J. Cent. South Univ. (2018) 25: 2971-2978

DOI: https://doi.org/10.1007/s11771-018-3967-5

Improving collecting performance of sodium oleate using a polyoxyethylene ether in scheelite flotation

CHEN Chen(陈臣)¹, ZHU Hai-ling(朱海玲)^{1, 2}, QIN Wen-qing(覃文庆)¹,CHAI Li-yuan(柴立元)², JIA Wen-hao(贾文浩)¹

1. School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China;

2. School of Metallurgy and Environment, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: In order to improve the scheelite flotation with sodium oleate (NaOL), the effect of a non-ionic polyoxyethylene ether (JFC-5) on the floatability of scheelite was investigated through flotation experiments at 10 °C, compared with 60 mg/L NaOL alone, the recovery of scheelite is improved from 22% to 85% in the presence of JFC-5 with a mass ratio of 20% at pH 10. Moreover, the resistance to Ca²⁺ of NaOL is increased. The adsorption mechanism was analyzed by zeta potential measurement, contact angle measurement and X-ray photoelectron spectroscopy (XPS) analysis. The results show that the adsorption of NaOL on scheelite surface is enhanced after adding JFC-5 due to the more negative zeta potentials and larger contact angles of scheelite. And the co-adsorption of NaOL and JFC-5 is confirmed by XPS analysis, so it is indicated that the adsorption of JFC-5 decreases the electrostatic repulsion between the oleate ions, resulting in the stronger adsorption of NaOL on scheelite surface. In short, the mixed NaOL/JFC-5 collector can effectively improve scheelite flotation.

Key words: scheelite; polyoxyethylene ether; synergistic effect; low temperature flotation; adsorption

Cite this article as: CHEN Chen, ZHU Hai-ling, QIN Wen-qing, CHAI Li-yuan, JIA Wen-hao. Improving collecting performance of sodium oleate using a polyoxyethylene ether in scheelite flotation [J]. Journal of Central South University, 2018, 25(12): 2971–2978. DOI: https://doi.org/10.1007/s11771-018-3967-5.

1 Introduction

After a long period of exploitation, the development of tungsten resources in China has been transferring from wolframite to scheelite, and great efforts have been devoted to the beneficiation of scheelite. Flotation is the most commonly used strategy for separating tungsten ores from gangue minerals [1–4], and fatty acids are generally used as the collector [5, 6]. However, their collecting performance is severely affected by slimes, metal ions and low temperatures [7, 8]. In industrial practice, the influence of temperature is particularly evident, how to improve the flotation results with fatty acids at low temperatures is a problem puzzling mineral processing field for many years.

Among various methods to enhance the low temperature flotation using fatty acids collectors, modified fatty acids and collector mixtures are two major research directions. It has been proved that chlorinated fatty acids, sulfated fatty acids and ether acids can enhance low temperature flotation, while they do not spread in industry because of the complex production process and relatively high cost [9, 10]. At present, the simplest and most efficient approach is using a collector mixture. The synergists, mainly including anionic surfactants and non-ionic surfactants, are beneficial to the flotation of non-sulphide minerals with fatty acids collectors [11]. On the one hand, the presence of synergists can decrease the influence of low temperature on collecting performance of fatty acids. BU et al [12] suggested that the addition of sodium dodecyl benzene sulfonate (SDBS) remarkably improved the solubility and dispersibility of oleic acid, thus enhancing its collecting capability and increasing the floatability of minerals at 10 °C. The non-ionic surfactants, such as NP-4, Tween-80 and MOA-9, have been successfully used to enhance the low temperature flotation with fatty acids [13–16]. On the other hand, it has been demonstrated that synergists can increase the flotation recovery and selectivity, enhance the adsorption of oleate ions at the liquid/air interface and to the mineral surface [17–21], and decrease the harmful effects of slime and Ca²⁺ on collecting performance [22].

