# Rare earth elements and uranium in Minjingu phosphate fertilizer products: Plant food for thought, Resources, Conservation and Recycling

Nils Haneklaus, Dennis A. Mwalongo, Jacob B. Lisuma, Aloyce I. Amasi, Jerome Mwimanzi, Tomislav Bituh, Jelena Ćirić, Jakub Nowak, Urszula Ryszko, Piotr Rusek, et al.

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## To cite this version:

Nils Haneklaus, Dennis A. Mwalongo, Jacob B. Lisuma, Aloyce I. Amasi, Jerome Mwimanzi, et al.: Rare earth elements and uranium in Minjingu phosphate fertilizer products: Plant food for thought, *Resources, Conservation and Recycling*. *Resources, Conservation and Recycling*, inPress, 2024.1016/j.resconrec.2024.107694. emse-04585731

HAL Id: emse-04585731

https://hal-emse.ccsd.cnrs.fr/emse-04585731

Submitted on 29 May 2024

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Rare earth elements and uranium in Minjingu phosphate fertilizer products:
Plant food for thought

Nils H. Haneklaus<sup>a,b,*</sup>, Dennis A. Mwalongo<sup>a,c</sup>, Jacob B. Lisuma<sup>d</sup>, Aloyce I. Amasi<sup>a,c</sup>,
Jerome Mwimanzi<sup>a,c</sup>, Tomislav Bituh<sup>e</sup>, Jelena Ćirić<sup>f</sup>, Jakub Nowak<sup>g</sup>, Urszula Ryszko<sup>h</sup>,
Piotr Rusek<sup>h</sup>, Ali Maged<sup>i,j</sup>, Essaid Bilal<sup>k</sup>, Hajar Bellefqih<sup>k</sup>, Khaoula Qamouche<sup>b</sup>,
Jamal Ait Brahim<sup>l</sup>, Redouane Beniaza<sup>l</sup>, Hamid Mazouz<sup>m</sup>, Elizabet M. van der Merwe<sup>n</sup>,
Wayne Truter<sup>o</sup>, Hilda D. Kyomuhimbo<sup>p</sup>, Hendrik Brink<sup>p</sup>, Gerald Steiner<sup>b,q</sup>, Martin Bertau<sup>r,s</sup>,
Raghav S. Soni<sup>t</sup>, Ashwin W. Patwardhan<sup>t</sup>, Pushpito K. Ghosh<sup>t</sup>, Thomas T. Kivevele<sup>a</sup>,
Kelvin M. Mtei<sup>a</sup>, Stanisław Wacławek<sup>u</sup>

ARTICLE INFO

Keywords:

Minjingu phosphate ore
Rare earth elements (REEs)
Uranium
Circular economy
Tanzania

ABSTRACT

Minjingu phosphate ore is Tanzania's sole domestic supply of phosphorus (P). The ore contains medium to high concentrations of naturally occurring P<sub>2</sub>O<sub>5</sub> (20–35 %) and relevant concentrations of uranium and rare earth elements (REEs) are also suspected to be present. Currently, neither uranium nor REEs are recovered. They either end up in mine tailings or are spread across agricultural soils with fertilizer products. This work provides a first systematic review of the uranium and REE concentrations that can be expected in the different layers of Minjingu phosphate ore, the way the ore is presently processed, as well as a discussion on alternative processing pathways with uranium/REE recovery. The study analyzed ten distinct Minjingu phosphate ore layers, four mine tailings, and five intermediate and final mineral fertilizer products from the Minjingu mine and processing plant located in northern Tanzania. The results confirm that the uranium concentrations and to a lesser degree, the REE concentrations are indeed elevated if compared to concentrations in other phosphate ores. The study does not identify a significant risk resulting from this. The development of techno-economic solutions for more

<footnote><sup>a</sup> Nelson Mandela African Institution of Science and Technology, School for Materials, Energy, Water, Environmental Science and Engineering, Arusha, Tanzania</footnote>

<footnote><sup>b</sup> Td-Lab Sustainable Mineral Resources, Universität für Weiterbildung Krems, Dr.-Karl-Dorrek-Straße 30, 3500 Krems an der Donau, Austria</footnote>

<footnote><sup>c</sup> Tanzania Atomic Energy Commission (TAEC), Directorate of Nuclear Technology and Technical Services, P. O. Box 743, Arusha, Tanzania</footnote>

<footnote><sup>d</sup> Tobacco Research Institute of Tanzania (TORITA), P.O. Box 431, Tabora, Tanzania</footnote>

<footnote><sup>e</sup> Institute for Medical Research and Occupational Health, Zagreb, Croatia</footnote>

<footnote><sup>f</sup> Institute of Meat Hygiene and Technology, Kaćanskog 13, 11000, Belgrade, Serbia</footnote>

<footnote><sup>g</sup> AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Mickiewicza 30 Av., 30-059, Kraków, Poland</footnote>

<footnote><sup>h</sup> Łukasiewicz Research Network–New Chemical Syntheses Institute, al. Tysiąclecia Państwa Polskiego 13a, 24-110, Puławy, Poland</footnote>

<footnote><sup>i</sup> Geology Department, Faculty of Science, Suez University, 43221, Suez, Egypt</footnote>

<footnote><sup>j</sup> Institute of Process Engineering, Johannes Kepler University Linz, Altenberger Straße 69, 4040, Linz, Austria</footnote>

<footnote><sup>k</sup> Ecole Nationale Supérieure des Mines de Saint Etienne, CNRS UMR, EVS, 5600, F42023, Saint Etienne, France</footnote>

<footnote><sup>l</sup> Institute of Science, Technology & Innovation (IST&I), Mohammed VI Polytechnic University (UM6P), Ben Guerir, 43150, Morocco</footnote>

<footnote><sup>m</sup> Expertise Center for Phosphate (CEPH), Mohammed VI Polytechnic University (UM6P), Ben Guerir, 43150, Morocco</footnote>

<footnote><sup>n</sup> Chemistry Department, University of Pretoria, Lynnwood Road, Pretoria, 0001, South Africa</footnote>

<footnote><sup>o</sup> University of the Free State, Natural and Agricultural Sciences, Bloemfontein, 9300, South Africa</footnote>

<footnote><sup>p</sup> Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa</footnote>

<footnote><sup>q</sup> Complexity Science Hub Vienna, Josefstädter Straße 39, 1080, Vienna, Austria</footnote>

<footnote><sup>r</sup> TU Bergakademie Freiberg, Institute of Chemical Technology, Leipzig Straße 29, Freiberg, Germany</footnote>

<footnote><sup>s</sup> Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Fraunhofer Technology Center for High-Performance Materials THM, Freiberg, Germany</footnote>

<footnote><sup>t</sup> Institute of Chemical Technology, Department of Chemical Engineering, Mumbai, 400019, India</footnote>

<footnote><sup>u</sup> Technical University of Liberec, Centre for Nanomaterials, Advanced Technologies and Innovation, Studentská 1402/2, 461 17, Liberec 1, Czech Republic</footnote>

<footnote>* Corresponding author.
E-mail address: nils.haneklaus@donau-uni.ac.at (N.H. Haneklaus).</footnote>
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Rare earth elements and uranium in Minjingu phosphate fertilizer products:
Plant food for thought

Nils H. Haneklaus<sup>a,b,*</sup>, Dennis A. Mwalongo<sup>a,c</sup>, Jacob B. Lisuma<sup>d</sup>, Aloyce I. Amasi<sup>a,c</sup>,
Jerome Mwimanzi<sup>a,c</sup>, Tomislav Bituh<sup>e</sup>, Jelena Ćirić<sup>f</sup>, Jakub Nowak<sup>g</sup>, Urszula Ryszko<sup>h</sup>,
Piotr Rusek<sup>h</sup>, Ali Maged<sup>i,j</sup>, Essaid Bilal<sup>k</sup>, Hajar Bellefqih<sup>k</sup>, Khaoula Qamouche<sup>b</sup>,
Jamal Ait Brahim<sup>l</sup>, Redouane Beniaza<sup>l</sup>, Hamid Mazouz<sup>m</sup>, Elizabet M. van der Merwe<sup>n</sup>,
Wayne Truter<sup>o</sup>, Hilda D. Kyomuhimbo<sup>p</sup>, Hendrik Brink<sup>p</sup>, Gerald Steiner<sup>b,q</sup>, Martin Bertau<sup>r,s</sup>,
Raghav S. Soni<sup>t</sup>, Ashwin W. Patwardhan<sup>t</sup>, Pushpito K. Ghosh<sup>t</sup>, Thomas T. Kivevele<sup>a</sup>,
Kelvin M. Mtei<sup>a</sup>, Stanisław Wacławek<sup>u</sup>

ARTICLE INFO

Keywords:

Minjingu phosphate ore
Rare earth elements (REEs)
Uranium
Circular economy
Tanzania

A B S T R A C T

Minjingu phosphate ore is Tanzania's sole domestic supply of phosphorus (P). The ore contains medium to high concentrations of naturally occurring P₂O₅ (20–35 %) and relevant concentrations of uranium and rare earth elements (REEs) are also suspected to be present. Currently, neither uranium nor REEs are recovered. They either end up in mine tailings or are spread across agricultural soils with fertilizer products. This work provides a first systematic review of the uranium and REE concentrations that can be expected in the different layers of Minjingu phosphate ore, the way the ore is presently processed, as well as a discussion on alternative processing pathways with uranium/REE recovery. The study analyzed ten distinct Minjingu phosphate ore layers, four mine tailings, and five intermediate and final mineral fertilizer products from the Minjingu mine and processing plant located in northern Tanzania. The results confirm that the uranium concentrations and to a lesser degree, the REE concentrations are indeed elevated if compared to concentrations in other phosphate ores. The study does not identify a significant risk resulting from this. The development of techno-economic solutions for more

<footnote><sup>a</sup> Nelson Mandela African Institution of Science and Technology, School for Materials, Energy, Water, Environmental Science and Engineering, Arusha, Tanzania</footnote>

<footnote><sup>b</sup> Td-Lab Sustainable Mineral Resources, Universität für Weiterbildung Krems, Dr.-Karl-Dorrek-Straße 30, 3500 Krems an der Donau, Austria</footnote>

<footnote><sup>c</sup> Tanzania Atomic Energy Commission (TAEC), Directorate of Nuclear Technology and Technical Services, P. O. Box 743, Arusha, Tanzania</footnote>

<footnote><sup>d</sup> Tobacco Research Institute of Tanzania (TORITA), P.O. Box 431, Tabora, Tanzania</footnote>

<footnote><sup>e</sup> Institute for Medical Research and Occupational Health, Zagreb, Croatia</footnote>

<footnote><sup>f</sup> Institute of Meat Hygiene and Technology, Kaćanskog 13, 11000, Belgrade, Serbia</footnote>

<footnote><sup>g</sup> AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Mickiewicza 30 Av., 30-059, Kraków, Poland</footnote>

<footnote><sup>h</sup> Lukasiewicz Research Network–New Chemical Syntheses Institute, al. Tysiąclecia Państwa Polskiego 13a, 24-110, Puławy, Poland</footnote>

<footnote><sup>i</sup> Geology Department, Faculty of Science, Suez University, 43221, Suez, Egypt</footnote>

<footnote><sup>j</sup> Institute of Process Engineering, Johannes Kepler University Linz, Altenberger Straße 69, 4040, Linz, Austria</footnote>

<footnote><sup>k</sup> Ecole Nationale Supérieure des Mines de Saint Etienne, CNRS UMR, EVS, 5600, F42023, Saint Etienne, France</footnote>

<footnote><sup>l</sup> Institute of Science, Technology & Innovation (IST&I), Mohammed VI Polytechnic University (UM6P), Ben Guerir, 43150, Morocco</footnote>

<footnote><sup>m</sup> Expertise Center for Phosphate (CEPH), Mohammed VI Polytechnic University (UM6P), Ben Guerir, 43150, Morocco</footnote>

<footnote><sup>n</sup> Chemistry Department, University of Pretoria, Lynnwood Road, Pretoria, 0001, South Africa</footnote>

<footnote><sup>o</sup> University of the Free State, Natural and Agricultural Sciences, Bloemfontein, 9300, South Africa</footnote>

<footnote><sup>p</sup> Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa</footnote>

<footnote><sup>q</sup> Complexity Science Hub Vienna, Josefstädter Straße 39, 1080, Vienna, Austria</footnote>

<footnote><sup>r</sup> TU Bergakademie Freiberg, Institute of Chemical Technology, Leipzig Straße 29, Freiberg, Germany</footnote>

<footnote><sup>s</sup> Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Fraunhofer Technology Center for High-Performance Materials THM, Freiberg, Germany</footnote>

<footnote><sup>t</sup> Institute of Chemical Technology, Department of Chemical Engineering, Mumbai, 400019, India</footnote>

<footnote><sup>u</sup> Technical University of Liberec, Centre for Nanomaterials, Advanced Technologies and Innovation, Studentská 1402/2, 461 17, Liberec 1, Czech Republic</footnote>

<footnote>* Corresponding author.
E-mail address: nils.haneklaus@donau-uni.ac.at (N.H. Haneklaus).</footnote>
comprehensive utilization of Minjingu ore is, however, strongly encouraged and suggestions on such processes are provided.

# 1. Introduction

Phosphate ores that are used to produce mineral fertilizers can show elevated concentrations of heavy metals and particularly cadmium has been identified as a potential health risk (McLaughlin et al., 2021). Radiotoxic uranium (Haneklaus, 2021a) as well as rare earth elements (REEs) (Chen and Graedel, 2015; Gaustad et al., 2021) can also be found in phosphate ores worldwide in relevant concentrations.

Phosphate ores can be of magmatic (or igneous) and sedimentary origin, and these ores tend to show very different concentrations of accompanying trace elements. In the case of igneous phosphate ores REEs can reach concentrations of 0.2 wt%, while the concentrations are usually lower in sedimentary phosphate ores. Globally, a REE concentration of approximately 0.05 wt% for phosphate ore is realistic (Wu et al., 2018a) since there are much more sedimentary than igneous phosphate ore reserves. Uranium can show concentrations of 0.01 to 0.02 % in Moroccan phosphate ores (Qamouche et al., 2021; Ulrich et al., 2014) that present more than 70 % of the currently known reserves (Cooper et al., 2011), while uranium concentrations are usually below 0.005 % in phosphate ores of magmatic origin.

Although the concentrations in phosphate ore can be considered moderate at best, the overall quantities of both REEs and uranium that could theoretically be recovered are impressive given that approximately 220 million metric tons (t) per year, phosphate ore is mined globally. Phosphate ore is among the 5th most mined materials on earth, and Hakkar et al. (2021) estimated that REEs recovered from phosphate ore mining in Morocco alone could cover 7–15 % of global demand. Zhang (2014) further showed that the United States could cover its whole demand if REEs were recovered during phosphate processing in Florida. Haneklaus (2021a) further estimated that uranium recovered during phosphate fertilizer production could have theoretically provided some 16–26 % or 8800–14,000 t of the world's commercial uranium requirements in 2018. More detailed country estimates are available for the United States: approximately 2116 t of 20,386 t uranium requirements could be recovered from phosphates in 2014 (Kim et al., 2016; Steiner et al., 2020), China: 648 t (approximately 10 % of the countries uranium demand) could be recovered during fertilizer production in 2016 (Shang et al., 2021), the European Union: approximately 334 t uranium (approximately 2 % of the demand) could have been recovered from phosphate ore imports in 2017 (Tulsidas et al., 2019), and Argentina: approximately 19 t uranium (some 8 % of the countries demand) could have been recovered in 2017 (López et al., 2019). Besides, Diwa et al. (2023) estimated that 14–26 t of uranium (12–21 % of projected near-term uranium demand) could be recovered from imported phosphate ores in the Philippines. Khan et al. (2023) did similar calculations for domestically mined phosphate ores in Saudi Arabia and concluded that there 413–551 t (11–15 % of the projected near-term, future natural uranium requirements) could be recovered during phosphate fertilizer production. Ramteke et al. (2022) further investigated the uranium concentrations in DAP fertilizers in India and found that approximately 1320 t uranium were distributed on agricultural soils between 2021 and 2022. If recovered, this uranium could significantly contribute to the natural uranium requirements of India.

Currently, both REEs and uranium are not industrially recovered during phosphate fertilizer production. Recovery of uranium from wet-process phosphoric acid (WPA), an intermediate liquid product in phosphate fertilizer production, was done on an industrial scale in Florida in the 1980s and 1990s before decreasing uranium prices made this practice uneconomic (Haneklaus et al., 2017a,b). Ye et al. (2019) pointed out that (for uranium) we will probably not see a choreographed recovery from phosphates at all >400 fertilizer plants around the world

starting anytime soon, but maybe recovery operations starting at phosphate mines that show elevated uranium concentrations to a degree that there is a pull-factor, in a way that uranium can be sold at economic profit, but also a push-factor, in a way that not recovering the uranium could result in lower final fertilizer prices as a result of the elevated heavy metal content (Scholz and Wellmer, 2018) or even limitations with regards to its sale as a result of potentially introduced uranium limits in fertilizers.

The Minjingu phosphate ore deposit in northern Tanzania is known for its relatively high concentration of naturally occurring uranium. It was in fact the increased radiation that led to the discovery of the deposit at the edge of Lake Manyara, now a national park and protected area in northern Tanzania, by a South African mining company in 1956 (Mchihiyo, 1991; Van Kauwenbergh, 1991).

