Titanium Extraction Metallurgy Developments and Control of Impurity Elements

Abstract

Titanium extraction metallurgy poses numerous challenges owing to a combination of various characteristics such as high chemical reactivity, high melting point, strong affinity towards oxygen and nitrogen, pyrophoricity of nascent sponge. Kroll process of magnesium reduction of has become the widely employed titanium sponge production technology. Sodium reduction of TiCl4 known as Hunter’s process has also been employed for the industrial production of titanium sponges for about two decades. Subsequently quoting techno-economic reasons, the Hunter sponge plants across the world have been closed. There have been several efforts over the years to evolve an alternative to the Kroll process mainly to achieve a simple and cost and energy-effective titanium extraction process. Control of impurity elements in the titanium metal during the metal extraction process assumes greater importance as thermodynamics does not favour any purification method to be employed for the metal. This chapter brings out historical developments in titanium extraction metallurgy and highlights recent developments as well to produce high-purity titanium sponges required for titanium alloys for the end applications across various sectors.

1. Introduction

Titanium is an element with atomic number 22 and placed in the Group IV B of the periodic table. Discovered by an English chemist, William Gregor in 1791, assumed its name ‘titanium’ when a German chemist, M.H. Klaproth found it in 1795 as a new element in the mineral rutile and called it ‘titanium’ (titans is the goddess of earth in Greek). Titanium possesses a melting point of 1663°C and a density of 4.5 g/cm³. Lightweight, high specific strength coupled with excellent corrosion resistance resulted in titanium emerging as the structural material of choice for a variety of applications in aerospace and many chemical industries. The titanium also has special properties of body compatibility, non-magnetism and non-toxicity. Titanium has different oxidation states and is highly reactive and forms alloys and compounds with a large number of elements. It is because of the outer thin oxide film, the metal exhibits superior corrosion resistance to a wide range of chemicals, seawater and aggressive media. The ability to form different types of attractive/lustrous shades of colours on the surface on anodizing led to its use in artificial jewellery and various types of eye-catching consumer durables.

Titanium exhibits allotropy and changes its crystal structure from hcp to bcc on heating at 882°C. Important properties of titanium of interest for a material scientist/metallurgist include its amenability to heat treatment to achieve tailor-made properties for different end applications. A large number of titanium alloys of type alpha, alpha plus beta, beta and gamma phases are developed and put into use for various types of applications up to a temperature of 550°C.

Titanium is plentifully available in nature. In the early days, it was referred to as ‘rare metal’ mainly because of its limited usage on account of its expensive nature. Constituting about 0.63% of the earth’s crust, titanium is the ninth most abundant element and fourth most abundant structural metal (after iron, magnesium and aluminium). The two most important and widely available titanium minerals are ilmenite (FeOTiO₂) and rutile (TiO₂). Ilmenite is subjected to chemical beneficiation or thermal smelting to prepare synthetic rutile or titania slag, which contains increased TiO₂ content.

2. Emergence of titanium extraction metallurgy

Extractive metallurgy of titanium is complicated due to the high melting point, high chemical reactivity and pyrophoric nature (catches fire under frictional forces) of nascent titanium metal. Vacuum or inter-gas atmosphere is essential for producing titanium metal. Historically preparation of pure titanium metal from the oxide concentrate has been a challenging task. Early efforts to prepare the metal directly from its oxide had been futile because of the large amount of oxygen retained in the product. Oxygen, nitrogen, carbon and iron are the most important impurity elements in titanium, which adversely affect the ductility and mechanical properties of the metal. Realizing the implications of preparing the metal directly from the oxide, early efforts shifted the focus to attempt metal separation from a non-oxygen-bearing compound. Thus, chloride metallurgy came into existence where a metal chloride is prepared by high-temperature chlorination of metal oxide and subsequently the metal chloride is reduced/electrolysed to prepare the metal.

