Phase Formation Features in the Metallothermal Reduction of Natural Coltan

Abstract

Phase formation characteristics during the thermochemical reduction of metals from natural coltan using aluminum and calcium–aluminum alloy at 1400–1450 °C were investigated to develop methods for extracting niobium and tantalum from rare metal raw materials. The studied coltan sample consists of a columbite–tantalite solid solution with the composition (Mn,Fe)(Nb,Ta)₂O₆, cassiterite Sn_0.9O₂, tapiolite (Ta,Nb)₂(Mn,Fe)O₆, and calcioolivine Ca₂SiO₄. This study established that the choice of reducing agent determines the sequence of oxide phase transformations. During the aluminothermic process, orthorhombic columbite–tantalite is completely reduced, while tetragonal tapiolite persists even at 1400 °C. The use of a calcium–aluminum alloy containing 69.4 wt.% Ca results in a reversal of this trend: tapiolite is reduced at the early stages (800–1250 °C) through an intermediate (Ta,Nb)O₂ phase, whereas the columbite–tantalite solid solution remains up to 1250 °C. Calcium, having a high affinity for oxygen, forms intermediate perovskite-type oxide phases that act as diffusion barriers, limiting the access of the reducing agent to residual mineral inclusions (mainly Nb-Ta minerals of the orthorhombic crystal system). A temperature rise to 1450 °C initiates the redistribution of oxide components: the content of CaNbO₃ decreases, the Ca₂(Nb,Ta)AlO₆ phase disappears, and its components are involved in the formation of Ca(Nb,Ta)_0.25MnO_2.74 and Ca₄Nb₂O₉. Diffusion constraints are reduced, and the residual columbite–tantalite solid solution is reduced, as confirmed by its complete absence in the products at 1450 °C. In the metallic phase, solid solutions of tantalum and niobium, Ta-Nb-Sn intermetallic compounds (Ta,Nb)₃Sn, titanium aluminide, and ferroalloys with an increased (Ta,Nb)/(Fe,Mn) ratio are formed. The phase transformations elucidated during metallothermic reduction of coltan using different reducing agents, together with the formation of metallic and intermetallic phases, establish a scientific foundation for the development of advanced rare metal extraction processes.

1. Introduction

Niobium and tantalum are classified as refractory rare metals of significant technological importance in modern industry. Due to their unique chemical and physical properties, such as corrosion and heat resistance, ductility, superconductivity, high electrical and magnetic permeability, as well as biocompatibility, these metals are widely used in various fields of modern technology, e.g., the manufacture of heat-resistant alloys and alloyed steels, electrovacuum devices and electrolytic capacitors, medical and chemical equipment resistant to aggressive media, etc. [1,2,3].

Niobium and tantalum commonly occur together in various ore minerals and concentrates due to their similar physicochemical properties. More than 50 niobium and tantalum minerals are known, represented predominantly by complex oxides. From the perspective of metal extraction, the three most important groups are: the columbite–tantalite solid solution series with the general formula (Fe,Mn)(Nb,Ta)₂O₆; pyrochlore—(Na,Ca,REE)₂(Nb,Ta,Ti)₂O₆(OH,F); and loparite—(Na,Ca,Ce)(Ti,Nb)O₃ [3,4]. In addition to the aforementioned minerals, ore assemblages may include tapiolite, euxenite, thoreaulite, and ilmenorutile, as well as minerals containing niobium as an isomorphic impurity (e.g., ilmenite, cassiterite, and wolframite).

In terms of niobium and tantalum content, coltan represents the most valuable raw material [5,6]. The largest coltan deposits are located in the Democratic Republic of the Congo [7,8]. The crystal structure of columbite–tantalite is described by the general formula AB₂O₆, where the A-site cation comprises an isomorphic mixture of Fe²⁺ and Mn²⁺, while the B-site is occupied by Nb⁵⁺ and Ta⁵⁺. The composition of the columbite–tantalite solid solution series varies widely, with Nb₂O₅ content ranging from 2 to 75 wt.% and Ta₂O₅ from 1 to 85 wt.% [4]. Minerals of this group crystallize in the orthorhombic crystal system and are chemically similar to tapiolite, which shares the same general formula but adopts a tetragonal structure [4,9].

Against the background of steadily increasing global demand for niobium and tantalum, the involvement of new deposits in industrial processing and the development of advanced technologies for manufacturing value-added products based on these metals have become critical priorities.

Numerous methods exist for extracting niobium and tantalum from ores and concentrates [10,11], as well as from industrial waste [12,13] and secondary raw materials [14,15]. Conventional coltan processing typically involves chemically aggressive conditions. The most common methods utilize hydrofluoric acid [16,17], mixed acid systems [18,19], or various alkalis [20,21]. Chlorination technology is also widely employed for processing mineral and technogenic Nb-Ta raw materials [22,23]. While these conventional methods have proven technologically and economically efficient for high-grade tantalum–niobium ores, they encounter significant difficulties when processing low-grade materials. Specifically, chlorination technology poses severe safety and environmental risks due to the use of large volumes of chlorine, rendering it economically unviable for ores with high gangue content. Similarly, hydrometallurgical processes inevitably generate significant volumes of fine-grained sludge requiring disposal or storage. Additional drawbacks include complex feed preparation, multi-stage process flows, energy-intensive evaporation operations, and wastewater neutralization requirements.