Minjingu phosphate ore shows natural uranium levels that would also qualify the deposit as a very low-grade uranium ore under the definition of the World Nuclear Association (WNA) (Haneklaus et al. 2017a). Mwalongo et al. (2023a) recently pointed out that the uranium concentration in Minjingu is indeed higher than at commercial uranium mines in Namibia on the other side of the African continent. Since fertilizer is more important for Tanzania's economy than uranium, the ore is mined for its elevated P₂O₅ content (20–25 % on average) and not its elevated uranium content (0.03–0.04 % on average).

Previously, some 10–15 years ago, Minjingu phosphate ore was after simple beneficiation that included sorting, sieving and rapid drying at 700–800 °C, applied directly as fine fertilizer powder on agricultural soils in East Africa. The fertilizer produced this way proved to be effective on the acidic soils found in East Africa, and the material was considerably less expensive than imported fertilizers from abroad. It was therefore historically not only used in Tanzania, but also in Kenya, Uganda, Burundi, Zambia and even South Africa as indicated in Fig. 1 (Mkangwa, 2003; Msolla et al., 2005; Nabahungu et al., 2007; Ndeleko-Barasa et al., 2021; Nyambati and Opola, 2014; Sarini et al., 2015;

<figure><img src="image_3.png" /><figcaption>Fig. 1. Location of the Minjingu phosphate rock deposit in northern Tanzania as well as countries to which Minjingu phosphate ore products have historically been exported to.</figcaption></figure>
comprehensive utilization of Minjingu ore is, however, strongly encouraged and suggestions on such processes are provided.

# 1. Introduction

Phosphate ores that are used to produce mineral fertilizers can show elevated concentrations of heavy metals and particularly cadmium has been identified as a potential health risk (McLaughlin et al., 2021). Radiotoxic uranium (Haneklaus, 2021a) as well as rare earth elements (REEs) (Chen and Graedel, 2015; Gaustad et al., 2021) can also be found in phosphate ores worldwide in relevant concentrations.

Phosphate ores can be of magmatic (or igneous) and sedimentary origin, and these ores tend to show very different concentrations of accompanying trace elements. In the case of igneous phosphate ores REEs can reach concentrations of 0.2 wt%, while the concentrations are usually lower in sedimentary phosphate ores. Globally, a REE concentration of approximately 0.05 wt% for phosphate ore is realistic (Wu et al., 2018a) since there are much more sedimentary than igneous phosphate ore reserves. Uranium can show concentrations of 0.01 to 0.02 % in Moroccan phosphate ores (Qamouche et al., 2021; Ulrich et al., 2014) that present more than 70 % of the currently known reserves (Cooper et al., 2011), while uranium concentrations are usually below 0.005 % in phosphate ores of magmatic origin.

Although the concentrations in phosphate ore can be considered moderate at best, the overall quantities of both REEs and uranium that could theoretically be recovered are impressive given that approximately 220 million metric tons (t) per year, phosphate ore is mined globally. Phosphate ore is among the 5th most mined materials on earth, and Hakkar et al. (2021) estimated that REEs recovered from phosphate ore mining in Morocco alone could cover 7–15 % of global demand. Zhang (2014) further showed that the United States could cover its whole demand if REEs were recovered during phosphate processing in Florida. Haneklaus (2021a) further estimated that uranium recovered during phosphate fertilizer production could have theoretically provided some 16–26 % or 8800–14,000 t of the world's commercial uranium requirements in 2018. More detailed country estimates are available for the United States: approximately 2116 t of 20,386 t uranium requirements could be recovered from phosphates in 2014 (Kim et al., 2016; Steiner et al., 2020), China: 648 t (approximately 10 % of the countries uranium demand) could be recovered during fertilizer production in 2016 (Shang et al., 2021), the European Union: approximately 334 t uranium (approximately 2 % of the demand) could have been recovered from phosphate ore imports in 2017 (Tulsidas et al., 2019), and Argentina: approximately 19 t uranium (some 8 % of the countries demand) could have been recovered in 2017 (López et al., 2019). Besides, Diwa et al. (2023) estimated that 14–26 t of uranium (12–21 % of projected near-term uranium demand) could be recovered from imported phosphate ores in the Philippines. Khan et al. (2023) did similar calculations for domestically mined phosphate ores in Saudi Arabia and concluded that there 413–551 t (11–15 % of the projected near-term, future natural uranium requirements) could be recovered during phosphate fertilizer production. Ramteke et al. (2022) further investigated the uranium concentrations in DAP fertilizers in India and found that approximately 1320 t uranium were distributed on agricultural soils between 2021 and 2022. If recovered, this uranium could significantly contribute to the natural uranium requirements of India.

Currently, both REEs and uranium are not industrially recovered during phosphate fertilizer production. Recovery of uranium from wet-process phosphoric acid (WPA), an intermediate liquid product in phosphate fertilizer production, was done on an industrial scale in Florida in the 1980s and 1990s before decreasing uranium prices made this practice uneconomic (Haneklaus et al., 2017a,b). Ye et al. (2019) pointed out that (for uranium) we will probably not see a choreographed recovery from phosphates at all >400 fertilizer plants around the world

starting anytime soon, but maybe recovery operations starting at phosphate mines that show elevated uranium concentrations to a degree that there is a pull-factor, in a way that uranium can be sold at economic profit, but also a push-factor, in a way that not recovering the uranium could result in lower final fertilizer prices as a result of the elevated heavy metal content (Scholz and Wellmer, 2018) or even limitations with regards to its sale as a result of potentially introduced uranium limits in fertilizers.

The Minjingu phosphate ore deposit in northern Tanzania is known for its relatively high concentration of naturally occurring uranium. It was in fact the increased radiation that led to the discovery of the deposit at the edge of Lake Manyara, now a national park and protected area in northern Tanzania, by a South African mining company in 1956 (Mchihiyo, 1991; Van Kauwenbergh, 1991).

Minjingu phosphate ore shows natural uranium levels that would also qualify the deposit as a very low-grade uranium ore under the definition of the World Nuclear Association (WNA) (Haneklaus et al. 2017a). Mwalongo et al. (2023a) recently pointed out that the uranium concentration in Minjingu is indeed higher than at commercial uranium mines in Namibia on the other side of the African continent. Since fertilizer is more important for Tanzania's economy than uranium, the ore is mined for its elevated P₂O₅ content (20–25 % on average) and not its elevated uranium content (0.03–0.04 % on average).

Previously, some 10–15 years ago, Minjingu phosphate ore was after simple beneficiation that included sorting, sieving and rapid drying at 700–800 °C, applied directly as fine fertilizer powder on agricultural soils in East Africa. The fertilizer produced this way proved to be effective on the acidic soils found in East Africa, and the material was considerably less expensive than imported fertilizers from abroad. It was therefore historically not only used in Tanzania, but also in Kenya, Uganda, Burundi, Zambia and even South Africa as indicated in Fig. 1 (Mkangwa, 2003; Msolla et al., 2005; Nabahungu et al., 2007; Ndeleko-Barasa et al., 2021; Nyambati and Opola, 2014; Sarini et al., 2015;

<figure><img src="image_5.png" /><figcaption>Fig. 1. Location of the Minjingu phosphate rock deposit in northern Tanzania as well as countries to which Minjingu phosphate ore products have historically been exported to.</figcaption></figure>
Szilas et al., 2007a,b, 2008). In all these regions similar acidic soils can be found.

15 years ago, all fertilizer powder has been further processed into granules, which constituted a tremendous improvement since the fertilizer produced this way can be distributed much better by hand on agricultural soils. In the last decade, the granules were further blended with urea, a source of nitrogen, to produce different kinds of compound fertilizers that show a similar agronomic response in East Africa if compared to that of DAP fertilizers (Benson et al., 2012).

At present, the fertilizer plant at the Minjingu mine produces some 50,000 t of fertilizer per year that is distributed to agro-dealers in Tanzania (Rutsaert et al., 2021). The Minjingu Mine and Fertilizer Ltd. intends to double production with a new rotary dryer for mechanical drying of the mined phosphate ore, as well as a larger granulation and blending plant (currently erected) in the next 1-2 years. Even after the Minjingu fertilizer plant extension, the combined output of 100,000 t of fertilizer per year will be far from covering the fertilizer need of Tanzania, which easily exceeds 400,000 t per year right now and may double over the next 5-10 years given the still relatively low crop yields in the country (Harou et al., 2022; Michelson et al., 2021).

Currently, Minjingu products, which are produced locally, receive substantial subsidies in Tanzania. As a result, they are predominantly utilized within the country, with only limited quantities being exported. It is worth noting that agriculture plays a significant role in Tanzania's economy, contributing to over 25 % of the GDP and employing about 75 % of the workforce. A major ammonia and urea fertilizer facility is under construction in the southern Tanzanian port town of Mtwara, aiming to offer more cost-effective fertilizers to local farmers in East Africa. Furthermore, there are several promising studies exploring the utilization of organic fertilizers like goat manure (Mhagama et al., 2023).

Due to the relatively high uranium concentrations at the Minjingu deposit (0.03–0.04 % on average) that are similar to the concentrations found at commercial uranium mining projects, such as the Manyoni Uranium Project (0.011–0.013 % uranium) (Loila et al., 2022) and the Mkuju River Project (0.026 % uranium) (Uranium One, 2023) in Tanzania, an economic case (pull-factor) can also be made for uranium recovery in Minjingu despite the relatively small phosphate ore processing operation of approximately 100,000 t per year. Large WPA units process 2–3 million t of phosphate ore per year, and often multiple units make up a whole phosphate fertilizer plant, as is, for instance, the case in Jorf Lasfar (Morocco), the largest WPA complex in the world (Arhouni et al., 2023). Given the average uranium concentration of 0.03–0.04 % or 300–400 ppm (parts per million), theoretically 30–40 t of uranium could be recovered per year at the Minjingu fertilizer plant, assuming no losses. Given a uranium price of USD 50 per lb of $U_3O_8$, some USD 3.9–5.2 million in revenue could theoretically be generated annually from selling co-recovered uranium during Minjingu fertilizer production.

The amount of co-produced uranium is obviously tiny in comparison to commercial uranium mines, such as the Mkuju River project in southern Tanzania, which, if started as planned, is expected to produce some 1600 t of uranium annually (Rweyemamu and Kim, 2020). It is noteworthy though, that a potential uranium recovery at the Minjingu fertilizer plant, will ideally result in a cleaner fertilizer product, and does not come with the potential environmental pollution that is presently discussed for in-situ leach (ISL) and open pit uranium mining in Tanzania (Banzi et al., 2015; Loila et al., 2022; Rweyemamu and Kim, 2020). It is further noteworthy that due to the ongoing uranium exploration and mining operations, Tanzania already has a regulatory framework for uranium production in place (Kimaro and Mdoe, 2018; Winde et al., 2017) that could ease the way for byproduct uranium recovery at the Minjingu deposit.

The (literally) million-dollar question is, what are realistic average uranium concentrations? What are realistic uranium recovery rates? Do the final fertilizer products still work as effectively as they do now? And are there other valuable minerals, such as REEs, present in sufficient

concentrations so that they could also be co-extracted and sold as well?

This review aims to provide an overview of what is presently known about the REEs and uranium concentrations at the Minjingu deposit and discuss for the first time current and potential alternative processing pathways to answer as many of the previously raised questions as possible. Section 2 of this review provides an overview of the work published on heavy metals in Minjingu phosphate ore. To the best of our knowledge, the REE content of Minjingu ore has not been systematically analyzed yet, and we did our own analysis which is presented here to fill this gap in the literature. The sampling and the methods used for the sample analysis are presented in Section 3. Section 4 presents and discusses the results of the sampling campaign and how the results may support alternative processing pathways for the Minjingu ore. Section 5 presents the conclusions of this review, discusses the limitations of the presented work, and offers policy recommendations based on the findings of this study.

## 2. Literature review

Radioactivity measurements can be conducted with relatively simple measurement equipment, while determining REEs and other trace elements in phosphate ores is more challenging and requires more sophisticated machines. Usually inductively coupled plasma mass spectrometry (ICP-MS) is used. We believe that this, and the fact that REEs are less relevant for Tanzania's economy, which is largely based on agricultural products, are the reasons why there are a relevant number of studies reporting the uranium content and radioactivity measurements of Minjingu phosphate ore, but we were not able to find a study that systematically analyzed the REE content of the material. There are further considerable differences in the reported uranium content. Interviews with researchers from the Tanzania Atomic Energy Commission (TAEC) who have been monitoring the Minjingu mine for more than 20 years and mining engineers of Minjingu Mining and Fertilizer Ltd. suggest that there is a large difference in the radioactivity of the Minjingu ore depending on the location and depths at which ore samples can be drawn, so that all measurements reported in this literature review, though different, seem indeed to be correct, but simply report on samples taken from different locations of the Minjingu deposit.

Bianconi (1987) documented that Minjingu ore contained a maximum uranium content of 680 ppm, or 0.068 %, when the material was still being transported to the Tanga WPA fertilizer plant for processing. This relatively high uranium concentration in comparison to other phosphate ore deposits was subsequently validated by Meza et al. (2015), who examined a total of 45 Minjingu phosphate ore samples from various locations and identified a peak uranium concentration of 650 ppm. It is noteworthy that, aside from one sample exhibiting this peak uranium concentration, all other Minjingu phosphate ore samples that were investigated showed uranium concentrations half of that value.

There is now no legislative restriction on the amount of uranium allowed in fertilizers while such limits exist for cadmium (Samrane et al., 2023; Ulrich, 2019). According to Kratz et al. (2016), the German Commission for the Protection of Soils recommended setting a legal limit of 50 mg of uranium per kilogram of $P_2O_5$ or 167 ppm for fertilizers containing 30 % $P_2O_5$. Similarly high values of 9,550 Bq kg$^{-1}$ $^{238}$U or 767 ppm eU (uranium equivalent) were reported by Mustonen and Annanmaki (1988) for the topmost phosphate ore layer, while a lower $^{226}$Ra activity concentration of 2,850 Bq kg$^{-1}$ or 232 ppm eU was noted for the lower phosphate ore layer. These results support Bianconi's earlier assessment.

Prior to the Tanga fertilizer plant's activities ceasing in the early 1990s, Makweba and Holm (1993) examined one ground phosphate ore sample, two fertilizer (triple superphosphate and single superphosphate) samples, and phosphogypsum samples produced at the plant utilizing Minjingu ore. Using gamma-ray spectrometry and alpha-spectrometry, the researchers observed ore concentrations of 337
Szilas et al., 2007a,b, 2008). In all these regions similar acidic soils can be found.

15 years ago, all fertilizer powder has been further processed into granules, which constituted a tremendous improvement since the fertilizer produced this way can be distributed much better by hand on agricultural soils. In the last decade, the granules were further blended with urea, a source of nitrogen, to produce different kinds of compound fertilizers that show a similar agronomic response in East Africa if compared to that of DAP fertilizers (Benson et al., 2012).

At present, the fertilizer plant at the Minjingu mine produces some 50,000 t of fertilizer per year that is distributed to agro-dealers in Tanzania (Rutsaert et al., 2021). The Minjingu Mine and Fertilizer Ltd. intends to double production with a new rotary dryer for mechanical drying of the mined phosphate ore, as well as a larger granulation and blending plant (currently erected) in the next 1-2 years. Even after the Minjingu fertilizer plant extension, the combined output of 100,000 t of fertilizer per year will be far from covering the fertilizer need of Tanzania, which easily exceeds 400,000 t per year right now and may double over the next 5-10 years given the still relatively low crop yields in the country (Harou et al., 2022; Michelson et al., 2021).

Currently, Minjingu products, which are produced locally, receive substantial subsidies in Tanzania. As a result, they are predominantly utilized within the country, with only limited quantities being exported. It is worth noting that agriculture plays a significant role in Tanzania's economy, contributing to over 25 % of the GDP and employing about 75 % of the workforce. A major ammonia and urea fertilizer facility is under construction in the southern Tanzanian port town of Mtwara, aiming to offer more cost-effective fertilizers to local farmers in East Africa. Furthermore, there are several promising studies exploring the utilization of organic fertilizers like goat manure (Mhagama et al., 2023).

Due to the relatively high uranium concentrations at the Minjingu deposit (0.03–0.04 % on average) that are similar to the concentrations found at commercial uranium mining projects, such as the Manyoni Uranium Project (0.011–0.013 % uranium) (Loila et al., 2022) and the Mkuju River Project (0.026 % uranium) (Uranium One, 2023) in Tanzania, an economic case (pull-factor) can also be made for uranium recovery in Minjingu despite the relatively small phosphate ore processing operation of approximately 100,000 t per year. Large WPA units process 2–3 million t of phosphate ore per year, and often multiple units make up a whole phosphate fertilizer plant, as is, for instance, the case in Jorf Lasfar (Morocco), the largest WPA complex in the world (Arhouni et al., 2023). Given the average uranium concentration of 0.03–0.04 % or 300–400 ppm (parts per million), theoretically 30–40 t of uranium could be recovered per year at the Minjingu fertilizer plant, assuming no losses. Given a uranium price of USD 50 per lb of $U_3O_8$, some USD 3.9–5.2 million in revenue could theoretically be generated annually from selling co-recovered uranium during Minjingu fertilizer production.

The amount of co-produced uranium is obviously tiny in comparison to commercial uranium mines, such as the Mkuju River project in southern Tanzania, which, if started as planned, is expected to produce some 1600 t of uranium annually (Rweyemamu and Kim, 2020). It is noteworthy though, that a potential uranium recovery at the Minjingu fertilizer plant, will ideally result in a cleaner fertilizer product, and does not come with the potential environmental pollution that is presently discussed for in-situ leach (ISL) and open pit uranium mining in Tanzania (Banzi et al., 2015; Loila et al., 2022; Rweyemamu and Kim, 2020). It is further noteworthy that due to the ongoing uranium exploration and mining operations, Tanzania already has a regulatory framework for uranium production in place (Kimaro and Mdoe, 2018; Winde et al., 2017) that could ease the way for byproduct uranium recovery at the Minjingu deposit.

