Pyrometallurgical Approach in the Recovery of Niobium and Tantalum

Abstract

The pyrometallurgical methods of the recovery of various critical metals have been established. Certain pyrometallurgical approaches for niobium (Nb) and tantalum (Ta) extraction have been studied and investigated by several researchers. For instance, the conventional reduction of Nb mineral or Nb2O5 to Nb metal has been conducted over the decades. Albeit, the success level of this process, it however involves the consumption of lots of energy, high cost of equipment/equipment maintenance, somewhat low Nb and Ta recovery and limited effectiveness on low grade minerals; and thus, considered cost intensive and inefficient. In addition, the inadequacies of pyrometallurgical extraction of these metals from their complex, low grade mineral ores due to its difficulty and large energy requirement in melting the elemental impurities and gangue minerals have been a major concern. On this premise therefore, the study will provide insights into recent pyrometallurgical techniques of Nb and Ta extraction as well as associated factors and challenges.

1. Introduction

The extraction and purification of metals from their mineral complex or concentrates, based on physicochemical changes occurring at elevated/somewhat high temperatures, is referred to as pyrometallurgy. Pyrometallurgical processes essentially involves certain heating procedures; usually employing dry methods conducted at high or elevated temperatures and may also involve the melting of the charge or feed material as well as high-temperature processes in which chemical reactions occur between gases, solids, and molten materials. Mineral solids composed of valuable metals are processed to make intermediate compounds for further processing or to their elemental or metallic state. Calcining, roasting and smelting operations are typical pyro-metallurgical processes that involve gases and solids, and may also result in molten products. The exothermic character of the chemical reactions occurring may be the source of the energy needed to sustain the high temperature pyro-metallurgical processes. Most frequently, fuel is being used to contribute energy to the process, or in the case of some smelting procedures, electrical energy is being applied directly. Roasting on the other hand may however involve thermal gas–solid processes, such as oxidation, reduction, chlorination, sulphation, and pyro-hydrolysis [1]. The pyrometallurgical methods of the recovery of various critical metals have been established over the decades. Certain pyrometallurgical approaches for niobium (Nb) and tantalum (Ta) extraction have been studied and investigated by several researchers in various parts of the world. For instance, the conventional reduction practice of Nb/Ta minerals or Nb₂O₅/Ta₂O₅ to Nb/Ta metals has been conducted for several decades. Albeit, the measure of success of this process, pyrometallurgical procedure was reported to involve high temperatures and high energy consumption, high equipment cost, use of sophisticated equipment and high cost of (equipment) maintenance, somewhat low Nb and Ta yield/recovery and limited effectiveness on low grade minerals; and therefore, the process was regarded cost intensive and inefficient. In addition, the inadequacies of pyrometallurgical extraction of these metals from their complex low grade mineral ores due to its difficulty and large energy requirement in melting the value metals as well as various elemental impurities/gangue minerals have shown great concerns [2]. Despite, the large energy requirement in melting the elemental impurities/gangue minerals as a result of the complexity and low-grade nature of the minerals, successful high temperature processes/applications have been recorded, to adequately/efficiently extract and recover Nb and Ta metals from their minerals. Therefore, on this premise, the study provides insights into recent pyrometallurgical techniques of the extraction and recovery of Nb and Ta, as well as certain associated influencing factors. Thus, this chapter is concisely based on the recent pyrometallurgical approaches relating to the beneficiation, Nb and Ta extraction and recovery from mineral complexes.

1.1 Niobium and tantalum chemistry

Niobium and tantalum were discovered by British scientist, Charles Hatchett in 1801; and Swedish scientist, Anders Ekeberg in 1802, respectively. Niobium (Nb, Z = 41) has very similar geochemical properties, electronic structures and behavior (small radius/atomic size and high charge) with tantalum (Ta, Z = 73) [3, 4] and as such the two metallic elements are regarded as geochemical twins [5, 6, 7], and hence similar beneficiation and extraction techniques are usually adopted and employed for both metal elements. Nb and Ta metals are both refractory, transition, weakly acidic, BCC solid metallic elements. A lot of research work and investigations have been reported on the extraction and recovery of Nb and Ta as well as the chemistry involved in extracting both metals. From studies conducted, the Nb and Ta chemistry, hydrolysis, solubility and complexes in aqueous media have been established by various researchers [2, 8, 9, 10, 11, 12, 13, 14, 15, 16]. From the reports by certain researchers, it can simply be established that oxygen (O₂⁻) ions have often shown very close similarities in ionic radii compared to that of fluorine (F⁻) ions. Thus, there can be somewhat easy/simple substitutions occurring between O₂⁻ and F⁻ anions in a matrix or complexing compounds [2, 11]. In addition therefore, the hydroxides (OH⁻) also possess somewhat similar chemical characteristics/properties as well as in geometry with F⁻ anions. This similarity is especially and essentially found in their charge and size; and thus, a justification (scientifically), of somewhat easy and feasible substitutions that may occur between their complexes/compounds in the chemical and/or thermodynamic reactions. Therefore, apart from O²⁻, Ta and Nb are hypothesized also, to be very liable of forming or producing soluble complexes or compounds with certain strong ligands, such as: OH⁻ and F⁻ [2, 17, 18]. More so, Nb and Ta are particularly somewhat soluble (in wt. % levels) with alkaline melts or solutions [2, 18, 19], and might even as well attain greater solubility with carbonatite molten solutions [2, 18, 20]. Figures 1 and 2 depicts the basic description and representation of Nb and Ta metals, respectively, indicating that they are refractory, transition, weakly acidic, BCC solid metallic elements.

