PROFESSIONAL TECHNICAL AND SCIENTIFIC PAPERS

 

Valorization potentials of phosphate tailings at Minjingu mines in Northern Tanzania

 

 

D.D. Mdachi; A.M. Rugaika; R.L. Machunda

The Nelson Mandela African Institution of Science and Technology, Tanzania

Correspondence

 

 

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ABSTRACT

Sedimentary and igneous rocks are the two primary sources of phosphate that are mined and beneficiated to fertilizer. During the beneficiation process, phosphate is lost into the tailings. We investigated phosphate concentrations in tailings dumps at Minjingu mine, Tanzania using energy-dispersive X-ray fluorescence spectrometry to quantify the chemical compositions. The phosphate content in the tailings varied from 12.91% phosphorus pentoxide (P₂O₅) in Tailings 2 to 19.61% in Tailings Dump 1. The naturally occurring phosphate concentration in rocks ranges from 3% to 35%, and phosphate tailings from various locations with P₂O₅ concentrations as low as 6.46-12.65% have been beneficiated to commercial fertilizer. Our investigation revealed that phosphate concentrations in Minjingu tailings may be sufficient to be recovered for commercial applications. Suitable recovery methods are discussed, and we recommend that beneficiation should be performed to minimize the loss of phosphate into tailings. Further research is needed to identify the optimal beneficiation methodology.

Keywords: phosphate tailings, valorization, Minjingu mine, phosphorus pentoxide, beneficiation

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Introduction

The main source of phosphorus and the primary raw material for manufacturing phosphate fertilizers is phosphate rock (Abouzeid, 2007; Farid et al., 2022). Most global phosphate ores originate from igneous and sedimentary deposits (Alsafasfeh and Alagha, 2017; Liang et al., 2018). Over 80% of phosphate fertilizers used globally are sourced from sedimentary phosphatic deposits (Derhy et al., 2020; El Bamiki et al., 2021). The concentration of phosphorus pentoxide (P₂O₅) in phosphate ores ranges from 3% to 35% (Notholt et al, 1979).

Phosphate deposits are mined using both opencast (or surface mining) and underground methods (Ptáček, 2016). The most common method is opencast mining, where the overburden is removed to uncover the phosphate reserve (Zhang, 2014). Bulldozers and excavators can be used to remove the topsoil, which can then be stored in stockpiles for later use, or used immediately at other reclamation sites (Mislevy et al., 2015; Toama, 2017).

The global demand for and production of phosphate fertilizers is increasing rapidly, while reserves continue to decrease (Oliveira et al., 2011; FAO, 2019; Safhi et al., 2022). Thus, the need to recover phosphate from tailings has become increasingly important (Jandieri, 2023). According to reserve assumptions and various scenarios for population growth, increasing demand for phosphate fertilizer has directly led to the depletion of phosphatic rock reserves (Wünscher, 2013). Gou et al (2019) reported that there is the possibility of phosphate being lost into tailings during the beneficiation process.

Phosphate tailings are industrial waste generated during the processing of phosphate ores into phosphate fertilizers (Chen et al., 2017). Tailings are widely recognized as a secondary source of phosphorus, which can be successfully utilized for the manufacture of phosphate fertilizer concentrates (Alsafasfeh and Alagha, 2017; Alsafasfeh et al., 2022).

Through beneficiation, the phosphate grade of the concentrate can be increased to between 28% and 35% P2O5 (Ravi et al., 2014; Boujlel et al., 2019; Alsafasfeh et al, 2022). Phosphatic rocks contain variable concentrations of phosphate minerals (Toama et al, 2015). Igneous phosphate rocks are typically low grade (< 5% P₂O₅) in comparison with sedimentary rock phosphates, but can be upgraded to 30% P₂O₅ (van Kauwenbergh, 2010).

Phosphatic rocks account for approximately 95% of global phosphate production. Some of the phosphate is lost in the tailings during beneficiation (Toamam et al, 2015). It has been demonstrated that up to 50% of the P2O5 can be lost during beneficiation (van Kauwenbergh, 2010). Taha et al. (2021) used column flotation to assess the efficiency of beneficiation at a fertilizer manufacturer in Brazil, and found that only 46.2% of the P₂O₅ was recovered.

