A Novel Continuous Ultrasound-Assisted Leaching Process for Rare Earth Element Extraction: Environmental and Economic Assessment

Abstract

Rare earth elements (REEs) make up integral components in personal electronics, healthcare instrumentation, and modern energy technologies. REE leaching with organic acids is an environmentally friendly alternative to traditional extraction methods. Our previous study demonstrated that batch ultrasound-assisted organic acid leaching of REEs can significantly decrease environmental impacts compared to traditional bioleaching. The batch method is limited to small volumes and is unsuitable for industrial implementation. This study proposes a novel approach to increase reaction volume using a continuous ultrasound-assisted organic acid leaching method. Laboratory experiments showed that continuous ultrasound-assisted leaching increased the leaching rate (µg/h) 11.3–24.5 times compared to our previously reported batch method. Techno-economic analysis estimates the cost of the continuous approach using commercially purchased organic acids is $9465/kg of extracted REEs and $4325/kg of extracted REEs, using gluconic acid and citric acid, respectively. The sensitivity analysis reveals that substituting commercially purchased organic acids with microbially produced biolixiviant can reduce the process cost by approximately 99% while minimally increasing energy consumption. Environmental assessment shows that most of the emissions stemmed from the energy required to power the ultrasound reactor. We concluded that increased leaching capacity using a continuous ultrasound-assisted approach is feasible, but process modifications are needed to reduce the environmental impact.

1. Introduction

Rare earth elements (REEs) possess unique magnetic and optical properties that make them especially useful in a wide range of technologies including healthcare instrumentation and electronic vehicles [1]. Despite the immense importance of REEs around the world, production of the global supply is dominated by only a few countries. A lack of domestic REE production raises concerns about supply chain security and geopolitical influence. Reflecting this urgent need, the U.S. Department of Defense (DoD) has invested over $439 million to establish a domestic REE supply chain since 2020 [2]. To secure a domestic supply of REEs and decrease reliance on foreign suppliers, sustainable domestic REE extraction must be implemented.

Traditional extraction and separation of REEs from ore involves the use of hazardous chemicals. For example, current methods used to recover REEs from ore include digestion (also known as “cracking”), leaching, and solvent extraction. REE-containing minerals are typically digested in strong sulfuric acid (H₂SO₄) or sodium hydroxide (NaOH) at high temperatures [3]. Rare earth sulfates resulting from digestion can be leached with water [4]. The pregnant leach solution (PLS) produced during leaching proceeds to a solvent extraction process that is often performed many times in tandem to achieve acceptable purities of REE products [5]. It is estimated that 2000 tons of toxic waste is produced for every ton of REEs produced using these processes [6].

Biological extraction of REEs from mineral sources, in addition to e-waste, can potentially provide a less hazardous alternative to these methods [7]. Bioleaching has been successfully implemented in the mining industry via oxidation of sulfide ores by autotrophic organisms. Although this method is generally not suitable for non-sulfidic ores that contain REEs (i.e., bastnasite and monazite), heterotrophic bioleaching is effective [8]. Heterotrophic organisms consume organic carbon compounds and produce organic acids and other metabolites, which solubilize REEs [9]. Despite a growing field of research into heterotrophic REE bioleaching of mineral and waste sources, it has not yet been commercially utilized. The economics of bioleaching operations is heavily reliant on process parameters (e.g., solids loading, reactor configuration, and temperature) [10]. Organic acid bioleaching is often performed under milder conditions than traditional REE extraction (i.e., lower temperatures and less acidic pH). This can reduce the energy inputs required for extraction, as well as decrease the amount of hazardous waste produced. In addition, organic acids can be produced from naturally replenishable sources, such as agricultural waste or by-products.

Multiple studies investigated the ability of organic acids to leach REEs from a variety of sources (

Table 1. Literature review of recent organic acid REE leaching studies.

