O.; Soucy, G. Titanium: An Overview of Resources and Production Methods. *Minerals* **2021**, *11*, 1425. https://doi.org/10.3390/

**Citation:** El Khalloufi, M.; Drevelle,

Academic Editors: Shuai Wang, Xingjie Wang and Jia Yang

min11121425

Received: 15 November 2021 Accepted: 12 December 2021 Published: 16 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

## **1. Introduction**

Titanium is a transition metal that is used more in the production of high-strength, corrosion-resistant, and thermally stable metal alloys for the aerospace and shielding industries. The titanium production cost has so far hindered the growth in the use of this metal relative to other base metals on the market, even though titanium is the fourth most abundant structural metal in the earth's crust with 0.6%. It comes after iron, magnesium, and aluminum, but remains exotic due to its prohibitive cost [1,2], which prevents the metal from reaching its full potential in marine and automotive industry applications. Older production technology, high energy losses, and loss of material or metal are some of the problems associated with the production of titanium metal [3]. Further, all base metals are inferior to titanium in terms of specific mechanical and chemical properties, but titanium has not yet been fully exploited [1,2,4]. Titanium exhibits unique properties, some of which are proprietary, which could help it replace common metals and alloys such as steel and aluminum in many applications [5].

The current commercial method of titanium production is the Kroll process, marketed by DuPont Germany in 1948 [4]. It is a discontinuous, energy-, and labor-intensive process whose strict conditions make it expensive; therefore, researchers around the world are investigating new methods for extracting titanium from its precursors. Titanium is mainly produced from minerals such as ilmenite FeTiO<sup>3</sup> and rutile (TiO2) while smaller quantities are produced from perovskite (CaTiO3) and titanite or sphene (CaTiSiO5) [6]. The main ilmenite deposits are located in Australia, China, Norway, Canada, Madagascar, India, South Africa, and Vietnam, while rutile deposits are found in Sierra Leone, the United States, India, and South Africa [7].

Ilmenite is a significant mineral [8], which contains between 40% and 65% titanium dioxide. The other elements are either ferrous oxide or ferric oxide and sometimes small amounts of vanadium, magnesium, and/or manganese. Ilmenite's main sources are the

heavy mineral sands (alluvial deposits) but are also commonly distributed in hard rock [9]. Currently, ilmenite accounts for 92% of the world's titanium mineral production. Rutile (TiO2) has a titanium dioxide content of 93–96% but is difficult to find in natural ilmenite deposits [7]. Thereby, the present review focuses on the alluvial deposit, more precisely the ilmenite, and the extraction of titanium from it.

Titanium metal production consumes a small proportion of total titanium reserves per year [7]. Low density and high tensile strength make titanium attractive for industrial applications, and give titanium-containing alloys the highest strength-to-weight ratio, an important property for metals in the steel industry. In addition, titanium is a valuable metal and can resist corrosion of both seawater and acids. As an alloyed metal, it can also resist corrosion better than copper and nickel alloys and has a low modulus of elasticity that is half that of steel and nickel alloys. The most common titanium alloy is Ti6A-4V (6% aluminum, 4% vanadium, 90% titanium) and is typically used in medical applications such as knee replacement implants. Metal is also one of the main elements in the aerospace industry, architecture, chemical, and automotive applications [10].

The traditional methods of manufacturing Ti follow the same general procedures as steel, including the primary metal production, the melting and casting of alloy ingots, forging and rolling to produce rolling products, and the manufacture of components or structures from rolling products [11]. Many powder production processes are at various stages of development. There are two general approaches for Ti metal production: electrochemical and thermochemical methods. The well-known example of the electrochemical methods is the Fray, Farthing, and Chen (FFC) or Cambridge process [12], which is based on the electrolysis reduction of titanium oxide. The Kroll process [13], is an example of the thermochemical way that is a commercially suitable process for primary Ti metal production today.

This review provides an overview of titanium resources, of which ilmenite is the main source, as well as it focuses on some effective methods for producing titanium powder through extractive metallurgy processes, and highlights a comprehensive view of the opportunities and challenges.

#### **2. Titanium Usage and Market**

Titanium, and particularly titanium alloys, have become economically accessible following a drop of nearly 30% in the price of commercially pure titanium over the last five years [14]. Titanium alloys have many physical (lightness, good mechanical properties, resistance to cryogenic temperatures) and chemical (resistance to electrochemical corrosion, biocompatibility) advantages [15,16], which make them an indispensable material for civil and military applications in fields as vast as in energy, transport, medical (MRI magnet for observing the human body), water treatment, and the transport of corrosive liquids and gases (Figure 1) [14].

