Experimentally Determined Ti-Al-Cl Phase Diagram at T = 150 to 400
Authors: Copland Evan , Webster Nathan 
Published: February 2020

Introduction

The titanium aluminum chlorine, Ti-Al-Cl, system encompasses a range of current industrial processes and a number of processes currently under development. The major industries and processes are: TiCl4 production via high temperature chlorination of titanium oxide ore for the pigment industry, Ziegler-Natta catalysts, β-TiCl3(AlCl3)1/3 produced by Al driven reduction of TiCl4(l) for use in polypropylene production [1],  Ti metal production via high temperature reduction of TiCl4 using metal reducing agents (Mg, Na, etc.) and Ti-Al alloy production for transport and chemical industries. Despite the importance of these processes, there are no published phase diagrams for the Ti-Al-Cl system and only limited thermodynamic data for TiCl3-AlCl3 and TiCl2-AlCl3 solution phases [2].

A number of issues make this system difficult to study. The salts compounds react with H2O, O2 and many organic compounds and they can only be handled in clean glove boxes. The salts are volatile, form complex molecular species that are non-ideal; which make it difficult to identify the molecular species and measure their partial pressures [3,8]. Titanium and aluminium have a range of oxidation states (Ti: 4+, 3+, 2+ and Al: 3+, 2+, 1+), the stability of each oxidation state depend on the local chemical environment and a series of disproportionation reactions allow conversion between ions [3,4]. The unique properties of AlCl3(s) and Al2Cl6(l) result in the formation of a range of complex solution phases with TiCl3 and TiCl2 [2,4]. Many container materials react with these salts and Ti-Alloys at temperatures above 200°C [5]. Finally, it is difficult to prepare metallographic samples containing salt and metal for characterization via scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction needed for a phase equilibria study. However, recent work has solved a number of these experimental problems allowing an initial investigation of the phase equilibria in Ti-Al-Cl system.

Experimental and Results

The experiments consisted of mixing different amounts of high purity TiCl3(s) and AlCl3(s), loading the mixtures into a container, where the “sample” is the salt mixture plus the container. Multiple samples were loaded into wells a metal block that was sealed and a controlled over pressure of static high purity argon was supplied to limit vapour loss. The sealed metal block was loaded into the isothermal zone of a vertical tube furnace for annealing. (All sample preparation, loading and unloading was done inside a clean glove box.) To study salt / metal equilibrium pure-aluminium, pure-titanium and various Ti-Alloy containers were used. To study the equilibrium between the TiCl3-AlCl3 and TiCl2-AlCl3 pseudo binaries an Al2O3 container was used together with a small amount of Al(s).

The samples were equilibrated at temperatures between 150 and 400°C for 50 ~ 200hr and quenched. After unloading the samples were mounted in epoxy and prepared for SEM and EPMA. All metallographic sample preparation and carbon coating was done inside a clean glove box. The prepared samples were transferred into the SEM / EPMA without exposure to air. Similarly, XRD sample preparation was performed inside a clean glove box, with powders mounted into a bespoke sample holder and sealed from exposure to air and moisture using Mylar film.

This worked focused on determining the solubility range of nominally binary salt phases, TiCl3(s), TiCl2(s) and AlCl3(s) and the stability of ternary salts like TiAlCl5(s) [7] and Ti(AlCl4)2(s,l) [2-4,6] and determining phase compositions in three phase fields between: 1) TiCl3-AlCl3 and TiCl2-AlCl3 and 2) TiCl2-AlCl3 and Ti-Al. Results from initial experiments are reported as a number of isothermal sections between 150 and 400°C.

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