Organic Acid	Acid Concentration	REE Source	Pulp Density	Percent REEs Leached	Reference
Citric	1 M	Nd magnet waste	3%	100%	[11]
Citric	1 M	Phosphogypsum	20%	83%	[12]
Citric	3 M	Phosphogypsum	5%	62%	[13]
Citric	1.45 M	REE magnet swarf	10%	90%	[14]
Oxalic	45 mM	Coal fly ash	5%	31%	[16]
Citric	5 M	Phosphogypsum	67%	63%	[15]
Gluconic	100 mM	Ion adsorption clay	67%	100%	[17]
Pyruvic	4 M	Coal fly ash	14%	74%	[18]
Citric	1 M	Coal fly ash	0.25%	27%	[19]

). For example, Gergoric et al. used both citric acid and acetic acid to leach REEs from neodymium (Nd) magnet waste. They found that citric acid was able to achieve nearly 100% leaching efficiency [11]. Gasser et al. observed that higher leaching efficiencies were achieved with phosphogypsum at higher citric acid concentrations. The maximum leaching yield of 83.4% was observed at the optimal concentration of 1 M. Interestingly, the leaching efficiency decreased when the concentration of citric acid was increased beyond optimums, similar to data reported by Romano et al., and Lütke et al. [12,13,14]. Zhang et al. used organic acids derived from Aspergillus niger culture filtrate to leach REEs from phosphogypsum [15]. They found that using the culture filtrate increased the leaching 63.3% compared to a simulated organic acid mixed solution, suggesting that other metabolites participate in REEs leaching [15]. Optimization of an A. niger culture producing mainly oxalic acid was able to achieve an REE leaching rate of 30.9% from coal fly ash [16]. Zhou et al. also used organic acids (e.g., gluconic, citric, and succinic acid) produced by A. niger to leach REEs from ion-adsorption clay [17]. The spent medium method resulted in the best leaching results, with approximately 100% yield [17]. Sakr et al. investigated the use of commercial organic acids to leach REEs from coal fly ash [18]. The highest leaching yield of 74.1% was achieved with pyruvic acid, followed by 68.8% yield with formic acid, and 66.4% with tartaric acid [18]. Zhang et al. investigated a novel column leaching method with citric acid [19]. They found that the column method increased REE recovery from coal fly ash (64.8%) compared to a conventional agitation method (<30%). In addition, they found that leaching of aluminum (Al), titanium (Ti), and iron (Fe) was less than 13.7% [19].

The structure of citric acid suggests that it has the highest chelation potential of organic acids due to its tricarboxylic nature [13]. Although this can be beneficial during leaching, high concentrations of citric acid may be inhibitory due to the tendency of REE/citric acid complexes to form insoluble species through polymerization [12].

Although bioleaching has the potential to reduce the use of hazardous chemicals and mitigate emissions, the slow rate of bioleaching is a barrier to industrial implementation [20]. Our prior experimental studies, using gluconic acid leaching suggest that ultrasound-assisted bioleaching can reduce carbon dioxide, methane, and nitrous oxide emissions by 91% compared to traditional bioleaching, in addition to reducing incubation time [21]. In this study, the performance of a continuous ultrasound-assisted organic acid leaching reactor was evaluated using both gluconic and citric acid. The results from the laboratory study were used to inform a life cycle assessment (LCA) and techno-economic analysis (TEA). These studies explore the potential for using a continuous ultrasound-assisted leaching technique to recover REEs from Idaho-sourced soil.

2. Materials and Methods

2.1. Sample Collection

The Diamond Creek prospect area is located 8 miles outside of Salmon, Idaho. Historic data estimates that the area holds about 75,500 tons of total rare earth oxides [22]. The REE veins in the Diamond Creek area are hosted by quartzite, siltite, and granite [23]. Monazite REE placer deposits that are derived from the weathering of intrusive of metasedimentary rock are also found in the area [24] Soil was collected from Diamond Creek and contained a mixture of soil and small-to-medium sized rocks that were scooped with an excavator bucket (Figure 1). The samples were reduced and homogenized using a ball mill.

2.2. Continuous Ultrasound-Assisted Organic Acid Leaching

A novel continuous ultrasound reactor was designed to improve the leaching efficiency of REEs from Diamond Creek surface soil. The soil sample contained an average of approximately 11.6 mg/g (1.16%) of mixed REEs. The mixed REE concentration of the soil was calculated using the sum of cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), neodymium (Nd), praseodymium (Pr), scandium (Sc), samarium (Sm), terbium (Tb), yttrium (Y), and ytterbium (Yb). Ultrasound treatment was performed using a custom-built rig using a modified LSP-500 horn sonicator (Industrial Sonomechanics, Miami, FL, USA). The sample is held in a 4000 mL reservoir and is stirred by a magnetic stir bar. The sample was pumped from the reservoir to the sonicator and back to the reservoir via a peristaltic pump (Figure 2). Samples were taken using an inline sample valve and the pressure generated during the sonication process was monitored with a pressure gauge.

