Flotation kinetics of molybdenite in common sulfate salt solutions

Minerals Engineering 148 (2020) 106182

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Short communication

Flotation kinetics of molybdenite in common sulfate salt solutions a

Hongjia Zhu , Yubiao Li a b

a,b,⁎

a

a

, Clement Lartey , Wanqing Li , Gujie Qian

T

b

School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, Hubei, China College of Science and Engineering, Flinders University, Bedford Park, Adelaide 5042, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Molybdenite Flotation Kinetics Sulfate salt

This study investigates the kinetics of molybdenite (MoS2) flotation in sulfate media (Na+, K+, Ca2+ and Mg2+) with a concentration of 10−4–10−2 M. Results indicate that both monovalent and divalent cations inhibit MoS2 flotation. Three flotation models were used to fit the flotation kinetic data. The Gamma model gives the best 2 (adjusted coefficient of determination) and RMSE (root mean square error), goodness of fit in terms of RAdj however, fails in predicting MoS2 flotation recovery in MgSO4 solution due to the contradictory to the actual flotation trend. In contrast, the rectangular model fits MoS2 flotation in all the sulfate solutions. The flotation rate constant k values decrease with increasing sulfate concentration, indicating that the increased sulfate concentration reduces MoS2 flotation rate. The lowest k was found in the MgSO4 media, suggesting that MgSO4 is most detrimental to MoS2 flotation recovery. In addition, the R∞ was decreased with increasing sulfate concentration in an order of Mg2+ > Ca2+ > K+ > Na+, with the minimum R∞ being obtained in 10−2 M MgSO4, probably due to the formation and adsorption of hydrophilic Mg(OH)2 onto MoS2 surface.

1. Introduction Molybdenite (MoS2) is the most important molybdenum (Mo) source and normally associated with copper sulfide ores (Hirajima et al., 2014; Li et al., 2018b; Song et al., 2012). Froth flotation, as an efficient physico-chemical technique to separate minerals, is widely used in MoS2 recovery (Abkhoshk et al., 2010; Ai et al., 2017; Castro et al., 2016; Hernáinz and Calero, 2001). As flotation consumes a massive amount of freshwater, the use of groundwater, recycled water or seawater has become increasingly important (Hirajima et al., 2016; Wang and Peng, 2014). Previous studies have reported mineral flotation in sea and saline water, e.g., Qiu et al. (2017) applied high-salinity water to separate pyrite (FeS2) from chalcopyrite (CuFeS2) and MoS2 under alkaline conditions. These alternative water sources contain various inorganic electrolytes including KCl, NaCl, Na2SO4, MgSO4, and CaCl2, which may have different effects on mineral flotation (Mu and Peng, 2019; Suyantara et al., 2018). The presence of divalent ions (i.e., Ca2+, Mg2+, SO42− and CO32−) in aqueous solution, can form colloidal precipitates under alkaline condition (Suyantara et al., 2018) and may have detrimental impacts on flotation. For example, Hirajima et al. (2016) reported that Mg2+ ions strongly decreased MoS2 flotation efficiency above pH 9 due to the adsorption of Mg(OH)2 precipitates. However, the flotation kinetics of MoS2 in salt solution especially in

⁎

sulfate solution have not attracted much attention. Our recent study (Li et al., 2018c) has revealed the effects of some common cations (i.e., Na+, K+, Ca2+ and Mg2+) and sulfate (SO42−) anions on MoS2 flotation recovery, but the flotation kinetics of MoS2 in sulfate solution were not examined. Therefore, it is essential to understand how the impurity ions present in sulfate solution affect the flotation kinetics of MoS2. In this study, MoS2 flotation was carried out in sodium sulfate (Na2SO4), potassium sulfate (K2SO4), calcium sulfate (CaSO4) and magnesium sulfate (MgSO4) aqueous media, with the flotation recovery being fitted using three models. The specific aim was to understand the influence of these common sulfate salts on the flotation kinetics of MoS2 and the related influencing mechanisms. 2. Experimental 2.1. Materials The MoS2 sample was obtained from Guilin, Guangxi province, China. Block samples were crushed and ground in a three head grinding machine (RK/XPM, Wuhan Rock Grinding Equipment Manufacturing Co., Ltd., Wuhan, China), and then wet sieved to a size fraction of 38–75 μm. The sized samples were washed thoroughly using ethanol, dried in a vacuum oven at 30 °C for 24 h, then sealed and stored in a freezer to avoid oxidation. X-ray diffraction (XRD, Fig. 1) analysis

Corresponding author. E-mail address: [email protected] (Y. Li).

https://doi.org/10.1016/j.mineng.2020.106182 Received 15 May 2019; Received in revised form 13 December 2019; Accepted 2 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.