Figure 1.

(a) Nb element; (b) samples of Nb metal [21, 22, 23].

Figure 2.

(a) Ta element; (b) samples of Ta metal [21, 23].

1.2 Niobium and tantalum minerals

Nb and Ta primary mineral ores are natural occurring complex oxide minerals, mostly of low grade deposits with chemical composition majorly composed of different content levels of Nb and Ta refractory metals’ penta-oxides (Nb₂O₅ and Ta₂O₅), iron (Fe₂O₃), manganese (MnO), tin (SnO₂) and titanium (TiO₂) oxides with other refractory, metallic oxides. Nb/Ta minerals occurring as penta-oxides (Nb₂O₅/Ta₂O₅) are often associated together in similar mineral structures. Over the years, a lot of researchers have consistently and extensively studied the recovery and extraction of Ta and Nb from their primary deposits or mineral ores as well as their secondary sources, applying various mineral processing steps and certain beneficiation/separation methods. More recently, Nb and Ta have been extracted from several primary rock deposits and mineral ores. Howbeit, these primary deposits/mineral ores of Ta and Nb are somewhat scarce in the world and are naturally deposited with a low average crustal abundance of about 2 and 20 mg/kg, respectively. Hence, this in their classification, are referred to as rare metals [15, 16, 23, 24]. Table 1 displays important Nb-Ta minerals along with their major characters.

Ore Mineral	Chemical formula	Locations	Major characteristics/properties
			Color	S.G	Structure	Deposition
Columbite-tantalite (Coltan)	(Fe,Mn)(Nb,Ta)2O6	Australia, Brazil, DRC, Ethiopia, Finland, France, Japan, Madagascar, Nigeria, Norway, Russia, USA	Reddish-brown to black	5.2–8.2	Orthorhombic	Peralkalines,Beryl pegmatites (granitic &syenitic), Carbonatites, Peraluminous granites
Pyrochlore	(Na,Ca)Nb2O6(O,OH,F)	Brazil, Canada, DRC, Norway	Pale yellow to dark black	4.2–5.8	Isometric; Octahedral	Pegmatites, Peralkalines,Carbonatites
Microlite	(Na,Ca)2Ta2O6(O,OH,F)	Australia, Brazil, Madagascar, Norway, Sweden, USA, Zimbabwe	Brown to pale yellow	5.5–6.4	Isometric; Cubic isomorphic	Pegmatites, Peralkalines,Carbonatites
Fergusonite	(REE)(Ba,Nb,Ta)O4	Australia, Madagascar, Norway, Rhodesia, Russia, Srilanka, Sweden, USA	Yellow to brown	4.3–5.8	Tetragonal; Monoclinic	Granite pegmatites
Samarskite	(Fe,Ca,U,Y,Ce)2(Nb,Ta)2O6	Canada, DRC, India, USA	Black to reddish-brown	4.2–6.2	Orthorhombic	Granitepegmatites
Mossite-Tapiolite	(Fe,Mn)(Nb,Ta,Ti)2O6	Morocco	Brownish-black to black	7.3–7.9	Tetragonal	Granitepegmatites

Table 1.

Major Nb and Ta mineral resources [2, 11, 23, 25, 26].

1.3 Niobium and tantalum extraction

The demand of Nb and Ta has consistently become somewhat higher than the supply. However, the extraction and recovery of Nb and Ta from their minerals and from other elements contained in the mineral composition, has become very tedious and difficult, involving very complex processes and techniques. Also, the choice selection and application variation of the mineral enrichment, beneficiation and extraction techniques has become somewhat tedious and complicated as they practically depend on the mineralogy of the mineral ore or concentrate, its nature and type, level of purity and impurity, number of safe beneficiation/extraction process steps and most importantly, on the physicochemical properties of the mineral with respect to the chemical composition (the physical and chemical nature/properties of the value and gangue minerals [2, 23]. Table 2 depicts certain major determinant or influencing factors to be considered when selecting suitable techniques for extraction/recovery of Nb/Ta.

Factors	Explanation
Technical	The mineralogy, ore type, nature, purity & impurity, scalability & number of beneficiation/extraction process steps
Economical	The cost price of mineral ore type, beneficiation cost & profit, cost per ton of metal produced, fuel availability & cost, quantity/quality of process products, market & energy cost
Environmental	The eco pollution, proper handling & storage of waste/by-products, environment & ISO 14001 guidelines
Quality	The implementation & maintenance of a quality management system in line with ISO 9001 & ISO 17025 standards
Safety	The beneficiation process facility requirements for the safety of workers, safe handling & storage of waste/by-products, maintaining health, safety & OHSAS 18001 standards

Table 2.

Factors considered when selecting Nb and Ta extraction/recovery process routes [16, 23, 27].

