Processes for Recycling

Jiayun Zhang, ... Fumitaka Tsukihashi, in Treatise on Process Metallurgy: Industrial

Processes, 2014
4.4.1.2.1 Conventional Kroll Process
Kroll process as a mature technique has been developed for seven decades and is
operated in industrial large scale. The process includes several steps, carbochlorination to
produce TiCl4, TiCl4 purification, the reduction of purified TiCl 4 by magnesium, as well as the
electrolysis of MgCl2 generated by the reduction to produce metallic Mg and chlorine.
During carbochlorination, carbon (oil–coke) and chlorine are used to react
with TiO2 producing TiCl4 (along with other volatile chlorides) and CO or CO 2 between 1073
and1173 K.
In the following treatment, the impurities of silicon, iron, aluminum and magnesium, calcium,
etc., are removed by fractional distillation to generate TiCl 4 (99.9% in purity). Then purified
TiCl4 is reduced by molten magnesium between 1073 and 1173 K in argon atmosphere to
produce sponge titanium.
By the subsequent cooling and vacuum distillation to remove the magnesium and
magnesium chloride in sponge titanium, purer sponge titanium (99.5–99.7%) can be
produced. MgCl2 obtained from reduction of TiCl4 is sent to electrolysis sector to produce
chlorine and magnesium. The Cl2 and Mg produced are, respectively, used for chlorination
of TiO2 and reduction of TiCl4, respectively.
It is noted the three main steps, TiO2 chlorination, magnesium reduction as well as
electrolysis, in the process are operating independently of each other. Both Mg and Cl 2 play
role as intermediaries. Although Cl2 can be used cyclically, however the loss of chlorine
during the transport and operation must be supplemented. Moreover, the chlorination of
Kroll process is a batch operation, and each of the process operation cycle lasts up to 10
days. All these make the process expensive, labor intensive, energy consuming, and
somewhat are polluting.
On the contrary, due to the low cost and no chlorine discharge accompanied, the latterly
reported electrochemical approaches have attracted close attention [27–30].

Advances in Titanium Production

D.J. Fray, in Encyclopedia of Materials: Science and Technology, 2006
In the Kroll process, the magnesium ingots are loaded into a steel reactor, the reactor is
welded shut and heated to about 950 °C; titanium tetrachloride, which boils at 136 °C, is fed
into the reactor. The reaction is allowed to continue for about three days and the
magnesium chloride byproduct is tapped off. Frequently, this is electrolyzed onsite to
produce magnesium for the reduction process and chlorine for the carbo-chlorination
reaction. At the end of the reduction process, the vessel is allowed to cool, the weld is
broken, and the titanium sponge removed as a porous cylinder; the remaining magnesium
chloride is volatilized at 950 °C under vacuum. The whole process from carbo-chlorination
to the final titanium sponge takes 17 days, is labor intensive, and the chemicals are toxic.
The largest Kroll reactor in the world can make only 10 ton batches. Subsequent processing
of the Kroll sponge into alloys, ingots, and sheets is expensive as every operation seems to
double the cost of the product. Alloying is particularly difficult with heavier elements such as
nickel and niobium, as the elements tend to segregate on melting. The flowchart for the
Kroll process is shown in Fig. 1 and this is the process that all new processes must be
compared against. The Kroll process has ruled supreme for many years although Wilhelm
Kroll, stated in the 1950s that his process will be superceded by an electrolytic route within
15 years.
Figure 1 . Flowchart for Kroll process (Sumitomo website) Reproduced with permission of Sumitomo.