The (literally) million-dollar question is, what are realistic average uranium concentrations? What are realistic uranium recovery rates? Do the final fertilizer products still work as effectively as they do now? And are there other valuable minerals, such as REEs, present in sufficient

concentrations so that they could also be co-extracted and sold as well?

This review aims to provide an overview of what is presently known about the REEs and uranium concentrations at the Minjingu deposit and discuss for the first time current and potential alternative processing pathways to answer as many of the previously raised questions as possible. Section 2 of this review provides an overview of the work published on heavy metals in Minjingu phosphate ore. To the best of our knowledge, the REE content of Minjingu ore has not been systematically analyzed yet, and we did our own analysis which is presented here to fill this gap in the literature. The sampling and the methods used for the sample analysis are presented in Section 3. Section 4 presents and discusses the results of the sampling campaign and how the results may support alternative processing pathways for the Minjingu ore. Section 5 presents the conclusions of this review, discusses the limitations of the presented work, and offers policy recommendations based on the findings of this study.

## 2. Literature review

Radioactivity measurements can be conducted with relatively simple measurement equipment, while determining REEs and other trace elements in phosphate ores is more challenging and requires more sophisticated machines. Usually inductively coupled plasma mass spectrometry (ICP-MS) is used. We believe that this, and the fact that REEs are less relevant for Tanzania's economy, which is largely based on agricultural products, are the reasons why there are a relevant number of studies reporting the uranium content and radioactivity measurements of Minjingu phosphate ore, but we were not able to find a study that systematically analyzed the REE content of the material. There are further considerable differences in the reported uranium content. Interviews with researchers from the Tanzania Atomic Energy Commission (TAEC) who have been monitoring the Minjingu mine for more than 20 years and mining engineers of Minjingu Mining and Fertilizer Ltd. suggest that there is a large difference in the radioactivity of the Minjingu ore depending on the location and depths at which ore samples can be drawn, so that all measurements reported in this literature review, though different, seem indeed to be correct, but simply report on samples taken from different locations of the Minjingu deposit.

Bianconi (1987) documented that Minjingu ore contained a maximum uranium content of 680 ppm, or 0.068 %, when the material was still being transported to the Tanga WPA fertilizer plant for processing. This relatively high uranium concentration in comparison to other phosphate ore deposits was subsequently validated by Meza et al. (2015), who examined a total of 45 Minjingu phosphate ore samples from various locations and identified a peak uranium concentration of 650 ppm. It is noteworthy that, aside from one sample exhibiting this peak uranium concentration, all other Minjingu phosphate ore samples that were investigated showed uranium concentrations half of that value.

There is now no legislative restriction on the amount of uranium allowed in fertilizers while such limits exist for cadmium (Samrane et al., 2023; Ulrich, 2019). According to Kratz et al. (2016), the German Commission for the Protection of Soils recommended setting a legal limit of 50 mg of uranium per kilogram of $P_2O_5$ or 167 ppm for fertilizers containing 30 % $P_2O_5$. Similarly high values of 9,550 Bq kg$^{-1}$ $^{238}$U or 767 ppm eU (uranium equivalent) were reported by Mustonen and Annanmaki (1988) for the topmost phosphate ore layer, while a lower $^{226}$Ra activity concentration of 2,850 Bq kg$^{-1}$ or 232 ppm eU was noted for the lower phosphate ore layer. These results support Bianconi's earlier assessment.

Prior to the Tanga fertilizer plant's activities ceasing in the early 1990s, Makweba and Holm (1993) examined one ground phosphate ore sample, two fertilizer (triple superphosphate and single superphosphate) samples, and phosphogypsum samples produced at the plant utilizing Minjingu ore. Using gamma-ray spectrometry and alpha-spectrometry, the researchers observed ore concentrations of 337
and 377 ppm eU and 408 and 481 ppm eU, respectively. Further observations of the radioactivity of surface water, phosphate ore, mine tailings, leaf vegetation, calf meat, poultry feed, and leaf vegetation, as well as background radiation measurements near the Minjingu mine were provided by Banzi et al. (2000). The estimated uranium concentrations for mine tailings and phosphate ores were 4,250 Bq kg⁻¹ ²²⁶Ra and 5,760 Bq kg⁻¹ ²²⁶Ra, or 468 ppm eU and 346 ppm eU, respectively. The study explored exposure pathways and risks to the local population. Upon publication, this research even led to a brief suspension of Minjingu phosphate ore processing, before it became evident that not having access to inexpensive domestically produced fertilizer is also an unsatisfable solution.

Another relevant study is the work by Semu and Singh (1995) that investigated the long-term accumulation of heavy metals (Cd, Zn, Mn, Cu, Ni, and Pb) in soils and plants after the use of Minjingu phosphate rock. The authors did not consider uranium but found that the relatively low levels of cadmium in Minjingu phosphate ore are an advantage compared to other fertilizers that can show higher cadmium concentrations.

Koleleni and Tafisa (2019) further investigated different vegetables and soil samples taken near Minjingu village using wavelength dispersive x-ray fluorescence. The study did not report the uranium content of the examined samples, but recommended the reduced consumption of several vegetables as a result of relatively high (other than uranium) heavy metal concentrations.

More recently, Mwalongo et al. (2023a) analyzed phosphate ores from Burundi, Kenya, Tanzania, and Uganda, as well as fertilizer products sold in these countries as well as Rwanda. Minjingu phosphate ore was reported to have a uranium content of 446 ppm, and fertilizers produced from Minjingu phosphate ore generally showed the highest concentrations of uranium. In the next step, Mwalongo et al. (2023b) investigated the influence of NPK fertilizers with varying uranium content on the radioactivity of tobacco plants in Kenya, Tanzania, and Uganda (all countries are major tobacco producers in East Africa). Uranium concentrations in Minjingu phosphate ore were not reported, but it was reported that Minjingu phosphate ore products (fertilizers produced with Minjingu phosphate ore) contain relatively high uranium concentrations if compared to other fertilizers, and the tobacco plants grown with these fertilizers also showed slightly higher levels of radioactivity (still within allowed limits) than tobacco plants grown with fertilizers that show lower levels of uranium. More investigations on the heavy metal content of Minjingu ore derived fertilizers were further recommended by Lisuma et al. (2022). The relatively high concentrations of uranium in Minjingu phosphate ore, if compared to other phosphate ores was further confirmed in a recent meta-analysis (Mwalongo et al., 2024).

Naturally occurring radioactivity in Minjingu phosphate ore is not only relevant for the fertilizers that are produced with this ore, but also affects the tailings of the fertilizer plants that need to be managed. Mdachi et al. (2024) recently assessed the Cu, Zn, Al, Mn, Ni, Fe, Pb and As concentrations as well as the radioactivity of Minjingu processing plant's tailings concluding that the radioactivity is above recommended levels, while heavy metals are of little concern.

The fact that there has been no recent systematic analysis of the present uranium and REEs content in Minjingu ore and related products led to this study, that for the first time aims to shed light on the unique composition of Minjingu ore as well as its products and discuss potential cleaner production pathways.

### 3. Sampling and sample analysis

Samples from 10 Minjingu phosphate ore layers, 4 Minjingu mine tailings, and 5 Minjingu fertilizer products were analyzed for rare earths and uranium contents using ICP-MS. In addition, Minjingu fertilizer powder was analyzed using X-ray fluorescence (XRF).

Globally, most (>90 %) phosphate ore is processed using the WPA

process, during which the majority (>80 %) of the uranium naturally occurring in the phosphate ore transfers to the liquid WPA, from where it can be recovered using industrially proven solvent extraction processes (Beltrami et al., 2014; Singh et al., 2016), while the majority of REEs (>80 %) transfer to the solid phosphogypsum fraction, from which recovery is economically challenging (Bilal et al., 2023; Rutherford et al., 1994; Santos et al., 2006).

The Minjingu phosphate ore undergoes a distinctive processing method illustrated in Fig. 2 that is different from the popularly used WPA process. Following mining, the ore undergoes a concentration process where unwanted materials are manually removed through handpicking. The remaining material is currently spread on the ground for natural sun drying (Fig. 2b). Efforts are underway to implement a coal-fired rotary dryer, allowing for ore drying regardless of weather conditions (Fig. 2c). After two to three weeks of sun drying, the dried ore is fed into an impact crusher, preliminarily sieved, and then passed into a hot air furnace where it is heated to temperatures ranging from 700 to 800 °C (Fig. 2d). The dried ore undergoes another round of sieving before entering gravity classification. This beneficiation process stands out in comparison to other thermal methods employed for phosphate ore concentration (Abouzeid, 2008; Ruan et al., 2019).

Fertilizer powder, already rich in P₂O₅ content, is extracted after final sieving, as indicated in Fig. 2e. Following gravity classification, the fertilizer powder is transported by truck to a granulation plant where it is transformed into fertilizer granules. Finally, these granules are blended to create the final fertilizer products, which are then packaged and transported by truck to agro-traders for final distribution. Fig. 2 offers a concise overview of the Minjingu phosphate ore processing flow, with the green boxes indicating the points where samples were gathered for this study. Samples were obtained from the phosphate ore (prior to handpicking) (see Fig. 2a), the mine tailings (see Fig. 2d), the fertilizer powder (see Fig. 2e), the fertilizer granules, and three final fertilizer products (see Fig. 2f).

Ten 100 g dry samples from each of the nine phosphate ore layers as well as surface samples were collected, crushed, and powdered before being dried in an oven at 100°C for 24 h to a consistent weight. These materials were then homogenized in a pulverizer after being sieved using 150 µm standard sieves. For each sample, the pulverizer was run for 30 min at a speed of 150 rpm. The pulverizer used three spherical balls with a radius of 3 mm each. 50 g of each typical sample (L0-L9) were taken after homogenization and submitted to Morocco's National Centre for Energy and Nuclear Science and Technology (CNESTEN) for ICP-MS analysis.

The materials were digested at CNESTEN using a microwave SPEEDWAVE4 (Berghof Products + Instruments GmbH, Eningen unter Achalm, Germany) and a solution of 3 mL HNO₃ (60 %) and 5 mL HF (40 %) (Merck, Darmstadt, Germany). After cooling, the liquid was transferred to 50 ml flasks and filled up with high quality water until the appropriate volume was reached. Thermo Fischer Scientific's (Waltham, USA) XSERIES 2 ICP-MS was used to conduct the measurements. The spectrometer was tuned to produce the lowest CeO²/Ce⁺ and Ba₂⁺/Ba⁺ ratios as well as the best analysis density. The National Institute of Standards and Techniques in Gaithersburg, Maryland, USA, provided "Western Rock Phosphate" to standardize the correlation coefficient for all calibration curves. Single-component CertiPrep SPEX solutions and lanthanoids from the ASTASOL combination (AN 9088 (MN)) were used for external calibration.

In the same way, two tailing samples and two surrounding soil samples (T1-T4) were prepared and analyzed. Samples from the fertilizer powder, fertilizer granules and final fertilizer products (F1-5) were digested directly and analyzed using ICP-MS. In addition, XRF analysis was conducted for the fertilizer powder (F1) at the Institute of Chemical Technology (ICT) in Mumbai, India.
and 377 ppm eU and 408 and 481 ppm eU, respectively. Further observations of the radioactivity of surface water, phosphate ore, mine tailings, leaf vegetation, calf meat, poultry feed, and leaf vegetation, as well as background radiation measurements near the Minjingu mine were provided by Banzi et al. (2000). The estimated uranium concentrations for mine tailings and phosphate ores were 4,250 Bq kg⁻¹ ²²⁶Ra and 5,760 Bq kg⁻¹ ²²⁶Ra, or 468 ppm eU and 346 ppm eU, respectively. The study explored exposure pathways and risks to the local population. Upon publication, this research even led to a brief suspension of Minjingu phosphate ore processing, before it became evident that not having access to inexpensive domestically produced fertilizer is also an unsatisfable solution.

Another relevant study is the work by Semu and Singh (1995) that investigated the long-term accumulation of heavy metals (Cd, Zn, Mn, Cu, Ni, and Pb) in soils and plants after the use of Minjingu phosphate rock. The authors did not consider uranium but found that the relatively low levels of cadmium in Minjingu phosphate ore are an advantage compared to other fertilizers that can show higher cadmium concentrations.

Koleleni and Tafisa (2019) further investigated different vegetables and soil samples taken near Minjingu village using wavelength dispersive x-ray fluorescence. The study did not report the uranium content of the examined samples, but recommended the reduced consumption of several vegetables as a result of relatively high (other than uranium) heavy metal concentrations.

More recently, Mwalongo et al. (2023a) analyzed phosphate ores from Burundi, Kenya, Tanzania, and Uganda, as well as fertilizer products sold in these countries as well as Rwanda. Minjingu phosphate ore was reported to have a uranium content of 446 ppm, and fertilizers produced from Minjingu phosphate ore generally showed the highest concentrations of uranium. In the next step, Mwalongo et al. (2023b) investigated the influence of NPK fertilizers with varying uranium content on the radioactivity of tobacco plants in Kenya, Tanzania, and Uganda (all countries are major tobacco producers in East Africa). Uranium concentrations in Minjingu phosphate ore were not reported, but it was reported that Minjingu phosphate ore products (fertilizers produced with Minjingu phosphate ore) contain relatively high uranium concentrations if compared to other fertilizers, and the tobacco plants grown with these fertilizers also showed slightly higher levels of radioactivity (still within allowed limits) than tobacco plants grown with fertilizers that show lower levels of uranium. More investigations on the heavy metal content of Minjingu ore derived fertilizers were further recommended by Lisuma et al. (2022). The relatively high concentrations of uranium in Minjingu phosphate ore, if compared to other phosphate ores was further confirmed in a recent meta-analysis (Mwalongo et al., 2024).

Naturally occurring radioactivity in Minjingu phosphate ore is not only relevant for the fertilizers that are produced with this ore, but also affects the tailings of the fertilizer plants that need to be managed. Mdachi et al. (2024) recently assessed the Cu, Zn, Al, Mn, Ni, Fe, Pb and As concentrations as well as the radioactivity of Minjingu processing plant's tailings concluding that the radioactivity is above recommended levels, while heavy metals are of little concern.

The fact that there has been no recent systematic analysis of the present uranium and REEs content in Minjingu ore and related products led to this study, that for the first time aims to shed light on the unique composition of Minjingu ore as well as its products and discuss potential cleaner production pathways.

### 3. Sampling and sample analysis

Samples from 10 Minjingu phosphate ore layers, 4 Minjingu mine tailings, and 5 Minjingu fertilizer products were analyzed for rare earths and uranium contents using ICP-MS. In addition, Minjingu fertilizer powder was analyzed using X-ray fluorescence (XRF).

Globally, most (>90 %) phosphate ore is processed using the WPA

process, during which the majority (>80 %) of the uranium naturally occurring in the phosphate ore transfers to the liquid WPA, from where it can be recovered using industrially proven solvent extraction processes (Beltrami et al., 2014; Singh et al., 2016), while the majority of REEs (>80 %) transfer to the solid phosphogypsum fraction, from which recovery is economically challenging (Bilal et al., 2023; Rutherford et al., 1994; Santos et al., 2006).

The Minjingu phosphate ore undergoes a distinctive processing method illustrated in Fig. 2 that is different from the popularly used WPA process. Following mining, the ore undergoes a concentration process where unwanted materials are manually removed through handpicking. The remaining material is currently spread on the ground for natural sun drying (Fig. 2b). Efforts are underway to implement a coal-fired rotary dryer, allowing for ore drying regardless of weather conditions (Fig. 2c). After two to three weeks of sun drying, the dried ore is fed into an impact crusher, preliminarily sieved, and then passed into a hot air furnace where it is heated to temperatures ranging from 700 to 800 °C (Fig. 2d). The dried ore undergoes another round of sieving before entering gravity classification. This beneficiation process stands out in comparison to other thermal methods employed for phosphate ore concentration (Abouzeid, 2008; Ruan et al., 2019).

Fertilizer powder, already rich in P₂O₅ content, is extracted after final sieving, as indicated in Fig. 2e. Following gravity classification, the fertilizer powder is transported by truck to a granulation plant where it is transformed into fertilizer granules. Finally, these granules are blended to create the final fertilizer products, which are then packaged and transported by truck to agro-traders for final distribution. Fig. 2 offers a concise overview of the Minjingu phosphate ore processing flow, with the green boxes indicating the points where samples were gathered for this study. Samples were obtained from the phosphate ore (prior to handpicking) (see Fig. 2a), the mine tailings (see Fig. 2d), the fertilizer powder (see Fig. 2e), the fertilizer granules, and three final fertilizer products (see Fig. 2f).

Ten 100 g dry samples from each of the nine phosphate ore layers as well as surface samples were collected, crushed, and powdered before being dried in an oven at 100°C for 24 h to a consistent weight. These materials were then homogenized in a pulverizer after being sieved using 150 µm standard sieves. For each sample, the pulverizer was run for 30 min at a speed of 150 rpm. The pulverizer used three spherical balls with a radius of 3 mm each. 50 g of each typical sample (L0-L9) were taken after homogenization and submitted to Morocco's National Centre for Energy and Nuclear Science and Technology (CNESTEN) for ICP-MS analysis.

