Fusion Extraction of Base Metals (Al, Cr, Fe, Ti and V) Using Ammonium Phosphate Salt as Flux

Abstract

The fusion method of using ammonium phosphate salt as flux was assessed for its ability to precipitate metaphosphate compounds containing trivalent ions M³⁺ = Al, Cr, Fe, Ti, and V as M(PO₃)₃ in inorganic salts and a certified reference material (CRM) mineral ore sample. Fusion analysis using mixtures of inorganic salts containing AlCl₃, CrCl₃, FeCl₃, and VCl₃ showed variable amounts of precipitates isolated as metaphosphate compounds in the order of iron (12%) < vanadium (13%) < chromium (30%) < (44%) aluminum. However, an analysis of the CRM (AMIS 0368) where magnetite (Fe³⁺/Fe²⁺) and ilmenite (Ti⁴⁺) are the dominant phases, showed that the obtained precipitates were in the order of chromium (less than 0.1%) < vanadium (1%) < aluminum (2%) < titanium (9%) < iron (68%). The metaphosphate compounds isolated via the use of this method were identified using XRD analysis. SEM–EDX analysis showed micro-crystalline particles from the inorganic salts that were irregular and clustered, contrary to the amorphous micro particles which were produced from the CRM. The degree of specificity improved considerably using CRM ore (AMIS 0368) with high iron content (~76%, Fe³⁺/Fe²⁺). This method was shown to be highly selective towards metals with a stable trivalent oxidation state. No other elements of a different oxidation state were precipitated.

1. Introduction

Recent trends in research have sparked a renewed interest in establishing methods which are efficient in processing ores and concentrates using hydrometallurgical and pyrometallurgical techniques [1]. Mineral processing is often complicated by the presence of impurities and the inability of the method to achieve clean separation. The efficiency of extraction in hydrometallurgical and pyrometallurgical techniques is mainly dependent on the degree of selectivity and the amount of impurities in the sample. In the hydrometallurgical extraction of platinum group elements (PGE) from chromite ore, impurities derived from refractive elements and other elements, such as aluminum, ferric iron, magnesium, and chromium, complicate the mineral processing by producing molten sticky products [2], which affect the normal operating conditions. Refractive elements occur in chromite ores in different quantities depending on the spinel structure (Mg²⁺, Fe²⁺) O (Cr³⁺, Al³⁺, Fe³⁺)₂O₃. The removal of base metals is essential to the PGE industry for the removal of interfering base metals, which often limit the normal operating conditions of the blast furnace by forming sticky melts, which cause high operational costs and frequent shutdown of plants.

In different studies, we investigated the isolation of chromium, vanadium, and tungsten from chromite, titanomagnetite (AMIS 0501), and wolframite mineral ore using ammonium phosphate salt as flux. The use of ammonium phosphate as flux was key in isolating these metals as insoluble micro-crystalline c-type metaphosphate M(PO₃)₃ particles (where M = Cr³⁺ and V³⁺⁾, and soluble phosphates compounds such as WO₂(PO₃)₂ [3]. From our previous studies, it was shown that the formation of metaphosphate compounds provided an alternative approach for separating chromium and vanadium [4] from samples with complex matrices, such as mineral ores [5]. However, elemental analysis of the isolated chromium and vanadium metaphosphate products revealed the presence of Al, Fe, or Ti as impurities. Metaphosphate products of chromium revealed the presence of 5% (Fe), 2% (Al), and <1% (Ti), whilst the metaphosphate product of vanadium revealed the presence of 21% (Fe), 3% (Al), and 45% (Ti). Both metaphosphate products revealed impurities with similar chemical compositions, which prompted a revisit of this technique.

A review of this fusion method was prompted by the desire to improve the selectivity of the ammonium phosphate fusion method and to increase the purity of the metaphosphate’s products. The key drive to this investigation was the need to determine the optimum conditions for the selective separation of pure metaphosphate compounds from samples with complex matrices. To achieve this, a review of the reported metaphosphate compounds of the transition elements was considered, as well as the conditions under which they were formed. This background information was critical to decipher the disparity between the unreacted starting material and the possible by-products. Although, due to the trivalent oxidation state of the transition metals, the formation of metaphosphate compounds is considered unusual in solid-state chemistry. These compounds are stabilized by a dense network of polyphosphate (PO₄)₃⁻ units, which forms an anionic framework that has a high degree of chemical, mechanical, and thermal stability [6].