In recent years, non-ionic surfactants have been widely used in minerals flotation because they show excellent properties in high solubility, good compatibility with other type surfactants, and good stability [23]. A large number of researches have shown that the mixed anionic and non-ionic surfactants show a lower critical micelle concentration (CMC) due to the synergistic effect in reducing the surface tension of water [24, 25], and thus they can exhibit better collecting abilities than individual surfactant. Nevertheless, researches mainly focus on the flotation technique and process in low temperature flotation system, the flotation performance of minerals, the interaction between anionic and non-ionic surfactants, as well as the adsorption mechanism of mixed anionic/non-ionic collectors on mineral surface at low temperatures have not been systemically studied.

Therefore, in this work, a polyoxyethylene ether (JFC-5) was considered as a synergist, its effect on flotation performance of scheelite with sodium oleate (NaOL) at 10 °C was investigated through flotation experiments, and the adsorption mechanism was studied by zeta potential measurement, contact angle measurement, and XPS analysis. The aim of this work is to improve the flotation performance of scheelite, and provide a theoretical foundation for the application of the mixed anionic/non-ionic collectors in flotation practice at low temperatures.

2 Experimental

2.1 Samples

The scheelite samples were obtained from Qinghai, China. The samples were crushed, ground and screened to collect the fractions of 38–74 μm size for micro-flotation experiments. Chemical analysis results show that the WO₃ grade is 75.71% with a high purity of 94.01%, and the gangue mineral is quartz.

2.2 Reagents

Sodium oleate (NaOL) of chemically pure was used as the collector, a iso-decanol polyoxyethylene ether C₁₀H₂₁O(C₂H₄O)₅H (JFC-5 for short) with the useful content of 99.5% was used as the synergist. The ratio of JFC-5 in the mixed collector is defined as the mass fraction of the used NaOL concentration. Solutions of HCl and NaOH were used to adjust the pH values of flotation pulp, deionized water was used in all experiments with the conductivity of 0.4–0.5 μS/cm.

2.3 Flotation tests

Micro-flotation tests were carried out using an XFG flotation machine of 40 mL (Jilin Exploration Machinery Factory, China). The specific process is shown in Figure 1. After flotation, the concentrate and tailing were filtered, dried and weighed to calculate the recovery independently.

Figure 1 Schematic of process for micro-flotation tests

2.4 Zeta potential measurement

Zeta potentials of scheelite were measured using a Malvern Zetasizer Nano ZEN36900 analyzer (England), and the effects of pH and NaOL concentration were investigated. For each measurement, the mineral suspension containing 0.1% solids was conditioned with the required reagents for 3 min, and an average of three measured values was recorded as the final result.

2.5 Contact angle measurement

Crystal scheelite sample was used for contact angle measurements using the MiniLab ILMS (GBX, France). The sample was first conditioned with deionized water, NaOL or the mixed NaOL/JFC-5 solution at different pH values respectively, then washed by deionized water and dried by nitrogen. When measuring, the deionized water with the volume of about 3.5 μL was dropped onto the scheelite surface. For each pH, we repeated at least three times at different sample locations, and then took an average.

2.6 X-ray photoelectron spectroscopy (XPS) analysis

In order to investigate the adsorption method of NaOL and JFC-5 on scheelite surface, XPS analysis of NaOL, JFC-5, scheelite treated with NaOL and the mixed NaOL/JFC-5 collector were conducted using a ESCALAB 250Xi spectrophotometer (Thermo Fisher Scientific Inc.). The scheelite samples less than 2 μm were treated with deionized water, NaOL and the mixed NaOL/ JFC-5 collector at pH 10, respectively, and then filtered and dried for entrusted tests. The peak fitting used summed Gaussian–Lorentzian (SGL) function with the mass fractions of 70% Gaussian and 30% Lorentzian.

3 Results and discussion

3.1 Flotation performance of scheelite

Figure 2 presents the floatability of scheelite as a function of pH using 60 mg/L NaOL as the collector at 10 °C. It can be seen that the scheelite recovery presents the similar trend with the increase of pH value, and a maximum recovery is obtained at pH 10. Moreover, the increase of JFC-5 ratio causes an increase in scheelite recovery. When JFC-5 ratio increases from 0 to 20%, the maximum recovery of scheelite increases from 22% to about 85%. The results clearly show that the mixed NaOL/JFC-5 collector is more effective in low temperature flotation of scheelite than NaOL alone. However, the JFC-5 ratio in the mixed collector has a suitable range, the previous research we have done showed that the recovery of scheelite will decrease when JFC-5 ratio is larger than 20% [26]. Therefore, the mixed NaOL/JFC-5 collector with a mass ratio of 20% is selected for further studies.

