Rare Earth Elements in Phosphate Ores and Industrial By-Products: Geochemical Behavior, Environmental Risks, and Recovery Potential

Abstract

Phosphate rock is a vital natural resource classified by the European Commission as a critical raw material (CRM), extensively mined for its agricultural, industrial, and technological applications. While primarily used in fertilizer production, phosphate deposits also contain significant concentrations of trace metals, notably rare earth elements (REE), which are essential for renewable energy, electronics, and defense technologies. In response to growing demand, the recovery of REE from phosphate ores and processing by-products, particularly phosphogypsum (PG), has gained international attention. This review provides a comprehensive analysis of the global phosphate industry, examining production trends, market dynamics, and the environmental implications of phosphate processing. Special focus is placed on the geochemical behavior and mineralogical associations of REE within phosphate ores and industrial residues, namely PG and purification sludge. Although often treated as waste, these by-products represent underexplored secondary resources for REE recovery. Technological advancements in hydrometallurgical, solvometallurgical, and bioleaching methods have demonstrated promising recovery efficiencies, with some pilot-scale studies exceeding 70%–80%. However, large-scale implementation remains limited due to economic, technical, and regulatory constraints. The circular economy framework offers a pathway to enhance resource efficiency and reduce environmental impact. By integrating innovative extraction technologies, strengthening regulatory oversight, and adopting sustainable waste management practices, phosphate-rich countries can transform environmental liabilities into strategic assets. This review concludes by identifying key knowledge gaps and suggesting future research directions to optimize REE recovery from phosphate deposits and associated by-products, contributing to global supply security, economic diversification, and environmental sustainability.

1. Introduction

Phosphate rock ranks among the top five most extensively mined ores on earth globally, with an estimated annual production of 240 million tons [1]. It is a vital non-renewable resource that plays an essential role in global food security, industrial applications, and strategic economic planning. The world’s supply of phosphate ore is derived predominantly from sedimentary phosphate rock deposits, which constitute around 95% of total production. Approximately 85% of extracted phosphate is utilized primarily in the manufacture of phosphoric acid (PA) and chemical fertilizers [2,3,4].

Phosphate-based fertilizers are essential for improving soil fertility and increasing crop yields [5,6]. The Food and Agriculture Organization (FAO) projects that global food production must increase by 70% between 2005 and 2050 to meet the demands of a growing population [7], highlighting the critical dependence on phosphate-based fertilizers. In addition to agriculture, phosphate derivatives, including phosphoric acid and fertilizers, are employed across various industrial sectors such as food additives, animal feed, detergents, and chemical manufacturing [8,9,10].

Phosphate reserves are geographically concentrated, rendering their supply geopolitically sensitive. China, Morocco, and the United States together account for over 60% of global production, with Morocco alone possessing approximately 70% of the world’s known phosphate reserves [1]. This regional concentration raises concerns regarding supply security, price volatility, and geopolitical tensions.

The phosphate valorization process comprises multiple stages, starting with ore extraction, followed by physical beneficiation techniques such as crushing, grinding, and water washing, aimed at increasing the ore’s phosphorus content [11]. The processed product, referred to as marketable phosphate, is subsequently converted into PA and fertilizers using the “wet process” [12,13]. In this method, fluorapatite reacts with sulfuric acid, yielding phosphoric acid and phosphogypsum (PG) as a by-product [14].

The high demand for phosphate fertilizers has resulted in the accumulation of significant PG waste worldwide. For every ton of phosphoric acid produced, approximately five tons of PG are generated [15,16]. Global PG production is estimated at 200–300 million tons annually, yet only 15% is recycled into applications such as construction materials or soil amendments. The remaining 85% is either stockpiled or discharged into marine environments, leading to environmental contamination [17,18,19,20,21]. The environmental hazards associated with PG disposal include the release of heavy metals, naturally occurring radionuclides, and rare earth elements (REE), which result in bioaccumulation in marine organisms and pose potential risks to human health [16,22,23,24,25,26,27,28].

In addition to phosphorus, phosphate deposits are enriched in various elements compared to the average shale and other sedimentary rocks, including cadmium (Cd), chromium (Cr), zinc (Zn), strontium (Sr), Uranium (U), and REE [29,30,31,32]. REE are classified by the European Commission as “technology-critical raw materials” due to their strategic importance in high-tech industries and renewable energy technologies, and are designated as critical raw materials (CRMs) [33] characterized by high economic importance and vulnerable supply chains (Figure 1).

As global demand for REE increases, phosphate deposits and their by-products, particularly PG and decadmiation sludge, have gained attention as alternative REE sources. During phosphate processing, up to 85% of REE are transferred to PG, promoting growing interest in their recovery [34,35]. It is estimated that global phosphate rock production contains nearly 100,000 tons of REE annually [36,37,38], positioning phosphate-related products as an underexploited source of these critical elements.

Figure 1. Conceptual relationships between rare earth elements (REE) with very high supply risk and key enabling technologies/fields. Colored arrows indicate where REE are used. Each arrow color links the field (left) to its corresponding technologies (right); green: Renewable Energy/Batteries/Fuel cells/Wind, blue: Electric vehicles/Traction motors/Robotics, orange: Defense Aerospace/Drones/Information Communication. Visualization created by the authors; information adapted from the European Commission Joint Research Centre (JRC), Critical Raw Materials for Strategic Technologies and Sectors in the EU: A Foresight Study (2020) [39].

Figure 1. Conceptual relationships between rare earth elements (REE) with very high supply risk and key enabling technologies/fields. Colored arrows indicate where REE are used. Each arrow color links the field (left) to its corresponding technologies (right); green: Renewable Energy/Batteries/Fuel cells/Wind, blue: Electric vehicles/Traction motors/Robotics, orange: Defense Aerospace/Drones/Information Communication. Visualization created by the authors; information adapted from the European Commission Joint Research Centre (JRC), Critical Raw Materials for Strategic Technologies and Sectors in the EU: A Foresight Study (2020) [39].

Several studies have investigated REE in phosphate ores, PG, and contaminated environments, revealing the need for further research into their distribution, traceability, speciation, and recovery potential from phosphate processing streams.

Given the strategic significance of phosphate-derived products and the environmental challenges posed by PG waste, this review aims to (i) provide a comprehensive overview of the global phosphate industry, and (ii) examine the geochemical behavior, mineralogical associations, and environmental mobility of REE in phosphate ores and industrial by-products. By synthesizing the existing literature, this review highlights critical knowledge gaps and outlines future research directions, particularly regarding REE speciation during phosphate processing, ascertaining that REE can occur in multiple oxidation states, each with distinct chemical reactivity that strongly influences their speciation, mobility, and partitioning in geochemical systems. Furthermore, this review emphasizes the need for deeper investigation into the fate of REE in contaminated environments and for the development of efficient and sustainable recovery technologies.

2. Global Overview of the Phosphate Industry

2.1. Global Production, Reserves, and Market Dynamics

Phosphate reserves are unevenly distributed across key producing countries (Figure 2). The United States Geological Survey [1] estimates that global phosphate rock production reached approximately 240 million tons in 2024. The global phosphate industry is highly concentrated, with China, Morocco, and the United States collectively dominating global supply, underscoring its geopolitical significance [8,40].

As shown in Figure 3, Morocco holds over 50 billion tons of phosphate rock reserves among these producers, accounting for more than 70% of the global total [1]. This extreme concentration of the reserve within a single country raises concerns about supply security and market volatility, especially in the context of rising global demand for phosphate fertilizers and industrial applications [43]. The dependence on phosphate imports for agricultural productivity exposes nations without domestic phosphate reserves to geopolitical risks and price fluctuations [44].

The global phosphate market is influenced by multiple factors, including agricultural demand, industrial applications, technological innovation, and international trade policies. The Food and Agriculture Organization (FAO) projects that global food production must increase by 70% by the year 2050 [7], directly correlating with rising fertilizer consumption; therefore, forecasts still suggest a 1.8% annual increase in fertilizer demand through 2050 [45].

According to Grand View Research, the phosphate industry is expected to grow steadily, with a projected compound annual growth rate (CAGR) of 3.2% from 2022 to 2030 [46], reaching USD 29.73 billion by 2030, driven by increasing demand from the fertilizer, food, and chemical industries. However, price volatility remains a defining feature of the market, as illustrated by the 2008 price spike, which was driven by biofuel demand, speculative trading, and supply chain disruptions [47].

International trade and supply chain dynamics are also pivotal in shaping the phosphate industry. Morocco, as the largest exporter of phosphate rock, exerts significant influence over global supply networks. Studies on global phosphorus flows highlight the role of trade agreements, export tariffs, and regional demand fluctuations on market stability [36].