Commercially purchased gluconic acid and citric acid were used for the leaching experiments as a model for microbially produced biolixiviant. Room temperature (21 °C) continuous ultrasound-assisted leaching was performed at 10% (w/v) solids loading in either 3.1 M gluconic acid or 3.2 M citric acid (Sigma-Aldrich, St. Louis, MO, USA) for 1, 2, or 3 h. The sonicator was allowed to run for 55 min continuously cooled for five minutes before running again. During incubation, the temperature of the samples was recorded. After incubation, the samples were centrifuged to separate the leachate from the solids. Three 50 mL aliquots of leachate were collected per time for further analysis. The remaining solids were dried, weighed, and re-loaded with fresh gluconic or citric acid and incubated for an additional three hours. Pearson correlation calculations were performed in Python (v3.12) and the codes are provided in Appendix B.

2.3. Analytical Methods

REE leachates and un-leached soil samples were digested in 70% nitric acid, filtered (0.22 µm), and acidified with ultra-pure nitric acid to a concentration of 2% (v/v). The concentration of the REE leachate was measured with inductively coupled plasma and mass spectrometry (ICP-MS). The concentration of mixed REEs was determined by adding together the concentrations of Ce, Dy, Er, Gd, Ho, La, Nd, Pr, Sc, Sm, Tb, Y, and Yb. Mixed REE leaching yields were calculated using Equations (1) and (2). The mass of mixed REEs solubilized during the leaching reaction (M, µg) was calculated using Equation (1), where C is the mixed REE concentration of the sample (µg/mL) and V is the total reaction volume (mL).

$$M=C\times V$$

(1)

The leaching yield (Yi, %) was calculated using Equation (2), where Original REE mass refers to the mass of mixed REEs (µg) present in the soil/ore used for leaching.

$$Yi={\displaystyle \frac{M\times 100}{Original REE mass}}$$

(2)

2.4. Environmental Impact Assessment

The LCA was performed using laboratory data reported in Section 3.1 in conjunction with previously reported data. The environmental impacts of the continuous ultrasound-assisted method were compared to our previously published data using a batch ultrasound-assisted leaching method [21]. There are four phases that make up the LCA study, including definition of the goal and scope, life cycle inventory analysis, life cycle impact assessment (LCIA), and data interpretation.

2.4.1. Goal and Scope

The LCA study herein investigates the gaseous emissions (CO₂, NO₂, and CH₄) environmental impacts of a novel continuous ultrasound-assisted organic acid leaching method. The functional unit was defined as the extraction of 1 kg of mixed REEs from Diamond Creek surface soil. This study was a gate-to-gate analysis of continuous ultrasound-assisted organic acid leaching to a mixed REE leachate (Figure 3). The organic acid leaching process utilized commercially purchased gluconic or citric acid to produce a mixed REE leachate.

2.4.2. Life Cycle Inventory Analysis

The inputs and outputs of the process were defined from experimental data (Section 3.1), other relevant studies, AGRIBALYSE, and OpenLCA databases. The REE extraction process comprises the following unit operations: (a) continuous ultrasound-assisted leaching and (b) centrifugation. The inputs of these operations include Diamond Creek soil, gluconic or citric acid, and energy. The outputs are mixed REE leachates, process emissions, and solid by-products. The REE-rich soil contained a mixed REE concentration of 1.16%.

2.4.3. Life Cycle Impact Assessment

The LCIA was conducted using OpenLCA LCIA methods (version 2.1.3), TRACI 2.1, developed by the U.S. Environmental Protection Agency (EPA). The nomenclature is presented in Abbreviations. Emission factors and gaseous emissions for the proposed process were calculated using Equations (3) and (4):

$$L_{EF}={ER}_{CO2}\times L_{EFCO2}+{ER}_{CH4}\times L_{EFCH4}+{ER}_{N2O}\times L_{EFN2O}$$

(3)

$$L_{GWP}=M_{RL}\times L_{EF}$$

(4)

2.4.4. Interpretation

The environmental impacts were calculated, and the major contributors were identified. The results were compared to previously reported data using batch ultrasound-assisted leaching [21] and bioleaching using Gluconobacter oxydans [25]. The results will be used to identify process modifications that can improve energy consumption and environmental impacts. The calculated environmental impacts were normalized to one kg of leached REEs to allow comparison to similar studies.