Production of Rare Metal Powders

Oleg D. Neikov, ... Irina B. Murashova, in Handbook of Non-Ferrous Metal Powders
(Second Edition), 2019
Reduction of Zirconium Tetrachloride by Magnesium
Reduction of zirconium tetrachloride by magnesium named the Kroll process [19] is
commercially the most widely used [20,21]. Zircon is generally used as precursor for the
manufacture of zirconium tetrachloride feedstock for the Kroll process. The zircon (ZrSiO4)
is carbochlorinated to produce zirconium tetrachloride (ZrCl 4), silicon tetrachloride (SiCl4),
and carbon monoxide (CO) according to equation:
ZrSiO4+4C+4Cl2→ZrCl4+SiCl4+4CO

The relatively impure ZrCl4 separated from the SiCl4 during the carbochlorination process,
still contains hafnium. In one of several methods to separate hafnium and zirconium [20],
hydrolysis of ZrCl4 is followed by liquid-liquid extraction as the oxychloride (ZrOCl2), followed
by precipitation as Zr(OH) 4 and thermal decomposition to ZrO2. The ZrO2 is then again
carbochlorinated, and the resulting ZrCl4 used as a feedstock in the Kroll process. In this
process, the ZrCl4 is reduced with magnesium metal in a sealed batch reactor at about
1123 K. Zirconium metal and magnesium chloride are formed in an exothermic reaction:
ZrCl4+2Mg→Zr+2MgCl2

The magnesium chloride and any excess magnesium used in the reaction are removed
from the reaction mixture by high-temperature vacuum distillation. The pyrophoric zirconium
sponge so obtained is crushed, sorted, and purified by vacuum arc remelting to yield the
metal as ingots. The metal can also be purified by the Van Arkel-De Boer process [20],
where it is allowed to react with a halogen (e.g., iodine), and the metal halide vapor is then
decomposed on a white-hot (1673 K) tungsten wire to yield so-called crystal bar:
Zr+2I2→ZrI4
ZrI4→Zr+2I2

Rare Earth, Titanium Group Metals, and Reactive Metals

Production
Osamu Takeda, ... Toru H. Okabe, in Treatise on Process Metallurgy: Industrial Processes,
2014
2.9.2.1.3.1 Hunter Process
The Hunter process shares the same steps as the Kroll process for producing TiCl4, but it
reduces TiCl4 with a sodium reductant. The operation mode in the Hunter process is either a
one- or two-step operation. In the one-step operation, the stoichiometric amount of sodium
metal necessary for the reduction of TiCl 4 is reacted in one installment, and TiCl 4 is reduced
directly to titanium metal in a single reaction container. In contrast, the two-step operation
starts by first reducing TiCl4 to titanium dichloride (TiCl2) with half of the stoichiometric
amount of sodium necessary. Next, TiCl 2 in molten sodium chloride (NaCl) is transferred to
another container and then reduced to titanium metal with additional sodium reductant. The
major reasons for employing the two-step operation are that the sodium reduction
generates a larger amount of heat compared to the use of magnesium and controlling the
vapor pressure of sodium metal is difficult.
The one-step operation proceeds as follows:
TiCl4g+4Nal→Tis+4NaCll

The two-step operation consists of these two reactions:

TiCl4g+2Nal→TiCl2l,inNaCl+2NaCll
TiCl2l,inNaCl+2Nal→Tis+2NaCll

Figure 2.9.20 shows a schematic diagram of the reduction container used in the Hunter
process. The titanium adheres to the inner wall of the container to a lesser extent than in
the Kroll process, and the level of contamination by iron and other elements originating from
the container wall is relatively low. Titanium is leached with dilute hydrochloric acid to
remove NaCl and trace amounts of the remaining sodium metal. Unlike the case of the Kroll
process, the titanium produced is in the form of a powder called sponge fines, which is
useful as an inexpensive raw material in powder metallurgy.

Figure 2.9.20 . Schematic diagram of reaction container used in Hunter process.

The main problem for the industrial application of the Hunter process is the separation of
the titanium produced and NaCl, as well as the reproduction of sodium metal. Figure
2.9.21 shows the vapor pressures of various chlorides. Since the vapor pressure of NaCl is
lower than the vapor pressure of MgCl2 (pNaCl = 3 × 10− 3 atm, pMgCl2 = 1 × 10− 2 atm at
1200 K [47]), it is difficult to efficiently separate NaCl by distillation. Therefore, NaCl should
be removed by leaching in an aqueous solution. Furthermore, recovering the by-product,
NaCl, from the aqueous solution requires additional energy. Because these industrial issues
have remained unresolved, with the exception of the production of some electronic
materials, large-scale titanium smelting using the Hunter process for industrial purposes has
been discontinued since 1993. However, the process remains attractive because of its
superiority to the Kroll process in terms of the form and purity of the metal deposit
produced. Thus, R&D into sodium reduction processes continues to this day [48].
Figure 2.9.21 . Vapor pressures of metals and chlorides.

