Leaching Kinetics of Rare-Earth Elements from Complex Ores by Acidic Solutions

Abstract

In this study, the leaching kinetics of rare-earth elements were investigated using HCl and HNO₃ solutions. A standard crushed complex ore was utilized as raw material. The complex ore (particle size of under 54 µm) contains REE (Ce, La, Nd, Pr, etc.). Different leaching durations were selected for the kinetic studies. The highest leaching efficiency was obtained in HCl solution, and the leaching mechanism was identified to diffusion through a product layer. The rate constants of Ce, La, Nd, and Pr were calculated as 1.08 × 10⁻², 0.86 × 10⁻², 1.87 × 10⁻², and 1.34 × 10⁻² h⁻¹, respectively.

Introduction

The rare earths elements (REEs) are a group of elements that contains the lanthanides, yttrium, and scandium . REEs are categorized into two sub-groups: the light rare earth elements (LREEs), and the heavy rare earths elements (HREEs) [1]. REEs can be used in several areas, such as permanent magnets , optics and lasers, catalysts, wind- and solar- energy applications, and rechargeable batteries . Due especially to the increasing applications in clean energy , usage of REEs in industry has increased significantly over last two decades [2]. The European Commission has identified raw materials which are important but having supplying issues as Critical Raw Materials (CRMs). LREEs and HREEs are both listed in the CRMs [3]. There are more than 250 REE containing minerals , but the most important minerals in industry are bastnaesite, monazite , xenotime, fluocerite, parasite, etc. [1]. The commonly used environmental production route for REEs is hydrometallurgical method contains leaching and solvent extraction , followed by precipitation , and reduction .

The leaching mechanism of REE containing ores is affected by the ore properties, reaction kinetics, hydrodynamic and mass transfer behaviors [4]. Leaching kinetics are important for designing a chemical process. Generally, the solid-liquid reactions are realized in a leaching process [5]. There are three major models (homogeneous, the shrinking core /particle, and grain models) that are used for calculating the leaching kinetics [6]. For a system containing non-porous particles and a reactant fluid, the shrinking core /particle model describes the leaching mechanism. In the shrinking core /particle model , there can be a solid layer of the product around the unreacted core or not [5]. For the system containing a homogenous pore distribution , the homogeneous model is utilized. However, if there is formation of compacted dense grains, the grain model is more suitable [6]. The uniform pore model is applied where the solid particles have uniform open cylindrical pores, while the random uniform pore model is applied where the solid particles have pores with different shapes. In all of these models, the shrinking model is accepted as the classical model . Experimentally, the shrinking models are more accurate than the other models, specifically in the leaching of ores [5].

There are three steps in a shrinking core model ; mass transfer of leachate through the surface of solid particle, pore diffusion and chemical reaction. The slowest step controls the process rate [5].

There are several studies that investigate the reaction kinetics of REEs containing raw materials. The studies focused on usage of different types of acids, ores, acid concentration and leaching durations. Bian et al. has studied a bastnasite concentrate containing 71.4 mass% rare-earth oxides. They investigated the effects of HCl concentration, leaching temperature , liquid to solid ratio, and particle size [7]. A roasted bastnasite concentrate (83.1% REO containing) was used by Feng et al. The effects of particle size of the raw material, stirring speed, sulfuric acid concentration and leaching temperature were investigated [8]. Walawalker at el. studied the secondary REE sources (mine tailing, metallurgical slags, and byproducts of phosphogypsum ), and they investigated the leaching kinetics using different acid concentrations [9]. Ferdowsi et al. also studied secondary sources containing REEs. An iron ore waste’s apatite was used as raw material, and the kinetic investigation was made using nitric acid solutions [10].

In this study a rare earth carbonatite ore containing about 10% REO was used. The ore is a certified reference material (AMIS0185) and comes from the Wigu Carbonate Complex in Tanzania. The investigation of reaction kinetics with using diluted HCl and HNO₃ acid solutions (3 M) was conducted using this material.

Experimental

Materials

In this study, a complex ore called Wigu Carbonatite Complex, obtained from Wigu Hill, Tanzania, was used as the raw material. The complex ore is bastnasite-rich with minor amounts of synchisite, parasite and monazite with traces of apatite . The chemical composition of obtained ore is given in Table 1. The leaching solution was prepared by mixing analytical grade acid solutions (obtained from Merck) with distilled water.