The extraction efficiency of Nb and Ta from mineral deposits greatly depends on the measure or degree of removal of associated impurities present in the crude Nb mineral ore [28]. Hence, for a successful Nb and Ta metal extraction (for both pyrometallurgical and pyro-hydrometallurgical procedures), the impurity content in the oxide mineral ore and the choice of beneficiation/extraction methods and/or extraction agents are key [29]. Thus, a successful beneficiation/separation process application is often rated on the basis of the obtained percentage recovery of the high-purity valuable minerals as well as the impurity content being reduced to a minimum with fewer processing/recovery steps [2, 16]. In addition, low-grade Nb/Ta minerals mined mechanically, such as columbites and tantalites have been reported to contain a lot of impurities and sometimes have less than 0.1% Nb₂O₅/Ta₂O₅ contained in the mineral ore. This increases the difficulty in the mineral processing and subsequent metal recovery. Thus, an efficient ore enrichment beneficiation process route is important to effectively improve the metal extraction and significantly eases up the subsequent downstream pyro- or hydro-metallurgical decomposition and separation processes of the mineral for successful Nb and Ta recoveries. Hence, the need for the minerals to undergo beneficiation/enrichment processes for ore upgrade to an industrially acceptable metallurgical grade of minimum composition of 25% Nb₂O₅ and Ta₂O₅ or 50% in their combine form, which is the required content in the ore for effectiveness/efficiency of subsequent downstream extraction process [30].

Howbeit, due to the complexity and presence of impurities such as: refractory/metallic oxides, rare earths and radioactive elements; Nb₂O₅ and Ta₂O₅ are usually hydrometallurgically extracted from its mineral ore source using hydrofluoric (HF) acid as the decomposition agent before separation and recovery. The traditional method of extraction of Nb and Ta from primary mineral resources using hydrometallurgical procedures namely; employing aqueous dissolution and subsequent solvent extraction (SX), ionic exchange (IX) or other separation/purification processes [31, 32] is well documented in literature. However, some of these process steps are usually employed under harsh aqueous media conditions of very toxic, concentrated, hazardous and corrosive chemicals with several complex separation steps [33]. This is chiefly due to the insolubility rate of Nb, Ta and other refractory metallic oxides in milder conditions [32, 34]. The chemical and physical similarities of Ta and Nb with other refractory elements make their separation from mineral ores a complex and difficult process [35]. Over the decades, several pyro- and hydro-metallurgical processes of the extraction/recovery of Nb and Ta have been established by various researchers. For instance, Jean Charles Galissard de Marignac developed a hydrometallurgical process in 1866, popularly referred to as the “Marignac process” for Nb and Ta extraction. This very method involved fractional crystallization process in order to separate the metals as potassium oxypentafluoroniobate monohydrate (K₂[NbOF₅] H₂O) and potassium heptafluorotantalate (K₂[TaF₇]), respectively. This in turn was also reduced in order to obtain the metals of certain high purity [2, 36], most times adopting the process of electro-winning in fused salts [2, 13]. Howbeit, the establishment of liquid–liquid extraction (LLE) in the 20th century replaced the Marignac method. This process was however developed by the Ames Laboratory and the U.S. Bureau of Mines in 1957. The method utilized the difference in acid solubility of Nb and Ta F⁻ ions in organic solvents at specific acid levels [2, 10, 37, 38, 39, 40].

In recent times, several researchers and investigators enhanced this Nb/Ta extraction method and also, various novel process applications have been studied and developed. The digestion of Ta and Nb mineral particles and subsequent stepwise/selective separation of the metals from their reaction complexes; however considering the recovery and purity level, cost and economic aspects, energy and reagent consumption [2, 41], waste and environment management issues. Among methods adopted by these researchers in order to achieve simultaneous digestion and decomposition of Ta and Nb minerals were chlorination process, alkali fusion method, alkali fusion-acid leaching, alkaline solution dissolution, ammonium fluoride (NH₄F) and ammonium bifluoride (NH₄HF₂) fusion, decomposition with direct H₂SO₄ acid or combined with HF acid, [2, 9, 42, 43, 44, 45]. Howbeit, the most successful method employs the initial halogenation and dissolution in HF acid or a combination of HF acid or a fluoride medium like fluoride salts (which forms oxy-fluorides) with mineral acids [2, 26, 39, 40, 43, 45, 46, 47, 48]. This is usually achieved at somewhat high temperatures and concentrations, and under harsh, hazardous and corrosive operational conditions [9, 26, 43, 49, 50] due to the resistance of Nb-Ta mineral ores to chemical/acid attack at mild conditions [32, 39, 40, 48, 51, 52]. Hence, with selective leachants like HF acid, hydrometallurgical techniques was completely adopted and recommended suitable, as a result of its lesser consumption of energy and higher recovery/grade purity of Nb products essentially from complex and low grade mineral ores [52, 53]. However, with stringent regulations on health, safety and environment (HSE) increasing significantly every day, adoption of this hydrometallurgical process has since then, suffered its own share of demerits due to toxicity and harmful nature of HF and other fluoride media [44, 52, 54, 55].