Numerous metaphosphate compounds have been reported as belonging to a series of polyphosphate compounds [7,8]. These polymorphic compounds can conform into different crystalline forms denoted as A, B, C, D, and E, of which the C-type is the predominant form [9]. The crystalline structure of chromium [10] and vanadium metaphosphate compounds obtained in our previous study belong to the C-type compounds. The C-type polyphosphate compounds (M(PO₃)₃ M = Cr³⁺ and V³⁺) are composed of infinite PO₃⁻ chains and a network of individual MO₆ octahedral structures connected through an infinite number of repeated PO₄ tetrahedral units, as shown in Figure 1 [11]. Studies have shown similarities in the chemical and physical properties of these isostructural compounds V(PO₃)₃ [12], Cr(PO₃)₃ [6,13,14], Al(PO₃)₃ [15], Ti(PO₃)₃ [16], and Fe(PO₃)₃ [6]. The formation of mixtures of metaphosphate products was considered highly inevitable in samples such as chromite (Mg²⁺, Fe²⁺)O (Cr³⁺, Al³⁺, Fe³⁺)₂O₃ and titanomagnetite Fe²⁺(Fe³⁺, Ti)₂O₄ mineral ore, which bear different M³⁺ ions. Studies have shown similarities in the magnetic properties, thermal stability, high melting points, etc., of these metaphosphate compounds, making separation a daunting task [17].

It is evident from the preceding analysis that the current fusion method using ammonium phosphate flux is susceptible to the formation of different metaphosphate products. The possibility of co-precipitation is highly probable in mineral ore samples due to the presence of impurities. Therefore, it was decided that alternative operational conditions were necessary to improve the selectivity of this method. The preliminary stage of this investigation involved a review of this fusion process to determine the optimum conditions necessary for the isolation of pure metaphosphate compounds. The reaction mechanism was also examined in an attempt to optimize the selectivity and purity of the products.

2. Experimental Section

2.1. Chemicals and Materials

Ammonium hydrogen phosphate, both (NH₄)₂HPO₄ and (NH₄)H₂PO₄, aluminum nitrate-9-hydrate (Al(NO₃)₃·9H₂O), vanadium (III) chloride (VCl₃), iron(III) chloride-6-hydrate (FeCl₃·6H₂O), chromium(III) chloride hexahydrate (CrCl₃·6H₂O), mineral acid (HNO₃ 65%) together with the original multi element ICP standard (1000 mg/L) were purchased from Merck Chemicals, Modderfontein, South Africa. The ceramic crucibles (capacity, 150 mL) were purchased from Terra Nova Ceramics (Johannesburg, South Africa), and all the reagents were used without further purification. Ultra-pure deionized water (0.01 µS/cm, conductivity) was used for wet chemical analysis and all analytical experimental measurements were reported as an average of three replicates.

2.1.1. Description of the Prepared Inorganic Sample

The inorganic sample was prepared by mixing Al(NO₃)₃·9H₂O, (0.701 g), VCl₃ (0.155 g), FeCl₃·6H₂O (0.243 g), and CrCl₃·6H₂O (0.147 g). The resultant mixture containing 50 mg of each element (Al, V, Fe and Cr) was added to the ammonium phosphate flux (25 g) and thoroughly homogenized. The final mixture was transferred in a ceramic crucible and placed in a preheated oven (800 °C) until a molten melt was formed (20 min). The melt was cooled at room temperature and dissolved in deionized water.