2.2. Emerging Trends and Future Prospects

Several emerging trends are reshaping the future of the global phosphate industry. One major trend is the growing demand for high-efficiency fertilizers, which offer enhanced nutrient content and controlled release, thereby maximizing crop yields while minimizing environmental impact. The rise in organic and bio-based fertilizers is also notable, driven by consumer preferences for sustainable agriculture [48]. The global phosphate industry is undergoing significant consolidation through mergers and acquisitions, as major players seek to expand their market share and secure resource access. Expansion initiatives in Brazil, Kazakhstan, Mexico, Russia, and South Africa may transform the global supply landscape, contingent on their success in bringing new production capacity. The phosphate industry stands at a critical juncture, facing challenges related to resource depletion, environmental sustainability, and evolving market dynamics. Nonetheless, technological innovation, sustainability-driven practices, and rising demand across sectors present opportunities for growth and transformation. The industry’s ability to navigate these challenges while meeting global demand will be decisive in shaping its future trajectory [44,49].

3. Economic Data and Forecasts

According to Technavio (2024), the phosphate rock market is projected to grow to USD 4.91 billion, with a compound annual growth rate (CAGR) of 3.79% from 2023 to 2028 [50]. The phosphate fertilizer market is expected to expand significantly, from USD 51.47 billion in 2024 to USD 73.04 billion in 2028, driven by increasing fertilizer demand, the rise in high-quality meat consumption, and rapid urbanization [50].

Economic forecasts for the phosphate industry remain optimistic. Issaoui et al. (2021) [51] suggest that Tunisia’s phosphate production could rise substantially by 2025, contingent on the successful implementation of modernization projects and improved labor relations. This aligns with government-led initiatives to revitalize the phosphate industry, including investments in equipment upgrades and production capacity.

In addition to production forecasts, value addition within the phosphate sector presents significant opportunities. These include expanding phosphoric acid and fertilizer production, enhancing existing processes, and developing technology to recover valuable by-products such as REE and other critical materials [51]. Such value-added products could enhance the economic viability of the phosphate industry and support broader economic diversification in phosphate-rich nations. However, the industry faces serious environmental challenges, particularly related to PG waste [52,53,54,55,56,57,58,59,60]. Addressing these issues is essential for ensuring long-term sustainability. In countries like Tunisia, where phosphate production has declined in recent years, revitalization efforts must tackle production inefficiencies, labor disputes, and environmental concerns while also adapting to global market shifts and exploring secondary uses of phosphate by-products, especially in the context of critical element recovery. Given the economic importance of phosphate production, the potential for REE recovery from phosphate by-products warrants further exploration. Understanding the geochemical context of associated REE, including their occurrence, mineralogical associations, and concentration patterns, is essential for developing viable recovery strategies. Therefore, prior to implementing extraction technologies, it is imperative to examine the global distribution, geochemistry, and host phases of REE in phosphate ores and their by-products.

4. Context and Importance of Rare Earth Elements in Phosphates

Rare earth elements, as defined by the International Union of Pure and Applied Chemistry (IUPAC), consist of the 15 lanthanides along with scandium (Sc) and yttrium (Y). They were first discovered in 1788 [61], remaining largely absent from industrial applications until the mid-20th century, with global annual production and consumption of rare earth oxides (REOs) staying below 5000 tons until the 1960s [61,62]. Since then, REE have become integral to a wide range of technologies, including television screens, petroleum refining, computer systems, and more recently, clean energy and defense applications [33,61,63,64]. Their role in emerging technologies such as electric vehicles, wind turbines, and energy-efficient lighting has positioned REE as critical materials in the global energy transition [65,66,67,68].

As global demand for REE continues to rise, secondary sources such as phosphate ores and their by-products have gained increasing attention, especially amid geopolitical constraints affecting traditional REE supply chains [69,70,71,72,73,74,75]. As projected by Elshkaki (2021) [76], demand for REE in clean technologies may exceed 1000 kilotons by 2050 (Figure 4), promoting renewed interest in phosphate deposits as alternative sources [30,37,70,71,77,78].

Phosphate rocks are increasingly recognized not only for their phosphorus content but also for their enrichment in REE and other critical raw materials, particularly in sedimentary phosphate deposits [30,31,79,80,81,82]. Table 1 provides an overview of REE and other critical materials occurring in phosphate deposits from leading phosphate-producing countries.

The presence of REE in phosphates was first described by Altschuler (1967) [83] and Baturin et al. (1972) [84], with subsequent studies confirming their affinity for fluorapatite, the dominant mineral phase in phosphates [85]. REE can substitute for calcium in the apatite crystal lattice [86,87,88,89], and may also occur in accessory minerals such as monazite (CaCeLaNdThPO₄) and xenotime (YPO₄) [90,91,92,93,94,95,96].

According to Emsbo et al. (2015) [30], REE concentrations in phosphate ores ranging from 100 to 800 mg/kg are sufficient to consider them as primary sources, especially given the relative ease of extraction compared to other deposit types [97,98,99]. The wet process used to produce phosphoric acid from phosphate rock, presented in Figure 5, also facilitated REE recovery as sulfuric acid dissolution can extract a significant portion of these elements [100,101,102,103,104,105,106]. Studies have demonstrated REE recovery efficiencies of up to 70% using 2 M sulfuric acid, and nearly 100% with dilute H₂SO₄ and HCl in certain U.S. phosphate ores [30,107]. Recent work by Salem et al. (2019) and Jang et al. (2024) showed that decadmiation sludge from phosphoric acid purification containing 1000 to 2000 mg/kg REE can yield up to 90% recovery [98,99].

Table 1. Concentrations of trace elements and REE in phosphate ores from different regions of the world.

