Development of the Fray-Farthing-Chen Cambridge Process: Towards the Sustainable Production of Titanium and Its Alloys
Development of the Fray-Farthing-Chen Cambridge Process: Towards the Sustainable Production of Titanium and Its Alloys
DI HU 0
ALEKSEI DOLGANOV 0
MINGCHAN MA 0
BIYASH BHATTACHARYA 0
MATTHEW T. BISHOP 0
GEORGE Z. CHEN 0
0 1.-Department of Chemical and Environmental Engineering, Energy Engineering Research Group, Faculty of Science and Engineering, University of Nottingham Ningbo China , Ningbo 315100 , China. 2.-International Doctoral Innovation Centre, University of Nottingham Ningbo China , Ningbo 315100 , China. 3.-Department of Chemical and Environmental Engineering, Advanced Materials Research Group, Faculty of Engineering, University of Nottingham , Nottingham NG7 2RD, UK. 4.-
The Kroll process has been employed for titanium extraction since the 1950s. It is a labour and energy intensive multi-step semi-batch process. The postextraction processes for making the raw titanium into alloys and products are also excessive, including multiple remelting steps. Invented in the late 1990s, the Fray-Farthing-Chen (FFC) Cambridge process extracts titanium from solid oxides at lower energy consumption via electrochemical reduction in molten salts. Its ability to produce alloys and powders, while retaining the cathode shape also promises energy and material efficient manufacturing. Focusing on titanium and its alloys, this article reviews the recent development of the FFC-Cambridge process in two aspects, (1) resource and process sustainability and (2) advanced post-extraction processing.
Titanium and its alloys exhibit excellent properties;
including high specific strength, biocompatibility, and
resistance to extreme conditions.1 However, their high
costs have placed them in niche markets such as
aerospace, medical implants, and offshore
applications.2 Affordable production of titanium and its alloys
has been pursued since the Kroll process was first
commercialised in the early 1950s.3 The throughput
from the Kroll Process for titanium extraction has
been increased to some extent compared to that of the
formerly used Hunter Process,4 and several
innovations have been applied to increase the efficiency.5
Nonetheless, this process is still an inherently
laborand energy-intensive (energy consumption: ca. 50
kWh/kg Ti6), environmentally unfriendly (> 2 kg CO2
per kg Ti) and a semi-batch process. Thus, there have
been continuous research and industrial efforts to
improve or replace the Kroll process7–22 as
summarised in the supplementary Table S1.
In addition to extraction, the fabrication of
titanium into its alloys and final products has
many obstacles to overcome, owing to titanium
and its alloys having high affinity to oxygen and
poor machinability.23 For example, the cost of
post-extraction processes (i.e., from arc melting
to fabrication) accounts for 62% of the total cost
for producing a 2.54 cm thick titanium alloy
plate as illustrated in supplementary Fig. S1.8
Due to the relatively low density of titanium,
some alloying elements tend to segregate and
multi-step remelting is necessary to achieve full
homogenisation of the final alloys at high costs.
Furthermore, the fabrication of titanium alloys
in complex shapes increases both the waste and
cost and calls for creative manufacturing
techniques such as additive manufacturing,24–26
nearnet-shape casting,27 spark plasma sintering
(SPS),28 and metal injection moulding.29 Most of
these advanced techniques are based on powder
metallurgy,30,31 and powder production requires
sophisticated pulverisation and spheroidisation
processes, which in turn adds extra costs to
the final products.
For a sustainable and affordable titanium
industry, process evolution has become necessary, which
may come from two directions:32 (1) resource and
process sustainable extraction of titanium and (2)
advanced manufacturing of titanium alloys and
their final products. This paper provides an
overview of one of the promising extractive
electrometallurgy techniques, i.e., the Fray-Farthing-Chen
(FFC) Cambridge process, focusing on the aspects
related to a sustainable titanium industry.