In 1910, Hunter developed sodium reduction of titanium tetrachloride (TiCl₄) to prepare high-purity titanium metal. The process was later on extensively studied and developed by Imperial Chemical Industries, UK, for industrial implementation. Some industries based on this process in UK and Japan had been operating till the early 90s. In 1925, van Arkel and de Boer developed the process of dissociation of titanium iodide (TiI₄) on a tungsten filament to produce high-purity titanium metal. In 1937, W J Kroll worked on magnesium reduction of TiCl₄ to produce high-purity metal and patented the process. The Kroll process was subsequently developed by the US Bureau of Mines for industrial-scale implementation. Simultaneously in the former USSR and Japan also similar developments took place. Fused salt electrolysis of TiCl₄ to prepare titanium was extensively studied by Dow Howmet USA, Reactive Metals Inc., UK, and Electrochemica Marco Ginatta, Italy, and pilot scale plants had been operating based on this process till recently. These three processes viz. Hunter process of sodium reduction, Kroll process of magnesium reduction and fused salt electrolysis of TiCl₄ are proven and are considered to be established methods of titanium sponge production. In all three processes, nascent metal formed is in the form of a porous aggregate of titanium metal particulates and termed as ‘titanium sponge’. Titanium sponge is the basic raw material for the manufacture of titanium/titanium alloys, which takes place through vacuum arc melting of sponge into ingot followed by conventional metal working techniques to obtain the desired product for end use. A detailed description of the processes along with historical developments in metal extraction is well documented in the literature [1, 2, 3]. Some of the experiences gained in the Kroll titanium sponge technology development program [4] successfully conducted at the Defence Metallurgical Research Laboratory, Hyderabad, India, are shared in this presentation.

Subsequently, there have been several efforts the world over to develop a simple, energy and cost-effective and environmentally friendly titanium extraction process. Most of these efforts focus to cut down the process steps, evolving a continuous process and/or directly obtaining titanium from TiO₂. The majority of these advanced processes result in titanium powder, which can be utilized in preparing the desired products through additive manufacturing or 3D printing.

3. Established methods of titanium sponge production

As discussed, the preparation of TiCl₄ is an essential step in the production of titanium sponges. TiCl₄ is mainly prepared by high-temperature carbo-chlorination of titanium mineral concentrate (rutile/beneficiated ilmenite) in a fluidized bed-type refractory lined furnace at a temperature of about 1000°C. The tetrachloride obtained on chlorination is relatively impure containing chlorides of other metals such as Fe, Sn, Si, Al, V. TiCl₄ is an aggressive chemical and readily reacts with atmospheric moisture and hydrolyzes forming thick fumes of HCl and oxy-chloride. Hence, handling and purification of TiCl₄ is hazardous and to be carried out cautiously. Before using for the production of titanium sponge, TiCl₄ is to be purified sufficiently, which is generally done by employing fractional distillation and precipitation techniques.

Thus, the established methods of titanium sponge production essentially involve the process steps (Figure 1) of chlorination of beneficiated ilmenite or rutile in the presence of carbon at a temperature of about 950C to produce TiCl₄, purification of the tetrachloride by fractional distillation to obtain high-purity TiCl₄ followed by either sodium reduction (Hunter process) or magnesium (Kroll process). Alternatively, titanium sponge can also be produced by fused salt electrolysis of TiCl₄ in LiCl-KCl melt. A brief description of these three processes is provided in the sequel before a discussion is taken up on ‘control of impurity elements’ in the product.

Figure 1.

Process flow diagram for established methods of titanium sponge production.

4. The Hunter process

In this Hunter process, TiCl₄ is reduced with sodium metal and the following chemical reaction represents the reduction process:

$${\text{TiCl}}_4+\text{4Na}=\mathrm{Ti}+\text{4NaCl},{{\text{∆H}}^{\mathrm{o}}}_{80\text{0C}}=-653.7\mathrm{kJ}/\text{mole}.$$E1

The reduction process is conducted in an inert gas atmosphere using a steel crucible. TiCl₄ is fed into the reaction crucible, which is holding a bed of molten sodium. The temperature of reduction needs to be precisely controlled within the small range of melting point of NaCl (801C) and boiling point of sodium metal (887C) for ensuring a smooth reduction process. The main product of the titanium sponge needs to be separated from adhered by-product (NaCl) and leftover reductant (sodium), which is done by water leaching of the reaction product Later on, however, the process underwent several modifications and improvements compared to the method originally employed by Hunter. Nippon Soda, Japan and Deeside Titanium, UK-operated titanium sponge production plants employing this process. However, the process has several setbacks such as very high exothermic heat generation, highly reactive species, very close window of process operating parameters, difficulty in recovering by-product/effluent disposal, etc., in addition to the hazardous nature of handling sodium. Currently, no industry is operating in the world based on this process for titanium sponge production.