2.1.2. Description of the Certified Reference Material (CRM)

The certified reference material (CRM) used is a titanomagnetite ore, (AMIS 0368) which was purchased from African Mineral Standards (AMIS). According to the analysis certificate, the reference material originated from Rhovan Vanadium Mine in Northwest Province of South Africa. Method of preparation: The CRM was crushed, dry milled, and air classified to less than 54 microns. Confirmation of particle size was conducted using wet sieve particle size analysis, which confirmed the material contained 98.5% of particles less than 54 microns. The certified elemental content of the major elements in the sample were reported as 3.64 ± 0.06% (Al₂O₃), 0.24 ± 0.02% (Cr₂O₃), 75.51 ± 0.86% (Fe₂O₃), 0.68 ± 0.02% (MgO), 2.65 ± 0.06% (SiO₂), 13.91 ± 0.18% (TiO₂), and 1.52 ± 0.04% (V₂O₃).

2.2. Experimental Procedures

2.2.1. Sample Preparation and Characterization

Fusion of the control sample together with the CRM (AMIS 0368) was performed using a Barnstead Thermolyne furnace. Eutech pH meter (CyberScan, pH 1500, Thermo Scientific, Shanghai, China) was used for the pH measurements of the filtrate solutions. Characterization of the precipitates obtained was performed using Bruker Fourier-Transform infrared (FT-IR, Billerica, MA, USA) spectrometer in the wavenumber range 400–4000 cm⁻¹. The spectrum was recorded via KBr pellet technique using Thermo Scientific Nicolet 6700, Scanning electron microscope (SEM, Tescan VEGA3, Brno-Kohoutovice, Czech Republic) equipped with an energy dispersive X-ray spectrometer (EDX, Oxford X-Max^N) from the Centre for Microscopy, using AZtec software (University of the Free State) and an X-ray powder diffraction spectrometer using Highscore software (XRD, Malvern Panalytical, Empyrean, Malvern, United Kingdom) from Geology department (University of the Free State). Wet chemical analysis was performed via a Perkin-Elmer Nexion (model 2000c) inductively coupled plasma-mass spectrometer (ICP-MS, Shelton, CT, USA) using the measurement conditions set out in

Table 1. Selected ICP-MS operating conditions for elemental analysis using the KED mode.

Parameter/Component	Value/Type/Mode
Torch position	X (−1.52 mm), Y (−0.33 mm) and Z (0.00 mm)
Auxiliary gas flow	1.2 L/min
Nebulizer gas flow	0.96 L/min
Plasma gas flow	15 L/min
Dwell time	50 ms
RF power	1100 W
Rinsing time	60 s
Integration time	1 ms
Replicates	3
Nebulizer	Meinhard concentric type A3
Scanning mode	Peak hopping
Spray chamber	Baffled quartz cyclonic
Mode of operation	Kinetic discrimination energy (KED) using Helium gas

.

2.2.2. Preparation of Multi-Element Calibration Standards

Multi-element calibration standards with a working range of 0–500 µg/L were prepared in separate volumetric flasks (100.0 mL) using a ‘Transferpette’ micro-pipette. The solutions were acidified using HNO₃ acid (5.0 mL; 65%) and filled to the mark using deionized water. The final solutions were homogenized before use.

2.2.3. Sample Preparation Using Ammonium and Sodium Phosphate Flux

Fusion of the inorganic salts and CRM (AMIS 0368) was conducted separately in triplicate analysis using ammonium phosphate flux (NH₄)₂HPO₄/NH₄H₂PO₄). The powdered samples (1.0 g) were thoroughly mixed in separate ceramic crucibles with an excess amount of ammonium phosphate flux (sample: flux ratio, 1:25). The resultant mixtures were heated in a furnace (800 °C) until molten melts were formed (±15 min). The red-hot molten melts were cooled at room temperature and deionized water (50 mL) was added. The solid melts were stirred until the all the melts formed mixtures. The resultant mixtures were filtered, and the solid product was air-dried at room temperature before being characterized using FT-IR, SEM–EDX, and XRD analysis, whilst the filtrate was analyzed using ICP-MS.

