An Overview of Thermochemical Reduction Processes for Titanium Production

Abstract

Titanium is one of the most abundant metals with superior properties such as excellent mechanical properties, high strength-to-weight ratio, and oxidation and corrosion resistance. However, it is commercially expensive to produce; hence, its use is limited. Currently, the Kroll process remains the most commercially exploited to produce titanium. Therefore, this paper thoroughly reviews some other proposed and developing thermo-reduction methods using the two main precursors titanium dioxide (TiO₂) and titanium chloride (TiCl₄) together with the environmental impacts they cause. The exorbitant production cost and environmental issues have resulted in enormous research and development to innovate more sustainable methods of titanium production. The various processes were comprehensively analyzed to assess whether they have the potential to expand to be economically viable. From this review, it is apparent that most of the methods still require further research to scale them up to an industrial and commercial level. Recent developments including the Council for Scientific and Industrial Research-Ti (CSIR-Ti), Titanium Reduction Oxide (TiRO), Preform Reduction Process (PRP), and hydrogen-assisted magnesiothermic reduction (HAMR) processes are auspicious for producing high-purity titanium sustainably.

1. Introduction

Titanium is among the most abundant elements on earth and possesses excellent mechanical properties, a high strength-to-weight ratio, and oxidation and corrosion resistance. It is mainly used in extreme conditions such as high-temperature applications and excessive corrosion environments, hence its application in aerospace, automobiles, and shipbuilding. The main sources of titanium ore that are economical to mine are rutile and ilmenite, which account for about 89% of titanium production [1]. The commercial method of production for titanium is very expensive as it is both labor- and energy-intensive. Recent changes in technology and improvements in materials used in different conditions have resulted in an increase in demand for titanium and its alloys, which is an urgent matter that the scientific research community must address to propose cheaper solutions.

Researchers have devised several methods to produce titanium that fall mainly into two groups: thermochemical and electrochemical [2,3]. The electrochemical techniques consist of the electrolysis of TiO₂ and TiCl₄ in molten salt, extracting sponge titanium using a solid oxygen-permeable, in situ reduction of TiO₂ by Ca metal obtained by electrolyzing CaO in CaCl₂ molten salt, and electrolysis of the electrolytic-soluble anode TiCxOy in molten salt [4,5]. The thermochemical reduction methods can be subdivided into two groups according to the raw materials used [6]. Some methods use TiO₂ as their raw material, while others use TiCl₄ after the inclusion of a chlorination step in the process flowsheet. There is magnesiothermic reduction [7], sodium-thermal reduction of TiCl₄ [8], alumina carbothermic reduction [9], and self-propagating high-temperature synthesis (SHS). In all these thermochemical processes, carbon is used as the reductant, which produces CO₂ emissions into the atmosphere. Several researchers have used other elements as reductants [10,11]. However, limited research on the direct reduction of titanium ores has been conducted using other reductants such as hydrogen between temperatures of 800 and 1100 degrees Celsius. Hydrogen has many environmental and economic benefits. It can be produced from water, which occupies 70% of our continent, and does not produce toxic emissions.

The current research aims to review thermo-reduction processes for titanium production and investigate the possibility of using emerging alternative processes to reduce the cost and environmental impacts caused by plants producing metallurgically acceptable quality titanium and its alloys. This article outlines various types of titanium ores and their applications, as well as thermochemical processes using TiCl₄ and TiO₂ as precursors. Furthermore, an environmental impact assessment and comprehensive analysis of the thermochemical methods are conducted. Figure 1 summarizes the current processes, precursors, and reductants used for titanium production.

2. Titanium Ores

Titanium is one of the most abundant elements in the earth’s crust. It is often found in oxide mineral matrices combined with iron, with ilmenite (FeTiO₃) being the most common titanium-bearing mineral [12]. Other titanium-bearing minerals are brookite (orthorhombic TiO₂), anatase (tetragonal, metastable, octahedrite TiO₂), leucoxene (Fe₂O₃TiO₂), perovskite, and rutile (tetragonal TiO₂). Most of the titanium produced economically comes mainly from leucoxene, rutile, and ilmenite [13]. Most ilmenite is found in beach sands, and 31% of the world’s titanium production comes from it, followed by leucoxene, while rutile is the purest natural source of TiO₂, reaching up to 95% purity [14]. The distribution of titanium ores is exhibited in Figure 2.

3. Application of Titanium and Its Alloys

Titanium and its alloys consist of five groups, which are α-Ti, near α-alloys, β-alloys, α+β alloys, and Ti-based intermetallic compounds [16]. The unique properties of titanium and its alloys compel its use in various applications. Titanium and its alloys have gained popularity in additive manufacturing due to the capability of manufacturing complex parts in a cost-effective, time-saving, and resource-saving way; hence, they are used in biomedical engineering [17]. According to Pandey [18], the metal and its alloys are used to make dental implants, orthopedic implants, and cardiovascular systems, as they possess a combination of mechanical properties, corrosion resistance, a low elastic modulus, and biocompatibility.