CONCEPT OF THE FFC-CAMBRIDGE
The FFC-Cambridge process was first established
on the electro-reduction of TiO2 to pure titanium in
molten calcium chloride (CaCl2),15 and now it has
been applied to reduce a variety of metal
compounds, particularly oxides, to their respective
metals, alloys and intermetallic compounds.6,33–48
In the process, the preformed metal compound (e.g.,
pellet of TiO2) is attached on a cathode which is then
electrolysed against a suitable anode under a cell
voltage that is high enough to ionise the oxygen in
the metal compound without decomposing the
electrolyte (e.g., molten CaCl2). The FFC-Cambridge
process can be represented by the following
reactions where M represents a metal.8
MOxðsÞ ¼ MðsÞ þ x=2O2ðgÞ ðusing an inert anodeÞ
nMOxðsÞ þ xCðsÞ ¼ nMðsÞ þ xCOnðgÞ
ðusing a carbon anode; n ¼ 1 or 2Þ
MOxðsÞ þ 2xe
¼ MðsÞ þ xO2
¼ x=2O2ðgÞ þ 2xe ðusing an inert anodeÞ
þ CðsÞ ¼ COnðgÞ þ 2ne
ðusing a carbon anode; n ¼ 1 or 2Þ
Figure 1a illustrates schematically the
FFC-Cambridge process49 and Fig. 1b shows the typical
porous and interconnected microstructure of the as
produced FFC titanium. This morphology results
from the reconstructive phase transformations
during the complex kinetic pathway for deoxidation of
TiO2, and the in situ sintering of the formed
titanium fine particles,50,51 which can be pulverised
to powders for further treatments or applications.52
TOWARDS HIGHER EFFICIENCY FOR
During electrolysis in molten CaCl2, once TiO2 is
partially reduced to TiOx (1 < x £ 2), the discharged
oxygen anions (O2 ) and nearby calcium cations
(Ca2+) tend to combine with it, either chemically or
electrochemically, to form perovskites (CarTiOx, x/
r ‡ 2).42,50 This intermediate step is named as
in situ perovskitisation.42 The suggested pathway
is given in supplementary materials (SM).50
The problem of in situ perovskitisation is that it
reduces the porosity and slows O2 transport
through the pores between the oxide particles.42,53
It is believed that CaO plays an important role in
this process.54,55 A low O2 concentration may force
the oxidation of chloride ions (Cl ) to chlorine gas on
the anode during initial electrolysis stages.50,54
Thus, an appropriate initial CaO concentration
could mitigate the transport limitation of O2 ions
and increase the current density.54 Moreover, an
intrinsic barrier to the electro-reduction of TiO2 to
titanium persists, i.e., the oxide-to-metal molar
volume ratio which is known as the
Pilling-Bedworth ratio (PBR) (cf. SM).56 This ratio is commonly
used for analysis of metal oxidation in hot air,
whilst it should also help understand the reversed
process, i.e., electro-reduction of metal oxides.57
Since these kinetic barriers persist in the
FFCCambridge process, despite an acceptable level of
energy consumption (ca. 33 kWh/kg Ti versus ca. 50
kWh/kg Ti for the Kroll process),57 the current
efficiency is still low (e.g., 15% to achieve
£ 3000 ppm oxygen in Ti)42,54,57 when compared
with that for chromium (Cr) extraction
(e.g., > 70%
@ < 2000 ppm oxygen in Cr)
49 and zirconium (Zr)
extraction (e.g., 45% @ 1800 ppm oxygen in Zr).58
To cope with these issues, some improvements in
the FFC-Cambridge process have been made,42,54,57
as elaborated below.
