TITANIUM Process Technologies When titanium became available after World War II, many thought that it would become as commonplace as aluminum. That has not happened because titanium remains very expensive, mostly due to the high cost of extraction. Steven J. Gerdemann Albany Research Center Albany, Oregon itanium has a unique set of properties: low density, high specific strength, high tem- perature strength, and exceptional resis- tance to corrosion. Titanium is the fourth Tmost common structural metal in the earth’s crust. Only iron, aluminum, and magnesium are more abundant. More titanium is available than nickel, copper, chromium, lead, tin, and zinc put together. However, the current titanium production system is extremely labor and capital intensive. Titanium is expensive only because the current process for re- fining the ore to metal is a multi-step, high tem- perature batch process. This article will first de- scribe current titanium technology, and will then discuss four of the most promising approaches to reduce the cost of titanium. These include the Kroll, The Guggenheim Museum in Bilbao, Spain, is finished with an ultrathin Hunter, Cambridge, and Armstrong processes. layer of titanium metal. its high purity and the fact that titanium is sepa- Titanium extraction today rated from oxygen. Any process that is proposed Currently all domestic titanium metal produc- to eliminate the chlorination step will have to find a tion begins with rutile (TiO2). Rutile ($0.48/lb Ti) way to replace these functions. is combined with petroleum coke and chlorinated in a fluid bed reactor at 1000°C (1830°F) to produce Kroll process TiCl4 (known as “tickle”). Today’s Kroll process has changed very little from the one DuPont developed to produce tita- TiO2 + 2Cl2 + C —> TiCl4 + CO2 nium in 1948. Briefly, it involves the following steps. A clean and dry stainless steel retort is pumped It is possible to start with other less costly ma- down and filled with argon. Then enough magne- terials such as ilmenite or slag, but both of these sium to reduce all the TiCl4 plus 15% to 30% excess contain more iron and other impurities. The liquid is introduced into the retort. The retort is heated to TiCl4 is purified and then approximately 90% of 800°C to 900°C (1470 to 1650°F), and TiCl4 is slowly the TiCl4 is oxidized back to TiO2 for the pigment fed into the retort. Magnesium reduces TiCl4 ac- industry. Titanium metal producers either purchase cording to the reaction: TiCl4 from a pigment manufacturer (Oremet, which is now shut down) or produce their own (Titanium TiCl4 + 2Mg —> Ti + 2MgCl2 Metals Corporation, Timet). TiCl4 ($1.35/lb Ti) is the starting point for all com- MgCl2 is periodically tapped off as the reduction mercial processes and most proposed new routes. proceeds. After several days, depending on the size The two major reasons for starting with TiCl4 are of the retort, the reaction stops and the retort pres- ADVANCED MATERIALS & PROCESSES/JULY 2001 41 sure rises. At this point, approximately 30% of the Heroult process had on aluminum. Before Hall- initial magnesium charge is still unreacted. The ti- Heroult, aluminum was produced by sodium re- From the tanium metal formed is a porous mass that resem- duction and was more expensive than gold. Now first, the bles a sponge. aluminum costs less than $1.00/lb. However, sig- The retort now contains titanium metal (sponge), nificant differences between aluminum and tita- Kroll unreacted magnesium, and some MgCl2. These im- nium make an electrolytic titanium process much process purities may be removed by either leaching or more difficult. has been vacuum distillation. Vacuum distillation removes For one thing, the melting point of titanium is the unreacted magnesium and MgCl2 by raising 1000°C (1800°F) higher than that of aluminum. Thus, criticized the temperature of the retort and applying a all electrolytic processes tried to date have produced as vacuum. This removes the more volatile magne- solid titanium that results in dendritic structures and sium and MgCl2, leaving behind the titanium a loss of electrolyte due to drag-out. In the electrolytic expensive sponge. Then the reactor is opened and the tita- bath, aluminum has only one stable valence state, and nium is pressed or jack-hammered out. The tita- but titanium has two. These multiple valence states inefficient. nium sponge is sheared to 0.6 cm (0.25 in.) chunks, cause a loss of electron efficiency. But the major any alloying metals are added, and perhaps some problem is that the electrolytic route may not be less scrap titanium. It is then melted to produce an ingot. expensive than the Kroll process because both begin To assure uniformity and remove inclusions, the with TiCl4 ($1.45/lb Ti). Some economic analyses ingot is remelted once or twice more. The