The uses of titanium have expanded based on its inherent properties as well as the development of new alloys. The main use is still in aerospace and aeronautics applications, such as engines, airframes, missiles, and spacecraft [16]. Aerospace applications are based on the low density (Table 1) and high strength-to-weight ratio of titanium alloyed at high temperatures. Titanium's corrosion resistance makes it a natural material for seawater, marine, and naval applications. In addition, titanium is largely used in seawater-cooled power plant capacitors [16].

**Figure 1.** Titanium applications of the Western world by market sector, 2017 [17] (Reprinted with permission from Elsevier, Copyright, (2020)).


**Table 1.** Some properties of pure titanium [15,16].

Titanium can also be utilized in petroleum refineries, paper, and pulp bleaching operations, nitric acid plants, and some organic synthesis production [4]. Moreover, Titanium has found its use in the medical field. In particular, depending on clinical needs, titanium and a multitude of its alloys offer high axial flexibility, good expansion behavior, radio-opacity, and hemocompatibility. Even of its sophisticated bio-applications, the main use of titanium in biomedicine is as a structural prosthesis [1,2,18].

## **3. Resources**

Titanium deposits are huge, with current estimates assuming a global reserve of 650 billion metric tons of titanium oxide. The minable deposits are found in South Africa at Namaqualand and Richards Bay, Australia, Canada, Norway, and Ukraine (Figure 2) [1,7]. The two main minerals being considered for use are ilmenite and rutile and although these are the minerals available for economic mining, TiO<sup>2</sup> is part of almost every mineral, sand, and rock [4].

The common titanium minerals are anatase, brookite, leucoxene, perovskite, titanite, rutile, and ilmenite. However, only ilmenite, leucoxene, and rutile have crucial commercial value. Ilmenite and rutile are the two main titanium minerals used in industrial applications, mainly for titanium metal production and pigment-grade titanium dioxide [19].

#### **4. Mineral Ilmenite**

In 1827, Adolph Theodor Kupffer discovered Ilmenite (titanoferrite) in the Ilmen Mountains of Russia. FeTiO<sup>3</sup> is the typical chemical formula of ilmenite, while its chemical composition is (40–65 wt.%) TiO<sup>2</sup> and (35–60 wt.%) Fe2O<sup>3</sup> [20]. Table 2 shows the properties of pure ilmenite.

**Table 2.** Basic properties of pure ilmenite (FeTiO<sup>3</sup> ) [21].


Ilmenite is an economically important mineral, mainly because of its role in the production of titanium oxide pigments. Its magnetic properties and those of ilmenitehematite solid solutions (Fe2O3;) are particularly important in commercial extraction by magnetic separation. The dependence of the ilmenite structure on temperature, pressure, and composition is strongly related to its electronic, magnetic, and optical properties [22]. Due to the coexistence of ilmenite with geikielite (MgTiO3) and pyrophanite (MnTiO3) in these rocks, the typical chemical formula of this mineral in magmatic rocks is (Fe, Mn, Mg) TiO<sup>3</sup> [21]. Ilmenite is often confused with other iron-bearing minerals such as hematite and magnetite because of its high magnetic susceptibility (Figure 3). Ilmenite has a hexagonal crystal structure that is similar to corundum (Al2O3) and different from other iron minerals and has lower magnetism compared to hematite and magnetite. The crystal presents an octahedral structure alternating and coordinated with iron and titanium layers [22,23].

**Figure 3.** Ilmenite sand (**left**) and grinded material (**right**) from the Metchib company, Quebec, Canada.

The Tellnes (Norway) mines produce 550,000 tons of ilmenite per year [24]. The largest Ti producers in the world are China, Australia, and South Africa (Figure 4). China produces ilmenite in significant quantities, while Australia and South Africa have the world's largest natural reserves for ilmenite and rutile [7,21].

**Figure 4.** Ilmenite mine production in different countries in 2020 [7].

## **5. Metallurgical Extraction of Titanium from Its Concentrates**

Many methods are used for the production of Ti metal powder. Ti powder is, therefore, the product of extraction processes that produce primary metal by using titanium tetrachloride (TiCl4) or titanium dioxide (TiO2) as feed material [25,26]. Processes for the manufacture of titanium powder directly as extractive metallurgical products include the manufacture of Ti from TiCl4, purified TiO2, and/or improved titanium slag (UGS) with a TiO<sup>2</sup> content greater than 90%. Upgraded titanium slag (UGS) is one of the fundamental

products of the carbothermal reduction of titanium ore such as ilmenite. Natural rutile and synthetic rutile are also included in this raw material category. These processes can be categorized: (1) thermochemical methods and (2) electrochemical methods [11].

#### *5.1. Thermochemical Processes*

#### 5.1.1. Kroll Process

The commercial production of primary Ti metal is generally made either by Kroll [13] or Hunter processes [27]. The standard process against which new technologies are compared is the Kroll process (Figure 5).