In the development of technologies for refractory rare metals, particularly tantalum and niobium, metallothermal recovery of valuable metals from low-grade complex ores and technogenic waste, followed by hydrometallurgical separation, represents a promising direction [24,25,26,27,28,29]. This concentration method relies on the selective reduction of element oxides present in the raw material, which is particularly advantageous for processing low-grade ores and concentrates. Furthermore, metallothermal reduction offers several advantages over processes such as carbothermal reduction, primarily due to its exothermic nature and simplified equipment design.

Reduction of Ta and Nb from their oxides can be achieved using reducing agents such as aluminum [24], magnesium [25,26], calcium [27,28], and silicon [29]. The metallothermal process is often conducted via self-propagating high-temperature synthesis (SHS) to obtain powders with a high specific surface area. In contrast, the aluminothermic process yields metal in the form of ingots or pellets. Aluminum is selected as the reducing agent for rare metal oxides due to its high reactivity and relatively low cost. If the heat generated during the reaction is insufficient, or to facilitate metal–slag separation, exothermic additives (e.g., iron oxide) and fluxes (CaO, CaF₂, and SiO₂) are added to the charge.

To enhance the reduction efficiency of tantalum and niobium oxides, the use of reducing agents with high chemical activity is recommended. Calcium, possessing significant reducing power, is one of the most efficient reagents for producing tantalum and niobium. However, its application in pure form is limited by its high reactivity with air, which leads to rapid oxide film formation, complicating industrial handling. An optimal solution is the use of a calcium–aluminum master alloy, which combines the high reducing potential of calcium with the oxidation resistance of aluminum. This approach offers additional advantages: firstly, the formation of low-melting-point eutectics in the CaO–Al₂O₃ system, facilitating metal–slag separation; secondly, achieving a high yield of high-quality metal due to the synergistic effect of the alloy components.

The industrial implementation of the calcium–aluminothermic process was carried out at NPM Silmet OÜ (Estonia) with the objective of reducing pure niobium and tantalum oxides [28]. Nevertheless, the utilization of this technology in relation to ore-based mineral feedstock, particularly coltan, remains to be the subject of adequate scholarly scrutiny. It is imperative that research is conducted into the patterns of phase formation during the reduction of coltan, in order to develop a scientifically sound technology for the extraction of rare metals. This work is of particular relevance in this context.

This paper presents the results of a study on phase formation during the metallothermal reduction of natural coltan using aluminum and a calcium–aluminum alloy.

2. Materials and Methods

A natural coltan sample from the Democratic Republic of the Congo was used in this study; its chemical composition is presented in

Table 1. Chemical composition of the natural coltan sample.

Mineral	Content, wt.%
	Nb	Ta	Fe	Mn	Sn	Si	Al	Cu	Mg	Ni	Ti	Zr
Coltan	21.18	26.74	8.29	4.86	5.90	0.81	0.62	0.36	0.78	0.34	1.19	0.11

. The mineral was crushed to a particle size of <0.1 mm prior to experimentation. Aluminum powder of PA-4 grade (Al ≥ 98.0 wt.%, Fe ≤ 0.35 wt.%, Si ≤ 0.40 wt.%, and particle size < 0.14 mm) and metallic calcium (Ca ≥ 98.7 wt.%, Cu ≤ 0.01 wt.%, and particle size < 1.0 mm) were employed as reducing agents. Additionally, a calcium–aluminum master alloy was prepared by melting high-purity metals in an electric resistance furnace under an argon atmosphere at 950–1000 °C. The Ca/Al mass ratio in the alloy was maintained at 2.27:1.0. Its phase composition was characterized by the intermetallic compounds Ca₈Al₃ and Ca₁₃Al₁₄, and the alloy was ground to a particle size of <0.1 mm for experimental use.

The chemical composition of the natural coltan sample was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a SPECTRO ARCOS spectrometer (SPECTRO Analytical Instruments GmbH, Kleve, Germany).

The phase composition and crystal structure of the natural coltan sample, as well as the products of its interaction with aluminum, calcium, or a calcium–aluminum master alloy, were determined by powder X-ray diffraction (XRD) using Rietveld refinement. Measurements were performed at room temperature on a D8 ADVANCE diffractometer («Bruker AXS GmbH», Karlsruhe, Germany) equipped with a VÅNTEC-1 position-sensitive detector («Bruker AXS GmbH», Karlsruhe, Germany), using Cu Kα radiation (34 kV, 40 mA) and a β-filter. Diffraction patterns were collected over a 2θ range of 5–100° with a step size of 0.021° and a counting time of 3284 s per step. Phase identification and structural analysis were carried out using the ICDD PDF-4+ database.

The microstructure and elemental composition of the natural coltan sample were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) using a TESCAN MIRA 3 LMU microscope (TESCAN ORSAY HOLDING, Brno, Czech Republic) equipped with an Oxford Instruments INCA Energy 350 system (Oxford Instruments plc, Abingdon, Oxfordshire, UK) and an X-max 80 silicon drift detector (TESCAN ORSAY HOLDING, Brno, Czech Republic). The phase identification for minerals was established based on the correlation of three independent datasets: (1) local elemental composition obtained by EDS; (2) morphology and contrast observed in BSE mode; and (3) crystallographic parameters determined by Rietveld refinement for the bulk sample.

Phase transformations during the metallothermal reduction process were investigated using simultaneous thermal analysis (STA) with NETZSCH STA 449 C Jupiter (NETZSCH-Gerätebau GmbH, Selb, Germany) and STA 449 F3 Jupiter instruments (NETZSCH-Gerätebau GmbH, Selb, Germany). Measurements were performed during heating to 800–1460 °C and subsequent cooling at a rate of 20 °C/min under a flow of high-purity argon (99.998%). The compositions of the samples used for thermal analysis are presented in

Table 2. Composition of coltan-reductant mixtures used for thermal analysis: heating at 20 °C/min to target temperatures (t_max) under flowing argon.

