The major processes for refining and purifying titanium are dependent on keeping the material isolated from the atmosphere under a vacuum to avoid unwanted high-temperature reactions with, and contamination by, air gases during refining.
Vacuum arc remelting and more recently induction skull melting have benefited from the continuing development of vacuum-pump technology and also from recent innovations in the design of specialized vacuum pumping systems. This article reviews the latest developments and techniques in vacuum pumping systems applied to the two main titanium-refining processes.
Vacuum Arc RemeltingThe vacuum arc remelting (VAR) furnace is used for purification of reactive metals such as titanium (Ti). A long electrode is assembled from purified Ti sponge, scrap and clean recycled Ti metal. It is then placed in a vacuum chamber above a water-cooled copper mold that is electrically grounded. A DC arc is struck between the two, which melts the end of the Ti electrode. The electrode is gradually consumed as molten metal drops into the copper mold, and a purified Ti ingot is built up below. The melting process is usually carried out twice to obtain the desired Ti purity and metallurgical structure.
Melting by electric arc under vacuum in VAR provides many advantages over induction melting techniques. There is complete elimination of contact with atmospheric air gases, efficient removal of dissolved gases (e.g., hydrogen and nitrogen) and removal of lighter metallic impurities. Directional solidification through the resulting purified ingot can also be achieved.
The operating pressure range inside the Ti VAR furnace must be between 1x10 -2to 1x10-3mbar or lower, and a substantial vacuum pumping speed must be provided to ensure the complete removal of all of the gaseous impurities arising. The process creates fine dust and particles that can be transported by the gas flow toward the vacuum pumps. Where significant amounts of raw Ti sponge material are used in the electrode there is also the problem of chloride-compound evolution, which can cause significant corrosion issues wherever the exhausted gases contact the atmosphere.
The classical Ti VAR vacuum pumping system includes vapor boosters, which hold very low operating pressures against the high gas loads and high particulate loads. Vapor boosters are a well-established and unique technology, but as “secondary” pumps they need to be backed by other vacuum pumps capable of exhausting pumped gases to atmospheric pressure. A typical Ti VAR system is shown in Figure 1. The backing pumps, typically a pair of mechanical booster plus dry primary pump combinations, are used for initial pumpdown (“roughing”) of the furnace to reach the pressures at which the vapor boosters can start to operate.
The key advantages of this classic system are that the vapor boosters are a cost-effective means of providing a consistently high pumping speed at very low pressures despite the dust contamination coming from the process. Dust particles are simply absorbed into the vapor booster’s oil and sink to the bottom of its oil boiler tank. Furthermore, the use of dry-mechanism primary vacuum pumps in the backing-pump combinations provides good dust handling, consistent vacuum performance and low maintenance compared to traditional oil-sealed (“wet”) primary vacuum pumps. Since dry primary pumps are hot-running, the risk of corrosion from condensation of chloride residues in the pump exhaust is also reduced. Pumping-speed curves for the vapor boosters and backing-pump combinations in a typical VAR vacuum-pump system are shown in Figure 2.
The assembly of the primary Ti electrode also happens under vacuum. The electrode can be new raw material (i.e. clean Ti sponge), together with appropriate alloying additions, or made from carefully inspected and cleaned Ti scrap and off-cuts (called “revert”). Usually, new sponge material is crushed into briquettes that are then welded together to form the long electrode assembly.
Alternatively, many larger revert pieces can be simply welded together into an electrode. Argon plasma welding in a chamber with an inert argon atmosphere is a common method to achieve electrode assembly. To avoid any risk of oxide or nitride formation at the high welding temperatures, it is essential to remove all the air from the chamber and the material before starting to weld. A typical plasma briquette-welding furnace has a vacuum pumping system to provide evacuation of air down to 0.1 mbar and also control chamber argon pressure during the process. A typical example is shown in Figures 3 and 4.
This dry pumping system has two sets of pumps on independent pipe lines with each set protected by a large filter (F1 and F2 in Fig. 3). The filter is a vessel partly filled with dry packing rings (e.g., Raschig or Pall rings), which are ideal for permitting high gas flows while still efficiently trapping the heavy particulate load arising from the plasma-welding process.
Because of the very high surface area of the briquette material, and consequently the very high outgassing load that results on evacuation, a pumping system is needed with a high pumping speed in the 10 mbar to 0.1 mbar region. Two sets of large mechanical booster pumps (HV8000) plus large dry primary pumps (IDX1000) are used.
Both full pump sets are required for the initial evacuation, and the pumpdown usually takes from 30-50 minutes. After this, the furnace is refilled with pure argon to a pressure slightly above atmosphere to ensure that no air can leak in again. The plasma-welding process then takes place, and this can last for several hours. During welding, only one dry primary pump is left running (IDX1000 in Line A in Fig. 3) so that as thermal expansion of the argon atmosphere in the furnace periodically opens the argon-line pressure-relief valve, the vented argon is safely exhausted by this running pump. Finally, at the completion of the process, this pump alone is used to re-evacuate the furnace to remove the argon atmosphere prior to venting with air.
