Innovative Application of Microwave Treatment for Recovering of Rare

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Innovative Application of Microwave Treatment for Recovering of Rare Earth Elements from Phosphogypsum Adrian Lambert, John Anawati, Mugdha Walawalkar, Jason Tam, and Gisele Azimi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03588 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Innovative Application of Microwave Treatment for Recovering of Rare Earth Elements from Phosphogypsum Adrian Lambert1†, John Anawati1†, Mugdha Walawalkar1, Jason Tam2, and Gisele Azimi1,2* 1

Laboratory for Strategic Materials, Department of Chemical Engineering and Applied

Chemistry, University of Toronto, 200 College St., Toronto, ON M5S 3E5, Canada 2

Department of Materials Science and Engineering, University of Toronto, 184 College St.,

Toronto, ON M5S 3E4, Canada †these authors contributed equally to this work *corresponding. [email protected]

ABSTRACT

Some rare earth elements (REEs) are classified as strategic materials because of their increasingly high demand, supply uncertainty, and near zero recycling. For tackling the sustainability challenges associated with REEs, their technospheric mining, i.e., recovery from secondary sources is imperative. Characterization results indicate that phosphogypsum, a

ACS Paragon Plus Environment

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byproduct of the fertilizer industry, contains about 0.03-0.4 wt% REEs. Here, a novel process was developed that utilizes microwave irradiation to enhance the leaching efficiency of REEs from phosphogypsum. Optimal REE leaching was achieved by either microwaving at low power (600 W) and short duration (5 min) or at high power (1200 W) and long duration (15 min). The former creates cracks and pores in the particles, enhancing the infiltration of lixiviant, with minimal conversion of gypsum into less soluble crystals. The latter results in thermal degradation of the PG particles and the release of REEs at the cost of changing the PG crystal structure to less soluble phases. In all cases microwave pretreatment had a positive effect (more than 20% increase) on REE leaching efficiency. At the optimum microwaving conditions (15 min irradiation (2.45 GHz) at 1200 W), 80% Nd, 99% Y, and 99% Dy leaching efficiency was achieved.

KEYWORDS Phosphogypsum, Rare earth elements, Microwave irradiation, Direct acid leaching, Secondary resource, Waste valorization

Introduction Phosphogypsum (PG) is the solid residue generated in the production of phosphoric acid through sulfuric acid digestion of phosphate rock. It is primarily composed of gypsum (CaSO4•2H2O) along with some phosphates, fluorides, sulfates, and trace components.1 Approximately 4.5–5.5 tonnes of PG are produced per tonne of P4O10, leading to an estimated annual global production of 100–280 million tonnes.1–4 However, only about 15% of this production capacity is employed productively, primarily in the construction and agriculture


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industries,1 with the remainder being landfilled, representing an unexploited opportunity for the valorization of this material, and a potential environmental risk. PG is considered a potential secondary source for rare earth elements (REE), a group of materials with unique physicochemical properties, which are used in many critical and emerging green technologies, namely wind turbines, electric and hybrid car batteries, automotive catalytic convertors and petroleum refining catalyst, among others. PG has a REE content of 0.01 to 0.40 wt%, because the phosphoric acid process partitions 65–85 % of the REE content of the starting phosphate rock (0.04–1.57 %wt REE) to the PG.5–7 Although this valuable REE content is only present in trace concentrations in the PG, the large worldwide stockpiles and production rates of this low-demand material represent an economical and sustainable feedstock alternative to the primary mining of REE ores. Concerns regarding REEs availability are twofold. First, the demand is increasing at a rate that their supply will fall behind, because it takes time to bring new production routes online. Second, geological scarcity, extraction complexities, and dependence on sources in politically volatile countries makes their supply insecure. REEs are not particularly rare geologically: according to United States Geological Survey (USGS),8 the global REE reserves are about 130 million tonnes; however, a large percentage of them are either located in areas that are not accessible and too expensive to mine or too low in concentration9. Also, opening new mines and building refineries would take several years and requires an enormous capital expenditure of US$106 million to US$2.5 billion10,11. Furthermore, there are environmental concerns associated with opening new mines because of environmental dangers like radioactive elements and mine tailings storage. Considering these aspects, it is imperative to explore alternative secondary sources to tackle REEs supply challenge.


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Technospheric mining of REEs from secondary sources can involve recycling of postconsumer products, such as nickel metal hydride batteries10, permanent magnets12, and urban solid waste13, and recovery from stocks of landfilled industrial waste, such as phosphogypsum6,14, bauxite residue15,16, mine tailings, and metallurgical slags9. There have been several studies on the extraction of REEs from PG, utilizing HNO3, HCl, and H2SO4 as the leachant – Table 1 presents an overview of some previous studies. For the most part, reported extraction efficiencies are relatively low and consume large quantities of acid. Table 1. Literature review of previously reported studies on the leaching of REEs from phosphogypsum Conditions Ref.

