In the United States, practically all zirconium metal is now being made by the Kroll process. This process was an adaptation to zirconium of a similar process for titanium developed by W. J. Kroll. The work of Kroll and metallurgists of the Albany, Oregon, station of the Bureau of Mines culminated in a plant to produce 135,000 kg zirconium/year at the station. A similar plant was operated by the Carborundum Metals Corporation, at Akron, New York. These have been superseded by the plant of the Teledyne Wah Chang Albany Company, at Albany, Oregon, with a capacity in 1978 of 3.4 million kg/year.

The form of the Kroll process used in this plant is believed to be generally similar to the process used at the Bureau of Mines plant, which has been described in detail by Shelton et al. [S3]. The principal steps in the Kroll process as practiced in the Bureau of Mines plant are diown in Fig. 7.11.

Figure 7.11 Kroll zirconium process as practiced at Albany, Oregon.

Production of ZrCl4. Zirconium oxide from the hafnium-separation step was mixed with carbon black, dextrin, and water in proportions 142 Zr02, 142 C, 8 dextrin, and 8 water. The mixture was pressed into small briquettes (3.8 X 2.5 X 1.9 cm) and dried at 120°C in a tray drier. The oxide briquettes were charged to the reaction zone of a vertical-shaft chlorinator lined with silica brick. The charge was first heated by carbon resistance strips until it became conductive. During production, the bed temperature was maintained at 600 to 800°C by an electric current passed directly through the bed. After steady conditions were reached, a reactor 66 cm in diameter produced about 25 kg ZrCLt/h. The ZrCl4 was condensed from the reaction products in two cyclone-shaped aftercondensers in series, and the chlorine off-gas was removed in a water scrubbing tower.

The Wah Chang plant is believed to use an electrically heated fluidized-bed chlorination reactor.

Reduction of ZrCl4. The furnace used for reducing ZrCU to zirconium metal is shown in Fig. 7.12. The outer shell is a stainless steel cylinder 178 cm high and 70 cm inside diameter. An annu­lar trough 5 cm wide and 23 cm deep is welded to the top lip of the cylinder. The top lid of the furnace carries a cylindrical ring that dips into this trough. The trough is filled with a low-melting lead alloy. This arrangement facilitates opening and closing the furnace.

The top lid also carries stainless steel cooling coils, through which air may be circulated, to control top temperatures and prevent loss of ZrCl4 vapor in gases discharged from the furnace. The furnace is provided with three external electric resistance heating elements to provide the heat of sublimation of ZrCU and control temperature distribution.

A stainless steel( reduction crucible rests on the bottom of the furnace. Before a run is started, this is charged with 55 kg of distilled magnesium.

Resting on the top of the crucible is an Inconel can charged with raw ZrCU. The total amount of ZrCU charged here and possibly present on the air-cooling coils from a previous run is 236 kg, an amount that provides a magnesium excess of 10 or 15 percent for the reduction reaction

.Evocuation Tube
в / ^Bleeding Valve

Top Plate Heating Element

eod Seal

eod Seal Heating Element

.Middle — Zone Heating Element Raw Chloride Spocers

etort

Reduction

Baffle

Crucible Lifting Bor

Reduction

Crucible

Lower — Zone Heating Element

Cooling Plugs

ZrCl4 + 2Mg -»• Zr + 2MgCl2

Inconel rods extending into the ZrCL» improve heat transfer.

The detailed operating cycle has been described by Shelton et al. [S3]. A brief summary of the procedure follows. With the reactor first at 300°C and cooling air flowing through the top coils, the furnace is evacuated and flushed with helium three times to remove gases originally present. Temperatures are then raised to 450°C while the internal pressure is kept near atmospheric by bleeding helium. This purges additional gas occluded in the ZrCI* charge. Top temperatures are then reduced to lower the ZrCl4 vapor pressure, while additional helium is fed to hold pressure near atmospheric. The temperature of the reduction crucible containing the magnesium is next raised to 825°C, at which reaction with ZrCLt vapor commences. The ZrCL* transport rate is controlled by the rate at which heat is supplied to the middle-zone heater. The rate is kept as high as possible without raising the temperature in the magnesium reaction zone over 875°C. Completion of reaction is indicated by a fall in pressure, which is countered by supplying additional helium. Heaters are then turned off and the vessel is cooled to 150°C by air blown over the outer reactor surface.

The total cycle time is around 40 h, of which 18 h is for the reduction reaction itself. The product of the reaction is a lower layer of spongy zirconium metal mixed with MgCl2, covered by a layer of frozen MgCl2.

Vacuum distillation of MgCl2. To remove MgCl2 from the zirconium sponge it is necessary to resort to vacuum distillation. Water leaching cannot be used because the finely divided zirconium sponge would become contaminated by oxide corrosion product.

The crucible containing the zirconium sponge and MgClj is transferred to the vacuum distillation retort, shown in Fig. 7.13, where it is supported, upside down, over a perforated, stainless steel funnel. The air in the retort is evacuated, and the crucible is heated to 900 to 920°C to melt the MgCl2, which partially drains off the sponge. Salt still wetting the sponge is distilled at this temperature to a water-cooled jacket inside the retort.

Arc melting. As a last step, the salt-free sponge is fed into an arc-melting furnace and cast into the desired shapes in a water-cooled, copper mold. The furnace atmosphere is helium.

Details of the Rroll process are given in papers by Kroll and his co-workers [КЗ, K4], Shelton [S3], and Lustman and Kerze [LI].

Reduction with sodium. A modified process in which ZrCL was reduced by sodium was used by National Distillers and Chemicals Corporation in Ashtabula, Ohio, during the late 1950s [С1].

Production of hafnium metal. Hafnium metal has been produced from HfOClj by the same methods used in making zirconium. Up to the vacuum distillation step, separate equipment was used than for zirconium, to avoid contaminating the zirconium.