original have shown savings over the Kroll process. How- melt adds approximately $1 per pound and each ever, companies that have built pilot scale electrolytic remelt another $0.50. plants (Dow-Howmet, RMI) have not seen these From the first, the Kroll process has been criti- savings in practice. cized as expensive and inefficient. It is a series of Electrolytic processes have been the major area batch steps, many of which are labor intensive. of active titanium extraction research. None of these However, after more than 50 years and many an- processes has resulted in a commercial production nounced new processes, nothing has replaced it. In of titanium. In fact, the pattern has been for com- fact, the Kroll process has changed very little. The panies to spend millions of dollars on a pilot plant major difference is that the retort size has become and then abandon the project. larger and the magnesium reduction and vacuum Part of the problem may be that the development distillation steps are carried out in the same reactor. time for a process is longer that the titanium market The Hunter process is very similar to the Kroll cycle, and none of the pilot plants have managed process, except that magnesium is replaced by to survive a downturn. Another problem may be sodium. Even though the process is similar, the that the market for titanium has grown so slowly Hunter process is slightly more expensive, and con- that there has never been a need for a completely sequently the Hunter process exists today only to new, from-the-ground-up plant. In fact, the number supply a small specialty high-purity powder market. of commercial producers has contracted. In 1958, Few theoretical studies have been made of the at least six U.S. companies were in various stages Kroll process, and consequently much is left to learn of titanium metal sponge production. Today there about it. Some interesting recent work by Okabe et is only one, Timet. This is a bit of a cyclic argument, al. has shown that the Kroll and Hunter reactions because the slow growth is due, in part, to the con- can be interpreted as electrochemical reactions. A sistently high price and cyclic unreliability of the better understanding would probably lead to in- demand. It could also be argued that the commit- cremental improvements in the Kroll process. Pos- ment to electrolytic extraction has been misplaced, sible improvements might include reducing the and that the electrolytic route will never be less amount of excess magnesium required, or reduc- costly than the Kroll process. tion in the amount of titanium downgraded due to The reality is that titanium can be made elec- contamination at the walls, or better control of ni- trolytically. The problem has always been one of tride inclusions. It might even be possible to modify economics. Marco Ginatta of GTT continues to the Kroll or Hunter process to make it continuous. pursue the chloride process and has built a large It is unlikely that a revolutionary improvement in pilot plant in Torino, Italy. It is possible that a com- the Kroll process would result in a dramatic re- mercial electrolytic plant may replace the Kroll duction of the price, but evolutional change re- process someday, but it seems unlikely that any sulting in many small improvements could lead to electrolytic process that begins with TiCl4 will dra- less costly titanium. matically reduce the price of titanium. Electrolytic processes The FFC Cambridge process In 1953, Kroll predicted that an electrolytic route A second, more radical approach toward elec- would produce titanium in 15 years. Electrolytic trolytic reduction announced by Derek Fray of processes were being developed concurrently with Cambridge has created a lot of excitement. In this the Kroll and Hunter process. Why is it that after process, TiO2 is pressed into pellets and becomes over 50 years of development, no commercial elec- the cathode in a 950°C (1740°F) calcium chloride trolytic titanium plants have yet been built? (CaCl2) bath. A graphite electrode is the anode. The promise of electrolytic processes has always When a current is applied, the oxygen is ionized been that they would have the same effect on the and dissolves into the CaCl2 bath. Because the price of titanium that the introduction of the Hall - monovalent oxygen is in solution, the problem of 42 ADVANCED MATERIALS & PROCESSES/JULY 2001 divalent titanium ions is eliminated. This method What will the oxygen content of the product be? has produced titanium with only 60 ppm oxygen How much will it cost to make titanium by the up to the kilogram scale. Armstrong process? Because it also begins with Since this route begins with rutile ($0.48/lb Ti) TiCl4, the raw material costs are the same as with it appears (on paper) that titanium could be pro- the Hunter process.