5. The Kroll process

The Kroll process of magnesio-thermic reduction of TiCl₄ to produce titanium sponge is represented by the following chemical reaction.

$${\text{TiCl}}_4+\text{2Mg}=\mathrm{Ti}+{\text{2MgCl}}_2,{{\text{∆H}}^{\mathrm{o}}}_{80\text{0C}}=-325\mathrm{kJ}/\text{mole}.$$E2

The reaction takes place between liquid magnesium and gaseous TiCl₄ (boiling point 136C) forming solid titanium. There is a possibility of a large number of reactions involving also lower chlorides of titanium viz. TiCl₃ and TiCl₂ as presented in Table 1. The standard enthalpy and entropy changes of the reactions which indicate the thermodynamic possibility and exothermic/endothermic nature of reactions are presented in the table. The overall reduction reaction is highly exothermic and necessitates external cooling of the reactor for controlling the reaction temperature.

S.No.	Reaction	ΔG^o1100K	ΔH^o1100K
1	½ TiCl4(g) + Mg (l) = ½ Ti(s) + MgCl2(l)	−155.8	−214.1
2	½ TiCl4(g) + ¼ Mg(l) = ½ TiCl3(g) + ¼ MgCl2(l)	−38.1	−45.1
3	½ TiCl4(g) + ½ Mg(l) = ½ TiCl2(s/l) + ½ MgCl2(l)	−168.7	−90.3
4	2/3 TiCl3(g) + 1/3 Mg(l) = 2/3 TiCl2(s/l) + 1/3 MgCl2(l)	−60.3	−174.2
5	TiCl2(l) + Mg(l) = Ti(s) + MgCl2(l)	−130.9	−90.7
6	½ TiCl4(g) + 1/6 Ti (s) = 2/3 TiCl3(g)	−8.2	20.6
7	½ TiCl4(g) + ½ Ti(s) = TiCl2(s/l)	−24.9	−123.4
8	2/3 TiCl3(g) + 1/3 Ti(s) = TiCl2(s/l)	−16.7	−144.0

Table 1.

List of possible reactions in the Mg-Ti-Cl system.

Physico-chemical aspects of reaction chemistry and some aspects of reaction mechanism and titanium sponge formation in a Kroll reduction reactor are discussed in detail in the literature [5, 6]. Also, phase equilibria in the system, Mg-Ti-Cl over a wide temperature range of 500–2000°C, were studied. The extent to which a given reaction would occur is influenced by several factors such as temperature, the physical state of the reactants, mutual solubilities of the substances, surface and interfacial phenomena, reaction kinetics and heat and mass transfer in the system.

From a thermodynamic analysis of various possible reactions involved in the magnesio-thermic reduction of TiCl₄, the following inferences were drawn:

• When gaseous TiCl₄ reacts with liquid magnesium, reduction of TiCl₄ to TiCl₂ is the most probable.

• When gaseous TiCl₄ reacts with magnesium vapour, reactions that result in the formation of TiCl₂ and TiCl₃ are the most probable.

• Formation of magnesium sub-chloride (MgCl) is also a possibility at high temperatures. MgCl is unstable under normal conditions but is identified in the gaseous phase at high temperatures.

• Among the secondary reactions of TiCl₄ with titanium, the reaction forming TiCl₂ is the most probable.

