Electrolytic reduction of mixed solid oxides in molten salts for energy efficient production of the TiNi alloy

Abstract

Direct electrochemical reduction of mixed TiO 2 and NiO powders to TiNi alloy has been successfully demonstrated in molten CaCl 2 at 900℃ by constant voltage electrolysis. The electrolysis energy consumption was as low as 23.4 kWh/kg-TiNi, although the current efficiency was 20.5% in the preliminary experiments. During the process, NiO was first reduced to Ni at high speed, accompanied by TiO 2 being perovskitized to CaTiO 3−x which was gradually reduced to Ni 3 Ti and TiNi, assisted by the depolarization of the preformed Ni. The cell voltage for preparation of the TiNi alloy was lower than that for Ti. Adjusting the cell voltage not only affected the reduction speed, but also offered a convenient access to the preparation of the nickel/perovskite composite.Fast and energy efficient technologies for material production are essential for the sustainable development of human society. The TiNi alloy is an important intelligent material [1] , having high abrasive resistance, superior elasticity and good shape memory. It is usually prepared by either of two methods: high temperature melting of mixed pure metals [2] or powder metallurgy [3] ( Fig. 1(a)). Both methods require high vacuum conditions supported by special equipment, expensive pure titanium and nickel as the feed, and a large amount of energy input. Currently, titanium metal is produced by the Kroll process ( Fig. 1(a)) which is a high temperature and batch method, involving complicated multistep operations and high energy consumption (~50 kWh/kg-Ti). The process uses the toxic chlorine, although the by-product magnesium chloride may be recycled through molten salt electrolysis. The production of nickel metal also involves electrolysis and is challenged by serious sulfuric acid pollution, leading to high energy consumption (8-10 kg Ce/kg Ni). Although titanium and its alloys have excellent material properties and are abundant in resource (titanium is the ninth most abundant element in the earth's crust), the high cost of production constrains directly its wider applications. Consequently, a novel production approach with low cost and low environmental impact is highly desirable.In 2000, Chen et al. [4] reported a new process in which titanium can be produced by direct electrochemical reduction of solid titania in molten salts. This new process, which is called the FFC Cambridge Process and has attracted international attention [5][6][7][8] , has also been successfully used for preparation of other metals [9][10][11][12][13][14] . In this laboratory, research has been carried out to increase the speed of the reduction of titania [15] , to reduce the insulator silica for the extraction of pure silicon [16] , and to prepare, by electrolysis, the very active heavy rare earth metals that find much more resources in China [17] . In comparison with traditional methods, the new method deoxidizes solid metal oxide to the metal in one operational step. It is much shorter in process and reduces significantly the energy consumption and environmental impact, thus offeri...

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

“…6,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] In the process, the preformed metal compound (e.g., pellet of TiO 2 ) 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 CaCl 2 ). The FFC-Cambridge process can be represented by the following reactions where M represents a metal.…”

“…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 (Ti-W) alloy can make effective implants because of their low cytotoxicity, superior wear resistance and strength, and relatively low elastic modulus.…”

“…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-W 34,35,81 Ti-Ni, 6,[39][40][41] TiFe, [42][43][44][45][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.…”

Production of Ti–W Alloys from Mixed Oxide Precursors via the FFC Cambridge Process

“…There has been work published on the Fe-Ti, Ni-Ti and Ti-Mo systems; Ma et al [5] reduced 2 g pellets of natural and synthetic ilmenite (FeTiO 3 ) to FeTi in 4 h, however the pellets were encased in a molybdenum mesh, effectively increasing the electro-active area of the pellet which accounts for the short reduction time. Zhu et al [6] reduced NiO-TiO 2 sintered precursors to form Ni-Ti with low quantities of Ni 3 Ti in 8…”

Electrochemical Preparation of Porous Ti–13Zr–13Nb Alloy and It's Corrosion Behavior in Ringer's Solution

“…For example, TiZr 18) , TiNbNi 19) as well as a biomedical alloy, Ti-29Nb-23Ta-4.6Zr (TNTZ) 20) with very low oxygen content were prepared by the method. Compared with other methods, the direct and indirect molten-salt electrochemical reduction methods only contain one step without merging the metal extraction and melting process, resulting in a lower energy consumption, i.e., 23.5 kWh/kg-TiNi alloy 21) and possibly low cost. There is rare report on the investigation of the corrosion behavior of the alloys, especially the as-prepared porous alloys in the potential application media.…”