Howbeit, the present limitations involved in the conventional use of HF acid for leaching and dissolution of Nb and Ta minerals ranges from the loss of HF acid through volatilization during the metals’ extraction, of about 6–10% Vol. (pressure of 763 mm Hg at room temp), the great amounts of HF acid wastes generated that contains fluoride salts and the serious challenge it poses to personnel and the environment with respect to waste management as well as the safe disposal of such waste fluoride salts. Thus, it can be related that the high corrosiveness, toxicity, volatility, chemical consumption and cost, generation of fluoride-bearing waste water, and other environmental related issues of HF acid, thus contribute to the difficulty and complexity of the extraction/separation process route, as well as the high equipment maintenance/operation cost. Due to these shortcomings as well as the low recovery yield of Ta and Nb, the demand for these metals is somewhat higher than their supply. This is as a result of their criticality and numerous engineering/technological applications, such as: the utilization in the production of rocket missiles, aircraft engines, telephones, solar cells, turbine blades, capacitors, HSLA/stainless steels, oil/gas pipelines, particle accelerators, nuclear reactors, super conductors, refractive index of lenses, heat resistant/cutting tools [2, 23, 56]. Thus, a great deal of research investigation is imperative for enhancing or developing simpler, cleaner, effective and less complicated Ta and Nb extraction/separation process route in order to obtain optimum recoveries and resource utilization. On this premise therefore, several researchers have established the need to develop fluoride-free mineral decomposition media under mild conditions as substitute or alternative extraction procedure for Nb and Ta recovery. Albeit several investigations conducted on Nb and Ta extraction process route from their various mineral resources, research is still on-going in order to completely establish and develop a more efficient process alternative or substitute route for the dissolution/decomposition of the minerals; with very significant efforts ascribed towards mitigating the draw-backs and shortcomings of the conventional extraction/separation of Nb and Ta adopting HF acid as the dissolution medium. As a result of the various challenges encountered in the separation of Ta and Nb from associated impurities in their minerals and from each other, it thus became necessarily important to explore/exploit several extraction and recovery processes. Therefore, it is only imperative that serious attention should also be attributed to the pyrometallurgical as well as the pyro-hydrometallurgical approaches for optimum/efficient Nb and Ta extraction/recovery from their minerals.

2. Pyrometallurgical approaches

Over the years, the pyrometallurgical approach of Nb and Ta extraction has been established. Several authors have reported their investigations on the conventional pyrometallurgical reduction of Nb/Ta penta-oxide to the metal [36]. However, this process often consumed a lot of energy, high reagent consumption, high cost of equipment/equipment maintenance, low Nb/Ta recovery and limited effectiveness on the Nb/Ta low grade minerals; and thus, certain researchers considered the process application inefficient and not cost effective [52]. Habashi [53] reported the inadequacies of the pyrometallurgical extraction of the metals from their complex and low grade minerals due to the extraction difficulty, high temperature and large energy requirement in melting the elemental constituents and gangue minerals. Hence, with selective leachants like hydrofluoric (HF) acid, hydrometallurgical techniques have been completely adopted and recommended suitable, as a result of its lesser consumption of energy and higher recovery/grade purity of Nb products essentially from complex low grade minerals [52, 53]. However, with stringent regulations on health, safety and environment (HSE) protection increasing significantly every day, the adoption of this hydrometallurgical process has since then, suffered its own share of demerits due to the high eco-unfriendliness, toxic and harmful nature of the adopted HF acid and other fluoride extraction media [44, 52, 54, 55], in comparison to the pyrometallurgical techniques. Recently, a lot of research works have been directed towards the pyrometallurgical extraction of Nb and Ta, especially as an adjunct process for optimum and efficient recovery of the metals. More so, despite the inadequacies and shortcomings involved in pyrometallurgical techniques, more especially the employment of elevated temperatures, certain positive effects however can be achieved during high temperature extractive procedures on complex minerals. Thus the following establishments:

• At elevated temperatures, less complex and less expensive reductive agents may be employed.

• There is accelerated rate of reaction at elevated temperatures, hence, leading to high metal recoveries/extractions or productions.

• The rate of reaction doubles after every 10°C increase in temperature; requiring lower activation energy and thus, aids in quicker chemical reactions.

• In employing high temperatures, there is the ability to thermally treat large amount of minerals or ore deposits in compact space giving rise to capital and processing cost effectiveness.

• High temperature extractive procedures such as: fusing can efficiently extract/separate reactive metals in the mineral composition from the mineral deposits.

• As a result of high temperatures, there is the feasibility of shift of reaction as well as the increase in solubility rate of the metals/metallic compounds in aqueous media, even in H₂O.

2.1 Pyrometallurgy and high temperature extraction techniques

Pyrometallurgical procedures on the treatment of Nb and Ta minerals or complexes may involve certain thermal and chemical processes and reactions which may lead to the modification and formation of entirely new mineral phases.

2.1.1 Roasting process

Roasting as a pyrometallurgical procedure involves gas–solid reactions at elevated temperatures with the aim of mineral/metal decomposition, separation and purification. Roasting of mineral complexes or concentrates is often regarded a thermo-chemical process where the chemical conversion of the mineral takes place with oxygen or some other elements or compounds employed at somewhat high temperatures, converting it into another chemical form or state. This is a route or step that has been adopted by several researchers in the processing of certain Nb and Ta complexes or mineral concentrates, usually conducted in order to prepare the mineral for an adjunct hydrometallurgical procedure. For instance, the feasibility of leaching or dissolution process is at an increased measure if the metals in question are in their less stable (oxide) forms and thus more soluble and easy to dissolute. This is usually the situation after the mineral complex has undergone roasting procedure, either on the Nb and Ta complexes or on other mineral impurities/gangue associated in the mineral matrix. Howbeit, before roasting is performed, the minerals often times undergo some physicochemical beneficiation and separation process to some extent, such as froth flotation, gravity, magnetic or electrostatic concentrations. The mineral concentrate may be mixed with certain reagents or materials in order to aid/enhance roasting. Mineral oxides can be somewhat easily reduced to their metallic state or elemental form compared to sulphide minerals. Typically, the roasting of Nb and Ta mineral complexes may often involve certain complex thermal and chemical gas–solid reactions taking place between the mineral/material solids and the furnace atmosphere; and this however consists of several process types.