Leachant

Temp. (°C)

Time (h)

L:S ratio

REE in Leaching PG efficiency (wt%) 57%

1.5 M HNO3 Walawalkar 1.5 M HCl et al.14 1.5 M H2SO4

80

20 min

8

0.03%

51% 23%

Lokshin et 22-30 wt% H2SO4 al.17

-

20-30 min

1.8-2.2

0.41%

69.471.4%

Lokshin et 4 wt% H2SO4 al.18

-

1248

-

0.73%

50.5%

Vorob’ev et H2SO4 al.19

-

-

-

-

30% 43%

3 M HNO3 Ismail al.20

et

2 M HCl

25

3

2

0.048%

4 M H2SO4 Jarosinski et al.21

5-10 wt% H2SO4

13% < 20

80% 6

10-15 wt% H2SO4

12%

2

40

0.6% 52%


4

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Preston al.22

et

1 M HNO3 + 0.5 M Ca(NO3)2 + 2.5 M 20 NH4NO3

Abramov et H2SO4-HNO3 ratio: 23 al. 3.2-1.2, 1-3 wt%

70

2

6.8%

76%

8-12 min

4-5

0.51.1%

85-86%

4

0.042%

58%

20/3

0.034%

49%

36 wt% HNO3 Al-Thyabat and Zhang24

90%vol -H2SO4 (96 72 wt%)

1

10%vol -H3PO4 (25 wt%)

HammasNasri et 10 % H2SO4 25 al.

60

1-2

1.3

0.022%

50%

Liang al.1

50

2

7

0.022%

52%

2

7.5

0.44%

72%

et

5% H2SO4 10-30 % H2SO4

Rychkov et +Grinding, Ultrasonic al.26 Activation, and Resin Sorption

The present study builds upon a previous study14 that investigated REE recovery from PG by direct acid leaching, investigating the use of different mineral acids (HCl, HNO3, and H2SO4) under various operating conditions in terms of leaching temperature, acid concentration, solid to liquid ratio (S/L), and residence time. This study concluded that the optimal process conditions were 1.5 M HCl, 80 °C, S/L = 1/8, and 20 min residence time, under which an average leaching efficiency of 51% was obtained. To further improve REE extraction, a novel pretreatment technique, i.e., microwave irradiation, was investigated to condition the PG for acid leaching. Microwave electromagnetic irradiation (0.3-300 GHz) is used as a heating technique in several applications.27 Notably, it induces heating within the bulk of irradiated samples by exciting their molecular dipoles at their resonant frequencies. Typically, microwave ovens are operated at 2.45


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GHz, achieving heating by the relaxation processes exerting torque on the water (or other microwave-absorbing condensed phase), by polarizing permanent dipoles with the oscillating electric field – the heat is produced by frictional losses from the reorientation and rotational diffusion of the dipoles.28 As gypsum is a poor absorber of microwave energy, while any moisture within the particles is a good absorber,29 this method of heating allows rapid and selective heating of samples, and the potential creation of localized heating zones within the solid phase, corresponding to heat generated in the areas with higher moisture content, creating fractures by thermal expansion or phase transitions. Several previously published studies have investigated microwave applications in mining and process metallurgy in an attempt to improve the yield of extracted metal, to improve process efficiency and to reduce processing time.27,30 Previous studies have demonstrated that microwave irradiation can be used to improve leaching of mineral samples by introducing fractures and micro-cracks, which assist grindability and leachate infiltration.31–33 This technique has also been applied to the enhancement of REE leaching. Huang et al.34 pretreated oven-dried REE mixed concentrate with NaOH (35.35 wt%) with microwave radiation (1000 W, 30 min) to improve the leaching efficiency of REEs with HCl. In addition, Reid et al.16 demonstrated that microwave heating (1000 W, 10 min) enhances the H2SO4 leaching of scandium and REEs from bauxite residue, another industrial waste reimagined as a valuable secondary resource. Previous studies have also specifically investigated the microwave heating of gypsum and PG, particularly for drying applications.29,35,36 Microwave irradiation has also been used to synthesize hydroxyapatite nanoparticles from PG in solution,37 and to condition PG for use in the adsorption of chromium (VI) ions from aqueous solutions.38


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In this study, the effect of the microwave pretreatment step was investigated and optimized, and thorough fundamental investigations were performed to propose a physicochemical mechanism for this process. The ultimate goal for this work is to develop an efficient and sustainable process for the valorization of otherwise waste phosphogypsum.