Figure 2 Effect of pH on floatability of scheelite in presence of NaOL or mixed collector

Figure 3 shows the effect of NaOL concentration on flotation performance of scheelite with or without Ca²⁺ using NaOL and the mixed NaOL/JFC-5 collector at pH 10. With the increase of NaOL concentration, the recovery of scheelitle with NaOL alone gradually increases to 80% at 120 mg/L NaOL, and then has no significant change. While for the mixed NaOL/JFC-5 collector, the recovery of scheelite is up to 84% when NaOL concentration is only 30 mg/L, but begins to decrease when NaOL concentration exceeds 60 mg/L. It is clearly proved that the addition of JFC-5 can greatly decrease the required NaOL concentration to obtain the same recovery as NaOL alone.

In the presence of 40 mg/L Ca²⁺, the recovery of scheelite with NaOL alone is dramatically reduced, and it is only around 20% at the NaOL concentration of 180 mg/L, showing the detrimental influence of Ca²⁺ on the collecting performance of NaOL. When using the mixed NaOL/JFC-5 collector, the recovery of scheelite decreases from 85% to 70% when NaOL concentration is 60 mg/L, and then it basically remains unchanged with the further increase of NaOL concentration. Thus it is shown that the addition of JFC-5 can also increase the resistance to Ca²⁺ of NaOL. In addition, compared with the mixed NaOL/JFC-5 collector without Ca²⁺, the different change trends obtained with Ca²⁺ maybe caused by the consumption of the excess NaOL in the pulp.

Figure 3 Effect of NaOL concentration on floatability of scheelite in absence and presence of Ca²⁺

According to the concentration logarithm diagram of 40 mg/L Ca²⁺ in Figure 4, it mainly exists in the form of Ca²⁺ at pH 10, which can react with oleate ions to produce calcium oleate precipitation, and reduce the effective concentration of NaOL in the pulp, thus decreasing the flotation recovery of scheelite. While in the presence of JFC-5, the EO groups in the molecular structure of polyoxyethylene ether could combine with Ca²⁺ and limit its transfer ability, so the concentration of Ca²⁺ reacted with oleate ions is decreased and then the probability of precipitation is decreased [27], thus the mixed NaOL/JFC-5 has strong calcium resistance capability.

Figure 4 Concentration logarithm diagram for hydrolysis components of Ca²⁺

3.2 Zeta potentials of scheelite

Figure 5 shows the zeta potentials of scheelite as a function of pH in different systems. The zeta potential of scheelite in deionized water is negative in the pH range of 2–12, and decreases with the increase of pH value, which agrees well with literatures [28, 29]. After treated with 60 mg/L NaOL, the zeta potential of scheelite decreases, and the reduced value increases with increasing pH, indicating a stronger chemical adsorption of NaOL on scheelite surface under alkaline conditions [30–32]. Compared with NaOL alone, scheelite gets more negative potential in the presence of the mixed NaOL/JFC-5 collector, and the zeta potential of scheelite decreases with increasing JFC-5 ratio. As a kind of nonionic surfactant, JFC-5 does not dissociate into charged cations or anions, but exists in the form of molecule or micelle in aqueous solution. Therefore, it is indicated that JFC-5 can enhance the adsorption of NaOL on scheelite surface, and thus promote the flotation behavior of scheelite.

