Elsevier

Minerals Engineering

Volume 155, 15 August 2020, 106440
Minerals Engineering

Stability of lanthanum in sulfate and phosphate systems and implications for selective rare earths extraction

https://doi.org/10.1016/j.mineng.2020.106440Get rights and content

Highlights

  • Phosphor activity during roasting of sulfated monazite ore affects extraction.

  • High P2O5 partial pressure promotes reconversion of REE sulfate into phosphate.

  • The formation of REE phosphate results in low REE extraction.

  • LaPO4 was formed when direct contact (liquid–solid) was promoted.

  • La(PO3)3 was formed when indirect contact (gas–solid) was promoted.

Abstract

The stability of Rare Earth Elements (REE), both sulfates and phosphates, was investigated using monazite ore, a residue from a synthetic rutile manufacturing process and a high-grade lanthanum sulfate sample. It was demonstrated that REE sulfates are converted into phosphates at temperatures within the range of 450–800 °C. This reaction depends on excess phosphorus present in compounds that are less stable than monazite. The presence of high partial pressures of P2O5 in the system will reconvert REE sulfates into insoluble REE polyphosphates – REE(PO3)3. Relatively high partial pressure may be attained by the decomposition of phosphorus compounds present in the sample, such as monocalcium phosphates and phosphoric acid generated by the addition of sulfuric acid. The direct contact with phosphoric acid leads, in turn, to the formation of monazite-like compounds. The gas–solid mechanism is believed to be the most suitable due to the easier access of the gas phase to the REE sulfate particles, thus making it possible to achieve the drastic decrease (conversion into phosphate) in REE extraction observed at 800 °C. This study’s results are expected to contribute to a better understanding of the factors favoring selectivity in REE extraction over iron from monazite ores.

Introduction

Rare earth elements (REE) are essential to modern industry. The main drivers for its consumption are the catalyst, magnets, polishing powders, and rechargeable battery electrodes. Other relevant applications for these elements can be found in the metallurgical, ceramics, phosphor, pigment, and glass industries (Lucas et al., 2014).

Rare earth elements are extracted from four main sources, namely, bastenaesite (Ce, La, Y)(CO3)F, monazite (Ce, La, Pr, Nd, Th, Y)PO4, xenotime (YPO4) and ion-adsorption clays (Gupta and Krishnamurthy, 2005). China is the major producer, with Bayan Obo being its main production site. Mountain Pass (USA) and Mount Weld (Australia) are also worth mentioning for their relevance and production capacities. Fig. 1 shows a simplified flowsheet for these three most relevant operations in the world. Mountain Pass is a typical bastnaesite deposit, while Bayan Obo processes a mixture of bastnaesite and monazite. Mount Weld, by contrast, is a weathered monazite deposit. All of these processing routes require a heating stage prior to the application of hydrometallurgical processes. This flowsheet configuration is also present in the processing design for new deposits, as most use sulfuric acid and require a heating stage as a preparation before leaching (Verbaan et al., 2015).

Bastnaesite ores often undergo physical concentration (flotation, magnetic concentration, gravity concentration) to about 60% Rare Earth Oxides (REO) prior to further processing. In China, the mixed concentrates are roasted with sulfuric acid (H2SO4) at approximately 500 °C in a rotary kiln. This condition promotes the release of CO2 and HF from the concentrates and the formation of water-soluble, REE sulfates. Mountain Pass also used to calcine the concentrate at a slightly higher temperature (650 °C). This process, when carried out under an oxidative atmosphere, promotes the release of CO2 from the concentrate while Ce (III) oxidizes to Ce (IV). The material leaving the furnace was leached in 30% (v/v) HCl solution at room temperature, a condition in which the REEs are dissolved, except Ce (IV), which remains in a solid phase (Gupta and Krishnamurthy, 2005). Following leaching, different approaches may be applied to solution purification, depending on the liquor composition. In Mountain Pass, the purification removed iron and thorium by precipitation, followed by solvent extraction (SX), thus producing a purified solution ready for the final separation of the REE. In Bayan Obo, the purification stage selectively separates the REE by precipitation as double sulfates, which are then converted into hydroxides and leached in an HCl solution, prior to SX separation.

Monazite is the second most important REE mineral and is usually found in complex ore bodies exposed to intensive weathering, thus resulting in refractoriness to conventional physical beneficiation due to very fine, micro-level association between the main REE-carrying mineral (monazite) and the gangue material (goethite and ilmenite, among others). This means that the solids fed to the leaching tanks often exhibit low REE content, but high content of acid-consuming contaminants, like iron. Mount Weld (Fig. 1) is an exception to this general trend, as the ore is amenable to some degree of physical concentration. After sulfation, the concentrate is roasted within a range of 400–600 °C and leached in water. Purification, REE precipitation, and final separation of the REEs are also reported.