Methods Chemicals and materials Phosphogypsum samples, derived from sedimentary apatite, were obtained from Nutrien Ltd.’s fertilizer operations, located in Alberta, Canada. The average particle size was measured to be 71 µm. A thorough characterization of this sample was performed and presented elsewhere.14 Hydrochloric acid (ACS Reagent grade, 36.5–38.0% assay) was used for the leaching process, and deionized water (0.055 µS, Millipore) was used to make all solutions. Microwave pretreatment process Microwave pretreatment (2.45 GHz) was performed using a multi-power microwave oven (Panasonic NN-ST775S). Dried (overnight at 50 °C) PG samples (approximately 30 g per trial) were treated at various nominal powers between 600 and 1200 W for 5, 10, 15, and 45 min in alumina crucibles, which are heat-resistant and do not interact with the microwave radiation as porcelain crucibles would. To prevent overheating and fire, the samples were heated with continuous supervision in intervals of 5-10 min, allowing 5 s to cool between rounds of irradiation. The crucibles were heated individually to ensure that the radiation was only being adsorbed by the studied sample. The microwave oven was purged continuously with 0.5 L/min of nitrogen gas and adequate ventilation was provided to collect any potential fumes produced. The microwave oven was equipped with a rotating stage to ensure even distribution of the radiation over the PG samples. After microwaving, solid samples were collected and


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characterized by X-ray diffraction (XRD) and secondary electron microscopy (SEM). The microwave-treated samples were then used as the feed for the acid leaching experiments. The visual changes occurring during microwave treatment are shown the Supporting Information Video S1. Acid Leaching Leaching experiments were conducted inside a 250 mL heated glass reaction vessel (temperature control within ±1 °C) agitated by a shaft stirrer (600 rpm). All test runs were conducted in 1.5 M HCl at a solid to liquid ratio of 1:15 (g:mL). Samples were withdrawn at 60 min, and the leachate was immediately filtered using 0.45 µm nylon syringe filters to halt the leaching reaction. The filtered samples were diluted with 5 wt% HNO3 prior to elemental analysis with ICP-OES (PerkinElmer Optima 8000) to determine the concentration of REEs in the leachate. Leaching efficiency was defined as the ratio of the mass of extracted REE per unit mass of the solid and the REE concentration in the unleached PG (eq. 1). ‫ݕ‬௜ =

௠೗೐ೌ೎೓ೌ೟೐,೔ ௠೔೙೔೟೔ೌ೗,೔

=

஼಺಴ುషೀಶೄ,೔ ×ி೏೔೗ೠ೟೔೚೙ ×௏೗೐ೌ೎೓ೌ೟೐ ௠ುಸ,೗೐ೌ೎೓೐೏ ×൤

೘೔ ൨ ೘ುಸ

[eq. 1]

Symbol definitions: yi: leaching efficiency of element i, mleachate,i: mass of element i in the final leachate, minitial,i: mass of element i in the starting PG, CICP-OES,i: concentration of element i measured by ICP-OES, Fdilution: ICP-OES dilution factor, Vleachate: volume of leachate in the leaching step, mPG,leached: mass of PG added to the leaching agent, [mi/mPG]: mass proportion of element i per unit mass of starting PG. Aqua regia digestion and ICP-OES characterization The elemental composition of the PG samples was quantified by inductively coupled plasma – optical emission spectroscopy (ICP-OES) following digestion with aqua regia (3 HCl: 1 HNO3 by volume) at 220 °C using an Ethos EZ Microwave Digestion System (the composition values were taken as the average of three independent experiments). Following digestion, the samples were diluted in 5 wt% HNO3 solution for ICP-OES characterization. Leachate samples were also


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analyzed by ICP-OES. The list of wavelengths used for quantification is given in the Supporting Information Table S1. Morphological and crystal structure analysis Morphological characterization of the samples was performed using SEM (Hitachi SU8230), and mineralogical analysis of the sample was performed with XRD (Philips PW1830). Experimental Design Two series of trials were performed: an orthogonal full factorial series of tests to determine the optimal microwave parameters, namely the microwave power (4 levels: 600, 800, 1000, and 1200 W) and irradiation duration (3 levels: 5, 10, 15 min) given identical leaching conditions, and a single-factor series of trials to determine the optimal leaching temperature (4 levels: 25, 45, 65, 85 °C). The results of leaching at the optimal microwave settings were compared with those of leaching unirradiated PG. The full list of runs performed is provided in Table 2. Empirical model building and optimization To determine the theoretical optimum microwave conditions, the leaching efficiency of neodymium (Nd), yttrium (Y), and dysprosium (Dy) were fit to the form given in eq. 2 by multiple linear least squares regression (mLLSR, eq. 3). ‫ݕ‬ො௜ = ߚመ଴ + ߚመଵ ܺଵ + ߚመଶ ܺଶ + ߚመଵଶ ܺଵ ܺଶ + ߚመଵଵ ܺଵଶ + ߚመଶଶ ܺଶଶ

