As part of the valorization of the Moroccan phosphate rock by extraction of rare earths, different experiments on natural Moroccan phosphate from the Gantour Basin (basin of phosphate in the Youssoufia area) were done in the REMATOP laboratory. The response surface methodology was used to study the effects of the influence of different parameters (acid concentration, solid/liquid ratio, duration of the reaction, stirring speed, and temperature) on the progress of the phosphate rock’s dissolution process to determine the optimal parameters for the extraction of rare earths. The analyses were made at the same time on the mineral matrix and the solutions resulting from the attack of this matrix by different concentrations of hydrochloric acid. The rare earth analyses done by the ICP-MS technique have shown an overall amount of rare earths (ΣREs) of 228.408 ppm with the existence of yttrium as the major element.
Rare earth elements (REEs) include the 15 elements of lanthanide group from lanthanum to lutetium, coupled with chemically similar yttrium and infrequently scandium [
At present, Morocco has the largest phosphate reserve in the world. This wealth is estimated at more than 90 billion m3 [
Various acids are used as leaching agents in recovery studies of rare earths from phosphate rock by hydrometallurgical processes such as H2SO4, HNO3, H3PO4, and HCL [
In addition to the nature of the acid “leaching agent”, the leaching of rare earth elements from phosphate rock is influenced by many factors. For this reason, experimental design has been used to control the different factors that influence in the leaching process in order to optimize experimental conditions. The influential factors are optimized using response surface methodology and multicriteria optimization with a Doehlert design and desirability function [
The experimental study is based on samples of the natural phosphate. The samples were provided by OCP group. The natural black phosphate samples chosen for this study originated from the fields of Youssoufia (Morocco). This phosphate is apatitic. The general properties of the Youssoufia phosphate and the structure of the fluorapatite, which is the most stable component in the natural state with a formula Ca10(PO4)6F2, have been described in previous works [
Local situation of Gantour and Oulad Abdoun sedimentary phosphate deposits [
Figure
The sum of the analytical techniques used is subdivided into two main types: Techniques illustrating the chemical and mineralogical characteristics of our sample (the phosphate rock of Youssoufia): X-Ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR) Analysis of phosphate and the resulting solution of hydrochloric leaching by ICP/MS to determine its rare earth content
The general characteristics of phosphate were determined using XRD mineralogical quantification. Table
Mineralogical composition of the phosphate.
Mineralogical characteristics | Percentages (%wt) |
---|---|
Apatite, Ca10(PO4)6F2 | 90 |
Calcite, CaCO3 | 2.7 |
Dolomite, CaMgCO3 | 1.7 |
Quartz, SiO2 | 1.4 |
Organic matter | 2.8 |
Amorphous silica | 0.2 |
Pyrite | 0.3 |
Figure
X-Ray diffraction pattern of the raw phosphate.
The chemical composition of phosphate found by using ICP/MS is given in Table
Chemical composition of the phosphate.
Chemical characteristics | Percentages (%wt) |
---|---|
P2O5 | 28 |
SiO2 | 3.25 |
Al2O3 | 1.47 |
Fe2O3 | 0.34 |
CaO | 45.05 |
MgO | 1.20 |
KO2 | 0.52 |
MnO | <0.01 |
TiO2 | 0.03 |
BPL | 61.18 |
BPL (bone phosphate of lime) = 2.185 P2O5.
Descriptive statistics of REEs (ppm) and minor elements (ppm) in natural phosphate rock of Youssoufia.
Element | ppm |
---|---|
Y | 143 |
La | 24.06 |
Ce | 13 |
Pr | 3.626 |
Nd | 17.52 |
Sm | 3.681 |
Eu | 1.008 |
Gd | 5.143 |
Tb | 0.8387 |
Dy | 5.672 |
Ho | 1.303 |
Er | 4.315 |
Tm | 0.6205 |
Yb | 4.2 |
Lu | 0.7843 |
ΣREEs | 228.408 |
ΣMEs | 3538.1 |
The presence of apatite phases and carbonate in bright phosphate is confirmed by infrared spectroscopy techniques. The large band between 3750 and 3000 cm−1 corresponds to the stretching mode of the absorbed water at 3515 cm−1 and the bending mode at 3450. It is a combination of the antisymmetric stretching mode, the symmetric stretching mode, and an overtone of the bending mode [ The 2361 cm−1 peak corresponds to the atmospheric CO2 [ The 1645 cm−1 peak corresponds to the third vibration mode of the adsorbed water molecule known as angular deformity [ The 1456 cm−1 double peak is assigned to carbonate v3 [ The peaks observed at 1040 cm−1 and 963 cm−1 are related to the stretching mode v3 and to the stretching mode v1 of The 885 cm−1 double peak is assigned to carbonate v2 [ The peaks around 604 cm−1 are related to the vibration mode v4 of
The group of
Infrared spectrum of natural phosphate.
The observation by a scanning electron microscope (SEM) shows that Youssoufia phosphate rock before attack with hydrochloric acid consists essentially of irregularly shaped phosphate particles and sometimes rounded (Figures
Morphological observation of the surface of natural phosphate before treatment.
Morphological observation of the surface of natural phosphate after treatment.
The homogeneity of the material and its particle size is an important parameter that should be well controlled. For this, we worked with a particle size <80
A defined volume of hydrochloric acid (HCl,
Experimental domain for the Doehlert experimental design.
Factor (XI) | HCl (%) | Temperature (°C) | Stirring (rpm) | S/L ratio (%) | Time (min) | |
---|---|---|---|---|---|---|
Level 1 | −1 | 13 | 25 | 100 | 20 | 30 |
Level 2 | 0 | 18 | 45 | 200 | 35 | 45 |
Level 3 | +1 | 23 | 65 | 300 | 50 | 60 |
Table
Experimental design and result.