Experiment No	Charge Composition, wt.%				tmax, °C
	Coltan	Al	Ca-Al Master Alloy	Ca
1	79.7	20.3	–	–	1400
2	77.8	17.8	–	4.4	1400
3	69.4	–	30.6	–	800
4					1250
5					1460

.

3. Results and Discussion

According to XRD data (Figure 1), the studied coltan sample consists of the following minerals: 80.0 wt.% columbite–tantalite solid solution with the composition (Mn,Fe)(Nb,Ta)₂O₆, 8.0 wt.% cassiterite Sn_0.9O₂, 7.5 wt.% tapiolite (Ta,Nb)₂(Mn,Fe)O₆, and 4.5 wt.% calcioolivine Ca₂SiO₄. The columbite–tantalite solid solution and calcioolivine crystallize in the orthorhombic crystal system with space groups Pbcn (ICDD PDF-4 04-005-8824) and Pmmn (ICDD PDF-4 04-012-6734), respectively, whereas cassiterite and tapiolite belong to the tetragonal crystal system (space group P4₂/mnm). The unit cell parameters for all identified minerals are listed in

Table 3. Phase composition and unit cell parameters of phases in the natural coltan sample.

No	Mineral/ Formula	Unit Cell Parameters				Content, wt.%	PDF4 Cards
		a (Å)	b (Å)	c (Å)	V (Å3)
1	Columbite–tantalite/ (Mn,Fe)(Nb,Ta)2O6	14.3003 (8)	5.7410 (3)	5.0982 (3)	418.55 (4)	80.0	04-005-8824
2	Calcioolivine/ Ca2SiO4	6.734 (8)	5.515 (7)	9.30 (1)	345.2 (7)	4.5	04-012-6734
3	Cassiterite/ Sn0.9O2	4.7387 (2)	–	3.1839 (2)	71.50 (1)	8.0	01-070-4176
4	Tapiolite/ (Mn,Fe)(Ta,Nb)2O6	4.7551 (4)	–	9.214 (1)	208.33 (4)	7.5	04-011-1021

. Based on the general formula AB₂O₆ for columbite–tantalite and the dependence of unit cell parameters on the Fe/Mn ratio [30], the proportions of iron and manganese were estimated to be 0.46 and 0.54, respectively.

SEM-EDS analysis revealed the chemical compositions of various mineral phases within the coltan sample corresponding to the columbite–tantalite group (Figure 2,

Table 4. Local elemental composition of identified mineral phases determined by EDS.

Points	Element Content, wt.% (Min-Max)/Mean Value									Mineral/Chemical Formula
	O	Al	Si	Ti	Mn	Fe	Nb	Sn	Ta
1	22.9–26.8	–	–	–	3.9–11.7	1.0–9.3	22.6–41.9	–	17.6–42.4	Columbite/ (Mn0.56Fe0.44)(Nb0.69Ta0.31)2O6
	24.9	–	–	–	7.8	5.2	32.3	–	30.0
2	21.7–23.5	–	–	–	3.7–8.1	3.8–8.7	13.8–26.5	0.0–1.4	37.6–51.8	Tantalite/ (Mn0.38Fe0.62)(Nb0.47Ta0.53)2O6
	22.6	–	–	–	5.9	6.3	20.2	0.7	44.7
3	19.1–19.2	–	–	–	1.4–2.1	8.3–9.5	–	–	69.9–70.4	Tapiolite/ (Mn0.17Fe0.83)Ta2O6
	19.2	–	–	–	1.8	8.9	–	–	70.2
4	21.6–23.1	–	–	0.0–0.6	2.9–5.3	3.3–6.1	2.6–7.1	12.4–13.1	47.8–54.1	Wodginite/ (Mn0.48Fe0.52)(Sn0.67Nb0.33)Ta2O8
	22.4	–	–	0.3	4.1	4.7	4.9	12.8	51.0
5	31.0–33.4	–	–	0.0–7.0	–	59.6–69.0	–	–	–	Hematite/ Fe2O3
	32.2	–	–	3.5	–	64.3	–	–	–
6	35.4–36.2	–	–	28.3–29.5	1.4–2.7	32.8–33.7	–	–	–	Ilmenite/ FeTiO3
	35.8	–	–	28.9	2.1	33.3	–	–	–
7	48.1–48.9	29.1–29.6	13.9–14.0	–	–	7.7–8.0	–	–	–	Cyanite/ Al2SiO5
	48.5	29.4	14.0	–	–	7.9	–	–	–
8	52.7–52.9	–	47.1–47.3	–	–	–	–	–	–	Quartz/ SiO2
	52.8	–	47.2	–	–	–	–	–	–
9	21.1–21.2	–	–	–	–	–	–	77.6–78.8	0.0–1.2	Cassiterite/ SnO2
	21.2	–	–	–	–	–	–	78.2	0.60
10 *	33.6	0.2	14.3	–	–	0.6	–	–	–	Zircon/ (Zr0.89Hf0.11)SiO4

* means 41.7% Zr, 9.6% Hf.