Metal injection molding (MIM) of titanium and titanium

alloys
T. Ebel, in Handbook of Metal Injection Molding (Second Edition), 2019
19.5.1 Novel production techniques
A high portion of the overall costs for titanium is caused by the reduction of the ore, which is
commonly performed by applying the Kroll process. This is a multi-step process in which
first the oxide is converted into chloride, and then the chloride is reduced to metal. Several
attempts have been made to replace this cost-intensive technique by other methods or by
developing new powder production techniques (Froes, Gungor, & Imam, 2007). Examples
are described later.
Plasma-quench process
The raw material TiCl4 is used, which is dissociated thermally by means of a plasma arc.
By rapid quenching, powder particles are formed. However, TiCl 4 is rather expensive and
the process is hard to control because several reactions are involved.
MHR, metal hydride reduction
This technique (Froes, 1998) applies calcium hydride for the direct reduction of titanium
oxide according to the chemical formula TiO2 + 2CaH2 → Ti + 2CaO + 2H2. The advantages
are that only one step is needed, the powder is chloride-free and it is possible also to
produce TiH2, which can be used as a raw material, as mentioned in Section 19.5.2.
Armstrong process
Basically, this is a modification of the Hunter process, working with sodium: TiCl 4 + Na
(molten) → Ti + NaCl. By a continuous process, production of both sponge and powder is
possible. The drawback is again the rather high costs of the raw materials.
TiRO process
Here, TiCl4 reacts with Mg-powder in a fluidized bed reactor (Doblin, Cantin, & Gulizia,
2016) and by vacuum distillation titanium is separated from MgCl 2. This process is
continuous and results in very fine powder particles forming agglomerates.
FCC-Cambridge process
The Fray-Farthing-Chen (FCC)-Cambridge process is an electrolytic process where a
cathode pressed from TiO2 pellets and a graphite anode are placed in a CaCl2 bath. Oxygen
ions diffuse from the cathode and CO2 is formed at the anode: TiO2 + C → Ti + CO2.
Sponge, which can be crushed to powder, can be made, but the cathode production is
rather cost intensive. In addition, high energy consumption has to be considered. However,
this technique is considered as being most suitable for providing a significant cost
reduction compared to Kroll processing and is commercially applied today (Mellor &
Doughty, 2016). By adding oxides of other chemical elements, it is also possible to produce
alloys.
To date, all processes still suffer from a high risk of impurity pickup and the costs for large-
scale production are not clear in all cases. However, development continues and there is
reasonable hope of producing lower cost powder of high quality in the future.

Production of Titanium and Titanium Alloy Powders

Oleg D. Neikov, Victor G. Gopienko, in Handbook of Non-Ferrous Metal Powders (Second
Edition), 2019
Abstract
In this chapter, the manufacturing technologies of titanium and titanium alloy powders,
including the sodium reduction method (Hunter process), the Armstrong process, the
magnesium reduction method (Kroll process), the calcium hydride reduction process, the
hydrogenation/dehydrogenation (HDH) process, the plasma-rotating electrode process
(PREP), gas atomization, the FFS-Cambridge process, the MER process, the Chinuka
process, the electrolytic refining process, and the TIRO method are considered. The CSIP
process, the method of disproportionation of lower titanium halogenides, the CSIR process,
the chemical synthesis of nanosized titanium powders, the amalgam metallurgy titanium
technique, and mechanical alloying are also described. Additive manufacturing for the
production of titanium and titanium alloy components is discussed. Different ranges of
titanium powder applications are shown. The main aspects in the titanium powder process
selection and titanium powder-based materials selection are considered.