1. As previously described, a sufficient amount of
O2 (which can be in the form of CaO) is
required during initial electrolysis stages for
electro-reduction of TiO2.54 Also, the releasing
rate of O2 during different electrolysis stages
must be carefully controlled to avoid local CaO
saturation at the cathode, which would slow or
even stall the electrolysis.54,55 Therefore, a
combination electrolyte, CaCl2 + 2 mol.% CaO,
has been utilised.54,59 It was reported that, for
16 h of electrolysis, the titanium samples with
2000 ppm to 5000 ppm oxygen were produced
with a current efficiency ranging from 10% to
2. In order to avoid in situ perovskitisation, ex situ
perovskitisation has been introduced.42 This
process is carried out by pre-mixing and
sintering TiO2 and CaO/calcium hydroxide (Ca(OH)2)
to form the perovskite precursor (e.g., calcium
titanate (CaTiO3)). It was found that direct
electro-reduction of the formed CaTiO3 (10 h of
electrolysis, 2100 ppm oxygen in titanium)42
was significantly faster than that of the TiO2
precursor (16 h of electrolysis, 2400 ppm oxygen
in titanium).54 However, this process releases
CaO to the melt, which requires purification
steps for industrial applications.
3. Despite the intrinsic barrier of PBR, titanium can
dissolve oxygen to form solid solutions. Although
the oxygen diffusion rate in titanium at a given
temperature is fixed, the removal of dissolved
oxygen can be accelerated by increasing the
porosity of the TiO2 precursor, which will enlarge the
titanium/molten CaCl2 interface.57 The increased
porosity can also mitigate the local saturation of
O2 ions in the pores of TiO2 and its subsequent
reactions to form CaO and perovskites (cf.
supplementary RS1 to RS5). Ammonium bicarbonate
(NH4HCO3) was utilised as a cheap and recyclable
fugitive pore forming agent to fabricate
highporosity TiO2 precursors.57 Nevertheless, the
increased porosity and subsequent high surface area
of the electrolytic product can increase the oxygen
content during washing to remove the solidified
salt. A two-step procedure was therefore
introduced, including a high voltage reduction step (e.g.,
electrolysis at 3.2 V for 3 h at 850 C) and a low
voltage in situ sintering step (e.g., electrolysis at
2.6 V for 3 h at 850 C).57 With this improvement,
the energy consumption and current efficiency for
extracting titanium with 1900 ppm oxygen were
21.5 kWh/kg and 32.3%, respectively.57
These improvements are summarised in
supplementary Table S2. A remaining challenge to the
FFC-Cambridge process for titanium extraction is
the lower current efficiency. This can be ascribed
partly to electronic conduction in the molten salt,
due to dissolved calcium metal in CaCl2 at less than
unit activity54,55,60–62 and the presence of
redoxactive impurities,63 although the latter can be
largely removed by pre-electrolysis. Further
understanding is still required to overcome these
obstacles and it is anticipated that by careful control of
the electrolysis conditions, the energy consumption
and current efficiency should reach 12.5 kWh/kg
and 50% to achieve 2000 ppm oxygen.60
TOWARDS RESOURCES SUSTAINABILITY
A More Sustainable Feedstock
For titanium production, the FFC-Cambridge
process commonly uses pigment grade TiO2 as the
feedstock, which is safer to handle and transport
than TiCl4 used by the Kroll process. Although the
price of pigment grade TiO2 is typically double of
that of TiCl4, it only needs 1.66 kg of TiO2 to
produce 1 kg of titanium whereas 4 kg of TiCl4 is
required for the same yield. However, pigment
grade TiO2 is produced by either the chloride or
sulphate process (cf. supplementary RS8 to RS9 and
RS10 to RS12), and both cause environmental
In particular, for TiCl4 production, the
carbochlorination process uses hazardous chemicals and
substantial quantities of energy, and emits carbon
oxides. It also requires high-grade natural rutile
which is rapidly depleting.64 Consequently, the
exploitation of a more sustainable and low-cost
resource for the FFC-Cambridge process has been