Table 1

Chemical composition of complex ore (mass fraction, %)

Compound	Σ REEs	Σ RE_xO_y	SiO₂	CaO	Fe₂O₃	MgO	Al₂O₃	LOI
Mass%	8.38	10.05	21.53	11.48	5.29	4.65	2.22	20.69

Methods

The leaching experiments were conducted in a 150 mL-glass beaker, using a magnetic stirrer at a speed of 400 rpm. In the experiments, 5 g of complex ore was added to 3 M acid solutions (100 ml), at room temperature (25 °C). The leaching durations selected were: 5–10–15–30–45–60 min. Solid-liquid separation was performed after the leaching period by using a filtering glassware-flask (multiple neck, 1000 mL). The obtained filtered cakes were dried for 24 h at room temperature . From the dried filter cake mass, the amount of dissolved ore was calculated by a precision scale (Sartorius, Grade 391). The pregnant solutions were stored in volumetric flasks (250 mL). The amount of rare earths in the leaching filtrate solution was analyzed by ICP-OES spectrometer (Agilent 720 series Radial). The filter cakes were analyzed by XRD spectrometer.

Result and Discussion

Leaching Efficiency

The leaching efficiencies of elements were calculated according to Eq. (1):

$\eta = \frac{{Me\, in \,solution \,(\text{mg}/\text{L})/(1000\,{\text{ml}}/250\,{\text{ml}}) }}{{5000 \,{\text{mg}}\, \times \,Me\,(\text{wt}.\% )}}$

(1)

The leaching efficiencies of the elements are given in Table 2. The amount of dissolved REEs in the acid solutions increased with increasing in leaching durations, for both acid solutions. The chemical compositions of the pregnant solutions and filtered cakes were measured for all of the REEs and base metals by ICP-OES. The highest recovery values (the leaching efficiencies) were obtained in HCl solution at 60 min leaching duration: 38.3%, 33.3%, 49.0%, and 41.1% for Ce, La, Nd, and Pr, respectively. The reason of lower REE efficiencies might be based on higher dissolution of non-REE elements. The average value of total REE contents were 515.6 mg/L, and the average contents of Fe, Al, Ca, Mg, Mn, Si were calculated as 461, 30, 1818, 790, 207, 38 mg/L, respectively. The results showed that leaching durations were not sufficient for higher recovery values.

Table 2

The recovery values of the leaching (obtained by ICP-OES)

Acid solution	Leaching duration (min)	Recovery values (leaching efficiency ), (%)
Acid solution	Leaching duration (min)	Ce	La	Nd	Pr
HCl	10	31.2	26.3	40.1	32.7
HCl	15	33.2	28.3	42.6	34.9
HCl	30	36.0	30.6	46.1	37.7
HCl	45	37.7	32.6	48.7	40.2
HCl	60	38.3	33.3	49.0	41.1
HNO₃	10	27.3	24.1	36.6	30.1
HNO₃	15	27.6	24.0	36.6	30.0
HNO₃	30	30.4	26.8	40.8	33.4
HNO₃	45	30.8	27.5	41.3	33.8
HNO₃	60	31.6	28.0	42.2	34.8

Leaching Kinetics

The leaching process of Wigu carbonatite ore could be explained by the shrinking core model [11]. The reaction rate can be controlled by diffusion through a product layer, and in this case kinetic calculations were made according to Eq. (2) [7]:

$1 - \frac{2}{3}x - (1 - x)^{2/3} = k_{d} t$

(2)

The reaction rate can also be controlled by surface reaction, in this case kinetic calculations were made according to Eq. (3) [7]:

$1 - \left( {1 - x} \right)^{1/3} = k_{r} t$

(3)

where x is fraction reacted; $k_{d}$ and $k_{r}$ are the rate of constant, respectively.

If the reaction is controlled by diffusion through a product layer or surface reaction, there must be a linear relationship between the left side of the equation and time. The slope of line is the apparent rate constant k_d or k_r, which is directly proportional to 1/r₀² or 1/r₀.

The Arrhenius equation is used to calculate the activation energy of a reaction. The Arrhenius equation can be represented as:

$k = A\exp [ - E /(RT)]$

(4)

where k is the reaction rate constant, A is the pre-exponential factor (depends on the reaction); R is the ideal gas constant, T is the temperature ; and E is the apparent activation energy [8].