Ti-Nb-Zr system and its surface biofunctionalization for

biomedical applications
M. Dinu, ... A. Vladescu, in Titanium in Medical and Dental Applications, 2018
2.4.3 Fabrication of titanium alloys
As compared with other alloys used for industrial purposes, Ti alloys exhibit high production
costs. Starting from the extraction and transformation of the raw material in the form
of ilmenite and rutile, which are reduced to metallic titanium using the Kroll process, up to
the final product with its specific properties according to each application, there is a complex
multistep process. Because high-purity titanium exhibits poor strength, during the
manufacturing process, the final alloy composition needed for certain properties and thus
for certain applications is adjusted by alloying with certain amounts of oxygen or elements
that stabilize the different component phases. However, due to its high reactivity, if
precautions are not taken in the melting process of titanium, this can lead to higher costs.
Using the common fabrication methods used for other metallic materials, products in
semifinished form are processed by casting and forming processes such as rolling, forging,
extrusion, etc. Furthermore, in order to obtain a final product, efforts are made to minimize
the machining costs (such as milling, turning, grinding, drilling, etc.), and thus, especially in
in the case of “near-net-shape” products, proper fabrication routes are selected in order to
avoid extensive postprocessing of components [39]. In order to obtain the desired
mechanical properties prior to service, surface modification processes should be applied.
Microstructures, preexisting defects, and residual stresses can lead to failure; by applying
suitable heat treatments, these service failures can be overcome [40].
Lately, new technologies for the fabrication of titanium alloy components have been
developed to meet the requirements of each application. In the biomedical field, by using
advanced manufacturing, complex shapes with surface finishes and porous structures can
be created to match the mechanical properties of the bone (similar elastic modulus) [41]. A
good example is powder metallurgy, which enables the production of complex titanium parts
starting from production of the powder up to the hot isostatic pressing. Studies showed that
the Ti-6Al-4V alloy obtained by powder metallurgy exhibited tensile properties higher than
the one obtained in the as-cast form but similar with the wrought alloy [42]. By tailoring the
porosity of Ti alloys, better biological properties can be achieved regarding the adhesion,
viability, differentiation, and growth of the cells. Moreover, Wang et al. [43] showed that the
mechanical properties of a porous Ti-10Zr-10Nb can be adjusted to resemble the cortical
bone, exhibiting an elastic modulus of 3.9 GPa and a compressive yield strength of 67 MPa
when the porosity was 69%.
Modern techniques also include the layer additive manufacturing process, which is a rapid
prototype process based on the melting of titanium powder using a laser or an electron
beam, thus building step by step the shape dictated by an AutoCAD model [44]. Another
approach in this field is the powder injection molding method, which implies the use of a
powder mixed with a binder that is injected into a die, followed by removing the binder and
sintering the part [45].
From traditional implant fabrication techniques to advanced manufacturing technologies
available today, there is a wide range of choices that enables the production of functional
materials with tailored and optimized properties at a reduced processing cost and waste.

Aircraft Materials
R. Boyer, in Encyclopedia of Materials: Science and Technology, 2001
1 Historical Development
Prior to World War II, aluminum was used almost exclusively in metal aircraft construction
with some steel in selected areas where its higher strength and stiffness was required, such
as landing gear and engine support structure components. With the development of
the Kroll process as a production means of economically extracting titanium from its ore,
titanium began being used for aerospace applications in the 1950s, driven primarily by the
engine companies. It is now a key material for both airframe and engine structure due to its
high specific strength and corrosion resistance relative to aluminum. In the early-to-mid
1960s, composite materials consisting of boron-based fibers impregnated with various
polymers were introduced into aerospace structure. These composites soon evolved into
the present-day carbon fiber composites, commonly referred to
as CFRP (carbon fiber reinforced plastic) or GR/EP (graphite/epoxy) or PMC
(polymer matrix composites). (The earlier boron fiber composites were very costly, and very
difficult to machine. Development of the graphite fibers then displaced the earlier boron
composites.)The polymeric matrices, graphite fibers, and processing have constantly
evolved to the point where cost and structurally efficient airframe structure can be produced.
They are attractive materials due to their low density, high strength and stiffness, and
excellent fatigue characteristics in conjunction with the ability to tailor the composite layup to
the specific requirements of the component. Thermoset epoxy-based resin systems typically
have dominated the airframe industry. However, more specialized systems have been used
for applications requiring higher operating temperatures, such as bismaleimides (BMI).
Significant advances have also been made with regard to the development and application
of aluminum (ARALL and GLARE) and titanium hybrid laminates (polymer matrix
composite/Ti), and aluminum and titanium metal matrix composites, though their usage has
been limited by high costs.

