Titanium metal particle growth through autocatalytic electroless deposition in a molten salt slurry reactor

Abstract

Near-net-shape manufacturing of titanium metal components through powder metallurgy confers various cost-saving benefits, from improved material utilisation to reduced energy consumption. Further savings can be realised by reducing the cost of the titanium metal powder that is used as feed material in powder metallurgy.

Titanium metal-powder production costs can be reduced by removing the steps currently needed (i.e. milling, vacuum arc re-melting and atomisation) to convert titanium Kroll sponge into a powder product that is suitable for use in powder metallurgy. This can be potentially achieved through a controlled metallothermic reduction of titanium tetrachloride to produce a titanium metal powder product that can be used directly in powder metallurgy.

Consequently, this thesis divides titanium metal-powder into two main categories, namely primary and secondary metal powder products.

Titanium metal-powder is classified as a primary product when it can be used directly as feed material in powder metallurgy. In contrast, a secondary metal-powder product requires extra processing steps after chemical reduction before it can be used in powder metallurgy.

To illustrate, titanium metal powder produced through plasma spheroidization is an example of a secondary metal powder product.

The metallothermic reduction reaction is known to have two dominant reaction mechanisms. These mechanisms have a characteristic product morphology that forms under specific reaction conditions. The first mechanism is responsible for the sponge-like morphology obtained from the Kroll process, while the second mechanism results in an ultrafine precipitate that has a surface area to volume ratio large enough to oxidise in air to the extent that it cannot be regarded as commercially pure.

Consequently, a primary titanium metal-powder product has to date not been realised as efforts to achieve particle growth on this ultrafine precipitate have been unsuccessful.

This thesis's main objective was to demonstrate that metal particle growth on suspended metal particles is indeed possible through a controlled metallothermic reduction in a molten salt reaction medium.

Subsequent efforts resulted in the postulation of a third reaction mechanism that would enable titanium metal particle growth. The postulated growth mechanism is electrochemical and referred to as “autocatalytic electroless deposition on suspended titanium metal particles”.

Theory development and modelling efforts indicated that the postulated growth mechanism is possible, but only in a particular and low concentration range where both reagents are present in a meta-stable equilibrium with each other in the molten chloride reaction medium. The concentration range is estimated to be in the range of parts per million for each reagent.

It was further shown that more than one product morphology is inherent in the conditions where the postulated mechanism is possible as there is no dominant reaction mechanism at such low reagent concentrations. Therefore, the metallothermic reduction reaction should be regarded as a system of reaction mechanisms at these conditions.

Experimental results substantiated the postulated growth mechanism's existence to the extent where β-titanium metal was deposited on the surface of metallised ilmenite particles.

The deposited layer was distinguishable from the substrate particle as ilmenite contains α-titanium (i.e. a hexagonally closed packed crystal system).

Therefore, controlled titanium metal particle growth is hypothetically possible through a mechanism known as “autocatalytic electroless deposition”.

However, further effort is still needed to demonstrate whether a viable primary titanium metal powder product can be produced.