The activity energy is also found in the graph ln k to 1/T. The slope of this graph gives us the value of –E/R (Eq. 5) [8]:

$\ln k = \ln A - E / (RT)$

(5)

The reaction rate values obtained after leaching with HNO₃ and HCl solutions are given in Figs. 1, 2, 3 and 4, respectively. The relationship between the kinetic rates and leaching durations were calculated by the Least-Squares Regression method [12]. The slope, interception points and R squared values were calculated for each of the REEs (La, Ce, Nd, and Pr) and their graphs were plotted by using Microsoft Excel software.

../images/468727_1_En_201_Chapter/468727_1_En_201_Fig1_HTML.gif — Fig. 1
Graph of $1 - \frac{2}{3}x - (1 - x)^{2/3} = k_{d} t$ change leaching duration for HNO₃

$$ 1 - \frac{2}{3}x - (1 - x)^{2/3} = k_{d} t $$ — Fig. 1
Graph of $1 - \frac{2}{3}x - (1 - x)^{2/3} = k_{d} t$ change leaching duration for HNO₃

../images/468727_1_En_201_Chapter/468727_1_En_201_Fig2_HTML.gif — Fig. 2
Graph of $1 - \left( {1 - x} \right)^{1/3} = k_{r} t$ change with leaching duration for HNO₃

$$ 1 - \left( {1 - x} \right)^{1/3} = k_{r} t $$ — Fig. 2
Graph of $1 - \left( {1 - x} \right)^{1/3} = k_{r} t$ change with leaching duration for HNO₃

../images/468727_1_En_201_Chapter/468727_1_En_201_Fig3_HTML.gif — Fig. 3
Graph of $1 - \frac{2}{3}x - (1 - x)^{2/3} = k_{d} t$ change leaching duration for HCl

../images/468727_1_En_201_Chapter/468727_1_En_201_Fig4_HTML.gif — Fig. 4
Graph of $1 - \left( {1 - x} \right)^{1/3} = k_{r} t$ change with leaching duration for HCl

The leaching results with 3 M HNO₃ solutions are presented in Figs. 1 and 2. The highest conversion fraction rate was detected in Nd recovery , followed by Pr, Ce, and La. Because of the R squared value, the leaching reactions were assumed to be diffusion controlled. The highest R squared value was calculated for La recovery , at 0.9111. The rate constants of Ce, La, Nd, and Pr were calculated as 0.65 × 10⁻², 0.53 × 10⁻², 1.29 × 10⁻², and 0.84 × 10⁻² h⁻¹, respectively.

The leaching results with 3 M HCl solutions are given in Figs. 3 and 4. The recovery values obtained were similar to those in the HNO₃ tests, although, higher R squared value were calculated for the HCl tests. The highest R squared value was calculated for Pr recovery , at 0.9538. The rate constants of Ce, La, Nd, and Pr were calculated as 1.08 × 10⁻², 0.86 × 10⁻², 1.87 × 10⁻², and 1.34 × 10⁻² h⁻¹, respectively. In the further studies different temperatures and the different stirring speeds will be carried out to determination of reaction mechanism , exactly. Due to these results, HCl solutions were more effective than HNO₃ solutions.

Conclusions

In this study, a complex ore called Wigu Carbonatite Complex, obtained from Wigu Hill, Tanzania, was used as the raw material. The conversion fractions and the leaching rate constants of dissolution reactions of REEs were measured by different leaching durations. The leaching conditions were fixed: 3 M acid solution (HNO₃ or HCl), 1/20 solid/liquid ratio and 400 rpm stirring speed. The highest leaching efficiency was obtained in HCl solution at the 60 min leaching duration, with recovery of 38.3% for Ce, 33.3% for La, 49.0% for Nd, and 41.1% for Pr. The highest rate constants for dissolutions of Ce, La, Nd, and Pr were calculated as 1.08 × 10⁻², 0.86 × 10⁻², 1.87 × 10⁻², and 1.34 × 10⁻² h⁻¹, respectively. The reaction control mechanism for leaching was identified to be diffusion through a product layer. In the further studies, the effects of particle size, leaching temperature and stirring rate of the leaching process on the conversion of REE dissolution and their efficiencies will be carried out. Their results can give the better understanding about the reaction kinetics and the leaching mechanism. The activation energy for the leaching can also be calculated depending on the results of leaching at different solution temperatures. The activation energies were not calculated, due to the fact the tests were all conducted at the same temperature .

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