5.2.2. Ono and Suzuki Process

The (OS) process invented by Ono and Suzuki et al. [54,82,83] is based on a reduction of TiO<sup>2</sup> by Ca. Ca is generated by electrolysis of CaO in molten salt CaCl2. A layer of calcium oxide that inhibits any further reaction was generated by the simple reaction of calcium with titanium dioxide generated (Figure 9); however, the ability of calcium chloride to dissolve significant amounts of calcium oxide may be effective in overcoming this problem [49].

**Figure 9.** Diagram of the cell used in the OS process [83] (Reprinted with permission from Elsevier, Copyright, (2015)).

5.2.3. Quebec Iron and Titane (QIT) Process

The QIT process, patented by François Cardarelli in 2009 [84], is a process for the electrolytic extraction of Ti metal from compounds containing TiO<sup>2</sup> in a liquid state. Specifically, this process uses a molten compound containing TiO<sup>2</sup> as the cathode on the bottom and a consumable carbon as the anode. A layer of electrolyte, such as molten salts (e.g., CaF2) or a solid-state ion conductor is used as the O−<sup>2</sup> ion carrier in the middle. The temperature ranges between 1700 ◦C and 1900 ◦C in this process [76].

There have been several reports on the development of continuous processes based on the same chemistry as the above processes. A new process was presented, which integrates carbochlorination and electrolyzation to elaborate metallic titanium in molten NaCl-CaCl<sup>2</sup> electrolyte [82]. This experiment was made at 577 ◦C using some specific precursors like carbon-doped TiO2. Likewise, a pre-treatment is performed to partially reduce TiO<sup>2</sup> to titanium sub-oxides with lower valence. The compacted pre-treated mass is electrolyzed at a temperature of 1000 ◦C in a molten CaCl<sup>2</sup> bath for 1–5 h [85].

An investigation of the CaCl<sup>2</sup> effect on the calcium vapor reduction process of Ti2O<sup>3</sup> has been performed [86]. The compound CaCl<sup>2</sup> plays a crucial role in the calciothermic reduction process of Ti2O<sup>3</sup> to prepare porous titanium [86]. Likewise, the generation of a low oxygen Ti powder was carried out by electrolytic reduction of CaTiO<sup>3</sup> in a CaCl2-CaO melt. By using different sizes of raw oxide particles, the concentration of residual oxygen in the cathode imposes a reduction mechanism using calcium from the cast iron [87]. Another method was introduced for the preparation of titanium metal by reduction of TiCl<sup>4</sup> in NaCl-KCl-NaF eutectic melts [88]. A new method for manufacturing porous titanium by in situ calcium vapor reduction of titanium sesquioxide was presented by Yang et al. [89]. By calcium vapor, Ti2O<sup>3</sup> and CaCl<sup>2</sup> powders have been reduced. The product was leached with hydrochloric acid and deionized water and the porous structure of Ti was obtained [89]. Furthermore, LiCl-KCl molten salt was used as an electrolyte due to its relatively low melting point to produce metallic titanium. The oxygen content in the titanium crystal is 1200 ppm [90].

Commercial TiO<sup>2</sup> has been reduced by Mg in a hydrogen atmosphere [91]. A mandatory deoxygenation process was added to ensure that the purity met standard specifications for titanium. The magnesium reduction and deoxygenation combination process is a holistic approach that produces Ti powders meeting ASTM specifications [91].

Otherwise, for patents published in this sense, Zhu et al. [92] invented a method for the electrowinning of titanium metal from a soluble anode molten salt containing titanium and concerns the technical field of non-ferrous metal metallurgy. In 2009, François Cardarelli [84] also invented a process for the electrowinning of Ti metal from compounds containing a liquid TiO<sup>2</sup> (Table 7). This process uses a molten compound containing TiO<sup>2</sup> as the cathode in the bottom. The anode can be a consumable carbon, a stable inert anode, or a gas diffusion anode powered by a combustible gas (e.g., H2, CO, etc.). The temperature reaches 1700 ◦C to 1900 ◦C for this process [76].