2.5. Case Study

To evaluate the TEA and LCA models, a case study of continuous ultrasound treatment for six hours with gluconic or citric acid was performed. The aim of the case study is to improve mixed REE leaching yield and decrease energy usage. The following assumptions are derived from our experimental data reported in Section 3.1 and other published studies.

• 100,000 kg (100 metric tons) of soil containing a mixed REE concentration of 1.16% was processed.
• 142 kg of mixed REEs were extracted from 100,000 kg of soil using gluconic acid (12.2% yield).
• 202 kg of mixed REEs were extracted from 100,000 kg of soil using citric acid (17.4% yield).
• Energy data was measured in the laboratory-scale experiments and scaled linearly. Energy data of the key continuous ultrasound-assisted leaching steps were included in this study (i.e., ultrasound leaching and centrifugation of the resulting slurry). The energy used to power the facility was not considered.
• The capital, overhead, and variable costs are not considered in this study. A comprehensive evaluation of these costs can be found in Brown et al., 2024 [21].
• Emission factors from the Intergovernmental Panel on Climate Change for a 100-year time horizon (28 kg CO₂ eq./kg CH₄ and 265 kg CO₂ eq./kg N₂O) were used to calculate process emissions [26].
• The emissions produced during continuous ultrasound-assisted leaching of 100,000 kg of soil were assumed to be 69,428,000 CO₂ eq./kg CO₂, 544 kg CO₂ eq./kg N₂O, and 2788 kg CO₂ eq./kg CH₄.
• The LCA study focuses on gaseous emissions and environmental impacts of the midstream ultrasound-assisted leaching process. The environmental impacts of upstream processes (collection and transportation) additional midstream processes (separation and refinement), and downstream processes (distribution) were not assessed in this study.

2.6. Techno-Economic Analysis

Experimental data was used for TEA calculations of the continuous leaching process only. Reagent prices included the cost of purchasing commercial gluconic or citric acid, and the energy price consisted of the electricity required to power the ultrasound-assisted leaching equipment. The total annual cost of continuous ultrasound leaching was calculated with Equation (5), which included the cost of reagents, Equation (6), and the energy cost required to power the equipment, Equation (7). The resulting data was normalized to one kg of leached REEs to facilitate comparison to similar studies. The process inputs and outputs are shown in

Table A1. Summary of inputs and outputs of the proposed continuous ultrasound-assisted leaching process.

Inputs	Quantity	Outputs	Quantity
Soil	100,000 kg	-	-
Gluconic acid, 50% (w/w)	2,232,000 kg
Citric acid, 100% anhydrous	1,107,000 kg	-	-
Water (citric acid)	1,107,000 kg	-	-
Electricity, medium voltage {US}	2,584,000 kWh	-	-
-	-	Mixed REEs (gluconic acid)	142 kg
-	-	Mixed REEs (citric acid)	202 kg
-	-	CO2 emissions	69,428,000 kg
-	-	N2O emissions	544 kg
-	-	CH4 emissions	2788 kg
-	-	Solid byproducts (gluconic acid)	99,798 kg
		Solid byproducts (citric acid)	99,858 kg

. The assumptions used in the calculations are shown in

Table A2. Assumptions used to estimate cost in the Techno-Economic analysis.

