Effect mechanism of nonane-1,1-bisphosphonic acid as an alternative collector in monazite flotation: Experimental and calculational studies

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Abstract

Monazite ((Ce, La)PO4) is one of the major types of light rare earth minerals from which the light rare earth elements cerium (Ce) and lanthanum (La) are economically extracted. Flotation is extensively used to recover fine-grained monazite. Sodium oleate (NaOL) is considered as the collector with strong collecting ability for monazite flotation. However, this study shows that its collecting ability is still limited. In this paper, a phosphonic acid, nonane-1,1-bisphosphonic acid (C9-BPA), was employed as the novel collector in place of NaOL. Flotation experiments show that even when the C9-BPA dosage is less than one-fifth of the NaOL dosage, the monazite recovery using C9-BPA as the collector is approximately 22 wt% higher than that using NaOL. The mechanism by which C9-BPA adsorbs on monazite was investigated using zeta potential, infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements as well as first-principles calculations. Zeta potential measurements show a more significant decrease in the zeta potentials of monazite after the addition of C9-BPA compared to those after the addition of NaOL. For C9-BPA-treated monazite, the characteristic peaks of C9-BPA are observed in the IR and C 1s XPS spectrum, whereas for monazite treated by NaOL, no characteristic peak of NaOL was observed. Experimental results show that C9-BPA has a stronger affinity towards the monazite surface than NaOL as confirmed by the higher adsorption energy of CP-BPA on the monazite surface (‒204.22 kJ/mol) than NaOL (‒48.48 kJ/mol). This study demonstrates an extensive application value and prospect of C9-BPA in monazite flotation and helps design novel collectors with strong collecting ability for monazite flotation.

Graphical abstract

Nonane-1,1-bisphosphonic acid (C9-BPA) has higher adsorption energy (–204.22 kJ/mol) and more bonds formed with the monazite surface than sodium oleate (NaOl, –48.48 kJ/mol). Hence, C9-BPA has stronger collecting ability towards monazite than NaOL.

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Introduction

Due to their unique properties, the light rare earth elements cerium (Ce) and lanthanum (La) are used in a wide range of applications such as additives for special optical glass,1 alloys,2, 3, 4 manufacturing of nickel metal hydride batteries, catalysts and reducing agents. Monazite ((Ce, La)PO4) is a major phosphate mineral containing Ce and La and is a major source for extracting Ce and La. Monazite is also the important source of thorium (Th).5,6 Th is an important and potential nuclear fuel. The recovery of light rare earth minerals including monazite is related to not only the national economic development, but also the national security.

Flotation is the most extensively used technique for the recovery of fine-grained monazite.7, 8, 9 It employs the difference in the surface hydrophobicity of minerals to separate valuable minerals from their associated gangue minerals.10,11 Flotation reagents are an important factor in the flotation process.12, 13, 14 Flotation collectors can selectively adsorb on the target mineral surfaces to increase their hydrophobicity.15,16

Hydroxamic acids are, generally, considered to be the most effective collectors for monazite flotation such as a modified naphthyl hydroxamate (known as H205) and benzohydroxamic acid (BHA).17 However, the hydroxamic acid collectors have the following disadvantages. Firstly, the performances of the hydroxamic acid collectors are sensitive to temperature.18 At low temperature (<approximately 35 °C), H205 adsorption to monazite is unstable and thus, it shows the limited collecting ability. As the pulp temperature increases to approximately 75 °C, the adsorption stability of H205 increases and thus, its collecting ability is enhanced.19 However, further increase in temperature is detrimental to the performance of H205. The flotation pulp temperature must be strictly controlled with H205 collector. Secondly, some hydroxamic acid collectors have the limited collecting ability towards the monazite flotation such as H205 and BHA. Higher dosages of H205 are required to guarantee the recovery of monazite. BHA exhibits the excellent selectivity in the oxide mineral flotation, while, it shows the limited collecting ability. Pb2+ ion is commonly used as the activator in BHA flotation of oxide minerals to enhance the collecting ability of BHA.13,20, 21, 22, 23 However, the use of Pb2+ ion causes the increase of production cost and the environmental pollution. To overcome the limited collecting ability of hydroxamic acid collectors, carboxylate collectors has been paid attention again.