**Figure 5.** Scheme and reactions of the Kroll process of titanium sponge production [28] (Reprinted with permission from Springer Nature, Copyright, (2020)).

> In the process, magnesium metal (the reducing agent) is injected into a retort filled with argon and heated to 800–900 ◦C. However, the oxides impurity contained in the Ti slag is also chlorinated, so refined TiCl<sup>4</sup> has been produced by purifying the crude TiCl<sup>4</sup> before the Mg reduction [13,29]. Although most of the by-product MgCl2, and excess magnesium, is drained during reduction, the product sponge contains residual magnesium and MgCl<sup>2</sup> in its porosity [30]. Magnesium and MgCl<sup>2</sup> are separated by vacuum distillation or helium sweeping followed by leaching. Part of the sponge must be decommissioned due to contamination of the autoclave wall [13,29].

> According to one estimate, 70% of the total energy consumption is considered for the distillation to produce the sponge metal. It shows that the cost of metal purification is one of the major cost drivers, in addition to the cost of the precursor and reductant [30].

## 5.1.2. Hunter Process

The popular process established on TiCl<sup>4</sup> reduction using Na is the Hunter process. The Hunter and Kroll processes are quite similar in that they are considered as thermochemical processes based on the reduction of TiCl<sup>4</sup> to produce Ti [27]. Economically, the Hunter process is considered non-competitive with the Kroll process. The main difficulty is that to produce one mole of Ti by reducing TiCl<sup>4</sup> requires four moles of Na, whereas only two moles of Mg are needed for 1 mole of Ti [11]. In addition, producing Na by electrolysis

is at least as expensive as that of Mg. These problems make the processing of Na more expensive than the use of Mg. However, the Hunter process can also produce Ti powder instead of Ti sponge [31].

In the Hunter process, TiCl<sup>4</sup> and Na are gradually introduced into the reactor. The process is generally performed over 800 ◦C [28,32]. Ti is formed at the surface of the molten bath, where the gas TiCl<sup>4</sup> is exposed to the Na. Ti crystals then form and are set at the bottom of the liquid bath. According to the operating parameters, some Ti particles can form Ti sponge, while others are deposited as Ti powder. The purity of the powder produced by the Hunter process is often higher (99%) [11,29]. Table 3 provides a comparison between Kroll and Hunter processes.

**Table 3.** Comparison between Kroll and Hunter processes [31] (Reprinted with permission from John Wiley and Sons, Copyrright, (2013)).


## 5.1.3. Armstrong Process

The Armstrong process is considered the most advanced process. It uses the same reactions as the Hunter process [33].

Therefore, the crucial advantage of the Armstrong process [34] is the continuity of the operation, pumping molten sodium in the reactor to react continuously with TiCl<sup>4</sup> gas (Table 4). Ti powder and the resulting NaCl are collected from the reactor by the sodium stream. Once the unreacted liquid Na is removed by filtration, as well as Ti powder is purified by washing out the salt. The Armstrong product can be described as mini sponges, i.e., microporous particles [35]. Figure 6 shows the Armstrong process aspects.

**Figure 6.** The Armstrong flow diagram [36].

## 5.1.4. TiRO Process

This process was developed by CSIRO (Commonwealth Scientific and Industrial Research Organization) in Australia and used the same reactions as the Kroll process but in a fluidized bed reactor in which gas–solid fluidization takes place, which considerably increases the reaction rate and reduces both operating and capital costs [37].

The TiRO process consists of two main steps (Figure 7): reduction of TiCl<sup>4</sup> in a fluidized bed with Mg powder and vacuum distillation to remove the by-products MgCl<sup>2</sup> and Mg [38].

**Figure 7.** Diagram of the process TiRO™ [39].

According to the information mentioned above, TiCl<sup>4</sup> can be reduced by sodium or magnesium. The main characteristics of the powder are well detailed in Table 4 below. All these processes below use TiO<sup>2</sup> as raw material to produce titanium powder with high purity.


#### 5.1.5. Metal Hydride Reduction (MHR) Process

The MHR process was first introduced in 1945, and the most remarkable work was reported by Borok [44], in 1965 and Froes et al. in 1998 [45]. Calcium hydride was used to directly reduce TiO2. In Russia, a commercial operation has been reported that uses the same procedure [46].

## 5.1.6. Electronically Mediated Reduction (EMR) Process

EMR process uses calcium as a reducing agent to produce titanium. The TiO<sup>2</sup> reduction is made with no direct contact with the reducing Ca-Ni alloy, and hence the contamination of titanium can be successfully avoided by using EMR. This approach can be exploited to develop another process to produce titanium powder continuously [47].