Parameter	Definition	Value (Gluconic Acid)	Value (Citric Acid)	Unit
CAT	Annual total cost of the proposed process	1,339,490	827,869	$/yr
CR	Annual total cost of reagents	1,055,250	588,629	$/yr
CE	Annual total cost of energy	284,240	284,240	$/yr
COA	Cost of purchasing organic acids	0.59	0.33	$/L
UOA	Annual organic acid utilization	1,800,000	1,800,000	L/metric ton
UL	Annual ultrasound-assisted leaching utilization	100	100	Metric tons/yr
CUP	Cost of utilities	0.11	0.11	$/kWh
UE	Annual utilization rate of energy	2,584,000	2,584,000	kWh/yr

. The nomenclature is provided in Abbreviations.

$$C_{AT}=C_R+C_E$$

(5)

$$C_R=C_{OA}\times {U_{OA}\times U}_L$$

(6)

$$C_E=C_{UP}\times U_E$$

(7)

2.7. Sensitivity Analysis

The effect of organic acid sources is considered for the sensitivity analysis. Energy consumption and process cost were calculated and compared for commercially purchased organic acids and microbially produced organic acids. Experimentally determined energy usage data and previously published reagent consumption data were used for the calculations [25,27].

3. Results

3.1. Laboratory Results

The temperature in the reaction vessel was measured and consistently increased over time between 22.0–46.8 °C (

Table A3. Temperature measured during continuous bioleaching using either 3.1 M gluconic or 3.2 M citric acid.

Organic Acid	Ore/Soil Type	Time (h)	Temperature (°C)
Gluconic acid	Virgin	0	22.0
	Virgin	1	35.0
	Virgin	2	40.3
	Virgin	3	45.9
	Recycled	1	36.9
	Recycled	2	41.2
	Recycled	3	46.8
Citric acid	Virgin	0	22.0
	Virgin	1	39.7
	Virgin	2	40.1
	Virgin	3	40.5
	Recycled	1	38.8
	Recycled	2	39.9
	Recycled	3	41.0

). The continuous leaching yield of virgin (untreated) REE-rich soil with gluconic acid remained relatively constant over the three-hour incubation time (1.8–2.1% yield per hour), while the yield with citric acid decreased over time. The yield of recycled (treated) soil increased over time in both gluconic and citric acid samples, with citric acid demonstrating better performance (Figure 4a). The cumulative leaching yield of citric acid (17.4%) was slightly higher than gluconic acid (12.2%) (Figure 4b). Compared to our previously previous results using a batch leaching method [21], the continuous process increased the leaching rate of gluconic acid by 11.3 times, and that of citric acid increased by 24.5 times (Figure 4c). Similarly, the mass of REEs recovered per mass of solids with gluconic acid increased by 2.5 times, and that of citric acid increased by 6.9 times (Figure 4d).

3.2. Data Analysis Results

Boxplots were created to visualize the relationship between acid type (Figure 4e), soil type (Figure 4f), and treatment time (Figure 4g). Citric acid exhibited a higher average yield and gluconic acid showed a wider yield range. Virgin soil showed a higher average yield compared to recycled soil. The first three hours of continuous ultrasound treatment resulted in the best performance and hour 4–6 showed lower yields. The presence of soil and small-to-medium sized rocks in our collected material suggests that there may be a mixture of placer deposits and REE-bearing monazite ore present in the samples. The small particle size of placer sands may be more amenable to leaching compared to the larger particle size of the rocks found in the sample. Higher leaching efficiency of the virgin soil may be due to preferential leaching of these small particles.

3.3. Environmental Impact Assessment Results

Table 2. Environmental impact analysis of continuous ultrasound-assisted leaching, using gluconic or citric acid to leach 1 ton of soil.

Impact Categories	Unit	Gluconic Acid	Citric Acid
Fossil fuel depletion	MJ surplus	9.10 × 104	9.03 × 104
Eutrophication	kg N eq.	4.59 ×102	4.56 ×102
Global warming	kg CO2 eq.	7.70 × 105	7.70 × 105
Acidification	kg SO2 eq.	4.91 × 102	4.87 × 102
Ozone depletion	kg CFC-11 eq.	8.07 × 10−3	8.02 × 10−3
Non-carcinogenics	CTUh	1.90 × 10−2	1.93 × 10−2
Smog	kg O3 eq.	3.38 × 103	3.36 × 103
Ecotoxicity	CTUe	6.10 × 105	6.06 × 105
Carcinogenics	CTUh	4.46 × 10−3	4.43 × 10−3
Respiratory effects	kg PM2.5 eq.	5.22 × 101	5.18 × 101

shows the environmental impacts of processing 385 kg of soil per day over 260 days of operation, normalized to one ton of soil. Based on this analysis, the major environmental concerns include global warming, ecotoxicity, and fossil fuel depletion. The environmental impacts of using citric acid are slightly lower than using gluconic acid. The results of this study showed 5.42 × 10⁵ and 3.81 × 10⁵ kg CO₂ eq./kg REEs leached was produced using continuous ultrasound-assisted leaching with gluconic and citric acid, respectively. Most of the gaseous emissions originated from the energy required to power the continuous ultrasound reactor equipment. A small portion of the energy consumption was attributed to the separation of solids and leachate (centrifugation).