Phosphate Deposit	Trace Elements	Average Concentrations (mg/kg)	REE	Average Concentrations (mg/kg)	TREO (mg/kg)	TREO/(U + Th)	References
Gafsa Basin, Tunisia	Cd; Cr; Pb; Sr; Zn; U	24.25; 175.5; 4.52; 1349.5; 196; 59.7	La; Ce; Nd; Sm; Eu; Gd; Tb; Yb	51.26; 79.31; 50.14; 9.65; 3.34; 10.7; 6.1; 5.52	256.7	--	Smida et al. (2021) [108]
Gafsa Basin, Tunisia	N.A.	N.A.	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	3.55; 123.5; 84; 123.5; 18.35; 79; 14.85; 3.39; 14.1; 2.02; 13.1; 2.84; 8.34; 1.15; 7.58; 1.19	600.2	--	El Zrelli et al. (2021) [32]
Gafsa Basin, Tunisia	Ba; Br; Cd; Cr; Cu; Mo; Ni; Sc; Sr; U; Y; Zn; Zr	39; 12.41; 34; 238; 9.32; 5.96; 17.9; 3.68; 1788; 30; 99; 228; 41	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	74; 108.16; 15.38; 62.58; 13; 2.78; 11.73; 1.8; 10.2; 2.31; 6.76; 0.95; 6.31; 1	376.6	--	Garnit et al. (2017) [109]
Gafsa Basin, Tunisia	Cd; Zn; Cr; Cu; Ni; Sr; Sc; Co; Rb; Sb; Cs; Ba; Hf; Ta; Th; U; As; Mo; Zr	43.68; 337.91; 226.12; 18.05; 25.21; 1501.35; 3.21; 1.69; 7.3; 0.5; 0.48; 57.45; 0.65; 0.24; 7.02; 23.88; 6.07; 14.92; 79.5	La; Ce; Nd; Sm; Eu; Gd; Tb; Dy; Yb	63.49; 101.92; 57.94; 10.7; 3; 12.86; 1.53; 10.43; 5.72	318.2	10.30	Galfati et al. (2010) [110]
Algeria	V; Cr; Li; Co; Ni; Cu; Zn; As; Rb; Sr; Zr; Nb; Mo; Cd; Cs; Ba; Hf; Ta; W; Tl; Pb; Th; U	100.52; 208.89; 5.87; 1.14; 28.56; 14.72; 209.82; 7.18; 4.64; 1834.58; 19.15; 1.81; 2.90; 28.06; 2.32; 57.61; 0.32; 0.16; 0.54; 1.38; 2.14; 5.20; 55.74	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Tb; Dy; Ho; Er; Tm; Yb; Lu; ∑REE	3.95; 190.35; 92.05; 78.95; 15.91; 68.81; 12.52; 3.28; 15.49; 2.18; 14.93; 3.38; 10.37; 1.42; 8.12; 1.54; 328.95	630.3	10.34	Kechiched et al. (2020) [111]
Morocco	Minor elements (MEs)	ΣMEs = 3538 mg/kg	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm Yb; Lu	143; 24.06; 13; 3.63; 17.52; 3.68; 1; 5.14; 0.84; 5.67; 1.3; 4.31; 0.62; 4.2; 0.79	282.2	--	Amine et al. (2019) [112]
South-West China	Cr; Zr; Co; Ni; Th; U; V; As; Sb; Mn; Sr	24; 32; 6; 106; 4; 28; 38.6; 63; 11.4; 599.33; 488.33	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	242.6; 160.6; 121; 30.4; 132; 24.5; 6.4; 29.7; 4; 24.3; 5; 13.13; 1.56; 7.6; 1	970.6	30.33	Zhang et al. (2021) [113]
South China	N.A.	N.A.	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	165.76; 198.77; 407.19; 53.26; 221.32; 39.72; 9.59; 36.32; 5.04; 24.5; 4.29; 11.46; 1.19; 5.88; 0.75	1426.2	--	Xin et al. (2015) [114]
South China	N.A.	N.A.	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm Yb; Lu	93.23; 33.11; 47.94; 10.48; 49.04; 11.20; 2.81; 12.05; 1.96; 11.43; 2.32; 6.20; 0.72; 3.7; 0.48	346.8	--	Xin et al. (2016) [115]
South China	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; Y	Total reserves (tons) 164536; 253849; 3355; 115949; 32901; 2975; 25758; 3641; 2003; 4291; 12599; 1803; 9442; 1431; 147387	--	--	Emsbo et al. (2015) [30]
USA	N.A.	N.A.	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	5.82; 110.33; 64.00; 105.14; 6.49; 74.12; 0.00; 8.05; 14.36; 1.73; 12.16; 0.00; 8.50; 3.03; 7.42; 1.19	503.4	--	Liang et al. (2017) [85]
Arkansas, USA	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	207.9; 566.37; 124.11; 790.62; 477.25; 314.96; 582.71; 70; 278.53; 82.77; 82.18; 8.7; 42; 5.91	4255	--	Murthy et al. (2004) [116]
Mountain Pass, USA	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; Y	REE total ore concentration (tons) 743495; 1042279; 88027; 247033; 18913; 2350; 4701; 351; 765; 90; 136; 46; 46; 27; 2642	--	--	Long et al. (2012) [117]
Love Hollow, USA	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; Y	REE concentrations (tons/100km2) 2009; 443543; 4074; 182148; 369; 8370; 4176; 5494; 32981; 6477; 1715; 1934; 10328; 1215; 242496	--	--	Emsbo et al. (2015) [30]
Egypt	U; Th	127.26; 22.942	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; ∑REE	3.71; 46.74; 34.81; 51.75; 6.59; 27.57; 5.12; 1.35; 6.09; 1.02; 5.978; 1.29; 4.21; 0.6; 4.16; 0.76; 151.3	239	1.59	Shahin et al. (2020) [118]
Jordan	Nb; Rb; Sr; Th; U; V; Zr; Mo; Cu; Pb; Zn; Ni; As; Cd; Sb; Cr; Ba; Co; Ga; Hf	0.63; 2.32; 1466.27; 0.68; 58.42; 160.11; 16.40; 8.64; 35.86; 1.82; 217.38; 50.80; 9.38; 19.48; 1.25; 170.90; 590.79; 1.58; 1.09; 0.30	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Ho; Er; Tm; Yb; Lu	37.4; 13.4; 8.1; 1.91; 8.2; 1.67; 0.44; 2.36; 0.66; 2.12; 0.32; 2.06; 0.36	96.5	1.63	Abed et al. (2016) [119]
Jordan	Ba; Cd; Cr; Cs; Cu; Ga; Hf; Mo; Nb; Ni; Rb; Sb; Sc; Se; Sr; Th; Tl; U; V; Zn; Zr	66.89; 19.18; 190.53; 0.07; 10.24; 0.70; 1.18; 3.68; 0.73; 50.21; 1.39; 0.39; 1.22; 0.81; 340.91; 0.61; 0.07; 24.48; 64.08; 113.84; 50.92	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; ∑ REE	13.91; 5.59; 6.33; 0.99; 4.03; 0.81; 0.21; 0.98; 0.15; 0.96; 0.24; 0.73; 0.11; 0.70; 0.12; 21.95	43.6	1.74	Amireh et al. (2019) [120]
Brazil	Ba; Zr; Th	6480; 2450; 103	La; Ce; Nd; Sm; Eu; Tb; Yb; Lu	2220; 5310; 1490; 256; 68; 13.0; 14.8; 0.65	11272.5	--	De Oliveira et al. (2007) [121]
Brazil	N.A.	N.A.	Sc; Y; La; Ce; Nd; Pr; Sm; Sc; Eu; Dy; Gd; Er; Yb; Ho; Tb; Lu	2.95; 41.95; 32.6; 44.3; 36.85; 8.05; 6.25; 1.9; 7.15; 8.15; 3.1; 2.65; 1.15; 1.35; 0.45	235.6	--	Silva et al. (2019) [122]

N.A.: not analyzed. TREOs are calculated from reported REE concentrations using standard oxide factors and summation: La–Lu as Ln₂O₃, Ce as CeO₂, Y as Y₂O₃. Where some REE were not reported in a source, TREOs reflect a partial sum over available REE. Rows that reported REE tonnage or areal inventories (rather than concentrations) are not converted to ppm and are omitted from the ratio analysis. Evidence pedigree: All entries are derived from peer-reviewed papers.

Table 1. Concentrations of trace elements and REE in phosphate ores from different regions of the world.