The reduction process is carried out in a steel/stainless steel crucible/reactor under an argon gas atmosphere. It is a batch process and based on the batch size, the required quantity of magnesium is taken into the reactor and heated to 780°C. TiCl₄ is then pumped into the reactor from the top closure of the reactor. The temperature of reduction is maintained in the range 780–830°C by removing exothermic heat through the circulation of air around the outer surface of the reactor. Titanium particulate formed in the reduction process tends to agglomerate in sizeable pieces along the reactor’s inner wall and grown pieces fall and are collected at the perforated bottom plate of the reactor vessel. The reaction by-product MgCl₂ (which melts at 712°C) is liquid under reactor conditions and being heavier than molten magnesium moves down paving the way for liquid magnesium to ascend to the top surface. MgCl₂ collected at the bottom of the reactor is periodically withdrawn from the reactor for accommodating the increased volume of the titanium sponge. The titanium sponge grows as a cylindrical cake inside the reactor. It is inevitable that some amounts of magnesium metal and magnesium chloride get entrapped in the pores of the sponge during their transportation through the sponge during the process. To compensate the loss of reductant (magnesium metal) that is entrapped in the pores, usually 50–60% excess (more than the stoichiometric requirement) magnesium is used in the sponge production campaigns. After completion of the reduction process, the sponge cake comprises un-reacted magnesium metal and magnesium chloride, which are entrapped in the pores of the titanium sponge.

Vacuum distillation of the reduced mass is carried out by taking out the reaction crucible containing the reduced mass into another reactor (Figure 2) and heated to about 1000°C under a dynamic vacuum of the order of 5–10 x 10⁻³ mbar, to distil out Mg/MgCl₂ resulting in titanium sponge freed from the entrapants. The basic principle in the vacuum distillation process is that at 1000°C, the vapour pressure of Mg and MgCl2 is higher whereas the vapour pressure of titanium is very insignificant. However, there are practical issues in the optimization of principle parameters of the distillation process such as distillation soak, temperature, monitoring of vacuum as fall in the vacuum inside the process reactor is overlapped with atmospheric leaks into the system. However, a realistic time of distillation time could be assessed through simultaneous observation of vacuum fluctuations in the process reactor/condenser vessel and electrical energy consumption pattern of heating of the reactor with reduced mass. After cooling to room temperature under vacuum/argon gas, the sponge cake is ejected out of the reactor at room temperature carefully employing appropriate equipment/tooling. Figure 3 shows a photograph of a 3 MT sponge cake produced at the DMRL titanium research centre. Any burnt or coloured particulates (formed due to oxidation) are manually removed before the cake is further handled and processed to prepare 2–25 mm size material for taking up ingot melting.

Figure 2.

Schematic of reactors for reduction and vacuum distillation processes in the conventional Kroll process.

Figure 3.

A 3 MT titanium sponge cake produced at DMRL titanium experimental facility by combined process technology.

The Kroll process has several advantages over the other two processes of sodium reduction and fused salt electrolysis in terms of scaling up product purity, and amenability to recycle magnesium metal from the by-product (MgCl₂) through electrolysis. Over the years there have been several technological advancements that taken place in the Kroll technology as mentioned below:

• Enlargement of batch size (from 2 to 4 MT at the beginning of the industry to 8–12 MT at present). Increased batch size is advantageous for enhanced productivity and high-quality yield.

• Combined process technology (wherein reduction and vacuum distillation processes are conducted in a single reactors assembly system (Figure 4). This significantly cuts down energy requirement and total cycle time of a production batch.

• Recycling of by-product MgCl₂ by fused salt electrolysis in multipolar cells to regenerate magnesium metal and chlorine for captive consumption in an integrated titanium sponge plant.

• Advanced process instrumentation and AI techniques for process control, sponge sorting, etc.

• Evolution of different types of reactor materials, innovative sponge quality evaluation techniques and so on.

Figure 4.

Schematic arrangement of reactors in the combined process technology: (a) side by side and (b) one over the other.

6. Fused salt electrolysis of TiCl₄

Electrolytic processes of metal extraction have many advantages such as simplicity, scope for producing high-purity metal and amenable to semi-continuous/continuous process operations and so on. Efforts on titanium metal extraction by fused salt electrolysis concept are very old and several researchers put in efforts to develop the same. Initially, electrolysis of TiO₂ in molten electrolytes of alkali borates and phosphates was pursued by International Research Inc., USA. However, there were many technical issues such as higher operating temperatures, corrosive electrolytes, product contamination by oxygen and difficulty in purifying the product. Subsequently, fused salt electrolysis of TiCl₄ in LiCl-KCl bath was considered to be more viable and extensively studied simultaneously by Imperials Chemicals Inc., UK, Dow Howmet and Reactive Metals Inc., USA, and Ginatta Marco, Italy. Consistent and sustained efforts put in by GTT, Italy, on Pilot plant studies of the electrolysis technology led to further development of the technology for industrial implementation [6, 7]. It is learnt that the multi-valency of titanium, covalent bonding in TiCl₄ (characterized by solubility limitation) and control of product purity have been found to be major challenges in working out a commercial model of the cell.