). The sample matrix predominantly consists of medium-grained columbite (Mn_0.56Fe_0.44)(Nb_0.69Ta_0.31)₂O₆ with a niobium content up to 41.9 wt.%, and tantalite (Mn_0.38Fe_0.62)(Nb_0.47Ta_0.53)₂O₆ enriched in tantalum (up to 51.8 wt.% Ta). Niobium-free tapiolite particles with the composition (Mn_0.17Fe_0.83)Ta₂O₆, containing iron (up to 9.5 wt.%) and manganese (up to 2.1 wt.%), were also identified. Additionally, medium-sized wodginite inclusions (Mn_0.48Fe_0.52)(Sn_0.67Nb_0.33)Ta₂O₈ containing approximately 13.0 wt.% Sn were detected. In addition to structurally bound tin, discrete cassiterite (SnO₂) particles were observed. Associated gangue minerals in the sample include hematite, ilmenite (Ti ≈ 29.0 wt.%), kyanite, quartz, and zircon (41.7 wt.% Zr, 9.6 wt.% Hf).

According to thermal analysis data, the DSC thermogram of the studied coltan sample exhibited two endothermic effects (Figure 3): the first, a weak effect with onset/peak temperatures of 1361/1409 °C, was attributed to the orthorhombic-to-tetragonal phase transition of columbite–tantalite [31], while the second, with a peak at 1467 °C, corresponded to mineral melting. Upon cooling, exothermic effects associated with crystallization and the reverse phase transition were observed at onset/peak temperatures of 1442/1437 °C and 1364/1355 °C, respectively.

Experimental investigation of the interaction between natural coltan and aluminum, with the aluminum content calculated for complete metal reduction (25.5 wt.% Al), demonstrated that heating of the reaction mixture on the DSC thermogram (Figure 4) is accompanied by an endothermic effect attributed to aluminum melting (onset/peak temperatures: 650/660 °C). Subsequent heating to 1400 °C resulted in an exothermic effect with onset/peak temperatures of 984/1168 °C, associated with the reduction of metal oxides. No pronounced thermal effects were detected during the cooling of the reaction products.

According to X-ray diffraction (XRD) data (Figure 5,

Table 5. Phase composition and unit cell parameters of phases in the aluminothermic reduction products of natural coltan (25.5 wt.% Al) at 1400 °C.

No	Phase	Spatial Group	Unit Cell Parameters			Content, wt.%	Cards PDF4
			a (Å)	c (Å)	V (Å3)
1	Al2O3	R-3c	4.758	12.993	254.77	43.3	04-010-6477
2	NbO	Pm-3m	4.210	–	74.60	7.4	04-008-3657
3	Sn	I41/amd	5.831	3.181	108.16	2.7	04-004-7747
4	(Ta,Nb)(Fe,Mn)Al	P63/mmc	4.997	8.112	175.42	11.0	04-001-5964
5	(Ta,Nb)3Sn	Pm-3n	5.278	–	147.05	5.9	04-023-4737
6	(Fe,Mn)(Nb,Ta)2O6	P42/mnm	4.757	9.220	208.67	8.2	01-074-2182
7	(Ta,Nb)ss	Im-3m	3.293	–	35.70	3.0	04-001-2738
8	(Nb,Ta)(Fe,Mn)2	P63/mmc	4.857	7.906	161.53	9.6	04-003-3920
9	Mg0.02CuCr0.98O2	R-3m	2.979	17.198	132.15	7.2	04-015-2954
10	(Zr,Nb)2Al	P63/mmc	4.681	5.783	109.74	1.7	04-003-9852

), the products of aluminothermic reduction of coltan after heating to 1400 °C comprise a multicomponent mixture including intermetallic compounds (Ta,Nb)FeAl and (Ta,Nb)Sn, a (Ta,Nb) solid solution, the (Nb,Ta)(Fe,Mn)₂ and (Zr,Nb)₂Al phases, as well as oxide compounds: corundum (Al₂O₃), tetragonal tapiolite ((Fe,Mn)(Nb,Ta)₂O₆), and metastable niobium monoxide (NbO).

The presence of NbO in the products indicates the stepwise nature of the reduction of niobium-containing phases, attributed to localized aluminum deficiency due to its consumption in aluminide formation.

As demonstrated in the Materials Project database and the relevant literature [32], the Nb-O system comprises multiple stable phases: Nb₂O₅, NbO₂, and NbO. The reduction of niobium pentoxide by aluminum is a process that occurs in stages.

Experimental evidence has been provided to confirm the following sequence of reactions: Nb₂O₅ → NbO₂ → NbO → Nb. During the aluminothermic reduction of Nb₂O₅, the sequential formation of NbO₂ (at ~550 °C) and NbO (at ~700 °C) was observed prior to the formation of the metal [32].

A key feature of the process is the differential behavior of the initial coltan mineral phases: orthorhombic columbite–tantalite (space group Pbcn) was completely reduced, forming metals, ferroalloys, and intermetallic compounds, whereas tetragonal tapiolite (space group P4₂/mnm) was retained at 8.2 wt.% with unit cell parameters close to the initial values (a = 4.757 Å, c = 9.220 Å). This indicates the increased resistance of tantalum-rich mineral domains to aluminothermic reduction. This behavior is attributed to the crystallochemical features of the phases: tapiolite, unlike columbite, is characterized by a high tantalum content (up to 85 wt.% Ta₂O₅ [4]) and, similar to pyrochlore [33], exhibits stronger Ta–O bonds compared to Nb–O bonds. Consequently, the reducibility of columbite–tantalite group minerals by aluminum increases with increasing niobium content, as manifested in the sequence: tapiolite → columbite. The observed trend is consistent with kinetic data on the carbothermic reduction of Ta,Nb-containing minerals, where tantalum-rich phases exhibit a lower degree of metal recovery compared to niobium-rich phases at 1200–1400 °C [34].