Powder metallurgy of titanium alloys

F.H. FroesSam, in Advances in Powder Metallurgy, 2013
8.3.3 ADMA Products hydrogenated titanium process
The use of titanium hydride powder instead of titanium sponge fines has led to the
achievement of essentialy100% density, using a simple cost-effective press-and-sinter
technique, in complex parts.20,21 In this work, hydrogenated non-Kroll powder (by cooling the
sponge produced in a Kroll process with hydrogen rather than the conventional inert gas, a
lower cost titanium hydride powder has been produced by ADMA Products) was utilized
along with 60:40 Al:V master alloy to produce components made from the Ti-6Al-4V alloy.
The press-and-sinter densities achieved using this novel fabrication technique are shown
in Fig. 8.11. The associated microstructure and typical mechanical properties are shown
in Fig. 8.12 and Table 8.4 (after cold pressing, sintering, forging and annealing),
respectively. The mechanical properties compare well with those exhibited by cast-and-
wrought product. The low cost of this process in combination with the attractive mechanical
properties make this approach well suited to the cost-obsessed automobile industry. The
parts shown in Fig. 8.13 have already been fabricated and a cost estimate of less than
US$3.00 for an 0.32 kg (0.705 lb) connection link has been made.23
8.11 . Density of Ti-6Al-4V compacts after sintering. Conditions 5 and 7 used hydrided powder and show by far

the highest and most uniform densities.

8.12 . Microstructure of sintered Ti-6Al-4V material.

Table 8.4 . Room temperature tensile properties of a hydrogenated titanium compact (after dehydrogenation)

PM Ti-6-AI-4V Ultimate tensile Yield strength Elongation Reduction

strength (Mpa) (Mpa) (ksi) (%) of area (%)
(ksi)
3.5 cm (1.376") 994–1028(144–149) 911–938 (132– 14.0–15.5 34–38
thick 136)
ASTM 897(130) 828(120) 10 25
8.13 . Ti-6Al-4V parts produced using a press-and-sinter approach and titanium hydride: (1) connecting rod

with big end cap, (2) saddles of inlet and exhaust valves, (3) plate of valve spring, (4) driving pulley of

distributing shaft, (5) roller of strap tension gear, (6) screw nut, (7) embedding filter, fuel pump, and (8)

embedding filter.

(courtesy Ukrainian Academy of Sciences)

In Kroll’s process, the removal of the Ti sponge from the retort and its subsequent crushing
is time and energy intensive. In comparison, ADMA’s process produces TiH 2 which, unlike
Ti sponge, is very friable (see Fig. 8.14) and easily removed from the retort with no need for
an expensive sizing operation. ADMA’s vacuum distillation processing time is also at least
80% less than in Kroll’s process since phase transformations/lattice parameter changes of
the hydride sponge, in the presence of hydrogen, accelerate the distillation removal of
MgCl2. Finally, atomic hydrogen is released during sintering–dehydriding of the TiH 2 powder
and acts as a scavenger for impurities (e.g. oxygen, chlorine, magnesium etc) resulting
in titanium alloys with low interstitials that at least meet the properties of ingot
metallurgy alloys.

8.14 . TiH  powder.

2

(courtesy of ADMA Products)

A comparison of the S–N fatigue behavior of BE and prealloyed material with cast-and-
wrought product is shown in Fig. 8.15.12

8.15 . Fatigue data scatterbands of conventional BE, low chloride BE, treated low chloride BE, and PA,

compared with wrought annealed material.

Powders can be subsequently fabricated to other product forms, such as titanium sheet,
(Fig. 8.16). Alloy sheet can be fabricated in a similar manner by adjusting the feedstock to a
mixture of titanium powder and alloying additions.
8.16 . Schematic of the process used to produce commercially pure titanium sheet at CSIRO. 19