In aerospace applications, airframes, gas turbines, and rocket engines are extensively made of titanium and its alloys [19]. The engines are made of high-temperature resistant intermetallic compounds made from TiAl. Ti and its alloys provide a better strength-to-weight ratio as compared to aluminum or steel, thereby increasing safety standards by reducing accidents and failures [20]. Ti has low thermal expansion, hence reducing the distortion or rupture of components.

In the automotive industry, titanium is used to manufacture shafts, discs, casings, and blades [21]. Busarac et al. [22] found that its strength is retained even at elevated temperatures, giving it an advantage over other metals and their alloys such as steel, aluminum, and magnesium. Using titanium to produce engine connecting rods increases fuel efficiency and reduces pollutants. The engine valves made from titanium have lower fuel consumption and longer life spans, increasing vehicle reliability. The intake valves are usually made of Ti-6Al-4V, while the exhaust valves are made of Ti-6242S. The use of a TiAl turbine rotor increases engine acceleration performance and durability due to the ability to withstand high temperatures inside turbochargers [23].

In addition to biomedical, aerospace, and automotive engineering, titanium is also used in corrosion and extreme environments such as saltwater desalination plants, heat exchangers, and electric power plants as a lightweight, robust, and corrosion-resistant material. CP-Ti (ASTM) grades 1–4 are commonly used for these purposes [24]. Figure 3 shows the titanium application by the market sector of the Western world.

4. Thermochemical Production of Titanium Using TiCl₄

The Ellingham diagram in Figure 4 exhibits that Al, Mg, Na, Li, K, and some rare earth metals can thermodynamically reduce TiCl₄. Various processes using these reductants are outlined throughout this section.

4.1. The Kroll Process

Currently, the Kroll process is the method used to produce titanium from its ores commercially. It consists of the chlorination stage, reduction stage, vacuum distillation stage, and finally the cutting/crushing stage, as depicted in Figure 5 [25]. The chlorine used is obtained from the electrolysis of MgCl₂ recycled from the magnesiothermic reduction stage. However, the electrolysis of MgCl₂ requires extensive equipment, which is very expensive and consumes enormous amounts of energy. To overcome this challenge, ref. [26] suggested a novel method to pyrolyze MgCl₂ using oxygen to produce chlorine and MgO. The MgO is then smelted to produce Mg for TiCl₄ reduction. The Kroll process is illustrated in Figure 3, and the chemical reactions that take place are presented by Equations (1)–(3). The carbochlorination represented by Equation (1) is conducted in a fluidized bed at 1000 degrees Celsius. Equation (2) represents the reduction stage, whereby the TiCl₄ is reduced by molten Mg at 800 degrees Celsius in a highly exothermic reaction in the absence of air for 36–50 h. To remove impurities from the sponge titanium, excessive magnesium is added to the magnesiothermic reduction stage, which causes low utilization of magnesium [9]. The vacuum distillation stage removes all impurities formed as byproducts during the magnesiothermic reduction stage. Impurities such as MgCl₂, if not removed, will interfere with the contact between molten Mg and TiCl₄. The MgCl₂ is dissociated by electrolysis according to Equation (3), and the products are recycled into the Kroll process.

TiO₂ + 2Cl₂ +2C = TiCl₄ + 2CO

(1)

TiCl₄ + 2Mg = Ti + 2MgCl₂

(2)

MgCl₂ = Mg + Cl₂

(3)

Though the Kroll process has been commercially acceptable, it is a complex process consisting of multiphase physiochemical processes such as adsorption, evaporation, dissolution, diffusion, and crystallization [27]. Moreover, the excess Mg added during the reduction stage leads to low-efficiency utilization, culminating in an increase in low-valent titanium compounds that prolong the production cycle and hence increase energy consumption and compromise quality. The process also incurs high labor costs and expensive equipment for purification.

