Reaction-Mediator-Based Chlorination for the Recycling of Titanium Metal Scrap Utilizing Chloride Waste

Abstract

In this study, a novel chlorination technique for metallic titanium (Ti) was devised in order to establish a recycling process that uses both Ti metal scrap and iron chloride (FeCl x ) waste, and its feasibility was demonstrated. Direct reaction between Ti and FeCl x has drawbacks such as slow kinetics of Ti chlorination and high volatilization of FeCl x . To overcome these, the authors proposed a chlorination technique utilizing a reaction mediator in molten salt. Based on thermodynamic analyses of lanthanoid chlorides, some fundamental experiments were carried out with samarium trichloride (SmCl 3 ) as a reaction mediator. It was experimentally demonstrated that SmCl 3 in molten magnesium chloride (MgCl 2 ) can smoothly chlorinate Ti metal into gaseous titanium tetrachloride (TiCl 4 ), and the by-product SmCl 2 in the molten salt can be regenerated into SmCl 3 by FeCl 2 . Thus, SmCl 3 in a molten salt works efficiently as a reaction mediator, and the newly proposed chlorination technique has the potential to make the Ti recycling process more efficient and environmentally friendly.

Chlorination-Volatilization Behavior of Titanium Metal Scraps during Recycling Using Reaction-Mediating Molten Salt

“…According to the literature, 30) the melting point of SmCl 2 is 1132 K, and its Gibbs energy of fusion has small positive value at 1100 K: (2) = −137 kJ at 1100 K Consequently, even in a molten salt system, Ti in scrap is po- Fig. 3 Chemical potential diagrams at 1100 K for the (a) Ti-Sm-Cl system, 17,27) (b) Fe-Sm-Cl system, 17,27,28) (c) Ni-Sm-Cl system, 27,29) and (d) Cr-Sm-Cl system. 27) (a) Potential point α represents TiCl 4 /SmCl 2 /SmCl 3 eq., (b) potential point β represents Fe/SmCl 2 /SmCl 3 eq., (c) potential point γ represents Ni/SmCl 2 /SmCl 3 eq., and (d) potential point δ represents CrCl 2 /SmCl 2 /SmCl 3 eq.…”

“…= 950 K) as a reaction-mediating molten salt. [16][17][18][19] However, these previous studies used commercially pure Ti rod as the feed material. The present study therefore aims to test the feasibility of using this reaction-mediator-based chlorination technique to recycle off-grade Ti sponge and Ti-6Al-4V alloy.…”

“…16,17) In this technique, which is shown in Fig. 2(b), the chlorination of Ti scrap by FeCl x waste can be separated into two distinct reactions that occur in the molten salt solvent: (A) the chlorination of Ti by a reaction mediator (RCl n ) that generates TiCl 4 gas and a subchloride of the reaction mediator (RCl n-δ ); and (B) the regeneration of the reaction mediator by FeCl x , with solid Fe being obtained as a by-product.…”

“…The present study therefore aims to test the feasibility of using this reaction-mediator-based chlorination technique to recycle off-grade Ti sponge and Ti-6Al-4V alloy. Particular focus is given to understanding the chlorination behavior of these materials through thermodynamic analyses and funda- 16,17) In this molten salt, Ti is chlorinated into TiCl 4 gas by reacting with a reaction mediator (RCl n ) (reaction (A)), with the reaction mediator then regenerated from its subchloride by FeCl x (reaction (B)) . In principle, TiCl 4 can be extracted ef ciently from Ti scrap because the deposition of Fe on the Ti surface is prevented.…”

“…[13][14][15][16][17][18][19] Recovery of Ti in the form of TiCl 4 is advantageous in terms of providing better control over the impurity level, and is therefore well suited to recycling low-grade Ti scrap. Furthermore, this process helps to minimize the problems associated with chloride wastes, 20) which are commonly generated and discarded in both the Koll process and the chloride process used for producing TiO 2 pigment.…”