As shown above, there are many methods and newly developed technologies to produce titanium powder. Kroll process the most utilized one can produce titanium with less oxygen content and metallic impurities, but its productivity is still very low, magnesium as a reductant has a high cost compared to other metals such as calcium and aluminum, and it is a labor-intensive batch process. Furthermore, the hunter process produces titanium powder purer than Kroll's powder but uses an expensive sodium reductant and heterogeneous exothermic reactions. For other processes, they present advantages at the production level but they are still at the laboratory scale and require more time to be developed in the industry. It is possible to develop a method to directly reduce oxides to efficiently produce Ti with low oxygen amount in the next few years. In principle, the production of high purity metallic Ti by direct reduction of Ti precursors is possible; hence, a method that relies on the melting of these oxides and is based on the principles of the Kroll process should be developed as a new approach to establish a low-cost Ti reduction process.

On the other hand, new technologies are developed as an alternative to the Kroll process. The electrochemical road is the best choice for many industries. The FCC Cambridge provides a product with almost 0.3 mass% oxygen, and it is a semi-continuous operation. However, this method exhibits many drawbacks: low current efficiency (20–40%), slow oxygen diffusion, difficult separation of metal/salt, and is costly for the TiO<sup>2</sup> pellet feed. On the other hand, the OS process can produce titanium directly from TiO<sup>2</sup> reduction in molten salt, but it still has some problem contamination because of the presence of carbon. Furthermore, many electrochemical procedures have been performed recently. They dictate a good enhancement in the cost and production issues, but they are difficult to scale up for the difficulty in facilitating a homogeneous reaction.



**Table 7.** Advantages and disadvantages of some patents in titanium production based on electrochemical technics.

#### **6. Conclusions**

Titanium has remained an essential metal because of its wide use in different fields. Its demand in the industry has prompted unprecedented technical progress. Natural ilmenite is the most abundant titanium-bearing mineral in the earth's crust. The mineral ilmenite has been significantly grown since his discovery. Nowadays, it is the most crucial ore of titanium. The presented review shows the presence of enormous techniques for manufacturing titanium powder and titanium metal. Many of these processes are at various stages of development. The incentive for the development of new processes is often firmly rooted in the ambition to achieve a low-cost alternative to the Kroll process for the production of primary Ti metal. Marketed chloride-based thermochemical processes, such as the Kroll and Hunter processes, are batch operations and require high-quality natural rutile or improved synthetic slag, such as feeding and using cost-sensitive chlorination and thermochlorination steps. Many improvements have been made to thermochemical processes, but they offer few opportunities for cost reduction beyond current technology. Several development methods have generated considerable interest and scale-up efforts, including the Armstrong process and the FFC Cambridge process. The former is an example of using a continuous process for the reduction of TiCl<sup>4</sup> with Na or Mg while the latter is an example of the various electrochemical methods that convert TiO<sup>2</sup> into Ti metal in molten salt. A recently developed method, named the HAMR process, is based on destabilizing the Ti-O system by using hydrogen as a temporary alloying element during the magnesium thermal reduction of TiO2. However, compared with the Kroll, these producers are expanding their production to meet the unprecedented demand for titanium. Overall, it is expected that it will take several years before any new process will be in commercial production and compete with the Kroll process.

**Author Contributions:** Conceptualization, M.E.K. and G.S.; formal analysis, M.E.K.; funding acquisition, G.S.; investigation, M.E.K.; methodology, M.E.K., G.S. and O.D.; resources, G.S.; supervision, G.S.; validation, M.E.K.; writing—original draft preparation, M.E.K.; writing—review and editing, G.S. and O.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by NSERC (Natural Sciences and Engineering Research Council of Canada) discovery grant (RGPIN-2018-06128) and Metchib company-Quebec, Canada.

**Acknowledgments:** Metchib Company supported this research. We thank Metchib Company in Canada for providing rock samples and supporting us.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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