes of flotation, the quartz recovery is 70% and 36% with and without talc nanoparticles, respectively. In this condition the true flotation recovery is about 35%.

Fig. 4 Recovery of quartz and water in the presence and absence of collector. Quartz size: −250 + 38 μm, pH = 1.5, frother A65: 75 ppm, talc nanoparticles 567 nm; 8 kg t⁻¹.

3.2.2. Effects of pH and talc dosage. The experimental results show that the recovery of quartz in different pH values versus talc dosages have similar pattern during 25 minutes of flotation. The maximum quartz recovery (−500 + 38 μm) is obtained at pH 1.5 and 8 kg t⁻¹ of talc. Moreover, the maximum recovery of quartz particles is 24%, 17% and 13% at pH 1.5, 6.0 and 10.5, respectively. Factors such as gravity forces, particle size and electrostatic repulsion low or high surface coverage by talc nanoparticles can cause low recovery particularly at pH 6.0 and 10.5.

3.2.3. Effects of quartz particle size. The experimental results show that the recovery of quartz for all different sizes is low in the first minute of flotation (data not presented). However, after 5 minutes of flotation, the recovery of quartz with −125 + 38 μm size rises dramatically to 53% when pH is 1.5 while talc dosage is 8 kg t⁻¹. In contrast, at these condition the recovery of coarser quartz particles (i.e. −250 + 38 μm and −500 + 38 μm) increases gradually to about 25% and 8%, respectively. In other words, the quartz flotation recovery increases as its particle size decreases.

3.2.4. Effects of talc particle size and dosage. Fig. 5 provides an overview of the effect of dosage of different talc nanoparticles on the quartz flotation recovery. At first glance, it is clear that much less amount of smaller talc nanoparticle is required to reach the maximum flotation recovery in each test.

Fig. 5 Total quartz recovery as a function of type and dosage of talc at different flotation time. The quartz particle size −125 + 38 μm and pH 1.5. The error bars represent the 95% confidence interval of the average values.

The quartz recovery is initially low (e.g. within 1 min), and also less amount of talc is needed (i.e. <20 kg t⁻¹). When large amount of talc is used (e.g. 80 kg t⁻¹), a steep rise in the quartz recovery is observed. In contrast, it can be seen that 90% recovery of quartz is achieved by using 45 kg t⁻¹ talc after 30 minutes. By further increasing the talc dosage to 80 kg t⁻¹, the recovery tends to decline gradually to reach the same level as of that obtained for the lowest dosage of talc (Fig. 5A). This may be due to the factors such as talc nanoparticles aggregation, changing the pulp viscosity, or longer bubbles resistance time.

Fig. 5B shows that for smaller talc nanoparticles, it is possible to decrease the talc consumption in shorter time of flotation. A sharp increase of quartz recovery is observed when the flotation time increases from 1 to 5 minutes. So the recovery reaches to the maximum level of 52% at 10 kg t⁻¹ of talc dosage, compared to 25 kg t⁻¹ for the maximum recovery at 1 minute. Afterwards, the recovery levels of quartz tend to decline steadily until reaching a plateau particularly when 30 kg t⁻¹ of talc is used. At least 92% of quartz recovery was obtained at a bit less than 10 kg t⁻¹ of talc dosage with talc particle size 350 nm (Fig. 5B). This is less than that needed to achieve 90% recovery for quartz when coarser talc was used (i.e. 45 kg t⁻¹ talc with 567 nm size, Fig. 5A). Therefore, reducing the size of talc nanoparticles can result in increasing the quartz recovery with using less amount of talc in shorter flotation time. Fig. 5C shows that by further reducing the talc size to 235 nm, the maximum quartz recovery can be obtained when only 2 kg t⁻¹ of talc is used. The similar pattern between all curves in Fig. 5 suggests that the initial adsorption of talc nanoparticles does not cause enough surface hydrophobicity for quartz particles to enhance flotation recovery. It should be noted that talc nanoparticles have a large active surface area which can promote their adsorption onto the quartz surface.^36–38

The particle size distribution of talc nanoparticles after grinding was found adequate to enable these particles to be adsorbed on the quartz surface, and promotes the flotation. However, investigations the effect talc nanoparticles preparation, for example producing more uniform size distribution, as well as surface treatment of talc particles are suggested for future work.