The materials were digested at CNESTEN using a microwave SPEEDWAVE4 (Berghof Products + Instruments GmbH, Eningen unter Achalm, Germany) and a solution of 3 mL HNO₃ (60 %) and 5 mL HF (40 %) (Merck, Darmstadt, Germany). After cooling, the liquid was transferred to 50 ml flasks and filled up with high quality water until the appropriate volume was reached. Thermo Fischer Scientific's (Waltham, USA) XSERIES 2 ICP-MS was used to conduct the measurements. The spectrometer was tuned to produce the lowest CeO²/Ce⁺ and Ba₂⁺/Ba⁺ ratios as well as the best analysis density. The National Institute of Standards and Techniques in Gaithersburg, Maryland, USA, provided "Western Rock Phosphate" to standardize the correlation coefficient for all calibration curves. Single-component CertiPrep SPEX solutions and lanthanoids from the ASTASOL combination (AN 9088 (MN)) were used for external calibration.

In the same way, two tailing samples and two surrounding soil samples (T1-T4) were prepared and analyzed. Samples from the fertilizer powder, fertilizer granules and final fertilizer products (F1-5) were digested directly and analyzed using ICP-MS. In addition, XRF analysis was conducted for the fertilizer powder (F1) at the Institute of Chemical Technology (ICT) in Mumbai, India.
<figure><img src="image_4.png" /><figcaption>Fig. 2. Overview of Minjingu phosphate ore processing and sample selection.</figcaption></figure>

# 4. Results and discussion

## 4.1. Rare earths concentrations

Table 1 shows the REE concentrations in Minjingu phosphate ore (L0-L9), surrounding soil (T1-T2), mine tailings (T3-T4) and fertilizer products (F1–F5) analyzed for this work. With regards to the REE concentration in Minjingu phosphate ore, considerable differences can be observed between the different rock layers from which samples were drawn (see details in Fig. 2). The fact that different rock layers show varying concentrations of elements is not unusual, and at the Minjingu

mine, only soft phosphate rock layers L2, L4, and L7 are presently mined and further processed. With the foreseen fertilizer plant extension, it is also planned to mine hard phosphate rock layers that can show even higher P₂O₅ concentrations (>25 %) than the presently mined soft phosphate rock layers (usual P₂O₅ concentrations of 20–25 %). Even the highest concentrations of REEs found in layer L7 (614 ppm) are not exceptionally high if compared to REE concentrations found in other major phosphate rock deposits (see Fig. 3). Measured average REE concentrations of 236 ppm in the investigated rock layers, and average REE concentrations of 374 ppm in the presently mined phosphate ore layers do not encourage the commercial recovery of REEs. The average
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Overview of Minjingu phosphate ore processing and sample selection.</figcaption></figure>

# 4. Results and discussion

## 4.1. Rare earths concentrations

Table 1 shows the REE concentrations in Minjingu phosphate ore (L0-L9), surrounding soil (T1-T2), mine tailings (T3-T4) and fertilizer products (F1–F5) analyzed for this work. With regards to the REE concentration in Minjingu phosphate ore, considerable differences can be observed between the different rock layers from which samples were drawn (see details in Fig. 2). The fact that different rock layers show varying concentrations of elements is not unusual, and at the Minjingu

mine, only soft phosphate rock layers L2, L4, and L7 are presently mined and further processed. With the foreseen fertilizer plant extension, it is also planned to mine hard phosphate rock layers that can show even higher P₂O₅ concentrations (>25 %) than the presently mined soft phosphate rock layers (usual P₂O₅ concentrations of 20–25 %). Even the highest concentrations of REEs found in layer L7 (614 ppm) are not exceptionally high if compared to REE concentrations found in other major phosphate rock deposits (see Fig. 3). Measured average REE concentrations of 236 ppm in the investigated rock layers, and average REE concentrations of 374 ppm in the presently mined phosphate ore layers do not encourage the commercial recovery of REEs. The average
<caption>**Table 1**</caption>

Rare earth element (REE) concentration in Minjingu phosphate ore, mine tailings, and fertilizer products measured in this study.

<table><thead><tr><th rowspan="2"></th><th colspan="10">Minjingu phosphate ore samples</th><th colspan="4">Tailings</th><th colspan="5">Fertilizer products</th></tr><tr><th>L0</th><th>L1</th><th>L2</th><th>L3</th><th>L4</th><th>L5</th><th>L6</th><th>L7</th><th>L8</th><th>L9</th><th>T1</th><th>T2</th><th>T3</th><th>T4</th><th>F1</th><th>F2</th><th>F3</th><th>F4</th><th>F5</th></tr></thead><tbody><tr><td>Y</td><td>22</td><td>5</td><td>1</td><td>7</td><td>20</td><td>21</td><td>27</td><td>144</td><td>10</td><td>31</td><td>21</td><td>51</td><td>10</td><td>10</td><td>13</td><td>27</td><td>11</td><td>13</td><td>46</td></tr><tr><td>La</td><td>23</td><td>8</td><td>1</td><td>31</td><td>12</td><td>4</td><td>71</td><td>52</td><td>15</td><td>14</td><td>36</td><td>100</td><td>15</td><td>14</td><td>13</td><td>20</td><td>9</td><td>11</td><td>33</td></tr><tr><td>Ce</td><td>54</td><td>22</td><td>4</td><td>77</td><td>30</td><td>12</td><td>143</td><td>86</td><td>41</td><td>37</td><td>82</td><td>236</td><td>42</td><td>38</td><td>35</td><td>53</td><td>26</td><td>33</td><td>92</td></tr><tr><td>Pr</td><td>6</td><td>2</td><td>0</td><td>8</td><td>3</td><td>1</td><td>23</td><td>13</td><td>4</td><td>4</td><td>8</td><td>31</td><td>4</td><td>3</td><td>3</td><td>5</td><td>2</td><td>3</td><td>8</td></tr><tr><td>Nd</td><td>41</td><td>19</td><td>3</td><td>56</td><td>22</td><td>10</td><td>176</td><td>95</td><td>31</td><td>29</td><td>60</td><td>219</td><td>28</td><td>25</td><td>24</td><td>35</td><td>19</td><td>22</td><td>61</td></tr><tr><td>Sm</td><td>9</td><td>4</td><td>1</td><td>10</td><td>5</td><td>2</td><td>35</td><td>20</td><td>6</td><td>7</td><td>12</td><td>41</td><td>5</td><td>5</td><td>5</td><td>7</td><td>4</td><td>5</td><td>13</td></tr><tr><td>Eu</td><td>2</td><td>1</td><td>0</td><td>2</td><td>1</td><td>1</td><td>6</td><td>4</td><td>1</td><td>1</td><td>2</td><td>7</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>3</td></tr><tr><td>Gd</td><td>9</td><td>5</td><td>4</td><td>9</td><td>7</td><td>4</td><td>28</td><td>22</td><td>6</td><td>9</td><td>12</td><td>38</td><td>9</td><td>6</td><td>6</td><td>8</td><td>5</td><td>6</td><td>14</td></tr><tr><td>Tb</td><td>1</td><td>0</td><td>0</td><td>1</td><td>1</td><td>0</td><td>2</td><td>2</td><td>0</td><td>1</td><td>1</td><td>3</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Dy</td><td>8</td><td>4</td><td>1</td><td>5</td><td>7</td><td>6</td><td>15</td><td>43</td><td>4</td><td>9</td><td>9</td><td>26</td><td>5</td><td>4</td><td>5</td><td>8</td><td>4</td><td>5</td><td>15</td></tr><tr><td>Ho</td><td>1</td><td>1</td><td>0</td><td>1</td><td>1</td><td>1</td><td>1</td><td>9</td><td>0</td><td>1</td><td>1</td><td>3</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>2</td></tr><tr><td>Er</td><td>5</td><td>4</td><td>1</td><td>2</td><td>5</td><td>6</td><td>5</td><td>57</td><td>2</td><td>9</td><td>6</td><td>12</td><td>3</td><td>3</td><td>4</td><td>7</td><td>3</td><td>4</td><td>11</td></tr><tr><td>Tm</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>6</td><td>0</td><td>1</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Yb</td><td>6</td><td>4</td><td>1</td><td>1</td><td>4</td><td>5</td><td>3</td><td>58</td><td>3</td><td>12</td><td>5</td><td>10</td><td>3</td><td>3</td><td>4</td><td>8</td><td>4</td><td>4</td><td>13</td></tr><tr><td>Lu</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>5</td><td>0</td><td>1</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td><strong>SUM</strong></td><td><strong>188</strong></td><td><strong>79</strong></td><td><strong>15</strong></td><td><strong>209</strong></td><td><strong>118</strong></td><td><strong>74</strong></td><td><strong>537</strong></td><td><strong>614</strong></td><td><strong>125</strong></td><td><strong>166</strong></td><td><strong>256</strong></td><td><strong>776</strong></td><td><strong>127</strong></td><td><strong>113</strong></td><td><strong>115</strong></td><td><strong>184</strong></td><td><strong>91</strong></td><td><strong>108</strong></td><td><strong>315</strong></td></tr><tr><td><strong>Av.</strong></td><td><strong>236</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Rare earth element (REE) concentrations in different phosphate ores. Data sources: Abbes et al. (2020); Aly and Mohammed (1999); Awadalla (2010); Christmann (2014); De Oliveira and Imbernon (1998); El-Zrelli et al. (2021); Habashi (1985); Kandil et al. (2010); Khater et al. (2016); Nie et al. (2013); Orabi et al. (2018); Pavón et al. (2022); Ramos et al. (2016); Roshdy et al. (2023); Soudry et al. (2002); Wang et al. (2010); Wu et al. (2018a); Zaitsev et al. (2015); Zhang (2014); Zielinski et al. (1993).</figcaption></figure>

REE concentration in the mine tailings of 120 ppm is even lower and similarly does not encourage REE extraction, although it should be mentioned that this material is already mined, ground, and stacked so that it is easily accessible and a large portion of the costs for mining are already covered as illustrated by Reitsma et al. (2018). The 776 ppm measured in the overburden (T2) is slightly higher, but does not seem to be a good representation of the overall Minjingu deposit, nor do we expect to find this REE concentration in large amounts of material on site.

In Fig. 3, the REE concentrations found in the Minjingu phosphate ore are compared to the REE concentrations found in other phosphate ores. REE concentrations in phosphate ores vary but are usually not higher than 1 % or 10,000 ppm (Habashi, 1985), with the exception of the Catalão Minerochemical Complex in Brazil for which Ramos et al. (2016) report, supported by independent analysis from De Oliveira and Imbernon (1998), REE concentrations as high as 16,650 ppm in selected samples. This is highly unusual, and most phosphate ores globally (particularly since most phosphate ores are of sedimentary origin) show concentrations of approximately 0.05 % or 500 ppm.

It can be noted that the REE concentrations in Minjingu phosphate ore are not particularly high and will most likely not make REE recovery an economical option. Similar observations were made for the analyzed mine tailings as well as the fertilizer products. In some of the processes

for uranium recovery from phosphates, a large share of the REEs, or lanthanides, that are chemically very similar to the actinides, are coextracted together (Habashi et al., 1986; Wu et al., 2018b), so that if such a process is applied REEs in Minjingu phosphate ore might also become a target for recovery (see more discussion on this in Chapter 4.4).

It is noteworthy that besides REEs, other valuable trace elements could potentially be co-recovered, as pointed out by Chen and Graedel (2015). Most notably, we measured vanadium concentrations of 21–201 ppm in the investigated samples. This concentration is again relatively low if compared to the concentration of primary vanadium ores in Africa (Boni et al., 2023), but co-recovery could again be profitable as a large share of the costs for mining and processing is already covered through the sales of fertilizer products.

It is further noteworthy that there is a much lower push-factor for REE extraction from phosphate ores and phosphate fertilizers than there is for cadmium and uranium, with some fertilizers even being enriched with REEs in an attempt to make them work more effectively (Silva et al., 2019; Ribeiro et al., 2021; Tommasi et al., 2020). Furthermore, REE extraction from phosphates (usually REEs are recovered from phosphogypsum) was not successfully practiced on an industrial scale historically (Diwa et al., 2022; Ramirez et al., 2022), as it was the case with uranium recovery (from WPA), so there are considerably higher
<table><caption>**Table 1**
Rare earth element (REE) concentration in Minjingu phosphate ore, mine tailings, and fertilizer products measured in this study.</caption><thead><tr><th rowspan="2"></th><th colspan="10">Minjingu phosphate ore samples</th><th colspan="4">Tailings</th><th colspan="5">Fertilizer products</th></tr><tr><th>L0</th><th>L1</th><th>L2</th><th>L3</th><th>L4</th><th>L5</th><th>L6</th><th>L7</th><th>L8</th><th>L9</th><th>T1</th><th>T2</th><th>T3</th><th>T4</th><th>F1</th><th>F2</th><th>F3</th><th>F4</th><th>F5</th></tr></thead><tbody><tr><td>Y</td><td>22</td><td>5</td><td>1</td><td>7</td><td>20</td><td>21</td><td>27</td><td>144</td><td>10</td><td>31</td><td>21</td><td>51</td><td>10</td><td>10</td><td>13</td><td>27</td><td>11</td><td>13</td><td>46</td></tr><tr><td>La</td><td>23</td><td>8</td><td>1</td><td>31</td><td>12</td><td>4</td><td>71</td><td>52</td><td>15</td><td>14</td><td>36</td><td>100</td><td>15</td><td>14</td><td>13</td><td>20</td><td>9</td><td>11</td><td>33</td></tr><tr><td>Ce</td><td>54</td><td>22</td><td>4</td><td>77</td><td>30</td><td>12</td><td>143</td><td>86</td><td>41</td><td>37</td><td>82</td><td>236</td><td>42</td><td>38</td><td>35</td><td>53</td><td>26</td><td>33</td><td>92</td></tr><tr><td>Pr</td><td>6</td><td>2</td><td>0</td><td>8</td><td>3</td><td>1</td><td>23</td><td>13</td><td>4</td><td>4</td><td>8</td><td>31</td><td>4</td><td>3</td><td>3</td><td>5</td><td>2</td><td>3</td><td>8</td></tr><tr><td>Nd</td><td>41</td><td>19</td><td>3</td><td>56</td><td>22</td><td>10</td><td>176</td><td>95</td><td>31</td><td>29</td><td>60</td><td>219</td><td>28</td><td>25</td><td>24</td><td>35</td><td>19</td><td>22</td><td>61</td></tr><tr><td>Sm</td><td>9</td><td>4</td><td>1</td><td>10</td><td>5</td><td>2</td><td>35</td><td>20</td><td>6</td><td>7</td><td>12</td><td>41</td><td>5</td><td>5</td><td>5</td><td>7</td><td>4</td><td>5</td><td>13</td></tr><tr><td>Eu</td><td>2</td><td>1</td><td>0</td><td>2</td><td>1</td><td>1</td><td>6</td><td>4</td><td>1</td><td>1</td><td>2</td><td>7</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>3</td></tr><tr><td>Gd</td><td>9</td><td>5</td><td>4</td><td>9</td><td>7</td><td>4</td><td>28</td><td>22</td><td>6</td><td>9</td><td>12</td><td>38</td><td>9</td><td>6</td><td>6</td><td>8</td><td>5</td><td>6</td><td>14</td></tr><tr><td>Tb</td><td>1</td><td>0</td><td>0</td><td>1</td><td>1</td><td>0</td><td>2</td><td>2</td><td>0</td><td>1</td><td>1</td><td>3</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Dy</td><td>8</td><td>4</td><td>1</td><td>5</td><td>7</td><td>6</td><td>15</td><td>43</td><td>4</td><td>9</td><td>9</td><td>26</td><td>5</td><td>4</td><td>5</td><td>8</td><td>4</td><td>5</td><td>15</td></tr><tr><td>Ho</td><td>1</td><td>1</td><td>0</td><td>1</td><td>1</td><td>1</td><td>1</td><td>9</td><td>0</td><td>1</td><td>1</td><td>3</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>2</td></tr><tr><td>Er</td><td>5</td><td>4</td><td>1</td><td>2</td><td>5</td><td>6</td><td>5</td><td>57</td><td>2</td><td>9</td><td>6</td><td>12</td><td>3</td><td>3</td><td>4</td><td>7</td><td>3</td><td>4</td><td>11</td></tr><tr><td>Tm</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>6</td><td>0</td><td>1</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td>Yb</td><td>6</td><td>4</td><td>1</td><td>1</td><td>4</td><td>5</td><td>3</td><td>58</td><td>3</td><td>12</td><td>5</td><td>10</td><td>3</td><td>3</td><td>4</td><td>8</td><td>4</td><td>4</td><td>13</td></tr><tr><td>Lu</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>5</td><td>0</td><td>1</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td><strong>SUM</strong></td><td><strong>188</strong></td><td><strong>79</strong></td><td><strong>15</strong></td><td><strong>209</strong></td><td><strong>118</strong></td><td><strong>74</strong></td><td><strong>537</strong></td><td><strong>614</strong></td><td><strong>125</strong></td><td><strong>166</strong></td><td><strong>256</strong></td><td><strong>776</strong></td><td><strong>127</strong></td><td><strong>113</strong></td><td><strong>115</strong></td><td><strong>184</strong></td><td><strong>91</strong></td><td><strong>108</strong></td><td><strong>315</strong></td></tr><tr><td><strong>Av.</strong></td><td><strong>236</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Rare earth element (REE) concentrations in different phosphate ores. Data sources: Abbes et al. (2020); Aly and Mohammed (1999); Awadalla (2010); Christmann (2014); De Oliveira and Imbernon (1998); El-Zrelli et al. (2021); Habashi (1985); Kandil et al. (2010); Khater et al. (2016); Nie et al. (2013); Orabi et al. (2018); Pavón et al. (2022); Ramos et al. (2016); Roshdy et al. (2023); Soudry et al. (2002); Wang et al. (2010); Wu et al. (2018a); Zaitsev et al. (2015); Zhang (2014); Zielinski et al. (1993).</figcaption></figure>

REE concentration in the mine tailings of 120 ppm is even lower and similarly does not encourage REE extraction, although it should be mentioned that this material is already mined, ground, and stacked so that it is easily accessible and a large portion of the costs for mining are already covered as illustrated by Reitsma et al. (2018). The 776 ppm measured in the overburden (T2) is slightly higher, but does not seem to be a good representation of the overall Minjingu deposit, nor do we expect to find this REE concentration in large amounts of material on site.