4.2. The Hunter Process

The hunter process follows a similar procedure as the Kroll process. The titanium ore is first chlorinated at elevated temperatures of over 800 degrees Celsius to change the TiO₂ to TiCl₄ solution, which undergoes purification to remove impurities from the ores. However, the Hunter process uses liquid sodium to reduce TiCl₄ for titanium production [28]. The processes can be summarized by the chemical reactions represented by Equations (4) and (5) using the one-step method. Ti crystals form at the surface and settle at the bottom of the bath. According to Reddy et al. [29], the purity of the titanium produced via the Hunter process is very high, reaching up to 99%, making it suitable for applications in the electronic industry. Depending on the parameters, the titanium formed may be a sponge or powder containing some NaCl salts entrapped in its pores [30]. After the reduction stage, the mixture of Ti and NaCl is chipped, crushed, leached, and washed. NaCl is electrolyzed and recycled back into the Hunter Process, while the Ti is vacuum dried [31]. The Hunter process produces a purer product compared to the Kroll Process. The Hunter process has been implemented commercially by the Deeside Titanium plant in the United Kingdom. However, the Hunter process presented some problems, which included the high cost of sodium, the limited solubility of sodium in molten sodium chloride, larger retort requirements, higher energy consumption, and the production of higher salt volumes during the reduction stage as compared to the Kroll process [32]. Moreover, it is impractical to drain all the NaCl produced during the reduction stage because of the solubility of titanium subchlorides in NaCl.

TiO₂ + 2Cl₂ + 2C = TiCl₄ + 2CO

(4)

TiCl₄ + 4Na = 4NaCl + Ti

(5)

4.3. The Armstrong Process

The Armstrong process is an upgrade to the Hunter process, making it continuous by employing a flowing stream of molten sodium-reducing gaseous TiCl₄ injected at sonic speed, which carries the produced Ti metal and by-products from the reactor [33]. The solution from the reactor is filtered to remove the unreacted Na and distilled to remove any residual Na as shown in Figure 6. Haapala et al. [34] found that the Ti powder obtained after washing can be of commercially pure (CP) 2 grade and utilizes 50% less energy than the Kroll Process. The Armstrong process occurs at relatively lower temperatures around 660–850 degrees Celsius and costs less than the Kroll process. Moreover, several metallic chlorides can be injected into the reactor, hence producing alloying powders. This process has been implemented in a commercial pilot plant, and further process design is still being gathered to scale it up to an industrial commercial plant [31]. Nevertheless, according to [35], Cristal Metals constructed a titanium powder production plant with a 2000-ton capacity in Ottawa, Illinois, USA, implementing the Armstrong process.

4.4. The TiRO Process

The TiRO Process is a continuous process developed by CSIRO Australia wielding a fluidized bed reactor to directly produce Ti powder as revealed in Figure 7. Mg powder reacts with TiCl₄ vapor at 690 degrees Celsius producing titanium powder and MgCl₂ [36]. Argon gas is used as the sweep gas to inject Mg and TiCl₄ into the bed reactor and continuously remove the products. Early trials have shown difficulties in magnesium chloride separation. The Ti powder is separated from the MgCl₂ by vacuum distillation [3]. Similarly to the Kroll process, the MgCl₂ is electrolyzed and recycled in the process. The TiRO process produces CP 2 Ti as well as Ti alloys by introducing the alloying elements as precursors in the fluidized bed reactor.

4.5. The CSIR-Ti Process

The CSIR-Ti is a multistage continuous process that uses a series of electrically isolated reactors and a molten salt such as NaCl or MgCl₂ as the reductant developed in South Africa [37]. To date, a pilot plant with a production capacity of 2 kg/h has been set up at CSIR Pretoria Campus. TiCl₄ is pre-reduced by a MgCl₂/NaCl powder to produce TiCl₂ bearing solution, which is transferred to a second reactor and further reduced by Mg in molten salt to form dispersed titanium powder suspended in the molten salt. The two-step reactions are indicated by Equations (6) and (7). HCl leaching separates the titanium powder from the molten salt, producing CP grade 4 Ti [38]. The salt will be electrolyzed and recycled in the process.

TiCl₄ + Mg = TiCl₂ + MgCl₂

(6)

TiCl₂ + Mg = Ti + 2MgCl₂

(7)

4.6. The ADMA Process

The ADMA process is a semicontinuous magnesiothermic reduction of TiCl₄ in a hydrogen atmosphere at 850 degrees Celsius to produce hydrogenated titanium powder [39]. A mixture of hydrogen gas and Mg is used as the reduction agent, thereby reducing the reduction time and increasing Mg utilization. After the reduction is complete, hydrogen gas preheated at 1000 degrees Celsius is supplied to the reactor [31]. Hydrogen decreases the temperature and the time to remove MgCl₂ during the vacuum distillation stage, therefore making the process faster and cheaper. The titanium is hydrogenated by supplying cold hydrogen gas to form porous TiH₂ powder, which is collected and crushed to a determined size. The method is cheaper than the Kroll process, but the dehydrogenation step adds an extra cost. According to a report by [40], the ADMA process was a success that needs only to be upgraded to a commercial level.

