Titanium sponge: what it is and how it is produced

The raw material

Titanium is an abundant element in the earth's crust, but it does not occur free — it occurs as an oxide. The principal ores are rutile (TiO₂) and ilmenite (FeTiO₃). Ilmenite is usually upgraded into synthetic rutile.

Titanium's affinity for oxygen is very high. Reduction from oxide to metal therefore cannot be done with carbon in a single step as it is for steel; carbothermic reduction would form titanium carbide. The industrial answer is to convert to a chloride first and then reduce with a metal.

The Kroll process step by step

1. Chlorination. Rutile or synthetic rutile is reacted with chlorine gas at around 900 °C together with coke, giving titanium tetrachloride (TiCl₄). The product is a liquid at room temperature.
2. Purification. Crude TiCl₄ is separated from impurities such as iron and vanadium chlorides by fractional distillation. The purity of the final titanium is largely decided at this step.
3. Reduction. Pure TiCl₄ reacts with molten magnesium in a steel retort under an argon atmosphere (roughly 800–900 °C). Titanium deposits in metallic form, with magnesium chloride (MgCl₂) as the by-product.
4. Separation. The MgCl₂ and residual magnesium inside the resulting mass are removed by vacuum distillation. What remains is the porous structure known as titanium sponge.
5. Crushing and classification. The sponge mass is crushed, sized and separated by zone. Regions close to the retort wall generally contain more iron.

The process is a batch process and one cycle takes days. The main reason titanium is expensive compared with steel or aluminium is not ore scarcity but how energy- and time-intensive this production chain is.

Why it is porous

During reduction, titanium grows as a branching network inside the magnesium chloride. When the salt is removed under vacuum, the hollow skeleton of that network is what remains. Because the porous structure increases the surface area, sponge must be stored protected from moisture and air.

Quality classes

Titanium sponge is classified by its impurity content. The determining elements are iron, oxygen, nitrogen, carbon and chlorine. In practice Brinell hardness is also used as a quality indicator: hardness rises as impurities rise.

Where the sponge goes

Sponge is not used directly. It is pressed into an electrode together with alloying elements and recycled scrap, then melted — usually two or three times — in a vacuum arc furnace (VAR) to cast an ingot. Alloys such as Ti-6Al-4V are created at this stage.

The resulting ingot passes through forging, rolling and machining to become aircraft structural parts, engine components, medical implants and chemical industry equipment.

Alternative routes

The Hunter process uses sodium instead of magnesium and gives a purer product, but at higher cost. Direct electrochemical reduction methods such as FFC Cambridge remain at the research stage. Today almost all world production goes through the Kroll process.

When you buy

• Write element-by-element upper limits rather than a class: O, Fe and N above all.
• State the particle size range; it varies with the VAR electrode pressing method.
• Require a limit on chlorine residue.
• Stipulate air- and moisture-tight packaging.
• Ask for a certificate of analysis for every lot and, where needed, independent verification.