[eq. 2]

ߚመ = ሺܺ ் ܺሻିଵ ሺܺ ் ܻ௜ ሻ

[eq. 3]

Symbol definitions: ŷi: model-predicted leaching efficiency of element i, Xj: coded parameter level for variable j (j = 1: Microwave power, j = 2: Microwave duration), ߚመ௝ : fitted model parameter for variable j, ߚመ : vector containing all fitted model parameters, X: experimental calculation matrix (Table S1), XT: matrix transpose of X, Yi: vector containing all experimental results for element i. The fitted empirical models were simplified at a confidence level of 95% using the tdistribution, and an analysis of variance (ANOVA) was performed to assess their ability to


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adequately explain the observed variance in the data. Further description of the empirical model building methodology is given in the Supporting Information.

Results and Discussion Chemical composition of phosphogypsum The chemical composition of PG was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) following aqua regia digestion. The main REEs present are yttrium (Y), lanthanum (La), neodymium (Nd), cerium (Ce), gadolinium (Gd), samarium (Sm), dysprosium (Dy), erbium (Er), and ytterbium (Yb) with a total concentration of 0.0317 wt% (Fig. 1). These levels are approximately equivalent to the levels reported for other phosphogypsums.1,25 Of these components, Y, Nd, and Dy were selected as markers to monitor REE extraction, as they belong to different subgroups of the REEs and can be expected to behave differently: Y is the most abundant REE and is not a part of the Lanthanide series; Nd is a critical and economically important REE, is relatively abundant in the PG, and is a representative of light REEs; and Dy is a critical and economically important REE and is a representative of the heavy REEs.


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Figure 1. REE composition of unleached PG. The PG samples were digested by aqua regia. REE concentrations were quantified by ICP-OES and represent the average of three digestions. The three main studied categories of REEs and their corresponding marker elements are highlighted. Optimization of microwave parameters The effect of the microwaving parameters in the pretreatment step was expected to have a highly non-linear dependence on the extraction,35 as such the primary microwave settings (microwave power and duration) were tested in a full factorial experimental design, which captured the quadratic effects of both of these variables, and the interactions between these variables (Table 2a). Table 2. Description of all experimental runs and their associated extraction results. a) Full factorial assessment of microwave pretreatment parameters. b) Additional test at high irradiation duration. c) Single factor assessment of the effect of leaching temperature.

Run ID

Leach Temperature

X1: X2: Extraction (%) Microwave Microwave


11


(oC)

Power

Duration

(W)

(min)

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(Nd, Y, Dy)