Phosphate Deposit	Trace Elements	Average Concentrations (mg/kg)	REE	Average Concentrations (mg/kg)	TREO (mg/kg)	TREO/(U + Th)	References
Gafsa Basin, Tunisia	Cd; Cr; Pb; Sr; Zn; U	24.25; 175.5; 4.52; 1349.5; 196; 59.7	La; Ce; Nd; Sm; Eu; Gd; Tb; Yb	51.26; 79.31; 50.14; 9.65; 3.34; 10.7; 6.1; 5.52	256.7	--	Smida et al. (2021) [108]
Gafsa Basin, Tunisia	N.A.	N.A.	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	3.55; 123.5; 84; 123.5; 18.35; 79; 14.85; 3.39; 14.1; 2.02; 13.1; 2.84; 8.34; 1.15; 7.58; 1.19	600.2	--	El Zrelli et al. (2021) [32]
Gafsa Basin, Tunisia	Ba; Br; Cd; Cr; Cu; Mo; Ni; Sc; Sr; U; Y; Zn; Zr	39; 12.41; 34; 238; 9.32; 5.96; 17.9; 3.68; 1788; 30; 99; 228; 41	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	74; 108.16; 15.38; 62.58; 13; 2.78; 11.73; 1.8; 10.2; 2.31; 6.76; 0.95; 6.31; 1	376.6	--	Garnit et al. (2017) [109]
Gafsa Basin, Tunisia	Cd; Zn; Cr; Cu; Ni; Sr; Sc; Co; Rb; Sb; Cs; Ba; Hf; Ta; Th; U; As; Mo; Zr	43.68; 337.91; 226.12; 18.05; 25.21; 1501.35; 3.21; 1.69; 7.3; 0.5; 0.48; 57.45; 0.65; 0.24; 7.02; 23.88; 6.07; 14.92; 79.5	La; Ce; Nd; Sm; Eu; Gd; Tb; Dy; Yb	63.49; 101.92; 57.94; 10.7; 3; 12.86; 1.53; 10.43; 5.72	318.2	10.30	Galfati et al. (2010) [110]
Algeria	V; Cr; Li; Co; Ni; Cu; Zn; As; Rb; Sr; Zr; Nb; Mo; Cd; Cs; Ba; Hf; Ta; W; Tl; Pb; Th; U	100.52; 208.89; 5.87; 1.14; 28.56; 14.72; 209.82; 7.18; 4.64; 1834.58; 19.15; 1.81; 2.90; 28.06; 2.32; 57.61; 0.32; 0.16; 0.54; 1.38; 2.14; 5.20; 55.74	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Tb; Dy; Ho; Er; Tm; Yb; Lu; ∑REE	3.95; 190.35; 92.05; 78.95; 15.91; 68.81; 12.52; 3.28; 15.49; 2.18; 14.93; 3.38; 10.37; 1.42; 8.12; 1.54; 328.95	630.3	10.34	Kechiched et al. (2020) [111]
Morocco	Minor elements (MEs)	ΣMEs = 3538 mg/kg	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm Yb; Lu	143; 24.06; 13; 3.63; 17.52; 3.68; 1; 5.14; 0.84; 5.67; 1.3; 4.31; 0.62; 4.2; 0.79	282.2	--	Amine et al. (2019) [112]
South-West China	Cr; Zr; Co; Ni; Th; U; V; As; Sb; Mn; Sr	24; 32; 6; 106; 4; 28; 38.6; 63; 11.4; 599.33; 488.33	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	242.6; 160.6; 121; 30.4; 132; 24.5; 6.4; 29.7; 4; 24.3; 5; 13.13; 1.56; 7.6; 1	970.6	30.33	Zhang et al. (2021) [113]
South China	N.A.	N.A.	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	165.76; 198.77; 407.19; 53.26; 221.32; 39.72; 9.59; 36.32; 5.04; 24.5; 4.29; 11.46; 1.19; 5.88; 0.75	1426.2	--	Xin et al. (2015) [114]
South China	N.A.	N.A.	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm Yb; Lu	93.23; 33.11; 47.94; 10.48; 49.04; 11.20; 2.81; 12.05; 1.96; 11.43; 2.32; 6.20; 0.72; 3.7; 0.48	346.8	--	Xin et al. (2016) [115]
South China	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; Y	Total reserves (tons) 164536; 253849; 3355; 115949; 32901; 2975; 25758; 3641; 2003; 4291; 12599; 1803; 9442; 1431; 147387	--	--	Emsbo et al. (2015) [30]
USA	N.A.	N.A.	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	5.82; 110.33; 64.00; 105.14; 6.49; 74.12; 0.00; 8.05; 14.36; 1.73; 12.16; 0.00; 8.50; 3.03; 7.42; 1.19	503.4	--	Liang et al. (2017) [85]
Arkansas, USA	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu	207.9; 566.37; 124.11; 790.62; 477.25; 314.96; 582.71; 70; 278.53; 82.77; 82.18; 8.7; 42; 5.91	4255	--	Murthy et al. (2004) [116]
Mountain Pass, USA	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; Y	REE total ore concentration (tons) 743495; 1042279; 88027; 247033; 18913; 2350; 4701; 351; 765; 90; 136; 46; 46; 27; 2642	--	--	Long et al. (2012) [117]
Love Hollow, USA	N.A.	N.A.	La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; Y	REE concentrations (tons/100km2) 2009; 443543; 4074; 182148; 369; 8370; 4176; 5494; 32981; 6477; 1715; 1934; 10328; 1215; 242496	--	--	Emsbo et al. (2015) [30]
Egypt	U; Th	127.26; 22.942	Sc; Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; ∑REE	3.71; 46.74; 34.81; 51.75; 6.59; 27.57; 5.12; 1.35; 6.09; 1.02; 5.978; 1.29; 4.21; 0.6; 4.16; 0.76; 151.3	239	1.59	Shahin et al. (2020) [118]
Jordan	Nb; Rb; Sr; Th; U; V; Zr; Mo; Cu; Pb; Zn; Ni; As; Cd; Sb; Cr; Ba; Co; Ga; Hf	0.63; 2.32; 1466.27; 0.68; 58.42; 160.11; 16.40; 8.64; 35.86; 1.82; 217.38; 50.80; 9.38; 19.48; 1.25; 170.90; 590.79; 1.58; 1.09; 0.30	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Ho; Er; Tm; Yb; Lu	37.4; 13.4; 8.1; 1.91; 8.2; 1.67; 0.44; 2.36; 0.66; 2.12; 0.32; 2.06; 0.36	96.5	1.63	Abed et al. (2016) [119]
Jordan	Ba; Cd; Cr; Cs; Cu; Ga; Hf; Mo; Nb; Ni; Rb; Sb; Sc; Se; Sr; Th; Tl; U; V; Zn; Zr	66.89; 19.18; 190.53; 0.07; 10.24; 0.70; 1.18; 3.68; 0.73; 50.21; 1.39; 0.39; 1.22; 0.81; 340.91; 0.61; 0.07; 24.48; 64.08; 113.84; 50.92	Y; La; Ce; Pr; Nd; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu; ∑ REE	13.91; 5.59; 6.33; 0.99; 4.03; 0.81; 0.21; 0.98; 0.15; 0.96; 0.24; 0.73; 0.11; 0.70; 0.12; 21.95	43.6	1.74	Amireh et al. (2019) [120]
Brazil	Ba; Zr; Th	6480; 2450; 103	La; Ce; Nd; Sm; Eu; Tb; Yb; Lu	2220; 5310; 1490; 256; 68; 13.0; 14.8; 0.65	11272.5	--	De Oliveira et al. (2007) [121]
Brazil	N.A.	N.A.	Sc; Y; La; Ce; Nd; Pr; Sm; Sc; Eu; Dy; Gd; Er; Yb; Ho; Tb; Lu	2.95; 41.95; 32.6; 44.3; 36.85; 8.05; 6.25; 1.9; 7.15; 8.15; 3.1; 2.65; 1.15; 1.35; 0.45	235.6	--	Silva et al. (2019) [122]

N.A.: not analyzed. TREOs are calculated from reported REE concentrations using standard oxide factors and summation: La–Lu as Ln₂O₃, Ce as CeO₂, Y as Y₂O₃. Where some REE were not reported in a source, TREOs reflect a partial sum over available REE. Rows that reported REE tonnage or areal inventories (rather than concentrations) are not converted to ppm and are omitted from the ratio analysis. Evidence pedigree: All entries are derived from peer-reviewed papers.

Sustainable REE development has also focused on recycling from industrial processes and end-of-life products, as well as mining residues and landfills, although most research remains at the academic level due to technical and economic constraints [38,100,123,124]. Additional studies have explored REE recovery from uranium-rich minerals [125,126,127,128], fluorite [129], marine sediments [130,131,132,133,134,135], and phosphate rock [38,73,100,136,137], with the latter considered among the most promising secondary sources.

With approximately 250 million tons of phosphate rock consumed annually, and average REE concentrations of 500 mg/kg, global phosphate deposits may contain 50 million tons of REE, translating to a potential annual yield of ~100,000 tons [97,138]. This underscores the need to purify phosphoric acid, PG, and associated phosphoric acid sludge to recover REE and other valuable elements.

In phosphate ores and their wet-process streams, REE commonly co-occur with naturally occurring radioelements such as uranium (U) and thorium (Th). This co-association has both operational and commercial significance: elevated U and Th concentrations can shift intermediate products above exemption limits, placing them under IAEA Class 7 transport and handling regulations. Such conditions have direct implications for flowsheet design, reagent selection, permitting, logistics, and unit costs [139]. Because the REE grade alone can be an unreliable indicator of technical and regulatory viability, each REE entry is paired with corresponding U and Th (when available), and the screening TREO/(U + Th) ratio is reported. This approach follows best-practice assessments for REE flowsheets, where monazite- and bastnäsite-bearing analogs exhibit contrasting radiological burdens [70,140].

Despite its high impurity content, PG has demonstrated significant economic potential, with studies estimating that 100 million tons of PG could be worth nearly USD 8.937 billion [141]. In Tunisia, particularly at the Gabès fertilizer plant, PG is enriched in REE, base metals (Al, Cu, and Zn), precious metals (Ag), and other strategic elements, including antimony (Sb), arsenic (As), chromium (Cr), magnesium (Mg), silicon (Si), tantalum (Ta), tungsten (W), and vanadium (V). El Zrelli et al. (2018) [57] estimated that recovering valuable elements from 5.26 million tons of PG annually could generate ~ USD 39.60 million [57], highlighting the dual benefit of environmental protection and economic contribution.

To contextualize the economic opportunities and commercial implications related to REE recovery from phosphate ores, Table 1 compiles trace element data with a focus on REE, specifically reported TREO grades paired with U and Th concentrations, as well as the screening ratio TREO/(U + Th); here, TREO represents the sum of La–Lu + Y expressed as oxides, while U and Th are listed as elemental concentrations (mg·kg⁻¹). Where U or Th are unavailable, entries are marked N.A. Pairing TREOs with U and Th provides a first-order assessment of radiological burden, potential IAEA Class 7 transport and handling implications, and the process intensity required to produce saleable REE products.

Table 2. Benchmark TREOs vs. U-Th for Mountain Pass and Mountain Weld REE Mines.