2.2.4. Analysis of the Ammonium Phosphate Precipitate

SEM–EDX Analysis

The precipitates from the inorganic sample and CRM (AMIS 0368) were mounted on aluminum pin stubs using double-sided carbon tape and coated with Iridium (±10 nm) using a Leica EM ACE600 sputter coater. Specimens were imaged and analyzed with a JSM-7800F Extreme-resolution Analytical Field Emission SEM (Tokyo, Japan), equipped with an Oxford Instruments X-Max 80 mm EDX detector, using AZtec software. Samples were imaged at 5 kV at a WD of 10 mm and EDX analysis were performed at 20 kV.

XRD Analysis

The XRD analysis was used for the phase identification of the precipitates from the inorganic and CRM (AMIS 0368) samples. The Bruker (Melvern Panalytical Empyrean) X-ray diffractometer, equipped with Cu Kα radiation of wavelength ≈ 0.154 nm, was used to record the XRD patterns. Rietveld refinement was performed using the FullProf software to calculate the structural parameters.

2.2.5. Analysis of the Ammonium Phosphate Filtrate Solutions

ICP-MS Analysis

The filtrate solutions from the inorganic sample and CRM (AMIS 0368) were analyzed using ICP-MS for the elemental content. The acid matrix of these samples was matched by adding HNO₃ (5 mL; 65%) to the samples. The ICP-MS operating conditions used for elemental analysis via the KED mode are listed in Table 1. The samples were homogenized and analyzed using the selected isotopic lines.

3. Results and Discussion

The analysis of the selective isolation of metaphosphate compounds from the inorganic and mineral ore sample (CRM) was first performed by probing the effects of temperature. Temperature was determined in our previous studies to be key in the formation of metaphosphate products and in achieving complete sample dissolution. Although various approaches for the synthesis of metaphosphate compounds have been used at different temperatures [19], it was yet to be established if temperature changes had an effect on the dissolution as well as the formation of metaphosphate compounds. The optimum temperature for the fusion extraction of metaphosphate compounds using various mineral ore samples was previously determined to be 800 °C. Preliminary investigations at lower temperatures (<800 °C) showed incomplete dissolution, and at higher temperatures (>800 °C), viscous molten melts were obtained, which solidified the melts so that they were difficult to dissolve. The fusion of pure salts (AlCl₃, CrCl₃, FeCl₃, and VCl₃) at 800 °C yielded solid melts which were glassy and crystalline (Figure 2). It is worth noting that all of the solid melts obtained using this method were hygroscopic, which made them easier to dissolve in water or any other polar solvents.

The different products obtained at temperatures above and below 800 °C were consistent with the research findings conducted by Magda et al. [20], which shows the decomposition stages of ammonium phosphate salt. The decomposition of ammonium phosphate salt (Equations (1)–(4)) forms different species depending on temperature. The uncertainty of forming a variety of products increased as the temperature increased beyond 800 °C and more phosphate species formed.

$${(\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{H}\mathrm{P}\mathrm{O}}_4\stackrel{∆ < 800 °\mathrm{C}}{\to }{\mathrm{N}\mathrm{H}}_4{\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4+{\mathrm{N}\mathrm{H}}_3$$

(1)

$${2\mathrm{N}\mathrm{H}}_4{\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\stackrel{∆ < 800 °\mathrm{C}}{\to }{(\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{H}}_2{\mathrm{P}}_2{\mathrm{O}}_7+{\mathrm{H}}_2\mathrm{O}$$

(2)

$${(\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{H}}_2{\mathrm{P}}_2{\mathrm{O}}_7\stackrel{∆ < 800 °\mathrm{C}}{\to }{2\mathrm{N}\mathrm{H}}_4{\mathrm{P}\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$$

(3)

$${2\mathrm{N}\mathrm{H}}_4{\mathrm{P}\mathrm{O}}_3\stackrel{∆ > 800 °\mathrm{C}}{\to }{\mathrm{P}}_2{\mathrm{O}}_5+{2\mathrm{N}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}$$

(4)