In the present study, the phase formation during the aluminothermic reduction of coltan at temperatures below 1400 °C was not the subject of investigation.

Therefore, it is not possible to determine the sequence of formation of intermediate oxides in the initial phase of the reaction directly. Nonetheless, in consideration of the DSC data (exothermic peaks in the 900–1168 °C range), the presence of suboxides (NbO) in the products at 1400 °C, and data from the literature [35], the following macro-mechanism of interaction can be proposed. The process is initiated at 660 °C, which is the temperature at which aluminum melts. This is accompanied by the formation of physical contact between the liquid reducing agent and the surface of the coltan mineral grains, as well as the wetting of the oxide surface by the aluminum melt.

Subsequently, active reduction and phase formation (660–1168 °C) occur. Within the temperature range corresponding to exothermic effects on the DSC curve (maximum at 1168 °C), a vigorous interaction is observed. The process is exothermic and rapid, and as a consequence, the principal reactions proceed in parallel, rather than strictly sequentially. The reduction process commences with phases that are enriched in niobium (columbite–tantalite solid solution), given that the Nb–O bonds are weaker than the Ta–O bonds.

The schematic equation for the initial reduction can be represented as follows:

(Mn,Fe)(Nb,Ta)₂O₆ + Al → NbO + (Nb,Ta) + (Mn,Fe) + Al₂O₃.

(1)

Concurrent reactions are also observed. As the metals are subjected to reduction, they react with each other and with aluminum according to the following mechanisms:

(Mn,Fe) + (Nb,Ta) → (Nb,Ta)(Fe,Mn)₂,

(2)

(Mn,Fe) + (Nb,Ta) → (Ta,Nb)(Fe,Mn)Al.

(3)

Simultaneously, the associated tin (cassiterite) undergoes a reduction process in stages and binds refractory metals, as evidenced by data from [36]:

3SnO₂ + 2Al = 3SnO + Al₂O₃,

(4)

3SnO + 2Al = 3Sn + Al₂O₃,

(5)

(Ta,Nb) + Sn → (Ta,Nb)3Sn.

(6)

The process is then said to reach completion, accompanied by a further reduction of oxides and the homogenization of the metal melt.

The presence of residual tapioite in the products, as the phase richest in tantalum, is consistent with the data from a study demonstrating the interaction of reducing agents with natural tantalite [35].

The partial replacement of aluminum with calcium (19.8 wt.% Ca) significantly alters the nature of the interaction between coltan and the reducing agent, as reflected in the DSC thermogram (Figure 6). The interaction of the components is initiated after aluminum melting and appears as an intense combined exothermic peak with an onset at 668 °C and a maximum at 704 °C. This effect is probably attributed to the formation of calcium–aluminum intermetallic compounds (e.g., Al₄Ca and Al₂Ca), which form at the boundary of the liquid Al-Ca phase [37]. Subsequently, the resulting mixture of intermetallic compounds and residual aluminum melt acts as an active reducing agent, providing the reduction of the oxide components of natural coltan. The reduction processes proceed through several stages, observed as exothermic peaks at 815 °C, 989 °C, 1200 °C, and 1310 °C, indicating the sequential course of complex multiphase reactions from partial reduction to the formation of final metallic and oxide phases.

According to XRD results (Figure 7,

Table 6. Phase composition and unit cell parameters of phases in the reduction products of natural coltan with an aluminum–calcium mixture (19.8 wt.% Ca) at 1400 °C.

No	Phase	Spatial Group	Unit Cell Parameters			Content wt.%	Cards PDF4
			a (Å)	c (Å)	V(Å3)
1	Al2O3	R-3c	4.756	12.990	254.47	39.1	04-015-8995
2	(Ta,Nb)3Sn	Pm-3n	5.272	–	146.54	4.7	04-004-5091
3	NbO	Pm-3m	4.208	–	74.51	6.5	04-005-6079
4	Sn	I41/amd	5.829	3.180	108.08	2.8	04-003-2197
5	(Ta,Nb)(Fe,Mn)Al	P63/mmc	4.984	8.146	175.20	5.2	04-001-5964
6	(Ta,Nb)O2	P42/mnm	4.759	3.078	69.71	9.0	04-002-3935
7	(Nb,Ta)(Fe,Mn)2	P63/mmc	4.859	7.905	161.65	5.1	04-001-7266
8	(Ta,Nb)7(Fe,Mn)6	R-3m	5.015	27.809	605.66	1.7	04-001-3106
9	(Ta,Nb)ss	Im-3m	3.295	–	35.77	3.8	04-003-5516
10	Ca12Al14O32.5	I-43d	11.992	–	1724.65	1.6	04-015-0819
11 *	(Mn,Fe)(Nb,Ta)2O6	Pbcn	14.194	5.133	417.72	4.2	04-012-3522
12	Nb6.15Cr1.40Fe5.45	R-3m	4.940	26.958	569.64	8.0	04-022-6431
13	Al(PO4)	P3121	5.005	10.609	230.13	4.6	04-009-5761
14	(Nb,Ta)(Fe,Mn)2	P63/mmc	4.848	7.855	159.87	2.1	04-017-9960
15	SiO2	P3121	4.922	5.363	112.53	1.7	04-005-4718

* b = 5.734 Å.