a) Microwave parameters A1

45

1200

15

69.8, 91.9, 92.1

A2

45

1200

10

56.7, 92.2, 89.6

A3

45

1200

5

62.1, 95.3, 86.2

A4

45

1000

15

53.4, 84.0, 91.1

A5

45

1000

10

54.0, 83.0, 90.3

A6

45

1000

5

57.5, 80.4, 89.0

A7

45

800

15

54.1, 81.2, 87.7

A8

45

800

10

55.0, 80.8, 84.2

A9

45

800

5

61.6, 75.3, 81.4

A10

45

600

15

59.0, 83.4, 95.1

A11

45

600

10

64.1, 82.6, 92.7

A12

45

600

5

67.0, 81.7, 89.8

1200

45

47.7, 73.0, 77.9

b) Extended microwave duration B1

45

c) Leaching temperature C1

25

0

0

45.4, 66.8, 68.8

C2

25

0

0

44.1, 67.3, 66.7

C3

45

0

0

51.8, 69.1, 72.4

C4

45

0

0

50.2, 70.8, 74.1

C5

65

0

0

61.7, 78.9, 84.1

C6

65

0

0

60.1, 78.7, 83.8

C7

85

0

0

65.4, 85.6, 87.3

C8

85

0

0

64.6, 84.4, 89.9

C9

25

1200

15

65.3, 89.0, 88.0


12

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A1

45

1200

15

69.8, 91.9, 92.1

C10

65

1200

15

74.7, 94.7, 95.9

C11

85

1200

15

79.8, 99.5, 99.1

From these extraction results, empirical models (95% confidence) were constructed to quantify the effect of these variables. The resulting model parameters are presented in Figure 2, the numerical values for the models and their associated ANOVA analysis are provided in Tables S3 and S4 in the Supporting Information. The three studied REEs exhibit unique responses to the microwave settings, which suggests that the effect of irradiation on these elements is based on different mechanisms. The baseline average extraction of Nd was approximately 30% lower than that of Y and Dy, likely due to the fact that Nd is the REE that incorporates most readily into PG, while Y and the heavy REEs incorporate appreciably less.39 The primary effect of microwave power did not have a significant effect on Nd and Dy extraction, but it had a strong positive effect on Y extraction. Moreover, this parameter had a strong quadratic effect on all three REEs. Microwave duration had a significant negative impact on Nd extraction, but it had a significant positive impact on Y and Dy extraction. Furthermore, this parameter had a quadratic impact on Nd extraction, but not on Y and Dy. Interestingly, the interaction effects between these parameters were different for the three REEs: they had a synergistic effect on Nd extraction, an antagonistic effect on Y extraction, and an insignificant effect on Dy extraction. The difference in the responses for each of the elements can be explained by the differences in both atomic radius and electromagnetic/dielectric properties, such as quality factor, which would both impact the response independently, but are not mutually correlated for the REEs. These unpredictable differences in microwave properties between the various REES has been previously reported by Cho et al.40 For example, Y and Dy have nearly identical atomic radii and bond strengths;


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however, Y has a dielectric quality factor more than twice that of Dy, while Nd has a much larger atomic radius, and much lower bond strength, but a relatively similar quality factor to Y. The complex interplay between these properties can explain the differences in response for the different species.

Figure 2. Ordered factor effect coefficient charts for the empirical extraction models. The error bars represent the estimated 95% (α = 0.05) confidence interval for each of the factors (n = 8, tdistribution, from the pooled standard deviation of the repeated direct leaching trials). The inset panels indicate the correlation between the empirical models and the experimental data, and the coefficient of determination for the models. Each panel represents a different analyte and its associated extraction model. a) Neodymium, b) Yttrium, c) Dysprosium. The combined response surfaces of these parameters are shown in Figure 3. In general, the moderate power settings result in a local minimum extraction, while the outer limits of the tested design space tend to result in high extractions. To select an overall optimum, the extraction model outputs for all three elements were combined and weighed according to their abundance in the starting PG and their approximate market values41 to assign a dollar value to the amount of extracted REEs as a function of microwave power and duration. Interestingly, the optimum


14

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extraction values occur at opposite extremes of the design space: at low power + short duration, and high power + long duration, which is primarily driven by Nd extraction, because of its relatively high value and abundance in the starting material; however, the global maximum occurs at the more elevated settings, because of the added economic benefit of enhanced Y and Dy extraction.


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Figure 3. Response surface plots for the generated empirical extraction models. The individual extraction efficiencies for each of the studied REEs (a: Nd, b: Y, c: Dy) and the value × abundance – weighted overall REE extraction (d) is also presented. Effect of leaching temperature Given the optimized microwave parameters, additional tests were performed to determine the optimal leaching temperature (Figure 4). As the leaching temperature was expected to affect extraction independently of the microwave settings, these tests were performed outside the trials for microwave parameters optimization. Furthermore, these tests were used to validate the use of microwave pretreatment in comparison with direct leaching of untreated PG. For all studied REEs, microwave pretreatment had a significant (α = 0.05) positive effect on extraction, improving extraction by 20% on average, and particularly enhancing extraction at lower leaching temperatures. In all cases, higher leaching temperatures resulted in enhanced REE extraction, which is consistent with previously reported results.14 To determine whether the observed increase in the extraction efficiency is attributed to microwave irradiation and it is not simply a heating effect, PG samples were heated in a conventional oven at 100 °C and 150 °C, two temperatures corresponding to the observed temperature extremes for microwaved PG. Conventional heating resulted in a decrease in the extraction efficiency (Supporting Information Figure S2); thus, the observed extraction enhancement for the microwaved PG is specifically due to the microwave irradiation step.


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Figure 4. Effect of leaching temperature on Nd, Y, and Dy extraction efficiency for microwave pretreated and untreated PG. The error bars represent the standard deviation for the repeated direct leaching trials (n = 2), the Microwaved trials were assumed to have equal variance to the Control trials. The stars (*) indicate the trials in which the Control and Microwaved trials were significantly different at 95 % confidence (t-distribution, α = 0.05) The leachate concentration, leaching time, and solids to liquids ratio was maintained constant for all tests, and the microwave parameters were held constant for all microwaved samples. On the basis of the combined results, the optimal process conditions were: microwave pretreatment at 1200 W for 15 min, followed by leaching in 1.5 M HCl at a S/L ratio of 1/15 g/mL at 85 °C for 60 min, which resulted in 80% Nd, 99% Y, and 99% Dy extraction efficiency. Effect of microwave power and treatment time on PG morphology and crystal structure As discussed above, microwave pretreatment had a significant and non-linear effect on the REE extraction, as such it follows that these effects are caused by changes in the microstructure and/or mineralogy of the PG. It has previously been demonstrated that the water heating and vaporization that occurs during microwave pretreatment can introduce nano-sized cracks and