A particular feature of the pump sets used in this application is the use of high-flow gas ballast on the IDX1000 primary pumps, using compressed air. Firstly, this provides a constant gas flow through the IDX1000 to keep the pump internals clean from residual dust and corrosive particles. Secondly, it provides a convenient and highly effective method of reducing the heating effect caused by argon compression and avoiding any risk of excessive pump operating temperatures.
Induction Skull MeltingThe induction skull melting (ISM) process offers several advantages, particularly for titanium investment casting. It has fast become an attractive, high-efficiency process for the lower-cost production of high-quality titanium castings. It is also able to use Ti scrap, revert and sponge in any proportion and produce high-quality results.
ISM uses a vacuum furnace that has a water-cooled, copper crucible inside made of electrically isolated segments that permit an induction field to be generated in the Ti feedstock inside the crucible without heating the crucible itself. Ti is melted and circulated by the induction field, forming a molten dome with solidification underneath, thus creating a solid, outer Ti “skull” containing pure, molten Ti inside. The pure liquid Ti is promptly cast into the desired mold, leaving the solid “skull” behind in the crucible. This skull of purified Ti can then be reused in the next cycle.
The vacuum levels required for ISM are not as demanding as for VAR, but ISM vacuum systems have some special requirements due to the very high purity of the Ti castings being made and to the technological and safety needs of the ISM furnace itself.
There is potential for large amounts of particles and dust, which contain corrosive chloride materials, to be generated. This means that initial turbulent-flow evacuation of the chamber at the start of the process can draw substantial amounts of contamination into the vacuum pumping system as the chamber pressure initially falls from atmospheric pressure down to around 200 mbar. As such, the chamber is evacuated down to 200 mbar using a separate, auxiliary liquid ring pump (LRP) system, since LRPs are suited to operating down to this “rough vacuum” pressure and are very tolerant to large amounts of particulate contamination.
However, to achieve the required deep vacuum for operating the ISM process (typically down to below 0.05 mbar) and to avoid any risks of furnace contamination by back-streaming of pump fluids at these low pressures, only dry-mechanism pumping systems can be considered for the main vacuum pumping system.
The overall system pumping capacity must be sufficient to hold the highest theoretical gas loads from the process to within the acceptable chamber maximum pressure. For a typical ISM furnace this means holding a pressure of less than 0.1 mbar at maximum system gas-flow rate. This requires a three-stage pumping solution. It is a particular challenge when the required pumping capacity is specified in terms of hydrogen (H2), which is a common contaminant of metals since dry-mechanism primary pumps, in particular, can have a low pumping efficiency with hydrogen unless special precautions are observed.
In recent state-of-the-art ISM installations, flexible vacuum pumping systems are employed that can be switched between various ISM and VAR furnaces depending on production demand. An example of such a flexible three-stage vacuum system – using two stages of mechanical booster pumps backed by dry claw mechanism primary pumps – is shown in Figures 5 and 6. This provides vacuum for a plant having one large ISM furnace together with one smaller ISM and a small VAR. The system is sized to provide the required vacuum pumping capacity for the large ISM when all pumps are used together. However, it can also be operated as two independent pumping skids (in Fig. 5 labeled Skid A and Skid B by closure of valves V1 and V2) so that the smaller ISM and the VAR can both be operated simultaneously if production demands require this.
Recent live test results on this system with all pumps running together confirm that an effective hydrogen pumping speed of over 25,000 m3/hour H2at 0.1 mbar is achieved with this design, which is very close to the theoretically predicted H2 pumping-speed value from computer modeling of this pumping system.
Other significant features of this special system include the provision of large dust filters in the gas stream in front of the primary dry pumps (Figure 5 labeled F1 and F2 in front of GV1 and GV2). This location for filtration to protect the primary pumps is ideal since gas pressures are comparatively high here, meaning that pumping-speed losses through the filters are minimal and any dust tends to pass easily through the two stages of mechanical boosters preceding the filters. To avoid long-term residual dust accumulation, there is also a shutdown air-purge cleaning system fitted to primary pumps GV1 and GV2, which admits atmospheric-pressure air through valves V51 and V52 and through the primary pumps for a few minutes prior to shutting them down at the end of the process.
SummaryThe use of the latest dry vacuum pumping technology, together with extensive practical vacuum-system experience on Ti refining processes, now enables more reliable and more cost-effective vacuum pumping solutions to be applied to modern Ti refining plants. This can provide the owner with more efficient and flexible plant operation, which can then lead to a more competitive supply of Ti materials. Such improvements in technology can ultimately provide enhanced opportunities for further diversification of the applications of this important metal.IH
For more information:Contact Dr. Simon Bruce, product marketing manager, Edwards Ltd, UK, Edwards Ltd, Dolphin Road, Shoreham-by-Sea, West Sussex BN43 6PB; e-mail: email@example.com; web: www.edwardsvacuum.com
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