REE Deposit	TREO (wt%)	U (mg·kg−1)	Th (mg·kg−1)	TREO/(U + Th)	Reference
Mountain Pass (USA)	9.3	17	175.8	414.1	Haxel et al. (2002) [142] Park et al.(2023) [143]
Mount Weld (Australia)	8.8	25.4	659.1	125.6	Hellman & Duncan (2018) [144] Lynas Rare Earths (2025) [145]

Where mines report U and Th as oxides, elemental values were computed using standard stoichiometry: U = U₃O₈ × 0.8480 and Th = ThO₂ × 0.8788. TREOs (ppm) are calculated as TREO(wt%) × 10,000. Ratios are then TREO(ppm)/[U(ppm) + Th(ppm)] (elemental basis). Evidence pedigree: All entries are derived from peer-reviewed papers.

benchmarks two operating REE mines—Mountain Pass (USA; bastnäsite-dominant carbonatite) and Mount Weld (Australia; monazite-rich regolith over carbonatite)—by reporting TREOs, U, Th, and TREO/(U + Th) values. These sites were selected because they are well-characterized, producing end-members in REE mineralogy and radiological profiles. Bastnäsite systems typically exhibit lower Th (vs. monazite with higher Th), with transparent public data that provide an operationally relevant baseline. Consistent with this contrast, bastnäsite systems typically show lower Th and, thus, higher TREO/(U + Th) (lighter radiological overhead). In contrast, monazite-rich systems carry higher Th, resulting in tighter processing and compliance margins. The availability of transparent public data for both sites provides an operationally relevant baseline. Comparing Table 1 and Table 2 thus helps identify phosphate ores closest to commercial viability and those requiring regulatory thresholds. To link the strategic importance of REE in phosphate systems with their practical recovery pathways, Figure 6 presents a conceptual mass-flow model tracing REE from deposit geology through mining, beneficiation, wet-process phosphoric acid production, and by-products, into environmental compartments and potential recovery nodes. The diagram illuminates (i) the distribution of REE across process streams; (ii) the implications of U–Th co-mobilization on radiological constraints; and (iii) the intervention points for selective leaching, solvent-extraction or ionic-liquid systems, ion exchange, and membrane-assisted separations. This overarching framework lies at the foundation of the comparative screening summarized in Table 1 and Table 2, and forms the conceptual basis for the process-selection logic elucidated in Section 8.

When considered collectively, Table 1 and Table 2 establish the techno-economic framework for the recovery of REE from phosphate rock. Across phosphate ores, the TREO value is generally modest, while U + Th concentrations are non-negligible. This yields low TREO/(U + Th) ratios, leading to higher radiological overhead (potential Class 7 implications), greater reagent intensity, and tighter permitting margins. Table 2 provides a foundation for these results by anchoring them against two producing end-members with transparent public data. Mountain Pass is characterized by bastnäsite carbonatite with relatively low Th, resulting in higher TREO/(U + Th) ratios and a comparatively lighter radiological burden. Conversely, Mount Weld has a monazite-rich regolith, exhibiting higher Th, producing lower TREO/(U + Th) ratios and a more stringent regulatory compliance envelope. This discrepancy underscores the necessity for distinguishing between phosphate entries in Table 1 that demonstrate commercial viability and those that do not. Ores in this category will require speciation-based flowsheets that prioritize early segregation and stabilization of U/Th, while avoiding the accumulation of radioelements in transportable intermediates. These flowsheets should also be integrated with existing assets to optimize utility use and reduce unit costs.

5. Rare Earth Elements in Phosphate Deposits and Phosphogypsum: Historical Progress, Global Initiatives, and Challenges

In phosphate systems, REE primarily occur through isomorphic substitution in fluorapatite, and, to a lesser extent, in accessory minerals such as monazite and xenotime, which together influence both grade and extractability [83,84,86,87,88,89,90,91,92,93,96,146].

In sedimentary phosphorites, light REE (LREE) enrichment is common, and Ce and Eu anomalies provide insights into redox conditions and hydrothermal inputs. Negative Ce anomalies typically indicate oxic depositional environments, whereas positive Eu anomalies may indicate hydrothermal activity or detrital feldspar input—factors in REE enrichment and extractability [147,148,149].

Figure 7 presents an overview of the historical efforts to explore the recovery potential of REE. Historical recovery efforts began in the 1930s with Soviet studies on REE recovery from phosphoric acid by-products [100,150]. In Romania, hydrochloric acid digestion yielded REO concentrates with ~87.9% purity, though recovery rates (~65%) were insufficient for commercial viability [37,151,152]. Between 1965 and 1972, Kemira Oy in Finland demonstrated commercial-scale feasibility [153], while Solvay in South Africa explored co-extracting uranium and REE from phosphoric acid in the 1970s and 1980s [154]. More recently, Russia and Finland launched joint initiatives to recover REE from PG [155], culminating in a 2016 industrial-scale proposal by Russia’s MISiS National Research and Technology University [100]. In Tunisia, Hammas-Nasri et al. (2019) achieved 84% REE recovery from phosphogypsum using a two-step leaching process [156], surpassing the 60.8% reported by Masmoudi-Soussi et al. (2020) [157].

In the U.S., the Department of Energy has supported REE recovery research on extraction in Florida as well as the Idaho phosphate operations, aiming to diversify global REE supply [85,153]. While these pilot studies confirmed technical feasibility, low REE concentrations and the presence of radioactive elements have limited commercial adoption [158].

In phosphate ores, REE occur either in accessory minerals such as monazite [(Ce, La, Nd, Th)PO₄] and xenotime (YPO₄) or by substituting for calcium in apatite [85]. During the wet process, these minerals react with sulfuric acid to form soluble REE sulfates and phosphoric acid, as shown in the following simplified reactions [85]:

2(Ce, La, Nd, Th) PO₄ + 3H₂SO₄ → (Ce, La, Nd, Th)₂(SO₄)₃ + 2H₃PO₄

(1)

2YPO₄ + 3H₂SO₄ → Y₂(SO₄)₃ + 2H₃PO₄

(2)

Typical phosphate ores contain 100–800 mg/kg REO, often exceeding concentrations in ion-adsorption clays. Since the wet process already dissolves apatite, REE co-leach and can be recovered from phosphoric acid, purification sludge, and PG [100,101,102,103,105,106,159,160]. Pilot studies report ~ 70% leaching efficiency using 2 M H₂SO₄ [106], near-complete recovery with dilute H₂SO₄/HCl in U.S. ores [30], and 90% recovery from phosphoric-acid purification sludge [98].

During fertilizer production, 70–85% of REE typically partition into PG [69,161,162,163,164]. With global PG stocks exceeding 7 billion tons and growing by 150–200 million tons annually [165], the economic potential is significant. REE concentrations in PG range from <100 mg/kg to as much as 60,000 mg/kg [161], with an estimated 21 million tons of REE globally contained in PG [85,166].

The Florida Industrial and Phosphate Research Institute, in collaboration with the Critical Materials Institute, has investigated REE recovery from waste clay, flotation tailings, phosphoric acid, PG, and sludge [159,167]. Researchers found that efficient REE recovery from these streams could increase the availability of critical materials and improve phosphoric acid production [100]. Liang et al. (2017) reported that ~ 90% of REE were released during leaching, though only ~52.82% entered the leachate under optimized conditions [85]. Al-Thyabat and Zhang (2015) achieved 58.1% REE recovery from cadmiferous sludge [123].

In China, phosphate deposits such as Xinhua phosphates contain up to 611.27 mg/kg REE [168]. In Morocco, REE concentrations of ~228.408 mg/kg have been reported in the Gantour Basin, and the Office Chérifien des Phosphates (OCP) is exploring PG reprocessing partnerships [74,160].

In contrast, Jordan’s phosphorites contain lower REE concentrations (1.28–116.74 mg/kg), leading to a focus on uranium extraction [119,120,169]. Russia’s Kola phosphorite ore, with up to 1% REE [170]., has yielded >85% recovery using sulfuric acid, and pilot-scale production of high-purity cerium carbonate, lanthanum oxide, and neodymium oxide has been demonstrated [161,171,172,173].

In Tunisia, Gafsa Basin phosphates average at 322 mg/kg REE, with some basins like Sra Ouertane exceeding 1700 mg/kg [32,79,109,174]. Despite this potential, large-scale REE recovery remains limited. Hammas-Nasri et al. (2016) [164] achieved high REE recovery from PG using a two-step leaching method [164]. Nevertheless, environmental pollution, particularly PG discharge into the Gulf of Gabès, remains a major challenge. El Zrelli et al. (2021) [32] emphasized the economic losses from untapped REE, calling for a robust research and development (R&D) and policy framework to transition from pilot studies to commercial-scale operations [32].

Numerous hydrometallurgical, solvometallurgical, and bioleaching techniques have been explored for REE extraction from phosphate ores and processing streams [100,106,141,161,175,176,177,178,179,180,181,182,183]. Bioleaching using microorganisms such as Aspergillus niger has shown promising results, with extraction efficiencies reaching up to 80% [184]. While several pilot-scale studies have reported recovery rates exceeding 70–80%, the transition to commercial-scale implementation remains constrained by economic feasibility, radioactivity waste management, and regulatory compliance [28,141,158,185]. Life cycle assessments have also raised concerns about the net environmental benefits, emphasizing the need to balance energy consumption, reagent use, and waste disposal [69].