In Fig. 3, the REE concentrations found in the Minjingu phosphate ore are compared to the REE concentrations found in other phosphate ores. REE concentrations in phosphate ores vary but are usually not higher than 1 % or 10,000 ppm (Habashi, 1985), with the exception of the Catalão Minerochemical Complex in Brazil for which Ramos et al. (2016) report, supported by independent analysis from De Oliveira and Imbernon (1998), REE concentrations as high as 16,650 ppm in selected samples. This is highly unusual, and most phosphate ores globally (particularly since most phosphate ores are of sedimentary origin) show concentrations of approximately 0.05 % or 500 ppm.

It can be noted that the REE concentrations in Minjingu phosphate ore are not particularly high and will most likely not make REE recovery an economical option. Similar observations were made for the analyzed mine tailings as well as the fertilizer products. In some of the processes

for uranium recovery from phosphates, a large share of the REEs, or lanthanides, that are chemically very similar to the actinides, are coextracted together (Habashi et al., 1986; Wu et al., 2018b), so that if such a process is applied REEs in Minjingu phosphate ore might also become a target for recovery (see more discussion on this in Chapter 4.4).

It is noteworthy that besides REEs, other valuable trace elements could potentially be co-recovered, as pointed out by Chen and Graedel (2015). Most notably, we measured vanadium concentrations of 21–201 ppm in the investigated samples. This concentration is again relatively low if compared to the concentration of primary vanadium ores in Africa (Boni et al., 2023), but co-recovery could again be profitable as a large share of the costs for mining and processing is already covered through the sales of fertilizer products.

It is further noteworthy that there is a much lower push-factor for REE extraction from phosphate ores and phosphate fertilizers than there is for cadmium and uranium, with some fertilizers even being enriched with REEs in an attempt to make them work more effectively (Silva et al., 2019; Ribeiro et al., 2021; Tommasi et al., 2020). Furthermore, REE extraction from phosphates (usually REEs are recovered from phosphogypsum) was not successfully practiced on an industrial scale historically (Diwa et al., 2022; Ramirez et al., 2022), as it was the case with uranium recovery (from WPA), so there are considerably higher
techno-economic risks associated with commercially recovering REEs from phosphates.

## 4.2. Uranium concentration in phosphate ore

Fig. 4 shows the reported uranium concentrations in phosphate ores globally. Here the situation is different, and it can be clearly seen that phosphate ore from Minjingu shows the highest concentrations of uranium. New Zealand, which is listed in Fig. 4, is not a major phosphate rock-producing country, and the reported uranium concentrations are presently more of a geological interest. Adam et al. (2014) reported uranium concentrations of 1055 ppm for the Uro phosphate deposit in Sudan that were not considered since they seem inflated and are not supported by other studies of the deposit (AbowSlama et al., 2014; Elmahdi and Wagialla, 2018). Besides, phosphate-uranium deposits that are not mined such as the Itataia deposit in Brazil that can show uranium concentrations of 816 ppm (Veríssimo et al., 2016) or the Matongo

deposit in Burundi that can show uranium concentrations of 632 ppm (Mwalongo et al., 2023a; Songore, 1991) were not considered as it is not clear yet if they can actually be used for fertilizer production.

## 4.3. Uranium concentration in phosphate fertilizers

Extended use of phosphate rock and phosphate fertilizer results in uranium build-up on agricultural soils, which is actively discussed by the scientific community (Schnug and Lottermoser, 2013; Sun et al., 2020a, b, Takeda et al., 2006). There is particular concern regarding uranium accumulation since the metal is radiotoxic and can endanger soil fertility, leach into groundwater (Liesch et al., 2015; Mathivanan et al., 2022), and be taken up by crops (Bigalke et al., 2017; Chen et al., 2021; Singh et al., 2005; Yamaguchi, 2009). At present, there is no legal limit for uranium in mineral fertilizers, though some organizations, such as the Soil Protection Commission of the German Federal Environment Agency (UBA), suggest formulating a legal limit. Kratz et al. (2016)

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Reported uranium concentrations in phosphate ore. Data sources: Abbes et al. (2020); Abed (2011); AbovSlama et al. (2014); Al-Bassam (2007); Al-Eshaikh et al. (2016); Altschuler et al. (1958), 1966, 1980; Azouazi et al. (2001); Banerjee et al. (1982); Baturin and Kochenov (2001); Bech et al. (2010); Bouabdallah et al. (2019); Bunus (2000); Cevik et al. (2010); Cook et al. (1990); Cook (1972); Chandrajith and Dissanayake (2009); Dar et al. (2014); El Bamiki et al. (2021); Falck and Wymer (2006); Gnandi and Tobschall (2003); Haneklaus et al. (2015); Hayumbu et al. (1995); Howard and Hough (1979); Jallad et al. (1989); Khater et al. (2016); Krea and Khalaf (2000); Menzel (1968); Onal and Atak (1992); Palattao et al. (2018); Pantelica et al. (1997); Parker (1984); Roselli et al. (2009); Ryszko et al. (2023); Saad et al. (2003); Sattouf (2007); Schnug et al. (1996); Scholten and Timmermans (1995); Shlewit and Alibrahim (2008); Soudry et al. (2002); Sun et al. (2020); Syers et al. (1986); Van Kauwenbergh (1997); Vogel et al. (2020); Weterings and Janssen (1985); Zanin and Zamirailova (2007).</figcaption></figure>
techno-economic risks associated with commercially recovering REEs from phosphates.

## 4.2. Uranium concentration in phosphate ore

Fig. 4 shows the reported uranium concentrations in phosphate ores globally. Here the situation is different, and it can be clearly seen that phosphate ore from Minjingu shows the highest concentrations of uranium. New Zealand, which is listed in Fig. 4, is not a major phosphate rock-producing country, and the reported uranium concentrations are presently more of a geological interest. Adam et al. (2014) reported uranium concentrations of 1055 ppm for the Uro phosphate deposit in Sudan that were not considered since they seem inflated and are not supported by other studies of the deposit (AbowSlama et al., 2014; Elmahdi and Wagialla, 2018). Besides, phosphate-uranium deposits that are not mined such as the Itataia deposit in Brazil that can show uranium concentrations of 816 ppm (Veríssimo et al., 2016) or the Matongo

deposit in Burundi that can show uranium concentrations of 632 ppm (Mwalongo et al., 2023a; Songore, 1991) were not considered as it is not clear yet if they can actually be used for fertilizer production.

## 4.3. Uranium concentration in phosphate fertilizers

Extended use of phosphate rock and phosphate fertilizer results in uranium build-up on agricultural soils, which is actively discussed by the scientific community (Schnug and Lottermoser, 2013; Sun et al., 2020a, b, Takeda et al., 2006). There is particular concern regarding uranium accumulation since the metal is radiotoxic and can endanger soil fertility, leach into groundwater (Liesch et al., 2015; Mathivanan et al., 2022), and be taken up by crops (Bigalke et al., 2017; Chen et al., 2021; Singh et al., 2005; Yamaguchi, 2009). At present, there is no legal limit for uranium in mineral fertilizers, though some organizations, such as the Soil Protection Commission of the German Federal Environment Agency (UBA), suggest formulating a legal limit. Kratz et al. (2016)

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Reported uranium concentrations in phosphate ore. Data sources: Abbes et al. (2020); Abed (2011); AbovSlama et al. (2014); Al-Bassam (2007); Al-Eshaikh et al. (2016); Altschuler et al. (1958), 1966, 1980; Azouazi et al. (2001); Banerjee et al. (1982); Baturin and Kochenov (2001); Bech et al. (2010); Bouabdallah et al. (2019); Bunus (2000); Cevik et al. (2010); Cook et al. (1990); Cook (1972); Chandrajith and Dissanayake (2009); Dar et al. (2014); El Bamiki et al. (2021); Falck and Wymer (2006); Gnandi and Tobschall (2003); Haneklaus et al. (2015); Hayumbu et al. (1995); Howard and Hough (1979); Jallad et al. (1989); Khater et al. (2016); Krea and Khalaf (2000); Menzel (1968); Onal and Atak (1992); Palattao et al. (2018); Pantelica et al. (1997); Parker (1984); Roselli et al. (2009); Ryszko et al. (2023); Saad et al. (2003); Sattouf (2007); Schnug et al. (1996); Scholten and Timmermans (1995); Shlewit and Alibrahim (2008); Soudry et al. (2002); Sun et al. (2020); Syers et al. (1986); Van Kauwenbergh (1997); Vogel et al. (2020); Weterings and Janssen (1985); Zanin and Zamirailova (2007).</figcaption></figure>
report that the legal limit (50 mg uranium per kg P₂O₅ for P-containing fertilizers) suggested by UBA was exceeded by all 161 phosphate fertilizer samples the researchers investigated. Further analysis of 414 phosphate fertilizers used in Europe (Verbeek et al., 2020), reported similar uranium concentrations but are still somewhat lower (all between 10 and 110 ppm for Germany by Kratz et al. (2016) and 0–242 ppm with a mean concentration of 41 ppm and a median at 23 ppm for the EU-27 + Norway and the United Kingdom by Verbeek et al. (2020)) than the uranium concentrations measured here in Minjingu phosphate ore products, which are roughly by a factor of 1.5 higher compared to the uranium concentrations measured in Europe. It is further noteworthy that the concentrations provided in Fig. 5 are often the result of radioactivity measurements and that the P₂O₅ concentration as well as other important factors of the fertilizers were not reported.

### 4.4. Alternative processing of Minjingu phosphate ore

Fig. 6 shows a direct comparison between the dry beneficiation process currently used to develop Minjingu phosphate ore (Fig. 6a) and the very different WPA process that is usually deployed to process phosphate ore (Fig. 6b). Globally, less than 0.5 % of mined phosphate ore is used directly on agricultural soil after simple beneficiation (Zhang, 2014). Both processes are simplified here, and the intention is to indicate how different they are and what the challenges of integrating uranium recovery in a dry phosphate ore beneficiation process may be.

It is unlikely that Minjingu phosphate ore will be processed using the WPA process or another wet-chemical process. The WPA- and, by extension, other chemical processes require significant amounts of water in addition to sulfuric or other acids. The Minjingu mine and processing plant are located in a semi-arid region some 550 km away from the closest seaport, making the supply of large amounts of water challenging. In addition, using the WPA process results in considerable amounts of fine-powdery phosphogypsum. If phosphate ore with low radioactivity and heavy metal concentrations is used to produce fertilizers, the resulting phosphogypsum can be utilized as a secondary raw material in construction (Rashad, 2017; Haneklaus et al., 2022) or in agriculture (Alcordo and Rech cigl, 1993). Since the Minjingu ore contains considerable amounts of uranium, this would not be the case, so that the overall environmental footprint of any wet-chemical process (even with uranium recovery) will most likely be significantly larger than that of the current dry beneficiation process (without uranium recovery).

During the WPA process with sulfuric acid (Fig. 6b), uranium can be recovered from the liquid WPA using solvent extraction. This process

has been used on an industrial scale in the past (Astley and Stana, 2014) and has been well documented in several authoritative reviews (Beltrami et al., 2014; Bunus, 2000; Singh et al., 2016).

Minjingu phosphate ore may profit from alternative dry processing such as electrostatic separation as described by Bittner et al. (2015) as well as Sobhy and Tao (2014), the improved hard process (IHP) presently under development in Florida (Walters, 2011), or other innovative dry concentration processes that have recently been reviewed by Sajid et al. (2021). It is noteworthy that the present process, though simple in its design, works extremely well. Calcination is energy-intensive, so reducing energy inputs, by introducing solar thermal calcination as proposed by Haneklaus et al. (2017c,d, 2021b) and recently investigated to pilot-plant scale by the EU SOLPART project (Esence et al., 2020; Moumin et al., 2019) might be a long term proposition for the Minjingu plant that is located in an area that offers the required high levels of solar radiation.

More importantly, uranium recovery should be considered during the processing of Minjingu phosphate ore as a result of the relatively high occurring concentrations that could make recovery economically profitable. Comparable high concentrations of REEs and other valuable elements could not be confirmed in this work. It is noteworthy, that although the concentrations of uranium can be considered elevated in the Minjingu ore (approximately 3x of more common concentrations), the resulting compound fertilizers do not show significantly higher (approximately 1.5x more common concentrations) uranium concentrations. Since traditionally uranium is recovered from the liquid WPA during wet chemical processing, new innovative solutions for uranium recovery during Minjingu phosphate ore beneficiation should be investigated. Theoretically, the uranium could be directly leached from the phosphate ore after primary sieving and sorting before calcination or from the fertilizer powder after calcination.

Abilash et al. (2009, 2013) and Mäkinen et al. (2019) proposed, for instance, bioleaching of a lower-grade uranium apatite ore in India and Finland. Al-Khaledi et al. (2019) and Roshdy et al. (2023) propose direct leaching of REEs and uranium from Egyptian phosphate ore and Guzmán et al. (1995) were probably the first to propose such an approach for phosphate ores from Mexico with the explicit aim of reducing dissipation of radiotoxic uranium. Gabriel et al. (2013a, b) rightfully point out that direct leaching of uranium from phosphates is presently not economically profitable, and this will most likely also be the case at the Minjingu fertilizer plant, given the relatively small size of the operation, as well as still fairly low uranium prices worldwide.

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Reported uranium concentrations in phosphate fertilizers. Data sources: Al-Shawi and Dahl (1995); Barisic et al. (1992); Hamamo et al. (1995); Khater, 2011; Khater and Al-Sewaidan (2008); Kratz et al. (2016); Lal et al. (1985); Otero et al. (2005); Palattao et al. (2018); Pfister et al. (1976); Qamouche et al. (2020); Ramteke et al. (2022); Verbeeck et al. (2020); Vucic and Ilic (1989); Yamazaki and Geraldo (2003).</figcaption></figure>
report that the legal limit (50 mg uranium per kg P₂O₅ for P-containing fertilizers) suggested by UBA was exceeded by all 161 phosphate fertilizer samples the researchers investigated. Further analysis of 414 phosphate fertilizers used in Europe (Verbeek et al., 2020), reported similar uranium concentrations but are still somewhat lower (all between 10 and 110 ppm for Germany by Kratz et al. (2016) and 0–242 ppm with a mean concentration of 41 ppm and a median at 23 ppm for the EU-27 + Norway and the United Kingdom by Verbeek et al. (2020)) than the uranium concentrations measured here in Minjingu phosphate ore products, which are roughly by a factor of 1.5 higher compared to the uranium concentrations measured in Europe. It is further noteworthy that the concentrations provided in Fig. 5 are often the result of radioactivity measurements and that the P₂O₅ concentration as well as other important factors of the fertilizers were not reported.

### 4.4. Alternative processing of Minjingu phosphate ore

Fig. 6 shows a direct comparison between the dry beneficiation process currently used to develop Minjingu phosphate ore (Fig. 6a) and the very different WPA process that is usually deployed to process phosphate ore (Fig. 6b). Globally, less than 0.5 % of mined phosphate ore is used directly on agricultural soil after simple beneficiation (Zhang, 2014). Both processes are simplified here, and the intention is to indicate how different they are and what the challenges of integrating uranium recovery in a dry phosphate ore beneficiation process may be.

It is unlikely that Minjingu phosphate ore will be processed using the WPA process or another wet-chemical process. The WPA- and, by extension, other chemical processes require significant amounts of water in addition to sulfuric or other acids. The Minjingu mine and processing plant are located in a semi-arid region some 550 km away from the closest seaport, making the supply of large amounts of water challenging. In addition, using the WPA process results in considerable amounts of fine-powdery phosphogypsum. If phosphate ore with low radioactivity and heavy metal concentrations is used to produce fertilizers, the resulting phosphogypsum can be utilized as a secondary raw material in construction (Rashad, 2017; Haneklaus et al., 2022) or in agriculture (Alcordo and Rech cigl, 1993). Since the Minjingu ore contains considerable amounts of uranium, this would not be the case, so that the overall environmental footprint of any wet-chemical process (even with uranium recovery) will most likely be significantly larger than that of the current dry beneficiation process (without uranium recovery).

During the WPA process with sulfuric acid (Fig. 6b), uranium can be recovered from the liquid WPA using solvent extraction. This process

has been used on an industrial scale in the past (Astley and Stana, 2014) and has been well documented in several authoritative reviews (Beltrami et al., 2014; Bunus, 2000; Singh et al., 2016).

Minjingu phosphate ore may profit from alternative dry processing such as electrostatic separation as described by Bittner et al. (2015) as well as Sobhy and Tao (2014), the improved hard process (IHP) presently under development in Florida (Walters, 2011), or other innovative dry concentration processes that have recently been reviewed by Sajid et al. (2021). It is noteworthy that the present process, though simple in its design, works extremely well. Calcination is energy-intensive, so reducing energy inputs, by introducing solar thermal calcination as proposed by Haneklaus et al. (2017c,d, 2021b) and recently investigated to pilot-plant scale by the EU SOLPART project (Esence et al., 2020; Moumin et al., 2019) might be a long term proposition for the Minjingu plant that is located in an area that offers the required high levels of solar radiation.

