Fertilizer International 518 Jan-Feb 2024
31 January 2024
Innovative magnesium removal technology
PHOSPHATE INDUSTRY INNOVATION
Innovative magnesium removal technology
Kevin De Bois of Prayon Technologies describes an innovative process for removing magnesium from phosphate rock. Increasingly, phosphoric acid producers are looking to consume low-grade phosphate rock as a feedstock due to the prohibitive costs of high-grade rock sources. This has potentially negative consequences as the presence of impurities such as magnesium can negatively affect both the phosphoric acid process and the quality of the acid produced.
Introduction
Two types of impurities are found in phosphate rock. The first type affects the quality of the final acid or phosphogypsum. These include heavy metals, such as Cd, As, Cr, Pb, and Hg, radionuclides, and elements such as fluorine that limit the range of applications of the final product.
The second type of impurities negatively affect the phosphoric acid and fertilizer production process. The three major impurities in this category are magnesium, aluminium, and iron. These three elements are collectively measured in phosphate rock, relative to P2 O5 content, as the minor element ratio (MER). This is calculated as follows:
MER = (MgO + Al2 O3 + Fe2 O3 ) / P2 O5
It is generally accepted that producing phosphoric acid from phosphate rock with a MER higher than 0.08 is problematic. This article will address this challenge by focusing on the removal of magnesium from phosphate rock.
Magnesium – a production plant poison!
While magnesium is a key element for plant health and growth, it is considered a ‘poison’ for phosphoric acid and fertilizer production. As Pierre Becker, one of the fathers of phosphoric acid technology, wrote1 : “With MgO > 0.6% in the phosphate rock, it is difficult to produce phosphoric acid.”
In phosphoric acid plants, excess magnesium in phosphate rock will:
1. Increase the viscosity of the phosphoric acid.
2. Reduce the P2 O5 yield.
3. Decrease the efficiency of gypsum crystal growth.
The primary effect of magnesium on phosphoric acid production is to increase viscosity. This, in turn, adversely impacts the filterability of phosphoric acid/gypsum slurries. The typical filterability of an acid with an MER of 0.04, for example, is around 8-10 tonnes P2 O5 /m², while the filterability of an acid with an MER of 0.15 is around 3-4 t P2 O5 /m². In practice, this means that, at higher MER values, the filter’s sur face area will need to be more than doubled to achieve the same filtration throughput.
Additionally, magnesium also affects the P2 O5 yield. This is because P2 O5 losses occur when magnesium partially precipitates as pyrophosphate magnesium during phosphoric acid concentration2 .
Although magnesium’s impact on gypsum crystallisation is not entirely clear, several studies suggest that magnesium affects this mechanism and it kinetics by inhibiting gypsum formation1,3,4 .
Previously, OCP and Jacobs Engineering have conducted pilot plant and lab tests to better understand the effects of the MgO content of phosphate rock on phosphoric acid production. Their results indicate that higher MgO levels in phosphate rock reduce filtration, and lower chemical yields due to the loss of water-soluble P2 O55 .
As well as its impact on phosphoric acid production, magnesium also affects the ability to granulate phosphate fertilizers, such as MAP, DAP, NPK, and NPS, and reduces the mechanical strength of the resulting granules.
Treating low-grade rock – Ecophos and GetMoreP
In recent years, Prayon has been extensively developing and perfecting the Ecophos and GetMoreP (GMP) processes (Fertilizer International 512, p38) to address the increasing demand for the treatment of low-grade and/or high MER phosphate rocks.
The underlying principle of both the GMP and Ecophos processes is a nuanced and selective approach to solubility. In general, these two processes proceed as follows:
• Firstly, a controlled acid attack maximises the solubilisation of P2 O5 from phosphate rock.
• This is followed by a step that gradually increases pH to induce the precipitation of aluminium, iron and, if necessary, fluorine.
• Following this pH adjustment, the slurry is filtered and the unwanted filter cake is disposed of to landfill.
• Simultaneously, a secondary increase in pH is used to precipitate P2 O5 from the valuable liquid fraction – which contains the majority of the P2 O5 – in the form of dicalcium phosphate (DCP).
Avoiding excessive pH adjustment is crucial as this prevents the precipitation of impurities like lead, cadmium and even magnesium with P2 O5 . Such elements are typically precipitated in a subsequent third step, depending on the processes involved.