No. of experiments |
|
|
|
|
|
|
---|---|---|---|---|---|---|
1 | 13 | 25 | 100 | 20 | 60 | 3.870 |
2 | 23 | 25 | 100 | 20 | 30 | 8.932 |
3 | 12 | 65 | 100 | 20 | 30 | 4.801 |
4 | 23 | 65 | 100 | 20 | 60 | 13.970 |
5 | 13 | 25 | 300 | 20 | 30 | 3.104 |
6 | 23 | 25 | 300 | 20 | 60 | 11.452 |
7 | 13 | 65 | 300 | 20 | 60 | 7.523 |
8 | 23 | 65 | 300 | 20 | 30 | 10.134 |
9 | 13 | 25 | 100 | 50 | 30 | 2.055 |
10 | 23 | 25 | 100 | 50 | 60 | 4.833 |
11 | 13 | 65 | 100 | 50 | 60 | 2.144 |
12 | 23 | 65 | 100 | 50 | 30 | 4.343 |
13 | 13 | 25 | 300 | 50 | 60 | 2.020 |
14 | 23 | 25 | 300 | 50 | 30 | 5.343 |
15 | 13 | 65 | 300 | 50 | 30 | 2.430 |
16 | 23 | 65 | 300 | 50 | 60 | 6.343 |
17 | 18 | 45 | 200 | 35 | 45 | 2.903 |
18 | 18 | 45 | 200 | 35 | 45 | 3.010 |
19 | 18 | 45 | 200 | 35 | 45 | 3.001 |
20 | 18 | 45 | 200 | 35 | 45 | 3.020 |
21 | 18 | 45 | 200 | 35 | 45 | 3.890 |
Note that during the attack of the crude phosphate with hydrochloric acid for experiments 10, 14, and 16 (Table
The response surface methodology (RSM) was used to optimize operating conditions allowing the maximum dissolution of the rare earths. This method allows one to seek optimum levels of various factors to achieve a desired response level. The five factors influencing phosphate rock dissolution (the concentration of HCl, temperature, stirring, solid/liquid ratio, and time) were coded respectively (
This design permits to represent the responses studied in all experimental domains of these two factors. The experimental design and responses are given in Table
Analysis of
From Table
The model presents a high determination coefficient
3D response surface plots were used to visualize the relationship between the response (
The Doehlert experimental design and experimental results are given in Table
Estimated values of coefficients for response
Coefficient | Value | F. inflation | Ecart-Type | t. exp. | Significance % |
---|---|---|---|---|---|
|
3.1648 | 0.1745858 | 18.13 | <0.01 |
|
|
2.7398 | 1.41 | 0.19519286 | 14.04 | <0.01 |
|
0.8526 | 1.13 | 0.16904198 | 5.04 | 0.395 |
|
−0.3290 | 1.41 | 0.19519286 | −1.69 | 15.3 |
|
−5.2174 | 1.23 | 0.2618787 | −19.92 | <0.01 |
|
1.5116 | 1.13 | 0.16904198 | 8.94 | 0.0291 |
t. exp indicates the experimental value of the student rapport calculated as follows: t. exp = value/Ecart-Type.
In addition to temperature, acid concentration, and time, our study also tested the influence of stirring and S/L ratio that were neglected in other leaching optimization work of rare earths from phosphate as the case of the study conducted by Kim et al. [
Except for the time of leaching, the other reaction parameters have not been studied over continuous intervals (in general, fixing three arbitrary values to be tested for each parameter), which is one of the major gaps that we have avoided by the adoption of RSM [
Figure
Response surface three-dimensional plots of the interaction of every two variables: (a) percentage of HCl (
The optimal operating conditions allowing the maximum extraction were studied by the response surface methodology. The attained solubilization of 75% for REE was obtained by the following conditions: acidification of the ore by % HCL = 23 for 45 min at a temperature of 44°C, a solid/liquid ratio of 43%, and a stirring speed of 233 rpm.
To determine the optimal conditions acid concentration, solid/liquid ratio, reaction time, stirring speed, and temperature in order to define the optimal parameters allowing the maximum dissolution of the rare earths, the responses are optimized simultaneously using the desirability function included in the NEMROD software. It is based on the transformation of all responses obtained from different scales into an identical scale of desirability (individual desirability). The global desirability function
The desirability function varies in the interval [0, 1]; the value 1 corresponds to the maximum satisfaction (desired value) and 0 corresponds to an unacceptable response [
Characteristics of maximum response
Response | Target value | Weight |
|
|
|
Cal. value | Exp. value |
---|---|---|---|---|---|---|---|
|
8 | 1 | 100.00 | 93.17 | 100.00 | 7.99 | 7.12 |
Desirability | — | — | 100.00 | 93.17 | 100.00 | — | — |
In summary, the use of the surface response methodology in our study for the optimization of rare earth leaching from phosphate offers an opportunity to invent a recovery process of these strategic elements. Knowing that the response surface methodology has already proved its ability to improve various industrial processes of different industries, such as the food industry and the wastewater industry [
Rare earth
Rare earth element
Bone phosphate of lime
Laboratoire de Réactivité des Matériaux et Optimisation des Procédés
Office chérifien des phosphates
Inductively coupled plasma mass spectrometry
Minor element
Response surface methodology.
The authors confirm that the data supporting the findings of this study are available within the article.
The authors declare that they have no conflicts of interest.
The authors gratefully thank Reminex Managem laboratories in Marrakech, Morocco, for the chemical analyses and the help provided as well as the Center of Analysis and Characterization (CAC) at Cady Ayyad University, Marrakech, Morocco.