Figure 5 Effect of pH on zeta potential of scheelite surface

Figure 6 shows the effect of NaOL concentration on the zeta potential of scheelite in the presence of 40 mg/L Ca²⁺ at pH 10. It is seen that the presence of Ca²⁺ causes the zeta potential of scheelite increase from –29.1 to –5.22 mV in distilled water, suggesting the adsorption of Ca²⁺ on scheelite surface. With the increase of NaOL concentration, the zeta potential of scheelite has a negative shift, and the change degree caused by the mixed NaOL/JFC-5 is larger than that by NaOL alone, so it is shown that more NaOL has adsorbed on scheelite surface when using the mixed NaOL/JFC-5 collector. In addition, the zeta potential of scheelite decreases by about 20 mV in the addition of 60 mg/L NaOL without Ca²⁺, while only 12 mV with Ca²⁺ at pH 10. This result clearly demonstrates that the presence of Ca²⁺ can decrease the adsorption of NaOL on scheelite surface, and the mixed NaOL/JFC-5 collector presents stronger resistance to Ca²⁺ than NaOL alone.

Figure 6 Effect of NaOL concentration on zeta potential of scheelite surface in presence of Ca²⁺

3.3 Contact angles of scheelite

The surface roughness value of scheelite sample used in this work was less than 0.05 μm, which was measured by AFM [33]. BUSSCHER et al [34] proved that surface roughness values less than 0.1 μm can not affect the wettability of the substrates. So it can be considered that the change in contact angles of scheelite is caused by surface chemical heterogeneities.

Figure 7 shows the effect of pH on the contact angles of scheelite in different systems. In deionized water, the contact angle of scheelite gradually increases to a maximum of approximately 45° at pH 7, and then decreases. After adding NaOL, the contact angle of scheelite has an evident increase, with the increase of pH value, the contact angle first increases and then maintains around 90° when the pH value is above 6. So it is indicated that the adsorption of NaOL has occurred on scheelite surface, thus increasing the hydrophobicity of scheelite surface to promote the flotation. In the presence of the mixed NaOL/JFC-5 collector, the beneficial effect of the non-ionic surfactant JFC-5 is observed in the entire experimental pH range of 2–12. The contact angle increases by about 10° in strong acid conditions (pH<4), so the flotation recoveries of scheelite in strong acid conditions are much higher than that in presence of NaOL alone. In a wide pH range of 5–10, there is about 3°–4° increase in contact angle of scheelite.

Figure 7 Effect of pH on contact angle of scheelite in different systems

The beneficial effect of secondary surfactant in the contact angle was usually ascribed to the more uniform distribution and adsorption of the main collector. LU et al [35] investigated the effect of a PEO containing nonionic polymer on the contact angle of a francolite (carbonatite apatite) crystal with sodium oleate, and the results showed that after adding 10% polymer, the contact angle of francolite increased with a maximum of 20°. Researches by SIS and CHANDER [8] showed that the contact angle of apatite can be increased (up to 5°) by adding NP-4 in the low oleate concentration range of 0.1–2 mg/L, while decreased with further increasing oleate concentration.

3.4 XPS analysis

XPS analysis has been widely used in qualitative elemental analysis and surface analysis, it can identify all elements in the periodic table except H and He according to the characteristic line of elements [36]. Since carbon atom has different existences in NaOL and JFC-5 molecules, XPS analysis was used to characterize the changes on the carbon atom of scheelite surface and analyze the adsorption of the mixed NaOL/JFC-5 collector, the C1s spectra of NaOL, JFC-5, as well as scheelite treated with NaOL in the absence and presence of JFC-5 at pH 10 are shown in Figure 8.

It can be seen that C—C and O=C—O are two forms of carbon atom in NaOL molecules, and the corresponding binding energies are 284.8 and 288.16 eV, respectively. The C1s peaks at 284.76, 286.11 and 288.82 eV occur in the spectrum of JFC-5, corresponding to C—C, C—O—C and O=C—O, respectively. For pure JFC-5, carbon atom exists in the two forms of C—C and C—O—C, while it can be oxidized in the air and some oxidation products, such as acetaldehyde and hydroperoxide, so the peak at 288.82 eV is observed. After treated with NaOL (60 mg/L), scheelite surface has two forms of carbon atom, which are C—C at 284.81 eV and O=C—O at 288.39 eV. After treated with the mixed NaOL/JFC-5 (60 or 12 mg/L) collector, three peaks are observed at 284.81, 286.01 and 288.38 eV, respectively corresponding to C—C, C—O—C and O=C—O. Therefore, the forms of carbon atom both in NaOL and JFC-5 molecules occur on scheelite surface, showing the co-adsorption of JFC-5 and NaOL. The above results of zeta potential and contact angle measurement show that the adsorption of NaOL on scheelite surface is enhanced by adding JFC-5, so it can be indicated that the adsorption of JFC-5 can decrease the electrostatic repulsion between the oleate ions, and make the oleate more easier to adsorb on scheelite surface, thus enhancing the hydrophobicity and floatability of scheelite.