4.7. The ARC Process

The continuous process was developed by the Albany Research Centre (ARC) to produce titanium powder in a bath of molten salt in a two-stage reduction method illustrated by Equations (8) and (9) using Na metal [3]. The reactants are diluted in a molten chloride salt, thereby inhibiting the interlocking of dendritic sponge in a stirred tank reactor. The titanium grows on small titanium particles until they are large enough to settle at the bottom of the reactor for removal. Regardless of the promising initial experiments, a review by Jena et al. [31] denotes that the ARC process could not reach the required purity and uniform particle size for commercialization.

TiCl₄ + 2Na = TiCl₂ + 2NaCl

(8)

TiCl₂ + 2Na = Ti + 2NaCl

(9)

4.8. The SRI (Stanford Research Institute) Process

The SRI process involves the reduction of metal chlorides such as TiCl₄ with hydrogen to produce powder of Ti and its alloys in a high-temperature fluidized bed reactor in a single step [41]. The product forms on a particulate substrate of the same material produced by crossing 1% of the product to a similar size and feeding it back to the reactor. The particulate titanium produces Ti powder. However, the process is still in the laboratory stage, and further research on the microstructure, impurity control, product recycling, and rate of growth is being conducted [42].

4.9. The ITT (Idaho Ti Technologies) Process

The ITT process produced very fine titanium hydride by thermal dissociation of TiCl₄ in an electric arc vacuum chamber over 3727 degrees Celsius forming plasma [43]. TiCl₄ dissociates into titanium and chlorine atoms according to Equation (10). Injected hydrogen reacts with condensed titanium particles and chlorine to form TiH₂ and HCL, respectively, as indicated by Equations (11) and (12). The combined effect of rapid cooling, the formation of HCL, and the reducing effect of hydrogen prevent the back reaction of titanium and chlorine [42].

TiCl₄ = Ti + Cl₂

(10)

Ti + H₂ = TiH₂

(11)

Cl₂ + H₂ = 2HCl

(12)

4.10. The JTS (Japan Titanium Society) Process

Toho Titanium Co., Ltd. and Sumitomo Titanium Corp developed the continuous JTS process, which involves the use of Ca dissolved in molten CaCl₂ as shown in Figure 8 [44]. Ti powder and some salts are produced and separated. A volume reaction occurs that has a greater reaction rate compared to the surface reaction in the Kroll process. A titanium ingot is then produced by plasma arc remelting of the titanium–salt mixture at a temperature below the boiling point of CaCl_2,, which is also recovered in the liquid state. However, Takeda et al. [35] found that this process requires an efficient cooling system for the reactor since so much heat is generated. The subprocesses should be thoroughly investigated to actualize an efficient process. Among these subprocesses is Ca electrolysis, which is yet to be clarified since the process has low current efficiency and hardly produces high-purity calcium, which should be available in sufficient amounts. Additionally, the behavior of Ca metal fog, which forms around the cathode during Ca electrolysis, should be researched in detail. To overcome these hurdles, Villechaise et al. [45] reported that industry–academic–government collaboration has been implemented to improve the process.

4.11. Vapor Phase Reduction Process

Vapor phase reduction involves the reduction of TiCl₄ with a reductant in its vapor phase. The Albany Research Centre has used magnesium vapor as the reducing agent and argon gas to carry the TiCl₄, making it a continuous process at 1000 degrees Celsius that forms a mixture of Ti, MgCl₂, and Mg, which is trapped by an electronic separator to separate it to obtain titanium powder [37,46]. However, the Ti powder produced during this process is of minute size, causing difficulties during the separation of Ti from the Ti, MgCl₂, and Mg mixture. Moreover, the oxygen content was too high, at around 2–3 wt%, due to the large specific surface area. The VARTECH process is another vapor phase reduction process. The VARTECH process proposes the direct manufacturing of titanium powder and its alloys at a very low cost with high energy efficiency using as a vapor phase reagent AlCl₃ or hydrogen in a specialized vapor phase reactor under atmospheric conditions, hence lowering the processing costs [47,48]. It can produce intermetallic TiAl, Ti₃Al, TiAl₃, and other alloys by reacting with TiCl₄ vapor. However, not much information about this process has been released, as it is still being developed under the Missile Defence Agency (MDA) SBIR contract.

