The Kroll Process

October 31, 2012
Jean Van Rensselar
Tip or Best Practice

The Kroll Process

Titanium has traditionally been produced by the Kroll process, a costly method made even more expensive by the fact that it wastes magnesium during the procedure. Steps include extraction, purification, sponge production, alloy creation and forming and shaping – with these steps usually carried out by different processors in different locations.

Extraction
The manufacture receives titanium concentrates from mines. The iron is removed until the product contains about 85% titanium dioxide. The material is heated to 1,652 f and reduced to impure titanium tetrachloride (TiCI4) and carbon monoxide

Purification
The reacted metal is put into large distillation tanks and heated. Impurities are separated using fractional distillation and precipitation. This action removes metal chlorides that include iron, vanadium, zirconium, silicon and magnesium.

Sponge Production
The purified titanium tetrachloride is transferred as a liquid to a stainless steel reactor vessel. Magnesium is added and the container is heated to about 2,012 F. Argon is then pumped into the container. This removes the air and prevents contamination with oxygen or nitrogen. The magnesium reacts with the chlorine to produce liquid magnesium chloride. Since the melting point of titanium is higher than that of the reaction, this leaves a pure titanium solid.

Titanium solid is removed by boring from the reactor and treated with water and hydrochloric acid to remove excess magnesium and magnesium chloride. The resulting solid is a porous metal called sponge.

Alloy Creation
The sponge is mixed with alloys and scrap metal with the exact proportion of sponge-to-alloy material formulated prior to production. The mass is then pressed into compacts and welded together. The result is a sponge electrode that is placed in a vacuum arc furnace for melting and ingot formation. In order to produce a commercially acceptable ingot, it is usually remelted 1-2 additional times. In the U.S., most ingots produced by the Kroll method weigh about 9,000 lbs. (4082 kg) and are 30 inches in diameter.

When the ingot is finished, it is inspected for defects and the surface may be conditioned per customer specifications. From there, the ingot will be shipped to a finished goods manufacturer where it is milled and fabricated.

From: http://www.madehow.com/Volume-7/titanium.html  

 
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