Minerals Engineering 148 (2020) 106182

H. Zhu, et al.

3. Results 3.1. Flotation experiment Fig. 2 shows the cumulative MoS2 recovery as a function of flotation time in four different sulfate salts. About 90% of MoS2 recovery was obtained in the absence of sulfate salts (pure water), in agreement with the results reported previously (Wan et al., 2017; Zanin et al., 2009). However, MoS2 flotation recovery was decreased to various degrees in the presence of sulfate salts within the concentration range investigated, showing an order of Mg2+ > Ca2+ > K+ > Na+. In addition, the MoS2 flotation recovery was significantly decreased with increasing sulfate salt concentration from 0 M to 10−2 M. 3.2. Flotation kinetics Table 2 shows the fitted kinetic parameters based on the cumulative recovery in salt solutions at various concentrations using Models 1 and 2, showing greater R∞ values for Model 2 (Li et al., 2018a; Ni et al., 2018). In addition, the R∞ value of Model 1 was lower than the actual cumulative recovery at 10 min, e.g., 89.67% vs. 90.12%, indicating that Model 1 might be not suitable for fitting the ultimate MoS2 flotation 2 recovery. It should be noted that, the Gamma model has greater RAdj and lower RMSE, as compared to Model 2.

Fig. 1. X-ray diffraction (XRD) pattern of MoS2 sample.

indicates a high purity of MoS2 sample used herein, with all the typical peaks being detected. Flotation solution was prepared by adding analytical-grade chemical reagents into the Milli-Q water (Millipore® ultrapure water; Billerica, MA, USA), including Na2SO4, K2SO4, CaSO4 and MgSO4. The slurry pH was adjusted using sodium hydroxide (NaOH) and hydrochloric acid (HCl) solutions when required.

4. Discussion Fig. 3 shows the flotation recovery and the fitted R∞ using Models 1 and 2. It is clearly that the predicted R∞ based on Model 1 in all the salt solutions is lower than that of the flotation recovery at 10 min, indicating that this Model is not suitable for predicting the ultimate flotation recovery of MoS2. In contrast, the predicted flotation recovery based on Model 2 is to some extents greater than that at 10 min. Therefore, Model 2 is more reasonable to fit the ultimate flotation recovery of MoS2 in the four types of solution with salt concentration ranging from 10−4 to 10−2 M. Fig. 4 shows the evolution of predicted R∞ in the case of MgSO4 solution based on Models 2 and 3. In contrast to the decreased R∞ when MgSO4 concentration is increased from 10−4 to 10−2 M, the R∞ fitted by Model 3 is increased with increased MgSO4 concentration, e.g., from 78.47% (10−4 M) to 90.7% (10−2 M). However, this does not agree with the actual flotation experiment herein, e.g., the MoS2 flotation recovery decreases with increased MgSO4 concentration. Actually, it has been commonly reported that seawater inhibits mineral flotation under alkaline conditions, primarily due to the adsorption of Mg-containing precipitates onto the mineal surface (Hirajima et al., 2016; Li et al., 2017; Qiu et al., 2016; Rebolledo et al., 2017; Suyantara et al., 2018). Specifically, colloidal Mg(OH)2 were formed under alkaline condition due to Mg2+ present in seawater and precipitated onto MoS2 surface, greatly reducing its hydrophobicity (Jeldres et al., 2017; Li et al., 2018b; Suyantara et al., 2018). Therefore, Model 2 (one-parameter) is better in fitting MoS2 flotation in sulfate solution, as compared to Model 3 (Gamma model).

2.2. Single mineral flotation tests All MoS2 flotation tests were performed in a XFGⅡmechanical flotation machine (Wuhan Exploration Machinery Factory, China). 0.2 g of MoS2 powder (38–75 µm) and 25 mL conditioned solution were added into a 40 mL flotation cell. The MoS2 particles were floated at pH 10 in the presence of 10−4 M–10−2 M of Na2SO4, K2SO4, CaSO4 and MgSO4 over 10 min. The froth products were collected every 10 s in the next 10 min for sampling, at an airflow rate of 1.2 cm/s and at 1200 rpm. Froth concentrates and residues were dried in a vacuum oven at 30 °C for 24 h and subsequently weighed to determine the cumulative recovery.

