The Manufacturing Process
Titanium is produced using the Kroll process. The steps involved include extraction, purification, sponge production, alloy creation, and forming and shaping. In the United States, many manufacturers specialize in different phases of this production. For example, there are manufacturers that just make the sponge, others that only melt and create the alloy, and still others that produce the final products. Currently, no single manufacturer completes all of these steps.
At the start of production, the manufacturer receives titanium concentrates from mines. While rutile can be used in its natural form, ilmenite is processed to remove the iron so that it contains at least 85% titanium dioxide. These materials are put in a fluidized-bed reactor along with chlorine gas and carbon. The material is heated to 1,652°F (900°C) and the subsequent chemical reaction results in the creation of impure titanium tetrachloride (TiCl4) and carbon monoxide. Impurities are a result of the fact that pure titanium dioxide is not used at the start. Therefore the various unwanted metal chlorides that are produced must be removed.
The reacted metal is put into large distillation tanks and heated. During this step, the impurities are separated using fractional distillation and precipitation. This action removes metal chlorides including those of iron, vanadium, zirconium, silicon, and magnesium.
Production of the sponge
Next, the purified titanium tetrachloride is transferred as a liquid to a stainless steel reactor vessel. Magnesium is then added and the container is heated to about 2,012°F (1,100°C). Argon is pumped into the container so that air will be removed and contamination with oxygen or nitrogen is prevented. The magnesium reacts with the chlorine producing liquid magnesium chloride. This leaves pure titanium solid since the melting point of titanium is higher than that of the reaction.
The titanium solid is removed from the reactor by boring and then treated with water and hydrochloric acid to remove excess magnesium and magnesium chloride. The resulting solid is a porous metal called a sponge.
The pure titanium sponge can then be converted into a usable alloy via a consumable-electrode arc furnace. At this point, the sponge is mixed with the various alloy additions and scrap metal. The exact proportion of sponge to alloy material is formulated in a lab prior to production. This mass is then pressed into compacts and welded together, forming a sponge electrode.
The sponge electrode is then placed in a vacuum arc furnace for melting. In this water-cooled, copper container, an electric arc is used to melt the sponge electrode to form an ingot. All of the air in the container is either removed (forming a vacuum) or the atmosphere is filled with argon to prevent contamination. Typically, the ingot is remelted one or two more times to produce a commercially acceptable ingot. In the United States, most ingots produced by this method weigh about 9,000 lb (4,082 kg) and are 30 in (76.2 cm) in diameter.
After an ingot is made, it is removed from the furnace and inspected for defects. The surface can be conditioned as required for the customer. The ingot can then be shipped to a finished goods manufacturer where it can be milled and fabricated into various products.
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