To support sustainable recovery, researchers increasingly advocate for integrated geochemical investigations to assess the speciation, distribution, and mobility of REE and associated trace metals in phosphate-based materials. Advanced analytical techniques, including synchrotron radiation, X-ray absorption spectroscopy, and isotopic tracing, are instrumental in identifying the chemical forms and host phases of REE, as well as co-occurring hazardous elements [168,169]. These insights are critical for developing selective and environmentally responsible extraction strategies, while also informing remediation efforts in contaminated environments.

The evolution of REE recovery from phosphate systems—from early theoretical studies in the Soviet Union to near-commercial pilot plants in several countries—demonstrates the growing recognition of phosphate by-products as strategic secondary resources. Harnessing these underexploited materials could enhance global supply chain resilience while generating additional revenue streams from phosphate-producing regions. However, technical, environmental, and socio-economic challenges persist. High processing costs, the presence of radioactive by-products, and the need for community engagement and transparent governance remain key barriers. Nevertheless, ongoing innovations in extraction technologies, coupled with supportive policies, could transform PG from an environmental liability into a valuable resource, advancing both REE supply security and circular economy objectives.

Given the scale of global phosphate production and the significant REE content in ores and by-products, a detailed understanding of the REE geochemical distribution and mineralogical associations is essential for designing viable and efficient recovery pathways.

6. Geochemistry of Rare Earth Elements in Phosphates, Global Distribution, Traceability, and Environmental Impact

6.1. Geochemistry

The chemical composition of phosphorites, including their REE content, is governed by lithology, depositional environment, and post-depositional processes [172,186]. Variability in the REE concentrations across phosphate deposits reflects the differences in redox conditions, sediment provenance, and diagenetic history. For instance, oxic-marine-shelf phosphorites typically exhibit negative cerium (Ce) anomalies due to oxidative removal of Ce³⁺ from seawater prior to the incorporation into phosphate minerals [147,148,149]; in contrast, anoxic or suboxic conditions may retain higher Ce content and lack such anomalies [148,149]. Non-marine phosphorites may present flatter REE patterns and weak Ce anomalies. Positive europium (Eu) anomalies often indicate hydrothermal influence or detrital input from Eu-rich feldspars [147,148], and may correlate with higher heavy REE (HREE) proportions. Post-depositional processes such as phosphatization, silicification, or carbonate replacement can further modify REE signatures, offering insights into the ore genesis and beneficiation potential [173].

Marine sedimentary phosphorites are generally enriched in light REE (LREE) due to the preferential incorporation of larger ionic LREE’ radii into the fluorapatite lattice [30,147]. In contrast, igneous or weathered phosphate deposits, and some non-marine lacustrine deposits may display more balanced LREE/HREE ratios, especially where xenotime is the dominant host phase [90,91,92,93,94,95,96].

Mineralogical controls are critical to REE abundance and extractability. In most sedimentary phosphorites, REE substitute for Ca²⁺ in fluorapatite (Ca₅(PO₄)₃F) [86,87,88,89]. Accessory minerals such as monazite ((Ce,La,Nd,Th)PO₄) and xenotime (YPO₄) can locally dominate the REE budget, particularly in detrital- or igneous-influenced settings [90,91,92,93,94,95,96]. REE may also be adsorbed onto Fe–Mn oxyhydroxide or clay minerals, which are more labile and environmentally mobile [147,148]. These mineralogical differences influence both the total REE content and the choice of extraction method.

During phosphate mineral precipitation, REE incorporation is influenced by pH, redox potential, and fluid composition [147,172,186]. Fractionation patterns in phosphorites typically show LREE enrichment, driven by ionic radius effects and aqueous complexation behavior [147]. Negative Ce anomalies are diagnostic of oxic conditions [147,148,149], while positive Eu anomalies suggest hydrothermal or detrital inputs [148,149].

6.2. Global Distribution

Phosphate deposits worldwide exhibit diverse REE concentrations and geochemical signatures, shaped by their depositional and diagenetic histories.

In Tunisia, phosphates from the Gafsa Basin are enriched in LREE, particularly La, Ce, and Nd, with occasional slight positive europium (Eu) and negative cerium anomalies [108,109,110,187] (Figure 8). These geochemical patterns suggest oxic marine shelf deposition, with minor detrital input [147,148,149]. Notably, the Sra Ouertane deposit, with REE concentrations exceeding 1700 mg/kg, reflects localized enrichment linked to variations in sedimentary facies and accessory mineral content [79,109]. Fluorapatite dominates as a REE host [86,87,88,89], indicating high leachability during wet-process phosphoric acid production, though monazite-rich zones may require more aggressive leaching strategies [90,91,92,93,94,95,96].

In Algeria, phosphate deposits from the Tébessa region contain over 1000 mg/kg, with a clear LREE dominance and slight negative Ce anomalies, consistent with marine sedimentary origins under oxic conditions [147,148,149]. The association of REE with fluorapatite [86,87,88,89] supports the use of acid leaching, although monazite inclusions necessitate radiological safety assessments [90,91,92,93,95,96].

In Morocco, Gantour Basin phosphates show moderate REE concentrations (~228 mg/kg) and negative Ce anomalies, indicating well-oxygenated shallow-marine deposition [112,147,148,149]. The LREE’ enrichment and fluorapatite dominance make sulfuric acid leaching feasible, though low ore grades suggest PG valorization may be more economically viable [69,85,98].

In China, southern and southwestern phosphate deposits contain some of the highest global REE concentrations, reaching up to 3000 mg/kg total rare earth oxides (TREOs) [113,114,115,168]. Variable Ce anomalies reflect a range of depositional settings [147,149]. The presence of monazite and xenotime contributes to HREE enrichment, requiring selective or multi-stage leaching strategies [74,97,138].

In the United States, Florida’s phosphate deposits and by-products (e.g., tailings, PG) are LREE-enriched [85,167], with negative Ce anomalies indicative of oxic marine deposition [147,148,149]. However, the Mountain Pass region exhibits elevated total REE concentrations and a more balanced LREE/HREE ratio in its primary bastnaesite mineral [117,188,189,190]. These mineralogical differences dictate processing routes: acid leaching for LREE-rich apatite, and alkaline cracking for HREE-rich ores.

Phosphate deposits in Egypt, particularly those at Abu Tartur, contain low-to-moderate REE concentrations, averaging at ~151 mg/kg. These deposits show LREE enrichment [118] and minor negative Ce anomalies, indicative of an oxic marine depositional environment with limited detrital input [147,148,149]. The dominance of fluorapatite in these ores suggests compatibility with wet-process phosphoric acid production; however, the relatively low REE grades may limit the economic feasibility of standalone REE recovery [69].

In Jordan, phosphate deposits such as Eshidiya exhibit some of the lowest REE concentrations, typically <100 mg/kg [120,169]. These deposits are characterized by LREE depletion and slight negative Ce anomalies, reflecting oxic marine conditions [147,149], as shown in Figure 8. Given these low grades, direct REE extraction from the ore is unlikely to be viable. Instead, more promising options may include phosphogypsum valorization or co-recovery with uranium [125,126,127,128].

In Brazil, phosphate deposits like Tapira show exceptionally high REE concentrations, reaching up to 5000 mg/kg TREOs [31,121,122]. The balanced LREE/HREE ratios observed in these deposits suggest mixed marine and igneous influence [147,172,186]. The occurrence of monazite and xenotime necessitates multi-stage beneficiation steps and leaching processes to recover the full REE spectrum [74,138].

6.3. Traceability and Environmental Impact

Understanding the REE traceability during phosphate processing is critical for evaluating both economic potential and environmental risks. During beneficiation and acidulation, REE redistribute across product and waste streams, including phosphoric acid, PG, decadmiation sludge, and contaminated environments [85].

Approximately 15–30% of the total REE dissolve into phosphoric acid, primarily due to their solubility in sulfuric acid solutions [175], while 70–85% of REE are retained in PG, where they substitute for calcium in the CaSO₄2H₂O crystal lattice [100,191]. Decadmiation sludge, also called acid purification sludge, accumulates REE along with heavy metals and radionuclides such as uranium and thorium [28,158]. These by-products represent valuable secondary sources, but also pose environmental challenges if not properly managed. Improper disposal of PG, particularly in marine environments like the Gulf of Gabès in Tunisia, can lead to leaching of REE and toxic metals into seawater, with documented bioaccumulation in sediments, fish, and shellfish, raising public health concerns [57,192].