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pores into minerals, allowing enhanced infiltration of leaching agent into the particle matrix.16 SEM images of the PG particles treated at 1200W are presented in Figure 5. It was observed that, as the irradiation duration increases, the surface roughness and particle porosity increases. Within the first 5 min of irradiation (Figure 5b), the general morphology of the particles appears to change, with separation occurring between the parallel planes along the particle edge, and fragments appearing on the face. After 10 min of irradiation, the faces of the particles appeared to become somewhat striated with grooves. After 15 min, the striations appeared to have progressed from the face into the bulk of the particle, forming bundle-like structures and leading to a considerable increase in apparent particle porosity. This non-linear progression of morphology with irradiation time is consistent with previous studies, which showed that the drying rate of microwaved gypsum depends primarily on the particle moisture content (a nonlinear trend) and the microwave power.35 The use of microwave irradiation to provide targeted heating is essential to achieve the observed morphology changes, as simply heating the PG samples in a conventional oven at 100 °C or 150 °C did not result in any morphological changes (Figure 5e and f).


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Figure 5. Change in PG microstructure during microwave irradiation. SEM micrographs were taken of samples irradiated at 1200 W for the specified duration: a) 0 min (Original PG), b) 5 min, c) 10 min, d) 15 min, e) 1 h conventional heating at 100 °C, f) 1 h conventional heating at 150 °C. As the PG particles expand and degrade with microwave irradiation, inner surfaces become exposed and introduce a higher active surface area accessible to the leaching agent. This observed expansion relative to the original PG explains the observed overall improvement of leaching achieved with microwave pretreatment. To determine if further increasing the irradiation duration would increase REE leaching efficiency at high microwave power, an additional test using 45 min microwave time was conducted (Supporting Information Figure S3). The extended irradiation time resulted in a decrease in the extraction of all three elements. Observation of the particle microstructure (Supporting Information Figure S4) revealed that the extended heating resulted in a considerable loss of particle porosity, possibly due to the


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agglomeration and sintering of particles, which reduces the particle porosity; thus, impeding leachate infiltration into the particles, reducing the leaching efficiencies.

To better understand the mechanism, X-ray diffraction was utilized to characterize changes in the crystal structure of PG samples as a function of irradiation power and time, and Rietveld refinement was utilized to quantify the relative amounts of gypsum (CaSO4•2H2O), hemihydrate (CaSO4•0.5H2O), and anhydrite (CaSO4). The changes in crystal structure are presented in Figure 6.

Figure 6. Rietveld refinement results from the XRD spectra of microwaved PG. The PG samples were microwave treated as indicated. The resulting crystal phases were then quantified by Rietveld analysis. The crystal structure of the three observed phases are also presented. The structural data was obtained from Boeyens & Ichharam,42 Hawthorne & Ferguson,43 and Bezou et al.44. Structures were visualized using the VESTA software.45


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Microwave exposure caused a distinct change in the crystal structure of the PG sample causing a shift from the doubly hydrated CaSO4•2H2O to hemihydrated CaSO4•0.5H2O to anhydrous CaSO4, indicating the vaporization of crystal water. Previous studies have shown that in solution, microwave irradiation can induce the formation of CaSO4•0.5H2O, which requires temperatures exceeding 107 °C, by creating localized hot spots,46 and in this work, during the microwave treating of dry PG, the temperature was raised to about 140 °C. When wet gypsum is heated in a microwave, moisture absorbs the radiation more readily than gypsum. In a previous study, when a sample of wet gypsum was exposed to 1400 W for 2 min, a small amount of CaSO4•0.5H2O was produced, with no CaSO4 production, and decreasing the power or duration resulted in drying without any observable phase transformation.29 In this study, increasing the amount of microwave energy delivered (Power × Duration) resulted in a sharp drop in CaSO4•2H2O content, accompanied by a proportional increase in CaSO4•0.5H2O and CaSO4, until 900 kJ, at which CaSO4•2H2O was depleted. However, in this transition region, the extent of conversion was highly variable, with all samples irradiated at above 1000 W showing complete conversion, and relatively no change in the crystal structure over time (Supporting Information Figure S5). The power density was observed to have a critical role in the phase composition of the treated PG – For example, trials A3 and A11 both had a total power input of 360 kJ, and A2 and A7 both had an input of 720 kJ – For the lower energy pair, the sample irradiated at 600 W comprised 33% gypsum, while the 1200 W counterpart had no gypsum content. At the higher energy input, the difference between the two trials was smaller; however, gypsum was still present in the low-power trial but not in the high-power counterpart. In the microwaved samples, the ratio of CaSO4•0.5H2O to CaSO4 was approximately constant at 1.0 ± 0.1. These XRD results suggest that at high microwave power, the dehydration of CaSO4•2H2O to CaSO4•0.5H2O and