In Tanzania, phosphate deposits are found in many different areas, and are of both igneous (Zizi, Ngualla, Panda Hill, Sangu-Ikola, and Nachendezwaya) and sedimentary (Minjingu, Chali Hill, and Chamoto) origin (Mchihiyo, 1991; Jama and van Straaten, 2006). The Minjingu deposit comprises two types of phosphates, soft phosphate and hard phosphate, which both contain > 20% P₂O₅ and can easily be upgraded to 30% P₂O₅ by dry screening, making the resultant material suitable for direct fertilizer application (van Kauwenbergh, 1991; Szilas et al., 2008; Mwalongo et al., 2022).

Bulldozers and hydraulic excavators are used in the open-pit mining process at Minjingu Mines and Fertilizers Limited (MMFL); both for the removal of overburden (topsoil, clay, and sand layers) and the excavation of ore. Owing to the soft rock at MMFL, no drilling or blasting is necessary. The phosphatic materials are delivered by dump trucks to the pre-drying area, where they are spread out, crushed, and mixed by bulldozers. The waste clays are dumped in enormous piles. Before transportation to the beneficiation facility, the phosphatic materials must be dried to < 15% moisture (Szilas, 2002). Beneficiation is performed via physical separation rather than chemical means.

Crushing, grinding, screening, scrubbing, heavy media separation, washing, roasting, calcination, and flotation techniques are used to beneficiate low-grade phosphate ore (Liu et al., 2016; Ruan et al, 2019). Arroug et al. (2021) reported that low-grade tailings containing 15.84% P₂O₅ could be upgraded to 30.7% P₂O₅ using an organic acid leaching method. Li et al (2021) reported that low-grade phosphate with approximately 12.65% P₂O₅ was upgraded to 28.68% P₂O₅ using direct and reverse flotation. Teague and Lollback (2012) found that ultrafine phosphate tailings could be upgraded from 6.46% to 34.7% P₂O₅ by flotation, and Alsafasfeh et al. (2022) obtained approximately 84.6% P2O5 recovery from tailings using direct froth flotation.

Khoshjavan and Rezai (2012), El-Midany et al (2013), Shariatiet al (2015), and Ismaila et al. (2020) have reported that phosphatic rocks with high CaO/P₂O₅ ratios can be beneficiated using calcination. Khoshjavan and Rezai (2012) upgraded low-grade phosphatic rock containing 11.9% P₂O₅ and 24.49% CaO to 31% P₂O₅ and 43.12% CaO via calcination and flotation. Ismaila et al. (2020) reported that calcination can be used to reduce the CaO/ P₂O₅ ratio from 2.5 to 1.65 Shariati et al (2015) indicated that low-grade phosphate deposits containing 9.16% P₂O₅ and 46.01% CaO could be upgraded to 30.77% P₂O₅ and 45.11% CaO using calcination and shaking table methods.

Little is known regarding the quantity of P₂O₅ present in Tanzanian phosphate tailings and how much may be recovered. Therefore, the purpose of this study was to evaluate the amount of phosphate in the tailings at MMFL in northern Tanzania.

 

Material and methods

The study area is located at Minjingu Hill, near Manyara Lake (3°42'21"-3°42'3" S, 35°54'56"-35°54'14" E). Samples were taken from tailings situated near to the open pit (Figure 1).

Samples the were collected at locations shown in Figure 2. There were five designated sample locations in each tailings dump. Three samples were collected at each designated sample location and composited. The samples were ground, dried, and split to obtain representative 50 g samples and packed into clean plastic bags (maximum capacity 1 kg) for analysis.

Sample preparation

All samples were sieved through a 60 μm mesh and dried in an oven at 100°C. A total of 4 g of each sample was mixed with 0.9 g of binder and pulverized for 10 min at 180 r/min. The pulverized material was placed in a cylindrical die of 32 mm diameter and pressed at a hydraulic pressure of 15 bar for 1 min to obtain a durable pellet for X-ray fluorescence (XRF) analysis.

X-ray analysis

Energy-dispersive XRF was used to identify the major elements in the pellets. The measurement time for all major elements in a given sample was approximately 900 s. Elemental concentrations were precisely calibrated using the International Atomic Energy Agency Certified Reference Material Soil 7 (CRM IAEA Soil 7).