3.1. XRD Characterization

Precipitates obtained from the pure salts, CRM mineral ore, and inorganic salts were analysed using the XRD technique to identify their composition as well as the purity of the isolated precipitates. The XRD pattern of the precipitates obtained from pure salts were evaluated and compared with those in the XRD data base. The differences in the XRD pattern confirmed the successful conversion of the starting material to products. It was confirmed that the patterns corresponded to the metaphosphate compounds of Al(PO₃)₃, Cr(PO₃)₃, Fe(PO₃)₃, and V(PO₃)₃ [12] (Figure 3a). The XRD patterns for the precipitates obtained from the inorganic salts showed that the following mixtures were present in the metaphosphate compounds: Cr(PO₃)₃ [11] Fe(PO₃)₃, Al(PO₃)₃, and Ti(PO₃)₃ [16]. The XRD pattern of the CRM mineral ore positively identified the presence of two major phases in the ore: magnetite (Fe₃O₄) and ilmenite (FeO·TiO₂). These phases in the mineral ore are the major sources of iron (Fe²⁺/Fe³⁺) and Ti³⁺ in the CRM mineral ore. The XRD pattern for the CRM showed dominant peaks in the region of 20–35° (2θ) corresponding to Fe(PO₃)₃, Al(PO₃)₃, and Ti(PO₃)₃, as shown in Figure 3b.

3.2. IR Characterization

The IR spectra of the precipitates from the inorganic salts and CRM mineral ore exhibit three strong peaks in the region of 1246–1273, 956–964, and 713–767 cm⁻¹, which were characteristic of the stretching vibration of P–O_ext and the asymmetric stretching frequencies of (P–O_int)PO₂ and P–O–P of the metaphosphates. The spectrum obtained from the CRM precepitate showed an additional sholder peak in the region of 1222 cm⁻¹, which also corresponds to the P–O_ext vibration of the metaphosphates. The three stretching bands in the region of 964, 767, and 713 cm⁻¹ were assigned to the stretching mode of the P–O–P bridge bonds. Both spectra of the two products exhibit bands with a poor resolution in the range of 470–615 cm⁻¹, which is mainly predominated by the antiasymetric stretching modes of the isolated tridimensional network of MO₆ units, and to a lesser extent, by the antisymmetric bending modes of the chain group (O–P–O). A summary of the stretching frequencies of both products is provided in

Table 2. Comparison between the stretching frequencies of selected bands of the experimentally determined precipitates from the inorganic salts and CRM mineral ore with the reported metaphosphates (M(PO₃)₃) M = Ti³⁺, V³⁺, Cr³⁺, and Fe³⁺. Values in bold denote experimental values.

Stretching Bands	Experimentally Determined		Reference [22]
	Inorganic Salts	CRM Mineral Ore	Ti3+	V3+	Cr3+	Fe3+
vas(P–Oext)|PO2|	1273 vs	1246 vs	1385 vs	1240 vs	1240 vs	1230 vs
	1147	1149	1115 s	1150 s	1160 s
	1107	1118			1130 s	1120 s
vs(P–Oext) |PO2|	1076	1080 vs	1090 s	1095 s	1095 vs	1075 s
v(P–Oext)|PO3|	1033	1033	1040 s		1050 s
	1010	1014		1020 s	1020 s	1020 s
vas(P–Oint) |P–O–P|	964 vs	956 vs	960 s	975 s	975 s	970 s
			770 m	770 m	775 m
	763	767 vs	750 m	765 m	760 w	750 m
vas(P–Oint) |P–O–P|	713 vs	713 vs	715 m	715 m
				725 m
						705 m
	675	675	690 w	685 m	685 w	675 m
vas(M–O)	621	617				615 w
					570 m	580 w
			560 m	560 m	560 m
sas(O–P–O)						545 m
					520 m
				510 m		505 m
					490 m
			470 m	480 m		470 m

vs—very strong; s—strong; m—medium; w—weak.

. The stretching fequences of both products are compared with those of the isostructural metaphosphate compounds belonging to the C-type. The IR spectra of the precipitates from the inorganic salts showed a number of similar peaks to those of the C-type chromium (III)metaphosphate compound, whilst the precipitate from CRM mineral ore was closely related to the C-type iron (III) metaphosphate. The splitting peaks which can be observed in the region of 665–765 cm⁻¹ of both spectra is attributable to the high crystallinity of the proucts [21]. This was confirmed by the sharp distinguished peaks of both precipitates and the complete lack of background noise compared to the pattern of the CRM mineral ore, which was amorphous.