Targeting magnesium removal
The GMP and Ecophos processes and their underlying principles are now well-established and understood. Colloboration with EuroChem, which has access to high magnesium deposits in Kazakhstan, led Prayon to an obvious follow-up question: could these procedures be streamlined for certain types of phosphate rock where the sole objective was magnesium removal?
To answer this question properly, the relevant process costs versus the leaching/solubility objectives need to be considered, particularly in terms of acid and base consumption (calcium carbonate or hydrated lime), as illustrated in Figure 1.
Prayon and EuroChem carried out a series of leaching experiments to look at the main factors affecting magnesium removal and P2 O5 yield. Numerous rocks were leached at varying acid/base ratios to assess their influence on magnesium and P2 O5 extraction.
The results obtained were surprisingly positive – as they revealed the potential to extract a substantial amount of magnesium while effectively still retaining the majority of P2 O5 in the solid phase (Figure 2). These promising findings highlighted the effectiveness and viability of Prayon and EuroChem’s joint leaching process and its ability to remove magnesium without compromising the integrity of solid-state phosphate.
Further tests were carried out subsequently to fine-tune the process via adjustments to variables such as residence time, temperature, the solids content, etc.
These supplementary investigations provided insights into the acid leaching process. Importantly, they confirmed conclusively that dissolving carbonate impurities with sulphuric acid is a promising and practical beneficiation method for phosphate rock.
Prayon and EuroChem’s results demonstrated that this targeted dissolution method is selective for carbonates, particularly dolomite (CaMg(CO3 )2 ) yet, valuably, leaves the phosphate minerals intact and unaltered. Following leaching, a meticulous filtration and washing procedure is used to separate the beneficiated solid rock from the acid solution containing the dissolved carbonates. The process generates gypsum (CaSO4·2H 2 O) and magnesium sulphate (MgSO4 ) as coproducts and releases CO2 gas.
The refined and upgraded phosphate rock obtained – which is now entirely devoid of magnesium – can proceed seamlessly to the phosphoric acid plant for downstream processing. The residual acid solution, meanwhile, undergoes further treatment to precipitate magnesium as magnesium hydroxide (Mg(OH)2 ). Finally, solid/liquid separation is used to recover the precipitated hydroxide and generate a water filtrate (Figure 3).
Process validation at Technophos
All of these experimental data required validation at pilot-scale and semi-industrial level. The was necessary to substantiate laboratory findings for this novel leaching process, define its key technical features and demonstrates its resilience at larger scale.
The necessary verification was carried out at Technophos, Prayon’s showcase pilot and semi-industrial demonstration plant in Varna, Bulgaria. Technophos is equipped with semi-industrial reactors, ranging from 1 to 10 m³ capacity, and various industrial solid/liquid separation units, including vacuum filters, press filters and settlers.
The versatile and advanced test unit at Varna was used to comprehensively assess a modular production process for the digestion of phosphate rock with dilute acid. Technophos uses a systematic three-stage approach to provide a gradual but comprehensive process evaluation:
• Phase 1: Laboratory tests
• Phase 2: Pilot tests
• Phase 3: Semi-industrial demonstration test.
Satisfactory results were obtained from the initial laboratory and pilot test campaigns at Technophos conducted and managed by specialists from Prayon and EuroChem – with the beneficiated rock produced meeting all the necessary requirements and specifications. Building on these successful outcomes, the decision was made to proceed with a demonstration test project to prove the feasibility of continuous beneficiated rock production at semi-industrial scale. The following findings were all provided by this semi-industrial trial.
Overall, the semi-industrial test results demonstrated the ability of the method to effectively process and upgrade phosphate rock with a magnesium content of up to 3.6 percent MgO. Phosphate rock containing this high level of magnesium would – without effective removal technology – be unsuitable for phosphoric acid production.
The following two modules were evaluated during the Technophos trial:
- Magnesium leaching module. A highly efficient vacuum filtration process was chosen to separate the beneficiated rock from the magnesium leaching solution. By meticulously selecting a filter cloth with a high air permeability, the larger rock particles, sand and gypsum generated by the acidulation reaction were retained effectively. The chosen filter, by ensuring a uniform cake distribution, enhanced both filterability and drying efficiency. The outcome of these choices was a beneficiated rock with an impressively low average moisture content (less than 15%). This also translated into a desirably low water consumption (one tonne of water per tonne of P2 O5 ).