More importantly, uranium recovery should be considered during the processing of Minjingu phosphate ore as a result of the relatively high occurring concentrations that could make recovery economically profitable. Comparable high concentrations of REEs and other valuable elements could not be confirmed in this work. It is noteworthy, that although the concentrations of uranium can be considered elevated in the Minjingu ore (approximately 3x of more common concentrations), the resulting compound fertilizers do not show significantly higher (approximately 1.5x more common concentrations) uranium concentrations. Since traditionally uranium is recovered from the liquid WPA during wet chemical processing, new innovative solutions for uranium recovery during Minjingu phosphate ore beneficiation should be investigated. Theoretically, the uranium could be directly leached from the phosphate ore after primary sieving and sorting before calcination or from the fertilizer powder after calcination.

Abilash et al. (2009, 2013) and Mäkinen et al. (2019) proposed, for instance, bioleaching of a lower-grade uranium apatite ore in India and Finland. Al-Khaledi et al. (2019) and Roshdy et al. (2023) propose direct leaching of REEs and uranium from Egyptian phosphate ore and Guzmán et al. (1995) were probably the first to propose such an approach for phosphate ores from Mexico with the explicit aim of reducing dissipation of radiotoxic uranium. Gabriel et al. (2013a, b) rightfully point out that direct leaching of uranium from phosphates is presently not economically profitable, and this will most likely also be the case at the Minjingu fertilizer plant, given the relatively small size of the operation, as well as still fairly low uranium prices worldwide.

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Reported uranium concentrations in phosphate fertilizers. Data sources: Al-Shawi and Dahl (1995); Barisic et al. (1992); Hamamo et al. (1995); Khater, 2011; Khater and Al-Sewaidan (2008); Kratz et al. (2016); Lal et al. (1985); Otero et al. (2005); Palattao et al. (2018); Pfister et al. (1976); Qamouche et al. (2020); Ramteke et al. (2022); Verbeeck et al. (2020); Vucic and Ilic (1989); Yamazaki and Geraldo (2003).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>(b) Wet-Phosphoric Acid (WPA) Process</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Comparison of (a) the present Minjingu phosphate rock beneficiation process and (b) the wet phosphoric acid process with sulfuric acid that is used at more than 85 % of phosphate fertilizer plants globally.</figcaption></figure>

4.5. Importance of Minjingu phosphate fertilizer products

It is essential to realize that Minjingu phosphate rock, and its fertil-
izer products substantially increase crop yield in Tanzania, as may best
be illustrated in Fig. 7 using data from the Global Forum on Agricultural
Research and Innovation (GFAR, 2018). The domestically produced
Minjingu fertilizer products are (especially given the national subsidies
in Tanzania) an affordable source of domestically produced fertilizer
that works well on the acidic soils found in the country, decreasing the
dependency on the international fertilizer market (Mew et al., 2023). If
uranium is to be recovered from Minjingu phosphate ore during mineral
fertilizer production, it is paramount that the quality of the final fertil-
izer product is not compromised.

5. Conclusions and policy recommendations

This work provides for the first time a systematic overview of Min-
jingu phosphate ore processing in Tanzania and puts the REE and ura-
nium concentrations found in the different ore layers, tailings, and
fertilizer products in context, by comparing them to concentrations of
other phosphates found globally. It was found that the REE concentra-
tions in Minjingu phosphate ore are too low to justify economic recov-
ery. The already suspected elevated uranium concentrations in the
Minjingu phosphate ore could be confirmed by the systematic analysis
presented here, and elevated uranium concentrations were further
found in the final fertilizer products, though in lower relative concen-
trations than in the ore. It could further be shown that there are

<figure><img src="image_10.png" /><figcaption>Fig. 7. Comparison of the crop yield in a village in Tanzania with no mineral fertilizer (Farmer’s Practice), Minjingu Phosphate Rock, Minjingu Mazao and DAP fertilizers.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>(b) Wet-Phosphoric Acid (WPA) Process</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Comparison of (a) the present Minjingu phosphate rock beneficiation process and (b) the wet phosphoric acid process with sulfuric acid that is used at more than 85 % of phosphate fertilizer plants globally.</figcaption></figure>

## 4.5. Importance of Minjingu phosphate fertilizer products

It is essential to realize that Minjingu phosphate rock, and its fertilizer products substantially increase crop yield in Tanzania, as may best be illustrated in Fig. 7 using data from the Global Forum on Agricultural Research and Innovation (GFAR, 2018). The domestically produced Minjingu fertilizer products are (especially given the national subsidies in Tanzania) an affordable source of domestically produced fertilizer that works well on the acidic soils found in the country, decreasing the dependency on the international fertilizer market (Mew et al., 2023). If uranium is to be recovered from Minjingu phosphate ore during mineral fertilizer production, it is paramount that the quality of the final fertilizer product is not compromised.

## 5. Conclusions and policy recommendations

This work provides for the first time a systematic overview of Minjingu phosphate ore processing in Tanzania and puts the REE and uranium concentrations found in the different ore layers, tailings, and fertilizer products in context, by comparing them to concentrations of other phosphates found globally. It was found that the REE concentrations in Minjingu phosphate ore are too low to justify economic recovery. The already suspected elevated uranium concentrations in the Minjingu phosphate ore could be confirmed by the systematic analysis presented here, and elevated uranium concentrations were further found in the final fertilizer products, though in lower relative concentrations than in the ore. It could further be shown that there are

<figure><img src="image_12.png" /><figcaption>**Fig. 7.** Comparison of the crop yield in a village in Tanzania with no mineral fertilizer (Farmer's Practice), Minjingu Phosphate Rock, Minjingu Mazao and DAP fertilizers.</figcaption></figure>
considerable differences between the different ore layers analyzed here as well as literature data on Minjingu ore. All this results in considerable uncertainties and limitations of this and other studies. Minjingu phosphate fertilizer products are important for the development and food security of Tanzania, and it is crucial that the final fertilizer products work as effectively as they do now. Despite this, it is still a good idea to look into ways to make the production of Minjingu fertilizer more sustainable, such as solar calcination or reducing dust and uranium recovery without affecting the quality of the final fertilizer products.

Declaration of Generative AI and AI-assisted technologies in the
writing process

The authors declare that no generative AI and AI-assisted technology
was used in the writing process.

CRediT authorship contribution statement

Nils H. Haneklaus: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Visualization, Writing – original draft, Writing – review & editing. Dennis A. Mwalongo: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Writing – review & editing. Jacob B. Lisuma: Formal analysis, Writing – review & editing. Aloyce I. Amasi: Formal analysis, Writing – review & editing. Jerome Mwimanzi: Formal analysis, Writing – review & editing. Tomislav Bituh: Formal analysis, Writing – review & editing. Jelena Ćirić: Formal analysis, Writing – review & editing. Jakub Nowak: Formal analysis, Writing – review & editing. Urszula Ryszko: Formal analysis, Writing – review & editing. Piotr Rusek: Formal analysis, Writing – review & editing. Essaid Bilal: Formal analysis, Funding acquisition, Writing – review & editing. Hajar Bellefqih: Formal analysis, Writing – review & editing. Khaoula Qamouche: Formal analysis, Writing – review & editing. Jamal Ait Brahim: Formal analysis, Writing – review & editing. Redouane Beniazza: Formal analysis, Funding acquisition, Writing – review & editing. Hamid Mazouz: Conceptualization, Formal analysis, Funding acquisition, Writing – review & editing. Elizabet M. van der Merwe: Formal analysis, Funding acquisition, Writing – review & editing. Wayne Truter: Formal analysis, Funding acquisition, Writing – review & editing. Hilda D. Kyomuhimbo: Formal analysis, Writing – review & editing. Hendrik Brink: Formal analysis, Funding acquisition, Writing – review & editing. Gerald Steiner: Formal analysis, Writing – review & editing. Martin Bertau: Formal analysis, Writing – review & editing. Raghav S. Soni: Formal analysis, Writing – review & editing. Ashwin W. Patwardhan: Formal analysis, Writing – review & editing. Pushpito K. Ghosh: Formal analysis, Writing – review & editing. Thomas T. Kivevele: Formal analysis, Writing – review & editing. Kelvin M. Mtei: Formal analysis, Writing – review & editing. Stanisław Wacławek: Formal analysis, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgments

This work was supported by the Tanzania Atomic Energy Commis-
sion (TAEC) and Nelson Mandela African Institution of Science and

Technology (NM-AIST). This work further received support from the Austrian Federal Ministry of Education, Science and Research (BMBWF) through Austria's Agency for Education and Internationalization (OeAD) [Grant Numbers: Africa UNINET P006, P056, and P058; APPEAR Project 341; CZ 12/2024; FR 12/2024; HR 14/2024; KOEF 01/2019; PL 12/2024]. Khaoula Qamouche further acknowledges BMBWF/ OeAD support through Ernst Mach Grant MPC-2023-00562. We thank the participants of the International Symposium on Uranium Raw Material for the Nuclear Fuel Cycle (URAM 2023) for valuable comments that further improved this work.

References

Abbes, et al., 2020. Thermal beneficiation of Sra Ouertane (Tunisia) low-grade phosphate rock. Minerals 10 (11), 937. https://doi.org/10.3390/min10110937.

Abed, 2011. Review of uranium in the Jordanian phosphorites: distribution, genesis, and industry. Jordan J. Earth Environ. Sci. 35-45. ISSN 1995-6681.

Abouzeid, 2008. Physical and thermal treatment of phosphate ores — an overview. Int. J. Miner. Process. 85 (4), 59–84. https://doi.org/10.1016/j.minpro.2007.09.001.

Abilash, et al., 2009. Dissolution of uranium from silicate-apatite ore by Acidithiobacillus ferrooxidans. Hydrometallurgy 95 (1–2), 70–75. https://doi.org/10.1016/j.hydromet.2008.04.013.

Abilash, et al., 2013. Comparative performance of uranium bioleaching from low grade indian apatite rock in column and bioreactor. Energy Procedia 39, 20–32. https://doi.org/10.1016/j.egypro.2013.07.188.

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considerable differences between the different ore layers analyzed here as well as literature data on Minjingu ore. All this results in considerable uncertainties and limitations of this and other studies. Minjingu phosphate fertilizer products are important for the development and food security of Tanzania, and it is crucial that the final fertilizer products work as effectively as they do now. Despite this, it is still a good idea to look into ways to make the production of Minjingu fertilizer more sustainable, such as solar calcination or reducing dust and uranium recovery without affecting the quality of the final fertilizer products.

## Declaration of Generative AI and AI-assisted technologies in the writing process

The authors declare that no generative AI and AI-assisted technology was used in the writing process.

## CRediT authorship contribution statement

**Nils H. Haneklaus:** Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Visualization, Writing – original draft, Writing – review & editing. **Dennis A. Mwalongo:** Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Writing – review & editing. **Jacob B. Lisuma:** Formal analysis, Writing – review & editing. **Aloyce I. Amasi:** Formal analysis, Writing – review & editing. **Jerome Mwimanzi:** Formal analysis, Writing – review & editing. **Tomislav Bituh:** Formal analysis, Writing – review & editing. **Jelena Ćirić:** Formal analysis, Writing – review & editing. **Jakub Nowak:** Formal analysis, Writing – review & editing. **Urszula Ryszko:** Formal analysis, Writing – review & editing. **Piotr Rusek:** Formal analysis, Writing – review & editing. **Essaid Bilal:** Formal analysis, Funding acquisition, Writing – review & editing. **Hajar Bellefqih:** Formal analysis, Writing – review & editing. **Khaoula Qamouche:** Formal analysis, Writing – review & editing. **Jamal Ait Brahim:** Formal analysis, Writing – review & editing. **Redouane Beniazza:** Formal analysis, Funding acquisition, Writing – review & editing. **Hamid Mazouz:** Conceptualization, Formal analysis, Funding acquisition, Writing – review & editing. **Elizabet M. van der Merwe:** Formal analysis, Funding acquisition, Writing – review & editing. **Wayne Truter:** Formal analysis, Funding acquisition, Writing – review & editing. **Hilda D. Kyomuhimbo:** Formal analysis, Writing – review & editing. **Hendrik Brink:** Formal analysis, Funding acquisition, Writing – review & editing. **Gerald Steiner:** Formal analysis, Writing – review & editing. **Martin Bertau:** Formal analysis, Writing – review & editing. **Raghav S. Soni:** Formal analysis, Writing – review & editing. **Ashwin W. Patwardhan:** Formal analysis, Writing – review & editing. **Pushpito K. Ghosh:** Formal analysis, Writing – review & editing. **Thomas T. Kivevele:** Formal analysis, Writing – review & editing. **Kelvin M. Mtei:** Formal analysis, Writing – review & editing. **Stanisław Wacławek:** Formal analysis, Writing – review & editing.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Acknowledgments

This work was supported by the Tanzania Atomic Energy Commission (TAEC) and Nelson Mandela African Institution of Science and

Technology (NM-AIST). This work further received support from the Austrian Federal Ministry of Education, Science and Research (BMBWF) through Austria's Agency for Education and Internationalization (OeAD) [Grant Numbers: Africa UNINET P006, P056, and P058; APPEAR Project 341; CZ 12/2024; FR 12/2024; HR 14/2024; KOEF 01/2019; PL 12/2024]. Khaoula Qamouche further acknowledges BMBWF/OeAD support through Ernst Mach Grant MPC-2023-00562. We thank the participants of the International Symposium on Uranium Raw Material for the Nuclear Fuel Cycle (URAM 2023) for valuable comments that further improved this work.

## References

Abbes, et al., 2020. Thermal beneficiation of Sra Ouertane (Tunisia) low-grade phosphate rock. Minerals 10 (11), 937. https://doi.org/10.3390/min10110937.

Abed, 2011. Review of uranium in the Jordanian phosphorites: distribution, genesis, and industry. Jordan J. Earth Environ. Sci. 35-45. ISSN 1995-6681.

Abouzeid, 2008. Physical and thermal treatment of phosphate ores — an overview. Int. J. Miner. Process. 85 (4), 59-84. https://doi.org/10.1016/j.minpro.2007.09.001.

Abilash, et al., 2009. Dissolution of uranium from silicate-apatite ore by Acidithiobacillus ferrooxidans. Hydrometallurgy 95 (1-2), 70-75. https://doi.org/10.1016/j.hydromet.2008.04.013.

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Diva, et al., 2023. Uranium supply potential from imported phosphate rocks for the Philippine nuclear power program. Extr. Ind. Soc. 15, 101303 https://doi.org/ 10.1016/j.exis.2023.101303.

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Haneklaus et al. (2017a) Phosphate rocks and nuclear proliferation. Science & Global Security 25(3), 143–158. https://doi.org/10.1080/08929882.2017.1394061.

Haneklaus, et al., 2017b. To extract, or not to extract uranium from phosphate rock, that is the question. Environ. Sci. Technol. 51 (2), 753–754. https://doi.org/10.1021/acs. est.6b05506.

Haneklaus, et al., 2017c. Economic evaluation of flameless phosphate rock calcination with concentrated solar power and high temperature reactors. Energy 140 (1), 1148–1157. https://doi.org/10.1016/j.energy.2017.08.123.

Haneklaus, et al., 2017d. Stop smoking—tube-in-tube helical system for flameless calcination of minerals. Processes 5 (4), 67. https://doi.org/10.3390/pr5040067.

Haneklaus, 2012a. Unconventional uranium resources from phosphates. In: Encyclopedia of Nuclear Energy, 286-291. https://doi.org/10.1016/B978-0-12- 819725-7.00152-5.

Haneklaus, 2021b. Calcination. In: Encyclopedia of Nuclear Energy, 131-138. https:// doi.org/10.1016/B978-0-12-819725-7.00161-6.

Haneklaus, 2021b. Closing the upcoming EU gypsum gap with phosphogypsum. Resour. Conserv. Recycl. 182, 106328 https://doi.org/10.1016/j. resconrec.2022.106328.

Harou, et al., 2022. The joint effects of information and financing constraints on technology adoption: evidence from a field experiment in rural Tanzania. J. Dev. Energy 155, 102707 https://doi.org/10.1016/j.jdeveco.2021.102707.

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Beltrami, et al., 2014. Recovery of uranium from wet phosphoric acid by solvent extraction processes. Chem. Rev. 114 (24), 12002. https://doi.org/10.1021/cr5001546.

Benson et al. (2012) The supply of inorganic fertilizers to smallholder farmers in Tanzania: evidence for fertilizer policy development. IFPRI Discussion Paper 01230. https://doi.org/10.2139/ssrn.2197893.

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Bigalke, et al., 2017. Accumulation of cadmium and uranium in arable soils in Switzerland. Environ. Pollut. 221, 85-93. https://doi.org/10.1016/j.envpol.2016.11.035.

Bilal, et al., 2023. Phosphogypsum circular economy considerations: a critical review from more than 65 storage sites worldwide. J. Clean. Prod. 414, 137561 https://doi.org/10.1016/j.jclepro.2023.137561.

Bittner et al. (2015) Electrostatic beneficiation of phosphate ores: review of past work and discussion of an improved separation system. https://steqtech.com/wp-content/uploads/2015/11/SYMPHOS-2015_Bittner-final.pdf. Accessed 2023-07-30.

Boni, et al., 2023. Vanadium ore resources of the African continent: state of the Art. Ore Geol. Rev. 157, 105423 https://doi.org/10.1016/j.oregeorev.2023.105423.