In 2006, it was demonstrated that titanium, with
< 3000 ppm oxygen and low metallic impurities,
can be extracted directly from titania dust (collected
from the floor near the rotary kiln in a titania
processing plant) and metatitanic acid (in solid
state), via the FFC-Cambridge process.43 Utilisation
of these low-cost feedstocks can reduce the
environmental impact, and are worth further research and
Moreover, titanium-rich slag,43,65 synthetic and
natural ilmenite ore, containing Fe, Si, Mg, Ca, Mn,
and Al,45,46,66 were also successfully reduced to
ferrotitanium alloys. The supplementary Fig. S3a
and S3b show the microstructures of the ground
natural ilmenite feedstock before and after
More recently, low-cost and novel titanium alloys
were produced directly from either synthetic rutile
(i.e., rutile produced from ilmenite, with a transition
metal element concentration of 3.7% and aluminium
content below 1%),21,67 or naturally occurring rutile
ore (beach sand)20 via the FFC-Cambridge process
(see supplementary Fig. S3c and S3d).67 The obtained
titanium alloy powder was spheroidised, and
fabricated into a billet via hot isostatic pressing (HIP) and
subjected to monotonic tensile testing.20 The test
result revealed that the ultimate tensile strength of
this material is close to that of Ti-6Al-4V.20
In summary, the FFC-Cambridge process can use
various cheap and recycled feedstocks to produce
titanium and specific alloys, making it a more
resource sustainable and environmentally friendly
process. These alternative and low-cost feedstocks
currently find little or no applications, but feeding
them into the FFC-Cambridge process surely
increases their values.
Regeneration and Cathodic Protection of
Titanium and Its Alloys
When subjected to hot processing in air, titanium
and its alloys can form a layer of solid oxygen solution,
i.e., the alpha-case underneath the surface oxide scale.
The alpha-case is brittle and can severely deteriorate
the performances of titanium components.68 Current
methods for removing this alpha-case include pickling
in acid, and grinding, which inevitably change the
original dimensions of the component, and add costs
and environmental burdens to titanium
manufacturing. It was demonstrated that under almost identical
conditions for electro-reduction of TiO2 (cf. Figs. 1a
and 2b) but replacing TiO2 by the alpha-case covered
titanium or its alloy samples, the alpha-case can be
effectively converted to a low-oxygen metal phase by
the FFC-Cambridge process as shown in Fig. 2a.69
This work implies an alternative, simpler and more
material efficient way to either regenerate spent
titanium components without affecting their
dimensions, or recycle titanium scraps.69
Recently, this cathodic refining concept was
adopted for cathodic protection of titanium alloys
from being oxidised in hot air using molten salt
fluxes as the electrolyte.70 In this work, the
titanium alloy was the solid or liquid cathode, coupled
with an oxygen-evolving anode of iridium (Ir) which
is inert under these working conditions.70 Further
development of this method has led to a novel idea
of laser welding titanium alloy in air without using
a protective gas.71
TOWARDS THE PROCESSES
A Carbon-Free Titanium Extraction Process
The use of carbon based anodes in the
FFCCambridge process leads to evolution of carbon
oxides (cf. R5) and carbon dioxides (CO2) can react
with O2 ions in the molten salt to form carbonate
ions (CO32 ) which can then transfer to, and be
reduced to carbon at the cathode.22,54,72 These
parasitic reactions lower the current efficiency and
cause contamination to the cathodic products via,
e.g., carbide formation.22 When evolving gases, the
carbon anode may also release carbon debris that
float on the molten salt surface and potentially short
circuit the cell, further impairing the current
efficiency.59 Thus, by replacing carbon with an inert
material, pure oxygen can evolve as the only anodic
off-gas (cf. R4), and current and energy efficiency
and product quality can all improve.
