A Closer Look at Titanium Processing
Milling and processing are the biggest cost factors in producing titanium. This metal is one of the most abundant in the earth, so, unlike some precious metals, its price is not about its rarity. Instead, the cost is directly related to the difficulty of dealing with the raw materials.
Over the years, significant advancements have been made in this field, but it’s generally still all based on the Kroll Process, which is founded on principles that were originally introduced back in 1938. The problem is that it is highly reactive with oxygen, nitrogen, and hydrogen in the air at elevated temperatures. In order to compensate for that, any fabrication is going to involve a complicated process.
Titanium processing can be divided into two basic parts, each with its own series of steps. Every part of this process, though, is important to produce pure titanium or ensure the strongest alloys.
Creating Sponge from the Ore
At the initial stages, this process is not even thinking about commercially viable materials yet. Instead, the recently extracted ore is used to create a sponge.
The titanium oxide ores must be reacted with chlorine in a bed of petroleum coke. This way, the oxygen will combine with the carbon in the coke and produce carbon monoxide and carbon dioxide. Once that has happened, the titanium will react with the chlorine and create the gaseous titanium tetrachloride (TiCl4).
The next step is to remove all the fine coke and titanium ore particles from the tetrachloride. After all that has been cleaned out, the TiCl4 is liquefied and sent through a distillation process to get even more impurities out.
It is then put through a second distillation process to make sure that all the impurities that can be removed are removed. This leaves us with a titanium tetrachloride that is very nearly 99.9% pure.
This pure form can be reduced with sodium or magnesium (depending on the processing method) to produce a very porous version of titanium. This is what is referred to as “sponge.”
In order to create this material, the titanium must be placed in a large, sealed vessel that is then heated to 1,000° C. It can have no contact with any air or moisture at this point, or the final metal will turn out brittle.
Finally, the sponge will be cleaned of any magnesium or sodium left in its pores so it can be crushed and filtered one more time to reclaim any leftover chlorine and other materials.
Creating Ingot from Sponge
There are still a number of steps to go after reaching the sponge stage. The titanium needs to be made into ingot before it is commercially usable.
The sponge must be put in a vacuum or argon environment and melted through either vacuum arc remelting (the most common and the most capable of producing high-performance alloys) or cold-hearth melting (generally considered better for handling recycled titanium).
This melting process will provide some usable ingots. However, they can also be processed another step to derive the mill products. These are the ingots that can be turned into bar, tube, plate, wire, and other products.
Despite the complexities, titanium processing is well established and produces high-quality, pure materials. Still, there are many factors that could adversely affect the final products, which is why great care must be employed at every step in the process to ensure the results match the expectations.