2.1.2 Roasting types

2.1.2.1 Oxidative roasting

This is the most common type of roasting process industrially practiced and it’s more often employed on sulphide complexes or mineral concentrates. In this type of roasting process, the (sulphide) complex is converted to oxides and sulfur is thereby given off as sulfur dioxide gas (SO₂). The complex is however heated to elevated temperatures in the presence of excess oxygen or air in order to completely or partially burn out or replace the impurity elements (which in most cases is usually sulfur) with oxygen. Here, introduced oxygen is supplied and hence replaces the sulfur being burnt off. However, this procedure is usually considered environmentally unfriendly as harmful sulfuric gases are released into the environment. Howbeit, it is essential to note that the sulfur gas released in the form of sulfur dioxide gas (SO₂) may be trapped and essentially utilized in the production of sulfuric acid (H₂SO₄). At the occurrence of complete or almost complete sulfur removal from the sulphide complex, the residue is referred to as dead roast. In this type of roasting, quartz and certain other gangue minerals may act as catalyzers. The general oxidative roasting chemical reaction is represented in Eqs. 1 to 2.

Metal sulphide —› Metal oxide.

$$M{S}_{\left(c\right)}+3/2O_{2\left(g\right)}\to M{O}_{\left(c\right)}+S{O}_{2\left(g\right)}$$E1

$$2M{S}_{\left(s\right)}+3O_{2\left(g\right)}\to 2M{O}_{\left(s\right)}+2S{O}_{2\left(g\right)}$$E2

2.1.2.2 Chloridizing roasting

This roasting process type involves the transformation or conversion of certain mineral complexes or metallic compounds to chloride forms either by employing reduction or oxidation processes or conditions. This type of roasting can be employed to extract and process certain metals from their chloride state or forms; such as Nb, Ta, Be, Ti, Zr, as well as U and certain rare earth elements (REE). Some overall chloridizing roasting forms is represented by the chemical reactions in Eqs. 3 and 4.The Eq. 3 represents the chlorination process of a sulphide mineral complex which involves an exothermic reaction; while Eq. 4 presents the involvement of an oxide mineral complex with the addition of elemental sulfur serving as a catalyst and facilitating the chemical reaction. More so, some carbonate mineral ores also react almost similarly as the oxide mineral ores after being decomposed or digested under certain elevated temperatures to their oxide state.

$$2NaCl+MS+2O_2\to N{a}_{2}S{O}_4+MC{l}_2$$E3

$$4NaCl+2MO+S_2+3O_2\to 2N{a}_{2}S{O}_4+2MC{l}_2$$E4

2.1.2.3 Volatilizing roasting

This roasting type is the volatilization or elimination of the metallic oxides such as ZnO, As₂O₂, Sb₂O₂ and some other oxides from the mineral complex. Volatilizing roasting however involve the careful oxidation of mineral ores at high temperatures in order to remove unwanted elements or impurities in their volatile oxide state or form. In this type of roasting process, the careful control of the amount of oxygen in the roaster is imperative as the excessive oxidation of the mineral complex may produce non-volatile oxide complexes.

2.1.2.4 Magnetic roasting

This type of roasting involves the controlled reduction of certain metallic mineral complexes, converting them into their metallic forms, in order to ease up subsequent beneficiation, separation and processing routes or steps. For instance, a non-magnetic iron oxide, such as hematite (Fe₂O₃) is reduced in a controlled reduction reaction to magnetite (Fe₃O₄), a more magnetic iron form or state, prior to magnetic separation and/or other concentration processes. This method of roasting has been employed for the concentration of certain Nb and Ta mineral complexes by several researchers with the aim of recovering or eliminating associated iron content.

2.1.2.5 Reductive roasting

This type of roasting process involves the partial reduction of an oxide mineral ore, in preparation to the actual smelting reduction process or hydrometallurgical extraction or recovery of the metal from the mineral ore to its metallic form or state. This is regarded as the most adopted and successful form of roasting on Nb and Ta minerals/complexes.

2.2 Calcination process/thermal treatment

Prior to calcination, a precipitation process is usually conducted on the Nb and Ta products/leachates obtained fromthe downstream hydrometallurgical procedures, generating certain intermediates of Nb and Ta as precipitates, and often in the form of pentaoxides or probably dry hydroxides. The structure of the washed/filtered precipitates may contain either two kinds of water. The first kind of water is incorporated within the Nb(V) and Ta(V) hydroxides, and can only be eliminated with calcination employed at somewhat higher temperatures within the range of 700 to 900°C. The other kind of water is known to be intrinsic moisture and may easily be eliminated by only employing a drying process at lower temperature range within 100 to 200°C.Calcination of the processed Nb and Ta complexes or precipitates may result in the yield of high purity Nb and Ta oxides. Calcination process however, also leads to the elimination of certain volatile constituents that were entrained in the precipitates, such as ammonium or fluoride [2, 25]. The calcination/water removal process may be simply presented as in Eqs. 5 and 6.