Bouabdallah, et al., 2019. Chemical, mineralogical and rare earth elements distribution study of phosphorites from Sra Ouertane deposit (Tunisia). J. Afr. Earth Sci. 157, 103505 https://doi.org/10.1016/j.jafrearsci.2019.05.013.

Bunus, 2000. Uranium and rare earth recovery from phosphate fertilizer industry by solvent extraction. Miner. Process. Extr. Metall. Rev. 21 (1-5), 81-478. https://doi.org/10.1080/08827500008914174.

Cevik, et al., 2010. Radiological and chemical assessment of phosphate rocks in some countries. J. Hazard. Mater. 182 (1-3), 531-535. https://doi.org/10.1016/j.jhazmat.2010.06.064.

Chandrajith, Dissanayake, 2009. Phosphate mineral fertilizers, trace metals and human health. J. Natl. Sci. Found. 37 (3), 153-165. https://doi.org/10.4038/jnsfrs. v3713.1219.

Chen, Graedel, 2015. The potential for mining trace elements from phosphate rock. J. Clean. Prod. 91, 337-346. https://doi.org/10.1016/j.jclepro.2014.12.042.

Chen, et al., 2021. Uranium (U) source, speciation, uptake, toxicity and bioremediation strategies in soil-plant system: a review. J. Hazard. Mater. 413, 125319 https://doi.org/10.1016/j.jhazmat.2021.125319.

Christmann, 2014. a forward look into rare earth supply and demand: a role for sedimentary phosphate deposits? Procedia 83, 19-26. https://doi.org/10.1016/j. proeng.2014.09.005.

Cook, 1972. Petrology and geochemistry of the phosphate deposits of Northwest Queensland, Australia. Econ. Geol. 67 (8), 1193-1213. https://doi.org/10.2113/ gsecongoe.67.8.1193.

Cook, et al., 1990. Neogene to holocene phosphorites of Australia. In: Burnett, W.C., Riggs, S.R. (Eds.), Phosphate Deposits of the World. Cambridge University Press, Cambridge, pp. 98-115.

Cooper, et al., 2011. The future distribution and production of global phosphate rock reserves. Resour., Conserv. Recycl. 57, 78-86. https://doi.org/10.1016/j. resconrec.2011.09.009.

Dar, et al., 2014. Uranium (U) concentration and its genetic significance in the phosphorites of the Paleoproterozoic Bijawar Group of the Lalitpur district, Uttar Pradesh, India. Arab. J.Geosci. 7 (6), 2237-2248. https://doi.org/10.1007/s12517- 013-0903-8.

De Oliveira, Imbernon, 1998. Weathering alteration and related REE concentration in the Catalão I carbonatite complex, central Brazil. J. South Am. Earth Sci. 11 (4), 379-388. https://doi.org/10.1016/S0895-9811(98)00024-8.

Diva, et al., 2022. Evaluating radiation risks and resource opportunities associated with phosphogypsum in the Philippines. J. Radioanal. Nucl. Chem. 331, 967-974. https://doi.org/10.1007/s10967-021-08142-8.

Diva, et al., 2023. Uranium supply potential from imported phosphate rocks for the Philippine nuclear power program. Extr. Ind. Soc. 15, 101303 https://doi.org/ 10.1016/j.exis.2023.101303.

El Bamiki, et al., 2021. Phosphate rocks: a review of sedimentary and igneous occurrences in Morocco. Minerals 11, 1137. https://doi.org/10.3390/min1101137.

Elmadhi, Wagialla, 2018. Jebel Kurun phosphate rock characteristics and technical viability to produce phosphoric acid. J. Miner. Mater. Character. Eng. 6 (5), 555-567. https://doi.org/10.4236/jmmce.2018.65040.

El Zrelli, et al., 2021. Rare earth elements characterization associated to the phosphate fertilizer plants of Gabes (Tunisia, Central Mediterranean Sea): geochemical properties and behavior, related economic losses, and potential hazards. Sci. Total Environ. 791, 148268 https://doi.org/10.1016/j.scitotenv.2021.148268.

Esence, et al., 2020. A shallow cross-flow fluidized-bed solar reactor for continuous calcination processes. Solar Energy 196, 389-398. https://doi.org/10.1016/j. solener.2019.12.029.

Falck, Wymer, 2006. Uranium in phosphate fertilizer production. In: Merkel, B.J., Hasche-Berger, A. (Eds.), Uranium in the Environment. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-28367-6-89.

Gabriel, et al., 2013a. A critical assessment of global uranium resources, including uranium in phosphate rocks, and the possible impact of uranium shortages on nuclear power fleets. Ann. Nucl. Energy 58, 213-220. https://doi.org/10.1016/j. anucene.2013.03.010.

Gabriel, et al., 2013b. Building future nuclear power fleets: the available uranium resources constraint. Resour. Policy 38 (4), 458-469. https://doi.org/10.1016/j. resourpol.2013.06.008.

Gaustad, et al., 2021. Rare earth metals from secondary sources: review of potential supply from waste and byproducts. Resour., Conserv. Recycl. 167, 105213 https:// doi.org/10.1016/j.resconrec.2020.105213.

GFAR (2018) In Tanzania, farmers restore faith in fertilizer. https://blog.gfar.net/2018/ 10/10/in-tanzania-farmers-restore-faith-in-fertilizer/, accessed 2023-09-22.

Gnandi, Tobschall, 2003. Distribution patterns of rare-earth elements and uranium in tertiary sedimentary phosphorites of Hahotoe-Kpogamé, Togo. J. Afr. Earth Sci. 37 (1-2), 1-10. https://doi.org/10.1016/j.jafrearsci.2003.08.002.

Guzmán, et al., 1995. Uranium in phosphate rock and derivatives. J. Radioanal. Nucl. Chem. 189, 301-306. https://doi.org/10.1007/BF02042609.

Habashi, 1985. The recovery of the lanthanides from phosphate rock. J. Chem. Technol. Biotechnol. 35 (1), 5-14. https://doi.org/10.1002/jctb.5040350103.

Habashi, et al., 1986a. The recovery of uranium and the lanthanides from phosphate rock. J. Chem. Technol. Biotechnol. 36 (6), 259-267. https://doi.org/10.1002/ jctb.280360604.

Hakkar, et al., 2021. Enhancing rare earth element transfer from phosphate rock to phosphoric acid using an inexpensive fly ash additive. Miner. Eng. 172, 107166 https://doi.org/10.1016/j.mineng.2021.107166.

Hamao, et al., 1995. Studies of radioactivity and heavy metals in phosphate fertilizer. J. Radioanal. Nucl. Chem. 194, 331-336. https://doi.org/10.1007/BF02038431.

Haneklaus, et al., 2015. Energy neutral phosphate fertilizer production using high temperature reactors: a Philippine case study. Philipp. J. Sci. 144 (1), 69-79. ISSN 0031-7683.

Haneklaus et al. (2017a) Phosphate rocks and nuclear proliferation. Science & Global Security 25(3), 143-158. https://doi.org/10.1080/08929882.2017.1394061.

Haneklaus, et al., 2017b. To extract, or not to extract uranium from phosphate rock, that is the question. Environ. Sci. Technol. 51 (2), 753-754. https://doi.org/10.1021/acs. est.6b05506.

Haneklaus, et al., 2017c. Economic evaluation of flameless phosphate rock calcination with concentrated solar power and high temperature reactors. Energy 140 (1), 1148-1157. https://doi.org/10.1016/j.energy.2017.08.123.

Haneklaus, et al., 2017d. Stop smoking—tube-in-tube helical system for flameless calcination of minerals. Processes 5 (4), 67. https://doi.org/10.3390/pr5040067.

Haneklaus, 2021a. Unconventional uranium resources from phosphates. In: Encyclopedia of Nuclear Energy, 286-291. https://doi.org/10.1016/B978-0-12- 819725-7.00152-5.

Haneklaus, 2021b. Calcination. In: Encyclopedia of Nuclear Energy, 131-138. https:// doi.org/10.1016/B978-0-12-819725-7.00161-6.

Haneklaus, 2021b. Closing the upcoming EU gypsum gap with phosphogypsum. Resour. Conserv. Recycl. 182, 106328 https://doi.org/10.1016/j. resconrec.2022.106328.

Harou, et al., 2022. The joint effects of information and financing constraints on technology adoption: evidence from a field experiment in rural Tanzania. J. Dev. Energy 155, 102707 https://doi.org/10.1016/j.jdeveco.2021.102707.

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Mwalongo, et al., 2024. Uranium dissemination with phosphate fertilizers globally: a systematic review with focus on East Africa. Sustainability 16 (4), 1496. https://doi.org/10.3390/su16041496.

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J. Radioanal. Nucl. Chem. 216 (2), 261-264. https://doi.org/10.1007/BF02033788.

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Qamouche, et al., 2020. Radiological characterization of phosphate rocks, phosphogypsum, phosphoric acid and phosphate fertilizers in Morocco: an assessment of the radiological hazard impact on the environment. Mater. Today: Proc. 27 (4), 3234-3242. https://doi.org/10.1016/j.matpr.2020.04.073.

Qamouche, et al., 2021. Uranium and other heavy metal sorption from Moroccan phosphoric acid with argan nutshell sawdust. Miner. Eng. 171, 107085 https://doi.org/10.1016/j.mineng.2021.107085.

Ramirez, et al., 2022. Rare earths in Philippine phosphogypsum: use them or lose them. Extr. Ind. Soc. 10, 101082 https://doi.org/10.1016/j.exis.2022.101082.

Ramos, et al., 2016. Rare earth elements in raw materials and products of the phosphate fertilizer industry in South America: content, signature, and crystalline phases. J. Geochem. Explor. 168, 177-186. https://doi.org/10.1016/j.gexplo.2016.06.009.

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Rashad, 2017. Phosphogypsum as a construction material. J. Clean. Prod. 166, 732-743. https://doi.org/10.1016/j.jclepro.2017.08.049.

Reitsma et al. (2018) On the sustainability and progress of energy neutral mineral processing. 10(1), 235. https://doi.org/10.3390/su10010235.

Ribeiro, et al., 2021. Rare earth elements (REEs) rich-phosphate fertilizers used in Brazil are more effective in increasing legume crops yield than their REEs-poor counterparts. Int. J. Plant Prod. 15, 1-11. https://doi.org/10.1007/s42106-021-00129-5.

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Ruan, et al., 2019. Review on beneficiation techniques and reagents used for phosphate ores. Minerals 9 (4), 253. https://doi.org/10.3390/min9040253.

Rutherford, et al., 1994. Environmental impacts of phosphogypsum. Sci. Total Environ. 149 (1-2), 1-38. https://doi.org/10.1016/0048-9697(94)90002-7.

Rutsaert, et al., 2021. The geography of agricultural input markets in rural Tanzania. Food Secur. 13, 1379-1391. https://doi.org/10.1007/s12571-021-01181-9.

Rweyemamu, Kim, 2020. Potential environmental hazard to the public from the operation of uranium mining and milling facility. Radiat. Prot. Dosimetry 192 (1), 75-88. https://doi.org/10.1093/rpd/nca195.

Ryszko, et al., 2023. Quality of phosphate rocks from various deposits used in wet phosphoric acid and P-fertilizer production. Materials. (Basel) 16 (2), 793. https://doi.org/10.3390/ma16020793.

Saad, et al., 2003. Determination of the uranium content of Egyptian phosphate ores by passive and active detectors. Radiat. Radiat. Meas. 36 (1-6), 561-565. https://doi.org/10.1016/S1350-4487(03)00203-8.

Sajid, et al., 2021. Synoptic view on P ore beneficiation techniques. Alex. Eng. J. 61 (4), 3069-3092. https://doi.org/10.1016/j.aej.2021.08.039.

Samrane, et al., 2023. Comprehensive analysis and relevant developments of cadmium removal technologies in fertilizers industry. Miner. Eng. 201, 108189 https://doi.org/10.1016/j.mineng.2023.108189.

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Lai, et al., 1985. Determination of uranium in fertilisers using fission track method. Fertilizer Res. 6, 85-89. https://doi.org/10.1007/BF01058167.

Liesch, et al., 2015. Uranium in groundwater — Fertilizers versus geogenic sources. Sci. Total Environ. 536, 981-995. https://doi.org/10.1016/j.scitotenv.2015.05.133.

Lisuma, et al., 2022. Suitability of blended Minjingu fertilisers for flue cured tobacco production in Tanzania's sandy soils. J. Mod. Agric. Biotechnol. 1 (3), 1-13. https://www.jagronline.org/10.53964/jmab.2022013.

Lolila, et al., 2022. Demarcation of pollution-prone areas around the Manyoni Uranium Project, Tanzania. Aerosol Air Qual. Res. 22 (12), 220214 https://doi.org/10.4209/aqr.220214.

López, et al., 2019. Uranium supply potential from phosphate rocks for Argentina's nuclear power fleet. Resour. Policy 62, 397-404. https://doi.org/10.1016/j.resourpol.2019.04.008.

Mäkinen, et al., 2019. Sequential bioleaching of phosphorus and uranium. Minerals 9 (6), 331. https://doi.org/10.3390/min9060331.

Makweba, Holm, 1993. The natural radioactivity of the rock phosphates, phosphatic products and their environmental implications. Sci. Total Environ. 133, 99-110. https://doi.org/10.1016/0048-9697(93)09115-M.

Mathivanan, et al., 2022. Mobilization and health risk assessment of fertilizer induced uranium in coastal groundwater. Environ. Res. 203, 111781 https://doi.org/10.1016/j.envres.2021.111791.

Mchihiyo, 1991. Phosphate potential in Tanzania. Fertilizer Res. 30, 177-180. https://doi.org/10.1007/BF01048650.

McLaughlin, et al., 2021. Chapter One - Managing cadmium in agricultural systems. Adv. Agron. 166, 1-129. https://doi.org/10.1016/bs.agron.2020.10.004.

Mdachi, et al., 2024. The assessment of heavy metals and natural radioactivity in the phosphate tailings at Minjingu mines in Tanzania. J. Ecol. Eng. 25 (1), 269-277. https://doi.org/10.12911/22998993/175249.

Menzel, 1968. Uranium, radium, and thorium content in phosphate rocks and their possible radiation hazard. Agric. Food Chem. 16 (2), 231-234. https://doi.org/10.1021/jf60156a002.

Mew, et al., 2023. Phosphate price peaks and negotiations – part 2: the 2008 peak and implications for the future. Resour. Policy 83, 103588. https://doi.org/10.1016/j.resourpol.2023.103588.

Meza, et al., 2015. A thorough investigation of the uranium concentration in phosphate mines: a case study of Minjingu phosphate mine, Arusha, United Republic of Tanzania. Int. J. Low Radiat. 10 (1), 74-92. https://doi.org/10.1504/IJLR.2015.071766.

Mhagama, et al., 2023. Optimization of goat manure against use of N-Minjingu Nafaka plus fertilizer for improved lablab growth and yield in semi-arid areas of Northern Tanzania. Environ. Challenges, 100751. https://doi.org/10.1016/j.envc.2023.100751.

Mkangwa, 2003. Effects of Minjingu phosphate rock and triple super phosphate as phosphorus sources for maize and groundnut in semi-arid lands of Central Tanzania. East. Afri. Agric. For. J. 69 (1), 81-88. https://doi.org/10.4314/eaafj.v69i11.1808.

Michelson, et al., 2021. Misperceived quality: fertilizer in Tanzania. J. Dev. Econ. 148, 102579 https://doi.org/10.1016/j.jdeveco.2020.102579.

Moumin, et al., 2019. Solar treatment of cohesive particles in a directly irradiated rotary kiln. Solar Energy 182, 480-490. https://doi.org/10.1016/j.solener.2019.01.093.

Msolla, et al., 2005. Hard Minjingu Phosphate Rock: an alternative P source for maize production on acid soils in Tanzania. Nutr. Cycl. Agroecosyst. 72, 299-308. https://doi.org/10.1007/s10705-005-6081-7.

Mustonen and Annanmaki (1988) Studies on the radiation exposure of workers in connection with processing of the Minjingu phosphate in Tanzania Supplementary Report to the Finnish Center for Radiation and Nuclear Safety 666/622/87.

Mwalongo, et al., 2023a. Uranium in phosphate rocks and mineral fertilizers applied to agricultural soils in East Africa. Environ. Sci. Pollut. Res. 30, 33898-33906. https://doi.org/10.1007/s11356-022-24574-5.

Mwalongo, et al., 2023b. Influence of phosphate fertilizers on the radioactivity of agricultural soils and tobacco plants in Kenya, Tanzania, and Uganda. Environ. Sci. Pollut. Res. https://doi.org/10.1007/s11356-023-27543-8.

Mwalongo, et al., 2024. Uranium dissemination with phosphate fertilizers globally: a systematic review with focus on East Africa. Sustainability 16 (4), 1496. https://doi.org/10.3390/su16041496.

Nabahungu, et al., 2007. Limestone, Minjingu Phosphate rock and green manure application on improvement of acid soils in Rwanda. In: Bationo, A., Wasba, B., Kihara, J., Kimetu, J. (Eds.), Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-5760-1-65.

Ndeleko-Barasa, et al., 2021. Agronomic and financial benefits of direct Minjingu phosphate rock use in acidic humic nitisols of Upper Eastern Kenya. Heliyon (11), e08332. https://doi.org/10.1016/j.heliyon.2021.e08332.

Nie, et al., 2013. Containing rare earth phosphate rock processing and rare earth enrichment in those processes. Adv. Mat. Res. 734-737, 1022-1028. https://doi.org/10.4028/www.scientific.net/AMR.734-737.1022.

Nyambati, Opala, 2014. The effect of Minjingu phosphate rock and triple superphosphate on soil phosphorus fractions and maize yield in western Kenya. ISRN Soil Sci. https://doi.org/10.1155/2014/920541.