3.3. Comparing talc with traditional collectors

In Fig. 6, the flotation of quartz in the best condition of the current study (i.e. talc with 235 nm size and 2 kg t⁻¹ dosage, flotation time 30 min, quartz size −125 + 38 μm, and pH 1.5) is compared with the reported values of quartz recovery using cationic diamine collector and sodium oleate.^39,40 It can be seen that it is experimentally possible to achieve more than 90% quartz recovery, but at different pH range. Talc and either diamine or sodium oleate collectors lead to increase quartz flotation recovery on the base of physical and chemical interaction. The pH range between the i.e.p. of quartz and talc is indeed the range that talc and quartz particles are oppositely charged. Therefore, there is a chance for talc particles to be electrostatically attached to the quartz surface (physical interaction). Chemically, the addition of diamine leads to an increase in the zeta potential of quartz causing the cationic collector to be adsorbed. When pH of the suspension is less than the i.e.p. of quartz (i.e. pH ≈ 4.7) the quartz surface is positively charged and diamine can be adsorbed on this mineral via hydrogen bonding. When pH of the suspension is higher than 4.7, electrostatic adsorption takes place between the quartz and collector since the surface of quartz is negatively charged.³⁹ Similar behaviour can be seen when sodium oleate is used.⁴⁰

Fig. 6 Comparing quartz recovery using diamine collector, 1 × 10⁻⁵ M quartz size −150 + 38 μm (ref. 39) and sodium oleate, 4 × 10⁻⁴ M (ref. 40) and talc nanoparticles 235 nm, 2 kg t⁻¹, quartz size −125 + 38 μm in different pH (present work).

In summary, pH is a key parameter in collector adsorption for both physical and chemical attraction. This kind of physical behaviour of talc nanoparticles as solid collector in flotation process is similar to the cationic diamine and oleate collectors. Therefore, talc nanoparticles introduce a new class of solid, natural hydrophobic collector to float hydrophilic minerals such as quartz.

3.4. Effects of contact angle

Fig. 7 shows the contact angle of water on the quartz surfaces at different pH values. The concentration of talc nanoparticles was 2 kg t⁻¹ in the contact angle measurement tests. The low contact angle of quartz without collector shows that it is extremely hydrophilic. Therefore, the natural floatability and hydrophobicity of quartz is not enough for flotation. The obtained contact angle data for quartz (i.e. 40°–50°) in the absence of collector is consistent with the previously reported data.^41,42 After addition of talc nanoparticles as collector, the contact angle of quartz surfaces increases significantly, especially in the acidic range. So, it can be concluded that talc nanoparticles are physically adsorbed on the quartz particles surfaces due to the different electrostatic charges. This results in a remarkable increase in the quartz hydrophobicity in the range of pH 1.0–2.5. Therefore, the hydrophobicity of quartz mineral sufficiently increases and they can be floated at pH 1.5. This is in agreement with what observed in zeta potential and flotation tests as previously discussed.

Fig. 7 Contact angle of quartz mineral with and without talc nanoparticles as collector.

It is often argued that there can be a link between contact angle values and maximum size of particles that can be floated. In this work, lower contact angles were found for the coarser quartz particles which may explain the difficulties with floating these large particles although the gravity force can be also influential. The work is in progress and results will be discussed in future publications.

3.5. Talc nanoparticles adsorption and quartz recovery

Fig. 8 shows the recovery of quartz at different conditions as a function of the amount of talc adsorbed on the quartz particles. It can be seen that the recovery of quartz increases when more talc nanoparticles are adsorbed until reaching a plateau.

Fig. 8 Influence of the talc nanoparticles coverage on quartz particle (size ranges −250 + 38 μm and −500 + 38 μm at pH 1.5) on the quartz recovery.

It should be also highlighted that as the size of quartz particles decreases the optimum talc dosage to reach the highest level of quartz recovery (the plateau levels as discussed) increases from 8 kg t⁻¹ to 45 kg t⁻¹. After that, although the amount of talc transferred to the concentrate increases, the quartz recovery decreases. This can be due to the entrainment, coagulation, particles aggregation and hydrodynamic forces which needs to be further investigated.