3.4. Techno-Economic Analysis Results

Continuous ultrasound-assisted leaching process would utilize 100 metric tons (100,000 kg) of soil and produce 142 kg and 202 kg of mixed REEs using gluconic and citric acid, respectively. These TEA results estimate the leaching cost to be $9465/kg extracted REEs using gluconic acid, with 79% of the cost derived from reagent cost and 21% derived from energy cost. The total annual cost of using citric acid is estimated to be $4325/kg extracted REEs, with 67% of the cost derived from reagent cost and 33% derived from energy cost. Gluconic acid is generally produced via microbial fermentation at the industrial scale. The fermentation medium must undergo multiple separation and purification steps to obtain high purity gluconic acid which lead to an expensive production cost [28]. Citric acid is also produced via fermentation at the industrial scale but has the benefit of higher productivity and yields compared to the fermentation of gluconic acid [29]. It should be noted that the economic data presented herein does not include a comprehensive total of all production costs (i.e., labor, waste management, debt service, capital, and overhead costs). The calculated costs of the continuous ultrasound-assisted leaching process only account for reagent and energy costs.

Table 3. Reagent and utility costs associated with continuous ultrasound-assisted leaching of 1 kg mixed REEs.

Item	Description	Quantity/kg REEs Leached	Unit Price	Cost/kg REEs Leached	Reference
Gluconic acid	-	12,719 L	$0.59/L	$7457	[30]
Citric acid	-	8917 L	$0.33/L	$2916	[31]
Electricity, medium voltage {US}|market group for|APOS, U	Gluconic	18,259 kWh	$0.11/kWh	$2008	[32]
	Citric	12,802 kWh	$0.11/kWh	$1408	[32]

shows detailed economic data.

3.5. Sensitivity Analysis Results

Energy consumption and reagent cost required for the leaching process was compared utilizing commercially purchased and microbially produced organic acids. The calculations only considered reagent and energy costs and excluded other associated production costs (i.e., labor, waste management, debt services, capital, and overhead costs). Only glucose and water were included as inputs for microorganism growth and biolixiviant production. The other nutrients required for organism growth were not included due to their insignificant quantity [21,27]. Using microbially produced organic acids reduced the cost of materials by approximately 99% but minimally increased the energy usage (

Table 4. Comparison of energy consumption and material cost from commercially purchased and microbially produced organic acid.

Organic Acid	Source	Energy Usage (kWh/kg REEs Leached)	Reagent Cost ($/kg REEs Leached)
Gluconic	Commercial	18,259	7457
Citric	Commercial	12,802	2916
Gluconic	Microbial	18,419	54
Citric	Microbial	12,915	38

).

4. Discussion

Continuous ultrasound-assisted leaching results indicate that citric acid can achieve a higher yield of mixed REEs compared to gluconic acid. The structure of citric acid (tri-carboxylic) suggests that it has a higher potential to form complexes with REEs than other organic acids. Despite the potential increase in REE leaching ability, studies suggest that citric acid concentrations over 1 M can be inhibitory [12]. The concentrations of citric acid (3.2 M) and gluconic acid (3.1 M) used in this study are much higher than can be achieved with microbial fermentation during a bioleaching process. Biologically relevant concentrations of organic acids in biolixiviants are in the range of 20–220 mM [8,33,34]. Multiple studies have shown that biolixiviants perform better than pure organic acids due to microbial secretions, such as enzymes and siderophores. The addition of these secretions would provide an advantage over commercially purchased acids, but the path forward for bioleaching using the proposed method would require additional processing steps to bridge this gap. Concentration of the biolixiviant via rotary evaporation or lyophilization can provide a higher concentration of microbially produced organic acids [35]. Furthermore, feedstock beneficiation can provide an increased percentage of REEs in the Diamond Creek soil [36]. The selection of additional methods relies heavily on the process parameters, such as microorganism type, direct vs. indirect bioleaching, and feedstock composition.