$$2Nb{\left(OH\right)}_5+Hea{t}_{(700-900}{o}_{C)}\to N{b}_2{O}_5+5H_{2}O$$E5

$$2Ta{\left(OH\right)}_5+Hea{t}_{(700-900}{o}_{C)}\to T{a}_2{O}_5+5H_{2}O$$E6

2.3 Thermal production of Nb and Ta metals

The yield or production of Nb and Ta as pure metals is essentially dependent on their forms which they were found in the metal intermediates. For instance, the Nb metal is often achieved by the reduction of its intermediates employing reducing chemical agents like Al, Ca, Mg, H, Na or any suitable electron donors. The most widely practiced/adopted approach is based on the use of Al and is referred to as aluminothermic process, consisting of reactions between the pentaoxides (Nb₂O₅) and the pure Al at certain temperatures >1100°C, as shown in Eq. 7. However, the Ta metal refining from potassium heptafluorotantalate is mostly obtained by its reduction with Na (in Eq. 8). It is recommended however, that in order to achieve high purity Ta metal, both the salt as well as the Na-bearing reducing agent should be of high purity. The chemical reaction is to take place at approximately 1200°C temperature inside the reactor that operates under an inert atmosphere or vacuum, in order to prevent the accumulation of oxygen during the reduction procedure. The chemical reaction that occurs is therefore exothermic and have been reported to release about an energy of about 2985 kJ/kg [25, 57].

$$N{b}_2{O}_5+\frac{10}{3}Al\to 2N{b}_{(s)}+\frac{5}{3}A{l}_2{O}_{3(s)};\Delta H=-890kJ/mol$$E7

$$K_{2}Ta{F}_7+5Na\to Ta+5NaF+2KF$$E8

2.4 Influencing factors

The selection of appropriate existing, developing or newly developed novel beneficiation and extraction process routes, decomposition and separation techniques that are suitable for complex Ta and Nb mineral matrices and concentrates is highly dependent on specific factors. Other than the physical and chemical factors or properties, which includes: the mineral type, nature, and chemical composition, there are other factors that may affect the selectivity/choice of the process steps and dictate the process application of extraction and separation process routes, such as: the technical, environmental, economical, safety and quality factors. This may also include the ability/feasibility of obtaining high purity/high yield recoveries, with minimal consumption of energy, number of process steps, by-products and environment pollution. Cost and scalability from laboratory to industrial/commercial scales are also considered major factors [16, 23]. The cost factor however may include the price cost of the mineral type, price cost of the processed materials, fuel cost and availability as well as product quality/quantity [23, 27]. More so, certain determinant factors or parameters may influence the degree of success as well as the decomposition rate of the roasting of Nb and Ta complexes. This includes: temperature, time, physicochemical condition of the mineral complex, chemical reagent type, availability/cost of the reagents or additives, mineral sample to reagent ratio, reagent concentration, mixing ratio, stirring speed, stirring time, equipment or furnace as well as the eco-friendly/unfriendly nature of the chemical reagents employed and the reaction process.

3. Literature overview of the pyro-hydrometallurgical recovery of Nb and Ta

Various pyrometallurgical processes have been adopted by several researchers for the extraction and recovery of Nb and Ta. Howbeit, the high temperature and high thermal energy consumption has poised a major setback. Thus this very process is usually adopted with certain adjunct processes, in order to reduce the temperature applied, thermal energy consumption as well as the cost implications. Moving forward, a lot of researchers have concurred that alkali fusion, an old and earliest established industrial method for mineral decomposition [43, 44, 45], and the alkali roasting-leaching decomposition process have been employed on Nb and Ta minerals with certain degree of success. The reaction chemistry, complexes and solubility rate or efficiency of Nb and Ta in alkaline media has thus, been investigated by various researchers [18, 19, 43, 44, 45, 47, 58, 59, 60, 61, 62, 63, 64, 65, 66]. Alkali roasting technique as a decomposition process has also been investigated by researchers [26]. Few investigations have been reported on the efficacy of alkali reductive roasting and dissolution of Nb/Ta ore minerals or concentrates. Nevertheless, there is still need to adopt mild concentrations/consumption of the alkali reagents with reduced/moderate temperatures. Also, most successful decomposition processes were however established effective to a certain measure on high-grade minerals but not effective on low-grade minerals [2, 4]. However, alkali roast and leach dissolution method employing KOH roasting and H₂O leaching on a low-grade Nb-Ta mineral ore has had great recoveries [44, 47]. KHSO₄ fusion as a pyrometallurgical process to improve the subsequent leaching efficiency of a low grade Nb/Ta polymineralized ore has also been adopted [62]. The study reported the feasibility of Ta/Nb decomposition and leach recovery efficiencies of alkali dissolution process on low-grade minerals. Hence, the reductive roast mineral treatment utilizing certain alkali agents was regarded an eco-friendly process and thus, a promising extraction route with moderate leachant and energy consumptions. Howbeit, high roasting temperatures and alkali reagent consumptions were main set-backs.

For instance, Ghambi et al. [56] investigated a novel pyrometallurgical procedure for extracting and purifying columbite and tantalite concentrates (29% Ta₂O₅ and 16% Nb₂O₅). A reductive roasting procedure was employed in the study. The Nb and Ta concentrates were reduced with solid carbon in the form of activated charcoal (carbothermic reduction) and alkali as the reducing atmosphere employing high temperatures ranging from 800 to 950°C. This was reported to aid the reduction of the iron oxides present in the concentrates to metallic iron as well as the subsequent magnetic separation. Suri et al. [58] investigated the alkaline decompositions for recovery of Nb and Ta from a cassiterite (Sn) bearing mineral, using Na₂CO₃ and K₂CO₃ salts. The authors reported successful recoveries of Nb and Ta employing alkaline fusion/reduction roasting-acid leaching processes. Phase transformations of pellets of Sn oxides such as SnO and SnO₂, and Na₂CO₃ alkaline salt when roasted under certain carbothermic (CO-CO₂ mixed gas) thermal atmospheres were investigated by Liu et al. [66]. The resulting highly pure pellets of Na₂SnO₃ reported by the researchers confirm the favorable reaction chemistry of certain alkaline salts on the pyrochlore mineral group. Nete et al. [67] however explained the feasibility of adopting the alkali flux decomposition process as a substitute digestion method. Albeit the low Nb and Ta recoveries of microwave digestion, their investigations displayed process simplicity and also showed that the process allowed smaller amounts of chemical reagents for digestion. The authors also concluded that flux fusion with lithium tetraborate had shorter time for digestion and high recoveries of >90%; however larger amounts of chemical reagents and time-consuming sample preparations were needed for digestion.