Lead isotope and REE fingerprinting techniques have proven effective in tracing contamination sources, distinguishing natural versus anthropogenic inputs, and supporting environmental monitoring efforts [147].

Trace metals and REE in phosphates influence biogeochemical cycles and ecosystem health. While elements like zinc and copper are beneficial at low concentrations, other metals such as cadmium and arsenic are toxic and require strict management to prevent environmental and health risks [193,194].

As discussed in Section 6.1, Ce and Eu anomalies provide valuable insights into depositional and post-depositional processes. Combined with LREE/HREE ratios, these anomalies help infer REE host phases—whether fluorapatite, amenable to acid leaching, or refractory monazite/xenotime phases requiring more aggressive treatment [83,93,95,96]. These mineralogical and geochemical patterns are central to evaluating both the environmental mobility and economic recovery potential of REE in phosphate systems.

7. Environmental and Sustainability Challenges in the Phosphate Industry

The phosphate industry is a vital component of global agriculture and industrial development. However, it faces significant environmental and sustainability challenges. This industry is among the most environmentally impactful sectors, with issues related to resource depletion, waste management, and ecosystem contamination.

Phosphate processing redistributes trace elements and critical metals across various streams, including phosphoric acid and fertilizers, PG, solid wastes, and surrounding environments [28,53,58,195,196]. Among these, PG is the most pressing concern as it contains heavy metals, radionuclides, and REE, and has been shown to contaminate soil, groundwater, and marine ecosystems, posing risks to ecological health and public safety [16,178,197,198,199,200].

The mobility and bioavailability of trace metals in the environment are influenced by pH, redox conditions, and organic matter content, which plants uptake and enter into food chains [201,202,203].

Globally, regions such as Florida (USA), Huelva (Spain), Brazil, China, Morocco, and Tunisia have reported PG-related contamination—including uranium, radium, and other toxic metals [28,192,198,200,204,205,206,207,208,209,210]—threatening groundwater, agriculture, and marine life. In Brazil, despite occasional PG use as a soil amendment, its high radionuclide and metal contents continue to endanger coastal ecosystems [211]. Similarly, China and Morocco face growing soil and water contamination linked to PG disposal [200,212,213,214,215,216].

In Tunisia, phosphate by-products have severely impacted the Gulf of Gabès, a biologically rich marine habitat and vital fishing zone [55,59]. Studies have documented the accumulation of heavy metals (e.g., cadmium, strontium, copper, and lead) and radionuclides in marine sediments and marine organisms [28,32,58,197,217,218], posing risks to biodiversity and human health [200,208]. These findings underscore the urgent need for stricter environmental regulations, improved waste management, and alternative PG uses [58,59].

Beyond environmental risks, the phosphate industry faces the challenge of “peak phosphorus”, raising concerns about the long-term availability of high-quality phosphate reserves [6,8,219]. As global demand for phosphate rises, PG production will increase, exacerbating waste-related issues [220,221,222]. Despite these challenges, many research studies offered a more optimistic perspective: PG could serve as a secondary source of REE, which are critical for clean energy and advanced technologies. Harnessing this potential could transform PG from an environmental liability into a strategic resource, aligning with circular economy goals and promoting sustainable development in phosphate-producing regions.

REE are essential for high-tech industries, prompting research into their recovery from industrial by-products and landfilled residues [71,177,223,224]. PG is emerging as a potential “urban mine” for REE due to its significant volume and the presence of critical elements [69,141,171]. Although REE concentrations in PG are relatively low, the global scale PG production—over 7 billion tons and growing by 150–200 million tons annually [165]—results in a substantial cumulative resource, with ~21 million tons of REE estimated in existing PG dumps [156,164,166]. PG also contains a diverse suite of metals, including Sr, Fe, Y, Ce, Cr, Zn, Cu, Pb, Cd, and U. This diversity enhances the economic potential of PG [225].

A variety of hydrometallurgical and solvometallurgical techniques have been explored for REE recovery from PG [100,161,178,226]. In Tunisia, Hammas-Nasri et al. (2019) achieved 84% REE recovery using a two-step process: washing PG with a sodium chloride solution, followed by sodium carbonate leaching at elevated temperatures [156]. Other innovative techniques, such as bioleaching with Aspergillus niger, have demonstrated extraction efficiencies of ~74%, indicating their potential for industrial-scale applications [227]. These recovery methods not only diversify revenue streams for the phosphate industry but also mitigate environmental impacts by reducing the volume and toxicity of PG waste.

In parallel, sustainable practices, including phytoremediation and habitat rehabilitation, are increasingly being implemented in regions affected by phosphate mining [228,229,230]. Certain plant species can absorb and accumulate heavy metals, thereby restoring contaminated ecosystems [231]. Valorization strategies, such as the reuse of PG in construction materials or as a soil amendment, provide additional opportunities for waste reduction and resource efficiency [15,199,232,233,234]. Collectively, these initiatives aim to improve sustainability throughout the phosphate supply chain, ensuring the resilience of the industry to both resource scarcity and environmental challenges.

Overall, the literature highlights the critical need for effective management of phosphate processing by-products. Phosphogypsum, in particular, poses significant environmental hazards, including soil and water contamination and marine ecosystem degradation. Yet, these same by-products contain valuable materials, especially REE, which could generate economic value while reducing environmental damage. Achieving this dual goal requires a comprehensive understanding of the impact of these elements, their source traceability, chemical fate and speciation throughout phosphate processing, and their impact on contaminated environments. Such knowledge is essential for assessing contamination risks, informing policy decisions on fertilizer use and waste management, and evaluating recovery potential. Additionally, linking REE’ geochemistry and the host minerals to extraction methods ensures that recovery strategies are both technically feasible and economically justified.

For countries like Tunisia, where phosphate resources are abundant but ecosystems are vulnerable, responsible management of PG and contaminated sites—particularly in marine environments—can serve as a model for balancing industrial growth with environmental stewardship. This approach can ensure both the sustainability of the phosphate sector and effective environmental safety management.

8. Opportunities and Strategies for REE Recovery from Phosphate Ores and Industrial By-Products

The rising demand for REE, coupled with the geopolitical and environmental challenges of conventional REE sources, has triggered growing interest in secondary resources, particularly phosphate deposits and phosphate processing residues such as PG [70,71,77,235,236].

Efforts to recover REE from these materials date back to the 1930s, when Soviet researchers explored REE recovery during phosphate rock processing [100]. Since the 1930s, pilot-scale and near-industrial initiatives have been launched globally to explore REE recovery from phosphate processing streams. For example, in the 1960s, Romania developed a multi-step REE concentration process using hydrochloric acid digestion of phosphate rock, reaching 87.9% purity but only 65% recovery, insufficient for industrial implementation [100,152]. Finland’s Kemira Oy demonstrated commercial-scale feasibility between 1965 and 1972 [153], and South Africa’s Solvay ran co-extraction pilot tests for uranium and REE from phosphoric acid in the 1970s and 1980s [154,175]. Several countries, including Russia, Finland, and the United States, have advanced this research through pilot programs and industrial proposals. Notably, a 2016 Russian initiative proposed an industrial-scale REE recovery route from PG [166], and the U.S. Department of Energy began funding programs to integrate REE extraction into phosphate rock and PG processing [153]. However, despite technical successes, the low REE content of most phosphate materials and the complications of managing radioactive by-products such as uranium and thorium have delayed commercial deployment [158].

In recent years, PG has emerged as a particularly promising secondary resource. With global stocks exceeding 7 billion tons and growing by 200–300 million tons annually, PG may contain up to 21 million tons of REE [156,165,166,237]. In addition to REE, PG contains valuable metals such as strontium (Sr), iron (Fe), yttrium (Y), cerium (Ce), chromium (Cr), titanium (Ti), zinc (Zn), copper (Cu), lead (Pb), vanadium (V), and cadmium (Cd) [232], further enhancing the economic viability of resource recovery.

A wide range of extraction techniques has been explored, including hydrometallurgical (acid leaching and solvent extraction), solvometallurgical, and bioleaching methods. Although pilot trials have achieved recovery rates exceeding 70–80%, full-scale implementation remains constrained by economic feasibility, waste stream management, and radiological safety concerns [28,74,226,238,239,240]. Case studies from Tunisia and Florida demonstrate that integrating REE recovery into existing phosphate processing workflows can enhance economic returns while reducing environmental impact. Liang et al. (2018) [138] achieved ~61% REE recovery from Florida phosphate flotation tailings using sulfuric acid under optimized leaching conditions. In China, Li et al. (2021) [97] reported recovery rates up to 94.3% from phosphate ore leached with phosphoric acid under optimized parameters (30% P₂O₅, L/S = 10:1).