 

Results and discussion

The accuracy of the XRF data was evaluated using the criterion for judging the acceptability of analytical methods (SR criterion) (Oscar et al., 2008):

where SR stands for standard random error, C_X represents the measured value, C_W is the certified value, and δ indicates the standard deviation of the experimental values.

According to this criterion, the difference between a certified value and an acquired analytical data-point can be separated into three categories: excellent (SR < 25%), acceptable (25% < SR < 50%), and unacceptable (SR > 50%). Table I presents data evaluated from three samples of the CRM IAEA Soil-7. The values for Mg, Al, Si, P, K, Ca, and Fe as determined by XRF were all in excellent agreement (SR < 25%), but that for Na was unacceptable (SR > 50%).

 

 

Our results revealed that Tailings dumps 1 and 2 had different concentrations of Na₂O, MgO, AhO₃, SiO₂, P₂O₅, K₂O, CaO, and Fe₂O₃. The data are shown in Tables II and III, respectively. Typically, a greater R² value (closer to unity) equates to a better match between a regression model and the data. In this context, the certified and measured concentrations were shown to exhibit strong correlations (Figure 3).

Composition of phosphate tailings

Our elemental analysis results demonstrated that the tailings are mainly composed of CaO, P₂O₅, SiO₂, Na₂O, MgO, AhOs, Fe₂Os, and Κ₂0, as shown in Tables II and III. The lowest concentration of P2O5 was found in sample TD21 (12.96 ± 0.02%), and the highest in TD13 (19.45 ± 0.14%). The average P₂O₅ concentrations in Tailings dumps 1 and 2 were 17.82 ± 0.29% and 14.92 ± 0.40%, respectively. Because phosphatic rocks are the main source of phosphorus, the P2O5 concentration largely determines the ore quality (Mwalongo et al., 2022). There are three different grades of phosphate ores based on P₂0₅ content: low grade (12%-16% P₂0₅), medium grade (17%-25% P₂O₅), and high grade (26%-35% P₂O₅ (Sengul et al., 2006). According to this classification, the samples from MMFL can be considered as low grade in Tailings dump 2 (Table III) and medium grade in Tailings dump 1 (Table II). The low levels of phosphates in the tailings reported herein are similar to previous findings in tailings from various other locations across the globe (Oliveira et al., 2011; Teague and Lollback, 2012; Shariati et al 2015; Arroug et al., 2021; Li et al 2021; Yang et al., 2021).

 

Conclusion

The tailings at Minjingu Mines and Fertilizers Limited (MMFL) contain elevated concentrations of Na₂O, P₂O₅, SiO₂, and CaO, with maximum values of 15.09 ± 0.24% (TD23), 19.45 ± 0.14% (TD13), 24.21 ± 0.20% (TD22), and 37.57 ± 0.26% (TD13), respectively. The lowest concentrations of Na₂O, P₂O₅, SiO₂, and CaO were 10.58 ± 0.19% (TD11), 12.96 ± 0.03% (TD21), 8.90 ± 0.05% (TD13), and 27.18 ± 0.14% (TD22), respectively. The present study, along with previous research conducted globally, indicates that the P2O5 concentration in MMFL tailings may be amenable to upgrading for the manufacture of commercial fertilizer. We recommend further research to explore an appropriate beneficiation technique for the recovery of phosphate fertilizers from these tailings.

 

Acknowledgements

The authors wish to express their appreciation to the Tanzania Mining Commission for financial support of this research. We also thank MMFL management for their cooperation in providing access to collect samples at their mine site. We thank the management of the Tanzania Atomic Energy Agency for the use of their laboratory facilities. Finally, we thank David Wacey, PhD, from Edanz (www.edanz.com/ac) for editing a draft of this manuscript.

Author contributions

D. Mdachi: Conceptualization, funding acquisition, formal analysis, investigation, writing original draft preparation, visualization A. Rugaika: Methodology, review and editing, supervision R. Machunda: Methodology, validation, formal analysis, review and editing, supervision. All authors have read and approved the final manuscript.

 

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Correspondence:
A.M. Rugaika
Email: anita.rugaika@nm-aist.ac.tz

Received: 22 Jun. 2023
Revised: 11 Oct. 2023
Accepted: 14 Jun. 2024
Published: July 2024

 

 

ORCID:  A.M. Rugaika http://orcid.org/0000-0002-6313-9136