2.3. Flotation kinetic models Three flotation kinetic models (Table 1) were examined to study the effects of sulfate salts on the flotation kinetics of MoS2. Models 1 and 2 are one-parameter models while Model 3 (Gamma model) is a two2 parameter model. Both RAdj (adjusted coefficient of determination) and RMSE (root mean square error) were used to assess the applicability of each model (Ahmed, 2013; Hernainz et al., 1996).

Table 1 Three flotation kinetic models applied. No.

Name of model

Formula

R (t ) = R∞ [1 − exp(−k1 t )]

1

Classical first-order model

2

First-order model with rectangular distribution

3

Gamma distribution

References

{ (1 −

R (t ) = R∞ 1 −

1 [1 k2 t

R (t ) = R∞

1 (1 + bt )a

− exp(−k2 t )]

)

Dowling et al. (1985)

}

Yuan et al (1996) Vinnett et al. (2019)

Note: t is the flotation time; R(t) is the cumulative recovery of minerals at time t; R∞ is the maximum cumulative recovery (t → ∞). In Model 1, k1 is a deterministic rate constant (originally proposed by Garcia-Zuñiga (1935) while k2 is the maximum rate constant in Model 2 (Huber-Panu et al., 1976). a and b in Model 3 are the Gamma shape and rate parameters, respectively. 2

Minerals Engineering 148 (2020) 106182

H. Zhu, et al.

100

(a)

80

Cumulative recovery (%)

Cumulative recovery (%)

100

60

40

20

0

Pure water

Na2SO4

Cumulative recovery (%)

100

K2SO4

CaSO4

(b)

80

60

40

20

0

MgSO4

Pure water

Na2SO4

K2SO4

CaSO4

MgSO4

(c)

80

60

40

10 min 8 min 5 min 3 min 1 min

20

0

Pure water

Na2SO4

K2SO4

MgSO4

CaSO4

Fig. 2. MoS2 recovery in: (a) 10−4 M, (b) 10−3 M and (c) 10−2 M sulfate solution at pH 10. Table 2 Fitted parameters of MoS2 flotation in sulfate solution using three flotation kinetics models. Sulfate salt

Pure water

−4

Concentration Model 1

Model 2

Model 3

Na2SO4

K2SO4

10

10

−3

10

−2

−4

CaSO4

10

10

−3

10

−2

−4

MgSO4

10

10

−3

−2

10

10−4

10−3

10−2

2 RAdj

0.9999

0.9999

0.9999

0.9995

0.9992

0.9961

0.9959

0.9992

0.9913

0.9980

0.9999

0.9871

0.9868

RMSE R∞ k1

0.478 89.67 1.211

0.464 86.96 1.177

0.399 83.67 1.047

0.816 79.48 0.936

1.059 84.20 1.156

2.228 80.39 1.022

1.965 69.13 0.964

1.023 81.65 1.141

3.103 74.63 1.099

1.296 65.09 1.005

0.381 78.46 1.080

3.333 65.97 0.742

2.964 59.02 0.468

2 RAdj

0.9983

0.9982

0.9978

0.9983

0.9992

0.9999

0.9999

0.9989

0.9985

0.9997

0.9971

0.9960

0.9909

RMSE R∞ k2

1.664 94.95 2.909

1.649 92.25 2.799

1.746 89.39 2.409

1.459 85.50 2.100

1.045 89.47 2.725

0.2087 86.14 2.32

0.363 74.32 2.163

1.209 86.79 2.687

1.304 79.70 2.523

0.464 69.79 2.276

1.892 83.67 2.501

1.853 71.81 1.629

2.461 65.99 0.962

2 RAdj

1

1

1

0.9999

1

1

0.9999

0.9999

0.9997

0.9999

0.9999

0.9993

0.9943

RMSE R∞ a b

0.1354 90.07 9.242 0.1397

0.1785 87.34 9.822 0.1271

0.122 84.00 12.75 0.08558

0.482 80.20 7.156 0.1401

0.1204 85.36 3.933 0.3381

0.1198 85.07 1.545 0.8727

0.3877 73.3 1.584 0.786

0.4407 82.61 4.464 0.2893

0.6347 86.13 0.6998 2.653

0.3349 67.02 2.591 0.4636

0.4430 78.47 178.9 0.006055

0.8843 87.93 0.4958 2.107

2.255 90.7 0.4325 1.154

hydrophobicity of MoS2, thereby decreasing its recovery. According to the fitting results of Model 2, the decreased R∞ and k are also related to the production and adsorption of these precipitates and complexes onto MoS2 surface. Fig. 6 shows the variation of the flotation rate constant k from fittings using Model 2 as a function of sulfate salt concentrations. The low k indicates poor flotation recovery of minerals (Yang et al., 2019). In this work, the k values decrease with increasing sulfate concentration, indicating that the increased sulfate concentration reduces MoS2 flotation rate. The lowest k was found in the MgSO4 media, suggesting that MgSO4 is most detrimental to MoS2 flotation recovery.