Among the various metallic and ceramic
materials tested, including cermets, doped tin oxide
(SnO2)73,74 and the solid solution of CaTiO3 and
calcium ruthenate (CaRuO3) (i.e.,
CaTixRu1 xO3)75,76 are reviewed here as the candidates
of a proper inert anode material for the
FFCCambridge process. The antimony oxide (Sb2O3)
(electrical conductivity enhancer) and copper oxide
(CuO) (densification enhancer) doped SnO277 was
initially tested for making an inert anode in
cryolite-alumina melts.78 Using the same anode,
successful reduction of tantalum pentoxide (Ta2O5) to
tantalum metal via the FFC-Cambridge process was
achieved, although tin contamination was observed
in the cathodic product.74 The use of the doped SnO2
anode can result in improved current efficiency and
a cleaner electrolyte when compared to that of a
carbon anode.74 However, an insulating layer of
calcium stannate (CaSnO3) formed on the anode
surface after 24 h electrolysis, which ultimately
terminated the operation.22,73,74
CaRuO3 was tested as the inert anode material to
evolve pure oxygen during electro-reduction of TiO2
and proven highly stable in chloride melts (see
supplementary Fig. S4a and S4b).59,75,76 However,
CaRuO3 alone is too expensive to use, whilst CaTiO3
is too resistive. Therefore, the cheaper CaTiO3 and
the highly conductive CaRuO3 were utilised to form
the solid solution of CaTiO3-CaRuO3, i.e.,
CaTixRu1 xO3, which was then made into the inert anode
for titanium and titanium-nickel (Ti-Ni) alloy
production.41,76 The CaTixRu1 xO3 inert anodes
exhibited no noticeable erosion/corrosion or formation of
an insulating layer after use (see supplementary
Fig. S4c and S4d). The corrosion rate of this inert
anode was calculated as only 0.0015 g/cm2/h in
molten CaCl2 containing 1 wt.% CaO.76
By combining an inert anode with the optimised
processing conditions, within a timeframe of
1416 h, the energy consumption and current efficiency
for titanium extraction via the FFC-Cambridge
process can be ca. 17 kWh/kg Ti and ca. 40%,
Additionally, the Solid Oxide Membrane (SOM)
process16 has also shown the ability to eliminate the
carbon related issues for titanium alloy production17
An Affordable Alloying Process
The FFC-Cambridge process can be fed with
mixed metal oxides at a predefined ratio to produce
an alloy in one step. Its simplicity over the
conventional process for Ti-Ni fabrication is exemplified in
supplementary Fig. S5.6
Due to its solid state reactions, the
FFC-Cambridge process can make alloys that are either
impossible, or challenging to make by conventional
processes, such as those with alloying elements with
vastly mismatching densities, melting points and
vapour pressures.60,79 The titanium-tungsten
(TiW) alloy can make effective implants because of
their low cytotoxicity, superior wear resistance and
strength, and relatively low elastic
modulus.34,35,80,81 However, fabrication of Ti-W alloys is
not viable by melt processing, as the melting point
of tungsten (3422 C) is higher than the boiling point
of titanium (3287 C). Additionally, tungsten has a
huge difference in density to titanium (19,250 kg/m3
versus 4505 kg/m3) which can cause segregation of
the alloying elements during melting and the
following liquid processing. Although powder
metallurgy has been used,80–82 in order to overcome the
extremely sluggish diffusion kinetics of tungsten
during consolidation, fine titanium and tungsten
particles have to be used which will inevitably
increase the oxygen content. To address these
issues, the FFC-Cambridge process has therefore
been successfully employed to fabricate Ti-W alloys
in one step.34,35,81
Since its initial conception, the FFC-Cambridge
process has been used to fabricate numerous
titanium alloys, such as Ti-6Al-4V,36 Ni-35Ti-15Hf,37
Ti-10V-2Fe-3Al,20,38 Ti-W34,35,81 Ti-Ni,6,39–41
TiFe,42–46 and Ti-Mo.20,47 It was also noted that the
a- and b-phases in the Ti-Zr alloys could be easily
tuned by controlling the electrolysis duration, which
adjusts the oxygen content in the Ti-Zr alloys.48
Most recently, the high-entropy alloys (e.g.,
TiNbTaZr and TiNbTaZrHf) have been fabricated
using the FFC-Cambridge process, which further
demonstrates its capabilities for alloy making.83
INCORPORATION WITH THE ADVANCED
One of the key areas of recent development within
the titanium industry is powder metallurgy (e.g., 3D
printing25,26 and near-net-shape manufacturing31).