Our previous work using a batch ultrasound-assisted leaching method showed a reduction in process energy and gaseous emissions compared to traditional bioleaching methods [21]. Although these results are promising, the batch ultrasound method cannot accommodate the high throughput required by the mining and processing industry. The results of continuous ultrasound-assisted leaching presented in this study demonstrate the feasibility of ultrasound-assisted leaching with larger reaction volumes. Our data shows that the leaching rate (µg/h) and mass of REEs leached are increased using the continuous ultrasound method compared to the batch method. Enhanced recovery of REEs can increase process economics and the feasibility of being implemented by the mining industry.

Although the proposed continuous ultrasound-assisted leaching process can improve leaching efficiency, the percentage yield remains relatively low (12.2–17.4%) compared to some previous leaching studies using organic acids. There are three main factors that can contribute to these lower yields: (1) lixiviant composition, (2) experimental conditions, and (3) feedstock characteristics. The commercial organic acids used as a lixiviant in our experiments generally show a lower yield than microbial biolixiviant. Multiple studies have demonstrated improved leaching efficiency when using both direct and indirect bioleaching compared to pure organic acids [8,15,34]. The improved performance is generally attributed to the presence of cellular metabolites (e.g., siderophores and enzymes) and the adsorption of REEs to microbial biomass during direct leaching.

Several experimental conditions may have contributed to lower leaching yields, such as temperature and pulp density. Multiple studies have reported higher leaching efficiencies at elevated temperatures, but these processes require higher energy consumption [37]. The continuous ultrasound-assisted leaching experiments were performed at room temperature (20 °C) to decrease process energy consumption. Although the pulp density varies widely across leaching studies, a lower solids percentage generally results in higher leaching yields. Our previous study showed that lower percent solids improved leaching yield but had negative economic outcomes [21]. The experiments reported herein utilized 10% solids to improve the process economics and energy consumption of the process.

The composition of the feedstock is also an important factor that influences leaching yield. The soil samples used in our experiments contained approximately 1% total REEs, which are present in crystalline monazite grains [38]. Although ore containing ≥1% REEs is considered feasible for mining and extraction, there are multiple beneficiation steps (e.g., froth flotation and density separation) that produce a rare earth concentrate with 50–60% total REEs [3]. There was no beneficiation applied to the Diamond Creek samples prior to leaching. We expect that leaching yield will be improved with the application of one or more beneficiation steps. Multiple previous studies utilized rare earth magnet waste with a high percentage of total REEs (~25%) which contributed to the high leaching yields observed [11,14]. Other feedstocks, like phosphogypsum and coal fly ash generally have total REE concentrations ≤0.5% but achieved high leaching yields [12,13,15,16,18,19]. The higher leaching yield of these feedstocks can be attributed to their physical and chemical composition. Most REEs present in phosphogypsum are adsorbed to its surface which makes them more accessible to lixiviants [39]. Similarly, the REEs in coal fly ash are incorporated into amorphous aluminosilicates that are more amenable to leaching than the crystalline phase found in monazite grains [40]. A major motivation behind our study is to improve leaching yield, while minimizing leaching time and energy consumption. To achieve this, we chose to forgo beneficiation, perform leaching at a higher pulp density (10%) and ambient temperature, and screen the feedstock’s performance using commercially purchased acids. The balance of costs and benefits must be considered when choosing leaching parameters and feedstock processing methods.

TEA results show that the process cost is heavily influenced by the source of organic acids. Production of biolixiant by microorganisms is significantly cheaper than commercially purchased organic acids. Microbially produced biolixiant contains a mixture of multiple organic acids and other cellular metabolites, while commercially purchased organic acids are sold in higher concentrations. Although biolixiant can be concentrated, this adds further energy-consuming steps that were not accounted for in this study. Furthermore, the LCA results show that energy usage contributes the most to environmental impacts of the continuous ultrasound process. Compared to previous REE bioleaching publications [25,27], the proposed continuous leaching process demonstrated increased emissions (

Table 5. Comparison of LCA results to similar studies.