Figure 8 C1s spectra of NaOL (a), JFC-5 (b), scheelite treated with NaOL (c) and mixed NaOL/JFC-5 collector (d)

4 Conclusions

The synergistic effects of a polyoxyethylene ether JFC-5 in scheelite flotation with NaOL were investigated. Flotation results show that the collector NaOL has poor collecting performance for scheelite at 10 °C, while in the addition of JFC-5, the recovery of scheelite is greatly increased, and the required NaOL concentration is too much lower to obtain the same recovery as NaOL alone. In addition, the mixed NaOL/JFC-5 collector has strong resistance to Ca²⁺.

The adsorption mechanism of the mixed collector was analyzed by ways of zeta potential measurement, contact angle measurement and XPS analysis. Compared with NaOL alone, the zeta potential of scheelite decreases in the addition of the mixed NaOL/JFC-5 collector whether Ca²⁺ exists or not. Since JFC-5 exists in the form of molecule or micelle in aqueous solution, the more negative zeta potentials of scheelite are caused by the increase of NaOL adsorption. Moreover, adding JFC-5 can also increase the contact angle of scheelite in the whole pH range, thus increasing the hydrophobicity and flotation recovery of scheelite. The results of XPS analysis indicate the co- adsorption of NaOL and JFC-5 on scheelite surface.

In a word, the mixed NaOL/JFC-5 collector is more effective in scheelite flotation due to the low temperature requirement, the low concentration and strong resistance to Ca²⁺. Since the polyoxyethylene ether is a kind of environmentally-friendly surfactant with extensive source, it has good industrial application and development prospect. Next, we will generalize this kind of synergist to the flotation of other minerals which are enriched by fatty acids collectors, and apply the mixed collector in flotation practice.

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(Edited by FANG Jing-hua)

中文导读

聚氧乙烯醚改善油酸钠在白钨矿浮选中的捕收性能

摘要：为了增强油酸钠体系下白钨矿的浮选行为，通过浮选试验考察了一种非离子型聚氧乙烯醚JFC-5对白钨矿在10 °C时可浮性的影响。与单独使用60 mg/L油酸钠时相比较，当pH=10，添加质量比为20%的JFC-5时，白钨矿的浮选回收率由22%提高到85%。此外，油酸钠的抗钙离子能力得以增强。通过动电位测试、接触角测量和X射线光电子能谱（XPS）考察了捕收剂在矿物表面的吸附机理，结果显示添加JFC-5后，白钨矿表面的动电位更负且接触角更大，表明油酸钠在白钨矿表面的吸附增强。XPS分析结果证实了JFC-5和油酸钠在白钨矿表面的共吸附，从而推测JFC-5在白钨矿表面的吸附减弱低了油酸根离子之间的静电斥力，使得油酸钠在白钨矿表面的吸附更强。总之，油酸钠/JFC-5组合捕收剂可以有效改善白钨矿浮选。

关键词：白钨矿；聚氧乙烯醚；增效作用；低温浮选；吸附

Foundation item: Project(2016RS2016) supported by Hunan Provincial Science and Technology Leader (Innovation Team of Interface Chemistry of Efficient and Clean Utilization of Complex Mineral Resources), China

Received date: 2017-10-23; Accepted date: 2018-05-02

Corresponding author: QIN Wen-qing, PhD, Professor; Tel: +86-731-88876843; E-mail: qinwenqing369@126.com; ORCID: 0000- 0001-5570-9680