As a result of the volatilization and loss of toxic/corrosive HF acid during dissolution process of Nb-Ta minerals, resulting to equipment corrosion and harm to human beings and environment, Yang et al. [45] established a novel process decomposition of low-grade Nb/Ta minerals. The researchers employed alkali fusion method, applying caustic soda (sodium hydroxide, NaOH) with minimum alkali consumption of alkali-sample ratio reduced from 3:1 to 1:1. A reduced alkali-sample ratio of 1:1, initial mineral particle size of 75 μm and 650°C temperature for 30mins had significant influence on the Nb/Ta mineral decomposition. The mineral was however converted to Na(Nb,Ta)O₃ complexing compounds and not that of Na₃(Nb,Ta)O₄, which is usually realized from the conventional alkali fusion method. Results however indicated mineral decomposition efficiencies of 98% cassiterotantalites and 99% pyrochlores in the Nb/Ta ore. Howbeit, K(Nb,Ta)O₃ and Na(Nb,Ta)O₃ compounds/complexes are somewhat not soluble in most mild solutions. Also, Berhe et al. [68] carried out the comparison of the performance and decomposition of the kentichamangano-tantalite ore by both HF-H₂SO₄ leaching and KOH fusion-leaching. The comparative study thus proposed KOH sub-molten salt as a substitute process method for the dissolution of Nb/Ta minerals with the intention of mitigating/eliminating HF acid pollution and associated harm or hazards. Results also explained that the dissolution rate is dependent on the proportions/concentrations of the HF-H₂SO₄ and KOH leaching agents. In addition therefore, higher ratios of HF to H₂SO₄ in the acidic system and higher concentrations of KOH during the alkaline fusion process step led to lesser amounts of leftover residues after the decomposition process thereby resulting to better or increased dissolution rates after H₂O leaching. Thus, the authors however concluded that KOH possess the potential of being a suitable alternative or substitute material to the conventionally utilized HF acid. Similarly, De Oliveira, De Souza and Lopes-Moriyama [69], conducted investigations on alkaline thermal treatments with subsequent acid leaching on a ferro-columbite mineral in order to obtain mixed proportions of Ta and Nb oxides. The study however reported good yield recoveries, employing alkali fusion with potassium bisulphate (KHSO₄) and subsequent HCl acid leaching. More so, mechanical pre-mixing process has been established on primary minerals and reported to aid proper distribution of the chemical reagents on the mineral particles, thus, enhancing pyrometallurgical decomposition [70]. Table 3 therefore summaries the major pyro(hydro)metallurgical extraction methods recently adopted by researchers.

Reference	Mineral deposit	Pyro(hydro)metallurgical technique employed	Characterization analysis	Positive results obtained
De Oliveira et al. [69]	Natural Ferro-columbite	KHSO4 fusion + HCl& HF-H2SO4 dissolution	XRF, XRD, BET, SEM, EDXS	Increased grade of 99.6% Nb
Sun et al. [71]	Nb-Ti synthetic ilmeno-rutile	Akali (NaOH) roasting + H2SO4 dissolution	XRD, XRPS, LPA, ICP-OES	Increased recovery of 96.68% Nb& Ti; Formation of a new Nb-Ti phase for feasible downstream treatment
Ghambi et al. [56]	Columbite& tantalite concs	Na2CO3-activated charcoal assisted alkali reductive roasting +2-stage wet MS + H2O & C2H2O4 dissolution + NaHSO4 roasting	XRPD, SEM, EDXS	Concs can attain a purity of 80% with further refining; May serve as alternative approach/substitute for HF dissolution, mitigating associated demerits
Purcell et al. [33]	Ferro-columbite	NH4HF2 fusion/leaching	XRF, ICP-OES	Recovery of 96.00% Nb& 91.00% Ta
Nete et al. [35]	HG tantalite	NH4HF2 fusion + SX with MIAK	ICP-OES	Recovery of ~ 100.00% Nb& 98.90% Ta
Wu et al. [72]	LG Nb-Ta ore	H2SO4 acid roasting/leaching	MLA, XRF, AAS	Recovery of 84.00% Nb
Permana et al. [73]	Sn slag	NaOHalkali roasting + water quenching + sieving	XRF, TGA, SEM	Recovery of 0.90% Nb& 0.40% Ta
		NaOHalkali roasting + HF/HCl acid leaching		Recovery of 2.09% Nb&2.01% Ta
Permana et al. [74]	Sn slag	NaOHalkali roasting + NaOH quenching	XRF, XRD	Recovery of 0.20% Nb& 0.14% Ta
		NaOHalkali roasting + HCl leaching		Recovery of 3.57% Nb& 3.75% Ta
Kitungwa et al. [75]	Columbite	NH4HF2-KOH fusion system	XRF, OPM, ICP-OES	Recovery of 95.00% Nb& 94.00% Ta
Habinshuti et al. [76]	Ferro-columbite	KOH-assisted alkali roasting + water-based leaching	XRF, SEM, EDS, ICP-OES	Recovery of 97.00% Nb& 85.00% Ta
Yang et al. [45]	LG Nb-Ta ore	NaOH fusion + water leaching	XRD, SEM, ICP-OES	Recovery of 99.00% Nb& 98.00% Ta
Wang et al. [44]	LG Nb-Ta ore	KOH alkali roasting + water leaching	XRD, ICP-OES	Recovery of 94.70% Nb& 93.60% Ta
Shikika et al. [77]	Coltan ore	Alkali (KOH) roasting + water leaching	XRD, XRF, SEM, EDS, PIXE, ICP-AES	Decomposition of Nb& Ta without the use of HF; Obtaining PLS suitable for downstream SX
Berhe et al. [68]	LG Mangano-tantalite	KOH fusion + water leaching	XRF, XRD, FT-IR	Recovery of 0.35% Nb& 14.20% Ta
Berhe et al. [78]	HG tantalite	KOH fusion + water leaching	EDXRF, XRD, FT-IR, ICP-OES	Recovery of 94.73% Nb& 75.80% Ta
		KOH fusion + water leaching + SX with EMIC/AlCl3		Recovery of 99.84% Nb& 90.81% Ta
Nete et al. [39]	Pure (Nb2O5 and Ta2O5) pentaoxides	NH4HF2 fusion + PPT with PPDA	ICP-OES	Recovery of 23.00% Nb& 73.00% Ta
		NH4HF2 fusion + single step SX with MIBK		Recovery of 2.00% Nb& 80.00% Ta
		NH4HF2 fusion + double step SX with MIBK		Recovery of ~ 100.00% Ta
		NH4HF2 fusion + IX with strong amberlite resin		Recovery of 91.69% Nb& 73.39% Ta
		NH4HF2 fusion + IX with weak dowex marathon resin		Recovery of 96.05% Nb& 52.34% Ta
Nete et al. [67]	Nb-Ta tantalite	Microwave assisted H2SO4 leaching	XRF, XRD, ICP-OES	Recovery of 90.25% Nb& 88.90% Ta
		Li2B4O7 fusion		Recovery of 98.50% Nb& ~ 100.00% Ta