Recent research emphasizes that the geochemical and mineralogical characteristics of phosphate ores are key determinants of recovery efficiency and technology selection. The choice of extraction method must be guided by mineralogical host phases and their REE patterns. For example, fluorapatite, the dominant REE host in many sedimentary phosphates, is typically enriched in LREE and exhibits negative Ce anomalies. These features align with sulfuric acid leaching, already integrated into phosphoric acid production, enabling co-recovery of REE with phosphorus. In contrast, monazite and xenotime, refractory minerals often rich in HREE, require more aggressive treatments such as alkaline cracking, hot acid digestion, or caustic fusion. These minerals are more prevalent in igneous-influenced phosphates such as those in Brazil, Mountain Pass (USA), and parts of China. Adsorbed and amorphous REE-bearing phases, often found in process residues such as PG, are amenable to bioleaching, ion exchange, or weak acid leaching. These approaches are more environmentally sustainable but generally slower. A summary of the relationship between dominant REE’ host minerals, their geochemical signatures, and corresponding extraction strategies is presented in

Table 3. Relationship between dominant REE’ host mineral, geochemical signature, and optimal extraction approach.

Dominant REE Host Mineral	Typical REE Pattern	Extraction Approach	Considerations
Fluorapatite	LREE-enriched; high LREE/HREE; often negative Ce anomaly [83,93,95]	Sulfuric acid leaching (wet-process), recovery from phosphoric acid, phosphogypsum, or purification sludge [100,101,102,103,104,105,106]	Recovery integrated into fertilizer production; REE co-leach with P2O5; possible selectivity
Monazite	LREE-rich, may show subdued anomalies; high Th/U [93,95,96]	Hot concentrated acid digestion	Requires radioelement management; high REE grade but higher processing cost
Xenotime	HREE-enriched; flatter REE profile; possible positive Eu anomaly [93,95,96]	Mixed-acid digestion	Strategic for HREE; more aggressive reagents needed
Adsorbed/ amorphous phases	Low total REE; variable patterns [99,184,203,241]	Bioleaching, ion exchange, or weak acid leaching [99,184,191,241]	Environmentally friendlier; slower kinetics; suitable for PG or process waters

Evidence pedigree: All entries are derived from peer-reviewed papers.

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Integrating geochemical fingerprinting (e.g., Ce anomalies and LREE/HREE ratios) with mineralogical analysis is essential for designing cost-effective and environmentally responsible recovery routes. For instance, a strong negative Ce anomaly coupled with high LREE content often points to fluorapatite-hosted REE, well-suited to acid leaching. Flatter REE profiles or positive Eu anomalies may instead indicate HREE-rich monazite or xenotime phases, requiring tailored leaching protocols. Selective solvent extraction and ion-exchange resins can further refine REE recovery, especially when designed to target the specific REE distribution of the leachate [73,135,136].

While the concept of “urban mining” REE from phosphate resources is technically viable, its large-scale adoption depends on reducing reagent and energy costs, ensuring regulatory compliance—particularly regarding radioactive co-contaminants—and securing public and stakeholder support. Recent advances in extraction technologies, coupled with policy frameworks promoting circular economy practices, could enable the large-scale transformation of PG from a waste by-product to a strategic asset that supports a more sustainable REE supply chain.

To operationalize the evaluation of recovery opportunities, a decision framework was developed linking ore mineralogy, extraction strategy, energy and reagent requirements, and economics. This framework enables rapid screening of phosphate ores based on REE grade, radiological burden, and process feasibility. Figure 9 presents a schematic summarizing the main decision stages and associated performance indicators derived from recent studies [30,74,97,156].

The decision tree indicates that fluorapatite-hosted REE can be processed with reduced energy and reagent demand compared to monazite or xenotime systems, although with moderate recoveries. Conversely, monazite-bearing phosphates exhibit higher TREO grades. They require energy-intensive cracking processes and radiological control measures due to elevated U and/or Th content. The framework thus assists in identifying favorable candidates for pilot-scale testing and challenging streams where energy or radiological burdens outweigh potential economic returns.

9. Executive Summary

Phosphate deposits and their processing by-products represent a significant yet underutilized secondary source of REE. Globally, phosphate ores contain 0.02–0.3 wt% TREOs, corresponding to 10–30 Mt REEO theoretically available in known reserves. However, only a small percentage of the total mineral content is technically recoverable. Laboratory and pilot studies have reported recovery rates ranging from 40% to 90%, depending on the mineralogy of the material (fluorapatite, monazite, or xenotime). Accounting for beneficiation losses and process efficiency, the technical potential corresponds to roughly 15–25% of the theoretical inventory.

The constraints imposed by environmental and radiological factors further limit the feasibility of this approach. Materials with high U and Th content are unlikely to meet industrial or regulatory limits for large-scale treatment. According to these criteria, only about half of the technically recoverable REE fraction globally is considered environmentally acceptable. This fraction is equivalent to 1–3 million tons of REE (REEO). Predominant environmentally acceptable resources are low-Th fluorapatite ores and phosphogypsum by-products, which can be processed using mild acid or solvent-extraction circuits integrated into existing fertilizer plants. The recovery of these residues could yield 5–10% of future global demand for REE, thereby reducing waste burdens.

In summary, phosphate resources have been demonstrated to possess substantial potential for REE recovery; however, the feasibility of this recovery is contingent upon the implementation of three distinct filters, namely: (1) geological availability, (2) technical extractability, and (3) environmental acceptability.

10. Conclusions

The phosphate industry plays a pivotal role in global agriculture while offering a potential secondary source of critical raw materials. However, the commercial production of REE from phosphate ores and phosphate streams has not yet achieved significant scale due to the unfavorable grade–reagent–radioactivity economics at industrial levels. The majority of phosphate ores/PG demonstrate modest TREO values, coupled with quantifiable U and Th, yielding low TREO/(U + Th) ratios. This results in increased reagent consumption per unit REO, the necessity of radiological controls (and, in certain instances, Class 7 logistics for intermediates), and elevated costs associated with effluent treatment and long-term stack stewardship. In comparison, the production of bastnäsite operations involves the pairing of higher TREOs with lower Th, thereby facilitating more straightforward permitting processes and reducing operating intensity.

Environmental constraints are of paramount importance. It is imperative that any flowsheet design avoids the co-mobilization of U/Th and co-occurring metals into process waters. Furthermore, it must prevent their concentration in transportable intermediates and deliver demonstrable, durable risk reduction for very large inventories. In coastal regions such as the Gulf of Gabès in Tunisia, the social license to operate will be contingent upon indisputable evidence that the recovery process mitigates the ecological burden, rather than merely displacing it within the site.

The current market structure further narrows the feasibility of such an outcome. At current prices, Ce and La offer limited standalone value and are often oversupplied. The robustness of a project is typically contingent on Nd-Pr (and, where accessible, Dy-Tb). Processes that extract “bulk TREOs” but lack the capability to upgrade or separate them into marketable products will encounter significant challenges in meeting economic thresholds, particularly in the presence of U/Th overheads.

The question of whether phosphate streams resolve the HREE/LREE issue remains a subject of debate. It has been observed that certain streams demonstrate slightly elevated Y-group proportions in comparison to their parent ores; however, it is noteworthy that these streams are predominantly characterized by a dominance of LREE. This could transform the field of economics by being complemented by advantages such as zero-cost feed (waste liability), integration with wet-process phosphoric acid assets, and co-remediation.

While there is a promising technological landscape, barriers to entry persist. Selective leaching of CaSO4·2H2O to limit U/Th co-dissolution, task-specific extractants/ionic liquids that favor Nd-Pr at low head grades, membrane-assisted separations to recycle acids/bases, and bioleaching consortia for labile REE pools have shown encouraging pilot-scale results. However, three barrier classes continue to dominate: technical (speciation-aware selectivity in strong acids, reliable U/Th partitioning and immobilization), economic (reagent and energy intensity), and regulatory/social (auditable radiological stewardship across the flowsheet and evidence of net risk reduction).

In Tunisia, for example, where phosphate resources are abundant but ecosystems are vulnerable, the viable path is speciation-led, integration-first, remediation-positive: flowsheets are designed around the actual REE hosts and U/Th behavior, embedded within the existing WPA infrastructure to share utilities and solids handling, and delivering quantifiable environmental gains (reduced metal/radionuclide flux, stabilized residues). In circumstances where the Nd-Pr upgrading process is deemed feasible, phosphate regions possess the potential to transform phosphate streams from a liability into a strategic, circular asset. Absent such alignment, low TREOs and radiological overheads will keep most phosphate-derived resources below the threshold for commercial REE production.