), the oxide component of the products from the interaction of natural coltan with an aluminum–calcium mixture after heating to 1400 °C under argon atmosphere was found to contain a mayenite phase (Ca₁₂Al₁₄O₃₃, space group I-43d) in addition to corundum (Al₂O₃). A key feature, in contrast to aluminothermic reduction, is the absence of tapiolite and the formation of a tetragonal oxide (Ta,Nb)O₂ (space group P4₂/mnm), while residual amounts of orthorhombic columbite–tantalite solid solution remain. The results obtained indicate a predominant interaction of calcium, as well as Ca-Al intermetallic compounds, with tetragonal Ta-Nb minerals. The formation of a tantalum–niobium oxide solid solution probably occurs during the reduction stage of tetragonal tapiolite. In contrast to aluminothermic reduction, the calcium–aluminothermic reducibility of columbite–tantalite group minerals increases with increasing tantalum content, as follows: columbite → tapiolite.

The metallic phase contained (Table 6) iron- and manganese-bearing tantalum–niobium aluminides ((Ta,Nb)(Fe,Mn)Al), tantalum–niobium solid solutions, a tantalum–niobium–tin intermetallic compound with the structure (Ta,Nb)₃Sn (space group Pm-3m), as well as ferroalloys with varying (Ta,Nb)/(Fe,Mn) ratios, specifically (Ta,Nb)(Fe,Mn)₂ and (Ta,Nb)₇(Fe,Mn)₆. The presence of the (Ta,Nb)₇(Fe,Mn)₆ phase when using a mixture of reducing agents, and its absence in the conventional aluminothermic process, indicates more efficient reduction of tantalum and niobium when employing an aluminum–calcium mixture.

When a calcium–-aluminum master alloy (69.4 wt.% Ca) was used as a reducing agent in combination with natural coltan, the DSC thermogram (Figure 8) exhibited an endothermic effect with onset/peak temperatures of 535/543 °C, attributed to melting of the eutectic component in the alloy. A combined exothermic effect with a pronounced maximum at 657 °C was attributed to phase transformations within the alloy and the reduction of tapiolite, resulting in the formation of tantalum–niobium oxide solid solutions. X-ray diffraction analysis of intermediates obtained by heating the reagent mixture to 800 °C (Figure 9,

Table 7. Phase composition of reduction products from natural coltan using a Ca–Al master alloy at different heating temperatures (800–1450 °C) under argon.

No	Phase	Spatial Group	The Content in the Products (wt.%) After Heating to a Temperature (°C)			Cards PDF4
			800	1250	1450
1	(Mn,Fe)(Nb,Ta)2O6	Pbcn	43.9	2.5	-	04-014-2992 04-012-3522
2	CaAl2	Fd-3m	23.7	-	-	04-012-6338
3	SnO2	P42/mnm	3.2	0.4	-	04-003-0974
4	Nb3MnO6	Immm	5.9	-	-	04-012-6976
5	(Ta,Nb)O2	P42/mnm	1.8	-	-	04-004-3018
6	CaSnO3	Pnma	6.3	-	-	04-007-8720
7	Fe0.7Al0.14Si0.16	Im-3m	1.4	-	-	04-018-7279
8	Ca(OH)2(CaO)	P-3m1	4.7	3.4	-	04-010-3117 (04-006-5940)
9	Mn3O4	I41/amd	1.4	-	-	04-015-2577
10	Mg0.55Ti0.45	Im-3m	2.9	-	-	04-017-5241
11	Ti3O5(OH)2	C2/m	4.9	7.2	12.5	04-021-8783
12	Ca12Al14O32.5	I-43d	-	28.7	9.5	04-015-0819
13	(Nb,Ta)(Fe,Mn)2	P63/mmc	-	9.0	7.4	04-017-9960 04-001-7266
14	(Ta,Nb)3Sn	Pm-3n	-	14.6	9.8	04-004-5091
15	Ca2(Nb,Ta)AlO6	P21/n	-	3.2	-	04-014-0990
16	Sn	I41/amd	-	1.2	-	04-004-7747
17	(Ta,Nb)ss	Im-3m	-	4.2	2.8	04-003-5516
18	Ti3Al	P63/mmc	-	4.8	-	04-003-0974
19	(Ta,Nb)7(Fe,Mn)6	R-3m	-	3.9	2.5	04-012-3522
20	CaNbO3	Pnma	-	16.9	1.6	04-001-9017
21	(Ta,Nb)(Fe,Mn)Al	P63/mmc	-	-	1.0	04-001-5964
22	Al(PO4)	P3121	-	-	2.5	04-004-6777
23	Ca3Al2O6	Pa-3	-	-	26.3	04-001-3106
24	(Ta,Nb)0.80(Fe,Mn)0.20	Fm-3m	-	-	2.9	04-007-3583
25	Ca(Nb,Ta)0.25MnO2.74	Pm-3m	-	-	12.9	04-020-8945
26	(Fe,Mn)ss	Im-3m	-	-	1.1	04-016-6734
27	Ca4Nb2O9	P21/c	-	-	1.8	04-017-5651
28	Ca0.5Zr2(PO4)3	R-3c	-	-	3.5	04-005-5489
29	Ta0.3W0.7O2.85	Pm-3m	-	-	1.5	04-008-4357

) revealed that, as a result of the interactions, the alloy composition shifted toward CaAl₂, indicating aluminum enrichment. Additionally, CaO formed during reduction reacted with SnO₂ to produce calcium stannate (CaSnO₃).