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CaSO4 is fast, because water readily absorbs microwave radiation, while the gypsum itself does not.29 Combining these results with the microstructure observations, we propose that the microwave irradiation causes dielectric heating of the water molecules present within the crystal structure of the gypsum phase and causes them to vaporize, resulting in a transition from gypsum to hemihydrate to anhydrite. This conversion can result in the observed formation of breaks and pores in the particles as the expanding liberated water vapor escapes,35,36 and as the particle experiences internal localized stresses caused by the rapid density shift of the crystals (ρCaSO4·2H2O = 2.3 g/cm3, ρCaSO4·0.5H2O = 2.7 g/cm3, ρCaSO4 = 3.0 g/cm3). The changes in microstructure caused by the phase conversions appear to be more important for REE extraction than the phases themselves, as there is no direct correlation between any of the three crystal phases and the extraction of the three monitored REEs (Supporting Information Figure S6); however, these changes can contribute to making the REEs more or less accessible. Furthermore, as the REEs can integrate either isomorphously with the gypsum lattice or as their own phases (explained further below), it is possible that the localized heating and irradiation provided by the microwaves affects the affinity of the REE ions to the gypsum lattice, as they have different electronic properties than the matrix Ca2+ ions, or induces specific changes to the REE-bearing phases. Because of the trace concentrations of the REEs, future investigation using advanced spectroscopic techniques would be required to observe these effects in detail.

Changes in particle crystal structure and morphology during leaching The solid residues from leaching were collected at different time intervals during the leaching process to observe the changes in the crystal phases (XRD – Supporting Information Figure S7)


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and morphology (SEM – Supporting Information Figure S8). During leaching, any hemihydrate is rapidly (< 6 min) either dissolved or converted to either gypsum or anhydrite, while gypsum and anhydrite levels stayed relatively constant over time, with the exception of the samples irradiated at 1200 W, in which the initially-low proportion of gypsum increased over time. In comparison with the unleached microwave pretreated PG, the leach residue samples were rougher and more irregularly shaped, with particle fragments on the surface, and with the number of fragments increasing with longer leaching times. These observations are consistent with the acid infiltrating into the PG particles and breaking them apart into smaller pieces from the inside, and thus releasing more REEs. The crystal structure of gypsum is held together primarily by sheets of weak hydrogen bonds,42 which would allow the acidic leaching agent to more readily break apart the gypsum crystals into smaller crystals, as was observed. Furthermore the rough surface morphology is consistent with the needle-like appearance of anhydrite.47

Proposed process mechanism Previous studies have shown that in CaSO4•2H2O, the lanthanides are present as isomorphous Ca2+ substitutions, where two REE3+ ions replace three Ca2+ ions, leaving one vacant Ca2+ lattice site.39,48 Conversely, other studies have suggested that the differences in ionic charge and radius between Ca2+ and REE3+ result in the formation of separate REE phases; for instance, it has been shown that in precipitation conditions similar to those found in the formation of PG, europium (Eu) adsorbs onto the calcium sulfate crystals as a metastable amorphous/nanocrystalline phosphate/sulfate precipitate.49 It has thus been suggested that both mechanisms can occur, in parallel, with REE ions forming both easily leachable separate REE phases, and incorporating directly into the gypsum crystal lattice, which would require degradation of the gypsum lattice to


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access.26 As such, one can expect that process conditions, which result in increased degradation of the gypsum lattice and/or allow for increased infiltration of the leaching agent, would result in higher REE extraction. We hypothesize that the seemingly contradictory observed extraction behaviors, in which REE extraction on average is improved by microwave pretreatment, but the microwave process settings that maximize this extraction are at the tested extremes (low power + short duration, and high power + long duration), are the result of two competing physicochemical mechanisms. In addition, the known differences in the integration of different REEs in gypsum39 and electromagnetic properties40 can explain the observed differences in the extraction behavior of Nd, Y, and Dy. Firstly, effective leaching requires the infiltration of leachate into the PG particles, to access the highest possible amount of active surface area for the leaching reaction. Leaching can thus be enhanced by the cracks, pores, and channels caused by both the expansion and escape of water vapor, and the contraction of the crystal lattice as it transitions from gypsum to hemihydrate to anhydrite. Antagonistically to this effect, the crystal lattice transitions from gypsum, which is held together by relatively weak hydrogen bonding and is therefore more easily breakable by the leachate, to hemihydrate and anhydrite that are mainly held together by stronger ionic bonding, and thus have lower solubility.50 Hence, the optimal REE extraction conditions correspond to either the mild microwave conditions, which allow some degradation of the PG particles while maintaining the readily leachable hydrated gypsum, or the severe conditions, which allow considerably more particle degradation, at the cost of converting the matrix to a less leachable form. A schematic diagram of this mechanism is presented in Figure 7.