4.12. Aluminothermic Reduction

Aluminothermic reduction of TiCl₄ has been implemented by CSIRO to produce various alloys such as Ti-6Al-4V, Ti-47Al-2.3Cr-2.3Nb, and gamma Ti-Al by adding corresponding chlorides of alloying elements [37]. Nevertheless, the process poses some challenges that include uncontrollable product phases, a low yield, and incomplete reaction at high temperatures; hence, it has been made into a two-step process. The first process occurs at 200 degrees Celsius and is presented by Equation (13) using AlCl₃ as a catalyst to form TiCl₃-Al-AlCl₃ [49]. The titanium subchlorides are heated to 1000 degrees Celsius in the second step to produce Ti-Al alloys as shown in Equation (14). The AlCl₃ can be dissociated by electrolysis to Al and Cl₂, which can be reacted by TiO₂ to generate TiCl₄.

$${\mathrm{TiCl}}_4+{\displaystyle \frac{1}{3}}\mathrm{Al}=\mathrm{Ti}{\mathrm{Cl}}_3+\mathrm{Al}{\mathrm{Cl}}_3$$

(13)

TiCl₃ + (x + 1) Al = Ti-Al_x + AlCl₃

(14)

5. Thermochemical Production of Titanium Using TiO₂

According to the Ellingham diagram in Figure 9 for the formation of oxides, only Mg, Al, Ce, La, Ca, Li, and Y can reduce TiO₂.

5.1. Combustion Synthesis (Magnesiothermic) Reduction

TiO₂ can be directly reduced by combustion using Mg as the reductant under pressure in an argon atmosphere. The first attempt at using this method was conducted by Frolov and Fetzov [50], during the 1980s. Nersisyan et al. [51] managed to produce Ti powder with 1.5 wt% oxygen after leaching the product combusted under pressure using Mg and Ca(OH)₂ in an argon atmosphere. The Ti powder was further deoxidized using Ca granules at 850–900 degrees Celsius for 2 h to decrease the oxygen content to 0.2–0.3 wt%.

5.2. Self-Propagating High-Temperature Synthesis (SHS)

The SHS method/combustion synthesis has two reaction modes, which are thermal explosion and wave propagation. External energy is used to induce local chemical reactions that form a front of chemical reactions that release heat causing combustion in the whole reaction system, hence synthesizing the required products [3]. Thermite SHS is where aluminum is used to reduce iron oxide in violent explosions and excessive heat generation reactions to produce liquid iron and liquid alumina. This method has been applied to the extractive metallurgy of titanium and its alloys. Titanium ore, which contains metal oxides other than those of titanium, aluminum, and flux, is blended, briquetted, and charged into the reaction furnace. According to Merzhanov [52], the reaction can only be spontaneous at an adiabatic temperature of 1800 K, and above this, additional heat has to be supplied to maintain the chemical process. However, it is quite a challenge to maintain the fluidity of the melt at these high temperatures due to the reactiveness of titanium with O, N, and C. These temperatures have also resulted in interfacial reaction melts and mold-promoting inclusions, which affect the mechanical properties of cast titanium alloys, hence hindering the application of the method. Fan et al. [53] also implemented SHS in a Mg-TiO₂ system in a multi-stage reduction to obtain a non-stochiometric low valent titanium oxide intermediate with an oxygen content of 13.93 wt%.

5.3. Hydrogen-Assisted Magnesium Reduction (HAMR) Process

High-purity titanium-containing oxygen of less than 0.15% wt can be produced by the HAMR process invented by Xia et al. [54]. Purified TiO₂ is combined with Mg as the reductant in a hydrogen atmosphere at temperatures between 600 and 800 degrees Celsius to produce TiH₂ particles that are heat treated to eliminate porosity and hydrogen to attain titanium particles with a high oxygen content. Final deoxidation of the titanium particles produces titanium powder that conforms to ASTTM-B299-13 [3]. The porosity and morphology of the titanium powder is affected by the relative density of the TiO₂ fed into the system, of which porous TiO₂ has proven to be advantageous. The hydrogen destabilizes the Ti-O system, increasing the thermodynamic force between the Mg and O₂ reaction. The process is cheaper compared to the Kroll process due to the elimination of the chlorination and the vacuum distillation stages.

5.4. Metal Hydride Reduction (MHR) Process

The MHR process involves the use of calcium hydride to directly reduce TiO₂. The reduction occurs at temperatures around 800–1100 degrees Celsius but can also be performed at low temperatures producing TiH₂ [15]. Likewise, magnesium hydride can be used instead according to Equation (15) but requires a higher temperature than calcium hydroxide to dehydrogenate. Mg also has a relatively lower melting point and higher vapor pressure than Ca [31]. The TiH₂ produced can be dehydrogenated to produce the titanium metal.

TiO₂ + 2MgH₂ = TiH₂ + 2MgO + H₂

(15)

5.5. Electronically Mediated Reduction (EMR) Process

EMR uses calcium as a reducing agent without direct contact between the TiO₂ and Ca-Ni alloy. This method is advantageous since no titanium contamination takes place, and hence, there is no need for a titanium purifying stage [29]. However, the process is not wholly thermochemical as it takes place in an electrolytic cell divided into two parts. The first part of the electrolytic cell melts CaCl₂ to generate Ca, which is dissolved in Ni to form a Ca-Ni alloy. The second part of the electrolytic cell connects TiO₂ to the Ca-Ni alloy to produce metallic Ti [55].

