*Article* **Maintenance of the Metastable State and Induced Precipitation of Dissolved Neodymium (III) in an Na2CO<sup>3</sup> Solution**

**Youming Yang 1,2, Xiaolin Zhang <sup>1</sup> , Kaizhong Li 1,\* , Li Wang <sup>3</sup> , Fei Niu 1,2, Donghui Liu <sup>1</sup> and Yuning Meng <sup>1</sup>**


**Abstract:** Rare earths dissolved in carbonate solutions exhibit a metastable state. During the period of metastability, rare earths dissolve stably without precipitation. In this paper, neodymium was chosen as a representative rare earth element. The effects of additional NaCl and CO<sup>2</sup> on the metastable state were investigated. The metastable state can be controlled by adding NaCl to the Na2CO<sup>3</sup> solution. Molecular dynamics studies indicated that the Cl− provided by the additional NaCl partially occupied the coordination layer of Nd3+, causing the delayed formation of neodymium carbonate precipitation. In addition, the additional NaCl decreased the concentration of free carbonate in the solution, thereby reducing the behavior of free contact between carbonate and Nd, as well as resulting in the delay of Nd precipitate formation. Consequently, the period of the metastable state was prolonged in the case of introduction of NaCl. However, changing the solution environment by introducing CO<sup>2</sup> can destroy the metastable state rapidly. Introduction of CO<sup>2</sup> gas significantly decreased the CO<sup>3</sup> <sup>2</sup><sup>−</sup> content in the solution and increased its activity, resulting in an increase of the free CO<sup>3</sup> <sup>2</sup><sup>−</sup> concentration of the solution in the opposite direction. As a result, the precipitation process was accelerated and the metastable state was destroyed. It was possible to obtain a large amount of rare earth carbonate precipitation in a short term by introducing CO<sup>2</sup> into the solution with dissolved rare earths in the metastable state to achieve rapid separation of rare earths without introducing other precipitants during the process.

**Keywords:** neodymium; metastable state; maintenance; induced precipitation

## **1. Introduction**

Rare earths are strategic metal resources that are used in a wide range of industries. For example, they can be found in the development of high-tech advanced materials for permanent magnets, luminescence, catalysis and hydrogen storage, as well as in basic industries such as metallurgy, machinery and petrochemicals in general [1–5].

Rare earth carbonate is a barely soluble substance, with a solubility in water of only 10−5–10−<sup>7</sup> mol·<sup>L</sup> −1 [6,7]. However, when rare earth ions are added to a higher concentration of alkali metal carbonate solution, there occurs the phenomenon of rare earth dissolution in the carbonate solution. The amount of rare earth dissolution increases with increased carbonate concentration. As early as 1963, Taketatsu [8,9] found that when a certain amount of rare earth chloride solution was gradually added to a concentrated K2CO<sup>3</sup> solution, sediment of the rare earth carbonate was generated first and then dissolved again with the passage of reaction time. The dissolution amount of rare earth increased with the increase of CO<sup>3</sup> <sup>2</sup><sup>−</sup> concentration and the atomic number of the rare earth (except for Ce and Y). Restricted by the situation of the industry at that time, rare earth resources were not as scarce as nowadays, but comparatively abundant. Therefore, the discovery of

**Citation:** Yang, Y.; Zhang, X.; Li, K.; Wang, L.; Niu, F.; Liu, D.; Meng, Y. Maintenance of the Metastable State and Induced Precipitation of Dissolved Neodymium (III) in an Na2CO<sup>3</sup> Solution. *Minerals* **2021**, *11*, 952. https://doi.org/10.3390/ min11090952

Academic Editors: Shuai Wang, Xingjie Wang, Jia Yang and Kenneth N. Han

Received: 19 July 2021 Accepted: 27 August 2021 Published: 31 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the regularity of dissolution of rare earths in carbonate solution did not attract the attention of the rare earth separation industry.

Vasconcellos et al. [10] carried out a feasibility study on selective dissolution, separation and enrichment of a rare earth in carbonate solution based on Taketatsu's regularity [8,9]. Low-Ce rare earth carbonate concentrates were selectively dissolved and successfully enriched with yttrium by using NH4HCO3/(NH4)2CO<sup>3</sup> solution as a carrier. The grade of yttrium increased from 2.4% to 81.0%. In addition, it was also found that the concentration of NH<sup>4</sup> + influenced the dissolution behavior of rare earth in the solution system in that a higher concentration of NH<sup>4</sup> + could enhance the solubility of rare earths. Reference to research on the equilibrium hydrochemical behavior of neodymium in a Na<sup>+</sup> -Cl−-CO<sup>3</sup> <sup>2</sup>−-HCO<sup>3</sup> − solution system [11], shows that there is a relationship between the solubility of rare earth in a carbonate solution and the concentration of the NaCl salt as an impurity, and the dissolved amount of rare earth increases with increasing concentration of NaCl in the solution. High concentration of NaCl results in high ionic strength of the solution. In addition, according to research data on rare earth adsorption in a water-bearing sand layer [12], at higher ionic strength of the solution, the more significant was the rare earth adsorption in the sand layer. In other words, a greater amount of rare earth loss was caused by dissolution. Thus, when the concentration of NH<sup>4</sup> + in the solution increased, this resulted in a greater amount of dissolved rare earths, as observed by Vasconcellos [10], which can be attributed to the influence of the ionic strength of the solution [13].