As illustrated by Fig. 1, the purification and precipitation stages following the REE dissolution consist of a number of unit operations, resulting in several possible process flowsheets (Silva et al., 2018). Therefore, approaches to minimize the dissolution of impurities during leaching would reduce the complexity and costs associated with the purification stages. Within this context, a selective process for REE extraction over iron and thorium was discussed by the authors (Teixeira et al., 2019). The process is based on a “temperature window” established by the differences in the decomposition conditions of sulfates, namely REEs, iron, and thorium sulfates during roasting. A thermodynamic analysis of the system indicated a temperature gap of about 150 °C: from 750 °C (ferric sulfate decomposition) to 900 °C (neodymium sulfate decomposition). However, experimental results obtained for a monazite ore from a phosphate mine demonstrated a much narrower gap, only 50 °C, between 750 °C and 800 °C. Two possible phenomena may be taking place during the heating stage, according to the literature. The first would be the formation of an intermediary phase, such as the oxysulfate phase (La2O2SO4) (Onal et al., 2015). These authors investigated the selective roasting of permanent magnet scrap, in an attempt to separate iron from neodymium and praseodymium. Thus, a maximum REE extraction at 750 °C was observed. The extraction decreases as temperature increases, reaching 95% for Nd and Pr at 800 °C and 85% at 850 °C. Onal et al. (2015) attributed this decrease to the formation of an oxysulfate phase, as represented by equation (1).REE2(SO4)3(s)ΔREE2O2SO4(s)+2SO2(g)+O2(g)

As the oxysulfate phase is not water soluble, the REE extraction decreases. The formation of lanthanum oxysulfate was confirmed by Thermogravimetric Analysis (TGA), as described elsewhere (Teixeira et al., 2019). However, it took place only around 1000–1100 °C, a temperature much higher than that of 850 °C reported by Onal et al. (2015). The decrease in REE extraction between 750 °C and 850 °C described by these authors was significantly milder than the drop to extraction levels as low as 10% that it was observed in ore roasting.

A second hypothesis to explain the extraction drop would be the formation of insoluble REE phosphates due to the reaction between REE sulfate and excess phosphorus present in the monazite ore, extracted from a phosphate mine. Phosphoric acid (H3PO4) can be formed by the reaction between monazite and sulfuric acid:2REEPO4(s)+3H2SO4(aq)2H3PO4(l)+REE2SO43(s)

This hypothesis was suggested by Demol et al. (2018) during their investigation on the effect of a baking temperature of a rich (>90%) monazite sample. This high monazite content favored the release of phosphorus compounds, making the reagent available for back reaction. The occurrence of a similar phenomenon in the typical, complex mineralogical assembly of actual ore samples, has yet to be explored.

The present work is part of a broader investigation focusing on the application of roasting for improving the selective extraction of REE. More specifically, the hypothesis that soluble REE sulfates are back transformed to REE phosphate during roasting is analyzed in detail. The experimental approach encompassed the analysis of the leaching residue of an iron and phosphorus-rich monazite ore, thermodynamic simulation, and roasting experiments with pure reagents designed to clarify the phases involved and the mechanism behind phase transformation. The results are expected to contribute to a better understanding of the factors favoring selectivity in REE extraction over iron from monazite ores.

Section snippets

Materials

Two distinct samples were used in this work: Ore samples received from a Brazilian phosphate mine and a solid residue obtained from the production process of synthetic rutile (TiO2). The ore sample was subjected to the REE extraction procedure, and the solid residue produced by step (v), as detailed below, was used in this investigation. The ore sample composition, described in detail elsewhere (Teixeira et al., 2019), showed REO, Fe, Th, and P ranging from 4.3 to 4.9%, 21.8–25.8%, 181–352 mg·kg

Results and discussion

Fig. 3 shows the REE extraction by leaching after sulfation and roasting at different temperatures. The results obtained from different lithotypes of a monazite ore are compared with those from a spray roaster residue obtained from anatase-rutile conversion. The composition of this residue is shown in Table 1. Compared to the ore, the residue shows lower iron (16%) and higher P (12%) and REE (10%) contents, being 96% of the REE comprised of La, Ce, Pr and Nd and 4% comprised of the remaining

Conclusions

In this work, the mechanism behind the selective REE extraction over iron by the roasting of sulfated monazite was clarified. The unexpected decrease in REE extraction from ores roasted at 800 °C was explained by analyzing the phosphorus behavior in the system during sulfation and roasting stages. It was demonstrated that excess phosphorus remaining in the sulfated material may cause the reconversion of REE sulfates to REE phosphates, as well as the presence of phosphoric acid will lead to the

CRediT authorship contribution statement

Leandro Augusto Viana Teixeira: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Ruberlan Gomes Silva: Validation, Writing - review & editing. Daniel Majuste: Writing - review & editing. Virginia S.T. Ciminelli: Supervision, Conceptualization, Writing - review & editing.

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.

Acknowledgments

The authors would like to thank Vale S.A., in special Cassia Souza, Keila Gonçalves and Patrice Mazzoni for providing samples, material support and permission for publishing the results of this investigation. The authors are also thankful to Wagner Soares, Luzia Chaves, Vale and CDM technicians, for their technical support in this work. V. Ciminelli and D. Majuste acknowledge the support from CNPqConselho Nacional de Desenvolvimento Científico e Tecnológico, CAPESCoordenação de

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      Bastnaesite ((Ce, La, Y)(CO3)F), monazite ((Ce, La, Pr, Nd, Th, Y)PO4), xenotime (YPO4), and ion-adsorption clays represent the relevant rare earth elements (REE) carriers. Dissolution is often carried out in sulfuric acid media following a step where rare earth minerals are converted into their respective water-soluble sulfates by mixing the solids with concentrate sulfuric acid (H2SO4) (Teixeira et al., 2020; Verbaan et al., 2015). This preparation step for leaching is named sulfation, sulfuric acid baking/bake, acid bake/digestion, or sulfation roast (Gontijo et al., 2020).

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