1. Introduction
The extraction and processing of mineral resources cause environmental pollution and deterioration in some regions. After the completion of the geological processes and formation of the Earth’s crust, all elements are in equilibrium. However, human intervention, even at the stage of geological prospecting, mobilizes the minerals concentrated in the Earth’s crust. A commercial component and associated minerals in the development zone are activated, affecting the human habitat. After the beginning of development, the human habitat transforms into a domain of impact. Accumulated and stored industrial wastes are the most hazardous to the environment [
1].
In the specific case of rare earth elements (REE), which are a group of chemical elements that include all the lanthanides (Ln), yttrium, and scandium, these elements are often divided into two categories: light rare earth elements (LREE), ranging from lanthanum to samarium, and heavy rare earth elements (HREE) ranging from europium to lutetium [
2,
3,
4]. Due to their unique properties, REE are widely used in applications such as permanent magnets, energy storage systems, superconductors, electronics, and metal alloys. The importance of REE is growing every day due to their applications in modern technology and their consequent role in the fourth industrial revolution [
5]. With the increasing global demand for REE in recent years, the traditional prospects of rare earth mining have started to be re-evaluated and new extraction possibilities have been considered [
6,
7,
8]. Research carried out in the region of the Bagre-Nechí mining district in Colombia indicates the presence of monazite, with concentrations of rare earth oxides between 55 and 63% in the tailings of the mining operations [
9,
10].
Commercial rare earth concentrates have concentrations between 55 and 65% of rare earth oxides [
11,
12]. Although the REE content in tailings may be lower than in primary sources, their processing can be justified if environmental benefits are considered, e.g., mine site remediation and land reclamation [
13]; In addition, the identification of mining and industrial wastes with potential for utilization is currently a very relevant topic [
1,
4,
14], as the development of innovative processes to extract elements of interest is not only a way to reduce the environmental impact of a company but also an opportunity to increase the useful life of the company after the depletion of reserves. Moreover, future REE supplies will likely rely on numerous unconventional resources other than classic ore deposits. Among these unconventional resources, low-grade deposits and tailings represent the next logical step for the mining industry, as shown by the decrease in minimum cut-off grades of all metals over time [
15,
16].
Monazite
is a rare earth element (REE) phosphate [
17] and is one of the most critical rare earth minerals in the world, serving as the main source of thorium and light rare earth elements (LREE) such as lanthanum, cerium, neodymium, and praseodymium [
18]. The recovery of REE from monazite is not an easy task as REE are found forming a chain of polyhedra, in which each REE is linked to nine oxygen atoms forming the
polyhedron, which in turn is linked to five tetrahedra of
[
19,
20] as shown in
Figure 1. In the crystal structure of monazite, the
–
O and
P–
O bonds are covalent bonds.
P–
O bonds have a short bond length and a high chemical valence which greatly affects the lattice energy. Consequently, the structure becomes thermally and chemically stable [
21]. Therefore, the presence of phosphate bonds in monazite ore hinders decomposition even at elevated temperatures, which affects overall REE recovery [
22].
REE ores such as monazite are generally processed in several stages (concentration, dephosphorization, leaching, and solvent extraction) [
23]. These separation processes dramatically increase the percentage of REE phosphates in the final concentrate. The removal of REE from monazite concentrates generally involves decomposition of the phosphate structure; typically, the ore is decomposed with sodium hydroxide or potassium hydroxide to produce rare earth oxides. The resulting products are subsequently leached [
24]; there are several hydrometallurgical treatments with inorganic acids to leach REE from their ores [
25,
26,
27]. Different comparative studies have been published for the leaching of REE with different inorganic acids (
), where the best leaching efficiencies were achieved with
and
compared to
[
20,
28].
Solvent extraction, or liquid-liquid extraction, is one of the most important separation processes in hydrometallurgy [
29]. Among the various solvent extraction approaches, the use of organophosphorus extractants for liquid-liquid extraction of REE has been highlighted [
30,
31,
32]. Di-2-ethylhexyl phosphoric acid
(
Figure 2a) belongs to the class of organic phosphonic acids and is the most important investigated extractant in the separation of rare earth elements since the pioneering work of Miranda [
33]. The main feature of
is its ability to form a hydrogen bond between the extractant molecules, leading to the formation of dimeric structures [
34], as can be seen in
Figure 3a. In recent years, research and development of extractants for REE separation has focused on phosphonic acids with lower pKa value and sterically higher chain [
35]. Accordingly, SOLVAY developed an extractant called
®, which is a mixture of phosphonic and phosphinic acids [
35,
36] (see
Figure 2b), possibly like the
extractant,
also has the ability to form dimetric structures between phosphonic acid and phosphinic acid molecules (
Figure 3b). The extensive use of organophosphorus extractants for the separation of rare-earth ions from aqueous solutions by solvent extraction is mainly due to their high sorption rate, chemical stability, and low aqueous solubility properties [
29,
37,
38].
Alluvial gold mining tailings from placer deposits in the Bagre-Nechí mining district region of Colombia are important sources of high economic value minerals such as monazite [
9,
10]. However, it can be observed that there are no detailed studies to determine the technical feasibility of extracting REE from these monazites, which is a highly relevant issue. Therefore, the purpose of this study was to determine the concentration, dephosphorization, and leaching conditions of monazite and to subsequently study the solvent extraction conditions of
,
, and
. To achieve this purpose, a monazite concentrate was first obtained from alluvial gold mining tailings from the Bagre-Nechí mining district in Colombia by combining techniques including gravimetric, magnetic, and electrostatic separation; subsequently, phosphate removal conditions were evaluated with
. Next, the dephosphorized product was leached with
, and finally, the extraction of
,
, and
from the leaching liquor was studied using
and
diluted in n-heptane as extractants. The dephosphorization conditions evaluated were treatment times and temperatures. In addition, an analysis of mineral species in the residue and monazite concentrate was performed; different parameters of the solvent extraction process were also evaluated systematically; the formation of organometallic phases was determined using log D vs. pH, slope analysis, and McCabe–Thiele diagrams. As
Ce,
La, and
Nd are in the majority, the main focus of the studies was on these three elements.