Kroll process
The Kroll process is invented by William Justin Kroll and patented in 1940, process for obtaining commercially pure titanium.
Ilmenite ( FeTiO3 ) is an important raw material for titanium production. To produce from the ilmenite rutile ( TiO2), is reduced in the arc with carbon. The molten iron is collected at the bottom and is tapped from time to time.
Frequently, however, is industrially directly from natural rutile or from synthetic rutile (obtained by HCl leaching of ilmenite ) from. Previously, the titanium slag (see below) had meaning.
TiO2 ( see: titanium dioxide ) is reacted at temperatures of 750-1000 ° C with chlorine and coke to titanium tetrachloride ( titanium ( IV) chloride ) ( reductive chlorination ).
The titanium chloride, among other things, by fractional distillation and usually a vanadium removal separate from the existing impurities in the ore. The starting material for the Kroll process is pure titanium tetrachloride, which is already in very large amounts for the production of white pigment available. Pure TiCl4 is reduced with magnesium at temperatures of about 800-900 ° C under a protective gas atmosphere (helium, argon) to the metallic titanium.
Magnesium chloride ( MgCl2) is tapped batchwise in the major amount. Based on the principle of the smallest forced more and more titanium is thus simulated. The trapped radicals of magnesium chloride and the residual unreacted magnesium, which is submitted in excess, are either removed from the titanium sponge with hydrochloric acid, or better, since a higher purity is obtained, removed by vacuum distillation.
The Kroll process provides the so-called titanium sponge, a hard, porous mass that needs to be worked out elaborately by the vacuum distillation means of turning or other machining processes from the process plant. Previously this was done even with the jackhammer. The same applies to the Hunter method.
Further processing to a first industrially useful product (ingot ): This titanium sponge must then be pressed into meter-long consumable electrodes (diameter depending on the furnace typically 500 to 1200 mm ), possibly alloying constituents ( such as Al, V) are welded as a strip outside. Most a triple -melting in a vacuum arc furnace is required in order to obtain technically usable, homogeneous Titaningots ( Gießbarren ). The wall material due to the high reactivity of hot and liquid titanium exclusively water-cooled copper can be used.
After the older Hunter method TiCl4 is reacted with Na instead of Mg. Both the formed NaCl and MgCl2 are thermodynamically stable, the reactions are virtually complete and very exothermic.
Technical titanium usually contains 0.2 to 0.3% Fe, and usually 0.2% O not included.
This remains particularly elaborate production with consumption of metallic magnesium also requires the high price.
The production of purer Ti takes place ( in small quantities ) - as with many other metals also - via the Van Arkel -de Boer process.