Since titanium and its alloys produced from the
FFC-Cambridge process are typically in a porous
structure (see Fig. 1b), they can be pulverised and
used as the feedstock for powder metallurgy. This
potential was investigated by MetalysisTM
(Rotherham, UK) through direct grinding of the electrolytic
titanium, hydriding-grinding-dehydriding, and
fusion and gas atomisation.52
Recently, MetalysisTM also attempted direct
electro-reduction of natural rutile powders to titanium
powders.20 Following plasma spheroidisation (see
Fig. 3a) and 3D printing of the electrolytic powders,
affordable titanium components were made.20,21
Figure 3b shows a 3D printed aerospace turbine
guide vane using MetalysisTM spherical titanium
powders.20,21 The workability of the electrolytic
titanium powders was also evaluated using
different shaping techniques, e.g., HIP (Fig. 3c), and SPS
Another feature of the FFC-Cambridge process is
that it proceeds in the solid state, and the
electrolytic products retain the shape closely to the
original shape of the oxide precursors, although
shrinkage would occur.36,58,84 By taking advantage
of this unique ability, different shapes of Ti-6Al-4V
components (such as hollow spheres, hollow
miniature golf club heads, and cylindrical cups) were
produced from their slip-cast oxide precursors.36
Figure 4 displays the photographs of different
nearnet-shape products from the FFC-Cambridge
The versatility of the FFC-Cambridge process for
near-net-shape production has been further
demonstrated by fabricating hierarchically structured
titanium foams for tissue scaffold applications,84
and Zr and Zr-2.5Nb tubes for nuclear reactor
Understanding of the mechanisms and kinetic
barriers of the FFC-Cambridge process has
progressed steadily in recent years, leading to the
production of titanium with < 2000 ppm oxygen at
32.3% in currently efficiency and 21.5 kWh/kg in
energy consumption. The process has the capability
to combine different metallurgical steps, including
metal extraction, alloying, and shaping, into one
step. This has been shown to dramatically improve
almost every aspect for sustainable and affordable
production of titanium and its alloys.
Regarding resource sustainability, the process
can handle different lower cost feedstocks, recycle
titanium wastes and offer cathodic protection of
titanium artefacts when subjecting these materials
to hot processing in air. Process sustainability may
be achieved via replacing the carbon anode by an
inert anode. Different materials have been
evaluated, showing CaTiO3-CaRuO3 to be most promising
in terms of service life and cost. In addition, using
the inert anode has also improved the current
efficiency and product purity, improving the process
Products from the process can be powdery or of a
similar shape as the oxide precursor. Subjected to
spheroidisation, the powder can be fed into 3D
printing, SPS, and HIP. The shape retention ability
has enabled direct conversion of metal oxide
precursors with complex shapes into final titanium
alloy components, i.e., near-net-shape production.
It should be pointed out that, like many
discoveries and inventions, the laboratory research that
led to the FFC-Cambridge process has been based
on, and benefited from many past research and
industrial achievements and failures.85–93 Now, the
FFC-Cambridge process has been in industrial trial
for over 16 years,94,95 and the developments are
steady and promising toward a bright future (cf.
MetalysisTM and GLABATTM in SM).
This work was supported by the Engineering and
Physical Sciences Research Council (grant number
EP/J000582/1, EP/F026412/1, EP/G037345/1, EP/
L016362/1); Zhejiang Provincial Applied Research
Programme for Commonweal Technology
, Ningbo Municipal Government (3315
Plan, 2014A35001-1, 2016A610114), the
International Doctoral Innovation Centre, Ningbo
Education Bureau, Ningbo Science and Technology
Bureau, and the University of Nottingham.
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The online version of this article (https://doi.org/
supplementary material, which is available to authorized
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