3.3. SEM–EDX and ICP-MS Analysis

3.3.1. Analysis of Products from a Mixture of Inorganic Salts

The morphology and the surface/interface characterization of the precipitates from the inorganic salts and CRM mineral ore sample (AMIS) was conducted using the SEM–EDX technique at a nanospace range of 0–20 keV. The objective of the analysis was to identify the physical and chemical phases in the precipitates, as well as the elemental composition. SEM images (Figure 4) of the precipitates from the inorganic salts showed micro-crystalline particles that were irregular and clustered. These particles were uniformly distributed throughout the sample and consisted of O (51.3 wt%), P (31.7 wt%), Cr (6.7 wt%), Al (4.6 wt%), Fe (1.8 wt%), and V (2.0 wt%). All the elements added as starting materials (i.e., Al, Cr, Fe, and V) in the form of inorganic salts were retained in the precipitate but in different elemental proportions. A mixture of these elements in the precipitate confirmed the inability of the ammonium phosphate fusion method to selectively precipitate a single product. The order of preference, according to the precipitated inorganic product, was Fe (1.8 wt%) < V (2.0 wt%) < Al (4.6 wt%) < Cr (6.7 wt%), which corresponds to the percentage conversion of 44.37% (Cr), 30.46% (Al), 13.25% (V), and 11.92% (Fe). It is interesting to note that iron was the least preferred amongst the M³⁺ metals despite its abundance. The results also suggest chromium has the highest likelihood of being precipitated first as chromium(III) metaphosphate. The aforementioned precipitation order, determined by EDX analysis, was attributed to the differences in the rate of reaction between the phosphate (PO₃⁻) ions and the M³⁺ ions (M = Al, Cr, Fe, and V).

The ICP-MS analysis of the filtrate solutions obtained after the removal of solid precipitates aimed to determine the elemental content of the unreacted M³⁺ ions and other ions in the sample. The percentage conversion of the M³⁺ ions to metaphosphate compounds was determined by analyzing the corresponding elemental content in the filtrate solutions.

Table 3. Summary of the elemental content of the precipitates and filtrate solutions obtained from the inorganic salts and CRM mineral ore.

Element	Inorganic Salts (Certified/Known conc. %)	Inorganic Salts		AMIS 0368 (Certified/Known conc. (%))	AMIS 0368
		SEM–EDX (Precipitate, %)	ICP-MS (Filtrate, %)		SEM–EDX (Precipitate, %)	ICP-MS (Filtrate, %)
Al	25	7.62	17.38	3.64	2.37	1.27
Cr	25	11.09	13.91	0.24	<0.1	0.24
Fe	25	2.98	22.02	75.51	68.45	7.06
V	25	3.31	21.69	1.52	0.98	0.54
Ti	-	-	<0.01	13.91	9.46	4.45
Na	-	-	<0.01	-	-	<0.01
Mg	-	-	<0.01	-	-	<0.01
K	-	-	<0.01	-	-	<0.01
Si	-	-	<0.01	2.65	-	1.75
Ca	-	-	<0.01	-	-	<0.01
Mn	-	-	<0.01	-	-	<0.01
Co	-	-	<0.01	-	-	<0.01
Ni	-	-	<0.01	-	-	<0.01
Cu	-	-	<0.01	-	-	<0.01
Ba	-	-	<0.01	-	-	<0.01

-—Element not present/no certified or known conc. Emboldened elements reflect the theme of the manuscript.

shows the quantitative results of the filtrate solutions. The matrix effects caused by the presence of polyatomic ions were circumvented using the KED helium mode. The percentage content of unreacted M³⁺ ions in the filtrate solutions were 91.30% (Fe), 88.82% (V), 71.52% (Al), 54.87% (Cr). The high recoveries of Fe in the filtrate solution was assumed to be the result of a slow interaction between the PO₃⁻ and Fe³⁺, which led to a reduction in product formation in comparison to the other trivalent ions Cr³⁺, Al³⁺, and V³⁺ ions.