Reference	kg CO2 eq./ton Material Processed	Mass REEs Leached (kg/ton Material)	kg CO2 eq./kg REEs Leached	REE Source	Leaching Method
[27]	1.00 × 102	26.10	3.84 × 100	FCC catalysts	Biolixiviant
[25]	5.80 × 101	3.24	1.79 × 101	FCC catalysts	Biolixiviant
[21]	1.90 × 105	11.18	1.70 × 104	REE-rich soil	Gluconic acid
This study	7.70 × 105	1.42	5.42 × 105	REE-rich soil	Gluconic acid
This study	7.70 × 105	2.02	3.81 × 105	REE-rich soil	Citric acid

). The leaching process utilized by Thompson et al. and Jin et al. was performed at an elevated temperature (30 °C) and longer incubation time (24 h), which contributes to a higher mass of REEs recovered. Thompson et al. and Jin et al. assumed a solid loading of 50%, while we used only 10% solid loading. The higher percentage loading used in these studies allowed them to use less energy per kg REEs leached. Additionally, the energy required to power the ultrasound reactor contributes heavily to the environmental impact of the continuous ultrasound-assisted process (2.5 × 10⁶ kWh or 99% of energy consumption). The leaching methods in Thompson et al. and Jin et al. only required power for the biolixiviant production and bioleaching process. Therefore, their energy requirements (9 and 121 kWh/kg REEs leached for Thompson et al. and Jin et al., respectively) are much lower compared to those presented in this study (18,259 and 12,802 kWh/kg REE leached for gluconic and citric acid, respectively). Compared to our previous published study using batch method ultrasound-assisted leaching [21], the continuous method has increased energy requirements and gaseous emissions due to the longer ultrasound treatment time required for the continuous method (6 h vs. 30 min). Conversely, this study utilized increased solids loading (10%) compared to the previously reported batch method (1%) and would require fewer ultrasound units needed for daily operation (38,500 units for the batch method and 962 units for the continuous method). Although a higher pulp density was used in this study, a larger volume of acid was used per sample due to the acid replenishment used for recycled solids.

To reduce the environmental impacts and increase economic feasibility of the continuous ultrasound-assisted leaching process, a reduction in energy consumption is necessary. Due to the increased scale of leaching utilized by the continuous ultrasound method, it would be extremely difficult to reduce the process energy without a low energy pretreatment step. Microwave-assisted leaching has been suggested to decrease process energy and reaction time during leaching [41,42]. These studies suggest that both ultrasound and microwave treatment can increase the porosity of the solid matrix and enhance the REE dissolution rate [41]. A combined microwave and ultrasound-assisted process has been utilized to improve zinc leaching [43] and the removal of ash constituents in coal [44]. To date, there have been no studies utilizing a combined microwave and ultrasound-assisted method for REE extraction.

The continuous ultrasound method presented herein can increase leaching efficiency, as well as leaching scale. Although these results are promising, further study is needed to increase the technology readiness level (TRL) for industrial implementation. Future studies should focus on the following topics:

• Exploration of ultrasound-assisted organic acid leaching with microbially produced biolixiviants (such as Gluconobacter oxydans and Aspergillus niger).
• Implementation of a mixed-organic acid leaching method.
• Improving organic acid leaching yield utilizing a combination of ultrasound and microwave-assisted methods.
• Further increasing the scale of ultrasound-assisted organic acid leaching to encourage industrial implementation.

5. Conclusions

The results of continuous ultrasound-assisted leaching showed an increase in the leaching rate and mass of REEs leached compared to the batch method. These results demonstrated the feasibility of ultrasound-assisted leaching with larger reaction volumes that are necessary for industrial extraction. Although these results are promising, the overall percentage REE yield remained relatively low using gluconic acid (12.2%) and citric acid (17.4%). LCA results showed that most of the environmental impacts stem from the energy required to power continuous ultrasound-assisted leaching reactor. A small portion of the energy requirement is attributed to the separation of solids and leachate (centrifugation). TEA results indicated that the source of organic acid used in the leaching process can significantly impact the process cost. Utilizing microbially produced biolixiviant over commercially purchased organic acids is a cheaper option, but there are tradeoffs regarding the concentration and purity of organic acids. Further innovation in the ultrasound-assisted organic acid leaching process is necessary to increase yield and make the process more appealing for industrial implementation.