Table 3.

Nb-Ta pyro(hydro)metallurgical extraction procedures adopted by various researchers.

Key: MLA = Mineral liberation analyzer;LG = Low grade; HG = High grade; SX = Solvent extraction; IX = Ionic exchange; MIBK = Methyl isobutyl ketone; MIAK = Methyl isoamyl ketone; EMIC = 1-ethyl-3-methyl imidazolium chloride; PPDA = ρ-phenylenediamine; LPA = Laser particle analyzer; PPT = Precipitation; EDXRF = Energy dispersive X-ray fluorescence; XRPS = X-ray photoelectron spectroscopy; XRPD = X-ray power diffractometry; Concs = Concentrates; SEM = Scanning electron microscopy; EDXS = Energy dispersive X-ray spectroscopy; OPM = Optical microscopy; AAS = Atomic absorption spectrophotometry; BET = Brunauer–Emmett–Teller; ICP-OES/AES = Inductively coupled plasma-optical/atomic emission spectroscopy; FT-IR = Fourier transform-infrared spectroscopy; TGA = Thermogravimetric analysis.

4. Conclusion

The separation and recovery of Nb/Ta metallic elements from mineral deposits has become somewhat challenging, involving several complex/complicated extraction process routes from the rest of the gangues/impurity elements composed in the mineral ore or deposit and from the value metal elements themselves. This can be chiefly attributed to the presence of numerous impurities such as: metal oxides, refractory, rare earth (REE) and radioactive elements, and hence the complexity of the metal extraction process. In that regard, Nb₂O₅ and Ta₂O₅haveconventionally been extracted hydrometallurgically, using HF acid as the dissolution agent, or a combination of HF and H₂SO₄ mineral acid dissolutions. Howbeit, the high volatility, corrosive and toxic nature, high chemical/reagent consumption, eco unfriendliness and waste generation of fluorides have contributed to Nb/Ta extraction complexity, high equipment maintenance and high operational cost. This, as a result has increased the extraction difficulty and low yield recovery/purity of the metals, coupled with their numerous industrial applications in engineering and technology. This study therefore has provided insights into recent adopted pyro- or pyrohydro-metallurgical Nb/Ta extraction process routes as well as its associated factors. Certain pyrometallurgical techniques, such as reductive roasting, carbothermic reductions, alkali roasting and alkali fusions, etc. have shown great success in Nb/Ta extraction. Regardless of the drawbacks encountered, such as elevated temperature and high energy/reagent consumption, the adoption of high temperature procedures is imperative as it plays key roles in the decomposition of these value metals, in preparation for subsequent downstream measures. Thus, the employment and development of pyrometallurgical extraction routes for efficient Nb/Ta recovery is therefore encouraged with certain process advances/improvements, proper process optimization, as well as adoption of adjunct techniques so as to mitigate/curtail such limitations.

Acknowledgments

We sincerely acknowledge the Faculty of Engineering and the Built Environment, Tshwane University of Technology, Pretoria, South Africa. Most especially, our special recognition and hearty appreciation goes to Adeline C. Nzeh and also to Dr. Swithin N. Nzeh (G.P, United Kingdom; MBBS, MRCGP, Dip. Paed, RCPathME) for their significant contributions towards the success and actualization of the present study.