Further heating to 1250 °C results in intensive reduction of metals from the oxide components of natural coltan, as evidenced by exothermic effects with maxima at 923 and 1108 °C. In the temperature range of 800–1250 °C (Figure 9, Table 7), solid solutions of tantalum and niobium, a tantalum–niobium–tin intermetallic compound (Ta,Nb)₃Sn, titanium aluminide Ti₃Al, as well as ferroalloys with varying (Ta,Nb)/(Fe,Mn) ratios are formed in the metallic phase. The formation of intermetallic compounds with an A15 (Nb₃Sn) structure is consistent with the data presented in [38], which investigated phase equilibria in the Al–Nb–Sn system at 900 and 1200 °C. The findings demonstrated the stability of the A15 phase within the specified temperature range and the high solubility of tin in Nb₂Al. As demonstrated in [39,40], the existence of the Laves phase Fe₂Nb and the μ-phase Fe₇Nb₆ at temperatures ranging from 1200 to 1450 °C is evidenced by the available data on ferroalloys and iron intermetallides. Despite a significant decrease in the content of orthorhombic columbite–tantalite solid solution in the reaction products, its residual amount remains, whereas tetragonal tapiolite is completely absent in the final products. In addition, a significant difference between calcium–aluminothermic reduction and the conventional aluminothermic process is the formation of intermediate oxide phases with a perovskite-type structure Ca₂(Nb,Ta)AlO₆ and CaNbO₃. These compounds serve as diffusion barriers that restrict the access of the reducing agent to the residual mineral inclusions of columbite–tantalite, thereby slowing down the final stage of reduction.

According to XRD data, the products of coltan heating with a calcium–aluminum master alloy to 1450 °C (Figure 9, Table 7) showed the complete disappearance of Ti₃Al, which was previously present in the reduction products at 1250 °C. However, there was an increase in the content of reduced titanium oxide in the form of Ti₃O₅(OH)₂, formed during storage as a result of air exposure. This phenomenon indicates that titanium aluminide can act as a reducing agent when heated to 1450 °C. The resulting alumina (Al₂O₃) is involved in the formation of calcium aluminates during reduction.

The most significant changes occur in the niobium-containing oxide subsystem. The CaNbO₃ content sharply decreases from 16.9 to 1.6 wt.%, accompanied by the complete disappearance of the residual columbite–tantalite solid solution. Instead, a Nb-poor, Mn- and Ca-rich phase with the composition Ca(Nb,Ta)_0.25MnO_2.74, and a calcium niobate with high CaO content, Ca₄Nb₂O₉, are formed. Within the oxide matrix, calcium oxide is redistributed to form more thermodynamically stable high-temperature compounds. The released CaO and newly formed alumina (Al₂O₃) initiate recrystallization of the aluminate bonding phase. A transformation of the mayenite phase (Ca₁₂Al₁₄O_32.5) was observed: its content decreased from 28.7 to 9.5 wt.%, while tricalcium aluminate (Ca₃Al₂O₆) formed in an amount of 26.3 wt.%.

In the context of a Ca-Al alloy, the process exhibits a slightly divergent macro-mechanism. In the initial reduction stage (up to 800 °C), the interaction between coltan and the Ca-Al alloy occurs via the formation of intermediate oxide phases. In the process of reduction, a portion of the niobium present in tapioite reacts with oxygen to form dioxide. This reaction subsequently combines with manganese, resulting in the formation of Nb₃MnO₆. The overall initial reaction is thus represented as follows (where the Ca-Al alloy is denoted as Ca_0.604Al_0.396):

(Mn,Fe)(Nb,Ta)₂O₆ + Ca_0.604Al_0.396 → (Ta,Nb)O₂ + Nb₃MnO₄ + (Fe,Al) + CaO + Al₂O₃.

(7)

Concurrently, cassiterite reacts with calcium oxide (CaO) according to the following reaction:

SnO₂ + CaO = CaSnO₃.

(8)

Concurrently, the alloy components are converted into CaAl₂, which subsequently acts as the primary reducing agent.

The following reaction equations (or diagrams) can be used to represent the process of heating the reaction mixture to 1250 °C:

(Mn,Fe)(Nb,Ta)₂O₆ + CaAl₂ → (Mn,Fe) + (Nb,Ta) + CaO + Al₂O₃

(9)

(Mn,Fe)(Nb,Ta)₂O₆ + CaAl₂ → (Mn,Fe) + (Nb,Ta) + Ca₂(Nb,Ta)AlO₆ + Al₂O₃

(10)

3CaSnO₃ + 2CaAl₂ = 3Sn + 5CaO + 2Al₂O₃

(11)

3(Nb, Ta) + Sn = (Nb, Ta)₃Sn,

(12)

(Nb, Ta) + (Fe, Mn) → (Nb, Ta)_x(Fe, Mn)_y,

(13)

where x and y represent the fractions of (Nb, Ta) and (Fe, Mn) in the ferroalloy composition, respectively.

Subsequent reduction is achieved through the involvement of the established intermetallic compounds of coltan impurity elements (Ti₃Al), aluminum dissolved in the solid solution (Nb, Ta), and calcium vapor. In [41], calcium sublimation above 1000 °C was observed during the study of the interaction between FeNb₂O₆ and Ca. A similar phenomenon appears to be characteristic of the interaction between coltan and the ligature. As indicated by XRF data, a substantial redistribution of components transpires at temperatures in excess of 1450 °C. A complete disappearance of titanium aluminide is observed, which can be schematically represented by the following equations:

The following reactions are possible:

(Mn,Fe)(Nb,Ta)₂O₆ + Ti₃Al → (Nb,Ta)_x(Fe,Mn)_y + Ti₃O₅ + Al₂O₃,

(14)

(Mn,Fe)(Nb,Ta)₂O₆ + Al → (Nb,Ta)_x(Fe,Mn)_y + (Ta,Nb)(Fe,Mn)Al + Al₂O₃.