Nowadays, a number of techniques have been developed for the separation of rare earths, such as solvent extraction [14], ion exchange, membrane separation and ionic liquids [15], as well as other methods. Among them, the most widely used is the traditional solvent extraction technique. The other methods are less used because of high cost. The core component of solvent extraction is the extractant. Currently, a number of high-performance extractants have been developed [16], but toxicity and loss of extractants are always the key weaknesses limiting the development of the technology. In addition, due to the increasingly stringent requirements of environmental protection, the high salt wastewater generated during the separation of rare earths remains a problem [17], and is also a bottleneck in the solvent extraction separation process [18]. For the healthy development of the rare earth industry, it is necessary and urgent to develop a new type of highly efficient and environmentally friendly separation technology.

A good and feasible green method to separate rare earths is by using the metastable state of the carbonate solution which dissolves them. In our previous study [19], a series of experiments on metastable states was carried out by choosing neodymium as an example of rare earth elements. Our results indicated that neodymium dissolved in a sodium carbonate solution exhibited some metastable properties. Among them, the most important one was that there is a limit to the dissolution of neodymium in a certain concentration of sodium carbonate solution, after which there is instantaneous saturated solubility. When the dissolved neodymium in the solution does not exceed its solubility, it is stable in the solution for a period (metastable period) without precipitating neodymium carbonate. Our previous study was not very comprehensive and limited by the length of the paper, so a follow-up study of metastable solution-induced precipitation was not carried out.

Now, combined with the idea of the solid-liquid separation of rare earths, we are continuing to consider the potential value of the metastable state. Rare earths are dissolved and enriched in the metastable period and precipitated and separated after exceeding this period. No other impurities are introduced in this process. In addition, the carbonate solution can be recycled. Therefore, this may be a potential method for green separation of rare earths. Hence, how to manually control the metastable state and the precipitation of rare earth carbonate is the core content of the present study.

In this study, the artificial control of metastable states is discussed in detail. Neodymium was chosen as a representative of rare earth elements. The effect of changing the solution environment, such as ion concentration, on the metastable state was studied, and the effective conditions for maintaining and destroying the metastable state were discovered.

#### **2. Experiment** discovered.

#### *2.1. Raw Materials and Equipment*

The rare earth material used in the experiment was a 10 g·L <sup>−</sup><sup>1</sup> dilute NdCl<sup>3</sup> solution obtained by diluting high purity NdCl<sup>3</sup> solution with deionized water. The high purity NdCl<sup>3</sup> solution was purchased from the rare earth smelting & separating plant in Longnan, Jiangxi Province, and its distribution is shown in Table 1. Solutions of Na2CO3, Na2CO3/NaCl with different concentration gradients and dilute hydrochloric acid for acidification were obtained by dissolving AR-grade Na2CO3, solid NaCl and HCl solution with deionized water. 2. Experiment 2.1. Raw Materials and Equipment The rare earth material used in the experiment was a 10 g·L−1 dilute NdCl3 solution obtained by diluting high purity NdCl3 solution with deionized water. The high purity NdCl3 solution was purchased from the rare earth smelting & separating plant in Longnan, Jiangxi Province, and its distribution is shown in Table 1. Solutions of Na2CO3, Na2CO3/NaCl with different concentration gradients and dilute hydrochloric acid for

the solution environment, such as ion concentration, on the metastable state was studied, and the effective conditions for maintaining and destroying the metastable state were


**Table 1.** Content of the high purity solution of NdCl<sup>3</sup> . acidification were obtained by dissolving AR-grade Na2CO3, solid NaCl and HCl solution with deionized water.

Minerals 2021, 11, x FOR PEER REVIEW 3 of 13

An experiment in which CO<sup>2</sup> gas was used to induce precipitation of a metastable state solution was carried out by using an autoclave with a CO<sup>2</sup> high-pressure cylinder, as shown in Figure 1. Other equipment used in experiments is shown in Table 2. An experiment in which CO2 gas was used to induce precipitation of a metastable state solution was carried out by using an autoclave with a CO2 high-pressure cylinder, as shown in Figure 1. Other equipment used in experiments is shown in Table 2.

<100 <100 <100 <100 <100 <100 <100

Figure 1. Schematic diagram of autoclave ventilation experiment: 1—CO2 pressure reducing valve; 2—high pressure cylinder; 3—rotating motor; 4—air inlet; 5—safety valve; 6—air outlet; 7 controller; 8—polytetrafluoroethylene tank; 9—agitator. **Figure 1.** Schematic diagram of autoclave ventilation experiment: 1—CO<sup>2</sup> pressure reducing valve; 2—high pressure cylinder; 3—rotating motor; 4—air inlet; 5—safety valve; 6—air outlet; 7—controller; 8—polytetrafluoroethylene tank; 9—agitator.


**Table 2.** Information of equipment.