5.6. Calciothermic Reduction

Calciothermic reduction takes place in molten calcium chloride to produce titanium powder in the presence of calcium metal floating above the molten salt at about 900 degrees Celsius [56]. The calcium metal that partially dissolves into the molten salt reacts with the TiO₂ below the molten salt, thereby reducing the thermodynamic activity of the byproduct and hence reducing the oxygen concentration. Spongy titanium powder is obtained with residual calcium content. The dissolved Ca is obtained from the salt by electrolysis. During the process, the reduction rate and final oxygen concentration are dependent on the particle sizes of the feed Celsius [31].

5.7. Preform Reduction Process (PRP)

The PRP was proposed by Okabe et al. [57]; Okabe [58], whereby a preform of TiO₂ is made by mixing it with a flux of CaO/CaCl₂ and a binder that is calcined before reduction to remove the binder as shown by Equation (16). The preform is sintered at 1073 K. Ca metal is put under preform without direct contact, inhibiting contamination of the final powder at 800–1000 degrees Celsius for 6 h in a Ca vapor environment as shown in Figure 10. The product is acid leached to separate the titanium metal from the impurities, resulting in a final powder having 0.28–0.66 wt% oxygen. The process produces homogenous fine powder when the flux composition and preform size are controlled. PRP has good anti-contamination ability due to the small contact area with the reactor material and a self-supporting structure preform [35].

TiO₂ + Ca → Ti + 2CaO

(16)

5.8. Aluminothermic Reduction

Aluminum has also been used as a reductant for titanium oxide. Chaikin et al. [59] performed a reduction process under pressure by charging, rutile, aluminum shavings, and quartz in a shaft furnace at 1000–1100 degrees Celsius. The titanium-alloy produced had to be separated from corundum by alkaline cleaning. The authors also found that the size of the feed material affects the reactivity of the materials, with the reduction using aluminum shavings producing better results than using aluminum powder as a reducing agent. Kirev et al. [60] were able to produce a titanium–aluminum alloy with a particle size in the range of 17–69 microns by aluminothermic reduction. However, further ultrasonic cleaning in a solution of glycerin and alcohol to separate the corundum from the titanium spheres had to be conducted. Aluminothermic reduction of TiO₂ can only be used to produce Ti-Al alloys but never Ti metal with a low oxygen content [61]. This is attributed to the high oxygen reactivity in the slag and the formation of stable intermetallic alloys.

6. Environmental Effects of Thermochemical Production of Titanium

The current review has focused on the sustainable production of titanium by thermochemical methods that require low energy consumption, improved recyclability, reduced waste generation, and efficient resource utilization. Titanium production has severe environmental impacts that include climate change (GWP), photochemical oxidant formation (POFP), human toxicity (HTP), particulate matter formation (PMFP), eutrophication (MEP), terrestrial acidification (AP), metal depletion (MDP), and fossil depletion (FDP) [62].

The Kroll process, Hunter process, Armstrong process Arc, CSIR-Ti, and other processes rely on TiCl₄ as their precursor. This requires a mandatory carbochlorination stage of rutile or upgraded slag to produce the precursor. Carbon is used as a reactant during the carbochlorination stage, resulting in the production of greenhouse gases such as carbon monoxide and carbon dioxide, which is detrimental to the environment [63]. Furthermore, during the reduction stage, some of the chloride gases escape as acidic gases, which can lead to acidic rainfall that is detrimental to crops and livestock. The chlorine gases also pose health hazards to the human body such as nausea, violent cough, headache, and chest pains. The chlorine used causes impurification in the final powder production, increasing the amount of energy used during the purification stage [64]. According to a sensitive analysis conducted by Gao et al. [63], the chlorination stage contributes most to photochemical oxidant formation and fossil depletion, accounting for 35.8% and 37.9%, respectively. Fossil depletion is due to the use of coke in the process.

Thermodynamically, the energy required to reduce a kg of titanium is low at 16.92 MJ/kg-Ti. However, the recycling of byproducts and the energy required to run the equipment contribute a significant portion to the energy consumption of the process, increasing it to 55–360 MJ/kg-Ti depending on the equipment, process, and reducing agents used. In total, 66% of the energy used in the Kroll process is electrical energy [65]. During power generation, there is the discharge of toxic substances and heavy metals, which causes human toxicity, especially in areas that use thermal electricity. A study by Gao et al. [63] indicated that the electrolysis of magnesium chloride constitutes 39.6% of the energy used in the Kroll process. The study also showed that electrolysis had the most environmental impact, as it had excessive power consumption and significant indirect emissions, climate change (GWP), human toxicity (HTP), terrestrial acidification (AP), and eutrophication (MEP).