(15)

The following reaction may occur with calcium pairs:

2Ca₂(Ta,Nb)AlO₆ + 5Ca = 2(Ta,Nb) + 9CaO + Al₂O₃.

(16)

3CaNbO₃ + Ca = Ca₄Nb₂O₉ + Nb.

(17)

Concurrently, a redistribution of components occurs within the CaO-Al₂O₃ system.

A generalized interaction diagram of natural coltan with various reducing agents heated to 1400 and 1450 °C, shown in Figure 10, illustrates the key differences in phase formation between aluminothermic and calcium–aluminothermic reduction processes. Elucidating these mechanisms enables the identification of critical factors determining the efficiency of niobium and tantalum extraction. The conventional aluminothermic process dictates the reduction sequence based on the strength of the metal–oxygen chemical bond. Since the Ta–O bond is thermodynamically more stable than the Nb–O bond, tetragonal tapiolite (enriched in tantalum) retains its structure even at 1400 °C, whereas orthorhombic columbite–tantalite is completely reduced. In contrast, when using a Ca–Al master alloy, an inversion of this process is observed. Calcium, with its high affinity for oxygen and ability to form stable niobates, alters the thermodynamic potential of the system. Tapiolite is reduced in the early stages (800–1250 °C) via an intermediate (Ta,Nb)O₂ oxide phase and is completely absent from the product mixture at 1250 °C. The role of perovskite-like phases as diffusion barriers in the Ca–Al alloy system is a pivotal factor in the observed inversion of the reduction sequence. It appears that, at the “coltan–perovskite” interface, the limiting step is the diffusion of the reducing agent through the layer of forming perovskite towards the reaction front. The observed delay in the reduction of columbite–tantalite at 1250 °C and its intensification at 1450 °C, when the perovskite phases undergo structural transformations, is consistent with the hypothesis. Simultaneously formed calcium niobate acts as a diffusion barrier, restricting the access of the reducing agent to residual inclusions of orthorhombic columbite–tantalite. Raising the temperature to 1450 °C initiates the redistribution of oxide components, accompanied by the completion of the reduction of residual columbite–tantalite.

The results obtained contribute to the development of theoretical understanding of the mechanisms of phase formation during metallothermal reduction of tantalum- and niobium-containing natural raw materials. The use of aluminum as a reducing agent is associated with several limitations. The high thermodynamic stability of the system promotes the formation of aluminides in the metallic phase, resulting in significant aluminum content in the final product and significantly complicating subsequent refining stages required to obtain the target metals.

The use of a calcium–aluminum master alloy as a reducing agent overcomes these limitations. Calcium, with its high chemical activity, ensures more complete extraction of tantalum and niobium into the metallic phase. Furthermore, the calcium oxide formed during the reduction process interacts with refractory corundum (α-Al₂O₃), forming low-temperature eutectics in the CaO–Al₂O₃ system.

4. Conclusions

• The mechanism of phase formation during the metallothermal reduction of niobium and tantalum from natural coltan using aluminum, a calcium–aluminum mixture, and a Ca–Al master alloy as reducing agents has been elucidated.
• It has been established that the reduction of coltan by aluminum during heating is limited by the crystallochemical stability of the initial minerals. Selective phase transformations are observed with stoichiometric consumption of the reducing agent: the orthorhombic columbite–tantalite solid solution is completely reduced, whereas tetragonal tapiolite remains preserved in the reduction products up to 1400 °C. The interaction products are a multicomponent mixture consisting of (Ta,Nb) metallic solid solutions, ferroalloys with a low (Ta,Nb)/(Fe,Mn) ratio, and aluminides of the (Ta,Nb)(Fe,Mn)Al system. The oxide component of the products consists mainly of corundum (Al₂O₃), niobium suboxide (NbO), and residual tapiolite.
• The use of a Ca–Al master alloy fundamentally changes the process kinetics, causing an inversion of the reduction sequence of columbite–tantalite group minerals. Unlike aluminothermy, tapiolite is reduced in the early stages (up to 1250 °C) via intermediate oxide solid solutions, whereas the complete decomposition of columbite–tantalite requires heating to 1450 °C. A distinctive feature of the process is the formation of intermediate perovskite-type niobate phases, which are absent during conventional aluminum reduction. At 1250 °C, the system is dominated by mayenite (Ca₁₂Al₁₄O_32.5) and calcium niobate (CaNbO₃). Increasing the temperature to 1450 °C leads to the final reduction of the residual columbite–tantalite solid solution with the release of manganese-containing phases, resulting in the formation of a perovskite-like phase Ca(Nb,Ta)_0.25MnO_2.74, accompanied by a parallel decrease in the CaNbO₃ content. The released components are redistributed: calcium oxide is consumed for the synthesis of high-temperature tricalcium aluminate (Ca₃Al₂O₆) instead of mayenite, as well as for the formation of calcium-enriched niobate Ca₄Nb₂O₉.
• The results obtained may be utilized for the development of metallothermal extraction processes for niobium and tantalum from natural columbite; however, quantitative assessment of process efficiency requires additional metallurgical testing with physical separation of the metallic and slag phases.
• The following areas have been identified as offering significant potential for research: The first stage of the research involved the detailed kinetic modeling of the process, including the calculation of activation energies and the identification of rate-limiting steps. The second stage involved the investigation of phase evolution using in situ high-temperature X-ray diffraction methods to monitor transformations in real time. The third stage involved conducting pilot-scale laboratory tests to optimize the charge composition (type of reducing agent and fluxing additives) and process operating parameters, in order to identify the optimal process operating parameters.