In addition, as shown in Fig. 5, the R∞ (Model 2) was decreased with increasing sulfate concentration, with the impact being in the order of Mg2+ > Ca2+ > K+ > Na+. The lowest R∞ was obtained from the flotation experiment with 10−2 M MgSO4, in line with the flotation recovery (Fig. 2). Li et al. (2018c) has reported that MoS2 can be oxidized or leached in Na2SO4 and K2SO4 solution, increasing the electrostatic repulsion between negatively charged bubbles and MoS2 surfaces, further reducing MoS2 flotation recovery. In addition, the negative effect of K2SO4 is stronger than that of Na2SO4, consistent with that shown in Fig. 3. Moreover, as reported in many other studies (Hirajima et al., 2016; Jeldres et al., 2016; Li et al., 2018c; Li et al., 2017), the CaMoO4, Mg(OH)2 and MgOH+ formed and adsorbed on the surface of MoS2 in the solution of CaSO4 and MgSO4, reduce the surface 3

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Fig. 3. Comparison among the flotation recovery at 10 min and fitted R∞ by Models 1 and 2.

Fig. 6. Effects of sulfate salt concentration on flotation rate k fitted by Model 2. 2 model generally gives the greatest RAdj and lowest RMSE, but not suitable in fitting MoS2 flotation in MgSO4 solution, due to the opposite prediction to the actual flotation recovery. In contrast, the Rectangular model is considered as the best model for fitting MoS2 flotation in sulfate solution. In addition, the flotation rate constant of MoS2 was decreased to various degrees with increasing sulfate salt concentration from 10−4 to 10−2 M, with the most pronounced decrease being observed in the MgSO4 solution controlled under alkaline condition, probably due to the formation and adsorption of Mg precipitates (i.e., Mg(OH)2) onto MoS2 surface.

CRediT authorship contribution statement Hongjia Zhu: Conceptualization, Methodology, Data curation, Writing - original draft. Yubiao Li: Writing - review & editing, Supervision, Project administration, Funding acquisition. Clement Lartey: Resources, Data curation, Investigation. Wanqing Li: Validation. Gujie Qian: Validation.

Fig. 4. The predicted R∞ in MgSO4 solution by Model 2 and Model 3.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge financial supports from the National Natural Science Foundation of China under the projects 51974215, 51604205 and 51774223. The financial support from the undergraduate research foundation for independent innovation from Wuhan University of Technology (2019-ZH-A1-02) is also gratefully acknowledged. References Abkhoshk, E., Kor, M., Rezai, B., 2010. A study on the effect of particle size on coal flotation kinetics using fuzzy logic. Expert Syst. Appl. 37, 5201–5207. Ahmed, M.M., 2013. Discrimination of different models in the flotation of Maghara coal. Miner. Process. Extract. Metall. 113, 103–110. Ai, G., Yang, X., Li, X., 2017. Flotation characteristics and flotation kinetics of fine wolframite. Powder Technol. 305, 377–381. Castro, S., Lopez-Valdivieso, A., Laskowski, J., 2016. Review of the flotation of molybdenite. Part I: Surface properties and floatability. Int. J. Miner. Process. 148, 48–58. Dowling, E.C., Klimpel, R.R., Aplan, F.F., 1985. Model discrimination in the flotation of a porphyry copper ore. Miner. Metall. Process. 2, 87–101. Garcia-Zuñiga, H., 1935. La recuperación por flotación es una función exponencial del

Fig. 5. Effects of sulfate salt concentration on R∞ fitted by Model 2.

5. Conclusions The influence of four common sulfate salts on the flotation kinetics of MoS2 was thoroughly explored and studied using three flotation kinetic models. The flotation results show that the recovery of MoS2 was decreased with increased concentration of Na2SO4, K2SO4, CaSO4 and MgSO4, in the order of Mg2+ > Ca2+ > K+ > Na+. The Gamma 4

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