Development of the Fray-Farthing-Chen Cambridge Process: Towards the Sustainable Production of Titanium and Its Alloys

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 post-extraction 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. Electronic supplementary material The online version of this article (10.1007/s11837-017-2664-4) contains supplementary material, which is available to authorized users.


Non-electrolytic process Products
The innovations in the Kroll Process itself [4] (i) Fluidized bed chlorination; (ii) increased batch size; (iii) combined process technology; (vi) sponge handling and evaluation techniques; (v) development of energy-efficient cells Sponge Preform Reduction Process (PRP) [5] Reduction of TiO2 by Ca vapour Powder compact Mechanochemical Process [6] Mechanical energy activated chemical reactions and structural changes. (i) Ambient temperature CaH2 reduction of TiO2; (ii) ambient temperature Mg reduction of TiCl4.

Powder
Armstrong Process [7] Liquid Na reduction of TiCl4 vapor Powder
The pathway of in situ perovskitisation [17]: The electrochemical reduction of TiO2 involves the consumption of CaO during the early stages, such as the incorporation of calcium ions into the oxide precursor (cf. RS1 to RS4) meanwhile discharging oxygen ions at the anode, and production of CaO during the late stages, such as decomposition of calcium titanite (CaTi2O4) to titanium monoxide (TiO) and CaO (cf. RS6).

Pilling-Bedworth Ratio (PBR)
Where the subscripts o and m represent the oxide and metal, respectively, V the molar volume, M the molar mass, ρ the density, and n the number of metal atoms in the oxide formula, e.g. n = 1 for TiO2, n = 2 for aluminium oxide (Al2O3).

Fig. S2.
An illustration shows the influence of PBR on electrochemical reduction of Cr2O3, TiO2, and MgO, respectively (reproduced from [18] with permission).
As illustrated in Fig. S2, when the PBR is greater than unity, i.e. Vo > Vm, a porous metal layer can be generated from the removal of oxygen. This porous layer is essential for the electrochemical reduction process as it could allow molten salt to access the underlying metal oxide. Thus, the reductiongenerated oxygen ions (O 2-) could be removed through the electrolyte in the pores of the metal layer, whist the solid state reduction of metal oxide to its metal could proceed (cf. chromium oxide (Cr2O3) to Cr as shown in Fig. S2, where PBR = 2). However, when the PBR is close to unity, the formed metal layer becomes less porous leading to a relatively low electrochemical reduction speed (cf. TiO2 to titanium in Fig. S2, where PBR = 1.33 based on TiO to titanium). When the PBR is much less than unity, deoxidation process will stall (cf. MgO to Mg in Fig. S2, where PBR = 0.81). It should be mentioned here, the final step in the process of electrochemical reduction of TiO2 is TiO to titanium (as described in RS7), and thereby PBR for TiO to titanium is used to evaluate the kinetic difficulty level presented in the process. Inert anode, 14 to 16 hours of electrolysis, and optimised electrolysis conditions [21]. N/A 17 40% Chloride process (using purer ore or rutile as the feedstock): TiO2 + C + 2Cl2 = TiCl4 + CO2 RS8a TiO2 + 2C + 2Cl2 = TiCl4 + 2CO RS8b TiCl4 + O2 = TiO2 + 2Cl2 RS9 Sulphate process (using ilmenite, i.e. FeTiO3, as the feedstock): FeTiO3 + 2H2SO4 = FeSO4 + TiOSO4 + 2H2O RS10 TiOSO4 + (n+1)H2O = TiO2·nH2O + H2SO4 RS11 TiO2·nH2O = TiO2 + nH2O RS12 Fig. S3. Scanning electron microscopic images of (a) ground natural ilmenite feedstock, (b) its electrolysis product (electrolysis at 3.0 V, 900 °C, for 12 hours) (reprinted from [22] with permission), (c) synthetic rutile feedstock, and (d) backscattered image of consolidated (via spark plasma sintering) product from electro-reduction of synthetic rutile (with oxygen content ca. 3500 ppm) (reprinted from [23] with permission).

Commercial Progresses
Since 2001, Metalysis TM has been focusing on the commercialisation of the FFC-Cambridge process for titanium and other metals/alloys production. The titanium production capacity has been steadily increased from grams (  [26]. The GEN 5 facility will be the multiple modules of GEN 4, whose production capability can be easily adjusted from 100s to 1,000s tonnes according to different demands [26]. On the other hand, the FFC-Cambridge process has also been commercialised for production of silicon nanowire based negative electrode materials for Lithium-ion batteries by GLABAT TM (a Chinese stateowned automotive battery research institute) [27].

FFC-Cambridge process
Conventional process