Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 2 2 ore in a physical form called sponge, which is consolidated to ingot and fur- ther processed to mill products in a manner similar to steelmaking. Tita­ nium metal has a dull silver luster. Powder metallurgy and additive manu- facturing have sparked additional inter- est in producing Ti metal over the past few years [18, 19] . PROPERTIES The two principal categories of ti- tanium alloys include corrosion-resis- tant alloys and structural alloys [4,12,13,16] . Corrosion-resistant alloys are based on a single phase (alpha) microstruc- ture with dilute additions of solid solution-strengthening and alpha-sta- bilizing elements such as O (interstitial), Pd, Ru, and Al (substitutional). These al- loys are used in the chemical, energy, paper, and food processing industries to produce highly corrosion-resistant tubing, heat exchangers, valve hous- ings, and containers. The single-phase alpha alloys provide excellent corrosion resistance, good weldability, and easy processing and fabrication, but have relatively low strength. The beta phase is stabilized by ad- ditions such as Mo, Va, Nb, Fe (substitu- tional), and H (interstitial). A dispersion of alpha in the beta matrix togeth- er with solid solution strengthening of both the alpha and beta phases leads to higher strength alloys referred to as structural alloys. They are divided into four categories: near-alpha alloys, al- pha plus beta alloys, beta alloys, and titanium aluminide (ordered) interme- tallics (based on Ti x Al where x = 1 or 3). Processing and microstructure con- trol are crucial to optimizing the me- chanical properties of structural Ti alloys [4,12,13,16] . In markets served by major U.S. ti- tanium producers, corrosion-resistant alloys comprise roughly 25% of the to- tal output; Ti-6Al-4V accounts for 60%; and all other structural alloys make up the remaining 15%. MANUFACTURING Commercial production of titani- um metal involves chlorination of ru- tile (TiO 2 ) in the presence of coke or another form of carbon [12] . The most im- portant chemical reaction involved is the resulting TiCl 4 known as tickle, pu- rified by distillation and chemical treat- ments, and subsequently reduced to titanium sponge using either the Kroll process (Mg) or Hunter process (Na). ORE CONCENTRATE REFINING The TiO 2 content of ore concen- trates determines further processing steps. High-grade ore such as rutile is refined to pigment-grade TiO 2 via chlo- rination. Lower grade ore is processed via the sulfate route. The chlorination process commercialized by DuPont in the early 1960s, which produces a bet- ter quality pigment, requires less pro- cessing energy than the sulfate process (1800 compared with 2500 kWh/t), and creates less waste discharge [17-19] . The sulfate process produces roughly 6 tons of waste/ton of TiO 2 , compared with 1 ton of waste produced through the chloride process [20] . However, high- grade ore is required for the chloride process; i.e., TiO 2 content > 70% with < 1% MgO and 0.2% CaO, because high MgO and CaO-containing ores clog the chlorinator. In the past, environmental problems forced the industry to shut down sulfate plants and install expen- sive pollution-control equipment. Due to the shortage of high-grade TiO 2 re- serves, the pigment industry must adapt the ore to the chloride process. Fig. 3 − Chloride process for treating high- grade titanium-oxide ore. The trend has been toward ore bene- ficiation. Ore containing 50-60% TiO 2 content is beneficiated by partial re- duction, then leached with sulfuric or hydrochloric acid to yield a concentrate containing > 90% TiO 2 , the so-called synthetic rutile [20, 21] . SULFATE PROCESS In the sulfate process (Fig. 2), il- menite ore is treated with sulfuric acid at a temperature between 150° and 180°C. Undissolved solids are removed and liquid is evaporated under vacuum and cooled. Precipitated FeSO 4 7H 2 O is filtered, and the filtrate is concentrated to 230 g/L. Heating to 90°C hydrolyzes titanyl sulfate to insoluble titanyl hy- droxide by the reaction: TiOSO 4 + 2 H 2 O → TiO (OH) 2 ↓ + H 2 SO 4 Seed crystals are added to ensure the rutile crystal form; otherwise ana- tase is obtained. The precipitate is thor- oughly washed using water and sulfuric acid to remove all traces of discolor- ing elements; e.g., Fe, Cr, V, and Mn. TiO(OH) 2 is finally calcined at 1000°C to TiO 2 [21] . CHLORIDE PROCESS In the chloride process (Fig. 3), high-grade titanium-oxide ore is chlo- rinated in a fluidized-bed reactor in the presence of coke at a temperature Fig. 2 − Sulfate process for treating ilmenite ore [22] .