The maximum coverage of randomly deposited non-interacting talc on the quartz surface is predicted to be less than 5%. Of course, some part of the floated talc nanoparticles can entrain along with the quartz particles. Therefore, it is assumed that the difference between the total floated talc and the total adsorbed talc are due to the entrainment. This result suggests that the majority of talc nanoparticles are physically adsorbed on the quartz surface. The smaller talc nanoparticles are able to recover more quartz particles compared to the larger particles. In fact, the maximum recovery for quartz with −250 + 38 μm and −125 + 38 μm size are respectively obtained when 2% and 27% of the total talc transferred into the froth. The maximum surface coverage of the talc nanoparticles which is distributed individually on the quartz particle surfaces (−250 + 38 μm) is achieved at around 9% adsorption after 12 minutes (Table 3). Other researchers have reported that less than 20% of mono layer coverage of conventional surfactant collectors can give a good flotation performance.⁷ Furthermore, surfactant collectors at low coverage are not uniformly distributed on the mineral surface. Instead, they are usually present as hemi micelles.^43,44

Table 3 Talc nanoparticle coverage, adsorption and quartz recovery during time, quartz size −250 + 38 μm, Talc-A: 8 kg t⁻¹, pH = 1.5

(t) min	(λT) %	(m) %w	(R) %	Predicted by model
				Eqn (3)	Eqn (4)
1	8.11	1.15	9.42	9.16	9.67
5	8.64	2.00	25.00	20.52	20.79
12	9.01	2.86	37.70	32.66	32.29
22	10.45	3.71	48.33	46.55	45.53
60	13.35	4.51	70.73	76.44	73.37
135	14.47	5.21	83.08	100.00	100.00
165	15.48	5.45	95.61	100.00	100.00

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Table 3 shows a good correlation between the quartz recovery, talc nanoparticle coverage and talc adsorption. In this table, the empirical parameter “λ_T”, depends on the actual percentage of total talc nanoparticle coated and presented on the quartz particle surfaces. “m” is defined as the actual weight percentage of the talc adsorbed on quartz from total added talc. “R” is the cumulative recovery of quartz particles during flotation time, “t”.

The relationship between cumulative quartz recovery within time has similar pattern to that of talc coverage within time , and also the wt% of adsorbed talc within time . In order to develop a model, the cumulative quartz recovery is defined as a function of flotation time relating to the surface coverage and talc adsorption. Therefore, the relationship between R, t, λ_T and m are defined as two function of R = f(λ_T,t) and R = g(m,t) by the following empirical equations:

The flotation recovery can be obtained in terms of λ_T, m and t:

also,

The simplest relationship between quartz recovery, surface coverage, and talc adsorption as well as flotation time defined for quartz with −250 + 38 μm size at pH 1.5 (with low error of around ±1% and 95% confidence interval). It is possible to predict the amount of total coverage by measuring “m” at a specific time using eqn (5). The above relationships can be developed as a generic model (eqn (6)–(8)) for other size ranges of quartz:

where a, b, c, k, α, β, Φ, Ψ and θ are model constants.

On the base of experimental results and the model, quartz recovery rate increases by increasing the surface coverage. For example, as the total coverage increases from around 8% to 9%, the total quartz flotation recovery increases from around 9% to 37% only during 10 minutes. Therefore, it is possible to predict how much talc nanoparticles from the total added talc is adsorbed on the quartz surface. According to the experimental data, only 4.5 wt% of the total Talc-A (about 360 g t⁻¹ from 8 kg t⁻¹) and about 13.5% surface coverage is needed to obtain more than 70% quartz recovery. This also suggests how the behaviour of talc as solid collector is different from the conventional liquid collectors.

4. Conclusion

This study aims to identify novel application of natural hydrophobic talc nanoparticles as a collector in quartz flotation. There is a pH range between the i.e.p. of quartz and talc which could be selected for flotation where talc and quartz particles are oppositely charged and electrostatic attraction is possible. Similar to conventional flotation of quartz where liquid organic collectors are used, the optimum particle size of quartz for flotation by talc nanoparticles as collector was found to be around −125 + 38 μm. The required dosage of talc nanoparticles was found to be directly related to their size and specific surface area. Reducing the talc nanoparticles size from 567 nm to 235 nm resulted in significant decrease in the talc consumption from 45 kg t⁻¹ to less than 2 kg t⁻¹ to obtain the similar flotation recovery. An empirical equation was developed to predict the recovery of quartz as a function of flotation time, talc coverage and talc adsorption. It was demonstrated that talc nanoparticles have important role as effective collector in flotation of hydrophilic minerals such as quartz.

Acknowledgements

The authors are grateful to mineral processing and geochemical Laboratories School of Mining Engineering, University of Tehran for their contribution on this research. Also, the authors would like to acknowledge the financial support of University of Tehran Science and Technology Park under grant number 94048.

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