Onal, Atak, 1992. Uranium production as a byproduct from Yarimca phosphoric acid plant. Miner. Process. Extr. Metall. Rev. 10 (1), 1-9. https://doi.org/10.1080/08827509208914071.

Orabi, et al., 2018. Extraction of rare earth elements from Abu-Tartour wet process phosphoric acid using synthesized salicylaldehyde azine. Miner. Eng. 122, 113-121. https://doi.org/10.1016/j.mineng.2018.03.037.

Otero, et al., 2005. Fertiliser characterisation: major, trace and rare earth elements. Appl. Geochem. 20 (8), 1473-1488. https://doi.org/10.1016/j.apgeochem.2005.04.002.

Palattao, et al., 2018. Recovery of uranium from Philippine wet phosphoric acid using D2EHPA-TOPO solvent extraction. Philipp. J. Sci. 147 (2), 275-284. ISSN 0031-7683.

Pantelica, et al., 1997. INAA of some phosphates used in fertilizer industries.

J. Radioanal. Nucl. Chem. 216 (2), 261-264. https://doi.org/10.1007/BF02033788.

Parker, 1984. Tennessee Valley Authority National Fertilizer Development Center Progress 83 (No. TVA/OACD-84/4). Tennessee Valley Authority, Muscle Shoals. Office of Agricultural and Chemical Development, AL (USA).

Pavón, et al., 2022. Iron(III) removal and rare earth element recovery from a synthetic wet phosphoric acid solution using solvent extraction. Miner. Eng. 182, 107569 https://doi.org/10.1016/j.mineng.2022.107569.

Pfister, et al., 1976. Population dose from natural radionuclides in phosphate fertilizers. Radiat. Environ. Biophys. 13, 247-261. https://doi.org/10.1007/BF01330769.

Qamouche, et al., 2020. Radiological characterization of phosphate rocks, phosphogypsum, phosphoric acid and phosphate fertilizers in Morocco: an assessment of the radiological hazard impact on the environment. Mater. Today: Proc. 27 (4), 3234-3242. https://doi.org/10.1016/j.matpr.2020.04.073.

Qamouche, et al., 2021. Uranium and other heavy metal sorption from Moroccan phosphoric acid with argan nutshell sawdust. Miner. Eng. 171, 107085 https://doi.org/10.1016/j.mineng.2021.107085.

Ramirez, et al., 2022. Rare earths in Philippine phosphogypsum: use them or lose them. Extr. Ind. Soc. 10, 101082 https://doi.org/10.1016/j.exis.2022.101082.

Ramos, et al., 2016. Rare earth elements in raw materials and products of the phosphate fertilizer industry in South America: content, signature, and crystalline phases. J. Geochem. Explor. 168, 177-186. https://doi.org/10.1016/j.gexplo.2016.06.009.

Ramekte, et al., 2022. Analyses of uranium and fluoride in diammonium phosphate fertilizers marketed in India during 2021-2022. Int. J. Manures and Fertilizers 10 (2), 1-10.

Rashad, 2017. Phosphogypsum as a construction material. J. Clean. Prod. 166, 732-743. https://doi.org/10.1016/j.jclepro.2017.08.049.

Reitsma et al. (2018) On the sustainability and progress of energy neutral mineral processing. 10(1), 235. https://doi.org/10.3390/su10010235.

Ribeiro, et al., 2021. Rare earth elements (REEs) rich-phosphate fertilizers used in Brazil are more effective in increasing legume crops yield than their REEs-poor counterparts. Int. J. Plant Prod. 15, 1-11. https://doi.org/10.1007/s42106-021-00129-5.

Roselli, et al., 2009. Radiological characterization of phosphate fertilizers: comparison between alpha and gamma spectrometry. Microchem. J. 91 (2), 181-186. https://doi.org/10.1016/j.microc.2008.10.003.

Roshy, et al., 2023. Leaching of rare earths from Abu Tartur (Egypt) phosphate rock with phosphoric acid. J. Mater. Cycles Waste Manag. 25, 501-517. https://doi.org/10.1007/s10162-022-01558-8.

Ruan, et al., 2019. Review on beneficiation techniques and reagents used for phosphate ores. Minerals 9 (4), 253. https://doi.org/10.3390/min9040253.

Rutherford, et al., 1994. Environmental impacts of phosphogypsum. Sci. Total Environ. 149 (1-2), 1-38. https://doi.org/10.1016/0048-9697(94)90002-7.

Rutsaert, et al., 2021. The geography of agricultural input markets in rural Tanzania. Food Secur. 13, 1379-1391. https://doi.org/10.1007/s12571-021-01181-9.

Rweyemamu, Kim, 2020. Potential environmental hazard to the public from the operation of uranium mining and milling facility. Radiat. Prot. Dosimetry 192 (1), 75-88. https://doi.org/10.1093/rpd/nca195.

Ryszko, et al., 2023. Quality of phosphate rocks from various deposits used in wet phosphoric acid and P-fertilizer production. Materials. (Basel) 16 (2), 793. https://doi.org/10.3390/ma16020793.

Saad, et al., 2003. Determination of the uranium content of Egyptian phosphate ores by passive and active detectors. Radiat. Radiat. Meas. 36 (1-6), 561-565. https://doi.org/10.1016/S1350-4487(03)00203-8.

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Samrane, et al., 2023. Comprehensive analysis and relevant developments of cadmium removal technologies in fertilizers industry. Miner. Eng. 201, 108189 https://doi.org/10.1016/j.mineng.2023.108189.

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Szilas, et al., 2007b. Establishment of an agronomic database for Minjingu phosphate rock and examples of its potential use. Nutr. Cycl. Agroecosyst. 78, 225–237. https://doi.org/10.1007/s10705-006-9086-y.

Szilas, et al., 2008. The reactivity of Tanzanian Minjingu phosphate rock can be assessed from the chemical and mineralogical composition. Geoderma 147 (3-4), 172–177. https://doi.org/10.1016/j.geoderma.2008.08.009.

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Walters, 2011. A technical review of the improved hard process. In: The Fertiliser Association of India International Technical Conference. New Delhi, India. April 21-22, 2011.

Wang, et al., 2010. Recovery of rare earths from wet-process phosphoric acid. Hydrometallurgy 101 (1-2), 41–47. https://doi.org/10.1016/j.hydromet.2009.11.017.

Weterings, Janssen, 1985. Recovery of uranium, vanadium, yttrium and rare earths from phosphoric acid by a precipitation method. Hydrometallurgy 15 (2), 173–190. https://doi.org/10.1016/0304-386X(85)90052-0.

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Wu, et al., 2018a. Recovery of rare earth elements from phosphate rock by hydrometallurgical processes – a critical review. Chem. Eng. J. 335, 774–800. https://doi.org/10.1016/j.cej.2017.10.143.

Wu, et al., 2018b. Simultaneous recovery of rare earths and uranium from wet process phosphoric acid using solvent extraction with D2EHPA. Hydrometallurgy 175, 109–116. https://doi.org/10.1016/j.hydromet.2017.10.025.

Yamaguchi, et al., 2009. Distribution of uranium in soil components of agricultural fields after long-term application of phosphate fertilizers. Science of the Total Environment 407 (4), 1383–1390. https://doi.org/10.1016/j.scitotenv.2008.10.011.

Yamazaki, Geraldo, 2003. Uranium content in phosphate fertilizers commercially produced in Brazil. Appl. Radiat. Isot. 59 (2-3), 133–136. https://doi.org/10.1016/S0969-8043(03)00159-3.

Ye, et al., 2019. Uranium resources in China's phosphate rocks - identifying low-hanging fruits. IOP Conf. Ser.: Earth Environ. Sci. 227, 052033 https://doi.org/10.1088/1755-1315/227/5/052033.

Zaitsev, et al., 2015. Rare earth elements in phosphorites and carbonatites of the Devonian Kola Alkaline Province, Russia: examples from Kovdor, Khibina, Vuoriyarivi and Turiy Mys complexes. Ore Geol. Rev. 64, 477–498. https://doi.org/10.1016/j.oregorev.2014.06.004.

Zanin, Zamirailova, 2007. Uranium in supergene phosphorites. Geochem. Int. 45, 32–46. https://doi.org/10.1134/S001670290701003X.

Zhang, 2014. Comprehensive recovery and sustainable development of phosphate resources. Procedia Eng. 83, 37–51. https://doi.org/10.1016/j.proeng.2014.09.010.

Zielinski, et al., 1993. Recovery of lanthanides from kola apatite in phosphoric acid manufacture. J. Chem. Technol. Biotechnol. 56 (4), 355–360. https://doi.org/10.1002/jctb.280560405.
Semu, Singh, 1995. Accumulation of heavy metals in soils and plants after long-term use of fertilizers and fungicides in Tanzania. Fertilizer Res. 44, 241–248. https://doi.org/10.1007/BF00750931.

Shang, et al., 2021. Unconventional uranium in China's phosphate rock: review and outlook. Renew. Sustain. Energy Rev. 140, 110740 https://doi.org/10.1016/j.rser.2021.110740.

Shlewit, Alibrahim, 2008. Recovery of uranium from phosphate by carbonate solutions. J. Radioanal. Nucl. Chem. 275, 97–100. https://doi.org/10.1007/s10967-007-6965-x.

Silva, et al., 2019. Inputs of rare earth elements in Brazilian agricultural soils via P-containing fertilizers and soil correctives. J. Environ. Manage. 232, 90–96. https://doi.org/10.1016/j.jenvman.2018.11.031.

Singh, et al., 2005. Uranium uptake studies in some plants. Radiat. Meas. 40 (2-6), 666-669. https://doi.org/10.1016/j.radmeas.2005.01.013.

Singh, et al., 2016. Recovery of uranium from phosphoric acid: a review. Solvent Extr. Ion Exch. 34 (3), 201–225. https://doi.org/10.1080/07366299.2016.1169142.

Sobhy, Tao, 2014. Innovative RTS technology for dry beneficiation of phosphate. Procedia Eng. 83, 111–121. https://doi.org/10.1016/j.proeng.2014.09.020.

Songore, 1991. The Matongo phosphate deposit in Burundi. Fertilizer Res. 30, 151–153. https://doi.org/10.1007/BF01048646.

Soudry, et al., 2002. Uranium oxidation state and related variations in geochemistry of phosphorites from the Negev (southern Israel). Chem. Geol. 189 (3-4), 213–230. https://doi.org/10.1016/S0009-2541(02)00144-4.

Steiner, et al., 2020. Making Uranium Recovery from Phosphates Great Again? Environ. Sci. Technol. 54 (3), 1287–1289. https://doi.org/10.1021/acs.est.9b07859.

Sun, et al., 2020. Co-evolution' of uranium concentration and oxygen stable isotope in phosphate rocks. Appl. Geochem. 114, 104476 https://doi.org/10.1016/j.apgeochem.2019.104476.

Sun, et al., 2020a. Critical accumulation of fertilizer-derived uranium in Icelandic grassland Andisol. Environ. Sci. Eur. 32, 92. https://doi.org/10.1186/s12302-020-00367-w.

Sun, et al., 2020b. Non-critical uranium accumulation in soils of German and Danish long-term fertilizer experiments. Geoderma 370, 114336. https://doi.org/10.1016/j.geoderma.2020.114336.

Syers, et al., 1986. Chemical and physical characteristics of phosphate rock materials of varying reactivity. J. Sci. Food Agric. 37 (11), 1057–1064. https://doi.org/10.1002/jsfa.2740371102.

Szilas, et al., 2007a. Can local Minjingu phosphate rock replace superphosphate on acid soils in Tanzania? Nutr. Cycl. Agroecosyst. 77, 257–268. https://doi.org/10.1007/s10705-006-9064-4.

Szilas, et al., 2007b. Establishment of an agronomic database for Minjingu phosphate rock and examples of its potential use. Nutr. Cycl. Agroecosyst. 78, 225–237. https://doi.org/10.1007/s10705-006-9086-y.

Szilas, et al., 2008. The reactivity of Tanzanian Minjingu phosphate rock can be assessed from the chemical and mineralogical composition. Geoderma 147 (3-4), 172–177. https://doi.org/10.1016/j.geoderma.2008.08.009.

Takeda, et al., 2006. Accumulation of uranium derived from long-term fertilizer applications in a cultivated Andisol. Sci. Total Environ. 367 (2-3), 924–931. https://doi.org/10.1016/j.scitotenv.2006.01.006.

Tommasi, et al., 2020. Review of rare earth elements as fertilizers and feed additives: a knowledge gap analysis. Arch. Environ. Contam. Toxicol. 81, 531–540. https://doi.org/10.1007/s00244-020-00773-4.

Tulsidas, et al., 2019. Uranium resources in EU phosphate rock imports. Resour. Policy 61, 151–156. https://doi.org/10.1016/j.resourpol.2019.02.012.

Ulrich, 2019. Cadmium governance in Europe's phosphate fertilizers: not so fast? Sci. Total Environ. 650 (1), 541–545. https://doi.org/10.1016/j.scitotenv.2018.09.014.

Ulrich, et al., 2014. Uranium endowments in phosphate rock. Sci. Total Environ. 478, 226–234. https://doi.org/10.1016/j.scitotenv.2014.01.069.

Uranium One (2023) Mkuju river project. http://www.uranium1.com/index.php/en/development/mkuju-river-tanzania, accessed 2023-07-17.

Van Kauwenbergh, 1991. Overview of phosphate deposits in East and Southeast Africa. Fertilizer Res. 30, 127–150. https://doi.org/10.1007/BF01048645.

Van Kauwenbergh, 1997. Cadmium and other minor elements in world resources of phosphate rock. In: Proceedings-Fertilizer Society No. 400. London.

Verbeeck, et al., 2020. Trace element concentrations in mineral phosphate fertilizers used in Europe: a balanced survey. Sci. Total Environ. 712, 136419 https://doi.org/10.1016/j.scitotenv.2019.136419.

Veríssimo, et al., 2016. The Itataia phosphate-uranium deposit (Ceara, Brazil) new petrographic, geochemistry and isotope studies. J. South Am. Earth Sci. 70, 115–144. https://doi.org/10.1016/j.jsames.2016.05.002.

Vogel, et al., 2020. Uranium and thorium species in phosphate rock and sewage sludge ash based phosphorus fertilizers. J. Hazard. Mater. 382, 121100 https://doi.org/10.1016/j.jhazmat.2019.121100.

Vučić, Ilić, 1989. Extraction and spectrophotometric determination of uranium in phosphate fertilizers. J. Radioanal. Nucl. Chem. 129, 113–120. https://doi.org/10.1007/BF02037574.

Walters, 2011. A technical review of the improved hard process. In: The Fertiliser Association of India International Technical Conference. New Delhi, India. April 21-22, 2011. https://doi.org/10.1016/j.hydromet.2009.11.017.

Wang, et al., 2010. Recovery of rare earths from wet-process phosphoric acid. Hydrometallurgy 101 (1-2), 41-47. https://doi.org/10.1016/j.hydromet.2009.11.017.

Weterings, Janssen, 1985. Recovery of uranium, vanadium, yttrium and rare earths from phosphoric acid by a precipitation method. Hydrometallurgy 15 (2), 173–190. https://doi.org/10.1016/0304-386X(85)90052-0.

Winde, et al., 2017. Uranium from Africa - an overview on past and current mining activities: re-appraising associated risks and chances in a global context. J. Afr. Earth Sci. 129, 759-778. https://doi.org/10.1016/j.jafrearsci.2016.12.004.

Wu, et al., 2018a. Recovery of rare earth elements from phosphate rock by hydrometallurgical processes - a critical review. Chem. Eng. J. 335, 774–800. https://doi.org/10.1016/j.cej.2017.10.143.

Wu, et al., 2018b. Simultaneous recovery of rare earths and uranium from wet process phosphoric acid using solvent extraction with D2EHPA. Hydrometallurgy 175, 109–116. https://doi.org/10.1016/j.hydromet.2017.10.025.

Yamaguchi, et al., 2009. Distribution of uranium in soil components of agricultural fields after long-term application of phosphate fertilizers. Science of the Total Environment 407 (4), 1383–1390. https://doi.org/10.1016/j.scitotenv.2008.10.011.

Yamazaki, Geraldo, 2003. Uranium content in phosphate fertilizers commercially produced in Brazil. Appl. Radiat. Isot. 59 (2-3), 133–136. https://doi.org/10.1016/S0969-8043(03)00159-3.

Ye, et al., 2019. Uranium resources in China's phosphate rocks - identifying low-hanging fruits. IOP Conf. Ser.: Earth Environ. Sci. 227, 052033 https://doi.org/10.1088/1755-1315/227/5/052033.

Zaitsev, et al., 2015. Rare earth elements in phosphorites and carbonatites of the Devonian Kola Alkaline Province, Russia: examples from Kovdor, Khibina, Vuoriyarivi and Turiy Mys complexes. Ore Geol. Rev. 64, 477–498. https://doi.org/10.1016/j.oregorev.2014.06.004.

Zanin, Zamirailova, 2007. Uranium in supergene phosphorites. Geochem. Int. 45, 32–46. https://doi.org/10.1134/S001670290701003X.

Zhang, 2014. Comprehensive recovery and sustainable development of phosphate resources. Procedia Eng. 83, 37–51. https://doi.org/10.1016/j.proeng.2014.09.010.

Zielinski, et al., 1993. Recovery of lanthanides from kola apatite in phosphoric acid manufacture. J. Chem. Technol. Biotechnol. 56 (4), 355–360. https://doi.org/10.1002/jctb.280560405.