Among the three processes as discussed above, the Kroll process withstood the test of time and has been the predominant method of titanium sponge production the world over. Though the electrolytic process is thought to be a potential alternative to the Kroll process, to date, it remained at pilot scale operations only.

7. World production of titanium sponge

Major titanium sponge-producing countries have been the USA, Japan, Russia, Kazakhstan, Ukraine, UK, and China. In the UK, production was closed down in the early ‘90s. In India, a small capacity (500 MT/year) titanium sponge plant was established with the technology developed at DMRL and has been producing titanium sponges since 2012 for meeting domestic needs. Recently, Saudi Arabia started a 15,000 MT/year capacity titanium sponge plant.

The present world production capacity for titanium sponge is placed at 3,50,000 MT per annum and China is contributing about 50% of the total [8]. It is to be noted that the Kroll process is used in the entire world production.

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8. Control of impurity elements in the Kroll process

In view of the wide application of the Kroll process for titanium sponge production, this chapter envisages to bring in a detailed discussion on the quality control aspects pertaining to this titanium extraction method. As already mentioned, titanium is highly reactive and the thermodynamics of the titanium-based systems do not permit any purification method that can be implemented for purifying sponge. Only control of impurities is a solution for preparing high-purity sponge. The important impurity elements in titanium sponge that adversely affect its properties are O, N, C, Fe, H, Ni, Cr, Mg and chlorides. Among this H impurity, Mg and chlorides are generally not of serious concern as they are driven out during the ingot melting. However, Mg and chloride contents are to be kept at a minimum to improve the shelf life of the sponge (before it is taken up for ingot melting).

The following are the major factors that influence the quality/purity of titanium sponge produced by the Kroll process:

• Purity of raw materials and other consumables (TiCl₄, Mg, argon gas, etc.)

• Cleanliness and pressure tightness of the reactor assembly

• Process operating conditions (Reduction temperature, TiCl₄ feed rate, exposure of reduced mass, order of vacuum and vacuum distillation soak, overall cycle time, etc.)

• Reactor material

• Care taken during sponge cake handling and size reduction processing to prevent contamination of sponge from equipment and tooling

• Sponge storage before melting

Mastery of high-purity titanium sponge production lies in all the above parameters. For example, though the raw materials are of high purity, other parameters mentioned above tend to cause impurity elements to join the sponge. In Table 2, various parameters that act as a source of various impurity elements are listed. In general, sponge material adjacent to the wall gets contamination by diffusion of an impurity element from reactor material and the content of that reduces towards the inner mass. The bottom portion of the sponge cake is highly contaminated due to diffusion of impurity elements from the false bottom plate of the reactor and also, all the impurity elements present in magnesium metal transfer into the sponge before the metal ascends to the top of the liquid surface inside the reactor. The top portions of the sponge are enriched with oxygen and chloride due to atmospheric leaks from the nozzles of the reactor lid and entrainment of the last distilled chloride material. Based on the experimental data involving analysis of the large number of samples collected from various sites across the sponge cake, a general pattern of impurities distribution is known. Typical distribution of impurity elements in titanium sponge is schematically shown in Figure 5. Thus, the sponge in the central regions/core assumes the highest purity and is separately harnessed while processing/size reduction of the sponge cake to prepare homogenous quality lots of very high purity. All titanium sponge producers evolve and follow systematic procedures for grading and quality evaluation of sponge cakes and prepare uniform quality sponge lots in finished size, which is suitable for subsequent ingot melting of Ti/Ti alloys [9].