7. Comprehensive Analysis of the Thermochemical Reduction of Titanium

When the precursor is TiCl₄, all the other processes using the same precursor are an improvement to the Kroll or Hunter process except the JTS, ITT, and SRI. The reviewed methods primarily aim to improve continuity and efficiency in the Kroll and Hunter processes by introducing innovations such as low-temperature operations or easy separation and transportation of products. The Armstrong process introduces a molten sodium that provides efficient removal of products, hence making the process fast. The CSIR-Ti makes the Hunter process continuous, as argon is used as a sweeping gas continuously removing products. The TiRO process uses the fluidized bed reactor, which expands the surface contact area and hence increases the rate of reaction. The ARC process is a multistage process of the Hunter process. The ARC process first reduces the TiCl₄ to low valent TiCl₃, which is then reduced to the titanium metal. The ADMA process includes the partial replacement of the reductant Mg with H. This increases Mg efficiency during the reduction stage. Furthermore, Mg contamination in the final product is reduced due to unreacted Mg. The distillation stage also takes less time and costs less since the amount of impurities to be removed is reduced. To overcome this barrier, with TiCl₄ as the precursor, the ITT method has been proposed, which turns hydrogen into plasma at very high temperatures and atomizes TiCl₄ into its atoms. To date, no practical process using hydrogen as a reductant has been successful. CSIR even abandoned research using H₂ as a reductant due to economic reasons.

Reduction processes that use TiO₂ as the precursor aim to produce Ti metal from raw materials without including the chlorination and distillation stages. However, the processes may result in additional stages such as deoxidation or dehydration stages. Furthermore, using TiO₂ as a precursor may be a difficult route since it is too stable to be reduced by either carbon or hydrogen and is resistant to chemicals. Thermodynamically, the metals Mg and Ca are capable of the direct reduction of TiO₂. However, magnesium cannot reduce the amount of oxygen to industrially acceptable levels, while calcium produces high heat, making it difficult to attain a homogenous reaction and at the same time contaminating the titanium metal. These reductants are preferred to be used in other states (molten or vapor) or in conjunction with another reductant such as hydrogen. Due to these problems, methods such as the PRP method emerged, which uses Ca vapor to produce titanium metal with 99% purity surrounded by CaO particles [58]. From Figure 1, it is evident that more research is focused on the use of Ca when TiO₂ is the precursor owing to the very strong reducing capability of the metal. The HAMR introduces an intermediate alloy by using hydrogen, which destabilizes the Ti-O bond during the magnesium thermal reduction of TiO₂. The development of reduction processes of titanium oxide using calcium metal as a reductant has advanced significantly. Thermodynamically, it is impossible to reduce TiO₂ into Ti metal using molecular hydrogen. However, using hydrogen as the reductant results in different intermediate compounds; hence, the research using hydrogen due to this thermodynamic barrier is limited. A comparison of the various processes is presented in

Table 1. Comparison of titanium production processes.

Process	Cost	Scalability	Environmental Impact	Product Purity	Advantages	Challenges	References
Hunter process	High	High	High (chemical waste)	High	Produces ultra-high-purity titanium	High cost of Na Batch process High energy consumption	[3,66]
Kroll process	High	High	High (Chlorine waste)	High	Lower cost of Mg than Na Industry standard, widely used	Batch process Energy-intensive, large carbon footprint	[3,66]
ARC	Medium	High	Moderate	High	Continuous production Controllable reaction speed Effective for melting and refining titanium alloys; scalable	High energy consumption; needs inert atmosphere to avoid contamination	[1]
Vapor-phase reduction process	High	Moderate	High (chemical waste)	Very high	Continuous production Produces high-purity titanium; versatile applications	Titanium powder has high oxygen, magnesium, or chlorine content; expensive infrastructure and high energy use	[3]
TiRo	Medium	High	Moderate	High	Efficient powder production, lower cost than Kroll	Limited adoption, oxygen contamination risks	[36]
EMR	Medium	Moderate	Low	Very high	Continuous production Energy efficient, scalable for certain applications	Onerous separation of metal and salt Requires extensive R&D for optimization	[3]
MHR	Medium-High	Moderate	Moderate	High	Single-step process Potential for lower cost titanium	Complex reactor design High energy consumption and pollution	[31,67]
SHS	Low	Low	Moderate	Moderate	Simple setup, low cost High efficiency	Uncontrollable process Limited scalability and product uniformity	[68]
HAMR	Low	Moderate	Low	High	Low-cost reduction process	Limited data on large-scale use	[31,69]
ADMA	Medium	High	Moderate	High	Uses novel techniques for cost-effective production	Relatively new, high setup cost	[70]
CSIR-Ti	Medium	Moderate	Moderate	High	Alternative to Kroll, promising for industrial use Continuous production	Oxygen content difficult to control Requires development for widespread adoption	[1,3]
JTS	High	Low	High	Very High	Ultra-high purity titanium for niche markets	Extremely high cost Harsh separation conditions	[71]
SRI	Medium	Moderate	Moderate	High	Lower energy process	Large gas recycling loop Requires process optimization for wider use	[71]
PRP	Low	Moderate	Low	Moderate	Simple, cost-efficient High reduction efficiency	Limited to small-scale production	[57]
Aluminothermic	Low	Moderate	Moderate	High	Cost-efficient for certain applications	Residual aluminum impurity risks	[37,72]
Calciothermic reduction	Medium	Moderate	Low	High	Low production cost High product purity	Handling calcium metal safely is challenging	[15,73]
Armstrong	Medium-High	Moderate	Low	High	Continuous production of titanium powder with uniform size Controllable reaction speed	Requires proprietary equipment, scalability challenge Expensive reductant and residual impurities	[3,33]
ITT	Medium	High	Moderate	High	Integrated production with lower cost and improved scalability	Harsh plasma process Requires significant technological integration for implementation	[74]