#### 5.1.7. Process for Reducing Preforms

Okabe et al. [48] have developed this process where TiO<sup>2</sup> is mixed with either CaO or CaCl<sup>2</sup> and sintered in the air. Calcium vapor reduces TiO<sup>2</sup> at temperatures between 800 and 1000 ◦C, with the calcium oxide dissolving in calcium chloride. A hydrochloric acid solution has been used for leaching the product [46]. Again, these experiments were only conducted in the laboratory [49].

#### 5.1.8. Hydrogen-Assisted Magnesium Reduction (HAMR) Process

The HAMR process is developed to produce a Ti powder with a very low oxygen content, by using a hydrogen atmosphere, molten salt, and deoxygenation step to guarantee that the oxygen amount in titanium powder is sufficiently low (less than 0.15% by weight) [50,51]. Table 5 shows a comparison between the characteristics of these processes based on TiO<sup>2</sup> reduction.

**Table 5.** Comparison between characteristics of TiO<sup>2</sup> reduction-based processes (reprinted from [11] Taylor and Francis, an open-access journal).


There have been several reports and trials to perform continuous processes based on the same chemistry as the above processes. Lu et al. [59] produced a fine titanium powder from a titanium sponge by the shuttle: the disproportionation reaction and its reverse reaction (proportioning reaction) of titanium ions in molten NaCl-KCl at 750 ◦C. Moreover, some experiments were performed to synthesize TiH<sup>2</sup> from the reaction between CaH<sup>2</sup> and TiCl<sup>4</sup> in the presence of Ni [60].

In another study, porous titanium was obtained in mixtures of molten CaO-CaCl<sup>2</sup> salts dissolved in Ca by self-sintering with the exothermic reaction between porous CaTiO<sup>3</sup> and calcium vapor at 1000 ◦C for 6 h under vacuum [61]. Development of a new method based on deoxidation of dissolved O from Ti. The process leads to the formation of YOCl using Mg as a deoxidizer at 1027 ◦C [62].

Daniel Spreitzer et al. [63] have used a laboratory fluidized bed reactor to cut down the hematite kinetic reaction to produce Fe by hydrogen around 600–800 ◦C by measuring the change in weight of the sample portion during reduction. Moreover, with combining magnesiothermal reduction of TiO<sup>2</sup> and a leaching purification process, titanium metal powder was obtained with only 2.98% of O [64]. The synthesis of a cermet based on Fe-TiC by carbothermal reduction of ilmenite was successfully produced [65]. This method was done in an atmosphere containing argon in a scope of 850–1350 ◦C. Another study has led to developing an environmentally friendly pre-treatment process to recycle titanium turning waste and ferrotitanium ingots with low levels of gaseous impurities [66]. The number of gaseous impurities in the titanium scrap before the removal of machining oils from the surface reached 2% [66]. Similarly, Nersisyan et al. [67] have developed a combustion synthesis route for single-phase titanium-based compounds (e.g., FeTi, TiC, TiB2, and TiFeSi2) from the precursor mixture FeTiO<sup>3</sup> (natural ilmenite)-αMg-C (B, Si)-kNaCl. The method allows the process temperature and the phase composition to be controlled by changing the number of moles of Mg and NaCl [67]. Furthermore, new research has been developed to study the effect of the degree of ilmenite reduction on the chemical and phase characteristics of ferrotitanium and slag produced by the SHS aluminothermic process, which is a highly exothermic thermite reaction [68]. Increasing reduction not only reduces the consumption of aluminum and the amount of slag produced in the preparation of ferrotitanium but also reduces the oxygen content and improves the titanium and iron qualities [68]. Further, the Panzhihua ilmenite carbothermal reduction with activated carbon has been studied by using isothermal trials between 1200 ◦C and 1500 ◦C [69]. By decreasing the pressure and increasing the temperature, the impurities (Mg, Mn) in the product have been removed [69]. The carbothermal reduction behavior of ilmenite at high temperatures was studied by thermodynamic calculations [70]. FeTi formation is generated at 1650 ◦C. By increasing the temperature, a clear increase of TiC is observed, which can also encourage the further reduction of ilmenite slag at high temperatures [70].

Otherwise, for patents published in this sense, Mu et al. [71] invented a method that aimed to improve the metallic titanium production with a low-energy titanium-containing material by a molten salt electrolysis process (Table 6).

Similarly, in 2016, Fang et al. [51] presented a research procedure for producing a titanium powder or sponge. For instance, the method may include obtaining a TiO2-rich material, reducing impurities to produce purified TiO2, reducing the purified TiO2, using a metallic reducer at the same temperature and pressure to produce a TiH<sup>2</sup> product [51]. RMI (now part of Arconic) has patented a process that carries out above 900 ◦C to reduce oxygen in Ti-6Al-4V powder [72].