S.No.	Parameter	Source of impurity elements
1.	Reactor material; Carbon steel; Austenitic stainless steel; Ferritic chromium steel	Fe, C, O; Ni, Fe, Cr; Cr, Fe
2.	Raw materials; TiCl4; Magnesium	O, Fe, Si, Sn, etc.; Fe, Al, Si, etc.
3.	Cleanliness & pressure tightness of reactor assembly	O, N, H
4.	Process operating conditions	Fe, O, N, chloride
5.	Equipment & tooling for sponge cake processing	C, Si, O, Fe
6.	Sponge storage	O, N

Table 2.

Sources of impurities in the Kroll process of titanium sponge production.

Figure 5.

Typical distribution of impurity elements in a titanium sponge cake.

Implementation of all quality control measures in titanium sponge production results in higher purity of the product with enhanced yield. It is claimed that industry giants prepare high pure sponges of 5 N (five nines, i.e., 99.999% Ti) and supply them for critical & electronic applications [10]. Similarly, sponge with very low iron and nickel (Fe + 3 Ni ≤ 120 ppm) is regularly prepared for the manufacture of critical aero-engine components.

Though ASTM MD 120 provides complete specifications on titanium sponge purity, it is advisable to consider sponges of much more purity for aerospace applications, in view of the high sensitivity of mechanical properties of titanium with respect to the impurity elements especially O, N, C and iron. Major sponge producers across the globe specify their own standards. MIDHANI is the major manufacturing unit of titanium and titanium alloys in India. In Table 3, MIDHANI, Japanese (SHOWA) and Russian specifications of high-purity titanium sponge are presented and a comparison is made with ASTM specification. Analysis of the best sponge produced at the DMRL research centre is also included in the table. The Brinnel Hardness Number (BHN) of a machined button melted out of a sponge sample acts as a very informative indicator of the purity of the sponge. Hence, a mention of BHN of the sponge is always found in the titanium sponge specifications.

Impurity element	CIS TG-90	SHOWA; S-90	ASTM; MD 120	DMRL lot; L001/2 K	MIDHANI
O	0.040	0.060	0.100	0.032	0.080
N	0.020	0.010	0.015	0.003	0.015
C	0.030	0.020	0.020	0.006	0.015
Fe	0.060	0.030	0.120	0.018	0.050
Ni	NS	NS	NS	<0.005	0.050
Cr	NS	NS	NS	0.009	NS
Mg	0.080	0.045	0.080	0.004	0.080
Chloride	0.080	0.080	0.120	0.005	0.100
H	NS	0.002	0.001	0.002	NS
Ti (by difference)	99.6	99.8	99.6	99.8	99.6
BHN	80–90	90	120	82	100

Table 3.

Specifications of high-purity titanium sponge—different standards (content in wt%).

9. Trends in the development of alternate titanium extraction processes

The Kroll process technology though widely found acceptance has been used for industrial-scale production of titanium sponges. It has the demerits of being highly capital and labour-intensive, having higher energy consumption and higher cost of production. Improvements over the years in technology implementation have reached stagnation. Established titanium giants too claim an energy requirement of as high as 30–32 kWh/kg of sponge. However, the process has been sustaining as there is no alternative method of sponge production is evolved so far. Worldwide over there have been several efforts and intensive research continuing on alternate processes of titanium metal extraction. During the last 2–3 decades, very encouraging developments have been taking place with a few new extractions processes taken forward to even pilot-scale production. Various process technologies that are being tried out for low-cost production of titanium sponge are summarized by Kraft [11].

It is well known that titanium metal usage is restricted mainly because of higher costs. The new alternate titanium metal extraction process mainly aims at bringing down the energy requirement and cost of production. The approach for the same mainly consists of simplifying the process flow sheet by cutting down the number of unit operations, direct production of end components by combining the extraction and forming processes, etc. Thus, the processes lead to the direct production of Ti/Ti alloy powders followed by near net shape forming/3D printing (Figure 6) assuming greater importance and are on the way of emerging as viable alternate titanium production methods.

Figure 6.

Current trends in the development of alternate titanium metal extraction processes.

The following innovative processes are mentioned worthy in this context and are on the verge of adaptation for industrial-scale metal production.