Carboxylates are the most widely used collectors in industrial practice and were traditionally the collector of choice for monazite flotation, prior to the development of hydroxamic acids collectors. Oleic acid (CH3(CH2)7CH=CH(CH2)7COOH) and its salts, such as sodium oleate (CH3(CH2)7CH=CH(CH2)7COONa, NaOL), are the most widely used carboxylate collectors for the monazite flotation.7,24,25 Oleic acid collectors have the stronger collecting ability than hydroxamic acids, while, they show the poor selectivity. Their poor selectivity limits the their use. A novel flotation process solves this disadvantage of them. The lack of selectivity of oleic acid collectors is overcome by only employing them in an initial unselective flotation stage, and subsequently using a hydroxamate to selectively recover monazite.26 The removal of fatty acid from the gangue mineral surfaces prior to a more selective flotation stage is imperative to facilitate selective monazite flotation. However, the flotation experiment results in this study show that the collecting ability of NaOL for monazite flotation is still limited.

Previous studies show that phosphonic acids have the strongest collecting ability for monazite flotation compared with carboxylates and hydroxamic acids.21 In this study, a phosphonic acid, nonane-1,1-bisphosphonic acid (C9-BPA), was employed as a new collector in the monazite flotation. C9-BPA shows the stronger collecting ability than NaOL. In the future, the combination use of C9-BPA and hydroxamic acid collectors (or novel depressants with excellent selectivity) has large application foreground in the monazite flotation. In this study, its effect mechanism in monazite flotation were investigated by the experimental methods and the first-principles calculations.

This manuscript is divided into three parts. In the first part, flotation experiments were used to demonstrate the stronger collecting ability of C9-BPA than NaOL. In the second part, experimental methods, including zeta potential, infrared spectroscopy and X-ray photoelectron spectroscopy measurements, were employed to investigate the adsorption behaviors of C9-BPA on the surface of monazite. In the third part, first-principles calculations were used to confirm the experimental results from the atomic level. This study has two objectives: (1) to prove the applicability and prospects of C9-BPA in monazite flotation; and (2) to support the design of novel collectors with strong collecting abilities for monazite flotation.

Section snippets

Materials and reagents

The pure mineral crystals of monazite used in all the experiments were obtained from a light rare earth ore in Guangxi province, China. The mineral samples were further purified by using shaking table separations.

The chemical composition of the monazite sample was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, ICP-8100, Shimazu Corporation, Japan), potassium dichromate titration and gravimetric method. The TiO2, Al2O3, SiO2, P2O5 and Fe2O3 grades were analyzed

Flotation tests

Fig. 2(a) shows the flotation recovery of monazite as a function of pH in the presence of C9-BPA and NaOL, respectively. At 3.0 × 10−5 mol/L dosage of C9-BPA, as the pH value increases from 3 to 7, the monazite recovery increases from approximately 40 wt% to a maximum value of approximately 80 wt% before it gradually decreases as the pH value continues to rise. At 1.6 × 10−4 mol/L dosage of NaOL, the monazite recovery increases to a maximum value of approximately 58 wt% as the pH value

Discussion

The results of flotation experiments show that the collecting ability of nonane-1,1-bisphosphonic acid (C9-BPA) for monazite is stronger than that of sodium oleate (NaOL) (Fig. 2), for which the reasons were investigated by experimental methods including zeta potential, infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) experiments, as well as first-principles calculations.

A more significant decrease in the zeta potential of monazite after the addition of C9-BPA than that

Conclusions

In this paper, a phosphonic acid, nonane-1,1-bisphosphonic acid (C9-BPA), was employed as the novel collector for monazite flotation. Flotation experiments show that the lower dosage of C9-BPA than sodium oleate (NaOL) can obtain the higher recovery of monazite, for which the reasons were investigated by experimental methods, including zeta potential, infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements as well as first-principles calculations.

Zeta potential

Acknowledgments

We thank Dr. Wencai Yi for his useful tool qvasp and Professor Guanghui Li (Mining and Coal Institute, Inner Mongolia University of Science & Technology) for his help in synthesis of nonane-1,1-bisphosphonic acid.

References (39)

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Foundation item: Project supported by the National Key Research and Development Program of China (2019YFC0408300), the Excellent Youth Foundation of IMUST (2017YQL05), the Key Program for International S & T Cooperation Projects of China (2019YFE012999), the Science Fund for Distinguished Young Scholars of Hunan Province (2020JJ2044), the Young Elite Scientists Sponsorship Program of Hunan Province, China (2018RS3011), Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgic Mineral Resources, the National Natural Science Foundation of China (U2067201, 51774328, 51674045, 51404300), the National 111 Project of China (B14034), and Inner Mongolia Natural Science Foundation (2020LH05027, 2019MS05039).

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