- Neutralisation module. The magnesium leaching module delivered highly positive results, efficiently precipitating P2 O5 , magnesium, and sulphate. The standard quality water generated was also suitable for recycling in the leaching process. Semi-industrial testing also highlighted the significance of temperature in the neutralisation module. Hydrated lime was used for logistical reasons, although the use of quicklime is generally recommended at industrial-scale to optimise operational expenditure (opex). Filter presses proved to be exceptionally efficient at separating the slurry generated in the neutralisation module, although alternative vacuum filtration methods will be explored in future. The resulting cake, which comprises of pure gypsum dihydrate and magnesium oxide, could be recycled at fertilizer production plants to enhance the magnesium content of specific fertilizers.
A sustainable beneficiation process
The above two modules can be operated in a loop as a sustainable process that generates minimal residues. The beneficiated rock product can be directly supplied as a feedstock for downstream phosphoric acid plants, while the residue from the neutralisation module can be valorised in the fertilizer plant, as suggested. The water consumed in the process is completely recyclable with no liquid waste discharge.
Concurrently, a pilot test was carried out with Prayon’s Mark IV di-hydrate (DH) process using the beneficiated rock from the semi-industrial Technophos trial as a feedstock. Results showed that 75 percent of the MgO present in the beneficiated rock passed into the phosphoric acid.
DH phosphoric acid production remained stable, albeit with slightly lower filterability rates than observed with low magnesium phosphate rock. The phosphoric acid generated was used to successfully manufacture standard-grade fertilizers such as NPK (15-15-15) and monoammonium phosphate (MAP) without any issues. The fertilizer granules obtained also exhibited excellent mechanical strength (above 15 MPa).
Conclusions
To conclude, the magnesium leaching process developed by Prayon and EuroChem provides several notable advantages:
1. High magnesium removal from phosphate rock: The initial magnesium extraction phase is remarkably efficient, achieving more than 60 percent magnesium removal.
2. High P2 O5 yield: P2 O5 losses are minimal (below 5%) with the process delivering a stable 98 percent yield. This high efficiency is crucial as it optimises the quality of the beneficiated rock.
3. Feasibility of treating high-magnesium rock: Results demonstrate the feasibility of beneficiating high magnesium phosphate rocks (1.9% Mg equivalent to 3.5% MgO) which – without effective Mg removal technology – are unsuitable for use in phosphoric acid production.
4. Low operational costs: The process is cost-effective with raw material consumption limited to water (one tonne per tonne of P2 O5 ) and quicklime (0.15 tonne per tonne of P2 O5 ).
5. Filtration process efficiency: Given the importance of filtration in the fertilizer industry, the successful use of undervacuum filtration in the magnesium leaching module, and the excellent filterability in the neutralisation module, gives confidence in the easy industrialisation of the process.
6. Sustainable process: The magnesium leaching and magnesium neutralisation modules can be operated in a loop as a sustainable process. Overall residues are minimal, as the beneficiated rock product obtained directly supplies the phosphoric acid plant, while the residue from the neutralisation module can be valorised in the fertilizer plant or elsewhere. Finally, the water consumed is entirely recyclable and the process therefore generates zero liquid discharge.
7. Validated for fertilizer production: The semi-industrial beneficiation test was supplemented by a pilot test on the beneficiated rock with the Prayon Mark IV di-hydrate (DH) process. The acid produced was successfully used to manufacture standard-grade fertilizers, such as NPK 15-15-15 and MAP. These granulated without any issues and exhibited excellent mechanical strength (above 15 MPa).
8. Easy integration into existing plants: The magnesium leaching process can be seamlessly integrated into existing plants, as a key intermediate step between mechanical beneficiation and phosphoric acid production, requiring only minor operational changes.
In summary, semi-industrial testing by Prayon and EuroChem has successfully demonstrated has successfully demonstrated the viability of magnesium leaching as an effective phosphate rock beneficiation process. The positive results obtained highlight its potential industrial application in high-quality fertilizer production. By optimising key parameters and improving process efficiency, the approach to beneficiation outlined in this article should contribute to the overall sustainability and environmental performance of the phosphoric acid industry.
EuroChem and Prayon have signed a licensing and patent agreement for this joint process and a joint international application has been submitted. As per the terms of this agreement, Prayon has the right to license and replicate this process worldwide with certain limitations.
References