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Figure 7. Schematic diagram of the microwave-assisted phosphogypsum leaching pretreatment mechanism. The schematic is presented for illustrative purposes and is not drawn to scale.

Conclusions In this investigation, a process for enhancing the leaching efficiency of rare earth elements from phosphogypsum was developed and optimized. By subjecting the PG particles to a microwave pretreatment, the atomic- and micro-structure of the PG is conditioned to more readily allow the infiltration of HCl leaching agent within the bulk of the particles. Optimal REE leaching can be achieved by either microwaving at low power (600 W) and short duration (5 min) or at high power (1200 W) and long duration (15 min). It was also demonstrated that, regardless of microwave parameters, the irradiation process enhances REE extraction, relative to


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unmicrowaved PG. The leaching of microwave pretreated PG can be further enhanced by leaching at elevated temperatures. This process technique is a component for a larger potential near-zero-waste process for the sustainable valorization of PG, a low cost and readily available alternative feedstock. Microwave pretreatment as a processing technique, represents a low capital and operating cost processing step, which results in considerable gains in extraction efficiency. However, several questions must be addressed before this process can be implemented at industrial scale. The main obstacles to scale-up are the optimization of the electric field pattern, development of a system that is robust to harsh industrial plant conditions (dust, dirt, rain, snow, remote location), and the development of material handling systems.51 Despite potential scale-up complications and increased process complexity, microwave pretreatment presents a considerable cost saving opportunity, as room-temperature leaching of pretreated PG outperforms direct leaching of asreceived PG at 85 °C. Bradshaw et al.52 estimated the cost of a comparable commercial ore microwave treatment process to be $0.18-0.94 per tonne of starting material, while the cost of heating the leachate from room temperature to 85 °C is estimated to be $8.3/tonne if heat exchange (10 °C approach temperature) is utilized, and up to $50/tonne if no heat is recaptured (assuming an energy cost of $13.28/GJ).53 Alternative extraction techniques have been explored for the valorization of PG, such as leaching with organic liquids, ultrasonic mechanoactivation, and in-situ ion-exchange adsorption.7,26 As microwave pretreatment acts to degrade the PG particles, making them more permeable to leaching agents, this complimentary technique could be adapted to enhance a wide variety of PG extraction processes.


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Next steps for this work will focus on the recovery of these valuable materials from the leachate solution, either by selective precipitation, ion exchange, solvent extraction, or other novel techniques, and on determining the extraction potential of the problematic radioactive contaminants. Another major focus of future work will be the recovery and recycling of HCl to develop the process into an essentially closed-loop system. Furthermore, towards the goal of creating a near-zero-waste process, the remaining PG solids must be assessed for their suitability to be employed in a value-added application, such as construction materials or agriculture. Because the solid residue remains in the form of gypsum, future work will include assessment of the leached residue to determine whether the leaching process lowers the radioactivity to levels permitting commercial use as gypsum. As a whole, the microwave pretreatment technique could help enable potential phosphogypsum complete valorization process, allowing extraction of valuable REEs, and purification of commercial-grade gypsum, turning what was formerly considered a problematic waste into a valuable secondary resource.

ASSOCIATED CONTENT Supporting Information: A PDF document containing additional information on the experimental and statistical methodology and supporting results and a Supplementary Video. These materials are available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *corresponding. [email protected] Author Contributions


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G.A. conceived and supervised the research. A.L. designed and performed the microwave pretreatment and leaching experiments and XRD analysis. J.A. analyzed and interpreted the results and drafted the manuscript. M.W. contributed to some experiments and XRD analysis. J.T. performed SEM characterizations. All authors contributed to revising the manuscript and have given approval to the final version.

ACKNOWLEDGMENT The authors acknowledge the financial support provided by Natural Sciences and Engineering Research Council of Canada (NSERC) (No. 498382). Also, Nutrien is gratefully acknowledged for providing us with phosphogypsum samples. We thank Dr. Connie Nichol for her collaboration throughout the project. We thank Mr. Kok Long Ng for his help with SEM imaging and Mr. Jiakai Zhang for his help with filming the microwave heating process. Access to the electron microscopy facility in the Canada Foundation for Innovation (CFI) funded Ontario Centre for the Characterization of Advanced Materials and the Walter Curlook Materials Characterization & Processing Laboratory is acknowledged.

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Synthesis, and Design of Chemical Processes, Fourth Edi.; Prentice Hall, 2012.

SYNOPSIS A novel process for the extraction of Rare Earth Elements from Phosphogypsum utilizing microwave irradiation pretreatment and direct acid leaching. TOC/Abstract graphic (For Table of Contents Use Only)


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Innovative Application of Microwave Treatment for Recovering of Rare

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