3.3.2. EDX Analysis of Precipitates from CRM (AMIS 06038) Mineral Ore Sample

In this part of the study, we investigate whether the elemental content in the ore has any bearing influence in the selective precipitation of Al, Cr, Fe, Ti, and V as metaphosphates. According to the certificate, the percentage content of these elements in the CRM were reported as Fe₂O₃ (75.51%), TiO₂ (13.91%), Al₂O₃ (3.64%), V₂O₃ (1.52%), Cr₂O₃ (0.24%). Magnetite (Fe²⁺(Fe³⁺)₂O₄) and rutile (TiO₂) were identified by the XRD pattern as the dominant phases in the mineral ore. The fusion of this CRM sample using ammonium phosphate flux (sample: flux, 1:25) yielded a grey powdered precipitate. SEM–EDX analysis of this product revealed micro-particles that were crystalline and irregular in shape and size. The elemental compositions of these particles, as revealed by EDX analysis (Figure 5), were in the order of Cr (<0.5 wt%) < V (0.5 wt%) < Al (2.6 wt%) < Ti (3.5 wt%) < Fe (17.8 wt%). Interestingly, the elemental contents in the precipitate were ordered according to the quantities in the CRM. Ions in high concentration, e.g., Fe and Ti, were precipitated in higher proportions compared to those with lower concentrations. The proportions of the metal concentrations precipitated from the CRM mineral ore were Fe (68.45%), Ti (9.46%), Al (2.37%), V (0.98%), Cr (<0.1%). These results confirmed the influence of (M³⁺) ion content (either formed or naturally occurring +3 ions) in the original sample towards the precipitation of metaphosphate compounds. The results obtained in this part of the study also demonstrated the influence of M³⁺ content on selectivity, i.e., the higher the M³⁺ ion content in a sample, the higher the metaphosphate product.

Analysis of the CRM filtrate solutions using ICP-MS revealed an average percentage recovery of Fe (7.06%), Ti (4.45%), Al (1.27%), V (0.54%), Cr (0.24%) from the original source. The percentage content of the M³⁺ ions retained in the filtrate solution differed from those obtained from the inorganic solutions. The higher percentage proportion of iron in the filtrate solution was presumed to be the result of Fe²⁺ ions from the starting material (Fe²⁺(Fe³⁺,Ti)₂O₄). Since iron (Fe³⁺/Fe²⁺) occupied over ~76% of the CRM, it was inevitable that most of the Fe³⁺ ions in solution reacted with the PO₃⁻ ions, thereby limiting the chance (~24%) of the other elements (Ti³⁺, Al³⁺, V³⁺, and Cr³⁺ ions) interacting with PO₃^- ions. Other elements identified in the CRM filtrate solution included Na, Mg, K, Ca, Si, Mn, Co, Ni, Cu, and Ba, and their respective percentage contents are listed in Table 3. The presence of only Fe, Ti, Al, V, and Cr in the precipitate further confirmed the preference (selectivity) of this method towards the ions with a trivalent oxidation state.

4. Conclusions

The selective precipitation of base metals (M = Al, Cr, Fe, Ti, and V) was achieved using the ammonium phosphate fusion method. This procedure was determined to be ideal for the extraction of base elements as metaphosphate compounds. The chemical composition of the isolated metaphosphate compounds contained metal ions in a trivalent oxidation state. Therefore, it can be concluded that this extraction technique was ideal for metals that can form a stable trivalent oxidation state, as no impurities with different oxidation states were found. The isolated metaphosphate products were insoluble in both polar and organic solvents. The fusion method was characterized by poor selectivity as mixtures of metaphosphate from the M³⁺ compounds were precipitated. Selectivity was shown to be dependent on the percentage content of M³⁺ ions in the original sample or formed during the fusion process. It was also observed that selectivity improved in samples, such as the CRM, which contained high iron content (75.51%, presumably Fe³⁺/Fe²⁺ ions).