9.1 The FFC (Fray Farthing & Chen) process

Titanium metal preparation directly from TiO₂ by a novel electrochemical reduction process was successfully developed and patented by Fray, Farthing and Chen of Cambridge University, UK in 1999 [12, 13]. The process essentially involves cathodic treatment of various types of TiO₂ pre-forms at high temperatures (950–980°C) in an argon gas atmosphere using molten calcium chloride as electrolyte and graphite as an anode. When DC voltage is applied, oxygen of the TiO₂ cathode is removed and transported through the electrolyte to the anode where CO/CO₂ is formed and let out. After the process, the sponge is subjected to water/acid leaching for removing adhered electrolyte material. Since the discovery of the process several developments/advancements has been taking place for an improved understanding of the mechanism of metallization, scaling up, enhanced product purity, applying process for alloy preparation and exploiting the principle for other possible metal oxides, etc. Basic research carried out at DMRL attempted to bring in an improved understanding of the reaction mechanism and the importance of monitoring CO₂ content in the vent gases [14, 15]. Excellent reviews by Mohan Das [16, 17] bring out details of developments that have been taking place in the FFC process over the years. Metalysis, UK, reportedly works on pilot-scale studies of the process and even exploring industrial-scale implementation [18, 19] .

10. Armstrong process

The Armstrong process is based on the sodium reduction of TiCl₄ (Hutner’s process) with many advancements brought in to produce high-quality titanium powder particles in a continuous manner. The process technology was developed by researchers experienced in handling molten sodium at International Titanium Powder LLC, IL, USA [20]. The process technology has been demonstrated for producing pure titanium powder as well as standard titanium alloy powders. Basically, the process involves the reduction of TiCl₄ vapour, which is injected into a molten pool of sodium in a titanium reactor. The product of titanium powder and sodium chloride is leached with water to remove the salt. Any excess sodium metal in the product is separated by distillation.

11. OS process

Calciothermic reduction of TiO₂ with several process improvements to obviate the theoretical and practical difficulties encountered in the conventional calico-thermic reduction has been constantly pursued by Ono & Suzuki of Kyoto University, Japan. In the OS process, TiO₂ is fed to molten calcium/CaCl₂ bath and the reduction process is continued with the dissolution of the reduction by-product CaO in the calcium chloride. Continuous supply of the reductant, calcium metal is ensured by in situ electrolysis of CaO that is generated as a by-product. Pilot plant scale studies were reportedly carried out to study and establish the feasibility of continuous production of titanium metal powder [21].

12. CSIRO process

Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia, pursued and experimented on gaseous phase reduction of TiCl₄ vapour with magnesium metal vapour (with the same reaction basis as the Kroll process) in a fluidized bed reactor [22]. The reaction by-product, MgCl₂ and un-reacted magnesium metal are removed by vacuum distillation, while high-purity titanium powder can be continuously withdrawn from the reactor. Based on the successful development of the process for continuous production of titanium powder, it is being taken forward to pilot-scale testing.

There are many other processes of titanium metal extraction that have been in experimentation and pursued on a lab scale. The processes include the metal hydride reduction process (MHR process) wherein TiO₂ is reduced by calcium hydride, electronically mediated reduction process (EMR) wherein TiO₂ is reduced by calcium metal without any contact of a reductant, hydrogen-assisted magnesium reduction of TiO₂ process, high temperature fused salt electrolysis of TiO₂ using metallic anode (being pursued by MIT, USA), combined magnesium thermal reduction of TiCl₄ electro-slag melting, etc., which promise a way ahead in the pursuit of evolving an alternated process of titanium metal extraction. A recent review on titanium production methods [23] covers many efforts that are being put in for developing a new process of titanium metal extraction. Thus, titanium extraction metallurgy provides a large scope for trying out different types of processes and techniques for producing the metal.

Acknowledgments

Author wishes to express sincere thanks and gratitude to Dr.G.Madhusudhan Reddy, Outstanding Scientist and Director, DMRL for according approval for the publication of this chapter on titanium extraction metallurgy developments. This rich experience gained by the author in the area of development & demonstration of Kroll titanium sponge production technology has been possible only because of the collective efforts of a large number of scientists, expert technical personnel and supporting staff involved in the program at DMRL. Financial support provided by DRDO, DAE and ISRO during various stages of development, demonstration and transfer of technology to the industry is gratefully acknowledged.