.

8. Recommendations and Future Outlooks

Various thermochemical reduction processes discussed in this review paper are still under development; therefore, these processes present some research gaps within the titanium production industry. According to Okabe [58], the currently utilized Kroll process is excellent in removing impurities such as iron and oxygen during the carbochlorination stage. Nevertheless, the Kroll process has a low reduction rate and high energy consumption; hence, more research needs to be conducted to accelerate the reduction rate and lower the energy consumption. Furthermore, all the processes utilizing TiCl₄ as the precursor should be investigated by employing alternative sources of carbon reductants during the carbochlorination stage such as biochar and charcoal as they are renewable resources that would enhance sustainability. The ADMA process produces titanium hydride requiring further processing to attain metal titanium; thus, scrutinization of the dehydrogenation techniques needs to be conducted to make the process economically feasible. The vapor phase reduction process manufactures titanium with 2%–3% weight oxygen, which is above the commercial grade with difficult separable particles [37]. Further studies on the process may decrease the oxygen content of the produced titanium and improve separability. Additionally, the ARC process still needs more probing to produce pure titanium of uniform particle size that can be commercialized. The Armstrong process managed to produce titanium metal using a flowing stream of liquid sodium and gaseous titanium chloride with a high surface area to volume ratio and low bulk density [33]. Still, more studies to increase its overall bulk density are recommended. Additionally, with the success of the Armstrong process at a pilot scale, Jena et al. [31] mentioned that further process design to reach commercial scale is required. The CSIRO two-step aluminothermic reduction of TiCl₄ generated Ti-Al metal with the amount of Al in the alloy dependent on the Al/TiCl₄ ratio in the charge material. Sun et al. [37] suggested that the process needs modification to become a continuous process. Furthermore, the aluminothermic reduction of TiO₂ produced Ti-Al with an oxygen content of <0.2 wt % after a slag remelting step; a high alumina slag can be recommended to be used as a feed for aluminum production to reduce the amount of waste produced. The hydrogen-assisted reduction of TiCl₄ occurs at very high temperatures, presenting a challenge wherein the chlorine and titanium react back to form TiCl₄ during cooling. Further study on how to collect solid titanium before the back reaction in the hydrogen-assisted reduction of TiCl₄ would be a great milestone when using this process. To date, the process has managed to produce titanium hydride and hydrochloric acid. According to (Sun et al. [37], only the partial reduction of TiO₂ using hydrogen at temperatures offered by plasma to titanium suboxides has been achieved, although the information is unavailable in the open literature. Further experimentation may make it possible to fully reduce the TiO₂ to titanium metal. Some of the processes including TiRo, CSIR-Ti, Armstrong, and PRP discussed have reached pilot scale and produced satisfactory results. Only further testing is proposed to acquire information that can be used for scaling up to a commercial level.

9. Conclusions

This review shows the vast techniques for the thermoreduction extraction methods of titanium that are at various stages of development, aiming to achieve a low cost and environmentally friendly production method. The Kroll process is the only commercially viable titanium extraction method to date despite its extensive energy use and environmental impacts. Research has been conducted over several decades with processes such as the Armstrong and CSIR-Ti reaching a pilot scale. Further research needs to be performed to develop these processes to a commercial level. Overall, this review offers valuable insights into the state-of-the-art thermoreduction processes for titanium production and analyzes the advances that have been made so far regarding the thermoreduction processes. Conclusively, it may take a few more decades before any new process is commercialized and competes with the Kroll process.